Transition metal ion-assisted synthesis of monodisperse, quasi-spherical gold nanocrystals via citrate reduction

Abstract

We demonstrate that monodisperse, quasi-spherical gold nanocrystals (Au NCs) can be produced via quick addition of an aqueous solution of sodium citrate containing Ag⁺ and Fe²⁺ or Cu⁺ ions into a boiling aqueous solution of HAuCl₄. The size of the resulting Au NCs increases from 12 nm to 60 nm with the decrease of citrate concentration. Compared with those obtained via a conventional Frens method, the NCs obtained via our approach have a fairly narrow distributed size and spherical shape. The significant improvement in the size polydispersity and shape sphericity is a result of the synergic effect of Ag⁺ and Fe²⁺ (or Cu⁺) ions. During the formation of Au NCs, the Ag⁺ ions are utilized to accelerate the nucleation via catalysis of the oxidation of citrate and regulate the NC isotropic growth, while the Fe²⁺ (or Cu⁺) ions significantly accelerate the reduction of Au³⁺ ions to Au⁰ atoms to speed up the nucleation when the concentration of citrate and silver ions was reduced to below 6.0 × 10⁻³ wt% and 8.5 × 10⁻⁴ wt%, respectively.