Heteropoly acid-incorporated TiO2 colloids as novel photocatalytic systems resembling the photosynthetic reaction center

TiO2 colloids are very useful photocatalytic systems, capable of converting solar energy to chemical or electrical energy and environmental cleaning. The key step in enhancing photocatalytic efficiency is improving photoinduced interfacial electron transfer like plant photosynthesis. It remains difficult to modify TiO2 particles as a real analogue of the photosynthetic reaction centers of green plants. We attempted to incorporate a photoreactive heteropoly acid (HPA) such as H3PW12O40 into TiO2 colloids in aqueous polyvinyl alcohol (PVA) (0.1%) solution, and found that two light reactions appear to operate in a series. Upon illumination of the HPA/TiO2 system with near-UV light (300-375 nm), interfacial electron transfer takes place from the conduction band of TiO2 to the incorporated HPA, which is also excited to catalyze photoreduction of Methyl Orange. The extent of the photoinduced reduction of the HPA adsorbed on TiO2 particles depends on the concentration ratio of the HPA and TiO2 colloids, irradiation wavelength, and intensity of radiation. It is found that the interfacial electron-transfer mechanism of HPA/TiO2 is quite analogous to the "Z-scheme" mechanism for plant photosynthetic systems. The photoinduced charge-carrier generation at the heterojunction is very efficient, and the photoreduction of Methyl Orange has been observed to be synergistically enhanced upon illumination of the HPA-incorporated TiO2 with visible light as well as near-UV light.