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 (∼0.84 nm) in the presence of PAA-Na. These clusters, which correspond to the size of the Au₁₃ cluster, coalesce to form nanocrystals. Our molecular dynamics simulations suggest that Au₁₃ 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 Au₁₃ 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.