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 waterin-oil microemulsions ofwater/AOT/isooctan e at 25 degreesC were described. The Au/Pt bimetallic nanoparticles prepare d using feeding solutions with the concentration ratios of [HAuCl4]/ [H2PtC l6] = 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 sug gested the formation of bimetallic nanoparticles. The EDX analysis on indiv idual particles confirmed the presence of two metals in a particle. It was indicated that the composition for each particle was roughly consistent wit h 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 thei r surface composition varied linearly with that of the feeding solution. Th e investigations on the size and formation kinetics of particles suggested that the reduction of AuCl4- and PtCl6-2 ions was almost finished before th e formation of nuclei, the size of the bimetallic nanoparticles was determi ned by number of the nuclei formed at the very beginning of reaction, and t he 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. Accordingl y, a formation process for the Au/Pt bimetallic nanoparticles was suggested .