One-Pot Hydrothermal Preparation of Fe3O4 Decorated Graphene for Microwave Absorption

Fe3O4 decorated graphene was synthesized for electromagnetic wave absorption via a facile one-pot hydrothermal approach. The structure and morphology of the as-prepared nanomaterials were systematically investigated. The graphene oxide (GO) was reduced and Fe3O4 nanoparticles were evenly decorated on the surface of reduced graphene oxide (rGO) nanosheets. The average particle size of Fe3O4 nanoparticles is about 15.3 nm. The as-prepared rGO-Fe3O4 nanocomposites exhibited a good microwave absorption performance because of the combination of graphene and magnetic Fe3O4. When the thicknesses are 1.6 mm and 6.5 mm, the reflection loss (RL) values are up to −34.4 dB and −37.5 dB, respectively. The effective bandwidths are 3.8 and 1.9 GHz.


Introduction
With the extensive usage of digital devices, the electromagnetic wave (EMW) is becoming a new environmental pollutant and threat. It not only seriously disrupts the operation of electronic equipment and satellite communication but also jeopardizes human health [1,2]. EMW absorbing materials are considered to be the most effective strategy to eliminate EMW pollution because they can absorb EMW and convert the electromagnetic energy into heat loss. Therefore, there is an urgent demand for developing high-performance EMW absorbing materials to counteract the adverse effects [3].
The ideal EMW absorbing materials should possess strong absorption ability, broadband, and low density [4]. To serve these purposes, various materials have been investigated, such as magnetic metal powders [5], ferrite [6], and titanates [7]. However, these materials cannot meet the increasing requirements of the EMW absorbing materials due to drawbacks like high density and being easily corroded or oxidized in harsh environments. To overcome these problems, carbon-based materials including graphite [8,9] and carbon nanotubes [10,11] have been developed to improve the EMW absorption because of low density, good thermostability, and corrosion resistance. As a representative of novel carbon-based nanomaterials, graphene is considered as an excellent EMW absorbing material due to its prominent carrier mobility, high surface area, ultra-light weight, and high thermal and

Characterization
The products were characterized through X-ray diffraction (XRD, Rigaku D/Max 2400 diffractometer, Tokyo, Japan) with Cu Kα source at a scanning speed of 5 • /min from 10 to 70 • . GO, rGO, Fe 3 O 4 , and rGO-Fe 3 O 4 were analyzed with a Fourier transform infrared spectrometer (FTIR, Thermo Fisher Scientific Nicolet iS 50, Waltham, MA, USA). Raman spectra (Thermo Scientific DXRxi, Waltham, MA, USA) were recorded by a Raman system with an excitation wavelength of 514.5 nm. Thermogravimetric analysis (TGA, PerkinElmer TGA 8000, Waltham, MA, USA) was carried out using a thermal analyzer at a heating rate of 10 • C/min under nitrogen atmosphere. The distribution of Fe 3 O 4 on the surface of rGO was observed by a transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan) at an accelerating voltage of 200 kV. The electromagnetic parameters were collected using a vector network analyzer (Agilent HP8753D, Santa Clara, CA, USA). All the samples (35% nanocomposites and 65% wax) were pressed to toroidal-shaped rings (D inner : 3 mm, D outer : 7 mm).

Results and Discussion
The crystalline structures of the obtained nanocomposites are investigated ( Figure 1). GO shows a typical diffraction peak at 11.7 • , which can be attributed to the introduction of oxygenic functional groups on the honeycomb carbon skeleton of graphene. After hydrothermal treatment for 12 h, a new peak appears at 23.3 • and the peak at 11.7 • disappears in the spectrum of rGO, implying the reduction of GO to graphene. For Fe 3

Results and Discussion
The crystalline structures of the obtained nanocomposites are investigated ( Figure 1). GO shows a typical diffraction peak at 11.7°, which can be attributed to the introduction of oxygenic functional groups on the honeycomb carbon skeleton of graphene. After hydrothermal treatment for 12 h, a new peak appears at 23.3° and the peak at 11.7° disappears in the spectrum of rGO, implying the reduction of GO to graphene. For Fe3O4, seven peaks located at 18.3°, 30.1°, 35.5°, 43.1°, 53.5°, 57.0° and 62.6° can be assigned to the reflections of (111), (220), (311), (400), (422), (511) and (440) planes of Fe3O4 (JCPDS, Card No. 75-0033) with the face-centered cubic structure [21]. The spectrum of rGO-Fe3O4 exhibits similar patterns with that of Fe3O4 except for a slightly broad peak at nearly 23.3°, because of the combination of Fe3O4 nanoparticles and rGO. The chemical structure of GO, rGO, Fe3O4, and rGO-Fe3O4 were studied through FTIR spectra ( Figure 2). The characteristic absorption bands located at 1717, 1033, 1578, 1398, and 3398 cm −1 are associated with C=O, C-O in the epoxide group, C=C in the aromatic ring, and the deformation and stretching vibration of -OH groups, respectively. The spectrum of rGO is consistent with that of GO except for the intensity of these peaks decrease greatly. It is implied that GO is reduced during the solvothermal process. Two bands appear at 2918 and 2867 cm −1 , corresponding to -CH2 and -CH3 groups in the residual polyethylene glycol. For Fe3O4, a characteristic absorption band appears at 535 cm −1 , corresponding to Fe-O vibrations of Fe3O4. rGO-Fe3O4 also exhibits the same curve as Fe3O4 and rGO, indicating Fe3O4 nanoparticles were effectively decorated on rGO nanosheets. The chemical structure of GO, rGO, Fe 3 O 4 , and rGO-Fe 3 O 4 were studied through FTIR spectra ( Figure 2). The characteristic absorption bands located at 1717, 1033, 1578, 1398, and 3398 cm −1 are associated with C=O, C-O in the epoxide group, C=C in the aromatic ring, and the deformation and stretching vibration of -OH groups, respectively. The spectrum of rGO is consistent with that of GO except for the intensity of these peaks decrease greatly. It is implied that GO is reduced during the solvothermal process. Two bands appear at 2918 and 2867 cm −1 , corresponding to -CH 2 and -CH 3 groups in the residual polyethylene glycol. For Raman is used to further investigate the structural changes of rGO-Fe3O4 composites. The results are shown in Figure 3. Except for Fe3O4, the other nanocomposites present the typical reflections of carbon at 1349 cm −1 (D band, the disordered structures of GO) and 1589 cm −1 (G band, the in-plane vibrations of sp 2 bonded carbon atoms). The ID/IG is used to measure the quantity of defects. The ID/IG ratio of rGO (0.95) is slightly lower than that of GO (0.98), indicating the reduction of GO. The existence of Fe3O4 nanoparticles may disorder the structure of graphene, so the ID/IG ratio of rGO-Fe3O4 increases up to 1.11. The typical peaks of Fe3O4 centered around 215, 276, 388, 481, and 585 cm −1 appear in Fe3O4 and rGO-Fe3O4 spectra, which is assigned to the Ag1(1), Eg2 + Eg3, Eg4, A1g(2), and Eg5 modes, respectively [22].  spectra, which is assigned to the A g1 (1), E g2 + E g3 , E g4 , A 1g (2), and E g5 modes, respectively [22]. Raman is used to further investigate the structural changes of rGO-Fe3O4 composites. The results are shown in Figure 3. Except for Fe3O4, the other nanocomposites present the typical reflections of carbon at 1349 cm −1 (D band, the disordered structures of GO) and 1589 cm −1 (G band, the in-plane vibrations of sp 2 bonded carbon atoms). The ID/IG is used to measure the quantity of defects. The ID/IG ratio of rGO (0.95) is slightly lower than that of GO (0.98), indicating the reduction of GO. The existence of Fe3O4 nanoparticles may disorder the structure of graphene, so the ID/IG ratio of rGO-Fe3O4 increases up to 1.11. The typical peaks of Fe3O4 centered around 215, 276, 388, 481, and 585 cm −1 appear in Fe3O4 and rGO-Fe3O4 spectra, which is assigned to the Ag1(1), Eg2 + Eg3, Eg4, A1g(2), and Eg5 modes, respectively [22].  The thermal stability of these samples are also investigated (Figure 4). The weight losses of GO are~10 wt.% (at around 100 • C) and~40 wt.% (at around 300 • C), indicating the removal of H 2 O and the pyrolysis of the oxygenic functional groups, respectively. The total weight loss of rGO is nearly 30 wt.% at around 400 • C, resulting from the decomposition of residual oxygenic functional groups. The improved thermal stability of rGO demonstrates the reduction of GO. Compared to GO and rGO, Fe 3 O 4 and rGO-Fe 3 O 4 exhibit better thermal stabilities, and the weight losses are 6 wt.% and 9 wt.% at 800 • C, suggesting the mass ratio of Fe 3 O 4 is much larger than that of graphene nanosheets in rGO-Fe 3 O 4 composites. It is concluded that the obtained rGO-Fe 3 O 4 nanocomposites will possess an excellent EMW absorption at low thickness because the magnetic Fe 3 O 4 nanoparticles are preponderant in mass.
Materials 2020, 13, x FOR PEER REVIEW 5 of 12 The thermal stability of these samples are also investigated ( Figure 4). The weight losses of GO are ~10 wt.% (at around 100 °C) and ~40 wt.% (at around 300 °C), indicating the removal of H2O and the pyrolysis of the oxygenic functional groups, respectively. The total weight loss of rGO is nearly 30 wt.% at around 400 °C, resulting from the decomposition of residual oxygenic functional groups. The improved thermal stability of rGO demonstrates the reduction of GO. Compared to GO and rGO, Fe3O4 and rGO-Fe3O4 exhibit better thermal stabilities, and the weight losses are 6 wt.% and 9 wt.% at 800 °C, suggesting the mass ratio of Fe3O4 is much larger than that of graphene nanosheets in rGO-Fe3O4 composites. It is concluded that the obtained rGO-Fe3O4 nanocomposites will possess an excellent EMW absorption at low thickness because the magnetic Fe3O4 nanoparticles are preponderant in mass. Typical transmission electron microscope (TME) of rGO-Fe3O4 are presents in Figure 5. Fe3O4 nanoparticles with spherical structures are dispersed on the rGO nanosheets (Figure 5a). The average diameter of Fe3O4 particles is 11.3 ± 1.8 nm based on the size distribution analysis with 100 arbitrarily selected nanoparticles (Figure 5b). In the hydrothermal process, Fe 3+ ions are anchored by the rGO nanosheets and grow into Fe3O4 nanoparticles. The rGO nanosheets play a confinement function to prevent the Fe3O4 nanoparticles from detaching and aggregating. From the high-resolution transmission electron microscopy (HRTEM) image in Figure 5c, the interplanar spacing (0.253 nm) is assigned to Fe3O4 (311) plane [23]. The selected area electron diffraction (SAED) in the bottom-left inset of Figure 5c implies the face-centered cubic (fcc) structure of polycrystalline Fe3O4 nanoparticles. Therefore, the TEM results further confirm the formation of Fe3O4 nanoparticles on rGO.  As shown in Figure 6, the complex permittivity ε r = ε − jε and complex permeability µ r = µ − jµ of Fe 3  As shown in Figure 6, the complex permittivity ɛr = ɛ′-jɛ″ and complex permeability μr = μ′-jμ″ of Fe3O4 and rGO-Fe3O4 were measured. The values of ɛ′ and ɛ″ for Fe3O4 and rGO-Fe3O4 are shown in Figure 6a,b. For rGO-Fe3O4, the ɛ′ decreased from 14.11 to 11.48, and ɛ″ values increased from 3.13 to 4.67 (in 2-18 GHz). It is obvious that the ɛ′ and ɛ″ for rGO-Fe3O4 are much larger than that of Fe3O4. Higher values of ɛ′ are attributed to the existence of conductive rGO nanosheets. The higher ɛ″ values are ascribed to the multiple dielectric loss behaviors of rGO-Fe3O4. Firstly, the high electric conductivity of rGO-Fe3O4 is facilitated to increase the dielectric loss. Besides, the Fe3O4 nanoparticles loaded on rGO and the residual -OH and -COOH groups of rGO can serve as dipoles to enhance the dipole polarization. Furthermore, the multiple interfaces between rGO and Fe3O4 nanoparticles can enhance the interfacial polarization. The above factors are beneficial for the enhancement of dielectric loss behaviors of rGO-Fe3O4. The Debye dipolar relaxation is regarded as a key reason to explain EMW absorption mechanism of materials [24]. The ɛr can be described as Equation (1) [25]: The Debye dipolar relaxation is regarded as a key reason to explain EMW absorption mechanism of materials [24]. The ε r can be described as Equation (1) [25]: where ε s and ε ∞ are the static and optical permittivity. τ is the relaxation time. Based on the Equation (1), ε and ε" are expressed as Equations (2) and (3): The relation of ε and ε" is expressed as Equations (4) and (5): From Equation (5), the ε versus ε curve is a semicircle. Every semicircle is considered as single Debye relaxation process. The ε -ε plots for Fe 3 O 4 and rGO-Fe 3 O 4 are presented in Figure 6c. The curve of Fe 3 O 4 is irregular, while the curve of rGO-Fe 3 O 4 exhibits four semicircles, and every semicircle is corresponding to a Debye dipolar relaxation. It suggests that the existence of multiple dielectric relaxations in the electromagnetic wave absorption processes. Besides, the Cole-Cole semicircles are distorted, which implys there are other processes (conductive loss, Maxwell-Wagner relaxation, and dipolar polarization) [26].
The sharply decrease with the increasing frequency between 2 GHz and 8 GHz. The peak is mainly ascribed to the natural resonance derived from Fe 3 O 4 nanoparticles [27]. The resonance frequencies of Fe 3 O 4 nanoparticles are usually less than 2 GHz [7]. The obtained nanocomposites with nanometer-scale sizes will possess enhanced anisotropy energies and their resonance frequencies can shifts to higher frequencies due to the small size effect [28].
The µ of Fe 3 O 4 is approximately constant at 8-18 GHz, while that of rGO-Fe 3 O 4 becomes relatively stabilized with some fluctuations and increases gradually from 14 GHz to 18 GHz. Eddy current loss is a key factor for magnetic loss. It is verified as follows Equation (6): where µ 0 is vacuum permeability, σ is conductivity, d is the thickness. If the eddy current loss is the main factor, the C 0 (C 0 = 2πµ 0 σd 2 /3) is independent of frequency. The C 0 curve is shown in Figure 6f. C 0 for Fe 3 O 4 sharply decrease at from 2 GHz to 8 GHz and remain constant at 8-18 GHz, indicating that eddy current loss is the key magnetic loss at X and Ku bands. The C 0 curve of rGO-Fe 3 O 4 is similar to that of Fe 3 O 4 except for the increase from 14 GHz to 18 GHz, implying that there is some other mechanism for rGO-Fe 3 O 4 . It is ascribed to the synergistic effect of rGO and Fe 3 O 4 nanoparticles, which is similar with Shu's work [29].
The tanδ ε and tanδ µ of Fe 3 O 4 and rGO-Fe 3 O 4 are calculated to describe the electromagnetic loss abilities (Figure 7). It is obviously that the plots of tanδ ε and tanδ µ have similar trends with those of ε" and µ". The values of tanδ ε for rGO-Fe 3 O 4 are much higher than that of Fe 3 O 4 especially at the Ku band because of rGO nanosheets. Besides, the tanδ µ value for rGO-Fe 3 O 4 is smaller than tanδ ε , implying the dielectric loss is preponderant in the electromagnetic wave absorption, as reported in other literature [30]. The enhancements of tanδ ε and tanδ µ at high frequencies are favorable for improving EMW absorbing properties in the high frequency regions.
Materials 2020, 13, x FOR PEER REVIEW 9 of 12 other literature [30]. The enhancements of tanδε and tanδμ at high frequencies are favorable for improving EMW absorbing properties in the high frequency regions. The reflection loss (RL) curves of Fe3O4 and rGO-Fe3O4 at varied thickness are presented in Figure 8. The minimum RL of bare Fe3O4 is −7.7 dB at 5.2 GHz with 6.5 mm. The EMW absorption performance of rGO nanosheets is also poor due to the absence of the magnetic component [31]. rGO-Fe3O4 displays the enhanced EMW absorption performance. The minimum RL are −34.4 dB at 1.6 mm and −37.5 dB at 6.5 mm. The corresponding effective bandwidths (≤−10 dB) are 3.8 GHz and 1.9 GHz. The EMW absorption properties of rGO-Fe3O4 composites in high frequency and low frequency are higher than other similar rGO-Fe3O4 materials, which are reported in other works [32,33]. Besides, the maximum absorption band moves towards the low frequency when absorber thickness increases. The relationship between reflection loss (RL) and the frequency are calculated by the Equations (7) and (8)  The reflection loss (RL) curves of Fe 3 O 4 and rGO-Fe 3 O 4 at varied thickness are presented in Figure 8. The minimum RL of bare Fe 3 O 4 is −7.7 dB at 5.2 GHz with 6.5 mm. The EMW absorption performance of rGO nanosheets is also poor due to the absence of the magnetic component [31]. rGO-Fe 3 O 4 displays the enhanced EMW absorption performance. The minimum RL are −34.4 dB at 1.6 mm and −37.5 dB at 6.5 mm. The corresponding effective bandwidths (≤−10 dB) are 3.8 GHz and 1.9 GHz. The EMW absorption properties of rGO-Fe 3 O 4 composites in high frequency and low frequency are higher than other similar rGO-Fe 3 O 4 materials, which are reported in other works [32,33]. Besides, the maximum absorption band moves towards the low frequency when absorber thickness increases.
Materials 2020, 13, x FOR PEER REVIEW 9 of 12 other literature [30]. The enhancements of tanδε and tanδμ at high frequencies are favorable for improving EMW absorbing properties in the high frequency regions. The reflection loss (RL) curves of Fe3O4 and rGO-Fe3O4 at varied thickness are presented in Figure 8. The minimum RL of bare Fe3O4 is −7.7 dB at 5.2 GHz with 6.5 mm. The EMW absorption performance of rGO nanosheets is also poor due to the absence of the magnetic component [31]. rGO-Fe3O4 displays the enhanced EMW absorption performance. The minimum RL are −34.4 dB at 1.6 mm and −37.5 dB at 6.5 mm. The corresponding effective bandwidths (≤−10 dB) are 3.8 GHz and 1.9 GHz. The EMW absorption properties of rGO-Fe3O4 composites in high frequency and low frequency are higher than other similar rGO-Fe3O4 materials, which are reported in other works [32,33]. Besides, the maximum absorption band moves towards the low frequency when absorber thickness increases. The relationship between reflection loss (RL) and the frequency are calculated by the Equations (7) and (8)  The relationship between reflection loss (RL) and the frequency are calculated by the Equations (7) and (8): Z in = Z 0 µ r ε r tanh j 2π f d c √ µ r ε r (8) where c is the speed of light, d is the thickness of the microwave absorbing material, and Z and Z 0 are the input impedance and free space impedance, respectively. Obviously, rGO-Fe 3 O 4 nanocomposites exhibit good EMW absorption abilities, which can be attributed to these factors. Firstly, the defects caused by Fe 3 O 4 nanoparticles and residual hydroxy and carboxyl groups on rGO can act as the dipoles to improve the polarization relaxation. Secondly, the introduction of magnetic Fe 3 O 4 nanoparticles improves the magnetic loss abilities according to natural resonance and eddy current effect mechanism. Finally, the abundant heterointerfaces between graphene and Fe 3 O 4 nanoparticles are conducive to the multiple reflections, scatting, and refraction of the incident electromagnetic wave, which results in improved electromagnetic attenuation.

Conclusions
rGO-Fe 3 O 4 nanocomposites were synthesized by the hydrothermal method. GO was reduced to rGO and Fe 3 O 4 nanoparticles and simultaneously loaded on rGO nanosheets. The obtained rGO-Fe 3 O 4 nanocomposites possessed optimal RL values of −34.4 dB at 1.6mm. The effective absorption bandwidth is 3.8 GHz. Another minimum RL value is −37.5 dB at 6.5 mm with an absorption bandwidth of 1.9 GHz. The conductivity loss, polarization loss, eddy current loss, and natural resonation are major mechanisms in the electromagnetic wave dissipations. It is believed that rGO-Fe 3 O 4 nanocomposites can be employed as a high-efficiency electromagnetic wave absorbing candidate.