## 4 Construction of Core-Shell Nanostructures
### Au@Ag Core-Shell Nanostructures  
Au@Ag core-shell nanostructures exhibit finely tuned LSPR and SERS properties based on the shape and size, and have potential applications in sensors [97, 98, 99, 91] and catalysis [100], thus many attempts have been made to prepare these nanostructures. The shape and size of the Au core is important for the evolution of Ag on Au nanoparticles. Many groups select Au nanorods as cores to construct the Au@Ag core-shell nanostructures to which excellent optical properties are ascribed. [PERSON] _et al_. [101] described two different routes to coat the Au nanorods with silver for the first time. One way is to reduce AgNO\({}_{3}\) by AsA on the surface of Au nanorods with sodium citrate used as stabilizer, but the repeatability of results is not good and it is difficult to take TEM pictures due to the high concentration of sodium citrate. In this regard, they chose PVP as stabilizer instead of sodium citrate. However, they found that Ag\({}^{+}\) ions cannot be reduced in low pH solution, because the redox potential of AsA varied with pH until it was sufficient to overcome that of Ag. Therefore, when NaOH was added or AsA was replaced by citric acid which is a weak acid, Ag shells formed. The results indicated that with the increased amount of AgNO\({}_{3}\) added, the thickness of the Ag shell increased and the blue-shift of the longitudinal plasmon mode of the nanorods was enhanced. Later on, [PERSON] _et al_. [102] demonstrated that Ag nanorods can be grown directly from Au decahedrons and nanorods. Benefiting from the success of [PERSON] in forming Au@Ag core-shell nanorods from Au decahedron seeds, we can get a deeper understanding of the growth mechanism of nanorods. In the initial stage, Ag\({}^{+}\) ions were reduced and deposited on the decahedral seeds. Then, Ag nanocrystals grew along the longitudinal direction with slight changes along the lateral direction assigned to the selective adsorption of PVP on the (100) facets. The results correspond to the growth mechanism of Ag nanorods proposed by [PERSON] _et al_. However, in the research of [PERSON] _et al_. [103], the morphology of Ag nanoparticles were octahedrons grown from Au nanorods. They explained that Au@Ag core-shell nanorods were transition modes, when more Ag was deposited, (100) and (110) facets began to disappear and the nanorods finally evolved into octahedrons. In addition, they can easily tune the size of Ag octahedrons by using Au nanorods with different aspect ratios or controlling the amount of AgNO\({}_{3}\) added to the reaction system. In the case of other Au nanorods as seeds, [PERSON] _et al_. [104] also produced Ag octahedron shelsl from Au nanorods in a different way. Compared with the work of [PERSON] _et al_. [103], they controlled the thickness of Ag shells from thin to thick not only by using different aspect ratios of Au nanorods, but also using methoxypoly(ethylene glycol)-thiol (mPEG-SH) as stabilizer which can bind at the Au nanorod tips to block the growth of Ag along the longitudinal direction leading to the formation of Ag octahedrons. Moreover, they demonstrated that the optical properties of these core-shell constructs can be finely tuned with the change of thickness of Ag shell and the Au@Ag core-shell octahedrons with thin shells showing optical properties similar to those of pure Ag octahedrons. As with the above-mentioned Ag nanoprisms and triangular synthesized by H\({}_{2}\)O\({}_{2}\) etching, additional seed-mediated growth method are often used to construct Au@Ag core-shell nanoprisms and triangulars. [PERSON] _et al_. [96] reported a photochemical method to form Au@Ag core-shell nanoprisms. In their experiments, the role of Au seed was as a photocatalyst leading to the growth of Ag triangular shells which slightly depended on the shape of the Au cores. However, the morphology of Ag triangles can be easily tuned by changing the excitation wavelength and size of the Au core.