Under the influence of light, certain materials catalyze chemical reactions such as the decomposition of organic substances. The photo-catalytic properties of titanium dioxide (TiO2) are used for self-cleaning surfaces, antibacterial materials and for air and water purification.
Organic molecules from the air collect on surfaces and form a thin film of grease. The self-cleaning effect of photo-catalytic surfaces is based on the fact that the photo-catalytically formed radicals catalyze the oxidation of the organic substances, thereby destroying the film of grease. Thus, dust particles that normally stick to the greasy film on surfaces and form the actual dirty film, can be easily washed away by rain for example.
Photo catalysts can also be used to remove odor-forming gases (e.g. cigarette smoke) from the air or to rid drinking water of organic contaminants.