Primary Processes During the Photodeposition of Ag Clusters on TiO₂ Nanoparticles

In-situ transient laser flash photolysis absorption studies were carried out on transparent colloidal TiO₂ suspensions in the presence of Ag⁺ ions and polyvinyl alcohol irradiated by an XeF laser (351nm wavelength) to investigate the early stages of Ag photodeposition on the TiO₂ nanoparticles in the nanosecond and millisecond timescales. Photon fluxes of 5 × 10^-5 and 1 × 10^-6 mol photon/L were used; these were equivalent to approximately 60 and 2 photons/TiO₂ particle, respectively. Metallic silver clusters were detected immediately after the laser pulse using both photon fluxes, with calculated quantum yields of 0.3 and 0.5, respectively. In both cases, the metallic silver clusters formed had the same characteristic absorption maxima, assigned to silver clusters containing ≥12 silver atoms. The formation of these larger than expected silver clusters, even at low photon fluxes, is believed to be a result of an electron transfer within the TiO₂ aggregates, similar to an antenna effect. It is proposed that the silver deposits form an Ohmic contact on the TiO₂ particle thus allowing the continuous electron transfer through the metallic silver nuclei and hence the further reduction of adsorbed Ag⁺ ions. This study reports the first observation of an antenna type effect for a photocatalytic reduction reaction.