The Investigation On The Structural Stability And Optical Properties Of Titanate Nanotubes

The titanate nanotubes have been attracted widely attention due to their strong absorption in the visible light region and potential application on the utilization of the solar energy. The titanate nanotubes prepared by the hydrothermal method have growing interest due to their simple operation, cheap fabrication and high yield. In the past decade, much research has been done on the composition, formation mechanism and physical and chemical properties. But so far, it is not clear that formation mechanism, the origin of specific photoelectric properties in the visible light region and structural stability. Our main investigation can be summarized as follows:We systemically studied the effect of the calcination temperature on the morphology, structural stability and optical properties of the titanate nanotubes from the thermodynamics degree. The results show that the sodium titanate nanotubes were thermally stable at T﹤600℃. But the titanic acid nanotubes were thermally unstable. It can be realized that the nanotubes have stronger absorption and emission in the visible light region by controlling the appropriate desiccation process. The three absorption peaks located at 515,575 and 675 nm, respectively.These specific absorption bands are in relation to the special energy levels. The specific photoelectric properties are determinated by the unique tubular structures and oxygen vacancies defects.We found that the titanic acid nanotubes transformed from orthorhombic to anatase at higher temperature.The mechanism of phase transition is that the dehydration of intralayered and interlayered groups in the titanate nanotubes under the thermodynamics conditions.After the orthorhombic structure was destroyed, the structure rearranged to form anatase lattice.We systemically studied the effect of the post treatment on the structure and optical properties of the titanate nanotubes from the dynamics degree.The results show that the photoluminescence intensity increases and the peak position shifts to shorter wavelength with the decrease of pH values under the visible light excitation. XPS results have verified that the specific optical properties are in relation to the elements under the different chemical environment.We further revealed that the formation mechanism of the nanotubes and the mechanism of the phase transition from the orthorhombic structure to anatase were controlled by the morphology change induced by the surface chemistry. At the same time we could obtain one-dimentional titanate materials by controlling the appropriate post treatment conditions, which has the structural stability and stronger absorption in the visible light region. In order to further investigate the formation mechanism of the nanotubes and the origin of specific photoelectric properties, we studied high pressure structural phase transitions of the sodium titanate nanotubes.In-situ high-pressure Raman and X-ray powder diffraction results show that two structural phase transitions were observed in the sodium titanate nanotubes in our pressure range. One is the phase transitions from the orthorhombic structure to lepidocate structure under the pressure of 5 GPa; the other is the phase transitions from lepidocate structure to the amorphous phase above the pressure of 16.7 GPa. At the same time the collapse of the tubular structure occurs. The structural phase transitions of the titanate nanotubes are irreversible. The high pressure metastable phase– the lepidocate structure can remain when the pressure recovers to ambient pressure. It demonstrates that Ti-O bonds play the important role in the structural phase transition of the titanate nanotubes.