【作者】 于立岩；

【导师】 崔作林；

【作者基本信息】 青岛科技大学 ， 材料学， 2009， 博士

【摘要】 纳米碳纤维和纳米碳管有许多独特的性能,这些独特的性能使其具有许多潜在的应用,如催化剂载体,储氢,复合材料,纳米电子和纳米机械装置,和场致发射装置。影响纳米碳纤维和纳米碳管生长的因素很多,如催化剂种类和粒子尺寸,反应温度,载体的类型以及碳源的种类,在所有这些因素中,催化剂粒子尺寸和反应温度是关键因素。在本论文中,我们利用不同纳米金属催化剂和各种催化剂前驱体,催化乙炔或乙烯制备了一维碳纳米材料,纳米碳纤维和纳米碳管,并且利用铁基底铜催化剂催化乙炔成功的制备了纳米碳纤维阵列。通过扫描电镜、透射电镜、X射线粉末衍射、原子力显微镜和热重分析仪等分析方法对合成的纳米碳纤维和纳米碳管进行了表征。重点研究了催化剂粒子尺寸和反应温度对所制备的纳米碳纤维和纳米碳管形貌的影响,以及铜催化剂催化乙炔垂直定向生长碳纤维需具备的因素,并且讨论了其生长机理。在前期科研的基础上,我们有了新的进展:采用物理方法制备的纳米金属粒子成功的合成了螺旋形貌的碳纤维,实验重复性好,并发现了螺旋碳纤维生长的小尺寸效应。分别采用惰性气体蒸发法制备的纳米铜粒子和氢电弧等离子体法制备的纳米A1₂Cu粒子为催化剂,催化乙炔制备了螺旋型和直线型两种不同形貌的纳米碳纤维。研究结果表明催化剂粒子尺寸对所制备的纳米碳纤维的形貌有很大的影响。通常粒径小于50nm的催化剂粒子生长螺旋纳米碳纤维,在一个纳米催化剂粒子上,总是只生长两根螺旋纳米纤维。它们具有绝对相反的螺旋旋向,但却具有相同的纤维直径,纤维截面,螺旋直径,螺旋圈数,螺旋间距,和螺旋长度。这两根螺旋纤维经常在相同螺旋圈数的位置同时发生螺旋旋向的改变,即螺旋反转。较大尺寸的催化剂粒子易催化生长直线型纳米碳纤维。纳米催化剂粒子在催化纳米碳纤维生长的过程中,经历了一个重要的形状变化过程。即由初始的不规则形状变成了规则的多面体形状。分析表明催化剂粒子形状改变的驱动力来源于乙炔气体对纳米粒子晶面的吸附引起的表面能变化。纳米粒子不同晶面活性差异是生长螺旋纤维的首要条件。利用硝酸铜、碱式碳酸铜和酒石酸铜作为催化剂前驱体,催化乙炔制备了螺旋/直线纳米碳纤维。同样观察到粒径较小的催化剂粒子易生长螺旋纳米碳纤维,粒径较大的催化剂粒子易催化生长直线型纳米碳纤维。利用铁基底铜催化剂催化乙炔成功的制备了纳米碳纤维阵列,在基底上纳米碳纤维排列整齐,催化剂粒子位于垂直生长的碳纤维中部位置。透射照片清楚地显示在一个催化剂粒子上生长出两根纳米碳纤维,纳米碳纤维的直径与催化剂粒径相当,两根纳米碳纤维的夹角为180°;催化剂粒子与反应前相比形貌发生了变化。利用玻璃基底镀的铜溶胶膜和使用蒸发法在玻璃基底上沉积的铜膜催化乙炔制备了纳米碳纤维膜,此方法中纳米碳纤维在玻璃基底上无垂直生长的现象,纳米碳纤维相互交叉和缠绕,定向性并不明显。铜催化剂催化乙炔垂直定向生长碳纤维需具备以下因素:（1）基底上的铜催化剂粒子排列致密;（2）铜催化剂粒径足够大（大于100nm）。生长过程如下:在一个铜催化剂粒子上生长出两根纳米碳纤维,分别沿垂直方向生长,由于基底上的催化剂粒子排列很致密,每根碳纤维在生长的过程中均受到周围碳纤维力的作用,互相支撑,使得纳米碳纤维在垂直方向上生长。玻璃基底镀的铜溶胶膜中的纳米铜粒子分布稀疏,由其催化乙炔生长碳纤维时,碳纤维之间缺乏相互支撑,无法保持垂直向上生长;蒸发法在玻璃基底上沉积的铜粒子粒径较小,在其上对称生长出两根螺旋纳米碳纤维呈“V”型,无法实现垂直生长。利用氢电弧等离子体法制备的纳米铁、钴、镍粒子为催化剂,以乙炔或乙烯为碳源,在不同反应温度下制备了纳米碳纤维和纳米碳管。实验结果表明,反应温度对碳产物的形貌有很大影响。一般在低温下,如铁在温度低于650℃时,镍在低于温度低于550℃时,钴在温度低于500℃时,催化产物为纳米碳纤维;在较高温度时,铁反应温度在710-800℃,镍反应温度在650-850℃之间时,钴在温度在550-850℃之间时,制得的碳产物是纳米碳管;当温度再继续升高后,制得碳产物为纳米碳颗粒。在较低温度下,碳源气体分子附着在纳米催化剂粒子表面,在催化作用下分解产生碳原子。碳原子在催化剂的某个晶面上开始堆垛沉积,不断伸长。随着反应的进行,长成为具有一定长度的纳米碳纤维。在较高温度时,碳源气体分子首先被吸附在金属催化剂粒子的某个晶面上,然后分解出碳原子,碳原子通过溶解反应进入到金属粒子内部后,再由吸附碳原子的一面扩散到另一面,并以碳管的形式在此面析出。首次以铜为催化剂,使用电弧法制备了竹节状的纳米碳管,每根纳米碳管的顶端有一个催化剂粒子。更多还原

【Abstract】 Carbon nanotubes and carbon nanofibers have many unique properties,such as high electric conductivity,high surface area,and high mechanical strength.These unique properties result in many potential applications,such as catalyst supports, hydrogen storage,composite materials,nano-electronic and nano-mechanical devices, and field emission devices.The synthesis parameters,such as types and particle sizes of catalysts,reaction temperatures,type of support materials,and type of carbon sources may greatly affect the growth process of carbon nanotubes and carbon nanofibers.Among these growth parameters,particle size of catalysts and reaction temperature undoubtedly play key roles in the structure properties of carbon nanotubes and carbon nanofibers.In this thesis,one-dimensional carbon nanomaterials,carbon nanofibers and carbon nanotubes,were synthesized by the pyrolysis of acetylene or ethylene using different metal nanoparticle catalysts or catalyst precursors.Vertically aligned carbon nanofibers were prepared successfully by the decomposition of acetylene with copper catalyst coated on an iron substrate.The products were characterized by SEM,TEM, AFM,XRD,DTA/TG,and so on.The effects of catalyst particle size and reaction temperature on the morphology of carbon nanofibers and carbon nanotubes were investigated.The determining factors in fiber alignment using copper catalyst were studied.The growth mechanism of the obtained carbon nanomaterials was also discussed.Helical carbon nanofibers were synthesized at low temperature by the catalytic decomposition of acetylene with metal nanoparticles prepare by a physical method. Good yields and reproducibility were obtained.Small size effect for synthesis of helical carbon nanofibers was found.Carbon nanofibers were synthesized by catalytic decomposition of acetylene with copper nanoparticles prepared by an evaporation method and Al₂Cu nanoparticles using a hydrogen-arc plasma method.Different morphologies（helical and straight） of carbon nanofibers were observed.The catalyst particle size had a considerable effect on the morphology of carbon nanofibers. Helical carbon nanofibers were grown on copper nanoparticles with a grain size less than 50 nm.The following characteristics of the helical carbon nanofibers were discovered.Firstly,there are always only two helical nanofibers symmetrically grown over a single catalyst nanoparticle,which is below 50 nm in size.Secondly,the two helical nanofibers have absolutely opposite helical senses,namely,one is left-handed coiled,and the other is right-handed coiled.Thirdly,the two helical nanofibers are identical in coil diameter,coil length,fiber diameter,cycle number,coil pitch and cross section.Finally,the fiber diameter of the helical nanofibers is approximately equal to the grain size of the nanocatalyst particle responsible for their growth,but the coil diameter almost twice the size of the catalyst nanoparticle.Therefore,based on the information mentioned above,we conclude that the two helical fibers follow a mirror-symmetric growth mode.When the catalyst particle size was larger than 50 nm, straight carbon nanofibers were obtained dominantly.This symmetric growth mode was induced by the shape changes in catalyst nanoparticles during catalyzing the chemical vapor deposition of acetylene.The shape changes were caused by the changes in surface energy resulting from the acetylene-adsorption on the nanoparticles.The catalytic activity anisotropy of the particle surfaces was the essential condition that fiber can be grown in a helical morphology.Helical and straight carbon nanofibers were synthesized by the catalytic decomposition of acetylene at a low temperature using Cu（NO₃）₂, CuCO₃·Cu（OH）₂·xH₂O and copper tartrate as catalyst precursors.Helical carbon nanofibers with a symmetric growth mode were grown on catalyst nanoparticles with a grain size less than 50 nm.If the catalyst particle size was in the range of 50-300 nm,straight carbon nanofibers were obtained dominantly.Vertically aligned carbon nanofibers were produced by the decomposition of acetylene with copper catalyst coated on an iron substrate.Interestingly,we find that catalyst particles are located at the middle of the formed carbon nanofibers.TEM images reveal that there are only two straight nanofibers grown over a single catalyst nanoparticle.The fiber diameter is approximately equal to the grain size,of the catalyst particle.The angles between the two straight nanofibers are 180.The catalyst particles undergo shape changes during the growth of vertically aligned carbon nanofibers.In contrast,the carbon nanofibers grow in a random orientation, when using copper nanoparticle catalysts coated on a glass plate by a sol-gel method or an evaporation method.The determining factors in fiber alignment using copper catalyst are as follows:（1） Dense copper catalyst particles on a substrate.（2） Catalyst particles are large enough.Two straight nanofibers vertically grown over a single catalyst nanoparticle on a substrate.Because of the dense copper catalyst particles, each fiber is supported by surrounding fibers in the growth process.Attributed to sparse distribution,the carbon nanofibers without supporting by surrounding fibers grow in random orientations on a glass substrate base coated by copper films using a sol-gel method.Because helical carbon nanofibers with a "V"-type mirror-symmetric growth mode were grown on copper nanoparticles with a small grain size,the carbon nanofibers grown in random orientations on glass substrates base coated by copper nanoparticles using an evaporation methodTwo types of carbon nanomaterials,including carbon nanofibers and carbon nanotubes,were synthesized at different temperatures by the catalytic pyrolysis of acetylene or ethylene with iron,cobalt,and nickel nanoparticles prepared using a hydrogen-arc plasma method.The structures of the products were closely related to the reaction temperature,and could be changed from fibers to tubes by simply increasing the reaction temperature.At low temperature,such as the growth with iron nanoparticles under 650℃,the growth with nickel nanoparticles under 550℃,and growth with cobalt nanoparticles under 500℃,carbon nanofibers were obtained. While at higher temperature,such as the growth with iron nanoparticles between 710℃-800℃,the growth with nickel nanoparticles between 650℃-850℃,and the growth with cobalt nanoparticles between 550℃-850℃,carbon nanotubes were the dominant products.The reaction temperature had a significant effect on the activities of the catalysts and carbon sources.At high temperature,the carbon source gas decomposed to produce carbon atoms,which dissolved at catalyst surface exposed to gas and diffused into metal nanoparticles and deposited on the rear side.As for the growth of carbon nanofibers at lower temperatures,we infer that,in this case,maybe there is no dissolution and diffusion of carbon atoms into the catalyst particles,and a surface catalytic polymerization reaction occurs.Namely,carbon atoms decomposed from the carbon source gas deposited on the surface of catalyst to form carbon nanofibers.Bamboo-like carbon nanotubes were synthesized by an arc method using copper as catalyst.The growth mechanism for bamboo-like carbon nanotubes was discussed.更多还原