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Hydrothermal synthesis of magnetic reduced graphene oxide sheets
Materials Research Bulletin 46 (2011) 2077–2083
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Hydrothermal synthesis of magnetic reduced graphene oxide sheets Jianfeng Shen, Min Shi, Hongwei Ma, Bo Yan, Na Li, Mingxin Ye * Center of Special Materials and Technology, Fudan University, Shanghai 200433, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 May 2011 Received in revised form 19 June 2011 Accepted 25 June 2011 Available online 2 July 2011
We demonstrated an environmental friendly and efficient route for preparation of magnetic reduced graphene oxide composite (MN-CCG). Glucose was used as the reducing agent in this one-step hydrothermal method. The reducing process was accompanied by generation of magnetic nanoparticles. The structure and composition of the nanocomposite was confirmed by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, thermal gravimetric analysis, atomic force microscopy and transmission electron microscopy. It was found that the prepared MN-CCG is highly water suspendable and sensitive to magnetic field. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Nanostructures B. Chemical synthesis D. Magnetic properties
1. Introduction Graphene, one-atom thick planar carbon nanosheet, has generated huge activity in many areas of science and engineering due to its unprecedented physical and chemical properties [1–8]. It is expected to find a variety of applications because of its distinctive properties [9–20]. The reliable technique to produce single layer graphene of high quality is the micromechanical cleavage method [21]. However, this route is not practical for large-scale generation of graphene. Nowadays, among known strategies for producing graphene-based materials, utilization of graphite oxide (GO) is the most versatile and scalable method [22–31]. GO is a strongly oxygenated and highly hydrophilic material that can be easily exfoliated in water to yield stable dispersions consisting mostly of graphene oxide sheets (GOS). These stable and flexible GOS are usually used as the starting material for the generation of graphene-based materials. However, though the chemical oxidation method is convenient to exfoliate GOS via solution based processes, it introduces functional groups such as carboxyl and epoxide. The presence of these groups makes the GOS suspendable in polar solvents but drastically decreases the conductivity as a result of loss in the conjugated sp2 network. Hydrazine has been widely used to reduce GOS to get chemically converted graphene (CCG) [32–35].
* Corresponding author at: Center of Special, Materials and Technology, Fudan University, 220 Handan Road, Shanghai 200433, China. Tel.: +86 021 55664095; fax: +86 021 55664094. E-mail address: [email protected] (M. Ye). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.06.042
Magnetic nanoparticles have attracted increasing interest in the material and colloid science community in recent years. Recent researches have been devoted to high-tech applications as in the fields of magnetic storage devices, biomedical engineering and drug delivery [36–38]. Composing graphene with other functional materials is a promising way to combine the effect of functional materials with that of graphene. As one kind of graphene-based hybrid material, graphene/nanoparticles composites have aroused extensive interest as hybridization improves the performance of both graphene and nanoparticles [29,39,40–48]. More recently, nanoparticles dispersed on graphene sheets have been prepared using hydrazine as a reducing agent. Unfortunately, the use of highly toxic and dangerously unstable hydrazine or dimethylhydrazine to reduce GOS remains a serious challenge for large-scale production of graphene nanosheets. Besides, these methods were shown to be difficult to control and hence not suitable for mass production. For these reasons, it is highly desirable to develop a low-cost, facile and ‘‘green’’ approach for preparing soluble RGO sheets in large quantities. It is shown that glucose can be used as a ‘‘green’’ agent to prepare RGO in large scale [49]. The combination of iron oxide nanostructure and graphene has potential for the use of magnetic graphene sheets in many applications ranging from electromagnetic devices to magnetically guided drug delivery systems [50]. Zhu et al. prepared RGO/Fe2O3 with a two-step synthesis by homogeneous precipitation and subsequent reduction of GO with hydrazine under microwave irradiation [51]. Liang et al. reported the fabrication of magnetic, electrically conducting, and flexible paper composed of graphene and Fe3O4 with hydrazine as the reducing agent [52]. Shi et al. reported a facile approach to synthesis Fe3O4 nanoparticles
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attached to RGO by a solvothermal process [53]. Zhang et al. employed microwave irradiation as a heat source to prepare magnetite graphene composites [54]. Zhou et al. prepared a wellorganized flexible interleaved composite of graphene decorated with Fe3O4 particles through in situ reduction of iron hydroxide between graphene sheets [55]. However, considering these results, despite the significant efforts, there still exist some challenges and problems in the field of MN-CCG composites: (1) most of the preparation protocol is relatively complex and many of them used toxic materials; (2) the integrated properties of both graphene and magnetic nanoparticles were rarely concerned; (3) the dispersibility of functionalized graphene sheets needs to be improved; and (4) the composites have rare functional groups for further chemical modification. In this work, we synthesized the composite of CCG decorated with iron oxide nanoparticles (MN-CCG) by a simple hydrothermal method. We used graphene oxide as a precursor for CCG and ferrous sulfate as a single-source precursor of iron oxide. The hydrothermal conversion method used in this study has several advantages over the common reduction processes: (1) the reducing ability of glucose is greatly enhanced with the hydrothermal condition, which is strong enough to reduce graphene oxide; (2) the process is simple and scalable, and is industrially compatible; (3) it is intrinsically pure since it utilizes only water and glucose; and (4) the closed system of relative high temperature and internal pressure promotes the recovery of pconjugation after dehydration, which is favorable for minimizing defects [56]. To the best of our knowledge, few studies on the preparation of MN-CCG using hydrothermal method have been reported so far. 2. Experimental procedures 2.1. Materials Pristine graphite was purchased from Qingdao BCSM. Co., Ltd. Ferrous sulfate, glucose and ammonium hydroxide was supplied by
Shanghai Chemical Reagent Company. All other reagents were at least of analytical reagent grade and used without further purification. 2.2. Preparation of MN-CCG GO was obtained by the modified Hummers method as described elsewhere [57–59]. 100 mg of ferrous sulfate was added to 5 mL of water. The above mixture was slowly dropped to the mixture of 20 mg NaOH/5 mL ethanol to form part A. 100 mg of GO was added to 50 mL water. The mixture was sonicated for 30 min followed by high-speed stirring for further 1 h. 100 mg glucose and 1 mL ammonium hydroxide was added to the GO solution to get part B. Subsequently, part A and part B were mixed. The mixture was put into an autoclave and heated at 160 8C for 4 h. The color of the suspension shifted from brown to black, which indicated the transition from GO to CCG. When the reduction reaction was finished, the as-synthesized product was isolated by centrifugation, washed with pure water and ethanol for several times, and dried at 90 8C for 12 h. The fabrication process is outlined in Fig. 1. In addition, CCG and iron oxide were synthesized in the same way as the composites in the absence of ferrous sulfate and GO, respectively. 2.3. Instruments and measurement Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a NEXUS 670 spectrometer. Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal microspectrometer with 514.5 nm laser excitation. Thermogravimetric analysis (TGA) was conducted with Netzsch TG 209F1 that was fitted to a nitrogen purge gas at 10 8C/min heating rate. X-ray diffraction (XRD) was taken on D/max-gB diffractometer using Cu Ka radiation. Transmission electron microscopy (TEM) was performed with a JEOL JEM-2100F. Atomic force microscopy (AFM) images were obtained using a Multimode Nano4 in the tapping mode. Water bath sonication was performed with a JL-60 DTH sonicator (50 W).
Fig. 1. Experimental procedure used in this study.
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Fig. 2. FTIR spectra of GO (a), CCG (b), iron oxide (c) and MN-CCG (d).
3. Results and discussion One purpose of this study is to find an efficient, scalable and green way to prepare CCG. The exfoliated single-layer GOS can be well dispersed in aqueous solutions, which provides a low temperature process to make single-layer graphene sheets. Besides, this solution-compatible process is potential for largearea application. In view of the fact that hydrazine is toxic and dangerously unstable, it is urgent to develop a green chemistry route for the reduction of graphene oxide. Though there are some researches on the green preparation of graphene, most of them are time-consuming with little productivity. Recognizing these difficulties, we present here for the first time a facile hydrothermal method for preparation of stable MN-CCG suspension with nontoxic agent in an alkaline medium. In the basic medium, subsequent carboxyl termination occurred with the evidence of color change from yellow-brown to black during the hydrothermal process as the exfoliated GOS with negatively charged oxygen functional groups can undergo fast deoxygenating in alkaline solution (pH 10). Fig. 2 shows the FTIR spectra of GO (a), CCG (b), iron oxide (c) and MN-CCG (d). The spectrum of GO (Fig. 2a) illustrates the presence of C–O (nC–O at 1100 cm 1), C5 5O in carboxylic acid and 1 carbonyl moieties (nC5 ). The broad, intense band at 5O at 1650 cm 3450 cm 1 can be attributed to O–H stretching vibration. For the CCG sample (Fig. 2b), the intensities of the bands associated to oxygen functional groups strongly decrease in relation to those of GO. Nevertheless, the elimination of these bands is not complete. In particular, the persistence of the band at about 1450 cm 1
Fig. 3. Raman spectra of raw graphite (a), GO (b), CCG (c), and MN-CCG (d).
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Fig. 4. TGA curves of raw graphite (a), GO (b), CCG (c), iron oxide (d) and MN-CCG (e).
(carboxy C–O) implies that a little fraction of hydroxyl functionalities still remain in the CCG sample. In the spectrum of MN-CCG (Fig. 2d), we find that most of the peaks in spectra of CCG and magnetic nanoparticles (Fig. 2c) also exist. Additional vibrational band at 1440 cm 1 appeared which can be assigned to the formation of monodentate complex between the carboxyl group and Fe on the surface of the iron oxide nanoparticles. Moreover, the peak at 600 cm 1 can be ascribed to lattice absorption of iron oxide [50]. These results support that the preparation of MN-CCG with our hydrothermal process is successful. Raman spectroscopy is a widely used tool to study the structure of conjugated carbon bonds because of their high Raman intensities. In contrast to the Raman spectrum of raw graphite (Fig. 3a), which shows a sharp Raman band of the graphite lattice at 1580 cm 1 (G band) and a weak disorder band caused by the graphite edges (D band), a strong D band appears at 1355 cm 1 in the Raman spectrum of GO (Fig. 3b). This D band is attributed to the sp3 amorphous carbons generated by oxidation. In addition, the G band becomes broader and blue shifts to 1600 cm 1. The Raman spectrum of the CCG (Fig. 3c) showed two peaks at 1605 cm 1 (in plane vibrational G band), and 1360 cm 1 (disordered vibrational D band). The G band of CCG shifts to a higher frequency in comparison with pristine graphite, which is attributed to the less ordered carbon and defects of the CCG sheets [60]. In the spectrum of MN-CCG, we can find that the there is a 5 cm 1 blue shift in G
Fig. 5. XRD patterns of raw graphite (a), GO (b), CCG (c), iron oxide (d) and MN-CCG (e).
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Fig. 6. AFM images of graphene oxide (a), CCG (b) and MN-CCG (c) with concentration of 0.05 mg/mL. Image dimensions are 1.2 mm 1.2 mm, 2.4 mm 2.4 mm and 1.0 mm 1.0 mm.
band position when compared to bulk graphite. This shift is attributed to the transformation of bulk graphite crystal to CCG sheets. It suggests a decrease in the average size of the sp2 domains upon reduction of the exfoliated GOS, but more numerous in numbers [61]. TGA is a complementary technique that can reveal the composition and changes in thermal stability of the samples. The successful functionalization of CCG with magnetic nanoparticles was also reflected in the TGA curves. TGA trace of pristine graphite (Fig. 4a) shows little weight loss, displaying a simple linear dependence after 500 8C, which is about 3% below 700 8C. In contrast, although GO (Fig. 4b) is thermally unstable and starts to lose mass upon heating even below 100 8C, the major mass loss occurs at about 200 8C, presumably due to pyrolysis of the labile oxygen-containing functional groups to yield CO, CO2 and steam. The TGA curve of CCG (Fig. 4c) shows a mass loss of 6% in the rage of 110–300 8C, while TGA trace of GO shows a mass loss of more than 25% in the same range, suggesting a much lower content of oxygencontaining groups in CCG than in GO. While in the curve of MNCCG (Fig. 4e), comparing with the curve of GO and magnetic nanoparticles (Fig. 4d), we can find that the weight loss of the sample is greatly restricted. The reason is that iron oxide can
impose restriction on mobilization of CCG, thus resulting in homogeneous heating and avoid heat concentration. This indicates that there is a strong interaction between iron oxide and CCG [62]. The exposed surface of each single GO layer established at a higher pH condition facilitates the sufficient contact of a GO layer with the iron ions, bring about the larger iron oxide content [63]. Considering the weight loss of the samples, the MN-CCG nanocomposite contains CCG and iron oxide, with weight ratio of 4:1. Fig. 5 shows the XRD patterns of raw graphite (a), GO (b), CCG (c), iron oxide (d) and MN-CCG (e). The raw graphite shows a very strong 0 0 2 peak at 2u = 268. As for GO, while a small change in the position of the principal reflection is observed with oxidation, the most striking difference is the intensity and broadness of the peak observed at 2u = 10.58, corresponding to an average interlayer spacing of 9.0 A´˚ . Because of the presence of oxygen-containing functional groups attached on both sides of the graphene sheet and the atomic scale roughness arising from structural defects (sp3 bonding) generated on the originally flat graphene sheet, individual GOS are expected to be thicker than individual pristine graphene sheets. As to the CCG sample, the substantial shift of the (0 0 2) reflection from 8.2 to 3.6 A´˚ after the reduction processing of
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Fig. 7. Representative TEM images of GO (left) and MN-CCG (right).
GO confirms the formation of CCG from GO. However, the interlayer spacing of CCG is slightly higher than that of well ordered graphite (3.34 A´˚ ), suggesting the presence of residual oxygen functionalities in CCG. Besides, the broad nature of the reflection indicates poor ordering of the sheets along the stacking direction, implying that the sample is composed mostly of single or few layers of CCG [64]. The XRD pattern of the free iron oxide obtained in the control experiment is shown in Fig. 5d. The dspacing values of the peaks match well with data from the JCPDS card (39-1346 19-0629) for g-Fe2O3 (the diffraction angels at 2u = 36.528, 62.648, 43.528, 53.548 can be assigned to (3 1 1), (4 4 0), (4 0 0), (4 2 2) crystal planes of g-Fe2O3, respectively). However, one should keep in mind that the real composition of the nanoparticles may not be 100% g-Fe2O3. It also contains a small amount of Fe3O4 (the diffraction angels at 2u = 30.28, 35.68, 43.38, 53.78, 57.38 and 62.88 can be assigned to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) crystal planes of Fe3O4, respectively). The average particle size of the prepared iron oxide nanoparticles was calculated to be ca. 10 nm based on Scherer’s equation. Fig. 5e shows the XRD pattern of the as-synthesized MN-CCG composite. The CCG nanosheets only show a weak 0 0 2 diffraction line. It was reported that the attached nanoparticles may prevent the restacking of carbon sheets, and therefore the characteristic diffraction peaks of the layered structure disappear [65]. AFM is one of the most direct methods of quantifying the degree of exfoliation to graphene sheet level after the dispersion of the powder in a solvent. The sample for AFM measurements was prepared by ultrasonic treatment of samples (in water) dispersion
of 0.05 mg/mL. Then, the samples were prepared through dropcasting on freshly cleaved mica surface and the mica was dried at ambient conditions for 24 h. It is known that the basal planes of the graphene sheets in GO are decorated mostly with epoxide and hydroxyl groups, in addition to carbonyl and carboxyl groups. These oxygen functionalities will alter the van der Walls interactions between the layers and make them hydrophilic, thus facilitating their exfoliation in aqueous media. On average, the height of the GO sheets is ca. 0.8 nm (Fig. 6a). While a pristine graphene sheet is atomically flat with a well-known thickness of ca. 0.34 nm, graphene oxide is expected to be thicker, mainly owing to the presence of functional groups above and below the graphene oxide plane. Thus, the exfoliation of GO to individual or double-layer graphene oxide sheets was indeed achieved in this study. As to the image of CCG (Fig. 6b), its average height is about 0.9 nm. Although CCG nanosheets have a similar structure to graphene, the oxidation of graphite introduces point defects in the two-dimensional (2D) carbon framework after the reduction. The studies on CCG indicate that an individual GO sheet comprises ordered pristine graphitic domains with a size of several nanometers surrounded by areas of oxidized carbon atoms or point defects. After the chemical reduction, the oxygenated groups are partially reduced to re-establish the conjugated graphite network. Considering the intrinsic monolayer ripples owing to thermal fluctuations, it can be concluded that the CCG sheets are mixtures of monolayer and bilayers. The MN-CCG nanocomposite was redispersed to form a metastable dispersion and drop-dried on a mica substrate for
Fig. 8. Magnetic properties of samples: (a) magnetic hysteresis loops measured at room temperature for iron oxide nanoparticles and MN-CCG; (b) separation from water solution under an external magnetic field. (Inset) Solutions before and after magnetic separation.
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AFM study. We find that even after a long time of sonication during the preparation of the AFM specimen, the magnetic nanoparticles are still strongly anchored on the surface of CCG sheets, suggesting the strong interaction between magnetic nanoparticles and CCG sheets. Magnetic nanoparticles can interact with the CCG sheets through physisorption, electrostatic binding or through charge transfer [66]. On the other hand, the magnetic nanoparticles on the surface of CCG sheets can act as spacers to efficiently prevent the restacking of CCG sheets, avoiding the loss of their high active surface area. From the image of MN-CCG (Fig. 6c), it is evident that almost all CCG sheets are separated from each other and coupled by magnetic nanoparticles. An important feature of the adhesion of nanoparticles to CCG is the inhibition of CCG sheets in dry state. By functioning as a spacer, the nanoparticles increase the distance between the CCG sheets, thereby making both faces of CCG accessible. On comparison with the AFM image of CCG, the surface of MN-CCG is much rougher, which might be attributed to the growth of magnetic nanoparticles on CCG sheets. The AFM image showed that the thickness of the MN-CCG composite was in the range of 10–12 nm. Since the size of the magnetic nanoparticles is about 10 nm (according to XRD result) and the thickness of the CCG is about 1 nm, it is reasonable to conclude that the composite consists of monolayer CCG and magnetic nanoparticles. To further characterize the exact structures of nanocarbons in the dispersions, we conducted TEM analysis. From TEM image of exfoliated GO (Fig. 7), large sheets were observed to be situated on the top of the grid, where they resembled silk veil waves. They were transparent and entangled with each other. The typical image of MN-CCG illustrates that almost all the CCG sheets are separated from each other and coupled by magnetic nanoparticles. Since the monolayer carbon nanosheets are extremely thin, it is hard to make a distinction between them and the carbon-supported films on the copper grid. The introduction of nanoparticles into the dispersion of graphene sheets impedes the formation of a stacked graphitic structure. By functioning as a ‘‘spacer’’, the nanoparticles increase the distance between the graphene sheets, thereby making both the faces of graphene accessible. From TEM image of MN-CCG, we can find that the average size of magnetic nanoparticles is about 10 nm. All our experiments confirm unambiguously that magnetic nanoparticles have been successfully attached onto the exfoliated CCG sheets. In a preliminary investigation of magnetic properties of samples, hysteresis loops were recorded at room temperature. The magnetization curves are shown in Fig. 8a. As we can see, MNCCG shows ferromagnetic characteristic, but the saturation magnetization is smaller than iron oxide due to dilute effect. Magnetic separability of MN-CCG is also tested in water by placing a magnet near the glass bottle. The black powder was attracted toward the magnet in a short period of time (Fig. 8b), demonstrating high magnetic sensitivity. 4. Conclusions In this study, we demonstrated the preparation of MN-CCG nanocomposite with a hydrothermal process. Glucose was used as the reducing agent and the reducing process was accompanied by generation of nanoparticles. Glucose was found to yield highly reduced suspensions with hydrothermal reaction in a way comparable to those provided by hydrazine. These findings, together with the non-toxicity of this natural product, suggest that glucose represents an ideal substitute for hydrazine in the large-scale production of solution-processable CCG. Besides being eco-friendly, this process has several advantages like simplicity, high productivity, low cost and short processing times. With the hydrothermal process, the reducing process can be accompanied
by generation of magnetic nanoparticles. We believe that MN-CCG is particularly attractive for the design of novel composites, and detailed studies on this application are underway. In addition, this hydrothermal method may be readily extended to the preparation of other classes of hybrids on CCG sheets for technological applications. References [1] J. Zhu, Nat. Nanotechnol. 3 (2008) 528–529. [2] H. Liu, S. Ryu, Z. Chen, M.L. Steigerwald, C. Nuckolls, L.E. Brus, J. Am. Chem. Soc. 131 (2009) 17099–17101. [3] C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int. Ed. 48 (2009) 7752–7777. [4] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [5] J. Wu, W. Pisula, K. Mu¨llen, Chem. Rev. 107 (2007) 718–747. [6] Z.H. Ni, H.M. Wang, J. Kasim, H.M. Fan, T. Yu, Y.H. Wu, Y.P. Feng, Z.X. Shen, Nano Lett. 7 (2007) 2758–2763. [7] E. Sykes, H. Charles, Nat. Chem. 1 (2009) 175–176. [8] M. Ishigami, J.H. Chen, W.G. Cullen, M.S. Fuhrer, E.D. Williams, Nano Lett. 7 (2007) 1643–1648. [9] J.C. Meyer, A.K. Geim, M.I. Katsnelson, K.S. Novoselov, T.J. Booth, S. Roth, Nature 446 (2007) 60–63. [10] L. Zhang, J. Xia, Q. Zhao, L. Liu, Z. Zhang, Small 6 (2010) 537–544. [11] X. Zhao, Q. Zhang, D. Chen, P. Lu, Macromolecules 43 (2010) 2357–2363. [12] J.I. Paredes, S. Villar-Rodil, A. Martinez-Alonso, J.M.D. Tascon, Langmuir 24 (2008) 10560–10564. [13] D. Konatham, A. Striolo, Nano Lett. 8 (2008) 4630–4641. [14] L. Qu, Y. Liu, J. Baek, L. Dai, ACS Nano 4 (2010) 1321–1326. [15] M. Liang, L. Zhi, J. Mater. Chem. 19 (2009) 5871–5878. [16] W. Yang, K.R. Ratinac, S.P. Ringger, P. Thordarson, J.J. Gooding, F. Braet, Angew. Chem. Int. Ed. 49 (2010) 2114–2138. [17] X. Du, P. Guo, H. Song, X. Chen, Electrochim. Acta 55 (2010) 4812–4819. [18] S. Myung, J. Park, H. Lee, K.S. Kim, S. Hong, Adv. Mater. 22 (2010) 2045–2049. [19] C. Guo, H. Yang, Z. Sheng, Z. Lu, Q. Song, C. Li, Angew. Chem. Int. Ed. 122 (2010) 3078–3081. [20] Z. Yin, S. Wu, X. Zhou, X. Huang, Q. Zhang, F. Boey, H. Zhang, Small 6 (2010) 307–312. [21] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666–669. [22] O. Akhavan, E. Ghaderi, J. Phys. Chem. C 113 (2009) 20214–20220. [23] O.C. Compton, S.T. Nguyen, Small 6 (2010) 711–723. [24] C. Lu, H. Yang, C. Zhu, X. Chen, G. Chen, Angew. Chem. Int. Ed. 48 (2009) 4785–4787. [25] X. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, H. Dai, Nano Res. 1 (2008) 203–212. [26] H. Li, J. Chen, S. Han, W. Niu, X. Liu, G. Xu, Talanta 79 (2009) 165–170. [27] H. Wu, J. Wang, X. Kang, C. Wang, D. Wang, J. Liu, I.A. Aksay, Y. Lin, Talanta 80 (2009) 403–406. [28] R.S. Ruoff, Nat. Nanotechnol. 3 (2008) 10–11. [29] X. Yang, X. Zhang, Y. Ma, Y. Huang, Y. Wang, Y. Chen, J. Mater. Chem. 19 (2009) 2710–2714. [30] A.J. Patil, J.L. Vickery, T.B. Scott, S. Mann, Adv. Mater. 21 (2009) 3159–3164. [31] J. Kim, L.J. Cote, F. Kim, W. Yuan, K.R. Shull, J. Huang, J. Am. Chem. Soc. 132 (2010) 8180–8186. [32] S. Park, R.S. Ruoff, Nat. Nanotechnol. 4 (2009) 217–224. [33] S. Park, J. An, I. Jung, R.D. Piner, S.J. An, X. Li, A. Velamakanni, R.S. Ruoff, Nano Lett. 9 (2009) 1593–1597. [34] D. Li, M.B. Mu¨ller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101–105. [35] S. Villar-Rodil, J.I. Paredes, A. Martı´nez-Alonso, J.M.D. Tasco´n, J. Mater. Chem. 19 (2009) 3591–3593. [36] M. Lattuada, T.A. Hatton, Langmuir 23 (2007) 2158–2168. [37] A. Lu, E.L. Salabas, F. Schueth, Angew. Chem. Int. Ed. 46 (2007) 1222–1244. [38] X. Lin, A.C.S. Samia, J. Magn. Magn. Mater. 305 (2006) 100–109. [39] R. Pasricha, S. Gupta, A.K. Srivastava, Small 5 (2009) 2253–2259. [40] R. Muszynski, B. Seger, P.V. Kamat, J. Phys. Chem. C 112 (2008) 5263–5266. [41] Z. Liu, D. He, Y. Wang, H. Wu, J. Wang, Sol. Energy Mater. Sol. Cells 94 (2010) 1196–1200. [42] E. Yoo, T. Okata, T. Akita, M. Kohyama, J. Nakamura, I. Honma, Nano Lett. 9 (2009) 2255–2259. [43] B. Seger, P.V. Kamat, J. Phys. Chem. C 113 (2009) 7990–7995. [44] G. Lu, S. Mao, S. Park, R.S. Ruoff, J. Chen, Nano Res. 2 (2009) 192–200. [45] Y. Kim, J. Han, B.H. Hong, Y. Kwon, Adv. Mater. 22 (2010) 515–518. [46] G.M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth, R. Mu¨lhaupt, J. Am. Chem. Soc. 131 (2009) 8262–8270. [47] R. Yuge, M. Zhang, M. Tomonari, T. Yoshitake, S. Iijima, M. Yudasaka, ACS Nano 2 (2008) 1865–1870. [48] B. Kong, J. Geng, H. Jung, Chem. Commun. (2009) 2174–2176. [49] C. Zhu, S. Guo, Y. Fang, S. Dong, ACS Nano 4 (2010) 2429–2437. [50] J. Shen, Y. Hu, M. Shi, N. Li, H. Ma, M. Ye, J. Phys. Chem. C 114 (2010) 1498–1503. [51] X. Zhu, Y. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, ACS Nano 5 (2011) 3333–3338. [52] J. Liang, Y. Xu, D. Sui, L. Zhang, Y. Huang, Y. Ma, F. Li, Y. Chen, J. Phys. Chem. C 114 (2010) 17465–17471.
J. Shen et al. / Materials Research Bulletin 46 (2011) 2077–2083 [53] W. Shi, J. Zhu, D.H. Sim, Y.Y. Tay, Z. Lu, X. Zhang, Y. Sharma, M. Srinivasan, H. Zhang, H.H. Hng, Q. Yan, J. Mater. Chem. 21 (2011) 3422–3427. [54] M. Zhang, D. Lei, X. Yin, L. Chen, Q. Li, Y. Wang, T. Wang, J. Mater. Chem. 20 (2010) 5538–5543. [55] G. Zhou, D. Wang, F. Li, l. Zhang, N. Li, Z. Wu, L. Wen, G.Q. Lu, H. Cheng, Chem. Mater. 22 (2010) 5306–5313. [56] Y. Zhou, Q. Bao, L.A.L. Tang, Y. Zhong, K.P. Loh, Chem. Mater. 21 (2009) 2950–2956. [57] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [58] J. Shen, M. Shi, N. Li, B. Yan, H. Ma, Y. Hu, M. Ye, Nano Res. 3 (2010) 339–349. [59] J. Shen, Y. Hu, C. Li, C. Qin, M. Ye, Small 5 (2009) 82–85. [60] A. Chunder, J. Liu, L. Zhai, Macromol. Rapid Commun. 31 (2010) 380–384.
2083
[61] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558–1565. [62] H. Hu, X. Wang, J. Wang, L. Wan, F. Liu, H. Zheng, R. Chen, C. Xu, Chem. Phys. Lett. 484 (2010) 247–253. [63] Z. Hu, M. Aizawa, Z. Wang, N. Yoshizawa, H. Hatori, Langmuir 26 (2010) 6681–6688. [64] A.V. Murugan, T. Muraliganth, A. Manthiram, Chem. Mater. 21 (2009) 5004–5006. [65] C. Xu, X. Wang, Small 5 (2009) 2212–2217. [66] F. Li, J. Song, H. Yang, S. Gan, Q. Zhang, D. Han, A. Ivaska, L. Niu, Nanotechnology 20 (2009) 455602.
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Hydrothermal synthesis of magnetic reduced graphene oxide sheets Jianfeng Shen, Min Shi, Hongwei Ma, Bo Yan, Na Li, Mingxin Ye * Center of Special Materials and Technology, Fudan University, Shanghai 200433, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 May 2011 Received in revised form 19 June 2011 Accepted 25 June 2011 Available online 2 July 2011
We demonstrated an environmental friendly and efficient route for preparation of magnetic reduced graphene oxide composite (MN-CCG). Glucose was used as the reducing agent in this one-step hydrothermal method. The reducing process was accompanied by generation of magnetic nanoparticles. The structure and composition of the nanocomposite was confirmed by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, thermal gravimetric analysis, atomic force microscopy and transmission electron microscopy. It was found that the prepared MN-CCG is highly water suspendable and sensitive to magnetic field. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Nanostructures B. Chemical synthesis D. Magnetic properties
1. Introduction Graphene, one-atom thick planar carbon nanosheet, has generated huge activity in many areas of science and engineering due to its unprecedented physical and chemical properties [1–8]. It is expected to find a variety of applications because of its distinctive properties [9–20]. The reliable technique to produce single layer graphene of high quality is the micromechanical cleavage method [21]. However, this route is not practical for large-scale generation of graphene. Nowadays, among known strategies for producing graphene-based materials, utilization of graphite oxide (GO) is the most versatile and scalable method [22–31]. GO is a strongly oxygenated and highly hydrophilic material that can be easily exfoliated in water to yield stable dispersions consisting mostly of graphene oxide sheets (GOS). These stable and flexible GOS are usually used as the starting material for the generation of graphene-based materials. However, though the chemical oxidation method is convenient to exfoliate GOS via solution based processes, it introduces functional groups such as carboxyl and epoxide. The presence of these groups makes the GOS suspendable in polar solvents but drastically decreases the conductivity as a result of loss in the conjugated sp2 network. Hydrazine has been widely used to reduce GOS to get chemically converted graphene (CCG) [32–35].
* Corresponding author at: Center of Special, Materials and Technology, Fudan University, 220 Handan Road, Shanghai 200433, China. Tel.: +86 021 55664095; fax: +86 021 55664094. E-mail address: [email protected] (M. Ye). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.06.042
Magnetic nanoparticles have attracted increasing interest in the material and colloid science community in recent years. Recent researches have been devoted to high-tech applications as in the fields of magnetic storage devices, biomedical engineering and drug delivery [36–38]. Composing graphene with other functional materials is a promising way to combine the effect of functional materials with that of graphene. As one kind of graphene-based hybrid material, graphene/nanoparticles composites have aroused extensive interest as hybridization improves the performance of both graphene and nanoparticles [29,39,40–48]. More recently, nanoparticles dispersed on graphene sheets have been prepared using hydrazine as a reducing agent. Unfortunately, the use of highly toxic and dangerously unstable hydrazine or dimethylhydrazine to reduce GOS remains a serious challenge for large-scale production of graphene nanosheets. Besides, these methods were shown to be difficult to control and hence not suitable for mass production. For these reasons, it is highly desirable to develop a low-cost, facile and ‘‘green’’ approach for preparing soluble RGO sheets in large quantities. It is shown that glucose can be used as a ‘‘green’’ agent to prepare RGO in large scale [49]. The combination of iron oxide nanostructure and graphene has potential for the use of magnetic graphene sheets in many applications ranging from electromagnetic devices to magnetically guided drug delivery systems [50]. Zhu et al. prepared RGO/Fe2O3 with a two-step synthesis by homogeneous precipitation and subsequent reduction of GO with hydrazine under microwave irradiation [51]. Liang et al. reported the fabrication of magnetic, electrically conducting, and flexible paper composed of graphene and Fe3O4 with hydrazine as the reducing agent [52]. Shi et al. reported a facile approach to synthesis Fe3O4 nanoparticles
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attached to RGO by a solvothermal process [53]. Zhang et al. employed microwave irradiation as a heat source to prepare magnetite graphene composites [54]. Zhou et al. prepared a wellorganized flexible interleaved composite of graphene decorated with Fe3O4 particles through in situ reduction of iron hydroxide between graphene sheets [55]. However, considering these results, despite the significant efforts, there still exist some challenges and problems in the field of MN-CCG composites: (1) most of the preparation protocol is relatively complex and many of them used toxic materials; (2) the integrated properties of both graphene and magnetic nanoparticles were rarely concerned; (3) the dispersibility of functionalized graphene sheets needs to be improved; and (4) the composites have rare functional groups for further chemical modification. In this work, we synthesized the composite of CCG decorated with iron oxide nanoparticles (MN-CCG) by a simple hydrothermal method. We used graphene oxide as a precursor for CCG and ferrous sulfate as a single-source precursor of iron oxide. The hydrothermal conversion method used in this study has several advantages over the common reduction processes: (1) the reducing ability of glucose is greatly enhanced with the hydrothermal condition, which is strong enough to reduce graphene oxide; (2) the process is simple and scalable, and is industrially compatible; (3) it is intrinsically pure since it utilizes only water and glucose; and (4) the closed system of relative high temperature and internal pressure promotes the recovery of pconjugation after dehydration, which is favorable for minimizing defects [56]. To the best of our knowledge, few studies on the preparation of MN-CCG using hydrothermal method have been reported so far. 2. Experimental procedures 2.1. Materials Pristine graphite was purchased from Qingdao BCSM. Co., Ltd. Ferrous sulfate, glucose and ammonium hydroxide was supplied by
Shanghai Chemical Reagent Company. All other reagents were at least of analytical reagent grade and used without further purification. 2.2. Preparation of MN-CCG GO was obtained by the modified Hummers method as described elsewhere [57–59]. 100 mg of ferrous sulfate was added to 5 mL of water. The above mixture was slowly dropped to the mixture of 20 mg NaOH/5 mL ethanol to form part A. 100 mg of GO was added to 50 mL water. The mixture was sonicated for 30 min followed by high-speed stirring for further 1 h. 100 mg glucose and 1 mL ammonium hydroxide was added to the GO solution to get part B. Subsequently, part A and part B were mixed. The mixture was put into an autoclave and heated at 160 8C for 4 h. The color of the suspension shifted from brown to black, which indicated the transition from GO to CCG. When the reduction reaction was finished, the as-synthesized product was isolated by centrifugation, washed with pure water and ethanol for several times, and dried at 90 8C for 12 h. The fabrication process is outlined in Fig. 1. In addition, CCG and iron oxide were synthesized in the same way as the composites in the absence of ferrous sulfate and GO, respectively. 2.3. Instruments and measurement Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a NEXUS 670 spectrometer. Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal microspectrometer with 514.5 nm laser excitation. Thermogravimetric analysis (TGA) was conducted with Netzsch TG 209F1 that was fitted to a nitrogen purge gas at 10 8C/min heating rate. X-ray diffraction (XRD) was taken on D/max-gB diffractometer using Cu Ka radiation. Transmission electron microscopy (TEM) was performed with a JEOL JEM-2100F. Atomic force microscopy (AFM) images were obtained using a Multimode Nano4 in the tapping mode. Water bath sonication was performed with a JL-60 DTH sonicator (50 W).
Fig. 1. Experimental procedure used in this study.
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Fig. 2. FTIR spectra of GO (a), CCG (b), iron oxide (c) and MN-CCG (d).
3. Results and discussion One purpose of this study is to find an efficient, scalable and green way to prepare CCG. The exfoliated single-layer GOS can be well dispersed in aqueous solutions, which provides a low temperature process to make single-layer graphene sheets. Besides, this solution-compatible process is potential for largearea application. In view of the fact that hydrazine is toxic and dangerously unstable, it is urgent to develop a green chemistry route for the reduction of graphene oxide. Though there are some researches on the green preparation of graphene, most of them are time-consuming with little productivity. Recognizing these difficulties, we present here for the first time a facile hydrothermal method for preparation of stable MN-CCG suspension with nontoxic agent in an alkaline medium. In the basic medium, subsequent carboxyl termination occurred with the evidence of color change from yellow-brown to black during the hydrothermal process as the exfoliated GOS with negatively charged oxygen functional groups can undergo fast deoxygenating in alkaline solution (pH 10). Fig. 2 shows the FTIR spectra of GO (a), CCG (b), iron oxide (c) and MN-CCG (d). The spectrum of GO (Fig. 2a) illustrates the presence of C–O (nC–O at 1100 cm 1), C5 5O in carboxylic acid and 1 carbonyl moieties (nC5 ). The broad, intense band at 5O at 1650 cm 3450 cm 1 can be attributed to O–H stretching vibration. For the CCG sample (Fig. 2b), the intensities of the bands associated to oxygen functional groups strongly decrease in relation to those of GO. Nevertheless, the elimination of these bands is not complete. In particular, the persistence of the band at about 1450 cm 1
Fig. 3. Raman spectra of raw graphite (a), GO (b), CCG (c), and MN-CCG (d).
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Fig. 4. TGA curves of raw graphite (a), GO (b), CCG (c), iron oxide (d) and MN-CCG (e).
(carboxy C–O) implies that a little fraction of hydroxyl functionalities still remain in the CCG sample. In the spectrum of MN-CCG (Fig. 2d), we find that most of the peaks in spectra of CCG and magnetic nanoparticles (Fig. 2c) also exist. Additional vibrational band at 1440 cm 1 appeared which can be assigned to the formation of monodentate complex between the carboxyl group and Fe on the surface of the iron oxide nanoparticles. Moreover, the peak at 600 cm 1 can be ascribed to lattice absorption of iron oxide [50]. These results support that the preparation of MN-CCG with our hydrothermal process is successful. Raman spectroscopy is a widely used tool to study the structure of conjugated carbon bonds because of their high Raman intensities. In contrast to the Raman spectrum of raw graphite (Fig. 3a), which shows a sharp Raman band of the graphite lattice at 1580 cm 1 (G band) and a weak disorder band caused by the graphite edges (D band), a strong D band appears at 1355 cm 1 in the Raman spectrum of GO (Fig. 3b). This D band is attributed to the sp3 amorphous carbons generated by oxidation. In addition, the G band becomes broader and blue shifts to 1600 cm 1. The Raman spectrum of the CCG (Fig. 3c) showed two peaks at 1605 cm 1 (in plane vibrational G band), and 1360 cm 1 (disordered vibrational D band). The G band of CCG shifts to a higher frequency in comparison with pristine graphite, which is attributed to the less ordered carbon and defects of the CCG sheets [60]. In the spectrum of MN-CCG, we can find that the there is a 5 cm 1 blue shift in G
Fig. 5. XRD patterns of raw graphite (a), GO (b), CCG (c), iron oxide (d) and MN-CCG (e).
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Fig. 6. AFM images of graphene oxide (a), CCG (b) and MN-CCG (c) with concentration of 0.05 mg/mL. Image dimensions are 1.2 mm 1.2 mm, 2.4 mm 2.4 mm and 1.0 mm 1.0 mm.
band position when compared to bulk graphite. This shift is attributed to the transformation of bulk graphite crystal to CCG sheets. It suggests a decrease in the average size of the sp2 domains upon reduction of the exfoliated GOS, but more numerous in numbers [61]. TGA is a complementary technique that can reveal the composition and changes in thermal stability of the samples. The successful functionalization of CCG with magnetic nanoparticles was also reflected in the TGA curves. TGA trace of pristine graphite (Fig. 4a) shows little weight loss, displaying a simple linear dependence after 500 8C, which is about 3% below 700 8C. In contrast, although GO (Fig. 4b) is thermally unstable and starts to lose mass upon heating even below 100 8C, the major mass loss occurs at about 200 8C, presumably due to pyrolysis of the labile oxygen-containing functional groups to yield CO, CO2 and steam. The TGA curve of CCG (Fig. 4c) shows a mass loss of 6% in the rage of 110–300 8C, while TGA trace of GO shows a mass loss of more than 25% in the same range, suggesting a much lower content of oxygencontaining groups in CCG than in GO. While in the curve of MNCCG (Fig. 4e), comparing with the curve of GO and magnetic nanoparticles (Fig. 4d), we can find that the weight loss of the sample is greatly restricted. The reason is that iron oxide can
impose restriction on mobilization of CCG, thus resulting in homogeneous heating and avoid heat concentration. This indicates that there is a strong interaction between iron oxide and CCG [62]. The exposed surface of each single GO layer established at a higher pH condition facilitates the sufficient contact of a GO layer with the iron ions, bring about the larger iron oxide content [63]. Considering the weight loss of the samples, the MN-CCG nanocomposite contains CCG and iron oxide, with weight ratio of 4:1. Fig. 5 shows the XRD patterns of raw graphite (a), GO (b), CCG (c), iron oxide (d) and MN-CCG (e). The raw graphite shows a very strong 0 0 2 peak at 2u = 268. As for GO, while a small change in the position of the principal reflection is observed with oxidation, the most striking difference is the intensity and broadness of the peak observed at 2u = 10.58, corresponding to an average interlayer spacing of 9.0 A´˚ . Because of the presence of oxygen-containing functional groups attached on both sides of the graphene sheet and the atomic scale roughness arising from structural defects (sp3 bonding) generated on the originally flat graphene sheet, individual GOS are expected to be thicker than individual pristine graphene sheets. As to the CCG sample, the substantial shift of the (0 0 2) reflection from 8.2 to 3.6 A´˚ after the reduction processing of
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Fig. 7. Representative TEM images of GO (left) and MN-CCG (right).
GO confirms the formation of CCG from GO. However, the interlayer spacing of CCG is slightly higher than that of well ordered graphite (3.34 A´˚ ), suggesting the presence of residual oxygen functionalities in CCG. Besides, the broad nature of the reflection indicates poor ordering of the sheets along the stacking direction, implying that the sample is composed mostly of single or few layers of CCG [64]. The XRD pattern of the free iron oxide obtained in the control experiment is shown in Fig. 5d. The dspacing values of the peaks match well with data from the JCPDS card (39-1346 19-0629) for g-Fe2O3 (the diffraction angels at 2u = 36.528, 62.648, 43.528, 53.548 can be assigned to (3 1 1), (4 4 0), (4 0 0), (4 2 2) crystal planes of g-Fe2O3, respectively). However, one should keep in mind that the real composition of the nanoparticles may not be 100% g-Fe2O3. It also contains a small amount of Fe3O4 (the diffraction angels at 2u = 30.28, 35.68, 43.38, 53.78, 57.38 and 62.88 can be assigned to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) crystal planes of Fe3O4, respectively). The average particle size of the prepared iron oxide nanoparticles was calculated to be ca. 10 nm based on Scherer’s equation. Fig. 5e shows the XRD pattern of the as-synthesized MN-CCG composite. The CCG nanosheets only show a weak 0 0 2 diffraction line. It was reported that the attached nanoparticles may prevent the restacking of carbon sheets, and therefore the characteristic diffraction peaks of the layered structure disappear [65]. AFM is one of the most direct methods of quantifying the degree of exfoliation to graphene sheet level after the dispersion of the powder in a solvent. The sample for AFM measurements was prepared by ultrasonic treatment of samples (in water) dispersion
of 0.05 mg/mL. Then, the samples were prepared through dropcasting on freshly cleaved mica surface and the mica was dried at ambient conditions for 24 h. It is known that the basal planes of the graphene sheets in GO are decorated mostly with epoxide and hydroxyl groups, in addition to carbonyl and carboxyl groups. These oxygen functionalities will alter the van der Walls interactions between the layers and make them hydrophilic, thus facilitating their exfoliation in aqueous media. On average, the height of the GO sheets is ca. 0.8 nm (Fig. 6a). While a pristine graphene sheet is atomically flat with a well-known thickness of ca. 0.34 nm, graphene oxide is expected to be thicker, mainly owing to the presence of functional groups above and below the graphene oxide plane. Thus, the exfoliation of GO to individual or double-layer graphene oxide sheets was indeed achieved in this study. As to the image of CCG (Fig. 6b), its average height is about 0.9 nm. Although CCG nanosheets have a similar structure to graphene, the oxidation of graphite introduces point defects in the two-dimensional (2D) carbon framework after the reduction. The studies on CCG indicate that an individual GO sheet comprises ordered pristine graphitic domains with a size of several nanometers surrounded by areas of oxidized carbon atoms or point defects. After the chemical reduction, the oxygenated groups are partially reduced to re-establish the conjugated graphite network. Considering the intrinsic monolayer ripples owing to thermal fluctuations, it can be concluded that the CCG sheets are mixtures of monolayer and bilayers. The MN-CCG nanocomposite was redispersed to form a metastable dispersion and drop-dried on a mica substrate for
Fig. 8. Magnetic properties of samples: (a) magnetic hysteresis loops measured at room temperature for iron oxide nanoparticles and MN-CCG; (b) separation from water solution under an external magnetic field. (Inset) Solutions before and after magnetic separation.
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AFM study. We find that even after a long time of sonication during the preparation of the AFM specimen, the magnetic nanoparticles are still strongly anchored on the surface of CCG sheets, suggesting the strong interaction between magnetic nanoparticles and CCG sheets. Magnetic nanoparticles can interact with the CCG sheets through physisorption, electrostatic binding or through charge transfer [66]. On the other hand, the magnetic nanoparticles on the surface of CCG sheets can act as spacers to efficiently prevent the restacking of CCG sheets, avoiding the loss of their high active surface area. From the image of MN-CCG (Fig. 6c), it is evident that almost all CCG sheets are separated from each other and coupled by magnetic nanoparticles. An important feature of the adhesion of nanoparticles to CCG is the inhibition of CCG sheets in dry state. By functioning as a spacer, the nanoparticles increase the distance between the CCG sheets, thereby making both faces of CCG accessible. On comparison with the AFM image of CCG, the surface of MN-CCG is much rougher, which might be attributed to the growth of magnetic nanoparticles on CCG sheets. The AFM image showed that the thickness of the MN-CCG composite was in the range of 10–12 nm. Since the size of the magnetic nanoparticles is about 10 nm (according to XRD result) and the thickness of the CCG is about 1 nm, it is reasonable to conclude that the composite consists of monolayer CCG and magnetic nanoparticles. To further characterize the exact structures of nanocarbons in the dispersions, we conducted TEM analysis. From TEM image of exfoliated GO (Fig. 7), large sheets were observed to be situated on the top of the grid, where they resembled silk veil waves. They were transparent and entangled with each other. The typical image of MN-CCG illustrates that almost all the CCG sheets are separated from each other and coupled by magnetic nanoparticles. Since the monolayer carbon nanosheets are extremely thin, it is hard to make a distinction between them and the carbon-supported films on the copper grid. The introduction of nanoparticles into the dispersion of graphene sheets impedes the formation of a stacked graphitic structure. By functioning as a ‘‘spacer’’, the nanoparticles increase the distance between the graphene sheets, thereby making both the faces of graphene accessible. From TEM image of MN-CCG, we can find that the average size of magnetic nanoparticles is about 10 nm. All our experiments confirm unambiguously that magnetic nanoparticles have been successfully attached onto the exfoliated CCG sheets. In a preliminary investigation of magnetic properties of samples, hysteresis loops were recorded at room temperature. The magnetization curves are shown in Fig. 8a. As we can see, MNCCG shows ferromagnetic characteristic, but the saturation magnetization is smaller than iron oxide due to dilute effect. Magnetic separability of MN-CCG is also tested in water by placing a magnet near the glass bottle. The black powder was attracted toward the magnet in a short period of time (Fig. 8b), demonstrating high magnetic sensitivity. 4. Conclusions In this study, we demonstrated the preparation of MN-CCG nanocomposite with a hydrothermal process. Glucose was used as the reducing agent and the reducing process was accompanied by generation of nanoparticles. Glucose was found to yield highly reduced suspensions with hydrothermal reaction in a way comparable to those provided by hydrazine. These findings, together with the non-toxicity of this natural product, suggest that glucose represents an ideal substitute for hydrazine in the large-scale production of solution-processable CCG. Besides being eco-friendly, this process has several advantages like simplicity, high productivity, low cost and short processing times. With the hydrothermal process, the reducing process can be accompanied
by generation of magnetic nanoparticles. We believe that MN-CCG is particularly attractive for the design of novel composites, and detailed studies on this application are underway. In addition, this hydrothermal method may be readily extended to the preparation of other classes of hybrids on CCG sheets for technological applications. References [1] J. Zhu, Nat. Nanotechnol. 3 (2008) 528–529. [2] H. Liu, S. Ryu, Z. Chen, M.L. Steigerwald, C. Nuckolls, L.E. Brus, J. Am. Chem. Soc. 131 (2009) 17099–17101. [3] C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int. Ed. 48 (2009) 7752–7777. [4] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [5] J. Wu, W. Pisula, K. Mu¨llen, Chem. Rev. 107 (2007) 718–747. [6] Z.H. Ni, H.M. Wang, J. Kasim, H.M. Fan, T. Yu, Y.H. Wu, Y.P. Feng, Z.X. Shen, Nano Lett. 7 (2007) 2758–2763. [7] E. Sykes, H. Charles, Nat. Chem. 1 (2009) 175–176. [8] M. Ishigami, J.H. Chen, W.G. Cullen, M.S. Fuhrer, E.D. Williams, Nano Lett. 7 (2007) 1643–1648. [9] J.C. Meyer, A.K. Geim, M.I. Katsnelson, K.S. Novoselov, T.J. Booth, S. Roth, Nature 446 (2007) 60–63. [10] L. Zhang, J. Xia, Q. Zhao, L. Liu, Z. Zhang, Small 6 (2010) 537–544. [11] X. Zhao, Q. Zhang, D. Chen, P. Lu, Macromolecules 43 (2010) 2357–2363. [12] J.I. Paredes, S. Villar-Rodil, A. Martinez-Alonso, J.M.D. Tascon, Langmuir 24 (2008) 10560–10564. [13] D. Konatham, A. Striolo, Nano Lett. 8 (2008) 4630–4641. [14] L. Qu, Y. Liu, J. Baek, L. Dai, ACS Nano 4 (2010) 1321–1326. [15] M. Liang, L. Zhi, J. Mater. Chem. 19 (2009) 5871–5878. [16] W. Yang, K.R. Ratinac, S.P. Ringger, P. Thordarson, J.J. Gooding, F. Braet, Angew. Chem. Int. Ed. 49 (2010) 2114–2138. [17] X. Du, P. Guo, H. Song, X. Chen, Electrochim. Acta 55 (2010) 4812–4819. [18] S. Myung, J. Park, H. Lee, K.S. Kim, S. Hong, Adv. Mater. 22 (2010) 2045–2049. [19] C. Guo, H. Yang, Z. Sheng, Z. Lu, Q. Song, C. Li, Angew. Chem. Int. Ed. 122 (2010) 3078–3081. [20] Z. Yin, S. Wu, X. Zhou, X. Huang, Q. Zhang, F. Boey, H. Zhang, Small 6 (2010) 307–312. [21] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666–669. [22] O. Akhavan, E. Ghaderi, J. Phys. Chem. C 113 (2009) 20214–20220. [23] O.C. Compton, S.T. Nguyen, Small 6 (2010) 711–723. [24] C. Lu, H. Yang, C. Zhu, X. Chen, G. Chen, Angew. Chem. Int. Ed. 48 (2009) 4785–4787. [25] X. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, H. Dai, Nano Res. 1 (2008) 203–212. [26] H. Li, J. Chen, S. Han, W. Niu, X. Liu, G. Xu, Talanta 79 (2009) 165–170. [27] H. Wu, J. Wang, X. Kang, C. Wang, D. Wang, J. Liu, I.A. Aksay, Y. Lin, Talanta 80 (2009) 403–406. [28] R.S. Ruoff, Nat. Nanotechnol. 3 (2008) 10–11. [29] X. Yang, X. Zhang, Y. Ma, Y. Huang, Y. Wang, Y. Chen, J. Mater. Chem. 19 (2009) 2710–2714. [30] A.J. Patil, J.L. Vickery, T.B. Scott, S. Mann, Adv. Mater. 21 (2009) 3159–3164. [31] J. Kim, L.J. Cote, F. Kim, W. Yuan, K.R. Shull, J. Huang, J. Am. Chem. Soc. 132 (2010) 8180–8186. [32] S. Park, R.S. Ruoff, Nat. Nanotechnol. 4 (2009) 217–224. [33] S. Park, J. An, I. Jung, R.D. Piner, S.J. An, X. Li, A. Velamakanni, R.S. Ruoff, Nano Lett. 9 (2009) 1593–1597. [34] D. Li, M.B. Mu¨ller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101–105. [35] S. Villar-Rodil, J.I. Paredes, A. Martı´nez-Alonso, J.M.D. Tasco´n, J. Mater. Chem. 19 (2009) 3591–3593. [36] M. Lattuada, T.A. Hatton, Langmuir 23 (2007) 2158–2168. [37] A. Lu, E.L. Salabas, F. Schueth, Angew. Chem. Int. Ed. 46 (2007) 1222–1244. [38] X. Lin, A.C.S. Samia, J. Magn. Magn. Mater. 305 (2006) 100–109. [39] R. Pasricha, S. Gupta, A.K. Srivastava, Small 5 (2009) 2253–2259. [40] R. Muszynski, B. Seger, P.V. Kamat, J. Phys. Chem. C 112 (2008) 5263–5266. [41] Z. Liu, D. He, Y. Wang, H. Wu, J. Wang, Sol. Energy Mater. Sol. Cells 94 (2010) 1196–1200. [42] E. Yoo, T. Okata, T. Akita, M. Kohyama, J. Nakamura, I. Honma, Nano Lett. 9 (2009) 2255–2259. [43] B. Seger, P.V. Kamat, J. Phys. Chem. C 113 (2009) 7990–7995. [44] G. Lu, S. Mao, S. Park, R.S. Ruoff, J. Chen, Nano Res. 2 (2009) 192–200. [45] Y. Kim, J. Han, B.H. Hong, Y. Kwon, Adv. Mater. 22 (2010) 515–518. [46] G.M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth, R. Mu¨lhaupt, J. Am. Chem. Soc. 131 (2009) 8262–8270. [47] R. Yuge, M. Zhang, M. Tomonari, T. Yoshitake, S. Iijima, M. Yudasaka, ACS Nano 2 (2008) 1865–1870. [48] B. Kong, J. Geng, H. Jung, Chem. Commun. (2009) 2174–2176. [49] C. Zhu, S. Guo, Y. Fang, S. Dong, ACS Nano 4 (2010) 2429–2437. [50] J. Shen, Y. Hu, M. Shi, N. Li, H. Ma, M. Ye, J. Phys. Chem. C 114 (2010) 1498–1503. [51] X. Zhu, Y. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, ACS Nano 5 (2011) 3333–3338. [52] J. Liang, Y. Xu, D. Sui, L. Zhang, Y. Huang, Y. Ma, F. Li, Y. Chen, J. Phys. Chem. C 114 (2010) 17465–17471.
J. Shen et al. / Materials Research Bulletin 46 (2011) 2077–2083 [53] W. Shi, J. Zhu, D.H. Sim, Y.Y. Tay, Z. Lu, X. Zhang, Y. Sharma, M. Srinivasan, H. Zhang, H.H. Hng, Q. Yan, J. Mater. Chem. 21 (2011) 3422–3427. [54] M. Zhang, D. Lei, X. Yin, L. Chen, Q. Li, Y. Wang, T. Wang, J. Mater. Chem. 20 (2010) 5538–5543. [55] G. Zhou, D. Wang, F. Li, l. Zhang, N. Li, Z. Wu, L. Wen, G.Q. Lu, H. Cheng, Chem. Mater. 22 (2010) 5306–5313. [56] Y. Zhou, Q. Bao, L.A.L. Tang, Y. Zhong, K.P. Loh, Chem. Mater. 21 (2009) 2950–2956. [57] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [58] J. Shen, M. Shi, N. Li, B. Yan, H. Ma, Y. Hu, M. Ye, Nano Res. 3 (2010) 339–349. [59] J. Shen, Y. Hu, C. Li, C. Qin, M. Ye, Small 5 (2009) 82–85. [60] A. Chunder, J. Liu, L. Zhai, Macromol. Rapid Commun. 31 (2010) 380–384.
2083
[61] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558–1565. [62] H. Hu, X. Wang, J. Wang, L. Wan, F. Liu, H. Zheng, R. Chen, C. Xu, Chem. Phys. Lett. 484 (2010) 247–253. [63] Z. Hu, M. Aizawa, Z. Wang, N. Yoshizawa, H. Hatori, Langmuir 26 (2010) 6681–6688. [64] A.V. Murugan, T. Muraliganth, A. Manthiram, Chem. Mater. 21 (2009) 5004–5006. [65] C. Xu, X. Wang, Small 5 (2009) 2212–2217. [66] F. Li, J. Song, H. Yang, S. Gan, Q. Zhang, D. Han, A. Ivaska, L. Niu, Nanotechnology 20 (2009) 455602.