Nanoscale material transformations, including nucleation, growth, dissolution, and phase transitions, correlated with heterogeneity, fluctuations, and structural ordering, are essential to many physical and chemical processes of materials. There has been significant interest in studying nanoscale material transformations in real-time. Understanding and further controlling them allow the opportunity to tailor materials' physical and chemical properties and facilitate the discovery of novel materials. In-situ transmission electron microscopy (TEM) is the most suitable technique for direct observation of material dynamics and nanoscale materials transformations with the necessary spatial and temporal resolution. However, imaging nanoscale materials transformations through liquids with transmission electron microscopy has been a significant challenge due to the fast dynamics and the difficulties in maintaining sufficient resolution while imaging. With our development of ultrathin-thin liquid cells, we are able to reveal materials' growth, dissolution, and phase transition with atomic resolution at various temperatures. My research contains three components centered on the development and applications of different types of liquid cells. Specifically, I apply these methods of development to the following three projects.
(1) Investigating the stacking order and phase transition is significant for the property and application of van der Waals layered nanocrystals. However, direct observation and microscopic understanding are still challenging. Here we have observed a temperature-dependent phase transition in the layered InCl3 nanocrystal with atomic resolution, which is enabled by our modified in-situ liquid cell TEM techniques applied at varying temperature. Under cryogenic temperature, InCl3 exhibits multiple stacking orders. Compared to the InCl3 crystal maintaining the phases during the process at 0 °C, the InCl3 crystal shows a distinct pathway during phase transition at room temperature. Layer-by-layer growth is also observed at the reaction front. This study provides microscopic insights into the transformation process of InCl3 and thus sheds light on the controllable synthesis and property tuning of layered materials for advanced applications.
(2) As a catalyst for CO2 reduction and other applications, Cu metal nanostructures have attracted extensive research interest because it is the only metal capable of converting CO2 into multi carbon-based compounds. Controlling the structure and morphology of copper nanostructures during the preparation process is crucial for enabling the functional design of materials. Here, we have investigated the effect of different temperatures on the morphology of Cu nanoparticles during electrochemical deposition. We find that reducing the temperature can significantly affect the size, nucleation density, and crystallinity of Cu nanoparticles. Electrodeposition at low temperature (-20 ºC) produced clusters of small copper nanoparticles, which are distinct from the large highly crystalline copper nanoparticles (~16 nm) obtained from the room temperature process. After analysis, we find that the differences in the morphology and crystallinity of the Cu nanoparticles are caused by changes in the reduction reaction rate and surface diffusion. The limitation of the reaction rate promotes the formation of multi nuclei, and the slow surface diffusion leads to the poor crystallinity of the particles. This work contributes to our understanding of low-temperature electrochemical processes and facilitates the design of catalytic materials with hierarchical structures.
(3) Designing corrosion-resistant materials requires a complementary understanding of corrosion mechanisms at multiple scales. Atomic insights into the corrosion behavior of metals are thus critical. Here, we have studied the pitting corrosion of a Sn nanocrystal with a nanoscale protection layer and the uniform corrosion of Sn metal in real-time using liquid cell TEM. Our observation shows that pitting corrosion starts from defective areas and pitting in different regions merges as the etching area becomes larger. The isotropic etching occurs first, followed by anisotropic etching during the pitting corrosion process. For uniform corrosion, surface diffusion layer and “creeping-like” etching behavior are observed in the thin liquid region. Understanding these corrosion behaviors and dynamics provides important fundamental insight connecting nanocrystal crystalline structure to the development of kinetically stabilized surface features and demonstrates the importance of developing new materials with corrosion resistance.
