Photocatalysis can be described as a redox reaction catalyzed by a semiconductor material excited to the surface of which the reactants are adsorbed. Photocatalysis is called heterogeneous when the catalyst (semiconductor) is not soluble in the reaction medium.
The most widely used semiconductor is titanium dioxide (TiO2). TiO2 has a lot of advantages compared to other semiconductors such as its efficiency, non-toxicity, stability and low cost.
The figure below summarizes all the reactions occurring at the surface of a semiconductor, TiO2, under the action of a suitable UV radiation