Site-Selective Carving and Co-Deposition: Transformation of Ag Nanocubes into Concave Nanocrystals Encased by Au–Ag Alloy Frames

We report a facile synthesis of Ag nanocubes with concave side faces and Au–Ag alloy frames, namely Ag@Au–Ag concave nanocrystals, by titrating HAuCl 4 solution into an aqueous mixture of Ag nanocubes, ascorbic acid (H 2 Asc), NaOH, and cetyltrimethylammonium chloride (CTAC) at an initial pH of 11.6 under ambient conditions. Different from all previous studies involving poly(vinylpyrrolidine), the use of CTAC at a sufficiently high concentration plays an essential role in carving away Ag atoms from the side faces through galvanic replacement. Concurrent co-deposition of Au and Ag atoms via chemical reduction at orthogonal sites on the surface of Ag nanocubes leads to the generation of Ag@Au–Ag concave nanocrystals with well-defined and controllable structures. Specifically, in the presence of CTAC-derived Cl – ions, the titrated HAuCl 4 is maintained in the AuCl 4 – species, enabling its galvanic replacement with the Ag atoms located on the side faces of nanocubes. The released Ag + ions can be retained in the soluble form of AgCl 2 – by complexing with the Cl – ions. Both the AuCl 4 – and AgCl 2 – in the solution are then reduced by ascorbate monoanion, a product of the neutralization reaction between H 2 Asc and NaOH, to Au and Ag atoms for their preferential co-deposition onto the edges and corners of the Ag nanocubes. Compared with Ag nanocubes, the Ag@Au–Ag concave nanocrystals exhibit much stronger SERS activity at an excitation of 785 nm, making it feasible to monitor the Au-catalyzed reduction of 4-nitrothiophenol by NaBH 4 in situ . When the Ag cores are removed, the concave nanocrystals evolve into Au–Ag nanoframes with controllable ridge thicknesses.