Synthesis of nitrogen doped graphene from graphene oxide within an ammonia flame for high performance supercapacitors

Abstract

This paper introduces a novel process for preparing nitrogen (N) doped graphene by using an ammonia flame treatment under ambient conditions, which is simple, effective, faster and economical. That is, when graphene oxide (GO) was treated in the ammonia flame, GO not only could be reduced to graphene, but also could be doped with nitrogen atoms simultaneously. Furthermore, due to the special atmosphere in the ammonia flame, the N-doped graphene exhibited differences from the N-doped graphene by using other processes, which indicated the special properties and potential applications. The experimental results revealed: (1) the N atom concentration was up to 3.97 at% in the N-doped graphene; (2) various nitrogen species including pyridinic-N, pyrrolic-N and quaternary-N were detected in the N-doped graphene; (3) the specific capacitance of the N-doped graphene was 246.4 F g⁻¹ at a current density of 1 A g⁻¹ with high cycle stability, which was about 2 times higher than that of regular graphene without N-doping. It was indicated that this N-doped graphene could be an excellent electrode material for supercapacitor applications.

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1. Introduction

Graphene is a two-dimensional single-layer nanostructure of sp²-hybridized conjugated carbon atoms.^1–3 Due to its extraordinary electrical characteristics, thermal characteristics, and superior mechanical properties, graphene has been widely investigated in the area of electrode materials for high performance electrochemical capacitors.^2–5 In general, graphene needs to be modified to improve its performance in practical applications.⁶ An efficient way is to dope graphene with substituent heteroatoms such as nitrogen (N) atom, which can increase electron mobility and leading to a larger capacitance, due to its atomic size and strong valence bonds.⁷ Recently, N-doped graphene has attracted wide attentions.^8,9

Some methods have been reported including chemical vapor deposition (CVD),^10,11 plasma treatment,^12,13 thermal conversion of nitrogen precursor,^14–17 arc discharge,¹⁸ hydrothermal,^6,19 flames^20,21 and some other methods.^22–24 Lu et al.¹¹ prepared CVD-derived few-layer N-doped graphene sheets using 1,3,5-triazinemolecules as the sole source of both carbon and nitrogen, and the doping concentration increasing via decreasing the growth temperature. Shao et al.¹³ prepared N-doped graphene by exposing graphene to nitrogen plasma. According to their study, the N-doped graphene exhibits much higher electrocatalytic activity toward oxygen reduction and H₂O₂ reduction than graphene. Hu et al.¹⁵ synthesized the N-doped graphene upon thermal reduction of graphene oxide with ammonia hydroxide. Gao et al.¹⁷ directed synthesis of N-doped graphene sheets via a thermal conversion of polyacrylonitrile thin film, and the N/C atomic ratio is ca. 2.7%. Panchakarla et al.¹⁸ prepared N-doped graphene by performing the arc discharge of graphite electrodes in the presence of H₂, He, and pyridine vapor. The boron doped graphene can be prepared by changing the gas composition with the same method. Du et al.⁶ used the ammonia water solution as the reducing agent and proved that graphene oxide could be reduced and functionalized with N-doping simultaneously. In this process, ammonia water was used as reducing agent, solvent, and nitrogen precursor.

In this paper, we introduce an alternative process for preparing nitrogen (N) doped graphene by using an ammonia flame treatment under ambient condition. That is, when graphene oxide (GO) was treated in the ammonia flame, GO not only could be reduced to graphene, but also be doped with nitrogen atoms simultaneously. Comparing to the other processes, present method is of advantages of simpleness, effectiveness, efficiency and economy. Furthermore, due to the special atmosphere in the ammonia flame, the N-doped graphene with various nitrogen species exhibited differences from the N-doped graphene by using other processes, which indicated the special properties and potential applications.

2. Experimental

2.1. Preparation of the N-doped graphene (NG)

(1) Graphene oxide (GO) was synthesized from natural graphite powder using a modified Hummers' method;³ (2) 100 mg GO solid was dispersed in 100 mL of H₂O with the aid of ultrasonication by using a high power ultrasonic pole for 2 hours, and a black-brown GO aqueous colloid was formed; (3) GO paper was prepared by filtration of 10 mL of the above colloid through a Millipore filter (50 mm in diameter and 0.45 mm in pore size), followed by washing, air drying, and peeling off from the filter; (4) the N-doped graphene was prepared by treating the GO paper in ammonia flames for one minute, and the liquid fuel was composited of 70% ethanol and 30% n-propylamine. For comparison, the reduced graphene oxide (rGO) was prepared by using thermal exfoliation GO at 700 °C in the argon atmosphere.

2.2. Characterizations

The phase structures of the samples were characterized by using a X-ray diffraction spectrometer (XRD) (D8 Advanced XRD; Bruker AXS, Karlsruhe, Germany) with Cu Kα radiation. The morphologies of the samples were observed by using a scanning electron microscope (SEM, S-4800; Hitachi High-Technologies Corporation, Japan) and a HRTEM (JEM 2010FEF HRTEM; JEOL, Japan). Raman spectra were measured in a laser scanning confocal micro-Raman spectrometer (LabRAM HR, HORIBA, France). The surface chemical species of the N-doped graphene was examined on a X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA) using Al Kα radiation of 1486.6 eV as the excitation source.

2.3. Electrode preparation and electrochemical measurements

The electrochemical tests were carried out in 6 M KOH aqueous electrolyte solution at room temperature. The electrochemical properties of the samples were investigated using a CHI660D Electrochemical Working Station (Shanghai Chenhua, China). All electrochemical measurements were carried out in a three-electrode system, wherein the sample modified nickel foam as the working electrode (WE), platinum as the counter electrode, and saturated calomel electrode (SCE) electrode as the reference electrode. The WE was prepared by mixturing the N-doped graphene (or rGO), conductive carbon black and PVDF with a mass ratio of 8 : 1 : 1. Then adding appropriate amount of DMF and grinding for one hour to obtain the homogeneous solution. The solution was then casted on nickel foam to obtain an electrode. The assembled pressed at 10 MPa for one minute and dried in a vacuum oven at 60 °C for 12 hours. The mass of active materials coated on each WE is about 1.5 mg.

The specific capacitance (C) was calculated from the slope of each discharge curve, according to the equation C = (I × Δt)/(ΔV × m), where I is the constant discharge current, Δt is the discharge time, ΔV is the voltage difference in discharge and m is the mass of NG (or rGO) coated on each WE.^2,4 Electrochemical impedance spectroscopy (EIS) measurements were made in the frequency range of 0.1–100 000 Hz by applying an AC voltage with 10 mV perturbation.

3. Results and discussion

Fig. 1 illustrates the XRD patterns of the samples. For GO, a weak peak at 11.5° indicated the significant increase of graphite layer spacing.⁶ After the flame treatment, the GO peaks disappeared and a weak peak emerged at 26°, which demonstrated that a significant deoxygenation process happened and GO has been reduced into the N-doped graphene.⁶ Fig. 2 and 3 show the SEM and HRTEM morphologies of the N-doped graphene. Clearly, the N-doped graphene exhibited a good lamellar structure, and rich wrinkles structures on the surface. The number of the carbon atom layers was exactly identified at the edge, which indicated that the N-doped graphene has about 5 layers, as shown in Fig. 3(b).

Fig. 1 X-ray diffraction patterns of GO and the N-doped graphene.

Fig. 2 (a) SEM morphology of GO; (b) SEM morphology of the N-doped graphene.

Fig. 3 (a) HRTEM images of the N-doped graphene; (b) the corresponding images at high magnification.

XPS is an effective surface chemical analysis technique, which is used to determine the species and chemical states of the elements in the surface of the materials. Fig. 4 illustrates the XPS results of the N-doped graphene. It could be seen that the N-doped graphene had a predominant C1s peak at ∼284.8 eV, an relative weak O1s peak at ∼531.9 eV and an obvious N1s peak at ∼399.9 eV. According to the analysis of the XPS spectrum, the N atom concentration was about 3.97% in the N-doped graphene.

Fig. 4 (a) XPS spectra of the N-doped graphene, (b) N1s spectra of the N-doped graphene.

In addition, the XPS spectrum of C1s can be divided in to three different peaks, which corresponded to the signals of C–C (∼284.8 eV), C=O (∼288.9 eV), C–N or C–OH (∼285.9 eV). It has been known that N1s spectrum usually can be deconvoluted into three individual peaks, which are assigned to pyridine N, pyrrolic N and graphite N.²⁵ Fig. 4(b) illustrates the high resolution N1s spectrum of the N-doped graphene. Obviously, the N1s peak became broad and asymmetry trend, which indicated various presence species of N atoms.

In generally, when a nitrogen atom is doped into graphene, it usually has three bonding configurations within the carbon lattice, that is, pyridinic N, pyrrolic N and quaternary N (or graphitic N).²⁵ Therefore, it was confirmed that nitrogen atoms have been doped into the graphene lattice successfully, and three nitrogen species were formed within the N-doped graphene. Especially, the pyrrolic N (∼399.9 eV) occupied the dominant portion, as shown in Fig. 4(b).

Table 1 summary the relationship of N atom concentration and species with variant N-doping methods. Clearly, the N atom concentration and species are related to the experimental conditions and/or N-doping methods. Comparatively, the present work provided a relative higher N atom concentration up to 3.97 at% than the methods such as arc discharge,¹⁸ thermal treatment,²⁶ and flames,²⁰ while it was of advantages of simpleness, effectiveness, efficiency and economy. In addition, the present work also provided abundant N atom species in the N-doped graphene, which indicated its special electrochemical properties.

Table 1 Nitrogen-doping methods and nitrogen concentration on N-doped graphene

Synthesis method	Precursors	N content, at%	Species of N atoms	Ref.
CVD	Cu foil as catalyst, acetonitrile	9	Pyridinic, pyrrolic, graphitic	10
CVD	Cu foil as catalyst, 1,3,5-triazine	2.1–5.6	Pyridinic, pyrrolic, graphitic	11
Plasma treatment	Graphene oxide, ammonia plasma	6–25	Pyridinic, pyrrolic, graphitic	12
Plasma treatment	Graphene, nitrogen plasma	8.5	Pyridinic, pyrrolic, graphitic	13
Catalyst-free thermal annealing method	Graphene oxide–melamine	6.6–10.1	Pyridinic, pyrrolic, graphitic	14
Arc discharge	Graphite–H2–He–pyridine vapor.	0.6–1.4	Pyridinic, graphitic	18
Solvothermal	Graphene oxide, ammonia water	4.4	Pyridinic, primary amine	6
Thermal treatment	Graphite oxide after thermal expansion, NH3–Ar	2.0–2.8	Pyridinic, pyrrolic, graphitic	26
Flame	Ni film as catalyst, amine + ethanol	1.4	Pyridinic, graphitic	20
Flame	Graphene oxide, amine + ethanol	3.97	Pyridinic, pyrrolic, graphitic	Present work

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Raman spectroscopy is a very important technology for characterizing graphene. Fig. 5 gives the Raman spectra of the samples GO, rGO and the N-doped graphene. The results revealed that after the flame treatment, 2D peak was weakened and D peak was enhanced for the N-doped graphene, when compared to GO and rGO. The reason was that the substitution of nitrogen dopants disordered the graphene lattice integrity.²⁷

Fig. 5 Raman spectra of GO, rGO and the N-doped graphene.

The electrochemical properties of the N-doped graphene were measured by using various techniques involving cyclic voltammetry (CV), galvanostatic charge–discharge curves (GCD) and electrochemical impedance spectroscopy (EIS). Fig. 6(a) illustrates the CV curves of the N-doped graphene and rGO at a scan rate of 100 mV s⁻¹ with a voltage range from −0.1 V to −1.1 V. It was revealed that the N-doped graphene and rGO show typical electric double layer capacitor. However, compared with the complete rectangular, the CV curves exhibited a certain deviation due to a certain amount of functional groups on surface of the N-doped graphene and rGO.^5,28 Obviously, the curve scale of the N-doped graphene was much larger than that of rGO, which demonstrated a better electrochemical property of the N-doped graphene. Fig. 6(b) shows the CV curves of the N-doped graphene with different scan rates. It was notable that the N-doped graphene had an excellent electrochemical stability in a wide range of scan rate, and the obvious increase of current density with the scan rates indicated good rate ability for the electrode.

Fig. 6 (a) CV curves of rGO and the N-doped graphene at a scan rate of 10 mV s⁻¹ in 6 M KOH solution; (b) CV curves of the N-doped graphene at different scan rates of 10, 20, 50, 100 and 200 mV s⁻¹.

Fig. 7(a) shows the GCD curves of rGO and the N-doped graphene at the charge–discharge current density of 1 A g⁻¹. The curve deviations from a perfect triangle were caused by faraday pseudo-capacitance, which was resulted from the partial oxidation functional group attached on the graphene surface.^5,7,28 There were no obvious voltage drops in the curves indicated small internal resistance of the electrode materials. According to the capacitance equation evaluated from the slopes of the discharge curves (shown in the Experimental and materials section), the specific capacitance of the N-doped graphene and rGO were calculated. That was, the N-doped graphene was of an excellent specific capacitance up to 246.4 F g⁻¹ at 1 A g⁻¹, while rGO only 133.7 F g⁻¹ at 1 A g⁻¹, which almost two times lower than the N-doped graphene. The results demonstrated that the N-doping acted as an important factor to the high specific capacitance. However, the performance of N-graphene could be ascribed to several factors. For example, the insertion of N-atoms into graphene layers changed the values of Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) in order to reduce the band gap, which therefore increased the electron mobility and lower the electron work function at the carbon/liquid interface when comparing to pure carbon, and resulted in a larger capacitance.^7,29 In addition, the surface defects and disordered morphology induced by doping N atoms could increase the electrode/electrolyte wettability,²⁵ and improved the electrochemical performance significantly.

Fig. 7 (a) GCD curve of rGO and the N-doped graphene at a current density of 1 A g⁻¹ in 6 M KOH solution; (b) GCD curves of the N-doped graphene at different current densities of 1, 2, 3, 4, 5 and 6 A g⁻¹ in 6 M KOH; (c) the specific capacitance variation curves with current density, of NG and rGO; (d) cycling stability of the N-doped graphene upon charging–discharging at a current density of 1 A g⁻¹.

Fig. 7(b) shows the GCD curves of the N-doped graphene at different charge–discharge current density in the range of 1 A g⁻¹ to 6 A g⁻¹. The specific capacitances of the N-doped graphene (or rGO) at different current densities (1, 2, 3, 4, 5 and 6 A g⁻¹) were calculated, as shown in Fig. 7(c). It could be seen that when the charge–discharge current density was increased from 1.0 to 6.0 A g⁻¹, the specific capacitance of the N-doped graphene decreased from 246.4 F g⁻¹ to 132.6 F g⁻¹. Cycle life is one of the most important electrochemical performances of supercapacitors. The cyclability of the N-doped graphene electrodes was carried out by using GCD measurement at a current density of 1 A g⁻¹ in the voltage window of −0.1 V to −1.1 V. Obviously, the specific capacitance of the N-doped graphene showed a very slight decrease to 89.6% for the first cycle after 2000 times' GCD test, as shown in Fig. 7(d), which indicated its excellent cycle stability for capacitor applications.

EIS is usually used to investigate the performance of electrochemical capacitors such as internal resistance, capacity, etc. The EIS data were analyzed by using Nyquist plots, which showed the frequency response of the electrode/electrolyte system and were the plots of the imaginary component (Z′′) of the impedance against the real component (Z′).^4,30 Fig. 8 shows the EIS curve of rGO and the N-doped graphene. The inset illustration shows that Nernst curve at high frequency region. The equivalent resistance of the electrode materials could be obtained by x-axis intercept of the curve. The value included the total combination of the ionic resistance of electrolyte, intrinsic resistance of active materials and contact resistance at the active material current collector interface.^30,31 As shown in Fig. 8, the equivalent resistance of the N-doped graphene and rGO were apparently about 1.0 Ω and 1.2 Ω, and these two electrodes exhibited very small inherent resistance. The curve of the low frequency showed the impedance of the electrode material, which was caused by the ion diffusion between graphene layers. Moreover, both spectra revealed a Warburg angle higher than 45°in low frequencies, which indicated that the electrodes were strongly controlled by ion diffusion/transport process. All of the results indicated that the N-doped graphene was suitable as an electrode material for supercapacitors.

Fig. 8 Nyquist plot of rGO and the N-doped graphene obtained in 6 M KOH (inset is the enlarged plot of the high-frequency regions).

4. Conclusions

In summary, a simple, effective, faster and economical ammonia flame treatment has been successfully employed to prepare the N-doped graphene under ambient conditions. The N atom concentration is up to 3.97 at% in the N-doped graphene with various nitrogen species. Electrochemical measurements indicated that the N-doped graphene material exhibited a specific capacitance of 246.4 F g⁻¹ at a current density of 1 A g⁻¹ and high cycle stability. The present process provides a possibility for mass production of N-doped graphene with low price, and exhibits an enormous potential for electrode material applications.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (nos 11174227, 51209023, J1210061), the Fundamental Research Funds for the Central Universities (no. 20142020205), National Key Technology R&D Program of the Hubei province (no. 2013BHE012) and Chinese Universities Scientific Fund.

References

1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos and A. A. Firsov, Nature, 2005, 438, 197–200.
2. Y. P. Zhang, C. Z. Luo, W. P. Li and C. X. Pan, Nanoscale, 2013, 5, 2616–2619.
3. Y. P. Zhang, D. L. Li, X. J. Tan, B. Zhang, X. F. Ruan, H. J. Liu, C. X. Pan, L. Liao, T. Zhai, Y. Bando, S. S. Chen, W. W. Cai and R. S. Ruoff, Carbon, 2013, 54, 143–148.
4. D. F. Sun, X. B. Yan, J. W. Lang and Q. J. Xue, J. Power Sources, 2013, 222, 52–58.
5. Y. Huang, J. J. Liang and Y. S. Chen, Small, 2012, 8, 1805–1834.
6. X. S. Du, C. F. Zhou, H. Y. Liu, Y. W. Mai and G. X. Wang, J. Power Sources, 2013, 241, 460–466.
7. W. Fan, Y. Y. Xia, W. W. Tjiu, P. K. Pallathadka, C. B. He and T. X. Liu, J. Power Sources, 2013, 243, 973–981.
8. H. M. Jeong, J. W. Lee, W. H. Shin, Y. J. Choi, H. J. Shin, J. K. Kang and J. W. Choi, Nano Lett., 2011, 11, 2472–2477.
9. G. X. Luo, L. Z. Liu, J. F. Zhang, G. B. Li, B. L. Wang and J. J. Zhao, ACS Appl. Mater. Interfaces, 2013, 5, 11184–11193.
10. A. L. M. Reddy, A. Srivastava, S. R. Gowda, H. Gullapalli, M. Dubey and P. M. Ajayan, ACS Nano, 2010, 4, 6337–6342.
11. Y. F. Lu, S. T. Lo, J. C. Lin, W. J. Zhang, J. Y. Lu, F. H. Liu, C. M. Tseng, Y. H. Lee, C. T. Liang and L. J. Li, ACS Nano, 2013, 7, 6522–6532.
12. G. Singh, D. S. Sutar, V. D. Botcha, P. K. Narayanam, S. S. Talwar, R. S. Srinivasa and S. S. Major, Nanotechnology, 2013, 24, 355704.
13. Y. Y. Shao, S. Zhang, M. H. Engelhard, G. S. Li, G. C. Shao, Y. Wang, J. Liu, I. A. Aksay and Y. H. Lin, J. Mater. Chem., 2010, 20, 7491–7496.
14. Z. H. Sheng, L. Shao, J. J. Chen, W. J. Bao, F. B. Wang and X. H. Xia, ACS Nano, 2011, 5, 4350–4358.
15. T. Hu, X. Sun, H. T. Sun, G. Q. Xin, D. L. Shao, C. S. Liu and J. Lian, Phys. Chem. Chem. Phys., 2014, 16, 1060–1066.
16. S. C. Hou, X. Cai, H. W. Wu, X. Yu, M. Peng, K. Yan and D. C. Zou, Energy Environ. Sci., 2013, 6, 3356–3362.
17. H. Gao, L. Guo, L. X. Wang and Y. F. Wang, Mater. Lett., 2013, 109, 182–185.
18. L. S. Panchakarla, K. S. Subrahmanyam, S. K. Saha, A. Govindaraj, H. R. Krishnamurthy, U. V. Waghmare and C. N. R. Rao, Adv. Mater., 2009, 21, 4726–4730.
19. X. R. Wang, X. L. Li, L. Zhang, Y. Yoon, P. K. Weber, H. L. Wang, J. Guo and H. J. Dai, Science, 2009, 324, 768–771.
20. Y. P. Zhang, B. Cao, B. Zhang, X. Qi and C. X. Pan, Thin Solid Films, 2012, 520, 6850–6855.
21. B. Cao, B. Zhang, X. D. Jiang, Y. P. Zhang and C. X. Pan, J. Power Sources, 2011, 196, 7868–7873.
22. H. L. Cao, X. F. Zhou, Z. H. Qin and Z. P. Liu, Carbon, 2013, 56, 218–223.
23. I. T. Kim and M. W. Shin, Mater. Lett., 2013, 108, 33–36.
24. J. W. Lang, X. B. Yan, X. Y. Yuan, J. Yang and Q. J. Xue, J. Power Sources, 2011, 196, 10472–10478.
25. H. B. Wang, T. Maiyalagan and X. Wang, ACS Catal., 2012, 2, 781–794.
26. D. S. Geng, Y. Chen, Y. G. Chen, Y. L. Li, R. Y. Li, X. L. Sun, S. Y. Ye and S. Knights, Energy Environ. Sci., 2011, 4, 760–764.
27. Z. Zafar, Z. H. Ni, X. Wu, Z. X. Shi, H. Y. Nan, J. Bai and L. T. Sun, Carbon, 2013, 61, 57–62.
28. A. Bagri, R. Grantab, N. V. Medhekar and V. B. Shenoy, J. Phys. Chem. C, 2010, 114, 12053–12061.
29. M. M. Yang, B. Cheng, H. H. Song and X. H. Chen, Electrochim. Acta, 2010, 55, 7021–7027.
30. G. H. Sun, K. X. Li and C. G. Sun, Microporous Mesoporous Mater., 2010, 128, 56–61.
31. L. Sun, L. Wang, C. G. Tian, T. X. Tan, Y. Xie, K. Y. Shi, M. T. Li and H. G. Fu, RSC Adv., 2012, 2, 4498–4506.

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