We report the seed-mediated transformation of the Ag nanocube into Ag@Au-Ag concave nanocrystals, a nanostructure with six concave side faces decorated by Au-Ag alloy frames at the edges and corners. The synthesis was performed by titrating aqueous HAuCl4 into an aqueous mixture of Ag nanocubes, ascorbic acid (H2Asc), NaOH, and cetyltrimethylammonium chloride (CTAC) at an initial pH of 11.6 under ambient conditions. The use of CTAC at sufficiently high concentration enabled the site-selective carving of the Ag atoms from the side faces through galvanic replacement. Specifically, the CTAC-derived Cl− ions played three key roles: (1) they would preferentially bind to the side faces of Ag nanocubes and direct the galvanic replacement reaction to that site; (2) they would maintain the Au(III) species as AuCl4− to facilitate its galvanic replacement with the Ag atoms; and (3) they would complex with the released Ag+ ions into AgCl2+ species that are soluble. Concurrently, both the AuCl4− and AgCl2− species would be reduced by the ascorbate monoanion, generated from the neutralization of H2Asc by NaOH, to Au and Ag atoms for their preferential co-deposition onto the edges and corners of the Ag nanocubes. As the Ag atoms continue to be carved out from the side faces and the Au and Ag atoms continue to deposit on the edges and corners of the Ag nanocube, we could eventually obtain the Ag@Au-Ag concave nanocrystals. We evaluated that the Ag@Au-Ag concave nanocrystals exhibit stronger SERS activity compared with Ag nanocubes at an excitation of 785 nm, making it feasible to monitor the Au-catalyzed reduction of 4-nitrothiophenol by NaBH4 in-situ. By removing the Ag cores from the concave nanocrystals, we could obtain Au−Ag nanoframes with controllable ridge thicknesses.

Site-selective carving and co-deposition: Transformation of Ag nanocubes into concave nanocrystals encased by Au-Ag alloy frames

2019 Fall National Meeting

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