**4.1 Fabrication of GBFs**

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

along the fiber axis by SP<sup>2</sup>

**4. Graphene-based fibers (GBFs)**

atoms densely packed in an SP2

cal strength (σint

· V−1 · s

cm<sup>2</sup>

it has very high mechanical strength and modulus. The tensile strength and modulus of CF can reach 7 and 700 GPa, which are much higher than those of glass fiber and Kevlar fiber. CF can withstand high temperature above 3000°C without contact with air. Therefore, CF has outstanding heat-resistant performance. The higher is the temperature, the greater is the fiber strength. After graphitization, the density of mesophase asphalt-based CF increases, and the carbon content exceeds 99%. Most of the carbon atoms in the fiber form a large area of graphite sheet structure

vibration. Therefore, the thermal conductivity of the graphite fiber can reach up to 1000 W/mK [45, 58, 59]. It is worth noting that the electrical properties of CF are not ideal, because of the inherent polycrystalline structure and a large number of grain boundaries inevitably formed during the pyrolysis of organic precursors [21].

Graphene is a two-dimensional (2D) crystalline sheet with a monolayer of carbon

a single layer of the graphitic film in graphite. Thus, graphene is the thinnest nanomaterial known [60, 61]. As shown in **Figure 4**, the length of carbon–carbon bond in graphene is about 0.142 nm; all carbon atoms are connected with three surrounding carbon atoms by σ bond; the remaining P electron orbit is perpendicular to the plane of graphene to form delocalized π bond because π electron can move freely in the plane, rendering graphene holding excellent electrical properties [62, 63].

Since graphene was found in 2004 [61], because of its unique physical and chemical characteristics, such as extraordinary thermal conductivity [64], mechani-

functional groups, graphene materials obtained from chemical methods such as graphene oxide and reduced graphene oxide (rGO) are highly maneuverable and reactive, which further inspires a wide range of research enthusiasm in preparation, chemical modification, and well-controlled assembly of advanced and macroscopic structures for various device applications [71–77]. To this end, graphene-based 3D aerogels (GBAs), 2D membranes (GBMs), and 1D fibers (GBFs) have been developed. Among them, GBAs hold the current world record for the lightest material,

*(a) Schematic diagram of a honeycomb crystal lattice of graphene, and (b) a single-layer suspended graphene* 

2D = 42 ± 4(N · m−1)) [65], and fast electron mobility (*μ* ≈ 10,000

−1) [66–70], it has aroused great interest. Due to the oxygen-containing

hybridization, which is very conducive to the phonon


**68**

**Figure 4.**

*sheet exhibits intrinsic microscopic roughening.*

At present, the manufacturing methods of GBFs are mainly influenced by traditional synthetic fiber production methods, including melt spinning and solution spinning [63]. However, due to the high-temperature stability of graphene, its melting temperature is even higher than that of fullerene and carbon nanotubes. Therefore, melt spinning is not the choice for manufacturing GBFs, while solution spinning is [81, 82]. Solution spinning mainly includes wet spinning, dry jet wet spinning, and dry spinning. In addition to these traditional solution spinning methods, some new methods, including electrophoresis, template hydrothermal method, and chemical vapor deposition-assisted assembly, have been developed recently. In this part, the common methods of preparing GBFs will be introduced in detail.

#### *4.1.1 Wet spinning*

Wet spinning is one of the main methods to prepare chemical fiber. The important step is to prepare a spinning solution. Because graphene is not easily dispersed in water or other organic solvents, it is difficult to prepare a spinning solution, so it is not possible to prepare fibers from graphene by wet spinning [83–85]. As an important precursor of graphene, graphene oxide can be well dispersed in polar solvents (such as water), so it is expected to prepare fibers by wet spinning [86]. The steps of preparing GBFs by wet spinning are as follows: first, GO dispersions are injected into a stable aqueous solution to form GO spinning dope and then injected into the coagulation bath to form a gel-like fiber to prepare GO dope. After solidification for a period of time, GO fiber can be obtained by extracting colloidal fiber and drying, and then GO fiber can be reduced to produce GBFs, as shown in **Figure 5**. An rGO fiber can be further produced by reducing the GO fiber when needed [86, 87]. To ensure uniform and continuous formation of gelatinous fibers, the fibers after solidification should be kept at a certain speed. They can be drawn through a rotating bath or using a collecting unit, as shown in **Figure 5**. The highest strength rGO fiber is made by the method shown in **Figure 5a**. This method includes an easy spin of a small amount of fiber, but it lacks accurate control of fiber moving speed. In contrast, the method shown in **Figure 5b** can provide constant traction and determined moving speed to synthesize fibers, so the method is more suitable for producing fibers with accurate tensile ratio and good scalability [88].

**Figure 5.**

*(a) The synthesis of graphene oxide fiber by wet spinning in rotating coagulation bath and (b) collection unit [86, 87].*

Zhen et al. prepared liquid crystal GO aqueous solution for the first time in 2011, taking NaOH/methanol solution as coagulation bath, obtaining GO fiber through wet spinning, and then reducing GO fiber in hydroiodic acid to produce GBFs. This method can make GO sheets form liquid crystals, which can enhance the strength and flexibility of GBFs. The tensile strength of the fiber is 140 MPa, and the conductivity is 2.5 × 104 S · m<sup>−</sup><sup>1</sup> [86]. Then, Zhen et al. further tried to increase the lamella of the raw material GO, using N,N-dimethylformamide (DMF) as the solvent, acetone, and ethyl acetate mixture as the coagulation bath. After that, the mechanical ability of GBFs was improved by spinning drafting and high-temperature treatment at 3000°C, making its strength reach 1.45 GPa [89]. On the other hand, the conductivity of GBFs can be improved by ion doping, and the conductivity of potassium doped GBFs can reach 2.24 × 107 S · m<sup>−</sup><sup>1</sup> [90]. In addition, Shaohua et al. prepared non-liquid crystal GO aqueous solution to achieve a high concentration of spinning solution to improve the fiber yield. The concentration of a spinning solution can reach 2%, and then GBFs were obtained through a similar wet spinning process and reduction by hydrocodone. The mechanical and electrical properties of the fiber were 208 MPa and 1.53 × 103 S · m<sup>−</sup><sup>1</sup> [91], respectively.

In addition to GBFs, graphene composite fibers can also be prepared by wet spinning, so as to effectively improve the fiber performance and expand the application field. Conducting polymer monomers are polymerized in situ during spinning to prepare composite fibers [92, 93], or oxides or other materials are added directly into the spinning solution to increase the capacity of fiber-shaped supercapacitors [94, 95]. Wujun et al. used GO to disperse the water-insoluble activated carbon in the aqueous solution, spinning and reducing to obtain graphene/activated carbon composite fiber. The fiber has a specific surface area of 1476.5 m2 · g<sup>−</sup><sup>1</sup> and a capacity of 43.8 F · g<sup>−</sup><sup>1</sup> [96]. Similarly, the graphene/manganese dioxide composite fiber can be spun by a similar process, and the capacity of the supercapacitor can reach 66.1 F · cm<sup>−</sup><sup>3</sup> [97]. In addition to inorganic materials, GO and polyvinyl alcohol (PVA) also have good compatibility. Adding sodium hydroxide to non-liquid crystal GO aqueous solution for a pH = 11. Then, adding PVA can significantly increase the affinity between fiber and electrolyte [98]. Similarly, the surface of the fiber with a large number of hydroxyl groups can significantly increase the hydrophilicity and strength of the fiber, which is caused by a large number of oxygen-containing functional groups on its surface [99]. Mochen et al. developed a method to improve the strength of graphene fiber. They spun GO and phenolic resin together. After carbonization under the condition on 1000°C, the C–C covalent bond was formed between graphene sheets, and the fiber strength reached 1.45 GPa [100].

**71**

**Figure 6.**

*Fiber Composites Made of Low-Dimensional Carbon Materials*

The tensile strength and elongation at break of graphene fiber with 10% phenolic resin are 1.45 GPa and 1.8%, respectively, which are better than most GBFs reported before. The increase of strength, toughness, and elongation can be attributed to the formation of a C–C bond between the graphene sheet and phenolic carbon, which provides sliding space for the graphene sheet before fracture. Yang et al. developed a simple but effective method for continuous manufacturing of neat, morphologically defined, graphene-based hollow fibers (HFs) with coaxial capillary spinning strategy. As shown in **Figure 6**, the preparation method of GO-HFs is to use coaxial

compressed air to replace the internal fluid of KCl/methanol solution to successfully prepare GO-HFs with necklace structure (nGO-HF). Experiments show that nGO-HF has a large elongation of about 6% when it breaks, which indicates that nGO-HF has a strong ability to bear compression, which is caused by the elastic deformation of hollow microspheres [101]. Therefore, the physical properties of GBFs can be

In the dry spinning of GBFs, GO dispersion (mainly dispersed in water) is also used as a spinning assistant rather than a coagulation bath. Instead, the GO dispersion is injected and sealed in a pipe, and the GO dispersion is precipitated in the form of gel state fiber at high temperature by heating or chemical reduction, and then dried rGO fibers can be obtained by further solvent removal. GO dispersions are considered to be colloids with large-size dispersants [102–104]. The study of Dong et al. and Yu et al. shows that high temperature can promote the rapid movement of GO dispersant and increase the possibility of collision and precipitation of GO plate. At the same time, high temperature or chemical reduction can also separate the

*Schematic of the setup that used a dual-capillary spinneret to directly spin GO-HFs [102].*

KCl methanol solution and use

*DOI: http://dx.doi.org/10.5772/intechopen.92092*

capillary spinneret to spray silk in 3 mol · L<sup>−</sup><sup>1</sup>

controlled by adjusting the spinning conditions.

*4.1.2 Dry spinning*

*Fiber Composites Made of Low-Dimensional Carbon Materials DOI: http://dx.doi.org/10.5772/intechopen.92092*

The tensile strength and elongation at break of graphene fiber with 10% phenolic resin are 1.45 GPa and 1.8%, respectively, which are better than most GBFs reported before. The increase of strength, toughness, and elongation can be attributed to the formation of a C–C bond between the graphene sheet and phenolic carbon, which provides sliding space for the graphene sheet before fracture. Yang et al. developed a simple but effective method for continuous manufacturing of neat, morphologically defined, graphene-based hollow fibers (HFs) with coaxial capillary spinning strategy. As shown in **Figure 6**, the preparation method of GO-HFs is to use coaxial capillary spinneret to spray silk in 3 mol · L<sup>−</sup><sup>1</sup> KCl methanol solution and use compressed air to replace the internal fluid of KCl/methanol solution to successfully prepare GO-HFs with necklace structure (nGO-HF). Experiments show that nGO-HF has a large elongation of about 6% when it breaks, which indicates that nGO-HF has a strong ability to bear compression, which is caused by the elastic deformation of hollow microspheres [101]. Therefore, the physical properties of GBFs can be controlled by adjusting the spinning conditions.

## *4.1.2 Dry spinning*

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

Zhen et al. prepared liquid crystal GO aqueous solution for the first time in 2011, taking NaOH/methanol solution as coagulation bath, obtaining GO fiber through wet spinning, and then reducing GO fiber in hydroiodic acid to produce GBFs. This method can make GO sheets form liquid crystals, which can enhance the strength and flexibility of GBFs. The tensile strength of the fiber is 140 MPa, and

*(a) The synthesis of graphene oxide fiber by wet spinning in rotating coagulation bath and (b) collection unit* 

the lamella of the raw material GO, using N,N-dimethylformamide (DMF) as the solvent, acetone, and ethyl acetate mixture as the coagulation bath. After that, the mechanical ability of GBFs was improved by spinning drafting and high-temperature treatment at 3000°C, making its strength reach 1.45 GPa [89]. On the other hand, the conductivity of GBFs can be improved by ion doping, and the conductiv-

et al. prepared non-liquid crystal GO aqueous solution to achieve a high concentration of spinning solution to improve the fiber yield. The concentration of a spinning solution can reach 2%, and then GBFs were obtained through a similar wet spinning process and reduction by hydrocodone. The mechanical and electrical properties of

In addition to GBFs, graphene composite fibers can also be prepared by wet spinning, so as to effectively improve the fiber performance and expand the application field. Conducting polymer monomers are polymerized in situ during spinning to prepare composite fibers [92, 93], or oxides or other materials are added directly into the spinning solution to increase the capacity of fiber-shaped supercapacitors [94, 95]. Wujun et al. used GO to disperse the water-insoluble activated carbon in the aqueous solution, spinning and reducing to obtain graphene/activated carbon

[96]. Similarly, the graphene/manganese dioxide composite fiber can

[97]. In addition to inorganic materials, GO and polyvinyl alcohol

(PVA) also have good compatibility. Adding sodium hydroxide to non-liquid crystal GO aqueous solution for a pH = 11. Then, adding PVA can significantly increase the affinity between fiber and electrolyte [98]. Similarly, the surface of the fiber with a large number of hydroxyl groups can significantly increase the hydrophilicity and strength of the fiber, which is caused by a large number of oxygen-containing functional groups on its surface [99]. Mochen et al. developed a method to improve the strength of graphene fiber. They spun GO and phenolic resin together. After carbonization under the condition on 1000°C, the C–C covalent bond was formed between graphene sheets, and the fiber strength reached 1.45 GPa [100].

[86]. Then, Zhen et al. further tried to increase

[91], respectively.

[90]. In addition, Shaohua

· g<sup>−</sup><sup>1</sup>

and a capacity

S · m<sup>−</sup><sup>1</sup>

ity of potassium doped GBFs can reach 2.24 × 107 S · m<sup>−</sup><sup>1</sup>

composite fiber. The fiber has a specific surface area of 1476.5 m2

be spun by a similar process, and the capacity of the supercapacitor can reach

the fiber were 208 MPa and 1.53 × 103 S · m<sup>−</sup><sup>1</sup>

the conductivity is 2.5 × 104

**Figure 5.**

*[86, 87].*

**70**

of 43.8 F · g<sup>−</sup><sup>1</sup>

66.1 F · cm<sup>−</sup><sup>3</sup>

In the dry spinning of GBFs, GO dispersion (mainly dispersed in water) is also used as a spinning assistant rather than a coagulation bath. Instead, the GO dispersion is injected and sealed in a pipe, and the GO dispersion is precipitated in the form of gel state fiber at high temperature by heating or chemical reduction, and then dried rGO fibers can be obtained by further solvent removal. GO dispersions are considered to be colloids with large-size dispersants [102–104]. The study of Dong et al. and Yu et al. shows that high temperature can promote the rapid movement of GO dispersant and increase the possibility of collision and precipitation of GO plate. At the same time, high temperature or chemical reduction can also separate the

oxygen-containing groups in GO and reduce the zeta absolute potential of GO dispersion. Finally, due to the lack of sufficient electrostatic repulsion, GO sheets' precipitate is assembled into fibers. The fibers in the gelatinous state expand in the solvent, but the diameter of the fibers can be reduced by about 80% after drying [105, 106].

In the process of dry spinning, the precipitate of GO sheet under the condition of 220–230°C is actually a solvothermal process (the water is used as the solvent to disperse GO). The process flow is shown in **Figure 7** [107], and the fibers made in this way are actually rGO fibers. It is reported that 27% of oxygen in GO can be removed at 180°C and most hydroxyl, epoxy, and carboxyl groups begin to separate at 200°C [108, 109]. Therefore, GBFs synthesized by the hydrothermal method has considerable conductivity, without post-reduction treatment [105, 110]. In addition to the hydrothermal method, Jihao et al. also use the chemical reduction method of CO to produce rGO fiber. First, the GO and vitamin C (VC) solution was injected into the polypropylene (PP) tube, then heated to 80°C, and kept for 1 h, while GO was reduced and assembled into gel-like rGO fibers. After extraction and drying, the fiber diameter decreased by 95–97%, which was due to the shrinkage of the fiber due to the removal of moisture. Finally, the conductivity of the rGO fiber is about 8 S · cm<sup>−</sup><sup>1</sup> [111].

In the dry spinning process, GO dispersion does not necessarily exist in the form of liquid crystal [106]. The randomly dispersed low-concentration GO dopes (8 mg · ML<sup>−</sup><sup>1</sup> ) composed of small-diameter GO (*d* = 0.5–4 μm) can also be used for the synthesis of GBFs [112, 113]. Compared with the wet spinning method, the preparation of graphene fiber by dry spinning does not need other auxiliary reagents, only needs high-temperature reduction or chemical reaction to get GBFs, and can get any shape of GBFs by pre-setting template, even hollow structure GBFs [105, 106, 112]. It is worth noting that after reduction, the strength of GO fiber obtained by dry spinning is lower than that by wet spinning, but its toughness is as high as 19.12 MJ · m<sup>−</sup><sup>3</sup> , which is expected to become a green processing method of GBFs in the future [107].

#### *4.1.3 Dry jet wet spinning*

Dry jet wet spinning is another important spinning method of conventional synthetic fiber. The results show that PAN-based carbon fibers can be spun with high concentration coating by this method and the mechanical properties of the fiber are better than that of wet spinning [52, 114]. Shayan et al. use dry jet wet spinning to improve the strength of the fiber. The existence of the air layer effectively reduces the speed gradient of the spinning liquid from the spinneret to the coagulation bath, so that the fiber has a better arrangement. However, if the air layer is too long, it will affect the tensile property of the fiber and control the diameter of the needle and the distance of the air layer. Then, the high-strength GBFs with circular cross section can be spun (**Figure 8a**) [115].

**73**

and dry spinning.

*cross-sectional images of a GBFs [103].*

**Figure 8.**

*4.1.4 Chemical vapor deposition-assisted assembly*

have a high conductivity of about 1000 S · m<sup>−</sup><sup>1</sup>

*Fiber Composites Made of Low-Dimensional Carbon Materials*

**Figure 8b** and **c** shows the surface and cross-sectional images of dry jet wet spinning fiber, which indicates that the GBFs with smooth surface and circular cross section can be produced by dry jet wet spinning with proper solvent coalescent pair (chlorosulfonic acid and diethyl ether), which is not realized in both wet spinning

*(a) A digital photo showing the setup for dry jet wet spinning of GBFs [115]. (b) and (c) Surface and* 

In the production of graphene made by chemical vapor deposition, the composition of graphene can be easily changed by changing the composition of the gas phase. Xinming et al. reported in 2011 a method of self-assembly of twodimensional CVD grown films into one-dimensional GBFs in ethanol, acetone, and other organic solvents through the change of surface tension. The resulting fibers

method of film assembly. They first scraped and coated the GO dispersion into multiple film strips, dried and twisted it to get GO fiber, and then put it through thermal reduction to get GBFs. The GO fiber made by this method has high elonga-

strength is low (9.7–85.9 MPa) due to many defects in the fiber section [117]. Jiali et al. also developed a method for preparing GBFs by film shrinkage, as shown in **Figure 9**. First, graphene was produced on copper foil by CVD with methane as a carbon source. In order to obtain a complete and independent graphene film, a layer of polymethyl methacrylate (PMMA) is spin-coated on the surface of graphene. The copper foil is etched with 1 M ammonium persulfate solution, and the PMMA layer is washed off with acetone to obtain the laminated graphene film. Second, the film is pulled out of the solution with tweezers to shrink to form GBFs with uniform diameter [118]. The graphene film can be directly used to prepare GBFs by film

tion at break (8.3–78.3%) and excellent fracture toughness (1.3–17.4 J · m<sup>−</sup><sup>3</sup>

[116]. Seyed et al. reported another

), but its

*DOI: http://dx.doi.org/10.5772/intechopen.92092*

*Fiber Composites Made of Low-Dimensional Carbon Materials DOI: http://dx.doi.org/10.5772/intechopen.92092*

#### **Figure 8.**

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

of moisture. Finally, the conductivity of the rGO fiber is about 8 S · cm<sup>−</sup><sup>1</sup>

In the dry spinning process, GO dispersion does not necessarily exist in the form of liquid crystal [106]. The randomly dispersed low-concentration GO dopes (8 mg ·

) composed of small-diameter GO (*d* = 0.5–4 μm) can also be used for the synthesis of GBFs [112, 113]. Compared with the wet spinning method, the preparation of graphene fiber by dry spinning does not need other auxiliary reagents, only needs high-temperature reduction or chemical reaction to get GBFs, and can get any shape of GBFs by pre-setting template, even hollow structure GBFs [105, 106, 112]. It is worth noting that after reduction, the strength of GO fiber obtained by dry spinning is lower than that by wet spinning, but its toughness is as high as 19.12 MJ · m<sup>−</sup><sup>3</sup>

which is expected to become a green processing method of GBFs in the future [107].

Dry jet wet spinning is another important spinning method of conventional synthetic fiber. The results show that PAN-based carbon fibers can be spun with high concentration coating by this method and the mechanical properties of the fiber are better than that of wet spinning [52, 114]. Shayan et al. use dry jet wet spinning to improve the strength of the fiber. The existence of the air layer effectively reduces the speed gradient of the spinning liquid from the spinneret to the coagulation bath, so that the fiber has a better arrangement. However, if the air layer is too long, it will affect the tensile property of the fiber and control the diameter of the needle and the distance of the air layer. Then, the high-strength GBFs with circular cross

*Schematic illustration of the dry spinning process with a concentrated organic dispersion of GO [108].*

[111].

,

oxygen-containing groups in GO and reduce the zeta absolute potential of GO dispersion. Finally, due to the lack of sufficient electrostatic repulsion, GO sheets' precipitate is assembled into fibers. The fibers in the gelatinous state expand in the solvent, but the diameter of the fibers can be reduced by about 80% after drying [105, 106]. In the process of dry spinning, the precipitate of GO sheet under the condition of 220–230°C is actually a solvothermal process (the water is used as the solvent to disperse GO). The process flow is shown in **Figure 7** [107], and the fibers made in this way are actually rGO fibers. It is reported that 27% of oxygen in GO can be removed at 180°C and most hydroxyl, epoxy, and carboxyl groups begin to separate at 200°C [108, 109]. Therefore, GBFs synthesized by the hydrothermal method has considerable conductivity, without post-reduction treatment [105, 110]. In addition to the hydrothermal method, Jihao et al. also use the chemical reduction method of CO to produce rGO fiber. First, the GO and vitamin C (VC) solution was injected into the polypropylene (PP) tube, then heated to 80°C, and kept for 1 h, while GO was reduced and assembled into gel-like rGO fibers. After extraction and drying, the fiber diameter decreased by 95–97%, which was due to the shrinkage of the fiber due to the removal

**72**

**Figure 7.**

ML<sup>−</sup><sup>1</sup>

*4.1.3 Dry jet wet spinning*

section can be spun (**Figure 8a**) [115].

*(a) A digital photo showing the setup for dry jet wet spinning of GBFs [115]. (b) and (c) Surface and cross-sectional images of a GBFs [103].*

**Figure 8b** and **c** shows the surface and cross-sectional images of dry jet wet spinning fiber, which indicates that the GBFs with smooth surface and circular cross section can be produced by dry jet wet spinning with proper solvent coalescent pair (chlorosulfonic acid and diethyl ether), which is not realized in both wet spinning and dry spinning.

### *4.1.4 Chemical vapor deposition-assisted assembly*

In the production of graphene made by chemical vapor deposition, the composition of graphene can be easily changed by changing the composition of the gas phase. Xinming et al. reported in 2011 a method of self-assembly of twodimensional CVD grown films into one-dimensional GBFs in ethanol, acetone, and other organic solvents through the change of surface tension. The resulting fibers have a high conductivity of about 1000 S · m<sup>−</sup><sup>1</sup> [116]. Seyed et al. reported another method of film assembly. They first scraped and coated the GO dispersion into multiple film strips, dried and twisted it to get GO fiber, and then put it through thermal reduction to get GBFs. The GO fiber made by this method has high elongation at break (8.3–78.3%) and excellent fracture toughness (1.3–17.4 J · m<sup>−</sup><sup>3</sup> ), but its strength is low (9.7–85.9 MPa) due to many defects in the fiber section [117]. Jiali et al. also developed a method for preparing GBFs by film shrinkage, as shown in **Figure 9**. First, graphene was produced on copper foil by CVD with methane as a carbon source. In order to obtain a complete and independent graphene film, a layer of polymethyl methacrylate (PMMA) is spin-coated on the surface of graphene. The copper foil is etched with 1 M ammonium persulfate solution, and the PMMA layer is washed off with acetone to obtain the laminated graphene film. Second, the film is pulled out of the solution with tweezers to shrink to form GBFs with uniform diameter [118]. The graphene film can be directly used to prepare GBFs by film

**Figure 9.** *Schematic illustration of GBFs prepared by film assembly [118].*

shrinkage method, and the obtained fiber generally has more pores. However, the CVD method needs a lot of instrument investment and strict gas conditions, and the cost is high, so it is difficult to promote.

## *4.1.5 Templated hydrothermal method*

Zelin et al. reported a template hydrothermal method to prepare GBFs. GO dispersion was injected into the stripping pipe, sealed at both ends, and then heat-treated in water at 230°C for 2 h to form continuous GBFs. The structure of GBFs can be adjusted by controlling the concentration of GO dispersion and the inner diameter of a glass tube. The graphene fiber has a porous structure, has a density of only 0.23 g · cm<sup>−</sup><sup>3</sup> , and has a good flexibility [105]. Yunming et al. used a simple low-temperature-induced self-assembly method to synthesize GBFs. They mixed GO and ascorbic acid evenly and sealed them in a specific straight glass tube. They carried out the hydrothermal reaction at 90°C and 120°C, respectively, until the fiber was completely formed and then obtained GBFs with layered porous structure. Its conductivity can reach 1.3 × 104 S · m<sup>−</sup><sup>1</sup> . After heating, it has excellent mechanical properties and can be easily woven into the spinning products [119]. Lizhi et al. further developed on the basis of previous methods, and the specific preparation process is shown in **Figure 10**.

First, the dispersion of GO is sprayed into liquid nitrogen through a spout to prepare a layer bridging GO dispersion - interconnected graphene oxide ribbons (IGOR). Then, a certain concentration of GO dispersion is uniformly mixed with IGOR dispersion and injected into a quartz capillary with an inner diameter of 0.4 mm. The two ends are sealed, heated at 230°C for 2 h, and finally dried in air for 12 h. The GBFs show higher strength and toughness [120].

In order to increase the length of GBFs prepared by the hydrothermal method, Dingshan et al. improved the above methods. The authors replaced the brittle glass tube with the flexible and high-temperature-resistant fused silica capillary column, injected the GO dispersion containing ethylenediamine into it, and sealed it. After that, they put it in the furnace at 220°C for 6 h, extruded it with nitrogen to form the fiber, dried it, and collected the long enough GBFs [106]. Although the GBFs with porous structure can be prepared by the hydrothermal method, it is difficult to achieve continuous production because of the need of closed space and long reaction time.

**75**

**Figure 11.**

supercapacitor [121].

**Figure 10.**

*Fiber Composites Made of Low-Dimensional Carbon Materials*

*4.1.6 Conductive substrate-assisted spontaneous reduction and assembly*

*Schematic illustration of graphene hybrid fibers prepared by hydrothermal method [120].*

The conductive substrate-induced spontaneous reduction and self-assembly of GO generally proceeds by putting metal substrates (e.g., Al, Fe, Cu) into GO solution for GBF preparation. As shown in **Figure 11**, Junjie et al. take copper wire as the substrate and adopt the three-electrode method to make the GO sheet continuously deposit on the surface of copper wire under the double induction of electrochemistry and template. Both GO and copper are simultaneously restored. Then, they etch and remove the copper wire in the FeCl3 solution to obtain the graphene hollow fiber with an oriented structure. The controllable preparation of the hollow fiber can be realized by controlling the diameter, length of the substrate, and the time of electrochemical deposition. The graphene hollow fiber has excellent flexibility and conductivity and can be used as the electrode material of

*Scheme of spontaneous reduction and assembly of graphene hollow fiber on active metals substrates [121].*

*DOI: http://dx.doi.org/10.5772/intechopen.92092*

*Fiber Composites Made of Low-Dimensional Carbon Materials DOI: http://dx.doi.org/10.5772/intechopen.92092*

**Figure 10.**

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

shrinkage method, and the obtained fiber generally has more pores. However, the CVD method needs a lot of instrument investment and strict gas conditions, and

Zelin et al. reported a template hydrothermal method to prepare GBFs. GO dispersion was injected into the stripping pipe, sealed at both ends, and then heat-treated in water at 230°C for 2 h to form continuous GBFs. The structure of GBFs can be adjusted by controlling the concentration of GO dispersion and the inner diameter of a glass tube. The graphene fiber has a porous structure, has a

simple low-temperature-induced self-assembly method to synthesize GBFs. They mixed GO and ascorbic acid evenly and sealed them in a specific straight glass tube. They carried out the hydrothermal reaction at 90°C and 120°C, respectively, until the fiber was completely formed and then obtained GBFs with layered porous

mechanical properties and can be easily woven into the spinning products [119]. Lizhi et al. further developed on the basis of previous methods, and the specific

First, the dispersion of GO is sprayed into liquid nitrogen through a spout to prepare a layer bridging GO dispersion - interconnected graphene oxide ribbons (IGOR). Then, a certain concentration of GO dispersion is uniformly mixed with IGOR dispersion and injected into a quartz capillary with an inner diameter of 0.4 mm. The two ends are sealed, heated at 230°C for 2 h, and finally dried in air for

In order to increase the length of GBFs prepared by the hydrothermal method, Dingshan et al. improved the above methods. The authors replaced the brittle glass tube with the flexible and high-temperature-resistant fused silica capillary column, injected the GO dispersion containing ethylenediamine into it, and sealed it. After that, they put it in the furnace at 220°C for 6 h, extruded it with nitrogen to form the fiber, dried it, and collected the long enough GBFs [106]. Although the GBFs with porous structure can be prepared by the hydrothermal method, it is difficult to achieve continuous production because of the need of closed space and long reaction time.

, and has a good flexibility [105]. Yunming et al. used a

. After heating, it has excellent

S · m<sup>−</sup><sup>1</sup>

the cost is high, so it is difficult to promote.

*Schematic illustration of GBFs prepared by film assembly [118].*

structure. Its conductivity can reach 1.3 × 104

preparation process is shown in **Figure 10**.

12 h. The GBFs show higher strength and toughness [120].

*4.1.5 Templated hydrothermal method*

density of only 0.23 g · cm<sup>−</sup><sup>3</sup>

**Figure 9.**

**74**

*Schematic illustration of graphene hybrid fibers prepared by hydrothermal method [120].*
