Recent Aspects of Photocatalytic Technologies for Solar Fuels, Self-Cleaning, and Environmental Cleanup

Increasingly severe climatic, energy, and environmental problems warrant the need to continue to develop greenhouse gas-mitigating, energy-producing, energysaving, environmentally-beneficial technologies. The closely related fields of semiconductor photoelectrochemistry and semiconductor photocatalysis, largely involving titanium dioxide, have blossomed during the past forty years since the publication of our initial work on photoelectrochemical water splitting.1 This highly cited paper has provided a foundation for steadily increasing numbers of works on a broad range of topics, including applications such as solar light-induced water splitting (hydrogen production)2, CO2 reduction to produce carbonaceous solar fuels,3 water purification, decontamination, and disinfection, as well as new materials and fundamental aspects. Our recent review summarizes a number of photocatalytic applications, including selfcleaning surfaces, anti-fogging surfaces, heat dissipation, corrosion prevention, and visible light-sensitive materials.4 Figure 1 illustrates such a broad range of applications. The topics of “designer” titanium dioxide materials with various levels of dimensionality5,6 and photocatalysis for environmental applications7 have also been investigated thoroughly in our laboratory. Our early work on photocatalytic3 and photoelectrochemical8,9 CO2 reduction is now continuing at present, in the laboratory of our colleague, Akihiko Kudo,10 and in our own laboratory, both at the Tokyo University of Science, as well as by a number of other groups around the world. At the outset, we would like to emphasize the essential unifying principles of photocatalysis before presenting some specific examples. After energetic photons are absorbed in the semiconductor, electrons and holes are generated. The mobile electrons are free to move around, reaching the surface of the solid, and then react with water or oxygen. Similarly, the mobile, highly energetic holes reach the surface and oxidize water and/or organic matter. Thus, there are four simple cases for reactions involving the electrons and holes: (Case 1) water-water, (Case 2) oxygenwater, (Case 3) water-organic, and (Case 4) oxygen-organic. Cases 3, 4, and 5 are all involved with photocatalytic decomposition of organics, whereas Case 1 is likely to be involved in the photoinduced hydrophilic effect (PIHE), as well as photocatalytic water splitting. Fig. 1. Overview of photocatalytic applications.