Perspectives on Chemical Modification of Carbon Nanomaterials Assisted by Microwave Radiation Mini

Carbon nanomaterials (CNMs) as graphene and carbon nanotubes (CNTs) have attracted attention of scientific community due to their high performance and applications. However, their potential applications as nanofillers, depends necessarily of their production which high structural quality, great compatibility with the host system. For instance, in polymeric matrices this issue involves several challenges due to their low dispersibility in water and organic solvents, thus different process of chemical modification of CNMs surfaces have been reported. In cases of functionalization with amino groups, most of them use in one of their chemical steps thionyl chloride (SOCl2) as intermediate reagent. In general, the use of SOCl2 limits the chemical process, due to of its dangerous nature and high level of toxicity. Another drawback found in this kind of methodology is the time spent on reflux lasting days and making it a very onerous process. In this context, the chemical modification of CNMs assisted by microwave radiation has been explored in synthesis, purification and chemical modification processes of these nanostructures resulting in simpler and faster procedures that allow its used on a large scale and with low environmental impact.


Mini Review
CMNs as graphene and carbon nanotubes exhibit exceptional physical properties that have generated great interest from the scientific and technological community [1][2][3][4]. Graphene can be understood as a single stable 2D graphite layer formed only by carbon atoms hybridized in sp 2 and arranged in hexagonal rings, that theoretically is considered as a fundamental block for the construction of other allotropic covalent carbon solids, such as graphite, fullerenes and CNTs [5]. In order, the development of several methods of production of graphene and carbon nanotubes has been gaining relevance because their nanostructure exhibits excellent electrical, thermal and mechanical high flexibility. In special, CNTs have been studied in approximately the last two decades by many researchers due to their great potential application in different fields [6]. Their important features such as light weight, high aspect ratio, and impressive tensile strength, thermal and electrical conductivity make them useful as nano additives in different polymers and ceramics systems [7]. On the other hand, it was observed that graphene presents high electrical mobility (200.000cm 2 V −1 s −1 ) [8], two orders of magnitude greater than the silicon and its electric transport occurs from ballistic form at room temperature [5]. Graphene showed thermal conductivity of approximately 5.000Wm −1 K −1 [9], and high surface area (2.630m 2 g −1 ) [10]. Moreover, it is important to mention that graphene presents a modulus of elasticity of 1.1GPa and tensile strength of 125GPa, 200 times greater than steel [11].
The ability to control the functionalities of CNMs surface is importance for exploring and building blocks in the engineering of supra-molecular structures, that imparting them specific solubility in specific solvents or host matrices [33]. In general, to produce a more reactive CNMs surface in different substrates, chemical groups may be covalently introduced on their surface [5,24,34]. In case of graphene, graphite and CNTs, this type of procedure needs to be controlled duo to the inherent re-hybridization process of carbon atoms sp 2 to sp 3 . This process is accompanied by the partial or total loss of π-π electronic conjugation that alter significantly their electrical properties [5,22,26,35,36]. To overcome this issue, one strategy adopted is the doping of the crystalline network of CNMs with hetexroatoms such as N, P and B without destroying their structural integrity, thus preserving their potential properties [20,22,37] however, this task is not easy.
Several studies have been carried out to develop effective CNMs fictionalization process, and most of them adopted the strategy based on processes of carboxylation. This reaction has advantages in that the carboxylated groups can be derivatized to ester, amides and isocyanates [33,38]. However, many works with amino-functionalization reaction in CMNs, report processes via SOCl2 (Figure 1). Niyogi et al. [34] reported an effective way of obtaining graphene oxide functionalized with octadecylamine (ODA) after the treatment of graphite oxide with sulfuric acid, followed by treatment with thionyl chloride (SOCl 2 ) and ODA, forming GO-ODA. Hu Y et al. [6] were able to functionalize CNMs with 4,4'-diaminodiphenylsulfono (DDS) in dimethylformamide (DMF) at 130 °C under inert atmosphere at reflux for 6 days. Silva et al. [7] modified covalently CNTs with triethylenetetramine groups (TETA). Initially, they used as the reaction mixture in the presence of distilled SOCl 2 and maintained under reflux for 24 hours. After reflux, all SOCl 2 was extracted from the medium by distillation. Then, excess TETA was added in volumetric flask and kept under reflux for 72 hours [7]. This classical amino-functionalization reaction with SOCl 2 has resulted in some difficulties and inconveniences, since thionyl chloride has a high degree of toxicity, and high carcinogenic potential. SOCl 2 reacts violently in contact with water, releasing acid gases and, in contact with metal surfaces, can generate flammable and explosive hydrogen. Generally, this methodology requires approximately 96 hours or up to 6 days, which makes the procedure very onerous [7,39].
On the other hand, several chemical modification methodologies assisted by microwave have been reported [16,19,20,[40][41][42][43][44][45][46]. In general, the irradiation process can be used in mild conditions, with controlled temperatures and reduced time reaction, when compared to conventional techniques ( Figure 1) [13,33,41,[46][47][48]. An advantage of use microwave radiation that only 3 minutes of irradiation in CNTs samples (in H 2 SO4/HNO 3 mixture) is sufficient to produce carboxyl groups promoting their dispersibility in water [48]. Amidation, esterification and cycle addition reactions in CNMs surface assisted by microwave in several solvent systems and ionic liquids have also been reported [49][50][51][52]. The introduction of amide groups in GO surface in only 30 minutes without the use of SOCl 2 , was reported by Ribeiro et al. [33]. Several other methodologies of chemical modification of CNMs assisted by microwave have led to the formation of CNMs functionalized with different groups in a smaller reaction time [46], some of them are summarized in the Table 1. Table 1: Synthesis and chemical functionalization methodologies of CNMs assisted by microwave radiation.

Nanomaterials Methodology Reference
Graphene Oxide The addition of a small amount graphene oxide leads to deoxygenation reaction of GO under microwave irradiation and graphene formation. [41] MWCNTs Multi-walled carbon nanotubes (MWCNTs) synthesized by thermal chemical vapor deposition have been successfully purified by high efficient microwave acid digestion method. [53] Carbon nanotubes Interaction study of carbon nanotubes and microwaves, responses, and applications [46] MWCNTs-NCO Microwave-assisted functionalization of carbon nanotubes with isocyanate groups allowed a reduction of functionalization time from 24h to 30 minute with no change in the degree of functionalization or in the nanotube characteristics. [38] Carbon black Carbon black surfaces were sequentially modified with ethylenediamine by microwave irradiation. [54] Graphite Versatile method to achieve the exfoliation and reduction of graphite oxide assisted by microwave reaction. [55] Graphene Chemical reduction method assisted by microwave irradiation for the synthesis of chemically converted graphene sheets and metal nanoparticles dispersed on the graphene sheets. [44] Graphene-chitosan Chitosan (CS) modified graphene nanosheets were prepared under microwave irradiation in N,N-dimethylformamide medium. [19] Mos2/graphene Graphene-based molybdenum disulfide catalyst was prepared via a microwave irradiation assisted route. [51] MWCNTs-bisphenol-A-polycarbonate MWNTs were grafted with bisphenol-A-polycarbonate in a solid state reaction at 110 °C using microwave irradiation. [56] These techniques have also been used in several CNMs grafting procedures in polymeric molecules [56,57], and in catalytic systems such as hydrodesulfurization reactions [51]. Thus, chemical manipulation processes of CNMs assisted by microwave have often used in methodologies for the synthesis, purification and functionalization of CNMs samples, providing simple, easy methodologies that allow us used it in large scale.

Conclusion
The microwave-activated chemical reactions cover a wide spectrum of potential applications for the chemical manipulation of CNMs, providing a fast, compact, simple, selective, and economical technique. In addition, the reaction system can evolve under mild conditions in the absence of toxic reagents (such as SOCl 2 ), and without the use of long heating time, as seen in the classical refluxes. Further studies on the behavior of CNMs as graphene and CNTs in different systems activated by microwaves should be explored opening new perspectives in their applications in different areas of science and technology.