Green Approach To Prepare Graphene-Based Composites with High

ARTICLE pubs.acs.org/JPCC

Green Approach To Prepare Graphene-Based Composites with High Microwave Absorption Capacity Xin Bai, Yinghao Zhai, and Yong Zhang* State Key Laboratory of Metal Matrix Composites, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 200240 Shanghai, P. R. China

bS Supporting Information ABSTRACT: Chemically reduced graphene (CR-G)/poly(ethylene oxide) (PEO) composites are prepared by a simple aqueous mixing method. Graphite oxide (GO) is prepared by a modified Hummers method and further dispersed in water to form graphene oxide (G-O). The as prepared G-O is mixed with PEO and in situ reduced by L-ascorbic acid. CR-G monolayers are ∼1 nm in thickness and ∼1.5 μm in both length and width as confirmed by AFM, indicating their large aspect ratio of about 1500. G-O is dispersed in PEO at the molecular level due to hydrogen bonding, and PEO acts as a barrier for CR-G layers to prevent agglomeration during the process of reduction. CR-G/PEO composites have high permittivity, resulting from the uniform dispersion of electrically conductive CR-G with high aspect ratio. CRG/PEO composite (2.6 vol %) shows high microwave absorbing capacity as its minimum reflection loss is
38.8 dB. CR-G sheets form a huge number of electrical pathways which can dissipate microwave energy into heat effectively as well as dielectric relaxation and interface scattering induced by large CR-G/PEO interfaces.

’ INTRODUCTION Graphene, two-dimensional planar sheets composed by sp2bonded carbon atoms, has drawn great attention due to its unique topological structures and interesting properties. A monolayer of graphene is a material endowed with the highest Young’s modulus (∼1000 GPa) based on theoretical and experimental results.1 Thermal conductivity of graphene is 5000 W m
1 K
1, comparable with the highest value of reported single-walled carbon nanotubes (SWNTs).2 In addition, electrical conductivity of graphene can be up to 6000 S cm
1,3 and its high aspect ratio can help to constitute effective network inside graphene composites at low graphene content, which promises its potential applications in microwave absorption (MA) fields. Microwave radiation is potentially harmful to biological systems, which are exposed to microwave for a considerable period of time.4,5 MA materials are of great significance for their capacity of suppressing microwave radiations. High permittivity is crucial for composites with electrically conductive or dielectric fillers to raise MA capacity. SWNT/polymer composites with high real and imaginary permittivity in the frequency of 0.5
2 GHz were reported by Grimes et al.6 Micheli et al.7 studied the effect of carbon inclusion size and geometry on electromagnetic properties of epoxy composites in X-band (8.2
12.4 GHz), in which SWNTs and carbon nanofibers showed high dielectric permittivity and MA capability, while granular graphite of microsize showed limited MA capacity. Liang et al.8 reported the high electromagnetic interference (EMI) shielding efficiency for epoxy resin filled with graphene. But up to now, few studies have been realized insofar as the MA capacity of graphene filled polymer composites is concerned. r 2011 American Chemical Society

So far, large volume production of high electrically conductive graphene remains a big challenge in graphene research field. Among various synthesis approaches toward graphene, chemical reduction approach is favorable for its scalable productivity and flexible functionalization of graphene. Hydrazine was much more frequently used to reduce graphene oxide (G-O) due to its high reduction efficiency.9
14 Unfortunately, hydrazine and their derivatives are toxic and harmful to human as well as the environment. Zhang et al.15 and Gao et al.16 reported a green method to reduce G-O with L-ascorbic acid (L-AA) in aqueous media, which is environmentally friendly. Their researches were both conducted in aqueous media showing environmentally friendly. Meantime, L-AA possessed higher reduction efficiency concerning the electrical conductivity of the as-prepared chemically reduced graphene (CR-G) in comparison with such reductants as ammonia and potassium hydroxide, etc.17 If G-O is highly reduced without surfactants, it will cause inevitable aggregation,11,18
20 making it difficult to be uniformly dispersed in polymers. In our research, composites of G-O and poly(ethylene oxide) (PEO) were prepared by a simple aqueous mixing method, and then G-O was in situ reduced by L-AA. X-ray diffraction (XRD) analysis showed CR-G sheets were dispersed in PEO matrix with exfoliated structure. Microwave measurement showed CR-G/PEO composites had high complex permittivity on account of large aspect ratio and uniform dispersion Received: March 15, 2011 Revised: May 10, 2011 Published: May 19, 2011 11673

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Figure 1. (a) A tapping mode AFM image of CR-G sheets with the height profile, (b) UV
vis spectra of CR-G, (c) the C 1s peak in the XPS spectra of GO, and (d) CR-G.

of CR-G along with large interface between CR-G and PEO. The composites had excellent MA capacity with reflection loss of
38.8 dB, which could be attributed to the pronounced conduction loss, dielectric relaxation, and interface scattering.

’ EXPERIMENTAL SECTION Preparation of Graphite Oxide (GO). GO was prepared using a modified Hummers method21 from flake graphite. Briefly, flake graphite (5 g) and NaNO3 (3 g) were put into a flask, and concentrated H2SO4 (120 mL, 98%) was added under stirring in an ice
water bath. KMnO4 (22.5 g) was slowly added to the above mixture over 1 h and followed by continuously stirring at 23 °C for 2 h. Then H2SO4 aqueous solution (700 mL, 5 wt %) was slowly added under stirring, and the temperature was kept at 98 °C. When the temperature was decreased to 60 °C, H2O2 aqueous solution (15 mL, 30 wt %) was added. The product was washed with HCl solution (5 wt %) and distilled water several times and freeze-dried. Preparation of G-O/PEO Composites. GO (25 mg) was dispersed in water (50 mL) in an ultrasonic bath for 1 h at 23 °C to yield a clear solution. In this process, GO was completely exfoliated down to individual sheets to form a stably dispersed G-O/H2O solution. Meanwhile, PEO (2.5 g, molecular mass, 1.5  106) was dissolved in water (80 mL) at room temperature. The G-O dispersion was gradually added to the PEO solution and sonicated for 1 h at 23 °C to form uniform solution of G-O/PEO composite (1/100 w/w). G-O/PEO composite (5/100 w/w) was prepared following the same procedure. The densities of GO and PEO are 2.2 and 1.2 g cm
3, so the volume ratios of G-O/PEO composites (1/100 w/w) and (5/100 w/w) are 0.54 and 2.6 vol %, respectively.

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Preparation of CR-G/PEO Composites. G-O was in situ reduced by L-AA in the G-O/PEO solution. Typically, L-AA (250 mg) was added into G-O/PEO (0.54 vol %) aqueous solution and stirred at 50 °C for 24 h. When the reaction ended, the color of the solution turned from yellow to dark black, indicating G-O was successfully reduced into CR-G. The solution was dried under vacuum at 60 °C until its weight kept unchanged. CR-G/PEO composites were press-molded at 80 °C under the pressure of 5 MPa for testing. Characterization. Atomic force microscopy (AFM) images of CR-G were taken in the tapping mode by carrying out on NanoNavi II, with samples prepared by spin-coating sample solutions onto freshly exfoliated mica substrates and dried at 80 °C under vacuum. Transmission electron microscopy (TEM) was carried out on a JEOL JEM-2010 electron microscope at 200 kV, and samples were prepared by depositing a drop of solution containing CR-G on a carbon-coated copper grid and dried at ambient temperature prior to analysis. UV
vis absorption spectra were recorded at room temperature on a Perkin-Elmer Lambda 20 UV
vis spectrophotometer. Raman spectra were measured on a Jobin Yvon LABRAM-1B multichannel confocal microspectrometer with 632.8 nm laser excitation. X-ray photoelectron spectroscopy (XPS) spectra were collected on a PerkinElmer PHI 5000C ESCA system using Al KR radiation. XRD spectra were acquired by D/max-2200/PC using Cu KR radiation. Scanning electronic microscopy (SEM) images were taken on a JSM-7401F field-emission SEM system. For microwave measurement, cylindrical toroidal specimens with an outer diameter of 7.0 mm, inner diameter of 3.04 mm, and thickness of 4.0 mm were set in a coaxial line. Microwave scattering parameters (S parameter, Sij, i, j = 1, 2) were measured using an Agilent 8722ES vector network analyzer in the frequency range of 2
18 GHz. Then complex permittivity (εr = ε0
jε00 ) was calculated from the S parameters according to the literature.22 Since no ferromagnetic materials were involved in our experiment, complex permeability was considered to be that of free space (μr = 1). Electrical conductivity test was performed on SB100A/2 digital four-point probe system (Shanghai, P. R. China).

’ RESULTS AND DISCUSSION In our study, GO was prepared by a modified Hummers method. After mild sonication, GO was homogenously dispersed in water to form G-O and further reduced by L-AA. AFM was applied to determine the aspect ratio of CR-G. An AFM image of typical CR-G sheets is shown in Figure 1a with the height profile. CR-G sheets were flat with a lateral size of ∼1.5 μm and thickness of ∼1 nm, implying CR-G sheets existed in water with exfoliated structure and the aspect ratio of CR-G sheets were ∼1500. UV
vis spectra are effective to trace the reduction process. As shown in Figure 1b, typical absorbing peaks appeared at 234 and 298 nm in the curve of G-O, corresponding to the π
π* transitions of aromatic CdC bonds and n
π* transitions of CdO bonds, respectively. The CR-G had a peak at 261 nm that was red-shifted from 234 nm for G-O. The absorption intensity of the entire spectrum increased dramatically, indicating that the G-O might be reduced and the aromatic structure might be restored gradually, which was in agreement with the reported article.12 The reduction process could also be proved by XPS. Figure 1c,d shows the C 1s XPS spectra of GO and CR-G. GO had three different peaks centered at 284.6, 286.5, 287.1, and 11674

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Figure 2. XRD patterns of PEO, CR-G/PEO composite (2.6 vol %), GO, and graphite.

288.4 eV, corresponding to CdC/C
C in aromatic rings, C
O (epoxy and alkoxy), CdO groups, respectively. Although the three types of carbon remained in Figure 1d, the intensity of the peaks corresponding to oxygen-containing groups decreased to a large extent compared with GO, showing the high reduction efficiency of L-AA. TEM with selected area electron diffraction (SAED) pattern of CR-G and Raman spectra of GO and CR-G results also proved the reduced exfoliated structure of CR-G when prepared with this approach (Supporting Information S1 and S2). To obtain CR-G composites, CR-G was prepared in the presence of PEO. G-O/PEO composites were prepared through a simple aqueous mixing method and further reduced by L-AA in situ as described in the Experimental Section. XRD is a powerful tool to determine the dispersion of fillers in polymer matrix.23,24 As shown in Figure 2, a diffraction peak of pristine graphite is observed at 26.4°, indicating the distance between layers of graphene was 0.35 nm according to the Bragg equation. After graphite was oxidized, the diffraction peak shifted to 11.0°, indicating the distance between layers was expanded to 0.8 nm, which made it possible for polymers to intercalate into GO sheets. PEO is a typical crystalline polymer showing strong peaks at about 19° and 24°. The peak at 24° was a combination of the (112) and (032) reflections, and the peak at 19° corresponded to the (120) reflection. Similar to poly(vinyl alcohol),23 PEO could interact with G-O through hydrogen bonding,25 giving rise to exfoliated structure of G-O in polymer matrix, and the XRD pattern of CRG/PEO composite (2.6 vol %) only showed the diffraction peaks similar to PEO. Neither the diffraction peak of graphite nor the peak of GO appeared, indicating CR-G sheets have been exfoliated into monolayers in the PEO matrix, which is in accordance with SEM results (Supporting Information S3). That may be attributed to PEO acting as a barrier for CR-G, which prevented the agglomeration of CR-G during the reduction process. As important parameters for microwave electromagnetic properties, the real part (ε0 ) and imaginary part (ε00 ) of complex permittivity of PEO and its composites were measured in the frequency range of 2
18 GHz (Figure 3a,b). The composites had much higher ε0 and ε00 than PEO. In X-band that was frequently studied, CR-G/PEO composite (2.6 vol %) had ε0 ranged from 8.0 to 7.3 and ε00 about 3.1, while PEO had ε0 ranged from 3.1 to 3.0 and ε00 about 0.4. CR-G/PEO composites had higher permittivity than most carbon nanotube/polymer composites, while high ε00 was of great importance for electrical loss.7,26 The ε0 of PEO and its composites decreased with increasing frequency in 2
18 GHz. The ε00 of PEO and CR-G/PEO composite (0.54 vol %) decreased with increasing frequency, but the ε00 of CR-G/PEO composite

Figure 3. Electromagnetic characteristics of CR-G/PEO composites in the 2
18 GHz range: (a) real part of complex permittivity, (b) imaginary part of complex permittivity, and (c) dielectric loss factor.

(2.6 vol %) kept unchanged around 3.1 after 11 GHz, which may be related to a resonance behavior that was reported for SWNT/polyurethane composites when the nanotube concentration was higher than 15 wt %.26 And a change in dielectric loss factor of composites filled with multiwalled carbon nanotubes was also attributed to resonance.27 The dielectric loss factor (tan δe = ε00 /ε0 ) indicates the inherent dissipation of electromagnetic energy for dielectric materials. The frequency dependency of tan δe of PEO and its composites is shown in Figure 3c. The composites have much higher tan δe than PEO. CR-G/PEO composite (2.6 vol %) and CR-G/PEO composite (0.54 vol %) had nearly the same tan δe in low frequencies (2
7.6 GHz), but the former had increasing tan δe while the latter had decreasing tan δe at frequencies over 7.6 GHz. The reflection loss (RL) of a metal-backed single absorb layer was calculated as follows:


Z
1


in ð1Þ RL ðdBÞ ¼ 20 log


Zin þ 1
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Figure 5. A schematic representation for the possible dissipation route of electromagnetic wave in the CR-G/PEO composites.

Figure 4. Reflection loss curves for the CR-G/PEO composites with different thickness in the frequency range of 2
18 GHz: (a) 0.54 vol % and (b) 2.6 vol %.

while the normalized input impedance (Zin) was calculated by   rffiffiffiffi μr 2πfdpffiffiffiffiffiffiffiffi μr ε r Zin ¼ tanh j ð2Þ c εr where f is the microwave frequency, d is the thickness of the absorb layer, c is the velocity of electromagnetic wave in vacuum, and εr and μr are the complex relative permittivity and permeability, respectively. RL is expected to be as low as possible at a given sample thickness. The calculated RL curves of the CR-G/PEO composites with different thickness are shown in Figure 4. Interestingly, the CR-G/PEO composite (0.54 vol %) had the minimum RL less than
10 dB, and CR-G/PEO composite (2.6 vol %) had the minimum RL of
38.8 dB at the optimal sample thickness of 1.8 mm and the
10 dB absorption frequency ranged from 13.9 to 18 GHz. The minimum RL of CR-G/PEO composite (2.6 vol %) at the thickness of 2.0 mm reached
32.4 dB, and the
10 dB absorption frequency ranged from 12.4 to 18 GHz. RL of CR-G/ PEO composite (2.6 vol %) with wide range of thickness (1.8
4.0 mm) was less than
25 dB, indicating its excellent MA capacity. Considering the low graphene content in the CRG/PEO composite (2.6 vol %), this kind of composites should be a promising light-weighted, high-absorption MA material with potential applications. The high MA capacity of CR-G/PEO composites is attributed to electrical conduction loss, dielectric relaxation, interface scattering, and multiple reflections (as shown in Figure 5), while the absorbed energy is dissipated as heat. When electromagnetic waves propagated within the composites, the directional motion of charge carriers on CR-G formed oscillatory current, and boundary charges induced dielectric relaxation and polarization.7 Besides, due to the difference in complex permittivity between

CR-G and PEO, pronounced interface scattering would be generated.28 The well-dispersed exfoliated CR-G sheets with high aspect ratio could constitute conduction network within the PEO matrix, resulting in high conduction loss. The dielectric relaxation and polarization were mainly induced by interfacial multipoles, which existed along the boundaries between CR-G sheets and PEO matrix.29,30 They were related to the interface area, which was the same as the interface scattering. High surface area of CR-G sheets with well dispersion could enhance the attenuation caused by relaxation and scattering. Furthermore, multiple reflections could be induced due to the huge aspect ratio and layered structure of CR-G (Figure 5); thus, the routes of electromagnetic waves propagate in the layer are extended, which brings more efficient absorption.

’ CONCLUSIONS A novel MA material CR-G/PEO composite was prepared through a green approach. G-O was in situ reduced by L-AA in PEO aqueous solution to form CR-G/PEO composites. CR-G was uniformly dispersed in the composites as exfoliated structure and conductive filler. The CR-G/PEO composites had high permittivity and very low microwave reflection loss, with great potential to be used in the field of protecting people from microwave radiation. ’ ASSOCIATED CONTENT

bS

Supporting Information. Raman spectra, TEM of CR-G, and SEM image of CR-G/PEO composite (0.54 vol %). This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: þ86 21 54743257. Fax: þ86 21 54741297.

’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 50773036 and No. 51073092). ’ REFERENCES (1) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385. (2) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Nano Lett. 2008, 8, 902. (3) Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Nature Nanotechnol. 2008, 3, 491. 11676

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