ID-12-P-2797 In situ Study of Oxidative Etching of Palladium Nanocrystals by Liquid Cell Electron Microscopy

      The development of liquid flow holder incorporated within a transmission electron microscope (TEM) enables the possibility to observe the size and shape change of nanocrystals in liquid through the electron-transparent viewing window^[1,2]. In this talk, we will report our observations on the dissolution process of palladium (Pd) nanocubes within a TEM (FEI Tecnai F20).

      Figure 1A shows time consequential HAADF-STEM images of a dissolving palladium nanocube during the oxidative etching process. At the beginning, the nanocube exhibited well-defined cubic structure terminated with {100} facets (Figure 1A, 0 s). As the chemical etching proceeded, the nanocube started to shrink firstly from its apexes and edges rather than the side facets, and then transformed itself into a round-shaped NC. This was an energy-favored process on considering the fact that those atoms located at the apexes and on the edges have lower coordination and therefore possess higher chemical reactivity. The mechanism of oxidative etching here was similar to that for conventional synthesis by introducing Br^- ions and O₂, where palladium was oxidized to Pd²⁺ ions. The introduction of Br^- ions could further promote such an oxidizing reaction by forming the complexes such as [PdBr4]^2-. Here instead of dissolved oxygen in the solution, the oxidizing agents such as OH•, HO2•, O could be produced in the liquid solution via the interaction between high-energy electrons and the water molecules. The whole process can be summarized as illuminated in Figure 2.

References
(1) Niels de Jonge^*, Diana B. Pekys et al. PNAS. 2008, VOL 106, NO. 7, 2159-2164
(2) Edward R. White, Scott B. Singer, Brian C. Regan^* et al. ACS NANO. 2012, VOL 6, NO. 7, 6308-6317