A photoluminescence (PL) band peaked at 840 nm is observed for an n-TiO2 (rutile) electrode in aqueous electrolyte solutions when illuminated under a slight anodic bias and was previously explained as arising from an intermediate of photooxidation reaction of water on the n-TiO2 electrode. The chemical structure and properties of the PL-emitting species have been investigated in the present work by measurements of photocurrent, photoluminescence, electroluminescence, and transient luminescence as well as inspection of the electrode surface by transmission electron microscopy. It is concluded that the PL band should be assigned to an electronic transition from the conduction band to the vacant level of HO• radicals present in a bulk defect (atomic gaps) near the n-TiO2 (rutile) surface and that the radicals act as an intermediate of the photooxidation reaction of water. The results give confirmative evidence to our previously proposed new mechanism that the surface Ti−OH group cannot be oxidized by photogenerated holes, and thus the reaction in acidic solutions is initiated by the oxidation of Ti−OH or OH- in the bulk defect near the surface.

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