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Co composites with hierarchical structure for electromagnetic absorption
Journal of Alloys and Compounds 826 (2020) 154203
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MOF-derived jujube pit shaped C/Co composites with hierarchical structure for electromagnetic absorption Chenglei Peng*, Yanan Zhang, Baoshan Zhang School of Electronic Science and Engineering, Nanjing University, 163 Xianlin Ave, Nanjing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2019 Received in revised form 4 February 2020 Accepted 5 February 2020 Available online 10 February 2020
A novel flake-assembled jujube pit shaped C/Co composites assembled by plentiful flakes has been successfully synthesized by annealing Co-based metal-organic frameworks (MOFs) at different temperatures. Owing to the optimal attenuation capacity, impedance matching and unique hierarchical structure, C/Co composites annealed at 600o C exhibited excellent reflection loss (RL) performance. The minimum RL value could reach 41 dB at 8.3 GHz with a matching thickness of 2.8 mm. Moreover, the effective bandwidth could reach 5.6 GHz (10.215.8 GHz) with a matching thickness of only 2.0 mm. This study not only provides a promising microwave absorber but also develops a facile method to synthesize C/Co composites with regular hierarchical structure. © 2020 Elsevier B.V. All rights reserved.
Keywords: Hierarchical structure Magnetic-carbon composites MOF-Derived Jujube pit shaped Microwave absorption
1. Introduction Considering the serious pollution caused by the electromagnetic wave, increasing efforts have been devoted to developing highly efficient microwave absorbers. As a kind of traditional microwave absorbers, magnetic materials including metals, alloys and have been intensively investigated [1e6]. Nevertheless, the drawbacks of high density and narrow bandwidth caused by the limited microwave absorption mechanism restricted their further application [7,8]. Numerous efforts have been devoted to conquer the abovementioned problems. Generally, there were several significant methods up till now. The first approach was the structure design of magnetic absorbers, especially hierarchical structures, which could not only decrease their density but also improve absorption efficiency. For example, Li et al. have proved that u-channelled spherical FeCoNi Janus colloidal particles own a broad effective bandwidth (RL -10 dB) of 9.2 GHz at thickness of 2.0 mm [9]. Liu et al. have prepared Fe3 O4 /C nanosheets via a low-cost carbothermal reduction process, and found that inner Fe3 O4 flakes show strong shape anisotropy effect, resulting in a higher natural resonance frequency and thus improved microwave absorption properties [10]. And reports about heterostructured nanorings of Fe
* Corresponding author. E-mail address: [email protected] (C. Peng). https://doi.org/10.1016/j.jallcom.2020.154203 0925-8388/© 2020 Elsevier B.V. All rights reserved.
Fe3 O4 @C hybrid and waxberry-like hierarchical Ni@C microspheres can also prove the important effects of hierarchical structures on impendence matching and strong interfacial polarization [11,12]. Based on the absorbing mechanism, combining magnetic materials with dielectric materials (like carbon materials) was another effective method. Moreover, massive studies have shown that the introduction of carbon materials can not only decrease the density of absorbers but also ameliorate the electromagnetic performance to some extent [13e15]. Furthermore, the better impedance matching and dielectric loss inherited from carbon materials equip the composites with extended working frequency [16,17]. Taking the advantages of two methods mentioned above, hierarchical structured magneticcarbon absorbers assembled by flakes would be a kind of promising candidates to meet the high requirements of strong absorption, light weight, thin thickness and broad bandwidth [18]. Recently, Cheng et al. discovered superior microwave attenuation performance in bamboo-like CNTs with CoNi nanoparticles in the end. Excellent effective bandwidth can be obtained in X band with filler ratio as low as 20 wt% [19]. And broad effective bandwidth of 6.0 GHz and 5.9 GHz could also be obtained by porous Fe3 O4 /C composite flowers and hierarchical porous Co/C crabapples respectively [20]. Liu et al. found that the permittivity, permeability and exchange resonance could be significantly enhanced by the special hierarchical structure of Co/C/Fe/C flowers with strawberrylike surface [21]. Despite an optimal absorbing performance
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obtained of this kind of material, the stepwise synthesis of flowerlike magnetic particles followed by carbothermal reduction processes may be tedious. Therefore, it is urgent to develop a facile method to obtain appropriate precursors for magnetic-carbon absorbers with hierarchical structure. Fortunately, the rapid developments of MOFs in varied fields might offer a promising way to solve the problem mentioned above. For example, wrinkled carbon foils can be fabricated from MOF nanosheets and applied in superior sodium storage [22]. By simply adsorbing melamine, conventional ZIF-67 has also been successfully employed to gain composites with abundant pores and high specific surface area [23]. Considering the outstanding virtues of MOFs and their composites, various hierarchical architectures could be designed and constructed by changing metal ions, organic ligands and composite structure [24]. As far as we are concerned, the reports about MOF-derived hierarchical composites assembled by flakes for electromagnetic wave absorption were rare. Hence, a facile method related with the MOFs has been put forward to synthesize the C/Co composites with hierarchical structure, which can be used to investigate the relations between structure and microwave absorbing performance. The as-synthesized composites were assembled by carbon flakes wrapped with Ni nanoparticles and exhibited the spindle-like morphology. With the changing calcining temperature, the structure of the composites maintains well. However, the microwave absorbing performance were different because of the variation of electromagnetic parameters. With the matching thickness of 2.8 mm, the RL could reach 40.1 dB. Moreover, the effective microwave absorption bandwidth (RL -10 dB) could be 5.6 GHz (10.2 15.8 GHz) at the thickness of only 2.0 mm. Therefore, we not only synthesize C/Co composites with unique structure, but also shed new light on the microwave absorbing properties control. 2. Experimental section 2.1. Synthesis of Co-MOF All chemicals were of analytical grade and used without further purification. In this work, The Co-MOF has been prepared by a solvothermal reaction. Firstly, 0.674 g sodium benzoate (5.5 mmol) was added into 30 mL N,N-Dimethylformamide (DMF), which was labeled as solution A. The solution containing cobaltous acetate tetrahydrate (1.94 g, 7.8 mmol) and DMF (30 mL) was labeled as solution B. Solution A was added dropwise into solution B and stirred for 12 h. After 1.83 g 1,4-benzenedicarboxylic acid was added and completely dissolved, the above mixtures were stirred for another 1 h and transferred into stainless steel autoclave with a capacity of 100 mL. After reacting for 10 h at 180o C, the precipitates were collected by centrifugation, rinsed with DMF and ethanol for three times, and dried in vacuum at 80o C. 2.2. Synthesis of flake-assembled jujube pit shaped Co@C composites Jujube pit shaped Co@C composites could be obtained by calcinating the above Co-MOF at 500o C, 600o C and 700o C for 2 h under Ar atmosphere, respectively. And as-prepared products were named as Co/C-500, Co/C-600 and Co/C-700. 2.3. Materials characterization The X-ray diffraction (XRD) patterns for phase analysis were recorded from Rigaku D/max-rC diffractometer using Cu Ka radiation (l ¼ 0.154718 nm with 40 kV scanning voltage and 40 mA scanning current). The detailed information of morphology and
structures of Co/C composites could be collected by scanning electron microscopy (FE-SEM, HITACHI S4800) and transmission electron microscopy (TEM, JEOL JEM-2100) in detail. The content of Co was measured by inductively coupled plasma-atomic emission spectroscopy (ICP, Optima 500 DV). By means of vector network analyzer (Agilent PNA N5224A), the electromagnetic parameters including complex permittivity and magnetic permeability could be determined in the frequency range of 218 GHz. Before testing, the as-prepared products should be mixed with paraffin uniformly and then pressed into toroidal rings with outer diameter of 7.0 mm and inner diameter of 3.04 mm afterwards. The mass fraction of asprepared production in toroidal ring was fixed at 40%. 3. Results and discussion As shown in Fig. 1(a), the typical peaks of three as-prepared products at 44:2o , 51:5o and 75:8o were corresponding to the (111), (200) and (220) planes of cubic Co (JCPDS 15e0806) respectively and there were no other peaks which could be observed. To further confirm the composition of C/Co-500, C/Co600 and C/Co-700, the Raman spectra were also provided (Fig. 1(b)). The D band at 1350 cm1 and G band at 1580 cm1 could be seen clearly, indicating the coexistence of amorphous and graphitic carbon [25]. In general, the graphitization degree could be estimated by the intensity ratio of D and G peaks (ID =IG ) [26]. The ID =IG values of C/Co-500, C/Co-600 and C/Co-700 were 0.95, 0.74 and 0.60, that have been summarized in Fig. 1(c), suggesting the increasing graphitization degree of three sample. This phenomenon may be caused by the better catalytic activity of transition metal at increased annealing temperatures [7]. To investigate the morphology and structures of three samples, the SEM and TEM images were given. According to relevant literature, the C/Co composites with 3D carbon framework were inherited from Co-MOF, which is usually built by the connection of Co2 þ , and 1,4-benzenedicarboxylic acid [23]. And sodium benzoate in this experiment was employed as a modulator to adjust the available MOF growth directions. The morphologies of Co-MOF were exhibited in Fig. S1. Obviously, the 3D framework assembled by nanosheets with smooth surface have been formed after solvothermal process. From the SEM images of C/Co-500, C/Co-600 and C/Co-700 (Fig. 2(a, b, c)), we could see that three samples maintained the same jujube pit-shaped morphology and the diameters of particles were mostly limited in the range of 78 mm. It is noteworthy that the structure of composites did not collapse at high calcination temperature, proving the outstanding stability of the typical structure compared with other structures such as nanorods and nanoparticles within rhombic dodecahedral shape [27e29]. On the basis of magnified images, the composites were assembled by massive flakes with the thickness of about 100 nm. A great deal of pores could also be observed on the surface of carbon sheets. As exhibited in TEM images of C/Co-600 (Fig. 2(f and g) and the inset in Fig. 2(e)), the nanoparticles with the diameter of 2030 nm were distributed on the flakes. Obvious boundary between carbon shell and Co nanoparticles could be viewed, as marked in the Fig. 2(h). The existence of carbon-magnetic structure could not only prevent magnetic particles from hostile environment, but also provide stronger interfacial polarization, benefitting the dissipation of electromagnetic wave [30,31]. The well-defined interplanar distance of the core is approximately 0.2079 nm, in line with the (111) planes of Co. As exhibited in elemental mapping in Fig. 2(i), C and Co elements were distributed uniformly. Therefore, by virtue of Co-MOF precursors, the unique jujube pit shaped structure assembled by carbon sheets embedded with Co nanoparticles were synthesized successfully, shown in Fig. 3(a). Furthermore, there were no significant differences among
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Fig. 1. (a) XRD patterns of C/Co-500, C/Co-600 and C/Co-700; (b) Raman Spectra of C/Co-500, C/Co-600 and C/Co-700; (c) the ID =IG values of C/Co-500, C/Co-600 and C/Co-700.
Fig. 2. (a, b, c) SEM images of C/Co-500, C/Co-600 and C/Co-700; (d, e) SEM images of C/Co-600; (f, g) TEM images of C/Co-600; (h, i) HRTEM image elemental mapping of C/Co-600.
morphologies of three samples. The nonmental element contents and Co content (obtained by ICP-AES) were further supplied. As shown in Table 1, the mass content of N element decreased at elevated temperature and held steady around at 7.5 wt% in this study. It is worth mentioning that
carbon element of composites increased slightly at high calcination temperature. And composites possessed at 600o C obtained the highest Co content. By the element analysis, we could conclude that annealing temperature did not seriously affect element content. To prove the high surface area of MOF-derived composites, the
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Fig. 3. (a) Schematic illustration of jujube pit shaped C/Co composites. (b) N2 adsorption-desorption isotherms of C/Co-500, C/Co/600 and C/Co/700; the inset is the average pore size of C/Co-500, C/Co-600 and C/Co-700.
Table 1 Element analysis results of the obtained C/Co composites. Sample
N (wt%)
C (wt%)
Co (wt%)
Others (wt%)
S500 S600 S700
10.7 7.47 7.6
31.42 32.99 35.49
37.2 40.66 38.17
20.68 18.88 18.74
N2 adsorption-desorption isotherms of C/Co-500, C/Co-600 and C/ Co-700 are provided in Fig. 3(b). The SBET of three samples are 197.4 m2 /g, 114.3 m2 /g and 92.7 m2 /g. And with the increasing temperature, the average pore size first rises then falls. It is not hard to understand that the precursors would have a complete decomposition gradually, and the slight collapse of micro-pore structure under high temperature [32]. According to Maxwell-Garnett theory, the pores in the composites may be conducive to impedance matching [33e35]. The microwave absorbing performance can be evaluated by RL which is determined by electeomagnetic parameters including complex permittivity (εr ) and complex permeability (mr ) of samples. The real and imaginary parts of complex permittivity (ε’; ε’’) represent the storage and attenuation capacity of dielectric energy. Similarly, the real and imaginary parts of complex permeability (m’; m’’) represent the storage and attenuation capacity of magnetic energy. Fig. 4(a) and (b) exhibit the real and imaginary part of permittivity of C/Co-500, C/Co-600 and C/Co-700. The ε’ values of three samples are in the range of 3.32.5, 13.0 8.0 and 62.9 11.7 at 218 GHz, respectively. The ε’’ values of C/Co-500 are less than 0.6 at 218 GHz. For C/Co-600, the maximum ε’’ value of 6.2 occurs at 2 GHz and the minimum value of 2.5 takes place at 16.44 GHz. The ε’’ values of C/Co-700 are restricted in the range of 48.3.10.7. It is obvious that the higher ε’ and ε’’ values of as-prepared products would be reached at the higher calcination temperature. Thus, C/Co-700 may own stronger ability of storing and attenuating electric energy. According to free electron theory, electrical conductivity (sAC ) is an important factor deciding ε’’ values, which can be described by the formula below [36].
ε’’ ¼
sAC 2pf ε0
(1)
where f is the microwave frequency and ε0 is dielectric constant of free space. The electrical conductivity of C/Co-700 originates in
highest graphitization degree in above-mentioned samples, resulting in superior attenuation capacity of electric energy. In other words, because of the minor differences of structure and element contents among as-prepared samples, graphitization degree may be the dominant factor affecting permittivity of composites. The C/Co-600 possesses highest m’ values at 26 GHz. And there are two obvious resonance peaks for C/Co-700 at 12 GHz and 16 GHz (Fig. S2). From Fig. 4(c) and (d), we may safely concluded that the dielectric loss played a dominant role in electromagnetic wave absorbing in C/Co composites. Generally, the microwave absorbing performance of materials is always evaluated by RL values. According to transmission line theory, RL is determined by electromagnetic parameters (εr ; mr ), frequency(f) and matching thickness (d).
Zin ¼ Zo
rffiffiffiffiffi mr 2pfd pffiffiffiffiffiffiffiffiffi tanh j mr εr c εr
Z Z o RL ¼ 20log in Zin þ Zo
(2)
(3)
where Zin is the input impedance of the absorber, Zo is the impedance of free space. The RL values of absorbers with the various matching thickness of 2.0, 2.2, 2.4, 2.8 and 3.2 mm in 218 GHz were shown in Fig. 5(a, b, c). As a qualified absorber, RL value less than 10 dB is of great necessity, which means materials can attenuate 90% of incident wave. However, the C/Co-500 and C/ Co-700 could hardly meet this requirement, their minimum RL values were greater than 5 dB and 6 dB with thickness larger than 3.2 mm. Fortunately, the sample C/Co-600 possessed the excellent electromagnetic absorbing performance and its minimum RL was 41 dB when the matching thickness was 2.8 mm. Besides minimum RL, effective bandwidth (RL -10 dB) is another important target for practical application. When the matching thickness was only 2.0 mm, the optimal effective bandwidth of C/Co-600 could be 5.6 GHz (10.2 15.8 GHz). From the three-dimensional (3D) RL representation of C/Co-600 exhibited in Fig. S3, the better effective bandwidth and RL could be observed at lower matching thickness. Consequently, C/Co-600 assembled by flakes could be regarded as a kind of promising candidates for attenuating electromagnetic wave. Compared with MOF-derived composites reported recently [37e42], C/Co composites were among the best, as shown in Table 2.
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Fig. 4. (a, b) The real and imaginary permittivity of C/Co-500, C/Co-600 and C/Co-700; (c, d) the dielectric loss tangents (tandε ¼ ε’’ =ε’ ) and magnetic loss tangents (tandm ¼ m’’ = m’ ) of C/Co-500, C/Co-600 and C/Co-700.
Fig. 5. (a, b, c) The reflection loss of C/Co-500, C/Co-600 and C/Co-700.
Table 2 Microwave absorption properties of some similar absorbing materials. Sample
RLmin (dB)
fe
d (mm)
Paraffin (wt%)
Ref.
NiCo/nanoporous carbon Fe3O4/ZIF-67@ wood aerogel C/Co Co/N-Decorated Carbon Co/C@V2O3 ferrite/Co/porous carbon C/Co
51 23.4 15.7 44.6 40.1 47.3 24.2
4.5 3.5 5.4 4.5 4.64 2.72 5.6
1.5 1.5 1.7 1.5 1.5 2.5 2.0
30% / 30% 20% 20% 70% 40%
37 38 39 40 41 42 Our work
To compare the comprehensive attenuation ability of as-prepared products more intuitively, the attenuation constant a is necessary (Fig. 6(a)), which can be expressed by the formula below [43].
pffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf ’’ ’’ ’ ’ ðm ε m ε Þ þ ðm’’ ε’’ m’ ε’ Þ2 þ ðm’’ ε’’ þ m’ ε’ Þ2 a¼ c
(4)
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Fig. 6. (a) The attenuation constant a of C/Co-500, C/Co-600,C/Co-700; (b, c, d) jZin =Zo j values for C/Co-500, C/Co-600 and C/Co-700.
where c is the velocity of light. Obviously, the highest a value of C/ Co-700 proved its optimal attenuation ability. However, it had a poor RL performance. This phenomenon can be explained by impedance matching, another significant factor influencing microwave absorption properties. In general, the impedance matching of materials can be described by jZin =Zo j values. When jZin = Zo j value is 1, namely the input impedance of the absorber Zin is equal to the impedance of free space Zo , the electromagnetic wave will enter into absorbers totally [43]. Therefore, the outstanding impedance matching is precondition of optimal RL. As exhibited in Fig. 6(b, c, d), the area marked by red dotted lines represents the jZin = Zo j values at the range of 0.81.2. Compared with the sample C/Co500, the jZin =Zo j values of C/Co-600 covers broader frequency at same matching thickness, manifesting the better impedance matching. Furthermore, the jZin =Zo j values of C/Co-700 are less than 0.45 at measured thickness and frequency, revealing that most of electromagnetic wave are reflected on the surface of absorbers. Although the high attenuation capacity exists, the poor impedance matching properties lead to the poor RL. By analyzing the electromagnetic parameters, dielectric loss contributes more in attenuation mechanism of C/Co-600. To further reveal the polarization processes, Cole-Cole plots of C/Co-600 were provided in Fig. S4. On the basis of Debye theory, there is a relationship between ε’ and ε’’ below.
ε’
ε ε 2 ε S þ ε ∞ 2 ∞ þ ðε’’ Þ2 ¼ S 2 2
(5)
It can be deduced that the plot of ε’’ versus ε’ should be a semicircle (cole-cole circle) and every circle corresponds to a polarization process. Evidently, three polarization processes exist in sample C/Co-600. On the one hand, a great deal of interfaces between carbon sheets and Co particles supply a stronger interfacial polarization. On the other hand, the gaps between carbon sheets have a significant effect on multiple reflection of electromagnetic wave. Furthermore, for the C/Co composites assembled by abundant flakes, adjusting electromagnetic parameters by changing annealing temperatures is an effective method to develop their application in the microwave absorbing field.
4. Conclusions The jujube pit shaped C/Co composites with hierarchical structure assembled by a great deal of flakes derived from MOFs precurors were successfully synthesized. By changing calcination temperatures, the optimal attenuation capacity and impedance matching property could be obtained. Combined with specific structure, the minimum reflection loss (RL) of C/Co-600 could reach 41 dB with the matching thickness of 2.8 mm. Moreover, when the matching thickness was only 2.0 mm, the effective bandwidth could be 5.6 GHz. In this study, we not only designed and obtained the high-efficiency absorbers but also provided a facile method to synthesize the jujube pit shaped C/Co composites assembled by flakes.
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Acknowledgements Financial support from the Aeronautical Science Foundation of China (2017ZF52066), the National Natural Science Foundation of China (No. 11575085), the Qing Lan Project, the Six talent peaks project in Jiangsu Province (No. XCL-035), the Jiangsu 333 Talent Project and the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology of Nanjing University are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154203. References [1] W. Dai, F. Chen, H. Luo, Y. Xiong, X. Wang, Y. Cheng, R. Gong, Synthesis of yolkshell structured carbonyl iron@ void@ nitrogen doped carbon for enhanced microwave absorption performance, J. Alloys Compd. 812 (2020) 152083. [2] Y. Zhang, Z. Yang, M. Li, L. Yang, J. Liu, Y. Ha, R. Wu, Heterostructured CoFe@C@ MnO2 nanocubes for efficient microwave absorption, Chem. Eng. J. 382 (2020) 123039. [3] G. Shao, J. Liang, W. Zhao, B. Zhao, W. Liu, H. Wang, B. Fan, H. Xu, H. Lu, Y. Wang, et al., Codecorated polymer-derived cSiCNeramic aerogel composites with ultrabroad microwave absorption performance, J. Alloys Compd. 813 (2020) 152007. [4] L. Wang, X. Yu, X. Li, J. Zhang, M. Wang, R. Che, MOF-derived yolk-shell Ni@C@ ZnO Schottky contact structure for enhanced microwave absorption, Chem. Eng. J. 383 (2020) 123099. [5] X. Wang, F. Pan, Z. Xiang, Q. Zeng, K. Pei, R. Che, W. Lu, Magnetic vorte coreshell Fe3O4@C nanorings with enhanced microwave absorption performance, Carbon 157 (2020) 130e139. [6] X. Cui, W. Liu, W. Gu, X. Liang, G. Ji, Two-dimensional MoS2modified using CoFe2O4 nanoparticles with enhanced microwave response in the anXd banKud, Inorg. Chem. Front. 6 (2019) 590e597. [7] Y. Zhang, B. Quan, W. Liu, X. Liang, G. Ji, Y. Du, A facile one-pot strategy for fabrication of carbon-based microwave absorbers: effects on annealing and paraffin content, Dalton Trans. 46 (2017) 9097e9102. [8] D. Xu, J. Qiao, N. Wu, W. Liu, F. Wang, L. Lv, J. Pan, Y. Dong, J. Liu, Facile synthesis of three-dimensional porous Co/MnO composites derived from bimetal oxides for highly efficient electromagnetic wave absorption, ACS Sustain. Chem. Eng. 7 (2019) 8687e8695. [9] H. Li, Z. Cao, J. Lin, H. Zhao, Q. Jiang, Z. Jiang, H. Liao, Q. Kuang, Z. Xie, Synthesis of u-channelled spherical Fex(CoyNi1y)100x Janus colloidal particles with excellent electromagnetic wave absorption performance, Nanoscale 10 (2018) 1930e1938. [10] Y. Liu, Y. Fu, L. Liu, W. Li, J. Guan, G. Tong, Low-cost carbothermal reduction preparation of monodisperse Fe3O4/C coreeshell nanosheets for improved microwave absorption, ACS Appl. Mater. Interfaces 10 (2018) 16511e16520. [11] X. Jian, X. Xiao, L. Deng, W. Tian, X. Wang, N. Mahmood, S. Dou, Heterostructured nanorings of FeFe3O4@C hybrid with enhanced microwave absorption performance, ACS Appl. Mater. Interfaces 10 (2018) 9369e9378. [12] D. Liu, Y. Du, P. Xu, N. Liu, Y. Wang, H. Zhao, L. Cui, X. Han, Waxberry-like hierarchical Ni@C microspheres with high-performance microwave absorption, J. Mater. Chem. C 7 (2019) 5037e5046. [13] Z. Lou, C. Yuan, Y. Zhang, Y. Li, J. Cai, L. Yang, W. Wang, H. Han, J. Zou, Synthesis of porous carbon matrix with inlaid Fe3C/Fe3O4 micro-particles as an effective electromagnetic wave absorber from natural wood shavings, J. Alloys Compd. 775 (2019) 800e809. [14] J. Qiao, X. Zhang, D. Xu, L. Kong, L. Lv, F. Yang, F. Wang, W. Liu, J. Liu, Design and synthesis of TiO2/Co/carbon nanofibers with tunable and efficient electromagnetic absorption, Chem. Eng. J. 380 (2020) 122591. [15] L. Ding, Y. Huang, X. Liu, Z. Xu, S. Li, J. Yan, P. Liu, Broadband and multilayer core-shell FeCo@C@mSiO2 nanoparticles for microwave absorption, J. Alloys Compd. 812 (2020) 152168. [16] W. Liu, J. Liu, Z. Yang, G. Ji, Extended working frequency of ferrites by synergistic attenuation through a controllable carbothermal route based on prussian blue shell, ACS Appl. Mater. Interfaces 10 (2018) 28887e28897. [17] G. Wu, Z. Jia, X. Zhou, G. Nie, H. Lv, Interlayer controllable of hierarchical MWCNTs@C@FexOy cross-linked composite with wideband electromagnetic absorption performance, Compos. Appl. Sci. Manuf. 128 (2020) 105687. [18] J. Lv, X. Liang, W. Liu, J. Chen, Z. Yang, G. Ji, Encapsulating metal nanoparticles inside carbon nanoflakes: a stable absorbent designed from free-standing sandwiched composites, Dalton Trans. 47 (2018) 11713e11721. [19] Y. Cheng, J. Cao, H. Lv, H. Zhao, Y. Zhao, G. Ji, In situ regulating aspect ratio of bamboo-like cnts via CoxNi1x-catalyzed growth to pursue superior
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
MOF-derived jujube pit shaped C/Co composites with hierarchical structure for electromagnetic absorption Chenglei Peng*, Yanan Zhang, Baoshan Zhang School of Electronic Science and Engineering, Nanjing University, 163 Xianlin Ave, Nanjing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2019 Received in revised form 4 February 2020 Accepted 5 February 2020 Available online 10 February 2020
A novel flake-assembled jujube pit shaped C/Co composites assembled by plentiful flakes has been successfully synthesized by annealing Co-based metal-organic frameworks (MOFs) at different temperatures. Owing to the optimal attenuation capacity, impedance matching and unique hierarchical structure, C/Co composites annealed at 600o C exhibited excellent reflection loss (RL) performance. The minimum RL value could reach 41 dB at 8.3 GHz with a matching thickness of 2.8 mm. Moreover, the effective bandwidth could reach 5.6 GHz (10.215.8 GHz) with a matching thickness of only 2.0 mm. This study not only provides a promising microwave absorber but also develops a facile method to synthesize C/Co composites with regular hierarchical structure. © 2020 Elsevier B.V. All rights reserved.
Keywords: Hierarchical structure Magnetic-carbon composites MOF-Derived Jujube pit shaped Microwave absorption
1. Introduction Considering the serious pollution caused by the electromagnetic wave, increasing efforts have been devoted to developing highly efficient microwave absorbers. As a kind of traditional microwave absorbers, magnetic materials including metals, alloys and have been intensively investigated [1e6]. Nevertheless, the drawbacks of high density and narrow bandwidth caused by the limited microwave absorption mechanism restricted their further application [7,8]. Numerous efforts have been devoted to conquer the abovementioned problems. Generally, there were several significant methods up till now. The first approach was the structure design of magnetic absorbers, especially hierarchical structures, which could not only decrease their density but also improve absorption efficiency. For example, Li et al. have proved that u-channelled spherical FeCoNi Janus colloidal particles own a broad effective bandwidth (RL -10 dB) of 9.2 GHz at thickness of 2.0 mm [9]. Liu et al. have prepared Fe3 O4 /C nanosheets via a low-cost carbothermal reduction process, and found that inner Fe3 O4 flakes show strong shape anisotropy effect, resulting in a higher natural resonance frequency and thus improved microwave absorption properties [10]. And reports about heterostructured nanorings of Fe
* Corresponding author. E-mail address: [email protected] (C. Peng). https://doi.org/10.1016/j.jallcom.2020.154203 0925-8388/© 2020 Elsevier B.V. All rights reserved.
Fe3 O4 @C hybrid and waxberry-like hierarchical Ni@C microspheres can also prove the important effects of hierarchical structures on impendence matching and strong interfacial polarization [11,12]. Based on the absorbing mechanism, combining magnetic materials with dielectric materials (like carbon materials) was another effective method. Moreover, massive studies have shown that the introduction of carbon materials can not only decrease the density of absorbers but also ameliorate the electromagnetic performance to some extent [13e15]. Furthermore, the better impedance matching and dielectric loss inherited from carbon materials equip the composites with extended working frequency [16,17]. Taking the advantages of two methods mentioned above, hierarchical structured magneticcarbon absorbers assembled by flakes would be a kind of promising candidates to meet the high requirements of strong absorption, light weight, thin thickness and broad bandwidth [18]. Recently, Cheng et al. discovered superior microwave attenuation performance in bamboo-like CNTs with CoNi nanoparticles in the end. Excellent effective bandwidth can be obtained in X band with filler ratio as low as 20 wt% [19]. And broad effective bandwidth of 6.0 GHz and 5.9 GHz could also be obtained by porous Fe3 O4 /C composite flowers and hierarchical porous Co/C crabapples respectively [20]. Liu et al. found that the permittivity, permeability and exchange resonance could be significantly enhanced by the special hierarchical structure of Co/C/Fe/C flowers with strawberrylike surface [21]. Despite an optimal absorbing performance
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obtained of this kind of material, the stepwise synthesis of flowerlike magnetic particles followed by carbothermal reduction processes may be tedious. Therefore, it is urgent to develop a facile method to obtain appropriate precursors for magnetic-carbon absorbers with hierarchical structure. Fortunately, the rapid developments of MOFs in varied fields might offer a promising way to solve the problem mentioned above. For example, wrinkled carbon foils can be fabricated from MOF nanosheets and applied in superior sodium storage [22]. By simply adsorbing melamine, conventional ZIF-67 has also been successfully employed to gain composites with abundant pores and high specific surface area [23]. Considering the outstanding virtues of MOFs and their composites, various hierarchical architectures could be designed and constructed by changing metal ions, organic ligands and composite structure [24]. As far as we are concerned, the reports about MOF-derived hierarchical composites assembled by flakes for electromagnetic wave absorption were rare. Hence, a facile method related with the MOFs has been put forward to synthesize the C/Co composites with hierarchical structure, which can be used to investigate the relations between structure and microwave absorbing performance. The as-synthesized composites were assembled by carbon flakes wrapped with Ni nanoparticles and exhibited the spindle-like morphology. With the changing calcining temperature, the structure of the composites maintains well. However, the microwave absorbing performance were different because of the variation of electromagnetic parameters. With the matching thickness of 2.8 mm, the RL could reach 40.1 dB. Moreover, the effective microwave absorption bandwidth (RL -10 dB) could be 5.6 GHz (10.2 15.8 GHz) at the thickness of only 2.0 mm. Therefore, we not only synthesize C/Co composites with unique structure, but also shed new light on the microwave absorbing properties control. 2. Experimental section 2.1. Synthesis of Co-MOF All chemicals were of analytical grade and used without further purification. In this work, The Co-MOF has been prepared by a solvothermal reaction. Firstly, 0.674 g sodium benzoate (5.5 mmol) was added into 30 mL N,N-Dimethylformamide (DMF), which was labeled as solution A. The solution containing cobaltous acetate tetrahydrate (1.94 g, 7.8 mmol) and DMF (30 mL) was labeled as solution B. Solution A was added dropwise into solution B and stirred for 12 h. After 1.83 g 1,4-benzenedicarboxylic acid was added and completely dissolved, the above mixtures were stirred for another 1 h and transferred into stainless steel autoclave with a capacity of 100 mL. After reacting for 10 h at 180o C, the precipitates were collected by centrifugation, rinsed with DMF and ethanol for three times, and dried in vacuum at 80o C. 2.2. Synthesis of flake-assembled jujube pit shaped Co@C composites Jujube pit shaped Co@C composites could be obtained by calcinating the above Co-MOF at 500o C, 600o C and 700o C for 2 h under Ar atmosphere, respectively. And as-prepared products were named as Co/C-500, Co/C-600 and Co/C-700. 2.3. Materials characterization The X-ray diffraction (XRD) patterns for phase analysis were recorded from Rigaku D/max-rC diffractometer using Cu Ka radiation (l ¼ 0.154718 nm with 40 kV scanning voltage and 40 mA scanning current). The detailed information of morphology and
structures of Co/C composites could be collected by scanning electron microscopy (FE-SEM, HITACHI S4800) and transmission electron microscopy (TEM, JEOL JEM-2100) in detail. The content of Co was measured by inductively coupled plasma-atomic emission spectroscopy (ICP, Optima 500 DV). By means of vector network analyzer (Agilent PNA N5224A), the electromagnetic parameters including complex permittivity and magnetic permeability could be determined in the frequency range of 218 GHz. Before testing, the as-prepared products should be mixed with paraffin uniformly and then pressed into toroidal rings with outer diameter of 7.0 mm and inner diameter of 3.04 mm afterwards. The mass fraction of asprepared production in toroidal ring was fixed at 40%. 3. Results and discussion As shown in Fig. 1(a), the typical peaks of three as-prepared products at 44:2o , 51:5o and 75:8o were corresponding to the (111), (200) and (220) planes of cubic Co (JCPDS 15e0806) respectively and there were no other peaks which could be observed. To further confirm the composition of C/Co-500, C/Co600 and C/Co-700, the Raman spectra were also provided (Fig. 1(b)). The D band at 1350 cm1 and G band at 1580 cm1 could be seen clearly, indicating the coexistence of amorphous and graphitic carbon [25]. In general, the graphitization degree could be estimated by the intensity ratio of D and G peaks (ID =IG ) [26]. The ID =IG values of C/Co-500, C/Co-600 and C/Co-700 were 0.95, 0.74 and 0.60, that have been summarized in Fig. 1(c), suggesting the increasing graphitization degree of three sample. This phenomenon may be caused by the better catalytic activity of transition metal at increased annealing temperatures [7]. To investigate the morphology and structures of three samples, the SEM and TEM images were given. According to relevant literature, the C/Co composites with 3D carbon framework were inherited from Co-MOF, which is usually built by the connection of Co2 þ , and 1,4-benzenedicarboxylic acid [23]. And sodium benzoate in this experiment was employed as a modulator to adjust the available MOF growth directions. The morphologies of Co-MOF were exhibited in Fig. S1. Obviously, the 3D framework assembled by nanosheets with smooth surface have been formed after solvothermal process. From the SEM images of C/Co-500, C/Co-600 and C/Co-700 (Fig. 2(a, b, c)), we could see that three samples maintained the same jujube pit-shaped morphology and the diameters of particles were mostly limited in the range of 78 mm. It is noteworthy that the structure of composites did not collapse at high calcination temperature, proving the outstanding stability of the typical structure compared with other structures such as nanorods and nanoparticles within rhombic dodecahedral shape [27e29]. On the basis of magnified images, the composites were assembled by massive flakes with the thickness of about 100 nm. A great deal of pores could also be observed on the surface of carbon sheets. As exhibited in TEM images of C/Co-600 (Fig. 2(f and g) and the inset in Fig. 2(e)), the nanoparticles with the diameter of 2030 nm were distributed on the flakes. Obvious boundary between carbon shell and Co nanoparticles could be viewed, as marked in the Fig. 2(h). The existence of carbon-magnetic structure could not only prevent magnetic particles from hostile environment, but also provide stronger interfacial polarization, benefitting the dissipation of electromagnetic wave [30,31]. The well-defined interplanar distance of the core is approximately 0.2079 nm, in line with the (111) planes of Co. As exhibited in elemental mapping in Fig. 2(i), C and Co elements were distributed uniformly. Therefore, by virtue of Co-MOF precursors, the unique jujube pit shaped structure assembled by carbon sheets embedded with Co nanoparticles were synthesized successfully, shown in Fig. 3(a). Furthermore, there were no significant differences among
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Fig. 1. (a) XRD patterns of C/Co-500, C/Co-600 and C/Co-700; (b) Raman Spectra of C/Co-500, C/Co-600 and C/Co-700; (c) the ID =IG values of C/Co-500, C/Co-600 and C/Co-700.
Fig. 2. (a, b, c) SEM images of C/Co-500, C/Co-600 and C/Co-700; (d, e) SEM images of C/Co-600; (f, g) TEM images of C/Co-600; (h, i) HRTEM image elemental mapping of C/Co-600.
morphologies of three samples. The nonmental element contents and Co content (obtained by ICP-AES) were further supplied. As shown in Table 1, the mass content of N element decreased at elevated temperature and held steady around at 7.5 wt% in this study. It is worth mentioning that
carbon element of composites increased slightly at high calcination temperature. And composites possessed at 600o C obtained the highest Co content. By the element analysis, we could conclude that annealing temperature did not seriously affect element content. To prove the high surface area of MOF-derived composites, the
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Fig. 3. (a) Schematic illustration of jujube pit shaped C/Co composites. (b) N2 adsorption-desorption isotherms of C/Co-500, C/Co/600 and C/Co/700; the inset is the average pore size of C/Co-500, C/Co-600 and C/Co-700.
Table 1 Element analysis results of the obtained C/Co composites. Sample
N (wt%)
C (wt%)
Co (wt%)
Others (wt%)
S500 S600 S700
10.7 7.47 7.6
31.42 32.99 35.49
37.2 40.66 38.17
20.68 18.88 18.74
N2 adsorption-desorption isotherms of C/Co-500, C/Co-600 and C/ Co-700 are provided in Fig. 3(b). The SBET of three samples are 197.4 m2 /g, 114.3 m2 /g and 92.7 m2 /g. And with the increasing temperature, the average pore size first rises then falls. It is not hard to understand that the precursors would have a complete decomposition gradually, and the slight collapse of micro-pore structure under high temperature [32]. According to Maxwell-Garnett theory, the pores in the composites may be conducive to impedance matching [33e35]. The microwave absorbing performance can be evaluated by RL which is determined by electeomagnetic parameters including complex permittivity (εr ) and complex permeability (mr ) of samples. The real and imaginary parts of complex permittivity (ε’; ε’’) represent the storage and attenuation capacity of dielectric energy. Similarly, the real and imaginary parts of complex permeability (m’; m’’) represent the storage and attenuation capacity of magnetic energy. Fig. 4(a) and (b) exhibit the real and imaginary part of permittivity of C/Co-500, C/Co-600 and C/Co-700. The ε’ values of three samples are in the range of 3.32.5, 13.0 8.0 and 62.9 11.7 at 218 GHz, respectively. The ε’’ values of C/Co-500 are less than 0.6 at 218 GHz. For C/Co-600, the maximum ε’’ value of 6.2 occurs at 2 GHz and the minimum value of 2.5 takes place at 16.44 GHz. The ε’’ values of C/Co-700 are restricted in the range of 48.3.10.7. It is obvious that the higher ε’ and ε’’ values of as-prepared products would be reached at the higher calcination temperature. Thus, C/Co-700 may own stronger ability of storing and attenuating electric energy. According to free electron theory, electrical conductivity (sAC ) is an important factor deciding ε’’ values, which can be described by the formula below [36].
ε’’ ¼
sAC 2pf ε0
(1)
where f is the microwave frequency and ε0 is dielectric constant of free space. The electrical conductivity of C/Co-700 originates in
highest graphitization degree in above-mentioned samples, resulting in superior attenuation capacity of electric energy. In other words, because of the minor differences of structure and element contents among as-prepared samples, graphitization degree may be the dominant factor affecting permittivity of composites. The C/Co-600 possesses highest m’ values at 26 GHz. And there are two obvious resonance peaks for C/Co-700 at 12 GHz and 16 GHz (Fig. S2). From Fig. 4(c) and (d), we may safely concluded that the dielectric loss played a dominant role in electromagnetic wave absorbing in C/Co composites. Generally, the microwave absorbing performance of materials is always evaluated by RL values. According to transmission line theory, RL is determined by electromagnetic parameters (εr ; mr ), frequency(f) and matching thickness (d).
Zin ¼ Zo
rffiffiffiffiffi mr 2pfd pffiffiffiffiffiffiffiffiffi tanh j mr εr c εr
Z Z o RL ¼ 20log in Zin þ Zo
(2)
(3)
where Zin is the input impedance of the absorber, Zo is the impedance of free space. The RL values of absorbers with the various matching thickness of 2.0, 2.2, 2.4, 2.8 and 3.2 mm in 218 GHz were shown in Fig. 5(a, b, c). As a qualified absorber, RL value less than 10 dB is of great necessity, which means materials can attenuate 90% of incident wave. However, the C/Co-500 and C/ Co-700 could hardly meet this requirement, their minimum RL values were greater than 5 dB and 6 dB with thickness larger than 3.2 mm. Fortunately, the sample C/Co-600 possessed the excellent electromagnetic absorbing performance and its minimum RL was 41 dB when the matching thickness was 2.8 mm. Besides minimum RL, effective bandwidth (RL -10 dB) is another important target for practical application. When the matching thickness was only 2.0 mm, the optimal effective bandwidth of C/Co-600 could be 5.6 GHz (10.2 15.8 GHz). From the three-dimensional (3D) RL representation of C/Co-600 exhibited in Fig. S3, the better effective bandwidth and RL could be observed at lower matching thickness. Consequently, C/Co-600 assembled by flakes could be regarded as a kind of promising candidates for attenuating electromagnetic wave. Compared with MOF-derived composites reported recently [37e42], C/Co composites were among the best, as shown in Table 2.
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Fig. 4. (a, b) The real and imaginary permittivity of C/Co-500, C/Co-600 and C/Co-700; (c, d) the dielectric loss tangents (tandε ¼ ε’’ =ε’ ) and magnetic loss tangents (tandm ¼ m’’ = m’ ) of C/Co-500, C/Co-600 and C/Co-700.
Fig. 5. (a, b, c) The reflection loss of C/Co-500, C/Co-600 and C/Co-700.
Table 2 Microwave absorption properties of some similar absorbing materials. Sample
RLmin (dB)
fe
d (mm)
Paraffin (wt%)
Ref.
NiCo/nanoporous carbon Fe3O4/ZIF-67@ wood aerogel C/Co Co/N-Decorated Carbon Co/C@V2O3 ferrite/Co/porous carbon C/Co
51 23.4 15.7 44.6 40.1 47.3 24.2
4.5 3.5 5.4 4.5 4.64 2.72 5.6
1.5 1.5 1.7 1.5 1.5 2.5 2.0
30% / 30% 20% 20% 70% 40%
37 38 39 40 41 42 Our work
To compare the comprehensive attenuation ability of as-prepared products more intuitively, the attenuation constant a is necessary (Fig. 6(a)), which can be expressed by the formula below [43].
pffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf ’’ ’’ ’ ’ ðm ε m ε Þ þ ðm’’ ε’’ m’ ε’ Þ2 þ ðm’’ ε’’ þ m’ ε’ Þ2 a¼ c
(4)
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Fig. 6. (a) The attenuation constant a of C/Co-500, C/Co-600,C/Co-700; (b, c, d) jZin =Zo j values for C/Co-500, C/Co-600 and C/Co-700.
where c is the velocity of light. Obviously, the highest a value of C/ Co-700 proved its optimal attenuation ability. However, it had a poor RL performance. This phenomenon can be explained by impedance matching, another significant factor influencing microwave absorption properties. In general, the impedance matching of materials can be described by jZin =Zo j values. When jZin = Zo j value is 1, namely the input impedance of the absorber Zin is equal to the impedance of free space Zo , the electromagnetic wave will enter into absorbers totally [43]. Therefore, the outstanding impedance matching is precondition of optimal RL. As exhibited in Fig. 6(b, c, d), the area marked by red dotted lines represents the jZin = Zo j values at the range of 0.81.2. Compared with the sample C/Co500, the jZin =Zo j values of C/Co-600 covers broader frequency at same matching thickness, manifesting the better impedance matching. Furthermore, the jZin =Zo j values of C/Co-700 are less than 0.45 at measured thickness and frequency, revealing that most of electromagnetic wave are reflected on the surface of absorbers. Although the high attenuation capacity exists, the poor impedance matching properties lead to the poor RL. By analyzing the electromagnetic parameters, dielectric loss contributes more in attenuation mechanism of C/Co-600. To further reveal the polarization processes, Cole-Cole plots of C/Co-600 were provided in Fig. S4. On the basis of Debye theory, there is a relationship between ε’ and ε’’ below.
ε’
ε ε 2 ε S þ ε ∞ 2 ∞ þ ðε’’ Þ2 ¼ S 2 2
(5)
It can be deduced that the plot of ε’’ versus ε’ should be a semicircle (cole-cole circle) and every circle corresponds to a polarization process. Evidently, three polarization processes exist in sample C/Co-600. On the one hand, a great deal of interfaces between carbon sheets and Co particles supply a stronger interfacial polarization. On the other hand, the gaps between carbon sheets have a significant effect on multiple reflection of electromagnetic wave. Furthermore, for the C/Co composites assembled by abundant flakes, adjusting electromagnetic parameters by changing annealing temperatures is an effective method to develop their application in the microwave absorbing field.
4. Conclusions The jujube pit shaped C/Co composites with hierarchical structure assembled by a great deal of flakes derived from MOFs precurors were successfully synthesized. By changing calcination temperatures, the optimal attenuation capacity and impedance matching property could be obtained. Combined with specific structure, the minimum reflection loss (RL) of C/Co-600 could reach 41 dB with the matching thickness of 2.8 mm. Moreover, when the matching thickness was only 2.0 mm, the effective bandwidth could be 5.6 GHz. In this study, we not only designed and obtained the high-efficiency absorbers but also provided a facile method to synthesize the jujube pit shaped C/Co composites assembled by flakes.
C. Peng et al. / Journal of Alloys and Compounds 826 (2020) 154203
Acknowledgements Financial support from the Aeronautical Science Foundation of China (2017ZF52066), the National Natural Science Foundation of China (No. 11575085), the Qing Lan Project, the Six talent peaks project in Jiangsu Province (No. XCL-035), the Jiangsu 333 Talent Project and the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology of Nanjing University are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154203. References [1] W. Dai, F. Chen, H. Luo, Y. Xiong, X. Wang, Y. Cheng, R. Gong, Synthesis of yolkshell structured carbonyl iron@ void@ nitrogen doped carbon for enhanced microwave absorption performance, J. Alloys Compd. 812 (2020) 152083. [2] Y. Zhang, Z. Yang, M. Li, L. Yang, J. Liu, Y. Ha, R. Wu, Heterostructured CoFe@C@ MnO2 nanocubes for efficient microwave absorption, Chem. Eng. J. 382 (2020) 123039. [3] G. Shao, J. Liang, W. Zhao, B. Zhao, W. Liu, H. Wang, B. Fan, H. Xu, H. Lu, Y. Wang, et al., Codecorated polymer-derived cSiCNeramic aerogel composites with ultrabroad microwave absorption performance, J. Alloys Compd. 813 (2020) 152007. [4] L. Wang, X. Yu, X. Li, J. Zhang, M. Wang, R. Che, MOF-derived yolk-shell Ni@C@ ZnO Schottky contact structure for enhanced microwave absorption, Chem. Eng. J. 383 (2020) 123099. [5] X. Wang, F. Pan, Z. Xiang, Q. Zeng, K. Pei, R. Che, W. Lu, Magnetic vorte coreshell Fe3O4@C nanorings with enhanced microwave absorption performance, Carbon 157 (2020) 130e139. [6] X. Cui, W. Liu, W. Gu, X. Liang, G. Ji, Two-dimensional MoS2modified using CoFe2O4 nanoparticles with enhanced microwave response in the anXd banKud, Inorg. Chem. Front. 6 (2019) 590e597. [7] Y. Zhang, B. Quan, W. Liu, X. Liang, G. Ji, Y. Du, A facile one-pot strategy for fabrication of carbon-based microwave absorbers: effects on annealing and paraffin content, Dalton Trans. 46 (2017) 9097e9102. [8] D. Xu, J. Qiao, N. Wu, W. Liu, F. Wang, L. Lv, J. Pan, Y. Dong, J. Liu, Facile synthesis of three-dimensional porous Co/MnO composites derived from bimetal oxides for highly efficient electromagnetic wave absorption, ACS Sustain. Chem. Eng. 7 (2019) 8687e8695. [9] H. Li, Z. Cao, J. Lin, H. Zhao, Q. Jiang, Z. Jiang, H. Liao, Q. Kuang, Z. Xie, Synthesis of u-channelled spherical Fex(CoyNi1y)100x Janus colloidal particles with excellent electromagnetic wave absorption performance, Nanoscale 10 (2018) 1930e1938. [10] Y. Liu, Y. Fu, L. Liu, W. Li, J. Guan, G. Tong, Low-cost carbothermal reduction preparation of monodisperse Fe3O4/C coreeshell nanosheets for improved microwave absorption, ACS Appl. Mater. Interfaces 10 (2018) 16511e16520. [11] X. Jian, X. Xiao, L. Deng, W. Tian, X. Wang, N. Mahmood, S. Dou, Heterostructured nanorings of FeFe3O4@C hybrid with enhanced microwave absorption performance, ACS Appl. Mater. Interfaces 10 (2018) 9369e9378. [12] D. Liu, Y. Du, P. Xu, N. Liu, Y. Wang, H. Zhao, L. Cui, X. Han, Waxberry-like hierarchical Ni@C microspheres with high-performance microwave absorption, J. Mater. Chem. C 7 (2019) 5037e5046. [13] Z. Lou, C. Yuan, Y. Zhang, Y. Li, J. Cai, L. Yang, W. Wang, H. Han, J. Zou, Synthesis of porous carbon matrix with inlaid Fe3C/Fe3O4 micro-particles as an effective electromagnetic wave absorber from natural wood shavings, J. Alloys Compd. 775 (2019) 800e809. [14] J. Qiao, X. Zhang, D. Xu, L. Kong, L. Lv, F. Yang, F. Wang, W. Liu, J. Liu, Design and synthesis of TiO2/Co/carbon nanofibers with tunable and efficient electromagnetic absorption, Chem. Eng. J. 380 (2020) 122591. [15] L. Ding, Y. Huang, X. Liu, Z. Xu, S. Li, J. Yan, P. Liu, Broadband and multilayer core-shell FeCo@C@mSiO2 nanoparticles for microwave absorption, J. Alloys Compd. 812 (2020) 152168. [16] W. Liu, J. Liu, Z. Yang, G. Ji, Extended working frequency of ferrites by synergistic attenuation through a controllable carbothermal route based on prussian blue shell, ACS Appl. Mater. Interfaces 10 (2018) 28887e28897. [17] G. Wu, Z. Jia, X. Zhou, G. Nie, H. Lv, Interlayer controllable of hierarchical MWCNTs@C@FexOy cross-linked composite with wideband electromagnetic absorption performance, Compos. Appl. Sci. Manuf. 128 (2020) 105687. [18] J. Lv, X. Liang, W. Liu, J. Chen, Z. Yang, G. Ji, Encapsulating metal nanoparticles inside carbon nanoflakes: a stable absorbent designed from free-standing sandwiched composites, Dalton Trans. 47 (2018) 11713e11721. [19] Y. Cheng, J. Cao, H. Lv, H. Zhao, Y. Zhao, G. Ji, In situ regulating aspect ratio of bamboo-like cnts via CoxNi1x-catalyzed growth to pursue superior
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