Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Chemistry

Supervisor

Sham, Tsun-Kong

Abstract

The one-dimensional (1D) TiO2 nanotubes (NTs) and their derivatives have been extensively studied due to their wide applications, such as photocatalysts, solar cell, paints and so on. Since TiO2 has a wide band gap (~3.0 eV), its photo-absorption as the photocatalyst only occurs in the UV region that wastes mostly solar energy. Therefore, the solution to higher photocatalytic efficiency has been sought for some time. This thesis presents a study of the size-dependent phase transitions of TiO2 NTs using synchrotron-based X-ray techniques. The chemical environment including local symmetry and the luminescence origin of the TiO2 NTs can be tracked by X-ray absorption near-edge structures (XANES) and X-ray excited optical luminescence (XEOL) technique. As a result, the anatase-to-rutile transition highly depended on the sizes of NTs with the same annealing treatment. Additionally, the shortest NTs require a lower temperature to start the amorphous-to-anatase transition.

Summary for Lay Audience

Since titania (TiO2) was discovered as a photocatalyst for splitting water by Fujishima and Honda in 1972, it has been developed and widely used in water splitting, supercapacitors, solar cells, photocatalytic degradation of pollutants, sensors and lithium-ion batteries. Furthermore, more attention has been concentrated on one-dimensional (1D) TiO2 nanotubes (NTs) among the TiO2 materials family, which is due to their low cost, non-toxicity, excellent stability. However, the large band gap (~3.0 eV) limits the photo-absorption of TiO2 to act as a photocatalyst, which only occurs in the near-ultraviolet range and wastes 95% of solar energy. Therefore, the solution to higher photocatalytic efficiency has been sought for some time and this thesis explores and combines two possible solutions. It presents a study of the size-dependent phase transitions of TiO2 NTs by synchrotron-based X-ray spectroscopy, which involves precise control of size (length, diameter, and thickness) by a two-step electrochemical anodization under various conditions and achievements of desirable phases of TiO2 NTs by subsequent thermal-annealing treatments with different temperatures. In the characterization, there are four techniques used in this thesis. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to track the surface morphology and the crystalline structure of the TiO2 NTs. Importantly, the synchrotron-based techniques are also applied to provide valuable structural information of TiO2 NTs. The synchrotron is the facility that accelerates electrons to nearly the speed of light and emits light with a continuous and wide range of wavelengths from the infrared to gamma rays, which is a powerful tool to probe the structural and electronic information of various materials. The chemical environment including local symmetry and the luminescence origin of the TiO2 NTs can be tracked by X-ray absorption near-edge structures (XANES) and X-ray excited optical luminescence (XEOL) technique. As a result, the size features of TiO2 NTs can be controlled by the electrochemical anodization method. The anatase-to-rutile transition highly depended on the sizes of NTs with the same annealing treatment. Additionally, the shortest NTs require a lower temperature to start the amorphous-to-anatase transition.

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