Understanding and Controlling the Chemistry in Liquid Cell Electron Microscopy to Study Nanocrystal Shape Transformations

Nanomaterials exhibit unique size-dependent properties, and colloidal, metallic nanocrystals have been utilized in catalytic, biomedical, and optical applications. Observing the dynamics of nanocrystals in their native liquid environment requires the high spatial resolutionof the electron microscope and encapsulation using a thin membrane such as graphene. By understanding and controlling the chemistry during liquid cell electron microscopy reactions, fundamental insights about nanoscale processes can be elucidated to improve colloidalsynthesis of nanocrystals.Oxidative etching of gold nanocrystals using a combination of preloaded FeCl3 and electron beam-generated radiolysis products was used as a model system for learning about the chemistry of the liquid cell environment. By collecting etching trajectories of multiplenanocubes and rhombic dodecahedra, with {100} and {110} facets respectively, as they transition through intermediate {hk0} facets, the initial FeCl3 concentration was found to control the chemical potential of the etching process. Using Monte Carlo simulations to help understand the trajectories, the chemical potential was found to change the intermediate facets when etching from cubes but not rhombic dodecahedra due to differences in coordination of inner surface atoms.The electron beam generates highly reactive radiolysis products during liquid cell imaging, so determining the electron beam's role on the chemistry of the liquid environment is useful for experimental design and reproducibility. Following the volume trajectories ofetching nanocrystals, it was found that the electron beam dose rate controls the etching rate independently of the FeCl3 control of the chemical potential. Further, at low dose rates, the electron beam generates sacrificial reductant hydrogen gas bubbles that prevent premature etching. Ex situ etching experiments using FeCl3 confirmed the dynamics observed in theliquid cell and provided additional insight into the liquid cell environment. Additional in situ TEM observations of the nanoscale Kirkendall effect of silver nanoparticles and electron stimulated desorption of NaCl nanocubes shows the wide range of processes that can be investigated. Although the chemistry of these liquid cell processes are not as well understood as the gold nanocrystal etching, useful insights about structural transformations were extracted.Finally, an undergraduate research program for first-year students was designed and implemented using liquid cell TEM data. The structure of the program is presented with feedback from the participants showing quantifiable gains in self-identication as scientist