The synthesis of Au/Pd bimetallic nanoparticles at various molar ratios by
the coreduction of HAuCl4 and H2PdCl4 with hydrazine in the reverse micelle
s 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 dia
meters of about 3 nm. Both the W-vis absorption spectra and X-ray diffracti
on patterns for the bimetallic systems and the physical mixtures of individ
ual metallic nanoparticles suggested the formation of bimetallic nanopartic
les. The energy dispersive X-ray analysis on the single particle showed tha
t the composition for each particle was roughly consistent with that of fee
ding solution. The X-ray photoelectron spectroscopy measurement revealed th
at Pd atoms were significantly enriched in the outer part of Au/Pd bimetall
ic 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 tha
t of Pd, (3) the size of bimetallic nanoparticles was determined by the num
ber of the nuclei formed at the very beginning of reaction, and (4) the nuc
lei for the bimetallic system might be composed of Au and Pd. According to
these suggestions, a formation process of Au/Pd bimetallic nanoparticles wa
s proposed.