**2. Preparation of graphene and functionalized graphene: prior art**

In the light of what is reported in Section 1, it can be easily understood that primary objectives of the research in the field of graphene are as follows:


**1. Introduction**

176 Graphene Materials - Structure, Properties and Modifications

reactions [19].

functionalization [24–26].

development

layers.

such as oxygen and nitrogen

Graphene [1–3] is the thinnest material on Earth and is commonly considered as a single layer of graphite. It has a two‐dimensional conjugated structure, good thermal stability, very high aspect ratio and specific surface area and has, as a consequence, outstanding electronic, thermal and mechanical properties. At low temperatures and high magnetic fields, quantum Hall effect has been observed in graphene layers for both electrons and holes [4, 5]. The in‐plane thermal conductivity of graphene is among the highest recorded for known materials, about 2000–4000 W m−1 K−1 at room temperature [6, 7]. A graphene sheet has theoretical elastic

In the light of such properties, impressive research activity is currently carried out for applying graphene in high‐performance materials [10], in fields such as nanoelectronics [3], energy storage and energy conversion [11]. Graphene composites allow contributing the exceptional properties of graphene to the macroscopic scale. In particular, light, flexible, robust and conductive graphene papers find a broad spectrum of applications such as electrochemical energy storage devices [12], catalyst supports and fuel cells [13], sensors and actuators [14],

Graphene and derivatives are also finding increasing applications in the field of catalysis [17, 18]. Indeed, carbocatalysis is largely used for promoting synthesis and transformation of organic or inorganic substrates: carbons favour reduction, oxidation and bond‐forming

All the applications mentioned above become successful if graphene or graphitic aggregates made by only few layers of graphene can be used. Large research efforts are thus made in

Moreover, high interest is on functionalization of graphene layers [24–33]. Graphene oxide is considered a stable carbon framework to be functionalized [24] and, as it will be discussed in the next paragraph, is the product of the first step of the oxidation‐reduction process aimed at preparing graphene starting from graphite. It is thus the subject of much of the research on

(i) To prepare graphene and few‐layer graphene through a simple, environmentally friend‐ ly, sustainable and economically viable method that could be applied for large‐scale

(ii) To introduce functional groups on graphene layers, containing in particular heteroatoms

(iii) To preserve the ideal structure of graphene, in graphene and functionalized graphene

In a nutshell, our main goal was the controlled functionalization of graphene layers.

modulus of over 1 TPa and Young modulus of about 1060 MPa [8, 9].

chemical filters and membranes [15] and structural composites [16].

order to prepare graphene and few‐layer graphene [3, 10, 20–23].

In our group, research was performed with the following objectives:

The present chapter summarizes the results of such a research.

Nowadays, single‐layer or few‐layer graphene are obtained through bottom‐up and top‐down approaches [3, 10, 20–23], such as epitaxial growth of graphene films, micromechanical cleavage and dilution in appropriate solvents.

Best practice, particularly in view of large scale applications, is considered the oxidation of graphite or graphitic nanofiller to graphite oxide (well known as GO), followed by thermal or chemical reduction. However, such a pathway is affected by several flaws, in the oxidation and reduction steps and also in consideration of the features of reduced GO.

Harsh and even dangerous reaction conditions are required for the oxidation of pristine graphitic material. First papers on carbon oxidation date back to the nineteenth century. Brodie reported the use of fuming HNO<sup>3</sup> and KClO<sup>3</sup> as intercalant and oxidant [34], in the frame of a multicycle process, that produced toxic and explosive gases such as NO<sup>2</sup> /N2 O4 and ClO<sup>2</sup> , respectively. Indeed, explosions have been documented [35, 36]. Towards the end of the century, Staudenmeier used a blend of H2 SO<sup>4</sup> and HNO<sup>3</sup> (2/1) with KClO<sup>3</sup> as oxidant, in a one‐step process that still produced explosive ClO<sup>2</sup> [37]. These methods of the eigh‐ teenth century cannot be taken into consideration for large‐scale production of GO. Towards the 1960s of the last century, Hummers reported the use of H2 SO<sup>4</sup> /NaNO<sup>3</sup> and KMnO<sup>4</sup> to intercalate and oxidize graphite [38]. The Hummers' method is considered promising for large‐scale production of GO as KMnO<sup>4</sup> is very efficient and reaction takes only few hours, explosive ClO<sup>2</sup> is not formed and the replacement of HNO<sup>3</sup> with NaNO<sup>3</sup> eliminates acidic smokes. However, NO<sup>2</sup> /N2 O4 are still formed and reaction products such as manganese ions can be hardly removed, only by washing with acids such as HCl which, in turn, remains strongly absorbed on the graphitic substrate. Finally, only partial oxidation is obtained [39–41]. Research has been thus dedicated to improve the Hummers' method [40–43]. NaNO<sup>3</sup> has been removed by increasing the amount of KMnO<sup>4</sup> and by using a H2 SO<sup>4</sup> /H3 PO<sup>4</sup> mixture [40]. NaNO<sup>3</sup> has been simply removed, without observing negative effects on the reaction [41]. However, only an incomplete conversion of graphite to GO was obtained. The pre‐oxidation of graphite with P2 O5 and K2 S2 O8 in H2 SO<sup>4</sup> adds a further step [44]. The complete conversion to GO has been reported [42] by using graphite flakes with sizes in the range of 3–20 μm. In another variation of the Hummers' method, the concentration of NaNO<sup>3</sup> and KMnO<sup>4</sup> and the residence times have been modified [43]. GO can be also prepared by solvent‐free mechano‐ chemical oxidation of graphite [45]. However, such a method can be hardly employed for large‐scale production.

The Hummers' method based on the use of NaNO<sup>3</sup> , applied by one of the authors of the present chapter is fully described in Ref. [46].

It is evident that the objective of developing a really simple method, suitable for large‐scale production, has not been achieved yet.

The reduction step is crucial in order to obtain graphene. Well‐known methods are based on the use of hydrazine [47–50] or hydrogen plasma [49]. To avoid reaggregation of graphitic flakes, a stabilizing agent should be added [9, 51]. Hydrazine is well known as a toxic reagent. Hence, eco‐friendly methods have been attempted. Quick deoxygenation of graphite oxide assisted by a base (NaOH, 0.1 M) has been performed at moderate temperatures (80°C) [52]. Ascorbic acid has been used as well [53]. Thermal [54] and flash [55] reductions have been reported.

However, it is widely acknowledged that reduction is still incomplete and that the ideal graphene structure is neither preserved nor restored [20, 56]. Hence, also this objective is not achieved yet.

However, the pathway that could lead to graphene through graphite oxidation gives the opportunity of preparing GO that could be a suitable building block for further reactions [24–26] as well as for catalytic applications [17, 18]. Moreover, functionalized single‐layer graphene sheets can be prepared by splitting graphite oxide [57]. The structure of GO has been investigated for decades, but it is substantially still unknown [20, 21, 58], and this makes GO not the ideal building block for further reactions. It was reported [59] that hydroxyl and epoxide groups are on the surface of basal planes and carbonyl and carboxyl groups are on the edges. Moreover, it is widely acknowledged that oxidation leads to extensive disruption of sp2 hybridization of graphene layers, that is, graphene properties are drastically damaged and GO loses the benefits of a graphitic structure.

This brief sum up allows to comment that important research efforts are required in order to achieve the objective of preparing graphene and few‐layer graphene (optionally containing functional groups) preserving the ideal graphene structure and through a simple, eco‐friendly, economically viable and scalable method.
