Abstract
The kinetics of the formation of gold nanoparticles on the surface of pre-illuminated TiO2 have been investigated using stopped-flow technique and steady state UV/Vis spectroscopy. Excess electrons were loaded on the employed nanosized titanium dioxide particles by UV-A photolysis in the presence of methanol serving as hole scavenger, stored on them in the absence of oxygen and subsequently used for the reduction of AuIII ions. The formation of gold nanoparticles with an average diameter of 5 nm was confirmed after mixing of the TiO2 nanoparticles loaded with electrons with aqueous solution of tetrachloroaureate (HAuCl4) by their surface plasmon absorbance band at 530 nm, as well as by XRD and HRTEM measurements. The rate of formation of the gold nanoparticles was found to be a function of the concentration of the gold ions and the concentration of the stored electrons, respectively. The effect of PVA as a stabilizer of the gold nanoclusters was also studied. The observed kinetic behavior suggests that the formation of the gold nanoparticles on the TiO2 surface is an autocatalytic process comprising of two main steps: 1) Reduction of the gold ions by the stored electrons on TiO2 forming gold atoms that turn into gold nuclei. 2) Growth of the metal nuclei on the surface of TiO2 forming the gold particles. Interestingly, at higher TiO2 electron loading the excess electrons are subsequently transferred to the deposited gold metal particles resulting in bleaching of their surface plasmon band. This bleaching in the surface plasmon band is explained by the Fermi level equilibration of the Au/TiO2 nanocomposites. Finally, the reduction of water resulting in the evolution of molecular hydrogen initiated by the excess electrons that have been transferred to the previously formed gold particles has also been observed. The mechanism of the underlying multistep electron-transfer process has been discussed in detail.