Two-Step Synthesis of Large Gold–Silver Alloy Nanoparticles via the Combination of Seeded Growth and Citrate Co-reduction

We report a novel two-step method for the synthesis of uniform quasi-spherical and monodisperse gold–silver alloy nanoparticles with diameters of approximately 80 nm. In step one, gold–silver alloy seeds (G0) with diameters of approximately 30 nm are synthesized by citrate co-reduction of gold acetate and silver nitrate. In step two, the seeds grow into large alloy nanoparticles with diameters of approximately 80 nm by the combination of seeded growth and citrate co-reduction of gold and silver precursors. Our method makes use of gold acetate (rather than chloroauric acid) as a gold precursor to avoid the formation of silver chloride precipitates during synthesis. To our knowledge, we are the first to prepare gold–silver alloy nanoparticles with diameters of above 70 nm using gold acetate as a gold precursor. The alloy seeds and seed-grown nanoparticles show good control of size and composition, which enables tuning of the plasmonic band of these gold–silver alloy nanoparticles. Gold and silver are distributed uniformly within the cores of all Au-Ag alloy nanoparticles, while the surface is enriched in silver. This silver-enriched surface layer is thicker and more prominent for 25% Au alloy nanoparticles than for 50% and 75% Au alloy nanoparticles.