Defect-Assisted Deposition of Au on Ag for the Fabrication of Core–Shell Nanocubes with Outstanding Chemical and Thermal Stability

We report the defect-assisted deposition of Au on Ag nanocubes for the generation of Ag@Au core–shell nanocubes with three to eight atomic layers. In a standard protocol, we disperse Ag nanocubes in an ethylene glycol (EG) solution containing poly(vinylpyrrolidone) and then titrate aqueous HAuCl4 at 110 °C. Initially, the galvanic replacement reaction between HAuCl4 and Ag allows the deposition of Au on the edges of the nanocubes with highest surface energy as the Ag atoms on the side faces are dissolved into the aqueous suspension in the form of Ag(I) ions to create surface defects. These defects then become preferential sites for the codeposition of Ag and Au atoms derived from the coreduction of both the Ag(I) and Au(III) ions in the reaction solution by EG. Once the defect sites have been filled, the additional Ag and Au atoms will rapidly diffuse across the entire surface of each nanocube for the generation of a nanocube with a relatively thin shell made of a Ag–Au alloy. Afterward, the chemical reduction of HAuCl4 by EG serves as a predominant pathway to generate Au atoms for their deposition on the nanocubes in a manner similar to conventional seeded growth. We also use surface-enhanced Raman scattering to characterize the transformation of the core–shell nanocubes with a Ag-dominated to a Au-enriched outermost surface as the Au shell thickness is increased. The as-obtained Ag@Au core–shell nanocubes with a Au shell of eight atomic layers are stable in aqueous 30% H2O2 for at least 12 h, together with remarkable thermal stability in EG at 110 °C for 6 h.