Mechanisms of Reactions Induced by Photocatalysis of Titanium Dioxide Nanoparticles

Photochemical reactions induced by TiO₂ nanoparticles share common mechanistic features where electron and hole pairs are formed, migrate to the surface, and their recombination competes with their reaction with various substrates. The main interest in TiO₂ photocatalysis is related to its potential application for decontamination of water and air. However, the absorption of TiO₂, which is limited to UV light, does not enable the use of natural or cheap light sources, and therefore tremendous effort has been invested in inducing visible-light activity via modification of TiO₂ including doping with nonmetals and metals, surface coating, and bi- and multicomponent assembling. In addition, much research has been carried out to inhibit the electron–hole recombination and enhance the reactions of holes and electrons with substrates. The basic mechanism of bare and modified TiO₂ and the main principles of the photocatalytic processes remain similar, although the excitation energy is different and the energies of the electrons and holes and their reaction kinetic parameters may vary. These photocatalytic processes are reviewed and discussed.