Carbon nanostructures include single-wall carbon nanotubes (SWNT), multi-wall carbon nanotubes (MWNT), fullerenes, nanodiamonds, and nanoonions, and such nanostructures can be manufactured in various manners.
For example, in one relatively common manner, nanotubes can be produced by electric arc discharge. Nanotubes formed by such a process are typically MWNT. To produce SWNT, various catalytic metals (e.g., cobalt) can be added to the graphite electrodes. Arc discharge typically provides relatively low yield. Moreover, the so formed nanotubes will have in most cases relatively large inhomogeneity in length and chirality. Fullerenes can be obtained in similar manner from soot prepared in an arc generator using a carbonaceous electrode (typically without catalyst). When the electrodes are immersed in water, nanoonions can be formed that float to the surface of the water. So formed fullerenes and nanoonions can then be processed (typically in a shockwave compression) to form nanodiamonds.
Alternatively, especially where increased yield or localized synthesis of nanostructures is desired, chemical vapor deposition (CVD) can be employed in which a feed gas (e.g., methane or ethylene) is decomposed in the presence of a metal catalyst to grow nanotubes. For example, numerous nanotubes can be grown at the same time on a silicon dioxide template (that can be patterned) in predetermined positions. Such process may further be modified by the choice of the particular catalyst to influence the type of nanotube that is to be produced. While CVD synthesis is directional and relatively simple, industrial significant yields are typically not achieved. CVD was reported to also yield nanodiamonds under certain conditions, however, other nanostructures are rarely formed using CVD.
In yet another manner, laser ablation may be employed in which a laser pulse evaporates a solid target of graphite that contains a small amount of metal catalyst (˜1 atomic % Ni and ˜1% Co). The ablated material is transferred into a background gas (e.g., Ar) which is gently flowing through a quartz tube inside a high temperature (e.g., 1000° C.) oven. Laser ablation generally allows for tighter control of reaction conditions, and with that tends to provide a more defined population of nanotubes. Furthermore, nanotubes (and also fullerenes under certain conditions) can be produced in relatively good quantities. However, such a process is relatively energy consuming, requires expensive equipment, and highly trained personnel.
Other less common methods of forming nanostructures include plasma based synthesis of nanotubes. Such methods advantageously allow for mass production of nanotubes, but generally require megawatt quantities of energy. Similarly, nanostructures have been produced by impulse heating of fluorinated graphite dust in a 27.12 MHz inductively coupled plasma. Again, which such method may yield a relatively high yield of SWNT, the energy demand in most cases is cost-prohibitive. “Two-dimensional” carbon nanostructures, and particularly graphene, were until recently thought to be difficult, if not even impossible to manufacture. However, advances in plasma assisted CVD have yielded doped carbon flakes as described in WO 2004/095494, and more recently, graphene layers were reported that were extracted as an individual plane from a graphite crystal (Novoselov et al., Electric Field Effect in Atomically Thin Carbon Films, Science 2004 306: 666-669).
Therefore, while various materials and methods for manufacture of carbon nanostructures are known in the art, all or almost all of them suffer from one or more disadvantages, especially where large quantities of carbon nanostructures are desired. Thus, there is still a need to provide improved compositions and methods for manufacture of carbon nanostructures.