Pressure-Induced Phase Transitions in TiO2 Rutile Nanorods

TiO2 nanorods (diameter of 6-8 nm) with the rutile-type structure have been investigated by Raman spectroscopy under high pressure. Additional bands in the Raman spectrum at ambient pressure are interpreted as resulting from defects that break the Raman selection rules as the simulated vibrational density of states (vDOS) is in good agreement with the experimental spectrum. Under pressure, the rutile structure transforms to the baddeleyite one at P similar to 16 GPa. After the pressure cycle, the alpha-PbO2 phase is recovered. The sequence of phases is discussed in terms of thermodynamics and kinetics based on crystallographic relationships. In contrast to other one-dimensional TiO2 nanomaterials that adopt a metastable structure, the nanorods studied in this work show little impact of the morphology on the phase transitions sequence compared to that of the bulk TiO2.