Rational Design of Superior Microwave Shielding Composites

Research Article pubs.acs.org/journal/ascecg

Rational Design of Superior Microwave Shielding Composites Employing Synergy of Encapsulating Character of Alginate Hydrogels and Task-Specific Components (Ni NPs, Fe3O4/CNTs) Nan Zhou,† Qingda An,*,† Zuoyi Xiao,† Shangru Zhai,*,† and Zhan Shi‡ †

Faculty of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China

‡

S Supporting Information *

ABSTRACT: Three-dimensional (3D) porous magnetic carbonaceous bead-like (MCB) composites (SA-Ni-(Fe3O4/CNTs)-X; SA stands for sodium alginate, CNTs means carbon nanotubes and X means Fe3O4/CNTs percentage) have been successfully fabricated through a facile one-step encapsulation process, followed by carbonization at 600 °C in nitrogen atmosphere. These magnetic nickel nanocrystals and Fe3O4/CNTs were uniformly dispersed into the entire porous carbon matrix without aggregation; and various techniques like scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and vector network analyzer (VNA) were conducted to demonstrate the morphology, structure, chemical content and electromagnetic parameters of the SA-Ni(Fe3O4/CNTs)-X composites. The effect of the Fe3O4/CNTs molar percentage on the electromagnetic parameters and electromagnetic wave absorbing properties of the SA-Ni-(Fe3O4/CNTs)-X composites were investigated in the frequency range of 2−18 GHz. It was proven that the composites would be superior lightweight microwave absorbers when the Fe3O4/CNTs molar percentages were relatively high. When a 25% Fe3O4/CNTs molar percentage was used, it could lead to a maximum reflection loss (RL) of −32 dB at 10.8 GHz even with a thickness of 2 mm; the effective microwave absorption bandwidth (RL < −10) reached 3.2 GHz (from 9.3 to 12.5 GHz). The superior electromagnetic wave absorbing properties could be assigned to the high attenuation, Debye relaxation, electric polarization, interfacial polarization and high conductivity of the task-specific components. It is thus considered that the newly synthesized SA-Ni-(Fe3O4/CNTs)-X composite could be a promising candidate for novel types of lightweight and high-performance electromagnetic wave absorbing materials with great potentiality in practice. KEYWORDS: Alginate, Encapsulation, Metal NPs, CNTs, Electromagnetic wave absorption

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INTRODUCTION An electromagnetic (EM) wave absorber, a kind of functional material, can absorb electromagnetic waves validly and transform electromagnetic energy into thermal energy or dispel microwave by interference. During the past decade, for the purpose of the military and commercial application, an increasing number of materials have been investigated as microwave absorbers, leading to extensive research on the development of microwave absorbers with lightweight, thin coating thickness, broad effective frequency and strong absorption properties and so on.1,2 Comparatively, in the past several years, 3D porous composites in the field of electromagnetic absorption have attracted particular attention, considering these matrices with a porous structure that would typically provide new interfacial effects, with complementary and synergetic behaviors between the host and guest species.3 Currently, the extensive studies on 3D porous composites in the field of electromagnetic waves © 2017 American Chemical Society

absorption can be largely associated with its special heterogeneous interfaces. The presence of the heterogeneous interface can greatly improve the electromagnetic wave absorption performance. For instance, as a result of multiinterfacial polarizations, the dielectric loss performance could be enhanced by heterogeneous interfaces.4 In addition, the heterogeneous interface can also provide more active sites to promote the scattering and reflection of electromagnetic waves. The electromagnetic waves would be scattered and absorbed back and forth inside the composites to further achieve the purpose of weaken electromagnetic waves.5 Therefore, the 3D porous composites would be hopeful to have a major breakthrough in the field of electromagnetic wave absorption. As was demonstrated, Zhang and co-workers synthesized Fe− Received: March 7, 2017 Revised: April 27, 2017 Published: May 15, 2017 5394

DOI: 10.1021/acssuschemeng.7b00711 ACS Sustainable Chem. Eng. 2017, 5, 5394−5407

Research Article

ACS Sustainable Chemistry & Engineering

potential for fabricating efficient electromagnetic wave absorbers at low filling levels.18,19 For instance, Liu et al. synthesized Fe3O4-doped MWCNTs composites and investigated their microwave absorption properties, finding that the composite exhibited great electromagnetic wave attenuation.20 Accordingly, on the basis of the above proposal and our experience on fabricating 3D composites using alginate,21,22 herein we present a simple fabrication method of one-step encapsulation and controllable carbonization process to synthesize a novel porous graphitic carbon composites with excellent microwave absorbing performance. First, the Fe3O4doped MWCNTs composites were mixed with sodium alginate and then cross-linked by Ni2+ ions to form a gel via an easy-tohandle encapsulation. Second, a carbonization process was carried out at 600 °C to obtain 3D SA-Ni-(Fe3O4/CNTs)-X. The structure, morphology, magnetic properties and interfacial interactions SA-Ni-(Fe3O4/CNTs)-X were fully investigated. The effect of different percentage of filled-Fe3O4@MWCNTs on the electromagnetic wave absorption properties of synthesized composites was thoroughly studied.

Co/nanoporous carbon (NPC) nanocomposites and studied their electromagnetic wave absorption properties.6 The maximum reflection loss of the composites reached −21.7 dB at 15.2 GHz and the absorption bandwidths more than −10 dB were 5.8 GHz with a thickness of 1.2 mm. Besides, the authors have synthesized three-dimensional (3D) hierarchical cabbagelike carbonaceous Fe/CCMs-X (X means treating temperature) composites by a hydrothermal reaction and carbonization processes, and the composite materials exhibited a wider absorption band and a greater reflection loss.7 The maximum reflection loss (RL) of the composites reached −22.9 dB and the absorption bandwidths were 4.32 GHz with a thickness of 2.5 mm. Additionally, Zhu et al. prepared a core−shell structured MnFe2O4@SiO2+polyvinylidene fluoride (PVDF) nanocomposite and researched its electromagnetic wave absorption properties.8 It was indicated that the absorption bandwidth with (RL) values less than −10 dB was up to 3.58 GHz, and the maximum reflection loss (RL) of the composites gained −25.73 dB. More recently, Wang et al. fabricated 3D net-like SnO 2 /Fe 3 O 4 /multiwalled carbon nanotubes (MWCNTs) composites, and this kind of composite materials exhibited a wider absorption band and a greater reflection loss.9 As proposed, the designed porous structure can lead to some salutary physical effects to promote electromagnetic wave absorption, including the interaction of magnetic properties and dielectric properties, reflection and scattering within the material, and multipolarization at the heterointerface.10−12 However, although several kinds of 3D porous absorbers for electromagnetic wave absorption have been reported in previous works, it is still a great challenge to control high dispersion of magnetic species in related 3D materials in a facile, controllable manner. More unfavorably, the high-cost, high-density, low-yield and complicated preparation processes are extra limiting factors for their potential applications. In view of these issues, researchers have turned to nature for new research ideas, among which is to make the biomass as a carbon-based material precursor. As a result, biomasses have been paid more attention in the preparation of useful carbonaceous functional materials. Among various agricultural/industrial byproducts, the seaweed-derived alginate has attracted increasing interest to fabricate various functional materials, due to its easy availability and large annual production.13−16 More interestingly, alginate macromolecules can be easily cross-linked with metal ions, such as Fe3+, Co2+ and Ni2+, etc. Actually, in our previous study,17 for a binary phase heterostructured electromagnetic wave absorber (SA-FeX), sodium alginate could be cross-linked with Fe3+ to form a gel, which was freeze-dried and carbonized at 600 °C, leading to a 3D porous composite material with favorable microwave absorption properties. The composite materials were found to exhibit a wider absorption band and a greater reflection loss. As compared to those related absorbers prepared by multisteps, this kind of 3D composite materials not only have a facile preparation process, low-density, high-yield and developed nanonetwork but also can effectively make the magnetic metal particles highly disperse into the carbonaceous matrix, hence improving the absorption of electromagnetic waves. However, to enhance further the absorption properties of this type of 3D composites, there is still significant improvement that can be achieved. To the best of our knowledge, MWCNTs are widely seen as the most competitive candidate because of its good conductivity, excellent mechanical strength and unparalleled tubular structure, which make them great

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EXPERIMENTAL SECTION

Materials. Sodium alginate, ammonia solution, nitric acid (HNO3), FeCl3·6H2O, FeCl2·4H2O and Ni(CH3COO)2·4H2O were purchased from Tianjin Kermel Chemical Reagent Co. The MWCNTs (diameter, 20−40 nm; length, ∼20 μm) were purchased from Shenzhen Nanoport Co. Ethanol was purchased from Tianjin Kermel Chemical Reagent Factory, China. All chemicals were of analytical grade and used without further purification. Preparation of Oxidized MWCNTs. In a typical synthesis, 1.0 g of the pristine MWCNTs was first dispersed in 100 mL of 68% HNO3 by exerting ultrasonic dispersion and refluxed at 120 °C in an oil bath with constant stirring for 6 h. After the mixture was cooled to room temperature, the treated MWCNTs was filtered and then washed with distilled water until the pH value reached neutral, the obtained MWCNTs was dried at 60 °C for further use. Preparation of MWCNTs/Fe3O4 Composite. Typically, 0.5 g of the above oxidized MWCNTs and 0.3 g of FeCl3·6H2O were dispersed in 100 mL of distilled water under mechanical stirring at 70 °C. Afterward, the reaction system was maintained for 15 min before N2 was purged into solution to remove oxygen. Then, 0.1 g of FeCl2· 4H2O was added and kept stirring under N2 atmosphere for 30 min. Subsequently, 3 mL of ammonia solution was added into the above reaction system. The reaction was allowed to proceed for another 40 min under N2 atmosphere, and the Fe3O4/MWCNTs composites were prepared. Finally, the composites were separated using a magnet and washed with distilled water several times, Fe3O4/MWCNTs composites were obtained and dried at 60 °C for further use. Preparation of MCB SA-Ni-(Fe3O4/CNTs)-X. Sodium alginate (SA) with determined amounts of MWCNTs/Fe3O4 (0%, 9%, 16.5%, 25%) was dispersed in 100 mL of distilled water and magnetic stirring for 12 h. The mixture was then added dropwise to a 5% Ni(CH3COO)2 solution for cross-linked hydrogels beads left undisturbed in Ni(II) solution for 6 h for complete cross-linking to obtain nickel-alginate hydrogels beads. Lastly, the hydrogels beads were washed with distilled water followed by ethanol to remove extra CH3COO− and Ni2+ ions. Then, the as-prepared hydrogels beads were transferred to a freezer at −50 °C for 12 h to obtain 3D SA-Ni(Fe3O4/CNTs)-X aerogels. Then, the obtained aerogels beads were placed in a tube furnace and heated at a rate of 5 °C min−1 to 600 °C and kept for 2 h in a N2 atmosphere. After being cooled in flowing N2, the obtained samples were recorded as SA-Ni-(Fe3O4/CNTs)-X (the number X refer to the mass percentage of Fe3O4/CNTs). Materials Characterization. The morphology and structure of the samples were investigated by high-resolution transmission electron microcopy (HRTEM, Hitachi H9000NAR) and scanning electron microscopy (SEM, JEM JEOL 2100) with energy dispersive 5395


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ACS Sustainable Chemistry & Engineering Scheme 1. Fabrication Process of Magnetic Carbonaceous Biomass Absorbing Material

spectroscopy (EDS) images, respectively. The crystal structure was determined by X-ray diffraction, the XRD patterns were obtained with a Rigaku model D/max-2500 diffractometer using Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA, scan rate (2θ) of 8° min−1 ranging from 10° to 70°. The chemical bonds were analyzed by Fourier transform infrared spectroscopy and recorded on a PerkinElmer FT-IR spectrophotometer with 400−4000 cm−1 resolution. The chemical composition was investigated by X-ray photoelectron spectroscopy (XPS) with measurements performed using the PHI 5000 Versa Probe system. The magnetic properties of the composites were measured by vibration sample magnetometry (VSM) at room temperature. Electromagnetic Measurements. The electromagnetic wave absorption properties of SA-Ni-(Fe3O4/CNTs)-X composites were researched by using a vector network analyzer (Agilent N5222A) in the range of 2−18 GHz. Coaxial specimens for electromagnetic parameters were fabricated by mixing paraffin with 15 wt % and pressing them into a cylindrical-shaped (ψout of 7.0 mm, ψin of 3.04 mm). The reflection loss (RL) was calculated according to the following equations:23

Z in = Z0(μr /εr)1/2 tan h[j(2πfd)/c(με )1/2 ] r r

(1)

RL = 20log|(Z in − Z0)/(Z in + Z0)|

(2)

spheres containing different percentages of Fe3O4/CNTs were placed in a tube furnace and carbonized at 600 °C under an N2 atmosphere, and the obtained carbonaceous samples would be ready for electromagnetic wave absorption performance testing. First, the morphologies and structures of the porous SA-Ni(Fe3O4/CNTs)-X and SA-Ni hybrids were characterized by using SEM and TEM. SEM and TEM images of SA-Ni-(Fe3O4/ CNTs)-X and SA-Ni are shown in Figure 1 and Figure 2. The morphology of the SA-Ni composites could be clearly presented from SEM images Figure 1a1,2. Clearly, the surface of the carbon matrix is rough and the carbon skeleton is seriously coacervated, which is not beneficial to the formation of the electromagnetic wave track, further hampering the diffusion of electromagnetic waves inside the carbon skeleton. However, when Fe3O4/CNTs were introduced into SA-Ni by encapsulation and carbonization, it was found that Fe3O4/ CNTs was thoroughly encapsulated by the carbon matrix, as shown in Figure 1b1,2. The structure was fluffy and the pore size increased, leading to the formation of a porous network. Nevertheless, with the increase of Fe3O4/CNTs content, the structure became more abundant, as shown in Figure 1c1,2. Hence, for SA-Ni-(Fe3O4/CNTs)-25%, the abundant channel structure might further promote the loss of electromagnetic waves.24 Moreover, elemental mapping by EDS (see inset of Figure 1d) wildly used in analyzing the composition of the SANi-(Fe3O4/CNTs)-25%, and furnished evidence that the distribution of C, Fe and Ni elements within the carbonaceous bead-like was homogeneous. Meanwhile, TEM was used to investigate further the microstructures of the SA-Ni-(Fe3O4/CNTs)-X and SA-Ni. The TEM image of the SA-Ni clearly shows that their shapes are spherical with diameters mainly from 17 to 27 nm and the particle distribution is uniform, without remarkable aggregation (see inset of Figure 2a). Furthermore, the C layer covers Ni and NiO nanoparticles (see inset of Figure 2b,c). Figure 2d shows a lattices the image of Ni, it can be clearly seen that an ordered

where Zin stands for the input impedance of the absorber, Z0 is the impedance of free space, d is the thickness of absorber, f is the frequency, c is the velocity of light, and μr (μr = μ′ − jμ″) and εr (εr = ε′ − jε″) are the relative permeability and permittivity, respectively.

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RESULTS AND DISCUSSION Characterization of SA-Ni-(Fe3O4/CNTs)-X. The manufacturing process of SA-Ni-(Fe3O4/CNTs)-X is illustrated in Scheme 1. First, the Fe3O4/CNTs were synthesized by a coprecipitation method, followed by mixing with sodium alginate to form a sol. Then, by electronic attraction, the carboxyl group of SA can form a complex with Ni(II). By virtue of this one-step encapsulation process, plenty of nickel ions could be bound by ion-exchange between sodium and nickel ions. Second, the as-prepared hydrogels beads were transferred to a freezer at −50 °C for 12 h to obtain dried 3D SA-Ni(Fe3O4/CNTs)-X aerogels. Then, the obtained aerogels 5396


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Figure 1. SEM images of SA-Ni (a1,2), SA-Ni-(Fe3O4/CNTs)-9% composites (b1,2) and SA-Ni-(Fe3O4/CNTs)-25% composites (c1,2), and the EDS elemental mapping images of SA-Ni-(Fe3O4/CNTs)-25% (d).

has no stationary interplanar spacing, indicating that the microstructure of carbon is composed of relatively disordered arrays, which would be further supported by the XRD results

array of crystal lattices with an interplanar spacing of 0.25 nm, corresponding to standard (111) surface Ni lattices. Nothing that, from Figure 2d, one can see that the covered carbon layers 5397


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Figure 2. TEM and HRTEM images of SA-Ni (a), (b), (c) and (d) HRTEM image of Ni lattice area of SA-Ni. TEM and HRTEM images of SA-Ni(Fe3O4/CNTs)-25% (e), (f), (g) and (h).

interface also increased, the loss of electromagnetic waves would be constantly strengthened. The structures and phase composition of the synthesized SANi-(Fe3O4/CNTs)-X and pure SA-Ni were further examined by XRD, and the results are presented in Figure 3. Clearly, as is

given below. Besides, in contrast to pure SA-Ni, being incorporated magnetic CNTs, the internal structure surely became more complex. Figure 2e displays the TEM image of as-prepared SA-Ni-(Fe3O4/CNTs)-25% composites. It can be observed that the CNTs are not intertwined each other but relatively evenly distributed in the carbon layer, and the metal particles (Ni) uniformly distributed into the network. The result in Figure 2f shows that Fe3O4 particles tightly adhere to the surface of the CNTs with a particle size distribution ranging from about 30 to 40 nm. Moreover, these Fe3O4 particles exposed on the surface of CNTs can easily attach to Ni particles, which are easy to form surface polarization and can promote the absorption of electromagnetic waves. Moreover, it is worth noting that the C layers contain a number of structural defects, such as C-layer breakage and severe mixing, as shown in Figure 2h. These defects are prone to form the uneven and terraced fields.25 However, the presence of defective sites in C layers facilitates microwave absorption. The defect site generates an additional energy state near the Fermi level, which can reduce the electron transition energy.26 In addition, the outlines of Fe3O4/C, Fe3O4/CNTs and Ni/CNTs interfaces can be clearly observed, as shown in Figure 2g,h. These interfaces would induce plenty of beneficial substances that can exert effects on microwave absorption, such as interfacial polarization, microwave scattering and reflecting. With increasing Fe3O4/CNTs content, this heterogeneous

Figure 3. XRD patterns of SA-Ni-(Fe3O4/CNTs)-X composites with different doped Fe3O4/CNTs percentage at 2θ = 10−70°. 5398


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stretching vibrations, indicating that SA-Ni produced unsaturated groups during the pyrolysis process. Besides, the appearance of peaks at ∼1384 and ∼2924 cm−1 can be corresponded to the CH symmetric deformation vibration and −CH2 group. When Fe3O4/CNTs was introduced into sodium alginate, the SA-Ni-(Fe3O4/CNTs)-X (X = 9%, 25%) composites exhibited similar FT-IR results as SA-Ni. However, in contrast to the SA-Ni and SA-Ni-(Fe3O4/CNTs)-X FT-IR spectra of the characteristic peaks, small yet distinct differences can be observed in the 559 cm−1. This difference of characteristic peaks between SA-Ni and SA-Ni-(Fe3O4/ CNTs)-X can be possibly due to the presence of FeO. It indicated that the Fe3O4/CNTs combined with SA forming SANi-(Fe3O4/CNTs)-X composites successfully. However, with the increase of Fe3O4/CNTs content, the FeO bond became stronger and stronger, which may be due to the high Fe3O4/ CNTs content. XPS is a very useful technique to investigate the surface states of materials, which can deliver the information with regard to the atomic compositions, also can identify the type of bonds between atoms. The XPS survey spectra of SA-Ni and SA-Ni(Fe3O4/CNTs)-25% composites are shown in Figure S1a. Clearly, three strong peaks at 284.6, 529.8 and 860 eV can be observed from the XPS survey spectra of SA-Ni composites, which can be ascribed to the C, O and Ni, respectively; and the atomic percentages of Ni, O and C in the surface were roughly estimated to be 1.91%, 8.6% and 89.49%, respectively. As a highly accurate result of XPS, the predominant presence of C in the surface (89.49%) indicates that Ni nanoparticles are well encapsulated by C layers, which can be deduced by the TEM results. In addition, the results of XPS analysis indicate that the atomic percentages of C, O, Ni and Fe in the surface of SA-Ni(Fe3O4/CNTs)-25% composites were 91.87%, 6.78%, 1.1% and 0.25%, respectively. Compared with SA-Ni composites, the content of C increases obviously, which may be attributed to the introduction of MWCNTs. The O 1s spectrum of SA-Ni(Fe3O4/CNTs)-25% shows four peaks, e.g., 528.1, 529.5, 531.5 and 530.6 eV, which can be assigned to CO, FeO, NiO and OCO,30 respectively (Figure S1b). The formation of this carboxyl group (OCO) is possibly due to the addition of the oxidized MWCNTs. As shown in Figure S1c, the spectrum of C 1s shows three peaks at 284.1, 284.6 and 285.3 eV, corresponding to the CC, CH and CO groups of SA-Ni-(Fe3O4/CNTs)-25%, respectively.31 The perfection degree of the carbon layers can be estimated by the intensity ratio of CC. As can be seen from Figure S1c, the strong peak represents a relatively imperfect array of carbon layers. In the Ni 2p spectra (Figure S1d), the fitting peak Ni 2p3/2 at 852 eV corresponds to the NiNi bond,28 whereas the Ni 2p1/2 peak at 866.7 eV can be assigned to Ni2+, which stems from the NiO.28 In addition, considering the X-ray diffraction peaks of γFe2O3 is very similar to XRD peaks of Fe3O4, an XPS measurement was conducted to ensure the valence states of the Fe element. As shown in Figure S1e, there are two peaks at binding energies of 709.2 and 723.5 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2, indicating the appearance of Fe3O4 in the sample. Meanwhile, the XPS also showed C, O and Ni elements in the SA-Ni, which is consistent with the EDS results. Accordingly, on the basis of the above analysis results, it is confirmed that the preparation of SA-Ni-(Fe3O4/CNTs)-X composites with tunable composition, controllable dispersion has been achieved.

shown by Figure 3a, the peaks at 2θ = 25.9° in pure SA-Ni show that the sample consists mainly of amorphous carbon, resulting from the carbonization of alginate organic polymers at high temperature treatment.27 Moreover, the characteristic diffraction peaks at around 2θ = 45°, 53° correspond to the (111), (200) planes to Ni (JCPDS card no. 04-0850). The other diffraction peaks at around 2θ = 42°, 47° can be corresponded to the NiO.28 When Fe3O4/CNTs were introduced into sodium alginate, besides the XRD diffraction peaks of Ni and NiO phases, there were also Fe3O4 peaks. Additionally, it was also observed that a broad diffraction peak at around 2θ = 26.1° was detected, which can be described to the (002) crystal plane of hexagonal graphite (JCPDS 656212). These results confirmed that MWCNTs species had been successfully encapsulated in the final samples,9 implying that SA-Ni-(Fe3O4/CNTs)-X composites were mainly composed of magnetic species of Ni crystals, Fe3O4 and carbonaceous substances of MWCNTs and alginate-derived carbonaceous network (Figure 3b). More interestingly, with the increase of Fe3O4/CNTs content introduced in the samples, the intensity of diffraction peaks of the Ni phase, Fe3O4 phase and MWCNTs phase increased significantly (see inset of Figure 3c,d). It was suggested that the Ni, Fe3O4 and MWCNTs in SA-Ni(Fe3O4/CNTs)-25% should be of a higher degree crystallinity. It is well-known that, for a typical magnetic loss material for EM wave absorption, the increment of Fe3O4 component would be beneficial to the EM wave absorption.29 Furthermore, according to the analysis of XRD results, it can also be deduced that the encapsulation of Fe3O4/CNTs species into SA-Ni had minor impact on the original crystal structure, which will be further supported by the following FT-IR results. Additionally, FT-IR results can provide information on the bonding of organic functional groups. The FT-IR spectra of the SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 25%) were recorded within the range of 4000−400 cm−1 and are displayed in Figure 4. It can be observed from the FT-IR spectrum of the SA-Ni that a band appeared at around ∼3433 and ∼1073 cm−1, which can be associated with O−H and CO stretching vibrations, indicative of the presence of numerous hydroxyl groups in SA. The bands at ∼1632 cm−1 could be associated with (CC)

Figure 4. FT-IR spectra of (a) SA-Ni, (b) SA-Ni-(Fe3O4/CNTs)-9% and (c) SA-Ni-(Fe3O4/CNTs)-25% composites. 5399


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ACS Sustainable Chemistry & Engineering Magnetic Properties. The above characterization results indicated that the porous SA-Ni-(Fe3O4/CNTs)-X composites were successfully fabricated by a one-step encapsulation process combined with heating treatment. The characteristics of the composite components and the porous structure of the prepared SA-Ni-(Fe3O4/CNTs)-25% composites are expected to possess good microwave absorption properties. To start, the magnetic properties of SA-Ni-(Fe3O4/CNTs)-X composites were researched at room temperature using a vector network analyzer (VSM). As shown in Figure 5, two samples showed

respectively, and the EM absorption properties were measured at room temperature in the frequency range of 2−18 GHz. The real parts (ε′ and μ′) represent the storage of electric and magnetic energy, respectively. The imaginary parts (ε″ and μ″) imply the loss of electric and magnetic energy. As illustrated in Figure 6a, it can be seen that the real parts of complex permittivity of the SA-Ni-(Fe3O4/CNTs)-25% and SA-Ni(Fe3O4/CNTs)-16.5% samples tend to decrease rapidly with increasing frequency, whereas the ε′ of the other two samples tend to decrease slowly, steadily with increasing frequency. According to those previous literature reports, this appearance can be attributed to the fact that the dipoles exist in the material that more and more hard to keep the phase orientation with the electric vector of the incident radiation.34 Furthermore, the high conductivity of MWCNTs is denotative of a comparatively high dipole density,35 with the increase of Fe3O4/CNTs content, the density of the dipole also increased, which cannot reorient itself in the wake of the applied electric field, and may be responsible for observing quickly reduce in ε′ of SA-Ni(Fe3O4/CNTs)-25% and SA-Ni-(Fe3O4/CNTs)-16.5% samples. It is worth noting that the ε′ values of the SA-Ni-(Fe3O4/ CNTs)-25% sample is higher than that of the other three samples. This manifests better energy storage and polarization. As shown in Figure 6b, with increasing Fe3O4/CNTs amount, the ε″ values tend to decrease from 12.9, 8.2, 2.7 and 2.3 to 6.2, 4.9, 1.7 and 1.5 for SA-Ni-(Fe3O4/CNTs)-X (X = 25%, 16.5%, 9%, 0%), respectively, in the frequency range of 2−18 GHz. It can be observed that the SA-Ni-(Fe3O4/CNTs)-25% sample was significantly larger than those of SA-Ni-(Fe3O4/CNTs)-X (X = 16.5%, 9%, 0%). As known, the ε″ values are in keep with the dielectric loss ability. A higher ε″ value also implies that this structure could be beneficial for generating higher dielectric loss. When the electromagnetic wave approaches to the heterogeneous interfaces, the interfacial polarization would take place and a number of polarized charges would amass at the heterogeneous interfaces, strengthen dielectric loss. The real part (μ′) and imaginary part (μ″) of the composites are shown in Figure 6c,d. It can be found that the μ′ values exhibit an increase tendency in the 2−18 GHz frequency range with slight fluctuations; only the μ′ values of the SA-Ni(Fe3O4/CNTs)-25% composite increase significantly in the range of 15−17 GHz. Although the corresponding μ″ values exhibit a decrease tendency in the frequency region over 2−18 GHz for SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 16.5%, 25%) respectively, among the four samples, the μ″ values are fluctuate around zero. The real part μ′ SA-Ni-(Fe3O4/CNTs)-X composites kept almost constant in the low frequency range and then gently increased with the increase of frequency. The imaginary part μ″ of the SA-Ni-(Fe3O4/CNTs)-X composites behaves in contrast with respect to frequency as that of real part μ′. Compared with the SA-Ni-(Fe3 O4 /CNTs)-0%, the imaginary part μ″ of the SA-Ni-(Fe3O4/CNTs)-X composites would be lower because the amount of Fe3O4/CNTs increase; this then resulted in the decrease of the amount of eddy currents induced by the electromagnetic waves in system.36 In general, magnetic loss is mainly due to the natural resonance, hysteresis, exchange resonance, domain-wall displacement and eddy current effect.37 However, the hysteresis loss in the weak field is negligible. The domain wall resonance is usually located at a much lower frequency range of the MHz frequency.38 Hence, eddy current effects and natural resonance can cause attenuation of EM waves in the 2−18 GHz frequency

Figure 5. Magnetization curves of SA-Ni-(Fe3O4/CNTs)-9% and SANi-(Fe3O4/CNTs)-25% composites at room temperature.

typical ferromagnetic hysteresis loops, mainly associating with the presence of metallic Fe3O4 and Ni nanoparticles. The saturation magnetization (MS), coercivity (Hc) and remnant magnetization (Mr) are 8.043 emu·g−1, 21.104 Oe and 2.312 emu·g−1 for the SA-Ni-(Fe3O4/CNTs)-9%, and 22.829 emu· g−1, 25.654 Oe and 0.9368 emu·g−1 for the SA-Ni-(Fe3O4/ CNTs)-25%, respectively. The low MS of the as-prepared SANi-(Fe3O4/CNT)-9% samples is mainly related to the lower Fe3O4/CNTs percentage, resulting in the relatively poor crystallinity. Moreover, the defects in the crystal structure surface may also lead to a significant decrease in the saturation magnetization of the porous carbon matrix.32 However, as compared to SA-Ni-(Fe3O4/CNTs)-9%, the SA-Ni-(Fe3O4/ CNT)-25% sample has an enhanced coercivity. With the increase of Fe3O4/CNTs amount in the composites, the coercivity increased also. Therefore, it can be deduced that the increase of coercivity could be closely attributable to the increasing ratio of the heterogeneous component of MWCNTs in the composites. The larger Hc value might have led to a larger magneto-crystalline anisotropy energy, which would be helpful to enhance the microwave absorption properties.33 Electromagnetic Wave Absorption Properties. Generally, high-performance electromagnetic wave absorption is usually derived from the complementarities between the effective permeability and the permittivity of the materials. Therefore, our independent measurement of complex permittivity (εr = ε′ − jε″) and permeability (μr = μ′ − jμ″) of these samples to are used to understand better their microwave absorption properties. Figure 6 shows the electromagnetic parameters (relative permittivity, εr = ε′ − jε″ and relative permeability, μr = μ′ − jμ″) of the composites containing 15 wt % of SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 16.5%, 25%), 5400


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Figure 6. Frequency dependence of (a) the real part (ε′) and (b) imaginary part (ε″) of the complex permittivity; (c) real part (μ′) and (d) imaginary part (μ″) of the complex permeability of the SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 16.5%, 25%).

range. The eddy current loss can be expressed by the following equation:39 μ″ = 2πμo (μ′)2 σd 2f /3

(3)

where μo represents the permeability of vacuum, d represents the thickness and σ represents the electrical conductivity of the composite. If magnetic loss only comes from eddy current effects, the values of Co = μ″(μ′)−2 f−1 should be constant when the frequency changes. From Figure 7, the values increased in the beginning, afterward decreased in the frequency range of 2−3 GHz and 3−4 GHz singly, and four weak peaks existed over the frequency range of 4−18 GHz. The values of Co = μ″(μ′)−2 f−1 are nearly constant, which means that the eddy current loss mainly affected the dissipation of microwave energy, particularly in the range of 5−18 GHz. However, another mechanism for magnetic loss is natural resonance, which can be expressed by the following equation:40

Ha = 4|K | /3μ0 MS

(4)

2πfr = γHa

(5)

Figure 7. Value of μ″(μ′)−2 f−1 of SA-Ni-(Fe3O4/CNTs)-X composites with different doped Fe3O4/CNTs percentages as a function of frequency.

The higher anisotropy energy would be greatly helpful to improve electromagnetic waves absorption properties. The two possible contributions to electromagnetic waves absorption are dielectric loss and magnetic loss.42 To study further which one is leading for the SA-Ni-(Fe3O4/CNTs)-X

where γ means the gyromagnetic ratio, Ha represents the anisotropy energy, and |K| represents the anisotropy coefficient. As known, the resonance frequency rest on the anisotropy field, which is related to the coercivity value of the materials.41 With the increase of Fe3O4/CNTs content, anisotropy increased also. 5401


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Figure 8. ε′−ε″ curves of SA-Ni-(Fe3O4/CNTs)-X composites with different doped Fe3O4/CNTs percentages.

the Debye theory and the relationship between ε′ and ε″ can be expressed by the following equation:45

(X = 0%, 9%, 16.5%, 25%) composites, the magnetic loss tangent (tan δm = μ″/μ′) and dielectric loss tangent (tan δe = ε″/ε′) are calculated and shown in Figure S2. Among the four samples, a decreasing tendency of dielectric and magnetic loss is generally observed as the frequency increases, respectively. Only the SA-Ni-(Fe3O4/CNTs)-16.5% tended to increase with frequency increasing from 1.9 to 0.78. In the high-frequency region, the appearance of the negative tan δm possibly resulted from the negative permeability. In the entire frequency range, the dielectric loss tangent is much larger than the magnetic loss tangent, implying that the dielectric loss played an important part in the EM absorption of these samples. As everyone knows, the dielectric loss of the EM energy principally roots in the interfacial polarization and the dipole polarization at microwave frequencies.43 In this process, the amount of the surface atoms with unsaturated bonds in Fe3O4/Ni particles will tremendously increase, resulting in the increase of the diploes. Therefore, a large amount of dipole polarization can be devoted to the dielectric loss. In addition, the fine dispersion of Fe3O4/ CNTs in the porous carbon matrix can introduce more additional interfaces and consequently interfacial polarization, leading to the interfacial polarization and dielectric loss.44 Furthermore, the space−charge polarization, happening among adjacent metal nanoparticles, probably could lead to the increase of the dielectric loss to a certain degree.42 As is stated above, the composite is an electromagnetic wave absorber of dielectric loss type. To understand better the mechanisms of dielectric loss of the electromagnetic absorber,

εr = ε′ + iε″ = ε∞ + (εs − εo)/(1 + iωτo)

(6)

where ε∞ represents the relative dielectric permittivity at highfrequency limit, εs means the static permittivity, ω is frequency and τ is relaxation time. According to the above eq 6, ε′ and ε″ can be deduced as given below. ε′ = ε∞+(εs − εo)/[1 + (ωτo)2 ]

(7)

ε″ = ωτo(εs − εo)/[1 + (ωτo)2 ]

(8)

According to eqs 7 and 8, the relationship between ε′ and ε″ can be described as [ε′ − (εs + ε∞)/2]2 + (ε″)2 = [(εs − ε∞)/2]2

ε′ = ε″ /(2πfτ ) + ε∞

(9) (10)

Based on eq 10, the curve of (ε′−ε″) is a single semicircle, which is defined as a Cole−Cole semicircle, a semicircle corresponds to a Debye relaxation process. As shown in Figure 8a, no evident semicircle can be found in SA-Ni-(Fe3O4/ CNTs)-X (X = 0.9%). For SA-Ni-(Fe3O4/CNTs)-16.5%, three small semicircles and a tiny semicircle are found in Figure 8b. Interestingly, a relatively large semicircle and five small semicircles are found for SA-Ni-(Fe3O4/CNTs)-25% sample (Figure 8c), indicating that there are six dielectric relaxation processes in the SA-Ni-(Fe3O4/CNTs)-25% hybrid composite. 5402


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Figure 9. (a) Frequency dependence of microwave RL values of SA-Ni-(Fe3O4/CNTs)-X (X = 25, 16.5, 9, 0%) composites. Three-dimension images of calculated RL values of (b) SA-Ni-(Fe3O4/CNTs)-25%, (c) 16.5%, (d) 9% and (e) 0% composites.

polarization possible. On commutative electronic field, the contact of negative charges and positive charges at the interface of the heterostructure would lead to the variation in the charge density and generate an electric dipole moment, causing interfacial relaxation.48 Therefore, the dielectric loss of porous SA-Ni-(Fe3O4/CNTs)-X composites could be attributed to dipole polarization, interfacial polarization and space−charge polarization together. Additionally, in the interior of absorber, EM-wave attenuation plays an important part in the damping of the electromagnetic waves. The attenuation constant α means the

It also demonstrates that the addition of Fe3O4/CNTs promotes the Debye dipolar relaxation process. According to previous literature, Ni nanoparticles possess high conductivity, when the electromagnetic wave is transmitted to the surface of the Ni particles, the high conductivity of metal Ni would make the significant skin effect happen.46 At the same time, it would result in its dielectric loss performance degradation. Nevertheless, the doped MWCNTs are a polar material with abundant electric dipole that shows a regular rotation in an alternating electronic field. This regular rotation would lead to a dielectric relaxation.47 Furthermore, the interfaces of the heterogeneous Fe3O4/CNTs nanocomposites make interfacial 5403


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ACS Sustainable Chemistry & Engineering attenuation effect of materials, as calculated according to the below formula:49

Table 1. Electromagnetic Wave Absorption Properties of Reported and Studied Porous Composites

α = ( √2πf /c) × √[(μ″ε″ − μ′ε′) + √[(μ″ε″ − μ′ε′)2 + (μ″ε″ + μ′ε′)2 ]]

(11)

where f represents the frequency of the electromagnetic waves and c is the velocity of electromagnetic waves in free space. As shown in Figure S3, the relying of attenuation constant α on frequency reveals that the SA-Ni-(Fe3O4/CNTs)-25% shows the largest α value among the four samples in the frequency range of 2−18 GHz. Therefore, it can be inferred that the SANi-(Fe3O4/CNTs)-25% sample might exhibit a more excellent electromagnetic wave absorption performance than those of the SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 16.5%,) samples. To investigate further the electromagnetic wave absorption performance of the as-synthesized SA-Ni-(Fe3O4/CNTs)-X composites, the reflection loss (RL) values are calculated from the relative complex permittivity (εr) and permeability (μr) at a given frequency (2−18 GHz) and different absorber thicknesses. Figure 9a−e depicts the calculated theoretical reflection loss (RL) curves of the composites-wax with different thicknesses in the frequency range of 2−18 GHz with a loading of 15 wt %. It can be observed that the microwave absorption performances of SA-Ni-(Fe3O4/CNTs)-X composites are better than those of pure SA-Ni hybrids. In particular, the SA-Ni-(Fe3O4/CNTs)-25% sample exhibits the best electromagnetic absorption properties and the maximum RL is −32 dB at 10.8 GHz with only a thickness of 2 mm. The effective microwave absorption bandwidth (RL < −10) is 3.2 GHz from 9.3 to 12.5 GHz. In our earlier study, the pure SA-Ni was successfully prepared and its maximum RL is −8.7 dB was obtained at 12.3 GHz with a thickness of 4 mm. Compared with the pure SA-Ni, the electromagnetic absorption properties of SA-Ni-(Fe3O4/CNTs)-X (X = 9%, 16.5%, 25%,) composites can be largely enhanced. Moreover, with the amount of Fe3O4/ CNTs increasing, the absorption properties of the sample enhanced significantly. Among the four samples, SA-Ni(Fe3O4/CNTs)-0% exhibited the worst electromagnetic wave absorption property with the maximum RL is −8.7 dB. That may be due to the weak polarization and poor weakening performance. However, with the increase of Fe3O4/CNTs content, these multi-interfaces result in significant interfacial polarization and multipolarization, and this process is accompanied by high conductance and a high attenuation constant, which would significantly enhance the dielectric loss. Above all, compared to other three samples, SA-Ni-(Fe3O4/ CNTs)-25% has a superior property. Moreover, a same method was also carried out for another two proportions prepared using 30% and 35%, respectively. Because of the nature of the material itself, the proportion of 35% of the sample in the postprocessing were broken. Simultaneously, the sample prepared with 30% also had a certain degree of crack in the preparation of the sample, which exerted a serious effect on the measurement of the data of absorption. Therefore, it can be concluded that a ratio of more than 25% does not exhibit good electromagnetic wave absorption performance. To demonstrate this, Figure S4 depicts the RL values for SA-Ni-(Fe3O4/CNTs)30% composites with different thicknesses in the frequency range of 2−18 GHz. According to the above analysis, it is shown that the SA-Ni-(Fe3O4/CNTs)-25% composite material possessed the largest microwave absorption properties. Meanwhile, for comparison purpose, Table 1 shows the microwave

Sample

RL (dB)

Thickness (mm)

Frequency range (GHz)

Fe-Co/nanoporous carbon Fe/CCMs-500 MnFe2O4@ SiO2+PVDF SnO2/Fe3O4/ MWCNTs Fe-SA-600 SA-Ni-(Fe3O4/ CNTs)-25%

−21.7

1.2

5.8

6

−22.9 −25.73

2.5 2.0

4.32 3.58

7 8

−42

1.9

2.8

9

−24 −32

1.5 2.0

4.8 3.2

15 This work

refs

absorption performance of the representative porous composites reported before; and it can be deduced that this newly synthesized composite is better than most of related carbonaceous absorbers. Based on the above-mentioned analysis, the possible microwave absorbing mechanism that could be explained by strong conduction loss, dielectric loss and multiple reflections in the porous structure SA-Ni-(Fe3O4/CNTs)-X composites is schematically shown in Scheme 2. We can conclude that the mechanism may come from the following areas: First, when the incident wave propagates into surface of materials, part of the electromagnetic energy enters into the materials and interacts with electrons and charged particles, and then energy is dissipated in the form of heat. Second, there are residual oxygen functional groups in the oxidized MWCNTs that can increase the absorption of electromagnetic energy.50 These functional groups and the small size of Ni, Fe3O4 nanoparticles can behave as a polarization centers, favorably enhancing the interfacial polarization.50 Third, the Debye relaxation process can enhance the attenuation of electromagnetic waves, and the high conductivity of MWCNTs plays an important part in the forming of conductive networks.51 Moreover, the presence of residual groups and defects of the Fe3O4/CNTs could produce multiple scattering and reflections, which would further strengthen the electromagnetic wave absorption capacity.52 Generally, the excellent electromagnetic wave absorption properties of the composite material can be associated with the synergistic effect between the components. By contrast, the higher value of attenuation and high conductivity make SA-Ni(Fe3O4/CNTs)-X composite possess highly enhanced microwave absorbing properties. As a consequence, it can be a promising candidate as lightweight and high efficiency EM wave absorbing material in practical processes.

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CONCLUSIONS A new kind of 3D SA-Ni-(Fe3O4/CNTs)-X composites have been successfully prepared via a combination of one-step encapsulation and carbonization processes, and the dependence of microwave absorption on varied encapsulated contents of Fe3O4/CNTs was investigated over the range of 2−18 GHz. It was demonstrated that the newly designed SA-Ni-(Fe3O4/ CNTs)-25% composite exhibited the highest microwave absorption due to the synergistic effects between the attenuation and interfacial polarization, electric polarization and conductive network. The microwave absorption properties of the SA-Ni-(Fe3O4/CNTs)-25% composites could reach −32 dB at 10.8 GHz with a thickness of only 2 mm and the effective absorption bandwidth (RL ≤ −10 dB) was 3.20 GHz (9.3− 5404


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ACS Sustainable Chemistry & Engineering Scheme 2. Possible Mechanism for the Microwave Absorption of SA-Ni-(Fe3O4/CNTs)-X composites

Preparative Chemistry of Jilin University (2016-04) are kindly acknowledged.

12.5 GHz). This synthetic process for 3D SA-Ni-(Fe3O4/ CNTs)-X composites is quite facile, and the usage of the enriched biomass-based precursor makes it possible to fabricate other porous metal/carbon composites with tunable compositions. As a result, this kind of synthetic 3D SA-Ni-(Fe3O4/ CNTs)-X composites can be potentially used as lightweight, flexible and strong absorption performance shielding materials in actual applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00711. XPS survey spectra, dielectric loss tangents and magnetic loss tangents curve, attenuation constant, SA-Ni-(Fe3O4/ CNTs)-30% absorbing performance measurement (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.-D. An). *E-mail: [email protected] (S.-R. Zhai). ORCID

Zhan Shi: 0000-0001-9717-1487 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21546008, 21676039) and the Opening Foundation of State Key Laboratory of Inorganic Synthesis and 5405


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