Photocatalytic •OH radical formation in TiO2 aqueous suspension studied by several detection methods

Photoinduced reaction with TiO2 semiconductor photocatalysts was investigated by using nitroxide radicals as spin probes for (OH)-O-. radicals. The effects of some additives such as I-, Cl-, ClO4-, methanol and 2-propanol on the photocatalytic decay of nitroxide radicals, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidine-1-oxyl and 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, were investigated. Among the additives, only iodide ions that can be oxidized by trapped holes prohibited significantly the decay of the nitroxide radicals, indicating that trapped holes oxidize nitroxide probe radicals. For several TiO2 photocatalysts, the apparent quantum efficiency of (OH)-O-. radical formation was calculated and compared with those obtained by the other detection methods, such as DMPO spin trapping and terephthalic acid fluorescence methods. The experimental observations suggest that the photocatalytic oxidation should be caused more preferably by trapped holes or adsorbed (OH)-O-. radicals, rather than the photoinduced valence band holes in semiconductor and the free (OH)-O-. radicals in solution.