Primary Events in the Photocatalytic Deposition of Silver on Nanoparticulate TiO2

Abstract

The photocatalytic deposition of silver from ethanol solution on TiO2 nanoparticles prepared with a chemisorbed surface alkoxide layer has been examined in real time by picosecond-resolved transient absorption spectroscopy. This photocatalyst formation of surface-trapped photoelectron states, hypothesized to be Ti(III), can be followed on the time scale of the experiment (≤10 ns). Loss of these electrons to recombination is inconsequential, presumably owing to sacrificial hole trapping by the surface alkoxide states. Silver deposition occurs on the same time scale, and the pseudo-first-order rate constant for growth of the silver(0) transient absorption is the same as for the disappearance of the Ti(III) states under these conditions. We infer that one-electron, inner sphere reduction of Ag(I) by Ti(III) is rate determining in the formation of the colloidal silver deposit. These particles must accordingly grow by a sequence of alternating electronic and ionic events analogous to those hypothesized to be involved in latent image formation in silver halide photography. The quantum yield for silver deposition under our conditions was estimated as ca. 0.8, much higher than that reported by other authors (refs 1, 2, and 6).