Mechanistic Roles of Hydroxide in Controlling the Deposition of Gold on Colloidal Silver Nanocrystals

This article describes a systematic study of the roles played by hydroxide in controlling the deposition of Au on Ag nanocubes for the fabrication of diversified Ag–Au bimetallic nanocrystals. The synthesis simply involves the titration of aqueous HAuCl₄ into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (H₂Asc), NaOH, and poly­(vinylpyrrolidone) at room temperature. The OH^– ions from NaOH can affect the reduction kinetics of the Au­(III) precursor in a number of ways and thereby the deposition pathways of the Au atoms. First of all, the OH^– can accelerate the reduction kinetics by neutralizing H₂Asc into ascorbate monoanion (HAsc^–), the true player behind the reduction power of ascorbic acid. Second, the OH^– can neutralize the added HAuCl₄ and progressively transform AuCl₄^– into AuCl₃(OH)⁻, AuCl₂(OH)₂^–, AuCl­(OH)₃^–, or Au­(OH)₄^– through ligand exchange, generating Au­(III) precursors with increasingly lower reduction potentials and thus lower probability for galvanic replacement reaction with Ag nanocubes than AuCl₄^–. Third, the OH^– can react with the Ag⁺ ions released from the galvanic reaction to generate Ag₂O patches at the corners of Ag nanocubes. Our results indicate that the deposition of Au on Ag nanocubes can follow two distinct pathways depending on the initial pH of the reaction solution. When the initial pH is controlled in the range of 10.3–11.9, the reduction of Au­(III) is initiated by Ag nanocubes but dominated by HAsc^– afterward, leading to the formation of Ag@Au core-frame and then core–shell nanocubes. In contrast, if the initial pH is controlled in the range of 3.2–4.8, both the galvanic replacement with Ag nanocubes and the chemical reduction by HAsc^– contribute to the conversion of Au­(III) to Au atoms. The Ag⁺ ions released from the galvanic replacement can also be reduced by HAsc^– to transform Ag nanocubes into Ag@Ag–Au concave nanocubes with hollow interiors and alloyed walls.