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An environment-friendly preparation of reduced graphene oxide nanosheets via amino acid
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Nanotechnology 22 325601 (http://iopscience.iop.org/0957-4484/22/32/325601) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 22 (2011) 325601 (7pp)
doi:10.1088/0957-4484/22/32/325601
An environment-friendly preparation of reduced graphene oxide nanosheets via amino acid Dezhi Chen1,2, Lidong Li1 and Lin Guo1 1 School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics, Beijing 100191, People’s Republic of China 2 School of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, People’s Republic of China
E-mail:
[email protected] and
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Received 24 April 2011, in final form 27 June 2011 Published 14 July 2011 Online at stacks.iop.org/Nano/22/325601 Abstract Chemically modified graphene has been studied in many applications due to its excellent electrical, mechanical, and thermal properties. Among the chemically modified graphenes, reduced graphene oxide is the most important for its structure and properties, which are similar to pristine graphene. Here, we introduce an environment-friendly approach for preparation of reduced graphene oxide nanosheets through the reduction of graphene oxide that employs L-cysteine as the reductant under mild reaction conditions. The conductivity of the reduced graphene oxide nanosheets produced in this way increases by about 106 times in comparison to that of graphene oxide. This is the first report about using amino acids as a reductant for the preparation of reduced graphene oxide nanosheets, and this procedure offers an alternative route to large-scale production of reduced graphene oxide nanosheets for applications that require such material. S Online supplementary data available from stacks.iop.org/Nano/22/325601/mmedia (Some figures in this article are in colour only in the electronic version)
graphene-like materials [3, 8]. Generally, hydrazine [9–15], dimethylhydrazine [2, 16], and NaBH4 [17] are used for the preparation of RGO, but they are either toxic or hazardous. Hence, it is very important to find an environment-friendly and effective reductant to reduce GO for the preparation of RGO [3, 18]. Recently, environment-friendly chemical agents, such as vitamin C [19–21], aluminum powder [22], reducing sugar [23], have been reported to produce RGO. However, RGO prepared by the reduction of GO via amino acids has not been reported in the literature up to now. For this purpose, we propose a facile method to prepare RGO nanosheets that utilizes L-cysteine as the reductant. L-cysteine is an amino acid which contains a thiol group. The thiol is susceptible to be oxidized to form the disulfide derivative cystine [24]. Due to the ability of thiols to undergo redox reactions, L-cysteine has antioxidant properties. L-cysteine’s antioxidant properties are typically expressed in the tripeptide glutathione, which occurs in humans as well as other organisms [25]. By
1. Introduction During the last half decade, chemically modified graphene (CMG) has been studied in the context of many applications, such as polymer composites, energy-related materials, sensors, ‘paper’-like materials, field-effect transistors (FETs), and biomedical applications, due to its excellent electrical, mechanical, and thermal properties [1–7]. Generally, the reactions of chemically modified graphene oxide can be classified into (i) reduction (removing oxygen groups from graphene oxide) and (ii) chemical functionalizations (adding other chemical functionalities to graphene oxide). Among the reactions of graphene oxide, the reduction process is of the utmost importance due to the similarities between reduced graphene oxide (RGO) and pristine graphene. For scientists and engineers endeavoring to use graphene in large-scale applications, chemical reduction of graphene oxide is the most obvious and desirable route to synthesize large quantities of 0957-4484/11/325601+07$33.00
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7.0. Finally, one part of the as-prepared product was dispersed in aqueous solution of pH = 10 (using NaOH aqueous solution of 0.1 mol l−1 to adjust the pH) or N,N -dimethylacetamide by ultrasonication (500 W) to prepare the suspension of RGO, and the other part was used to produce the powder of RGO by drying at 50 ◦ C for 24 h under vacuum.
virtue of the antioxidant nature of L-cysteine, we successfully synthesized RGO nanosheets in aqueous solution under the mild conditions.
2. Experimental details 2.1. Materials
2.4. Characterization
Graphite powder, natural briquetting grade, 100 mesh, 99.9995% (metals basis), was purchased from Alfa Aesar. L-cysteine (purity: 97%) was purchased from Sigma. Analytical-grade NaOH, K2 S2 O8 , P2 O5 , KMnO4 , N,N dimethylacetamide, anhydrous ethanol, 98% H2 SO4 , 36% HCl and 30% H2 O2 aqueous solution were purchased from the Beijing chemical reagents company and used directly without further purification. All aqueous solutions were prepared with deionized water.
Ultraviolet–visible (UV–vis) spectra were obtained using a Cintra 10e spectrophotometer (GBC Scientific Equipment Pty Ltd, Australia). The aqueous suspension of GO and RGO was used as the UV–vis samples, and the deionized water was used as the reference. Raman spectra were recorded from 1000 to 1900 cm−1 on a LabRAM HR800 laser Raman spectroscope (HORIBA Jobin Yvon CO. Ltd, France) using a 514.5 nm argon ion laser. All samples were deposited on silicon wafers in powder form without using any solvent. Fourier transform infrared (FT-IR) spectra of the samples were recorded on an Avatar 360 spectrophotometer (Thermo Nicolet, USA). The test specimens were prepared by the KBr disc method. XRD analyses were carried out on an x-ray diffractometer (D/MAX-1200, Rigaku Denki Co. Ltd, Japan). The XRD ˚ at 40 kV patterns with Cu Kα radiation (λ = 1.5406 A) and 40 mA were recorded in the range of 2θ = 5◦ –80◦ . Thermogravimetric analysis (TGA) was performed under a nitrogen flow (100 ml min−1 ) using a Pyris Diamond TG/DTA (Perkin Elmer, Inc., USA). The samples were heated from 50 to 800 ◦ C at 5 ◦ C min−1 . The x-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantera xray photoelectron spectroscope (ULVAC-PHI, Inc., Japan), and the binding energy was calibrated with C 1s = 284.8 eV. Atomic force microscopy (AFM) images were acquired in a tapping mode with a commercial multimode Nanoscope IIIa (Veeco Co. Ltd). Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained using a JEM-2100F transmission electron microscope (JEOL Ltd, Japan) operated at 200 kV. The electrical conductivity was measured using a SDY-6 digital four-point probe system (Guangzhou, PR China).
2.2. Preparation of the GO GO was synthesized using graphite powder by a modified Hummers’ method [26–28]. In brief, 6 g of graphite powder were added to an 80 ◦ C solution of 25 ml concentrated H2 SO4 , 5 g of K2 S2 O8 , and 5 g of P2 O5 . The mixture was reacted for 6 h, after which it was diluted with 1 liter of water, and washed using a 0.22 μm Nylon Millipore filter to remove the residual acid. Afterward, this pre-oxidized graphite was put into ice cold (0 ◦ C) concentrated H2 SO4 (230 ml). 30 g of KMnO4 were then added gradually under stirring and the temperature of the mixture was controlled below 10 ◦ C. Successively, this mixture was stirred at 35 ◦ C for 2 h, after which 500 ml of distilled water were added slowly to keep the temperature below 50 ◦ C. After further reaction for 2 h, 1.4 liter of water and 20 ml of 30% H2 O2 were added, and the color of the mixture changed into brilliant yellow along with bubbling. The mixture was then centrifuged and washed with a total of 3 liter of 10% HCl solution followed by 3 liters of water to remove the acid. The resulting solid was subjected to dialysis for a week to remove the remaining metal ions and acids. Finally, the product was dried at 50 ◦ C for 24 h under vacuum. 50 mg graphite oxide was exfoliated into deionized water (100 ml) by ultrasonication (500 W) to form GO aqueous suspension at room temperature. The as-obtained yellow-brown 0.5 mg ml−1 aqueous suspension of GO (figure S1-a available at stacks. iop.org/Nano/22/325601/mmedia) was stored in a volumetric flask, and used for the further characterizations and chemical reduction.
3. Results and discussion The UV–vis absorption spectrum of GO shown in figure 1(a) is characterized by the π –π ∗ of the C=C plasmon peak around 230 nm and a shoulder around 300 nm which is often attributed to n –π ∗ transitions of the carbonyl groups [29]. While reduced by L-cysteine (figures 1(b)–(f)), the plasmon peak gradually red-shifts to ∼270 nm with the increase of the reduction time, reflecting increased π -electron concentration and structural ordering, which is consistent with the restoration of sp2 carbon and possible rearrangement of atoms [30]. It implies that the GO might be reduced and the aromatic structure might be restored gradually, and the degree of reduction was gradually improved with the increase of reaction time. Similar features and trends are observed for the reduction of GO by L-ascorbic acid [19, 21]. The Raman spectra further support the structural change before and after the reduction of GO. Figure 2 shows the
2.3. Reduction of the GO Typically, 0.2 g of L-cysteine was put into 20 ml GO aqueous suspension of 0.5 mg ml−1 . The mixture was kept in a tightly sealed glass bottle and stirred for 12, 24, 48, and 72 h respectively at room temperature (26 ± 2 ◦ C). Firstly, the black product was isolated by centrifugation at 4000 rpm, and then 20 ml NaOH aqueous solution of 0.1 mol l−1 was added into the product to dissolve L-cystine. Then, the solution was centrifuged at 10 000 rpm, and the obtained black slurry was washed with adequate deionized water and ethanol up to pH = 2
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Figure 1. UV–vis spectra of aqueous dispersion of GO before (a) and after being reduced by L-cysteine for different reduction times 12 h (b), 24 h (c), 48 h (d), 72 h (e).
Figure 3. FT-IR of the GO before (a) and after reduction by L-cysteine for different reduction times 12 h (b), 24 h (c), 48 h (d), 72 h (e).
respectively. Aromatic C=C stretching vibration shows at 1625 cm−1 , and the peaks at 1220 cm−1 and 1053 cm−1 can be attributed to the epoxy C–O stretching vibration and the alkoxy C–O stretching vibration, respectively [32]. While being reduced by L-cysteine, the peaks for oxygen functional groups gradually decreased with reaction time, and some of them disappeared completely. These observations confirmed that most oxygen functionalities in the GO were removed [20]. The curve is very similar to the curve of RGO by vitamin C [19], indicating the high efficiency of reduction by Lcysteine. However, the peak at 1625 cm−1 attributed to the aromatic C=C group still exists. It suggests that the frame of sp2 carbon atoms after reduction by L-cysteine is retained well, as before. The distance between two layers is an important parameter to evaluate the structural information of the graphene. The XRD patterns of graphite and GO are compared with those of the samples reduced by L-cysteine for different reduction times in figure 4. Owing to 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 atomically flat graphene sheet [33], the d -spacing of the GO (figure 4(b)) is about 0.78 nm (2θ ≈ 11.3◦ ), which is significantly larger than the d(002) value of graphite (figures 4(a), d ≈ 0.34 nm, 2θ ≈ 26.2◦ , thickness single-layer pristine graphene). With the increase of reaction time, the (002) peak of GO gradually disappears whereas the broad diffraction peak at 24.0◦ (d ≈ 0.37 nm) progressively becomes prominent (figures 4(c)–(f)). This shift in the interlayer spacing can be attributed to the reduction of the GO, where the reduction makes the RGO pack tighter than the GO [11]. Though there is a decrease in the interlayer spacing compared with GO, the basal spacing of RGO is higher than that of well-ordered graphite (single-layer pristine graphene). The higher basal spacing may be due to the presence of residual oxygen functional groups, indicating incomplete reduction of GO. The fact that (002) reflection
Figure 2. Raman spectra of graphite (a), GO before (b) and after reduction by L-cysteine for different reduction times 12 h (c), 24 h (d), 48 h (e), 72 h (f).
typical Raman spectra of graphite, GO, and after reduction by L-cysteine (figure 2). The Raman spectrum of the pristine graphite, as expected, displays a prominent G (the E2g mode of sp2 carbon atoms) peak as the only feature at 1582 cm−1 . In the Raman spectrum of GO after reduction by L-cysteine, the G band is broadened and shifted to around 1597 cm−1 . In addition, the D band (the symmetry A1g mode) becomes prominent. Noticeably, the Raman spectrum of GO after reduction by L-cysteine shows a higher D/G intensity ratio than GO (0.94), and the D/G intensity ratio increases (0.98, 1.02, 1.08 and 1.17) with the increase of reduction time (12, 24, 48 and 72 h) step by step. The variation of the relative intensities of G band to D band in the Raman spectra of the GO during the reduction usually reveals the change of the electronic conjugation state. This change suggests an increase in the number of sp2 domains with the reduction of GO [10, 31]. Figure 3 shows the FT-IR spectra of GO after reduction by L-cysteine. For GO, the characteristic peaks for C=O stretching vibration appear at 1746 cm−1 , and for O–H the stretching and deformation vibration appear at 3420 cm−1 and 1395 cm−1 , 3
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The above data suggest that we can produce RGO nanosheets through the reduction of GO by L-cysteine for 72 h. TGA measurements provide further proof of the preparation of RGO. Figure 5 shows the TGA curves for GO and RGO. On the one hand, GO is thermally unstable. There are two steps for mass loss with increasing temperature. The first mass loss is about 10% around 100 ◦ C, which can be attributed to the removal of adsorbed water. The second mass loss around 200 ◦ C is about 30% because of the decomposition of labile oxygen functional groups [35]. On the other hand, the removal of the thermally labile oxygen functional groups by L-cysteine results in much increased thermal stability for the RGO, and only 10% of the mass was lost at around 200 ◦ C [10]. This result is also supported by XPS measurements (figure 6), which show that the C/O ratio in the GO increases remarkably after reduction by L-cysteine for 72 h (figure 6(a)). Furthermore, a significant decrease of oxygenated carbon-related signals at 286–289 eV after reduction (figure 6(b)) reveals that most of the epoxide, hydroxyl, and carboxyl functional groups are removed after the reduction. AFM is a powerful tool to measure the thickness of the samples. Figure 7 shows the typical AFM height images of GO and the RGO nanosheets prepared by L-cysteine. The average thickness of the GO and RGO nanosheets obtained is about 1.0 nm and 0.8 nm respectively. The data match well with the previous reports [9, 10, 19, 36]. Figure 8 presents TEM images and a SAED pattern of GO and RGO nanosheets. Figure 8(a) displays that GO nanosheets are corrugated, but the SAED pattern (in the inset) indicates a crystalline structure. The SAED pattern contains information from many GO grains. A typical sharp, polycrystalline ring pattern is obtained. The first ring comes from the (1100) plane, and the second ring comes from the (1120) plane. Strong diffraction spots are observed on the ring. These results imply that the GO nanosheets before reduction are not randomly oriented with respect to one another, and the inter-layered coherence is not destroyed at this stage [37]. The TEM image of the RGO nanosheets shows a wrinkled paper-like structure. The SAED pattern in the inset of figure 8(b) shows a typical sharp polycrystalline ring pattern composed of many diffraction spots, indicating the loss of long range ordering between the RGO nanosheets.
Figure 4. XRD spectra of graphite (a), GO before (b) and after reduction by L-cysteine for different reduction times 12 h (c), 24 h (d), 48 h (e), and 72 (f).
Figure 5. Normalized TGA plots for GO (a) and RGO (b).
in these samples is very broad suggests that the samples are very poorly ordered along the stacking direction. It indicates that these samples comprise largely free RGO nanosheets (figure 4(f)) [34].
Figure 6. XPS spectra of GO and RGO, survey scan (a); C1S (b). The Na signal in RGO is attributed to residual NaOH.
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Figure 7. AFM images and section analysis of GO (a) and RGO (b) nanosheets absorbed on freshly cleaved mica.
Figure 8. TEM image and a typical SAED pattern (inset) of the GO (a) and RGO (b) nanosheets.
To determine the extent to which the chemical reduction of exfoliated GO nanosheets restores the electrical properties of the graphitic network, we measured the room-temperature electrical conductivities of compressed-powder samples of GO and the RGO via L-cysteine for 72 h. The conductivity of compressed-powder samples of GO is 7.97 × 10−7 S m−1 , while the RGO is as high as 0.124 S m−1 . It reveals that the conductivity of RGO by L-cysteine increases about 106 times in comparison to that of GO. Therefore, it indicates that RGO produced in this way has unique electrical properties that are the same as those produced via other methods [20]. The mechanism for the chemical reduction of GO is still an open question. A possible mechanism for the chemical reduction of GO by L-cysteine is shown in scheme 1. We
speculate that it should be a two-step SN2 nucleophilic reaction followed by a thermal elimination. L-cysteine is an amino acid which contains a thiol group. The thiol is susceptible to oxidization and ready to release a proton, functioning as a nucleophile [24]. The proton has commonly high binding affinity to the oxygen-containing groups, such as hydroxyl and epoxide groups on GO to form H2 O molecules [19]. Finally, the L-cysteine is oxidized into L-cystine, leading to the formation of RGO nanosheets. Actually, the XRD spectra (figure S2 available at stacks.iop.org/Nano/22/325601/ mmedia) of the RGO sample without any washing also confirms the above deduction. It is well known that the L-cystine is an amino acid insoluble in water and alcohol. Practically, however, it can 5
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Scheme 1. Possible mechanism of reduction of GO with L-cysteine.
be dissolved in dilute solutions of alkali hydroxides. Hence, a proper amount of NaOH aqueous solution of 0.1 mol l−1 was added into the product to remove L-cystine. Surprisingly, the NaOH aqueous solution can also disperse the precipitation of the RGO nanosheets for being less hydrophilic as a result of oxygen removal. With the aid of ultrasonication, we can prepare well dispersed RGO nanosheets in an aqueous solution of pH = 10 (figure S1-b available at stacks.iop.org/ Nano/22/325601/mmedia). This may be attributed to residual neutral carboxylic groups being converted to negatively charged carboxyl groups in alkaline solution [9, 38], and the electrostatic repulsion makes the RGO nanosheets’ dispersions stable. The presence of the remaining carboxyl groups on RGO nanosheets indicates the ineffective and incomplete reduction which greatly compromises their conductivity. Actually, the conductivity reported here is not high. Although a few of the RGO nanosheets were precipitated after two weeks, we still observed other RGO nanosheets dispersed well at the top of liquid. Furthermore, the RGO nanosheets also can be dispersed well in N,N -dimethylacetamide (figure S1-c available at stacks.iop.org/Nano/22/325601/mmedia). These results indicate that we can prepare uniformly dispersed and conductive RGO nanosheets through gentle reduction of GO by L-cysteine without any stabilizer.
Acknowledgments This work was supported by the National Basic Research Program of China (2010CB934700) and the National Natural Science Foundation of China (50725208, 20973019).
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4. Conclusions In summary, we have introduced an environmentally friendly approach for the reduction of GO by using L-cysteine as the reductant. During the reduction of GO, the thiol group of L-cysteine releases protons, and the protons commonly have high binding affinity to the oxygen-containing groups of GO, such as hydroxyl and epoxide groups, to form water molecules. This is the first report about the use of amino acid as reductant for the preparation of RGO sheets. The conductivity of RGO produced in this way increases by about 106 times as compared with that of GO. Furthermore, this reduction method avoids the use of any toxic reagents, and both reductant and the coproduct during the reduction of GO are biocompatible. This procedure offers an alternative route to large-scale RGO production for applications that require such material. 6
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