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 WO ₃ 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 L₃-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.