Amorphous-Phase-Mediated Crystallization of Ni Nanocrystals Revealed by High-Resolution Liquid-Phase Electron Microscopy.

Nonclassical features of crystallization in solution have been recently identified both experimentally and theoretically. In particular, an amorphous-phase-mediated pathway is found in various crystallization systems as an important route, different from the classical nucleation and growth model. Here, we utilize high-resolution in situ transmission electron microscopy with graphene liquid cells to study amorphous-phase-mediated formation of Ni nanocrystals. An amorphous phase is precipitated in the initial stage of the reaction. Within the amorphous particles, crystalline domains nucleate and eventually form nanocrystals. In addition, unique crystallization behaviors, such as formation of multiple domains and dislocation relaxation, are observed in amorphous-phase-mediated crystallization. Theoretical calculations confirm that surface interactions can induce amorphous precipitation of metal precursors, which is analogous to the surface-induced amorphous-to-crystalline transformation occurring in biomineralization. Our results imply that an unexplored nonclassical growth mechanism is important for the formation of nanocrystals.