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

Abstract

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 react...