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graphene with superior electromagnetic wave absorption performances
Author’s Accepted Manuscript Facile synthesis of a novel flower-like BiFeO3 microspheres/graphene with superior electromagnetic wave absorption performances Xiang Gao, Yan Wang, Qiguan Wang, Xinming Wu, Wenzhi Zhang, Meng Zong, Lijuan Zhang www.elsevier.com/locate/ceri
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S0272-8842(18)33063-3 https://doi.org/10.1016/j.ceramint.2018.10.243 CERI19955
To appear in: Ceramics International Received date: 9 September 2018 Revised date: 27 October 2018 Accepted date: 29 October 2018 Cite this article as: Xiang Gao, Yan Wang, Qiguan Wang, Xinming Wu, Wenzhi Zhang, Meng Zong and Lijuan Zhang, Facile synthesis of a novel flower-like BiFeO3 microspheres/graphene with superior electromagnetic wave absorption p e r f o r m a n c e s , Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.243 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile synthesis of a novel flower-like BiFeO3 microspheres/graphene with superior electromagnetic wave absorption performances
Xiang Gaoa, Yan Wanga*, Qiguan Wanga, Xinming Wua, Wenzhi Zhanga, Meng Zongb, Lijuan Zhanga
a
School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, PR China
b
Department of Applied Chemistry and The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi'an 710072, PR China
*
Corresponding author. Tel.: +86 13909249145. [email protected] (Y. Wang)
Abstract In recent years, porous or layered magnetic materials have received increasing attention due to their low density and lightweight. In this work, porous BiFeO3 microspheres and three-dimensional porous BiFeO3 microsphere-reduced graphene oxide (RGO) composite (3D porous BiFeO3/RGO) were prepared by one-step etching processing using pure BiFeO3 particles as precursors. The precursor undergoes dissolution-recrystallization/reduction process, resulting in large amount of BiFeO3 fragments and graphene hybrid product, which forms 3D porous BiFeO3/RGO composite. Electromagnetic (EM) absorption performance measurements exhibit that at low thickness of 1.8 mm, porous BiFeO3/RGO composite can achieve reflection loss (RL) value up to -46.7 dB and absorption bandwidth (defined by RL <-10 dB) exceeding 4.7 GHz (from 12.0 to 16.7 GHz), testifying outstanding microwave absorbing performance. Compared with pure
porous BiFeO3, improved EM wave absorption ability of as-prepared porous BiFeO3/RGO composite is attributed to interfacial polarization, multiple reflections, scattering, and appropriate impedance matching.
Keywords: Graphene; BiFeO3; Three-dimensional composite; Electromagnetic wave absorption
1. Introduction With the increasing use of electromagnetic (EM) waves in radars, advanced detectors, and precision weapons, the EM wave pollution has become increasingly serious.1-4 Therefore, there is an urgent need in investigating EM wave absorbing materials. EM wave absorbing material is a kind of functional material that can effectively absorb the incident EM waves and convert the EM energy into heat or other forms of energy.5, 6 The ideal wave-absorbing material would combine high absorption, wide frequency band, low density, thin thickness, and good environmental stability.7-11 However, the traditional ferrite-based wave-absorbing material can difficultly meet the above-mentioned requirements because of their high density, narrow absorption bands and too weak absorption.12-14 The development of graphene has opened up a new research field for absorbing materials.15 The unique two-dimensional structure of graphene gives it a large specific surface area that promotes scattering and multiple reflections of EM waves, and improves their absorption.16-18 At the same time, when graphene is prepared by the reduction method, defects generated on the graphene sheets and residual oxygen-containing functional groups are prone to generate polarization relaxation and electric dipole relaxation under the action of an external
electric field, so that the dielectric loss increases.19, 20 The absorption mechanism on a single graphene sheet mainly relies on dielectric loss.21, 22 According to the EM complementarity theory, except for the single dielectric loss or magnetic loss, the impedance matching between permittivity and permeability also affects the microwave absorption performances of a material. Nevertheless, despite of its high permittivity, RGO results in a poor EM wave absorption abilities because of its weak impedance matching.23-26 Plenty of studies have confirmed that single-loss type or single-component absorbers can difficultly ensure high absorption in the 2-18 GHz range. Multi-loss matching and multi-component absorption are good strategies to possibly solve this problem.27-29 Nowadays, one of the most effective ways to improve the absorbing properties is to combine graphene with porous or layered magnetic materials, such as Fe3O4,30-32 Co3O4,33, 34 Co,35, 36
NiFe2O4,37 Bi2Fe4O9,38 or Fe2O3,39 CoFe2O4,40 or Fe3O4/Fe.41 Thanks to its large specific surface
area, high surface atomic ratio and high number of dangling bonds, the porous or layered magnetic material ensures interfacial polarization and multiple scattering of the EM wave. Specifically, the incidence of an EM wave on the porous magnetic material surface generates interfacial polarization and multiple scattering that will rapidly lower its intensity.42-45 The graphene prepared by the chemical reduction method has a good deal of oxygen-containing functional groups on its surface, which can easily be combined with other magnetic loss absorbents.46, 47 The resulting chemical modification of the magnetic loss absorbent can give rise to synergistic effects with the graphene and improve the impedance matching, thus increasing the magnetic loss of the composite.48-51 In this work, we used N,N-dimethylformamide (DMF) as a solvent and reducing agent
(hydrazine) to prepare porous BiFeO3/RGO composite under N2 environment by simultaneous dissolution-recrystallization/reduction method, as shown in scheme 1. The porous BiFeO3/RGO with a thickness of 1.8 mm shows strongest RL value of -46.7 dB at 13.8GHz with the absorption bandwidth of 4.7 GHz (12.0 GHz-16.7 GHz). The 3D porous structure of BiFeO3/RGO results in an excellent EM absorption even at very low thickness. This property is ascribed to the high electrical conductivity and high specific surface area of RGO as well as the 3D structure of porous BiFeO3 microspheres.
2. Experimental 2.1 Preparation of porous BiFeO3/RGO The pure BiFeO3 nano-particles were prepared by the hydrothermal reaction as reported previously.52 Graphene oxide (GO) was obtained by the improved Hummers method.53 The 3D porous BiFeO3/RGO composite was prepared by a facile etching method. Pure BiFeO3 (0.5 g) and GO (0.5 mg) nanosheets were dissolved into DMF (150 ml) under ultrasounds. Afterwards, the solution was added into 6 mL hydrazine and 1.5 mL methyl mercaptoacetate under N2 protection for 30 min. Then, the mixed solution was placed in a water bath at 80°C for 45 min. Finally, the reaction was ended by cold ethanol addition, and the precipitate was washed multiple times with ethanol and deionized water, and then dried at 60℃. Meanwhile, 3D porous BiFeO3 microspheres were also obtained using the above route without adding GO nanosheets. 2.2 Characterization The as-obtained products were investigated by X-ray diffraction (XRD, German Bruker D8), X-ray photoelectron spectroscopy (XPS, ESCALAB 250), field-emission scanning electron microscope (FESEM, Quanta 600FEG), transmission electron microscope (TEM, JTM-2100),
Brunner-Emmet-Teller (Quad-rasorb-SI instrument) and vibrating sample magnetometer (VSM, Lake Shore7307). The electromagnetic (EM) parameters were measured by a vector network analyzer (HP8720ES) at 2–18 GHz. 30 wt% samples and 70 wt% paraffin were pressed into a cylindrical shape specimen (φin=3.04 mm and φout=7 mm) for EM measurement.
3. Result and discussion The structure and phase composition of all samples were tested by XRD. Fig 1a displays the diffraction pattern of pure BiFeO3 nanoparticles, which is consistent with the standard card (JCPDS no.20-0169).54 Fig 1b and 1c exhibit the XRD patterns of porous BiFeO3 and porous BiFeO3/RGO composite, respectively. We can evidently observe that all the diffraction peaks of pure BiFeO3 nanoparticles are preserved in the porous BiFeO3 and porous BiFeO3/RGO composite. Nevertheless, the intensities of all peaks are significantly weakened, which may be attributed to the presence of a large amount nanoscale debris in the etched BiFeO3.55 After the addition of graphene, no striking diffraction peaks of RGO are observed in the XRD pattern of the composite, which may be covered by the strong diffraction peaks of BiFeO3 in the porous BiFeO3/RGO composite. Furthermore, no impurity-related peaks are found, confirming the high purity of the porous BiFeO3/RGO composite. In order to further study the chemical composition and surface state of porous BiFeO3/ RGO composite, XPS analysis was carried out, as shown in Fig 2. The porous BiFeO3/RGO composite consists mainly of Bi, C, O and Fe, with the respective core level spectra located at 158, 283, 531 and 719 eV, respectively (Fig 2a). The high-resolution C1s spectrum (Fig 2b) shows two major peaks at 284.4 eV and 286.3 eV, relating to the C-C/C=C and C-O groups, respectively, demonstrating the successful adding of RGO in the composite. The Bi 4f spectrum of composite
(Fig 2c) presents two main peaks at 158 and 163.8 eV, corresponding to the Bi 4f7/2 and Bi 4f5/2, respectively. The high-resolution Fe 2p spectrum (Fig 2d) exhibits a doublet peak at 711.8 eV (Fe 2p3/2) and 719.2 eV (Fe 2p1/2). 56 The O1s spectrum (Fig 2e) in the porous BiFeO3/RGO composite is split into three peaks at 530.1, 531.6 and 532.9 eV, which are attributed to surface-adsorbed H2O and oxygen-containing functional groups in graphene.55 The morphology and surface structure of all products were analyzed by FESEM (Fig 3). Fig 3a and 3b display the SEM images of pure BiFeO3 particles and most of the particles exhibit a uniform cylindrical shape. After etching, the porous BiFeO3 microspheres (Fig 3c, d) exhibit uniform flower-like nanoporous structure. As depicted in Fig 3e, f, porous BiFeO3 microspheres, with diameter of around 0.5-1 μm, are uniformly covered or wrapped on the graphene sheets. Particularly, the microstructure of porous BiFeO3 microspheres is not destroyed by the graphene addition. The above results fully demonstrate successful preparation of the 3D porous BiFeO3/ RGO composite, whose structure is highly beneficial to the multiple reflection of EM wave, improving the EM wave absorbing performances of the material. The detailed morphologies of porous BiFeO3 and porous BiFeO3/RGO composite were further characterized by TEM images. As shown in Fig 4a and 4b, the etched BiFeO3 microspheres present a typical spherical porous structure, which is agreement with SEM analysis. From TEM images (Fig 4c and 4d), it can be observed that the BiFeO3 microspheres in the BiFeO3/RGO display a flower-like structure and are dispersed on thin graphene sheets. To illustrate the porosity of etched BiFeO3, the samples were characterized by BET method. From Fig 5a, the samples show a type IV isotherm, revealing the existence of mesoporous. By BET calculating, the SBET of BiFeO3 particles, porous BiFeO3 microspheres are 1.39 m2/g and 33.5 m2/g. The pore size demonstrated in Fig 5b shows that
porous BiFeO3 microspheres have pore size of around 20-100 nm, which is larger than that of BiFeO3 particles. The pore volume for BiFeO3 particles, porous BiFeO3 microspheres is about 0.0072 cm3/g and 0.106 cm3/g, respectively. Fig 6 exhibits hysteresis curves of porous BiFeO3 and porous BiFeO3/RGO at room temperature, respectively. Owing to the non-magnetic graphene and magnetic BiFeO3, the magnetic properties of the composite mainly result from BiFeO3 microspheres. Hysteresis curves from porous BiFeO3 and porous BiFeO3/RGO show S-shaped (Fig 6), with negligible coercivity and residual magnetization, which demonstrate that the porous BiFeO3 and the porous BiFeO3/RGO possess superparamagnetism as well as the saturation magnetization of 1.46 and 0.71 emu g-1, respectively. Meanwhile, the saturation magnetization of the porous BiFeO3/RGO is lower than that of the porous BiFeO3 microspheres, which is ascribed to the presence of the non-magnetic graphene. The real (ε', µ'), imaginary (ε'', μ'') part of the permittivity and permeability, the dielectric loss (tan δε=ε''/ε') and magnetic loss (tan δμ=μ''/µ') of the porous BiFeO3 and porous BiFeO3/RGO are shown in Fig 7. Theoretically, ε' and μ' represent the storage ability of electric and magnetic energy, while ε'' and μ'' are related to loss capability of electric and magnetic energy.57, 58 As shown in Fig 7a, b, the ε' and ε'' values of porous BiFeO3/RGO are larger than porous BiFeO3 at 2~18 GHz. Based on Maxwell-Wagner theory, multiple polarizations generally include interfacial polarization and dipole polarization due to the layered porous BiFeO3 structure with unsaturated coordination bonds and the interface between the two materials, as well as dipole and defect polarization caused by some residual oxygen functional groups such as epoxy, hydroxyl and carbonyl in graphene.38 The above factors act as polarization centers, resulting in the enhancement
of ε'. Furthermore, both ε' and ε" values exhibit similar fluctuations within the range of 2-15 GHz and 4-16 GHz, respectively. The large number of vacancies present in the porous BiFeO3 microspheres generates the defects, resulting in multiple reflections and dipole polarization.52 It is clearly observed from Fig 7e that tanδε and ε'' values of the porous BiFeO3/RGO have a similar fluctuation tendency, and the tanδε values of porous BiFeO3/RGO are much larger than porous BiFeO3 in the range of 2.5-3.6 GHz, 6.5-10.6 GHz, 12.2-14.3 GHz and 15.4-18GHz, respectively, indicating that the dielectric loss is beneficial to enhancing the microwave absorption.59 In Fig 7c, the μ' values of two samples display almost the same tendency in the frequency range of 2-18 GHz, with similar fluctuation peaks in the range of 4.6-17.6 GHz. As demonstrated in Fig 7d and 7f, the μ'' and tanδμ values decrease significantly with the increasing frequency. Meanwhile, the μ'' and tanδμ values of porous BiFeO3/RGO are larger than porous BiFeO3 in the range of 2.5-18 GHz, revealing the stronger magnetic loss dielectric loss.60 Attenuation constant (α) is a key parameter to evaluate the EM wave attenuation capability. It can be calculated from the equation.61, 62
=
2 f ( " " ' ') ( " " ' ') 2 ( ' " " ') 2 c
(1)
It should be noted that the attenuation constant α represents the overall attenuation capability. As shown in Fig 7g, the attenuation constant α of porous BiFeO3/RGO is higher than porous BiFeO3 microspheres, which is ascribed to higher tanδε, tanδμ and further improves EM wave attenuation ability of porous BiFeO3 microspheres. Meanwhile, impedance matching is also a crucial parameter for the EM wave absorption. When the input impedance is close to the free impedance (Z=1), the EM wave can enter the interior of the absorber, meaning a superior EM wave absorption performance. The equation is follows 63, 64;
Z
Zin
Z0
r
2 fd r tanh j c
r r
(2)
Zin is the input impedance and Z0 (Z0= (μ0/ε0)1/2=377Ω) is the impedance of free space, Z is the normalized input impedance. From Fig 7h, we can obviously observe that the Z value of composite is equal to 1.0 at 14.5 GHz, complying with the position of maximum RL. To further characterize the EM wave absorption properties, we investigated the reflection losses (RL) of porous BiFeO3 and porous BiFeO3/RGO by the following equations; 65
RL (dB) 20log
Zin Z 0
( Zin Z 0 )
( Zin Z 0 )
2 fd r r r tanh j r c
(3)
(4)
Where f is the EM wave frequency, Zin is the input impedance, Z0 is the impedance of the free space, d is the thickness of absorber and c is the velocity of light. In Fig 8a, the porous BiFeO3 displays weak absorption properties and the maximum RL is only -17.6 dB at 9.7 GHz with the thickness of 4 mm. It can be observe from Fig 8b that the porous BiFeO3/RGO possesses a stronger EM wave absorption compared with the porous BiFeO3 in the range of 1.5-4 mm, which is due to the intensive interfacial polarization and impedance matching after the introduction of the graphene nanosheets.66 The maximum RL of porous BiFeO3/RGO can reach -46.7 dB at 14.5 GHz with an optimal thickness of only 1.8 mm and the corresponding bandwidth (RL≤-10 dB) is 4.7 GHz (12.0-16.7 GHz). Fig 8c exhibits the comparison of the maximum RL of EM waves at different thicknesses for porous BiFeO3 and porous BiFeO3/RGO. Compared with porous BiFeO3, porous BiFeO3/RGO shows a much higher RL at different thicknesses. It is observed clearly from Fig 8d that absorption bandwidth (RL≤-10 dB) of porous BiFeO3/RGO is 2.8, 4.7, 3.7, 4.5, 4.5, 2.1, and 1.6 GHz at the thickness of 1.5, 1.8, 2.0, 2.5, 3.0, 3.5, and 4.0 mm, respectively. The
absorption bandwidth of porous BiFeO3 is only 0.8, 1.7, 2.2, and 4.0 GHz at the thickness of 2.5, 3.0, 3.5, and 4.0 mm respectively. The above results show that, compared with porous BiFeO3, the porous BiFeO3/RGO exhibits a significant increase of the RL value and the absorption bandwidth. Fig 9a and 9b summarize the relation of maximum RL, bandwidth, and absorber thickness for some typical EM absorbing materials. It can be distinctly observed that the improving microwave absorption properties, with strong absorption, thin thickness and broad absorption band, make the porous BiFeO3/RGO be highly competitive with carbon materials and magnetic absorbers. Thus, the porous BiFeO3/RGO is a promising lightweight and high-performance absorber.
Conclusion In summary, the porous BiFeO3 and porous BiFeO3/RGO composite were successfully obtained by a facile etching method. Electromagnetic absorption measurements show that the porous BiFeO3/RGO composite possesses strong absorption and broad effective bandwidth even at very thin thickness. The maximum RL of porous BiFeO3/RGO composite can reach up to -46.7 dB at 14.5 GHz, and the corresponding bandwidth (RL≤-10 dB) is 4.7 GHz (12.0-16.7 GHz) at only 1.8 mm. Obviously, the remarkably improved EM absorption performance, in comparison with porous BiFeO3, is ascribed to effective complementarities, electron polarization, interfacial polarization, and unique 3D porous nanostructures. Hence, the porous BiFeO3/RGO composite possesses broad prospects as high-performance absorbers.
Acknowledgements
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No 61701386 and No 51303147), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No 2017JQ5060) and President’s Fund of Xi’an Technological University (project No. XAGDXJJ18008).
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Fig.1. XRD patterns of pure BiFeO3 particles (a), porous BiFeO3 microspheres (b) and porous BiFeO3/RGO (c) Fig.2. Survey scan (a), C 1s (b), Bi 4f (c), Fe 2p (d), O 1s (e) of porous BiFeO3/RGO Fig.3. FESEM images of pure BiFeO3 particles (a, b), porous BiFeO3 microspheres (c, d) and porous BiFeO3/RGO (e, f)
Fig.4. TEM images of porous BiFeO3 microspheres (a, b) and porous BiFeO3/RGO (c, d) Fig.5. N2 adsorption-desorption isotherms (a) and pore size distribution (b) of the samples Fig.6. Magnetic hysteresis curves of porous BiFeO3 microspheres (a) and porous BiFeO3/RGO (b) Fig.7. Complex permittivity (a, b), complex permeability (c, d), dielectric loss tangent (e), magnetic loss tangent (f), attenuation constant (g) and impedance matching (h) of the samples Fig.8. Calculated RL value of the porous BiFeO3 (a), porous BiFeO3/RGO (b), the comparison of the RL value (c) and the absorption bandwidth by the RL value exceeding -10 dB (d) of the porous BiFeO3 and porous BiFeO3/RGO Fig.9. Reflection loss versus bandwidth (a), Reflection loss versus thickness (b) for the typical EMW absorbing materials reported in recent literatures. Scheme 1 Scheme for the fabrication of porous BiFeO3 microspheres and porous BiFeO3/RGO by the etching process.
Fig. 1
Fig. 2a
Fig. 2b
Fig. 2c
Fig. 2d
Fig. 2e
Fig. 3
Fig. 4
Fig. 5a
Fig. 5b
Fig. 6
Fig. 7a
Fig. 7b
Fig. 7c
Fig. 7d
Fig. 7e
Fig. 7f
Fig. 7g
Fig. 7h
Fig. 8a
Fig. 8b
Fig. 8c
Fig. 8d
Fig. 9a
Fig. 9b
Scheme. 1
PII: DOI: Reference:
S0272-8842(18)33063-3 https://doi.org/10.1016/j.ceramint.2018.10.243 CERI19955
To appear in: Ceramics International Received date: 9 September 2018 Revised date: 27 October 2018 Accepted date: 29 October 2018 Cite this article as: Xiang Gao, Yan Wang, Qiguan Wang, Xinming Wu, Wenzhi Zhang, Meng Zong and Lijuan Zhang, Facile synthesis of a novel flower-like BiFeO3 microspheres/graphene with superior electromagnetic wave absorption p e r f o r m a n c e s , Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.243 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile synthesis of a novel flower-like BiFeO3 microspheres/graphene with superior electromagnetic wave absorption performances
Xiang Gaoa, Yan Wanga*, Qiguan Wanga, Xinming Wua, Wenzhi Zhanga, Meng Zongb, Lijuan Zhanga
a
School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, PR China
b
Department of Applied Chemistry and The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi'an 710072, PR China
*
Corresponding author. Tel.: +86 13909249145. [email protected] (Y. Wang)
Abstract In recent years, porous or layered magnetic materials have received increasing attention due to their low density and lightweight. In this work, porous BiFeO3 microspheres and three-dimensional porous BiFeO3 microsphere-reduced graphene oxide (RGO) composite (3D porous BiFeO3/RGO) were prepared by one-step etching processing using pure BiFeO3 particles as precursors. The precursor undergoes dissolution-recrystallization/reduction process, resulting in large amount of BiFeO3 fragments and graphene hybrid product, which forms 3D porous BiFeO3/RGO composite. Electromagnetic (EM) absorption performance measurements exhibit that at low thickness of 1.8 mm, porous BiFeO3/RGO composite can achieve reflection loss (RL) value up to -46.7 dB and absorption bandwidth (defined by RL <-10 dB) exceeding 4.7 GHz (from 12.0 to 16.7 GHz), testifying outstanding microwave absorbing performance. Compared with pure
porous BiFeO3, improved EM wave absorption ability of as-prepared porous BiFeO3/RGO composite is attributed to interfacial polarization, multiple reflections, scattering, and appropriate impedance matching.
Keywords: Graphene; BiFeO3; Three-dimensional composite; Electromagnetic wave absorption
1. Introduction With the increasing use of electromagnetic (EM) waves in radars, advanced detectors, and precision weapons, the EM wave pollution has become increasingly serious.1-4 Therefore, there is an urgent need in investigating EM wave absorbing materials. EM wave absorbing material is a kind of functional material that can effectively absorb the incident EM waves and convert the EM energy into heat or other forms of energy.5, 6 The ideal wave-absorbing material would combine high absorption, wide frequency band, low density, thin thickness, and good environmental stability.7-11 However, the traditional ferrite-based wave-absorbing material can difficultly meet the above-mentioned requirements because of their high density, narrow absorption bands and too weak absorption.12-14 The development of graphene has opened up a new research field for absorbing materials.15 The unique two-dimensional structure of graphene gives it a large specific surface area that promotes scattering and multiple reflections of EM waves, and improves their absorption.16-18 At the same time, when graphene is prepared by the reduction method, defects generated on the graphene sheets and residual oxygen-containing functional groups are prone to generate polarization relaxation and electric dipole relaxation under the action of an external
electric field, so that the dielectric loss increases.19, 20 The absorption mechanism on a single graphene sheet mainly relies on dielectric loss.21, 22 According to the EM complementarity theory, except for the single dielectric loss or magnetic loss, the impedance matching between permittivity and permeability also affects the microwave absorption performances of a material. Nevertheless, despite of its high permittivity, RGO results in a poor EM wave absorption abilities because of its weak impedance matching.23-26 Plenty of studies have confirmed that single-loss type or single-component absorbers can difficultly ensure high absorption in the 2-18 GHz range. Multi-loss matching and multi-component absorption are good strategies to possibly solve this problem.27-29 Nowadays, one of the most effective ways to improve the absorbing properties is to combine graphene with porous or layered magnetic materials, such as Fe3O4,30-32 Co3O4,33, 34 Co,35, 36
NiFe2O4,37 Bi2Fe4O9,38 or Fe2O3,39 CoFe2O4,40 or Fe3O4/Fe.41 Thanks to its large specific surface
area, high surface atomic ratio and high number of dangling bonds, the porous or layered magnetic material ensures interfacial polarization and multiple scattering of the EM wave. Specifically, the incidence of an EM wave on the porous magnetic material surface generates interfacial polarization and multiple scattering that will rapidly lower its intensity.42-45 The graphene prepared by the chemical reduction method has a good deal of oxygen-containing functional groups on its surface, which can easily be combined with other magnetic loss absorbents.46, 47 The resulting chemical modification of the magnetic loss absorbent can give rise to synergistic effects with the graphene and improve the impedance matching, thus increasing the magnetic loss of the composite.48-51 In this work, we used N,N-dimethylformamide (DMF) as a solvent and reducing agent
(hydrazine) to prepare porous BiFeO3/RGO composite under N2 environment by simultaneous dissolution-recrystallization/reduction method, as shown in scheme 1. The porous BiFeO3/RGO with a thickness of 1.8 mm shows strongest RL value of -46.7 dB at 13.8GHz with the absorption bandwidth of 4.7 GHz (12.0 GHz-16.7 GHz). The 3D porous structure of BiFeO3/RGO results in an excellent EM absorption even at very low thickness. This property is ascribed to the high electrical conductivity and high specific surface area of RGO as well as the 3D structure of porous BiFeO3 microspheres.
2. Experimental 2.1 Preparation of porous BiFeO3/RGO The pure BiFeO3 nano-particles were prepared by the hydrothermal reaction as reported previously.52 Graphene oxide (GO) was obtained by the improved Hummers method.53 The 3D porous BiFeO3/RGO composite was prepared by a facile etching method. Pure BiFeO3 (0.5 g) and GO (0.5 mg) nanosheets were dissolved into DMF (150 ml) under ultrasounds. Afterwards, the solution was added into 6 mL hydrazine and 1.5 mL methyl mercaptoacetate under N2 protection for 30 min. Then, the mixed solution was placed in a water bath at 80°C for 45 min. Finally, the reaction was ended by cold ethanol addition, and the precipitate was washed multiple times with ethanol and deionized water, and then dried at 60℃. Meanwhile, 3D porous BiFeO3 microspheres were also obtained using the above route without adding GO nanosheets. 2.2 Characterization The as-obtained products were investigated by X-ray diffraction (XRD, German Bruker D8), X-ray photoelectron spectroscopy (XPS, ESCALAB 250), field-emission scanning electron microscope (FESEM, Quanta 600FEG), transmission electron microscope (TEM, JTM-2100),
Brunner-Emmet-Teller (Quad-rasorb-SI instrument) and vibrating sample magnetometer (VSM, Lake Shore7307). The electromagnetic (EM) parameters were measured by a vector network analyzer (HP8720ES) at 2–18 GHz. 30 wt% samples and 70 wt% paraffin were pressed into a cylindrical shape specimen (φin=3.04 mm and φout=7 mm) for EM measurement.
3. Result and discussion The structure and phase composition of all samples were tested by XRD. Fig 1a displays the diffraction pattern of pure BiFeO3 nanoparticles, which is consistent with the standard card (JCPDS no.20-0169).54 Fig 1b and 1c exhibit the XRD patterns of porous BiFeO3 and porous BiFeO3/RGO composite, respectively. We can evidently observe that all the diffraction peaks of pure BiFeO3 nanoparticles are preserved in the porous BiFeO3 and porous BiFeO3/RGO composite. Nevertheless, the intensities of all peaks are significantly weakened, which may be attributed to the presence of a large amount nanoscale debris in the etched BiFeO3.55 After the addition of graphene, no striking diffraction peaks of RGO are observed in the XRD pattern of the composite, which may be covered by the strong diffraction peaks of BiFeO3 in the porous BiFeO3/RGO composite. Furthermore, no impurity-related peaks are found, confirming the high purity of the porous BiFeO3/RGO composite. In order to further study the chemical composition and surface state of porous BiFeO3/ RGO composite, XPS analysis was carried out, as shown in Fig 2. The porous BiFeO3/RGO composite consists mainly of Bi, C, O and Fe, with the respective core level spectra located at 158, 283, 531 and 719 eV, respectively (Fig 2a). The high-resolution C1s spectrum (Fig 2b) shows two major peaks at 284.4 eV and 286.3 eV, relating to the C-C/C=C and C-O groups, respectively, demonstrating the successful adding of RGO in the composite. The Bi 4f spectrum of composite
(Fig 2c) presents two main peaks at 158 and 163.8 eV, corresponding to the Bi 4f7/2 and Bi 4f5/2, respectively. The high-resolution Fe 2p spectrum (Fig 2d) exhibits a doublet peak at 711.8 eV (Fe 2p3/2) and 719.2 eV (Fe 2p1/2). 56 The O1s spectrum (Fig 2e) in the porous BiFeO3/RGO composite is split into three peaks at 530.1, 531.6 and 532.9 eV, which are attributed to surface-adsorbed H2O and oxygen-containing functional groups in graphene.55 The morphology and surface structure of all products were analyzed by FESEM (Fig 3). Fig 3a and 3b display the SEM images of pure BiFeO3 particles and most of the particles exhibit a uniform cylindrical shape. After etching, the porous BiFeO3 microspheres (Fig 3c, d) exhibit uniform flower-like nanoporous structure. As depicted in Fig 3e, f, porous BiFeO3 microspheres, with diameter of around 0.5-1 μm, are uniformly covered or wrapped on the graphene sheets. Particularly, the microstructure of porous BiFeO3 microspheres is not destroyed by the graphene addition. The above results fully demonstrate successful preparation of the 3D porous BiFeO3/ RGO composite, whose structure is highly beneficial to the multiple reflection of EM wave, improving the EM wave absorbing performances of the material. The detailed morphologies of porous BiFeO3 and porous BiFeO3/RGO composite were further characterized by TEM images. As shown in Fig 4a and 4b, the etched BiFeO3 microspheres present a typical spherical porous structure, which is agreement with SEM analysis. From TEM images (Fig 4c and 4d), it can be observed that the BiFeO3 microspheres in the BiFeO3/RGO display a flower-like structure and are dispersed on thin graphene sheets. To illustrate the porosity of etched BiFeO3, the samples were characterized by BET method. From Fig 5a, the samples show a type IV isotherm, revealing the existence of mesoporous. By BET calculating, the SBET of BiFeO3 particles, porous BiFeO3 microspheres are 1.39 m2/g and 33.5 m2/g. The pore size demonstrated in Fig 5b shows that
porous BiFeO3 microspheres have pore size of around 20-100 nm, which is larger than that of BiFeO3 particles. The pore volume for BiFeO3 particles, porous BiFeO3 microspheres is about 0.0072 cm3/g and 0.106 cm3/g, respectively. Fig 6 exhibits hysteresis curves of porous BiFeO3 and porous BiFeO3/RGO at room temperature, respectively. Owing to the non-magnetic graphene and magnetic BiFeO3, the magnetic properties of the composite mainly result from BiFeO3 microspheres. Hysteresis curves from porous BiFeO3 and porous BiFeO3/RGO show S-shaped (Fig 6), with negligible coercivity and residual magnetization, which demonstrate that the porous BiFeO3 and the porous BiFeO3/RGO possess superparamagnetism as well as the saturation magnetization of 1.46 and 0.71 emu g-1, respectively. Meanwhile, the saturation magnetization of the porous BiFeO3/RGO is lower than that of the porous BiFeO3 microspheres, which is ascribed to the presence of the non-magnetic graphene. The real (ε', µ'), imaginary (ε'', μ'') part of the permittivity and permeability, the dielectric loss (tan δε=ε''/ε') and magnetic loss (tan δμ=μ''/µ') of the porous BiFeO3 and porous BiFeO3/RGO are shown in Fig 7. Theoretically, ε' and μ' represent the storage ability of electric and magnetic energy, while ε'' and μ'' are related to loss capability of electric and magnetic energy.57, 58 As shown in Fig 7a, b, the ε' and ε'' values of porous BiFeO3/RGO are larger than porous BiFeO3 at 2~18 GHz. Based on Maxwell-Wagner theory, multiple polarizations generally include interfacial polarization and dipole polarization due to the layered porous BiFeO3 structure with unsaturated coordination bonds and the interface between the two materials, as well as dipole and defect polarization caused by some residual oxygen functional groups such as epoxy, hydroxyl and carbonyl in graphene.38 The above factors act as polarization centers, resulting in the enhancement
of ε'. Furthermore, both ε' and ε" values exhibit similar fluctuations within the range of 2-15 GHz and 4-16 GHz, respectively. The large number of vacancies present in the porous BiFeO3 microspheres generates the defects, resulting in multiple reflections and dipole polarization.52 It is clearly observed from Fig 7e that tanδε and ε'' values of the porous BiFeO3/RGO have a similar fluctuation tendency, and the tanδε values of porous BiFeO3/RGO are much larger than porous BiFeO3 in the range of 2.5-3.6 GHz, 6.5-10.6 GHz, 12.2-14.3 GHz and 15.4-18GHz, respectively, indicating that the dielectric loss is beneficial to enhancing the microwave absorption.59 In Fig 7c, the μ' values of two samples display almost the same tendency in the frequency range of 2-18 GHz, with similar fluctuation peaks in the range of 4.6-17.6 GHz. As demonstrated in Fig 7d and 7f, the μ'' and tanδμ values decrease significantly with the increasing frequency. Meanwhile, the μ'' and tanδμ values of porous BiFeO3/RGO are larger than porous BiFeO3 in the range of 2.5-18 GHz, revealing the stronger magnetic loss dielectric loss.60 Attenuation constant (α) is a key parameter to evaluate the EM wave attenuation capability. It can be calculated from the equation.61, 62
=
2 f ( " " ' ') ( " " ' ') 2 ( ' " " ') 2 c
(1)
It should be noted that the attenuation constant α represents the overall attenuation capability. As shown in Fig 7g, the attenuation constant α of porous BiFeO3/RGO is higher than porous BiFeO3 microspheres, which is ascribed to higher tanδε, tanδμ and further improves EM wave attenuation ability of porous BiFeO3 microspheres. Meanwhile, impedance matching is also a crucial parameter for the EM wave absorption. When the input impedance is close to the free impedance (Z=1), the EM wave can enter the interior of the absorber, meaning a superior EM wave absorption performance. The equation is follows 63, 64;
Z
Zin
Z0
r
2 fd r tanh j c
r r
(2)
Zin is the input impedance and Z0 (Z0= (μ0/ε0)1/2=377Ω) is the impedance of free space, Z is the normalized input impedance. From Fig 7h, we can obviously observe that the Z value of composite is equal to 1.0 at 14.5 GHz, complying with the position of maximum RL. To further characterize the EM wave absorption properties, we investigated the reflection losses (RL) of porous BiFeO3 and porous BiFeO3/RGO by the following equations; 65
RL (dB) 20log
Zin Z 0
( Zin Z 0 )
( Zin Z 0 )
2 fd r r r tanh j r c
(3)
(4)
Where f is the EM wave frequency, Zin is the input impedance, Z0 is the impedance of the free space, d is the thickness of absorber and c is the velocity of light. In Fig 8a, the porous BiFeO3 displays weak absorption properties and the maximum RL is only -17.6 dB at 9.7 GHz with the thickness of 4 mm. It can be observe from Fig 8b that the porous BiFeO3/RGO possesses a stronger EM wave absorption compared with the porous BiFeO3 in the range of 1.5-4 mm, which is due to the intensive interfacial polarization and impedance matching after the introduction of the graphene nanosheets.66 The maximum RL of porous BiFeO3/RGO can reach -46.7 dB at 14.5 GHz with an optimal thickness of only 1.8 mm and the corresponding bandwidth (RL≤-10 dB) is 4.7 GHz (12.0-16.7 GHz). Fig 8c exhibits the comparison of the maximum RL of EM waves at different thicknesses for porous BiFeO3 and porous BiFeO3/RGO. Compared with porous BiFeO3, porous BiFeO3/RGO shows a much higher RL at different thicknesses. It is observed clearly from Fig 8d that absorption bandwidth (RL≤-10 dB) of porous BiFeO3/RGO is 2.8, 4.7, 3.7, 4.5, 4.5, 2.1, and 1.6 GHz at the thickness of 1.5, 1.8, 2.0, 2.5, 3.0, 3.5, and 4.0 mm, respectively. The
absorption bandwidth of porous BiFeO3 is only 0.8, 1.7, 2.2, and 4.0 GHz at the thickness of 2.5, 3.0, 3.5, and 4.0 mm respectively. The above results show that, compared with porous BiFeO3, the porous BiFeO3/RGO exhibits a significant increase of the RL value and the absorption bandwidth. Fig 9a and 9b summarize the relation of maximum RL, bandwidth, and absorber thickness for some typical EM absorbing materials. It can be distinctly observed that the improving microwave absorption properties, with strong absorption, thin thickness and broad absorption band, make the porous BiFeO3/RGO be highly competitive with carbon materials and magnetic absorbers. Thus, the porous BiFeO3/RGO is a promising lightweight and high-performance absorber.
Conclusion In summary, the porous BiFeO3 and porous BiFeO3/RGO composite were successfully obtained by a facile etching method. Electromagnetic absorption measurements show that the porous BiFeO3/RGO composite possesses strong absorption and broad effective bandwidth even at very thin thickness. The maximum RL of porous BiFeO3/RGO composite can reach up to -46.7 dB at 14.5 GHz, and the corresponding bandwidth (RL≤-10 dB) is 4.7 GHz (12.0-16.7 GHz) at only 1.8 mm. Obviously, the remarkably improved EM absorption performance, in comparison with porous BiFeO3, is ascribed to effective complementarities, electron polarization, interfacial polarization, and unique 3D porous nanostructures. Hence, the porous BiFeO3/RGO composite possesses broad prospects as high-performance absorbers.
Acknowledgements
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No 61701386 and No 51303147), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No 2017JQ5060) and President’s Fund of Xi’an Technological University (project No. XAGDXJJ18008).
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Fig.1. XRD patterns of pure BiFeO3 particles (a), porous BiFeO3 microspheres (b) and porous BiFeO3/RGO (c) Fig.2. Survey scan (a), C 1s (b), Bi 4f (c), Fe 2p (d), O 1s (e) of porous BiFeO3/RGO Fig.3. FESEM images of pure BiFeO3 particles (a, b), porous BiFeO3 microspheres (c, d) and porous BiFeO3/RGO (e, f)
Fig.4. TEM images of porous BiFeO3 microspheres (a, b) and porous BiFeO3/RGO (c, d) Fig.5. N2 adsorption-desorption isotherms (a) and pore size distribution (b) of the samples Fig.6. Magnetic hysteresis curves of porous BiFeO3 microspheres (a) and porous BiFeO3/RGO (b) Fig.7. Complex permittivity (a, b), complex permeability (c, d), dielectric loss tangent (e), magnetic loss tangent (f), attenuation constant (g) and impedance matching (h) of the samples Fig.8. Calculated RL value of the porous BiFeO3 (a), porous BiFeO3/RGO (b), the comparison of the RL value (c) and the absorption bandwidth by the RL value exceeding -10 dB (d) of the porous BiFeO3 and porous BiFeO3/RGO Fig.9. Reflection loss versus bandwidth (a), Reflection loss versus thickness (b) for the typical EMW absorbing materials reported in recent literatures. Scheme 1 Scheme for the fabrication of porous BiFeO3 microspheres and porous BiFeO3/RGO by the etching process.
Fig. 1
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Scheme. 1