The Photocatalyst
In 1967, a Japanese scientist, Dr. Akira Fujishima, discovered a very important photocatalytic phenomena; it is just like the reverse process of photosynthesis of plants that converts CO2 and H2O to organics. He observed that certain semi-conductors such as titanium dioxide (TiO2) and znic oxide (ZnO) when received UV radiation (Ea), they will generate a pair of electron and electron-hole. They are very unstable and react with water and oxygen to form oxygen and hydroxyl radicals (O2-, OH·). These radicals are very aggressive intermediates; they react with the surrounded organics and convert them to carbon dioxide and water.
Diagram of an Exothermic Reaction with an External Activation Energy, Ea
In order to make the photocatalyst active, an external energy Ea is required:
Ea = h·c/λ
Ea = 3.2 e-V ; h = 6.626 X 10-34J·s, Planck's Constant ; c= 2.998 X 108m/s, speed of light ; λ ≦ 380 nm (wavelength) which is in the UV light region.
Therefore, for photcatalyst to function, some UV light is required to activate the mechanism.
You may visit http://en.wikipedia.org/wiki/TiO2 for more information.