Preparation, Microstructure Tuning, Properties And Applications Of Porous TiO₂ Materials

Since the early twentieth century, titanium dioxide (TiO2) has been widely used as a pigment and in sunscreen, toothpaste, ointment, etc. In 1972, Fujishima and Honda discovered the phenomenon of photocatalytic splitting of water on a TiO2 electrode under ultraviolet light. Since then, enormous efforts have been devoted to the research of TiO2 materials, which have led to many promising applications such as photocatalysis, photovoltaics, electrochromics, etc. It is demonstrated that these applications of TiO2 depend not only on the properties of the TiO2 material itself but also on the microstructure tuning of the TiO2 material. Therefore, it is of great significance to establish the relationship between microstructure, property and application of TiO2 material for the improvement of the old functions and the exploitation of new potential applications.This thesis focuses on the controllable preparation of porous TiO2 materials and the establishment of microstructure-property relationship. The porous TiO2 materials take advantage of the unique integration of the porosity and the properties of TiO2 itself, and thus exhibit some novel and fascinating applications.First, transition-metal-doped titanium glycolates (M-TQ with M=Fe, Mn), which are the first non-stoichiometric heterometal alkoxides, have been synthesized through a solvothermal doping approach. The inhibition effect of Fe3+ and Mn2+ on the crystallisation of M-TG has been explained on basis of a classical nucleation model. Furthermore, the as-synthesized M-TG materials are used directly as single-source precursors for the preparation of metal-doped titania (M-TiO2) through a simple thermal treatment process. The as-prepared M-TiO2 materials maintain the rod-like morphology of the precursors and possess a mesoporous structure with high crystallinity. The photocatalytic activities of the M-TiO2 materials obtained are evaluated by testing the degradation of phenol under UV irradiation. From the photocatalytic results, it is concluded that high crystallinity, large surface area and appropriate transition-metal-doping are all beneficial to the enhancement of the photocatalytic performance of the doped TiO2 material. To elucidate the effects of the doped transition metals (iron and manganese) on the photocatalytic activity of the resultant TiO2 material, electron-transfer behaviour in M-TiO2 under UV-irradiation is investigated by in situ EPR spectrascopy.Second, titanium glycolate (TG) has been found to be a photoactive inorganic-organic hybrid semiconductor for the first time, and it can be directly converted into porous TiO2 with huge surface area (534 m2g-1) under UV-irradiation. More importantly, the porous TiO2 obtained through this unusual approach exhibits a superior electron-storing capacity (1.4 mmol electrons in 1 g TiO2). It is demonstrated that the introduction of porous structure significantly enhances the electron-storing efficiency of TiO2 material. On the other hand, it is feasible to exploit the material for applications related to the stored electrons, again due to the presence of the pores and hence the accessibility of the stored electrons in the material. The reduction of nitrobenzene by the stored electrons shows that the porous TiO2 with stored electrons can function as an efficient and green reducing agent, and the release of a stored electron to an electron acceptor is companied by the removal of a proton on the surface of the porous TiO2. The magnetic investigation shows that the porous TiO2 containing stored electrons is a room-temperature ferromagnetic semiconductor.Third, amorphous titanium glycolate spheres, synthesized at room temperature, are also a photoactive semiconducting material, which can be transformed to the mesoporous TiO2 spheres under UV-irradiation. The whole synthetic process of the mesoporous TiO2 spheres is free from thermal treatment and is rather facile, controllable and reproducible. X-ray photoelectron (XPS) and IR spectroscopies indicate that the as-obtained porous TiO2 possesses rich surface hydroxyl groups. More importantly, it is remarkable to find that the as-obtained mesoporous TiO2 spheres are able to convert urea to graphitic carbon nitride (C3N4) efficiently under a mild condition. Control experiments show that the TiO2 spheres are unique for the direct conversion of urea to C3N4 because other materials such as OH-poor TiO2, OH-rich silica and simple hydroxides (NaOH) all fail to catalyze this conversion reaction. The uniqueness of the TiO2 spheres lies in their high surface area in combination with rich titanol groups which play a pivotal role in the de-oxygenation of urea and the subsequent polymerization of the intermediates (such as cyanamide). The successful formation of C3N4 from urea opens the exciting possibility to use oxygen-containing compounds for direct preparation of C3N4 materials.Finally, bulk-reduced TiO2 nanorods have been successfully prepared using porous amorphous titania as precursor in the presence of HCl and imidazole. The porous feature of the TiO2 precursor renders it easy for the TiO2 precursor and the reducing gases, which are in situ produced via combustion of imidazole, to interact, leading to the growth of the bulk-reduced TiO2 nanorods. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and Raman spectroscopy reveal that the as-prepared TiO2 nanorods are anisotropically grown along the [001] direction, and they are dominated by the rutile phase with a trace of anatase phase. Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) confirm that the self-doping species (Ti3+ ions and electrons trapped in oxygen vacancies) are present in the bulk of the sample. Furthermore, the separation behaviors of photogenerated charges in the bulk-reduced TiO2 nanorods, the bulk-reduced TiO2 nanoparticles and the stoichiometric rutile TiO2 nanoparticles have been investigated through surface photovoltage spectroscopy (SPS) and transient photovoltage (TPV) technique. It is demonstrated that the integration of the rod-like shape and the bulk-reduced feature significantly enhances the separation of photogenerated charges in the TiO2 material.