ABSTRACT: 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 rollup 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.