A Versatile Route toward the Electromagnetic Functionalization of

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A versatile route towards the electromagnetic functionalization of MOF-derived 3D nanoporous carbon composites Wei Liu, Lei Liu, Zhihong Yang, Junjie Xu, Yanglong Hou, and Guangbin Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00320 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Materials & Interfaces

A versatile route towards the electromagnetic functionalization of MOF-derived 3D nanoporous carbon composites Wei Liu,† Lei Liu,† Zhihong Yang,† Junjie Xu‡, Yanglong Hou*,‡ and Guangbin Ji*,†

† College

of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, P. R. China.

‡ Beijing

Key Laboratory for Magnetoelectric Materials and Devices (BKLMMD), BIG-EAST,

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China.

*Corresponding Author: Prof. Dr. Guangbin Ji E-mail: [email protected] Prof. Dr. Yanglong Hou E-mail: [email protected]

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ABSTRACT Designable electromagnetic parameters accompanied by low density of metal-organic frameworkderived (MOF-derived) metal/carbon composites are essential prerequisites for excellent microwave absorbing materials. However, conventional route is confined to slight modification on physicochemical properties of metal species and carbon which also restricts the functionalization of MOF-derived materials. Here, a facile technique has been improved by making full use of highly porous structure to uniformly introduce metallic Co nanoparticles into carbon matrix derived from Cu3(btc)2. Through changing the starting amount of Co sources, the composition of the final products can be tuned, offering an effective route to control electromagnetic properties. Multiple attenuation mechanisms are employed to realize excellent reflection loss performance which can be clarified by modified equivalent circuit mode. Effective frequency bandwidth (fe) over whole X band can be obtained by optimizing interfacial polarization through changing interface area and electrical conductivity. Broad fe covering almost whole Ku band from 12.3 to 18 GHz with a thin thickness of 1.85 mm can be gained through improving impedance matching and enhancing conduction loss. The present work not only sheds light on the easy fabrication of high-performance lightweight microwave absorbing materials but also paves the way for extending functionalities of MOF-derived carbon composites. Keywords: metal-organic frameworks, insertion technique, Cu/Co/C composites, microwave absorption, functionalization


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1. INTRODUCTION Extensive efforts worldwide have been stimulated by the ever increasing detrimental effects of electromagnetic wave on biological systems and electronic equipment.1 Various techniques have been employed to overcome the pollution caused by electromagnetic radiations, in which the utilization of microwave absorbing materials has been regarded as one of the most promising ways.2, 3 The main superiority lies in that incident microwave can be absorbed through diverse attenuation mechanisms or interfered by reflected waves. Increasingly stringent performance requirements of novel microwave absorbing materials have been put forward, including having thin thickness, being lightweight and having strong absorption intensity in a broad frequency range.4 According to transmission line theory, reflection loss (RL) value can be calculated by the following equations.

[

Z in = µ r / ε r tanh − j ( 2πfd / c ) µ r / ε r RL = 20 log

Z in − Z 0 Z in + Z 0

]

(1) (2)

where Zin, µr, εr, d, f, c and Z0 represent input impedance, complex permeability, complex permittivity, coating thickness of the absorber, incident microwave frequency, velocity of light and impedance of free space.5-7 Therefore, the key task of designing high-performance microwave absorbing materials is tuning µr and εr in desired frequency range when the thickness is restricted by total weight. In conclusion, designable electromagnetic parameters accompanied by low density are essential prerequisites for excellent microwave absorbing materials. Fortunately, the rapid development of a novel kind of materials, metal-organic frameworks (MOFs) may provide solutions of acquiring high-performance microwave absorber. MOFs, also called as porous coordination polymers are composed of metal ions as nodes and organic ligands as linkers. Their intrinsic nature of ultrahigh surface area, ordered accessible cavities and tunability of composition and structure offers enormous possibilities in the synthesis of lightweight porous carbon materials with specific function including microwave absorption (MA).8,

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Among a wide variety of MOF-derived

carbon materials, magnetic metal/carbon composites may be one of the most promising candidate for ACS Paragon Plus Environment

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excellent microwave absorber, due to the intrinsic advantages gained from each components and extra advantages inherited from MOFs: (1) lightweight features can be achieved which should be attributed to the existence of carbon and highly porous nature of the frameworks; (2) optimal electromagnetic parameters can be obtained through changing composition and structure which ensures excellent RL performance; (3) deep insights into the attenuation process can be gained from investigation on MOF derivatives with delicately designed nanostructures. Howbeit, research can be further promoted in developing advanced technique and ascertaining detailed attenuation mechanisms. On one hand, recent route is limited to optimizing pyrolysis process of MOFs which would only bring about slight change in electromagnetic parameters. Many efforts have been devoted to improving graphitization degree of carbon matrix and modifying physicochemical properties of metal species.10-12 Developing an effective technique to obtain metal species with designed content and type in carbon matrix is quite important for tuning the µr and εr in a wide range. On the other hand, attenuation mechanisms should be verified on basis of experimental results. In the microwave frequency range, main attenuation mechanisms consist of conduction loss, interfacial polarization, natural resonance, eddy current loss and interference cancellation for MOF-derived porous magnetic metal/carbon composites.13 It should be noted that conduction loss exists in whole frequency range which plays a vital role in broadband microwave absorption. However, rare descriptions have been given for composites rather than single component (carbon).14 Thus, the role of both metal and carbon in whole conduction loss needs to be figured out. Under the premise of limited electrical conductivity, researcher’s attention has been focused on interfacial polarization to further improve dielectric loss ability.11, 12, 15 Nevertheless, rare work provides clear and effective ways to tune interfacial polarization relaxation loss and corresponding theories are insufficient, too.16 Magnetic loss including natural resonance and eddy current loss is always much weaker than dielectric loss for MOF-derived carbon materials. But, increase of µr is in favor of optimizing interference cancellation and impedance matching. Although interference cancellation works only in specific frequency range restrict by εr, µr and thickness, it should be stressed and employed to weaken incident waves in designed frequency.17 ACS Paragon Plus Environment

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Besides, excellent impedance matching which can be reached by equating εr and µr is important for effective attenuation of incident microwave. In a word, physiochemical properties of MOF-derived materials should be modified to fulfill task in varied fields. Although numerous reports of metal-organic-frameworks-derived (MOF-derived) materials can be found in various fields including electrocatalysis, lithium-ion batteries, sensors, gas storage and separation, the application of MOF-derived materials is still constrained due to finite types of elements in the precursors and products.18 As far as we are concerned, conventional technique mainly concentrates on encapsulating functional fillers into MOFs or enwrapping MOFs with desired components. Both routes suffer from limited types, irregular dispersion state of additional components and complicated synthesis procedures.19-23 Thus, universal and facile technique is required to introduce designated functional components uniformly within matrix inherited from MOFs. As a result, extending functionalities of porous carbon frameworks derived from MOFs through compositional design would be possible. To address the challenge of optimizing the electromagnetic properties and extending the functionalities of MOF-derived carbon materials, a facial technique has been improved to uniformly introduce tunable amount of magnetic Co nanoparticles into porous Cu/C composites derived from Cu3(btc)2 by make full use of its highly porous structure. Through changing initial content of Co species, µr and εr have been controlled in a wide range. By analyzing the microwave absorption performance of as-prepared samples, attenuation mechanisms are also discussed. Effective frequency bandwidth (fe) over whole X band can be obtained by optimizing interfacial polarization through modifying interface area and electrical conductivity. Broad fe covering almost whole Ku band from 12.3 to 18 GHz with a thin thickness of 1.85 mm can be gained through improving impedance matching and enhancing conduction loss. Besides, the technique may provide useful hints to introduce various metal species by changing experiment conditions and more pores by chemical etching into MOF-derived porous carbon composites. And it is essential to achieve the full potential of this technique in extending functionalities of MOF-derived materials. ACS Paragon Plus Environment

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2. EXPERIMENTAL SECTION Synthesis of Cu3(btc)2. All of the chemicals were obtained from Nanjing Chemical Reagent Co., Ltd. with analytically pure grade which can be used directly. Cu3(btc)2 was prepared through a facile stirring method according to previous report.24 In detail, a certain amount of Cu(NO3)2‧3H2O and 1,3,5benzenetricarboxylate (BTC) were dissolved in water (100 mL) and ethanol (100 mL), respectively, to get the solutions with concentration of 80 mmol‧l-1. Afterwards, BTC solution was poured into metal salt solution and the mixtures were stirred for 18 h under room temperature. Then, blue crystals were obtained by filtration and washed by ethanol for three times. After dried in vacuum at 60 oC for 12h, pure blue Cu3(btc)2 powder was gained. Synthesis of Cu3(btc)2/Co2+ Mixtures. A series of Co2+ solutions were prepared by dissolving Co(NO3)2·6H2O (1,3,5,7 g) in ethanol (5 mL). Then, Co2+ solutions (1 mL) were injected into dried Cu3(btc)2 (1 g) in an agate mortar. After thoroughly ground, dry Cu3(btc)2/Co2+ mixtures were obtained and labeled as SP1, SP2, SP3 and SP4, respectively, based on Co2+ initial concentration from low to high. More information can be obtained from Figure S1. Synthesis of Porous Cu/C and Cu/Co/C Composites. 1.5 g of Cu3(btc)2 was put into a porcelain boat which was then heated in a quartz tube under the protection of N2 flow. The sample was incubated at 700 oC for 2 h with a ramping rate 5 oC‧min-1. After naturally cooling down, black powders can be gathered after grinding and the obtained Cu/C composite was named as SC. Similarly, SP1, SP2, SP3 and SP4 were thermally treated in same condition and the products were named as SCC1, SCC2，SCC3 and SCC4, respectively. Characterizations. Thermalgravimetric (TG) analysis of Cu3(btc)2 and Cu3(btc)2/Co2+ mixtures were carried out by a PerkinElmer Diamond TG/DTA apparatus at a ramping rate of 10 oC·min-1. Powder XRD patterns were recorded with a Bruker D8 ADVANCE diffractometer by using Cu-Ka radiation. Weight contents of Cu and Co were gained from ICP-AES. Raman spectra analysis was conducted on a Renishaw in Via 2000 Raman microscopes. Scanning electron microscope images were taken by a ACS Paragon Plus Environment

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Hitachi S4800 and TEM images were taken by a JEOL JEM 2100 TEM at an accelerating voltage of 200 kV. The BET surface area are determined by a micromeritics ASAP 2020 system at 77 K. Hystersis loops of SCCs were obtained from a Lakeshore 7400 vibration magnetometer. Complex permittivity and permeability are determined by an Agilent PNA N5244A vector network analyzer. The toroidal ring samples were prepared by mixing paraffin (60 wt%) with as-prepared powders and then pressed into a mould with φout of 7.0 mm and φin of 3.04 mm.

3. RESULTS AND DISCUSION Scheme 1. Formation of porous Cu/C and Cu/Co/C composites through a MOF-derived adsorptioncalcination route.

Co2+ ethanol

Co2+ adsorption

Cu3(btc)2

Cu3(btc)2/Co2+

2

2

N

N porous carbon matrix

Cu nanoparticles

porous carbon matrix

Co nanoparticles

Cu/C

Cu/Co/C

Typical route of this adsorption-calcination technique is shown in Scheme 1. Synthetic method and application of Cu3(btc)2 have been well studied while improvements can still be made to extend its application area. Cu3(btc)2 is made up of dimer Cu paddle wheels linked by carboxyl groups in 1, 3, 5benzenetricarboxylates. After high temperature thermal treatment in N2, One-dimensional rod-like Cu3(btc)2 prepared via a facile aqueous stirring method would be converted to porous carbon frameworks embedded with Cu nanoparticles (Cu/C composites, labeled as SC).25 Interestingly, asACS Paragon Plus Environment

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prepared Cu/C composites possess unexpected dielectric loss properties especially in high frequency range which is rarely reported.26 However, it is still far from satisfying for practical application. As illustrated above, one of the most efficient methods to improve the RL performance of MOF-derived carbon materials is introducing magnetic metal nanoparticles. Inspired by previous reports, strong adsorption capabilities of Cu3(btc)2 are utilized to induce Co2+ into channels and pores of the MOFs. During the grinding process, Co2+ would enter into ordered pores within the flow of ethanol. After evaporation of ethanol, injected Co sources would be confined in the ordered pores which avoid unwanted aggregation. After carbonization of Cu3(btc)2/Co2+ mixture, Co nanoparticles are successfully dispersed into the Cu/C composites (Co/Cu/C composites, labeled as SCC1, SCC2, SCC3 and SCC4 with increased initial content of Co(NO3)2‧6H2O) and the content of Co can be easily controlled though changing the adsorbed amount of Co2+.

a

b

100

♣ ♣ Cu

Cu3(btc)2

Intensity (a.u.)

Weight (%)

o

356 C 60 Cu2+/Cu0: 0.3419 eV

40

♦ ♣ ♦ ♣♦

♦ Co 2+

♣

♣

N2

305 oC

SC ♣

2+

Cu3(btc)2/Co

80

♣ ♦

0

SCC1

♣

♣

SCC2

♣♦

♣♦

♣♦

♣♦

♣♦

♣

SCC3 SCC4

Co /Co : -0.28 eV 20 100

200

300

400

500

600

700

800

30

40

50

o

c

d 70

70.16 59.18

57.85

50 42.01

Cu Co

40 30

42.99

39.2

38.81

27.23

20 10

70

ID/I G:0.891

Intensity (a.u.)

60

60

80

2θ θ (°)

Temperature ( C)

Weight (%)

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

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SC

0.904

SCC1

0.897

SCC2

0.925

SCC3

0.842

SCC4

6.69

SC

SCC1 SCC2 SCC3 SCC4

800

1000 1200

1400

1600

1800

2000

2200

-1

Raman shift (cm ) ACS Paragon Plus Environment

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Figure 1. (a) TG curves of Cu3(btc)2 and Cu3(btc)2/Co2+ mixtures.(b) XRD patterns of as-prepared Cu/C and Cu/Co/C composites.(c) Weight content of Cu and Co decided by ICP. (d) Raman spectra of obtained Cu/C and Cu/Co/C materials. On account of the existence of Co sources in Cu3(btc)2, different pyrolysis process can be observed in Figure 1a. As for Cu3(btc)2, three weight loss steps centered in 130 oC, 240 oC and 356 oC can be obviously seen. It can be inferred that the MOFs structures are severely damaged at 370 oC and the products in this stage may be copper oxides and carbon with residue organic groups. With increasing temperature, copper oxides would be reduced by carbon which leads to weight loss from 370 oC.25 Distinct weight loss behavior of Cu3(btc)2/Co2+ mixture should be noticed, too. More weight decrease centered in about 130 oC and 240 oC should be related with the evaporation of ethanol and dehydration of mixtures. It is necessary to point out there is a sharp decline in 305 oC, indicating structure collapse of Cu3(btc)2. The phenomenon also illustrates that Co2+ brings instability to Cu3(btc)2.27 Another interesting trend is that at about 430 oC, there is a unique decrease step which can be ascribed to the reduction of Co2+. According to previous research, Co2+ owns lower reduction potential (-0.28 eV) than Cu2+ (0.3419 eV), resulting in harsher reduction environment.28 To confirm the composition of the final products gained at 700 oC, powder XRD patterns are displayed in Figure 1b. Composites derived from Cu3(btc)2 show only three strong diffraction peaks in 43.3o, 50.5o and 74.2o, corresponding to (111), (200) and (220) planes of cubic Cu (PDF#65-9026), respectively.25 While materials gained from Cu3(btc)2/Co2+ mixtures all have diffraction peaks at 44.2o, 51.5o and 75.8o which can be indexed to (111), (200) and (220) planes of cubic Co (PDF#15-0806).29 No other peaks can be found, thus we may speculate that SC is composed of Cu and carbon, while the other samples are consist of Cu, Co and carbon. The results also suggest that magnetic Co has been successfully introduced into Cu/C composites through adsorption and calcination procedures. To further confirm the contents of each component in the final composites, ICP-AES is employed to identify the precise contents of metal elements which are summarized in Figure 1c. It can be anticipated that the content of Co would increase as the increase of starting Co sources. As expected, SCC3 owns 9 ACS Paragon Plus Environment


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higher Co content of 42.01 wt.% than SCC1 (6.69 wt.%) and SCC2 (27.23 wt.%). Nevertheless, SCC4 possesses less Co of 38.81 wt.% which may be related with the less soluble metal element caused by denser carbon coating. We also should point out both SCC3 and SCC4 own equal content of Cu and Co. The fact can be explained as the adsorbed capacity of Cu3(btc)2 may be reached. It is interesting that the Cu content goes up before falling down, resulting from the more consumption of carbon during the carbothermal reduction of more Co2+. To supply indirect evidence, the content of carbon can be roughly estimated as 42.15, 23.15, 13.59, 15 and 21.99 wt.% for SC, SCC1, SCC2, SCC3 and SCC4, respectively. To sum up, ICP results further prove that Co has been introduced into the composites and the content has also been effectively optimized through altering the Cu3(btc)2/Co2+ mixtures. Raman spectra are given to characterize the bonding state of carbon (Figure 1d). D-band in 1350 cm-1 and G-band in 1580 cm-1 can be clearly seen for all samples, which reveals the existence of carbon.27 Similar with previous work on MOFs-derived composites, high-temperature catalytic effect of magnetic metal is in favor of formation of sp2 hybridized carbon.30 However, nonmagnetic SC also displays noticeable G-band. We hypothesize that either Cu has catalytic graphitization effect or high temperature thermal treatment plays a key role. It has well documented that the intensity ratio of ID/IG can be cited to describe the graphitization degree.31 In this case, ID/IG values are 0.891, 0.904, 0.897, 0.925 and 0.842 for SC, SCC1, SCC2, SCC3 and SCC4, respectively, which means the graphitization degree are all close with each other. This also proves that optimizing thermal treatment conditions may be more important than increasing catalysts contents for increasing graphitization degree of MOFderived carbon frameworks. Meanwhile, similar graphitization degree may lead similar εr of carbon which is helpful to understand the relations between metal species and εr.


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a

c

b amorphous carbon Cu/C (SC)

Cu/Co/C (SCC3)

amorphous carbon

200 nm

20 nm

200 nm

d

SCC3

e

f

Co

Cu

C

O

0.205 nm (111) cubic Co 20 nm

5 nm

Figure 2. (a, b) TEM images of Cu nanoparticles embedded carbon matrix (SC). (c, d) TEM images of Cu and Co nanoparticles embedded carbon matrix (SCC3). (e) HRTEM image of SCC3 indicating the successful insertion of Co nanoparticles. (f) Elemental mapping revealing the uniform dispersion of Co, Cu, C and O elements of Cu/Co/C composites (SCC3). Unique MOF-derived micro/nano-structure is characterized by using SEM and TEM. SEM images of Cu3(btc)2, as-prepared Cu/C and Cu/Co/C composites are given (Figure S2). With increased Co contents, one-dimensional rod-like structures gradually collapsed and aggregated into irregular threedimensional frameworks. This also reveals the instability of precursors induced by Co sources. Figure 2 exhibits more structural information of Cu/C and Cu/Co/C composites. From Figure 2a, we can see numerous Cu nanoparticles with size range of 10~500 nm are well dispersed in the carbon matrix. Most of the nanoparticles are about 20 nm while some are grown up to outshoot with large size of 500 nm. We should point out that Cu/Co/C composites share similar nanostructure with Cu/C composites. For ACS Paragon Plus Environment

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one thing, lattice fringes of cubic Co can be observed in Figure 2e which confirms the effectiveness of the technique to successfully insert Co nanoparticles into carbon matrix. For another, Co nanoparticles are well isolated by carbon which confirms the advantage of exploiting intrinsic highly ordered pores within MOFs.32 Remarkable difference lies in the amount of nanoparticles for Cu/C and Cu/Co/C composites. It is easy to understand that with similar size, the amount should be decided by the content of metal. Therefore, it is no doubt denser nanoparticles are viewed in Cu/Co/C composites. More information can be gathered from TEM images of other samples (Figure S3, S4a-c). Besides, no graphene layers can be found around metal nanoparticles, indicating that carbon matrix is mainly amorphous. It is interesting that on the basis of Raman study, the ID/IG values are lower than some reported MOF-derived composites. Namely, graphitization degrees of as-synthesized carbon materials are higher. This may be ascribed to increased amount and active surface area of metal nanoparticles than reported MOF-derived composites which increase the graphitization of whole carbon matrix simultaneously by high-temperature catalysis. To confirm the larger interface area, namely dispersion state of nanoparticles, mapping results of SCC3 are supported in Figure 2f. Most importantly, homogeneous distribution of Cu and Co nanoparticles is definite. O element can also be detected in the whole composites which may originate in the oxides in the surface of metal nanoparticles.33 For instant, lattice fringes belong to CuO can be found in HRTEM image of SC (Figure S2d).


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a O2

SC: 200.6 m ⋅g

80

0

20

40

80

1 20

Pore size (nm)

2

SCC3: 151.2 m ⋅g 0.0

0.2

0.4

0.6

d

1.0

0.8

1.0

SCC1 SCC2 SCC3 SCC4

60 51.7 36.5 28.4 5.1

0.5

0 -20

S-CC1 S-CC2 S-CC3 S-CC4

-40 -60

M (emu⋅⋅g-1)

20

-1

Relative pressure (P/P0 )

c 40

-1

40

limited protection

inside metal species

2

SC SCC3

3.0 0

QA (cm3⋅g-1)

120

1.72 9 .22

27.8 2

pores in carbon

1 06.9 6

b

M (emu⋅⋅g-1 )

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


Mr

0.0

1.74 8.45 7.65 12.11 Hc

-0.5

352 234 234 206 -1.0

-10000

-5000

0

5000

10000

-400

H (Oe)

-200

0

200

400

H (Oe)

Figure 3. (a) Schematic illustration of nanostructures around metal species. (b) N2 adsorption/desorption isotherms of Cu/C (SC) and Cu/Co/C (SCC3) composites, inset exhibits pore size distribution of SC and SCC3. (c, d) Hysteresis loops and enlarged hysteresis loops of magnetic Cu/Co/C composites. From our point of view, the porous structures should be the main reason why oxidation of metal nanoparticles happens when exposed in air. As shown in Figure 3a, there may be some pores in the carbon matrix which lead to direct contact between metal and air, thus weak the protection against oxidation. N2 sorption isotherms of Cu/C and Cu/Co/C composites are provides in Figure 3b. BET specific surface area of SC is 200.6 m2‧g-1 which is larger than 151.2 m2‧g-1 of SCC3. It is not hard to understand that pores mainly exist in carbon and SCC3 owns less carbon due to additional Co nanoparticles. Speaking of pore size, pores with 1.72 and 9.22 nm diameter mainly build the porous structure of SC. It is common for MOF-derived metal/carbon composites. After insertion, the pores of ACS Paragon Plus Environment

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SCC3 are mainly composed of meso-pores (3.00 and 27.82 nm) and macro-pore (106.96 nm). We can clearly see that pores tend to be larger and new pores may be formed due to the extra Co nanoparticles. To draw a conclusion, Cu3(btc)2 derived Cu/C frameworks are composed of even-distributed Cu nanoparticles and porous carbon matrix in which slight oxidation of Cu may happen. As for Cu/Co/C frameworks, Co nanoparticles are uniformly inserted into Cu/C composites which would cause decreased content as well as pores of carbon matrix. Changes in composition and structure also affect the magnetic properties which can be verified from hysteresis loops (Figure 3c, d). Clear rising trend of Ms values can be viewed. In detail, Ms values of SCC1, SCC2, SCC3 and SCC4 are 5.1, 28.4, 36.5 and 51.7 emu‧g-1, respectively. Interestingly, Hc values exhibit shrinking trend. Namely, Hc values decrease from 352 Oe of SCC1 to 234 Oe of SCC2 and SCC3. Then, it keeps falling down to 206 Oe for SCC4. Besides, two aspects should be mentioned to illustrate the variation of magnetic properties of SCCs. Increased content of Co nanoparticles should be the main reason and oxidation state of metallic Co cannot be ignored, too. Diverse composition and structure induced by Co nanoparticles are benefit for the optimization of electromagnetic parameters as well as microwave absorption properties. εr can be effectively controlled (Figure S5a-d). In general, real part (ε′) and imaginary part (ε″) of εr increase with the increasing content of Co. In detail, ε′ values of SC, SCC1, SCC2, SCC3 and SCC4 decrease from 8.95 to 7.02, from 9.00 to 5.25, from 13.82 to 8.31, from 12.92 to 7.44 and from 33.19 to 14.84, respectively in the range of 218 GHz. Meanwhile, this trend maintains for the ε″ values. Specially, ε″ values of SC increase from 1.25 at 2 GHz to 2.33 at 18 GHz with obvious relaxation peak centered in 9.72 GHz and small fluctuations around 15 GHz. Similarly, ε″ values of SCC1 go down from 2.36 at 2 GHz to 2.27 at 18 GHz with obvious relaxation peak in same 9.72 GHz and fluctuations in about 15 GHz. With more content of Co, ε″ values of SCC2 and SCC3 fluctuate in a narrow range of 4.77 to 4.51 and 4.34 to 3.78, respectively. SCC4 displays highest ε″ values from 15.86 to 11.31 in the measuring frequency range, too. Based on free electron theory, ε″ is related with electrical conductivity (σAC) to some extent: ACS Paragon Plus Environment

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σ AC = ε 0ε ' '2πf

(3)

where ε0 is the dielectric constant of free space, and f is the microwave frequency.34 In this case, we may infer that σAC from high to low values are SCC4, SCC2, SCC3, SCC1 and SC. As we know, metal owns much higher σAC than amorphous carbon and in this case carbon of all samples may possess similar σAC caused by similar graphitization degree. Thus, we should focus on the effect of metal species on electrical conductivity. Metal contents of SCC4, SCC2, SCC3, SCC1 and SC are 78.01, 86.41, 85, 76.85 and 57.85 wt.%, respectively. We may deduce that larger ε″ value is caused by larger metal content. Abnormal behavior of SCC4 may be related to another critical factor: oxidation state of metal which decides the real content of metal. In our opinion, porous structures are severely damaged in SCC4 which hinder the direct contact to air. Microstructures also need to be cited to illustrate why SCC4 possesses much higher ε″ than SCC2 and why ε″ value of SC is close to SCC1. As far as we are concerned, metal nanoparticles act as nodes and highly porous carbon materials act as linkers which are in favor of constructing interconnected electron transmission network.

Hence, larger size carbon

frameworks encapsulating metal nanoparticles would show superiority in constructing conductive pathway than smaller size one which is insulated by more adhesion agent. As for SC and SCC4, onedimensional and three-dimensional structures with large size further improve the σAC. We may conclude that remarkable improvement of ε″ induced by Co may correspond to mainly intrinsic high electrical conductivity of metal and unique cross-linked structure which offers pathways for electron transport. µr has also been successfully enhanced (Figure S5a-d). SC is nonmagnetic material, indicating that no magnetic loss abilities can be anticipated. After 6.69 wt.% Co has been added into the Cu/C composites, µ′ values decline from 1.08 at 2 GHz to 1.00 at 18 GHz with a resonance peak located in about 5.32 GHz. Likewise, three resonance peaks appear in 5.2 GHz, 11.36 GHz and 16.36 GHz with peak values of 0.11, 0.08 and 0.04, respectively. The increase of µr demonstrates that magnetic loss has been activated caused by magnetic Co nanoparticles. With larger content of Co, the µ′ values quickly drops from 1.10 to 1.00 at 7.2 GHz, from 1.13 to 1.00 at 8.6 GHz and from 1.22 to 0.97 at 9.2 GHz for SCC2,


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SCC3 and SCC4, respectively. Furthermore, broadened resonance peaks in 4 GHz can be distinguished in µ″ values of SCC2, SCC3 and SCC4. It can be noted that the sequence of µ″ value follows the content of Co from most to least except for SCC4. This may be ascribed weakest oxidation state of Co nanoparticles in SCC4. Dielectric loss tangents (tan δε) and magnetic loss tangents (tan δµ) are calculated to present the attenuation abilities of as-prepared materials (Figure S5e, f). In the range of 2-8 GHz, SCC4 owns both strongest dielectric and magnetic loss abilities while SCC3 and SCC2 show weaker but similar tan δε and tan δµ values. Just as obviously, SCC1 is even weaker and SC is the weakest based on the loss tangents. In the range of 8-12 GHz, tan δε values of SCC1, SCC2 and SCC3 are almost the same in which SCC1 offers a bit stronger dielectric loss abilities due to the relaxation happened in this frequency range.35 And tan δµ values of SCC1, SCC2 and SCC3 are quite close and small, too. As for the range of 12-18 GHz, the sequence of tan δε values from high to low is：SCC4, SCC2, SCC3, SCC1 and SC, which corresponds to the change of ε″ discussed above. However, only SCC4 provides strong magnetic loss abilities in this frequency range. To draw a conclusion, attenuation including dielectric and magnetic loss abilities has been successfully improved through the addition of Co nanoparticles. MA performance is further supplied to describe the vital role of varied composition and structure cause by magnetic Co nanoparticles (Figure S6). It is interesting that SC shows common performance rather than bad performance. RL values of -15.1 dB can be reached at 17.72 GHz with a thin thickness of 1.7 mm. Broad effective frequency bandwidth (fe) of 3.6 GHz can be realized at thickness of 1.9 mm. We should point out that a broaden area of fe can be found near 11 GHz and almost no fe would be gained below 6 GHz. As for SCC1, much better performance can be observed. RL value of -52.5 dB at 11.88 GHz and fe of 5.44 GHz have been fulfilled at thickness of 2.8 mm. It is clear that there is a broadened area of fe near 11 GHz which resembles RL behaviors of SC. Better performance especially in high frequency has been completed by SCC2. RL value of -21.23 dB at 13.72 GHz with a thickness of 1.9 mm, accompanied by fe of 5.2 GHz is better than most of previous materials. Owing to similar εr


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and µr, the MA performance of SCC3 is even better than SCC2 that RL value of -25.0 dB at 13.72 GHz and fe of 5.36 GHz with a thickness of 1.95 mm have been achieved. Due to the impedance mismatching cause by too high εr, the RL performance of SCC4 is far from satisfactory.

a

b

2

2

RL < -5 3.16

µ′

4

4 4.6 5 6 7

1.13 1.10

1.08 1.03

8

SC

1.22

2 GHz

3.16

frequency range

Frequency (GHz)

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SC

SCC1 SCC2 SCC3 SCC4

c

SCC1 SCC2 SCC3 SCC4

d SCC1 SCC2 SCC3 SCC4

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Thickness (mm)

0.05

µ ′′(µ′)−2f−1

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


0.03 0.02 0.01 0.00

10

4

6

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dm

SCC3

dexp

8 6 same d

4 2

2

dm dexp

SC

same f 2

4

6

8

10

12

14

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18

Frequency (GHz)

Frequency (GHz)

Figure 4. (a, b) Frequency range of RL values less than -5 dB and µ′ values at 2 GHz for all samples. (c) Calculated C0 values of magnetic Cu/Co/C composites. (d) Frequency dependence of matching thickness (dexp) and calculated thickness (dm) for Cu/C (SC) and Cu/Co/C (Cu/Co/C) composites. Detailed discussions would be developed in three regions: 2-8 GHz, 8-12 GHz (X band) and 12-18 GHz (Ku band) based on unique main loss mechanisms. Figure 4a shows the frequency range of RL value less than -5 dB, considering that -10 dB can hardly be well covered in 2-8 GHz, due to the relatively low magnetic loss abilities of all samples. It is widely accepted that most magnetic loss ACS Paragon Plus Environment

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mechanisms happened in low frequency which make magnetic materials good low frequency microwave absorbing materials. Thanks to the introduction of magnetic Co nanoparticles, -5 dB can be achieved from 4 to 8 GHz for SCC1. The lower limit extends to 3.16 GHz for SCC2 and SCC3 with stronger magnetic loss abilities. It can be further extended to 2 GHz for SCC4. We may build relations between these behaviors and the µ′ values at 2 GHz. From Figure 4b, we can learn that µ′ values at 2 GHz rise from 1.03, 1.08, 1.10 and 1.13 to 1.22 for SC, SCC1, SCC2, SCC3 and SCC4, respectively. Therefore, larger µ′ is benefit for better RL performance in this frequency range. In other words, stronger magnetic loss abilities lead to better low frequency RL performance. According to previous research, there are relations among complex permeability and magnetic parameters:

µi =

M s2 akH c M s + bλξ

(4)

where λ is magnetostriction constant, ξ stands for an elastic strain parameter of the crystal, a and b are two composition-related constants, κ represents a proportion coefficient.36 Hence, complex permeability can be enhanced by increasing saturation magnetization (Ms) and decreasing coercivity (Hc). On the basis of hysteresis loops, we can easily found that the improved µ′ originates in enhanced Ms and decreased Hc. To further investigate the magnetic loss mechanisms, C0 constants are calculated by following equation:

C0 = µ ' ' ( µ ' ) −2 f −1

(5)

The value of C0 would keep the same in whole frequency range if magnetic loss only arises from eddy current effect. As shown in Figure 4c, C0 values of Cu/Co/C composites vary with frequency, thus natural resonance also contributes to magnetic loss.37-41 Although magnetic loss is essential for low frequency attenuation, interference cancellation should be main mechanism in this case. According to widely accepted quarter-wavelength matching model, peak frequency (fm) is decided by absorber thickness (dm) and electromagnetic parameters (εr and µr):

d m = nc /( 4 f m ( ε r µ r )1/ 2 ) ACS Paragon Plus Environment

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where n is constant (n=1, 3, 5, … ).17 In Figure 4d, solid dots represent peak frequency and corresponding thickness based on experimental results, while open dots show simulation results. It can be inferred from the good agreement between simulation and experimental results (dexp) that interference cancellation plays a key role in the attenuation of incident microwave especially for SC in low frequency with rather weak dielectric loss and no magnetic loss abilities. Interference cancellation can be also controlled by optimizing εr and µr. Larger εr and µr induced by Co not only reduce fm at same thickness, but also decrease dm at same frequency. In brief, introduction of Co leads to both enhanced magnetic loss ability and optimized interference cancellation which improve the MA performance.

a

b

5 optimal fe at varied thickness 4 3.84

8-12 GHz 4 RL < -10

enhanced by polarization 0.5

tan δ ε (a.u.)

2 1

2.6 mm

3.1 mm

2.32

2.45 mm

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2.7 mm

fe (GHz)

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SCC2

SCC3

strong conduction loss

0.4

SC SCC1 SCC2 SCC3

0.3

average value: 0.27 0.48 0.44 0.46

0

SC

SCC1

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9

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Frequency (GHz)

c

d fe

3.48 GHz 5.64 GHz 5.28 GHz 5.68 GHz

-10

-20

SC SCC1 SCC2 SCC3

1.85 mm 2.3 mm 1.8 mm 1.85 mm

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12

0.6

0.5

tan δε (a.u.))

0

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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


SC SCC1 average value: SCC2 0.28 0.42 0.52 0.48 SCC3

0.4

0.3

0.2 8

14

16

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12

13

Frequency (GHz)

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Figure 5. (a, b) Optimal effective frequency bandwidth (RL ≤ -10 dB) and corresponding dielectric loss tangents of Cu/C (SC) and Cu/Co/C (SCC1, SCC2 and SCC3) composites in X band. (c, d) ACS Paragon Plus Environment

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Microwave reflection loss curves and corresponding dielectric loss tangents of Cu/C (SC) and Cu/Co/C (SCC1, SCC2 and SCC3) composites in Ku band. Figure 5a summarizes the fe in the range of 8-12 GHz (X band). 2.32 GHz of fe can be reached with a thickness of 2.7 mm for SC which is worse than any other samples. The whole X band can be covered by SCC1 while SCC2 and SCC3 are able to cover more than 85%. Dielectric, magnetic loss and interference cancellation should be main attenuation mechanisms in this range. Difference in microwave absorption behaviors should originate in varied dielectric and magnetic loss abilities. It is obvious that magnetic loss of Cu/Co/C composites is helpful to the attenuation of incident microwave. However, according to the loss tangents, dielectric loss contributes more than magnetic loss. Stronger dielectric loss abilities of Cu/Co/C than Cu/C composites give rise to broader fe in this frequency range. Generally, conduction loss and polarization are main dielectric loss mechanisms in microwave frequency range. Conduction loss of Cu/C composites should be weaker than Cu/Co/C composites due to lower electrical conductivity. In view of the fact that SCC1 owns smallest ε″, it is abnormal that SCC1 possesses largest average tan δε value (Figure 5b). Thus, polarization may be the true reason why SCC1 possesses broadest fe. The effect of polarization process can be seen in the quick drop of ε′ and resonance peak of ε″ from 8 to12 GHz for SCC1. It can be deduced that interfacial polarization among Cu, Co and carbon plays a key role in the polarization process.26,42 The fact proves that interfacial polarization is able to effectively strengthen dielectric loss abilities when the increase of electrical conductivity is limited. Figure 5c concludes the fe in the frequency range of 12-18 GHz (Ku band). In this frequency band, conduction loss, polarization, interference cancellation and magnetic loss are all required to be considered. Similar with above discussions, dielectric loss becomes more and more important. Besides, no or weak resonance peaks of ε″ can be found. Thus, conduction loss should be paid more attention to than polarization process (Figure 5d). Weaker conduction loss of SC leads to bad performance: maximum fe of 3.48 GHz is got at thickness of 1.85 mm. On the contrary, stronger conduction loss of Co/Cu/C composites ensures excellent absorption performance. However, we should notice that


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attenuation abilities of all samples are all strong enough to provide good microwave absorption performance in this frequency range. Therefore, impedance matching has been cited to investigate the origin of varied microwave absorption performance. |Zin/Z0| values of samples in the optimized conditions are given (Figure S7). When the value is 1, perfect impedance matching is realized and when the value is away from 1, finite impedance matching is reached. In our opinion, |Zin/Z0| values should approximately be less than 1.96 and large than 0.52 to ensure effective absorption (RL≤-10) which may be named as effective impedance matching. By comparing Figure S6 with Figure S7, we can find that effective absorption region lies strictly in the range given by effective impedance matching for all samples. This may be a guide line for optimizing impedance matching.


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a mould

EAA

EI

EAM

air

sample/wax ring

EA

similar composition

absorber

metal plate

b

electron

migrating

hopping

Co

Cu

dipole

heat external E

Rh >> Rm

e-

d

600 SC SCC1 SCC2 SCC3 SCC4

500 400 300

magnetic loss

200

FeSn2@C Ref. 50

3.5

SCC1 (Cu/Co/C)

Thickness (mm)

c

α (a.u.))

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conduction loss interfacial polarization

100

Co/C-500 Ref. 10

2.5

SCC3 (Cu/Co/C)

2.0

2

4

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16

18

S2 (Fe/C) Ref. 11

Co/CNTs Ref. 49 1.5

S600 (Ni/C) Ref. 15

CNTs/Co-900 Ref. 12 Ni@C/Si-R Ref. 48 Fe-Co/NPC-2.0 Ref. 47

1.0

0

S15 (Fe/C) Ref. 46

3.0

6

Frequency (GHz)

8

10

12

14

16

18

Frequency (GHz)

Figure 6. (a) Schematic illustration of related attenuation mechanisms in simulated situation (left side shows the toroidal ring samples and mould, right side gives the description of interference cancellation model (b) Possible attenuation mechanisms and equivalent circuit mode in external electromagnetic field (E). (c) Attenuation constants α of all samples. (d) Summary of previous reports about MOFderived carbon-based microwave absorbing materials. ACS Paragon Plus Environment

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Detailed attenuation mechanisms are depicted in Figure 6a. Testing ring-like samples are prepared by thoroughly mixing as-synthesized powders with paraffin through a customized mould. And it shares similar composition with conventional MA coatings which are composed of MA powders and binding agent which also lead to analogous attenuation process. From the view point of energy, incident wave possesses EI, reflected wave from air-absorber interface possess EAA, reflected wave from absorbermetal interface possesses EAM and energy consumed in absorber is labeled as EA. A part of electromagnetic wave energy of EAA and EAM would be dispersed due to the interference cancellation in specific frequency which contributes to the microwave stealth.43 As for EA, Cu/C and Cu/Co/C composites show distinct attenuation process. According to Cao’s work, an equivalent circuit mode can be cited in which amorphous carbon frameworks can be regarded as resistance for hopping electrons (Rh) and metal nanoparticles should be modeled as resistance for migrating electrons (Rm).14 Electrons would be stimulated and moved in external electromagnetic field which eventually leads to induced micro-current. And the energy would be consumed in the form of Joule heat on basis of Ohm’s law.44 In this case, two factors should be stressed: increased content of free electrons and enlarged Rm induced by more metal nanoparticles. It should be noted that Rh is much larger than Rm due to the amorphous and porous nature of carbon. Thus, more free electrons within metal nanoparticles may be the true reason why conduction loss is enhanced for Cu/Co/C composites. Another key dielectric loss mechanism should be interfacial polarization loss. Although much attention has been paid to interfacial polarization loss, effective control and related theory are absent. Thus, we may speculate that interface area and gap of electrical conductivity are most important factors to tune interfacial polarization. Charge unbalance between metal nanoparticles and amorphous carbon matrix ensures strong interfacial polarization loss which can be seen for Cu/C composites.45 Thanks to the increased interface area induced by Co, more dipoles would be formed which further enhance the interfacial polarization loss. This can be proved by the unique relaxation behavior of Cu/C and Cu/Co/C composites. Besides, magnetic loss of Co also contributes to the consumption of incident energy. To comprehensively illustrate the role of Co in improving attenuation ability, attenuation constant α is cited.41 As we can see in Figure 6b, the values of ACS Paragon Plus Environment

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α of all samples basically increase with initial Co content. Enhanced multiple mechanisms working in distinct frequency range induced by Co nanoparticles should be the main reason for stronger attenuation ability. This also proves that introducing additional metal nanoparticles is an effective method to further control the electromagnetic properties of MOF-derived carbon materials. Previous reports of MOFderived metal/carbon are summarized in Figure 6c.10-12, 15 ,46-50 In brief, thin thickness and low frequency microwave absorption performance should be emphasized. For instance, fe of 5.68 GHz can be reached by SCC3 with thickness of 1.85 mm which is better than most reports. 8-12 GHz can be covered by SCC1 with thickness of 3.1 mm which shed light on extending lower limit of frequency.

4. CONCLUSIONS A facial and effective technique has been developed to introduce desired amount of magnetic Co nanoparticles into porous Cu/C composites derived from Cu3(btc)2. Conduction loss has been enhanced in whole frequency range by more free electrons caused by metal nanoparticles and conductive network offered by amorphous carbon. Interfacial polarization has also been strengthened in specific frequency range by increased interface area and huge gap between metal nanoparticles and carbon matrix. Interference cancelation and magnetic loss have been stressed in low frequency range due to the improved magnetic properties. Besides, Co nanoparticles also optimize the impedance matching of Cu/C composites. Control of multiple attenuation mechanisms leads to excellent reflection loss performance in different frequency range. Owing to strongest magnetic properties, SCC4 behaves best in low frequency range and -5 dB can be realized from 2 to 8 GHz. Extended fe in 8-12 GHz of SCC1 should be ascribed strong interfacial polarization loss. Thanks to strong attenuation and good impedance matching, fe of 5.68 GHz can be obtained at a thin thickness of 1.85 mm for SCC3. Besides, the technique may provide useful hints to introduce metal, metal oxides or other metal compounds by changing experiment conditions or generate more pores by chemical etching MOF-derived porous carbon composites. It is essential to achieve the full potential of this adsorption-calcination route in extending functionalities of MOF-derived materials. ASSOCIATED CONTENT ACS Paragon Plus Environment

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Supporting Information SEM and TEM images of Cu3(btc)2, Cu/C and Cu/Co/C composites. Digital graphs of Cu3(btc)2/ethanol solution and precursors for thermal treatment. Complex permittivity and permeability, reflection loss values and impedance matching behaviors are provided, too. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *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 thank the financial supports from the National Nature Science Foundation of China (No.: 11575085, 51631001, 51590882, 51602154), the National Key R&D Program of China (2017YFA0206301), 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.

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Table of Contents

A versatile route towards the electromagnetic functionalization of MOF-derived 3D nanoporous carbon composites ACS Paragon Plus Environment

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Wei Liu,† Lei Liu,† Zhihong Yang,† Junjie Xu‡, Yanglong Hou*,‡ and Guangbin Ji*,†

† College

of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, P. R. China.

‡ Beijing

Key Laboratory for Magnetoelectric Materials and Devices (BKLMMD), BIG-EAST,

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China.

dipoles

heat Cu

eporous carbon matrix

Cu Co

Insertion technique

magnetic loss Co


Extended functionality

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A Versatile Route toward the Electromagnetic Functionalization of

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