Question

If TiO 2 powder is placed in a small beaker containing polluted water and
irradiated with sunlight, a photocatalytic process occurs whereby the pollutants are
broken down. The photocatalyst (sensitiser) simultaneously adsorbs the organic
pollutant and oxygen, and oxidation and reduction occur on the surface of the
semiconductor through the photogenerated holes and electrons, respectively. TiO 2 is
an excellent sensitiser for treatment of water on account of its being nontoxic, a good
catalyst and inert under the ambient conditions of the process. Explain the conditions
necessary for photosensitised oxidation and reduction to occur on the surface of a
TiO 2 semiconductor.

Answer 1

Titanium dioxide photosensitization may be achieved in various ways, involving surface modification with appropriate species. The photosensitization process requires a visible light-induced electron or hole injection into conduction or valence band, respectively. Efficiency of this process depends on electronic interaction between the photosensitizer moiety (surface complex) and TiO2 particle. At least two types of the charge injection mechanisms may be distinguished—in the first, charge is transferred from the excited state of the sensitizer molecule to the conduction or valence band while the second mechanism involves a direct molecule-to-band charge transfer (MBCT). The MBCT process can be realized by surface titanium(IV) complexes with various organic and sometimes inorganic ligands.