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Highly cuboid-shape heterobimetallic metal–organic frameworks derived from porous Co/ZnO/C microrods with improved electromagnetic wave absorption capabilities Qiang Liao, Man He, Yuming Zhou, Shuangxi Nie, Yongjuan Wang, Saichun Hu, Haiyong Yang, Haifang Li, and Yuan Tong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09093 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Applied Materials & Interfaces

Highly cuboid-shape heterobimetallic metal–organic frameworks derived

from

porous

Co/ZnO/C

microrods

with

improved

electromagnetic wave absorption capabilities Qiang Liao, † , ‡ Man He,*, † Yuming Zhou,*, † Shuangxi Nie, ‡ Yongjuan Wang, † Saichun Hu†, Haiyong Yang†, Haifang Li† and Yuan Tong†. †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing

211189, China ‡

Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control,

College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China Abstract MOF-derived porous metal/C composites have drawn considerable attention from the microwave absorption field owing to their large pore volumes and surface areas (MOF: metal organic framework). Exploring single-MOF-derived materials with high-intensity and broadband absorption is largely needed, but remains a challenge. Here, porous Co/ZnO/C (CZC) microrods were fabricated easily from cuboid-shape heterobimetallic MOFs. CZC provides an efficient platform for integrating different semiconductors (ZnO), magnetic metal (Co) and carbon sources into one particle, which enhances the electromagnetic wave absorbing ability. The carbonization temperature which is critical for electromagnetic parameters was studied in detail. CZC annealed at 700 °C outperformed those obtained at 600 or 800 °C in terms of microwave wave absorbing properties. The reflection loss (RL) was optimized to -52.6 (or -20.6) dB at 1

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12.1 (or 14.8) GHz with effective bandwidth (RL≤-10 dB) of 4.9 (or 5.8) GHz at the coating thickness of 3.0 (or 2.5) mm. Such enhancement of EM wave absorbing capabilities is ascribed to the well-built porous structure, dielectric loss and magnetic loss. This work offers a new way to prepare porous magnetic metal/C composites with excellent microwave absorbing properties starting from heterobimetallic MOFs. Keywords: Heterobimetallic MOF; Porous microrods; carbon; Co/ZnO/C composites; EM wave absorption * Corresponding author. E-mail: [email protected] (M. He); [email protected] (Y. Zhou) 1. Introduction The development of electronic technology in modern digital society has led to a new urge for electronic equipment and wireless devices.1 However, the resulting electromagnetic (EM) wave pollution has become increasingly serious in civil and military fields, which severely threaten biological systems and information safety.2-7 Microwave absorbers, as efficient solutions to such pollution, are used to weaken sharply EM energy and convert it into thermal energy.8,9 Thus, it is urgent to design promising thin and low-weight EM wave absorbents with broad frequency and strong absorption.10 Porous carbon is a representative porous material with low density, large surface, high conductivity and low filler loading ratio, which account for its potential applications in diverse fields,11,12 such as gas adsorption/storage,13 electrode materials14 catalysts,15-16 and especially microwave absorption.17-19 For instance, the 2


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pristine ordered mesoporous carbons carbonized at different temperatures could considerably enhance the reflection loss (RL) performance.20 Qiu et al. fabricated nano-porous carbons via carbonization and KOH activation of walnut shells and found the porous biomass carbon was an ideal lightweight absorber of EM waves.21 Porous carbon, a representative dielectric loss material, is better at EM absorption.22,23 However, based on its absorption mechanism, the only dielectric loss weakens the EM impedance matching.24,25 The addition of metals into porous carbon can further strengthen the microwave absorption. In the porous Co/C composite with a Co-MOF as a template (MOF: metal organic framework), the RL maximized to -35.3 dB with effective absorption bandwidth of 5.8 GHz and coating thickness of 4 mm.4 The porous Ni/C derived from Ni-based MOF possessed an optimal RL of -51.8 dB with a broad bandwidth of 3.48 GHz and coating thickness of 2.6 mm.19The novel Fe/C composite with MOF as a template self-sacrificing precursor showed the minimum RL of -29.5 dB with a broad bandwidth of 4.3 GHz and coating thickness of 2.5 mm.26 However, there is rare research about heterobimetallic MOF absorbers, which are significantly more adjustable than single MOFs in metal centers and ligand structures. Constructing heterobimetallic MOFs is an effective way to adjust the EM parameters of carbon absorbers.27-29 Heterobimetallic MOF cuboids are ideal self-sacrificial templates to fabricate porous carbon with high use efficiency for microwave absorption. Many methods have been proposed to synthesize nano- and micro-scale MOFs, which may bring about new characteristics and expand the template options. However, the size- and shape-controlled synthesis is yet a challenge, owing to the severe 3



dependence on organic solvents or base solutions, which are environmentally unfriendly. Inspired by previous works, we synthesized novel heterobimetallic MOF cuboids from the reaction between Co2+/Zn2+ mixture and 2-methylimidazole in deionized water at room temperature. This synthetic way is outstanding with environmental friendliness, low costs and high productivity.30 Specifically, we prepared porous Co/ZnO/C (CZC) microrods using a CoZn-MOF as a template/precursor and through pyrolysis at desired temperature under N2, and comprehensively investigated their EM wave absorbing performance and loss mechanism. The heterobimetallic porous CZC microrods significantly outperformed single MOF-derived magnetic metal/C porous composites in terms of microwave absorption. 2. Experimental 2.1 Chemicals Nitrate hexahydrates [(X(NO3)2·6H2O), X=Zn, Co], 2-methylimidazole (Hmim, C4H6N2) and polyvinylpyrrolidone (PVP, K90) (all analytical grade). 2.2 Construction of cuboids Zn/Co MOF In a typical procedure of synthesizing cuboids Zn/Co MOF, Zn(NO3)2·6H2O (0.446 g) and Co(NO3)2·6H2O (0.146 g) were put into 40 mL of deionized water for 30 min under magnetic stirring. The resulting solution was added into 40 mL of deionized water containing 1.3 g of Hmim and 0.3 g of PVP under stirring for 5 min, followed by storage at 25 °C for 4 h. Powder was obtained after centrifugation, washing with deionized water, and drying at 60 °C. 2.3 Construction of porous Co/ZnO/C (CZO) microrods Porous CZC microrods were prepared by thermally treating the cuboids 4


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ZnCo-MOF at a rate of 2 °C·min-1 for 1 hour at 600, 700 or 800 °C and kept in N2. The microrods were marked as CZC-x, where x= 600, 700 or 800 °C was the calcinating temperature. 2.4 Material characterization The samples were investigated on a Smart Lab X-ray diffractometer (XRD; Rigaku) with Cu Ka radiation and a JEM-2100 high-resolution transmission electron microscope (HRTEM) coupled with copper grids. Their surface morphology was observed on an FEI Inspect F50 device. Element distributions were explored via element mapping.

N2

adsorption-desorption

isotherms

were

obtained

by

the

Brunauer-Emmett-Teller (BET) method. Static magnetics was characterized on a Model 7407 vibrating sample magnetometer (VSM, Lake Shore). Other instruments included a Thermo Fisher Raman microscope and a Thermo Scientifc Escalab 250 Xi X-ray photoelectron spectroscope (XPS, spot size = 650 µm, pass energy = 30.0 eV). 2.5 EM measurements Complex permittivity and magnetic permeability were measured by a vector network analyzer (Agilent PNA N5224A) at frequency 2–18 GHz. Samples were prepared by homogeneously mixing paraffin wax (70 wt%) with products (30 wt%) in paraffin wax and then compressed into toroidal rings (outside diameter = 7.00 mm; inner diameter = 3.04 mm). 3.Results and discussion In this process (Scheme 1), the CoZn-MOF cuboids were gained via a facile one-stage room-temperature reaction among Co2+/Zn2+, Hmim and PVP in water, and 5



subsequent thermal annealing in N2 for carbonization, thus transforming CoZn-MOF cuboids into porous CZC microrods. In fact, during nucleation and growth processes of CoZn-MOF, the O and N atom of the of PVP is weekly coordinated to Zn2+ and Co2+ ions, consequently, PVP provided steric stabilization. Formation of the PVP-protected CoZn-MOF is schematized (Scheme S1). Typical SEM and TEM images of CoZn-MOF (Figure 1a-c) show the cuboid shape crystals are smooth-surfaced with ∼1.5 µm in width and ∼7.5 µm in length. The TEM image (magnified images in the inset of Figure 2c) also clearly exhibits the cuboid-shape of CoZn-MOF. XRD of precursors (Figure 1d) shows all diffraction peaks agree well with a previous report.31 To clarify the phase structure evolution from CoZn-MOF to porous CZC microrods during the carbonization, we recorded the XRD patterns of the metal/porous carbon particles (CZC-x, Figure 2a). The enhanced XRD peaks indicate the gradual intensification of crystallinity with the temperature rise. The broad peak at ~ 25° corresponds to the (002) plane of graphitic carbon.32 The three main peaks at 44.2°, 51.5° and 75.8° are ascribed to the (111), (200) and (220) planes of metal Co, respectively (JCPDS No. 15-0806).33 Some weak peaks of ZnO 34,35 also appear in the XRD patterns of the three samples. The degree of graphitization was detected via Raman spectroscopy (Figure 2b). A smaller ID/IG suggests higher degree of graphitization,36 where the D band around 1340 cm−1 and the G band around 1580 cm−1 are characteristic of disordered carbon and graphitic carbon, respectively. The ratios of CZC-600, CZC-700 and CZC-800 are 0.96, 0.94 and 0.91, respectively. Obviously, CZC-800 has the highest ID/IG ratio, which is attributed to the catalysis effect of transition metal (Co).37 6


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Figure 3a-c show typical SEM images of CZC-x (x=600, 700 and 800). Clearly, the three samples retained their original cuboid shapes after the calcination, which was certified by the TEM images (Figure 3d-f). Moreover, the TEM images show that the CZC composites possess highly porous structure due to the escape of 2-methylimidazole, release of small molecules (CO, CO2, etc.) and evaporation of Zn metals during pyrolysis. More details can be given by HRTEM images (Figure 3g-i), as the three samples have well-resolved lattice fringe with interplanar spacing of 0.21 and 0.18 nm, which consist with the (111) and (200) planes of Co, respectively.38 This fringe spacing of ZnO (~0.25 nm) perfectly matches with the (002) plane.39 Meanwhile, a coating of porous carbon matrix was clearly observed around these nanoparticles.40 The SEM image and elemental mapping of CZC-700 (Figure 4) show homogeneous distributions of C, Co, Zn, O and N elements, demonstrating the porous carbon matrix was anchored with these nanoparticles, and the actual contents of Co, ZnO and carbon in the as-prepared CZC-700 samples was determined by EDS (Figure S1). The porous structures of the products were further studied by nitrogen adsorption-desorption isotherms (Figure 5a-b). The carbonization of the MOFs generated abundant mesopores, leading to unique permittivity behaviors compared with traditional materials. All the samples mainly belong to typical type-IV isotherms, which may result from their microporous structures.41 Remarkably, the BET specific surface areas of CZC-600, CZC-700 and CZC-800 are 127.4, 252.7 and 401.9 m2·g-1, respectively (Table 1). The dramatic increment of surface areas may be attributed to evaporation of metal Zn during pyrolysis.42,43 As discussed above, the porous structure not only endows the composites with large specific surface area and total pore volume, but also causes 7



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multiple reflections and scattering for energy dissipation, converting microwave energy to heat or other energy during prolonged propagation. Moreover, the porous structure contributes to reducing the weight density of the absorber. The electronic structure and chemical valence of CZC composites were investigated via XPS. With CZC-700 as an example, the elements and valence were examined in detail. The characteristic peaks of Zn, Co, C, N and O (Figure 6a) are very consistent with the elemental mappings. The binding energies (BE) of the Co 2p3/2 and Co 2p1/2 peaks are 779.7 and 796.1 eV, respectively (Figure 6b), further confirming Co2+ in the HMOF was reduced to Co0 by the carbonization in N2.44,45 The Zn 2p XPS spectrum has two strong peaks at 1022.2 and 1045.2 eV (Figure 6c), which are ascribed to Zn 2p3/2 and Zn 2p1/2, respectively, indicating the existence of Zn as Zn2+.46 The three samples present typical ferromagnetic hysteresis loops measured at ∼298 k (Figure 6d), due to magnetic Co nanoparticles. The saturation magnetization (Ms) values of CZC-600, CZC-700 and CZC-800 are around 6.6, 15.8 and 30.7 emu·g–1, respectively and they are smaller than the Ms value of bulk Co

47

owing to

the presence of nonmagnetic porous carbon and ZnO in the composites. It is roughly deduced that the amount of ZnO decreases but the amount of Co increases with increasing temperature of calcination.48 Clearly, the Ms of CZC-800 is much higher compared with CZC-600 and CZC-700, which is attributed to the decreased Zn content.2 As reported, ferromagnetism contributes to improving electromagnetic wave absorption.9, 19 3. EM absorption capabilities 8


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To investigate the microwave absorption mechanism, we evaluated the relative complex

permittivity

( ε r = ε ′ − jε ′′ )

and

relative

complex

permeability

( µ r = µ ′ − j µ ′′ ) at 2–18 GHz. The real permittivity ( ε ′ ) and real permeability ( µ ′ ) can be expressed as the storage ability of electrical and magnetic energy, while the imaginary parts ( ε ′′ and µ ′′ ) as the dissipation ability.49,50 To determine the microwave absorption capabilities, we investigated their EM parameters with the mass ratio 30 wt% CZC in paraffin. Moreover, complex permittivity and complex permeability were investigated at the frequency of 2-18 GHz. The ε ′ of CZC-600, CZC-700 and CZC-800 generally declines, while ε ′′ changes slightly from 0.5 to 0.39, 1.03 to 1.23, and 2.03 to 2.73, respectively, with the rising frequency (Figure 7a-b). With the rise of annealing temperature, CZC-800 shows the highest complex permittivity, ε ′ of

6.58 and ε ′′ of 2.14. Apparently, CZC-800 shows the largest

dielectric loss tangent (tan δε = ε ′′ / ε ′ ) (Figure 7c), indicating the dielectric loss ability is improved with the temperature rise, which is in agreement with expansion of the complex permittivity. The presence of resonant peaks suggests that multiple polarization relaxation processes occur in the composites under alternating EM field. Dielectric loss is mainly induced by loss of conductivity and polarization loss. This work shows the conductive loss is decided by the electrical conductivity, which can be determined from graphitization. Raman spectra (Figure 2b) confirm high carbonization temperature intensifies the graphitization of carbonaceous skeleton in CZC, which results in higher conductivity and further promotes complex permittivity according to the free electron theory2,8. At the range of microwave frequency, 9



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polarization loss may result from the interface and dipole polarization. Multiple interface polarization occurs at the Co/carbon, ZnO/carbon and carbon/paraffin interfaces, while dipole polarization can generally be caused by dipole redirection and EM interaction. Magnetic loss is another key factor in EM wave absorption.51,52 µ ′ and µ ′′ fluctuate remarkably and tend to decrease with the increasing frequency (Figure 7d-e). Clearly, the pyrolysis temperature slightly affects µ ′ , which declines slowly from 1.07 to 0.97, 1.07 to 0.93, and 1.08 to 0.94 in CZC-600, CZC-700 and CZC-800 respectively.53 In the gigahertz range, magnetic loss mainly originates from natural ferromagnetic resonance, and eddy current loss due to the eddy current effect ′′ ′ −2 −1 .26 ,48 If the magnetic loss mainly depends on the expressed as C（ 0 C0 = µ ( µ ) f ）

eddy current loss, C0 is constant regardless of frequency. However, the C0 of CZC composites varies from 2.0 to 18.0 GHz (Figure 8). Thus, the magnetic loss can be attributed to natural ferromagnetic resonance and exchange resonance. As we know, a smaller difference between dielectric loss tangent (tan δε ) and magnetic loss tangent (tan δ µ ) indicates the better impedance matching. For CZC-700, the similarity between tan δ ε and tan δ µ is beneficial for the impedance matching, resulting in the highest EM wave absorption ability. We evaluated the RL of the products by experimentally determining the complex permittivity and complex permeability as follows: 54,55 Z in = Z 0 ( µr / ε r )1/2 tanh  j (2π fd / c)(µr ε r )1/2 

10


(1)

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RL ( dB ) = 20 lg

Z in − 1 Z in + 1

(2)

where Z in is the input impedance of the absorber, Z0 is the impedance of free space, f is the microwave frequency, d is the absorber thickness, and c is the velocity of EM waves in free space. Generally, RLs of EM materials should be less than 10 dB (90% microwave absorption). Figure 9(a)-(c) show the RLs of the CZC composites with varying thicknesses at 2−18 GHz. Clearly, CZC-700 shows better EM absorbing properties, in which the optimum RL at 12.1 (or 14.8) GHz is -52.6 (-20.6) dB with effective bandwidth (RL≤-10 dB) of 4.9 (5.8) GHz at thickness of 3.0 (2.5) mm. Noticeably, CZC-700 exhibits a wider effective bandwidth of 12.2 (from 5.8 to 18.0) GHz with the change of thickness. For CZC-800, the smallest RL is -21.6 dB at 5.6 GHz. At the coating thickness of 2.0 mm, the optimum RL is -15.6 dB at 10.6 GHz with effective bandwidth of 4.7 GHz. Moreover, the effective bandwidth is 13.2 GHz with the adjustment of thickness. In contrast, CZC-600 shows the weakest EM absorption ability with the smallest RL below -16.2 dB and narrower effective bandwidth at the thickness of 3.5 mm. CZC-600 fails to create the strongest RL because of poor impedance matching. The above analyses confirm the vital role of temperature in the EM wave absorption. Interestingly, the RL peak converts to low frequency with the growing layer thickness. The quarter-wavelength attenuation law can be explained by the relation between absorber thickness ( tm ) and peak frequency ( fm ) as follows: 56,57 11



tm =

nλ nc ( n = 1, 3, 5,...) = 4 4 fm µ r ε r

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(3)

where λ is the wavelength of the EM wave, c is the light velocity in free space, and n =1, 3, 5…. Figure 10 shows the CZC-700 simulated curves of the RL peak frequency and the matching thickness, which well fits the quarter-wavelength matching conditions. Additionally, the quarter-wave law efficiently and significantly assists the thickness design of EM wave absorbers, when the corresponding permittivity and permeability are measured. Figure 11 is a schematic illustration of proposed fundamental mechanism underlying the microwave absorption by CZC absorbers. The excellent EM wave absorption capabilities of CZC might depend on the porous structure, dielectric loss (including interfacial and dipole polarization, conduction loss) and magnetic loss. Firstly, porous structure can lead to multiple reflections and scattering to dissipate energy, converting microwave energy to heat or other forms of energy during prolonged propagation. In our case, the entering microwave would undergo a series of scatter and reflection in the porous structure.3 Secondly, the introduction of ZnO and Co nanoparticles into the carbon matrix brings in more interfaces and multiple interface polarization, which occurs at the Co/carbon, ZnO/carbon and carbon/paraffin interfaces, much increasing dielectric loss.58,59 Thirdly, the presence of numerous small-sized ZnO nanoparticles in the carbon layer will cause dipole polarization, resulting in dielectric loss. Fourthly, the well-separated carbon layers and the associated ZnO and Co provide more conductive paths, causing conduction loss 12


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(conductive paths are explained in supporting information). Finally, Co nanoparticles can induce magnetic loss that attenuates EM energy.9, 35 The porous CZC with smaller RL and wider bandwidth at lower filler loading outperforms similar materials (Table 2). Therefore, this porous CZC is suitable for EM wave absorption applications.

4. Conclusions Porous Co/ZnO/C (CZC) microrods were synthesized from the cuboid-shape heterobimetallic MOF. The CZC composites inherited the advantages of single CO- and Zn-MOF derivatives, and exhibited excellent microwave absorption performances owing to the porous structure and dielectric loss & magnetic loss. Especially, the composite annealed at 700 °C performed the best with the smallest RL of -52.6 dB at 12.1 GHz and effective bandwidth of 4.9 GHz at the coating thickness of 3.0 mm. Our work offers as easy way to design porous magnetic/dielectric microwave absorbers. The CZC hybrids are expected to be a superior EM wave absorber.

Acknowledgements This work was supported by the National Nature Science Foundation of China (51673040), the Natural Science Foundation of Jiangsu Province (BK20171357), the Prospective Joint Research Project of Jiangsu Province (BY2016076-01), Opening Project of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control (KF201605), the Fundamental Research Funds for Central Universities (2242018k30008), Scientific Innovation Research Foundation of College Graduate in Jiangsu Province (KYLX16_0266), Project Funded by the Priority Academic Program 13



Development of Jiangsu Higher Education Institutions (1107047002), and Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (BA2016105).

Supporting Information PVP as surfactant was used to regulate the size and shape of cuboid-shape CoZn-MOF, EDS of the as-prepared CZC-700 samples, Schematic illustration of conductive paths constructed by Co/ZnO/C.

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Figure captions Scheme 1. Schematic illustration of synthesis process of porous CZC composites. Figure 1 (a and b) SEM image, (c) TEM image, (d) XRD pattern of cuboid-shape CoZn-MOF.

Figure 2 (a) XRD patterns and (b) Raman spectra of CZC-600, CZC-700 and CZC-800.

Figure 3 SEM images (upper), TEM images (middle) and HRTEM images (lower) of 23



CZC: (a), (d), (g) CZC-600, (b), (e), (h) CZC-700 and (c), (f), (i) CZC-800

Figure 4 Elemental mappings of CZC-700. Figure 5 (a) N2 sorption isotherms and (b) pore size distributions of CZC-600, CZC-700 and CZC-800.

Figure 6 (a) XPS spectra of CZC-700, (b) Co 2p, (c) Zn 2p. (d) hysteresis loops of CZC composites prepared at different annealing temperatures.

Figure 7 Complex permittivity (upper) and complex permeability (lower) of three samples: (a), (d) real part, (b), (e) imaginary part, (c) dielectric loss tangent, and (f) magnetic loss tangent.

Figure 8 C0 curves of CZC composites. Figure 9 Reflection loss in the frequency range of 2-18 GHz: (a) CZC-600, (b) CZC-700, (c) CZC-800.

Figure 10 Dependence of 1/4 λ matching thickness on RL peak frequency for CZC-700.

Figure 11 Schematic illustration of EM wave absorption mechanisms for CZC composites.

Table captions: Table 1. Specific surface areas and total pore volume of CZC-600, CZC-700 and CZC-800.

Table 2. Comparison of EM wave absorption with similar materials.

24


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Scheme 1. Schematic illustration of synthesis process of porous CZC composites.

Figure 1 (a and b) SEM images, (c) TEM image, (d) XRD pattern of cuboid-shape CoZn-MOF

25



Figure 2 (a) XRD patterns and (b) Raman spectra of CZC-600, CZC-700 and CZC-800

Figure 3 SEM images (upper), TEM images (middle) and HRTEM images (lower) of CZC: (a), (d), (g) CZC-600, (b), (e), (h) CZC-700 and (c), (f), (i) CZC-800

26


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Figure 4 Elemental mappings of CZC-700.

Figure 5 (a) N2 absorption isotherms and (b) pore size distributions of CZC-600, CZC-700 and CZC-800. Table 1. Specific surface areas and total pore volumes of CZC-600, CZC-700 and CZC-800.

Sample

SBET(m2g-1)

SLangmuir(m2g-1) 27


Vpore(cm3g-1)


Page 28 of 33

CZC-600

127.4

168.9

0.0580

CZC-700

252.7

335.8

0.1225

CZC-800

401.9

535.4

0.1991

Figure 6 (a) XPS spectra of CZC-700, (b) Co 2p, (c) Zn 2p. (d) hysteresis loops of CZC composites prepared at different annealing temperatures.

28


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Figure 7 Complex permittivity (upper) and complex permeability (lower) of three samples: (a), (d) real part, (b), (e) imaginary part, (c) dielectric loss tangent, and (f) magnetic loss tangent.

Figure 8 C0 curves of CZC composites.

29



Figure 9 Reflection loss in the frequency range of 2-18 GHz: (a) CZC-600, (b) CZC-700, (c) CZC-800.

30


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Figure 10 Dependence of 1/4 λ matching thickness on RL peak frequency for CZC-700.

Figure 11 Schematic illustration of EM wave absorption mechanisms for CZC composites. 31



Page 32 of 33

Table 2. Comparison of EM wave absorption with similar materials.

Filler Sample

Matrix

Minimum

Layer

Effective

loading

RL

thickness bandwidth

(wt%)

(dB)

(mm)

(GHz)

Ref.

Flower-like Co/CoO

Paraffin

50

-50

3.5

4.5

60

Co@NPC@TiO2

Paraffin

50

-51.7

1.65

4.2

61

Co@Fe

Paraffin

60

-47.3

1.5

4.8

25

Co/C-800

Paraffin

30

-32.4

2.0

3.8

37

ZnO/NPC@Co/NPC

Paraffin

50

-28.8

1.9

4.2

52

Fe3O4@ZnO

Epoxy

30

-40

5.0

∼2.0

62

Porous CZC-700

Paraffin

30

-52.6

3.0

4.9

This work

Porous CZC-800

Paraffin

30

-15.6

2.0

4.7

This work

32


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TOC graphic

33


Highly cuboid-shape heterobimetallic metal ... - ACS Publications

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