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Metal-organic framework nanoparticles decorated with graphene: A high-performance electromagnetic wave absorber
Author’s Accepted Manuscript Metal-organic framework nanoparticles decorated with graphene: A high-performance electromagnetic wave absorber Yan Wang, Wenzhi Zhang, Xinming Wu, Chunyan Luo, Tan Liang, Gang Yan www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(16)30537-6 http://dx.doi.org/10.1016/j.jmmm.2016.04.093 MAGMA61422
To appear in: Journal of Magnetism and Magnetic Materials Received date: 21 February 2016 Revised date: 3 April 2016 Accepted date: 28 April 2016 Cite this article as: Yan Wang, Wenzhi Zhang, Xinming Wu, Chunyan Luo, Tan Liang and Gang Yan, Metal-organic framework nanoparticles decorated with graphene: A high-performance electromagnetic wave absorber, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.04.093 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.
Metal-organic framework nanoparticles decorated with graphene: a high-performance electromagnetic wave absorber Yan Wang1, Wenzhi Zhang, Xinming Wu, Chunyan Luo, Tan Liang, Gang Yan School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, PR China
Abstract: A novel metal organic framework (MOF) coated RGO was fabricated by a one-step method. The morphology and microstructure of MOF-53(Fe)/RGO composite were characterized by XRD and TEM. The electromagnetic parameters indicate that MOF-53(Fe)/RGO composite shows enhanced electromagnetic absorption properties compared with MOF-53(Fe). The maximum RL can reach -25.8 dB at 15.4 GHz and the absorption bandwidth with the reflection loss exceeding -10 dB is 5.9 GHz (from 12.1 to 18 GHz) with the thickness of 2 mm. The possible absorption mechanism was also investigated in detail. Our results indicate the potential application of MOF/RGO composite as a more efficient microwave absorber. Keywords: Metal-organic frameworks; Carbon materials; Porous structure; Magnetic materials; Microwave absorption properties;
1．Introduction Electromagnetic waves have become a serious problem in various fields which may not only interrupt electronic devices, but also threatens human life [1]. To solve these problems, many researchers are paying great attention to the development of electromagnetic absorption materials. Recently, graphene has attracted considerable attention for achieving high-performance 1
Corresponding author: Tel: +86 13909249145
E-mail address: [email protected]
electromagnetic absorber due to the residual defects, larger interface, high permittivity and low density [2-3]. Nevertheless, the sole graphene suffers from impedance mismatch owing to high conductivity, which results in weak absorption and narrow absorption frequency [4]. Therefore, various magnetic materials such as Fe3O4 [5], Ni [6], NiFe2O4 [7] and CoFe2O4 [8], etc have been used to decorate reduced graphene oxide (RGO) to enhance electromagnetic attenuation, which contributes to a proper impedance matching between permittivity and permeability. Meanwhile, carbon materials such as MWCNT [9] and PANI [10] also have been added into the surface of RGO to improve the microwave absorption due to the formation of conducting network and unique structural characteristics. Recently, researchers have studied the microwave absorption properties of the multi-element composites due to their multi-functional electrical and magnetic properties, as well as multiple reflections and interfacial polarizations, such as rGO/Carbon Microspheres/rGO Composite [11] and CuS/Magnetically Decorated Graphene [12]. Metal organic frameworks (MOF) are a class of porous materials, which has attracted considerable attention because of high surface area, low density, diverse structural topologies as well as many functionalities [13]. The fascinating properties make MOF used in a variety of application areas, including gas storage, catalysts, sensors and supercapacitors [14-17]. The incorporation of MOF into graphene materials is an effective approach to improve the microwave absorbing properties of graphene. However, to the best of our knowledge, microwave absorption properties of MOF decorated with graphene have never been reported. In this work, we directly grow MOF-53(Fe) on the surface of graphene by a one-step hydrothermal strategy. Structural and morphological of MOF-53(Fe)/RGO composite have been investigated. The MOF-53(Fe)/RGO composite can obtain good electromagnetic properties
compared with MOF-53(Fe).
2．Experimental The formation mechanism of MOF-53(Fe)/RGO composite can be explained as Fig. 1. Firstly, Graphene oxide (GO) was prepared by Hummer’s method [18]. Secondly, GO was dispersed in 64.7 ml DMF (1 mg/ml) by sonication treatment, then 0.81 g FeCl3·6H2O and 0.5 g terephthalic acid were added and stirred for 30 min. Lastly, the solution was transferred into a 100 ml Teflon-lined stainless steel autoclave and kept in an oven at 150 ℃ for 12 h. For comparison, MOF-53(Fe) was prepared via a similar hydrothermal process without GO. After cooling down to room temperature, the products were washed with deionized water and ethanol, then dried at 150 ℃ under vacuum for 8 h. The structure was analyzed by X-ray diffraction (XRD) patterns (German Bruker D8 with Cu-Kα radiation). Transmission electron microscopy (TEM, American FEI F30 G2) was employed to analyze the morphology and the size of the samples. The N2 adsorption-desorption isotherms were measured on a Quad-rasorb-SI instrument, and the specific surface area was determined by the Brunauer-Emmett-Teller (BET) method. The magnetic properties were measured using a vibrating sample magnetometer (VSM, Lake Shore7307) with a maximum applied field of 13500 Oe. The electromagnetic parameters were analyzed by using a HP8720ES vector network analyzer in the range of 2-18 GHz.
3. Results and discussion The morphology of sample was investigated by TEM, as shown in Fig. 2 (a-d). The MOF-53(Fe) particles consist entirely of polyhedrons with an average edge length of around 500 nm which indicates the well crystalline nature of MOF-53(Fe). The MOF-53(Fe) polyhedrons are
high dispersible and anchored on graphene surface. The RGO nanosheets are transparent and wrinkled, indicating that RGO is a few atomic layers in thickness and is of good quality. Furthermore, no bare MOF-53(Fe) particles or RGO can be observed, which confirms the strong interfacial bonding between MOF-53(Fe) particles and RGO. Fig. 2e shows the XRD patterns of GO、RGO、MOF-53(Fe) and MOF-53(Fe)/RGO. The XRD patterns of both MOF-53(Fe) particles and MOF-53(Fe)/RGO composite can be perfectly assigned to the phase of MOF-53(Fe) [19]. The diffraction peak of MOF-53(Fe)/RGO composite had remained unchanged after incorporation of the graphene, which indicates that there was no apparent loss of crystallinity and decomposition of the framework structure. Furthermore, the broad peak of RGO at 24.50 can be attributed to the graphite-like structure, and no diffraction peaks resulting from GO (at 10.50) can be found, which means that oxygen groups have been removed and GO is effectively reduced to RGO [20]. To confirm the microstructure of porous MOF-53(Fe), nitrogen adsorption-desorption measurements were performed to investigate the pore diameter and surface area. Fig. 2 (f-g) shows the nitrogen adsorption-desorption isotherm at 77 K and pore diameter distribution of MOF-53(Fe). It can be seen in Fig. 2f that the hysteresis loop of MOF-53(Fe) is formed between 0.8P/P0 and 1.0 P/P0, indicating the existence of meso-pore (2-50 nm) and macro-pore (>50 nm). So, the synthesized MOF-53(Fe) is a kind of the IV-type material [21]. From Fig.2g, the average pore diameter, volume and BET surface area are 134.22 nm, 0.11 cm3 g-1, 22.36 m2 g-1, respectively. Fig. 2h shows the typical magnetization curves of MOF-53(Fe) and MOF-53(Fe)/RGO composite measured at room temperature. The magnetization hysteresis loops of MOF-53(Fe) and MOF-53(Fe)/RGO composite show S-like, indicating the nature of typical superparamagnetic material. The saturation magnetization (Ms) is 13.38 emu/g for MOF-53(Fe), and 6.21 emu/g for
the MOF-53(Fe)/RGO composite respectively. The Ms value of MOF-53(Fe)/RGO is lower than that of the MOF-53(Fe), which can be attributed to the existence of non-magnetic RGO. Fig.3a shows the real part (ε΄) and imaginary part (ε˝) of MOF-53(Fe) and MOF-53(Fe)/ RGO composite. It can be observed that the ε΄ and ε˝ values of MOF-53(Fe)/RGO decrease gradually from 11.3 to 5.1 and 6.2 to 2.6, respectively, with several fluctuations in the frequency range of 2-18 GHz. Moreover, the ε΄ and ε˝ values of MOF-53(Fe) have almost no change. The ε˝ values of MOF-53(Fe)/RGO are higher than MOF-53(Fe), implying that the strong dielectric loss is responsible for electromagnetic wave absorption properties of MOF-53(Fe)/RGO. In Fig.3b, we can see that the µ΄ values of all samples exhibit several variations at 2-18 GHz. Meanwhile, the imaginary part (µ˝) of MOF-53(Fe)/RGO declines gradually at 2-12 GHz, and then exhibits broad resonance peaks at 12-15.5 GHz with a maximum value of 0.41 at 14.8 GHz. The imaginary part (µ˝) of MOF-53(Fe) has a similar curve with MOF-53(Fe)/RGO and the broad resonance peak locates at 12.9-15.5 GHz with a maximum value of 0.41 at 14.1 GHz. According to the naturalresonance equation [22]: 2πfr=rHa
(1)
Ha=4｜K1｜/3µ0MS
(2)
where r is the gyromagnetic ratio, Ha is the anisotropy energy, and ｜K1｜is the anisotropy coefficient. The high resonance frequencies of MOF-53(Fe)/RGO and MOF-53(Fe) are attributed to the small size effect and the confinement effect. It is believed that the anisotropy energy of nanometer scale size materials would be remarkably increased due to the surface anisotropic field by the small size effect. The µ˝ values of MOF-53(Fe) are slightly larger than MOF-53(Fe)/RGO, which indicates a higher magnetic loss. From Fig.3(c-d), we can see that the tanδε values of MOF-53(Fe)/RGO are much higher than MOF-53(Fe) over 2-18 GHz, while the tanδm values of
MOF-53(Fe) are slightly larger than MOF- 53(Fe)/RGO, indicating that the microwave absorption mechanism of MOF-53(Fe)/RGO composite is mainly dependent on the dielectric loss. The phenomenon can be explained in the following factors. In terms of the electromagnetic theory, the dielectric loss of MOF-53(Fe)/RGO composite may be attributed to the layered structure, natural resonance, Debye dipolar relaxation and electron polarization etc [23]. On one hand, according to the free electron theory, ε˝=1/2ε0πρf, where ε0 is the permittivity of a vacuum, ρ is the resistivity, f is the frequency [24]. The conductivity of RGO is high, which enables a reduction of the resistivity, and results in the increase of the dielectric loss. On the other hand, RGO nanosheets form a conducting network, which migrating and hopping electron may enhance eddy current between RGO and MOF-53(Fe). This is why MOF-53(Fe)/RGO composite has a higher dielectric loss. The excellent electromagnetic wave absorption performance is mainly ascribed to two key factors: electromagnetic wave attenuation and impedance matching. The high dielectric and magnetic loss only suggest electromagnetic wave can be absorbed by materials, which means enhanced electromagnetic wave attenuation. The impedance matching indicates that electromagnetic wave can maximum enter material interior without being reflected by material surface and the characteristic impedance of the absorbing materials should be close to that of the free space (377 sq-1) to achieve zero reflection on the surface of the materials [25]. Only to satisfy the two conditions at the same time, microwave absorbing materials can achieve broad band and strong absorption peak. In order to investigate the microwave absorption properties of the samples, the reflection losses (RL) of MOF-53(Fe) and MOF-53(Fe)/RGO composite are calculated as follows [26]:
R(dB)=20 lg |
Zin 1 | Zin 1
(3)
Zin
r 2fd tanh( j r r ) r c
(4)
where Zin is the input impedance of the absorber, f is the frequency, d is the thickness of the absorb layer, c is the velocity of electromagnetic wave in vacuum. In Fig.3e, we can see that MOF-53(Fe) exhibits poor microwave absorbing properties with the thickness of 1.5-4 mm, the maximum RL is only -13.7 dB at 14.4 GHz. As shown in Fig.3f, after the introduction of graphene, the maximum RL reaches -25.8 dB at 15.4 GHz and the absorption bandwidth corresponding to RL at -10 dB is 5.9 GHz (from 12.1 to 18 GHz) for a layer of 2 mm thickness. Since the absorber with broad peak (RL values less than -10 dB are 5.9 GHz) can be designed to attenuate microwave, the MOF53(Fe)/RGO composite are very promising for new types of microwave absorption materials. In addition, it is noted that the attenuation peaks transfer to the low frequency with the increasing of thickness. It can be explained as follows [27]:
d nm / 4 m
(n=1, 3, 5, 7, 9……)
c f
(5) (6)
where ε and µ are the complex relative permittivity and permeability of the absorber, respectively. So, the matching thickness of the composites has an important influence on the microwave attenuation. These results demonstrate that the microwave absorption properties of MOF-53(Fe)/ RGO composite are superior to those of MOF-53(Fe). Fig.4 shows the schematic illustration of the absorption mechanism for MOF-53(Fe)/RGO composite. The excellent microwave absorbing performance of MOF-53(Fe)/RGO composite can be explained by the following facts. Firstly, the introduction of RGO has enhanced the εr of the MOF-53(Fe), and increased the tanδε, which helps to improve the impedance matching. Secondly,
the enormous aspect ratio, layered structure, residual defects and groups of the MOF-53(Fe)/RGO can cause multiple reflections and scattering, which will further enhance the microwave absorption properties [20]. Meanwhile, the high surface area of RGO can also provide more active sites for dissipating and scattering of the electromagnetic wave [28]. Thirdly, the interfacial polarization, dipole polarization and charge transfer between MOF-53(Fe) and RGO can convert electromagnetic energy into heat energy [2]. The porous MOF-53(Fe) particles possess many pores, which can be served as point defects and induces dipole moment, which provides enhanced interfacial dipole polarizations to increase dielectric relaxations and is beneficial to microwave absorption [29]. Furthermore, there are innumerable interfaces between MOF-53(Fe) and RGO and leads to extra interfacial polarization [30]. Lastly, the special porous structure of MOF-53(Fe) also makes a contribution to better impedance matching with free space than the corresponding solid materials and the MOF-53(Fe) with an interior space can also lead to multiple scattering to enlarge energy attenuation, which is vital for the microwave absorption performance. Besides this, porous structure of MOF-53(Fe) is beneficial to suppressing the eddy current loss of magnetic particles and maintaining high permeability at high frequency [31]. In general, the enhanced microwave absorption property of MOF-53(Fe)/RGO composite is attributed to the complementary effect of the MOF-53(Fe) and RGO.
4. Conclusion In summary, we developed a facile route to synthesize MOF/RGO composite and the microwave absorption properties have been investigated. The synthesized porous MOF/RGO composite exhibited excellent electromagnetic absorption properties. The maximum RL is up to -25.8 dB at 15.4 GHz and the absorption bandwidths exceeding -10 dB are 12.2 GHz (from 5.8 to
18 GHz) with a thickness in the range of 1.5-4 mm. Notably, the MOF-carbon based materials can be developed as an attractive candidate for applications as a microwave absorber.
Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No 51303147 and No 21506167) and the National college students’ innovative training plan (Grant No 201510702015).
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[16] W.H. Li, X.F. Wu, N. Han, et al. Sens Actuators B 225 (2016) 158-166. [17] Y. Zhao, Z.X. Song, X. Li, et al. Energy Storage Mater 2 (2016) 35-62. [18] Y. Wang, Y. Huang, Q.F. Wang, et al. Powder Technol 249 (2013) 304-308. [19] J. Gordon, H. Kazemian, S. Rohani. Mater Sci Eng C 47 (2015) 172-179. [20] Y. Wang, X.M. Wu, W.Z. Zhang, et al. Synth Met 210 (2015) 165-170. [21] Q.F. Wang, Y. Huang, Y. Zhao, et al. Surf Interface Anal 45 (2013) 1297-1303. [22] Y.J. Chen, P. Gao, R.X. Wang, et al. J Phys Chem C 113 (2009) 10061-10064. [23] D.Z. Chen, G.S. Wang, S. He, et al. J Mater Chem A 1 (2013) 5996-6003. [24] Y.J. Chen, G. Xiao, T.S. Wang, et al. J Phys Chem C 115 (2011) 13603-13608. [25] B. Zhao, G. Shao, B.B. Fan, et al. Mater Lett 121 (2014) 118-121. [26] X. Bai, Y.H. Zhai, Y. Zhang. J. Phys. Chem. C 115 (2011) 11673-11677. [27] A.N. Yusoff, M.H. Abdullah, S.H. Ahmad, et al. J Appl Phys 92 (2002) 876-882. [28] C.W. Zou, Y.D. Yao, N.D. Wei, et al. Composites Part B 77 (2015) 209-214. [29] Y.Y. Ren, L. Yang, L.D. Wang, et al. Powder Technol 281 (2015) 20-27. [30] D.P. Sun, Q. Zou, Y.P. Wang, et al. Nanoscale 6 (2014) 6557-6562. [31] C.Z. He, S. Qiu, X.Z. Wang, et al. J Mater Chem 22 (2012) 22160-22166.
Figure captions Fig. 1. Schematic illustration of the synthesis procedure of MOF-53(Fe)/RGO composite Fig. 2. TEM images of MOF-53(Fe) (a-b) and MOF-53(Fe)/RGO composite (c-d), XRD patterns (e) of GO、RGO、MOF-53(Fe) and MOF-53(Fe)/RGO composite, Nitrogen adsorption-desorption isotherm (f) and pore diameter distribution (g) of MOF-53(Fe), magnetization curves (h) of
MOF-53(Fe) and MOF-53(Fe)/RGO composite Fig. 3. The ε΄ and ε˝ values (a), µ΄ and µ˝ values (b), tanδε (c), tanδm (d) of MOF-53(Fe) and MOF-53(Fe)/RGO composite, the reflection loss curves of MOF-53(Fe) (e) and MOF-53(Fe)/ RGO composite (f) Fig. 4. Schematic illustration of the absorption mechanism for MOF-53(Fe)/RGO composite
Highlights • MOF-53(Fe)/RGO composite was fabricated by a one-step hydrothermal strategy. • The morphology, microstructure, magnetic and electromagnetic properties were investigated. • The maximum reflection loss of MOF-53(Fe)/RGO composite can reach -25.8 dB at 15.4 GHz with the thickness of 2 mm. • The absorption bandwidths exceeding -10 dB are 12.2 GHz (from 5.8 to 18 GHz) in the range of 1.5-4 mm.
Graphical abstract
PII: DOI: Reference:
S0304-8853(16)30537-6 http://dx.doi.org/10.1016/j.jmmm.2016.04.093 MAGMA61422
To appear in: Journal of Magnetism and Magnetic Materials Received date: 21 February 2016 Revised date: 3 April 2016 Accepted date: 28 April 2016 Cite this article as: Yan Wang, Wenzhi Zhang, Xinming Wu, Chunyan Luo, Tan Liang and Gang Yan, Metal-organic framework nanoparticles decorated with graphene: A high-performance electromagnetic wave absorber, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.04.093 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.
Metal-organic framework nanoparticles decorated with graphene: a high-performance electromagnetic wave absorber Yan Wang1, Wenzhi Zhang, Xinming Wu, Chunyan Luo, Tan Liang, Gang Yan School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, PR China
Abstract: A novel metal organic framework (MOF) coated RGO was fabricated by a one-step method. The morphology and microstructure of MOF-53(Fe)/RGO composite were characterized by XRD and TEM. The electromagnetic parameters indicate that MOF-53(Fe)/RGO composite shows enhanced electromagnetic absorption properties compared with MOF-53(Fe). The maximum RL can reach -25.8 dB at 15.4 GHz and the absorption bandwidth with the reflection loss exceeding -10 dB is 5.9 GHz (from 12.1 to 18 GHz) with the thickness of 2 mm. The possible absorption mechanism was also investigated in detail. Our results indicate the potential application of MOF/RGO composite as a more efficient microwave absorber. Keywords: Metal-organic frameworks; Carbon materials; Porous structure; Magnetic materials; Microwave absorption properties;
1．Introduction Electromagnetic waves have become a serious problem in various fields which may not only interrupt electronic devices, but also threatens human life [1]. To solve these problems, many researchers are paying great attention to the development of electromagnetic absorption materials. Recently, graphene has attracted considerable attention for achieving high-performance 1
Corresponding author: Tel: +86 13909249145
E-mail address: [email protected]
electromagnetic absorber due to the residual defects, larger interface, high permittivity and low density [2-3]. Nevertheless, the sole graphene suffers from impedance mismatch owing to high conductivity, which results in weak absorption and narrow absorption frequency [4]. Therefore, various magnetic materials such as Fe3O4 [5], Ni [6], NiFe2O4 [7] and CoFe2O4 [8], etc have been used to decorate reduced graphene oxide (RGO) to enhance electromagnetic attenuation, which contributes to a proper impedance matching between permittivity and permeability. Meanwhile, carbon materials such as MWCNT [9] and PANI [10] also have been added into the surface of RGO to improve the microwave absorption due to the formation of conducting network and unique structural characteristics. Recently, researchers have studied the microwave absorption properties of the multi-element composites due to their multi-functional electrical and magnetic properties, as well as multiple reflections and interfacial polarizations, such as rGO/Carbon Microspheres/rGO Composite [11] and CuS/Magnetically Decorated Graphene [12]. Metal organic frameworks (MOF) are a class of porous materials, which has attracted considerable attention because of high surface area, low density, diverse structural topologies as well as many functionalities [13]. The fascinating properties make MOF used in a variety of application areas, including gas storage, catalysts, sensors and supercapacitors [14-17]. The incorporation of MOF into graphene materials is an effective approach to improve the microwave absorbing properties of graphene. However, to the best of our knowledge, microwave absorption properties of MOF decorated with graphene have never been reported. In this work, we directly grow MOF-53(Fe) on the surface of graphene by a one-step hydrothermal strategy. Structural and morphological of MOF-53(Fe)/RGO composite have been investigated. The MOF-53(Fe)/RGO composite can obtain good electromagnetic properties
compared with MOF-53(Fe).
2．Experimental The formation mechanism of MOF-53(Fe)/RGO composite can be explained as Fig. 1. Firstly, Graphene oxide (GO) was prepared by Hummer’s method [18]. Secondly, GO was dispersed in 64.7 ml DMF (1 mg/ml) by sonication treatment, then 0.81 g FeCl3·6H2O and 0.5 g terephthalic acid were added and stirred for 30 min. Lastly, the solution was transferred into a 100 ml Teflon-lined stainless steel autoclave and kept in an oven at 150 ℃ for 12 h. For comparison, MOF-53(Fe) was prepared via a similar hydrothermal process without GO. After cooling down to room temperature, the products were washed with deionized water and ethanol, then dried at 150 ℃ under vacuum for 8 h. The structure was analyzed by X-ray diffraction (XRD) patterns (German Bruker D8 with Cu-Kα radiation). Transmission electron microscopy (TEM, American FEI F30 G2) was employed to analyze the morphology and the size of the samples. The N2 adsorption-desorption isotherms were measured on a Quad-rasorb-SI instrument, and the specific surface area was determined by the Brunauer-Emmett-Teller (BET) method. The magnetic properties were measured using a vibrating sample magnetometer (VSM, Lake Shore7307) with a maximum applied field of 13500 Oe. The electromagnetic parameters were analyzed by using a HP8720ES vector network analyzer in the range of 2-18 GHz.
3. Results and discussion The morphology of sample was investigated by TEM, as shown in Fig. 2 (a-d). The MOF-53(Fe) particles consist entirely of polyhedrons with an average edge length of around 500 nm which indicates the well crystalline nature of MOF-53(Fe). The MOF-53(Fe) polyhedrons are
high dispersible and anchored on graphene surface. The RGO nanosheets are transparent and wrinkled, indicating that RGO is a few atomic layers in thickness and is of good quality. Furthermore, no bare MOF-53(Fe) particles or RGO can be observed, which confirms the strong interfacial bonding between MOF-53(Fe) particles and RGO. Fig. 2e shows the XRD patterns of GO、RGO、MOF-53(Fe) and MOF-53(Fe)/RGO. The XRD patterns of both MOF-53(Fe) particles and MOF-53(Fe)/RGO composite can be perfectly assigned to the phase of MOF-53(Fe) [19]. The diffraction peak of MOF-53(Fe)/RGO composite had remained unchanged after incorporation of the graphene, which indicates that there was no apparent loss of crystallinity and decomposition of the framework structure. Furthermore, the broad peak of RGO at 24.50 can be attributed to the graphite-like structure, and no diffraction peaks resulting from GO (at 10.50) can be found, which means that oxygen groups have been removed and GO is effectively reduced to RGO [20]. To confirm the microstructure of porous MOF-53(Fe), nitrogen adsorption-desorption measurements were performed to investigate the pore diameter and surface area. Fig. 2 (f-g) shows the nitrogen adsorption-desorption isotherm at 77 K and pore diameter distribution of MOF-53(Fe). It can be seen in Fig. 2f that the hysteresis loop of MOF-53(Fe) is formed between 0.8P/P0 and 1.0 P/P0, indicating the existence of meso-pore (2-50 nm) and macro-pore (>50 nm). So, the synthesized MOF-53(Fe) is a kind of the IV-type material [21]. From Fig.2g, the average pore diameter, volume and BET surface area are 134.22 nm, 0.11 cm3 g-1, 22.36 m2 g-1, respectively. Fig. 2h shows the typical magnetization curves of MOF-53(Fe) and MOF-53(Fe)/RGO composite measured at room temperature. The magnetization hysteresis loops of MOF-53(Fe) and MOF-53(Fe)/RGO composite show S-like, indicating the nature of typical superparamagnetic material. The saturation magnetization (Ms) is 13.38 emu/g for MOF-53(Fe), and 6.21 emu/g for
the MOF-53(Fe)/RGO composite respectively. The Ms value of MOF-53(Fe)/RGO is lower than that of the MOF-53(Fe), which can be attributed to the existence of non-magnetic RGO. Fig.3a shows the real part (ε΄) and imaginary part (ε˝) of MOF-53(Fe) and MOF-53(Fe)/ RGO composite. It can be observed that the ε΄ and ε˝ values of MOF-53(Fe)/RGO decrease gradually from 11.3 to 5.1 and 6.2 to 2.6, respectively, with several fluctuations in the frequency range of 2-18 GHz. Moreover, the ε΄ and ε˝ values of MOF-53(Fe) have almost no change. The ε˝ values of MOF-53(Fe)/RGO are higher than MOF-53(Fe), implying that the strong dielectric loss is responsible for electromagnetic wave absorption properties of MOF-53(Fe)/RGO. In Fig.3b, we can see that the µ΄ values of all samples exhibit several variations at 2-18 GHz. Meanwhile, the imaginary part (µ˝) of MOF-53(Fe)/RGO declines gradually at 2-12 GHz, and then exhibits broad resonance peaks at 12-15.5 GHz with a maximum value of 0.41 at 14.8 GHz. The imaginary part (µ˝) of MOF-53(Fe) has a similar curve with MOF-53(Fe)/RGO and the broad resonance peak locates at 12.9-15.5 GHz with a maximum value of 0.41 at 14.1 GHz. According to the naturalresonance equation [22]: 2πfr=rHa
(1)
Ha=4｜K1｜/3µ0MS
(2)
where r is the gyromagnetic ratio, Ha is the anisotropy energy, and ｜K1｜is the anisotropy coefficient. The high resonance frequencies of MOF-53(Fe)/RGO and MOF-53(Fe) are attributed to the small size effect and the confinement effect. It is believed that the anisotropy energy of nanometer scale size materials would be remarkably increased due to the surface anisotropic field by the small size effect. The µ˝ values of MOF-53(Fe) are slightly larger than MOF-53(Fe)/RGO, which indicates a higher magnetic loss. From Fig.3(c-d), we can see that the tanδε values of MOF-53(Fe)/RGO are much higher than MOF-53(Fe) over 2-18 GHz, while the tanδm values of
MOF-53(Fe) are slightly larger than MOF- 53(Fe)/RGO, indicating that the microwave absorption mechanism of MOF-53(Fe)/RGO composite is mainly dependent on the dielectric loss. The phenomenon can be explained in the following factors. In terms of the electromagnetic theory, the dielectric loss of MOF-53(Fe)/RGO composite may be attributed to the layered structure, natural resonance, Debye dipolar relaxation and electron polarization etc [23]. On one hand, according to the free electron theory, ε˝=1/2ε0πρf, where ε0 is the permittivity of a vacuum, ρ is the resistivity, f is the frequency [24]. The conductivity of RGO is high, which enables a reduction of the resistivity, and results in the increase of the dielectric loss. On the other hand, RGO nanosheets form a conducting network, which migrating and hopping electron may enhance eddy current between RGO and MOF-53(Fe). This is why MOF-53(Fe)/RGO composite has a higher dielectric loss. The excellent electromagnetic wave absorption performance is mainly ascribed to two key factors: electromagnetic wave attenuation and impedance matching. The high dielectric and magnetic loss only suggest electromagnetic wave can be absorbed by materials, which means enhanced electromagnetic wave attenuation. The impedance matching indicates that electromagnetic wave can maximum enter material interior without being reflected by material surface and the characteristic impedance of the absorbing materials should be close to that of the free space (377 sq-1) to achieve zero reflection on the surface of the materials [25]. Only to satisfy the two conditions at the same time, microwave absorbing materials can achieve broad band and strong absorption peak. In order to investigate the microwave absorption properties of the samples, the reflection losses (RL) of MOF-53(Fe) and MOF-53(Fe)/RGO composite are calculated as follows [26]:
R(dB)=20 lg |
Zin 1 | Zin 1
(3)
Zin
r 2fd tanh( j r r ) r c
(4)
where Zin is the input impedance of the absorber, f is the frequency, d is the thickness of the absorb layer, c is the velocity of electromagnetic wave in vacuum. In Fig.3e, we can see that MOF-53(Fe) exhibits poor microwave absorbing properties with the thickness of 1.5-4 mm, the maximum RL is only -13.7 dB at 14.4 GHz. As shown in Fig.3f, after the introduction of graphene, the maximum RL reaches -25.8 dB at 15.4 GHz and the absorption bandwidth corresponding to RL at -10 dB is 5.9 GHz (from 12.1 to 18 GHz) for a layer of 2 mm thickness. Since the absorber with broad peak (RL values less than -10 dB are 5.9 GHz) can be designed to attenuate microwave, the MOF53(Fe)/RGO composite are very promising for new types of microwave absorption materials. In addition, it is noted that the attenuation peaks transfer to the low frequency with the increasing of thickness. It can be explained as follows [27]:
d nm / 4 m
(n=1, 3, 5, 7, 9……)
c f
(5) (6)
where ε and µ are the complex relative permittivity and permeability of the absorber, respectively. So, the matching thickness of the composites has an important influence on the microwave attenuation. These results demonstrate that the microwave absorption properties of MOF-53(Fe)/ RGO composite are superior to those of MOF-53(Fe). Fig.4 shows the schematic illustration of the absorption mechanism for MOF-53(Fe)/RGO composite. The excellent microwave absorbing performance of MOF-53(Fe)/RGO composite can be explained by the following facts. Firstly, the introduction of RGO has enhanced the εr of the MOF-53(Fe), and increased the tanδε, which helps to improve the impedance matching. Secondly,
the enormous aspect ratio, layered structure, residual defects and groups of the MOF-53(Fe)/RGO can cause multiple reflections and scattering, which will further enhance the microwave absorption properties [20]. Meanwhile, the high surface area of RGO can also provide more active sites for dissipating and scattering of the electromagnetic wave [28]. Thirdly, the interfacial polarization, dipole polarization and charge transfer between MOF-53(Fe) and RGO can convert electromagnetic energy into heat energy [2]. The porous MOF-53(Fe) particles possess many pores, which can be served as point defects and induces dipole moment, which provides enhanced interfacial dipole polarizations to increase dielectric relaxations and is beneficial to microwave absorption [29]. Furthermore, there are innumerable interfaces between MOF-53(Fe) and RGO and leads to extra interfacial polarization [30]. Lastly, the special porous structure of MOF-53(Fe) also makes a contribution to better impedance matching with free space than the corresponding solid materials and the MOF-53(Fe) with an interior space can also lead to multiple scattering to enlarge energy attenuation, which is vital for the microwave absorption performance. Besides this, porous structure of MOF-53(Fe) is beneficial to suppressing the eddy current loss of magnetic particles and maintaining high permeability at high frequency [31]. In general, the enhanced microwave absorption property of MOF-53(Fe)/RGO composite is attributed to the complementary effect of the MOF-53(Fe) and RGO.
4. Conclusion In summary, we developed a facile route to synthesize MOF/RGO composite and the microwave absorption properties have been investigated. The synthesized porous MOF/RGO composite exhibited excellent electromagnetic absorption properties. The maximum RL is up to -25.8 dB at 15.4 GHz and the absorption bandwidths exceeding -10 dB are 12.2 GHz (from 5.8 to
18 GHz) with a thickness in the range of 1.5-4 mm. Notably, the MOF-carbon based materials can be developed as an attractive candidate for applications as a microwave absorber.
Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No 51303147 and No 21506167) and the National college students’ innovative training plan (Grant No 201510702015).
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Figure captions Fig. 1. Schematic illustration of the synthesis procedure of MOF-53(Fe)/RGO composite Fig. 2. TEM images of MOF-53(Fe) (a-b) and MOF-53(Fe)/RGO composite (c-d), XRD patterns (e) of GO、RGO、MOF-53(Fe) and MOF-53(Fe)/RGO composite, Nitrogen adsorption-desorption isotherm (f) and pore diameter distribution (g) of MOF-53(Fe), magnetization curves (h) of
MOF-53(Fe) and MOF-53(Fe)/RGO composite Fig. 3. The ε΄ and ε˝ values (a), µ΄ and µ˝ values (b), tanδε (c), tanδm (d) of MOF-53(Fe) and MOF-53(Fe)/RGO composite, the reflection loss curves of MOF-53(Fe) (e) and MOF-53(Fe)/ RGO composite (f) Fig. 4. Schematic illustration of the absorption mechanism for MOF-53(Fe)/RGO composite
Highlights • MOF-53(Fe)/RGO composite was fabricated by a one-step hydrothermal strategy. • The morphology, microstructure, magnetic and electromagnetic properties were investigated. • The maximum reflection loss of MOF-53(Fe)/RGO composite can reach -25.8 dB at 15.4 GHz with the thickness of 2 mm. • The absorption bandwidths exceeding -10 dB are 12.2 GHz (from 5.8 to 18 GHz) in the range of 1.5-4 mm.
Graphical abstract