Electromagnetic Wave Absorption Properties of Reduced Graphene

Article pubs.acs.org/JPCC

Electromagnetic Wave Absorption Properties of Reduced Graphene Oxide Modified by Maghemite Colloidal Nanoparticle Clusters Luo Kong, Xiaowei Yin,* Yajun Zhang, Xiaoyan Yuan, Quan Li, Fang Ye, Laifei Cheng, and Litong Zhang Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, West Youyi Rd., No. 127, Xi’an, Shaanxi 710072, People’s Republic of China ABSTRACT: Graphene is highly desirable as an electromagnetic wave (EM) absorber because of its large interface, high dielectric loss, and low density. Nevertheless, the conductive and electromagnetic parameters of pure graphene are too high to meet the requirement of impedance match, which results in strong reflection and weak absorption. In this paper, we report a facile solvothermal route to synthesize reduced graphene oxide (RGO) nanosheets combined with surface-modified γ-Fe2O3 colloidal nanoparticle clusters. The obtained two-dimensional hybrids exhibit a relatively low EM reflection coefficient (RC) and wide effective absorption bandwidth, which are mainly attributed to the unique microstructure of colloidal nanoparticle clusters assembled on RGO. The nanoparticle clusters have more interfaces. The interfacial polarization within nanoparticle clusters and conductivity loss of RGO plays an important role in absorbing EM power. The minimum RC reaches −59.65 dB at 10.09 GHz with a matching thickness of 2.5 mm. The special integration of some metal oxide semiconductor crystals assembled on RGO sheets provides an effective avenue to design metal oxide semiconductor/carbon hybrids as EM absorbing materials.

1. INTRODUCTION As a latent environmental pollution, electromagnetic wave (EM) radiation is potentially harmful to biological systems, information safety, and electromagnetic compatibility and, in recent years, has attracted more and more attention. EM absorbing materials have aroused great interests in civil and military fields. The EM absorption properties are evaluated using reflection coefficient (RC, dB), which is based on the metal back-panel model. RC expresses the ratio of the total reflected EM power against the incident EM power, and a lower RC means better EM absorption performance. X-band (8.2− 12.4 GHz) is often used for civil and military applications, and one of the greatest challenges is to design an EM absorbing material exhibiting a RC lower than −40 dB at a sample thickness smaller than 3 mm in X-band. The designing principia of EM absorbing material need to consider both EM absorption capability and impedance matching characteristic. For the dielectric absorbing materials, strengthening of EM absorption capability can be attributed to a strong conductivity loss, and it can be optimized by phase component and microstructure with an interfacial polarization capability. The impedance matching characteristic can be determined by equivalent dielectric constant, which can be adjusted by tailoring host material, electrical loss phase, scale, concentration, distribution and porosity of host materials, and the EM absorbent.1−4 Recently, new carbon materials, including carbon nanotubes (CNTs) and graphene and carbon hybrid materials have drawn great attention due to their unique microstructures and properties, which promote their extensive potential application in many fields.5,6 Usually, a moderate conductivity material is © 2013 American Chemical Society

suitable for using as EM absorbing material. CNTs have high conductivity, which is more suitable as EM shielding materials than EM absorbing materials.7 However, the dielectric properties of CNTs can be adjusted by combination with other materials. Fe encapsulated within CNTs as the EM absorbing materials has been reported.1 The crystalline Fe is confined in carbon nanoshells, and the EM absorption capability derives mainly from magnetic effects. In X-band, the real part (ε′) and imaginary part (ε″) of the permittivity are approximately equal to 30 and 45, and the real part (μ′) and imaginary part (μ″) of the permeability are equal to 2 and 1.6, respectively. The dielectric loss and RC are slightly larger than the ideal case, and the EM absorption properties can be further improved. When the surfaces of multiwalled CNTs (MWCNTs) were modified by zinc oxide (ZnO) particles,8 ε′ and ε″ of the composites with 40 wt % MWCNTs/ZnO were approximately equal to 7 and 2, respectively, and the minimum RC (RCmin) reached −30 dB in X-band. A resistor− capacitor model could well describe the relationships between the above microstructure and EM absorption properties. In comparison with CNTs, graphene as a new unique twodimensional material has been extensively researched in recent years. Sometimes, graphene can be abstracted as unzipped CNTs,9 which have a greater interface compared to CNTs. Graphene is highly desirable as an EM absorber because of its large interface, high dielectric loss, and low density. NeverReceived: June 13, 2013 Revised: August 28, 2013 Published: August 28, 2013 19701

dx.doi.org/10.1021/jp4058498 | J. Phys. Chem. C 2013, 117, 19701−19711

The Journal of Physical Chemistry C

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Table 1. Summary of EM Absorption Properties Reported in Recent Papers X-band EM absorbing material A/B/C

absorbent content

−/RGO/−

ε′

ε″

μ′

μ″

∼30

∼20

∼0.9

∼-0.1

optimum frequency (GHz) 7

optimum thickness (mm) 2

RCmin (dB) −7

paraffin/T-CNCs/−

15 wt %

∼6

3−4

13.5

2.5

−28

SiO2/MWCNT/−

10 wt %

8−11

7.5−11

10.1

2.5

−20

silica/CFs/− epoxy resin/SWCNT/−

20 wt % 3 wt %

19−24 ∼5.5

∼11 ∼1.3

9.9 10

5 9.7

−10 −19

paraffin/Fe3O4/−

70 wt %

∼23

∼2.5

∼0.8

∼0.25

11

1.5

−20

paraffin/γ-Fe2O3/−

70 wt %

∼28

∼1

∼0.8

∼0.2

9

4

−50

PEO/RGO/− epoxy resin/CNT/Fe epoxy silicone/MWCNT/CI

2.6 vol % 17 wt % 0.5−50 vol %

7−8 ∼30 ∼7

∼3 ∼45 ∼4

∼2 ∼1.1

∼1.6 ∼2.8

9 11 10.5

3 1.2 1.5

−27 −25 −17

paraffin/C/Fe3O4

55 wt %

∼7

∼0.6

∼1

∼0.3

15

2

−28

paraffin/RGO/PANI

20 wt %

∼6.5

∼3.5

12.9

2.5

−45

epoxy resin/CCTO/CI

40−40 wt %

∼11

∼0.2

∼1.4

∼0.9

8.4

2

−47

paraffin/RGO/γ-Fe2O3

45 wt %

∼10

∼4

∼0.95

∼0.02

10.09

2.5

−59.65

ref 16 (2011) 17 (2008) 18 (2009) 7 (2010) 19 (2010) 20 (2011) 20 (2011) 6 (2011) 1 (2004) 21 (2010) 22 (2011) 10 (2012) 23 (2013) this work

Figure 1. Fabrication process for γ-Fe2O3/RGO hybrid. (a) Stable suspension of GO, iron ions, and sodium acetate dispersed in a vial, (b) primary nanocrystals nucleate in a supersaturated solution, and (c) nanocrystals aggregate into larger colloidal nanocrystal clusters on the RGO surface.

get the minimum RC at 10 GHz.3 For improvement of the EM absorption capability, an A−B−C type hierarchical material architecture is designed. A low dielectric loss phase, which is denoted as the C phase, is added into the EM absorbing materials composed of an insulating matrix (A) and electrically loss (B) phases. The C phase has larger permittivity than the A phase, but smaller than the B phase,3 which may enhance the interface polarization capability. The two-dimensional nanosheets coated by nanoparticles have more nanointerface than the one-dimensional nanotube, which may lead to a higher dielectric loss. Thus, RGO modified by semiconductor particles has a promising prospect as an effective EM absorbing material. On the basis of the above analysis, the idea comes from whether or not the semiconductor particles can be assembled on RGO. Iron oxide-based microsized or nanosized materials20,27 are highly desirable because of their hierarchical dendrite-like microstructure. They are also used as an EM absorber to attenuate EM. Recently, G/Fe3O4, G/Fe, and G/ Fe3O4@Fe/ZnO nanocomposites as the EM absorbing materials have been reported. Nanoparticles are uniformly

theless, the conductive and electromagnetic parameters of pure graphene are too high to meet the requirement of impedance match, resulting in strong reflection and weak absorption. On the other hand, through the chemical preparation method, the properties of graphene oxide (GO) and mildly reduced graphene oxide (RGO) can be better tailored than the graphene due to the formation of the hydroxyl, carbonyl, and carboxyl groups on the graphene surface. These make GO and RGO more easily modified by other materials.6,10−15 Therefore, semiconductor particles assembled on the surface of GO may provide a new way to design EM absorbing material. Table 1 summarizes the phase composition, electromagnetic parameters, optimum thickness, optimum frequency, and RC of the absorption materials in the recent literature.1,6,7,10,16−23 To obtain improved EM absorption properties, absorbing materials need to have a suitable permittivity, dielectric loss, and an appropriately high conductivity.24−26 At a sample thickness of 2.86 mm, the optimal ε′ and ε″ should be close to 7.3 and 3.3 to get the minimum RC at 10 GHz. If the sample thickness decreases to 2.5 mm, ε′ and ε″ should be close to 9.4 and 3.8 to 19702



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samples used for dielectric properties measurement were prepared by mixing the as-synthesized powders with paraffin. On the basis of the model of metal backplane, the RC of the sample is determined from the measured relative complex permeability and permittivity according to the following equations:

deposited on the graphene, and good EM absorption properties can be achieved by the reasonable construction of heteronanostructures.28−30 Inspired by this idea, we synthesize the RGO modified by γ-Fe2O3 colloidal nanoparticle clusters via solvothermal method, while the RGO still preserve the twodimensional planar structure. The growth of γ-Fe2O3 clusters follows the well-documented two-stage growth model, in which primary nanocrystals nucleate first in a supersaturated solution and then aggregate into larger secondary particles on the RGO surface. The nanoparticle clusters have more interfaces than a single particle, and the interfacial polarization is a benefit for attenuation of EM power. The fabrication process for the γFe2O3/RGO hybrid is shown in Figure 1. The dielectric and EM absorption properties of the γ-Fe2O3/RGO/paraffin hybrid materials are investigated. Hence, it is valuable to explore a new kind of EM absorber that shows strong absorption capability at thin thickness and investigate the relationship between the microstructure and dielectric properties.

RC/dB = 20 log|(Z in − 1)/(Z in + 1)|

(1)

μ tanh(j2π με fd /c) ε

(2)

Z in =

where Zin, d, and μ are the normalized input impedance, thickness, and permeability of the material, respectively; c is the light velocity in vacuum; and f is the microwave frequency.31 When the RC is smaller than −10 dB, only 10% of the microwave energy is reflected while 90% of that is absorbed. The corresponding frequency range over which the RC is smaller than −10 dB is defined as the effective absorption bandwidth.

2. EXPERIMENTAL SECTION 2.1. Preparation Process. All of the chemicals in our experiments were analytical grade reagents and directly used without further treatment. The following outlines the preparation of γ-Fe2O3/RGO hybrids: first, GO (XF Nano Materials Tech Co., Nanjing, China) was prepared using a modified Hummers method from flake graphite. A 15 mg amount of GO was dispersed in 30 mL of ethylene glycol (EG) by sonication for 2 h. To this suspension, 0.405 g of ferric chloride and 0.615 g of anhydrous sodium acetate were added. The mixture was stirred for 2 h to form a black solution with a concentration of 0.05 mol/L, and then the solution was placed in a Teflon-sealed autoclave and maintained at 160 °C for 12 h. The black product was isolated by magnetic separation, repeatedly washed with absolute ethanol, and dried at 50 °C in air. Preparation of γ-Fe2O3 particle is given as follows: γFe2O3 particles were also prepared by solvothermal method, and the process was the same as that of γ-Fe2O3/RGO hybrids except for the absence of GO. 2.2. Phase Composition and Microstructure Characterization. The crystal structure of the as-synthesized samples was identified by X-ray diffractometer (X’ Pert Pro, Philips, Almelo, The Netherlands), using Cu Kα (λ = 1.54 Å) radiation. Raman spectra were taken on a Renishaw Ramoscope (Confocal Raman Microscope, inVia, Renishaw, Gloucestershire, U.K.) equipped with a He−Ne laser (λ = 514 nm). X-ray photoelectron spectra (XPS) were measured using a Thermo Scientific X-ray photoelectron spectrometer (XPS, K-Alpha, Thermo Scientific, Waltham, MA, USA). The morphology of the as-synthesized samples was observed by a scanning electron microscope (S-4700, Hitachi, Tokyo, Japan). A transmission electron microscope (G-20, FEI-Tecnai, Hillsboro, USA) operating at 200 KV accelerating voltage was used for transmission electron microscopy (TEM) analysis. 2.3. Dielectric and EM Absorption Property Characterization. The relative complex permittivity (εr) and permeability (μr) of the as-received samples with dimensions of 22.86 mm × 10.16 mm × 2.5 mm were measured by a vector network analyzer (VNA, MS4644A, Anritsu, Atsugi, Japan) using the waveguide method in the X-band. Direct current (DC) conductivity measurements were performed with a standard two lines method using the DC source (Precise Current Source by Keithley, 6220, Cleveland, OH, USA). The

3. RESULTS AND DISCUSSION 3.1. Crystalline Phase and Morphology. Figure 2a shows the XRD patterns of the GO, γ-Fe2O3 particles, and the γFe2O3/RGO hybrid materials. The diffraction peak of GO is observed at 2θ = 11°, indicating the distance between atomic layers of graphite (002) is expanded to 0.80 nm. The complete oxidation of graphite into the GO makes it possible for γ-Fe2O3 to be assembled on GO sheets.32 The diffraction peaks of the γFe2O3/RGO hybrid materials are exhibited in Figure 2. All of the peaks can be assigned to both the magnetite Fe3O4 (JCPDS card 65-3107) and the maghemite γ-Fe2O3 (JCPDS card 391346). Remarkably, no apparent diffraction peaks could be identified at 2θ = 11° and/or 20−30°, indicating that γ-Fe2O3 particles are efficiently assembled on the surface of GO, suppressing the stacking of graphene layers and destroying the (002) interplanar periodic structure.33 The diffraction peaks of the γ-Fe2O3 particles prepared using the same conditions but without adding GO are also shown in Figure 2a. The diffraction peaks are the same as those of γ-Fe2O3/RGO hybrids. Raman spectra of the GO, γ-Fe2O3 particles, and the γ-Fe2O3/RGO composite are shown in Figure 2b. The Raman spectrum of the GO shows the D and G characteristic bands of carbon at around 1364 and 1600 cm−1. The fundamental Raman scattering peak is observed at 707 cm−1, corresponding to the characteristic band of γ-Fe2O3.34,35 It should be reasonable to postulate that the sample is composed of γ-Fe2O3. The Raman spectrum of the γ-Fe2O3/RGO composite shows a typical peak of maghemite and characteristic peaks of the D and G bands. It is well-known that there exists a satellite peak at around 719.2 eV characteristic of γ-Fe2O3.36,37 In our case, the binding energies of Fe 2p3/2 and Fe 2p1/2 are 710.4 and 724 eV, respectively, and a satellite peak at 719 eV (indicated by the arrow) is ascribed to γ-Fe2O3 (Figure 2c), which is in good agreement with the literature.36,37 The results of XPS patterns are in good agreement with the XRD and Raman spectra. We also investigate the dispersion stability of the γ-Fe2O3 particles and γ-Fe2O3/RGO hybrids. When the content of product is 5 mg/mL, they can be easily dispersed in ethanol, while remaining stable for 2 h at room temperature, and they can be easily separated from the dispersant by using a magnet, which is the characteristic for ferromagnets, as clearly shown in the insets of Figure 2c. 19703



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of CC bonds in the composite increases from 49.2 to 82.9%. On the other hand, although the three types of heterocarbon remain in Figure 3b, the absorbance intensities of the hydroxyl carbon, the carbonyl carbon, and the carboxyl carbon decrease sharply. The percentages decrease from 17.1 to 6.7%, from 21.0 to 2.8%, and from 12.7 to 7.6%, respectively. These results indicate that most of the oxygen functional groups in sample GO are removed and the graphitic structure of reduced GO through the EG reduction are remarkably restored. All of the above results clearly reveal that GO is effectively deoxidized by this simple solvothermal synthesis method. Furthermore, their O1s spectra are also investigated in detail, which is particularly important to confirm or disprove the existence of O−C bonds. In Figure 3c, the O1s peak of GO is composed of two peaks centered at 532.4 and 531 eV. The dominating peak at 532.4 eV should be attributed to epoxy C− O groups, while the weak one at 531 eV corresponds to carbonyl oxygen in O−CO groups. The O1s peak of γFe2O3/RGO (Figure 3d) can be fitted to three peaks at 533, 531.3, and 529.8 eV. The peak at 533 eV should be attributed to the original oxygen in RGO, while the one at 529.8 eV should arise from γ-Fe2O3.. The peak at 531.3 eV should be attributed to the bonds of Fe−O−C formed between γ-Fe2O3 and RGO, which can also be confirmed from the results in the literature showing that the binding energy of O1s in Fe−O−C bond can be present in the range of 521−533 eV.40 The microstructure of the γ-Fe2O3, and γ-Fe2O3/RGO hybrids were characterized by scanning electron microscopy (SEM) and TEM. Parts a and b of Figure 4 show the morphologies of the GO and γ-Fe2O3 particles, respectively. In Figure 4a, the stripelike wrinkle is the lattice fringes of GO’s (002) facet. Because the lattice periodic structure of GO is destroyed thanks to the preparation process of GO, lattice fringes are distorted and many stripelike wrinkles can be observed. In Figure 4b, we can see that the shape of the γFe2O3 particle is spherical, and the diameter is approximately 250 nm. Figure 4c shows the morphology of γ-Fe2O3/RGO hybrids. It is obvious that RGO sheets are transparent and the size is larger than 10 μm. The γ-Fe2O3 particles are all uniformly distributed on both sides of the RGO sheets and the particle size is 250 nm without any change. The TEM image (Figure 4d) reveals that these γ-Fe2O3 particles are firmly attached to the RGO sheets even after the ultrasonication used to disperse the γ-Fe2O3/RGO for TEM characterization. The γFe2O3 particles adhere to RGO sheets, which prevent γ-Fe2O3 from agglomeration and enable a good dispersion of these oxide particles over the RGO substrate. The higher magnification TEM image (Figure 4e) indicates that the obtained maghemite particles are colloidal nanoparticle clusters.41 The small primary nanocrystals seem to be connected with each other by an amorphous matrix as a bridge.42,43 Figure 4f shows a high-resolution TEM (HRTEM) image where the bottom inset is a fast Fourier transform (FFT) pattern of a γ-Fe2O3 cluster. The FFT pattern indicates that the ring patterns are composed of individual barely visible spots, which is characteristic of a polycrystalline structure. The HRTEM image and FFT pattern suggest that the γ-Fe2O3 clusters are composed of nanoparticle clusters. 3.2. Dielectric and EM Absorption Properties. 3.2.1. GO, γ-Fe2O3, and γ-Fe2O3/RGO Hybrids. Figure 5 illustrates the relative complex permittivity (εr= ε′ − jε″) and permeability (μr= μ′ − jμ″) of GO, γ-Fe2O3, and γ-Fe2O3/RGO hybrid samples in X-band, which were prepared by mixing the

Figure 2. (a) XRD profiles and (b) Raman spectra of GO, γ-Fe2O3, and γ-Fe2O3/RGO, and (c) Fe 2p core-lever XPS spectra of γ-Fe2O3 and γ-Fe2O3/RGO.

Furthermore, the C1s spectra of GO and γ-Fe2O3/RGO hybrids are shown in Figure 3 to elucidate their surface compositions. In Figure 3a, GO possesses the highest heterocarbon components such as the functional groups of C−O (hydroxyl and epoxy, 286.4 eV), CO (carbonyl, 287.0 eV) groups, and O−CO (carboxyl, 288.3 eV) groups.38,39The above result suggests that GO contains large numbers of functional groups on its surface. These functional groups can be effectively removed by EG reduction, so the significant decrease of heterocarbon components is observed by C1s spectra in Figure 3a,b. After EG reduction, the percentage 19704



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Figure 3. C1s XPS spectra of (a) GO and (b) γ-Fe2O3/RGO, and O1s XPS spectra of (c) GO and (d) γ-Fe2O3/RGO.

bandwidth of the γ-Fe2O3/RGO hybrid is 2.04 GHz (from 8.2 to 10.24 GHz). A schematic diagram is presented in Figure 7, which gives a visual demonstration of the EM absorbing mechanism. In Figure 7, a resistor−capacitor circuit model is used to explain the relationship between the γ-Fe2O3/RGO hybrid structure and corresponding electrical properties. Pure RGO resistors (RRGO) and γ-Fe2O3/RGO resistors (RRGO+γ‑Fe2O3) are the basic resistors in the circuit. A capacitor at the interface of γ-Fe2O3 nanoparticles (Cγ‑Fe2O3) also plays an important role in the system. It is well-known that the permittivity and permeability mainly originate from electronic polarization, ion polarization, interface polarization, and their magnetic properties, on which the crystal structure and special geometrical morphology may have an important influence.20 In our case, EM absorption properties of γ-Fe2O3/RGO hybrid may be ascribed to conductivity loss of RGO and interfacial polarization within nanoparticle clusters. First, GO is effectively deoxidized by solvothermal synthesis. The conductivity is significantly increased because the oxygen functional groups are removed and the graphitic structure is remarkably restored. It is beneficial for EM attenuation. Second, when the γ-Fe2O3 clusters are adhered to RGO sheets, it can prevent agglomeration of γ-Fe2O3 clusters. Due to the stronger interfacial interaction of γ-Fe2O3 nanoparticles, interface polarization plays an increasingly important role in absorbing EM.

as-synthesized powders with paraffin with a weight ratio of 50 wt %. For γ-Fe2O3 and GO samples, the ε′ values are in the ranges of 4.88−4.71 and 4.85−4.35, while the ε″ values are approximately equal to 0.33 and 1.6, respectively. For the γFe2O3/RGO hybrid sample, the complex relative permittivity increases remarkably and the ε′ and ε″ values are in the ranges of 12.90−11.39 and 5.38−4.97, respectively (Figure 5a,b). Moreover, the μ′ values of γ-Fe2O3 and γ-Fe2O3/RGO increase obviously with the frequency increased from 0.83 to 0.95 and from 0.90 to 0.96, respectively. The μ″ value should be equal to 0. Since the GO is a nonmagnetic material, the complex relative permeability should be equal to (1, 0) (Figure 5c,d). To reveal the EM absorption properties, RC is determined at a given frequency and sample thickness, using the relative complex permeability and permittivity according to eqs 1 and 2. Figure 6 shows the RC of the three samples at a sample thickness of 2.5 mm. As shown in Figure 6, we find that the three samples exhibit different EM absorption properties. For γFe2O3 particles, the EM absorption capability is weak and there is no obvious absorption peak in X-band. The minimum RC is −2.09 dB at 12.4 GHz. GO also has a weak absorption capability, and the minimum RC is −6.70 dB at 12.4 GHz. Nevertheless, the γ-Fe2O3/RGO hybrid exhibits better EM absorption properties. There is a sharp peak at 8.89 GHz with the minimum RC = −18.19 dB. The effective absorption bandwidth is one of the most important parameters for EM absorbing materials. It is observed that the effective absorption 19705



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Figure 4. (a) TEM images of GO, (b) SEM images of γ-Fe2O3 particles, (c) SEM image of γ-Fe2O3/RGO hybrids, (d and e) TEM images of γFe2O3/RGO hybrids, and (f) HRTEM image where the bottom inset shows a FFT pattern of γ-Fe2O3 cluster.

3.2.2. Effect of γ-Fe2O 3/RGO Hybrids Content. An important fact is that the measured dielectric spectra in this study are of samples which are comprised of γ-Fe2O3/RGO hybrids embedded in a paraffin matrix. Therefore, the measured data are collective behaviors or an integration of the intrinsic dielectric response of one single cube, and the mass fraction of absorbent is an important factor. To further reveal the EM absorption properties of the γ-Fe2O3/RGO hybrid materials, we

prepared the samples with different mass fractions (from 40 to 60 wt %), and Figure 8 illustrates the complex relative permittivity and permeability in X-band. As shown in Figure 8a,b, with the increase of the mass fraction, both ε′ and ε″ show a monotone and increased trend across the whole X-band. The ε′ of samples increase from 7.89 to 18.36, and the ε″ of samples increase from 2.45 to 9.32. In Figure 8c, μ′ shows a general increasing tendency in X-band, while their absolute values 19706



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Figure 5. Frequency dependence of (a) real and (b) imaginary parts of relative complex permittivity and (c) real and (d) imaginary parts of relative complex permeability of GO, γ-Fe2O3, and γ-Fe2O3/RGO hybrids dispersed in paraffin with 50 wt %.

Figure 7. Schematic illustration of possible EM absorption mechanisms and resistor−capacitor circuit model in the γ-Fe2O3/ RGO hybrids.

resonance behaviors of microwave magnetic permeability also have been observed in porous Fe3O4/carbon core/shell nanorods.22 It implies that the γ-Fe2O3/RGO hybrids would be a strong dielectric loss and weak magnetic loss type EM absorbent in X-band. However, too high permittivity is harmful to the impedance matching and results in strong reflection and weak absorption.16 When the appropriate permittivity value can meet the requirement of impedance matching, it makes a better EM absorption capability. Therefore, the optimization of EM parameters is the key to improve the EM absorption properties. As is known, microwave electrical conductivity of a dielectric material can be evaluated by using the following equation,

Figure 6. Reflection coefficient of GO, γ-Fe2O3, and γ-Fe2O3/RGO hybrids dispersed in paraffin with thickness of 2.5 mm as a function of frequency.

decrease with increasing mass fraction. Figure 8d shows that the μ″ is weak. The shape of the present permeability spectra is similar to that of Fe3O4/RGO absorbent in epoxy resin.44 The complex permeability is attributed to the motion of the domain wall at lower frequencies, the spin rotation is at higher frequencies, and there is no strong resonance in X-band.21 Thus, the complex permeability is weak in X-band. These 19707



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Figure 8. Frequency dependence of (a) real and (b) imaginary parts of relative complex permittivity and (c) real and (d) imaginary parts of relative complex permeability of γ-Fe2O3/RGO hybrids dispersed in paraffin with different mass fraction.

γ-Fe2O3/RGO hybrids. For the lower concentrations, the resistivity is dominated by that of the host matrix, with negligible conducting path formed by γ-Fe2O3/RGO hybrids. However, the conductivity increases monotonically when the concentration increases to enough extent. In our case, the conductivities of samples with mass fractions of 40, 45, 50, 55, and 60 wt % are 2.46 × 10−6, 1.84 × 10−5, 6.65 × 10−5, 2.61 × 10−4, and 3.21 × 10−4 (Ω·cm)−1, respectively. Figure 9 also shows the mass fraction dependence of the average complex permittivity (in X-band). It indicates the same increasing tendency with conductivity. Figure 10 shows the RC of the sample at a thickness of 2.5 mm. The minimum RC and effective absorption bandwidth first increase and then decrease with increasing mass fraction of γ-Fe2O3/RGO hybrids. When the mass fraction is 45 wt %, the effective absorption bandwidth is 3 GHz and the minimum RC reaches −59.65 dB. When the mass fraction is more than 45 wt %, the EM absorption properties decrease due to the inappropriate permittivity. The larger mass fraction of γFe2O3/RGO hybrids destroys the requirement of impedance matching and results in strong EM reflection at the surface of the sample, which hinders EM power from transforming into heat. 3.2.3. Relationship between Frequency, Sample Thickness, and RC. The EM absorption mechanism of γ-Fe2O3/RGO hybrids is based mainly on dielectric loss. It is revealed that high EM absorption capability requires a low real part of permittivity and an appropriately high dielectric loss.3,45,46 The relation

where σ is the electric conductivity (S/m), ε0 is the free space permittivity (8.854 × 10−12 F/m), and f is the frequency (Hz). It is clear that the conductivity increases as an increase of the imaginary part of permittivity. σ σ = ε″ = ωε0 2πfε0 (3) Figure 9 shows the conductivity of the γ-Fe2O3/RGO hybrids/paraffin samples as a function of mass fraction. It is observed that conductivity rises with increasing mass fraction of

Figure 9. Conductivity and complex permittivity of the γ-Fe2O3/RGO hybrids/paraffin as a function of mass fraction. 19708



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RC decreases with the increase of frequency, which can be seen from the gray line in Figure 11b. The complex permittivity of the minimum RC is reduced from 13.80−4.65j to 6.26−3.04j, when the frequency rises from 8.2 to 12.4 GHz. The three-dimensional graphic for RC, frequency, and thickness of the 45 wt % γ-Fe2O3/RGO hybrid sample is given in Figure 11c. The thickness of the EM absorbing materials which is an odd multiple of a quarter-wavelength, results in a sharp destructive interference. This is caused by the inverse phase angle of the reflection EM from the upper and bottom surfaces.3 When the EM absorbing materials are thick enough, the RC tends to be a stable value. Figure 11d is the top view of Figure 11c. It shows the optimal thickness of the EM absorbing materials which is decreasing with the increase of frequency. It can be attributed to the decrease of EM wavelength, when the EM speed remains constant.

Figure 10. Reflection coefficient of γ-Fe2O3/RGO hybrids/paraffin composite with thickness of 2.5 mm as a function of frequency.

4. CONCLUSION γ-Fe2O3/RGO hybrids were prepared via solvothermal synthesis. γ-Fe2O3 colloidal nanoparticle clusters were assembled on RGO sheets, which lead to an effective optimization about electromagnetic parameters. The appropriate permittivity value can meet the requirement of impedance matching and make a lower EM reflection and stronger EM absorption. There are stronger interfacial interactions between RGO and γ-Fe2O3. Enhancement of interfacial polarization within nanoparticle cluster plays an important role in absorbing EM power. The obtained two-dimensional hybrids exhibit a relatively low RC and wide effective absorption bandwidth. The minimum RC reaches −59.65 dB, and the effective absorption bandwidth reaches 3 GHz in X-band, when the thickness of absorber is 2.5 mm. Some metal oxides colloidal nanoparticle clusters

between permittivity and RC calculated by eq 1 at a frequency of 10 GHz and a sample thickness of 2.5 mm is exhibited in Figure 11a. Figure11b is the top view of Figure 11a. The optimum real and imaginary parts of the permittivity, which leads to the minimum value of RC, are equal to 9.40 and 3.79, respectively. Electromagnetic parameters of samples with different mass fraction are transformed into equivalent nonmagnetic parameters, and they are marked on Figure 11b. It can be noticed that the sample with a mass fraction of 45 wt % possesses the best complex permittivity leading to a minimum RC. The complex permittivity value is 9.50−3.65j and the corresponding RC is −33.89 dB at 10 GHz. It is interesting that the permittivity corresponding to a minimum

Figure 11. (a) Relationship between complex permittivity and RC and (b) the top view, and (c) RC of the EM absorber at different thickness and (d) the top view. 19709



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assembled on RGO sheets provide an ingenious and effective method to design EM absorbing materials.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 029 88494947. Fax: +86 029 88494620. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by funding from the State Key Laboratory of Solidification Processing in NWPU (Grant No. KB200920), the Natural Science Foundation of China (Grant No. 51332004), the program for New Century Excellent Talents in University, and the 111 Project (Grant No. B08040).

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REFERENCES

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Electromagnetic Wave Absorption Properties of Reduced Graphene

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