【作者】 周斌；

【导师】 郭万林；

【作者基本信息】 南京航空航天大学 ， 工程力学， 2008， 博士

【摘要】 碳纳米管的独特结构和优异的物理力学性能使它成为纳米科技领域中构筑纳尺度器件和系统的重要基础,为纳米科技领域的创新提供着持续强劲的原动力。碳纳米管具有大的比表面积和优美的空心结构,可以容易地吸附各种气体,这不仅会显著改变碳纳米管的物理力学性能,而且为强化化学反应提供了一种纳尺度受限环境。碳纳米管与小分子的作用以及碳纳米管内外化学反应的差异性是碳纳米管研究和应用中最受关注的问题之一。本文基于严谨的密度泛函理论,利用万亿次计算集群的强大计算能力,对H2、O2、H2O分子在碳纳米管表面的复杂的化学吸附过程及吸附后系统的性质开展了系统深入的计算模拟与分析,取得以下三个方面的新进展:1.揭示了碳纳米管轴向变形、电荷注入对化学吸附氢分子过程的重要影响。通过第一性原理计算研究了利用轴向变形、电荷注入等方法降低氢化学分子吸附能垒,从而利用氢分子化学吸附提高碳纳米管储氢能力的可能性。通过对氢分子在具有轴向变形的(8,0)碳纳米管表面化学吸附的计算,发现合适的轴向变形能显著地降低氢分子直接吸附过程的能垒。但是,拉伸压缩变形对各化学吸附构型之间的转化过程的调控不明显。通过研究(3,3),(5,5),(5,0),(8,0)等一系列带电碳纳米管吸附氢分子的能垒,发现注入电荷对化学吸附能垒有明显的影响,吸附能垒的变化规律依赖于注入电荷的符号(电子或空穴)、电量以及碳纳米管手性、直径等综合因素。特别地,本文发现注入空穴可以有效降低zigzag碳纳米管的氢分子吸附能垒;相对地,注入电子对降低armchair碳纳米管吸附氢分子的能垒更为有效。研究结果表明,通过施加轴向变形或注入电荷有望提高碳纳米管储氢性能。2.发现氧分子在碳纳米管表面的吸附、解离及扩散显著依赖于内外表面的凹凸性,并提出碳纳米管氧化的机械调控原理。通过第一性原理计算,对单线态氧分子在碳纳米管内外表面的吸附、解离及扩散等动力学行为进行了系统研究,发现碳纳米管内外表面的凹凸性对氧分子吸附、解离和扩散过程都有十分显著的影响。研究表明,单线态氧分子在碳纳米管外表面化学吸附远比在内表面上容易。但是,由于氧原子与内表面的结合较弱并且存在外表面上不具有的独特稳定吸附构型,氧原子在内表面上的扩散路径与外表面上的显著不同,并且具有比在外表面上低大约0.8eV的扩散能垒。通过对单线态氧分子在内突变形的碳纳米管上吸附的研究,发现使碳纳米管发生内外凹凸改变的机械变形能显著增强内表面吸附氧分子的能力。表明碳纳米管的反复变形可能交替增强氧在碳纳米管表面的吸附和扩散过程,对碳纳米管氧化防护、利用以及碳纳米管器件的设计有重要指导意义。3.计算模拟了碳纳米管化学吸附水分子的过程及其对碳纳米管电性的显著影响。通过第一性原理计算研究了水分子在(8,0)碳纳米管表面的化学吸附对碳纳米管电学性质的影响,同时也研究了水分子在(8,0)碳纳米管表面可能的化学吸附过程。结果表明,物理吸附的水分子对碳纳米管的电学性质影响微弱,而化学吸附的水分子能显著减小碳纳米管的能隙。尽管水分子在理想(8,0)碳纳米管表面化学吸附比较困难。但是,当存在外加横向电场或掺杂N原子时,水分子在(8,0)碳纳米管表面吸附的能垒都有明显地降低。由于在实际使用的碳纳米管可能会不同程度地含有各种杂质(如制备碳纳米管的金属催化剂等)或者受到一定的横向电场作用,因此,水分子在碳纳米管表面化学吸附并改变碳纳米管的电学性质是可能的。本研究为文献报道的湿度影响碳纳米管电学性质的实验现象提供了一种可能的解释。更多还原

【Abstract】 Carbon nanotubes (CNTs) exhibit unique electronic, mechanical, and chemical properties that make them attractive building blocks in many fields of nanotechnology. In particular, the large surface area and hollow geometry of carbon nanotubes make them easy to adsorb small molecules and sensitive to environmental exposure. Although great efforts have been made on these issues, there are still many concerns remaining elusive. In this thesis, the first-principles calculations are systematically performed to study the chemisorption behaviors of the molecular hydrogen, oxygen, and water on CNTs. The following advances have been achieved:1. Axial deformation and the net charge remarkably influence the H2 chemisorption on CNTs. The tensile and compressive axial deformation of the (8, 0) CNT can effectively influence the energy barrier for the direct chemisorption of H2. It is shown that, for the optimal chemisorption/desorption pathway, about a 7.05% compressive deformation of the CNT can lower the chemisorption barrier from 1.98 to 1.56 eV, and the desorption barrier from 1.64 to 1.20 eV. Besides, the chemisorption configurations would transform to an energetically more favorable configuration, through the diffusion of H on the CNT. However, the tensile or compressive axial deformation can not effectively modulate the energy barriers for the transitions between chemisorption configurations. The study on chemisorption of H2 on a series of charged CNTs reveals that the net charge can remarkably change the chemisorption barrier of H2. Generally, the negative charge can more remarkably lower the chemisorption barrier for the armchair CNTs, while the positive charge can more efficiently lower the barrier for the zigzag CNTs. Therefore, our results show that it is possible to improve the hydrogen storage capacity of carbon nanotube by applying the axial strain or injecting the charge to the carbon nanotubes.2. The kinetics properties of chemisorption, dissociation and diffusion of oxygen on the CNTs are strongly sensitive to the surface concavity-convexity. The first-principles calculations show that the chemisorption (cycloaddition) of a singlet O2 on the outer surface of the (8, 0) CNT has an energy barrier about 0.94eV, while the chemisorption barrier for the inner surface is much higher. On both the inner and outer surfaces of the CNT, the cycloadded O2 is likely to dissociate into two isolated O with an energy barrier of about 0.6~1.3 eV. Becausse of the different bonding strength on inner and outer surfaces and the presence of unique configuration with oxygen on the inner surface, the pathways of oxygen diffusion on the inner surface are completely different from that on the outer surface. And the dissociated O on the inner surface can diffuse easily with a small energy barrier of 0.33eV, while the diffusion on the outer surface has a relatively high energy barrier of 1.11 eV. It is also found that the radial deformations of CNT can remarkably enhance the chemisorption of O2 even on the inner surface. It is could be deduced that, when the CNTs are deformed by the alternating mechanical force, the protruding site can chemisorb O2 easily and the following concave surface can facilitate the diffusion. Therefore, the oxidation can be strongly enhanced.3. The water chemisorption is found to remarkably influence the electronic property of CNTs and water dissociative adsorption on the CNT may occur easily when an external electric field or doping atoms being introduced. The influence of water chemisorption on the electronic propertis of the (8, 0) CNT and the kinetics behavior of water chemisorption is studied by density founctional calcualtions. The results show that the water molecule dissociative adsorption on the (8, 0) CNT can obviously decrease the energy gap. Using the elastic band method, the chemisorption of water on the (8, 0) CNT is found to be very difficult, and the smallest energy barrier of the chemisorptions is 2.89 eV in our work, with the H and OH dissociated from water bonding at the opposite sites of carbon hexagon. However, the external electric field in tranverse direction or the substitution atom N on the CNT surface can obviously lower the energy barrier for water chemisorption. Therefore, the foreign atoms, such as the iron as the catalyst for growing CNTs, or the external electric field may lead the water can easy to chemisorb on the CNTs and further influence the electronic properties as the previously reported experiment.更多还原