Fei Linfeng, Gan Xianglai, Ng Sheung Mei, Wang Hui, Xu Ming, Lu Wei, Zhou Yanchun, Leung Chi Wah, Mak Chee-Leung, Wang Yu
Department of Applied Physics , The Hong Kong Polytechnic University , Hong Kong SAR, China.
School of Materials Science and Engineering , Nanchang University , Nanchang , Jiangxi 330031 , China.
ACS Nano. 2019 Jan 22;13(1):681-688. doi: 10.1021/acsnano.8b07864. Epub 2018 Nov 30.
The nucleation of crystals from ubiquitous solid-state reactions impacts a wide range of natural and synthetic processes and is fundamental to physical and chemical synthesis. However, the microscopic organization mechanism of amorphous precursors to nanoscale clusters of ordered atoms (nucleus) in an all-solid environment is inaccessible by common experimental probes. Here, by using in situ transmission electron microscopy in combination with theoretical simulations, we show in the reactive formation of a metal carbide that nucleation actually occurs via a two-step mechanism, in which a spinodal-structured amorphous intermediate reorganizes from an amorphous precursor and precedes the emergence of a crystalline nucleus, rather than direct one-step nucleation from classical consideration. We further isolated a series of sophisticated dynamics during formation and development of the nucleus in real-space and interpreted them by thermodynamic favorability. We anticipate that such an indirect organization mechanism which contains a metastable intermedium among the free energy gap between precursors and nanocrystals has its chance in underlying most solid-state crystallizations, whereas the as-established experimental method represents a step forward in exploring fundamentals in chemical reaction, material engineering, etc.
普遍存在的固态反应中晶体的成核影响着广泛的自然和合成过程，是物理和化学合成的基础。然而，在全固态环境中，从无定形前驱体到有序原子的纳米级团簇（晶核）的微观组织机制无法通过常规实验探针进行探究。在这里，通过结合原位透射电子显微镜和理论模拟，我们发现在金属碳化物的反应形成过程中，成核实际上是通过两步机制发生的，其中旋节线结构的无定形中间体从无定形前驱体中重组，并先于晶核出现，而不是从经典角度直接一步成核。我们进一步在实空间中分离出了晶核形成和发展过程中的一系列复杂动力学，并通过热力学有利性对其进行了解释。我们预计，这种在前驱体和纳米晶体之间的自由能间隙中包含亚稳中间体的间接组织机制，在大多数固态结晶过程中都有可能存在；而所建立的实验方法代表了在探索化学反应、材料工程等领域的基础方面向前迈出的一步。
