Controllable Synthesis And Simulation Of Carbon Nanotube In Electric Field And Physical And Chemical Properties

With the development and advanced research in nano-science and technology, carbon nanotubes (CNTs) have attracted a lot of attention over the last decade due to their unique properties and potential in broad applications. On the one hand, continuous progresses has been achieved on further control and precise assembly in CNTs growth, deep investigation of growth mechanism; on the other hand, a lot of devices have been fabricated based on the unique properties of CNTs. However, there are still many technique problems to be overcome. It is an ultimate goal for CNTs'growth to control the growth direction and microstructure by using simple and efficient methods so as to obtain tunable properties. This will present significant opportunities to basic science and nanotechnology.In the present work, a simple and scalable synthesis method-ethanol flame method was used to produce multiwalled carbon nanotubes (MWCNTs). Flaming process has some advantages, such as simple experiment set-up, convenience of applying electric field, negligible influence of flame on electric field. Electric field was applied to produce well-aligned MWCNTs on a large surface area with high repeatability and stability. Simulations based on finite element method were carried out to investigate the growth mechanism of MWCNTs in electric field. In addition, the applied electric field can also improve the crystallinity of graphite sheets and transform the microstructure of MWCNTs, which would therefore greatly influence the physical and chemical properties. The efforts of precise control on the growth direction and microstructure of MWCNTs make flame method more suitable for industrial process because flame method has some unique advantages such as low cost, easy to control and high efficiency. At the same time, the study of electronic, optical and electrochemical properties provide valuable data for potential applications in electronic devices, optical switch and electrochemical sensors.This dissertation includes ten chapters. Chapter one is the introduction, where the origin of the subject, significance and the major contribution by the author are introduced briefly. The progress and situation of MWCNTs, aligned MWCNTs and synthesis of aligned MWCNTs in the electric field were reviewed, with the emphasis on the development and advantages of flame method.Chapter two described the basic principle and experimental method of controllable synthesis of MWCNTs. The characterization method of morphology and microstructure, the measurements of electronic property, photoluminescence and electrochemistry were also introduced.In chapter three, well-aligned MWCNTs were successfully synthesized in ethanol flames by applying an external electric field. It is found that the alignment is guided by the electrostatic force acted on the catalyst particles at the tips of the MWCNTs. The electric field control not only aligned the MWCNTs successfully but also improved the diameter uniformity and the crystallinity of graphite sheets.In chapter four, microstructural transformation MWCNTs from the"herringbone"into highly crystalline structure in an electric field was experimentally observed by using a high-resolution transmission electron microscope and the growth mechanism was modeled and discussed by using a finite element method. It is found that the MWCNTs microstructures can be changed by an electric field through the influence of the electrostatic force on the carbon surface and bulk diffusion on/in a deformed catalyst particle. Preliminary experiment revealed that an isomeric"tubular––fishbone––tubular––fishbone……"MWCNTs could be synthesized when a pulsed electric field was applied, which is expected to exhibit special properties and promising applications. In chapter five, finite element method was adopted to simulate the growth of MWCNTs in electric field and calculate the electrostatic forces acting upon CNT itself and the catalyst particle at the MWCNT tip systematically. The numerical results revealed that: 1) the electrostatic attractive force along the field direction acting on the catalyst particle at CNT tip is much larger than that on the tube, which plays a key role for vertical growth of MWCNTs; 2) the value of the electrostatic attractive force acting on the nanotube and catalyst particle is related to height, density and diameter of MWCNTs.In chapter six, a novel process for synthesizing unentangled well-aligned MWCNTs in ethanol flames was introduced. The results showed that a large area, vertically and unentangled growing, uniform density aligned MWCNTs were produced in high repeatability by controlling the thickness of Ni nanocrystalline layer and sampling time in flame. It is found that small aspect ratio is indispensable for growing unentangled aligned carbon nanotubes.In chapter seven, the electronic properties of controllably synthesized MWCNTs were investigated. The measurements of electronic conductivity revealed that MWCNTs with different microstructures show totally different electronic behaviors.In chapter eight, the photoluminescence (PL) of MWCNTs synthesized with ethanol flames was investigated at temperatures from 30 K to 300 K. Broad band PL emission in the near-infrared region was observed at temperatures below 240 K. The PL intensity was abruptly boosted up at 240 K, and then quickly became quenched and undetectable as the temperature was further increased. The PL emission of the MWCNTs only appeared in the first cycle of the temperature rising. However, it could be recovered by exposing the sample in an oxygen-abundant environment. It is proposed that the PL emission of the MWCNTs is resulted from the carbon oxyhydride-like fluorophors bound to the MWCNTs surface.In chapter nine, solid-cored carbon nanofibers (CNFs) were used to prepared supercapacitors. Their porosity, chemical properties and electrochemical activity were compared with the MWCNTs synthesized by chemical vapor deposition. The specific surface area of CNFs was comparable to that of MWCNTs due to a larger amount of micropores on the surface. Electrochemical characterization showed that CNFs had a much larger power density and greater capacitance than that of MWCNTs. The capacitance of CNFs came from both double layer capacitance and the pseudo-capacitance. Abundant, electroactive functional groups on the surface of CNFs contributed the majority (59.3%) of its total capacitance. These results demonstrate a promising approach to make electrode materials for electrochemical capacitors with both enhanced pseudo-capacitance and high double layer capacitance.Chapter ten is the conclusions of all the research work mentioned in this dissertation.