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Mechanisms of Phase Transformations of TiO2 Nanotubes and Nanorods

Andrei Buin, Styliani Consta*, and Tsun-Kong Sham
Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7
J. Phys. Chem. C, 2011, 115 (45), pp 22257–22264

Phase transformations of titanium dioxide ($\mathrm{TiO_2}$) 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, $\mathrm{TiO_2}$ nanotubes with amorphous (amT) and anatase structures (anT) as well as $\mathrm{TiO_2}$ 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 $\mathrm{TiO_4}$ and $\mathrm{TiO_5}$ 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 $\mathrm{TiO_5}$ and TiO6 complexes. The evolution of the number of $\mathrm{TiO}_x$ ($x = 4, 5, 6, 7$) 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 $\mathrm{TiO_2}$ nanotubes and nanorods where simulations are performed beyond ${\mu}s$.