Mechanism of the OH Radical Generation in Photocatalysis with TiO2 of Different Crystalline Types

The difference in the OH radical generation through photocatalytic reaction with distinct crystalline types of TiO2 in aqueous suspension was explored by means of a fluorescence probe method. By employing two kinds of probe molecules with different adsorptivities, that is, coumarin and coumarin-3-carboxylic acid, we could detect OH radicals located near the TiO2 surface and those in the bulk solution individually. The amount of the OH radicals near the TiO2 surface was much larger than that in the solution, which diffused from the TiO2 surface to the bulk solution. Rutile TiO2 produced a much smaller amount of OH radicals as compared to anatase and anatase-contained TiO2. On the addition of H2O2, the OH radical generation for pure anatase TiO2 decreased but increased for rutile and rutile-contained TiO2. This difference could be explained as follows: at the surface of anatase TiO2, trapped holes become adsorbed OH radicals, while at the rutile surface, Ti-peroxo (Ti-OO-Ti) produced by the combination of two trapped holes which correspond to the adsorbed H2O2, could work as a catalyst for the OH radical generation from water.