Synthesis of Au/Pd bimetallic nanoparticles in reverse micelles

The synthesis of Au/Pd bimetallic nanoparticles at various molar ratios by the coreduction of HAuCl4 and H2PdCl4 with hydrazine in the reverse micelles of water/sodium bis(2-ethylhexyl) sulfosuccinate/isooctane at 25 degreesC was investigated. Transmission electron microscopy photographs showed that the Au/Pd bimetallic nanoparticles obtained at the feeding compositions of [HAuCl4]/[H2PdCl4] = 9/1 to 1/9 essentially had uniform sizes and mean diameters of about 3 nm. Both the W-vis absorption spectra and X-ray diffraction patterns for the bimetallic systems and the physical mixtures of individual metallic nanoparticles suggested the formation of bimetallic nanoparticles. The energy dispersive X-ray analysis on the single particle showed that the composition for each particle was roughly consistent with that of feeding solution. The X-ray photoelectron spectroscopy measurement revealed that Pd atoms were significantly enriched in the outer part of Au/Pd bimetallic nanoparticles. The characterization and formation kinetics of particles suggested that (1) most of AuCl4- and PdCl42- ions were reduced before the formation of nuclei, (2) the nucleation rate of Au was much faster than that of Pd, (3) the size of bimetallic nanoparticles was determined by the number of the nuclei formed at the very beginning of reaction, and (4) the nuclei for the bimetallic system might be composed of Au and Pd. According to these suggestions, a formation process of Au/Pd bimetallic nanoparticles was proposed.