Catalytic Synthesis of Carbon Nanotubes and Nanofibers

Carbon nanotubes and nanofibers are graphitic filaments/whiskers with diameters ranging from 0.4 to 500 nm and lengths in the range of several micrometers to millimeters. Carbon nanofibers and nanotubes are grown by the diffusion of carbon (via catalytic decomposition of carbon containing gases or vaporized carbon from arc discharge or laser ablation) through a metal catalyst and its subsequent precipitation as graphitic filaments [1–6]. Three distinct structural types of filaments have been identified based on the angle of the graphene layers with respect to the filament axis [5, 7], namely stacked, herringbone (or cup-stacked [8]), and nanotubular [9] as shown in Figure 1. It can be seen that the graphite platelets are perpendicular to the fiber axis in the stacked form, the graphene platelets are at an angle to the fiber axis in the herringbone form, and tubular graphene walls are parallel to the fiber axis in the nanotube. In the literature today, the common practice is to classify the stacked and herringbone forms of graphitic filaments under the general nomenclature of “nanofibers” whereas “nanotube” is used to describe the case where tubular graphene walls are parallel to the filament axis. Other definitions used today are that highly crystallized tubular structures are nanotubes whereas defective ones are nanofibers, or tubular structures ∼20 nm or below in diameter are nanotubes but larger diameter filaments are fibers. In this work, we prefer to use the term nanotube to describe carbon filaments with tubular graphene walls parallel to the axis and use the term nanofiber for carbon filaments with graphene layers at other angles. This is because special physical properties arise from the “nanotube” structure which distinguish it from the “nanofiber” structure, which itself has other advantageous properties, as will be described later. Carbon nanofibers and nanotubes have been synthesized since the 1960s, but why has one particular form (i.e. the nanotube) received so much attention recently? In 1991, Iijima reported that highly graphitized carbon nanotubes, formed from the arc discharge of graphite electrodes, contained several coaxial tubes and a hollow core [9]. This important discovery led to the realization that with graphene tubes parallel to the filament axis, these highly crystallized tubular carbon structures would inherit several important properties of “intraplane” graphite. In particular, a nanotube exhibits high electrical conductivity, thermal conductivity, and mechanical strength along its axis. As there are very few open edges and dangling bonds in the structure, nanotubes are also very inert and species tend