Generation and deactivation processes of superoxide formed on TiO2 film illuminated by very weak UV light in air or water

Kinetics of photogenerated O-2(-) under very weak light illumination (1 mu W cm(-2)) at the air-TiO2 interface as well as the water-TiO2 interface were studied quantitatively by measuring the chemiluminescence from luminol dropped an TiO2 film-type photocatalysts. The total number of steady-state O-2(-) on TiO2 surface was determined from the chemiluminescence intensity: about 1 x 10(14) cm(-2) at the air-TiO2 interface and about 2 x 1014 cm(-2) at the water-TiO2 interface. From the initial relation between illuminated number of photons and generated number of O-2(-), the generation yield of O-2(-) was estimated to be about 40% at the air-TiO2 interface and about 80% at the water-TiO2 interface. The generation and deactivation kinetics suggest that O-2(-) is recombined with a trapped hole on the surface and releases an electron to TiO2, producing free oxygen, and that most O-2(-) do not migrate into the air or water bulk, but exist as adsorbed form on TiO2.