Huang Xing, Liu Zhongqiang, Millet Marie-Mathilde, Dong Jichen, Plodine Milivoj, Ding Feng, Schlögl Robert, Willinger Marc-Georg
Department of Heterogeneous Reactions , Max Planck Institute for Chemical Energy Conversion , 45470 Mülheim an der Ruhr , Germany.
Department of Inorganic Chemistry , Fritz Haber Institute of Max Planck Society , Faradayweg 4-6 , 14195 Berlin , Germany.
ACS Nano. 2018 Jul 24;12(7):7197-7205. doi: 10.1021/acsnano.8b03106. Epub 2018 Jun 26.
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 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 core-shell configuration to a Janus structure at the atomic scale. The transformation occurs through dewetting and crystallization of the NiP shell and is accompanied by surface segregation of Ag. Further temperature increase leads to a complete sublimation of Ag and formation of individual NiP 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 and the interfacial energy between NiP 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.
纳米晶体的性质高度依赖于它们的形态、组成和结构。通过对这些参数进行定制合成，已成功应用于生产具有特定应用所需特性的纳米晶体。然而，为了完全控制其性质，需要了解纳米晶体在外部刺激和应用条件下的行为。在此，我们以Ag-NiP纳米晶体为模型系统，通过原位像差校正扫描透射电子显微镜研究热处理后的结构演变。实时成像与元素分析相结合，能够在原子尺度上观察从Ag-NiP核壳结构到Janus结构的转变。这种转变通过NiP壳层的去湿和结晶发生，并伴随着Ag的表面偏析。进一步升高温度会导致Ag完全升华并形成单个NiP纳米晶体。基于密度泛函理论计算的理论模型对这种转变进行了合理化解释。我们的模型表明，这种转变是由NiP表面能以及NiP与Ag之间的界面能变化驱动的。对热处理诱导的结构改性过程中原子动力学的直接观察，将有助于理解诸如老化或在催化等应用中与反应气相相互作用随时间引发的更复杂转变。
