S.N. Jha | Homi Bhabha National Institute (HBNI, BARC, MUMBAI) (original) (raw)
Papers by S.N. Jha
JBIC Journal of Biological Inorganic Chemistry, 2021
The role of the pentapeptide, NHSFM, derived from the surface exposed part of the metal ion bindi... more The role of the pentapeptide, NHSFM, derived from the surface exposed part of the metal ion binding loop of the subunit II of cytochrome c oxidase on the maturation of the binuclear purple CuA center of the enzyme has been investigated using several experimental and computational methods. The copper ion was found to form 1:1 complex of the pentapeptide with a binding constant ~ 104 M-1 to 105 M-1, where a 4 ligand coordination from the peptide in a type 2 copper center was revealed. The pH dependence of the metal-peptide was associated with a [Formula: see text] of ~ 10 suggesting deprotonation of the N-terminal amine. EXAFS studies as well as DFT calculations of the metal-peptide complexes revealed pH dependent changes in the metal-ligand bond distances. Spectroscopic properties of the metal peptides calculated from TDDFT studies agreed with the experimental results. Restrained molecular dynamics (RMD) simulations indicated coordination of a carbonyl oxygen from the asparagine (N) side chain and of water molecules apart from histidine (H), methionine (M) and terminal amine of asparagine (N) in a distorted square planar geometry of Cu-NHSFM. Analyses of the backbone distances as well as B-factors for the metal peptide suggested that the peptide backbone becomes more compact and rigid on binding of the metal ion. This indicated that binding of copper ion to this pentapeptide in the protein possibly cause movement of the protein backbone bringing other coordinating residues closer to the copper ion, and thus helping in sequential uptake of copper ions to the protein.
ACS Applied Materials & Interfaces, 2021
The present work elaborates the high-energy-density, stable, and flexible supercapacitor devices ... more The present work elaborates the high-energy-density, stable, and flexible supercapacitor devices (full-cell configuration with asymmetric setup) based on a two-dimensional tungsten oxide/selenium (2D WO3/Se) nanocomposite. For this, the 2D WO3/Se nanocomposite synthesized by a hydrothermal method followed by air annealing was coated on a flexible carbon cloth current collector and combined separately with both 0.1 M H2SO4 and 1-butyl-3-methyl imidazolium tetrafluoroborate room temperature ionic liquid (BmimBF4 RTIL) as electrolyte. Different physicochemical characterization techniques, viz., transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, are utilized for phase confirmation and morphology identification of the obtained samples. The electrochemical analysis was used to evaluate charge storage mechanism. The half-cell configuration (three electrode system) in 0.1 M H2SO4 shows a specific capacitance of 564 F g-1 at 6 A g-1 current density, whereas with ionic liquid as electrolyte, a higher specific capacitance of 1650 F g-1 was obtained at a higher current of 40 mA and working potential of 4 V. Importantly, the asymmetric flexible supercapacitor device with PVA-H2SO4 electrolyte shows a working voltage of 1.7 V. A specific capacitance of 858 mF g-1 is obtained for the asymmetric electrode system with an energy density of 47 mWh kg-1 and a power density of 345 mW kg-1 at a current density of 0.2 A g-1.
Here, we report a reproducible method for the synthesis of SA-Ms on the surface of the electroact... more Here, we report a reproducible method for the synthesis of SA-Ms on the surface of the electroactive bacterium, Geobacter sulfurreducens (GS). Our approach is based on the extracellular electron transfer capability of GS to reduce extracellular metal ions by harvesting the metabolically formed electrons inside the cell. The formation of SA-Ms on the surface of GS was achieved by creating an MN3 moiety (where M=Ir, Pt, Co, Ru, Cu, and Pd, and N = nitrogen), identified by X-ray absorption fine structure analysis. Interestingly, our bio-derived SA-M@GS can be directly used as OER and HER electrocatalysts upon drying without further heat treatments such as pyrolysis or hydrothermal approaches.
Scientific Reports, 2020
Thin films of β-W are the most interesting for manipulating magnetic moments using spin–orbit tor... more Thin films of β-W are the most interesting for manipulating magnetic moments using spin–orbit torques, and a clear understanding of α to β phase transition in W by doping impurity, especially oxygen, is needed. Here we present a combined experimental and theoretical study using grazing incidence X-ray diffraction, photoelectron spectroscopy, electron microscopy, and ab initio calculations to explore atomic structure, bonding, and oxygen content for understanding the formation of β-W. It is found that the W films on SiO2/Si have 13–22 at.% oxygen in A15 β structure. Ab initio calculations show higher solution energy of oxygen in β-W, and a tendency to transform locally from α to β phase with increasing oxygen concentration. X-ray absorption spectroscopy also revealed local geometry of oxygen in β-W, in agreement with the simulated one. These results offer an opportunity for a fundamental understanding of the structural transition in α-W and further development of β-W phase for device...
New Journal of Chemistry, 2020
The formation of O-vacancies along with their local environment and locus of formation for the O-... more The formation of O-vacancies along with their local environment and locus of formation for the O-vacancies has been understood using EXAFS and ab initio techniques in a doped (Zn2+) anatase lattice.
ACS Applied Materials & Interfaces, 2020
Energy storage and conversion driven by electro-or photocatalyst is a highly exciting field of re... more Energy storage and conversion driven by electro-or photocatalyst is a highly exciting field of research, and generations of effective and durable oxide catalysts have received much attention in this field. Here, we report A-site lanthanum-doped oxygen-rich quinary oxide CaLaScRuO 6+δ synthesized by adopting the solid-state reaction method and characterized by various techniques such as powder X-ray diffraction, neutron diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and temperature-programmed reduction in the presence of a hydrogen atmosphere (H 2-TPR). X-ray absorption study confirms the existence of mixed valent Ru ions in the structure, which enhances the oxygen stoichiometry for the partial balance of an extra cationic charge. Neutron powder diffraction and reduction of the material in a hydrogen atmosphere (H 2-TPR) can confirm the oxygen overstoichiometry of the catalyst. The present material works as an efficient and robust oxygen bifunctional electrocatalyst for ORR/ OER (oxygen evolution reaction/oxygen reduction reaction) followed by four-electron transfer pathway in a strong (1 M KOH) alkaline medium. The catalytic nature of the designed structural and chemical flexible perovskite is a novel example of an electrocatalyst for the oxygen bifunctional activity.
Inorganic chemistry, Jan 2, 2018
Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol-gel combustion rout... more Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol-gel combustion route, which can be used both as a blue-emitting phosphor material and a white-emitting phosphor material, depending on the annealing temperature during synthesis. The material also has the potential to be used in optical thermometry. These fascinating color-tunable emission characteristics can be linked with the various defect centers present inside the matrix and their changes upon thermal annealing. Various defect centers, such as anionic vacancy, cationic vacancy, antisite defect, etc., create different electronic states inside the band gap, which are responsible for the multicolor emission. The color components are isolated from the complex emission spectra using time-resolved emission spectroscopy (TRES) study. Interestingly, the lifetime values of the various defect centers were found to change significantly from milliseconds to microseconds upon thermal annealing, which makes the pho...
ChemistrySelect, 2017
MoS 2 nanospheres were formed using a template free hydrothermal process, which exhibit high cata... more MoS 2 nanospheres were formed using a template free hydrothermal process, which exhibit high catalytic activity towards hydrogen evolution reaction (HER). The extend of defect sites are probed by extended X-ray absorption fine structure which found decrease in coordination number at Mo site rather than at S site. DFT calculations identified an uneven strain and defect distribution between two S planes of curved MoS 2. Based on hydrogen adsorption on various sites, we identify a new pathway called "extended activity @ shielded defects", for Volmer-Tafel and Volmer-Heyrovsky mechanisms, where H adsorption occurs at exposed S layer driven by defects in underneath S layer of nanosphere. Having higher defect concentration it exhibited excellent HER activity with overpotential of À0.12 V, Tafel slope of 90 mV/decade, and higher turnover frequency. Our findings provide an avenue to design and engineer advanced nanostructures for catalysis, electronic devices, and other potential applications.
Scientific Reports, 2017
Graphene has been studied intensively in opto-electronics, and its transport properties are well ... more Graphene has been studied intensively in opto-electronics, and its transport properties are well established. However, efforts to induce intrinsic optical properties are still in progress. Herein, we report the production of micron-sized sheets by interconnecting graphene quantum dots (GQDs), which are termed ‘GQD solid sheets’, with intrinsic absorption and emission properties. Since a GQD solid sheet is an interconnected QD system, it possesses the optical properties of GQDs. Metal atoms that interconnect the GQDs in the bottom-up hydrothermal growth process, induce the semiconducting behaviour in the GQD solid sheets. X-ray absorption measurements and quantum chemical calculations provide clear evidence for the metal-mediated growth process. The as-grown graphene quantum dot solids undergo a Forster Resonance Energy Transfer (FRET) interaction with GQDs to exhibit an unconventional 36% photoluminescence (PL) quantum yield in the blue region at 440 nm. A high-magnitude photocurren...
Journal of Materials Chemistry C, 2017
Ferromagnetism originating from oxygen vacancies could be observed in Th1−xBixO2−δ (0 < x ≤ 0.... more Ferromagnetism originating from oxygen vacancies could be observed in Th1−xBixO2−δ (0 < x ≤ 0.3) solid solutions. The saturation magnetization was found to increase with Bi concentration as confirmed by magnetic and EPR investigations.
Journal of Materials Chemistry A, 2017
A highly efficient, low-cost (precious-metal-free), highly stable nanohybrid electrocatalyst cont... more A highly efficient, low-cost (precious-metal-free), highly stable nanohybrid electrocatalyst containing carbon-supported molybdenum carbide and nitride nanoparticles of size ranging from 8 to 12 nm exhibit excellent HER catalytic activity.
ACS Applied Materials & Interfaces, 2016
Oxygen reduction reaction (ORR) is increasingly being studied in oxide systems due to advantages ... more Oxygen reduction reaction (ORR) is increasingly being studied in oxide systems due to advantages ranging from cost effectiveness to desirable kinetics. Oxygen-deficient oxides like brownmillerites are known to enhance ORR activity by providing oxygen adsorption sites. In parallel, nitrogen and iron doping in carbon materials, and consequent presence of catalytically active complex species like C-Fe-N, is also suggested to be good strategies for designing ORR-active catalysts. A combination of these features in N-doped Fe containing brownmillerite can be envisaged to present synergistic effects to improve the activity. This is conceptualized in this report through enhanced activity of N-doped Ca2Fe2O5 brownmillerite when compared to its oxide parents. N doping is demonstrated by neutron diffraction, UV-vis spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Electrical conductivity is also found to be enhanced by N doping, which influences the activity. Electrochemical characterization by cyclic voltammetry, rotating disc electrode, and rotating ring disk electrode (RRDE) indicates an improved oxygen reduction activity in N-doped brownmillerite, with a 10 mV positive shift in the onset potential. RRDE measurements show that the compound exhibits 4-electron reduction pathways with lower H2O2 production in the N-doped system; also, the N-doped sample exhibited better stability. The observations will enable better design of ORR catalysts that are stable and cost-effective.
Inorganic Chemistry, 2017
Considering the fact that pyrophosphate-based hosts are in high demand for making highly efficien... more Considering the fact that pyrophosphate-based hosts are in high demand for making highly efficient luminescence materials, we doped two visible lanthanide ions, viz. Sm 3+ and Eu 3+ , in Zn 2 P 2 O 7. Interestingly, it was oberved that pure Zn 2 P 2 O 7 displayed blue-green dual emission on irradiation with ultraviolet light. Emission and lifetime spectroscopy shows the presence of defects in pyrophosphate samples which are responsible for such emission. DFT calculations clearly pinpointed that the electronic transitions between defect states located at just below the conduction band minimum (arises due to V O 1+ and V O 2+ defects) and valence band maximum, as well as impurity states situated in the band gap, can lead to dual emission in the bluegreen region, as is also indicated by emission and lifetime spectra. X-ray absorption near edge spectroscopy (XANES) shows the stabilization of europium as well as samarium ion in the +3 oxidation state in α-Zn 2 P 2 O 7. The fact that α-Zn 2 P 2 O 7 has two different coordination numbers for zinc ions, i.e. fiveand six-coordinate, the study of dopant ion distribution in this particular matrix will be an important step in realizing a highly efficient europium-and samarium-based red-emitting phosphor. Time resolved photoluminescence (TRPL) shows that both of these ions are heterogeneously distributed between fiveand sixcoordinated Zn 2+ sites and it is the six-coordinated Zn 2+ site which is the most favorable for lanthanide ion doping. Extended Xray absorption fine structure (EXAFS) measurements also suggested that a six-coordinated zinc ion is the preferred site occupied by trivalent lanthanide ions, which is in complete agreement with TRPL results. It was observed that there is almost complete transfer of photon energy from Zn 2 P 2 O 7 to Eu 3+ , whereas this transfer is inefficient and almost incomplete in case of Sm 3+ , which is indeed important information for the realization of pyrophosphate-based tunable phosphors.
Dalton Transactions, 2016
During the formation of Bi2−2xLa2xUO6 (x = 0–0.05), La+3 ions replace Bi+3 in the Bi2UO6 lattice ... more During the formation of Bi2−2xLa2xUO6 (x = 0–0.05), La+3 ions replace Bi+3 in the Bi2UO6 lattice with the creation of some oxygen vacancies.
Inorganic Chemistry, 2016
SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematical... more SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematically using X-ray diffraction and time-resolved photoluminescence techniques. A detailed analysis of the optical properties of Tb(3+) ions in SrZrO3 was performed to correlate them with the local environment of the lanthanide ions in this perovskite. Photoluminescence (PL) spectroscopy showed that emission spectrum consists of host as well as Tb(3+) emission indicating the absence of complete host-dopant energy transfer. On the basis of emission spectrum and PL decay study it was also observed that Tb(3+) is not homogeneously distributed in SrZrO3 perovskite; rather, it is occupying two different sites. It is corroborated using extended X-ray absorption fine structure studies that Tb(3+) is stabilized on both six-coordinated Zr(4+) and eight-coordinated Sr(2) site. The energies calculated using density functional theory (DFT) indicates that Tb occupation in Sr site is energetically more favorable than Zr site. The analysis of valence charge distribution also substantiated our structural stability analysis of site-selective Tb doping in SrZrO3. Time-resolved emission spectroscopy is employed to elucidate the difference in the spectral feature of Tb(3+) ion at Sr(2+) and Zr(4+) site. DFT-calculated density of states analysis showed that energy mismatch of Tb-d states with Zr-d and O-p states of SZP makes the energy transfer from host SZP to Tb(3+) ion difficult.
The Journal of Physical Chemistry C, 2015
ABSTRACT In this work, five types of MnO2 nanostructres (nano-wires, nano-tubes, nano-particles, ... more ABSTRACT In this work, five types of MnO2 nanostructres (nano-wires, nano-tubes, nano-particles, nano-rods and nano-flowers) were synthesized with a fine control over their -crystallographic form by hydrothermal method. The electrocatalytic activities of these materials were examined towards oxygen reduction reaction (ORR) in alkaline medium. Numerous characterizations were correlated with the observed activity by analyzing their crystal structure (TGA, XRD, TEM), material morphology (FE-SEM), porosity (BET), inherent structural nature (IR, Raman, ESR), surfaces (XPS) and electrochemical properties (Tafel, Koutecky-Levich plots and % of H2O2 produced). Moreover, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) analysis were employed to study the structural information on the MnO2 coordination number as well as interatomic distance. These combined results show that the electrocatalytic activities are significantly dependent on the nano-shapes, and follow an order nano-wire nano-rod nano-tube nano-particle nano-flower. -MnO2 nano-wires possess enhanced electrocatalytic activity compared to other shapes even though the nano-tubes possess a much higher BET surface area. In the ORR studies, -MnO2 nano-wires displayed Tafel slope of 65 mV/decade, n-value of 3.5 and 3.6% of hydrogen peroxide production. The superior ORR activity was attributed to the fact that it possesses active sites composed with two shortened Mn-O bonds along with a Mn-Mn distance of 2.824 Å which provides an optimum requirement for the adsorbed oxygen in a bridge mode favouring the direct 4 electron reduction. According to the first principles based density functional theory (DFT) the enhancement in ORR activity was due to the stronger interaction between the surface and adsorbates for the (211) surface than all other surfaces.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2007
Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO 4 :Dy... more Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO 4 :Dy phosphor samples at the Dy L 3 edge with synchrotron radiation. Measurements were carried out on a set of samples which were subjected to post-preparation annealing at different temperatures and for different cycles. The EXAFS data have been analysed to find the Dy-S and Dy-O bond lengths in the neighbourhood of the Dy atoms in a CaSO 4 matrix. The observations from EXAFS measurements were verified with XANES and XPS techniques. On the basis of these measurements, efforts were made to explain the loss of thermoluminescence sensitivity of CaSO 4 :Dy phosphors after repeated cycles of annealing at 400°C in air for 1 h.
Journal of Synchrotron Radiation, 2016
An innovative scheme to carry out continuous-scan X-ray absorption spectroscopy (XAS) measurement... more An innovative scheme to carry out continuous-scan X-ray absorption spectroscopy (XAS) measurements similar to quick-EXAFS mode at the Energy-Scanning EXAFS beamline BL-09 at INDUS-2 synchrotron source (Indore, India), which is generally operated in step-by-step scanning mode, is presented. The continuous XAS mode has been implemented by adopting a continuous-scan scheme of the double-crystal monochromator and on-the-fly measurement of incident and transmitted intensities. This enabled a high signal-to-noise ratio to be maintained and the acquisition time was reduced to a few seconds from tens of minutes or hours. The quality of the spectra (signal-to-noise level, resolution and energy calibration) was checked by measuring and analysing XAS spectra of standard metal foils. To demonstrate the energy range covered in a single scan, a continuous-mode XAS spectrum of copper nickel alloy covering both Cu and NiK-edges was recorded. The implementation of continuous-scan XAS mode at BL-09 w...
International Journal of Engineering Research, 2014
A new X-ray absorption fine structure (EXAFS) data acquisition and data analysis program is devel... more A new X-ray absorption fine structure (EXAFS) data acquisition and data analysis program is developed using LabVIEW. It runs under window operating system and offers several unique features. It has a user friendly graphical user environment and is capable of saving data in variety of formats. The availability of online data presentation is particularly useful for data examination during the EXAFS experiments as well as analysing quality of the sample. Transmission and Fluorescence mode are implemented to characterize variety of samples.
JBIC Journal of Biological Inorganic Chemistry, 2021
The role of the pentapeptide, NHSFM, derived from the surface exposed part of the metal ion bindi... more The role of the pentapeptide, NHSFM, derived from the surface exposed part of the metal ion binding loop of the subunit II of cytochrome c oxidase on the maturation of the binuclear purple CuA center of the enzyme has been investigated using several experimental and computational methods. The copper ion was found to form 1:1 complex of the pentapeptide with a binding constant ~ 104 M-1 to 105 M-1, where a 4 ligand coordination from the peptide in a type 2 copper center was revealed. The pH dependence of the metal-peptide was associated with a [Formula: see text] of ~ 10 suggesting deprotonation of the N-terminal amine. EXAFS studies as well as DFT calculations of the metal-peptide complexes revealed pH dependent changes in the metal-ligand bond distances. Spectroscopic properties of the metal peptides calculated from TDDFT studies agreed with the experimental results. Restrained molecular dynamics (RMD) simulations indicated coordination of a carbonyl oxygen from the asparagine (N) side chain and of water molecules apart from histidine (H), methionine (M) and terminal amine of asparagine (N) in a distorted square planar geometry of Cu-NHSFM. Analyses of the backbone distances as well as B-factors for the metal peptide suggested that the peptide backbone becomes more compact and rigid on binding of the metal ion. This indicated that binding of copper ion to this pentapeptide in the protein possibly cause movement of the protein backbone bringing other coordinating residues closer to the copper ion, and thus helping in sequential uptake of copper ions to the protein.
ACS Applied Materials & Interfaces, 2021
The present work elaborates the high-energy-density, stable, and flexible supercapacitor devices ... more The present work elaborates the high-energy-density, stable, and flexible supercapacitor devices (full-cell configuration with asymmetric setup) based on a two-dimensional tungsten oxide/selenium (2D WO3/Se) nanocomposite. For this, the 2D WO3/Se nanocomposite synthesized by a hydrothermal method followed by air annealing was coated on a flexible carbon cloth current collector and combined separately with both 0.1 M H2SO4 and 1-butyl-3-methyl imidazolium tetrafluoroborate room temperature ionic liquid (BmimBF4 RTIL) as electrolyte. Different physicochemical characterization techniques, viz., transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, are utilized for phase confirmation and morphology identification of the obtained samples. The electrochemical analysis was used to evaluate charge storage mechanism. The half-cell configuration (three electrode system) in 0.1 M H2SO4 shows a specific capacitance of 564 F g-1 at 6 A g-1 current density, whereas with ionic liquid as electrolyte, a higher specific capacitance of 1650 F g-1 was obtained at a higher current of 40 mA and working potential of 4 V. Importantly, the asymmetric flexible supercapacitor device with PVA-H2SO4 electrolyte shows a working voltage of 1.7 V. A specific capacitance of 858 mF g-1 is obtained for the asymmetric electrode system with an energy density of 47 mWh kg-1 and a power density of 345 mW kg-1 at a current density of 0.2 A g-1.
Here, we report a reproducible method for the synthesis of SA-Ms on the surface of the electroact... more Here, we report a reproducible method for the synthesis of SA-Ms on the surface of the electroactive bacterium, Geobacter sulfurreducens (GS). Our approach is based on the extracellular electron transfer capability of GS to reduce extracellular metal ions by harvesting the metabolically formed electrons inside the cell. The formation of SA-Ms on the surface of GS was achieved by creating an MN3 moiety (where M=Ir, Pt, Co, Ru, Cu, and Pd, and N = nitrogen), identified by X-ray absorption fine structure analysis. Interestingly, our bio-derived SA-M@GS can be directly used as OER and HER electrocatalysts upon drying without further heat treatments such as pyrolysis or hydrothermal approaches.
Scientific Reports, 2020
Thin films of β-W are the most interesting for manipulating magnetic moments using spin–orbit tor... more Thin films of β-W are the most interesting for manipulating magnetic moments using spin–orbit torques, and a clear understanding of α to β phase transition in W by doping impurity, especially oxygen, is needed. Here we present a combined experimental and theoretical study using grazing incidence X-ray diffraction, photoelectron spectroscopy, electron microscopy, and ab initio calculations to explore atomic structure, bonding, and oxygen content for understanding the formation of β-W. It is found that the W films on SiO2/Si have 13–22 at.% oxygen in A15 β structure. Ab initio calculations show higher solution energy of oxygen in β-W, and a tendency to transform locally from α to β phase with increasing oxygen concentration. X-ray absorption spectroscopy also revealed local geometry of oxygen in β-W, in agreement with the simulated one. These results offer an opportunity for a fundamental understanding of the structural transition in α-W and further development of β-W phase for device...
New Journal of Chemistry, 2020
The formation of O-vacancies along with their local environment and locus of formation for the O-... more The formation of O-vacancies along with their local environment and locus of formation for the O-vacancies has been understood using EXAFS and ab initio techniques in a doped (Zn2+) anatase lattice.
ACS Applied Materials & Interfaces, 2020
Energy storage and conversion driven by electro-or photocatalyst is a highly exciting field of re... more Energy storage and conversion driven by electro-or photocatalyst is a highly exciting field of research, and generations of effective and durable oxide catalysts have received much attention in this field. Here, we report A-site lanthanum-doped oxygen-rich quinary oxide CaLaScRuO 6+δ synthesized by adopting the solid-state reaction method and characterized by various techniques such as powder X-ray diffraction, neutron diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and temperature-programmed reduction in the presence of a hydrogen atmosphere (H 2-TPR). X-ray absorption study confirms the existence of mixed valent Ru ions in the structure, which enhances the oxygen stoichiometry for the partial balance of an extra cationic charge. Neutron powder diffraction and reduction of the material in a hydrogen atmosphere (H 2-TPR) can confirm the oxygen overstoichiometry of the catalyst. The present material works as an efficient and robust oxygen bifunctional electrocatalyst for ORR/ OER (oxygen evolution reaction/oxygen reduction reaction) followed by four-electron transfer pathway in a strong (1 M KOH) alkaline medium. The catalytic nature of the designed structural and chemical flexible perovskite is a novel example of an electrocatalyst for the oxygen bifunctional activity.
Inorganic chemistry, Jan 2, 2018
Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol-gel combustion rout... more Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol-gel combustion route, which can be used both as a blue-emitting phosphor material and a white-emitting phosphor material, depending on the annealing temperature during synthesis. The material also has the potential to be used in optical thermometry. These fascinating color-tunable emission characteristics can be linked with the various defect centers present inside the matrix and their changes upon thermal annealing. Various defect centers, such as anionic vacancy, cationic vacancy, antisite defect, etc., create different electronic states inside the band gap, which are responsible for the multicolor emission. The color components are isolated from the complex emission spectra using time-resolved emission spectroscopy (TRES) study. Interestingly, the lifetime values of the various defect centers were found to change significantly from milliseconds to microseconds upon thermal annealing, which makes the pho...
ChemistrySelect, 2017
MoS 2 nanospheres were formed using a template free hydrothermal process, which exhibit high cata... more MoS 2 nanospheres were formed using a template free hydrothermal process, which exhibit high catalytic activity towards hydrogen evolution reaction (HER). The extend of defect sites are probed by extended X-ray absorption fine structure which found decrease in coordination number at Mo site rather than at S site. DFT calculations identified an uneven strain and defect distribution between two S planes of curved MoS 2. Based on hydrogen adsorption on various sites, we identify a new pathway called "extended activity @ shielded defects", for Volmer-Tafel and Volmer-Heyrovsky mechanisms, where H adsorption occurs at exposed S layer driven by defects in underneath S layer of nanosphere. Having higher defect concentration it exhibited excellent HER activity with overpotential of À0.12 V, Tafel slope of 90 mV/decade, and higher turnover frequency. Our findings provide an avenue to design and engineer advanced nanostructures for catalysis, electronic devices, and other potential applications.
Scientific Reports, 2017
Graphene has been studied intensively in opto-electronics, and its transport properties are well ... more Graphene has been studied intensively in opto-electronics, and its transport properties are well established. However, efforts to induce intrinsic optical properties are still in progress. Herein, we report the production of micron-sized sheets by interconnecting graphene quantum dots (GQDs), which are termed ‘GQD solid sheets’, with intrinsic absorption and emission properties. Since a GQD solid sheet is an interconnected QD system, it possesses the optical properties of GQDs. Metal atoms that interconnect the GQDs in the bottom-up hydrothermal growth process, induce the semiconducting behaviour in the GQD solid sheets. X-ray absorption measurements and quantum chemical calculations provide clear evidence for the metal-mediated growth process. The as-grown graphene quantum dot solids undergo a Forster Resonance Energy Transfer (FRET) interaction with GQDs to exhibit an unconventional 36% photoluminescence (PL) quantum yield in the blue region at 440 nm. A high-magnitude photocurren...
Journal of Materials Chemistry C, 2017
Ferromagnetism originating from oxygen vacancies could be observed in Th1−xBixO2−δ (0 < x ≤ 0.... more Ferromagnetism originating from oxygen vacancies could be observed in Th1−xBixO2−δ (0 < x ≤ 0.3) solid solutions. The saturation magnetization was found to increase with Bi concentration as confirmed by magnetic and EPR investigations.
Journal of Materials Chemistry A, 2017
A highly efficient, low-cost (precious-metal-free), highly stable nanohybrid electrocatalyst cont... more A highly efficient, low-cost (precious-metal-free), highly stable nanohybrid electrocatalyst containing carbon-supported molybdenum carbide and nitride nanoparticles of size ranging from 8 to 12 nm exhibit excellent HER catalytic activity.
ACS Applied Materials & Interfaces, 2016
Oxygen reduction reaction (ORR) is increasingly being studied in oxide systems due to advantages ... more Oxygen reduction reaction (ORR) is increasingly being studied in oxide systems due to advantages ranging from cost effectiveness to desirable kinetics. Oxygen-deficient oxides like brownmillerites are known to enhance ORR activity by providing oxygen adsorption sites. In parallel, nitrogen and iron doping in carbon materials, and consequent presence of catalytically active complex species like C-Fe-N, is also suggested to be good strategies for designing ORR-active catalysts. A combination of these features in N-doped Fe containing brownmillerite can be envisaged to present synergistic effects to improve the activity. This is conceptualized in this report through enhanced activity of N-doped Ca2Fe2O5 brownmillerite when compared to its oxide parents. N doping is demonstrated by neutron diffraction, UV-vis spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Electrical conductivity is also found to be enhanced by N doping, which influences the activity. Electrochemical characterization by cyclic voltammetry, rotating disc electrode, and rotating ring disk electrode (RRDE) indicates an improved oxygen reduction activity in N-doped brownmillerite, with a 10 mV positive shift in the onset potential. RRDE measurements show that the compound exhibits 4-electron reduction pathways with lower H2O2 production in the N-doped system; also, the N-doped sample exhibited better stability. The observations will enable better design of ORR catalysts that are stable and cost-effective.
Inorganic Chemistry, 2017
Considering the fact that pyrophosphate-based hosts are in high demand for making highly efficien... more Considering the fact that pyrophosphate-based hosts are in high demand for making highly efficient luminescence materials, we doped two visible lanthanide ions, viz. Sm 3+ and Eu 3+ , in Zn 2 P 2 O 7. Interestingly, it was oberved that pure Zn 2 P 2 O 7 displayed blue-green dual emission on irradiation with ultraviolet light. Emission and lifetime spectroscopy shows the presence of defects in pyrophosphate samples which are responsible for such emission. DFT calculations clearly pinpointed that the electronic transitions between defect states located at just below the conduction band minimum (arises due to V O 1+ and V O 2+ defects) and valence band maximum, as well as impurity states situated in the band gap, can lead to dual emission in the bluegreen region, as is also indicated by emission and lifetime spectra. X-ray absorption near edge spectroscopy (XANES) shows the stabilization of europium as well as samarium ion in the +3 oxidation state in α-Zn 2 P 2 O 7. The fact that α-Zn 2 P 2 O 7 has two different coordination numbers for zinc ions, i.e. fiveand six-coordinate, the study of dopant ion distribution in this particular matrix will be an important step in realizing a highly efficient europium-and samarium-based red-emitting phosphor. Time resolved photoluminescence (TRPL) shows that both of these ions are heterogeneously distributed between fiveand sixcoordinated Zn 2+ sites and it is the six-coordinated Zn 2+ site which is the most favorable for lanthanide ion doping. Extended Xray absorption fine structure (EXAFS) measurements also suggested that a six-coordinated zinc ion is the preferred site occupied by trivalent lanthanide ions, which is in complete agreement with TRPL results. It was observed that there is almost complete transfer of photon energy from Zn 2 P 2 O 7 to Eu 3+ , whereas this transfer is inefficient and almost incomplete in case of Sm 3+ , which is indeed important information for the realization of pyrophosphate-based tunable phosphors.
Dalton Transactions, 2016
During the formation of Bi2−2xLa2xUO6 (x = 0–0.05), La+3 ions replace Bi+3 in the Bi2UO6 lattice ... more During the formation of Bi2−2xLa2xUO6 (x = 0–0.05), La+3 ions replace Bi+3 in the Bi2UO6 lattice with the creation of some oxygen vacancies.
Inorganic Chemistry, 2016
SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematical... more SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematically using X-ray diffraction and time-resolved photoluminescence techniques. A detailed analysis of the optical properties of Tb(3+) ions in SrZrO3 was performed to correlate them with the local environment of the lanthanide ions in this perovskite. Photoluminescence (PL) spectroscopy showed that emission spectrum consists of host as well as Tb(3+) emission indicating the absence of complete host-dopant energy transfer. On the basis of emission spectrum and PL decay study it was also observed that Tb(3+) is not homogeneously distributed in SrZrO3 perovskite; rather, it is occupying two different sites. It is corroborated using extended X-ray absorption fine structure studies that Tb(3+) is stabilized on both six-coordinated Zr(4+) and eight-coordinated Sr(2) site. The energies calculated using density functional theory (DFT) indicates that Tb occupation in Sr site is energetically more favorable than Zr site. The analysis of valence charge distribution also substantiated our structural stability analysis of site-selective Tb doping in SrZrO3. Time-resolved emission spectroscopy is employed to elucidate the difference in the spectral feature of Tb(3+) ion at Sr(2+) and Zr(4+) site. DFT-calculated density of states analysis showed that energy mismatch of Tb-d states with Zr-d and O-p states of SZP makes the energy transfer from host SZP to Tb(3+) ion difficult.
The Journal of Physical Chemistry C, 2015
ABSTRACT In this work, five types of MnO2 nanostructres (nano-wires, nano-tubes, nano-particles, ... more ABSTRACT In this work, five types of MnO2 nanostructres (nano-wires, nano-tubes, nano-particles, nano-rods and nano-flowers) were synthesized with a fine control over their -crystallographic form by hydrothermal method. The electrocatalytic activities of these materials were examined towards oxygen reduction reaction (ORR) in alkaline medium. Numerous characterizations were correlated with the observed activity by analyzing their crystal structure (TGA, XRD, TEM), material morphology (FE-SEM), porosity (BET), inherent structural nature (IR, Raman, ESR), surfaces (XPS) and electrochemical properties (Tafel, Koutecky-Levich plots and % of H2O2 produced). Moreover, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) analysis were employed to study the structural information on the MnO2 coordination number as well as interatomic distance. These combined results show that the electrocatalytic activities are significantly dependent on the nano-shapes, and follow an order nano-wire nano-rod nano-tube nano-particle nano-flower. -MnO2 nano-wires possess enhanced electrocatalytic activity compared to other shapes even though the nano-tubes possess a much higher BET surface area. In the ORR studies, -MnO2 nano-wires displayed Tafel slope of 65 mV/decade, n-value of 3.5 and 3.6% of hydrogen peroxide production. The superior ORR activity was attributed to the fact that it possesses active sites composed with two shortened Mn-O bonds along with a Mn-Mn distance of 2.824 Å which provides an optimum requirement for the adsorbed oxygen in a bridge mode favouring the direct 4 electron reduction. According to the first principles based density functional theory (DFT) the enhancement in ORR activity was due to the stronger interaction between the surface and adsorbates for the (211) surface than all other surfaces.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2007
Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO 4 :Dy... more Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO 4 :Dy phosphor samples at the Dy L 3 edge with synchrotron radiation. Measurements were carried out on a set of samples which were subjected to post-preparation annealing at different temperatures and for different cycles. The EXAFS data have been analysed to find the Dy-S and Dy-O bond lengths in the neighbourhood of the Dy atoms in a CaSO 4 matrix. The observations from EXAFS measurements were verified with XANES and XPS techniques. On the basis of these measurements, efforts were made to explain the loss of thermoluminescence sensitivity of CaSO 4 :Dy phosphors after repeated cycles of annealing at 400°C in air for 1 h.
Journal of Synchrotron Radiation, 2016
An innovative scheme to carry out continuous-scan X-ray absorption spectroscopy (XAS) measurement... more An innovative scheme to carry out continuous-scan X-ray absorption spectroscopy (XAS) measurements similar to quick-EXAFS mode at the Energy-Scanning EXAFS beamline BL-09 at INDUS-2 synchrotron source (Indore, India), which is generally operated in step-by-step scanning mode, is presented. The continuous XAS mode has been implemented by adopting a continuous-scan scheme of the double-crystal monochromator and on-the-fly measurement of incident and transmitted intensities. This enabled a high signal-to-noise ratio to be maintained and the acquisition time was reduced to a few seconds from tens of minutes or hours. The quality of the spectra (signal-to-noise level, resolution and energy calibration) was checked by measuring and analysing XAS spectra of standard metal foils. To demonstrate the energy range covered in a single scan, a continuous-mode XAS spectrum of copper nickel alloy covering both Cu and NiK-edges was recorded. The implementation of continuous-scan XAS mode at BL-09 w...
International Journal of Engineering Research, 2014
A new X-ray absorption fine structure (EXAFS) data acquisition and data analysis program is devel... more A new X-ray absorption fine structure (EXAFS) data acquisition and data analysis program is developed using LabVIEW. It runs under window operating system and offers several unique features. It has a user friendly graphical user environment and is capable of saving data in variety of formats. The availability of online data presentation is particularly useful for data examination during the EXAFS experiments as well as analysing quality of the sample. Transmission and Fluorescence mode are implemented to characterize variety of samples.