S. N. Jha

Growth of block copolymer stabilized metal nanoparticles probed simultaneously by in situ XAS and UV–Vis spectroscopy

The growth of Au and Pt nanoparticles from their respective chloride precursors using block copolymer-based reducers has been studied by simultaneous in situ measurement of XAS and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at INDUS-2 SRS at RRCAT, Indore, India. While the XANES spectra of the precursor give real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed at the intermediate stages of growth. The growth kinetics of both types of nanoparticles are found to be almost similar and are found to follow three stages, though the first stage of nucleation takes place earlier in the case of Au than in the case of Pt nanoparticles due to the difference in the reduction potential of the respective precursors. The first two stages of the growth of Au and Pt nanoparticles as obtained by in situ XAS measurements could be corroborated by simultaneous in situ measurement of UV-Vis spectroscopy also.

Chemical shift of Mn and Cr K-edges in X-ray absorption spectroscopy with synchrotron radiation

Mn and Cr K X-ray absorption edges were measured in various compounds containing Mn in Mn2u2009+u2009, Mn3u2009+u2009 and Mn4u2009+u2009 oxidation states and Cr in Cr3u2009+u2009 and Cr6u2009+u2009 oxidation states. Few compounds possess tetrahedral coordination in the 1st shell surrounding the cation while others possess octahedral coordination. Measurements have been carried out at the energy dispersive EXAFS beamline at INDUS-2 Synchrotron Radiation Source at Raja Ramanna Centre for Advanced Technology, Indore. Energy shifts of ~ 8–16xa0eV were observed for Mn K edge in the Mn-compounds while a shift of 13–20xa0eV was observed for Cr K edge in Cr-compounds compared to values in elemental Mn and Cr, respectively. The different chemical shifts observed for compounds having the same oxidation state of the cation but different anions or ligands show the effect of different chemical environments surrounding the cations in determining their X-ray absorption edges in the above compounds. The above chemical effect has been quantitatively described by determining the effective charges on Mn and Cr cations in the above compounds.

Investigation of Fe doped ZnO thin films by X-ray absorption spectroscopy

Fe doped ZnO thin films with varying Fe doping concentrations have been deposited by rf magnetron sputtering technique on c-Si substrates. X-ray Diffraction (XRD) measurements show the pure wurtzite structure of all the samples. X-ray near edge spectroscopy (XANES) measurements confirm that Fe is present in the samples in Fe2+ and Fe3+ oxidation states. Extended X-ray Absorption Fine Structure (EXAFS) measurements at Zn and Fe K edges prove that Zn atoms are being substituted by Fe dopants, Zn vacancies are created in the samples near the dopant sites and also support the presence of mixed oxidation states obtained by XANES measurements. Oxygen K-edge XANES results support the conjecture that Fe goes to Zn sites in tetrahedral symmetry. Magnetization measurements show the presence of room temperature ferromagnetism (RTFM) in the samples and saturation magnetization is found to increase as Fe doping concentration increases. The observed ferromagnetism seems to be intrinsic in nature and it is explained in the light of XANES and EXAFS observations.

Nature of WO4 tetrahedra in blue light emitting CaWO4 probed through the EXAFS technique

Blue emission due to self trapped exciton recombination in CaWO4 is believed to be very sensitive to the nature of the WO4 structural units. The present manuscript deals with the probing of structural differences existing in WO4 tetrahedra of CaWO4 particles having varying average crystallite sizes and different blue light emission characteristics. Based on XRD, W L1 edge XANES and W L3 edge EXAFS studies, it is inferred that factors like average W–O and Ca–O bond lengths, average number of oxygen atoms around W6+ ions and disorder in WO4 tetrahedra do not have any effect on the blue luminescence intensity from the samples. The lifetime value of excitons is lower for the nanocrystals/nanoparticles of CaWO4 compared to the bulk sample. The lower lifetime of self trapped excitons and the associated decrease in blue luminescence intensity for nanoparticles/nanocrystals (compared to bulk) has been explained based on competing non-radiative processes involving the interaction of holes with surface hydroxyl groups and an associated decrease in the extent of radiative exciton recombination.

Photocatalytic H2 evolution from water-methanol system by anisotropic InFeO3(ZnO)(m) oxides without cocatalyst in visible light.

InFeO3(ZnO)m series of oxides are found to give unprecedented H2 evolution from water-methanol mixtures without using any cocatalysts. This family of compounds has an anisotropically layered structure in which Zn/FeOn polyhedra are sandwiched between InO6 octahedral layers. Local structure characterization by X-ray absorption spectroscopy reveals that Zn coordination changes from pentacoordinated to tetrahedral geometry across the series, whereas Fe geometry remains trigonal bipyramidal in all the compounds. This peculiar structure is conducive for a spatial separation of photogenerated charges reducing recombination losses. Band gap energies calculated from absorption spectra indicate potential visible light activity, and this may be due to the orbital mixing of Fe 3d and O 2p as revealed by pre-edge features of X-ray absorption spectra. Band positions are also advantageously placed for a visible light H2 generation and is indeed found to be the case in methanol-assisted water splitting with standardized hydrogen evolution of ∼19.5 mmol g(-1) h(-1) for all the catalysts.

Chemical shift of U L3 edges in different uranium compounds obtained by X-ray absorption spectroscopy with synchrotron radiation

Uranium L3 X-ray absorption edge was measured in various compounds containing uranium in U4+, U5+ and U6+ oxidation states. The measurements have been carried out at the Energy Dispersive EXAFS beamline (BL-08) at INDUS-2 synchrotron radiation source at RRCAT, Indore. Energy shifts of ~ 2–3 eV were observed for U L3 edge in the U-compounds compared to their value in elemental U. The different chemical shifts observed for the compounds having the same oxidation state of the cation but different anions or ligands show the effect of different chemical environments surrounding the cations in determining their X-ray absorption edges in the above compounds. The above chemical effect has been quantitatively described by determining the effective charges on U cation in the above compounds.

Correlation of Mo dopant and photocatalytic properties of Mo incorporated TiO2: an EXAFS and photocatalytic study

The present study addresses the quantitative estimation and understanding of the nature of phases present in Mo-incorporated titania through extensive EXAFS measurements (at Mo K-edge and Ti K-edge) and to decipher their role in photodegradation of Methylene Blue (MB) dye under UV and visible irradiation. EXAFS results revealed the presence of both MoO3 nano heterophase phase and substitutional Mo dopants in the TiO2 lattice, the extent of the latter was reduced with the increasing Mo content of the sample. The presence of MoO3 in the Mo-incorporated titania was also revealed by FT-IR and TEM studies. Photocatalytic studies have shown considerable adsorption of the cationic MB-dye, perhaps due to the electronic interaction between the dye and catalytic surface. Under visible irradiation, the photocatalytic activity followed the trend: Mo-5 > Mo-2 > Mo-1 > Mo-10 ≫ TiO2, while the trend for photodegradation of MB dye under UV irradiation was as follows: Mo-5 > Mo-2 > Mo-1 > TiO2 > Mo-10. These results have been explained in the light of the structural properties of the Mo–TiO2 system obtained from EXAFS measurements. It has been observed that the relative ratio of substitutional Mo-dopant to the MoO3 phase in this tri-phasic photocatalyst plays a crucial role in augmenting its oxidative photocatalytic property.

Investigating structural aspects to understand the putative/claimed non-toxicity of the Hg-based Ayurvedic drug Rasasindura using XAFS.

XANES- and EXAFS-based analysis of the Ayurvedic Hg-based nano-drug Rasasindura has been performed to seek evidence of its non-toxicity. Rasasindura is determined to be composed of single-phase α-HgS nanoparticles (size ∼24u2005nm), free of Hg(0) or organic molecules; its structure is determined to be robust (<3% defects). The non-existence of Hg(0) implies the absence of Hg-based toxicity and establishes that chemical form, rather than content of heavy metals, is the correct parameter for evaluating the toxicity in these drugs. The stable α-HgS form (strong Hg-S covalent bond and robust particle character) ensures the integrity of the drug during delivery and prevention of its reduction to Hg(0) within the human body. Further, these comparative studies establish that structural parameters (size dispersion, coordination configuration) are better controlled in Rasasindura. This places the Ayurvedic synthesis method on par with contemporary techniques of nanoparticle synthesis.

Role of compensating Li/Fe incorporation in Cu0.945Fe0.055−xLixO: structural, vibrational and magnetic properties

Doped transition metal oxides, like CuO, are spintronic materials. An increase of magnetic moment has been reported in Fe-doped CuO. Additional secondary doping elements such as Li may further modify magnetism in transition metal oxides due to changes in valence state and size. Such complex doping may generate secondary impurity phases, restricting the success of such applications. Enhancement of magnetic properties strengthens applicability of the materials in devices. Single phase monoclinic Cu0.945Fe0.055−xLixO crystalline powders without any impurity are synthesized. Structural studies, electronic valence state, fine structure, and local geometry of constituent elements confirm the purity of the materials and provide clues to probable reasons leading to enhanced magnetism. Ferromagnetic coupling between neighboring spins is most likely dependent on the spin and separation between the spin species.

EXAFS studies on Gd-doped ZrO 2 thin films deposited by RF magnetron sputtering

ZrO2 thin films with 0%, 7%, 9%, 11%, and 13% Gd doping have been prepared by RF magnetron sputtering and have been characterized by grazing incidence x-ray diffraction, spectroscopic ellipsometry, and optical transmission measurements to probe their structural and optical properties. Extended x-ray absorption fine structure (EXAFS) measurements have also been carried out on the samples at the Zr K- and Gd L3-edges. It has been observed that Gd goes to Zr sites up to 9%-11% doping concentration, and for Gd doping concentrations beyond 11%, Gd precipitates out as a separate Gd2O3 phase. The local structure information surrounding the Zr and Gd sites obtained from the analysis of the EXAFS studies have also been used to explain the macroscopic optical properties of the samples.