Mechanisms for photooxidation reactions of water and organic compounds on carbon-doped titanium dioxide, as studied by photocurrent measurements

Photocatalytic activity of carbon-doped anatase TiO2 (C-TiO2) particles was investigated. Observation of a low anodic photocurrent for a C-TiO2 thin film electrode in 0.5 M Na2SO4 (pH = 6.4) under visible light irradiation suggested the occurrence of water oxidation by photogenerated holes in a C-induced midgap level of C-TiO2, though a C-TiO2 particulate suspension system in the presence of an electron acceptor (Fe3+) gave no detectable O-2 evolution under the visible light irradiation. The photocurrent measurements also showed that the holes in the midgap level of C-TiO2 were able to oxidize efficiently methanol and formic acid. Interestingly, the oxidation of methanol was explained reasonably by a mechanism that it proceeded indirectly via a surface intermediate of the water oxidation such as Ti-O center dot, in harmony with the abovementioned assumption of the water photooxidation by the midgap-level holes, whereas the oxidation of formic acid was interpreted to occur by a direct reaction of the midgap-level holes. On the other hand, the holes generated in the O2p valence band by the UV irradiation caused the oxidation of both methanol and formic acid via the direct mechanism. All the experimental results were thus explained on the basis of a recently reported new mechanism for the water photooxidation on TiO2 that it was initiated by a nucleophilic attack of a water molecule (Lewis base) on a surface-trapped hole (Lewis acid).