Transformation of Ag Nanocubes into Ag–Au Hollow Nanostructures with Enriched Ag Contents to Improve SERS Activity and Chemical Stability

We report a strategy to complement the galvanic replacement reaction between Ag nanocubes and HAuCl₄ with co-reduction by ascorbic acid (AA) for the formation of Ag–Au hollow nanostructures with greatly enhanced SERS activity. Specifically, in the early stage of synthesis, the Ag nanocubes are sharpened at corners and edges because of the selective deposition of Au and Ag atoms at these sites. In the following steps, the pure Ag in the nanocubes is constantly converted into Ag⁺ ions to generate voids owing to the galvanic reaction with HAuCl₄, but these released Ag⁺ ions are immediately reduced back to Ag atoms and are co-deposited with Au atoms onto the nanocube templates. We observe distinctive SERS properties for the Ag–Au hollow nanostructures at visible and near-infrared excitation wavelengths. When plasmon damping is eliminated by using an excitation wavelength of 785 nm, the SERS activity of the Ag–Au hollow nanostructures is 15- and 33-fold stronger than those of the original Ag nanocubes and the Ag–Au nanocages prepared by galvanic replacement without co-reduction, respectively. Additionally, Ag–Au hollow nanostructures embrace considerably improved stability in an oxidizing environment such as aqueous H₂O₂ solution. Collectively, our work suggests that the Ag–Au hollow nanostructures will find applications in SERS detection and imaging