The microwave absorption technique is a key issue of modern research because of the requirement to reduce pollution to the environment and subsequent damage to human beings. Microwave absorption is a process in which the wave cannot permeate through or be reflected by a material.^[1] The absorption of the material depends on the frequency complex permittivity, complex permeability, and layer thickness.^[2] Magnetic losses and dielectric losses are two important electromagnetic properties of microwave absorbers. There are two ways to enhance the absorption of incident waves: one is to enhance magnetic losses and the other is to improve dielectric losses.^[3] Polyaniline (PANI) is a promising material for microwave absorption applications because it is inexpensive, environmentally stable, easy to process, and it exhibits excellent electrical conductivity.^{[4, 5]} However, a single polymer is not enough to produce larger electric or magnetic losses. Therefore, PANI-based composites, such as carbon nanotubes, carbon fibers, and graphene have been popularly synthesized to further develop the electrical conductivity.^{[6, 7]} Due to the high surface area, excellent conductivity, and remarkable mechanical stiffness, graphene is an outstanding carbon material that is used to form a composite with PANI.^{[8, 9]}

Mie theory (the classical electromagnetic theory of spherical particles) was developed nearly a century ago and it plays a crucial role in describing the optical properties of metal nanoparticles, especially silver and gold particles.^[10] Metallic nanoparticles exhibit strong and tunable surface plasmon resonance (SPR) when interacting with a microwave.^{[11, 12]} In small metallic nanoparticles, localized surface plasmon polaritons can be excited at discrete frequencies.^[13] This was experimentally demonstrated by Ritchie.^[14] Due to their unique optical and electrical properties, metallic nanoparticles can be used in biosensors, ^[15] surface-enhanced Raman spectroscopy (SERS), ^[16] and microwave absorption.^{[17, 18]} Peng et al. synthesized a core– shell structure by silver nanoparticles coated with Ni_0.5Zn_0.5Fe₂O₄ spinel ferrites, which exhibit good reflection losses (− 32.871  dB).^[2] Melvin et al. grafted silver nanoparticles onto the surface of carbon nanotubes, which also have good reflection loss (− 21.9  dB).^[19] Basavaraja et al. reported a polyaniline– gold/graphene oxide composite, whose electromagnetic interference shielding effectiveness is enhanced.^[18] However, the studies of gold nanoparticles for the applications in microwave absorbers are very limited.

In this research, we synthesize GO/PANI/GAu_nano (GPAu_nano) nanocomposite by using graphene oxide (GO) as a substrate to adsorb aniline monomer. GO/PANI/GAu_nano has a great improvement in relative complex permittivity and permeability compared with the GO/PANI nanocomposite. In our work, the maximum reflection loss of GPAu_nano is up to − 24.61  dB at 15.45  GHz, and the bandwidth corresponding to RL at − 10  dB can reach 4.08  GHz (from 13.92  GHz to 18.00  GHz) for a 2-mm-thick-layer.

Figure  1 shows the TEM images of GO, GP, and GAu_nano nanocomposites. Figure  1(a) is the TEM image of pure GO sheet. After composite reaction, aniline monomer is adsorbed on the surface of GO due to the electrostatic attraction, as shown in Fig.  1(b) and the inset image with high resolution. Fig.  1(c) shows that gold nanoparticles are adsorbed on the surface of graphene, which prevents their particles from being agglomerated. It can be seen in Fig.  1(d) that the gold nanoparticles are uniformly distributed on the composite. In addition, GO sheets are covered by PANI arrays evenly, indicating that the nucleation and growth processes occur only on the surface of the GO sheet.

Fig.  1.

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	Fig.  1. TEM images of GO (a), GP (b), inset image with a different resolution, GAunano (c), GPAunano (d) nanocomposites.

Figure  2 shows the FTIR spectra of GO, PANI, GP, and GPAu_nano nanocomposites. In the FTIR spectrum of GO, absorption peaks at 1732  cm^{− 1}, 1617  cm^{− 1}, 1225  cm^{− 1}, and 1053  cm^{− 1} correspond to C= O characteristic absorption of − COOH, O– H flexural vibrations attributable to the vibrations of the residual water, C– OH stretching vibration and C– O stretching vibration, respectively.^[22] In the spectrum of GP shown in Fig.  2, absorption peaks centering at 792  cm^{− 1}, 1123  cm^{− 1}, 1291  cm^{− 1}, 1486  cm^{− 1}, and 1557  cm^{− 1} are attributed to the flexural vibrations inside and outside aromatic C– H plane, C– N in PANI, the aromatic C= C stretching vibration, and the vibration of C= N respectively, ^[23] among which 1123  cm^{− 1} is the characteristic peak of conductive PANI. The characteristic peak of GO at 1732  cm^{− 1} and the disappearance of 1617  cm^{− 1} prove the polymerization of aniline monomers by the adsorption on the surface of GO and the participation of the surface functional groups of GO in the polymerization process. 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.

Figure  3 shows the XRD patterns of GO, GP, GPAu _nano nanocomposites. The XRD pattern of the GO shows a sharp peak at 2θ = 10.54° , corresponding to the (001) reflection of GO.^[22] In the cases of GP and GPAu_nano, diffraction peaks corresponding to both GP and Au can clearly be seen. The diffraction peaks at 9.41° , 14.91° , 20.72° , 25.55° are derived from PANI and those at 38.32° , 45.58° , 64.76° , 77.81° come from Au, ^[24] while the characteristic diffraction peak of GO disappears.

The microwave absorption property of material is generally determined by the complex relative permittivity and permeability. Figure  4 shows the values of complex relative permittivity real part (ε ′ ) (a), relative permittivity imaginary part (ε ′ ) (b), relative permeability real part (μ ′ ) (c), relative permeability imaginary part (μ ′ ) (d), dielectric loss tangent (tanδ _ε = ε ′ / ε ′ ) (e), and magnetic loss tangent (tanδ _μ = μ ′ / μ ′ ) (f), for GP and GPAu_nano. From Figs.  4(a) and 4(b), it can be seen that both ε ′ and ε ′ values of GP are higher than those of GPAu_nano in a range of 4  GHz– 18  GHz. But, ε ′ of GPAu_nano has a minimal peak at 16.3  GHz and ε ′ of GPAu_nano has a maximal peak at 15.45  GHz. The μ ′ of GPAu_nano has a maximal peak at 15.62  GHz as shown in Fig.  4(d). This shows that a resonance absorption is produced due to the introduction of gold nanoparticles.^[12] When gold nanoparticles absorb microwaves, a closely induced current will be produced inside the material, which is in accordance with the work of Peng et al.^[2] An electromagnetic field is then formed around the gold nanoparticles before the increasing electromagnetic loss. We can also see that the values of tanδ _μ are higher than the values of tanδ _ε are nearly at a frequency of 15.45  GHz. This indicates that the magnetic loss plays a main role in the microwave absorption nearly at the frequency of 15.45  GHz.

To further study the microwave absorption properties, the reflection loss (R_L) is used to represent the microwave absorbing properties. The R_L values of GP and GPAu_nano are calculated as follows:

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

In Fig.  5(a), it can be observed that the maximum R_L of GP is − 13.26  dB at 14.94  GHz and the bandwidth corresponding to R_L at − 10  dB can reach 2.72  GHz (from 13.92  GHz to 16.64  GHz) for a layer of 2  mm in thickness. But, for GPAu_nano (Fig.  5(b)), the maximum R_L value is − 24.61  dB at 15.45  GHz and the bandwidth corresponding to R_L at − 10  dB can reach 4.08  GHz (from 13.92  GHz to 18.00  GHz) for a layer of 2  mm in thickness. The GPAu_nano composite exhibit an excellent microwave absorption performance in comparison with GP (Fig.  5(a)), which is due to the microwave-induced resonance effect produced by gold nanoparticles that increases the microwave absorption and reduces the reflection losses.

Fig.  2.

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	Fig.  2. FTIR spectra of pristine GO, PANI, GP, and GPAunano nanocomposite.

Fig.  3.

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	Fig.  3. XRD patterns of pristine GO (with inset patterns), GP, and GPAunano nanocomposites.

Fig.  4.

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	Fig.  4. Complex relative permittivity real parts (a), relative permittivity imaginary parts (b), relative permeability real parts (c), relative permeability imaginary parts (d), values of dielectric loss tangent (e), and values of magnetic loss tangent (f) for GP and GPAunano.

Fig.  5.

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	Fig.  5. Reflection loss curves of GP (a) and GPAunano (b).

The excellent microwave absorption performance of the GO/PANI/ Au_nano nanocomposite is attributed mainly to the following crucial factors: gold nanoparticles can absorb microwaves and a closely induced current will be produced inside the material, this generates an electromagnetic field, ^[2] thus increasing the electromagnetic loss. Ultimately, the microwave absorption capacity of the material is improved.

The ternary composites of GO/PANI/GAu _nano are successfully prepared via a co-blend method. The obtained nanocomposite shows the enhanced microwave absorption properties compared with GO/PANI. The maximum R_L of a 2-mm-thick GO/PANI/GAu_nano layer is up to − 24.61  dB at 15.45  GHz, which is more than − 13.26  dB at 14.94  GHz of GO/PANI. The bandwidth corresponding to R_L of a 2-mm-thick GO/PANI/GAu_nano layer at − 10  dB can reach 4.08  GHz (from 13.92  GHz to 18.00  GHz). The introduction of gold nanoparticles significantly enhances the microwave absorption of the composite. This demonstrates that GO/PANI/GAu_nano can be a promising material with excellent microwave absorption properties.