Au@Ag Nanoparticles: Halides Stabilize {100} Facets

Seed-mediated growth is the most efficient methodology to control the size and shape of colloidal metal nanoparticles. In this process, the final nanocrystal shape is defined by the crystalline structure of the initial seed as well as by the presence of ligands and other additives that help to stabilize certain crystallographic facets. We analyze here the growth mechanism in aqueous solution of silver shells on presynthesized gold nanoparticles displaying various well-defined crystalline structures and morphologies. A thorough threedimensional electron microscopy characterization of the morphology and internal structure of the resulting core−shell nanocrystals indicates that {100} facets are preferred for the outer silver shell, regardless of the morphology and crystallinity of the gold cores. These results are in agreement with theoretical analysis based on the relative surface energies of the exposed facets in the presence of halide ions. SECTION: Plasmonics, Optical Materials, and Hard Matter G control of metal particles at the nanoscale is a crucial step in the development of nanoplasmonic materials and devices. Among all the reported synthetic routes, the seeded growth method can be considered as the most versatile one, since it allows fine-tuning of both the morphology and size of the nanoparticles by simply reducing additional metal ions on preformed nanoparticle seeds. Such seeded growth process can be implemented either in aqueous solution or in organic solvents such as N,N-dimethylformamide (DMF) or polyols, usually mediated by surfactants or polymers, respectively. Typically, reduction in organic solvents (which are the actual reducing agents) requires different experimental conditions, such as higher temperatures or the presence of (apparently) spectator ions. Although both approaches cannot be easily compared, it is widely accepted that the final particle shape, regardless of the synthetic approach, is mainly dictated by the crystalline structure of the seed, as well as by the effect of ligands (surfactants/polymers) and other additives (ions in general). The preferential adsorption of ligands or additives onto the seeds makes certain crystallographic facets thermodynamically more stable, reducing their surface free energy. As a consequence, the relative free energies for different facets and thus their relative growth rates may change as compared to surface energies in vacuum. Although the fast development of spectroscopic techniques has allowed us to learn progressively more about the growth mechanism, the explicit role of the different additives has not been unequivocally discerned. In nonaqueous solvents, seedmediated growth has been widely explored for silver, gold, and other metals, allowing in each case to propose a possible growth mechanism. However, in aqueous solution, the method has been mainly developed to generate gold nanoparticles in a wide range of morphologies, including Platonic solids, prisms, and other shapes containing high-index facets. Different approaches have also been recently proposed for silver nanoparticles, although shape-controlled synthesis still requires deeper insights in this case. Focusing on the seeded growth approach in aqueous media, control over particle growth has been achieved in the presence of surfactants (most frequently cetyltrimethylammonium bromide, CTAB) containing halides and silver ions as key additives. Generally, the detailed analysis of the growth process and the crystalline structure of the particles have been used to help establish the corresponding growth mechanisms. A general method has been recently proposed to explain the shape evolution of small Received: June 19, 2013 Accepted: June 20, 2013 Letter