ENVIRONMENTAL PHOTOCATALYSIS

Photocatalysis over semiconductors [1-4] is based on the excitation of the semiconductor by photons whose energy is at least equal to that of the band gap. The electrons thus promoted to the conduction band can be transferred to surface/adsorbed electron-acceptor groups, molecules or ions, and the associated electron vacancies/positive holes in the valence band can be filled by electron-donor surface/adsorbed species. Therefore redox reaction can be produced. However, the charge carriers and the radicals or radical ions resulting from the capture of electrons and holes coexist in or on the same semiconductor particles. Consequently, the probability of recombination of charges and radicals or of back electron transfers is high. This draws the attention on the main flaw of photocatalysis over semiconductors which is the low rates of the desired chemical transformations with respect to the photonic power employed. Methods to enhance these rates are indicated below. Regarding the species generated by interfacial capture of electrons it is generally admitted that in air or aerated water, dioxygen can act as an electron-acceptor yielding superoxide or hydroperoxyl radicals. Cations can also scavenge electrons provided their redox potential is lower than the conduction band energy level. Hole transfers are believed to produce hydroxyl radicals from adsorbed water molecules or surface basic hydroxyl groups or hydroxide anions. However, direct electron transfer from electron