Letter pubs.acs.org/NanoLett
Ultrastrong Graphene-Based Fibers with Increased Elongation Mochen Li,†,‡ Xiaohong Zhang,‡ Xiang Wang,‡ Yue Ru,‡ and Jinliang Qiao*,†,‡ †
College of Materials Science and Engineering, Beijing University of Chemical and Technology, Beijing 100029, China SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, China
‡
S Supporting Information *
ABSTRACT: A new method to prepare graphene-based fibers with ultrahigh tensile strength, conductivity, and increased elongation is reported. It includes wet-spinning the mixture of GO aqueous dispersion with phenolic resin solution in a newly developed coagulation bath, followed by annealing. The introduced phenolic carbon increased densification of graphene fibers through reducing defects and increased interfacial interaction among graphene sheets by forming new C−C bonds, thus resulting in the increasing of stiffness, toughness, and conductivity simultaneously.
KEYWORDS: Graphene fiber, graphene oxide, phenolic carbon, strength, conductivity reducing the diameter of single filament to 1.6 μm and annealing at 3000 °C under argon protection.19 However, the strength increase was based on the sacrificing of elongation. In fact, there is no exception that a strong fiber has an increased strength but compromised elongation at break.20 Application of the strong fibers will be limited if their elongation at break is too short. Many scientists have been working on developing strong fibers with large elongation at break for a long time. For example, the spider silk with both high strength and elongation has been a dream of scientists for decades to prepare in lab.21 Therefore, it is an important research area and also a big challenge to increase the strength and elongation of strong graphene fiber simultaneously. Based on previous fundamental studies, tremendous approaches have been applied to increase the strength of graphene fibers, among which reducing defects in the fibers is extremely promising and widely adopted. According to the Griffith’s criterion,22 tensile strengths of brittle materials are determined by the randomly distributed defects, defects size, and their location. Carbon fibers with smaller diameter have been proved to have lower probability of forming defects and naturally have high strength;23 and graphene fibers that acquire the highest reported tensile strengths are also due to defect reducing.19 As a consequence, the tensile strength of graphene fibers can be increased whenever defects in the fibers are reduced. However, there is no published paper describing how to increase elongation of a graphene fiber while its tensile strength is increased. Fortunately, fracture mechanism of the
G
raphene, a monolayer material of sp2-hybridized carbon atoms arranged in a two-dimensional lattice, has attracted tremendous attention due to its highest Young’s modulus of 1.1 TPa,1 fracture strength of 130 GPa,1 and carrier mobility of 200 000 cm2 V−1 s−1.2 In order to transfer the excellent properties of individual graphene sheet to its macroscopic assemblies, considerable efforts have been dedicated to prepare macroscopic materials by using graphene or graphene oxide (GO) as building blocks. These macroscopic materials, including one-dimensional fiber, two-dimensional film and paper, and three-dimensional hydrogel and aerogel, have been found excellent in extensive applications.3−6 Graphene fiber with very high strength and conductivity of heat and electricity has attracted scientists from both academy and industry worldwide because it has exhibited application potentials in many fields, including energy storage materials,7 fire-resistant conductors,8 etc. Graphene fibers are usually prepared from GO dispersion, and the liquid crystalline behavior of GO dispersion can lead orientation of GO sheets in shear field,9,10 which can form ordered assembly in macroscopic fibers. In 2011, Gao et al. reported the first fabrication method of graphene fibers by using traditional wet-spinning technique and tensile strength of the fibers reached to 102 MPa.11 Since then, many efforts have been devoted to improve the mechanical properties of graphene fibers, such as introducing chemical bonds to cross-link GO sheets,12−14 elevated reduction temperature,15 preparing graphene/CNT hybrid fibers,16,17 and intercalating small-sized graphene sheets into graphene fibers consisted of large-sized graphene sheets.18 Recently, a graphene fiber with the highest tensile strength of 1.78 GPa and fracture elongation of 0.5% (measurement gauge length: 5 mm) was prepared by reducing defects inside graphene fibers, such as © 2016 American Chemical Society
Received: July 26, 2016 Revised: September 22, 2016 Published: September 29, 2016 6511
DOI: 10.1021/acs.nanolett.6b03108 Nano Lett. 2016, 16, 6511−6515
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Nano Letters
Figure 1. Stress−strain curves of graphene fiber and G-PC fibers (a). Schematic illustration of the mechanism of phenolic carbon modifying graphene fiber (b). Densities of graphene fibers and G-PC fibers with different phenolic carbon content (c).
graphene fiber prepared by reducing defects (1.0 GPa and 1.4%).19 It is well-known that high tensile strength and fracture elongation means high toughness since the area under the stress−strain curve can be used to express the toughness of a material. The area under the stress−strain curve of G-PC10 is 14.5 MJ m−3, which is more than 110% higher than that of graphene fibers without PC in this work (6.9 MJ m−3). It is interesting to know why phenolic carbon could increase the strength, toughness, and elongation of graphene fibers simultaneously. The mechanism that phenolic carbon can improve the properties of G-PC fiber is proposed as shown in Figure 1b. It is supposed that phenolic carbon could not only enhance the densification of graphene fibers via reducing defects but also increase elongation via forming new C−C bonds between graphene sheets and phenolic carbon. The newly formed C−C bonds can link graphene layers and provide longer slipping distance of graphene sheets before breaking, thus the fracture elongation of G-PC fibers could be increased. The newly introduced C−C bonds by amorphous phenolic carbon inside G-PC fibers can also increase the tensile strength of graphene fibers as well. Therefore, the introduced phenolic carbon increases the strength and elongating of graphene fibers simultaneously. The proposed mechanism has been proven by the following experimental results. The densification of G-PC fibers was confirmed by densities testing. Phenolic carbon is known as an amorphous carbon with lower density than graphene; therefore, G-PC fibers should have lower density than graphene fibers. Surprisingly, densities testing results show that all G-PC fibers have higher density than graphene fiber as illustrated in Figure 1c, indicating that phenolic carbon can effectively increase the density of G-PC fibers via reducing defects in graphene fibers. However, excessive phenolic carbon cannot increase the density of GPC fibers further after most of the defects have been restored. Hence, the densities of G-PC15 and G-PC20 demonstrate to be lower than that of G-PC5 and G-PC10 fibers. The fracture surface of GO fibers, graphene fibers, and G-PC fibers was observed by scanning electron microscopy (SEM). The effect of phenolic carbon on defects reducing is clearly shown in Figure 2. It can be found that both GO fiber and graphene fiber contain many defects in their fracture surface, while G-PC fibers show reduced voids and defects. It also can be found that voids and defects do not reduce further with the increase of phenolic carbon content after the sample G-PC10, which is consistent with density testing result and mechanical properties. Polarized optical microscope (POM), Fourier transform infrared spectroscopy (FTIR), Raman spectra, and XPS
graphene fibers could also be described by the tension-shear model. According to this model, the graphene sheets endure a pulling force to slide from the stacked graphene blocks under the tensile force in the fiber axial direction.12 Therefore, controlling the sliding among graphene sheets could prevent fracture of graphene fibers, i.e., improve the strength and elongation simultaneously. In this communication, a new method including a newly developed coagulation bath for fabricating graphene-based fibers with ultrahigh tensile strength and increased elongation is reported. The high performance graphene-based fibers, actually phenolic carbon modified graphene (G-PC) fibers, were prepared by wet-spinning the mixture of GO aqueous dispersion with nonwater-soluble phenolic resin (PR) solution and followed by annealing. The G-PC fibers prepared from small-sized GO sheets along and annealed at only 1000 °C could have amazing tensile strength of 1.45 GPa, which is close to the highest reported GF.19 Typical stress−strain curves and mechanical properties of graphene fibers, G-PC fibers, and GO fibers are presented in Figure 1a, Figure S1, and Table 1. As Table 1. Mechanical Properties of GO Fiber, Graphene Fiber, and G-PC fibers sample GO fiber graphene fiber G-PC5 G-PC10 G-PC15 G-PC20
tensile strength GPa
elongation at break %
Young’s modulus GPa
0.22 0.68
13.4 1.3
4.2 57
1.07 1.45 0.83 0.67
1.8 1.8 1.3 1.0
70 120 73 62
presented in Figure 1a, the tensile strength and fracture elongation of G-PC fibers has been improved simultaneously compared with graphene fibers. The tensile strength and fracture elongation of G-PC10 fiber, which phenolic content is 10 wt % of GO, increases by 113% from 0.68 to 1.45 GPa and 38% from 1.3% to 1.8%, respectively. The fracture elongation of 1.8% is also much longer than that of the graphene fibers with the highest tensile strength (0.5%). In fact, the tensile strength of G-PC10 fiber (1.45 GPa) is the second highest reported data for graphene fiber and both tensile strength and Young’s modulus of G-PC10 fiber are much higher than that of most of graphene fibers as summarized in Table S1. In addition, tensile strength and fracture elongation of G-PC10 fiber (1.45 GPa and 1.8%) are superior to most of graphene fibers with high strength, such as the graphene fiber consisted of large- and small-sized graphene sheets (1.08 GPa and 1.45%)18 and the 6512
DOI: 10.1021/acs.nanolett.6b03108 Nano Lett. 2016, 16, 6511−6515
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Figure 2. SEM images of GO fiber (a,b), graphene fiber (c,d), and G-PC fibers with different PR content: GO-PR5 (e,f), GO-PR10 (g,h), GO-PR15 (i,j), and GO-PR20 (k,l).
PC fibers. The increased ID/IG ratio of G-PC fibers from 0.93 to 0.95 is obviously caused by the introduced phenolic carbon.24,25 Details of the chemical changes during the annealing of GO fibers and GO-PR fibers and the chemical state of carbon atoms were further elucidated by XPS measurements. As expected, the C/O atomic ratio is significantly increased upon annealing as shown in Figure S2a, suggesting the successful reduction of GO fibers to graphene fibers. The high-resolution C 1s XPS spectra for graphene fibers and G-PC10 fibers as shown in Figure 3e,f can be fitted into three different characteristic peaks corresponding to different bonding energy of the carbon atoms. The main peak at 284.8 eV can be assigned to the sp2 bonded carbon atoms, whereas the peak at 285.9 eV originates from sp3 carbon atoms and the 286.5 eV corresponds to the carbon atoms in C−O bonds.26,27 Through calculation by using the area of the corresponding peak in the deconvoluted C 1s XPS spectra, it can be found that the fraction of sp3 carbon in G-PC10 fibers is increased from 6.5% of graphene fiber to 10.9% after annealing. It also can be found that about 1.6% carbon atoms have formed new sp3 carbon−carbon bonds in GPC10 fibers after excluding the sp3 carbon in graphene fibers and PC as shown in Figure S2b. This result suggests that new C−C bonds are formed in G-PC fibers between graphene sheets and PC after thermal annealing. Thus, we could conclude that phenolic carbons should exist among graphene sheets in G-PC fibers, and the C−C bonds between graphene sheets and phenolic carbon could be formed because phenolic carbon is located among the graphene sheets. The newly formed C−C bonds between graphene sheets and phenolic carbon would greatly improve the interactions between graphene sheets and control the sliding of highly aligned and compacted graphene layers, thus endowing G-PC fibers with ultrastrong tensile strength and increased elongation. Based on the above analysis, densified G-PC fibers with reduced defects and increased number of C−C bonds should
measurement are applied to evidence that phenolic carbon can form C−C bonds between PC and graphene and enhance the interfacial interactions between graphene sheets via characterizing the liquid crystal structure of spinning solutions, −CO stretching vibration and ID/IG ratio. The liquid crystal structures of GO dispersion and GO-PR spinning solution as shown in Figure 3a,b exhibit stable birefringence, which originated from oriented structures of local asymmetrical GO sheets under shear force. Compared with Figure 3a, more black and white stripes could be observed in Figure 3b, which implies that thin phenolic resin layers exist among liquid crystal layers of graphene oxide sheets. The local long birefringence of GO-PR spinning solution shows that agglomeration of GO sheets has not been caused by the addition of small amount of ethanol. The maintained liquid crystal of GO-PR spinning solution ensures a good spinnability for fabricating GO-PR fibers. Therefore, phenolic resin should locate among GO sheets and phenolic carbon could remain among graphene sheets after annealing. The interfacial interaction between phenolic resins and GO sheets was investigated by FT-IR. As depicted in Figure 3c, stretching vibration peak of −CO at 1718 cm−1 in GO shifts to a higher frequency of 1726 cm−1 in GO-PR fibers, indicating that significant interfacial interactions, such as hydrogen bonds, must exist between −CO groups on GO sheets and −OH groups in PR chains. This experimental result indicates that phenolic resin has been successfully introduced into GO-PR fibers and locates among the graphene sheets. The introduced phenolic resin should be transformed to amorphous carbon, while GO is reduced to graphene sheets during annealing. Meanwhile, new C−C bonds should be formed between phenolic layers and graphene sheets, while the C−C bonds are formed among graphene sheets during annealing, which further increase the interaction among graphene sheets. Raman spectra as shown in Figure 3d were applied to support the conclusion that phenolic carbon has been successfully introduced into G6513
DOI: 10.1021/acs.nanolett.6b03108 Nano Lett. 2016, 16, 6511−6515
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Figure 3. Photographs of GO aqueous dispersion (a) and GO-PR spinning solution (b) at concentration of 7 mg/mL. FTIR spectra of GO, PR, and GO-PR fibers (c). Raman spectra of GO fiber, GO-PR fibers, graphene fiber, G-PC fibers, and phenolic carbon (d). Deconvoluted XPS C 1s spectra of graphene fibers (e) and G-PC10 fibers (f).
also possess outstanding electrical properties. The exceptional conductivities of G-PC fibers are demonstrated in Figure 4. Rather high conductivity of 8.4 × 104 S/m is acquired in GPC10 fibers, which is 40% higher than that of graphene fiber (6.0 × 104 S/m). In summary, ultrastrong graphene-based fibers with increased elongation and exceptional conductivity have been prepared by utilizing graphene oxides and phenolic resin as building blocks. The introduced phenolic carbons, produced from phenolic resin during annealing, played the most important role in the modification of graphene fibers. The phenolic carbons could enhance the densification of graphene fiber by reducing defects and form new C−C bonds inside graphene fibers. The newly developed graphene-fiber with high tensile strength, Young’s modulus, conductivity, and reasonable fracture elongation may have many potential applications, such as wearable sensors, supercapacitors devices, lightweight conductive cables, and solar cell textiles in the future.
Figure 4. Electrical conductivity of G-PC fibers with different PR content.
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(25) Luo, D.; Zhang, G.; Liu, J.; Sun, X. J. Phys. Chem. C 2011, 115 (23), 11327−11335. (26) Hong, C.; Tu, J.; Gu, C.; Zheng, X.; Liu, D.; Li, R.; Mao, S. X. Adv. Eng. Mater. 2010, 12 (9), 920−925. (27) Yang, X.; Haubold, L.; DeVivo, G.; Swain, G. M. Anal. Chem. 2012, 84 (14), 6240−6248.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b03108. More experimental details, characterization , and discussion (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Ph.D. Programs Foundation of SINOPEC Beijing Research Institute of Chemical Industry.
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