- Email: [email protected]
Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors
G Model JIEC 4712 No. of Pages 7
Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors Tae Ann Kima,1, Jun Beom Pyoa,1, Sang-Soo Leea , Min Parka,b,* a b
Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Seoul 02447, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 June 2019 Received in revised form 26 July 2019 Accepted 4 August 2019 Available online xxx
Developing materials that can absorb large amounts of strain without fracture or significant degradation in their electronic properties is essential for the realization of stretchable electronics. Here, stretchable conductors made from reduced graphene oxide/polydimethylsiloxane (r-GO/PDMS) composite were fabricated to achieve stable resistance changes under uniaxial stretching. By controlling the weight fraction of GO in water, highly aligned and porous GO structure was obtained over 0.5 wt% of GO solution. After thermal and chemical reduction of GO to r-GO film, PDMS was infiltrated into pores of the film to make free-standing and elastic composites. Electrical percolation of r-GO in the composite was achieved at the r-GO concentration over 0.3 wt% along with improvement of electrical conductivity. However, highly aligned r-GO/PDMS composite only exhibited notable suppression of resistance change under the large deformation of 100% strain. The resistance change of 0.3 wt% r-GO/PDMS composites at 100% strain was over 7 times higher than that of 0.7 wt% r-GO/PDMS composites, exhibiting the dramatic effect of rGO alignments. Even after repeated stretching/releasing cycles, the variation in resistance from 0 to 70% of strain was stable. The stretchable conductor based on the aligned r-GO/PDMS composite was successfully applied as a resistive strain sensor. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Reduced graphene oxide PDMS Self-assembly Conductive composite Stretchable conductors Strain sensors
Introduction Graphene, a 2D sp2 hybridized carbon allotrope, has been one of the well-studied nanofillers in nanocomposites for various applications due to its interesting properties including ballistic charge carrier mobility and exceptional mechanical properties [1–3]. Among different kinds of graphene, graphene oxide (GO), prepared by oxidizing graphite with a strong acid, has several advantages as nanofillers. First, the preparation of the material is a scalable solution process, deploying higher commercial values. Second, GOs have facile process-ability because they could be highly dispersive in polar solvents [4–7]. Third, graphene’s high electrical conductivity can be recovered by reducing GO via thermal, chemical, electrochemical or optical treatments [8,9]. Having such advantages, GO and reduced form of GO (r-GO) have been studied as nanofillers to create conductors [10–16], catalyst support [17,18], and more recently to develop stretchable
* Corresponding author at: Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea. E-mail address: [email protected] (M. Park). 1 These two authors made equal contributions.
conductors [19–21], an essential component in the next-generation soft electronic devices. One of the key factors that control the performance of nanocomposites is the nanoscale morphology of fillers [22–25]. When GO/r-GO is used as filler materials, its highly anisotropic dimensions from 2D planar shape play an important role in determining properties of the composite. The effect is amplified as graphene exhibits different in-plane and out-of-plane physical properties [26,27]. For instance, a composite employing randomly dispersed graphenes showed isotropic electrical conductivity [28]. Meanwhile, a composite employing aligned graphenes exhibited anisotropic conductivity of five orders of magnitude between the direction of alignment and its perpendicular [26]. Therefore, controlling the dispersion and orientation of graphene fillers is critical in maximizing the electrical conductivity of graphene nanocomposite. As a stretchable conductor, a composite material should exhibit little change in resistance upon applied strains. To achieve this property, several groups have demonstrated the importance of filler alignment in the composites under deformation. Kim et al. showed the stress-induced organization of conductive nanoparticles (NPs) in polymers along the stretching direction, which enables to retain good electron
https://doi.org/10.1016/j.jiec.2019.08.018 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
2
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Characterization
transport between NPs even at high strains [29]. Zhang et al. fabricated uniaxially aligned carbon nanotube (CNT) bundles, embedded in poly(dimethylsiloxane) (PDMS). The composite has stable electrical conductivity under a repetitive tensile strain up to 100% when the strain was applied parallel to the orientation of the CNTs [30]. These results imply that fine control of GO/r-GO assembly is important not only for achieving high conductivity in nanocomposites but also for maintaining conductive pathways under deformation. Geometrical engineering can further attain stable conductive pathways and enhance stretchability. For instance, intact graphenes lose their conducting behavior over 25% strain [31], while foam-like 3D graphene network exhibits a better resilience and a little change in resistance up to 50% of strain [32,33]. Perforation of composite films also improves stretchability without deteriorating their conductivity [34], since the holes in the film reduce the effective Young’s modulus and the yield strength of the film [35– 37]. To develop an ideal graphene composite for the stretchable conductor, a process that simultaneously aligns graphene and develops porous structures is required. Numerous approaches to fabricate self-assembled or porous 2D materials have been reported including Langmuir technique [38], electrospinning [39], phase separation [40,41], three-dimensional printing [42], and freeze-drying method [43,44]. However, few studies reported a process that creates aligned-porous GO or r-GO network and alignment effects of 2D nanofillers in composite under tensile strain. Here, we report a facile fabrication method to prepare highly aligned r-GO/PDMS composites as stretchable conductors, using the self-assembly behavior of anisotropic 2D materials and freeze-drying method. We characterized morphological changes of r-GO structures by varying the weight fraction of GO in solution and examined electromechanical properties of r-GO/PDMS composites under uniaxial tension. In addition, the resulted r-GO/PDMS composite was utilized as a resistive strain sensor.
To examine the morphology of GO, 0.1 wt% of GO aqueous dispersion was prepared and spin-coated on a piranha-treated silicon wafer. Then, the thickness and size distribution of GO were characterized by an atomic force microscope (AFM, MFP-3D, Asylum Research). The measurement of mean diameter and area of GO, conducted for more than 100 GOs, were performed with Image J software. The morphologies of the GOs, the r-GO structures, and the cross-section of r-GO/PDMS composites were observed using a FESEM (JSM-6701F, Jeol). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI 5000 VersaProbe (UlvacPHI) using monochromatic Al Kα radiation. X-ray diffraction measurements were performed by Rigaku Dmax2500 using Cu Kα radiation. Anisotropic characteristics of highly aligned r-GOs were examined by confocal Raman spectroscopy (Horiba LabRAM HR 3D) with visible laser excitation at 633 nm. Free-standing r-GOs were vertically and horizontally orientated from a glass substrate to obtain Raman spectra. The BET surface area of the r-GOs was measured by nitrogen adsorption experiments at 77 K on a Belsorp-Max (Bel Japan), after the sample was evacuated at room temperature for 12 h. The sheet resistance was measured using a four-point-probe measurement system (CRESBOX, Napson). A precise length-controlling system was used to apply tensile strain to characterize the change in electrical resistance under various tensile strains, measured by a potentiostat (Parstat 2273, Princeton Applied Research). To demonstrate the r-GO/PDMS composite as a strain sensor, it was encapsulated with an elastomeric adhesive film (VHB 4905, 3M) and attached to an elbow. The bending angle of the elbow was measured by Inertial Measurement Units (EBIMU-9DOFV3, E2BOX) with a receiver (EBIMU24GV3, E2BOX). The change in resistance while angular displacement was recorded by a digital multimeter (651, Keithley). For the resistance measurements, copper strips or wires were attached to each end of the r-GOs with conductive metals before infiltration with PDMS prepolymer to fabricate the electrodes with small contact resistance.
Experimental methods
Results and discussion
Fabrication of highly aligned r-GO structures and their composites
Preparation of r-GO porous films
GO was synthesized by a modified Hummers’ method [45]. Aqueous GO dispersions with various concentration of GO (0.1, 0.3, 0.5, 0.7 wt%) were prepared by sonication for less than 1 min and vigorous shaking. Highly aligned and porous GO structure was obtained in the following procedure: First, a glass substrate (piranha-treated) was immersed in a 3% aqueous solution of 3aminopropyltryethoxysilane (3-APTES, Sigma-Aldrich) for 12 h, followed by thorough rinsing with deionized water and drying with a nitrogen gun [45]. The GO dispersion was deposited on 3APTES modified glass substrates by the doctor-blade technique and quickly frozen by placing it on a liquid nitrogen cooled plate. The quenched samples were then moved to the freeze-dryer (FD-1000, Eyela) and sublimed at 50 C under 10 Pa for 12 h. Reduction of GOs was achieved in the hydrazine vapor environment at 200 C for 6 h, finally obtaining r-GO films.
We developed a facile preparation method to produce a highly aligned and porous r-GO film for a stretchable conductor as presented in Fig. 1. GO solution was first fabricated into a film by
Fabrication of a r-GO/PDMS resistive strain sensor Before infiltrating PDMS prepolymer into the r-GO film, metal wires were attached to the film with eutectic gallium-indium (EGaIn, Sigma-Aldrich) as electrical contacts. PDMS was infiltrated and cured as described previously. Acquired free-stranding r-GO/ PDMS composite was encapsulated with a stretchable and adhesive film (VHB 4905, 3M) before attaching the device to an elbow for better adhesion to the body.
Fig. 1. Fabrication process of r-GO/PDMS composite conductors.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
bar-coating on a cooled metal plate to immediately freeze the film as soon as GO was cast. Solvent from the frozen film was sublimated without destroying the aligned GO structure using a freeze-drying method. Prepared GO film was then reduced to recover the conductivity of graphene, which was finally infiltrated with PDMS to achieve stretchability. GOs, synthesized by a modified Hummer’s method, have 1 nm of thickness indicating single-layered sheets, 2.53 mm of mean diameter, and 20.1 mm2 of average area (Fig. S1). Sufficient shear force and adhesion/wetting between GO solution and a substrate are essential for uniform deposition of a dispersed GO film. The bar-coating method, which provides shear forces, was selected to cast the GO solution on a low-temperature glass substrate. Piranha-cleaned glass substrates were functionalized with 3aminopropyltryethoxysilane (3-APTES) to provide strong attraction with negatively charged GO [46], which was confirmed by the appearance of nitrogen (N) 1s peak on the XPS spectrum of the 3APTES treated glasses (Fig. S2). Although GO is advantageous in developing self-assembled and porous structures due to the high dispersibility in water, the resulting freeze-dried GO film should be reduced to restore electrical conductivity of graphene. Hydrazine is one of the most common reducing agents to convert GO to its reduced form [8]. By combining hydrazine with heat treatment, the sheet resistance of obtained r-GO can be significantly decreased [47,48]. To achieve highly conductive r-GO films, we optimized the hydrazine reduction condition by varying the temperature (50 C, 200 C) and the annealing time. High temperature (200 C) treatment under hydrazine can lower the sheet resistance of the samples by one order of magnitude compared to low temperature (50 C) treated ones (Fig. 2). Higher conductivity was achieved at longer exposure time for both conditions. However, the conductivity did not exhibit any notable changes after 12 h in the case of 200 C, which was chosen as an optimum annealing time. The reduction of GO was further confirmed by XPS and XRD. Fig. S3 shows the XPS survey scan and high-resolution C1s spectra of GO and r-GO. After thermal and chemical reduction, O1s peak significantly decreased indicating the removal of oxygen-containing functional groups from the GO. The high-resolution C1s spectra were de-convoluted into three peaks at 284.7 eV, 286.2 eV, and 289.0 eV, corresponding to sp2-hybridized carbon bond, carbon atoms directly bonded to oxygen, and carbonyl groups, respectively [49–51]. After reduction, the intensity ratio of each distinct peak was changed from 2.96:3.30:1 to 6.81:1.92:1, suggesting carboncarbon bonds were restored while oxygen functional groups were
Fig. 2. The change of sheet resistance as a function of annealing time under different temperatures.
3
removed. Fig. S4 exhibits the XRD patterns of GO and r-GO. The (001) peak of GO was observed at 2u = 10.6 , corresponding to an interlayer spacing (d) of 0.834 nm. After the reduction, the sharp peak at 10.6 completely disappeared and a broad peak centered at around 22.6 (d = 0.393 nm) was only observed. This value is similar to the d value (0.336 nm) of graphite, implying partial recovery of graphitic structure [52–54]. Internal structures of r-GO film and r-GO/PDMS composite Fig. 3a–c show the cross-sectional images of freeze-dried r-GO films. At low concentration (below 0.3 wt%), the r-GO sheets showed no directional order and only had interconnected structures. Above 0.5 wt%, observed were the highly aligned structures of r-GO, whose alignment planes were mostly parallel to the glass substrate. However, the surface morphologies were not affected by the concentration of r-GO. All r-GO films derived from GO solutions of 0.3, 0.5, 0.7 wt% seem to be well aligned parallel to substrates because the shearing orientation occurred during the casting process, which had affected only on the near-surface region (Fig. S5). The alignment of r-GOs was further confirmed by the polarized Raman spectroscopy. A significant intensity change in Raman signals has been observed in highly aligned carbon materials depending on the orientation of the sample with respect to the incident beam [26,55,56]. As shown in Fig. 3d–f, r-GO films exhibiting aligned structures (concentration above 0.5 wt%) from the SEM images, also had an anisotropic scattering behavior in the Raman spectra. At high graphene concentrations (0.5 wt% and 0.7 wt%), both the D (around 1320 cm 1) and the G (around 1610 cm 1) bands showed strong intensities when the incident light was parallel to the aligned direction (in-plane) while the scattering was significantly suppressed when the laser was perpendicular to the aligned direction (through the thickness). At low concentration (0.3 wt%), no significant changes in the scattering intensity were observed in terms of the sample orientation. We investigated the porous feature of freeze-dried r-GO films using nitrogen adsorption-desorption measurements. All the samples exhibited characteristic features of type-IV isotherms with a hysteresis loop, indicating the presence of large macropores and mesopores (Fig. 4). The formation of the hysteresis loop is involved in filling and emptying of mesopores, having widths in the range of 2–50 nm, by capillary condensation/evaporation [57–59]. The specific surface area (SSABET), mean diameters and size distribution of mesopores (dBJH) calculated by BET analysis and BJH method are summarized in Fig. S6. Both SSA and diameters exhibited a steady falling tendency as GO concentration was increased. Assuming r-GO sheets as rigid circular plates with a very thin thickness, the SSA was inversely proportional to their thickness, implying that r-GO sheets composing porous structures tended to be more aggregated and stacked at a higher concentration which resulted in a reduction of both SSA and pore diameters. The r-GO film was infiltrated with PDMS prepolymer, followed by heat-curing, to preserve the aligned r-GO morphology in composite and consequently increase mechanical durability and stretchability without loss of conductivity. After infiltration and curing, the cross-sectional morphologies of the composites were examined (Fig. 5). As shown in the SEM examinations, no voids were observed, evidencing successful infiltration of PDMS into the r-GO structures. Also, all the composites exhibited characteristic patterns, which were comparable with pristine r-GO structures, shown in the inset figures of Fig. 5. The composites derived from the r-GO with highly aligned structure also exhibited similar aligned patterns in their cross-sections. This implied that polymer infiltration did not alter the alignment of r-GO and retained the original r-GO structure. The r-GO/PDMS composites exhibited good
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
4
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Fig. 3. Cross-sectional morphologies of freeze-dried r-GO films: (a) 0.3 wt%, (b) 0.5 wt%, and (c) 0.7 wt% (Scale bar = 100 mm). Brown disks and a grey rectangle represent r-GOs and a substrate, respectively. Raman spectra of r-GO films in parallel and perpendicular orientations with respect to the incident laser: (d) 0.3 wt%, (e) 0.5 wt%, and (f) 0.7 wt% (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
stretchability due to the presence of rubbery PDMS matrix and moderate transparency due to the porous structures of r-GO (Fig. S7). The change in resistance under strain
Fig. 4. Nitrogen adsorption (ADS, closed circles)—desorption (DES, open circles) isotherms of freeze-dried r-GO films.
The fabricated stretchable r-GO/PDMS composites were employed as elastic electrical wirings, as shown in Fig. 6a. We measured the change in resistance under strain with those composites (Fig. 6b). The conductivity measurement samples prepared from 0.3, 0.5 and 0.7 wt% of GO solutions showed electrical conduction, while those from 0.1 wt% GO were not electro-conductive, evidencing that conductive percolation pathway could be developed from 0.3 wt% of GO solution. At every 10% strain, current–voltage (I–V) curves were measured to calculate the change in resistance. The current increased linearly with the applied voltage, indicating the presence of good ohmic contact between the composites and electrodes [60,61].
Fig. 5. Cross-sectional morphologies of r-GO/PDMS composites (a) 0.3 wt%, (b) 0.5 wt%, and (c) 0.7 wt% (Inset: SEM images of r-GO films before infiltration).
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
5
Fig. 6. (a) r-GO/PDMS composites used as stretchable conductors for lighting a LED lamp, (b) resistance of r-GO/PDMS composites made from GO solutions with different concentrations as a function of tensile strain and (c) the stability of the resistance of r-GO/PDMS (0.7 wt%) under multiple stretching/releasing cycles with 70% of applied strain.
The resistance of r-GO/PDMS composites with 0.3 wt% r-GO structures continuously increased with strain. At small deformation range, less than 30% of strain, the resistance increased by about 4 times of the initial resistance. The relatively small increase in resistance might result from the well-connected network structures of r-GO which can provide additional stretchability without breaking its conductive pathway. At higher deformation region, however, the resistance increased by 20 times of the starting value. When the composite was elongated to 100%, the conductivity was completely lost, implying that conductive networks of r-GO in the composite were significantly broken. Highly aligned r-GOs (0.5 wt%, 0.7 wt%) showed interesting electro-conductive behaviors when stretched. First, at 100% of strain, both of samples were still conductive and their resistances were much smaller than that of 0.3 wt% of r-GO samples. This suggests that highly aligned r-GOs developed more rigid conducting networks against uniaxial tension than randomly connected ones. Also, the resistance did not increase continuously as the strain built up but several plateau regions appeared where the resistance did not change at all. This can be related with the sliding effects between r-GO sheets similar with stretchable electrodes made of gold nanosheets [62]: Aligned r-GO can slide with each other under stretching while keeping their contact points so that their conductivity can be retained. Samples fabricated from 0.7 wt % of r-GO, having more aligned structures, exhibited 30% less resistance change compared to 0.5 wt% of r-GO. In addition, after 10% of strain, the resistance of 0.7 wt% samples only increased by up to 20% of previous resistance while that of 0.5 wt% increased up to 56%. Lastly, we tested the stability of our conductors under repeated stretching/releasing cycles with the composites of 0.7 wt % r-GO. The change of initial resistance and the resistance under 70% of strain was recorded at multiple cycles, shown in Fig. 6c. Even after fifty cycles, the initial resistance and the final resistance under 70% of strain was always recovered to the pristine resistance values from the first stretching/releasing.
Application to resistive strain sensors Based on the r-GO/PDMS (0.7 wt% r-GO) composite, we fabricated a resistive strain sensor, a device that requires an electrode that reversibly changes resistance upon repeated strains. The strain sensitivity (gauge factor, GF) of the composite, defined
Fig. 7. (a) Photos presenting movements of an arm with a r-GO/PDMS strain sensor and IMUs attached. (b) Summary of measured resistance by r-GO/PDMS strain sensor and the respective angles between IMUs due to arm movements.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
6
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
by the slope of the relative change of the resistance versus applied strain, is 3.6 0.2 with up to 70% of strain value. Conventional metal-foil based strain gauges have similar GFs in the range of 2–5 but they do not show high stretchability [63]. We attached the rGO/PDMS composite to an elbow to monitor applied strains by measuring resistance change during arm folding motion. Two inertial measurement units (IMUs) were attached to an arm to measure the angles in between the IMUs as shown in Fig. 7a to quantify the movements. By simultaneously measuring both resistance and angles during the movement of an arm, we were able to test the reliability of the fabricated sensor more accurately. It is notable that overlay comparison of the resistance and angle data were closely related as presented in Fig. 7b. In other words, the degree of arm movements can be deduced by measuring the resistance. We strongly believe that an aligned r-GO morphology would endure repeated strains by reversibly sliding and recovering the conductive path when stress is released. Conclusions We prepared stretchable and conductive PDMS composites with highly aligned and porous r-GO as filler. Due to the concentrationdependent self-assembly property of GOs, freeze-dried GO solutions containing more than 0.5 wt% of GO exhibited highly aligned structures. The combined thermal and chemical reduction of GO under hydrazine vapor gave us enhanced conductivity compared to hydrazine reduction at low temperature. The samples fabricated from highly aligned r-GO exhibited unique conducting behavior when the strain was applied. The variation in resistance sharply decreased compared to those from randomly connected structures. Also, they had step-like resistance changes due to the sliding effect of r-GO sheets. Even though the resistance increased by about 4 times the starting value, stable variation was observed up to 50 times of stretching/releasing cycles. The prepared r-GO films were applied to strain sensors to demonstrate their potential application for soft electronics. The alignment of disk-shaped fillers with our unique methods can be applied to other anisotropic fillers which require the stable variation in performance under stretching. Acknowledgment This work was financially supported by the internal grant of Korea Institute of Science and Technology (KIST). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jiec.2019.08.018. References [1] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183, doi:http://dx.doi.org/ 10.1038/nmat1849. [2] C. Lee, X. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385, doi:http://dx.doi.org/ 10.1126/science.1157996. [3] W.-C. Tan, W.-H. Shih, Y.F. Chen, Adv. Funct. Mater. 24 (2014) 6818, doi:http:// dx.doi.org/10.1002/adfm.201401421. [4] Y. Xu, W. Hong, H. Bai, C. Li, G. Shi, Carbon 47 (2009) 3538, doi:http://dx.doi. org/10.1016/j.carbon.2009.08.022. [5] N.V. Medhekar, A. Ramasubramaniam, R.S. Ruoff, V.B. Shenoy, ACS Nano 4 (2010) 2300, doi:http://dx.doi.org/10.1021/nn901934u. [6] J. Kim, L.J. Cote, F. Kim, W. Yuan, K.R. Shull, J.X. Huang, J. Am. Chem. Soc. 132 (2010) 8180, doi:http://dx.doi.org/10.1021/Ja102777p. [7] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101, doi:http://dx.doi.org/10.1038/nnano.2007.451. [8] S. Stankovich, D.A. Dikin, R.D. Piner, Ka. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558, doi:http://dx.doi.org/10.1016/j. carbon.2007.02.034. [9] T. Kuila, A.K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Nanoscale 5 (2013) 52, doi: http://dx.doi.org/10.1039/c2nr32703a.
[10] H. Chen, M.B. Müller, K.J. Gilmore, G.G. Wallace, D. Li, Adv. Mater. 20 (2008) 3557, doi:http://dx.doi.org/10.1002/adma.200800757. [11] T. Zhou, F. Chen, C. Tang, H. Bai, Q. Zhang, H. Deng, Q. Fu, Compos. Sci. Technol. 71 (2011) 1266, doi:http://dx.doi.org/10.1016/j.compscitech.2011.04.016. [12] H. Wu, L.T. Drzal, Mater. Chem. Phys. 182 (2016) 110, doi:http://dx.doi.org/ 10.1016/j.matchemphys.2016.07.011. [13] H. Yang, X.F. Yao, Z. Zheng, L.H. Gong, L. Yuan, Y.N. Yuan, Y.H. Liu, Compos. Sci. Technol.167 (2018) 371, doi:http://dx.doi.org/10.1016/j.compscitech.2018.08.022. [14] J. Phiri, L.S. Johansson, P. Gane, T. Maloney, Compos. B: Eng. 147 (2018) 104, doi: http://dx.doi.org/10.1016/j.compositesb.2018.04.018. [15] J.W. Jo, J.U. Lee, W.H. Jo, Polym. Int. 64 (2015) 1676, doi:http://dx.doi.org/ 10.1002/pi.4981. [16] J. Liu, H.J. Choi, L.Y. Meng, J. Ind. Eng. Chem. 64 (2018) 1, doi:http://dx.doi.org/ 10.1016/j.jiec.2018.02.021. [17] X.Y. Zhu, Z.S. Lv, J.J. Feng, P.X. Yuan, L. Zhang, J.R. Chen, A.J. Wang, J. Colloid Interface Sci. 516 (2018) 355, doi:http://dx.doi.org/10.1016/j.jcis.2018.01.047. [18] H. Bin Meng, X.F. Zhang, Y.L. Pu, X.L. Chen, J.J. Feng, D.M. Han, A.J. Wang, J. Colloid Interface Sci. 543 (2019) 17, doi:http://dx.doi.org/10.1016/j.jcis.2019.01.110. [19] M. Chen, L. Zhang, S. Duan, S. Jing, H. Jiang, C. Li, Adv. Funct. Mater. 24 (2014) 7548, doi:http://dx.doi.org/10.1002/adfm.201401886. [20] C. Feng, Z. Yi, L.F. Dumée, F. She, Z. Peng, W. Gao, L. Kong, Carbon 139 (2018) 672, doi:http://dx.doi.org/10.1016/j.carbon.2018.07.016. [21] X. Wang, S. Meng, M. Tebyetekerwa, Y. Li, J. Pionteck, B. Sun, Z. Qin, M. Zhu, Compos. A: Appl. Sci. Manuf. 105 (2018) 291, doi:http://dx.doi.org/10.1016/j. compositesa.2017.11.027. [22] S.H. Xie, Y.Y. Liu, J.Y. Li, Appl. Phys. Lett. 92 (2008), doi:http://dx.doi.org/ 10.1063/1.2949074. [23] T. Ramanathan, F.T. Fisher, R.S. Ruoff, L.C. Brinson, Chem. Mater. 17 (2005) 1290, doi:http://dx.doi.org/10.1021/cm048357f. [24] Y.J. Kwon, Y. Kim, H. Jeon, S. Cho, W. Lee, J.U. Lee, Compos. B: Eng. 122 (2017) 23, doi:http://dx.doi.org/10.1016/j.compositesb.2017.04.005. [25] G. Mittal, V. Dhand, K.Y. Rhee, S.J. Park, W.R. Lee, J. Ind. Eng. Chem. 21 (2015) 11, doi:http://dx.doi.org/10.1016/j.jiec.2014.03.022. [26] N. Yousefi, X. Lin, Q. Zheng, X. Shen, J.R. Pothnis, J. Jia, E. Zussman, J.K. Kim, Carbon 59 (2013) 406, doi:http://dx.doi.org/10.1016/j.carbon.2013.03.034. [27] N. Yousefi, X. Sun, X. Lin, X. Shen, J. Jia, B. Zhang, B. Tang, M. Chan, J.-K. Kim, Adv. Mater. 26 (2014) 5480, doi:http://dx.doi.org/10.1002/adma.201305293. [28] Y. Li, Y.A. Samad, K. Polychronopoulou, S.M. Alhassan, K. Liao, Sci. Rep. 4 (2015) 1, doi:http://dx.doi.org/10.1038/srep04652. [29] Y. Kim, J. Zhu, B. Yeom, M. Di Prima, X. Su, J.G. Kim, S.J. Yoo, C. Uher, N.A. Kotov, Nature 500 (2013) 59, doi:http://dx.doi.org/10.1038/nature12401. [30] Y. Zhang, C.J. Sheehan, J. Zhai, G. Zou, H. Luo, J. Xiong, Y.T. Zhu, Q.X. Jia, Adv. Mater. 22 (2010) 3027, doi:http://dx.doi.org/10.1002/adma.200904426. [31] T. Shao, B. Wen, R. Melnik, S. Yao, Y. Kawazoe, Y. Tian, J. Chem. Phys. 137 (2012), doi:http://dx.doi.org/10.1063/1.4766203. [32] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.-M. Cheng, Nat. Mater. 10 (2011) 424, doi: http://dx.doi.org/10.1038/nmat3001. [33] Z. Wang, X. Liu, X. Shen, N.M. Han, Y. Wu, Q. Zheng, J. Jia, N. Wang, J.K. Kim, Adv. Funct. Mater. 28 (2018) 1, doi:http://dx.doi.org/10.1002/adfm.201707043. [34] T. Sekitani, Y. Noguchi, K. Hata, T. Fukushima, T. Aida, T. Someya, Science 321 (2008) 1468, doi:http://dx.doi.org/10.1126/science.1160309. [35] K.Y. Chun, Y. Oh, J. Rho, J.H. Ahn, Y.J. Kim, H.R. Choi, S. Baik, Nat. Nanotechnol. 5 (2010) 853, doi:http://dx.doi.org/10.1038/nnano.2010.232. [36] Á. Kovács, A. Kovács, U. Mescheder, Comput. Mater. Sci. 43 (2008) 59, doi: http://dx.doi.org/10.1016/j.commatsci.2007.07.023. [37] C. Van Rijn, M. Van Der Wekken, W. Nijdam, M. Elwenspoek, J. Microelectromechanical Syst. 6 (1997) 48, doi:http://dx.doi.org/10.1109/84.557530. [38] X. Luo, K. Ma, T. Jiao, R. Xing, L. Zhang, J. Zhou, B. Li, Nanoscale Res. Lett. 12 (2017), doi:http://dx.doi.org/10.1186/s11671-017-1864-8. [39] X. Huang, R. Wang, T. Jiao, G. Zou, F. Zhan, J. Yin, L. Zhang, J. Zhou, Q. Peng, ACS Omega 4 (2019) 1897, doi:http://dx.doi.org/10.1021/acsomega.8b03615. [40] C. Schugens, V. Maquet, C. Grandfils, R. Jerome, P. Teyssie, Polymer (Guildf) 37 (1996) 1027, doi:http://dx.doi.org/10.1016/0032-3861(96)87287-9. [41] C. Schugens, V. Maquet, C. Grandfils, R. Jerome, P. Teyssie, J. Biomed. Mater. Res. 30 (1996) 449, doi:http://dx.doi.org/10.1002/(Sici)1097-4636(199604) 30:4<449::Aid-Jbm3>3.0.Co;2-P. [42] H. Seyednejad, D. Gawlitta, R.V. Kuiper, A. de Bruin, C.F. van Nostrum, T. Vermonden, W.J.A. Dhert, W.E. Hennink, Biomaterials 33 (2012) 4309, doi: http://dx.doi.org/10.1016/j.biomaterials.2012.03.002. [43] R. Xing, W. Wang, T. Jiao, K. Ma, Q. Zhang, W. Hong, H. Qiu, J. Zhou, L. Zhang, Q. Peng, ACS Sustain. Chem. Eng. 5 (2017) 4948, doi:http://dx.doi.org/10.1021/ acssuschemeng.7b00343. [44] R. Guo, T. Jiao, R. Li, Y. Chen, W. Guo, L. Zhang, J. Zhou, Q. Zhang, Q. Peng, ACS Sustain. Chem. Eng. 6 (2018) 1279, doi:http://dx.doi.org/10.1021/acssuschemeng.7b03635. [45] J. Zhao, S. Pei, W. Ren, L. Gao, H.-M. Cheng, ACS Nano 4 (2010) 5245, doi:http:// dx.doi.org/10.1021/nn1015506. [46] Q. He, H.G. Sudibya, Z. Yin, S. Wu, H. Li, F. Boey, W. Huang, P. Chen, H. Zhang, ACS Nano 4 (2010) 3201, doi:http://dx.doi.org/10.1021/nn100780v. [47] V.H. Pham, T.V. Cuong, S.H. Hur, E.W. Shin, J.S. Kim, J.S. Chung, E.J. Kim, Carbon 48 (2010) 1945, doi:http://dx.doi.org/10.1016/j.carbon.2010.01.062. [48] P.G. Ren, D.X. Yan, X. Ji, T. Chen, Z.M. Li, Nanotechnology 22 (2011), doi:http:// dx.doi.org/10.1088/0957-4484/22/5/055705. [49] L.G. Guex, B. Sacchi, K.F. Peuvot, R.L. Andersson, A.M. Pourrahimi, V. Ström, S. Farris, R.T. Olsson, Nanoscale 9 (2017) 9562, doi:http://dx.doi.org/10.1039/ c7nr02943h.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx [50] Y.C. Shi, A.J. Wang, X.L. Wu, J.R. Chen, J.J. Feng, J. Colloid Interface Sci. 484 (2016) 254, doi:http://dx.doi.org/10.1016/j.jcis.2016.09.008. [51] Y.C. Shi, J.J. Feng, X.X. Lin, L. Zhang, J. Yuan, Q.L. Zhang, A.J. Wang, Electrochim. Acta 293 (2019) 504, doi:http://dx.doi.org/10.1016/j.electacta.2018.10.068. [52] C. Hu, X. Zhai, L. Liu, Y. Zhao, L. Jiang, L. Qu, Sci. Rep. 3 (2013) 1, doi:http://dx. doi.org/10.1038/srep02065. [53] J.H. Cheng, Y.H. Chen, Y.S. Yeh, S. Hy, L.Y. Kuo, B.J. Hwang, Electrochim. Acta 197 (2016) 146, doi:http://dx.doi.org/10.1016/j.electacta.2015.12.116. [54] H. Ham, T. Van Khai, N.H. Park, D.S. So, J.W. Lee, H.G. Na, Y.J. Kwon, H.Y. Cho, H. W. Kim, Nanotechnology 25 (2014), doi:http://dx.doi.org/10.1088/0957-4484/ 25/23/235601. [55] N. Yousefi, M.M. Gudarzi, Q. Zheng, S.H. Aboutalebi, F. Sharif, J.-K. Kim, J. Mater. Chem. 22 (2012) 12709, doi:http://dx.doi.org/10.1039/c2jm30590a. [56] B. Kwon, J. Park, W. Jeong, G. Jeong, H.S. Ryu, P. Paoprasert, S.Y. Park, I. In, Carbon Lett. 27 (2018) 26, doi:http://dx.doi.org/10.5714/CL.2018.27.026.
7
[57] K.S.W. Sing, J. Porous Mater. 2 (1995) 5, doi:http://dx.doi.org/10.1007/ BF00486564. [58] P. Chang, Z. Qin, Carbon Lett. 25 (2018) 117, doi:http://dx.doi.org/10.5714/ CL.2018.25.117. [59] Y.W. You, E.H. Moon, I. Heo, H. Park, J.S. Hong, J.K. Suh, J. Ind. Eng. Chem. 45 (2017) 164, doi:http://dx.doi.org/10.1016/j.jiec.2016.09.019. [60] T.A. Kim, S.-S. Lee, H. Kim, M. Park, RSC Adv. 2 (2012) 10717, doi:http://dx.doi. org/10.1039/c2ra20792c. [61] Y. Kim, Y.J. Kwon, J.Y. Hong, M. Park, C.J. Lee, J.U. Lee, J. Ind. Eng. Chem. 68 (2018) 399, doi:http://dx.doi.org/10.1016/j.jiec.2018.08.015. [62] G.D. Moon, G.-H. Lim, J.H. Song, M. Shin, T. Yu, B. Lim, U. Jeong, Adv. Mater. 25 (2013) 2707, doi:http://dx.doi.org/10.1002/adma.201300794. [63] M. Amjadi, K.U. Kyung, I. Park, M. Sitti, Adv. Funct. Mater. 26 (2016) 1678, doi: http://dx.doi.org/10.1002/adfm.201504755.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors Tae Ann Kima,1, Jun Beom Pyoa,1, Sang-Soo Leea , Min Parka,b,* a b
Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Seoul 02447, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 June 2019 Received in revised form 26 July 2019 Accepted 4 August 2019 Available online xxx
Developing materials that can absorb large amounts of strain without fracture or significant degradation in their electronic properties is essential for the realization of stretchable electronics. Here, stretchable conductors made from reduced graphene oxide/polydimethylsiloxane (r-GO/PDMS) composite were fabricated to achieve stable resistance changes under uniaxial stretching. By controlling the weight fraction of GO in water, highly aligned and porous GO structure was obtained over 0.5 wt% of GO solution. After thermal and chemical reduction of GO to r-GO film, PDMS was infiltrated into pores of the film to make free-standing and elastic composites. Electrical percolation of r-GO in the composite was achieved at the r-GO concentration over 0.3 wt% along with improvement of electrical conductivity. However, highly aligned r-GO/PDMS composite only exhibited notable suppression of resistance change under the large deformation of 100% strain. The resistance change of 0.3 wt% r-GO/PDMS composites at 100% strain was over 7 times higher than that of 0.7 wt% r-GO/PDMS composites, exhibiting the dramatic effect of rGO alignments. Even after repeated stretching/releasing cycles, the variation in resistance from 0 to 70% of strain was stable. The stretchable conductor based on the aligned r-GO/PDMS composite was successfully applied as a resistive strain sensor. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Reduced graphene oxide PDMS Self-assembly Conductive composite Stretchable conductors Strain sensors
Introduction Graphene, a 2D sp2 hybridized carbon allotrope, has been one of the well-studied nanofillers in nanocomposites for various applications due to its interesting properties including ballistic charge carrier mobility and exceptional mechanical properties [1–3]. Among different kinds of graphene, graphene oxide (GO), prepared by oxidizing graphite with a strong acid, has several advantages as nanofillers. First, the preparation of the material is a scalable solution process, deploying higher commercial values. Second, GOs have facile process-ability because they could be highly dispersive in polar solvents [4–7]. Third, graphene’s high electrical conductivity can be recovered by reducing GO via thermal, chemical, electrochemical or optical treatments [8,9]. Having such advantages, GO and reduced form of GO (r-GO) have been studied as nanofillers to create conductors [10–16], catalyst support [17,18], and more recently to develop stretchable
* Corresponding author at: Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea. E-mail address: [email protected] (M. Park). 1 These two authors made equal contributions.
conductors [19–21], an essential component in the next-generation soft electronic devices. One of the key factors that control the performance of nanocomposites is the nanoscale morphology of fillers [22–25]. When GO/r-GO is used as filler materials, its highly anisotropic dimensions from 2D planar shape play an important role in determining properties of the composite. The effect is amplified as graphene exhibits different in-plane and out-of-plane physical properties [26,27]. For instance, a composite employing randomly dispersed graphenes showed isotropic electrical conductivity [28]. Meanwhile, a composite employing aligned graphenes exhibited anisotropic conductivity of five orders of magnitude between the direction of alignment and its perpendicular [26]. Therefore, controlling the dispersion and orientation of graphene fillers is critical in maximizing the electrical conductivity of graphene nanocomposite. As a stretchable conductor, a composite material should exhibit little change in resistance upon applied strains. To achieve this property, several groups have demonstrated the importance of filler alignment in the composites under deformation. Kim et al. showed the stress-induced organization of conductive nanoparticles (NPs) in polymers along the stretching direction, which enables to retain good electron
https://doi.org/10.1016/j.jiec.2019.08.018 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
2
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Characterization
transport between NPs even at high strains [29]. Zhang et al. fabricated uniaxially aligned carbon nanotube (CNT) bundles, embedded in poly(dimethylsiloxane) (PDMS). The composite has stable electrical conductivity under a repetitive tensile strain up to 100% when the strain was applied parallel to the orientation of the CNTs [30]. These results imply that fine control of GO/r-GO assembly is important not only for achieving high conductivity in nanocomposites but also for maintaining conductive pathways under deformation. Geometrical engineering can further attain stable conductive pathways and enhance stretchability. For instance, intact graphenes lose their conducting behavior over 25% strain [31], while foam-like 3D graphene network exhibits a better resilience and a little change in resistance up to 50% of strain [32,33]. Perforation of composite films also improves stretchability without deteriorating their conductivity [34], since the holes in the film reduce the effective Young’s modulus and the yield strength of the film [35– 37]. To develop an ideal graphene composite for the stretchable conductor, a process that simultaneously aligns graphene and develops porous structures is required. Numerous approaches to fabricate self-assembled or porous 2D materials have been reported including Langmuir technique [38], electrospinning [39], phase separation [40,41], three-dimensional printing [42], and freeze-drying method [43,44]. However, few studies reported a process that creates aligned-porous GO or r-GO network and alignment effects of 2D nanofillers in composite under tensile strain. Here, we report a facile fabrication method to prepare highly aligned r-GO/PDMS composites as stretchable conductors, using the self-assembly behavior of anisotropic 2D materials and freeze-drying method. We characterized morphological changes of r-GO structures by varying the weight fraction of GO in solution and examined electromechanical properties of r-GO/PDMS composites under uniaxial tension. In addition, the resulted r-GO/PDMS composite was utilized as a resistive strain sensor.
To examine the morphology of GO, 0.1 wt% of GO aqueous dispersion was prepared and spin-coated on a piranha-treated silicon wafer. Then, the thickness and size distribution of GO were characterized by an atomic force microscope (AFM, MFP-3D, Asylum Research). The measurement of mean diameter and area of GO, conducted for more than 100 GOs, were performed with Image J software. The morphologies of the GOs, the r-GO structures, and the cross-section of r-GO/PDMS composites were observed using a FESEM (JSM-6701F, Jeol). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI 5000 VersaProbe (UlvacPHI) using monochromatic Al Kα radiation. X-ray diffraction measurements were performed by Rigaku Dmax2500 using Cu Kα radiation. Anisotropic characteristics of highly aligned r-GOs were examined by confocal Raman spectroscopy (Horiba LabRAM HR 3D) with visible laser excitation at 633 nm. Free-standing r-GOs were vertically and horizontally orientated from a glass substrate to obtain Raman spectra. The BET surface area of the r-GOs was measured by nitrogen adsorption experiments at 77 K on a Belsorp-Max (Bel Japan), after the sample was evacuated at room temperature for 12 h. The sheet resistance was measured using a four-point-probe measurement system (CRESBOX, Napson). A precise length-controlling system was used to apply tensile strain to characterize the change in electrical resistance under various tensile strains, measured by a potentiostat (Parstat 2273, Princeton Applied Research). To demonstrate the r-GO/PDMS composite as a strain sensor, it was encapsulated with an elastomeric adhesive film (VHB 4905, 3M) and attached to an elbow. The bending angle of the elbow was measured by Inertial Measurement Units (EBIMU-9DOFV3, E2BOX) with a receiver (EBIMU24GV3, E2BOX). The change in resistance while angular displacement was recorded by a digital multimeter (651, Keithley). For the resistance measurements, copper strips or wires were attached to each end of the r-GOs with conductive metals before infiltration with PDMS prepolymer to fabricate the electrodes with small contact resistance.
Experimental methods
Results and discussion
Fabrication of highly aligned r-GO structures and their composites
Preparation of r-GO porous films
GO was synthesized by a modified Hummers’ method [45]. Aqueous GO dispersions with various concentration of GO (0.1, 0.3, 0.5, 0.7 wt%) were prepared by sonication for less than 1 min and vigorous shaking. Highly aligned and porous GO structure was obtained in the following procedure: First, a glass substrate (piranha-treated) was immersed in a 3% aqueous solution of 3aminopropyltryethoxysilane (3-APTES, Sigma-Aldrich) for 12 h, followed by thorough rinsing with deionized water and drying with a nitrogen gun [45]. The GO dispersion was deposited on 3APTES modified glass substrates by the doctor-blade technique and quickly frozen by placing it on a liquid nitrogen cooled plate. The quenched samples were then moved to the freeze-dryer (FD-1000, Eyela) and sublimed at 50 C under 10 Pa for 12 h. Reduction of GOs was achieved in the hydrazine vapor environment at 200 C for 6 h, finally obtaining r-GO films.
We developed a facile preparation method to produce a highly aligned and porous r-GO film for a stretchable conductor as presented in Fig. 1. GO solution was first fabricated into a film by
Fabrication of a r-GO/PDMS resistive strain sensor Before infiltrating PDMS prepolymer into the r-GO film, metal wires were attached to the film with eutectic gallium-indium (EGaIn, Sigma-Aldrich) as electrical contacts. PDMS was infiltrated and cured as described previously. Acquired free-stranding r-GO/ PDMS composite was encapsulated with a stretchable and adhesive film (VHB 4905, 3M) before attaching the device to an elbow for better adhesion to the body.
Fig. 1. Fabrication process of r-GO/PDMS composite conductors.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
bar-coating on a cooled metal plate to immediately freeze the film as soon as GO was cast. Solvent from the frozen film was sublimated without destroying the aligned GO structure using a freeze-drying method. Prepared GO film was then reduced to recover the conductivity of graphene, which was finally infiltrated with PDMS to achieve stretchability. GOs, synthesized by a modified Hummer’s method, have 1 nm of thickness indicating single-layered sheets, 2.53 mm of mean diameter, and 20.1 mm2 of average area (Fig. S1). Sufficient shear force and adhesion/wetting between GO solution and a substrate are essential for uniform deposition of a dispersed GO film. The bar-coating method, which provides shear forces, was selected to cast the GO solution on a low-temperature glass substrate. Piranha-cleaned glass substrates were functionalized with 3aminopropyltryethoxysilane (3-APTES) to provide strong attraction with negatively charged GO [46], which was confirmed by the appearance of nitrogen (N) 1s peak on the XPS spectrum of the 3APTES treated glasses (Fig. S2). Although GO is advantageous in developing self-assembled and porous structures due to the high dispersibility in water, the resulting freeze-dried GO film should be reduced to restore electrical conductivity of graphene. Hydrazine is one of the most common reducing agents to convert GO to its reduced form [8]. By combining hydrazine with heat treatment, the sheet resistance of obtained r-GO can be significantly decreased [47,48]. To achieve highly conductive r-GO films, we optimized the hydrazine reduction condition by varying the temperature (50 C, 200 C) and the annealing time. High temperature (200 C) treatment under hydrazine can lower the sheet resistance of the samples by one order of magnitude compared to low temperature (50 C) treated ones (Fig. 2). Higher conductivity was achieved at longer exposure time for both conditions. However, the conductivity did not exhibit any notable changes after 12 h in the case of 200 C, which was chosen as an optimum annealing time. The reduction of GO was further confirmed by XPS and XRD. Fig. S3 shows the XPS survey scan and high-resolution C1s spectra of GO and r-GO. After thermal and chemical reduction, O1s peak significantly decreased indicating the removal of oxygen-containing functional groups from the GO. The high-resolution C1s spectra were de-convoluted into three peaks at 284.7 eV, 286.2 eV, and 289.0 eV, corresponding to sp2-hybridized carbon bond, carbon atoms directly bonded to oxygen, and carbonyl groups, respectively [49–51]. After reduction, the intensity ratio of each distinct peak was changed from 2.96:3.30:1 to 6.81:1.92:1, suggesting carboncarbon bonds were restored while oxygen functional groups were
Fig. 2. The change of sheet resistance as a function of annealing time under different temperatures.
3
removed. Fig. S4 exhibits the XRD patterns of GO and r-GO. The (001) peak of GO was observed at 2u = 10.6 , corresponding to an interlayer spacing (d) of 0.834 nm. After the reduction, the sharp peak at 10.6 completely disappeared and a broad peak centered at around 22.6 (d = 0.393 nm) was only observed. This value is similar to the d value (0.336 nm) of graphite, implying partial recovery of graphitic structure [52–54]. Internal structures of r-GO film and r-GO/PDMS composite Fig. 3a–c show the cross-sectional images of freeze-dried r-GO films. At low concentration (below 0.3 wt%), the r-GO sheets showed no directional order and only had interconnected structures. Above 0.5 wt%, observed were the highly aligned structures of r-GO, whose alignment planes were mostly parallel to the glass substrate. However, the surface morphologies were not affected by the concentration of r-GO. All r-GO films derived from GO solutions of 0.3, 0.5, 0.7 wt% seem to be well aligned parallel to substrates because the shearing orientation occurred during the casting process, which had affected only on the near-surface region (Fig. S5). The alignment of r-GOs was further confirmed by the polarized Raman spectroscopy. A significant intensity change in Raman signals has been observed in highly aligned carbon materials depending on the orientation of the sample with respect to the incident beam [26,55,56]. As shown in Fig. 3d–f, r-GO films exhibiting aligned structures (concentration above 0.5 wt%) from the SEM images, also had an anisotropic scattering behavior in the Raman spectra. At high graphene concentrations (0.5 wt% and 0.7 wt%), both the D (around 1320 cm 1) and the G (around 1610 cm 1) bands showed strong intensities when the incident light was parallel to the aligned direction (in-plane) while the scattering was significantly suppressed when the laser was perpendicular to the aligned direction (through the thickness). At low concentration (0.3 wt%), no significant changes in the scattering intensity were observed in terms of the sample orientation. We investigated the porous feature of freeze-dried r-GO films using nitrogen adsorption-desorption measurements. All the samples exhibited characteristic features of type-IV isotherms with a hysteresis loop, indicating the presence of large macropores and mesopores (Fig. 4). The formation of the hysteresis loop is involved in filling and emptying of mesopores, having widths in the range of 2–50 nm, by capillary condensation/evaporation [57–59]. The specific surface area (SSABET), mean diameters and size distribution of mesopores (dBJH) calculated by BET analysis and BJH method are summarized in Fig. S6. Both SSA and diameters exhibited a steady falling tendency as GO concentration was increased. Assuming r-GO sheets as rigid circular plates with a very thin thickness, the SSA was inversely proportional to their thickness, implying that r-GO sheets composing porous structures tended to be more aggregated and stacked at a higher concentration which resulted in a reduction of both SSA and pore diameters. The r-GO film was infiltrated with PDMS prepolymer, followed by heat-curing, to preserve the aligned r-GO morphology in composite and consequently increase mechanical durability and stretchability without loss of conductivity. After infiltration and curing, the cross-sectional morphologies of the composites were examined (Fig. 5). As shown in the SEM examinations, no voids were observed, evidencing successful infiltration of PDMS into the r-GO structures. Also, all the composites exhibited characteristic patterns, which were comparable with pristine r-GO structures, shown in the inset figures of Fig. 5. The composites derived from the r-GO with highly aligned structure also exhibited similar aligned patterns in their cross-sections. This implied that polymer infiltration did not alter the alignment of r-GO and retained the original r-GO structure. The r-GO/PDMS composites exhibited good
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
4
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Fig. 3. Cross-sectional morphologies of freeze-dried r-GO films: (a) 0.3 wt%, (b) 0.5 wt%, and (c) 0.7 wt% (Scale bar = 100 mm). Brown disks and a grey rectangle represent r-GOs and a substrate, respectively. Raman spectra of r-GO films in parallel and perpendicular orientations with respect to the incident laser: (d) 0.3 wt%, (e) 0.5 wt%, and (f) 0.7 wt% (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
stretchability due to the presence of rubbery PDMS matrix and moderate transparency due to the porous structures of r-GO (Fig. S7). The change in resistance under strain
Fig. 4. Nitrogen adsorption (ADS, closed circles)—desorption (DES, open circles) isotherms of freeze-dried r-GO films.
The fabricated stretchable r-GO/PDMS composites were employed as elastic electrical wirings, as shown in Fig. 6a. We measured the change in resistance under strain with those composites (Fig. 6b). The conductivity measurement samples prepared from 0.3, 0.5 and 0.7 wt% of GO solutions showed electrical conduction, while those from 0.1 wt% GO were not electro-conductive, evidencing that conductive percolation pathway could be developed from 0.3 wt% of GO solution. At every 10% strain, current–voltage (I–V) curves were measured to calculate the change in resistance. The current increased linearly with the applied voltage, indicating the presence of good ohmic contact between the composites and electrodes [60,61].
Fig. 5. Cross-sectional morphologies of r-GO/PDMS composites (a) 0.3 wt%, (b) 0.5 wt%, and (c) 0.7 wt% (Inset: SEM images of r-GO films before infiltration).
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
5
Fig. 6. (a) r-GO/PDMS composites used as stretchable conductors for lighting a LED lamp, (b) resistance of r-GO/PDMS composites made from GO solutions with different concentrations as a function of tensile strain and (c) the stability of the resistance of r-GO/PDMS (0.7 wt%) under multiple stretching/releasing cycles with 70% of applied strain.
The resistance of r-GO/PDMS composites with 0.3 wt% r-GO structures continuously increased with strain. At small deformation range, less than 30% of strain, the resistance increased by about 4 times of the initial resistance. The relatively small increase in resistance might result from the well-connected network structures of r-GO which can provide additional stretchability without breaking its conductive pathway. At higher deformation region, however, the resistance increased by 20 times of the starting value. When the composite was elongated to 100%, the conductivity was completely lost, implying that conductive networks of r-GO in the composite were significantly broken. Highly aligned r-GOs (0.5 wt%, 0.7 wt%) showed interesting electro-conductive behaviors when stretched. First, at 100% of strain, both of samples were still conductive and their resistances were much smaller than that of 0.3 wt% of r-GO samples. This suggests that highly aligned r-GOs developed more rigid conducting networks against uniaxial tension than randomly connected ones. Also, the resistance did not increase continuously as the strain built up but several plateau regions appeared where the resistance did not change at all. This can be related with the sliding effects between r-GO sheets similar with stretchable electrodes made of gold nanosheets [62]: Aligned r-GO can slide with each other under stretching while keeping their contact points so that their conductivity can be retained. Samples fabricated from 0.7 wt % of r-GO, having more aligned structures, exhibited 30% less resistance change compared to 0.5 wt% of r-GO. In addition, after 10% of strain, the resistance of 0.7 wt% samples only increased by up to 20% of previous resistance while that of 0.5 wt% increased up to 56%. Lastly, we tested the stability of our conductors under repeated stretching/releasing cycles with the composites of 0.7 wt % r-GO. The change of initial resistance and the resistance under 70% of strain was recorded at multiple cycles, shown in Fig. 6c. Even after fifty cycles, the initial resistance and the final resistance under 70% of strain was always recovered to the pristine resistance values from the first stretching/releasing.
Application to resistive strain sensors Based on the r-GO/PDMS (0.7 wt% r-GO) composite, we fabricated a resistive strain sensor, a device that requires an electrode that reversibly changes resistance upon repeated strains. The strain sensitivity (gauge factor, GF) of the composite, defined
Fig. 7. (a) Photos presenting movements of an arm with a r-GO/PDMS strain sensor and IMUs attached. (b) Summary of measured resistance by r-GO/PDMS strain sensor and the respective angles between IMUs due to arm movements.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
6
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
by the slope of the relative change of the resistance versus applied strain, is 3.6 0.2 with up to 70% of strain value. Conventional metal-foil based strain gauges have similar GFs in the range of 2–5 but they do not show high stretchability [63]. We attached the rGO/PDMS composite to an elbow to monitor applied strains by measuring resistance change during arm folding motion. Two inertial measurement units (IMUs) were attached to an arm to measure the angles in between the IMUs as shown in Fig. 7a to quantify the movements. By simultaneously measuring both resistance and angles during the movement of an arm, we were able to test the reliability of the fabricated sensor more accurately. It is notable that overlay comparison of the resistance and angle data were closely related as presented in Fig. 7b. In other words, the degree of arm movements can be deduced by measuring the resistance. We strongly believe that an aligned r-GO morphology would endure repeated strains by reversibly sliding and recovering the conductive path when stress is released. Conclusions We prepared stretchable and conductive PDMS composites with highly aligned and porous r-GO as filler. Due to the concentrationdependent self-assembly property of GOs, freeze-dried GO solutions containing more than 0.5 wt% of GO exhibited highly aligned structures. The combined thermal and chemical reduction of GO under hydrazine vapor gave us enhanced conductivity compared to hydrazine reduction at low temperature. The samples fabricated from highly aligned r-GO exhibited unique conducting behavior when the strain was applied. The variation in resistance sharply decreased compared to those from randomly connected structures. Also, they had step-like resistance changes due to the sliding effect of r-GO sheets. Even though the resistance increased by about 4 times the starting value, stable variation was observed up to 50 times of stretching/releasing cycles. The prepared r-GO films were applied to strain sensors to demonstrate their potential application for soft electronics. The alignment of disk-shaped fillers with our unique methods can be applied to other anisotropic fillers which require the stable variation in performance under stretching. Acknowledgment This work was financially supported by the internal grant of Korea Institute of Science and Technology (KIST). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jiec.2019.08.018. References [1] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183, doi:http://dx.doi.org/ 10.1038/nmat1849. [2] C. Lee, X. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385, doi:http://dx.doi.org/ 10.1126/science.1157996. [3] W.-C. Tan, W.-H. Shih, Y.F. Chen, Adv. Funct. Mater. 24 (2014) 6818, doi:http:// dx.doi.org/10.1002/adfm.201401421. [4] Y. Xu, W. Hong, H. Bai, C. Li, G. Shi, Carbon 47 (2009) 3538, doi:http://dx.doi. org/10.1016/j.carbon.2009.08.022. [5] N.V. Medhekar, A. Ramasubramaniam, R.S. Ruoff, V.B. Shenoy, ACS Nano 4 (2010) 2300, doi:http://dx.doi.org/10.1021/nn901934u. [6] J. Kim, L.J. Cote, F. Kim, W. Yuan, K.R. Shull, J.X. Huang, J. Am. Chem. Soc. 132 (2010) 8180, doi:http://dx.doi.org/10.1021/Ja102777p. [7] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101, doi:http://dx.doi.org/10.1038/nnano.2007.451. [8] S. Stankovich, D.A. Dikin, R.D. Piner, Ka. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558, doi:http://dx.doi.org/10.1016/j. carbon.2007.02.034. [9] T. Kuila, A.K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Nanoscale 5 (2013) 52, doi: http://dx.doi.org/10.1039/c2nr32703a.
[10] H. Chen, M.B. Müller, K.J. Gilmore, G.G. Wallace, D. Li, Adv. Mater. 20 (2008) 3557, doi:http://dx.doi.org/10.1002/adma.200800757. [11] T. Zhou, F. Chen, C. Tang, H. Bai, Q. Zhang, H. Deng, Q. Fu, Compos. Sci. Technol. 71 (2011) 1266, doi:http://dx.doi.org/10.1016/j.compscitech.2011.04.016. [12] H. Wu, L.T. Drzal, Mater. Chem. Phys. 182 (2016) 110, doi:http://dx.doi.org/ 10.1016/j.matchemphys.2016.07.011. [13] H. Yang, X.F. Yao, Z. Zheng, L.H. Gong, L. Yuan, Y.N. Yuan, Y.H. Liu, Compos. Sci. Technol.167 (2018) 371, doi:http://dx.doi.org/10.1016/j.compscitech.2018.08.022. [14] J. Phiri, L.S. Johansson, P. Gane, T. Maloney, Compos. B: Eng. 147 (2018) 104, doi: http://dx.doi.org/10.1016/j.compositesb.2018.04.018. [15] J.W. Jo, J.U. Lee, W.H. Jo, Polym. Int. 64 (2015) 1676, doi:http://dx.doi.org/ 10.1002/pi.4981. [16] J. Liu, H.J. Choi, L.Y. Meng, J. Ind. Eng. Chem. 64 (2018) 1, doi:http://dx.doi.org/ 10.1016/j.jiec.2018.02.021. [17] X.Y. Zhu, Z.S. Lv, J.J. Feng, P.X. Yuan, L. Zhang, J.R. Chen, A.J. Wang, J. Colloid Interface Sci. 516 (2018) 355, doi:http://dx.doi.org/10.1016/j.jcis.2018.01.047. [18] H. Bin Meng, X.F. Zhang, Y.L. Pu, X.L. Chen, J.J. Feng, D.M. Han, A.J. Wang, J. Colloid Interface Sci. 543 (2019) 17, doi:http://dx.doi.org/10.1016/j.jcis.2019.01.110. [19] M. Chen, L. Zhang, S. Duan, S. Jing, H. Jiang, C. Li, Adv. Funct. Mater. 24 (2014) 7548, doi:http://dx.doi.org/10.1002/adfm.201401886. [20] C. Feng, Z. Yi, L.F. Dumée, F. She, Z. Peng, W. Gao, L. Kong, Carbon 139 (2018) 672, doi:http://dx.doi.org/10.1016/j.carbon.2018.07.016. [21] X. Wang, S. Meng, M. Tebyetekerwa, Y. Li, J. Pionteck, B. Sun, Z. Qin, M. Zhu, Compos. A: Appl. Sci. Manuf. 105 (2018) 291, doi:http://dx.doi.org/10.1016/j. compositesa.2017.11.027. [22] S.H. Xie, Y.Y. Liu, J.Y. Li, Appl. Phys. Lett. 92 (2008), doi:http://dx.doi.org/ 10.1063/1.2949074. [23] T. Ramanathan, F.T. Fisher, R.S. Ruoff, L.C. Brinson, Chem. Mater. 17 (2005) 1290, doi:http://dx.doi.org/10.1021/cm048357f. [24] Y.J. Kwon, Y. Kim, H. Jeon, S. Cho, W. Lee, J.U. Lee, Compos. B: Eng. 122 (2017) 23, doi:http://dx.doi.org/10.1016/j.compositesb.2017.04.005. [25] G. Mittal, V. Dhand, K.Y. Rhee, S.J. Park, W.R. Lee, J. Ind. Eng. Chem. 21 (2015) 11, doi:http://dx.doi.org/10.1016/j.jiec.2014.03.022. [26] N. Yousefi, X. Lin, Q. Zheng, X. Shen, J.R. Pothnis, J. Jia, E. Zussman, J.K. Kim, Carbon 59 (2013) 406, doi:http://dx.doi.org/10.1016/j.carbon.2013.03.034. [27] N. Yousefi, X. Sun, X. Lin, X. Shen, J. Jia, B. Zhang, B. Tang, M. Chan, J.-K. Kim, Adv. Mater. 26 (2014) 5480, doi:http://dx.doi.org/10.1002/adma.201305293. [28] Y. Li, Y.A. Samad, K. Polychronopoulou, S.M. Alhassan, K. Liao, Sci. Rep. 4 (2015) 1, doi:http://dx.doi.org/10.1038/srep04652. [29] Y. Kim, J. Zhu, B. Yeom, M. Di Prima, X. Su, J.G. Kim, S.J. Yoo, C. Uher, N.A. Kotov, Nature 500 (2013) 59, doi:http://dx.doi.org/10.1038/nature12401. [30] Y. Zhang, C.J. Sheehan, J. Zhai, G. Zou, H. Luo, J. Xiong, Y.T. Zhu, Q.X. Jia, Adv. Mater. 22 (2010) 3027, doi:http://dx.doi.org/10.1002/adma.200904426. [31] T. Shao, B. Wen, R. Melnik, S. Yao, Y. Kawazoe, Y. Tian, J. Chem. Phys. 137 (2012), doi:http://dx.doi.org/10.1063/1.4766203. [32] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.-M. Cheng, Nat. Mater. 10 (2011) 424, doi: http://dx.doi.org/10.1038/nmat3001. [33] Z. Wang, X. Liu, X. Shen, N.M. Han, Y. Wu, Q. Zheng, J. Jia, N. Wang, J.K. Kim, Adv. Funct. Mater. 28 (2018) 1, doi:http://dx.doi.org/10.1002/adfm.201707043. [34] T. Sekitani, Y. Noguchi, K. Hata, T. Fukushima, T. Aida, T. Someya, Science 321 (2008) 1468, doi:http://dx.doi.org/10.1126/science.1160309. [35] K.Y. Chun, Y. Oh, J. Rho, J.H. Ahn, Y.J. Kim, H.R. Choi, S. Baik, Nat. Nanotechnol. 5 (2010) 853, doi:http://dx.doi.org/10.1038/nnano.2010.232. [36] Á. Kovács, A. Kovács, U. Mescheder, Comput. Mater. Sci. 43 (2008) 59, doi: http://dx.doi.org/10.1016/j.commatsci.2007.07.023. [37] C. Van Rijn, M. Van Der Wekken, W. Nijdam, M. Elwenspoek, J. Microelectromechanical Syst. 6 (1997) 48, doi:http://dx.doi.org/10.1109/84.557530. [38] X. Luo, K. Ma, T. Jiao, R. Xing, L. Zhang, J. Zhou, B. Li, Nanoscale Res. Lett. 12 (2017), doi:http://dx.doi.org/10.1186/s11671-017-1864-8. [39] X. Huang, R. Wang, T. Jiao, G. Zou, F. Zhan, J. Yin, L. Zhang, J. Zhou, Q. Peng, ACS Omega 4 (2019) 1897, doi:http://dx.doi.org/10.1021/acsomega.8b03615. [40] C. Schugens, V. Maquet, C. Grandfils, R. Jerome, P. Teyssie, Polymer (Guildf) 37 (1996) 1027, doi:http://dx.doi.org/10.1016/0032-3861(96)87287-9. [41] C. Schugens, V. Maquet, C. Grandfils, R. Jerome, P. Teyssie, J. Biomed. Mater. Res. 30 (1996) 449, doi:http://dx.doi.org/10.1002/(Sici)1097-4636(199604) 30:4<449::Aid-Jbm3>3.0.Co;2-P. [42] H. Seyednejad, D. Gawlitta, R.V. Kuiper, A. de Bruin, C.F. van Nostrum, T. Vermonden, W.J.A. Dhert, W.E. Hennink, Biomaterials 33 (2012) 4309, doi: http://dx.doi.org/10.1016/j.biomaterials.2012.03.002. [43] R. Xing, W. Wang, T. Jiao, K. Ma, Q. Zhang, W. Hong, H. Qiu, J. Zhou, L. Zhang, Q. Peng, ACS Sustain. Chem. Eng. 5 (2017) 4948, doi:http://dx.doi.org/10.1021/ acssuschemeng.7b00343. [44] R. Guo, T. Jiao, R. Li, Y. Chen, W. Guo, L. Zhang, J. Zhou, Q. Zhang, Q. Peng, ACS Sustain. Chem. Eng. 6 (2018) 1279, doi:http://dx.doi.org/10.1021/acssuschemeng.7b03635. [45] J. Zhao, S. Pei, W. Ren, L. Gao, H.-M. Cheng, ACS Nano 4 (2010) 5245, doi:http:// dx.doi.org/10.1021/nn1015506. [46] Q. He, H.G. Sudibya, Z. Yin, S. Wu, H. Li, F. Boey, W. Huang, P. Chen, H. Zhang, ACS Nano 4 (2010) 3201, doi:http://dx.doi.org/10.1021/nn100780v. [47] V.H. Pham, T.V. Cuong, S.H. Hur, E.W. Shin, J.S. Kim, J.S. Chung, E.J. Kim, Carbon 48 (2010) 1945, doi:http://dx.doi.org/10.1016/j.carbon.2010.01.062. [48] P.G. Ren, D.X. Yan, X. Ji, T. Chen, Z.M. Li, Nanotechnology 22 (2011), doi:http:// dx.doi.org/10.1088/0957-4484/22/5/055705. [49] L.G. Guex, B. Sacchi, K.F. Peuvot, R.L. Andersson, A.M. Pourrahimi, V. Ström, S. Farris, R.T. Olsson, Nanoscale 9 (2017) 9562, doi:http://dx.doi.org/10.1039/ c7nr02943h.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018
G Model JIEC 4712 No. of Pages 7
T.A. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx [50] Y.C. Shi, A.J. Wang, X.L. Wu, J.R. Chen, J.J. Feng, J. Colloid Interface Sci. 484 (2016) 254, doi:http://dx.doi.org/10.1016/j.jcis.2016.09.008. [51] Y.C. Shi, J.J. Feng, X.X. Lin, L. Zhang, J. Yuan, Q.L. Zhang, A.J. Wang, Electrochim. Acta 293 (2019) 504, doi:http://dx.doi.org/10.1016/j.electacta.2018.10.068. [52] C. Hu, X. Zhai, L. Liu, Y. Zhao, L. Jiang, L. Qu, Sci. Rep. 3 (2013) 1, doi:http://dx. doi.org/10.1038/srep02065. [53] J.H. Cheng, Y.H. Chen, Y.S. Yeh, S. Hy, L.Y. Kuo, B.J. Hwang, Electrochim. Acta 197 (2016) 146, doi:http://dx.doi.org/10.1016/j.electacta.2015.12.116. [54] H. Ham, T. Van Khai, N.H. Park, D.S. So, J.W. Lee, H.G. Na, Y.J. Kwon, H.Y. Cho, H. W. Kim, Nanotechnology 25 (2014), doi:http://dx.doi.org/10.1088/0957-4484/ 25/23/235601. [55] N. Yousefi, M.M. Gudarzi, Q. Zheng, S.H. Aboutalebi, F. Sharif, J.-K. Kim, J. Mater. Chem. 22 (2012) 12709, doi:http://dx.doi.org/10.1039/c2jm30590a. [56] B. Kwon, J. Park, W. Jeong, G. Jeong, H.S. Ryu, P. Paoprasert, S.Y. Park, I. In, Carbon Lett. 27 (2018) 26, doi:http://dx.doi.org/10.5714/CL.2018.27.026.
7
[57] K.S.W. Sing, J. Porous Mater. 2 (1995) 5, doi:http://dx.doi.org/10.1007/ BF00486564. [58] P. Chang, Z. Qin, Carbon Lett. 25 (2018) 117, doi:http://dx.doi.org/10.5714/ CL.2018.25.117. [59] Y.W. You, E.H. Moon, I. Heo, H. Park, J.S. Hong, J.K. Suh, J. Ind. Eng. Chem. 45 (2017) 164, doi:http://dx.doi.org/10.1016/j.jiec.2016.09.019. [60] T.A. Kim, S.-S. Lee, H. Kim, M. Park, RSC Adv. 2 (2012) 10717, doi:http://dx.doi. org/10.1039/c2ra20792c. [61] Y. Kim, Y.J. Kwon, J.Y. Hong, M. Park, C.J. Lee, J.U. Lee, J. Ind. Eng. Chem. 68 (2018) 399, doi:http://dx.doi.org/10.1016/j.jiec.2018.08.015. [62] G.D. Moon, G.-H. Lim, J.H. Song, M. Shin, T. Yu, B. Lim, U. Jeong, Adv. Mater. 25 (2013) 2707, doi:http://dx.doi.org/10.1002/adma.201300794. [63] M. Amjadi, K.U. Kyung, I. Park, M. Sitti, Adv. Funct. Mater. 26 (2016) 1678, doi: http://dx.doi.org/10.1002/adfm.201504755.
Please cite this article in press as: T.A. Kim, et al., Highly aligned and porous reduced graphene oxide structures and their application for stretchable conductors, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.018