The properties of nanocrystals are highly dependent 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 obtain full control over the properties, the behavior of nanocrystals under external stimuli and application conditions needs to be understood. Herein, using Ag-NiP x nanocrystals as a model system, we investigate the structural evolution upon thermal treatment by in situ aberration-corrected scanning transmission electron microscopy. A combination of real-time imaging with elemental analysis enables the observation of the transformation from a Ag-NiP x core-shell configuration to a Janus structure at the atomic scale. The transformation occurs through dewetting and crystallization of the NiP x 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 modeling based on density functional theory calculations. Our model suggests that the transformation is driven by changes of the surface energy of NiP x and the interfacial energy between NiP x and Ag. The direct observation of atomistic dynamics during thermal-treatment-induced structural modification will help to understand more complex transformations that are induced by aging over time or the interaction with a reactive gas phase in applications such as catalysis.
Keywords: Ag-NiPx nanocrystal; atomic-scale observation; in situ STEM; structural transformation; surface energy; theoretical modeling.