materials Article
Controllable Fabrication of Fe3O4/ZnO Core–Shell Nanocomposites and Their Electromagnetic Wave Absorption Performance in the 2–18 GHz Frequency Range Xiaodong Sun 1 , Guangyan Ma 2 , Xuliang Lv 1 , Mingxu Sui 1 , Huabing Li 2, *, Fan Wu 3, * and Jijun Wang 4, * 1
2 3 4
*
Key Laboratory of Science and Technology on Electromagnetic Environmental Effects and Electro-Optical Engineering, The Army Engineering University of PLA, Nanjing 210007, China;
[email protected] (X.S.);
[email protected] (X.L.);
[email protected] (M.S.) College of Field Engineering, The Army Engineering University of PLA, Nanjing 210007, China;
[email protected] School of Mechanical Engineering, Nanjing University of Science & Technology, Nanjing 210094, China Research Institute for National Defense Engineering of Academy of Military Science PLA China, Beijing 100036, China Correspondence:
[email protected] (H.L.);
[email protected] (F.W.);
[email protected] (J.W.)
Received: 16 April 2018; Accepted: 9 May 2018; Published: 11 May 2018
Abstract: In this study, Fe3 O4 /ZnO core–shell nanocomposites were synthesized through a chemical method of coating the magnetic core (Fe3 O4 ) with ZnO by co-precipitation of Fe3 O4 with zinc acetate in a basic medium of ammonium hydroxide. The phase structure, morphology and electromagnetic parameters of the Fe3 O4 /ZnO core–shell nanocomposites were investigated. The results indicated that the concentration of the solvent was responsible for controlling the morphology of the composites, which further influenced their impedance matching and microwave absorption properties. Moreover, Fe3 O4 /ZnO nanocomposites exhibited an enhanced absorption capacity in comparison with the naked Fe3 O4 nanospheres. Specifically, the minimum reflection loss value reached −50.79 dB at 4.38 GHz when the thickness was 4.5 mm. It is expected that the Fe3 O4 /ZnO core–shell structured nanocomposites could be a promising candidate as high-performance microwave absorbers. Keywords: core–shell structure; electromagnetic absorption; interfacial polarization; Fe3 O4 ; ZnO
1. Introduction In recent decades, advanced electromagnetic (EM) applications have taken on a fundamental role in areas such as satellite communication, radar systems, and wireless networks [1–5]. However, the problem of powerful electromagnetic interference (EMI) is becoming serious. EMI pollution certainly hinders the extensive utilization of electromagnetic wave (EMW) devices and has many negative effects on the environment and human health. Many efforts have been devoted to investigating efficient solutions for eliminating EMI pollution. Microwave absorption materials (MAMs) are a kind of functional material that can effectively absorb the energy of EMW on their surface and then transform that EMW energy into thermal energy [6–9]. The development of efficient MAMs is being pursued with high demand, and a considerable number of concepts have been actively investigated in order to develop MAMs with properties including light weight, low price, low thickness, wide absorption bandwidth capability, strong absorption intensity, and anti-oxidation [10–12]. Traditional MAMs, including ferrite [13], inorganic metal salts [14], carbonyl iron [15], graphene [16], and conducting polymers [17], have been widely employed in various applications. Materials 2018, 11, 780; doi:10.3390/ma11050780
www.mdpi.com/journal/materials
Materials 2018, 11, 780
2 of 12
However, these materials are hardly able to satisfy all of the requirements of qualified MAMs. Typically, permittivity (dielectric property) and permeability (magnetic property) are the key factors influence the absorption property of MAMs. Much research has focused on the synthesis and complementation of different components in order to avoid poor impedance matching [18,19]. According to the EMW absorption mechanism, the microwaves can be absorbed on a large scale and dissipated into thermal energy through magnetic losses and dielectric losses if the characteristic impedance of the absorber is well matched [20]. The composition of magnetic and dielectric materials is significant in improving the impedance matching between permeability and permittivity. Additionally, the employment of different micro-structures in the absorbers could influence their properties. Among the many existing micro-structures, core–shell structures, designed with magnetic components and dielectric components, have attracted a great deal of attention due to their superior microwave absorption properties, which benefit from induced interfacial polarization, as well as improved impedance matching [21]. One-dimensional (1D) zinc oxide (ZnO)-related nanomaterials have attracted enormous attention in recent decades as dielectric absorbents because of their light weight and dielectric semiconductive properties [22]. To date, many types of ZnO-based materials have been reported that confirm that the absorption property can be modified by compositing ZnO with magnetic materials [22–24]. Additionally, employing ordinary magnetic-dielectric materials as surrogates for rare metals is cost-effective and utilitarian. Considering the composite synthetic technique of ZnO, many progressive methods have been reported in previous works, such as Zn/ZnO [25], Cu/ZnO [25], and reduced graphene oxide/ZnO [26]. Previous studies have confirmed that good EM impedance matching and the efficient complementarity between relative permittivity and permeability can be realized by the synergistic effect of the magnetic and the dielectric compositions. In the present work, taking this principle into consideration, we chose ferroferric oxide as the magnetic counterpart and synthesized Fe3 O4 /ZnO core–shell structured nanocomposites with Fe3 O4 cores and ZnO shells. The morphologies and EMW absorption properties were investigated in detail. This work provides a lead for designing dielectric-magnetic absorbers via a facile method. Moreover, the as-synthesized Fe3 O4 /ZnO core–shell structured nanocomposites exhibited an enhanced absorption property, which may be expected to be useful in building a novel platform in advanced EMW absorbers. 2. Materials and Methods 2.1. Materials Ferric chloride (FeCl3 ·6H2 O), sodium citrate (Na3 C6 H5 O7 ·2H2 O), sodium acetate (NaOAc), and zinc acetate (Zn(OAc)2 ) were commercially obtained from Aladdin Chemical Reagent, China. Ammonium Hydroxide (NH3 ·H2 O), ethylene glycol (EG), and absolute ethanol were purchased from Xilong Chemical Reagent Co. Ltd. (Guangzhou, China). All the reagents were used without further purification. Deionized water was produced in our laboratory and used for all experiments. 2.2. Synthesis of Fe3 O4 Nanoparticles Fe3 O4 nanoparticles (NPs) were prepared by a solvothermal method as reported previously [27]. FeCl3 ·6H2 O (0.016 mol) and Na3 C6 H5 O7 ·2H2 O (0.004 mol) were dissolved in EG (70 mL) under magnetic stirring. Then, NaOAc (0.005 mol) was slowly introduced into the mixture solution, generating a transparent suspension. The resulting solution was then transferred into a Teflon-lined stainless-steel autoclave (100 mL capacity). Subsequently, upon sealing, the autoclave was maintained at 200 ◦ C for 10 h. After cooling down to room temperature, the precipitate was collected by the magnet and washed with absolute ethanol and deionized water several times, then dried in a vacuum oven at 50 ◦ C for 12 h.
Materials 2018, 11, x FOR PEER REVIEW
3 of 11
by the magnet and washed with absolute ethanol and deionized water several times, then dried in a Materials 2018, 11, 780 3 of 12 vacuum oven at 50 °C for 12 h. 2.3. 4/ZnO Nanocomposites 2.3.Synthesis SynthesisofofFe Fe3O 3 O4 /ZnO Nanocomposites Fe nanocomposites were prepared through the chemical method of coating the Fe33OO4/ZnO 4 /ZnO nanocomposites were prepared through the chemical method of coating the magnetic magnetic core (Fe 4) with ZnO by co-precipitation of Fe3O4 with Zn(OAc)2 in a basic medium of core (Fe3 O4 ) with3O ZnO by co-precipitation of Fe3 O4 with Zn(OAc)2 in a basic medium of NH3 ·H2 O. NH 3·H2O. Briefly, the as-prepared Fe3O4 NPs (0.25 mmol) were dissolved in deionized water (50 mL), Briefly, the as-prepared Fe3 O4 NPs (0.25 mmol) were dissolved in deionized water (50 mL), Zn(OAc)2 Zn(OAc) 2 (2 mmol) was dissolved in deionized water (20 mL), then the solutions were mixed together (2 mmol) was dissolved in deionized water (20 mL), then the solutions were mixed together by by ultrasonic dispersal min. Subsequently, mixed solution was mechanically stirred ultrasonic dispersal forfor 1515 min. Subsequently, thethe mixed solution was mechanically stirred forfor 0.50.5h. h. themeantime, meantime,a acertain certainamount amountofofNH NH·3H ·H2O O was was added added to to the the suspension. suspension. Then InIn the Then the the resultant resultant 3 2 solution was loaded into a 100 mL Teflon-lined stainless-steel autoclave and kept at 120 ◦ Cfor solution was loaded into a 100 mL Teflon-lined stainless-steel autoclave and kept at 120°C for15 15h.h. The The resulting resulting bronzing bronzing product product was was collected, collected, washed washed with with absolute absoluteethanol ethanoland and deionized deionizedwater water several times by centrifugation, and then dried in a vacuum oven at 50 °C overnight. the convenience ◦ several times by centrifugation, and then dried in a vacuum oven at 50 C overnight. the convenience of 4/ZnO nanocomposites prepared in 3 mL NH3·H2O and 2 mL NH3·H2O will be ofdiscussion, discussion,the theFe Fe3O 3 O4 /ZnO nanocomposites prepared in 3 mL NH3 ·H2 O and 2 mL NH3 ·H2 O will denoted as sample A and sample B, respectively. be denoted as sample A and sample B, respectively. 2.4. 2.4.Characterization Characterization The Thecrystalline crystallinestructure structureand andphases phasesof ofthe thesamples sampleswere wereperformed performedby byX-ray X-raydiffraction diffraction(XRD, (XRD, Rigaku Denki Co. Ltd., Tokyo, Japan) using a Cu Kα radiation (λ = 0.15418 nm) in a scattering range Rigaku Denki Co. Ltd., Tokyo, Japan) using a Cu Kα radiation (λ = 0.15418 nm) in a scattering (2θ) of (2θ) 10–80° at an ◦accelerating voltage voltage of 40 kV.ofX-ray spectroscopy (XPS) studies range of 10–80 at an accelerating 40 kV.photoelectron X-ray photoelectron spectroscopy (XPS) were performed using the ESCALAB 250Xi (Thermo Fisher Fisher Scientific, Waltham, MA, USA). The studies were performed using the ESCALAB 250Xi (Thermo Scientific, Waltham, MA, USA). morphologies of the as-synthesized samples were characterized by scanning electron microscopy The morphologies of the as-synthesized samples were characterized by scanning electron microscopy (SEM, (SEM, JSM-7500F, JSM-7500F, JEOL, JEOL, Beijing, Beijing, China) China) and and transmission transmission electron electron microscopy microscopy (TEM, (TEM, JEM-2100 JEM-2100 microscope with an accelerating voltage of 200 kV, JEOL, Beijing, China). The EM parameters microscope with an accelerating voltage of 200 kV, JEOL, Beijing, China). The EM parameters of of complex complex relative relative permeability permeability (μ (µrr == μ′ µ0 −−jμ″) jµ”)and andpermittivity permittivity(ε(εrr==ε′ε0−−jε″) jε”)ininthe thefrequency frequencyrange rangeof of 2–18 2–18GHz GHzwere wereperformed performedby byvector vectornetwork networkanalyzer, analyzer,Agilent, Agilent,N5230A N5230A(Agilent (AgilentTechnologies TechnologiesInc., Inc., Santa SantaClara, Clara,CA, CA,USA, USA,as asshown shownin in Figure Figure 1a). 1a). The Theas-prepared as-preparedsamples sampleswere weremixed mixedwith withparaffin paraffin (different samples (inner (inner diameter diameter φφinin == 3.04 (different mass mass percentages) percentages) and pressed pressed into toroidal-shaped samples 3.04 mm, mm, outer = 7.00 mm, as shown in Figure 1b). outerdiameter diameterφφout = 7.00 mm, as shown in Figure 1b). out
Figure 1. The coaxial waveguide instrumentation (a) and the toroidal–shaped sample (b). Figure 1. The coaxial waveguide instrumentation (a) and the toroidal–shaped sample (b).
3. Results and Discussion 3. Results and Discussion To confirm the phases and structures of the as-prepared samples, the corresponding XRD To confirm the phases and structures of the as-prepared samples, the corresponding XRD pattern pattern of Fe3O4/ZnO composites is shown in Figure 2. As for Fe3O4/ZnO composites, the existence of of Fe3 O4 /ZnO composites is shown in Figure 2. As for Fe3 O4 /ZnO composites, the existence of major diffraction peaks corresponding to the (220), (311), (400), (422), (511), and (440) planes can be major diffraction peaks corresponding to the (220), (311), (400), (422), (511), and (440) planes can be observed. These planes can be readily indexed to standard cards of JCPDS No.88-0866, revealing that observed. These planes can be readily indexed to standard cards of JCPDS No.88-0866, revealing that the crystallinity of Fe3O4 remains unchanged after coating. Six diffraction peaks were assigned to the the crystallinity of Fe3 O4 remains unchanged after coating. Six diffraction peaks were assigned to the (100), (101), (102), (110), (103), and (112) planes, which is consistent with ZnO (JCPDS No.36-1451). (100), (101), (102), (110), (103), and (112) planes, which is consistent with ZnO (JCPDS No.36-1451). Therefore, the XRD patterns confirmed the coexistence of Fe3O4 and ZnO. The surface elemental states Therefore, the XRD patterns confirmed the coexistence of Fe3 O4 and ZnO. The surface elemental of Fe3O4/ZnO nanocomposites were further analyzed by XPS, and the results are presented in Figure states of Fe3 O4 /ZnO nanocomposites were further analyzed by XPS, and the results are presented in Figure 3. From the typical survey spectrum, the existence of Fe, Zn, C and O elements can be found.
Materials 2018, 11, x FOR PEER REVIEW Materials FOR PEER REVIEW Materials 2018, 2018, 11, 11, x780
4 of 11 44 of of 11 12
3. From the typical survey spectrum, the existence of Fe, Zn, C and O elements can be found. In Figure 3. From the typical survey spectrum, the existence of Fe, Zn, C and O elements can be found. In Figure 3b, the high-resolution spectrum of Fe is given; two peaks appeared at 710.9 and 724.3 eV, In Figure 3b, the high-resolution spectrum Fe is given; peaks appeared at 710.9 and724.3 724.3eV, eV, 3b, the high-resolution spectrum of Fe isofgiven; two two peaks appeared at 710.9 and corresponding to the band energies of Fe 2p3/2 and Fe 2p1/2, respectively [28]. This indicates the corresponding to the band energies of Fe 2p and Fe 2p , respectively [28]. This indicates the corresponding band energies of Fe 2p3/2 3/2 and Fe 2p1/2 1/2, respectively This indicates generation of oxide of Fe(II) and Fe(III), which is in good agreement with the literature and is consistent and Fe Fe(III) whichisisiningood goodagreement agreementwith withthe theliterature literature and and is is consistent consistent generation of oxide of Fe(II) (II) and , ,which (III) with Fe3O4 [29]. The existence of the Fe element indicates that the shell of ZnO may be in porous O44 [29]. [29]. The Theexistence existenceof ofthe theFe Fe element element indicates indicates that that the the shell shell of of ZnO ZnO may may be be in porous with Fe33O condition. Figure 3c displays the high-resolution spectrum of Zn. The peaks at 1021.8 and 1044.8 eV condition. Figure 3c displays displays the the high-resolution high-resolution spectrum of Zn. The peaks at 1021.8 and 1044.8 eV 1044.8 eV correspond to Zn 2p3/2 and Zn 2p1/2, respectively. Hence, the composites are composed of Fe3O4 and correspond to to Zn Zn 2p 2p3/2 andZnZn2p2p , respectively. Hence, composites composed correspond 1/2,1/2 respectively. Hence, the the composites are are composed of Feof3OFe 4 and 3 O4 3/2and ZnO. and ZnO. ZnO.
Figure 2. XRD patterns of and Fe3O4/ZnO composites. Figure 2. 2. XRD XRD patterns patterns of of and and Fe Fe33O O44/ZnO Figure /ZnOcomposites. composites.
Figure3.3.XPS XPS spectraofofFe FeO 3O4/ZnO /ZnO nanocomposites: nanocomposites: (a) survey spectrum; spectrum; (b) (b) Fe Fe 2p 2pbinding bindingenergy energy Figure (a) survey Figure 3. XPS spectra spectra of Fe33O 44/ZnO nanocomposites: (a) survey spectrum; (b) Fe 2p binding energy spectrum; and (c) Zn 2p binding energy spectrum. spectrum; and and (c) (c) Zn Zn 2p 2p binding binding energy energy spectrum. spectrum; spectrum.
The scanning electron microscopy (SEM) image of Fe3O4 NPs is shown in Figure 4a. It can be scanning electron microscopy (SEM) image of Fe33O O44 NPs NPs is is shown shown in in Figure 4a. It It can be The scanning seen that Fe3O4 NPs have a relatively uniform spherical shape, and the Fe 3O4 NPs with smooth seen that Fe33O O44 NPs NPs have have aa relatively uniform spherical shape, and the Fe33O44 NPs NPs with smooth seen surfaces have diameters in the range of 250–300 nm. Figure 4b,c shows the as-synthesized Fe3O4/ZnO surfaces have diameters in in the the range range of of 250–300 250–300 nm. nm. Figure Figure 4b,c 4b,cshows showsthe theas-synthesized as-synthesizedFe Fe3 3OO44/ZnO /ZnO nanocomposites under different experimental conditions. Sample A is shown in Figure 4b; it is visible nanocomposites under different experimental experimental conditions. conditions. Sample Sample A is shown in Figure 4b; it is visible visible that the products had a disorderly composition comprising a few scattered Fe3O4 NPs and short ZnO that the products had a disorderly composition comprising comprising aa few few scattered scattered Fe Fe33O44 NPs and short ZnO nanorods. The presence of disordered nanorods and NPs suggests that ZnO particles failed to nanorods. The Thepresence presenceofof disordered nanorods NPs suggests thatparticles ZnO particles to disordered nanorods and and NPs suggests that ZnO failed to failed generate generate chemical bonds with the Fe3O4 NPs and grew into short rod shapes alone with the chemical chemical bonds with the Fewith O NPs and grew into short rod shapes alone with the introduction of generate bonds the Fe 3 O 4 NPs and grew into short rod shapes alone with the 3 4 introduction of a larger amount of ammonium hydroxide (3 mL). When the amount of ammonium introduction of aof larger amounthydroxide of ammonium hydroxide (3 mL). When the amounthydroxide of ammonium a larger amount ammonium (3 mL). When the amount of ammonium was hydroxide was reduced to 2 mL (sample B) in the mix solution, the product exhibited a spherical hydroxide B) in the solution, the product exhibited spherical reduced to was 2 mLreduced (sampletoB)2inmL the(sample mix solution, the mix product exhibited a spherical shape a(Figure 4c), shape (Figure 4c), and the diameters were a bit larger than those of the Fe3O4 NPs in Figure 4a. We shape 4c), and diameters were a bit those of the Fe 3 O 4 NPs in Figure 4a. We and the(Figure diameters werethe a bit larger than those oflarger the Fethan O NPs in Figure 4a. We deduced that Fe 3 4 3 O4 deduced that Fe3O4 NPs were uniformly covered by the ZnO shells in a spherical shape. The magnetic deduced Fe3O4 NPs were uniformly covered byathe ZnO shells in aThe spherical shape. The NPs werethat uniformly covered by the ZnO shells in spherical shape. magnetic NPs aremagnetic utilized NPs are utilized as a seed-mediated growth mechanism to grow a layer of ZnO on their surfaces, thus NPs utilized as a growth seed-mediated growth mechanism grow layer of ZnO on their surfaces, as a are seed-mediated mechanism to grow a layertoof ZnOa on their surfaces, thus makingthus the making the surface much rougher than the naked Fe3O4 NPs. The morphology and distribution of the making the surface much than Fe3Omorphology 4 NPs. The morphology and distribution of the surface much rougher thanrougher the naked Fethe NPs. The and distribution of the Fe3 O4 /ZnO 3 O4naked Fe3O4/ZnO core–shell structured nanocomposites are clearly recognizable from the lowFe 3O4/ZnOstructured core–shell structured nanocomposites are clearly from TEM the image lowcore–shell nanocomposites are clearly recognizable from therecognizable low-magnification magnification TEM image in Figure 4d. It can be discerned that the nanocomposites are nearly magnification in Figure It can be discerned thatspherical the nanocomposites nearly in Figure 4d. It TEM can beimage discerned that the4d. nanocomposites are nearly in shape, with are a diameter spherical in shape, with a diameter distribution of 280–330 nm, which is consistent with the SEM spherical in of shape, withnm, a diameter distribution of the 280–330 nm, which is consistent with the SEM distribution 280–330 which is consistent with SEM image in Figure 4c. The high-resolution image in Figure 4c. The high-resolution TEM image of one typical core–shell structured Fe3O4/ZnO image in Figure 4c.typical The high-resolution TEM image one typical core–shell structured 3O4/ZnO TEM image of one core–shell structured Fe3 Oof /ZnO composite is demonstrated inFeFigure 4e. 4 composite is demonstrated in Figure 4e. The distinction between the transparent boundary and the composite is demonstrated in Figure 4e. The distinction between the transparent boundary and the
Materials 2018, 11, 780
5 of 12
Materials 2018, 11, x FOR PEER REVIEW
5 of 11
The distinction between the transparent boundary and the dark core confirms the growth of ZnO on the Fe NP. It can observed that on a thin layer ZnO is growing on the oflayer Fe3 Oof 3 O4confirms 4 NP dark core thebegrowth of ZnO the Fe 3O4 of NP. It can be observed thatedge a thin ZnOatisa thicknesson of the ~15edge nm. Figure alsoatpresents the diffraction by the inserted SAED growing of Fe 3O4f 4 NP a thickness of ~15 nm.profile Figuregenerated 4f also presents the diffraction pattern and confirms the structure of ZnO and Fe O . The SEM and TEM results clearly indicate 3 confirms 4 profile generated by the inserted SAED pattern and the structure of ZnO and Fe 3O4. The that the possess a core-shell structure, andpossess that theainner Fe3 O4type NPsstructure, cores are SEM andnanocomposites TEM results clearly indicate that thetype nanocomposites core-shell successfully wrapped the uniformed ZnO shells. and that the inner Fe3Owith 4 NPs cores are successfully wrapped with the uniformed ZnO shells.
Figure 4. The SEM images of Fe3O4 (a); sample A (b); and sample B (c); TEM image (d); HRTEM image Figure 4. The SEM images of Fe3 O4 (a); sample A (b); and sample B (c); TEM image (d); HRTEM image (e) of sample A and SAED pattern (f) of sample B, respectively. (e) of sample A and SAED pattern (f) of sample B, respectively.
In order to explore the microwave absorption properties of Fe3O4/ZnO nanocomposites, the In complex order topermeability explore the(μmicrowave absorption properties of Fe3 O−4jε″) /ZnO nanocomposites, relative r = μ′ − jμ″) and relative permittivity (εr =ε′ were measured by a 0 0 the relative complex permeability (µr = µrange − jµ”) permittivity were measured r =ε − jε”) vector network analyzer in the frequency ofand 2–18relative GHz. The measured(εsamples were prepared by by a vector network analyzer in the frequency range of 2–18 GHz. The measured samples were uniformly mixing with paraffin (in mass fractions of 30%, 50%, and 70%) at 85 °C, pressed into toroidal◦ C, pressed prepared by uniformly mixing with paraffin line (in mass fractions of 30%, 50%, andevaluated 70%) at 85 shaped samples. According to transmission theory, EM properties can be based on the into toroidal-shaped samples. According to transmission line theory, EM properties can be evaluated relative complex permeability (μr = μ′ − jμ″) and relative permittivity (εr =ε′ − jε″). μ′ and ε′ represent the 0 based to on store the relative complex permeability and relative permittivity (εr =ε0 which − jε”). r = µ − jµ”) ability EM energy, whereas μ″ and ε″(µrepresent the inner dissipation of EM energy, 0 and ε0 represent the ability to store EM energy, whereas µ” and ε” represent the inner dissipation of µ originates from the relaxation and resonance mechanisms. The relative complex permittivity (ε′, ε″) and EM energy, which originates(μ′, from relaxation and resonance mechanisms. The relative complex relative complex permeability μ″)the of Fe 3O4, sample A and sample B measured in the frequency range 0 , ε”) and relative complex permeability (µ0 , µ”) of Fe O , sample A and sample B permittivity (ε 4 ε′ of Fe3O4 is in the range of 2–18 GHz are plotted in Figure 5a–d. From Figure 5a,b, it can be found3 that measured theε″ frequency range ofrange 2–18 GHz are plotted Figure 5a–d. From Figure it can of 4.67–5.03,inand of Fe3O4 is in the of 0.40–1.02. Afterinbeing composited with ZnO,5a,b, the values 0 of Fe O is in the range of 4.67–5.03, and ε” of Fe O is in the range of 0.40–1.02. be found that ε 3 4 3 4 of ε′ and ε″ show a sharp growth. As for sample A, the values of ε′ and ε″ are in the range of 7.12–8.94 0 and ε” show a sharp growth. As for sample A, the After being composited with ZnO, the the values of εof and 0.70–1.67, respectively. Meanwhile, values ε′ and ε″ raise to the range of 9.62–16.60 and 1.17– 0 and ε” are in the range of 7.12–8.94 and 0.70–1.67, respectively. Meanwhile, the values of ε0 values of ε 10.23, respectively, after the Fe3O4 NPs were coated with the ZnO shell (sample B). The values of ε′ and and raise toB the range more of 9.62–16.60 and great 1.17–10.23, the Fe3frequency O4 NPs were coated ε″ forε”sample fluctuate and exhibit changerespectively, in the main after measuring region. As 0 with the shell The values of ε aand ε” for trend sample B fluctuate more and exhibit shown in ZnO Figure 5a, (sample the ε′ of B). sample B presents declining with increasing frequency, whilegreat the change main measuring frequency shown Figure 5a, the ε0 The of sample trend of in ε″ the is the contrary, and some peaksregion. appear As in the highin frequency region. curvesBofpresents ε′ and ε″a declining trend with increasing frequency, while the trend of ε” is the contrary, and some peaks appear indicate that the ZnO shell can greatly improve the dielectric properties of the material. Figure 5c,d 0 in the high frequency region.permeability The curves of and ε” indicate that can greatly improve the shows the relative complex of εthe three materials. Asthe forZnO Fe3Oshell 4 NPs, the value of μ′ drops dielectric properties material.range Figure showsand the then relative complex permeability the three sharply from 1.27 in of thethe frequency of 5c,d 2–6 GHz, shows a fluctuating trendof versus the 0 drops sharply from 1.27 in the frequency range of 2–6 GHz, materials. As for Fe O NPs, the value of µ 4 the Fe3O4/ZnO (sample A and B) composites display a similar variation trend changing frequency,3 and and then shows a fluctuating versusrange. the changing frequency,may andresult the Fe (sample A 3 O4 /ZnO throughout the entire measuredtrend frequency This phenomenon from the eddy current and B) composites display a similar variation trend throughout the entire measured frequency range. effect. Compared to the value of μ″, differences in the μ″ values of Fe3O4 and Fe3O4/ZnO composites are This phenomenon may result from the eddy current to the of µ”, of differences in distinguished at low frequency. Meanwhile, in the effect. range Compared of 8–18 GHz, thevalue μ″ curves Fe3O4 and Fe3O4/ZnO composites change to become similar; one μ″ value peak of sample B is observed at ~11 GHz, which may be attributable to the dissipation of EM energy. Furthermore, it is noticed that negative
Materials 2018, 11, 780
6 of 12
the µ” values of Fe3 O4 and Fe3 O4 /ZnO composites are distinguished at low frequency. Meanwhile, in Materials 11, x GHz, FOR PEER 6 of 11 the range2018, of 8–18 the REVIEW µ” curves of Fe3 O4 and Fe3 O4 /ZnO composites change to become similar; one µ” value peak of sample B is observed at ~11 GHz, which may be attributable to the dissipation of values of μ″Furthermore, occur in the high frequency due to calibration or sensitivity issues of the experimental EM energy. it is noticed thatrange negative values of µ” occur in the high frequency range due toset-up. calibration or sensitivity issues of the experimental set-up.
Figure5.5.Frequency Frequencydependence dependenceon onthe the(a) (a)real realpart partand and(b) (b)imaginary imaginarypart partofofrelative relativecomplex complex Figure permittivity; (c) real part and (d) imaginary part of relative complex permeability. permittivity; (c) real part and (d) imaginary part of relative complex permeability.
The theoretical reflection loss (RL) of the composite absorber at different thicknesses was The theoretical reflection loss (RL) of the composite absorber at different thicknesses was calculated using the following equations [30–32]: calculated using the following equations [30–32]: (1) 𝑅𝐿 = 20 𝑙𝑜𝑔 |(𝑍𝑖𝑛 − 𝑍0 )⁄(𝑍𝑖𝑛 + 𝑍0 )| RL = 20 log|( Zin − Z0 )/( Zin + Z0 ) | (1) 𝜇𝑟 2𝜋𝑓𝑑 √𝜇𝑟 𝜀𝑟 (𝑗 √ 𝑍𝑖𝑛 = r 𝑍0 √ 𝑡𝑎𝑛ℎ (2) 2π f d 𝑐µr ε r ) µr𝜀𝑟 Zin = Z0 tanh j (2) εr c Here, Z0 is the impedance of free space, Zin is the normalized input impedance of the absorber, Here, Z0 is the impedance of free space, Zin is the normalized input impedance of the absorber, d is the thickness, C is the velocity of EMW in free space, and f is the frequency of the incident wave. d is the thickness, C is the velocity of EMW in free space, and f is the frequency of the incident RL values of −10 dB and −20 dB correspond to 90% and 99% attenuation of the incident EMW energy, wave. RL values of −10 dB and −20 dB correspond to 90% and 99% attenuation of the incident and the frequency range where RL is smaller than −10 dB is defined as the effective absorption EMW energy, and the frequency range where RL is smaller than −10 dB is defined as the effective bandwidth. absorption bandwidth. Figure 6a–e shows the plots of RL versus the frequency of the Fe3O4 NPs and two samples of Figure 6a–e shows the plots of RL versus the frequency of the Fe3 O4 NPs and two samples of Fe3O4/ZnO composites at different thicknesses. As for Fe3O4 NPs, the minimum RL of −7.28 dB is Fe3 O4 /ZnO composites at different thicknesses. As for Fe3 O4 NPs, the minimum RL of −7.28 dB is observed at 15.68 GHz with the thickness of 2.5 mm, which indicates that the naked Fe 3O4 NPs have observed at 15.68 GHz with the thickness of 2.5 mm, which indicates that the naked Fe3 O4 NPs have a a weak EMW absorption property. Furthermore, with the doping of the dielectric component, the weak EMW absorption property. Furthermore, with the doping of the dielectric component, the EMW EMW absorption property of sample A can be improved slightly. As shown in Figure 6b, the absorption property of sample A can be improved slightly. As shown in Figure 6b, the minimum minimum RL is −13.91 dB at 5.52 GHz with the thickness of 4.5 mm. This is because the ZnO particles RL is −13.91 dB at 5.52 GHz with the thickness of 4.5 mm. This is because the ZnO particles failed failed to generate chemical bonds with the Fe3O4 NPs and grew into short rod shapes alone; therefore, to generate chemical bonds with the Fe3 O4 NPs and grew into short rod shapes alone; therefore, the composites were unable to obtain a good impedance match and interfacial polarization. As for the composites were unable to obtain a good impedance match and interfacial polarization. As for the sample B loaded with 30 wt % (Figure 6c), because of the high dispersion in the paraffin matrix, the sample B loaded with 30 wt % (Figure 6c), because of the high dispersion in the paraffin matrix, the Fe3O4/ZnO nanoparticles failed to generate conductive interconnections, so the EMW absorption the Fe3 O4 /ZnO nanoparticles failed to generate conductive interconnections, so the EMW absorption performance did not show an enhancement in comparison to pure Fe3O4. It is noticed that sample B performance did not show an enhancement in comparison to pure Fe3 O4 . It is noticed that sample loaded with 50 wt % shows an enhanced EMW absorption property (Figure 6d). Specifically, the B loaded with 50 wt % shows an enhanced EMW absorption property (Figure 6d). Specifically, the minimum RL value of −50.79 dB can be achieved at 4.38 GHz with the thickness of 4.5 mm. Based on minimum RL value of −50.79 dB can be achieved at 4.38 GHz with the thickness of 4.5 mm. Based on the results of Figure 4d–f, we deduced that the incorporation of the dielectric ZnO into the Fe 3O4 NPs the results of Figure 4d–f, we deduced that the incorporation of the dielectric ZnO into the Fe3 O4 NPs may generate a high dielectric constant and loss due to the effective interfaces between the dielectric and magnetic materials, giving them an advantage in terms of matching complex permittivity and permeability. The enhanced EM absorption properties benefit from the uniform core–shell structures, which induce an intensification of interfacial polarization. It is worth noting that the core–shell structured Fe3O4/ZnO composites are able to achieve an enhanced absorption property in both low
Materials 2018, 11, 780
7 of 12
may generate a high dielectric constant and loss due to the effective interfaces between the dielectric and magnetic materials, giving them an advantage in terms of matching complex permittivity and permeability. The enhanced EM absorption properties benefit from the uniform core–shell structures, Materials 2018, x FOR PEER REVIEW 7 of Materials 2018, 11,11, xan FOR PEER REVIEW 7 of 1111 which induce intensification of interfacial polarization. It is worth noting that the core–shell structured Fe3 O4 /ZnO composites are able to achieve an enhanced absorption property in both low andhigh highfrequency frequencybands; bands;such suchdual dualabsorption absorptionregions regionsare arealso alsocompetitive competitiveinincomparison comparisontotoother other and and high frequency bands; such dual absorption regions are also competitive in comparison to other materials. It can be observed from Figure 6d that the minimum RL values all shift toward the lower materials. It can be observed from Figure 6d that the minimum RL values all shift toward the lower materials. It can be observed from Figure 6d that the minimum RL values all shift toward the lower frequencyregion regionwith withincreasing increasingthickness, thickness,which whichcan canbebeexplained explainedbybythe thequarter-wavelength quarter-wavelengthmatch match frequency frequency region with increasing thickness, which can be explained by the quarter-wavelength match principle [18]: principle [18]: principle [18]: 𝑛𝜆 𝑛𝑐𝑛𝑐 nλ 𝑡𝑚= =𝑛𝜆= == pnc 1,3,5……) ) (3) (𝑛(𝑛==1,3,5 (3) (3) tm𝑡𝑚 = 𝑚 √(|𝜀𝑟𝜇𝑟|)( n = 1, 3, 5 . . . ) 44 4𝑓4𝑓 √(|𝜀𝑟𝜇𝑟|) 𝑚 4 4 f m (|εrµr |) where tm isthe the absorberthickness, thickness, μr isthe the complexpermeability permeability atfmf,m,and and εr isthe the complex where where ttmm is is the absorber absorber thickness, μµrr is is the complex complex permeability at at fm , and εεrr is is the complex complex permittivity atfmf.mThe . The frequencydependence dependence oftmtm(n(n= =1,1,3)3)isiscalculated calculated andplotted plotted onthe the contour permittivity permittivity at at fm . The frequency frequency dependence of of tm (n = 1, 3) is calculated and and plotted on on the contour contour mapsininFigure Figure7.7.ItItcan canbebenoticed noticedthat thatallallthe thepoints pointsofofRL RL min lieononthe thecurves curvesofoftmtmfor forsample sampleB.B. maps min lie maps in Figure 7. It can be noticed that all the points of RLmin lie on the curves of tm for sample B. Thus, itisisdemonstrated demonstrated thatthe the quarter-wavelengthmatch match principleisisananeffective effective toolthat that provides Thus, Thus, it it is demonstrated that that the quarter-wavelength quarter-wavelength match principle principle is an effective tool tool that provides provides crucial guideininthe the thicknessdesign design ofabsorbers. absorbers. aa acrucial crucial guide guide in the thickness thickness design of of absorbers.
Figure6.6.RL RLcurves curvesofofparaffin paraffinsamples samplescontaining containing5050 3O4 (a) andsample sampleA (b);RL RLcurves curvesofof Figure RL wtwt %% FeFe (a) and and sample AA(b); (b); RL 33O44 (a) paraffinsamples samplescontaining containing3030wtwt (c); and sample respectively. paraffin sample B,B, respectively. %% (c); 5050 wtwt %% (d)(d) and 7070 wtwt %% (e)(e) sample B, respectively.
Figure 7.RL RL 2-Dcontour contourmap maprepresentations representations inthe thefrequency frequencyrange rangeofof2–18 2–18GHz GHzloaded loadedwith with Figure Figure 7. 7. RL 2-D 2-D contour map representations ininthe frequency range of 2–18 GHz loaded with 50 wt % 50 wt % of sample B. 50 wt % ofB. sample B. of sample
Typically,the themagnetic magneticloss lossisisimplied impliedbybythe theimaginary imaginarypart partofofpermeability permeabilityand andmainly mainly Typically, originates from hysteresis loss, domain wall displacement, natural resonance, and eddy current originates from hysteresis loss, domain wall displacement, natural resonance, and eddy current resonance.InIngeneral, general,hysteresis hysteresisloss lossisismainly mainlycaused causedbybythe thetime timelag lagofofthe themagnetization magnetizationvector vector resonance. behindthe theexternal externalEM-field EM-fieldvector vectorand andwill willalways alwaysbebenegligible negligibleinina aweak weakapplied appliedfield, field,while while behind domain wall resonance loss takes place in the MHz frequency range. The following equation is used domain wall resonance loss takes place in the MHz frequency range. The following equation is used determinewhether whethereddy eddycurrents currentscontribute contributetotothe themagnetic magneticloss loss[33]: [33]: totodetermine
Materials 2018, 11, 780
8 of 12
Typically, the magnetic loss is implied by the imaginary part of permeability and mainly originates from hysteresis loss, domain wall displacement, natural resonance, and eddy current resonance. In general, hysteresis loss is mainly caused by the time lag of the magnetization vector behind the external EM-field vector and will always be negligible in a weak applied field, while domain wall resonance loss takes place in the MHz frequency range. The following equation is used to determine whether eddy currents contribute to the magnetic loss [33]: Materials 2018, 11, x FOR PEER REVIEW
C0 = µ00 µ0
−2
f −1
8 of 11
(4)
magneticloss lossonly onlystems stemsfrom from the eddy current, 0 should be equal to a constant value 2πμ02d2σ IfIfmagnetic the eddy current, C0Cshould be equal to a constant value 2πµ0 d σ (d is the thickness of the MAMs, σ is the electrical conductivity, andµ μ0isisthe thepermeability permeabilityinina a (d is the thickness of the MAMs, σ is the electrical conductivity, and 0 vacuum) and would be independent of frequency; if not, the magnetic loss is ascribed natural vacuum) and would be independent of frequency; if not, the magnetic loss is ascribed totonatural resonance.From FromFigure Figure8a, 8a,we wefind findthat thatthe thevalue valueofofCC0varies varieswith withthe thefrequency frequencyand andpresents presentsaasharp sharp resonance. 0 declining tendency in the frequency range of 2–8 GHz. However, when the frequency is in the range declining tendency in the frequency range of 2–8 GHz. However, when the frequency is in the range of of ~8–12 GHz, the value C0 closes to a constant. Based on this phenomenon, it can be concluded ~8–12 GHz, the value of Cof 0 closes to a constant. Based on this phenomenon, it can be concluded that that magnetic loss results from the natural and exchange resonance and the current eddy current magnetic loss results from the natural and exchange resonance and the eddy effect. effect.
Figure8.8.Frequency Frequencydependence dependenceofofCC 0 (a) and values of attenuation constant of α (b) of sample B in Figure 0 (a) and values of attenuation constant of α (b) of sample B in therange rangeofof2–18 2–18GHz. GHz. the
According to transmission line theory, EMW absorption properties can be expressed by the According to transmission line theory, EMW absorption properties can be expressed by the attenuation constant of α. The attenuation constants of sample B with 30, 50, and 70 wt % were attenuation constant of α. The attenuation constants of sample B with 30, 50, and 70 wt % were calculated using the following expression [34,35]: calculated using the following expression [34,35]: √2𝜋𝑓 r (5) √(𝜇 ” 𝜀 ” − 𝜇 ′ 𝜀 ′ )2 + (𝜇 ′ 𝜀 ” + 𝜇 ” 𝜀 ′ )2 α√= × √(𝜇 ” 𝜀 ” − 𝜇 ′ 𝜀 ′ ) +q 2π f𝑐 2 2 0 0 0 0 0 0 00 00 00 00 00 00 (5) α= × (µ ε − µ ε ) + (µ ε − µ ε ) + (µ ε + µ ε ) where c is the velocity ofc light in a vacuum. Figure 8b shows the plot of the attenuation constant of α versusc is frequency. It can be seen the sample 50 wtthe % filler loading has the largest value where the velocity of light in a that vacuum. Figurewith 8b shows plot of the attenuation constant of αof α; thus, we supposed that the sample with 50 wt % filler loading possesses greater EMW attenuation versus frequency. It can be seen that the sample with 50 wt % filler loading has the largest value of α; and impedance matching the other samples. Figure 9 shows the dielectric loss (tanδattenuation ε = ε″/ε′) and thus, we supposed that thethan sample with 50 wt % filler loading possesses greater EMW 0 ) and magnetic tangent loss (tanδ = μ″/μ′) Fe3O4/ZnO and9 Fe 3O4 NPs, respectively. tanδεε and tanδ μ are and impedance matching thanμ the otherofsamples. Figure shows the dielectric loss (tanδ = ε”/ε 0 two possible contributors for EMW absorption, and are commonly used to describe material loss magnetic tangent loss (tanδµ = µ”/µ ) of Fe3 O4 /ZnO and Fe3 O4 NPs, respectively. tanδε and tanδ µ capacity. Therefore, we calculated the tangent loss and based the data in Figure 5. Specifically, sample are two possible contributors for EMW absorption, areon commonly used to describe material loss B has aTherefore, higher tanδ value than nakedloss Febased 3O4 NPs, indicating that the ZnO shell sample obviously capacity. weε calculated thethe tangent on the data in Figure 5. Specifically, B improves the dielectric properties of the composites. Additionally, Figure 7 clearly shows that the has a higher tanδε value than the naked Fe3 O4 NPs, indicating that the ZnO shell obviously improves magnetic lossproperties factor (tanδ is much higher than the dielectric loss7 factor ε) inthat the the lowmagnetic frequency the dielectric ofμ)the composites. Additionally, Figure clearly(tanδ shows range (~2–7 GHz), which indicates that magnetic loss plays a vital role in EMW absorption in this loss factor (tanδµ ) is much higher than the dielectric loss factor (tanδε ) in the low frequency range region. Meanwhile, in the high frequency range (~8–12 GHz), the value of tanδ ε is higher than tanδμ, (~2–7 GHz), which indicates that magnetic loss plays a vital role in EMW absorption in this region. which indicates dielectric lossrange is the(~8–12 main loss in this frequency region. Such than a complementarity Meanwhile, in thethat high frequency GHz), the value of tanδ tanδµ , which ε is higher between dielectric loss and magnetic loss demonstrates the Fe 3 O 4 /ZnO composites possess indicates that dielectric loss is the main loss in this frequency region. Such a complementaritytobetween promising absorption properties. dielectric lossEMW and magnetic loss demonstrates the Fe3 O4 /ZnO composites to possess promising EMW
absorption properties.
magnetic loss factor (tanδμ) is much higher than the dielectric loss factor (tanδε) in the low frequency range (~2–7 GHz), which indicates that magnetic loss plays a vital role in EMW absorption in this region. Meanwhile, in the high frequency range (~8–12 GHz), the value of tanδε is higher than tanδμ, which indicates that dielectric loss is the main loss in this frequency region. Such a complementarity between dielectric Materials 2018, 11, 780 loss and magnetic loss demonstrates the Fe3O4/ZnO composites to possess 9 of 12 promising EMW absorption properties.
Figure (a) and and magnetic magneticloss losstangent tangent(b) (b)ofofthe theFe FeO 3O4 and Fe3O4/ZnO (sample Figure 9. 9. Dielectric Dielectric loss loss tangent tangent (a) 3 4 and Fe3 O4 /ZnO (sample B), respectively. B), respectively.
In Table 1, the recently reported EMW absorption performances of typical Fe3 O4 material-based composites, as well as the Fe3 O4 /ZnO composites prepared in this work, have been plotted. In comparison with the reported composites in Table 1, it can be observed that the Fe3 O4 /ZnO composites have a wide effective absorption bandwidth and a promising negative RL value among these composites. It can be concluded that the as-fabricated Fe3 O4 /ZnO nanocomposites with enhanced EMW absorption properties confirm the presence of an efficient complementarity between magnetic and dielectric loss. The above-mentioned advantages indicate that this special core–shell structured absorber is able to meet the requirements of ideal MAMs. Table 1. EMW absorption performances of typical Fe3 O4 -based composites reported in this work and recent literature. Sample SnO2 /Fe3 O4 /MWCNTs Fe3 O4 /SiO2 /rGO Fe2 O4 /MnO2 Fe3 O4 @C FePc-Fe3 O4 -BF Fe3 O4 /ZnO
wt (%)
Optimum Frequency (GHz)
Minimum RL Value (dB)
Ref.
70 20 40 66.7 75 50
10.90 9.70 16.80 16.20 5.90 4.38
−42.00 −26.60 −41.50 −22.60 −31.10 −50.79
[36] [37] [38] [39] [40] This work
4. Conclusions In summary, Fe3 O4 /ZnO nanocomposites were synthesized via a chemical method of coating magnetic cores (Fe3 O4 ) with ZnO by co-precipitation of Fe3 O4 with zinc acetate in a basic medium of ammonium hydroxide, and the morphology and the microwave absorption properties were investigated in detail. It is suggested that the amount of ammonium hydroxide plays a key role in controlling the morphologies of the composites, and the SEM and TEM results further confirmed that ZnO shell generated chemical bonds with the Fe3 O4 NPs. Owing to the core–shell structure, an efficient complementary balance was achieved between dielectric loss and magnetic loss. Moreover, the enhanced microwave absorption properties benefitted from the core–shell structure, which induces intensified interfacial polarization. Specifically, the minimum RL value of −50.79 dB can be achieved at 4.38 GHz when the thickness is 4.5 mm. The mechanism of designing neoteric structures with magnetic and dielectric materials in order to broaden the effective absorption bandwidth would open up a promising domain in designing composites with high EM absorption performance. As a result, our Fe3 O4 /ZnO nanocomposites are expected to form a novel platform for advancing EMW absorbers. Author Contributions: X.S. conceived and designed the experiments; G.M. and X.L. performed the experiments; M.S. and H.L. analyzed the data; F.W. and J.W. wrote the paper. Funding: This research was funded by Natural Science Foundation of Jiangsu Province (BK20161466).
Materials 2018, 11, 780
10 of 12
Conflicts of Interest: The authors declare no conflict of interest.
References 1.
2.
3.
4.
5.
6. 7.
8.
9. 10.
11. 12. 13. 14.
15.
16.
17.
Yuan, J.; Liu, Q.; Li, S.; Lu, Y.; Jin, S.; Li, K.; Chen, H.; Zhang, H. Metal organic framework (MOF)-derived carbonaceous Co3 O4 /Co microframes anchored on RGO with enhanced electromagnetic wave absorption performances. Synth. Met. 2017, 228, 32–40. [CrossRef] Hu, Q.; Qi, X.; Cai, H.; Xie, R.; Long, L.; Bai, Z.; Jiang, Y.; Qin, S.; Zhong, W.; Du, Y. Preparation of porous Fe2 O3 nanorods-reduced graphene oxide nanohybrids and their excellent microwave absorption properties. Sci. Rep. 2017, 7, 11213. [CrossRef] [PubMed] Dom, R.; Subasri, R.; Hebalkar, N.Y.; Chary, A.S.; Borse, P.H. Synthesis of a hydrogen producing nanocrystalline ZnFe2 O4 visible light photocatalyst using a rapid microwave irradiation method. RSC Adv. 2012, 2, 12782. [CrossRef] Jian, X.; Chen, X.; Zhou, Z.; Li, G.; Jiang, M.; Xu, X.; Lu, J.; Li, Q.; Wang, Y.; Gou, J.; et al. Remarkable improvement in microwave absorption by cloaking a micro-scaled tetrapod hollow with helical carbon nanofibers. Phys. Chem. Chem. Phys. 2015, 17, 3024–3031. [CrossRef] [PubMed] Qiang, R.; Du, Y.; Chen, D.; Ma, W.; Wang, Y.; Xu, P.; Ma, J.; Zhao, H.; Han, X. Electromagnetic functionalized Co/C composites by in situ pyrolysis of metal-organic frameworks (ZIF-67). J. Alloys Compd. 2016, 681, 384–393. [CrossRef] Feng, J.; Wang, Y.; Hou, Y.; Li, L. Tunable design of yolk–shell ZnFe2 O4 @RGO@TiO2 microspheres for enhanced high-frequency microwave absorption. Inorg. Chem. Front. 2017, 4, 935–945. [CrossRef] Wan, G.; Wang, G.; Huang, X.; Zhao, H.; Li, X.; Wang, K.; Yu, L.; Peng, X.; Qin, Y. Uniform Fe3 O4 coating on flower-like ZnO nanostructures by atomic layer deposition for electromagnetic wave absorption. Dalton Trans. 2015, 44, 18804–18809. [CrossRef] [PubMed] Hu, Q.; Tong, G.; Wu, W.; Liu, F.; Qian, H.; Hong, D. Selective preparation and enhanced microwave electromagnetic characteristics of polymorphous ZnO architectures made from a facile one-step ethanediamine-assisted hydrothermal approach. CrystEngComm 2013, 15, 1314. [CrossRef] Sano, E.; Akiba, E. Electromagnetic absorbing materials using nonwoven fabrics coated with multi-walled carbon nanotubes. Carbon 2014, 78, 463–468. [CrossRef] Liu, P.; Huang, Y.; Yan, J.; Yang, Y.; Zhao, Y. Construction of CuS Nanoflakes Vertically Aligned on Magnetically Decorated Graphene and Their Enhanced Microwave Absorption Properties. ACS Appl. Mater. Interfaces 2016, 8, 5536–5546. [CrossRef] [PubMed] Lv, H.; Zhang, H.; Ji, G.; Xu, Z.J. Interface Strategy to Achieve Tunable High Frequency Attenuation. ACS Appl. Mater. Interfaces 2016, 8, 6529–6538. [CrossRef] [PubMed] Lu, B.; Dong, X.L.; Huang, H.; Zhang, X.F.; Zhu, X.G.; Lei, J.P.; Sun, J.P. Microwave absorption properties of the core/shell-type iron and nickel nanoparticles. J. Magn. Magn. Mater. 2008, 320, 1106–1111. [CrossRef] Qiang, R.; Du, Y.; Zhao, H.; Wang, Y.; Tian, C.; Li, Z.; Han, X.; Xu, P. Metal organic framework-derived Fe/C nanocubes toward efficient microwave absorption. J. Mater. Chem. A 2015, 3, 13426–13434. [CrossRef] Li, X.; Feng, J.; Du, Y.; Bai, J.; Fan, H.; Zhang, H.; Peng, Y.; Li, F. One-pot synthesis of CoFe2 O4 /graphene oxide hybrids and their conversion into FeCo/graphene hybrids for lightweight and highly efficient microwave absorber. J. Mater. Chem. A 2015, 3, 5535–5546. [CrossRef] Sui, M.; Lü, X.; Xie, A.; Xu, W.; Rong, X.; Wu, G. The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption. Synth. Met. 2015, 210, 156–164. [CrossRef] Hu, C.; Mou, Z.; Lu, G.; Chen, N.; Dong, Z.; Hu, M.; Qu, L. 3D graphene-Fe3 O4 nanocomposites with high-performance microwave absorption. Phys. Chem. Chem. Phys. 2013, 15, 13038–13043. [CrossRef] [PubMed] Li, B.; Weng, X.; Wu, G.; Zhang, Y.; Lv, X.; Gu, G. Synthesis of Fe3 O4 /polypyrrole/polyaniline nanocomposites by in-situ method and their electromagnetic absorbing properties. J. Saudi Chem. Soc. 2017, 21, 466–472. [CrossRef]
Materials 2018, 11, 780
18.
19. 20. 21.
22. 23.
24.
25.
26.
27. 28.
29.
30.
31.
32.
33.
34. 35. 36. 37.
11 of 12
Quan, B.; Liang, X.; Xu, G.; Cheng, Y.; Zhang, Y.; Liu, W.; Ji, G.; Du, Y. A permittivity regulating strategy to achieve high-performance electromagnetic wave absorbers with compatibility of impedance matching and energy conservation. New J. Chem. 2017, 41, 1259–1266. [CrossRef] Zhang, Z.; Liu, X.; Wang, X.; Wu, Y.; Liu, Y. Electromagnetic and microwave absorption properties of Fe–Sr0.8 La0.2 Fe11.8 Co0.2 O19 shell-core composites. J. Magn. Magn. Mater. 2012, 324, 2177–2182. [CrossRef] Shen, J.; Chen, K.; Li, L.; Wang, W.; Jin, Y. Fabrication and microwave absorbing properties of (Z-type barium ferrite/silica)@polypyrrole composites. J. Alloys Compd. 2014, 615, 488–495. [CrossRef] Ren, Y.; Zhu, C.; Zhang, S.; Li, C.; Chen, Y.; Gao, P.; Yang, P.; Ouyang, Q. Three-dimensional SiO2 @Fe3 O4 core/shell nanorod array/graphene architecture: Synthesis and electromagnetic absorption properties. Nanoscale 2013, 5, 12296–12303. [CrossRef] [PubMed] Deng, J.; Wang, Q.; Zhou, Y.; Zhao, B.; Zhang, R. Facile design of a ZnO nanorod–Ni core–shell composite with dual peaks to tune its microwave absorption properties. RSC Adv. 2017, 7, 9294–9302. [CrossRef] Najim, M.; Modi, G.; Mishra, Y.K.; Adelung, R.; Singh, D.; Agarwala, V. Ultra-wide bandwidth with enhanced microwave absorption of electroless Ni-P coated tetrapod-shaped ZnO nano- and microstructures. Phys. Chem. Chem. Phys. 2015, 17, 22923–22933. [CrossRef] [PubMed] Liu, Y.; Yu, L.; Hu, Y.; Guo, C.; Zhang, F.; Wen David Lou, X. A magnetically separable photocatalyst based on nest-like gamma-Fe(2 )O(3 )/ZnO double-shelled hollow structures with enhanced photocatalytic activity. Nanoscale 2012, 4, 183–187. [CrossRef] [PubMed] Cheng, Y.-F.; Bi, H.; Wang, C.; Cao, Q.; Jiao, W.; Che, R. Dual-ligand mediated one-pot self-assembly of Cu/ZnO core/shell structures for enhanced microwave absorption. RSC Adv. 2016, 6, 41724–41733. [CrossRef] Feng, W.; Wang, Y.; Chen, J.; Guo, L.; Ouyang, J.; Jia, D.; Zhou, Y. Microwave absorbing property optimization of starlike ZnO/reduced graphene oxide doped by ZnO nanocrystal composites. Phys. Chem. Chem. Phys. 2017, 19, 14596–14605. [CrossRef] [PubMed] Sun, Y.; Xiao, F.; Liu, X.; Feng, C.; Jin, C. Preparation and electromagnetic wave absorption properties of core–shell structured Fe3 O4 –polyaniline nanoparticles. RSC Adv. 2013, 3, 22554. [CrossRef] Zhang, Y.; Zhang, M.; Yang, J.; Ding, L.; Zheng, J.; Xu, J.; Xiong, S. Formation of Fe3 O4 @SiO2 @C/Ni hybrids with enhanced catalytic activity and histidine-rich protein separation. Nanoscale 2016, 8, 15978–15988. [CrossRef] [PubMed] Yu, M.; Liang, C.; Liu, M.; Liu, X.; Yuan, K.; Cao, H.; Che, R. Yolk–shell Fe3 O4 @ZrO2 prepared by a tunable polymer surfactant assisted sol–gel method for high temperature stable microwave absorption. J. Mater. Chem. C 2014, 2, 7275. [CrossRef] Zhang, Y.; Quan, B.; Liu, W.; Liang, X.; Ji, G.; Du, Y. A facile one-pot strategy for fabrication of carbon-based microwave absorbers: Effects on annealing and paraffin content. Dalton Trans. 2017, 46, 9097–9102. [CrossRef] [PubMed] Zhao, B.; Guo, X.; Zhou, Y.; Su, T.; Ma, C.; Zhang, R. Constructing hierarchical hollow CuS microspheres via a galvanic replacement reaction and their use as wide-band microwave absorbers. CrystEngComm 2017, 19, 2178–2186. [CrossRef] Xie, A.; Sun, M.; Zhang, K.; Jiang, W.; Wu, F.; He, M. In situ growth of MoS2 nanosheets on reduced graphene oxide (RGO) surfaces: Interfacial enhancement of absorbing performance against electromagnetic pollution. Phys. Chem. Chem. Phys. 2016, 18, 24931–24936. [CrossRef] [PubMed] Sui, M.; Sun, X.; Lou, H.; Li, X.; Lv, X.; Li, L.; Gu, G. Synthesis of hollow Fe3 O4 particles via one-step solvothermal approach for microwave absorption materials: Effect of reactant concentration, reaction temperature and reaction time. J. Mater. Sci. Mater. Electron. 2018, 29, 7539–7550. [CrossRef] Liu, T.; Xie, X.; Pang, Y.; Kobayashi, S. Co/C nanoparticles with low graphitization degree: A high performance microwave-absorbing material. J. Mater. Chem. C 2016, 4, 1727–1735. [CrossRef] Liu, X.; Nie, X.; Yu, R.; Feng, H. Design of dual-frequency electromagnetic wave absorption by interface modulation strategy. Chem. Eng. J. 2018, 334, 153–161. [CrossRef] Wang, L.; Xing, H.; Liu, Z.; Shen, Z.; Sun, X.; Xu, G. Facile synthesis of net-like Fe3 O4 /MWCNTs decorated by SnO2 nanoparticles as a highly efficient microwave absorber. RSC Adv. 2016, 6, 97142–97151. [CrossRef] Pan, Y.-F.; Wang, G.-S.; Yue, Y.-H. Fabrication of Fe3 O4 @SiO2 @RGO nanocomposites and their excellent absorption properties with low filler content. RSC Adv. 2015, 5, 71718–71723. [CrossRef]
Materials 2018, 11, 780
38. 39. 40.
12 of 12
Liu, X.; Wu, N.; Cui, C.; Bi, N.; Sun, Y. One pot synthesis of Fe3 O4 /MnO2 core–shell structured nanocomposites and their application as microwave absorbers. RSC Adv. 2015, 5, 24016–24022. [CrossRef] Li, W.; Lv, B.; Wang, L.; Li, G.; Xu, Y. Fabrication of Fe3O4@C core–shell nanotubes and their application as a lightweight microwave absorbent. RSC Adv. 2014, 4, 55738–55744. [CrossRef] Guo, H.; Zhan, Y.; Chen, Z.; Meng, F.; Wei, J.; Liu, X. Decoration of basalt fibers with hybrid Fe3 O4 microspheres and their microwave absorption application in bisphthalonitrile composites. J. Mater. Chem. A 2013, 1, 2286–2296. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
