An Amorphous-to-Anatase\nPhase Transition Study of\nTiO<sub>2</sub> Nanotubes by Temperature-Dependent Synchrotron-Based\nIn Situ X‑ray Diffraction

TiO<sub>2</sub> nanotubes (NTs) with\namorphous and crystalline\nstructures have attracted interest due to their wide range of applications,\nparticularly black TiO<sub>2</sub>, which addresses the limitations\nof conventional TiO<sub>2</sub> (a wide band gap of 3.0–3.2\neV). Understanding the amorphous-to-anatase phase transition is crucial\nfor phase control of crystalline TiO<sub>2</sub>. This study investigates\nthe size-dependent phase transition behavior of TiO<sub>2</sub> and\nblack TiO<sub>2</sub> NTs using temperature-dependent synchrotron-based\nX-ray diffraction. Thermal treatments reveal that phase transitions\noccur in the ranges of 210–240 °C for normal NTs and 270–300\n°C for black NTs. The onset temperature and crystallization growth\nare dependent on size, especially NT length, at least in the system\nunder investigation. We observe anisotropic crystallization in quantum\nconfinement, with the unit cell undergoing compression in the <i>a</i>-axis and expansion in the <i>c</i>-axis during\ncrystallization. The longest black NTs (BV50T4) show a significant\ndifference in the <i>c</i>/<i>a</i> ratio. Defects,\nsuch as oxygen vacancies, localize on specific NT planes.