Synthesis of Carbon Nanostructures by Microwave Irradiation

Abstract

Carbon nanotubes (CNT) have been synthesized with various techniques of which the most common ones are laser ablation, electric arc discharge, and chemical vapor deposition. These methods produce CNTs with different characteristics, sometimes involving complex experimental setups that add to their cost of production. It is of current general interest the development of new techniques for the efficient and selective synthesis of CNTs and other carbon nanostructures at the cheapest possible cost. One such possibility is the use of microwave radiation, which over the past few years has played an important role as a thermal tool in organic synthesis due to considerable advantages over conventional methods (Lidstrom, et al., 2001). The use of microwave radiation in the synthesis and functionalization of carbon nanotubes or other nanostructures is advantageous because it provides a fast and uniform heating rate that can be selectively directed towards a targeted area. The first report of the production of carbon nanostructures with microwaves was made by Ikeda et al (Ikeda et al., 1995), who synthesized fullerenes from microwave-induced naphthalene-nitrogen plasma at atmospheric pressure inside a cylindrical coaxial cavity. O. Kharissova has reported the synthesis of vertically aligned carbon nanotubes using a domestic microwave oven (Kharissova, 2004). Graphite is a good microwave radiation absorber. It has been used in military applications as radar-absorbing material and in anti-electromagnetic interference coatings for civil purposes. Milled flake graphite and carbon nanotubes have microwave absorption maxima in the 10-15 GHz frequency range (Fan et al., 2009). Microwave radiation can heat or cause arcing in many objects and powdered samples can absorb such radiation and be heated efficiently. Short-time direct exposure to microwave irradiation has been used to produce exfoliated graphite as well as to reduce graphite oxide (Zhu et al., 2010). In graphite powder, absorbed microwave radiation is converted into heat via dielectric loss and conductive loss mechanisms. Graphite powder is oxidized by long exposure to ambient air and may become partly electrically insulating. Microwaves are absorbed with energy dissipation through the coupling of the radiation electric field with local electric dipoles associated with structural defects in graphite powder particles such as particle edges, dangling bonds, C-O bonds, impurities and others. The electric field of microwaves also drives electric currents with efficient generation of heat due to the highly diffusive transport