Fe nanocomposites

Fe nanocomposites

Materials Research Bulletin 48 (2013) 3362–3366 Contents lists available at SciVerse ScienceDirect Materials Research Bulletin journal homepage: www...

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Materials Research Bulletin 48 (2013) 3362–3366

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Electromagnetic absorption properties of graphene/Fe nanocomposites Yujin Chen a,*, Zhenyu Lei a, Hongyu Wu a, Chunling Zhu b, Peng Gao b,*, Qiuyun Ouyang a, Li-Hong Qi a, Wei Qin c,* a

Key Laboratory of In-Fiber Integrated Optics, Ministry Education of China, College of Science, Harbin Engineering University, Harbin 150001, China College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China c School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 February 2013 Received in revised form 7 May 2013 Accepted 7 May 2013 Available online 15 May 2013

Graphene (G)/Fe nanocomposites with ferromagnetic properties at room temperature were fabricated by a facile and green method. Transmission electron microscope (TEM) and atomic force microscopy (AFM) amylases reveal that the a-Fe nanoparticles with a diameter of only about 10 nm were uniformly dispersed over the surface of the graphene sheets. Compared with other magnetic materials and the graphene, the nanocomposites exhibited significantly enhanced electromagnetic absorption properties. The maximum reflection loss to electromagnetic wave was up to 31.5 dB at a frequency of 14.2 GHz for G/Fe nanocomposites with a thickness of 2.5 mm. Importantly, the addition of the nanocomposites is only about 20 wt.% in the matrix. The enhanced mechanism is discussed and it is related to high surface areas of G/Fe nanocomposites, interfacial polarizations between graphene and iron, synergetic effect and efficient dispersity of magnetic NPs. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Composites A. Interfaces B. Chemical synthesis D. Dielectric properties D. Magnetic properties

1. Introduction Graphene (G) has attracted much attention because of its combination of excellent mechanical, electrical, thermal, and optical properties with ultrathin thickness [1]. Recently, electromagnetic wave (EM) absorbing and shielding materials have attracted much attention owing to the expanded EM interference problems. Due to its large surface area and high conductivity, graphene has shown excellent EM shieding properties [2]. However, the carrier mobilities of graphene with a high quality at room temperature is harmful to its EM absorption properties in terms of the impedance match mechanism [3]. Thus how to design and prepare good EM absorbing materials based on graphene still remains a challenge [4]. Recently, graphene-based heteronanostructures have been developed to attenuate EM wave energy. Cao et al. fabricated Ni/G nanocomposites by one-step VCD method. The maximum reflection loss of the mixture containing 20 wt.% of Ni/G nanocomposites in paraffin was about 13 dB as the thickness of the absorbing materials was in range of 1.5–3 mm [5]. Singh et al. reported that 10 wt.% of graphene in nitrile butadiene rubber had the reflection loss less than 10 dB over a wide frequency range [6]. Bai et al. studied the EM absorption properties of G/poly(ethylene oxide) (PEO), and found that the G/PEO nanocomposites

* Corresponding author. Tel.: +86 451 82519754; fax: +86 451 82519754. E-mail addresses: [email protected] (Y. Chen), [email protected] (P. Gao), [email protected] (W. Qin). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.05.020

containing 2.6 vol% of graphene had a minimum refletion loss of about 38.8 dB [7]. Very recently, we fabricated G/polyaniline nanorod arrays, G/[email protected]/ZnO quaternary nanocomposites and G/Fe3O4 nanohybrids [8–10]. These materials exhibited very good EM absorption properties. In terms of the results above, constructing graphene-based heterostructures by rational designation is an efficient strategy to attenuate EM wave energy. However, to our best of knowledge, synthesis and EM absorption properties of G/Fe nanocomposites have not been reported. Herein we developed a facile and green method to synthesize G/ Fe nanocomposites for applications in EM absorption field. The nanocomposites not only have enhanced EM absorption properties, but also are lightweight owing to high surface areas, interfacial polarizations and efficient separation of magnetic NPs. The minimum reflection loss reaches 31.5 dB at 14.2 GHz for the absorber with thickness of 2.5 mm, and the absorption bandwidth with the reflection loss below 15 dB is up to 10.9 GHz (from 5.8 to 16.7 GHz) for the absorber with the thickness of 2– 5 mm.

2. Experimental Graphene used in this work was fabricated by a modified method [10,11]. 0.1 g of the obtained graphene sheets was dispersed into 300 mL water, and then 1.0 g of Fe(NO3)39H2O was added. The mixture above was kept at 50 8C for 2 h under stirring. The precipitates were separated by centrifugation, washed with distilled water and absolute ethanol, dried under vacuum. At

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the stage, G-b-FeOOH nanocomposites were prepared [10]. Finally, G/Fe nanocomposites were obtained after the dried G-b-FeOOH nanocomposites were annealed at 693 K for 6 h under an Ar/H2 flow. The morphology and the size of the synthesized samples were characterized by scanning electron microscopy (SEM) JEOL-JSM6700F, and transmission electron microscope (TEM) JEOL 2010. The crystal structure of the sample was determined by X-ray diffraction (XRD) D/max 2550 V, Cu Ka radiation. The magnetic properties were measured by a vibrating sample magnetometer (VSM) at room temperature. Atomic force microscopy (AFM) measurements were done using a AJ III scanning probe microscope. AFM samples were prepared by spin-coating sample solutions onto freshly clean silicon substrates. Surface properties of the samples were studied by the Brunauer–Emmett–Teller (BET) methods via nitrogen adsorption and desorption measurements. The composite samples used for EM absorption measurement were prepared by mixing 20 wt.% G/Fe nanocomposites with wax. The complex permittivity and permeability of the composites were measured by an Anritsu 37269D vector network analyzer using the coaxial method. 3. Results and discussion The structural characterizations for graphene sheets and G-bFeOOH nanocomposites were shown in the previous literature

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[10]. Fig. 1(a) shows a typical XRD pattern of G/Fe nanocomposites, in which only the diffraction peaks from a-Fe (JCPDs, 06-0696, cell parameters: a = 2.86 A´˚ ) are observed, indicating the high crystalline purity of a-Fe. Low-resolution TEM image (Fig. 1(b)) shows that a-Fe particles with a very small size are uniformly covered over the surface of graphene. Magnified TEM images display that the average diameter of a-Fe nanoparticles (NPs) is about 10 nm, as shown in Fig. 1(c). Clear lattice fringes from an individual NP (Fig. 1(d)) are also observed, suggesting a single crystal nature of aFe NPs. Energy dispersity spectroscopy demonstrates that the atomic ratio of C to Fe is about 35.2:1 in G/Fe nanocomposites (not shown) [10]. Fig. 2(a) shows a typical AFM image of G/Fe nanocomposites with lateral dimensions of several micrometers. The surface of the graphene sheet in the nanocomposites has an average thickness of about 21 nm, suggesting that both sides of the graphene sheet are coated with the magnetic NPs [12]. High-resolution AFM image (Fig. 2(b)) shows the average diameter of the magnetic particles is about 11 nm, which is in accordance with the TEM observations. Surface properties of the samples were studied by the BET methods via nitrogen adsorption and desorption measurements. Fig. 3 shows the nitrogen adsorption and desorption isotherms of G/Fe nanocomposites. The BET surface area of the G/Fe nanocomposites is 174.6 m2/g. The field-dependent magnetization of G/Fe nanocomposites was measured by a vibrating sample magnetometer at room temperature, as shown in Fig. 4. Significant hysteresis

Fig. 1. (a) XRD pattern, (b) low-resolution TME image, (c) magnified TEM image, and (d) HRTEM image of G/Fe nanocomposites.

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Fig. 2. (a) AFM image and (b) high-resolution AFM image of the G/Fe nanocomposites.

loops in the M–H curves indicate the ferromagnetic characteristics of the materials. The saturation magnetization (Ms), coercivity (Hc) and retentivity (Mr) are 21.59 emu/g, 254.66 Oe and 4.30 emu/g for G/Fe nanocomposites. The values of Ms are largely lower than those

of their bulk materials due to the presence of graphene in the materials [13,14]. In general, ferromagnetic nanostructures, especially for NPs, are easily aggregated except that appropriate surfactant active agents were introduced into the reaction system

Fig. 3. The nitrogen adsorption–desorption isotherms of G/Fe nanocomposites.

Fig. 4. Magnetization hysteresis loop of the G/Fe nanocomposites.

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during the synthetic process [15,16]. However, the ferromagnetic a-Fe NPs are dispersed very well over the surface of graphene. It implies that those NPs are tightly integrated with graphene and the trend of the aggregation among NPs is efficiently inhibited. Ferromagnetic G/Fe nanocomposites may have good EM absorption characteristics due to multidomain wall dominant in the magnetic NPs [17–26]. The electromagnetic parameters (relative complex permittivity, er ¼ e0  je00 and relative complex permeability, mr ¼ m0  jm00 ) of the wax composites containing 20 wt.% of G/Fe nanocomposites were shown in Fig. 5(a) shows the curves of the G/Fe nanocomposites. The real part (e0 ) and the imaginary part (e00 ) vary in the range of 5.68–9.39 and 2.88–6.38 for G/Fe nanocomposites at the frequency region over 2–18 GHz, respectively. Compared with other magnetic absorbing materials [18–20], the imaginary parts of G/Fe nanocomposites are very high, indicating that they have strong dielectric loss to EM wave. Importantly, the values of the dielectric tangent loss (tan de ¼ e00 =e0 ) of G/Fe nanocomposites are 0.49–0.68 over 2– 18 GHz, as shown in Fig. 5(c). The values are significantly larger than those of other magnetic materials [18–20]. Furthermore, they have a little fluctuation over 2–18 GHz, which is also different from other magnetic materials. For example, Fe3O4/SnO2 and Fe3O4/TiO2 nanostructures have strong dielectric loss either only at low-frequency range (2–9 GHz) or only at high-frequency range (9–18 GHz) [18–20]. Thus, the dielectric loss for the G/Fe nanocomposites is not only strong but also very efficient over all the tested frequency range (2–18 GHz). Fig. 5(b) shows the complex permeability measured for the G/Fe nanocomposites. The values of m0 are in the range of 0.74–0.95 for G/Fe nanocomposites. The values of m00 increase firstly, then decrease gradually and exhibit a very weak resonance peak at about 2.3 GHz for the magnetic materials. According to the naturalresonance equations [27], 2p f r ¼ rHa Ha ¼

(1)

4jK 1 j 3m0 MS

(2)

where r is the gyromagnetic ratio, Ha is the anisotropy energy, and K1 is the anisotropy coefficient. The resonance frequency is around several tens of megahertz for bulk a-Fe [25,26,28]. It demonstrates that the resonance frequency of the magnetic materials shifts to higher frequency compared to the bulk iron. In general, the anisotropy energy of small size materials, especially on nanometer scale, would be remarkably increased due to the increased surface anisotropic field induced by the small size effect [21–23]. The increase of the anisotropy energy results in the shift. In addition, the maximum value of the magnetic tangent loss is 0.26 for G/Fe nanocomposites, indicating that they have magnetic loss to EM wave to some degree. G/Fe nanocomposites have very strong and efficient dielectric loss over all the tested frequency range as well as the magnetic loss at low-frequency range, suggesting that they must have good EM absorption properties. According to the transmit-line theory, the reflection loss (RL) can be calculated by the following equations: Z in ¼



mr er

1=2

  j2p fd ðmr er Þ1=2 tan h c

Z  1 RL ðdBÞ ¼ 20 log in Z in þ 1

(3)

(4)

where Zin is the input impedance of the absorber, c is the velocity of electromagnetic waves in free space, f is the frequency of microwaves, and d is the thickness of the absorber. Fig. 6 shows

Fig. 5. (a) The complex permittivity, (b) the complex permeability, and (c) tangent loss of G/Fe nanocomposites.

the calculated reflection loss of G/Fe nanocomposites with different thicknesses. It can be found that the maximum reflection loss reaches 31.5 dB at 14.2 GHz for the absorber with thickness of 2.5 mm, and the absorption bandwidth with the reflection loss below 15 dB is up to 10.9 GHz (from 5.8 to 16.7 GHz) for the absorber with the thickness of 2–5 mm. Compared with other magnetic or nonmagnetic materials [18,24,29,30], G/Fe nanocomposites exhibit significantly enhanced EM absorbing ability. Furthermore, the added quantity in the wax matrix for other magnetic materials is more than 40 wt.%, whereas it is only 20 wt.% for the G/Fe nanocomposites. In addition, the EM absorption properties of the G/Fe nanocomposites exhibited significantly improved EM absorption properties compared to the as-obtained graphene sheets [10]. In addition, even if the addition amount of the G/Fe nanocomposites in wax was decreased to 15 wt.%, they still show strong EM absorption characteristics. The absorption

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the surface of graphene and their sizes is about 10 nm. Compared to the graphene and other magnetic materials, the G/Fe nanocomposites exhibit enhanced EM absorption properties. The minimum reflection loss reaches 31.5 dB at 14.2 GHz for the absorber with thickness of 2.5 mm, and the absorption bandwidth with the reflection loss below 15 dB is up to 10.9 GHz for the absorber with the thickness of 2–5 mm. Thus, the deposition of magnetic nanostructures on the surface of graphene is an efficient way to fabricate lightweight materials for applications in the EM absorption field. Acknowledgements

Fig. 6. The reflection loss of the G/Fe nanocomposites with a content of 20 wt. % in wax.

We thank the National Natural Science Foundation of China (Grant Nos. 51072038, 51272050, 61205113 and 21001035), Program for New Century Excellent Talents in University (NECT10-0049), Outstanding Youth Foundation of Heilongjiang Province (Grant No. JC201008), the 111 project (B13015) of Ministry Education of China to the Harbin Engineering University, and also the Fundamental Research Funds for the Central Universities of China for the financial support of this research. References

Fig. 7. The reflection loss of the G/Fe nanocomposites with a content of 15 wt.% in wax.

bandwidth with the reflection loss below 15 dB is 10.5 GHz for the G/Fe nancomposites, as shown in Fig. 7. The maximum reflect loss of the G/Fe nancomposites can be up to 46.8 dB as the absorber thickness is 7 mm. The results above demonstrate that G/ Fe nanocomposites not only have strong absorption characteristics and wide absorption frequency, but also are lightweight. The enhanced absorption properties may be related to the following factors. The G/Fe nanocomposites have a high surface area (174.6 m2/g), which results in the presence of more diploes. The dipole polarizations will contribute to the enhanced EM absorption properties. The interfaces existed between graphene and Fe NPs, and the interfacial polarization and the associated relaxation will result in the enhanced EM absorption properties [18–20]. In addition, the magnetic Fe NPs were separate from each other and dispersed very well over the surface of the graphene sheets. The heat, produced as the nanocomposites irradiated by EM wave, will be rapidly transferred into graphene due to its high thermal conductivity. 4. Conclusions G/Fe nanocomposites were fabricated by a facile and green method. The magnetic a-Fe NPs were uniformly anchored on

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