C Microspheres with Enhanced Microwave

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MOFs-Derived Hollow Co/C Microspheres with Enhanced Microwave Absorption Performance Zhennan Li, Xijiang Han, Yan Ma, Dawei Liu, Yahui Wang, Ping Xu, Chaolong Li, and Yunchen Du ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01270 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Sustainable Chemistry & Engineering

MOFs-Derived Hollow Co/C Microspheres with Enhanced Microwave Absorption Performance Zhennan Li†, Xijiang Han*,†, Yan Ma†, Dawei Liu†, Yahui Wang†, Ping Xu†, Chaolong Li*,‡, Yunchen Du*,†,§ †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, NO. 92 West Da-Zhi Street, Nangang District, Harbin 150001, China ‡

Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, NO. 266 Fangzheng Avenue, Shuitu Hi-tech Industrial Park, Beibei District, Chongqing 400714, China §

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the

People's Republic of China, Heilongjiang University, NO. 74 Xuefu Road, Nangang District, Harbin 150080, PR China * Corresponding authors. Prof. Yunchen Du, email: [email protected] Prof. Xijiang Han, email: [email protected] Prof. Chaolong Li, email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT:

Rational

construction

of

profitable

microstructure

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in

carbon-based

electromagnetic composites is becoming a promising strategy to reinforce their microwave absorption performance. Herein, the microstructure design is innovatively coupled with metalorganic frameworks (MOFs)-derived method to produce hollow Co/C microspheres (Co/C-HS). The resultant composites combine the advantages of hollow microstructure and good chemical homogeneity. It is found that the pyrolysis temperature plays an important role in determining the electromagnetic properties of these hollow Co/C microspheres, where high pyrolysis temperature will increase relative complex permittivity and decrease relative complex permeability. When the pyrolysis temperature is 600 °C, the sample (Co/C-HS-600) will show improved impedance matching and good attenuation ability, and thus an excellent microwave absorption performance with strong reflection loss (-66.5 dB at 17.6 GHz) and wide response bandwidth (over -10 dB, 3.7-18.0 GHz) can be achieved. By comparing with Co/C composites derived from conventional ZIF-67, it can be validated that hollow microstructure is greatly helpful to upgrade the performance by boosting dielectric loss ability and suppressing negative interaction between carbon matrix and incident electromagnetic waves, as well as providing multiple reflection behaviors. We believe that this study may open up a new avenue to promote the electromagnetic applications of MOFs-derived carbon-based composites.

KEYWORDS: Hollow microstructure, Co/C microspheres, MOFs-derived, ZIF-67, microwave absorption, impedance matching.

INTRODUCTION As a universal strategy in dealing with electromagnetic pollution, electromagnetic interference, and military stealth, microwave absorption has received considerable attention and aroused


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widespread interest in the past decade.

1-3

Dielectric loss and magnetic loss are two common

pathways that can attenuate the incident electromagnetic waves.4 Therefore, constructing composites with dual loss mechanisms has been widely accepted as an effective route to upgrade the performance of microwave absorbing materials (MAMs).5,

6

Among various types of

composites, magnetic carbon-based MAMs are always residing at the frontier of microwave absorption due to their significant synergistic effects and tunable electromagnetic functions.7-11 In the recent advances of magnetic carbon-based composites, the direct pyrolysis of metalorganic frameworks (MOFs) has become a highly attractive preparation technology,12, 13 and the resultant products also demonstrated their great superiority in the dissipation of electromagnetic energy.14, 15 On one hand, the pyrolysis of MOFs can induce in situ formation of magnetic metal nanoparticles in carbon matrix, which may account for enhanced magnetic loss ability because these magnetic metal nanoparticles have larger saturation magnetization, unlimited Snoek's effect, and distinguishable permeability in the frequency range of gigahertz as compared with conventional magnetic ferrites;16,

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on the other hand, the periodic arrangements of metal

coordination centers and organic ligands in crystalline MOFs provide a congenital advantage for good chemical homogeneity of magnetic metal/carbon composites, which are very favorable for the uniformity of electromagnetic property and the accumulation of multiple polarization resonances.18 To date, the fabrication of various magnetic metal/carbon composites, including Fe/C, Co/C, Ni/C, FeNi/C, and FeCo/C, has been successfully achieved by MOFs-derived method.19-28 In addition to the intrinsic nature of MAMs, microstructure also plays an important role in determining their microwave absorption performance. Some unique configurations, e.g. porous structure, hollow microstructure, yolk-shell structure, sandwich-like structure, have been


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successfully constructed in various carbon-based MAMs, and manifested their effectiveness for the enhanced performance by improving characteristic impedance matching, inducing multiple reflection behaviors, and intensifying interfacial polarizations.29-35 For example, Yuan et al. fabricated 3-D ordered arrays of core-shell microspheres consisting of Fe3O4 cores and ordered mesoporous carbon shells, and confirmed that the strong reflection loss characteristics benefited from ordered microsphere arrays and mesoporous channels;30 Zhou et al. reported that hollow carbon nanospheres with an outer diameter of ∼70 nm and inner diameter of ∼30 nm could exhibit a minimum reflection loss (RL) of -50.8 dB at 13.5 GHz with a thickness of 1.9 mm and an response bandwidth of 4.8 GH.31 Our group recently designed yolk-shell Fe3O4@C and C@C microspheres as lightweight MAMs, respectively, and the results validated that the distinctive core@void@shell configuration was greatly helpful for the consumption of incident electromagnetic waves, leading to a stronger reflection loss and a wider response bandwidth as compared with their solid counterparts.36, 37 In view of these achievements, it can be expected that the creation of some profitable microstructures in those MOFs-derived magnetic metal/carbon composites will further reinforce their microwave absorption performance. However, literature review indicates that there are very few successful examples accessible, because it is very difficult to manipulate the growth of MOFs.38 In this article, we employ hollow ZIF-67 assemblies as the precursor to pioneer the synthesis of hollow Co/C microspheres via a MOFs-derived strategy. The obtained composites combine the merits of chemical homogeneity and structure effect. It is found that the desirable hollow microstructure grants these Co/C microspheres better reflection loss characteristics than those Co/C polyhedrons derived from pristine ZIF-67, which can be attributed to their improved impedance matching and attenuation ability.


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EXPERIMENTAL SECTION Synthesis of hollow Co/C Composites Hollow ZIF-67 assemblies were prepared through a one-step template synthesis according to a previous report.39 Typically, 1.021 g of cetyltrimethylammonium bromide (CTAB) was dissolved in 200 mL of deionized water, and then 1.05 mL aqueous solution of Co(NO3)2 (0.5 mol/L) was introduced. This mixture was stirred for 15 min before instantaneous addition of 21.0 mL aqueous solution of 2-methylimizole (1.096 mol·L-1). The solution was stirred continuously for another 3 h, and the resultant purple products were collected by centrifugation, washed with N, N-Dimethylformamide (DMF), and then dried in an oven at 60 °C overnight. The as-prepared ZIF-67 assemblies were pyrolyzed in a horizontal tubular furnace under Ar atmosphere at designated temperature for 3 h with a heating rate at 2 °C/min. The final Co/C composites were denoted as Co/C-HS-x, where x referred to the pyrolysis temperature. For comparison, a control sample, Co/C-600, was also prepared by pyrolyzing pristine ZIF-67 polyhedrons under Ar atmosphere at 600 °C for 3 h. Characterization Scanning electron microscope (SEM) images were obtained on an HELIOS NanoLab 600i (FEI). Transmission electron microscope (TEM) images were obtained on a Tecnai F20 operating at an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) data were recorded on a Rigaku D/MAXRC X-ray diffractometer with a Cu Kα radiation source (45.0 kV, 50.0 mA). The thermogravimetric (TG) analysis was carried out on a SDT Q600 TGA (TA Instruments) in the temperature range of room temperature to 800 °C at a heating rate of 10 °C/min under air atmosphere. Raman spectra were recorded on a confocal Raman spectroscopic system (Renishaw, In Via) using a 633 nm laser. The magnetic hysteresis loops were measured


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with a LakeShore 7404 (LakeShore, USA) vibrating sample magnetometer (VSM). An Agilent PNA-N5244A vector network analyzer (Agilent, USA) was used to determine the relative permeability and permittivity in the frequency range of 2.0-18.0 GHz for the calculation of reflection loss. A sample containing 30 wt% of the as-prepared product was pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm for electromagnetic wave measurement with paraffin wax was used as the binder. RESULTS AND DISCUSSION The strategy for synthesizing hollow Co/C microspheres is schematically depicted in Figure 1. Firstly, Co2+ can be enriched around the vesicles of CTAB due to its strong interaction with ammonium headgroups, as proved in a previous study.39 When 2-methylimizole is introduced, the nucleation of ZIF-67 will preferentially occur on the surface of the vesicles, and the subsequent growth of these nucleation sites will lead to the formation of a polycrystalline ZIF-67 shell. After washing with DMF and high-temperature pyrolysis, a hollow carbon shell containing Co nanoparticles (Co/C-HS) can be finally obtained. SEM images reveal that the as-prepared ZIF-67 assemblies are composed of well-dispersed microspheres with an average diameter of about 500 nm and a very rough surface (Figure 2a and 2b), and their hollow microstructure can be directly observed in TEM image (Figure 2c). The crystalline phase of these ZIF-67 assemblies is also investigated by wide-angle XRD, and the diffraction peaks are found to be precisely matched with the simulation results of standard ZIF-67 crystals in the 2θ rage of 5-40° (Figure 2d),39 indicating that the change of microstructure will not affect the crystallization of ZIF-67 units in these assemblies.


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Figure 1. Schematic illustration of preparing hollow Co/C microspheres via a MOFs-derived strategy.

Figure 2. SEM images (a and b), TEM image (c), and XRD pattern (d) of hollow ZIF-67 assemblies. Figure 3 shows SEM and TEM images of Co/C-HS-500, Co/C-HS-600, and Co/C-HS-700. It is very interesting that these carbon-based composites can inherit the basic spherical profile of ZIF-67 assemblies (Figure 3a-c), while their average sizes are much smaller and present a monotonous decrease with increasing the pyrolysis temperature. Besides, some visible pores also appear on the surface of these Co/C microspheres, which may be attributed to the gas release


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during the decomposition of organic ligands. TEM images identify that there are many heterogeneous nanoparticles in the carbon matrix, whose average particle sizes are ~20 nm, ~30 nm, and ~85 nm (Figure 3d-f), respectively. A high-resolution TEM (HR-TEM) image indicates that these nanoparticles have well-resolved lattice fringes with an interspacing of 0.20 nm (Figure S1), corresponding to the (111) plane of metallic Co with a face-centered cubic (fcc) structure. If we remove these Co nanoparticles by acid etching, the hollow microstructure of these Co/C microspheres will become clear (Figure S2a). It is more important that the hollow microstructure has good mechanical stability to survive from the pressing process for electromagnetic measurement (Figure S2b). These results indicate that hollow ZIF-67 assemblies herein may be taken as a kind of advanced MOFs-related precursors for carbon-based composites, which can promise good dispersion of Co nanoparticles and desirable hollow microstructure simultaneously. However, it has to mention that the hollow carbon shells show relatively weak inhibition on the agglomeration of Co nanoparticles, so that Co/C-HS-700 contains much larger Co nanoparticles than its counterpart from conventional ZIF-67.20 As reported in previous studies, high-temperature pyrolysis always induces a continuous graphitic carbon layer on the surface of Co nanoparticles (Figure S1),21,

40

and therefore, the exact

microstructure of these Co/C microspheres can be understood as a hollow carbon shell decorated uniformly by core-shell Co@graphitic carbon nanoparticles. Elemental mapping images confirm the homogeneous distribution of C and Co atoms, while N and O atoms are also detected in these Co/C composites (Figure S3). N atoms undoubtedly come from the organic ligand of ZIF-67.41 To make clear the specific reason for the presence of O atoms, the chemical composition of these composites is further analyzed by XPS measurements. As observed, XPS results again verify that C, Co, N, and O are four main elements in these composites (Figure S4a), while the contents of


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N and O are gradually decreased with the increase of pyrolysis temperature. C 1s spectra reveal that there are abundant C-O and C=O bands (Figure S4b), which may be attributed to the formation of O-containing functional groups on the surface of carbon matrix. Co 2p spectra identify zero-valence and divalent signals at the same time (Figure S4c), and the weak signals of zero-valence metallic Co is caused by the limited detection depth of XPS technique.42 These results indicate that O atoms in these composites are generated by the surface oxidation on both carbon matrix and Co nanoparticles, and high pyrolysis temperature is not only favorable for the removal of O-containing functional groups, but also helpful to suppress the spontaneous oxidation through stimulating the growth of Co nanoparticles.

Figure 3. SEM images (a, b and c), TEM images (d, e and f) of hollow Co/C composites pyrolyzed at different temperature. Figure 4a shows XRD patterns of Co/C-HS-500, Co/C-HS-600, and Co/C-HS-700. All these samples exhibit three well-resolved characteristic peaks at 2θ=44.2, 51.6 and 76.0°, which can indexed to (111), (200), and (220) planes of fcc-Co (JCPDS No. 15-0806).43 Although metallic


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Co has two stable crystal structures, close-packed hexagonal (hcp) phase and fcc phase, the diffraction peaks assigned to hcp-Co are not detectable in these composites, because the fcc structure is thermodynamically preferred above 450 °C.44 Even if the signals of Co2+ have been detected by XPS spectra (Figure S4c), the corresponding characteristic peaks of cobalt oxides are still absent in the patterns of these composites, indicating that most Co2+ species in the precursor has been transformed into zero-valence metal nanoparticles and the spontaneous oxidation on the surface of Co nanoparticles is rather limited. It is worth noting that the characteristic peaks of fcc-Co gradually become narrow and strong with increasing the pyrolysis temperature. This phenomenon means the growth of Co nanoparticles, in good agreement with TEM results (Figure 3d-f). However, XRD patterns still fail to present the characteristic peaks of graphitic carbon that have been identified by HR-TEM image (Figure S1), which may be rationally explained by the facts that carbon components in these hollow Co/C microspheres are overall amorphous and the graphitization only occurs on the surface of Co nanoparticles due to their catalytic effect.45 TG analysis is widely utilized to determine the specific carbon content in carbon-based composites, as carbon species will be completely burned off in air at 800 °C.18, 21 As shown in Figure 4b, Co/C-HS-500 displays a very weak weight loss in the temperature range of 100-150 °C owing to the removal of trace adsorbed water and surface groups, and a weak increase induced by the oxidation of Co nanoparticles in the temperature range of 150-250 °C. With further increasing the temperature, the combustion of carbon species will account for a sharp weight loss until the residual mass is unchanged above 500 °C. TG curves of Co/C-HS-600 and Co/C-HS-700 are a little different from that of Co/C-HS-500, where the initial weight changes in the temperature range of 100-250 °C disappear and only a sharp weight loss can be observed. In addition, one can also found that the onset of the weight decrease slightly shifts to high


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temperature region from Co/C-HS-500 to Co/C-HS-700. These phenomena imply that a relatively high carbonization degree is achieved in Co/C-HS-600 and Co/C-HS-700, and higher pyrolysis temperature is helpful to improve the thermal stability of these hollow Co/C microspheres. By considering that the oxidation of Co nanoparticles into final Co3O4 will compensate the combustion of carbon species, and thus the carbon content should be estimated according to the following equation, %carbon = (1 − % − %water)

( ) ()

(1)

where wt%R is the remaining weight percentage after combustion, and M refers to the molecular weights of Co and Co3O4. The calculation results indicate that the relative carbon contents of Co/C-HS-500, Co/C-HS-600 and Co/C-HS-700 are 50.1%, 47.8% and 41.4%, respectively. Raman spectroscopic technique is further employed to discern the bonding state of carbon atoms in these hollow Co/C microspheres (Figure 4c). As observed, all samples exhibit two distinct bands at about 1330 cm-1 and 1590 cm-1, respectively. The former is generally defined as D band, corresponding to the surface defects and disorder in zone-edge A1g symmetry of sp2 lattice; the latter, related to the E2g vibration induced by sp2 bond, is always described as G band.46 According to the three-stage model proposed by Ferrari and Robertson, the intensity ratio of D band to G band, ID/IG, can be taken as a criterion to evaluate the ordering of carbon atoms (graphitization degree of carbon materials).47 When amorphous carbon materials are transformed into nanocrystalline graphite, decreasing defects will increase the number of ordered rings and consolidate the intensity of D band, while G band will retain its intensity because it relates only to bond stretching of sp2 pairs. As a result, ID/IG will increase with decreasing amorphization.47 In our case, the values of ID/IG for Co/C-HS-500, Co/C-HS-600 and Co/C-HS-700 are 0.80, 0.85 and 0.96, respectively. In view of their amorphous nature (Figure 4a), the increased ID/IG values


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signify that high pyrolysis temperature can make a positive contribution to enhancing the graphitization degree of carbon components. Figure 4d shows the magnetic hysteresis loops of Co/C-HS-500, Co/C-HS-600, and Co/C-HS700. Based on these curves, it can be concluded that the presence of high-density Co nanoparticles endows these composites with typically ferromagnetic behaviors. By considering that carbon species will not be magnetized under current conditions, saturation magnetization (MS) is calculated in terms of the real Co content determined by TG curves (Figure 4b). The values of MS and coercivity (HC) for Co/C-HS-500, Co/C-HS-600 and Co/C-HS-700 are 114.6 emu/g and 364 Oe, 125.4 emu/g and 398 Oe, and 143.8 emu/g and 141 Oe, respectively. The differences in magnetic properties can be attributed to the crystallinity and size of Co nanoparticles in these composites. Generally speaking, high pyrolysis temperature can induce large size and good crystallinity of Co nanoparticle, as proved by TEM images and XRD patterns, and both of them will be favorable for the enhanced MS.48, 49 Therefore, Co/C-HS-700 performs the largest MS among these three composites. Although HC of metallic Co is usually associated with its grain size and magnetic anisotropy,50 the grain size may play a more decisive role in our case because Co nanoparticles herein do not present unique morphology and high-index facets. Leslie-Pelecky and Rieke found that the dependence of HC on grain size was similar to a downward parabola and the critical grain size for metallic Co was about 70 nm.51 If the grain size is less than 70 nm, there will be a positive correlation between HC and grain size, and a negative correlation will be observed once the grain size is beyond 70 nm. As indicated by TEM images (Figure 3d-f), the sizes of Co nanoparticles in Co/C-HS-500 and Co/C-HS-600 are obviously smaller than 70 nm, and thus Co/C-HS-600 performs larger HC than Co/C-HS-500. However,


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when the pyrolysis temperature is 700 °C, most Co nanoparticles will harvest a remarkable increase in grain size over 70 nm, which is responsible for the apparently decreased HC.

Figure 4. XRD patterns (a), TG curves under air atmosphere (b), Raman spectra (c), and fileddependent magnetization (d) of hollow Co/C composites pyrolyzed at different temperature. Inset in (d) is a magnification of magnetic hysteresis loops. According to the transmission line theory, the microwave absorption performance of an absorber is highly dependent on its relative complex permittivity (ɛr=ɛr'-jɛr") and complex permeability (µr=µr'-jµr"), and is generally evaluated by the concept of reflection loss (RL) that can be deduced from the following equations,52 #$% &'

(dB) = 20log "

"

#$% ('

1π

)*+ = , - tanh /0( )345µ6 ε6 7 ε 2 µ

-

(2) (3)

where Zin refers to the normalized input impedance of a metal-backed microwave absorbing layer, d is the layer thickness, c and f are the velocity and frequency of electromagnetic waves,


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respectively. Figure 5 shows the real parts (ɛr' and µr') and imaginary parts (ɛr" and µr") of Co/CHS-500, Co/C-HS-600, and Co/C-HS-700 in the frequency range of 2.0-18.0 GHz. It is widely accepted that ɛr' and µr' are related to the storage capability of electric and magnetic energy, and ɛr" and µr" express the loss ability of electric and magnetic energy.53 Co/C-HS-500 gives almost unchanged relative complex permittivity in the studied frequency range, whose ɛr' and ɛr" are close to 5.0 and 0.4 (Figure 5a and 5b), respectively, indicating its weak dielectric loss ability. With the increase of pyrolysis temperature, both ɛr' and ɛr" can be subsequently enhanced and display typical frequency dispersion behaviors. For example, the ɛr' values of Co/C-HS-600 and Co/C-HS-700 decrease from 13.5 to 8.2 and 19.3 to 10.6, respectively, and their corresponding ɛr" values decline from 6.6 to 3.6 and 18.3 to 6.4, respectively. In general, dielectric loss ability originates from conductivity loss and polarization loss.54, 55 Based on the free electron theory, ɛr"=1/2πρfε0 (ρ and ε0 are the resistivity and the dielectric constant of free space, respectively),56 it can be predicted that low resistivity (i.e. high conductivity) will be beneficial to the formation of strong dielectric loss. Polarization loss can be divided into ionic polarization, electronic polarization, dipole orientation polarization and interfacial polarization, while dipole orientation polarization and interfacial polarization are two main mechanisms that work for the loss of electromagnetic waves in gigahertz range, because ionic polarization and electronic polarization usually occur at much higher frequency region (103-106 GHz).57, 58 The results of Raman spectra confirm that carbon components in these composites have an incremental graphitization degree from Co/C-HS-500 to Co/C-HS-700, which suggests that the conductivity will be dependently reinforced. However, it has to mention that high pyrolysis temperature may diminish the defects and interfaces, resulting in the degraded contribution from dipole orientation polarization and interfacial polarization. By considering the experimental data (Figure 5a and 5b), conductivity


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loss plays a dominant role in determining the dielectric loss ability of these hollow Co/C microspheres. The variations of relative complex permeability are quite different from those of relative complex permeability. The µr' curve of Co/C-HS-500 starts from 1.08 at 2.0 GHz and ends with 0.96 at 18.0 GHz, while it performs it maximum of 1.10 at 3.0 GHz and three obvious fluctuations in the frequency ranges of 2.0-8.0 GHz, 8.0-14.0 GHz, and 14.0-18.0 GHz (Figure 5c). Although Co/C-HS-600 displays a similar µr' curve, there is an intersecting point at 4.6 GHz between these two curves, where Co/C-HS-500 has higher µr' values in the range of 2.0-4.6 GHz and inverse change can be distinguished over 4.6-18.0 GHz. This difference in the storage capability of magnetic energy may be attributed to their different content, crystallinity, and dispersion of Co nanoparticles. Higher pyrolysis temperature fails to impact µr' values greatly, so that Co/C-HS-700 almost presents an identical µr' curve to Co/C-HS-600. It is very interesting that the effect of pyrolysis temperature on the magnetic loss ability becomes much clearer (Figure 5d), and the specific µr" values are in the order of Co/C-HS-500>Co/C-HS-600>Co/CHS-700. As previously reported, eddy current effect and natural ferromagnetic resonance are two main factors responsible for magnetic loss in gigahertz range.18, 59 If magnetic loss only comes from eddy current effect, the parameter of C0 associated with relative complex permeability and frequency, C0=µr"(µr')-2f-1, will be constant and independent on the frequency.55, 59 As plotted in Figure S5, C0 values of these three samples keep changing from 2.0 to 18.0 GHz, and thus eddy current effect is not the only mechanism for magnetic loss. Actually, the well matched fluctuations between µr' and µr" curves have demonstrated that there were considerable contribution from natural ferromagnetic resonance. Of note is that Co/C-HS-700 with large MS and small HC exhibits the poorest magnetic loss ability, which is opposite to the prediction of


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initial permeability for ferromagnetic MAMs.28, 60 This is because carbon materials with good conductivity favor the formation of an AC electric field and an induced magnetic field through the interaction with incident electromagnetic waves, and the induced magnetic field will result in negative µr" values due to the radiation of magnetic energy.61, 62 In our case, carbon substrate in Co/C-HS-700 has high graphitization degree, and thus its good conductivity can produce more negative µr" values as compared with Co/C-HS-500 and Co/C-HS-600. Although strong magnetic response of Co/C-HS-700 predicts its good magnetic loss ability, the negative µr" values derived from its carbon substrate pull down the overall performance.

Figure 5. Relative complex permittivity (a, b) and relative complex permeability (c, d) of hollow Co/C composites pyrolyzed at different temperature. Figures 6a-c show the calculated RL maps of Co/C-HS-500, Co/C-HS-600 and Co/C-HS-700 on the basis of the measured electromagnetic parameters (the maximum value of RL maps is artificially truncated to -25.0 dB for clarity). It can be seen that their RL properties are highly sensitive to the pyrolysis temperature. Co/C-HS-500 harvests negligible microwave absorption


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ability owing to its weak dielectric loss, and the strongest RL value is only -4.8 dB at 10.1 GHz with the absorber thickness of 3.49 mm. By comparison, the RL characteristics of Co/C-HS-600 are significantly enhanced, and its maximum RL value reaches up to -66.5 dB at 17.6 GHz with the absorber thickness of 1.53 mm and its response bandwidth (over -10.0 dB, 90% absorption) can cover the frequency range of 3.7-18.0 GHz by manipulating the absorber thickness from 1.0 to 5.0 mm. When the pyrolysis temperature is settled at 700 °C, the RL characteristics for Co/CHS-700 will be unexpectedly degraded, where the maximum value and response bandwidth are 16.6 dB (18.0 GHz, 1.29 mm) and 6.6-18.0 GHz, respectively. In addition, the RL curves of these three composites with the absorber thickness of 2.00 mm are also plotted (Figure 6d), which virtually validate the superior performance of Co/C-HS-600 to those of Co/C-HS-500 and Co/C-HS-700 in both RL intensity and response bandwidth. To make clear the essential reasons for these different microwave absorption properties, two important parameters, attenuation constant (α) and impedance matching, are further investigated.63 The former represents the overall attenuation ability toward incident electromagnetic waves under the joint actions of dielectric loss and magnetic loss, and it can be expressed by the following equation. 9=

√1π; 2

× ,(=6 ′′ ε6 ′′ − =6 ′ ε6 ′ ) + 5(=6 ′′ ε6 ′′ − = ′ ε6 ′ )1 + (=6 ′′ ε6 ′′ + =6 ′ ε6 ′ )1

(4)

The latter determines the transmission of incident electromagnetic waves, and poor impedance matching implies the intensive reflection at the front surface of MAMs or direct penetration without any profitable consumption.64 As shown in Figure 7a, α value demonstrates strong dependence on both frequency and pyrolysis temperature. High frequency and pyrolysis temperature are favorable for creating strong attenuation ability. For example, the α values of Co/C-HS-500, Co/C-HS-600, and Co/C-HS-700 monotonously increase from 5.2, 40.1, and 84.8 at 2.0 GHz to 33.6, 141.0, and 217.2 at 18.0 GHz. It can be found that the change of α value at a


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specific frequency point is consistent with relative complex permittivity rather relative complex permeability (Figure 5), indicating the dominance of dielectric loss in these composites. Ma et al. ever proposed a delta-function method to compare the matching degree of characteristic impedance of different MAMs,65 |∆| = |sinh1 (C34) − D|

(5)

where K and M can be deduced by εr and µr as C= D=

HH

H

HH

H

O PO QR PR EF ,G-H I-H ∙KLMN - - - - T S 2∙UKVI-HH ⁄I-H X∙UKVG-HH ⁄G-H X

E G-H I-H UKVI-HH ⁄I-H XUKVG-HH ⁄G-H X S

HH

H

HH

H

S

O PO _R PR ZG-H UKVI-HH ⁄I-H X&I-H UKVG-HH ⁄G-H X[ (\]^MN - - - - T` ZG-H UKVI-HH ⁄I-H X(I-H UKVG-HH ⁄G-H X[ S

(6)

S

(7)

According to these equations, the delta-value maps of Co/C-HS-500, Co/C-HS-600, and Co/CHS-700 are calculated in Figure 7b-d, where a small delta value always suggests good impedance matching. It is clear that the map of Co/C-HS-600 contains a larger area close to zero (|∆| ≤ 0.4) than those of Co/C-HS-500 and Co/C-HS-700, which intuitively validates its better impedance matching. The poor impedance matching in Co/C-HS-500 can be attributed to its negligible loss ability that cannot afford sufficient conversion toward incident electromagnetic energy. Although Co/C-HS-700 provides superior loss ability (Figure 7a), its large difference between relative complex permittivity and complex permeability results in the overwhelming reflection at the front surface rather than the interior consumption.32, 64 As a result, Co/C-HS-700 also performs an inferior matching degree of impedance to Co/C-HS-600. That is to say, the excellent RL characteristics of Co/C-HS-600 should be linked with its good impedance matching that can account for less reflection at the front surface, together with its good attenuation ability that can convert incident electromagnetic energy effectively.


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Figure 6. Reflection loss maps of Co/C-HS-500 (a), Co/C-HS-600 (b) and Co/C-HS-700 (c) with various absorber thicknesses in the frequency range of 2.0-18.0 GHz, and their reflection loss curves with an absorber thickness of 2.0 mm (d).

Figure 7. Frequency-dependent α values of hollow Co/C composites pyrolyzed at different temperature (a), and calculated delta value maps of Co/C-HS-500 (b), Co/C-HS-600 (c), and Co/C-HS-700 (d).


19


Page 20 of 35

To address the advantages of hollow microstructure in Co/C composites, a control composite, Co/C-600, directly derived from ZIF-67 polyhedrons are also studied. It is found that Co/C-600 possesses quite similar crystalline phase, chemical composition, graphitization degree of carbon matrix, and magnetic properties to those of Co/C-HS-600 (Figure S6), and thus the difference in microstructure will be responsible for their different electromagnetic properties and RL characteristics. Figure S7 compares the relative complex permittivity and complex permeability of Co/C-600 and Co/C-HS-600 in the frequency range of 2.0-18.0 GHz. Although ɛr' curve of Co/C-600 has a similar variation trend to that of Co/C-HS-600, its specific values are much smaller, especially in the frequency range of 2.0-16.1 GHz. Correspondingly, Co/C-600 also exhibits inferior ɛr" values to Co/C-HS-600. These results demonstrate that hollow microstructure boosts the dielectric loss ability of Co/C-HS-600, which may be explained by the following reasons. Firstly, hollow microstructure can stimulate the increase of the volume per unit mass in Co/C composite, and thus carbon matrix in Co/C-HS-600 will gain a greater opportunity to produce cross-linked conductive network, making substantial contribution to conductivity and relative complex permittivity.56 Secondly, hollow microstructure creates more interfaces and bring intensive interfacial polarization.65 Similar function of hollow microstructure can be also observed in some previous MAMs.66 The curves of µr' and µr" further recognize the effects of hollow microstructure on magnetic storage and magnetic loss. On one hand, good dispersion of Co nanoparticles in Co/C-HS-600 promises better magnetic storage capability; on the other hand, the internal cavity in Co/C-HS-600 suppresses the interaction between carbon matrix and incident electromagnetic waves, and results in rather limited radiation of magnetic energy in high-frequency range. Moreover, hollow microstructure is considered to be favorable for multiple reflection behaviors, which promotes the consumption of


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incident electromagnetic energy.67 Figure S8 shows the RL maps of Co/C-600, and its strongest RL value and response bandwidth are -15.3 dB (4.8 GHz, 5.0 mm) and 4.3-18.0 GHz, respectively. Both of them signify that Co/C-600 has relatively weak microwave absorption performance as compared with Co/C-HS-600. The α curve and delta-value map reveal that the superiorities of hollow microstructure will be directly related to enhanced attenuation ability and improved impedance matching (Figure S9 and Figure S10). In addition, we also list RL characteristics of some typical Co/C composites published in recent years (Table 1) ,8, 22, 68-74 and it can be found that Co/C-HS-600 herein displays an excellent performance that can be comparable to those of top-level MAMs ever reported, which again confirms the importance of rational design on the microstructure of MOFs-derived carbon-based composites.


21

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

Page 22 of 35

Table 1. Microwave absorption performance of similar materials. Absorbers

Absorber thickness (mm)

RL (dB) (Frequency, GHz)

Bandwidth below -10 dB (range, GHz)

Integrated thickness (mm)

RL (dB) (frequency, GHz; thickness, mm)

Bandwidth below 10 dB (range, GHz)

Ref.

Co@C Microspheres

2.0

-30.2 (8.5)

3.0 (7.1-10.1)

1.0-5.0

-68.7 dB (10.6, 2.3)

15.3 (2.7-18.0)

8

CNTs/Co-900

2.0

-34.9 (10.8)

3.6 (8.6-12.2)

1.0-5.0

-60.4 dB (15.1, 1.81)

13.9 (4.1-18.0)

22

Co-Carbon ball

2.0

-15.8 (10.1)

2.5 (9.0-11.5)

1.3-3.0

-20.6 dB (16.2, 1.0)

12.1 (5.9-18.0)

68

Co/C nanoparticles

2.0

-40.5 (16.9)

8.2 (9.8-18.0)

1.4-3.0

-43.4 dB (16.8, 2.3)

11.6 (6.4-18.0)

69

Co-CNTs

3.0

-39.3 (15.7)

3.5 (14.-17.5)

3.0-4.0

-39.3 dB (15.7, 3.0)

5.7 (11.8-17.5)

70

RHPC/Co

2.0

-31.0 (9.6)

2.2 (8.5-10.7)

1.1-4.0

-40.1 dB (10.7, 1.8)

14.1 (3.9-18.0)

71

Co/CNTs

1.8

-17.5 (9.8)

2.9 (8.4-11.3)

1.0-5.0

-36.5 dB (4.1, 1.8)

13.5 (2.9-16.4)

72

C(Co)

2.5

-27.5 (9.6)

2.3 (8.1-10.4)

1.5-4.5

-40.0 dB (4.2, 5.0)

12.6 (3.7-16.3)

73

Co-C/MWCNTs

2.0

-23.5 (15.8)

4.5 (13.5-18.0)

1.0-4.0

-58.9 dB (9.0, 2. 99)

12.3 (5.7-18.0)

74

Co/C-600

2.0

-14.7 (13.4)

2.9 (12.1-15.0)

1.0-5.0

-15.3 dB (4.8, 5.0)

13.7 (4.3-18.0)

Herein

Co/C-HS-600

2.0

-31.3 (12.8)

4.4 (11.0-15.4)

1.0-5.0

-66.5 dB (17.6, 1.53)

14.3 (3.7-18.0)

Herein

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CONCLUSIONS A series of hollow Co/C microspheres pyrolyzed at different temperature have been successfully fabricated by employing hollow ZIF-67 assemblies as a new kind of self-sacrificing template. In situ transfomation of organic ligands makes hollow microstructure well preserved in the resultant carbon matrix, and simultaneous carbothermal reduction can promise good dispersion of Co nanoparticles. The pyrolysis temperature has a great impact on the chemical compositions, graphitization degree of carbon matrix, grain size of Co nanoparticle, and especially on electromagnetic properties. When the pyrolysis temperature is 600 °C, an improved impedance matching in cooperation with good attenuation ability will endow the composite, Co/C-HS-600, with strong reflection loss and wide response bandwidth. Its excellent microwave absorption performance is better than many Co/C composites in previous studies. By comparing with Co/C composites derived from conventional ZIF-67, the advantages of hollow microstructure in boosting dielectric loss, amending magnetic loss, and enhancing attenuation ability can be clearly addressed. These results verify that rational design on the microstructure of MOFs-derived carbon-based composites is an effective strategy to construct high-performance MAMs in the future.


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Page 24 of 35

ASSOCIATED CONTENT Supporting Information. HRTEM image of Co/C-HS-600; TEM images of Co/C-HS-600 after being etched by acid (a) and after being molded in the wax; Elemental mapping images of Co/C-HS-600; XPS spectra of Co/C-HS composites; C0 curves of Co/C-HS-x and Co/C-600; XRD, TG, Raman and magnetic hysteresis loop of Co/C-600; Relative complex permittivity and relative complex permeability of Co/C-600 and Co/C-HS-600; Reflection loss and Calculated delta value map of Co/C-600. AUTHOR INFORMATION Corresponding Author *

Corresponding authors. E-mail: [email protected] (Y. Du); [email protected] (X.

Han). Tel: +86-(451)-86418750; fax: +86-(451)-86413702. *

Corresponding authors. E-mail: [email protected] (C. Li).

Tel: +86-(023)-65935099; fax: +86-(023)-65935000. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (21676065, 21776053, and 21571043), and Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education.


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Composite

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Zeolitic

Imidazolate

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TOC:

Synopsis: MOFs-derived hollow Co/C microspheres demonstrate a promising strategy for highperformance microwave absorbing materials toward sustainable precaution on electromagnetic pollution.


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C Microspheres with Enhanced Microwave

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