Growing Mesocrystals Through Nanoplatelet Oriented Sliding and Attachment

Pacific Northwest National Laboratory (PNNL)

In oriented attachment, already formed nanoscale crystals come together to create a larger single crystal—a process that significantly differs from classic crystal growth. Despite being observed in many materials systems, the fundamental forces that govern oriented attachment are still overall poorly understood. Researchers from the Ion Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center studied the formation of gibbsite crystals through the oriented attachment of nanoplatelets. The team observed the sliding motion in real time using transmission electron microscopy (TEM), which showed how the nanoplatelets shifted and moved as they attached together. Combining this observation with simulations, the researchers determined that a staggered arrangement of platelets is the lowest energy option, leading to the structure of the observed larger crystals.

Gibbsite is an aluminum-based material found in legacy nuclear waste within the United States. Understanding how gibbsite nanomaterials behave has implications for the management of this legacy nuclear waste. Beyond waste processing, this work provides important details about the forces behind oriented attachment. This knowledge can be used to help develop models that predict oriented attachment and material stability in both gibbsite and other systems.

Oriented attachment is a critical, yet poorly understood, pathway in which larger crystals grow through the self-assembly of nanocrystals. In oriented attachment, particles separated by solvent align and join via precise rotation and translation, driven by atomic-scale forces. In particular, the forces that facilitate observed uniform stacking and superlattice formation remain unclear. IDREAM researchers showed how macroscopic gibbsite mesocrystals form through the directional sliding of nanoplates. TEM and X-ray scattering data support the formation of a monoclinic superlattice structure based on nanoplate stacking with a uniform ≈50-degree stagger along the gibbsite [010] direction. In situ liquid-cell TEM captured preferential sliding along the gibbsite [010] direction, which slows as particle overlap increases. Molecular dynamics simulations highlight that the staggered arrangement corresponds to a global free-energy minimum. The simulations also confirm that sliding along the [010] direction is energetically favored, providing insight into the role of interfacial water in achieving long-range ordered assemblies. These insights highlight the energy landscape’s role in oriented attachment, with implications for material synthesis and hierarchical structures in nature.