Electrodeposited nanostructured WO3 thin films for photoelectrochemical applications

Thin films of WO3 were deposited on FTO-coated glass substrates by electrodeposition using aqueous solutions of peroxotungstic acid. The effects of varying the tungsten concentration of peroxotungstic acid and deposition time on the mineralogical, microstructural, morphological, optical, and photoelectrochemical properties were determined using X-ray diffraction, scanning electron microscopy, focused ion beam milling, UV-vis spectrophotometry, and linear potentiodynamic voltammetry, respectively. The films consisted of monoclinic WO3 of grain sizes in the range 77-122 nm and thicknesses in the range 258-1394 nm; the true porosities were <5%. These microstructural and morphological parameters depended largely upon the tungsten concentration and deposition time. Some preferred orientation was observed and this was considered to result from crystallographic and microstructural factors. The optical transmission data revealed significant decreases in the optical indirect band gap, from 3.05 eV to 2.60 eV, as a function of increasing film thickness. This was considered to result from differential contributions from the surface and bulk band gap components as well as compressive stress. The voltammetry data and associated Butler plot revealed the establishment of a Schottky depletion layer and a flat-band potential of +0.2 V to +0.3 V versus Ag/AgCl. Although the calculated photoconversion efficiencies were in the range 0.02-0.14%, which is commensurate with the use of a tungsten-halogen light rather than xenon, there was a trend of increasing efficiency as a function of increasing film thickness. This was attributed to decreasing band gap and increasing light absorption. The shape of the curve of the preceding data supports the conclusion of differential contributions from the surface and bulk band gap components. Finally, evidence of photolysis in the absence of an external applied potential suggests the importance of the effect of grain size on the pH and its alteration of the flat-band potential. (C) 2012 Elsevier Ltd. All rights reserved.