One-dimensional Carbon Nanomaterials And TiO₂ Nanomaterials: Controlled Synthesis, Characterizations, Properties And Application

As a kind of important nanomaterials, one-dimensional carbon nanomaterials (including carbon nanotubes and carbon nanofibers) have excellent electrical and mechanical properties. In the one-dimensional carbon nanomaterials research, there are still many problems to be solved, the main obstacle hinder one-dimensional carbon nanomaterials applied in practical application is controlled synthesis. Micro-morphology and micro-structure of one-dimensional carbon nanomaterials greatly affect the electrical, mechanical and other properties. Controlled synthesis of one-dimensional carbon nanomaterials include:purity controlled, crystallization controlled, alignment controlled, length controlled and diameter controlled. The key of controlled synthesize one-dimensional carbon nanomaterials is to find a simple, effective and repeatable controlled method. Applied electric field and magnetic field in the synthesis process has been shown to be an effective way to achieve controlled synthesis. In this paper, electric field and magnetic field were applied in synthesis of one-dimensional carbon nanomaterials to control the order, crystalline, diameter and other characteristics, and theoretical explanation is given by simulation calculation. These researches deepen the controlled synthesis using electric and magnetic fields.Transition metal oxide TiO2 with advantages of nontoxic, efficient photocatalysis, stability, and lower cost, is an ideal photocatalyst materials with a great prospect, has aroused great interest of researchers. However, since inherent defects of TiO2 can not effectively absorb visible light due to the wide band gap, and low electron-hole separation efficiency, the current study focused on improve the photocatalytic performance of TiO2 by doping, composite, and dye-sensitized modification to increase the visible light absorption and electron-hole separation efficiency. However, there was little research reveal the TiO2 photocatalytic mechanisms, in particular observation of photocatalytic process in the atomic-scale. In this research, the photocatalytic progress of TiO2 was studied using HRTEM by observed the crystal structure variation of TiO2 during degradation of methylene blue, rhodamine B and methyl orange, and a new photocatalytic mechanism was developed. On the other hand, TiO2 was modified from a total of co-doping, nano compound, and exposure of activity surface to enhance the photocatalytic ability.This dissertation is divided into nine chapters. The first chapter is the introduction, describes the research background, origin, meaning and importance of this work. The first half introduces the basic knowledge, research status of one-dimensional carbon nano-materials, and then introduces the research progress of electric fields and magnetic fields control synthesis of one-dimensional carbon nano-materials. The basics of TiO2 and photocatalysis were introduced in the second half, and then from the photocatalysis mechanism of TiO2, electrospun TiO2 applied in photocatalysis, synthesis and research progress of (001) high activity surface exposed TiO2, and improve the photocatalytic ability of TiO2 by co-doped of these four aspects in the study is summarized and concluded.The experimental methods, characterization methods and test equipment in the various parts of work are described in the second chapter. It includes several parts:controlling CNTs array growth and micro-structure of that in flame synthesis process using a magnetic field; applied a larger electric field in the CVD process to control the diameter and other characteristics of obtained CNFs; synthesizing Cu2O/TiO2 sub-micron-fiber composite combined with electrospinning progress and alcohol-based chemical solution deposition method; by adding NH4F, anatase mixed rutile TiO2 nanosheets were successfully obtained through a one-step hydrothermal progress; Mo+C co-doped TiO2 were synthesized by thermal oxidation of TiC and MoO3 mixture, and the photocatalysis ability of that was studied.In the third chapter, the influence to the morphology and microstructure of carbon nanotubes by applied a constant magnetic field in flames during growth was studied. It is found that magnetic field can not only induce the array growth of carbon nanotubes, but also affects the micro-structure:to promote the direction of the graphite layer arranged along the nanotube axis, and improve the crystallization of graphite. By simulation and calculation, it is revealed that there is induced force acting upon the tube of carbon nanotubes in magnetic field to promote its growth direction along the magnetic field, which is different with electric field force acting upon the catalyst particles on the top of carbon nanotubes. So the magnetic field induced force acting upon the tube of carbon nanotubes is the main reason for induction. Furthermore, magnetic field can affect the deposition pattern of carbon atoms: improve the order of deposition, and promote which tends to grow along the axis of graphite layer.On the basis of research on low electric field inducing in the carbon nanotubes growth, in the fourth chapter, the influence of applying high electric field on the growth of carbon nanofibers in CVD system was studied. It is found that instead of inducing "the bottom growth" carbon nanofibers array growth as the previous work, the size of catalyst particles can be changed by a high electric field, as a result, the diameter of carbon nanofibers can be controlled by adjusting the strength of the external electric field. In addition, the diameter distribution of carbon nanofibers is more uniform when the strength of electric field enhanced, and the applied electric field can also affect the deposition of carbon atoms, so that it can not form a "hollow" structure.In the fifth chapter, the crystal structure changes of TiO2 during degradation of methylene blue, rhodamine B and methyl orange were studied with the HRTEM observation. Photocatalytic process and the mechanism of anatase TiO2 were researched in the atomic-molecular scale, and proposed a "hotocatalytic degradation theory based TiO2 lattice distortion driving force". The main point of view can be described as:first, dye molecules adsorb on the surface of anatase TiO2 and formatted strong chemical bonds, which can displace anatase TiO2 surface atom, result in lattice structure distortion. Then under illumination, distorted lattice tends to return to a lower free energy of the normal lattice state. This recovery can be called as "lattice distortion driving force", its effect is to break the bonds of adsorbed molecules, split a large molecule into several parts of small molecules, and degradation of the dye molecules is achieved coupled with the oxidation of free carboxyl group. This distortion of the surface atoms and recovery can be observed by judging HRTEM lattice image from clarity to fuzzy. Compared with the "electron-hole theory", this theory can also explain the failure of TiO2.By choosing TiO2 electrospun fibers with a little research as a composite system, in the sixth chapter, Cu2O/TiO2 sub-micron-fiber composites were synthesized combined with electrospinning progress and an alcohol-based chemical solution deposition method. It is found that the size of Cu2O particles greatly influences the complex synergies between Cu2O and TiO2 which proved to take effect only when the Cu2O particles size less than 100 nm, and photocatalytic activity of the composites can be much higher than TiO2 sub-micron-fibers. Different from other work, the relationship between microstructure and properties of composites is studied in this paper, and it is confirmed that the microstructure greatly influences the composites properties.In the seventh chapter, by adding NH4F, (001) surface exposed anatase mixed rutile TiO2 nanosheets were successfully synthesized after one step hydrothermal process. And the relationship between amount of added HF and the ratio of (001) exposed surface, photocatalytic properties was studied; the influence of NH4F content on the percentage of rutile in mixed crystal, the ratio of (001) exposed surface, photocatalytic properties was also revealed. In the best parameter conditions, the photocatalytic ability of (001) surface exposed anatase mixed rutile TiO2 nanosheets can be four times as high as the P25. Significance of this study is to combine two means of exposing (001) activity surface and mixed crystal, through a simple one-step hydrothermal method, obtained extremely efficient photocatalyst, which create a good future for the design and preparation of new catalysts.Compared with single-doped, co-doping, on one hand, can enhance the visible light absorption, and on the other hand reduce the electron-hole recombination center formation bring by single-doping. In the eighth chapter, based on the theoretical prediction of Mo+C co-doped, Mo+C co-doped TiO2 were obtained by thermal oxidation of a mixture of TiC and MoO3, and the influence of C, Mo doping on the band and photocatalytic ability of TiO2 was in-depth studied. Experiments confirmed that C doping narrows band gap of TiO2, makes it absorb visible light; Mo doping has little effect on the band gap of TiO2, but can reduce the electron-hole recombination center formation bring by C doping, and improve the photocatalytic ability. The study verifies the theoretical prediction and obtained an efficient photocatalyst.Chapter nine is a full summary. Finally, a brief introduction of published papers and participated project in the graduate were given.