TiO2 photocatalysis

It is known that TiO2 exhibits good photocatalytic properties which could be used in various commercial applications such as water and air purification or photo-induced biodegradation of microbes. TiO2 occurs in nature in the form of three crystal polymorphs: anatase, brookite and rutile. For photocatalytic applications only anatase and rutile are of interest, however anatase shows greater photocatalytic activity. The ability to carry out photocatalytic reactions arises from TiO2 band gap. When TiO2 is irradiated with light of energy equal or higher to its band gap energy, an electron is promoted from the valence band to the conduction band leaving behind a positive hole. These photogenerated charge carriers can be trapped by surface adsorbed molecules, form strong radicals and oxidize organic compounds. However, a drawback in TiO2 photocatalysis is that it absorbs mainly ultraviolet light. Therefore many efforts are made to prepare visible light-active TiO2. Photocatalytic activity of titania (TiO2) depends on its crystal phase, crystallinity, particle size and/or specific surface area. High crystalline TiO2 exhibits higher photocatalytic activity due to less bulk defects, which represent recombination centers for photogenerated charge carriers. On the other hand, high crystalline particles are usually large and the photogenerated electrons and holes undergo volume recombination before they can be trapped by surface adsorbed molecules and form radicals which are very important for photocatalytic activity. Powders consisting of small particles exhibit high specific surface area and offer more active sites for photocatalytic reactions [1], [2].