Cooperative interactions among CTA⁺, Br^– and Ag⁺ during seeded growth of gold nanorods

We have carried out a comprehensive study on the formation mechanism of Au nanorods (AuNRs) in binary surfactant mixtures composed of quaternary ammonium halide and sodium oleate (NaOL). We identify the cetyltrimethyl ammonium (CTA)-Br-Ag⁺ complex as the key ingredient in directing the anisotropic growth of AuNRs. Based on the improved understanding of the cooperative interactions among CTA⁺, Br^– and Ag⁺, we further demonstrate that AgBr, which is readily solubilized by the cetyltrimethyl ammonium bromide (CTAB) or cetyltrimethyl ammonium chloride (CTAC) micelles, can be employed as the combined source of Ag⁺ and Br^– for the preparation of AuNRs. The growth of high-quality AuNRs can be completed within 15 min under extremely low bromide content (0.1 mM).