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.