异型碳纳米管和富勒烯吸附性能及其生长机理的计算模拟研究

【作者】 吴红丽；

【导师】 邱介山；

【作者基本信息】 大连理工大学 ， 化学工艺， 2005， 博士

【摘要】 纳米科技是21世纪技术革命的前沿领域。其中,以富勒烯和碳纳米管为代表的纳米碳材料倍受科研工作者的关注。纳米碳材料的吸附性能尤其是储氢性能的研究及其生长机制的研究是其中两大异常活跃的领域。从理论上揭示碳纳米管和富勒烯的吸附性能以及碳纳米管生长机制是倍受人们关注的重要课题。 迄今为止,有关碳纳米管之吸附性能的研究主要局限于直线型碳纳米管。已有的研究结果表明,碳纳米管是一种优良的气体吸附材料,其吸附性能在很大程度上依赖于碳纳米管的形貌和结构。本文采用分子力学（MM）和分子动力学（MD）方法,对几种不同形态的碳纳米管,即三种理想的单壁Y型碳纳米管和三种理想的单壁L型碳纳米管对小分子（H₂,O₂,NH₃,H₂O,CH₄）的吸附性能进行了计算模拟研究,并与相应的直线型碳纳米管之吸附性能进行了对比,系统地考察了碳纳米管的形态、直径、螺旋性、结构缺陷以及吸附温度等因素对吸附过程的影响,并对吸附机理进行了探讨。模拟研究结果表明:（1） 氢分子在端口开放的异型碳纳米管上通过自然的物理吸附得到的储氢量相当有限,即使在低温时也无法达到美国能源部（DOE）的储氢标准;（2） 端口封闭的异型碳纳米管之储氢量高于相应直线型碳纳米管的储氢量,且Y型碳纳米管的储氢量略高于L型碳纳米管的储氢量;在本论文研究范围之内,对于任何一种给定的异型碳纳米管,降低温度和增大碳纳米管的直径均有助于提高异型碳纳米管的储氢量;且椅式异型碳纳米管之储氢性能略优于齿式异型碳纳米管的储氢性能;（3） 异型碳纳米管对NH₃分子和H₂O分子的吸附明显优于对CH₄、O₂和H₂分子的吸附;在吸附H₂O、NH₃和O₂分子后,碳管和小分子之间的电荷转移较大;（4） 碳纳米管或者其吸附体系的前线轨道主要来自于七元环及其附近碳原子的贡献,说明Y型碳纳米管上七元环及其周围碳原子可能成为化学修饰和化学反应的活性位点,与本论文中有关Y型碳纳米管化学加氢的计算结果相吻合;（5） 在吸附小分子后,异型碳纳米管最高占有轨道（HOMO）与最低未占轨道（LUMO）之间的能差明显减小,这一趋势对于椅式异型碳纳米管而言更为明显。 采用从头算方法在HF/6-31G水平上比较了氢分子进入几种富勒烯及其氧化物(C₆₀,C₆₀O,C₆₀O₂,C₇₀)笼内的难易程度,考察了富勒烯碳笼的大小、碳笼上的氧原子以及氢分子进入碳笼的路径对这一过程的影响。计算结果表明:（1） 对于C₆₀和C₆₀O,由于五元环弯曲程度高于六元环,氢分子垂直五元环进入笼内的能垒小于垂直六元环进入笼内的相应能垒,即氢分子容易沿着垂直于富勒烯上的五元环进入笼内;而对于C₆₀O₂,由于两个氧原子的存在,得到的计算结果与C₆₀和C₆₀O相反;（2） 对于C₇₀,氢分子垂直于端部五元环进入笼内的能垒与垂直于端部六元环进入笼内的能垒相接近,但氢分子垂直于赤道六元环进入笼内的能垒明显升高;（3） 氢分子进入较大碳笼C₇₀的能垒小于进入富勒烯C₆₀内的能垒,说明大的富勒烯碳笼有利于氢分子在笼内的存储。更多还原

【Abstract】 Nano science and technology （NST） are at the forefront of technological revolutions in the 21st century, among which carbon nanomaterials have been the hot topics of many researchers, with special attention to the fullerenes and carbon nanaotubes （CNTs）. The investigations on the adsorption properties especially associated with hydrogen storage properties and the growth mechanism for fullerenes and carbon nanaotubes are two most active fields, in which the theoretical study holds its distinguishable position as this in turn will contribute to experimental processes even practical applications.A large body of open literatures has addressed the issues related to the absorption properties of straight CNTs and the results showed that the absorption properties are highly dependent of their structure and morphology. Unfortunately, to our best knowledge, little coverage has been given to hetero-junctioned CNTs such as Y-junction CNTs that are available in fairly large amount depending on the methods and precursors used. To assess the possibility of non-straight or hetero-junctioned carbon nanotubes as adsorbents for several typical molecules, such as H₂, O₂, NH3, H2O, CH4, theoretical studies were performed on Y-shaped, L-shaped and straight CNTs via molecular mechanics （MM） and molecular dynamics （MD） simulations. For each shape, three different ideal single-walled structures are of particular interest. The factors that will influence the absorption properties of CNTs have been taken into account, including the shape, diameter and helicity of CNTs, the defects as well as the adsorption temperature. The mechanism involved in adsorption process is another focus. The main results are summarized as below:（1） By natural adsorption, the hydrogen amount in each open-ended CNT is quite limited, and seems unlikely to meet the standard set by department of energy （DOE） of America even at 80 K; （2） Hydrogen storage capacity in close-ended hetero-junctioned CNTs is higher than in straight CNTs under the same conditions, and Y-CNTs are also in preference to L-CNTs. For each type of CNT, larger diameter, lower temperature and smaller defect are beneficial for hydrogen storage, and the armchair-branched CNTs are more favorable to hydrogen storage than zigzag-branched CNTs; （3） Of all the gas molecules involved, NH3 and H₂O show higher tendency to be absorbed on hetero-junctioned CNTs, which can be interpreted in terms of more charge transfer; （4） For both adsorption and non-absorption state, the frontier orbitals of hetero-junctioned CNTs arise mainly from the carbon atoms on heptagons and their neighbors. This enables us to suggest that the carbon atoms on heptagons and their neighbors may function as active sites for further chemical reaction, which is confirmed by the calculations of chemical addition of hydrogen atoms; （5） The energy gaps between two frontier orbitals for each hetero-junctioned CNTs diminish considerably after the molecule was adsorbed, and this phenomenon is more pronounced for armchair hetero-junctioned CNTs.The possibility of hydrogen storage in several fullerenes and their oxides （C₆0, C₆0O, C₆0O₂, C70） was also a concern of this work. By ab initio method, the effects of the cage size,更多还原