Ultrastrong Graphene–Copper Core–Shell Wires ... - ACS Publications

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Ultrastrong Graphene−Copper Core−Shell Wires for High-Performance Electrical Cables Sang Jin Kim,† Dong Heon Shin,†,‡ Yong Seok Choi,‡ Hokyun Rho,† Min Park,† Byung Joon Moon,† Youngsoo Kim,§ Seuoung-Ki Lee,† Dong Su Lee,† Tae-Wook Kim,†,∥ Sang Hyun Lee,†,∥ Keun Soo Kim,⊥ Byung Hee Hong,*,‡ and Sukang Bae*,† †

Applied Quantum Composites Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Jeollabuk-do 55324, Republic of Korea ‡ Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea ∥ Department of Nanomaterials and Nano Science, Korea University of Science and Technology (UST), Daejeon 34113, Republic of Korea § Graphene Square Inc., Inter-university Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea ⊥ Department of Physics & Astronomy, Sejong University, Seoul 05006, Republic of Korea S Supporting Information *

ABSTRACT: Recent development in mobile electronic devices and electric vehicles requires electrical wires with reduced weight as well as enhanced stability. In addition, since electric energy is mostly generated from power plants located far from its consuming places, mechanically stronger and higher electric power transmission cables are strongly demanded. However, there has been no alternative materials that can practically replace copper materials. Here, we report a method to prepare ultrastrong graphene fibers (GFs)−Cu core−shell wires with significantly enhanced electrical and mechanical properties. The core GFs are synthesized by chemical vapor deposition, followed by electroplating of Cu shells, where the large surface area of GFs in contact with Cu maximizes the mechanical toughness of the core−shell wires. At the same time, the unique electrical and thermal characteristics of graphene allow a ∼10 times higher current density limit, providing more efficient and reliable delivery of electrical energies through the GFs−Cu wires. We believe that our results would be useful to overcome the current limit in electrical wires and cables for lightweight, energy-saving, and high-power applications. KEYWORDS: graphene fibers, copper, electroplating, tensile strength, ampacity (maximum current density) ultimate tensile strength as high as ∼475 MPa, which is 2.6 times higher than that of commercial Cu wires. The maximum current density of the composite material is measured to be ∼1.0 × 106 A/cm2, which is 10 times and 100 times higher than those of commercial Cu wire and pristine GFs, respectively.

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opper (Cu) has many advantages such as outstanding electrical properties and low cost, which are being used universally in electrical device applications. However, the limit in electrical and mechanical performance of Cu hinders its applications to lightweight high-power cables useful for electrical vehicles, motors for drones, etc.1 Recently, metal and carbon-based composite materials have received significant attention as alternative wire materials. For example, carbon nanotubes and graphene-based materials were used to enhance the performance of electrical wires.2−10 In particular, the fiber types of graphene oxides (GOs) synthesized via chemical approaches were explored to enhance the mechanical property.11,12 However, the conductivity of the GO-based fibers was not as good as that of chemical vapor deposition (CVD) graphene fibers.13−15 Here, we report a method to prepare ultrastrong CVD graphene fibers (GFs) electroplated with Cu (EP Cu). The resulting GFs−Cu wires show excellent © 2018 American Chemical Society

RESULTS AND DISCUSSION Figure 1 illustrates the schematic fabrication process of graphene fiber and copper core−shell wires. Graphene on nickel (Ni) films, which were synthesized using the CVD method, consisted of ∼10 layers and had a uniform electrical distribution (Figure S1). The samples were line-patterned using photolithography and oxygen plasma treatment (Figure 1b). Received: January 3, 2018 Accepted: March 6, 2018 Published: March 6, 2018 2803

DOI: 10.1021/acsnano.8b00043 ACS Nano 2018, 12, 2803−2808

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Figure 1. Schematic fabrication of graphene fibers (GFs)−Cu core−shell wires. (a) Graphene growth on Ni films (300 nm) using the CVD method. (b) Patterning of graphene using photolithography and oxygen plasma treatment. (c) The graphene sheets are rolled and shrunk in a hydrophilic solution during the Ni film etching process. (d) Extraction of graphene fiber using a roller. (e) Electroplating of copper on a graphene fiber surface. (f) Wire consisting of core GFs and shell electroplated Cu.

Figure 2. Structural changes of CVD-GFs. (a−c) SEM image of a single knotted CVD-GF. (a) Loosely knotted, (b) slightly loosely knotted, and (c) tightly knotted GFs. (scale bar: 50 μm). (d) Large-scale GFs compared with a commercial 15 μm Cu wire. (e) Weaving GFs into fabrics. The inset shows an infrared scan of GFs in fabrics attached to the surface of a bottle, while applying microwaves for 5 s. (f) Demonstration of LEDs connected via GFs (operating voltage, 9 V). These results indicate that GFs are mechanically strong and electrically conducting.

Figure 2 shows that graphene fibers can maintain their structure without experiencing failure with the application of bending or deforming forces. GFs were tightly pulled into a knot, and no significant damage or defects were observed (Figure 2a−c). The extracted very long GFs were simply attached to a metal frame or stitched into a fabric (Figure 2d,e). The inset of Figure 2e shows IR scanning of GFs in fabrics attached to the surface of a bottle irradiated with an electromagnetic (EM) wave at 2.45 GHz.17−19 The heat is generated on GFs via Joule heating, and diamagnetism effects were accumulated. The relative maximum temperature of the GFs was approximately 78 °C during the 5 s EM process. In addition, a light-emitting diode (LED) connected using GFs

Then, the samples were soaked in Ni etchant and EtOH cosolvent to etch the Ni films (Figure 1c and Figure S2a). Separated graphene sheets from bare substrates were scrolled up and shrunk in solution (Figure 1d and Figure S2b).16 Scrolled graphene sheets were extracted using a roller to change the structure of graphene from sheet to monolithic fiber type (Figure 1e and Figure S2c), and the diameters of GF can be controlled by varying the width of graphene stripes or by twisting (Figure S3). After several rinsing cycles with distilled water and EtOH solution, the samples were dried in air for 2 h to remove the moisture (Figure S2d). To obtain the GF and Cu core−shell wires, GFs were electroplated in a CuSO4 solution for 100−600 s (Figure 1f). 2804

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Figure 3. Time dependence of electroplated Cu on CVD-GFs. (a) Graphene fibers, (b) 100 s electroplated Cu, (c) 200 s electroplated Cu, (d) 300 s electroplated Cu, (e) 400 s electroplated Cu, and (f) 500 s electroplated Cu on CVD-GFs, showing a change of Cu particles into Cu thin films on the GF surface with an increase in electroplating time (scale bar: 50 μm, the scale bar of the inset is described in each image).

Figure 4. Mechanical properties of graphene fibers and electroplated Cu−GFs. (a) Electrical resistance variation of GFs at a bend radius up to 11.5 mm. The inset shows the resistance change of GFs as a function of bending cycles. (b) Ashby plot of maximum tensile strength versus strain curves for various graphene-based fiber materials, which yielded a tensile strength of EP Cu−GFs that is more than 2.5 times greater than that of commercial Cu. (c) Cross-section SEM images of 500 s EP Cu−GFs, showing the core (GFs)−shell (Cu) structure with embedded Cu particles in GFs.

was turned on at 9 V. This demonstrates that the GFs have an extremely high mechanical and electrical stability (Figure 2f). The surface of GFs was treated using an electroplating method with a CuSO4 solution to fabricate core−shell wires with various electroplating times (Figure 3). Figure 3b and c show that Cu nanoparticles were formed on the surface of 100 and 200 s electroplated GFs. However, in the case of 300 s and

further electroplated GF samples, Cu particles were connected to the surface and formed a uniform film structure (Figure 3d− f). We optimized the electroplating process to study the surface properties and chemical composition using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Pristine GFs and 100 s Cu−GFs showed clear Raman spectral peaks of graphene at 1580 cm−1 (G band) and 2700 cm−1 (2D 2805

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Figure 5. Electrical and thermal properties of electroplated Cu−GFs. (a) Variation of conductivity with Cu electroplating time, showing enhancement of electrical conductivity with Cu electroplating time. (b) Variation of ampacity with Cu electroplating time, showing that 500 s Cu electroplated GFs have a higher ampacity value compared to other times of Cu electroplating and commercial Cu. Electrothermal (or Joule) heating effect in EP Cu−GFs induced by high current density shown in the inset image. (c) Change of temperature difference between the near middle (highest temperature) of fibers and the end of the fibers with input power. (d) Thermal conductivities of various fibers.

from cross-section SEM images (Figure 4c). The tightly bound core−shell wires show a higher ultimate tensile strength because the core-GFs supported the shell-Cu well when tensile stress was applied. In addition, a higher mechanical strength and a slightly lower strain of EP Cu−GFs originate from high resistance of graphene against deformation due to the strong interaction between tightly stacked layers of CVD graphene and the polycrystalline structure of Cu. The conductivities of EP Cu−GFs were increased to 3.7 × 105 S cm−1 at ambient conditions, as measured using the fourprobe method, which yielded an electrical property of EP Cu− GFs that was 2 orders higher than that of pristine GFs (∼3.5 × 103 S cm−1) with an increment of Cu volume fraction (Figure 5a). The electrical conductivity of 3.7 × 105 S cm−1 for 500 s EP Cu−GFs is similar to 2.8 × 105 S cm−1 for the commercial 50 μm Cu wire. In the case of conversion on conductivity per weight value, the graphene-only fiber (before electroplating) shows the conductivity/weight values of 1.1 × 107 S/(cm·mg), and the graphene fibers electroplated with Cu for 200−500 s exhibit (3.1−4.4) × 106 S/(cm·mg), which are considerably higher than commercially available Cu wires (∼1.0 × 106 S/ (cm·mg)), as shown in Figure S7. The ampacity (maximum current density) values of EP Cu−GFs were enhanced to ∼1.0 × 106 A/cm2 with an increase in electroplating time and were approximately 10 times higher than that of commercial Cu wire (Figures 5b and S7). The current density values of 100 and 200 s EP Cu−GFs are lower than that of commercial Cu due to the particle structure of Cu on GFs, which has an insufficient effect

band). For the further electroplated Cu−GFs samples, complete coverage of Cu films on the surface of GFs was confirmed by the absence of G and a 2D band with only the Cu fluorescence background being detected (Figure S4a). Figure S4b and c show the X-ray photoelectron spectroscopy spectra of 500 s Cu−GFs, which confirm the existence of C 1s (sp2hybridized carbon orbital) and Cu 2p (Cu 2p3/2 and Cu 2p1/2 bonding states). Thus, Cu films cover the GFs’ surface well during the electroplating process. The atomic force microscope (AFM) image was taken to confirm the surface of EP Cu−GFs (Figure S4d). The grain sizes of resulting Cu were roughly measured by electron backscatter diffraction (EBSD), which are smaller than a few micrometers (Figure S4e and f). The flexibility of GFs on the PET substrate was demonstrated by measuring resistances with respect to bending radii (Figure 4a). Little variation in resistances is confirmed up to the bending radius of 11 mm, and resistance values are almost recovered after the release. The initial resistance can be recovered even for the bending radius of 11.5 mm and cycle test. Figures 4b and S5 show the mechanical properties of EP Cu−GFs that were reinforced with electroplating treatment. The ultimate tensile strength of 500 s EP Cu−GFs is approximately 475 MPa, which is higher than that of commercial Cu wire, graphene, and GO-based fibers.20−25 GO-based materials show a relatively higher strain and easily slide between small grains during mechanical deformation.19 However, the mechanical strengthening of EP Cu−GFs is associated with a core−shell structure, which can be confirmed 2806

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extracted from solution using a sharp syringe and dried at atmospheric conditions for 1 h to remove moisture. Electroplating Process. During the electroplating process, the electrolytic bath was 200 g/L copper sulfate and was adjusted to pH ∼1 using H2SO4. The cathode was made of graphene fibers, and the anode was made of copper balls in a titanium basket; a constant 1.5 A/ dm2 was applied. Characterization. The Raman spectra were obtained using a Raman spectrometer (Invia-Reflex, Renishaw, 514 nm). The optical transmittance of the graphene sheets was measured using a UV−vis/ NIR spectrophotometer (V-670, Jasco). The sheet resistance was measured using a four-point probe resistivity meter (FPP-RS8, CTI Korea). XPS analyses were carried out using a Thermo Scientific KAlpha (small-spot X-ray photoelectron spectrometer system). SEM images and cross-section images were acquired using a Nova NanoSEM 450 (FEI) and Helios NanoLab 650 (FEI), respectively. The AFM image was measured by a noncontact mode (Park System, XE-100). Conductivity was measured using a nanovoltmeter (Keithley 2182A), and ampacity was measured using a source meter (Keithley 2440) at atmospheric conditions. Tensile strength was measured using an automatic single-fiber test system (Favimat+, Textechno). Thermal conductivities were measured using a thermal emission microscope (Themos mini C10614-02, Hamamatsu) and source meter (Keithley 2440) at vacuum conditions.

on the wire structures. After 300 s of electroplating time, the well-covered film structure of Cu on GFs shows enhanced current density values. We believe that GFs contributed to the higher current flow through the core−shell wires due to its outstanding charge mobility and thermal conductance while maximum current was applied.26−30 In other words, the GFs spread the heat more effectively, thereby increasing the maximum current value of EP Cu−GFs, suppressing an increase in electrical resistance due to heat generation by the Joule-heating effect.31−35 In addition, we measured thermal conductivity on GFs, EP Cu−GFs, and commercial Cu wire to confirm the heat spreading effect of GFs. The average thermal conductivity of GFs, commercial Cu, and EP Cu−GFs is 1236, 166, and 354 W m−1 K−1, respectively, implying that the enhancement in the thermal conductivity of EP Cu−GFs is attributed to the efficient heat spreading by GFs (Figures 5c,d and S8).36 The mechanical and electrical properties of graphene and Cu are synergistically combined on the resulting wires. Graphene is a densely packed material that is less defective compared with loosely bonded metal.37 In addition, graphene has a better electrical property, which provides a much higher ampacity. We can also expect a much enhanced maximum current density of nanoscale graphene−metal hybrid materials using further study on the fine pattern fabrication process of graphene and metal hybrid structures such as 1D nanostructures.38

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b00043. Detailed information on the optical and electrical properties of graphene films, extraction of CVD-GFs method, controlled diameters of graphene fibers, surface analysis of GFs and Cu-electroplated GFs, mechanical, electrical, and thermal properties of electroplated Cu− GFs (PDF)

CONCLUSION We have demonstrated an approach to fabricate CVD graphene fibers−metal core−shell wires using electroplating methods that can be utilized to enhance the allowable electrical current and mechanical strength. The electrical properties of large-scale GFs and Cu core−shell wires are controlled by changing the shape of CVD graphene and the Cu electroplating time. Using graphene fibers as an electric wire during the electroplating process, Cu can be uniformly electroplated on a graphene surface to create a homogeneous Cu−GFs core−shell wire. The mechanical strength of the graphene-based core−shell wires was enhanced due to the compactly bound structure of GFs and polycrystalline Cu. In addition, the ampacity of the resulting EP Cu−GFs composite is ∼10 times higher than that of commercial Cu wires. Thus, we believe that our method of preparing graphene−Cu core−shell wires and electroplated metal composites will be very useful for overcoming the current limitation of electrical wires for various lightweight and highpower electrical applications.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Tae-Wook Kim: 0000-0003-2157-732X Sang Hyun Lee: 0000-0002-7784-5939 Byung Hee Hong: 0000-0001-8355-8875 Notes

The authors declare no competing financial interest.

METHODS ACKNOWLEDGMENTS This work was supported by the Korea Institute of Science and Technology (KIST) Institutional Program and Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science (2017M3A7B4049167), ICT, and Future Planning (2016M3A7B4910458) and the Industrial Strategic Technology Development Program (10079969, 10079974) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). K.S.K. acknowledges the Priority Research Centers Program (Grant 2010-0020207) by the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.

Graphene Synthesis and Patterning. Graphene sheets were synthesized on high-purity nickel films (300 nm, 99.999%)/SiO2/Si substrates via the atmospheric pressure chemical vapor deposition (APCVD) method with argon (1000 sccm), H2 (350 sccm), and CH4 (250 sccm) gases at 970 °C. After coating a photoresist (AZ5214) on the surface of the as-grown sample, the synthesized graphene sheets were line-patterned using photolithography and oxygen plasma (20 mTorr, 15 s). The sample was soaked in acetone to remove the photoresist layer. Graphene Fiber Fabrication. Without any polymer coating, linepatterned graphene sheets on Ni films were dipped in a mixture of H2SO4-based nickel etchant and EtOH. Ni films were etched using the etchant, and graphene sheets were separated from bare silicon substrates during the etching process. Separated graphene sheets were rolled and changed into the fiber structure. Graphene fibers were 2807

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DOI: 10.1021/acsnano.8b00043 ACS Nano 2018, 12, 2803−2808