The Fabrication, Characterization And Properties Of Carbon Nanofibers Over Water-soluble Alkali Metal Salt

Since Iijima’s paper on helical carbon nanotubes in 1991, carbon nanomaterials (CNMs) such as carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene have attracted widespread attention. As a unique kind of CNMs, CNFs have high intensity, low density, large specific area, great adsorbability and electrical conductivity. It has been suggested that CNFs can be utilized in combination machinery, new energy storage, electrode materials, field emitting devices and microwave absorption. For CNFs generation, methods such as arc discharge, electrospinning, polymer blend spinning and catalytic chemical vapor deposition (CVD). Nonetheless, it is common to have metal impurities in the products, which hinder further characterizations and applications of CNFs. It is tedious and costly to remove impurities from CNFs. Furthermore, unexpected defects or contaminants could be introduced into the CNFs during purification procedures. As a traditional method, CVD has its advantages such as controllable reaction conditions and mild reaction process. So CVD has been widely used to obtain CNFs with high purity and selectivity. Nevertheless, there are disadvantages such as outgrowth of desired product. As for the growth mechanism of CNFs in CVD processes, there are still controversies.Based on the reasons above, we have conducted the research on "The fabrication, Characterization and Properties of Carbon Nanofibers over water-soluble alkali metal salt". The research works of this thesis mainly include the following aspects:(1) Through the pyrolysis of acetylene, large quantities of carbon nanocoils (CNCs) or CNFs can be synthesized by using NaxKy catalyst. We found that the temperature adopted for pyrolysis is an important factor for selectively obtaining the different kinds of products. Because the catalysts we used are water-soluble, they can be easily removed, and the CNMs can be harvested with undamaged in high purity. Based on the results, we conducted discussion on the role of catalyst, effect of pyrolysis temperature, and formation mechanism of the CNMs.(2) Using acetylene as reactant and Na2CO3 powder as catalyst, CNFs and CNTs were selectively synthesized in this study at acetylene decomposition temperature of 450 and 500℃, respectively. Because Na2CO3 is water-soluble, it can be removed from the products through repeated washing by deionized water and CNMs can be harvested with undamaged in high purity.(3) NaNO3 powder was used as catalyst for the synthesis of CNMs. By controlling the decomposition temperature (400 and 450℃), liner CNFs and helical CNFs can be selectively synthesized. It is worth pointing out that the use of water-soluble salts bypasses the scabrous problems related to CNMs purification.(4) Using acetylene as carbon source, ammonia as nitrogen source, and Na2CO3 powder as catalyst, we synthesized nitrogen-doped CNFs and CNCs selectively at 450 and 500℃, respectively. The approach is simple, inexpensive, and environment-benign, which can be used for controlled production of nitrogen-doped CNFs or nitrogen-doped CNCs. We report the role of catalyst, the effect of pyrolysis temperature, and the photoluminescence properties of the as-harvested nitrogen-doped CNFs or nitrogen-doped CNCs.