Structure Matters – Direct In‐situ Observation of Cluster Nucleation at Atomic Scale in a Liquid Phase

The formation of almost all solid crystalline materials starts with a nucleation reaction, where a handful of atoms come together to form the initial crystal seed, which then grows to a larger crystal. Thus, understanding and controlling nucleation is essential for the synthesis and manufacturing of most material systems, especially nanomaterials. Despite this, little is known from direct experimental observations about the initial steps of nucleation, the formation of sub‐nanometer sized clusters. Here, we directly study the atomic nucleation reactions of such sub‐nm clusters of Pt in‐situ in a liquid phase. We do this by inducing nucleation in suspended nanofilms and supported nanodroplets of an ionic liquid (1‐butyl‐3‐methyl imidazolium chloride, 5–50 nm thickness), which is observed at atomic resolution by scanning transmission electron microscopy. We can observe Pt atoms to nucleate into few‐atom clusters, which coalesce and grow into cluster agglomerates or nanoparticles, or redissolve. When comparing nucleation in nanofilms and carbon‐supported nanodroplets, nucleation is rarely observed in nanofilms, while easily observable at high nucleation rate in nanodroplets. This is due to the presence of the pre‐existing liquid‐solid interface, resulting in heterogeneous nucleation in the nanodroplets while there is only homogeneous nucleation in the nanofilms. Ultimately, our results show that the nucleation pathways of nanoparticles are not just determined by the local chemical environment, but are also influenced by size and structure of the initially formed clusters.