Mechanisms of Phase Transformations of TiO2 Nanotubes and Nanorods

Phase transformations of titanium dioxide (TiO2) nanotubes and nanorods at elevated temperatures are studied using molecular dynamics (MD) and replica exchange molecular dynamics (REMD) utilized here in the same way as simulated annealing. In the study, TiO2 nanotubes with amorphous (amT) and anatase structures (anT) as well as TiO2 amorphous nanorods (amR) that are amenable to experimental investigation are considered at various temperatures. It is found that amT and amR transform into a rutile rod, while anT transforms into a brookite nanotube. It is demonstrated that transformation of anT starts from TiO4 and TiO5 complexes found in the surfaces of the system, in contrast to amT and amR, where initial grains of the new phase may develop throughout the entire system starting from TiO5 and TiO6 complexes. The evolution of the number of TiOx (x = 4, 5, 6, 7) complexes indicates that the transformation of amT and amR occurs almost suddenly relative to the transformation of anT. The initial grains of transformation of amT have a structure close to rutile, while those of anT have brookite features. To our knowledge, we report the first simulations of phase transformations of TiO2 nanotubes and nanorods where simulations are performed beyond mu s.