Effects of synthetic conditions and heat-treatment on the structure of partially ion-exchanged titanate nanotubes

TiO2-derived nanotubes prepared by hydrothermal treatment in NaOH (aq.) have been widely studied with regard to their composition and their formation mechanism. There exist some contradictions among previous reports, e.g. proposed crystal structure and thermal stability. Some difference of processing conditions and remnant sodium in nanotubes in previous works lead to the confusions on nanotubes' structure, thermal stability, and so on. In this paper, microstructural change during the hydrothermal and post-heat-treatments was studied for well-defined sodium contained (i.e. partially ion-exchanged) nanotubes, with nominal composition of ∼H1.5Na0.5Ti3O7. Under the hydrothermal treatment at 120 °C, the nanotubes grew longer until 12 h, and became almost constant length for more than 12 h. By the post-heat-treatment above 350 °C, some of the nanotubes began to break into particles of anatase phase, and the others remained as nanotubes where a lot of Na existed, indicating the remnant Na stabilized nanotube structure. At higher heat-treatment temperatures, the particles changed into rutile phase, and the nanotubes converted into Na2Ti6O13 nanorods. In situ high-temperature SEM study also clarified their structural change during heating.