Solution-Phase Synthesis of Sub-10 nm Au−Ag Alloy Nanoparticles

Gold−silver alloy nanoparticles were synthesized via reduction of varying mole fractions of HAuCl4 and AgNO3 by sodium borohydride in the presence of sodium citrate as a capping agent, in water. Solution concentrations were adjusted to avoid the precipitation of AgCl during the course of the reaction. The plasmon absorption band of Ag−Au nanoparticles shifts linearly to the red with increasing Au content. Due to the direct dependence of the metal salt ratio to the shift of the absorbance peak and the lack of an apparent core−shell structure in transmission electron microscope images, it is concluded that the nanoparticles are alloys, rather than core−shell composites.