【作者】 张宇；

【导师】 李廷举； 温斌；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2010， 博士

【摘要】 碳纳米管是一种新型的一维纳米碳材料,其独特的几何结构使其拥有优异的力学、电学和化学性能,从而在场发射、纳米电子器件、储氢材料以及高强度复合材料等方面有着良好的应用前景。研究发现碳纳米管依据手性和直径的不同而呈现出不同的导电性,而单一手性和直径的碳纳米管目前还无法通过实验获得,从而极大地限制了其应用。在碳纳米管中进行异质元素(硼/氮)掺杂可以有效控制碳纳米管的晶体结构和电子结构,产生优于纯碳纳米管的物理化学性质,因此成为目前研究的热点本文对化学气相沉积法合成氮掺杂碳纳米管的各个工艺参数进行了系统研究。通过改变反应物中氮原子浓度研究了氮原子掺杂对碳纳米管生长的影响。并且通过优化工艺参数,研究了反应温度、反应物种类以及生长促进剂对生成物形貌和电子性质的影响,为氮掺杂碳纳米管的可控生长打下基础。最后,研究了氮掺杂碳纳米管的电化学性能并对其作用机理进行了分析。以吡啶和乙炔为碳氮源合成了氮掺杂碳纳米管,通过调整反应物中碳氮原子比实现了碳纳米管的可控生长。生成物的最大氮掺杂浓度为3.32 at.%,竹节长度为～40 nm,随着反应物中氮原子浓度的降低,生成物的氮掺杂浓度逐渐降低为1.77 at.%,竹节长度增大到140 nm。氮原子的掺杂会引起碳纳米管六边形拓扑结构的畸变,因此随着氮掺杂浓度的增大,碳纳米管由较为平直变得弯曲,同时管壁由于缺陷的增多而变得粗糙。TGA测试表明,氮掺杂降低了生成物的热稳定性。XPS测试表明氮元素在碳纳米管中以吡啶、吡咯和石墨三种形式存在,随着掺杂浓度的增大吡啶式的氮元素浓度逐渐增多。通过在750-950℃的温度范围内合成氮掺杂碳纳米管,研究了温度对生成产物的产量、形貌、氮掺杂浓度及热稳定性影响。结果表明,温度通过影响碳、氮原子在催化剂颗粒中的扩散系数,从而影响生成物的产量、形貌。生成物产量随着生长温度的升高线形增加。900℃时合成的氮掺杂碳纳米管具有最高的氮掺杂浓度(4.6 at.%)。当反应温度为950℃时,制备出一种具有独特的“鼓形”结构的氮掺杂碳纳米管,分析认为催化剂的变形和吡咯式氮原子比例的增多是产生变形的主要原因。TGA测试结果表明,产物的热稳定性是由氮掺杂浓度和生长温度共同决定的,950℃时合成氮掺杂碳纳米管的初始氧化温度为535℃,比其他温度下合成产物拥有更优良的热稳定性。实验以吡啶、乙二胺和二乙胺等不同含氮有机物为反应前驱体合成了氮掺杂碳纳米管。研究了反应物中化学基团和自身氮原子浓度对生成碳管的形貌、产量、氮掺杂浓度和成键性能的影响。研究发现,反应物所含化学基团是影响生成物的氮掺杂浓度的主要原因。对反应物按照生成物的氮掺杂浓度排序为吡啶>乙二胺>二乙胺。以乙二胺为反应物生成的氮掺杂碳纳米管具有较多的缺陷,具有大内径,薄管壁的结构。XPS测试发现,在这种缺陷较多的碳纳米管结构中,吡啶式氮原子含量较多,这是因为吡啶氮主要与边缘和开口处碳原子相连接。研究认为,乙二胺中含有的-NH2基团具有腐蚀性,对催化剂颗粒和碳管壁进行腐蚀形成这一现象。通过对样品进行TGA测试发现,产物的热稳定性除了与氮掺杂浓度有关,与反应物种类也有关。由于-NH2具有腐蚀性能,乙二胺制备的产物具有较多缺陷,当其与吡啶制备的碳管具有相同的氮掺杂浓度时,其热稳定性劣于后者。为提高氮掺杂碳纳米管的产量和氮掺杂浓度,通过在反应物中添加少量乙硼烷气体研究了异质元素的添加对产物的影响。结果发现,少量硼原子的添加可以对催化剂起活化作用提高产量；随着硼原子添加量的增多,会对催化剂起毒害作用,降低其产量。同时随着硼添加量的增多,生成物管壁扭曲。硼原子添加可以提高氮原子浓度,当乙硼烷流量为3 mL/min时,生成氮掺杂浓度为4.7 at.%,由于硼碳氮三元物的抗氧化能力,产物的热稳定性提高,当乙硼烷流量为4 mL/min时,生成物的初始氧化温度为475℃。基于氮原子掺杂对碳纳米管导电性提高的观点,实验制备了氮掺杂碳纳米管超级电容,并将其电化学性能与未掺杂碳纳米管进行比较。研究发现,氮掺杂提高了碳纳米管的超级电容性能。在恒流充放电测试中,当充放电电流为1 mA时,无掺杂碳纳米管和800、850、900、950℃时制备的氮掺杂碳纳米管的比电容分别为19.9、44.3、42.4、38.8、31.2 F/g。在循环伏安测试中,随着扫描速率的增加,氮掺杂碳纳米管在大电流下仍能保持良好的矩形,这说明氮掺杂碳纳米管电极稳定性好、效率高,在大电流下具有更好的响应速率,是一种更优良的超级电容电极材料。直流阻抗谱测试发现氮掺杂降低了碳纳米管电极的等效串联电阻和接触内阻,提高了响应频率。通过机理研究证明了碳纳米管比表面积的增大是其比电容增大的最主要原因。由于氮原子的掺杂提高了碳纳米管的导电性,从而降低了碳纳米管超级电容的等效串联电阻、接触内阻并提高了超级电容的“特征频率”。此外,氮原子的掺杂改变了碳纳米管的极性,使得电解液更容易浸润在碳纳米管网络中,这一特点对电化学性能的提高也有着良好的作用。更多还原

【Abstract】 Carbon nanotubes (CNTs) possess the excellent mechanical, electrical and chemical properties for their unique quasi-one-dimensional geometry structures, which may lead to various applications in field emission, nanoelectronic devices, hydrogen storage materials and high strength nanocomposites. However, CNTs show a variety of electronic behaviors from metallic to semiconducting, depending on the tube diameter and the chirality. Controlling these parameters during the synthesizing process is still a challenge for the current research, which seriously restrict the future applications of CNTs. Doping CNTs with other chemical elements (B or N) provide an effective way to solve this problem. Such nanotubes exhibit the advantage that their electronic properties mainly depend on the composition and are relatively easy to control.In this thesis, free-standing N-doped MWNTs have been synthesized in a large quantity by the chemical vapor deposition method using nano-Ni/bergmeal powder as catalyst. The effect of synthesis parameters (such as the growth temperature, the nitrogen content of the precursors, the stability of C and N precursors and the promoter) on the yield, structure, themal stability and bonding character of N-doped MWNTs were studied in details. The electrochemical property of N-doped MWNTs has been studied and the mechanism involved was discussed.A controllable synthesis of N-doped MWNTs with various N contents was carried out by the pyrolysis of pyridine and acetylene. C/N atom ratios of reactants were adjusted by the continuously variation of C2H2 flow rates, which resulted in the products with different N contents. The maximum N content of 3.32 at.% was obtained in MWNTs with the bamboo distiance of～40 nm. When the N content decreased to 1.77 at.%, the bamboo distance increased to～140 nm accordingly. The incorporation of N atomes into MWNTs can distorted its hexagonal lattice and the increasing N content can result in a more flexural tube with the coarse surface. TGA test showed that the thermal stability was degraded with the increasing N content. XPS test results revealed that N atoms inserted into the graphite network in the forms of "pyridine-like", "pyrrole-like" and "graphite-like" bonding characters. With the increase of N content, the relative content of "pyridine-like" N atoms gradually increased.In order to explore the effect of the growth temperature on the structure, morphology, N content and thermal stability of the products, N-doped MWNTs were synthesized in the temperature range of 750-950℃. The experimental results showed that the diffusion coefficients of C and N atoms in the catalyst particles could be affected by the temperature, which resulted in the various yields and morphologies. The yield of the resultant increased linear with the increasing growth temperature. The maximum N content (4.6 at.%) in MWNTs has been obtained from the sample grown at 900℃. N-doped MWNTs synthesized at 950℃possessed the unique "drum-like" morphology. The deformation of the catalyst particle at highe temperature was proposed to be responsible for the formation of the "drum-like" structure. XPS results showed that the "drum-like" N-doped MWNTs possessed the highest relative content of "pyrrole-like" N atoms, which was also a reason for the formation of the "drum-like" structure. TGA test showed that the thermal stability of the products depended on both the N content and the growth temperature. The sample prepared at 950℃possesses the highest oxidizing temperature of 535℃.A series of organic compound with various N contents, such as pyridine, ethylenediamine and diethylamine, were designed as C and N sources to prepared N-doped MWNTs. The influences of the chemical group and the N/C atoms ratio of the precursors on the morphology, yield, N contents and bonding configuration of N-doped MWNTs were investigated. The experimental result showed that the chemical groups of the precursors had more important effects than their C/N atoms ratio. According to the N contents in the products, the precusors were arranged as pyridine> ethylenediamine> diethylamine. When ethylenediamine was used as reactant, the as-prepared N-doped MWNTs contained more defects, which exhibited a specific structure with the big inner diameter and thin tube wall. According to the XPS measurements, it was found that the N-doped MWNTs with more defects possessed more abundant relative content of "pyridine-like" N atme. It deduced that this phenomena occurs because the-NH2 group in ethylenediamine would corrode the catalyst particles and the tube walls. Based on the results of TGA measurements, the thermal stability of the product depended not only on the N contents but also on the procursor type. Because of the corrosivity of the-NH2 group, the product synthesized with ethylenediamine processes more defects, whose thermal stability was worse than that synthesized with pryidine when MWNTs had the similar N contents.In order to synthesize N-doped MWNTs with high yield and N content, B atom was chosen as the growth promoter in the CVD method process. The results showed that a few additions of B atoms could enhance the yield of the product due to its activation for the catalyst. With the increasing addition of the B atom, the yield reduced because B atom had a harmful effect on the catalystic particles. Besides, the tube wall of the as-prepared MWNTs was distorted with the increasing addition of B atoms. It was found that the addition of B promoter could promot the N content of MWNTs. When flow rate of B2H6 was 3mL/min, the N content was measured to be 4.7 at.% in the product. The thermal stability of the product could be improved due to the antioxidant capacity of the B-C-N compound. When flow rate of B2H6 was 4mL/min, the initinal oxidizing temperature was 475℃.Based on their good conductance, the electrochemical properties of N-doped MWNT supercapacitors have been investigated and compared with that of the undoped ones. The galvanostatic charge/discharge, cyclic voltammetry (CV) and AC impedance spectroscopy were used to evaluate the eleletrochemical capacitive performance of this new type carbon nanotube, using 6M KOH as the electrolyte. In the galvanostatic charge/discharge measurement, when the charge/discharge current was 1mA, the calculated specific capacitances of MWNTs was 19.9 F/g, and those of N-doped MWNTs synthesized at 800、850、900 and 950℃are 44.3、42.4、38.8 and 31.2 F/g, respectively. As can be seen from the CV and AC impedance spectroscopy measurements, N-doped MWNT surpercapacitors functioned like a ideal capacitor with better rate performance, lower equivalent series resistance (ESR) and contacet resistance, higher knee frequency and higher Csp than the undoped one. The reason was that N-doped MWNTs possessed larger specific surface areas and pore volumes than the undoped ones. The enhanced conductivity of N-doped MWNTs was proposed to responsible for their lower ESR and higher rate capability. Moreover, substituted N doping in graphitic carbon lattices can enhance the surface wettability of MWNTs in aqueous electrolyte, which was important to maximize the access of the electrolyte to the surface of carbon.更多还原