Enriching Silver Nanocrystals with a Second Noble Metal.

Noble-metal nanocrystals have received considerable interests owing to their fascinating properties and promising applications in areas including plasmonics, catalysis, sensing, imaging, and medicine. As demonstrated by ample examples, the performance of nanocrystals in these and related applications can be augmented by switching from monometallic to bimetallic systems. The inclusion of a second metal can enhance the properties and greatly expand the application landscape by bringing in new capabilities. Seeded growth offers a powerful route to bimetallic nanocrystals. This approach is built upon the concept that preformed nanocrystals with uniform, well-controlled size, shape, and structure can serve as seeds to template and direct the deposition of metal atoms. Seeded growth is, however, limited by galvanic replacement when the deposited metal is less reactive than the seed. The involvement of galvanic replacement not only makes it difficult to control the outcome of seeded growth but also causes degradation to some properties. We have successfully addressed this issue by reducing the salt precursor(s) into atoms with essentially no galvanic replacement. In the absence of self-nucleation, the atoms are preferentially deposited onto the seeds to generate bimetallic nanocrystals with controlled structures. In this Account, we use Ag nanocubes as an example to demonstrate the fabrication of Ag@M and Ag@Ag-M (M = Au, Pd, or Pt) nanocubes with a core-frame or core-shell structure by controlling the deposition of M atoms. A typical synthesis involves the titration of M (a precursor to M) ions into an aqueous suspension containing Ag nanocubes, ascorbic acid, and poly(vinylpyrrolidone) under ambient conditions. In one approach, aqueous sodium hydroxide is introduced to increase the initial pH of the reaction system. At pH = 11.9, ascorbic acid is dominated by ascorbate monoanion, a much stronger reductant, to suppress the galvanic replacement between M and Ag. In this case, the M atoms derived from the reduction by ascorbate monoanion are sequentially deposited on the edges, corners, and side faces to generate Ag@M core-frame and then core-shell nanocubes. The other approach involves the use of ascorbic acid as a relatively weak reductant while M is cotitrated with Ag ions in the absence of sodium hydroxide. At pH = 3.2, when the molar ratio of Ag to M is sufficiently high, the added Ag ions can effectively push the galvanic reaction backward and thus inhibit it. As a result, coreduction of the two precursors by ascorbic acid produces Ag and M atoms for the generation of Ag@Ag-M core-frame nanocubes with increasingly thicker ridges. The Ag@Ag-Pd core-frame nanocubes can serve as a dual catalyst to promote the stepwise reduction of nitroaromatics to aminoaromatics and then oxidation to azo compounds. The consecutive reactions can be monitored using surface-enhanced Raman scattering (SERS). The Ag@Au core-shell nanocubes with Au shells of three or six atomic layers exhibit plasmonic peaks almost identical to those of the Ag nanocubes while the chemical stability and SERS activity are substantially augmented. For both types of bimetallic nanocubes, the Ag cores can be selectively removed to generate nanoframes and nanoboxes.