PHOTOELECTROPHORESIS OF COLLOIDAL SEMICONDUCTORS .2. TRANSIENT EXPERIMENTS ON TIO2 PARTICLES

We report experimental data and a second-order kinetic model for the time dependence of photo-induced changes in electrophoretic mobilities of aqueous dispersions of TiO2 semiconductor particles when irradiated with photons of energy corresponding to greater than their band gap (ca. 3.2 eV), in the absence of hole and electron scavengers. This is the first time that (laser Doppler) electrophoresis has been used to obtain kinetic information, in this case for analysing photocatalytic processes relevant to solar energy harvesting for chemical synthesis/destruction. The second-order rate constant for intra-particle electron-hole recombination, k(r') was determined as > 10(6) times greater than either the pseudo-first-order rate constants for charged surface site generation from holes and electrons, k'A and k'B, respectively; this indicates that recombination was a dominant process in determining the efficiency of surface charge generation in the absence of charge carrier scavengers. The rate constants for positive and negative ionic surface charge removal by electrons and holes k(e) and k(h), respectively, were ca. 1-20 m3 mol-1 s-1, and compare very closely with the value of k(r') indicating that these processes compete with direct electron-hole recombination. The quantum yield, phi, for hole/electron capture by oxidizable/reducible sites on the particle surface was in the range (5.2-7.9) x 10(6).