An understanding of how facets of a nanocrystal develop is critical for controlling
nanocrystal shape and designing novel functional materials. However, the atomic pathways
of nanocrystal facet development are mostly unknown because of the lack of direct
observation. We report the imaging of platinum nanocube growth in a liquid cell using
transmission electron microscopy with high spatial and temporal resolution. The growth
rates of all low index facets are similar until the {100} facets stop growth. The
continuous growth of the rest facets leads to a nanocube. Our calculation shows that
the much lower ligand mobility on the {100} facets is responsible for the arresting of {100}
growing facets. These findings shed light on nanocrystal shape-control mechanisms
and future design of nanomaterials.