*3.1. Nomenclature and Fundamental Aspects*

Graphene, discovered by Geim and Novoselov in 2004, is relatively a new two dimensional (2D) sheet-like material in which a honeycomb or hexagonal structure with a flat lattice configuration, completely composed of sp2 hybridized carbon atoms that are covalently bound, is densely packed[102,103]. Graphene, an atomic layer of graphite, is the unique carbon's allotrope, where each atom is tightly linked to its neighbors by an only electronic cloud in which a C–C bond distance is 0.142 nm [104]. It is considered as a fundamental basis for all carbon allotropes and as the mother of a graphitic family for all the dimensions.

Graphene-based nanomaterials (GNMs), the first materials reported as examples of 2D nanocarbons, can be classified based on the number of sheet layers, surface modifications, total oxygen content or orientation [105]. Graphene is highly hydrophobic and is prone to agglomeration, owing to the strong van der Waals' interactions between the 2D graphene sheets, leading to low surface area and ineffective use of its outstanding features [106]. These latter are also closely dependent on the graphene availability as a single layer because if the layers are in close vicinity to each other, they are likely to restack or agglomerate due to π–π interactions. Hence, its functionalization is commonly required to surpass these issues. Typically, three types of functionalization approaches through covalent (nucleophilic substitution, electrophilic substitution, condensation and addition), noncovalent (π–π bonding, electrostatic attraction and hydrogen bonding, etc.) or a combination of both interactions can be used, where the aromaticity of graphene can be either lost or preserved [107]. As shown in Figure 3a–e, GNMs can be found in various forms for which the most important ones that will be the focus of the present review are graphene nanosheets (GNS), graphene nanoplatelets (GNP), graphene

oxide (GO), reduced graphene oxide (RGO) and graphene and graphene oxide quantum dots (GQD). In the frame of the present review, the acronym GN will encompass GNS and GNP.

**Figure 3.** Some common forms of graphene: (**a**) graphene oxide, (**b**) pristine graphene, (**c**) functionalized graphene, (**d**) graphene quantum dot and (**e**) reduced graphene oxide. Reproduced with permission from Reference [46]. Licensed under a Creative Commons Attribution 3.0 International License (https://creativecommons.org/licenses/by/3.0/); (**f**) Different properties of graphene and its applications. Reproduced with permission from Reference [108]. Copyright ©2019, Elsevier.

Graphene oxide (GO), commonly prepared from the oxidation of graphite, consists of a few- or a single-layer sheet. GO sheets are rich in various oxygen-containing groups such as hydroxyl, epoxy, carboxyl, carbonyl, phenol, lactone and quinone, which can change the van der Waals interactions. The two former chemical groups are mostly present on the basal plane, whereas the others with small quantities are found at the sheet edges. These functional groups in GO can deeply influence its electrochemical, mechanical and electronic features. Despite the aromaticity of graphene is lost in GO, owing to exploitation of π electrons in the covalent bonding of these oxy groups on graphene backbone, the carbonyl, carboxyl and so forth groups at the edge render them more dispersible in both organic solvents and water [44,107]. The hydrophobic aromatic frameworks and the hydrophilic oxygen-containing groups make GO amphiphilic, allowing its interaction with inorganic and organic molecules.

Reduced graphene oxide (RGO), obtained by the reduction of GO [109], contains fewer oxygen atoms, hence, is less negatively charged [106]. During the reduction, RGO recovers the graphitic arrangements (partial recuperation of the sp2 from sp3 hybridization of GO) through the elimination of the oxygen-containing groups, which have been inserted in the oxidation step, thus, restoring the electronic properties of graphene [110]. This partial reduction and the exposure to some chemicals allow tailoring the conductivity, band-gap and optical features of the material [111,112].

Graphene quantum dots (GQDs), which can be found as single- or multiple layers, display interesting features such as good chemical stability, high surface area, tunable physical characteristics, stable photoluminescence and low toxicity [113,114]. They can be used in optoelectronic, electronic, biomedical, sensors and energy storage. They usually consist of up to 10 layers of 10–60 nm size RGO [46].

Graphene-based nanomaterials possess exceptional electrical, optical, mechanical, electrochemical and thermal features that make them versatile for a wide range of applications and have drawn worldwide attention in both academic and engineering fields [44,105]. They can be employed in industrial applications such as biomedical, solar cells, biosensors, supercapacitors, electromagnetic absorbers, optical devices, integrated circuit, protective coatings, organiclight-emitting diodes, sound transducers, petroleumindustry, automobile components, aerospace, energy storage, nanocomposites and contamination purification in wastewater management, to cite a few (Figure 3f) [107,115,116].
