In recent years, with the rapid development of the modern communication technologies, especially 5G wireless communication, electronic components such as miniature filters, integrated inductor, microchip antenna, and electromagnetic interference (EMI) devices have been required to have the characteristics such as miniaturization, high-speed, low energy-consumption, good electromagnetic wave absorption, and flexibility.^[1–5] There is a growing demand for the magnetic materials with high resonance frequency (f_r), large permeability (μ), and suitable magnetic loss in a GHz frequency range to be used as inductor cores, antenna substrates, and a wide range of other microwave devices in size reduction and minimizing the electromagnetic energy trapped in the devices.^[6] For example, magnetic materials can be used to reduce the volume of microchip antennas because it is roughly inversely proportional to the refractive index , in which and are the real part of permittivity and permeability, respectively.^[7,8] To fabricate the electromagnetic (EM) absorption material with large EM absorption ratio, the material should have a strong absorption capability and good impedance matching of permittivity and permeability. The high electronic resistivity of magnetic nanomaterials dispersed into a polymeric matrix will also reduce the eddy current loss significantly, and potentially increase the quality factor (Q factor) of a fabricated component. This opens the way for fabricating the miniaturized and flexible integrated antennas and inductors at high frequencies.^[9]

In addition, carbon nanotube (CNT)- and graphene-based composites have become new candidates for electromagnetic functional materials including microwave absorbing materials.^[10,11] Generally, the enhanced microwave absorption of these materials can be mainly attributed to the following aspects: the residual defects on the surface of the graphene oxide (GO) sheets, the large aspect ratio and high conductivity, interfacial polarizations in these composites. Good EM absorption properties of reduced graphene oxide (rGO) or its related materials have been demonstrated. Zhang et al. have reported that an ultralight and highly compressible graphene foam has a broadband and tunable high-performance microwave absorption property.^[12] Chen et al. have reported that novel reduced GO (RGO) composites with hematite nanoparticles embedded in RGO layers and wrapped by RGO sheets exhibited remarkably improved electromagnetic absorbing performance.^[13] Many other composites, such as rGO/α-Fe₂O₃,^[14] PVDF/GO,^[15] and rGO/ZnO hollow spheres^[16] have been reported as EM absorption materials. Though they usually have good EM absorption properties, poor dispersity and stability, and large loading content of GO or rGO, are the severe drawbacks to impede their practical applications. In addition, most of researchers use paraffin as matrix to measure the EM absorption, which cannot provide the practical EM parameters in the electronic devices to demonstrate their real EM absorption properties.

Here in this work, we prepare monodisperse Fe₃O₄ nanoparticles (NPs) and fabricate large-area rGO/Fe₃O₄ NPs/PU flexible film by using a facile solution-processable method. Fe₃O₄ NPs has recently attracted much attention due to its high Curie temperature ( ), high magnetic moment, and relatively good air-stability.^[17–20] PU is widely used in electronic area due to its good mechanical properties, anti-aging properties, and suitable dielectronic properties.^[21,22] GO and PU can sufficiently disperse in dimethylformamide and form the strong interfacial bonding among rGO, Fe₃O₄ NPs, and PU. The flexible composited film shows excellent microwave absorption properties due to the interfacial polarization, charge transfer effect, and good impedance matching.

The x-ray diffraction (XRD) data were collected on a D2 PHASER x-ray diffractometer (Cu radiation, λ = 0.154 nm) at 40 kV and 30 mA. The morphology of the sample was investigated by transmission electron microscopy (TEM, JEM-2100F with operation voltage 200 kV). Raman spectra were obtained with a Renishaw Raman system model in Via-Reflex spectrometer and a LabRam HR 800 spectrometer of HORIBA Co. The 532-nm radiation from a solid-state laser was used as an excitation source. For magnetic and dielectric spectrum measurements, the rGO/Fe₃O₄ NPs/PU film was processed into an annulus film with an outer diameter and an inner diameter . The scattering parameters (S₁₁ and S₂₁) were measured by a vector network analyzer (Agilent N5224A) through using a coaxial transmission–reflection method in a frequency range of 0.1 GHz–18 GHz. The complex permeability ( and permittivity ( were determined from the scattering parameters by using the Nicolson models.

Figure 1(a) shows the typical TEM images of few-layered GO prepared by a mild sonic exfoliation with the oxidization in advance to increase the interspace of graphite. The bright spots shown in the selected area electron diffraction (SAED) pattern indicate that the GO has a good crystalline structure after being exfoliated. The Fe₃O₄ NPs prepared by the high-temperature organic solution method have a narrow size distribution with an average diameter of 7.0 ± 0.5 nm as shown in Fig. 1(b). Such monodisperse Fe₃O₄ NPs with uniform size and shape can have uniform electromagnetic response. In Fig. 1(c), we can observe that the Fe₃O₄ NPs directly form monolayer assembly on the surface of GO sheets with a low cover density, when Fe₃O₄ NPs with a low concentration of 0.75 mg/mL in hexane were mixed with the DMF dispersion of GO sheets under a mild sonication. It should be noted that the multiply-layer assembly phenomena of Fe₃O₄ NPs on GO sheets can be observed by mixing the chloroform solution of Fe₃O₄ NPs and the DMF dispersion of GO sheets. It appeared that mixing two immiscible solutions in hexane and DMF via sonication was an essential step to realize monolayer assemble of Fe₃O₄ NPs on GO sheets.^[26] If the concentration of Fe₃O₄ NPs in hexane increases to 1.5 mg/mL, the monolayer assembly of Fe₃O₄ NPs with a high stacking density can be observed, which demonstrates that this method can realize the high-density loading on the GO sheets.

Figure 1.

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	Figure 1. TEM images of (a) GO sheet, with inset showing SAED pattern; (b) monodisperse Fe3O4 NPs; (c) low- and (d) high-stacking-density monolayer assembly of Fe3O4 NPs on GO nanosheet.

To improve the interface binding strength and permittivity properties, GO/Fe₃O₄ NPs are annealed at moderate temperatures to form rGO / Fe₃O₄ NPs. If being annealed at above 600 °C, Fe₃O₄ NPs will aggregate and result in a wide size distribution. As shown in Figs. 2(a) and 2(b), the GO/Fe₃O₄ NPs are annealed at 300 °C and 500 °C, respectively. After being annealed at 300 °C, the Fe₃O₄ NPs keep their size unchanged, but undergo a slight change in shape: from the sphere to polygon. Since the shape observed in TEM is a project two-dimensional (2D) image, the Fe₃O₄ NPs should change their shape from truncated icosahedron to polyhedron after being annealed at 300 °C. After being annealed at 500 °C, the Fe₃O₄ NPs shows their size obviously increases to 9.0 ± 0.5 nm with some big NPs and their shape also becomes polyhedron. Figure 2(c) shows, the XRD patterns of the as-prepared and annealed GO, together with the pristine graphite. The (002) peak intensity of graphite is much large and the other peaks cannot well be shown if the whole (002) peak is included in Fig. 2(c). The diffraction peak around 26.48 for graphite disappears in the pattern of GO and a broad peak is present at 2θ =10.98, indicating the successful oxidation of raw graphite into graphite oxide.^[25,27,28] After annealing, the diffraction peaks at 2θ=10.98 disappears and a broad peak forms and moves closer to 2θ=26.48 after 500 °C annealing than after 300 °C annealing. This indicates that the GO nanosheets can be reduced to rGO only by the simple high-temperature annealing. Figure 2(d) shows the Raman spectrum of the GO and rGO/Fe₃O₄ composites. There are two characteristic peaks located at around 1350 cm⁻¹ and 1597 cm⁻¹ which are attributed to the D and G band of GO. Comparing with pure GO, two weak peaks appear at 281 cm⁻¹ and 678 cm⁻¹ for the rGO/Fe₃O₄, which is due to the existence of Fe₃O₄. The intensity ratio of the D band to the G band for GO and rGO/Fe₃O₄ annealed at 300 °C and 500 °C are 0.92, 0.86, and 0.81, respectively, indicating that the annealing treatment can remote some oxygen-containing groups and reduce the disordered stacking of GO sheets.^[29] Meanwhile, the location of D and G band are observed to shift to the lower wavenumbers, recovering the hexagonal structures of carbon atoms. It should also be noted that the saturation magnetization (M_s) of rGO/Fe₃O₄NPs can be increased from 55 emu/g of the as-prepared sample to 72 emu/g of 500 °C annealed sample. The rGO/Fe₃O₄ NPs with higher can achieve larger since /2 showing that increases with M_s increasing. In the expression, K_u is the magnetic anisotropy.

Figure 2.

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	Figure 2. TEM images of rGO/Fe3O4 NPs annealed at (a) 300 °C and (b) 500 °C. (c) The XRD patterns of pristine graphite, as-prepared GO, GO annealed at 300 °C and 500 °C. (d) Raman spectra of GO, rGO/Fe3O4 NPs annealed at 300 °C and 500 °C.

Figure 3(a) shows the picture of the smooth thin PU film with a thickness of 0.5 mm and a length of 15 cm containing 5-wt% rGO/Fe₃O₄ NPs annealed at 300 °C. The thickness of the film can well be controlled in a range from 0.1 mm to 2 mm by adjusting the height of the scraper in our home-made linear glue spreading machine controlled with a precise stepping motor as shown in Fig. 3(b). The film has good flexible properties with a bending angle of 180° as shown in Fig. 3(c). Moreover, the film can be cut into different shapes, which is greatly convenient for the practical application.

Figure 3.

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	Figure 3. Pictures of (a) 15-cm-long rGO/Fe3O4 NPs/PU film, (b) our home-made scraping film facility, and (c) freely folding rGO/Fe3O4 NPs/PU film.

Figures 4(a) and 4(b) show the frequency dependence of the permittivity and permeability for the pure PU film and PU film containing 5 wt% rGO/Fe₃O₄ NPs annealed at 300 °C and 500 °C (named as rGO/Fe₃O₄ NPs/PU-300 and rGO/Fe₃O₄ NPs/PU-500). Figure 4(a) shows that the real part ( ) of relative complex permittivity ( ) for the pure PU film, rGO/Fe₃O₄ NPs/PU-300, and rGO/Fe₃O₄ NPs/PU-500 decrease from 4.3 to 3.0, 8.2 to 3.9, and 26.1 to 5.9, with the increasing frequency in the range of 0.1 GHz–18 GHz, respectively. The value of significantly increases with the rise of annealing temperature of GO in the composites because the higher temperature leads the quantity of GO to decrease more greatly, which results in the increase of the dipolar polarization and electrical conductivity. The value of the imaginary part ( ) corresponding to pure PU, rGO/Fe₃O₄ NPs/PU-300, and rGO/Fe₃O₄ NPs/PU-500 are in a range of 0.02–0.33, 1.69–0.73, and 19.26–2.14, respectively. The larger value of means that the better EM wave absorption property. From Fig. 4(b), it can be seen that the value of of PU is nearly a constant in the whole frequency range from 0.1 GHz to 18 GHz. The value of of rGO/Fe₃O₄ NPs/PU-500 is larger than that of rGO/Fe₃O₄ NPs/PU-300 below 0.5 GHz. While the rGO/Fe₃O₄ NPs/PU-300 shows that the value of decreases slower with frequency increasing to above 0.5 GHz and the cut-off frequency of about 5 GHz is higher than 3 GHz of rGO/Fe₃O₄ NPs/PU-500 whose value of decreases fast due to the large Fe₃O₄ particles sizes of rGO/Fe₃O₄ NPs/PU-500. According to our previous research,^[30] Fe₃O₄ NPs with large sizes have low blocking resonance frequency due to the superparamagnetic behavior. It should be noted that the value of of rGO/Fe₃O₄ NPs/PU film is 5 wt% larger than that of pure Fe₃O₄ NPs dispersed in the PU with the same mass ratio since the higher stacking density on the GO can achieve the strong dipolar–dipolar magnetic interaction.

Figure 4.

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	Figure 4. (a) Frequency-dependent relative complex permittivity and (b) frequency-dependent relative complex permeability. Frequency-dependent RL for the rGO/Fe3O4 NPs/PU films with different thicknesses, annealed at 300 °C (c) and at 500 °C (d).

According to the transmission line theory, for a single layer absorber with a backed metal plate, the refection loss (RL) curves are simulated from the electromagnetic parameters at various sample thickness values by means of the following expressions^[31,32]

Table 1.

Table 1.

Table 1. Comparison of EM absorption properties of rGO/Fe3O4 NPs/PU film with other reported results. . Materials Max. Max. Max. Max. Maximum RL/dB Ref. rGO/Fe2O3 11 at 2 GHz 3.5 at 2 GHz–16 GHz 1 near to 0 −38 dB at 14.8 GHz/2 mm; −25 dB at 4.5 GHz/5 mm with 70% weight ratio [14] N-doped GC 11.7 at 2 GHz 3.7 at 2 GHz–16 GHz 1 near to 0 −25 dB at 14.2 GHz/2 mm; −32 dB at 6.0 GHz/4 mm with 10% weight ratio [10] Hollow Fe@C 6 at 2 GHz 5.5 at 2 GHz–16 GHz 1.1 0.18 −54.4 dB at 8.8 GHz/4.5 mm; with 30% weight ratio [29] N-rGA/Ni 16 at 2 GHz 5 at 2 GHz–16 GHz 1 Less than 0 −30 dB at 12.0 GHz/2 mm; −20 dB at 4.0 GHz/5 mm with 10% weight ratio [30] rGO/Fe3O4 NPs/PU film 12.5 at 2 GHz 6.0–2.1 at 2 GHz–18 GHz 1.5 at 2 GHz 0.3 −17.5 dB at 16 GHz/2 mm; −35 dB at 4.5 GHz/5 mm with 5% weight ratio this work

Table 1. Comparison of EM absorption properties of rGO/Fe3O4 NPs/PU film with other reported results. .

In this work, monodisperse magnetic Fe₃O₄ NPs are prepared by a facile high-temperature organic solution method and loaded on the GO with a high monolayer stacking density. A simple and robust solution-processable method is used for preparing the rGO/Fe₃O₄ NPs/PU flexible films. The rGO and PU can form a strong hydrogen bonding due to a lot of hydroxy groups, which results in a well-dispersed rGO in PU instead of agglomeration or restack. The permittivity value of the composites in a frequency range 0.1 GHz–18 GHz increases with annealing temperatures of GO increasing. For 5-wt% rGO/Fe₃O₄ NPs/PU, the maximum RL of over −35 dB appears at 4.5 GHz when the thickness of film increases to 5 mm. The main microwave absorption mechanism of rGO/Fe₃O₄ NPs/PU film is dielectric loss produced by relaxation process that includes interfacial polarization and electronic dipole polarization between rGO and PU, and good impedance match from the introduce of permeability of Fe₃O₄ NPs. Such an rGO/Fe₃O₄ NPs/PU flexible film can be used as effective microwave absorption material in the future flexible electronic devices.