  We determined a full 3D atomic structure of a dumbbell-shaped Pt nanoparticle
formed by a coalescence of two nanoclusters using deep learning assisted atomic
electron tomography. Formation of double twin boundary was clearly observed at
the interface, while substantial anisotropy and disorder were also found
throughout the nanodumbbell. This suggests that the diffusion of interfacial
atoms mainly governed the coalescence process, but other dynamic processes such
as surface restructuring and plastic deformation were also involved. A full 3D
strain tensor was clearly mapped, which allows direct calculation of the oxygen
reduction reaction activity at the surface. Strong tensile strain was found at
the protruded region of the nanodumbbell, which results in an improved
catalytic activity on {100} facets. This work provides important clues
regarding the coalescence mechanism and the relation between the atomic
structure and catalytic property at the single-atom level.
