Preparation of Au/Pt bimetallic nanoparticles in water-in-oil microemulsions

The preparation and characterization of bimetallic nanoparticles at various molar ratios of Au/Pt by the coreduction of chloroauric and chloroplatinic acid with hydrazine in the water-in-oil microemulsions of water/AOT/isooctane at 25 °C were described. The Au/Pt bimetallic nanoparticles prepared using feeding solutions with the concentration ratios of [HAuCl ₄]/ [H ₂PtCl ₆] = 9/1-1/9 essentially were monodispersed and had mean diameters of 3-4.5 nm. Both the UV/vis absorption spectra and XRD patterns of the bimetallic systems and the physical mixtures of individual metallic nanoparticles suggested the formation of bimetallic nanoparticles. The EDX analysis on individual particles confirmed the presence of two metals in a particle. It was indicated that the composition for each particle was roughly consistent with that of the feeding solution. The XPS analysis showed that the resultant Au/Pt bimetallic nanoparticles had a structure of Au-core/Pt-shell and their surface composition varied linearly with that of the feeding solution. The investigations on the size and formation kinetics of particles suggested that the reduction of AuCl ₄ ^- and PtCl ₆ ^-2 ions was almost finished before the formation of nuclei, the size of the bimetallic nanoparticles was determined by number of the nuclei formed at the very beginning of reaction, and the nucleation rate of Au was much faster than that of Pt. Furthermore, the nuclei for the bimetallic system might be composed of Au and Pt. Accordingly, a formation process for the Au/Pt bimetallic nanoparticles was suggested.