Photocatalysis with RECAM
Photocatalysis is a phenomenon by which light can lower the activation energy of a chemical reaction. In a semiconductor, the energy levels available for the electrons are divided into a valence band (VB), which is completely filled with electrons, and an empty conduction band (CB). The bands are separated by a forbidden energy gap (“Energy gap”: Eg). When a photon with hv energy, which is higher than the Eg value of any given semiconductor, is absorbed by the latter, the “promotion' of an electron from the valence band to the conduction occurs. This leads to the formation of two charge carriers, namely the electron in the conduction band, eˉCB, and an empty positive electron within the valence band, which is known as a 'hole' and generally indicated with h+VB. Thus, a charge carrier is generated.
Electrons in excited state within the conduction band and holes within the valence band may recombine and dissipate the initial energy in the form of heat. They could also be trapped in metastable surface states or react with the accepting electrons and with donating electrons and absorbed on the semiconductor surface.
Through this mechanism, excited states are generated within the material. These can start chain processes such as redox reactions and molecular transformations in molecules with which the material comes into contact. h+vb is a strong oxidant which can either oxidize a compound directly or react with donating electrons such as water or hydroxide ions to form hydroxyl radicals and reacts with organic contaminant compounds. The oxygen molecule is a good acceptor of electrons, as it can form a radical anionic superoxide O2 after capturing the electron
Titanium dioxide (TiO2) is a well-known semiconductor material which is particularly interesting for its photocatalytic properties. Amongst different crystalline forms of titanium dioxide (or the amorphous), anatase and rutile are those which have a photocatalytic activity. Both have a high refractive index, n, respectively equivalent to 2,52 (anatase) and 2,72 (rutile), which determines a high level of light 'trapping' in the material structure, and thus a higher probability of photon absorption.
Furthermore, the photocatalytic degradation processes may be accelerated by hydrogen peroxide. During the reaction, the peroxide can produce hydroxyl radicals by reacting with O2- or by direct photolysis. In addition, it can act as a conductive band to accept electrons such as oxygen and form hydroxyl radicals.