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 solution into an aqueous mixture of Ag nanocubes, ascorbic acid (HAsc), 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 is maintained in the AuCl 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 by complexing with the Cl ions. Both the AuCl and AgCl in the solution are then reduced by ascorbate monoanion, a product of the neutralization reaction between HAsc 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 in situ. When the Ag cores are removed, the concave nanocrystals evolve into Au-Ag nanoframes with controllable ridge thicknesses.

We report a facile synthesis of Ag@Au concave cuboctahedra by titrating aqueous HAuCl4 into a suspension of Ag cuboctahedra in the presence of ascorbic acid (AA), NaOH, and poly(vinylpyrrolidone) (PVP) at room temperature. Initially, the Au atoms derived from the reduction of Au(3+) by AA are conformally deposited on the entire surface of a Ag cuboctahedron. Upon the formation of a complete Au shell, however, the subsequently formed Au atoms are preferentially deposited onto the Au{100} facets, resulting in the formation of a Ag@Au cuboctahedron with concave structures at the sites of {111} facets. The concave cuboctahedra embrace excellent SERS activity that is more than 70-fold stronger than that of the original Ag cuboctahedra at an excitation wavelength of 785 nm. The concave cuboctahedra also exhibit remarkable stability in the presence of an oxidant such as H2O2 because of the protection by a complete Au shell. These two unique attributes enable in situ SERS monitoring of the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH4 through a 4,4'-dimercaptoazobenzene (trans-DMAB) intermediate and the subsequent oxidation of 4-ATP back to trans-DMAB upon the introduction of H2O2.

Noble-metal nanoframes consisting of interconnected, ultrathin ridges have received considerable attention in the field of heterogeneous catalysis. The enthusiasm arises from the high utilization efficiency of atoms for significantly reducing the material loading while enhancing the catalytic performance. In this review article, we offer a comprehensive assessment of research endeavors in the design and rational synthesis of noble-metal nanoframes for applications in catalysis. We start with a brief introduction to the unique characteristics of nanoframes, followed by a discussion of the synthetic strategies and their controls in terms of structure and composition. We then present case studies to elucidate mechanistic details behind the synthesis of mono-, bi-, and multimetallic nanoframes, as well as heterostructured and hybrid systems. We discuss their performance in electrocatalysis, thermal catalysis, and photocatalysis. Finally, we highlight recent progress in addressing the structural and compositional stability issues of nanoframes for the assurance of robustness in catalytic applications.

This paper describes a new approach to site-selective sulfuration at the corner sites of Ag nanocrystals including triangular nanoplates and nanocubes. The reaction simply involved mixing an aqueous suspension of the Ag nanocrystals with an aqueous solution of polysulfide at room temperature. As a precursor to elemental S, polysulfide is highly soluble in water and can directly react with elemental Ag upon contact to generate Ag(2)S in the absence of oxygen. The reaction was easily initiated at the corner sites and then pushed toward the center. By controlling the reaction time and/or the amount of polysulfide added, the reaction could be confined to the corner sites only, generating Ag-Ag(2)S hybrid nanocrystals with greatly improved stability against aging at 80 and 100 °C in air than their counterparts made of pure Ag.

We report a facile synthesis of Au-based cubic nanoboxes as small as 20 nm for the outer edge length, together with well-defined openings at the corners and walls fewer than 10 atomic layers (or <2 nm) in thickness. The success relies on the selective formation of Ag2O at the corners of Ag nanocubes, followed by the conformal deposition of Au on the side faces in a layer-by-layer fashion. When six atomic layers of Au are formed on the side faces to generate Ag@Au6L core-shell nanocubes, we can selectively remove the Ag2O patches at the corner sites using a weak acid, making it possible to further remove the Ag core by H2O2 etching without breaking the ultrathin Au shell. This synthetic approach works well for Ag nanocubes of 38 and 18 nm in edge length, and the wall thickness of the nanoboxes can be controlled down to 2 nm. The resultant Au nanoboxes exhibit strong plasmonic absorption in the near-infrared region, consistent with computational simulations.