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Understanding the mechanism of nucleation and growth of nanocrystals is crucial for designing and regulating the structure and properties of nanocrystals. However, the process from molecules to nanocrystals remains unclear because of the rapid and complicated dynamics of evolution under reaction conditions. Here, the complete evolution process of solid‐phase chloroplatinic acid during the electron beam irradiation triggered reduction and nucleation of platinum nanocrystals is recorded. Aberration‐corrected environmental transmission electron microscopy is used for direct visualization of the dynamic evolution from H2PtCl6 to Pt nanocrystals at the atomic scale, including the formation and growth of amorphous clusters, crystallization, and growth of clusters, and the ripening of Pt nanocrystals. At the first two stages, there exists a critical size of ≈2.0 nm, which represents the start of crystallization. Crystallization from the center and density fluctuation are observed in the second stage of the crystallization of a few clusters with a size obviously larger than the critical size. The work provides valuable information to understand the kinetics of the early stage of nanocrystal nucleation and crystallization at atomic scale. [ABSTRACT FROM AUTHOR]

The electronic metal-support interaction (EMSI) of heterogenous catalysts has attracted much attention for its enhancing electrocatalytic efficiency by impacting adsorption and desorption energy of reaction intermediates. Herein, we synthesize the monodispersed single-crystal Pt nanoparticles (~3 nm) decorated on MoS 2 nanosheets with 2 H phase (Pt-MoS 2) by a universal wet chemical method. The morphology and structure characterizations show that the loaded Pt single-crystals are approximately half truncated octahedral shapes enclosed by {1 1 1} and {2 0 0} facets. Benefitting from interfacial electronic structure modulation, the Pt-MoS 2 exhibits a lower overpotential (67.4 mV at the current desity of 10 mA·cm−2), smaller Tafel slope (76.2 mV·dec−1), and robuster HER durability than those of pristine MoS 2 nanosheets and prepared Pt NPs. The theoretical simulations further reveal that the Pt atom at the interface, which belong to {2 0 0} facets and adjoin S atoms, own the smallest hydrogen adsorption free energy by manipulating the d -band center of the Pt metal surface. This work provides crystalline structure and electronic configuration engineering strategy towards to design and develop of metal-support heterostructure with high HER activity and stability. A heterostructure of Pt nanoparticles supported by MoS 2 nanosheets (Pt-MoS 2) is fabricated as a hydrogen evolution reaction (HER) electrocatalyst with high HER activity and stability. The enhanced performance is proved to be ascribed to the interfacial electronic structure modulation. [Display omitted] • HER performance of Pt-MoS 2 heterostructure is significantly enhanced. • Atomic-level structure characterization of Pt-MoS 2 is investigated. • The enhanced catalysis activity is attributed to the interfacial electronic structure modulation. [ABSTRACT FROM AUTHOR]