CATALYTIC ROLE OF LATTICE-DEFECTS IN THE PHOTOASSISTED OXIDATION OF WATER AT (001) N-TIO2 RUTILE

We report experimental data on the photocurrent-time dependence observed during the photoassisted oxidation of water at a H-2 reduced (001) TiO2 rutile single crystal, as a function of the concentration of generated donor centers. The observed photocurrent transients are fitted with a kinetic model previously proposed, according to which the photocurrent time decay at high band-bending is controlled by the second-order rate constant, k2, for H2O2 generation from the recombination of surface-bound OH-degrees(s) radicals, photogenerated by hole trapping at Ti-coordinated OH- ions of basic character. k2 is found to increase linearly with the concentration of donor centers, N(D), in the range (1.5 - 43) x 10(17) cm-3, from 7.0 x 10(-15) to 8.5 x 10(-14) cm2 s-1. The apparent increase of the catalytic activity of TiO2 surface in the photooxidation of H2O to H2O2 is attributed to the formation of Ti2O3 corundum type lattice defects during TiO2 reduction process in H-2. The catalytic properties of these defects are thought to be associated with the short distance between Ti3+ pairs, which makes easy covalent bonding between adsorbed OH-degrees(s) radicals. The k2 rate constant at corundum catalytic units is estimated to be about 5 orders of magnitude greater than that corresponding to nondefective TiO2 rutile areas (i.e., almost-equal-to 5 x 10(-10) vs almost-equal-to 5 x 10(-15) cm2 s-1).