Inhibition of Electron‐Hole Recombination in Substitutionally Doped Colloidal Semiconductor Crystallites

The subtitutional doping of 120‐Å‐ sized TiO 2 Particles with Fe( III )ions has a profound effect on the charge carrier recombination time in this colloidal semiconductor. In undoped particles, the mean lifetime of an electronhole pair is ca. 30 ± 15 ns. Doping with 0.5% Fe( III ) drastically augments the charge‐carrier lifetime which is extended to minutes or hours. The slow character of the recombination dynamics in Fe( III )‐doped colloids was confirmed by laser photolysis using the characteristic optical of electrons in TiO 2 to monitor the time course of the reaction. EPR studies showed the Fe( III ) ions to enter the host lattice on Ti( IV ) sites, charge compensation taking place through the formation of oxygen vacancies. Valence‐band holes produced under band‐gap excitation react with these centers it the bulk forming Fe( IV ), the conduction band electrons being trapped by Ti( IV ) at the particle surface. Presumably, the spatial separation of the trapped electron and hole sites inhibits their recombination.