After the development of a method for macroscopic production of fullerenes, in particular the C60 molecule, a lot of attention has been given to the optical and collective properties of C60 in gas phase, solid and film. The more recent development in fullerene related research is the synthesis of coaxial carbon sheets, usually referred to as carbon tubes or nano tubes.
Nanoscale Carbon Onion

Carbon nanomaterials with curved surface have gained great interests because of their novel mechanical and electronic properties. One such material is carbon onions, which consist of concentric spherical graphitic sheets and is an important member of the fullerene family. These spherical concentric graphitic shells called carbon onions have opened the doorway to a new area of materials science.Carbon nanoonion consists of spherical C60 (buckyball) core surrounded by onion-like nested spherical graphite layers and hence it is named based on its unique molecular structure. Creation of carbon onions and coils are produced at low temperature in near-critical benzene when irradiated with an ultraviolet laser.

Carbon onions can be synthesized by several different methods. The main methods to fabricate carbon onions are electron beam exposure, arc discharge, chemical vapor deposition and ball milling method. Electron beam exposure can only produce trace amount of carbon onions; arc discharge needs very high temperature(4000K) and carbon onion is produced as byproduct; chemical vapor deposition demands to put in catalyst and its products are hard to purification; ball milling method ’s product is non-homogeneous and at low graphitization. Among these methods, DC arc charge method is an economical method to produce carbon onions in bulk quantities. Carbon onions have some fascinating structures and properties due to their high degree of symmetry. Studies dealing with carbon onions still remain low in comparison to the intense research on single-layer fullerenes or nanotubes, partly because of the complicated spherical multi-layer structures and the broad size distribution of produced onions. Carbon onions are produced in both sub critical and supercritical benzene, whereas carbon coils are produced on an alloy catalyst composed of (Fe:Cr:Ni = 74:18:8) in supercritical benzene irradiated with a laser beam. The operational temperature is much lower than that used in conventional carbon structure synthesizing methods and a large number of carbon structures, such as carbon onions and coils, are produced in supercritical benzene.

In another process carbon black collected from gas carburizing furnace can be used as raw material as carbon black is composed of polyhedral carbon and nano-carbon spheres consisting of three layers. Carbon black can be converted into carbon onions easily by vacuum heat treatment at relatively low temperature.

Carbon Onion has many orders of magnitude higher sorption capacity than traditional sorbents like active carbon. It is much cheaper than other commercially available carbon-based nanomaterials like C60 fullerene and carbon nanotubes. Carbon onions have manifestoes potential application prospect in areas of conductivity、catalyst and nano-compound materials.