【作者】 马杰；

【导师】 王健农；

【作者基本信息】 上海交通大学 ， 材料学， 2009， 博士

【摘要】 单壁碳纳米管(SWCNT)具有独特的结构,优异的力学性能和理化性能,在复合材料、纳米电子器件、催化剂载体等各大领域都具有巨大的应用前景。但对实际应用而言,还存在一些技术难题。例如:SWCNT的电子结构与其直径有关,文献中所报道的SWCNT总是不同直径的SWCNT的混合体,导致金属型和半导体型SWCNT混合存在。有效地增大SWCNT的直径进而控制其电学性能成为SWCNT基础研究和应用的瓶颈。又如,电弧法、激光法、催化热解法所制备的原始SWCNT样品中,都含有大量的杂质(如催化剂颗粒),需要利用强氧化方法才能除去,但不可避免地对SWCNT结构产生破坏。如何大规模无损伤纯化SWCNT是个迫切需要解决的难题。再如,碳纳米管表面缺陷少、缺乏活性基团,碳管之间存在较强的范德华引力,易于形成团聚或缠绕,不能在溶液中均匀分散,影响和制约单壁碳纳米管的应用。本文以SWCNT制备--纯化--分散--应用为研究路线,对以上的问题进行深入的研究和探讨。本文以乙醇为碳源,二茂铁作为催化剂,噻吩为添加剂成功制备了大直径单壁碳纳米管。在保持其他工艺参数不变的条件下,仅仅通过改变噻吩的含量,有效地增大了SWCNT的平均直径,可从1 nm增加到5.8 nm,而且实现了样品的连续制备,SWCNT原始样品的产量可达1g/h。根据实验结果,提出了增大SWCNT直径的生长模型:噻吩高温下分解出的硫原子被吸附在纳米铁催化剂颗粒的表面,所形成的液相区成为SWCNT的成核点,不同硫的添加量,对应不同大小的成核点以及不同直径的SWCNTs。电学性能测试表明:不同直径的SWCNT样品具有不同的电学性能,大直径(LD)SWCNT的电阻率为0.1 m·cm,小直径(SD)SWCNT的电阻率为0.5 m·cm,相差近5倍。伏安测试进一步表明,LD-SWCNTs呈现线性电阻特征,而SD-SWCNT呈现非线性电阻特性,其原因是由于SD-SWCNT样品中可能存在较多的半导体型SWCNT,随着电压的增大,占主要比例的半导体型碳管由于温度效应的影响,使得半导体型碳管中参加导电的载流子数目增多,宏观纤维样品的电阻率随之下降,呈现非线性电阻特征。本文探索出一种纯化效果显著对结构无损伤的SWCNT纯化方法。即将原始样品中的金属Fe催化剂颗粒在空气气氛下,低温氧化成氧化铁组织,然后将样品置于氩气气氛下进行高温热处理,使氧化铁与外层包裹的碳发生还原反应,形成金属铁颗粒,最后采用弱酸酸洗反应剩余物,去除金属Fe颗粒,最后达到纯化SWCNT的目的。对纯化前后SWCNT进行了Raman、近红外和热重分析,结果表明:本纯化方法对SWCNT的石墨结构没有明显的破坏。SWCNT具有极大的长径比,长度达到微米级,直接制备获得的碳管易缠绕,分散性差。本文制备的LD-SWCNTs样品纯化后不需进行任何复杂表面官能团修饰处理,可以直接均匀分散于溶液中,而SD-SWCNT无法均匀分散,以悬浮状大颗粒的形式存在于溶液中。结合红外光谱和X射线光电子能谱分析测试表明:SD-SWCNT和LD-SWCNT表面均含有相同性质和相同摩尔数的官能团,从而排除碳管样品表面官能团差异性对分散的影响,可以推断纯化后小直径、大直径SWCNT样品在溶液中分散性的差异是由于直径差异所致。本论文首次提出低温冷冻法处理SD-SWCNT。以乙醇水溶液为溶剂,液氮作为冷却介质,将获得的冷冻固体高速粉碎,成功地获得SD-SWCNT均匀分散的溶液。低温冷冻处理后SD-SWCNT样品在乙醇溶液中均匀分散,纳米激光粒度分布仪测试结果表明SD-SWCNT在乙醇溶液中分散的颗粒度为165.1nm,较未处理的初始样品,平均分散粒度有了三个数量级的明显减小。开展了使用SWCNT作为燃料电池催化剂载体的研究。采用乙二醇还原的方法制备出碳负载型金属Pt颗粒的催化剂。研究了不同直径SWCNT作为载体的Pt颗粒催化剂的催化性能的差异。测试结果表明:LD-SWCNT负载的Pt催化剂具有最高的电化学比表面积,相对于SD-SWCNT和多壁碳纳米管有近一倍的提高。比Johnson Matthey公司的商业化催化剂的催化活性也提高了近50%,这可能归属于LD-SWCNT具有优良的导电性能和分散性能。更多还原

【Abstract】 Single-walled carbon nanotubes (SWCNTs) have potential applications in many fields, such as composite materials, nano-electronic devices, and catalyst supports, due to their unique structure and good mechanical, physical and chemical properties. However, there exist some technical hurdles for the applications. For example, the electronic structure of a SWCNT is related to its tube diameter. The currently available techniques for CNT synthesis always produce tubes with a wide range of diameters. As a result, metallic and semiconducting SWCNTs are always coexistent. As a matter of fact, it has been a bottleneck for many fundamental studies and applications how to control the electric properties by increasing the diameter of SWCNTs. The second hurdle is that initial SWCNTs synthesized by arc-discharge, laser vaporization, and chemical vapor deposition (CVD) methods contain a large quantity of impurities (catalytic nano-particles). These impurities need to be purified by strong oxidation methods, but during such purification process, structural destruction of SWCNTs is inevitable. Thus, it is necessary to develop techniques for non-destructive purification. A third hurdle concerns dispersion, Current SWCNTs are bound into entangled ropes or masses with bad dispersion. They have to be separated from each other and have good dispersion for many applications. In this dissertation, the research route follows“controllable synthesis-purification- dispersion-application”, and new approaches are proposed to resolve the above-mentioned issues with SWCNTs.A CVD method is used for synthesizing SWCNTs in this study. Ethanol is used as the carbon feedstock, ferrocene as the catalyst, and thiophene as the growth promoter. Under keeping the other experimental condition, the diameter of SWCNTs is increased effectively only by changing the amount of thiophene addition. SWCNT samples with different average diameters from 1 to 5.8 nm can be synthesized continuously, with a production rate of about 1 g h-1. Based on experimental results, a growth model is proposed: S atoms from thiophene decomposition are absorbed on the surface of an Fe particle. The absorption leads to the formation of a liquidus region on the metal surface, and this liquidus region becomes the nucleation core of SWCNTs. With changing thiophene addition, the size of the liquidus region and thus the tube diameter change.The electrical properties of SWCNTs have been studied. Results show that different-diameter SWCNTs have different electrical properties: the electric resistivity of SD-SWCNTs is 0.5m·cm and that for LD-SWCNTs is 0.1m·cm. Additionally, voltammetry curves suggest that the resistance behavior of SD-SWCNTs is non-linear, but that of LD-SWCNTs is of a linear nature. This may be attributed to the possibility that the SD-SWCNT sample contains a high proportion of semiconducting variants, the number of carrier increases for temperature effects with the increasing of the voltage, and thus the electric resistivity decreases.In this dissertation, a high-effective and non-destructive purification method is proposed. Fe catalytic particles are heated to be oxidized at a low temperature in air, and then the sample is heat-treated under the protection of Ar gas at a high temperature to induce the reduction of Fe2O3 by the encapsulating carbon to Fe. Finally, the naked Fe or FeO particles are dissolved easily by HCl. During this purification process, little damage is induced to the graphitic structure of SWCNTs as evidenced by TG, Raman and NIR spectroscopic studies.SWCNTs have high length-diameter ratios, and thus are usually bundled and entangled together with low dispersion in any solutions. The LD-SWCNTs, however, are found to be dispersible uniformly in water or ethanol without any modification after purification. In contrast, the purified SD-SWCNTs could not be dispersed. Instead, they tend to form large aggregates in solutions. Fourier transform infrared spectrometer and X-ray photoelectron spectroscopy results indicate that SD- and LD-SWCNTs have the same types and the same quantities (mol) of functional groups on their surfaces. Thus, it may be concluded that the good dispersion observed for LD-SWCNTs is a result of their large diameters.To improve the dispersion of SD-SWCNTs, a cryogenic freezing and crushing method is proposed in this dissertation. Ethanol and water were used as solvents, and liquid nitrogen as a freezing medium. The frozen mixture of ethanol, water, and SWCNTs was crushed with high-speed blades. After dissolution and filtering, the obtained SD-SWCNTs could be dispersed uniformly in ethanol. The results of nano-laser particle size analyzer indicate that the average particle size of such SWCNTs is 165.1 nm, which are 3 orders of magnitude smaller than the SD-SWCNTs without the treatment.The application of SWCNTs as a catalyst support for fuel cells has been studied. Pt nanoparticles are deposited on LD- and SD-SWCNTs by a glycol reduction method. Electrochemical tests show that the catalytic activity of Pt particles on LD-SWCNTs is 2 times of that for Pt particles on SD-SWCNTs or multi-walled CNTs and also much higher than the commercial catalyst from Johnson Matthey Co. The excellent activity observed for LD-SWCNTs may be attributed to their good electrical conductivity and dispersion property.更多还原