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

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.