Faculty Publications
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Item Photo- and Electrocatalytic Reduction of CO2 over Metal-Organic Frameworks and Their Derived Oxides: A Correlation of the Reaction Mechanism with the Electronic Structure(American Chemical Society, 2022) Payra, S.; Ray, S.; Sharma, R.; Tarafder, K.; Mohanty, P.; Roy, S.A Ce/Ti-based bimetallic 2-aminoterephthalate metal-organic framework (MOF) was synthesized and evaluated for photocatalytic reduction of CO2 in comparison with an isoreticular pristine monometallic Ce-terephthalate MOF. Owing to highly selective CO2 adsorption capability, optimized band gaps, higher flux of photogenerated electron-hole pairs, and a lower rate of recombination, this material exhibited better photocatalytic reduction of CO2 and lower hydrogen evolution compared to Ce-terephthalate. Thorough probing of the surface and electronic structure inferred that the reducibility of Ce4+ to Ce3+ was due to the introduction of an amine functional group into the linker, and low-lying Ti(3d) orbitals in Ce/Ti-2-aminoterephthalate facilitated the photoreduction reaction. Both the MOFs were calcined to their respective oxides of Ce1-xTixO2 and CeO2, and the electrocatalytic reduction of CO2 was performed over the oxidic materials. In contrast to the photocatalytic reaction mechanism, the lattice substitution of Ti in the CeO2 fluorite cubic structure showed a better hydrogen evolution reaction and consequently, poorer electroreduction of CO2 compared to pristine CeO2. Density functional theory calculations of the competitive hydrogen evolution reaction on the MOF and the oxide surfaces corroborated the experimental findings. © 2022 American Chemical Society.Item Investigation of CdSe and ZnSe as Potential Back Surface Field Layers for CdTe-Based Solar Cells: A Study from First Principles Calculations(John Wiley and Sons Inc, 2023) Ray, S.; Tarafder, K.A class of II–VI semiconductors, especially CdTe, is a highly photo-reactive compound that would be suitable for photovoltaic applications. However, being a highly resistive material, CdTe produces considerable contact resistance and drastically reduces the efficiency of photovoltaic devices. Introducing a back surface field layer at the contact region may significantly improve the device's performance. This work investigates the suitability of using ZnSe and CdSe layer as a back-surface-field layer in CdTe-based solar cells through accurate electronic structure calculations using the hybrid-density functional theory method. The calculations show that both ZnSe/CdTe and CdSe/CdTe behave as type-II heterojunctions with band gaps of 2.0 and 1.1 eV, respectively. The Mulliken electronegativity method is used to determine the correct band edge positions concerning the vacuum level for all the pristine semiconductors and their interfaces. Calculation shows that a significant charge redistribution in the interface leads to the formation of an effective local field near the contact region for both ZnSe/CdTe and CdSe/CdTe heterostructures. This local field may help to separate the photogenerated electron–hole pairs in the active layer by pushing the opposite charges into the two different sections of the heterojunction. Additionally, the heterojunctions also exhibit better light-absorption characteristics in the visible light range. © 2023 The Authors. Advanced Theory and Simulations published by Wiley-VCH GmbH.Item Distal Synergistic Effect in Bimetal-Organic Framework for Superior Catalytic Water Oxidation(American Chemical Society, 2023) Bhoi, U.; Ray, S.; Bhand, S.; Ninawe, P.; Roy, D.; Rana, S.; Tarafder, K.; Ballav, N.Metal-organic frameworks (MOFs) are emerging as promising electro-catalysts for the oxygen evolution reaction (OER). The bimetallic design strategy was further adopted in MOFs to elevate the OER performance by a synergistic effect. The proximal metal-oxygen-metal bonding configuration with typical 3dπ-2pπ-3dπ interaction was apparently essential for an effective electronic coupling between the metal centers. Here, we report an example of distal synergy in a bimetal-organic framework exhibiting a better OER activity than the monometallic counterparts, as well as the conventional proximal synergy. To achieve a current density of 10 mA·cm-2, our electrodeposited bimetallic MOF, Co-Ni(TCNQ)2(H2O)2 (TCNQ = 7,7,8,8-tetracyanoquinodimethane), on a glassy-carbon electrode required an overpotential value of 220 mV. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed distinctive electronic coupling between the Co(II)-3d7 and Ni(II)-3d8 centers, despite being 9 Å apart, leading to an overall charge delocalization in the structure via TCNQ. © 2023 American Chemical Society.Item Mechanistic insight and first principle analysis of cation-inverted zinc ferrite nanostructure: A paradigm for ppb-level room temperature NOx sensor(Elsevier B.V., 2024) Nath, V.G.; Ray, S.; Rodney, J.D.; Prakasha Bharath, S.; Roy, S.; Tarafder, K.; Subramanian, A.; Chul Kim, B.Herein, we adopted a new paradigm for developing a high-performance gas sensor by leveraging the mixed spinel ZnFe2O4 structure (mZFO) to enhance the adsorption of NOx molecules. Material characterization reveals the formation of the mZFO due to the cation inversion in lattice sites. The estimated value of the inversion degree is observed to shift from 0.78 to 0.39 with an increase in the calcination temperature. The mZFO nanoparticles calcined at 500 °C show exceptional sensing performance due to their suitable grain size (∼2 times Debye length), neck diameter, and surface area. The sensing studies conducted at various NOx concentrations indicate that the sensor can detect ppb level of NOx with a detection limit of about 9 ppb at room temperature. The detailed sensing mechanism is elucidated based on the density functional theory calculations (DFT) and Bader charge analysis. The outstanding sensor performance is attributed to the formation of a mixed spinel structure, wherein the adsorption energy of NOx (∼-0.6 eV) in the presence of surface adsorbed oxygen is higher than that of the normal spinel structure (∼-0.1 eV). Furthermore, the sensor exhibited a fast response and recovery times (7 and 92 s at 800 ppb NO2), excellent stability, and selectivity. The practical suitability of the mZFO sensor was studied by analyzing the vehicle exhaust emissions. We strongly believe this work would pave a novel approach to developing a high-potential gas sensor by modifying the cation distributions in the spinel ferrites. © 2024 Elsevier B.V.Item Electrochemical insights into manganese-cobalt doped ?-Fe2O3 nanomaterial for cholesterol detection: a comparative approach(Royal Society of Chemistry, 2025) Sushmitha, S.; Ray, S.; Rao, L.; Nayak, M.P.; Carva, K.; Badekai Ramachandra, B.R.Herein, a self-assembled hierarchical structure of hematite (?-Fe2O3) was synthesized via a one-pot hydrothermal method. Subsequently, the nanomaterial was doped to obtain MxFe2?xO3 (M = Mn-Co; x = 0.01, 0.05, and 0.1) at precise concentrations. An electrode was fabricated by coating the resulting nanocomposite onto a nickel foam (NF) substrate. Electrochemical characterization demonstrated the excellent performance of cobalt-doped ?-Fe2O3, among which Co0.05Fe0.95O3 (CF5) exhibited a superior performance, showing a two-fold increase in sensitivity of 1364.2 ?A mM?1 cm?2 (±0.03, n = 3) in 0.5 M KOH, a limit of detection (LOD) of ?0.17 mM, and a limit of quantification (LOQ) of ?0.58 mM. The Density Functional Theory (DFT) was performed to understand the doping prompting in the reduced bandgap. The fabricated electrode displayed a rapid response time of 2 s and demonstrated 95% stability, excellent reproducibility, and selectivity, as confirmed by tests with several interfering species. A comprehensive evaluation of the electrode's performance using human blood serum highlighted its robustness and reliability for cholesterol detection in clinical settings, making it a promising tool for clinical and pharmaceutical applications. © 2025 The Royal Society of Chemistry.Item Temperature-dependent in situ Cd substitution at Zn sites in Cu2ZnSnS4 thin films via sol–gel method: Experimental and DFT insights(Elsevier B.V., 2025) Chennangod, S.; Ray, S.; P, A.S.; Tarafder, K.; Bhat, T.N.We report a systematic study of in situ cadmium (Cd) substitution at Zinc (Zn) sites in Cu2ZnSnS4 (CZTS) thin films synthesized via a scalable sol–gel route, with sulfurization carried out at 300 °C, 400 °C, and 500 °C. X-ray diffraction and Raman spectroscopy demonstrate that higher sulfurization temperatures along with increased Cd content progressively suppress the secondary Cu2SnS3 phase, while field-emission SEM and atomic force microscopy reveal enhanced grain growth and a smoother granular surface. UV–Vis absorption measurements show a continuous band-gap reduction from 1.43 eV in undoped CZTS to 1.20 eV at the highest Cd level, corroborated by a red shift in photoluminescence emission. X-ray photoelectron spectroscopy and density functional theory (GGA-PBE and HSE06) with orbital-projected density of states (p-DOS) analyses attribute this narrowing to localized Cd-induced states near the conduction band minimum and lattice expansion effects. Additionally, preliminary photovoltaic characterization demonstrated improved device performance for the Cd:CZTS solar cell compared to the pristine CZTS cell, exhibiting higher photocurrent density and enhanced external quantum efficiency. These results confirm that precise control of sulfurization temperature and Cd incorporation not only tailors the electronic structure and band gap but also suppresses undesirable secondary phases, offering a promising route to optimize kesterite thin films for high-efficiency photovoltaic applications. © 2025 Elsevier B.V.