Electronic Thesis and Dissertation Repository

Degree

Master of Science

Program

Chemistry

Supervisor

Yang Song; Tsun-Kong Sham

Abstract

One dimensional (1D) titanium dioxide (TiO2) nanomaterials have been extensively studied in recent years due to their superior electrical, optical mechanical and chemical properties compared with their bulk counterparts. Two different kinds of 1D TiO2 nanomaterials, TiO2 nanowires (TiO2 NW) and TiO2 nanotubes (TiO2 NT), are studied in this thesis by using various techniques.

In one study, TiO2 NW synthesized by hydrothermal method and a series of calcinated TiO2 NW were investigated by using absorption near edge structures (XANES), X-ray diffraction (XRD) and X-ray excited optical luminescence (XEOL). It is found that TiO2 NW as synthesized undergo a phase transition under calcination: TiO2-B phase transformed to anatase phase and then to rutile phase. Moreover, a green photoluminescence (PL) started showing up at a wavelength of 470 nm at 650 ℃, then shifts to 550 nm at 750 ℃ and finally at 1000 ℃ the green PL disappears and near infrared (IR) PL was observed. Also optical behaviours are consistent with the phase transformation sequence. The green PL at 550 nm and near IR PL at 830 nm is attributed to oxygen vacancies of anatase phase and the bulk defects of rutile phase respectively. However, a new phenomenon is observed, the green PL observed at 470 nm. Considering this green PL is consistent with the presence of the TiO2-B phase, it is proposed that the green PL at 470 is due to the TiO2-B phase.

In another study, the high-pressure behaviour of anatase TiO2 NT is studied using Raman and synchrotron X-ray diffraction at increasing pressure up to 31.1 GPa. Upon compression, anatase phase directly transforms to baddeleyite phase at ~ 12.2 GPa, Besides, we found in the compression of anatase phase, (1) a higher compressibility along c-axis is observed compared to a-axis, which may be due to crystal structure and growth direction of TiO2 NT, and (2) and the bulk modulus is 164 GPa which is much lower than that of other TiO2 nanomaterials. Upon decompression, the α-PbO2 phase is retrieved at ambient pressure and a compression-decompression cycle is finished. These results indicate that the 1D morphology of TiO2 NT may be responsible for its high-pressure transition and bulk modulus.


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