Figure 4.

Superoxide anion radical (O₂^−•), equation (1) and hydroxyl radical (^•OH), equation (2) formation resulting from the photo-excitation of TiO₂. An electron transfer from photo-excited TiO₂ to molecular oxygen leads to production of the superoxide anion radical. Hydroxyl radicals can be formed by electron release from water catalyzed by photo-excited TiO₂. By reoxidation of the Ti³⁺ ions back to Ti⁴⁺ ions, the process can start again. Similar generation of superoxide anion and hydroxyl radicals occurs in the case of ZnO.