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.

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Active sites and catalytic activity of heterogeneous catalysts is determined by their surface atomic structures. However, probing surface structure at atomic resolution is difficult especially for solution ensembles of catalytic nanocrystals which consist of heterogeneous particles with irregular shapes and surfaces. Here, we constructed 3D maps of coordination number (CN) and generalized CN ( CN ) for individual surface atoms of sub-3 nm Pt nanocrystals. Our results reveal that the synthesized Pt nanocrystals are enclosed by islands of atoms with non-uniform shapes that lead to complex surface structures, including a high ratio of low-coordination surface atoms, reduced domain size of low-index facets, and various types of exposed high-index facets. 3D maps of CN are directly correlated to catalytic activities assigned to individual surface atoms with distinct local coordination structures, which explains the origin of high catalytic performance of small Pt nanocrystals in important reactions such as oxygen reduction reaction and CO electro-oxidation.

The 3D atomic arrangements of materials determine the free energy landscape, thus governing the physical and catalytic properties of those materials. The 3D structures of nanoparticles can deviate from the periodic atomic arrangement of their bulk counterparts due to the dominance of surface dangling bonds, defects, and dislocations. One approach to understand the structure of nanoparticles and their resulting unique properties involves precise probing of the 3D positions of all constituent atoms of individual nanoparticles. The 3D electron tomography and Brownian one particle reconstruction allow investigation of the 3D atomic positions of nanoparticles. Both methods use transmission electron microscopy (TEM) or scanning TEM (STEM) images of nanoparticles with different projection angles and collect their phase information in reciprocal space to reconstruct the 3D structure of the particles. The thus‐reconstructed 3D maps of metal nanoparticles are highly resolved, facilitating the determination of their atomic coordinates. Grain boundary, dislocation, and lattice expansion are observed on the 3D atomic maps. On the basis of the 3D atomic maps, the physical properties of individual nanoparticles can be accurately predicted, enabling purpose‐driven synthesis.