Preparation, Characterization, And Metal Modification Of Nanosize TiO₂ Microspheres With Controllable Crystalline Structures

It is the key target of photocatalysis research to explore highly effective photocatalytic materials. Due to its low cost, chemical stability, non-toxicity, and favorable physical/chemical properties, nano-TiO2 has been considered to be a promising photocatalysts in the applications of photocatalysis, especially in the decomposition of environmental contaminants. In this thesis, high photocatalytic activity TiO2 were prepared by selecting appropriate preparation condition to control the crystalline phase and morphology of TiO. Then, metal and rare earth doping was used to not only restrain the recombination of photogenerated electrons and holes, but also extend the light response of TiO2 into the visible light region. Above mentioned relative study is favorable for the practical application of TiO2-based photocatalysis.Nano-TiO2 samples were prepared with sol-gel and hydrothermal methods. By changing the type and concentration of used acids as well as the hydrothermal temperature, pure rutile (R), bicrystalline mixture of anatase (A) and brookite (B), and tricrystalline mixture with different ratio of A, R, and B were prepared. The samples were characterized by the XRD and TEM. In the HCl and HNO3 aqueous solutions, the hydrothermal crystallization process can be controlled by changing the concentration of acid and the temperature of hydrothermal treatment. The proportion of different crystal phases are determined by the thermodynamic and kinetic stabilities of different crystal phases, while the concentration of acid can influence the route of hydrolysis and the amount of charges adsorbed on the surface of TiO2 crystal nuclei. In H2SO4 solution, the obtained samples consist of bicrystalline mixture of A and B, and the ratio of anatase and brookite is independent of the concentration of H2SO4 and the temperature of hydrothermal treatment. The presence of SO42- ions can increase the steric repulsion and electrostatic repulsion among different nuclei, consequently promoting the formation of more crystal, restraining its growth, and preventing the formation of rutile.Anatase TiO2 microspheres with porous frameworks have been prepared by controlled hydrolysis with water generated "in situ" via an esterification reaction under ultrasound irradiation, followed by hydrothermal treatment. The samples were characterized by XRD, TEM, XPS, EPR, AAS, FT-IR, DRS, and nitrogen adsorption-desorption methods. It was found that the obtained TiO2 microspheres consist of disordered wormhole-like porous and mesoporous structures. The formation of porous structure might be related to the slow 'in situ' hydrolysis under ultrasound irradiation. The formation of unique anatase should be attributed to the presence of SO42- ions.On the basis of preparing anatase nano-TiO2 microspheres, a series of Fe-doped TiO2 catalysts were prepared, and characterized by XRD, TEM, XPS, EPR, AAS, UV-vis DRS, and nitrogen adsorption-desorption. The results indicated that Fe3+ ions can be incorporated into the crystal lattice of TiO2 to substitute Ti4+, leading to the formation Ti-O-Fe bond. Similar to the undoped TiO2, Fe-TiO2 also exhibits anatase phase, porous structure, and spherical shape. The energy level of Fe3+/Fe4+ is above the valence band edge of TiO2 and the energy level for Fe3+/Fe2+ is below the conduction band edge of TiO2. Therefore, Fe3+ ions, acting as both electrons and holes traps, can restrain the recombination of photo-generated electrons and holes, resulting in the increase of photocatalytic activity. However, when the doping amount of Fe3+ ions is too large, Fe3+ ions can act as the recombination centers of the photo-generated electrons and holes, resulting in the decrease of photocatalytic activity. The enhancement of light absorption of Fe-TiO2 in the visible region is resulted from the electronic transition from the dopant energy level (Fe3+/Fe4+) to the conduction band of TiO2.On the basis of preparing anatase nano-TiO2 microspheres, rare earth Ce-doped TiO2 catalysts were prepared, and characterized by XRD, UV-vis DRS, SEM-EDS, TEM, AAS, and XPS. The results revealed that Ce4+ ions did not substitute Ti4+ into the crystal lattice of TiO2, but were doped into the interstitial sites of TiO2 octahedra or adsorbed on the crystal faces of TiO2 crystallites. Similar to the undoped TiO2, Ce-TiO2 also remained anatase phase, porous structure, and spherical shape. However, Ce modification can decrease the crystallite size and increase the surface area of TiO2. Ce modification improves the photocatalytic activity of TiO2 under both UV and visible light irradiation. Under visible light irradiation, electrons can be excited from the valence band of Ce-TiO2 into Ce 4f level or be excited from the ground state of Ce2O3 into Ce 4f level. Under UV light irradiation, Ce4+ modification not only promotes the formation of hydroperoxy radicals but also restrains the recombination of electrons and holes by trapping the photogenerated electrons, resulting in the improvement of quantum yield.