Generation and Deactivation Processes of Superoxide Formed on TiO2 Film Illuminated by Very Weak UV Light in Air or Water

Abstract

Kinetics of photogenerated O2- under very weak light illumination (1 μW cm-2) at the air−TiO2 interface as well as the water−TiO2 interface were studied quantitatively by measuring the chemiluminescence from luminol dropped on TiO2 film-type photocatalysts. The total number of steady-state O2- on TiO2 surface was determined from the chemiluminescence intensity; about 1 × 1014 cm-2 at the air−TiO2 interface and about 2 × 1014 cm-2 at the water−TiO2 interface. From the initial relation between illuminated number of photons and generated number of O2-, the generation yield of O2- 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 O2- is recombined with a trapped hole on the surface and releases an electron to TiO2, producing free oxygen, and that most O2- do not migrate into the air or water bulk, but exist as adsorbed form on TiO2.