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Bi hollow porous microrods by controlling their composition
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
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Tailoring impedance match and enhancing microwave absorption of Fe3O4/ Bi24Fe2O39/Bi hollow porous microrods by controlling their composition Lin Liu, Na He, Jiacheng Sun, Panbing Hu, Rujia He, Jinxiu Cheng, Weifu Tian, Guoxiu Tong
⁎
College of Chemistry and Life Sciences, Zhejiang Key Laboratory for Reactive Chemistry on Solid Surface, Zhejiang Normal University, Jinhua 321004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fe3O4/Bi24Fe2O39/Bi composites Synthesis Hollow porous microrods Magnetic property Composition-dependent microwave absorption
A self-assembly/precipitate conversion/decomposition process was developed for the controllable synthesis of Fe3O4/Bi24Fe2O39/Bi hollow porous microrods (HPMRs). The results demonstrated that the crystal size, component, and performances of HPMRs could be effectively modulated via changing Fe2+/Bi3+ molar ratio (γ). Fe3O4/Bi24Fe2O39/Bi HPMRs exhibited ferromagnetic behavior at room temperature. As Bi and Bi24Fe2O39 contents increased with γ, the saturation magnetization Ms and attenuation constantly decreased, whereas coercivity Hc and impedance matching ratio increased. Compounding Fe3O4 with small quantities of Bi and Bi24Fe2O39 into HPMRs can significantly enhance microwave absorption. Fe3O4/Bi24Fe2O39/Bi HPMRs formed at γ = 1:0.25 exhibited the optimum microwave absorption performance. The minimum RL was − 47.3 dB at 8.72 GHz, corresponding to 2.4 mm sample thickness. The absorption band with the reflection loss below − 20 dB was up to 14.0 GHz for the absorber with a thickness of 1.4 − 8.0 mm. The results demonstrate that the introduction of electromagnetic transparent materials (Bi24Fe2O39 or Bi) can improve the microwave absorption performances of Fe3O4 composites owing to enhanced impedance matching rather than attenuation constant.
1. Introduction The study on applications of magnetic nanomaterials for solving serious electromagnetic (EM) pollution problems has been focused. [1,2]. Magnetic nanomaterials, as EM microwave absorbent (EMWA), can effectively retards EM pollution and strengthen the defense capability of weapon system [1–5]. Two crucial factors for promoting the practical application of EMWA include improvement of attenuation constant (α) and impedance matching. The attenuation mechanism of EM wave absorbing materials is mainly based on high magnetic loss and dielectric loss. According to the electromagnetic wave energy conversion principle, the proper impedance matching between permittivity and permeability determines the reflection and attenuation characteristics of EMWA. To our best knowledge, absorbers with a single structure and component can’t meet the aforementioned high demands. Therefore, heterostructured magnetic nanomaterials must be developed to enhance the absorption properties of EM waves. The design of one-dimensional (1D) ferromagnetic nanomaterials (i.e., nanotubes, nanorods, nanobelts, chains, and nanowires) has been the subject of the research in the past few years because of their potential use in biomedicine, magnetic recording, spin electronics, and microwave absorption. In particular, a great deal of research has been
given to 1D heterostructured, hollow magnetic nanomaterials because of their high shape anisotropy, hollow structure, high specific area, and unique interface. One major advantage of these tubular structures over solid particles is the possibility of obtaining multifunctional particles by utilizing the presence of inner and outer surfaces. Many studies have confirmed that excellent microwave absorbing performances, such as light weight and broad bandwidth, can be actualized by reasonable framing 1D hollow heterostructures. For example, Che et al. [3] prepared CNT/Fe nanocomposites as an excellent microwave absorber owing to the magnetic effects and confinement role of crystalline Fe in carbon nanoshells. Zhu et al. [4] found that Fe3O4/TiO2 core/shell nanotubes exhibited enhanced microwave absorption owing to the decreased eddy current effect and increased anisotropy energy of the core/shell nanotubes. Cao et al. [5] fabricated 3D Fe3O4-multiwall carbon nanotube composites to improve absorption capacity via the synergy of dielectric loss and magnetic loss and the enhanced multiple interfaces. The ability to control the component and interface of such materials is crucial for adjusting material properties. Different strategies have been used for synthesis of these 1D magnetic materials; these methods include template method [6], coaxial electrospinning [7], gas bubbles-directed self-assembly [8], magnetic field induced-Oswald ripening, [9] and a tartrate assisted hydrothermal method [10]. Although
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (G. Tong). https://doi.org/10.1016/j.pnsc.2018.08.008 Received 12 March 2018; Received in revised form 29 August 2018; Accepted 29 August 2018 1002-0071/ © 2018 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Liu, L., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2018.08.008
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at 284.8 eV. The surface morphologies and microstructures of the samples were observed under field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, 5 kV) and high resolution-transmission electron microscope (HR-TEM, JEM-2100F, 200 kV). A textural characteristic was analyzed on a nitrogen adsorption apparatus (ASAP 2020 M Micromeritics Instruments, USA).
most scholars have focused on creating 1D solid magnetic particles, few studies have shown potential in creating heterostructured, hollow magnetic structures. Magnetite (Fe3O4) with cubic spinel structure is an important soft magnetic material for various applications in industry and technology, such as spin electronic devices, catalysis, sewage treatment, healthcare, magnetic fluids, and so on, Fe3O4 is suitable for EM wave absorption because of its large saturation magnetization, high permeability, and excellent chemical stability. However, the low impedance match of this compound limits its application as a broadband and lightweight EMWA. For this reason, we developed strategies to improve impedance match properties, such strategies include modulation of morphology, size [11], and component [10]; adjustment of surface and interfacial performance [12–16], or ion substitution [17]. Thus far, Fe3O4 is generally compounded with other materials (ZnO [18], TiO2[4], SnO2, Fe [15], Au, Ni [14], and C [13,15,19].) into heterostructured absorbers. Bismuth-based functional materials were widely investigated for optoelectronics and photocatalytic applications [20,21]. However, few works have reported on the introduction of electromagnetic transparent materials (Bi24Fe2O39 or Bi) to improve the microwave absorbing properties of Fe3O4 composites. To adjust impedance match properties, we synthesize Fe3O4/ Bi24Fe2O39/Bi hollow porous microrods (HPMRs) for the first time via a self-assembly/precipitate conversion/decomposition process. To verify our original idea, we systematically studied the static magnetic and microwave absorption performances of the HPMRs. We found that Bi24Fe2O39 and Bi, which were introduced as an EM transparent component, can be used to adjust not only magnetic performance but also the impedance match and dielectric performances based on composition modulation or interfacial interaction between the three components. Our results provide new insight into the composition-dependent impedance match and microwave absorption performances of HPMRs.
2.3. Measurement of the properties A Model 7404 vibrating sample magnetometer (LakeShore, USA) was used to study the magnetic performances of the samples. Based on the coaxial line method, an Agilent N5230 vector network analyzer was used to measure the relative complex permeability (εr = ε′ + jε″) and permittivity ( μr = μ′ + jμ″) of the sample-wax composites. The cylindrical samples with 3.0 mm in inner diameter, 7.0 mm in outer diameter and a thickness of ca. 3.5 mm were fabricated by uniformly mixing the absorbents with paraffins in a mass fraction of 65 − 70% and then pressed into cylindrical compacts. 3. Results and discussion 3.1. XRD analysis XRD analysis was conducted to reveal the effects of Fe2+/Bi3+ molar ratio on the phase structure, composition, and crystal size (D) of the samples. In Fig. 1a, the peaks marked by ♥ match well with tetragonal Bi24Fe2O39 [a = 0.7705 nm; space group P-421c(114); JCPDS No. 42–0201][22]. The peaks badged by ♣ can be well be appointed to the cubic inverse spinel structure Fe3O4 [a = 0.83905 nm; space group Fd3m (227); JCPDS No. 65–3107][15]. The other peaks marked by ♦ can be identified as rhombohedral Bi [a = 0.4547 nm; space group R3 m(166); JCPDF No. 44–1246][23]. These findings confirm the existence of the rhombohedra crystal structure in the samples. The XRD data revealed that pure Fe3O4 was obtained at γ = 1:0, and the composites of Fe3O4, Bi24Fe2O39, and Bi were produced at
2. Experimental 2.1. Preparation of Fe3O4/Bi24Fe2O39/Bi HPMRs All chemical reagents were analytical grade and were used without any purification. In a typical experimental procedure, 2 mmol sodium dodecyl sulfate (SDS) was dissolved in a mixed solvent of 10 mL of ethylene glycol (EG) and 4 mL of H2O. The resulting mixture was stirred at 45 °C for 30 min until it was clear. Two identical solutions were obtained by dissolving 2 mmol (NH4)2Fe(SO4)2 and 2 mmol (NH4)2C2O4. Both solutions were magnetically stirred at 45 °C for 1 h. After the reaction was completed, the resulting yellow precipitates were centrifugally separated and washed three times with H2O and C2H5OH to obtain rod-shaped FeC2O4·2H2O precursors. The precursors were transferred to 20 mL of 0.5 mmol Bi(NO3)3·5H2O solution. The mixture was heated in a water bath at 45 °C for 2 h. The composition of the precursors was controlled by changing the Fe2+/Bi3+ molar ratio (γ = 1:0, 1:0.25, 1:0.5, 1:0.75, 1:1, 1:2) via keeping FeC2O4·2H2O precursors fixed and adjusting [Bi3+] concentration (0 M, 0.001 M, 0.002 M, 0.003 M, 0.004 M, 0.008 M, respectively). Finally, the precursors were calcined at 500 °C for 3 h under the protection of nitrogen and heating rate of 5 °C·min−1 to obtain Fe3O4/Bi24Fe2O39/Bi HPMRs. 2.2. Characterization The phases structure of the samples was analyzed by X-ray diffraction (XRD, D/MAX-IIIA, CuKα radiation, λ = 0.15406 nm, 10°/ min). A Horiba, EX-250 energy dispersive X-ray spectrometer (EDX) was used to determine the element components of the samples. An ESCALAB 250Xi X-ray photoelectron spectrometer (XPS) was employed to gain further insight into the chemical composition and the oxidation states of Bi, O, and Fe. The binding energies obtained in the XPS analysis are standardized for specimen charging using C1s as the reference
Fig. 1. (a) XRD patterns and (b) phase mass fraction of the products formed at various Fe2+/Bi3+ molar ratios (γ). 2
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Table 1 Phase content (β), lattice parameter (a), average crystal size (D), and microwave absorbing properties of the BixFe3-xO4/Bi hollow microrods obtained under various Fe3+/Bi3+molar ratios γ. γ
1:0 1:0.25
1:0.50
1:0.75
Phase content /%
Fe3O4 Fe3O4 Bi Bi24Fe2O39 Fe3O4 Bi Bi24Fe2O39 Fe3O4 Bi Bi24Fe2O39
100 36.60 55.68 7.72 11.80 84.21 3.99 3.03 79.30 17.67
d
a /Å
D311 /nm
Microwave-absorbing properties Optimal RL value (dB)
f (GHz) (optimal RL)
dw (mm) (RL < –20 dB)
Frequency range (GHz) (RL < –20 dB)
0.276 0.164
6.291 6.287
33.5 47.9
− 13.23 − 47.32
10.96 8.32
/ 1.4 − 8.0
/ 2.0 − 16.0
0.139
6.293
67.8
− 15.66
16.40
/
/
0.081
/
105.5
− 47.90
17.92
6.8 − 7.0
17.44 − 18.0
γ = 1:0.25–1:0.5. These results confirm the formation of Fe3O4/ Bi24Fe2O39/Bi composites. As γ varies (1:0, 1:0.25, 1:0.50, 1:0.75), the peak intensities from Fe3O4 gradually weakened, whereas the peak intensities from Bi first increased and then decreased. Such changes are due to the variation in the contents and crystallinity of Fe3O4 and Bi. The quantitative phase of samples was determined by the relative intensity ratio (RIR) method based on the XRD data [24]. Quantitative calculations were carried out using the following equation:
⎡ ⎤ 1 Xi = (Ihkli )(Ki, n ) ⎢ m ⎥ ∑ j = 1 (Ihkl ′ j )(Kj, n ) ⎣ ⎦
(1)
where Xi is the fraction of phase i which needs to be measured; Ki,n and Kj,n are the RIR for diffraction line n of phase i and j; Ii,n and Ij,n are the intensities of diffraction line n of phase i and j, respectively; and m is the number of phases in the samples (Fig. 1b and Table 1). As shown in Fig. 1b, when γ varies (1:0, 1:0.25, 1:0.5, 1:0.75), Fe3O4 content gradually decreases from 100% to 3.03%, whereas the Bi content first increases from 0% to 84.21% and then decreases to 79.30%. Conversely, Bi24Fe2O39 content first increases from 0% to 7.72% and then decreases to 3.03%. Such changes may be owing to the decreased size of the precursors, causing the decreased interface area between Fe3O4 and Bi. The aforementioned results are consistent with the subsequent XPS and EDX data. Average crystalline size (D) was calculated from the XRD311 peak width based on the Scherrer formula: D = 0.89λ/βcosθ, where θ is the Bragg angle; λ is the wavelength of the X-ray; and β is the half-height of angle diffraction. The obtained values are given in Table 1. Increase in D is seen from 33.5 nm to 67.8 nm at γ = 1:0–1:0.5, and decrease is seen at γ = 1:0.75. These results indicate that the component, phase, and crystalline size of the samples can be adjusted by controllingγ.
Fig. 2. Mass fractions of Fe and Bi elements are functions of Fe2+/Bi3+ molar ratio (γ).
further (i.e., 1:0.75∼ 1:2), Fe and Bi contents have persisted with so little change. This may be because Bi2 (C 2 O4 )3⋅7H 2 O crystallizing out and absorbing on the surface of FeC 2 O4⋅2H 2 O hindered the diffusion of Bi3+. Similar phenomena were also reported in literature [9,15,16]. Therefore, regulation of Fe2+/Bi3+ molar ratio via controlling [Bi3+] can effectively control the content of the phase in the heterogeneous structure. 3.3. XPS characterization XPS was used to analyze the surface state and elemental composition of the HPMRs obtained under differentγ. As shown in Fig. 3, binding energies of all elements have been calibrated with the binding energy of C 1 s (284.8 eV). In the full spectra (Fig. 3a), Fe, Bi, O, and C elements were found for the samples. The peaks centered at binding energies of 680.2, 466.5, 442.6, 186.7, 161.0, 96.4, and 21.0 eV correspond to Bi 4p3/2, Bi 4d3/2, Bi 4d5/2, Bi 4f5/2, Bi 4f7/2, Bi 5p, and Bi 5d, respectively. The other peaks at binding energies of 56.2, 285.2, 530.8, 711.2, and 724.5 eV are ascribed to Fe 3p, C 1 s, O 1 s, Fe 2p3/2, and Fe 2p1/2, respectively. When γ varies from 1:0.25 to 1:0.75, the peak intensities of Fe and O are decreased, whereas those of Bi are increased. Such changes could be related to the increased Bi content and decreased Fe and O contents, consistent with the XRD and EDX data. The Bi 4 f spectra contain four peaks (Fig. 3b). Two peaks centered at 164.5 and 159.2 eV correspond to Bi 4f5/2 and Bi 4f7/2 of Bi3+, respectively, demonstrating that chemical state of Bi is trivalent oxidation in Bi24Fe2O39[25]. The other two peaks at 157.2 eV (4f7/2) and 162.6 eV (4f5/2) correspond to Bi°, indicating the presence of metallic Bi in the composites [26]. As seen from Fig. 3b, the peak intensity of Bi° increases with γ varying from 1:0.25 to 1:0.5, corresponding to the increased Bi° content. However, two peaks from Bi3+ are significantly
3.2. EDX analysis The elemental composition of the samples obtained at various γ is characterized by EDX. The Fe and Bi contents in the products measured by EDX were plotted as shown in Fig. 2. When γ varies from 1:0–1:2, the mass fraction of Fe reduces from 100 wt% to 0 wt%; the mass fraction of Bi increases from 0 wt% to 100 wt%. To reveal the relationship between element (Fe, Bi) content and γ, we fit experimental data with two Boltzmann equations, corresponding to two curves (Fig. 2); one is associated with the dissolution of FeC 2 O4⋅2H 2 O , and the other is related to Bi3+ ions diffusion and crystal growth of Bi2 (C 2 O4 )3⋅7H 2 O . When Fe2+/Bi3+ molar ratio is very low (i.e., 1:0 to 1:0.50), low supersaturation is detrimental to forming Bi2 (C 2 O4 )3⋅7H 2 O . When Fe2+/Bi3+ molar ratio is increased to 1:0.50, lower solubility products of Bi2 (C 2 O4 )3⋅7H 2 O than that of FeC 2 O4⋅2H 2 O promotes the dissolution of FeC 2 O4⋅2H 2 O , thus accelerating the diffusion of Bi3+ ions and crystal growth of Bi2 (C 2 O4 )3⋅7H 2 O . If Fe2+/Bi3+ molar ratio is increased 3
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Fig. 3. XPS spectra of full survey (a), Bi 4 f (b), O 1 s (c), and Fe 2p (d) for samples formed at various γ.
higher than those of metallic Bi°, demonstrating higher content of Bi24Fe2O39 than that of Bi° in the composites. The O 1 s spectra were deconvoluted into three photoemission peaks (Fig. 3c), corresponding to three distinct oxygen species; the peaks at 530.4, 531.7, and 533.1 eV are assigned to Fe–O/Bi-O [27], OH−, and H2O, respectively. The Fe 2p spectra can be fitted with four peaks, as shown in Fig. 3d. The peaks centered at 710.5 and 724.0 eV are characteristic peaks of Fe2+, whereas peaks centered at 712.2 and 726.4 eV correspond to Fe3+, consistent with literature [19], XPS data further confirm the formation of Fe3O4/Bi24Fe2O39/Bi composites.
image (Fig. 4j), the interplanar spacing of 0.328 nm (left up) and 0.296 nm (right down) correspond to (012) facet of Bi and (220) facet of Fe3O4, respectively. Between them, short range ordered lattice fringes with interplanar spacing of 0.318 nm can be assigned as (201) facets of Bi24Fe2O39. XRD and TEM results verify that the current materials are composites of Fe3O4, Bi shell, and Bi24Fe2O39. Interface plays an important role in the electronic and magnetic properties of Fe3O4/ Bi24Fe2O39/Bi HPMRs. The Fe3O4/Bi24Fe2O39/Bi HPMRs were synthesized by a self-assembly/precipitate conversion/ decomposition process [14]. Some reactions occur in the above three-step process as follows [28]:
3.4. SEM and TEM analysis
Fe 2 + + C 2 O42 − + 2H 2 O ↔ FeC 2 O4⋅2H 2 O
The effect of γ on the size, morphology, and microstructure of the sintered samples can be studied by SEM, TEM, and SAED. As seen from Figs. 4a and 4b, the samples formed at γ = 1:0 were microrods with a solid rectangular section and 3D sponge-like porous structures. Their length and aspect ratios were 3.53–6.21 µm and 3–10, respectively. These porous structures were composed of massive nanorods ranging from 30 nm to 40 nm in diameter, with smooth surface and a large number of pores. When γ was 1:0.25, the microrods became longer and thinner with 1.55–6.62 µm in length and 0.2–0.4 µm in diameter (Fig. 4c). The microrods consisted of nanorods with 200–400 nm in length and 50–100 nm in diameter; a large number of pores generated among nanorods. A splitting microrod (Fig. 4d) reveals the hollow structure with wall thickness of 50–100 nm. The hollow porous structure still was kept at γ = 1:0.5 (Figs. 4e–4f); however, only irregular nanoparticles were produced at γ = 1:0.75–1:2 (Fig. S1). The mirostructures of the sample were further observed by TEM. Seen from the low-power TEM images, the samples formed at γ = 1:0.25 were rectangular microrods with porous structures; massive nanoparticles form a cross-linked network (Figs. 4g–4h). The various contrasts in edges and centers reveal the hollow structures. The wall thickness was about 50–100 nm. Some clear rings consisting of diffraction spots were found in the SAED patterns, indicating high crystallinity (Fig. 4i). In the HR-TEM
2Bi3 +
+ 3FeC 2 O4⋅2H 2 O + H2 O ↔ Bi2 (C 2 O4 )3⋅7H 2 O + Δ
3FeC 2 O4⋅2H 2 O⟶Fe3 O4 + 2CO2 + 4CO + 6H2 O Δ
Bi2 (C 2 O4 )3⋅7H 2 O⟶Bi + 2CO2 + 2H2 O
(2)
3Fe 2 +
(3) (4) (5)
Δ
2FeC 2 O4⋅2H 2 O + Bi2 (C 2 O4 )3⋅7H 2 O⟶Bi24 Fe2 O39 + 2CO2 + 4CO + 6H2 O (6) In the first step, rod-like ferrous oxalate precursors directly precipitate from the solution by coordinated self-assembly at room temperature [29,30]. FeC2O4·2H2O nuclei generates via the co-precipitation of Fe2+ and C2O42- (Reaction 2). Fe2+ and C2O42- produce the infinite chain arrangement via chelation reaction; each Fe2+ ion interacts with two H2O molecules to form the octahedral FeO6 unit. Nanocrystals preferentially grow along the b-axis direction via coordinated self-assembly causing the formation of 1D nanomaterials. The second step is the precipitate conversion process from FeC2O4·2H2O (3.2 ×10−7) to Bi2(C2O4)3·7H2O to form rod-like ferrous oxalate/bismuth oxalate precursors. This is because the solubility products of Bi2(C2O4)3·7H2O (4.0 ×10–36) [31] is lower than that of FeC2O4·2H2O (3.2 ×10−7) (Reaction 3). Bi2(C2O4)3·7H2O nanocrystals anchor on FeC2O4·2H2O's surface by Fe-O-Bi bonds. Some decomposition reactions 4
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Fig. 4. (a–f) SEM, (g–h) TEM, (i) SAED pattern, and (j) HR–TEM image of the samples obtained under various γ: (a–b) 1:0, (c–d, g–j) 1:0.25, and (e–f) 1:0.5.
HPMRs. The mesopores stem from the primary aggregation of the nanoparticles insides; the larger macropores result from the hollow structure. This is in conformity with SEM results. The BET specific surface area (SBET), pore volume, and pore size of the HPMRs originated from the nitrogen adsorption–desorption isotherm are 8.9954 m2·g−1, 0.014069 cm3·g−1, and 62.5593 Å, respectively.
occur in the third step, as shown in Reaction 4–6. Under the protection of N2 and/or Ar, precursors decompose and release a large amount of heat and gases (e.g., CO, H2O, and CO2), resulting in the formation of numerous pores. Meanwhile, the released gases drive the remaining metal ions toward the surface [32] and thus induce the formation of hollow porous Fe3O4/Bi24Fe2O39/Bi microrods. Also, the sintering temperature, time, and ambience can be applied to adjust the crystal size, phase structure, and textural characteristics of the products. Further studies are under way.
3.6. Static magnetic properties The vibrating sample magnetometer was used to analyze the magnetic properties of the HPMRs at room temperature. In Fig. 6a, all samples exhibit a typical S-type hysteresis loop, indicating that the samples are ferromagnetic. All of the samples reach the saturation magnetization at about 10 kOe. From Fig. 6b, pure Fe3O4 PMRs exhibit the highest saturation magnetization Ms of 84.66 emu g–1 and the lowest coercivity Hc of 225.45 Oe. After Fe3O4 was combined with Bi24Fe2O39 and Bi, the Ms values of Fe3O4/Bi24Fe2O39/Bi HPMRs gradually decreased from 36.78 emu g–1 at γ = 1:0.25 to 0.019 emu·g1 at γ = 1:2. This demonstrates that coating Fe3O4 with Bi and Bi24Fe2O39 is not beneficial in improving Ms. Ms generally depends on the component, interface, and crystal size of composites. The current HPMRs are a composite of ferromagnetic Fe3O4, nonferromagnetic Bi shell, and the inter-diffused layer of antiferromagnetic Bi24Fe2O39. Obviously, an increase in the content of antiferromagnetic Bi24Fe2O39 and nonferromagnetic Bi leads to a decrease in Ms. The Hc values of Fe3O4/Bi24Fe2O39/Bi HPMRs almost remained unchanged at γ = 1:0 – 1:0.5 and then sharply increased from 225.05 Oe at γ = 1:0.5 to 14191 Oe at γ = 1:2, which is significantly higher than that of pure Fe3O4 (225.45 Oe) and bismuth ferrite (4470 Oe) [34]. The enhanced Hc value is generally determined by crystal anisotropy, shape anisotropy, stress anisotropy, and exchange anisotropy. Obviously, crystal size is not a critical factor in determining Hc. Fe3O4 nanoparticles have multidomain structures because of the bigger average crystal size (D) than the multidomain/monodomain critical size (DC) (50 nm for Fe3O4). As a result, Hc decreases with the increase of D. In this study, the significant enhancement in Hc should be owed to the surface pinning role of antiferromagnetic Bi24Fe2O39 and nonferromagnetic Bi on the surface of ferromagnetic Fe3O4.
3.5. Textural characteristics Fig. 5 is the nitrogen physical adsorption and desorption isotherms and the corresponding pore size distribution curves for the typical Fe3O4/Bi24Fe2O39/Bi HPMRs formed at γ = 1:0.25. According to BDDT classification [33], the HPMRs display the type II isotherms and H3 hysteresis loops. The hysteresis loop of type H3 corresponds to the partial retention of slitlike large pore. Seen from the inset in Fig. 5, multimodal pore size distribution is found in the mesoporous and macroporous regions, owing to the hollow porous heterostructure of the
Fig. 5. Nitrogen adsorption and desorption isotherms and (the inset) the corresponding pore size distribution curve of the typical samples. 5
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Besides, as for Fe3O4/Bi24Fe2O39/Bi HPMRs, the interfacial magnetic coupling exists between Fe3O4 and Bi24Fe2O39 with magnetically disordered spins, which increases with Fe3O4 and Bi24Fe2O39 contents, thereby causing enhanced Hc. A similar phenomenon was found in isostructural spinel interfaces of Ni and NiFe2O4, caused by a thin interdiffused layer at the interface [14]. These data demonstrate that controlling the composition, structure, and interface can tune magnetic performances of Fe3O4/Bi24Fe2O39/Bi HPMRs. 3.7. Microwave absorbing properties Reflection loss (RL) generally was used to describe the EM wave absorption properties of the samples. Using the transmission line theory, the RL values were calculated based on the as-obtained complex permeability and permittivity:
RL (dB ) = 20 log (Zin − Z0)/(Z in+Z0)
(7)
Z in= μr / εr tanh[j (2πfd/ c ) μr εr ]
(8)
Z0 =
(9)
μ 0 / ε0
Where, f is frequency, d is sample thickness, c is the light velocity, Zin is the input impedance between free space and the materials, and Z0 is the characteristic impedance of free space. The effects of γ, f, d, and mass fraction on RL were investigated to elucidate the microwave absorption mechanism of Fe3O4/Bi24Fe2O39/Bi HPMRs. The composition-dependent microwave absorption properties of the prepared samples were studied, as shown in Fig. 7 and Table 1. Altering γ (1:0, 1:0.25, 1:0.50, and 1:0.75) can tune the composition
Fig. 6. Fe2+/Bi3+ molar ratio (γ) versus magnetic hysteresis loops. The lower right inset shows expanded regions around origin.
Fig. 7. (a1, b1, c1, d1) calculated 3D plots, (a2, b2, c2, d2) reflection loss curves, (a3, b3, c3, d3) dependence of matching thickness (tm) on matching frequency (fm) at wavelengths of λ/4, 3λ/4, and 5λ/4, and (a4, b4, c4, d4) effective bandwidth (RL < −10 dB) of the paraffin composites containing 70% wt. samples formed under various λ (1:0 for a1−a4, 1:0.5 for b1 −b4, 1:0.75 for c1−c4, and 1:1 for d1−d4). 6
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Fig. 8. (a) Calculated reflection loss curves, (b) the calculated matching thickness according to the λ/4, 3λ/4, and 5λ/4 models. 3D reflection loss curves, (c) the corresponding 3D plots, and (d) the effective bandwidth (RL ≤ −10 dB) of the paraffin composites containing 65% wt. Fe3O4/Bi24Fe2O39/Bi HPMRs (S2) formed under λ = 1:0.25.
7d1 shifted to the 3/4 wavelength, corresponding to the 3/4 wavelength cancellation model. The bandwidth (RL ≤ −10 dB) shifted to higher frequency ranged with thicker sample thicknesses (Fig. 7c4−7d4). This demonstrates that proper Bi24Fe2O39 and Bi contents in Fe3O4/Bi24Fe2O39/Bi HPMRs favors the improvement of microwave absorbing properties. Mass fraction of Fe3O4/Bi24Fe2O39/Bi HPMRs was optimized for the best EM wave absorption performance. The composite of paraffin and 65 wt% Fe3O4/Bi24Fe2O39/Bi HPMRs obtained at λ = 1:0.25 display the best property (Fig. 8). The matching thickness (tm) and matching frequency (fm) were calculated based on the following equation: tm = nc /(4f μr εr ) . As seen in Fig. 8a, fm gradually moved toward m the low frequency with the increase of tm. The minimum RL reaches − 47.3 dB at 8.72 GHz, corresponding to 2.4 mm matching thickness. The absorbing band (RL ≤ −20 dB, corresponding to 99% attenuation) reached 14.0 GHz (at the frequency ranges of 2.0 −16.0 GHz) for composites with thickness range of 1.4 − 8.0. Fig. 8b shows three curves (tm versus fm) at n = 1, 3, 5, corresponding to wavelengths λ/4, 3λ/4, and 5λ/4, respectively. RL reaches the peak value if the tm and fm satisfy the abovementioned equation. In the Fig. 8a, the main absorbing peak (RL ≤ −20 dB) corresponds to the quarter wavelengths (Fig. 8b) and is over 2 − 18 GHz. This indicates that Fe3O4/Bi24Fe2O39/Bi HPMRs have good absorption performance and obey the λ/4 cancellation model. Fig. 8c further shows that the absorption band (RL ≤ −10 dB, 90% attenuation) is over 2 − 18 GHz, with sample thickness of 1.6–9.0 mm. The effective bandwidth (RL ≤ −10 dB) is 3.3 GHz (Fig. 8d). In particular, the absorbing band (RL ≤ −20 dB) of 14.0 GHz is broader than those reported in literature (i.e., spongy Fe3O4 (8.91 GHz) [30], ZnO/Ni/ZnxNiyFe3-x-yO4 microhexahedra (13.1 GHz)[29], Fe3O4/ TiO2 nanotubes (0.3 GHz)[34], Fe3O4/C nanospindles (4.5 GHz)[35],
Fig. 9. Attenuation constant (α) of the samples formed at various γ.
and microwave absorption performances of the HPMRs. As for the Fe3O4 PMRs obtained at γ = 1:0, the minimum RL value of − 13.23 dB was observed at 10.96 GHz with a sample thickness of 1.6 mm (Fig. 7a1−a3). The absorbing band (RL ≤ −20 dB) was 0 GHz. The effective bandwidth (RL ≤ −10 dB) was below 2 GHz (Fig. 7a4). At γ = 1:0.25, Fe3O4/Bi24Fe2O39/Bi HPMRs showed the best microwave absorption performances (Fig. 7b1−b3). A strong and wide absorbing band was found over 2 − 18 GHz, corresponding to the quarter-wavelength cancellation model. The minimum RL value of − 30.29 dB reached at 2.62 GHz with a sample thickness of 5.9 mm (Fig. 7b1). The absorbing band (RL ≤ −20 dB) was 3.1 GHz (Fig. 7b2). The largest effective bandwidth (RL ≤ −10 dB) is 3.28 GHz corresponding to 1.5 mm sample thickness (Fig. 7b4). Further varying γ (1:0.5, 1:0.75), the absorption band (RL ≤ −20 dB) decreased to 0 GHz (Fig. 7c1−7d1). The short and narrow absorption band in Fig. 7c1 and 7
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Fig. 10. Modulus of the normalized impedance Z versus frequency for the samples formed at various γ.
14–18 GHz stems from exchange resonance. With the value of γ varying (1:0, 1:0.25, 1:0.5, 1:0.75), the Fe3O4 content decreases, and the contents of Bi24Fe2O39 and Bi increase. Such changes cause the decreased attenuation constant. Impedance match between permeability and permittivity is another critical factor to determine mirowave absorption. High impedance match ratio represents the ability of the microwave to enter into the absorber and convert to thermal energy or be dissipated through interference. Impedance match requires that permittivity and permeability satisfy the equation: Z = Zin/ Z0 ∝ 1, where Z is the modulus of the normalized characteristic impedance, Zin is the input impedance between free space and the materials, and Z0 is the characteristic impedance of free space. When the value of Z is equal or close to 1, it is beneficial in improving microwave absorption. The plots of Z versus frequency are shown in Fig. 10. With varying γ from 1:0 to 1:0.75, Z values become closer to 1, implying an increasing impedance match. Our data demonstrate that increasing the contents of Bi and Bi24Fe2O39 favor an improved impedance match. The enhanced match should be ascribed to the hollow and porous heterostructure. On one hand, Bi and interdiffused layer Bi24Fe2O39 with low conductivity and nonferromagnetic properties can decrease the permittivity and permeability, therefore making tan δE close to tan δM. On the other hand, air with low permittivity inside the hollow and porous structures can improve impedance matching. From these analyses, it is deduced that the impedance matching enhancement rather than the attenuation constant is a central factor in the improved microwave absorption.
Fe3O4/C nanorings (9.55 GHz)[16], BiFeO3 (0 GHz) [36], and CuxFe3xO4 @Cu core-shell hollow spherical chains (9.5 GHz)[17]. Seen from Table 1, Fe3O4/Bi24Fe2O39/Bi HPMRs exhibit light weight, higher absorption, and broader bandwidth than Fe3O4 PMRs. Our data demonstrate that the introduction of electromagnetic transparent materials (Bi24Fe2O39 or Bi) can significantly improve the microwave absorbing properties of Fe3O4. It is generally accepted that attenuation constant (α) and impedance matching are two important factors in determining the microwave absorption. α and impedance matching were investigated to reveal the enhanced microwave absorption mechanism of Fe3O4/Bi24Fe2O39/Bi HPMRs. The attenuation constant α is used to analyze the attenuation ability of the absorber, which can be calculated using the following equation [37,38]:
α=
2 πf c
(μ″ε″ − μ′ε′) +
(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2
(10)
In Fig. 9, Fe3O4 PMRs show two strong attenuation peaks at 4 − 10 GHz and 12 − 18 GHz. After combining Fe3O4 with Bi and Bi24Fe2O39, the obtained Fe3O4/Bi24Fe2O39/Bi HPMRs exhibit lower α values. Moreover, α values are gradually decreased with the increasing contents of Bi and Bi24Fe2O39. This indicates that increasing the Bi and Bi24Fe2O39 contents suppresses improvement of attenuation ability. The decreased attenuation constant can be interpreted as follows: in this study, Fe3O4/Bi24Fe2O39/Bi HPMRs consist of Fe3O4, Bi24Fe2O39, and Bi. Fe3O4 is a kind of ferromagnetic absorbers, whereas Bi24Fe2O39 and Bi are nonmagnetic materials with low conductivity. On the one hand, the low conductivity restrains the local movement of bound charges and the variation of dipole moments in the alternating EM field, causing the decreased dielectric relaxations and dielectric loss (Fig. S2c–S2f). On the other hand, an increase in the content of antiferromagnetic Bi24Fe2O39 and nonferromagnetic Bi will decrease Ms, resulting in low permeability and magnetic loss (Fig. S3c). In Fig. S3d, the eddy current loss shows a strong peak at less than 8 GHz and a weak peak at 14–18 GHz. The former is generally defined as natural resonance; the latter is generally defined as “exchange mode” resonance, as a result of the surface effect, small size effect, and spin wave excitations. According to the skin effect criterion,[39] magnetic loss for Fe3O4/ Bi24Fe2O39/Bi HPMRs at 2–8 GHz stems from natural resonance and at
4. Conclusions Fe3O4/Bi24Fe2O39/Bi HPMRs have been prepared via a self-assembly/precipitate conversion/decomposition process. Changing Fe2+/ Bi3+ molar ratio can handily adjust their composition and properties. In Fe3O4/Bi24Fe2O39/Bi HPMRs, the increase of Bi and Bi24Fe2O39 contents with Fe2+/Bi3+ molar ratio leads to the decreased Ms and increased Hc. Compounding Fe3O4 PMRs with Bi and Bi24Fe2O39 can enhance impedance matching and decrease the attenuation constant. An optimum mirowave absorption performance is shown by Fe3O4/ Bi24Fe2O39/Bi HPMRs obtained at γ = 1:0.25, with the absorbing band (RL ≤ −20 dB) of 14.0 GHz. The minimum RL reaches − 47.3 dB at 8
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8.72 GHz, corresponding to a 2.4 mm matching thickness. Compared with Fe3O4 PMRs, Fe3O4/Bi24Fe2O39/Bi HPMRs exhibit higher absorption, lighter weight, and wider bandwidth. The enhanced microwave absorbing properties should be owned to enhanced impedance matching rather than attenuation constant caused by electromagnetic transparent materials (Bi24Fe2O39 or Bi). Our investigations provide useful guidance for designing and preparing other hollow, porous, heterostructured absorbers.
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Acknowledgements This work was supported by the National Natural Scientific Foundation of China (51672252), National Innovation and Entrepreneurship Training Program of Undergraduates (201710345010), and Xinmiao Talent Project of Zhejiang Province (2017R404007). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.pnsc.2018.08.008. References [1] B. Wen, M.S. Cao, M.M. Lu, W.Q. Cao, H.L. Shi, J. Liu, X.X. Wang, H.B. Jin, X.Y. Fang, W.Z. Wang, J. Yuan, Adv. Mater. 26 (2014) 3484–3489. [2] W.Q. Cao, X.X. Wang, J. Yuan, W.Z. Wang, M.S. Cao, J. Mater. Chem. C 3 (2015) 10017–10022. [3] R.C. Che, L.M. Peng, X.F. Duan, Q. Chen, X.L. Liang, Adv. Mater. 16 (2004) 401–405. [4] C.L. Zhu, M.L. Zhang, Y.J. Qiao, G. Xiao, F. Zhang, Y.J. Chen, J. Phys. Chem. C 114 (2010) 16229–16235. [5] Y.H. Chen, Z.H. Huang, M.M. Lu, W.Q. Cao, J. Yuan, D.Q. Zhang, M.S. Cao, J. Mater. Chem. A 3 (2015) 12621–12625. [6] P.S. Antonel, C.L.P. Oliveira, G.A. Jorge, O.E. Perez, A.G. Leyva, R.M. Negri, J. Nanopart. Res 17 (2015) 294. [7] Z.H. Wang, L. Zhao, P.H. Wang, L. Guo, J.H. Yu, J. Alloy. Compd. 687 (2016) 541–547. [8] G.X. Tong, J.G. Guan, Q.J. Zhang, Adv. Funct. Mater. 23 (2013) 2405–2413. [9] Y.N. Li, T. Wu, K.D. Jiang, G.X. Tong, K.Y. Jin, N.X. Qian, L.H. Zhao, T.X. Lv, J. Mater. Chem. C 4 (2016) 7119–7129. [10] B. Zhao, G. Shao, B.B. Fan, Y.J. Xie, R. Zhang, J. Magn. Magn. Mater. 372 (2014) 195–200. [11] Y. Liu, T.T. Cui, Y.N. Li, Y.C. Ye, G.X. Tong, Mater. Chem. Phys. 173 (2016) 152–160. [12] K.D. Jiang, Y. Liu, Y.F. Pan, R. Wang, P.B. Hu, R.J. He, L.L. Zhang, G.X. Tong, Appl.
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Tailoring impedance match and enhancing microwave absorption of Fe3O4/ Bi24Fe2O39/Bi hollow porous microrods by controlling their composition Lin Liu, Na He, Jiacheng Sun, Panbing Hu, Rujia He, Jinxiu Cheng, Weifu Tian, Guoxiu Tong
⁎
College of Chemistry and Life Sciences, Zhejiang Key Laboratory for Reactive Chemistry on Solid Surface, Zhejiang Normal University, Jinhua 321004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fe3O4/Bi24Fe2O39/Bi composites Synthesis Hollow porous microrods Magnetic property Composition-dependent microwave absorption
A self-assembly/precipitate conversion/decomposition process was developed for the controllable synthesis of Fe3O4/Bi24Fe2O39/Bi hollow porous microrods (HPMRs). The results demonstrated that the crystal size, component, and performances of HPMRs could be effectively modulated via changing Fe2+/Bi3+ molar ratio (γ). Fe3O4/Bi24Fe2O39/Bi HPMRs exhibited ferromagnetic behavior at room temperature. As Bi and Bi24Fe2O39 contents increased with γ, the saturation magnetization Ms and attenuation constantly decreased, whereas coercivity Hc and impedance matching ratio increased. Compounding Fe3O4 with small quantities of Bi and Bi24Fe2O39 into HPMRs can significantly enhance microwave absorption. Fe3O4/Bi24Fe2O39/Bi HPMRs formed at γ = 1:0.25 exhibited the optimum microwave absorption performance. The minimum RL was − 47.3 dB at 8.72 GHz, corresponding to 2.4 mm sample thickness. The absorption band with the reflection loss below − 20 dB was up to 14.0 GHz for the absorber with a thickness of 1.4 − 8.0 mm. The results demonstrate that the introduction of electromagnetic transparent materials (Bi24Fe2O39 or Bi) can improve the microwave absorption performances of Fe3O4 composites owing to enhanced impedance matching rather than attenuation constant.
1. Introduction The study on applications of magnetic nanomaterials for solving serious electromagnetic (EM) pollution problems has been focused. [1,2]. Magnetic nanomaterials, as EM microwave absorbent (EMWA), can effectively retards EM pollution and strengthen the defense capability of weapon system [1–5]. Two crucial factors for promoting the practical application of EMWA include improvement of attenuation constant (α) and impedance matching. The attenuation mechanism of EM wave absorbing materials is mainly based on high magnetic loss and dielectric loss. According to the electromagnetic wave energy conversion principle, the proper impedance matching between permittivity and permeability determines the reflection and attenuation characteristics of EMWA. To our best knowledge, absorbers with a single structure and component can’t meet the aforementioned high demands. Therefore, heterostructured magnetic nanomaterials must be developed to enhance the absorption properties of EM waves. The design of one-dimensional (1D) ferromagnetic nanomaterials (i.e., nanotubes, nanorods, nanobelts, chains, and nanowires) has been the subject of the research in the past few years because of their potential use in biomedicine, magnetic recording, spin electronics, and microwave absorption. In particular, a great deal of research has been
given to 1D heterostructured, hollow magnetic nanomaterials because of their high shape anisotropy, hollow structure, high specific area, and unique interface. One major advantage of these tubular structures over solid particles is the possibility of obtaining multifunctional particles by utilizing the presence of inner and outer surfaces. Many studies have confirmed that excellent microwave absorbing performances, such as light weight and broad bandwidth, can be actualized by reasonable framing 1D hollow heterostructures. For example, Che et al. [3] prepared CNT/Fe nanocomposites as an excellent microwave absorber owing to the magnetic effects and confinement role of crystalline Fe in carbon nanoshells. Zhu et al. [4] found that Fe3O4/TiO2 core/shell nanotubes exhibited enhanced microwave absorption owing to the decreased eddy current effect and increased anisotropy energy of the core/shell nanotubes. Cao et al. [5] fabricated 3D Fe3O4-multiwall carbon nanotube composites to improve absorption capacity via the synergy of dielectric loss and magnetic loss and the enhanced multiple interfaces. The ability to control the component and interface of such materials is crucial for adjusting material properties. Different strategies have been used for synthesis of these 1D magnetic materials; these methods include template method [6], coaxial electrospinning [7], gas bubbles-directed self-assembly [8], magnetic field induced-Oswald ripening, [9] and a tartrate assisted hydrothermal method [10]. Although
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (G. Tong). https://doi.org/10.1016/j.pnsc.2018.08.008 Received 12 March 2018; Received in revised form 29 August 2018; Accepted 29 August 2018 1002-0071/ © 2018 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Liu, L., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2018.08.008
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at 284.8 eV. The surface morphologies and microstructures of the samples were observed under field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, 5 kV) and high resolution-transmission electron microscope (HR-TEM, JEM-2100F, 200 kV). A textural characteristic was analyzed on a nitrogen adsorption apparatus (ASAP 2020 M Micromeritics Instruments, USA).
most scholars have focused on creating 1D solid magnetic particles, few studies have shown potential in creating heterostructured, hollow magnetic structures. Magnetite (Fe3O4) with cubic spinel structure is an important soft magnetic material for various applications in industry and technology, such as spin electronic devices, catalysis, sewage treatment, healthcare, magnetic fluids, and so on, Fe3O4 is suitable for EM wave absorption because of its large saturation magnetization, high permeability, and excellent chemical stability. However, the low impedance match of this compound limits its application as a broadband and lightweight EMWA. For this reason, we developed strategies to improve impedance match properties, such strategies include modulation of morphology, size [11], and component [10]; adjustment of surface and interfacial performance [12–16], or ion substitution [17]. Thus far, Fe3O4 is generally compounded with other materials (ZnO [18], TiO2[4], SnO2, Fe [15], Au, Ni [14], and C [13,15,19].) into heterostructured absorbers. Bismuth-based functional materials were widely investigated for optoelectronics and photocatalytic applications [20,21]. However, few works have reported on the introduction of electromagnetic transparent materials (Bi24Fe2O39 or Bi) to improve the microwave absorbing properties of Fe3O4 composites. To adjust impedance match properties, we synthesize Fe3O4/ Bi24Fe2O39/Bi hollow porous microrods (HPMRs) for the first time via a self-assembly/precipitate conversion/decomposition process. To verify our original idea, we systematically studied the static magnetic and microwave absorption performances of the HPMRs. We found that Bi24Fe2O39 and Bi, which were introduced as an EM transparent component, can be used to adjust not only magnetic performance but also the impedance match and dielectric performances based on composition modulation or interfacial interaction between the three components. Our results provide new insight into the composition-dependent impedance match and microwave absorption performances of HPMRs.
2.3. Measurement of the properties A Model 7404 vibrating sample magnetometer (LakeShore, USA) was used to study the magnetic performances of the samples. Based on the coaxial line method, an Agilent N5230 vector network analyzer was used to measure the relative complex permeability (εr = ε′ + jε″) and permittivity ( μr = μ′ + jμ″) of the sample-wax composites. The cylindrical samples with 3.0 mm in inner diameter, 7.0 mm in outer diameter and a thickness of ca. 3.5 mm were fabricated by uniformly mixing the absorbents with paraffins in a mass fraction of 65 − 70% and then pressed into cylindrical compacts. 3. Results and discussion 3.1. XRD analysis XRD analysis was conducted to reveal the effects of Fe2+/Bi3+ molar ratio on the phase structure, composition, and crystal size (D) of the samples. In Fig. 1a, the peaks marked by ♥ match well with tetragonal Bi24Fe2O39 [a = 0.7705 nm; space group P-421c(114); JCPDS No. 42–0201][22]. The peaks badged by ♣ can be well be appointed to the cubic inverse spinel structure Fe3O4 [a = 0.83905 nm; space group Fd3m (227); JCPDS No. 65–3107][15]. The other peaks marked by ♦ can be identified as rhombohedral Bi [a = 0.4547 nm; space group R3 m(166); JCPDF No. 44–1246][23]. These findings confirm the existence of the rhombohedra crystal structure in the samples. The XRD data revealed that pure Fe3O4 was obtained at γ = 1:0, and the composites of Fe3O4, Bi24Fe2O39, and Bi were produced at
2. Experimental 2.1. Preparation of Fe3O4/Bi24Fe2O39/Bi HPMRs All chemical reagents were analytical grade and were used without any purification. In a typical experimental procedure, 2 mmol sodium dodecyl sulfate (SDS) was dissolved in a mixed solvent of 10 mL of ethylene glycol (EG) and 4 mL of H2O. The resulting mixture was stirred at 45 °C for 30 min until it was clear. Two identical solutions were obtained by dissolving 2 mmol (NH4)2Fe(SO4)2 and 2 mmol (NH4)2C2O4. Both solutions were magnetically stirred at 45 °C for 1 h. After the reaction was completed, the resulting yellow precipitates were centrifugally separated and washed three times with H2O and C2H5OH to obtain rod-shaped FeC2O4·2H2O precursors. The precursors were transferred to 20 mL of 0.5 mmol Bi(NO3)3·5H2O solution. The mixture was heated in a water bath at 45 °C for 2 h. The composition of the precursors was controlled by changing the Fe2+/Bi3+ molar ratio (γ = 1:0, 1:0.25, 1:0.5, 1:0.75, 1:1, 1:2) via keeping FeC2O4·2H2O precursors fixed and adjusting [Bi3+] concentration (0 M, 0.001 M, 0.002 M, 0.003 M, 0.004 M, 0.008 M, respectively). Finally, the precursors were calcined at 500 °C for 3 h under the protection of nitrogen and heating rate of 5 °C·min−1 to obtain Fe3O4/Bi24Fe2O39/Bi HPMRs. 2.2. Characterization The phases structure of the samples was analyzed by X-ray diffraction (XRD, D/MAX-IIIA, CuKα radiation, λ = 0.15406 nm, 10°/ min). A Horiba, EX-250 energy dispersive X-ray spectrometer (EDX) was used to determine the element components of the samples. An ESCALAB 250Xi X-ray photoelectron spectrometer (XPS) was employed to gain further insight into the chemical composition and the oxidation states of Bi, O, and Fe. The binding energies obtained in the XPS analysis are standardized for specimen charging using C1s as the reference
Fig. 1. (a) XRD patterns and (b) phase mass fraction of the products formed at various Fe2+/Bi3+ molar ratios (γ). 2
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Table 1 Phase content (β), lattice parameter (a), average crystal size (D), and microwave absorbing properties of the BixFe3-xO4/Bi hollow microrods obtained under various Fe3+/Bi3+molar ratios γ. γ
1:0 1:0.25
1:0.50
1:0.75
Phase content /%
Fe3O4 Fe3O4 Bi Bi24Fe2O39 Fe3O4 Bi Bi24Fe2O39 Fe3O4 Bi Bi24Fe2O39
100 36.60 55.68 7.72 11.80 84.21 3.99 3.03 79.30 17.67
d
a /Å
D311 /nm
Microwave-absorbing properties Optimal RL value (dB)
f (GHz) (optimal RL)
dw (mm) (RL < –20 dB)
Frequency range (GHz) (RL < –20 dB)
0.276 0.164
6.291 6.287
33.5 47.9
− 13.23 − 47.32
10.96 8.32
/ 1.4 − 8.0
/ 2.0 − 16.0
0.139
6.293
67.8
− 15.66
16.40
/
/
0.081
/
105.5
− 47.90
17.92
6.8 − 7.0
17.44 − 18.0
γ = 1:0.25–1:0.5. These results confirm the formation of Fe3O4/ Bi24Fe2O39/Bi composites. As γ varies (1:0, 1:0.25, 1:0.50, 1:0.75), the peak intensities from Fe3O4 gradually weakened, whereas the peak intensities from Bi first increased and then decreased. Such changes are due to the variation in the contents and crystallinity of Fe3O4 and Bi. The quantitative phase of samples was determined by the relative intensity ratio (RIR) method based on the XRD data [24]. Quantitative calculations were carried out using the following equation:
⎡ ⎤ 1 Xi = (Ihkli )(Ki, n ) ⎢ m ⎥ ∑ j = 1 (Ihkl ′ j )(Kj, n ) ⎣ ⎦
(1)
where Xi is the fraction of phase i which needs to be measured; Ki,n and Kj,n are the RIR for diffraction line n of phase i and j; Ii,n and Ij,n are the intensities of diffraction line n of phase i and j, respectively; and m is the number of phases in the samples (Fig. 1b and Table 1). As shown in Fig. 1b, when γ varies (1:0, 1:0.25, 1:0.5, 1:0.75), Fe3O4 content gradually decreases from 100% to 3.03%, whereas the Bi content first increases from 0% to 84.21% and then decreases to 79.30%. Conversely, Bi24Fe2O39 content first increases from 0% to 7.72% and then decreases to 3.03%. Such changes may be owing to the decreased size of the precursors, causing the decreased interface area between Fe3O4 and Bi. The aforementioned results are consistent with the subsequent XPS and EDX data. Average crystalline size (D) was calculated from the XRD311 peak width based on the Scherrer formula: D = 0.89λ/βcosθ, where θ is the Bragg angle; λ is the wavelength of the X-ray; and β is the half-height of angle diffraction. The obtained values are given in Table 1. Increase in D is seen from 33.5 nm to 67.8 nm at γ = 1:0–1:0.5, and decrease is seen at γ = 1:0.75. These results indicate that the component, phase, and crystalline size of the samples can be adjusted by controllingγ.
Fig. 2. Mass fractions of Fe and Bi elements are functions of Fe2+/Bi3+ molar ratio (γ).
further (i.e., 1:0.75∼ 1:2), Fe and Bi contents have persisted with so little change. This may be because Bi2 (C 2 O4 )3⋅7H 2 O crystallizing out and absorbing on the surface of FeC 2 O4⋅2H 2 O hindered the diffusion of Bi3+. Similar phenomena were also reported in literature [9,15,16]. Therefore, regulation of Fe2+/Bi3+ molar ratio via controlling [Bi3+] can effectively control the content of the phase in the heterogeneous structure. 3.3. XPS characterization XPS was used to analyze the surface state and elemental composition of the HPMRs obtained under differentγ. As shown in Fig. 3, binding energies of all elements have been calibrated with the binding energy of C 1 s (284.8 eV). In the full spectra (Fig. 3a), Fe, Bi, O, and C elements were found for the samples. The peaks centered at binding energies of 680.2, 466.5, 442.6, 186.7, 161.0, 96.4, and 21.0 eV correspond to Bi 4p3/2, Bi 4d3/2, Bi 4d5/2, Bi 4f5/2, Bi 4f7/2, Bi 5p, and Bi 5d, respectively. The other peaks at binding energies of 56.2, 285.2, 530.8, 711.2, and 724.5 eV are ascribed to Fe 3p, C 1 s, O 1 s, Fe 2p3/2, and Fe 2p1/2, respectively. When γ varies from 1:0.25 to 1:0.75, the peak intensities of Fe and O are decreased, whereas those of Bi are increased. Such changes could be related to the increased Bi content and decreased Fe and O contents, consistent with the XRD and EDX data. The Bi 4 f spectra contain four peaks (Fig. 3b). Two peaks centered at 164.5 and 159.2 eV correspond to Bi 4f5/2 and Bi 4f7/2 of Bi3+, respectively, demonstrating that chemical state of Bi is trivalent oxidation in Bi24Fe2O39[25]. The other two peaks at 157.2 eV (4f7/2) and 162.6 eV (4f5/2) correspond to Bi°, indicating the presence of metallic Bi in the composites [26]. As seen from Fig. 3b, the peak intensity of Bi° increases with γ varying from 1:0.25 to 1:0.5, corresponding to the increased Bi° content. However, two peaks from Bi3+ are significantly
3.2. EDX analysis The elemental composition of the samples obtained at various γ is characterized by EDX. The Fe and Bi contents in the products measured by EDX were plotted as shown in Fig. 2. When γ varies from 1:0–1:2, the mass fraction of Fe reduces from 100 wt% to 0 wt%; the mass fraction of Bi increases from 0 wt% to 100 wt%. To reveal the relationship between element (Fe, Bi) content and γ, we fit experimental data with two Boltzmann equations, corresponding to two curves (Fig. 2); one is associated with the dissolution of FeC 2 O4⋅2H 2 O , and the other is related to Bi3+ ions diffusion and crystal growth of Bi2 (C 2 O4 )3⋅7H 2 O . When Fe2+/Bi3+ molar ratio is very low (i.e., 1:0 to 1:0.50), low supersaturation is detrimental to forming Bi2 (C 2 O4 )3⋅7H 2 O . When Fe2+/Bi3+ molar ratio is increased to 1:0.50, lower solubility products of Bi2 (C 2 O4 )3⋅7H 2 O than that of FeC 2 O4⋅2H 2 O promotes the dissolution of FeC 2 O4⋅2H 2 O , thus accelerating the diffusion of Bi3+ ions and crystal growth of Bi2 (C 2 O4 )3⋅7H 2 O . If Fe2+/Bi3+ molar ratio is increased 3
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Fig. 3. XPS spectra of full survey (a), Bi 4 f (b), O 1 s (c), and Fe 2p (d) for samples formed at various γ.
higher than those of metallic Bi°, demonstrating higher content of Bi24Fe2O39 than that of Bi° in the composites. The O 1 s spectra were deconvoluted into three photoemission peaks (Fig. 3c), corresponding to three distinct oxygen species; the peaks at 530.4, 531.7, and 533.1 eV are assigned to Fe–O/Bi-O [27], OH−, and H2O, respectively. The Fe 2p spectra can be fitted with four peaks, as shown in Fig. 3d. The peaks centered at 710.5 and 724.0 eV are characteristic peaks of Fe2+, whereas peaks centered at 712.2 and 726.4 eV correspond to Fe3+, consistent with literature [19], XPS data further confirm the formation of Fe3O4/Bi24Fe2O39/Bi composites.
image (Fig. 4j), the interplanar spacing of 0.328 nm (left up) and 0.296 nm (right down) correspond to (012) facet of Bi and (220) facet of Fe3O4, respectively. Between them, short range ordered lattice fringes with interplanar spacing of 0.318 nm can be assigned as (201) facets of Bi24Fe2O39. XRD and TEM results verify that the current materials are composites of Fe3O4, Bi shell, and Bi24Fe2O39. Interface plays an important role in the electronic and magnetic properties of Fe3O4/ Bi24Fe2O39/Bi HPMRs. The Fe3O4/Bi24Fe2O39/Bi HPMRs were synthesized by a self-assembly/precipitate conversion/ decomposition process [14]. Some reactions occur in the above three-step process as follows [28]:
3.4. SEM and TEM analysis
Fe 2 + + C 2 O42 − + 2H 2 O ↔ FeC 2 O4⋅2H 2 O
The effect of γ on the size, morphology, and microstructure of the sintered samples can be studied by SEM, TEM, and SAED. As seen from Figs. 4a and 4b, the samples formed at γ = 1:0 were microrods with a solid rectangular section and 3D sponge-like porous structures. Their length and aspect ratios were 3.53–6.21 µm and 3–10, respectively. These porous structures were composed of massive nanorods ranging from 30 nm to 40 nm in diameter, with smooth surface and a large number of pores. When γ was 1:0.25, the microrods became longer and thinner with 1.55–6.62 µm in length and 0.2–0.4 µm in diameter (Fig. 4c). The microrods consisted of nanorods with 200–400 nm in length and 50–100 nm in diameter; a large number of pores generated among nanorods. A splitting microrod (Fig. 4d) reveals the hollow structure with wall thickness of 50–100 nm. The hollow porous structure still was kept at γ = 1:0.5 (Figs. 4e–4f); however, only irregular nanoparticles were produced at γ = 1:0.75–1:2 (Fig. S1). The mirostructures of the sample were further observed by TEM. Seen from the low-power TEM images, the samples formed at γ = 1:0.25 were rectangular microrods with porous structures; massive nanoparticles form a cross-linked network (Figs. 4g–4h). The various contrasts in edges and centers reveal the hollow structures. The wall thickness was about 50–100 nm. Some clear rings consisting of diffraction spots were found in the SAED patterns, indicating high crystallinity (Fig. 4i). In the HR-TEM
2Bi3 +
+ 3FeC 2 O4⋅2H 2 O + H2 O ↔ Bi2 (C 2 O4 )3⋅7H 2 O + Δ
3FeC 2 O4⋅2H 2 O⟶Fe3 O4 + 2CO2 + 4CO + 6H2 O Δ
Bi2 (C 2 O4 )3⋅7H 2 O⟶Bi + 2CO2 + 2H2 O
(2)
3Fe 2 +
(3) (4) (5)
Δ
2FeC 2 O4⋅2H 2 O + Bi2 (C 2 O4 )3⋅7H 2 O⟶Bi24 Fe2 O39 + 2CO2 + 4CO + 6H2 O (6) In the first step, rod-like ferrous oxalate precursors directly precipitate from the solution by coordinated self-assembly at room temperature [29,30]. FeC2O4·2H2O nuclei generates via the co-precipitation of Fe2+ and C2O42- (Reaction 2). Fe2+ and C2O42- produce the infinite chain arrangement via chelation reaction; each Fe2+ ion interacts with two H2O molecules to form the octahedral FeO6 unit. Nanocrystals preferentially grow along the b-axis direction via coordinated self-assembly causing the formation of 1D nanomaterials. The second step is the precipitate conversion process from FeC2O4·2H2O (3.2 ×10−7) to Bi2(C2O4)3·7H2O to form rod-like ferrous oxalate/bismuth oxalate precursors. This is because the solubility products of Bi2(C2O4)3·7H2O (4.0 ×10–36) [31] is lower than that of FeC2O4·2H2O (3.2 ×10−7) (Reaction 3). Bi2(C2O4)3·7H2O nanocrystals anchor on FeC2O4·2H2O's surface by Fe-O-Bi bonds. Some decomposition reactions 4
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Fig. 4. (a–f) SEM, (g–h) TEM, (i) SAED pattern, and (j) HR–TEM image of the samples obtained under various γ: (a–b) 1:0, (c–d, g–j) 1:0.25, and (e–f) 1:0.5.
HPMRs. The mesopores stem from the primary aggregation of the nanoparticles insides; the larger macropores result from the hollow structure. This is in conformity with SEM results. The BET specific surface area (SBET), pore volume, and pore size of the HPMRs originated from the nitrogen adsorption–desorption isotherm are 8.9954 m2·g−1, 0.014069 cm3·g−1, and 62.5593 Å, respectively.
occur in the third step, as shown in Reaction 4–6. Under the protection of N2 and/or Ar, precursors decompose and release a large amount of heat and gases (e.g., CO, H2O, and CO2), resulting in the formation of numerous pores. Meanwhile, the released gases drive the remaining metal ions toward the surface [32] and thus induce the formation of hollow porous Fe3O4/Bi24Fe2O39/Bi microrods. Also, the sintering temperature, time, and ambience can be applied to adjust the crystal size, phase structure, and textural characteristics of the products. Further studies are under way.
3.6. Static magnetic properties The vibrating sample magnetometer was used to analyze the magnetic properties of the HPMRs at room temperature. In Fig. 6a, all samples exhibit a typical S-type hysteresis loop, indicating that the samples are ferromagnetic. All of the samples reach the saturation magnetization at about 10 kOe. From Fig. 6b, pure Fe3O4 PMRs exhibit the highest saturation magnetization Ms of 84.66 emu g–1 and the lowest coercivity Hc of 225.45 Oe. After Fe3O4 was combined with Bi24Fe2O39 and Bi, the Ms values of Fe3O4/Bi24Fe2O39/Bi HPMRs gradually decreased from 36.78 emu g–1 at γ = 1:0.25 to 0.019 emu·g1 at γ = 1:2. This demonstrates that coating Fe3O4 with Bi and Bi24Fe2O39 is not beneficial in improving Ms. Ms generally depends on the component, interface, and crystal size of composites. The current HPMRs are a composite of ferromagnetic Fe3O4, nonferromagnetic Bi shell, and the inter-diffused layer of antiferromagnetic Bi24Fe2O39. Obviously, an increase in the content of antiferromagnetic Bi24Fe2O39 and nonferromagnetic Bi leads to a decrease in Ms. The Hc values of Fe3O4/Bi24Fe2O39/Bi HPMRs almost remained unchanged at γ = 1:0 – 1:0.5 and then sharply increased from 225.05 Oe at γ = 1:0.5 to 14191 Oe at γ = 1:2, which is significantly higher than that of pure Fe3O4 (225.45 Oe) and bismuth ferrite (4470 Oe) [34]. The enhanced Hc value is generally determined by crystal anisotropy, shape anisotropy, stress anisotropy, and exchange anisotropy. Obviously, crystal size is not a critical factor in determining Hc. Fe3O4 nanoparticles have multidomain structures because of the bigger average crystal size (D) than the multidomain/monodomain critical size (DC) (50 nm for Fe3O4). As a result, Hc decreases with the increase of D. In this study, the significant enhancement in Hc should be owed to the surface pinning role of antiferromagnetic Bi24Fe2O39 and nonferromagnetic Bi on the surface of ferromagnetic Fe3O4.
3.5. Textural characteristics Fig. 5 is the nitrogen physical adsorption and desorption isotherms and the corresponding pore size distribution curves for the typical Fe3O4/Bi24Fe2O39/Bi HPMRs formed at γ = 1:0.25. According to BDDT classification [33], the HPMRs display the type II isotherms and H3 hysteresis loops. The hysteresis loop of type H3 corresponds to the partial retention of slitlike large pore. Seen from the inset in Fig. 5, multimodal pore size distribution is found in the mesoporous and macroporous regions, owing to the hollow porous heterostructure of the
Fig. 5. Nitrogen adsorption and desorption isotherms and (the inset) the corresponding pore size distribution curve of the typical samples. 5
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Besides, as for Fe3O4/Bi24Fe2O39/Bi HPMRs, the interfacial magnetic coupling exists between Fe3O4 and Bi24Fe2O39 with magnetically disordered spins, which increases with Fe3O4 and Bi24Fe2O39 contents, thereby causing enhanced Hc. A similar phenomenon was found in isostructural spinel interfaces of Ni and NiFe2O4, caused by a thin interdiffused layer at the interface [14]. These data demonstrate that controlling the composition, structure, and interface can tune magnetic performances of Fe3O4/Bi24Fe2O39/Bi HPMRs. 3.7. Microwave absorbing properties Reflection loss (RL) generally was used to describe the EM wave absorption properties of the samples. Using the transmission line theory, the RL values were calculated based on the as-obtained complex permeability and permittivity:
RL (dB ) = 20 log (Zin − Z0)/(Z in+Z0)
(7)
Z in= μr / εr tanh[j (2πfd/ c ) μr εr ]
(8)
Z0 =
(9)
μ 0 / ε0
Where, f is frequency, d is sample thickness, c is the light velocity, Zin is the input impedance between free space and the materials, and Z0 is the characteristic impedance of free space. The effects of γ, f, d, and mass fraction on RL were investigated to elucidate the microwave absorption mechanism of Fe3O4/Bi24Fe2O39/Bi HPMRs. The composition-dependent microwave absorption properties of the prepared samples were studied, as shown in Fig. 7 and Table 1. Altering γ (1:0, 1:0.25, 1:0.50, and 1:0.75) can tune the composition
Fig. 6. Fe2+/Bi3+ molar ratio (γ) versus magnetic hysteresis loops. The lower right inset shows expanded regions around origin.
Fig. 7. (a1, b1, c1, d1) calculated 3D plots, (a2, b2, c2, d2) reflection loss curves, (a3, b3, c3, d3) dependence of matching thickness (tm) on matching frequency (fm) at wavelengths of λ/4, 3λ/4, and 5λ/4, and (a4, b4, c4, d4) effective bandwidth (RL < −10 dB) of the paraffin composites containing 70% wt. samples formed under various λ (1:0 for a1−a4, 1:0.5 for b1 −b4, 1:0.75 for c1−c4, and 1:1 for d1−d4). 6
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Fig. 8. (a) Calculated reflection loss curves, (b) the calculated matching thickness according to the λ/4, 3λ/4, and 5λ/4 models. 3D reflection loss curves, (c) the corresponding 3D plots, and (d) the effective bandwidth (RL ≤ −10 dB) of the paraffin composites containing 65% wt. Fe3O4/Bi24Fe2O39/Bi HPMRs (S2) formed under λ = 1:0.25.
7d1 shifted to the 3/4 wavelength, corresponding to the 3/4 wavelength cancellation model. The bandwidth (RL ≤ −10 dB) shifted to higher frequency ranged with thicker sample thicknesses (Fig. 7c4−7d4). This demonstrates that proper Bi24Fe2O39 and Bi contents in Fe3O4/Bi24Fe2O39/Bi HPMRs favors the improvement of microwave absorbing properties. Mass fraction of Fe3O4/Bi24Fe2O39/Bi HPMRs was optimized for the best EM wave absorption performance. The composite of paraffin and 65 wt% Fe3O4/Bi24Fe2O39/Bi HPMRs obtained at λ = 1:0.25 display the best property (Fig. 8). The matching thickness (tm) and matching frequency (fm) were calculated based on the following equation: tm = nc /(4f μr εr ) . As seen in Fig. 8a, fm gradually moved toward m the low frequency with the increase of tm. The minimum RL reaches − 47.3 dB at 8.72 GHz, corresponding to 2.4 mm matching thickness. The absorbing band (RL ≤ −20 dB, corresponding to 99% attenuation) reached 14.0 GHz (at the frequency ranges of 2.0 −16.0 GHz) for composites with thickness range of 1.4 − 8.0. Fig. 8b shows three curves (tm versus fm) at n = 1, 3, 5, corresponding to wavelengths λ/4, 3λ/4, and 5λ/4, respectively. RL reaches the peak value if the tm and fm satisfy the abovementioned equation. In the Fig. 8a, the main absorbing peak (RL ≤ −20 dB) corresponds to the quarter wavelengths (Fig. 8b) and is over 2 − 18 GHz. This indicates that Fe3O4/Bi24Fe2O39/Bi HPMRs have good absorption performance and obey the λ/4 cancellation model. Fig. 8c further shows that the absorption band (RL ≤ −10 dB, 90% attenuation) is over 2 − 18 GHz, with sample thickness of 1.6–9.0 mm. The effective bandwidth (RL ≤ −10 dB) is 3.3 GHz (Fig. 8d). In particular, the absorbing band (RL ≤ −20 dB) of 14.0 GHz is broader than those reported in literature (i.e., spongy Fe3O4 (8.91 GHz) [30], ZnO/Ni/ZnxNiyFe3-x-yO4 microhexahedra (13.1 GHz)[29], Fe3O4/ TiO2 nanotubes (0.3 GHz)[34], Fe3O4/C nanospindles (4.5 GHz)[35],
Fig. 9. Attenuation constant (α) of the samples formed at various γ.
and microwave absorption performances of the HPMRs. As for the Fe3O4 PMRs obtained at γ = 1:0, the minimum RL value of − 13.23 dB was observed at 10.96 GHz with a sample thickness of 1.6 mm (Fig. 7a1−a3). The absorbing band (RL ≤ −20 dB) was 0 GHz. The effective bandwidth (RL ≤ −10 dB) was below 2 GHz (Fig. 7a4). At γ = 1:0.25, Fe3O4/Bi24Fe2O39/Bi HPMRs showed the best microwave absorption performances (Fig. 7b1−b3). A strong and wide absorbing band was found over 2 − 18 GHz, corresponding to the quarter-wavelength cancellation model. The minimum RL value of − 30.29 dB reached at 2.62 GHz with a sample thickness of 5.9 mm (Fig. 7b1). The absorbing band (RL ≤ −20 dB) was 3.1 GHz (Fig. 7b2). The largest effective bandwidth (RL ≤ −10 dB) is 3.28 GHz corresponding to 1.5 mm sample thickness (Fig. 7b4). Further varying γ (1:0.5, 1:0.75), the absorption band (RL ≤ −20 dB) decreased to 0 GHz (Fig. 7c1−7d1). The short and narrow absorption band in Fig. 7c1 and 7
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Fig. 10. Modulus of the normalized impedance Z versus frequency for the samples formed at various γ.
14–18 GHz stems from exchange resonance. With the value of γ varying (1:0, 1:0.25, 1:0.5, 1:0.75), the Fe3O4 content decreases, and the contents of Bi24Fe2O39 and Bi increase. Such changes cause the decreased attenuation constant. Impedance match between permeability and permittivity is another critical factor to determine mirowave absorption. High impedance match ratio represents the ability of the microwave to enter into the absorber and convert to thermal energy or be dissipated through interference. Impedance match requires that permittivity and permeability satisfy the equation: Z = Zin/ Z0 ∝ 1, where Z is the modulus of the normalized characteristic impedance, Zin is the input impedance between free space and the materials, and Z0 is the characteristic impedance of free space. When the value of Z is equal or close to 1, it is beneficial in improving microwave absorption. The plots of Z versus frequency are shown in Fig. 10. With varying γ from 1:0 to 1:0.75, Z values become closer to 1, implying an increasing impedance match. Our data demonstrate that increasing the contents of Bi and Bi24Fe2O39 favor an improved impedance match. The enhanced match should be ascribed to the hollow and porous heterostructure. On one hand, Bi and interdiffused layer Bi24Fe2O39 with low conductivity and nonferromagnetic properties can decrease the permittivity and permeability, therefore making tan δE close to tan δM. On the other hand, air with low permittivity inside the hollow and porous structures can improve impedance matching. From these analyses, it is deduced that the impedance matching enhancement rather than the attenuation constant is a central factor in the improved microwave absorption.
Fe3O4/C nanorings (9.55 GHz)[16], BiFeO3 (0 GHz) [36], and CuxFe3xO4 @Cu core-shell hollow spherical chains (9.5 GHz)[17]. Seen from Table 1, Fe3O4/Bi24Fe2O39/Bi HPMRs exhibit light weight, higher absorption, and broader bandwidth than Fe3O4 PMRs. Our data demonstrate that the introduction of electromagnetic transparent materials (Bi24Fe2O39 or Bi) can significantly improve the microwave absorbing properties of Fe3O4. It is generally accepted that attenuation constant (α) and impedance matching are two important factors in determining the microwave absorption. α and impedance matching were investigated to reveal the enhanced microwave absorption mechanism of Fe3O4/Bi24Fe2O39/Bi HPMRs. The attenuation constant α is used to analyze the attenuation ability of the absorber, which can be calculated using the following equation [37,38]:
α=
2 πf c
(μ″ε″ − μ′ε′) +
(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2
(10)
In Fig. 9, Fe3O4 PMRs show two strong attenuation peaks at 4 − 10 GHz and 12 − 18 GHz. After combining Fe3O4 with Bi and Bi24Fe2O39, the obtained Fe3O4/Bi24Fe2O39/Bi HPMRs exhibit lower α values. Moreover, α values are gradually decreased with the increasing contents of Bi and Bi24Fe2O39. This indicates that increasing the Bi and Bi24Fe2O39 contents suppresses improvement of attenuation ability. The decreased attenuation constant can be interpreted as follows: in this study, Fe3O4/Bi24Fe2O39/Bi HPMRs consist of Fe3O4, Bi24Fe2O39, and Bi. Fe3O4 is a kind of ferromagnetic absorbers, whereas Bi24Fe2O39 and Bi are nonmagnetic materials with low conductivity. On the one hand, the low conductivity restrains the local movement of bound charges and the variation of dipole moments in the alternating EM field, causing the decreased dielectric relaxations and dielectric loss (Fig. S2c–S2f). On the other hand, an increase in the content of antiferromagnetic Bi24Fe2O39 and nonferromagnetic Bi will decrease Ms, resulting in low permeability and magnetic loss (Fig. S3c). In Fig. S3d, the eddy current loss shows a strong peak at less than 8 GHz and a weak peak at 14–18 GHz. The former is generally defined as natural resonance; the latter is generally defined as “exchange mode” resonance, as a result of the surface effect, small size effect, and spin wave excitations. According to the skin effect criterion,[39] magnetic loss for Fe3O4/ Bi24Fe2O39/Bi HPMRs at 2–8 GHz stems from natural resonance and at
4. Conclusions Fe3O4/Bi24Fe2O39/Bi HPMRs have been prepared via a self-assembly/precipitate conversion/decomposition process. Changing Fe2+/ Bi3+ molar ratio can handily adjust their composition and properties. In Fe3O4/Bi24Fe2O39/Bi HPMRs, the increase of Bi and Bi24Fe2O39 contents with Fe2+/Bi3+ molar ratio leads to the decreased Ms and increased Hc. Compounding Fe3O4 PMRs with Bi and Bi24Fe2O39 can enhance impedance matching and decrease the attenuation constant. An optimum mirowave absorption performance is shown by Fe3O4/ Bi24Fe2O39/Bi HPMRs obtained at γ = 1:0.25, with the absorbing band (RL ≤ −20 dB) of 14.0 GHz. The minimum RL reaches − 47.3 dB at 8
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8.72 GHz, corresponding to a 2.4 mm matching thickness. Compared with Fe3O4 PMRs, Fe3O4/Bi24Fe2O39/Bi HPMRs exhibit higher absorption, lighter weight, and wider bandwidth. The enhanced microwave absorbing properties should be owned to enhanced impedance matching rather than attenuation constant caused by electromagnetic transparent materials (Bi24Fe2O39 or Bi). Our investigations provide useful guidance for designing and preparing other hollow, porous, heterostructured absorbers.
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