Fabrication of High-Concentration and Stable Aqueous Suspensions

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Fabrication of High-Concentration and Stable Aqueous Suspensions of Graphene Nanosheets by Noncovalent Functionalization with Lignin and Cellulose Derivatives Qiang Yang,† Xuejun Pan,*,† Fang Huang,‡ and Kecheng Li‡ Department of Biological Systems Engineering, UniVersity of Wisconsin-Madison, 460 Henry Mall, Madison, Wisconsin 53706, and Department of Chemical Engineering, UniVersity of New Brunswick, P.O. Box 69000, Fredericton, N.B., Canada, E3B 6C2 ReceiVed: October 26, 2009; ReVised Manuscript ReceiVed: January 26, 2010

Stable aqueous suspensions of graphene (G) nanosheets with high concentration (0.6-2 mg/mL) were prepared through chemical reduction of exfoliated graphite oxide (EGO) with the aid of sodium lignosulfonate (SLS), sodium carboxymethyl cellulose (SCMC), and pyrene-containing hydroxypropyl cellulose (HPC-Py). The noncovalently functionalized graphene nanosheets with a 3.3 ( 1.4 nm average thickness were characterized with use of UV-vis spectroscopy, fluorescence spectroscopy, atomic force microscopy, attenuated total reflectance micro-Fourier transform infrared spectroscopy, and Raman spectroscopy. Introduction Graphene (G) is a one-atom-thick planar sheet of sp2-bonded carbon atoms densely packed together in a honeycomb hexagonal lattice. It has been attracting substantial attention in recent years because of its excellent mechanical1 and electronic2 properties. So far, the graphene has been prepared by chemical vapor deposition,3 epitaxial growth,4 micromechanical cleavage,2 liquid-phase exfoliation,5 hydrothermal dehydration,6 and chemical reduction of exfoliated graphite oxide (EGO) suspension.7 Chemical reduction of the EGO suspension is the most used process to produce graphene. Dispersing agents are usually required during the reduction in most cases; otherwise, the graphene freshly formed from the reduction of the EGO aggregates readily and precipitates irreversibly from solvents. Noncovalentfunctionalization8–15 andcovalentfunctionalization16–21 of the graphene can be used to prevent irreversible aggregation of the freshly produced graphene. For example, aromatic and isocyanate organic molecules have been used as the noncovalent and covalent functionalization reagents, respectively. Shi et al. prepared stable aqueous suspensions of graphene sheets using water-soluble 1-pyrenebutyrate.9 Stankovich et al. reported that stable graphene sheets in polar solvent could be obtained from chemical reduction of the isocyanate-treated EGO.20 Polysaccharides such as gum arabic,22 helical amylose,23 and starch24 have been successfully used as the dispersants for carbon nanotubes. Recently, we successfully dispersed carbon nanotubes in solvents using water-soluble pyrene-modified hydroxypropyl cellulose and chitosan.25,26 To our knowledge, no work has been done in the stabilization of the graphene using natural polymeric dispersants, such as lignin and cellulose derivatives. They are environmental friendly, renewable, biologically compatible, and biodegradable. In this paper, three natural polymers including sodium lignosulfonate (SLS), sodium carboxymethyl cellulose (SCMC), and pyrene-containing hydroxypropyl cellulose (HPC-Py) were found to be excellent stabilizers for the preparation of high-concentration and stable graphene aqueous suspensions. * To whom correspondence should be addressed. Tel: 608-262-4951. Fax: 608-262-1228. E-mail: [email protected]. † University of Wisconsin-Madison. ‡ University of New Brunswick.

Experimental Section Materials. Graphite microcrystalline (powder, 300 mesh, 75-82% C, 18-25% ash) was purchased from Alfa Aesar. Sodium carboxymethyl cellulose (SCMC, Mw ) 250 000, DS ) 0.9), sodium lignosulfonate (SLS, low sulfonate content, Mw ) 60 000), hydroxypropyl cellulose (HPC, white powder, average Mn ) 10 000), 4-(dimethylamino) pyridine (DMAP), N,N-dicyclohexylcarbodiimide (DCC), 1-pyrenebutyric acid (Py), potassium permanganate (KMnO4), phosphorus pentoxide (P2O5), potassium persulfate (K2S2O8), and sodium borohydride (NaBH4) were obtained from Sigma-Aldrich. Hydrogen peroxide solution (H2O2, 30%), hydrochloric acid (HCl), 96% sulfuric acid (H2SO4), and anhydrous dimethylformamide (DMF, dried with 4 Å molecular sieves before use) were bought from Fisher Scientific. The HPC-Py weight percentage of pyrene moieties is 2.2, DS ) 0.02, calculated from the 1H NMR spectrum by comparing the pyrene proton signals to the glucose unit proton signals.25,26 Instruments. 1H NMR spectra were taken on a Bruker DRX360 NMR (360.13 MHz, Bruker, Rheinstetten, Germany) spectrometer fitted to a 5 mm 1H-broadband gradient probe with inverse geometry, using CDCl3 as solvent. Sonication experiments were carried out on FS 110 D (Fisher scientific). Reduction experiments were carried out on a shaker (Thermo Scientific, MAXQ 4450). UV-vis spectra were collected from 200 to 600 nm, using a viable-temperature UV-vis spectrophotometer (Cary 50 Bio, Varian). Fluorescence spectra (λexcitation ) 325 nm) were recorded from 350 to 650 nm on a MOS-250 fluorescence spectrometer (Biologic, Claix, France). Attenuated total reflectance micro-Fourier transform infrared (ATR-FTIR) spectra were recorded from 800 to 4000 cm-1 on a PerkinElmer Spectrum 100 Series FT-IR spectrophotometer with a universal ATR sampling accessory (Waltham, MA). Raman spectra were collected from 800 to 2000 cm-1 on a Horiba Jobin-Yvon LabRAM ARAMIS Raman confocal microscope (632.8 nm, Aramis CRM, Horiba Jobin Yvon, Edison, NJ). Typical tappingmode atomic force microscopy (AFM) measurements were performed with an MFP-3D AFM (AsylumResearch, USA), using TESP-SS (Veeco, USA) super sharp probes at 320 kHz frequency. Samples were first sonicated in a sonic mixer (Bransonic, B-2200R-4) at 30 °C, and then transferred onto a

10.1021/jp910232x  2010 American Chemical Society Published on Web 02/12/2010

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Figure 1. UV-vis absorption (a) and ATR-FTIR (b) spectra of the graphite oxide.

SCHEME 1: Chemical Structures of the SLS, SCMC, and HPC-Py

Figure 2. Digital photos of the G (A), SLS/G (B), SCMC/G (C), and HPC-Py/G (D) aqueous suspensions after 4-month storage.

freshly cleaved mica (9.9 mm diameter, Ted Pella, INC) surface and allowed to air-dry at ambient temperature for AFM observation. Preparation of Graphite Oxide. Graphite oxide (GO) was prepared with the method reported by Hummers and Shi9,27 with modification. Graphite powder (3 g), P2O5 (2.5 g), and K2S2O8 (2.5 g) were mixed in 96 wt % H2SO4 (12 mL). The mixture reacted at 80 °C overnight. The resulting preoxidized graphite was collected by centrifugation, washed five times with water and once with acetone, and dried at vacuum. The as-prepared preoxidized graphite was added into 96% sulfuric acid (120 mL) on an ice bath. Then, KMnO4 (15 g) was gradually added into the mixture within 1 h. The resulting mixture was kept at 35 °C for 2 h and gradually diluted with 200 mL of ice water within 30 min on an ice bath (keep the temperature below 40 °C). After the dilution, the mixture was magnetically stirred at room temperature for 2 h. To stop the oxidization reaction, deionized water (400 mL) and 30% H2O2 (50 mL) were added into the reaction mixture. When bubbling stopped, the lightly yellow product was filtered and washed with 10% HCl aqueous solution and then 10 times with water, then dried under vacuum, thus obtaining the graphite oxide. Fabrication of Graphene Nanosheets Suspensions. Graphene nanosheets were fabricated by reducing the graphite oxide prepared above. Specifically, for the SLS/G suspension, graphite oxide (6 mg, 12 mg or 20 mg) and SLS (60 mg) were dispersed in 10 mL of deionized water under sonication for 1 h, using FS 110 D (Fisher scientific). For the SCMC/G suspension, graphite oxide (40 mg) and SCMC (40 mg, 100 mg or 200 mg) were dispersed in 20 mL of deionized water under sonication for 1 h. For the HPC-Py/G suspension, graphite oxide (13 mg) and the HPC-Py (60 mg) were dispersed in 20 mL of dry DMF under sonication for 1 h. The resulting yellow exfoliated graphene oxide suspensions were then reduced with NaBH4. Briefly, NaBH4 (20 mg) was added into each of the exfoliated

graphene oxide suspensions. The reduction was carried out on a shaker (Thermo Scientific, MAXQ 4450) at 400 rpm and 80 °C for 24 h. At the end of the reduction, the polymer/graphene nanosheets (SLS/G, SCMC/G, and HPC-Py/G) were filtered out from corresponding suspensions and washed twice with water. Results and Discussion The prepared graphite oxide (GO) was characterized by UV-vis and ATR-FTIR spectroscopy. Figure 1 shows the UV-vis absorption (a) and ATR-FTIR (b) spectra of the GO. The GO has a characteristic peak at 231 nm, corresponding to π-π* transitions of carbon-carbon bonds, and a shoulder at 300 nm, due to n-π* transitions of carboxyl bonds, as shown in Figure 1a.28 The characteristic peaks of the GO, including O-H stretching at 3286 cm-1, CdO stretching at 1724 cm-1, skeletal vibration of unoxidized graphitic domains at 1626 cm-1, O-H deformation at 1407 cm-1, C-OH stretching at 1223 cm-1, and C-O stretching at 1004 cm-1, were clearly observed in the ATR-FTIR spectrum, Figure 1b. These peaks are in good agreement with those in the literature.9 The stable graphene nanosheets suspension was prepared by two steps. First, the graphite oxide was exfoliated under sonication in the presence of the polymeric dispersant. Successively, the homogeneous exfoliated graphene oxide suspension was reduced by NaBH4. In the present study, sodium lignosulfonate (SLS), sodium carboxymethyl cellulose (SCMC), and pyrene-containing hydroxypropyl cellulose (HPC-Py) (their chemical structures are shown in Scheme 1) were used as the polymeric dispersants. All graphene suspensions stabilized by these polymeric dispersants were stable in water for four months without conspicuous aggregates observed, as shown in Figure 2. On the contrary, without addition of the polymeric stabilizers, the newly produced (reduced) graphene readily formed irreversible aggregates in water, as shown in Figure 2A. Sodium lignosulfonate (SLS) is an amphiphilic polyelectrolyte with hydrophobic alkyl backbone chains linked to aromatic rings and hydrophilic sodium sulfonate groups at the R-carbon. Liu

Aqueous Suspensions of Graphene Nanosheets et al. indicated that the SLS is an excellent dispersant for multiwalled carbon nanotubes with a solubility as high as 1.5 mg/mL in water and a good storage stability.29 In the present study, we used the SLS (Mw ) 60 000) with low sulfonate content to stabilize the graphene nanosheets in water for the first time. Our results showed that the SLS at different weight ratios (10:1, 5:1, and 3:1, SLS/GO, respectively) was able to stabilize the graphene nanosheets within the range of concentrations from 0.6 to 2 mg/mL in water. When poly(sodium 4-styrenesulfonate) (PSS) was used, a higher weight ratio (10: 1, PSS/GO) was necessary to obtain a stable aqueous graphene nanosheets suspension with a concentration of 1 mg/mL.12 Similarly, a weight ratio of 7.5 was required to stabilize the graphene nanosheets (1 mg/mL) in water with sulfonated polyaniline (SPANI).13 The excellent stabilizing ability of the SLS toward the graphene nanosheets is attributed to its unique chemical structure. First, its alkyl chains can be strongly and irreversibly adsorbed onto the graphene surface by strong hydrophobic attraction.29 More importantly, its aromatic rings can presumably interact with the graphene surface by π-π stacking. Meanwhile, its sulfonic groups (-SO3Na) can supply enough electrostatic repulsion force against van der Waals attraction of the graphene nanosheets. Sodium carboxymethyl cellulose (SCMC) is an important polysaccharide anionic polyelectrolyte, which has proven to be the best polymeric dispersant so far for single-walled carbon nanotubes because of good isolation and stability at high concentration.30–32 In addition, the carboxymethyl cellulosegrafted carbon nanotubes showed good solubility and stability in water.33 In this study, the SCMC with higher molecule weight (Mw ) 250 000) were first used at varied weight ratios (1:1, 2.5:1, and 5:1; SCMC/GO) to stabilize the graphene nanosheets at a concentration of 2 mg/mL in water. At high weight ratios (2.5:1 and 5:1), the obtained aqueous suspensions of the graphene nanosheets were very stable. Even when the weight ratio of SCMC/GO was as low as 1, only a trace of graphene sediments was observed. The good dispersing ability of the SCMC toward the graphene nanosheets should be attributed to its electrostatic repulsion force from the -COONa groups disrupting the van der Waals interaction. Hydroxypropyl cellulose (HPC) is water-soluble and apparently able to disperse graphene, but the attempt to stabilize graphene using unmodified hydroxypropyl cellulose failed. On the other hand, our previous research showed that pyrenemodified hydroxypropyl cellulose (HPC-Py) could disperse multiwalled carbon nanotubes well in aqueous and organic solvents by noncovalent functionalization,26 in which the HPCPy was attached on the carbon nanotubes by the π-π stacking between the pyrene moieties and the carbon nanotubes. Therefore, the HPC-Py was expected to be a good dispersant for graphene nanosheets in water as well. To avoid the aggregation of the HPC-Py in water above 45 °C,26 dry DMF was used as solvent of the HPC-Py and graphene oxide in the present study. The concentration of graphite oxide was 0.65 mg/mL and the weight ratio (HPC-Py/GO) was 4.6. After the reduction of graphene oxide, only traces of sediments were observed in the HPC-Py/G suspension; the sediments were removed by centrifugation. The obtained black suspensions of graphene nanosheets were filtered, then washed twice with water. The resulting graphene nanosheets are readily redispersible in water with the help of sonication for 10 min. The HPC-Py stabilized the graphene nanosheets presumably by steric repulsion force from its long polymer chains.

J. Phys. Chem. C, Vol. 114, No. 9, 2010 3813 A UV-vis spectrometer is widely used to verify the formation of stable graphene aqueous suspension. After reduction of the graphite oxide, the characteristic absorption peak at 231 nm usually red-shifts to about 260-270 nm,34 and the absorption peak at 300 nm disappears, which indicates that highly conjugated electronic structure is restored in the resulting graphene. As shown in Figure 3a, no absorption peak at 300 nm was observed, but the characteristic peak of the graphene at 260-270 nm was not visible either in the SLS/G spectrum. Since SLS has a strong absorption peak at 283 nm, it probably overshadowed the absorption peak of the graphene between 260 and 270 nm. Usually, the linear relationship between absorbance at the characteristic peak and the corresponding concentration (the Beer’s law) is used as an indicator of the good solubility of graphene.35,36 Here, considering the fact that the characteristic absorption peak of SLS/G at ∼265 nm was not observed, the relationship between the absorbance at 283 nm and concentration of the SLS/G was used as an indirect indicator of the solubility of the graphene. As described in the Experimental Section, excessive SLS was washed away during the cleaning of SLS/G product, and the residual SLS was associated with the graphene nanosheets. Therefore, the SLS concentration should be proportional to the graphene concentration. The inset of Figure 3a indicates that there is a good linear relationship between absorbance at 283 nm and the concentration of SLS/G. In addition, if the SLS/G was not well-dispersed (dissolved) in water, the absorbance would not be linearly correlated to its concentration. Thus, the SLS/G suspension was proved to have good solubility/dispersibility in water. On the contrary, the characteristic peak of the graphene was clearly observed at 266 nm in the SCMC/G suspension (see Figure 3b) since the SCMC does not have a characteristic peak as SLS does that interferes with the graphene peak. Similarly, its absorbance is linearly correlated with the concentration of SCMC/G in the suspension. As expected, the characteristic peak of the graphene in HPCPy/G was also overshadowed by the absorbance peaks of the pyrene moieties in the HPC-Py,9,10 as shown in Figure 3c. As a whole, the absorption spectrum of the HPC-Py/G was similar to that of the HPC-Py, except for weaker absorption intensities and red-shifts in the same wavelength regions. In the presence of graphene, corresponding absorption peaks of the HPC-Py red-shifted to 268, 279, 330, and 347 nm, respectively. In the fluorescence spectrum (see Figure 3d), the HPC-Py had a characteristic peak at 485 nm.10 However, when the HPC-Py was attached to the graphene surface, its fluorescence was completely quenched due to electronic or energy transfer.9,10 This observation indicates that there existed strong π-π stacking between the pyrene moieties and the graphene, which helped the HPC-Py attach to the graphene surface and resulted in the formation of a stable HPC-Py/G suspension in water with good solubility/dispensability. Atomic force microscopy (AFM) is usually utilized to measure the thickness of polymer-functionalized graphene nanosheets. In theory, the thickness of a single-sheet graphene oxide is 0.34 nm.7 However, the thickness of the single-sheet graphene oxide is usually found 0.9-1.3 nm from AFM analysis.37 The variation in the thickness is attributed to the presence of oxygen-containing functional groups and to the fact that the graphene sheet was generally not lying perfectly flat on the mica surface prior to AFM analysis.38,39 In addition, polymer-coated graphene nanosheets produced from the reduction of the exfoliated graphite oxide are typically a combination of several single graphene layers with a thickness of 2.5-4 nm.12–14 Figure 4 shows the tapping-mode AFM image and

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Figure 3. (a) UV-vis absorption spectra of the SLS/G aqueous suspension (inset: correlation curve of absorbance at 283 nm versus concentration) at varied concentrations. (b) UV-vis absorption spectra of the SCMC/G aqueous suspension (inset: correlation curve of absorbance at 266 nm versus concentration) at varied concentrations. (c) UV-vis spectra of the HPC-Py and HPC-Py/G aqueous suspension. (d) Fluorescence spectra of the HPC-Py and HPC-Py/G aqueous suspension.

Figure 4. Tapping-mode AFM image and cross-section height profile of a typical graphene nanosheet. The sample was prepared on a freshly cleaved mica sheet.

cross-section height profile of a graphene nanosheet (SLS/G) prepared in this study. The graphene nanosheet had a 3.3 ( 1.4 nm mean thickness. Considering that the polymer chains

can be inserted between the graphene layers,15 the graphene nanosheets were constructed of 2-3 layers of a single graphene sheet.

Aqueous Suspensions of Graphene Nanosheets

J. Phys. Chem. C, Vol. 114, No. 9, 2010 3815 nanoshets than SCMC. The ATR-FTIR and Raman analyses indicate that the graphene nanosheets are associated with the corresponding polymeric dispersants. Conclusion Three natural polymers (lignin and cellulose derivatives) were successfully used as dispersants in preparing grapheme nanosheets through chemical reduction of exfoliated graphite oxide. The resulting suspensions of the polymer-stabilized graphene nanosheets are stable in water at high concentrations (0.6-2 mg/mL).

Figure 5. ATR-FTIR spectra of the HPC-Py/G (a), SLS/G (b), GO (c), and SCMC/G (d).

Acknowledgment. The authors gratefully acknowledge the starting-up support from the Department of Biological Systems Engineering and the College of Agriculture and Life Science at the University of Wisconsin-Madison and USDA McIntireStennis Fund. References and Notes

Figure 6. Raman spectra of the GO (a), SCMC/G (b), HPC-Py/G (c), and SLS/G (d)

Chemical structures of these graphene nanosheets were analyzed by ATR-FTIR spectrometer. ATR-FTIR spectra of the graphite oxide, SLS/G, SCMC/G, and HPC-Py/G are shown in Figure 5. After reduction of the graphite oxide, the CdO characteristic peak of graphite oxide at 1724 cm-1 (except for the HPC-Py/G) disappeared and characteristic peaks of the polymers were observed in the graphene nanosheets. Specifically, the C-H stretching at 2930 cm-1 and S-O stretching at 1039 cm-1 were observed in the SLS/G.40 The carboxyl absorption peak of the COONa at 1634 cm-1 and cellulose backbone absorptions at 2920 cm-1 (the C-H stretching) and 1107 cm-1 (the C-O-C deformation), respectively, were observed in the SCMC/G.41 In the spectrum of the HPC-Py/G, the CdO stretching at 1724 cm-1, C-H stretching at 2974, 2921, and 2863 cm-1, and pyrene ring stretching and deformation at 1623 and 840 cm-1, respectively, were observed.42 The structures of these graphene nanosheets were further analyzed by Raman spectrometer. Figure 6 shows the Raman spectra of the GO (a), SCMC/G (b), HPC-Py/G (c), and SLS/G (d). The graphite oxide displays two characteristic peaks at 1270 (Dband, C-C) and 1598 cm-1 (G-band, CdC), respectively. Compared with the GO, there are no obvious shifts observed in the SCMC/G, HPC-Py/G, and SLS/G. However, ID/IG ratios (intensity ratio of the D-band to G-band) of the graphene nanosheets are higher than that of the graphite oxide. Specifically, the ID/IG ratios of the GO, SCMC/G, HPC-Py/G, and SLS/G are 0.92, 0.96, 1.01, and 1.00, respectively. The enhancement of the ID/IG ratio is attrubited to the increase in sp3-hybridized sidewall carbons by the functionalization. Additionally, the ratios of the HPC-Py/G and SLS/G are higher than that of the SCMC/G, probably because the HPC-Py and SLS had stronger interactions with the graphenes and therefore resulted in higher polymer contents in the resulting graphene

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