Fe3C Nanoparticles Embedded in Nitrogen-Doped

Jan 4, 2019 - Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, Heilongjiang Univers...
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Pea-like Fe/Fe3C Nanoparticles Embedded in Nitrogen-Doped Carbon Nanotubes with Tunable Dielectric/Magnetic Loss and Efficient Electromagnetic Absorption Zhan Xu,† Yunchen Du,*,†,‡ Dawei Liu,† Yahui Wang,† Wenjie Ma,† Ying Wang,† Ping Xu,† and Xijiang Han*,†

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†

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China ‡ Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, PR China S Supporting Information *

ABSTRACT: One-dimensional microstructure has been regarded as one of the most desirable configurations for magnetic carbon-based microwave absorbing materials (MAMs). Herein, pea-like Fe/Fe3C nanoparticles embedded in nitrogen-doped carbon nanotubes (Fe/Fe3C@NCNTs) are successfully prepared through a direct pyrolysis of the mixture of FeCl3·6H2O and melamine under inert atmosphere. The chemical composition and microstructural feature of these Fe/ Fe3C@NCNTs composites are highly dependent on the pyrolysis temperature. As a result, their electromagnetic properties can be also manipulated, where dielectric loss gradually decreases with the increasing pyrolysis temperature and magnetic loss presents a reverse variation trend. When the pyrolysis temperature reaches 600 °C, the as-obtained composite, Fe/Fe3C@NCNTs-600 can perform a maximum reflection loss of −46.0 dB at 3.6 GHz with a thickness of 4.97 mm and a qualified bandwidth of 14.8 GHz with the integrated thickness from 1.00 to 5.00 mm. It is very interesting that the microwave absorption performance of this new kind of composites is not so susceptible to the pyrolysis temperature as those common magnetic carbon-based MAMs because there is an effective balance between dielectric loss and magnetic loss, which accounts for a very stable attenuation ability when the pyrolysis temperature range changes from 600 to 700 °C. These favorable characteristics, including low-cost raw materials, easy preparation, and stable performance, may render Fe/Fe3C@NCNTs composites as a novel kind of MAMs in the future. KEYWORDS: Fe/Fe3C nanoparticles, carbon nanotubes, one-dimensional structure, dielectric loss, magnetic loss, microwave absorption

1. INTRODUCTION The extensive application of electromagnetic (EM) technology in different fields has brought new social problems, because the radiation of EM waves not only disturbs the electronic communication and increases the risk in information security but also directly threatens the physical health of human being. These adverse effects feature EM pollution as the fourth pollution source after water pollution, air pollution, and noise pollution, and meanwhile, its positive precaution also becomes a hot topic in the scientific community.1−4 Shielding and absorption are two popular strategies that may weaken/resist the negative impact from those undesirable EM waves.5−7 Particularly, EM absorption has gained more and more attention in recent years because of its low reflection and high consumption toward incident EM waves, which offers an advanced and sustainable alternative in the conversion of EM energy.8,9 The efficiency of EM absorption is highly dependent on the intrinsic EM properties of microwave-absorbing © XXXX American Chemical Society

materials (MAMs). Although some single-component MAMs, for example, carbon, magnetic metals, metal oxides, and conductive polymers, can work for the attenuation of incident EM waves to some extent,9−12 there is still a mainstream trend in constructing all kinds of composites to enhance absorption efficiency through profitable synergistic effects between different components.13−15 Among various composites, magnetic carbon-based MAMs are always considered as the most promising candidates for their relatively low density, improved chemical stability, and diverse forms, as well as compatible/ tunable magnetic loss and dielectric loss.16−19 For example, Wang et al. manipulated EM properties of Fe3O4/N-dope graphene composites through tailoring the growth of Fe3O4 clusters and realized powerful absorption, broad bandwidth Received: November 2, 2018 Accepted: January 3, 2019 Published: January 4, 2019 A

DOI: 10.1021/acsami.8b19201 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces response, and selective-frequency characteristic;20 Quan et al. reported a facile method for CoxNiy/C composites by pyrolyzing Co2+/Ni2+-containing alginate gels, where the feeding Co2+/Ni2+ ratio was confirmed to be quite important for the reflection loss characteristics of final composites.21 Our group previously designed Fe/C nanocubes with Prussian blue as the self-sacrificing template and found that the uniform distribution of Fe nanoparticles in carbon frameworks could account for multiple relaxation processes and upgrade the microwave absorption performance significantly.22 Recent advances in the fabrication of MAMs indicate that rational design on the microstructure may also play a crucial role in reinforcing their absorption efficiency.23−27 Onedimensional MAMs, with high aspect ratio, special shape anisotropy, and geometric effect, have demonstrated their unique superiority for the dissipation of EM waves through electron/phonon vibration along the axial direction.28−30 For example, Yang et al. revealed that BaTiO3 nanowires could act as isotropic antennas and create some discontinuous networks, which would strengthen the permeation of EM waves into numerous conductive stings and induce the dissipative current responsible for energy attenuation.31 Similar functions of onedimensional microstructure were also identified in Ni nanofibers, Co/CoO nanofibers, and MoO2/C nanowires.32−34 These facts greatly promote the development of onedimensional magnetic carbon-based composites as highperformance MAMs, thanks to the positive combination of functional and structural advantages.30,35 A popular strategy is to decorate some carbon substances, for example, carbon nanotubes (CNTs) and carbon nanofibers, with different magnetic components, while such magnetic carbon-based composites easily suffer from insufficient loading and serious agglomeration of magnetic particles, resulting in poor chemical homogeneity and degraded microwave absorption performance.36 The embedment of magnetic particles inside CNTs may be a desirable configuration to restrain their agglomeration.37,38 More importantly, Che et al. ever pointed out that when magnetic metal Fe was confined inside CNTs, the involved multiple domain wall displacements along the axial direction of CNTs would consolidate natural magnetic resonance and magnetic loss effectively.39 It is unfortunate that the loading of abundant magnetic particles inside CNTs cannot be achieved easily. Although a chemical vapor deposition (CVD) technique could encapsulate crystalline magnetic nanoparticles with CNTs, the generalization and application of such a method were still restricted by its complex procedures and low yield.37,40 Therefore, it still remains a challenge to construct one-dimension magnetic carbon-based MAMs in a simple way. In previous studies, it has been demonstrated that when melamine was utilized as carbon precursor, one-dimensional CNTs could be easily generated in the presence of catalytic Fe particles through high-temperature pyrolysis.41,42 Herein, we obtain one-dimensional composites with Fe/Fe3C nanoparticles embedded in CNTs (Fe/Fe3C@NCNTs) through direct pyrolysis of the mixture of FeCl3·6H2O and melamine under inert atmosphere. On the one hand, homogeneous-like Fe3+ can be transformed into small Fe nanoparticles, which can fulfill their catalytic effectiveness and produce uniform CNTs; on the other hand, highly dispersed Fe nanoparticles are very helpful for their embedment inside CNTs rather than agglomeration outside CNTs. Compares with the CVD technique, this preparation precept is more simple and

efficient. When their EM properties and microwave absorption performances are comprehensively, it can be found that the novel configuration of Fe/Fe3C@NCNTs not only suppresses the agglomeration of magnetic particles and consequent skin effect but also provides good chemical homogeneity and sufficient interfaces between Fe/Fe3C nanoparticles and CNTs. More interestingly, there is a benign complementarity between dielectric loss and magnetic loss in Fe/Fe3C@ NCNTs, and thus, their attenuation ability is less sensitive to the pyrolysis temperature and they can maintain their efficient EM absorption in the temperature range of 600−700 °C. Compared with those reported magnetic carbon-based MAMs, Fe/Fe3C@NCNTs composites herein have some fascinating advantages, for example, ordered microstructure, stable absorption efficiency, easy preparation, and low cost, which will render them as promising candidates for EM pollution precaution in the future.

2. EXPERIMENT 2.1. Synthesis of the Fe/Fe3C@NCNTs. Fe/Fe3C@ NCNTs composites were fabricated via a facile pyrolysis process. In a typical procedure, 3.0 g of FeCl3·6H2O was dissolved in 20.0 mL ethanol, and then, 3.0 g of melamine was added under continuous stirring to form a suspension. The resultant suspension was then placed into an oven at 80 °C for 48 h to produce tawny solid. The dried powder was pyrolyzed in a quartz tube furnace at designated temperature for 6 h under a nitrogen flow of 50.0 mL min−1. The final products were named Fe/Fe3C@NCNTs-X, where X referred to the pyrolysis temperature. 2.2. Characterization. Scanning electron microscopy (SEM) images and energy-dispersive spectroscopy (EDS) images were obtained on a Quanta 200S (FEI), and the samples were mounted on aluminum studs by using a adhesive graphite tape and sputter-coated with gold before analysis. Transmission electron microscopy (TEM) images were obtained on a Tecnai F20 operating at an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) data were obtained on a Rigaku D/Max RC X-ray diffractometer with a Cu Kα radiation source (45.0 kV, 50.0 mA). The XPS spectrum was recorded in a PHI 5000 VersaProbe with an Al Kα X-ray source operating at 150 W. Raman spectra were recorded on a confocal Raman spectroscopic system (Renishaw, InVia) using a 633 nm laser. A thermogravimetric analysis (SDT Q600 TGA) was used to measure the composite in the temperature range 50−800 °C at a heating rate of 10 °C min−1. The magnetic hysteresis loops were made using a LakeShore 7404 (LakeShore, USA) vibrating sample magnetometer (VSM). A four-probe resistivity meter (RTS-9, Guangzhou 4-probes Technology Co., Ltd, China) was utilized to measure the electronic conductivity of these samples, where a mixture containing 50 wt % of composites and 50 wt % of wax was pressed into a disc with a diameter of 1.5 cm and a thickness of 0.3 mm before the measurements. An Agilent N5234A vector network analyzer (Agilent, USA) was applied to determine the relative permeability and permittivity in the frequency range of 2.0−18.0 GHz for the calculation of reflection loss. A sample containing 30 wt % of the obtained composite was pressed into a ring with an outer diameter of 7.00 mm, an inner diameter of 3.00 mm, and a thickness of 2.00 mm for microwave measurement in which paraffin wax was used as the binder. B

DOI: 10.1021/acsami.8b19201 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. SEM images and statistical diameter distribution of Fe/Fe3C@NCNTs-600 (a,d), Fe/Fe3C@NCNTs-700 (b,e), and Fe/Fe3C@NCNTs800 (c,f).

3. RESULTS AND DISCUSSION During the preparative process, we find that the weight ratio of melamine to FeCl3·6H2O may greatly affect the morphology of the final composites. As shown in Figure S1, we manipulate their weight ratio from 2:1 to 1:1, 2:3, and 1:2 in the mixture. Although CNTs can be detected in all composites after hightemperature pyrolysis (700 °C), most composites contain a large amount of impurities (Figure S1a,c,d). Only in the case of proper melamine/FeCl3·6H2O ratio (1:1), the composite will be fully consisted of CNTs (Figure S1b). When melamine is excessive, insufficient catalyst cannot realize complete transformation of melamine into CNTs, and thus, there will be a large amount of disordered carbon species in the composites. When FeCl3·6H2O is excessive, the obtained CNTs cannot accommodate all Fe particles, resulting in the agglomeration of Fe particles outside CNTs. By considering poor chemical homogeneity of these disordered composites, we fix the weight ratio of melamine to FeCl3·6H2O at 1:1 in the following study. SEM images reveal that the pyrolysis temperature plays another important role in the morphology evolution of Fe/ Fe3C@NCNTs. When the pyrolysis temperature is designated at 500 °C, the resultant product displays a completely disordered microstructure (Figure S2), indicating that current temperature is not enough for the formation of CNTs. The desirable one-dimensional microstructure can be identified until the pyrolysis temperature reaches 600 °C (Figure 1a−c). Melamine is employed as a carbon source in this study, and thus, these CNTs are essentially doped with a small amount of N atoms, as supported by the results of EDS (Figure S3). Moreover, one can also find that the diameter of these CNTs gradually increases with the pyrolysis temperature, and the average values for Fe/Fe3C@NCNTs-600, Fe/Fe3C@NCNTs700, and Fe/Fe3C@NCNTs-800 are 98, 154, and 174 nm (Figure 1d−f), respectively. The change in the diameter can be associated with the formation mechanism of these CNTs. There is a consensus that the decomposition product of melamine may preferentially be on the surface of Fe nanoparticles and then transformed into CNTs with their catalytic effect.41,42 High pyrolysis temperature is undoubtedly favorable for the growth of Fe nanoparticles, and thus, the average diameter of CNTs will become large with the pyrolysis temperature. Of note is that such a unique microstructure will not be destroyed unless the pyrolysis temperature reaches 1000 °C (Figure S4). A lot of existing references indicate that the optimum pyrolysis temperature for carbon-based MAMs will be in the range of 600−800 °C,22,33 and thus, we pay much

more attention to the structure−activity relationship of Fe/ Fe3C@NCNTs-600, Fe/Fe3C@NCNTs-700, and Fe/Fe3C@ NCNTs-800. TEM images confirm that almost all Fe species are encapsulated inside the melamine-derived NCNTs, and no nanoparticles are attached on the external surface of these NCNTs (Figure 2a−c). It is very interesting that these

Figure 2. TEM images of Fe/Fe3C@NCNTs-600 (a), Fe/Fe3C@ NCNTs-700 (b), Fe/Fe3C@NCNTs-800 (c), and high-resolution TEM images of Fe/Fe3C@NCNTs-600 (d,e).

nanoparticles are discontinuously dispersed inside NCNTs, resulting in a pea-like microstructure. A closer inspection indicates that the lattice fringe of internal nanoparticles is 0.20 nm (Figure 2d), which corresponds to the (110) plane of metal Fe with a body-centered cubic (bcc) structure. In addition, there is a continuous carbon coating on the surface of these nanoparticles, and both the carbon coating and the NCNTs wall have identical interplanar spacing (0.34 nm, Figure 2e), suggesting that they are composed of graphitic carbon. Therefore, the specific microstructure of these pea-like composites can be depicted as core−shell nanoparticles with Fe-based nanoparticles and graphitic carbon coating embedded inside graphitic NCNTs. This unique configuration will be significantly helpful to improve the microwave absorption performance of these composites, as it not only provides sufficient heterogeneous interfaces between Fe-based nanoC

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pyrolysis temperature. In addition, one can also find a small hump at 2θ ≈ 26° for Fe/Fe3C@NCNTs-600, Fe/Fe3C@ NCNTs-700, and Fe/Fe3C@NCNTs-800, which confirms the graphitization of carbon components and is in good agreement with the result of high-resolution TEM image (Figure 2e). The XPS spectrum is conducted to study the elemental composition on the surface of the Fe/Fe3C@NCNTs-X composites surface. It is obvious that the survey spectra record the signals of Fe, O, C, and N (Figure 4). The weak signal of

particles and carbon components but also restrains the formation of Fe-based conductive networks that may account for strong reflection toward incident EM waves and reduce magnetic loss through possible skin effect.38,43 Figure 3 shows XRD patterns of Fe/Fe3C@NCNTs obtained at different pyrolysis temperature. As observed,

Figure 4. XPS spectra of Fe3C@NCNTs composites at different temperatures.

Fe element was ascribed to the fact that most Fe/Fe3C nanoparticles were closely wrapped by graphitic carbon shells and incapable of being effectively analyzed because of the limited detection depth of XPS spectra, and O element mainly comes from residual gas molecules and trace surface groups. The presence of N element is consistent with the result of EDS (Figure S3). To avoid the influences of Fe and O elements, we utilize the atomic ratio of N/C to evaluate the change of chemical composition on their surface. It is found that N/C ratios for Fe/Fe3C@NCNTs-600, Fe/Fe3C@NCNTs-700, and Fe/Fe3C@NCNTs-800 are 0.089, 0.079, and 0.060, respectively. The doping of N element is always linked with defect sites in the carbon matrix,45 and thus, the gradual decrease in N/C ratio means that high pyrolysis temperature is favorable for good crystallization of CNTs in these composites. However, for MAMs, defect sites can act as polarization centers under applied EM field to reinforce energy dissipation,1,45 and thus, there may be strong dielectric loss in Fe/Fe3C@NCNTs-600 owing to its high N doping concentration. TG curves are utilized to determine the specific content of carbon components in Fe/Fe3C@NCNTs-600, Fe/Fe3C@ NCNTs-700, and Fe/Fe3C@NCNTs-800. As shown in Figure 5, all three samples undergo a negligible weight loss between 50 and 300 °C, implying their good thermal stability. The onset of weight increase for these three samples is found at about 300 °C, which can be attributed to the oxidation of Fe/ Fe3C nanoparticles. The weight decrease for Fe/Fe3C@ NCNTs-600, Fe/Fe 3 C@NCNTs-700, and Fe/Fe 3 C@ NCNTs-800 starts at about 396, 428, and 442 °C, and ends at about 610, 650, and 670 °C, respectively. The change in temperature range for weight loss implies that high pyrolysis temperature can further improve the stability. The complete combustion of carbon components and the oxidation of Fe/ Fe3C nanoparticles in air at high temperature (>700 °C) make the final product be only Fe2O3, and thus, the relative carbon content in these composites can be calculated in terms of the following equation by combining the weight loss and the results of XRD refinement

Figure 3. XRD patterns of Fe/Fe3C@NCNTs composites at different temperatures.

there are no any characteristic diffraction peaks that can be indexed to metal iron or iron oxides in Fe/Fe3C@NCNTs500, indicating that the carbothermal reduction of Fe3+ has not been triggered under such conditions. This phenomenon also explains that the disordered microstructure of Fe/Fe3C@ NCNTs-500 (Figure S2) should be attributed to the absence of catalytic metal Fe nanoparticles. When the pyrolysis temperature reaches 600 °C, Fe/Fe3C@NCNTs-600 gives a set of distinguishable peaks, where those at 44.6°, 65.0°, and 82.3° can be precisely matched with the (110), (200), and (211) planes of bcc-Fe (JCPDS 06-0696), and the others at 37.6°, 37.7°, 42.9°, 43.7°, 44.6°, 45.0°, 45.9°, and 49.1° belong to the typical pattern of Fe3C crystals (JCPDS 35-0772). It is well known that Fe3C is usually commensal with metal Fe from carbothermal reduction due to the occupation of C atoms in the octahedral voids of bcc-Fe crystals.22,36,44 However, the peak of (031) plane in Fe3C (the strongest peak) is very close to that of (110) plane in bcc-Fe, and thus, high-resolution TEM image cannot distinguish them exactly (Figure 2d). The similar properties of Fe and Fe3C make it difficult to determine their relative contents by some conventional methods, and thus, the software of TOPAS 4.2 (Bruker Corporation) is applied for the Rietveld quantitative phase analysis. The peak shape is fitted by the pseudo-Voigt function, and Obs and Cal are the experimental spectrum and calculated contour, respectively. With the assistance of XRD refinement, the weight ratio of Fe to Fe3C in Fe/Fe3C@NCNTs-600 is deduced as 25.7:74.3. The relative intensities of characteristic peaks of Fe3C will decrease significantly in Fe/Fe3C@NCNTs700 and Fe/Fe3C@NCNTs-800, and the corresponding weight ratios are 64.2:35.8 and 82.7:17.3, respectively. These results suggest that the chemical composition of magnetic components in these composites can be easily regulated by the D

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0.91 to 0.57, validating that the graphitization degree of carbon components is gradually improved with the increasing pyrolysis temperature. Figure 6b exhibits the magnetic hysteresis loops of Fe/Fe3 C@NCNTs-600, Fe/Fe3 C@ NCNTs-700, and Fe/Fe3C@NCNTs-800 at room temperature, and the corresponding saturation magnetization (MS) values are 73.6, 99.0, and 116.9 emu/g, respectively. The incremental MS values can be explained from two aspects. On the one hand, carbon is nonferromagnetic, and thus, low carbon content at high pyrolysis temperature (Figure 5) is favorable for a large MS value; on the other hand, Fe3C has a much weaker magnetic response than metal Fe, and thus, the diminishing Fe3C content (Figure 3) will also promote MS value. Differently, coercivity (HC) provides a negatively correlated relationship with the pyrolysis temperature, where the specific HC values for Fe/Fe3C@NCNTs-600, Fe/Fe3C@ NCNTs-700, and Fe/Fe3C@NCNTs-800 are 359.3, 161.7, and 104.7 Oe, respectively (Figure S5). The gradual decrease in HC values may be attributed to the size effect of magnetic particles, and large particles beyond the critical size (14 nm) will produce small HC value.48 According to the prediction of initial permeability, large MS and low HC are greatly favorable for strong magnetic loss,40,49 and thus, the order of magnetic loss may be Fe/Fe3C@NCNTs-800 > Fe/Fe3C@NCNTs-700 > Fe/Fe3C@NCNTs-600. In the field of microwave absorption, relative complex permittivity (εr = εr′ − jεr″) and complex permeability (μr = μ′ − jμ″) are usually considered as two extremely important parameters that are related to reflection loss characteristics, and their corresponding tangents, tan δe = ε″r /ε′r and tan δm = μ″r /μ′r , may act as the indexes for dielectric loss and magnetic loss, respectively.50−52 Figure 7a,b show real parts (εr′) and imaginary parts (εr″) of relative complex permittivity of Fe/ Fe3C@NCNTs composites in the frequency range of 2.0−18.0 GHz. Fe/Fe3C@NCNTs-600 displays the largest ε′r and ε″r values among these three composites, whose εr′ decreases from 18.9 at 2.0 GHz to 14.3 at 18.0 GHz, and εr″ decreases from 5.6 at 2.0 GHz to 3.9 at 11.4 GHz and then increases to 5.9 at 18.0 GHz. Compared with Fe/Fe3C@NCNTs-600, both ε′r and ε″r values of Fe/Fe3C@NCNTs-700 present moderate decreases in the studied frequency range, while they keep similar variation trends. It is very interesting that Fe/Fe3C@NCNTs800 generates the lowest ε′r and ε″r values, and especially, its ε′r values are almost frequency-independent and fluctuate between 1.2 and 1.7. By deducing dielectric tangents (Figure 7c), one can find that the dielectric loss ability of Fe/Fe3C@ NCNTs-600 is a little higher than that of Fe/Fe3C@NCNTs700 but much higher than that of Fe/Fe3C@NCNTs-800. As we know, dielectric loss comes from polarization loss and conductivity loss.53,54 Dipole orientation and interfacial polarizations are two typical modes in common MAMs that can work for the attenuation of incident EM waves in the frequency range of 2.0−18.0 GHz because the other two modes, electronic and ionic polarizations, are too fast (103−106 GHz) and essentially elastic with negligible energy consumption.46,55 Actually, the concave curve of ε″r values is usually linked with the signal of polarization relaxation in previous studies.56,57 The measurements on a four-probe resistivity meter suggest that the conductivities of the mixture of paraffin wax with Fe/Fe3C@NCNTs-600, Fe/Fe3C@ NCNTs-700, and Fe/Fe3C@NCNTs-800 are 4.49 × 10−4, 3.88 × 10−4, and 4.32 × 10−4 S/cm, respectively. By considering the small size effect, the conductivity of Fe/Fe3C

Figure 5. TGA curves of Fe/Fe3C@NCNTs composites under air atmosphere.

ij y jja + 3MFe zzzijj 100 − wt % carbon yzz = wt % R × 2MFe jj zj z j MFe3C zzk a+1 MFe2O3 { k {

(1)

where a is the weight ratio of Fe to Fe3C, wt % R is the remaining percentage after complete combustion, and wt % carbon is the weight percentage of carbon components (including NCNTs and carbon shells around Fe/Fe3C nanoparticles). As deduced, the relative content of carbon components in Fe/Fe3C@NCNTs-600, Fe/Fe3C@NCNTs700, and Fe/Fe3C@NCNTs-800 can be finally verified as 49.0, 38.1, and 36.1%, respectively. Apart from the relative content of carbon components, their graphitization degree will also play an important role in the dielectric loss of carbon-based composites, where high graphitization can reinforce the conductivity loss of carbon components and further lead to a significant enhancement in total dielectric loss. 3,43 To discern the difference in graphitization degree, Raman spectra of Fe/Fe3C@NCNTs600, Fe/Fe3C@NCNTs-700, and Fe/Fe3C@NCNTs-800 are presented in Figure 6a. It is clear that all three composites

Figure 6. Raman spectra (a) and field-dependent magnetization curves (b) of Fe/Fe3C@NCNTs composites from different pyrolysis temperature.

display two distinguishable peaks assigned to D-band and Gband at about 1330 and 1580 cm−1, respectively. Generally, Dband refers to the disordered arrangement of carbon atoms and G-band is produced by the stretching vibration of carbon atoms with sp2 hybridization (not only in graphite).18 TEM images and XRD patterns have confirmed that carbon frameworks in these composites were highly crystalline (Figures 2 and 3), and thus, G-band herein should be preferentially associated with graphitic sp2 sites, which means that we can employ the intensity ratio of D-band to G-band (ID/IG) to evaluate the change in graphitization degree.46,47 It can be found that from Fe/Fe3C@NCNTs-600 to Fe/Fe3C@ NCNTs-800, the values of ID/IG monotonously decrease from E

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Figure 7. Frequency-dependent εr′ (a), εr″ (b), tan δe (c), μr′ (d), μr″ (e), and tan δm (f) of Fe/Fe3C@NCNTs composites from different pyrolysis temperature.

are not as large as those in ε′r and ε″r values, it can be still observed that both μ′r and μ″r values will ascend gradually with the pyrolysis temperature. The magnetic tangents further reveal that magnetic loss ability of these composites has the same order as their complex permeability (Figure 7f), which is in good agreement with the prediction of VSM results. It is widely accepted that magnetic loss is generally composed of domain wall resonance, hysteresis loss, eddy current effect, and natural ferromagnetic resonance.59,60 In most cases, the contribution from domain wall resonance and hysteresis loss is rather limited in gigahertz range because they mainly become active in weak field or low frequency.16,61 If natural ferromagnetic resonance works for magnetic energy consumption of incident EM waves, it will produce a highly frequency-dependent C0 values (C0 = μ″r /(μ′r )2f, f refers to the frequency), and otherwise, C0 values may be approximate constant in some frequency intervals.61,62 After plotting C0 curves (Figure S6), one can find that C0 values of these three composites always keep changing in the frequency range of 2.0−18.0 GHz, indicating that eddy current effect and natural ferromagnetic resonance are together responsible for the process of magnetic loss. Notably, Fe/Fe3C@NCNTs-600 exhibits negative μr″ and magnetic tangent values in the frequency range of 12.9−18.0 GHz. This strange phenomenon has been observed in some carbon-based composites and taken as a radiation of magnetic energy.63−65 According to Maxwell equations, the motion of charges in carbon frameworks with good conductivity may induce an alternating electric field under the applied EM field, which can generate an internal magnetic field and release some magnetic energy in turn. Herein, Fe/Fe3C@NCNTs-600 has relatively high carbon content and weak magnetic response, and the intrinsic magnetic loss cannot counteract the radiation of magnetic energy in high-frequency range, resulting in the negative μ″r and magnetic tangent values. On the basis of the results of EM parameters (Figure 7), it can be concluded that pyrolysis temperature has quite opposite effects on dielectric loss and magnetic loss of Fe/Fe3C@ NCNTs composites. To evaluate the total loss ability, attenuation constant (α), a concept features the amplitude attenuation of EM waves, is further calculated using the following equation58,66

nanoparticles will not be the same order of magnitude as highly crystalline carbon frameworks, which indicates that the conductivity of these composites may be resulted from the balance between carbon content and graphitization degree. As revealed by Raman spectra and TG curves (Figures 6a and 5), Fe/Fe3C@NCNTs-600 has a relatively low graphitization degree, while its carbon content is much higher than those of Fe/Fe3C@NCNTs-700 and Fe/Fe3C@NCNTs-800. Obviously, the carbon content dominates the conductivity of Fe/ Fe3C@NCNTs-600. Although Fe/Fe3C@NCNTs-700 and Fe/Fe3C@NCNTs-800 have similar carbon content and distinguishable graphitization degree, Fe/Fe3C@NCNTs-800 only produces a slight enhancement in conductivity. This is because Fe/Fe3C@NCNTs-800 has larger nanotube size than Fe/Fe3C@NCNTs-700, which means that its packing density in the wax matrix will be less than that of Fe/Fe3C@NCNTs700 under the same loading condition. Dipole orientation polarization is usually resulted from the frustrated reorientation of dipoles under high-frequency EM field.51,58 In many cases, bound charges in defect sites and residual groups in MAMs can act as dipoles to achieve dipole orientation polarization.16,17 XPS spectra detect temperature-dependent N atoms in these composites (Figure 4), implying that the defect content may be gradually decreased at high pyrolysis temperature. Therefore, it is believable that the order of dipole orientation polarization should be Fe/Fe3C@NCNTs-600 > Fe/Fe3C@ NCNTs-700 > Fe/Fe3C@NCNTs-800. Moreover, the incremental nanotube size from Fe/Fe3C@NCNTs-600 to Fe/ Fe3C@NCNTs-800 induces larger Fe/Fe3C nanoparticles and less nanotubes, which more or less depress the contribution from interfacial polarization. In view of these facts, it can be concluded that large relative complex permittivity and strong dielectric loss of Fe/Fe3C@NCNTs-600 should be benefited from its high conductivity and positive polarization loss, including both dipole orientation polarization and interfacial polarization. For the other two composites, the contribution of polarization relaxation in Fe/Fe3C@NCNTs-700 offsets its shortcomings in conductivity effectively, leading to larger complex permittivity than Fe/Fe3C@NCNTs-800. Figure 7d,e shows real parts (μ′r ) and imaginary parts (μ″r ) of relative complex permeability of Fe/Fe3C@NCNTs composites in the frequency range of 2.0−18.0 GHz. Although the differences in μ′r and μ″r values among these three composites F

DOI: 10.1021/acsami.8b19201 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces α= ×

loss curves at some certain thicknesses are provided in Figure S7. For intuitive comparison, the maximum values of the ordinates in these maps are artificially limited to −30.0 dB. Fe/ Fe3C@NCNTs-600 exhibits a remarkable microwave absorption performance, and its maximum reflection loss value even reach up to −46.0 dB at 3.6 GHz with an absorber thickness of 4.97 mm (the corresponding reflection loss curve can be found in Figure S8). Fe/Fe3C@NCNTs-700 and Fe/Fe3C@NCNTs800 performs much inferior reflection loss intensities, and their maximum values are only −23.9 dB (3.9 GHz, 5.00 mm) and −19.3 dB (15.2 GHz, 4.98 mm), respectively. However, it has to point out that the real gap in absorption efficiency is not as large as that in reflection loss value because −20.0 dB of reflection loss means 99.0% of absorption efficiency and −40.0 dB equals 99.99% of absorption efficiency. Actually, the qualified absorption toward incident EM waves is usually defined as −10.0 dB (90.0% of absorption efficiency), and the qualified bandwidth over −10.0 dB is a more important evaluation indicator in microwave absorption.67,68 In this regard, Fe/Fe3C@NCNTs-600 and Fe/Fe3C@NCNTs-700 can be taken as excellent MAMs with similar effectiveness, because their qualified bandwidths are 14.8 GHz (from 3.2 to 18.0 GHz) and 14.6 GHz (from 3.4 to 18.0 GHz), respectively. It is unfortunate that Fe/Fe3C@NCNTs-800 only creates a qualified bandwidth at 3.8 GHz (from 14.2 to 18.0 GHz). It is well known that the loading content of MAMs in paraffin wax may affect their microwave absorption performance remarkably,5,11 and thus, we also investigate the reflection loss characteristics of Fe/Fe3C@NCNTs-600, Fe/Fe3C@NCNTs700, and Fe/Fe3C@NCNTs-800 at different loading contents. As shown in Figure S9, when the loading content is decreased to 10 wt %, all these composites have quite small relative complex permittivity and complex permeability, resulting in their negligible attenuation toward incident EM waves. With the increasing loading content to 50 wt %, their EM parameters will be visibly enhanced, while they still present poor microwave absorption (Figure S10). This is because the gap between relative complex permittivity and complex permeability is too large in such a situation, leading to mismatched characteristic impedance and strong reflection of EM waves at the front surface of MAMs.12,19 Therefore, the optimum loading content for these composites should be 30 wt %. We further compare the qualified bandwidths of Fe/Fe3C@ NCNTs-600 and Fe/Fe3C@NCNTs-700 with those of some typical one-dimensional MAMs ever reported (Table 1).28−31,34,38,46,50,63,69 As observed, Fe/Fe3C@NCNTs-600 and Fe/Fe3C@NCNTs-700 indeed demonstrate their promising prospects as novel MAMs. It is worth noting that most carbon-based MAMs in previous studies were highly susceptible to the pyrolysis temperature, and a small temperature span could affect their reflection loss characteristics greatly.22,54,67 However, compared with those

2 πf c (μr″εr″ − μr′εr′) +

(μr″εr″ − μr′εr′)2 + (μr′εr″ + μr″εr′)2 (2)

As shown in Figure 8, Fe/Fe3C@NCNTs-600 and Fe/ Fe3C@NCNTs-700 give very similar α values in the frequency

Figure 8. Frequency-dependent α values of Fe/Fe3C@NCNTs composites from different pyrolysis temperature.

range of 2.0−18.0 GHz, whereas there is an obvious gap in α value between Fe/Fe 3 C@NCNTs-600 and Fe/Fe 3 C@ NCNTs-800, especially in high-frequency range. These results indicate that the total loss ability of Fe/Fe3C@NCNTs composites is less impacted when the pyrolysis temperature changes from 600 to 700 °C because as compared with Fe/ Fe3C@NCNTs-600, the increase in magnetic loss of Fe/ Fe3C@NCNTs-700 makes up its corresponding decrease in dielectric loss. Although Fe/Fe3C@NCNTs-800 also displays an increase in magnetic loss to some extent, the decrease in dielectric loss is much more remarkable, resulting in a total degradation in attenuation ability. The microwave absorption performance of MAMs is usually estimated by their reflection loss (RL) characteristics, which can be calculated using the following equation RL (dB) = 20 log

Zin − 1 Zin + 1

(3)

Therein, Zin is the normalized input impedance and given by ÄÅ ÉÑ ÅÅ i 2π y ÑÑ μr ÑÑ tanhÅÅÅjjjj zzzfd με Zin = r ÑÑ r εr ÅÅÇ k c { ÑÖ (4) where c is the velocity of EM waves in free space and d is the thickness of MAMs layer. Figure 9 shows three-dimensional reflection loss maps of Fe/Fe3C@NCNTs composites in the frequency range of 2.0−18.0 GHz with integrating absorber thicknesses from 1.00 to 5.00 mm, and some typical reflection

Figure 9. Reflection loss maps of Fe/Fe3C@NCNTs-600 (a), Fe/Fe3C@NCNTs-700 (b), and Fe/Fe3C@NCNTs-800 (c) in the frequency range of 2.0−18.0 GHz. G

DOI: 10.1021/acsami.8b19201 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Table 1. Microwave Absorbing Properties of Various One-Dimensional MAMs in Previous References and in This Work absorbers

integrated thickness (mm)

maximum RL (frequency, thickness)

bandwidth over −10 dB (range, GHz)

refs

Ni nanochains MnO2@Fe nanorods FeNi/C nanofibers BaTiO3 nanowires MoO2/C nanowires FexCyNz/N-CNT SiCw@C nanorods MOFs/SiC nanowires Fe/C nanofibers Fe/SiC fibers Fe/Fe3C@NCNTs-600 Fe/Fe3C@NCNTs-700

2.00−5.00 1.50−3.50 1.00−5.00 2.50−5.00 1.00−5.00 2.00−5.00 0.50−5.00 2.00−6.00 1.10−5.00 1.00−4.00 1.00−5.00 1.00−5.00

−19.6 dB (16.7 GHz, 4.00 mm) −15.0 dB (12.1 GHz, 3.00 mm) −24.8 dB (9.4 GHz, 2.70 mm) −24.6 dB (9.0 GHz, 3.00 mm) −47.6 dB (11.1 GHz, 2.00 mm) −25.1 dB (4.5 GHz, 4.00 mm) −61.2 dB (9.0 GHz, 4.16 mm) −47.0 dB (9.3 GHz, 3.00 mm) −67.5 dB (16.6 GHz, 1.30 mm) −46.3 dB (6.4 GHz, 2.25 mm) −46.0 dB (3.6 GHz, 4.97 mm) −23.9 dB (3.9 GHz, 5.00 mm)

7.5 (4.0−7.5 & 13.4−17.4) 2.2 (10.8−13.0) 14.2 (3.8−18.0) 7.0 (4.9−11.9) 14.2 (3.8−18.0) 8.2 (3.4−11.6) 12.8 (5.2−18.0) 14.3 (3.7−18.0) 15.1 (2.9−18.0) 6.9 (3.2−8.0 & 8.8−10.9) 14.8 (3.2−18.0) 14.5 (3.5−18.0)

28 29 30 31 34 38 46 50 63 69 herein herein

counterparts, Fe/Fe3C@NCNTs composites harvest insusceptible microwave absorption efficiencies in the temperature range of 600−700 °C, which benefits from the positive balance between dielectric loss and magnetic loss. The unique structure−performance relationship and simple preparative method, as well as good microwave absorption performance, may be considerably helpful for the popularization of Fe/ Fe3C@NCNTs composites in industrial applications.

4. CONCLUSION With the mixture of FeCl3·6H2O and melamine as the precursor, a series of magnetic carbon-based composites have been prepared through a simple pyrolysis under inert atmosphere. Once the pyrolysis temperature exceeds 600 °C, metal Fe nanoparticles derived from high-temperature carbothermal reduction may act as the catalytic sites for the formation of CNTs, resulting in a unique pea-like configuration. The uniform dispersion of metal Fe and commensal Fe3C nanoparticles not only suppresses the undesirable skin effect in conventional bulky Fe particles but also creates sufficient heterogeneous interfaces that may favor the dissipation of incident EM energy. High pyrolysis temperature can reduce the relative contents of carbon components and Fe3C species and further lead to a degraded dielectric loss and an enhanced magnetic loss. The composite Fe/Fe3C@ NCNTs-600 pyrolyzed at 600 °C displays a good microwave absorption performance, whose maximum reflection loss is −46.0 dB at 3.6 GHz with a thickness of 4.97 mm and qualified bandwidth is 14.8 GHz (from 1.0 to 5.0 mm). It is very interesting that Fe/Fe3C@NCNTs-700 harvests a very similar qualified bandwidth. The EM analysis reveals that in the pyrolysis temperature range of 600−700 °C, the enhancement in magnetic loss can make up the decrease in dielectric loss effectively, and thus, the total attenuation ability can be well maintained.

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NCNTs composites; frequency-dependent C0 values of Fe/Fe3C@NCNTs composites from different pyrolysis temperature in the frequency range 2−18 GHz; twodimensional typical reflection loss maps of the Fe/ Fe3C@NCNTs-600 (a), Fe/Fe3C@NCNTs-700 (b), and Fe/Fe3C@NCNTs-800 (c); reflection loss curves of Fe/Fe3C@NCNTs-600 with the absorber thickness of 4.97 mm; complex permittivity, complex permeability, and reflection loss of Fe/Fe3C@NCNTs-600 (a−c), Fe/ Fe3C@NCNTs-700 (d−f) and Fe/Fe3C@NCNTs-800 (g−i) with 10 wt % loadings; and complex permittivity, complex permeability, and reflection loss of Fe/Fe3C@ NCNTs-600 (a−c), Fe/Fe3C@NCNTs-700 (d−f), and Fe/Fe3C@NCNTs-800 (g−i) with 50 wt % loadings (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.D.). *E-mail: [email protected]. Phone: +86-(451)-86418750. Fax: +86-(451)-86413702 (X.H.). ORCID

Yunchen Du: 0000-0002-5157-2290 Ping Xu: 0000-0002-1516-4986 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the financial support from Natural Science Foundation of China (21676065 and 21776053) and Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b19201. SEM images of pyrolysis products of precursors with different FeCl3·6H2O contents; SEM image of Fe/ Fe 3 C@NCNTs-500; EDS spectra of Fe/Fe 3 C@ NCNTs-600 (a), Fe/Fe3C@NCNTs-700 (b), and Fe/ Fe3C@NCNTs-800 (c); SEM images of Fe/Fe3C@ NCNTs-900 (a) and Fe/Fe3C@NCNTs-1000 (b); local enlargement magnetic hysteresis loops of Fe/Fe3C@ H

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DOI: 10.1021/acsami.8b19201 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX