Figure 8. Consecutive steps in the chemical synthesis of rGO using ascorbic acid as a reducing agent. (
a) Oxidation and exfoliation of graphite using Hummer’s method. (
b) Reduction and conversion of Mn (VII) ions to soluble Mn (II) ions by the addition of ascorbic acid. (
c) Color transition of the exfoliated graphite oxide from greenish yellow to black in the early stage of reduction. (
d) Loss of hydrophilicity of GO when stirring is paused. (
e) Precipitation of rGO after completion of the reduction stage and cooling down to room temperature. (
f) Filtration of rGO using cellulose filter paper. (
g) rGO powder after freeze-drying. (Reproduced with permission from ref. [
31]. Copyright 2015 Springer Nature Publications).
3. Flexible Composites and Applications of GO/rGO
The designed flexible composites should have not only excellent flexibility but also a certain strength which would enable the composites to withstand external environmental factors. Recently, a layered composite of GO/PVC with large mechanical strength fabricated by the vacuum filtration method was reported [
58]. These flexible composites composed of GO and rGO are useful in multiple applications including energy storage, water purification, textiles, and robotics. Polymerization was used to prepare PANI nanowires on a GO sheet composite. The obtained products showed excellent performance when used as supercapacitor electrodes [
59,
60]. Water purification has become another issue in the past few decades. Ongoing research has tackled this problem. For example, a GO-based TiO
2 composite membrane could be used as a filtration membrane for the removal of water impurities. The composite was fabricated by vacuum filtration and allowed a moderate water purification [
61]. Although chemically modified graphene or rGO itself is not very appealing in terms of its properties, these properties can be enhanced in forming composite materials with conductive polymers. For instance, an rGO/polypyrrole nanowires composite fabricated in situ showed better performances than rGO and can be used in the fabrication of portable electronic devices [
62]. Yarns were used to produce an electronic textile fabric by coating rGO through electrostatic self-assembly in the presence of adhesive bovine serum albumin. The preparation of the material is shown in
Figure 15 [
63].
Figure 15. Illustration of the three steps used to prepare rGO/nano yarns (rGO/NYs). (Reproduced with permission from ref. [
63]. Copyright 2013 Wiley Publications).
E-textile has revolutionized the whole flexible and portable device industry with the introduction of additional properties. In addition, a pressed composite of rGO–MnFe
2O
4 and polyvinylidene fluoride fabricated by a simple sonication method turned out to be a good absorber of harmful microwaves in the electromagnetic spectrum [
64]. Within energy applications, flexible composites of V
2O
5/polyindole and activated carbon cloth were used as cathodic and anodic electrodes of an asymmetric supercapacitor. V
2O
5 nanostructures were constructed on a carbon cloth by in situ polymerization and showed good cyclic stability on testing (
Figure 16) [
65].
Figure 16. CVs of (
A) rGO@actived carbon cloth (rGO@ACC) and (
C) V
2O
5/polyindole@ACC. Galvanostatic charge–discharge curves of (
B) rGO@ACC and (
D) V
2O
5/polyindole@ACC. (Reproduced with permission from ref. [
65]. Copyright 2016 ACS Publications).
Paper electrodes for energy application are receiving great attention. A GO solution was prepared by Hummer’s method. Then, GO-based paper electrodes which can act as flexible substrates, actuators, supercapacitor electrodes etc., were fabricated through the steps of vacuum infiltration, spin coating, and drop casting.
A composite of nickel cobalt oxide/GO was tested as a supercapacitor electrode and revealed a large capacitance of 1211 Fg
-1. Its fabrication was achieved by coprecipitation using sodium dodecyl sulfate as the template and ammonia as the precipitant [
66]. rGO obtained from GO by the hydrothermal route and titanium carbide obtained by selective etching of aluminum were combined by ultra-sonication and filtration, yielding the rGO/titanium carbide composite. CV, GCD, and EIS analysis proved it to be outstanding for electrochemical performance in supercapacitors [
67]. Additionally, other carbon-based materials like CNT are extraordinary products due to several characteristics when hybridized with conducting polymers such as MnO
x and rGO composites developed by spray coating and electrodeposition. They show a high capacitive behavior and improve the cyclic stability for supercapacitors [
68]. Notably, graphene and its derivatives are analogous to other carbon-based materials, providing new perspectives to research. The fabrication techniques are also being modified. Recently, the metal-organic framework template-assisted method was utilized on a large scale and showed great potential for energy applications. In this regard, as shown in
Figure 17, rGO/MoO
3 was reported to be an excellent composite for energy storage in supercapacitors as an electrode [
69].
Figure 17. Preparation process of rGO/MoO
3 composites. (Reproduced with permission from ref. [
69]. Copyright 2015 Wiley Publications).
Organic substrates have been widely used in recent studies, but metallic substrates remain an important choice. Copper metallic foil acted as a substrate for the growth of rGO/Cu
2O through the hydrothermal technique and showed moderate advantages as an electrode in supercapacitors [
70]. However, the sol–gel approach is also simple and has been used for the fabrication of rGO-based composites. rGO paper was obtained by a modified Hummer’s method followed by evaporation drying. ZnO was deposited in the form of layers on rGO paper by using a stabilizer through a synthesis process. The composite ZnO/rGO/ZnO has been utilized for supercapacitor electrodes [
71]. The choice of the material for positive or negative electrodes plays a vital role in energy devices. A compatible negative electrode material in a supercapacitor for Fe
2O
3 nanoparticle clusters/rGO paper was investigated. The composite was synthesized through the hydrothermal technique [
72]. Chong et al. [
73] prepared an MnO
2/rGO nanocomposite by a facial one-step electrochemical method. MnO
2 nanoparticles were uniformly distributed on rGO nanosheets and acted as spacers to prevent rGO nanosheets from restacking. This unique structure provided MnO
2/rGO with high specific capacitance. Furthermore, the MnO
2/rGO composite also showed high conductivity and excellent potential cycling stability, and has potential as electrode material for highly stable supercapacitors. In addition to MnO
2, tungsten oxide (WO
x) is widely studied as electrode material for supercapacitors. Recently, a W
18O
49 nanowires (NWs)/rGO nanocomposite, which can act as the negative electrode in asymmetric supercapacitor devices, was prepared from the precursors WCl
6 and GO by the solvothermal method [
74]. The asymmetric supercapacitor W
18O
49 NWs-rGO//rGO showed high specific capacitance and excellent cycling stability. For the fabrication of paper-based electrode, the incorporation of celluloses and pulps is desirable to attain flexibility and stability. GO-based nanocomposite of nanocrystalline cellulose acetate fabricated via stirring and a solvent casting method showed high thermal stability and good mechanical strength [
75]. Likewise, cotton pulp was mixed with LiCl in addition with anhydrous DMAc by stirring. Further addition of a GO suspension to cotton pulp resulted in the formation of a cellulose-based composite useful for energy and memory storage [
76]. An outstanding anodic electrode material was designed by fabricating a composite of GO and TiO
2, whereas further reduction of the composite to rGO/TiO
2 was obtained by stirring and drying. Anatase TiO
2 exhibits higher power and energy density than other conventional metal oxides [
77]. In comparison with cellulose, the residual paper pulp is more stable. As a consequence, it can be used in the fabrication of rGO-based flexible composites. First, the paper pulp was stirred in stable solvent and then it was mixed with GO. Next, the suspension was infiltrated with and reduced by hydrazine vapors at a certain temperature via the drop casting technique. The obtained composite possessed better performance compared with cellulous-based composites when applied in flexible electrodes [
78]. Altogether, natural fiber-based GO/rGO paper composites have been proven to have excellent performance in multiple applications, especially in energy storage and conversion devices in the modern portable device industry.