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.

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The viscometric properties and structure of concentrated sheared colloidal gels, formed at a pH = 8 from a commercial aqueous sol of 7 nm diameter silica particles, were investigated. As the system gels under a constant shear rate, the viscosity first increases, then peaks when the shear stress reaches a shear-rate-independent threshold value of about 350 Pa, and finally decreases asymptotically to approximately one tenth the peak value. This low-viscosity state can apparently be held indefinitely by maintaining the shear, but the system gels once the shear is removed. The structural origins of this viscometric behaviour were studied using in situ small-angle neutron scattering (SANS). An abrupt change in the character of the SANS intensity was observed at the time corresponding to the peak in the viscosity. Samples of the gelling system were extracted and diluted at various times after gel initiation. Effective hydrodynamic diameters of these extractants were estimated by dynamic light scattering, and their Guinier diameters were estimated by SANS when appropriate. We conclude that the drop in viscosity corresponds to a structural densification of growing clusters of the colloidal particles, and that this transformation occurs once the critical stress is reached.