Observing the Growth of Pb3O4 Nanocrystals by in Situ Liquid Cell Transmission Electron Microscopy.

Understanding the growth behaviors of nanomaterials during liquid-phase synthesis will be beneficial in designing and applying many functional nanodevices. However, the growth pathways regarding the nanocrystal facet development remain largely unknown as direct observation is lacking. Herein, the in situ study of Pb3O4 nanocrystals' growth is reported by using the liquid cell transmission electron microscopy with high spatial and temporal resolution. The findings indicate that Pb3O4 nanocrystals' growth follows distinct trajectories with shape evolution when the growth pathways are varied. Three growth pathways are observed, including the monomer growth of Pb3O4 nanocrystals, the coalescence growth of four stationary Pb3O4 nanocrystals, and the oriented attachment growth of Pb3O4 nanocrystal pairs and multiple randomly dispersed Pb3O4 nanocrystals. It is the first observation that Pb3O4 nanocrystals with a regular quadrilateral shape are formed, in which nanocrystal facets preferentially grow along the [002] direction of Pb3O4. Theoretical analysis confirms in this study that the surface energy and physical driving force play key roles in the growth of nanocrystals in a liquid. Such understanding of the growth pathways and quantification of formation kinetics are important for the design of hierarchical nanomaterials and the control of nanocrystal self-assembly for functional devices.