Development of Hybrid Tungsten Oxide Photoanodes Admixed with Borododecatungstate-Polyanion Modified-Hematite: Enhancement of Water Oxidation upon Irradiation with Visible Light

A typical procedure based on sol-gel aggregation for the fabrication of mesoporous WO3 photoanodes was modified and used produce modified tungsten oxide films by utilizing the tungstic acid sol admixed with Keggin-type borotungstic acid (H5BW12O40) or the borotungstic acid-stabilized hematite (Fe2O3). Physicochemical properties (structure, morphology and spectroscopic identity) of the resulting hybrid (composite) WO3 films were assessed using X-ray diffraction, scanning electron microscopy, as well as UV-Vis and Raman spectroscopies. During photoelectrochemical experiments with a solar cell system operating in 0.5 mol dm−3 H2SO4 in three-electrode configuration, the composite 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), relative to the performance of analogous bare (pristine) mesoporous tungsten oxide films. Independent voltammetric diagnostic experiments with Pt microdisk electrode implied the appearance of oxygen during photooxidation of water. The enhancement effect was the most pronounced for WO3 films admixed with borotungstic acid-stabilized hematite. The observed phenomena could be rationalized either in terms of the presence of new conduction band structures in composite WO3 films or a marked increase in the degree of hydration of the composite oxide structures regardless of the annealing at high temperatures (450 °C). It was also apparent from our experiments in 0.5 mol dm−3 NaCl, the systems were also applicable to sea-water-type environments.