@article { ISI:000304888700036, title = {Visible-light-induced photosplitting of water over gamma {\textquoteleft}-Fe4N and gamma {\textquoteleft}-Fe4N/alpha-Fe2O3 nanocatalysts}, journal = {Journal of Physical Chemistry C}, volume = {116}, number = {22}, year = {2012}, month = {JUN}, pages = {12156-12164}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

The application of gamma{\textquoteright}-Fe4N, a noble-metal-free, low-cost catalyst, in the photosplitting of neat water into stoichiometric amounts of H-2 and O-2 under visible-light irradiation is reported for the first time. The catalyst showed optical absorption and photoluminescence emission bands in the entire visible region. The photocatalytic water-splitting activity was wavelength-dependent, the quantum efficiency for H-2 evolution being ca. 1.7 and 0.7\% at excitation wavelengths of 450 and 500 nm, respectively. Addition of electron donor/acceptor sacrificial reagents considerably affected the yield and stoichiometry of H-2 and O-2. At the same time, the product yield increased in a composition-dependent manner for (gamma{\textquoteright}-Fe4N)(x) + (alpha-Fe2O3)(1-x) nanocomposites. This activity augmentation is ascribed to the better dispersion of the active component gamma{\textquoteright}-Fe4N and also to the availability of more active surface sites at Fe4N/Fe2O3 contacts. Moreover, the proximity of the valence band potential of the component photosystems promotes the preferential transfer/entrapment of photoexcited hole carriers. We envisage that the defect/impurity-induced interband-gap energy states may play a vital role in these charge-transfer processes, leading thereby to more effective e(-)-h(+) separation and the enhanced rate of the water-splitting reaction. First-principles electronic structure analysis suggests that the extraordinary photocatalytic and optical properties of intermetallic gamma{\textquoteright}-Fe4N may arise from the particle-size-dependent changes in electronic structure.

}, issn = {1932-7447}, doi = {10.1021/jp303255f}, author = {Dhanasekaran, P. and Salunke, Hemant G. and Gupta, Narendra M.} } @article { ISI:000323842900082, title = {Preparation-method-dependent morphological, band structural, microstructural, and photocatalytic properties of noble metal-GaNbO4 nanocomposites}, journal = {RSC Advances}, volume = {3}, number = {37}, year = {2013}, month = {JUL}, pages = {16817-16828}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {

We report the distinct physicochemical and photophysical properties of gallium niobate photocatalysts (bandgap: similar to 3.1 eV), prepared by a solid-state (SS) reaction and sol-gel (SG) method and dispersed with a noble metal (similar to 0.5\% of Pt, Au, or RuOx) cocatalyst. SG-GaNbO4 comprised smaller size particles (similar to 20-50 nm) and a larger surface area (similar to 160 m(2) g(-1)) compared to SS-GaNbO4 (particle size similar to 30-150 nm, surface area similar to 27 m(2) g(-1)). XRD patterns revealed a preparation-dependent variation in the relative intensity of prominent reflections. In TEM examination, SG samples exhibited small-range grain boundaries and heterogeneous metal/substrate interfacial contacts, while SS-GaNbO4 had long-range ordering. Laser-Raman and thermoluminescence investigations revealed that lattice distortion, defect-induced inter-bandgap charge trapping states, and the local environment around the metal/semiconductor interfaces may also depend on the preparation method. Metal-GaNbO4 nanocomposites showed no activity for the dissociation of pure water under UV (\>250 nm) irradiation, despite the favourable conduction and valence band potentials. This was attributed to the sharp Ga and Nb d-levels in the narrow conduction band of GaNbO4, as confirmed by ab initio electronic structure calculation. These photocatalysts, however, showed good activity for semiconductor-mediated photo-dissociation of aqueous methanol to produce H-2; a cocatalyst-dependent activity trend, Pt \> RuOx \> Au, was observed. Doping of S at similar to 5\% of the oxygen sites led to decreased photoactivity, ascribed to the presence of localized S 3p states just above the O 2p valence level. In conclusion, besides band characteristics, certain morphological and microstructural properties play a crucial role in the photoactivity of the metal/oxide nanocomposites.

}, issn = {2046-2069}, doi = {10.1039/c3ra43477j}, author = {Devi, Suman and Kelkar, Sarika A. and Kashid, Vikas and Salunke, Hemant G. and Gupta, Narendra M.} } @article { ISI:000341287200041, title = {Photophysical, bandstructural, and textural properties of o-FeNbO4 in relation to its cocatalyst-assisted photoactivity for water oxidation}, journal = {RSC Advances}, volume = {4}, number = {63}, year = {2014}, month = {JUL}, pages = {33435-33445}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {

In this study, a relationship between physicochemical, photophysical and photocatalytic properties of hydrothermally synthesized orthorhombic iron niobate (FeNbO4) is investigated. o-FeNbO4 displayed a multi-regime optical absorbance, which was ascribed to at least two distinct phenomena: (i) bandgap (similar to 3.4 eV) excitation giving rise to UV absorbance and (ii) energy transitions involving disorder-induced sub-bandgap donor or acceptor states leading to visible light absorbance. The preparation-dependent distortion in the crystal lattice and the existence of closely spaced inter-bandgap energy states were corroborated by powder X-ray diffraction, photoluminescence, thermoluminescence, and Raman spectroscopy studies. The first principles electronic structure elucidation and photoelectrochemical measurements supported a wide bandgap for FeNbO4, in contrast to the narrow bandgap reported previously. Correspondingly, a small photocurrent density was observed for FeNbO4 (similar to 2 to 3 mu A cm(-2)) under 1 sun illumination, suggesting the availability of a smaller cross section of photogenerated charge pairs. Following these band characteristics, while no H-2 evolution was observed, FeNbO4 gave rise to particle size-dependent O-2 evolution during visible light irradiation of water in the presence of electron scavengers, the samples loaded with NiO as cocatalyst showing better activity. Further, the transmission electron microscopy examination revealed the dominant exposure of (011) facets of FeNbO4, besides a significant heterogeneity of inter-domain boundaries. Overall, our results confirm that the photoactivity of metal/oxide nanocomposites is governed by a combination of factors, such as: grain morphology, microstructure, surface adsorption states, and the localized inter-bandgap energy states. Our study also reveals that, in contrast to prevalent assumptions, the wavelength at the absorption edge may not represent the true band-to-band energy gap of metal oxide semiconductors, which is relevant to their photocatalytic activity.

}, issn = {2046-2069}, doi = {10.1039/c4ra06059h}, author = {Babu, Robin and Kelkar, Sarika A. and Kashid, Vikas and Achary, Srungarpu N. and Salunke, Hemant G. and Gupta, Narendra M.} }