Graphene, the allotropic form of carbon comprised of sp²-bonded carbon atoms in a sheet-

like hexagonal lattice arrangement in a two-dimensional plane. Three atomic orbitals, from

carbon atoms viz. 2s, 2px, and 2py orbitals are hybridized to form sp²-hybridized orbitals,

which form covalent bonds with the neighboring carbon atoms. These arrangements of

hybridized orbitals lead to a hexagonal honeycomb lattice planar structure in graphene. The

2pz orbital is oriented perpendicular to the planar structure and form π bond and these π

bonds are hybridized together to form the π-band which contributes to the astonishing

electrical conductivity of graphene. Thus, graphene is composed of a closely packed single

layer of carbon atoms, creating a 2D honeycomb lattice plane. In single-layer graphene,

carbon atoms bond with adjacent carbon atoms with sp² hybridization forming a benzene

ring in which each atom donates an unpaired electron. Graphene is theoretically a non-

metal, but is frequently described as a quasi-metal due to its properties being like that of a

semi-conducting metal.

The graphene carbon atoms are bonded to only three other carbon atoms, although

they can bond to a fourth carbon atom. This ability with high tensile strength and high

surface area to volume ratio brands graphene as one of the promising materials in

the fabrication of composites. The inimitable physical properties such as appallingly

high carrier mobility, mechanical strength, flexibility, and thermal conductivity

positioned graphene as a supreme material [23]. This wonderful material has pro­

mising application in the field of bioelectronics owing to its superb electromechanical

properties.

The oxidized form of graphene is known as graphene oxide (GO) and the surface of GO

is decorated with oxygen-bearing functional groups such as hydroxyl (-OH) and epoxy

(

) groups on sp³ hybridized carbon, on the basal carbon plane and carbonyl(-C=O)

and carboxyl(C-OOH) groups were attached at the edge’s sheets of sp² hybridization

carbon. The presence of these functional groups enhances the hydrophilicity of graphene

and widens their applications in biological fields such as sensing, drug delivery, and

implantable devices [24]. The structures of graphene-based materials are shown in

Figure 16.1.

FIGURE 16.1

Structures of graphene-based materials: (a) the pristine graphene (pure-arranged carbon atoms) with sp²-

hybridized carbon atoms, and the chemically modified graphene, including (b) graphene oxide (GO); (c) re­

duced graphene oxide (RGO); and (d) graphene quantum dot (GQD).

Source: (Reproduced from Sensors 2017, 17, 2161; doi: 10.3390/s17102161: licensed under creative commons

attribution (CCBY) ( http://creativecommons.org/licenses/by/4.0/).

Graphene Nanostructures

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