Synthesis of titanate nanotubes under hydrothermal conditions and their acid stability

Products with different nanostructures are prepared by hydrothermal treatment on commercial TiO₂ nanopowders in NaOH solution and followed by post washing. The hydrothermal treatment from 0 h to 72 h affects not only the extent of particle-to-nanotube conversion but also the extent of nanotube-to-nanofiber conversion. After 36 h of hydrothermal reaction, P25 nanopowders transform into nanotubes. Nanofibers are the major products if the reaction proceeds up to 72 h. The surface area of the nanotubes reaches ca. 357 m²/g at 36 h and decreases to ca. 21 m²/g at 72 h. For structural analysis, FE-SEM and TEM are used. Results show that post washing treatment is a key factor in controlling the morphologies of the final products; intermediate nanosheets transform into nanotubes or nanofibers during post washing process. XRD analysis indicates that post-treatment can also affect the chemical composition of the final nanoproducts. The structural stability of the nanotubes in acid aqueous solution is also studied. The local structure of as-prepared nanotubes gradually collapses as the proof the backwashing solution decreases. This finding demonstrates that titanate nanotube is acid sensitive material.