All chemical reagents used in this study were analytical reagents and used without further purification. Aniline was distilled under the protection of high-purity N₂ and then kept in a refrigerator before using. Graphene– oxide was synthesized from natural graphite oxidation using the modified Hummers method.^[15]

First, the purified GO (0.027 g) was added into 75 ml of 1 mol/L aqueous HCL solution, and the mixture was ultrasonicated until GO was fully dispersed. Afterward, 25-ml ethanol was added into the reaction solution to keep the solution from freezing. Then aniline monomer was added into the above solution. The mixture was stirred for 1 h at − 10 ° C to form a uniform mixture. The oxidant, (NH₄)₂S₂O₈ (APS), was dissolved in 25-ml aqueous HCL solution (the molar ratio of aniline to APS is 1.5) and the mixture was cooled to − 10 ° C. The polymerization was performed by the rapid addition of the precooled oxidant solution, and the mixture was stirred for 24 h at − 10 ° C. Emerald flocculent precipitates were obtained and washed with a large amount of 1-mol/L HCL, ethanol, and DI water. The resulting precipitates were diluted into 100-ml DI water. For comparison, PANI was synthesized in the absence of GO via the procedure mentioned above.

The 0.029-g Fe₃O₄ (synthetized by the titration hydrolyzation method) was added into 10-ml of ethanol and ultrasonicated until Fe₃O₄ was fully dispersed, then mixed with the prepared GO/PANI solution. After mechanical stirring for 24 h, the product was dried at 40 ° C for 48 h under a vacuum.

The morphologies of the samples were characterized by scanning electron microscopy (SEM) (Hitachi S-4800), transmission electron microscopy (TEM) (JEM-1400 operating at 120 kV) and Fourier transform infrared spectroscopy (FTIR) (Thermo Fisher Nicolet 6700). The crystallographic structures of the samples were determined by X-ray diffraction (XRD) (BRUKER) equipped with Cu Kα radiation (k = 0.15406 nm). The electromagnetic (EM) parameters of the composite were measured by an HP8722ES network analyzer.

Figure 1 shows the SEM images of GO, GP, and GPF nanocomposites. GO shows a paper-sheet-like shape. While GP nanocomposites exhibit multiple shapes, mainly flakes along with some fibrillar morphologies, and the sheets are thicker. Aniline monomers are adsorbed on the surfaces of GO flakes due to the electrostatic attraction.^[13] With the polymerization reactions proceeding on the surfaces of GO flakes, the resulting nanocomposites display a flaky structure. PANI arrays are evenly covered on GO sheets, indicating that the nucleation and growth processes occur only on the surfaces of GO sheets. During the growth stage of PANI on the GO sheets, the linear nature and expanded chain structure of the PANI enable the PANI chains to act as a molecular template; ^[16] this structure could shorten the electronic conductive pathways. As shown in Fig. 1(c), Fe₃O₄ particles scatter on the surfaces of GO/PANI nanocomposites. These images above confirm that the ternary nanocomposites are synthesized successfully.

Fig. 1.

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	Fig. 1. SEM images of GO (a), GO/PANI (b), GPF (c) nanocomposites.

The XRD patterns of GO, GP, Fe₃O₄, and GPF nanocomposites are presented in Fig. 2. The XRD pattern of GO shows an intense, sharp peak that corresponds to the (0 0 1) plane at 2θ = 10.49° , corresponding to the interlayer spacing of 0.843 nm between GO sheets. In the case of GP, peaks at 9.4° , 14.5° , 20.5° , 25.3° corresponding to (0 0 1), (0 1 1), (0 2 0), and (2 0 0) crystal planes are the characteristic peaks of PANI in its emeraldine salt form, ^[17] while the characteristic diffraction peak of GO disappears, implying the full use of the GO in PANI as the substrate. In the case of Fe₃O₄, a series of diffraction peaks at 2θ = 30.4° , 35.9° , 43.4° , 53.5° , 57.6° , and 63.2° are attributed to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) reflections.^[4] In the XRD pattern of GPF, the diffraction peaks of GP, the diffraction peaks of Fe₃O₄ particles can also be observed, indicating that the Fe₃O₄ particles exist in the nanocomposites.

Fig. 2.

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	Fig. 2. Typical XRD patterns of (a) Fe3O4, GP, GPF, and (b) GO.

The composite structure is further indicated by spectroscopic measurement. In the FTIR spectrum of GO, absorption peaks emerge at 1732 cm^{− 1}, 1617 cm^{− 1}, 1225 cm^{− 1}, and 1053 cm^{− 1}, corresponding to C= O characteristic absorptions in the – COOH, O– H flexural vibrations attributable to the vibrations of the residual water, C– OH stretching vibration and C– O stretching vibration, respectively, ^[18] which are consistent with previously reported results. In the spectrum of PGO shown in Fig. 3, absorption peaks centered at 792 cm^{− 1}, 1121 cm^{− 1}, 1290 cm^{− 1}, 1482 cm^{− 1}, and 1556 cm^{− 1} are attributed to the flexural vibrations inside and outside the aromatic C– H plane, C– N in PANI, the aromatic C= C stretching vibration, and the vibration of C= N respectively, ^[18] among which 1121 cm^{− 1} is the characteristic peak of conductive PANI. That the characteristic peaks of GO at 1732 cm^{− 1} and 1617 cm^{− 1} disappear proves not only that aniline monomers are polymerized by adsorption on the surface of GO, but also that the surface functional groups of GO participate in polymerization. It is possible that – COOH on the surface of GO, as a kind of proton doping agent, could be connected to N atoms in PANI.^[19] In the case of GPF, all the characteristic peaks of PANI are found, proving that Fe₃O₄ particles are successfully dispersed into GO/PANI.

Fig. 3.

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	Fig. 3. FTIR spectra of pristine GO, PANI, GP, and GPF nanocomposite.

The reflectivity R is used to represent the microwave absorbing property. The reflection losses (RLs) of GO, PANI, PGO, and PGF each can be calculated from

In the above equations, ε _r and μ _r are the relative complex permittivity and permeability respectively, f is the microwave frequency, d is the layer thickness, and c is the velocity of microwave in free space.^[7]

In Fig. 4(a), it can be observed that the maximum R_L of GO is only − 3.4 dB at 9.3 GHz when its thickness is 5.5 mm. For PANI in Fig. 4(b), the maximum R_L values are less than − 10.6 dB when the thickness values range from 0.5 mm to 5.5 mm. As shown in Fig. 4(c), the maximum R_L of GO/PANI is − 14 dB at 15.8 GHz when the thickness is 5.5 mm. For GO/PNAI/Fe₃O₄ in Fig. 4(d), it is noted that there is one sharp and strong wave absorption peak at 14 GHz, the maximum R_L is up to − 27 dB with its thickness being 2 mm, and the absorption bandwidths exceeding − 10 dB are more than 11.2 GHz (from 6.8 GHz to 18 GHz) with thickness values being in the range 1.5 mm– 4 mm. The results demonstrate that GO/PNAI/Fe₃O₄ exhibits excellent microwave absorption properties than . , and GN/Fe₃O₄.^[7]

Fig. 4.

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	Fig. 4. Reflection loss curves of GO (a), PANI (b), GP (c), and GPF (d).

In order to investigate the possible microwave absorption mechanism of the composite sample, the values of complex permittivity real part (ε ′ ), permittivity imaginary part (ε ″ ), dielectric loss tangent (tanδ _ε = ε ″ /ε ′ ), and magnetic loss tangent (tanδ _μ = μ ″ /μ ′ ) of GO/PANI/Fe₃O₄ are shown in Figs. 5(a)– 5(c). We also investigate the permeability real part (μ ′ ) and imaginary part (μ ″ ) to compare the magnetic properties with those of GO, PANI, and GP, and they are shown in Fig. 5(d) and Fig. 5(e).

Fig. 5.

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	Fig. 5. Variations of complex relative permittivity (a), permeability (b), and the loss tangent (c) with frequency of GO/PANI/Fe3O4; comparisons of permeability real part (d) and imaginary part (e) among GO, PANI, GP, and GPF.

In Fig. 5(a), it is clear that the ε ′ decreases from 9.5 to 7.0 and the ε ″ increases from 1.35 to 3.2 with several fluctuations in the frequency range of 1 GHz– 18 GHz, which is due to electric dipolar polarization. This result suggests that the GO/PNAI/Fe₃O₄ composites have dielectric loss properties. In Fig. 5(b), the μ ′ values are in the range 1.0– 1.1 and the μ ″ values are less than 0.2 in the frequency range 1 GHz– 18 GHz. Moreover, from Fig. 5(c), it can be observed that tanδ _μ and tanδ _ε values each have a similar increase trend over the frequency range 1 GHz– 18 GHz and these tendencies are very close to each other, indicating an improved impedance matching. As is well known, the μ ′ and μ ″ show the ability to indicate the magnetic energy loss. It is noticed from Figs. 5(d) and 5(e) that the μ ′ and μ ″ of GPF are slightly higher than those of GO, PANI, and GP, indicating a better impedance matching between dielectric and magnetic loss.^[20]

Firstly, the oxygen functional groups at the interface of GO and the interaction between π − π * promote the adoption of aniline monomer, which is conducive to the transmission electron and the improvement of conductivity.^[13] Secondly, GO and PANI are dielectric loss absorbers, ^{[21, 22]} and Fe₃O₄ particles are magnetic loss absorbers.^[23] The enhanced impedance matching has a significant effect on improving the microwave absorption properties.^[24] Thirdly, electronic spin and charge polarization play an important role in the microwave absorption properties due to the electronic transmission between Fe²⁺ and Fe³⁺ ions.^[2]