Abstract:
TiO2 nanotubes synthesized by Ti anodization have attracted significant and continually increasing research interest over the past years with the high potential for technological applications. TiO2 nanotubes provide a unique combination of wide band gap semiconductor properties with a large surface area and precisely controlled morphologies, making them promising candidates for solar cells, water splitting, environment remedies, sensors, sterilization, surface wettability control, and more. In order to optimize the properties for different applications, the microstructures of anodized TiO2 should ideally be designed and tailored at the nanoscale. A crucial step toward this goal is a developed understanding of the formation mechanisms and morphology evolution. The dynamic competitions between oxidation and chemical dissolution during Ti anodization have been identified in this thesis. This demonstrates experimentally that the inside diameter of nanotubes gradually increases towards the tube apex, due to the longer period of dissolution of the inside tube-wall. When the dynamic balance between oxidation and chemical dissolution is reached, nanowires can be synthesized at the top region. The morphology evolution of anodized TiO2 has been investigated and discussed. It reveals that the applied voltage determines the pattern of etching pits, which in turn governs the anodized TiO2, including the tube diameter and hexagonal features. With this developed understanding, several novel hierarchical morphologies are synthesized. The obtained microstructures provide a convenient case to evaluate the influence of morphology on surface wettability, proving that surface morphology acts as an amplifier for surface wettability. By tailoring the microstructures and organic modification, the surface wettability of anodized TiO2 can be well tuned from superhydrophilicity to super-hydrophobicity, with the maximum water contact angle up to 172o. The potential application for aquatic devices is also explored on a treated Ti mesh, which shows strong floating stability. The formation of anodized TiO2 is a dynamic process involving the oxidation process at the interface between oxide layer and Ti substrate, which suffers from the insufficient supply of oxygen. Ti anodization is thus applied to synthesise defective TiO2-x with controllable oxygen vacancy. The obtained black TiO2-x shows ultrahigh absorbance over the visible light region (400-800 nm), doubling the highest absorbance previously reported on hydrogenated TiO2. Electron paramagnetic resonance analysis shows the controllable levels of oxygen vacancies, and transmission electron microscopy reveals its partial crystallised structure - both of which can be easily controlled by post-annealing. The preliminary works have shown its high photocatalytic activity under visible light, and further revealed its ability to absorb microwave energy. This research on defective black TiO2-x, although preliminary, has shown tremendous potential of this material, pointing out a new direction that crystal defects might be the key to developing visible-light sensitive and microwave-absorbing materials.