Precise three-dimensional (3D) atomic structure determination of individual nanocrystals is a prerequisite for understanding and predicting their physical properties. Nanocrystals from the same synthesis batch display what are often presumed to be small but possibly important differences in size, lattice distortions, and defects, which can only be understood by structural characterization with high spatial 3D resolution. We solved the structures of individual colloidal platinum nanocrystals by developing atomic-resolution 3D liquid-cell electron microscopy to reveal critical intrinsic heterogeneity of ligand-protected platinum nanocrystals in solution, including structural degeneracies, lattice parameter deviations, internal defects, and strain. These differences in structure lead to substantial contributions to free energies, consequential enough that they must be considered in any discussion of fundamental nanocrystal properties or applications.
Nucleation in atomic crystallization remains poorly understood, despite advances in classical nucleation theory. The nucleation process has been described to involve a nonclassical mechanism that includes a spontaneous transition from disordered to crystalline states, but a detailed understanding of dynamics requires further investigation. In situ electron microscopy of heterogeneous nucleation of individual gold nanocrystals with millisecond temporal resolution shows that the early stage of atomic crystallization proceeds through dynamic structural fluctuations between disordered and crystalline states, rather than through a single irreversible transition. Our experimental and theoretical analyses support the idea that structural fluctuations originate from size-dependent thermodynamic stability of the two states in atomic clusters. These findings, based on dynamics in a real atomic system, reshape and improve our understanding of nucleation mechanisms in atomic crystallization.
Defining the redox
activity of different surface facets of ceria
nanocrystals is important for designing an efficient catalyst. Especially
in liquid-phase reactions, where surface interactions are complicated,
direct investigation in a native environment is required to understand
the facet-dependent redox properties. Using liquid cell TEM, we herein
observed the etching of ceria-based nanocrystals under the control
of redox-governing factors. Direct nanoscale observation reveals facet-dependent
etching kinetics, thus identifying the specific facet ({100} for reduction
and {111} for oxidation) that governs the overall etching under different
chemical conditions. Under each redox condition, the contribution
of the predominant facet increases as the etching reactivity increases.
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