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

Graphene is a two-dimensional (2D) crystalline sheet with a monolayer of carbon atoms densely packed in an SP2 -bonded honeycomb lattice and can be considered as 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], mechanical strength (σint 2D = 42 ± 4(N · m−1)) [65], and fast electron mobility (*μ* ≈ 10,000 cm<sup>2</sup> · V−1 · s −1) [66–70], it has aroused great interest. Due to the oxygen-containing 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,

#### **Figure 4.**

*(a) Schematic diagram of a honeycomb crystal lattice of graphene, and (b) a single-layer suspended graphene sheet exhibits intrinsic microscopic roughening.*

**69**

*Fiber Composites Made of Low-Dimensional Carbon Materials*

and 3D macroscopic architectures for various applications.

[78], and have demonstrated good capability in the

removal of spilled oils [78, 79]. GBMs, which are usually fabricated by infiltration or CVD, have also found to have various applications in the field of energy storage and conversion [80]. Compared with GBAs and GBMs, GBFs possess not only outstanding mechanical property and high conductivity but also good valuable flexibility that can be curved, knotted, and even woven into flexible conductive fabric, which are considered capable of improving the practical applications of GBFs. The development of high-performance GBFs could inspire more engineering applications of graphene. However, assembling microscopic graphene sheets into 1D fiber remains as an unusual challenge because of the irregular shape and size and the movable stacked layers of graphene sheets compared with the highly tangled CNT assemblies [63]. Nevertheless, the assembly of graphene sheets into macroscopic fibers has attracted wide interest due to the lightweight, lower cost, shareability, ease of functionalization, and practical importance of GBFs in contrast to CNTs and CF. Beyond that, 1D GFs with mechanical flexibility is particularly important for wearable textile devices and can serve as the building blocks for constructing 2D

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.

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].

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

with a density of 0.16 mg · cm<sup>−</sup><sup>3</sup>

**4.1 Fabrication of GBFs**

*4.1.1 Wet spinning*

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

with a density of 0.16 mg · cm<sup>−</sup><sup>3</sup> [78], and have demonstrated good capability in the removal of spilled oils [78, 79]. GBMs, which are usually fabricated by infiltration or CVD, have also found to have various applications in the field of energy storage and conversion [80]. Compared with GBAs and GBMs, GBFs possess not only outstanding mechanical property and high conductivity but also good valuable flexibility that can be curved, knotted, and even woven into flexible conductive fabric, which are considered capable of improving the practical applications of GBFs. The development of high-performance GBFs could inspire more engineering applications of graphene. However, assembling microscopic graphene sheets into 1D fiber remains as an unusual challenge because of the irregular shape and size and the movable stacked layers of graphene sheets compared with the highly tangled CNT assemblies [63]. Nevertheless, the assembly of graphene sheets into macroscopic fibers has attracted wide interest due to the lightweight, lower cost, shareability, ease of functionalization, and practical importance of GBFs in contrast to CNTs and CF. Beyond that, 1D GFs with mechanical flexibility is particularly important for wearable textile devices and can serve as the building blocks for constructing 2D and 3D macroscopic architectures for various applications.
