Exfoliated Graphite Oxide Decorated by PDMAEMA Chains and

Jul 14, 2009 - McAllister , M. J., Li , J.-L., Adamson , D. H., Schniepp , H. C., Abdala , A. A., Liu , J., Herrera-Alonso , M., Milius , D. L., Car ,...
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Exfoliated Graphite Oxide Decorated by PDMAEMA Chains and Polymer Particles Yongfang Yang,*,‡ Jie Wang,† Jian Zhang,† Jinchuan Liu,† Xinglin Yang,§ and Hanying Zhao*,† †

Key Laboratory of Functional Polymer Materials, Ministry of Education, Department of Chemistry, Nankai University, Tianjin 300071, P. R. China, ‡Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China, and §Institute of Polymer Chemistry, Nankai University, Tianjin 300071, P. R. China Received April 22, 2009. Revised Manuscript Received June 28, 2009

Exfoliated graphite oxide (GO) sheets with hydroxyl groups and amine groups on the surface were prepared by modification of graphite. Atom transfer radical polymerization (ATRP) initiator molecules were grafted onto the GO sheets by reactions of 2-bromo-2-methylpropionyl bromide with hydroxyl groups and amine groups. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) chains on the surface of GO sheets were synthesized by in-situ ATRP. X-ray photoelectron spectroscopy, thermogravimetric analysis, and transmission electron microscopy (TEM) results all demonstrated that polymer chains were successfully produced. After grafting of PDMAEMA, the dispersity of GO sheets in solvents was improved significantly. Poly(ethylene glycol dimethacrylate-co-methacrylic acid) particles were deposited on GO sheets via hydrogen bonding between MAA units on polymer particles and amine groups of PDMAEMA. TEM and scanning electron microscopy were used to characterize the structure of the nanocomposites.

Introduction Graphene-based materials have attracted considerable attention because of their unique properties such as the excellent mechanical, optical, and electrical properties.1-3 These materials may find applications as field-effect and ultrahigh-frequency transistors,4-6 ultrasensitive sensors,7 electromechanical resonators,8 and mechanically reinforced composites.9 Generally speaking, there are two routes to prepare graphene sheets: mechanical exfoliation techniques and synthetic methods.10,11 The exfoliation techniques can generate a large amount of graphene sheets at one time; however, the chemical methods yield graphene sheets with smaller size (containing as few as 60 carbon atoms) compared to those obtained by exfoliation techniques. Aksay and co-workers reported a method to produce functionalized single graphene sheets in bulk quantities through thermal expansion of graphite oxide. The hydroxyl, epoxy, and carboxylic acid functional groups on the graphene sheets can be created by oxidation and thermal expansion *To whom correspondence should be addressed. E-mail: [email protected]. edu.cn (Y.Y.); [email protected] (H.Z.).

(1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Wu, J.; Pisula, W.; M€ullen, K. Chem. Rev. 2007, 107, 718. (3) Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (4) Gilje, S.; Song, H.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394. (5) Brey, L.; Fertig, H. A. Phys. Rev. B: Condens. Matter. Mater. Phys. 2006, 73, 235411. (6) Son, Y. W.; Cohen, M. L.; Louie, S. G. Phys. Rev. Lett. 2006, 97, 216803. (7) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652. (8) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. M.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 317, 490. (9) 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. (10) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (11) Yang, X.; Dou, X.; Rouhanipour, A.; Zhi, L.; Rader, H. J.; M€ullen, K. J. Am. Chem. Soc. 2008, 130, 4216. (12) Schniepp, H. C.; Li, J.-L.; McAllister, M. J.; Sai, H.; Herrera-Alonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 110, 8535.

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of graphite,12,13 which enable graphene to be further modified by chemical reactions, and thus many different functional groups or functional particles can be introduced onto the graphene sheets. For example, Li and co-workers produced graphene sheets in aqueous solution by taking advantage of electrostatic interaction.14 Xu and co-workers prepared stable aqueous dispersions of graphene sheets by using a water-soluble pyrene derivative, 1-pyrenebutyrate, as a stabilizer.15 Lomeda and co-workers functionalized graphene sheets by treatment with aryl diazonium salts, allowing these nanosheets to be dispersed in polar aprotic solvents.16 Stankovich and co-workers prepared conducting graphene-polymer nanocomposites by solution mixing of exfoliated functionalized graphite oxide sheets with polystyrene, acrylonitrile-butadiene-styrene, and styrene-butadiene copolymers.9 In this paper, we reported fabrication of exfoliated graphite oxide (GO) decorated by poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and polymer particles. Amine groups were introduced to the GO sheets by a reaction of the carboxylic acid groups at the edge of the sheets with a coupling agent. Atom transfer radical polymerization (ATRP) initiator was grafted onto the sheets by reactions of 2-bromo-2-methylpropionyl bromide with amine groups and hydroxyl groups on the surface. PDMAEMA were prepared on the surface by in-situ ATRP. Poly(ethylene glycol dimethacrylateco-methacrylic acid) (poly(EGDMA-co-MAA)) particles with active carboxylic acid groups on the surface were deposited on the GO sheets via hydrogen bonding. The process is shown in Scheme 1.

Experimental Section 1. Materials. Natural flake graphite with an average particle size of 40 μm (99%) was purchased from Qingdao Guangyao (13) McAllister, M. J.; Li, J.-L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Chem. Mater. 2007, 19, 4396. (14) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101. (15) Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. J. Am. Chem. Soc. 2008, 130, 5856. (16) Lomeda, J. R.; Doyle, C. D.; Kosynkin, D. V.; Hwang, W. F.; Tour, J. M. J. Am. Chem. Soc. 2008, 130, 16201.

Published on Web 07/14/2009

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Scheme 1. Synthesis of PDMAEMA Chains on Exfoliated Graphite Oxide (GO) Sheets by ATRP and Decoration of GO Sheets by Polymer Particles

Graphite Co. Ltd. Fuming nitric acid (>90%) was purchased from Tianjin FengChuan Chem. Co. Sulfuric acid (98%), potassium chlorate (98%), and hydrochloric acid (37%) were purchased from Tianjin Institute of Chemical Agents and used as received. 2-(Dimethylamino)ethyl methacrylate (DMAEMA, 99.0%) was purchased from Acros. Before use, it was distilled at a reduced pressure. Ethylene glycol dimethacrylate (EGDMA) was purchased from Aldrich Chemical Co. and used without further treatment. Methacrylic acid (MAA) was provided by Tianjin Chemical Reagent Co. and purified by distillation at a reduced pressure. Divinylbenzene was purchased from Shengli Chem. Co.; before use it was washed with 5% aqueous solution of sodium hydroxide and water and then dried over anhydrous magnesium sulfate. 1,3-Diaminopropane (98%) was provided by Beijing Chemical Reagents Co. N-Hydroxysuccinimide (NHS) and N-(3-(dimethylamino)propyl)-N0 -ethylcarbodiimide hydrochloride (EDC 3 HCl) were purchased from Shanghai Medpep Co. Copper(I) bromide (CuBr) was purchased from Guo Yao Chemical Co. and was purified by washing with glacial acetic acid. N,N,N,N,N-Pentamethyldiethylenetriamine (PMDETA, 99%) and 2-bromo-2-methylpropionyl bromide (98%) were purchased from Aldrich and were used as received. 2,20 -Azobis(isobutyronitrile) (AIBN) was analytical grade available from Chemical Factory of NanKai University and was recrystallized from methanol. Acetonitrile (Tianjin Chemical Reagents Co.) was dried over 4 A˚ molecular sieves and purified by distillation before use. All the solvents were distilled before use.

2. Preparation of Poly(EGDMA-co-MAA) Particles. Poly(EGDMA-co-MAA) particles were synthesized by a method as we reported in a previous paper.17 The details were described as follows: EGDMA (0.80 mL, 4.2 mmol), methacrylic acid (1.20 mL, 14.1 mmol), and AIBN (0.04 g, 0.24 mmol) were (17) Bai, F.; Yang, X.; Li, R.; Huang, B.; Huang, W. Polymer 2006, 47, 5775.

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dissolved in 80 mL of acetonitrile in a dried 100 mL two-necked flask, attached with a fractionating column, a Liebig condenser, and a receiver. The flask was submerged in a heating mantle, and the reaction mixture was heated to boiling temperature within 20 min, and a part of solvent was distilled. The initial homogeneous reaction mixture became milky white in 10 min. The reaction was stopped after 40 mL of acetonitrile was distilled. After the polymerization, the resultant poly(EGDMA-co-MAA) particles were purified by repeated cycles of centrifugationdecantation-suspension using ethanol and acetone. The particle size was determined by transmission electron microscopy (TEM). 3. Expansion and Exfoliation of Graphite. Exfoliated GO was prepared according to the Staudenmaier method.12,18 Graphite (5 g) was reacted with concentrated nitric acid (45 mL) and sulfuric acid (87.5 mL) in the presence of potassium chlorate (55 g). On completion of the reaction, the mixture was added to excess water and washed with HCl solution (5%) and water until the pH of the filtrate was neutral. The dried sample was stored in a vacuum oven at 60 °C before use. Thermal exfoliation of GO was conducted by placing GO in a Muffle furnace preheated to 950 °C and held in the furnace for 30 s. The exfoliated GO was dispersed in ethanol by high shear mixing for 30 min, followed by bath sonication for 24 h. 4. Preparation of ATRP-Initiator/GO Composites. GO sheets (1.0 g) were added to 250 mL of dry DMF. After 24 h of ultrasonication, NHS (3.42 g) and EDC 3 HCl (5.75 g) were added to the solution at 0 °C. After stirring for 2 h, 3.8 mL of 1,3diaminopropane was added, and the solution was stirred overnight at room temperature. The GO sheets modified by 1,3diaminopropane were washed by water and ethanol. After drying in a vacuum oven at 40 °C, GO sheets were dispersed in a mixture of 250 mL of toluene and 10.5 mL of triethylamine, and 6.2 mL of (18) Staudenmaier, L. Ber. Dtsch. Chem. Ges. 1898, 31, 1481.

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Article 2-bromo-2-methylpropionyl bromide was added to the solution. After stirring at 0 °C for 2 h, the solution was stirred at 90 °C overnight, and after which the solids was filtered, washed by toluene, and dried in a vacuum oven at 40 °C. 5. Preparation of GO/PDMAEMA Nanocomposites. A typical ATRP of DMAEMA was described as follows. Under a nitrogen atmosphere, 16 mg (0.115 mmol) of CuBr, 67 μL (0.32 mmol) of PMDETA, and 1.5 mL of DMAEMA monomer (8.9 mmol) were added into a Schlenk flask. After CuBr was dissolved totally, the solution was transferred to another Schlenk flask with 0.2 g of ATRP initiator-modified GO. The flask was purged by vacuum and then flushed with nitrogen. The polymerization was conducted at 60 °C overnight. After polymerization, the polymer nanocomposite was dispersed in THF and was centrifugated to remove the possible “free” polymer. In order to determine the molecular weight and molecular weight distribution of polymer chains on the surface of GO sheets, a controlled experiment was conducted. In the experiment sacrificial ATRP initiator was added into a polymerization system, and the molecular weight and the molecular weight distribution of the obtained polymer were determined by gel permeation chromatography (GPC).

6. Deposition of Poly(EGDMA-co-MAA) Particles on GO Sheets. Poly(EGDMA-co-MAA) particles (20 mg) were dispersed in 2 mL of ethanol, and the solution was added dropwise to a dispersion of GO/PDMAEMA nanocomposites (2 mg) in 10 mL of ethanol. After stirring for 2 days at room temperature, the nanocomposite was characterized by TEM and scanning electron microscopy (SEM). 7. Characterization. The thermal properties of the nanocomposites were measured by thermogravimetric analysis (TGA). The samples were heated to 800 °C at a heating rate of 10 K/min under a nitrogen atmosphere on a Netzsch TG 209. High-resolution TEM observations were carried out on a Tecnai G2 20 S-TWIN electron microscope equipped with a Model 794 CCD camera (512  512). TEM specimens were prepared by dipping copper grids into solutions and drying in air. X-ray diffraction (XRD) study were carried out on a D/max-2500 diffractometer with Cu KR radiation (λ = 1.5406 A˚). SEM observations were conducted on a JEOL JSM-5600. The gold-coating samples were used in the measurements. The grafting density of ATRP initiator on GO sheets was characterized by ion chromatography. The apparent molecular weight and molecular weight distribution of PDMAEMA were determined with GPC equipped with a Waters 717 autosampler, a Waters 1525 HPLC pump, three Waters Ultra Styragel columns with 5K-600K, 500-30K, and 100-10K molecular ranges, and a Waters 2414 refractive index detector. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Kratos Axis Ultra DLD spectrometer employing a monochromated Al KR X-ray source (hν = 1486.6 eV), hybrid (magnetic/ electrostatic) optics, and a multichannel plate and delay line detector. All XPS spectra were recorded using an aperture slot of 300  700 μm, survey spectra were recorded with a pass energy of 160 eV, and high-resolution spectra were recorded with a pass energy of 40 eV. Atomic force microscopy (AFM) images were collected on a Nanoscope IV atomic force microscope (Digital Instruments). The microscope was operated in tapping mode using Si cantilevers with a resonance frequency of 320 kHz. The voltage was between 2 and 3 V, and a tip radius was less than 10 nm. A drive amplitude of 1.2 V and a scan rate of 1.0 Hz were used. Raman spectra were recorded at different locations of the sample using a RM 2000 microscopic confocal Raman spectrometer (Renishaw PLC, England) with a 514 nm Ar laser.

Results and Discussion At the edge of GO sheets there are carboxylic acid groups, and on the surface there are hydroxyl groups. The carboxylic acid groups can be converted to amine groups by reactions with NHS and 1,3-diaminopropane. ATRP initiator molecules were attached 11810 DOI: 10.1021/la901441p

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to GO sheets via reactions of 2-bromo-2-methylpropionyl bromide with hydroxyl groups and amine groups. PDMAEMA chains on the sheets were prepared by ATRP of DMAEMA. Polymer particles were deposited on GO sheets via hydrogen bonding between PDMAEMA and particles (Scheme 1). An exfoliation process of the graphite flakes depends on the structures of the graphite (including the size and the crystal structure) and time of the oxidation process. The size of the graphite used in this research is about 40 μm. After 72 h of oxidation, the graphite was washed and dried, and then it was placed into a Muffle furnace preheated to 950 °C and held in the furnace for 30 s to cause a rapid expansion. The expanded graphite was dispersed in ethanol under sonication. After drying, the product was characterized by XRD. Figure 1 shows XRD spectra of natural flake graphite (curve a) and GO (curve b). The (002) diffraction peak of graphite appears at 2θ = 26.4°, corresponding to a d-spacing of 3.35 A˚. However, for the exfoliated GO the diffraction peak basicly disappears, which indicates an amorphous structure, although this does not necessarily require that all the stacking is lost. The disruption of the layered structure greatly reduces the attractive interactions between the layers, which allows this material to be further modified. Figure 2 shows a tapping mode AFM image of exfoliated GO sheets. The sample was prepared by depositing GO dispersion in ethanol (0.1 mg/mL) onto a new cleaved mica surface and dried under vacuum at room temperature. The cross-sectional view of the AFM image indicated that the height of the sheets was about 2.7 nm, which indicates that the flake of GO has two or three layers. It is also noticed that there are some sheets with thickness of 5.6 nm corresponding to five or six layers. Raman spectroscopy is a useful tool to characterize GO. Raman spectrum of graphite exhibits a peak at 1581 cm-1 (Figure S1 in the Supporting Information). After oxidation the Raman spectrum of GO shows two prominent peaks at 1583 and 1355 cm-1 (Figure S2), which correspond to the G and D bands. The appearance of D band at 1355 cm-1 indicates the introduction of the sp3-type structural disorder.19 Exfoliated GO, ATRP initiator modified GO, and GO/PDMAEMA nanocomposite were analyzed by TGA (Figure 3). TGA of the exfoliated GO was found to have 13 wt % weight loss in the range between 100 and 800 °C (curve a in Figure 3), which was attributed to the loss of the functional groups such as COOH and OH groups on the sheets. Upon grafting of ATRP initiator, the composite was found to have 32% weight loss (curve b in Figure 3). To determine the quantity of the ATRP initiator attached to the sheets, the bromine content was characterized by ion chromatography. The elemental result shows that the Br content is 0.45 wt %, which means the concentration of bromine on the surface is about 0.056 mmol/g. The GO/ATRP initiator composite was also characterized by XPS. The peak at 70.5 eV corresponding to binding energy of Br3d was observed on the XPS spectrum of GO/ ATRP (Figure S4). The TGA result of GO/PDMAEMA nanocomposite shows 56% weight loss (curve c in Figure 3), which indicates that the weight percentage of PDMAEMA is about 24%. In order to prove the grafting of PDMAEMA on the surface of GO, FT-IR measurement was conducted. The FT-IR spectrum of GO/PDMAEMA is presented in Figure 4. A wide band at 30003650 cm-1 is attributed to the hydroxyl stretching vibration of the C-OH group of the GO.20 In the spectrum, the typical absorption peaks of PDMAEMA include an absorption at 2937 cm-1 due to C-H symmetric and asymmetric stretching of methyl and (19) Kim, K.; Park, H.; Woo, B.-C.; Kim, K. J.; Kim, G. T.; Yun, W. S. Nano Lett. 2008, 8, 3092. (20) Wang, X.; Zhi, L.; M€ullen, K. Nano Lett. 2008, 8, 323.

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Figure 1. XRD spectra of natural flake graphite (spectrum a) and exfoliated GO (spectrum b).

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Figure 3. TGA curves of (a) exfoliated GO, (b) GO/ATRP initiator, and (c) GO sheets with PDMAEMA chains on the surface.

Figure 4. FT-IR spectrum of GO/PDMAEMA nanocomposites.

Figure 2. Typical AFM image and height profile of exfoliated GO.

methylene groups, absorptions at 2825 and 2767 cm-1 from C-H stretching of the -N(CH3)2 groups, an absorption at 1151 cm-1 from C-N stretching of -N(CH3)2 groups, and a sharp absorption at 1726 cm-1 from the CdO stretch of the ester group of PDMAEMA.21 The XPS spectrum provides information on the type and number of different species of a given atom in the molecules. GO and GO/ PDMAEMA nanocomposite were characterized by XPS. Figure 5 shows wide XPS spectra of GO and GO/PDMAEMA nanocomposite. For GO/PDMAEMA, there is a peak at 399.8 eV corresponding to the N1s binding energy of PDMAEMA chains. However, the XPS spectrum of GO does not show a peak at 399.8 eV, which implies that the surfaces of the flakes of GO/PDMAEMA have been successfully functionalized with PDMAEMA. According to XPS result, the percentage of nitrogen was calculated to be (21) Ye, Q.; Wang, X.; Hu, H.; Wang, Da.; Li, S.; Zhou, F. J. Phys. Chem. C 2009, 113, 7677.

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Figure 5. XPS results of GO (spectrum a) and GO/PDMAEMA nanocomposite (spectrum b).

3.19 wt %, so the percentage of PDMAEMA in the nanocomposite was about 35 wt %. Figure 6 shows C1s binding energy of GO and GO/PDMAEMA nanocomposite. The C1s XPS spectrum of GO showed binding energies at 284.6 eV (C-C in GO), 285.7 eV (COH), 286.7 eV (C-O), 288 eV (CdO), and 289.1 eV (O-CdO).22,23 The C1s XPS spectrum of GO/PDMAEMA nanocomposite also (22) Shan, C.; Yang, H.; Song, J.; Han, D.; Ivaska, A.; Niu, L. Anal. Chem. 2009, 81, 2378. (23) Park, S.; An, J.; Piner, R.; Jung, I.; Yang, D.; Velamakanni, A.; Nguyen, S.; Ruoff, R. Chem. Mater. 2008, 20, 6592.

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Figure 6. C1s XPS spectra of GO (a) and GO/PDMAEMA nanocomposite (b). The dark lines are experimental lines, the red lines are fitting lines, and the green lines represent XPS spectra of C1s corresponding to different binding energies.

showed all the energy peaks of C1s, but the percentage of the area of the peak at 289.1 eV (O-CdO) increased from 0.97% to 3.85%, which also proves the grafting of PDMAEMA chains onto the surface of GO sheets. Our calculation result on the basis of the C1s XPS results showed that the percentage of PDMAEMA was about 32 wt %. This result keeps accordance to the value calculated according to the N1s binding energy. One way to determine the molecular weight of polymer chains on the surface of solids prepared by living/controlled free radical polymerization was to add sacrificial initiator molecules into the polymerization system and measure the molecular weight of the free polymer chains in the solution, because it was reported that free polymers initiated by the sacrificial initiator molecules in solution have almost the same molecular weights as those formed on the solid substrates.24-27 The apparent molecular weight (Mn) and molecular weight distribution of free PDMAEMA were 27K and 1.72, respectively. The morphology of the exfoliated GO was studied by TEM. The GO sheets were dispersed in ethanol solution under sonication. A TEM specimen was prepared by dipping a copper grid (24) Jayachandran, K. N.; Raymond, N. J.; Brooks, D. E. Macromolecules 2004, 37, 734. (25) Feng, W.; Chen, R.; Brash, J. L.; Zhu, S. Macromol. Rapid Commun. 2005, 26, 1383. (26) Liu, T.; Jia, S.; Kowalewski, T.; Matyjaszewski, K. Macromolecules 2006, 39, 548. (27) Zhang, M.; Liu, L.; Wu, C.; Fu, G.; Zhao, H.; He, B. Polymer 2007, 48, 1989.

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Figure 7. (a) A TEM image of exfoliated GO, (b) a magnified TEM image showing the edge of GO, and (c) a TEM image of GO/ PDMAEMA nanocomposite after stained by OsO4.

into the solution and drying in air. Figure 7a is a TEM image of exfoliated GO at low magnification. On the image big GO sheets can be observed. Figure 7b is a magnified TEM image showing Langmuir 2009, 25(19), 11808–11814

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Figure 8. Photograph of exfoliated GO dispersed in water at pH=1 (A) and in methanol (B) and GO/PDMAEMA nanocomposite dispersed in water at pH=1 (C) and in methanol (D).

the edge of GO. On the image it is clearly seen that the flake has five layers and the thickness is about 5 nm, which suggests that the height of a single layer is about 1.0 nm. This value is consistent with those reported by other groups.28-30 For example, Wu and co-workers found that single graphene sheets produced by hydrogen arc discharge exfoliation method have a topographic height of 0.9-1.1 nm.31 The thickness of single-layered graphene is thicker significantly than pristine graphene sheet (0.34 nm) because of the presence of oxygen-containing functional groups on both sides of the graphene sheets. A TEM image of GO/PDMAEMA nanocomposite is presented in Figure 7c. The nanocomposite was dispersed in CHCl3, and a dipped specimen was observed after stained by OsO4. The dark dots on GO sheets represent PDMAEMA nanosized domains. In solution the grafted PDMAEMA chains are extended due to the solubility of the polymer chains in the solvent; however, after drying PDMAEMA chains collapse onto the surface of the GO sheets forming nanosized domains.32,33 The average size of PDMAEMA domains is about 4 nm. It is useful to prepare well-dispersed GO sheets in solvents with or without surfactants. After grafting of PDMAEMA chains, the dispersity of GO sheets in a solvent is significantly improved. The GO/PDMAEMA can be readily dispersed in water at pH = 1 or in methanol after a very short time sonication. However, for pure GO no homogeneous suspension can be produced even after as long as 2 h of sonication; precipitated particles or aggregated structures can always be observed. Figure 8 shows a photograph of GO and GO/PDMAEMA nanocomposite in water (pH = 1) and in methanol after 3 min of sonication. GO cannot disperse in both solvents; however, GO/PDMAEMA can disperse well. PDMAEMA is a pH-sensitive polymer. Under acidic conditions, the amine groups on the side chains are protonated, causing the polymer to be hydrophilic and soluble in water. The dispersity of GO/PDMAEMA is attributed to the solubility of the grafting PDMAEMA in methanol or acidic water. Polymer particles with carboxylic acid groups on the surface were prepared by distillation copolymerization of methacrylic (28) Paredes, J. I.; Villar-Rodil, S.; Martı´ nez-Alonso, A.; Tascon, J. M. D. Langmuir 2008, 24, 10560. (29) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, 3499. (30) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463. (31) Wu, Z.-S.; Ren, W.; Gao, L.; Zhao, J.; Chen, Z.; Liu, B.; Tang, D.; Yu, B.; Jiang, C.; Cheng, H. ACS Nano 2009, 3, 411. (32) Zhao, H.; Farrell, B. P.; Shipp, D. A. Polymer 2004, 45, 4473. (33) Yang, Y.; Liu, L.; Zhang, J.; Li, C.; Zhao, H. Langmuir 2007, 23, 2867.

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Figure 9. FT-IR spectrum of polymer particles poly(EGDMA-coMAA) (EGDMA/MAA = 60/40) with active carboxylic acid groups.

Figure 10. TEM image of exfoliated GO decorated by poly(EGDMA-co-MAA) particles.

Figure 11. SEM image of GO decorated by poly(EGDMA-coMAA) particles.

acid and ethylene glycol dimethacrylate (EGDMA).17 The FTIR spectrum of poly(EGDMA-co-MAA) particles is shown in Figure 9. A strong and broad peak at 3400 cm-1 due to the stretching vibration of OH bond can be observed clearly. DOI: 10.1021/la901441p

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Meanwhile, there is a strong peak at 1697 cm-1 corresponding to the stretching vibration of the carbonyl groups of the acid. These results demonstrated that the carboxylic acid groups were successfully incorporated into the copolymer particles by distillation precipitation polymerization. Figure S5 shows a TEM image of the particles, the polymer particles are narrow dispersed, and the size of the particles ranges from 120 to 180 nm with an average value at about 150 nm. Poly(EGDMA-co-MAA) particles with carboxylic acid groups on the surfaces were dispersed in ethanol and added to the dispersion of GO/PDMAEMA in ethanol. Figure 10 is a TEM image of GO sheets with polymer particles on the surface. The spheres on the sheets can be observed clearly. The size of the microspheres was kept unchanged. The hydrogen bonding between MAA units on poly(EGDMA-co-MAA) particles and amine groups of PDMAEMA on the surface of GO sheets is responsible for the deposition of polymer particles. Figure 11 is a SEM image of GO sheets decorated by polymer particles. The particles are densely distributed on the GO sheets. It is noted that on TEM and SEM image GO sheets with polymer particles on the surface tend to aggregate together. This can be explained by the fact that one polymer microsphere is able to interact with PDMAEMA chains on different sheets and connect them together. Because many different kinds of functional materials can be loaded on polymer particles, the research reported in this

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article provides a versatile method to produce functional polymer/GO nanocomposites.

Conclusions We prepared PDMAEMA on the surface of exfoliated GO sheets by using in situ ATRP. After grafting of PDMAEMA, the dispersity of the nanocomposites in solvents was improved significantly. Poly(EGDMA-co-MAA) particles were deposited on GO sheets via hydrogen bonding between MAA units on polymer particles and amine groups of PDMAEMA. This research provides a direct way to modification of GO and preparation of functional polymer/GO composites, which will accelerate the development and applications of the GO-based materials. Acknowledgment. This project was supported by Doctoral Fund of Hebei University of Technology and National Natural Science Foundation of China under Contracts 20544001 and 20574037. Supporting Information Available: Raman spectra for graphite, GO, and GO/PDMAEMA using 514 nm laser excitation, XPS spectrum of GO/ATRP initiator composite, and TEM image of poly(EGDMA-co-MAA) (EGDMA/ MAA = 60/40) particles. This material is available free of charge via the Internet at http://pubs.acs.org.

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