Photo-Electrochemical Properties of WO₃ and a-Fe₂O₃ Thin Films

Abstract

Iron oxide (a-Fe2O3) in hematite crystalline structure and tungsten trioxide have recently attracted much attention as possibly convenient materials to be used for hydrogen production via photoelectrochemical water splitting. Thius is due to their favorable properties such as band gaps between 2.0 - 2.2 eV (a-Fe2O3) and 2.5–2.8 eV (WO3) which allows absorbing of a substantial fraction of solar spectrum. FTO glass substrates were used for both types of films. Tungsten trioxide films were prepared by sedimentation of WO3 particles and further annealing at different temperatures to improve adhesion. Iron oxide (a-Fe2O3) hematite films were prepared by advanced pulsed plasma deposition method of High Power Impulse Magnetron Sputtering (HiPIMS). The films were evaluated on the basis of physical properties such as crystalline structure, surface topography and electrical behavior. The functional properties were investigated under simulated photoelectrochemical (PEC) water splitting conditions. Different excitation lights were used: monochromatic (very narrow single peak at light spectra) and the standard solar illumination conditions (AM 1.5 G). Also the influence of the electrolyte/electrode and substrate/electrode illumination of layers was studied. As deposited WO3 films have rather small photocurrents. Higher annealing temperature results in better adhesion of particles and increase in photocurrent. Optimum annealing temperature is 450-500 °C. Increase of the annealing temperature to 600 °C caused the formation of undesirable crystal phases (produced by the reaction of WO3 and FTO layer) and significant decrease in photocurrent. Despite confirmed hematite phase of as-deposited films, these were almost photoelectrochemically inactive. The main reason is probably the high density of defects and imperfections in crystalline structure, and thus a high extend of backward electron-hole pair recombination. Annealing in air at 650 °C significantly improved photoefficiency which can be explained by the diffusion of tin from the FTO substrate into hematite resulting in the extrinsic doping of hematite improving its electronic properties.