A catalyst that accelerates a photochemical reaction. This type of catalyst works if the energy of the photons of light is greater than the energy between the valence band and the conduction band (see energy bands) of the material. If this is the case then electron-hole pairs are produced. The free electrons produce free radicals, such as the hydroxyl radical, which promote chemical reactions. There are many applications and potential applications of photocatalysis, such as adding titanium oxide to concrete; this keeps the concrete clean by producing radicals that react with dirt so that it is dissolved by rain.