The formation of reactive singlet oxygen molecule ( 1 O 2 ), the a 1 D g state of excited O 2 , in the photocatalytic TiO 2 aqueous suspension system was shown to be evident by detecting the nearinfrared phosphorescence at 1270 nm by means of a gated photon counting method. The lifetimes of O 2 ( 1 D g ) in three different media (water, ethanol, 1 : 1 water-ethanol mixture) were elucidated and compared with those reported in corresponding homogeneous media. Based on the comparison of the lifetime with those of the other reactive species such as OH radicals and trapped holes, the contribution of 1 O 2 to the actual photocatalytic procedure was discussed.

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