A Facile Vapor-Phase Hydrothermal Method for Direct Growth of Titanate Nanotubes on a Titanium Substrate via a Distinctive Nanosheet Roll-Up Mechanism

We present a facile vapor-phase hydrothermal approach for direct growth of vertically aligned titanate nanotubes on a titanium foil substrate. The resultant nanotubes display external diameters of 50-80 nm and walls with an average thickness of 10 nm that consist of more than 10 titanate layers. This is in strong contrast to the titanate nanotubes obtained from alkaline liquid-phase hydrothermal methods, which are generally smaller than 12 nm in external diameter and have walls consisting of less than five titanate layers. Importantly, the investigation confirmed that under vapor-phase hydrothermal conditions, the nanotubes were formed via a distinctive nanosheet roll-up mechanism that differs remarkably from those of conventional liquid-phase hydrothermal processes. For the first time, a coaxial circular cylinder crystal structure of the resultant nanotubes was confirmed.