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C composites derived from MOFs toward high-performance electromagnetic wave absorption
Journal of Magnetism and Magnetic Materials 487 (2019) 165334
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Research articles
Porous flower-like Ni/C composites derived from MOFs toward highperformance electromagnetic wave absorption
T ⁎
Zilong Zhang, Yangyang Lv, Xiqiao Chen, Zhuang Wu, Yaoyi He, Lei Zhang, Yanhong Zou
Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education & Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha 410082, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flower-like structure Impedance matching Electromagnetic properties
The porous flower-like Ni/C composites were simply prepared through the pyrolysis of Zn-doped metal organic frameworks (MOFs) under a N2 atmosphere. The method is facile, controllable and without surface active agents. The microstructure, composition, and electromagnetic parameters can be easily controlled by varying thermal decomposition temperature to achieve the high electromagnetic loss. Interestingly, the Ni/C composites calcined at 800 °C have strong electromagnetic absorption with the maximum reflection loss (RL) value of −52.4 dB. In addition, the effective absorption band width (< −10 dB) of absorbing coating is as high as 5 GHz, and it is noteworthy that the composites have a very thin matching thickness of 1.6 mm. The excellent electromagnetic attenuation performance can be ascribed to the novel flower structures, synergistic effect between nickel and carbon, and remarkable impedance matching. This designed flower-like composites supply an impactful pathway to achieve potential candidate materials as highly effective microwave absorbers.
1. Introduction Recently, with the rapid arising of electronic equipment, high speed processors and personal digital assistants, electromagnetic pollution has brought great harm to human beings and the environment [1–5]. Thus, high-efficiency electromagnetic absorption materials, which can turn electromagnetic energy into heat energy by electromagnetic loss, have attracted tremendous attention over the past years [6–8]. High-performance microwave absorption materials should possess strong absorption, light weight, thin thickness, and broad frequency [9–12]. In addition, it is expected that the electromagnetic energy can be absorbed strongly in a wider frequency range by improving the composition and structure of the absorbing materials to adjust their electromagnetic parameters [13]. As known to all, the electromagnetic absorbing performance of the absorbers strongly depend on their microstructure and morphology [14]. Therefore, considerable efforts have been devoted to obtaining submicron or nanometer scales structures with various morphologies, including one dimensional (1-D) (nanowires [15], nanofibers [16]), two dimensional (2-D) (nanosheets [17,18]) and three dimensional (3-D) [19,20] with various methods. For example, Guan et al. fabricated 1-D α-MnO2 nanorods through water-bathing chemical deposition at less temperature. With the increase of reaction time, the prepared products were transformed from dendritic microspheres to nanorods, and they ⁎
found that the morphological structure of product largely affects its electromagnetic properties [21]. Similarly, Fu et al. reported the preparation of Fe hexagonal microflakes through hydrogen-thermal reduction. Compared with 0-D and 1-D structures, this 2-D microsheet structure has better microwave absorption performance due to large surface area and low weight, and its maximum RL is −15.3 dB with effective absorption band of 4.4 GHz [22]. Moreover, Ren and coworkers prepared 3-D graphene/γ-Fe2O3 nanosheet arrays by seed-assisted technique. The electromagnetic absorption capability of the arrays were significantly higher than those of some magnetic materials and graphene sheets, and the maximum RL is −64.1 dB with the thickness of 4.92 mm [23]. Flower-like absorbers have been brought to spotlight due to their unique advantages. These especial 3-D flower-like structures consist of massive ultrathin flakes, which may disperse electromagnetic waves between flower layers and improve microwave absorption performance [24–26]. Lv et al. reported a facile method to synthesize multihole flower-like structure Co/CoO by hydrothermal and annealing. The maximum RL of Co/CoO compound material is −50 dB at 7.2 GHz when the coating thickness is 3.5 mm [26]. Moreover, Liu et al. fabricated flower-like Co20Ni80 alloy microspheres by surfactant-directing method. The obtained Co20Ni80 alloy microspheres have good microwave absorption performance with a broadband absorption of 5.5 GHz [27]. Unfortunately, the preparation process of these materials usually
Corresponding author. E-mail address: [email protected] (Y. Zou).
https://doi.org/10.1016/j.jmmm.2019.165334 Received 17 February 2019; Received in revised form 16 April 2019; Accepted 20 May 2019 Available online 21 May 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 487 (2019) 165334
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Fig. 1. (a) XRD result of Zinc-doped MOF precursor. SEM images of (b) MOF precursor and (c, d) Zinc-doped MOF precursor.
2.2. Characterization
needs complex precursors or surface active agents, such as Co (CH3COO)2, glycerol and sodium citrate. As far as we know, these reagents not only add the cost of production, but also introduces foreign substances, resulting in a decrease in absorption strength [28]. Therefore, it is still a big challenge for preparation of flower-like absorption materials by simple and reliable methods. In this study, we prepare the porous flower-like Ni/C composites by calcining the as-prepared Zn-doped Ni-MOF under N2 atmosphere. The preparation method is simple, low-cost and steerable. The surface microstructures and electromagnetic performance of these Ni/C composites could be simply adjusted through changing the pyrogenic decomposition temperature. More remarkably, the optimized Ni/C composite has strong electromagnetic absorption capacity with a maximum RL of −52.4 dB and the coating thickness is only 1.6 mm. In addition, the qualified absorption bandwidth of absorbing coating material is 5 GHz.
The powder X-ray diffraction information were tested through Model D8 ANVANCE diffractometer (Cu-Ka irradiation, 40 kV voltage, 30 mA current). Scanning electron microscopy (FSEM, Hitachi S-4800, 5 kV) was used to study the morphology and microstructure of samples. Element states are analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB, 250Xi). Specific surface was obtained through BrunauerEmmett-Teller (BET) means with a Tri-Star 3020 system. The mixture of paraffin wax and 30 wt% samples was evenly pressed into annular mould with size of 7.0 × 3.0 × 2.0 mm. The electromagnetic parameters were measured by a network analyzer (AV3629, CETC41, China). 3. Results and discussion The XRD result of the Zn-doped MOF is displayed in Fig. 1(a). Through XRD analysis results of the precursor, the diffraction peaks are better coincide with the former results [29], demonstrating the successful synthesis of Zn-doped MOF. The morphology characteristic of the as-prepared MOF precursors are characterized by SEM. Fig. 1(b) shows that the MOF without Zn doping presents layered structure. There are a large number of nanosheets and the size is about 3 μm. Fig. 1(c) and (d) show the morphological features of Zn-doped MOF precursor. With the introduction of Zn ions, the flower-like microspheres composed of abundant sheet-like structures were formed, as shown in Fig. 1(c). It can be clearly seen that the size of nanosheet is about 1 μm, from the high magnification SEM image (Fig 0.1(d)). The Fig. 2(a) displays the XRD patterns of NC600, NC700, NC800, and NC900 at different calcination temperature. The three primary diffraction peaks locating at 42.8°, 49.8°, and 73.1° are well assigned to Ni3ZnC0.7 (JCPDS No: 28-0713). The other three primary diffraction peaks locating at 44.5°, 51.8°, and 76.4° are well assigned to Ni of JCPDS No: 04-0850. When the calcination temperature keeps rising, the diffraction peaks of the Ni/C composites tend to be sharper and stronger, which indicates that the crystallinity is increased [30]. Nevertheless, as the calcining temperature rises to 800 °C, this Ni3ZnC0.7 diffraction peaks disappear in the sample. The phenomenon may be interpreted through the fact that Ni2+ is reduced almost
2. Experiment section Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), p-benzenedicarboxylic acid (PTA, 99%), N, N-dimethylformamide (DMF), and sodium hydroxide were acquired from Shanghai Aladdin Reagent Corporation.
2.1. Preparation of the Zn-doped MOF and Ni/C nanoparticles The Zn-doped MOF was prepared through an easy solvothermal method [29]. In brief, 1 mmol of PTA, 0.6 mmol of ZnCl2 and Ni (NO3)2·6H2O were added into 60 ml DMF. The next step is that 4 ml sodium hydroxide water solution (0.4 M) was cautiously appended dropwise to above solution. The prepared solution was moved to the 100 ml Teflon-lined stainless autoclave kept for 8 h at 100 °C. Finally, collected green precipitate were washed three times using ethanol and deionized water, and drying at 70 °C for 48 h. The Zn-doped MOF precursors were heated to 600 °C, 700 °C, 800 °C, and 900 °C maintaining for 2 h under nitrogen gas atmosphere, respectively. The heating rate is 3 °C min−1. The obtained products are marked in NC600, NC700, NC800, and NC900, respectively. 2
Journal of Magnetism and Magnetic Materials 487 (2019) 165334
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Fig. 2. (a) XRD results of the as-synthesized composites. (b) N2 adsorption–desorption isotherms, (c) XPS survey spectra, (d) C1s spectrum, (e) Ni 2p spectrum, and (f) Zn 2p spectrum of the NC800.
C]C, C–OH and O–C]C, respectively [32]. Fig. 2(e) displays that the binding energies peaks at 852.3 and 869.8 eV should be assigned to Ni 2p3/2 and Ni 2p1/2, respectively. The fitting peaks at 861.1 and 879.8 eV are assigned as satellite peaks. These features indicate the existence of Ni2+ [33,34]. As shown in Fig. 2(f), there are two peaks at 1021.7 (Zn 2p3/2) and 1044.9 eV (Zn 2p1/2) in Zn 2P spectrum. This phenomenon shows that the existence of Zn element in the shape of Zn2+ [35]. The representative SEM images of the flower-like composites with different reaction temperatures are shown in Fig. 3. Obviously, 3-D flower-like structures can be observed from the images. Moreover, the surface structure of Ni/C composites was significantly affected by the calcination temperature. The precursor (Fig. 1(b)) exhibits a typical flower-like structure with smooth surface. However, massive holes appeared on the surface of petals after pyrolysis processes. The average size of flower-like structure was about 5 μm. With the further increase of temperature, the holes on the flower surface increase continuously. Some flower shapes collapsed and the size of flower body decreases from about 5 μm to about 3 μm in NC900 (Fig. 3(d)). This may be related to the increase of pore size, the expansion of holes dimension could not brace the framework of flower-like structure. Moreover, this porous flower-like framework further promotes the microwave absorption ability by enhancing the scattering of electromagnetic waves.
entirely to Ni. Moreover, porous structures should also be emphasized. Fig. 2(b) exhibits the N2 adsorption–desorption isotherm of NC800. Obviously, the curve of this sample coincides with the typical type-IV isotherms, showing mesoporous characteristics [31]. The SBET of the sample is 60.1 m2 g−1, which demonstrates the characteristics of porous structure. In addition, the EDS characterization was performed on the all Ni/C composites calcined at 600 °C, 700 °C, 800 °C, and 800 °C. The contents of each element in Ni/C composites are shown in Fig. S2. With the increase of pyrolysis temperatures, the contents of Zn is gradually reduced from 20.8 wt% to 0.67 wt%. This is because that zinc elements may be evaporated from the Ni/C composite at high pyrolysis temperature. As the calcination temperature increases, the contents of both Ni and C elements show a significant increase. The EDS results in Fig. S2 reveals that the weight contents of Ni of NC600, NC700, NC800, and NC900 are 38.17%, 43.24%, 46.35%, and 53.18%, respectively. For the C elements, the weight contents are 19.54%, 24.57%, 24.03%, and 37.45%, respectively. To further explore surface element state of this composites, the XPS measurement is applied. The presence of C, O, Ni and Zn elements of the composites are confirmed by survey spectrum (Fig. 2(c)). The C1s spectrum (Fig. 2(d)) can be decomposed into four parts, which indicates that there are four kinds of carbon in the sample. The four peaks locating at 284.4, 284.8, 285.6 and 288.4 eV correspond to C–C, C–C/ 3
Journal of Magnetism and Magnetic Materials 487 (2019) 165334
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Fig. 3. SEM images of 3-D flower-like composites after different temperature calcination: (a) NC600; (b) NC700; (c) NC800; (d) NC900.
Fig. 4. Real prats of complex permittivity (a) and permeability (d), imaginary prats of complex permittivity (b) and permeability (e), dielectric (c) and magnetic (f) loss tangents of 3-D flower-like composites after different temperature calcination.
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Fig. 5. Plots ε' versus ε'' for different flower-like composites: (a) NC600, (b) NC700, (c) NC800, and (d) NC900.
phenomenon [37]. The ε′ and ε″ of the flower-like composites (NC600, NC700 and NC800) both increase with the increasing reaction temperature. While the calcination temperature continues to rise, the complex permittivity of NC900 shows a significant drop. Because of the highest temperature pyrolysis, a large number of defects and functional groups disappear from the NiC900. These defects acting as polarization centers can generate polarization relaxation, while the functional groups could produce electron dipole polarization [13]. As shown in Fig. 4(b), the imaginary part of complex permittivity decreases overall with the increase of frequency. Moreover, there are two obvious resonant peaks at about 10 and 17 GHz, especially for NC800 and NC900. For the NC800 with reaction temperature at 800 °C, it has maximum complex permittivity, where the values of ε′ decreases from 12.5 to 9. While the values of ε″ present two distinct resonance peaks at about 9 and 17 GHz. These resonant peaks have been proved to be helpful for microwave absorption, which are considerably associated with multiple polarization relaxation processes [38]. The importance of these resonant peaks is further highlighted by the tanδε curves (Fig. 4c), where some representative peaks are obviously shown in the same frequency range. According to Debye theory, ε′ and ε″ meet the formula (1): [39,40]
Fig. 6. Frequency dependences of μ″(μ′)−2f−1values of as-prepared Ni/C composites after different temperature calcination.
In order to evaluate the magnetic properties of the Ni/C composites, their magnetic hysteresis loops are measured at room temperature as shown in Fig. S1. From Figs. S1(c–f), it can be observed that the value of saturation magnetization (Ms) gradually increases with increasing calcination temperature. The Ms values of four Ni/C composites are 36.65, 38.09, 48.23, 58.29, respectively. While the Hc of 15.09, 15.50, 14.48, and 16.01 belong to NC600, NC700, NC800 and NC900, respectively. When a magnetic rotor is placed beside a small bottle filled with Ni/C composites dispersed in ethyl alcohol, the nanoparticles rapidly move and accumulate near the magnetic rotor shown in inset of Fig. S1. The microwave absorption performances are greatly dependent on electromagnetic parameters, where the real and imaginary part of complex permittivity and complex permeability describe the storage and loss ability of electromagnetic energy, respectively [36]. In addition, dielectric loss tangents are also important indicators for describing dielectric attenuation ability of flower-like composites. Remarkably, the real permittivity (Fig. 4(a)) of the composite materials decreases with rising frequency, which exhibits a typical frequency dispersion
(ε′ − ε∞)2 + (ε″)2 = (εs − ε∞)2
(1)
where εs and ε∞ are the static permittivity and relative permittivity at the high-frequency limit, respectively. According to this equation, we could deduced that the graph of ε′ to ε″ should be a unitary semicircle, which expresses a Debye relaxation process. As observed, the ε′-ε″ curves of NC700 and NC800 show more semicircles than those of NC600. In addition, the ε′-ε″ curve values of NC800 are the biggest, which indicates the strongest Debye relaxation. In addition, Fig. 5 shows that the NC800 possesses high values of ε′ and ε″, which indicating the strong Debye relaxation to dielectric loss [10,13]. Because of the great quantity of Ni nanoparticles in flower-like Ni/C composites, magnetic attenuation is unquestionably essential element for evaluating their electromagnetic wave absorbing properties. The μ′ values of NC600 and NC700 gradually decrease from 1.25 to 0.9 and from 1.29 to 0.93 with the increase frequency, respectively. Remarkably, the μ′ values of NC800 and NC900 drop sharply around 5
Journal of Magnetism and Magnetic Materials 487 (2019) 165334
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Fig. 7. RL values of all flower-like Ni/C composites (a) NC600, (b) NC700, (c) NC800, and (d) NC900.
4 GHz, as shown in Fig. 4(d). The μ′ values of four samples show two resonance peaks at about 9 and 11.8 GHz. At the same time, the μ″ values of the Ni/C composites in 2–10 GHz fluctuate sharply and strong resonance peaks are observed in Fig. 4(e). The high magnetic loss may come from natural ferromagnetic resonance. The natural ferromagnetic resonance, as one of the main factors causing magnetic attenuation, could be further supported through the peaks in tanδμ graph [41]. In addition, eddy current effect also makes an important contribution to magnetic attenuation. We calculated the relationship between μ″(μ′)−2f−1 [42] and frequency, as shown in Fig. 6. If the eddy current effect is the only cause of magnetic loss, the value of μ″(μ′)−2f−1 should be a constant [43]. Obviously, the μ″(μ′)−2f−1 values of all flower-like Ni/C composites vary with frequency in 2–10 GHz. Nevertheless, the μ″(μ′)−2f−1 values have changed little in 10–18 GHz, which indicates eddy current effect contributes prominently to magnetic attenuation in 10–18 GHz. According to transmission line theory, the microwave attenuation capacity could usually be expressed through RLs, which could be conclude by the following formulas: [44]
Zin = Z0 (μr / εr )1/2tan h [j (2πfd )(μr εr )1/2 / c ]
(2)
RL = 20log |(Zin − Z0)/(Zin + Z0)|
(3)
where Z0 represents the impedance of free space, Zin represents the input impedance, f is the electromagnetic wave frequency, d is the coating thickness, and c is the velocity of electromagnetic wave. Fig. 7 presents theoretical RL values of flower-like composites with different thickness in 2–18 GHz. Remarkably, we can conclude that the calcination conditions have important influences on the microwave absorption capabilities of these flower-like Ni/C composites. As shown in Fig. 7(a), the microwave absorption ability of NC600 is very weak with −12 dB and the absorption bandwidth is narrow. With the increase of temperature, the electromagnetic wave absorbing properties of flower-like Ni/C nanocomposites have been significantly improved. From Fig. 7(c), we found that the NC800 possesses the best microwave absorption performance. Compared with other flower-like Ni/C composites, the NC800 achieved the strongest absorption with maximum
Fig. 8. (a) Attenuation constant α and (b) relative input impedance (|Zin/Z0|) of the Ni/C composites with different frequency.
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RL of −52.4 dB. More importantly, the corresponding coating thickness is only 1.6 mm. However, with further increase of temperature, the RL of composites begins to decrease. The maximum RL of NC900 is only −36 dB with the corresponding thickness of 3.5 mm, as shown in Fig. 7(d). In addition, compared with other MOF-derived carbon materials, flower-like Ni/C composites show superior comprehensive properties, as shown in Table S1. In general, good impedance matching characteristic is the prerequisite condition for electromagnetic wave attenuation. When the |Zin/Z0| value is close to 1.0, there will be no reflected electromagnetic waves [44]. According to the equation (3), we calculated the |Zin/Z0| values of the composite at different calcination temperature, as shown in Fig. 8(b). The Ni/C composite calcined at 800 °C displays the best impedance matching, which indicate that the microwave can enter the interior of materials with minimum reflection. In order to further understand the absorbing mechanism, the schematic diagram is shown in Fig. S3 After the electromagnetic wave entered absorbing materials, the electromagnetic energy was converted into heat energy by the dielectric loss and magnetic loss. The dielectric loss derive from the interfacial polarization, dipole polarization, migration, and hopping of electrons, while the magnetic loss from natural resonance and exchange resonance of Ni nanoparticles. Moreover, these porous flower-like structures could produce multiple scatterings between petals, and the porous structure further increases the scattering strength, which are beneficial to the microwave attenuation. For confirm the above results, we computed the attenuation constant (α) of the Ni/C composites: [45,46]
α=
2πf c
×
(μ″ε″ − μ′ε′) +
(μ″ε″ − μ′ε′)2 + (μ′ε″ − μ″ε′)2
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165334. References [1] S.B. Cao, H.B. Liu, L. Yang, Y.H. Zou, X.H. Xia, H. Chen, The effect of microstructure of graphene foam on microwave absorption properties, J. Magn. Magn. Mater. 458 (2018) 217–224. [2] X.Q. Chen, Z. Wu, Z.L. Zhang, L.Y. Heng, S. Wang, Y.H. Zou, Impedance matching for omnidirectional and polarization insensitive broadband absorber based on carbonyl iron powders, J. Magn. Magn. Mater. 476 (2019) 349–354. [3] S. Sugimoto, T. Maeda, D. Book, T. Kagotani, K. Inomata, M. Homma, H. Ota, Y. Houjou, R. Sato, GHz microwave absorption of a fine α-Fe structure produced by the disproportionation of Sm2Fe17 in hydrogen, J. Alloy Comp. 330 (330) (2002) 301–306. [4] A. Hua, D.S. Pan, Y. Li, J. Luan, Y. Wang, J. He, D.Y. Geng, W. Liu, S. Ma, Z.D. Zhang, Fe3Si-core/amorphous-C-shell nanocapsules with enhanced microwave absorption, J. Magn. Magn. Mater. 471 (2019) 561–567. [5] Y.H. Zou, P. Tassin, T. Koschny, C.M. Soukoulis, Interaction between graphene and metamaterials: split rings vs. wire pairs, Opt. Expr. 20 (11) (2012) 12198–12204. [6] Y. Zhang, Y. Huang, H.H. Chen, Z.Y. Huang, Y. Yang, P.S. Xiao, Y. Zhou, Y.S. Chen, Composition and structure control of ultralight graphene foam for high-performance microwave absorption, Carbon 105 (2016) 438–447. [7] Z.L. Zhang, X.Q. Chen, Z.L. Wang, L.Y. Heng, S. Wang, Z.X. Tang, Y.H. Zou, Carbonyl iron/graphite microspheres with good impedance matching for ultrabroadband and highly efficient electromagnetic absorption, Opt. Mater. Exp. 8 (11) (2018) 3319–3331. [8] X.H. Li, J. Feng, Y.P. Du, J.T. Bai, H.M. Fan, H.L. Zhang, Y. Peng, F.S. Li, One-pot synthesis of CoFe2O4/graphene oxide hybrids and their conversion into FeCo/graphene hybrids for lightweight and highly efficient microwave absorber, J. Mater. Chem. A 3 (10) (2015) 5535–5546. [9] S.F. Lai, Y.H. Wu, J.J. Wang, W. Wu, W.H. 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Li, Enhanced microwave absorption of Fe nanoflakes after coating with SiO2 nanoshell, Nanotechnology 21 (9) (2010) 095708. [18] X.K. Zhang, T. Ekiert, K.M. Unruh, J.Q. Xiao, High frequency properties of polymer composites consisting of aligned Fe flakes, J. Appl. Phys. 99 (8) (2006) 08M914–08M916. [19] B. Zhao, G. Shao, B.B. Fan, Y.Q. Chen, R. Zhang, Effect of The TiO2 amounts on microwave absorption properties of Ni/TiO2 heterostructure composites, Phys. B: Cond. Matter 454 (1) (2014) 120–125. [20] J.W. Liu, J. Cheng, R.C. Che, J.J. Xu, M.M. Liu, Z.W. Liu, Double-shelled yolk−shell microspheres with Fe3O4 cores and SnO2 double shells as high-performance microwave absorbers, J. Phys. Chem. C 117 (1) (2013) 489–495. [21] H.T. Guan, G. Chen, J. Zhu, Y.D. Wang, Facile synthesis and microwave absorption properties of α-MnO2 nanorods, Functl. Mater. Lett. 5 (4) (2012) 1250043. [22] L.S. Fu, J.T. Jiang, C.Y. Xu, L. 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(4)
From the curves of attenuation constants, we observed that the composites calcined at 800 °C possess the greatest attenuation, especially at high frequencies. From Fig. 4(c), it is obvious that the NC800 possesses the highest tanδε in the frequency range of 2–18 GHz. It is indicated that the NC800 has the strongest dielectric loss. In addition, from Fig. 4(c) and (f), dielectric loss tangent is much larger than the magnetic loss tangent, indicating that the loss mechanism of the Ni/C composite is mainly based on dielectric loss. Therefore, the NC800 has the best microwave absorption properties because of optimum impedance matching characteristic and the greatest attenuation capacity.
4. Conclusion In summary, the 3-D flower-like Ni/C composites were fabricated by synthesis of Zn-doped Ni-MOF precursor in a N2 atmosphere. Compared with previous strategies for controllable synthesis of 3-D flower structures, this method is simple, low-cost and without surface active agents. We find that the reaction temperature have important influences on microstructure, composition, and microwave absorbing characteristic. These 3-D flower-like structures have massive porous and large spacing flakes, which increases the electromagnetic wave scatter. The optimized flower-like Ni/C composites display high electromagnetic absorption performances. Interestingly, the optimum one calcined at 800 °C shows strong electromagnetic attenuation with an optimal RL value of −52.4 dB. Remarkably, the effective absorption bandwidth (< −10 dB) of absorbing coating is 5 GHz and the matching thickness is only 1.6 mm. We believe that the 3-D flower-like composites derived from MOF may supply a good strategy for the preparation of novel electromagnetic absorber.
Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant Nos. 61378002) and the Key Research and Development Plan of Hunan Province (Grant No. 2017NK2121). 7
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(2017) 2005–2014. [26] H.L. Lv, X.H. Liang, G.B. Ji, H.Q. Zhang, Y.W. Du, Porous three-dimensional flowerlike Co/CoO and its excellent electromagnetic absorption properties, ACS Appl. Mater. Interfaces 7 (18) (2015) 9776–9783. [27] Q.H. Liu, X.H. Xu, W.X. Xia, R.C. Che, C. Chen, Q. Cao, J.G. He, Dependency of magnetic microwave absorption on surface architecture of Co20Ni80 hierarchical structures studied by electron holography, Nanoscale 7 (5) (2015) 1736–1743. [28] G.X. Tong, J.H. Yuan, W.H. Wu, Q. Hu, H.S. Qian, L.C. Li, J.P. Shen, Flower-like Co superstructures: morphology and phase evolution mechanism and novel microwave electromagnetic characteristics, CrystEngComm 14 (6) (2012) 2071. [29] J. Yang, C. Zheng, P.X. Xiong, Y.F. Li, M.D. Wei, Zn-doped Ni-MOF material with a high supercapacitive performance, J. Mater. Chem. A 2 (2014) 19005–19010. [30] S. Tyagi, H.B. Baskey, R.C. Agarwala, V. Agarwala, T.C. Shami, Development of hard/soft ferrite nanocomposite for enhanced microwave absorption, Ceram. Int. 37 (7) (2011) 2631–2641. [31] P. Schneider, Adsorption isotherms of microporous-mesoporous solids revisited, Appl. Catal. A: Gen. 129 (2) (1995) 157–165. [32] T.T. Chen, F. Deng, J. Zhu, C.F. Chen, G.B. Sun, S.L. Ma, X.J. Yang, Hexagonal and cubic Ni nanocrystals grown on graphene: phase-controlled synthesis, characterization and their enhanced microwave absorption properties, J. Mater. Chem. 22 (30) (2012) 15190–15197. [33] M. Mukoyoshi, H. Kobayashi, K. Kusada, M. Hayashi, T. Yamada, M. Maesato, J.M. Taylor, Y. Kubota, K. Kato, M. Takata, T. Yamamoto, S. Matsumura, H. Kitagawa, Hybrid materials of Ni NP@MOF prepared by a simple synthetic method, Chem. Commun. 51 (62) (2015) 12463. [34] P. Wen, P.W. Gong, J.F. Sun, J.Q. Wang, S.R. Yang, Design and synthesis of NiMOF/CNT composites and rGO/carbon nitride composites for an asymmetric supercapacitor with high energy and power density, J. Mater. Chem. A 3 (2015) 13874. [35] W. Liu, L. Liu, G.B. Ji, D.R. Li, Y.N. Zhang, J.N. Ma, Y.W. Du, Composition design and structural characterization of MOF-derived composites with controllable electromagnetic properties, ACS Sust. Chem. Eng. 5 (2017) 7961–7971. [36] H. Sun, R. Che, X. You, Y. Jiang, Z. Yang, J. Deng, L. Qiu, H. Peng, Cross-stacking
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Research articles
Porous flower-like Ni/C composites derived from MOFs toward highperformance electromagnetic wave absorption
T ⁎
Zilong Zhang, Yangyang Lv, Xiqiao Chen, Zhuang Wu, Yaoyi He, Lei Zhang, Yanhong Zou
Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education & Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha 410082, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flower-like structure Impedance matching Electromagnetic properties
The porous flower-like Ni/C composites were simply prepared through the pyrolysis of Zn-doped metal organic frameworks (MOFs) under a N2 atmosphere. The method is facile, controllable and without surface active agents. The microstructure, composition, and electromagnetic parameters can be easily controlled by varying thermal decomposition temperature to achieve the high electromagnetic loss. Interestingly, the Ni/C composites calcined at 800 °C have strong electromagnetic absorption with the maximum reflection loss (RL) value of −52.4 dB. In addition, the effective absorption band width (< −10 dB) of absorbing coating is as high as 5 GHz, and it is noteworthy that the composites have a very thin matching thickness of 1.6 mm. The excellent electromagnetic attenuation performance can be ascribed to the novel flower structures, synergistic effect between nickel and carbon, and remarkable impedance matching. This designed flower-like composites supply an impactful pathway to achieve potential candidate materials as highly effective microwave absorbers.
1. Introduction Recently, with the rapid arising of electronic equipment, high speed processors and personal digital assistants, electromagnetic pollution has brought great harm to human beings and the environment [1–5]. Thus, high-efficiency electromagnetic absorption materials, which can turn electromagnetic energy into heat energy by electromagnetic loss, have attracted tremendous attention over the past years [6–8]. High-performance microwave absorption materials should possess strong absorption, light weight, thin thickness, and broad frequency [9–12]. In addition, it is expected that the electromagnetic energy can be absorbed strongly in a wider frequency range by improving the composition and structure of the absorbing materials to adjust their electromagnetic parameters [13]. As known to all, the electromagnetic absorbing performance of the absorbers strongly depend on their microstructure and morphology [14]. Therefore, considerable efforts have been devoted to obtaining submicron or nanometer scales structures with various morphologies, including one dimensional (1-D) (nanowires [15], nanofibers [16]), two dimensional (2-D) (nanosheets [17,18]) and three dimensional (3-D) [19,20] with various methods. For example, Guan et al. fabricated 1-D α-MnO2 nanorods through water-bathing chemical deposition at less temperature. With the increase of reaction time, the prepared products were transformed from dendritic microspheres to nanorods, and they ⁎
found that the morphological structure of product largely affects its electromagnetic properties [21]. Similarly, Fu et al. reported the preparation of Fe hexagonal microflakes through hydrogen-thermal reduction. Compared with 0-D and 1-D structures, this 2-D microsheet structure has better microwave absorption performance due to large surface area and low weight, and its maximum RL is −15.3 dB with effective absorption band of 4.4 GHz [22]. Moreover, Ren and coworkers prepared 3-D graphene/γ-Fe2O3 nanosheet arrays by seed-assisted technique. The electromagnetic absorption capability of the arrays were significantly higher than those of some magnetic materials and graphene sheets, and the maximum RL is −64.1 dB with the thickness of 4.92 mm [23]. Flower-like absorbers have been brought to spotlight due to their unique advantages. These especial 3-D flower-like structures consist of massive ultrathin flakes, which may disperse electromagnetic waves between flower layers and improve microwave absorption performance [24–26]. Lv et al. reported a facile method to synthesize multihole flower-like structure Co/CoO by hydrothermal and annealing. The maximum RL of Co/CoO compound material is −50 dB at 7.2 GHz when the coating thickness is 3.5 mm [26]. Moreover, Liu et al. fabricated flower-like Co20Ni80 alloy microspheres by surfactant-directing method. The obtained Co20Ni80 alloy microspheres have good microwave absorption performance with a broadband absorption of 5.5 GHz [27]. Unfortunately, the preparation process of these materials usually
Corresponding author. E-mail address: [email protected] (Y. Zou).
https://doi.org/10.1016/j.jmmm.2019.165334 Received 17 February 2019; Received in revised form 16 April 2019; Accepted 20 May 2019 Available online 21 May 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) XRD result of Zinc-doped MOF precursor. SEM images of (b) MOF precursor and (c, d) Zinc-doped MOF precursor.
2.2. Characterization
needs complex precursors or surface active agents, such as Co (CH3COO)2, glycerol and sodium citrate. As far as we know, these reagents not only add the cost of production, but also introduces foreign substances, resulting in a decrease in absorption strength [28]. Therefore, it is still a big challenge for preparation of flower-like absorption materials by simple and reliable methods. In this study, we prepare the porous flower-like Ni/C composites by calcining the as-prepared Zn-doped Ni-MOF under N2 atmosphere. The preparation method is simple, low-cost and steerable. The surface microstructures and electromagnetic performance of these Ni/C composites could be simply adjusted through changing the pyrogenic decomposition temperature. More remarkably, the optimized Ni/C composite has strong electromagnetic absorption capacity with a maximum RL of −52.4 dB and the coating thickness is only 1.6 mm. In addition, the qualified absorption bandwidth of absorbing coating material is 5 GHz.
The powder X-ray diffraction information were tested through Model D8 ANVANCE diffractometer (Cu-Ka irradiation, 40 kV voltage, 30 mA current). Scanning electron microscopy (FSEM, Hitachi S-4800, 5 kV) was used to study the morphology and microstructure of samples. Element states are analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB, 250Xi). Specific surface was obtained through BrunauerEmmett-Teller (BET) means with a Tri-Star 3020 system. The mixture of paraffin wax and 30 wt% samples was evenly pressed into annular mould with size of 7.0 × 3.0 × 2.0 mm. The electromagnetic parameters were measured by a network analyzer (AV3629, CETC41, China). 3. Results and discussion The XRD result of the Zn-doped MOF is displayed in Fig. 1(a). Through XRD analysis results of the precursor, the diffraction peaks are better coincide with the former results [29], demonstrating the successful synthesis of Zn-doped MOF. The morphology characteristic of the as-prepared MOF precursors are characterized by SEM. Fig. 1(b) shows that the MOF without Zn doping presents layered structure. There are a large number of nanosheets and the size is about 3 μm. Fig. 1(c) and (d) show the morphological features of Zn-doped MOF precursor. With the introduction of Zn ions, the flower-like microspheres composed of abundant sheet-like structures were formed, as shown in Fig. 1(c). It can be clearly seen that the size of nanosheet is about 1 μm, from the high magnification SEM image (Fig 0.1(d)). The Fig. 2(a) displays the XRD patterns of NC600, NC700, NC800, and NC900 at different calcination temperature. The three primary diffraction peaks locating at 42.8°, 49.8°, and 73.1° are well assigned to Ni3ZnC0.7 (JCPDS No: 28-0713). The other three primary diffraction peaks locating at 44.5°, 51.8°, and 76.4° are well assigned to Ni of JCPDS No: 04-0850. When the calcination temperature keeps rising, the diffraction peaks of the Ni/C composites tend to be sharper and stronger, which indicates that the crystallinity is increased [30]. Nevertheless, as the calcining temperature rises to 800 °C, this Ni3ZnC0.7 diffraction peaks disappear in the sample. The phenomenon may be interpreted through the fact that Ni2+ is reduced almost
2. Experiment section Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), p-benzenedicarboxylic acid (PTA, 99%), N, N-dimethylformamide (DMF), and sodium hydroxide were acquired from Shanghai Aladdin Reagent Corporation.
2.1. Preparation of the Zn-doped MOF and Ni/C nanoparticles The Zn-doped MOF was prepared through an easy solvothermal method [29]. In brief, 1 mmol of PTA, 0.6 mmol of ZnCl2 and Ni (NO3)2·6H2O were added into 60 ml DMF. The next step is that 4 ml sodium hydroxide water solution (0.4 M) was cautiously appended dropwise to above solution. The prepared solution was moved to the 100 ml Teflon-lined stainless autoclave kept for 8 h at 100 °C. Finally, collected green precipitate were washed three times using ethanol and deionized water, and drying at 70 °C for 48 h. The Zn-doped MOF precursors were heated to 600 °C, 700 °C, 800 °C, and 900 °C maintaining for 2 h under nitrogen gas atmosphere, respectively. The heating rate is 3 °C min−1. The obtained products are marked in NC600, NC700, NC800, and NC900, respectively. 2
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Fig. 2. (a) XRD results of the as-synthesized composites. (b) N2 adsorption–desorption isotherms, (c) XPS survey spectra, (d) C1s spectrum, (e) Ni 2p spectrum, and (f) Zn 2p spectrum of the NC800.
C]C, C–OH and O–C]C, respectively [32]. Fig. 2(e) displays that the binding energies peaks at 852.3 and 869.8 eV should be assigned to Ni 2p3/2 and Ni 2p1/2, respectively. The fitting peaks at 861.1 and 879.8 eV are assigned as satellite peaks. These features indicate the existence of Ni2+ [33,34]. As shown in Fig. 2(f), there are two peaks at 1021.7 (Zn 2p3/2) and 1044.9 eV (Zn 2p1/2) in Zn 2P spectrum. This phenomenon shows that the existence of Zn element in the shape of Zn2+ [35]. The representative SEM images of the flower-like composites with different reaction temperatures are shown in Fig. 3. Obviously, 3-D flower-like structures can be observed from the images. Moreover, the surface structure of Ni/C composites was significantly affected by the calcination temperature. The precursor (Fig. 1(b)) exhibits a typical flower-like structure with smooth surface. However, massive holes appeared on the surface of petals after pyrolysis processes. The average size of flower-like structure was about 5 μm. With the further increase of temperature, the holes on the flower surface increase continuously. Some flower shapes collapsed and the size of flower body decreases from about 5 μm to about 3 μm in NC900 (Fig. 3(d)). This may be related to the increase of pore size, the expansion of holes dimension could not brace the framework of flower-like structure. Moreover, this porous flower-like framework further promotes the microwave absorption ability by enhancing the scattering of electromagnetic waves.
entirely to Ni. Moreover, porous structures should also be emphasized. Fig. 2(b) exhibits the N2 adsorption–desorption isotherm of NC800. Obviously, the curve of this sample coincides with the typical type-IV isotherms, showing mesoporous characteristics [31]. The SBET of the sample is 60.1 m2 g−1, which demonstrates the characteristics of porous structure. In addition, the EDS characterization was performed on the all Ni/C composites calcined at 600 °C, 700 °C, 800 °C, and 800 °C. The contents of each element in Ni/C composites are shown in Fig. S2. With the increase of pyrolysis temperatures, the contents of Zn is gradually reduced from 20.8 wt% to 0.67 wt%. This is because that zinc elements may be evaporated from the Ni/C composite at high pyrolysis temperature. As the calcination temperature increases, the contents of both Ni and C elements show a significant increase. The EDS results in Fig. S2 reveals that the weight contents of Ni of NC600, NC700, NC800, and NC900 are 38.17%, 43.24%, 46.35%, and 53.18%, respectively. For the C elements, the weight contents are 19.54%, 24.57%, 24.03%, and 37.45%, respectively. To further explore surface element state of this composites, the XPS measurement is applied. The presence of C, O, Ni and Zn elements of the composites are confirmed by survey spectrum (Fig. 2(c)). The C1s spectrum (Fig. 2(d)) can be decomposed into four parts, which indicates that there are four kinds of carbon in the sample. The four peaks locating at 284.4, 284.8, 285.6 and 288.4 eV correspond to C–C, C–C/ 3
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Fig. 3. SEM images of 3-D flower-like composites after different temperature calcination: (a) NC600; (b) NC700; (c) NC800; (d) NC900.
Fig. 4. Real prats of complex permittivity (a) and permeability (d), imaginary prats of complex permittivity (b) and permeability (e), dielectric (c) and magnetic (f) loss tangents of 3-D flower-like composites after different temperature calcination.
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Fig. 5. Plots ε' versus ε'' for different flower-like composites: (a) NC600, (b) NC700, (c) NC800, and (d) NC900.
phenomenon [37]. The ε′ and ε″ of the flower-like composites (NC600, NC700 and NC800) both increase with the increasing reaction temperature. While the calcination temperature continues to rise, the complex permittivity of NC900 shows a significant drop. Because of the highest temperature pyrolysis, a large number of defects and functional groups disappear from the NiC900. These defects acting as polarization centers can generate polarization relaxation, while the functional groups could produce electron dipole polarization [13]. As shown in Fig. 4(b), the imaginary part of complex permittivity decreases overall with the increase of frequency. Moreover, there are two obvious resonant peaks at about 10 and 17 GHz, especially for NC800 and NC900. For the NC800 with reaction temperature at 800 °C, it has maximum complex permittivity, where the values of ε′ decreases from 12.5 to 9. While the values of ε″ present two distinct resonance peaks at about 9 and 17 GHz. These resonant peaks have been proved to be helpful for microwave absorption, which are considerably associated with multiple polarization relaxation processes [38]. The importance of these resonant peaks is further highlighted by the tanδε curves (Fig. 4c), where some representative peaks are obviously shown in the same frequency range. According to Debye theory, ε′ and ε″ meet the formula (1): [39,40]
Fig. 6. Frequency dependences of μ″(μ′)−2f−1values of as-prepared Ni/C composites after different temperature calcination.
In order to evaluate the magnetic properties of the Ni/C composites, their magnetic hysteresis loops are measured at room temperature as shown in Fig. S1. From Figs. S1(c–f), it can be observed that the value of saturation magnetization (Ms) gradually increases with increasing calcination temperature. The Ms values of four Ni/C composites are 36.65, 38.09, 48.23, 58.29, respectively. While the Hc of 15.09, 15.50, 14.48, and 16.01 belong to NC600, NC700, NC800 and NC900, respectively. When a magnetic rotor is placed beside a small bottle filled with Ni/C composites dispersed in ethyl alcohol, the nanoparticles rapidly move and accumulate near the magnetic rotor shown in inset of Fig. S1. The microwave absorption performances are greatly dependent on electromagnetic parameters, where the real and imaginary part of complex permittivity and complex permeability describe the storage and loss ability of electromagnetic energy, respectively [36]. In addition, dielectric loss tangents are also important indicators for describing dielectric attenuation ability of flower-like composites. Remarkably, the real permittivity (Fig. 4(a)) of the composite materials decreases with rising frequency, which exhibits a typical frequency dispersion
(ε′ − ε∞)2 + (ε″)2 = (εs − ε∞)2
(1)
where εs and ε∞ are the static permittivity and relative permittivity at the high-frequency limit, respectively. According to this equation, we could deduced that the graph of ε′ to ε″ should be a unitary semicircle, which expresses a Debye relaxation process. As observed, the ε′-ε″ curves of NC700 and NC800 show more semicircles than those of NC600. In addition, the ε′-ε″ curve values of NC800 are the biggest, which indicates the strongest Debye relaxation. In addition, Fig. 5 shows that the NC800 possesses high values of ε′ and ε″, which indicating the strong Debye relaxation to dielectric loss [10,13]. Because of the great quantity of Ni nanoparticles in flower-like Ni/C composites, magnetic attenuation is unquestionably essential element for evaluating their electromagnetic wave absorbing properties. The μ′ values of NC600 and NC700 gradually decrease from 1.25 to 0.9 and from 1.29 to 0.93 with the increase frequency, respectively. Remarkably, the μ′ values of NC800 and NC900 drop sharply around 5
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Fig. 7. RL values of all flower-like Ni/C composites (a) NC600, (b) NC700, (c) NC800, and (d) NC900.
4 GHz, as shown in Fig. 4(d). The μ′ values of four samples show two resonance peaks at about 9 and 11.8 GHz. At the same time, the μ″ values of the Ni/C composites in 2–10 GHz fluctuate sharply and strong resonance peaks are observed in Fig. 4(e). The high magnetic loss may come from natural ferromagnetic resonance. The natural ferromagnetic resonance, as one of the main factors causing magnetic attenuation, could be further supported through the peaks in tanδμ graph [41]. In addition, eddy current effect also makes an important contribution to magnetic attenuation. We calculated the relationship between μ″(μ′)−2f−1 [42] and frequency, as shown in Fig. 6. If the eddy current effect is the only cause of magnetic loss, the value of μ″(μ′)−2f−1 should be a constant [43]. Obviously, the μ″(μ′)−2f−1 values of all flower-like Ni/C composites vary with frequency in 2–10 GHz. Nevertheless, the μ″(μ′)−2f−1 values have changed little in 10–18 GHz, which indicates eddy current effect contributes prominently to magnetic attenuation in 10–18 GHz. According to transmission line theory, the microwave attenuation capacity could usually be expressed through RLs, which could be conclude by the following formulas: [44]
Zin = Z0 (μr / εr )1/2tan h [j (2πfd )(μr εr )1/2 / c ]
(2)
RL = 20log |(Zin − Z0)/(Zin + Z0)|
(3)
where Z0 represents the impedance of free space, Zin represents the input impedance, f is the electromagnetic wave frequency, d is the coating thickness, and c is the velocity of electromagnetic wave. Fig. 7 presents theoretical RL values of flower-like composites with different thickness in 2–18 GHz. Remarkably, we can conclude that the calcination conditions have important influences on the microwave absorption capabilities of these flower-like Ni/C composites. As shown in Fig. 7(a), the microwave absorption ability of NC600 is very weak with −12 dB and the absorption bandwidth is narrow. With the increase of temperature, the electromagnetic wave absorbing properties of flower-like Ni/C nanocomposites have been significantly improved. From Fig. 7(c), we found that the NC800 possesses the best microwave absorption performance. Compared with other flower-like Ni/C composites, the NC800 achieved the strongest absorption with maximum
Fig. 8. (a) Attenuation constant α and (b) relative input impedance (|Zin/Z0|) of the Ni/C composites with different frequency.
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RL of −52.4 dB. More importantly, the corresponding coating thickness is only 1.6 mm. However, with further increase of temperature, the RL of composites begins to decrease. The maximum RL of NC900 is only −36 dB with the corresponding thickness of 3.5 mm, as shown in Fig. 7(d). In addition, compared with other MOF-derived carbon materials, flower-like Ni/C composites show superior comprehensive properties, as shown in Table S1. In general, good impedance matching characteristic is the prerequisite condition for electromagnetic wave attenuation. When the |Zin/Z0| value is close to 1.0, there will be no reflected electromagnetic waves [44]. According to the equation (3), we calculated the |Zin/Z0| values of the composite at different calcination temperature, as shown in Fig. 8(b). The Ni/C composite calcined at 800 °C displays the best impedance matching, which indicate that the microwave can enter the interior of materials with minimum reflection. In order to further understand the absorbing mechanism, the schematic diagram is shown in Fig. S3 After the electromagnetic wave entered absorbing materials, the electromagnetic energy was converted into heat energy by the dielectric loss and magnetic loss. The dielectric loss derive from the interfacial polarization, dipole polarization, migration, and hopping of electrons, while the magnetic loss from natural resonance and exchange resonance of Ni nanoparticles. Moreover, these porous flower-like structures could produce multiple scatterings between petals, and the porous structure further increases the scattering strength, which are beneficial to the microwave attenuation. For confirm the above results, we computed the attenuation constant (α) of the Ni/C composites: [45,46]
α=
2πf c
×
(μ″ε″ − μ′ε′) +
(μ″ε″ − μ′ε′)2 + (μ′ε″ − μ″ε′)2
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From the curves of attenuation constants, we observed that the composites calcined at 800 °C possess the greatest attenuation, especially at high frequencies. From Fig. 4(c), it is obvious that the NC800 possesses the highest tanδε in the frequency range of 2–18 GHz. It is indicated that the NC800 has the strongest dielectric loss. In addition, from Fig. 4(c) and (f), dielectric loss tangent is much larger than the magnetic loss tangent, indicating that the loss mechanism of the Ni/C composite is mainly based on dielectric loss. Therefore, the NC800 has the best microwave absorption properties because of optimum impedance matching characteristic and the greatest attenuation capacity.
4. Conclusion In summary, the 3-D flower-like Ni/C composites were fabricated by synthesis of Zn-doped Ni-MOF precursor in a N2 atmosphere. Compared with previous strategies for controllable synthesis of 3-D flower structures, this method is simple, low-cost and without surface active agents. We find that the reaction temperature have important influences on microstructure, composition, and microwave absorbing characteristic. These 3-D flower-like structures have massive porous and large spacing flakes, which increases the electromagnetic wave scatter. The optimized flower-like Ni/C composites display high electromagnetic absorption performances. Interestingly, the optimum one calcined at 800 °C shows strong electromagnetic attenuation with an optimal RL value of −52.4 dB. Remarkably, the effective absorption bandwidth (< −10 dB) of absorbing coating is 5 GHz and the matching thickness is only 1.6 mm. We believe that the 3-D flower-like composites derived from MOF may supply a good strategy for the preparation of novel electromagnetic absorber.
Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant Nos. 61378002) and the Key Research and Development Plan of Hunan Province (Grant No. 2017NK2121). 7
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