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"Govindaraj, A."
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Importance of trivalency and the e(g)(1) configuration in the photocatalytic oxidation of water by Mn and Co oxides
Prompted by the early results on the catalytic activity of LiMn2O4 and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn2O3, LaMnO3, and MgMn2O4 are found to be very good catalysts with turnover frequencies of 5 × 10(-4) s(-1), 4.8 × 10(-4) s(-1), and 0.8 × 10(-4) s(-1), respectively. Among the cobalt oxides, Li2Co2O4 and LaCoO3--especially the latter--exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10(-4) s(-1) and 1.4 × 10(-3) s(-1), respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co(3+) in the Co oxides is in the intermediate t2g(5)e(g)(1) state whereas Mn(3+) is in the t2g(3e(g)(1) state. The presence of the e(g)(1) electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.
Journal Article
Chemical storage of hydrogen in few-layer graphene
Birch reduction of few-layer graphene samples gives rise to hydrogenated samples containing up to 5 wt % of hydrogen. Spectroscopic studies reveal the presence of sp³ C-H bonds in the hydrogenated graphenes. They, however, decompose readily on heating to 500 °C or on irradiation with UV or laser radiation releasing all the hydrogen, thereby demonstrating the possible use of few-layer graphene for chemical storage of hydrogen. First-principles calculations throw light on the mechanism of dehydrogenation that appears to involve a significant reconstruction and relaxation of the lattice.
Journal Article
Sensors for the nitrogen oxides, NO2, NO and N2O, based on In2O3 and WO3 nanowires
Sensing characteristics of ZnO, In2O3 and WO3 nanowires have been investigated for the three nitrogen oxides, NO2, NO and N2O. In2O3 nanowires of ∼20 nm diameter prepared by using porous alumina membranes are found to have a sensitivity (defined as the ratio of the sensor resistance in the gas concerned to that in air) of about 60 for 10 ppm of all the three gases at a relatively low temperature of 150 °C. The response and recovery times are around 20 s. The sensitivity of these In2O3 nanowires is around 40 for 0.1 ppm of NO2 and N2O at 150 °C. WO3 nanowires of 5–15 nm diameter, prepared by the solvothermal process show a sensitivity of 20–25 for 10 ppm of the three nitrogen oxides at 250 °C. The response and recovery times are 10 s and 60 s, respectively. The sensitivity is around 10 for 0.1 ppm of NO2 at 250 °C. The sensitivity of In2O3 and WO3 nanowires is not affected by humidity even up to 90% relative humidity. The study also reveals that the sensing mechanism for the three nitrogen oxides have a commonality in that the desorption of oxygen is a crucial step in all the cases.
Journal Article
Unipolar assembly of zinc oxide rods manifesting polarity-driven collective luminescence
Oriented assemblies of small crystals forming larger structures are common in nature and crucial for forthcoming technologies as they circumvent the difficulties of structural manipulation at microscopic scale. We have discovered two distinctive concentric assemblies of zinc oxide rods, wherein each rod has an intrinsically positive and a negative polar end induced by the noncentrosymmetric arrangement of Zn and O atoms. All the rods in a single assembly emanate out of a central core maintaining a single polar direction. Due to growth along the two polar surfaces with different atomic arrangements, these assemblies are distinct in their intrinsic properties and exhibit strong UV luminescence in the exterior of Zn-polar assemblies, unlike the O-polar assemblies. Although novel applications can be envisioned, these observations suggest that hierarchical organization with respect to internal asymmetry might be widespread in natural crystal assemblies.
Journal Article
Nanotubes and nanowires
This book provides a comprehensive and up-to-date survey of the research areas of carbon nanotubes, inorganic nanotubes and nanowires including: Synthesis; Characterisation; Properties; Applications Nanotubes and Nanowires includes an extensive list of references and is ideal both for graduates needing an introduction to the field of nanomaterials as well as for professionals and researchers in academia and industry.
eBook
Importance of trivalency and the eg¹ configuration in the photocatalytic oxidation of water by Mn and Co oxides
Prompted by the early results on the catalytic activity of LiMn ₂O ₄ and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn ₂O ₃, LaMnO ₃, and MgMn ₂O ₄ are found to be very good catalysts with turnover frequencies of 5 × 10 ⁻⁴ s ⁻¹, 4.8 × 10 ⁻⁴ s ⁻¹, and 0.8 ×10 ⁻⁴ s ⁻¹, respectively. Among the cobalt oxides, Li ₂Co ₂O ₄ and LaCoO ₃—especially the latter—exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10 ⁻⁴ s ⁻¹ and 1.4 × 10 ⁻³ s ⁻¹, respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co ³⁺ in the Co oxides is in the intermediate t ₂g⁵e g¹ state whereas Mn ³⁺ is in the t ₂g³e g¹ state. The presence of the e g¹ electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.
Journal Article
Synthesis, Characterization and Properties of Single-Walled Carbon Nanohorns
Single-walled nanohorns (SWNHs) have been prepared by sub-merged arc discharge of graphite electrodes in liquid nitrogen. The samples were examined by scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. Nitrogen and boron doped SWNHs have been prepared by the sub-merged arc discharge method using melamine and elemental boron as precursors. Intensification of Raman D-band and stiffening of G-band has been observed in the doped samples. The electrical resistance of the SWNHs varies in opposite directions with nitrogen and boron doping. Functionalization of SWNHs through amidation has been carried out for solubilizing them in non-polar solvents. Water-soluble SWNHs have been produced by acid treatment and non-covalent functionalization with a coronene salt. SWNHs have been decorated with nanoparticles of Au, Ag and Pt. Interaction of electron donor (tetrathiafulvalene, TTF) and acceptor molecules (tetracyanoethylene, TCNE) with SWNHs has been investigated by Raman spectroscopy. Progressive softening and stiffening of Raman G-band has been observed respectively with increase in the concentration of TTF and TCNE.
Journal Article
Oxide nanotubes prepared using carbon nanotubes as templates
Hollow nanotubes of SiO2, Al2O3, V2O5, and MoO3 have been prepared using carbon nanotubes as templates. The procedure involves coating the carbon nanotubes with tetraethylorthosilicate, aluminum isopropoxide, or vanadium pentoxide gel, followed by calcination and heating at higher temperatures in air to oxidize the carbon. SiO2 nanotubes containing transition metal ions have been prepared by this procedure since such materials may be of use in catalysis.
Journal Article
Graphene composites containing chemically bonded metal oxides
Composites of graphene involving chemically bonded nano films of metal oxides have been prepared by reacting graphene containing surface oxygen functionalities with metal halide vapours followed by exposure to water vapour. The composites have been characterized by electron microscopy, atomic force microscopy and other techniques. Magnetite particles chemically bonded to graphene dispersible in various solvents have been prepared and they exhibit fairly high magnetization.
Journal Article