Preparation Of Titanium Dioxide Nanofibers And Their Photocatalytic Activity Research

As one kind of photocatalyst, TiO₂ (nanomaterials) has a vast range of prospects for waste water treatment, gas purification and hydrogen preparation, thus a great deal of attention has aroused. But the TiO₂ nanoparticles suspension, which is widely studied now, has many disadvantages: it is easily to flocculate and deactivate and it is difficult to be recovered, the efficiency of using photoenergy is low, therefore the practical application of this photocatalytic technique is restricted. On the bases of comprehensive review of the photocatalysis technology and the approaches for improving photoactivity, TiO₂ nanofibers (TF) were prepared and investigated systematically to meet obliged requirements for practical applications of TiO₂ in waste water treatments and environmental management.At first, TiO₂ nanopowder was prepared by sol-microemulsion-gel technique and was characterized by X-ray diffraction spectra (XRD), transmission electron microscope images (TEM), fourier transform infrared spectra (FT-IR), Raman spectra and X-ray photoelectron spectroscopy spectra (XPS). Comparing with TiO₂ nanopowder prepared by sol-gel method, TiO₂ nanopowder synthesized by sol-microemulsion-gel method shows smaller particle size, larger surface area, lower aggregation degree, higher binding energy and blue shift of ultraviolet absorption. When pH value is 8.5 and the mole ratio of surfactant to distilled water (W) is 1.08, the TiO₂ particle size reaches the least (7nm), the granularity distribution is more uniform and the surface area is the largest (273m2/g). Increasing heat-treatment temperature and prolonging the heat-treatment time, TiO₂ powders transform from amorphous, anatase and finally transform to rutile, the crystallite size increases. The process of TiO₂ crystalline growth accords with the model of crystalline growth kinetics which was suggested by Eastman.Taking the above TiO₂ powder as precursor, TF was prepared successfully with the simple hydrothermal method and heat treatment. The influences of reaction temperature, reaction time, alkali content, washing conditions and heat treatment temperature on the structure and morphology of the product were investigated. Prolonging the reaction time, the length of nanofibers and its yield increase gradually, after 72h all the precursors transform into fibers, the length of them reaches micron degree, when the reaction temperature is lower then 130℃, the transformation of TiO₂ powder is incomplete, the appropriate temperature is 150℃. If the temperature is higher than 170℃, the morphology of the product changes into band shapes; when the KOH concentration is lower than 8mol/L, the yield of fibers is little. When the concentration is over 12mol/L, the shape of the products becomes flakes or short poles and cannot turns to be fibers. Washing conditions do not affect the morphology of the fibers, but make great influences on the structure and chemical component of TF.When acid washing is not enough and the ion exchange is incomplete, there is more K2Ti6O13 component in TF. The optimum calcination temperature is 400℃, at this temperature, the morphology of the product is uniform, the crystallite is complete. With increasing the calcination temperature, the agglomeration degree of fibers increases gradually. When the temperature reaches 550℃, fibers collapse evidently. When the heat treatment temperature is less than 550℃, the sample phase is anatase. When the heat treatment temperature is over 650℃, the sample phase is the mixture of anatase and rutile. When the heat treatment temperature reaches 850℃, the anatase in the sample turns to be rutile completely.High-resolution transmission electron microscope images (HRTEM), X-ray photoelectron spectra (XPS) and diffuse reflection spectra (DRS) show that TiO₂ fibers have solid-layer structure and are consisted of Ti, O and C elements comparing with TiO₂ powder. Their crystalline sizes are small and the red shift appears in the ultraviolet absorption band. The synthesis mechanism of TF can be described as follows: under the effect of strong alkali, TiO₂ nanoparticles of anatase turn to be the nuclei of K2Ti6O13, which may grow along the crystalline faces to be K2Ti6O13 nanofibers gradually according the mechanism of dissolution-crystallization -growth. By ionic exchange and heat treatment, K2Ti6O13 nanofibers are changed into TiO₂ nanofibers with anatase.From the fundamental characteristics of photocatalytic reaction, the influences of photocatalytic reaction conditions on photocatalytic degradation of MB were investigated. The optimum conditions are 56mL/s of aerating rate, 2.0g/L of photocatalyst concentration, 40W of UV-lamp power and 6.0 pH value. The kinetics model is established which contains initial concentration of organic compound, UV-lamp power, photocatalyst concentration etc, i.e.With the same experimental condition, TF show better disinfection performance than P25. The disinfection mechanism can be described as follows: in the course of TF disinfection, the outer membrane of cells are destroyed and holes are made by the reactive species (·O2-,·OH and h+) produced by TiO₂ photocatalysis, which results in the change of the permeability membrane and enables the reactive species to reach the cytoplasmic membrane easily. The cytoplasmic membrane is attacked by reactive species, which leads the peroxidation of membrane lipid and the death of cells.Ag-TF fibers were prepared by in-situ method and immersion method. Their phase structure, microscopic morphology and chemical component were characterized by TEM, XRD, DRS, and XPS in order to analyze the effects of different-doping methods on the structures and performances of the products, The results show that the morphology of the fibers prepared by two methods is evidently different. The surface of Ag-TF fibers prepared by in-situ method is smooth, but on the surface of Ag-TF fibers prepared by immersion method, there are spheric grains with size range of 2 to 5nm which have Ag2O characteristic X-radiation peak. However, fibers prepared by an in-situ method do not show diffraction peak Ag2O, which suggest Ag+ implant into the crystal lattice of TiO₂ and do not exist on the fibers surface to form grains. Comparing with the Ag-TF fibers prepared by in-situ method, the fibers prepared by immersion method show red shift of DRS.Doping with certain amount of Ce4+, the photoactivity of TF increases. The optimum content is 0.5wt% Ce4+. With the adequate heat-treatment temperature and doping concentration, the photocatalytic rate of prepared Ce-TF to MB degradation is higher than that of P25. Ce-TF(0.5) was characterized by correlation analysis methods. The results show that TiO₂ nanoparticle size of Ce-TF(0.5) fibers is less than that of TF, and Ce4+ has barrier effect on the phase transition from anatase to rutile in fibers. Ce-TF(0.5) fibers show low fluorescence intensity and high photoquantum yield. It is attributed to the fact that the doping Ce4+ is the capturing places for electron/hole pairs, which decreases the recombination rate of electron/hole pairs.Through the research of TF, it is showed that TF has very high photocatalytic degradation activity and a very long continuously-utilizing life. It has a very important role in the treatment of waste water and a vast range of prospects.