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Graphite and Graphene Nanoplatelets (GNP) Filled Polymer Matrix Nanocomposites

Marc PONÇOT, Adrien LETOFFE, Stéphane CUYNET, Sébastien FONTANA and Lucie SPEYER

Institut Jean Lamour, University of Lorraine, Nancy, France

1.1. General information on graphene

1.1.1. Definition and structure

Graphene is a plane of carbon atoms in sp2 hybridization. It is the basic constituent of graphite, carbon nanotubes and fullerenes. Figure 1.1 shows a graphene plane.

Graphene can present some structural defects, although its low dimensionality reduces the number of possible imperfections. For instance, the graphene plane may not be strictly composed of hexagons of carbon atoms: these are Stone-Wales defects, where four hexagons are transformed into two pentagons and two heptagons when one of the bonds rotates. Single and double gaps can appear, as well as dislocations, atoms and impurities of substitution. These defects have a strong influence on the physical properties of graphene. In addition, some defects generate dangling bonds that increase the chemical reactivity of graphene (Banhart et al. 2011).

Figure 1.1. Representation of a graphene plane

1.1.2. Structures associated with graphene

As will be discussed later in this chapter, a single graphene plane is difficult to obtain and synthesis methods most often result in the stacking of multiple planes, often referred to in the literature as graphene itself. In fact, a specific terminology for carbonaceous materials with a two-dimensional character has been established (Bianco et al. 2013) and the following structures should be distinguished from the single graphene plane:

• - double-wall or triple-wall graphene: structure in free suspension or resting on a substrate, composed of a stack of two or three graphene planes of extended lateral dimension;
• - few-layer graphene (FLG): structure in free suspension or resting on a substrate for a stack of two to five planes;
• - multi-layer graphene (MLG): structure in free suspension or resting on a substrate for a stack of six to 10 planes;
• - graphite nanosheets: two-dimensional graphitic materials with one dimension not exceeding 100 nm;
• - nanoribbon graphene: graphene plane as defined above, with lateral dimensions not exceeding 100 nm and 10 nm, respectively;
• - graphenic carbonaceous materials: include all of the carbonaceous materials made up of sp2-hybridized atoms, like the structures defined above, or the disordered carbons (carbon blacks, activated carbons) and graphite.

1.1.3. Graphene properties

The calculated and measured properties of graphene are unique and remarkable, which explains the interest of the scientific community. Wallace (1947) showed the particular electronic structure of graphene. Thus, graphene is considered as a zero-gap semiconductor. The mobility of charge carriers is high and has been measured up to 230,000 m2·V-1·s-1 (Bolotin et al. 2008). This value corresponds to the highest value for a semiconductor. Its thermal conductivity is also remarkable. It reaches 5,300 W·m-1 ·K-1 (Rutter et al. 2007), which corresponds to the highest value for a material. In comparison, diamond, which is well known for its ability to evacuate heat, has a thermal conductivity of only 2,000 W·m-1·K-1. Its network of sp2-hybridization atoms gives graphene a unique set of mechanical properties. The combination of a hardness equivalent to that of diamond, close to 1 TPa, a very high flexural strength, a high specific stress at break of 48,000 kN·m·kg-1 and its elongation at break of 25% makes it unique (Lee et al. 2008; Zhu et al. 2010). In comparison, steel has a specific stress at break of 154 kN·m·kg-1 (Zhu et al. 2010). Graphene has a transparency of 97.7% in the visible spectrum (Rutter et al. 2007; Zhu et al. 2010). Graphene is considered to be the most impermeable material in the world. This is due to the combination of its sp2-hybridization carbon atom lattice with high electron density in its aromatic rings and its structure (covalent bonds and rupture stress) (Berry 2013). Another important property of graphene is that its theoretical specific surface area is 2,630 m2·g-1 (Ferrari et al. 2015), which is very high for a carbonaceous material.

1.2. Graphene preparation methods

Graphene is an amazing material because of the combination of exceptional properties conferred by its two-dimensional character and sp2-hybridization lattice of carbon atoms. The properties are particularly noticeable in the case of a single plane of free single-crystal graphene. Many works have been dedicated to graphene synthesis since the research by Novoselov and Geim (2004). Several so-called major synthesis methods have been developed and allow very different samples to be obtained according to several criteria:

• - the structural quality and chemical purity of the sample;
• - the presentation of the sample: graphene planes on metallic substrate, SiC or other, or dispersion;
• - the cost price and the industrializable nature of the method.

Figure 1.2 shows the materials obtained by the synthesis methods described in this chapter, according to their structural quality and their estimated cost for industrial production.

Figure 1.2. Quality and industrial cost of graphene samples obtained by different synthesis techniques (Novoselov et al. 2012). For a color version of this figure, see www.iste.co.uk/bai/nanocomposites.zip

Mechanical exfoliation produces an excellent structural quality but, although very simple in principle, would generate too high a manufacturing cost. Liquidphase exfoliation, although based on the same principle, provides graphene dispersions of lower quality, but it is much easier to obtain large quantities of material. The supported growth techniques, which will not be detailed in this chapter, are expensive techniques (energy cost, price of substrates and silicon carbide wafers, reaction chambers), but the material is of high quality, and some techniques make it possible to obtain very large surface areas (Bae et al. 2010). In graphite, the cohesion of the crystal along the axis is ensured by Van der Waals bonds. The basic principle of exfoliation techniques is to break these bonds, by the so-called dry method or in liquid phase, in order to obtain graphene. It is also possible to exfoliate other graphite-based materials, such as intercalation compounds and graphite oxide (GO).

1.2.1. Graphite exfoliation

1.2.1.1. Dry exfoliation

Mechanical exfoliation is the synthesis technique made famous by Novoselov and Geim (2004). The principle is very simple and consists of applying adhesive tape on graphite (typically pyrolytic graphite because of its superior crystalline quality), peeling off the tape by isolating a certain thickness of graphene planes, and repeating the operation on the film thus obtained until the thinnest possible thickness is reached (Novoselov et al. 2004). The material is then usually transferred to a SiO2/Si substrate to allow for the selection of the single walls by way of characterization techniques, such as optical microscopy. To date, mechanical exfoliation remains the technique that enables the best electronic and structural quality to be obtained. However, it is not suitable for industrial production: its main application remains basic research and the development of prototypes for various applications (Novoselov et al. 2012). Other dry exfoliation techniques have been developed, motivated by the crystalline quality of graphene obtained by mechanical exfoliation and in attempts to solve the problem of large-scale production. Laser ablation of graphite (Dhar et al. 2011) or anodic exfoliation (Shukla et al. 2009) have been performed and also yield high-quality material.

1.2.1.2. Liquid-phase exfoliation by ultrasound

Liquid-phase exfoliation of graphite is based on graphite dispersion in solvent and on the transition from dispersion to ultrasounds. Due to the hydrodynamic forces, the ultrasounds will generate a phenomenon of cavitation, namely the formation of bubbles. These bubbles will be able to break the Van der Waals bonds between the graphene planes, and the solvent must be able to stabilize the planes in suspension to prevent their reaggregation. The fundamental characteristic of the solvent to be considered in this perspective is the surface tension. Indeed, a solvent with adapted surface tension will be able to minimize the interfacial tension between the graphene sheets and the solvent molecules, and thus prevent their reaggregation. The ideal surface tension is 40-50 mJ·m-2. Graphite exfoliation is thus carried out in solvents such as N-methylpyrrolidone (NMP) or dimethylformamide (DMF). A centrifugation step needs to be added to remove the thickest particles (Hernandez et al. 2008). This process allows dispersions of micrometer-sized particles to be obtained, generally less than 5 planes thick, with a final...