Abstract
The reaction of electrons stored on TiO2 nanoparticles with silver ions in aqueous solution has been studied employing the stopped flow technique. Prior to the kinetic experiments, nanosized TiO2 particles were loaded with electrons by UV (A) photolysis in the presence of methanol. The formation of silver nanoparticles is detected by their typical surface plasmon (SP) absorbance band at 400 nm. Multiphase kinetic decay curves were observed for the electron absorbance as well as for the build-up of the plasmonic absorbance of the silver nanoparticles. This kinetic behavior is attributed to the multistep formation mechanism of the silver particles on the surface of TiO2 followed by the transfer of excess electrons to the deposited silver particles. The mechanism of the formation and growth of the silver particles on the TiO2 surface is proposed to be as following: (i) reduction of silver ions to form silver atoms which in turn form the nuclei for the metal particles, (ii) growth of the silver nuclei to form silver particles, and (iii) coalescence of the formed silver particles to form even bigger particles. Following the reduction of all silver ions present in solution, the remaining excess electrons are then transferred to the deposited silver particles resulting in a slight blue shift of the surface plasmon band. Subsequently, the stored electrons on the silver particles are used for the reduction of adsorbed H+ to produce H-2 gas. The effect of Polyvinyl alcohol (PVA) as a stabilizer for the deposited metal nanoparticles as well as the effect of molecular oxygen on the unstabilized silver deposits have also been investigated. It could be shown that molecular oxygen acts as an electron acceptor, resulting in the partial oxidation of the deposited silver particles and thus a red shift and a damping of the surface plasmon absorbance band. The rate constants of the decay of the TiO2 electron absorbance as well as of the build-up of the plasmon absorbance of the silver nanoparticles have been measured.