Reductive energy generated at a TiO2 photocatalyst under UV light can be stored in WO3 by coupling them together, and the stored energy can be used after dark. However, the reduction of WO3 requires cation intercalation for charge neutralization. Thus, behavior of the TiO2-WO3 composite on an ITO electrode was examined in nonelectrolytic media. When the TiO2 and WO3 were close to each other (less than 1 mm), WO3 could be reduced even in pure water or humid air (relative humidity g25%), by irradiating the composite with UV light. In dry air, WO3 was not reduced efficiently, even if the TiO2 and WO3 nanoparticles were mixed well. These results suggest that protons generated at the TiO2 surface as a result of photocatalytic oxidation of water are intercalated into WO3, and therefore ionic conductivity of the medium or the composite film surface is important. The composite film charged in air exhibited almost the same electrode potential as that of the film charged in aqueous NaCl.

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