High temperature Raman spectroscopy of titanate nanotubes

Titanate nanotubes were synthesized by hydrothermally treating the TiO2 powder with NaOH solution. Their formation and thermal stability was discussed in the view of different TiO2 precursors and Na+ content. The starting precursors for nanotube synthesis were either anatase or the mixture of anatase, rutile and high-pressure TiO2 II phase (TiO2 II) as a major component. The samples with various Na/Ti ratios were prepared by ion exchange of interlayer Na+ cations with H+ ions under controlled pH conditions. The thermal stability and the structural changes of nanotubes were studied in situ at high temperatures using Raman spectroscopy (RS) and transmission electron microscopy (TEM).We found that hydrothermal treatment of mixture of TiO2 phases (anatase, rutile and TiO2 II) leads to the formation of titanate nanotubes with structure and morphology similar to these obtained from pure anatase. However, their temperature stability was reduced, so transform to anatase at temperatures around 80 °C. In the case of nanotubes obtained from anatase precursor, Raman bands characteristic for titanate nanotubes were visible up to 300 °C. At 500 °C titanate nanotubes in H-form, H2Ti3O7, were completely transformed to anatase, while Na-form, Na2Ti3O7, showed phase transition to hexatitanate, Na2Ti6O13, nanowires.