*4.1. CNC*/*GN*

GN possesses large surface area, exceptional electrocatalytic activity, high mechanical strength, good electronic transport characteristics, excellent optical properties and thermal performance, which has motivated its broad application prospect in several fields such as functional composites, electrochemical sensors and catalysis, among others [136]. To expand the number of applications of GN and enhance its inherent properties, numerous GN composites have been successfully produced and applied in several fields. Recently, nanocomposites of CNC/GN have attracted widespread attention, owing to their exceptional features and synergetic effects that develop new ways and opportunities for the production, characterization and application of new materials in nanotechnology. It has been recently demonstrated that the preparation of CNC/GN can be carried out with and without chemical functionalization for which the water-based dispersion is the common starting approach to produce composites with/without a combination with various types of materials such as metallic and ceramic nanoparticles or natural and synthetic polymers. Several processes can be further applied such as filtration, hot pressing, deposition and drying to generate a wide range of advanced materials. Thus, such CNC/GN-based materials hold a great promise for several applications ranging from packaging to biomedical fields. Nevertheless, the optimization of the composite compositions and tailoring of their properties can extend the number of applications and reduce the production cost for eventual scalability.

Carrasco at al. have employed CNC as an effective graphene stabilizer in aqueous dispersion at high concentration for which the exfoliation of graphite to generate graphene flakes has been carried out using a tip sonication [137]. Such an approach based on CNC-assisted liquid-phase exfoliation (LPE) produced graphene flakes decorated with CNC stabilizers with interesting properties. The authors proved that such hybrids could be employed in different applications such as composites and supercapacitors. In another study by Cui et al. an interesting efficient one-step mechanical-chemical method to in-situ produce CNC/GN hybrid, with rigid 2D structure and improved interfacial interactions, from micro fibrillated cellulose and graphite using ball milling has been developed [138]. A schematic illustration of the composite preparation is given in Figure 5A. This hybrid was successfully dispersed within poly(propylene carbonate) (PPC) with strong interfacial interactions which can increase its glass transition temperature (*T*g) and enhance its mechanical and electrical features for practical uses. The obtained PPC/CNC/GN composite displayed a *T*<sup>g</sup> of 51.3 ◦C, which is higher than that of pure PPC (34.0 ◦C). The percolation threshold considerably decreased from 15 to 5 wt.%, whereas the tensile strength and the Young's modulus reached 52.8 MPa and 731.2 MPa, respectively.

**Figure 5.** (**A**) Schematic illustration of the production of poly(propylene carbonate) (PPC)/cellulose nanocrystals (CNC)/aminated graphene (GN) composites and the available hydrogen bonding. Reproduced with permission from Reference [138]. Copyright ©2018, Elsevier; (**B**) Schematic presentation of the SPI-based nanocomposite film. Reproduced with permission from Reference [139]. Licensed under a Creative Commons Attribution 3.0 International License (https://creativecommons. org/licenses/by/3.0/); (**C**) Transmission electron microscopy (TEM) and optical micrographs of the CNC/GN solution showing good dispersion. Reproduced with permission from Reference [140]. Copyright ©2015, Elsevier; (**D**) Schematic synthesis of Au@CNC-GN catalyst. Reproduced with permission from Reference [141]. Copyright ©2018, The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC; (**E**) Preparation procedure of chitosan/WN/GN hydrogel. Reproduced with permission from Reference [142]. Copyright ©2020, Elsevier; (**F**) TEM images of GN and CNC/GN sol mixtures containing 2 wt.% of GN. Reproduced with permission from Reference [143]. Licensed under a Creative Commons Attribution 3.0 International License (https://creativecommons.org/licenses/by/3.0/).

Montes et al. have recently demonstrated the existence of a synergetic reinforcement of poly(vinyl alcohol) (PVA) nanocomposites with CNC-stabilized graphene [144]. They produced CNC/GN hybrid using a CNC-assisted LPE that allows the stabilization of the resulting GN in aqueous dispersion. Such hybrid was incorporated into a PVA aqueous solution by a direct blending to obtain a nanocomposite after casting evaporation. It was mentioned that the thermal stability of the composite is improved through the addition of 1 wt.% of CNC/GN hybrid nanofiller. Moreover, the mechanical features have been also enhanced compared to the neat PVA (20% improvement in tensile strength and 50% in Young's modulus). It was claimed that CNC played a dual role, where it acts as GN stabilizer and PVA reinforcement. Moreover, the synergetic effect of CNC/GN hybrid is notable, where interesting thermal, mechanical and electrical features can be attained through the tailoring of the nanofiller loading. Recently, this research group has studied the effect of CCN/GN hybrid on the properties of poly(lactic acid) (PLA) based film [145]. The composite was prepared using a melt blending method, a conventional technique for plastic compounding, at a total loading of 1wt.% and then processed by

hot pressing to generate the film. Compared to the baseline, the film PLA/CNC/GN exhibited high thermal stability and better mechanical features with an increase of 11 and 8% in the tensile strength and Young's modulus, respectively. The investigation of the gas barrier properties as well as the antifungal activity of the prepared film revealed significant improvements, which make it a potential candidate in food packaging trays and agricultural film applications.

A few years ago, an interesting composite based on soy protein isolate (SBI) and CNC/GN has been developed by Li et al. as food packaging material [139]. A schematic presentation of the nanocomposite preparation is shown in Figure 5B. The authors exploited the high aspect ratio of 1D CNC with the flexible and strength 2D GN to manufacturing active interfacial adhesion laminate nanocomposites. To obtain a stable aqueous graphene dispersion via sonication, the negatively charged sulfate ester groups of CNC were firstly modified through the incorporation of positively charged surface functionalities using the cationic polyethyleneimine. Such modification enhanced the strong ionic interactions with negatively charged GN for efficient dispersion and later-by-layer assembly with SBI. The obtained composite film displayed interesting mechanical features and improved surface hydrophobicity for which the tensile strength increased from 3.75 to 7.49 MPa and the water contact angle augmented from 39◦ to 54◦ compared to the control film. Better thermal stability, water resistance and UV-visible light barrier ability were also exhibited by such composite film, making it a potential candidate as food packaging material.

Valentini et al. produced polymer solar cells using optically transparent conductive GN and CNC film [146]. They reported that the mixture containing 10 mL of CNC suspension (0.5 wt.%) and 10 mL of GN solution (1 wt.%), which was prepared in an ultrasonic bath at room temperature for 20 min and evaporated under a nitrogen stream, was the best composition. The obtained NG/CNC layer, which has a low surface roughness, was optically transparent and enabled light to go through. The measurement of the contact angle of GN/CNC demonstrated a lower contact angle value when compared to those of the neat glass or CNC film, which was assigned to the flatter surface morphology of the GN/CNC film. These authors produced a photovoltaic device by spin coating. The thickness of the spin-cast photosensitive layer was about 100 nm, as determined by atomic force microscopy. The manufactured polymer solar cell reached a higher short-circuit current density value, revealing its improved electron blocking action. The enhanced mechanical properties, the optical transparency as well as the electrical conductivity of the hybrid layer will certainly allow the development of the next generation of flexible and foldable printed optoelectronic devices. In another work, Wang et al. combined GN and CNC to produce flexible, electrically and thermally conductive hybrid thin film using a water-based approach and vacuum filtration [140]. Figure 5C shows the transmission electron microscopy (TEM) and optical micrographs of the obtained GN of about 15 layers as well as CNC/GN solution, revealing that GN was uniform without the appearance on any segregation after mixing with hydrophilic CNC. The better particle alignment with the removing of the internal pores was promoted by the use of the hot-press process. It was found that the hot-pressed 25 wt.% CNC hybrid paper exhibited interesting mechanical features for which the modulus was improved by 57% and tensile strength by 33% with respect to the neat GN paper. The electrical conductivity was negatively affected by the increase of the amount of CNC and the optimum CNC loading was 15%. Such hybrids can find application in heat and electrical-conducting fields.

The employment of CNC/GN hybrid as a supporting material to produce supported metal catalysts, which can be used as dispersing, capping or reducing agents, using a clean, simple and effective process has been reported. Wang et al. have deposited mono-dispersed gold nanoparticles (Au NPs) on multifunctional CNCN/GN hybrid sheets to generate catalysts with efficient catalytic activity, flexibility and stability [141]. The production procedure is briefly illustrated in Figure 5D. The hybrid structure allowed the reduction, growth and immobilization of Au NPs. The OH-groups of CNC coordinated with GN permitted creating narrow nanosized Au NPs anchored onto the surface of the hybrid through the in-situ reduction of Au3<sup>+</sup>. Such a catalyst has been revealed to be effective for one-pot reaction of an alkyne, an amine and an aldehyde in water since it can minimize the

environmental pollutions caused by heavy metallic ions and organic solvents. It can be reused for several times without significant deactivation. It is also expected to be employed in a wide range of applications such as energy storage and catalysis.

On the other hand, stimuli-responsive hydrogels, such as 3D polymeric networks, are considered prominent intelligent drug delivery systems to selectively release the drug at the desired sites. The emerging of CNC/GN hybrids has pushed the scientific community to develop a new generation of efficient hydrogels. Omidi et al. have successfully developed a pH-responsive hydrogel containing aminated CNC (WN), aminated graphene (GN) and chitosan via Schiff base reaction by a synthetic dialdehyde in a few minutes [142]. The preparation procedure is schematized in Figure 5E. The prepared hydrogel exhibited better sensitivity to different external stimuli encompassing pH and amino acids. More specifically, it displayed a pH-responsive release behavior for anticancer drugs. Also, the hydrogel presented strong antibacterial activity against gram-positive bacteria, revealing the efficiency of such hydrogel as a potential candidate for the localized drug delivery systems.

To extend the application of CNC/GN hybrids as anti-static or electromagnetic interference shielding materials, Liu et al. have prepared sandwiched films of epoxy resin and GNC/GN paper. Firstly, a hybrid paper of GN containing 10 wt.% of CNC was produced using ultrasonication process in aqueous suspension [147]. This hybrid displayed an electrical conductivity of 16,800 S/m and tensile strength of 31.3 MPa. To manufacture the sandwiched film of epoxy and CNC/GN, a dip coating method was applied through the introduction of the paper into epoxy resin solution followed by a curing process at ambient temperature. The authors revealed that the moduli of the films were about 300 folds and the tensile strength increased by two-folds concerning the pure resin. The glass transition of the composite increased as well when compared to that of the neat resin. Besides, the coated CMC/GN hybrid by epoxy resin displayed better dimensionality integrity after sonication in water for two hours. In another work, a composite containing water-born polyurethane/CNC/GN has been recently prepared through one-step sol process by Yang et al. as a thermosetting coating material for wood-based composites, which exhibited better energy-saving characteristics [143]. The better dispersion of CNC/GN has been optimized as shown in a TEM image in Figure 5F. The properties of the prepared composites have been improved through the incorporation of CNC/GN, where the thermal conductivity, abrasion resistance and hardness were enhanced and meanwhile, the coating adhesion was maintained at an acceptable level. The authors claimed that such findings can promote the development of wooden heating material with better-energy saving characteristics.

In another study, Nie et al. have introduced a small amount of CNC (also named CNWs) to GN (CNWs/GN = 1/20 *w*/*w*) to improve its uniform dispersion in a waterborne epoxy polymeric matrix (WEP), which is still challenging at a high GN loading, using a solution-casting approach [148]. A schematic illustration of the preparation of the composite is depicted in Figure 6. The obtained film at a GN loading of 1.0 wt.% achieved enhanced mechanical properties with a higher Young's modulus of 2820 MPa compared to the neat epoxy (2034 MPa). The glass transition of such composite increased by 4.3 ◦C when compared to the pure resin. The better dispersion of GN on the surface of epoxy owing to the effect of CNWs led to the increase of the water contact angle, confirming the improvement of the water-barrier behavior of the composite CNWs/GN/WEP. The authors have assessed the anticorrosion effectiveness of the prepared coating composite using potentiodynamic polarization and electrochemical impedance spectroscopy tools. The results demonstrated that CNWs played a double role through improving the dispersion of GN and the corrosion resistance for mild steel.

**Figure 6.** Neat water (**a**); aqueous dispersion of CNC (CNWs) (**b**); dispersion of GN in water (**c**); GN in CNWs aqueous dispersions (**d**); dispersion stability of GN in water and CNWs aqueous dispersions after the settlement of 30 min (**e**); dispersion of 1.0% GN/CNWs in waterborne epoxy polymeric matrix (WEP), (**f**); schematic of CNWs with negative charges (**h**); schematic of negatively charged CNWs adsorbed on graphene sheets (**i**); schematic of GN sheets stabilized in WEP assisted with CNWs (**j**); Field emission scanning electron microscopy (FE-SEM) micrograph of CNWs adsorbed on graphene sheet (**k**). Reproduced with permission from Reference [148]. Copyright ©2019, Wiley.
