Enhancement of Water Oxidation at Tungsten Oxide Photoanodes Doped with Borotungstate-Polyanion Modified-Hematite

Abstract

A procedure based on sol-gel aggregation for the fabrication of mesoporous WO3 photoanaodes was adopted to produce the doped oxide films by admixing the tungstic acid sol with Keggin-type borotungstic acid (H5BW12O40) or the borotungstic acid–stabilized hematite (Fe2O3). Such physicochemical properties as structure, morphology and spectroscopic identity of the resulting hybrid (doped) WO3 films were assessed using X-ray diffraction, scanning electron microscopy, as well as UV-Vis and Raman spectrocopies. When using a solar cell system operating in 0.5 mol dm-3 H2SO4 in the three-electrode configuration, the doped WO3 films acting as photoanodes yielded (following illumination with visible light) significantly (up to 80%) larger water oxidation photocurrents (at potentials higher than 0.75 V vs. RHE), in comparison to the analogous undoped mesoporous tungsten oxide films. The effect was the most pronounced in a case of the WO3 film doped with borotungstic acid–stabilized hematite. The observed enhancement effects could be rationalized either in terms of appearance of new conduction band structures in doped WO3 films or a marked increase in the degree of hydration of the doped oxide structures regardless annealing at high temperatures (450 oC). Judging from our experiments in 0.5 mol dm-3 NaCl, the systems are also applicable in the sea-water-type environments. Present results are consistent with the view that the doped tungsten oxide structures do not exhibit significant undesirable electron-hole recombination effects. Optimization of the system is underway aiming at further improvement of the photocurrent efficiency and the systems' utility under different conditions. Any increase in the photoelectrochemical efficiency of WO3–based films is of importance to the development of photoanodes for the visible-light driven water splitting.