to the formation of well-dispersed graphene colloids [21]. Likewise, Athanasios and coworkers investigated the synthesis of reduced graphene oxide by engaging the use of using sodium borohydride [22]. Apart from these, there are other chemical routes employing the use of hydroquinone [23] and strong alkaline stock solutions [24], thermal reduction and solvothermal methods have also been vastly studied. The thermal reduction of GO to form rGO utilizes the thermal treatment (at high temperatures) to eradicate the functional oxide moieties from the surface of GO [25]. Chen and coworkers reported the thermal reduction of GO with the assistance of microwaves and reaction time being only a few minutes. The rGO prepared was effectively soluble in N,N-dimethylacetamide solvent [25]. In the same way, Dubin and coworkers detailed a solvothermal method in their work to yield rGO dispersion. This approach resulted in the deoxygenation of GO yielding rGO layers by refluxing the GO in N-methyl-2-pyrrolidinone (NMP) at 200°C [26].
3.4 Graphene-Based Composites
Graphene is a stimulating substrate for the immobilization of numerous polymeric compounds and nanoparticles of inorganic materials including metal, metal oxide or semiconductor. The decoration of graphene sheets with polymeric and metallic nanoparticles possibly offers a new area of research to investigate the potential in catalytic, magnetic, and optoelectronic applications. Given literature, the metallic nanoparticle decorated graphene sheets offer the ability of their significant use in numerous applications, namely, chemical sensory platforms, hydrogen storage systems, photo-electronic devices, etc. The synthetic routes for the preparation of graphene-inorganic material-based composites are largely categorized into ex-situ and in-situ methodologies as represented in Figure 3.3. In ex-situ synthesis, initially, surface-functionalization of graphene layers is carried out such that they can chelate by non-covalent interactions or covalent linkages, followed by the blending graphene with synthesized nanocrystals of inorganic materials. Such as, the encapsulation of the positively charged SiO2 or Co3O4 nanoparticles onto negatively charged GO through electrostatic interaction, which were further utilized for the Li-ion battery application [27].
Figure 3.3 Types of Graphene-based composites and their synthesis approaches.
While, in in-situ synthesis, the processes encompass nucleation at controlled conditions, thereby, giving upswing to the uniform surface covering of graphene layers. Like as, the chemical reduction process is used to reduce HAuCl4 with NaBH4 in an rGO-octadecylamine (ODA) solution, thus forming graphene-Au composites [28]. Inline to this, hydrothermal treatment is also well investigated in which the functional groups on graphene offer nucleation and anchoring sites for the growth of nanoparticles NPs for the synthesis of Ni(OH)2 nanostructure onto graphene [29]. As well, resembling this, the solvothermal process is also employed for the synthesis of CdS–rGO composites by hydrothermally treating blend of GO and cadmium acetate at 180°C for 12 hours [30]. The results revealed that this in-situ method outcome in high yield formation of single-layer graphene and inhibits their aggregation. Additionally, sol-gel process is engaged for the synthesis of TiO2-GO/rGO composites by the chemical reaction of titanium isopropoxide and GO/rGO sheets which ensued the surface hydroxyl groups on GO/rGO that necessitate nucleation sites [31]. The polymer-graphene composites are formed by the 3D arrangement of polymers onto graphene sheets and on this basis has been classified into three broad categories, namely, graphene-filled, graphene-layered and graphene-functionalized polymer composites. The graphene-filled polymer composites are usually formed by solution mixing, sonication, electropolymerization or melt compounding. Such as, polyaniline has been electro-deposited on graphene paper with an electrolytic solution comprising of aniline monomers by electropolymerization, which outcomes in the random distribution of graphene fillers in the polyaniline polymer matrices [32].
Figure 3.4 Schematic illustration of layer-by-layer deposition of GO sheets onto films of polyelectrolyte PAH (adapted with permission from reference [33]).
Alternatively, the graphene-layered polymer composites are formed by layer-by-layer accumulation with Langmuir–Blodgett (LB) technique. Like as, layer-by-layer deposition of GO sheets onto films of polyelectrolyte poly(allylamine hydrochloride) (PAH) by LB technique, thereby resulting in GO-layered-PAH composites as represented in Figure 3.4 [33]. These GO-layered-PAH composites-based PEMs can be easily suspended over quite a few millimeter apertures and withstand enormous mechanical bends and deformations. As a substitute to filling and layering of graphene onto polymer matrix, an alternative approach of polymer functionalization onto graphene by employing covalent and non-covalent functionalization came to existence. After that, with covalent functionalization of graphene with PVA lead to the formation of GO–PVA composite sheets, centred on the chemical reaction between the surface moieties of the polymers and the oxygenated functionalities on the GO sheets has been demonstrated [34].
3.5 Graphene-Based Hybrid Nanocomposites
Apart from the graphene-based inorganic composites, there are graphenebased hybrid nanocomposites which employ the use of nanostructures namely metal-organic frameworks, carbon nanotubes, biomaterials, etc., and have been essentially investigated for various applications (e.g. Li-ion batteries, supercapacitors, fuel cells, photovoltaic devices, etc.) as represented in Figure 3.5. Metal-organic frameworks (MOF)-graphene based nanocomposites, has been investigated for storage applications and gas purification. Like as, MOF-5 (a zinc-based MOF) and graphite oxide (GO) nanocomposites have been synthesized, further been investigated for ammonia adsorption, and the results revealed their higher dispersive forces than MOF alone which attributed for more adsorption of gas [35].
Figure 3.5 Types of Graphene-based hybrid nanocomposites and their synthesis approaches.
Similarly, HKUST-1(a copper-based MOF) and GO, has been synthesized, further been investigated for NO2 adsorption under dry and moist conditions, and the results revealed their high-performance efficiency is owing to their amplified porosity and the reactive adsorption sites on copper-based MOF [36]. Also, Zr-based metal-organic framework (Zr-MOF) and reduced graphene oxide (rGO), has been synthesized by in-situ approach and further been investigated for hydrogen storage applications. The presented experimental studies displayed upsurges surface area characteristics for the rGO/Zr-MOF nanocomposite concerning pristine Zr-MOF, which resulted in boosted hydrogen storage capacity of rGO/Zr-MOF nanocomposite [37]. Furthermore, biomaterials like DNA, aptamers, antibodies, enzymes etc. has been decorated onto graphene to form hybrid graphene nanocomposites and has potential applications in sensing, cell-imaging and probing living cells. An example of biomaterial, i.e. single-stranded DNA (ss-DNA)-graphene nanocomposite has been synthesized, and its potential application in electrochemistry is investigated. The ss-DNA/Graphene nanocomposite offers a novel biographene hybrid electrochemical platform for the biological determination of redox enzymes [38]. Similarly, double-strand DNA (ds-DNA) and gold nanorods (GNRs) has been decorated onto graphene oxide, thereby fabricating a sandwich-modified electrode composed of GNRs/ds-DNA/GO for the electrochemical sensing of diazinon (DZ), a pesticide based on amperometry [39]. Carbon nanotubes-graphene
