Some semiconductor materials, such as titanium dioxide or zinc oxide, can initiate oxidation-reduction reactions in the case of irradiation with light of the appropriate wavelength. That can decompose organic molecules or even simple organisms such as viruses, bacteria, algae or fungi to basic compounds, in particular, carbon dioxide and water, or weak mineral acids.
The principle of photocatalysis (see image below) is based on the absorption of photons of ultraviolet radiation by a semiconductor, while the energy of the captured photon causes the electron to jump from the valence to the conduction band. The specificity of photocatalytic materials is the appropriate width of the bandgap between these two energy states, which is small enough for incident photons to excite valence electrons. At the same time large enough to limit recombination (electron return from the valence band to its original energy level). Upon excitation, highly reactive electron-hole pairs are formed, which initiate subsequent redox reactions. Positively charged electron vacancies ("holes") oxidize water molecules to hydroxide anions and subsequently to hydroxyl radicals, while excited electrons reduce oxygen molecules to form superoxide radicals. These further react with water to produce hydrogen peroxide and hydroxyl radicals. Thanks to their strong oxidizing properties, these products initiate the oxidative degradation of adsorbed organic substances and molecules leading to their decomposition into harmless products - carbon dioxide and water. A significant benefit is that the semiconductor itself participates in these reactions only as a catalyst (it uses the energy of the incident photons to generate electron-hole pairs), while it is not transformed or consumed in any way.