Microscopic view of nucleation in the anatase-to-rutile transformation

We use molecular simulation techniques to investigate the anatase-to-rutile transformation in TiO ₂ nanocrystals. A thermodynamic analysis indicates that edge and corner atoms significantly influence the critical size at which rutile nanocrystals become energetically preferred over anatase. We use molecular dynamics simulations to probe kinetics of the transformation in individual anatase nanocrystals as well as in nanocrystal aggregates. We follow structural evolution using simulated X-ray diffraction. Additionally, we develop a local order parameter to distinguish individual Ti ions as anatase, rutile, or anatase {112} twin-like. We apply our local order parameter to track the formation and growth of rutile nuclei. Anatase {112} twins form easily at surfaces and interfaces of nanocrystal aggregates, and we observe that rutile forms among the twins. Stable rutile nuclei maintain {101} facets during growth as a result of nucleation from layers of alternating anatase {112} twins. Our results are in agreement with experiment and indicate the central role of {112} twin-like anatase in the transformation.