Preparation And Properties Of Hollow Titania Microspheres By Alcohothermal Method

Titania (TiO₂) has previously been studied over the years. In the nanoscale regiment, it often exhibits very unusual electric, optical, magnetic and chemical properties, which cannot be achieved by their bulk counterparts. These unique physical and chemical properties are of technological importance and scientific research interests. So far, the most actively pursued research on TiO₂ is its use for photo-assisted degradation of a variety of toxic chemicals, as a promising electrode material in dye-sensitized solar cells, and as gas sensor. Therefore, it is very significant to research and apply of many characteristics of TiO₂ in analytical chemistry, such as photoluminescence, chemical stability, photosensitive and gas-sensitive, et al.The silica-based phases are the most widely used materials in HPLC. Silica supports are superior to other supports in terms of efficiency, rigidity and performance. However, there are several disadvantages with silica-based materials such as severe peak tailing in the chromatography of basic compounds and limited pH stability. The defects of silica limit its application in some important fields. There has been an increasing importance in application of titania-based HPLC packings for their excellent chemical, high-temperature, mechanical stability as well as bioanalysis suitability. But titania prepared by the traditional methods has some deficiency, such as less specific surface area and hole volume, imperfect hole structure , wider range of size distribution, etc.. Therefore, it is essential that develop a new method to prepare titania-based packing materials,which should have the good features such as larger specific surface area, proper pore volume, narrow hole scope distribution and more homogeneous particle size distribution.In this paper nanostructured TiO₂ hollow microspheres were prepared via a facile alcohothermal method using tetra-n-butyl titanate as a precursor, including subsequent thermal treatment. The products were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope. The effects of experimental and environment conditions on hollow microspheres have been investigated, and the further discussion has been carried out to understand the forming mechanism of nanostructured TiO₂ hollow microspheres. Meanwhile, the chromatogram performance of hollow TiO₂ microspheres was studied. At the same time, TiO₂ nanoparticles were prepared by alcohothermal method. An abnormal fine re-dispersibility and stability of them in aqueous medium was observed during phase transformation from Anatase to Rutile. Compared with TiO₂ nanoparticles prepared by hydrolysis of titanium tetrachloride (TiCl₄) in an acid aqueous solution, this phenomenon had nothing to do with their preparation methods.The main work includes the following aspects:1. Preparation of nanostructured TiO₂ hollow microspheresA solution containing absolute ethanol, concentrated H2SO4 and purified water was stirred heavily in a purified nitrogen flushed system at room temperature for 30 min. After that, Ti(OC4H9n)4 was added slowly via a dropping pipette under vigorous stirring during another 30 min. The final reaction mixture was sealed in a Teflon-lined stainless steel 30 ml autoclave, which was employed to obtain a high temperature and pressure for the nucleation and growth of TiO₂ hollow microspheres. As shown in the SEM images, TiO₂ microspheres with a relatively uniform particle size distribution of 5-10μm present in our transparent sol. In addition, since these TiO₂ microspheres have small crystal grains, which present a process of amorphous-crystalline transition tendency during thermal treatment according to XRD images, they might have good potential application in catalyst supports, coatings and catalysis in wastewater treatment, etc.2. Effects of experimental and environment conditions on hollow microspheresA serious of experimental conditions, such as time and temperature of thermal treatment, species of acid, the molar ratio of reactants etc., have been investigated as factors influencing upon the configuration of TiO₂ hollow microspheres. During the investigation, the results clearly indicate that the molar ratio of H2O / Ti(OC₄H₉ⁿ)₄ and H₂SO₄/ Ti(OC₄H₉ⁿ)₄ are the two main factors influencing upon the configuration of TiO₂ hollow microspheres. The size and the shell thickness of the hollow TiO₂ microspheres can be controlled by altering the molar ratio of H2O / Ti(OC₄H₉ⁿ)₄ and H₂SO₄/ Ti(OC₄H₉ⁿ)₄. On the basis of mentioned results, an auto-orientation assembly mechanism to form hollow TiO₂ microspheres is proposed.3. Investigation of the parameters of TiO₂ hollow microspheresPorous TiO₂ microspheres (surface area: 146.1 m2·g-1, average pore volume: 0.31 cm3·g-1, pore diameter: 7.4 nm, microspheres diameter: 5-7μm) with a narrow particle size distribution, were characterized by N₂ adsorption method. The results show that the physical and chemical properties of TiO₂ microspheres are better than that of the microspheres prepared by the other methods. The data of potentiometric titration demonstrate that the acid-base value of TiO₂ microspheres were lower than that of SiO₂. At the same time, the hydrophobic selectivity and sensitivity of TiO₂ microspheres as HPLC stationary phase were studied by benzene, toluene and ethyl benzene as solutes and acetonitrile solution as a mobile phase. The results show that the packing support exhibits a little of reversed-phase behavior. The good chromatographic performances and high chemical stability were showed when it was used to separate the neutral and basic solutes using acetonitrile solution as a mobile phase.4. The dispersion stability of different nanocrystalline TiO₂Although two typical TiO₂ nanoparticles were prepared by different methods, an abnormal fine re-dispersibility and stability of them in aqueous medium, which has nothing to do with their preparation methods, was observed during phases transformation from Anatase to Rutile. FT-IR and TEM data show that increasing surface curvature occurred during A-R transformation is advantageous to water adsorption on the surface of the TiO₂ nanoparticles, which induces the stronger repulsive interaction. It is believed that this hydration forces that displace upon particles aggregation result in the abnormal fine re-dispersibility and stability of TiO₂ nanoparticles in aqueous medium. The distinct zeta potentials of the different particles manifest that the surface electrostatic forces are not crucial factors in abnormal fine re-dispersibility of TiO₂ nanoparticles observed during A-R phases transformation. Consequently, it would be quite helpful for understanding the water adsorption behaviors on TiO₂ surfaces (especial for mixture of rutile and anatase), preventing aggregation of TiO₂ nanoparticles and explaining some phenomena observed in their photocatalytic activities.