In Situ Preparation of Cobalt Nanoparticles Decorated in N-Doped

Jun 11, 2018 - The solid Co/N-C NFs exhibit excellent electromagnetic wave absorbability, ... Carbonized Design of Hierarchical Porous Carbon/Fe3O4@Fe...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 22591−22601

In Situ Preparation of Cobalt Nanoparticles Decorated in N‑Doped Carbon Nanofibers as Excellent Electromagnetic Wave Absorbers Huihui Liu,† Yajing Li,† Mengwei Yuan,† Genban Sun,*,†,‡ Huifeng Li,† Shulan Ma,† Qingliang Liao,*,‡ and Yue Zhang*,‡

ACS Appl. Mater. Interfaces 2018.10:22591-22601. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/11/19. For personal use only.

†

Beijing Key Laboratory of Energy Conversion and Storage Materials and College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: Electrospinning and annealing methods are applied to prepare cobalt nanoparticles decorated in N-doped carbon nanofibers (Co/N-C NFs) with solid and macroporous structures. In detail, the nanocomposites are synthesized by carbonization of as-electrospun polyacrylonitrile/cobalt acetylacetonate nanofibers in an argon atmosphere. The solid Co/N-C NFs have lengths up to dozens of microns with an average diameter of ca. 500 nm and possess abundant cobalt nanoparticles on both the surface and within the fibers, and the cobalt nanoparticle size is about 20 nm. The macroporous Co/N-C NFs possess a hierarchical pore structure, and there are macropores (500 nm) and mesopores (2−50 nm) existing in this material. The saturation magnetization (Ms) and coercivity (Hc) of the solid Co/N-C NFs are 28.4 emu g−1 and 661 Oe, respectively, and those of the macroporous Co/N-C NFs are 23.3 emu g−1 and 580 Oe, respectively. The solid Co/N-C NFs exhibit excellent electromagnetic wave absorbability, and a minimum reflection loss (RL) value of −25.7 dB is achieved with a matching thickness of 2 mm for solid Co/N-C NFs when the filler loading is 5 wt %, and the effective bandwidth (RL ≤ −10 dB) is 4.3 GHz. Moreover, the effective microwave absorption can be achieved in the whole range of 1−18 GHz by adjusting the thickness of the sample layer and content of the dopant sample. KEYWORDS: nanocomposites, N-doped carbon nanofibers, hierarchical pore structure, microwave absorption, ferromagnetism (e.g., Fe, Co, Ni, and their alloys).18−20 Generally, the efficient complementarity between the complex permittivity and complex permeability (namely, impedance matching) cannot be achieved by using only magnetic materials.21−23 The combination of magnetic and dielectric absorbers has been considered as an effective way to reach the impedance matching,24−26 such as reduced graphene oxide−Fe3O4, Fe3O4−Fe and graphene,27,28 Fe3O4 and multiwalled carbon nanotube (MWCNT), 26 Fe 3 C and carbon nanotubes (CNTs),29 porous CNTs/cobalt nanoparticles,30 and Fe3O4 and carbon nanofibers (CNFs).31 As can be seen, most of the dielectric materials are graphene and carbon nanotubes, and the preparation method is relatively complicated. Taking one of them as an example, the CoNi/nitrogen-doped graphene hybrids reached the lowest reflection loss (RL) values (−22 dB) at 10 GHz with a matching thickness of only 2.0 mm.23 According to the authors, the CoNi/nitrogen-doped graphene

1. INTRODUCTION In recent decades, with the increasing use of electronic and wireless communication equipment, electromagnetic (EM) pollution has become more and more serious.1−3 The EM wave absorbing material can effectively absorb the EM wave and convert the electromagnetic energy into heat energy or other energy through various loss mechanisms of the material.4−7 Thus, a large number of research studies have focused on the design and manufacture of effective EM wave absorbing materials from both theoretical and experimental perspectives.8−10 The absorbing materials can be divided into dielectric loss materials and magnetic loss materials. Dielectric loss materials attenuate or absorb EM waves via ion polarization, electron polarization, molecular polarization, dipole polarization, and interfacial polarization.8,11−15 It is well known that the magnetic loss is mainly due to eddy current effects, natural resonance, and exchange resonance.16,17 Although the magnetic properties are mainly characterized by magnetic permeability, the sizes, crystal structures, and morphologies of the materials also have impacts. Common magnetic loss materials are metallic soft magnetic materials © 2018 American Chemical Society

Received: March 31, 2018 Accepted: June 11, 2018 Published: June 11, 2018 22591

DOI: 10.1021/acsami.8b05211 ACS Appl. Mater. Interfaces 2018, 10, 22591−22601

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Synthesis of Solid and Macroporous Co/N-C NFs

absorbing areas by adjusting the thickness of the sample layer and the content of the dopant sample.

hybrids show improved microwave absorption properties, compared with the single CoNi nanocrystals and graphene oxide. Among a variety of dielectric absorbers, carbon nanofibers have attracted considerable attention because of their lightweight, large specific surface area, good electrical conductivity, and low cost.31,32 For example, hollow carbon nanofibers consisting of nanometric Fe3O4 particles were successfully synthesized by electrospinning process and showed a lowest reflection loss of −44 dB for a nanocomposite with 7.5 wt % Fe3O4.31 Park and co-workers prepared Ni−Fecoated carbon nanofibers by electrospinning methods and tested their microwave absorbing properties. Their results showed that the materials have the absorbing bandwidth of 3.7 GHz (8.3−12.0) in the X-band with the content of 40 wt % (2.4 mm thickness), and this is about 26% increased result compared with the optimal CNF/epoxy composites.33 Few literature studies have reported on the cobalt/carbon nanofiber composites. On the other hand, compared with the bulk cobalt, cobalt nanoparticles have a small size, large specific surface area, and high surface atomic ratio, which are beneficial to interfacial polarization and multiple reflections, and thus, are favorable for microwave absorption.34,35 In this work, we are committed to achieve uniform dispersion of cobalt nanoparticles, meanwhile, they are decorated in N-doped carbon nanofibers. Electrospinning technology is one of the main techniques in preparing nanofibers due to the advantages of simple, high spinning material variety, low cost and process control. The preparation of full-band absorbing materials with a low doping amount is a huge challenge. In the present work, solid and macroporous Co/N-C NFs are controllably prepared on a large scale by electrospinning and annealing methods and have shown excellent magnetic properties, with the fiber length of dozens of micrometers, the diameter of about 500 nm, and the cobalt nanoparticle size of dozens of nanometers. The macroporous Co/N-C NFs possess a hierarchical pore structure with macropores (500 nm) and mesopores (2−50 nm) existing in this material. This kind of special structure is rarely seen in the reported literature.36−38 Therefore, this type of macroporous nanofibers used for electromagnetic wave absorption is novel and original. The simple electrospinning and annealing method used to prepare the macroporous Co/N-C NF materials can be extended to the preparation of other macroporous materials. The two nanomaterials with 5 wt % filler loading showed a superior EM absorption performance in the intermediate frequency and high frequency, and the materials with 10 wt % filler loading have achieved effective microwave absorption in the intermediate and low frequency. Therefore, these materials are promising for practical applications in different EM

2. EXPERIMENTAL SECTION 2.1. Raw Materials. Polyacrylonitrile (PAN, Mw = 150 000) was purchased from the Sigma-Aldrich Company. Cobalt acetylacetonate (97%) was purchased from Aladdin Reagent Co. Ltd. N,N-Dimethyl formamide (DMF, 99.5%), sodium hydroxide (NaOH, 96%), ammonium hydroxide (NH3·H2O, 25%), and absolute ethanol (CH3OH, 99.7%) were purchased from Beijing Chemical Reagent Co. Ltd. Ethyl orthosilicate (TEOS) was purchased from Tianjin Damao Chemical Reagent Co. Ltd. All reagents were of analytical grade and used without further purification. 2.2. Synthesis of Silica Microspheres. Detailed information about the synthesis of template (silica spheres) has been described by Stöber et al.39,40 Briefly, 7.5 mL (TEOS) and 75 mL of absolute ethanol were stirred for 20 min, then 10 mL of ammonia and 50 mL of anhydrous ethanol were added and efficiently stirred for 20 min. The uniform mixture was added dropwise to a three-necked flask, then placed in a water bath at 40 °C, and stirred for 3 h before washing with ethanol three times and drying at 80 °C. Finally, silica spheres with the particle size of about 500 nm were obtained. 2.3. Synthesis of Solid Co/N-C NFs. As shown in Scheme 1, cobalt acetylacetonate (1.2 g) and 1.0 g of PAN were added to 10 mL of DMF and the mixture solution was stirred for 24 h at room temperature. The solution was then electroporated with a syringe for spinning. The spinning conditions were as follows: positive high pressure of 15 kV, negative high pressure of −3 kV, receiving distance of 18 cm, and feed rate of 0.08 mL min−1. The precursor nanofibers were stabilized at 280 °C for 3 h followed by carbonization in a tube oven at 800 °C under an Ar atmosphere. 2.4. Synthesis of Macroporous Co/N-C NFs. The macroporous Co/N-C NFs were also prepared using the same synthesis route (Scheme 1) with the addition of 1.2 g of the as-prepared silica spheres (ca. 500 nm) to the spinning solution. The produced carbonized sample was treated with 2 mol L−1 of sodium hydroxide solution for 2 h to remove the template of silica spheres before washing with deionized water and drying at 80 °C for overnight. 2.5. Synthesis of Pure N-C NFs. The pure N-C NFs were prepared using the same synthesis route as solid Co/N-C NFs preparation without the use of cobalt acetylacetonate. 2.6. Characterization. The crystal structures of pure N-C NFs, solid and macroporous Co/N-C NFs were analyzed by powder X-ray diffraction (PXRD) using a Phillips X’pert ProMPD diffractometer at 40 mA and 40 kV with monochromatic Cu Kα radiation. The morphologies of the samples were studied by field-emission scanning electron microscopy (FE-SEM; acceleration voltage of 5 kV, S-8010; Hitachi). The microstructure, size distribution, and energy dispersive X-ray (EDX) element mapping were studied by high-resolution transmission electron microscopy (HRTEM) using JEM-2010, JEOL, and FEI Tecnai G2 F20 at an acceleration voltage of 200 kV. The Raman spectra were recorded from 1000 to 2000 cm−1 using a microscopic confocal Raman spectrometer with an excitation line of 532 nm. In addition, the Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface areas of the as-prepared 22592

DOI: 10.1021/acsami.8b05211 ACS Appl. Mater. Interfaces 2018, 10, 22591−22601

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ACS Applied Materials & Interfaces samples by a N2 adsorption apparatus (BET; JW-BK112). The pore size distribution was investigated with the Barrett−Joyner−Halenda method. The magnetic properties were characterized by the superconducting quantum interference device (SQUID-MPMS3). The EM wave absorption properties were studied through a vector network analyzer (HP-E8362B, Agilent) in the frequency range of 1− 18 GHz. The specimens applied for the EM measurements were prepared by homogeneous blending of the samples with paraffin wax (the weight ratio of the solid and macroporous Co/N-C NFs were 5 and 10 wt %, respectively. The weight ratio of the pure N-C NFs was 5 wt %), and then the mixture was pressed into a ring shape (an outer diameter of 7 mm and an inner diameter of 3.04 mm).

2b, a large scale of the elemental cobalt nanoparticles with a size of about 20 nm can be observed. The HRTEM image for solid Co/N-C NFs shown in Figure 2c confirms the unidirectional fringe pattern and the measured interlayer spacing of 0.210 nm, corresponding to the (111) plane of cobalt. The corresponding selected area electron diffraction (SAED) pattern in this region is shown in the inset of Figure 2c, which shows that the clear diffraction rings can be indexed to the (111) and (220) planes. Moreover, the elemental distribution maps (Figure 2d−f) verified the presence of elemental C, N, and Co in the solid Co/N-C NFs. From the TEM and HRTEM images of macroporous Co/N-C NFs shown in Figure 2h, we can see the macroporous carbon and cobalt in the composite nanomaterials, and the size of the cobalt particle is very small, only several nanometers in diameter. The HRTEM image of macroporous Co/N-C NFs shown in Figure 2i confirms the unidirectional fringe pattern and the measured interlayer spacing of 0.210 nm, corresponding to the (111) plane of cobalt. The corresponding selected area electron diffraction (SAED) pattern in this region is shown in the inset of Figure 2i, which shows that the clear diffraction rings can be indexed to the (111) and (220) planes, which are consistent with the XRD data. Moreover, the EDX mapping results (elemental distribution of C, N, and Co) further confirm that the cobalt nanoparticles decorated on both the surface and within the macroporous Co/N-C NFs (Figure 2j−l) is consistent with the above SEM and HRTEM results. The surface composition of pure N-C NFs, solid, and macroporous Co/N-C NFs were further confirmed by X-ray photoelectron spectroscopy (XPS) measurements. As shown in the survey XPS spectra (Figure 3a), the characteristic peaks of C 1s, O 1s, and N 1s can be clearly found in pure N-C NFs, whereas in the XPS spectra (Figure 3b,c), the main peaks observed in the survey scans of the two composites are C 1s, O 1s, N 1s, and Co 2p peaks centered at ca. 285, 530, 400, and 778−790 eV, respectively, indicating that the presence of Ndoped carbons and cobalt particles in the composites is consistent with the EDX mapping results (Figure 2), and the presence of O may be due to the sample surface being partly oxidized. The XPS spectrum of C 1s for two composites are shown in Figure 3d,e, the peaks at around 284.8, 286.0, and 290.3 eV can be indexed to C−C, C−O, and C−N, respectively.42 The N 1s spectra of solid and macroporous Co/N-C NFs are shown in Figure 3f,g, which displays four types of N species, pyridinic-N (398.2 eV), pyrrolic-N (399.3 eV), graphitic-N (400.1 and 401.1 eV), and oxidized-N (403.1 eV).43,44 As shown in Figure 3h,i, the peaks located at ca. 781 and 796 eV belong to the binding energy of Co 2p3/2 and Co 2p1/2. The peaks at 778.1 and 778.6 eV can be indexed to metallic Co for the composites. Meanwhile, the peaks at 781.0 and 786.0 eV (the satellite peak) can be indexed to the Co(II) oxidation state (Figure 3h).45 As shown in Figure 3i, the Co(II) oxidation state peaks are located at 780.7 and 788.0 eV (the satellite peak). The oxides were formed due to the surface being partly oxidized. The results implied the coexistence of metallic Co and Co2+ in the solid and macroporous Co/N-C NFs due to the partial oxidation of the Co surfaces. The specific surface area and pore size distribution of the solid and macroporous Co/N-C NFs were measured by a BET specific surface instrument. As shown in Figure 4a, the solid Co/N-C NFs exhibit the typical IV N2 adsorption isothermals and an H2-type hysteresis loop, demonstrating the presence of

3. RESULTS AND DISCUSSION The phase composition and the crystal structure of the asprepared samples were studied by XRD and Raman spectroscopy. Figure 1 show the XRD patterns and Raman spectra of

Figure 1. (a) XRD patterns and (b) Raman spectra of macroporous and solid Co/N-C NFs, and pure N-C NFs.

pure N-C NFs, solid and macroporous Co/N-C NFs. This shows that the solid and macroporous Co/N-C NFs have the same XRD patterns (Figure 1a). The wide diffraction peak appearing at 2θ = 26° is from the (111) crystal face of graphite carbon (JCPDS card no. 41-1487) with a layer spacing d = 0.337 nm. The results implied the existence of graphite carbon in the as-prepared samples. The peaks at 44.2, 51.5, and 75.8° were attributed to (111), (200), and (220) planes of the cobalt metal (JCPDS card no. 15-0806), respectively, indicating the presence of the cobalt element in the composites. It is well known that the Raman spectra usually exhibit two broad peaks around 1332 cm−1 known as the D peak and at 1580 cm−1 known as the G peak.41 The higher the ratio of the intensity value of D to G (ID/IG), the more defects exist. As shown in Figure 1b, the D and G bands of pure N-C NFs are 1347 and 1576 cm−1, however, the D and G bands of the solid Co/N-C NFs and macroporous Co/N-C NFs are 1347, 1595 and 1337, 1591 cm−1, respectively. We can also calculate the value of ID/ IG to be 1.03:1 for pure N-C NFs, 1.11:1 for solid Co/N-C NFs, and 1.05:1 for macroporous Co/N-C NFs. Compared with pure carbon powders, increased intensity ratios of D/G for the composites were observed, indicating the decrease in the average size of the sp2 domains in the two composites. Figure 2 presents the morphologies and structures of solid and macroporous Co/N-C NFs analyzed by SEM, TEM, and HRTEM measurements. The SEM images (Figure S1) shows that the lengths of these nanofibers can reach dozens of microns. The average diameters are 500 nm and 1 μm for solid and macroporous Co/N-C NFs, respectively (Figure 2a,g). From the TEM images of solid Co/N-C NFs shown in Figure 22593

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Figure 2. (a, g) SEM, (b, h) TEM, and (c, i) HRTEM images of solid and macroporous Co/N-C NFs, respectively; the insets of (c) and (i) are the selected area electron diffraction; EDX mapping images of (d−f) solid and (j−l) macroporous Co/N-C NFs.

ferromagnetic properties at 300 K. As can be seen from Figure 5a,b, the saturation magnetic susceptibility of the solid and macroporous Co/N-C NFs are 28.4 and 23.3 emu g−1, respectively, which is much lower than that of the bulk cobalt (162.5 emu g−1).35,50 In addition, the presence of a large amount of nonmagnetic carbon in the composite material and the defects on the surface of the material may also reduce the saturation susceptibility. The solid Co/N-C NFs has larger saturation magnetization than macroporous Co/N-C NFs, the difference in magnetization values is related to its higher cobalt content (Table 1). It is well known that the coercivity force (Hc) is a vital parameter for assessing the magnetic properties, and the absorbent with a high Hc value may cause a highfrequency resonance.51 Moreover, the prepared cobalt particles with the particle size of around 20 nm exhibit ferromagnetism, and are conducive to the improvement of the coercive force (Hc) of the material.52 Herein, the solid Co/N-C NFs with a higher cobalt content has a larger coercive force of 661 Oe at 300 K, compared with the macroporous Co/N-C NFs (580 Oe) (inset pictures in Figure 5a,b), which is significantly larger than the coercive force of the bulk material (10 Oe).53 The EM wave absorption mechanism can be summarized in the following equations39,41

mesoporous structures in the material, and the macroporous Co/N-C NFs also shows a mesoporous structure in the material. Thus, there are two types of pores (macropores and mesopores) existing in the macroporous Co/N-C NFs. The specific surface area of the solid Co/N-C NFs is 190.1 m2 g−1 and the pore sizes were mainly distributed at 3.7 nm (Figure 4b). In contrast to solid Co/N-C NFs, the specific surface area of the macroporous Co/N-C NFs is 188.2 m2 g−1 and the mesoporous pores are mainly distributed in a size of 4 nm (Figure 4c,d). As shown in Figure S2, the specific surface area of pure N-C NFs is 28.1 m2 g−1. The Co/N-C NF materials are much larger than that of the pure N-C NFs, showing that the cobalt content has a great influence on the specific surface area of these materials.46,47 As for the macroporous Co/N-C NFs, the cobalt content of this material is lower than the solid Co/ N-C NFs, and the 500 nm macropores it contains has no significant effect on the specific surface area of the material,48,49 so, the specific surface area of the macroporous Co/NC NFs is smaller than the solid Co/N-C NFs. Figure 5 shows the magnetic hysteresis loop of the solid and macroporous Co/N-C NFs at 300 K. The magnetic saturation of the applied magnetic field is about 10 kOe. As shown in Figure 5, the two materials show typical and strong 22594

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Figure 3. Survey scans for XPS spectra of (a) pure N-C NFs, (b) solid, and (c) macroporous Co/N-C NFs. (d, e) C 1s peaks, (f, g) N 1s peaks, (h, i) Co 2p peaks of solid and macroporous Co/N-C NFs, respectively.

Figure 4. (a, c) Nitrogen adsorption/desorption isotherms, (b, d) the pore size distributions of solid and macroporous Co/N-C NFs, respectively.

Zin = Z0 μr /εr tanh[j(2πfd /c) μr /εr ]

(1)

RL (dB) = 20 log|(Zin − Z0)/(Zin + Z0)|

(2)

the relative complex permeability and permittivity, respectively, Z0 is air impedance, Zin is the input impedance of the absorber, and RL is the reflection loss value. Generally, when the reflection loss value of the material is equal to −10 dB, 90% of the EM wave is absorbed by the absorbing material, indicating

where, f is the microwave frequency, d is the thickness of the absorber, c is the velocity of light (3 × 108 m s−1), μr and εr are 22595

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Figure 5. Magnetic hysteresis loops of (a) solid and (b) macroporous Co/N-C NFs at room temperature.

λ/4 model, the relationship between the absorber thickness (tm) and the peak frequency (f) can be given by the following equation55,56

Table 1. Co Atom Content Investigated by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) sample

Co (wt %)

solid Co/N-C NFs macroporous Co/N-C NFs

10.1 9.1

tm = nc /[(4f )(|εr||μr |)1/2 ]

(3)

Figure 7a′−d′ display the frequency-dependent λ/4 of the asprepared materials. Apparently, all of the samples obey the λ/4 model. It is obvious that the absorption band of the Co/N-C NF materials with 10 wt % filler loading shifted to a lower frequency range, compared with the materials with 5 wt % filler loading. By adjusting the absorber thickness and doping amount, the Co/N-C NFs can achieve better microwave absorption at low frequency, which are widely used for Wi-Fi devices, mobile phones, and radar systems.57,58 Therefore, it is important to develop absorbers with high efficiency at low frequency bands. Compared with the solid Co/N-C NFs, the macroporous Co/N-C NFs has the lower microwave absorption properties, this may be ascribed to the fact that macroporous Co/N-C NFs has a relatively low Co content as investigated by ICP-AES (Table 1). According to the relative paper, the prepared cobalt particles with a particle size of 20 nm exhibit ferromagnetism, and are conducive to the improvement of the coercive force (Hc) of the material.5,12 It is well known that Hc is a vital parameter for assessing the magnetic properties, and the absorbent with a high Hc value may cause a high-frequency resonance and an enhanced microwave absorption.51,53 Herein, the macroporous Co/N-C NFs with a lower coercive force have low microwave absorption properties. The minimum reflection loss (RL) value of the obtained solid Co/N-C NFs (−25.7 dB) and macroporous Co/N-C NFs (−14.5 dB) are not very prominent, but the filler loading of our samples are only 5 wt %, which are very small compared with other Co-based microwave absorbers. For example, the Co/AC balls with 30 wt % filler loading show a maximum reflection loss of −20.6 dB at 16.1 GHz. 59 The multiwalled carbon nanotubes (MWCNTs)/Co with 60 wt % filler loading reached the lowest RL values (−37 dB) with a matching thickness of 5.3 mm.60 The Co/C-PW composite with 50 wt % nanoparticles reached the lowest RL values of −43.4 dB.35 We also compared our samples with other absorbents that exhibit high-microwave absorption performances at low filler loadings in the latest reports. For example, Zhao and co-workers prepared the Ni/carbon foam, the maximum reflection loss is −45 dB at 13.3 GHz with Ni nanoparticle loading (42 wt %) and the filler content of 10 wt %.56 They also prepared the Ni/ carbon aerogels, the lowest RL value (−57 dB) is obtained at the Ni content (18.5 wt %) and the filler content of 10 wt %.54

that the absorbing material has the practical application value.6,45 To evaluate the microwave absorption properties of the as-prepared materials, all of the samples were mixed with paraffin wax and assembled into an electromagnetic wave absorber. Figure 6 shows the change of the reflection loss value of the samples versus frequency. As show in Figure 6a, for solid Co/N-C NFs with 5 wt % filler loading, when the thickness is 2, 3, and 4 mm, the strongest EM wave absorption is obtained at 14.3 GHz for −25.7 dB, 9.0 GHz for −18.2 dB, and 6.6 GHz for −13.6 dB. The lowest reflection loss value reaches −25.7 dB with the thickness of 2.0 mm at 14.3 GHz, and the effective bandwidth is 4.3 GHz (12.5−16.8 GHz) for the thickness of 2.0 mm. By adjusting the thickness of the sample layer, the effective microwave absorption can be achieved in whole frequency (1−18 GHz). However, when the filler loading is 10 wt %, the minimum RL of the material is −24.5 dB with the thickness of 3.0 mm, and the effective bandwidth is 3.3 GHz (9.9−13.2 GHz) for the thickness of 2.0 mm (Figure 6b). In Figure 6c, it can be clearly seen that macroporous Co/N-C NFs with 5 wt % filler loading has a minimum RL of −14.5 dB at 16.6 GHz with the thickness of 6.0 mm. The broad effective absorption bandwidth is 2.9 GHz (10.7−13.8 GHz) while the thickness is 3.0 mm, and the material has a good microwave absorption in the high frequency and intermediate frequency. But for macroporous Co/N-C NFs with 10 wt % filler loading, the lowest reflection loss value reaches −11.8 dB with the thickness of 3.0 mm at 5.9 GHz, and the effective bandwidth is 1.7 GHz (8.8−10.5 GHz) for the thickness of 2.0 mm (Figure 6d), and the effective microwave absorption can be achieved in the intermediate frequency and low frequency. The RL values for the pure N-C NFs with 5 wt % filler loading (Figure 6e) cannot reach −10 dB within the thickness range of 2.0−6.0 mm, and the minimum RL is −5.7 dB obtained at 18.0 GHz with a thickness of 6.0 mm, which is meaningless for practical applications. The absorption frequency of the minimum loss value moves towards the low frequency and multiple absorption bands (when the thickness is more than 4 mm, there are two absorption bands in the 1.0−18.0 GHz range) appeared as the absorber thickness of the samples increased. This is consistent with many literature findings.36,54 To better understand this, the frequency dependence of quarter-wavelength (λ/4) of the samples was investigated (Figure 7). In the 22596

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Figure 6. Reflection loss in the frequency range of 1−18 GHz for solid Co/N-C NFs with a filler loading of (a) 5 wt %, (b) 10 wt %; macroporous Co/N-C NFs with a filler loading of (c) 5 wt %, (d) 10 wt %; (e) pure N-C NFs with 5 wt % filler loading.

Compared with the two materials, the as-prepared Co/N-C NFs have a smaller metal loading (10.1 and 9.1 wt %) and a smaller doping amount (5 wt %). Besides, the solid Co/N-C NFs achieve a wide effective bandwidth (4.3 GHz) at 2.0 mm.

This wide bandwidth is very important, as the corresponding absorption efficiency is within the range of 90−100%. The enhanced EM absorption mechanism for the Co/N-C NFs can be speculated as the following aspects. First, the appropriate 22597

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Figure 7. (a−d) Reflection loss in the frequency range of 1−18 GHz, (a′−d′) simulations of the absorber thickness (tm) versus peak frequency ( f) of the solid Co/N-C NFs with 5, 10 wt % filler loading, macroporous Co/N-C NFs with 5, 10 wt % filler loading under l/4 conditions, respectively.

Figure 8. Electromagnetic parameters of solid and macroporous Co/N-C NFs with 5 and 10 wt % filler loading in the frequency range of 1−18 GHz: (a) real part of permittivity, (b) imaginary part of permittivity, (c) real part of permeability, and (d) imaginary part of permeability.

basically the same as ε′. As for solid and macroporous Co/N-C NFs, as the content of the sample increases in paraffin, the value of ε′ and ε″ increases. This can be interpreted that the complex permittivity of a composite is strongly affected by the concentration of the dielectric material.61 According to the formula ε″ = σ/(2πε0 f),62 the higher conductivity value leads to a larger complex permittivity. For the solid Co/N-C NFs, due to the high concentration of the metallic Co and dielectric carbon when the filler loading is 10 wt %, the conductivity increased with the decreasing paraffin wax content. So, the absorber with 10 wt % filler loading have a higher ε′ value and ε″ value than the absorber with 5 wt % filler loading. In addition, solid Co/N-C NFs has a better microwave absorption performance when compared with macroporous Co/N-C NFs, and we find that its ε′ value and ε″ value is in a moderate value, which is consistent with the findings of Du and co-workers, in which they found that a moderate complex permittivity was more beneficial to the microwave absorption performance of an absorber.63,64 As shown in Figure 8c,d, the

conductivity of the N-doped carbon nanofibers endows the absorbers with medium conductivity loss at the low filler loading (5 wt %). Second, the magnetic properties of the cobalt nanoparticles can effectively improve the impedance matching between the complex permeability and permittivity, further to contribute EM absorption.54 It is exciting that these Co/N-C NFs display a relatively good microwave absorption property with the smallest fiber loadings. From the above analysis we can conclude that the composite materials in the whole frequency (1−18 GHz) have effective microwave absorption. To explore the possible mechanism of the microwave absorption of the above-mentioned solid and macroporous Co/N-C NFs with 5 and 10 wt % filler loading, the complex relative permittivity and permeability of the four samples with a thickness of 2 mm were measured, respectively. As shown in Figure 8a,b, the ε′ values of the four samples are decreasing in the range of 1.0−18.0 GHz, decrease from 10.4 to 7.6, 17.7 to 10.6, 7.1 to 5.0 and 27.2 to 13.5, respectively, the trend of ε″ is 22598

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Figure 9. Frequency dependences of the dielectric loss tangent (a) and magnetic loss tangent (b) for the solid and macroporous Co/N-C NFs with 5, 10 wt % filler loading.

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μ′ and μ″ of the four samples are relatively stable in the range of 1−18 GHz, and we supposed that the complex permittivity is the main factor affecting the microwave absorbing properties of the four samples. In addition, the dielectric loss tangents (tan dε = ε″/ε′) and magnetic loss tangents (tan dμ = μ″/μ′) of the solid and macroporous Co/N-C NFs with 5, 10 wt % filler loading at different frequencies are shown in Figure 9. It can be seen from Figure 9a that for the solid Co/N-C NFs, the frequency dependence of tan dε exhibits a variation trend very similar to the frequency dependence of the ε″ curves (see Figure 8b) in the range of 6−18 GHz. In Figure 9b, the frequency dependences of tan dμ are quite similar to the frequency dependences of μ″ curves (see Figure 8d) in the range of 1−18 GHz, demonstrating that both dielectric loss and magnetic loss contribute to the microwave absorption of this sample, that is, achieved impedance matching. However, macroporous Co/NC NFs did not show a similar result, and some results show that a proper impedance matching, which enhances the microwave absorption, can be achieved via adjusting the composition of the absorber.11,34,41 Therefore we proposed that the proper impedance matching is accomplished solid Co/ N-C NFs in the composite. Compared with macroporous Co/ N-C NFs, the solid Co/N-C NFs has the lowest reflection loss value and broader effective bandwidth, this may be ascribed to the fact that the solid Co/N-C NFs has the higher Co content investigated by ICP-AES, which results in a larger coercive force. Besides, the proper impedance matching is also important for its excellent microwave absorption properties.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05211.

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SEM images of solid Co/N-C NFs and macroporous Co/N-C NFs; nitrogen adsorption/desorption isotherms and the pore size distributions of pure N-C NFs (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.S.). *E-mail: [email protected] (Q.L.). *E-mail: [email protected] (Y.Z.). ORCID

Genban Sun: 0000-0001-9005-8123 Shulan Ma: 0000-0002-8326-3134 Yue Zhang: 0000-0001-7772-3280 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Overseas Expertise Introduction Projects for Discipline Innovation (B14003), the National Key Research and Development Program of China (No. 2016YFA0202701), the National Natural Science Foundation of China (Nos. 21771024, 21421003, 51672026, 51527802, 51372020, and 51232001), Beijing Municipal Science & Technology Commission (No. Z161100002116027).

4. CONCLUSIONS The solid and macroporous Co/N-C NFs have been successfully synthesized by electrospinning and simple annealing methods. The two nanocomposites show excellent ferromagnetic properties at room temperature. The solid Co/ N-C NFs exhibit the superior microwave absorption performance in the range of 1−18 GHz. It has the minimum RL value of −25.7 dB at a thickness of 2 mm with only 5 wt % filler loading, and the absorption bandwidth for RL ≤ −10 dB is as large as 4.3 GHz. The excellent microwave absorption properties can be ascribed to the proper impedance matching via adjusting the composition of the absorber and higher Co content. We proposed that these Co/N-C NFs are promising for practical applications as high-performance microwave absorbers. The macroporous Co/N-C NFs possess a special hierarchical pore structure and a small amount of magnetic metal cobalt loading. It is expected to become lightweight, less metal-loaded, and full-band microwave absorbing materials.

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