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 WO 3 electrodes, deposited by using a one-step sol–gel method on conductive oxide F-SnO 2 substrates, are dramatically improved following additional higher-temperature (ca. 700 °C) annealing. Largely reduced recombination of charge carriers photogenerated in activated WO 3 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-μm-thick photoanode—approximately 2 times larger than on the electrodes of the same thickness only annealed at 550 °C. The relative enhancement of the photocurrent induced by the further annealing at 700 °C scaled up with decreasing the film thickness with a 3-fold increase observed for the thinnest tested, 0.25-μm-thick WO 3 electrode that reaches 2.75 mA cm –2 . We obtained such high photocatalytic water splitting performance without depositing any additional water oxidation catalyst.