synthesized by solution blending or CVD, and have been functional in applications like Li-ion batteries, supercapacitors, fuel cells, conductors, photovoltaic devices, etc. Like as synthesis of graphene nanosheets-supported carbon-nanotube-Sn nanorods based nanocomposite was engaging in-situ CVD method. This nanocomposite represents extraordinary energy storage abilities and outstanding performance like as efficient anodes for Li-ion battery (LIB) applications [40]. Similarly, nitrogen-doped graphene/carbon nanotube nanocomposites have been synthesized by hydrothermal treatment of graphene oxide, oxidized multiwalled carbon nanotube and ammonia at 180°C. This nanocomposite reveals it boosted electrochemical activities and acts as an effectual electrocatalyst for the oxygen reduction reaction with positive onset, large current, and robust durability [41].
3.6 The Mechanics of Graphene Nanocomposites
The mechanics in nanomaterials encompasses the physical aspects of mechanical engineering namely elastic moduli, contact forces, friction, etc. which are largely influenced by chemical reactions based on inter-molecular, thermal fluctuations, chemical bonds, etc. [42]. The mechanics of graphene nanocomposites have been extensively discussed based on the periodic hexagonal chemical structure of graphene. The distinctiveness of material characteristics of graphene sheets is influenced the size of graphene sheets, L, which should be far larger than the size of the carbon ring, its smallest element, a = 2.46 Å [43]. The nano-mechanics arising in graphene sheets is constituted by the sliding friction, which comprises surface-to-surface interactions, sliding induced excitation of the atomic lattice vibrations, i.e., phonons and bonding interaction of phonons namely electrostatic interactions and π -π bonding. The interfacial sliding in graphene sheets focusses on the spatial exclusion of electrons (ESEE) at the interface of two graphene sheets, which can be observed by Pauli’s exclusion principle. Also, the oscillations at the nanoscale in graphene are characteristic to the thermodynamic nature of carbon atoms. These oscillations are associated with the vibrations at the atomic scale and the scattering of low energy phonons.
3.7 Functionalization
Owing to the augmented research interest of using graphene as filler to design multifunctional nanomaterials, a diversity of approaches for the surface modification of graphene by functionalization have been investigated. For functionalization, the essential aspects that play role include, the chemical nature of the bonding has noteworthy inferences, and the dispersion approaches yield composites that are non-covalent assemblies through comparatively weak dispersive forces [44]. The nature of functionalized graphene either by covalent or non-covalent bond largely illustrates stable dispersion simplifying synthesis of composite with enhanced mechanical, thermal and conductivity properties [45].
3.7.1 Covalent Functionalization
The covalent functionalization in graphene is commonly headed by chemical oxidation of the graphite with the effect of strong acids and oxidants to acquire oxygen-rich functional moieties that assist as pioneers for the chelation of organic molecules. As a detailed synthesis of GO outcomes in extremely functionalized oxygen surface moieties, thereby reaching the C/O fraction of 2:1 [46]. The generic reactions of covalent functionalization in graphene comprises of two methods:
• The covalent bond amongst C=C moieties of graphene and free radicals
• Covalent bonds between –O moieties of GO and functional surface moieties of organic molecules.
Like as the covalent functionalization of GO by imidazolium has been reported in the literature. Here, it is witnessed the covalent chelation via amide bonds between the 1-(3-aminopropyl)-imidazole and carboxylic moieties of GO. Consequently, the transformation was observed in the heterocyclic ring employing N-methylation to its ionic imidazolium derivatives [47]. Similarly, the covalent functionalization is witnessed between GO and 3-aminopropyl triethoxysilane (APTS) by epoxy moieties. The chemical reaction comprised of silicon-functionalized GO (Si−GO), which was uniformly amalgamated with silica matrix and excess of APTS to endure covalent chelation [48].
3.7.2 Non-Covalent Functionalization
Graphene can also form hybrid nanostructures by means of non-covalent bonds, namely by Van der Waals, ionic bonds, etc., that is largely owed to its negative charged surface characteristics due to oxygen-rich functional moieties. Additionally, graphene’s graphitic assemblage with delocalized π-orbitals which necessitate π-π interactions generally promote non-covalent functionalization. This functionalization has chief merit that it does not interrupt the π-conjugation; in contrary, the covalent functionalization builds huge defects on the graphene sheet [49]. The non-covalent functionalization is widely employed potentially in various fields, like as sensors (i.e. for heavy metals, pollutants) and biomedical applications. Pyrene derivatives are largely preferred for non-covalent functionalization as they devise strong chelation with the basal plane of graphene utilizing π-π interactions [50]. Like as the non-covalent functionalization of gold nanoparticles@DNA onto graphene nanoplatelets. This type of functionalization of graphene nanoplatelets with gold nanoparticles@DNA was proficient by chemical oxidation of graphite which transformed it to GO, followed by chemical reduction by hydrazine [51]. Similarly, the non-covalent functionalization via π−π bonding is observed between conjugated tri-block copolymer (PEG-OPE) and GO, thereby forming amphiphilic rGO nanoplatelets. Owing to the amphiphilic nature of the tri-block copolymer, the resultant composite PEG-OPE@rGO is fundamentally soluble in a diverse solvent [52].
3.8 Thermal Properties
Owing to the fundamental concept that thermal conductivity of any material (say bulk or nanomaterial) is directed by the influence of the lattice vibrations, i.e. phonons [53]. The diverse carbon allotropes, namely, 3D: graphite, diamond, and 1D: carbon nanotubes, have presented greater thermal conductivity characteristics owed to witnessed strong covalent bonds in their structure and phonon scattering processes. Formerly 1D: carbon nanotubes are recognized for their utmost thermal conductivity characteristics at room temperature. However, these agonize from large thermal contact resistance [54]. In recent times, the maximum room temperature thermal conductivity acquired was up to ~5000 W/mK for the single-layered graphene nanostructure, which has been described. While, for supported graphene, the maximum thermal conductivity, observed is ~600 W/mK, globally recognizing graphene as an outstanding material in numerous polymer matrix to augment heat transport mechanistic [55]. Additionally, there are two major factors which affect the thermal conductivity of graphene, namely, defect-edge scattering processes and isotopic doping, as these factors are unfavourable due to phonon scattering losses [53].
3.9 Conclusions
Of late, graphene, being the multifunctional carbon nanomaterial has exceptional properties including electrical, thermal, mechanical, optical, and long electron means free paths made it compelling for various engineering applications. Research efforts have been fervent for reconnoitring the essential physics, chemistry and nano-mechanics of graphene. The characteristic features of graphene, namely, Hall Effect, utmost charge transport and highest thermal conductivity, have not yet been witnessed in any materials. The potential applications of graphene include the fabrication of transparent & flexible electrodes, its polymer composites for mechanical engineering applications, energy storage, chemical-sensors and biomedical engineering. Consequently, the future strategic efforts for advanced graphene-era directs to the requisite extensive fundamental research that is essential to offer a basic understanding and their nano-engineering potential.
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