Preparation of RGO/Fe₃O₄ Nanocomposites as a Microwave Absorbing Material

Abstract

The hydrophobic nanocomposites of reduced graphene oxide (RGO) and Fe₃O₄ (RGO/Fe₃O₄) were prepared by a one-pot process through co-precipitation under alkaline conditions. The microwave absorption performance of the RGO/Fe₃O₄ nanocomposites was analyzed according to their electromagnetic parameters. The results showed that the RGO/Fe₃O₄ nanocomposites displayed better absorbing performance than the pristine Fe₃O₄ nanoparticles, owing to the synergistic effect of Fe₃O₄ and RGO. The maximum reflection loss (RL) of the RGO/Fe₃O₄ nanocomposites with a thickness of 2 mm reached −45.7 dB at 13.3 GHz, and the bandwidth (RL < −10 dB) ranged from 11.5 to 16.5 GHz. However, the maximum RL of the Fe₃O₄ nanoparticles with a thickness of 5 mm only reached −5.3 dB at 5.7 GHz. The RGO/Fe₃O₄ nanocomposites have a great potential application in high-performance electromagnetic microwave absorbing.

1. Introduction

As a carrier of information, the microwave (in a frequency range of 0.3–300 GHz) is increasingly used in communications, medical care, navigation, remote control, industry and other fields. At the same time, it has produced many negative effects in the fields of communication, information technology, radar detection and human health [1]. Therefore, developing high-performance microwave-absorbing materials is of extraordinary significance and has aroused the increasing interest of many researchers. At present, the research on absorbing materials mainly includes the following aspects: ferrites, nano-sized absorbing materials, ultrafine metal powders, conductive polymers, dielectric ceramic materials, smart stealth materials, fibrous absorbing materials, etc. [2]. However, single-absorbing materials have some intrinsic disadvantages, and the electromagnetic parameters are difficult to optimize, which limits their application range [3]. At present, new electromagnetic absorbing materials are being developed for composite materials [4].

Nanocrystalline magnetic absorbing materials have a large magnetic loss and excellent microwave absorption properties, owing to the characteristics of exchange coupling, small-dimension effect, surface effect and large lattice distortion. Fe₃O₄ nanoparticles are of an inverse spinel-type ferromagnetic material and have been mostly studied among many well-known magnetic nanoparticles because of their low toxicity, environmental friendliness and superparamagnetism [5]. Fe₃O₄ is a double complex dielectric material and possesses both magnetic and dielectric losses for electromagnetic waves in the microwave wave band, which is widely used in a good deal of electrical equipment and microwave equipment. However, Fe₃O₄ has some disadvantages, such as a small dielectric constant, poor high-temperature characteristics and high density, resulting in poor absorbing performance as an absorbing material. The practical applications of sole Fe₃O₄ in microwave absorption are thereby restricted.

To solve this problem, combining magnetic nanoparticles with functionalized carbon materials has been preferentially researched to obtain an ideal material with excellent absorbing performance. For example, Cheng et al. prepared the core-shell hybrid of Fe₃O₄ nanocrystals/mesoporous carbon hollow spheres (MCHS) [6]. The maximum RL of the hybrid composite reaches −60.2 dB at 15.5 GHz and has an effective bandwidth (RL < −10 dB) of 5.7 GHz at 2.3 mm of thickness [6]. Zhang et al. prepared Fe₃O₄/graphene nanocomposites with a flower-like structure by a solvothermal method. The maximum RL of the nanocomposites could reach −53.2 dB, and its effective absorption bandwidth was 7.9 GHz (8.1–16 GHz) with a thickness of 2.5 mm [1].

Reduced graphene oxide (RGO) is a typical two-dimensional (2D) carbon material, which is characterized by its high specific surface area, lightweight, high conductivity, considerable defects, and residual oxidizing groups [7,8]. These properties are advantageous to the attenuation and absorption of electromagnetic waves, which makes RGO a potential electromagnetic absorber that has attracted wide attention [9].

In this work, hydrophobic RGO/Fe₃O₄ nanocomposites were synthesized by co-precipitation in the GO suspension using oleic acid as the surfactant to prepare the hydrophobic nanocomposites with excellent absorbing performance. The planar structure of GO provides an ideal platform for the growth of Fe₃O₄ nanoparticles, which can alleviate the agglomeration of Fe₃O₄ magnetic nanoparticles. At the same time, Fe₃O₄ nanoparticles on the surface of GO could inhibit the re-stacking of GO nanosheets in the reduction process. Furthermore, oleic acid as a surfactant provided steric stabilization and prevented the agglomeration of RGO/Fe₃O₄ nanocomposites [10]. One end of the oleic acid molecule is the hydrophilic group, which is bonded to the surface of the RGO/Fe₃O₄ nanocomposites, and the other end is the lipophilic group, which is exposed on the surface of the particle [11,12], resulting in the hydrophobic of the nanocomposites. The prepared hydrophobic RGO/Fe₃O₄ nanocomposites possess a broader absorbing band and show stronger electromagnetic absorption due to the synergy effect between Fe₃O₄ and GO.

2. Results and Discussion

2.1. Morphology and Chemical Composition Analysis

TEM was used to observe the size and morphology of the particles. Comparing the TEM images of Fe₃O₄ nanoparticles and RGO/Fe₃O₄ nanocomposites, the size growth of the Fe₃O₄ nanocrystals on RGO sheets can be clearly observed (Figure 1). Fe₃O₄ is uniformly deposited on the RGO nanosheets, with almost no agglomeration and irregular shapes. The average particle size is 20~30 nm. The bare Fe₃O₄ nanoparticles aggregate more seriously, and the particle size is even larger. This indicates that RGO is a good substrate for the nucleation and growth of Fe₃O₄ nanoparticles. Moreover, RGO is transparent, and its edges with folds can be seen, as shown in Figure 1b, indicating that the graphene sheet is super thin, and the specific surface area of RGO is increased to some extent, which is beneficial not only to increase the load of the magnetic nanoparticles but also to alleviate the agglomeration of nanoparticles due to the nano-sized effects, and thus improve the absorbing properties of nanocomposites [13].

Figure 2 shows the X-ray diffraction peaks of Fe₃O₄ nanoparticles and RGO/Fe₃O₄ nanocomposites, confirmed by the standard JCPDS card (PDF # 79-0418). It clearly shows that the Fe₃O₄ nanoparticles are spinel structures. The significant peaks at 2θ values of 21.4°, 35.3°, 41.4°, 50.8°, 63.1°, 67.5° and 74.5° were assigned to the (111), (220), (311), (400), (422), (333) and (440) planes [14]. No additional peak was observed in the XRD pattern, suggesting that no other phases except Fe₃O₄ are present. The RGO/Fe₃O₄ nanocomposites have the same diffraction peak as Fe₃O₄ nanoparticles. Another broad peak, around 2θ = 25°, can be observed with a black line, corresponding to the (002) plane, and the (001) peak of GO disappears, which indicates that the GO has been transformed into RGO during the co-precipitation process [15]. The results suggest that RGO/Fe₃O₄ nanocomposites have been successfully prepared after the reaction. The crystalline sizes of Fe₃O₄ and RGO/Fe₃O₄ were estimated for the (311) peak using the Scherrer equation [16],

$$D=\frac{0.89\lambda }{B\cos \theta }$$

(1)

where D refers to the size of the nanoparticles, λ is the wavelength of the X-ray, B is the FWHM of the characterization peak, and θ is the Bragg diffraction angle. The calculated particle sizes for Fe₃O₄ and RGO/Fe₃O₄ were 17.8 nm and 16.4 nm, respectively. When deposited on the RGO nanosheets, the size of the Fe₃O₄ particle decreases, which is consistent with the results of TEM. XRD analysis proved the formation of Fe₃O₄ particles in the nanometer range.

In order to further determine the phase structure, the sample was analyzed by infrared spectroscopy. The FT-IR spectra of Fe₃O₄ nanoparticles and RGO/Fe₃O₄ nanocomposites are shown in Figure 3. Five bands at 570, 1630, 2922, 2852 and 3430 cm⁻¹ appear in the FT-IR spectrum of Fe₃O₄ nanoparticles. The absorbance bands at 2852 and 2922 cm⁻¹ are ascribed to the vCH₂ symmetric and asymmetric stretching vibrations of oleic acid, respectively [5,17]. The peak at 570 cm⁻¹ could be assigned to the Fe-O-Fe stretching vibrations [18]. The presence of the peaks at 1630 and 3430 cm⁻¹ indicates that the surfaces of the nanoparticles adsorb a large number of hydroxyl groups in an aqueous environment [19]. Compared with the Fe₃O₄ nanoparticles, a new peak at 1735 cm⁻¹ appears in the FT-IR spectrum of RGO/Fe₃O₄ nanocomposites, which belongs to the C=O group of RGO [20]. Furthermore, the strong band at 3430 cm⁻¹ is associated with the stretching vibration of the O-H bond in the RGO/Fe₃O₄ [21]. The absorption peaks at 800 and 1350 cm⁻¹ are due to the C-O-H bending vibration and C-H vibration, respectively [22]. It indicates that RGO, Fe₃O₄ and oleic acid are successfully connected by chemical bonds.

The XPS was used to analyze the elements and their valence states in the RGO/Fe₃O₄ nanocomposites. Figure 4a clearly shows three different peaks at 284.3 eV, 530 eV and 711.9 eV of the RGO/Fe₃O₄ composites, corresponding to C 1s, O 1s and Fe 2p, respectively. The results displayed that the RGO/Fe₃O₄ nanocomposites contain three elements of C, O and Fe. In addition, the detailed element distribution can be obtained in the whole spectrum. The atom contents of C, O, and Fe of the RGO/Fe₃O₄ nanocomposites are 26.8%, 46.8%, and 26.4%, respectively. In the high-resolution spectrum of Fe 2p (Figure 4b), the peaks at 710.6 and 724.0 eV could be assigned to the band energies of Fe 2p_3/2 and Fe 2p_1/2, respectively [23]. Furthermore, the charge transfer satellite of Fe (2p_3/2) cannot be found at around 720 eV, further confirming the successful preparation of Fe₃O₄ nanoparticles [24]. In the high-resolution spectrum of O 1s (Figure 4c), the peaks due to Fe-O, O-C/OH and C=O are observed at the binding energies of 529.7, 530.91 and 531.71 eV, respectively [25]. All the above analyses confirm that the RGO/Fe₃O₄ nanocomposites have been successfully prepared.

The Raman spectra of the GO and RGO/Fe₃O₄ nanocomposites are shown in Figure 5. As is known, the D-band is associated with the breathing mode of A_1g, representing disorder or defects in GO. The G-band is usually assigned to the E_2g mode and caused by in-plane vibration, and the 2D band represents the second-order scattering of E_2g phonons [26]. The intensity ratio of the D- to G-band (I_D/I_G) indicates the degree of defects in the carbonaceous material [27]. The Raman peaks at 1583 cm⁻¹ and 1347 cm⁻¹ correspond to the G-band and D-band, respectively. The ratio (I_D/I_G) of GO, 0.91, is lower than the ratio (I_D/I_G) of the RGO/Fe₃O₄ nanocomposite, at 0.98. A variety of internal reflections are produced, mainly due to the lattice defects and residual functional groups on the surface of the RGO/Fe₃O₄ nanocomposites. These reflections contribute to enhancing the microwave absorption capability of the nanocomposites. Furthermore, in the RGO/Fe₃O₄ spectrum, the 2D band appearing at 2670 cm⁻¹ suggests that GO has been reduced, and the peak of 680 cm⁻¹ indicates the presence of Fe₃O₄ [14], as confirmed by the XRD pattern aforementioned.

2.2. Magnetic Properties

Figure 6 shows the magnetic hysteresis loops of different samples at room temperature. Both samples exhibit typical magnetic hysteresis behaviors. The RGO/Fe₃O₄ nanocomposites possess the superparamagnetic properties as well as the Fe₃O₄ nanoparticles, indicating that the nanocrystals own the ultra-small size. The saturation magnetization (Ms) values of Fe₃O₄ and RGO/Fe₃O₄ are 75.1 emu/g and 61.5 emu/g, respectively. Compared with the Fe₃O₄ nanoparticle, it is found that the Ms value of RGO/Fe₃O₄ is smaller, which is attributed to the magnetic dilution effect caused by the non-magnetic material of RGO. The coercivity of the Fe₃O₄ nanoparticles and RGO/Fe₃O₄ nanocomposites is 4.5 Oe and 4.0 Oe, respectively. The results reveal that the coercivity of the composites is little affected by a proper amount of RGO, given that the morphology of Fe₃O₄ is similar in the two samples.

2.3. Hydrophobic Properties

Generally, the absorbing performance of an absorber in a humid environment will be weakened; therefore, hydrophobic properties are essential for most EM wave absorbers in practical applications [28]. Wettability is an important property of a material’s surface that is related to the chemical composition and microscopic morphology of the surface [29]. The RGO/Fe₃O₄ nanocomposites were tableted, and the water static contact angle of the surface was measured. It can be seen from Figure 7 that the water static contact angles of the RGO/Fe₃O₄ nanocomposites are 147.2°, 147.9° and 146.5° for three respective measurements, and the average value is 147.2°. This confirms the excellent hydrophobicity of the RGO/Fe₃O₄ nanocomposites.

2.4. Analyses of Electromagnetic Parameters and Absorbing Properties

Materials with excellent electromagnetic wave absorption have two important characteristics: impedance matching and attenuation. Only when the wave impedance of the material matches the wave impedance of free space can the incident electromagnetic wave enter the material as much as possible. Here, the attenuated abilities of the composites depend on their magnetic and dielectric losses. The corresponding microwave absorption mechanism can be described as follows [6].

RL is the main evaluation index of the ability of the electromagnetic absorption of absorbers. Based on the transmission line law, the RL value can be calculated by using the following formulae [30,31]:

$$RL=20 \log \left|\frac{Z-Z_0}{Z+Z_0}\right|$$

(2)

$$Z=\sqrt{\frac{{\mu }_r{\mu }_0}{{\epsilon }_r{\epsilon }_0}}\tanh \left[j\left(2\pi fd/c\right)\sqrt{{\mu }_r{\epsilon }_r}\right]$$

(3)

where Z₀ and Z refer to the impedance of free space and the absorber, respectively; ε_r and μ_r represent the relative permittivity and permeability, respectively; f refers to the frequency of the electromagnetic wave; d means the absorber thickness; and c is the light velocity.

Figure 8a,b exhibits the simulation RL of the Fe₃O₄ nanoparticles and RGO/Fe₃O₄ nanocomposites in the range of 2~18 GHz, respectively. Compared with Fe₃O₄ nanoparticles, the RL values of the RGO/Fe₃O₄ nanocomposites are significantly increased. The maximum RL can reach −45.7 dB at a thickness of 2 mm, and the effective frequency bandwidth is 5 GHz (11.5~16.5 GHz), which is due to interface polarization [32]. The absorbing properties of Fe₃O₄ nanoparticles are very poor over the entire frequency region. All the RL values of Fe₃O₄ nanoparticles do not reach −10 dB with different thicknesses, and the maximum RL value is only −5.3 dB. When adding a tiny amount of GO, the magnetic loss, as well as the dielectric loss, are influenced to a great extent, which greatly improves the absorbing properties of the RGO/Fe₃O₄ nanocomposites. It can also be observed that the maximum RL value decreases substantially with the increasing layer thickness, the peak position continuously moves to the low frequency, and the corresponding optimal matching thickness appears, which reveals that the microwave absorbing performance of the RGO/Fe₃O₄ nanocomposites at varied frequencies can be adjusted by controlling the thickness. The phenomenon can be explained by the principle of quarter wavelength theory [33].

Figure 8c,d exhibits the three-dimensional image of the RL values and corresponding two-dimensional projection map of the RGO/Fe₃O₄ nanocomposites. The RL value of −10 dB is the critical point for measuring the electromagnetic wave absorption capability of the material. When the RL value is lower than −10 dB, the absorbing material can reflect less than 10% of electromagnetic waves. Therefore, the effective absorption bandwidth (RL < −10 dB) should be as wide as possible in more frequency ranges [34]. The effective absorption band increased from 2 GHz to 18 GHz with different thicknesses, from 2.4 to 4.7 mm. The result reveals that the RGO/Fe₃O₄ nanocomposite has remarkable electromagnetic wave absorption ability, which can achieve more than 90% of the absorption effect, and can achieve electromagnetic wave absorption in different absorption bands through adjusting the coating thickness. Compared with the other similar electromagnetic wave absorbers, as shown in

Table 1. The electromagnetic wave absorption or shielding property of relative absorbers.

Samples	RLmin or Shielding Effectiveness(dB)	Bandwidth (RL < −10 dB, GHz)	Thickness (mm)	Ref.
FeNR@rGO	−23.1	3.9	4.0	[2]
Fe3O4/rGO	22.7	3.1	3.0	[35]
Cu9S5/Fe3O4	−39.5	2.2	4.3	[36]
Fe3O4@PEDOT/rGO	−48.8	4.3	2.9	[37]
Fe3O4/NRC	−29.6	4.32	1.5	[38]
MnO2/GNRs	−57	5.6	3.0	[39]
Conducting ferrofluid composite	41	4	-	[40]
Iron oxide-RGO	33.0	−5.6	-	[41]
RGO/Fe3O4	−45.7	5	2.0	this work

, obtained RGO/Fe₃O₄ nanocomposite shows competitive electromagnetic wave absorption properties, both in absorption efficiency and the effective absorption band.

The electromagnetic parameters of Fe₃O₄ nanoparticles and RGO/Fe₃O₄ nanocomposites were obtained during 2~18 GHz. The real part of relative complex permittivity (ε′) and complex permeability (μ′) refers to the storage capacity of the electrical and magnetic energy of the material, respectively. Meanwhile, the imaginary part of complex permittivity (ε″) and complex permeability (μ″) stands for the inner dissipative ability in the absorbers [42]. The data curves are shown in Figure 9. Both the values of ε′ and ε″ of the Fe₃O₄ nanoparticles are very low due to the relatively poor conductivity of Fe₃O₄. The ε′ values remain as a constant, at around 4.4 over the whole frequency range, while ε″ increases slowly from 0.1 to 0.3 during 2~18 GHz, revealing a poor ability of dielectric loss. The dielectric loss increased significantly with the GO addition. The ε′ of the RGO/Fe₃O₄ nanocomposites dropped from 13 to 7, and the ε″ fluctuated around 3.6 with the measured frequency. According to the theory of free electrons, ε″ ≈ 1/2πε₀ρf, RGO has high conductivity and forms a huge conductive network, which reduces the resistivity (ρ) of the nanocomposite. Therefore, by adding RGO, the dielectric constant of the composite is improved [43]. As shown by the XRD and Raman results, the reduction of graphene oxide leads to an increase in conductivity. Moreover, interfacial polarization is also an effect that should be considered. Due to the different dielectric properties, a large number of electrons accumulated at the heterointerface between Fe₃O₄ and GO. Under an external electric field, these free electrons will follow the variation and hop across the interface repeatedly, resulting in strong interfacial polarization relaxation, which converts the electromagnetic energy of the microwave into heat energy [44,45].

Figure 9c shows the real part (μ′) of the relative complex permeability of Fe₃O₄ nanoparticles and RGO/Fe₃O₄ nanocomposites with frequency. The values of μ′ for both samples are found to show a similar tendency for variation with frequency, and μ′ decreases gradually with an increase in frequency over 2~7 GHz and increases slowly around 7~18 GHz. As shown in Figure 9d, the imaginary part (μ″) of the relative complex permeability of the RGO/Fe₃O₄ nanocomposites gradually decreases at 2~9 GHz and almost keeps constant over 9~18 GHz as well as Fe₃O₄ nanoparticles. Furthermore, the μ″ values of the RGO/Fe₃O₄ nanocomposites are smaller than those of Fe₃O₄ nanoparticles with a measured frequency.

To further study the electromagnetic parameters of the RGO/Fe₃O₄ nanocomposites, the values of tan δ_ε and tan δ_μ of the Fe₃O₄ nanoparticles and RGO/Fe₃O₄ nanocomposites are measured, as shown in Figure 10.

The tangent angle of electric loss is tan δ_ε = ε″/ε′. The tangent angle of magnetic loss is tan δ_μ = μ″/μ′ [46,47]. The electrical loss tan δ_ε of Fe₃O₄ nanoparticles increased from 0.01 to 0.06, while the electrical loss tan δ_ε of the RGO/Fe₃O₄ nanocomposite increased from 0.3 to 0.49 over 2~18 GHz, indicating Fe₃O₄ has a negligible dielectric loss for microwave absorption (Figure 10a). As the resistance loss of the composite increased, the larger the macro current caused by the carrier (the eddy current caused by the electric field and the change in the magnetic field caused by the electric field), which is conducive to the conversion of electromagnetic energy into heat energy [48].

Figure 10b shows the sample’s tan δ_μ variation during 2~18 GHz. Obviously, the tan δ_μ values of the two samples are similar in the test frequency band. The tan δ_μ values decrease significantly at a low frequency (2~9 GHz) and remain constant at a high frequency (9~18 GHz), showing that the magnetic loss occurs mainly in the low-frequency range. The tan δ_μ values of the RGO/Fe₃O₄ nanocomposites are smaller than those of Fe₃O₄ nanoparticles during 2–18 GHz.

For electromagnetic wave absorption, a magnetic loss is another important mechanism. Generally, the magnetic loss is mainly dominated by eddy current loss, hysteresis loss and domain wall resonance [49]. If the magnetic loss is only due to the eddy current loss, the eddy current coefficient is represented by C₀ [50,51]:

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

(4)

Eddy current loss refers to the eddy current in the ferromagnetic body that generates heat in the magnetic core, and the electromagnetic energy is converted into heat energy. Whether or not it exhibits eddy current loss can usually be judged by analyzing whether the C₀ value is constant in the corresponding frequency range [52]. Figure 10c exhibits the curve of C₀ of the RGO/Fe₃O₄ nanocomposites. The values decrease significantly with increasing frequency until 10 GHz, and the values are nearly constant, around 10–18 GHz. This implies that the magnetic loss at a low frequency is mainly due to the resonance effect, but eddy current loss at a high frequency.

To further probe the dielectric loss mechanism of the RGO/Fe₃O₄ nanocomposite, the Debye equation is used to explain the polarization relaxation [53]. The relationship between ε′ and ε″ can be deduced from the Debye theory as follows [54]:

$$\left[{\epsilon }^{\prime }-\frac{1}{2}{\left({\epsilon }_s+{\epsilon }_{\infty }\right)}^2\right]+{\left({\epsilon }^{″}\right)}^2=\frac{1}{4}{\left({\epsilon }_s+{\epsilon }_{\infty }\right)}^2$$

(5)

where ε_s and ε_∞ denote the static dielectric constant and the relative dielectric constant, respectively.

The plot of ε″ versus ε′ would be a Cole–Cole semicircle, and each semicircle corresponds to a Debye relaxation process [55]. The ε″−ε′ curves of RGO/Fe₃O₄ nanocomposites are shown in Figure 10d. It is clear that three superimposed Cole–Cole semicircles appear from 7.5 to 13 GHz, revealing that the RGO/Fe₃O₄ nanocomposite has multiple Debye relaxation processes. The relaxation process may be due to some defects in RGO, interfacial polarization among Fe₃O₄ nanoparticles and RGO, and the polarization of residual functional groups [56]. Moreover, the Cole–Cole semicircle is not a regular semicircle, indicating that besides the Debye relaxation, there are multiple loss mechanisms, Maxwell–Wagner relaxation, electron polarization and interfacial polarization [57].

An effective absorbing agent depends on electromagnetic impedance matching and attenuation capability. Proper impedance matching means that more incident waves can enter the absorbing agent, while stronger attenuation can dissipate the incident waves. The impedance matching (Z) and attenuation constant (α) of electromagnetic waves in the material are described as follows [58].

$$Z=\left|Z_{in}/Z_0\right|=\left|{\left({\mu }_r/{\epsilon }_r\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\tanh \left[j\left(2\pi fd\left({\mu }_r{\epsilon }_r\right){}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}/c\right)\right]\right|$$

(6)

$$\alpha =\frac{\sqrt{2}\pi f}{c}\times \sqrt{\left({\mu }^{″}{\epsilon }^{″}-{\mu }^{\prime }{\epsilon }^{\prime }\right)+\sqrt{{\left({\mu }^{″}{\epsilon }^{″}-{\mu }^{\prime }{\epsilon }^{\prime }\right)}^2+{\left({\epsilon }^{\prime }{\mu }^{″}+{\epsilon }^{″}{\mu }^{\prime }\right)}^2}}$$

(7)

Figure 11 shows the calculated values of the Z and α with measured frequency. The α value of RGO/Fe₃O₄ nanocomposites is higher than that of Fe₃O₄ nanocrystals, around 2~18 GHz. This is due to the introduction of RGO, whereby increasing dielectric loss of the composite improves impedance matching and results in easier entry of the incident microwave into the composite. However, the Z value of RGO/Fe₃O₄ nanocomposites is smaller compared to Fe₃O₄ nanocrystals. This shows that only proper impedance matching and an attenuation coefficient can help enhance the microwave absorption of nanocomposites.

3. Materials and Methods

3.1. Materials

Ferrous sulfate heptahydrate (FeSO₄·7H₂O), Iron(III) chloride hexahydrate (FeCl₃·6H₂O) and anhydrous ethanol were received from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The oleic acid (C₁₈H₃₄O₂) was provided by Aladdin. Ammonium hydroxide (NH₄OH, 25 wt%) was purchased from Tianjin Hengxing Chemical Reagent Co., Ltd., Tianjin, China. All reagents in the experiments were of analytical grade and used without further purification. The deionized water was prepared in the laboratory.

3.2. Preparation of Hydrophobic RGO/Fe₃O₄ Nanocomposites

GO was prepared based on the reported literature [59]. The hydrophobic RGO/Fe₃O₄ was prepared by chemical co-precipitation. An amount of 0.15 g of GO was dispersed in 120 mL of deionized water by ultrasonication for 30 min. An amount of 5.1 g of FeCl₃·6H₂O and 3.0 g of FeSO₄·7H₂O were then added into the GO suspension under N₂ protection. An amount of 25 mL of NH₃·H₂O was diluted to 50 mL with deionized water, and then the ammonia mixture was added slowly into the above solution. Furthermore, 0.7 mL of oleic acid was also added to the solution while being mechanically stirred for 5 min. After vigorously stirring mechanically at 80 °C for 1 h, the black precipitate was deposited at the bottom of the three-necked flask. After cooling to room temperature, the hydrophobic RGO/Fe₃O₄ nanocomposites were collected by a magnet, washed several times with deionized water and anhydrous ethanol, and then dried in a vacuum oven at 60 °C for 24 h. The content of Fe₃O₄ in the RGO/Fe₃O₄ nanocomposite is about 95.8 wt%.

3.3. Characterization

The morphology was observed with transmission electron microscopy (TEM, Tecnai G220, FEI) and the crystal structure was measured by X-ray diffraction (XRD, PANalytical Corporation, Almelo, The Netherlands) using a co-target (λ = 1.5418 Å) in the range (2θ) of 10~80°. Fourier transform infrared spectroscopy (FT-IR) was recorded by a Thermo Nicolet 670 spectrometer in the range of 400~4000 cm⁻¹. The surface chemical composition of the nanocomposites was identified by ESCALAB 250 (Thermo Fisher Scientific, Waltham, MA, USA) X-ray photoelectron spectroscopy (XPS). Raman spectra were taken by an Invia Reflex Raman microscope with a 532 nm laser excitation from 400 to 2400 cm⁻¹. The magnetic properties were measured by a vibrating sample magnetometer (VSM-PPMS-9, Quantum Design, San Diego, CA, USA) at room temperature. The sample was tableted, and the water contact angle was examined with a contact angle measuring machine (JY-82B) at room temperature. The average value of the contact angle was achieved by 3 parallel measurements. Electromagnetic parameters, including the relative complex permittivity and permeability of the samples, were obtained by a vector network analyzer (VNA, PNA-A5244A) in the range of 2~18 GHz. The tested samples were fabricated by mixing RGO/Fe₃O₄ with paraffin wax (weight ratio = 1:1) to form uniform composites. The mixtures were then compressed into a ring shape (outer diameter: 7 mm, inner diameter: 3.04 mm) with a thickness of 2~5 mm.

4. Conclusions

In this work, the hydrophobic RGO/Fe₃O₄ nanocomposites have been successfully synthesized by in situ co-precipitation. Nanoparticles were chemically decorated with graphene oxide sheets to obtain superparamagnetic composite materials. The RGO/Fe₃O₄ nanocomposites have strong synergy and complementarity, which can enhance microwave absorption. The reflectance is significantly larger than that of pure Fe₃O₄ nanocrystals. The maximum RL of RGO/Fe₃O₄ composites with a thickness of 2 mm reached −45.7 dB at 13.3 GHz, and the effective bandwidth (RL < 10 dB) reached 5 GHz (11.5~16.5 GHz). In contrast, the maximum RL of the Fe₃O₄ nanoparticles only reached −5.3 dB at 5.7 GHz with 5 mm. Moreover, the hydrophobic RGO/Fe₃O₄ nanocomposites are more stable and versatile. The results of this work guide the further development of new composites with high microwave absorption performance and hydrophobic properties.