Low dimensional inorganic materials as electrocatalysts: Experimental and theoretical perspectives
Thapa R., Ghorai U.K., Dey R.S., Jena P., Kawazoe Y.
Editorial, Catalysis Today, 2026, DOI Link
Pyridinic-N Seized Co in Biphasic Nanoarchitecture for Reversible Oxygen Electrocatalysis Enabling Longevous (>1200 h) Aqueous and Dual-Anion Kosmotropic Electrolyte Stabilized High Power Quasisolid-State Zn–Air Battery
Das S., Bolar S., Siddharthan E.E., Pathak A., Thapa R., Phadikar U., Kolya H., Kang C.-W., Kuila T., Murmu N.C., Kundu A.
Article, Small Methods, 2025, DOI Link
View abstract ⏷
Integration of different active sites by heterostructure engineering is pivotal to optimize the intrinsic activities of an oxygen electrocatalyst and much needed to enhance the performance of rechargeable Zn–air batteries (ZABs). Herein, a biphasic nanoarchitecture encased in in situ grown N-doped graphitic carbon (MnO/Co-NGC) with heterointerfacial sites are constructed. The density functional theory model reveals formation of lattice oxygen bridged heterostructure with pyridinic nitrogen atoms anchored Co species, which facilitate adsorption of oxygen intermediates. Consequently, the well-designed catalyst with accessible active sites, abundant oxygen vacant sites, and heterointerfacial coupling effects, simultaneously accelerate the electron/mass transfer and thus promotes the trifunctional electrocatalysis. The assembled aqueous ZAB delivers maximum power density of ≈268 mW cm−2 and a specific capacity of 797.8 mAh gzn−1 along with excellent rechargeability and extremely small voltage gap decay rate of 0.0007 V h−1. Further, the fabricated quasisolid-state ZAB owns a remarkable power density of 163 mW cm−2 and long cycle life, outperforming the benchmark air-electrode and many recent reports, underlining its robustness and suitability for practical utilization in diverse portable applications.
Unveiling of Unpaired Surface Spins Regulated Magnetism in 2D α-Te Nanosheets: an Implication on Magnetoelectric Driven Hydrogen Evolution
Saini D., Krishankant, Mishra H.K., Mondal B., Naskar S., Kanupriya, Prajapati A., Iqbal A., Roy D., Thapa R., Ram S., Mandal D.
Article, Advanced Materials, 2025, DOI Link
View abstract ⏷
Quasi-2D tellurium (Te) unlocks surface spins (of valence 5p4 electrons) of tunable ferromagnetic order and response to strain-engineered electronic properties of widespread applications. In spin–orbit coupling, the inversion symmetry is broken in a 1S0 → 3S1 spin-transition of the ground electronic state, a synergetic pathway to charge spin-order under applied driving forces. The surface magnetism, combined with the ferroelectricity, gives a giant magnetoelectric response (absent in 1S0 bulk Te state) that is explored to boost the H2 evolution reaction (HER) with 2D α-Te as a synergetic catalyst. High-quality 2D α-Te synthesized as nanosheets is ordered primarily along (001) facets at duly enhanced d001 atomic spacing in the 5s2-Te lone pair electrons (diamagnetic) are spaced (Coulomb repulsion) via the 5p2 unpaired spins. Poled 2D α-Te in small fields, such as 30 mT, presents a HER overpotential that is decreased up to 100 mV, while the Tafel slope is declined up to 138 from 211 mV dec−1 for the bulk sample. The electrochemical stability of 2D α-Te is found quite impressive with 93% current retention (71% if non-magnetized) under chronoamperometric conditions. The results present that the 2D α-Te plays a game-changing role towards sustainable energy technologies, spintronics, and next-generation magnetoelectric devices.
Atom-Scale Charge Reorganization for MOF-Driven Electrocatalytic Switching
Kumar H., Dewan A., Dargily N.C., Nayak B., Sk M., Thapa R., Annadata H.V., Mendhe R.M., Ghosh B., Ottakam Thotiyl M.
Article, Advanced Functional Materials, 2025, DOI Link
View abstract ⏷
Achieving dynamic and reversible control over electrocatalytic reactions underpins the chemistry of next-generation energy devices. This work reveals a unique mechanism, atom-scale charge reorganization within a deliberately engineered metal-organic framework (MOF), that enables electrocatalytic switching during dioxygen redox processes. By precisely modulating atomic-level electronic structures, oxidation states and localized charge distributions through interfaces with nitrogen-rich supports, this work realizes a switchable bifunctional catalytic pathway that lowers the oxygen evolution (OER) and reduction (ORR) voltage gap to an exceptionally low 0.77 V. Notably, this modulation facilitates a mechanistic transition from a two- to a four-electron pathway during ORR, significantly enhancing reaction efficiency. This charge-driven reorganization mechanism translates into a high-performance rechargeable air battery, delivering superior power density, cycling stability, and energy efficiency over 100 h of continuous operation, surpassing noble metal-based systems. This work introduces localized charge reorganization as a powerful design principle for reconfigurable and high-efficiency MOF-based electrocatalysts in next-generation energy devices.
Local Enhanced Electric Field Assisted Electrocatalytic Nitrate Reduction to Ammonia Using Ni(TCNQ)2/NF Nanostructures
Mukherjee N., Adalder A., Paul S., Barman N., Thapa R., Mitra K., Urkude R., Ghorai U.K.
Article, Advanced Functional Materials, 2025, DOI Link
View abstract ⏷
Environmentally sustainable electrocatalytic nitrate reduction (NO3RR) is a very promising method for the synthesis of ammonia at room temperature via the complex eight-electron/nine-proton transfer mechanism. Herein, the local electric field-assisted electrochemical NO3RR process is proposed to identify the origin of catalytic activity and charge transfer kinetics resulting from different morphologies of the electrocatalyst. Accordingly, Ni(TCNQ)2/NF nanorods (NRs) and nanotips (NTs) are fabricated on Ni foam as electrocatalysts for the NO3RR. The Ni(TCNQ)2/NF NTs exhibits an impressive ammonia yield of up to 11286.9 µg h−1 cm−2 and a Faradaic efficiency (FE) of 83.7% at −1.0 V versus RHE, representing nearly a 2.2-fold increase in yield compared to the Ni(TCNQ)2/NF NRs. This greater performance is attributed to the local enhanced electric field (LEEF) generated at the tip-like Ni(TCNQ)2/NF NTs. Furthermore, a Zn–NO3− battery is developed here, and Ni(TCNQ)2/NF NTs shows a maximum power density of 2.15 mW cm−2. Experimental and computational findings demonstrate that the geometric and electrical properties of the nanostructures' shape significantly influence the electrochemical NO3RR by enhancing the kinetics of charge transfer. This study seeks to advance research on morphology-dependent electrochemical NO3RR through the strategic control of local electric field intensity in electrocatalysts.
Electronic and energy descriptors for SACs as tri-functional catalysts towards urea formation and unveiling the C–N coupling mechanism
Barman N., Majumder C., Thapa R.
Article, Chemical Science, 2025, DOI Link
View abstract ⏷
Single atom catalysts (SACs) have rapidly emerged as a cutting-edge trend in electro-catalysis for synthesizing nitrogen-based products such as ammonia, nitric acid and urea. In the present study, we considered NO2− as a specific N-based molecule, which participated in the simultaneous reduction with CO2 towards urea formation for 77 SACs. Our investigation demonstrates that among possible nitrogen-containing intermediates generated during the simultaneous electrochemical reduction of CO2 and NO2−, only the NH2 intermediate effectively couples with CO to form urea. In the case of simultaneous reduction towards urea formation, the NH2 free energy serves as an effective energy descriptor for identifying suitable catalysts, exhibiting a strong linear correlation with the limiting potential (R2 = 0.93). Additionally, we observed a strong Brønsted–Evans–Polanyi (BEP) relationship (R2 = 0.99) between the NH2 adsorption energy and the activation energy of the coupling intermediate (CONH2). Furthermore, the out of plane d-sub orbitals (dxz, dyz and dz2) of the transition metal (TM) were analysed to uncover the electronic origins of urea reactivity. Owing to the strong interaction between the d-sub orbitals of the TM and the sp3 hybrid orbitals of NH2, occupancy of the dyz orbitals plays a significant role in determining catalytic activity. This is evidenced by a linear correlation (R2 = 0.73) between orbital occupancy of dyz and NH2 adsorption energy for all systems, identifying it as the electronic origin of urea reactivity. Our interpretation regarding the descriptor is that NH2 sp3 (HOMO) donates a σ-electron to the TM d-orbital (LUMO), while the TM d-orbital (HOMO) donates a π*-electron back to NH2 sp3 (LUMO).
Electronic Descriptor to Identify the Activity of SACs for E-NRR and Effect of BF3 as Electrolyte Ion
Barman N., Kapse S., Thapa R.
Article, ChemSusChem, 2025, DOI Link
View abstract ⏷
Electrochemical nitrogen reduction reaction (e-NRR) is an eco-friendly alternative approach to generate ammonia under ambient conditions, with very low power supply. But, developing of an efficient catalyst by suppressing parallel hydrogen evolution reaction as well as avoiding the catalysts poisoning either by hydrogen or electrolyte ion is an open question. So, in order to screen the single atom catalysts (SACs) for the e-NRR, we proposed a descriptor-based approach using density functional theory (DFT) based calculations. We investigated total 24 different SACs of types TM−Pc, TM-N3C1, TM-N2C2, TM-NC3 and TM-N4, considering transition metal (TM). We have considered mainly BF3 ion to understand the role of electrolyte and extended the study for four more electrolyte ions, Cl, ClO4, SO4, OH. Herein, to predict catalytic activity for a given catalyst we have tested 16 different electronic parameters. Out of those, electronic parameter dxz↓ occupancy, identified as electronic descriptor, is showing an excellent linear correlation with catalytic activity (R2=0.86). Furthermore, the selectivity of e-NRR over HER is defined by using an energy parameter ▵G*H-▵G*NNH. Further, the electronic descriptor (dxz↓ occupancy) can be used to predict promising catalysts for e-NRR, thus reducing the efforts on designing future single atom catalysts (SACs).
Regulating the electronic structure of CoMoO4via La doping for efficient and durable electrochemical water splitting reactions
Arumugam B., Siddharthan E.E., Mannu P., Thapa R., Dong C.-L., Jeffery A.A., Kim S.-C.
Article, Journal of Materials Chemistry A, 2025, DOI Link
View abstract ⏷
Metal molybdates (M′MoO4, M = Fe, Co, and Ni) are recognized as active catalysts for water-splitting reactions. However, their poor electronic conductivity and low intrinsic activity hamper overall water-splitting activity and durability, limiting their widespread applications. Herein, the influence of lanthanum doping on the electrocatalysis of CoMoO4 toward overall water-splitting activity and durability in high-pH media was investigated. Varying La-dopant percentages in the CoMoO4 lattice tuned the electrocatalytic activity, and optimal performance was achieved at 5% La_CoMoO4 for hydrogen (η20 at 0.219 VRHE) and oxygen evolution reactions (η20 at 0.272 VRHE). Notably, La doping in CoMoO4 mitigated significant MoO42− leaching in the electrolyte, maintaining excellent structural integrity and demonstrating high durability for over 45 h in a two-electrode system, demanding a cell potential of only 1.68 V toward overall water splitting in 1 M KOH. Structural characterizations and in situ Raman studies established a dynamic surface reconstruction of active components toward the HER/OER. DFT analyses proved the modified electronic structure of CoMoO4 through La doping, effectively optimizing adsorption energies of reactive hydrogen and oxygen intermediates and boosting the intrinsic activity of CoMoO4 toward hydrogen and oxygen evolution reactions (HER/OER). This work depicts the prospect of rare-earth metal incorporation in non-noble metal-based electrocatalysts to design highly efficient and durable electrocatalysts for electrochemical applications.
Controlling electrocatalytic nitrate reduction efficiency by utilizing dπ-pπ interactions in parallel stacking molecular systems
Bhowmick S., Adalder A., Maiti A., Kapse S., Thapa R., Mondal S., Ghorai U.K.
Article, Chemical Science, 2025, DOI Link
View abstract ⏷
Electrochemical reduction of nitrate to ammonia using electrocatalysts is a promising alternative strategy for both wastewater treatment and production of green ammonia. Numerous tactics have been developed to increase the electrocatalyst's NO3RR activity. Herein, we report a unique molecular alignment-dependent NO3RR performance using α-CuPc and β-CuPc nanostructures as effective electrocatalysts for the ambient synthesis of ammonia. The well-aligned β-CuPc demonstrated an impressive ammonia yield rate of 62 703 μg h−1 mgcat−1 and a Faradaic efficiency of 96%. In contrast, the less well-aligned α-CuPc exhibited a yield rate of 36 889 μg h−1 mgcat−1 and a Faradaic efficiency of 61% at −1.1 V vs. RHE under the same conditions. Scanning tunneling microscopy/spectroscopy (STM/S) confirms that the well-aligned β-CuPc exhibits superior transport properties due to optimal interaction of the Cu atom with the nitrogen atom of parallel molecules (dπ-pπ) in its one-dimensional nanostructure, which is clearly reflected in the electrocatalytic performance. Furthermore, theoretical research reveals that the NO3RR is the predominant process on the β-CuPc catalyst in comparison to the hydrogen evolution reaction, which is verified by gas chromatography, with β-CuPc exhibiting weaker binding of the *NO intermediate at the copper site and a lower overpotential, hence facilitating the NO3RR relative to α-CuPc.
Decoding Dual-Functionality in N-doped Defective Carbon: Unveiling Active Sites for Bifunctional Oxygen Electrocatalysis
Bhardwaj S., Pathak A., Das S.K., Das P., Thapa R., Dey R.S.
Article, Small, 2025, DOI Link
View abstract ⏷
Oxygen electrocatalysis plays a pivotal role in energy conversion and storage technologies. The precise identification of active sites for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for developing an efficient bifunctional electrocatalyst. However, this remains a challenging endeavor. Here, it is demonstrated that metal-free N-doped defective carbon material derived from triazene derivative exhibits excellent bifunctional activity, achieving a notable ΔE value of 0.72 V. Through comprehensive X-ray photoelectron spectroscopy and Raman spectroscopic analyses, the active sites responsible for oxygen electrocatalysis are elucidated, resolving a long-standing issue. Specifically, pyridinic-N sites are crucial for ORR, while graphitic-N are good for OER. A predictive model utilizing π-electron descriptors further aids in identifying these sites, with theoretical insights aligning with experimental results. Additionally, in situ ATR-FTIR spectroscopy provides clarity on reaction intermediates for both reactions. This research paves the way for developing metal-free, site-specific electrocatalysts for practical applications in energy technologies.
From MOF to terbium-doped MOF: Investigating the role of bimetals in hybrid environment towards the sensing mechanism of antibiotic in water
Yazhini C., Rajamani M., Rafi J., ES E., Thapa R., Neppolian B.
Article, Applied Materials Today, 2025, DOI Link
View abstract ⏷
The rate of tetracycline (TC) production and consumption has tremendously increased lately for the treatment of infectious diseases in humans and animals. Their release into the environment through overuse and improper disposal practices has raised serious concerns for the ecosystem deliberating the significance of easy detection approaches towards TC. By virtue of the excellent physical and chemical attributes of Metal-Organic frameworks (MOFs), a Tb-doped MOF was developed for the selective and accurate detection of TC. Remarkably, the strong green luminescence of Tb3+ is quenched efficiently with TC even in the presence of other antibiotics. The sensor material exhibits a highly sensitive turn-off response with an exceptionally low limit of detection of 0.07 μM and a quenching constant value of 1.68 × 104 M-1. The mechanisms for selective quenching of fluorescence towards TC are investigated in detail using theoretical calculations and simulations to demonstrate the occurrence of electron transfer within the system. The detection behavior is also tested with river water samples collected from Chennai rivers which exhibited an excellent recovery of results qualifying Tb@ZB as a promising candidate to be developed into a prototype device to enable facile rapid analysis in real-time samples. The sensing approach provides a crucial ground for monitoring the presence of TC with 87–101 % reliability in wastewater systems.
Graphitic-carbon nitride immobilized Schiff base Palladium(II): Highly efficient electrocatalyst for hydrogen evolution reaction and density functional theory calculations
Nesaragi A.R., Dongre S S., Iqbal A., Thapa R., Balakrishna R.G., Patil S.A., R S.
Article, International Journal of Hydrogen Energy, 2025, DOI Link
View abstract ⏷
Utilizing renewable sources of energy to produce hydrogen by electrocatalytic water splitting has emerged as an achievable answer to the issues associated with fossil fuels. Designing an efficient electrocatalyst for hydrogen evolution reaction (HER) is an important research area and it is essential to develop the catalyst that is electrocatalytically active, stable and economical to overcome the use of high-cost Pt for HER reaction. In this regard, we have synthesized a novel graphitic carbon nitride-based Schiff base modified silane linked palladium (II) nanocatalyst (g-C3N4-Scb@Pd). The g-C3N4-Scb@Pd evaluated for electrocatalytic hydrogen evolution reaction showed excellent catalytic activity exhibiting lowest overpotential of 40.3 mV at 10 mA/cm2 compared to its other counterparts such as g-C3N4-Pd and g-C3N4. The enhanced activity of g-C3N4-Scb@Pd can be ascribed to higher tendency for charge transfer contributed by Schiff base modified silane bonding between Pd and g-C3N4 forming efficient pathway for improved electron migration in addition to synergistic effect of Pd and g-C3N4. Further, the DFT analysis identifies the active sites, electronic structures and analyses the underlying phenomena to elucidate the possible reason for improving the HER performance from g-C3N4 towards Pd@g-C3N4. This work introduces a new approach for designing and engineering the g-C3N4 based efficient electrocatalysts.
Low Potential Electrochemical CO2 Reduction to Methanol over Nickel-Based Hollow 0D Carbon Superstructure
Chongdar S., Chatterjee R., Reza S., Pal S., Thapa R., Bal R., Bhaumik A.
Article, Advanced Energy Materials, 2025, DOI Link
View abstract ⏷
Electrochemical carbon dioxide reduction reaction (CO2RR) to valuable fuels and chemical feedstock is a sustainable strategy to lower the anthropogenic CO2 concentration, thereby dynamising the carbon cycle in the environment. CH3OH on the other hand is undoubtedly the most desirable C1 product of CO2RR. However, selective electroreduction of CO2-to-CH3OH is very challenging and only limited catalysts are reported in literature. Pyrolyzing metal-organic frameworks (MOFs) to generate carbon matrix impregnated with metal nanoparticles, heralds exciting electrocatalytic properties. This study unveiled the morphological evolution of a mixed-ligand Ni-MOF (Ni-OBBA-Bpy) during pyrolysis, to generate Ni nanoparticles anchored 0D porous hollow carbon superstructures (Pyr-CP-800 and Pyr-CP-600). This unique morphology invokes high specific surface area and surface roughness to the materials, which synergistically facilitates the selective electroreduction of CO2-to-CH3OH. In comparison to most of the previously reported Ni electrocatalysts that mainly produced CO, Pyr-CP-800 selectively yielded CH3OH with Faradaic efficiency (FE) of 32.46% at −0.60 V versus RHE (reversible hydrogen electrode) in 1.0 M KOH solution, which is highest among other reported Ni-based electrocatalysts in the literature, to best of our knowledge. Additionally, insights from density functional theory (DFT) calculations revealed that Ni (111) plane to be the active site toward the electrochemical. CO2-to-CH3OH formation.
Architecture of imidazolium-based poly(ionic liquid)s-cobalt hexagonal thin nanosheets for high-energy density, using membrane electrolytes
Narayanan A., Pavan T., Barman N., Naik N.S., Thapa R., Rout C.S., Padaki M.
Article, Journal of Materials Chemistry A, 2025, DOI Link
View abstract ⏷
Transition-metal hydroxides have attracted significant interest as electrode materials for supercapacitors due to their abundant redox activity and excellent electrical conductivity. Herein, we present a novel design and engineering of a hexagonal thin nanosheet of cobalt hydroxide (Co(OH)2) with enveloped imidazolium-based poly(ionic liquid)s (PIL-Br, poly(1-butyl-3-vinylimidazolium bromide)). The presence of PILs in Co(OH)2 influenced morphogenesis control and a high capacitance of 1758 F g−1 at a current density of 2 A g−1 in a three-electrode system. A solid-state free-standing device was developed with a unique electrolyte configuration comprising EMIM-TFSI/PVDF-HFP, which further enhanced device performance. Achieving a high energy density of 212 W h kg−1 at a power density of 1499 W kg−1 underscored its capability to deliver stored energy effectively. Most notably, the device demonstrated exceptional durability, maintaining a capacity retention of 97% even after undergoing 10 000 cycles at 5 A g−1. Density functional theory also indicated the presence of PILs active sites in the composites, thereby promising a new in situ strategy for energy-storage applications.
Exploring electronic and energy descriptors to identify the dual metal center catalyst for the CO2ER towards C2 products
Sk M., Pathak A., Thapa R.
Article, Journal of Materials Chemistry A, 2025, DOI Link
View abstract ⏷
The electrocatalytic CO2 reduction reaction (CO2ER) has sparked immense interest due to its potential to generate valuable multi-carbon (C2) products. The innovative dual active site catalysts (DACs) feature dual-atom sites that create the perfect geometric environment for two CO molecules to bond simultaneously. In this study, we spotlight transition metal (TM) dimers anchored on nitrogen-doped graphene, referred to as TM1TM2@NGr, as our primary focus. Analysing 54 candidates, we evaluate their stability via negative binding energy and ICOHP values, enhanced by ab initio molecular dynamics (AIMD) calculations. Three systems namely WIr@NGr, WFe@NGr, and WW@NGr demonstrate remarkable selectivity for ethanol production due to their low free energy difference (ΔG*CO dimer-2*CO) which offers low overpotentials of 0.47, 0.49, and 0.5 V, respectively. We analysed 71 electronic parameters to identify key factors influencing the free energy difference (ΔG*CO dimer-2*CO) and found 12 electronic descriptors strongly correlated due to the Pearson correlation coefficient (r) being larger than 0.8. Among these, the occupancy of the dxz orbital (dxz_occ) and the downward channels of dxz and dz2 orbitals (dxz_down_occ and dz2_down_occ) were the most effective for predicting the energy difference, demonstrating the highest r values. This highlights their importance as key descriptors for ΔG*CO dimer-2*CO and corresponding C2 production. Furthermore, three DACs have been identified as highly effective for hydrogen evolution reactions (HER), while thirteen DACs show CO2ER to methane production. Our research offers vital insights into the catalytic mechanisms of DACs, paving the way for discovering cost-effective candidates for efficient CO2 conversion into valuable C2 products.
Single-Crystalline Fe2O3 on Reduced Graphene Oxide as an Anode Material for All-Solid-State Supercapacitors
Saxena M., Patil S.A., Reza S., Das A., Thapa R., Misra P.K.
Article, ACS Applied Nano Materials, 2025, DOI Link
View abstract ⏷
Research often focuses on improving cathode materials to boost the energy density of charge storage devices to levels comparable to batteries, while anode materials have been less frequently explored. This work presents the iron oxide/reduced graphene oxide (Fe2O3/rGO) composite synthesis by a one-pot hydrothermal method for possible utilization as an anode material in battery applications. The crystalline Fe2O3 nanoparticles were allowed to grow over two-dimensional (2D) rGO sheets as a growth template using the hydrophilic groups as nucleation centers. The unique structure of Fe2O3/rGO composites significantly enhances ion transport efficiency, optimizing active materials’ utilization and thus improving overall performance in energy storage applications. The optimized Fe2O3/rGO-3h composite electrode demonstrates an excellent specific capacity of 233 mAh g-1 at a current density of 2 A g-1. The flexible ASSC device shows a high capacity of 41 mAh g-1 at a current density of 1 A g-1 with an energy density of 27.8 Wh kg-1 at a power density of 750 W kg-1. Furthermore, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves of flexible devices remained stable upon bending up to 180° and during series or parallel connections. A laboratory prototype of a CR-2032 coin-type ultracapacitor can power a blue, red, green, and yellow LED to run continuously on just one charge. Further, density functional theory (DFT) was employed to investigate the charge transfer, electronic structure, and binding interactions of Fe2O3/rGO composites, elucidating their potential as high-performance anode materials for supercapacitors. This work unlocks an avenue for designing and fabricating Fe2O3/rGO composites as promising anode materials by utilizing rGO as a template for the next generation of supercapacitors with improved energy storage performance.
Structural engineering of bimetallic NiMoO4 for high-performance supercapacitors and efficient oxygen evolution reaction catalysts
Patil S.A., Jagdale P.B., Iqbal A., Reza S., Jinagi M., Rajput P., Sfeir A., Royer S., Thapa R., Samal A.K., Saxena M.
Article, Journal of Materials Chemistry A, 2025, DOI Link
View abstract ⏷
Advancing energy storage and conversion research on 2D nanostructures hinges on the critical development of bifunctional electrodes capable of effectively catalyzing oxygen evolution reactions and facilitating charge storage applications. Although metal oxide materials have been shown to be promising electrode materials for energy storage and conversion, an easy and reliable synthesis strategy for achieving a 2D morphology to fully utilize their electrochemical potential has not yet been achieved. Herein, we report the synthesis of NiMoO4 self-assembled, ultrathin nanosheets through ionic layer epitaxy with precise control over the Ni : Mo composition ratio. X-ray absorption spectroscopy reveals a uniform radial distance shift in NiMoO4, indicating the homogeneous distribution of Ni and Mo in equal proportions. The optimized 1 : 1 NiMoO4 nanosheet device exhibits a high areal capacitance of 4.93 mF cm−2 with promising stability (20 000 cycles). Furthermore, the OER activity of ultrathin 1 : 1 NiMoO4 exhibits an overpotential (η10) of 318 mV and a Tafel value of 51 mV dec−1, suggesting fast reaction kinetics. This investigation reveals a promising possibility for developing high-performance electrode materials using 2D metal oxides, thereby achieving high material efficiency.
Harnessing Bio-Inspired Axial Coordination to Boost Synergistic Effects for Enhanced Bifunctional Oxygen Electrocatalysis
Samui S., Iqbal A., Thapa R., Dey R.S.
Article, Small, 2025, DOI Link
View abstract ⏷
Strategic alteration of the chelating atoms around the metal center can modify the electronic band structure of the electrocatalyst, improving its performance in oxygen evolution and reduction reactions (OER/ORR). Advancements in the development of catalysts with heteroatoms and axial modifications in the coordination sphere are mostly limited to single-molecule electrocatalysts or elevated temperature-mediated pyrolysis approaches for oxygen electrocatalysis. Inspired by biological catalytic systems with axial coordination, a pyrolysis-free strategic methodology is adopted for the synthesis of an iron-metaled covalent organic polymer matrix axially laminated over cobalt-based metal-organic framework through an imidazole moiety. Precise engineering of coordination atoms in synthesized core-shell material, featuring dual metal sites with distinct neighboring atom exhibits mutual synergy due to the presence of bridging imidazole moiety between two metal sites. Modulated synergism navigates the electronic structure such that it favors specific reactant adsorption on specific metal sites during bifunctional O2 electrocatalysis as confirmed through in situ Raman spectroscopy and in situ attenuated total reflection infrared (ATR-IR) spectroscopy. Through dynamic correlation between the in-situ studies and modified d-band center obtained theoretically, the pivotal role of axial coordination linkage mediated synergism favoring ORR/OER process via target-specific reactant adsorption is demonstrated.
Harnessing self-powered and photoresponsive biomechanical activity sensors by exploring the piezo-phototronic effect in lead-free layered halide perovskite/PVDF composites
Kumar P., Paul T., Sahoo A., Singh M., Pathak A., Thapa R., Banerjee R.
Article, Journal of Materials Chemistry A, 2025, DOI Link
View abstract ⏷
Developing flexible, wearable, efficient, and self-powered electronic devices based on piezoelectric nanogenerators aspires to be a sustainable solution to renewable energy harvesting and storage. We report on a lead-free halide perovskite Cs3Sb2I9 and polyvinylidene fluoride (PVDF) based composite device capable of scavenging energy from routine biomechanical activities. Regulated incorporation and optimization of Cs3Sb2I9 into the PVDF matrix increased the electroactive phase of the device to ~82% with a piezoelectric coefficient of 7.48 pm V-1. The champion device produced an open circuit output voltage of 85 V and a current of 2.6 µA. Furthermore, the device generated approximately ~1.26 µW cm-2 of power density when connected to a 0.8 MO resistor, sufficient to operate portable electronic gadgets. We tested the device for its energy generation capabilities under simple human biomechanical movements such as hand hammering, finger tapping, elbow bending, knee bending, and toe pressing. To demonstrate the versatility of the nanogenerator device, we also tested its energy generation and storage capabilities by charging capacitors up to ~2.2 V. The device exhibited impressive durability and repeatability over 10 000 cycles, underscoring its potential as a promising solution for addressing the energy demand of portable and Internet of Things (IoT) devices through piezoelectric nanogenerators. Work function calculations using density functional theory demonstrated that the composite exhibited a reduced work function compared to individual components, indicating favorable electron emission characteristics. We also realized the piezo-phototronic effect in the composite using a self-powered photodetector, which exhibited an increment of 63% in the photocurrent, offering potential for piezotronic and optoelectronic devices.
Ternary Heteroatom-Doped Carbon As a High-Performance Metal-Free Catalyst for Electrochemical Ammonia Synthesis
Bhardwaj S., Alli S.J., Barman N., Thapa R., Dey R.S.
Article, ACS Applied Materials and Interfaces, 2025, DOI Link
View abstract ⏷
The electrochemical nitrogen reduction reaction (NRR) has garnered much attention, but the major challenge remains with efficient electrocatalysts. Metal-free carbonaceous materials, doped with heteroatoms and structural defects, present a promising alternative to metal-based catalysts. This study introduces a novel strategic stepwise synthesis strategy of defective nitrogen-doped carbon material, further doped with secondary heteroatoms boron and fluorine (FBDG). These secondary atoms in combination create additional active sites for nitrogen adsorption and activation and suppress the hydrogen evolution reaction (HER). The synergistic effect of three heteroatoms and induced defects in the catalyst enhances electron-donor behavior, improving π bonding within the carbon framework and facilitating the electron transfer processes during NRR, resulting in a significantly high Faradaic efficiency of 38.1% in the case of metal-free electrocatalysts. The theoretical calculation reveals that FBDG possesses a sufficient charge density to reduce nitrogen at a low overpotential following an alternating free energy pathway. The reaction intermediates are thereby identified by in situ ATR-FTIR studies. For the rapid screening of ammonia, we used a rotating ring disk system (RRDE) and did a kinetic study. The high efficiency, stability, and cost-effectiveness of FBDG position it as a strong contender for sustainable ammonia production and pave the way for future advancements in NRR.
ZIF-67 templated Co3O4/NiCo2O4@Mn0.2Cd0.8S p-n heterojunction with a perfect d-band alignment boost photocatalytic hydrogen evolution reaction
Mehmood S., Kengne Fotso L.E., Sk S., Iqbal A., Thapa R., Pal U.
Article, International Journal of Hydrogen Energy, 2025, DOI Link
View abstract ⏷
Designing suitable heterojunctions effectively addresses challenges like rapid electron-hole recombination, limited mobility, restricted absorption, and insufficient active sites. Thus to improve the photocatalytic performance, we synthesized a p-n heterojunction photocatalyst, Co3O4/NiCo2O4@Mn0.2Cd0.8S, by coupling ZIF-67-derived p-type Co3O4/NiCo2O4 double-shelled nanocages with n-type Mn0.2Cd0.8S nanoneedles via a template-assisted method. Analytics revealed the judicious anchoring of the Mn0.2Cd0.8S over the Co3O4/NiCo2O4 surface, reinforcing the photocatalytic activity. The resultant heterostructure exhibits superior photocatalytic performance, achieving an HER rate of ∼14.34 mmol h−1 g−1 with an AQY of ∼35.1 %. This represents an enhancement of ∼72-fold compared to pristine Mn0.2Cd0.8S. The synergistic interplay within the heterostructure, facilitated by abundant active sites, enhanced light absorption, and an efficient charge transfer channel at the p-n heterojunction interface, promotes efficient photoexcited charge separation and transfer. Furthermore, the DFT calculations reveal that the incorporation of NiCo2O4 and Mn0.2Cd0.8S into the Co3O4 framework significantly reduces the HER overpotential from |ΔGH| = 0.32 eV for pristine Co3O4 to 0.20 eV for the Co3O4/NiCo2O4@Mn0.2Cd0.8S heterostructure. This enhancement is attributed to the optimized charge distribution at the active sites and a downward shift in the d-band centre from −2.15 eV to −2.30 eV, which weakens the adsorption of reaction intermediates, thereby accelerating HER kinetics.
Unlocking the Oxygen Evolving Activity of Molybdenum Nickel Bifunctional Electrocatalyst for Efficient Water Splitting
Nsanzimana J.M.V., Jose V., Sk M., Reddu V., Xiaogang L., Dangol R., Hao R., Huang Z., Yan Q., Thapa R., Maiyalagan T., Wang X., Lee J.-M.
Article, Small, 2025, DOI Link
View abstract ⏷
Earth-abundant transition metal-based catalysts with exceptional bifunctionality for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are greatly desired. Alloyed catalysts, such as molybdenum-nickel (MoNi), are known to demonstrate enhanced HER activity, yet suffer from low OER performance. To realize improved functionality, elemental doping can be an effective approach, giving rise to synergistic interactions between incorporated metal species, optimizing surface adsorption of target intermediates, and promoting reaction. Herein, the enhanced OER performance of the MoNi catalyst while simultaneously boosting HER activity via incorporating a small amount of iron and chromium into MoNi (Mo-Ni(FeCr)) is demonstrated. For an optimized Mo-Ni(FeCr) catalyst, in 1.0 m potassium hydroxide electrolyte, an overpotential of only 11 and 179 mV for HER and OER, respectively, are required to afford a current density of 10 mA cm−2. For the overall water splitting, a current density of 20 mA cm−2 is reached at 1.489 V. The DFT calculations demonstrated that the inclusion of Fe and Cr in a molybdenum-nickel catalyst reduced the limiting potentials for both OER and HER, unlocking efficient bifunctionality activity for water splitting. These findings signify the improved electrocatalytic performance of, amongst the most active bifunctional electrocatalysts.
Atomically Dispersed Cu-Ni Dual-Metal Sites on g-C3N4 for Synergistic Enhancement of Photocatalytic Hydrogen Evolution
Islam H., Jaksani B., Iqbal A., Varangane S., Annadata H.V., Ghosh B., Sarma B.B., Thapa R., Pal U.
Article, ACS Applied Energy Materials, 2025, DOI Link
View abstract ⏷
Single-atom catalysts (SACs) offer a promising strategy to enhance light utilization and charge carrier dynamics in photocatalytic hydrogen evolution. However, the uniform dispersion and stabilization of single-atom active sites remain significant challenges. Herein, we report the fabrication of atomically dispersed Cu and Ni dual-metal sites anchored on graphitic carbon nitride (CuNi-g-C3N4), achieving an outstanding hydrogen evolution rate of 1275 μmol g-1 h-1 under visible light irradiation. The synergistic interaction between Cu and Ni bimetallic dual-atom catalysts (DACs) modulates the electronic structure of g-C3N4, creating active sites and suppressing charge recombination, as confirmed by density functional theory (DFT) calculations. X-ray absorption spectroscopy (XAS) analysis verifies the atomic dispersion of the Cu and Ni sites, revealing their interactions with the carbon nitride framework. Photoluminescence (PL) spectroscopy and electrochemical impedance spectroscopy (EIS) further demonstrate enhanced charge separation and reduced recombination in CuNi-g-C3N4. The catalyst exhibits excellent stability and maintains its photocatalytic activity over multiple reaction cycles, underscoring the potential of DACs for sustainable hydrogen production. This study provides insights into the rational design of dual single-atom catalysts for the efficient conversion of solar energy to hydrogen.
Advanced tarnish-resistant silver alloys using Cu, Al, Zn, and Be: composition optimization and surface passivation
Kozhakkattil H., Sk M., Thapa R., VinodKumar G.S.
Article, Applied Surface Science Advances, 2025, DOI Link
View abstract ⏷
A novel class of tarnish-resistant silver alloys containing Cu, Al, Zn, and Be was developed. While Cu is a conventional alloying element in sterling silver (Ag-7.5 wt. %Cu), the addition of Al, Zn, and Be was aimed at forming stable surface oxides to inhibit Ag₂S formation that tarnishes silver alloy surface. The silver alloys produced were subjected to Passivation Heat Treatment (PHT) under oxygen atmosphere, promoting the formation of protective oxide layers. The XRD, SEM/EDX, and XPS characterization confirmed the formation of oxides contributing to tarnish resistance. Accelerated tarnish tests and UV–Visible reflectance spectroscopy demonstrated that Ag-3.5Cu-2Zn-1.9Al-0.1Be alloy exhibited strong resistance to tarnishing, having maximum reflectance values in the range of 60–70 %. The trace addition of Be was pivotal in controlling oxidation by creating a barrier for the diffusion of oxygen during PHT, preventing CuO related fire stains and ensuring tarnish resistance. The adsorption energy ratios of sulphur and oxygen of the silver alloys were studied computationally. The lower value of the ratio is implicative of a preference for oxidation over sulphidation. The value obtained is 0.373 for Ag-3.5Cu-2Zn-1.9Al-0.1Be, which is the least, and it is due to the presence of appropriate amounts of Zn, Al, and Be in the composition.
Ultrathin 2D Ni/Co Hydroxide Heterostructures for High Energy Density Flexible Microsupercapacitor
Patil S.A., Jagdale P.B., Barman N., Sfeir A., Pathak M., Royer S., Thapa R., Samal A.K., Saxena M.
Article, ChemSusChem, 2025, DOI Link
View abstract ⏷
Assembling 2D ultrathin nanosheets into vertical heterostructures offers significant potential for advanced energy storage due to enhanced active sites, improved ion diffusion, and increased electrical conductivity, leading to superior ion/electron transport, higher energy density, and improved rate performance. In case of transition-metal hydroxides, to overcome with challenges such as random assembly, complex synthesis, instability, and poor interfacial contact is critical. This study synthesizes large area, ultrathin, 2D Nickel/Cobalt hydroxide vertical heterostructures (nickel as the top layer) using a wet chemical process, achieving 32% higher areal charge storage compared to cobalt/nickel hydroxide vertical heterostructures, 57% higher than Ni(OH)2 and 330% higher than individual Co(OH)2. The synergistic interaction between nickel and cobalt hydroxides contributes to a high volumetric capacity (710 mAh cm−3) and energy density (285 mAh cm−3) in symmetric devices. The flexible microsupercapacitor retains 75% capacitance after 15,000 cycles and demonstrates stability under bending up to 135°, with a volumetric capacity of 393 mAh cm−3. Density functional theory simulations complement experiments, revealing interaction energy and electronic state redistribution near the Fermi level. This integrated approach serves as a guide for enhancing electrochemical properties in 2D heterostructures, aiding in the development of next-generation, high-performance energy storage materials.
Synthesis of Asymmetric Multidentate Azo-Based Redox-Active Ligand and Its Coordination Compounds with Ruthenium(II) for the Development of Efficient Electrocatalysts for Enhanced Hydrogen Evolution Reaction
Sharma S., Ghosh S., Jangra R., Siddharthana E.E., Sankar M., Thapa R., Bandyopadhyay A.
Article, ACS Applied Energy Materials, 2025, DOI Link
View abstract ⏷
This research focuses on the strategic design of efficient Ru-based electrocatalysts for hydrogen evolution reactions in an acidic medium. An asymmetric azo ligand was synthesized and used as a building block to prepare two Ru(II) compounds by varying the metal:ligand ratio. All of the newly developed compounds were characterized through different characterization techniques to confirm their structures and to understand the structure–property relationship. The newly synthesized azo-based ligand and both Ru(II) compounds demonstrated reversible electrochemical properties, which are prime requirements for developing a new electrocatalyst. The electrocatalytic performance of the Ru(II) Complex and Ru(II) Polymer for the hydrogen evolution reaction (HER) was evaluated using linear sweep voltammetry (LSV), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and chronopotentiometry in a 0.5 M aqueous H2SO4solution. It is possible to vary the metal:ligand ratio to produce a Ru(II) Complex (L:M::1:0.5), where a single Ru(II) center is coordinated to two ligands and a Ru(II) Polymer (L:M::1:3), where the ditopic ligand is working as a bridge between 2/3 different Ru(II) metal centers. This synthetic route was chosen to investigate the influence of bridging ligands and alterations in the coordination sphere surrounding the central metal ion as well as the impact of higher concentrations of the metal center on electrocatalytic performance, as the metal centers serve as active sites for the transfer of electrons to H+ions in an acidic medium. The Ru(II) Polymer, with more metal centers than the Ru(II) complex, demonstrated a lower onset potential (−0.162 V vs −0.340 V), lower overpotential at 10 mA/cm2(−0.202 V vs −0.435 V), and a smaller Tafel slope (60 vs 96 mV/dec), showing easier electron transport and a different HER mechanism. EIS experiments show that the Ru(II) Polymer has a lower HER electron transfer resistance than the Ru(II) Complex.
Pyridine Homopolymers Axial Ligation on Cobaloxime for Efficient Hydrogen Production
Shelake S.P., Iqbal A., Pal M., Ghorai S., Kshirsagar S.D., Indla N.R., Thapa R., Sainath A.V.S., Dutta A., Pal U.
Article, Small, 2025, DOI Link
View abstract ⏷
Photocatalytic hydrogen (H2) production by mimicking natural light-harvesting complexes offers a sustainable route to solar fuel generation. Here, a pyridine-based homopolymer-chelated cobalt complex is reported as an efficient and stable catalyst for H2 production via water splitting. This study introduces a series of reversible addition-fragmentation chain transfer (RAFT)synthesized homopolymers with pyridine moieties that axially coordinate to a cobaloxime core. This study investigates the effect of linker position (meta vs para) on proton reduction affinity in cobaloxime catalysts. The catalysts demonstrate potential for H2 generation under visible light, with repetitive runs and maintain morphological integrity. Both photochemical and electrochemical H2 production results align with and reinforce the catalytic efficacy of the system, achieving an H2 generation rate of up to 42.99 mmol h−1 g−1 for P4VP-Co and 29.58 mmol h−1 g−1 for P3MP-Co in a neutral aqueous solution with a sacrificial electron donor. The superior electrochemical activity of P4VP-Co over P3MP-Co is due to the higher electroactive surface area in P4VP-Co. The density functional thoery (DFT) studies reveal the active site for the reaction and examine the role of homopolymers in enhancing H2 evolution reaction. This study presents a unique example of a cobaloxime core chelated with electron-donating polymers for renewable energy conversion applications.
Unlocking the potential of quinoline-based glycopolymers for photoreforming hydrogen production
Shelake S.P., Iqbal A., Indla N.R., Sutar D.N., Saha S., Raghava Reddy K., Thapa R., Aminabhavi T.M., Sainath A.V.S., Pal U.
Article, Applied Catalysis B: Environmental, 2025, DOI Link
View abstract ⏷
Hydrogen gas is considered a clean fuel generated from renewable energy-rich resources such as natural biomass (carbohydrates, plastics, and food waste) through photoreforming process. However, reports on artificially tuned photoreforming substrates for hydrogen production are limited. Herein, synthesis of new homopolymers and diblock copolymers via a facile route is proposed using reversible addition-fragmentation chain transfer (RAFT) method. These homopolymers and diblock copolymers contain quinoline-based segments and glucose and maltose monomers as building blocks. These glycopolymers are characterized by NMR, FT-IR spectra, molecular weights by size exclusion chromatography (SEC) and thermal properties by TGA and DSC techniques. The regulated morphology of Cd-ZIF-8 photocatalyst drives the photoreforming of polymers exhibiting a 5-fold higher H2 production compared to pristine ZIF-8 with hydrophilic deacetylated glycopolymer. Cd-ZIF-8 in the presence of PAMQ-b-PMDG copolymer exhibited an improved H2 production of 1.26 mmol g−1 h−1 with an AQY (apparent quantum yield) of 3.09 % under simulated sunlight, closely aligned with copolymer hole-scavenging properties. Density functional theory (DFT) identified the modification of electronic structure of ZIF-8 by incorporing Cd create active sites and enhance the interactions with quinoline-glucose polymers as SEDs, which improved the H2 production performance, while isotopic studies confirmed the source of H2. The current research proved that synthetically tuned photoreforming substrates could transform renewable energy conversion.
Electrochemical Synthesis of Urea-Ammonium-Nitrate (UAN) Fertilizer via Dual Reduction of CO2 and Nitrate
Adalder A., Mitra K., Barman N., Thapa R., Urkude R., Das S., Ghorai U.K.
Article, Small, 2025, DOI Link
View abstract ⏷
The urea-ammonium-nitrate (UAN) fertilizers are the optimal supply of nutrients, which promotes healthy plants and reduces the need for frequent applications. Using manganese phthalocyanine (MnPc) supported by amine-functionalized graphene (AFG) catalyst via a unique electrochemical method that greatly improves the catalytic performance for the UAN synthesis process. During CO2 and NO3− co-reduction (CO2 + 2NO3− + 18H+ + 16e− → CO(NH2)2 + 7H2O), the MnPc-AFG catalyst demonstrated the urea production rate of 397.61 µg h−1 mgcat−1, as well as a 27.06% Faradaic efficiency (FE) at –1.0 V versus RHE. Simultaneously, the yield rate of ammonia synthesis from NO3− reduction [NO3− + 9H+ + 8e− → NH3 + 3H2O] is 433.95 µg h−1 mgcat−1 and exhibiting a 52.13% Faradaic efficiency at –1.0 V versus RHE. The electrolyte still contains unreacted nitrate, which raises the possibility of selective UAN production. In situ Fourier transform infrared spectroscopy (FTIR) confirmed the formation of C─N and N─H bonds during electrolysis. Density functional theory (DFT) analysis provided the overall reaction mechanism and free energy profile for the formation of UAN production. This sustainable development opens up new possibilities for the manufacture of electrochemical UAN fertilizers.
Acousto-Electric Conversion by the Piezoelectric Nanogenerator of a Molecular Copper(II) Complex
Haldar R., Naskar S., Mondal B., Iqbal A., Thapa R., Mandal D., Shanmugam M.
Article, Advanced Materials, 2025, DOI Link
View abstract ⏷
The conversion of sound waves into electrical energy holds immense potential in various real-life applications, particularly biomedical devices, smart security sensors, and noise pollution detectors. Yet, the field is largely underexplored, due to the limited availability of materials that operate efficiently at low frequencies of sound waves. Piezoelectric nanogenerators (PENGs), which generate electric charges through deformations caused by sound-induced pressure variations, emerge as promising candidates for acoustoelectric conversion. However, the rigidity and toxicity, of traditional piezoelectric bulk oxide-based PENGs make them unsuitable for wearable electronics and healthcare monitoring devices. As an alternative, we present an efficient, flexible PENG and acoustic nanogenerator (AcNG) based on a molecular ferroelectric [Cu2(L-phe)2(bpy)2(H2O)] (BF4)2.2H2O (1) complex with an impressive output peak-to-peak voltage of 4.94 V and an acoustoelectric conversion of 40 mV from 60 Hz soundwave is disclosed. Leveraging the sensitive low-frequency detection limit of this AcNG combined with a Machine Learning (ML) approach, voices can be distinguished with a surprising accuracy of 95%. Additionally, these devices enable rapid capacitor charging (within 10 s), highly sensitive pressure sensing (low as 4 kPa), and detecting human physiological motion, holding promise for their applications in biometric voice recognition, enhanced national security (AI-driven voice-recognition), and biomedical diagnostics.
High-Performance All-Pseudocapacitive Asymmetric Supercapacitor Device Based on a Spinel Co3O4/MWCNT Nanocomposite with Theoretical Insights
Saxena M., Pathak M., Patil S.A., Sk M., Thapa R., Sahoo S.K., Rout C.S., Misra P.K.
Article, ACS Applied Energy Materials, 2025, DOI Link
View abstract ⏷
The increasing global dependence on energy consumption makes exploring innovative high-performance energy storage solutions more crucial than ever. Supercapacitors are ideal for bridging the gap between traditional capacitors and batteries. A straightforward hydrothermal synthesis approach was used to fabricate the Co3O4/MWCNT nanocomposite as an electrode material. Electrochemical studies show that the Co3O4/MWCNT composite delivers a high specific capacitance of 1000 F/g at a current density of 16 A/g current density. An all-pseudocapacitive asymmetric configuration using Fe2O3-rGO as the anode demonstrates a high specific capacitance of 93.35 F/g at 4 A/g, along with an energy density of 38 Wh/kg and a power density of 7612 W/kg. The asymmetric device exhibits improved cycling stability, with 84% retention in capacitance and a Coulombic efficiency of 97% over 5000 cycles. Density functional theory was employed for a theoretical analysis of the energy storage potential of pure Co3O4and Co3O4/MWCNT composite structures, focusing on structural and electrical properties. Combining Co3O4nanostructures with a 1D MWCNT produces synergistic effects and provides a scaffold conducive to high-performance energy storage devices. Co3O4exhibits a wide range of electrochemical properties, with various forms, porosities, and textures. The surface morphology, increased surface area, and porosity, along with cubic crystal structure features, are critical for the electrochemical performance of Co3O4-based electrodes. The study aims to improve electrode stability and efficiency by optimizing the morphology, porosity, and surface characteristics of Co3O4. It offers key insights into the structure–property relationship and supports the development of scalable, durable electrode materials for next-generation hybrid supercapacitors with high energy and power densities.
Acetylene-Linked Octupolar Conjugated Microporous Polymers for CO2 Photoreduction to Methanol: Effect of Acceptor and π-Bridge Modulation
Chatterjee R., Reza S., Samui S., Thapa R., Bhaumik A.
Article, Angewandte Chemie - International Edition, 2025, DOI Link
View abstract ⏷
The development of efficient, metal-free photocatalysts for solar-driven CO2 reduction to methanol is promising for alleviating energy and environmental issues, but achieving high selectivity and conversion efficiency without sacrificial agents or co-catalysts remains a challenge. In this work, we report a series of acetylene-linked specialized donor–acceptor (D–A) type conjugated microporous polymers (CMPs) designed with tailored electronic structures to investigate their efficacy in photocatalytic CO2 reduction to methanol in an aqueous NaOH solution under visible light irradiation. Significantly, the optimized porous polymer TTT-DEBP, featuring a strong electron-accepting triazine ring and an extended π-conjugated diethynyl biphenyl (DEBP) system, achieved a higher CH3OH production rate of 30.8 µmol g−1h−1 with 90.3% selectivity and excellent recyclability. Experimental and theoretical investigations revealed that the synergistic effect of triazine, biphenyl, and acetylene moieties of the porous network reduces exciton binding energy, enhancing charge separation and transfer, and reduces charge recombination for improved photocatalytic performance.
Palladium-enhanced copper nitride anti-perovskite nanocrystals boosting CO2 electroreduction to C2 products
Chen J., Chowdhury M.H., Ghosh S., Thapa R., Bertino M., Tibbetts K.M., Wang W.-N.
Article, Chemical Engineering Journal, 2025, DOI Link
View abstract ⏷
Addressing the dual challenges of energy sustainability and climate change, carbon dioxide electrochemical reduction (CO2 ECR) emerges as a promising technology for converting CO2 into valuable hydrocarbons. This study presents a palladium-enhanced copper nitride (Cu3N) catalyst that significantly boosts the efficiency of CO2 ECR towards multicarbon (C2+) products with only 0.3 % Pd loading. By leveraging the structural stability of anti-perovskite Cu3N, known for its ability to accommodate dopants, we hypothesized that doping in anti-perovskite Cu3N would modulate its electronic structure and improve catalytic activity. Guided by tolerance factor analysis for anti-perovskite Cu3N, we incorporated Pd into anti-perovskite Cu3N in varying amounts to modulate intermediate adsorption during the CO2 ECR reaction. Characterization of the synthesized nanocrystals by transmission electron microscopy and X-ray diffraction confirmed the uniform incorporation of palladium and the preservation of the Cu3N structure. Electrochemical tests and density functional theory (DFT) calculations demonstrated that palladium doping significantly shifts the selectivity towards ethylene and ethanol, with a notable increase in the faradaic efficiency for C2 products at low palladium loadings. These findings underscore the potential of palladium-enhanced Cu3N nanocrystals as a superior catalyst for the sustainable production of high-value chemicals and fuels from CO2. This study not only elucidates the impact of bimetallic interactions in CO2 electroreduction but also showcases the precise control over product selectivity offered by strategically doped electrocatalysts.
Engineering CoNi₂S₄/Ni₃S₂ heterostructures: A joint experimental-theoretical study for high-performance solid-state supercapacitors
Sengupta S., Reza S., Arulraj R., Thapa R., Kundu M.
Article, Chemical Engineering Journal, 2025, DOI Link
View abstract ⏷
Supercapacitors have emerged as promising energy storage devices due to their high-power density and long cycle life, making them attractive alternatives to conventional energy storage systems. Among various electrode materials, transition metal sulfides, particularly cobalt nickel sulfides, have gained significant attention due to their superior electrical conductivity and rich redox chemistry. In this study, we synthesized cobalt nickel sulfides (CoNi₂S₄/Ni₃S₂) on nickel foam via a facile two-step hydrothermal method to investigate their electrochemical performance in two different morphologies: nanosheets and nanoarrays. Electrochemical characterization demonstrated that CoNi₂S₄/Ni₃S₂ nanoarrays exhibited enhanced performance compared to their nanosheet counterparts, with an areal specific capacitance of 8203.63 mF cm−2 at 30 mA cm−2 over 6000 cycles. The superior performance of the nanoarrays was attributed to their unique vertical alignment, providing more electroactive sites and reducing ion diffusion resistance. To further elucidate the electronic properties of CoNi₂S₄/Ni₃S₂, density functional theory (DFT) calculations were performed. The results indicated that the heterostructure exhibited an increased density of states near the Fermi level compared to pristine CoNi₂S₄, leading to improved charge storage capability. Quantum capacitance analysis revealed that the CoNi₂S₄/Ni₃S₂ heterostructure possessed higher capacitance values over a range of electrode potentials, corroborating the enhanced electrochemical properties observed experimentally. Furthermore, work function analysis suggested improved electronic conductivity in the hybrid structure, further supporting its superior charge transfer characteristics. The hybrid solid-state device fabricated by coupling of CoNi₂S₄/Ni₃S₂ electrodes with activated carbon, delivered an energy density of 0.204 mWh cm−2 at a power density of 8 mW cm−2. Impressively, even at a high-power density of 40.0 mW cm−2, it retained an areal energy density of 0.039 mWh cm−2.
S- and N-Enriched Cyclophosphazene- and Triazine-Based Inorganic–Organic Hybrid Nanoporous Electrocatalyst for Nitrate Upcycling to Ammonia
Balhara S., Adak M.K., Kumar P., Sk M., Thapa R., Das J., Mohanty P.
Article, Energy and Fuels, 2025, DOI Link
View abstract ⏷
Heteroatoms-enriched (S = 46 wt % and N = 24 wt %) cyclophosphazene- and triazine-moiety-based hybrid nanoporous material (CTHM-2) is reported as an efficient metal-free catalyst for the electrochemical nitrate (NO3–) upcycling to ammonia (NH3). The electrocatalyst exhibits a high NH3yield rate of 43 μg h–1mgcat–1(2.6 mmol h–1gcat–1) and a faradaic efficiency (FE) of 42%, at a low potential of −0.8 V vs RHE at the mass loading of 1.6 mg. A fast kinetics marked by a low Tafel slope of 48 mV dec–1in the CTHM-2 electrocatalyst is attributed to the Hads-mediated pathway, validated using TBA as a H* scavenger. Density functional theory simulations validate the mechanism with NO* as a key intermediate, confirming the reaction pathway for NO3RR. The sulfur enrichment in the framework enhances charge delocalization, which stabilizes Hadsspecies to facilitate NO3–and intermediate adsorption. CTHM-2 demonstrates consistent NO3RR activity up to 5 cycles and sustained 17 h of continuous operation under varied experimental conditions.
Boosting the Simultaneous Conversion of Glycerol and CO2 to Lactate and Formate Using ZrO2-Supported NiO Catalyst
Bhattacharjee S., Pulikkeel U., Amoli V., Sk M., Thapa R., Chowdhury B., Muller T.E., Chinthala P.K., Bhaumik A.
Article, Advanced Functional Materials, 2025, DOI Link
View abstract ⏷
Glycerol, a by-product of biodiesel production, and CO2, a major greenhouse gas, are abundant but underutilized feedstocks. Their simultaneous conversion into formic acid and lactic acid presents an innovative and sustainable approach to addressing environmental challenges. Formic acid, a versatile compound in multiple industries, and lactic acid, a versatile platform chemical used in food, pharmaceuticals, and biodegradable plastics, hold immense commercial value. In this work, a NiO─ZrO2 catalyst synthesized through incipient wetness impregnation is employed to achieve the simultaneous conversion of CO2 and glycerol in an alkaline medium. Comprehensive characterisation of the catalyst using PXRD, Raman spectroscopy, XPS, BET surface area, CO2/NH3-TPD, H2-TPR, and UHR-TEM analysis revealed its unique properties, including weak Lewis acid sites critical to its performance. Under optimal reaction conditions, 200 °C, 40 bar CO2, and KOH as the base, the catalyst achieved yields of 3.26 mmol of formate and 11.20 mmol of lactate. The synergistic interaction between NiO and ZrO2, along with the in situ formation of carbonate salts, is key to the high efficiency. An initial economic assessment demonstrates the commercial viability of co-producing formic and lactic acid, with the glycerol price and the efficiency of converting the formate and lactate salts to the corresponding acids being critical factors for the economic feasibility of the process.
Low Temperature Complexation Approach for Immobilization of Single Copper Atom Catalyst in Stacked Polytriazine for Click Cycloaddition Reaction
Giri P.K., Rawat A., Sk M., Swain B., Thapa R., Mohanty P.
Article, Small Methods, 2025, DOI Link
View abstract ⏷
A significant research gap in the field of synthesis of single atom catalysts (SACs) is addressed by developing a low-temperature complexation approach to stabilize the single metal atoms on stacked polytiazine matrix (g-C3N4) with a good metal loading. Unlike conventional high-energy (400–700 °C) and time-intensive (120–300 min) methods typically used for embedding SACs in g-C3N4 matrices, the present synthesis utilizes a facile, microwave-assisted method that operates at a low temperature of 140 °C and completes within 30 min. Comprehensive analysis reveal that complexation of the Cu2+/Cu+ ions with nitrogen in the polytriazine structure facilitates layer stacking. Specifically, Cu⁺ ions promote sheet formation in co-ordination with two nearby N atoms, while Cu2+ ions stabilize the stacked layers of the polytriazine framework through co-ordination with four N atoms. The resulting SAC exhibits a Cu metal loading up to 3.5 wt.%, with a specific surface area (SABET) of 330 m2 g−1 and pore size distribution centered at 1.9 and 5 nm. The SAC demonstrates excellent catalytic performance for click cycloaddition reactions under base-free conditions, with a high turnover frequency (TOF) of 120 h−1, a broad substrate scope, and reusability across seven cycles without detectable Cu leaching, making it a promising SAC for triazole synthesis.
Chromium Cation-Induced Self-Reconstruction of a Stable and High Performance Boride-derived Electrocatalyst for Oxygen Evolution Reaction
Ogolla C.O., Kasper M., Puthiyaparambath M.F., Farahbakhsh N., Thapa R., Thandavarayan M., Killian M.S., Butz B., Nsanzimana J.M.V.
Article, Small, 2025, DOI Link
View abstract ⏷
The rational design of efficient electrocatalysts for the oxygen evolution reaction (OER), holds the key to advancing the overall electrolytic water splitting performance. Here, a scalable one-pot synthesis of a stable chromium─iron nickel boride (Cr─FeNiB) electrocatalyst is reported for OER in which nickel-boron sites are micro-environmentally modified through interactions with iron and chromium. Comprehensive, correlative electrochemical, structural, and chemical analyses reveal the formation of amorphous-crystalline core-shell structures that transform into nanosheets upon activation with enhanced water oxidation catalytic activity. The enhanced catalytic performance is attributed to the chromium-induced chemical self-reconstruction of the catalyst, which facilitates favorable OER kinetics, increased turnover frequency, and a synergistic effect between metal and boron constituents. Density Functional Theory (DFT) calculation showed that chromium incorporation effectively shifts the d-band centers (ɛd) closer to the Fermi level and narrows the metal-d/boron-p band center gap (Δɛd-p) (Ni2B 1.10 eV → FeNiB 0.71 eV → Cr─FeNiB 0.55 eV) ultimately enhancing OER activity. Accordingly, the Δɛd-p is established as a key electronic descriptor for predicting and optimizing OER performance. These findings pave the way for a better understanding of metal boride-derived electrocatalysts and also contribute to the development of efficient, stable, earth-abundant, non-noble metal catalysts for water oxidation.
Enhancing host-guest interactions through interfacial modulation of IRMOF-MXene hybrids: A detailed study on the significance of accessible functional groups in electrochemical detection
Daniel M., Rafi J., Reza S., Thapa R., Mathur S., Neppolian B.
Article, Materials Research Bulletin, 2025, DOI Link
View abstract ⏷
Metal-organic frameworks (MOFs) offer excellent structural tuneability that enables selective host-guest interactions, but integrating them with conductive substrates like Ti3C2Tx MXene can reduce accessibility to functional groups. This work empirically analyzes the influence of synthesis strategy on the electrochemical performance of amino-functionalized IRMOF 3-MXene hybrids for dopamine (DA) sensing, restricting their availability for DA interaction and reducing sensing efficiency. In contrast, the post-synthetic hybrid retains free –NH₂ groups, enabling effective DA preconcentration, which allows subsequent electron transfer to the conductive Ti3C2Tx. This results in enhanced electrocatalytic response, with a synergistic index of 1.12, high sensitivity (263.8 µA mM−1 cm−2), and a low detection limit (56.4 nM) towards DA detection. The Schottky barrier formed at the MXene/IRMOF 3 interface modulates the charge transfer dynamics. Theoretical adsorption energy calculations further validate the experimental observations, highlighting the critical role of free and accessible functional groups in optimizing host-guest interactions for enhanced electrochemical performance.
Low-frequency hydromechanical energy triggered piezocatalytic activity of MoS2 nanosheets for sustainable removal of Cr (VI)
Ghosh T., Pathak A., Dhang S., Mondal J., Paul S., Adalder A., Thapa R., Saha S.
Article, Nano Energy, 2025, DOI Link
View abstract ⏷
Piezocatalysis is an emerging technology where a piezoelectric material under mechanical stress in water produces reactive oxygen species (ROS) that are effectively utilized for environmental remediation applications. However, unavailability of high-power mechanical energy source like ultrasound in nature, restricts the wide-spread deployment of piezocatalysis for real-life applications. Herein, we demonstrate successful utilization of low-power hydromechanical energy for ROS generation and consequent reduction of Cr (VI) through MoS2 nanosheets based piezocatalyst. MoS2 nanosheets are grown on cotton fabric (MoS2@CF) via facile solvothermal method and further characterized by XRD, XPS, FESEM and HRTEM techniques. With the application of mechanical force, the developed MoS2 nanosheets based piezocatalyst exhibits high Cr (VI) reduction efficiency, achieving complete reduction of 100 ppm Cr (VI) in just 50 min. A piezocatalytic filter is designed with MoS2@CF for mimicking piezocatalysis under real-life low-frequency water flow. The designed piezocatalytic filter demonstrates almost 99 % reduction of Cr (VI) in 9 hrs at a flow rate of 2 L/min. Piezocatalytic Cr (VI) reduction activity has been elucidated in light of mechanical stress induced band bending and the free carrier separation phenomena. The undertaken strategy highlights the great potential of piezocatalysis technology for harnessing low-power mechanical energy source in nature for wastewater treatment.
Corrigendum to “Design and fabrication of nickel lanthanum telluride microfibers for redox additive electrolyte-based flexible solid-state hybrid supercapacitor” [Journal of Energy Storage 65 (2023) 107286] (Journal of Energy Storage (2023) 65, (S2352152X23006837), (10.1016/j.est.2023.107286))
Bhol P., Jagdale P.B., Barman N., Thapa R., Saxena M., Samal A.K.
Erratum, Journal of Energy Storage, 2025, DOI Link
View abstract ⏷
The authors regret the incorrect image “Fig. 2” was inserted during the submission process. We would like to request the replacement of this figure with the correct version of Fig. 2, which is attached for your kind consideration. Please note that this correction does not affect the results, interpretation, or conclusions of the paper.[Figure presented] The authors would like to apologise for any inconvenience caused.
A phosphite-incorporated crystalline–amorphous cobalt-based electrocatalyst via surface-confined chemical reduction for efficient hydrogen evolution reaction
Nsanzimana J.M.V., Ogolla C.O., Puthiyaparambath M.F., Farahbakhsh N., Sakalli Y., Hepp M., Frohne J., Scheld W.S., Thapa R., Killian M.S., Butz B.
Article, Journal of Materials Chemistry A, 2025, DOI Link
View abstract ⏷
Targeted phase engineering of nanomaterials through non-equilibrium synthesis strategies provides a rich platform for the development and tuning of nanostructured catalysts with outstanding properties and advanced stability. Unconventional phases or even complex heterostructures play a crucial role in the performance of catalysts for efficient water electrolysis. By utilizing a controlled, iterative reduction synthesis route, a tailored binder-free cobalt boride-phosphite electrode (CoxB-[0.2]P–O) with mixed crystalline–amorphous phases is developed. The tailored cobalt boride chemistry by phosphites enables the alteration of the d electron distribution of cobalt, bringing the H* adsorption energy into the optimal range and thereby enhancing the HER activity. Comprehensive microstructure and spectroscopic analyses proved the success of the one-pot strategy to modulate the microenvironment chemistry of cobalt by incorporating boron and phosphite. Moreover, the tailored formation of nanostructures with locally varying morphology by the co-existence of amorphous and crystalline phases on the nanometer scale is confirmed. This approach facilitates the rational design for tuning the metal boride-based catalysts' activity for hydrogen evolution by tailored chemistry of the metal active centers and thus the phase engineering of similar nanomaterials while avoiding the necessity of any thermal post-treatment processes.
Harnessing the Trade-Off between CoFe/Fe3C Interfacial Junction with Unparalleled Potential Gap of 0.58 V for Reversible Oxygen Electrocatalysis: Application toward Liquid and Solid-State Zn-Air Batteries
Das S., Pathak A., Phadikar U., Kuila C., Maji A., Kuila T., Murmu N.C., Thapa R., Kundu A.
Article, Advanced Functional Materials, 2024, DOI Link
View abstract ⏷
Effective integration of multiple active moieties and strategic engineering of coordinated interfacial junctions are crucial for optimizing the reaction kinetics and intrinsic activities of heterogeneous electrocatalysts. Herein, a simple integrated heterostructure of biphasic Co0.7Fe0.3/Fe3C embedded on in situ grown N-doped carbon sheets is constructed. Rationally designed CoFe/Fe3C-T2 owns more accessible active sites and interfacial junction effects, cooperatively boosting the electron and mass transfer, needed for multifunctional electrocatalysis. Leveraging the synergistic effect of dual active sites, CoFe/Fe3C-T2 demonstrates outstanding oxygen electrocatalytic activity in alkaline medium with an ultra-low potential gap of 0.58 V, surpassing the recently available state-of-the-art catalysts. Moreover, CoFe/Fe3C-T2 air-electrode achieves a high peak power density of 249 mW cm−2, a large specific capacity of 808 mAh g−1 and excellent cycling stability for aqueous Zn-air batteries. Remarkably, the solid-state flexible ZAB also exhibits satisfactory performance, showcasing an open-circuit voltage of 1.43 V and a peak power density of 66 mW cm−2. These outstanding results push this catalyst to the top of the list of non-noble metal-based electrode materials. This work offers a viable method for using the active-site-uniting strategy to create double-active-site catalysts, which may find real-time applications in energy conversion/storage devices.
C2 Product Formation over the C1 Product and HER on the 111 Plane of Specific Cu Alloy Nanoparticles Identified through Multiparameter Optimization
Iqbal A., Tripathi A., Thapa R.
Article, Inorganic Chemistry, 2024, DOI Link
View abstract ⏷
C2 products are more desirable than C1 products during CO2 electroreduction (CO2ER) because the former possess higher energy density and greater industrial value. For CO2ER, Cu is a well-known catalyst, but the selectivity toward C2 products is still a big challenge for researchers due to complex intermediates, different final products, and large space of the catalyst due to its morphology, plane, size, host surface etc. Using density functional theory (DFT) calculations, we find that alloying of Cu nanoparticles can help to enhance the selectivity toward C2 products during CO2ER with a low overpotential. By a systematic investigation of 111 planes (which prefer the C1 product in the case of bulk Cu), the alloys show the generation of C2 products via *CO-*CO dimerization (* indicates adsorbed state). It also suppresses the counter-pathway of hydrogenation of *CO to *CHO, which leads to C1 products. Further, we find that *CH2CHO is the bifurcating intermediate to distinguish between ethanol and ethylene as the final product. We have used simple graphical construction to identify the catalyst for CO2ER over HER, and vice versa. We have also defined the case of hydrogen poisoning and projected a parity plot to recognize the catalyst for C2 product evolution over the C1 product. Our study reveals that Cu-Ag and Cu-Zn catalysts selectively promote ethanol production on 111 planes. Moreover, an edge-doped 2SO2 graphene nanoribbon as the host layer further lowers the barrier and selectively promotes ethanol on Cu38- and Cu79-based alloys. This work provides new theoretical insights into designing Cu-based nanoalloy catalysts for C2 product formation on the 111 plane.
Advanced electrocatalysts for NRR and HER: Experimental and computational design and development
Thapa R., Bhaumik A., Ghorai U.K., Jena P.
Editorial, Catalysis Today, 2024, DOI Link
N-Heterocyclic Carbene Moiety in Highly Porous Organic Hollow Nanofibers for Efficient CO2 Conversions: A Comparative Experimental and Theoretical Study
Bhattacharjee S., Tripathi A., Chatterjee R., Thapa R., Mueller T.E., Bhaumik A.
Article, ACS Catalysis, 2024, DOI Link
View abstract ⏷
Global warming and climate change are two severe environmental dangers brought on by the steady rise in the carbon dioxide (CO2) concentration in the atmosphere. Thus, in order to reduce this problem, it is essential to find an efficient material for high CO2 capture that can simultaneously exhibit good catalytic activity for CO2 utilization into useful chemicals. Herein, we report the synthesis of N-heterocyclic carbene-based porous organic polymers (NHC-01 and NHC-02) using the Friedel-Crafts reaction with the imidazolium salt and bi-phenyl. Among the two porous polymers, NHC-01 exhibited outstanding stability, high flexibility, and high BET surface area (1298 m2 g-1). NHC-01 material displayed a high CO2 uptake capacity of 2.85 mmol g-1 under 1.0 bar pressure at 273 K. NHC-01/02 has been utilized as a metal-free organocatalyst for the CO2 conversion reaction due to its high surface area, high CO2 absorption capacity, and as it bears the NHC moiety in the organic network. NHC-01 selectively reduced CO2 to methanol via hydrosilylation with complete conversion of silane under atmospheric CO2 pressure. Furthermore, the catalyst also shows good catalytic activity toward N-formylation and reductive cyclization reactions, which showed good yields up to at least four catalytic cycles. The reaction mechanisms are also studied by theoretical simulation using density functional theory (DFT), which shows that intermediates have the appropriate free energy level for the catalyst to promote the reaction with a low energy barrier.
Engineering lithium nickel cobalt manganese oxides cathodes: A computational and experimental approach to bridging gaps
Rajkamal A., Sharma A., Pullagura B.K., Thapa R., Kim H.
Review, Chemical Engineering Journal, 2024, DOI Link
View abstract ⏷
Lithium-ion batteries (LIBs) have transformed our envisioned future into a reality where induction motor engines power electric vehicles (EVs). While LIBs offer impressive advantages compared to other energy storage systems for EVs, they face practical deployment challenges in performance, cost, and scalability. One critical component of LIBs that has garnered significant attention is the cathode, primarily due to its high cost, stemming from expensive cobalt metals and limited capacity, which cannot meet the current demand. However, layered lithium nickel cobalt manganese oxide (NCM) materials have achieved remarkable market success. Despite their potential, much current research focuses on experimental or theoretical aspects, leaving a gap that needs bridging. Understanding the surface chemistry of these oxides and conducting operando observations is crucial. Combining advanced surface analysis techniques with theoretical calculations (viz., quantum mechanics) is proposed to bridge this knowledge gap. This review delves into recent performance achievements (viz., projected driving performance, current EVs model, and battery specifications), challenges, and opportunities associated with various NCM materials as cathode materials. It also explores cutting-edge developments in experimental and theoretical techniques that analyze battery operations, address frontier challenges, and provide novel insights. Furthermore, the review comprehensively discusses the concept of single-crystal (SC) NCM and its practical implications in EVs. Finally, the review provides an outlook on future guidelines for designing NCM cathodes for LIBs, emphasizing the convergence of experimental and computational/theoretical approaches to achieve superior performance.
Restricting Anion Migrations by Atomic Layer-Deposited Alumina on Perovskite Nanocrystals while Preserving Structural and Optical Properties
Rathod R., Kapse S., Pal D., Das M.R., Thapa R., Santra P.K.
Article, Chemistry of Materials, 2024, DOI Link
View abstract ⏷
Lead halide perovskite (LHP) nanocrystals (NCs) offer an easy tunability of band gap via anion exchanges, enabling their usage in tandem optoelectronic devices with graded band gaps. However, instantaneous anion migrations at the interfaces of different LHP layers impede the formation of well-defined interfaces. We deposited an ultrathin alumina (Al2O3-x) layer at the interface of CsPbBr3-CsPbI3 NC films by using atomic layer deposition (ALD) and demonstrated that low-temperature ALD alumina has negligible impact on the structural or optical properties of CsPbBr3 NCs except agglomeration. ALD alumina can restrict anion migration for months but cannot be prevented permanently. The rate of anion migration significantly decreases with an increase in the Al2O3-x layer thickness on CsPbBr3 NC films, which follows first-order kinetics. Density functional theory (DFT) calculations showed that the iodide ion can migrate through oxygen vacancies in the Al2O3-x layer with an activation energy of 1.54 eV. This strategy provides new insight into fabricating halide perovskite-based tandem optoelectronics devices.
Crystallinity and interfacial Mo–N–C bond engineered MoS2 embedded graphitic nitrogen doped carbon hollow sphere for enhanced HER activity
Chanda K., Bairi P., Maiti S., Tripathi A., Thapa R., Ghosh S., Panigrahi K., Roy D., Sarkar R., Chattopadhyay K.K.
Article, International Journal of Hydrogen Energy, 2024, DOI Link
View abstract ⏷
To restrain fossil fuel depletion, the need of the hour is the development of efficient, durable, and non-precious electrocatalysts for the hydrogen evolution reaction (HER). The hierarchical nanostructure consisting of transition metal dichalcogenides and graphitic heteroatom-doped carbon with an abundant interfacial M–N–C catalytic site is highly demanding for electrolytic applications. Herein, we report crystallinity-engineered ultrathin MoS2 nanosheets hierarchy over nitrogen-doped graphitic carbon (NC) hollow spheres as a promising material for HER. The well optimized NC@MoS2 demonstrates superior HER activity with a low onset overpotential of 9 mV and an overpotential of 145 mV at a current density of −10 mA cm−2. It exhibits low Tafel slope of 39 mV dec−1 and excellent chronoamperometric stability. Superior HER activity originates from interfacial Mo–N–C bonds. Density functional theory (DFT) calculations unveil that Mo–N–C bonds between MoS2 and NC matrix ease electronic transportation and further diminish Gibbs free energy for HER.
Deciphering the bridge oxygen vacancy-induced cascading charge effect for electrochemical ammonia synthesis
Biswas A., Barman N., Nambron A., Thapa R., Sudarshan K., Dey R.S.
Article, Materials Horizons, 2024, DOI Link
View abstract ⏷
Oxygen vacancy engineering has recently been gaining much interest as the charging effect it induces in a material can be used for varied applications. Usually, semiconductor materials act poorly in electrocatalysis, particularly in the nitrogen reduction reaction (NRR), owing to their inherent charge deficit and huge band gap. Vacancy introduction can be a viable material engineering route to make use of these materials for the NRR. However, a detailed investigation of the vacancy-type and its role for the structural reorientation and charge redistribution of a material is lagging in the field of NRRs. This work thus focuses on the synthesis of oxygen vacancy-engineered SnO2 with a gradual structural transformation from in-plane (iov) to bridge-type oxygen vacancy (bov) density. Consequently, the electron occupancy of the sp3d hybrid orbital changes, leading to an upshifted valence band maxima towards the Fermi level. This has a profound effect on the nature of N2 adsorption and the extent of N ≡ N bond polarization. Sn atoms adjacent to the bov are found to have a fair density of dangling charges that accomplish the NRR process at a comparatively low overpotential and determine the binding strength of the intermediates on the active site. The obscured yet stable reaction intermediates are thereby identified with in situ ATR-IR studies. A restricted hydrogen evolution reaction Faradaic on the Sn-site (favored over O-atoms) results in a Faradaic efficiency of 48.5%, which is better than that reported in all the literature reports on SnO2 for the NRR. This study thus unveils sufficient insights into the role of oxygen vacancies in a crystal as well as electronic structural alteration of SnO2 and the effect of active sites on the rate kinetics of the NRR.
Understanding the photo-sensitive essence of organic-inorganic hybrids for the targeted detection of azithromycin
Yazhini C., E.S E., Thapa R., Neppolian B.
Article, Chemosphere, 2024, DOI Link
View abstract ⏷
Being a macrolide antibiotic, the antiviral and anti-inflammatory properties of azithromycin (AZM) were taken advantage of during the COVID-19 pandemic which led to the overuse of AZM resulting in excessive release and accumulation in the waterways and ecosystem causing unpleasant threats to humankind. This demands the necessity for a highly sensitive material being capable of recognizing AZM in wastewater. Mindful of the optical attributes of organic ligand structures, we have constructed a hybrid material by chelating Zn2+ with pyridyl benzimidazole (PBI). The prepared sensor material ZnPBI was characterized using various microscopic and spectroscopic techniques including XRD, FT-IR, HR-SEM, HR-TEM, etc. The proposed sensor material exhibited proficient detection performance selectively towards AZM with a very low detection limit of 72 nM. Two linear ranges between 0 – 70 μM and 70–100 μM were observed corresponding to two different mechanistic pathways. To the best of our knowledge, the utilization of a metal-organic complex (MOC) for the fluorometric detection of AZM has not been explored so far. It is creditworthy to cite that the long-term structural stability of the sensor material was maintained for 100 days in water and it can be reused three times without any depreciation in the sensing activity. A combination of energy transfer routes, adsorption and electrostatic interactions for AZM detection are described experimentally and theoretically which provides insights into the role of MOC as sensing probes.
Iron phthalocyanine hollow architecture enabled ammonia production via nitrate reduction to achieve 100 % Faradaic efficiency
Sarkar S., Adalder A., Paul S., Kapse S., Thapa R., Ghorai U.K.
Article, Applied Catalysis B: Environmental, 2024, DOI Link
View abstract ⏷
Eight electron nitrate (NO3–) reduction to ammonia (NH3) offers a cost–effective and energy efficient route than the Haber–Bosch process. The state of art electrocatalysts for nitrate reduction shows potential activity, albeit suffering from poor Faradaic efficiency, kinetically sluggish multi electron–proton process, and competitive hydrogen evolution reaction. Herein, we present a hollow iron phthalocyanine (FePc) rectangular nanotube (RNTs) electrocatalyst with 100 % Faradaic efficiency and 35067.09 µg h–1 mgcat–1 ammonia yield, which is 3.5 times higher than that of FePc nanorods. One-of-a-kind hollow nanostructure has Fe-N4 active motif sites necessary for NO3– activation, dissociation, specific intermediate formation, and interaction, resulting in the energy-efficient generation of NH3, according to in-situ research combined with theoretical analysis of molecular scale reaction mechanism. These unique results, coupled with the monitoring of pH changes in real time during electrolysis, open up new possibilities for progressive ammonia production with reduced carbon footprint.
Site specific descriptors for oxygen evolution reaction activity on single atom catalysts using QMML
Siddharthan E.E., Ghosh S., Thapa R.
Article, Journal of Materials Chemistry A, 2024, DOI Link
View abstract ⏷
Descriptors are properties or parameters of a material that are used to explain any catalytic activity both computationally and experimentally. Such descriptors aid in designing the material's properties to obtain an efficient catalyst. For transition metals, the d-band center is a well-known descriptor that shows a Sabatier type relationship for several catalytic reactions. However, it fails to explain the activity when considering the same metal active site with a varying local environment. To address this, density functional theory was used for single atom catalysts (SACs) embedded on armchair and zigzag graphene nanoribbons (AGNR and ZGNR). By varying the anchoring nitrogen atoms' orientation and considering pristine and doped cases, 432 active sites were used to test the oxygen evolution reaction (OER) activity. It was observed that S and SO2 dopants help in reducing the overpotential on Co-SAC (η = 0.28 V). Along with the d-band center, a total of 105 possible descriptors were individually tested and failed to correlate with the OER activity. Furthermore, PCA was employed to narrow down unique descriptors, and machine learning algorithms (MLR, RR, SVR, RFR, BRR, LASSO, KNN and XGR) were trained on the two obtained descriptors. Among the models, SVR and RFR models showed the highest performances with R2 = 0.89 and 0.88 on test data. This work shows the necessity for a multi-descriptor approach to explain OER catalytic activity on SACs and the approach would help in identifying similar descriptors for other catalytic reactions as well.
Nanoconfinement Effect in Water Processable Discrete Molecular Complex-Based Hybrid Piezo- and Thermo-Electric Nanogenerator
Kumar A., Haldar R., Siddharthan E.E., Thapa R., Shanmugam M., Mandal D.
Article, Nano Letters, 2024, DOI Link
View abstract ⏷
Water-processable hybrid piezo- and thermo-electric materials have an increasing range of applications. We use the nanoconfinement effect of ferroelectric discrete molecular complex [Cu(l-phe)(bpy)(H2O)]PF6·H2O (1) in a nonpolar polymer 1D-nanofiber to envision the high-performance flexible hybrid piezo- and thermo-electric nanogenerator (TEG). The 1D-nanoconfined crystallization of 1 enhances piezoelectric throughput with a high degree of mechano-sensitivity, i.e., 710 mV/N up to 3 N of applied force with 10,000 cycles of unaffected mechanical endurance. Thermoelectric properties analysis shows a noticeable improvement in Seebeck coefficient (∼4 fold) and power factor (∼6 fold) as compared to its film counterpart, which is attributed to the enhanced density of states near the Fermi edges as evidenced by ultraviolet photoelectric spectroscopy and density functional based theoretical calculations. We report an aqueous processable hybrid TEG that provides an impressive magnitude of Seebeck coefficient (∼793 μV/K) and power factor (∼35 mWm−1K−2) in comparison to a similar class of materials.
Direct Electroplating Ruthenium Precursor on the Surface Oxidized Nickel Foam for Efficient and Stable Bifunctional Alkaline Water Electrolysis
Li C., Kim B., Li Z., Thapa R., Zhang Y., Seo J.-M., Guan R., Tang F., Baek J.-H., Kim Y.H., Jeon J.-P., Park N., Baek J.-B.
Article, Advanced Materials, 2024, DOI Link
View abstract ⏷
Water electrolysis to produce hydrogen (H2) using renewable energy is one of the most promising candidates for realizing carbon neutrality, but its reaction kinetics is hindered by sluggish anodic oxygen evolution reaction (OER). Ruthenium (Ru) in its high-valence state (oxide) provides one of the most active OER sites and is less costly, but thermodynamically unstable. The strong interaction between Ru nanoparticles (NPs) and nickel hydroxide (Ni(OH)2) is leveraged to directly form Ru–Ni(OH)2 on the surface of a porous nickel foam (NF) electrode via spontaneous galvanic replacement reaction. The formation of Ru─O─Ni bonds at the interface of the Ru NPs and Ni(OH)2 (Ru–Ni(OH)2) on the surface oxidized NF significantly enhance stability of the Ru–Ni(OH)2/NF electrode. In addition to OER, the catalyst is active enough for the hydrogen evolution reaction (HER). As a result, it is able to deliver overpotentials of 228 and 15 mV to reach 10 mA cm−2 for OER and HER, respectively. An industry-scale evaluation using Ru–Ni(OH)2/NF as both OER and HER electrodes demonstrates a high current density of 1500 mA cm−2 (OER: 410 mV; HER: 240 mV), surpassing commercial RuO2 (OER: 600 mV) and Pt/C based performance (HER: 265 mV).
Large-area ultrathin 2D Co(OH)2 nanosheets: a bifunctional electrode material for supercapacitor and water oxidation
Jagdale P.B., Patil S.A., Sfeir A., Barman N., Iqbal A., Royer S., Thapa R., Samal A.K., Ghosh D., Saxena M.
Article, Materials Today Energy, 2024, DOI Link
View abstract ⏷
Ultrathin film of active materials having thickness <5 nm emerged as a promising candidate for miniaturized energy storage and conversion devices. However, the small lateral size restricts its real device applications. Herein, we report large area, 2D, ultrathin (thickness: 4.3 ± 0.3 nm) Co(OH)2 nanosheets as bifunctional electrode material for supercapacitor and oxygen evolution reaction developed by ionic layer epitaxy method at the water-air interface. The symmetric supercapacitor device exhibited excellent volumetric capacitance of 2313 F/cm3 at 0.4 mA/cm2 current density. Additionally, it exhibited a remarkable volumetric energy density of 0.205 Wh/cm3 at a power density of 0.145 W/cm3, which is better than reported 2D electrode materials. Furthermore, the cobalt hydroxide (Co(OH)2) ultrathin-film electrodes showed improved oxygen evolution reaction electrocatalytic activity with low overpotential (ƞ10, 330 mV) and Tafel slope (47 mV/dec). The structural and morphological investigations after long-term operations show substantial stability of the electrode material. The theoretical investigations of electronic structures, quantum capacitance, and free energy profiles for the oxygen evolution reaction mechanism corroborate the experimental results.
In situ cascade steric stabilization of poly(ionic liquid) mediated hexagonal nickel hydroxide morphogenesis for high-performance flexible supercapacitors
Narayanan A., Naik N.S., Kapse S., Thapa R., Balakrishna R.G., Rout C.S., Padaki M.
Article, Journal of Materials Chemistry A, 2024, DOI Link
View abstract ⏷
A hybrid material comprising nickel hydroxide (Ni(OH)2) enveloped in a poly(ionic liquid) (PIL-Br), poly(1-butyl-3-vinylimidazolium bromide), is effectively employed as an additive in this study, wherein PIL-Br provides precise control over the shape and arrangement of Ni(OH)2 crystals. The PIL can chemically couple with the as-synthesized Ni(OH)2, imparting a modified surface electronic structure. This innovation introduces a fresh perspective for energy storage applications. Extensive characterization, along with density functional theory calculations, has explored the impact of PIL polymer groups. The findings indicate that these polymers facilitate a more favourable redox response on the surface of Ni(OH)2. The desired morphology of the nanoarchitectonics leads to a high performing solid-state free-standing device with 245 F g−1 specific capacitance at 1 A g−1. When integrated with an activated carbon electrode and a unique polyvinyl 1-butyl-3-methylimidazolium tetrafluoroborate/polyvinyl alcohol (BMIMBF4/PVA) configuration, the hybrid device achieves an ED of 86.2 W h kg−1 at a PD of 800 W kg−1. Profoundly, it retains a capacity retention of 96% even after undergoing 10 000 cycles at 5 A g−1. The utilization of stabilized nanoarchitectonics incorporating redox-active polymers marks a novel direction in various energy applications.
Transient Electro-Graphitization of MOFs Affecting the Crystallization of Ruthenium Nanoclusters for Highly Efficient Hydrogen Evolution
Karim G.M., Patra A., Deb S.K., Upadhya H., Das S., Mukherjee P., Ahmad W., Barman N., Thapa R., Dambhare N.V., Rath A.K., Das J., Manna U., Urkude R.R., Oh Y., Maiti U.N.
Article, Advanced Functional Materials, 2024, DOI Link
View abstract ⏷
Fine control over the graphitization level of carbonized nanostructures can play a strategic role in tuning the crystallization of supported nanocatalysts, thereby modulating the kinetics of catalysis. However, realizing the synergistic interplay of graphitization-tunable support and supported catalysts poses a significant challenge. This study proposes a current pulse-induced ultrafast strategy for developing MOF-derived graphitic nano-leaves (GNL) and supported ultrafine ruthenium nanoclusters exhibiting selective crystallization states depending on the tunable graphitization level of GNL. The resulting ultrafine (≈0.7 nm) amorphous-ruthenium nanoclusters linked with GNL (a-Ru@GNL500) exhibit state-of-the-art performance in the hydrogen evolution reaction (HER), requiring very low overpotentials of only 23.0 and 285.0 mV to achieve current densities of 10 and 500 mA cm−2, respectively. Furthermore, a-Ru@GNL500 demonstrates exceptional operational stability for 100 h under high HER currents of 200 and 400 mA cm−2. Density functional theory reveals that the unique electronic structure of a-Ru and the cooperative effect of cobalt embedded in the graphitic layer lower the occupancy of the antibonding orbital, resulting in an accelerated HER process. Additionally, the unique electronic structure, highly conducting GNL, and efficient bubble release dynamics of super-aerophobic a-Ru@GNL500 contribute to reduced overpotentials, particularly at high HER current densities.
Coordination Structure Modulation in Group-VIB Metal Doped Ag3PO4 Augments Active Site Density for Electrocatalytic Conversion of N2 to NH3
Biswas A., Sharma M.D., Kapse S., Samui S., Thapa R., Gupta S., Sudarshan K., Dey R.S.
Article, Small, 2024, DOI Link
View abstract ⏷
Doping is considered a promising material engineering strategy in electrochemical nitrogen reduction reaction (NRR), provided the role of the active site is rightly identified. This work concerns the doping of group VIB metal in Ag3PO4 to enhance the active site density, accompanied by d-p orbital mixing at the active site/N2 interface. Doping induces compressive strain in the Ag3PO4 lattice and inherently accompanies vacancy generation, the latter is quantified with positron annihilation lifetime studies (PALS). This eventually alters the metal d-electronic states relative to Fermi level and manipulate the active sites for NRR resulting into side-on N2 adsorption at the interface. The charge density deployment reveals Mo as the most efficient dopant, attaining a minimum NRR overpotential, as confirmed by the detailed kinetic study with the rotating ring disk electrode (RRDE) technique. In fact, the Pt ring of RRDE fails to detect N2H4, which is formed as a stable intermediate on the electrode surface, as identified from in-situ attenuated total reflectance-infrared (ATR-IR) spectroscopy. This advocates the complete conversion of N2 to NH3 on Mo/Ag3PO4-10 and the so-formed oxygen vacancies formed during doping act as proton scavengers suppressing hydrogen evolution reaction resulting into a Faradaic efficiency of 54.8% for NRR.
Magneto-Electrochemical Ammonia Synthesis: Boosting Nitrite Reduction Activity by the Optimized Magnetic Field Induced Spin Polarized System
Adalder A., Mitra K., Barman N., Thapa R., Bhowmick S., Ghorai U.K.
Article, Advanced Energy Materials, 2024, DOI Link
View abstract ⏷
Using low and optimized magnetic field along with electric field is a novel strategy to facilitate electrochemical nitrite reduction reaction (NO2RR). Herein, the magnetic field assisted electrocatalytic ammonia synthesis employing spin-thrusted β-MnPc at 95 mT magnetic field is explored. The calculated rate of ammonia generation is 16603.4 µg h−1 mgcat −1, which is almost twice that of the nonpolarized manganese phthalocyanine (MnPc) catalyst. Additionally, the Faradaic efficiency (FE) at –0.9 V versus RHE is found to be 92.9%, significantly higher compared to the nonpolarized MnPc catalyst. In presence of external magnetic field, MnPc catalysts provide a better electron transfer channel which results in a lower charge transfer resistance and hence better electrochemical performances. Density functional theory (DFT) result further verifies that magnetic field induced β-MnPc has a lower potential barrier (0.51 eV) for the protonation of NO* than nonpolarized β-MnPc (1.08 eV), which confirms the enhanced electrochemical nitrite reduction to ammonia.
Boosting Selective Nitrogen Oxidation to Nitric Acid by Synergizing Cobalt Phthalocyanine on Carbon Nitride Surface
Paul S., Adalder A., Barman N., Thapa R., Bera A., Mitra K., Ghorai U.K.
Article, Advanced Functional Materials, 2024, DOI Link
View abstract ⏷
The Ostwald process, which is producing HNO3 for commercial use, involves the catalytic oxidation of NH3 and a series of chemical reactions conducted under severe operating conditions. Due to their energy-intensive nature, these activities play a major role in greenhouse gas emissions and global energy consumption. In response to the urgent requirements of the global energy and environmental sectors, there is an increasingly critical need to develop novel, highly efficient, and environmentally sustainable methods. Herein, CoPc/C3N4 electrocatalyst, integrating CoPc nanotubes with C3N4 nanosheets, is shown. The CoPc/C3N4 electrocatalyst demonstrates yield rate of 871.8 µmol h−1 gcat−1 at 2.2 V, with corresponding Faradaic efficiency (FE) of 46.4% at 2.1 V, which notably surpasses that of CoPc. Through a combination of experimental investigations and density functional theory (DFT) calculations, this study shows that CoPc anchored on C3N4 effectively simplifies the adsorption and activation of chemically inactive nitrogen molecules. The improved catalytic activity for composite system may be the reason of re-distribution of charges over the CoPc, tuning the valence orbital of Co center due to the presence of 2D layer of C3N4. This mechanism significantly lowers the energy barrier required for critical breaking of inert N2, ultimately leading to a significant improvement in N2 oxidation efficiency.
Ultrathin, large area β-Ni(OH)2 crystalline nanosheet as bifunctional electrode material for charge storage and oxygen evolution reaction
Ashok Patil S., Jagdale P.B., Barman N., Iqbal A., Sfeir A., Royer S., Thapa R., Kumar Samal A., Saxena M.
Article, Journal of Colloid and Interface Science, 2024, DOI Link
View abstract ⏷
Bifunctional electrode materials are highly desirable for meeting increasing global energy demands and mitigating environmental impact. However, improving the atom-efficiency, scalability, and cost-effectiveness of storage systems, as well as optimizing conversion processes to enhance overall energy utilization and sustainability, remains a significant challenge for their application. Herein, we devised an optimized, facile, economic, and scalable synthesis of large area (cm2), ultrathin (∼2.9 ± 0.3 nm) electroactive nanosheet of β-Ni(OH)2, which acted as bifunctional electrode material for charge storage and oxygen evolution reaction (OER). The β-Ni(OH)2 nanosheet electrode shows the volumetric capacity of 2.82 Ah.cm−3(0.82 µAh.cm−2) at the current density of 0.2 mA.cm−2. The device shows a high capacity of 820 mAh.cm−3 with an ultrahigh volumetric energy density of 0.33 Wh.cm−3 at 275.86 W.cm−3 along with promising stability (30,000 cycles). Furthermore, the OER activity of ultrathin β-Ni(OH)2 exhibits an overpotential (η10) of 308 mV and a Tafel value of 42 mV dec-1 suggesting fast reaction kinetics. The mechanistic studies are enlightened through density functional theory (DFT), which reveals that additional electronic states near the Fermi level enhance activity for both capacitance and OER.
Unveiling the performance of ultrathin bimetallic CoxNi1−x(OH)2 nanosheets for pseudocapacitors and oxygen evolution reaction
Jagdale P.B., Patil S.A., Pathak A., Sk M., Thapa R., Sfeir A., Royer S., Samal A.K., Saxena M.
Article, Journal of Materials Chemistry A, 2024, DOI Link
View abstract ⏷
The rational design of highly efficient and stable electrodes is necessary for energy storage and electrocatalysis. Herein, we developed a nanometre thin bimetallic ultrathin CoxNi1−x(OH)2 nanosheet with a large lateral size by the ionic layer epitaxy (ILE) technique as an efficient bifunctional electrode material for pseudocapacitors and the oxygen evolution reaction. Its electrochemical performance was readily tuned by controlling the Co/Ni ratio. The nanosheet with a 1 : 3 Co : Ni ratio (termed Co1Ni3-NS) showed an excellent volumetric (areal) capacitance of 3783 F cm−3 (3 mF cm−2) at 0.3 mA cm−2 with 336 mW h cm−3 energy density at 256 W cm−3 power density and excellent stability, substantially outperforming other monometallic and bimetallic NSs. Moreover, as an electrocatalyst, Co1Ni3-NS delivered a lower overpotential (η10 = 318 mV) and Tafel slope (61 mV dec−1) in an alkaline environment. In situ Raman spectroscopy was employed to demonstrate the dynamic structural evolution of the catalyst during the OER process. Furthermore, DFT investigations further revealed that Co1Ni3-NS is a promising electrode with higher quantum capacitance and lower overpotential compared to other Co/Ni ratios. These findings pave a new way for controlled synthesis of highly efficient, bimetallic, and bifunctional electrode materials for pseudocapacitors and the OER.
Beyond Conventional Catalysts: Monoelemental Tellurium as a Game Changer for Piezo-Driven Hydrogen Evolution
Mishra H.K., Ankush, Barman N., Mondal B., Jha M., Thapa R., Mandal D.
Article, Small, 2024, DOI Link
View abstract ⏷
The increasing demand for clean hydrogen production over fossil fuels necessitates the development of sustainable piezoelectrochemical methods that can overcome the limitations of conventional electrocatalytic and photocatalytic approaches. In this regard, existing piezocatalysts face challenges related to their low piezoelectricity or active site coverage for hydrogen evolution reaction (HER). Driven by global environmental concerns, there is a compelling push to engineer practical materials for highly efficient HER. Herein, monoelemental 2D tellurium (Te) is presented as a class of layered chalcogenide with a non-centrosymmetric crystal structure (P3121 space group). The refined Te nanosheets demonstrate an unprecedented highly efficient H2 production rate ≈9000 µmol g−1 h−1 under ultrasonic mechanical vibration due to built-in piezo-potential in the system. The remarkable piezocatalytic performance of Te nanosheets arises from a synergistic interplay between their semi-metallic nature, favorable free energy landscape, enhanced electrical conductivity and outstanding piezoelectricity. As a proof of concept, the theoretical approach based on Density Functional Theory (DFT) validates the findings due to the gradual exposure of active sites on the Te nanosheets leading to a self-optimized catalytic performance for hydrogen generation. Therefore, mechanically driven Te emerges as a promising piezocatalyst with the potential to revolutionize highly efficient and sustainable technology for futuristic applications.
Morphology-Dependent Enhancement of Electrocatalytic Nitrogen Reduction Activity Using Iron Phthalocyanine Nanostructures
Sarkar S., Mukherjee N., Alli S.J., Bhabak P., Adalder A., Mukherjee S., Thapa R., Ghorai U.K.
Article, ACS Applied Energy Materials, 2024, DOI Link
View abstract ⏷
Ammonia is one of the most essential raw materials for daily life applications. As an alternative to the Haber-Bosch process, scientists are focusing on an important domain of electrocatalysis for ammonia production. Herein, we approached a morphological adaptation of the electrocatalyst (iron phthalocyanine, FePc) based on hollow nanotube and rod types; the catalyst showed different N2-to-NH3 productivity. Under ambient conditions, FePc nanorods showed a good ammonia yield rate and Faradaic efficiency (FE) of 323.44 μg h-1 mgcat.-1 and 23.33%, respectively, at −0.4 V vs RHE in 0.05 M H2SO4. However, when the rod was adapted to a hollow nanotube structure by control of the temperature and time parameters, the ammonia productivity further improved. Under the same conditions, FePc nanotubes showed an excellent ammonia yield rate of 425.46 μg h-1 mgcat.-1 and a corresponding FE of 23.61% at −0.4 V vs RHE. In addition to experimental observations, theoretical analysis using density functional theory is also provided to establish the reaction mechanism of ammonia synthesis from nitrogen reduction reaction (NRR) using an FePc electrocatalyst. This work opens an avenue showing geometric structural induction of electrocatalytic activity toward future sustainable ammonia production.
Size-, shape-, facet- and support-dependent selectivity of Cu nanoparticles in CO2 reduction through multiparameter optimization
Article, Nanoscale, 2024, DOI Link
View abstract ⏷
This study investigates the limited selectivity of the Cu111 surface for C-C bond formation during CO2 reduction and explores the factors influencing selectivity using Cu nanoparticles smaller than 2 nm. The optimal nanoparticle size for C-C bond formation on the 111 facet with minimal overpotential is determined using density functional theory. A suitable supporting surface to enhance the stability and catalytic performance of the Cu-based nanoparticles is identified. Various Cu catalyst geometries, including planar surfaces and cuboctahedral, icosahedral, and truncated octahedral Cu nanoparticles, are considered. Size-dependent effects on the binding energies of reaction intermediates and hydrogen atoms are examined. Carbon-based surfaces, particularly 2SO2-doped graphene nanoribbons, are stable hosts for the Cu nanoparticles and help in retaining the activity for CO2 reduction. Scaling relations between the binding energies of the intermediates suggest COOH binding energy as an energy descriptor. Through multiparameter optimization and with the help of parity line and graphical construction, Cu38 and Cu79 are found to be the most promising surface for C2 product generation. This study provides insights into the factors influencing the selectivity and catalytic performance of Cu nanoparticles, aiding the development of efficient catalysts for CO2 reduction.
Leveraging Soft Acid-Base Interactions Alters the Pathway for Electrochemical Nitrogen Oxidation to Nitrate with High Faradaic Efficiency
Singh R., Biswas A., Barman N., Iqbal M., Thapa R., Dey R.S.
Article, Small, 2024, DOI Link
View abstract ⏷
Electrocatalytic nitrogen oxidation reaction (N2OR) offers a sustainable alternative to the conventional methods such as the Haber–Bosch and Ostwald oxidation processes for converting nitrogen (N2) into high-value-added nitrate (NO3−) under mild conditions. However, the concurrent oxygen evolution reaction (OER) and inefficient N2 absorption/activation led to slow N2OR kinetics, resulting in low Faradaic efficiencies and NO3− yield rates. This study explored oxygen-vacancy induced tin oxide (SnO2-Ov) as an efficient N2OR electrocatalyst, achieving an impressive Faradaic efficiency (FE) of 54.2% and a notable NO3− yield rate (22.05 µg h−1 mgcat−1) at 1.7 V versus reversible hydrogen electrode (RHE) in 0.1 m Na2SO4. Experimental results indicate that SnO2-Ov possesses substantially more oxygen vacancies than SnO2, correlating with enhanced N2OR performance. Computational findings suggest that the superior performance of SnO2-Ov at a relatively low overpotential is due to reduced thermodynamic barrier for the oxidation of *N2 to *N2OH during the rate-determining step, making this step energetically favorable than the oxygen adsorption step for OER. This work demonstrates the feasibility of ambient nitrate synthesis on the soft acidic Sn active site and introduces a new approach for rational catalyst design.
An ultrathin 2D NiCo-LDH nanosheet decorated NH2-UiO-66 MOF-nanocomposite with exceptional chemical stability for electrocatalytic water splitting
Sk S., Madhu R., Gavali D.S., Bhasin V., Thapa R., Jha S.N., Bhattacharyya D., Kundu S., Pal U.
Article, Journal of Materials Chemistry A, 2023, DOI Link
View abstract ⏷
Utilization of bifunctional high-efficiency non-precious electrocatalysts for stable and effective water splitting is crucial to the growth of the clean energy industry. Topologically, the predetermined ordered structures of metal-organic frameworks (MOFs) can be contrived through the judicious assembly of a tailor-made synthesis strategy of layered double hydroxide (LDH) films. Aiming at NiCo-LDH@NH2-UiO-66 as a model system, for the first time, we examine the 2-methyl imidazole-induced ultrathin 2D NiCo-LDH nanosheet arrays in NH2-UiO-66 as an effective bifunctional electrocatalytic system for overall H2O splitting with marvellous performance and robustness in alkaline environments. The progressively tuned NiCo-LDH@NH2-UiO-66 catalyst demands overpotential values of 296 and 224 mV to deliver a current density of 50 mA cm−2 for the O2 evolution reaction (OER) and H2 evolution reaction (HER) in 1 M KOH aqueous solution, respectively. Tafel studies also revealed favorable reaction kinetics during electrochemical processes. The NiCo-LDH@NH2-UiO-66 bifunctional electrode displayed superior activity exhibiting a voltage of 1.65 V at a benchmarking current density of 10 mA cm−2 towards overall water splitting. Importantly, the NiCo-LDH@NH2-UiO-66 electrode shows an excellent specific capacitance of 0.00364 mF cm−2 with remarkable durability of the capacitor after 1000 cycles. To compare with the experimental result, we have performed density functional theory (DFT)-based calculation to estimate the HER and OER activity of the NiCo-LDH@NH2-UiO-66 heterostructure. From the HER free energy profile and Bader charge analysis, we have confirmed that the presence of NH2-UiO-66 helps in H2 production with 0.10 eV free energy of H2 adsorption (GH*). From the OER free energy profile, the estimated overpotential (η) is about 0.96 eV, which confirms that the electrochemical reaction towards the OER is also possible on the NiCo-LDH@NH2-UiO-66 structure. This study will bestow a beneficial blueprint for the utilization of an effective, durable, and economical MOF-based bifunctional catalytic system for overall water splitting.
Identification of ORR activity of random graphene-based systems using the general descriptor and predictive model equation
Kapse S., Barman N., Thapa R.
Article, Carbon, 2023, DOI Link
View abstract ⏷
Carbon based electrocatalysts are well known promising candidates for the oxygen reduction reaction (ORR), but the random approach to find the best catalyst using experimental method delayed the screening process and it leads to a huge cost with less proficiency. Using Quantum Mechanics followed by Machine Learning (QM/ML) approach, we can predict the best catalyst faster way and the origin of the cause can be identified for further development of carbon-based catalyst. Using the π electronic descriptor unveiled using density functional theory, we applied the analytical simple fit method and six different machine learning algorithms to develop a highly effective predictive model to estimate ΔGOH. Furthermore, structural relations of ZGNR and AGNR are demonstrated to estimate the Dπ(EF), R-Oπ, and ΔGOH of different widths of ribbon that reduces additional DFT calculations. By applying both SVR predictive model and structural relations, we predicted the ORR performance of 2500 sites of GNRs and listed a few most ideal active carbon sites with lower overpotential (η < 0.5V). To validate our study, we predicted the ORR performance of different sites in 0D, 1D, 2D doped graphene systems using SVR model and confirmed the values with the DFT computed results.
Synergetic effect of Ru @ octahedral site of Fe3O4 and charge transfer from rGO to Ru/Fe3O4 for improved hydrogen evolution reaction-experimental and DFT studies
Shwetha K.R., Nagaraju D.H., Kapse S., Thapa R., Budagumpi S., Yhobu Z.
Article, Materials Letters, 2023, DOI Link
View abstract ⏷
This work presents decoration of Ru @ Fe3O4 nanoparticles over conductive supports such as reduced Graphene Oxide (rGO) and Super P Carbon Black (SPCB) for hydrogen evolution reaction (HER). It is impressive that, low over potential of 148 mV at a current density 10 mA cm−2 was achieved for the Ru@Fe3O4/rGO/SPCB composite. Density functional theory (DFT) endorses that, charge transfer can occur from underneath rGO layer to the Ru @ octahedral site of Fe3O4 due to lower work function and higher Fermi energy level of rGO than Ru/Fe3O4. Density of state calculations reveals that increase in the density of states of Ru/Fe3O4 due to the layer of rGO in Ru/Fe3O4/rGO system.
Scrutinizing the role of tunable carbon vacancies in g-C3N4 nanosheets for efficient sonophotocatalytic degradation of Tetracycline in diverse water matrices: Experimental study and theoretical calculation
Preeyanghaa M., Erakulan E.S., Thapa R., Ashokkumar M., Neppolian B.
Article, Chemical Engineering Journal, 2023, DOI Link
View abstract ⏷
Metal-free polymeric graphitic carbon nitride (CN) materials are robust and stable visible-light-driven photocatalysts that have recently piqued interest in photocatalytic applications. Its photocatalytic performance is restricted remarkably due to moderate oxidation ability and fast charge carrier recombination rate. To address these issues, we engineered carbon-vacant CN (FCN) using a facile formalin-assisted thermal polymerization of molten CN precursor in which the carbon vacancies (Cv) were regulated by altering formalin dosage. Consequently, FCN catalysts revealed Cv concentration-dependent sonophotocatalytic degradation of Tetracycline (TC) antibiotics over diverse water matrices. The optimal FCN exhibited complete TC degradation efficiency within 60 min with a synergy index of 1.4, which is approximately 2.6 times higher than that of pristine CN. The enhanced sonophotocatalytic performance was mainly due to the synergistic effect of ultrasound and light irradiation. The Cv formation also resulted in enhanced charge carrier transportation and facilitated oxygen adsorption at the Cv site of FCN - supported by both experimental study and theoretical calculation. Subsequently, FCN generated abundant reactive active oxygen species including, •O2–, as well as indirectly •OH which played a significant role in the degradation pathway and mineralisation of the TC molecules. This study provides insight into understanding the correlation between controllable defects and sonophotocatalytic degradation properties of the self-doped and deficient FCN.
First-principles identification of CO oxidation via LH mechanism over ER mechanism on metal-boron centered single-metal dual site catalyst
Tripathi A., Kawazoe Y., Thapa R.
Article, Molecular Catalysis, 2023, DOI Link
View abstract ⏷
CO production has grown throughout time because of unwanted chemical reactions; therefore, the design of efficient and stable catalysts with minimal metal loading is in demand in the automobile industry for CO oxidation. Herein, density functional theory is used to study the CO oxidation mechanism on single-atom catalysts (SACs) and metal-boron centered single metal dual site catalysts (SM-DSCs). It is observed that CO oxidation could follow Eley-Rideal (ER) mechanism on single-atom catalysts, such as MN4, MN3, MN2, and MN2op systems, where M is a 3d transition metal. Whereas in the case of SM-DSCs, the inclusion of Boron will make the Langmuir-Hinshelwood (LH) mechanism possible over ER mechanism with less chance of CO poisoning. The Brønsted−Evans−Polanyi (BEP) relations are proposed (as a function of oxygen atom binding) to identify the energy barriers (Ea1 and Ea2) for the release of the first and second CO2 and O2 adsorption energies for any new SM-DSCs system. Further microkinetic model is implemented on 30 SM-DSCs and found that PtBN2op and AgBN2 SM-DSCs own best catalytic activity with high Sabatier activity and Turnover frequency for CO2 production. Hence, this work provides an approach toward the possibility of LH mechanism in the class of single-metal atom-based catalysts.
Elucidating the oxygen reduction reaction kinetics on defect engineered nanocarbon electrocatalyst: interplay between the N-dopant and defect sites
Bhardwaj S., Kapse S., Dan S., Thapa R., Dey R.S.
Article, Journal of Materials Chemistry A, 2023, DOI Link
View abstract ⏷
The active sites of electrocatalysts play a crucial role in the material design and mechanistic exploration of an electrocatalytic reaction. Defect-tailored heteroatom-doped carbon-based electrocatalysts for oxygen reduction reaction (ORR) have been much explored, but there is ambiguity in the prediction of active sites responsible for the performance of the material. To find the origin of the activity of this class of catalysts towards ORR, in this work, we use the quantum mechanics/machine learning (QM/ML) approach to derive energy-optimized models with both N-atoms and 5-8-5 defect sites which manifest exemplary ORR activity. Following this approach, we synthesized defect-engineered graphene (DG) using the zinc template method at 1050 °C to achieve optimum N-dopants and intrinsic (5-8-5) defects. The obtained electrocatalyst exhibits hierarchical porosity, high surface area, low nitrogen content, good stability and a satisfying ORR performance with a half-wave potential (E1/2) of 0.82 V, comparable to that of commercial Pt/C (E1/2 = 0.82 V). Further, the full energy profile was deduced using density functional theory and the charge redistribution in the material cross-verified a reduced overpotential for ORR. This work provides a strategy for the synthesis of noble-metal-free high-performance electrocatalysts for energy conversion.
Pd encapsulated core-shell ZIF-8/ZIF-67 for efficient oxygen evolution reaction
Varangane S., Jamma A., Prabhu Y.T., Karmakar A., Kundu S., Tripathi A., Thapa R., Bhasin V., Jha S.N., Bhattacharyya D., Pal U.
Article, Electrochimica Acta, 2023, DOI Link
View abstract ⏷
Tailored metal–organic frameworks with tunable physicochemical properties not only render integration of electroactive species but also facilitate the accessibility of catalytic sites, yet much less discussed for electrocatalysis. Herein, we report a rational shaping of core-shell Zeolitic Imidazolate Frameworks in a scalable seed mediated approach. The Pd encapsulated robust ZIF-8/ZIF-67 nanoarchitectures exhibit highly efficient electrocatalytic oxygen evolution reaction (OER) with low Tafel slope of 85 mV/dec and a 340 mV overpotential. The heterojunction favours a well-matched band energy, efficient charge mobility with exceptional OER with 10 mA cm−2 anodic current density, high turnover frequency (TOF: 1.21 sec−1), high Faradaic efficiency (91.88 %) and long-term durability as well as stability. The local structure and electrical information of Pd nanoparticles as well as their beneficial cooperative impact on ZIF-8/ZIF-67 affirmed by EXAFS and XANES analyses. Furthermore, DFT studies are harmonious with the experiments, that highly active molecules have a strong synergistic effect for the overall electrocatalytic OER performance exhibited by the catalysts, which elucidate the role of active Pd and Co sites and corroborate the experimental findings.
Strengthening the Metal Center of Co-N4 Active Sites in a 1D-2D Heterostructure for Nitrate and Nitrogen Reduction Reaction to Ammonia
Paul S., Sarkar S., Adalder A., Kapse S., Thapa R., Ghorai U.K.
Article, ACS Sustainable Chemistry and Engineering, 2023, DOI Link
View abstract ⏷
Ammonia forms the fundamental agricultural constituent and vital energy provenance of a clean hydrogen mediator. Ammonia production leads to immense energy utilization and drastic environmental repercussion. It is a daunting task to design and synthesize competent catalysts for reduction of nitrogenous species (nitrogen or nitrates, by the nitrogen reduction reaction (NRR) or nitrate reduction reaction (NO3RR) process, respectively) into ammonia. Cobalt(II) phthalocyanine (CoPc) nanotubes were effectively wrapped by 2D graphene sheets to produce a (1D-2D) heterostructure catalyst, which plays the role of a competent electrocatalyst for the NRR as well as NO3RR. The electrocatalyst showed an ammonia yield rate and a Faradaic efficiency of 58.82 μg h-1 mg-1cat and 95.12%, respectively, for the NO3RR and for NRR 143.38 μg h-1 mg-1cat and 43.69%, respectively. Bader charge investigation revealed the transport of charge to Co-N4 active sites from reduced graphene oxide (RGO), which aids during the production of intermediates NNH* for nitrogen reduction and *NOH for nitrate reduction along with suppression of the parasitic HER, thereby demonstrating good selectivity and Faradaic efficiency. This work showcases new mechanistic discernment about the role of work function, interfacial charge transport, and electrocatalytic overpotential for the nitrogen/nitrate reduction reaction.
Trimetallic Oxide Foam as an Efficient Catalyst for Fixation of CO2 into Oxazolidinone: An Experimental and Theoretical Approach
Srinivasappa P.M., Prasad D., Chaudhari N.K., Samal A.K., Thapa R., Siddharthan E.E., Jadhav A.H.
Article, ACS Applied Materials and Interfaces, 2023, DOI Link
View abstract ⏷
The excess anthropogenic CO2 depletion via the catalytic approach to produce valuable chemicals is an industrially challenging, demanding, and encouraging strategy for CO2 fixation. Herein, we demonstrate a selective one-pot strategy for CO2 fixation into “oxazolidinone” by employing stable porous trimetallic oxide foam (PTOF) as a new catalyst. The PTOF catalyst was synthesized by a solution combustion method using transition metals Cu, Co, and Ni and systematically characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), N2 sorption, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS) analysis. Due to the distinctive synthesis method and unique combination of metal oxides and their percentage, the PTOF catalyst displayed highly interconnected porous channels along with uniformly distributed active sites on its surface. Well ahead, the PTOF catalyst was screened for the fixation of CO2 into oxazolidinone. The screened and optimized reaction parameters revealed that the PTOF catalyst showed highly efficient and selective activity with 100% conversion of aniline along with 96% selectivity and yield toward the oxazolidinone product at mild and solvent-free reaction conditions. The superiority of the catalytic performance could be due to the presence of surface active sites and acid-base cooperative synergistic properties of the mixed metal oxides. A doubly synergistic plausible reaction mechanism was proposed for the oxazolidinone synthesis experimentally with the support of DFT calculations along with bond lengths, bond angles, and binding energies. In addition, stepwise intermediate formations with the free energy profile were also proposed. Also, the PTOF catalyst displayed good tolerance toward substituted aromatic amines and terminal epoxides for the fixation of CO2 into oxazolidinones. Very interestingly, the PTOF catalyst could be significantly reused for up to 15 consecutive cycles with stable activity and retention in physicochemical properties.
Optimizing CO2RR selectivity on single atom catalysts using graphical construction and identification of energy descriptor
Article, Carbon, 2023, DOI Link
View abstract ⏷
The electrocatalytic reduction of CO2 (CO2RR) into value-added hydrocarbons is limited due to high limiting potential (UL) and competing hydrogen evolution reaction (HER). To find the best catalyst for CO2 reduction the concept of hydrogen poisoning was not considered in the catalyst screening process. Herein, we present a simple screening method and graphical construction using multiparameter optimization for the design of highly active and selective single-atom catalysts (SAC) using density functional theory calculations. A series of SAC namely, MN4, MBN3 and H@MBN3 (M: metal) are investigated for CO2RR. Our results revealed that MN4 and MBN3 SAC are not favorable for CO2RR due to high UL > −0.85 V and hydrogen poisoning (ΔGH* < 0), respectively. H@MBN3 SAC (stable compounds forming H–B bonds) are identified as efficient catalysts with a low value of UL and significantly hinder the competitive HER. Among these, H@CoBN3 and H@FeBN3 SAC show excellent CO2RR activity with limiting potential −0.30 and −0.44 V respectively for CH4 production and no chance of HER. Scaling relations reveal the importance of *COOH/*CHO binding energy (Eb) as an energy descriptor to evaluate the catalytic performance. This work provides a new theoretical perspective to design a highly selective catalyst for CO2RR.
Integrating Ultrasmall Pd NPs into Core-Shell Imidazolate Frameworks for Photocatalytic Hydrogen and MeOH Production
Varangane S., Yendrapati T.P., Tripathi A., Thapa R., Bojja S., Anand P., Perupogu V., Pal U.
Article, Inorganic Chemistry, 2023, DOI Link
View abstract ⏷
The construction of photoactive units in the proximity of a stable framework support is one of the promising strategies for uplifting photocatalysis. In this work, the ultrasmall Pd NPs implanted onto core-shell (CS) metal organic frameworks (MOFs), i.e., CS@Pd nanoarchitectures with tailored electronic and structural properties are reported. The all-in-one heterogeneous catalyst CS@Pd3 improves the surface functionalities and exhibits an outstanding hydrogen evolution reaction (HER) activity rate of 12.7 mmol g-1 h-1, which is 10-folds higher than the pristine frameworks with an apparent quantum efficiency (AQE) of 9.02%. The bifunctional CS@Pd shows intriguing results when subjected to photocatalytic CO2 reduction with an impressive rate of 71 μmol g-1 h-1 of MeOH under visible-light irradiation at ambient conditions. Spectroscopic data reveal efficient charge migrations and an extended lifetime of 2.4 ns, favoring efficient photocatalysis. The microscopic study affirms the formation of well-ordered CS morphology with precise decoration of Pd NPs over the CS networks. The significance of active Pd and Co sites is addressed by congruent charge-transfer kinetics and computational density functional theory calculations of CS@Pd, which validate the experimental findings with their synergistic involvement in improved photocatalytic activity. This present work provides a facile and competent avenue for the systematic construction of MOF-based CS heterostructures with active Pd NPs.
Identification of Borophosphene/graphene heterostructure as anode for Li-ion Batteries and its origin
Article, Journal of Power Sources, 2023, DOI Link
View abstract ⏷
The development of two-dimensional (2D) material or heterostructure as an anode material is necessary to enhance the electrochemical performance of Li-ion batteries (LIBs) but finding the correct combination is a challenge. In the present work, using First principles study, we have proposed borophosphene (BP-ML) and graphene-based multilayer heterostructure as a possible anode material. We have found that BP-ML and graphene-based heterostructure are conductive in nature. Here, we have also investigated the role of Pz (π) and Py (σ) atomic orbital bands of BP-ML and graphene. On Li intercalation, charge transfer is mainly site and interface definitely which helps to improve the specific capacity. The specific capacity of the proposed heterostructure varies from 546 to 427 mA h/g. For maximum Li confirmation, the volume expansion of these heterostructures is about 14–16%. The presence of graphene helps to maintain the open-circuit voltage (OCV) of heterostructure on an average 0.7 V. Also, helps to support the diffusion barrier energy in the range of 0.27–0.71 eV. This proposed 2D heterostructure could be the future material for the LIB's anode material.
Design and fabrication of cobaltx nickel(1-x) telluride microfibers on nickel foam for battery-type supercapacitor and oxygen evolution reaction study
Bhol P., Patil S.A., Barman N., Ekambaram Siddharthan E., Thapa R., Saxena M., Altaee A., Samal A.K.
Article, Materials Today Chemistry, 2023, DOI Link
View abstract ⏷
Tellurium (Te) is a metal with the ability to function as a self-sacrificing template and exhibits strong chemical reactivity with counter metals. By the simple inward diffusion of metal ions on its surface, it can form long one-dimensional architect. Utilizing this, a one-dimensional cobaltx nickel(1-x) telluride (CoxNi(1-x)Te) microfibers (MFs) on nickel foam substrate has been constructed for a bifunctional electrode application. The theoretical and experimental investigations on the CoxNi(1-x)Te validates the Co0.75Ni0.25Te superiority over other ratios. The synergistically caused effects at 3:1 Co/Ni ratio reactivity with Te enhance the microstructure bifunctional electrode property with excellent stability during long cycle operation. The density functional theory calculations of Co0.75 Ni0.25Te revealed a better quantum capacitance due to the increased density of states near Fermi levels and achieved the enhanced oxygen evolution reaction activity because of increasing OH coverage. The assembled device Co0.75Ni0.25Te MF//activated carbon achieves outstanding energy storage performance with a maximum energy density of 50.8 Wh Kg−1 (58.4 μWh cm−2) at a power density of 672.7 W Kg−1 (773.5 μW cm−2) and sustains the performance up to 10,000 cycles, with a capacity retention of 90.1%. As an electrocatalyst, Co0.75Ni0.25Te MF/nickel foam requires a low overpotential (η) of 289 mV to reach a current density (j) of 10 mA cm−2 in 0.1 M KOH for oxygen evolution reaction.
Functionalized Silver Nanocubes for the Detection of Hazardous Analytes through Surface-Enhanced Raman Scattering: Experimental and Computational Studies
Basavaraja B.M., Bantwal R.P., Tripathi A., Hegde G., John N.S., Thapa R., Hegde G., Balakrishna R.G., Saxena M., Altaee A., Samal A.K.
Article, ACS Sustainable Chemistry and Engineering, 2023, DOI Link
View abstract ⏷
The functionalization of nanomaterials offers a significant impact on environmental protection. Three silver composites, silver nanocubes (Ag NCs) with cellulose (Ag-CE), Ag NCs with chitosan (Ag-CH), and Ag NCs with neutral alumina (Ag-NA), were synthesized with the incorporation of a very low concentration of Ag NCs. The synthesized Ag-composites were used for the detection of hazardous analytes, 4-mercaptobenzoic acid (4-MBA), rhodamine 6G (R6G), and methylene blue (MB), via a highly sensitive surface-enhanced Raman scattering (SERS) technique. The enhancement factors for the detection of MB were found to be 1.2 × 106, 1.4 × 106, and 3.7 × 105 using Ag-CE, Ag-CH, and Ag-NA, respectively. Limits of detection of 1 nM, 100 pM, and 100 μM were achieved for MB using Ag-CE, Ag-CH, and Ag-NA, respectively. The Ag-CH composite achieved excellent sensitivity and enhancement for the detection of MB compared to the other two Ag-composites. The order of detection efficiency of MB using Ag-composites was measured theoretically and follows the order Ag-CH > Ag-CE > Ag-NA. A real-time filtration unit showed excellent efficiency for MB removal. The present method can be employed in commercial filtration processes for the benefit of the environment and sustainable development.
Selective Electrocatalytic Oxidation of Nitrogen to Nitric Acid Using Manganese Phthalocyanine
Adalder A., Paul S., Ghorai B., Kapse S., Thapa R., Nagendra A., Ghorai U.K.
Article, ACS Applied Materials and Interfaces, 2023, DOI Link
View abstract ⏷
Ammonia is produced through the energy-intensive Haber-Bosch process, which undergoes catalytic oxidation for the production of commercial nitric acid by the senescent Ostwald process. The two energy-intensive industrial processes demand for process sustainability. Hence, single-step electrocatalysis offers a promising approach toward a more environmentally friendly solution. Herein, we report a 10-electron pathway associated one-step electrochemical dinitrogen oxidation reaction (N2OR) to nitric acid by manganese phthalocyanine (MnPc) hollow nano-structures under ambient conditions. The catalyst delivers a nitric acid yield of 513.2 μmol h-1 gcat-1 with 33.9% Faradaic efficiency @ 2.1 V versus reversible hydrogen electrode. The excellent N2OR performances are achieved due to the specific-selectivity, presence of greater number of exposed active sites, recyclability, and long period stability. The extended X-ray absorption fine structure confirms that Mn atoms are coordinated to the pyrrolic and pyridinic nitrogen via Mn-N4 coordination. Density functional theory-based theoretical calculations confirm that the Mn-N4 site of MnPc is the main active center for N2OR, which suppresses the oxygen evolution reaction. This work provides a new arena about the successful example of one step nitric acid production utilizing a Mn-N4 active site-based metal phthalocyanine electrocatalyst by dinitrogen oxidation for the development of a carbon-neutral sustainable society.
Halide Tunablility Leads to Enhanced Biomechanical Energy Harvesting in Lead-Free Cs2SnX6-PVDF Composites
Paul T., Sahoo A., Maiti S., Gavali D.S., Thapa R., Banerjee R.
Article, ACS Applied Materials and Interfaces, 2023, DOI Link
View abstract ⏷
The main challenges impeding the widespread use of organic-inorganic lead halide perovskites in modern-day technological devices are their long-term instability and lead contamination. Among other environmentally convivial and sustainable alternatives, Cs2SnX6 (X = Cl, Br, and I) compounds have shown promise as ambient-stable, lead-free materials for energy harvesting, and optoelectronic applications. Additionally, they have demonstrated tremendous potential for the fabrication of self-powered nanogenerators in conjunction with piezoelectric polymers like polyvinylidene-fluoride (PVDF). We report on the fabrication of composites constituting solvothermally synthesized Cs2SnX6 nanostructures and PVDF. The electroactive phases in PVDF were boosted by the incorporation of Cs2SnX6, leading to enhanced piezoelectricity in the composites. First-principles density functional theory (DFT) studies were carried out to understand the interfacial interaction between the Cs2SnX6 and PVDF, which unravels the mechanism of physisorption between the perovskite and PVDF, leading to enhanced piezoresponse. The halide ions in the inorganic Cs2SnX6 perovskites were varied systematically, and the piezoelectric behaviors of the respective piezoelectric nanogenerators (PENGs) were investigated. Further, the dielectric properties of these halide perovskite-based hybrids are quantified, and their piezoresponse amplitude, piezoelectric output signals, and charging capacity are also evaluated. Out of the several films fabricated, the optimized Cs2SnI6_PVDF film shows a piezoelectric coefficient (d33) value of ∼200 pm V-1 and a remanent polarization of ∼0.74 μC cm-2 estimated from piezoresponse force microscopy and polarization hysteresis loop measurement, respectively. The optimized Cs2SnI6_PVDF-based device produced an instantaneous output voltage of ∼167 V, a current of ∼5.0 μA, and a power of ∼835 μW across a 5 MΩ resistor when subjected to periodic vertical compression. The output voltage of this device is used to charge a capacitor with a 10 μF capacitance up to 2.2 V, which is then used to power some commercial LEDs. In addition to being used as a pressure sensor, the device is employed to monitor human physiological activities. The device demonstrates excellent operational durability over a span of several months in an ambient environment vouching for its exceptional potential in application to mechanical energy harvesting and pressure sensing applications.
Dual Vacancy Passivation in CsPbCl3 Perovskite Nanocrystals: Implications on Optoelectronic Applications
Dongre S S., Siddharthan E.E., Thapa R., Ramu S., Balakrishna R.G.
Article, ACS Applied Nano Materials, 2023, DOI Link
View abstract ⏷
Despite numerous advantages over the traditional light absorbing materials, colloidal cesium lead halide (CsPbX3, X = Cl, Br, or I) perovskite nanocrystals (NCs) suffer from enormous defect density, leading to shorter lifetime of charge carriers and material instability. A large number of positively and negatively charged ionic defects are inevitably formed from crystallization via high temperature. Herein, we have studied a simple post-synthesis defect passivation of blue emitting CsPbCl3 NCs using monovalent metal ion LiCl as a dual-passivating agent. The observed effect (on optical properties) went up by leaps and bounds. Photoluminescence (PL) quantum yield increases from 2.8 to 47.6%, while PL life time increases from 0.56 to 20.79 ns. Various other chloride salts (CaCl2, NH2Cl, KCl, and NaCl) and Li salts (LiBr and LiI) with different cation and anion combinations, respectively, did not give this effect. All these together with the enhanced overall stability of NCs suggest the synergistic effect of dual passivation and deep defect passivation that leads to significant suppression of non-radiative recombination. An X-ray photoelectron spectroscopy study also reveals that this simple strategy promotes simultaneous passivation of both defects (vacancies) formed from negatively (chlorine) and positively charged ions (lead) of CsPbCl3. Theoretical study and experimental analysis in this work, together delivers a perceptive understanding of cationic and anionic vacancy healing by LiCl in CsPbCl3 NCs, thus enhancing its utilization as efficient blue light emitters.
Engineering hydrophobic-aerophilic interfaces to boost N2 diffusion and reduction through functionalization of fluorine in second coordination spheres
Bhardwaj S., Das S.K., Biswas A., Kapse S., Thapa R., Dey R.S.
Article, Chemical Science, 2023, DOI Link
View abstract ⏷
Ammonia is a crucial biochemical raw material for nitrogen containing fertilizers and a hydrogen energy carrier obtained from renewable energy sources. Electrocatalytic ammonia synthesis is a renewable and less-energy intensive way as compared to the conventional Haber-Bosch process. The electrochemical nitrogen reduction reaction (eNRR) is sluggish, primarily due to the deceleration by slow N2 diffusion, giving rise to competitive hydrogen evolution reaction (HER). Herein, we have engineered a catalyst to have hydrophobic and aerophilic nature via fluorinated copper phthalocyanine (F-CuPc) grafted with graphene to form a hybrid electrocatalyst, F-CuPc-G. The chemically functionalized fluorine moieties are present in the second coordination sphere, where it forms a three-phase interface. The hydrophobic layer of the catalyst fosters the diffusion of N2 molecules and the aerophilic characteristic helps N2 adsorption, which can effectively suppress the HER. The active metal center is present in the primary sphere available for the NRR with a viable amount of H+ to achieve a substantially high faradaic efficiency (FE) of 49.3% at −0.3 V vs. RHE. DFT calculations were performed to find out the rate determining step and to explore the full energy pathway. A DFT study indicates that the NRR process follows an alternating pathway, which was further supported by an in situ FTIR study by isolating the intermediates. This work provides insights into designing a catalyst with hydrophobic moieties in the second coordination sphere together with the aerophilic nature of the catalyst that helps to improve the overall FE of the NRR by eliminating the HER.
Photon driven nitrogen fixation via Ni-incorporated ZrO2/Bi2O3: p-n heterojunction
A. R. S.C.L., Thapa R., B N.
Article, Catalysis Today, 2023, DOI Link
View abstract ⏷
One of the promising approaches to synthesize ammonia in ambient conditions is through artificial photocatalytic nitrogen fixation. In order to overcome the hassles of charge carrier recombination and finite light response of photocatalysts, high charge separation efficiency and a wide window for optical absorption are imperative. Herein, Ni-incorporated ZrO2/Bi2O3 has been fabricated with p-n heterojunction and a bandgap aligned to absorb visible light up to 560 nm which play an important role in enhancing the production rate of ammonia. The catalyst exhibits a maximum ammonia generation rate of 9668.2 µmol h−1 g−1. Further, systematic characterization studies confirm the formation of the desired photocatalyst. This work presents a propitious strategy to configure appropriate visible light responsive photocatalyst that efficiently reduces nitrogen.
Design and fabrication of nickel lanthanum telluride microfibers for redox additive electrolyte-based flexible solid-state hybrid supercapacitor
Bhol P., Jagdale P.B., Barman N., Thapa R., Saxena M., Samal A.K.
Article, Journal of Energy Storage, 2023, DOI Link
View abstract ⏷
In this article, we describe the design of a flexible solid-state hybrid supercapacitor (FSSHSC) using mixed metal telluride NiLaTe microfibers (MFs) atop Ni foam (NF). The goal of the study is to examine the effect of counter metal ions Ni and La, in addition to Te, in the formation of longer one-dimensional (1-D) architecture. The NiTe MFs and La2Te3 microrods (MRs) were synthesized and compared with bimetallic telluride. La exhibit's slower reaction kinetics than Ni when reacting with Te due to the different crystal structural stability of La2Te3 (cubic phase) and Te (hexagonal phase). Three-electrode investigations incorporating aqueous potassium ferrocyanide (KFC) reveal that the electrochemical performance of NiLaTe MFs is doubled compared to the typical aqueous KOH electrolyte alone. The presence of the redox pair [Fe(CN)6]3−/[Fe(CN)6]4− allows for faster ion transport inside the active electrode surface, which improves electrochemical performance. The FSSHSC device was fabricated, comprising NiLaTe as the positive, activated carbon (AC) as the negative electrode, and PVA-KOH-KFC gel, which acts as an electrolyte and a separator. The device performance was compared to that of a liquid system as well as one without a redox additive gel system. The significance of polymer-based gel and KFC for device construction is briefly reviewed. The final built device NiLaTe//AC in gel + KFC system has a maximum areal capacity of 65.6 μAh cm−2 (60.0 mA h g−1) at a current density of 2 mA cm−2, a maximum energy density of 45.5 Wh Kg−1 (52.36 μWh cm−2), and a maximum power density of 5488.7 W Kg−1(6312.0 μW cm−2). Furthermore, the device outperforms the cyclic stability for 10,000 cycles with an 80.1 % capacity retention and exhibits excellent flexibility at different bending angles. The assembled FSSHSCs device successfully illuminates an LED bulb for 70 s.
An interfacially stacked covalent porous polymer on graphene favors electronic mobility: ensuring accelerated oxygen reduction reaction kinetics by an in situ study
Kumar G., Das S.K., Siddharthan E.E., Biswas A., Bhardwaj S., Das M., Thapa R., Dey R.S.
Article, Journal of Materials Chemistry A, 2023, DOI Link
View abstract ⏷
The oxygen reduction reaction (ORR) is largely influenced by material conductivity as well as electron transfer mobility. Usually, covalent porous polymers are fascinating in terms of the surface area and availability of abundant functionalities serving as active sites. However, this class of materials largely suffers from electronic conductivity issues, which limits their extensive application in electrocatalytic reactions. To overcome this long-standing issue, herein, we have developed a metallo [Fe(ii)]-porphyrin-pyrene based pi-conjugated porous polymer (FePP), which was further modified with electrophoretically exfoliated graphene (FePP@G30/3/7). The interfacing of these two units via π-π interaction introduces the flexibility of >C-C< bond rotation resulting in-plane flipping of the bridging -Ph ring from out of plane orientation. The ring flipping-induced co-planarity was investigated through experimental and computational studies. In situ FTIR and operando Raman studies reveal that the ORR process with the FePP@G30/3/7 catalyst follows a 4e− reduction pathway. This phenomenon drives axial as well as equatorial charge mobility within the system influencing the active FeN4 site toward lowering the overpotential for the ORR.
Bond Exchange Mechanism: Unveiling the Volmer-Tafel Pathway and an Electronic Descriptor for Predicting Hydrogen Evolution Reaction Activity of Borophene
Siddharthan E.E., Ghosh S., Thapa R.
Article, ACS Applied Energy Materials, 2023, DOI Link
View abstract ⏷
Hydrogen as a fuel is a promising alternative to harmful fossil fuels, thereby its production by electrocatalytic hydrogen evolution reaction (HER) is an important research problem. Using density functional theory, H2 evolution through the Volmer-Tafel (V-T) pathway is studied on borophene sheets. The results reveal that the sheets exhibit high activity, comparable to platinum. A better theoretical understanding of the mechanism that explains the experimental results is an existing issue. To address this issue, we have proposed that H2 evolves through a bond-exchange mechanism in the Tafel step. The activation energy through this mechanism is drastically reduced, compared to the existing model of direct H2 evolution and matches well with the experimental results in terms of activation energy for HER. It is also tested and observed on other HER catalysts, such as nitrogen-doped graphene, 2H-MoS2, and Pt(111), which proves the proposed mechanism is correct for HER. An electronic property-based descriptor is proposed that shows a good correlation with HER activity on the borophene sheets. This work sheds insights into theoretical modeling of HER and can further lead to better methods toward the understanding of the reaction. Also the electronic descriptor can be tested on similar boron allotropes and reactions.
Controlling the Metal-Ligand Coordination Environment of Manganese Phthalocyanine in 1D-2D Heterostructure for Enhancing Nitrate Reduction to Ammonia
Adalder A., Paul S., Barman N., Bera A., Sarkar S., Mukherjee N., Thapa R., Ghorai U.K.
Article, ACS Catalysis, 2023, DOI Link
View abstract ⏷
Eight-electron nitrate reduction (NO3RR) offers a cost-effective and environmentally friendly route of ammonia production and wastewater remediation. However, identification and reinforcement of the metal-ligand interaction responsible for the catalytic activity in transition-metal phthalocyanine-based heterostructures still remain unclear due to their complexity. Herein, directed by computation, we present a heterostructure approach to couple 2D graphene sheets with 1D manganese (II) phthalocyanine to produce a pyrrolic-N coordinated electron-deficient Mn center that interacts to generate the vital intermediates of the NO3RR process. The catalyst system delivers an ammonia yield rate of 20,316 μg h-1 mgcat-1, a faradaic efficiency (FE) of 98.3%, and an electrocatalytic stability of 50 h. Mechanistic investigations verified by FTIR spectroscopy and theoretical calculations to identify Mn coordinated pyrrolic-N as the active sites in MnPc and RGO reinforce the active sites by orbital interaction for enhancing the charge transfer in the formation of *NOH @ NO3RR intermediates while suppressing the competitive hydrogen evolution reaction (HER), resulting in high selectivity and FE.
Origin of high stability, enhanced specific capacity, and low Li diffusion energy in boron doped Li3V2(PO4)3
Gavali D.S., Abhijitha V.G., Nanda B.R.K., Thapa R.
Article, Journal of Energy Storage, 2023, DOI Link
View abstract ⏷
Li3V2(PO4)3 (LVP) is a well-known cathode material of Li-ion batteries, whereas the specific capacity is limited by the maximum Lithium (Li) de-intercalation probability associated with vacancy energy. The reason behind this limitation is not known yet, which needs to be address, and the modification needs to be done in the electronic structure to get a more specific capacity. In this work, using First-principles calculation, we have found that Li1 (the tetrahedra geometry) is having a higher Li vacancy energy as compared to Li2 and Li3, which limits the Li de-intercalation below x = 1.8, (where x is Li concentration). The change in bandgap of LVP structure after boron (B) substitution is directly correlated to the change in the oxygen (O) occupancy due to B substitution. As of comparing formation energy plot of pristine LVP and B substituted LVBP structure, we have found that in case of LVBP structure formation of x > 2 Li de-intercalation is possible, which help to enhance the specific capacity up to 205 mAh/g. Overall, we identify the cause of the less specific capacity of LVP and provide the solution by suggestion B doping, which also provides less Li diffusion barrier energy.
Octahedral Pd3Cu7 Catalysts on Diverse Support Materials for Efficient Hydrogen Evolution: Theoretical Investigation and Mechanistic Perspective
Swain S., Iqbal A., Patil S.A., Thapa R., Saxena M., Jadhav A.H., Samal A.K.
Article, ACS Applied Materials and Interfaces, 2023, DOI Link
View abstract ⏷
This work showcases a novel strategy for the synthesis of shape-dependent alloy nanostructures with the incorporation of solid substrates, leading to remarkable enhancements in the electrocatalytic performance. Herein, an aqueous medium approach has been used to synthesize an octahedral PdXCuY alloy of different Pd:Cu ratios to better comprehend their electrocatalytic potential. With the aim to outperform high activity and efficient stability, zirconium oxide (ZrO2), graphene oxide nanosheets (GONs), and hexagonal boron nitride nanosheets (hBNNs) solid substrates are occupied to decorate the optimized Pd3Cu7 catalyst with a minimum 5 wt % metal loading. When compared to the counterparts and different ratios, the Pd3Cu7@hBNNs catalyst exhibited an optimal activity for hydrogen evolution reaction (HER). The lower overpotential and Tafel values observed are 64 and 51 mV/dec for Pd3Cu7@hBNNs followed by Pd3Cu7@ZrO2, which showed a 171 mV overpotential and a 98 mV/dec Tafel value, respectively. Meanwhile, the Pd3Cu7@GONs were found to have a 202 mV overpotential and a 110 mV/dec Tafel value. The density functional theory, which achieves a lower free energy (ΔGH*) value for Pd3Cu7@hBNNs than the other catalysts for HER, further supports its excellent performance in achieving the Volmer-Heyrovsky mechanism path. Moreover, the superior HER activity and sturdier resilience after 8 h of stability may be due to the synergy between the metal atoms, monodisperse decoration, and the coordination effect of the support material.
Fe(TCNQ)2 nanorod arrays: an efficient electrocatalyst for electrochemical ammonia synthesis via the nitrate reduction reaction
Mukherjee N., Adalder A., Barman N., Thapa R., Urkude R., Ghosh B., Ghorai U.K.
Article, Journal of Materials Chemistry A, 2023, DOI Link
View abstract ⏷
The electrochemical reduction of nitrate to ammonia (NO3RR) catalyzed by metal organic frameworks (MOFs) is a promising and efficient method for reducing nitrate pollution in water while simultaneously producing a valuable product, ammonia. Herein, we report the 3D nanoarray architecture of the metal organic complex Fe-(tetracyanoquinodimethane)2 Fe(TCNQ)2 as an efficient electrocatalyst that exhibits a high ammonia yield rate of 11 351.6 μg h−1 cm−2 and faradaic efficiency (FE) of 85.2% at −1.1 V vs. RHE and excellent catalytic stability up to 2 days. The excellent catalytic performance is evaluated by ATR-FTIR spectroscopy and a series of control experiments. Density functional-based theoretical calculations are carried out to identify Fe-N4 active sites in metal-organic network structures. This study showcases the advancement of transition metal-based organic frameworks as very effective electrocatalysts for the reduction of nitrate to ammonia (NH3).
Modulated Ultrathin NiCo-LDH Nanosheet-Decorated Zr3+-Rich Defective NH2-UiO-66 Nanostructure for Efficient Photocatalytic Hydrogen Evolution
Sk S., Jamma A., Gavali D.S., Bhasin V., Ghosh R., Sudarshan K., Thapa R., Pal U.
Article, ACS Applied Materials and Interfaces, 2023, DOI Link
View abstract ⏷
Defect engineering through modification of their surface linkage is found to be an effective pathway to escalate the solar energy conversion efficiency of metal-organic frameworks (MOFs). Herein, defect engineering using controlled decarboxylation on the NH2-UiO-66 surface and integration of ultrathin NiCo-LDH nanosheets synergizes the hydrogen evolution reaction (HER) under a broad visible light regime. Diversified analytical methods including positron annihilation lifetime spectroscopy were employed to investigate the role of Zr3+-rich defects by analyzing the annihilation characteristics of positrons in NH2-UiO-66, which provides a deep insight into the effects of structural defects on the electronic properties. The progressively tuned photophysical properties of the NiCo-LDH@NH2-UiO-66-D-heterostructured nanocatalyst led to an impressive rate of HER (∼2458 μmol h-1 g-1), with an apparent quantum yield of ∼6.02%. The ultrathin NiCo-LDH nanosheet structure was found to be highly favored toward electrostatic self-assembly in the heterostructure for efficient charge separation. Coordination of Zr3+ on the surface of the NiCo-LDH nanosheet support through NH2-UiO-66 was confirmed by X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy techniques. Femtosecond transient absorption spectroscopy studies unveiled a photoexcited charge migration process from MOF to NiCo-LDH which favorably occurred on a picosecond time scale to boost the catalytic activity of the composite system. Furthermore, the experimental finding and HER activity are validated by density functional theory studies and evaluation of the free energy pathway which reveals the strong hydrogen binding over the surface and infers the anchoring effect of the ultrathin layered double hydroxide (LDH) in the vicinity of the Zr cluster with a strong host-guest interaction. This work provided a novel insight into efficient photocatalysis via defect engineering at the linker modulation in MOFs.
Ag nanoparticles immobilized over highly porous crystalline organosilica for epoxidation of styrene using CO2 as oxidant
Chatterjee S., Das S., Bhanja P., E. S. E., Thapa R., Ruidas S., Chongdar S., Ray S., Bhaumik A.
Article, Journal of CO2 Utilization, 2022, DOI Link
View abstract ⏷
Selective epoxidation of olefins is a very important reaction as epoxides are widely been used as platform chemical in polymer and pharmaceutical industries. Unlike conventional oxidants like molecular O2 or peroxides, the use of CO2 as a soft oxidant for the oxidation of olefins is very challenging as it offers the utilization of waste and plentiful CO2 together with the potential for the mitigation of its harmful environmental effect. Thus, this process is cost effective and environmentally challenging. Herein, we report the synthesis of a new crystalline porous organosilica material TSOS-1 with orthorhombic crystal structure by using a tailor made bridging organoslilane precursor prepared through the Schiff base condensation of p-terphenyl-4,4′'-dialdehyde and 3-aminopropyl-trimethoxysilane. This novel crystalline material TSOS-1 has been synthesized hydrothermally in the absence of any structure-directing agent and it showed high BET surface area (220 m2 g−1) and nanoscale porosity. TSOS-1 is used as a support for stabilizing tiny AgNPs to obtain a robust nanocatalyst Ag@TSOS-1, which efficiently catalyses the conversion of styrene into predominantly styrene oxide (SO) using CO2 as a soft oxidant in an autoclave reactor under mild reaction conditions.
Descriptors and graphical construction for in silico design of efficient and selective single atom catalysts for the eNRR
Kapse S., Narasimhan S., Thapa R.
Article, Chemical Science, 2022, DOI Link
View abstract ⏷
The electrochemical nitrogen reduction reaction (eNRR) offers the possibility of ammonia synthesis under mild conditions; however, it suffers from low yields, a competing hydrogen evolution reaction pathway, and hydrogen poisoning. We present a systematic approach toward screening single atom catalysts (SACs) for the eNRR, by focusing on key parameters computed from density functional theory and relationships between them. We illustrate this by application to 66 model catalysts of the types, TM-Pc, TM-NXCY, and TM-N3, where TM is a 3d transition metal or molybdenum. We identified the best SACs as Sc-Pc, Cr-N4, Mn-Pc, and Fe-N2C2; these show eNRR selectivity over the HER and no hydrogen poisoning. The catalysts are identified through multi-parameter optimization which includes the condition of hydrogen poisoning. We propose a new electronic descriptor Oval, the valence electron occupancy of the metal center that exhibits a volcano-type relationship with eNRR overpotential. Our multi-parameter optimization approach can be mapped onto a simple graphical construction to find the best catalyst for the eNRR over the HER and hydrogen poisoning.
Improved Oxygen Redox Activity by High-Valent Fe and Co3+Sites in the Perovskite LaNi1- xFe0.5 xCo0.5 xO3
Sheelam A., Balu S., Muneeb A., Bayikadi K.S., Namasivayam D., Siddharthan E.E., Inamdar A.I., Thapa R., Chiang M.-H., Isaac Huang S.-J., Sankar R.
Article, ACS Applied Energy Materials, 2022, DOI Link
View abstract ⏷
Tuning the electronic structure of perovskite oxides via aliovalent substitution is a promising strategy to attain inexpensive and efficient electrocatalysts for energy conversion and storage devices. Herein, following the d-band center positions and using a simple sol-gel method followed by a pyrolysis step, LaNi1-xCo0.5xFe0.5xO3 (LNFCO-x; x = 0.0, 0.4, 0.5, and 0.6) electrocatalysts are designed and synthesized for oxygen redox reactions in 1 M KOH. Among them, LNFCO-0.5 has exhibited the lowest overpotential and the highest charge transfer kinetics in oxygen redox reactions. Overall, a 90 mV lower overpotential was observed in oxygen redox activity of LNFCO-0.5 compared to that of pristine LaNiO3. The mass activity of LNFCO-0.5 in the oxygen reduction reaction (at 0.7 V vs RHE) and oxygen evolution reaction (1.60 V vs RHE) was calculated to be 2.5 and 2.13 times higher than that of LaNiO3, respectively. The bifunctionality index (potential difference between the oxygen evolution at a current density of 10 mA cm-2 and the oxygen reduction at a current density of -1 mA cm-2) of LNFCO-0.5 was found to be 0.98. The substitution of Fe and Co for the Ni-site shifted the d-band center close to the Fermi level, which can increase the binding strength of the *OH intermediate in the rate-determining step. Also, the surface was enriched with Fe3+Δ, Co3+, and partially oxidized Ni3+ states, which is susceptible to tune the eg-orbital filling for superior oxygen redox activity.
Origin of pure and C doped borophene stability and its activity for OER
Article, Applied Surface Science, 2022, DOI Link
View abstract ⏷
In borophene, vacancy is a major reason for stability. However, a detailed understanding relating to vacancy and electronic property remains unexplored. Using Density Functional Theory (DFT) the effect of vacancy and doping on stability, electronic and catalytic properties of borophene is addressed in this work. It is shown how vacancy increases σ-electrons and decreases π-electrons of the neighboring boron atoms with the magnitude decreasing from 4 to 6-coordinated atoms, thereby contributing to their stability. The role of single and dual carbon doping on σ- and π-occupancy is explored. In addition, we have shown that dual carbon doping on β12-borophene analogue reduces the overpotential for oxygen evolution reaction (OER). We observed that the charge deficient boron atom helps in the reduction of oxygen binding and can decrease the overpotential towards OER. Overall, this work would help in fundamental understanding towards borophene stability and aid in choosing the suitable dopant for catalytic applications considering the stability.
Nanoribbons of 2D materials: A review on emerging trends, recent developments and future perspectives
Shinde P.V., Tripathi A., Thapa R., Sekhar Rout C.
Review, Coordination Chemistry Reviews, 2022, DOI Link
View abstract ⏷
The exclusive characteristic properties of two-dimensional (2D) materials have enticed most of the researchers and scientific community to employ a plethora of pioneering stratagems in order to puzzle out modern-day problems. Some of the possibilities are currently are at theoretical phases, while some of them are well-explored experimentally. Nanoribbon-like structures of the 2D materials are ancillary entrants for 2D nanosheets with tunable and enhanced properties. Protuberant properties like abundant and highly exposed active sites on the edges facilitated electron transfer, and superior charge mobility, etc. make them poles apart from their bulk counterparts. Nowadays, these nanoribbon-like structures are contributing to countless applications such as solar cells, photodetectors, sensing, field emission, energy storage, and catalysis. Accordingly, we have put an overview of recent advancements in the properties, preparations, and applications of nanoribbon-like structures based on 2D materials. The contemporary review thoroughly recapitulates the numerous properties of nanoribbon-like materials including crystal structure, bandgap, optical, electrical, magnetic, and catalytic properties. Moreover, the review also addresses the tuning approaches used to optimize the properties of nanoribbon materials. The most recent synthesis approaches used for the preparation of nanoribbon-like structures are also highlighted along with their exceptional implementation in various intriguing applications. Furthermore, this comprehensive review put the finishing touches on a discussion of the present-day outlook and some of the crucial scientific challenges which have to be addressed to improve the performances of nanoribbons of 2D materials-based devices.
Structural Metamorphosis and Band Dislocation of Trirutile NiTa2O6 under Compression
Karmakar S., Mukherjee B., Garg A.B., Gavali D.S., Thapa R., Banerjee S., Mukherjee G.D., Haque A., Behera D.
Article, Journal of Physical Chemistry C, 2022, DOI Link
View abstract ⏷
Trirutile NiTa2O6 has been studied under high pressure by in situ Raman and angle-dispersive synchrotron X-ray diffraction techniques. It undergoes a new quenchable phase at high pressures above 11.8 GPa accompanied by softening of the internal modes ν1(A1g), ν1(Eg), and ν6(Eg), and it is denser by 15% compared with its ambient phase. Various Raman-active modes of NiTa2O6 diminished at high pressures due to the distortion of edge-sharing TaO6 octahedra, which was further confirmed by X-ray diffraction and density functional theory results. The equation of state has been determined using the second-order Birch-Murnaghan equation, and the obtained bulk modulus is 199(4) GPa. The pressure and volume dependence of optical lattice vibrational frequencies and their corresponding Grüneisen parameters are calculated, indicating the inconsistency of the trirutile structure at high pressures, which was accompanied by the strong deformation of TaO6 octahedra. Pressure-induced structural metamorphosis and soft-mode-driven displacive transition related to the mechanical instability of NiTa2O6 are examined and decompression results recommend the transition is irreversible.
Quasi-one-dimensional van der Waals TiS3nanosheets for energy storage applications: Theoretical predications and experimental validation
Patra A., Kapse S., Thapa R., Late D.J., Rout C.S.
Article, Applied Physics Letters, 2022, DOI Link
View abstract ⏷
To cease the ever-increasing energy demand, additional enthusiastic focus has been given to generate more sustainable energy from alternative renewable sources. Storage of these energies for future usage solely banks on energy storage devices. A diversity of electrode materials based on two-dimensional (2D) transition metals and their derivatives have enticed the whole world owing to their tunable properties. Transition metal trichalcogenides (MX3 type) are the emergent class of 2D materials, which gathered a lot of interest because of their quasi-one-dimensional anisotropic properties with the van der Waals force of attraction in between the layers. Herein, TiS3 being a MX3-type of material is preferred as the battery type-supercapacitor electrode for energy storage applications with detailed theoretical predications and experimental validations. The highest capacitance attained for TiS3 is found to be 235 F/g (105 C/g) at 5 mV/s with a battery type of charge storage mechanism. The asymmetric hybrid device is fabricated using Ti3C2Tx MXene nanosheets as a negative electrode, and a brilliant 91% of capacitance retention is accomplished with an extensive potential window of 1.5 V. The investigational discoveries are substantiated by theoretical simulation in terms of the quantum capacitance assessment and charge storage mechanisms.
Highly efficient catalysts of ruthenium clusters on Fe3O4/MWCNTs for the hydrogen evolution reaction
Ramachandra S.K., Nagaraju D.H., Marappa S., Kapse S., Thapa R.
Article, New Journal of Chemistry, 2022, DOI Link
View abstract ⏷
Producing molecular hydrogen (H2) using water provides a sustainable approach for developing clean energy technologies. Herein, we report highly active ruthenium (Ru) clusters supported on iron oxide (Ru/Fe3O4) and Fe3O4/multi-walled carbon nanotubes (Ru/Fe3O4/MWCNTs) by simple electrochemical deposition in a neutral aqueous medium. The supported catalyst exhibits good hydrogen evolution reaction (HER) activity in an acidic environment. Cyclic voltammograms (CVs) of potassium ferrocyanide (K4[Fe(CN)6]) confirm that MWCNTs enhance the electron transfer process by decreasing the redox formal potential. The overpotentials of Ru/Fe3O4/MWCNTs and Ru/Fe3O4 electrocatalysts versus the reversible hydrogen electrode (RHE) were found to be 101 mV and 306 mV to reach a current density of 10 mA cm−2. As prepared, the catalyst displays good stability and retain its HER activity even after 1000 cycles. Furthermore, the stability of Ru/Fe3O4/MWCNTs was studied using the chronopotentiometric (CP) technique for 12 h and found negligible loss in the catalytic activity towards the HER. To explore the role of Ru and underneath MWCNTs in improving the catalytic performance of Fe3O4, density functional theory (DFT) calculations were carried out. DFT calculations indicate that the octahedral site of Ru/Fe3O4 favors the HER with a low overpotential. However, Ru/Fe3O4/MWCNTs are more efficient towards the HER, which could be due to the availability of both octahedral and tetrahedral catalytic sites.
Understanding the role of lithium bonds in doped graphene nanoribbons as cathode hosts for Li-S batteries: A first-principles study
Sinthika S., Pushpa Selvi M., Nimma Elizabeth R., Gavali D.S., Thapa R.
Article, International Journal of Energy Research, 2022, DOI Link
View abstract ⏷
Using first-principles calculations, we investigate a family of doped graphene nanoribbons (GNRs) for their suitability as cathode hosts in lithium-sulfur batteries. We probe the role played by the lone pairs of the dopants in confining the lithium polysulfides (LiPS) to understand the mechanism of binding. Our results show that the Li bond between the polysulfides and the doped GNRs is analogous to a hydrogen bond and also dipole-dipole interactions play a key role in anchoring the polysulfides. A critical donor-Li-acceptor angle of 180° is found to be essential for proper adsorption of LiPS, highlighting the importance of the directionality of lone pairs. The charge lost by the sulfur atom of the polysulfide upon adsorption and shape of the lone pair basins and the value of Electron Localization Function (ELF) at the dopant position can provide a quick estimate of the strength of the bond. Significant contractions in the ELF profiles are also observed upon Li2S adsorption, further providing evidence for the hydrogen bond-like nature of the Li bond. Our results corroborate the fact that all acceptors capable of forming hydrogen bonds can be employed as suitable dopants for carbon-based cathode hosts in Li-S batteries.
First-principles identification of interface effect on Li storage capacity of C3N/graphene multilayer heterostructure
Gavali D.S., Kawazoe Y., Thapa R.
Article, Journal of Colloid and Interface Science, 2022, DOI Link
View abstract ⏷
The design and development of new and light weight two-dimensional (2D) heterostructures as anode materials to enhance the electrochemical properties for Li-ion batteries (LIB's) is a challenge. In this work, using first-principles study, we have demonstrated that the ratio of two-dimensional polyaniline (C3N) and graphene in the multilayer heterostructures plays a major role to define the Li storage properties and to provide metallicity for easy conduction of electrons. We have found that charge transfer between Li and the host depends on the interface and site, which helps in the improvement in specific capacity. The proposed heterostructures shows specific capacity varies from 558 mAh/gm to 423 mAh/gm. The specific capacity is high for heterostructures with more graphene in ratio which is correlated to higher charge accumulation in the host. Also, graphene helps to minimize the open-circuit voltage (OCV) of C3N and maintained an average of 0.4 V. The volume expansion for fully lithiated heterostructures is within 22 %. Li diffusion barrier energy varies in the range of 0.57 to 0.25 eV. The proposed 2D heterostructures could be a future material for anode in LIB's and the description of the interface effect on Li storage properties will help for further development of 2D heterostructure materials.
Low-Basis Weight Polyacrylonitrile/Polyvinylpyrrolidone Blend Nanofiber Membranes for Efficient Particulate Matter Capture
Lakshmanan A., Gavali D.S., Venkataprasanna K.S., Thapa R., Sarkar D.
Article, ACS Applied Polymer Materials, 2022, DOI Link
View abstract ⏷
Particulate matter (PM) in air frequently poses a serious threat to human health. Smaller PM can easily enter into the alveolus and blood vessels with airflow. This work reports the first polyacrylonitrile (PAN)/polyvinylpyrrolidone (PVP) polymer blend nanofiber filter media for effectively capturing PM. Density functional theory (DFT) calculations are used to investigate the effect of the blending of two polymers on the dipole moment and the electrostatic potential. Based on the DFT calculations of the intermolecular interactions between nanofibers and PM, the PAN/ PVP heteromolecular percentage is considered for experimental synthesis, which can provide better performance in the filtration of pollutants. The composite PAN/PVP fiber network was successfully developed and optimized to cope with complex environments during the actual filtration process. The role of the blending ratio of PAN and PVP in wt % was explored on PM2.5 capture, and the refined ratio overcame the conflict between high filtration efficiency and low air pressure resistance. The air filter medium PAN/PVP (6:2) possesses an extremely high air filtration efficiency of 92% under a very low pressure drop of 18 Pa for a 0.5 g m−2 basis weight. Both polar and nonpolar functional groups in blend nanofibers promoted significantly the electrostatic attraction and improved the filtration efficiency under static and dynamic airflow. The PAN/PVP nanofiber membranes maintain outstanding air filtration under different temperature and humidity conditions. This study will shed light on the fabrication of high-efficiency low-basis weight nanofiber filter media as an end product.
Strategic Modulation of Target-Specific Isolated Fe,Co Single-Atom Active Sites for Oxygen Electrocatalysis Impacting High Power Zn-Air Battery
Sarkar S., Biswas A., Siddharthan E.E., Thapa R., Dey R.S.
Article, ACS Nano, 2022, DOI Link
View abstract ⏷
An effective modulation of the active sites in a bifunctional electrocatalyst is essentially desired, and it is a challenge to outperform the state-of-the-art catalysts toward oxygen electrocatalysis. Herein, we report the development of a bifunctional electrocatalyst having target-specific Fe-N4/C and Co-N4/C isolated active sites, exhibiting a symbiotic effect on overall oxygen electrocatalysis performances. The dualism of N-dopants and binary metals lower the d-band centers of both Fe and Co in the Fe,Co,N-C catalyst, improving the overpotential of the overall electrocatalytic processes (ΔEORR-OER = 0.74 ± 0.02 V vs RHE). Finally, the Fe,Co,N-C showed a high areal power density of 198.4 mW cm-2 and 158 mW cm-2 in the respective liquid and solid-state Zn-air batteries (ZABs), demonstrating suitable candidature of the active material as air cathode material in ZABs.
A Unique Bridging Facet Assembly of Gold Nanorods for the Detection of Thiram through Surface-Enhanced Raman Scattering
M. B. B., B. R.P., Tripathi A., Yadav S., John N.S., Thapa R., Altaee A., Saxena M., Samal A.K.
Article, ACS Sustainable Chemistry and Engineering, 2022, DOI Link
View abstract ⏷
Concerns have grown in recent years about the widespread use of the pesticide thiram (TRM), which has been linked to negative effects on local ecosystems. This highlights the critical need for quick and accurate point-of-need pesticide analysis tools for real-time applications. The detection of TRM using gold nanorods (Au NRs) with a limit of detection of 10-11M (10 pM) and an enhancement factor of 2.8 × 106along with 6.2% of signal homogeneity (with respect to the peak at 1378 cm-1) is achieved through surface-enhanced Raman scattering (SERS). The formation of an Au-S bond emphasizes the adsorption of TRM on Au NRs. The addition of Au NRs to TRM of higher and lower concentrations yields a side-by-side assembly (SSA) and a bridging facet assembly (BFA), respectively, and exhibited excellent hotspots for the ultralow detection of TRM. Bridging facets of Au NRs, such as (5 12 0) and (5 0 12) planes, are mainly responsible for the BFA. This kind of interaction is observed for the first time and not reported elsewhere. The detailed facets of Au NRs, namely, side facets, bridging facets, and pyramid facets were demonstrated with the 3D model of Au NRs. The computational studies confirming the SSA and BFA for Au NRs with varying concentrations of TRM are in well agreement with the experimental results. The interaction of Au NRs with TRM is highly sensitive, and the ultralow detection of hazardous TRM through SERS is an ideal technique for environmental protection, real-time applications, and analysis of one-of-a-kind materials.
Low-Temperature Spin-Canted Magnetism and Bipolaron Freezing Electrical Transition in Potential Electron Field Emitter NdNiO3
Karmakar S., Gavali D.S., Mistari C.D., Thapa R., More M.A., Behera D., Haque A.
Article, ACS Applied Electronic Materials, 2022, DOI Link
View abstract ⏷
The orthorhombic nanostructured NdNiO3 is prepared by the sol-gel auto-combustion method, and its temperature-dependent magnetic and electrical transport properties are studied. The electric field emission with density functional theory and current voltage characteristics are also investigated at room temperature. The low-temperature magnetic measurement (magnetization with field and temperature) shows that NdNiO3 undergoes a magnetic phase transition (TN) near 176 K from paramagnetic to spin-canted antiferromagnetic state. The temperature-dependent magnetic susceptibility (χ) reinforced the signature of magnetic phase transition, and it is fitted by the modified Curie-Weiss law. A metal to insulator (MIT) phase transition (∼178 K) is observed above TN from temperature- and frequency-dependent conductivity measurement. It originates due to higher distortion of NiO6 octahedra and bandwidth constriction of NdNiO3 nanostructured compound. The variation of the frequency exponent (n) with temperature illustrates the continuous-time random walk conduction model with bipolaron condensation near MIT and the non-overlapping small polaron tunneling model above room temperature. The spin-resolved density of states calculation exhibits the room temperature paramagnetic phase and metallic nature and helps us to calculate local work function (φ) ∼5.44 eV. Low turn-on field at 1 μA/cm2 ∼10.5 V/μm and high field emission current density 203 μA/cm2 at 21 V/μm are observed for layered NdNiO3 with a field enhancement factor (β) ∼1230, which promotes NdNiO3 as an efficient field emitter. The current-voltage characteristics of NdNiO3/p-Si heterostructures are also explored for future technological applications.
Hydrogen-Bonded Organic Framework Structure: A Metal-Free Electrocatalyst for the Evolution of Hydrogen
Giri L., Mohanty B., Thapa R., Jena B.K., Pedireddi V.R.
Article, ACS Omega, 2022, DOI Link
View abstract ⏷
The hydrogen-bonded organic frameworks (HOFs) have gained significant attention due to their various alluring applications in the fascinating field of supramolecular chemistry. Herein, we report the electrocatalytic activity of HOFs toward the hydrogen evolution reaction (HER) by utilizing the molecular adduct of cyanuric and trithiocyanuric acid with various organic substrates (melamine and 4,4′-bipyridine). Both the experimental and theoretical findings provide insights and validate the electrocatalytic activity toward HER applications. This work contributes significantly to designing novel highly efficient metal-free HOF-based electrocatalysts for the HER.
Understanding the Site-Selective Electrocatalytic Co-Reduction Mechanism for Green Urea Synthesis Using Copper Phthalocyanine Nanotubes
Mukherjee J., Paul S., Adalder A., Kapse S., Thapa R., Mandal S., Ghorai B., Sarkar S., Ghorai U.K.
Article, Advanced Functional Materials, 2022, DOI Link
View abstract ⏷
Green synthesis of urea under ambient conditions by electrochemical co-reduction of N2 and CO2 gases using effective electrocatalyst essentially pushes the conventional two steps (N2 + H2 = NH3 and NH3 + CO2 = CO(NH2)2) industrial process at high temperature and high pressure, to the brink. The single step electrochemical green urea synthesis process has hit a roadblock due to the lack of efficient and economically viable electrocatalyst with multiple active sites for dual reduction of N2 and CO2 gas molecules to urea. Herein, copper phthalocyanine nanotubes (CuPc NTs) having multiple active sites (such as metal center, Pyrrolic-N3, Pyrrolic-N2, and Pyridinic-N1) as an efficient electrocatalyst which exhibits urea yield of 143.47 µg h–1mg–1cat and faradaic efficiency of 12.99% at –0.6 V versus reversible hydrogen electrode by co-reduction of N2 and CO2 are reported. Theoretical calculation suggests that Pyridinic-N1 and Cu centers are responsible to form CN bonds for urea by co-reduction of N2 to NN* and CO2 to *CO, respectively. This study provides the new mechanistic insight about the successful electro-reduction of dual gases (N2 and CO2) in a single molecule as well as rational design of efficient noble metal-free electrocatalyst for the synthesis of green urea.
2D-black phosphorus/polyaniline hybrids for efficient supercapacitor and hydrogen evolution reaction applications
Namsheer K., Kapse S., Manoj M., Thapa R., Rout C.S.
Article, Sustainable Energy and Fuels, 2022, DOI Link
View abstract ⏷
With the emergence of wearable and portable electronics, solid-state supercapacitors are considered as a promising candidate to power electronic devices because of their interesting features compared to existing energy storage devices. Polyaniline is a promising candidate for energy storage and conversion applications due to its high theoretical capacitance and intrinsic conductivity. Still, relatively sluggish rate capability and energy storage performance should be addressed to compete with existing high-performance electrode materials. Here, we fabricated a solid-state supercapacitor electrode with an organic-inorganic hybrids of polyaniline (PANI) and black phosphorus (BP) to overcome the sluggish electrochemical performance. The fabricated supercapacitor device provides an exceptional specific capacitance of 350 mF cm−2 (116 F g−1) at a current density of 0.4 mA cm−2 and displays a high energy density of 31.1 μW h cm−2 at a power density of 330 mW cm−2 along with good cycling stability of 83.3% after 10 000 charge-discharge cycles. Also, this material shows a decent electrocatalytic HER performance with an overpotential value of 128 mV at a current density of 10 mA cm−2, a Tafel slope of 71 mV dec−1, and outstanding stability up to 24 h. Finally, DFT studies confirmed that the PANI/BP hybrid is a more promising electrode material for supercapacitor applications with higher CQ due to a larger density of states near the Fermi level as compared to pristine BP and PANI.
All-Solid-State Supercapacitor Based on Advanced 2D Vanadium Disulfide/Black Phosphorus Hybrids for Wearable Electronics
Sharma A., Kapse S., Verma A., Bisoyi S., Pradhan G.K., Thapa R., Rout C.S.
Article, ACS Applied Energy Materials, 2022, DOI Link
View abstract ⏷
Vanadium disulfide-black phosphorus (VS2-BP) hybrids were synthesized by a one-pot hydrothermal-assisted method to achieve enhanced electrochemical activity for supercapacitor applications. The concentration of BP was optimized to prevent the restacking nature of VS2and to enrich the active edges for electrolytic ion intercalation. The charge storage kinetics of the best-performing VS2-BP as an active electrode has demonstrated the dominance of the pseudocapacitive nature of the material. Furthermore, by sandwiching with a PVA/K2SO4gel electrolyte, an all-solid-state (ASS) vanadium disulfide-black phosphorus-50 mg (VS2-BP-50) symmetric device was developed on highly conductive carbon paper. The ASS VS2-BP-50 symmetric device displays the highest specific areal capacitance of 203.25 mF/cm2and exhibits the maximum areal energy density of 28.22 μW h cm-2at an areal power density of 596.09 mW cm-2, outperforming the previous literature. To understand the origin of the high quantum capacitance, we used density functional theory (DFT) and found that the charge accumulation region between VS2and BP monolayers and the charge transfer are the origin of the improved density of states in the VS2-BP hybrid. Moreover, exceptional mobility of K+ions and a higher diffusion rate were observed using the DFT method.
Lewis acid–dominated aqueous electrolyte acting as co-catalyst and overcoming N2 activation issues on catalyst surface
Biswas A., Kapse S., Ghosh B., Thapa R., Dey R.S.
Article, Proceedings of the National Academy of Sciences of the United States of America, 2022, DOI Link
View abstract ⏷
The growing demands for ammonia in agriculture and transportation fuel stimulate researchers to develop sustainable electrochemical methods to synthesize ammonia ambiently, to get past the energy-intensive Haber-Bosch process. However, the conventionally used aqueous electrolytes limit N2 solubility, leading to insufficient reactant molecules in the vicinity of the catalyst during electrochemical nitrogen reduction reaction (NRR). This hampers the yield and production rate of ammonia, irrespective of how efficient the catalyst is. Herein, we introduce an aqueous electrolyte (NaBF4), which not only acts as an N2-carrier in the medium but also works as a full-fledged “co-catalyst” along with our active material MnN4 to deliver a high yield of NH3 (328.59 μg h21 mgcat21) at 0.0 V versus reversible hydrogen electrode. BF3-induced charge polarization shifts the metal d-band center of the MnN4 unit close to the Fermi level, inviting N2 adsorption facilely. The Lewis acidity of the free BF3 molecules further propagates their importance in polarizing the N≡N bond of the adsorbed N2 and its first protonation. This push-pull kind of electronic interaction has been confirmed from the change in d-band center values of the MnN4 site as well as charge density distribution over our active model units, which turned out to be effective enough to lower the energy barrier of the potential determining steps of NRR. Consequently, a high production rate of NH3 (2.45 × 1029 mol s21 cm22) was achieved, approaching the industrial scale where the source of NH3 was thoroughly studied and confirmed to be chiefly from the electrochemical reduction of the purged N2 gas.
Role of Intrinsic Defects in Enhancing the Photoabsorption Capability of CuZn2AlSe4
Jyothirmai M.V., Thapa R.
Article, ACS Omega, 2022, DOI Link
View abstract ⏷
As a promising candidate for low-cost and eco-friendly thin-film photovoltaics, the emerging quaternary chalcogenide based solar cells have experienced rapid advances over the past decade. Here, we propose quaternary semiconducting chalcogenides CuZn2AlSe4 (CZASe) through cross-substitutions (cation mutations). The nonexistence of imaginary modes in the entire Brillouin zone of CZASe represents the inherent dynamic stability of the system. The electronic, optical, and defect properties of stannite CZASe quaternary semiconducting material was systematically investigated using density functional theory calculations. We have found that the chemical-potential control is very important for growing good-quality crystals and also to avoid secondary-phase formations such as ZnSe, Al2ZnSe4, and Cu3Se2. The observed p-type conductivity is mainly due to antisite defect CuZn, which has the lowest formation energy with a relatively deeper acceptor level than that of the Cu vacant site (VCu). The electronic band structures of vacancies and antisite defects by means of hybrid functional calculations show energy band shifting and energy band narrowing or broadening, which eventually tunes the optical band gap and improves the solar energy-conversion performance of semiconducting CZASe. Our results suggest that the stannite CZASe quaternary chalcogenides could be promising candidates for the efficient earth-abundant thin-film solar cells.
First-Principles Study of Two-Dimensional B-Doped Carbon Nanostructures for Toxic Phosgene Gas Detection
Parey V., Abraham B.M., Gaur N.K., Thapa R.
Article, ACS Applied Nano Materials, 2022, DOI Link
View abstract ⏷
The diverse coordination environment on the surface of carbon-based nanomaterials contributes significantly to their unique adsorption properties. Here, we perform first-principles calculations to determine the sensitivity and selectivity of pristine, 1B, and homonuclear 2B-doped graphdiyne, pentagraphene, and phagraphene structures toward toxic phosgene (COCl2) gas molecule. The strength of the phosgene gas adsorption on the perfect surfaces is negligible, while the substitution of homonuclear boron on the studied substrates can make inert carbon allotropes into an active material for capturing the COCl2gas molecule. Further, the charge density difference and Lowdin charge analysis were computed to provide additional insights for understanding the phenomenon clearly, and the results are completely consistent with the observed trends. The prominent changes in the electronic structure of homonuclear boron-doped surfaces indicate strong reactivity toward phosgene gas molecules, thereby inducing significant variations in the conductivity or resistivity of the sensing device. The πelectron occupancy is correlated with the sensing of carbon materials toward phosgene. Overall, homonuclear 2B-doped graphidyne and 1B/2B-doped pentagraphene display high selectivity and sensitivity with better performance, which makes them potential candidates toward target phosgene gas molecules. These fundamental atomic-scale insights may furnish novel outcomes into the rational design of defect engineered carbon-based nanomaterials for the detection of toxic phosgene gas molecules.
Hierarchical architecture of the metallic VTe2/Ti3C2Tx MXene heterostructure for supercapacitor applications
Sree Raj K.A., Barman N., Radhakrishnan S., Thapa R., Rout C.S.
Article, Journal of Materials Chemistry A, 2022, DOI Link
View abstract ⏷
Layered two-dimensional (2D) materials demonstrate exceptional performance as supercapacitor electrodes due to their unique intrinsic properties. A hybrid 2D/2D heterostructured electrode material can synergize the individual energy storage features of each of the layered 2D materials. Ti3C2 MXene is an emergent 2D material with enriched energy storage capabilities but suffers from certain vulnerabilities during the charge storage process. Vanadium ditelluride (VTe2) is an interesting yet unexplored layered material within the 2D transition metal dichalcogenide (TMD) family. Owing to the unique structural features, VTe2 showcases certain advantages in energy storage. The formation of the VTe2/Ti3C2 MXene heterostructure for energy storage applications has not been reported until now. Herein, we report a facile and simple hydrothermal synthesis approach to prepare bare VTe2 and the VTe2/MXene heterostructure for supercapacitor applications. The energy storage mechanism in the heterostructure is systematically analyzed. The synergistically induced interplaying effects enhance the specific capacitance of the heterostructure to 250 F g−1 along with excellent durability during long cycle operation. Consequently, an asymmetric system is constructed with VTe2/MXene as the positive electrode and MoS2/MXene as the negative electrode. The 2D/2D heterostructure-based asymmetric supercapacitor delivers excellent performance with an energy density of 46.3 W h kg−1 and a highest power density of 6400 W kg−1. Furthermore, the density functional theory calculations predict that the enhancement of electronic Te 5P states near the Fermi level due to MXene leads to improved performance of the VTe2/Ti3C2 hybrid for supercapacitor applications.
CrSe2/Ti3C2 MXene 2D/2D hybrids as promising candidates for energy storage applications
Raj S.K.A., Barman N., Namsheer K., Thapa R., Rout C.S.
Article, Sustainable Energy and Fuels, 2022, DOI Link
View abstract ⏷
Two dimensional material based heterostructures have been widely investigated for high performance energy storage applications due to their distinctive advantages. These heterostructures remove most of the congenital shortcomings belonging to each individual structure. In this study, electrochemical energy storage performance of the chromium diselenide (CrSe2)/Ti3C2 MXene heterostructure is examined. CrSe2 is a lesser explored layered TMD and is a promising candidate for energy storage applications. MXene on the other hand is an emerging material and is being extensively studied for its unique intrinsic energy storage abilities. The CrSe2/Ti3C2 MXene heterostructure electrode displays an enhanced energy storage performance in a basic electrolyte medium. The multi-dimensional hierarchical architecture of CrSe2/Ti3C2 MXene results in an increased number of active sites for electrochemical activities and improves the electrochemical stability. The synergistic interplayed effect triggered by fast ionic/electronic transportation, reduced agglomeration and volume expansion, etc. can be accounted for the enhanced electrochemical performance of the heterostructure. An all-solid-state supercapacitor is fabricated to utilize and demonstrate the energy storage capability of CrSe2/Ti3C2 MXene in real time applications. The fabricated device showed a compelling performance during the electrochemical assessment.
Inner filter effect on amino-functionalized metal-organic framework for the selective detection of tetracycline
Yazhini C., Rafi J., Chakraborty P., Kapse S., Thapa R., Neppolian B.
Article, Journal of Cleaner Production, 2022, DOI Link
View abstract ⏷
The use of tetracycline (TC) for the treatment of infectious diseases caused by bacteria has been humongous over the past few decades. However, the presence of untreated TC in freshwater leads to antimicrobial resistance. To prevent this concern from damaging the freshwater system, a stable sensor with high selectivity and rapid detection towards TC is desirable. Metal-Organic Frameworks (MOFs) are organic-inorganic hybrid structures with excellent stability, multifunctional ability, and tuneable pore structure that find extensive applications in detection techniques. The structural adaptability, excellent porosity and large surface area of MOFs prove advantageous in terms of adsorption of antibiotics. With this insight, a highly stable Zn-based Luminescent Metal-Organic Framework (LMOF) for the selective detection of tetracycline was developed. It is worth mentioning that, the LMOF-based sensor showed no depreciation in luminescence activity and can be reused more than 10 times. Moreover, a very low detection limit of 0.11 μM and a Stern-Volmer quenching constant (Ksv) value of 1.16 × 104 M−1 indicate its sensitivity precision towards TC detection. A significant inner filter effect of TC was observed in addition to adsorption of TC on the MOF through hydrogen bonding. Further, test strips based on IRMOF-3 showed satisfactory recovery in real-time samples. Thus, the stimuli-responsive sensor reports a simple strategy for excellent sensing performance towards TC.
Facile synthesis of alkyl- and arylboronate esters enabled by a carbon nanotube supported copper catalyst
Saini S., Gavali D.S., Bhawar R., Thapa R., Dhayal R.S., Bose S.K.
Article, Catalysis Science and Technology, 2022, DOI Link
View abstract ⏷
An efficient synthesis of alkylboronate esters via alkyl halide borylation catalysed by copper nanoparticles stabilised on nitrogen-doped carbon nanotubes (N-CNT) is reported. This nanocatalyst provides practical access to alkylboronate esters at room temperature in 1 h, with good functional group tolerance. The procedure is also applicable to the borylation of benzyl chlorides and bromides. Radical clock experiments suggest that the reaction involves a radical pathway. The catalyst can be recycled up to ten runs without appreciable loss in the activity. In addition, we demonstrated the use of this supported copper catalyst for the anti-Markovnikov-selective hydroboration of vinylarenes and borylation of aryl halides with B2pin2, providing alkyl- and arylboronate esters, respectively, in good to excellent yields.
Visible Light-Driven Metal-Organic Framework-Mediated Activation and Utilization of CO2for the Thiocarboxylation of Olefins
Saini S., Chakraborty D., Erakulan E.S., Thapa R., Bal R., Bhaumik A., Jain S.L.
Article, ACS Applied Materials and Interfaces, 2022, DOI Link
View abstract ⏷
Visible light-mediated photoredox catalysis has emerged to be a fascinating approach for the activation of CO2 and its subsequent fixation into valuable chemicals utilizing renewable and inexhaustible solar energy. Although great progress has been made in CO2 photoreduction, visible light-assisted organic synthesis using CO2 as a reactive substrate is rarely explored. Herein, we report an efficient, facile, and economically viable photoredox-mediated approach for the synthesis of important β-thioacids via carboxylation of olefins with CO2 and thiols over a porous functionalized metal-organic framework (MOF), Fe-MIL-101-NH2, as a photocatalyst under ambient conditions. This multicomponent reaction offers wide substrate scope, mild reaction conditions, easy work-up, cost-effective and reusable photocatalysts, and higher product selectivity. Computational studies suggested that CO2 interacts with the thiophenol-styrene adduct to facilitate the synthesis of β-thioacids in almost quantitative yields.
Oxygen Functionalization-Induced Charging Effect on Boron Active Sites for High-Yield Electrocatalytic NH3 Production
Biswas A., Kapse S., Thapa R., Dey R.S.
Article, Nano-Micro Letters, 2022, DOI Link
View abstract ⏷
Ammonia has been recognized as the future renewable energy fuel because of its wide-ranging applications in H2 storage and transportation sector. In order to avoid the environmentally hazardous Haber–Bosch process, recently, the third-generation ambient ammonia synthesis has drawn phenomenal attention and thus tremendous efforts are devoted to developing efficient electrocatalysts that would circumvent the bottlenecks of the electrochemical nitrogen reduction reaction (NRR) like competitive hydrogen evolution reaction, poor selectivity of N2 on catalyst surface. Herein, we report the synthesis of an oxygen-functionalized boron carbonitride matrix via a two-step pyrolysis technique. The conductive BNCO(1000) architecture, the compatibility of B-2p z orbital with the N-2p z orbital and the charging effect over B due to the C and O edge-atoms in a pentagon altogether facilitate N2 adsorption on the B edge-active sites. The optimum electrolyte acidity with 0.1 M HCl and the lowered anion crowding effect aid the protonation steps of NRR via an associative alternating pathway, which gives a sufficiently high yield of ammonia (211.5 μg h−1 mgcat−1) on the optimized BNCO(1000) catalyst with a Faradaic efficiency of 34.7% at − 0.1 V vs RHE. This work thus offers a cost-effective electrode material and provides a contemporary idea about reinforcing the charging effect over the secured active sites for NRR by selectively choosing the electrolyte anions and functionalizing the active edges of the BNCO(1000) catalyst.[Figure not available: see fulltext.].
Anisotropic phenanthroline-based ruthenium polymers grafted on a titanium metal-organic framework for efficient photocatalytic hydrogen evolution
Gonuguntla S., Sk S., Tripathi A., Thapa R., Jonnalagadda G., Nayak C., Bhattacharyya D., Jha S.N., Sesha Sainath A.V., Perupogu V., Pal U.
Article, Communications Chemistry, 2022, DOI Link
View abstract ⏷
Conjugated polymers and titanium-based metal-organic framework (Ti-MOF) photocatalysts have demonstrated promising features for visible-light-driven hydrogen production. We report herein a strategy of anisotropic phenanthroline-based ruthenium polymers (PPDARs) over Ti-MOF, a tunable platform for efficient visible-light-driven photocatalytic hydrogen evolution reaction (HER). Several analytical methods including X-ray absorption spectroscopy (XAS) revealed the judicious integration of the surface-active polymer over the Ti-MOF reinforcing the catalytic activity over the broad chemical space. PPDAR-4 polyacrylate achitecture led to a substantial increase in the H2 evolution rate of 2438 µmolg−1h−1 (AQY: 5.33%) compared to pristine Ti-MOF (238 µmol g−1 h−1). The separation of photogenerated charge carriers at the PPDAR-4/Ti-MOF interface was confirmed by the optical and electrochemical investigations. The experimental, as well as theoretical data, revealed their physical and chemical properties which are positively correlated with the H2 generation rate. This offers a new avenue in creating polymer-based MOF robust photocatalysts for sustainable energy.
Synergetic effect of localized and delocalized π electron on Li storage properties of Si/C heterostructures
Article, Carbon, 2021, DOI Link
View abstract ⏷
The composition of two different categories of anode materials, i.e. layered and alloy type is the future of anode material in Li-ion batteries. In our work, using First-principles approach, we proposed multilayer heterostructure of graphene/graphite (layered type) with silicon monolayer (Si-ML) (origin is alloy type), as a potential anode material for LiB's. The synergetic effect of the delocalized π electron of carbon and localized π electron of Si-ML plays a crucial role in maintaining the electron and ion conductivity increases the stability and specific capacity with moderate open circuit voltage. The model structures proposed in this work shows high specific capacities in the range of 748 to 438 mAh-gm−1. This indicates that we can tune the specific capacity using the different ratio of carbon and silicon layers. The localized π electron helps to restrict the volume expansion within 15% for a fully lithiated model structure. The low interlayer diffusion energy of Li-ion makes all the heterostructure as a potential anode material. The proposed study will help to understand the Li storage properties of the carbon/silicon based composite to develop the anode materials with a certain approach.
Charge trapping characteristics of sputter-AlOx/ALD Al2O3/Epitaxial-GaAs-based non-volatile memory
Mahata C., Ghosh S., Chakraborty S., Patro L.N., Tripathi A., Thapa R., Ramakrishna S., Kim S., Dalapati G.K.
Article, Journal of Materials Science: Materials in Electronics, 2021, DOI Link
View abstract ⏷
In this work, a novel memory capacitor structure has been presented with AlOx/Al2O3 bilayer dielectrics on high mobility Epitaxial-GaAs substrate. We have demonstrated the chemical and electrical properties of metal–electrode/AlOx/Al2O3/epi-GaAs-based memory device in detail. Sputter-grown non-stoichiometric AlOx has been used for both the charge trapping layer and blocking layer due to its intrinsic charge trapping capability and high bandgap. Ultra-thin tunneling layer of thicknesses 5 nm and 15 nm were prepared by atomic layer deposition technique and memory properties were compared on promising high mobility Epitaxial-GaAs/Ge heterostructure. The proposed device shows excellent charge trapping properties with a maximum memory window of 3.2 V at sweep voltage of ± 5 V, with good endurance and data retention properties. Oxygen-deficient AlOx layer acted as a charge trapping layer without any additional blocking layer which is impressive for non-volatile memory application on high mobility epi-GaAs substrate. In addition, density Functional Theory (DFT) has been employed to understand the physical origin of the intrinsic charge trapping defects in AlOx dielectric layer.
Synthesis of CTAB-Functionalized Large-Scale Nanofibers Air Filter Media for Efficient PM2.5Capture Capacity with Low Airflow Resistance
Lakshmanan A., Gavali D.S., Thapa R., Sarkar D.
Article, ACS Applied Polymer Materials, 2021, DOI Link
View abstract ⏷
Ambient particulate matter air pollution has become a serious environmental issue and poses grave threats to public health globally. The indoor and outdoor air protection could be achieved by filtering devices and facial masks. The development of air filter to eliminate particulate matter pollution from the air is necessary for human safety. To realize this, here a class of nanofiber air filter is reported with high efficiency and very low pressure drop. By controlling the surface chemistry through cetyltrimethylammonium bromide, it is achieved a >99.9% removal efficiency under extreme hazardous air-quality conditions for PM2.5 with quality factor of 0.469 Pa-1 and low ∼11 Pa pressure drop. The dipole moment and intermolecular interaction between the nanofibers and PM2.5 are investigated by density functional theory calculations. A long-term 15 day filtration test has proven that the nanofiber air filter maintains an excellent efficiency of 99%. This work pushes forward a significant step toward the design and development of high efficiency and a very low pressure drop air filter for various applications.
Novel Carbene Anchored Molecular Catalysts for Hydrogen Evolution Reactions
Brinda K.N., Malecki J.G., Yhobu Z., Nagaraju D.H., Budagumpi S., Erakulan E.S., Thapa R.
Article, Journal of Physical Chemistry C, 2021, DOI Link
View abstract ⏷
The development of low-cost molecular electrocatalysts for the ER from water remains scarce. The efficient electrocatalytic hydrogen volution reactions (HER) with a series of sterically encumbered carbene igated silver(I), gold(I) and nickel(II) complexes established here demonstrate he potential of molecular catalyst in hydrogen production from water. Tuning he benzannulation on the coumarin-substituted N-heterocyclic carbene (NHC) ligands afforded six new silver(I) (5-10), two new gold(I) (11 and 2), and two reported nickel(II) (13 and 14) NHC complexes, which differ by he steric bulk around the metal atom and the counterion. Benefiting from the esirable structure and appropriate porous morphology, complexes 6, 9, 10, and 11 exhibited significant electrocatalytic HER activity in acidic medium with n overpotential of-226.4,-445,-243 and-310 mV vs RHE, respectively, to rive a current density of 10 mA/cm2 when immobilized on glassy carbon lectrode, which is analogous to that of several transition metal-based anomaterials. The kinetic parameters such as Tafel slope value and exchange current density for the active complexes and the ormer observation authenticated the Volmer-Heyrovsky mechanism. Theoretical studies advocate the potential of developing novel eries of carbene-ligated HER electrocatalysts based on the controlled ligand field following scalable and viable protocols.
Cu2O/CuO heterojunction catalysts through atmospheric pressure plasma induced defect passivation
Dey A., Chandrabose G., Damptey L.A.O., Erakulan E.S., Thapa R., Zhuk S., Dalapati G.K., Ramakrishna S., Braithwaite N.S.J., Shirzadi A., Krishnamurthy S.
Article, Applied Surface Science, 2021, DOI Link
View abstract ⏷
A novel route to fabricate Cu2O/CuO heterojunction electrodes using an atmospheric pressure plasma jet (APPJ) is demonstrated. This process promotes favourable band alignment and produces nanoscale CuO surface features from Cu2O with low density of interfacial defects. This electrode can operate without any transparent current collector, showing remarkable currents and stability towards oxygen evolution reaction (OER) (6 mA cm−2 for 2 h at pH14) as well as photocatalytic hydrogen evolution reaction (HER) activity (−1.9 mA cm−2 for 800 s at pH7). When the electrocatalytic oxygen evolution (OER) activity was measured for Cu2O/CuO electrode deposited on FTO substrate the currents increased to ~40 mA cm−2 at 0.8 V vs SCE in 1 M KOH without compensating for the electrode electrolyte surface resistance (iR correction). The composite films also exhibited a high rate towards photo degradation of Methylene Blue (MB) and phenol in the visible spectra, indicating efficient charge separation. We modelled the electronic structure of this epitaxially grown Cu2O/CuO heterojunction using density functional theory. The calculations revealed the distinctive shifts towards Fermi level of the p-band centre of O atom in Cu2O and d-band centre of Cu atom in CuO at the interface contribute towards the increased catalytic activity of the heterostructure. Another factor influencing the activity stems from the high density of excited species in the plasma introducing polar radicals at the electrode surface increasing the electrolyte coverage. This work presents the potential of APPJ functionalization to tune the surface electronic properties of copper oxide based catalysts for enhanced efficiency in OER and HER water splitting.
Scalable Production of Cobalt Phthalocyanine Nanotubes: Efficient and Robust Hollow Electrocatalyst for Ammonia Synthesis at Room Temperature
Ghorai U.K., Paul S., Ghorai B., Adalder A., Kapse S., Thapa R., Nagendra A., Gain A.
Article, ACS Nano, 2021, DOI Link
View abstract ⏷
Electrocatalytic ammonia (NH3) synthesis through the nitrogen reduction reaction (NRR) under ambient conditions presents a promising alternative to the famous century-old Haber-Bosch process. Designing and developing a high-performance electrocatalyst is a compelling necessity for electrochemical NRR. Specific transition metal based nanostructured catalysts are potential candidates for this purpose owing to their attributes such as higher actives sites, specificity as well as selectivity and electron transfer, etc. However, due to the lack of a well-organized morphology, lower activity, selectivity, and stability of the electrocatalysts make them ineffective at producing a high NH3 yield rate and Faradaic efficiency (FE) for further development. In this work, stable β-cobalt phthalocyanine (CoPc) nanotubes (NTs) have been synthesized by a scalable solvothermal method for electrochemical NRR. The chemically synthesized CoPc NTs show excellent electrochemical NRR due to high specific area, greater number of exposed active sites, and specific selectivity of the catalyst. As a result, CoPc NTs produced a higher NH3 yield of 107.9 μg h-1 mg-1cat and FE of 27.7% in 0.1 M HCl at -0.3 V vs RHE. The density functional theory calculations confirm that the Co center in CoPc is the main active site responsible for electrochemical NRR. This work demonstrates the development of hollow nanostructured electrocatalysts in large scale for N2 fixation to NH3.
Energy parameter and electronic descriptor for carbon based catalyst predicted using QM/ML
Kapse S., Janwari S., Waghmare U.V., Thapa R.
Article, Applied Catalysis B: Environmental, 2021, DOI Link
View abstract ⏷
Descriptor based model can be efficient in identifying an optimal carbon-based catalyst for oxygen evolution reaction (OER). Here, we correlate the O-atom adsorption strength with the OER activity of graphene nanoribbon systems and define the energy parameters (ΔGO-ΔGOH) to identify the overpotential (ɳ). The π electron based descriptor can predict the catalytic activity of the graphene surfaces. Machine learning algorithms like Multiple Linear Regression, Random Forest Regression and Support Vector Regression (SVR) are trained on the data generated by density functional theory to predict the overpotential. An optimal active site for OER using proposed SVR model is identified with overpotential (0.29 V) and then validate through DFT calculations. To generalize the study, we used SVR model on N doped GNR to predict the site-specific activity towards OER. Such a combined approach can be extended to estimate the site-specific OER activity of different carbon catalysts at a dramatically reduced computational cost.
Controlled Loading of MoS2on Hierarchical Porous TiO2for Enhanced Photocatalytic Hydrogen Evolution
Tiwari A., Gautam A., Sk S., Gavali D.S., Thapa R., Pal U.
Article, Journal of Physical Chemistry C, 2021, DOI Link
View abstract ⏷
In this study, we garnered three important factors simultaneously, namely, wormhole mesoporosity of TiO2with well-designed interfaces for effective charge transfers, precise loading of MoS2for plasmon induction, and increased surface area with exposed surface atoms and active sites. The controlled loading of MoS2on porous TiO2(MPT) forms a heterojunction that effectively modulates the interface engineering and thereby greatly enhances hydrogen evolution. The synthesis of a photocatalyst is based on a simple hydrothermal process that is well characterized. The resulting composite materials were tested for hydrogen evolution reactions. At optimum loading, MPT10induced a maximum hydrogen evolution rate of 1376 μmol h-1g-1with 2.28% apparent quantum yield (AQY), which was 10-fold higher compared to the MCT10(MoS2-commercial TiO2) H2evolution rate of 138 μmol h-1g-1with 0.23% AQY under similar reaction conditions. The shorter decay component, lower emission intensity, and higher estimated lifetime of MPT10suggest its superiority over other materials. Density functional theory (DFT) calculations have further revealed the active sites of MPT and hierarchical porous TiO2(HPT) to support the experimental hydrogen evolution reaction (HER). This study suggests an avenue to design an efficient noble-metal-free photocatalyst for solar fuel productions.
Stable and boosted oxygen evolution efficiency of mixed metal oxide and borate planner heterostructure over heteroatom (N) doped electrochemically exfoliated graphite foam
Borah M., Sikdar A., Kapse S., Majumdar A., Dutta P., Karim G.M., Deb S., Thapa R., Maiti U.N.
Article, Catalysis Today, 2021, DOI Link
View abstract ⏷
Support-catalyst interface plays a critical role in electrocatalytic processes as the rates of involved reactions are directly linked with the interfacial charge transfer efficiency. Here, we are presenting nitrogen doped exfoliated graphite foil (NGF) as the 3D support which offers electronically coupled interface to catalytic heterostructure comprises of nickel-iron based oxide and its borate. As developed hybrid (NGF/NiFe/Borate) show excellent oxygen evolution reaction (OER) under alkaline condition with low overpotential of 281 mV at current density of 10 mA cm−2 and Tafel slope 62 mv dec−1. The results beat most nonprecious metal catalysts and noble commercial IrO2 deposited over same NGF substrate with outstanding long-term durability over 10 h at high current density 300 mA cm−2. Efficient electron injection from NGF support and its low resistance leads to enhancement in current density by more than 136 % at operative potential of 1.55 V, in comparison to un-doped exfoliated graphite foil support. Beside electronically tuned support, high catalytic efficiency is also linked with planner growth of borate nanosheets that helps in fast interfacial charge transfer in its transverse direction to the catalytic site. Our work highlights the importance of electronic tuning of catalyst support to enhance the performance of supported borate catalyst and to rationally design low cost, highly active OER catalyst.
Computationally exploring the role of S-dopant and S-linker in activating the catalytic efficiency of graphene quantum dot for ORR
Banerjee P., Das G.P., Thapa R.
Article, Catalysis Today, 2021, DOI Link
View abstract ⏷
To enhance the catalytic efficiency of graphene quantum dot (GQD) for ORR, the role of S-dopant and S-linker is explored here by utilizing the first-principles density functional theory-based approach. The ORR efficiency is found to be changed on different active sites with the variation of dopant position and dopant configuration on the sulfurized GQD. Charge transfer from the dopant site to the active sites and to the intermediates, as well as the binding energy of intermediates on the active sites play an important role to realize the potential determining step, onset potential and the overpotential for ORR. The increase in the doping concentration of S or the presence of oxygen is found to drastically enhance the ORR efficiency of GQD. Furthermore, S-linkers are exploited to connect two GQDs entangled or twisted with each other and resulting in a rather complex system. The combined effect of entangling, S-linkers and presence of a single N dopant at the meta position of the active site resulted in the adsorption of the intermediates with optimum binding strength. Consequently, the ORR activity is found to be enhanced, thereby resulting in a metal-free and cost-effective electrocatalyst with efficiency similar to the Pt-based metal catalysts.
Promoting reactivity of graphene based catalysts to achieve LH mechanism for CO oxidation
Article, Catalysis Today, 2021, DOI Link
View abstract ⏷
In heterogeneous catalysis, the LH (Langmuir Hinshelwood) mechanism is more efficient and recommended over any other mechanism adopted for CO oxidation. However, the LH mechanism over a carbon surface is a challenge and is paused to be applied to any carbon surface in practical application. In this work, we carried out density functional theory to study how the incorporation of nitrogen, boron atom (C→N, C→B) and co-doping on graphene nanoribbons can modify the catalytic activity of the surface and the preference between ER (Eley-Ridel) and LH mechanism is explored. Boron plays a crucial role in the adsorption of both CO and O2, whereas nitrogen doping can only activate O2 molecule through altering the triplet ground state. Considering the activation energy for the first CO2 formation and CO binding strength, we have defined a window to identify the operation of the LH mechanism in the catalysis. 3B edge doped AGNR (armchair graphene nanoribbon doped with three boron atoms at its edge) is identified as an active catalyst for the CO oxidation through the LH mechanism, with the SA (Sabatier activity) and TOF (turnover frequency) values of 0.90 and 2.47, respectively. This approach will help to search for metal-free catalysts for the CO oxidation with the efficient LH mechanism.
Advanced catalyst
Editorial, Catalysis Today, 2021, DOI Link
One-pot solvothermal synthesis of Co2P nanoparticles: An efficient HER and OER electrocatalysts
Jebaslinhepzybai B.T., Partheeban T., Gavali D.S., Thapa R., Sasidharan M.
Article, International Journal of Hydrogen Energy, 2021, DOI Link
View abstract ⏷
Development of an inexpensive electrocatalyst for hydrogen evolution (HER) and oxygen evolution reactions (OER) receives much traction recently. Herein, we report a facile one-pot ethyleneglycol (EG) mediated solvothermal synthesis of orthorhombic Co2P with particle size ~20–30 nm as an efficient HER and OER catalysts. Synthesis parameters like various solvents, temperatures, precursors ratios, and reaction time influences the formation of phase pure Co2P. Investigation of Co2P as an electrocatalyst for HER in acidic (0.5 M H2SO4) and alkaline medium (1.0 M KOH), furnishes low overpotential of 178 mV and 190 mV, respectively to achieve a 10 mA cm−2 current density with a long term stability and durability. As an OER catalyst in 1.0 M KOH, Co2P shows an overpotential of 364 mV at 10 mA cm−2 current density. Investigation of Co2P NP by XPS analysis after OER stability test under alkaline medium confirms the formation of amorphous cobalt oxyhydroxide (CoOOH) as an intermediate during OER process.
Unveiling the genesis of the high catalytic activity in nickel phthalocyanine for electrochemical ammonia synthesis
Murmu S., Paul S., Kapse S., Thapa R., Chattopadhyay S., N. A., Jha S.N., Bhattacharyya D., Ghorai U.K.
Article, Journal of Materials Chemistry A, 2021, DOI Link
View abstract ⏷
Electrochemical ammonia synthesis by the nitrogen reduction reaction (NRR) using an economically efficient electrocatalyst can provide a substitute for the Haber-Bosch process. However, identification of active sites responsible for the origin of catalytic activity in transition metal phthalocyanine is a difficult task due to its complex structure. Herein, density functional theory (DFT) is applied to identify the probable active sites of nickel phthalocyanine (NiPc) for the NRR as well as the origin of catalytic activity which is associated with the d band center and density of states (DOS) of Ni in NiPc. Accordingly, NiPc nanorods (NRs), synthesized by a solvothermal method in large scale, exhibit an NH3yield rate about 85 μg h−1mgcat−1and a faradaic efficiency (FE) of 25% at −0.3 Vvs.RHE. Moreover, the catalyst shows long term stability up to 30 hours while maintaining the NH3yield and FE. The isotopic labelling experiment and other control investigation led to validation of the nitrogen source in NH3formation. This study provides brand new insightful understanding of the active sites and the origin of the catalytic activity of NiPc for their NRR applications.
Nitrogen vacancy and hydrogen substitution mediated tunable optoelectronic properties of g-C3N4 2D layered structures: Applications towards blue LED to broad-band photodetection
Ghosh A., Saini H., Sarkar A., Guha P., Samantara A.K., Thapa R., Mandal S., Mandal A., Behera J.N., Ray S.K., Goswami D.K.
Article, Applied Surface Science, 2021, DOI Link
View abstract ⏷
Graphitic carbon nitride (g-C3N4), a 2D-organic semiconductor, has rapidly emerged as a potential alternative to the 2D-inorganic semiconductors in photocatalysis, but rare studies have been made hitherto about its applicability in optoelectronic devices. Considering the specific requirements of light-emitting diodes with efficient recombination of injected-carriers and photodetector devices with better charge separation, this work deals with synthesizing two variants of g-C3N4 samples with exclusively modified optical/electronic properties while keeping its basic structural framework. One sample is two-coordinated nitrogen deficient g-C3N4 (Nd-gCN) having very high photoluminescence (PL) and the other is hydrogen substituted g-C3N4 (H-gCN) exhibiting vanishingly low PL and ≈0.66 eV smaller bandgap than Nd-gCN. Role of nitrogen-vacancy and hydrogen substitution towards modulating optical/electronic properties of g-C3N4 are studied by combining experiments and density functional theory. Following strong luminescence, Nd-gCN sample manifests visibly blue emission in light-emitting devices; contrarily H-gCN sample shows potential in demonstrating efficient broadband photodetection. Besides moderate self-powered feature, photodetectors perform best at –5.0 V, corresponding to the highest responsivity Rλ=0.34A/W, EQEλ=59% and response time (0.18/0.29 sec). Efficient broadband photodetection performance of the heterojunction-devices is ascribed to the conjunct effects of drastic reduction in photogenerated carrier recombinations (PL quenching) and broadening of absorption regime facilitated by reduced bandgap and Si self-absorption.
Enhanced energy storage performance and theoretical studies of 3D cuboidal manganese diselenides embedded with multiwalled carbon nanotubes
Samal R., Bhat M., Kapse S., Thapa R., Late D.J., Sekhar Rout C.
Article, Journal of Colloid and Interface Science, 2021, DOI Link
View abstract ⏷
The burst of energy produced from the sustainable energy sources need to be harnessed by energy storage systems. Development of novel and advanced energy storage devices such as supercapacitors discover an enormous future ahead. Recently, hybrid supercapacitors (electric double layer capacitor (EDLC) and pseudocapacitors) trend to be used as energy storage interfaces for their improved efficacy in energy density without altering the power density. In the ongoing workplan, transition metal selenides MnSe2 and its hybrid with multiwalled carbon nanotubes (MWCNTs) are synthesized by a simplistic hydrothermal protocol. Certainly, cubic phases of MnSe2-MWCNT(MS/CNT) manifested superior electrochemical performance in both symmetric and asymmetric full cell configurations in contrast to prestine MnSe2(MS). The asymmetric MS/CNT cell achieved an excellent charge storage capability with an high energy density of 39.45 Wh kg−1 at a power density of 2.25 kW kg−1 maintaining an energy density of 14.5 Wh kg−1 at a high power density of 4.5 kWh kg−1 and also revealed long term stability over 5000 consecutive charge/discharge cycles (capacitance retention of 95.2%). Furthermore, the preferable growth along (2 0 0) direction in the presence of MWCNTs favoured in enriching the supercapacitive property of MS. The quantum capacitance of MnSe2 surfaces and MS/CNT heterostructure has been estimated using density functional theory simulation to confirm the experimental outcomes. Theoretical investigation simultaneously exposed the contribution of (2 0 0) plane of MnSe2 and MWCNTs cultured in enhanced DOS (density of states) near the Fermi level that remarkably promoted the energy storage efficiency of MS/CNT.
Ternary VS2/ZnS/CdS hybrids as efficient electrocatalyst for hydrogen evolution reaction: Experimental and theoretical insights
Shinde P.V., Gavali D.S., Thapa R., Singh M.K., Rout C.S.
Article, AIP Advances, 2021, DOI Link
View abstract ⏷
Widely used precious metal (i.e., Pt, or Pd) electrocatalysts need to be replaced with other cost-effective and earth-abundant materials for economical water splitting applications. Recently, two-dimensional (2D) transition metal dichalcogenides (MoS2, VS2, WS2, etc.) have emerged as ideal electrocatalysts for the hydrogen evolution reaction (HER) due to their tunable physicochemical properties and rich catalytic active sites. In this regard, we propose a strategy to achieve improved HER performance of VS2 by fabricating a hybrid material with transition metal (Zn and Cd)-based sulfides. A facile hydrothermal approach is employed to prepare a VS2/ZnS/CdS hybrid catalyst that exhibits remarkable electrocatalytic performance for the HER in acidic media with a small overpotential of 86 mV at 10 mA/cm2 and a Tafel slope of 74.4 mV/dec. This inferred the Volmer-Heyrovsky mechanism with electrochemical desorption of hydrogen as the rate-limiting step. High performance is attributed to the abundance of catalytically active sites and the synergistic interactions between the materials. Theoretical calculations reveal that the VS2/ZnS/CdS hybrid shows favorable HER activity owing to its low hydrogen adsorption free energy of about 0.35 eV. We believe that this work on designing 2D VS2/ZnS/CdS will offer a new pathway to discover an efficient H2 generation electrocatalyst.
Metal-Free Triazine-Based 2D Covalent Organic Framework for Efficient H2 Evolution by Electrochemical Water Splitting
Ruidas S., Mohanty B., Bhanja P., Erakulan E.S., Thapa R., Das P., Chowdhury A., Mandal S.K., Jena B.K., Bhaumik A.
Article, ChemSusChem, 2021, DOI Link
View abstract ⏷
Hydrogen evolution reaction (HER) by electrochemical water splitting is one of the most active areas of energy research, yet the benchmark electrocatalysts used for this reaction are based on expensive noble metals. This is a major bottleneck for their large-scale operation. Thus, development of efficient metal-free electrocatalysts is of paramount importance for sustainable and economical production of the renewable fuel hydrogen by water splitting. Covalent organic frameworks (COFs) show much promise for this application by virtue of their architectural stability, nanoporosity, abundant active sites located periodically throughout the framework, and high electronic conductivity due to extended π-delocalization. This study concerns a new COF material, C6-TRZ-TFP, which is synthesized by solvothermal polycondensation of 2-hydroxybenzene-1,3,5-tricarbaldehyde (TFP) and 4,4′,4′′-(1,3,5-triazine-2,4,6-triyl)tris[(1,1′-biphenyl)-4-amine]. C6-TRZ-TFP displayed excellent HER activity in electrochemical water splitting, with a very low overpotential of 200 mV and specific activity of 0.2831 mA cm−2 together with high retention of catalytic activity after a long duration of electrocatalysis in 0.5 m aqueous H2SO4. Density functional theory calculations suggest that the electron-deficient carbon sites near the π electron-donating nitrogen atoms are more active towards HER than those near the electron-withdrawing nitrogen and oxygen atoms.
Nitrogen doping derived bridging of graphene and carbon nanotube composite for oxygen electroreduction
Marbaniang P., Kapse S., Ingavale S., Thapa R., Kakade B.
Article, International Journal of Energy Research, 2021, DOI Link
View abstract ⏷
In this work, we report a cost-effective electro-catalyst for oxygen reduction reaction (ORR) by developing a composite between graphene oxide and carbon nanotubes and simultaneously doping with a nitrogen atom. The nitrogen-doped carbon nanotube/graphene oxide composites were prepared by chemical oxidation method followed by annealing process. Such composite enhances the active adsorption sites, electrical conductivity and corrosion resistance leading to better impact toward ORR. The N-graphene oxide/functionalized carbon nanotube catalyst that furnishes both pyridinic-N and pyridinic-N-O (or oxidic-N) shows higher efficiency toward electrocatalytic ORR performance in alkaline conditions. More interestingly, as-prepared electrocatalyst shows durability test up to 20 000 potential cycles without loss in its half-wave potential (Ehalf) while commercial Pt/C catalyst shows a huge loss in Ehalf value under similar conditions, indicating sustainability in the performance of former, especially compared to the reported metal or metal-free electrocatalysts. The origin of high catalytic activity of oxidic-N is correlated with π-electron density at the Fermi level studied by density functional theory and considered as an electronic descriptor.
Design principle of MoS2/C heterostructure to enhance the quantum capacitance for supercapacitor application
Article, Journal of Energy Storage, 2021, DOI Link
View abstract ⏷
1T Molybdenum disulfide (1T-MoS2) has been widely studied experimentally as an electrode for supercapacitors due to its excellent electrical and electrochemical properties. Whereas the capacitance value in MoS2 is limited due to the lower density of electrons near the Fermi level, and unable to fulfill the demand of industry i.e. quantum capacitance preferably higher than 300 μF/cm2. Here, we investigated the performance of 2H, 1T, and 1T′ phases of MoS2 in its pristine form and heterostructures with carbon-based structures as an electrode in the supercapacitors using density functional theory. Specifically, we reported that the underneath carbon nanotube (CNT) is responsible for the structural phase transition from 1T to 1T′ phase of MoS2 monolayer in 1T′-MoS2/CNT heterostructure. This is the main reason for a large density of states near Fermi level of 1T′-MoS2/CNT that exhibits high quantum capacitance (CQ) of 500 μF/cm2 at a potential of 0.6 V. Also, we observed that the nitrogen doping and defects in the underneath carbon surface amplify the CQ of heterostructure for a wider range of electrode potential. Therefore, the 1T′-MoS2/N doped CNT can be explored as an electrode for next-generation supercapacitors.
Structural, dielectric, electrical properties of Nd doped double perovskite ceramics and variation of density of states upon doping
Ray A., Basu T., Behera B., Gavali D.S., Thapa R., Vajandar S., Osipowicz T., Nayak P.
Article, Materials Chemistry and Physics, 2020, DOI Link
View abstract ⏷
BiFeO3-based composite materials are important due to their versatile multiferroic properties which can be further tuned upon doping. 0.5BiNdxFe1-xO3-0.5PbZrO3 (x = 0.05, 0.1, and 0.2) polycrystalline ceramic samples were prepared by solid-state reaction technique at high temperature. X-ray diffraction (XRD) and Rietveld refinement process confirm the presence of mainly rhombohedral (R3c) phase. Dielectric studies reveal the presence of Maxwell-Wagner type polarization. Electric impedance and modulus values of the samples were studied over a wide range of temperature and frequency. Complex impedance and modulus studies shows the presence of non-Debye type of relaxation in the materials. ac conductivity results can be fitted with Jonscher's power law and indicates the dominance of correlated barrier hopping mechanism in charge transport. Furthermore, density of states calculated from ac conductivity shows variation as a function of dopant content which is further corroborated from density functional theory-based results.
Microporous networks of NiMn2O4 as a potent cathode material for electric field emission
Karmakar S., Mistari C.D., Parey V., Thapa R., More M.A., Behera D.
Article, Journal of Physics D: Applied Physics, 2020, DOI Link
View abstract ⏷
The electric field-induced sterling electron emission of NiMn2O4 microporous networks synthesized via the sol-gel auto combustion route was investigated. Some primary characterization techniques such as x-ray diffraction, Fourier-transform infrared spectroscopy, and Raman spectroscopy were performed to confirm the pure crystallinity and metal oxide (Ni-O and Cr-O) stretching vibrations and also to provide a molecular fingerprint of the NiMn2O4 porous network. The distinct field emission (FE) properties of the NiMn2O4 microporous network was observed which was correlated with an electric field induced electron tunneling F-N (Fowler-Nordheim) model from a nearly planner conducting emitter surface with triangular potential-energy barrier approximation. A low turn-on field of 4.15 V µm-1 and threshold field of 5.25 V µm-1 were detected to draw emission current densities of 1 µA cm-2 and 10 µA cm-2 respectively. The local work function (Φ) of 5.509 eV for the NiMn2O4 porous network was computed using density functional theory (DFT) and it exhibits an impressive field enhancement factor (β) of 3381 with good FE current stability. These results demonstrate the potential application of this material for future vacuum micro/nanoelectronics and FE panel display applications.
Role of van der Waals interaction in enhancing the photon absorption capability of the MoS2/2D heterostructure
Saini H., Jyothirmai M.V., Waghmare U.V., Thapa R.
Article, Physical Chemistry Chemical Physics, 2020, DOI Link
View abstract ⏷
van der Waals (vdW) interaction-based heterostructures are known for enhanced photon absorption. However, the origin of these phenomena is not yet completely understood. In this work, using first-principles calculations, we provide a comprehensive study to show the effect of vdW interactions on the optical and electrical characteristics of the device and its origin. Herein, MoS2/2D (where 2D varies as graphene, black and blue phosphorene, and InSe) vdW heterojunctions are considered as model structures. The change in the band gap of the heterostructures is because of hybridisation and the non-linearity of the exchange-correlation functional. Hybridisation is correlated with strain and the difference in interstitial potential between layers of the heterostructure and the vacuum level. Significantly, the estimated values of energy conversion efficiency are high in the case of MoS2/InSe and MoS2/BlackP vdW heterostructures as compared to MoS2/GR and MoS2/BlueP, suggesting their potential application in efficient and atomically thick excitonic solar cell devices.
Electric field emission and anomalies of electrical conductivity above room temperature in heterogeneous NiO-SnO2 nano-ceramic composites
Karmakar S., Parey V., Mistari C.D., Thapa R., More M.A., Behera D.
Article, Journal of Applied Physics, 2020, DOI Link
View abstract ⏷
Microstructural NiO-SnO2 nano-ceramic matrix was synthesized via a solgel auto-combustion technique with a perspective to investigate its noteworthy electric field emission and temperature-induced conduction anomaly. Exceptional field emission performance of nickel-tin oxide composites was discovered with a low turn-on field of 3.9 V/μm and a threshold field of 5.30 V/μm with a good field emission current density of 110.44 μA/cm2 and current stability. Density functional theory was employed to estimate its local work function (φ) 3.365 eV, and the field enhancement factor (β) was obtained as 1570 by Fowler-Nordheim plot. The anomalies in conductivity spectra at 523 K were detected by a number of physical properties measurement including impedance, conductivity, dielectric, and differential scanning calorimetry with thermal expansion. These phenomena can be rationalized in terms strain-dependent thermal hysteresis effects and localized/delocalized e g electron with a transition from inferior conductive linkage [Ni2+-O2--Ni2+] and [Sn2+/Sn4+-O2--Sn2+/Sn4+] to higher conductive linkage [Ni2+-Ni3+] and [Sn2+-Sn4+] of coupled NiO-SnO2 matrix. The temperature dependence frequency exponent (n), ln τ, Rg, Rgb, Cg, and Cgb support additionally the conduction anomaly behavior, and the variation of dielectric constant (ɛr) and loss (tan δ) with temperature around 523 K has been explained in terms of the reduction of space charge layers due to reversal movement of delocalized e g electrons from the grain boundary limit. The frequency dispersing impedance, conductivity, and dielectric spectra with elevated temperature were also demonstrated to comprehend its conduction mechanism with theoretical correlation.
Stress-Induced Electronic Structure Modulation of Manganese-Incorporated Ni2P Leading to Enhanced Activity for Water Splitting
Sarkar S., Dheer L., Vinod C.P., Thapa R., Waghmare U.V., Peter S.C.
Article, ACS Applied Energy Materials, 2020, DOI Link
View abstract ⏷
The cornerstone of the emerging hydrogen economy is hydrogen production by water electrolysis with concomitant oxygen generation. Incorporating a third element in metal phosphides can tune the crystalline and electronic structure, hence improving the electrocatalytic properties. In this work, Mn-doped Ni2P with varying ratios of Mn and Ni has been explored as excellent catalysts for water splitting. A complete cell made of the best catalyst Ni1.5Mn0.5P electrodes showed low voltage of 1.75 V at a current density of 10 mA cm-2 due to enhanced electrical conductivity, induction of tensile stress, enhanced electrochemical surface area, and increased electric dipole upon Mn incorporation.
Fowler–Nordheim Law Correlated with Improved Field Emission in Self-Assembled NiCr2O4 Nanosheets
Karmakar S., Parey V., Mistari C.D., Thapa R., More M.A., Behera D.
Article, Physica Status Solidi (A) Applications and Materials Science, 2020, DOI Link
View abstract ⏷
Electric field emission (FE) properties are measured on self-assembled NiCr2O4 nanosheets in a planner “diode” arrangement at a base pressure of ≈1.0 × 10−8 mbar. The turn-on field at FE current density 1 μA cm−2 and the threshold field at FE current density 10 μA cm−2 are observed as 4.10 and 4.94 V μm−1, respectively. The local work function (Φ) is calculated as 4.358 eV, using density functional theory (DFT) in Quantum Espresso code and field enhancement factor (β) intensified up to 2074 from the sharp edges of the NiCr2O4 nanosheet arrays’ emitter surface. An exemplary FE current stability is observed from the current–time plot over a period of 4.5 h. The field enhancement factor (β), inferior turn-on field, and superior stability compared with any other pristine nanosheet compound recommend its potential application in flat panel display and vacuum micro-/nanoelectronics.
B2H6 splitting on catalytic surfaces and role of BH3 towards hydrogen spillover
Erakulan E.S., Kumar E.M., Jena P., Thapa R.
Article, Journal of Power Sources, 2020, DOI Link
View abstract ⏷
A fundamental understanding of the spillover mechanism is an open and challenging problem and plays an important role in catalysis. In particular, bond-exchange spillover mechanism is considered to be effective for reversible storage and release of hydrogen at near ambient conditions. For this, three critical steps are needed: finding the right support (acceptor), the right catalyst to split H2, and ensuring that once H2 is split, the H atoms can migrate on the surface with the help of secondary catalysts and eventually hydrogenate the entire material. In this paper we address these challenges using density functional theory. We show that BH3, a secondary catalyst, can be produced by symmetrically splitting its stable precursor, B2H6, on doped metal-free surfaces such as graphene and h-BN as well as on MOF5. In addition, to reduce computational cost, we develop structural descriptor and predictive model equation to effectively screen potential BH3 binding sites. Symmetrical splitting of B2H6 on different types of materials can address the hydrogen spillover challenge, making efficient storage of hydrogen possible.
Defect-Engineered MoO2Nanostructures as an Efficient Electrocatalyst for Oxygen Evolution Reaction
Guha P., Mohanty B., Thapa R., Kadam R.M., Satyam P.V., Jena B.K.
Article, ACS Applied Energy Materials, 2020, DOI Link
View abstract ⏷
This article presents the experimental and theoretical insights into defect-engineered MoO2 nanostructures (NSs) in terms of oxygen vacancy and OH- occupancy toward oxygen evolution reaction (OER). Two categories of β-MoO2 NSs are grown on a silicon substrate via a hydrogenation process from pregrown α-MoO3 structures. The postgrown MoO2 system gets OH- occupancy after 7 h of annealing (MoO2+OH-). On increasing the annealing duration to 9 hrs, both oxygen vacancies and OH- occupancy have been made into the MoO2 system (MoO2-x+OH-). The as-grown materials have been assessed for promising energy conversion applications toward electrocatalytic OER. The as-grown MoO2-x+OH- very efficiently catalyzes the OER at a lower overpotential and yields a higher current density compared to the as-grown MoO2+OH- and commercial MoO2. Both the oxygen vacancy and OH- occupancy in the MoO2 system play a synergistic role in enhancing the OER properties. The experimental observations are validated theoretically and plausibly explained with the help of a state-of-the-art density functional study. The simulation calculations reveal that the introduction of oxygen vacancy and OH- occupancy lowers the overpotential of OER. The OH- ions act passively on the surfaces of MoO2 that decrease the binding of reaction intermediates and aid in easy desorption of O2 molecules. Besides, the oxygen defect sites reduce the charge-transfer resistance, which eventually reduces the OER overpotential. Our empirical findings with theoretical supports render a significant shrewdness to the electrocatalytic performances of the defect-engineering MoO2 systems toward OER applications.
Superior field emission and alternating current conduction mechanisms for grains and grain boundaries in an NiO-[CdO]2 nanocomposite
Karmakar S., Raviteja B., Mistari C.D., Parey V., Thapa R., More M.A., Behera D.
Article, Journal of Physics and Chemistry of Solids, 2020, DOI Link
View abstract ⏷
A hierarchical NiO-[CdO]2 nanocomposite has been synthesized by a sol–gel auto-combustion route and characterized with a view to studying the electric field emission and conduction mechanism therein. The structural features, surface morphologies, and elemental compositions of the as-prepared samples have been characterized by XRD, Raman, FESEM, and TEM techniques. A low turn-on field (4.50 V/μm) and threshold field (5.04 V/μm) were found to be sufficient to draw emission current densities of 1 μA/cm2 and 10 μA/cm2 from NiO-[CdO]2-modified cathodes. A maximum emission current density of 121 μA/cm2 at a low applied electric field of 6.5 V/μm and long emission current stability were achieved at a preset value of 5 μA. The field enhancement factor (β) was determined as 1854 in the high-field region by computing the local work function (φ) through density functional theory (DFT), and the entire field emission (FE) performances have been compared with those of various pristine compounds. The temperature-dependent electrical conduction mechanism has been further explained with the help of impedance analysis over the temperature range 323–623 K and a wide frequency range from 5 Hz to 1 MHz. The grain and grain boundary contributions were well distinguished by impedance and a modulus formalism, with respective activation energies of Eg = 0.25–0.26 eV and Egb = 0.31–0.32 eV. The temperature-dependent frequency exponents for grains (n1) and grain boundaries (n2) demonstrate two different conduction mechanisms, namely quantum mechanical tunneling for grains, and correlated barrier hopping for grain boundaries. Maxwell–Wagner-type dielectric polarizations are explained by our experimental results, and the highest real dielectric constant (εr) 1893 was calculated at 623 K.
CO oxidation on Pt based binary and ternary alloy nanocatalysts: Reaction pathways and electronic descriptor
Tripathi A., Hareesh C., Sinthika S., Andersson G., Thapa R.
Article, Applied Surface Science, 2020, DOI Link
View abstract ⏷
The design of stable, highly active heterogeneous nanocatalyst with no CO poisoning is a challenge for the catalytic converter industry. This can be achieved by defining electronic descriptor and by understanding the bonding mechanism. We present results using density functional theory to design optimal metal nanoalloy catalysts for CO oxidation. The adsorption configuration of O2 and O on different active sites is studied in detail to find the efficient reaction pathway for CO oxidation. The varying extent of back-donation of charge is found to be the factor deciding the CO tolerance. The trade-off between the activity and CO tolerance signifies that there is an upper limit for alloying Pt with Ni and Co. An electronic descriptor based on the occupancy of the d orbital and d band center of the host atom is defined to explain the site dependent activity of nanocatalysts. The role of underneath carbon surface on the CO oxidation activity of metal sites and CO poisoning is described.
Pressure-induced octahedral tilting distortion and structural phase transition in columbite structured NiNb2O6
Karmakar S., Garg A.B., Sahu M., Tripathi A., Mukherjee G.D., Thapa R., Behera D.
Article, Journal of Applied Physics, 2020, DOI Link
View abstract ⏷
High-pressure behavior of the technologically important compound NiNb2O6 adopting a columbite-type orthorhombic structure at ambient pressure and temperature conditions was investigated using synchrotron x-ray powder diffraction, Raman spectroscopic measurements, and first-principles calculations. The x-ray diffraction data indicate the occurrence of irreversible pressure-induced structural phase transition in the studied compound beyond 9 GPa. The high-pressure phase is found to be monoclinic with space group P2/m. The large volume collapse (∼4.4%) at the transition indicates the nature of the transition to be of the first order. There is a change in oxygen anion coordination number around Nb from 6 to 8; however, the coordination number around Ni remains 6. The experimental pressure-volume data when fitted to the Birch-Murnaghan equation of states yield the value of ambient pressure bulk modulus (B 0) as 178.7 (17) GPa for the orthorhombic phase and 244 (6) for the high-pressure monoclinic phase. The changes in Raman spectra indicate the distortion of NbO6 octahedra resulting in structural phase transitions. The logarithmic variation of unit cell volume (V) with optical lattice mode frequency (ν) helped us to calculate their respective Grüneisen parameters (γ) and it also supports the instability of the orthorhombic columbite structure of NiNb2O6 beyond 9 GPa, originated due to strong octahedral deformation of NbO6 octahedra. The variation of structural, electronic, and optical properties with pressure has also been discussed through first-principles calculations based on density functional theory using the revised Perdew-Burke-Ernzerh of generalized gradient approximation. The theoretical bandgap collapses and the distortion of NbO6 octahedra through the O chains with pressure are emphasized from the density of states data.
2D/3D heterostructure of h-BN/reduced graphite oxide as a remarkable electrode Material for supercapacitor
Patil I.M., Kapse S., Parse H., Thapa R., Andersson G., Kakade B.
Article, Journal of Power Sources, 2020, DOI Link
View abstract ⏷
We employed a facile approach to synthesize a ‘two dimensional/three dimensional (2D/3D)’ heterostructure of hexagonal boron nitride and reduced graphite oxide (h-BN/rGO). Interestingly, 2 wt% h-BN loaded heterostructure (i.e. BN/rGO-2) exhibits a superior capacitive performance, including specific capacitance (Csp) of 304 and 226 F g−1 at 1 A g−1 in alkaline and acidic conditions respectively with an excellent rate capability (~98% retention @10k cycles). Importantly, the electrochemical analysis confirms the accumulation of charge solely on the surface/near-surface reactions (capacitive contribution) and not due to the diffusion-limited processes. The solid-state symmetric supercapacitor cell exhibits specific energy of 1.25 Wh kg−1 corresponding to the high power density of 1800 W kg−1. The enhancement in the Csp is mainly attributed to the non-hierarchical assembly of a 2D/3D heterostructure, which provides a special interface to the electroactive species. Furthermore, the mechanically activated GO (A-GO) plays a crucial role by enhancing the specific surface area and mesoporosity, thus establishing a positive synergistic effect on capacitive properties, upon composite formation with h-BN. Our theoretical assessment shows that the surface functionalities of GO, as well as h-BN, help to enhance the quantum capacitance of graphene-related materials.
Dendritic Ferroselite (FeSe2) with 2D Carbon-Based Nanosheets of rGO and g-C3N4as Efficient Catalysts for Electrochemical Hydrogen Evolution
Shwetharani R., Kapse S., Thapa R., Nagaraju D.H., Balakrishna R.G.
Article, ACS Applied Energy Materials, 2020, DOI Link
View abstract ⏷
Nanostructured transition metal dichalcogenides are demonstrated to be potential catalysts to produce molecular hydrogen through electroreduction of water. Finding an efficient and cost-effective catalyst as a substitute for a platinum-based catalyst for sustainable hydrogen production is still a major issue, more so for large-scale production. Herein, we have designed dendritic ferroselite (FeSe2) hybrid nanocomposites with 2D g-C3N4 and reduced graphene oxide (rGO) nanosheets, that is, FeSe2/g-C3N4 and FeSe2/rGO as electrocatalysts for hydrogen evolution reaction (HER). Interestingly, FeSe2/rGO exhibited higher performance compared to FeSe2/g-C3N4. The highly conductive 2D FeSe2/rGO hybrid with an aligned curvy rippling surface and dendritic morphology demonstrates an onset potential of 218 mV at a current density of 10 mV/cm2 versus reversible hydrogen electrode in comparison to that of FeSe2/g-C3N4 showing an onset potential of 437 mV. The detailed density functional theory (DFT) calculations were performed to investigate the intrinsic catalytic sites and Gibbs free energy (ΔGH*) of hydrogen adsorption for the HER process. The DFT calculations displayed 0.33 V less overpotential for carbon atoms of g-C3N4 (0.97 V) compared to rGO (1.3 V). In contrast, hybrids of FeSe2/rGO (0.86 V) display lower overpotential when compared to FeSe2/g-C3N4 (1.63 V), which is in agreement with experimental results. Electrochemical impedance spectroscopy reveals lower charge transfer resistance (Rct) for FeSe2/rGO. The high hydrogen evolution activity of FeSe2/rGO is due to the electrocatalytic synergistic effect of iron diselenide and rGO, contributing to the optimum free energy for HER and improved electron mobility.
Homonuclear B2/B3 doped carbon allotropes as a universal gas sensor: Possibility of CO oxidation and CO2 hydrogenation
Parey V., Jyothirmai M.V., Kumar E.M., Saha B., Gaur N.K., Thapa R.
Article, Carbon, 2019, DOI Link
View abstract ⏷
Carbon allotropes are known to be promising materials as chemical gas sensors. Proper modification in the electronic structure is required using suitable heteroatom doping to acquire better sensing property. We present the homonuclear B2 (B–B) and B3 (B–B–B) doped carbon allotropes for the detection of toxic gases such as NO2, CO2, NH3, H2S and CO using the density functional theory. The sensitivity and selectivity of pure and boron doped C60 surfaces towards toxic gas molecules are estimated. With a new perception, the gas sensitivity has been correlated with π electron occupancy of the host materials. The adsorption behaviour of toxic gases on the host surface during the presence of H2O, O2 and O molecule/atom is estimated. The concept of homonuclear B sites is extended to 585 divacancy graphene and 585 divacancy carbon nanotube (CNT) system and verified that the phenomena are general for sp2 hybridized carbon allotrope. We also tested the CO oxidation and CO2 hydrogenation on B2 and B3 doped: C60 cage, 585 DV graphene and 585 DV CNT. The CO oxidation occurs via Langmuir–Hinshelwood (LH) mechanism. Overall we have found that the presence of homonuclear bond can change the inert carbon allotropes into highly active sensor material.
Charge transfer induced ferromagnetism and anomalous temperature increment of coercivity in ultrathin α-Fe2O3 decorated graphene 2D nanostructures
Bhattacharya S., Dinda D., Kumar E.M., Thapa R., Saha S.K.
Article, Journal of Applied Physics, 2019, DOI Link
View abstract ⏷
To overcome the detrimental effect of charge transfer from a transition metal to 2D substrates like graphene, we have grown ultrathin antiferromagnetic α-Fe2O3 layers on both sides of the graphene surface. Anomalous magnetic behavior, viz., coercivity and exchange bias, increases with increasing temperature with strong ferromagnetic ordering. The highest values of coercivity and large exchange bias are obtained as 3335 Oe and 2361 Oe, respectively. Large enhancement (646%) in exchange bias is observed with an increase in temperature from 2 K to 70 K. Interlayer exchange coupling between the ferromagnetic layers becomes strongest at 300 K to achieve an ultralow coercivity of 22 Oe by growing an α-Fe2O3 phase on both sides of the graphene surface. A 32% negative magnetoresistance is observed as a result of exchange bias which changes with temperature. All these results are explained on the basis of the charge transfer effect at the interface of the graphene/α-Fe2O3 nanostructure at the low temperature region and the spin canting effect of surface states at the higher temperature region. Theoretical Density Functional Theory calculation is also done to understand the interface interaction, quantitative evaluation of charge transfer, and density of states.
First-principles identification of the origin for higher activity of surface doped carbon nanohorn: Impact on hydrogen storage
Banerjee P., Thapa R., Rajkamal A., Chandrakumar K.R.S., Das G.P.
Article, International Journal of Hydrogen Energy, 2019, DOI Link
View abstract ⏷
Presence of curvature is considered as a tuning parameter to activate the hydrogen storage capability of carbon nanostructures. Here, we explicate the role of ‘intra-curvature’ in a set of single-walled carbon nanohorns (SWCNHs), to adsorb light metal ad-atoms (M) e.g. Li, Na, Ca and subsequently explore the metal-doped systems for hydrogen storage application using density functional theory. The binding strength of ad-atoms on SWCNHs of different curvature is correlated with the π electron occupancy of the corresponding carbon ring. Higher π electron occupancy causes significantly high binding energy of the metal ad-atoms (M), thereby indicating high stability of those M−C bonds for intra-curvature values more than 11⁰, even at a higher temperature. After full hydrogenation, Li-doped SWCNHs are found to contain a maximum of 7.5 wt % of hydrogen. Overall, our results indicate that Li-doped SWCNHs with intra-curvature values higher than 11⁰, is a potential candidate for hydrogen storage.
Carbon Allotropes as Anode Material for Lithium-Ion Batteries
Review, Advanced Materials Technologies, 2019, DOI Link
View abstract ⏷
Anode materials that exhibit high energy density, high power density, long life cycle, and better safety profile for lithium-ion batteries are necessary for the development of electric vehicles. Computational and experimental studies to describe the relevant aspects of carbon allotropes as anode materials are discussed, toward the significant improvement of specific power and energy capacity. The role of types of carbon ring and mixed hybridization (sp, sp2, and sp3) in carbon-based anode materials for Li storage explored. An overview is provided on the procedures used to analyze the storage properties of anode materials using first-principles theoretical methods such as intercalation energy, volume expansion, and open circuit voltage. Finally, the progress, importance, design, and the challenges of carbon-based anode materials are comprehensively discussed.
Screening of suitable cationic dopants for solar absorber material CZTS/Se: A first principles study
Jyothirmai M.V., Saini H., Park N., Thapa R.
Article, Scientific Reports, 2019, DOI Link
View abstract ⏷
The earth abundant and non-toxic solar absorber material kesterite Cu2ZnSn(S/Se)4 has been studied to achieve high power conversion efficiency beyond various limitations, such as secondary phases, antisite defects, band gap adjustment and microstructure. To alleviate these hurdles, we employed screening based approach to find suitable cationic dopant that can promote the current density and the theoretical maximum upper limit of the energy conversion efficiency (P(%)) of CZTS/Se solar devices. For this task, the hybrid functional (Heyd, Scuseria and Ernzerhof, HSE06) were used to study the electronic and optical properties of cation (Al, Sb, Ga, Ba) doped CZTS/Se. Our in-depth investigation reveals that the Sb atom is suitable dopant of CZTS/CZTSe and also it has comparable bulk modulus as of pure material. The optical absorption coefficient of Sb doped CZTS/Se is considerably larger than the pure materials because of easy formation of visible range exciton due to the presence of defect state below the Fermi level, which leads to an increase in the current density and P(%). Our results demonstrate that the lower formation energy, preferable energy gap and excellent optical absorption of the Sb doped CZTS/Se make it potential component for relatively high efficient solar cells.
Designing of stable and highly efficient ordered Pt2CoNi ternary alloy electrocatalyst: The origin of dioxygen reduction activity
Lokanathan M., Patil I.M., Navaneethan M., Parey V., Thapa R., Kakade B.
Article, Nano Energy, 2018, DOI Link
View abstract ⏷
We report an ordered Pt2CoNi ternary alloy nanoelectrocatalyst, synthesized via simple molten salt synthesis (MSS) procedure and also defined theoretically a new descriptor to explain the origin of exceptional oxygen reduction reaction (ORR) activity. The catalyst consists of a very thin layer of carbon, since the seed-growth of such ordered Pt2CoNi nanoelectrocatalyst has been originated through the pores of high surface area carbon during a MSS. Electrocatalytic ORR activity of Pt2CoNi nanoelectrocatalyst is 5–6 times higher than that of commercial Pt/C catalyst with fascinating stability behavior in acidic media. Most interesting behavior of this Pt2CoNi has been observed after 15,000 and 25,000 durability cycles, where 16 times activity enhancement is achieved at 0.9 V. Furthermore, after 25,000 cycles, a specific activity of 0.605 mA/cm2Pt (22 times higher than its own) has been observed at 1.0 V for the first time. Density functional theory (DFT) based calculation is used to estimate the onset potential by plotting the free energy profile for ORR of each active site of nanocatalyst. New descriptors are proposed to explain the origin of exceptional catalytic activity of ternary metal nanoelectrocatalyst.
Chemical modulation of valance band in delafossite structured CuFeO2 thin film and its photoresponse
Bera A., Deb K., Sinthika S., Thapa R., Saha B.
Article, Materials Research Express, 2018, DOI Link
View abstract ⏷
Using simple spin coating process we report the development of delafossite structured CuFeO2 ceramic thin film on florine doped tin oxide (FTO) coated glass substrate and found improved electrical conductivity, through possible modulation of valance band with high photoresponse of these structures. The valance band of CuFeO2 predominantly comprising of localized Cu 3d and O 2p orbitals, has been chemically modulated through post annealing of the film in oxygen rich atmosphere in order to obtain delocalized holes as carriers. During post annealing of the film in the oxygen rich environment for substantially long time (8, 16 and 24 h) oxygen atoms are introduced in the crystal as interstitials, and thus brings a chemical modulation of valance band without any external doping. The crystal structure, optical band gap and p-type conductivity have been studied experimentally, and theoretical first-principle based density functional calculations estimate that the oxygen atoms create mid gap states and are responsible for the states in the conduction band. A fivefold increase in the electrical conductivity was observed upon 24 h of annealing. More interestingly an excellent photoresponse behavior of the CuFeO2 films in its J-V characteristics have been observed and reported in this article, which must appear very significant in exploring its prospect of application as a p type semiconductor in optoelectronic devices with appropriate energy band gap.
Structural and Electronic Descriptors of Catalytic Activity of Graphene-Based Materials: First-Principles Theoretical Analysis
Sinthika S., Waghmare U.V., Thapa R.
Article, Small, 2018, DOI Link
View abstract ⏷
Characteristic features of the d-band in electronic structure of transition metals are quite effective as descriptors of their catalytic activity toward oxygen reduction reaction (ORR). With the promise of graphene-based materials to replace precious metal catalysts, descriptors of their chemical activity are much needed. Here, a site-specific electronic descriptor is proposed based on the pz (π) orbital occupancy and its contribution to electronic states at the Fermi level. Simple structural descriptors are identified, and a linear predictive model is developed to precisely estimate adsorption free energies of OH (ΔGOH) at various sites of doped graphene, and it is demonstrated through prediction of the most optimal site for catalysis of ORR. These structural descriptors, essentially the number of ortho, meta, and para sites of N/B-doped graphene sheet, can be extended to other doped sp2 hybridized systems, and greatly reduce the computational effort in estimating ΔGOH and site-specific catalytic activity.
Ring type and π electron occupancy decides the Li-ion storage properties of Phagraphene: An example of sp2 hybridized carbon structure
Rajkamal A., Sinthika S., Andersson G., Thapa R.
Article, Carbon, 2018, DOI Link
View abstract ⏷
Graphite, a sp2 hybridized layered material is used as an anode material in commercial Lithium ion batteries (LIB's). It is desirable to improve the Li/C ratio in the sp2 hybridized carbon allotropes to increase the specific energy capacity. Using First-principles calculations bulk Phagraphene (contains penta-, hepta- and hexa ring) an example of sp2 hybridized carbon allotropes is identified as a potential high capacity anode material for rechargeable LIB's. The Li adsorption site preference is attributed to the combined effect of the pz electrons of the carbon atoms constituting a ring. It is found that the Li diffusion is favoured during both adsorbed and intercalated state. Bulk-Phagraphene shows desirable negative formation energy, high specific capacity of 558 mAh/g, and stable positive open circuit voltage profile for high Li intercalation. Overall the asymmetry in the pz electron occupancies of the distinct carbon atoms in Phagraphene is identified to be a major contributing factor for the higher activity of Phagraphene as compared to the graphene. This indicates that we can tune the property of sp2 based carbon structures by changing the π electron environments.
Origin of spin polarization in an edge boron doped zigzag graphene nanoribbon: A potential spin filter
Chakrabarty S., Wasey A.H.M.A., Thapa R., Das G.P.
Article, Nanotechnology, 2018, DOI Link
View abstract ⏷
To realize a graphene based spintronic device, the prime challenge is to control the electronic structure of edges. In this work we find the origin of the spin filtering property in edge boron doped zigzag graphene nanoribbons (ZGNRs) and provide a guide to preparing a graphene based next-generation spin filter based device. Here, we unveil the role of orbitals (p-electron) to tune the electronic, magnetic and transport properties of edge B doped ZGNRs. When all the edge carbon atoms at one of the edges of ZGNRs are replaced by B (100% edge B doping), the system undergoes a semiconductor to metal transition. The role of passivation of the edge with single/double atomic hydrogen on the electronic properties and its relation with the p-electron is correlated in-depth. 50% edge B doped ZGNRs (50% of the edge C atoms at one of the edges are replaced by B) also show half-metallicity when the doped edge is left unpassivated. The half-metallic systems show 100% spin filtering efficiency for a wide range of bias voltages. Zero-bias transmission function of the other configurations shows asymmetric behavior for the up and down spin channels, thereby indicating their possible application potential in nano-spintronics.
Induced ferromagnetism and metal-insulator transition due to a charge transfer effect in silver nanoparticle decorated Mo S2
Bag S., Bhattacharya S., Dinda D., Jyothirmai M.V., Thapa R., Saha S.K.
Article, Physical Review B, 2018, DOI Link
View abstract ⏷
MoS2 sheets are decorated by silver nanoparticles of size 10-12 nm. Localized holes are generated in Ag 4d levels carrying a magnetic moment due to the charge transfer effect at the interface and formation of Ag-S bonds. Temperature-dependent charge transport shows a metal-insulator (M-I) transition; the metallic phase is supported by the transition from negative to positive magnetoresistance and enhanced polarizability near the M-I transition. Observed ferromagnetism arises due to dominant coupling among the spins of localized holes in Ag nanoparticles at lower temperature. Density functional theory calculation is carried out to establish the charge transfer effect. The amount of charge transferred from Ag to MoS2 is evaluated from Bader charge analysis and the temperature effect is verified by a Nosé thermostat with the NVT ensemble method.
Resonant energy transfer in a van der Waals stacked MoS2-functionalized graphene quantum dot composite with: Ab initio validation
Roy R., Thapa R., Biswas S., Saha S., Ghorai U.K., Sen D., Kumar E.M., Kumar G.S., Mazumder N., Roy D., Chattopadhyay K.K.
Article, Nanoscale, 2018, DOI Link
View abstract ⏷
Graphene-based van der Waals (vdW) heterostructures can facilitate exciting charge transfer dynamics in between structural layers with the emission of excitonic quasi-particles. However, the chemical formation of such heterostructures has been elusive thus far. In this work, a simple chemical approach is described to form such van der Waals (vdW) heterostructures using few layer MoS2 sheet embedded quantum dots (QDs) and amine-functionalized graphene quantum dots (GQDs) to probe the energy transfer mechanism for tunable photoluminescence (PL). Our findings reveal an interesting non-radiative Förster-type energy transfer with the quenching of functional GQD PL intensity after GQD/MoS2 composite formation, which validates the existing charge transfer dynamics analogous to 0D and 2D systems. The non-radiative type of energy transfer characteristic from GQD into the MoS2 layer through vdW interactions has been confirmed by photoluminescence, time decay analyses and ab initio calculations with the shifting of the Fermi level in the density of states towards the conduction band in the stacked configuration. These results are encouraging for the fundamental exploration of optical properties in other chemically prepared QD/2D based heterostructures to understand the charge transfer mechanism and fingerprint luminescence quenching for future optoelectronic device and optical sensing applications.
Role of Gd-doping in conduction mechanism of BFO-PZO nanocrystalline composites: Experimental and first-principles studies
Ray A., Basu T., Behera B., Kumar M., Thapa R., Nayak P.
Article, Journal of Alloys and Compounds, 2018, DOI Link
View abstract ⏷
In this paper, in the conduction behaviour of Gd-doped 0.4BiGdxFe(1-x)O3–0.6PbZrO3 (BFO-PZO) with x = 0.0, 0.05, 0.10, 0.15, 0.20 composites, synthesized by solid-state reaction (mixed oxide) technique, was investigated. X-ray diffraction study with Rietveld refinement method revealed the formation of rhombohedral (R3c) phase. Dielectric constant and dielectric loss studies as a function of frequency reveal dispersion due to Maxwell-Wagner type of interfacial polarization while weak ferroelectric hysteresis loops have been recorded for all the samples. Complex impedance spectroscopy technique-based impedance, electrical modulus, and electrical conductivity of the composites revealed non-Debye type relaxation mechanism. Correlated barrier hopping (CBH) mechanism dominates in all the composites exhibiting high value of density of states (1023 eV−1 cm−1) which increased further after doping with Gd. It was seen that doping of Gd affects the activation energy of these composites which was complemented by performing density functional theory calculations. Bader charge calculation was performed to understand the chemical environment and charge transfer upon doping. In addition, a small change in the bandgap was found which may cause the change in activation energy.
Role of oxygen functionality on the band structure evolution and conductance of reduced graphene oxide
Roy R., Thapa R., Chakrabarty S., Jha A., Midya P.R., Kumar E.M., Chattopadhyay K.K.
Article, Chemical Physics Letters, 2017, DOI Link
View abstract ⏷
Here we report, structural and electrical transport properties of reduced graphene oxide as a function of oxygen bonding configuration. We find that mainly epoxy (COC) and carbonyl (CO) functional groups remain as major residual components after reduction using three different reducing agents. We calculate the band structure in the presence of epoxy and carbonyl groups and defects. Finally, we calculate the theoretical band mobility and find that it is less for the carbonyl with epoxy system. We correlate the distortion of linear dispersion and opening of bandgap at K-point with conductance for different graphene system in presence of oxygen moieties.
First-principles identification of site dependent activity of graphene based electrocatalyst
Nandhini S., Rajkamal A., Saha B., Thapa R.
Article, Molecular Catalysis, 2017, DOI Link
View abstract ⏷
Graphene based metal free electrocatalysts have been considered as potential candidate for efficient oxygen reduction reaction (ORR) in fuel cell systems. Using density functional theory we investigated the site dependent ORR activity of nitrogen doped and divacancy (DV_555-777) graphene structures considering free energy calculations and correlate with the occupancy of pz(π) electrons. The dioxygen adsorption strength differs for each carbon site in case of N doped and DV_555-777 graphene system. Range of overpotential and on-set potential has been estimated for same system. Among the sites and systems considered, the C2′ site of 2N doped graphene system is most active towards oxygen reduction with lowest overpotential. We have found that the DV_555-777 is more active than the pure system. In this case also we have observed a range of overpotential considering various C sites. We have been estimated the occupancy of pz(π) electrons of each C site of different systems. The occupancy of pz(π) electrons of C atoms increase near the dopant site due to back-donation mechanism in N doped graphene system. A strong correlation has been identified in-between activity of each C site with the occupancy of pz(π) electrons of corresponding site. Overall we concluded that each C site of N doped and defective graphene has different on-set potential and pz(π) electrons play a major role to define the activity.
Antiferro-ferromagnetic transition in ultrathin Ni(OH)2 layer grown on graphene surface and observation of interlayer exchange coupling in Ni(OH)2/graphene/Ni(OH)2 nanostructures
Bhattacharya S., Mathan Kumar E., Thapa R., Saha S.K.
Article, Applied Physics Letters, 2017, DOI Link
View abstract ⏷
The major limitation of using graphene as a potential spacer element in interlayer exchange coupling (IEC) might be due to destruction of ferromagnetism as a result of the charge transfer effect at the interface if a transition metal based ferromagnetic layer is grown on the graphene surface. To overcome this problem, we have used the antiferromagnetic Ni(OH)2 layer grown on the graphene surface to convert it ferromagnetic due to the charge transfer effect. By growing thin layers of Ni(OH)2 on both sides of the graphene surface, strong antiferromagnetic IEC with ultra-low coercivity (7 Oe) is observed. By lowering the nickel content, an ultrathin layer of Ni(OH)2 is grown on either side of graphene and shows complete ferromagnetism with a giant coercivity of 4154 Oe. Ab initio calculations have been done to substantiate this kind of charge transfer effect at the interface of Ni(OH)2 and graphene. Magnetotransport of the composite material is also investigated to understand the role of IEC in transport properties.
Ag nanoparticle decorated molybdenum oxide structures: Growth, characterization, DFT studies and their application to enhanced field emission
Guha P., Ghosh A., Thapa R., Kumar E.M., Kirishwaran S., Singh R., Satyam P.V.
Article, Nanotechnology, 2017, DOI Link
View abstract ⏷
We report a simple single step growth of α-MoO3 structures and energetically suitable site specific Ag nanoparticle (NP) decorated α-MoO3 structures on varied substrates, having almost similar morphologies and oxygen vacancies. We elucidate possible growth mechanisms in light of experimental findings and density functional theory (DFT) calculations. We experimentally establish and verified by DFT calculations that the MoO3(010) surface is a weakly interacting and stable surface compared to other orientations. From DFT study, the binding energy is found to be higher for (100) and (001) surfaces (∼-0.98 eV), compared to the (010) surface (∼-0.15 eV) and thus it is likely that Ag NP formation is not favorable on the MoO3(010) surface. The Ag decorated MoO3 (Ag-MoO3) nanostructured sample shows enhanced field emission properties with an approimately 2.1 times lower turn-on voltage of 1.67 V μm-1 and one order higher field enhancement factor (β) of 8.6 ×104 compared to the MoO3 sample without Ag incorporation. From Kelvin probe force microscopy measurements, the average local work function (Φ) is found to be approximately 0.47 eV smaller for the Ag-MoO3 sample (∼5.70 ±0.05 eV) compared to the MoO3 sample (∼6.17 ±0.05 eV) and the reduction in Φ can be attributed to the shifting Fermi level of MoO3 toward vacuum via electron injection from Ag NPs to MoO3. The presence of oxygen vacancies together with Ag NPs lead to the highest β and lowest turn-on field among the reported values under the MoO3 emitter category.
Flexible diode of polyaniline/ITO heterojunction on PET substrate
Bera A., Deb K., Kathirvel V., Bera T., Thapa R., Saha B.
Article, Applied Surface Science, 2017, DOI Link
View abstract ⏷
Hybrid organic-inorganic heterojunction between polyaniline and ITO film coated on flexible polyethylene terephthalate (PET) substrate has been prepared through vapor phase polymerization process. Polaron and bipolaron like defect states induced hole transport and exceptional mobility makes polyaniline a noble hole transport layer. Thus a p-n junction has been obtained between the hole transport layer of polyaniline and highly conductive n-type layer of ITO film. The synthesis process was carried out using FeCl 3 as polymerizing agent in the oxidative chemical polymerization process. The prepared polyaniline has been found to be crystalline on characterization through X-ray diffraction measurement. X-ray photoelectron spectroscopic measurements were done for compositional analysis of the prepared film. The UV–vis-NIR absorbance spectra obtained for polyaniline shows the characteristics absorbance as observed for highly conductive polyaniline and confirms the occurrence of partially oxidized emeraldine form of polyaniline. The energy band gap of the polyaniline has been obtained as 2.52 eV, by analyzing the optical transmittance spectra. A rectifying behavior has been observed in the electrical J-V plot, which is of great significance in designing polymer based flexible electronic devices.
Electron doped C 2 N monolayer as efficient noble metal-free catalysts for CO oxidation
Chakrabarty S., Das T., Banerjee P., Thapa R., Das G.P.
Article, Applied Surface Science, 2017, DOI Link
View abstract ⏷
Using state-of-the-art density functional theory (DFT) based approach; we investigated the catalytic activity of electron doped C 2 N monolayer (O → N) for CO oxidation. Large surface-to-volume ratio and uniformly distributed holes of recently synthesized planar 2D C 2 N have made it a potential candidate as noble metal-free catalyst. However, pristine C 2 N monolayer is chemically inert and hinders the adsorption of O 2 and CO molecule on it. Oxygen doping in C 2 N brings additional electrons to the system and introduces donor state below E F . Thus the reactivity of O-doped C 2 N (2OC 2 N) monolayer gets significantly enhanced, thereby opening up the possibility of its usage as a catalyst. This reactive 2OC 2 N surface adsorbs an incoming O 2 molecule along with the elongation of O[sbnd]O bond, making it chemically active. Presence of this pre-adsorbed active O 2 greatly impedes the adsorption of another incoming CO, favoring Eiley-Rideal (ER) mechanism for CO oxidation.
Exploring the catalytic activity of pristine T6[100] surface for oxygen reduction reaction: A first-principles study
Banerjee P., Chakrabarty S., Thapa R., Das G.P.
Article, Applied Surface Science, 2017, DOI Link
View abstract ⏷
The electrocatalytic activity of T6[100] surface containing both sp 3 (C 1 ) and sp 2 (C 2 ) hybridized carbon atoms is explored using first-principles density functional theory based approach. The top layered C 1 atom of the surface is found to be more active towards the oxygen reduction reaction (ORR), as compared to that of C 2 atom. This is attributed to the presence of dangling σ bond in the corresponding C 1 atom, leading to the high electron density near the Ferrmi level. Whereas, the π electron in the top layered C 2 atom forms a weak out of plane network. As estimated from free energy profile, the overpotential is much lower when C 1 is considered as the active site and the final step i.e desorption of final OH − ion is found to be the potential determining step. We have also reported the effect of Si dopant on the catalytic activity of T6[100] surface and explained the origin of high overpotential value in this case. Thus in this report, we propose a new metal-free catalyst i.e T6[100] surface, having both sp 2 (maintains the high metallicity needed to reduce ohmic loss) and sp 3 (helps in capturing the upcoming molecules) hybridized carbon atoms, as a potential candidate for ORR.
Schottky diode behaviour with excellent photoresponse in NiO/FTO heterostructure
Saha B., Sarkar K., Bera A., Deb K., Thapa R.
Article, Applied Surface Science, 2017, DOI Link
View abstract ⏷
Delocalization of charge carriers through formation of native defects in NiO, to achieve a good metal oxide hole transport layer was attemted in this work and thus a heterojunction of p-type NiO and n-type FTO have been prepared through sol-gel process on FTO coated glass substrate. The synthesis process was stimulated by imparting large number of OH − sites during nucleation of Ni(OH) 2 on FTO, so that during oxidation through annealing Ni vacancies are introduced. The structural properties as observed from X-ray diffraction measurement indicate formation of well crystalline NiO nanoparticles. Uniform distribution of NiO nanoparticles has been observed in the images obtained from scanning electron microscope. The occurrence of p-type conductivity in the NiO film was stimulated through the formation of delocalized defect carriers originated from crystal defects like vacancies or interstitials in the lattice. Ni vacancy creates shallow levels with respect to the valance band maxima and they readily produce holes. Thus a native p-type conductivity of NiO originates from Ni vacancies. NiO was thus obtained as an auspicious hole transport medium, which creates an expedient heterojunction at the interface with FTO. Excellent rectifying behavior was observed in the electrical J–V plot obtained from the prepared heterojunction. The results are explained from the band energy diagram of the NiO/FTO heterojunction. Remarkable photoresponse has been observed in the reverse characteristics of the heterojunction caused by photon generated electron hole pairs.
Asian consortium on computational materials science theme meeting on “first principles analysis & experiment: Role in energy research” 22-24 september 2016, SRM University, Kattankulathur, Chennai, India (ACCMS-TM 2016)
Conference paper, Applied Surface Science, 2017, DOI Link
Effect of Mg substitution in delafossite structured CuFeO2 thin film deposited on FTO coated glass substrate and its diode characteristics
Bera A., Deb K., Bera T., Sinthika S., Thapa R., Saha B.
Article, Thin Solid Films, 2017, DOI Link
View abstract ⏷
In an attempt to introduce delocalized electronic states in the valance band of p-type delafossite crystals of CuFeO2, substitution with Mg in its lattice has been done in this work. The delafossite type crystal structure has been achieved at a comparatively low temperature of 723 K through the simple chemical process of sol-gel spin coating technique. It was deposited on Florine doped tin oxide (FTO) coated glass substrate and subsequently studied with X-ray diffraction (XRD) measurement, transmission electron microscopic (TEM) measurement, atomic force microscopic (AFM) measurements, X-ray photoelectron spectroscopy (XPS). The structural, morphological, compositional, optical and electrical properties of the Mg doped CuFeO2/FTO heterostructure as investigated in this work, show significant development and response with Mg doping concentration. The density functional calculations also indicate such effects upon Mg substitution, arising from impurity of Mg 3s state, created near the upper edge of the valence band along with a shift in the Fermi level towards the valence band. The hole-doping by Mg2+ substituting Fe3+ enhances the quantum fluctuations and bond disorder providing enhanced p-type conductivity. Most importantly the heterostructure shows excellent diode like rectifying character in the current-voltage (I-V) plot and significantly modified under Mg doping concentrations. Chemically stable, environment friendly, p-type semiconductor material of CuFeO2 with improved functionality through Mg substitution will find its significant place in recent development of semiconductor processing technology.
Screening based approach and dehydrogenation kinetics for MgH2: Guide to find suitable dopant using first-principles approach
Kumar E.M., Rajkamal A., Thapa R.
Article, Scientific Reports, 2017, DOI Link
View abstract ⏷
First-principles based calculations are performed to investigate the dehydrogenation kinetics considering doping at various layers of MgH2 (110) surface. Doping at first and second layer of MgH2 (110) has a significant role in lowering the H2 desorption (from surface) barrier energy, whereas the doping at third layer has no impact on the barrier energy. Molecular dynamics calculations are also performed to check the bonding strength, clusterization, and system stability. We study in details about the influence of doping on dehydrogenation, considering the screening factors such as formation enthalpy, bulk modulus, and gravimetric density. Screening based approach assist in finding Al and Sc as the best possible dopant in lowering of desorption temperature, while preserving similar gravimetric density and Bulk modulus as of pure MgH2 system. The electron localization function plot and population analysis illustrate that the bond between Dopant-Hydrogen is mainly covalent, which weaken the Mg-Hydrogen bonds. Overall we observed that Al as dopant is suitable and surface doping can help in lowering the desorption temperature. So layer dependent doping studies can help to find the best possible reversible hydride based hydrogen storage materials.
Effect of surface doping on the band structure of graphene: a DFT study
Iyakutti K., Kumar E.M., Lakshmi I., Thapa R., Rajeswarapalanichamy R., Surya V.J., Kawazoe Y.
Article, Journal of Materials Science: Materials in Electronics, 2016, DOI Link
View abstract ⏷
Various techniques, like doping, vacancy creation, strain engineering are tried to open a gap in the bandstructure of graphene and in some cases the gap has opened up. However, when the gap opens up the Dirac cones disappear. Without Dirac cones, graphene loses all its novelty. So opening a gap in graphene, retaining Dirac cones has become a challenging task. We, through first principles study using Density Functional theory, have done band gap tuning investigations. We have succeeded in opening the band gap, retaining the Dirac cones. Surface doping (adsorption) of various elements are tried and finally surface doping of sulfur is found to induce band gap opening in graphene. The Dirac cones are retained and the graphene is now a semiconductor with fast moving massless Dirac Fermions. We are reporting this type of calculations for the first time.
Tuning the work function of randomly oriented ZnO nanostructures by capping with faceted Au nanostructure and oxygen defects: Enhanced field emission experiments and DFT studies
Ghosh A., Guha P., Thapa R., Selvaraj S., Kumar M., Rakshit B., Dash T., Bar R., Ray S.K., Satyam P.V.
Article, Nanotechnology, 2016, DOI Link
View abstract ⏷
The lowering of the work function (Φ) can lead to a better field emission (FE) behavior at lower threshold fields. We report on enhanced FE from randomly oriented and faceted Au-capped ZnO hetero-nanostructures (HNs) having more oxygen defects. Large-area arrays of non-aligned, faceted Au-capped ZnO HNs, such as nanowires (NWs) and triangular nanoflakes (TNFs) are grown using the chemical vapor deposition (CVD) method. Enhanced FE properties from the TNF sample resulted in a turn-on field as low as 0.52 V μm-1 at a current density of 0.1 mA cm-2 and a field enhancement factor (β) as high as ≈5.16 × 105. Under similar experimental conditions, drawing the same current density from an NW specimen needs a higher turn-on field (0.86 V μm-1) and to exhibit nearly four times less field enhancement factor compared to the TNFs samples. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements confirm the presence of more oxygen defects in the TNF samples compared to the NW samples. Kelvin probe force microscopy (KPFM) measurements show the average local work function to be 4.70 ± 0.1 eV for the TNF sample, which is ≈ 0.34 eV lower than the NW sample. Using density functional theory (DFT) calculations, the estimated Φ values are found to be 4.98 eV for ZnO(0001), 4.17 eV for Au(001)/ZnO(0001) and 3.91 eV for Au(001)/Ovac-ZnO(0001) surfaces. The DFT results are qualitatively in agreement with our experimental results. The presence of Au nanostructures on top of O-deficient and sharp-tipped TNFs results in enhanced FE performance following their reduced tunneling barrier via pinning of effective Φ.
CO Oxidation Prefers the Eley-Rideal or Langmuir-Hinshelwood Pathway: Monolayer vs Thin Film of SiC
Sinthika S., Vala S.T., Kawazoe Y., Thapa R.
Article, ACS Applied Materials and Interfaces, 2016, DOI Link
View abstract ⏷
Using the first-principles approach, we investigated the electronic and chemical properties of wurtzite silicon carbide (2H-SiC) monolayer and thin film structures and substantiated their catalytic activity toward CO oxidation. 2H-SiC monolayer, being planar, is quite stable and has moderate binding with O2, while CO interacts physically; thus, the Eley-Rideal (ER) mechanism prevails over the Langmuir-Hinshelwood (LH) mechanism with an easily cleared activation barrier. Contrarily, 2H-SiC thin film, which exhibits a nonplanar structure, allows moderate binding of both CO and O2 on its surface, thus favoring the LH mechanism over the ER one. Comprehending these results leads to a better understanding of the reaction mechanisms involving structural contrast. Weak overlapping between the 2pz(C) and 3pz(Si) orbitals of the SiC monolayer system has been found to be the primary reason to revert the active site toward sp3 hybridization, during interaction with the molecules. In addition, the influences of graphite and Ag(111) substrates on the CO oxidation mechanism were also studied, and it is observed that the ER mechanism is preserved on SiC/G system, while CO oxidation on the SiC/Ag(111) system follows the LH mechanism. The calculated Sabatier activities of the SiC catalysts show that the catalysts are very efficient in catalyzing CO oxidation.
Colossal magnetoresistance in amino-functionalized graphene quantum dots at room temperature: Manifestation of weak anti-localization and doorway to spintronics
Roy R., Thapa R., Kumar G.S., Mazumder N., Sen D., Sinthika S., Das N.S., Chattopadhyay K.K.
Article, Nanoscale, 2016, DOI Link
View abstract ⏷
In this work, we have demonstrated the signatures of localized surface distortions and disorders in functionalized graphene quantum dots (fGQD) and consequences in magneto-transport under weak field regime (∼1 Tesla) at room temperature. Observed positive colossal magnetoresistance (MR) and its suppression is primarily explained by weak anti-localization phenomenon where competitive valley (inter and intra) dependent scattering takes place at room temperature under low magnetic field; analogous to low mobility disordered graphene samples. Furthermore, using ab-initio analysis we show that sub-lattice sensitive spin-polarized ground state exists in the GQD as a result of pz orbital asymmetry in GQD carbon atoms with amino functional groups. This spin polarized ground state is believed to help the weak anti-localization dependent magneto transport by generating more disorder and strain in a GQD lattice under applied magnetic field and lays the premise for future graphene quantum dot based spintronic applications.
Mixed phase delafossite structured p type CuFeO2/CuO thin film on FTO coated glass and its Schottky diode characteristics
Bera A., Deb K., Chattopadhyay K.K., Thapa R., Saha B.
Article, Microelectronic Engineering, 2016, DOI Link
View abstract ⏷
Delafossite structures are of significant importance in the recent context of development of structured and engineered materials because of their natural super lattice structure. The delafossite structured CuFeO2 mixed with crystalline CuO phase has been prepared by the simple chemical method of sol-gel technique on fluorine doped tin oxide (FTO) coated glass substrate. The mixed phase of delafossite structured CuFeO2 with crystalline CuO was appeared on annealing the film at 723 K. The prepared film was characterized with X-ray diffraction measurements, atomic force microscopy, UV-Vis-NIR spectrophotometry and electrical I-V measurements. The optical energy band gap of 2.63 eV was obtained from the UV-Vis-NIR spectrophotometric measurements. On electrical characterization of the film, the (I-V) measurements show a Schottky diode like characteristics.
Si doped T6 carbon structure as an anode material for Li-ion batteries: An ab initio study
Rajkamal A., Kumar E.M., Kathirvel V., Park N., Thapa R.
Article, Scientific Reports, 2016, DOI Link
View abstract ⏷
First-principles calculations are performed to identify the pristine and Si doped 3D metallic T6 carbon structure (having both sp 2 and sp 3 type hybridization) as a new carbon based anode material. The ' € electron of C 2 atoms (sp 2 bonded) forms an out of plane network that helps to capture the Li atom. The highest Li storage capacity of Si doped T6 structure with conformation Li 1.7 Si 1 C 5 produces theoretical specific capacity of 632 mAh/g which substantially exceeding than graphite. Also, open-circuit voltage (OCV) with respect to Li metal shows large negative when compared to the pristine T6 structure. This indicates modifications in terms of chemical properties are required in anode materials for practical application. Among various doped (Si, Ge, Sn, B, N) configuration, Si doped T6 structure provides a stable positive OCV for high Li concentrations. Likewise, volume expansion study also shows Si doped T6 structure is more stable with less pulverization and substantial capacity losses in comparison with graphite and silicon as an anode materials. Overall, mixed hybridized (sp 2 + sp 3) Si doped T6 structure can become a superior anode material than present sp 2 hybridized graphite and sp 3 hybridized Si structure for modern Lithium ion batteries.
Effect of multiple defects and substituted impurities on the band structure of graphene: a DFT study
Iyakutti K., Mathan Kumar E., Thapa R., Rajeswarapalanichamy R., Surya V.J., Kawazoe Y.
Article, Journal of Materials Science: Materials in Electronics, 2016, DOI Link
View abstract ⏷
In graphene, band gap opening and tuning are important technological challenges for device applications. Various techniques have been suggested to this technologically complicated problem. Here, we present an ab initio study on the band gap opening in graphene through vacancy, adding impurity atom in the vacancy and substitutional co-doping. In the case of graphene with single vacancy a direct band gap of ~1 eV is obtained. This is a spin polarized state. The graphene system with two monovacancies gives rise to an effective indirect band gap (pseudo gap) of ~1 eV. The graphene substitutionally doped with B and N is co-doped (tri-doped) with S. This tri-doped graphene has turned into a semiconductor (band gap ~1 eV). These graphene semiconductors are better than the other semiconductor because of the presence of massless Dirac fermions in addition to normal electrons. This will have lot of application in device industry compared to a pristine graphene because of the presence of a gap and Dirac fermions. This type of band gap opening, with this type of defects and impurities, we are reporting for the first time.
Field emission properties of spinel ZnCo2O4 microflowers
Ratha S., Khare R.T., More M.A., Thapa R., Late D.J., Rout C.S.
Article, RSC Advances, 2015, DOI Link
View abstract ⏷
ZnCo2O4 microflowers were synthesized by a simple low temperature hydrothermal route. A single three-dimensional microflower consists of hundreds of self-assembled petals, with a thickness of several nanometers. These microflowers have exceptionally thin edges with a few petal layers. The ZnCo2O4 microflowers appeared to be stable and good field emitters.
An oxygen reduction catalytic process through superoxo adsorption states on n-type doped h-BN: A first-principles study
Shin D., Thapa R., Park N.
Article, Current Applied Physics, 2015, DOI Link
View abstract ⏷
Dioxygen adsorption and activation on metal-ligand systems are the key elements for biological oxidative metabolisms and also catalyst design for the oxygen reduction reaction (ORR). We show, through first-principles calculations, that similar dioxygen adducts can form on metal-free n-type doped hexagonal boron nitride (h-BN) nanostructures. The density of electron donors determines the charge state of dioxygen, either in superoxo and peroxo, which exactly correlates with the 'end-on' and 'side-on' configurations, respectively. Activated O2 in the superoxo state shows a better catalytic performance possibly mediating the direct four-electron reduction. The formation of hydrogen peroxide (H2O2) is practically eliminated, and thus we suggest that a surface coated with the n-type doped h-BN can be the basis for an ORR catalyst with increased stability.
Influence of enolate/epoxy configuration, doping and vacancy on the catalytic activity of graphene
Article, RSC Advances, 2015, DOI Link
View abstract ⏷
Using density functional theory based electronic structure analysis we substantiate that the bonding type of atomic oxygen (epoxide or enolate) and adsorption strength of molecular oxygen play vital roles in determining the overpotential of oxygen reduction reactions (ORR) of n and p doped graphene-based electrocatalysts. The presence of localized pz states influences the electron accepting and donating characteristics of the carbon atoms of the DV (555-777) defective graphene. We probe the origin of dopant-induced enolate and epoxide formation in both pristine and DV graphene based on the occupation of pz orbital of active site after doping. In spite of the slightly higher tendency of DV to adsorb molecular oxygen than pristine graphene, the enhancement in binding strengths of O2 with the introduction of dopants is higher in the pristine case when compared to the DV. The donation and back-donation interaction of dopant with nearby carbon atoms is inspected in detail. We examine the effects of boron and nitrogen co-doping and find that a doping configuration of a boron atom bonded to two nitrogen atoms (B2N) possesses moderate binding with atomic and molecular oxygen, suggesting it to be a better catalyst for oxygen reduction reaction with lowest overpotential among the systems considered. Moreover the possibility of CO poisoning has been tested for all the surfaces. The approach considered in this work is general and can be extended to understand the catalytic activity of other carbon/graphene based systems.
First principles guide to tune h-BN nanostructures as superior light-element-based hydrogen storage materials: Role of the bond exchange spillover mechanism
Kumar E.M., Sinthika S., Thapa R.
Article, Journal of Materials Chemistry A, 2015, DOI Link
View abstract ⏷
We investigate the interaction of molecular hydrogen with light-element-based n-doped hexagonal boron nitride (h-BN) nanostructures and moreover explore the bond exchange mechanism for spillover of atomic hydrogen using dispersion-corrected density functional theory (DFT-D) calculations. A number of doped configurations were tested and it has been found that co-doping of C and O on h-BN sheet significantly increases the adsorption energy of molecular H2. The charge transfer from the n-doped h-BN surface to H2 is found to be the reason for the higher interactions that boosted the binding energy. In addition, the doped h-BN surfaces act as catalysts and dissociate the H2 molecule with a very low activation barrier, but the migration of the resulting H atoms on the surface requires high energy. In order to facilitate easy and fast migration of H atoms, we introduce the bond exchange mechanism using external mediators i.e. borane (BH3) and gallane (GaH3) molecules which serve as secondary catalysts and help in lowering the migration barrier, leading to the formation of a hydrogenated surface. The partially hydrogenated surface in turn can also act as a hydrogen storage material, with a higher propensity to adsorb hydrogen molecules when compared to the unhydrogenated surface. Hence the surface proposed in this work can be used to store a substantial quantity of hydrogen as an energy source with easy adsorption and desorption kinetics.
Magnetic, elastic and optical properties of zinc peroxide (ZnO2): First principles study
Thapa R., Ghosh S., Sinthika S., Mathan Kumar E., Park N.
Article, Journal of Alloys and Compounds, 2015, DOI Link
View abstract ⏷
Using first principles method we elaborately discuss the magnetic, elastic and optical properties of pure, Zn and O vacant ZnO2. It is found that the electronic structure and band gap of ZnO2 is not sensitive to the active on-site Coulomb interaction term Ud, but found to be depending on the term Up. The role of orbitals subject to the correlation is thus completely opposite for the case of ZnO2 in respect of ZnO. Interestingly, the Zn vacancy converts ZnO2 as "d0 magnet". Indeed, our analysis show that, Zn vacancy transmuted O22- state into O2δ +2- state, indicating the partially filled π∗ states are the governing reason for the d0 magnetism. Both HSE06 and PBE0 functional confirm the same. The similar phenomena has been observed for other peroxide materials XO2 (X = Mg, Ca, Sr, Ba) studied here. Our results suggest that this class of materials can be studied further to exploit its potential in spintronic devices. Further the elastic properties have been estimated for pure ZnO2 at different pressures and for Zn and O vacant ZnO2 to know the stability of the system. Zn vacancy in ZnO2 also tunes optical properties, indicating its potential application in other areas.
Electronic properties of heterostructures of graphene with boron, nitrogen and boron nitride – A first principles study
Iyakutti K., Mathan Kumar E., Thapa R., Surya V.J., Kawazoe Y.
Article, International Journal of ChemTech Research, 2015,
View abstract ⏷
New graphene like 2D materials such as BN, MoS2and WS2have exotic electronic properties. Forming heterostructures of these 2D materials with graphene influence and alter the electronic character of graphene. But how the added 2D materials influence the electronic properties of graphene is not fully understood. We in this first principles study try to understand this problem and help the experimentalists to identify useful heterostructures. We have computationally designed the heterostructures of graphene by combining it with Boron, Nitrogen and Boron Nitride. Out of all the heteronanostructures designed, the Gra-BN heterostructure turns out to be interesting and has more applications than the other heterostuctures. This heterostructure behaves like a graphene and has improved electronic properties. Also this design paves way for the design of 3D graphene.
Efficient field emission from vertically aligned Cu2O1-δ(111) nanostructure influenced by oxygen vacancy
Nandy S., Thapa R., Kumar M., Som T., Bundaleski N., Teodoro O.M.N.D., Martins R., Fortunato E.
Article, Advanced Functional Materials, 2015, DOI Link
View abstract ⏷
In the architecture described, cuprous oxide (Cu2O) is tamed to be highly (111) plane oriented nanostructure through adjusting the deposition postulate by glancing angle deposition technique. In the controlled atmosphere oxygen vacancy is introduced into the Cu2O crystal subsequently fostering an impurity energy state (Eim) close to the conduction band. Our model of Cu2O electronic structure using density functional theory suggests that oxygen vacancies enhance the electron donating ability because of unshared d-electrons of Cu atoms (nearest to the vacancy site), allowing to pin the work function energy level around 0.28 eV compared to the bulk. This result is also complemented by Kelvin probe force microscopy analysis and X-ray photoelectron spectroscopy method. Oxygen vacancy in Cu2O (Cu2O1-δ) exhibits promising field emission properties with interesting field electron tunneling behavior at different applied fields. The films show very low turn-on and threshold voltages of value 0.8 and 2.4 V μm-1 respectively which is influenced by the oxygen vacancy. Here, a correlation between the work function modulation due to the oxygen vacancy and enhancement of field emission of Cu2O1-δ nanostructure is demonstrated. This work reveals a promising new vision for Cu2O as a low power field emitter device.
Facile synthesis of Ag nanowire-rGO composites and their promising field emission performance
Samantara A.K., Mishra D.K., Suryawanshi S.R., More M.A., Thapa R., Late D.J., Jena B.K., Rout C.S.
Article, RSC Advances, 2015, DOI Link
View abstract ⏷
Crystalline, ultra long silver nanowires (Ag NWs), few-layered rGO (reduced graphene oxide) and their rGO-Ag NW nanocomposite have been synthesized using a polyol reflux technique under optimized experimental conditions. The field emission performance of the rGO-Ag NW nanocomposite, rGO and Ag NW emitters was investigated. The turn on field required to draw an emission current density of ∼1 μA cm<sup>-2</sup> was found to be ∼5.00, 3.92 and 2.40 V μm<sup>-1</sup> for the Ag NW, rGO and rGO-Ag NW nanocomposite emitters, respectively. The combined contribution of the sharp edges of the thin graphene sheets and high aspect ratio of the Ag nanowires, and their synergetic effect in the rGO-Ag NW nanocomposite, are responsible for the enhanced field emission behavior. First-principles density functional calculations show that the enhanced field emission may also be due to the overlapping of the electronic structures of the Ag NWs and rGO nanosheets.
In plane conducting channel at the interface of CdO-ZnO isotype thin film heterostructure
Bera A., Thapa R., Chattopadhyay K.K., Saha B.
Article, Journal of Alloys and Compounds, 2015, DOI Link
View abstract ⏷
Electrical transport properties of CdO-ZnO thin film heterostructure have been studied in this work. Highly conducting CdO thin film is deposited on glass substrate by radio frequency magnetron sputtering technique. ZnO thin film was deposited by employing the same technique on CdO coated glass substrate to prepare an isotype heterostructure of these two n-type metal oxide semiconductors. The CdO thin film was of very high electrical conductivity induced by oxygen deficient point defects. The films were characterized by X-ray diffraction measurements, X-ray photoelectron spectroscopic measurements, field emission scanning electron microscopic measurements and electrical conductivity measurements. Carrier diffusion and carrier tunneling through the interface potential lead to an outstanding conducting channel at the ZnO layer of the isotype thin film heterostructure modifying the band structure at the interface.
Hydrogen spillover on DV (555-777) graphene – Vanadium cluster system: First principles study
Kumar E.M., Sabarikirishwaran P., Thapa R.
Conference paper, AIP Conference Proceedings, 2015, DOI Link
View abstract ⏷
Using dispersion corrected density functional theory (DFT+D), the interaction of Vanadium adatom and cluster with divacancy (555-777) defective graphene sheet has been studied elaborately. We explore the prospect of hydrogen storage on V 4 cluster adsorbed divacancy graphene system. It has been observed that V 4 cluster (acting as a catalyst) can dissociate the H 2 molecule into H atoms with very low barrier energy. We introduce the spillover of the atomic hydrogen throughout the surface via external mediator gallane (GaH 3 ) to form a hydrogenated system.
First principle identification of SiC monolayer as an efficient catalyst for CO oxidation
Sinthika S., Reddy C.P., Thapa R.
Conference paper, AIP Conference Proceedings, 2015, DOI Link
View abstract ⏷
Using density functional theory, we investigated the electronic properties of SiC monolayer and tested its catalytic activity toward CO oxidation. The planar nature of a SiC monolayer is found to stable and is a high band gap semiconductor. CO interacts physically with SiC surface, whereas O 2 is adsorbed with moderate binding. CO oxidation on SiC monolayer prefers the Eley Rideal mechanism over the Langmuir Hinshelwood mechanism, with an easily surmountable activation barrier during CO 2 formation. Overall metal free SiC monolayer can be used as efficient catalyst for CO oxidation.
First principles design of divacancy defected graphene nanoribbon based rectifying and negative differential resistance device
Chakrabarty S., Wasey A.H.M.A., Thapa R., Das G.P.
Article, AIP Advances, 2015, DOI Link
View abstract ⏷
We have studied using density functional theory and non-equilibrium Green's function based approach, the electronic structures of 555-777 divacancy (DV) defected armchair edged graphene nanoribbons (AGNR) as well as the transport properties of AGNR based two-terminal devices constructed with one defected electrode and one N doped electrode. Introduction of 555-777 DV defect into AGNR results in shifting of the π and π bands towards the higher energy value indicating a downward shift of the Fermi level. Formation of a potential barrier, analogous to that of conventional p-n junction, has been observed across the junction of defected and N-doped AGNR. The two terminal devices show diode like property with high rectifying efficiency for a wide range of bias voltages. The devices also show robust negative differential resistance with very high peak-to-valley ratio. Shift of the electrode energy states and modification of the transmission function with applied bias have been analyzed, in order to gain an insight into the nonlinear and asymmetric behavior of the current-voltage characteristics. Variation of the transport properties on the width of the ribbons has also been discussed.
Spectroscopic Studies on Interaction of Congo Red with Ferric Chloride in Aqueous Medium for Wastewater Treatment
Debnath A., Thapa R., Chattopadhyay K.K., Saha B.
Article, Separation Science and Technology (Philadelphia), 2015, DOI Link
View abstract ⏷
Absorption spectroscopic studies of Congo red (CR) in interaction with FeCl3 in aqueous medium have been reported in this article. The interaction of CR with FeCl3 has been investigated with an aim to explore a possible low cost and efficient way to remove CR from industrial waste water. The removal mechanism is based on the formation of a complex of iron with anionic part of the CR molecule in interaction with FeCl3. An exceptionally high removal rate of 2670 mg/g has been observed at 100 mg/L initial CR concentration and enhanced removal rate was found with increasing initial CR concentration.
Enhanced electron field emission from NiCo2O4 nanosheet arrays
Naik K.K., Khare R.T., Gelamo R.V., More M.A., Thapa R., Late D.J., Rout C.S.
Article, Materials Research Express, 2015, DOI Link
View abstract ⏷
Electron emission properties of electrodeposited spinel NiCo2O4 nanosheet arrays grown on Ni foam have been studied. The work function of NiCo2O4 was calculated by density functional theory using the plane-wave basis set and used to estimate the field enhancement factor. The NiCo2O4 nanosheet arrays exhibited a low turn-on field of 1.86 V μm.1 at 1 μA cm-2 and current density of 686 μA cm-2 at 3.2 V μm-1, with field enhancement factor β.=.1460 and good field emission current stability. The field emission properties of the NiCo2O4 nanosheet arrays showed enhanced performance compared to chemically prepared NiCo2O4 nanosheets. Hence, the nanosheet arrays have great potential as robust high performance vertical structure electron emitters for future flat panel displays and vacuum electronic device applications.
Spillover of hydrogen on SiC-ML surface: Doping effect and bond exchange mechanism
Kumar E.M., Prajapat B., Saha B., Thapa R.
Article, International Journal of Hydrogen Energy, 2015, DOI Link
View abstract ⏷
The dispersion corrected density functional theory has been employed in this work to explore the binding and splitting of H2 molecule on pure and N doped silicon carbide monolayer (SiC-ML) and spillover of H atoms. Small charge transfer from the donor level of N-doped SiC-ML surface to the σ∗ orbital helps to anchor the hydrogen molecule. The N-doped SiC-ML surface showed excellent catalytic behavior through lowering the HeH bond breaking energy to 0.87 eV, and thus helping in the facile spillover process. It is shown that the migration of H atoms on the surface to form a partially hydrogenated SiC-ML surface requires very high activation barrier. Here we have explained the need and role of borane (BH3 ) molecule as secondary catalysts for lowering of the migration energy barrier via bond exchange mechanism to facilitate easy and fast migration of H atoms. The nature of interaction of BH3 molecules with the various surfaces has been studied in detail. This work ensures the possibility of the spillover mechanism, and takes us a step ahead towards the implementation of the light-weight material as Hydrogen storage medium.
Self-Size-Limiting Nanoscale Perforation of Graphene for Dense Heteroatom Doping
Maiti U.N., Thapa R., Lim J., Li D.J., Kim K.H., Kim S.O.
Article, ACS Applied Materials and Interfaces, 2015, DOI Link
View abstract ⏷
A scalable and controllable nanoscale perforation method for graphene is developed on the basis of the two-step thermal activation of a graphene aerogel. Different resistance to the thermal oxidation between graphitic and defective domains in the weakly reduced graphene oxide is exploited for the self-limiting nanoscale perforation in the graphene basal plane via selective thermal degradation of the defective domains. The resultant nanoporous graphene with a narrow pore-size distribution addresses the long-standing challenge for the high-level doping of graphene with lattice-mismatched large-size heteroatoms (S and P). Noticeably, this novel heteroatom doping strategy is demonstrated to be highly effective for oxygen reduction reaction (ORR) catalysis. Not only the higher level of heteroatom doping but also favorable spin and charge redistribution around the pore edges leads to a strong ORR activity as supported by density functional theory calculations.
Activation of CO and CO2 on homonuclear boron bonds of fullerene-like BN cages: First principles study
Sinthika S., Kumar E.M., Surya V.J., Kawazoe Y., Park N., Iyakutti K., Thapa R.
Article, Scientific Reports, 2015, DOI Link
View abstract ⏷
Using density functional theory we investigate the electronic and atomic structure of fullerene-like boron nitride cage structures. The pentagonal ring leads to the formation of homonuclear bonds. The homonuclear bonds are also found in other BN structures having pentagon line defect. The calculated thermodynamics and vibrational spectra indicated that, among various stable configurations of BN-60 cages, the higher number of homonuclear N-N bonds and lower B:N ratio can result in the more stable structure. The homonuclear bonds bestow the system with salient catalytic properties that can be tuned by modifying the B atom bonding environment. We show that homonuclear B-B (B2) bonds can anchor both oxygen and CO molecules making the cage to be potential candidates as catalyst for CO oxidation via Langmuir-Hinshelwood (LH) mechanism. Moreover, the B-B-B (B3) bonds are reactive enough to capture, activate and hydrogenate CO2 molecules to formic acid. The observed trend in reactivity, viz B3 > B2 > B1 is explained in terms of the position of the boron defect state relative to the Fermi level.
Amino-functionalized graphene quantum dots: Origin of tunable heterogeneous photoluminescence
Sandeep Kumar G., Roy R., Sen D., Ghorai U.K., Thapa R., Mazumder N., Saha S., Chattopadhyay K.K.
Article, Nanoscale, 2014, DOI Link
View abstract ⏷
Graphene quantum dots are known to exhibit tunable photoluminescence (PL) through manipulation of edge functionality under various synthesis conditions. Here, we report observation of excitation dependent anomalous m-n type fingerprint PL transition in synthesized amino functionalized graphene quantum dots (5-7 nm). The effect of band-to-band π*-π and interstate to band n-π induced transitions led to effective multicolor emission under changeable excitation wavelength in the functionalized system. A reasonable assertion that equi-coupling of π*-π and n-π transitions activated the heterogeneous dual mode cyan emission was made upon observation of the PL spectra. Furthermore, investigation of incremented dimensional scaling through facile synthesis of amino functionalized quantum graphene flakes (20-30 nm) revealed it had negligible effect on the modulated PL pattern. Moreover, an effort was made to trace the origin of excitation dependent tunable heterogeneous photoluminescence through the framework of energy band diagram hypothesis and first principles analysis. Ab initio results suggested formation of an interband state as a manifestation of p orbital hybridization between C-N atoms at the edge sites. Therefore comprehensive theoretical and experimental analysis revealed that newly created energy levels can exist as an interband within the energy gap in functionalized graphene quantum structures yielding excitation dependent tunable PL for optoelectronic applications. © The Royal Society of Chemistry.
Rules of boron-nitrogen doping in defect graphene sheets: A first-principles investigation of band-gap tuning and oxygen reduction reaction catalysis capabilities
Sen D., Thapa R., Chattopadhyay K.K.
Article, ChemPhysChem, 2014, DOI Link
View abstract ⏷
Introduction of defects and nitrogen doping are two of the most pursued methods to tailor the properties of graphene for better suitability to applications such as catalysis and energy conversion. Doping nitrogen atoms at defect sites of graphene and codoping them along with boron atoms can further increase the efficiency of such systems due to better stability of nitrogen at defect sites and stabilization provided by B-N bonding. Systematic exploration of the possible doping/codoping configurations reflecting defect regions of graphene presents a prevalent doping site for nitrogen-rich BN clusters and they are also highly suitable for modulating (0.2-0.9 eV) the band gap of defect graphene. Such codoped systems perform significantly better than the platinum surface, undoped defect graphene, and the single nitrogen or boron atom doped defect graphene system for dioxygen adsorption. Significant stretching of the O-O bond indicates a lowering of the bond breakage barrier, which is advantageous for applications in the oxygen reduction reaction. Who needs to be replaced? Various boron and nitrogen doping/codoping schemes on double-vacancy defect graphene sheets are systematically explored by the means of first-principles calculations. The oxygen reduction reaction capabilities of such structures are investigated in detail (see picture). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
A first-principles investigation of oxygen reduction reaction catalysis capabilities of As decorated defect graphene
Sen D., Thapa R., Chattopadhyay K.K.
Article, Journal of the Chemical Society. Dalton Transactions, 2014, DOI Link
View abstract ⏷
Single and multiple As adatom adsorptions on double vacancy (DV) defect graphene sheets are extensively analyzed using dispersive force corrected density functional theory (DFT). Defect pentagonal and heptagonal bridge sites and the immediate neighborhood of the defect center are found to be most favorable for this purpose. Quantitative analysis of electronic structures revealed the As–C bonding to be mostly ionic in nature with some covalency arising from the overlap of As p states and C p states. For multiple As adatoms adsorption in close vicinity, ionicity of the As–C bonds are found to decrease to support As–As cohesion; the net result of which is manifested as better binding of dioxygen molecule with them and additional weakening of the O–O bond in the adsorbed state. Free energy profile of oxygen reduction reaction (ORR) cycle using multiple As atom adsorbed DV graphene as electrocatalyst predicts high affinity towards four electron process and forbids the formation of H2O2 via two electron process. Other traits, such as no intermediate O–O–H formation and high stability of the catalytic system throughout the reaction process indicate As adatoms adsorbed on DV graphene system to be efficient and highly stable as an alternate Pt free ORR electrocatalyst. © 2014 The Partner Organisations.
Doped h-BN monolayer as efficient noble metal-free catalysts for CO oxidation: The role of dopant and water in activity and catalytic de-poisoning
Sinthika S., Kumar E.M., Thapa R.
Article, Journal of Materials Chemistry A, 2014, DOI Link
View abstract ⏷
Using a first principles approach, we investigated the catalytic activity of a noble metal-free n-doped (C → B, O → N) hexagonal boron nitride (h-BN) monolayer for CO oxidation. The CO adsorption ability and hence the preferred Eiley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanism for CO oxidation is dopant-dependent: CO is chemisorbed on O-doped h-BN (OBN) while it physically interacts with the C-doped h-BN (CBN) surface. Even though both C and O doping create similar donor states below the Fermi level (Ef), O doping results in a larger bond length of O-B1 (one of the nearest B atom), out-of-plane displacement of the B1 atom, and less positive charge on the B1 atom, synergistically contributing to higher atomic activity. The presence of a pre-adsorbed O2 molecule on both types of surfaces eliminates any chances of CO poisoning of the surface, and CO oxidation prefers to proceed via the ER mechanism with a small activation barrier. The high values of Sabatier activities suggest that the doped h-BN surface is superior to Au55and Pt55nanoclusters. In case of CO oxidation by means of the LH mechanism, a stable O2⋯CO intermediate is produced, which requires a high barrier energy to break the O-O bond. However, the presence of a H2O molecule increases the activity of the catalyst and helps in catalytic CO de-poisoning. © 2014 the Partner Organisations.
Ab initio study of thin oxide-metal overlayers as an inverse catalytic system for dioxygen reduction and enhanced CO tolerance
Shin D., Sinthika S., Choi M., Thapa R., Park N.
Article, ACS Catalysis, 2014, DOI Link
View abstract ⏷
Using first-principles density functional theory calculations, we used a thin oxide overlayer, such as MgO, on a metal surface as an inverse catalyst for dioxygen reduction. Surface distortions in the oxide layer, combined with the tunneling of electron from the underneath metal, activated the adsorbed O2 in the form of a superoxo or peroxo. On the other hand, the thin MgO overlayer readily prevents the π-back-bonding between CO and the metal surface, thereby efficiently mitigating the affinity of the metal surface for CO. The operating potential and overpotential for the oxygen reduction reaction (ORR) process have been estimated for various combinations of thin insulators and metals. The strongest binding intermediate in the overall reaction pathway influenced the overpotential. We show that for a Ag(100)-supported MgO surface, the ORR commences with a low overpotential, which is comparable to that of the Pt(111) surface. This suggests that an optimally chosen insulator-metal overlayer structure can yield a sharply tuned free energy profile for ORR.
Field emission properties of ZnO nanosheet arrays
Naik K.K., Khare R., Chakravarty D., More M.A., Thapa R., Late D.J., Rout C.S.
Article, Applied Physics Letters, 2014, DOI Link
View abstract ⏷
Electron emission properties of electrodeposited ZnO nanosheet arrays grown on Indium tin oxide coated glass substrates have been studied. Influence of oxygen vacancies on electronic structures and field emission properties of ZnO nanosheets are investigated using density functional theory. The oxygen vacancies produce unshared d electrons which form an impurity energy state; this causes shifting of Fermi level towards the vacuum, and so the barrier energy for electron extraction reduces. The ZnO nanosheet arrays exhibit a low turn-on field of 2.4V/μm at 0.1 μA/cm2 and current density of 50.1 μA/cm2 at an applied field of 6.4 V/lm with field enhancement factor, β = 5812 and good field emission current stability. The nanosheet arrays grown by a facile electrodeposition process have great potential as robust high performance vertical structure electron emitters for future flat panel displays and vacuum electronic device applications.
Promotion of oxygen reduction by a bio-inspired tethered iron phthalocyanine carbon nanotube-based catalyst
Cao R., Thapa R., Kim H., Xu X., Kim M.G., Li Q., Park N., Liu M., Cho J.
Article, Nature Communications, 2013, DOI Link
View abstract ⏷
Electrocatalysts for oxygen reduction are a critical component that may dramatically enhance the performance of fuel cells and metal-air batteries, which may provide the power for future electric vehicles. Here we report a novel bio-inspired composite electrocatalyst, iron phthalocyanine with an axial ligand anchored on single-walled carbon nanotubes, demonstrating higher electrocatalytic activity for oxygen reduction than the state-of-the-art Pt/C catalyst as well as exceptional durability during cycling in alkaline media. Theoretical calculations suggest that the rehybridization of Fe 3d orbitals with the ligand orbitals coordinated from the axial direction results in a significant change in electronic and geometric structure, which greatly increases the rate of oxygen reduction reaction. Our results demonstrate a new strategy to rationally design inexpensive and durable electrochemical oxygen reduction catalysts for metal-air batteries and fuel cells. © 2013 Macmillan Publishers Limited. All rights reserved.
Small Pd cluster adsorbed double vacancy defect graphene sheet for hydrogen storage: A first-principles study
Sen D., Thapa R., Chattopadhyay K.K.
Article, International Journal of Hydrogen Energy, 2013, DOI Link
View abstract ⏷
Stability and electronic properties of small Pdn clusters (n = 1-5), adsorbed on different types of double vacancy (DV) defect graphene sheets are thoroughly investigated by both density functional theory (DFT) and molecular dynamics (MD). Defect bridge sites of DV(555-777) defect graphene sheet are identified to be the most favorable for Pd4 cluster adsorption. MD calculations, performed using a canonical ensemble, showed this system to be highly stable up to 800 K. Much better hybridization between C 2p and Pd 4d and 5s orbitals near Fermi level as well as higher charge transfer to graphene sheet was found to be the governing reason for enhanced stability of Pd4 cluster on DV(555-777) defect site. Comparative analysis of H2 storage on Pd4 cluster adsorbed pristine and DV(555-777) defect graphene sheet showed, while adsorption energy/H2 molecule for both cases lie well within desirable energy window for a hydrogen storage media, the later is much more efficient energetically as distorted in plane sp2 hybridization reduces the saturations of C-C bonds in the defect regions, making more electron density available for bonding; which leads to higher net charge gain of Pd4 cluster and higher charge sharing with H2 molecule.
Optical and vibrational properties of hydrogenated BN-sheet: First principles study
Article, Applied Surface Science, 2013, DOI Link
View abstract ⏷
In this work IR, Raman and optical properties of three conformers (Chair, Boat and Stirrup) of hydrogenated BN (BHNH) sheet are presented in detail. The blue shift of Raman peak corresponding to the BN vibrational mode (E 2g ) within h-BN layers can be attributed to shrinkage of lattice parameter. The phonon dispersion and density of phonon states are studied in depth to explore the nature of vibrational bonding in the three conformers of BHNH sheet. Vibrational study reveals that all the three conformers are very much stable. The dielectric and absorption spectra are different for Chair, Boat and Stirrup conformer and also compared with h-BN sheet. The estimated results indicate that IR spectra and Raman shift can be a useful technique to investigate the formation of BHNH sheet and to make a distinction between the three conformers. © 2013 Elsevier B.V. All rights reserved.
Site dependent metal adsorption on (3 × 3) h-BN monolayer: Stability, magnetic and optical properties
Sen D., Thapa R., Bhattacharjee K., Chattopadhyay K.K.
Article, Computational Materials Science, 2012, DOI Link
View abstract ⏷
Cu, Pd and Au metal adatom adsorption on different adsorption sites of graphene analog h-BN monolayer is presented. The results demonstrate that the atop N (AN) being the most favorable site for Cu while atop B (A B) for Pd/Au; as well as occurrence of chemisorption is also found in these sites. A general model has been proposed which essentially indicate electronegativity (χ) to be the governing reason regarding the choice of adsorption sites on h-BN sheet in a way such that the adatoms with χ < 2.04 are the most stable on AN site while adatoms with χ lying in the range 2.04-3.04 are the most stable on AB site. A detail study regarding magnetic properties reveal 100% spin polarization at Fermi level i.e. half metallic characteristics and 1μB/supercell magnetic moment in case of Cu and Au adatom adsorption at the most favorable sites. For Cu adsorbed h-BN system, the half metallic characteristics and strong chemical bonds arise from good hybridization at 0 eV between the outermost 2p orbital of nitrogen and the outermost 4s orbital of copper and at -1.49 eV between Cu 3d and 4s orbital with N 2p orbital. A thorough optical study on the above mentioned systems exhibits evolution/disappearance of different hump/shoulder peaks in the calculated absorption coefficient vs. energy plot which may be useful for experimental identification of the adsorbed systems. © 2011 Elsevier B.V. All rights reserved.
Structural transformation from Mn 3O 4 nanorods to nanoparticles and band gap tuning via Zn doping
Jha A., Thapa R., Chattopadhyay K.K.
Article, Materials Research Bulletin, 2012, DOI Link
View abstract ⏷
Synthesis of undoped and Zn doped hausmannite Mn 3O 4 nanorods was achieved through a simple hydrothermal route. Scanning electron microscopic studies showed that due to in situ Zn doping a structural deformation from Mn 3O 4 nanorods to a mixture of Mn 3O 4 nanorods and nanoparticles occurred. The amount of nanoparticles in the mixture increased with the increase of doping percentage. X-ray diffraction studies, transmission electron microscopy and selected area electron diffraction pattern revealed that both the nanorods and the nanoparticles were hausmannite Mn 3O 4. X-ray photoelectron spectroscopic studies confirmed successful zinc doping in Mn 3O 4. Microscopic studies revealed that the average diameters of Mn 3O 4 nanorods and nanoparticles were of 200 nm and 70 nm, respectively. The possible growth mechanism and the reasons behind the formation of nanoparticles along with nanorods are discussed briefly. UV-vis spectroscopic studies showed a continuous increase in the energy bandgap of Mn 3O 4 with the increase in Zn doping percentage. © 2011 Elsevier Ltd. All rights reserved.
First-principles identification of iodine exchange mechanism in iodide ionic liquid
Article, Journal of Physical Chemistry Letters, 2012, DOI Link
View abstract ⏷
We investigated the microscopic mechanism of ion transport in iodide ionic liquid, using first-principles calculations. We show that the desorption barrier of polyiodides (I 3 - or I 5 -) from the cation is in a similar energy range as or higher than the barrier for the bond dissociation and ensued desorption of neutral iodine (I 2). This suggests that, instead of the physical diffusion of such a negatively charged multiatomic species, the exchange of neutral iodine (I 2) between the polyiodides can be an easier channel for the movement of polyiodide. For the transport of the monoiodide anion (I -), we suggest the contribution of the Grotthuss-type ion exchange through the intermediately formed even-member anion (I 2n -), in addition to drift and diffusion. As a result, we suggest that, instead of the commonly cited diffusion of the triiodide/iodide (I 3 -/I -) redox couple, the exchange of neutral iodine (I 2) and the Grotthuss-type transport (I -) constitute the dominant ion transport mechanism. © 2012 American Chemical Society.
Anatase TiO 2 nanoparticles synthesis via simple hydrothermal route: Degradation of Orange II, Methyl Orange and Rhodamine B
Thapa R., Maiti S., Rana T.H., Maiti U.N., Chattopadhyay K.K.
Article, Journal of Molecular Catalysis A: Chemical, 2012, DOI Link
View abstract ⏷
We report the synthesis of anatase TiO 2 nanoparticles of average size 12 nm (determined by field emission scanning electron microscope and high resolution transmission electron micorscope) and the degradation of Orange II, Methyl Orange and Rhodamine B dyes using ultraviolet (UV) light source. The TiO 2 nanoparticles are well crystalline having anatase phase as confirmed by X-ray diffraction technique. The indirect and direct band gaps of TiO 2 nanoparticles were measured to be about 3.09 eV and 3.74 eV, respectively. The increase in the band gap from its bulk value may be accounted for the quantum confinement effect. After UV exposure, the Orange II (20 min), Methyl Orange (25 min) and Rhodamine B (150 min) concentrations were found to degrade more than 99%, 98% and 99%, respectively. Furthermore, the synthesized TiO 2 nanoparticles are efficient photocatalyst in comparison to the other reported TiO 2 nanostructures and composites (with other materials) due to its small volume/less electron-hole recombination, oxygen vacancy and good crystallinity. The LDA, CA-PZ functional and ultrasoft pseuodopotential were used to find the band gap of anatase TiO 2 theoretically. The PDOS signifies the formations of excitons are mainly in between O 2p and Ti 3d orbitals. © 2012 Elsevier B.V.
Palladium atoms and its dimers adsorbed on graphene: First-principles study
Thapa R., Sen D., Mitra M.K., Chattopadhyay K.K.
Article, Physica B: Condensed Matter, 2011, DOI Link
View abstract ⏷
In this work we have studied the stabilty, electronic and magnetic properties of Pd adatoms and dimers adsorbed on graphene system using first-principles calculations. The adsorption energies for Pd adatom and its dimer have been found to range from -0.986 to -1.135 eV and -0.165 to -1.101 eV, respectively, which signify stable configuration and future utilization of this system in catalysis. A shift but no separation of π and π bands at the Dirac point has been observed in case of Pd dimer adsorption in perpendicular configuration, which can be accounted for the breaking of symmetry of the graphene structure due to adsorption. 6468% spin polarization P(EF) and 1.9441.990 μB magnetic moment have been observed for Pd dimers adsorbed on graphene in perpendicular configuration for different sites. The unequal values of partial density of states for 4d and 5s orbitals of Pd dimers at Fermi level have been found to be responsible for the generation of high spin polarization. © 2010 Elsevier B.V. All rights reserved.
Quantum size effect on the optical properties of rf magnetron sputtered nanocrystalline cadmium oxide thin films
Saha B., Thapa R., Das N.S., Chattopadhyay K.K.
Conference paper, International Journal of Nanoscience, 2011, DOI Link
View abstract ⏷
CdO thin film with different thickness and different particle size are prepared through radio frequency magnetron sputtering technique. Quantum confinement effect causes the significant changes in their optical properties showing significant changes in the optical band gap. The CdO films are very highly conducting and transparent. Transparent and conducting thin films of CdO with effectively increased optical band gap are very useful for different device applications like solar cell, optoelectronic devices. © 2011 World Scientific Publishing Company.
Spectroscopic ellipsometric studies on the optical properties of phosphorus doped nanocrystalline NiO thin films
Das N.S., Chattopadhyay K.K., Saha B., Thapa R.
Conference paper, International Journal of Nanoscience, 2011, DOI Link
View abstract ⏷
Undoped and phosphorus doped nanocrystalline nickel oxide thin films have been synthesized on silicon and glass substrates by RF magnetron sputtering technique in pure Ar atmosphere. Proper phase formation was confirmed by X-ray diffraction analysis. Energy band gaps were determined using UV-Vis spectra. Formation of NiO nanoparticle of dimension ∼15 nm was confirmed using HRTEM. Doping of phosphorus as an impurity was confirmed from EDX spectra and XPS studies. Spectroscopic ellipsometric studies were performed on such films and the spectra were analyzed with a suitable model. Optical constants were determined and refractive indices were found to increase with increase of phosphorus doping percentages. © 2011 World Scientific Publishing Company.
Temperature-dependent ac conductivity and dielectric response of vanadium doped CaCu3Ti4O12 ceramic
Sen A., Maiti U.N., Thapa R., Chattopadhyay K.K.
Article, Applied Physics A: Materials Science and Processing, 2011, DOI Link
View abstract ⏷
Successful incorporation of vanadium dopant within the giant dielectric material CaCu3Ti 4O12 (CCTO) through a conventional solid-state sintering process is achieved and its influence on the dielectric as well as electrical properties as a function of temperature and frequency is reported here. Proper crystalline phase formation together with dopant induced lattice constant shrinkage was confirmed through X-ray diffraction. The temperature dependence of the dielectric constant at different constant frequencies was investigated. We infer that the correlated barrier hopping (CBH) model is dominant in the conduction mechanism of the ceramic as per the temperature-dependent ac conductivity measurements. The electronic parameters such as density of the states at the Fermi level, N(Ef) and hopping distance, Rω of the ceramic were also calculated using this model. © Springer-Verlag 2011.
Optical and electrical properties of p-type transparent conducting CuAlO2 thin film synthesized by reactive radio frequency magnetron sputtering technique
Saha B., Thapa R., Jana S., Chattopadhyay K.K.
Article, Indian Journal of Physics, 2010, DOI Link
View abstract ⏷
Thin films of p-type transparent conducting CuAlO2 have been synthesized through reactive radio frequency magnetron sputtering on silicon and glass substrates at substrate temperature 300deg;C. Reactive sputtering of a target fabricated from Cu and Al powder (1:1.5) was performed in Ar+O 2 atmosphere. The deposition parameters were optimized to obtain phase pure, good quality CuAlO2 thin films. The films were characterized by studying their structural, morphological, optical and electrical properties. © 2010 IACS.
Study of field emission and dielectric properties of AlN films prepared by DC sputtering technique at different substrate temperatures
Thapa R., Saha B., Goswami S., Chattopadhyay K.K.
Article, Indian Journal of Physics, 2010, DOI Link
View abstract ⏷
Nanocrystalline AlN thin films were prepared via DC sputtering technique at different substrate temperature. The crystal orientation and particle size of aluminum nitride thin films were investigated by XRD analysis. Study indicated that the sample contained pure phase hexagonal AlN nanoparticles with a single peak corresponding to the (100) planes. The peak at 665 cm-1 in the FTIR spectrum of film was assigned to the LO phonon of hexagonal AlN. The particle size of the film, prepared at substrate temperature 200°C was about 9.5 nm, as investigated by atomic force microscope. Field emission study indicated that it can be used as a good field emitter. Turn-on field (E to) of 15.02 V/μm was observed for the AlN films synthesized at substrate temperature 200 °C. Dielectric constant of the AlN film was found nearly independent of frequencies in the measured frequency range 1 KHz to 1 MHz, i.e. in the audio frequency range. The values of dielectric constant (ε) were 10.07, 9.46 and 8.65 for the film prepared at 70°C, 150°C and 200°C, respectively, at frequency 1 KHz. © 2010 IACS.
Band gap widening of nanocrystalline nickel oxide thin films via phosphorus doping
Das N.S., Saha B., Thapa R., Das G.C., Chattopadhyay K.K.
Article, Physica E: Low-Dimensional Systems and Nanostructures, 2010, DOI Link
View abstract ⏷
Phosphorus doped nanocrystalline NiO thin films were synthesized using radio frequency magnetron sputtering of a prefabricated target on glass and silicon substrates in argon atmosphere. X-ray diffraction studies confirmed the good crystallinity and proper phase formation. Phosphorus doping in NiO films was confirmed from the binding energy determination by X-ray photoelectron spectroscopic studies. Morphological information was obtained from the atomic force microscopic measurement. Determination of band gaps from the UV-vis-NIR spectrophotometric measurement showed that it increased from 3.66 to 3.81 eV corresponding to undoped and 10% P doped NiO thin films. © 2009 Elsevier B.V. All rights reserved.
Intentionally incorporated defect and its consequences in oxide thin film through Radio Frequency Magnetron Sputtering Technique
Saha B., Thapa R., Das N.S., Chattopadhyay K.K.
Article, Indian Journal of Physics, 2010, DOI Link
View abstract ⏷
Radio Frequency Magnetron Sputtering Technique has been employed to prepare metal oxide thin film of ZnO and CdO. The films were deposited in such condition that some point defects like oxygen vacancies have been Intentionally incorporated. The defects appeared with significant modification in the properties of the thin films. The prepared films were characterized by studying with X-ray diffraction study, X-ray photoelectron spectroscopic measurement, optical transmittance measurement, and electrical study. The electrical properties are found to change profoundly with the defect concentration. Consequently the optical properties also have been changed. © 2010 IACS.
Self filling of Ni nanoparticles in amorphous AlN nanotubes and its field emission property
Thapa R., Saha B., Das N.S., Maiti U.N., Chattopadhyay K.K.
Article, Applied Surface Science, 2010, DOI Link
View abstract ⏷
Unique aluminum nitride amorphous nanotubes filled with Ni nanoparticles have been successfully synthesized through the reaction of NH 3 over Ni-Al thin film at 1000 °C, which is similar to the extended vapor-liquid-solid technique. The X-ray diffraction and high-resolution transmission electron microscopic results are in good agreement with the amorphous nature of AlN nanotubes and crystallinity of Ni nanoparticles. The AlN nanotubes were having average diameter 35 nm and length ∼4 μm, whereas the Ni nanoparticles were having 5 nm in diameter. The unique structure showed excellent field emission property and high electrical conductivity ∼0.43 kmho/m at room temperature. The mechanism of good field emission property has been explained in detail. © 2010 Elsevier B.V. All rights reserved.
First principles analysis on V3+ doped aluminum nitride
Thapa R., Saha B., Chattopadhyay K.K.
Article, Computational Materials Science, 2010, DOI Link
View abstract ⏷
Using the spin density-functional-theory method and supercell approach we investigated the effect of vanadium doping on the magnetic and optical properties of aluminum nitride. The energy dependence of the imaginary part of the dielectric function and the absorption coefficient of vanadium doped AlN were calculated. The results showed the appearance of some new peaks and shoulders at various energies due to doping of vanadium. Also it was found that the total energy of the antiferromagnetic state was 64 meV higher than that of ferromagnetic state. The Curie temperature of V doped AlN was calculated based on the analysis of Cu doped AlN by Jia et al. (2007) [1] and found to be very high (>350 K). The magnetization was produced mainly due to the unpaired electrons in d-orbital of vanadium atom and the magnetic moment was found to be 2 μB for 6.25% V doped in a (2 × 2 × 2) supercell of AlN. © 2010 Elsevier B.V. All rights reserved.
Effect of vanadium doping on the dielectric and nonlinear current-voltage characteristics of CaCu3Ti4O12 ceramic
Sen A., Maiti U.N., Thapa R., Chattopadhyay K.K.
Article, Journal of Alloys and Compounds, 2010, DOI Link
View abstract ⏷
Here we report the effect of vanadium doping on the dielectric and electrical properties of giant dielectric material CaCu3Ti 4O12 (CCTO), synthesized through conventional solid-state reaction process. Proper crystalline phase formation together with dopant induced lattice constant shrinkage was confirmed through X-ray diffraction studies. The X-ray photoelectron spectroscopic studies confirmed vanadium doping with V4+ replacing Ti4+ at its lattice site. The grain boundary resistivity was found to decrease monotonically with the increase of V doping percentages as revealed by impedance spectroscopic measurement and furthermore the grain size was found to follow the similar trend. The reduced grain boundary resistivity was found to be responsible for the overall variation of current density-electric field (J-E) characteristics. © 2010 Elsevier B.V. All rights reserved.
Flexible cold cathode with ultralow threshold field designed through wet chemical route
Maiti U.N., Maiti S., Thapa R., Chattopadhyay K.K.
Article, Nanotechnology, 2010, DOI Link
View abstract ⏷
A flexible cold cathode based on a uniform array of ZnO nanowires over carbon fabrics was designed via a simple wet chemical route. The structural parameters of the nanowires (i.e. length, diameter) as well as their arrangement over the carbon fibers were tailored by adjusting nutrient solution composition and growth duration. The optimized arrays of ZnO nanowires exhibit excellent electron emission performance with ultralow turn-on as well as threshold fields of 0.27 and 0.56 V μm-1. This threshold field value is the lowest compared to any of the previous zinc-oxide-based cold cathodes realized through either chemical or vapor phase processes. In addition, the current density can reach an exceptionally high value of ∼11 mA cm-2 at an applied electric field of only 0.8 V μm-1. Flexible electronic devices based on a field emitter cold cathode may thus be realized through chemical processing at low budget but having high efficiency. © 2010 IOP Publishing Ltd.
A novel route for the low temperature synthesis of p-type transparent semiconducting CuAlO2
Saha B., Thapa R., Chattopadhyay K.K.
Article, Materials Letters, 2009, DOI Link
View abstract ⏷
CuAlO2 is an excellent p-type semiconductor material which usually needs very high temperature for its preparation. In this letter we report a novel synthesis procedure of p-type CuAlO2 powders through a very convenient chemical process at temperature much lower than needed for conventional solid state reaction method. CuAlO2 powders were prepared using Cu2O/CuO and Al2O3 powders of appropriate amounts as Cu and Al sources in molten NaOH at 360 °C. The prepared CuAlO2 powder was phase pure and was characterized by studying X-ray diffraction, Fourier transformed infrared spectroscopy, field emission scanning electron microscopy and X-ray photoelectron spectroscopy. © 2008 Elsevier B.V. All rights reserved.
Enhanced field emission from Si doped nanocrystalline AlN thin films
Thapa R., Saha B., Chattopadhyay K.K.
Article, Applied Surface Science, 2009, DOI Link
View abstract ⏷
Si doped and undoped nanocrystalline aluminum nitride thin films were deposited on various substrates by direct current sputtering technique. X-ray diffraction analysis confirmed the formation of phase pure hexagonal aluminum nitride with a single peak corresponding to (1 0 0) reflection of AlN with lattice constants, a = 0.3114 nm and c = 0.4986 nm. Energy dispersive analysis of X-rays confirmed the presence of Si in the doped AlN films. Atomic force microscopic studies showed that the average particle size of the film prepared at substrate temperature 200 °C was 9.5 nm, but when 5 at.% Si was incorporated the average particle size increased to ∼21 nm. Field emission study indicated that, with increasing Si doping concentration, the emission characteristics have been improved. The turn-on field (E to ) was 15.0 (±0.7) V/μm, 8.0 (±0.4) V/μm and 7.8 (±0.5) V/μm for undoped, 3 at.% and 5 at.% Si doped AlN films respectively and the maximum current density of 0.27 μA/cm 2 has been observed for 5 at.% Si doped nanocrystalline AlN film. It was also found that the dielectric properties were highly dependent on Si doping. © 2008 Elsevier B.V. All rights reserved.
Synthesis of cubic aluminum nitride by VLS technique using gold chloride as a catalyst and its optical and field emission properties
Thapa R., Saha B., Chattopadhyay K.K.
Article, Journal of Alloys and Compounds, 2009, DOI Link
View abstract ⏷
Thin films of nanocrystalline cubic AlN were synthesized by vapor-liquid-solid (VLS) route on Si and fused silica substrates at appropriate conditions. The formation of cubic AlN was confirmed by X-ray diffraction studies. We also observed that AuCl3 plays an important role as a catalyst in the synthesis of well crystalline cubic phase of AlN. Energy dispersive analysis of X-rays confirmed the presence of nearly stoichiometric aluminum and nitrogen in the films. Optical properties of the cubic AlN films deposited on the fused silica substrates were studied by measuring the transmittance versus wavelength. The films showed good transparency and the allowed indirect band gap, measured from the transmittance spectra, was 3.83 eV. Fourier transformed infrared spectra showed characteristics absorptions of different vibrational frequencies. The cubic AlN nanostructured films also showed good electron field emission properties. The turn on field was found to be 11.9 V/μm and the maximum current density was ∼1 A/m2. © 2008 Elsevier B.V. All rights reserved.
Bandgap widening in highly conducting CdO thin film by Ti incorporation through radio frequency magnetron sputtering technique
Saha B., Thapa R., Chattopadhyay K.K.
Article, Solid State Communications, 2008, DOI Link
View abstract ⏷
Transparent and highly conducting thin films of cadmium oxide (CdO) with titanium doping were synthesized by using radio frequency magnetron sputtering technique. The thin films were deposited on glass and silicon substrates with different percentages of titanium at a fixed substrate temperature 473 K and a fixed pressure of 0.1 mbar in Ar atmosphere. The deposited films were characterized by studying their crystallographic structure, optical and electrical properties. X-ray diffractometer, atomic force microscope, UV-Vis-NIR spectrophotometer, and X-ray photoelectron spectrophotometer were used for different characterizations. All the films have a rock-salt structure. A systematic increase in the optical bandgap was found for the CdO thin films with Ti doping, so that it can be considered as a candidate material for different optoelectronic device applications. Electrical conductivity was also found to increase with Ti doping concentration. © 2007 Elsevier Ltd. All rights reserved.
Optical and dielectric properties of PVA capped nanocrystalline PbS thin films synthesized by chemical bath deposition
Jana S., Thapa R., Maity R., Chattopadhyay K.K.
Article, Physica E: Low-Dimensional Systems and Nanostructures, 2008, DOI Link
View abstract ⏷
Nanoparticles of lead sulfide (PbS) have been grown within the pores of polyvinyl alcohol (PVA) matrix on glass substrates by chemical bath deposition at and below room temperature (30 °C). Lead acetate and thiourea, dissolved in an alkaline medium, were taken as the sources of lead and sulfur. X-ray diffraction and selected area electron diffraction studies confirmed the cubic nanocrystalline PbS phase formation. Transmission electron micrograph of the films revealed the particle size lying in the range 10-20 nm. X-ray photoelectron spectroscopic studies confirmed the presence of lead and sulfur in the films, and their atomic ratios were found to be dependent on the deposition temperature. UV-vis spectrophotometric measurement showed a direct allowed band gap lying in the range 2.40-2.81 eV, which is much higher than the bulk value (0.41 eV). The band gap decreases with the increase of deposition temperature. The dielectric constant of the PVA-capped nanocrystalline PbS was in the range 155-265 at higher frequencies, which is much higher compared to only PVA and bulk PbS. © 2008 Elsevier B.V. All rights reserved.
Wide range tuning of electrical conductivity of RF sputtered CdO thin films through oxygen partial pressure variation
Saha B., Thapa R., Chattopadhyay K.K.
Article, Solar Energy Materials and Solar Cells, 2008, DOI Link
View abstract ⏷
The effect of oxygen partial pressure variation on the electrical conductivity and the optical transparency of CdO thin films, deposited through RF magnetron sputtering were studied in detail. Thin films of CdO have been deposited through radio frequency magnetron sputtering of a prefabricated CdO target at a fixed pressure 0.1 mbar and at a substrate temperature 523 K. It was found that the electrical conductivity of the CdO films could be varied over three decades for a variation of oxygen partial pressure of 0-100%, without introducing any extrinsic dopants. X-ray diffraction (XRD) studies showed that the films were polycrystalline in nature with a preferential orientation along (1 1 1) plane. Compositional information was obtained by X-ray photoelectron spectroscopic studies. This wide range of variation of electrical properties was explained through the oxygen vacancies formation. © 2008 Elsevier B.V. All rights reserved.
Improvement of electrical and thermoelectric properties of CdO thin film by aluminum doping
Saha B., Thapa R., Chattopadhyay K.K.
Conference paper, Proceedings of the 14th International Workshop on the Physics of Semiconductor Devices, IWPSD, 2007, DOI Link
View abstract ⏷
CdO thin films doped with Al have been prepared by radio frequency magnetron sputtering technique. The films were deposited on glass substrates at a temperature of 473 K and at a pressure of 0.1 mbar in (Ar + O2) atmosphere. The deposited films were characterized by studying X-ray diffraction, AFM image, XPS spectra, electrical conductivity and thermoelectric properties. Films with different atomic % of Al have the same cubic structure, good uniformity and high transparency. The Al doped CdO films deposited with optimized sputtering parameters are found to have high electrical conductivity along with very good thermoelectric power. ©2007 IEEE.
Electro-oculogram changes at the switch in a manic-depressive patient
Hanna S.M., Jenner F.A., Souster L.P.
Article, British Journal of Psychiatry, 1986, DOI Link
View abstract ⏷
The correlations between eye movements on the EOG of a patient with a 48 hour cycle of manic-depressive type are described. They are used to confirm the fact that his switch from one state to another occurred at the same time of night whether he was awake or asleep.
