Synthesis and characterization of noble-metal nanostructures containing gold nanorods in the center.

2010 WILEY-VCH Verlag Gm Figure 1. A) A schematic image showing the formation of Au@Ag core–shell nanocrystals. B,C) TEM images of (B) the Au nanorods Noble-metal nanocrystals have received increasing interest because they display unique catalytic and optical properties sought for applications such as catalysis, diagnosis, plasmonics, and surface-enhanced Raman spectroscopy (SERS). The catalytic and optical properties of a noble-metal nanocrystal can be tailored by controlling its size, shape, elemental composition, as well as the internal and surface structures. Most recently, special attention has been paid to core–shell, bimetallic nanocrystals because they provide a new system with tunable catalytic and optical properties. In this case, the core–shell structure can be achieved through deposition of a metal on the surface of core made of another metal or via a galvanic replacement reaction between the core and a salt precursor to the second metal. In both cases, the metal nanocrystal serving as the core can have a specific geometric shape and this shape might be able to sustain during the coating or galvanic process. In general, it is a combination of the facets expressed on the core nanocrystal and the degree of lattice mismatch between the two metals that dictates the shape or morphology of the final product. Many attempts have been made to explore Au nanorods as the starting material to construct nanocrystals with a core–shell structure. For example, the ends and the side surface of Au nanorods have been coated with other metals such as Pd, Pt, Ag, Ni, and Au to add new functions to the Au nanorods. Besides coating, other strategies have been reported to generate Au nanocrystals with new geometric shapes by templating against Au nanorods. Specifically, Au nanorods have been transformed into various shapes via overgrowth on the entire surface or the side surface of Au nanorods. However, this strategy has not been widely extended to noble metals other than Au. Only recent studies by Xiang et al. and Becker et al. showed a system, where Ag could grow on specific sides of Au nanorods to generate nanocrystals with a semicircular or triangular morphology. Overall, it remains a challenge to find a simple way to control the overgrowth of other metals on Au nanorods in an effort to generate bimetallic nanocrystals with a core–shell structure and the desired properties. In this Communication, we present a facile method for the synthesis of Ag nanocrystals containing Au nanorods in the center, which will be denoted as Au@Ag for simplicity. Figure 1A shows schematically how the core–shell bimetallic nanocrystals are formed. The final products are mostly octahedrons with a few in other shapes (e.g., decahedrons). Since we used Au nanorods with a uniform size distribution, the core–shell nanocrystals were fairly uniform in terms of both size and shape. In general, the dimensions of the core–shell nanocrystals could be easily tuned by using Au nanorods with different aspect ratios and/or by controlling the amount of AgNO3 added into the reaction system. In a subsequent step, the Au@Ag nanocrystals can also be converted into hollow nanostructures made of Au, Pt, and Pd via galvanic replacement reactions, with Au nanorods encapsulated in the center.