High-Temperature Formation of Titanate Nanotubes and the Transformation Mechanism of Nanotubes into Nanowires

Large-scale pure titanate nanotubes were synthesized through the hydrothermal reaction between TiO2 powders and concentrated NaOH under an unexpected high temperature of 240 °C, while it was generally claimed that it is impossible to form nanotubes at temperatures higher than 180 °C. The titanate nanotube was found to be an inevitable intermediate product, which finally transformed into a nanowire upon increasing the hydrothermal treatment duration. It was proven that the successive appearance of nanosheets, nanotubes, and nanowires are three unavoidable kinetic products of the reaction. Increasing the temperature could only accelerate the nanotube−nanowire transformation process but could not affect the sequence of the reaction events. The transformation kinetics from nanotubes to nanowires under different reaction temperatures was studied. Detailed studies indicate that this transformation process was accompanied by a coarsening process induced by both oriented attachment (OA) and Ostwald ripening (OR) mechanisms simultaneously; thereafter, the OA−OR cooperative mechanism was proposed.