Synthesis of rGO/p-Fe3O4@PANI three-phase nanomaterials and electromagnetic wave absorption properties

A novel rGO/p-Fe3O4@PANI three-phase nanomaterial was prepared by coating conductive polyaniline (PANI) on reduced graphene oxide (rGO) and porous Fe3O4 (p-Fe3O4), which can increase the reflection interface of electromagnetic waves and extend the reflection path of electromagnetic waves propagating inside the material, and between them. Three kinds of rGO/p-Fe3O4@PANI samples were prepared. The mass percentage of PANI in the samples was established. The mass ratio of rGO to p-Fe3O4 was changed to ameliorate the impedance matching and attenuation constant. Compared with rGO/p-Fe3O4, rGO/p-Fe3O4@PANI exhibits excellent electromagnetic wave absorption performance. When the mass ratio of rGO to p-Fe3O4 in the 40 wt% three-phase material is 1:5 and the thickness is 2.5 mm, the minimum RL of rGO/p-Fe3O4@PANI is −41.38 dB at 9.75 GHz and the effective absorption bandwidth to electromagnetic wave reaches 3.08 GHz (from 8.40 to 11.48 GHz), indicating that is a potentially attractive candidate for high efficiency electromagnetic wave absorbers. The combination of the conductive polymer, porous magnetic nanoparticles and rGO improves the impedance matching condition to some extent, and effectively increases the attenuation constant of the material.


Introduction
With the rapid development of the modern society, electromagnetic wave has been extensively used in the field of wireless communications, but they have also brought problems such as electromagnetic interference and information leakage [1][2][3][4][5][6]. In addition, in the military field, there are demands for electromagnetic shielding technology and stealth technology such as in the field of military aircraft [7][8][9]. These have led to continuous and widespread attention on electromagnetic wave absorbing materials and there is an urgent need to develop an electromagnetic wave absorbing material that is thin in thickness, light in weight, wide in frequency bandwidth and strong in absorption [10,11].
Carbon nanomaterials represented by graphene and carbon nanotubes are favorable candidates for electromagnetic wave absorbing materials owing to their highly stable and unique structure and high electrical conductivity. Graphene oxide has excellent electromagnetic wave absorption performance owing to its low density, low resistivity, high conduction loss, polarization relaxation of defects and dipole polarization relaxation groups [12][13][14][15]. The existence of these dipole polarization relaxation groups is beneficial to the microwave absorption of materials, but these groups may carry a large number of oxygen atoms, which will weaken the polarization and conductivity of graphene oxide, and then affect its polarization/conduction loss as a microwave absorption material [16]. In order to take advantage of the existence of dipolar polarization relaxation groups and avoid the disadvantage of carrying a large number of oxygen atoms, the researchers reduced graphene oxide to rGO, which can reduce the oxygen atom content of rGO. In addition, graphene oxide has high electron mobility, making it difficult to meet the impedance matching of electromagnetic wave absorption [17]. In order to solve this problem, the researchers have proposed many solutions, but the most effective one is to combine graphene and magnetic particles-Fe 3 O 4 [18], Co 3 O 4 [19], CoFe 2 O 4 [20], NiFe 2 O 4 [21] etc-into a two-phase material.
Among many magnetic materials, Fe 3 O 4 is considered to be the most ideal absorbing material in the GHz range owing to its high magnetic permeability, low toxicity, low cost and availability [22,23]. Wang et al [24] uniformly dispersed 25 nm Fe 3 O 4 nanoparticles on the surface of graphene by depositing and annealing in Ar gas. Compared with pure magnetic particles and graphene, the electromagnetic wave absorption properties of the nanocomposite were significantly improved. The material with a thickness of 5 mm has a minimum RL of −40.36 dB, which is a good candidate of lightweight and strong electromagnetic wave absorption. However, conventional Fe 3 O 4 for absorbing materials still has some disadvantages such as high density and poor high temperature stability [25,26]. In view of those, researchers generally attempted to prepare nano-Fe 3 O 4 with various microstructures that can enhance the absorption of electromagnetic wave. This means to increase the performance of electromagnetic wave absorbing per unit mass of Fe 3 O 4 -reduce the amount of Fe 3 O 4 [27][28][29]. At the same time, researchers also tried to design a protective layer on the surface of nano-Fe 3 O 4 to form a coreshell structure. The usual methods are in situ polymerization and surface oxidation [30]. In-situ polymerization is usually carried out by coating a layer of conductive polymer on the surface of nano-Fe 3 O 4 , which is stable and helpful to enhance electromagnetic absorption properties, thereby improving the stability and electromagnetic wave absorption properties of nano-Fe 3 O 4 [31]. In order to reduce the oxygen content of graphene oxide, we reduced graphene oxide to rGO, and distributed p-Fe 3 O 4 with multiple micro-interfaces on the surface of rGO. Finally, PANI was coated on the surface of p-Fe 3 O 4 , rGO and between them, which was rare to report. When the material absorbs electromagnetic wave, PANI can enhance the absorption and play a role in the smooth transition of dielectric behavior of p-Fe 3 O 4 and rGO. At the same time, it can protect p-Fe 3 O 4 and rGO from oxidation to some extent.
In this paper, we designed and synthesized rGO/p-Fe 3 O 4 @PANI three-phase nanocomposites. Firstly, p-Fe 3 O 4 was prepared by simple hydrothermal method. Secondly, rGO/p-Fe 3 O 4 with different mass ratios was prepared by solution mixing and dispersion under ultrasonic stirring. Then, rGO/p-Fe 3 O 4 was modified by PANI through in situ polymerization of aniline monomers, resulting that a layer of PANI was coated on the surface of rGO, p-Fe 3 O 4 and between them. Finally, rGO/p-Fe 3 O 4 @PANI nanoparticles were mixed with paraffin to prepare rGO/p-Fe 3 O 4 @PANI electromagnetic wave absorbing material samples. The structure, morphology, dielectric properties, magnetic properties and electromagnetic wave absorption properties of rGO/p-Fe 3 O 4 @PANI three-phase nanocomposites were studied.

Synthesis of magnetic porous Fe 3 O 4 microspheres
The magnetic porous Fe 3 O 4 in this paper was prepared by the previously reported method [32]. The typical preparation process was as follows: 2.0 g of FeCl 3 ·6H 2 O, 5.4 g of sodium acetate and 1.5 g of polyethylene glycol were added to 60 mL of ethylene glycol and vigorously stirred into a homogeneous solution, and then transferred to a Teflon-lined stainless steel reaction vessel. After reacting at 210°C for 8 h, it was recovered by magnetic washing and dried to obtain black solid particles. Secondly, 0.69 g of the black solid particles, 1.5 g of glucose, 4.5 g of urea were uniformly dispersed in 60 ml of deionized water by ultrasonic dispersion, and then transferred to a Teflon-lined stainless steel reaction vessel, after reacting at 210°C for 10 h, it was recovered by washing and dried to obtain black particles.

Synthesis of rGO/p-Fe 3 O 4
The rGO/p-Fe 3 O 4 in this paper is given in table 1. Taking No.1 as an example, the preparation process was as follows: firstly, the purchased GO was ultrasonically dispersed into 300 ml of deionized water and transferred to a 500 ml three-necked flask. After magnetic stirring for 20 min, 146 μl of hydrazine hydrate was added, and after reducing for 100 min at 100°C, After filtration and washing with deionized water and ethanol, then drying in a vacuum oven at 60°C for 6 h, the rGO was collected. Secondly, ultrasound dispersion of 1.0 g of the black particles prepared in step 2.2 into 50 ml ethanol and 0.2 g rGO into 50 ml DMF were carried out. After ultrasonic dispersion for 30 min, the two were transferred to a 250 ml three-necked flask and mechanically stirred for 2 h. Finally, distillation under reduced pressure was carried out and the ethanol in the system was removed at a temperature of 60°C. After the ethanol was removed, the temperature was raised to 100°C to reduce the DMF in the system. A portion of the DMF was left in the three-necked flask for transfer of the product. The remaining mixture was poured into the glass surface dish and dried in a vacuum oven at 100°C for 5 h to obtain the black granules.

Modified rGO/p-Fe 3 O 4
1.0 g of black granules prepared in step 2.3 was added to 20 ml mixed liquids (ethanol: water=4:1) and mechanically stirred at room temperature for 1 h, then 3 mL of ammonia water and 3 ml of ethyl orthosilicate were added slowly. The mixture was mechanically stirred at room temperature for 12 h, off-white particles were obtained by washing and centrifugation.

Synthesis of rGO/p-Fe 3 O 4 @PANI
1.0 g of the off-white particles prepared in step 2.4 and 0.2 g of PVP were added to 50 ml of deionized water and mechanically stirred for 2 h in an ice water bath environment, then hydrochloric acid of 0.6 mol l −1 of 48 ml and 1.0 g aniline was added. After mechanically stirred for 1 h in an ice water bath environment, ammonium persulfate deionized water solution of 0.15 g ml −1 of 40 ml was added slowly. After mechanically stirred in an ice water bath environment for 8 h, the green particles were obtained by washing and centrifugation.

Characterization
The Fourier-transform infrared (FTIR) spectra of p-Fe 3 O 4 , rGO/p-Fe 3 O 4 , modified rGO/p-Fe 3 O 4 and rGO/p-Fe 3 O 4 @PANI were characterized by Fourier-transform infrared spectroscopy (Nicolet 6700). The crystal properties of p-Fe 3 O 4 , rGO/p-Fe 3 O 4 and rGO/p-Fe 3 O 4 @PANI were characterized by x-ray diffraction (XRD) diffractometer (Bruker D8 Advance, with Cu Kα source, λ=0.154056 nm). The microscopic morphology and structure of p-Fe 3 O 4 , rGO/p-Fe 3 O 4 and rGO/p-Fe 3 O 4 @PANI were characterized by scanning electron microscopy (SEM, Zeiss Ultra Plus). The surface functional groups of the product were measured by a x-ray photoelectron spectroscopy (XPS) instrument (Thermo Scientific K-Alpha). The room temperature magnetic properties of the p-Fe 3 O4, rGO/p-Fe 3 O 4 and rGO/p-Fe 3 O 4 @PANI were recorded by a vibrating sample magnetometer (VSM, JDAW-2000D) (the external magnetic field was −10000 to 10000 Oe). The electromagnetic parameters of the samples were measured using a vector network analyzer (VNA, Agilent N5247A) at an electromagnetic frequency ranging from 1 to 18 GHz.   1100 cm −1 , so it can be judged that the SiO 2 modified layer has been perfectly coated on the surface of p-Fe 3 O 4 through chemical bonds. Figure 1(d) is the infrared spectrum of rGO/p-Fe 3 O 4 @PANI. It contains all the absorption peaks of 1 (c). In addition, the peaks at 1580 cm −1 and 1500 cm −1 belong to the stretching vibration absorption peak of C=C in quinone ring and benzene ring; the peak at 1295 cm −1 belongs to the bending vibration absorption peak of C-N in benzene ring; the peak at 948 cm −1 corresponds to the vibration absorption peak of C-H, which is belong to the benzene ring after 1, 4 substitution [34,35]. Those all indicated that the in situ polymerization of polyaniline on TEOS modified rGO/p-Fe 3 O 4 surface was successful. Figure 2 shows the XRD spectra of p-Fe 3 O 4 , rGO/p-Fe 3 O 4 and rGO/p-Fe 3 O 4 @PANI. As shown in figure 2(a), the strong peaks at 30.076°,35.426°,43.053°,56.935°and 62.520°are the characteristic diffraction peaks of Fe 3 O 4 , which correspond to (220), (311), (400), (511) and (440) crystal surfaces respectively, and are completely consistent with the standard XRD card data (PDF#99-0073), indicating the existence and good crystallinity of Fe 3 O 4 [36]. As shown in figures 2(b) and (c), a characteristic diffraction peak appears near 2θ=20°−30°, which corresponds to the (002) surface of rGO [37]. Characteristic diffraction peaks of Fe 3 O 4 also appear in figures 2(b) and (c), but the intensity of characteristic diffraction peaks of Fe 3 O 4 decreases significantly compared with figure 2(a), especially in figure 2(c). For figure 2(b), the reason why the peak intensity of characteristic diffraction peaks of Fe 3 O 4 decreases is that the mass percentage of p-Fe 3 O 4 decreases when rGO is added. For figure 2(c), because the PANI shell is very thin and amorphous, it has no effect on the composite crystal structure, so there is no obvious characteristic diffraction peaks of PANI. The peak intensity of the characteristic diffraction peak of p-Fe 3 O 4 decreases because the mass percentage of p-Fe 3 O 4 decreases further after the rGO/p-Fe 3 O 4 coating PANI. Consistently, compared with figure 2(b), the peak intensity of the characteristic diffraction peak of rGO near 2θ=20°−30°also decreases. Combining these with Fouriertransform infrared spectra, it can be concluded that rGO/p-Fe 3 O 4 @PANI is successfully prepared. respectively, due to C=C/C-C in the aromatic ring, C-N and C=O (carbonyl group) [38]. As shown in figure 3(d), the N1s spectrum of rGO/p-Fe 3 O 4 @PANI can be divided into three peaks. The peaks at 398.93 eV and 399.68 eV correspond to quinone diamine (−N=) and phenylenediamine (−NH−) structure, the peak at 400.38 eV is attributed to the protonated imine (−N + =) structure, further demonstrating the successful synthesis of PANI [39].

Results and discussion
As can be seen from figure 4(a)-the scanning electron microscopy (SEM) of p-Fe 3 O 4 , the prepared p-Fe 3 O 4 exhibits irregular spherical shape and its surface is uneven with errors from top to bottom. Compared with regular spherical shape, the prepared p-Fe 3 O 4 has a higher specific surface area that can increase the reflection interface of electromagnetic waves and extend the reflection path of electromagnetic waves propagating inside the material. It also can be seen that the dispersion of p-Fe 3 O 4 is good. The porous structure of p-Fe 3 O 4 can be obviously seen from figure 4(b)-the transmission electron microscopy (TEM) of p-Fe 3 O 4 . As shown in figure 4(c), modified p-Fe 3 O 4 dispersed on the surface of rGO sheet does not exhibit a regular spherical shape, but the roughness of the surface and the amplitude of concavity and convexity is lower than that of the single p-Fe 3 O 4 , but it is still uneven and the structure is still favorable for the in situ polymerization of aniline. In addition, it can be seen from the red region of figure 4(c) that the SiO 2 layer also modified rGO and connected rGO and p-Fe 3 O 4 to some extent. As shown in figure 4(d), after in situ polymerization of aniline monomers on the surface of rGO/p-Fe 3 O 4 , PANI covers the whole p-Fe 3 O 4 surface with a very dense coating, which roughens the surface of p-Fe 3 O 4 particles and forms a dense core-shell structure of rGO/p-Fe 3 O 4 @PANI three-phase composite. After PANI is coated on the surface of p-Fe 3 O 4 , rGO and between them (red region of figure 4(d)), the interfacial relaxation can be increased and the electron polarization can be easily formed under the action of external electric field in the process of transmission, which can improve the dielectric loss of the material to electromagnetic wave, then improve the absorption performance of the material to electromagnetic wave. The magnetic properties of p-Fe 3 O 4 ( figure 5(a)), rGO/p-Fe 3 O 4 ( figure 5(b)), rGO/p-Fe 3 O 4 @PANI (figure 5 (c)) were studied by applying the maximum magnetic field of +10 KOe at room temperature using VSM. As shown in figure 5, the saturation magnetization of rGO/p-Fe 3 O 4 is reduced from 57.9 emu g −1 to 33.4 emu g −1 and the coercive force is reduced from 120 Oe to 80.4 Oe, compared with pure p-Fe 3 O 4 . In addition, it is apparent to figure 5(c) that the saturation magnetization of rGO/p-Fe 3 O 4 @PANI is reduced to  In order to characterize and understand the electromagnetic wave absorption mechanism of the prepared materials, the real parts (ε′) of the permittivity, the imaginary parts (ε″) of the permittivity, the real parts (μ′) of the permeability and the imaginary parts (μ″) of the permeability of each rGO/p-Fe 3 O 4 @PANI and rGO/p-Fe 3 O 4 samples are measured in the range of the electromagnetic wave frequency of 1-18 GHz and are shown in figures 6(a)-(d).
As shown in figure 6(a) (1:5), although ε′ of the former is slightly smaller than the latter, the variation with frequency of ε′ of the former is significantly smaller than the latter. The cause is that PANI is polymerized with rGO/p-Fe 3 O 4 at a mass ratio of 1:1. The conductivity of PANI is not as good as rGO, so the ε′ of rGO/p-Fe 3 O 4 (1: 5)@PANI is smaller than that of rGO/p-Fe 3 O 4 (1:5). However, after introducing conductive PANI into rGO/p-Fe 3 O 4 system, PANI can play a role in coordinating the interface between rGO and p-Fe 3 O 4 resulting in improving the ε′ of rGO/p-Fe 3 O 4 (1:5)@PANI to some extent. So the ε′ of rGO/p-Fe 3 O 4 (1:5)@PANI is only slightly smaller As shown in figure 6( 5) is approximately similar to that of rGO/p-Fe 3 O 4 (1:10)@PANI. The reason may be that even if rGO has a greater loss to electromagnetic waves than PANI, the mass percentage of PANI in rGO/p-Fe 3 O 4 @PANI system is significantly larger than that of rGO and the role of coordinating system of PANI, the loss to electromagnetic wave can be improved and the value of ε″ increases, but the effect of these is still limited.
As  As shown in figure 6(e), the dielectric loss tangent of rGO/p-Fe 3 O 4 @PANI is larger than that of rGO/p-Fe 3 O 4 on the whole. Although the dielectric loss tangent of two kinds of sample change with the frequency in the same trend, the change amplitude of the dielectric loss tangent of rGO/p-Fe 3 O 4 @PANI with the frequency is obviously smaller than that of rGO/p-Fe 3 O 4 , which also shows that the introduction of PANI into the rGO/p-Fe 3 O 4 system can not only increase the dielectric loss to electromagnetic wave to some extent, but also stabilize the dielectric loss of material to electromagnetic wave within frequency of 1-18 GHz. The reason is that PANI is coated on the surface of p-Fe 3 O 4 , rGO and between them, which can play a coordination and transition role between p-Fe 3 O 4 and rGO. For rGO/p-Fe 3 O 4 @PANI samples, with the decrease of the mass percentage of rGO (i.e., the increase of the mass percentage of p-Fe 3 O 4 ), the dielectric loss tangent of rGO/p-Fe 3 O 4 @PANI samples to electromagnetic wave has no significant change, probably because the mass percentage of rGO is relatively small to PANI, even if the mass percentage of rGO changes, the dielectric loss tangent of material to electromagnetic wave has no significant change. As shown in figure 6(f), for rGO/p-Fe 3 O 4 @PANI, because the mass percentage change of p-Fe 3 O 4 is small, their magnetic loss tangent values have no significant difference. The change range of magnetic loss tangent of rGO/p-Fe 3 O 4 @PANI is smaller than that of rGO/p-Fe 3 O 4 , mainly because of the introduction of PANI, which reduces and stabilizes the magnetism of p-Fe 3 O 4 to some extent.
In order to study that the three-phase nanocomposite material may have better electromagnetic wave absorption performance than the two-phase composite material, the reflection loss (RL) of each rGO/p-Fe 3 O 4 @PANI and rGO/p- Among them, Z in is the characteristic input impedance of the absorbing material, Z 0 ≈377Ω is the characteristic impedance of free-space, f is the frequency of electromagnetic wave, c is the speed of light, d is the thickness of the samples of absorbing material and ε r and μ r is the complex relative permittivity and permeability, respectively. Since the thickness of the sample has a significant influence on the attenuation characteristics of the electromagnetic wave, the RL of each rGO/p-Fe 3 O 4 @PANI three-phase and rGO/p-Fe 3 O 4 (1:5) two-phase composites of different thicknesses (2 to 3.5 mm) were compared and shown in figure 7. Generally, a material has an RL value which is less than −10 dB (90% of electromagnetic waves are absorbed) is considered to be an effective electromagnetic wave absorbing material. As shown in figure 7, as the thickness increases, the reflection peak gradually moves toward the lower frequency, satisfying the quarter-wavelength model [41].   An electromagnetic wave absorbing material with excellent performance needs to meet two basic requirements: when electromagnetic waves are projected onto the surface of the material, the electromagnetic wave should enter the material as much as possible, thereby there is a need to minimize direct reflection of electromagnetic wave as much as possible when electromagnetic waves are projected onto the surface of the material [42]. To achieve this goal, it is necessary to match the wave impedance of the composite sample to that of free-space. According to equation (1), the input impedance Z in is required to be equal to the free-space wave impedance Z 0 (377 Ω), that is, the imaginary part of Z in is close to zero and the real part of Z in is close to 377 Ω. Figure 8 presents the complex input impedance of each rGO/p-Fe 3 O 4 @PANI three-phase composite and rGO/p-Fe 3 O 4 (1:5) two-phase composite at a thickness of 2.5 mm.
It can be seen from figure 8 that the imaginary part of Z in of rGO/p-Fe 3 O 4 (1:20)@PANI sample is 4.10 Ω at 12.9 GHz and the real part of Z in is 411.0 Ω; the imaginary part of Z in of rGO/p-Fe 3 O 4 (1:10)@PANI sample is 5.03 Ω at 10.775 GHz, the real part of Z in is 401.8 Ω; the imaginary part of Z in of rGO/p-Fe 3 O 4 (1:5)@PANI sample is −1.47 Ω at 12.815 GHz, corresponding Z in is 302.3 Ω; the imaginary part of Z in of rGO/p-Fe 3 O 4 (1:5) sample is 4.11 Ω at 12.815 GHz and the real part of Z in is 418.59 Ω. It can be seen that the rGO/p-Fe 3 O 4 (1:10)@PANI sample have the best impedance matching and impedance matching performance of the rGO/p-Fe 3 O 4 (1:5)@PANI is relatively good. Therefore, introducing PANI and adjusting the mass percentage of rGO can improve the impedance matching performance of the rGO/p-Fe 3 O 4 @PANI material. However, the merits of determining the performance of electromagnetic wave absorbing of material need not only satisfy the impedance matching.
On the other hand, its electromagnetic energy should be attenuated through dielectric loss and magnetic loss mechanism of material when the electromagnetic wave propagating inside the material. A material having excellent performance of electromagnetic wave absorbing needs to prevent electromagnetic wave leaking out due to incomplete attenuation, resulting in affecting the performance of electromagnetic wave absorbing of the material. The absorption performance is given by the attenuation constant α, which can be determined as [43]: ( ( ) ( ) ) () Figure 9 shows the attenuation constant of each rGO/p-Fe 3 O 4 @PANI three-phase composite and rGO/p-Fe 3 O 4 (1:5) two-phase composite at a thickness of 2.5 mm. The attenuation constant of rGO/p-Fe 3 O 4 @PANI composites increases with the increase of the mass percentage of rGO. The attenuation constant of rGO/p-Fe 3 O 4 (1:10)@PANI is similar to rGO/p-Fe 3 O 4 (1:5), but all are smaller than rGO/p-Fe 3 O 4 (1:5)@PANI. Therefore, PANI, introduced into rGO/p-Fe 3 O 4 system, can improve the attenuation constant of materials. Therefore, rGO/p-Fe 3 O 4 (1:5)@PANI has the best performance of electromagnetic wave absorbing, which is a consequence of relatively good impedance matching and excellent attenuation constant.

Conclusions
In summary, rGO/p-Fe 3 O 4 @PANI was successfully synthesized by three steps. The impedance matching and attenuation constant of the samples were adjusted by changing the mass percentage of rGO and p-Fe 3 O 4 in rGO/p-Fe 3 O 4 @PANI. Compared with rGO/p-Fe 3 O 4 , rGO/p-Fe 3 O 4 @PANI exhibits excellent electromagnetic wave absorption performance. When the mass ratio of rGO to p-Fe 3 O 4 in the 40 wt% three-phase material is 1:5 and the thickness is 2.5 mm, the minimum RL of rGO/p-Fe 3 O 4 @PANI is −41.38 dB at 9.75 GHz and the  effective absorption bandwidth to electromagnetic wave reaches 3.08 GHz (from 8.40 to 11.48 GHz). This result proves that rGO/p-Fe 3 O 4 @PANI can be used as an excellent electromagnetic wave absorbing material.