MWCNTs Composite Derived from

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Article

Magnetically Aligned Co-C/MWCNTs Composite Derived from MWCNTs Interconnected Zeolitic Imidazolate Frameworks for Lightweight and Highly Efficient Electromagnetic Wave Absorber Yichao Yin, Xiaofang Liu, Xiaojun Wei, Ya Li, Xiaoyu Nie, Ronghai Yu, and Jianglan Shui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10067 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 54

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

ACS Applied Materials & Interfaces

Magnetically Aligned Co-C/MWCNTs Composite Derived from MWCNTs Interconnected Zeolitic Imidazolate Frameworks for Lightweight and Highly Efficient Electromagnetic Wave Absorber Yichao Yin,† Xiaofang Liu,*,† Xiaojun Wei,‡ Ya Li,† Xiaoyu Nie,

†

†

Ronghai Yu,*,† Jianglan Shui†

School of Materials Science and Engineering, Beihang University, Beijing 100191, China ‡

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

KEYWORDS: Electromagnetic wave absorption, zeolitic imidazolate framework, magnetic orientation, lightweight, high performance.

ABSTRACT: Developing lightweight and highly efficient electromagnetic wave (EMW) absorbing materials is crucial but challenging for anti-electromagnetic irradiation and interference. Herein, we used MWCNTs as templates for growth of Co-based zeolitic imidazolate frameworks (ZIFs), and obtained Co-C/MWCNTs composite by post pyrolysis. The MWCNTs interconnected the ZIF-derived Co-C porous particles, constructing a conductive network for electron hopping and migration. Moreover, the Co-C/MWCNTs composite was aligned in paraffin matrix under an external magnetic field, which led to the stretch of MWCNTs along the magnetic field direction. Due to the anisotropic permittivity of MWCNTs, the magnetic alignment considerably increased the dielectric loss of CoC/MWCNTs composite. Benefiting from the conductive network, the orientation-enhanced dielectric loss, and the synergistic effect between magnetic and dielectric components, the

ACS Paragon Plus Environment

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

Page 2 of 54

magnetically aligned Co-C/MWCNTs composite exhibited extremely strong EMW absorption with a minimum reflection loss (RL) of −48.9 dB at a filler loading as low as 15 wt%. The specific RL value (RL/filler loading) of the composite was superior to the previous MOFderived composite absorbers. It is expected that the proposed strategy can be extended to the fabrication of other lightweight and high-performance EMW absorbing materials.

1. INTRODUCTION Growing concerns about electromagnetic (EM) interference and pollution issues have driven an imperative requirement to develop high-performance EM wave (EMW) absorbing material which has the ability of dissipating EMW by converting EM energy into thermal energy.1-6 The new-generation EMW absorber should satisfy the demands of strong absorption, light weight, broad bandwidth, and small thickness.7-9 Among various innovative options, the construction of composite absorber consisting of both dielectric and magnetic components is stimulating considerable interest, because the EMW absorbing properties can theoretically benefit from the complementarities between permittivity and permeability of materials. So far, intensive efforts have been devoted to exploiting dual-loss composite absorbers such as Ni/C10, Fe3O4@C11, graphene/CoFe2O412,

Co/TiO213,

(Fe,

Ni)/C14,

FeSiAl/MWCNTs15,

and

graphene/Fe3O4/SiO2/NiO16 to promote EMW absorbing performance. However, it remains a great challenge for most composite absorbers to achieve the excellent EMW absorbing performance with a low filler loading in matrix. The large filler loading definitely results in the high specific gravity of the resultant absorbing coating, which greatly restrict their practical applications.


2

Page 3 of 54

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


To fulfill the lightweight requirement, rational design and fabrication of composite absorbers with porous/hollow structure and large specific surface area have become a hot topic. Alternatively, various carbonaceous materials such as carbon sphere17, carbon nanotubes18-20, carbon fibers21-22, graphene23-24, have been widely used as lightweight dielectric component in dual-loss composite absorbers. Recently, it is noticed that metal-organic framework (MOF)derived carbon materials can integrate all these desirable properties, and have emerged as a promising candidate for ideal EMW absorber. For example, Lü and co-workers reported a porous Co/C composite derived from pyrolyzed Co-based zeolitic imidazolate framework (ZIF–67, a subclass of MOF), which displayed good EMW absorbing properties with a minimum reflection loss (RL) of –35.3 dB at a Co/C loading of 60 wt%.25 Qiang et al. achieved a strong EMW absorption in Fe/C nanocubes fabricated by pyrolyzing Prussian blue. When the filler loading of Fe/C nanocubes was 40 wt%, an optimized RL value reached –20.3 dB.26 Ji’s group synthesized yolk-shell structured Co-C/void/carbonyl iron absorber by annealing ZIF–67/carbonyl iron core-shell particles. With a filler loading of 40 wt%, such absorber achieved a minimum RL value of −49.2 dB.27 The strong EMW absorption of the previous MOF-derived absorbers attracts more and more attention. Unfortunately, most MOFderived absorbers still failed to achieve excellent EMW absorbing performance at a filler loading below 30 wt%, partially due to the poor graphitization degree of carbon species even through high-temperature calcination, which was unfavorable to electron hopping and migration. To solve this problem, herein, we proposed to use multiwalled carbon nanotubes (MWCNTs) as template for growth of ZIF–67 particles. After a high-temperature pyrolysis, the CNT-templated ZIF–67/MWCNTs was converted into Co-C/MWCNTs, in which the ZIF–67


3


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

Page 4 of 54

derived Co-C particles were strung into a network by MWCNTs, thus building an electronconducting pathway (as illustrated in Scheme 1a). Moreover, it is well known that the EMW absorbing property of a material is strongly dependent on the EM parameters i.e. complex permittivity and complex permeability. As to CNT, the one-dimensional tubular structure leads to the higher electrical conductivity along the tube axis than that along the radical direction.28 The large discrepancy of electrical conductivity undoubtedly results in a large anisotropy of high-frequency complex permittivity. Therefore, it is possible to modulate the EWM absorbing property of the Co-C/MWCNTs composite by adjusting the carbon tube orientation. Meanwhile, the presence of magnetic Co nanoparticles (NPs) in composite could offer a unique opportunity to align the MWCNTs by an external magnetic field. In this work, the Co-C/MWCNTs composite was aligned in paraffin matrix under an external magnetic field which considerably enhanced the dielectric loss. Taking advantage of the constructed conductive network by MWCNTs, the orientationenhanced dielectric loss, and the synergistic effect between magnetic Co NPs and dielectric carbon species, the magnetically aligned Co-C/MWCNTs achieved extremely strong EMW absorption with an optimum RL value of –48.9 dB at a quite low filler loading of 15 wt%. To the best of our knowledge, this is the first report on improving the EMW absorbing properties of MOF-derived absorber by magnetic field modulation. The aligned Co-C/MWCNTs possessed the largest specific reflection loss (SRL=RL/filler loading) among the previous reported MOF-derived absorbers, which implies its potential application as a lightweight and highly efficient EMW absorbing material.


4

Page 5 of 54

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


Scheme 1. (a) Synthesis of Co-C/MWCNTs, (b) alignment of Co-C/MWCNTs in paraffin matrix under an external magnetic field, (c) measurements of EM parameters for unoriented and oriented samples.

2. EXPERIMENTAL SECTION Cobalt nitrate hexahydrate (Co(NO3)2 • 6H2O, 99%), 2–methylimidazole (98%), methanol (99%), and ethanol (99%) were purchased from J&K Scientific Ltd.. The above reagents were of analytical grade and used without further purification. MWCNTs were purchased from Shenzhen Nanotech PortCo. Ltd.. 2.1. Pretreatment of MWCNTs. Before using MWCNTs as templates to fabricate ZIF– 67/MWCNTs composite, the purchased MWCNTs were pretreated to functionalize surface with active groups. Typically, 5.0 g of MWCNTs and 150 mL of concentrated nitric acid were added into a round-bottomed flask and ultrasonicated for 1 h. Afterwards, the mixture was heated to 120 ºC and kept for 24 h with continuous stirring in an oil bath. After filtering the


5


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

Page 6 of 54

suspension, the pretreated-MWCNT precipitates were washed with deionized water several times to reach neutral pH, and dried in vacuum. 2.2. Synthesis of ZIF–67/MWCNTs. Briefly, 48 mmol of 2–methylimidazole was dissolved in a mixed solution containing 40 mL of methanol and 40 mL of ethanol. Subsequently, 50 mg of pretreated MWCNTs and 12 mmol of Co(NO3)2•6H2O were added in another 80 mL of mixed methanol-ethanol solution and ultrasonicated to produce a homogenous suspension. The above two solutions were quickly mixed under vigorously stirring, and aged for 24 h at room temperature. Finally, the precipitates were collected by centrifugation, washed with ethanol, and dried overnight. For comparison, the pure ZIF–67 particles were obtained as mentioned above without the addition of MWCNTs. 2.3. Synthesis of Co-C/MWCNTs composites. The Co-C/MWCNTs composite was obtained by pyrolysis of ZIF–67/MWCNTs in Ar atmosphere at 700 °C for 4 h. The heating rate was maintained at 2 °C/min. Additionally, pure ZIF–67 were annealed under the same condition to obtain Co-C composite. 2.4. Characterization. The crystal structure of the samples was analyzed using X-ray diffractometer (XRD, Rigaku D/MAX-2500) with Cu Kα irradiation (λ=1.54178 Å, 40.0 kV, 150.0 mA). Scanning electron microscopy (SEM, JEOL-JSM7500) and transmission electron microscopy (TEM, JEOL-JEM2100) were used to observe the morphology and microstructure of the samples. Raman spectra were measured by a Cryogenic matrix isolated Raman spectroscopic system (HORIBA Jobin Yvon, LabRAM ARAMIS) using a 532 nm laser. Magnetization curves of the samples were tested on a Quantum Design physical property measurement system (PPMS-9) at room temperature. The chemical composition and elemental


6

Page 7 of 54

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


valence state of the samples were analyzed by an X-ray photoelectron spectrometer (XPS, VG ESCALab 220i-XL). 2.5. Property Measurement. The relative complex permittivities and relative complex permeabilities of Co-C and Co-C/MWCNTs composites were measured using an AV3672C network analyzer in the frequency range of 2–18 GHz. For testing, Co-C and Co-C/MWCNTs composites were uniformly dispersed in paraffin matrix with a filler loading of 20 wt% and 25 wt% (denoted as Co-C 20 wt%, Co-C 25 wt%, Co-C/MWCNTs 20 wt%, and Co-C/MWCNTs 25 wt%). Finally, the Co-C/paraffin and Co-C/MWCNTs/paraffin mixtures were pressed into coaxial rings with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm. 2.6. Magnetic Alignment. The Co-C/MWCNTs composite with a filler loading of 15 wt% was uniformly dispersed in molten paraffin. As illustrated in Scheme 1b, an external magnetic field was applied on the mixture until the paraffin solidified. Afterwards, the magnetically aligned Co-C/MWCNTs/paraffin mixture was pressed into coaxial rings with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm. The relative complex permittivity and relative complex permeability of the oriented Co-C/MWCNTs/paraffin mixture were measured using the network analyzer with the electric field vector (E) of the incident EMW parallel to MWCNT alignment, as illustrated in Scheme 1c. 3. RESULTS AND DISCUSSION Crystal structure. The crystal structure of Co-C and Co-C/MWCNTs composites were characterized by XRD. As shown in Figure 1a, the XRD pattern of Co-C/MWCNTs resemble that of Co-C. The three sharp diffraction peaks at 44.2 °, 51.6 ° and 76.0 ° could be well indexed to the (111), (200) and (220) planes of metallic Co with cubic structure (JCPDS No. 89–4307), respectively. And the broad hump peak at 20–30 ° originated from the amorphous


7


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

Page 8 of 54

carbon. Besides, a weak but sharp diffraction peak at ~26.1°, superimposed on the broad amorphous peak, appeared in the XRD pattern of Co-C/MWCNTs composite. This peak belongs to the (002) plane of graphitic carbon (JCPDS No. 26–1076) from MWCNTs. Based on Scherrer equation, we calculated the average crystal size of Co NPs in Co-C and CoC/MWCNTs composites according to the full width at half maximum of Co (111) peak. It is found that the Co NPs in the two composites have similar crystal size of ~11 nm. Detailed structural characterizations of the carbon component were further carried out using Raman spectroscopy. As shown in Figure 1b, the Raman spectra present two typical peaks at ~1345 cm–1 (D band) and 1585 cm–1 (G band), respectively.29-30 The D band is associated with disordered carbon or defective graphitic structure, while the G band is a characteristic feature of graphitic layers.31-32 Generally, the graphitization degree of carbon materials can be evaluated based on the intensity ratio of D band to G band (ID/IG).29 The calculated ID/IG value for Co-C and Co-C/MWCNTs composites is 1.11 and 1.03, respectively. The lower ID/IG value for Co-C/MWCNTs is due to the addition of MWCNTs with higher graphitization degree.

Figure 1. (a) XRD patterns and (b) Raman spectra of Co-C and Co-C/MWCNTs composites. Morphology and microstructure. The morphology and microstructure of freestanding ZIF–67 and ZIF–67/MWCNTs composites were investigated by SEM and TEM. It is seen in


8

Page 9 of 54

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


Figure 2a and b that the freestanding ZIF–67 exhibited rhombic dodecahedron with smooth surface. After the addition of MWCNTs in the synthesis, the –COOH functional groups on MWCNT surface could induce the site-specific nucleation of ZIF–67 crystals.33-36 As observed in Figure 2f and h, the MWCNTs penetrated through the ZIF–67 particles and strung the particles together constructing a 3D network. Although the MWCNT-strung ZIF–67 particles retained the dodecahedral morphology, their particle size obviously decreased compared with that of pure ZIF–67 particles. This phenomenon was also observed in previous report by Lin et al.35 As shown in Figure 2b and h, the typical edge length of pure ZIF–67 particle was ~1.5 µm, while the edge length of the strung ZIF–67 particles decreased to ~300 nm. After pyrolysis process, both the freestanding and strung Co-C particles exhibited a good retention of the original shape, whereas the dodecahedron volume shrank and the surface became rough and concave because of the decomposition of 2–methylimidazole (Figure 2 c, d, i-l). The resultant Co-C particles displayed a porous structure with nanoscale Co NPs uniformly embedded within carbon matrix (Figure 2d and l). It is clear in Figure 2j–k that the formed Co-C particles did not fall off from the MWCNTs, indicating the high stability of the 3D framework connected by MWCNTs. The EDS result reveals that the Co content in the Co-C/MWCNTs composite is ~37 wt% (Figure S1 in supporting information).


9


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

Page 10 of 54

Figure 2. (a) SEM and (b) TEM images of ZIF–67; (c) SEM and (d) TEM images of ZIF–67 derived Co-C composite; (e, f) SEM images and (g, h) TEM images of ZIF–67/MWCNTs; (i, j) SEM images and (k, l) TEM images of Co-C/MWCNTs composite. Composition and elemental valence state. The surface composition and element valence state of the Co-C/MWCNTs composites were investigated by XPS. The survey scan in Figure 3a confirms the presence of C, N, O, and Co elements in the composites. As shown in Figure 3c, the deconvolution of C 1s spectrum reveals four carbon-containing functional groups that correspond to the C–C/C=C (284.6 eV), C–N (285.4 eV), C–O (286.3 eV) and C–O=C (289.0 eV), respectively.37-39 In Figure 3d, the high-resolution Co 2p XPS spectrum was fitted into three Co species, i.e. Co0 with binding energies at 778.0 and 793.0 eV, Co3+ ions with binding energies at 779.8 and 795.1 eV, and Co2+ ions with binding energies at 782.6 and 798.1 eV.34 The multiple valence states of Co suggest the surface oxidation of Co NPs in air atmosphere.38,


10

Page 11 of 54

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


40

As seen in the survey scan spectrum and Figure 3b, the Co-C/MWCNTs composite

contained a large amount of N elements derived from the decomposition of 2– methylimidazole. Similar to the previous reports, these N elements could be readily doped in the porous carbon matrix and MWCNTs at high temperature.34, 41 The N 1s XPS spectrum contained primary pyridinic–N (398.4 eV) and pyrrolic–N (399.7 eV) species plus a small ratio of graphitic–N (400.7 eV) and oxidized–N (402.8 eV) species.34

Figure 3. XPS spectra of Co-C/MWCNTs composite: (a) survey spectrum, deconvolution of (b) N 1s spectrum, (c) C 1s spectrum, and (d) Co 2p spectrum. Static

magnetic

and

electromagnetic

properties. The field dependence of

magnetizations for Co-C and Co-C/MWCNTs composites were measured at room temperature, as shown in Figure S2. The two samples exhibited a typical ferromagnetic behavior with obvious hysteresis loops in M−H curves. The magnetization was saturated with an applied magnetic field of 10 kOe. It is clear that the saturated magnetization (Ms) and the coercivity (Hc) of Co-C composite are 42.87 emu/g and 397.4 Oe, respectively. Whereas the addition of nonmagnetic MWCNTs caused the decrease of Ms and Hc to 34.96 emu/g and 365.1 Oe, demonstrating the weakening of ferromagnetism.


11


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

Page 12 of 54

Figure 4. Frequency dependence of complex permittivity for (a) Co-C and (b) Co-C/MWCNTs composites. Frequency dependence of complex permeability for (c) Co-C and (d) CoC/MWCNTs composites. The EMW absorbing property of a material is generally evaluated by reflection loss which can be calculated using relative complex permittivity (εr=ε′–jε″) and relative complex permeability (µr=µ′–jµ″) according to the transmission line theory.42 Therefore, we measured the EM parameters of Co-C and Co-C/MWCNTs composites with filler loadings of 20 wt% and 25 wt% in paraffin matrix. As observed in Figure 4a and b, the ε′ and ε″ values for Co-C and Co-C/MWCNTs composites increased as the filler loading raised, which is consistent with the effective medium theory. More importantly, it is seen that the Co-C/MWCNTs composites possessed much larger ε′ and ε″ values than those of Co-C composites with the same filler loadings. The enhancement of ε′ and ε″ implies the increase of polarization and dielectric-loss capabilities, respectively, which is ascribed to the addition of MWCNTs. It is noteworthy that


12

Page 13 of 54

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


the ε″ curves of Co-C and Co-C/MWCNTs composites exhibited several resonant peaks in the entire frequency range, indicating the multiple relaxation processes. In gigahertz frequency, the dielectric relaxations should originate from dipole polarization and interfacial polarization.43 Based

on

(ε ′ − ε ∞ ) + (ε ′′) 2

2

Debye

theory,

the

ε′

and

ε″

follow

the

equation:44-45

= ( ε s − ε ∞ ) , where εs and ε∞ are the static permittivity and the relative 2

permittivity at the high-frequency limit, respectively. Therefore, each Cole-Cole semicircle in ε′–ε″ plot represents one Debye relaxation process.46 To clarify the effects of MWCNTs on polarization modes, the Cole-Cole semicircles (ε′ versus ε″) were analyzed and shown in Figure S3. Compared with Co-C composites, the plots of ε′ versus ε″ for Co-C/MWCNTs composites with the same filler loadings obviously shifted to higher values and the semicircle radius increased. These findings indicate that the addition of MWCNTs enhanced the contribution of Debye relaxation to dielectric loss.47 Figure S3b–e presents the enlarged ε′–ε″ plot of each sample. It is clear that the plots of Co-C/MWCNTs composites display more distorted Cole-Cole semicircles than those of Co-C composites, demonstrating the increase of Debye relaxation processes caused by MWCNT addition. As well known, space charges could accumulate at the heterointerfaces between different dielectric media in composite, and cause macroscopic dipole moments which produce Debye-like relaxation processes under an alternating EM field. Therefore, the Co-C/MWCNTs composite not only possessed the interface polarization relaxations at Co/C and C/paraffin interfaces, which are the same as those of Co-C composite; but also created additional interface polarization relaxations at C/MWCNTs and MWCNTs/paraffin interfaces. Besides, the prior work demonstrated that the MWCNTs usually contained numerous defects in carbon network and abundant functional groups on surface.34, 48 The defects including doped N elements and carbon vacancies in carbon


13


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

Page 14 of 54

network could be considered as dipoles and worked as polarization centers that generate dipole polarization relaxations under an alternating EM field. Similarly, the oxygen-containing functional groups such as the –COOH on MWCNT surface could act as dipoles due to the different electronegativity between carbon and oxygen atoms, and produced polarization relaxations. Hence, the addition of MWCNTs generated more Debye relaxation processes in Co-C/MWCNTs composite, which induced more Cole-Cole semicircles in ε′–ε″ plots. In addition to the polarization relaxation, the conductive current also makes large contribution to the overall dielectric loss according to the equation:21

ε '' =

εs − ε∞ σ ωτ + 2 2 1+ ω τ ωε 0

(1)

where σ is electrical conductivity, ω is angular frequency, ε0 is the permittivity of free space, and τ is relaxation time. Based on the equation, the dielectric loss from conductive current contribution is directly proportional to the electrical conductivity. For Co-C/MWCNTs composite, the unique network constructed by MWCNTs could promote the electrical conductivity and thus increased the current induced-dielectric loss.49-51 According to the above analysis, the creation of multiple dipole and interfacial polarizations as well as the enhancement of conductivity led to the increase of dielectric loss for Co-C/MWCNTs composite, as illustrated in Scheme 2c–e. Magnetic loss is another crucial factor determining the EMW absorption because of the large amount of magnetic Co NPs in Co-C and Co-C/MWCNTs composites. Figure 4c and d presents the dependences of complex permeabilities for Co-C and Co-C/MWCNTs composites on frequency. As shown in Figure 4c, the µ′ curves of Co-C composites varied in the range of 0.90–1.30, and the µ″ values were 0.01–0.29 in 2–18 GHz. In comparison, the complex permeabilities (µ′ and µ″) of Co-C/MWCNTs composites slightly decreased in low-frequency


14

Page 15 of 54

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


region due to the addition of nonmagnetic MWCNTs. Whereas the µ′ and µ″ of both Co-C and Co-C/MWCNTs composites in high-frequency region decreased approximately to 1.0 and 0.0, respectively. It is found that obvious resonant peaks appeared in the µ″ curves of Co-C and CoC/MWCNTs composites. The low-frequency resonance corresponds to natural ferromagnetic resonance, while the other resonances mainly originate from non-uniform exchange resonance modes at high frequency.52 The obvious exchange-resonance peaks in µ″ curves could be assigned to the enhanced exchange resonance because the crystal size of Co NPs in composites is close to the exchange length of ~10 nm.53-54 The appearance of natural resonant peak in 2–4 GHz suggests the breakthrough of Snoek’s limits.55 According to the equation fr = γHa/2π (γ is gyromagnetic ratio), the resonant frequency fr could be well tuned by changing the anisotropy field Ha that is sensitive to the size and morphology of magnetic materials. The small size effect of Co NPs remarkably increased the anisotropy constant K, and consequently resulted in the increase of Ha (K=2πMsHa).56 According to the previous study, the K value of Co NPs with size of 15–50 nm is in the range of 5×106–30×106 erg/cm3, much higher than that of bulk Co (2.7×106 erg/cm3).57 Therefore, the fr of these binary and ternary composites appeared in gigahertz frequency while the fr of bulk cobalt is just located in megahertz frequency. Additionally, the EM energy might be attenuated in the form of eddy current. To evaluate the contribution of eddy current to magnetic loss, we analyzed the dependence of µ"(µ′)–2f–1 on frequency according to the equation µ" =2πµ0(µ′)2σt2f/3 (µ0 is the permeability in vacuum, and t is the thickness of absorber).58 If µ"(µ′)–2f–1 values keep constant with the change of frequency, eddy current loss will be the sole reason for magnetic loss.59 As shown in Figure S4, however, the µ"(µ′)–2f–1 values for all the samples intensively varied with increasing frequency. Thus the


15


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

Page 16 of 54

eddy current effect could be excluded, and the magnetic loss primarily originated from natural ferromagnetic resonance and exchange resonance. Figure 5 provides the calculated dielectric loss tangent (tanδε=ε″/ε′) and magnetic loss tangent (tanδµ=µ″/µ′) of Co-C and Co-C/MWCNTs composites. It is obvious that the tanδε values increased with increasing frequency whereas the tanδµ values showed an opposite variation trend. Because of the strong natural ferromagnetic resonance in 2–4 GHz, the tanδµ values of the composites were larger than the tanδε values, demonstrating the dominant effect of magnetic loss on EMW attenuation in low-frequency region. In the following 4–18 GHz, the dielectric loss played more important role in dissipating EMW due to the relatively larger tanδε values. Compared with Co-C composites, the Co-C/MWCNTs composites displayed comparable tanδµ values and enhanced tanδε values. It is further confirmed that the construction of conductive network was favorable to promoting the dielectric loss which could potentially improve the EMW absorbing properties.

Figure 5. Frequency dependences of (a) tanδε and (b) tanδµ for Co-C and Co-C/MWCNTs composites in 2−18 GHz.


16

Page 17 of 54

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


Electromagnetic wave absorption. The reflection losses of Co-C and Co-C/MWCNTs composites were calculated using the measured complex permittivity and complex permeability based on the equations:60-63

Zin = Z ο ( µ r ε r ) tan h  j ( 2π ƒt )( µ rε r )  12

RL = 20log

12

c 

Zin − Zο Zin + Z ο

(2)

(3)

where Z0 is the characteristic impedance of free space, Zin is the input impedance of the absorber and c is the velocity of light. Figure 6 illustrates the reflection-loss properties of Co-C and Co-C/MWCNTs composites with layer thicknesses of 1.0–4.0 mm in the frequency range of 2−18 GHz. As shown in Figure 6a and b, the Co-C composites presented weak EMW absorption at low filler loadings. Such poor performance is as expected because similar ZIFderived Co-C composite exhibited strong EMW absorption with filler loading as high as 60 wt%.25 Figure 6c and d demonstrates that the addition of MWCNTs significantly enhanced the EMW absorbing properties of the composites. Interestingly, the Co-C/MWCNTs composites with filler loadings of only 20 wt% and 25 wt% achieved 99 % attenuation of incident EMW (i.e. RL< –20 dB) at thin layer thickness. Especially, the Co-C/MWCNTs composite with filler loading of 25 wt% displayed extremely strong EMW absorption in X band with RL value up to −47.5 dB. The above comparison confirms the advantage of the unique conductive framework established by MWCNTs in improving EMW absorbing performance.


17


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

Page 18 of 54

Figure 6. Frequency dependence of reflection loss for Co-C and Co-C/MWCNTs composites: (a) Co-C 20 wt%, (b) Co-C 25 wt%, (c) Co-C/MWCNTs 20 wt% and (d) Co-C/MWCNTs 25 wt%. Magnetic orientation. Although the Co-C/MWCNTs composite with filler loading of 25 wt% possessed strong EMW absorbing properties, it is desirable to further decrease the filler loading of the composite absorber to fulfill the lightweight requirement. Herein, we applied a magnetic modulation strategy to promote the EMW absorption of Co-C/MWCNTs composite with a much lower filler loading. As illustrated in Scheme 1b, the Co-C/MWCNTs with a filler loading of only 15 wt% was aligned in paraffin matrix under an external magnetic field. To demonstrate the magnetic response of Co-C/MWCNTs, firstly, we dispersed them in ethanol and dropped on a glass slide. When the magnetic field was applied during the evaporation of the solvent, the Co-C/MWCNTs were quickly aligned along the plane of the glass slide, as observed in Figure 7a. The fast magnetic response ensured the orientation of Co-C/MWCNTs in paraffin matrix. Afterwards, we further investigated their alignment in paraffin matrix by


18

Page 19 of 54

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


observing the cross section of the Co-C/MWCNTs/paraffin sample using SEM. Figure 7b and c provides the cross-sectional SEM images of the oriented and unoriented CoC/MWCNTs/paraffin samples, respectively. In contrast to the random distribution of the CoC/MWCNTs without magnetic modulation (Figure 7c), it is clear in Figure 7b that the bundles of Co-C/MWCNTs were stretched along the magnetic field direction due to the dipolar interactions of magnetic particles on the MWCNT surface.

Figure 7. (a) Optical photograph of magnetic alignment of Co-C/MWCNTs on glass slide. SEM images of Co-C/MWCNTs dispersed in paraffin (b) with a magnetic field and (c) without a magnetic field. Figure 8a and b shows the frequency dependences of complex permittivity and complex permeability for the oriented Co-C/MWCNTs composite. Even at a very low filler loading of 15 wt%, the ε′ and ε″ of the oriented composite still kept high values varying in the range of 9.65–4.92 and 2.85–3.98, respectively, which are comparable to those of the unoriented CoC/MWCNTs with a high filler loading of 25 wt%. Previous studies confirmed that the permittivity along the MWCNT axis is much larger than that along the radical direction.64-65 Therefore, the parallel alignment of MWCNTs led to a large anisotropy of permittivity. When the electric field vector (E) of the incident EMW was parallel to the MWCNT alignment (as illustrated in Scheme 1c), the Co-C/MWCNTs would exhibit higher permittivity as shown in


19


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

Page 20 of 54

Figure 8a. In addition, Figure S5 presents the plot of ε'–ε" of the oriented Co-C/MWCNTs composite, which displayed equal number of Cole-Cole semicircles as observed in Figure S3e, revealing the dielectric polarization relaxation did not change after magnetic orientation. As to the dynamic magnetic behavior in high frequency, the µ′ and µ″ for the oriented Co– C/MWCNTs with 15 wt% loading show little change compared with the random CoC/MWCNTs with higher loading (Figure 8b), because the magnetic alignment did not change the random dispersion of magnetic Co NPs in matrix. Meanwhile the weak magnetic loss usually resulted in the insensitivity of complex permeability on filler loading, as observed in prior work.10,

18, 48

From the above analysis, it is concluded that the magnetic orientation

strategy could greatly promote the dielectric loss of the Co-C/MWCNTs. To further evaluate the attenuation capability of incident EMW inside the oriented CoC/MWCNTs sample, the attenuation constant α was calculated based on the following equation taking into account of both dielectric loss and magnetic loss:66

α=

2πƒ 2 2 × ( µ ″ε ″- µ ′ε ′)+（µ ″ε ″- µ ′ε ′） +（µ ′ε ″+µ ″ε ′） c

(4)

Figure S6a compares the α values of the Co-C (20 wt% and 25 wt%), unoriented CoC/MWCNTs (20 wt% and 25 wt%), and oriented Co-C/MWCNTs composites (15 wt%) in 2– 18 GHz. In general, the absorbing layer with a high filler loading benefits the attenuation of entered EMW. However, it is seen that the attenuation constant of the oriented CoC/MWCNTs composite with a low filler loading of 15 wt% could achieve comparable values to the random Co-C/MWCNTs composite with a filler loading of 25 wt%, which is due to the greatly enhanced dielectric loss by magnetic alignment. The large α value of the oriented CoC/MWCNTs composite implies good EMW absorbing performance as proved below.


20

Page 21 of 54

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


Figure 8. Frequency dependences of (a) ε′ and ε″, (b) µ′ and µ″, (c) reflection loss of oriented Co-C/MWCNTs composite; (d) comparison of specific reflection loss (SRL) of Co-C, unoriented and oriented Co-C/MWCNTs composites. Figure 8c shows the RL curves of the oriented Co-C/MWCNTs composite with a filler loading of 15 wt% at layer thickness of 1.0–4.0 mm. It is attractive to find that this sample exhibited an excellent EMW absorbing property with the optimum RL value up to –48.9 dB at 2.99 mm, which was even stronger than that of the unoriented Co-C/MWCNTs with 25 wt% filler loading. Nowadays, lightweight EMW absorbing material is a necessity for many applications such as aerospace and electronic devices. When considering a material’s density, RL alone is not a sufficient parameter for evaluating the comprehensive property. Therefore, specific reflection loss (SRL), which is obtained by dividing RL by filler loading, is a more appropriate criterion to evaluate the EMW absorbing effectiveness when comparing different materials. Figure 8d compares the SRL values of Co-C, unoriented Co-C/MWCNTs and oriented Co-C/MWCNTs samples. It is clear that the SRL value of the oriented Co-


21


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

Page 22 of 54

C/MWCNTs is much higher than those of Co-C and unoriented Co-C/MWCNTs. It demonstrates that the construction of conductive network and the magnetic orientation are effective methods to improve the comprehensive absorbing properties. Furthermore, we compared the SRL value of the oriented Co-C/MWCNTs with the state-of-the-art MOFderived absorbers, and the results are shown in Figure 9.13, 25-27, 31, 53, 67-74 The detailed property data for each absorber was listed in Table S1. Attractively, the oriented Co-C/MWCNTs exhibited higher SRL than other MOF-derived absorbers, which suggests its superior EMW absorbing behavior with light weight and highly efficient absorption.

Figure 9. Comparison of specific reflection loss of oriented Co-C/MWCNTs with the previous a

b

c

MOF-derived absorbers. NPC: nanoporous carbon composite, 500: calcined at 500 oC, CN: d

e

carbon-nanopolyhedron, CI: carbonyl iron, 800: calcined at 800 oC, fMOF (Fe): Fe-based MOF. As well known, EMW absorbing property is not exclusively dependent on the attenuation capability (α) of incident EMW inside the absorber by dielectric loss and magnetic loss.31, 40 Another crucial parameter i.e. impedance matching that determines the reflection of incident


22

Page 23 of 54

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


EMW at the air-absorber interface, must be taken into account. Therefore, we calculated the |Zin/Z0| value according to equation (2), which is a common parameter to evaluate the impedance matching. Ideal impedance matching requires the |Zin/Z0| value equal to 1.0, which implies the incident EMW enters the absorber with zero reflection.42, 44 As seen in Figure S6b, the |Zin/Z0| value of the oriented sample at a layer thickness of 2.99 mm fluctuated around 1.0 in 6–10 GHz implying most incident EMW could enter the absorber, and the strongest absorption peak was situated within this frequency region. The entered EMW was then converted into thermal energy by strong dielectric loss and magnetic loss as mentioned above. The primary loss models including the interfacial polarization relexation, the dipole polarization relaxation, the migration and hopping of electrons in the conductive framework, the micro-current loss, and the magnetic loss, are illustrated in Scheme 2a, c–e. Besides, the multiple reflection and scattering of EMW among Co-C particles can increase the EMW transmission paths, which also benefits the EMW attenuation (Scheme 2b). Furthermore, the magnetic alignment of MWCNTs considerably enhanced the dielectric loss of Co-C/MWCNTs composite, which is crucial for improving the comprehensive absorbing properties.


23


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

Page 24 of 54

Scheme 2. Scheme of primary EMW attenuation models in Co-C/MWCNTs composite: (a) magnetic loss, and electron hopping and migration, (b) multiple reflection and scattering, (c) interfacial polarization loss, (d) dipole polarization loss, and (e) micro-current loss. 4. CONCLUSIONS With a MWCNT template-directed strategy, Co-C/MWCNTs composite, in which Co-C particles were strung by MWCNTs, was fabricated by pyrolysis of ZIF–67/MWCNTs precursor. The conductive network constructed by MWCNTs effectively increased the dielectric loss of the composite by providing abundant paths for electron transportation. Under an external magnetic field, the Co-C/MWCNTs composite could be well aligned in paraffin matrix which considerably enhanced the effective permittivity due to the large anisotropic permittivity of MWCNTs. The magnetically oriented Co-C/MWCNTs composite exhibited superior comprehensive performance with the highest specific reflection loss compared with previous MOF-derived absorbers. When the filler loading of Co-C/MWCNTs in paraffin matrix was as low as 15 wt%, the minimum RL value could reach −48.9 dB. This study highlights the important effect of conductive network on improving EMW absorbing property,


24

Page 25 of 54

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


and simultaneously the magnetic orientation strategy could be extended to fabricate other types of lightweight and high-performance EMW absorber.

ASSOCIATED CONTENT Supporting Information SEM image and EDS result of Co–C/MWCNTs composite, room-temperature magnetization curves of Co-C and Co-C/MWCNTs composite, ε′–ε″ plots of all composites, frequency dependences of µ″(µ′)-2f-1 value for Co-C and unoriented Co-C/MWCNTs, attenuation constant of all composites, optimum RL and relative input impedance of oriented Co-C/MWCNTs, comparison of EMW absorbing properties of various MOF-derived composite absorbers.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. ORCID Xiaofang Liu: 0000-0002-2023-9890 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


25


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

Page 26 of 54

This work was financially supported by the Beijing Municipal Natural Science Foundation (2172031), Beijing Municipal Science and Technology Project (Z161100002116029), National Natural Science Foundation of China (51671010), Aeronautical Science Foundation of China (2016ZF51049), and Fundamental Research Funds for the Central Universities. 1.

REFERENCES Watts, C. M.; Liu, X.; Padilla, W. J., Metamaterial Electromagnetic Wave Absorbers.

Adv. Mater. 2012, 24, OP98-OP120. 2.

Liu, Q.; Cao, Q.; Bi, H.; Liang, C.; Yuan, K.; She, W.; Yang, Y.; Che, R., CoNi@SiO2

@TiO2 and CoNi@Air@TiO2 Microspheres with Strong Wideband Microwave Absorption. Adv. Mater. 2016, 28, 486-490. 3.

Hou, Y.; Cheng, L.; Zhang, Y.; Yang, Y.; Deng, C.; Yang, Z.; Chen, Q.; Wang, P.;

Zheng, L., Electrospinning of Fe/SiC Hybrid Fibers for Highly Efficient Microwave Absorption. ACS Appl. Mater. Interfaces 2017, 9, 7265-7271. 4.

Feng, J.; Hou, Y.; Wang, Y.; Li, L., Synthesis of Hierarchical ZnFe2O4@SiO2@RGO

Core-Shell Microspheres for Enhanced Electromagnetic Wave Absorption. ACS Appl. Mater. Interfaces 2017, 9, 14103-14111. 5.

Gholipur, R.; Khorshidi, Z.; Bahari, A., Enhanced Absorption Performance of Carbon

Nanostructure Based Metamaterials and Tuning Impedance Matching Behavior by An External AC Electric Field. ACS Appl. Mater. Interfaces 2017, 9, 12528-12539. 6.

Zhou, J.; He, J.; Li, G.; Wang, T.; Sun, D.; Ding, X.; Zhao, J.; Wu, S., Direct

Incorporation of Magnetic Constituents within Ordered Mesoporous Carbon-Silica Nanocomposites for Highly Efficient Electromagnetic Wave Absorbers. J. Phys. Chem. C 2010, 114, 7611-7617.


26

Page 27 of 54

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


7.

Wang, H.; Wu, L.; Jiao, J.; Zhou, J.; Xu, Y.; Zhang, H.; Jiang, Z.; Shen, B.; Wang, Z.,

Covalent Interaction Enhanced Electromagnetic Wave Absorption in SiC/Co Hybrid Nanowires. J. Mater. Chem. A 2015, 3, 6517-6525. 8.

Lv, H.; Guo, Y.; Wu, G.; Ji, G.; Zhao, Y.; Xu, Z. J., Interface Polarization Strategy to

Solve Electromagnetic Wave Interference Issue. ACS Appl. Mater. Interfaces 2017, 9, 56605668. 9.

Li, Y.; Wu, T.; Jiang, K.; Tong, G.; Jin, K.; Qian, N.; Zhao, L.; Lv, T., Mn2+ Induced

Structure Evolution and Dual-Frequency Microwave Absorption of MnxFe3−xO4 Hollow/Porous Spherical Chains Made by a One-Pot Solvothermal Approach. J. Mater. Chem. C 2016, 4, 7119-7129. 10.

Qiu, S.; Lyu, H.; Liu, J.; Liu, Y.; Wu, N.; Liu, W., Facile Synthesis of Porous

Nickel/Carbon Composite Microspheres with Enhanced Electromagnetic Wave Absorption by Magnetic and Dielectric Losses. ACS Appl. Mater. Interfaces 2016, 8, 20258-20266. 11.

Wu, T.; Liu, Y.; Zeng, X.; Cui, T.; Zhao, Y.; Li, Y.; Tong, G., Facile Hydrothermal

Synthesis of Fe3O4/C Core–Shell Nanorings for Efficient Low-Frequency Microwave Absorption. ACS Appl. Mater. Interfaces 2016, 8, 7370-7380. 12.

Zong, M.; Huang, Y.; Wu, H.; Zhao, Y.; Wang, Q.; Sun, X., One-pot Hydrothermal

Synthesis of RGO/CoFe2O4 Composite and Its Excellent Microwave Absorption Properties. Mater. Lett. 2014, 114, 52-55. 13.

Zhang, X.; Ji, G.; Liu, W.; Zhang, X.; Gao, Q.; Li, Y.; Du, Y., A Novel Co/TiO2

Nanocomposite Derived From a Metal–organic Framework: Synthesis and Efficient Microwave Absorption. J. Mater. Chem. C 2016, 4, 1860-1870.


27


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

14.

Page 28 of 54

Liu, X. G.; Li, B.; Geng, D. Y.; Cui, W. B.; Yang, F.; Xie, Z. G.; Kang, D. J.; Zhang, Z.

D., (Fe, Ni)/C Nanocapsules for Electromagnetic-Wave-Absorber in the Whole Ku-Band. Carbon 2009, 47, 470-474. 15.

Huang, L. N.; Liu, X. F.; Chuai, D.; Chen, Y. X.; Yu, R. H., Flaky FeSiAl Alloy-

Carbon Nanotube Composite with Tunable Electromagnetic Properties for Microwave Absorption. Sci Rep-Uk 2016, 6, 35377. 16.

Wang, L.; Huang, Y.; Sun, X.; Huang, H.; Liu, P.; Zong, M.; Wang, Y., Synthesis and

Microwave

Absorption

Enhancement

of

Graphene@Fe3O4@SiO2@NiO

Nanosheet

Hierarchical Structures. Nanoscale 2014, 6, 3157-3164. 17.

Xu, H.; Yin, X.; Zhu, M.; Han, M.; Hou, Z.; Li, X.; Zhang, L.; Cheng, L., Carbon

Hollow Microspheres with A Designable Mesoporous Shell for High-Performance Electromagnetic Wave Absorption. ACS Appl. Mater. Interfaces 2017, 9, 6332-6341. 18.

Chen, Y. H.; Huang, Z. H.; Lu, M. M.; Cao, W. Q.; Yuan, J.; Zhang, D. Q.; Cao, M. S.,

3D Fe3O4 Nanocrystals Decorating Carbon Nanotubes to Tune Electromagnetic Properties and Enhance Microwave Absorption Capacity. J. Mater. Chem. A 2015, 3, 12621-12625. 19.

Wang, Z.; Wu, L.; Zhou, J.; Jiang, Z.; Shen, B., Chemoselectivity-Induced Multiple

Interfaces in MWCNT/Fe3O4@ZnO Heterotrimers for Whole X-Band Microwave Absorption. Nanoscale 2014, 6, 12298-12302. 20.

Tian, X.; Meng, F.; Meng, F.; Chen, X.; Guo, Y.; Wang, Y.; Zhu, W.; Zhou, Z.,

Synergistic Enhancement of Microwave Absorption Using Hybridized Polyaniline@helical CNTs with Dual Chirality. ACS Appl. Mater. Interfaces 2017, 9, 15711-15718. 21.

Yan, L.; Hong, C.; Sun, B.; Zhao, G.; Cheng, Y.; Dong, S.; Zhang, D.; Zhang, X., In

Situ Growth of Core-Sheath Heterostructural SiC Nanowire Arrays on Carbon Fibers and


28

Page 29 of 54

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


Enhanced Electromagnetic Wave Absorption Performance. ACS Appl. Mater. Interfaces 2017, 9, 6320-6331. 22.

Xiang, J.; Li, J.; Zhang, X.; Ye, Q.; Xu, J.; Shen, X., Magnetic Carbon Nanofibers

Containing Uniformly Dispersed Fe/Co/Ni Nanoparticles as Stable and High-Performance Electromagnetic Wave Absorbers. J. Mater. Chem. A 2014, 2, 16905-16914. 23.

Wang, C.; Han, X.; Xu, P.; Zhang, X.; Du, Y.; Hu, S.; Wang, J.; Wang, X., The

Electromagnetic Property of Chemically Reduced Graphene Oxide and Its Application as Microwave Absorbing Material. Appl. Phys. Lett. 2011, 98, 072906. 24.

Sun, X.; He, J.; Li, G.; Tang, J.; Wang, T.; Guo, Y.; Xue, H., Laminated Magnetic

Graphene with Enhanced Electromagnetic Wave Absorption Properties. J. Mater. Chem. C 2013, 1, 765-777. 25.

Lu, Y.; Wang, Y.; Li, H.; Lin, Y.; Jiang, Z.; Xie, Z.; Kuang, Q.; Zheng, L., MOF-

Derived Porous Co/C Nanocomposites with Excellent Electromagnetic Wave Absorption Properties. ACS Appl. Mater. Interfaces 2015, 7, 13604-13611. 26.

Qiang, R.; Du, Y.; Zhao, H.; Wang, Y.; Tian, C.; Li, Z.; Han, X.; Xu, P., Metal Organic

Framework-Derived Fe/C Nanocubes Toward Efficient Microwave Absorption. J. Mater. Chem. A 2015, 3, 13426-13434. 27.

Quan, B.; Liang, X.; Ji, G.; Ma, J.; Ouyang, P.; Gong, H.; Xu, G.; Du, Y., Strong

Electromagnetic Wave Response Derived from the Construction of Dielectric/Magnetic Media Heterostructure and Multiple Interfaces. ACS Appl. Mater. Interfaces 2017, 9, 9964-9974. 28.

Ugarte, D.; Bacsa, W. S.; Doudin, B.; Forro, L.; De Heer, W. A., Graphitic Nanotubes

and Aligned Nanotubes Films. Braz. J. Phys. 1996, 26, 123-127.


29


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

29.

Page 30 of 54

Liu, T.; Xie, X.; Pang, Y.; Kobayashi, S., Co/C Nanoparticles with Low Graphitization

Degree: A High Performance Microwave-Absorbing Material. J. Mater. Chem. C 2016, 4, 1727-1735. 30.

Fang, J.; Liu, T.; Chen, Z.; Wang, Y.; Wei, W.; Yue, X.; Jiang, Z., A Wormhole-like

Porous Carbon/Magnetic Particles Composite as An Efficient Broadband Electromagnetic Wave Absorber. Nanoscale 2016, 8, 8899-8909. 31.

Qiang, R.; Du, Y.; Chen, D.; Ma, W.; Wang, Y.; Xu, P.; Ma, J.; Zhao, H.; Han, X.,

Electromagnetic Functionalized Co/C Composites by In Situ Pyrolysis of Metal-Organic Frameworks (ZIF-67). J. Alloys Compd. 2016, 681, 384-393. 32.

Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X., A Metal–Organic

Framework-Derived Bifunctional Oxygen Electrocatalyst. Nat. Energy 2016, 1, 15006. 33.

Wu, C.; Yang, Y.; Dong, D.; Zhang, Y.; Li, J., In Situ Coupling of CoP Polyhedrons

and Carbon Nanotubes as Highly Efficient Hydrogen Evolution Reaction Electrocatalyst. Small 2017, 13, 1602873-1602881. 34.

Yang, F.; Zhao, P.; Hua, X.; Luo, W.; Cheng, G.; Xing, W.; Chen, S., A Cobalt-Based

Hybrid Electrocatalyst Derived from A Carbon Nanotube Inserted Metal–Organic Framework for Efficient Water-Splitting. J. Mater. Chem. A 2016, 4, 16057-16063. 35.

Lin, R.; Ge, L.; Liu, S.; Rudolph, V.; Zhu, Z., Mixed-Matrix Membranes with Metal-

Organic Framework-Decorated CNT Fillers for Efficient CO2 Separation. ACS Appl. Mater. Interfaces 2015, 7, 14750-14757. 36.

Wang, Y.; Chen, B.; Zhang, Y.; Fu, L.; Zhu, Y.; Zhang, L.; Wu, Y., ZIF-8@MWCNT-

Derived Carbon Composite as Electrode of High Performance for Supercapacitor. Electrochim. Acta 2016, 213, 260-269.


30

Page 31 of 54

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


37.

Wang, L.; Huang, Y.; Li, C.; Chen, J.; Sun, X., Hierarchical Graphene@Fe3O4

Nanocluster@Carbon@MnO2 Nanosheet Array Composites: Synthesis and Microwave Absorption Performance. Phys. Chem. Chem. Phys. 2015, 17, 5878-5886. 38.

Pan, G.; Zhu, J.; Ma, S.; Sun, G.; Yang, X., Enhancing the Electromagnetic

Performance of Co Through the Phase-Controlled Synthesis of Hexagonal and Cubic Co Nanocrystals Grown on Graphene. ACS Appl. Mater. Interfaces 2013, 5, 12716-12724. 39.

Zhang, C.; Hao, R.; Liao, H.; Hou, Y., Synthesis of Amino-Functionalized Graphene as

Metal-Free Catalyst and Exploration of the Roles of Various Nitrogen States in Oxygen Reduction Reaction. Nano Energy 2013, 2, 88-97. 40.

Zhang, Z.; Dou, M.; Liu, H.; Dai, L.; Wang, F., A Facile Route to Bimetal and

Nitrogen-Codoped 3D Porous Graphitic Carbon Networks for Efficient Oxygen Reduction. Small 2016, 12, 4193-4199. 41.

Liu, Q. T.; Liu, X. F.; Feng, H. B.; Shui, H. C.; Yu, R. H., Metal Organic Framework-

Derived Fe/Carbon Porous Composite with Low Fe Content for Lightweight and Highly Efficient Electromagnetic Wave Absorber. Chem. Eng. J. 2017, 314, 320-327. 42.

Liu, X.; Chen, Y.; Cui, X.; Zeng, M.; Yu, R.; Wang, G. S., Flexible Nanocomposites

with Enhanced Microwave Absorption Properties Based on Fe3O4/SiO2 Nanorods and Polyvinylidene Fluoride. J. Mater. Chem. A 2015, 3, 12197-12204. 43.

Yang, H. J.; Cao, W. Q.; Zhang, D. Q.; Su, T. J.; Shi, H. L.; Wang, W. Z.; Yuan, J.;

Cao, M. S., NiO Hierarchical Nanorings on SiC: Enhancing Relaxation to Tune Microwave Absorption at Elevated Temperature. ACS Appl. Mater. Interfaces 2015, 7, 7073-7077.


31


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

44.

Page 32 of 54

Wang, Y.; Chen, D.; Yin, X.; Xu, P.; Wu, F.; He, M., Hybrid of MoS2 and Reduced

Graphene Oxide: A Lightweight and Broadband Electromagnetic Wave Absorber. ACS Appl. Mater. Interfaces 2015, 7, 26226-26234. 45.

Du, Y.; Liu, W.; Qiang, R.; Wang, Y.; Han, X.; Ma, J.; Xu, P., Shell Thickness-

Dependent Microwave Absorption of Core-Shell Fe3O4@C Composites. ACS Appl. Mater. Interfaces 2014, 6, 12997-12306. 46.

Dong, X. L.; Zhang, X. F.; Huang, H.; Zuo, F., Enhanced Microwave Absorption in

Ni/Polyaniline Nanocomposites by Dual Dielectric Relaxations. Appl. Phys. Lett. 2008, 92, 013127. 47.

Liu, X. F.; Hao, C. C.; Jiang, H.; Zeng, M.; Yu, R. H., Hierarchical

NiCo2O4/Co3O4/NiO Porous Composite: A Lightweight Electromagnetic Wave Absorber with Tunable Absorbing Performance. J. Mater.Chem. C 2017, 5, 3770-3778. 48.

Lu, M. M.; Cao, M. S.; Chen, Y. H.; Cao, W. Q.; Liu, J.; Shi, H. L.; Zhang, D. Q.;

Wang, W. Z.; Yuan, J., Multiscale Assembly of Grape-Like Ferroferric Oxide and Carbon Nanotubes: A Smart Absorber Prototype Varying Temperature to Tune Intensities. ACS Appl. Mater. Interfaces 2015, 7, 19408-19415. 49.

Tong, G.; Liu, Y.; Liu, F.; Guan, J., Easy Gas-Flow-Induced CVD Synthesis and

Tunable Electromagnetic Characteristics of Centipede-Shaped Iron/Cementite/Multiwalled Carbon Nanotube (Fe/Fe3C/MWCNT) Heterostructures. Surf. Coat. Technol. 2015, 283, 286297. 50.

Tong, G.; Liu, F.; Wu, W.; Du, F.; Guan, J., Rambutan-Like Ni/MWCNT

Heterostructures: Easy Synthesis, Formation Mechanism, and Controlled Static Magnetic and Microwave Electromagnetic Characteristics. J. Mater. Chem. A 2014, 2, 7373-7382.


32

Page 33 of 54

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


51.

Tong, G.; Wu, W.; Hua, Q.; Miao, Y.; Guan, J.; Qian, H., Enhanced Electromagnetic

Characteristics of Carbon Nanotubes/Carbonyl Iron Powders Complex Absorbers in 2–18GHz Ranges. J. Alloys Compd. 2011, 509, 451-456. 52.

Zhao, B.; Guo, X.; Zhao, W.; Deng, J.; Fan, B.; Shao, G.; Bai, Z.; Zhang, R., Facile

Synthesis of Yolk–Shell Ni@Void@SnO2(Ni3Sn2) Ternary Composites via Galvanic Replacement/Kirkendall Effect and Their Enhanced Microwave Absorption Properties. Nano Res. 2016, 10, 331-343. 53.

Yin, Y.; Liu, X.; Wei, X.; Yu, R.; Shui, J., Porous CNTs/Co Composite Derived from

Zeolitic

Imidazolate

Framework:

A

Lightweight,

Ultrathin,

and

Highly Efficient

Electromagnetic Wave Absorber. ACS Appl. Mater. Interfaces 2016, 8, 34686-34698. 54.

Yan, L.; Liu, J.; Zhao, S.; Zhang, B.; Gao, Z.; Ge, H.; Chen, Y.; Cao, M.; Qin, Y.,

Coaxial Multi-Interface Hollow Ni-Al2O3-ZnO Nanowires Tailored by Atomic Layer Deposition for Selective-Frequency Absorptions. Nano Res. 2016, 10, 1595-1607. 55.

Li, X.; Zhang, B.; Ju, C.; Han, X.; Du, Y.; Xu, P., Morphology-Controlled Synthesis

and Electromagnetic Properties of Porous Fe3O4 Nanostructures from Iron Alkoxide Precursors. J. Phys. Chem. C 2011, 115, 12350-12357. 56.

Chen, Y. J.; Gao, P.; Wang, R. X.; Zhu, C. L.; Wang, L. J.; Cao, M. S.; Jin, H. B.,

Porous Fe3O4/SnO2 Core/Shell Nanorods: Synthesis and Electromagnetic Properties. J. Phys. Chem. C 2009, 113, 10061-10064. 57.

Chen, J. P.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C., Magnetic Properties

of Nanophase Cobalt Particles Synthesized in Inversed Micelles. J. Appl. Phys. 1994, 76, 6316-6318.


33


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

58.

Page 34 of 54

Liu, Y.; Li, Y.; Jiang, K.; Tong, G.; Lv, T.; Wu, W., Controllable Synthesis of Elliptical

Fe3O4@C and Fe3O4/Fe@C Nanorings for Plasmon Resonance-Enhanced Microwave Absorption. J. Mater. Chem. C 2016, 4, 7316-7323. 59.

Liu, X. F.; Chen, Y. X.; Hao, C. C.; Ye, J. R.; Yu, R. H.; Huang, D. Q., Graphene-

Enhanced Microwave Absorption Properties of Fe3O4/SiO2 Nanorods. Composites Part A 2016, 89, 40-46. 60.

Liu, X.; Wang, L. S.; Ma, Y.; Zheng, H.; Lin, L.; Zhang, Q.; Chen, Y.; Qiu, Y.; Peng,

D. L., Enhanced Microwave Absorption Properties by Tuning Cation Deficiency of Perovskite Oxides of Two-Dimensional LaFeO3/C Composite in X-Band. ACS Appl. Mater. Interfaces 2017, 9, 7601-7610. 61.

Yang, R.; Wang, B.; Xiang, J.; Mu, C.; Zhang, C.; Wen, F.; Wang, C.; Su, C.; Liu, Z.,

Fabrication of NiCo2-Anchored Graphene Nanosheets by Liquid-Phase Exfoliation for Excellent Microwave Absorbers. ACS Appl. Mater. Interfaces 2017, 9, 12673-12679. 62.

Biswas, S.; Arief, I.; Panja, S. S.; Bose, S., Absorption-Dominated Electromagnetic

Wave Suppressor Derived from Ferrite-Doped Cross-Linked Graphene Framework and Conducting Carbon. ACS Appl. Mater. Interfaces 2017, 9, 3030-3039. 63.

Zhao, Y.; Liu, L.; Jiang, K.; Fan, M.; Jin, C.; Han, J.; Wu, W.; Tong, G., Distinctly

Enhanced Permeability and Excellent Microwave Absorption of Expanded Graphite/Fe3O4 Nanoring Composites. RSC Adv. 2017, 7, 11561-11567. 64.

Hekmatara, H.; Seifi, M.; Forooraghi, K., Microwave Absorption Property of Aligned

MWCNT/Fe3O4. J. Magn. Magn. Mater. 2013, 346, 186-191. 65.

Kim, I. T.; Tannenbaum, A.; Tannenbaum, R., Anisotropic Conductivity of Magnetic

Carbon Nanotubes Embedded in Epoxy Matrices. Carbon 2011, 49, 54-61.


34

Page 35 of 54

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


66.

Chuai, D.; Liu, X. F.; Yu, R. H.; Ye, J. R.; Shi, Y. Q., Enhanced Microwave

Absorption Properties of Flake-Shaped FePCB metallic Glass/Graphene Composites. Composites Part A 2016, 89, 33-39. 67.

Zhang, X.; Ji, G.; Liu, W.; Quan, B.; Liang, X.; Shang, C.; Cheng, Y.; Du, Y., Thermal

Conversion of An Fe3O4@Metal–Organic Framework: A New Method for An Efficient Fe– Co/Nanoporous Carbon Microwave Absorbing Material. Nanoscale 2015, 7, 12932-12942. 68.

Liang, X.; Quan, B.; Ji, G.; Liu, W.; Cheng, Y.; Zhang, B.; Du, Y., Novel Nanoporous

Carbon Derived From Metal–Organic Frameworks with Tunable Electromagnetic Wave Absorption Capabilities. Inorg. Chem. Front. 2016, 3, 1516-1526. 69.

Ma, J.; Zhang, X.; Liu, W.; Ji, G., Direct Synthesis of MOF-Derived Nanoporous

CuO/Carbon Composites for High Impedance Matching and Advanced Microwave absorption. J. Mater. Chem. C 2016, 4, 11419-11426. 70. Coated

Wang, Y.; Zhang, W.; Wu, X.; Luo, C.; Wang, Q.; Li, J.; Hu, L., Conducting Polymer Metal-Organic

Framework

Nanoparticles:

Facile

Synthesis

and

Enhanced

Electromagnetic Absorption Properties. Synth. Met. 2017, 228, 18-24. 71.

Zeng, X.; Yang, B.; Zhu, L.; Yang, H.; Yu, R., Structure Evolution of Prussian Blue

Analogues to CoFe@C Core–Shell Nanocomposites with Good Microwave Absorbing Performances. RSC Adv. 2016, 107, 105644-105652. 72.

Yuan, J.; Liu, Q.; Li, S.; Lu, Y.; Jin, S.; Li, K.; Chen, H.; Zhang, H., Metal Organic

Framework (MOF)-Derived Carbonaceous Co3O4/Co Microframes Anchored on RGO with Enhanced Electromagnetic Wave Absorption Performances. Synth. Met. 2017, 228, 32-40. 73.

Liu, W.; Pan, J.; Ji, G.; Liang, X.; Cheng, Y.; Quan, B.; Du, Y., Switching the

Electromagnetic Properties of Multicomponent Porous Carbon Materials Derived from


35


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

Page 36 of 54

Bimetallic Metal-Organic Frameworks: Effect of Composition. Dalton Trans. 2017, 46, 37003709. 74.

Liang, X.; Quan, B.; Sun, Y.; Ji, G.; Zhang, Y.; Ma, J.; Li, D.; Zhang, B.; Du, Y.,

Multiple Interfaces Structure Derived from Metal-Organic Frameworks for Excellent Electromagnetic Wave Absorption. Part. Part. Syst. Char. 2017, 34, 1700006.

TOC


36

Page 37 of 54

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


TOC 31x12mm (300 x 300 DPI)



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

Scheme 1. (a) Synthesis of Co-C/MWCNTs, (b) alignment of Co-C/MWCNTs in paraffin matrix under an external magnetic field, (c) measurements of EM parameters for unoriented and oriented samples. 73x38mm (300 x 300 DPI)


Page 38 of 54

Page 39 of 54

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


Figure 1. (a) XRD patterns and (b) Raman spectra of Co-C and Co-C/MWCNTs composites. 46x18mm (300 x 300 DPI)



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

Figure 2. (a) SEM and (b) TEM images of ZIF–67; (c) SEM and (d) TEM images of ZIF–67 derived Co-C composite; (e, f) SEM images and (g, h) TEM images of ZIF–67/MWCNTs; (i, j) SEM images and (k, l) TEM images of Co-C/MWCNTs composite. 104x78mm (300 x 300 DPI)


Page 40 of 54

Page 41 of 54

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


Figure 3. XPS spectra of Co-C/MWCNTs composite: (a) survey spectrum, deconvolution of (b) N 1s spectrum, (c) C 1s spectrum, and (d) Co 2p spectrum. 50x18mm (300 x 300 DPI)



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

Figure 4. Frequency dependence of complex permittivity for (a) Co-C and (b) Co-C/MWCNTs composites. Frequency dependence of complex permeability for (c) Co-C and (d) Co-C/MWCNTs composites. 88x65mm (300 x 300 DPI)


Page 42 of 54

Page 43 of 54

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


Figure 5. Frequency dependences of (a) tanδε and (b) tanδµ for Co-C and Co-C/MWCNTs composites in 2−18 GHz. 46x17mm (300 x 300 DPI)



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

Figure 6. Frequency dependence of reflection loss for Co-C and Co-C/MWCNTs composites: (a) Co-C 20 wt%, (b) Co-C 25 wt%, (c) Co-C/MWCNTs 20 wt% and (d) Co-C/MWCNTs 25 wt%. 86x62mm (300 x 300 DPI)


Page 44 of 54

Page 45 of 54

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


Figure 7. (a) Optical photograph of magnetic alignment of Co-C/MWCNTs on glass slide. SEM images of CoC/MWCNTs dispersed in paraffin (b) with a magnetic field and (c) without a magnetic field. 44x16mm (300 x 300 DPI)



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

Figure 8. Frequency dependences of (a) ε′ and ε″, (b) µ′ and µ″, (c) reflection loss of oriented Co-C/MWCNTs composite; (d) comparison of specific reflection loss (SRL) of Co-C, unoriented and oriented Co-C/MWCNTs composites. 85x60mm (300 x 300 DPI)


Page 46 of 54

Page 47 of 54

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


Figure 9. Comparison of specific reflection loss of oriented Co-C/MWCNTs with the previous MOF-derived absorbers. aNPC: nanoporous carbon composite, b500: calcined at 500 oC, cCN: carbon-nanopolyhedron, dCI: carbonyl iron, e800: calcined at 800 oC, fMOF (Fe): Fe-based MOF. 67x61mm (300 x 300 DPI)



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

Scheme 2. Scheme of primary EMW attenuation models in Co-C/MWCNTs composite: (a) magnetic loss, and electron hopping and migration, (b) multiple reflection and scattering, (c) interfacial polarization loss, (d) dipole polarization loss, and (e) micro-current loss. 72x37mm (300 x 300 DPI)


Page 48 of 54

Page 49 of 54

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


Figure S1. SEM image and EDS result of Co–C/MWCNTs composite. 37x18mm (300 x 300 DPI)



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

Figure S2. Room-temperature magnetization curves of Co-C and Co–C/MWCNTs composites. Inset: photograph of magnetic separation for Co-C and Co–C/MWCNTs composites. 53x38mm (300 x 300 DPI)


Page 50 of 54

Page 51 of 54

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


Figure S3. (a) Comparison of ε′–ε″ plots for Co-C and Co–C/MWCNTs composites. Enlarged ε′–ε″ plots for (b) Co-C (20 wt%), (c) Co-C (25 wt%), (d) Co–C/MWCNTs (20 wt%), and (e) Co–C/MWCNTs (25 wt%). 172x211mm (300 x 300 DPI)



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

Figure S4. Frequency dependences of µ″(µ′)-2f-1 values for Co-C and Co-C/MWCNTs composites in 2−18 GHz. 58x45mm (300 x 300 DPI)


Page 52 of 54

Page 53 of 54

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


Figure S5. ε′–ε″ plot of oriented Co-C/MWCNTs composite (15 wt%). 55x41mm (300 x 300 DPI)



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

Figure S6. Frequency dependences of (a) attenuation constant for all samples and (b) optimum RL value, relative input impedance (│Zin/Z0│), attenuation constant of oriented Co-C/MWCNTs composites (15 wt%). 45x16mm (300 x 300 DPI)


Page 54 of 54

MWCNTs Composite Derived from

Recommend Documents