*4.3. CNC*/*RGO*

The production of RGO, which is commonly performed by the exfoliation of pristine graphene followed by oxidation and reduction, may generate numerous defects such as grain boundaries, vacancies, Stone-Wales defects and macroscopic defects. These defects not only restrict its production at the industrial scale but also limit the full exploitation of its outstanding properties. Another obstacle in the practical use of RGO is the formation of irreversible agglomeration caused by the strong van der Waals interactions between graphene planes. Therefore, various attempts were made to reduce these drawbacks through RGO functionalization or incorporation of other additives such as vitamin C, green tea, protein bovine serum albumin and CNC, among others, to improve the properties and performance of the final derived nanocomposites and increase the number of its applications in several fields [28,173].

Nowadays, various unmodified CNC/RGO hybrids were actively explored for different applications such as sensors, flexible electronics, supercapacitors and photonic devices [174]. Several approaches used to produce some unmodified CNC/RGO hybrids have been reported for which the common method used to produce CNC/GO can be applied. Wan Khalid prepared COC/RGO nanocomposite by dispersion/ultrasonication of 1 mg RGO in ethanol and 1 mg of CNC in deionized water [175]. The supernatant was eliminated by centrifugation to recover the final hybrid. This latter, re-dispersed in ethanol, was drop-coated onto an electrode surface, which was intended to be used for electrochemical sensing of methyl paraben. The authors revealed that the obtained sensor exhibited good stability, reproducibility, selectivity toward methyl paraben and reusability compared to RGO-based sensor. Similarly, Nan et al. produced iridescent RGO/CNC film with advanced optical properties [176]. They prepared a suspension containing 1 wt.% of CNC and RGO floccules, which underwent an ultrasound treatment and dried using vacuum-assisted self-assembly (VASA) technique. The obtained films displayed regularly metallic iridescence owing to the homogeneous dispersion of RGO within the chiral nematic liquid crystals of CNC. The key factors to tune such behavior were the duration of ultrasonic treatment and the drying process. This iridescent hybrid exhibited better electrical properties in addition to the reversible change in color during the adsorption/desorption of water. The authors claimed that such a hybrid might find applications in photonic devices and biosensors. Recently, Wang et al. adopted a facile one-pot technique to prepare CNC/GO nanocomposite that was followed by a reduction using L-ascorbic acid to form CNC/RGO conductive paper [177]. The process, compared to the well-known ones, is schematically represented in Figure 12. Briefly, the exfoliation of graphite and the hydrolysis of cellulose occurred simultaneously in the reaction system, followed by subsequent reduction using green L-ascorbic acid. A conductive paper (CP) with high conductivity, excellent mechanical properties and thermal stability was then formed using ultrafiltration. It was stated that such CP can be used in implantable biosensors, smart textiles and portable micropower devices. In another research activity, Chen et al. proposed a new method to produce CNC/RGO hybrid, which was based on non-liquid-crystal spinning followed by a reduction using hydrogen iodide (HI), as schematized in Figure 13 [130]. The authors revealed that the incorporation of an alkaline media during the dispersion of CNC/RGO caused the electrostatic repulsion between CNC and GO sheets, leading to weaker hydrogen-bonding interaction and rendering the flowing process during spinning more homogeneous and easier. The authors demonstrated that the strength of RGO/CNC hybrid (230.6 MPa) was improved compared to pure RGO (157.5 Mpa). The hydrophilicity of the hybrid was also improved in addition to the high capacitive performance and conductivity. After that, such a hybrid was immersed in a polyvinyl alcohol acidic solution to fabricate flexible all-solid-state supercapacitor. The assembled supercapacitor achieved excellent bending stability, better flexibility, high energy density (5.1 mW h cm<sup>−</sup>3) and power density (496.4 mW cm−3). The authors claimed that the prepared hybrid easily meets the requirements of flexible or even wearable supercapacitor.

**Figure 12.** (**a**) A schematic flow diagram illustrates CNC/GO (GNCC) production using the one-pot method and further reduced to form conductive paper (CP, CNC/RGO). (**b**) A comparison of the present one-pot process with the conventional approach in literature. Reproduced with permission from Reference [177]. Copyright ©2019, Springer.

**Figure 13.** Schematic illustration of the preparation of CNC/RGO hybrid fiber. Reproduced with permission from Reference [130]. Copyright ©2018, Elsevier.

To further extend the number of applications, improve the different properties of CNC/RGO hybrids as well as their efficiency, numerous modifications of either CNC, RGO or both of them have been recently assessed. For instance, Zhao et al. produced electro-conductive nanocomposite based on CNC and TiO2-RGO. Firstly, GO prepared by the modified Hummers method was subjected to the photocatalytic reduction via TiO2. The obtained TiO2-RGO suspension was mixed with CNC suspension under ultrasonication. CNC/TiO2-RGO was then vacuum-filtered and dried. The obtained flexible transparent hybrid displayed improved electro-conductivity (9.3 S/m) with enhanced elastic modulus (3998 MPa) and tensile strength (18.1 MPa), stipulating that it can be used as a transparent flexible substrate for future electronic devices. In another work, Zhang et al. demonstrated the feasibility of the spinning of conductive filaments from oppositely charged nano-species, that is, cationic CNC and anionic RGO using interfacial nanoparticle complexation [178]. Initially, 2,3 dialdehyde cellulose was prepared by periodate oxidation, subjected to cationization with Girard's reagent or aminoguanidine hydrochloride and passed through the double-chamber system of a microfluidizer to form cationic CNC. Droplets of aqueous suspensions (cationic CNC and anionic GN), placed adjacent to each other, generated continuous CNC/GO filaments, as shown in Figure 14. These latter were immersed into hydrogen iodide solution, washed and dried, to afford CNC/RGO hybrid filaments. These hybrids displayed an electrical conductivity of 3298 ± 167 S/m and tensile strength of 190.3 ± 8 MPa.

**Figure 14.** (**a**) suspensions of cationic CNC and GO and a dual precipitated complex after simple mixing; (**b**–**e**) CNC/GO hybrid filament drawing process; (**f**) a single dried CNC/GO hybrid filament with a diameter of ~33 μm and a length of 53 cm; (**g**) a scheme illustrating the CNC/GO hybrid filament drawing process. Reproduced with permission from Reference [178]. Licensed under a Creative Commons Attribution 3.0 International License (https://creativecommons.org/licenses/by/3.0/).

Kabiri and Namari described another interesting process for the preparation of CNC/RGO hybrid. They functionalized RGO with CNC via "click" coupling between terminated propargyl-functionalized CNC (PG-CNC) and azide-functionalized GO (GO-N3) [179]. After the surface azidation of GO, the "click" reaction between GO-N3 and PG-CNC, already synthesized by the Peng method [180], was performed using copper-catalyzed azide-alkyne cycloaddition. The reduction of the final nanocomposite dispersed in deionized water under sonication was carried out using hydrazine at 70 ◦C. The obtained free-dried CNC/RGO hybrid exhibited interesting physicochemical properties and thermal stability. In another study done by Sadasivyni et al., a transparent and eco-friendly CNC/RGO film for proximity sensing was developed [181]. The authors employed layer-by-layer spraying of modified CNC/GO nanocomposite, which was obtained as detailed in Figure 15, on lithographic patterns of interdigitated electrodes on polymer substrates. The modified nanocomposite was reduced using anhydrous hydrazine at 80 ◦C to generate a hydrophobic CNC/RGO hybrid. The obtained sensitive sensor allowed detecting a human finger interface within a distance of 6 mm with interesting response and recovery time interval. This sensor has potential to be used in various applications such as robotics, punching machines, smart phones, electronics and optoelectronics.

On the other hand, several CNC/RGO-based polymer composites have been produced and assessed in several applications such as sensors, scaffolds in tissue engineering, food and drug packaging. The polymeric matrices tested include polyvinylidene chloride (PVDC) [173], natural rubber (NR) [182], polyethylene oxide [183], poly-lactic acid (PLA) [184,185] and polyamide 6 [186]. It was demonstrated that the incorporation of CNC/RGO conferred to the polymeric nanocomposites outstanding mechanical and thermal properties, interesting barrier features, low toxicity, high conductivity and so forth [183,184,186]. For instance, Cao et al. prepared a 3D interconnected CNC/RGO/NR network using a latex assembly method for which NR latex was incorporated into a CNC/RGO suspension [182]. The solid formed through the co-coagulation induced by an acidic solution was vacuum filtered. A schematic illustration of the process is provided in Figure 16. The obtained conductive structure displayed higher electric conductivity and better mechanical features with superior resistivity responses for organic liquids. Such nanocomposite can find application in sensing to discriminate various solvents leakage in chemical industries and environmental monitoring.

**Figure 15.** Reaction mechanism involved in modified CNC/GO synthesis. Reproduced with permission from Reference [181]. Copyright ©2015, Elsevier.

**Figure 16.** Schematic presentation of the synthesis of CNC/RGO/NR nanocomposite. Reproduced with permission from Reference [182]. Copyright ©2016, Elsevier.

In other research, Pal et al. assessed the combined effect of CNC and RGO in PLA nanocomposite as a scaffold in tissue engineering [185]. They initially prepared CNC and RGO via acid hydrolysis and modified Hummer's method, respectively and then employed a solution casting approach to produce CNC/RGO/PLA hybrid. The detailed preparation procedure is schematized in Figure 17. Compared to pristine PLA, the developed hydrophilic hybrid film revealed higher thermal stability, significantly increased tensile strength up to 23% with and enhancement in elongation at break, showing its ductile behavior. Moreover, the antibacterial activity against both Gram-negative *Escherichia coli* (*E. coli*) and Gram-positive *Staphylococcus aureus*(*S. aureus*) bacterial strains was highlighted. The *in-vitro* cytotoxicity assay indicated the non-toxicity of the nanocomposite film toward fibroblast cell line (NIH-3T3) as well. More recently, interesting research has been conducted by You et al. [173], who introduced the hybrid CNC/RGO to the solution of PVDC (Figure 18). The precipitated sample was vacuum dried to produce the CNCN/RGO/PVDC nanocomposite. The transparency of the CNC/RGO/PVDC coated on PET substrate was determined as 84% at 550 nm wavelength by UV–visible spectrometer in the regular transmission mode (Figure 18). It was proved that the utilization of stable dispersion of CNC/RGO enabled the fabrication of optically clear and thermostable nanohybrid film with improved barrier characteristics against water and oxygen. The authors claimed that the developed approach to produce CNCN/RGO/PVDC nanocomposite was effective and the obtained hybrid film is considered a potential candidate for food and drug packaging.

**Figure 17.** Schematic illustration of (**a**) the synthesis of reduced graphene oxide (RGO) from GO, (**b**) the method employed to prepare nanocomposite films. Reproduced with permission from Reference [185]. Copyright ©2017, Elsevier.

**Figure 18.** Dispersion stability of nanofillers. CNC, RGO and 1CNC:2RGO (1C:2R) hybrid in (**a**) THF:DMF co-solvent and (**b**) PVDC nanocomposite solutions of 0.1 wt.% fillers loading to PVDC. (**c**) Transmittance results of 0.1 wt.% PVDC/CNC, RGO and 1C:2R nanocomposite films. The 10 mm thick nanocomposite films were deposited on 125 mm thick PET substrates. (**d**) Large area (17 cm × 21 cm) PVDC/1C:2R–0.1 wt% nanocomposite film was obtained. Reproduced with permission from Reference [173]. Copyright ©2020, Elsevier.
