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Thesis Format

Integrated Article

Lu YaoFollow

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Sham, Tsun-Kong.

Abstract

The one-dimensional (1D) TiO2 nanotubes (NTs), and their derivatives have been extensively studied due to their potential use in water-splitting, solar cells, and lithium-ion batteries. Since TiO2 has a large band gap (~3.2 eV for anatase), there has been a search for higher photocatalytic efficiency by shifting the band gap into the visible range. This thesis presents a study of black TiO2 NTs and ZnO-TiO2 heterostructures using synchrotron-based X-ray spectroscopy and X-ray diffraction techniques. It involves the transformation from as-prepared TiO2 NTs to black TiO2 NTs via an electrochemical reduction method. ZnO-TiO2 NT heterostructures, designed to modify functionality with various preparation methodologies, are also explored. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to track the morphology and crystalline structure of samples, respectively. Furthermore, the local and electronic structures of the elements in the samples are determined by X-ray absorption near-edge structure (XANES). Combined with X-ray excited optical luminescence (XEOL), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS) techniques, the electronic structure of the valence band, conduction band, and band gap, as well as the luminescence with respect to specific elemental and chemical environments (defects), can be explored.

Summary for Lay Audience

Since its discovery as a photocatalyst for water splitting by Fujishima and Honda in 1972, titania has undergone extensive development and found widespread applications. These include its use in water splitting to produce hydrogen and oxygen, as well as in supercapacitors, solar cells, the photocatalytic degradation of pollutants, sensors, biological materials, and lithium-ion batteries. Among the various forms of TiO2, one-dimensional (1D) TiO2 nanotubes (NTs) have garnered significant attention within the materials community. This interest stems from their advantageous characteristics such as low cost, non-toxicity, high-rate performance, and excellent stability and cyclability. However, a significant limitation of TiO2 NTs lies in their large band gap (~3.2 eV), which restricts their photo-absorption to the near-ultraviolet range, consequently wasting approximately 95% of solar energy in the visible spectrum.

This thesis delves into the exploration and integration of morphology engineering and heterostructure strategies to enhance the potential photocatalytic activity of TiO2 NTs with optimization: black TiO2 and ZnO-TiO2 heterostructure. Highly ordered amorphous TiO2 NT arrays are synthesized followed by the electrochemical reduction method to achieve the transformation to black phase. Furthermore, ZnO-TiO2 heterostructures are synthesized via various methods to enhance the photocatalytic activity of TiO2 NTs. Importantly, synchrotron-based techniques are employed in this research. Synchrotrons are facilities capable of accelerating electrons to nearly the speed of light, emitting light across a broad spectrum from infrared to gamma rays, making them invaluable tools for generating X-rays. In this thesis, X-ray absorption near-edge structures (XANES) is utilized to determine the local and electronic structures of elements within TiO2 NTs. Additionally, X-ray excited optical luminescence (XEOL), X-ray emission spectroscopy (XES), resonant inelastic X-ray scattering (RIXS), and X-ray photoelectron spectroscopy (XPS) techniques are employed to further elucidate the implications of these findings for the functionality of black TiO2 and ZnO-TiO2 heterostructure.

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