Co Composite Derived from Zeolitic Imidazolate

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Porous CNTs/Co Composite Derived from Zeolitic Imidazolate Framework: A Lightweight, Ultrathin, and Highly Efficient Electromagnetic Wave Absorber Yichao Yin,† Xiaofang Liu,*,† Xiaojun Wei,‡ Ronghai Yu,*,† and Jianglan Shui† †

School of Materials Science and Engineering, Beihang University, Beijing 100191, China Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

S Supporting Information *

ABSTRACT: Porous carbon nanotubes/cobalt nanoparticles (CNTs/Co) composite with dodecahedron morphology was synthesized by in situ pyrolysis of the Co-based zeolitic imidazolate framework in a reducing atmosphere. The morphology and microstructure of the composite can be well tuned by controlling the pyrolysis conditions. At lower pyrolysis temperature, the CNTs/Co composite is composed of well-dispersed Co nanoparticles and short CNT clusters with low graphitic degree. The increase of pyrolysis temperature/time promotes the growth and graphitization of CNTs and leads to the aggregation of Co nanoparticles. The optimized CNTs/Co composite exhibits strong dielectric and magnetic losses as well as a good impedance matching property. Interestingly, the CNTs/Co composite displays extremely strong electromagnetic wave absorption with a maximum reflection loss of −60.4 dB. More importantly, the matching thickness of the absorber is as thin as 1.81 mm, and the filler loading of composite in the matrix is only 20 wt %. The highly efficient absorption is closely related to the well-designed structure and the synergistic effect between CNTs and Co nanoparticles. The excellent absorbing performance together with lightweight and ultrathin thickness endows the CNTs/Co composite with the potential for application in the electromagnetic wave absorbing field. KEYWORDS: zeolitic imidazolate framework, CNTs/Co, electromagnetic wave absorption, lightweight, ultrathin

1. INTRODUCTION With the rapid development of wireless communication and the wide application of electronic devices, electromagnetic (EM) radiation and interference have caused serious EM pollution, which brings great threats to people’s health and information safety.1−5 High-performance EM wave (EMW) absorbers therefore aroused worldwide attention in recent years with the aim of solving this increasingly serious problem by converting EM energy into thermal energy.6−12 Nowadays, strong EMW absorption has been achieved in ferrites and magnetic metals. However, their narrow absorbing bandwidth, large thickness, and density restrict practical applications.13,14 To solve these issues, an efficient method is to combine the magnetic materials with lightweight dielectric materials, taking advantage of the synergistic effect between the two components.15−17 Typically, carbonaceous materials are con© 2016 American Chemical Society

sidered as promising candidates because of their attractive advantages of high dielectric loss, low density, low cost, and easy preparation. By optimizing the constituent and architecture of the composite absorber, strong absorption of EM energy in a wide frequency range is expected to be achieved via adjusting the permittivity and permeability of the composite while significantly reducing the weight. The development of nanotechnology promotes the structural diversity of the nanocomposites. Metal−organic frameworks (MOFs), constructed with metals (clusters) and ligands having huge diversity, recently became an ideal precursor/template for fabricating carbon-based functional materials.18,19 The compoReceived: September 25, 2016 Accepted: November 28, 2016 Published: November 28, 2016 34686

DOI: 10.1021/acsami.6b12178 ACS Appl. Mater. Interfaces 2016, 8, 34686−34698

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthetic Progress of CNTs/Co Composite from ZIF-67 Precursor

tubular structure can form an interconnected network which provides additional conductive paths. In this work, we synthesized a porous CNTs/Co composite by in situ pyrolysis of ZIF-67 under Ar/H2 atmosphere. This synthetic strategy is facile, controllable, and low-cost. The microstructure and EM properties of the composites can be well tuned by changing the pyrolysis conditions. Interestingly, the optimized CNTs/Co composite exhibited strong EMW absorbing capability with RL values up to −60.4 dB, which is much stronger than those of the previous ones. More importantly, the matching thickness of the absorber was only 1.81 mm, and the filler loading of the CNTs/Co composite in paraffin matrix was as low as 20 wt %. The CNTs/Co composite with much stronger EMW absorbing capability, quite lower filler loading, and extremely small thickness could be applied as a lightweight, ultrathin, and highly efficient EMW absorber for practical applications.

sition and microstructure of the MOF-derived materials can be altered by simply controlling the thermal decomposition conditions of the MOFs. Furthermore, the periodic arrangement of metal atoms in crystalline MOFs provides a prerequisite condition for obtaining well-dispersed metal nanoparticles (NPs). In addition, the porous/hollow structure of the MOF-derived materials is favorable to decreasing the weight density of absorber. Very recently, MOF materials with magnetic elements as coordinated metals have been used to prepare magnetic metal NP-decorated porous carbon absorbers. For example, Qiang and co-workers prepared a porous Co/ C composite using zeolitic imidazolate framework-67 (ZIF-67, a subclass of MOFs) as precursor. The obtained Co/C composite displayed good EMW absorbing properties with a maximum reflection loss (RL) value of −39.6 dB at a thickness of 2.0 mm.20 Similarly, Lü et al. achieved strong EMW absorption in a porous Co/C composite derived from pyrolysis of ZIF-67 at 500 °C. An optimized RL of −35.3 dB at a thickness of 2.5 mm was obtained, as the filler loading of Co/C composite in matrix was 60 wt %.21 In addition, the Xu group reported the synthesis of porous Fe/C nanocubes through high-temperature pyrolysis of Prussian blue. When Fe/C was mixed with paraffin at a mass percentage of 40%, an RL value of −22.6 dB was achieved at a coating thickness of 2.0 mm.22 Despite the strong EMW absorption, the filler loading of the reported MOF-derived composites is usually higher than 30 wt % in the matrix while the layer thickness commonly exceeds 2.0 mm. The large density and thickness of these absorbers fail to satisfy the high demands for ideal EMW absorbers. Thus, it is highly desirable to develop new strategies to improve their comprehensive performances. Previous MOF-derived carbon-based absorbers mostly have a poor graphitization degree and contain a much higher ratio of amorphous carbon even through high-temperature calcination (900 °C), which is unfavorable to electron transport and thus weakens the attenuation of EMW. Therefore, it is expected that the increase in conductivity could offer a unique opportunity to improve the EMW absorbing performance. Previously, Yang et al. reported the synthesis of CNTs through the pyrolysis of Zn−Fe−ZIFs with dicyandiamide serving as an inducer of graphitic structure.23 This finding inspired us to improve the EMW absorbing performance of MOF-derived absorbers by constructing magnetic NP-decorated CNTs composites. The high graphitic degree of the CNTs benefits the increase in conductivity, and meanwhile the CNTs with one-dimensional

2. EXPERIMENTAL SECTION Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%), 2-methylimidazole (mIM, 98%), methanol (99%), and ethanol (99%) were purchased from J&K Scientific Ltd. All reagents were of analytical grade and used without further purification. 2.1. Synthesis of ZIF-67 Precursor. ZIF-67 polyhedrons were prepared by a simple precipitation reaction according to a previous report.19 Briefly, 48 mmol of 2-methylimidazole was dissolved in a mixed solution containing 40 mL of methanol and 40 mL of ethanol. Twelve millimoles of Co(NO3)2·6H2O was dissolved in another mixed methanol−ethanol solution. The above two solutions were quickly mixed under vigorously stirring and followed by aging for 24 h at room temperature. Finally, the purple precipitate was collected by centrifugation, washed with ethanol, and dried overnight. 2.2. Synthesis of Porous CNTs/Co Composite. As illustrated in Scheme 1, the porous CNTs/Co composite was obtained by heating the above ZIF-67 precursor in a reducing atmosphere with Ar/H2 flow (95%/5% in volume). During the heating process, the as-prepared precursor was first preheated at 350 °C for 1.5 h and then calcined at the desirable temperature for different times, i.e. 500 °C/3.5 h, 700 °C/3.5 h, 900 °C/3.5, and 900 °C/5 h, respectively. The heating rate was maintained at 2 °C/min. The tube furnace was cooled to room temperature naturally. 2.3. 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) with energy-dispersive Xray spectroscopy (EDS) analysis and transmission electron microscopy (TEM, JEOL-JEM2100) were used to observe the morphology and microstructure of the samples. Raman spectra were measured by a 34687


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ACS Applied Materials & Interfaces Cryogenic matrix isolated Raman spectroscopic system (HORIBA Jobin Yvon, LabRAM ARAMIS) using a 532 nm laser. Nitrogen adsorption and desorption isotherms were obtained at 77 K on a Quantachrome surface area and pore size analyzer (BET, NOVA3200e). XPS analysis was performed on X-ray photoelectron spectrometer (VG ESCALab 220i-XL). Thermogravimetry (TG) curves of the CNTs/Co composite were recorded on a NETZSCH TG209F3 thermal gravimetric analyzer under air in the temperature range of 50−800 °C with a heating rate of 5 °C min−1. 2.4. Property Measurement. The relative complex permittivities and relative complex permeabilities of the CNTs/Co composites were measured using an Agilent N5230C network analyzer in the frequency range of 2−18 GHz. For testing, the CNTs/Co composite was uniformly dispersed in paraffin matrix with a weight percentage of 20%. The mixture was then pressed into coaxial rings with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm.

increases from 18.9 to 44.8 nm. In addition, the evolution of carbon structure can be deduced from the enlarged XRD patterns in the region of 15−30°. For the CNTs/Co composite calcined at 500 °C, only a hump centered at ∼20.5° is observed in the XRD curve, which is the characteristic of amorphous carbon. When the calcination temperature increases to 700 °C, a weak peak appears at ∼26.1° corresponding to the (002) plane of graphitic carbon (JCPDS no. 26-1076). The intensity of this peak apparently increases as the calcination temperature rises, suggesting the enhancement of graphitization degree of CNTs. Detailed structural characterizations of the carbon component in CNTs/Co composites were further carried out using Raman spectroscopy. As shown in Figure 2b, the Raman spectra present two typical peaks at ∼1340 cm−1 (D band) and 1575 cm−1 (G band), respectively.24−26 The D band is associated with disordered carbon or defective graphitic structure, while the G band is a characteristic feature of graphitic layers. Generally, the intensity ratio of D band to G band (ID /IG ) is calculated to evaluate the degree of graphitization.24 As the calcination temperature increases and heating time extends, the value of ID/IG gradually decreases from 1.27 to 0.78 due to the removal of defects and functional groups in CNTs at higher pyrolysis temperature. The enhanced degree of graphitization improves the conductivity of the composite, which benefits increasing the dielectric loss. TG analysis is an ideal tool to determine the content of Co NPs in the CNTs/Co composite. Figure 2c presents the representative TG curve of the CNTs/Co composite (900 °C/ 3.5 h) recorded in air. It is clear that the TG curve shows a slight decrease below 250 °C because of the mass loss of adsorbed water in the composite and the removal of functional groups on the graphitic carbon. In the temperature region from 250 °C to 340 °C, the weight of the composite increases, mainly originating from the oxidation of metallic Co NPs in air. Subsequently, a sharp drop of TG curve appears between 340 °C and 600 °C. In this temperature zone, the carbon component was decomposed to CO2 and meanwhile the oxidation of Co NPs continued. As confirmed by the XRD pattern in Figure S1, the final product after TG measurement was Co3O4 with negligible carbon residue. Therefore, the content of Co NPs in CNTs/Co composite can be estimated to be ∼43 wt % based on the remaining weight after combustion. Figure 3 presents the typical SEM images of the CNTs/Co composites after different pyrolysis processes. The CNTs/Co composites maintain the original dodecahedral morphology of ZIF-67 after pyrolysis at 500−900 °C, while the edge length of the dodecahedron gradually shrinks and the surface becomes rough as the pyrolysis temperature increases. High density of short and curved CNTs distribute on the dodecahedron surface. During the pyrolysis processes, ZIF-67 served as C and Co sources as well as the growth template. In the presence of H2, a large amount of metallic Co NPs was generated in situ and then catalyzed the growth of CNTs. At a low temperature of 500 °C, only tiny CNT clusters can be observed on the particle surface (Figure 3a−c), which are mainly attributed to the incomplete pyrolysis of ZIF-67 and the insufficient growth of CNTs. The increase of calcination temperature promotes the pyrolysis of ZIF-67, leading to the formation of longer CNTs with higher density (Figure 3d−f). These CNTs are intertwined, constructing an interconnected network. For the CNTs/Co composites calcined below 900 °C, Co NPs are not distinguishable on the dodecahedron surface, implying their

3. RESULTS AND DISCUSSION Figure 1a,b shows the morphology of the as-prepared ZIF-67 which exhibits rhombic dodecahedron with smooth surface.

Figure 1. (a, b) SEM images and (c) XRD patterns of ZIF-67 precursor.

The particles have a narrow size distribution with an average edge length of ∼1.57 μm. The crystal structure of ZIF-67 was characterized by XRD and is shown in Figure 1c. All the diffraction peaks can be precisely matched with the simulated ZIF-67 phase with zeolite-type structure, confirming the high purity of the product. Figure 2a shows the XRD patterns of the CNTs/Co composites calcined under different conditions. All the samples display three primary diffraction peaks at 44.2°, 51.6°, and 76.0°, which can be assigned to the (111), (200), and (220) planes of metallic Co with cubic structure (JCPDS no. 150806), respectively. As calcination temperature increases and heating time extends, the three diffraction peaks become sharper and more intense, indicating the improvement in crystallinity of Co NPs. Meanwhile, the full width at halfmaximum (fwhm) of the (111) peak obviously decreases. According to the Scherrer equation, the average size of Co NPs 34688


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ACS Applied Materials & Interfaces

Figure 2. (a) XRD patterns and (b) Raman spectra of different CNTs/Co composites. (c) TG curve of CNTs/Co composite calcined at 900 °C for 3.5 h.

Figure 3. SEM images of the CNTs/Co composites obtained under different pyrolysis temperatures and times: (a−c) 500 °C/3.5 h, (d−f) 700 °C/ 3.5 h, (g−i) 900 °C/3.5 h, and (j−l) 900 °C/5.0 h.

small size and good dispersity. When the temperature rises to 900 °C, numerous Co particles could be clearly observed in Figure 3g−l. These large-sized Co particles derive from the aggregation of tiny Co NPs at high temperature. In addition,

the EDS characterization was performed on the representative CNTs/Co composite calcined at 900 °C for 3.5 h. The EDS result in Figure S2 reveals that the Co content in the composite is ∼37 wt %, which is close to the result of TG measurement. 34689


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Figure 4. TEM images of the CNTs/Co composites obtained under different pyrolysis temperatures and times: (a, b) 500 °C/3.5 h, (c, d) 700 °C/ 3.5 h, (e, f) 900 °C/3.5 h, and (g, h) 900 °C/5.0 h.

Figure 5. XPS spectra of the CNTs/Co composite calcined at 900 °C for 3.5 h. (a) Survey spectrum, (b) deconvolution of the C1s spectrum, (c) deconvolution of the O1s spectrum, and (d) the Co2p spectrum.

with tiny size evenly disperse in the CNTs/Co composites obtained at 500 °C and 700 °C. The uniform distribution of Co NPs benefits from the periodic arrangement of Co atoms in crystalline ZIF-67. The magnified images in the inset of Figure 4b,d demonstrate that many Co NPs are encapsulated by carbon nanotubes and situated at the tip of the tubes, further confirming that Co NPs acted as catalysts for the growth of

The morphology and microstructure of the CNTs/Co composites were further investigated by TEM and are shown in Figure 4. Clearly, the CNTs/Co composites possess highly porous structure due to the pyrolysis of imidazole and the construction of CNTs. The pyrolysis temperature and time have significant effects on the particle size and distribution of Co NPs. As shown in Figure 4a−d, a high density of Co NPs 34690


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Figure 6. N2 adsorption−desorption isotherms of CNTs/Co composites: (a) 500 °C/3.5 h, (b) 700 °C/3.5 h, (c) 900 °C/3.5 h, and (d) 900 °C/5.0 h; (e) Pore-size distributions derived from the adsorption branch by the BJH method. (f) Variation of specific surface area and pore volume of CNTs/Co composites obtained at different pyrolysis temperatures and times.

Figure 7. Frequency dependences of (a) real parts of complex permittivities and complex permeabilities, and (b) imaginary parts of complex permittivities and complex permeabilities of CNTs/Co composites.

CNTs. When the pyrolysis temperature increases to 900 °C (Figure 4e,f), the Co particles evidently grow up through the aggregation of adjacent small-sized Co NPs. When the annealing time extends to 5.0 h (at 900 °C), more serious agglomeration of Co NPs occurs as illustrated in Figure 4g,h. Additionally, the diameter of the CNTs increases from ∼10 to 40 nm as the pyrolysis temperature increases from 500 °C to 900 °C. The surface composition and element valence of the CNTs/ Co composite calcined at 900 °C for 3.5 h were investigated by XPS. The survey scan in Figure 5a confirms the presence of C, O, and Co elements in the composite. The deconvolution of the C1s spectrum consists of three components that correspond to carbon atoms in different functional groups: C−C/CC (284.8 eV), C−O (285.6 eV), and CO (288.1 eV) (Figure 5b).17,27 The high-resolution O1s spectrum in

Figure 5c reveals three peaks centered at 530.2, 531.7, and 533.5 eV, which can be assigned to the Co−OH, Co−O, and C−O species, respectively. The Co2p XPS spectrum in Figure 5d shows two primary peaks at 779.9 (Co2p3/2) and 795.3 eV (Co2p1/2), along with satellite peaks (denoted as S) at the higher binding energy region.28 These features belong to the characteristics of Co2+.29 Because the CoO phase was not detected in the XRD measurement, we presume that only a thin surface layer of metallic Co NPs was oxidized in the environment.17,30 Nitrogen adsorption−desorption measurements were performed to determine the porous structure of the CNTs/Co composites. As shown in Figure 6a−d, the four samples display typical type-IV isotherms with a distinct hysteresis loop at a relative pressure range of 0.4−1.0, suggesting the mesoporous structure of the composites. The pore-size distribution 34691


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Figure 8. Plots of ε′−ε″ for CNTs/Co composites: (a) 500 °C/3.5 h, (b) 700 °C/3.5 h, (c) 900 °C/3.5 h, and (d) 900 °C/5.0 h in the frequency range of 2−18 GHz.

imaginary parts (ε″ and μ″) are connected with the energy dissipation or loss.32,33 As observed in Figure 7a, the ε′ of CNTs/Co composites shows a typical frequency dispersion behavior; that is, it gradually decreases with increasing frequency. This phenomenon can be attributed to the increased lagging of polarization with respect to electric-field change at high frequency.34 As calcination temperature increases from 500 °C to 900 °C, the ε′ values of the composites vary in the range of 9.3−6.0, 14.2− 10.1, and 12.3−7.6, respectively, and a sharp decrease of ε′ (5.5−5.2) occurs when the calcination time extends to 5.0 h. The similar variation trend of ε″ with calcination temperature/ time can be also observed in Figure 7b. Differently, the ε″ curves of the CNTs/Co composites intensively fluctuate in the whole frequency range with several obvious resonant peaks. The presence of resonant peaks suggests that multiple polarization relaxation processes occur in the composites under alternating EM field. In the gigahertz frequency region, dipole polarization and interfacial polarization can considerably influence the permittivity. On the basis of Debye relaxation theory, ε′ and ε″ follow the equation:35,36

according to the Barrett−Joyner−Halenda (BJH) method indicates that the pore size of the composites is mainly less than 10 nm (Figure 6e). As the pyrolysis temperature and time increase, the specific surface area of the samples displays volcano-type variation, while the total pore volume is maintained at ∼0.2 cm3/g. The effect of calcination temperature and time on surface area of the composite chiefly derive from two aspects. On one hand, the increase of pyrolysis temperature and time promotes the carbonization of precursor and the growth of CNTs, which lead to the increase of surface area; on the other hand, Co NPs readily grow and aggregate during the high-temperature and/or long-time calcination, which inversely decrease the surface area of the composite. Therefore, the CNTs/Co composite calcined at 700 °C for 3.5 h possesses the largest surface area of 374.9 m2/g. Generally, the porous material can be treated as an effective medium as a mixture of air and components of the material. Hence, the porous structure can decrease the effective permittivity based on Maxwell−Garnett theory, which benefits the impedance matching.31 Additionally, the porous structure is also favorable to decreasing the weight density of the absorber. The EMW absorbing properties of a material are highly dependent on its EM parameters, i.e. relative complex permittivity (εr = ε′ − jε″) and relative complex permeability (μr = μ′ − jμ″). We measured the EM parameters of a paraffin mixture containing 20 wt % of CNTs/Co composite via VNA in the frequency range of 2−18 GHz. Figure 7 shows the frequency dependences of complex permittivity and complex permeability of the CNTs/Co composites. The real parts of permittivity (ε′) and permeability (μ′) represent the storage capabilities of electric and magnetic energies, while the

(ε′ − ε∞)2 + (ε″)2 = (εs − ε∞)2

(1)

where εs and ε∞ are the static permittivity and relative permittivity at the high-frequency limit, respectively. Thus, it is deduced from the equation that the plot of ε′ versus ε″ would be a single semicircle, generally regarded as the Cole−Cole semicircle. Each semicircle represents one Debye relaxation process.37 Figure 8 shows the plots of ε′−ε″ for the CNTs/Co composites in the frequency range of 2−18 GHz. It is clear that 34692


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ACS Applied Materials & Interfaces the plots of CNTs/Co composites calcined at 500 °C and 700 °C display more distorted Cole−Cole semicircles than those of the other two composites calcined at 900 °C. For CNTs/Co composite, interfacial polarization could be generated at the interfaces between Co NPs and CNTs as a result of the accumulation of charges at the interfaces under alternating EM field. Especially, the small size and uniform distribution of Co NPs in the composites calcined at 500 °C and 700 °C lead to the formation of abundant heterointerfaces between Co NPs and CNTs, thus producing strong interfacial polarizations. Besides, previous Raman analysis demonstrate that the CNTs contain numerous defects and functional groups. On one hand, the defects in carbon network could act as polarization centers, which would generate polarization relaxation under altering EM field. On the other hand, the oxygen-containing chemical bonds such as CO and C−O on the surface of CNTs could produce electronic dipole polarization due to the different electronegativity between carbon atom and oxygen atom. The multiple dipole and interfacial polarizations correspond to the Cole−Cole semicircles. As calcination temperature rises, the defects and functional groups in CNTs may gradually disappear, and meanwhile the aggregation of Co NPs could result in a decreased area of heterogeneous interfaces between Co NPs and CNTs. Therefore, the number of Cole−Cole semicircles in CNTs/Co composites calcined at 900 °C decreases relative to the others calcined at lower temperature. In addition, it is seen from Figure S3 that the plot of ε′ versus ε″ for CNTs/Co calcined at 900 °C for 5.0 h obviously shifts to a lower value, which suggests a reduced contribution of Debye relaxation to dielectric loss. In addition to the polarization relaxation-induced dielectric loss, the conductive current can also contribute alot to the dielectric loss according to the equation:38 ε −ε σ ε″ = s 2∞2 ωτ + ωε0 1+ωτ (2)

Figure 9. Frequency dependences of tan δε and tan δμ of CNTs/Co composites.

presents the dependences of μ′ and μ″ of the CNTs/Co composites on frequency, respectively. It is found that the pyrolysis conditions have little effect on the μ′ of the composites, and all the μ′ curves vary in the range of 0.85− 1.05. However, the μ″ of the CNTs/Co composites seems sensitive to the pyrolysis temperature. Compared with other composites, the CNTs/Co calcined at 700 °C has the largest μ″ value and shows a strong resonant peak at ∼15 GHz. In general, the magnetic loss of a material in the gigahertz range mainly derives from natural ferromagnetic resonance, exchange resonance, and eddy current effect.39 The resonant peaks in μ″ curves in low-frequency and high-frequency regions can be assigned to the natural resonance and exchange resonance, respectively. Unlike the bulk Co single crystal which possesses the natural resonance frequency ( f r) at megahertz frequency, the f r of the CNTs/Co shifts to gigahertz frequency range. According to the equation f r = γHa/2π, where γ is gyromagnetic ratio, and Ha is anisotropy field, it is possible to tune the resonant frequency by changing the anisotropy field of cobalt which is associated with its size and morphology. When the size of Co particles decreases to nanometer scale, the anisotropy constant K of Co NPs, which is directly proportional to Ha (K = 2πMsHa), will remarkably increase due to the small size effect and the confinement effect.40 Based on Chen’s study, the K value of Co NPs with size of 15−50 nm is in the range of 5 × 106 to 30 × 106 erg/cm3, much higher than that of bulk Co (2.7 × 106 erg/cm3).41 Therefore, the large Ha leads to the shift of f r of the CNTs/Co to gigahertz range. The CNTs/Co composite calcined at 700 °C exhibits an enhanced resonant peak in highfrequency range exclusively. This phenomenon can be attributed to the small size of the Co NPs, which is close to the exchange length.42 However, the enhancement of exchange resonance does not occur in the CNTs/Co calcined at lower and higher temperature, which is presumably because of the lower crystallinity and larger particle size of the Co particles, respectively. The contribution of eddy current to magnetic loss can be evaluated by analyzing the dependence of μ″(μ′)−2 f−1 on frequency.43,44 If the μ″(μ′)−2 f−1 value keeps constant with the change of frequency, the eddy current loss will be the sole reason for the magnetic loss of CNTs/Co composites. As shown in Figure 10, however, the μ″(μ′)−2 f−1 values for all the samples intensively vary with increasing frequency. Thus, the

where σ is electrical conductivity, ω is angular frequency, and τ is relaxation time. The first term in the equation represents the contribution from the above-mentioned polarization relaxation, while the second term corresponds to the conductive loss which is directly proportional to the electrical conductivity of the composite. The CNTs with one-dimensional structure can build an interconnected conductive network spreading over the CNTs/Co dodecahedron for electron hopping and migrating. The above Raman spectra and SEM observation proved that high pyrolysis temperature significantly improved the graphitization degree of CNTs and promoted the growth of CNTs. In this case, the conductive loss of the CNTs/Co composites progressively increases as the pyrolysis temperature/time increases. In contrast to polarization loss which becomes weak with increasing temperature/time, the conductive loss exhibits the reverse variation trend. Hence, the overall dielectric loss should have the optimal value among the four samples. The dielectric-loss properties of the CNTs/Co composites are further evaluated by the loss tangent (tan δε = ε″/ε′) as displayed in Figure 9. It is clear that the composite calcined at 900 °C for 3.5 h possesses the highest tan δε value of ∼0.58 in high-frequency region, demonstrating the largest dielectric loss whereas the CNTs/Co calcined at 900 °C for 5.0 h has the lowest tan δε value in the whole frequency range. Because of the large amount of Co NPs in CNTs/Co composites, magnetic loss is definitely another crucial factor for determining the EMW absorbing capability. Figure 7a,b 34693


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ACS Applied Materials & Interfaces Z in = Zo(μr /εr)1/2 tanh[j(2πfd)(με )1/2 /c] r r

RL = 20 log

Z in − Z0 Z in + Z0

(3)

(4)

where Z0 is the characteristic impedance of free space, Zin is the input impedance of the absorber, ε0 is the permittivity of the free space, and c is the velocity of light, respectively. Figure 11 shows the reflection-loss properties of the CNTs/Co composites with coating thicknesses of 1.0−8.0 mm in the frequency range of 2−18 GHz. As shown in Figure 11a−d, the pyrolysis temperature and time have important effects on the EMW absorbing performances of the CNTs/Co composites. Except the composite calcined at 900 °C for 5.0 h, the other three CNTs/Co composites exhibit strong EMW absorbing capability with |RL| > 20 dB when the coating thickness exceeds 2.0 mm. With increasing pyrolysis temperature, the maximum RL value of the CNTs/Co increases while the matching thickness decreases, indicating the improvement of EMW absorbing properties. The CNTs/Co calcined at 500 °C possesses the maximum RL of −24.6 dB at 8.5 GHz with a thickness of 3.5 mm. As the calcination temperature rises to 700 °C, the maximum RL reaches −54.5 dB at 7.5 GHz with a thickness of 3.04 mm. Interestingly, the maximum RL of the composite obtained at 900 °C for 3.5 h is up to −60.4 dB at 15.0 GHz, and the effective absorption bandwidth (|RL| > 10 dB, corresponding to the 90% attenuation of EMW) covers from 12.8 to above 18.0 GHz. More importantly, the matching thickness of the absorber is as thin as 1.81 mm. The ideal EMW absorber is required to have strong absorption, wide absorbing bandwidth, small thickness, and be lightweight. To evaluate the absorbing performance of the CNTs/Co composite, we list the reflection-loss properties of various MOF-derived absorbers reported in previous references in Table 1.20−22,48,49 In comparison with other MOF-derived

Figure 10. Frequency dependences of μ″(μ′)−2 f−1 values for CNTs/ Co composites.

eddy current effect could be excluded, and the magnetic loss primarily originates from the natural ferromagnetic resonance and exchange resonance. Figure 9 shows the calculated magnetic loss tangent (tan δμ = μ″/μ′) of the CNTs/Co composites. Among the four samples, the CNTs/Co calcined at 700 °C possesses the largest magnetic loss with tan δμ of 0.17 in high-frequency range. It is obvious that the tan δε values of the CNTs/Co composites are much larger than the tan δμ values. This finding suggests that the dielectric loss plays a dominant role in the attenuation of EM energy. The EMW absorbing performance of the CNTs/Co composites can be evaluated by the reflection loss (RL), which is calculated using the measured relative complex permittivity and relative complex permeability on the basis of transmission line theory:45−47

Figure 11. Frequency dependences of reflection losses of CNTs/Co composites: (a) 500 °C/3.5 h, (b) 700 °C/3.5 h, (c) 900 °C/3.5 h, and (d) 900 °C/5.0 h. (e−h) Dependence of matching thickness (tm) on matching frequency ( f m) of CNTs/Co composites at wavelengths of λ/4 and 3λ/4. 34694


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ACS Applied Materials & Interfaces Table 1. Comparison of EMW Absorbing Properties of Various MOF-Derived Composite Absorbers absorption bandwidth

a

sample-Ta

filler loading (wt %)

matching thickness d (mm)

matching frequency f (GHz)

max. RL value (dB)

thickness d (mm)

|RL| ≥ 10 dB (GHz)

ref

Co/C-800 Co/C-500 Fe/C-650 C-ZIF-67/TiO2 Fe−Co/NPCb CNTs/Co-900

30 40 40 50 50 20

2.55 2.5 2.0 1.65 1.2 1.81

9.60 5.80 13.50 13.8 15 15.04

−39.6 −35.3 −20.3 −51.7 −21.7 −60.4

2.0 4.0 2.0 − 1.2 1.81

3.8 (10.7−14.5) 5.8 (8.4−14.2) 7.2 (10.8−18) − 5.8 (12.2−18) >5.2 (12.8−18)

20 21 22 48 49 this work

T: calcination temperature. bNPC: nanoporous carbon composite.

Figure 12. Frequency dependences of (a) relative input impedance (|Zin/Z0|) and (b) attenuation constant α of CNTs/Co composites.

Figure 13. Scheme of primary EMW attenuation processes in CNTs/Co composite absorber. (a) Multiple reflections and scatterings, (b) dipole polarization and interfacial polarization, and (c) conductive network constructed by CNTs.

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The excellent EMW absorbing performance of CNTs/Co composite should derive from the synergistic effects between Co NPs and CNTs, which lead to the good impedance matching and strong EMW attenuation in the interior of the absorber. Good impedance matching is the prerequisite condition for the EMW absorption, which requires that the value of relative input impedance |Zin/Z0| is equal or close to 1.0 to achieve zero-reflection at the front surface of the absorber.52 According to eq 3, the |Zin/Z0| values of the CNTs/ Co composites at a representative thickness of 2.0 mm were calculated and displayed in Figure 12a. Among the four absorbers, the CNTs/Co obtained at 900 °C for 3.5 h shows the optimal impedance matching, which implies that the incident EMW can enter this absorber with minimum reflection. After entering the absorber, the EM energy was then converted to heat energy by strong dielectric loss caused by the interfacial polarization, dipole polarization, migration, and hopping of electrons, and the magnetic loss from natural resonance and exchange resonance of Co NPs (as visually summarized in Figure 13b,c). In addition, the multiple reflections and scatterings among CNTs/Co particles can increase the propagation paths for EMW, which also benefits the attenuation of EMW (Figure 13a). To determine the EMW attenuation inside these absorbers, we calculated the attenuation constant (α) of the CNTs/Co composites:52,53

absorbers, the optimized CNTs/Co absorber obtained in this work exhibits superior comprehensive performance with extremely strong EMW absorbing capability, ultrathin thickness, and quite low filler loading. In view of these attractive advantages, it is believed that the CNTs/Co composite is a promising candidate for a high-performance EMW absorber. In addition, it is noticeable that the matching frequency ( f m) of CNTs/Co shifts toward the low-frequency region with increasing thickness. This phenomenon can be explained by the quarter-wavelength cancelation law, tm = nλ/4 = nc /(4fm |μr ||εr| ) (n = 1, 3, 5, ...), where tm is the matching thickness.50,51 When tm and f m satisfy this equation, the two reflected EMWs from the air−absorber interface and absorber−conductive background interface are out of phase by 180°, leading to an extinction of them on the air−absorber interface. In this case, the RL reaches the maximum value. Figure 11e−h displays a simulation of the tm (marked as tsim m ) versus f m for the CNTs/Co composites according to the above equation (green and orange curves). Simultaneously, the experimental matching thicknesses (texp m ) versus the peak frequency were directly extracted from the RL curves in Figure 11a−d and denoted as red asterisks and blue rhombus. Obviously, all the scatter symbols are exactly located around the λ/4 and 3λ/4 curves, which suggests that the relationship between the matching thickness and peak frequency for the EMW absorption of the CNTs/Co composites obey the quarter-wavelength matching conditions. α=

2 πf × c

(μ″ε″ − μ′ε′) +

(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2

As shown in Figure 12b, all the CNTs/Co composites have strong EMW attenuation capability in the high-frequency region. The α values of the composites over the whole frequency range increase in the following sequence: α(700 °C− 3.5 h) > α(900 °C−3.5 h) > α(500 °C−3.5 h) > α(900 °C−5.0 h). Hence, the best impedance matching together with large attenuation capability endows the CNTs/Co composite calcined at 900 °C for 3.5 h the best EMW absorbing performance.

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4. CONCLUSIONS

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(5)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12178. XRD pattern of the final product after TG measurement. SEM image and EDS result of CNTs/Co composite calcined at 900 °C for 3.5 h. Plots of ε′−ε″ for CNTs/ Co composites in the frequency range of 2−18 GHz (PDF)

In summary, we demonstrated a facile strategy for controllable synthesis of CNTs/Co composite using Co-based ZIF-67 as precursor. The CNTs/Co composite had a dodecahedron morphology with porous structure. The size and distribution of Co NPs as well as the graphitic degree and size of CNTs could be well tuned by the pyrolysis temperature and duration. Compared with previous MOF-derived absorbers, the CNTs/ Co composite calcined at 900 °C for 3.5 h exhibited superior comprehensive performances in the aspects of absorbing capability, layer thickness, and weight density. When the filler loading of CNTs/Co in paraffin matrix was as low as 20 wt %, the maximum RL value reached −60.4 dB at an ultrathin thickness of 1.81 mm. The excellent performance was attributed to the well-designed structure of the composite, and the synergistic effect between CNTs and Co NPs. This study provides a simple and well controllable method to synthesize lightweight, ultrathin, and highly efficient EMW absorber from metal−organic framework.

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.

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ACKNOWLEDGMENTS This project was financially supported by the National Natural Science Foundation of China (51102006 and 51271009). REFERENCES

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