Facile Fabrication of Three-Dimensional Lightweight RGO/PPy Nanotube/Fe3O4 Aerogel with Excellent Electromagnetic Wave Absorption Properties.

In this article, a three-dimensional chemically reduced graphene oxide/polypyrrole nanotubes (PPy nanotubes)/Fe3O4 aerogel (GPFA) was fabricated by a simple one-step self-assembly process through hydrothermal reduction. The addition of both PPy nanotubes and Fe3O4 nanoparticles is aimed to avoid the aggregation of graphene sheets, effectively adjust the permittivity, and make better impedance matching between dielectric loss and magnetic loss of the composite aerogel to gain excellent electromagnetic (EM) wave absorption performance. The EM wave-absorbing results indicate that the ternary composite with an ultralow density of about 38.3 mg/cm3 shows an improved EM wave-absorbing property with a maximum reflection loss of -49.2 dB at the frequency of 11.8 GHz, with an effective absorption bandwidth below -10 dB reaching 6.1 GHz (9.8-15.9 GHz) at a thickness of 3.0 mm. Such an outstanding EM wave absorption behavior can be attributed to the multiple reflections, polarizations, and relaxation processes in the aerogel.

Introduction

Electromagnetic (EM) wave-absorbing materials are becoming significantly important because of the increasing use of electronic devices.[1,2] Nowadays, EM wave-absorbing materials with intense absorption ability, broad absorption bandwidth, ultralight weight, and thin thickness are in high demand.[3,4] Recently, n class="Chemical">graphene has been widely used as an EM wave-absorbing material because of its excellent properties, such as low density, high specific surface area, large aspect ratios, and versatile processing.[5,6] According to the EM energy conversion principle, a proper matching between the dielectric loss and the magnetic loss determines the reflection and attenuation characteristics of EM wave absorbers. Nevertheless, graphene is found to be nonmagnetic, and the EM wave absorption properties are mostly attributed to the dielectric loss. Moreover, the EM wave absorption of graphene materials can also be limited by their improper electrical conductivity. Therefore, composites of graphene with other lossy materials have been widely explored as an approach to enhance the EM wave-absorbing performance.[7−10]

One of the effective ways to solve this problem is to couple n class="Chemical">graphene with magnetic materials, such as Fe3O4,[11−13] Co3O4,[14] and NiFe2O4.[15] In addition, conducting polymers with excellent physical and chemical properties can also be used as EM wave absorbers. Over the past decades, considerable attention has been paid to the EM wave absorption properties of conducting polymers.[16−18] Polypyrrole (PPy), one of the most extensively studied conducting polymers, has attracted great interest in EM wave absorber construction for its lightweight, high electrical conductivity, and favorable physicochemical properties.[19−21] Recently, the ternary composites of graphene/Fe3O4/polyaniline (PANI),[22] reduced graphene oxide (RGO)–PPy–Co3O4,[23] and RGO–poly(3,4-ethylenedioxythiophene) (PEDOT)–Co3O4[18] have been studied because of their multifunctional EM wave dissipation paths. However, the fabrication process is complicated, and there may be aggregation problem because of the strong π–π interaction between graphene sheets in the reduction process. The aggregation can reduce the surface area and decrease the conductivity of graphene and thus lead to inferior EM wave-absorbing properties.

Construction of a three-dimensional (3D) graphene network from individual n class="Chemical">graphene sheets paves a new way to address the aggregation problems.[24−26] Furthermore, 3D graphene still maintains super properties of graphene sheets, which will guarantee the promising applications including EM wave absorption. For instance, Wang et al. fabricated graphene aerogels (GAs) constructed from interconnected graphene nanosheet-coated carbon fibers via a simple dip-coating method, and the composite aerogels showed a minimum reflection loss (RL) value of −30.53 dB at 14.6 GHz with the bandwidth of RL less than −10 dB of 4.1 GHz at a thickness of 1.5 mm. Song et al. prepared a 3D hierarchical RGO foam decorated with in situ grown ZnO nanowires (ZnOnws), and the ZnOnws/RGO foam/polydimethylsiloxane (PDMS) composite with 3.3 wt % absorber loading attained a minimum RL value of −27.8 dB at 9.57 GHz at a thickness of 4.8 mm, possessing a wide effective absorption bandwidth of 4.2 GHz covering the whole X band. In this paper, the vacuum infusion method was employed to transfer wax into 3D RGO/PPy nanotube/Fe3O4 networks to avoid damaging the 3D structure, as for instance when mechanically mixing fillers in a matrix. Meanwhile, it can be anticipated that the introduction of PPy nanotubes can not only act as a spacer to avoid the stacking of graphene sheets in the reduction process but also adjust the dielectric properties of GA. Moreover, the addition of Fe3O4 nanoparticles is aimed to make better impedance matching between the dielectric loss and the magnetic loss. What is more, the RGO/PPy nanotube/Fe3O4 aerogels (GAFAs) can be facilely fabricated through a simple one-step self-assembly process through hydrothermal reduction. It shows promising EM wave-absorbing performance with an ultralow density of 38.3 mg/cm3, and the results and mechanisms of microwave absorption of the composite aerogel are discussed in detail in this paper.

Experimental Section

Materials

GO was prepared by a modified Hummers’ method.[27] All the chemicals and reagents were of analytical grade and were used as received without any purification. Deionized water was produced in our laboratory and was used for all experiments.

Synthesis of Fe3O4 Nanoparticles

The Fe3O4 nanoparticles were prepared according to the procedure reported in refs (28) and (29). In detail, 5.2 g of n class="Chemical">FeCl3 (32 mmol) and 3.18 g of FeCl2·4H2O (16 mmol) were dissolved into 25 mL of deionized water with 0.86 mL of HCl, and then the mixture was added dropwise into 250 mL of 1.5 M NaOH aqueous solution under vigorous stirring. A black precipitate was obtained by centrifugation at 6000 rpm (3 min), and then the precipitate was washed three times with deionized water. Subsequently, 250 mL of a 0.01 M HCl solution was added to neutralize the anionic charges on the nanoparticles. The cationic Fe3O4 nanoparticles were collected by centrifugation at 10 000 rpm (10 min) and then freeze-dried for 48 h.

Preparation of PPy Nanotubes

PPy nanotubes were prepared according to the procedure reported in the lietraure.[3,30] In a typical procedure, FeCl3 (1.944 g) was dissolved into 250 mL of MO (sodium 4-[4′-(dimethylamino)phenyldiazo]phenylsulfonate (CH3)2n class="Chemical">NC6H4N=NC6H4SO3Na) (0.392 g) deionized water solution. A flocculent precipitate appeared immediately. Then, a certain amount of pyrrole monomer (0.84 mL) was added dropwise into it, and the mixture was softly stirred at room temperature for 24 h. The as-prepared precipitate was obtained by filtering and washed several times with ethanol and water alternately to remove impurities. After that, the product was freeze-dried for 48 h and then dried at 60 °C for 12 h to finally obtain the PPy nanotubes.

Fabrication of RGO/PPy Nanotube/Fe3O4 Aerogels (GPFAs)

As schematically shown in Figure , the GPFAs were prepared by a simple one-step self-assembly process through hydrothermal reduction. First, a certain amount of PPy nanotubes (0.12 g) was added into 20 mL of GO solution (6 mg/mL), followed by vigorous stirring for 6 h. Then, the prepared n class="Chemical">Fe3O4 nanoparticles (0.12 g) were added into the mixture and stirred for another 6 h to form a uniform compound. After dropping hydrazine hydrate (300 μL) into the above suspension, the composite was sealed into a 25 mL glass vial and placed in an oven heated to 95 °C for 12 h. Subsequently, the resulting hydrogel was taken out and washed several times with ethanol and deionized water to remove impurities and then freeze-dried for 48 h to obtain the GAFA. For comparison, GA, RGO/Fe3O4 aerogel (GFA), and RGO/PPy nanotube aerogel (GPA) were synthesized under the same condition as described above. The calculated densities of GA, GFA, GPA, and GPFA were approximately 16.6, 31.2, 29.0, and 38.3 mg/cm3, respectively.

Figure 1

Schematic fabrication process of GPFA.

The paraffin wax with a melting point of about 52–54 °C was used as the matrix. The 3D n class="Chemical">GPFA/wax samples were prepared by a vacuum-assistant filling method. First, the paraffin wax was heated to 90 °C and completely melted. Then, the 3D GPFA was immersed into the melted wax until the network was saturated with liquid wax. After that, the sample was put under vacuum for 10 min to remove the air bubbles trapped in the composites. Finally, the 3D GPFA/wax samples were cooled down and kept at room temperature for 5 h. For GA, GFA, GPA, and GPFA composites, their weight ratios were calculated as 1.81, 3.35, 3.12, and 4.08 wt %, respectively.

Characterization and Testing

X-ray diffraction (XRD) spectra were acquired by D/MAX2550/PC using Cu Kα radiation from 8° to 70° at a scan rate of 5°·min–1 under 35 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) was recorded using a Kratos AXIS Ultra DLD spectrometer. Raman spectra were taken on a SENTERRA R200 Raman spectrometer with a 532 nm laser excitation. n class="Chemical">Nitrogen absorption and desorption measurements were performed with an Autosorb iQ instrument. The surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. Fourier transform infrared (FT-IR) spectroscopy was carried out by an FTIR spectrophotometer (Bruck EQUINOX55) using the KBr method over a frequency range of 400–4000 cm–1. The morphology of samples was characterized by the transmission electron microscope (JEM-2100F, JEOL, Tokyo, Japan) and the field-emission scanning electron microscope (FEI-Sirion 200). The composite specimens prepared for EM wave absorption measurement were toroidal-shaped samples with an outer diameter of 7.00 mm and an inner diameter of 3.00 mm. The complex permittivity and permeability values were recorded using an Agilent 85050D vector network analyzer in the frequency range of 2–18 GHz.

Results and Discussion

The morphology and structure of the fabricated composite aerogels were observed by scanning electron microscope (SEM) and transmission electron microscopy (TEM), respectively, as shown in Figures and 3. Figure a shows the spectra of the prepared GPFA, which can be fabricated into any shape to meet the specific requirement in practical applications depending on the vessels used. We have tested the mechanical property of the composite aerogel, as shown in Figure S1. The results indicate that it can be at least 153 times larger than its own weight, thus resulting in good compression strength. Moreover, the magnetic property of the n class="Chemical">GPFA composite was also tested, and the results are shown in Figure S3. It can be observed that the as-prepared PPy nanotubes exhibit a diameter of about 50–100 nm with a length of 5–10 μm (Figures b and 3a). In Figure d, it can be found that the prepared GPFA exhibits a 3D interconnected porous network structure with an average pore diameter of about several to tens of micrometers. The PPy nanotubes are uniformly attached on the surface of RGO sheets, and meanwhile, some are dispersed between RGO sheets, which is helpful to avoid the aggregation of RGO sheets in the hydrothermal reduction process (Figures c,e and 3c). The Fe3O4 nanoparticles with some clusters are anchored homogeneously on the surface of RGO sheets and PPy nanotubes with a diameter of about 10–30 nm (Figures e and 3b–d). To verify the crystalline structure of Fe3O4 particles, high-resolution TEM (HRTEM) image of GPFA is presented in Figure d. It is clear that the lattice fringe space of Fe3O4 particles is about 0.253 nm, corresponding to the (311) plane of Fe3O4, which is in agreement with the XRD analysis, and the selected area electron diffraction pattern (inset in Figure d) further demonstrates their polycrystalline feature. The 3D porous structure and the uniform distribution of PPy nanotubes and Fe3O4 nanoparticles can be favorable for the composite aerogel’s EM wave absorption behavior.

Figure 2

(a) Photo of the fabricated GPFA; the SEM images of (b) PPy nanotubes, (c) GPA, and (d,e) GPFA; (f) XRD spectra of different samples.

Figure 3

TEM images of (a) PPy nanotubes, (b) GFA, and (c) GPFA and HRTEM image of (d) Fe3O4 nanoparticles.

(a) Photo of the fabricated GPFA; the SEM images of (b) PPy nanotubes, (c) GPA, and (d,e) n class="Chemical">GPFA; (f) XRD spectra of different samples.

TEM images of (a) PPy nanotubes, (b) GFA, and (c) n class="Chemical">GPFA and HRTEM image of (d) Fe3O4 nanoparticles.

The structures of the synthesized GO, RGO, PPy nanotubes, Fe3O4 nanoparticles, and n class="Chemical">GPFA were investigated by XRD, as shown in Figure f. A broad peak at about 25° is observed for RGO, when compared with GO, whose XRD peak appears at about 10.3°, which indicates that GO has been effectively reduced to graphene.[31,32] The synthesized PPy nanotubes possess an amorphous structure showing a broad XRD peak around 21.8°.[17,21] For Fe3O4 nanoparticles, diffraction peaks at 30.21, 35.61, 43.31, 57.31, and 62.91 are observed and can be assigned to the (220), (311), (400), (511), and (440) planes of Fe3O4, which is similar to the standard pattern of Fe3O4 (JCPDS card no. 19-0629).[33] The relatively weak intensity demonstrates that the prepared Fe3O4 particles are small. For GPFAs, the Fe3O4 XRD peaks can be found as described above, and only a slight broad peak for RGO and PPy can be observed, which may be due to their relatively low diffraction intensity.

Figure a shows the XPS spectra of GPFA, indicating the presence of C, n class="Chemical">N, O, and Fe elements in the composite aerogel. Figure b shows the C 1s XPS spectra of GO and RGO. For GO, four different peaks centered at 284.8, 286.6, 287.6, and 289.1 eV, which correspond to C=C/C–C, C–O, C=O, and O–C=O, respectively, can be observed, and for RGO, the intensity of the oxygen-containing peaks greatly decreases, demonstrating a considerable reduction of GO.[34] Meanwhile, a new peak centered at 285.7 eV, which is attributed to the C–N group, can be found, suggesting the presence of PPy nanotubes.[23] The N 1s peak can be divided into three Gaussian peaks with binding energies of 398.2, 399.7, and 401.4 eV, respectively (Figure c), which are from the neutral and imine-like structure (−C=N−), the neutral and amine-like structure (−N–H−), and positively charged nitrogen atoms (−NH+−) of the PPy nanotubes as described in the literature.[18,33,35] As shown in Figure d, the core-level binding energies at 709.8 and 723.9 eV are the characteristic doublets of Fe 2p3/2 and Fe 2p1/2, respectively.[33] These results indicate the successful fabrication of GPFAs.

Figure 4

(a) Full XPS spectrum and (b) C 1s, (c) N 1s, and (d) Fe 2p XPS core-level spectra of GPFA.

(a) Full XPS spectrum and (b) C 1s, (c) N 1s, and (d) n class="Chemical">Fe 2p XPS core-level spectra of GPFA.

The Raman spectra of RGO and GPFA are shown in Figure a. For the n class="Chemical">GPFA, the characteristic Raman scattering peak for Fe3O4 powder appears at around 673 cm–1 (inset figure), which corresponds to the A1g mode.[29] For PPy nanotubes, the broad peak obtained near 1065 cm–1 corresponds to the C–H in-plane deformation and two small peaks near 921 and 980 cm–1 are associated with the quinoid polaronic and bipolaronic structure, indicating the successful formation of PPy.[23,41] The peaks at 1343 and 1578 cm–1 are the characteristic peaks of the D and G bands from graphene, which are associated with disordered carbon and the sp2-hybridized carbon, respectively.[23,41] Compared to the pure RGO aerogel, the shift of the peaks can be found for both D and G bands, indicating a significant charge transfer between the graphene nanosheets and Fe3O4 nanoparticles.[29] Moreover, the enhanced intensity of GPFA for the G band around 1575 cm–1 reflects the interaction between the π-conjugated nanotubes and aromatic graphene basal plane.[23,41] The interactions among graphene sheets, PPy nanotubes, and Fe3O4 nanoparticles are helpful in improving its EM wave-absorbing performance.

Figure 5

(a) Raman spectra of RGO and GPFA and (b) FT-IR spectra of GO, RGO, PPy nanotube, Fe3O4 nanoparticle, and GPFA.

(a) Raman spectra of RGO and GPFA and (b) FT-IR spectra of GO, RGO, PPy nanotube, n class="Chemical">Fe3O4 nanoparticle, and GPFA.

The FT-IR spectra of the prepared GO, RGO, PPy nanotubes, Fe3O4 nanoparticles, and n class="Chemical">GPFA are shown in Figure b. For pure GO, the curve is very similar to that obtained in the previous work,[23,36] and the peaks centered at 1729, 1223, and 1053 cm–1 are associated with group C=O, epoxy C–O, and alkoxy C–O stretching vibration, respectively. The broad band of −OH stretching vibration appearing at 3419 cm–1 can be attributed to the vibration of the adsorbed water molecules. For RGO, most of the peaks related with the oxygen-containing functional groups disappear, indicating an effective reduction of GO. For PPy nanotubes, the strong peaks near 1314 and 1038 cm–1 are ascribed to C–N stretching and C–H deformation vibrations, and the peaks centered at 1181 and 912 cm–1 indicate the doping state of PPy.[17,23] For Fe3O4, the characteristic absorption peaks at 633 and 582 cm–1 corresponding to the Fe–O bond can be observed.[37,38] The main peaks of PPy and Fe3O4 can all be found in the spectra of GPFA, indicating the successful preparation of GPFA. Furthermore, for Fe3O4, the corresponding peaks are upshifted about 33 and 36 cm–1 and for the PPy nanotube, the corresponding peaks are upshifted about 24, 7, 23, and 27 cm–1. This phenomenon is probably attributed to the π–π interactions and hydrogen bonding among the RGO, aromatic PPy rings, and the Fe3O4 nanoparticles, which is beneficial for the enhancement of the composite aerogel’s EM wave absorption property.

The porous structure of the RGO aerogel and GPFA is further validated by n class="Chemical">nitrogen physisorption measurements, and the results are shown in Figure . Clearly, for both GA and GPFA, the N2 adsorption–desorption isotherms exhibit the type II hysteresis loop, characteristic to pores with different pore sizes.[39,40] The BET surface areas of the GA and GPFA are determined to be 149 and 305 m2 g–1, respectively. Moreover, the pore size distribution of GA calculated by the density functional theory method indicates a large proportion of mesopores with distribution from 2.5 to 20 nm with a peak pore diameter of approximately 2.8 nm (Figure b). For GPFA, the relative quantity of pore diameter exceeding 3 nm is largely increased with a maximum pore diameter of about 2.7 nm. It can be found that with the addition of PPy nanotubes, GPFA possesses larger surface area and pore diameter compared with GA, which proves that the PPy nanotubes can act as a spacer between graphene sheets and could effectively reduce the aggregation of graphene sheets during the chemical reduction process.

Figure 6

(a) Nitrogen adsorption and desorption isotherm and (b) pore size distribution plot of GA and GPFA.

(a) Nitrogen adsorption and desorption isotherm and (b) pore size distribution plot of n class="Chemical">GA and GPFA.

As is well-known, the EM wave-absorbing properties of the materials can be estimated by the RL curves. According to the classical transmission line theory, the RL values can be calculated by the following equations:[2]where c is the velocity of light, d is the thickness of the absorber, f is the wave frequency, and εr and μr are the measured relative complex permittivity and permeability of the composite absorber, respectively.[26]

Figure a–d shows the RL curves of GA, n class="Chemical">GFA, GPA, and GPFA with different thicknesses at 2–18 GHz. The maximum RL of the samples obviously shifts to lower frequency with the thickness increasing from 2.0 to 5.0 mm. As is known, the matching conditions are given as follows:where dm is the best matching thickness, c is the velocity of light in free space, and fm is the best matching frequency. Therefore, the EM wave absorption properties can be tuned by adjusting the thickness of the samples according to specific applications.[3] In Figure , the maximum RL values of GA, GFA, GPA, and GPFA are −37.8 dB at 7.8 GHz with a thickness of 4.5 mm, −47.1 dB at 8.15 GHz with a thickness of 4.5 mm, −40.2 dB at 9.35 GHz with a thickness of 3.5 mm, and −49.2 dB at 11.8 GHz with a thickness of 3 mm, and moreover, the bandwidths of RL below −10 dB are 3.7, 4.3, 3.95, and 6.1 GHz in the range of 6.2–9.9, 6.65–10.95, 7.75–11.7, and 9.8–15.9 GHz, respectively. It can be found that the addition of both Fe3O4 nanoparticles and PPy nanotubes is helpful to improve the EM wave-absorbing property of the RGO aerogel, and furthermore, the ternary GPFA exhibits the best EM wave absorption performance because of the introduction of multiple dissipation ways. For comparison, we crushed the GPFA into powders and mixed with wax to obtain the sample GPFA-S, which possessed the same mass ratio as the sample GPFA. The EM wave absorption properties of the sample GPFA-S were tested, and the results are shown in Figure S2. In addition, the EM wave-absorbing behavior of GPFA is compared with other previous works as listed in Table , and the results indicate that the composite aerogel can be used as good EM wave absorption materials.

Figure 7

RL curves of samples (a) GA, (b) GFA, (c) GPA, and (d) GPFA at different thicknesses from 2 to 5 mm in the frequency range of 2–18 GHz.

Table 1

Microwave Absorption Properties of Typical Materials Reported in This Work and Recent Reports

graphene-based composites	mass ratio (wt %)	thickness (mm)	max RL (dB)	RL below –10 dB (GHz)	performance from fillers or composites	refs
GN/PPy/Fe3O4		5.3	–56.9	about 3.0	filler	(33)
PPy–RGO–Co3O4	50 (wax)	2	–33.5	about 6.5	filler	(23)
RGO–PANI–Co3O4	50 (wax)	3.3	–44.5	4.3	filler	(18)
RGO–PPy–Co3O4		3.2	–43.5	6.4
RGO–PEDOT–Co3O4		3.1	–46.5	2.1
RGO–Co3O4	50 (wax)	3.3	–43.7	4.6	filler	(14)
3D graphene/Fe3O4	10 (wax)	4.0	–27.0	4.8	filler	(1)
RGO/porous Fe3O4/PANI	30 (wax)	1.0	–29.5	4.2	filler	(7)
GA	20 (wax)	1.5	–30.53	4.1	filler	(24)
PEDOT–GN–NiFe2O4	50 (wax)	2	–45.4	4.6	filler	(9)
RGO/MCNTs/Fe3O4	8 (wax)	2	–36	11.4 (2–4 mm)	filler	(42)
ZnOnws/RGO foam	3.3 (PDMS)	4.8	–27.8	4.2	composite	(25)
GPFA	4.08 (wax)	3	–49.2	6.1	composite	this work

RL curves of samples (a) GA, (b) n class="Chemical">GFA, (c) GPA, and (d) GPFA at different thicknesses from 2 to 5 mm in the frequency range of 2–18 GHz.

As described in the above equations, the RL is significantly influenced by relative complex permittivity (εr) and permeability (μr) of the test sample. To investigate the possible EM wave absorption mechanism of the above samples, we measured the relative complex permittivity and permeability of the samples, and the variation curves of complex permittivity real parts (ε′) and imaginary parts (ε″) and complex permeability real parts (μ′) and imaginary parts (μ″) are shown in Figure . As shown in Figure a, for samples n class="Chemical">GA, GFA, GPA, and GPFA, the values of ε′ decrease with increasing frequency from 8.38 to 3.22, 7.46 to 3.62, 9.42 to 4.56, and 8.66 to 4.02. For GPFA, the addition of highly conductive PPy nanotubes can obviously increase the ε′ values when compared with GA and GFA. Moreover, compared to GPA, the addition of Fe3O4 nanoparticles can decrease GPFA’s ε′ value to some extent, which is helpful to improve the impedance match and lead to weak reflection and good absorption.[23,33] In Figure a, the aerogels exhibit similar ε″ values, which decrease with increasing frequency with fluctuations between 4.36 and 1.84 in the frequency range of 2–18 GHz.

Figure 8

(a) Real part (εr′) and imaginary part (εr″) of the complex relative permittivity, (b) real part (μr′) and imaginary part (μr″) of the complex relative permeability, and (c) the loss tangent of different aerogels in the range of 2–18 GHz.

As far as we know, there are two types of contributions for EM wave absorption: dielectric loss and magnetic loss.[14]Figure b shows the real and imaginary parts of the complex permeability measured for the aerogels. For all samples, the values of both the real and imaginary parts of the complex permeability are small when compared with complex permittivity, indicating minor magnetic loss contributions to the EM wave absorption observed above.[18,23] On the basis of the EM parameters, the dielectrical dissipation factor (tan δε) and the magnetic dissipation factor (tan δμ) could be calculated. Generally, apart from dielectric loss and magnetic loss, another important concept relating to excellent EM wave absorption is the efficient complementarity between the relative permittivity and permeability.[23,33] As shown in Figure c, the ternary GPFA consisting of RGO sheets, PPy nanotubes, and n class="Chemical">Fe3O4 nanoparticles shows better impedance matching between the dielectric loss and the magnetic loss than other samples, which leads to best EM wave absorption properties. To further study the EM wave absorption performance of GPFAs, samples with different amounts of PPy nanotubes were prepared, and their EM wave absorption properties are compared with GPFA, as shown in Figures S4 and S5.

The incident EM wave is dissipated into heat by the 3D porous GPFA, as shown schematically in Figure . The 3D porous structure of the aerogel can easily trap the incident EM wave inside and make a longer propan class="Chemical">gation path in the absorber by multiple reflections and more scattering among the graphene sheets, PPy nanotubes, and Fe3O4 nanoparticles, which results in more dissipation of EM energy.[2,3,17,20] In addition, the introduction of PPy nanotubes and Fe3O4 nanoparticles not only leads to more electron polarization, interfacial polarization, and magnetic loss but also generates an efficient complementarity between the relative permittivity and permeability, which plays an important role in increasing the EM absorption properties.[18,33] In addition, PPy nanotubes and Fe3O4 nanoparticles attached on twisted graphene sheets provide RGO sheets with more defects, and thus, the composite will introduce more defect polarization relaxation processes.[3] Because the calculated density of GPFA is about 38.3 mg/cm3, it can be used as good ultralight EM wave absorption materials.

Figure 9

Schematic illustration of the EM wave-dissipated mechanism for the GPFA composite.

Conclusions

In this article, a 3D n class="Chemical">GPFA was fabricated by a simple one-step reduction self-assembly process. The PPy nanotubes can act as a spacer to avoid the aggregation of graphene sheets and adjust the permittivity of the composite aerogel. Moreover, the addition of Fe3O4 nanoparticles can make better impedance matching between the dielectric loss and the magnetic loss of the aerogel to gain excellent EM wave absorption performance. The EM wave-absorbing results indicate that the ternary aerogel with an ultralow density of about 38.3 mg/cm3 shows an improved EM wave absorption performance with a maximum RL of −49.2 dB at the frequency of 11.8 GHz, and the effective absorption bandwidth below −10 dB can reach 6.1 GHz (9.8–15.9 GHz) at a thickness of 3.0 mm. Such an outstanding EM wave absorption behavior can be attributed to the multiple reflections, polarizations, and relaxation processes in the aerogel. Therefore, GPFA can be used as an attractive candidate for EM wave absorbers.