Green Carbon Nanostructures for Functional Composite Materials: Comparison
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Graphene is a two-dimensional (2D) material composed of sp2 carbon monolayer arranged into a hexagonal network. Reduced graphene oxide (rGO), a carbon nanostructure from the graphene derivatives family, has been incorporated in composite materials due to its remarkable electrical conductivity, mechanical strength capacity, and low cost. Graphene oxide (GO) is typically synthesized by the improved Hummers’ method and then chemically reduced to obtain rGO.

  • reduced graphene oxide
  • clays
  • hydrothermal carbons
  • supported carbons
  • polymer composites

1. Introduction

The discovery of graphene, a two-dimensional (2D) material composed of sp2 carbon monolayer arranged into a hexagonal network, had a tremendous impact in carbon materials research [1]. Graphene was isolated from graphite by mechanical exfoliation with an adhesive tape for the first time in 2004 by Geim et al. [2]. The relevance of this work was recognized by the attribution of the Nobel Prize in Physics in 2010. The ideal single-layer graphene has exceptional physical properties, such as ultrahigh charge-carrier mobility (200,000 cm2 V−1 s−1 at room temperature), high Young’s modulus (∼1.0 TPa), high specific surface area (theoretical value of 2630 m2 g−1), absorption of only 2.3% of visible light, and high thermal conductivity (∼500 W m−1 K−1). Therefore, graphene is a promising material for many distinct areas, such as energy, medicine, or electronics [3][4][5].
Eighteen years after the graphene isolation, the 2D carbon materials research is greatly developed. However, the word “graphene” has been widely misused to designate distinct 2D carbon materials when it should be reserved for the graphene sheets [1]. The intense research around graphene led to a diversification of the synthesis methods and graphene materials synthesized. Different synthesis techniques produce graphene derivatives with distinct features, varying in number of layers, lateral size, yield, type of defects, and consequently, properties [6].
Graphene derivatives, including graphene nanoplatelets (GNP) [7], graphene oxide (GO) [8], and reduced graphene oxide (rGO) [9], are suitable fillers for the development of polymer composites. However, synthesis challenges associated with difficulties to scale up turn the graphene derivatives expensive nanomaterials [10].

2. Chemical Reduction of Graphene Oxide

rGO is the most used 2D carbon material for the development of electrically conductive and mechanically reinforced polymer composites [3]. Graphite, constituted by graphene layers bonded by strong van der Waals forces, is the bulk starting material to synthesize rGO. First, graphite is oxidized to produce GO. After that, GO suffers a reduction step to produce rGO, as shown in Figure 1.
Figure 1. The chemical production of graphene oxide from graphite.
Hummers’ method and its variations are currently the most used procedures to synthesize GO [11]. The chemical exfoliation of graphite is achieved using strong acids, as concentrated sulfuric and phosphoric acids, which promote the graphene layers’ separation. The oxygenation of the separated graphene layers is accomplished using oxidants, such as hydrogen peroxide and potassium permanganate. The reduction process consists in the partial removal of oxygen functionalities present in the GO structure, namely, tertiary alcohols and epoxides attached to sp3 carbons, and hydroxyl and carboxylic groups attached to the sp2 lattice. This process converts the hydrophilic and insulator GO (yellow) into the hydrophobic and electrically conductive rGO (black) [12]. The extent of oxygen removal relies on the efficiency of the reducing agent. However, the deoxygenation is incomplete and the remnant oxygen functionalities promote the rGO dispersion and functionalization and may interact with polymers, being an advantage for the preparation of composites.

3. Carbon Structures Derived from Biomass

Biomass typically presents a carbon content between 45% and 50%. The isolation of carbon from other chemical elements is accomplished by thermochemical treatments, such as pyrolysis, hydrothermal carbonization (HTC), or a combination of both processes. The materials obtained from biomass conversion present distinct properties that mainly rely on the starting carbon precursor and processing strategies [13][14].
Pyrolysis is the decomposition of biomass in temperatures typically between 350 and 1100 °C under inert atmosphere. Conventional pyrolysis is performed in a tubular furnace, where the heat generated by electricity is transferred to the biomass. The alternative microwave-assisted pyrolysis generates localized heat, being an energy efficient and time-saving method [15]. This method improves the surface area of materials. For example, hay-derived activated biochar produced by microwave pyrolysis showed a surface area 30% higher in comparison to conventional pyrolysis [16].
Starbon® is a patented technology that uses conventional pyrolysis to convert polysaccharides into carbonaceous mesoporous materials, commercially designated as Starbons [17]. Starch was the first polysaccharide used as a precursor for Starbons technology. The conventional preparation route comprises several sequential steps: (i) starch gelatinization, (ii) starch retrogradation, (iii) solvent exchange, (iv) drying, and (v) carbonization. For example, corn starch was gelatinized in heated distilled water and recrystallized by cooling down at 5 °C. Water from the retrograded starch gel was removed by solvent exchange with ethanol and dried to prevent collapse of the structure. The resulting material was pyrolyzed between 150 and 700 °C, after being treated with p-toluene sulfonic acid to catalyze the carbonization and keep the porous structure. The expanded starch showed a Brunauer–Emmett–Teller specific surface area (SBET) of approximately 180 m2 g−1 and a narrow pore volume of 0.4–0.6 cm3 g−1. The hydrophobicity of these materials was controlled by the degree of carbonization, producing more hydrophilic materials at lower temperatures [18]. The application of Starbons technology to alginic acid kept the first four steps in agreement with the starch processing, but the alginic acid gel was dried with supercritical CO2 and pyrolyzed in a broader temperature range of 200–1000 °C. The mesoporous materials showed a SBET of 200 m2 g−1, but the different pyrolysis temperatures did not influence the specific surface area. Nevertheless, the higher temperatures produced carbon materials with more graphitic domains, as demonstrated by an increment C/O ratio from XPS and elemental analysis techniques [19].
HTC is a thermochemical conversion method alternative to pyrolysis, in which biomass is processed inside a sealed autoclave at mild conditions using water as solvent. The mild processing temperatures, typically between 120 and 280 °C, generate supercritical water that promotes the biomass conversion. HTC consumes less energy compared with pyrolysis, being more sustainable from an ecological point of view. In this context, the microwave-assisted HTC is an alternative way that saves even more time and energy. For example, spherical carbon particles with a carbon content >90% were prepared by processing glucose only during 15 min by microwave-assisted HTC [20].
The HTC of carbohydrates involves complex chemical reactions, which can be divided into five general stages: (i) hydrolysis, (ii) dehydration, (iii) decarboxylation, (iv) polymerization, and (v) aromatization [14]. Figure 2 shows the proposed mechanism for the HTC of cellulose.
Figure 2. Proposed mechanism for HTC of cellulose [21].
Titirici et al. [22] investigated the structure and morphology of materials processed by HTC at 180 °C during 24 h using different mono- and polysaccharides as carbon sources. Hexoses-containing compounds (glucose, maltose, sucrose, amylopectin, and starch) and hexose derivative 5-hydroxymethyl-furfural-1-aldehyde produced interconnected particles and agglomerated spheres. The hexoses dehydrate into hydroxymethyl furfural and condense to form carbonaceous materials with a similar structure and composition. The interconnected hexose-derived structures result from the good water solubility of hydroxymethyl furfural. On the other hand, xylose and furfural, a pentose-containing compound and a pentose derivative, respectively, produced well-dispersed spheres. Xylose dehydrates to form furfural, which has a limited water solubility, and polymerizes, forming carbon structures identical to the ones obtained from pure furfural. The materials obtained from mono- and polysaccharides were identical. For example, cellulose at the water/cellulose interface hydrolyzes to glucose, following the mechanism of hexoses, as shown in Figure 2. On the other hand, raw cellulose follows a reaction mechanism associated with pyrolysis yielding highly aromatic materials even at mild conditions, since high pressure destabilizes the cellulose structure. The same study compares the temperature of biomass decomposition by HTC and pyrolysis processes. Rye straw biomass submitted to HTC decomposed between 240 and 280 °C, while during pyrolysis decomposition only started at 350 °C. The lower temperature required for rye straw decomposition by HTC is attributed to the high pressure involved in the process. Another advantage of the HTC process is the possibility to control the chemical composition of carbonaceous materials such as furan-to-arene ratio [23].
The hydrothermal treatment introduces oxygen-containing groups to the carbon structures, typically producing materials with reduced electrical conductivity. The addition of GO to glucose, used as carbon source, before HTC increases the electrical conductivity of the hydrothermal species obtained [24]. In this context, the HTC followed by a pyrolysis step is an efficient strategy to improve the electrical conductivity, as demonstrated with the conversion of sugar cane into an electrically conductive aerogel. The aerogel conductivity increased from 0.4 to 1.3 S cm−1, with the increment of HTC time before pyrolysis [25]. Similarly, chitosan treated by HTC followed by pyrolysis produced carbon structures with high electrical conductivity, having the advantage of maintaining the nitrogen atoms available for further functionalization. Post-pyrolysis transforms the sp3 hybridized carbons into sp2 carbons, being a fundamental step to increase the graphitization of the hydrothermal carbons [26].
HTC produces carbonaceous materials with a very low surface area and undeveloped porosity. Zhong et al. [27] proposed a vapor-phase alternative HTC treatment to carbonize monosaccharides. Sucrose was treated in a glass vial placed inside an autoclave, while the gap between the autoclave and the vial was filled with water during 24 h at 200 °C. This strategy produced spongelike mesoporous carbons, in opposition to the nonporous carbon material typically obtained by conventional HTC of sucrose. The combination of HTC followed by pyrolysis also creates porosity, reinforcing the benefits of using both thermochemical processes [28][29]. In this regard, the use of templates to shape the carbon materials during biomass conversion is a powerful method to tune the porosity and surface area.
The carbon nanostructures derived from biomass are sustainable and low-cost alternatives to graphene derivatives. HTC is an economical and eco-friendly technique since it uses mild temperatures, self-generated pressure, and water as solvent. Pyrolysis uses higher temperatures in comparison with HTC; however, it can still be considered a relatively economical technique. The combination of HTC and pyrolysis techniques may lower the energy consumption required for pyrolysis, making the process more economical and eco-friendlier. The porosity and mechanical and electrical properties of these graphitic structures can be modified by the processing conditions, tailoring their properties to become fillers of polymeric composite materials.

4. Graphitic Materials Supported on Lamellar Structures

Clay minerals are natural and abundant resources, adequate for the sustainable development of ecological materials. The porosity and functional groups present in natural or synthetic clays turn them into suitable platforms to adsorb diverse types of molecules [30][31], leading to a wide variety of uses, including hybrid materials for advanced applications [32]. Therefore, clays have been used as porous templates to produce nanostructured carbon materials. In this context, the use of clays can be done in two different approaches: as molds or templates that are removed after carbonization of a carbon precursor [33][34] or, alternatively, as supports maintained after nanocomposite synthesis [35][36].
Sepiolite is a natural hydrated magnesium silicate showing a microfibrous morphology that has been deeply investigated to prepare carbon nanomaterials and nanocomposites as either template or support. The structure of this clay mineral is organized in alternate Mg–silicate blocks and intracrystalline nanopores aligned in the fiber direction. This structural organization, forming interior cavities (tunnels) and exterior channels, turns sepiolite into an attractive template [37]. Acrylonitrile was adsorbed into sepiolite pores, polymerized to obtain polyacrylonitrile, and thermally treated by pyrolysis at 750 °C under N2 flow. The resulting carbon–clay nanocomposites were electrically conductive, maintaining the silicate template, which was removed with acid treatments, and free carbon fibers with 1 µm length and 20 nm diameter were obtained [30]. Carbon–sepiolite nanocomposites derived from cellulose were synthesized by HTC using sepiolite pretreated with hydrochloric acid. These acid treatments increase the amount of surface silanol groups (Si–OH) present at the clay surface due to the extraction of Mg2+ ions from its structure. The resulting carbon–sepiolite nanocomposites showed an increased adsorption capacity towards organic compounds, such as methylene blue and phenols, in comparison with pristine sepiolite [38]. However, these treatments could introduce deep alterations in the crystal order of the starting sepiolite generating silica-based materials [39][40][41].
Graphene-like materials were also prepared using sucrose or gelatin supported on sepiolite clay. The carbon–clay bionanocomposites obtained after pyrolysis at 800 °C under N2 atmosphere presented an electrical conductivity in the range of 0.01–1 S cm−1. The use of gelatin biopolymer as carbon precursor produced N-modified materials, being advantageous for further functionalization [35].
In contrast to 2D clay minerals, such as montmorillonite, sepiolite does not have swelling properties and the formation of intercalated compounds is not possible. In this case, caramel is presumed to fill the sepiolite pores and cover the external surface in agreement with N2 adsorption isotherms of the resulting nanocomposites [42]. The formation of graphitic material into porous silicate templates is represented in Figure 3. The graphitic material can be formed in the interior of the pores by an endogenic mechanism or at the silicate surface by an exogenic mechanism [43]. Al-pillared montmorillonite and glucose were treated by HTC followed by pyrolysis to synthesize another family of carbon–clay nanocomposites. The thermal treatments converted glucose into carbon clusters located in the montmorillonite layers and surface. Free carbon microspheres were also formed due to the HTC process. The montmorillonite pillaring strategy improved the SBET from 27.1 m2 g−1 to 129.6 m2 g−1 due to an increase in montmorillonite layers’ separation. The introduction of carbon and its conversion resulted into nanocomposites with a SBET of 162.6 m2 g−1, a pore volume inferior to 0.1 cm3 g−1, and an average pore size of 4.3 nm [44]. These SBET and pore volume are lower compared with the values obtained for mesoporous carbons prepared by the removal of a laponite clay template [45].
Figure 3. Schematic representation of graphene-like materials formed inside sepiolite pores (endogenic regions) and epitaxially grown on the sepiolite surface (exogenic regions) [43].
The removal of a clay template is advantageous to produce carbon nanomaterials with a large surface area. However, this strategy may be time-consuming and nonsustainable due to the use of toxic chemicals [34][45]. On the other hand, the maintenance of the clay template results in carbon–clay hybrid nanocomposites. The incorporation of these materials as fillers into insulating polymer matrices to produce composites can make them electrically conductive and improve their barrier and mechanical properties. The template maintenance is also advantageous for further material functionalization due to the presence of clay functional groups [42][43][46]. Nevertheless, despite the advantages of this strategy, it produces materials with inferior textural properties compared with the clay template removal [44][45].
MXenes are a large family of transition metal carbides, carbonitrides, and nitrides showing a general formula, Mn+1XnTx, where M is an early transition metal (e.g., Ti), X is carbon and/or nitrogen, and Tx represents termination groups (e.g., OH) [47]. Interestingly, they exhibit colloidal and surface properties in close relation to clay minerals but show useful additional properties, such as metallic electrical conductivity. These materials are excellent candidates to form carbon-based nanocomposites and, for instance, porous carbon nanospheres generated by pyrolysis of chitosan could be assembled to Ti3C2Tx MXene, as recently reported [48]. The resulting carbon-nanostructured materials (Figure 4) provide an elevated specific surface area (>1800 m2 g−1) and improved adsorption properties tested in dye adsorption from aqueous solutions. For instance, the adsorption capacity of crystal violet is close to 2750 mg g−1, which appears to be the highest adsorbed amount of dye per mass unit never reported for carbon-based materials [48].
Figure 4. Schematic representation of the assembly of hydrothermal carbon spheres (HCS) from chitosan (CS) and Ti3C2Tx MXene [48].

5. Polymer Composites Containing rGO

Carbon nanostructures are used to reinforce the mechanical, electrical, thermal, and optical properties of polymeric matrices. Therefore, biocomposites containing carbonaceous materials have application in many distinct fields. To the best of the author’s knowledge, among the carbon nanostructures reviewed, only rGO was applied to prepare polymer-based composites. However, the carbons derived from biomass and the clay-supported carbons have potential for the development of polymer-based composites. One advantage is the selection of the carbon precursor according to the applications. For example, a carbon precursor containing N- or S- functional groups may not only reinforce the composite through the establishment of interactions with the polymer but also improve their performance on the application. Similarly, the electrical conductivity and porosity of the carbon nanostructures can be tailored by an appropriate selection of the methodology. Therefore, the alternative green carbon nanostructures are promising materials for the development of polymer-based composites.
Table 1 presents the polymer composite materials prepared using the rGO reduced by the green methodologies. The applications found for these materials were corrosion protection [49][50][51], gas diffusion barriers [52], sensing [53], supercapacitors [54], environmental remediation [55], and food packaging [9][56]. Recently, the researchers revised the use of graphene derivatives in biopolymer composite nanostructures for food packaging applications, which is an example of application where these green carbon nanostructures can be employed [57].
Table 1. Applications of polymer-based composites prepared using the green carbon nanostructures previously reviewed.

Application

Carbon Nanostructure

Polymer Composite

Results

Ref.

Corrosion protection

rGO (Urtica dioica leaf)

Polyurethane/rGO (0.15 wt%) coatings (tested on mild steel)

Resistance against accelerated weathering condition; improved UV shielding and corrosion protection efficiency.

[49]

Corrosion protection

rGO (Peganum harmala seed)

Epoxy resin/rGO-Zn (0.15 wt%) coatings (tested on steel)

Dual active and barrier corrosion protection.

[50]

Corrosion protection

rGO (Peganum harmala seed)

Epoxy ester resin/rGO-Zn (0.15 wt%) coating (tested on steel)

Improved tensile strength (78%), Young’s modulus (102%) and fracture energy (83%); improved thermal stability (62%).

[51]

Gas diffusion barrier

rGO (elemental sulfur)

Polyimide/rGO (0.5–5 wt%) films

Improved tensile strength and Young’s modulus; 95% reduction of oxygen permeability.

[52]

Sensing

rGO (Bougainvillea glabra flower)

Nafion/rGO solution drop-casted on a carbon working electrode

Sensor electrode used for Pb2+ detection; improved sensitivity and ultralow limit of detection.

[53]

Supercapacitors

rGO (eucalyptus bark)

Nafion/rGO solution drop-casted on a glassy carbon electrode

High specific capacitance (239 F g−1) and high energy density (71 W h kg−1) at a current density of 2 A g−1.

[54]

Environmental remediation

rGO (Pseudoalteromonas sp.)

Sodium alginate/rGO solution dripped into CaCl2 solution to obtain spheres

MB and CR dye adsorption from water. Reusable absorbent with adsorption efficiency of the MB and CR 77.91% and 68.27% after 4 adsorption–desorption cycles.

[55]

Food packaging

rGO (HTC/caffeic acid)

Chitosan/rGO (50%) film

Electrically conductive film to sterilize food by in-pack PEF; electrical conductivity of 0.7 S m−1 and 2.1 × 10−5 S m−1 in-plane and through-plane, respectively.

[9]

Food packaging

rGO (HTC/ZnO)

Alginate/sepiolite/ZnO-rGO (50%)

Antimicrobial and electrically conductive film for food packaging. E. coli and S.

Inhibition of

aureus growth; electrical conductivity of 0.1 S m−1 and 7.5 × 10−5 S m−1 in-plane and through-plane, respectively.

[56]

Not mentioned

rGO (BM/Zn)

Epoxy resin/rGO (0.1–0.3%) composites

Improvement of thermomechanical properties.

[58]

rGO: reduced graphene oxide. UV: ultraviolet. MB: methylene blue. CR: Congo red. HTC: hydrothermal carbonization. PEF: pulsed electric field. BM: ball milling.

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