Chapter 1

An Introduction to Graphene

Konstantinos Spyrou and Petra Rudolf

1.1 Brief History of Graphite

Carbon takes its name from the latin word carbo meaning charcoal. This element is unique in that its unique electronic structure allows for hybridization to build up sp3, sp2, and sp networks and, hence, to form more known stable allotropes than any other element. The most common allotropic form of carbon is graphite which is an abundant natural mineral and together with diamond has been known since antiquity. Graphite consists of sp2 hybridized carbon atomic layers which are stacked together by weak van der Waals forces. The single layers of carbon atoms tightly packed into a two-dimensional (2D) honeycomb crystal lattice is called graphene. This name was introduced by Boehm, Setton, and Stumpp in 1994 [1]. Graphite exhibits a remarkable anisotropic behavior with respect to thermal and electrical conductivity. It is highly conductive in the direction parallel to the graphene layers because of the in-plane metallic character, whereas it exhibits poor conductivity in the direction perpendicular to the layers because of the weak van der Waals interactions between them [2]. The carbon atoms in the graphene layer form three σ bonds with neighboring carbon atoms by overlapping of sp2 orbitals while the remaining pz orbitals overlap to form a band of filled π orbitals – the valence band – and a band of empty π* orbitals – the conduction band – which are responsible for the high in-plane conductivity.

The interplanar spacing of graphite amounts to 0.34 nm and is not big enough to host organic molecules/ions or other inorganic species. However several intercalation strategies have been applied to enlarge the interlayer galleries of graphite from 0.34 nm to higher values, which can reach more than 1 nm in some cases, depending on the size of the guest species. Since the first intercalation of potassium in graphite, a plethora of chemical species have been tested to construct what are known as graphite intercalation compounds (GICs). The inserted species are stabilized between the graphene layers through ionic or polar interactions without influencing the graphene structure. Such compounds can be formed not only with lithium, potassium, sodium, and other alkali metals, but also with anions such as nitrate, bisulfate, or halogens.

In other cases the insertion of guest molecules may occur through covalent bonding via chemical grafting reactions within the interlayer space of graphite; this results in structural modifications of the graphene planes because the hybridization of the reacting carbon atoms changes from sp2 to sp3. A characteristic example is the insertion of strong acids and oxidizing reagents that creates oxygen functional groups on the surfaces and at the edges of the graphene layers giving rise to graphite oxide. Schafheutl [3] first (1840) and Brodie [4] 19 years later (1859) were the pioneers in the production of graphite oxide. The former prepared graphite oxide with a mixture of sulfuric and nitric acid, while the latter treated natural graphite with potassium chlorate and fuming nitric acid. Staudenmaier [5] proposed a variation of the Brodie method where graphite is oxidized by addition of concentrated sulfuric and nitric acid with potassium chlorate. A century later (1958) Hummers and Offeman [6] reported the oxidation of graphite and the production of graphite oxide on immersing natural graphite in a mixture of H2SO4, NaNO3, and KMnO4 as a result of the reaction of the anions intercalated between the graphitic layers with carbon atoms, which breaks the aromatic character. The strong oxidative action of these species leads to the formation of anionic groups on graphitic layers, mostly hydroxylates, carboxylates, and epoxy groups. The out of planar C–O covalent bonds increase the distance between the graphene layers from 0.35 nm in graphite to about 0.68 nm in graphite oxide [7]. This increased spacing and the anionic or polar character of the oxygen groups formed impart to graphene oxide (GO) a strongly hydrophilic behavior, which allows water molecules to penetrate between the graphene layers and thereby increase the interlayer distance even further. Thus graphite oxide becomes highly dispersible in water. The formation of sp3 carbon atoms during oxidation disrupts the delocalized π system and consequently electrical conductivity in graphite oxide deteriorates reaching between 103 and 107 Ω cm depending on the amount of oxygen [2, 8].

1.2 Graphene and Graphene Oxide

For several decades the isolation of graphene monolayer seemed to be impossible on the basis of, among other things, theoretical studies on the thermodynamic stability of two-dimensional crystals [9]. An important step in this direction was made by a research group in Manchester guided by Geim and Novoselov in 2004 [10] who reported a method for the creation of single layer graphene on a silicon oxide substrate by peeling the graphite by micromechanical cleavage (scotch tape method). Graphene exhibited outstanding structural [11], electrical [12], and mechanical properties [13] and 6 years later Novoselov and Geim were honored with the Nobel Prize in Physics “for groundbreaking experiments regarding the two-dimensional material graphene.” During this time a number of methods for the production of graphene monolayers have been developed. These methods can be divided into different categories depending on the chemical or physical process employed to obtain the single layer graphene. The next three sections describe the three types of chemical methods.

1.2.1 Preparation of Graphene from Graphene Oxide

Although the report on single sheets of GO [14, 15] obtained by procedures established by Staudenmaier and Hummers and Offeman [4–6] had been published, the scientific community largely continued to consider graphite oxide a layered graphitic material. It was not until after the isolation of pristine graphene by micromechanical cleavage that the question was reexamined and it was ascertained that the method developed by Hummers and Offeman produces exfoliated oxidized single graphene layers by the dispersion of graphite oxide in water. These chemically prepared monolayers of GO can be considered the precursors for the production of graphene by the removal of the oxygen groups. The precise structure of GO depends on the oxidation process and is still a subject of debate. The most accepted models are the Lerf–Klinowski and the Dékány models [16, 17]. Recently Ajayan et al. confirmed that for GO prepared with the protocol that resulted in the Lerf–Klinowski model, ring lactols are present at the edges of the GO sheets (Figure 1.1) [18].

Figure 1.1 Atomic force microscopy (AFM) image and structural model of graphene oxide (GO) sheets. (a) An AFM image of GO sheets on a silicon substrate. (b) A structural model of GO introduced by Ajayan et al. (Reproduced with permission from [18].)

The first dispersion of single graphene layers was presented in 2006 by Ruoff's group, which used hydrazine hydrate for the reduction of GO prepared by Hummers method [19, 20]. Although several reductive procedures have been applied by several research groups (see Table 1.1 and references therein) in the following years, none achieved full reduction of the GO monolayers into graphene. This agrees with the theoretical finding that a reduction of GO from 75% to 6.25% (C:O ratio 16 : 1) coverage is relatively easy but further reduction seems to be rather difficult [21]. For this reason the final isolated carbon monolayers derived from the reduction of GO are usually called partially reduced graphene oxide (rGO) or chemically converted graphene (CCG). The results of the various reductive procedures that have been developed are summarized in the following table.

Table 1.1 Summary of reduction agents for chemical reduction of graphene oxide [22].