The Effect of Thermal and Ultrasonic Treatment on the Formation of Graphene-oxide Nanosheets

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1 Journal of the Korean Physical Society, Vol. 56, No. 4, April 2010, pp The Effect of Thermal and Ultrasonic Treatment on the Formation of Graphene-oxide Nanosheets Won-Chun Oh, Ming-Liang Chen, Kan Zhang and Feng-Jun Zhang Department of Advanced Materials & Science Engineering,, Hanseo University, Seosan-si Won-Kweon Jang Div. of Electronic, Computer, and Communication Engineering, Hanseo University, Seosan-si Feng-Jun Zhang Anhui Key Laboratory of Advanced Building Materials, Anhui University of Architecture, Anhui Hefei, P. R. China (Received 6 January 2010, in final form 17 March 2010) Thermal and ultrasonic treatments of graphene oxide (GO) are proposed to produce grapheneoxide layers. These layers were comprehensively characterized by using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM), Raman spectroscopy, and Atomic Force Microscopy (AFM). The XRD patterns of the GO layers produced by using thermal treatment showed a broad peak at around 26.09, and those of the GO layers produced by using ultrasonic treatment showed a distinct peak at The FT-IR spectra indicated that the ultrasonic treatments for the sample GO did not change the functional group; however, all oxygencontaining functional groups in the GO layers produced by using thermal treatment were nearly completely removed. The EDX results showed that only 8.6 wt.% oxygen still existed in the sample of GO layers produced by using thermal treatment. From the TEM images, monolayer graphene oxide could be found in a flake form in the GO layers by thermal treatment. The visible D peak, the clear G peak and the characterized 2D band in the Raman spectra indicate the existence of defect-free monolayer and few-layer graphenes. AFM results showed that a single layer of thermally treated graphene oxide had been produced. PACS numbers: Fb Keywords: Graphene oxide, Thermal and ultrasonic treatment, TEM, Raman, AFM DOI: /jkps I. INTRODUCTION As a one-atom-thick two-dimensional crystal, graphene has been considered as a basic building block for all sp 2 graphitic materials including fullerenes, carbon nanotubes, and graphite [1]. Since Geim et al. peeled a few graphene sheets from high-crystalline graphite by using a scotch tape method in 2004 [2], graphene has increasingly attracted attention owing to its fascinating physical properties, including its unique electronic [3-10], thermal [11], and mechanical properties [12,13]. These unique properties hold great promise for potential applications in many technological fields, such as nanoelectronics, sensors, nanocomposites, batteries, supercapacitors, and hydrogen storage [13 16]. wc oh@hanseo.ac.kr In order to turn graphene applications into reality, one must fabricate the material on a large scale. Graphene was isolated by using micromechanical cleavage of graphite crystals. Recently, several approaches to synthesize large-scale graphene have been reported, including epitaxial growth on SiC, chemical reduction of graphite oxide, and exfoliation of graphene sheets [17-19]. The properties of two-dimensional graphene depend largely on its synthesis method, and they rely on the surface roughness and graphene-substrate interaction [20]. In addition, the properties of graphene materials are very sensitive to the number of graphene layers. Graphene s remarkable properties extend to bilayer and multi-layer graphene. However, producing graphene on a large scale by using existing mechanical methods is not still feasible. Searching for alternative simple approaches is an urgent matter [21]. However, the hydrophobic nature of

2 Journal of the Korean Physical Society, Vol. 56, No. 4, April 2010 graphene and its strong tendency to agglomerate in solvents present a great challenge to the development of fabrication methods and severely restrict its promising applications. Although the mechanism involved remains unproven, the chemical reduction of readily available exfoliated graphite oxide (GO) with reducing agents such as hydrazine and dimethylhydrazine is a promising strategy for large-scale production of graphene [22,23]. Unfortunately, the reducing agents involved are very hazardous, and the graphene obtained presents irreversibly agglomerated features in solvents. Herein, we report simple routes for the synthesis of processable graphene on a large scale by using two different processes: thermal and ultrasonic treatments of graphene oxide. II. EXPERIMENTAL 1. Preparation of Graphene Oxide Graphite (KS-6) was selected as the starting material. Graphene oxide (GO) was prepared from graphite according to the Hummers-Offeman method [24]. In brief, graphite powder (10 g) was dispersed in cold concentrated sulphuric acid (230 ml, 98 wt%, dry ice bath), and potassium permanganate (KMnO 4, 30 g) were gradually added with continuous vigorous stirring and cooling to prevent the temperature from exceeding 293 K. The dry ice bath was removed and replaced by a water bath, and the mixture were heated to 308 K for 30 min with gas release under continuous stirring, followed by slow addition of deionized water (460 ml), which produced a rapid increase in solution temperature up to a maximum of 371 K. The reaction was maintained for 40 min in order to increase the oxidation degree of the GO product; then the resultant bright-yellow suspension was terminated by addition of more distilled water (140 ml), followed by a hydrogen peroxide solution (H 2 O 2, 30%, 30 ml). The solid products was separated by centrifugation at 3000 rpm, was washed initially with 5% HCl until sulphate ions were no longer detectable with barium chloride (BaCl2), were then washed three times with acetone, and were air dried overnight at 338 K. 2. Thermal and Ultrasonic Treatment of Graphene Oxide Thermal and ultrasonic treatments of graphene oxide were performed as follows: After ca 0.5 grams of the graphene oxide powder had been heated at 1273 K for 1 h and then cooled to room temperature, sample of thermally treated of graphene oxide (TTG) was produced. Twenty-five milligrams of the graphene oxide powder was placed in a cup, and 200 ml of de-ionized water was then added. Ten minutes of magnetic stirring at 200 rpm yielded an inhomogeneous brown suspension. The resulting suspensions were further treated with ultrasonication (30 min, J), and after having been dried at 373 K, the sample of ultrasonically treated graphene oxide (UTG) was produced. 3. Characterizations of Samples X-ray diffraction (XRD) measurements were performed for GO, TTG, and UTG samples at room temperature. XRD patterns were obtained with a diffractometer (Shimata XD-D1, Japan) using Cu Kα radiation. Scanning electron microscopy (SEM) was used to observe the surface state and the structure of the samples (JOEL, JSM-5200, Japan). Energy dispersive X-ray (EDX) spectroscopy was for the elemental analysis of the composites. Raman spectra were utilized to detect possible structural defects in the graphene flakes. The measurements were carried out by using a Horiba Jobin Yvon LabRAM system with a 100 objective lens and a 532 nm laser excitation. Thin films ( 30 nm) were prepared by using vacuum filtration of the dispersions through porous alumina membranes. The state of the dispersed graphenes was observed using transmission electron microscopy (TEM, JEOL, JEM-2010, Japan). TEM at an acceleration voltage of 200 kv was used to investigate the number and the stacking state of graphene oxide layers on various samples. TEM specimens were prepared by placing a few drops of sample solution on a carbon grid. Fouriertransform-infrared (FT-IR) spectroscopy (FTS 3000MX, Biored Co.) was used to characterize the composite materials. The GO samples were also characterized by using atomic force microscopy (AFM, Tip mode, frequency Hz, Veeco NanoScope IIIa Multimode, DI, USA). AFM observations were conducted for the graphene oxide layers, and the samples for AFM analyses were precisely prepared by depositing a hydrosol of graphene oxide on freshly cleaved mica surfaces. III. RESULTS AND DISCUSSION Figure 1 illustrates the FT-IR spectra of powdery GO, UTG, and TTG. The most characteristic features in the FT-IR spectrum of GO are the adsorption bands corresponding to the C=O carbonyl stretching mode at 1728 cm 1, the C-OH stretching mode at 1226 cm 1, and the C-O stretching mode at 1050 cm 1 [25 27]. Beside the broad spectra at 3400 cm 1 is due to the stretching vibrations of structural OH groups, and the resonance at 1619 cm 1 can be assigned to the vibrations of the adsorbed water molecules, but the spectra may also contain components from the skeletal vibrations of unoxidized

3 The Effect of Thermal and Ultrasonic Treatment on the Formation of Graphene-oxide Won-Chun Oh et al Fig. 1. FTIR spectra of GO, UTG, and TTG. Fig. 2. XRD patterns of GO, UTG, and TTG. graphitic domains [25,28,29]. These peaks in the spectrum of UTG are very similar to these in the spectrum of GO, indicating that the ultrasonic treatment of the GO sample did not change the functional groups. However, in the spectrum of TTG, these peaks had almost totally disappeared, suggesting that all oxygen-containing functional groups in TTG layers had been nearly completely removed during the thermal treatment at 1273 K. Figure 2 shows the XRD patterns of powdery GO, UTG and TTG. The characteristic 2θ peak of GO appearing at corresponds to a d-spacing of approximately Å, which is consistent with the interlayer spacing of GO sheets reported in the literature, due to the existence of oxygen-rich groups on both sides of the sheets and water molecules trapped between the sheets [30,31]. The XRD pattern of UTG shows a distinct peak at corresponding to a d-spacing of Å, which might be attributed to the stacking of GO layers by ultrasonic treatment. The XRD pattern of TTG shows a broad peak at around corresponding to a d- Fig. 3. Raman spectra of some graphene-oxide nanosheets; (a) GO, (b) UTG, and (c) TTG. spacing of Å, which may be due to a modification of GO layers by thermal treatment. The obtained powdery GO, UTG, and TTG were further investigated by Raman spectroscopy. As shown in Fig. 3, the Raman spectrum of powdery GO displays two prominent peaks: 1356 and 1596 cm 1, which correspond to the well-documented D- and G-band, respectively. The Raman spectra of the obtained UTG and TTG also show both D- and G-bands at 1347 and 1596 cm 1 with D/G intensity ratios comparable to that

4 Journal of the Korean Physical Society, Vol. 56, No. 4, April 2010 Table 1. EDX elemental microanalyses of (a) GO, (b) UTG, and (c) TTG. Sample GO UTG TTG Fig. 4. SEM images of some graphene-oxide nanosheets; (a) GO, (b) UTG, and (c) TTG. of powdery GO, which suggests that the skeleton structure of GO remains in the UTG and the TTG. The D peak ( 1350 cm 1 ) and the G peak ( 1596 cm 1 ) are clearly seen in all cases. The characteristics of the 2D band ( 2700 cm 1 ) vary, along with the number of layers in the graphene flakes, from a single peak (monolayer) to a band with four component peaks (bilayer) to a broadened band (<five layers), and then finally to a graphite-type band with two component peaks (>five layers) [32]. As shown in Fig. 3, for the TTG, the shape of the 2D band indicates less than five layer of graphene, which further implies that only a few layers aggregate during the thermal treatment. The visible D peak, the clear G peak, and the characterized 2D band, witness the existence of defect-free monolayer graphene and fewlayer graphene in the TTG sample [33,34]. Figure 4 shows the SEM images of GO, UTG, and TTG composites. The samples show platelike forms without any amorphous or other kinds of crystallized C Element (wt.%) O C/O Fig. 5. EDX elemental microanalysis: (a) GO, (b) UTG, and (c) TTG. phase particles. The morphology of GO is observed to have flaky texture, reflecting its layered microstructure, as shown in Fig. 4(a). The larger interspaces of the layer and the thinner layer edges of GO can be clearly seen in Fig. 4(a). Fig. 4(a) demonstrates the structure of stacked GO layers, which results in a larger particle size in comparison with UTG (Fig. 4(b)). UTG composites show a relatively close surface morphology, which may be due to the exfoliation of the GO layers during the ultrasonic treatment process in an aqueous media. In the case of TTG composites, an exfoliated structure morphology can be found (Fig. 4(c)). TTG composites are thought to have separated into flakes, scales, or layers during the thermal treatment process. The morphological changes of the TTG composites compared with pristine GO might be ascribed to the thermal treatment. Figure 5 shows the EDX spectra of the prepared GO, UTG, and TTG composites. The results of the EDX elemental microanalyse for C and O elements are listed in Table 1. The C contents of the GO, UTG, and TTG are

5 The Effect of Thermal and Ultrasonic Treatment on the Formation of Graphene-oxide Won-Chun Oh et al Fig. 6. TEM images of some graphene-oxide nanosheets ; (a) GO and (b) TTG , 52.23, and 85.2%, respectively. The C contents of the TTG in the composites are expected to be increased due to the thermal treatment. Moreover, the O contents of TTG are only 8.59% and the mass ratio of C/O is 10, which indicates that some oxygen-containing functional groups still exist in the sample of TTG. The TEM image of TTG is compared with that of pristine GO in Fig. 6. The morphology of GO, consisting of thin stacked flakes and having a well-defined multilayered structure at the edge, can be clearly seen in Fig. 6(a). Monolayer fraphene and few-layer graphene is clearly seen in the sample of TTG (Fig. 6(b)). The fact that TTG was dressed on the surface of GO layers (Fig. 6(b)) may be interpreted by using Fig. 3(c). We propose that the electronegativity of oxygen atoms of the -OH, and the -COOH groups on the GO layer surface facilitates the further oriented aggregation of cation radicals of carbon. In addition, the surface oxygen-containing groups of GO might form H-bonds with the carbon atoms of TTG. Thus TTG grows into monolayer graphene in a flake form. The surface morphologies were characterized by using the average thickness of the sheets, the intervals between the sheets and the roughness parameters such as Ra. The parameter Ra is the mean roughness, that is, the mean value of the surface relative to the center plane (the plane for which the volume enclosed by the image above and below this plane are equal) [35]. The AFM images showed representative 3-D surface morphologies of the TTG composites. As expected, the surface morphologies were quite different, and the minimum height of the obtained layers (Fig. 7, bottom) was 3.09 nm Fig. 7. AFM image of TTG ; (a) histogram, (b) line profile, and (c) statistics. (Ra). The measured thickness of pristine graphene obtained by using mechanical cleavage was 1.6 nm because of the change in the tip-sample interaction as the tap-

6 Journal of the Korean Physical Society, Vol. 56, No. 4, April 2010 ping tip scanned over the surface and other factors [23]. Given the existence of residual oxygen-containing functional groups [36], the TTG layers should be relatively thicker. Thus, we speculate that a single layer of TTG layers had been produced. IV. CONCLUSION In this study, thinner graphene oxide layers were produced by using thermal and ultrasonic treatments. FT- IR spectroscopy showed that the ultrasonic treatment did not change the functional groups of GO; however, all oxygen-containing functional groups in the TTG layers were nearly completely removed during the thermal treatment. The visible D peak, the clear G peak, and the characterized 2D band, witness the existence of defectfree monolayer graphene and few-layer graphene in the sample of TTG. Monolayer and few-layer graphene were clearly observed in the TEM image of TTG. The minimum height of the obtained layers (AFM) was 3.09 nm. ACKNOWLEDGMENTS This work was supported by the ACT support project (No ) supervised from The Ministry of Knowledge Economy in The authors are grateful to The Ministry of Knowledge Economy for financial support. REFERENCES [1] A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183 (2007). [2] K. S. Novoselov et al., Science 306, 666 (2004). [3] K. S. Novoselov et al., Nature 438, 197 (2005). [4] K. S. Novoselov et al., Science 315, 1379 (2007). [5] C. Gomez-Navarro, R. T. Weitz, A. M. Bittner, M. Scolari, A. Mews, M. Burghard and K. Kern, Nano Lett. 7, 3499 (2007). [6] H. B. Heersche, P. Jarillo-Herrero, J. B. Oostinga, L. M. K. Vandersypen and A. F. Morpurgo, Nature 446, 56 (2007). [7] Y. Kopelevich and P. Esquinazi, Adv. Mater. 19, 4559 (2007). [8] G. M. Rutter, J. N. Crain, N. P. Guisinger and T. Li, Science 317, 219 (2007). [9] N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman and B. J. van Wees, Nature 448, 571 (2007). [10] J. B. Oostinga, H. B. Heersche, X. L. Liu, A. F. Morpurgo and L. M. K. Vandersypen, Nat. Mater. 7, 151 (2008). [11] A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao and C. N. Lau, Nano Lett. 8, 902 (2008). [12] D. A. Dikin et al., Nature 448, 457 (2007). [13] S. Stankovich et al., Nature 442, 282 (2006). [14] X. Wang, L. J. Zhi and K. Mullen, Nano Lett. 8, 323 (2008). [15] J. S. Wu, W. Pisula and K. Mullen, Chem. Rev. 107, 718 (2007). [16] J. H. Chen, M. Ishigami, C. Jang, D. R. Hines, M. S. Fuhrer and E. D. Williams, Adv. Mater. 19, 3623 (2007). [17] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes and Y. Jia, Carbon 45, 1558 (2007). [18] M. J. McAllister et al., Chem. Mater. 19, 4396 (2007). [19] D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol. 3, 101 (2008). [20] M. Ishigami, J. H. Chen, W. G. Cullen, M. S. Fuhrer and E. D. Williams, Nano Lett. 7, 1643 (2007). [21] R. Ruoff, Nat. Nanotechnol. 3, 10 (2008). [22] Y. Si and E. T. Samulski, Nano Lett. 8, 1679 (2008). [23] S. J. Park, J. H. An, R. D. Piner, I. Jung, D. X. Yang, A. Velamakanni, S. T. Nguyen and R. S. Ruoff, Chem. Mater. 20, 6592 (2008). [24] W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc. 80, 1339 (1958). [25] T. Szabo, O. Berkesi and I. Dekany, Carbon 43, 3186 (2005). [26] C. Hontoria-Lucas, A. J. Lopez-Peinado, J. D. Lopez- Gonzalez, M. L. Rojas-Cervantes, R. M. Martin-Aranda, Carbon 33, 1585 (1995). [27] G. I. Titelman, V. Gelman, S. Bron, R. L. Khalfin, Y. Cohen and H. Bianco-Peled, Carbon 43, 641 (2005). [28] M. Mermoux, Y. Chabre and A. Rousseau, Carbon 29, 469 (1991). [29] S. Stankovich, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Carbon 44, 3342 (2006). [30] D. D. Zhang, S. Z. Zu and B. H. Han, Carbon 47, 2993 (2009). [31] S. J. Park, J. H. An, I. H. Jung, R. D. Piner, S. J. An and X. S. Li, Nano Lett. 9, 1593 (2009). [32] J. Wang, Y. Hernandez, M. Lotya, J. N. Coleman and W. J. Blau, Adv. Mater. 21, 1 (2009). [33] Y. Hernandez et al., Nat. Nanotechnol. 3, 563 (2008). [34] A. C. Ferrari et al., Phys. Rev. Lett. 97, (2006). [35] C. Khulbe, B. Kruczek, G. Chowdhury, S. Gagne and T.Matsuura, J. Appl. Polym. Sci. 59, 1151 (1996). [36] P. Nemes-Incze, Z. Osváth, K. Kamarás and L.P. Biró, Carbon 46, 1435 (2008).