论文题目：定向生长碳纳米管薄膜的研究
作者简介：曹安源，男,1974年07月出生，1996年09月师从于清华大学吴德海教授，于2002年01月获博士学位。
摘
要
碳纳米管是新型的一维纳米材料，在1991年由日本的电镜专家发现并提出了其结构模型。碳纳米管是由石墨的六边形网状层面卷曲闭合形成的。它们的直径很小，通常在几十纳米以下，最小的单壁碳纳米管直径只有0.4 nm；但是长度可达几十甚至数百微米。如此高的长径比，以及其独特的中空管状结构，使得碳纳米管一经发现，就立刻引起了科学界的浓厚兴趣。研究结果表明，碳纳米管代表了纳米材料优异的性能。它们具有最高的轴向强度和导热系数，并且随着石墨层片卷曲方式的不同，既可以是导体，也可以是半导体。单壁碳纳米管还具有量子传输的特性。所有这些使得碳纳米管在许多领域具有潜在的应用，例如作为复合材料增强相、纳米机械装置、场发射材料、场效应管、显微探针、化学传感器、储氢材料以及微电子器件等。
碳纳米管最佳的力学、热学和电学性能都是沿着它们的轴向（即位于石墨的基面），可以设想，如果将大量的碳纳米管按照同样的方向排列起来，就能够充分地体现和发挥它们的优异性能。然而，人们通常采用化学气相沉积或电弧放电法所制备的样品中，碳纳米管都是呈无序排列的。这是因为碳纳米管的长径比极大，导致它们在生长过程中总是倾向于弯曲并且互相缠绕在一起，给进一步的研究与应用造成很大的困难。因此如何使得这种纳米级的一维材料沿着轴向呈定向地生长，最终形成一个规则排列的有序整体，就具有迫切的现实意义。
因此，本文首要的研究内容，就是通过实验方法的设计和工艺参数的优化，直接得到定向生长的碳纳米管薄膜。并且使其生长成为可控，即能够对所制备的定向碳纳米管薄膜的形态和质量进行控制。在此基础上，建立了碳纳米管定向生长的物理模型，进行了热力学与动力学分析。此外，本文在多壁碳纳米管薄膜中定向生长出了性能更加优越的单壁碳纳米管，这是实现单壁碳纳米管的定向生长的重要一步。论文最后研究了定向碳纳米管薄膜作为场发射和太阳能的工程应用，结果表明，定向生长的碳纳米管在这两个方面显示了良好的应用前景。
本文采用化学气相沉积法制备定向生长的碳纳米管薄膜。以二茂铁作催化剂、二甲苯为碳源，使二甲苯在合适的反应温度下裂解出碳原子簇，然后在二茂铁分解出的纳米级铁颗粒上重组形成碳纳米管结构。采取精确控制的方法，通过宏观调控参数实现了对微观纳米材料的排列。对本实验样品的检测结果表明，定向碳纳米管薄膜的生长包括形核期、高速生长期以及饱和期三个阶段，在反应30分钟后，其厚度可达90 mm。系统地研究了本实验方法下反应温度、碳源种类及进给速率、催化剂浓度、生长基底及其预处理等工艺参数对所制备样品的定向性、生长速率以及碳纳米管直径的影响规律，得到了优化的工艺参数。并且，利用碳纳米管在不同基底上（石英和金膜）的选择性生长制备了模块化的定向碳纳米管薄膜阵列。定向碳纳米管薄膜中残存有大量的催化剂（铁）颗粒，很多是位于碳纳米管的管腔内，采用后处理的办法很难去除。本文采用两阶段法，即在反应初始阶段进给一定量的催化剂，当形成厚度较小的一薄层定向碳纳米管薄膜后，就停止催化剂，只进给碳源，这时候定向碳纳米管薄膜仍然能够持续生长变厚，而薄膜中的铁颗粒含量却明显减少。对定向碳纳米管薄膜的净化和碳纳米管的开口也是棘手的问题。常规的方法是将样品分散于酸溶液中（硝酸或硫酸）煮沸，但这不可避免破坏了碳纳米管原来的定向性。本文采取在定向碳纳米管薄膜的生长过程中引入氧化性介质（水蒸汽、二氧化碳），有效去除了附着在碳纳米管管壁上的非晶碳，并经氧化打开了其端帽，从而直接得到了纯净、开口的定向碳纳米管薄膜。直接、可控地生长定向的碳纳米管薄膜把研究碳纳米管带到一个崭新的阶段，使人们可以从宏观、整体的角度来研究这些一维纳米材料的有序排列所产生的新性能。
本文发现了XRD衍射谱中（002）峰强度随着碳纳米管薄膜定向性的提高而单调递减的规律，并提出了相应的衍射模型。利用XRD衍射结果可以从宏观上、定性地确定碳纳米管的总体定向程度。
本文在实验和扫描电镜观察的基础上，建立了定向碳纳米管薄膜生长的物理模型，提出了碳纳米管的开口生长机制。由二茂铁分解出的大量微小铁颗粒在基底上的密集、均匀分布使碳纳米管形核时互相支撑、垂直生长，是形成定向碳纳米管薄膜的关键因素。催化剂的持续供给使碳纳米管在生长过程中始终保持开口，从而具有较高的化学活性，有利于其连续生长。热力学研究表明，碳纳米管更易于沿轴向（而不是径向）呈有序地定向生长。动力学分析表明，定向碳纳米管薄膜的平均生长速率为3 mm/min，瞬时最大速率可达6 mm/min。
上述的定向碳纳米管薄膜含有的都是多壁碳纳米管（含有多层管壁）。人们更加关心的是单壁碳纳米管，因为它们只有一层管壁，是最简单、最典型的一维纳米材料，但是性能却远远优于多壁碳纳米管。由于单壁碳纳米管的直径仅为1 nm左右且生长条件更为苛刻，其定向生长要比多壁碳纳米管困难得多。本文通过实验研究，实现了单壁碳纳米管在多壁碳纳米管薄膜中的定向生长，并且提出了单壁碳纳米管在定向碳纳米管薄膜中的受约束生长模型。直立的多壁碳纳米管对单壁碳纳米管的生长起支撑作用，它们之间的狭长空间构成了单壁碳纳米管定向生长的天然模板。同时，单壁碳纳米管较高的强度和良好的韧性是保证其持续地、呈“S”形生长的前提。高分辨透射电子显微镜观察表明，定向生长的单壁碳纳米管呈现两种形态：独立的平直单壁管束（在多壁管之间生长）和依附于多壁碳纳米管外壁上的单壁管束（和多壁管同步生长）。
碳纳米管薄膜的定向生长开辟了广阔的工程应用前景。它是理想的场发射材料，由于定向碳纳米管薄膜内的碳纳米管尖端一律向上，这些尖端可以被看作是无数的密集排列的发射尖锥。场发射研究结果表明，定向碳纳米管薄膜具有良好的场发射性能，在较低的场强下（< 2 V/mm）即可达到较高的发射电流密度（5 mA/cm2），是极有希望的平板显示器件的阴极材料。本文还实现了大面积（90 ´ 25 mm2）定向碳纳米管薄膜的场发射，并且通过控制定向碳纳米管薄膜的厚度及疏密程度提高了场发射的均匀性。
由于碳纳米管的密集垂直排列，定向碳纳米管薄膜对太阳光谱的可见光和近红外线显示出强烈的吸收作用（吸收比可达99 ％），在太阳能集热器方面有潜在的应用。通过制备模块化的定向碳纳米管薄膜阵列，使具有红外高反射特性的黄金薄膜和碳纳米管相结合，可以提高定向碳纳米管薄膜吸收太阳光的选择性。
关键词：碳纳米管，薄膜，定向生长
Abstract
Carbon nanotubes (CNTs), a new one-dimensional nano-materials, were firstly discovered by S. Iijima in 1991 and their tube-structure was proposed. CNTs can be formed by rolling up a graphitic sheet consisting of carbon-hexagons. They have very small diameters (the smallest single-walled nanotube is 0.4 nm), but their length usually reaches hundreds of micrometers. The large aspect ratios and the unique hollow structure make CNTs attractive to many scientists throughout the world since their discovery. Research results revealed that CNTs represented the best properties of nano-materials. They showed the highest tensile strength and thermal conductivity, and can be semi-conductive or metallic depending on how the graphitic sheets are rolled up. Furthermore, single-walled carbon nanotubes (SWNTs) are ideal one-dimensional quantum wires due to the extremely small sizes. The excellent properties make CNTs potential candidates in many fields, such as reinforcement in composites, nanometer mechanical system, field emitters, field-effect transistors, micro-probes of AFM, chemical sensors, hydrogen storage and micro electronic devices.
As the excellent mechanical, electrical and thermal properties of CNTs are all along their axis direction (in the basal plane of graphite), they can be fully explored only when a large number of CNTs are ordered arranged along the same direction. However, CNTs in the samples that were prepared by conventional Chemical Vapor Deposition (CVD) or electric arc method are always randomly distributed. CNTs have a strong tendency to curl and entangled with each other during the growth process because of their high aspect ratios, which heavily inhibit further applications of them. Therefore it is of great importance and much significance to be able to make CNTs grow along a particular direction, so that to obtain a well-ordered group consisting of parallel CNTs.
The first of all task of this dissertation is to synthesize well-aligned carbon nanotubes, which possess a self-oriented growth, by a controllable way, that is, the desired sample morphology and quality can be obtained easily. The physical model of the aligned growth of CNTs was established based on the experimental results, accompanied by a thermaldynamic and kinetic analysis. Particularly, the author firstly obtained large quantity of SWNTs, which grow upward unanimously also, in the aligned multi-walled nanotube (MWNTs) films. This should be a critical step toward the direct synthesis of aligned pure SWNTs. Finally, some engineering applications of aligned CNT films have been explored, including field emission, solar energy utilization and hydrogen storage. The results showed that aligned CNTs are surely promising candidates in these three aspects.
CVD method was used to synthesize well-aligned CNT films on plane quartz substrate, in which ferrocene was taken as the catalyst precursor and xylene as the carbon source. Ferrocene would decompose into numerous micro Fe particles at the reaction temperature (800 ~ 850 °C), and then carbon clusters coming from xylene arrive at these Fe particles and form the tube structure on the surface of them. Experimental results showed that the growth of aligned CNT films went through three sequential stages: nucleation, rapid growth followed by saturation. The film thickness can reach 90 mm after 30 minutes. The influence of the reaction temperature, different kinds of carbon sources and their feeding rate, ferrocene percentage in the xylene solution, growth substrates and their pre-treatments on the sample alignment, growth rate and the CNT diameters was studied systematically and the optimal experimental parameters proposed. Furthermore, patterned aligned CNT arrays were obtained on a substrate combining quarts and Au due to the selective growth of CNTs on these two kinds of materials. After the nanotube growth process, a large quantity of catalytic particles (Fe) will remain in the aligned CNT films, most of which stay inside the inner nanotube cavities, and can not be removed easily through post-treatments. A two-stage feeding way, feeding both ferrocene and xylene at the first stage until a short aligned nanotube film formed and then (at the second stage) feeding only xylene to keep its continuous growth, was adopted and the Fe particle content was found greatly reduced. Researchers concern also the purification of the aligned samples and how to open the CNT ends, which can not be treated by conventional ways such as boiling with acids (nitric or sulphuric acid) because the original nanotube alignment would be damaged inevitably. Oxidative media, such as water vapor or carbon dioxide, was introduced into the reaction furnace during the growth process and the amorphous carbon on the nanotube walls was removed effectively. Also, a certain percentage of the ends of CNTs was found open due to the oxidation on graphitic sheets. Introduction of oxidative media provides a feasible way to directly grow purified CNT films with open nanotube ends. A direct, controlled production of aligned CNT films brings the opportunity to fully study the new phenomenon that may emerge from these ordered nano-materials through a macroscopic point.
X-Ray Diffraction Characterization was carried out on the aligned CNT films, and it was firstly found that the intensity of (002) peak (the basal plane of graphite) decreased monotonically while the aligned degree of the samples increased. A XRD model was proposed to explain this phenomenon. XRD characterization is an easy method to determine the aligned degree of CNT films qualitatively and macroscopically.
A physical growth model was established for aligned CNT films, and the open-end growth mechanism of CNTs was proposed based on the examination by scanning electronic microscopy (SEM). The dense, even distribution of catalyst particles throughout the substrate surface is a critical factor for the formation of aligned CNT films, as CNTs formed on them are closely contacted and can support each other, resulting in the vertical growth from the substrate. The continuous catalyst feeding ensures open ends and high chemical activity of CNTs during their growth process. CNTs tend to grow orderly along the axis rather than the radius in thermodynamic aspect. The kinetic analysis showed that the average growth rate was about 3 mm/min by our way, and the can reach an instantaneous value of as high as near 6 mm/min.
The aligned CNT films mentioned above are composed of MWNTs, which have many layers of tube-walls. SWNTs, consisting of just one layer of graphitic sheet, are of more concern because they are the most simple and exact one-dimensional model in so many nano-materials, but of much better properties. However, the diameters of SWNTs are only about 1 nm and the growth of them require more strict conditions. So to achieve aligned SWNTs is much more difficult than to synthesize aligned MWNTs. Oriented growth of SWNTs among vertically aligned Multi-walled carbon nanotubes (MWNTs) was at the first time reported by the author through an improvement of the original CVD way. A confined-growth model of SWNT bundles among aligned CNT films was proposed. The larger and straight MWNTs can readily support the SWNT bundles, which have strong tendency to be curled during the growth process, and the narrow and long spacing between adjacent parallel MWNTs is a natural template for SWNT growth. The high strength and good flexibility of SWNTs ensure that they can grow upward continuously and appear a “S” shape. Examination by high-resolution transmission electronic microscopy (HRTEM) showed that there were mainly two kinds of SWNT bundles owning an aligned growth. One is the separated and straight bundles growing between MWNTs, and the other is the flexible SWNT bundles depending on the out-walls of adjacent MWNTs, which grow with them simultaneously.
The aligned growth of CNTs has brought a promising future in their engineering applications. Aligned CNT films are desired field emission materials, in which all the nanotube tips point upward unanimously and can act as numerous emission micro-cones with a high density. Field emission testing of them showed excellent performance, achieving a current density as high as 5 mA/cm2 under a relatively low applied voltage of 2 V/mm, which can readily act as cold cathodes of flat panel displays. A large area of 90 ´ 25 mm2 field emission was also obtained by direct synthesizing such large CNT films. The uniformity of field emission can be improved by preparing shorter CNT films with a larger inter-nanotube spacing.
The aligned CNT films showed strong absorption of sun light in the visible and near-infrared range (the absorptance reaches 99 %) due to the close arrangement of parallel CNTs, which showed potential applications in solar thermal collectors. The selectivity can be improved by combining the high visible absorption of CNTs and high infrared reflection of Au films.
Keywords: carbon nanotubes, films, aligned growth
