Facile Preparation of Platelike Tungsten Oxide Thin Film Electrodes with High Photoelectrode Activity

Tungsten trioxide (WO₃) thin film electrodes with platelike structures were prepared by a facile hydrothermal reaction of tungsten sheets in a dilute nitric acid solution at 100–180 °C and subsequent calcination at 450 °C. The calcination step facilitated the transformation of the crystal structure from tungsten oxide hydrates (WO₃·H₂O or WO₃·2H₂O) to monoclinic WO₃ without significant modification to the platelike structures. The photoelectrochemical performance of the thin film electrodes for water splitting that took place in a dilute sulfuric acid was strongly dependent on both temperature and the time used for the hydrothermal reaction. This suggests that the thickness of the film influences the process of photoexcited electron transport. The time required for the hydrothermal reaction under higher temperatures was reduced in the generation of thin film electrodes with high photoelectrode activity, because the crystal growth is accelerated at high temperatures and the electron transport is restricted by a relatively thick compact layer that is comprised of WO₃ nanoparticulates. The electrode exhibited sensitivity to the violet portion of the visible light spectrum due to the bandgap of 2.8 eV and high photoelectrode efficiency, as well as an incident photon-to-current conversion efficiency (IPCE) of 66.2%, for the photoelectrochemical oxidation of water.