posted on 2018-06-20, 00:00authored byXing Huang, Zhongqiang Liu, Marie-Mathilde Millet, Jichen Dong, Milivoj Plodine, Feng Ding, Robert Schlögl, Marc-Georg Willinger
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-NiPx 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-NiPx core–shell configuration to a Janus structure at the
atomic scale. The transformation occurs through dewetting 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 modeling based on density functional theory 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 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.