Characteristics of Titanate Nanotube and the States of the Confined Sodium Ions

The titanate nanotubes (TNTs) prepared by the reaction of titanium dioxide (TiO2) with NaOH were treated with acid (HNO3) solutions of various concentrations and/or heat treated at different temperatures. The properties of the treated TNTs were investigated in relation to the photocatalytic activities. The state and the role of Na ions confined in the TNTs were analyzed by means of Na-23 and H-1 NMR spectroscopy. It was revealed that there exist two states of Na ions, i.e. strongly and weakly interacting with the TNT system. The Na ions strongly bound to the interlayer of TNT play an important role to stabilize the tube structure. On the other hand, Na ions weakly interacting with the surface of the TNT suppress the photocatalytic activity but can be easily removed by a weak acid treatment with 50 mM HNO3 solution to lead the optimum photocatalytic activity. In addition, the Na ions on the surface of the TNT interfere with the crystalline transition to anatase on calcinations. Although the photocatalytic activity of TNT has been believed to be lower than that of conventional TiO2 photocatalysts of nanoparticles, it was also found that TNT showed specifically high photocatalytic activities, even higher than P25 TiO2 (Degussa P25), for some cationic reactant such as trimethylamine.