Photocatalytic Properties of TiO2 Nanoparticles
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The first step in heterogeneous photocatalysis by semiconductors is the generation of electron-hole pairs in the semiconductor particles. Unlike metals, semiconductors possess an energy region where there are no electron energy levels. This region extends from the highest occupied molecular orbital (HOMO) of the filled valence band, to the lowest unoccupied molecular orbital (LUMO) of the vacant conduction band, and is called the band gap of the semiconductor. When a semiconductor absorbs light with energy equal to, or greater than its band gap, an electron is excited from the valance band to the conduction band leaving a hole in the valance band, thereby creating an electron-hole pair. After excitation, the electrons and holes generated can follow several pathways. The migration of electrons and holes to the semiconductor surface allows for electron and/or holes to be transferred to the adsorbed organic or inorganic species. The semiconductor can donate electrons to reduce an electron acceptor (photocatalytic reduction). Alternatively, when holes migrate to the surface and interact with an electron donor, oxidation of the donor (photocatalytic oxidation) occurs. In competition with charge transfer to adsorbed species is electron/hole recombination. Excitons (electron/hole pairs generated in pure/bulk semiconductors) have a very short lifetime (nanoseconds) because of charge recombination. Recombination of the separated electron and hole can occur in the volume of the semiconductor or on the surface, resulting in the emission of a photon (light) or phonon (heat). In order to reduce volume recombination, it is necessary to minimize the volume of the particle through the use of nanoparticles. However, nanoparticles have a very high surface to volume ratio. Therefore, surface recombination becomes more probable, offsetting the decrease in volumetric recombination. Therefore, the particle size must be optimized to minimize both surface and volume recombinations. Doping with metal ions (Nd, Pd, Co, Pt, etc.) or electronegative atoms (C, N, S, F, P, etc.) of TiO2 offers a way to trap charge carriers and extend the lifetime of one or both of the charge carriers, enhancing the efficiency of the photocatalyst.