Seeded Growth Synthesis of Composition and Size-Controlled Gold-Silver Alloy Nanoparticles

We propose a novel methodology to synthesize monodisperse gold-silver alloy nanoparticles (NPs) with fine control of size and composition. The synthesis is based on a combination of the coreduction of gold and silver salts for the formation of alloys and the seeded growth approach for the formation of size-controlled NPs. While the simple use of coreduction gives alloy NPs limited to similar to 30 nm, the combination of both methods yields spherical alloy NPs with a size that can be controlled between similar to 30 and similar to 150 nm, with a coefficient of variation smaller than 15%. The alloy NPs can be synthesized to any composition between pure silver and pure gold. We also show that the alloy composition is nonhomogeneous, with a gold-rich core and a silver-rich surface. The transition in the alloy composition is gradual from the core to the surface, resulting in optical properties very similar to the optical properties of a homogeneous alloy, except for the smaller (similar to 30 nm) NPs. A multilayer Mie model has been introduced to study the effect of the nonhomogeneous alloy profile on the optical properties of the NPs. The inhomogeneous alloy structure is likely caused by galvanic replacement of Ag atoms at the surface by Au ions during the growth of the NPs.