Atomic Scale Mechanisms Underlying Thermal Reshaping of Anisotropic Gold Nanocrystals Revealed by in Situ Electron Microscopy

Abstract

Understanding the shape transformation processes of anisotropic nanocrystals is an essential step toward fully exploiting their remarkable shape-dependent properties. Here, we employ environmental transmission electron microscopy with controlled heating to directly visualize the thermal reshaping dynamics of gold nanorods and triangular nanoplates at the atomic level. Upon heating above 180 degrees C in 1 mbar of oxygen, thiol ligand molecules that are stabilizing these metastable nanostructures desorb to allow the subsequent migration of surface atoms. As a consequence, nanorods undergo rounding into ellipsoids, with indiscriminate surface migration of vertex atoms to the sides being mediated by the development of multiple high-index facets. By contrast, triangular nanoplates exhibit truncation of their vertices, which proceeds via selective layerby-layer migration of vertex atoms to the triangular faces until a hexagonal shape is attained. Although the thermodynamic driving force is the minimization of the fraction of undercoordinated surface atoms, with the final structure being an isotropic sphere, both surface curvature and surface faceting play a role in determining the pathway to be taken. The peculiar morphology of triangular nanoplates allowed the transition to hexagonal nanoplates to occur with the structure remaining enclosed by stable {111} facets on all faces. Our results demonstrate how in situ electron microscopy can afford fundamental insights into the reshaping mechanisms of anisotropic nanocrystals.