Treatment
2.2 Graphene Synthesis Routes
The method adopted for graphene synthesis is important in connection with graphene properties and, consequently, with projected applications of this novel carbon allotrope. Graphene synthesis has been carried out through two major routes depending on whether it is derived from graphite or other carbon sources [15]. They are known as “top-down” and “bottom-up” routes, respectively. Figure 2.1 shows an illustrative scheme of representative methods of each of these routes and main applications of the graphene derived.
Figure 2.1 Schematic illustration of top-down and bottom up routes for graphene synthesis, representative methods, and applications.
In the bottom-up route, graphene is directly synthesized from small organic molecules or atoms by chemical processes. Epitaxial growth on electrically insulating surfaces such as silicon carbide, chemical vapor deposition, either thermal or plasma enhanced on various metal substrates, and solvo-thermal process are representative examples of the bottom-up route [15–18]. This route leads to highly defect-free graphene nanosheets, especially suitable for electronic applications, even though at the expense of low yields and high processing costs.
On the other hand, the top-down route involves graphite sources as starting material and physical or chemical methods to yield a mixture of single and few layer graphene nanosheets (Figure 2.1). Graphene was obtained for the first time through this route by applying micromechanical exfoliation from a graphite piece, known as the “Scotch” tape method [7].
The most commonly applied top-down method, with great potential for large-scale production due to its simplicity and high yield, is based on the reduction of highly oxidized GO nanosheets, a nonconductive hydrophilic carbon material. The Hummers method is first used to generate graphite oxide through the addition of KMnO4 to a solution of flake graphite, NaNO3, and concentrated H2SO4 acid [19]. The acid is used to intercalate graphite with the assistance of NaNO3, and KMnO4 to oxidize the acid- intercalated graphite [15, 20]. The strong oxidizing agents introduce functional groups that increase the distance between nanolayers and facilitate their isolation.
Graphite oxide is subsequently peeled off usually by ultrasonic exfoliation in water to obtain GO followed by centrifugation. Thermal expandable exfoliation, static exfoliation, and chemical exfoliation are other techniques applied [21–24]. The supernatant from water exfoliation is colloidal and contains few- and single-layer sheets of GO. Successive washing of the supernatant with water is performed to remove the oxidizing agents. H2O2 is often added to reduce the remaining KMnO4. Oxygen-containing functionalities in the resulting GO sheets include carboxyl and carbonyl groups at the sheet edges, and hydroxyl and epoxy (1,2-ether) functional groups on the basal plane, that can alter van der Waals’ interactions leading to a range of solubility in water and organic solvents [25]. GO has attracted special attention not only as a precursor for large-scale production of
graphene but also for adsorption applications because of its large theoretical surface area, oxygen surface groups, high water dispersibility, stability, and ease of synthesis [26].
Improvements to the Hummers method in order to make it more efficient and environmentally friendly have been explored. For instance, exclusion of NaNO3, increase in KMnO4 amount, and use of 9:1 mixture of concentrated H2SO4/H3PO4 led to enhance the efficiency of the oxidation process, thus providing a higher yield of hydrophilic oxidized graphene with fewer defects in comparison with the conventional Hummers method. Besides, NaNO3 elimination avoids the release of toxic gases (NO2, N2O4) [27]. Additional incorporation of H3PO4 but keeping all the other reagents has also been reported [28]. Besides, GO with different oxidation degrees was synthesized by varying the amount of KMnO4 [29]. Formation of different types of oxygen-containing functional groups in GO and their influence on the nanostructure were examined. The results revealed a disruption of the graphitic amido black (AB) stacking order with the increase in the oxidation level, and also the formation of hydroxyl and carboxyl groups at lower oxidation levels and epoxide groups at the higher ones.
Other changes explored concern extra steps to improve the removal of impurities, such as acids, manganese salts, resulting from graphite oxidation.
In this direction, additional multi-washing with different solvents, dead-end filtration, and/or dialysis [30–32] have been adopted. To favor purification of the large amounts of GO required at industrial-scale, cross-flow filtration has been proposed [20, 33]. Nevertheless, all these methods require a previous centrifugation stage to separate unoxidized graphite from the mixture.
Recently, Chen et al. [20] developed an improved Hummers’ method using graphite with small flake sizes (3–20 μm) to produce GO with a significantly higher yield and simplicity in purification with respect to those involving large graphite flakes (10–100 μm). Purification involves dialysis for 1 week using a dialysis membrane with a molecular weight cutoff of 8,000–14,000 g mol–1 to remove the remaining acid and metal species. Due to the high maximized yield of GO, this method avoids centrifugation for separating the unoxidized graphite, thus reducing costs and favoring full-scale GO production.
In order to obtain graphene, GO is chemically reduced to RGO in a second stage by employing different reducing agents, being hydrazine hydrate the reagent mostly used currently [15]. Other reducing agents include sodium borohydryde (NaBH4), p-phenylene diamine, hydroquinone, and sodium hydrosulfite [34, 35]. All these agents are hazardous to human health and to the environment. Accordingly, alternative environmental friendly chemicals, such as ascorbic acid, baker’s yeast, and aluminum powder, polyphenol present in green tea solution, among others, have been tested [12].
Hydrothermal reduction has also been proposed [36, 37]. A main drawback of chemical reduction of GO is that the pristine graphene structure is not completely restored. Defects introduced into the nanostructure during oxidation cannot be completely removed by subsequent reduction, thus weakening graphene properties and restricting some potential applications [15, 27]. It should also be mentioned that the oxidation degree in both GO and RGO obtained through the Hummers method is very variable even following the same procedure, thus generating structural variability and changes in the proportion of graphene single- or multilayers.
Other chemical methods following the top-down route include intercalation compounds exfoliation and reduction of fluoride graphite, using either the commercial product or samples obtained by reaction of F2 and graphite [22].
Liquid-phase exfoliation, which is based on the use of surfactants or solvents that intercalate between graphite layers to facilitate graphene nanosheets separation, is another example of the top-down methods. Thorough founded reviews of major methods for graphene synthesis along with their advantages and drawbacks may be found elsewhere [11, 14, 15, 38, 39].