Tungsten oxide thin film photo-anodes by reactive RF diode sputtering

Tungsten oxide (WO3) thin films were deposited both on silicon, soda-lime glass and W foils by Ar/O2 plasma diode sputtering process from a W metal target. The influence of O2 content percentage on the structural and optical properties of the films, as well as the effects of post treatment annealing both in vacuum (400 °C) and in air (600 °C) have been investigated. X-ray diffraction studies revealed that the as-grown films are amorphous-like regardless of the oxygen percentage. The degree of crystallinity of films is increased by a post-growth thermal-annealing procedure. With respect to other plasma sputtering recipes, here a lower stress state is favoured by the slower deposition rate and the multi-step deposition. The optical band gap deduced from the absorbance spectra ranges from 3.1-3.3 eV for the amorphous samples and it decreases to 2.3-2.5 for the more crystalline films. The photoelectrochemical activity of WO3 samples annealed at 600 °C in air have been investigated as a function of the O2 content.