Core–shell gold/silver nanoparticles: Synthesis and optical properties

[Display omitted] ► Stable dispersions of Au–Ag core–shell nanoparticles were prepared in aqueous medium. ► The thickness of the uniform Ag shell was varied from 1.0 to 8.5nm. ► Below a shell thickness of ∼3nm the Au cores preserve their optical properties. ► Above 4.0nm, the resulting sol displays optical properties typical for pure Ag. ► The Cl− ions concentration in solution affects the stability of the bimetallic sol. Highly dispersed gold–silver core–shell nanoparticles were synthesized in a two-step process. The stabilizer-free gold core particles with an average diameter of ∼30nm were first precipitated by rapid reduction of HAuCl4 with l-ascorbic acid. Thin continuous silver shells of variable thickness were subsequently obtained by reducing controlled amounts of silver nitrate added in the gold sol. The plasmon band of gold gradually blue-shifted and a peak characteristic for silver eventually emerged as the amount of deposited silver increased. A strong and well-defined silver absorption band was recorded when the Ag content exceeded 60wt.%. It is shown that the concentration of Cl− ions in the gold precursor solution plays a critical role in the stability of the bi-metallic sol and the structure of the deposited silver shell.