Vapor treatment of nanocrystalline WO3 photoanodes for enhanced photoelectrochemical performance in the decomposition of water

WO3 mesoporous films were prepared using a sol-gel method to serve as the photoanode for water cleavage in a 1 M HClO4 solution with Pt as the counter electrode. Post-treatment of the as-synthesized WO3 film with methanol and ethanol vapors, especially with methanol, improves the photocurrent during photoelectrolysis of water. Water and hexane vapors have a deleterious influence on the WO3 film. Scanning electron microscopy and X-ray diffraction analyses showed that treatment with methanol vapors did not alter the configuration of the nanocrystalline framework or the WO3 mesoscopic structure. Optical absorption and W L-3-edge X-ray absorption near-edge structural analyses also revealed that the W-ion electronic and oxidation states remained unchanged after methanol treatment. However, extended X-ray absorption fine structure analysis of the W L-3-edge showed that the coordination number of W6+ sites in the WO3 film significantly increased with the methanol treatment, indicating a corresponding decrease in the defect-state density of the film. The observed increase in the coordination number resulted in a 25% increase in the electron transit rate of the WO3 film and enhanced solar energy conversion by 32% for the photoelectrolysis of water. We conclude that post-treatment with methanol vapor remedies the defect region in nanocrystalline WO3 films, and improves the film's electron transport performance in a photoelectrochemical cell.