Kinetics of Oriented Attachment of Mica Crystals.

Oriented attachment (OA), that is, the coalescence of crystals through attachment on coaligned crystal faces, is a nonclassical crystal growth process. Before attachment, a mesocrystal consisting of coaligned parallel crystals but with liquid separating them was observed. Fundamental questions such as why OA is kinetically favored and whether a mesocrystal stage is a prerequisite for OA are raised. Through combining brute-force molecular dynamics simulations and path samplings based on extensive umbrella simulations, we address these questions with a case study on the OA of a mica nanocrystal onto a mica crystal substrate in water. Brute-force simulations show that if two mica crystals are attached but largely misaligned, coalignment hardly appears. Thus, if OA is possible, then coalignment must appear before the attachment between crystals. Electrophoresis of the nanocrystal toward the substrate surface is spontaneous, but mesocrystal formation is occasional, also shown by brute-force simulations. Free energies along different pathways show that OA is spontaneous and kinetically favored over non-OA, and a mesocrystal formation is just a bifurcation in the pathway. OA is through a pathway in which the nanocrystal is tilted with respect to the substrate. Part of the nanocrystal is attached to the substrate first, and then, OA is gradually completed. Once a mesocrystal is occasionally formed, then a jump event is needed for the nanocrystal to get back to the OA pathway. The sampling technique here can hopefully guide the design of nanostructured materials facilitated by OA.