Irradiation of titanium dioxide generates both singlet oxygen and superoxide anion

Although photoexcited TiO2 has been known to initiate various chemical reac tions, such as the generation of reactive oxygen species, precise mechanism and chemical nature of the generated species remain to be elucidated. The present work demonstrates the generation of singlet oxygen by irradiated Ti O2 in ethanol as measured by ESR spectroscopy using 2,2,6,6-tetramethyl-4-p iperidone (4-oxo-TMP) as a O-1(2)-sensitive trapping agent. Under identical conditions, the superoxide ion was also detected by spin trapping agent 5, 5-dimetbyl-pyrroline-N-oxide (DMPO). Kinetic analysis in the presence of bo th 4-oxo-TMP and DMPO revealed that singlet oxygen is produced directly at the irradiated TiO2 surface but not by a successive reaction Involving supe roxide anion. The basis for this view is the fact that DMPO added in the mi xture increased the signals responsible for 4-oxo-2,2,6,6-tetramethyl-1-pip eridinyl (4-oxo-TEMPO), a reaction product of 4-oxo-TMP and O-1(2). The det ailed mechanism for the generation of O-1(2) and superoxide ion by irradiat ed TiO2 and reactions between these species and DMPO are discussed. (C) 199 9 Elsevier Science Inc.