C microspheres derived from Ni-metal organic framework for electromagnetic wave absorption

Journal Pre-proofs Hollow Ni/C microspheres derived from Ni-metal organic framework for electromagnetic wave absorption Yun Qiu, Ying Lin, Haibo Yang, Lei Wang, Mengqi Wang, Bo Wen PII: DOI: Reference:

S1385-8947(19)32619-1 https://doi.org/10.1016/j.cej.2019.123207 CEJ 123207

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

1 August 2019 7 October 2019 16 October 2019

Please cite this article as: Y. Qiu, Y. Lin, H. Yang, L. Wang, M. Wang, B. Wen, Hollow Ni/C microspheres derived from Ni-metal organic framework for electromagnetic wave absorption, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123207

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Hollow Ni/C microspheres derived from Ni-metal organic framework for electromagnetic wave absorption Yun Qiu, Ying Lin, Haibo Yang, Lei Wang, Mengqi Wang, Bo Wen School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an 710021, China *Corresponding *Corresponding

* *

authors. [email protected]

authors. [email protected] (Haibo Yang)

Corresponding author. Tel: +86-29-86168688; Fax: +86-29-86168688. Email: [email protected] Corresponding author. Tel: +86-29-86168688; Fax: +86-29-86168688. Email: [email protected]

Abstract: Hollow nickel/carbon (Ni/C) microsphere was synthesized by in situ pyrolysis of the Ni-based trimellitic acid framework (Ni-MOF) at argon atmosphere. The morphology of Ni-MOF precursor can be easily controlled by adjusting the volume ratio of DMF and H2O in solvothermal process. When the volume ratio of DMF: H2O is 2: 1, hollow Ni-MOF microspheres can be obtained and the corresponding Ni/C composite exhibits extremely strong electromagnetic wave absorption. The maximum reflection loss is up to -57.25 dB with the matching thickness of only 1.8 mm, while the corresponding effective absorption bandwidth (reflection loss below -10 dB) achieves 5.1 GHz. The excellent electromagnetic wave absorbing capabilities is related to the hollow structure and the synergistic effect between carbon and nickel nanoparticles. This work confirms that the as-prepared Ni/C composite with superior electromagnetic wave absorption properties is a promising candidate for lightweight electromagnetic wave absorbing materials. Keywords: Hollow; microsphere; Ni/C composite; electromagnetic wave absorption

1. Introduction With the rapid development of communication technology and the wide application of electronic products, electromagnetic radiation is seriously polluting the environment. The radiation effect of electromagnetic wave (EMW) not only interferes with the normal operation of the circuit, but also impairs the health of human beings. Currently, it has become a universal phenomenon that researchers make extensive efforts to develop the efficient EMW absorbing materials which possess strong adsorption, broad bandwidth, lightweight and thin matching thickness[1-3]. Common EMW absorbing materials include magnetic loss materials[4] and dielectric loss materials[5, 6]. In the past decades, magnetic metals[7] and ferrites[8] have received tremendous attention. However, the disadvantages of them like narrow absorbing bandwidth and high density restrict their practical applications seriously[9]. Combining the magnetic materials with lightweight dielectric materials such as carbonaceous materials may be a good solution to solve these problems by utilizing the synergistic effect of two components[10-12]. Examples included CoFe/carbon fiber composite[13], CoFe2O4/rGO composite[14], and carbon/Fe3C composite nanofibers[15]. Those studies indicate that constructing magnetic metals/carbon composites is an effective method to introduce dual loss mechanisms and enhance the EMW absorbing efficiency. Normally, optimizing the architecture of EMW absorbing materials is another effective approach to enhance their EMW absorbing properties[16, 17]. Many previous studies have proved that EMW absorption materials with yolk-shell[18] and

hollow[19] structure displayed noticeably EMW absorption performance. The hollow structure not only facilitates multiple reflection and scatters the incident EMW but also endows the materials with lightweight[20]. Metal-organic frameworks (MOFs) that consist with metals and ligands have been considered to be promising precursors or templates to obtain porous carbon-based materials through in-situ thermal carbonization process[21]. MOFs have high surface areas and effective porosities[22]. Therefore, they are ideal template materials to prepare porous carbon-based materials. Recently, the magnetic metal nanoparticles/carbon composites for EMW absorbing materials have been prepared by pyrolysising MOFs at inert atmosphere. For instance, Lü et al. successfully prepared the porous rhombic dodecahedral carbon/Co nanocomposites derived from ZIF-67 with a maximum reflection loss (RLmax) of -35.3 dB with the effective absorption bandwidth (RL ≤ −10 dB) of 5.8 GHz in 2.5 mm thickness[23]. Wang et al. reported that the two-dimensional Co/C composites derived from nanosheet-like ZIF-67 exhibited the RLmax of -39.3 dB at the matching thickness of 2.4 mm and the corresponding effective bandwidth is 5.1 GHz[7]. Liu and coworkers synthesized MOFs derived carbon-wrapped Ni composites[24]. The RLmax of Ni/C composite reached -51.8 dB at the thickness of 2.6 mm. Thus, using MOFs as precursors to obtain porous carbon-based EMW absorbing materials by in-situ thermal carbonization is worth further studying. Inspired by the above work, we prepared porous hollow Ni/C microsphere by in situ pyrolysis of Ni-MOF under Ar atmosphere. The microstructure of Ni-MOF

precursor can be controlled by adjusting the volume ratio of DMF/H2O mixed solvent in the solvothermal process. The hollow Ni/C microsphere exhibits a substantially enhanced EMW absorption performance with RLmax of -57.25 dB at 16.1 GHz when thickness is 1.8 mm and the corresponding effective bandwidth is 5.1 GHz. The results reveal that the hollow architecture endows material with strong attenuation ability of incident EMW. 2. Experimental procedure 2.1

Materials.

Nickel

nitrate

hexahydrate

(Ni(NO3)2•6H2O,

98%),

polyvinylpyrrolidone (PVP-K30, Mw = 40000, GR), benzene-1, 3, 5-tricarboxylic acid (H3BTC, 98%), ethanol (CH3CH2OH, ≥99.7%) and N,N-dimethylformamide (DMF, ≥99.5 %) were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2 Preparation of hollow Ni-MOF microspheres. The Ni-MOF was first synthesized according to the literature[25] with some modification. In typically procedure, 0.864 g Ni(NO3)2•6H2O, 0.3 g H3BTC, and 3 g PVP were dissolved into the deionized water and DMF mixed solution at a volumetric ratio of (60-x : x) (x is the volume of H2O, 0≤x≤30 mL). After magnetic stirring for 30 minutes, the homogeneous green solution was transferred into Teflon-lined stainless steel autoclave (100 mL) and maintained at 150 oC for 10h, and then the Ni-MOF microspheres were obtained. After cooling down to room temperature, the obtained products were washed with ethanol by centrifugation several times and dried under vacuum overnight. The obtained samples with different amounts of H2O (0, 10, 20, and 30 mL) were denoted as Ni-MOF-0, Ni-MOF-10, Ni-MOF-20 and Ni-MOF-30,

respectively. 2.3 Fabrication of Ni/C composites. The Ni/C composites were synthesized after heating Ni-MOF microspheres at 600 oC for 2h under Ar atmosphere at a heating rate of 2 oC/min. The obtained samples with different Ni-MOF precursors (Ni-MOF-0, Ni-MOF-10, Ni-MOF-20 and Ni-MOF-30) were denoted as Ni/C-0, Ni/C-10, Ni/C-20 and Ni/C-30, respectively. The fabrication procedure of Ni/C composites is illustrated in Scheme 1. 2.4 Materials characterization. The synthesized Ni/C composites were characterized by scanning electron microscope (SEM, Hitachi, S-4800), transmission electron microscopy (TEM, JEM-2100), X-ray diffraction (XRD, D8 Advance, Germany), Raman spectra (Raman, DXRxi), Brunner-Emmet-Teller (BET, ASAP 2020, Micromeritics) and X-ray photoelectron spectroscopy (XPS, AXIS SUPRA). 2.5 Electromagnetic measurements. The electromagnetic parameters of all the samples were measured on a vector network analyzer (VNA, HP8720ES, Agilent, USA) in transmission-reflection mode at 2-18 GHz. The Ni/C composites (30 wt%) were uniformly mixed with paraffin (70 wt%) at 80 oC and pressed into standard rings for the measurement of EMW absorption. The outer diameter, inner diameter and thickness of toroidal-like specimen were set to be 7.00, 3.04 and 3.00 mm, respectively. Paraffin has relatively low permittivity, which means that it is almost transparent to EMW. The microwave absorption properties of samples mixed with paraffin can be used to characterize the nature of materials[26]. 3. Results and discussion

3.1 Characterization of the materials. Figure 1(a-d) are the SEM images of Ni-MOF precursors with different DMF/H2O volume ratios. It can be seen that Ni-MOF precursors have a spherical morphology with a uniform size. In this system, PVP plays a vital role in regulating the morphology of Ni-MOF, because it is favorable to form regular spherical materials[27]. PVP is an organic polymer containing hydrophilic and hydrophobic groups and it is always used to synthesize nanomaterial as a steric stabilizer[16] or a capping agent with a major role to protect the product from agglomeration[28]. As the volume ratio of DMF/H2O decreasing, the diameter of the prepared Ni-MOF spheres becomes larger and larger. In general, rapid nucleation rates will lead to the formation of small grain sizes of MOF and vice versa[29]. The deprotonation rate of the organic linker in the solvent controls the nucleation rate and crystal growth rate[30]. H3BTC has higher deprotonation rate in DMF than that in water. Thus, H3BTC is more beneficial for the coordination of organic ligand with nickel ions, when the solvent is only DMF[31]. H3BTC has less solubility in water, which can be used to slow down the nucleation rate. Therefore, the deprotonation rate of H3BTC becomes slow in DMF/H2O mixed solvent. Consequently, the grain size of Ni-MOF is directly proportional to the increase of water volume. The yield of products is very low when the volume ratio of DMF/H2O is 1/2 because of the slow deprotonation rate of H3BTC in water. Figure 1(e-h) show the SEM images of Ni/C composites. The morphology of Ni-MOF after calcining at 600 oC at Ar atmosphere are well maintained, but the size

is significantly shrank due to the loss of organic ligand and PVP at high temperatures. The SEM images at high magnification (Figure S1 Supporting information) indicate that the surfaces of these spheres are composed of small particles. Figure 1(i-l) present the size distribution of Ni/C composites. Notably, the sizes of Ni/C composite spheres changes a lot with the volume ratio of DMF/H2O decreasing. The average size of Ni/C-0, Ni/C-10, Ni/C-20 and Ni/C-30 is ~0.35, ~0.55, ~1.1 and ~1.35 μm, respectively. It is known that the small size of material is beneficial to the microwave absorption due to the larger specific surface area. From this view of point, the Ni/C-0 sample might have better microwave absorption properties. TEM is used to further investigate the internal structural information of the Ni/C composites, as shown in Figure 2(a-d). Apparently, the large-magnification TEM images (Figure S2) of Ni/C composites confirm that they consists of many small particles. The high-resolution magnification TEM (HR-TEM) images (Figure 2(e-h)) identify that the lattice fringes of particles is 0.2 nm corresponding with the (111) plane of nickel, which implies that the pyrolysis process under Ar flow induces the formation of crystalline nickel alloy particles. These Ni nanoparticles are completely embedded by carbon, forming a core-shell Ni@C that can effectively prevent Ni particles from being oxidized by air. In order to further investigate the main influencing factors of hollow structure， the Ni-MOF precursors were prepared without PVP in DMF solvent (named as NP-MOF0) and DMF/H2O (40 mL/20 mL) mixed solvent (named as NP-MOF20), respectively. Figure S3 shows the SEM and TEM images of NP-MOF0 and

NP-MOF20. Evidently, PVP can be beneficial to obtain spherical Ni-MOF. Without PVP, one can only obtain irregular solid sphere and hollow sphere in DMF solution (Figure S3a and c) and DMF/H2O mixed solvent (Figure S3b and d), respectively. Therefore, both the PVP and DMF/H2O mixed solvent are crucial for the formation of uniform hollow microsphere structure. The kinetic process of the solvothermal reaction was studied to better understand the formation mechanism of this special structure. Ni-MOF-20 was prepared for different hydrothermal reaction time (1h, 3h, 5h and 10h). As presented in Figure S4(a and e), spherical Ni-MOF appeares after reaction for 1h but spheres are solid and nonuniform. When the reaction time is extended to 3~5h, the spheres become hollow from the center (Figure S4 b, c, f and g). Eventually, the hollow structure is obtained when the reaction time is over 10h (Figure S4 d and h). Based on the above experimental observations, nickel ions first coordinate with trimesic acid ligands to form solid spheres with the presence of PVP. Then the inner part of the sphere gradually dissolves during growing. This suggests that Ostwald ripening process controls the formation of such MOF nanostructures[32]. The crystal structure of Ni-MOF precursor is investigated by X-ray diffraction (XRD) patterns, as shown in Figure S4(a), which is well matched with the Ni-btc[33, 34]. The XRD patterns of all the Ni/C composites are displayed in Figure 3(a). Three diffraction peaks at about 44.5o, 51.8o and 76.4o can be assigned to the planes of (111), (200) and (220), respectively, which correspond well with face-centered cubic (fcc) nickel (JCPDS card No. 70-1849). It confirms that nickel ions in the Ni-MOF

precursor have been successfully transformed into nickel metal nanoparticles. Three other apparent diffraction peaks at virtually 2θ = 41.8o, 47.5o and 62.1o in the XRD pattern of Ni/C-0 are well indexed to crystalline carbon (JCPDS card No. 80-0004), which is a transition state between amorphous carbon and graphite[34]. After water participates in the reaction, these peaks disappear. Interestingly, the Ni/C-20 composite exhibits the highest peak intensity indicating that it has the best crystallinity. Nevertheless, the diffraction peaks of carbon are very weak, suggesting that the carbon may be amorphous. The Raman spectra further identifies the present of carbon and investigates the degree of graphitization of composites. As displayed in Figure 3b, all the Ni/C composites have two peaks at about 1350 cm-1 (D band) and 1590 cm-1 (G band), revealing that the organic ligands of H3BTC has been converted into carbon after the calcination under inert atmosphere. The D band and G band presents the vibration of sp2 hybridization, and sp3 defects and disorder, respectively[35, 36]. Generally, the intensity ratio (ID/IG) of the D band to the G band is used as an index for evaluating the degree of graphitization of the carbon material[37]. The ratios of Ni/C-0, Ni/C-10, Ni/C-20 and Ni/C-30 are 0.6945, 0.7487, 0.8251 and 0.7285, respectively, implying that Ni/C-20 has more defects among the other samples[38]. The element content and valence of Ni/C-20 composite was investigated by X-ray photoelectron spectroscopy (XPS). The survey spectra identify the presence of C, O and Ni elements, as shown in Figure S4(b), and the corresponding contents are 79.63%, 19.92% and 0.45%, respectively. The high-resolution C 1s spectrum consists

of three components corresponding the C-C/C=C (284.6 eV), C-O (286.2 eV), and C=O (288.45 eV)[39]. The high-resolution Ni 2p spectrum presents two typical peaks at 851.41 eV and 868.74 eV, conforming the metallic state of Ni[40]. The nitrogen absorption-deposition isotherms were used to study the surface area of the Ni/C composites and the Barrett-Joyner-Halenda (BJH) pore size distribution measurement was performed to measure the porosities of samples, as shown in Figure 4(a-e). The isotherms of all Ni/C composites display typical type-IV hysteresis according to the IUPAC classification, indicating the presence of mesoporous in Ni/C composites[41]. Figure 4(f) manifests that the pore size of Ni/C-20 composite is smaller than those of others. Apparently, the Ni/C-20 composite has the highest BET surface area of 194.85 m2 g-1 and the lowest total pore volume of 0.29 cm3 g-1. In general, porous materials can be treated as effective media with the mixtures of air and material components. Therefore, the porous structure can reduce the effective dielectric constant according to Maxwell-Garnett theory, which is conducive to impedance matching[16]. The increase of the specific surface area can promote the scattering of electromagnetic waves, which is beneficial to the absorption and attenuation of electromagnetic waves, thereby improving the absorption performance. In addition, the porous structure can also reduce the weight density of the absorber. 3.2 Electromagnetic wave absorption properties The electromagnetic parameters of Ni/C composites were measured by vector network analyze between 2 GHz and 18 GHz at room temperature. The microwave

absorption performance of materials can be reflected by their reflection loss (RL) that is evaluated via the following equations[42, 43]:

Zin = Z0

(

2πfd μrεr tanh j ε

μr r

|

c

|

𝑍𝑖𝑛 ― 1

RL(dB) = 20 log𝑍𝑖𝑛 + 1

)

(1)

(2)

where Zin is the normalized input impedance of the absorber, Z0 is the impedance of free space, f is the microwave frequency, c is the velocity of EMWs in free space and d is the thickness of absorbers. Figure 5(a-d) shows the calculated RL and the corresponding three-dimensional contour maps of RL (Figure 5e-f) of the Ni/C composites with thickness of 1.5-3 mm at 2-18 GHz. Generally, RLs of EMW materials should be less than -10 dB, which means absorption efficiency reaches about 90%. An outstanding absorbing material should meet strong absorption ability and wide absorption range. The RL characteristics of Ni/C composites are sensitive to the Ni-MOF precursors prepared in DMF/H2O mixed solvent. Although Ni/C-0, Ni/C-10, Ni/C-20 and Ni/C-30 are all effective for the attenuation of incident EMWs, their specific reflection loss values are greatly distinguishable. The Ni/C-0 composite has almost no effective absorption loss when the thickness is less than 2 mm. As the volume ratio of DMF/H2O decrease, the obtained Ni/C composites gradually have effective RL (< -10 dB) when thickness is less than 2 mm. Furthermore, the Ni/C-20 composite (Figure 5c) has excellent EMW absorbption properties that the optimum RL is -57.25 dB at 16.1 GHz with effective absorption bandwidth (RL≤ -10 dB) exceeds 5.1 GHz (12.9-18 GHz) and corresponding thickness is only 1.8 mm. The maximum RL value for Ni/C-30 is -31.46 dB with a matched thickness of 1.7 mm.

The above results suggest that mixed solvent has a positive effect with Ni/C composite especially when the volume ratio of DMF: H2O is 2: 1 owing to the hollow structure and high specific surface area. Good absorption performance is affected by many factors such as the special internal structures, specific surface area, electromagnetic parameters and impedance matching. This is the reason why Ni/C-30 composite has small size and same structure compared with Ni/C-20 composite but has relative weak effective reflection loss. Figure 6 presents the complex permittivity (εr = ε′ - jε″) and permeability (μr = μ′ jμ″) of the Ni/C composites. The real parts of permittivity (ε′) and permeability (μ′) imply the energy storage and the imaginary parts (ε″ and μ″) represent the dissipation capability for electric and magnetic energy[36,44]. Dielectric loss tangent ((tanδε = ε″/ε') and magnetic loss tangent (tanδμ = μ″/μ') are used to measure the degree of dielectric and magnetic dissipations, respectively. In Figure 6(a), the ε′ gradually decreases with the increasing of frequency, which is a normal behavior of dielectrics[45-47]. With the volume of H2O in synthetic process increasing from 0 mL to 30 mL, the ε′ values of Ni/C composites obviously increase in the whole frequency range (2-18 GHz). At the same time, the ε″ possesses a similar variation trend with frequency, as shown in Figure 6(b). The ε″ value of Ni/C-0 composite keeps constant within the frequency range of 2-18 GHz, indicating that its dielectric loss capacity is very weak. The ε′ and ε″ values of Ni/C-0 composite are smaller than those of others. According to the free electron theory[48], the ε″ enlarges with the improvement of electrical conductivity, which means the DMF/H2O mixed solvent influents electric

polarization of Ni/C composites. For the dielectric loss tangent curve, all the composites synthesized in DMF/H2O mixed solvent perform high dielectric loss values at 2-10 GHz, suggesting a strong dielectric loss ability. Moreover, it is interesting that the tanδε plot of Ni/C-10, Ni/C-20 and Ni/C-30 have several resonance peaks in the whole frequency range (2-18 GHz), which means that multiple polarization relaxations causes the dielectric loss of Ni/C composite[49]. Figure 6(d) and (e) depict the frequency-dependent real permeability (μ') and imaginary permeability (μ″), respectively. According to the μ' curves (Figure 6d), there is a small peak occurring at around 3.5 GHz, followed by a decline first and then a rise, and the resonance peak appears at virtually 16 GHz. From Figure 6(e) it can be observed that μ″ curves have a small peak around 4 GHz and then decrease in the range of 4-9 GHz and following increase in the range of 9-18 GHz with slight fluctuation. The magnetic loss tangent of all Ni/C composite are displayed in Figure 6(f). The tanδμ value of Ni/C-0 composites is lower than those of other Ni/C composites. In addition, it has several resonant peaks in magnetic loss, as shown in Figure 6f. The resonant peaks in low frequency may correspond to natural resonance[50] and the resonant peaks in high frequency correspond to exchange resonance[51]. The EMW absorption performance of Ni/C composites is may determined by both complex dielectric constant and magnetic permeability. The magnetic loss of Ni/C composites is mainly due to the contribution of nickel nanoparticles. Theoretically, the magnetic loss of magnetic materials is mainly derived from hysteresis loss, domain wall resonance, eddy current loss, natural

resonance and exchange resonance[7, 52]. Hysteresis loss is negligible in weak electromagnetic field and domain wall resonance loss does not occur in GHz band. The relationship between eddy current coefficient C0=μ″(μ')-2f -1 and frequency can be used to analyze whether the eddy current loss contributes to the magnetic loss[53]. If the C0 changes a little or almost keeps a constant with the increase of frequency, the magnetic loss main comes from eddy current loss[54]. From Figure 7 the coefficients of Ni/C composites change over the whole frequency range, demonstrating the nonexistence of eddy current effect. It can be concluded that the magnetic loss is derived from natural resonance and exchange resonance. To verify that Ni particles have a great contribution to the EMW absorption properties of Ni/C composite, pure carbon (C-20) was prepared by using etching method to remove Ni particles of Ni/C-20 composite. The detailed preparation process is shown in the supporting materials. The electromagnetic parameters were also measured with the same filler loading as Ni/C-20 at 2-18 GHz. As displayed in Figure S6(a), C-20 still keep hollow structure after Ni particles are removed. From Figure S6(c and d), it can be seen that the electromagnetic energy attenuation of C-20 is mainly based on dielectric loss and it almost doesn’t has magnetic loss. Furthermore, C-20 doesn’t has any effective reflection loss, as shown in Figure S6b. These results prove that Ni particles are very important for electromagnetic absorption. According Debye theory, dielectric loss possesses polarization and conductance loss. Cole-Cole curve (ε″-ε') can be utilized to study the polarization process. The relationship between ε′ and ε″ follows the following equation[55]:

(ε ― '

εs + ε∞ 2

εs - ε∞ 2

) + (ε ) = ( ) '' 2

2

(3)

where εs and ε∞ are the static permittivity and relative permittivity in higher frequency region, respectively. If ε″-ε' satisfies the Equation (3), the curve would be a semicircle (Cole-Cole semicircle), proving that there is polarization relaxation process, and one semicircle is associated with one polarization process. Figure 8 reveals the Cole-Cole plots of Ni/C composites in the frequency range of 2-18 GHz. Obviously, all the curves of Ni/C composites have multiple semicircles, demonstrating multi relaxation processes exist in composites. The Debye circle of the sample is not regular, which means that in addition to Debye relaxation process, other dielectric loss mechanisms such as electron polarization, dipole polarization and other polarizations also exist in the Ni/C complex. Under alternating EM field, interfacial polarization could be generated at the interface between Ni particles and carbon shell as well as the interface between Ni/C composite and paraffin matrix due to the charge accumulation. Furthermore, the composites with porous structure is advantageous for scattering of incident waves and increasing the dipole amount. Meanwhile, as revealed by XPS (Figure S4c) there are many defects in composite such as C=O and C-O which will produce electronic dipole polarization due to the different electronegativity between carbon atom and oxygen atom[56]. The attenuation constant α is a crucial factor to evaluate whether the microwave entering the absorber interior and it is calculated by the following equation[57]: α=

2𝜋𝑓 𝑐

2 2 × (𝜇′′𝜀′′ ― 𝜇′𝜀′) + (𝜇′′𝜀′′ ― 𝜇′𝜀′) + (𝜇′𝜀′′ ― 𝜇′′𝜀′)

(4)

where f is the frequency and c is the light velocity. Evidently, Ni/C composites prepared in mixed solvent display improved attenuation constant α in comparison with the Ni/C-0 composite while Ni/C-10 reveals the largest attenuation constant α in the entire frequency range, as presented in Figure 9. However, this result is inconsistent with the RLs analysis. The EMW absorption performance is more likely to be limited by impedance matching. Good EMW absorption properties require well matched characteristic impedance. When the impedance of microwave absorption materials close to that of free space (377 Ω), incident EMWs will transmit into the interior of microwave absorption materials and then be consumed by their intrinsic attenuation characteristic[58]. In contrast, microwave absorption materials cannot consume EMWs even if they have excellent intrinsic attenuation capability. Recently, a delta-founction method is proposed to describe the matching degree of characteristic impedanceand it can be calculated by the following equations[59]: |∆| = |sinh2(𝐾𝑓𝑑) ―𝑀| K=

4𝜋 𝜇′𝑟𝜀′𝑟 sin

M=

(5)

𝛿𝑒 + 𝛿𝑚 2

(6)

𝑐.cos𝛿𝑒cos 𝛿𝑚 4𝜇′𝑟cos𝛿𝑒 𝜀′𝑟cos 𝛿𝑚

(𝜇′𝑟cos 𝛿𝑒 ― 𝜀′𝑟cos 𝛿𝑚)2 +

[tan (

𝛿𝑚 2

―

𝛿𝑒 2

2

)] (𝜇 cos 𝛿 + 𝜀 cos 𝛿 ) ′𝑟

𝑒

′𝑟

(7) 2

𝑚

The delta-function represents the impedance matching degree. When the delta value close to zero (| △ |≤0.4), desirable impedance matching will be achieved[60]. Figure 10 presents the delta value maps for Ni/C-0, Ni/C-10, Ni/C-20 and Ni/C-30 composites by manipulating f (2-18 GHz) and d (1.5-3 mm). The matching region of Ni/C-20 composite is largest than those of other Ni/C composites, implying it

possesses better impedance. This result can also explain that why Ni/C-10 composite with superior attenuation constant, produces inferior reflection loss characteristics to Ni/C-20 composite. As a result, the excellent microwave absorption performance of Ni/C-20 is attributed to its matched characteristic impedance which can reduce the reflection probability of incident EMWs, and good attenuation ability that achieves the effective conversion of electromagnetic energy. Impedance matching performance is not only related to electromagnetic parameters, but also influenced by material structure. The nickel particle and carbon core-shell structure as well as porous and hollow structure can optimize impedance matching performance. By using the mixed solvent, not only the electromagnetic parameters of the composite can be adjusted, but also its microstructure can be controlled, and then a series of properties such as density, loss and impedance matching can be optimized to obtain the best reflection loss. The proposed EMW absorption mechanism for the enhanced EMW absorption of Ni/C-20 composite can be ascribed in Scheme 2. Firstly, many defects in carbon matrix such as C=O and C-O, introduce some dipole polarization[7]. Secondly, the interfaces among C/C, C/air and C/Ni induce lots of interfacial polarization. Good impedance matching allows electromagnetic waves to enter the interior of the material as much as possible. Thirdly, the combination of Ni alloys and carbon provides dielectric loss and magnetic loss. The magnetic loss including exchange and natural resonance comes from the Ni alloys[53]. Finally, the porous and hollow structure can lead to multiple reflections which convert microwave energy to other forms of energy

such as thermal energy[35]. In this work, the Ni/C composites inherit the porous structure of Ni-MOF after pyrolysis, which is good for the scatter and reflection of microwave. Table 1 summarized the EMW absorption performance of some typical nickel based microwave materials. It can draw a conclusion that Ni/C composites in this work have outstanding microwave absorption performance owing to the thin thickness, broaden effective frequency bandwidth and strong absorption capacity. 4. Conclusions Hollow Ni/C microsphere was fabricated by using hollow Ni MOF as self-sacrificing template. The DMF/H2O mixed solvent in solvothermal process plays an important role in controlling the structure of Ni-MOF. Specifically, when the volume ratio of DMF: H2O is 2: 1, the composite (Ni/C-20) exhibits excellent EMW absorption performance. Improved impedance matching and good attenuation enable composite with high reflection loss (-57.25 dB) and wide effective absorption bandwidth (5.1 GHz). These results demonstrate that rational design of the microstructure of MOF-derived carbon-based composites is an effective strategy for building high-performance materials. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51772177), the Shaanxi Science & Technology Co-ordination & Innovation Project of China (Grant No. 2017TSCXL-GY-08-05) and the Science Fund for Distinguished Young Scholars of Shaanxi Province (Grant No. 2018JC-029). References [1] P.B. Liu, Y.Q. Zhang, J. Yan, Y. Huang, L. Xia, Z.X. Guang, Synthesis of

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Figure caption Scheme 1. Schematic illustration of the formation of Ni/C composites. Scheme 2. Schematic illustration of the possible EMW absorption mechanism for the Ni-MOF derived porous Ni/C composite. Figure 1. SEM images of (a) MOF-0, (b) Ni-MOF-10, (c) Ni-MOF-20, (d) Ni-MOF-30, (e) Ni/C-0, (f) Ni/C-10, (g) Ni/C-20, (h) Ni/C-30; Size distribution of (i) Ni/C-0, (j) Ni/C-10, (k) Ni/C-20, (l) Ni/C-30. Figure 2. TEM and HR-TEM images of Ni/C composites (a) Ni/C-0, (b) Ni/C-10 (c) Ni/C-20, (d) Ni/C-30. Figure 3. XRD patterns (a) and Raman spectrum (b) of Ni/C composites. Figure 4. N2 adsorption−desorption isotherms of Ni/C composites: (a) Ni/C-0, (b) Ni/C-10, (c) Ni/C-20, and (d) Ni/C-30; (e) Pore-size distributions derived from the adsorption branch by the BJH method. (f) Variation of specific surface area and pore volume of Ni/C composites obtained at different volume ratio of DMF/H2O. Figure 5. (a-d) Reflection loss curves and (e-h) corresponding three-dimensional contour maps of (a, e) Ni/C-0, (b, f) Ni/C-10, (c, g) Ni/C-20 and (d, h) Ni/C-30 composite. Figure 6. Electromagnetic parameters of Ni/C composites (the real ε' (a) and imaginary ε″ (b) parts of the complex permittivity, (c) dielectric loss tan δε, the real μ' (d) and imaginary μ″ (e) parts of the complex permeability, and (f) magnetic loss tanδμ). Figure 7. μ″(μ')-2f -1 values of Ni/C composites.

Figure 8. Cole-Cole plots of Ni/C composites: (a) Ni/C-0, (b) Ni/C-10, (c) Ni/C-20, and (d) Ni/C-30 in the frequency range of 2-18 GHz. Figure 9. Attenuation constants (α) of Ni/C composites. Figure 10. Calculated delta maps of: (a) Ni/C-0, (b) Ni/C-10, (c) Ni/C-20, and (d) Ni/C-30 in the frequency range of 2-18 GHz with thickness of 1.5 mm to 3 mm. Table caption Table 1. Microwave absorption of typical nickel-based materials reported in recent literatures.

Scheme 1. Schematic illustration of the formation of Ni/C composites.

Scheme 2. Schematic illustration of the possible EMW absorption mechanism for the Ni-MOF derived porous Ni/C composite

Figure 1. SEM images of (a) MOF-0, (b) Ni-MOF-10, (c) Ni-MOF-20, (d) Ni-MOF-30, (e) Ni/C-0, (f) Ni/C-10, (g) Ni/C-20, (h) Ni/C-30; Size distribution of (i) Ni/C-0, (j) Ni/C-10, (k) Ni/C-20, (l) Ni/C-30

Figure 2. TEM and HR-TEM images of Ni/C composites (a, e) Ni/C-0, (b, f) Ni/C-10 (c, g) Ni/C-20, (d, h) Ni/C-30

Figure 3. XRD patterns (a) and Raman spectra (b) of Ni/C composites.

Figure 4. N2 adsorption−desorption isotherms of Ni/C composites: (a) Ni/C-0, (b) Ni/C-10, (c) Ni/C-20, and (d) Ni/C-30; (e) Pore-size distributions derived from the adsorption branch by the BJH method. (f) Variation of specific surface area and pore volume of Ni/C composites obtained at different volume ratio of DMF/H2O.

Figure 5. (a-d) Reflection loss curves and (e-h) corresponding three-dimensional contour maps of (a, e) Ni/C-0, (b, f) Ni/C-10, (c, g) Ni/C-20 and (d, h) Ni/C-30 composite.

Figure 6. Electromagnetic parameters of Ni/C composites (the real ε' (a) and imaginary ε″ (b) parts of the complex permittivity, (c) dielectric loss tan δε, the real μ' (d) and imaginary μ″ (e) parts of the complex permeability, and (f) magnetic loss tanδμ).

Figure 7. μ″(μ')-2f -1 values of Ni/C composites.

Figure 8. Cole-Cole plots of Ni/C composites: (a) Ni/C-0, (b) Ni/C-10, (c) Ni/C-20, and (d) Ni/C-30 in the frequency range of 2-18 GHz.

Figure 9. Attenuation constants (α) of Ni/C composites

Figure 10. Calculated delta maps of: (a) Ni/C-0, (b) Ni/C-10, (c) Ni/C-20, and (d) Ni/C-30 in the frequency range of 2-18 GHz with thickness of 1.5 mm to 3 mm.

Table 1. Microwave absorption of typical nickel-based materials reported in recent literatures. Materials

Filler loading (wt %)

Matching thickness (mm)

RLmax (dB)

Effective

Ni-10% MXene

---

1.75

-49.90

2.10

[61]

Ni@MnO2

50

3.60

-37.55

11.2

[62]

NiCo@g-C3N4

20

2.00

-35.63

4.80

[63]

Ni/C

30

1.50

-17.60

4.80

[64]

Ni/C microsphere

75

1.80

-28.40

4.90

[65]

Ni@C nanorods

60

1.70

-18.00

5.00

[66]

Ni/C composite

40

2.60

-51.80

3.48

[24]

Ni/C hollow microspheres(Ni/C-20)

30

1.80

-57.25

5.10

This work

bandwidth (GHz)

Refs

Highlights 1. Ni/C composite derived from Ni-metal organic framework was synthesized. 2. Hollow structure contributed to the electromagnetic absorption. 3. The optimal RL of Ni/C composite reached -57.25 dB. 4. Effective absorption bandwidth achieved 5.1 GHz with a thin thickness of 1.8 mm.