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

Abstract

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 HAuCl₄ solution 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. 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 HAuCl₄ is maintained in the AuCl₄^- 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 AgCl₂^- by complexing with the Cl^- ions. Both the AuCl₄^- and AgCl₂^- in the solution are then reduced by ascorbate monoanion, a product of the neutralization reaction between H₂Asc 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 NaBH₄ in situ. When the Ag cores are removed, the concave nanocrystals evolve into Au-Ag nanoframes with controllable ridge thicknesses.

Keywords: bimetallic nanocrystals; co-reduction; concave nanocrystals; galvanic replacement; surface capping; surface-enhanced Raman scattering.