Enhanced Photocatalytic Water Splitting on Very Thin WO3 Films Activated by High-Temperature Annealing

Further advancement in sunlight-driven splitting of water as a means of producing hydrogen and oxygen is mainly hampered by the availability of easy-to-prepare, inexpensive n-type semiconductor materials able to operate as stable and efficient photoanodes in a water photoelectrolysis cell. Here, we demonstrate that photocatalytic water oxidation currents on thin-layer semitransparent WO3 electrodes, deposited by using a one-step sol-gel method on conductive oxide F-SnO2 substrates, are dramatically improved following additional higher temperature (ca. 700 degrees C) annealing. Largely reduced recombination of charge carriers photogenerated in activated WO3 associated with enhanced light absorption yields at 1.23 V vs RHE, under simulated solar AM 1.5G irradiation (100 mW cm(-2)), water photo-oxidation currents close to 4.2 mA cm-2 on a 1.2-mu m-thick photoanode-approximately 2 times larger than on the electrodes of the same thickness only annealed at 550 degrees C. The relative enhancement of the photocurrent induced by the further annealing at 700 degrees C scaled up with decreasing the film thickness with a 3-fold increase observed for the thinnest tested, 0.25-mu m-thick WO3 electrode that reaches 2.75 mA cm(-2). We obtained such high photocatalytic water splitting performance without depositing any additional water oxidation catalyst.