Atomic-scale investigation of a new phase transformation process in TiO₂ nanofibers

Crystallography of phase transformation combining transmission electron microscopy (TEM) with in situ heating techniques and X-ray diffraction (XRD) can provide critical information regarding solid-state phase transitions and the transition-induced interfaces in TiO₂ nanomaterials theoretically and experimentally. Two types of reduced titanium oxides (Ti₃O₅, Ti₆O₁₁) are found during ex situ and in situ heating of TiO₂ (B) nanofibers with a specific morphology of the {100} single form (SF) in air and vacuum. The results indicate that the phase transformation process from TiO₂ (B) follows the TiO₂ (B) → Ti₃O₅ → Ti₆O₁₁ → anatase sequence for the nanofibers with the {100} SF. The occurrence of such a phase transition is selective to the morphology of TiO₂ (B) nanofibers. The corresponding orientation relationships (COR) between the four phases are revealed according to the TEM characterization. Four types of coherent interfaces, following the CORs are also found. They are TiO₂ (B)/Ti₃O₅, TiO₂ (B)/Ti₆O₁₁, Ti₆O₁₁/anatase and TiO₂ (B)/anatase respectively. The habit plane for the TiO₂ (B) to Ti₃O₅ transition is calculated as the {100}_TB by using the invariant line model. The detailed atomic transformation mechanism is elucidated based on the crystallographic features of the four phases.