Reduced Graphene Oxide-Wrapped Super Dense Fe3O4 Nanoparticles with Enhanced Electromagnetic Wave Absorption Properties

The efficient preparation of electromagnetic wave absorbing materials with low density and excellent electromagnetic wave absorption remains a considerable challenge. In this study, reduced graphene oxide (RGO) wrapped Fe3O4 nanoparticles (NPs) were synthesized based on one-step reaction by the reduction of graphene oxide (GO), and the generation of super-fine Fe3O4 NPs was achieved. The phase structure, chemical composition, micromorphology, and magnetism were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM), respectively. The electromagnetic characteristics were evaluated on a vector network analyzer by the coaxial line method. The results showed that super-fine Fe3O4 NPs with an average size of 6.18 nm are densely distributed on the surface of graphenes. The RGO/Fe3O4 nanocomposites exhibited excellent microwave absorption properties with a minimum reflection loss (RL) of up to −55.71 dB at 6.78 GHz at 3.5 mm thickness and the highest effective absorption bandwidth with RL values exceeding −10 dB is 4.76 GHz between 13.24 and 18 GHz at 1.7 mm thickness. This work provides a concise method for the development of RGO supported super dense Fe3O4 nanocomposites for high performance electromagnetic absorption applications.


Introduction
Due to the rapid development of electromagnetic wave detection technology, high performance microwave absorption materials have attracted more and more attention in the civil and military industries [1][2][3]. Ferrites have been widely used as electromagnetic wave (EW) absorbing agents due to their high saturation magnetization, low technological threshold, and cost [4,5]. Unfortunately, traditional ferrite absorbers have revealed shortcomings such as easy aggregation, high weight, and an inefficient EW absorption capability in practical applications. In general, these shortcomings are confined to the effects of magnetic loss when attenuating EW signals for Fe 3 O 4 nanoparticles (NPs) due to their high resistivity, but a single loss mechanism is not beneficial for the achievement of ideal EW absorption performance. Thus, it is necessary to introduce some other types of electromagnetic loss mechanisms to alleviate this dilemma [6][7][8]. Recently, graphene nanosheets (GNs) have gained a lot of attention as microwave absorbers by virtue of their excellent conductivity and light weight [9][10][11]. Researchers have found that reduced graphene oxide (RGO) has attractive microwave absorbing ability owning to its high dielectric loss. Nevertheless, single RGOs could not achieve ideal microwave absorption performance due to their high permittivity and low permeability, which would lead to unfavorable electromagnetic impedance matching. In order to mitigate the dilemma, a lot of research has been conducted by combining RGO with magnetic components, such as FeCo [12], Ni [13], NiCoP [14], CoFe 2 O 4 [15], and Fe 3 O 4 [16]. For example, Xu et al. synthesized RGO/Ni hybrids with different mass ratios to obtain an optimal reflection loss value of −39.03 dB at 13 GHz [13]. Xue et al. synthesized NiCoP/RGO nanocomposites by one-pot reaction in order to improve dielectric and magnetic loss and thus enhance the reflection loss [14]. Chu et al. synthesized α-Fe 2 O 3 /RGO with a maximum reflection loss of up to −42.8 dB at a thickness of 1.8 mm [16]. Therefore, decorating magnetic metal NPs onto the large surface of GNs is a flexible strategy for improving microwave absorption properties by combining dielectric and magnetic loss mechanisms into a micro-nano composite structure, which can also improve their aggregation resistance and reduce their weight [17][18][19][20][21][22][23][24][25][26].
Herein, we report an easy and efficient method for the synthesis of graphene wrapped super dense Fe 3 O 4 NPs via one-step reaction in order to enhance their microwave absorption properties. The phase structure, chemical composition, micromorphology, and magnetism of RGO/Fe 3 O 4 nanocomposites are investigated, and the electromagnetic parameters and microwave absorption performance of RGO/Fe 3 O 4 is evaluated.
Graphene oxide (GO) was synthesized using modified Hummers method [27]. The source materials (2 g of graphene powder, 60 mL of concentrated H 2 SO 4, and 7 g of KMnO 4 ) were successively put into a three-necked flask while undergoing mechanical stirring in an ice water bath. The mixture was heated to 35 • C while undergoing mechanical stirring for 3 h, and then diluted with distilled water (100 mL) dropwise. Afterwards, the mixture was heated to 90 • C while undergoing strong mechanical stirring for 30 min. Finally, distilled water (180 mL) and H 2 O 2 (20 mL, 30%) were added dropwise and then the mixture was kept undisturbed for 24 h. The obtained precipitation was washed with HCl solution and distilled water through centrifugation until the decantate became neutral. Finally, the resulting graphene oxides (GOs) were obtained by ultrasonic treatment in water followed by freeze-drying.
The as-obtained GOs were firstly dissolved in 70 mL DEG, and 400 mg FeCl 3 was added while the mixture was being stirred, then the suspension was heated to 220 • C while undergoing continuous stirring for 1 h with the protection of argon. Afterwards, NaOH solution was quickly poured into the suspension while undergoing stirring for another 0.5 h at 220 • C. Finally, the reaction system was cooled down to room temperature and the obtained RGO/Fe 3 O 4 was separated and purified by centrifugation, washing, and drying. For comparison, pure Fe 3 O 4 NPs was prepared using similar methods.
The chemical composition was characterized by X-ray photoelectron spectroscopy (XPS) performed on a Thermo ESCALAB 250 (Thermo Fisher Scientific Inc., Waltham, MA, USA) with Al-K α radiation. The micromorphology was observed by transmission electron microscopy (TEM) conducted on a Tecna G2 F20 S-TWIN electron microscope (FEI Inc., Hillsborough, OR, USA) operated at 200 kV. The hysteresis loop was recorded on a SQUID-VSM vibrating sample magnetometer (Quantum Design Inc., San Diego, CA, USA). Electromagnetic parameters, including relative complex permittivity and permeability, were measured in the frequency range of 1-18 GHz using the coaxial line method on an AV3629D Vector Network Analyzer (CETI Co., Qingdao, China) by mixing the samples with paraffin wax (weight ratio of 1:1) and pressing them into a standard cylindrical shape mold with an inner diameter of 3 mm, an outer diameter of 7 mm, and a thickness of 3 mm.

Chemical Composition and Morphology
The chemical composition of the RGO/Fe 3 O 4 nanocomposite was identified by XPS as shown in Figure 1. Figure 1a shows the XPS full spectrum of RGO/Fe 3 O 4 . It can be observed that the peaks located at around 56, 285, 532, and 711.3 eV belong to Fe3p, C1s, O1s and Fe2p, respectively, which indicates that RGO/Fe 3 O 4 consists of three major elements including C, O and Fe. In the Fe2p high resolution XPS spectra shown in Figure 1b, the peaks located at 711 and 723 eV are assigned to Fe 2p 3/2 and Fe 2p 1/2 , respectively, which is consistent with the characteristic peaks of Fe 3 O 4 . Nanomaterials 2019, 9,845 3 of 11 cylindrical shape mold with an inner diameter of 3 mm, an outer diameter of 7 mm, and a thickness of 3 mm.

Chemical Composition and Morphology
The chemical composition of the RGO/Fe3O4 nanocomposite was identified by XPS as shown in Figure 1. Figure 1a shows the XPS full spectrum of RGO/Fe3O4. It can be observed that the peaks located at around 56, 285, 532, and 711.3 eV belong to Fe3p, C1s, O1s and Fe2p, respectively, which indicates that RGO/Fe3O4 consists of three major elements including C, O and Fe. In the Fe2p high resolution XPS spectra shown in Figure 1b, the peaks located at 711 and 723 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively, which is consistent with the characteristic peaks of Fe3O4.  It can be seen from Figure 2a,b that the wrinkled surface of graphene nanosheets, which are capable of supplying a large loading area for NP growth, are homogeneously decorated with super dense spherical Fe3O4 NPs. The tiny Fe3O4 NPs, with an average size of 6.18 nm, are well distributed on the surface of the graphenes. In the loading process, GOs were employed as a flexible substrate for the in situ anchoring of Fe 3+ and its growth into Fe3O4 NPs, so they played a confinement function to prevent the Fe3O4 NPs from detaching and aggregating. In the HRTEM image shown in Figure 2c, the interplanar distance of the NPs is 0.25 nm, which is in accordance with the lattice spacing of the (311) plane of cubic magnetite Fe3O4, further confirming the formation of Fe3O4 nanocrystals on the surface of RGO inferred from the XPS results.   Figure 3a shows the hysteresis loops of different samples collected by a magnetometer at room temperature. The saturation magnetization (Ms) value of the RGO/Fe3O4, RGO, and Fe3O4 NPs are 36, 0.06, and 59 emu/g, and the corresponding coercivity (Hc) values are 25, 0, and 25 Oe, respectively. It can be observed that the Ms values for Fe3O4 NPs are higher than those of RGO/Fe3O4 and RGOs, indicating that the magnetism of RGO/Fe3O4 is introduced by loading magnetic Fe3O4 NPs onto the surface of nonmagnetic RGOs. Meanwhile, the Hc values of RGO/Fe3O4 and Fe3O4 NPs are the same, suggesting that the loading process has no effect on the intrinsic magnetic properties of Fe3O4 NPs. To further illustrate the magnetic properties, the RGO/Fe3O4 were dispersed in an ethanol solution (Figure 3b), which has favorable dispersibility and stability. After being attracted by a magnet (Figure  3c), the RGO/Fe3O4 dispersed in alcohol were quickly gathered together and attached to the bottle wall. Therefore, the graphenes were successfully magnetized by the loading of super dense Fe3O4 NPs.

Electromagnetic Characteristics
In order to find out the essential reasons for microwave absorption mechanisms, the electromagnetic parameters, including the complex permittivity and permeability of Fe3O4 NPs, RGO, and RGO/Fe3O4 nanocomposites, were measured. The real parts (ε' and μ') symbolize the storage capacity of electric and magnetic energy, and the imaginary parts (ε'' and μ'') symbolize the energy loss, respectively. The dielectric loss (tanδε = ε''/ε') and magnetic loss tangent (tanδμ = μ''/μ') give the balance between the real and imaginary parts in an absorbing structure. Figure 4a-c show the frequency dependence of the real part (ε') and the imaginary part (ε'') of complex permittivity, and the dielectric loss tangent (tanδε) for different samples. It is clear that the ε', ε'', and tanδε values for both RGO/Fe3O4 and RGO are larger than those of Fe3O4 NPs. The ε' of RGO/Fe3O4 declines from 13.69 to 6.98 with increasing frequency, and the ε'' remains relatively stable, changing from 5.55 to 3.04. The tanδε curve also exhibits a moderate growth trend ranging from 0.33 to 0.65 with some fluctuation, particularly in the high frequency region. Compared with pure RGOs, the RGO/Fe3O4 have a similar tendency in ε', but it is slightly lower in ε'' and tanδε. The enhanced ε', ε'', and tanδε of RGO/Fe3O4 is attributed to multiple dielectric loss behaviors derived from dielectric RGOs and magnetic Fe3O4 NPs. Firstly, the RGOs with high electric conductivity can form conducting networks, which is in favor of dielectric loss, thereby playing a main role in the substantial increase in ε', ε'', and tanδε values. From the ε'' versus ε' plot of the RGO/Fe3O4 (Figure 4d), it can be observed that there are multi-arcs for RGO/Fe3O4 and RGO, while there are no obvious arcs with increasing frequency for Fe3O4 NPs, indicating that debye dipolar relaxation is the main dielectric loss mechanism for RGO based nanostructure. In addition, the introduction of Fe3O4 NPs would create

Electromagnetic Characteristics
In order to find out the essential reasons for microwave absorption mechanisms, the electromagnetic parameters, including the complex permittivity and permeability of Fe 3 O 4 NPs, RGO, and RGO/Fe 3 O 4 nanocomposites, were measured. The real parts (ε and µ ) symbolize the storage capacity of electric and magnetic energy, and the imaginary parts (ε" and µ") symbolize the energy loss, respectively. The dielectric loss (tanδ ε = ε"/ε ) and magnetic loss tangent (tanδ µ = µ"/µ ) give the balance between the real and imaginary parts in an absorbing structure. Figure 4a-c show the frequency dependence of the real part (ε ) and the imaginary part (ε") of complex permittivity, and the dielectric loss tangent (tanδ ε ) for different samples. It is clear that the ε , ε", and tanδ ε values for both RGO/Fe 3 O 4 and RGO are larger than those of Fe 3 O 4 NPs. The ε of RGO/Fe 3 O 4 declines from 13.69 to 6.98 with increasing frequency, and the ε" remains relatively stable, changing from 5.55 to 3.04. The tanδ ε curve also exhibits a moderate growth trend ranging from 0.33 to 0.65 with some fluctuation, particularly in the high frequency region. Compared with pure RGOs, the RGO/Fe 3 O 4 have a similar tendency in ε , but it is slightly lower in ε" and tanδ ε . The enhanced ε , ε", and tanδ ε of RGO/Fe 3 O 4 is attributed to multiple dielectric loss behaviors derived from dielectric RGOs and magnetic Fe 3 O 4 NPs. Firstly, the RGOs with high electric conductivity can form conducting networks, which is in favor of dielectric loss, thereby playing a main role in the substantial increase in ε , ε", and tanδ ε values. From the ε" versus ε plot of the RGO/Fe 3 O 4 (Figure 4d), it can be observed that there are multi-arcs for RGO/Fe 3 O 4 and RGO, while there are no obvious arcs with increasing frequency for Fe 3 O 4 NPs, indicating that debye dipolar relaxation is the main dielectric loss mechanism for RGO based nanostructure. In addition, the introduction of Fe 3 O 4 NPs would create defects on the RGO surface, which would act as polarization centers for increasing dielectric loss. Secondly, although the sole Fe 3 O 4 NPs with ε" and tanδ ε approaching zero have hardly any dielectric loss, loading Fe 3 O 4 NPs onto the surface of RGOs can introduce extra dielectric polarization behaviors. The interfacial polarization might be strengthened by a multi-interface between Fe 3 O 4 NPs and graphenes, and the different electric potential between the two would induce charge accumulation at both ends, thus enhancing the space-charge polarization. The super-tiny Fe 3 O 4 NPs have unsaturated bonds, which can serve as dipoles, thus the dipole polarization is enhanced [28]. The above mentioned polarization processes are beneficial for the improvement of dielectric loss and for the better dissipation of microwave energy. Nanomaterials 2019, 9,845 6 of 11 defects on the RGO surface, which would act as polarization centers for increasing dielectric loss. Secondly, although the sole Fe3O4 NPs with ε'' and tanδε approaching zero have hardly any dielectric loss, loading Fe3O4 NPs onto the surface of RGOs can introduce extra dielectric polarization behaviors. The interfacial polarization might be strengthened by a multi-interface between Fe3O4 NPs and graphenes, and the different electric potential between the two would induce charge accumulation at both ends, thus enhancing the space-charge polarization. The super-tiny Fe3O4 NPs have unsaturated bonds, which can serve as dipoles, thus the dipole polarization is enhanced [28]. The above mentioned polarization processes are beneficial for the improvement of dielectric loss and for the better dissipation of microwave energy.   ) and imaginary (μ'') parts of the relative complex permeability, and the magnetic loss (tanδμ) for the different samples. It is seen that the μ' values for RGO/Fe3O4 and Fe3O4 NPs sharply decrease initially and then become relatively stabilized with some fluctuation as the frequency increases. The μ'' and tanδμ for Fe3O4 NPs have obvious resonance peaks at 2-6 GHz, while there is a decreasing trend with increasing frequency in the μ'' and tanδμ curve for RGO/Fe3O4, which are favorable for enhancing magnetic loss at low frequencies [29]. The multiple resonance peaks are mainly attributed to natural resonance derived from magnetic Fe3O4 NPs. When the spherical Fe3O4 NPs are smaller, the anisortropy constant is higher, and the natural resonance is stronger. Meanwhile, exchange resonance may also contribute to magnetic loss by a small amount and to the anisotropy of magnetic NPs. In addition, the μ''(μ') −2 ƒ −1 values have obvious fluctuations at 1-6 GHz but remain relatively stable subsequently (Figure 4h), indicating that the eddy-current loss may come into action after 6 GHz. Figure 5 displays the changes in reflection loss (RL) versus frequency for the samples at different thicknesses. Figure 5a shows that the absorption performance of Fe3O4 NPs is so poor that the minimum RL is merely −4.41 dB at 13.07 GHz at a thickness of 3.3 mm. For RGO, shown in Figure 5b, the absorption performance gets better, with the minimum RL increasing to −26.87 dB at a thickness of 3.9 mm and shifting to a lower frequency of 4.31 GHz. It is implied from Figure 5c that the incorporation of RGOs can shift the minimum RL of Fe3O4 NPs to a lower frequency region with enhanced microwave absorption and an enlarged effective bandwidth. The reflection loss of RGO/Fe3O4 nanocomposites is greatly enhanced, with the minimum RL value reaching up to −55.71 dB at 6.78 GHz with a thickness of 3.5 mm, and the highest effective absorption bandwidth with RL values lower than −10 dB is 4.76 GHz between 13.24 and 18 GHz at a thickness of 1.7 mm (Figure 5d). For comparison, the microwave absorption properties of dielectric/magnetic nanocomposites studied in similar works are displayed in Table 1.  4e-g show the real (µ ) and imaginary (µ") parts of the relative complex permeability, and the magnetic loss (tanδ µ ) for the different samples. It is seen that the µ values for RGO/Fe 3 O 4 and Fe 3 O 4 NPs sharply decrease initially and then become relatively stabilized with some fluctuation as the frequency increases. The µ" and tanδ µ for Fe 3 O 4 NPs have obvious resonance peaks at 2-6 GHz, while there is a decreasing trend with increasing frequency in the µ" and tanδ µ curve for RGO/Fe 3 O 4 , which are favorable for enhancing magnetic loss at low frequencies [29]. The multiple resonance peaks are mainly attributed to natural resonance derived from magnetic Fe 3 O 4 NPs. When the spherical Fe 3 O 4 NPs are smaller, the anisortropy constant is higher, and the natural resonance is stronger. Meanwhile, exchange resonance may also contribute to magnetic loss by a small amount and to the anisotropy of magnetic NPs. In addition, the µ"(µ ) −2 ƒ −1 values have obvious fluctuations at 1-6 GHz but remain relatively stable subsequently (Figure 4h), indicating that the eddy-current loss may come into action after 6 GHz.  Figure 5a shows that the absorption performance of Fe 3 O 4 NPs is so poor that the minimum RL is merely −4.41 dB at 13.07 GHz at a thickness of 3.3 mm. For RGO, shown in Figure 5b, the absorption performance gets better, with the minimum RL increasing to −26.87 dB at a thickness of 3.9 mm and shifting to a lower frequency of 4.31 GHz. It is implied from Figure 5c that the incorporation of RGOs can shift the minimum RL of Fe 3 O 4 NPs to a lower frequency region with enhanced microwave absorption and an enlarged effective bandwidth. The reflection loss of RGO/Fe 3 O 4 nanocomposites is greatly enhanced, with the minimum RL value reaching up to −55.71 dB at 6.78 GHz with a thickness of 3.5 mm, and the highest effective absorption bandwidth with RL values lower than −10 dB is 4.76 GHz between 13.24 and 18 GHz at a thickness of 1.7 mm (Figure 5d). For comparison, the microwave absorption properties of dielectric/magnetic nanocomposites studied in similar works are displayed in Table 1. Based on the above analysis, the enhanced microwave absorption properties of RGO/Fe 3 O 4 nanocomposite can be attributed to multiple dielectric and magnetic loss mechanisms illustrated in Figure 6. The multi-interface introduced by super dense Fe 3 O 4 NPs brought about extra polarization behaviors and magnetic loss, such as interfacial polarization, dipole polarization, space-charge polarization, eddy current loss, debye dipolar relaxation, natural resonance, and exchange resonance. All these processes improve the microwave absorption properties. Based on the above analysis, the enhanced microwave absorption properties of RGO/Fe3O4 nanocomposite can be attributed to multiple dielectric and magnetic loss mechanisms illustrated in Figure 6. The multi-interface introduced by super dense Fe3O4 NPs brought about extra polarization behaviors and magnetic loss, such as interfacial polarization, dipole polarization, space-charge polarization, eddy current loss, debye dipolar relaxation, natural resonance, and exchange resonance. All these processes improve the microwave absorption properties.

Conclusions
In summary, we have successfully synthesized RGO wrapped super dense Fe3O4 NPs via onestep reaction. The magnetic Fe3O4 NPs with an average size of 6.18 nm are well distributed on the surface of the graphenes. The RGO/Fe3O4 nanocomposites have shown excellent electromagnetic wave absorption properties. The minimum RL reaches up to −55.71 dB at 6.78 GHz at 3.5 mm thickness. The highest effective absorption bandwidth is 4.76 GHz between 13.24 and 18 GHz at 1.7 mm thickness. The multi-interface introduced by super dense Fe3O4 NPs brought about extra polarization behaviors and magnetic loss, both of which improved the microwave absorption

Conclusions
In summary, we have successfully synthesized RGO wrapped super dense