Structure of Carbon Onions

It has already been discussed in the chapter on fullerenes that the formation of a closed, cage-like structure from a planar, hexagonal lattice requires the presence of at least 12 pentagons. This rule is deduced directly from Euler s Theorem (Eq. 

1 that correlates the numbers of faces (F), corners (C), and edges (F) in a polyhedron  

Assuming now the atoms to be distributed in a way to give a sphere, it is obvious that its surface has to equal that of its constituent deformed five- and six-mem-bered rings (Eq. 4.3)  

One may further assume that due to the constant bond length of all shells, the density of atoms per unit surface is equal for aU shells of a carbon onion. Hence follows 12A5= lOAfi. With the surface ofahexagonof A = a ( /3/2), Eq. (4.4) is obtained  

This allows for calculating the difference of numbers of atoms between neighboring shells (Eq. 4.5)  

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Alexandrou I, Wang H, Sano N et al (2004) Structure of carbon onions and nanotubes formed by arc in liquids. J Chem Phys 120 1055-1058... 

Numerous spectroscopic methods have been applied to examine the physical properties and to elucidate the structure of carbon onions. They include IR- and Raman spectroscopy. X-ray diffraction, electron energy loss spectroscopy (EELS), absorption, and photoluminescence spectroscopy and NMR-spectroscopy. Each of these methods gives account of certain aspects of the geometric and electronic structure, so altogether quite a detailed picture is obtained of the situation in carbon onions and related materials. There is, however, a strong dependency on... 

McDonough, J. K., A. I. Frolov, V. Presser et al. 2012. Influence of the structure of carbon onions on their electrochemical performance in supercapacitor electrodes. Carbon... 

In this chapter, we discuss the structure of carbon onions and provide an overview of the various methods by which carbon onion-like materials may be produced. Also, some of the most important physical properties of carbon onion-like structures are reviewed with an phasis on those properties, which may be relevant for future applications of this material. 

The discovery of perfect geodesic dome closed structures of carbon, such as C o has led to numerous studies of so-called Buckminster fullerene. Dislocations are important features of the structures of nested fullerenes also called onion skin, multilayered or Russian doll fullerenes. A recent theoretical study [118] shows that these defects serve to relieve large inherent strains in thick-walled nested fullerenes such that they can show faceted shapes. 

Hollow carbon nanostructures are exciting new systems for research and for the design of potential nano-electronic devices. Their atomic structures are closely related to their outer shapes and are described by hex-agonal/pentagonal network configttrations. The surfaces of such structures are atomically smooth and perfect. The most prominent of these objects are ftil-lerenes and nanotubesjl]. Other such novel structures are carbon onions[2] and nanocones[3]. 

Very little is known about the physical properties of carbon onions. Electron spin resonance measurements on macroscopic quantities of onions, with 3-10 nm sizes, show that these structures have a Pauli-like spin susceptibility close to that of graphite [181]. It demonstrates that carbon onions also belong to the family of conducting carbon structures. 

The choice of the organic starting component largely influences the kind of carbon species generated. This becomes evident, for example, in the pyrolysis of metal complexes of some dehydro[n]annulenes Apart from the desired nanotubes, a series of other structures like carbon onions, graphite, or amorphous carbon is observed. 

In HRTEM pictures of some samples of carbon onions, spiral patterns are observed that correspond to three-dimensional, nauhlus-like spiral objects. These are especially found as an intermediate in the generahon of concentric nanoonions from other forms of carbon. A more detailed discussion of these structures and their relevance is found in Section 4.3 on the mechanisms of onion formahon. 

A thermal treatment of different carbon forms can lead to the formation of onionlike species as well. As for diamond particles, their surface structure plays an important role for the actual outcome of the process. If it is covered with functional groups, the bonding sites are saturated which renders a graphitization more difficult From danghng bonds, on the other hand, graphitized domains will arise that can serve as nucleation center to the formation of carbon onions. In this process, a suitable orientation of lattice planes as well as a small particle size that... 

Figure 4.31 The accretion of curved graphitic structures and of annealed aromatic compounds also leads to a formation of carbon onions. The inner shells are not always completely closed, which explains for defects of the onion structure ( ACS 1986).

Figure 4.38 Model explaining the electrical conductivity of a carbon material consisting of carbon onions and related structures. Conductive channels develop with increasing graphitization of the material ( MRS 2002).

Multiwalled nanotubes suggest themselves as a comparison structure for onions the same as the fullerenes do. like with the MWNT, the poor solubUity of carbon onions poses problems in chemical conversions. Further compUcations arise from the inhomogeneities regarding diameter and number of shells as instead of defined products, there will always be a mixture that is much harder to characterize. 

Apart from the irradiation with high-energy electrons, the conversion of carbon onions into diamond also succeeds by bombardment with ions like Ne. The latter are 36000 times heavier than the rather light-weight electrons. Consequently, they require far less velocity and thus smaller accelerator voltages to bear the same effect Diamond-like structures can further be generated by thermal treatment in air at 500 °C or by irradiation with a C02-laser. 

Figure 4.42 Operation principle of carbon onions in tribological applications. The lubricating effect is conserved even upon destruction of the onion structure.

Figure 12.7 Structure of carbon fiber, (a) A schematic iiiustralion of tree trunk or onion skin structure (ieft) and radial structure (right), (b) A typicai opticai micrograph of carbon fiber cross sections under polarized light in crossed nicols condition showing maltose cross patterns. Polarizer and analyzers are parallel to picture edges. Source Reprinted from Nyo H, Heckler AJ, Hoemschemeyer DL, Characterizing the structures of PAN based carbon fibers, 24 Nat Symposium, San Francisco, 179, 51-60, May 8-10.

Carbon onions are a member of the family of nanometer-scale graphite-like aU-carbon allotropes, the emergence of which was catalyzed by the Nobel Prize-winning discovery of the first member, the fullerene, by Kroto et al. in 1985. Initially, carbon onions were observed by lijima in 1980, and were brought to popular attention by the experiments of Ugarte in 1992. Structurally, they consist of concentric spherically closed carbon shells and receive their name from the close resemblance between their nanoscale structure and the more familiar concentric layered structure of an onion. Closely related to carbon onions is a class of material known as onion-like carbons (OLCs), which include polyhedral nanostructures such as ideal nested fullerenes. This material, rather than ideal spherical carbon onions, can be currently produced in macroscopic quantities, and, hence, be used for future applications. 

A new approach to the synthesis of carbon onions was proposed by Du et al. They used radio-frequency and microwave plasma to produce OLC particles from coal and carbon black. Optimization of synthesis conditions in their experiments enabled Du et al. to produce carbon onions with a minimum of structural defects. The growth of carbon onions with a diameter of about 30 nm was achieved by Shimizu et al. by the application of inductively coupled microplasma. However, both these plasma-based techniques resulted in the formation of carbon onions mixed with other carbons, which is a significant issue as the problem of carbon onions purification remains unsolved. 

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