Synthesis of single-crystalline TiO2 nanotubes

In this work, we demonstrate experimentally that the classification of inorganic fullerene-like structures can be extended from transition metal sulfides to include metal oxides. Single-crystalline TiO2 nanotubes have been synthesized for the first time with an unconstrained solution growth method by hydrolyzing TiF4 under acidic condition at 60 T. With SEM/HRTEM/XRD methods, it is found that the walls of TiO2 tubes are formed with stacking of {101} planes (anatase polymorph). Both HRTEM and XRD indicate an interplanar spacing of d(101) = 0.36 +/- 0.01 nm in the wall structure. Although the {101} planes can be curved into concentric cylinders, straight segments of {101} planes are also observed. The inner diameters of the tubes are in the range of 2.5-5 nm, and outer diameters are in the range of 20-40 nm with lengths up to a few hundred nanometers. Similar to the well-known fullerene-like nanotubes formed from layered materials (graphite, MoS2, and WS2) spherical and polyhedral closings for the TiO2 nanotubes are observed. Anodic aluminum oxide nanochannels and alumina nanoparticles have been used to differentiate growth modes and to address the role of solid substances in TiF4 hydrolysis. It is revealed that TiO2 can still be synthesized in the form of fullerene-like inorganic nanotubes, although it is not a layered compound. Future directions in this research area are also discussed.