K.Y. Rajpure

Physical properties of hematite -Fe2O3 thin films: application to photoelectrochem- ical solar cells

The physical properties and photoelectrochemical characterization of aluminium doped hematite α-Fe2O3, synthesized by spray pyrolysis, have been investigated in regard to solar energy conversion. Stable Al-doped iron (III) oxide thin films synthesized by a spray pyrolysis technique reveals an oxygen deficiency, and the oxide exhibits n-type conductivity confirmed by anodic photocurrent generation. The preparative parameters have been optimized to obtain good quality thin films which are uniform and well adherent to the substrate. The deposited iron oxide thin films show the single hematite phase with polycrystalline rhombohedral crystal structure with crystallite size 20–40 nm. Optical analysis enabled to point out the increase in direct band-gap energy from 2.2 to 2.25 eV with doping concentration which is attributed to a blue shift. The dielectric constant and dielectric loss are studied as a function of frequency. To understand the conduction mechanism in the films, AC conductivity is measured. The conduction occurs by small polaron hopping through mixed valences Fe2+/3+ with an electron mobility 300 K of 1.08 cm2/(Vs). The α-Fe2O3 exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as a photoanode for various electrolytes using white light to obtain I — V characteristics. The Al-doped hematite exhibited a higher photocurrent response when compared with undoped films achieving a power conversion efficiency of 2.37% at 10 at % Al:Fe2O3 thin films along with fill factor 0.38 in NaOH electrolyte. The flat band potential Vfb (−0.87 VSCE) is determined by extrapolating the linear part to C−2 = 0 and the slope of the Mott-Schottky plot.

Zinc oxide mediated heterogeneous photocatalytic degradation of organic species under solar radiation.

The photocatalytic decomposition of eco-persistent toluene, salicylic acid and 4-chlorophenol with sun light in an oxygenated aqueous suspension has been studied under nanocrystalline hexagonal ZnO photocatalyst. The effect of substrate temperature onto the structural, morphological and photoactive properties has been investigated. The degradation of toluene, salicylic acid and 4-chlorophenol were achieved using a photoelectrochemical reactor module equipped with synthesized ZnO electrodes. Kinetic parameters have been investigated in terms of a first order rate equation. The rate constant (-k) for this heterogeneous photocatalysis was evaluated as a function of the initial concentration of original species. Substantial reduction in concentrations of toluene, salicylic acid and 4-chlorophenol was achieved as analyzed from COD and TOC studies. The mechanism for the degradation of toluene, salicylic acid and 4-chlorophenol could be explained on the basis of Langmuir-Hinshelwood mechanism.

Optoelectronic properties of sprayed transparent and conducting indium doped zinc oxide thin films

Indium doped zinc oxide (IZO) thin films are grown onto Corning glass substrates using the spray pyrolysis technique. The effect of doping concentration on the structural, electrical and optical properties of IZO thin films is studied. X-ray diffraction studies show a change in preferential orientation from the (0 0 2) to the (1 0 1) crystal planes with increase in indium doping concentration. Scanning electron microscopy studies show polycrystalline morphology of the films. Based on the Hall-effect measurements and analysis, impurity scattering is found to be the dominant mechanism determining the diminished mobility in ZnO thin films having higher indium concentration. The addition of indium also induces a drastic decrease in the electrical resistivity of films; the lowest resistivity (4.03 × 10−5 Ω cm) being observed for the film deposited with 3 at% indium doping. The effect of annealing on the film properties has been reported. Films deposited with 3 at% In concentration have relatively low resistivity with 90% transmittance at 550 nm and the highest value of figure of merit 7.9 × 10−2 Ω−1.

Gallium doping in transparent conductive ZnO thin films prepared by chemical spray pyrolysis

Zinc oxide (ZnO) and ZnO : Ga films have been deposited by the spray pyrolysis method onto preheated glass substrates using zinc acetate and gallium nitrate as precursors for Zn and Ga ions, respectively. The effect of Ga doping on the structural, morphological, optical and electrical properties of sprayed ZnO thin films were investigated using x-ray diffraction (XRD), scanning electron microscopy, optical absorption, photoluminescence (PL) and Hall effect techniques. XRD studies reveal that films are polycrystalline with hexagonal (wurtzite) crystal structure. The thin films were oriented along the (0 0 2) plane. Room temperature PL measurements indicate that the deposited films exhibit proper doping of Ga in ZnO lattice. The average transparency in the visible range was around ~85–95% for typical thin film deposited using 2 at% gallium doping. The optical band gap increased from 3.31 to 3.34 eV with Ga doping of 2 at%. The addition of gallium induces a decrease in electrical resistivity of the ZnO : Ga films up to 2 at% gallium doping. The highest figure of merit observed in this present study was 3.09 × 10−3 cm2 Ω−1.

Photocatalytic degradation of toluene using sprayed N-doped ZnO thin films in aqueous suspension

Thin films of N-doped ZnO are synthesized via spray pyrolysis technique in aqueous medium using zinc acetate and ammonium acetate as precursors. Influence of N doping onto photochemical, structural, morphological, optical and thermal properties have been investigated. Structural analysis depicts hexagonal (wurtzite) crystal structure. The effect of N doping on the photocatalytic activity of N-doped ZnO in the degradation of toluene is studied and results are compared with pure ZnO. The results show that the rate of degradation of toluene over N-doped ZnO is higher as compared to that of pure ZnO and increases with increasing N doping up to 10 at.% and then decreases. The enhancement of photocatalytic activity of N-doped ZnO thin films is mainly due to their capability for reducing the electron hole pair recombination. The photocatalytic mineralization of toluene in aqueous solution has been studied by measuring COD and TOC. Possible reaction mechanism pathways during toluene degradation over N-doped ZnO has been proposed.

Effect of the substrate temperature on the properties of spray deposited Sb–Se thin films from non-aqueous medium

Abstract Semiconducting Sb–Se thin films are prepared on glass substrates from a non-aqueous medium using a spray pyrolysis technique. The films are deposited at a fixed solution concentration of 0.1 M and at various substrate temperatures. The film thickness is of the order of 0.5 μ m and is found to be relatively higher for the film deposited at 175°C substrate temperature. X-ray diffraction studies reveal that the as deposited films are amorphous in nature, while after annealing in the N 2 atmosphere at 325°C for 2 h, the films deposited at 200°C turn into polycrystalline ones. The analysis of the absorption coefficient data reveals that as the substrate temperature increases, the optical bandgap value of the material increases. It has also been found, for the film deposited at 200°C and annealed in N 2 atmosphere, that the polycrystalline material follows the direct optical transition with energy gap Eg opt equal to 2.14 eV. The electrical resistivity at room temperature (300 K) is of the order of 10 6 –10 7 Ω cm, which changes slightly after annealing.

Kinetic Analysis of Heterogeneous Photocatalysis: Role of Hydroxyl Radicals

This article provides the current research activities that concentrate on the role of hydroxyl radicals in heterogeneous photocatalysis by transition metal oxides for different nanostructures. We devote most attention to Al-based Fe2O3 nanostructures that have been synthesized using chemical methods. The visible light photocatalytic activity of nanocrystalline pure and Al-based Fe2O3 photocatalysts for degradation of Salicylic acid, 4-Cholorphenol, and Acid orange 7 (Azo dye) is reported. The catalytic activity and selectivity for organic species are remarkably influenced by the size of the different photocatalyst. Utilization of various structures, advanced oxidation processes, heterojunction between Al-based Fe2O3 and TiO2, and application of solar energy for heterogeneous photocatalysis of water impurities were discussed. Extent of complete mineralization of such compounds by measuring COD and TOC was discussed. We conclude this review with personal perspectives on the directions towards which future research on this new class of nanostructured materials for photocatalysis might be directed.

Physical properties of sprayed antimony doped tin oxide thin films: The role of thickness

Transparent conducting antimony doped tin oxide (Sb:SnO2) thin films have been deposited onto preheated glass substrates using a spray pyrolysis technique by varying the quantity of spraying solution. The structural, morphological, X-ray photoelectron spectroscopy, optical, photoluminescence and electrical properties of these films have been studied. It is found that the films are polycrystalline in nature with a tetragonal crystal structure having orientation along the (211) and (112) planes. Polyhedrons like grains appear in the FE-SEM images. The average grain size increases with increasing spraying quantity. The compositional analysis and electronic behaviour of Sb:SnO2 thin films were studied using X-ray photoelectron spectroscopy. The binding energy of Sn3d5/2 for all samples shows the Sn4+ bonding state from SnO2. An intensive violet luminescence peak near 395 nm is observed at room temperature due to oxygen vacancies or donor levels formed by Sb5+ ions. The film deposited with 20 cc solution shows 70 % transmittance at 550 nm leading to the highest figure of merit (2.11 × 10−3Ω−1). The resistivity and carrier concentration vary over 1.22 × 10−3 to 0.89 × 10−3Ωcm and 5.19 × 1020 to 8.52 × 1020 cm−3, respectively.

A comparative study of the properties of spray-deposited Sb2Se3 thin films prepared from aqueous and nonaqueous media

Semiconducting antimony triselenide thin films were obtained by spray-depositing aqueous and nonaqueous media, at optimized substrate temperature and solution concentration, onto optically flat glass substrates. Film thickness was of the order of 0.5 {micro}m. X-ray diffraction studies revealed that the as-deposited films prepared from the aqueous medium were polycrystalline, while those prepared from the nonaqueous medium were amorphous. The optical gap of the Sb{sub 2}Se{sub 3} thin films prepared from the aqueous medium was 1.88 eV, due to direct interband transition. In contrast, the optical gap for the films prepared from the nonaqueous medium was 1.73 eV. Room-temperature electrical resistivity for the films prepared from both media was found to be of approximately the same order, 10{sup 7} {Omega}-cm. Thermoelectric power (TEP) measurement studies revealed that the films prepared from both media showed p-type conductivity, with Seebeck coefficients of 46.2 and 18.3 {micro}V/C for the polycrystalline and amorphous Sb{sub 2}Se{sub 3} thin films, respectively.

Visible light catalysis of rhodamine B using nanostructured Fe2O3, TiO2 and TiO2/Fe2O3 thin films

The Fe(2)O(3), TiO(2) and TiO(2)/Fe(2)O(3) composite films are deposited using spray pyrolysis method onto glass and FTO coated substrates. The structural, morphological, optical and photocatalytic properties of Fe(2)O(3), TiO(2) and TiO(2)/Fe(2)O(3) thin films are studied. XRD analysis confirms that films are polycrystalline with rhombohedral and tetragonal crystal structures for Fe2O3 and TiO(2) respectively. The photocatalytic activity was tested for the degradation of Rhrodamine B (Rh B) in aqueous medium. The rate constant (-k) was evaluated as a function of the initial concentration of species. Substantial reduction in concentrations of organic species was observed from COD and TOC analysis. Photocatalytic degradation effect is relatively higher in case of the TiO(2)/Fe(2)O(3) than TiO(2) and Fe(2)O(3) thin film photoelectrodes in the degradation of Rh B and 98% removal efficiency of Rh B is obtained after 20min. The photocatalytic experimental results indicate that TiO(2)/α-Fe(2)O(3) photoelectrode is promising material for removing of water pollutants.