General Data on Carbon Allotropes

Abstract

Carbon, the 6th element in the periodic table denoted by the letter “C” and true element of life, provides the chemical basis for life on Earth due to its ability to form stable bonds with other carbon atoms, oxygen, nitrogen, sulfur, and many other elements in Mendeleev’s Periodic Table. Carbon is found almost everywhere, and it is one of the most abundant materials on earth. It is the 4th most common element in the universe and 15th most common on earth’s crust. All life on Earth contains various forms of carbonic structures, from proteins to the tallest trees [1]. Existence of a host of carbon inorganic forms is the also responsibility of stable single and multiple carbon-carbon covalent bonds. This process is called catenation, in which an element can bond with itself to form long chains. During much time, only two conventional carbon allotropes, graphite (black, soft, and conductive) and diamond (shiny, transparent, and extremely hard), have been known. Only in the last few decades have new synthetic carbon allotropes such as carbon nanotubes, fullerenes (buckminsterfullerene C₆₀, smaller and higher fullerenes), and graphene been discovered. Their outstanding properties, current and potential applications, testify their unique scientific and technological importance [2]. In addition, a host of other carbon structures, both obtained and still predicted, have been reported up to date.

Variety of carbon allotropes, hybridization, and dimensionality. (The image above is reproduced with permission of MDPI (Materials, 2015, 8, 3068–3100).)

Carbon, the 6th element in the periodic table denoted by the letter “C” and true element of life, provides the chemical basis for life on Earth due to its ability to form stable bonds with other carbon atoms, oxygen, nitrogen, sulfur, and many other elements in Mendeleev’s Periodic Table . Carbon is found almost everywhere, and it is one of the most abundant materials on earth. It is the 4th most common element in the universe and 15th most common on earth’s crust. All life on Earth contains various forms of carbonic structures, from proteins to the tallest trees [1]. Existence of a host of carbon inorganic forms is the also responsibility of stable single and multiple carbon-carbon covalent bonds. This process is called catenation , in which an element can bond with itself to form long chains. During much time, only two conventional carbon allotropes, graphite (black, soft, and conductive) and diamond (shiny, transparent, and extremely hard), have been known. Only in the last few decades have new synthetic carbon allotropes such as carbon nanotubes, fullerenes (buckminsterfullerene C₆₀, smaller and higher fullerenes), and graphene been discovered. Their outstanding properties, current and potential applications, testify their unique scientific and technological importance [2]. In addition, a host of other carbon structures, both obtained and still predicted, have been reported up to date.

Carbon exists in the form of many allotropes, which can be classified according to the character of chemical bond related with hybridization (sp, sp², sp³): zero-dimensional sp² fullerenes,¹ the two-dimensional sp² honeycomb lattice of graphene (parent to graphite and carbon nanotubes), or three-dimensional sp³crystals – diamond, lonsdaleite, and C₈ (Fig. 1.1) [3], as well as several mixed sp²-sp³ forms (e.g., Q-carbon or amorphous carbon), whose contributions sp²-sp³ can be measured [4]. Carbon can function either as a conductor, in sp chains, carbynes, and sp² planar structures, graphene and graphite, or as a wide-gap insulator in sp³ tethrahedral coordination − e.g., diamond and alkanes. The origin of these properties can be traced directly to the type of hybridization: sp, sp², or sp³. The first two have the potential to form bonds that are electrically conducting, while sp³ has insulating properties [5].

Fig. 1.1

Schematic illustration of the three kinds of hybridization of carbon. (Reproduced with permission of the NCBI)

Each allotrope has notably different electronic and mechanical properties. For instance, graphene has the characteristic semimetal electronic structure with a linear band dispersion and an extraordinarily high electron mobility. In contrast, diamond is a wide-band-gap insulator and one of the hardest natural materials known. Carbon allotropes from sp³-hybridized carbon (diamond) and sp²-hybridized carbon (graphite, graphene, fullerenes, nanotubes) have well-established properties, many of which are technologically relevant. A carbon allotrope constructed from sp-hybridized carbon atoms (carbyne), on the other hand, remains rare and presents a challenging synthetic goal.² The properties of carbon allotropes are quite different; thus, as a simplest example, among carbon allotropes and forms, the carbon nanofoam (0.002–0.020 g/cm³) and aerogel (0.020–1.00) possess minimal densities, being compared with others (nanodiamonds (2.97), graphitic cones (1.96), graphite (2.27), MWCNTs (1.98), carbon black (1.91), carbon xerogel (1.73), activated carbon (2.05), and ordered mesoporous carbon (1.63)) [6]. High surface area carbon materials mainly include activated carbon, porous carbon, carbide-derived carbon, carbon nanoonions, carbon aerogels, carbon nanotubes, carbon shell, graphene, and graphene quantum dots. New carbon forms are in a permanent search [7]; the discovery of novel carbon allotropes or carbon nanostructures [8], exhibiting unique structural and physical properties, has attracted intensive attention due to their fundamental and technological interests. A diagram of carbon nanostructures according to their hybridization state is shown in Fig. 1.2.

Fig. 1.2

Ternary diagram of carbon nanostructures according to their hybridization state [9]. (Reproduced with permission of the Royal Society of Chemistry)

There are many other classifications of carbon allotropes. On the basis of their dimensionality [10, 11], low-dimensional carbon nanomaterials can be divided into categories of ranging from zero-dimensional (0-D) to one-dimensional (1-D) and two-dimensional (2-D) depending on their nanoscale range (<100 nm) in different spatial directions. These carbon nanomaterials are focused on fullerene, onion-like carbon, carbon-encapsulated metal nanoparticles, nanodiamond (0-D), carbon nanofibers, carbon nanotubes (1-D), graphene, and carbon nanowall (2-D). Diamond, lonsdaleite, and C₈ are considered as 3D structures, sometimes together with graphite. According to one of other classifications, carbon has basically six allotropes, namely, (1) diamond; (2) graphite; (3) lonsdaleite (hexagonal diamond); (4) fullerenes C₆₀ (buckminister fullerene or bucky ball), C₇₀, C₈₂, C₅₄₀, and others; (5) amorphous carbon; and (6) carbon nanotubes (CNTs; buckytube). At the same time, several other additional carbon forms have been recently discovered or predicted, as well as various hybrids between them [12], for instance, the 3D sp²-hybridized graphene-CNT composite (Fig. 1.3) [13]. A generalization of most important carbon allotropes is given in a Table 1.1 and Fig. 1.4. For the nanosized carbon allotropes, the nomenclature “Carbon Nanotropes ” was also assigned [14], but this term is currently practically unknown. Reports about new DFT-predicted carbon allotropes appear frequently, introducing such intriguing and exotic carbon structures as, for example, protomene (Fig. 1.5) [15], consisting of a hexagonal crystal structure, with a fully-relaxed primitive cell involving 48 atoms, or a proposed series of carbon allotropes called novamene [16]. In whole, around 500 hypothetical 3-periodic allotropes of carbon are known at present. In general descriptions of carbon allotropes in online encyclopedias [17], the following carbon forms are mainly present in distinct orders:

Diamond	Chaoite
Graphite	Metallic carbon
Graphene	bcc-Carbon (cubic carbon)
Graphenylene	bct-Carbon
Amorphous carbon	M-Carbon
Buckminsterfullerenes	Q-Carbon
Carbon nanotubes	T-Carbon
Carbon nanobuds	Prismane C8
Glassy carbon (vitreous carbon)	Laves graph or K4 crystal
Atomic and diatomic carbon	Penta-graphene [18]
Carbon nanofoam	Haeckelites
Carbide-derived carbon3 (CDC)	Phagraphene
Lonsdaleite (hexagonal diamond)	Novamene
Linear acetylenic carbon (LAC) (or carbon-atom wires [9])	Protomene

Fig. 1.3

Schematic presentation of dimensionality of sp²-hybridized carbon nanostructures. (a) Fullerene (0D), (b) carbon nanotube (CNT) (1D), (c) graphene nanoribbon (1D), (d) graphene sheet (2D), (e) graphite (3D), (f) 3D CNT networks (3D), (g) hybrid of graphene with horizontal CNT (hCNT) (3D), (h) hybrid of graphene with vertical CNT (vCNT) (3D), and (i) graphene triple periodical minimal surfaces (or schwarzites 3D graphene). (Reproduced with permission of the IOP Science)

Table 1.1 Selected representative carbon allotropes

Fig. 1.4

Eight allotropes of carbon: (a) diamond, (b) graphite, (c) lonsdaleite, (d) C₆₀ buckminsterfullerene, (e) C₅₄₀, fullerite, (f) C₇₀, (g) amorphous carbon, and (h) single-walled carbon nanotube. (Source: Wikimedia Commons)

Fig. 1.5

Natural tiling for the protomene structure : (a) top view down from the ẑ direction (b) side view in the ŷ direction. (Reproduced with permission of the Elsevier Science)

Obviously, this set of carbon forms is incomplete. For example, if we would like to indicate production methods of carbon allotropes, we may add, to the CDC, the “MOF-derived carbon(s), ” a relatively new research area, dedicated to the use of metal-organic frameworks for preparation of metallated nanocarbons from MOFs as precursors by pyrolysis. Indeed, because of a lot of synthesis techniques for carbon allotropes (e.g., CVD and spray pyrolysis, laser ablation, or hydrothermal methods), this classification would not be useful. At the same time, a host of less-common nanocarbons, described below, are currently out of serious attention of researchers, for instance, carbon nanoworms or nanobrushes , among many others, despite potential useful applications. Carbon allotropes can also be doped with nitrogen and decorated/immobilized with metals [20, 21]. Structure, bonding, and mineralogy of carbon at extreme conditions are also attractive research areas [22]. So, the main objective of this book is to show for readers all possible available carbon allotropes, forms, and nanostructures, both discovered and theoretically predicted. At the same time, the coordination and organometallic chemistry [23] of carbon allotropes and their composites/materials [24] requires a major attention due to many applications.