6.1. Photocatalysis and Dark Photocatalysis
TiO2 is the most photocatalytically active material for the decomposition of organic materials (for example, it is used for degradation of organic pollutants).[28,38,197,198,385–387] The reason for this high activity are the band-edge positions relative to typical environments (such as water). The basic principles involved in the photocatalytic mechanism are shown in Figure 18. UV light promotes electrons from the valence band to the conduction band; holes and electrons will be separated (under the field of the Schottky junction with the environment), reach the semiconductor–environment interface, and react with appropriate redox species in the environment. Several highly reactive species are generated for example from the surrounding water by charge exchange at the valence band (H2O + h+!OHC) and at the conduction band (O2 + e!O2). These radicals and peroxo ions are able to virtually oxidize all organic materials to CO2 and H2O. Furthermore, at the valence band, direct h+ transfer to adsorbed species to initiate decomposition may also be considered.