Nanoscale

PAPER

Cite this: Nanoscale, 2019, 11, 12637

Received 15th September 2018,Accepted 28th May 2019

DOI: 10.1039/c8nr07527a

rsc.li/nanoscale

Multifunctional reduced graphene oxide-CVDgraphene core–shell fibers†

Yong Seok Choi,‡a Chang-su Yeo,‡b,c Sang Jin Kim,d Jin-Young Lee,e

Youngsoo Kim,f Kang Rae Cho,g Sanghyun Ju, h Byung Hee Hong *a,b andSang Yoon Park*b

The insufficient electrical conductivity and mechanical stretchability of conventional graphene fibers

based on reduced graphene oxide liquid crystals (rGO-LCs) has limited their applications to numerous

textile devices. Here, we report a simple method to fabricate multifunctional fibers with mechanically

strong rGO cores and highly conductive CVD graphene shells (rGO@Gr fibers), which show an outstand-

ing electrical conductivity as high as ∼137 S cm−1 and a failure strain value of 21%, which are believed to

be the highest values among polymer-free graphene fibers. We also demonstrate the use of the rGO@Gr

fibers for high power density supercapacitors with enhanced mechanical stability and durability, which

would enable their practical applications in various smart wearable devices in the future.

Introduction

Graphene, the thinnest two-dimensional material in the worldwith a one carbon atom hexagonal honeycomb structure, hasbeen an important research model in various fields of sciencesuch as electronics, physics, and materials science, and inenergy storage applications and bioapplications owing to itsexceptional properties.1–6,17–19,27 In recent years, graphene-based materials for smart fiber applications7 have been inten-sively studied utilizing their outstanding mechanical and elec-trical properties.3,8,22–26 In particular, the self-assembly of gra-phene oxide liquid crystal fibers fabricated by the wet-spinningmethod, first reported by the Gao group, has enabled the fabri-cation of super-strong fibers that have exceeded mechanical

strength compared to carbon fibers.9–15 However, compared toCVD graphene fibers, the electrical conductivity of GO or rGOfibers is still limited because of the intrinsic defects inevitablyformed during severe chemical oxidation and reduction pro-cesses.31 On the other hand, highly conducting CVD graphenefibers are not mechanically strong enough to be used as stand-alone fibers.24 Thus, a supporting layer such as poly(vinylalcohol) (PVA) has been used to compensate it, but the insulat-ing nature of PVA considerably reduces the conductivity.21

Thus, we introduce new graphene-based fiber structures withrGO cores and CVD graphene shells (rGO@Gr), taking advan-tages of the outstanding mechanical strength of rGO fibers aswell as the high electrical conductivity of CVD fibers. Theresulting fibers exhibit conductivity as high as ∼137 S cm−1

and maximum strain as high as 21%, which are believed to bethe best values among polymer-free graphene-based fibersreported so far. The optical, mechanical, electrical, andelectrochemical properties of rGO@Gr fibers have been care-fully characterized by Raman spectroscopy, scanning electronmicroscopy, I–V measurement, cyclic voltammetry, etc. Finally,application in high-performance fiber supercapacitors hasbeen successfully demonstrated.

Results and discussionThe fabrication process of rGO@Gr fibers

Fig. 1a shows the experimental scheme of the fabrication pro-cesses of CVD graphene-coated graphene oxide (GO@Gr)fibers. GO fibers (GOF) are fabricated by a roll-to-roll wet spin-ning process. The hydrophilic GO fiber is soaked in a solvent

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr07527a‡These authors contributed equally to this work.

aDepartment of Chemistry, Seoul National University, Gwanak_599, Gwanak-ro,

Gwanak-gu, Seoul 151-747, Republic of Korea. E-mail: cyscell@snu.ac.kr,

byunghee@snu.ac.kr; Fax: +02-889-1568; Tel: +02-880-6569bAdvanced Institute of Convergence Technology, Seoul National University, Suwon-si,

Gyeonggi-do 16229, Republic of Korea. E-mail: toycs112@snu.ac.kr,

yoonpark77@snu.ac.krcDepartment of Physics, Sungkyunkwan University, Suwon, KoreadApplied Quantum Composites Research Center, Institute of Advanced Composite

Materials, Korea Institute of Science and Technology, Republic of KoreaeDepartment of Chemical Engineering & Biotechnology, Korea Polytechnic University,

Republic of KoreafGraphene Square Inc., Inter-University Semiconductor Research Center, Seoul

National University, Seoul 08826, Republic of KoreagDepartment of Research and Development, Puritech Inc., Pyeongtaek 459-040, KoreahDepartment of Physics, Kyonggi University, Suwon 443-270, Korea

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mixture of water and ethanol (DI-water : EtOH = 9 : 1) while thehydrophobic graphene film is floating due to surface tension.The graphene film is self-assembled around the GO fiber as itis drawn out of the solvent. It should be noted that the selec-tion of solvents and their mixture ratio are crucial factors tofabricate GO@Gr fibers.20 The color of rGO@Gr fibers reducedby soaking in 28% HI solution (Sigma-Aldrich) is dark asshown in Fig. 1b, indicating that the graphene-covered GOFcan react with HI. The surface of the rGO@Gr fiber appearssmoother as the CVD graphene is coated on the rough surface.We suppose that the increased hydrophobicity of rGO afterreduction enhances the adhesion at the interface between CVDgraphene and rGO.

Raman spectroscopy analysis was performed to confirm thehybridization of GOF/rGOF and CVD graphene and the reductionof the core GOF to rGOF. The GOF exhibits rather broad D and Gpeaks, but the rGOF shows clear peaks as the amorphouscarbons are ordered into sp2 structures during the reduction

process (Fig. 1c).14 The GO@Gr and rGO@Gr fibers wrappedwith CVD graphene exhibit clear 2D peaks (blue and red lines,respectively). The narrow distribution of 2D/G peak intensityratios indicates that the core fibers are uniformly reduced andtightly attached to the CVD graphene shells (Fig. 1d and e).

Electrical properties

We found that the electrical conductivity of rGO@Gr fibers isproportional to the number of graphene layers (Fig. 2a). Theelectrical conductivity of an rGO@Gr (3) fiber coated with3 layers of CVD graphene is measured to be 137.02 ± 7.36S cm−1, which is 8.8 times greater than that of pristine rGOF(15.51 ± 4.77 S cm−1). We also determined the electrical con-ductivity per density (174 S g−1 cm−2) as it is crucial for light-weight electronic textile applications, which is comparable tostainless steel fibers. In addition, the porous structures ofrGO@Gr fibers with a higher surface area would be advan-tageous for numerous multifunctional wearable devices such

Fig. 1 (a) Schematic representation of the fabrication process of GOF@Gr. GOF is synthesized by the wet spinning method (right) and CVD gra-phene floating on a solution is transferred to the GOF surface by direct drawing processes (left). (b) Optical microscopy (OM) images (1st raw) andscanning electron microscopy (SEM) images (2nd raw) of GOF, GO@Gr and rGO@Gr fibers. The 3rd raw shows the cross-sectional SEM images. (c)Representative Raman spectra of GO, rGO, GO@Gr, and rGO@Gr fibers and CVD graphene film. The inset shows the Raman spectra of rGO and GO.(d) Raman intensity ratios I(2D)/I(G) of rGOF, GO@Gr and rGO@Gr fibers. (e) Schematic representation of the reduction process of GOF@Gr torGOF@Gr.

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as fiber supercapacitors. The electrical conductivity ofrGO@Gr fibers compared with previously reported graphene-based core–shell fibers (Fig. 2b) shows that the CVD grapheneplays an important role in increasing the electrical conduc-tivity of rGO@Gr fibers.29–35 In short, the current flow ofrGO@Gr fibers is enhanced as the CVD graphene shellefficiently dissipates the heat generated by the Joule-heatingeffect. To confirm the heat transfer effect, we measured the

thermal conductivity of fibers.41 The average thermal conduc-tivities of rGOF, rGO@Gr (1), rGO@Gr (2) and rGO@Gr (3) are430, 470, 565 and 691 W m−1 K−1, respectively, implying thatthe enhancement in the thermal conductivity of rGO-CVD gra-phene core–shell fibers is attributed to the heat transfer by theCVD graphene shell. The results are in accordance with theprevious reports that thermal conductivity tends to increasewith the electrical conductivity.42–45

Mechanical properties

In the case of rGOF, the strain and stress at breaking point are1.76% and 49.2 mN per tex, respectively. As the number of gra-phene layers increases, the stress of the fibers decreases mono-tonically down to 25.3 mN per tex, whereas the strain is con-siderably increased up to 21.2% (Fig. 3a and b), which is attrib-uted to the unique stretchability of CVD graphene.28 Thus, weconclude that the CVD graphene enables the high stretchabil-ity and high electrical conductivity of the fibers at the sametime, whereas other fibers incorporated with stretchable poly-mers exhibit a very low conductivity.

The mechanical flexibility was investigated by measuringthe electrical resistance change against mechanical defor-mation (Fig. 3c–f ). As shown in Fig. 3c, the resistance ofrGO@Gr and GOF@Gr doesn’t change for a bending radiusfrom 4 cm to 0.27 cm. In the case of the pure CVD graphenefiber, the electrical resistance is increased by 2% at a bendingradius of 1.3 cm before rupturing. These results indicate thatthe CVD graphene coated on rGOF synergistically enhancesthe flexibly as well as electrical conductivity. A micro-light-emitting diode (LED) is attached to rGO@Gr fibers andstitched to a fabric, showing that the conductivity can be main-

Fig. 2 Electrical properties of rGOF and rGOF@Gr fibers. (a) Electricalconductivity (blue) and electrical conductivity per density (red) of thefibers with respect to the increasing number of graphene layers. (b)Electrical conductivity of rGO@Gr compared with other graphene-based fibers with core–shell structures. (c) Representative temperaturechange between the near middle (highest temperature) of fibers and theend of the fibers with respect to the increasing input power (d) Thermalconductivities of fibers with respect to the increasing number of CVDgraphene layers.

Fig. 3 (a, b) Mechanical properties of rGOF and rGO@Gr fibers with the increasing number of graphene layers. (c) Plot of relative resistance changewith respect to the bending radius. The thickness of the PET substrate is 100 μm. (d) Demonstration of a light-emitting diode (LED) connectedthrough the rGO@Gr before and after bending. (e) Plot of relative resistance change with respect to the bending cycles. (f ) Photographs showingthe rGO@Gr fibers stitched to a fabric to connect a micro-LED.

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tained even with high mechanical deformation in wearableapplications (Fig. 3f). The cyclic bending test has been carriedout (Fig. 3e), showing that the electrical resistance change isnegligible for more than 25 000 cycles.

Electrochemical characterization

The electrochemical characterization was performed in a con-ventional three-electrode cell with an Ag/AgCl electrode (3 MNaCl, BAS Inc.) and a Pt wire (diameter of 0.5 mm) as the refer-ence and counter electrodes, respectively. All potentials referto the Ag/AgCl reference electrode operated in a 1 M aqueousNa2SO4 electrolyte. Fig. 4a and b show the cyclic voltammetry(CV) curves of the rGO@Gr fiber and the bare rGO fiber withthe scanning rate from 10 to 900 mV s−1 at a potential windowfrom 0 to 0.8 V in the 1 M Na2SO4 aqueous electrolyte. Boththe fibers exhibited nearly rectangular shape CV curves whichare indicative of ideal supercapacitive behavior. Within allscan rate ranges, the rGO@Gr fiber electrode shows a highercapacitance than that of the bare rGOF electrode. The calcu-lated specific capacitance (Csp) value of the rGO@Gr fiber is125.34 F g−1, at a scan rate of 10 mV s−1 which is enhancedalmost six times higher than that of rGOF (20.9 F g−1). SincerGOF has many crack and defect sites, its electrical perform-ance is poor and insufficient for capacitive applications.However, the CVD graphene on rGO@Gr fibers facilitates theformation of a conductive pathway and makes the electrontransportation easier and faster than that of rGOF. The galva-nostatic charge/discharge measurement of the rGO@Gr fiberelectrodes at different current densities shows that the chargecurves and discharge curves are nearly triangular and almost

symmetrical, which is suitable for a supercapacitor (Fig. 4c). Inaddition, as shown in Fig. 4d, there was no significant capaci-tance change during 10 000 cycles, exhibiting a superiorelectrochemical stability of rGO@Gr fiber electrodes. Forfurther evaluation of rGO@Gr fiber electrodes, specific energydensity (Esp) and specific power density (Psp) were calculated(Fig. 4e). The values of the energy density and power density ofrGO@Gr fiber electrodes are 11.14 Wh kg−1 at a scan rate of10 mV s−1 and 9088.73 W kg−1 at a scan rate of 900 mV s−1,respectively. However, rGOF electrodes exhibited values of 1.85Wh kg−1 at a scan rate of 10 mV s−1 and 1394.22 W kg−1 at ascan rate of 900 mV s−1. Thus, the characteristics of GO@Grfiber electrodes fall into the ultracapacitor region, showing thehighest power density among the graphene-based fibers.36–40

Conclusion

In summary, we devised a new graphene-based fiber structurewith rGO cores and CVD graphene shells, showing an out-standing electrical conductivity as high as ∼137 S cm−1 and afailure strain value of 21%, which are believed to be thehighest values among polymer-free graphene fibers. TheRaman spectra show that the quality of CVD graphene can bemaintained after the transfer onto rGOF, and the overall con-ductivity is proportional to the number of CVD graphenelayers. The mechanical strength of the rGO@Gr fiber decreaseswith the number of CVD graphene layers, but the maximumstrain considerably increases, which is expected to be moreadvantageous for stretchable applications. We also demon-strate the use of the rGO@Gr fiber for high power density

Fig. 4 Electrochemical characteristics of rGOF and rGO@Gr fibers. (a, b) CV curves for rGO@Gr and pure rGO fibers at different scan rates (10, 50,100, 150, 200, 250, 300, and 400 to 900 mV s−1) in 1 M Na2SO4 solution, respectively. (c) Representative galvanostatic charge/discharge curves ofrGO@Gr at different current densities (3, 4, 5, 6, 7, 10 and 13 A g−1). (d) Variation of the specific capacitance of the rGO@Gr electrode with respect tothe number of cycles (∼10 000). (e) Ragone plots related to the energy density and power density of rGO@Gr and rGOF to compare with other gra-phene based energy storage fibers.

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supercapacitors with enhanced mechanical stability and dura-bility. The energy density was increased from 1394.22 W kg−1

to 9088.73 W kg−1 by employing the outstanding electro-chemical performance of rGO@Gr fibers. Thus, we expect thatthe rGO@Gr fiber with excellent conductivity along withremarkable stretchability would be an important componentfor smart wearable applications in the future.

ExperimentalMaterials

Graphene oxide was synthesized using the modified Hummers’method, with the following materials: graphite, hydrogen iodide,phosphorus pentoxide, potassium persulfate and sodium sulfatewhich were purchased from Sigma-Aldrich. Fuming nitric acid,sulfuric acid, potassium permanganate, hydrogen peroxide,hydrochloric acid and ethylene glycol were supplied by Daejung,South Korea. All the chemicals were used as received.

Preparation of the CVD graphene film

Graphene was synthesized by the typical chemical vapor depo-sition (CVD) method on a highly pure copper foil (Alfa Aesar,99.95%) according to a previous study.16 The growth was per-formed with flowing 50 sccm H2 and 5 sccm CH4 gas at1000 °C.

GO fiber fabrication

The GO fiber was fabricated by using a co-flow wet-spinningmachine (Invisible In., Korea). A high purity GO dope(20 mg mL−1) was extruded through a single spinneret (24 G).The GO solution was injected at a rate of 400 μL min−1 intothe cetrimonium bromide (CTAB) coagulation bath, which wasrotating at 20 rpm, to produce GO fibers. The sample was keptin the bath for 20 min and then the gel GO fibers were dried atroom temperature.

Graphene-GO fiber fabrication

The CVD graphene film was wrapped around a GO fiber withthe direct drawing method and self-assembled to fabricateGOF@Gr structure fibers. After that, the GOF@Gr fibers wereplaced in a reduction solution (HI : AcOH = 2 : 5 v/v) at roomtemperature for 24 h, and then the rGOF@Gr fibers were putinto acetone to remove PMMA and washed with water.

Thermal conductivity

We measured the thermal conductivity of the suspended fibersusing a thermal emission microscope (Themos mini C10614-02, Hamamatsu) at vacuum conditions. A “Keithley2440” wasemployed as a source meter. The thermal conductivity wasobtained by the following equation:

κ ¼ εVIL4AcðT0 � TaÞ

where κ is the thermal conductivity, ε is the emissivity of thematerial, V is the applied voltage, I is the measured current,

L is the half length of the fiber, Ac is the cross-sectional area ofthe fiber, T0 is the highest temperature near the middle of thefiber, and Ta is the ambient temperature.

Characterization

Optical microscopy (OM) images were captured by using aNIKON ECLIPSE LV 100 ND OM installed with the NISElements D 4.20.00 software. The SEM images were obtainedby field-emission scanning electron microscopy (FESEM,AURIGA Carl Zeiss). A Raman spectrometer was used to charac-terize the reduction of fibers and the uniformity of CVD gra-phene coating (RM 1000-Invia Renishaw, 514 nm with a spotsize of 1 μm), and the electrical properties of the fibers weremeasured with a 4-point probe nanovoltmeter (Keithley 6221)and an Agilent B2912A. The tensile mechanical properties ofthe fibers were examined by using a Thermal MechanicalAnalyzer (TMA) of HITACHI at a loader speed of 10 μm min−1

at room temperature. Bending and mechanical durability testswere performed by using a home-built bending machine. Forthe analysis of energy storage properties, electrochemicalexperiments were carried out at room temperature in a three-electrode electrochemical cell connected to a potentiostat(660E, CHI Instruments, Inc.).

Conflicts of interest

There are no conflicts to declare

Acknowledgements

This research was supported by the Technology InnovationProgram and Industrial Strategic Technology DevelopmentProgram (No. 10079969 and 10044410) funded By the Ministryof Trade, Industry & Energy (MOTIE, Korea). This work wasalso supported by National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and FuturePlanning (No. 2016M2A7B4910458, 2018M3A7B4070990,2017M3C1A9069591, 2015R1A2A1A15053268) funded by theMinistry of Science, ICT, and Future Planning of KoreaGovernment and by the grant (AICT-2014-0006) from theAdvanced Institutes of Convergence Technology (AICT) andPuritech Company.

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