Preparation And Electrochemical Properties Of Nitrogen Doped Multi-walled Carbon Nanotubes

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.