Visualization of Colloidal Nanocrystal Formation and Electrode-Electrolyte Interfaces in Liquids Using TEM.

Transmission electron microscopy (TEM) has become a powerful analytical tool for addressing unique scientific problems in chemical sciences as well as in materials sciences and other disciplines. There has been a lot of recent interest in the development and applications of liquid phase environmental TEM. In this Account, we review the development and applications of liquid cell TEM for the study of dynamic phenomena at liquid-solid interfaces, focusing on two areas: (1) nucleation, growth, and self-assembly of colloidal nanocrystals and (2) electrode-electrolyte interfaces during charge and discharge processes. We highlight the achievements and progress that have been made in these two topical areas of our studies. For example, tracking single platinum particle growth trajectories revealed that two different pathways of growth, either by monomer attachment or coalescence between nanoparticles, led to the same particle size. With the improved spatial resolution and fast electron detection, we were able to trace individual facet development during platinum nanocube platinum nanocube growth. The results showed that different from the surface energy minimization rule prediction, the growth rates of all low-energy facets, such as {100}, {110}, and {111}, were similar. The {100} facets stopped growth early, and the continuous growth of the rest facets resulted in a nanocube. Density functional theory calculations showed that the amine ligands with low mobility on the {100} facets blocked the further growth of the facets. The effect of the ligand on nanoparticle shape evolution were further studied systematically using a Pt-Fe nanoparticle system by changing the oleylamine concentration. With 20%, 30%, or 50% oleylamine, Pt-Fe nanowires or nanoparticles with different morphologies and stabilities were achieved. Real-time imaging of nanoparticles in solution also enabled the study of interactions between nanoparticles during self-assembly. We further compared the study of noble-metal nanoparticles and transition-metal oxides in a liquid cell to elucidate the nanoparticle formation mechanisms. In the second part of this Account, we review the study of electrolyte-electrode interfaces by the development of electrochemical liquid cell TEM. The formation of single-crystalline Pb dendrites from polycrystalline branches and Li dendrite growth in a commercial electrolyte for Li ion batteries were observed. We also studied lithiation reactions of MoS and Au electrodes. MoS nanoflakes on the Ti electrode underwent irreversible decomposition, resulting in the vanishing of the MoS active nanoflakes. More detailed study using nanobeam diffraction indicated that the MoS nanoflakes were broken down into small nanoparticles as a result of the fast discharge. For the lithiation of Au electrodes, three distinct types of morphology changes during reactions were revealed, including gradual dissolution, explosive reaction, and local expansion/shrinkage. Additionally, we studied electrolyte decomposition reactions such as bubble formation and solid electrolyte interphase formation. At the end, our perspective on the challenges and opportunities in the applications of liquid phase environmental TEM for the study of liquid chemical reactions is provided.