S.R. Bharadwaj

Improved photocatalytic activity of indium doped cadmium sulfide dispersed on zirconia

A novel composite photocatalyst of indium doped cadmium sulfide dispersed on zirconium oxide has been synthesized, which shows enhanced photocatalytic activity for hydrogen generation from water. In this system, cadmium sulfide exists as a separate dispersed phase on the zirconia support. Optical absorption spectra indicate a blue shift of absorption edge for CdS and In doped CdS dispersed on ZrO2 compared to pure CdS and indium doped CdS. Among the supported CdS, In doped CdS exhibits better optical absorption property. Photocatalytic studies for hydrogen generation from water show an enhanced activity for CdS dispersed on ZrO2 and indium doping in CdS enhances the activity further. Fluorescence lifetime studies indicate that, in the supported CdS, the charge carriers have higher lifetime than that in the unsupported CdS. Photocurrent response experiments show a relatively higher current output for the In doped CdS dispersed on ZrO2 support. The enhanced photocatalytic activity of this composite sample is attributed to a combination of factors like enhanced lifetime of the photogenerated charge carriers, increased photoresponse and improved surface area. The present study leads to a new observation that the photocurrent response and photocatalytic activity of CdS and indium doped CdS are enhanced when they are dispersed on a support like ZrO2. These composites with Pd as co-catalyst exhibit a large increase in the photocatalytic activity due to the increased availability of electrons on the metal surface by the interfacial transfer of electrons from CdS to Pd, when irradiated.

Thermodynamic stability of barium thorate, BaThO3, from a Knudsen effusion study

Abstract The Gibbs energy of formation of barium thorate was determined using the Knudsen effusion forward collection technique. The evaporation process could be represented by the equation BaThO 3 (s)=ThO 2 (s)+BaO(g) The vapour pressure of BaO(g) over the two-phase mixture of BaThO3(s) and ThO2(s) was obtained from the rate of effusion of BaO(g) and could be represented as ln (p/ Pa ) (±0.39)=−50526.5/T/ K +26.95 (1770≤T/ K ≤2136) The Gibbs energy of formation of BaThO3(s) could be derived from this data and represented as Δ f G°( BaThO 3 (s) )/ kJ mol −1 ±8.0=−1801.75+0.276T/ K

Enhanced photocatalytic hydrogen generation over ZrO2–TiO2–CdS hybrid structure

Hybrid photocatalysts with suitable band structures are expected to show enhanced photocatalytic activity as compared to their constituent single phase compounds due to their improved physico-chemical properties. Here, we report an enhanced photocatalytic activity of a new composite photocatalyst comprising of ZrO2, TiO2, and CdS. This hybrid catalyst exhibits increased photocatalytic activity for hydrogen generation from water as compared to their constituent compounds. The photocatalytic activity decreases in the order: ZrO2-TiO2-CdS>TiO2-CdS>ZrO2-CdS>CdS>ZrO2-TiO2≈TiO2>ZrO2. An apparent quantum efficiency of 11.5% is obtained for ZrO2-TiO2-CdS with Pd as co-catalyst. Absorption edge of the composite is slightly blue shifted compared with that of pure CdS. Photoluminescence lifetime studies indicate an increased lifetime for the charge carriers in the composite sample as compared to that of pure CdS. Transmission electron microscopy images reveal that the particle size of the composite is much less than that of single phase CdS. The enhanced photocatalytic activity of the composite is attributed to the decreased particle size of CdS and increased lifetime of the charge carriers resulting from the efficient interfacial transfer of photogenerated electrons at the CdS/TiO2 and CdS/ZrO2 interface.

X-ray absorption spectroscopy of doped ZrO2 systems

ZrO2 samples with 11% Nd and La doping and with 7, 9, 11, and 13% Gd doping have been prepared by co-precipitation route followed by sintering at 700 °C and 1100 °C, for potential application as high conductivity electrolytes in solid oxide fuel cells. The samples have been characterized by x-ray diffraction with laboratory x-ray source of Cu Kα radiation and extended x-ray absorption fine structure (EXAFS) spectroscopy measurement at Zr K edge with synchrotron radiation. The XRD spectra have been analyzed to determine the structure of the samples and the EXAFS data have been analyzed to find out relevant local structure parameters of the Zr-O and Zr-Zr shells, viz., bond distances, co-ordinations, and disorder parameters. The effect of change in ionic radius as well as concentration of the dopants on the above parameters has been thoroughly studied. The experimental results, in some cases, have also been corroborated by first principle calculations of the energetics of the systems.

Thermodynamic stability of solid SrThO3

Abstract The Gibbs energy of formation of strontium thorate was determined by the Knudsen effusion forward collection technique. The evaporation process from a mixture of tungsten and strontium thorate in the Knudsen cell could be represented by the following heterogeneous equilibria: 5SrThO3(s)+W(s)=Sr2WO5(s)+5ThO2(s)+3Sr(g), 1670 Δ f G 0 ( SrThO 3 ( s )) (±5.0 kJ mol −1 )=−1953.6+0.367·T , (1670 Δ f G 0 ( SrThO 3 ( s )) (±7.0 kJ mol −1 )=−1960.2+0.369·T , (2135

The standard free energy of formation of ZrMo2O8 by the transpiration method

The standard free energy of formation for zirconium molybdate (ZrMo2O8) was derived from its vapour pressure measured in the temperature range 1029 to 1142 K employing the transpiration technique. ZrMo2O8 vaporizes incongruently according to the reaction n〈ZrMo2O8〉 → n〈ZrO2〉 + 2(MoO3)n, n = 3,4,5. The standard free energy of formation of ZrMo2O8 calculated from the partial pressure of (MoO3)3 in the vapour above ZrMo2O8 + ZrO2 mixture could be expressed by the relation ΔfG°〈ZrMo2O8〉 =(−2525.5±4.9) +(0.6115±0.0045)Tkj/mol (1029) < T/K < 1142). The values of ΔfG°〈ZrMo2O8〉 obtained in this work agree very well with those reported recently from solid electrolyte galvanic cell measurements.

Mechanistic insight by in situ FTIR for the gas phase photo-oxidation of ethylene by V-doped titania and nano titania.

Vanadium-doped titania is found to be a better photocatalyst for gas phase photo-oxidation of ethylene than nano titania. In situ FTIR studies were undertaken to elucidate the mechanistic pathway for ethylene oxidation on these two catalyst surfaces. Vanadium doping leads to formation of more chemisorbed hydroxyl species, which makes it a better photocatalyst. The labile hydroxyls which were responsible for the reduction of V(5+) to V(4+) during the process of calcination were also ascertained. The ethylene decomposition occurs via formation of ethoxy groups, transformed to acetaldehyde or enolates, subsequently to acetates/formates, and then to CO(2). The enolates were more stabilized on the TiO(2) surface, leading to formation of formates along with the acetates. On vanadium-doped TiO(2), acetaldehyde was more stabilized than its enol tautomer, leading to the formation of labile acetic acid and acetates. The formation of the labile acetic acid, adsorbed acetates, and the adsorbed acetate -M salts led to easier oxidation of them to provide higher yield of CO(2). The higher positive charge density over Ti in Ti(0.95)V(0.05)O(2) with respect to nano TiO(2) makes the acetate (stronger nucleophile) a more stable intermediate on it.

Photocatalytic H2 generation over In2TiO5, Ni substituted In2TiO5 and NiTiO3 – a combined theoretical and experimental study

We report here the role of Ni substitution in modifying the crystal structure, optical absorption properties and electronic properties of indium titanate, In2(1−x)Ni2xTiO5−δ (0.0 ≤ 2x ≤ 0.4) and its consequent effect on the photocatalytic properties for H2 generation. Rietveld refinement of observed XRD patterns of the titanates revealed that Ni2+ substitution has led to a decrease in lattice cell parameters and cell volume, contraction of InO6 octahedra and consequently improved charge carrier properties. Furthermore, the conduction band maximum (CBM) was found to be a hybrid state between Ni, Ti and In orbitals in 10% Ni-doped sample, which suggests that the photo-induced charges can be better transported in the substituted samples from zigzag chains of [·Ni–O–Ti⋯In–O–Ti----]. The UV-visible diffuse reflectance spectra exhibited that the band gap of the indium titanate phase decreased sequentially with an increase in the extent of Ni substitution. The underlying cause for band gap narrowing on Ni substitution was evaluated from plane wave based DFT calculations using the GGA + U approach. The decreasing order of photocatalytic activity (as a percentage of Ni substitution) for hydrogen generation from water–methanol mixture is as follows: 10% > 5% > indium titanate > 15% > 20%. The fall in activity below indium titanate coincided with the appearance of ilmenite NiTiO3 phase. Plane wave based DFT calculations performed on NiTiO3 revealed that strong intermixing of Ni-3d with O-2p orbitals occurred in the valence band of NiTiO3 and resulted in generation of a pseudo band gap of 0.3 eV at 1.4 eV below the Fermi level. This pseudo band gap might act as a hindrance and may contribute to weakening the intensity of the electronic transition due to Ni2+ → Ti4+ charge transfer. We propose here that an optimal concentration of 10% Ni substitution in indium titanate modifies the structural and electronic properties favorably leading to better photocatalytic activity by reducing the band gap, enhancing of the electron–hole separation and improving charge carrier properties.

Extended X-ray absorption fine structure study of Gd doped ZrO2 systems

ZrO2 samples with 7%, 9%, 11%, and 13% Gd doping have been prepared by co-precipitation route followed by high temperature sintering for potential application as high conductivity electrolytes in solid oxide fuel cells. The samples have been characterised by extended x-ray absorption fine structure (EXAFS) spectroscopy measurement around Gd L3 edge with Synchrotron radiation. The EXAFS spectra have been analysed to find out relevant local structure parameters of the Gd-O and Gd-Gd(Zr) shells, viz., bond distances, co-ordination numbers, and disorder parameters. The results were explained in the light of our earlier propositions from EXAFS measurements on the samples at Zr edge that Gd doping creates oxygen vacancies near the Zr sites and doping ∼9% is optimum for generation of oxygen vacancies in the ZrO2 matrix. The experimental results have also been corroborated by ab-initio first principle calculations of the energetics of the systems.

Determination of the Gibb's energy of formation of CaTeO3 and CaTe2O5 by the transpiration technique

Abstract The vapour pressure of two compounds, 〈CaTeO 3 〉 and 〈CaTe 2 O 5 〉, in the pseudo-binary CaOTeO 2 system was measured employing the microthermogravimetric transpiration assembly built in our laboratory. Both compounds vaporized incongruently, giving TeO 2 vapour according to the reactions 〈CaTeO 3 〉 = 〈CaO〉 + (TeO 2 ) and 〈CaTe 2 O 5 〉 = 〈CaTeO 3 〉 + (TeO 2 ) respectively. The vapour pressure of (TeO 2 ) above the mixtures of 〈CaTeO 3 + CaO〉 was measured in the temperature range 1169 to 1247 K. The Gibbs energy of formation of CaTeO 3 derived from the vapour pressure data could be expressed as a function of temperature by the equation Δ f G° 〈 CaTeO 3 〉 (±12.83 kJ mol −1 ) = − 975.08 + 0.242T (1169 T K CaTe 2 O 5 exhibited the reversible crystallographic phase transition at 1077±1 K as recorded by DTA. The Gibbs energy of formation of CaTe 2 O 5 derived from its vapour pressure measured below and above this phase transition could be expressed as a function of temperature in terms of the following equations: Δ f G° 〈 CaTe 2 O 5 〉 (±13.11 kJ mol −1 ) = − 1357.3 + 0.464T (1007 T K Δ f G° 〈 CaTe 2 O 5 〉 (±13.26 kJ mol −1 ) = − 1313.9 + 0.423T (1082 T K The phase transition temperature T tr and the enthalpy of transition ( ΔH ° T tr ) deduced from these equations were found to be 1048±30 K and 43.4±10 kJ mol −1 respectively.