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 μW cm -2 ) at the air−TiO 2 interface as well as the water−TiO 2 interface were studied quantitatively by measuring the chemiluminescence from luminol dropped on TiO 2 film-type photocatalysts. The total number of steady-state O 2 - on TiO 2 surface was determined from the chemiluminescence intensity; about 1 × 10 14 cm -2 at the air−TiO 2 interface and about 2 × 10 14 cm -2 at the water−TiO 2 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−TiO 2 interface and about 80% at the water−TiO 2 interface. The generation and deactivation kinetics suggest that O 2 - is recombined with a trapped hole on the surface and releases an electron to TiO 2, producing free oxygen, and that most O 2 - do not migrate into the air or water bulk, but exist as adsorbed form on TiO 2 .