Revealing Au13 as Elementary Clusters During the Early Formation of Au Nanocrystals

Understanding the formation mechanism of nanocrystals in solution is fundamental to the development of materials science. For a metal nanocrystal, the cluster-mediated formation mechanism is still poorly understood. In particular, identifying what types of clusters are dominant and how they evolve into a nanocrystal in the early nucleation stage remains a great challenge. Here, using liquid-cell transmission electron microscopy, we directly observe the formation of ultrasmall Au clusters (similar to 0.84 nm) in the presence of PAA-Na. These clusters, which correspond to the size of the Au-13 cluster, coalesce to form nanocrystals. Our molecular dynamics simulations suggest that Au, in an aqueous environment has greater stability when compared to other cluster sizes and provide atomistic details of growth by cluster coalescence. Collectively, our demonstration of Au-13 as the dominant species with an elaboration of their coalescence kinetics sheds light on nonclassical nanocrystal formation mechanisms and offers useful guidelines for designing innovative pathways for the synthesis of nanomaterials.

Automated summary

This study directly observed the formation of ultra-small Au clusters in the presence of PAA-Na, which coalesce to form nanocrystals corresponding to the size of Au-13 cluster. Molecular dynamics simulations revealed that gold has greater stability in an aqueous environment compared to other cluster sizes, providing atomistic details of growth by cluster coalescence.