Excellent Microwave Absorption Properties Derived from the Synthesis of Hollow Fe₃o₄@Reduced Graphite Oxide (RGO) Nanocomposites

Abstract

Magnetic nanoparticles, such as Fe₃O₄ and Co₃O₄, play a vital role in the research on advanced microwave absorbing materials, even if problems such as high density and narrow band impedance matching are still unsolved. Herein, the study of lightweight hollow Fe₃O₄@reduced graphite oxide (RGO) nanocomposites synthesized via the solvothermal method is presented. The microstructure and crystal morphology of the materials were characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses. Single crystalline hollow Fe₃O₄ spheres were grown onto RGO flakes, leading to the formation of heterojunction, which further influenced the microwave absorption properties. The latter were evaluated by standard microwave characterization in the frequency range of 2–18 GHz. It was found that, for a specific Fe₃O₄@0.125 g RGO composite, the minimum reflection loss can reach −41.89 dB at 6.7 GHz, while the reflection loss was less than −10 dB from 3.4 GHz to 13.6 GHz for a nanocomposite sample thickness in the range of 1–4 mm. The combination of these two materials thus proved to give remarkable microwave absorption properties, owing to enhanced magnetic losses and favorable impedance matching conditions.

1. Introduction

With the rapid advancement of science and technology, more and more electrical equipment and information systems have been used in various fields. However, such systems cause electromagnetic (EM) radiation, which is harmful to people’s health and affects the normal operation of electronic equipment. Therefore, the research on microwave absorbing materials (MAMs) has attracted extensive attention [1]. As an effective means to improve the survivability and penetration ability of weapon systems, the utilization of MAMs is extremely important for absorbing EM waves, reducing EM radiation, and improving the human living environment [2,3]. Nowadays, exploring new lightweight, reduced-thickness MAMs to meet the requirements of a wide absorption bandwidth, strong attenuation property with a good impedance match is very important [4,5,6].

In general, magnetite materials, including different types of ferrites [7], carbonyl iron [8], and inorganic magnetic microspheres [9], have been widely investigated and applied into practice for the merits of high-saturation magnetization and high coercivity, which contribute to magnetic energy loss [10]. Owing to Snoek’s limit at a high frequency, inverse spinel-type Fe₃O₄ with excellent magnetic properties hardly achieves the absorbing requirements in the gigahertz frequency range [11,12,13,14]. To enhance the microwave absorbing properties of Fe₃O₄ in the high frequency region, combining dielectric loss materials with Fe₃O₄ composite absorbers has been an effective way to improve the absorption properties of EM waves.

Recently, graphene [15], as a novel dielectric material, has attracted increasing attention owing to its low density and good physical and chemical properties [16]. Graphene oxide (GO) is a two-dimensional (2D) structure obtained by modifying the graphene sheet whose basal plane has hydroxyl groups and epoxide groups [17,18,19], which has a widespread application in chemistry [20], energy [21], catalysis [22], and environmental pollution control [23]. Nevertheless, GO materials that attenuate EM energy by means of dielectric loss are not favorable for EM absorption because of their poor impedance matching mechanism. It is well known that, based on the energy dissipation mechanism of the absorber, the microwave loss mechanism can be sorted into dielectric loss and magnetic loss, so effective complementarity between these factors is necessary to enhance microwave absorbing properties. Zong et al. [24] prepared a reduced graphene oxide-CoFe₂O₄ heterostructure composite synthesized by a facile route. The maximum absorption could reach −44.1 dB at 15.6 GHz and the absorption bandwidth with a reflection loss below −10 dB reached up to 4.7 GHz. Feng and co-workers prepared heterostructure Fe-Fe₃O₄@C core-shell composites, which displayed excellent microwave absorption via polymerization and calcination. The results showed that the optimal reflection loss can reach −29.3 dB at 12.6 GHz under a thickness of 3.9 mm [25]. Yang et al. [26] synthesized Fe₃O₄/Ppy/carbon nanotube (CNT) composites through the mingling of CNTs with Fe₃O₄/Ppy composites. The maximum absorption could reach −25.9 dB, and the effective bandwidth with a reflection loss less than −10 dB was about 4.5 GHz.

Herein, we successfully prepared hollow Fe₃O₄@RGO nanocomposites by the solvothermal method, in order to obtain a composite with better EM absorption properties. The crystalline structure, morphology, microwave absorption properties, and absorption mechanism were systematically investigated. Furthermore, the electromagnetic impedance matching performance and the microwave attenuation ability were discussed.

2. Experimental

2.1. Synthesis of Hollow Fe₃O₄@RGO Nanocomposites

A schematic illustration of the hollow Fe₃O₄@RGO composites is described in Figure 1. In this experiment, all chemicals were used as received, and the preparation process is demonstrated below [27,28]: First, 0.5 g of graphite oxide (GO) was dissolved in 80 mL of ethylene glycol (EG). After that, the solution was sonicated for 30 min, then 5.4 g of FeCl₃·6H₂O (Shanghai Aladdin Technology Co., Ltd., Shanghai, China) and 0.4 g of polyvinyl-pyrrolidone (PVP) were added. Then, after being stirred for 30 min, 2.5 g of urea (in a molar ratio Fe³⁺: urea = 1:2) was added. The obtained solution was transferred to a 100-mL Teflon-lined stainless-steel autoclave at 200 °C for 20 h. After that, the precipitates were washed with deionized water three times and dried at 60 °C for 12 h in a vacuum drying oven. As a comparison, 0.25 g of GO, 0.125 g of GO was dissolved in 80 mL of EG while maintaining other conditions, respectively.

2.2. Characterization and Measurement

An X-ray diffractometer (XRD, D8A Advance, BRUKER) was utilized to characterize the crystal structure of samples. X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo Fisher 0ESCALAB 250Xi. The morphology, size, and microstructure of the nanocomposites were examined on an FEIXL30 scanning electron microscope (SEM) and a G2F20 transmission electron microscope (TEM) [29]. The electromagnetic parameters were collected by a network analyzer (VNA, N5242A PNA-X, Agilent) through the classical coaxial measurement method by measuring in the frequency span from 10 MHz to 26.5 GHz. For the measurement of electromagnetic wave absorption properties, the samples were dispersed in paraffin homogeneously with a weight (wt%) ratio of 1:1 (sample/paraffin), and then the mixture was pressed into a cylindrical shape (ϕ_out = 7 mm and ϕ_in = 3 mm) with a thickness of only 3 mm. High requirements were needed for the test system to prepare samples, such as a smooth surface, flat without burrs or scratches, and no gaps between the inner and outer surfaces of the samples.

3. Results and Discussion

3.1. XRD and XPS Analyses

For the sake of clarification, the XRD patterns of hollow Fe₃O₄ and hollow Fe₃O₄@RGO are illustrated in Figure 2. As could be observed, the typical peaks at 29.9°, 35.5°, 42.9°, 53.1°, 56.8°, and 62.5° could be readily indexed to the (220), (311), (400), (422), (511), and (440) planes of hollow Fe₃O₄; moreover, the face-centered cubic phase structure (JCPDS no. 89-2355) could also be seen [30]. Obviously, all peaks in the hollow Fe₃O₄@RGO curve are slightly weaker than those in the hollow Fe₃O₄ curve, which could be ascribed to the graphene content in the composites. In addition, the (311) peak of hollow Fe₃O₄@RGO shifted toward the lower angle of 35.3°, which is in contrast to that of the hollow Fe₃O₄ shown in the magnified picture of XRD patterns in Figure 2b.

To confirm the phases and structures of the hollow Fe₃O₄@RGO composites, XPS spectra were measured. Elements, including C, O, and Fe were found to be correlated with various atom contents, as illustrated by the large energy range spectrum (Figure 3a). In Figure 3b, the high-resolution spectrum of Fe could be deconvoluted into two peaks, related to Fe 2p3/2 and Fe 2p1/2, which correspond to the band energies of 710.9 eV and 724.3 eV, respectively, indicating that the mixing ratio of oxides of Fe (II) and Fe (III) is consistent with the reported value of Fe₃O₄ [31]. Figure 3c shows the O1s peaks at 529.1 eV and 532.1 eV, which could be attributed to the O element in Fe₃O₄ and RGO, respectively [32]. Moreover, the C1s peak (Figure 3d) at 285.1 eV could be attributed to the C element in RGO. Hence, the composites are suggested to be composed of Fe₃O₄ and RGO, revealing that hollow Fe₃O₄ was successfully grown on the RGO multilayer.

3.2. Morphology Analysis

At the top of Figure 4, SEM and TEM images of the materials are shown, which contribute to the visualization of the morphology of both the hollow Fe₃O₄ and hollow Fe₃O₄@RGO nanocomposites. As presented in Figure 4a, all products have a hollow spherical structure with a uniform size, and the inner hollow structure can be observed in several broken pellets. Clearly, the cavity size is 130 nm, and the corresponding shell thickness is 35 nm, as can be seen in Figure 4d. Figure 4b–d provide representative images of the hollow Fe₃O₄@RGO, which suggest that the hollow Fe₃O₄ microspheres are adhered to the flaky RGO. Figure 4e–h display the overall structure of the hollow Fe₃O₄@RGO. Typically, the white area covered by gray or black areas is indicative of the hollowness of Fe₃O₄ microspheres, while the sample edge indicates the existence of RGO. In particular, it can be observed from Figure 4h that the RGO had coated the hollow Fe₃O₄. Notably, the Fe₃O₄ spheres in the surface of the hollow Fe₃O₄@RGO composites could improve the interaction between Fe₃O₄ and RGO. For the hollow Fe₃O₄@RGO composites, many charge carriers accumulated onto the interfaces, as shown in the inset of Figure 4h, which would lead to increased dielectric loss. Typically, the incident electromagnetic microwaves could be reflected and scattered between the multilayer interfaces, which would give rise to the dissipation of electromagnetic radiation energy [33].

3.3. Microwave Absorption Properties

Generally, the electromagnetic microwave absorption mechanism is investigated by parameters such as the relative complex dielectric permittivity (ε_r = ε’ – jε”), complex magnetic permeability (μ_r = μ’ – jμ”), and corresponding tangents, as displayed in Figure 5. Among them, ε’ and μ’ are well known to represent the storage ability of electromagnetic energy, whereas ε” and μ” are linked with energy dissipation [34]. Furthermore, both dielectric loss and magnetic loss are responsible for energy attenuation in electromagnetic microwave absorption, which can be characterized by tanδ_ε and tanδ_μ, respectively [35].

As can be observed from Figure 5a,b, the ε’ and ε” values of the hollow Fe₃O₄@RGO composites are higher than those of the hollow Fe₃O₄, owing to the addition of dielectric loss materials, which is particularly true for the hollow Fe₃O₄@0.5 g RGO composites. In detail, the ε’ values show a declining trend versus the changing frequency, whereas the ε” values exhibit a fluctuant variation trend, since the variation peaks appear at 6.5, 10, and 15 GHz across the whole range. Moreover, the permittivity curves indicate that the dielectric properties of the hollow Fe₃O₄ are greatly improved with the addition of RGO. Figure 5d,e show the μ’ and μ” values of the samples, among which the μ’ value presents a declining trend. Specifically, the μ’ value of the hollow Fe₃O₄ decreases dramatically in the range of 2–8 GHz, which is less than that of the hollow Fe₃O₄@RGO at the range of 4–18 GHz. Besides, the μ” values remain almost constant with the change in RGO concentration, except for the hollow Fe₃O₄. Figure 5c,f present the dielectric loss tangent and magnetic loss tangent of the samples, respectively. Notably, the dielectric loss tangent is less than 0.15, except for the hollow Fe₃O₄@0.25 g RGO in the range of 13–17 GHz, while the magnetic loss is mostly above 0.2. Obviously, the hollow Fe₃O₄@RGO composites possess a higher tanδ_μ at 2–18 GHz, which indicates that the magnetic loss contributes to the electromagnetic microwave absorption.

Based on the measured relative complex permeability and permittivity, the reflection loss (RL) of samples could be obtained according to the expressions listed below [36]:

$$Z_{in}=Z_0\sqrt{\frac{{\mu }_r}{{\epsilon }_r}}\tan \mathrm{h}[j\frac{2\pi fd}{c}\sqrt{{\mu }_r{\epsilon }_r}]$$

(1)

$$R_L=20\log [\left|\frac{Z_{in}-Z_0}{Z_{in}+Z_0}\right|]$$

(2)

$$Z_0=\sqrt{\frac{u_0}{{\epsilon }_0}}$$

(3)

where Z_in, Z₀ are the normalized input and free space characteristic impedance, respectively; ${\epsilon }_0$, μ₀ are the permittivity and permeability of vacuum, respectively; c is the light velocity; d is the thickness of absorber; and f represents the microwave frequency. Figure 6a–d display the three-dimensional correlations among the RL, frequency, and thickness of the hollow Fe₃O₄, hollow Fe₃O₄@0.5 g RGO, hollow Fe₃O₄@0.25 g RGO, and hollow Fe₃O₄@0.125 g RGO composites. Notably, the minimum RL of the hollow Fe₃O₄ composites (Figure 6a) is only −19.01 dB, which indicates that the hollow Fe₃O₄ composites have a lower microwave absorption. In addition, it could be observed from Figure 6b that the minimum RL of the hollow Fe₃O₄@0.5 g RGO composites reached −35.85 dB at around 8.4 GHz, and the corresponding frequency bandwidth below −10 dB was 2.1 GHz (ranging from 7.4 GHz to 9.5 GHz). For the hollow Fe₃O₄@0.25 g RGO composites, the minimum RL was as high as −32.93 dB at 4.8 GHz at 3 mm, and the corresponding bandwidth of the RL below −10 dB was 1.8 GHz, as shown in Figure 6c. For the hollow Fe₃O₄@0.125 g RGO composites in Figure 6d, the minimum RL was −41.89 dB at 6.7 GHz at 2.5 mm. Moreover, the RL increased with the decrease in RGO, and the peak shifted to a higher frequency. It could be clearly observed when comparing the minimum RL of three samples that the hollow Fe₃O₄@0.125 g RGO composites displayed the optimal composite mode with a reduced thickness. More importantly, the top-level microwave absorption performances of the hollow Fe₃O₄@0.125 g RGO composites further underlined the strong magnetic loss of the material. All composites exhibited higher RLs compared with those of the single hollow Fe₃O₄, which proves that the combination of hollow Fe₃O₄ and RGO would induce a better performance of microwave absorption, thus demonstrating that RGO could greatly improve the absorption property owing to the dielectric loss.

Microwave absorption properties are greatly associated with thickness; typically, the RL of the hollow Fe₃O₄@RGO is calculated for an absorber thickness of 1–4 mm by Equations (1)–(3), as shown in Figure 7a–c. Remarkably, the bandwidths (RL below −10 dB) cover the range of 2.7–13.8 GHz for thicknesses ranging from 1 to 4 mm within the overall frequency for all the composites. Moreover, it can be noticed that for higher thicknesses the minimum RL shifts towards lower frequencies, while the absorption peaks of the hollow Fe₃O₄@RGO composites become sharper. The RL reached a maximum of −34.3 dB for the 3-mm thick hollow Fe₃O₄@0.25 g RGO composite. Noticeably, for lower thicknesses, higher losses occur for lower RGO contents within the composites, while an opposite trend can be established at higher thicknesses. A sort of ‘turning point’ can be thus conceived at a thickness of 3 mm (green curves in Figure 7) regarding the interaction between microwaves and this typology of RGO-reinforced composites. Specifically, the frequency was found to be partly related to the maximum RL at a certain thickness, as displayed on the quarter-wavelength cancellation model [37]. The matching equation is expressed below:

$$t_m=\frac{n\lambda }{4}=nc/(4f_m\sqrt{\left|{\mu }_r\right|\left|{\epsilon }_r\right|})$$

(4)

where t_m, λ, and f_m are the matching thickness of the absorber, the wavelength, and the absorption frequency of the electromagnetic microwave. The calculated matching thickness and the peak frequency are plotted below the RL curves, where the red squares represent the specific thickness in the range of 1–4 mm corresponding to the peak frequency. In addition, the simulation of t_m vs. frequency for the hollow Fe₃O₄@RGO composites are also depicted based on the above equation. The red squares appear on the simulation curve, suggesting that the absorber thickness could be tailored to design the microwave absorption materials.

Furthermore, better impedance matching maximizes the penetration of electromagnetic waves into the material and reduces the direct reflection on the surface of the material [38]. When the value of the impedance matching characteristics (Z) is equal or close to 1, zero reflection is achieved, which is expressed as the following equation:

$$Z_{in}=\sqrt{\left|\frac{{\mu }_r}{{\epsilon }_r}\right|}\tan \mathrm{h}[\left(j\frac{2\pi fd}{c}\right)\sqrt{{\mu }_r{\epsilon }_r}]$$

(5)

The Z value of hollow Fe₃O₄@0.125 g RGO composites (Figure 7c) is almost equal to 1. Figure 7 reveals the amount of electromagnetic waves entering into the absorber and the good impedance matching in comparison with hollow Fe₃O₄@0.5 g RGO and hollow Fe₃O₄@0.25 g RGO composites.

Furthermore, magnetic loss is also closely correlated with natural resonance, eddy current resonance, hysteresis loss, and domain wall displacement. Generally, the eddy current coefficient (C₀) is almost a constant in accordance with the following equation when only magnetic loss is caused by the eddy current at the frequency range of 2–18 GHz.

$$C_0={\mu }^{″}{\left({\mu }^{\prime }\right)}^{-2}{f}^{-1}$$

(6)

As shown in Figure 8, the C₀-f curve presents an obviously declining trend at the frequency range of 2–18 GHz, which means that the attenuation of electromagnetic microwaves is not caused by eddy current resonance. However, the hysteresis loss is always inoperative in the weak field, while the domain wall resonance loss commonly appears in the MHz frequency range [39]. Hence, a conclusion could be drawn that the magnetic loss is mainly produced by natural resonance.

Table 1. Microwave absorption performances of various Fe₃O₄ nanoparticles in previous reports compared with hollow Fe₃O₄@RGO composites.

Sample	RL (dB)	Effective Bandwidth (GHz) (RL < 10 dB)	Thickness (mm)	Wt (%)	Reference
Fe3O4/5 wt% CNTs	−51.3	3.9	4.4	50	[40]
RGO/Fe3O4/ZnO	−57.0	5	2	33	[41]
CNTs/Fe3O4/rGO/C	−54.43	3.2	1.9	20	[42]
graphene@Fe3O4@PANI@TiO2	−41.8	3.5	1.6	50	[43]
graphene@Fe3O4@WO3@PANI	−46.7	1.8	1.5	30	[44]
Fe3O4/SiO2/graphene	−27.1	2.5	1.5	50	[45]
RGO/CoFe2O4/MWCNT	−46.8	3.4	1.6	50	[46]
PANI/Fe3O4/RGO	−36.5	5	2.5	40	[47]
hollow Fe3O4@RGO	−41.89	4.2	2.5	50	This work

lists some reported microwave absorption composites of the representative Fe₃O₄ material-based, graphene material-based, and Fe₃O₄@0.125 g RGO composite prepared in this work. Notably, the hollow Fe₃O₄@0.125 g RGO composites not only had a wide effective absorption bandwidth, but also displayed a promising negative RL value owing to this special hollow structure. Furthermore, the as-fabricated hollow Fe₃O₄@RGO properties could enhance the absorption performance and suit the requirements of ideal MAMs.

The microwave absorption performance of the hollow Fe₃O₄@RGO could be ascribed to the following reasons. (a) The outstanding magnetic hollow Fe₃O₄ sphere would induce a magnetic loss. (b) The main loss mechanism derives from dielectric loss rather than magnetic loss. Normally, dielectric loss is related to electronic dipole polarization and interfacial polarization. Firstly, electron migration, such as Fe²⁺ and Fe³⁺ ions, would induce dipole polarization in the composites. Secondly, the different neighboring phases, including dielectric constant and conductivity, have a significant effect on interfacial polarization. For the hollow Fe₃O₄@RGO composites, many charge carriers accumulated onto the interfaces, as shown in the TEM images, which aggravated the dielectric loss. (c) The interaction of dielectric loss and magnetic loss is also an important factor for the improvement of the microwave absorption performance; as a result, the RGO nanosheets modified by magnetic hollow Fe₃O₄ spheres have excellent absorption abilities.

4. Conclusions

In summary, hollow Fe₃O₄@RGO composites were successfully synthesized by the one-step solvothermal reaction route, which exhibited excellent microwave absorption properties in terms of the maximum RL value and the absorption bandwidth in the range of 2–18 GHz through tuning the hollow [Fe₃O₄]/[RGO] ratio. The surfaces of RGO were densely covered with hollow Fe₃O₄ spheres ~130 nm in diameter, with a corresponding shell thickness of ~35 nm, as shown by the SEM and TEM images. Specifically, the maximum RL could reach −41.89 dB at 6.7 GHz and the absorption bandwidth below −10 dB was as wide as 4.2 GHz at a thickness of 2.5 mm. The excellent performance of the novel composites could be ascribed to the strong magnetic loss and favorable impedance matching, and such microwave absorbers with strong absorption and a wide frequency band have shown great application potential in military and commercial fields.