Framing Silver Nanocrystals with a Second Metal to Enhance Shape Stability and Expand Functionality

Abstract

ConspectusSilver nanocrystals embrace fascinating properties for a wide variety of applications, but their performance tends to deteriorate because of shape instability arising from the dissolution of Ag atoms from high-energy sites such as edges and vertices. This issue can be addressed by framing the particle with a more stable metal M for the generation of a Ag@M core-frame nanocrystal. In addition to the improvement in shape stability, the inclusion of metal M expands the functionality and capability of the Ag nanocrystals. The first part of this Account introduces two strategies for the rational synthesis of Ag-based core-frame nanocubes. In the first strategy, a precursor to metal M is cotitrated with Ag+ ions into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (H2Asc, reducing agent) and poly(vinylpyrrolidone) (PVP, colloidal stabilizer) under ambient conditions. The M and Ag atoms derived from the two precursors are preferentially codeposited along the edges of Ag nanocubes for the creation of Ag@M-Ag core-frame nanocubes. The second strategy combines the carving of Ag from the side faces of Ag nanocubes and the concurrent deposition of M and Ag atoms on the edges in an orthogonal fashion. In one protocol, the precursor to M is titrated into an aqueous suspension of Ag nanocubes in the presence of H2Asc, sodium hydroxide (pH modifier), and cetyltrimethylammonium chloride (colloidal stabilizer and surface capping agent) under ambient conditions. In another protocol, the precursor is titrated into a mixture of Ag nanocubes, PVP (colloidal stabilizer and surface capping agent), and ethylene glycol (solvent and reducing agent) at an elevated temperature. In both cases, Ag atoms are carved from the side faces via oxidative etching while M and Ag atoms derived from the chemical reduction are codeposited on the edges for the generation of Ag@M-Ag core-frame concave nanocubes. The second part of this Account showcases the augmented properties of the Ag-based core-frame nanocrystals, in addition to some new functionality. The first example demonstrates how to preserve the shape of Ag nanocubes at an elevated temperature by passivating the vulnerable edges with Ir frames. The second example highlights the use of Ag–Pd core-frame nanocubes as a SERS probe for in situ monitoring the Pd-catalyzed reduction of 4-nitrothiophenol to 4-aminothiophenol by NaBH4 and the subsequent Ag-catalyzed oxidation of 4-ATP to trans-4,4′-dimercaptoazobezene by the oxygen from air. The third example establishes a method for the transformation of Ag@Au–Au core-frame concave nanocubes into trimetallic cage cubes through a site-selective galvanic replacement reaction. Altogether, these studies demonstrate that the shape stability of Ag nanocrystals can be enhanced while introducing new functionality by framing their edges with a different metal.