Monodisperse Colloidal Metal Nanoparticles to Core–Shell Structures and Alloy Nanosystems via Digestive Ripening in Conjunction with Solvated Metal Atom Dispersion: A Mechanistic Study

Abstract

Co-digestive ripening of two distinct metal nanoparticles is an exceptional method for the colloidal synthesis of core–shell heterostructures. In this report, a detailed investigation of the underlying mechanism by which surfactant molecule assisted interatomic transfer between two metal nanoparticles occurs has been described using gold/silver as a model system. Core–shell nanoparticles with gold in the core and silver in the shell in the size regime of 6.9 ± 1.8 nm were obtained by conducting the codigestive ripening of polydispersed particles of Au–pentanone and Ag–pentanone colloids in the presence of hexadecylamine as a capping agent used in the molar ratio of 1:30 with respect to metal. The progress of the formation of core–shell nanoparticles has been monitored using UV–visible spectroscopy and transmission electron microscopy. Detailed analysis of the nature of Au@Ag core–shell nanoparticles has been done with the help of HAADF-STEM technique, point-EDS analysis, elemental line scanning technique, and X-ray photoelectron spectroscopy. Variation of experimental conditions such as concentration of capping agent, molar ratio of Au and Ag, and temperature in the codigestive ripening process led to the realization that core–shell nanoparticles with thicker shell of silver can be obtained under certain reaction conditions. Alteration of the reaction conditions was also noted to affect the final Au@Ag nanoparticles with respect to average particle size and polydispersity. The as-prepared Au@Ag nanoparticles could be transformed to Au–Ag nanoalloys on being exposed to ultraviolet radiation of 254 nm. We have also attempted to elucidate the factors which dictate the formation of core–shell nanoparticles by comparing with the findings of theoretical studies from the literature with our experimental results.