Understanding Particle-Mediated Growth Pathways by using Advanced Transmission Electron Microscopy and X-Ray Scattering Techniques

Particle aggregation involves the phenomenon of oriented attachment (OA), where crystalline particles join together by attaching to specific crystal faces with lattice matching, is a prevalent mechanism in crystal growth and has been extensively utilized in the development of hierarchically structured crystalline materials, which has been applied to catalysis, energy storage, environmental conservation, biological medicine, etc. Using a combination of advanced transmission electron microscopy (TEM) and small-angle X-ray scattering/wide-angle X-ray scattering (WAXS) techniques and computational methods, we are investigating these phenomena for several metal oxide systems and their relationship to interfacial structure in vacuum and water vapor. Here we discuss three cases. In the first, we use high-resolution TEM and scanning TEM (STEM) to explore the aggregation behaviors of hematite nanocrystals with different exposed facets including {001}, {012}, {104} and {116}. The experimental data indicates that hematite nanoparticles aggregation-based crystallization is orientation dependent (along the [001] direction), not relies on exposed facet. We then compare the results to the predictions of density functional theory (DFT) to relate the behavior to surface interactions. In the second case, we report the formation of gibbsite mesocrystals in pure water. By evaporating the suspension of monodisperse gibbsite nanoplates with a diameter of around 100 nm, plate-like mesocrystals with a diameter of up to a hundred micrometers were formed. The single crystal XRD pattern and SAXS/WAXS of the mesocrystals matched that of bulk gibbsite crystal along the specific zone axis, indicating well-aligned particles in the monocrystals. Further analysis through scanning electron microscopy (SEM) and TEM revealed a hexagonal columnar superstructure. Moreover, the <i>in situ</i> liquid phase TEM was conducted and observed the OA of a few gibbsite nanoplates in pure water, and the result matched well with ex <i>situ</i> characterizations. The MD simulation was used to investigate the energy-structure relationship for the sliding motion of two co-planar gibbsite nanoplatelets along the (010) direction. The results indicated that to obtain the first perfectly aligned configuration, sliding in the (010) direction exhibits a smaller energy barrier than sliding in the (100) direction (0.89 vs. 2.49 kcal/mol). In the third case, the measurement of anisotropic forces between rutile TiO₂ (001) nanocrystals as a function of their azimuthal orientation and surface hydration extent using a combined environmental TEM-atomic force microscopy (AFM) technique. At tens of nanometers of separation, the attractive forces are weak and show no dependence on azimuthal alignment nor surface hydration. At separations of approximately one hydration layer, attractive forces are strongly dependent on azimuthal alignment and systematically decrease as intervening water density increases. Measured forces closely agree with predictions from Lifshitz theory and show that dispersion forces are capable of generating a torque between particles to align them.