Photocatalytic self-cleaning
Photocatalytic self-cleaning describes a property of surfaces that have been coated with photocatalysts, for example nanoparticles made of titanium dioxide (TiO 2 ). When exposed to (sun) light , organic materials are decomposed on the surface. The surfaces stay clean and have an antimicrobial effect . On some of these surfaces, water does not form droplets, but a thin layer , so that the eye cannot see any fogging on these surfaces (“ superhydrophilic surface”).
Mode of action using the example of titanium dioxide
The method is based on photocatalysis . Titanium dioxide (TiO 2 ) is a semiconductor ; Light creates electron-hole pairs in it if the energy of the photons is greater than the band gap (internal photoelectric effect ). The electrons or holes in the titanium dioxide can diffuse to the surface and generate radicals there , which lead to the decomposition of organic substances. The holes in particular have a high oxidative effect; OH radicals are formed from water (H 2 O) . Organic substances are thereby decomposed; In many cases, the end products are CO 2 and water.
The band gap of anatase , the most efficient form of TiO 2 for photocatalysis , is 3.2 eV (with the less efficient rutile crystal structure approx. 3.0 eV), this corresponds to a light wavelength of approx. 390 nm . This means that ultraviolet light is required for photocatalysis . Since the UV range only makes up a small part of sunlight , efforts are being made to reduce the band gap of anatase by doping and thus to utilize a larger range of the sunlight spectrum.
The superhydrophilic properties of the surfaces come about through oxygen vacancies on the TiO 2 surface. OH groups are bound at these points , which lead to good wetting with water.
Applications
Photocatalytically self-cleaning surfaces have been manufactured industrially in Japan for several years. One of the main areas of application are rear-view mirrors for motor vehicles; However, self-cleaning or antimicrobial tiles and photocatalytic emulsion paints are also produced.
So-called self-cleaning photocatalytic titanium oxide filters are also used in some air cleaning devices. These are irradiated with small UV lamps.