porous carbon microwave absorbing material based on ferrite@metal–organic framework

Accepted Manuscript Efficient ferrite/Co/porous carbon microwave absorbing material based on ferrite@metal–organic framework Lixi Wang, Yongkang Guan, Xu Qiu, Hongli Zhu, Shibing Pan, Mingxun Yu, Qitu Zhang PII: DOI: Reference:

S1385-8947(17)30957-9 http://dx.doi.org/10.1016/j.cej.2017.06.006 CEJ 17087

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

8 May 2017 1 June 2017 2 June 2017

Please cite this article as: L. Wang, Y. Guan, X. Qiu, H. Zhu, S. Pan, M. Yu, Q. Zhang, Efficient ferrite/Co/porous carbon microwave absorbing material based on ferrite@metal–organic framework, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.06.006

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Efficient ferrite/Co/porous carbon microwave absorbing material based on ferrite@metal–organic framework Lixi Wanga,b,*, Yongkang Guana, Xu Qiu a,b, Hongli Zhuc, Shibing Panc, Mingxun Yu c, Qitu Zhang a,b,* a

College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China

b

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing 210009, China

c

Shandong Institute of Nonmetallic Materials, Jinan 250031, China

*corresponding author: Lixi Wang; Qitu Zhang E-mail: [email protected]; [email protected]; [email protected]

Abstract A series of ferrite/Co/porous carbon materials were prepared by the situ-thermal carbonization of ferrite/ZIF-67 under N2 atmosphere, and the microwave absorption properties have also been investigated. The magnetoelectric synergistic microwave absorbing material is promising as an efficient microwave absorbing material. The component absorbers with unique porous carbon structure make great contribution to the impedance match, interface polarization, magnetoelectric synergistic effect, and multiple reflection and scattering loss. The carbonization temperature indicates a crucial effect on the porous structure, and too high temperature will result in the collapse of the porous structure. The ferrite/Co/porous carbon fabricated at 500 °C (FC500) exhibits the most enhanced microwave absorbing performance. The maximum reflection loss (RL) of FC500 reaches −31.05 dB at 14.32 GHz, and the effective absorption bandwidth (RL ≤ −10 dB) is 4.8 GHz (12.24 GHz−17.04 GHz) corresponding to a thickness of 1.5 mm. The maximum reflection loss (RL) can reach −47.31 dB at 8.4 GHz, and absorption bandwidth (RL ≤ −10 dB) is 2.72 GHz (6.80 GHz−9.52 GHz) with a thickness of 2.5 mm. Thus, the component absorption materials can significantly decrease the weight of ferrite absorbers, ascribing to the thinner optimum thickness. Key words: Lightweight; Porous carbon; MOFs template; Microwave absorbing

1. Introduction The electromagnetic interference has become a serious problem in various areas,

interrupting the electronic devices and threatening the human health [1]. Microwave absorbing materials (MAMs) have been proved to play an important role in solving this problem [2]. The single component MAMs have the disadvantages of narrow absorption band, weak reflection attenuation, thick coating, and high density, which hinders their practical application [3, 4]. The combination of absorbing materials with different absorption mechanism is favour to achieve the ideal control of microwave absorbing properties. Thus, much attention has been paid in the design of complex MAMs with multiple loss mechanisms to achieve wideband microwave absorption [5-8]. In the past decades, numerous research have focused on ferrites [9, 10], carbon nanotubes or nanocoils [11, 12], mesoporous carbon and graphene [13-16], nano metal particles [17, 18], and so on. These microwave absorbing materials have their disadvantage being used alone. The nano metal particle such as Fe, Co, Ni, and their alloys can obtain strong absorbing intensity only in a narrow frequency range. In addition, although the ferrites can exhibit efficient microwave absorbing performance, they cannot satisfy the requirement of lightweight. Among these kinds of MAMs, nano carbon absorbers are promising candidates for high-efficient MAMs, due to their light weight, high conductivity, high aspect ratio, good resistance against corrosion, and excellent mechanical properties [19, 20]. However, nano carbon absorbers also need to be combined with other absorbers to achieve wide absorbing band (below -10 dB) for their poor impedance matching [21-24]. The ferrite-MWCNTs composites exhibited excellent absorbing performance than ferrites and MWCNTs [25]. Meanwhile, novel ideas have also been addressed in tailoring porous nanoscale architecture to realize efficient microwave absorption other than only paying attention on the synthesis of MAMs with one-dimensional or two-dimensional [1, 8]. Micro-nano multi-level absorbers with tailoring porous nanoscale structures are effective to enhance microwave absorption with multi-loss mechanisms. Firstly, the microwave absorption has usually been enhanced by magnetoelectric synergistic effect from the combination of magnetic loss and dielectric loss; Secondly, the whole absorber has good impedance matching and the particles belong to micron scale for the wave length of microwave, which makes more microwave enter into the absorber; At last, porous nanoscale

structures have been tailored to achieve multiple internal reflection and scattering. In hence, the designed micro-nano structure absorbers can obtain tunable wide absorption range, strong absorption intensity, and light weight. Lately, porous carbon has been applied in super capacitor, battery anodes, electrocatalysts and so on [26-31]. In addition, sacrifice template method has been a convenient strategy to obtain porous carbon. Metal-organic frameworks (MOFs) with high surface area and tunable chemical structures have attracted tremendous attention. Nowadays, MOFs composed of metal−oxygen coordinating clusters as secondary building units and aromatic organic linkers have been proven to be ideal templates for fabricating porous carbon materials by in situ pyrolysis [32-36]. Surprisingly, a design on the structure and size of porous carbon using nanostructure tailoring can be achieved by a fascinating route that nanostructure units are arbitrarily combined as building blocks. In this work, we report an in situ pyrolysis method of ferrite/Co/porous carbon absorber materials (Fig. 1). The design of ferrite/Co/porous carbon absorber based on ferrite@metal–organic framework makes it a highly promising approach for obtaining high efficiency MAMs, opening a potentially novel method for frequency adjustment and absorption enhancement. The well-defined three-dimensional structure of ferrite/Co/porous carbon absorber has many attractive advantages as follows: 1) a complex micro-nano absorber with magnetic loss of ferrite and nano Co particles, and dielectric loss of nano porous carbon; 2) the well-defined 3D structure generally possess uniform micro-nano structure, well dispersed metal nanoparticles and nano porous carbon, exhibiting better performances as compared with those fabricated by conventional methods; 3) the lightweight carbon materials can not only enhance the performance but also decrease the usage of high density ferrites.

2. Experimental 2.1 Synthesis of W-ferrite particles (Sample F) Samples of Ba0.85Sm0.15Co 2Fe16O27 hexaferrite (Sample F) were synthesized by citrate–EDTA complexing method [37]. In a typical synthesis, Ba(NO3)2 was first

dissolved in an EDTA–NH3·H2O solution, and then the calculated amounts of Fe(NO3)3, Co(NO3)2 and Sm(NO3)3 were added to the solution. After stirring for several hours, proper amount of citric acid was introduced, the mole ratio (EDTA acid: citric acid: total metal ions) was controlled around 1.05:1:1. Precipitation might occur after citric acid addition, NH3· H2O was then added to adjust pH value to 6-7. EDTA–NH3 and citrate formed a buffering solution, so the pH value of the system successfully sustained during the whole water-evaporation process at 80 °C. With the evaporation of water, a dark yellow gel was obtained. The gel was then pretreated at 120 °C for several hours to make primary powders, which were calcined at 1100 °C for 5 h to obtain the product with final composition. 2.2 Fabrication of Ba 0.85Sm0.15Co 2Fe16O27@ZIF-67 template (Sample FZ) Ba0.85Sm0.15Co 2Fe16O27@ZIF-67 template was assembled by an in situ crystallization procedure. In this process, cetyltrimethyl ammonium bromide was used as surfactant agent for the loading of ZIF-67 on the surface of ferrite. Co(NO3)2.6H2O and 2-methylimidazole were chosen as starting materials for the synthesis of ZIF-67. The Co2+ ions and 2-methylimidazole were vertexes and organic framework, respectively. In addition, the chemical reaction process of ZIF-67 is shown in Fig. 1. 40mg Ba0.85Sm0.15Co 2Fe16O27 particles were first dispersed in 60 ml deionized water with 0.2 ml cetyltrimethyl ammonium bromide (CTAB, 0.01mol/L) as a surfactant. The ferrite solution was ultrasonic dispersed for 1 h. 0.45 g Co(NO3)2.6H2O was dissolved in 3 ml deionized water, and then the Co(NO3)2 solution was dropwised into the ferrite solution with magnetic stirring for 30 min. 5.5 g 2-methylimidazole was dissolved in 20 ml deionized water, and then the methylimidazole solution was dropwised into the above mentioned mixed solution with magnetic stirring for 1h. Then the solution was aged at room temperature for 24 h. In addition, another 5.5 g 2-methylimidazole and 0.45 g Co(NO3)2.6H2O were used for the preparation of pure ZIF-67 (Sample Z), and the pure porous carbon was obtained through the pyrolysis of pure ZIF-67 (Sample ZC). 2.3 Preparation of Ba0.85Sm0.15Co2Fe16O27/Co/porous carbon absorber (Sample FC) The as-prepared template solution was centrifugal separated and washed for several times with deionized water and ethanol. The wet product was dried in vacuum oven for

1h, and then the dried powders were calcined at 500 oC, 550 oC, and 650 oC under N2 atmosphere for 2 h with a heating rate of 2 oC min-1. Further, the as-prepared powders were cooled with the furnace. The ferrite/Co/ porous carbon materials were obtained through the pyrolysis of Ba0.85Sm0.15Co2Fe16O27@ZIF 67 template at 500 oC (Sample FC500), 550 oC (Sample FC550), and 650 oC (Sample FC650). The crystalline phases of the samples were characterized by powder X-ray diffraction (smart lab TM 9kW, Rigaku, Japan) with Cu Kα radiation (λ=1.5406 Å). The continuous scanning rate (2θ ranging from 5° to 80°) was 5° (2θ) min−1, and the step size was 0.02°. The crystal structure was obtained using the Crystal Impact Diamond software. The morphologies of the samples were observed using aberration corrected (S) transmission electron microscopy (FEI Titan 80-300 S/TEM, FEI, USA) and scanning electron microscopy (S-3400N, Hitachi, Japan). The Raman spectra were measured using Raman spectrometer (Labram HR800, Horiba, Japan) with a 514 nm laser beam as the light source. The pyrolysis processes of ZIF-67 was studied using thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) method (DSC204, Netzsch, Germany) under a nitrogen atmosphere at a heating rate of 10 °C min-1. Nitrogen adsorption isotherms and Brunauer–Emmett–Teller (BET) surface area were obtained by automatic three-station surface and pore distributor (Belsorp-Max, MicrotracBEL, Japan). The samples were prepared for measurements by degassing at 100 °C. Pore size distributions were calculated using the Barrett–Joyner–Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) was recorded using a Thermo ESCALAB 250Xi spectrometer. The magnetic hysteresis loops were measured using a vibrating sample magnetometer (VSM) (LakeShore 7404, USA) at room temperature. The scatting parameters (S11, S21) were measured on the toroidal-shape composites by a network analyzer (Agilent PNA 8363B, America) using the T/R coaxial line method in the range of 2−18 GHz [10]. The complex permittivity and permeability were determined from the scattering parameters. The as-prepared ferrite/Co/porous carbon composites were mixed with paraffin wax in a weight ratio of 7: 3, and the mixture was stirring for 0.5 h. Then the mixture was put into a stainless steel mold and compressed into toroidal-shape samples. The measurement samples were with an inner diameter of

3.05 mm, outer diameter of 7.00 mm, and thickness of 3.0−3.5 mm. The as-prepared toroidal-shape samples were put into the air chamber of a network analyzer for measurement. The reflection loss (RL) performance was calculated using the complex electromagnetic parameters and the absorber thickness [38]. The RL values were calculated according to the equations as follows:
 =
 ( /ε ) / tanh [(2/)( ε ) / ]  = 20log [(
 −
 )/(
 +
 )]

(1)

(2)

Here,
 is the input impedance of the absorber,
 is the impedance of free space,  and # are the relative complex permeability and permittivity, respectively. In addition, f is the frequency of the electromagnetic wave, d is the thickness of an absorber, and c is the velocity of electromagnetic waves in free space.

3. Results and discussion The crystal structure of BaCo2Fe16O27 is shown in Fig. 2(a). It is of hexagonal crystal symmetry and P 63/mmc space group, with unit cell parameters as a= b=5.899 Å and c= 32.846 Å. The BaCo2Fe16O27 with basal plane anisotropy is a soft magnetic material at room temperature. The BaCo2Fe16O27 hexaferrite has a crystalline structure and its unit cell is composed of the sequence of RSSR*S*S* between S and S* and between R and R*, 180° rotation about the c-axis occurs [39]. It has higher natural resonance frequency and large magnetic permeability loss, which results in its wide use in GHz frequency range [40]. It is proved that the Sm3+ ions replaced partly in the Ba2+ position could adjust the microwave absorbing performance of BaCo 2Fe16O27 hexaferrite [40]. The crystal structure of ZIF-67 is also shown in Fig. 2(b). The ZIF-67 is a kind of metal−organic framework material, which composes of Co2+ coordinating clusters as secondary building units and 2-methylimidazole organic linkers. It is of cubic crystal symmetry and I-43m space group, with unit cell parameters as a= b= c= 16.9589 Å. It is shown in Fig. 2(b) that the crystal is a dodecahedron with Co2+ ions as the vertexes, with a branch of pores, which makes sure its usage as a template of porous carbon materials [35]. The powder XRD analysis has been conducted and the XRD profiles of

ZIF-67, ferrite/ZIF-67 and ferrite/porous carbon samples are also shown in Fig. 2. It is found in Fig. 2(c), the diffraction peaks of ZIF-67 relates to (002), (112), and (222) at 12.76°, 14.66° and 18.1°, respectively [41]. The ferrite sample clearly indicates the main diffraction peaks of BaCo2Fe16O27 with fine crystallization. The diffraction peaks of ferrite correspond to (-120), (0110), (-126) and (024) at 30.28°, 32.38°, 34.51°, and 36.82°, respectively [42]. The ferrite/ZIF-67 samples exhibit the diffraction peaks of both the BaCo2Fe16O27 ferrite and ZIF-67. Moreover, after the deposition under N2, the sample shows a weak, broad peak at approximately 44.18°, which may attribute to the peak of graphitic carbon materials with a low degree of graphitization. The pyrolysis process of ZIF-67 was also studied using thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) method. The TG-DSC profile is shown in Fig. S1. It is indicated that there are three exothermic peaks corresponding to 437 oC, 508 o

C and 545 oC, ascribing to the decomposition of ZIF-67. In the while, there are two

degradation processes in the TG curve. It is revealed that there is only tiny weight loss up to 200 °C, concerning to the removal of moisture, guest molecules, and un-reacted species. The main weight loss is corresponding to the temperature from 400 oC-800 oC, confirming to the decomposition of ZIF-67. Thus the calcination temperatures of 500 oC, 550 oC and 650 oC have been chosen to confirm the decomposition of ZIF-67. The X-ray photoelectron spectrum was also performed to analyze the form of Co. It can be seen in Fig. 2(d) that, the binding energy values of Co 2p3/2 locate at around 779.7 and 781.5 eV can be ascribed to Co with valence of 0 and +2, which indicates the Co particle of ZC500 sample and Co2+ ions of ZIF-67 [43-45]. It is also demonstrated that the metal ions are reduced in situ in the thermal decomposition of ZIF-67 under N2 atmosphere. Additionally, the binding energy values are between 779.7 and 781.5 eV for the co-existence of Co particle and Co2+ in the ferrite. The morphologies and structures of ZIF-67, ferrite/ZIF-67, and ferrite/porous carbon yielded by calcinations at 500 °C were thoroughly investigated by SEM and TEM. The SEM images, TEM images, and SAED patterns are shown in Fig. 3. The ZIF-67 crystals reveal regular rhombic dodecahedral morphology composed of well-defined rhombus faces and straight edges, which was verified in the preparation of nitrogen, phosphorus,

and

sulfur

co-doped

hollow

carbon

shells

based

on

ZIF-67@

poly(cyclotriphosphazene-co-4,4’-sulfonyldiphenol) template [27]. In addition, regular rhombic dodecahedral ZIF-67 has also been obtained in fabrication of excellent electrocatalysts [29-30] Fig. 3(a) shows that the diffraction rings corresponds to the characteristic crystal plane of ZIF-67, which agrees well with the XRD patterns. It is exhibited in Fig. 3(b) that, the ZIF-67 crystals have been successfully synthesized on the surface of the ferrite particles evenly via the in situ crystallization route in the formation process. Polycrystal diffraction rings can be distinguished in Fig. 3(b) for ferrite/ZIF-67 samples, relating to both the ferrite and ZIF-67. After the carbonization treatment of ZIF-67 crystals under a N2 atmosphere, the products mainly retain the dodecahedral morphology with remarkable shrinkage as shown in Fig. 3(c). However, the characteristic diffraction rings disappear in Fig. 3(c) for ferrite/Co/C samples, attributing to the carbon formation after the thermal decomposition of ZIF-67. As the corresponding aberration corrected (S) TEM images shown in Fig. 3(d), the ferrite particles are in hexagonal plane shape, which is beneficial for microwave absorption. It is also obviously in Fig. 3(d-f), the ferrite particles are covered by carbon/Co particles. The TEM images (Fig. 3(d-f)) confirm that the porous carbon/Co particles on the surface of ferrites are in the size of 5-15 nm. Furthermore, the lattice spacing of the samples has been investigated in HRTEM images for confirming the existence of Co particles and carbon. The HRTEM image in Fig. 3(f) exhibits that the lattice spacing of the core is 0.205 nm corresponding to the Co (111) crystal plane and the lattice spacing of the shell is 0.344 nm relating to the carbon (002) crystal plane [46]. The nano architecture is intimately and densely decorated with numerous Co nanoparticles embedded in the carbon matrix during carbonization treatment. In the thermal decomposition of ZIF-67 under N2 atmosphere, organic components of ZIF-67 can be directly carbonized and metal ions are reduced in situ, thereby forming ferrite/Co/C materials. The carbonization of pure ZIF-67 has also been demonstrated by Y. Y. Lu et al [32, 47]. The samples have been also probed with Raman spectroscopy, which has been a standard non-destructive method to identify carbon complex materials and sensitive to

the subtle structural variations [48-50]. The peaks in the range of 200 cm−1-800 cm−1 are the characteristics of hexaferrite. As shown in Fig. 4(a) and (b), the peaks observed at around 467 cm−1 and 616 cm−1 are ascribed to A1g vibrations of Fe-O bonds at the octahedral 2a, 12k, and 4f2 sites, which has been proved in M-type ferrite [51]. The peaks appear at around 689 cm−1 and 713 cm−1 attribute to A1g vibrations of bipyramidal 2b and tetrahedral 4f1 sites, which has also been exhibited in M-type ferrite [51]. In addition, the peak at around 410 cm−1 corresponds to octahedral 12k, which has also been confirmed in M-type ferrite [51]. The peak at around 332 cm−1 and 525 cm−1 is ascribed to E1g vibrations and E2g vibrations at octahedral 12k site, respectively [51-53]. It is shown in Fig. 4(a-b), these lower wave numbers peaks appear due to the metal-oxygen characteristic vibration at crystal site of the hexaferrite [54]. The peaks at 1318.9 cm−1 and 1460.5 cm−1 in Fig.3(c) correspond to the ZIF-67 on the surface of ferrites. In Fig. 4(d), the broad peak at 1338.2 cm−1 indicates that significant structure defects/disorder, corresponding to D band of carbon [50, 55-57]. It means that the ZIF-67 on the surface of ferrite has been decomposed into porous carbon with structure defects/disorder. The peak at 1589.8 cm−1 shifts a little compared with that of graphite (~1580 cm−1), corresponding to the G band of carbon and amorphization of graphite structures ascribed to the structure defects [48, 50, 55-56]. In detail, these two peaks can be attributed to E2g mode of graphite related to the vibration of sp 2-bonded carbon atoms in a 2D hexagonal lattice and an A1g mode of graphite related to the disorder due to the finite particle size effect or lattice distortion of the graphite crystals [57, 58]. The saturation magnetization (Ms) and the coercivity (Hc) of F, FC500, FC550, and FC650 samples were measured, and the room temperature hysteresis loops curves are shown in Fig. 5. The Ms is 3.933 emu / g, 7.089 emu / g, 4.539 emu / g and 1.393 emu / g, respectively. Further, the Hc is 0.027 KOe, 0.164 KOe, 0.164 KOe, and 0.027 KOe, respectively. These results will be mentioned to discuss the permeability next. The as-prepared samples were mixed with wax matrix (weight ratio 7:3), and the electromagnetic parameters (permittivity and permeability) of the composites were measured in the frequency range of 2-18 GHz. It is well know that the real part of permittivity (ε′) and permeability (µ′) relates to the ability to store electric and magnetic

energy within the medium, respectively. In addition, the imaginary part of permittivity (ε″) and permeability (µ″) corresponds to the dissipation (or loss) of electric and magnetic energy [59, 60]. The dielectric and magnetic dissipation factors tan δE and tan δM were calculated by the ratio of imaginary part to real part of permittivity and permeability, respectively [43]. The electromagnetic parameters (permittivity and permeability) of the composites are shown in Fig. 6. The BaCo2Fe16O27 ferrite is a kind of magnetic loss type absorber, thus the ε″ and tan δE is almost 0. The ε′ remains around 4.5 and independent on the frequency. The pure porous carbon sample possess high real part of permittivity (ε′) and imaginary part of permittivity (ε″), demonstrating a kind of electric loss type absorber. It is shown in Fig. 6 that for the ZC, FC500, FC550, and FC650 samples, the value of ε′ is in the range of 13.65-31.36, 10.58-14.06, 13.98-18.84, and 13.19-16.74, respectively. In the while, the values of ε′ are dependent on the frequency. In Fig. 6(b), the value of ε″ is in the range of 14.57–35.01, 1.24–3.80, 0.85–4.49, and 0.16–5.09, respectively. It is obvious that, the ε″ of ZC is significantly higher than that of FC samples, and the tan δE also displays the similar tendency. It is indicated that ZC exhibits higher storage and loss capabilities for electric energy as compared to FC samples, and the FC550 shows higher permittivity than other FC samples. It is demonstrated that the electric polarization, electric conductivity and interfacial polarization are favourable for high complex permittivity according to the free electron theory [61]. It is reported that nano metal particles, defects in amorphous carbon, conducting graphite, and core–shell interfaces can act as the polarization centers under microwave irradiation [61, 62-65]. Thus, the increase of permittivity for FC samples as compared to ferrite is attributed to the increase of electric polarization, electric conductivity, and the interface polarization between ferrite and porous carbon. Furthermore, the nano Co particles embedded in the porous carbon will also arouse the improved conductivity and interfacial polarization between Co nanoparticles and the porous carbon, which may partly be responsible for the improvement of permittivity or conductivity. Moreover, it is exhibited that the much higher ε′ for FC samples as compared to ferrite is also caused by low conductive percolation thresholds, which has

been proved in other carbon-based nanoabsorbents [66-70]. Structure and morphology may make an important effect on the interface polarization. More interface polarization will be originated from the dangling-bonded atoms and defects [71]. Thus, the defects of nanoporous structure in the ferrite/Co/C absorbers have already been discussed in the Raman patterns. The real and imaginary part of permeability and magnetic loss tan δM of the as-prepared samples are indicated in Fig. 6(d-f). It is shown in Fig. 6(d) that for the F, ZC, FC500, FC550, and FC650 samples, the values of µ′ is in the range of 0.64–2.0, 0.53–1.37, 1.02–1.35, 0.71–1.53, and 0.78–1.99, respectively. The µ′ values of ZC decrease monotonously with the increase of frequency. The FC500 has the highest µ′ values among the four samples, exhibiting the highest storage capability for magnetic energy. It is exhibited in Fig. 6(e) that, the ferrite samples displayed the highest µ″ value in the frequency range of 2-8 GHz. Nevertheless, the µ″ value of FC samples is higher than that of ferrite in 8-18 GHz. The porous carbon has almost no contribution on the enhancement of µ″, while the embedded magnetic Co particles may relate to the improvement. Novelty, the incorporation of nanoporous carbon embedded with Co nanoparticles into the ferrite absorber makes it more efficient in dissipating magnetic energy, which improves the magnetic loss performance in 8-18 GHz frequency range. Furthermore, the combination of ferrite and porous carbon is the synergistic effect of electric loss and magnetic loss, which will attribute to better matching of input impedance with reduction of skin depth. Thus, the microwave absorption will be improved for the synergistic effect. For ferromagnetic MAMs, high initial permeability (µi) usually predicts strong magnetic loss ability, and it can be expressed by the equation as follow [72]:  = '()

$% &

* $% +,-.

(3)

Here, a, b are two constants which relate to the MAMs composition, respectively. In addition, λ, ξ are a magnetostriction constant and an elastic strain parameter of the crystal, respectively. Ms and Hc are saturation magnetization and coercivity, respectively. It is shown in Fig. 5 that sample FC500 can obtain higher Ms than F, FC550, and FC650,

which will originate higher permeability. Finally, the microwave absorption performance should be analyzed to discuss the effect on the incorporation of nanoporous carbon embedded with Co nanoparticles. The calculated reflection loss (RL) performance from the electromagnetic parameters (permittivity and permeability) of the as-prepared samples is shown in Fig. 7. For ZC500 sample, the RL maximum is -25dB at 13.9GHz when thickness is 2mm with an effective absorption bandwidth (RL ≤ −10 dB) of 3.5 GHz. Additionally, the ZF500 exhibits more excellent absorption performance than pure porous carbon sample in GHz frequency band. The RL maximum of FC500 reaches −31.05 dB at 14.32 GHz, and the effective absorption bandwidth (RL ≤ −10 dB) is 4.8 GHz (12.24 GHz−17.04 GHz) corresponding to a thinner thickness of 1.5 mm. Furthermore, the maximum reflection loss (RL) can also reach −47.31 dB at 8.4 GHz, and absorption bandwidth (RL ≤ −10 dB) is 2.72 GHz (6.80 GHz−9.52 GHz) with a thickness of 2.5 mm. However, the maximum RL of ferrite can reach -47.8 dB only when the thickness is 5.5mm, which limits its application in lightweight microwave absorbing materials. In addition, when the thickness is 1.5mm, the absorption bandwidth (RL ≤ −10 dB) ranges from 10.32 to 13.04 GHz and from 10.32 to 13.04 GHz for FC550 and FC650, respectively. The increasing carbonization temperature can cause the skeleton collapse of frame work without the organic linkers, resulting in the attenuation of microwave absorption. Overall, the incorporation of nanoporous carbon embedded with Co nanoparticles might attribute to the broadening of microwave absorption band and adjust of the absorption peak, which has been mentioned before. Among composite absorbers, carbon based composites have exhibited efficient microwave absorption. Magnetic nanoparticles confined within ordered mesoporous carbons have been reported as efficient microwave absorbers [73]. The maximum reflection loss (RL) values of -32 dB at 11.3 GHz and a broad absorption band (over 2 GHz) with RL values ≤10 dB are obtained [73]. Multi-wall carbon nanotubes decorated with ZnO nanocrystals have highly efficient and thermally stable microwave absorption coupled with a broad attenuation bandwidth, which almost covers the full X-band for RL ≤10 dB [74]. It is reported that ordered mesoporous carbon filled poly(methyl

methacrylate) composite films and ordered mesoporous carbon/ordered mesoporous silica/silica composites could both display excellent EMI shielding efficiency in 8.0–12.0 GHz (X-band) [75, 76]. In addition, the microwave absorbing performance of other typical carbon based materials are listed in Table S1. The reported works exhibit good RL performance in the carbon based absorbing materials [47, 77-82]. It is indicated that the as-prepared materials can obtain good microwave absorption performance with thinner thickness, which corresponds to a promising potential for efficient and lightweight microwave absorbing materials. The N2 adsorption-desorption isotherms were performed to analyze the specific surface area and total pore volume of the samples. The results are depicted in Fig. 8 and Table 1. Remarkably, the ZIF-67 sample has the highest Brunauer-Emmett-Teller (BET) specific surface area (SSA) of 1554.7 m2g-1. The SBET decreases to 475.7 m2g-1 for sample FZ, attributing to the low SBET of ferrite. Additionally, the SBET of sample FC500 (69.3 m2g-1) is lower than that of sample FZ (475.7 m2g-1), suggesting the shrink of the frame structure after the thermal decomposition. In the while, the total pore volume also exhibits a decrease tendency as shown in Table 1. The schematic illustration of possible microwave absorption mechanisms of the ferrite/Co/C absorbers is shown in Fig. 9. All in all, the synergistic effect of electric loss and magnetic loss is benefit for the wide absorption performance. Additionally, the enhanced microwave absorption performance is also ascribed to the nanostructure of porous carbon and embedded Co nanoparticles, which results in the multiple reflection and scattering. It is well explained by Cao model that dielectric loss will be enhanced by polarization induced by interface, defect or chemical bonds in/on the absorbers and conductivity loss [83-85]. Thus, the conductivity loss of the composites filled with higher porous carbon concentrations plays the main role on the dielectric loss. Moreover, the enhanced dielectric loss is also respect to the enhanced interfacial polarization of carbon structure/Co nanoparticles and ferrite/carbon structure. The Cao model has given a good guide to discuss this electron hopping mechanism [83-85]. It is proved that an electromagnetic wave absorber should achieve two aspects: first, the reflection at the interface should be decreased for the entrance of more electro wave; second, improved

dielectric and magnetic loss is required to increase the absorption [86, 87]. In this work, addtional interface polarization is originated from the dangling-bonded atoms and defects of nanoporous structure in the ferrite/Co/C absorbers. It is also reported that enhanced relaxation might tune microwave absorption, and the relaxation mainly originated from the defect dipole polarization and the interfacial polarization [88]. In addition, the Co nanoparticles are embedded in the amorphous carbon structure, which will resolve the eddy current loss of metal nanoparticles absorber. All in all, the ferrite/Co/porous carbon is a kind of multi-level microwave absorbing material, which will enhance the performance with impedance match, interface polarization, magneto-electric synergistic effect and multiple reflection and scattering loss.

4. Conclusion In summary, we have reported a simple route to obtain ferrite/Co/porous carbon microwave absorbing material based on ferrite@metal–organic framework as electromagnetic microwave absorption material. The magneto-electric synergistic absorbing structures present a porous feature with a specific surface area (SSA) of 69.3 m2g-1. The broad peaks at 1338.2 cm−1 and 1589.8 cm−1 in Raman patterns indicate that significant structure defects/disorder and amorphization of graphite structures, corresponding to D and G band of carbon. It means that the ZIF-67 on the surface of ferrite has been decomposed into porous carbon with structure defects/disorder. In addition, the lattice spacing of the core in HRTEM images is 0.205 nm corresponding to the Co (111) crystal plane and the lattice spacing of the shell is 0.344 nm, which relates to the carbon (002) crystal plane. When the carbonization temperature was 500 oC, the component absorber exhibited the most enhanced microwave absorbing performance. The maximum reflection loss (RL) of FC500 reaches −31.05 dB at 14.32 GHz, and the effective absorption bandwidth (RL ≤ −10 dB) is 4.8 GHz (12.24 GHz−17.04 GHz) corresponding to a thickness of 1.5 mm. Furthermore, the increasing carbonization temperature caused the skeleton collapse of framework without the organic linkers, resulting in the attenuation of microwave absorption. The enhanced microwave absorption performance of ferrite/Co/porous carbon is attributed to synergistic effect of

electric loss and magnetic loss, multiple reflection and scattering, and interface polarization originated from the dangling-bonded atoms and defects of nanoporous structure in the ferrite/Co/C absorbers.

Acknowledgments The authors acknowledge the generous financial support received from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the National Natural Science Foundation of China (51202111).

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Supporting Information Efficient ferrite/Co/porous carbon microwave absorbing material based on ferrite@metal–organic framework Lixi Wanga,b,*, Xu Qiu a,b, Yongkang Guana, Hongli Zhuc, Shibing Panc, Mingxun Yu c, Qitu Zhanga,b,* a

College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China

b

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing 210009, China

c

Institute 53 of China's Ordnance Industry, Jinan 250031, China

*corresponding author: Lixi Wang; Qitu Zhang E-mail: [email protected]; [email protected]; [email protected]

Fig. S1 TG-DSC profile of ZIF-67 sample

Table S1 Microwave absorbing performance of typical carbon based materials Maximum RL value (dB)

Maximum peak position (GHz)

Thickness (mm)

Frequency range (GHz) (RL<-10dB)

Efficient bandwidth (GHz) (RL<-10dB)

−31.05

14.32

1.5

12.24 −17.04

4.8

−47.31

8.4

2.5

6.80 −9.52

2.72

-40

4.2

5

3.5-5.0

1.5

[77]

-37

2.77

5.25

2.35-3.51

1.2

[78]

-16

9

2

8-10.1

2.1

[79]

Paraffin wax

-43

4.5

4

2.5-7

4.5

[80]

Paraffin wax

-31

9.9

2.5

7-13

6

[80]

Epoxy resin

-32.3

11.4

2.3

10-13.4

3.4

[47]

Ni/C microspheres

Epoxy resin

-28.4

15.4

1.8

13-17.2

4.2

[81]

α-Fe2O3/NiFe2O4/ porous activated carbon ball

Paraffin wax

-17.5

4.2

5.5

3.9-4.5

0.6

[82]

Absorbing agent

Matrix

ferrite/Co/porous carbon ferrite/Co/porous carbon

Paraffin wax Paraffin wax Paraffin wax Epoxy resin Epoxy resin

Co/porous carbon Co/MWCNTs Co/CNTs core–shell carbon–magnetite porous nanorods core–shell carbon–magnetite porous nanorods Co3O4/porous graphitic carbon nanosheets

Refs This work This work

Fig. 1 Schematic illustration of the fabrication of ferrite/Co/porous carbon microwave absorbing material Fig. 2 Crystal structure schematic (a, b), XRD patterns (c) and XPS patterns of ZIF-67, ferrite/ZIF-67 and ferrite/porous carbon Fig. 3 SEM images and SAED patterns of ZIF-67(a), ferrite/ZIF-67 (b), ferrite/porous carbon (c) and HRTEM images of ferrite/porous carbon (d-f) Fig. 4 Raman spectra of ferrite/ZIF-67(a, c) and ferrite/Co/C (b, d) Fig. 5 The room temperature hysteresis loops curves of the samples Fig. 6 Frequency dependence of electromagnetic parameters of the samples (a) the real part (ε′) and (b) imaginary part (ε″) of complex permittivity, (c) dielectric loss (tan δE), (d) real part (µ′), and (e) imaginary part (µ″) of complex permeability, (f) magnetic loss (tan δM) Fig. 7 Calculated results of the reflection loss vs frequency for samples with different thicknesses ((a) F, (b) ZC500, (c) FC500, (d) FC550, and (e) FC650 Fig. 8 (a) Nitrogen adsorption-desorption isotherms and (b) pore size distributions of the as-prepared samples Table 1 the specific surface area and total pore volume of the samples Fig. 9 Schematic illustration of possible microwave absorption mechanisms of the ferrite/Co/C absorbers

Fig. 1 Schematic illustration of the fabrication of ferrite/Co/porous carbon microwave absorbing material

(b) ZIF-67

(a) BaCo2 Fe16O27

(c)

(d) Fig. 2 Crystal structure schematic (a, b), XRD patterns (c) and XPS patterns of ZIF-67, ferrite/ZIF-67 and ferrite/porous carbon

(a)

(b)

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(c)

(d)

(e)

(f)

Ferrite Porous carbon (002) 0.344nm

(a)

(b)

(c)

(d)

Co particle (111) 0.205nm

Fig. 4 Raman spectra of ferrite/ZIF-67(a, c) and ferrite/Co/C (b, d)

Fig. 5 The room temperature hysteresis loops curves of the samples

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Fig. 6 Frequency dependence of electromagnetic parameters of the samples (a) the real part (ε′) and (b) imaginary part (ε″) of complex permittivity, (c) dielectric loss (tan δE), (d) real part (µ′), and (e) imaginary part (µ″) of complex permeability, (f) magnetic loss (tan δM)

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Fig. 7 Calculated results of the reflection loss vs frequency for samples with different thicknesses ((a) F, (b) ZC500, (c) FC500, (d) FC550, and (e) FC650

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Fig. 8 (a) Nitrogen adsorption-desorption isotherms and (b) pore size distributions of the as-prepared samples Table 1 the specific surface area and total pore volume of the samples Sample

SBET(m2g-1)

ZIF-67 FZ FC500

1554.7 475.7 69.3

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Total pore volume(cm3g-1) 0.6594 0.2368 0.0722

Fig. 9 Schematic illustration of possible microwave absorption mechanisms of the ferrite/Co/C absorbers

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Highlights

• Efficient and lightweight microwave absorbing material based on ferrite/Co/porous carbon. • Enhanced performance by synergistic effect, multiple reflection and scattering, and interface polarization. • Carbonization temperature of MOFs can adjust the absorbing performance.

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