Photocatalysis in use
Titanium dioxide (TiO2) is a semiconductor. Light generates electron-hole pairs on its surface if the energy of the photons is bigger than the band gap Eg (inner photoelectric effect).
Photocatalytic coating with self-cleaning
In titanium dioxide the electrons or holes can diffuse to the surface and generate radicals. That leads to decomposition of organic substances. Especially the holes have a high oxidative effect. Out of water OH-radicals are formed and organic substances are decomposed by them. In many cases CO2 and water are end products.
The crystalline structure Anatase, which is the most efficient form of photo catalysis has a band gap Eg of TiO2 3.2 eV. This energy has a light-wavelength of ca. 390 nm, so in this unchanged form of TiO2 only ultraviolet light is effective. Nowadays the band gap of Anatase can be reduced by doping of TiO2, so a bigger area of the light spectrum is usable.
One of the main characteristics of the photocatalysis is hydrophilicity. The word “hydrophilic” has Greek origins and means "water love". A water drop spreads flat on a hydrophilic surface, whereas water drops roll off a hydrophobic surface. A surface is called super-hydrophilic if a water drop dissolves on the surface. A hydrophilic surface enhances the self-cleaning when it is moistened by rain. Photo catalytic titanium dioxide coating becomes super-hydrophilic if irradiated by light. Photo catalytic titanium dioxide coating becomes super-hydrophilic if irradiated by light. Such a surface has decisive advantages: The coating acts by its hydrophilic property like a detergent; each rain washes the dirt off the surface. The surface obtains a self-cleaning effect.
Water spreads as a thin film on the surface. No disruptive drops are visible on a window pane during the rain. The view through a coated window pane is better than through an uncoated window pane. The thin film of water on the surface supports the evaporation if it is hot and enhances the cooling of the building, e.g. on coated glass facades. This saves energy and protects the environment.