J. Phys. Chem. C 2010, 114, 19885–19890
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Chemical Reduction of Graphene Oxide to Graphene by Sulfur-Containing Compounds Wufeng Chen,† Lifeng Yan,*,† and P. R. Bangal‡ Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, 230026, People’s Republic of China, and Inorganic and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Tarnaka, Hyderabad-500607, India ReceiVed: March 31, 2010; ReVised Manuscript ReceiVed: October 14, 2010
Instead of hydrazine, a series of sulfur-containing compounds such as NaHSO3, Na2SO3, Na2S2O3, Na2S · 9H2O, SOCl2, and SO2, were used as reducing agents to reduce graphene oxide to graphene. Fourier transform infrared spectrometry, atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, elemental analysis, and thermogravimetric analysis confirmed the formation of graphene under chemical reduction at 95 °C. The results reveal that the reducing ability of NaHSO3 is comparable to that of hydrazine. This newly found reducing agent is of low toxicity and nonvolatile, which makes the reduction much safer than hydrazine. A possible mechanism of the reduction has been suggested. The electrical conductivity of the graphene paper prepared using a NaHSO3 reducing agent is found to be 6500 S m-1, while it is observed to be 5100 S m-1 for hydrazine-reduced graphene paper. These studies also confirmed that SOCl2 can be a good candidate as a reducing agent to compete with hydrazine. Introduction Graphene, the single sheet of carbon atoms patterned in the honeycomb lattice form, has recently attracted much attention for its unique electronic properties, excellent mechanical properties, and superior thermal properties.1-3 As a base, the preparation of graphene by simple or safe methods is still an attractive topic.4-6 One important method for graphene preparation is the chemical reduction of graphene oxide (GO) by a suitable reducing agent.7 The reported chemical agents for preparation of chemical-reduced graphene (CRG) include hydrazine and its derivatives,8 hydroquinone,9 amino acid,10 NaBH4,11 NaOH,12 vitamin C,13 and so forth. Thermal14 and microwave irradiation15 and electrochemical reduction16 of GO are other simple methods to prepare graphene. Among the methods of chemical reduction, hydrazine or hydrazine hydrate are well-accepted reducing agents to date for their high reduction efficiency. However, N-doping in the as-prepared graphene makes it difficult to get highly pure and conductive graphene. In addition, hydrazine is very toxic and explosive, and it should be avoided in an actual process, especially in large-scale processes. Although NaBH4 showed excellent ability in the reduction of GO, its hydrolysis property made it difficult to get a stable NaBH4 aqueous solution, resulting in low efficiency in reduction. The other reducing reagents, such as NaOH, hydroquinone, and phenylenediamine, can reduce GO, but the ability of deoxidization is much weaker than that with hydrazine. Recently, Paredes et al. reported the efficient reducing of grphene oxide by vitamin C, which was similar to that of hydrazine.17 The development of new reducing agents for the efficient reduction of GO is indispensible, especially in water. It is well-known that many kinds of sulfur-containing compounds serve as efficient reducing agents, and some of them are slightly poisonous and explosive, such as NaHSO3 and Na2S. Now, the question that comes to mind is, is it possible to use * To whom correspondence should be addressed. E-mail: lfyan@ustc. edu.cn. Fax: +86-551-3603748. Tel: +86-551-3606853. † University of Science and Technology of China. ‡ Indian Institute of Chemical Technology.
sulfur-containing compounds as the reducing agent to reduce GO in aqueous solution? The studies on the reduction mechanism of GO by hydrazine reveal that the reduction includes hydrazine de-epoxidation and thermal dehydroxylation, decarbonylation, and decarboxylation steps. For the de-epoxidation, three possible routes are suggested, and all of them include the opening of the ring of epoxide by N2H4 and the removal of water and diazene.18 Therefore, both the nucleophile of the reducing agent and the properties of the oxidized product are the key factors to control the extent of reduction of GO. Since the bond energy of a C-S bond is weaker than that of a C-N bond, use of sulfur-containing compounds as the reducing agent may allow the de-epoxidation of GO more easily than would hydrazine. Here, a series of sulfur-containing compounds, such as NaHSO3, Na2S · 9H2O, Na2S2O3, SOCl2, and SO2, have been tested as reducing agents to reduce GO in aqueous solution or in the mixture of N,N-dimethylacetamide (DMAc) and water. Some cheap and safe reducing agents for the preparation of graphene preparation are suggested. The extent of the effect of this kind of sulfur-containing compounds to the reduction of GO is also discussed. Experimental Section Materials. Graphite powder, natural briquetting grade, 100 mesh, 99.9995% (metals basis), was purchased from Alfa Aesar. Analytical-grade NaHSO3, Na2S · 9H2O, SOCl2, Na2SO3, Na2S2O3, NaNO3, KMnO4,DMAc, and 85% N2H4 · H2O, 98% H2SO4, 30% H2O2 aqueous solution were purchased from Shanghai Chemical Reagents Company and used directly without further purification. Ultrapure water (18 MΩ) was produced by a Millipore System (Millipore Q, U.S.A.). Preparation of GO. Graphite oxide was prepared from natural graphite by the well-known Hummers method,15,19 and the as-prepared graphite oxide powder was obtained after freezedrying the suspension. Generally, GO refers to the exfoliated graphite oxide, existing in the form of single or few layers sheets. Notably, graphite oxide can be efficiently exfoliated to
10.1021/jp107131v 2010 American Chemical Society Published on Web 11/09/2010
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Figure 1. Photographs of GO powder (a) and the CRG powder reduced by 2.7 mmol NaHSO3 for 3 h (b).
produce aqueous colloidal suspensions of GO sheets by simple sonication20 and by stirring the water/graphite oxide mixture for a long enough time.21 Preparation of Graphene by Chemical Reduction of GO. Here, some sulfur-containing compounds work as the reducing agents to reduce GO in water. Chemically reduced graphene (CRG) was prepared by a general route. At first, 50 mg of GO powder was dispersed into 50 mL of water under the assistant of powerful ultrasound for 5 min (300 W, JY92-2D, Xinzhi), then 1.35 or 2.7 mmol of a reducing agent such as NaHSO3, Na2S · 9H2O, Na2S2O3, SOCl2, or N2H4 · H2O was added to the aqueous solution under the assistance of weak ultrasound or stirring. The mixture was then heated at 95 °C under stirring for 3 h. After that, it was filtrated, and the obtained solid was washed with 50 mL of deionized water four times. The as-prepared product was freeze-dried, and black powder of CRG was obtained. A binary mixture solvent (VH2O/VDMAc ) 2:3) was used in view to study the effect of solvent in the reduction of GO. SO2 gas was also used as a reducing agent to reduce GO, and it was prepared in situ by dropping 50 mL of NaHSO3 aqueous solution (20 wt %) into 50 mL of 98% concentrated sulfur acid at 90 °C in an oil bath. The as-prepared SO2 gas was directly input to the GO aqueous solution at the reaction temperature for 3 h. Preparation of Chemical-Reduced Graphene Suspension. The reduced graphene oxide suspension in the mixture of DMAc/H2O was prepared under the assistance of mild ultrasound. At first, 0.05 mg/mL of GO suspension was reduced by 1.35 mmol NaHSO3 in a binary mixture solvent (VH2O/VDMAc ) 2:3), as mentioned above, and then, the raw mixture was filtrated to remove the salts with washing by distilled water. Then, the as-prepared CRG powder was redispersed in a DMAc/water mixture solvent (VH2O/VDMAc ) 1:6) directly under the assistance of mild ultrasound, resulting in a homogeneous suspension of chemical-reduced graphene. This suspension was used for the AFM and TEM measurements. Preparation of GO and CRG Papers. GO or CRG powder was dispersed into DMAc under the assistance of powerful
ultrasound, and then, the suspensions were left to settle for 2 days. The upper layer of the suspension was filtrated to prepare GO or graphene paper according the method reported elsewhere22,23 and dried at 50 °C for 2 days. Characteristics of the Samples. Fourier transform infrared (FT-IR) spectra of the samples were recorded by a Bruker vector-2 spectrophotometer (Germany) using KBr-disk method. The thermal properties of the samples were characterized by a thermogravimeter (TGA, DTA-50, Shimazu, Japan), and all of the measurements were carried out under nitrogen gas over a temperature range of 30-700 °C with a ramp rate of 5 °C min-1. A commercial atomic force microscope (AFM, Nanoscope IIIa; Digital Instruments, Santa Barbra, CA), equipped with a J scanner was used to measure the morphology of the samples. A Si3N4 tip (Nanoprobes, Digital Instruments Inc.) was used in the contact mode. The scan rates were between 1.0 and 2.4 Hz. XPS spectra were recorded by an Escalab MK II photoelectron spectrometer (VG Scientific Ltd., United Kingdom). Raman spectra were taken by a RAMAMLOG 6 (Spex, U.S.A.) with a 50× objective lens and 514.5 nm laser excitation. TEM images were carried out on a Hitachi H-800 microscope at 200 kV. The surface structures of the CRG papers were measured using a Shimadzu SEM (Superscan SSX-550, Japan). The conductivity property of graphene papers was measured by a four-point method. C, H, N, and S contents of the samples were measured on an element analyzer (Elementar Analysensyeteme GMBH vatio EL III, Germany). Results and Discussion The as-prepared GO was well-dispersed into water to form a stable yellow-brown suspension, and NaHSO3 was dissolved into the suspension. The color of the GO and NaHSO3 mixture turned black when it was heated for 10 min at 95 °C. After a while, a black precipitate settled at the bottom of the flux. Upon filtration, washing, and freeze-drying, some black powder was obtained. Figure 1 shows the photographs of both the GO and as-prepared CRG powders. The color change is an indicator for the reduction of GO. The degree of reduction of GO can be judged by the ratio of C/O measured by elemental analysis. Table 1 shows the elemental content of the CRGs reduced either by N2H4 · H2O or NaHSO3 at different concentrations, different reaction times, and in different solvents. The CRG reduced by N2H4 · H2O in the DMAc and water mixture has a C/O ratio of 10.67, while the ratio of C/(N+O) is 7.57. These values indicate that there is some nitrogen left in the CRG which reduced the degree of conjugation in CRG. Therefore, we considered the determination of the ratio of C/(N+O) to be an alternative way to judge the degree of reduction of GO in place of the measurement of the C/O ratio. Similarly, the ratios of C/O and C/(N+O) were, respectively, found to be 7.89 and 7.66 when GO was reduced
TABLE 1: Elemental Analysis of Reduced GO by N2H4 · H2O or NaHSO4 H (mass%) N (mass%) C (mass%) S (mass%) O (mass%) C/O (mole) C/(N+O) (mole) C/(S+O) (mole) a
GO
N2H4a
NaHSO3a
N2H4b
NaHSO3c
NaHSO3b
1.310 0.57 44.84 2.74 50.54 1.18 1.17 1.16
0.77 3.74 84.51 0.36 10.62 10.67 7.57 10.47
0.52 0.22 81.76 0.68 16.83 6.48 6.39 6.35
0.63 4.25 84.87 0.34 9.91 11.4 7.66 11.2
0.43 0.18 82.33 0.62 16.44 6.68 6.59 6.55
0.77 0.37 83.92 0.75 14.19 7.89 7.66 7.69
1.35 mmol, 3 h, in H2O. b 2.7 mmol, 3 h, in DMAc/H2O. c 2.7 mmol, 3 h, in H2O. d 1.35 mmol, 24 h, in H2O.
NaHSO3 0.64 0.33 82.97 0.79 15.27 7.24 7.07 7.06
d
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Figure 2. High-resolution C1s XPS spectra of GO and CRG using NaHSO3 as a reducing agent. Figure 4. (a) Raman spectra of GO and the CRG reduced by NaHSO3. (b) Enlarged view of the same spectra showing the shift of the D-band and the G-band.
Figure 3. FT-IR spectra of GO (curve 1), CRG reduced by NaHSO3 (curve 2), and N2H4 · H2O (curve 3).
by 2.7 mmol NaHSO3 in a DMAc/water mixture, which clearly indicates that the degree of reduction of GO by NaHSO3 is close to that of N2H4 · H2O. To understand the role of solvent and the concentration of the reducing agent in the reduction process, we carried out the reaction in pure water with 1.35 and 2.7 mmol NaHSO3. The C/O and C/(N+O) ratios were 6.48 and 6.39 for 1.35 mmol, while they were 6.68 and 6.59 for the 2.7 mmol concentration. The degree of reduction was found to increase upon our increasing the amount of the reducing agent. In our previous work, we showed that addition of DMAc into the aqueous suspension of GO (DMAc/H2O ) 6:1) could promote the reduction of GO.15 Similarly, here, we observed that the reduction of GO is promoted by addition of DMAc during the chemical reduction of GO by NaHSO3. Graphene is dissolved into DMAc and may affect the chemical balance of the reduction and hence promote the reaction. Table 1 also shows that the degree of reduction of GO can be enlarged by extending the reaction time. The content S in the samples was also measured,
Figure 5. Normalized remaining mass of GO (curve 1), CRG reduced by N2H4 · H2O (curve 2), and NaHSO3 (curve 3).
and the results are summarized in Table 1. For GO, the content of S is 2.74, and the ratio of C/(S+O) is 1.16, indicating that there are some sulfide residues during the process of GO preparation which involving sulfide acid. For the CRG reduced by 2.7 mmol NaHSO3 in DMAc/H2O, the ratio of C/(S+O) is 7.69, very close to the ratio of C/O, and the content of S is 0.75 wt %. At the same time, the content of N in the CRG is 0.37 wt %, much lower than that of the CRG reduced by N2H4 · H2O. XPS measurements could provide direct evidence of the reduction of GO. Figure 2 shows the C1s XPS spectra of both GO and CRG reduced by NaHSO3 (2.7 mmol). Both curves were fitted considering the following contributions: CdC (sp2; peak 1), C-C (sp3; peak 2), C-O/C-O-C (hydroxyl and epoxy groups; peak 3), CdO (carbonyl groups; peak 4), and O-CdO
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Figure 6. AFM height images of GO (a) and CRG reduced by NaHSO3. (b-d) Section line analyses of (a) and (b), as indicated; the size of the scale bar is 1 µm.
(carboxyl groups; peak 5).24 Figure 2a shows the C1s peaks of GO, which consists of three main components arising from C-O (hydroxyl and epoxy, 286.7 eV), CdO (carbonyl, 287.4 eV), and CdC/C-C (284.7 eV) species, and a minor component of the OdC-O (carboxyl, 288.8 eV) species. After the chemical reduction, the oxygen species of C-O (hydroxyl and epoxy, 286.5 eV), CdO (carbonyl, 287.9 eV), and OdC-O (carboxyl, 289.1 eV) reduced significantly, as shown in Figure 2b, which indicates an efficient deoxidization. The major species remaining were CdC (284.7 eV) and C-C (285.5 eV). Figure 3 shows the typical FT-IR spectra of GO and the CRG reduced by NaHSO3 (S-CRG) and N2H4 · H2O (N-CRG). For GO, the characteristic peaks appear for carbonyl CdO (1734 cm-1), aromatic CdC (1624 cm-1), carboxy C-O (1415 cm-1), epoxy C-O (1228 cm-1), and C-O (1076 cm-1).25 After chemical reduction, the peaks for the oxygen functional groups were reduced significantly, and finally the peak at 1414 cm-1 for carboxy C-O entirely vanished. The remaining peak at 1577 cm-1 is attributed to the aromatic CdC group.25 Figure 4 shows the typical Raman spectra of GO and its relative CRG reduced by NaHSO3. Both spectra show the existence of the D, G, and 2D bands. For GO, the G band is located at 1602 cm-1, while for the CRG, the G band moved to 1588 cm-1, which is close to the value of the pristine graphite, and it confirms the reduction of GO during the chemical treatment. However, the existence of the D band at 1355 and 1352 cm-1, corresponding to GO and CRG both, also predicts the defect of the sample and the size of the in-plane sp2 domain.25 The intensity ratio of the D and G band (ID/IG) varies from 0.95 to 1.22, and the phenomena is unexpected. Paredes et al.,26 Villar-Rodil et al.,27 Kaner et al.,8 and Stankovich et al.7 also found similar results for graphene produced by the chemical reduction process. Stankovich et al.7 suggested that reduction of GO increases the number of aromatic domains of smaller average size in graphene, which could lead to an increase of the ID/IG ratio. Figure 5 shows the typical TG curves for the CRGs reduced by N2H4 · H2O and NaHSO3. Generally for GO, there are two major steps to loss of mass upon the increase of temperature. The loss of mass at around 100 °C, the boiling point of water, can be ascribed to the removal of absorbed water, and at around 200 °C, it can be attributed to the decomposition of labile oxygen functional groups.15 However, for the S-CRG and N-CRG, the loss of mass at around 200 °C is small, indicating the efficient
Figure 7. TEM image of the CRG reduced by NaHSO3, and the inset is the relative selected area electron diffraction pattern (SAED); the size of the scale bar is 200 nm.
removal of oxygen functional groups after reduction. In addition to this, it is interesting to note here that the thermal stability of S-CRG is a little better than that of N-CRG. Figure 6 shows the AFM height images and their analyses for GO and the CRG reduced by NaHSO3. As shown in Figure 6a and c, the size of the GO sheet is of micrometers order, and the thickness is 0.79 nm, which confirms that GO is a single layer.26 After reduction by NaHSO3, the size of the products (CRG) is still on the order of micrometers with a thickness of 0.87 nm, as shown in Figure 6b and d. This result indicates that the product is also a single layer. Figure 7 shows the TEM image of the CRG. Large graphene nanosheets were observed with wrinkled structures. The inset in Figure 7 displays a selected area electron diffraction (SAED) pattern of the CRG, and the clear six-spot patterns match those expected for individual graphene-based sheets.2,9 The papers of GO and the chemically reduced product S-CRG and N-CRG were prepared by a simple filtration of relative
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Figure 8. Photographic image of the CRG paper reduced by NaHSO3 (a) and SEM image of its cross section (b) (the size of the scale bar is 2 µm).
SCHEME 1: Possible Reduction Mechanism for GO by NaHSO3
suspensions. Figure 8a shows the photograph of the prepared S-CRG paper, which is light black in color. The layered structure of the graphene sheets can be viewed in the SEM image at the cross section of the S-CRG paper, as shown in Figure 8b. The electrical conductivity of the S-CRG and N-CRG papers was measured by the Van der Pauw method, and it was found that the conductivity is 5100 S m-1 for N-CRG paper, while it is 6500 S m-1 for S-CRG paper. This result reveals that NaHSO3 is an efficient reducing agent for the preparation of graphene with high conductivity. A density functional theory study is essential to understanding the reduction mechanisms of GO by NaHSO3.18 Here, we propose a possible mechanism, as shown in Scheme 1. We believe it is a two-step SN2 nucleophilic reaction followed by a thermal elimination. The oxidation of HSO3- to SO42- is believed to take place in the process. In order to support this argument, the SO42- content in the solution as a possible product of HSO3- was determined by the BaSO4 precipitation method. After the reduction reaction, the CRG product was separated. Then, 20 mL of aqueous solution of 0.5 M BaCl2 was added into the CRG product, and a sufficient amount of white-colored BaSO4 precipitation was observed. This result strongly supports the above argument. However, the transition state of the reduction should be observed, and we plan it soon. Do the other sulfur-containing compounds show similar reducing ability for GO? With a view to answering this question, we studied here a series of sulfur-containing compounds such as Na2S, Na2SO3, Na2S2O3, and SOCl2 exactly in a similar fashion. Surprisingly, all of them exhibit good reducing ability for GO to CRG, even if the SO2 gas also shows efficient reduction ability for GO. Table 2 lists the elemental analyses for the CRGs prepared by using different sulfur-containing
TABLE 2: Elemental Analysis of Reduced GO by the Other Sulfur-Containing Compounds Na2Sa Na2SO3a Na2S2O3a SO2 SOCl2a SOCl2b H (mass%) N (mass%) C (mass%) S (mass%) O (mass%) C/O (mole) C/(N+O) (mole) C/(S+O) (mole) a
0.91 0.32 78.21 1.98 18.58 5.61 5.51 5.33
0.41 0.41 61.86 1.82 35.50 2.32 2.29 2.27
0.56 0.62 72.70 1.13 24.99 3.88 3.77 3.79
0.60 0.50 81.06 1.20 16.64 6.49 6.28 6.27
0.52 0.21 82.23 0.79 16.25 6.75 6.65 6.59
0.65 0.28 84.89 0.84 13.35 8.48 8.29 8.22
5 mmol, 3 h. b 2.7 mmol, 3 h.
compounds, and it can be found that the ratios were C/O ) 8.48, C/(N+O) ) 8.29, and C/(S+O) ) 8.22 when SOCl2 was used as the reducing agent (2.7 mmol, 3 h). The reducing ability of SOCl2 is better than that of N2H4 · H2O and NaHSO3, but it is not safer than NaHSO3 since it can react with water acutely during the formation of HSO3-. The reaction could be described as SOCl2 + 2H2O ) H2SO3 + 2HCl. Therefore, the deoxidization mechanism of SOCl2 is similar to that of NaHSO3 with the high concentration of HSO3- formed in situ (Scheme 1). For SO2 gas, there were plenty of HSO3- production when it was bubbled into water, and the reducing mechanism is similar to that of NaHSO3. However, for Na2S, Na2SO3, and Na2S2O3, different reducing mechanisms may exist along with the HSO3mechanism. However, the reducing abilities of Na2S, Na2SO3, and Na2S2O3 were found to be less than that of NaHSO3. Conclusions A new reduction route of GO to graphene was reported. It was found that some sulfur-containing compounds, such as NaHSO3, SO2, SOCl2, Na2S2O3, and Na2S, could reduce GO to
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CRG in aqueous solution. Especially, NaHSO3 and SOCl2 show similar reducing ability to that of N2H4 · H2O. Hence, we conclude that NaHSO3 should be accepted as a better reducing agent for GO as it is not explosive and is less toxic than N2H4 · H2O. The analysis of the product revealed that after the reduction of GO, NaHSO3 was oxidized to Na2SO4, and a possible mechanism of reduction was suggested. This study explored the possibility that sulfur-containing compounds are also good reducing agents for the reduction of GO. Acknowledgment. This work is supported by the National Basic Research Program of China (No. 2010CB923302), the National Major Specific Project for Innovation of New Pharmaceuticals (2009ZX09103-715), and the Fundamental Research Funds for the Central Universities. References and Notes (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. (2) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282–286. (3) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652–655. (4) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. ReV. 2010, 39, 228–240. (5) Loh, K. P.; Bao, Q. L.; Ang, P. K.; Yang, J. X. J. Mater. Chem. 2010, 20, 2277–2289. (6) Gao, W.; Alemany, L. B.; Ci, L. J.; Ajayan, P. M. Nature Chem. 2009, 1, 403–408. (7) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565. (8) Tung, V. C.; Allen, M. J.; Yang, Y.; Kaner, R. B. Nat. Nanotechnol. 2009, 4, 25–29.
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