Nucleation reaction dynamics of Pt nanoparticles observed by the heterodyne transient grating method

The nucleation reaction dynamics of platinum nanoparticles in the photoreduction process of H2Pt(IV)Cl-6 solution were investigated by the heterodyne transient grating (HD-TG) method. The formation mechanism of platinum nanoparticles was considered, supported by information obtained from UV/VIS absorption spectroscopy during the reaction and SEM images of the generated nanoparticles. In particular, the roles of poly(N-vinyl-2-pyrrolidone) (PVP) as a protective polymer and ethanol as a solvent were studied. The chemical species involved in the reaction can be identified from the diffusion coefficients obtained from HD-TG measurements; the species observed by UV pulse irradiation were assigned to H2Pt(IV)Cl-6 as a reactant species and H2Pt(II)Cl-4 and Pt nuclei as product species. It was observed that the amounts of the reactant and product species increased, and many homogeneous nanoparticles were generated, by an increase in PVP concentration. The addition of ethanol to the solvent showed a larger effect on the enhancement of the reduction of H2Pt(IV)Cl-6 than that of PVP; however, it did not lead to Pt nuclei formation in the order of seconds. Nevertheless, because nanoparticle formation was confirmed by UV/VIS absorption spectroscopy and SEM images, the formation of nanoparticles following nuclei formation must have proceeded via a slow reaction. Therefore, nucleation and nanoparticle formation are considered to occur on a longer time scale than 10 s in water/ethanol solvent.