Molecular dynamics simulation of the aggregation of titanium dioxide nanocrystals: preferential alignment.

We use classical molecular-dynamics simulations to study the aggregation of various titanium dioxide (anatase) nanocrystals in vacuum. In all cases, we observe a strong tendency for the nanocrystals to aggregate with certain preferred orientations in a "hinge" mechanism. Although some of the nanocrystals possess significant dipole moments, dipole-dipole interactions do not direct aggregation, implying that higher-order multipole moments are the driving force for preferential alignment. These high-order multipole moments originate from under-coordinated O and Ti surface atoms on the edges between nanocrystal facets, which create localized regions of positive and negative charge. The observed mechanism for preferential alignment may be a driving force for oriented attachment and the growth of anisotropic structures during crystallization.