【作者】 张峻；

【导师】 潘春旭；

【作者基本信息】 武汉大学 ， 材料物理与化学， 2011， 博士

【摘要】 作为一种重要的纳米材料,一维碳纳米材料(包括碳纳米管和碳纳米纤维)具有优越的电学、力学性能。而在一维碳纳米材料的研究中,还存在许多有待解决的问题,阻碍一维碳纳米材料的基础研究及实际应用的主要障碍在其可控制备。一维碳纳米材料的微观形貌及微结构等特性极大地影响着其电学、力学等性能。一维碳纳米材料的可控制备包括：纯度控制、结晶控制、排列控制、长度控制、直径控制等。一维碳纳米材料可控制备的关键是找到一种简便有效、重复性好的控制方法。在制备过程中外加电场磁场已经被证实是一种实现可控制备的有效方法。本文在制备一维碳纳米材料时加入电场磁场,以调控其排列、结晶度、直径等特征,并通过模拟计算给出理论解释,深化了电场磁场可控制备的研究。过渡金属氧化物TiO2具有无毒无害、催化效率高,稳定性好,成本低廉等优点,是一种较为理想的光催化剂材料,具有巨大的应用前景,因而引起了研究者的巨大兴趣。但由于Ti02有禁带过宽、不能有效吸收可见光以及光生电子-空穴分离效率低等固有缺陷,目前对TiO2的研究主要集中于通过掺杂、复合、染料敏化等手段对其进行改性,以提高其对可见光的吸收和光生电子-空穴的分离效率,进而提高光催化性能。而对于TiO2的光催化机制,特别是从原子尺度观察光催化过程、揭示光催化的机理方面的研究不多。本文利用HRTEM技术观测研究了Ti02光催化降解亚甲基蓝、罗丹明B以及甲基橙三种有机物的过程中TiO2晶体结构的变化,提出了一种崭新的光催化机理,此外也从共掺杂、纳米复合,暴露活性面等角度对TiO2进行改性以提升其光催化性能。本论文分为九章。第一章是绪论部分,介绍了本论文工作的研究背景、来源、意义及其重要性,在前半部分介绍了一维碳纳米材料的基础知识、研究现状,然后介绍了电场磁场控制制备一维碳纳米材料的研究进展。后半部分首先介绍了光催化以及TiO2的基础知识,然后从Ti02光催化机理研究、电纺Ti02应用于光催化研究、(001)高活性面暴露Ti02制备与研究进展以及掺杂Ti02提高光催化性能这四个方面对TiO2的研究现状进行了概括和总结。本论文各部分工作的实验方法、表征测试手段及仪器设备在第二章中详述。主要包括几个内容：在火焰法制备CNTs过程中施加磁场实现对其阵列性及微结构的调控；在CVD法制备CNFs(?)过程中外加较大电场实现对直径等特征的调控；结合电纺法及基于酒精水溶液的化学沉淀法,制得Cu2O/TiO2亚微米纤维复合物：通过加入NH4F的方法,经过水热过程成功地一步制备出锐钛矿金红石混品纳米TiO2；通过热氧化TiC与MoO3的混合物制备出Mo+C共掺杂TiO2以及对合成物性能的测试。在第三章中,在火焰法制备碳纳米管时外加一个恒定磁场,系统研究了外加磁场对碳纳米管微观形貌及微结构的影响。发现外加磁场能诱导碳纳米管阵列生长,同时影响其微结构：促使石墨层沿碳纳米管轴心方向排列,并提高了石墨化结晶。通过模拟计算发现外加磁场对碳纳米管管身有一个诱导力作用,促使其沿磁场方向生长,相比于电场力作用于碳纳米管顶端的催化剂颗粒,碳纳米管管身所受磁场力是其诱导的主因。而磁场还能影响活性碳原子的沉积方式,使其沉积有序化、石墨层倾向于沿轴向生长。在前期研究低电场对碳纳米管生长的影响基础上,在第四章中,通过在CVD法中施加偏压,研究了高电场对碳纳米纤维生长的影响,发现与前期工作相比高电场不能使“底端生长”的碳纳米纤维形成阵列,但能通过改变催化剂颗粒的方式来影响碳纳米纤维的直径,从而实现通过调节外电场大小调控生成的碳纳米纤维的直径；此外,碳纳米纤维的直径分布也更加均一,并且外加电场还能影响活性碳原子的沉积,使其不能形成“空心”结构。在第五章中,用HRTEM对比观察了TiO2光催化降解亚甲基蓝、罗丹明B以及甲基橙三种有机物的过程中TiO2晶体结构的变化,从原子-分子角度对TiO2光催化过程和机理进行了初步的研究,提出了一个“基于晶格畸变驱动力的TiO2光催化降解理论”,其主要观点认为：降解物首先吸附在锐钛矿TiO2表面,并形成较强的化学键,并使锐钛矿TiO2表面原子产生位移、晶格结构发生畸变。在光照作用下,畸变的晶格倾向于恢复到自由能较低的正常晶格状态,这种恢复作用产生的驱动力可以称之为晶格畸变驱动力,它的作用是使吸附分子的分子键断裂,使一个较大分子裂解成几部分小分子,再配合自由羟基的氧化作用将其降解。这种表面原子的畸变与恢复,可以通过HRTEM晶格像的模糊与清晰程度进行观察和判断。与公认的“光生电子-空穴”理论相比,该理论还能解释TiO2的失效过程。基于一个研究不多的TiO2电纺纤维作为复合体系,在第六章中,结合电纺法及基于酒精水溶液的化学沉淀法,制得Cu2O/TiO2亚微米纤维复合物。研究发现Cu2O的颗粒尺寸极大地影响着复合物的协同效果,结果证实颗粒尺寸在100 nm以内的纳米级的Cu20颗粒才能与Ti02发生协同作用,使其具有比不复合Cu2O的TiO2亚微米纤维更高的光催化活性。与一般的复合研究不同,本工作研究了复合物微观形貌与性能的关系,证实了微观形貌极大地影响了复合的效果。在第七章中,通过加入NH4F的方法,经过水热过程成功地一步制备出锐钛矿金红石混晶纳米TiO2。并且系统研究了HF的添加量对(001)面暴露比例、光催化性能的影响；以及NH4F添加量对混晶中金红石所占比例、(001)面所占比例以及光催化性能的影响。在一个最佳参数条件下,制备的锐钛矿金红石混晶纳米Ti02光催化性能可达到P25的四倍。本研究的意义在于结合(001)活性面制备与混晶两种手段、通过简单的一步水热法制得一种极其高效的光催化剂,这为新型催化剂的设计及制备创造了一个良好的前景。相比于单掺杂体系,共掺杂能一方面增强可见光吸收,另一方面减少单掺杂带来的电子-空穴复合中心的形成。在第八章中,以理论预测的Mo+C共掺杂设计思想为指导,通过热氧化TiC与M003的混合物制备出Mo+C共掺杂Ti02,并深入研究C,Mo掺杂对TiO2的能带以及光催化性能的影响。实验证实C掺杂能缩小Ti02的禁带宽度,使其对可见有有所吸收；Mo掺杂在对Ti02能级影响不大的情况下能消除单独C掺杂带来的光生电子-空穴复合中心,进一步提高光催化效率,在验证理论预测的同时也从实验上得到一种高效催化剂。第九章是全文总结。最后简要介绍了作者在研究生期间发表的论文及参与课题等科研情况。更多还原

【Abstract】 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.更多还原