Characteristics of water adsorbed on TiO2 photocatalytic surfaces as studied by 1H NMR spectroscopy

This account is a review of the research on the characteristics of adsorbed water molecules and the photocatalytic reaction sites of incorporated organic molecules on various TiO2 photocatalysts in powder and film forms by means of H-1 NMR spectroscopy. The adsorbed water can be categorized into four distinctively different water species: i) rigid water species with restricted mobility near the solid surface; ii) less mobile water molecules in the intermediate water layer; iii) relatively mobile water molecules in the outer water layer; iv) very loosely adsorbed water molecules which exchange slowly with gaseous water molecules in air. The temperature dependence of the NMR signal of a small quantity of ethanol molecules incorporated in TiO2 suggests that the actual photocatalytic reaction site can be these adsorbed water regions. The surface conversion of hydrophilic properties of TiO2 films induced by photoirradiation is well reflected in the NMR signal change. The enhancement of water molecularly adsorbed on TiO2 films is evidently detected on UV irradiation; this phenomenon has been difficult to elucidate so far with alternative spectroscopic techniques.