Semiconductor metal oxides find application in dye-sensitised solar cells and as photocatalysts for a range of environmentally and industrially useful reactions. In both technologies, the systems are driven by the initial absorption of photons to form charge carriers. These charge carriers may subsequently recombine or diffuse to the oxide surface where they may undergo interfacial electron transfer. In the case of modern solar cells, this involves transfer of the photogenerated charge from the sensitising dye to the semiconductor matrix; in the case of photocatalysis, this involves transfer of the photogenerated charge from the semiconductor to solution. In solar cells, the semiconductor is most often employed in the form of a mesoporous layer; in photocatalysis, it may be in form of either a mesoporous layer or as nanoparticles.
Since 1972, the main foci of photocatalysis have been the photodestruction of organic pollutants and the splitting of water for hydrogen generation. Our studies have focussed on new applications of photocatalysis beyond these areas, in particular the applications of photocatalysis in nuclear fuel reprocessing; the development of novel, magnetic nanocomposite photocatalysts; and the production and characterisation, for sensor applications, of conducting mesoporous metal oxide films that exhibit high degrees of photo-induced superhydrophilicity.
This lecture will present an overview of these studies, concentrating on our work on superhydrophilic materials – the onset of superhydrophilicity in metal oxides being thought to be due to the photogeneration of oxygen vacancies within the semiconductor lattice. We are currently using Quartz Crystal Microbalance-based photo-induced condensation experiments (Fig 1, the first time such a phenomena has been reported) to study these systems and shall report on our attempts to correlate the degree of condensation within the metal oxide mesopores with photo-induced surface energy changes on the metal oxide by use of the Kelvin Equation for capillary condensation.