1 Introduction

The extraordinary ability of the chemical element carbon to combine with itself and other chemical elements in different ways is the basis of organic chemistry and of life. This chemical versatility also gives rise to a rich diversity of structural forms of solid carbon. This introductory chapter is an attempt to survey the very wide range of carbon materials that is now available with emphasis on chemical bonding and microstructure. The materials reviewed include: (i) crystalline forms of carbon: diamond, graphite, Fullerenes and carbynes; (ii) amorphous carbon films and diamond films; (iii) carbon nanoparticles, including carbon nanotubes; (iv) engineering carbons with more-or-less disordered microstructures based on that of graphite that are the main focus of this book.

1.1 Bonding between carbon atoms

Here, the bonding between carbon atoms is briefly reviewed; fuller accounts can be found in many standard chemistry textbooks, e.g., [1], The carbon atom [ground state electronic configuration (ls2)(2s22px2py)] can form sp\ sp2 and sp1 hybrid bonds as a result of promotion and hybridisation. There are four equivalent 2sp3 hybrid orbitals that are tetrahedrally oriented about the carbon atom and can form four equivalent tetrahedral a bonds by overlap with orbitals of other atoms. An example is the molecule ethane, C2H6, where a Csp3-Csp3 (or C-C) ct bond is formed between two C atoms by overlap of sp3 orbitals, and three Csp3-Hls ct bonds are formed on each C atom, Fig. 1, Al.

A second type of hybridisation of the valence electrons in the carbon atom can occur to form three 2sp2 hybrid orbitals leaving one unhybridised 2p orbital.

The sp2 orbitals are equivalent, coplanar and oriented at 120° to each other and form ct bonds by overlap with orbitals of neighbouring atoms, as in the molecule ethene, C2H4, Fig. 1, A2. The remaining p orbital on each C atom forms a n bond by overlap with the p orbital from the neighbouring C atom; the bonds formed between two C atoms in this way are represented as Csp2=Csp2, or simply as C=C.

Al, ethane A2, ethene A3,ethyno

Bl, benzene B2, coronene B3,ovalene

Fig. 1. Some molecules with different C-C bonds. Al, ethane, C2H6 (sp3); A2, ethene, C2H4 (sp2); A3, ethyne, C2H2 (sp1); Bl, benzene, C6H6 (aromatic); B2, coronene, C24H12; B3, ovalcne, C32HM.

In the third type of hybridisation of the valence electrons of carbon, two linear 2sp' orbitals are formed leaving two unhybridised 2p orbitals. Linear a bonds are formed by overlap of the sp hybrid orbitals with orbitals of neighbouring atoms, as in the molecule ethyne (acetylene) C2H2, Fig. 1, A3. The unhybridised p orbitals of the carbon atoms overlap to form two n bonds; the bonds formed between two C atoms in this way are represented as Csp^Csp, or simply as C=C.

It is also useful to consider the aromatic carbon-carbon bond exemplified by the prototypical aromatic molecule benzene, C6H6. Here, the carbon atoms are arranged in a regular hexagon which is ideal for the formation of strain-free sp2 ct bonds. A conventional representation of the benzene molecule as a regular hexagon is in Fig. 1, Bl. The ground state n orbitals in benzene are all bonding orbitals and are fully occupied and there is a large délocalisation energy that contributes to the stability of the compound. The aromatic carbon-carbon bond is denoted as Car~Car. Polynuclear aromatic hydrocarbons consist of a number, n, of fused benzene rings; examples are coronene, C24H12, (n = 7) and ovalene, C32H14, (n = 10), Fig. 1 B2, B3, where délocalisation of n electrons extends over the entire molecule. Note that the C:H atomic ratio in polynuclear aromatic hydrocarbons increases with increasing n. Dehydrogenative condensation of polynuclear aromatic compounds is a feature of the carbonisation process and eventually leads to an extended hexagonal network of carbon atoms, as in the basal plane of graphite (see Sections 2.2 and 6.1).

For carbon-carbon bonds the mean bond enthalpy increases and bond length decreases with increasing bond order, Table 1. When considering bond lengths in disordered carbon materials, particularly those containing significant amounts of heteroelements, it is useful to note that the values in Table 1 arc mean, overall values. Carbon-carbon bond lengths depend upon the local molecular environment. Table 2 lists some values of carbon-carbon bond lengths obtained from crystals of organic compounds. In general, bond length decreases as the bond order of adjacent carbon-carbon bonds increases.

Table 1. Some properties of carbon-carbon bonds

Bond Bond order

Bond length

Mean bond enthalpy

0 0

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