Enhanced Low-Frequency Electromagnetic Properties of MOF

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Functional Nanostructured Materials (including low-D carbon)

Enhanced Low Frequency Electromagnetic Properties of MOF-derived Cobalt through Interface Design Wei Liu, Shujuan Tan, Zhihong Yang, and Guangbin Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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ACS Applied Materials & Interfaces

Enhanced Low Frequency Electromagnetic Properties of MOF-derived Cobalt through Interface Design Wei Liu, Shujuan Tan, Zhihong Yang* and Guangbin Ji* College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, P. R. China.

*Corresponding Author: Prof. Dr. Guangbin Ji E-mail: [email protected] Dr. Zhihong Yang Emial: [email protected]

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ABSTRACT It is still a formidable challenge to ameliorate low frequency electromagnetic property of conventional microwave absorbing materials, which may be conquered by the coexistence of both strong dielectric and magnetic loss ability in low frequency range and perfect balance between complex permittivity and permeability with the help of structural design. Herein, by virtue of appropriate composition and structure of Co3[HCOO]6·DMF parallelepipeds, one-dimensional sponge-like metallic Co can be directly synthesized for the first time with strong magnetic loss in low frequency range. Furthermore, attenuation ability and impedance matching condition have been improved through the construction of interfacial structures between inner cobalt and surface carbon. With the structure of carbon changed from fragments to vertically aligned nanoflakes and eventually to thick layer with extra fragments, the dielectric loss would be continuously strengthened while the magnetic loss maintains well followed by a remarkable decline. A perfect balance between dielectric and magnetic loss has been achieved by sample S-Co/C-0.3 with minimum reflection loss value around -20 dB and effective absorption frequency range about 3.84 GHz in C band. Excellent microwave absorption performance can also be realized in X and Ku band. In addition, as-prepared Co and Co/C composites can also be potentially applied in electromagnetic shielding. The findings may pave the way for the manufacture of metal-based MOF-derivatives and the design of lightweight low-frequency electromagnetic materials. Key words: Porous cobalt, cobalt/carbon composites, metal-organic frameworks, microwave absorption, low frequency


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1. INTRODUCTION In order to eliminate the pollution caused by redundant electromagnetic wave or endow military targets with electromagnetic stealth property, microwave absorbing materials (MAMs) which can convert incident electromagnetic energy into thermal energy have been widely utilized.1 Although great achievement has been made so far, it is still a formidable challenge to realize effective low-frequency absorption by current MAMs. This should be attributed to the lack of strong attenuation process in low frequency as far as we are concerned. The consumption of incident microwave is achieved through distinct processes which can be divided into magnetic and dielectric loss.2,3 It is generally accepted that strong magnetic loss is one of the most important prerequisites for low frequency electromagnetic attenuation. This is highly dependent on related magnetic loss processes, especially natural resonance at low frequency. Current low frequency MAMs are mainly constituted by carbonyl iron and ferrites, resulting from strong enough magnetic loss at low frequency.4,5 However, they usually suffer from high density, instability in harsh environment and inadequate electrical conductivity.6,7 Therefore, lots of researchers are dedicated to integrate dielectric material with magnetic material to ameliorate the comprehensive performance. A facile route has been developed by Yuan et al. to fabricate Ni particles encapsulated in few-layer N-doped graphene supported by N-doped graphite sheets. With optimal Ni content, a broad fe up to 8.5 GHz (mainly in Ku band) can be reached at 3.0 mm.8 A series of olive-like γ-Fe2O3@C@α-MnO2 spindles with varied interior volumes have been prepared by chemically etching of γ-Fe2O3@C core-shell spindles. It can be inferred that strong magnetic loss contributes to the ultra-wide fe of 9.2 GHz (from 7.82 to 17.02 GHz) at 2.0 mm.9 Najim et al. successfully coated T-ZnO with nickel-phosphorus through a facile route and as-fabricated Ni-P coated T-ZnO owns an ultra-wide fe from 4 to 14 GHz which should stem from synergistic attenuation of dielectric ZnO and magnetic Ni.10 Although satisfying breakthrough has been made, we should point out that the attenuation ability of most reported MAMs cannot afford the requirement of efficient low frequency microwave absorption. ACS Paragon Plus Environment

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Rather than the limited magnetic loss of current MAMs, researchers have sought help from dielectric loss.11-13 Generally, conduction loss, polarization loss and interference cancellation should be taken into consideration for dielectric MAMs.14,15 First, interference cancellation only takes place in specific frequency, which has weak relation with broadband absorption. According to Qiao’s work, interference cancellation is highly confined in a narrow frequency range, thus contributes more to the peak values rather than effective frequency bandwidth.16 Second, polarization processes (usually mean interfacial polarization in microwave range) are always accountable for increased real complex permittivity (ε′) and presented as resonance peaks. Based on current reports, resonance peaks always occur at higher frequency and result in dual or multiple absorption peaks.17,18 Consequently, polarization loss contributes marginally to the wide-band low frequency absorption. Third, conduction loss has been considered as major dielectric loss process at low frequency. Moreover, it can be easily elevated by improving electrical conductivity.19,20 However, deteriorated impedance matching condition caused by too high electrical conductivity strictly restricts the strengthening of dielectric loss (mainly conduction loss) at low frequency.21-23 In brief, to achieve excellent low frequency electromagnetic wave absorption performance, researchers should overcome following obstacles: (1) How to obtain strong enough magnetic loss which should be main attenuation ability at low frequency.24-26 (2) How to introduce dielectric loss at low frequency including conduction loss, polarization loss and interference cancellation.27,28 (3) How to ensure good impedance matching condition. Currently, the most effective technique should be composition regulation and structure modulation.29 For example, Ni microspheres coated by long oriented MWCNTs has been prepared by Tong et al. which exhibit improved µr and magnetic loss.30 Elliptical Fe3O4/C core−shell nanorings can be synthesized via a facile one-pot hydrothermal route with enhanced microwave absorption performance caused by improved dielectric properties and impedance matching.31


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Inspired by previous work, we have developed a modified MOFs route to prepare Co/C composites with tunable interface structures, which can be employed as good low frequency MAMs. First, strong magnetic loss has been obtained by one-dimensional porous metallic cobalt in this work, which is prepared through elaborately selecting MOFs precursors. Recently, MOF-derived materials have become a hotspot in various fields due to their tunable composition and structure. Reported MOFderivatives can be roughly categorized into porous carbon, metal oxides, metal chalcogenides, metal carbides, metal phosphides and their composites.32 N-doped microporous carbon derived from ZIF-8 can be applied as a superior anode for sodium-ion battery after the decoration of amorphous red phosphorus.33 A series of metal oxides with highly complex mixed shells have been fabricated from MOFs and can be utilized in hybrid supercapacitors.34 NiCo-LDH/Co9S8, Co3InC0.75 and CoP@BCN are all well investigated with unique advantages.35-37 Thanks to the possibilities offered by numerous tyes of MOFs, excellent low frequency MAMs with desired composition and structure would be obtained.38,39 Herein, one-dimensional porous metallic Co has been successfully obtained derived from direct pyrolysis of Co3[HCOO]6·DMF which guarantees the strong magnetic loss at low frequency. As far as we are concerned, it is the first time that pure metal can be prepared from MOFs which may open a new avenue for the academic research and industrial application of metal material.40 Then, both dielectric loss and impedance matching condition have been optimized through the construction of unique interface structure between as-prepared Co and surface carbon, which can be facially adjusted by tuning the content of carbon source. With the structure of carbon changed from fragments to vertically aligned nanoflakes and eventually to thick layer with extra fragments, the dielectric loss would be continuously strengthened while the magnetic loss maintains well followed by a remarkable decline. Excellent impedance matching can also be realized in this route. Besides, asprepared Co and Co/C composites may be potentially utilized in electromagnetic shielding. Schematic illustration of experiment procedures and objectives of experiment design are summarized in Figure 1.


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This work may give inspiration and paradigm to the development of MOF-derivatives and promote the design of excellent low frequency electromagnetic materials.

Figure 1. Schematic illustration of experiment procedures and objectives of experiment design.

2. EXPERIMENTAL SECTION Synthesis of Co3[HCOO]6·DMF. Co3[HCOO]6·DMF parallelepipeds were synthesized by a modified solvothermal method.41 Typically, 8 g of Co(NO3)2·6H2O was dissolved in 56 mL of DMF. Subsequently, 7 mL of HCOOH was added into the mixture, followed by being magnetic stirred for 30 min. Then, the purple solution was transferred into a 100 mL Teflon-lined autoclave which was then incubated at 100 oC for 24 h. Pink precipitates can be gathered by filtration and should be washed by DMF for several times. After thoroughly dried in vacuum at 60 oC and being ground, pink Co3[HCOO]6·DMF powders were obtained. Synthesis of MOFs/glucose mixtures. The injection of glucose was realized according to a previous report.42 In a typical process, 0.1 g of glucose and 0.5 g of MOFs powders were soaked in 20 mL ethanol and stirred for 1 h. The resultant mixture was allowed to stand for 24 h. MOFs/glucose mixtures would be collected by filtration and washed by ethanol for several times, which were then dried at 60 o

C. This sample was named as S-MG-0.1. The other samples (S-MG-x, x means the weight of glucose)

were prepared by replacing 0.1 g glucose with 0.2 g, 0.3 g, 0.4 g and 0.5 g glucose, respectively.


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Synthesis of Co and Co/C composites. Metallic Co was obtained from direct pyrolysis of MOFs under inert atmosphere. In detail, 1.5 g of MOFs powders were loaded into a tube furnace and heated at 500 oC for 5 h under N2 atmosphere with a ramping rate of 5 oC·min-1. After naturally cooling down to room temperature, gray powders would be got after washing with ethanol for 3 times which was labeled as S-Co. Similar samples were also obtained at 400 oC and 600 oC, which were named as S-Co-400 and S-Co-600. The Co/C composites have been fabricated by annealing S-MG-x in the same condition. Pyrolysis products of S-MG-x were labeled as S-Co/C-x. Characterization. Thermogravimetric analysis (TGA) was carried out in N2 atmosphere with a heating rate of 10 oC·min-1. A Bruker D8 X-ray diffractometer with Cu Kα radiation with a working voltage of 40 kV and working current of 40 mA was employed to characterize the composition and phase of samples. Precise content of Co was decided by inductively coupled plasma-atomic emission spectrometer (ICP-AES). Before test, samples were thoroughly treated in aqua regia. Chemical state of each element was evaluated by a PHI 5000 VersaProbe device. A Lakeshore 7400 vibration magnetometer was used to provide hysteresis loops of all samples. Morphology was determined by scanning electron microscopy (SEM, Hitachi S4800). Porous structures of as-prepared Co/C composites were evaluated by a micromeritics ASAP 2020 system. Electromagnetic parameters were recorded from an Agilent PNA N5244A vector network analyzer by an installed software via a coaxial line method. The testing toroidal rings for microwave absorption test were made up of 50 wt% of samples and 50 wt% of paraffin with Φout of 7.0 mm and Φin of 3.04 mm. The other rings for electromagnetic shielding test were constituted by 80 wt% of samples and 20 wt% of paraffin.

3. RESULTS AND DISCUSION Digital photographs of Co3[HCOO]6·DMF, S-MG-0.1, S-MG-0.3 and S-MG-0.5 are given in Figure S1. Negligible differences can be distinguished which can be ascribed to the well dispersion of glucose. Pyrolysis process of as-prepared Co3[HCOO]6·DMF was researched by thermogravimetric analysis (TGA) under N2 atmosphere. As we can see in Figure 2a, there exist two obvious weight-loss steps in ACS Paragon Plus Environment

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the range of 150-190 oC and 290-360 oC. In our opinion, MOFs firstly undergoes a weight loss of guest molecules, DMF, whose boiling point is approximate 153 oC. We should point out that the calculated weight ratio of DMF is about 14.0 wt% which is consistent with our deduction.41 Next step should be the decomposition process of MOFs which is also in line with previous report. The weight ratio of product is 43.45 wt% at 360 oC and detailed information would be discussed later. It should be noted that a rising trend of weight can be observed with increasing temperature. In contrast, thermal stability of MOFs/glucose mixtures has also been evaluated by TGA. Take the case of S-MG-0.3, clear difference can be identified in Figure 2b. In detail, a slow weight loss step exists in the range of 100-290 o

C which is related with the evaporation of DMF. When the temperature is higher than 290 oC,

remarkable weight-loss process would take place. Only 34.65 wt% of product can tolerate high temperature up to 340 oC. The rather low residue weight ratio of S-MG-0.3 has close contact with the extra pyrolysis of glucose. In addition, the weight continues to decrease with elevated temperature, resulting from complete decomposition of glucose. To verify the composition of products, precise contents of Co have been shown in Figure 2c. The weight ratios of S-Co, S-Co/C-0.1 and S-Co/C-0.3 are 99.8%, 99.56% and 99.5%, respectively. Considering the possible oxidation process, we may safely conclude that S-Co is mainly pure Co while S-Co/C-0.1, 0.3 contain a small amount of carbon or unavoidable oxides. Intriguingly, S-Co/C-0.5 only possesses 62.77 wt% of Co. It is easy to understand that more glucose would lead to more residue carbon. Powder XRD patterns have been cited as Figure 2d. As for the product (S-Co) derived from direct pyrolysis of MOFs precursor, only three peaks can be distinguished which lie in 44.2o, 51.5o and 75.8o. They can be well matched with standard XRD pattern of cubic Co (PDF#15-0806).43,44 XPS spectra further provides more information about the surface elemental state of two kinds of samples. Only signals of Co, C and O can be detected in Figure 2e, indicating the high purity of the samples. In detail, signals of Co 3p, Co 3s, C 1s, O 1s, Co 2p and Co 2s can be seen. In this case, element C and O may originate from surface carbon and cobalt oxides. High resolution XPS spectra of Co 2s and C 1s are ACS Paragon Plus Environment

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supplied in order to investigate and compare the chemical state of Co and C. As for S-Co, the Co 2p spectrum can be deconvoluted into five peaks as displayed in Figure 2f. Peaks at 781 eV and 796.6 eV belong to Co2+ and corresponding satellite peaks can also be seen.45 This may originate in the oxidation in air or incomplete reduction during pyrolysis process. We can also view the signal of metallic Co at 778 eV.46 Similarly, high-resolution Co 2p peaks of S-Co/C-0.3 can be fitted into four peaks in Figure 2g, including Co2+ peaks (780.9 eV, 796.5 eV) and satellite peaks. No signal of metallic Co can be observed in the spectrum that can be explained as: layers of cobalt oxides and carbon on surface may make the inner Co undetected. High resolution C 1s spectrum of S-Co/C-0.3 is also given as Figure 2h. Clear two peaks can be assigned to C-(C, H) (284.8 eV) and O=C-O (289.1 eV). First peak should belong to surface carbon, while the second peak pertains to residue carboxyl groups.47 On the basis of above research, we speculate that a possible decomposition process of Co3[HCOO]6·DMF can be divide into following steps: (1) First, the evaporation of guest molecules, DMF. (2) Second, the intermediate products should contain CoO and organics. (3) Third, CoO would be totally reduced by organics and organics would all be converted to CO2.


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Figure 2. (a) TG curves of Co3[HCOO]6·DMF. (b) TG curves of S-MG-0.3. (c) Precise contents of Co derived from ICP. (d) XRD patterns of as-prepared Co and Co/C composites. (e) XPS full spectra of SCo and S-Co/C-0.3. (f) Co 2p high resolution spectrum of S-Co. (g) Co 2p high resolution spectrum of S-Co/C-0.3. (h) C 1s high resolution spectrum of S-Co/C-0.3. Morphology evolution has also been studied by SEM images shown in Figure 3. MOFs precursor mainly consists of parallelepipeds whose length is about several ten microns. It would benefit the construction of conductive network and the enhancement of attenuation of electromagnetic energy if large-size one-dimensional structure can be preserved during pyrolysis.47 Fortunately, S-Co basically shares similar appearance with precursors except for the size and the subunit. For one thing, violent ACS Paragon Plus Environment

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volume shrinkage should be the result of the loss of nonmetallic elements. Therefore, the length of S-Co is only about several micrometers which is only half of length of precursor. For another, precursor and S-Co are built up by different subunits. In this case, the parallelepipeds of precursor should contain large quantities of Co3[HCOO]6·DMF crystals which possesses highly ordered nanoporous structure. While micro-rods of S-Co are comprised of coadjacent Co nanoparticles with some macro-pores. We can also assume that during pyrolysis, a part of nonmetallic elements firstly leaves the parallelepipeds with soft organics-wrapped Co elements left. Then with totally removal of organics, residue hard Co elements are inclined to connect with each other to grow larger. Former process majorly leads to shrinkage and latter procedure results in sponge-like structure. We should point out that the temperature plays a key role in the latter procedure, which leads to the obvious change of porous structure of Co as shown in Figure S2. In detail, at lower temperature, porous structure can be maintained well. While at higher temperature, only rod-like structure could be preserved. The influence of porous structure on the electromagnetic properties of MOF-derived Co would be discussed later. Regardless of the morphology change of MOFs, the morphology evolution of carbon has also been elucidated. As shown in Figure 3c, almost no difference can be noticed between S-Co and S-Co/C-0.1, due to the rather low content of carbon. Limited amount of sp2 carbon precursors derived from glucose cannot afford the further growth of carbon on cobalt, thus leading to the island-like structure of carbon. As expected, the amount of “island” increases for S-Co/C-0.2 as exhibited in Figure S3. Interestingly, vertically aligned carbon nanoflakes appear on the surface of Co as shown in Figure 3d. Carbon nanoflakes first appears on the surface due to the catalytic effect of Co.48 Owing to the great difference of structure and expansion coefficients between Co and carbon, stress is created in nanoflakes which switches the growth direction from parallel to perpendicular to the Co substrate.49 However, the surface of Co would be covered tightly by carbon, if the carbon content is enough. This can be confirmed by the SEM images of S-Co/C-0.4 in Figure S3. It is worth mentioning that rod-like structure may be destroyed to some extent, which closely correlates with the higher mobility of Co-containing intermediates in micro-rods caused by more organics provided by glucose. As a consequence, rod-like structure of S-Co/C-0.5 with more carbon ACS Paragon Plus Environment

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source has been severely damaged as displayed in Figure 3e. Excessive amount of carbon leads to a dense coating layer with extra island on Co. Changing trend of surface carbon is summarized in Figure 3f, revealing the process from fragments (S-Co/C-0.1) to vertically aligned nanoflakes (S-Co/C-0.3) and finally to thick layer with extra fragments (S-Co/C-0.5).

Figure 3. (a) SEM image of Co3[HCOO]6·DMF parallelepipeds. (b) SEM image of S-Co. (c-e) SEM images of S-Co/C-0.1, S-Co/C-0.3 and S-Co/C-0.5. (f) Schematic illustration and SEM images of Co/C composites to determine the morphology evolution of carbon. Electromagnetic properties of S-Co are further characterized by complex permittivity (εr) and permeability (µr). As exhibited in Figure 4a, typical magnetic metallic behavior can be observed. In detail, ε′ value quickly drops from 54.32 at 2 GHz to 27.00 at 6 GHz and then declines slightly to 18.45 ACS Paragon Plus Environment

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at 18 GHz. It is noteworthy that ε″ manifests dielectric relaxation with a platform in the range of 2-6 GHz. Then, the ε″ value falls slowly from 24.48 at 6 GHz to 13.34 at 18 GHz. On the basis of conventional theory, ε′ and ε″ value represent storage and attenuation ability of incident electromagnetic energy, respectively.50 Therefore, S-Co possesses strong storage and attenuation ability which benefit from highly electrical conductivity of pure metal. Electromagnetic parameters of S-Co-400 and S-Co600 are supplied in Figure S4, from which the impact of porous structure can be learned. In our opinion, porous structure contributes to the construction of percolation networks and the enhancement of electrical conductivity. Moreover, porous structure is also conducive to the improvement of surface area, which offers more polarization sites. Briefly, εr should be remarkably boosted by the porous structure. Lowest εr values of S-Co-600 can be cited as evidence. Besides, the lower εr values of S-Co400 are decided by other inherent properties including crystallinity and surface state. The superiority of magnetic metal lies in the high µr value. As for µ′ value of S-Co, 1.38 can be reached at 2 GHz. Similarly, µ′ value continues to go down to 0.89 at 18 GHz. In the meantime, maximum and minimum values of µ″ are 0.40 at 2 GHz and 0.25 at 18 GHz, respectively. Microwave absorption performance has been evaluated according to following equations: = ( / ) / ℎ(2/)( ) / RL = 20log|( − )/( + )|

(1) (2)

Herein, Zin, d, Z0, f and c stand for input impedance, thickness of microwave absorbing material, impedance of air, frequency of incident electromagnetic wave and velocity of light, respectively.51 RL values of S-Co are summarized in Figure 4b. State-of-the-art MAMs should work well in whole frequency range (2-18 GHz) with coating thickness as thin as possible. Generally, RL value less than 10 dB is the minimum requirement for practical applications. Howbeit, in the testing range, no effective RL value can be gained and only -5 dB is attainable at high frequency range with thickness less than 2 mm. In this regard, the RL performance is restricted by impedance mismatching of S-Co. It is


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paramount to achieve the highest possible µr value which ensures broadband absorption and excellent impedance matching. Optimal µ′ values of some previous MOF-derived MA materials are summarized in Figure 4c. It is generally accepted that initial µi can be enlarged by promoting saturation magnetization (Ms) and depressing coercive force (Hc) which can be described as: =

#

%$&'() *+,-

(3)

where a and b are two constants related with composition, κ stands for a proportion coefficient, λ represents magnetostriction constant, ξ is an elastic strain parameter of the crystal.52 Porous Co/C composites derived from ZIF-67 own µ′ value of 1.3 (with loading ratio of 60 wt%) whose Ms and Hc values are 64.0 emu·g-1 and 159.6 Oe, respectively. The strong magnetic property is credited with 60 wt% of Co nanoparticles with 10 nm diameter.53 Fe/C nanocubes derived from prussian blue has higher µ′ value of 1.5 (with loading ratio of 40 wt%) with higher Ms (118.7 emu·g-1) and larger Hc value (383 Oe). 67.1 wt% of Fe ensures the higher Ms value while larger size of Fe nanoparticles may bring about unexpected Hc value.54 Stronger magnetic properties can be fulfilled by NiFe@C nanocubes/graphene with lower filling ratio of 30 wt%. Notwithstanding Ms value is 95.4 emu·g-1, lower Hc value of 135 Oe arms sample G800 with high µ′ value of 1.5.55 Our previous work about FeCo/C composites possesses higher µ′ value of 1.7 by the virtue of inherent strong magnetic property of FeCo alloy.56 To sum up, we may obey following empirical rules: (1) The type of magnetic components should be selected elaborately and metal or alloy is preferred. (2) Higher content of magnetic components should be pursued by adding magnetic elements or removing nonmagnetic elements. (3) The size of magnetic components should also be controlled to be much larger or smaller than critical particle diameter for the sake of low Hc value. In this research, metallic Co with promising magnetic property has been chosen and the content has been successfully increased to 99.8 wt% which can even be regarded as pure Co. Moreover, the size of Co is clearly larger than several hundred nanometers. As a result, µ′ value of S-Co with loading ratio of 50 wt% can be up to 1.38. ACS Paragon Plus Environment

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Figure 4. (a) Electromagnetic parameters of S-Co. (b) 3D RL representation of S-Co. (c) Comparison of optimal µi values of samples with different filling ratio according to previous reports on MOF-derived MAMs.39,44,53-64 Despite the pretty high µr value of S-Co, too high εr value induced by strong electrical conductivity of metal may hinder its further application. Researchers would always combine high resistance material with high conductive magnetic metal or alloy to achieve a balance between attenuation and impedance matching.65 But µr value would always decrease in this route. It is one of the major hurdles that simultaneously decreasing the too high εr value and retaining high µr value. Figure 5a proves the effectiveness of our technique on protecting the strong magnetic property. We can find that µ′ value of S-Co/C-0.1 ranges from 1.33 at 2 GHz to 0.86 at 18 GHz. S-Co/C-0.3 exhibits almost the same trend with µ′ value varying from 1.31 at 2 GHz to 0.84 at 18 GHz. Furthermore, µ″ values of above two samples are all less than 0.4 and larger than 0.17. Unfortunately, µ′ value of S-Co/C-0.5 is trending ACS Paragon Plus Environment

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down slowly from 1.16 at 2 GHz to 1.02 at 18 GHz while µ″ value fluctuates between 0.08 and 0.17. It can be inferred that about 40 wt% of carbon greatly weaken the magnetic performance of S-Co/C-0.5. Magnetic loss tangents (tan δµ) are calculated to illustrate the change of magnetic loss ability in Figure 5b. Predictably, S-Co possesses highest tan δµ value, which is larger than 0.3 in nearly whole frequencies. Strikingly, tan δµ value of S-Co/C-0.1 and S-Co/C-0.3 maintains well around 0.25, revealing the superiority of this route. Too high content of carbon in S-Co/C-0.5 leads to a low tan δµ value of about 0.1. As mentioned above, static magnetic parameters should be referred to. Hysteresis loops of all samples are shown in Figure5c. Ms values of S-Co, S-Co/C-0.1, S-Co/C-0.3 and S-Co/C-0.5 are 93.5, 82.3, 118.8 and 43.6 emu·g-1, respectively. Furthermore, Hc values turn from 132.9, 124, 138.2 to 477 Oe with increasing carbon content. Several conclusions can be drawn relying on varied magnetic parameters. (1) The lower Ms value of S-Co than bulk Co should primarily be ascribed to the unavoidable oxidation, resulting from enlarged surface area. (2) When carbon content is low, the protection effect plays a more important role that can be confirmed by higher Ms value of S-Co/C-0.3 than S-Co/C-0.5. (3) When carbon content exceeds a specific value, the dominate role is changed to composite effect which leads to descendant Ms value. (4) The size shrinkage of S-Co/C-0.5 is blamed for the obviously increased Hc value. (5) The sequence of obtained initial permeability matches well with the tested magnetic parameters. Generally, natural resonance and eddy current loss should be main attenuation mechanisms for magnetic MAMs in testing frequency range. Eddy current loss can be described as: .. = 2 (′) 0 /3 2 = ′′(′)3 3

(4) (5)

Here, σ represents the electrical conductivity and µ0 is the permeability in vacuum.6 If eddy current loss is the sole attenuation mechanism, the C0 value should be a constant. As illustrated in Figure 5d, the C0 values of all samples are changeable in whole frequency range. So multiple magnetic loss ACS Paragon Plus Environment

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mechanisms including natural resonance and eddy current loss contribute to the attenuation of incident microwave.

Figure 5. (a) Complex permeability of Co/C composites. (b) Magnetic loss tangents of all samples. (c) Hysteresis loops of all samples. (d) C0 values of as-made samples. The role of carbon on the variation of εr is depicted in Figure 6a. ε′ values of S-Co/C-0.3 fluctuates between 10.3 at 2 GHz and 10.13 at 18 GHz. As for S-Co/C-0.5, ε′ values decrease from 7.01 at 2 GHz to 5.39 at 6 GHz. We should point out ε′ values of S-Co/C-0.1 is close to 8.2 with a relaxation peak centered in about 12.75 GHz. Meanwhile, ε″ values of S-Co/C-0.1 are near 0.25 with a peak value of 1.4 at 12.8 GHz. As for S-Co/C-0.3 and S-Co/C-0.5, ε″ values are about 0.7 and 1.1. It has been well documented that ε″ value is mainly decided by electrical conductivity.66 Obviously, Co/C composites show much lower ε″ value than elemental Co. Moreover, we may conclude that the sequence of electrical conductivity from high to low is S-Co/C-0.5, S-Co/C-0.3 and S-Co/C-0.1. It may contradict our expectation, because addition of amorphous carbon would decrease the electrical conductivity of ACS Paragon Plus Environment

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Co/C composites. The reason should be the construction of conductive network in sample/wax mixtures. The decrease of size may be beneficial for the even dispersion of sample in wax, thereby facilitating the hopping and migrating of electrons. Previous reports confirm that ε′ value is mainly determined by polarization process and can be seen as a criterion for dielectric energy storage ability.67 It can be deduced that carbon has negative impact on dielectric energy storage ability of Co. S-Co/C-0.3 has highest ε′ value which can be explained by the unique architecture. Vertically aligned carbon nanoflakes not only ensure large exposed surface of carbon, but also reduce the cover of Co. Interface between carbon and Co can be preserved, too. This fact would all be conducive to the strengthened polarization process. Dielectric loss tangents (tan δε) of all samples are compared in Figure 6b. Unsurprisingly, tan δε values of Co are all higher than 0.6 while Co/C composites only lower than 0.2. A trend can be seen that with the increase of carbon content, tan δε values also rises, indicating the enhanced dielectric loss capability. In brief, we successfully reduce the εr value with maintained µr value which would undoubtedly ameliorate the impedance matching condition. |Zin/Z0| values have been employed to directly describe the impedance matching. On the basis of our previous work, perfect impedance matching can be achieved when the |Zin/Z0| value is 1 and effective impedance matching can be got when the |Zin/Z0| value is constrained in the range of approximate 0.5-1.9.64 As can be seen in Figure 6c, all the values of S-Co are lower than 0.4 which would only bring about unwanted reflection. With the help from carbon, effective impedance matching can be realized in a wide range shown in Figure 6d. Thanks to the improved impedance matching, RL performance of Co/C composites becomes much better, particularly when the thickness is thinner than 2 mm.


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Figure 6. (a) Complex permittivity of Co/C composites. (b) Dielectric loss tangents of all samples. (c,d) |Zin/Z0| values of S-Co and S-Co/C-0.3 in testing frequency range. Figure 7a shows the RL values of S-Co/C-0.1 in which we can see that effective absorption can be realized in the frequency range of about 12-18 GHz. When the thickness is only 1.85 mm, fe of 2.6 GHz can be gained. A broadened area can also be observed near 13 GHz that is the typical behavior of relaxation absorption. In general, dual peaks would occur for MAMs with dielectric resonance behavior. One peak should be the synergetic results of interference cancellation and microwave attenuation, which would be affected by thickness and frequency.16 The other peak should be the direct consequence of dielectric realaxation, which occurs at fixed frequency.65 It should be noted that these two peaks could be merged into one peak at relaxation frequency at suitable thickness (about 2.15 mm as shown in Figure 7a). In addition, the relaxation absorption peaks should be generated by strong interfacial polarization in S-Co/C-0.1 from our point of view. The island-like dispersion of carbon not only ensures large interface area, but also keeps the huge gap of electrical conductivity between cobalt and carbon.


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Due to stronger attenuation, S-Co/C-0.3 exhibits even better performance. Effective absorption area has shifted to both thinner thickness and lower frequency. When the thickness is only 1.6 mm, a broad fe of 4.12 GHz can be reached with RL peak value of -19.86 dB. In our opinion, the content of carbon is restricted by minimum required attenuation ability and the carbon content of S-Co/C-0.5 goes beyond the limits, thereby weakening the attenuation ability. Attenuation mechanisms are also described in Figure 7d. Considering magnetic loss tangent is larger than dielectric loss tangent, magnetic loss mechanisms including natural resonance and eddy current loss occurred in Co may contribute more to the efficient absorption. As for dielectric loss mechanisms, conduction loss and polarization loss should be focused on. According to our previous work, induced current in carbon nanoflakes with higher resistance would consume more energy than that in metallic Co.64 Vertical aligned carbon nanoflakes may provide longer electron traveling paths, which would lead to stronger conduction loss. Polarization loss can be categorized into dipole and interfacical polarization.68 As mentioned above, defect or surface groups in both Co and carbon would be polarization centers and larger surface area may make S-Co/C0.3 possess stronger dipole polarization loss. Moreover, abundant interface and different electrical conductivity between carbon nanoflakes and Co would also induce interfacial polarization, which promotes the conversion from electromagnetic energy to heat.


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Figure 7. (a-c) 3D RL representation of as-synthesized Co/C composites. (d) Schematic illustration of possible attenuation process (conduction loss (CL), interfacial polarization loss (IPL), dipole polarization loss (DPL), natural resonance (NR), eddy current loss (ECL)). Moreover, RL peak values of all samples in low frequency range (2-8 GHz) with thickness from 1-5 mm are surveyed in Figure 8a. Peak values of S-Co keep constant at -5 dB that is far from application requirements. Co/C composites behave much better and S-Co/C-0.3 is the most promising candidate among all samples with RL values less than -10 from 4 to 8 GHz. Furthermore, fe (RL < -5 dB) in 4-8 GHz (C band) of Co/C composites is analyzed in Figure 8b. It is delightful to point out that S-Co/C-0.1 possesses fe of 3.68 GHz at 4.0 mm which almost covers all C band. S-Co/C-0.3 performs even better which owns fe of 3.84 GHz at 3.8 mm and it may be an excellent MAMs in C band. S-Co/C-0.5 still has poorer performance with a fe of 3.44 GHz at 4.9 mm. As illustrated above, RL performance is rooted on attenuation and impedance matching condition. Figure 8c shows the dielectric and magnetic loss tangents in 2 GHz. S-Co possesses highest tan δε and tan δµ values, confirming the advantage of ACS Paragon Plus Environment

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magnetic metal served as effective low-frequency MAMs. With the addition of carbon, S-Co/C-0.1 and S-Co/C-0.3 show almost the same tan δµ values, indicating the well preservation of magnetic property. However, S-Co/C-0.5 owns much weaker magnetic loss capability, due to the coexistence of too much carbon. With the increase of carbon content, dielectric loss ability would be enhanced and optimal combination can be made by S-Co/C-0.3 with strong magnetic loss and proper dielectric loss. This may provide a guideline for designing electromagnetic parameters of low-frequency MAMs. Attenuation constants α in 2-8 GHz can be cited in Figure 8d to confirm the modulation of multiple mechanisms.56 α=

5 67 8

9( .. .. − . . ) + 5( .. .. − ′ ′) + ( . .. + ′′ ′)

(6)

Obviously, S-Co possesses highest α values in the given frequency range which is highly associated with conductive nature of metal. The reason why S-Co exhibits poor low frequency RL performance should be impedance mismatching. With the decoration of carbon species, impedance matching condition can be optimized. In the meantime, attenuation mechanisms have been modulated in order to maintain strong enough attenuation ability in low frequency. As for Co/C composites, α values keep increasing with frequency, suggesting the enhancement in attenuation capability. As a result, RL performance in C band is better than that in S band. By tuning the content and morphology of carbon species, attenuation ability can be strengthened, which can be proved by larger α values of S-Co/C-0.3. This also guarantees the best RL performance of S-Co/C-0.3 with thinnest thickness and widest effective frequency bandwidth. Figure S5 further provides α value in the frequency range of 8-18 GHz from which similar conclusions can be drawn. Preserving magnetic loss and enhancing dielectric loss may be an effective way to realize low frequency absorption currently. Future work should be done to further enhance magnetic loss with matched dielectric loss.


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Figure 8. (a,b) RL peaks values and effective frequency bandwidth (RL < -5 dB) in the frequency range of 2-8 GHz. (c) Dielectric and magnetic loss tangents at 2 GHz. (d) Attenuation constant α of all samples in the frequency range of 2-8 GHz. Moreover, as-prepared Co and Co/C composites may be served as promising candidates for electromagnetic shielding materials. Figure S6 depicts microwave absorbing model and electromagnetic shielding model. We should point out that absorption ability is vital for both shielding and absorbing performance. Therefore, our findings would benefit the design of electromagnetic shielding material through improving absorption part of shielding effectiveness. To confirm our deduction, total shielding effectiveness (SET), shielding effectiveness by reflection (SER) and by absorption (SEA) are summarized in Figure S7 according to measured S parameters using following equations.69 SER = 10 log (1/(1 − >S ? ))

(7)

SEA = 10 log ((1 − >S ? )/>S ? ) SET = 10 log (1/>S ? )

(8)

(9)


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As for S-Co, SET value gradually increases from 6.12 dB to 18.12 dB. And it is obvious that absorption loss dominates the attenuation process except at frequency lower than 4.08 GHz. After the construction of interface structure between cobalt and carbon, the shielding effectiveness has been greatly changed. When island-like structure of carbon is built, SET value has been obviously improved to 17.15 dB at 2 GHz and 43.29 dB at 18 GHz which should be ascribed to the enhance absorption loss in whole frequency. When carbon exists in the form of vertical nanoflakes, SET value is similar with that of S-Co. We also find that absorption is stronger than reflection at frequency above 3.4 GHz, indicating the promotion of attenuation ability at low frequency. S-Co/C-0.5, which owns thick layer of carbon with extra fragments, shows worse shielding performance than S-Co which is due to the weak attenuation ability of carbon. In the meantime, the change of porous structure of Co/C composites would obviously affect the SER. BET surface areas of S-Co/C-0.1, S-Co/C-0.3 and S-Co/C-0.5 are 1.9, 28.1 and 90.1 m2‧g-1 (Figure S8). It can be inferred that the porous structure is determined by carbon species in this case. With the increase of carbon contents and the evolution of microstructures, the surface areas would keep increasing. This also can be proved by the pore size distribution as shown in Figure S8. Pore size concentrates at about 5 nm, which is similar with reported porous carbon frameworks derived from MOFs.38,44,56,63 As for SER values, S-Co/C-0.1 owns poor performance, resulting from smallest surface area and least pores. With larger surface area, SER values of S-Co/C-0.3 are larger than SEA at frequency below 3.4 GHz, which should be ascribed to more inter-connected carbon flakes. Similarly, reflection also contributes to the attenuation of electromagnetic wave for S-Co/C-0.5. In brief, we may safely conclude that shielding performance can be optimized by interface structure design between carbon and cobalt.70 Besides, conduction loss and dipole polarization loss, multiple reflection and magnetic loss should be main absorption mechanisms as illustrated above. The vital role of enhancing attenuation ability by interface structure design should be stressed for both electromagnetic shielding and microwave absorbing materials.

4. CONCLUSIONS


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Strong magnetic loss, enhanced dielectric loss and improved impedance matching are beneficial to the effective electromagnetic attenuation at low frequency. (1) One-dimensional sponge-like metallic Co has been successfully prepared from direct pyrolysis of Co3[HCOO]6·DMF parallelepipeds. Assynthesized Co possesses strong magnetic property and its mixtures with 50 wt% wax own a real permeability of 1.38 at 2 GHz, which ensures strong magnetic especially at low frequency. (2) Co/C composites with distinct interface structures have been made to overcome the challenge of developing high-performance lightweight low-frequency MAMs. With the increase of carbon content, the morphology of carbon would change from fragments to vertically aligned nanoflakes and finally to thick layer with extra fragments. As a consequence, the dielectric loss is continuously strengthened and the magnetic loss maintains well followed by a remarkable decline. (3) A perfect balance between dielectric and magnetic loss has been achieved by S-Co/C-0.3. As a result, minimum RL value of -19.86 dB can be reached with a broad fe of 4.12 GHz at only 1.6 mm. In the frequency range of 2-8 GHz, minimum RL values of S-Co/C-0.3 approach -20 dB with thickness from 1 to 5 mm. Moreover, RL values less than -5 dB can be gained in almost whole C band. This work may give inspiration and reference to the development of MOF-derivatives and shed light on the fabrication of excellent low-frequency microwave absorbing materials and good electromagnetic shielding materials. ASSOCIATED CONTENT Supporting Information Digital photographs of Co3[HCOO]6·DMF and Co3[HCOO]6·DMF/glucose mixtures. SEM images of S-Co-400 and S-Co-600. SEM images of S-Co/C-0.2 and S-Co/C-0.4. Electromagnetic parameters of SCo-400 and S-Co-600. Attenuation constants of all samples in the frequency range of 8-18 GHz. Schematic illustration of electromagnetic shielding and microwave absorbing models. Shielding effectiveness of as-prepared Co and Co/C composites. N2 sorption isotherms and pore size distribution of as-prepared Co/C composites. This material is available free of charge via the Internet at http://pubs.acs.org. ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors approved the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are thankful for financial support from the National Nature Science Foundation of China (Nos. 11575085, 51602154), the Aeronautics Science Foundation of China (2017ZF52066), Qing Lan Project, Six talent peaks project in Jiangsu Province (No. XCL-035), Jiangsu 333 talent project, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. REFERENCES (1) Lv, H.; Yang, Z.; Wang, P. L.; Ji, G.; Song, J.; Zheng, L.; Zeng, H.; Xu, Z. J. A Voltage-Boosting Strategy Enabling a Low-Frequency, Flexible Electromagnetic Wave Absorption Device. Adv. Mater. 2018, 1706343. (2) Lv, H.; Guo, Y.; Yang, Z.; Guo, T.; Wu, H.; Liu, G.; Wang, L.; Wu, R. Doping Strategy To Boost the Electromagnetic Wave Attenuation Ability of Hollow Carbon Spheres at Elevated Temperatures. ACS Sustainable Chem. Eng. 2018, 6, 1539-1544. (3) Feng, A. L.; Jia, Z. R.; Zhao, Y.; Lv, H. L. Development of Fe/Fe3O4@C Composite with Excellent Electromagnetic Absorption Performance. J. Alloys. Compd. 2018, 745, 547-554.


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Enhanced Low-Frequency Electromagnetic Properties of MOF

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