ELECTRON-PARAMAGNETIC RESONANCE STUDY OF PHOTOGENERATED RADICALS IN TITANIUM-DIOXIDE POWDER AND ITS AQUEOUS SUSPENSIONS

On UV irradiation of TiO2 powder at 293 K, paramagnetic centres are generated as indicated by electron paramagnetic resonance (anisotropic singlet at a g value of 2.009). The formal rate constant of formation of paramagnetic centres during irradiation is 0.038 s-1 and their rate of decay after irradiation is 0.011 s-1. On irradiation of an aqueous TiO2 suspension saturated with air or oxygen, the formation of hydroxyl radicals is observed using the spin trapping technique (5,5-dimethyl-1-pyrroline-N-oxide as spin trap). No radical formation is observed in argon-saturated suspensions. Oxygen acts as an electron acceptor, separating the photogenerated charge carriers, and enabling the formation of hydroxyl radicals.