Enriching Silver Nanocrystals with a Second Noble Metal:
Silver is perhaps the best choice of material for plasmonics and related applications owing to its relatively low cost and favorable dielectric functions. Over the past two decades, significant progress has been made in the synthesis of Ag nanocrystals with controlled shapes and sizes to tailor their properties and thus optimize their performance in a range of applications. In particular, Ag nanocrystals have been prepared with sharp features on the surface to drastically augment their surface-enhanced Raman scattering (SERS) activity. However, the sharp features tend to vanish due to the high susceptibility of Ag towards oxidative etching. As another pitfall, Ag is limited in terms of catalytic application as it only shows activity towards oxidation reactions such as epoxidation, not reduction reactions.
One can address the aforementioned limitations of Ag nanocrystals by introducing a second noble metal (M) such as Au, Pd, or Pt to generate Ag-M bimetallic nanocrystals in the core-frame and core-shell structure. When the M atoms are selectively deposited on the edges of a Ag nanocrystal, for example, a Ag@M core-frame nanocrystal is formed. In this structure, the excellent plasmonic and SERS properties of the Ag core are still retained while the deposited M brings in new catalytic capabilities. Alternatively, when the M atoms are conformally deposited on the entire surface, a Ag@M core-shell nanocrystal is created.10 In this case, the M shell can greatly improve the chemical stability of the particle, in addition to the new catalytic properties associated with M. If the shell is kept below 1–2 nm thick, the excellent plasmonic and SERS properties of the Ag core can still be leveraged. Significantly, both SERS and catalytic properties can be integrated in the core-frame and core-shell nanocrystals to offer a unique probe for in situ detection and analysis of catalytic reactions by SERS.