Formation and characterization of Au-Ag bimetallic nanoparticles in water-in-oil microemulsions

Gold-silver bimetallic nanoparticles with varying mole fractions were synthesized in water-in-oil microemulsions of water/Aerosol OT/isooctane by the co-reduction of HAuCl4 and AgNO3 with hydrazine at 25 degreesC. TEM analysis revealed that the bimetallic nanoparticles were essentially monodisperse and had a mean diameter of 4-22 nm, increasing with an increase in the molar ratio of water to Aerosol OT (omega(0)) and Ag content. The UV/VIS absorption spectra of their solutions exhibited only one plasmon absorption and the absorption maximum of the plasmon band red-shifted almost linearly from 400 to 520 nm with increasing Au : Ag molar ratio, revealing the formation of an alloy. Although the characteristic peaks for Au and Ag were too close to distinguish, the XRD analysis showed that the characteristic peaks for a Au-Ag bimetallic systems became broader and accordingly suggested the formation of bimetallic nanoparticles. The EDX analysis confirmed directly the formation of Au-Ag bimetallic nanoparticles. It showed that the composition of Au-Ag bimetallic nanoparticles was in good agreement with that of the feed solution but the outer layer of the particles was enriched in Ag. The HRTEM study indicated the resultant Au-Ag bimetallic nanoparticles contained single and multiple twins as well as stacking faults, and no mismatch was present. In addition, it was found that the formation rate of Au nanoparticles was much faster than that of Ag nanoparticles. This satisfactorily accounted for the composition distribution within a Au-Ag bimetallic nanoparticle.