Assembly of molecular clusters and nanoparticles in solution is now recognized as an important mechanism of crystal growth in many materials, yet the assembly process and attachment mechanisms are poorly understood. To achieve this understanding we are investigating nucleation and assembly of iron oxide and calcium carbonate nanoparticles. In-situ TEM using a custom-designed holder and fluid cell to obtain sub-nanometer resolution shows that, in the iron oxide system, primary particles of ferrihydrite can interact with one another through translational and rotational diffusion until a near-perfect lattice match is obtained either with true crystallographic alignment or across a twin plane. Oriented attachment (OA) then occurs through a sudden jump-to-contact, which demonstrates the existence of an attractive potential that drives the OA process.
Following OA, the resulting interface expands through ion-by-ion attachment at a curvature-dependent rate. When there is a significant mismatch between the particle sizes and an attachment event does not occur during extended periods of particle interaction, the larger crystal can still grow in size due through Ostwald ripening, resulting in the disappearance of the smaller particle. In contrast to the clear role played by OA in the case of ferrihydrite, analysis of the assembly of akaganeite nanorods to form single crystal hematite spindles shows that attachment does not result in co-alignment, rather the initial mesocrystal is disordered and recrystallizes to a single crystal over time. Finally, in the calcium carbonate system, nanoparticles do interact and undergo aggregation events, however, the smallest particles often appear to be amorphous, with crystallinity presumably arising as a result of attachment. These results highlight the wide range of styles possible in mesocrystal formation, as well as the importance of in situ or temporally resolved studies when attempting to decipher the underlying pathways and mechanisms.