Preparation of micron sized graphite using a spark plasma technique

Abstract

A simple spark plasma technique is applied to prepare micron sized graphite from natural graphite in water. The as-prepared micron sized graphite was characterized by scanning electron microscopy, X-ray spectroscopy and X-ray photoelectron spectroscopy, which corroborated a successful oxidation of the natural graphite with abundant oxygen-containing functional groups, such as hydroxyl, epoxide and carboxyl groups, in high quality.

---

Micron sized graphite with outstanding physicochemical properties has been widely used in industry,¹ especially in the production of graphenes by Hummers method.² Currently, micron sized graphite is mainly synthesized by grinding methods based on mechanical ball-milling techniques,^3,4 with the disadvantages of long grinding time, high energy operation, and difficulty in controlling the thickness of the graphite sheets. While other methods, such as detonation on natural graphite and ultrasonic irradiation on treated graphite, have also been developed to synthesize micron sized graphite,^5,6 they were carried out by chemical modifications with harsh reaction conditions.⁷

To date, many carbon-related materials, such as carbon black,^8,9 fullerenes,¹⁰ carbon nanotubes,¹¹ graphene sheets^12–14 and graphene quantum dots,¹⁵ have been prepared by different plasma techniques due to their high and abundant energetic condition and easy work-up process. Plasma technique may also provide a promising method to prepare micron sized graphite¹⁶ because of the aromatic carbons at the edge of the natural graphite, which can be easily modified with versatile functional groups.¹⁷ Spark plasma is a method based on high temperature plasma momentarily generated in the gaps between powder materials by electrical discharge at the beginning of ON–OFF DC pulse energizing.¹⁸ The in situ generated activated species in high temperature could react with the aromatic carbons at the edge of the adjacent graphite particles, and further unzip the relatively large graphite to micro/nano particles. However, until now, there is very few paper mentioned the application of this technique in micro/nano graphite fabrications. Herein, we report a simple spark plasma technique to prepare micron sized graphite from natural graphite in aqueous solutions.

50 g natural graphite pieces (which were obtained from by Jinrilai Electronic Materials Factory, Qingdao, China) with average diameters within 5 mm (Fig. 1a, bottle A) were first dispersed at the bottom of a plasma reactor containing 150 mL water (Fig. S1,† the reactor). The spark plasma was triggered at a voltage of 12 kV, a current of 150 A, and a pulse width of 500 ns. After 10 min plasma etching, the micron sized graphite was obtained with a high dispersion property in aqueous solution (Fig. 1a, bottle B). A comparison among the natural graphite species, commercial graphite powders and our prepared graphite powders was displayed in Fig. S2,† which clearly indicated a better dispersion property of our prepared graphite powders than the commercial graphite powders after setting aside for 5 min.

Fig. 1 (a) Water dispersion of natural (A) and prepared graphite (B), (b and c) SEM images of prepared micron sized graphite.

The SEM measurements were conducted on a JEOL JSM-6330F operated at the beam energy of 15.0 kV. X-ray diffraction (XRD) patterns were recorded in reflection mode (Cu Kα radiation, λ = 1.5418 Å) using a Scintag XDS-2000 diffractometer. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCA Lab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation.

The morphologies of the as-prepared micron sized graphite were observed by SEM images (Fig. 1b and c), which displayed as sheet-like structures with variable two-dimensional planes at micron scales and nanoscale thickness. The particles were roughly flaky in shape and less than 20 μm in size (Fig. 1b). The thickness was measured directly from the as-prepared micron graphite sheets with a value of about dozens of nanometers (Fig. 1c).

The powder X-ray diffraction (XRD) was used to compare the patterns of the as-prepared micron sized graphite with the natural graphite, as depicted in Fig. 2. In the case of natural graphite, the strong (002) and (004) planes were a characteristic perpendicular direction (c-axis) to the graphite hexagonal planes, which decreased in intensity for micron sized graphite, suggesting the successful fabrication of the micron layered morphologies as expected. This result further corroborated the SEM observations.¹⁹ To further illustrate the size change of graphite, the average grain size, D, was calculated from the Scherrer formula, , where λ is 0.15406 nm, θ is the Bragg angle, and B is the full width at half maximum (FWHM).²⁰ While the D value of the natural graphite was calculated to be ∼42.71 nm, it reduced to ∼15.87 nm for the micron sized graphite. Both graphites were observed with distinct diffraction peaks at 26.3°, corresponding to a d₀₀₂ basal spacing of approximately 0.336 nm, which indicated a reserved interlayer spacing after spark plasma etching with a slight decrease in intensity.⁵ The weak peaks at 2θ = 43–44° were attributed to the unknown impurities in the as-prepared micron sized graphite, resulting in a lower degree of graphitization of 97.2% as compared with the natural graphite (99.7%), a potential risk of structure change or destruction.^21,22

Fig. 2 XRD patterns of natural graphite and prepared micron sized graphite.

The typical XPS survey spectra of the natural graphite and the as-prepared micron sized graphite were shown in Fig. 3. The O 1s intensity of the as-prepared micron sized graphite was nearly twice as that of the natural graphite, indicating an increased oxygen-containing functional groups after spark plasma etching (Fig. 3a). The high-resolved C 1s and O 1s spectra were displayed in Fig. 3b and c, respectively. The C 1s spectrum was resolved into five component Gaussian peaks: (1) the peak at 284.6 ± 0.2 eV corresponded to the sp² hybridized graphite-like carbon atom (C=C); (2) the peak at 285.7 ± 0.2 eV was attributed to the sp³ hybridized carbon atom (C–C); and (3) the peaks at 286.7 ± 0.2, 287.2 ± 0.2 and 288.5 ± 0.2 eV were considered as hydroxyl groups (C–OH), carbonyl groups (C=O) and carboxylate groups (COO⁻), respectively.^11,23 The curve fitting results are listed as a inset table in Fig. 3b. As for the O 1s spectrum, it can be resolved into three components at 532.1 ± 0.2, 533.6 ± 0.2 and 531.3 ± 0.2 eV for C=O, C–O and COO⁻ functional groups, respectively. These observations clearly indicated the oxidation of the aromatic edge carbons by introducing oxygen-containing functional groups, such as hydroxyl, carbonyl and carboxylate groups.¹¹ As a result, the zeta potential of the as-prepared micron sized graphite is lower than that of commercial graphite power due to those more oxygen-containing functional groups (Fig. S3†).

Fig. 3 (a) XPS wide-scan spectra of natural graphite and prepared micron sized graphite; high resolved (b) XPS C 1s and (c) O 1s spectra of prepared micron sized graphite. The inset tables are the curve fitting results of C 1s and O 1s spectra of the prepared micron sized graphite (BE: binding energy; FWHM: full widths at half maximum).

While the specific mechanism about the spark plasma technique is still unknown, a plausible mechanism is proposed and illustrated in Fig. 4a. Different from the fabrication of metal nanoparticles by melting techniques from high temperature plasma, the spark plasma technique used here should be described and acted as a physical peeling-off technique. Spark plasma, spark impact pressure, joule heating, and an electrical field diffusion effect were generated between the adjacent natural graphite powders.²⁴ The interval high pressure and heat were generated and released periodically, which provided pushing forces to break down the graphite flakes, and further to decompose or split the graphite powders into 2D planes at microscale levels. Moreover, the activated free radicals (such as HO˙, O˙) were generated simultaneously from water by the spark plasma discharging process, which oxidized edging carbon atoms by introducing extra oxygen-containing functional groups to generate the as-prepared micron sized graphite (Fig. 4b).²⁵

Fig. 4 (a) Schematic illustration of proposed mechanism; (b) proposed oxygen-containing functional groups on the surface of prepared micron sized graphite.

Conclusions

The present work proved a facile fabrication method for the synthesis of the micron sized graphite from the natural graphite by a spark plasma technique. The oxidation of the aromatic edge carbons were oxidized by introducing abundant oxygen-containing functional groups, which was confirmed by the XPS measurement. This simple reactor and mild reaction condition make the micron sized graphite easily to be produce with high quality at large-scale within a short time and a low price.

Acknowledgements

The authors acknowledge the financial support from National Natural Science Foundation of China (No. 21272236, 21477133), the Chinese National Fusion Project for ITER (2013GB110005) and the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection and the Priority Academic Program Development of Jiangsu Higher Education Institutions are acknowledged.

References

1. T. S. M. Enoki and M. Endo, Graphite intercalation compounds and applications, Oxford University, New York, 2003, pp. 6–41.
2. L. Y. Jiao, L. Zhang, X. R. Wang, G. Diankov and H. J. Dai, Nature, 2009, 458, 877–880.
3. Y. Kuga, M. Shirahige, T. Fujimoto, Y. Ohira and A. Ueda, Carbon, 2004, 42, 293–300.
4. M. Hentsche, H. Hermann, T. Gemming, H. Wendrock and K. Wetzig, Carbon, 2006, 44, 812–814.
5. J. H. Zhu, M. J. Chen, Q. L. He, L. Shao, S. Y. Wei and Z. H. Guo, RSC Adv., 2013, 3, 22790–22824.
6. Y. F. Li, J. H. Zhu, S. Y. Wei, J. Ryu, Q. Wang, L. Y. Sun and Z. H. Guo, Macromol. Chem. Phys., 2011, 212, 2429–2438.
7. G. L. Sun, X. J. Li, H. H. Yan, J. S. Qiu and Y. J. Zhang, Carbon, 2008, 46, 476–481.
8. G. H. Chen, W. G. Weng, D. J. Wu, C. L. Wu, J. R. Lu, P. P. Wang and X. F. Chen, Carbon, 2004, 42, 753–759.
9. K. S. Yun, B. R. Kim, W. S. Kang, S. C. Jung, S. T. Myung and S. J. Kim, J. Nanosci. Nanotechnol., 2013, 13, 7381–7385.
10. J. G. You, S. P. Xu, S. Q. Liu, D. M. Tian and J. A. Chi, New Carbon Mater., 2003, 18, 144–150.
11. R. Dubrovskya, V. Bezmelnitsyna and A. Eletskiib, Carbon, 2004, 42, 1063–1066.
12. S. Iijima, Nature, 1991, 354, 56–58.
13. G. X. Zhao, D. D. Shao, C. L. Chen and X. K. Wang, Appl. Phys. Lett., 2011, 98, 183114.
14. J. Kim, S. B. Heo, G. H. Gu and J. S. Suh, Nanotechnology, 2010, 21, 095601.
15. D. Van Thanh, L. J. Li, C. W. Chu, P. J. Yen and K. H. Wei, RSC Adv., 2014, 4, 6946–6949.
16. J. Kim and J. S. Suh, ACS Nano, 2014, 8, 4190–4196.
17. H. Lee, M. A. Bratescu, T. Ueno and N. Saito, RSC Adv., 2014, 4, 51758–51765.
18. T. Fukunaga, K. Nagano, U. Mizutani, H. Wakayama and Y. Fukushima, J. Non-Cryst. Solids, 1998, 232, 416–420.
19. G. H. Chen, D. J. Wu, W. G. Weng and C. L. Wu, Carbon, 2003, 41, 579–625.
20. V. T. Dang, H. C. Chen, L. J. Li, C. W. Chu and K. H. Wei, RSC Adv., 2013, 3, 17402–17410.
21. M. Stingaciu, R. K. Kremer, P. Lemmens and M. Johnsson, J. Appl. Phys., 2011, 110, 044903.
22. W. Kohs, H. J. Santner, F. Hofer, H. Schrottner, J. Doninger, I. Barsukov, H. Buqa, J. H. Albering, K. C. Moller, J. O. Besenhard and M. Winter, J. Power Sources, 2003, 119, 528–537.
23. C. L. Chen, A. Ogino, X. K. Wang and M. Nagatsu, Diamond Relat. Mater., 2011, 20, 153–156.
24. J. Carrey, H. B. Radousky and A. E. Berkowitz, J. Appl. Phys., 2004, 95, 823.
25. H. S. Uhm, J. H. Kim and Y. C. Hong, Appl. Phys. Lett., 2007, 90, 211502.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07710b

---