Enhanced Photoelectrochemical Activity of Sol−Gel Tungsten Trioxide Films through Textural Control

Novel mesoporous tungsten trioxide films with enhanced incident photon-to-current conversion efficiencies have been prepared by a sol−gel route from an aqueous precursor solution containing peroxopolytungstic acid (PPTA). For films heated in air at 500 °C, it was found that film texture depended in a precise and reproducible manner on adjustment of the pH of this precursor solution by addition of a small volume of a selected mineral acid. Mesoporous micrometer-thick transparent films were obtained from PPTA without pH adjustment while mesoporous semi-transparent films resulted when the pH was lowered. The transparent films had specific surface areas of 18 m2/g, average pore diameters of 7.3 nm, and average crystallite sizes of 30 nm. The semi-transparent films possessed specific surface areas of 30 m2/g, average pore diameters of 12.5 nm, and average crystallite diameters of 17 nm. In the case of the semi-transparent films, electron microscopy indicated that the fundamental crystallites formed part of larger 200−300 nm aggregates which were in turn interconnected to form an open micrometer-length scale porous network. The transparent films did not show this type of porous hierarchy with the absence of micrometer-scale porosity. Photoelectrochemical studies of the films indicated that the hierarchical semi-transparent films exhibited a considerably enhanced photo-response relative to transparent films due to increases in both the interface area and light scattering. After calcination of the semi-transparent films at 500 °C, anodic photocurrents up to an equivalent of 1.4 mA/cm2 under Air Mass 1.5 equivalent solar irradiation were measured. Our results suggest that film texture is a major factor in determining the performance of the films, and the method reported here provides a simple and convenient means for modulation of this texture.