Letter www.acsami.org
Coupling Hollow Fe3O4−Fe Nanoparticles with Graphene Sheets for High-Performance Electromagnetic Wave Absorbing Material Bin Qu,†,#,§ Chunling Zhu,*,‡ Chunyan Li,† Xitian Zhang,# and Yujin Chen*,† †
Key Laboratory of In-Fiber Integrated Optics, Ministry of Education, and College of Science, Harbin Engineering University, Harbin 150001, China ‡ College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China # Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, and School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China § Department of Applied Chemistry, College of Science, Northeast Agricultural University, Harbin 150030, China S Supporting Information *
ABSTRACT: We developed a strategy for coupling hollow Fe3O4−Fe nanoparticles with graphene sheets for highperformance electromagnetic wave absorbing material. The hollow Fe3O4−Fe nanoparticles with average diameter and shell thickness of 20 and 8 nm, respectively, were uniformly anchored on the graphene sheets without obvious aggregation. The minimal reflection loss RL values of the composite could reach −30 dB at the absorber thickness ranging from 2.0 to 5.0 mm, greatly superior to the solid Fe3O4−Fe/G composite and most magnetic EM wave absorbing materials recently reported. Moreover, the addition amount of the composite into paraffin matrix was only 18 wt %. KEYWORDS: hollow nanoparticles, graphene, composite, electromagnetic wave, absorption property
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which showed the minimal reflection loss toward EM wave of ca.−28 dB at 9 GHz and the absorption bandwidth with the reflection loss less than −10 dB of about 9 GHz.12 Li et al. prepared Fe3O4−graphene hybrids, which showed that the minimal reflection loss was −30.1 dB at a thickness of the absorbing film (d) of 1.48 mm, and the absorption bandwidth with reflection loss less than −10 dB was 10.5 GHz at d ranging from 1.48 to 3 mm.10 Zheng et al. reported that Fe3O4/ graphene composite exhibited the minimal reflection loss of −8.75 dB at 8.11 GHz.13 Sun et al. prepared porous Fe3O4decorated graphene, showing the minimal reflection loss of −20 dB at 17.2 GHz.14 Nevertheless, the addition amount of the composite in the matrix usually exceeded 20 wt %, and thus it still remains a great challenge to design and fabricate lightweight EM absorbing materials with strong absorption properties. Hollow nanostructures with controlled interior void and shell thickness are of great interest for scientists because of their interesting chemical and physical properties as well as their wide-ranging applications. The hollow nanostructures have lower density than that of their solid counterparts, affording their lightweight features under the condition of identical
lectromagnetic (EM) wave absorbing materials have attracted much attention because of increasing EM interference and radiation problems. Ideal EM wave absorbing materials are required to have not only a wide absorption frequency range and strong absorption properties, but also a lightweight feature.1 The traditional absorbers, such as ferrite, have strong absorption properties, but the addition amount into the matrix required is too high. Graphene (G), a new class of two-dimensional carbon nanostructure, has not only a stable structure but also a very high specific surface area.2 Thus, graphene may be used as a lightweight EM wave absorber; however, it usually exhibited low absorption property because of its attenuation toward EM wave only dependent on the dielectric loss as well as its poor impedance matching characteristic.3 Recently, it had been found that coupling nanostructured materials with the graphene sheets could improve the EM wave absorption properties significantly. These nanostructured materials included the dielectric loss materials such as ZnO,4 NiO,5 Co3O4,6 and Fe2O3,7 and the magnetic loss materials, such as Fe50Ni50,8 Co,9 Ni,10,11 Fe3O4,12−14CoFe2O4,15 NiFe2O4,16 etc. Among them, G/Fe3O4 heteronanostructures had attracted wide great attention because of the low cost and good antioxidant capability of Fe3O4, and the strong absorption characteristics after the hybridization.12−14 For example, Song et al. prepared Fe3O4/graphene heterostructural nanofillers, © 2016 American Chemical Society
Received: December 30, 2015 Accepted: February 1, 2016 Published: February 1, 2016 3730
DOI: 10.1021/acsami.5b12789 ACS Appl. Mater. Interfaces 2016, 8, 3730−3735
Letter
ACS Applied Materials & Interfaces volume. Thus, the hollow nanostructures may exhibit better EM wave absorption properties compared to their solid counterparts. Recently, several hollow materials applied in EM wave absorption field have been reported.17−21 For example, the minimal refection loss of hollow microsphere/ titania/M-type Ba ferrite was −30.1 dB at 8.5 GHz.17 Wang et al. reported that the minimum refection loss of Fe3O4-coated hollow glass sphere/reduced graphene oxide composite (HGS@Fe3O4@G) was −15.8 dB at 11.9 GHz when the absorber thickness is 2.5 mm.18 Cao et al. synthesized the hollow CdSe nanospheres with the optimal reflection loss of −31 dB at 17.8 GHz.19 Zhao et al. prepared flowerlike CuS hollow microspheres, showing the absorption bandwidth with reflection loss less than −10 dB of 3.6 GHz at d = 1.8 mm.20 Han et al. synthesized graphene-wrapped ZnO hollow spheres and the composite exhibited the minimum reflection loss of −45.05 dB at 9.7 GHz.21 The results above demonstrated that the hollow materials might be used as lightweight EM absorbing materials. However, most of the aforementioned hollow structures have a size in micrometer scale. The previous studies have proved that when the grain size of the absorbing materials is decreased to nanometer scale, their EM wave absorption properties will be improved sharply.5−7,9,12 Therefore, it is highly desirable to design and fabricate hollow nanostructured materials for high-performance EM wave absorbers. Herein we developed a strategy for coupling hollow Fe3O4− Fe nanoparticles with graphene sheets for high-performance electromagnetic wave absorbing material. The composite employed as EM wave absorbing material possessed following advantages: (i) The average diameter and shell thickness of the hollow Fe3O4−Fe nanoparticles were only 20 and 8 nm, respectively. Such small size would induce more dipoles to form around the interior and outermost shells of the hollow nanoparticles, leading to enhancement of the dipole polarizations. The enhanced dipole polarizations were helpful to the improvement EM wave absorption properties of the composite; (ii) Numerous interfaces existed in the composite, and thus the interfacial polarization and the associated relaxation facilitated the EM wave absorption properties compared to single component; (iii) The high thermal conductivity of graphene with uniform dispersion of the hollow nanoparticles could make the heat produced by EM wave irradiation diffuse outward rapidly; (iv) The nanoparticles had hollow feature and allowed the composite to employ as lightweight absorber. As a consequence, the hollow Fe3O4−Fe nanoparticle/graphene (Fe3O4−Fe/G) composite exhibited excellent electromagnetic wave absorption property. The minimal reflection loss (RL) value was up to −58 dB at 5.2 GHz, and the RL values could reach −30 dB at the absorber thickness range of 2.0−5.0 mm. Such EM wave absorption properties of the hollow Fe3O4−Fe/ G composite were greatly superior to the solid Fe3O4−Fe/G composite and most magnetic EM wave absorbing materials recently reported. Moreover, the addition amount of the hollow Fe3O4−Fe/G composite into paraffin matrix was only 18 wt %. Thus, the hollow Fe3O4−Fe/G composite could be applied as a lightweight EM wave absorber with strong absorption property for the practical applications. The hollow Fe3O4−Fe/G composite was fabricated using hollow Fe2O3/G composite as precursor (Supporting Information).22,23 Figure 1 shows the XRD pattern of the as-prepared hollow Fe3O4−Fe/G composite. The diffraction peaks at 2θ = 18.2, 30.2, 35.5, 37.1, 43.2, 53.6, 57.1, and 62.8° can be
Figure 1. XRD pattern of the hollow Fe3O4−Fe/G composite.
assigned, respectively to (111), (220), (311), (222), (400), (422), (511), and (440) reflections of the Fe3O4 (JCPDS no. 72−2303), whereas the peak at 2θ = 44.7° can be indexed to (110) plane of the Fe (JCPDS no. 87−0722). The XRD result reveals that the composite contains crystalline Fe3O4 and Fe. The broad diffraction peaks indicates that the particles on the graphene sheets have small sizes. According to the Scherrer equation (D = 0.9λ/Bcos θ, where D is the crystallite size, B is full width at half-maximum, θ is diffraction angle, and λ is the wavelength of X-ray) the crystal size of Fe3O4 particles are estimated to be 20 nm. Note that the intensities of the diffraction peaks from Fe3O4 are greatly higher than those from Fe, suggesting the Fe content is great smaller than the Fe3O4 content in the composite. The surface composition of the composite and valence state of Fe were characterized by X-ray photoelectron spectroscopy (XPS). The survey spectrum (Figure 2a) shows that hollow Fe3O4−Fe/G composite consists of Fe, O and C elements. Figure 2b shows the Fe 2p core-level XPS spectra of the hollow Fe3O4−Fe/G composite. Two main peaks at the positions of 711.5 and 724.9 eV can be assigned to Fe 2p3/2 and Fe 2p1/2, respectively, which are closed to the reported values for Fe3O4.22 Moreover, no shakeup satellite peaks, which are the fingerprints of the electronic structures of iron oxides such as αFe2O3 and γ- Fe2O3, can be identified, demonstrating that the Fe2O3 phase does not exist in the composite. In addition, a very weak peak at 707.0 eV corresponds to Fe0, further confirming the small amount of Fe in the composite.25 O 1s XPS spectrum (Figure 2c)) can be deconvoluted into three peaks located at 530.0, 531.5, and 533.1 eV, which are attributed to oxygen in the lattice (Fe−O),26 oxygen atoms in the surface hydroxyl groups (H−O), and oxygen in the lattice (C−O), respectively.27 Figure 2d shows C 1s XPS spectrum of the hollow Fe3O4−Fe/G composite. It clearly displays a considerable degree of oxidation with four components that correspond to carbon atoms in different functional groups: C−C/CC (284.6 eV), C−O (286.5 eV), CO (288.3 eV), and O−C O (289.1 eV) groups.28 The morphology and structure of the hollow Fe3O4−Fe/G composite were further investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM) measurements. A low-resolution SEM image reveals that the hollow Fe3O4−Fe/G composite inherit the morphology of the graphene sheet, as shown in Figure 3a. Highresolution SEM image (Figure 3b) clearly shows that the hollow particles with an average diameter typically ca. 20 nm are uniformly anchored on the graphene substrates. The hollow 3731
DOI: 10.1021/acsami.5b12789 ACS Appl. Mater. Interfaces 2016, 8, 3730−3735
Letter
ACS Applied Materials & Interfaces
Figure 2. (a) Survey spectrum, (b) Fe 2p, (c) O 1s, and (d) C 1s XPS spectra of the hollow Fe3O4−Fe/G composite.
Fe3O4−Fe nanoparticles with the graphene can be realized by our present method, which may be expended as a general method to synthesize other kinds of hollow magnetic nanoparticle/G composites. The field dependence of magnetization for the hollow Fe3O4−Fe/graphene composite was measured at room temperature by a vibrating sample magnetometer, as shown in Figure S1 (Supporting Information). Significant hysteresis loops in the M−H curves indicate the ferromagnetic behavior of the hollow Fe3O4−Fe/graphene composite. The saturation magnetization (Ms), coercivity (Hc), and retentivity (Mr) are 47.1 emu/g, 66.7 Oe, and 5.529 emu/g, respectively, for the hollow Fe3O4−Fe/ graphene composite. The magnetic measurements above demonstrate that the composite is ferromagnetic, facilitating the attenuation of the EM irradiation energy.8−16 The hollow Fe3O4−Fe/graphene composite possesses several advantages as EM absorbing material. First, numerous interfaces existed in the composite, and thus the interfacial polarization and the associated relaxation facilitated the improvement of the EM wave absorption properties compared to single component. Second, the average shell thickness of the hollow nanoparticles was as small as 8 nm. Such small size would induce more dipoles to form around the interior and outermost shells of the hollow nanoparticles, leading to enhancement of the dipole polarizations. The enhanced dipole polarizations were very helpful to the improvement EM wave absorption properties of the composite. Third, the high thermal conductivity of graphene with uniform dispersion of the hollow nanoparticles facilitated the diffusion of the heat produced as EM wave irradiated. The RL value, reflected the attenuation ability of the absorbing materials, can be calculated by the following equations24
Figure 3. (a, b) SEM, (c) TEM and (d) HRTEM images of hollow Fe3O4−Fe/G composite. The inset in c shows the corresponding SAED pattern.
feature of the nanoparticles is confirmed by a low-resolution TEM image (Figure 3c). On the basis of the measurements on more than 100 nanoparticles by means of TEM observation, we found that more than 73% of nanoparticles had hollow interior space, and their average shell thickness was around 8 nm. The inset of Figure 3c shows a selected area electron diffraction (SAED) pattern, where the labeled diffraction rings can be indexed to the (311), (220), (400), and (440) planes of the cubic Fe3O4 phase. Figure 3d shows a high-resolution TEM (HRTEM) image of individual hollow nanoparticle with a shell thickness of approximately 6 nm. Multiple crystal domains with different orientations are observed in the HRTEM image. In terms of the TEM, XRD and XPS analyses, coupling hollow
Z in = Z0(μr /εr)1/2 tanh[j 2πfd /c(με )1/2 ] r r
RL (dB) = 20log 3732
Z in − Z0 Z in + Z0
(1)
(2) DOI: 10.1021/acsami.5b12789 ACS Appl. Mater. Interfaces 2016, 8, 3730−3735
Letter
ACS Applied Materials & Interfaces
Fe3O4−Fe/G composite with addition amount of 18 wt % in the paraffin matrix at different thicknesses (d = 1−5 mm). It can be found that the hollow Fe3O4−Fe/G composite exhibits a minimal RL value is up to −58 dB at 5.2 GHz at d = 4.6 mm. Furthermore, the RL values exceeding −30 dB are achieved for the absorber at the thickness of the absorber ranging from 2 to 5 mm (Figure S3, Supporting Information). The absorption bandwidths with RL exceeding −10 dB are about 1.6 and 6.2 GHz at the thicknesses of 4.6 and 2.0 mm, respectively (Table 1). The EM wave absorption property of the hollow Fe3O4− Fe/G composite is greatly superior to the graphene and the solid Fe3O4−Fe/G composite22 (Table 1), demonstrating that the hollow structures facilitate the improvement of the EM wave absorption ability of the materials. Note that the hollow Fe3O4−Fe/G composite also exhibit significantly improved EM wave absorption properties compared to other magnetic materials (Table 1). Furthermore, the addition amount of the magnetic materials in the matrix was more than 20 wt %, whereas it was only 18 wt % for the hollow Fe3O4−Fe/G composite. Thus, the hollow Fe3O4−Fe/G composite can be used as a lightweight EM wave absorber with strong absorption property for the practical applications. Recently, mechanisms such as zero reflection and the quarter-wavelength matching model have been proposed to explain the EM wave absorption properties of the absorbers. For zero reflection, the εr and μr are required to have equal values in terms of EM wave theory.29 However, the permittivity of the hollow Fe3O4−Fe/G composite is much higher than their permeability (Figure S2). Therefore, the good EM wave absorption properties of the hollow Fe3O4−Fe/G composite cannot be explained by the zero reflection mechanism. As for the quarter-wavelength matching model, it usually takes place when the incident and reflected waves in the absorbers are out of phase by 180°. In the model, the relationship between the absorber thickness (tm) and the peak frequency ( f m) can be described by the following equation30
where Z0 is the impedance of free space, Zin is the input impedance of the absorber, εr = ε′−jε″ the relative complex permittivity, μr = μ′−jμ″ the relative complex permeability, c the velocity of electromagnetic waves in free space, f the frequency of microwaves and d the thickness of the absorber. Figure S2a shows the relative complex permittivity of the hollow Fe3O4−Fe/graphene composite in the frequency range of 2−18 GHz. The ε′values are in the range of 5.8−12.6 over 2−18 GHz, whereas the ε″ values gradually decrease from 4.9 to 3.2 with the increase of the frequency. Figure S2b shows the relative complex permeability of the hollow Fe3O4−Fe/G composite. The μ′ values are in the range of 1.16−1.23, whereas the μ″ values increase first, and then decrease gradually. The dielectric loss tangent values (tan δm = ε″/ε′) (Figure S2c) are greatly larger than the magnetic loss tangent values (tan δe = μ″/μ′) (Figure S2d), indicating that the EM wave absorption property of the hollow Fe 3O4−Fe/G composite may mainly originate from its dielectric loss. Figure 4a shows the frequency-dependent RL curves of the hollow
tm = nc /(4fm (|εr||μr |)1/2 , (n = 1, 3, 5, ...)
(3)
where |εr| and |μr| are the modulus of the measured εr and μr at f m, respectively. According to the model, if the matching thickness of the absorber satisfies the eq 3, the two emerging reflected EM waves from the air−absorber interface and absorber−metal interface are out of phase by 180°, leading to an extinction of them on the air−absorber interface. In this case, the absorber will exhibit excellent EM wave absorption property due to quarter-wavelength interference phenomenon.
Figure 4. (a) RL values of the hollow Fe3O4−Fe/G composite at d = 1.0−5.0 mm, (b) comparison of the calculated matching thickness (tmfit) under n = 1 to the tmexp obtained from RL values shown in Figure 4a), and (c) the modulus of the normalized characteristic impedance (Z = | Zin/Z0 |) for the hollow Fe3O4−Fe/G composite.
Table 1. Comparison of EM Wave Absorption Properties of Hollow Fe3O4−Fe/G Composite with Those of Other Magnetic Materialsa
a
materials
RL min. (dB)
d (mm)
absorption bandwidth (GHz) (
