Crystal Structures of Titanate Nanotubes: A Raman Scattering Study

Crystal structures of titanate nanotubes prepared from a NaOH treatment of TiO2 with subsequent acid washing were discussed from a viewpoint of vibrational spectroscopy. The correlation between the vibrational feature and the polymerization nature of the TiO6 octahedron was established by analyzing Raman scattering data of crystalline TiO2 (anatase and rutile) and layered protonic titanates. Then, the polymerization nature of TiO6 octahedra in the titanate nanotubes was identified by comparing their Raman scattering spectra with those of the crystalline TiO2 and layered protonic titanates. It demonstrated that the titanate nanotubes consist of two-dimensional TiO6 octahedral host layers with a lepidocrocite (gamma-FeOOH)-type layered structure. This conclusion was confirmed further by considering the Raman scattering properties of a restacked titanate prepared by assembling TiO6 octahedral layers derived from the original scroll-like titanate nanotubes. Our findings offered a convenient approach to validate the crystal structures of the products from an alkaline treatment of TiO2 under different experimental conditions.