The effect of hydrothermal conditions on the mesoporous structure of TiO2 nanotubes

A systematic analysis of the influence of preparation conditions in the alkali hydrothermal synthesis on the morphology of TiO2 nanotubes is performed using HRTEM and low temperature nitrogen adsorption. The possible mechanisms of nanotube formation are reviewed and a mechanism based on the key stage of wrapping of intermediate multilayered titanate nanosheets is suggested. The driving force for wrapping is considered to be the mechanical stress arising during crystallisation/dissolution. The average diameter of the nanotubes was found to depend on the temperature and on the ratio of weight of TiO2 to the volume of sodium hydroxide solution. An increase in the temperature from 120 to 150 °C results in an increase in the average nanotube diameter. Subsequent increases in the temperature result in the formation of non-hollow TiO2 nanofibers with an average diameter of 75 nm, a wide distribution in diameter and a length in excess of 10 µm. The increase of the TiO2 : NaOH molar ratio results in an increase in the average diameter of nanotubes and a decrease of surface area. The average inner diameter of TiO2 nanotubes varied between 2 and 10 nm. The pore-size distribution was evaluated from TEM, and low-temperature nitrogen adsorption data using the BJH method. It was shown that nitrogen adsorption is a suitable method for characterisation of the pore morphology of nanotubes.