The Synthesis And Properties Of Novel Carbon-based Nanomaterials

The existence of carbon nanotubes (CNTs) was firstly predicted by Kroto and later confirmed experimentally by Iijima. Since then, CNTs have captured worldwide attention. The material is a carbon allotrope, having diameter and length, respectively, in the nano-and micro-range. The structure CNTs can be visualized as having graphene sheets (several or more) rolled up into concentric cylinders. Since the landmark paper by Iijima in 1991, the utilization of carbon nanomaterials (CNMs) such as CNTs and carbon nanocomposites have received much attention. So far, the techniques for CNM production can be divided into three major categories:a) laser ablation, b) arc discharge, and c) catalytic chemical vapor deposition (CCVD). It has been found that CNMs can be produced in large quantities by CCVD processes, and hence at reduced cost. The possibility of generating CNMs cheaply and efficiently is a good foundation for wide application of CNMs. Being high in surface area and in tube form, CNTs have been investigated with much interest in the area of composites. The core/shell structure of metal (or metal oxide) encapsulated in CNMs, for example, can be viewed as a special kind of nanocomposites. The carbon shell not only protects the core material but also optimizes its property. This kind of carbon nanocomposites has good prospects in areas of magnetic recording, microwave-absorption, catalysis and supercapacitors.Moreover, through methods of theoretical calculation and molecule simulation, Dunlaph and Ihara independently proved that the introduction of pentagonal or heptagonal carbon rings in the network of hexagonal rings could result in the formation of helical carbon nanomaterials (HCNMs) that can be categorized as a kind of chiral materials. Besides having the properties of CNMs, the material exhibits helicity characteristics; e.g., it performed well when functioned as energy-absorption materials. The electronic transport properties of CNMs are helicity dependent as well: CNMs exhibit metal or semiconductor properties as governed by the nature of helicity. With such properties, HCNMs potentially can be used as nanosprings, bio-activators and composite additive of high capacity, as well as can be utilized in the fabrication of nano-inductance and gas-storage devices. Furthermore, HCNMs belong to a new generation of materials for microwave absorption:they are flexible, lightweight, thermally and chemically stable, and show good absorption properties in a wide frequency range. However, despite the recent progress in theoretical and experimental investigations, the production of HCNMs in high selectivity and large quantity is still not satisfactory.At present, the CVD process is the most adopted method for producing CNMs in large scale. It can be conducted at relatively low temperature without the use of complicated equipment. Usually, a catalyst made of transition metal(s) (Fe. Co, Ni and their alloy) is employed in the CVD process to decompose a hydrocarbon for CNM production. The removal of catalyst particles from the product could be troublesome. The purification process could cause damage to the obtained CNMs and increase the cost of production. Therefore, finding a water-soluble catalyst to replace the transition-metal catalyst has been emphasized in recent years. The works reported in this thesis were conducted to address such an issue:(Ⅰ) Synthesis of CNMs:properties and formation mechanism(i) Using NiO nanoparticles generated by sol-gel method as catalyst precursor, carbon nanorods (CNRs) and CNTs can be selectively synthesized in large quantities in benzene pyrolysis (350-500℃). Through proper selection of catalyst as well as its preparation method, CNRs and CNTs were mass produced at relatively low temperatures. The formation mechanism of CNRs and CNTs were proposed based on the results of our experiments.(ii) For even higher production of CNMs, an additive was added to the catalyst of transition metal. Over Fe/SnO2 nanoparticles generated by means of a combined sol-gel and hydrogen reduction method, ultrahigh yield of carbon nanofibers (CNFs) was achieved in the pyrolysis of acetylene at a certain temperature. By controlling the decomposition temperature, it is possible to achieve controllable synthesis of CNFs, bamboo-like CNTs and chains of carbon nanospheres in ultrahigh yield. The microwave absorption properties and formation mechanism of these CNMs have been studied and discussed in details. (iii) Over Fe-Cu nanoparticles derived from sol-gel synthesis followed by hydrogen reduction, carbon nanocomposites and carbon nanobelts (CNBs) were selectively synthesized in large quantities in acetylene decomposition at selected temperatures. The two products (different in color) could be harvested easily as they deposited on different locations of the ceramic plate on which the Fe-Cu nanoparticles were placed. By careful control of experimental conditions, the effect of reduction temperature and content of catalyst on the final products were studied and discussed in details. The optimal growth conditions were analyzed and possible formation mechanism of CNMs discussed. Moreover, the obtained carbon nanocomposites and CNBs show-good microwave absorption properties.(Ⅱ) Synthesis and property of HCNMsUsing Fe2O3 prepared by a coprecipitation method as catalyst precursor, helical carbon nanotubes (HCNTs) can be mass produced in a simple CVD route by the pyrolysis of acetylene at 450 C. By controlling the introduction time of hydrogen during pyrolysis, different structures of HCNTs were synthesized in a controllable manner. Also, CNTs of different helicities can be selectively produced in large quantities by varying the hydrogen flow rate. The simple CVD route effectively solves the problem of HCNM production. Besides, due to the special core/shell structure of catalyst nanoparticle and HCNTs, the obtained samples exhibit good magnetic performance and microwave-absorption properties.(Ⅲ) Water soluble substances as catalyst for CNM growth.Using water soluble substances derived from a simple chemical method as the catalysts, CNMs of different structures can be mass produced selectively by controlling the decomposition temperature and catalyst category. The results demonstrate that CNMs can be produced in high purity and in high quantity. The results also challenge the idea that only transition metal materials can be used as catalyst for CNM formation. The overall results have opened up another research field of new catalysts for CNM synthesis. With the problem related to catalyst removal from CNMs solved, it is getting more feasible to utilize CNMs in a wider sense.