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

Abstract

WO 3 mesoporous films were prepared using a sol-gel method to serve as the photoanode for water cleavage in a 1 M HClO 4 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 WO 3 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 WO 3 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 W 6+ sites in the WO 3 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 WO 3 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 WO 3 films, and improves the film's electron transport performance in a photoelectrochemical cell.Scheme 1 Processes of charge transport, back transfer, and chemical interaction in a photoelectrochemical cell for water cleavage. The n-type semiconducting photoanode is positively biased for generation of O 2 gas, with corresponding H 2 gas generation on the counter electrode. V CB , V VB , V FB , and E F represent the conduction-band, valence-band, flatband, and Fermi-energy levels of the electrodes.

Enhancing Majority Carrier Transport in WO3Water Oxidation Photoanode via Electrochemical Doping

“…The observed onsets of +1.85 V vs RHE for water oxidation, which agrees with the literature value of +1.9 V for a pristine monoclinic WO 3 film, 14,18,37,39 remain independent on the electrochemical reduction treatment. The groups of Lewis 43 and Choi, 44 recently reported that water oxidation is occurring in competition with sulfate oxidation at the illuminated WO 3 surface.…”

“…An indirect bandgap of 2.6 eV and a direct bandgap of 2.9 eV are observed, in good agreement with the literature values reported for monoclinic WO 3 . 38,39 Besides, an additional broad absorption peak in the infrared region is observed and is attributed to the electron transfer from W 6+ to W 5+ at the oxygendeficient sites in the structure. 10 This oxygen deficiency makes WO 3 nonstoichiometric and intrinsically n-type.…”

Semiconducting Metal Oxide Nanostructures for Water Splitting and Photovoltaics

“…Solvent treatment of WO 3 films (thickness about 4 µm) pre pared by sol-gel resulted as well useful in enhancing the photo electrochemical performances: [168] while not influencing the mesoscopic structure of the films, vapor treatments resulted in deterioration of the WO 3 nanocrystalline framework. Methanol treated samples, in particular, exhibited enhanced photocur rent, attributed to suppression of interfacial recombination of photogenerated electrons.…”

Effect of Surface Passivation on Photoelectrochemical Water Splitting Performance of WO3 Vertical Plate-Like Films

“…At the same time, the photocurrent values of WO3 and HfO2/WO3 decrease 29.0% (from 0.81 to 0.57 mA/cm 2 ) and 24.4% (from 0.94 to 0.71 mA/cm 2 ), respectively, after irradiation of 1180 s. This indicates that the HfO2/WO3 film shows better stability compared to the bare WO3 film. The photoelectrochemical performances of both photoanodes are also estimated by the light energy to chemical energy efficiency, which is calculated with the following equation [31,32].…”