Haimei Zheng
Lawrence Berkeley National Laboratory
Seminar: Noon, Thursday, March 19th, Koch Institute Conference Room (76–156)

An understanding of how materials grow and transform in their working environments is essential to the development of functional materials for various applications. My group studies a variety of physical and chemical processes of materials in liquids including gases by the development and applications of in situ transmission electron microscopy (TEM). In this talk, I will first present our recent study of shape evolution of colloidal nanocrystals in a liquid cell using TEM. An understanding of nanocrystal shape controlling mechanisms is significant since nanoparticle catalytic and other surface-enhance properties are highly dependent on their shape. However, how facets develop during growth is largely unknown due to the lack of direct observation. Using in situ liquid cell TEM, we have been able to identify unique growth mechanisms of nanocrystals and have discovered rules applied to bulk systems break down at the nanoscale. For instance, the facet development of Pt nanocubes during growth drastically differs from what is predicted by the widely accepted surface energy minimization rule, i.e., Wulff construction. We found that the growth rates of the {100}, {110} and {111} facets are similar until the {100} facets stop growth. Subsequently, the {110} facets are eliminated when two adjacent {100} facets meet. Eventually, the growth of {111} facets fills the corners to complete a nanocube. Theoretical calculation suggests oleylamine ligand mobility on the facet is likely responsible for the arresting of {100} growing facets. In the second part of the talk, I will present the in situ study of electrochemical processes for battery applications using electrochemical liquid cells. Dissolution-deposition at the electrode-liquid electrolyte interfaces including phase transformations of the precipitates during charge cycles in a Li‑S system will be discussed