How Nanoalloys Are Formed: A Case Study of Solution-Phase Synthesis of Au-Ag Nanoalloys Using Co-digestive Ripening

Digestive ripening has emerged as an outstanding solution-based methodology to obtain colloidal bimetallic nanomaterials with excellent control over their size and composition. Solvated metal atom dispersion (SMAD) in conjunction with digestive ripening has been used as a synthetic protocol to derive colloidal gold-silver nanoalloys. A comprehensive study has been carried out to elucidate the mechanistic pathways through which nanoalloying of gold and silver takes place via digestive ripening. The stoichiometric ratio between the individual metal nanoparticles, metal:capping agent ratio, and reaction temperature were found to be the determining factors to realize nanoalloying. A detailed mechanism of alloying in the nanosize regime has been established by examining the intermediate species using a combination of UV-visible spectroscopy and transmission electron microscopy (HAADF-STEM technique, point EDS analysis, and elemental line scanning technique). From this study, important insights into the nature of the "building blocks" of nanoalloys could be obtained, which augments our understanding of nanoscale interatomic diffusion processes that dictate the formation of various nanostructured materials, e.g., core-shell structures, alloys, and intermetallics. In addition, the functional group present in the capping agent molecules was found to be influential in determining the structure of bimetallic nanomaterials obtained via digestive ripening.