Sandwich-like graphene nanosheets decorated with superparamagnetic CoFe₂O₄ nanocrystals and their application as an enhanced electromagnetic wave absorber

Abstract

Sandwich-structured CoFe₂O₄/graphene hybrids are fabricated through a facile one-pot polyol route, and the electromagnetic wave absorption properties are investigated. TEM images and elemental mapping indicate that the graphene nanosheets are decorated with numerous tiny CoFe₂O₄ nanocrystals with a relatively uniform size of 7.8 nm, forming sandwich-like nanostructures. Magnetization measurement reveals that the CoFe₂O₄/graphene hybrids possess superparamagnetism at room temperature with zero coercivity. Investigations of the electromagnetic properties indicate that the complex permittivity of CoFe₂O₄/graphene hybrids is significantly improved in comparison with that of pure CoFe₂O₄ nanocrystals, leading to enhanced electromagnetic wave absorption properties of the CoFe₂O₄/graphene hybrids. The maximum reflection loss of the CoFe₂O₄/graphene hybrids is up to −36.4 dB at 12.9 GHz with the matching thickness of 2.5 mm, and the absorption bandwidth with reflection loss values below −10 dB is in the range of 5.4–18 GHz when the matching thicknesses are only 1.5–4.0 mm. These results suggest that the sandwich-like CoFe₂O₄/graphene hybrids with enhanced electromagnetic wave absorption properties and wide absorption bandwidth are an ideal candidate for electromagnetic wave absorption applications in the future.

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1. Introduction

During the past few years, the phenomenon of electromagnetic interference (EMI) and electromagnetic compatibility (EMC) have become serious problems, owing to the ever-increasing development of telecommunications and electronic devices working in the gigahertz (GHz) range.^1–3 The accompanying electromagnetic pollution can not only interrupt the operation of electronic devices but is also harmful to human health, which creates a necessity for electromagnetic interference shielding. Electromagnetic wave absorbing materials have drawn tremendous attention in wireless devices, industrial protection and military fields, owing to the fact that they can absorb electromagnetic waves and convert them into thermal energy or dissipate them by interference.^4,5 Electromagnetic wave absorbing materials with a strong adsorption value, wide adsorption frequency, thin matching thickness and lightweight characteristics are urgently required.^5,6 Much effort has been focused on the fabrication of electromagnetic wave absorbing materials, such as ferrites, magnetic metals, dielectrics and so on.^7–10 However, the large density and high matching thickness of these materials greatly restrict their technical applications. The exploration of highly efficient and lightweight electromagnetic wave absorbing materials with thin thickness is still a big challenge.

Nowadays, graphene has attracted much scientific interest both in the fundamentally theoretical and experimental scientific research communities because of its remarkable chemical, physical and mechanical properties.¹¹ The high dielectric loss and low density make graphene a promising material for electromagnetic wave absorption properties. However, the maximum reflection loss of graphene is only −6.9 dB,¹² which is unsuitable for practical applications. Owing to its large surface area and special layered structure, graphene has been promised to be nanoscale building blocks for novel hybrid materials.^13–16 Recently, decorating graphene nanosheets with magnetic nanoparticles, such as Fe₃O₄,^4,5 NiFe₂O₄,^17,18 Co₃O₄,^19,20 Fe,^21,22 Ni,²³ γ-Fe₂O₃,²⁴ etc., has been widely applied for electromagnetic wave absorption, which is possibly resulted from the synergistic effect of their high magnetic loss from magnetic nanomaterials and electric loss from light-weight graphene, leading to well impedance matching and high-performance electromagnetic wave attenuation. However, the low loading density of magnetic nanoparticles on the graphene nanosheets makes the complex permeability of the hybrids hard to improve, which possibly limits the electromagnetic wave-absorbing applications of these hybrids. To our best knowledge, few papers have been reported on the fabrication of sandwich-like graphene-based hybrids with large coverage density for electromagnetic wave absorption applications.

Magnetic CoFe₂O₄ spinel ferrite has large saturation magnetization, large magnetocrystalline anisotropy and high Snoek's limit, which is promised to have high complex permeability in a wide frequency range. These characteristics suggest that CoFe₂O₄ can be used as thin electromagnetic wave absorbers working at high frequency range.^25,26 However, the large density of CoFe₂O₄ make it useless in the applications requiring lightweight mass.²⁶ In addition, the CoFe₂O₄ nanocrystals reported before show large coercivity, which restricts their applications in high frequency. Recently, novel CoFe₂O₄-based electromagnetic wave absorption materials have been investigated, such as CNTs/CoFe₂O₄,²⁵ ZnO/CoFe₂O₄ ²⁶ and polyaniline/CoFe₂O₄.²⁷ Furthermore, CoFe₂O₄/graphene hybrids have been used for microwave absorption applications, which were fabricated through hydrothermal route in combination with annealing treatment or adjusting the reaction parameters.^28,29 However, these reported hydrothermal route is energy-intensive and needs rigorous reaction condition, which may hinder their industrial applications. Besides, it is still a big challenge to fabricate large area of graphene nanosheets uniformly decorated by magnetic nanocrystals without aggregation. To our best knowledge, no paper has been focused on the fabrication of sandwich-like graphene nanosheets decorated with superparamagnetic CoFe₂O₄ nanocrystals for electromagnetic wave-absorbing applications.

In this paper, sandwich-like graphene nanosheets decorated with superparamagnetic CoFe₂O₄ nanocrystals was fabricated by a facile one-pot polyol route. The structure and morphology were characterized at nanoscale. Electromagnetic wave absorption investigations showed that the sandwich-structured superparamagnetic CoFe₂O₄/graphene hybrids exhibited excellent electromagnetic wave absorbability than pure graphene and CoFe₂O₄ nanocrystals. The results indicate that the sandwich-structured CoFe₂O₄/graphene hybrids are ideal candidate for future electromagnetic wave absorption applications.

2. Experimental section

2.1 Synthesis of sandwich-structured CoFe₂O₄/graphene hybrids

Graphene oxide (GO) was prepared through a modified Hummers method.³⁰ The sandwich-structured CoFe₂O₄/graphene hybrids were synthesized by an one-pot polyol method. In a typical procedure, 40 mg GO was dispersed into 100 ml of ethylene glycol (EG) and ultrasounded for 3 h. 8 mmol FeCl₃·6H₂O and 4 mmol CoCl₂·6H₂O were added into the above suspension of GO and ultrasounded for another 3 h. 60 mmol NaAc was dissolved into the above solution. The solution was stirred and refluxed for 10 h. The black products were washed by water and ethanol for several times, and then dried in the atmospheric environment. For comparison purposes, pure CoFe₂O₄ nanocrystals were fabricated in the same approach without GO.

2.2 Characterization

The morphology and microstructure of the products were observed by high-resolution transmission electron microscope (HRTEM, FEI Tecnal G² F20) embedded with energy-dispersive X-ray spectroscopy (EDX, Oxford Instrument), high angle annular dark field (HAADF) and scanning transmission electron microscopy (STEM). The crystal structure was performed by X-ray powder diffraction instrument with Cu K_α radiation (λ = 1.5418 Å) (XRD, X'pert powder, Philips). The surface composition and oxidation state were obtained by the X-ray photoelectron spectroscopy (XPS, ESCALAB210). The magnetic properties were studied by vibrating sample magnetometer (VSM, Lake Shore 7304). The electromagnetic parameters were analyzed by a network analyzer (Agilent Technologies E8363B), in which the powders were mixed with paraffin with 70 wt% samples and pressed into toroidal shape (ψ_out: 7.00 mm, ψ_in: 3.04 mm).

3. Results and discussion

Fig. 1 shows the XRD patterns of GO and CoFe₂O₄/graphene hybrids. GO exhibits a strong diffraction peak at 13.4°, corresponding to the (001) reflection with an interlayer spacing of 0.65 nm, which is larger than that of graphite (0.34 nm). The enhancement of interlayer spacing is due to the formation of oxygenic functional groups between the graphite layers.³¹ For the CoFe₂O₄/graphene hybrids, the diffraction peaks at 2θ = 30.1°, 35.3°, 43.1°, 53.2°, 56.9°, 62° and 74.3° can be respectively indexed to the (220), (311), (400), (422), (511), (440) and (533) crystal planes of the pure spinel CoFe₂O₄ with a face-centered cubic (FCC) structure (Fd3̄m, JCPDS card no. 22-1086), suggesting the formation of CoFe₂O₄ nanocrystals. It is clear that the CoFe₂O₄/graphene hybrids show broadening diffraction peaks, suggesting that the crystalline grain is small. Based on the Scherer's formula, the average grain size of the CoFe₂O₄ nanocrystals is about 7.2 nm.

Fig. 1 XRD patterns of GO and CoFe₂O₄/graphene hybrids.

The morphologies of GO and CoFe₂O₄/graphene hybrids were characterized by TEM observations (Fig. 2). The layered GO (Fig. 2a and b) was transparent with several wrinkles at the edges. The SAED pattern of GO (inset of Fig. 2a) shows a set of six fold patterns, indicating that GO is few-layer with a hexagonal closed-packed (hcp) structure.³¹ HRTEM image (inset of Fig. 2b) indicates that each GO nanosheet is about 3 layers. Fig. 2c–f show the TEM images of the CoFe₂O₄/graphene hybrids with different magnification. It is clearly seen that tiny CoFe₂O₄ nanocrystals with diameters of 7.8 nm covered evenly on the whole surface of graphene, forming sandwich-like structures. The SAED pattern of CoFe₂O₄/graphene hybrids (inset of Fig. 1c) shows the standard ring patterns resulted from the cubic spinel structure of CoFe₂O₄, which is consistent with the XRD result. The HRTEM image of CoFe₂O₄ nanocrystals (inset of Fig. 2d) shows clear lattice fringes with an interplanar distance of 0.297 nm, which can be assigned to the (220) plane of CoFe₂O₄.

Fig. 2 TEM images of (a and b) GO and (c–f) CoFe₂O₄/graphene hybrids. Inset of (a) and (c) show the SAED patterns of GO and CoFe₂O₄/graphene hybrids, respectively; inset of (b) and (d) show the HRTEM images of GO and CoFe₂O₄/graphene hybrids, respectively.

The chemical composition of the samples was characterized by XPS spectra (Fig. 3). In the full scan XPS spectrum (Fig. 3a), several sharp peaks with the binding energy of 285, 530, 711 and 790 eV, were attributed to C 1s, O 1s, Fe 2p and Co 2p, respectively, indicating the existence of C, O, Fe and Co elements in the CoFe₂O₄/graphene hybrids. To further investigate the electronic states of the elements for the samples, high-resolution spectra were analyzed. In the O 1s spectrum (Fig. 3b), the intensity of oxygen peak for CoFe₂O₄/graphene hybrids was visibly reduced in comparison with that of GO, revealing that the oxygen-containing groups in CoFe₂O₄/graphene hybrids are partly removed. Besides, the binding energy associated with O 1s shifted from 532.6 eV in GO to 530.4 eV in CoFe₂O₄/graphene hybrids, which is originated from the lattice oxygen in M–O (M = Fe, Co). The Fe 2p spectrum (Fig. 3c) exhibits two peaks at 712.23 and 725.62 eV, corresponding to Fe 2p_3/2 and Fe 2p_1/2, respectively. The Co 2p spectrum (Fig. 3d) consists of two spin–orbit doublets characteristics of Co 2p_3/2 (778.6 eV, Co²⁺ in octahedron site; 784.2 eV, Co²⁺ in tetrahedron site) and Co 2p_1/2 (794.0 eV, Co²⁺ in octahedron site; 800.8 eV, Co²⁺ in tetrahedron site). These results suggest the formation of CoFe₂O₄ nanocrystals. The C 1s spectrum of GO (Fig. 3e) consists of four deconvoluted peaks, arising from the C–C/C=C (285.0 eV), C–OH (286.1 eV), C–O–C (287.1 eV) and C=O (288.9 eV) groups. For the C 1s spectrum of CoFe₂O₄/graphene hybrids (Fig. 3f), the peak is mainly the non-oxygenated carbon groups (C–C/C=C) and the relative contribution of oxygenated functional groups (C–O–C/C=O) decreased obviously, suggesting that most of the oxygenated functional groups were partly removed during the reaction process. The area ratios of the deconvoluted peaks for carbon-based functional groups of GO and CoFe₂O₄/graphene hybrids are displayed in Table 1. The XPS results further indicate that GO was reduced, during which CoFe₂O₄ nanocrystals were formed and anchored on the graphene nanosheets. The reduction of GO could significantly improve the electronic conductivity of CoFe₂O₄/graphene hybrids, making the graphene nanosheets as conductive channels between CoFe₂O₄ nanocrystals, which is favorable for electromagnetic wave absorption applications.³²

Fig. 3 XPS spectra: (a) wide scan of GO and CoFe₂O₄/graphene hybrids, (b) O 1s spectra of GO and CoFe₂O₄/graphene hybrids, (c) Fe 2p spectra of CoFe₂O₄/graphene hybrids, (d) Co 2p spectra of CoFe₂O₄/graphene hybrids, (e) C 1s spectra of GO and (f) C 1s spectra of CoFe₂O₄/graphene hybrids.

Table 1 The peak area ratios of the carbon-based groups for GO and CoFe₂O₄/graphene hybrids

Chemical band	C–C	C–OH	C–O–C	C=O
Binding energy (eV)	285.0	286.1	287.1	288.9
Relative area of GO (%)	45.2	10.1	36.0	8.7
Relative area of CoFe2O4/graphene hybrids (%)	57.0	29.7	13.3	0

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To further determine the situation of CoFe₂O₄ nanocrystals supported on graphene, HAADF-STEM and EDX elemental mapping techniques were used to investigate the morphology and element distributions of CoFe₂O₄/graphene hybrids. HAADF-STEM images, called Z-contrast images (Z is the atomic number), make it ideal and powerful for the characterization of heterogeneous nanocomposites with components of different atomic numbers.³³ It is believed that this Z-contrast images could offer a better distinction between CoFe₂O₄ (average Z ≈ 16) and C (Z = 6). Fig. 4a shows the HAADF-STEM image for the CoFe₂O₄/graphene hybrids. The CoFe₂O₄ nanocrystals were very tiny and anchored on the surface of graphene, forming honeycomb and sandwich-like structures. Fig. 4b displays the EDX spectrum of the CoFe₂O₄/graphene hybrids. The spectrum demonstrates the existence of carbon, oxygen, iron and cobalt in the samples, which is coincident with the XRD results. Moreover, quantitative analysis of the EDX spectrum indicates that the atomic ratio of Co–Fe is nearly 1 : 2, inferring a CoFe₂O₄ composition. Fig. 4c–f shows the corresponding EDX elemental mapping of C (K_α, 0.28 keV), O (K_α, 0.52 keV), Fe (K_α, 6.4 keV) and Co (K_α, 6.9 keV). It is clear that the elements C (Fig. 4c), O (Fig. 4d), Fe (Fig. 4e) and Co (Fig. 4f) is evenly distributed throughout the whole HAADF-STEM image perturbed only by a thickness contrast, which suggests that tiny CoFe₂O₄ nanocrystals covered on the graphene nanosheets and form sandwich-like structures. This result is accordant with the above TEM observations.

Fig. 4 Elemental mappings of CoFe₂O₄/graphene hybrids: (a) HAADF-STEM image; (b) EDX spectrum; (c) carbon mapping; (d) oxygen mapping; (e) iron mapping; (f) cobalt mapping.

Fig. 5 shows the magnetization versus magnetic field (M–H) curve of the pure CoFe₂O₄ nanocrystals and CoFe₂O₄/graphene hybrids measured at room temperature. The saturation magnetization (M_s) value of the pure CoFe₂O₄ nanocrystals and CoFe₂O₄/graphene hybrids is 63.8 and 45.3 emu g⁻¹, respectively, which is smaller than that of the corresponding bulk CoFe₂O₄ (74 emu g⁻¹). These results are ascribed to the possible effect of surface spin canting and/or dead magnetic layer on the CoFe₂O₄ nanocrystals.³⁴ Moreover, the smaller M_s of the CoFe₂O₄/graphene hybrids in respect to the pure CoFe₂O₄ nanocrystals is originated from the weight-fraction of non-magnetic graphene. Both of the samples show S-like shape with zero coercivity and remanence, demonstrating a typical superparamagnetic characteristic which is favorable for their usage as high-performance electromagnetic wave absorbers working at high frequency range.

Fig. 5 The M–H curve of the samples measured at room temperature.

Fig. 6 shows the frequency dependence of complex permeability and complex permittivity for the pure CoFe₂O₄ nanocrystals and CoFe₂O₄/graphene hybrids. No significant changes of the complex permeability are observed between the pure CoFe₂O₄ nanocrystals and CoFe₂O₄/graphene hybrids (Fig. 6a). The real part (μ′) of complex permeability for both the samples presents a similar frequency dependence and the values decrease slowly with the increase of frequency. It is clearly seen that the μ′ value of CoFe₂O₄/graphene hybrids is slightly decreased in comparison with the pure CoFe₂O₄ nanocrystals. This phenomenon is mainly attributed to the additional nonmagnetic GO in the hybrids. The imaginary part (μ′′) of complex permeability is almost the same, suggesting a negligible difference of magnetic loss between both the samples. The resonance peak around 12 GHz is possibly ascribed to exchange resonance of the magnetic nanocrystals.³⁵ The real part (ε′) and imaginary part (ε′′) of complex permittivity for the pure CoFe₂O₄ nanocrystals (Fig. 6b) are almost independent on frequency, which are nearly constant in the whole frequency range with an inconspicuous undulation (ε′ ≈ 4.6 and ε′′ ≈ 0.33). For the CoFe₂O₄/graphene hybrids, the complex permittivity is obviously enhanced and declines with the increase of frequency. The values of ε′ and ε′′ decline from 18.6 to 4.5 and 6.7 to 1.3, respectively, 3.5 and 24.7 times larger than that of the pure CoFe₂O₄ nanocrystals at 0.1 GHz. This result is attributed to the enhanced electric polarization and electrical conductivity by additional graphene, which could lead to enhanced dielectric dispersion and increased dielectric loss in comparison with the pure CoFe₂O₄ nanocrystals. In addition, the fluctuation of complex permittivity about 10 GHz for the CoFe₂O₄/graphene hybrids is probably due to the dipolar polarization dominated in the insulator–conductor composites.³⁵

Fig. 6 The frequency dependence of (a) complex permeability and (b) complex permittivity for the pure CoFe₂O₄ nanocrystals and CoFe₂O₄/graphene hybrids.

Fig. 7 shows the frequency dependence of dissipation factors represented by the magnetic loss tangent (tan δ_μ = μ′′/μ′) and dielectric loss tangent (tan δ_ε = ε′′/ε′) for the pure CoFe₂O₄ nanocrystals and CoFe₂O₄/graphene hybrids. The values of tan δ_μ and tan δ_ε for the pure CoFe₂O₄ nanocrystals are smaller than 0.2 in the whole frequency range, indicating that the magnetic loss and dielectric loss are considerably lower, which suggests that the pure CoFe₂O₄ nanocrystals may represent poor electromagnetic wave absorption properties. It is clear that the values of tan δ_ε for the CoFe₂O₄/graphene hybrids are obviously enhanced in comparison with that of the pure CoFe₂O₄ nanocrystals. This result indicates that the lightweight graphene play an important role to increase the dielectric loss, which is significant for the electromagnetic wave absorption. Besides, the reduction process of GO in this work may generate lots of defect sits in the 2D lattice of graphene, which can lead to an increase in the complex permittivity and enhance the dielectric loss of the electromagnetic wave absorption materials.³⁶ Moreover, the values of tan δ_ε are distinctly larger than that of the tan δ_μ, suggesting that the electromagnetic wave attenuation mechanism of the CoFe₂O₄/graphene hybrids is mainly attributed to the dielectric loss.

Fig. 7 Frequency dependence of the loss tangent of (a) pure CoFe₂O₄ nanocrystals and (b) CoFe₂O₄/graphene hybrids.

To further investigate the electromagnetic wave absorption properties of CoFe₂O₄/graphene hybrids, the reflection loss (RL) curves were calculated according the transmit line theory by the following equations:⁵

where Z_in is the input impendence of absorber, Z₀ is the impedance of air, f is the frequency of electromagnetic wave, d is the thickness of a electromagnetic wave absorber and c is the velocity of light in vacuum. Fig. 8 displays the relationship between RL and frequency for the pure CoFe₂O₄ nanocrystals and CoFe₂O₄/graphene hybrids. The RL values for the pure CoFe₂O₄ nanocrystals (Fig. 8a) cannot reach −10 dB with the thickness range of 1.0–4.0 mm and the maximum RL value is only −4.4 dB, revealing weak electromagnetic wave absorption ability. However, when the superparamagnetic CoFe₂O₄ nanocrystals are anchored on the graphene nanosheets, the electromagnetic wave absorption properties of CoFe₂O₄/graphene hybrids are obviously enhanced. The RL value of CoFe₂O₄/graphene hybrids (Fig. 8b) below −10 dB can be achieved in the 5.4–18 GHz range with the thickness of 1.5–4.0 mm, and the maximum RL value of −36.4 dB can be obtained at 12.9 GHz with a matching thickness of only 2.5 mm. It is clear that the graphene nanosheets are significantly important for the electromagnetic wave absorption properties of CoFe₂O₄/graphene hybrids. In comparison with the pure graphene and CoFe₂O₄ nanocrystals, the enhanced electromagnetic wave absorption performance of CoFe₂O₄/graphene hybrids is ascribed to the synergistic effect between superparamagnetic CoFe₂O₄ nanocrystals and light-weight graphene. Accompanying with the low density of graphene, these results suggest that the CoFe₂O₄/graphene hybrids are promised to be used as light weight and high-performance electromagnetic wave absorber for electromagnetic wave absorption applications in the future.

Fig. 8 The relationship between RL and frequency for the (a) pure CoFe₂O₄ nanocrystals and (b) CoFe₂O₄/graphene hybrids.

4. Conclusion

In summary, we developed a facile one-pot polyol strategy to fabricate sandwich-structured graphene nanosheets decorated by superparamagnetic CoFe₂O₄ nanocrystals, and their nanostructures and morphologies were characterized at nanoscale. Electromagnetic wave absorption properties show that the maximum RL of superparamagnetic CoFe₂O₄/graphene hybrids is −36.4 dB at 12.9 GHz for the thickness of 2.5 mm and the absorption bandwidth with the RL below −10 dB is 12.6 GHz (5.4–18 GHz) for the thickness of 1.5–4.0 mm. Investigations reveal that the introduction of graphene could significantly enhance electromagnetic wave absorption properties of the CoFe₂O₄ nanocrystals. The enhanced electromagnetic wave absorption performance of superparamagnetic CoFe₂O₄/graphene hybrids is ascribed to the synergistic effect between remarkable magnetic loss from the superparamagnetic CoFe₂O₄ nanocrystals and high electric loss from the light-weight graphene. This work suggests that the sandwich-like CoFe₂O₄/graphene hybrids may be attractive candidate for light weight and enhanced electromagnetic wave absorption applications in the future.

Acknowledgements

We thank the support from the grants from the National Natural Science Foundation of China (no. 61177059), Introduction Talent Project of Northwest University (pr13100), the Natural Science Foundation of Shannxi Province, China (2014JQ1040), the Scientific Research Foundation of Northwest University (13NW13), the Basic Scientific Research Business Expenses of the Central University, Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education from Lanzhou University and the Open Projects from Institute of Photonics and Photo-Technology of Northwest University (China).

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Footnote

† These authors contributed equally to this work.

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