Eventually, some of the electron and hole pairs migrating to the surface get involved in redox reactions even during their short lifetime (on the order of nanoseconds). Titanium (IV) is reduced to titanium (III), which is finally transformed to titanium (IV) combined with superoxide radical anions, if electron acceptors such as oxygen are available on the surface (Reactions 4 and 5) [3,9].
Meanwhile, the generated holes are utilized for the generation of hydroxyl radicals and direct oxidation of organics, R (Reactions 6-8) [3,9] or they can be combined with the electron from a donor species, depending on the reaction mechanism:
Because of the short lifetime of photo carriers, the prerequisite for Reactions 6-8 is the adsorption of substances such as water and organic molecules on the TiO2 surface and lattice oxygen (Oj-) (Reactions 9-11) [3,9]. This facilitates the redox reaction at the interface of TiO2 solid and the water:
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