OH Radical Formation at Distinct Faces of Rutile TiO2 Crystal in the Procedure of Photoelectrochemical Water Oxidation

It has been believed that the photogenerated OH radicals are major active species which cause photocatalytic oxidation of water. To investigate the actual contribution of OH radicals to the photocatalytic O-2 generation, the amount of the OH radicals was measured for the three kinds of rutile TiO2 electrodes having (100), (110), and (001) crystalline surfaces. The current efficiencies for O-2 generation measured with an oxygen sensor were almost 100% for all electrodes. However, the current efficiencies for OH radical formation estimated by means of a coumarin fluorescence probe method were less than 1%. Thus, it was experimentally elucidated that the contribution of OH radicals to the O-2 production is negligibly small. The amount of OH radical production decreased in the order of (100) > (110) > (001), along with the increase in the efficiency of the O-2 production. A plausible mechanism of OH radical formation as a byproduct in the O-2 generation process was proposed.