Difference in TiO₂ photocatalytic mechanism between rutile and anatase studied by the detection of active oxygen and surface species in water.

Various kinds of TiO2 photocatalysts have been practically applied in various fields. Knowing the exact surface properties is a prerequisite to develop further and efficient applications. However, the cause of the essential difference in the activities of the two polymorphs of TiO2, rutile and anatase, has not been clearly elucidated yet. We tried to clarify the cause in terms of active oxygen species (˙OH, ˙O2(-), and H2O2) photogenerated on the surfaces, which are considered practically involved in the photocatalytic reactions. It was revealed that for anatase the rate of ˙OH generation was high, but it decreased in the presence of H2O2. On the other hand, for rutile, ˙OH generation was very low but it increased in the presence of H2O2. The formation rate of ˙O2(-) for rutile was higher than that for anatase, suggesting that the photoinduced reduction process should not be accountable for the higher photocatalytic activity of anatase. Since the Ti-Ti distance on a rutile surface is smaller than that for anatase, rutile is capable of forming a surface structure such as Ti-OO-Ti, leading to readily form O2. The mechanism of fast coupling of two photoinduced conduction band holes to form Ti-OO-Ti was proposed, which is accountable for the lower reactivity of rutile. This mechanism was verified by the analysis of surface species with ATR-IR spectroscopy.