Single Nanocrystal Etching by 4D Graphene Liquid Cell Electron Microscopy

Etching of nanocrystals is a process in which surface-exposed atoms on nanocrystals are detached in specific chemical environments, predominating the structures and properties of nanocrystals, and occurs ubiquitously when nanocrystals are applied in applications such as thermal catalysis, electrochemical energy conversion, and drug delivery. However, surface structures of chemically-etched nanocrystals can differ from those of the bulk due to the high population of less-coordinated surface atoms and their preferential detachment during the etching. Structural transformations such as surface strains and phase transitions are predominant in crystals as small as a few nanometers and may further affect the structures of nanocrystals during etching. Thus, the structural changes in nanocrystal surfaces during etching should be understood beyond the scope established for bulk crystals. Here, we introduce a new method for investigating the atomic structure changes of single nanocrystals in liquid, namely “4D Brownian one-particle reconstruction” method, using graphene liquid cell (GLC) electron microscopy. We applied this method to track the atomic structures of Pt nanocrystals during their etching in an oxidative liquid medium. The reconstructed time-resolved three-dimensional (3D) atomic maps reveal that the Pt nanocrystals undergo a transformation from the ordered face-centered cubic (fcc) to a disordered structure. The transformation initiates on the surface via emerging hexagonal closed packed (hcp)-like disordered states, stabilizing less-coordinated surface-exposed atoms during the etching.