When a semiconductor is illuminated by light of suitable energy, i.e. greater than the band gap of the semiconductor:
electrons (e-) are promoted from the valence band (vb) to the conduction band F (cb) and they have the reducing power of the cb energy;
positive holes (h+) are created in the vb and they have the oxidation power of F the vb energy.
Evolution of the photoproduced pairs:
recombination (emission of thermal energy and/or luminescence);
reaction with electron acceptor or donor species (redox reaction, trapping mechanism).
Note: The prevalence of (ii) enhances the photoactivity of the semiconductor, but if the trapping mechanism involves the components of the semiconductor, photocorrosion can occur (ZnO, CdS).