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Catalyst and doping methods for arc graphene
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 445601 (http://iopscience.iop.org/0957-4484/25/44/445601) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 445601 (7pp)
doi:10.1088/0957-4484/25/44/445601
Catalyst and doping methods for arc graphene Hyunjin Cho1,6, InSeoup Oh1,6, JungHo Kang1, Sungchan Park1, Boncheol Ku2, Min Park1, Soonjong Kwak3, Partha Khanra4, Joong Hee Lee4 and Myung Jong Kim1,5 1
Soft Innovative Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeollabuk-do, 565-905, Korea 2 Carbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeollabuk-do, 565-905, Korea 3 Photoelectronic Hybrid Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Wolsong-gil 5, Seongbuk-gu, Seoul, 136-791, Korea 4 Department of BIN Fusion Technology, Chonbuk National University, 567, Baekjedaero, Deokjin-gu, Jeonju-si, Jeollabuk-do, 561-756, Korea 5 Nanomaterials Science and Engineering, Korea University of Science and Technology (UST), Daejeon 305-350, South Korea E-mail: [email protected] Received 11 August 2014 Accepted for publication 8 September 2014 Published 16 October 2014 Abstract
Nitrogen-doped graphene synthesis with ∼g scale has been accomplished using the arc discharge method. The defects formed in the synthesis process were reduced by adding various metal catalysts, among which Bi2O3 was found to be the most effective. Adding dopants to the starting materials increased the electrical conductivity of the graphene product, and the doping concentration in graphene was tuned by adjusting the amount of nitrogen dopants. A step-wise technique to fabricate graphene thin films was developed, including dispersion, separation, and filtering processes. The arc graphene can also find its potential application in supercapacitors, taking advantage of its large surface area and improved conductivity by doping. Keywords: arc discharge, graphene, catalyst, doping (Some figures may appear in colour only in the online journal) Introduction
synthetic methods used to produce large amounts of highquality graphene, however, are in high demand in order to bring this new material into practical applications. The arc discharge method was successfully adopted to produce highly crystalline, SWNTs for transparent conductive films [5]. High growth temperature and the role of catalysts were the key factors required to produce nearly defect-free SWNTs in the arcdischarge method [3]. On the other hand, arc discharge for graphene is in its infancy compared to that of CNTs. Undoped and doped arc-graphene-synthesis methods have been previously reported. Arc graphene was grown by vaporizing a graphite target in different gas environments such as H2 [6], He/H2 [7], CO2 [8], N2 [9], air [10] and NH3 [11]. As an alternative, arc discharge was used in order to acquire
The technique to vaporize elemental carbon to form nanostructures opened a new branch of science and technology (i.e. nanoscience and nanotechnology). C60 (buckyball: zerodimensional nanocarbon) was first synthesized by a laser ablation technique in 1985 [1]. Carbon nanotubes (CNTs: one-dimensional nanocarbon) were discovered [2] and developed to meet industrial standards using the arc discharge method [3]. Graphene, a two-dimensional (2D) nanocarbon, is now under intensive research efforts due to its remarkable electronic, thermal, and mechanical properties [4]. The 6
These authors contributed equally.
0957-4484/14/445601+07$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 445601
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Figure 2. SEM images of the graphene samples synthesized under
various buffer gases. Gas flow rates of (a) H2 400 sccm, (b) H2/N2, 400/400 sccm, (c) H2/He 400/400 sccm, and (d) N2 400 sccm. Scale bar is 500 nm.
550 torr in the absence of catalysts, and the discharge current was maintained at 150 A. The nitrogen-doped graphene (as above) was then annealed at 1000 °C in Ar or N2 atmosphere for 7 h. The morphologies of arc graphene (both doped and undoped) were characterized by transmission electron microscopy (TEM, JEM-2200FS). To obtain arc-graphene film, the products were dispersed in dimethylformamide by ultra-sonication for 5 h. The top liquid layer obtained by subsequently performed centrifugation (15 000 rpm, 30 m) was filtered to deposit graphene film on an anodic alumina oxide (AAO) filter paper (pore size 0.2 μm). To separate a pure graphene film, AAO was etched away by a 98% sodium hydroxide solution, and then electric conductivity, film thickness, and doping concentration of the obtained film were measured by a four-point probe measurement (Dasol ENG, FPP-RS8), a surface profiler (Surfcorder, ET200), and x-ray photoelectron spectroscopy (Ulvac-PHI 5000 Versa Probe II), respectively. The defect density of the graphene products after adding various metallic catalysts was determined by Raman spectroscopy (Horiba), as it is often indicated by the ID/IG ratio. The procedure for making arc-graphene materials and their films are graphically presented in the supporting information (figure S1).
Figure 1. TEM images of arc graphene synthesized under different
buffer gases: (a) H2 400 sccm, (b) H2/N2, 400/400 sccm, (c) H2/He 400/400 sccm, and (d) N2 400 sccm.
graphene sheets by the exfoliation and reduction of graphene oxide [12]. Catalysts and doping strategies considered to be critical for CNT applications, however, have not been proved effective for graphene. In this work, we demonstrate a synthesis technique for arc graphene with reduced defects and thus enhanced conductivity by controlling the amount of catalysts and dopants. In addition, pyridine, pyrrole, amine, and quaternary-type nitrogen containing surface-functional groups have formed on the basal plane, which increases electrochemical performance due to the delocalized π orbitals.
Experimental method Arc graphene synthesis and characterization
Graphene was synthesized by the arc discharge method using a hollow graphite rod with 6 mm diameter. The optimized recipe for nitrogen-doped graphene includes powder-form graphite (5 g), bismuth oxide (catalyst, 0.01 g), and 4-aminibenzoic acid (dopant, 0.1 g), with each placed into the hole. H2/He (400/400 sccm) was used as a buffer, and the pressure in the reaction chamber and the discharge current were maintained at 550 torr and 150 A, respectively. Synthesis time for ∼30 cm long graphite rod was 10–20 min, depending on synthesis conditions. For testing buffer gases, the synthesis was carried out at 550 torr and 150 A without adding any catalysts and dopants. For testing catalysts, a H2/He (400/400 sccm) mixture was used as a buffer gas at 550 torr in the absence of dopants, and the discharge current was maintained at 150 A. The weight of each catalyst was 0.01 g. For testing nitrogen dopants, a H2/ He (400/400 sccm) mixture was used as a buffer gas at
Arc graphene application into supercapacitors
Nitrogen-doped graphene was synthesized and subsequently annealed according to the above recipe. Electrochemical measurements were performed with the CHI660D (Chenhua, Shanghai) system. The working electrode was prepared by mixing 95 wt% as-prepared arc graphene with 5 wt% Poly Vinylidene Fluoride (PVDF) solution (in N-Methyl-2-pyrrolidone (NMP)). They were mixed together in an agate mortar until the mixture turned to a black, homogeneously thick solution. The resulting solution was cast on nickel foam at an 8 mg cm−2 ratio. It was dried at 60 °C and, finally, the electrode was pressed at 10 MPa. The electrochemical 2
Nanotechnology 25 (2014) 445601
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Figure 3. (a) Raman spectra and (b) the ID/IG ratio of graphene synthesized under various buffer gases.
measurements were carried out on the triple-electrode system in 6 M KOH solutions. Arc-graphene-coated nickel foam and Ag/AgCl were used as a working electrode, a counter electrode, and a reference electrode, respectively. Galvanometric charge–discharge measurements were performed at a current density of 100 mA g−1.
Results and discussion Arc-graphene synthesis was attempted with different gases without adding any catalysts and dopants: H2 (400 sccm), H2/ N2 (400/400 sccm), H2/He (400/400 sccm), and N2 (400 sccm). As shown in TEM images (figure 1), a fewlayered graphene structure was formed only in H2-containing atmospheres. SEM images (figure 2) showed the formation of pure and large-area graphene in pure H2 and H2/He environments. The graphene sheet was approximately a few micrometers in size, as shown in figure S2 (supporting information). As previously addressed, hydrogen atoms can passivate carbon dangling bonds, thus preventing the formation of closed carbon structures such as carbon nanotubes and fullerene-like structures [6]. A pure N2 environment was also tested to grow arc graphene with the same growth parameters. Only amorphous carbon was observed without any indication of graphitic structures as shown by TEM (figure 1(d)) and SEM (figure 2(d)) images due to the absence of hydrogen. The formation of amorphous structures is related to nitrogen, which could facilitate pentagon formation in sp2 carbon networks [13]. Graphene and an amorphous carbon mixture was formed in a H2/N2 environment, as shown in figure 2(b). Raman spectra were collected with a 514 nm excitation laser from the arc graphene grown in H2, H2/N2, and H2/He environments. The G peak (∼1580 cm−1) indicates the inplane C–C bond-stretching mode (E2g mode), and the D peak (∼1350 cm−1) originates from defects or disorder in all sp2 carbon structures. The ID/IG ratio is a frequently used measure of defect density in graphitic nanostructures. Ar+ ion-induced defects were studied by scanning tunneling microscopy by
Figure 4. TEM images of the graphene samples synthesized with
various metal catalysts including (a) Cu, (b) CuO, (c) Bi, and (d) Bi2O3.
Lucchese et al and it was reported that defect density was estimated by (ID/IG ratio)/102 [14]. The graphene grown in H2/He has the lowest defect density, while the graphene grown in H2/N2 has the highest defect density estimated from the ID/IG ratio, as shown in figure 3(b). Pentagon or amorphous formation in sp2 structures induced by nitrogen could account for the highest ID/IG ratio. The highest 2D peak (double resonance peak with two phonons) was observed from the graphene grown in H2/N2. This might be caused by the folded structure of graphene that could occur when it was mixed with amorphous carbon during formation [15]. The folded structures are also formed by buckling between two graphene domains due to pentagon rings [13]. The reason that the graphene grown in H2/He has lower ID/IG ratio than in 3
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Figure 5. (a) The ID/IG ratio of graphene synthesized with a variety of metal catalysts, and (b) The dependency of ID/IG on the amount of bismuth oxide.
pure H2 is not clearly understood. At growth temperatures (∼4000 °C), large amount of H2 are dissociated into atomic hydrogen that passivate dangling bonds at the edges of graphene. Excessive amount of atomic hydrogen, however, could damage graphene at high temperatures. The results acquired with a variation of growth-gas environment were in good agreement with the previous work by Shen et al [7], but we clearly supports the points with SEM, TEM, and Raman spectra. Additionally, we observed amorphous carbon formation in pure a N2 environment. We further improved the quality and conductivity of graphene with catalysts and controlled doping followed by annealing. The influence of catalysts on physical properties of arc graphene was studied systematically. The test involved 0.01 g of each catalyst in powder, while other parameters were fixed as described in the experimental methods. Transition metals, known as good catalysts for CNTs, such as Fe, FeS, Ni, and Co were attempted for arc graphene, but they tend to synthesize single-walled or multi-walled carbon nanotubes due to the formation of catalytic nanoparticles during the growth process. In order to avoid nanotube formation, catalysts without carbon solubility, such as Cu, CuO, Bi, and Bi2O3, were attempted to grow arc graphene. The graphene grown from four different catalysts showed clean and 2D sheet-like structures as shown in the SEM images (figure S3). There was no indication of CNT growth. Three metal catalysts including Bi, Bi2O3, and Cu reduced the defect density of arc graphene as shown in the lowered ID/IG ratio in figure 5(a). Bi and Bi2O3 favored thin graphene (3–6 layers) formation, while Cu and CuO usually yielded thicker films (∼10 layers) as shown in the TEM images in figure 4. Bi2O3 was found to be the most effective catalyst as the ID/IG ratio becomes lowest, and the optimized amount of Bi2O3 was determined based on the ID/IG ratio of the graphene film as presented in figures 5(a) and (b). Bismuth was previously used as an additive for growing SWNTs and was found to be a promoter, leading to the high-yield production of SWNTs with high crystallinity by facilitating sp2 carbon–carbon bond formation [16].
Table 1. Doping concentration and conductivity of nitrogen-doped
(N-doped) graphene synthesized with different amounts of 4aminobenzoic acid. dopant (g) 0 2 7
N-doping Concentration (%)
Conductivity (S m−1)
0.03 1.25 1.65
1 9 48
Copper foil is also known as the best catalyst layer to form single-layered graphene via CVD methods. Single-layered graphene can be assembled from dissociated carbon atoms on the copper surface due to catalytic activity of copper and its limited solubility of carbon [17]. In arc-graphene growth, bismuth or copper atoms can be considered to position at growing graphene edges and thus terminate dangling bonds. They also facilitate linkage of carbon hexagons into the graphene structure by lowering a kinetic barrier similar to the scooter mechanism in SWNT growth by laser ablation [18]. The catalytic activity order (Bi2O3 > Cu > Bi) and thin layer formation with Bi2O3 or Bi require further studies. As shown in figure 5(b), the amount of bismuth oxide was optimized based on the ID/IG ratio. An excessive amount of bismuth oxide catalyst has been proven to be harmful. It appears that agglomerated catalyst particles are no longer effective for growing arc graphene. In order to enhance electrical conductivity, in situ nitrogen doping was attempted. The chemicals included 4-aminobenzoic acid (C7H7NO2), bismuth nitrate (Bi5(OH)9(NO2)4, and NH3, which were tested for doping graphene without adding catalysts. 4-amninobenzoic acid was found to be an effective dopant, while (Bi5(OH)9(NO2)4, and NH3 gas did not show noticeable changes in the electrical conductivity of the arc graphene film. As the dose of 4-aminobenzoic acid increased from 0 to 7g, the nitrogen doping concentration and the corresponding electrical conductivity of the graphene films also increased 4
Nanotechnology 25 (2014) 445601
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Figure 6. (a) A schematic of the bonding configurations of the nitrogen dopants, and (b) XPS analysis of nitrogen-doped arc graphene.
based on the 4-point probe measurement and XPS analysis as summarized in table 1. The doping concentration was maximized to 1.65 atom % at the addition of 7 g of 4-aminobenzoic acid. It showed a higher nitrogen concentration compared to reports from other groups. It is generally accepted that the nitrogen-doping concentration is hardly higher than 1 atom % for graphene or SWNTs by an arc discharge method [9, 11, 19]. Nitrogen atoms can be differently positioned in the hexagonal carbon network as illustrated in figure 6(a). From XPS data (figure 6(b)), the nitrogen positions were determined as pyridinic (398.2 eV), amide (399 eV), pyrrolic (400.2 eV) and quaternary (401 eV) [20]. Positioning nitrogen to quaternary position rather than nitrogen termination is highly desirable for enhancing electrical properties, but it is still challenging because high growth temperature in arc discharge prefers the high-entropy structure. Additionally, the role of chamber pressure during the reaction was briefly investigated. The chamber pressure was varied, while other parameters were fixed according to the optimized recipe described in the experimental methods. Arc graphene grown at higher pressure (550 torr) exhibited improved conductivity compared to that grown at lower pressure (210 torr). Higher growth temperature induced by higher pressure could make such a difference due to the annealing effect at a given temperature [21]. Arc graphene grown at different pressure levels was subsequently annealed at 1000 °C in order to enhance the electrical conductivity by reducing defects. As presented in figure S5 (supporting information), annealing in Ar or N2 atmosphere leads to the enhancement of electrical conductivity, and the highest value (16 100 s m−1) was observed from the N2-annealed arc graphene grown at 550 torr pressure. This conductivity value is higher than that of the reduced graphene oxide (rGO) treated with hydrazine [22, 23], and comparable to the one treated with iodine-containing chemicals [24, 25]. Thermal annealing in N2 is a more efficient way to achieve higher conductivity due to the additional doping effect.
Figure 7. Cyclic voltammetry curves of arc graphene.
Graphene has a larger surface area than any known materials, which makes it a great candidate for supercapacitor electrodes [26], so we studied the electrochemical properties of arc graphene. The specific surface area of nitrogen-doped arc graphene was 117 m2 g−1 as measured by the Brunauer– Emmett–Teller method. The electrochemical performance and capacitance has been determined using the three electrode system in aqueous 6.0 M KOH electrolyte. The cyclic voltammograms (figure 7) of the nitrogen-doped arc graphene show approximately rectangular-like hysteresis, which is the characteristic of double layer capacitors. In addition, a light redox peak has appeared in the C–V curve, which may arise due to the presence of nitrogen and oxygen moieties on the surface of arc graphene. In particular, during arc discharge, nitrogen has been doped on the surface of graphene, and thus little redox peaks have appeared in the C–V curves, supported by the XPS analysis. The XPS analysis indicates that nitrogen-containing groups are positioned as pyridine, pyrrole, amine, and quaternary-structure in a hexagonal carbon network. It is noteworthy that the quinine and carboxylic-type 5
Nanotechnology 25 (2014) 445601
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Figure 8. (a) Galvanometric charge–discharge curves in different current densities, (b) capacitance values with different current densities, (c)
impedance spectroscopy, and (d) the Ragone plot.
reported to be 49 Fg−1 [29]. Nitrogen atoms incorporated in arc graphene seem to increase capacitance value. Electrochemical stability is crucial for the supercapacitor application. The nitrogen-doped arc graphene shows excellent cyclic stability of 92% up to 3000 cycles. Moreover, the maximum and minimum energy density are 12.04 and 2.08 Whkg−1, respectively, and the power densities are 124.91 and 4992 Wkg−1, respectively, as shown in figure 8(d). The electrochemical impedance spectroscopy was studied at open circuit voltage potential over the range of 1 MHz to 1 mHz. Figure 8(c) shows the Nyquist plot of the arc graphene. In the high-frequency region, a very small semicircle has formed, which indicates that the charge transfer process is faster due to less redox and high conductivity of the arc graphene. Moreover, in the low-frequency region, the fact that the imaginary part is almost parallel to the y-axis indicated the capacitance behaviour at 6M KOH solution. The equivalent series resistance (ESR) has been determined by extrapolation of the imaginary to the junction point of the real axis. The ESR of arc graphene is 0.70 Ω, which is attributed to that the high conductivity of arc graphene, which has reduced the series resistance. Thus, arc-discharge nitrogen-doped graphene is a promising candidate for the supercapacitor application.
oxygen-containing groups formed on the edge of graphene sheets increase the hydrophilicity and facilitate redox reaction up to a high scan rate. Furthermore, the significant appearance of redox peak and negligible voltage delay up to a high scan rate indicate good capacitive performance with uniform porosity for ion transition [27]. In order to get insight on the supercapacitive performance of arc graphene, galvanometric charge–discharge was carried out at different current densities. As shown in figure 8(a), the smooth charging and discharging indicates that the arc graphene performed as a good capacitive material. The nearly triangular shaped charge–discharge indicates slow redox reaction and good porosity of the arc graphene, which made the easy ion transition from electrode to electrolyte. Sridhar, et al showed that a longer exponential discharge curve may appear due to the Faradic reaction [28]. In our present study, at low current density, long exponential discharge curves have appeared, which showed that the Faradic reaction is caused by the oxygen moieties and nitrogen-doped graphene surface. The measured capacitance values were 86.75 and 35 F g−1 at current densities of 0.250 and 5 A g−1, respectively (figure 8(b)). In contrast, the capacitance value of chemically synthesized reduced graphene in a 1 M H2SO4 solution was 6
Nanotechnology 25 (2014) 445601
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Conclusions
[11] Li N, Wang Z Y, Zhao K K, Shi Z J, Gu Z N and Xu S K 2010 Large scale synthesis of N-doped multi-layered graphene sheets by simple arc-discharge method Carbon 48 255 [12] Wu Z S, Ren W C, Gao L B, Zhao J P, Chen Z P, Liu B L, Tang D M, Yu B, Jiang C B and Cheng H M 2009 Synthesis of graphene sheets with high electrical conductivity and good thermal stability by hydrogen arc discharge exfoliation ACS Nano 3 411 [13] Sjostrom H, Stafstrom S, Boman M and Sundgren J E 1995 Superhard and elastic carbon nitride thin films having fullerenelike microstructure Phys. Rev. Lett. 75 1336 [14] Lucchese M M, Stavale F, Ferreira E H M, Vilani C, Moutinho M V O, Capaz R B, Achete C A and Jorio A 2010 Quantifying ion-induced defects and Raman relaxation length in graphene Carbon 48 1592 [15] Hao Y, Wang Y, Wang L, Ni Z, Wang Z, Wang R, Koo C K, Shen Z and Thong J T L 2010 Probing layer number and stacking order of few-layer graphene by Raman spectroscopy Small 6 195 [16] Kiang C H, Goddard W A, Beyers R, Salem J R and Bethune D S 1996 Catalytic effects of heavy metals on the growth of carbon nanotubes and nanoparticles J. Phys. Chem. Solids 57 35 [17] Li X et al 2009 Large-area synthesis of high-quality and uniform graphene films on copper foils Science 324 1312 [18] Thess A et al 1996 Crystalline ropes of metallic carbon nanotubes Science 273 483 [19] Glerup M, Steinmetz J, Samaille D, Stephan O, Enouz S, Loiseau A, Roth S and Bernier P 2004 Synthesis of N-doped SWNT using the arc-discharge procedure Chem. Phys. Lett. 387 193 [20] Li X L, Wang H L, Robinson J T, Sanchez H, Diankov G and Dai H J 2009 Simultaneous nitrogen doping and reduction of graphene oxide J. Am. Chem. Soc. 131 15939 [21] Keidar M, Waas A M, Raitses Y and Waldorff E I 2006 Modeling of the anodic arc discharge and conditions for single-wall carbon nanotube growth J. Nanosci. Nanotechnol. 6 1309 [22] Stankovich S, Dikin D A, Piner R D, Kohlhaas K A, Kleinhammes A, Jia Y, Wu Y, Nguyen S T and Ruoff R S 2007 Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide Carbon 45 1558 [23] Ren P G, Yan D X, Ji X, Chen T and Li Z M 2011 Temperature dependence of graphene oxide reduced by hydrazine hydrate Nanotechnology 22 055705 [24] Xu Y X, Sheng K X, Li C and Shi G Q 2011 Highly conductive chemically converted graphene prepared from mildly oxidized graphene oxide J. Mater. Chem. 21 7376 [25] Park O K, Hahm M G, Lee S, Joh H I, Na S I, Vajtai R, Lee J H, Ku B C and Ajayan P M 2012 In Situ synthesis of thermochemically reduced graphene oxide conducting nanocomposites Nano Lett. 12 1789 [26] Stoller M D, Park S J, Zhu Y W, An J H and Ruoff R S 2008 Graphene-based ultracapacitors Nano Lett. 8 3498 [27] Frackowiak E and Béguin F 2001 Carbon materials for the electrochemical storage of energy in capacitors Carbon 39 937 [28] Sridhar V, Kim H J, Jung J H, Lee C, Park S and Oh I K 2012 Defect-engineered three-dimensional graphene-nanotubepalladium nanostructures with ultrahigh capacitance ACS Nano 6 10562 [29] Kuila T, Mishra A K, Khanra P, Kim N H, Uddin M E and Lee J H 2012 Facile method for the preparation of water dispersible graphene using sulfonated poly(ether-etherketone) and its application as energy storage materials Langmuir 28 9825
Large-scale synthesis of nitrogen-doped graphene has been demonstrated using an arc-discharge method. Defects formed during growth have been reduced by adding metal or metal oxide catalysts, and the electrical conductivity was improved by adding nitrogen-containing dopants followed by an annealing procedure. The electrochemical measurements of the doped arc graphene show its potential for energy storage materials.
Acknowledgements This work was supported by grants from the Korea Institute of Science and Technology (KIST) Institutional Program, the Converging Research Center Program funded by the Ministry of Science, ICT & Future Planning Technology (2014M3C1A8054009), the Fusion R&D Program funded by the Korea Research Council for Industrial Science and Technology, and the Graphene Materials/Components Development Project (10044366) through the Ministry of Trade, Industry, and Energy (MOTIE), Republic of Korea.
References [1] Kroto H W, Heath J R, O’Brien S C, Curl R F and Smalley R E 1985 C60: buckminsterfullerene Nature 318 162 [2] Iijima S 1991 Helical microtubules of graphitic carbon Nature 354 56 [3] Journet C, Maser W K, Bernier P, Loiseau A, de la Chapelle M L, Lefrant S, Deniard P, Lee R and Fischer J E 1997 Large-scale production of single-walled carbon nanotubes by the electric-arc technique Nature 388 756 [4] Geim A K 2009 Graphene: status and prospects Science 324 1530 [5] Han J T, Kim J S, Jeong H D, Jeong H J, Jeong S Y and Lee G W 2010 Modulating conductivity, environmental stability of transparent conducting nanotube films on flexible substrates by interfacial engineering ACS Nano 4 4551 [6] Subrahmanyam K S, Panchakarla L S, Govindaraj A and Rao C N R 2009 Simple method of preparing graphene flakes by an arc-discharge method J. Phys. Chem. C 113 4257 [7] Shen B S, Ding J J, Yan X B, Feng W J, Li J and Xue Q J 2012 Influence of different buffer gases on synthesis of fewlayered graphene by arc discharge method Appl. Surf. Sci. 258 4523 [8] Wu Y P, Wang B, Ma Y F, Huang Y, Li N, Zhang F and Chen Y S 2010 Efficient and large-scale synthesis of fewlayered graphene using an arc-discharge method and conductivity studies of the resulting films Nano Res. 3 661 [9] Guan L, Cui L, Lin K, Wang Y Y, Wang X T, Jin F M, He F, Chen X P and Cui S 2011 Preparation of few-layer nitrogendoped graphene nanosheets by DC arc discharge under nitrogen atmosphere of high temperature Appl. Phys. A 102 289 [10] Wang Z Y, Li N, Shi Z J and Gu Z N 2010 Low-cost and largescale synthesis of graphene nanosheets by arc discharge in air Nanotechnology 21 175602 7
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Catalyst and doping methods for arc graphene
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 445601 (http://iopscience.iop.org/0957-4484/25/44/445601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 193.140.28.22 This content was downloaded on 08/11/2014 at 12:13
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 445601 (7pp)
doi:10.1088/0957-4484/25/44/445601
Catalyst and doping methods for arc graphene Hyunjin Cho1,6, InSeoup Oh1,6, JungHo Kang1, Sungchan Park1, Boncheol Ku2, Min Park1, Soonjong Kwak3, Partha Khanra4, Joong Hee Lee4 and Myung Jong Kim1,5 1
Soft Innovative Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeollabuk-do, 565-905, Korea 2 Carbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeollabuk-do, 565-905, Korea 3 Photoelectronic Hybrid Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Wolsong-gil 5, Seongbuk-gu, Seoul, 136-791, Korea 4 Department of BIN Fusion Technology, Chonbuk National University, 567, Baekjedaero, Deokjin-gu, Jeonju-si, Jeollabuk-do, 561-756, Korea 5 Nanomaterials Science and Engineering, Korea University of Science and Technology (UST), Daejeon 305-350, South Korea E-mail: [email protected] Received 11 August 2014 Accepted for publication 8 September 2014 Published 16 October 2014 Abstract
Nitrogen-doped graphene synthesis with ∼g scale has been accomplished using the arc discharge method. The defects formed in the synthesis process were reduced by adding various metal catalysts, among which Bi2O3 was found to be the most effective. Adding dopants to the starting materials increased the electrical conductivity of the graphene product, and the doping concentration in graphene was tuned by adjusting the amount of nitrogen dopants. A step-wise technique to fabricate graphene thin films was developed, including dispersion, separation, and filtering processes. The arc graphene can also find its potential application in supercapacitors, taking advantage of its large surface area and improved conductivity by doping. Keywords: arc discharge, graphene, catalyst, doping (Some figures may appear in colour only in the online journal) Introduction
synthetic methods used to produce large amounts of highquality graphene, however, are in high demand in order to bring this new material into practical applications. The arc discharge method was successfully adopted to produce highly crystalline, SWNTs for transparent conductive films [5]. High growth temperature and the role of catalysts were the key factors required to produce nearly defect-free SWNTs in the arcdischarge method [3]. On the other hand, arc discharge for graphene is in its infancy compared to that of CNTs. Undoped and doped arc-graphene-synthesis methods have been previously reported. Arc graphene was grown by vaporizing a graphite target in different gas environments such as H2 [6], He/H2 [7], CO2 [8], N2 [9], air [10] and NH3 [11]. As an alternative, arc discharge was used in order to acquire
The technique to vaporize elemental carbon to form nanostructures opened a new branch of science and technology (i.e. nanoscience and nanotechnology). C60 (buckyball: zerodimensional nanocarbon) was first synthesized by a laser ablation technique in 1985 [1]. Carbon nanotubes (CNTs: one-dimensional nanocarbon) were discovered [2] and developed to meet industrial standards using the arc discharge method [3]. Graphene, a two-dimensional (2D) nanocarbon, is now under intensive research efforts due to its remarkable electronic, thermal, and mechanical properties [4]. The 6
These authors contributed equally.
0957-4484/14/445601+07$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 445601
H Cho et al
Figure 2. SEM images of the graphene samples synthesized under
various buffer gases. Gas flow rates of (a) H2 400 sccm, (b) H2/N2, 400/400 sccm, (c) H2/He 400/400 sccm, and (d) N2 400 sccm. Scale bar is 500 nm.
550 torr in the absence of catalysts, and the discharge current was maintained at 150 A. The nitrogen-doped graphene (as above) was then annealed at 1000 °C in Ar or N2 atmosphere for 7 h. The morphologies of arc graphene (both doped and undoped) were characterized by transmission electron microscopy (TEM, JEM-2200FS). To obtain arc-graphene film, the products were dispersed in dimethylformamide by ultra-sonication for 5 h. The top liquid layer obtained by subsequently performed centrifugation (15 000 rpm, 30 m) was filtered to deposit graphene film on an anodic alumina oxide (AAO) filter paper (pore size 0.2 μm). To separate a pure graphene film, AAO was etched away by a 98% sodium hydroxide solution, and then electric conductivity, film thickness, and doping concentration of the obtained film were measured by a four-point probe measurement (Dasol ENG, FPP-RS8), a surface profiler (Surfcorder, ET200), and x-ray photoelectron spectroscopy (Ulvac-PHI 5000 Versa Probe II), respectively. The defect density of the graphene products after adding various metallic catalysts was determined by Raman spectroscopy (Horiba), as it is often indicated by the ID/IG ratio. The procedure for making arc-graphene materials and their films are graphically presented in the supporting information (figure S1).
Figure 1. TEM images of arc graphene synthesized under different
buffer gases: (a) H2 400 sccm, (b) H2/N2, 400/400 sccm, (c) H2/He 400/400 sccm, and (d) N2 400 sccm.
graphene sheets by the exfoliation and reduction of graphene oxide [12]. Catalysts and doping strategies considered to be critical for CNT applications, however, have not been proved effective for graphene. In this work, we demonstrate a synthesis technique for arc graphene with reduced defects and thus enhanced conductivity by controlling the amount of catalysts and dopants. In addition, pyridine, pyrrole, amine, and quaternary-type nitrogen containing surface-functional groups have formed on the basal plane, which increases electrochemical performance due to the delocalized π orbitals.
Experimental method Arc graphene synthesis and characterization
Graphene was synthesized by the arc discharge method using a hollow graphite rod with 6 mm diameter. The optimized recipe for nitrogen-doped graphene includes powder-form graphite (5 g), bismuth oxide (catalyst, 0.01 g), and 4-aminibenzoic acid (dopant, 0.1 g), with each placed into the hole. H2/He (400/400 sccm) was used as a buffer, and the pressure in the reaction chamber and the discharge current were maintained at 550 torr and 150 A, respectively. Synthesis time for ∼30 cm long graphite rod was 10–20 min, depending on synthesis conditions. For testing buffer gases, the synthesis was carried out at 550 torr and 150 A without adding any catalysts and dopants. For testing catalysts, a H2/He (400/400 sccm) mixture was used as a buffer gas at 550 torr in the absence of dopants, and the discharge current was maintained at 150 A. The weight of each catalyst was 0.01 g. For testing nitrogen dopants, a H2/ He (400/400 sccm) mixture was used as a buffer gas at
Arc graphene application into supercapacitors
Nitrogen-doped graphene was synthesized and subsequently annealed according to the above recipe. Electrochemical measurements were performed with the CHI660D (Chenhua, Shanghai) system. The working electrode was prepared by mixing 95 wt% as-prepared arc graphene with 5 wt% Poly Vinylidene Fluoride (PVDF) solution (in N-Methyl-2-pyrrolidone (NMP)). They were mixed together in an agate mortar until the mixture turned to a black, homogeneously thick solution. The resulting solution was cast on nickel foam at an 8 mg cm−2 ratio. It was dried at 60 °C and, finally, the electrode was pressed at 10 MPa. The electrochemical 2
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Figure 3. (a) Raman spectra and (b) the ID/IG ratio of graphene synthesized under various buffer gases.
measurements were carried out on the triple-electrode system in 6 M KOH solutions. Arc-graphene-coated nickel foam and Ag/AgCl were used as a working electrode, a counter electrode, and a reference electrode, respectively. Galvanometric charge–discharge measurements were performed at a current density of 100 mA g−1.
Results and discussion Arc-graphene synthesis was attempted with different gases without adding any catalysts and dopants: H2 (400 sccm), H2/ N2 (400/400 sccm), H2/He (400/400 sccm), and N2 (400 sccm). As shown in TEM images (figure 1), a fewlayered graphene structure was formed only in H2-containing atmospheres. SEM images (figure 2) showed the formation of pure and large-area graphene in pure H2 and H2/He environments. The graphene sheet was approximately a few micrometers in size, as shown in figure S2 (supporting information). As previously addressed, hydrogen atoms can passivate carbon dangling bonds, thus preventing the formation of closed carbon structures such as carbon nanotubes and fullerene-like structures [6]. A pure N2 environment was also tested to grow arc graphene with the same growth parameters. Only amorphous carbon was observed without any indication of graphitic structures as shown by TEM (figure 1(d)) and SEM (figure 2(d)) images due to the absence of hydrogen. The formation of amorphous structures is related to nitrogen, which could facilitate pentagon formation in sp2 carbon networks [13]. Graphene and an amorphous carbon mixture was formed in a H2/N2 environment, as shown in figure 2(b). Raman spectra were collected with a 514 nm excitation laser from the arc graphene grown in H2, H2/N2, and H2/He environments. The G peak (∼1580 cm−1) indicates the inplane C–C bond-stretching mode (E2g mode), and the D peak (∼1350 cm−1) originates from defects or disorder in all sp2 carbon structures. The ID/IG ratio is a frequently used measure of defect density in graphitic nanostructures. Ar+ ion-induced defects were studied by scanning tunneling microscopy by
Figure 4. TEM images of the graphene samples synthesized with
various metal catalysts including (a) Cu, (b) CuO, (c) Bi, and (d) Bi2O3.
Lucchese et al and it was reported that defect density was estimated by (ID/IG ratio)/102 [14]. The graphene grown in H2/He has the lowest defect density, while the graphene grown in H2/N2 has the highest defect density estimated from the ID/IG ratio, as shown in figure 3(b). Pentagon or amorphous formation in sp2 structures induced by nitrogen could account for the highest ID/IG ratio. The highest 2D peak (double resonance peak with two phonons) was observed from the graphene grown in H2/N2. This might be caused by the folded structure of graphene that could occur when it was mixed with amorphous carbon during formation [15]. The folded structures are also formed by buckling between two graphene domains due to pentagon rings [13]. The reason that the graphene grown in H2/He has lower ID/IG ratio than in 3
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Figure 5. (a) The ID/IG ratio of graphene synthesized with a variety of metal catalysts, and (b) The dependency of ID/IG on the amount of bismuth oxide.
pure H2 is not clearly understood. At growth temperatures (∼4000 °C), large amount of H2 are dissociated into atomic hydrogen that passivate dangling bonds at the edges of graphene. Excessive amount of atomic hydrogen, however, could damage graphene at high temperatures. The results acquired with a variation of growth-gas environment were in good agreement with the previous work by Shen et al [7], but we clearly supports the points with SEM, TEM, and Raman spectra. Additionally, we observed amorphous carbon formation in pure a N2 environment. We further improved the quality and conductivity of graphene with catalysts and controlled doping followed by annealing. The influence of catalysts on physical properties of arc graphene was studied systematically. The test involved 0.01 g of each catalyst in powder, while other parameters were fixed as described in the experimental methods. Transition metals, known as good catalysts for CNTs, such as Fe, FeS, Ni, and Co were attempted for arc graphene, but they tend to synthesize single-walled or multi-walled carbon nanotubes due to the formation of catalytic nanoparticles during the growth process. In order to avoid nanotube formation, catalysts without carbon solubility, such as Cu, CuO, Bi, and Bi2O3, were attempted to grow arc graphene. The graphene grown from four different catalysts showed clean and 2D sheet-like structures as shown in the SEM images (figure S3). There was no indication of CNT growth. Three metal catalysts including Bi, Bi2O3, and Cu reduced the defect density of arc graphene as shown in the lowered ID/IG ratio in figure 5(a). Bi and Bi2O3 favored thin graphene (3–6 layers) formation, while Cu and CuO usually yielded thicker films (∼10 layers) as shown in the TEM images in figure 4. Bi2O3 was found to be the most effective catalyst as the ID/IG ratio becomes lowest, and the optimized amount of Bi2O3 was determined based on the ID/IG ratio of the graphene film as presented in figures 5(a) and (b). Bismuth was previously used as an additive for growing SWNTs and was found to be a promoter, leading to the high-yield production of SWNTs with high crystallinity by facilitating sp2 carbon–carbon bond formation [16].
Table 1. Doping concentration and conductivity of nitrogen-doped
(N-doped) graphene synthesized with different amounts of 4aminobenzoic acid. dopant (g) 0 2 7
N-doping Concentration (%)
Conductivity (S m−1)
0.03 1.25 1.65
1 9 48
Copper foil is also known as the best catalyst layer to form single-layered graphene via CVD methods. Single-layered graphene can be assembled from dissociated carbon atoms on the copper surface due to catalytic activity of copper and its limited solubility of carbon [17]. In arc-graphene growth, bismuth or copper atoms can be considered to position at growing graphene edges and thus terminate dangling bonds. They also facilitate linkage of carbon hexagons into the graphene structure by lowering a kinetic barrier similar to the scooter mechanism in SWNT growth by laser ablation [18]. The catalytic activity order (Bi2O3 > Cu > Bi) and thin layer formation with Bi2O3 or Bi require further studies. As shown in figure 5(b), the amount of bismuth oxide was optimized based on the ID/IG ratio. An excessive amount of bismuth oxide catalyst has been proven to be harmful. It appears that agglomerated catalyst particles are no longer effective for growing arc graphene. In order to enhance electrical conductivity, in situ nitrogen doping was attempted. The chemicals included 4-aminobenzoic acid (C7H7NO2), bismuth nitrate (Bi5(OH)9(NO2)4, and NH3, which were tested for doping graphene without adding catalysts. 4-amninobenzoic acid was found to be an effective dopant, while (Bi5(OH)9(NO2)4, and NH3 gas did not show noticeable changes in the electrical conductivity of the arc graphene film. As the dose of 4-aminobenzoic acid increased from 0 to 7g, the nitrogen doping concentration and the corresponding electrical conductivity of the graphene films also increased 4
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Figure 6. (a) A schematic of the bonding configurations of the nitrogen dopants, and (b) XPS analysis of nitrogen-doped arc graphene.
based on the 4-point probe measurement and XPS analysis as summarized in table 1. The doping concentration was maximized to 1.65 atom % at the addition of 7 g of 4-aminobenzoic acid. It showed a higher nitrogen concentration compared to reports from other groups. It is generally accepted that the nitrogen-doping concentration is hardly higher than 1 atom % for graphene or SWNTs by an arc discharge method [9, 11, 19]. Nitrogen atoms can be differently positioned in the hexagonal carbon network as illustrated in figure 6(a). From XPS data (figure 6(b)), the nitrogen positions were determined as pyridinic (398.2 eV), amide (399 eV), pyrrolic (400.2 eV) and quaternary (401 eV) [20]. Positioning nitrogen to quaternary position rather than nitrogen termination is highly desirable for enhancing electrical properties, but it is still challenging because high growth temperature in arc discharge prefers the high-entropy structure. Additionally, the role of chamber pressure during the reaction was briefly investigated. The chamber pressure was varied, while other parameters were fixed according to the optimized recipe described in the experimental methods. Arc graphene grown at higher pressure (550 torr) exhibited improved conductivity compared to that grown at lower pressure (210 torr). Higher growth temperature induced by higher pressure could make such a difference due to the annealing effect at a given temperature [21]. Arc graphene grown at different pressure levels was subsequently annealed at 1000 °C in order to enhance the electrical conductivity by reducing defects. As presented in figure S5 (supporting information), annealing in Ar or N2 atmosphere leads to the enhancement of electrical conductivity, and the highest value (16 100 s m−1) was observed from the N2-annealed arc graphene grown at 550 torr pressure. This conductivity value is higher than that of the reduced graphene oxide (rGO) treated with hydrazine [22, 23], and comparable to the one treated with iodine-containing chemicals [24, 25]. Thermal annealing in N2 is a more efficient way to achieve higher conductivity due to the additional doping effect.
Figure 7. Cyclic voltammetry curves of arc graphene.
Graphene has a larger surface area than any known materials, which makes it a great candidate for supercapacitor electrodes [26], so we studied the electrochemical properties of arc graphene. The specific surface area of nitrogen-doped arc graphene was 117 m2 g−1 as measured by the Brunauer– Emmett–Teller method. The electrochemical performance and capacitance has been determined using the three electrode system in aqueous 6.0 M KOH electrolyte. The cyclic voltammograms (figure 7) of the nitrogen-doped arc graphene show approximately rectangular-like hysteresis, which is the characteristic of double layer capacitors. In addition, a light redox peak has appeared in the C–V curve, which may arise due to the presence of nitrogen and oxygen moieties on the surface of arc graphene. In particular, during arc discharge, nitrogen has been doped on the surface of graphene, and thus little redox peaks have appeared in the C–V curves, supported by the XPS analysis. The XPS analysis indicates that nitrogen-containing groups are positioned as pyridine, pyrrole, amine, and quaternary-structure in a hexagonal carbon network. It is noteworthy that the quinine and carboxylic-type 5
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Figure 8. (a) Galvanometric charge–discharge curves in different current densities, (b) capacitance values with different current densities, (c)
impedance spectroscopy, and (d) the Ragone plot.
reported to be 49 Fg−1 [29]. Nitrogen atoms incorporated in arc graphene seem to increase capacitance value. Electrochemical stability is crucial for the supercapacitor application. The nitrogen-doped arc graphene shows excellent cyclic stability of 92% up to 3000 cycles. Moreover, the maximum and minimum energy density are 12.04 and 2.08 Whkg−1, respectively, and the power densities are 124.91 and 4992 Wkg−1, respectively, as shown in figure 8(d). The electrochemical impedance spectroscopy was studied at open circuit voltage potential over the range of 1 MHz to 1 mHz. Figure 8(c) shows the Nyquist plot of the arc graphene. In the high-frequency region, a very small semicircle has formed, which indicates that the charge transfer process is faster due to less redox and high conductivity of the arc graphene. Moreover, in the low-frequency region, the fact that the imaginary part is almost parallel to the y-axis indicated the capacitance behaviour at 6M KOH solution. The equivalent series resistance (ESR) has been determined by extrapolation of the imaginary to the junction point of the real axis. The ESR of arc graphene is 0.70 Ω, which is attributed to that the high conductivity of arc graphene, which has reduced the series resistance. Thus, arc-discharge nitrogen-doped graphene is a promising candidate for the supercapacitor application.
oxygen-containing groups formed on the edge of graphene sheets increase the hydrophilicity and facilitate redox reaction up to a high scan rate. Furthermore, the significant appearance of redox peak and negligible voltage delay up to a high scan rate indicate good capacitive performance with uniform porosity for ion transition [27]. In order to get insight on the supercapacitive performance of arc graphene, galvanometric charge–discharge was carried out at different current densities. As shown in figure 8(a), the smooth charging and discharging indicates that the arc graphene performed as a good capacitive material. The nearly triangular shaped charge–discharge indicates slow redox reaction and good porosity of the arc graphene, which made the easy ion transition from electrode to electrolyte. Sridhar, et al showed that a longer exponential discharge curve may appear due to the Faradic reaction [28]. In our present study, at low current density, long exponential discharge curves have appeared, which showed that the Faradic reaction is caused by the oxygen moieties and nitrogen-doped graphene surface. The measured capacitance values were 86.75 and 35 F g−1 at current densities of 0.250 and 5 A g−1, respectively (figure 8(b)). In contrast, the capacitance value of chemically synthesized reduced graphene in a 1 M H2SO4 solution was 6
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Conclusions
[11] Li N, Wang Z Y, Zhao K K, Shi Z J, Gu Z N and Xu S K 2010 Large scale synthesis of N-doped multi-layered graphene sheets by simple arc-discharge method Carbon 48 255 [12] Wu Z S, Ren W C, Gao L B, Zhao J P, Chen Z P, Liu B L, Tang D M, Yu B, Jiang C B and Cheng H M 2009 Synthesis of graphene sheets with high electrical conductivity and good thermal stability by hydrogen arc discharge exfoliation ACS Nano 3 411 [13] Sjostrom H, Stafstrom S, Boman M and Sundgren J E 1995 Superhard and elastic carbon nitride thin films having fullerenelike microstructure Phys. Rev. Lett. 75 1336 [14] Lucchese M M, Stavale F, Ferreira E H M, Vilani C, Moutinho M V O, Capaz R B, Achete C A and Jorio A 2010 Quantifying ion-induced defects and Raman relaxation length in graphene Carbon 48 1592 [15] Hao Y, Wang Y, Wang L, Ni Z, Wang Z, Wang R, Koo C K, Shen Z and Thong J T L 2010 Probing layer number and stacking order of few-layer graphene by Raman spectroscopy Small 6 195 [16] Kiang C H, Goddard W A, Beyers R, Salem J R and Bethune D S 1996 Catalytic effects of heavy metals on the growth of carbon nanotubes and nanoparticles J. Phys. Chem. Solids 57 35 [17] Li X et al 2009 Large-area synthesis of high-quality and uniform graphene films on copper foils Science 324 1312 [18] Thess A et al 1996 Crystalline ropes of metallic carbon nanotubes Science 273 483 [19] Glerup M, Steinmetz J, Samaille D, Stephan O, Enouz S, Loiseau A, Roth S and Bernier P 2004 Synthesis of N-doped SWNT using the arc-discharge procedure Chem. Phys. Lett. 387 193 [20] Li X L, Wang H L, Robinson J T, Sanchez H, Diankov G and Dai H J 2009 Simultaneous nitrogen doping and reduction of graphene oxide J. Am. Chem. Soc. 131 15939 [21] Keidar M, Waas A M, Raitses Y and Waldorff E I 2006 Modeling of the anodic arc discharge and conditions for single-wall carbon nanotube growth J. Nanosci. Nanotechnol. 6 1309 [22] Stankovich S, Dikin D A, Piner R D, Kohlhaas K A, Kleinhammes A, Jia Y, Wu Y, Nguyen S T and Ruoff R S 2007 Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide Carbon 45 1558 [23] Ren P G, Yan D X, Ji X, Chen T and Li Z M 2011 Temperature dependence of graphene oxide reduced by hydrazine hydrate Nanotechnology 22 055705 [24] Xu Y X, Sheng K X, Li C and Shi G Q 2011 Highly conductive chemically converted graphene prepared from mildly oxidized graphene oxide J. Mater. Chem. 21 7376 [25] Park O K, Hahm M G, Lee S, Joh H I, Na S I, Vajtai R, Lee J H, Ku B C and Ajayan P M 2012 In Situ synthesis of thermochemically reduced graphene oxide conducting nanocomposites Nano Lett. 12 1789 [26] Stoller M D, Park S J, Zhu Y W, An J H and Ruoff R S 2008 Graphene-based ultracapacitors Nano Lett. 8 3498 [27] Frackowiak E and Béguin F 2001 Carbon materials for the electrochemical storage of energy in capacitors Carbon 39 937 [28] Sridhar V, Kim H J, Jung J H, Lee C, Park S and Oh I K 2012 Defect-engineered three-dimensional graphene-nanotubepalladium nanostructures with ultrahigh capacitance ACS Nano 6 10562 [29] Kuila T, Mishra A K, Khanra P, Kim N H, Uddin M E and Lee J H 2012 Facile method for the preparation of water dispersible graphene using sulfonated poly(ether-etherketone) and its application as energy storage materials Langmuir 28 9825
Large-scale synthesis of nitrogen-doped graphene has been demonstrated using an arc-discharge method. Defects formed during growth have been reduced by adding metal or metal oxide catalysts, and the electrical conductivity was improved by adding nitrogen-containing dopants followed by an annealing procedure. The electrochemical measurements of the doped arc graphene show its potential for energy storage materials.
Acknowledgements This work was supported by grants from the Korea Institute of Science and Technology (KIST) Institutional Program, the Converging Research Center Program funded by the Ministry of Science, ICT & Future Planning Technology (2014M3C1A8054009), the Fusion R&D Program funded by the Korea Research Council for Industrial Science and Technology, and the Graphene Materials/Components Development Project (10044366) through the Ministry of Trade, Industry, and Energy (MOTIE), Republic of Korea.
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