"With respect to... TiO2, photocatalytic reactions are initiated by the absorption of illumination with energy equal to or greater than the band gap of the semiconductor. This produces electron-hole (e−/h+) pairs as in...TiO2+hv=e-cb(TiO2)+h+vb(TiO2)where cb is the conduction band and vb is the valence band. [A]s a result... the TiO2 particle can behave either as an electron donor or acceptor for molecules in contact... The electron and hole can recombine... On the other hand, they can participate in redox reactions with adsorbed species as the valence band hole is strongly oxidizing while the conduction band electron is strongly reducing. ...[T]he excited electron and the hole can participate in redox reactions with water, hydroxide ion (OH−), organic compounds or oxygen leading to mineralization of the pollutant. ...[R]eactions with water are predominant since the water molecules are more abundant ...[O]xidation of water or OH- by the hole produces the hydroxyl radical (·OH), a powerful oxidant. ...OH radicals are able to rapidly attack pollutants on the semiconductor surface and... are the most important radicals formed in TiO2 photocatalysis. An important reaction of the conduction band electron is reduction of adsorbed oxygen to oxygen radicals... [T]his prevents the electron from recombining with the hole and results in an accumulation of oxygen radicals... also... degrading contaminants in solution..."