Fe3O4@Fe

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Carbonized Design of Hierarchical Porous Carbon/FeO@Fe Derived from Loofah Sponge to Achieve Tunable High-Performance Microwave Absorption Huagao Wang, Fanbin Meng, Jinyang Li, Tian Li, Zijian Chen, Huabin Luo, and Zuowan Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02089 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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ACS Sustainable Chemistry & Engineering

Carbonized Design of Hierarchical Porous Carbon/Fe3O4@Fe Derived from Loofah Sponge to Achieve Tunable High-Performance Microwave Absorption Huagao Wang, Fanbin Meng*, Jinyang Li, Tian Li, Zijian Chen, Huabin Luo, Zuowan Zhou* Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, 111, North Section I, No 2 Ring Road, Jinniu District, Chengdu 610031, Sichuan, P. R. China *Corresponding author. E-mail addresses: [email protected] (FB Meng) E-mail addresses: [email protected] (ZW. Zhou) Abstract Recently, three-dimensional (3D) porous carbon materials derived from biomass have been arising more interesting as promising microwave absorbers due to the low cost, vastly available and sustainable of biomasses. Herein, a novelty strategy of utilizing loofah sponge as 3D hierarchical porous carbon precursors and ferric nitrate as magnetic precursor to prepare magnetic hierarchical porous carbon composites, which exhibit tunable high-performance microwave absorption (MA). During the carbonization process, the 3D-bundled microtube structure of loofah sponge changes into interconnected networks with hierarchical porous structures, and the precursor ferric nitrate converts into magnetic Fe3O4@Fe nanoparticles. As expected, the as-obtained loofah sponges-derived 3D porous carbon/Fe3O4@Fe composites treated at 600 oC exhibit outstanding MA performance. It displays the minimum reflection loss (RL) of -49.6 dB with a thickness of 2 mm and the effective absorption bandwidth (RL≤ 10 -dB) can reach 5.0 GHz (from 13 GHz to 18 GHz). 1

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The 3D hierarchical porous structures, interfacial polarization, synergistic enhancement between dielectic loss and magnetic loss, multiple reflections and scatterings make enhancement to the MA capability. Our research might provide an effective and facility strategy to prepare magnetic porous carbon derived from biomass for MA applications. Keywords: Loofah sponge; Carbonized temperature; Microwave absorption; Interface polarization; Impedance matching INTRODUCTION Recently, taking 3D porous carbonaceous materials as microwave absorbers to eliminate electromagnetic interference pollution has arised more attention for electronic safety and defense stealth technology, because of their lightweight, large specific surface area and tunable dielectric loss.1-6 Particularly, the 3D hierarchical porous structures can generate numerous MA sites of scattering and multi-reflections, which will extend the propagation path of electromagnetic wave in the absorber and achieve enhanced MA performance.7-9 And the porous carbon in the shapes of fibers, tubes or spheres, and graphene foams are fabricated to achieve high MA performance.3, 10-15 Nevertheless, fabrication of porous carbon-based microwave absorbing materials is complicated and needs relatively high costs, extensively using templating approaches and etching methods.16-19 Furthermore, the carbon resources (such as graphene and carbon nanotubes) are in high cost for industril production. Therefore, it is necessary to develop low-cost carbon-based high-performance microwave absorbing materials. Delightedly, applying the abundant and sustainable biomass to fabricate porous carbon materials is an effective approach to address the problems. The numerous 2


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inherent micro-scale porous structures in the biomasses as well as physical and chemical activation produce numerous porous structures are favorable to raising solid-void interfaces, which can induce the interfacial polarization in the presence of microwave. Meanwhile, the numerous pores contribute to multiple reflections and diffractions of microwave, leading to the attenuation of microwave energy.10,

20

Therefore, porous carbon materials derived from nature biomass possess the potential to be used as attractive microwave absorbing materials. As a kind of natural biomass, loofah sponge mainly consists of cellulose (66.59%), hemicellulose (17.44%), lignin (15.46%) and ash (0.66%),21 containing relatively high carbon content, and it also has unique structure, including many micrometer-scale hollow multichannels with dense and parallel arrangement,22 making it a prospective raw material to prepare hierarchical porous carbon for MA. Moreover, loofah sponge possesses number of functional groups (e.g. –COOH and –OH), which can chelate with Fe3+ ions with empty diorbitals.23-25 Therefore, loofah sponge can easily react with magnetic metal precursors to form a series of magnetic porous carbon through a facile pyrolysis treatment. In this work, the loofah sponge chelated with Fe3+ was directly transformed into hierarchical porous magnetic carbon materials with both micropore and mesopore structures by a simple process including carbonization, and KOH activation process. After carbonization, the relatively uniform porous structures were introduced and the Fe3+ ions were reduced to Fe3O4@Fe embedded in the porous carbon, which is not only benefitted to improve impedance matching, but also favorable to enhancing the 3



interface polarization and magnetic loss ability.26 As expected, the optimized magnetic porous carbon exhibits strong MA, and the minimum RL reaches -49.6 dB with a thinner thickness of 2.0 mm, and the bandbroad (RL≤ -10 dB) achieves 5 GHz at the range of 13-18 GHz, which is superior to most other carbon materials.27-29 EXPERIMENTAL SECTION Preparation of magnetic porous carbon composites The loofah sponge (from local agricultural market in Chengdu) was rinsed with NaOH solution (4 wt%) to remove fat, and then washed by water following being dried at 110 o

C for 24 h. Then the loofah sponges were completely impregnated with 0.1 M

Fe(NO3)3·9H2O aqueous under magnetically stirring at 80 oC so that Fe3+ ions can be absorbed on the loofah sponge as many as possible, followed by adding KOH (1.0 mol/L) and getting stirred at 100

o

C until water was absolutely evaporated.

Subsequently, the as-received carbon precursors were carbonized at desired temperatures (500-800 oC) maintaining 2 h under Ar atmosphere to obtain magnetic porous carbon composites. The as-prepared samples were washed by water to remove K+, and then dried at 60 oC. The resulting composites with various carbonized temperatures (500, 600, 700 and 800 oC) are denoted as MPC500, MPC600, MPC700 and MPC800, respectively. For comparison, we also prepared the magnetic carbon annealing at 600 oC without addition of KOH denoted as MC600. Materials characterization The structures of MPC composites were carried out on X-ray diffraction (XRD, Philips X'Pert PRO X-ray diffract to meter with CuKα radiation, λ=0.154 nm). The 4


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Raman analysis were measured on a Laser Raman spectroscopy (InVia, RENISHAW) using a 532 nm argon ion laser. X-ray photoelectron spectroscopy (XPS, VG Microtech, ESCA 2000) was performed to investigate the surface chemical state and composition. The morphology of the MPC composites were characterized on field emission

scanning

electron

microscopy

(FE-SEM,

JEOL,

JSM-7001F)

and

transmission electron microscope with high resolution transmission electron microscope (TEM and HRTEM, JEOL, JEM-2100F). The pore structures of the MPC composites were carried out by nitrogen adsorption–desorption isotherms at 77 K (JW-BK132F). The pore size distributions of the MPC composites were evaluated from adsorption branch based on Barrett-Joyner-Halenda (BJH) method and the pore diameters were collected from 0 to 300 nm, and the total pore volume was estimated at P/P0 = 0.99. The mercury intrusion measurement was also tested on Automated Mercury Intrusion Porosimeter (PoreMaster 33). Electromagnetic measurements The electromagnetic parameters of the MPC samples were performed on a vector network analyzer (AV3618, CETC) over 2-18 GHz. The mixtures were prepared by mixed 30 wt% of species with wax and pressed in the toroidal shape (the outer diameter: 7.0 mm, and the inner diameter: 3.04 mm). RESULTS AND DISCUSSION 3D magnetic porous carbon composites can be obtained by a one-pot carbonization method using loofah sponge chelated with Fe(NO3)3·9H2O as the MPC. As the addition of KOH into loofah sponge immersion with Fe(NO3)3·9H2O aqueous, the Fe3+ can react with OH- to form 5



Fe(OH)3. After carbonization, the Fe(OH)3 can convert into FeO(OH) and then ferric oxide. Meanwhile, the gas generated in the reaction can promote the formation of porous structures. XRD measurement is applied to identify the crystal structures of as-obtained MPC composites. When the temperature increased to 500-700 oC, the peaks at 30.2, 35.5, 37.1, 43.2, 53.6, 57.1, and 62.8 o should be ascribed to the (220), (311), (222), (400), (422), (511), and (440) crystal planes of Fe3O4 (NO. 88-0315). When the carbonization temperature raises up to 800 oC, there exist only two peaks at about 44.7o and 64.9o, corresponding to the (110) and (220) crystal plane of Fe (JCPDS. 06-0696), demonstrating that the Fe3+ ions are reduced to single Fe by carbon or the produced reduced gas (H2 or CO) during the carbonization process. In addition, the peak intensities of Fe3O4 in MPC500 are notably higher than those of Fe, indicating the Fe3O4 content is greatly larger than that of Fe in the MPC500 composites.29 In order to further evaluate the graphitization of the loofah sponge, Raman characterization is applied (Figure 1b). The peaks at around 1340 and 1590 cm-1 are owing to D and G band, which are related to the defects, disorder and graphitic of carbon materials, respectively.30 As the heating temperature increasing from 500-700 oC, the intensity ratio (ID/IG) of the MPC increases from 1.21 to1.53 , indicating that KOH activation can destroy graphite in the porous carbon materials.31 Based the phenomenological three-stage model, the rasied value of ID/IG of MPC is actually in the transitional state from amorphous carbon to nanocrystalline graphite.10 When the carbonization temperature goes up to 800 °C, the ID/IG value decreases to 1.32 instead, and there exist a weak 2D peak around 2700 cm-1 of MPC800. This is due to the enlarged sp2 carbon domain under higher carbonization temperature, thus leading to the transformation from nanocrystalline graphite to graphite carbon, which is in accordance with 6


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the previous reports.32 XPS measurements are also employed to determine the surface chemical composition and the valence of Fe in the MPC composites. From the XPS spectra (Figure 1c), it can be seen that three major peaks located around 284.5, 531, and 712 eV, which are assigned to the C 1s, O 1s and Fe 2p, respectively. Furthermore, the high-resolution Fe 2p XPS spectra of MPC600 presents two main peaks at 712.1 and 724.2 eV (Figure 1d), owning to the 2p3/2 and 2p1/2 levels, respectively. Moreover, no satellite peak can be found, suggesting no Fe2O3 phase exists in the MPC600. This indicates the formation of Fe3O4 in the composites during the carbonization process.33 Besides, a very weak peak located around 706.8 eV is also observed corresponding to α-Fe phase, indicating that a small amount of Fe is in the MPC600.34 Thus the XPS results further confirm the existence of Fe3O4 and a little Fe in MPC600 in accordance with the XRD analysis.

Figure 1. (a) XRD patterns of MPC composites, (b) Raman spectra of all the samples, (c) XPS spectrum of the composites and (d) XPS spectrum of Fe 2p. 7



Figure 2. SEM images of the obtained porous carbon: (a) Raw loofah sponge, (b) MPC500, (c) MPC600, (d) the corresponding elements mappings of MPC600, (e) MPC700 and (f) MPC800. It is noteworthy that the loofah sponge exhibits a 3D-bundled microtube-like with a diameter of about 10-20 µm (Figure 2a and inset). Importantly, there exist numerous functional groups, which can provide a lot of active surface sites for immobilization of Fe3+ irons. After a carbonization treatment, the MPC composites with 3D hierarchical porous structures are generated as shown in Figure 2b-2f. It is notably seen that abundant pores with a size of about 1-2 µm are generated, whose diameter size can increase with the carbonization temperature increasing. Furthermore, when the temperature is up to 500-700 oC (Figure 2b, 2c and 2e), there exist a large number of highly interconnected networks with porous structures, which is mainly due to the activation of KOH.35 It is beneficial for microwave scattering and multiple reflections. Meanwhile, the Fe3+ irons are converted into Fe3O4 nanoparticles anchored on the wall of the interconnected pores. Moreover, the corresponding

8


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element mappings of C, O, and Fe confirm the uniform distribution of Fe3O4 nanoparticles within the interconnected networks (Figure 2d). Additionally, the interconnected porous structures were disappeared and replaced by opened micropores for MPC800 (Figure 2f), which is related to the activation of potassium hydroxide.35-37 TEM and HRTEM are carried out to further investigate the microstructures of as-obtainbed MPC600 (Figure 3). As observed in Figure 3a, numerous nanopores can be observed in MPC600, and the diameters of the as-generated Fe3O4@Fe nanoparticles in the range of 80-100 nm are homogeneously distributed in the porous carbon. The TEM image further indicates the Fe3O4 nanoparticles are closely attached to the amorphous porous carbon. Meanwhile, the well-resolved crystal lattice with d-spacing values of 0.25 nm showed in the HRTEM (Figure 3b) should be attributed to the (311) plane of Fe3O4, which is consistent with the XRD analysis (Figure 1a). In addition, the lattice fringes of graphite carbon (0.33 nm) can also be observed, suggesting the existence of graphite carbon. This indicates that the generated Fe3O4@Fe nanoparticles can serve as catalyst and further lead to the formation of graphitization carbon.

Figure 3. TEM image of (a) the MPC600 and (b) the corresponding HRTEM image 9



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Moreover, the porous structures of MPC500-800 are further characterized by Brunauer-Emmett-Teller (BET) test and mercury intrusion porosimetry. The N2 adsorption–desorption isotherms curves of the MPC composites display a similar tendency, showing a rapid N2 uptake when the relative pressure is very low (P/P0 < 0.08) and then a continuous

increasing

in

the

other

test

range,

suggesting

the

presence

of

micro-/meso-/macropores (Figure S1a).18, 36 The carbonization and KOH activation can lead to large micropore volume and high specific surface area (Figure S1b and Table S1). Furtherore, the macropore size distribution and porosity of the MPC500-800 are measured using mercury intrusion porosimetry (Figure S1c), and the most probable macropore sizes are 1.44, 0.75, 0.71 and 0.56 µm for MPC500, MPC600, MPC700 and MPC800, respectively. It indicates higher carbonization temperature will decrease the macropore sizes. The hierarchically 3D porous structures with nano and micro-scale pores are benefit to the MA performance. Because the nano and micro-scale cavities acting as dihedral angles can improve the reflection of microwave inside the absorber and result in the loss of electromagnetic energy by enchancing the propagation path of microwave wave.10, 18 Based on the above results, the formation process of the 3D magnetic hierarchical porous carbon composites is illustrated as follows. Firstly, Fe(OH)3 can be obtained by the reaction between Fe3+ and OH- (equation 1). During the carbonization process, the Fe(OH)3 was firstly converted into FeO(OH) and then transformed into Fe2O3 (equation 2). At 400 oC, the KOH began to react with C to form metallic K and potassium carbonate (equation 3), and the KOH was completely transformed into K2CO3, and during this processes many intermediates were produced, such as H2, CO and CO2. Potassium carbonate began to decompose into 10


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potassium oxide and carbon dioxide above 700 oC, and then was consumed completely at 800 o

C (equation 4). The formed K can intercalate into the carbon lattices of the biomass carbon,

resulting in the expansion of carbon lattices.38 Meanwhile, the as-produced gases can not only make the loofah sponge introduce many pores (equation 3-7), but also act as the reducing agent for conversion of Fe2O3 into Fe3O4@Fe at 500-700 oC (equation 8).

Fe3+ + 3OH− → Fe ( OH )3

(1)

Fe ( OH )3 → FeO ( OH ) → Fe 2 O 3

(2)

6KOH + 2C → 2K + 3H 2 + 2K 2 CO3

(3)

2K 2CO3 → K 2O + CO2

(4)

CO2 + C → 2CO

(5)

K 2 CO 3 + 2C → 2K + 3CO

(6)

K 2 O + C → 2K + CO

(7)

3Fe2O3 + ( C, H2 , CO) → 2Fe3O4 + Fe ( CO, H2O, CO2 )

(8)

To investigate the MA capacity of the as-obtained specimens, 3D plots of reflection loss (RL) value against thickness and frequency are evaluated from the measured electromagnetic parameters on the basis of the transmission line theory based on expression (9) and (10).39-41

RL = 20lg ( Zin − Z o ) / ( Zin − Z o )   2π fd     c 

Z in = Z o µr / ε r tanh  j 

(9) 

µrε r 

(10)



Where Zin represents the input impedance of the microwave absorbing materials; Zo indicates the impedance value of free space, f stands for the frequency of electromagnetic wave; c refers to the speed of light; d indicates the thickness of the microwave absorbing materials; µr 11



and εr present the relative complex permittivity and complex permeability, respectively.42, 43 It is well known that an ideal electromagnetic wave absorbing material should not only possess strong absorption, but also hold the features of light, thin thinness, as well as wide bandwidth. When the value of RL is below -10 dB, indicating 90% of the electromagnetic wave energy will be absorbed. In general, the RL values under -10 dB can be considered as an effective bandwidth. As can be observed from Figure 4a-4b, MPC500 and MPC600 samples exhibit excellent MA performance with a low thickness. In detail, when the carbonization temperature is maintained at 500 oC, the minimum RL of MPC500 can reach -32 dB with a thickness of 2.0 mm, and the effective bandwidth is 4.4 GHz (13.6-18 GHz). As carbonization temperature increases to 600 oC, the minimum RL of MPC600 is up to -49.6 dB with a thinner thickness of 2.0 mm with corresponding effective bandwidth is 5 GHz (13-18 GHz). However, with the carbonization temperature continues to increase, the MPC700 exhibits worse MA performance than those of MPC500 and MPC600. Its minimum RL is only -15.3 dB throughout the region from 10.5 to 16.3 GHz with a thinner thickness of 2.0 mm (Figure 4c). For the MPC800, as shown in Figure 4d, the MA performance significantly drops and the minimum RL only reaches -8 dB, which might be ascribed to the collapse of carbon skeleton and the reduction of Fe3O4 to Fe due to the further activation of potassium hydroxide. In addition, in order to verify the activation of the KOH, the RL of MC600 sample is shown in Figure S2. The minimum RL is only -10.6 dB with an effective bandwidth of 2.3 GHz, which demonstrates the KOH plays a significant role in the MA performance. In summary, the above results demonstrate that the carbonization temperature and activating agent play an important impact on the MA performance. 12


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Figure 4. 3D plots of RL of the MPC materials (a) MPC500, (b) MPC600, (c) MPC700, (d) MPC800, and (e) 3D plots of minimum RL evaluation of all the samples. In order to comprehensively understand the MA performance of the obtained specimens, the minimum RL and broadwidth with different thickness are displayed in Figure 4e. It is obviously seen that the MPC600 exhibits high MA performance in the all samples. For comparison, some representative porous microwave absorbing materials and their 13



performances have been summarized in Table S2. Compared with other literatures, there is no doubt that the composites MPC600 with stronger absorption intensity, much more effective absorption bandwidth and lower thickness is the best candidate for ideal absorbers. We purpose that the excellent MA and adjustment of the absorption peaks of the MPC600 materials may be attributed the incorporation of porous carbon embedded with Fe3O4@Fe and the superior impedance matching. To better investigate the MA performance of the MPC samples, the relationship between electromagnetic parameters against the frequency is interpreted further. The electromagnetic parameters of the MPC500-800 composites have been considered and displayed in Figure 5. Although all samples are immersed in the ferric nitrate solution for the same period of time and the mass of KOH is also equal, the electromagnetic parameters showed significant differences, which suggested that the annealing temperature has a vital influence on the electromagnetic parameters. As can be found from Figure 5, the ε′ and ε″ decrease continuously covering the measured frequency, which was associated with the raised polarization lagging in the range of high frequency ( Figure 5a and 5b).44 When the annealing temperature remained at 800 oC, the ε′ and ε″ of MPC800 are larger than those of other samples and MPC600 shows relatively low values of ε′ and ε″, respectively, which may be attributable to the higher Fe content and the improvement of graphitization from the increasing carbonization temperature. These suggest MPC800 composites possess a higher energy storage and dissipation capability.45 The dielectric tangent loss (tan δε) of the MPC composites is shown in Figure 5c, it is easily observed that the MPC800 has the strongest dielectric loss, and the MPC600 exhibits the feature of no frequency dependency, which is 14


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benefitical to the broadband absorption.46 The higher ε″ value and tan δε value of MPC800 indicate a better dielectric loss capability, however a higher dielectric loss ability will lead to the impedance mismatching.20 This implies that lots of microwave can be reflected on the surface of the MPC800 absorbers, thus leading to the worse MA performance. Therefore, a moderate complex permittivity instead of a higher or lower one is more propitious to the MA performance.47 The dielectric properties of MPC materials could be explained based on Debye theory. The Cole-Cole expression illustrateing the relationship between ε′ and ε″ can be described in the following equation.28, 38 2

εs + ε∞  2   εs + ε∞   ε '−  + ( ε '' ) =   2    2 

2

(11)

Here, εs indicates the static permittivity and ε∞ presents the relative dielectric constant at the high-frequency limit. When a single semicircle exhibits in the plots of ε″ and ε′, it indicates that there exists a Debye relaxation behavior. However, in this current work, the Cole-Cole plots of all MPC specimens only exhibit slight fluctuations without evident Debye peak (Figure 5), demonstrating that the dipolar polarization can be negligible while the interfacial polarization may play a major part in the polarization process. The results are consistent with the previous literature that negated the presence of dipolar polarization in biomass based porous carbon above 500 °C.10 The interfacial polarization loss mainly includes the multiple interficial polarization existed in the different interfaces of the MPC composites, such as amorphous carbons/Fe, amorphous carbons/Fe3O4 and Fe3O4@Fe, and the interfaces from the abundant porous structure with large specific surface area. Additionaly, the increase of polarization loss may also results from the hierarchically porous structures with meso/ macro 15



porous pores, and the details are shown in Figure S2-S4, which can enhance the dielectric loss and thus is benefit to the MA performance. Fianlly, the raise of ε′ may be due to the improved interfacial polarization generating from the more massive accumulation of electrons at larger interfaces. And the increasing values of ε″ may be caused by improved the polarization loss and conduction loss induced by raised conductivity.10 In addition, Figure 5d and 5e illustrate the permeability against frequency of the specimens, where the µ′ of MPC500-700 samples decrease as the frequency raises. In detail, the µ′ values decrease with some fluctuations from 1.03 to 0.95, 1.12 to 1.02, and 1.17 to 1.03 for MPC500, MPC600, and MPC700 respectively. µ″ with magnetic resonance peaks increases rapidly with the raise of frequency, especially for MPC700 and MPC800. For the magnetic loss (Figure 5f), the tanδµ of all samples shows a similar tendency with multi resonance behaviours and the MPC700 possesses the highest tan δµ value.

Figure 5. Electromagnetic parameters of MPC specimens: (a) ε′, (b) ε″and (c) dielectric loss; (d) µ′, (e) µ″ and (f) magnetic loss. 16


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Figure 6. Cole-Cole plots of (a) MPC500, (b) MPC600, (c) MPC700, and (d) MPC800. Generally speaking, magnetic loss usually derives from natural resonance, exchange resonance and eddy current loss in a microwave frequency band.26 The resonance peaks from 2 to 8 GHz can be considered as resonance, because the low-frequency resonance peak usually originates from natural resonance. If the magnetic loss just generates from the eddy current loss, the value of µ″(µ′)-2f--1 should keep a constant value as frequency increases.6, 9 As is depicted in Figure 7, the values of µ″(µ′)−2f--1 for MPC composites are not eternally fixed constant owing to the natural resonance that might come from the geometrical configuration effect, while for MPC500, 600, 700 and 800, the values keep relatively constant in the range of 6.2-11.9, 8.0-13.0, 8.0-12.0 and 8.0-10.5 GHz, respectively, indicating that the magnetic loss partly derives from the eddy current loss. Besides, the resonance at higher frequency should be attributed to the exchange resonance based on 17



Aharoni’s theory which has been proven in ferromagnetic nanoparticles.48 Thus, the magnetic loss exhibited in the MPC composites is primarily coming from natural resonance in the range of 2-4 GHz, eddy currents effects at 11-15 GHz and exchange resonance at 15-18 GHz.

Figure 7. The plots of µ″(µ′)−2f--1 curves at different carbonization temperatures. Generally, the outstanding MA performance of MPC600 should be attributed to the synergistic effects between the magnetic Fe3O4@Fe and the dielectric amorphous carbons components, which result in the better impedance matching and strong EM wave attenuation in the inside of the microwave absorption materials. Therefore, a good impedance matching of the microwave absorption materials should be equal or close to that of free space, fulfilling zero reflection at the front surface of the microwave absorption materials. A delta function can be proposed to estimate the impedance matching of the absorbers according to the following expression.49 18


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∆ = sinh 2 ( Kfd ) − M

(12)

Where K and M can be calculated through the relative complex permittivity and complex permeability as presented following expression.  δε + δ m    2 

4π µ r'ε r' × sin  K=

(13)

c × cos δε × cos δ m

 δε + δ m  4 µ r'ε r' cos δε × cos δ m µ r' ε r' × sin    2  M= 2  δε − δ m   2 2 ( µ r' cos δε − ε r' cos δ m ) +  tan    ( µ r' cos δε + ε r' cos δ m )   2 

(14)

Generally, a perfect impedance matching is desired at a small delta value (|∆| < 0.4) and the area is expected to be as large as possible in the delta value maps. Figure 8 exhibits the contour maps of the calculated delta value with thickness ranging from 0.5 to 5.0 mm against the tested frequency over 2 to 18 GHz. Obviously, the MPC500 and MPC600 possess a larger region with delta value closed to zero comparing with other specimens, indicating an excellent impedance matching, which is related to their superior MA performance. However, when the annealing temperature is up to 700 and 800 oC, the region with delta value closed to zero is smaller than those of MPC500 and MPC600. This indicates a worser impedance matching, which is due to their larger values of the complex permittivity. Therefore, the MPC600 composites possess the highest impedance matching degree, indicating its MA performance is the best among the MPC composites, which is confirmed by the results of reflection loss.

19



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Figure 8. Calculated delta value maps of MPC composites: (a) MPC500, (b) MPC600, (c) MPC700 and (d) MPC800. Additionally, EM wave attenuation in the inner part of the microwave absorption materials is another key factor for high performance absorbers. To clarify the possible MA mechanism of the MPC materials, the attenuation constant (α) is taken into considerations, which presentsthe interior attenuation capability of the obtained specimen. The value of the constant (α) can be calculated by the following expression.6

a=

2 πf c

(ε ′′µ ′′ − ε ′µ ′) + (ε ′′µ ′′ − ε ′µ ′)2 + (ε ′′µ ′ + ε ′µ ′′)2

(15)

As shown in Figure 9, the attenuation ability of MPC600 possesses the higher value of α among all the specimens, demonstrating that the MPC600 has superior MA ability than those of other MPC samples. 20


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Figure 9. The values of attenuation constant for all MPC samples. Taken together, the 3D porous carbon materials are beneficial to attenuate electromagnetic wave. Firstly, after the loofah sponges were carbonization with KOH and Fe(NO3)3·9H2O, many defects have been introduced in the MPC, which can serve as the polarized centers to improve the MA performance. Meanwhile, the MPC composites possess numerous hierarchical porous structures, which can raise solid–void interfaces and thus benefit the multiple reflections and diffractions of microwave. Moreover, the massive solid–void interfaces can also bring about interfacial polarization, and electromagtic wave energy can be weakened through interfacial polarization relaxation loss. In addition, the interfaces between iron oxide and porous carbon can also work as the polarization centres, which could generate excessive polarization relaxations. Thus the above mentioned polarization losses are benefit to the dielectric loss. On the other hand, the introduced magnetic Fe3O4@Fe nanoparticles 21



can lead to the magnetic loss. Therefore, the enhanced dielectric loss and introduced magnetic loss can result in good impedance match, which is benefit to the MA performance. Additionally, the highly porous structures also can bring about the multiple reflections and scatterings of microwave, which can further enhance the MA performance. Therefore, the improved MA performance of MPC600 is ascribe to the good impedance matching, multiple reflections and scatterings among the hierarchical porous structures of MPC composites.

CONCLUSIONS In conclusion, we have successfully prepared series of MPC composites using loofah sponge biomass and Fe(NO3)3·9H2O through a facile and annealing process. The porous amorphous carbon/Fe3O4@Fe with multiple interfaces can be obtained. When the annealing temperature is 600 oC, the sample exhibits excellent MA performance. The minimal RL of MPC600 reaches -49.6 dB at 15.9 GHz with a thinner thickness of 2 mm, and the effective absorption bandbroad reaches 5.0 GHz. Moderate dielectric loss and magnetic loss can determinate the remarkable MA performance of MPC600. It is believed that this strategy would open a new route to the fabrication of high performance absorbers from renewable nature biomass for MA applications.

ASSOCIATED CONTENT Supporting Information Brunauer-Emmett-Teller (BET) test and mercury intrusion porosimetry of MPC500-800; Specific surface area (SBET), total pore volume, and the most probable pore size of all the MPC samples obtained by BET measurement; 3D plots of the calculated reflection loss of MC600; SEM image of MC600; The electromagnetic parameters of MC600; MA 22


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performance of porous materials reported in the recent literatures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (FB Meng); [email protected] (ZW Zhou).

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51573149), the Fundamental Research Funds for the Central Universities (No. 2682016CX069) and the Science and Technology Planning Project of Sichuan Province (No. 2018GZ0132 and No. 2018GZ0427).

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TABLE OF CONTENTS (TOC) GRAPHIC:

Synopsis: Hierarchical porous carbon/Fe3O4@Fe composites derived from loofah sponges chelated with Fe3+ have been successfully fabricated for tunable high-performance microwave absorption.

31



Loofah sponges chelated with Fe3+ are directly transformed into hierarchical porous carbon/Fe3O4@Fe composites under carbonized design, exhibiting high-performance microwave absorption. 47x33mm (600 x 600 DPI)


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Fe3O4@Fe

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