In Situ Atomic-Scale Observation of Surface Tension Induced Structural Transformation of Ag-NiP_x Core-Shell Nanocrystals

The properties of nanocrystals are highly depending on their morphology, composition and structure. Tailored synthesis over these parameters is successfully applied for the production of nanocrystals with desired properties for specific applications. However, in order to get a full control over the properties, the behavior of nanocrystals under external stimuli and application conditions needs to be understood. Herein, using Ag-NiPx nanocrystals as a model system, we investigate the structural evolution upon thermal-treatment by in situ aberration-corrected scanning transmission electron microscopy (STEM). A combination of real-time imaging with elemental analysis enables the observation of the transformation from a Ag-NiPx core-shell configuration to a Janus structure at the atomic scale. The transformation occurs through de-wetting and crystallization of the NiPx shell and is accompanied by surface segregation of Ag. Further temperature increase leads to a complete sublimation of Ag, and formation of individual Ni12P5 nanocrystals. The transformation is rationalized by theoretical modelling based on density functional theory (DFT) calculations. Our model suggests that the transformation is driven by changes of the surface energy of NiPx and the interfacial energy between NiPx and Ag. The here presented direct observation of atomistic dynamics during thermal-treatment induced structural modification will help to understand more complex transformations that are induced by ageing over time or the interaction with a reactive gas phase in applications such as catalysis