Facile One-Pot Solvothermal Synthesis of the RGO/MWCNT/Fe3O4 Hybrids for Microwave Absorption.

How to effectively regulate the electromagnetic parameters of magnetic composites to achieve better microwave absorption (MA) performances is still a serious challenge. Herein, we constructed nanocomposites composed of magnetic constituents and carbon materials to obtain high-efficiency electromagnetic wave absorbers. Self-assembled, multi-interfacial, and porous RGO/MWCNT/Fe3O4 hybrids (GMFs) were synthesized via in situ one-pot solvothermal method. The growth mechanism of the GMFs would be that the defects on reduced graphene oxide (RGO) provide sites for the crystallization of Fe3O4. Also, the RGO and Fe3O4 were further linked by the cross-connection of multiwalled carbon nanotubes (MWCNTs), which acted as a bridge. The MA mechanism of GMFs was studied while considering the synergistic effects between the three components (RGO, MWCNT, and raspberry-shaped Fe3O4) and their multi-interfacial and porous structure. Also, the MA performance of the GMFs was conducted. The GMFs exhibited a maximum reflection loss (RL) value of -61.29 dB at 10.48 GHz with a thickness of 2.6 mm when the contents of RGO and MWCNT were 6.3 and 1.3 wt %, respectively. The RL values (≤-10 dB) were observed to be in the range of 8.96-12.32 GHz, and the effective microwave absorption bandwidth was tunable from 3.52 to 18 GHz by changing the sample thickness. The results revealed that the multi-interfacial and porous structure of the GMFs is beneficial to MA performance by inducing multiscatterings. Since no toxic solvents were used, this method is environmentally friendly and has potential for large-scale production. The prepared GMFs may have a wide range of applications in MA materials against electromagnetic interference pollution.

Introduction

With the rapid development and application of electromagnetic wave technology, many electronic devices running on electromagnetic wave offer countless conveniences in communication. However, the electromagnetic interference pollution comes soon afterward, which is harmful to the equipment operation and biological processes, including human health.[1−4] To solve this problem, excellent microwave absorption (MA) materials are in demand in both military and civilian fields.[5,6]

Carbon-based materials, including graphene, carbon nanotube (CNT), carbon black, and carbon fiber, show absorption performance with lightweight, high specific surface area, and tunable properties of permittivity, electrical, and thermal conductivity.[7−9] In addition, the carbon-based materials show many other applications, such as membrane materials,[10] cathodes,[11] supercapacitors,[12] carbon-reinforced polymer composites,[13] uranium adsorption materials,[14,15] anticorrosion materials,[16] dye-sensitized solar cells,[17] lithium-ion batteries,[18] biosensors,[19] thermal conductive materials,[20] antiwear materials,[21] and ceramic nanocomposites.[22] Nevertheless, carbon materials are unilateral dielectric materials with high permittivity value and possess no magnetism, resulting in impedance mismatch,[23] which limits the improvement of the MA performance.

Magnetic particles, such as magnetic element, alloy (Fe, Co, Ni, and FeCo),[9] and their oxides (Fe3O4,[24] γ-Fe2O3,[25] Co@CoO,[26] Co3O4,[27] NiO,[28] and CoFe2O4[29]), have been reported in the field of MA materials owing to their dielectric loss and magnetic loss properties.[30−32] Except for the influences of the microstructures of electromagnetic wave absorbers, the balance between the complex permeability and complex permittivity (so-called impedance matching) plays a crucial role in the absorption performance.[33] It is difficult for conventional microwave-absorbing materials to meet the requirements of lightweight, high stability, and strong absorption for high-efficiency microwave absorbers.[9] Thus, the condition of high density of pure magnetic particles is not satisfied by the design of lightweight MA materials.

Recently, constructing nanocomposites composed of magnetic constituents and carbon materials has been regarded as an effective approach to obtain high-efficiency electromagnetic wave absorbers.[33] For instance, Co/C,[33] Fe3O4/graphene,[34] Fe3O4/CNTs,[35] NiFe2O4/RGO,[36] Ni/C,[37] Ni/NiO-C,[38] and MWCNT/NiO-Fe3O4[39] have shown excellent MA performance. The dielectric loss, magnetic loss, and impedance-matching properties are tunable by complex methods, the mass ratio of different substances, and the size of nanoparticles.[40−42]

In this study, we developed a facile solvothermal route for the one-step synthesis of RGO/MWCNT/Fe3O4 hybrids (GMFs) via an in situ self-assembly growth. The influences of the RGO content and the reaction time of the solvothermal process on the microwave absorption performance of the GMFs were studied. Structures and morphologies of the GMFs were characterized. The MA mechanisms were studied while considering of the synergistic effects between the three components and their structural characteristics in detail.

Results and Discussion

Structure and Morphology of GMFs

The phase structure of the samples was determined by X-ray diffraction (XRD). Figure depicts the XRD patterns of the RGO, MWCNT, Fe3O4, and GMFs. The diffraction peaks for both Fe3O4 and GMFs appeared at about 18.3, 30.1, 35.4, 37.1, 43.1, 53.4, 56.9, 62.5, and 73.9°, which corresponded to the (111), (220), (311), (222), (400), (422), (511), (440), and (533) lattice planes of the face-centered cubic (fcc) spinel phase of Fe3O4 (Powder Diffraction File (PDF) No. 19-0629), respectively. The RGO showed a very broad diffraction peak at 24.0°. The MWCNT showed peaks at 25.8 and 43.0°. The peaks of Fe3O4 can be observed in the XRD pattern of GMF5. However, the characteristic peaks of pure RGO and MWCNT at 24.0 and 25.8° cannot be seen, which may be due to the low contents of RGO and MWCNT in GMF5. XRD patterns of GMFs with different amounts of RGO (GMF0, GMF1, GMF3, GMF5, GMF7, and GMF15) and different reaction times of GMF5-8h are also displayed in the Supporting Information (Figure S1). All their peaks were completely consistent with each other.

Figure 1

XRD patterns of pure RGO, MWCNT, Fe3O4, and GMF5.

The morphologies of the samples were examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The size of pure Fe3O4 was about 300–700 nm according to the FESEM images (Figure a). Also, pure Fe3O4 were self-assembled cluster crystals. Pure RGO sheets were prepared by the same hydrothermal thermal condition of GMF5 as a control experiment. As for the GMF5, Fe3O4 were composed of the RGO sheets and MWCNT uniformly. The size of Fe3O4 was about 200–450 nm, which is smaller than that of pure Fe3O4 according to Figure c, which may be because the RGO and MWCNT affected the progress of growth (from about 450 to 700 nm) of Fe3O4. The RGO sheets in GMF5 were thinner than the pure RGO sheets, which may be due to the presence of Fe3O4 and MWCNT, which restrained the agglomeration of RGO. Also, the formation of nanohybrids of GMF5 would restrict the growth of Fe3O4 when the sizes of Fe3O4 was about 450 nm. The porous structure of Fe3O4 was more obvious on the surface of GMF5. Also, Fe3O4 were rough balls just like raspberries rather than smooth balls, which further explained that Fe3O4 were self-assembled cluster crystals rather than large single crystals. We guess that the low contents of RGO and MWCNT affected the self-assembly process of Fe3O4, or some small flaws of RGO and MWCNT acted as the crystal nucleus of Fe3O4.

Figure 2

(a) FESEM images of the pure Fe3O4, (b) pure RGO prepared under the same conditions from GO, and (c, d) GMF5.

The GMF5 was detached from the Cu foil in ethanol through ultrasonication and used for TEM characterization. It could be further confirmed that Fe3O4 were combined with RGO sheets uniformly interspersed with MWCNTs. Fe3O4 were porous and assembled by small nanoparticles in three dimension (3D), not only on the surface. The RGO was plicate and thin, which can be clearly observed though TEM, and few MWCNTs were interspersed in the RGO and Fe3O4. From the high-resolution TEM (HRTEM) image, it can be clearly observed that Fe3O4 was attached to RGO and MWCNTs (Figure g). In addition, in Figure g, the interplanar distance of 0.49 nm corresponds to the (111) plane of the cubic phase of GMF5. According to the HRTEM image (Figure c), pure Fe3O4 shows obvious lattices with the interplanar distance of 0.25 nm, corresponding well to the (311) plane of the cubic phase of Fe3O4. Also, Figure d shows a series of clear diffraction rings in the selected-area electron diffraction (SAED) pattern, which can be accurately assigned to the (220), (311), (422), (440), (533), and (731) facets of Fe3O4. As to GMF5, Figure h shows a series of clear diffraction rings in the SAED pattern, which can accurately be assigned to the (111), (220), (311), (400), (440), and (511) facets of Fe3O4.

Figure 3

(a, b) TEM images, (c) HRTEM image, and (d) SAED pattern of the pure Fe3O4 nanoparticles. (e, f) TEM images, (g) HRTEM image, and (h) SAED pattern of the GMF5.

Raman spectrum was used to characterize the structures of Fe3O4, GO, RGO, and GMFs. Figure shows that GO and RGO have two broad peaks at about 1345.2 cm–1 (D band) and about 1586.5 cm–1 (G band), respectively. Also, MWCNTs have three broad peaks at 1345.2 cm–1 (D band), 1586.5 cm–1 (G band), and 2701.4 cm–1 (2D band), which were associated with the defect and the graphitic structure of the carbon atoms.[5,43,44] Fe3O4 has no D band or G band but a characteristic peak at 693.3 cm–1, which was also shown in GMF5. For GMF5, the peaks were at 1347.2 cm–1 (D band) and 1600.9 cm–1 (G band). The peak intensities of D band and G band were lower, which may be due to the low content of RGO. Compared to GO or RGO, a shift in the peaks can be found for both D and G bands, indicating a significant charge transfer between the graphene nanosheets and Fe3O4. The charge transfer among RGO, MWCNT, and Fe3O4 was helpful for improving their electrochemical performance.[45−49] This may facilitate the formation of the nanohybrids with more uniform RGO, MWCNT, and Fe3O4. In addition, the ID/IG values of GO, RGO, MWCNT, and GMF5 were 1.12, 1.53, 0.99, and 1.71, respectively. It can be figured out that the value of ID/IG of RGO was higher than that of GO because the defect density was enlarged in the reduction progress of RGO.[50−52] Also, the value of ID/IG of GMF5 was higher than those of MWCNT, Fe3O4, GO, and RGO, which may be because GMF5 cannot be simply mixed with RGO, MWCNT, and Fe3O4; it is more like a kind of nanohybrid. Raman patterns of GMFs with different amounts of RGOs (GMF0, GMF1, GMF3, GMF5, GMF7, and GMF15) and different reaction times of GMF5-8h are also displayed in the Supporting Information (Figure S2). Figure S2 shows the Raman spectra of GMFs. The value of ID/IG of GMF5-8h was lower than that of GMF5, and it was the lowest. The reason was that the reaction time of GMF5-8h was less than that of GMF5 and others (24 h), which means that the reaction time may affect the value of ID/IG of the GMFs. The values of ID/IG of GMF0, GMF1, GMF3, GMF5, GMF7, and GMF15 were 2.46, 1.79, 1.93, 1.71, 1.42, and 1.53, respectively.

Figure 4

Raman patterns of MWCNT, synthesized Fe3O4, GO, RGO, GMF5-8h, and GMF5.

X-ray photoelectron spectroscopy (XPS) was further utilized to determine the surface composition of the products and the oxidation states of Fe and C. Figure a shows the typical XPS spectra of GMF0 and GMF5. It shows the signals of the three elements Fe, O, and C in GMF5. Figure b–d shows the Fe 2p, C 1s, and O 1s peaks of GMF5. Two broad peaks (Figure b) centered at 724. 6 and 711.2 eV were assigned to the spin–orbit split doublet of Fe 2p3/2 and Fe 2p1/2, respectively, in accordance with the reported values for Fe3O4.[1,49] The C 1s spectrum (Figure c) was divided into three components. Also, the binding energy peaks at 284.8, 286.4, and 288.6 eV were attributed to C 1s of C=C, C–O, and C=O, respectively.[53,54] The O 1s spectrum (Figure d) was divided into three components. The binding energy peaks at 529.6, 531.1, and 532.4 eV were attributed to the O (1s) of Fe3O4, OH, and H2O molecules, respectively.[55]

Figure 5

XPS survey scans of GMF0 and GMF5 (a). Fe 2p spectra (b), C 1s spectra (c), and O 1s spectra (d) of GMF5.

Magnetic Properties

To investigate the magnetic properties of RGO, Fe3O4, and GMF5, the vibrating sample magnetometer (VSM) test was employed.

Figure displays the room-temperature hysteresis loops of RGO, Fe3O4, and GMF5 in the presence of the applied magnetic field ranging from −15 000 to +15 000 Oe. The specific magnetic parameters including the saturation magnetization (Ms), coercivity (Hc), and remanent magnetization (Mr) are listed in Table .

Figure 6

Hysteresis loops of RGO, Fe3O4, and GMF5 at room temperature.

Table 1

Ms, Hc, and Mr values of RGO, Fe3O4, and GMF5

samples	Ms (emu/g)	Hc (Oe)	Mr (emu/g)
RGO	0	0
Fe3O4	79.64	46.62	6.46
GMF5	79.84	75.45	10.69

Both the saturation magnetization (Ms) and coercivity (Hc) of RGO show zero values, which means that the RGO had no paramagnetic characteristic. The values of saturation magnetization (Ms) of Fe3O4 and GMF5 were 79.64 and 79.84 emu/g, respectively, and display a paramagnetic characteristic, suggesting that pure Fe3O4 nanoparticles and GMF5 were actually formed by small nanoparticles, which was highly consistent with the results of SEM and TEM.[56−60]

Microwave Absorption of the GMFs

To study the microwave absorption of GMFs in detail, we investigated the frequency dependence of electromagnetic parameters using wax as a matrix containing 40 wt % of GMFs, as shown in Figure . It was well known that the real part and imaginary part of the electromagnetic parameters stand for the storage ability and loss ability of microwave, respectively. Figure shows that the GMFs have the typical dispersion characteristics, namely, the ε′ values decrease as the measured frequency increases. The dispersion characteristic may be ascribed to the orientation polarization of electric dipoles lagging behind the period change of the electric field.[5,61,62]

Figure 7

(a, b) Real and imaginary parts of the permittivity, (c, d) real and imaginary parts of the permeability, (e) dielectric loss tangent (tan δε), and (f) magnetic loss tangent (tan δμ) of GMFs.

Permittivity and permeability are known to mainly originate from polarization (orientation, interfacial, atomic, and space charge) and magnetic properties. Also, they may be significantly influenced by size, composition, structure, and geometrical configuration.[1] In addition, Figure a,b shows that from GMF0 to GMF15, the permittivities (real ε′ and imaginary ε″) accordingly increase positively with the increase of the RGO content with the frequency in the range of 2–18 GHz.

In detail, the value of ε′ of GMF7 was less than that of GMF5 from 2 to 6 GHz and the value of ε″ of GMF7 approaches that of GMF1. As for GMF5-8h, the value of ε′ was much bigger than that of GMF5 from 2 to 18 GHz; however, the value of ε′′ was a bit less than that of GMF5 from 2 to 5 GHz and was close to that of GMF5 from 5 to 7 GHz but a little larger than that of GMF5 from 7 to 18 GHz. Under the same reaction conditions, the increase of the content of RGO had a positive correlation with the real part of the dielectric constant of the composite, and different reaction times will also affect the change of the real part of the dielectric constant. The imaginary part of the dielectric constant of GMF7 does not increase with the increase of the RGO content. A comparison of GMF5 and GMF5-8h shows that the reaction time had little effect on the imaginary part of the dielectric constant.

Figure c,d shows that the permeability (real μ′ and imaginary μ″) of all the GMFs approach each other with the frequency in the range 2–18 GHz. Figure e,f shows the dielectric loss tangent (tan δε) and magnetic loss tangent (tan δμ) of GMFs. The values of tan δε of GMF0, GMF1, GMF3, and GMF5 were of the order of “GMF0 < GMF1 < GMF3 < GMF5” with the frequency in the range 2–18 GHz. The value of tan δε of GMF7 was bigger than that of GMF0 and less than that of GMF1 with the frequency in the range 2–16 GHz and also less than that of GMF0 with the frequency in the range 16–18 GHz. The value of tan δε of GMF15 was less than that of GMF5 with the frequency in the range of 2–6 GHz and more than that of GMF5 with the frequency in the range of 6–18 GHz. In addition, the value of tan δε of GMF5-8h was less than that of GMF5 with the frequency in the range of 2–18 GHz. It is worth noting that the values of μ″ are negative in some frequency regions (Figure d), suggesting that the GMFs might be used as metamaterials with negative electromagnetic parameters, which usually show unique physical properties different from naturally occurring materials.[37,63] Similar phenomena have also been found in Fe3O4/C composites.[5,55]

For practical application, the reflection loss values need to be lower than −10 dB, which means that over 90% of the incident microwave can be absorbed.[5]Figures –10 show the reflection loss curves of GMF0, GMF1, GMF3, GMF5, GFM5-8h, GMF7, and GMF15. For GMF0, GMF1, GMF3, GMF5, GMF7, and GMF15, the maximum reflection loss (RL) and the wide effective absorption bandwidth changed with the increase of the RGO content. The GMF5 showed a wide effective absorption bandwidth at 8.96–12.32 GHz and reached the maximum RL value of −61.29 dB at 10.48 GHz with a thickness of only 2.6 mm. GMF7 shows two peaks, wide effective absorption bandwidths at 1.94 and 1.54 GHz, and reached the maximum RL values of −51.74 dB at 4.80 GHz and −22.62 dB at 15.6 GHz with a thickness of 5.0 mm. GMF5-8h showed a wide effective absorption bandwidth at 2.64 GHz and reached the maximum RL value of −39.49 dB at 11.76 GHz with a thickness of 1.9 mm.

Figure 8

(a, b) Frequency-dependent RL curves and 3D maps of GMF0, (c, d) GMF1, and (e, f) GMF3.

Figure 10

(a, b) Frequency-dependent RL curves and 3D maps of GMF7 and (c, d) GMF15.

(a, b) Frequency-dependent RL curves and 3D maps of GMF5 and (c, d) GMF5-8h.

Both the wide effective absorption bandwidth and the maximum RL value of GMF5 were better than those of GMF5-8h. With the same ratio between the three components (RGO, MWCNT, and Fe3O4), GMF5 shows better MA performance than GMF5-8h because GMF5 was prepared with a longer reaction time. The reaction time affects the nanohybrids by affecting the self-assembly process of GMFs, thus the structures of GMFs were affected. The longer reaction time could be beneficial to the growth of Fe3O4 cluster crystals on RGO, and the hybridization process of the three components of GMF5 would be better than that of GMF5-8h.

In addition, MA properties of the other GMFs are described below. GMF0 shows two peaks, the wide effective absorption bandwidth at 2.08 and 1.76 GHz, and reached the maximum RL value of −18.36 dB at 5.28 GHz and −19.10 dB at 17.04 GHz with a thickness of 5.5 mm. GMF1 shows two peaks, the wide effective absorption bandwidths at 4.00–6.32 GHz (2.32 GHz) and 15.12–17.28 GHz (2.16 GHz), and reached the maximum RL value of −34.91 dB at 5.04 GHz and −19.77 dB at 16.24 GHz with a thickness of 5.5 mm. GMF3 showed a wide effective absorption bandwidth at 5.04–7.68 GHz (2.64 GHz) and reached the maximum RL value of −44.03 dB at 6.48 GHz with a thickness of 4.4 mm. GMF15 showed a wide effective absorption bandwidth at 12.80–17.04 GHz (4.24 GHz) and reached the maximum RL value of −19.55 dB at 14.64 GHz with a thickness of 1.5 mm.

The proper RGO content, MWCNT content, and assembly process of Fe3O4 may induce the porous structure and multiple interfaces of the GMFs. The porous media and interfaces can cause multiple scatterings and reflections of the incident microwave.[5,64] For GMF5, the multiple interfaces of Fe3O4 cluster crystals, the multiple combined interfaces of Fe3O4, RGO, and MWCNT, the porous structure of GMFs, and the appropriate contents of RGO and MWCNT regulate the electromagnetic parameters, which were beneficial for wave attenuation, energy conversion, and fit to an excellent MA property.[65,66] The microwave absorption properties of other GMFs were less than those of GMF5, but their microwave absorption properties were not bad, which may be due to the porous structure and multiple interfaces.

To show the superiority of GMFs, Table summarizes the microwave absorption performance of the recently reported graphene-based microwave absorption materials[34,62,67−72] and the GMFs in this work. Compared with most of these absorbers, the GMF5 has low contents of RGO and MWCNT, low cost, and simple synthesis process, as well as strong microwave absorption intensity, and the RL value was −61.29 dB at 10.48 GHz with a thickness of only 2.6 mm. The current work gives better results, and the reason was the synergistic effects among the three components (RGO, MWCNT, and raspberry-shaped Fe3O4) and their multi-interfacial porous structure.

Table 2

Microwave Absorption Properties for Some of the Recently Reported Graphene-Based Microwave Absorption Materials

absorber	maximum RL (dB)	optimum thickness (mm)	bandwidth (RL< −10 dB) (GHz)	absorber content (wt %)	ref
Fe3O4/graphene	–40	4.5	∼4.0 (4.6 to ∼8.6)	50	(72)
MoS2/RGO	–41.9	2.4	5.1 (12.9–18)	30	(71)
RGO/Fe3O4	–53	4.0	2.0 (4.5–6.5)	40	(70)
MWCNT/graphene foam	–39.5		∼16.0		(62)
graphene microflowers	–42.9	4.0	5.59	10	(69)
graphene aerogels@Ni	–52.3	3.0	6.5 (11.3–17.8)		(34)
graphene/BaFe12O19/CoFe2O4	–32.4	5.0	3.0		(68)
RGO/SiO2/Fe3O4	–56.4	4.5	4.1 (6.5–10.6)		(67)
GMF5	–61.3	2.6	3.4 (8.9–12.3)	40	this work
GMF5-8h	–39.5	1.9	2.7 (10.6–13.3)	40	this work

The mechanisms contributing to the MA of GMFs are discussed here. On the one hand, the large specific surface area of RGO and self-assembled, porous Fe3O4 formed the multiple interfaces of GMFs. With the connection of linear MWCNT, the GMFs formed the whole hybrids, which were beneficial to the transfer and conduction of electrons, thus improving the MA performance. On the other hand, the RGO and MWCNT effectively regulated the permittivity and permeability properties of GMFs, thereby regulating the MA performance. In addition, the excellent conductivity and thermal conductivity of RGO were beneficial for transferring the electrons and heat, further improving the GMFs’ efficiency of microwave loss for MA performance.

Conclusions

In summary, in this work, we fabricated GMFs via facile one-step solvothermal reaction method. The method was facile with no toxic solvents and conducive to large-scale preparation. The change of RGO content can regulate the electromagnetic parameters of the GMFs. The MWCNTs act as a bridge to connect with the RGO and Fe3O4 and helps the transfer of the electron. Fe3O4 were porous clusters composed of many small single crystals, which provided a three-dimensional absorbing space; thus, the electromagnetic wave would experience multiple reflections in the 3D porous structure. Thus, the overall specific surface area was large, which was conducive to the better absorption of electromagnetic waves. When the mass ratio of GO, MWCNT, and Fe3O4 was 5:1:73 and the reaction time of solvothermal reaction was 24 h, the maximum RL value of GMF5 was −61.29 dB at 10.48 GHz with a thickness of only 2.6 mm, and its effective microwave absorption bandwidth (RL < −10 dB) was at 8.96–12.32 GHz. By tuning the thickness of GMF5, the effective microwave absorption bandwidth was in the range from 3.52 to 18 GHz.

Experimental Section

Materials

The graphite powder was purchased from Qingdao Dongkai Graphite Co., Ltd. The potassium permanganate (KMnO4, A.R.) was purchased from Xilong Scientific Co., Ltd. The concentrated sulfuric acid (H2SO4, A.R., 95.0–98.0%), hydrogen peroxide (H2O2, A.R., 30 vol %), ethylene glycol (EG, A.R.), iron chloride hexahydrate (FeCl3·6H2O, A.R.), sodium acetate anhydrous (NaAc, A.R.), and concentrated hydrochloric acid (HCl, A.R., 36.0–38.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. The multiwalled carbon nanotubes (MWCNTs, purity >98 wt %, OD = 10–20 nm, length = 10–30 μm) were purchased from Time Nano Co., Ltd. (Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences). The ultrapure water was used in all the experiments.

Preparation of Graphene Oxide

The graphene oxide (GO) was prepared from graphite powder by a modified Hummers method.[73,74] In brief, 2.0 g of graphite powder (8000 mesh) was stirred in 100 mL of concentrated H2SO4 for about 6 min at 11 °C. Then, 12.0 g of KMnO4 was added and the mixture stirred by a magnetic stirrer for 30 min maintained below 20 °C; then, the temperature of the mixture was maintained at 35 °C for 2 h. One hundred fifty milliliters of ultrapure water and about 5.0 mL of H2O2 (30 vol %) were added dropwise. When the temperature of the mixture dropped to room temperature, about 500 mL of ultrapure water and about 1.85 vol % HCl solution were added. The mixture was made to stand overnight and the supernatant was poured out. If the mixture cannot be layered, it was washed and centrifuged by ultrapure water and 5 vol % HCl solution until it was neutral (pH = 7); then, it was freeze-dried for 48 h.

Preparation of Fe3O4 Nanoparticles

Fe3O4 were prepared by solvothermal process.[30] In detail, 10 mmol FeCl3·6H2O and 61 mmol NaAc were dissolved in 20 and 40 mL of ethylene glycol, respectively. They were mixed, and the mixture was stirred by a magnetic stirrer until the suspension spread evenly. Next, the mixture was sonicated for 5 min. Then, the mixture was poured into a 150 mL Teflon-lined stainless steel autoclave and the temperature was maintained at 180 °C for 24 h. The black products were washed five times with ethanol and ultrapure water and then freeze-dried for 48 h.

Preparation of GMFs

The GMFs were prepared facilely by an in situ one-pot solvothermal method. The schematic of the synthesis procedure of the GMFs was shown in Scheme . First, 0.01 g of MWCNT and a series of GO (0.00, 0.01, 0.03, 0.05, 0.07, and 0.15 g) were dispersed in 10 mL of ethylene glycol by sonicating for 5 min. Then, 10 mmol FeCl3·6H2O and 61 mmol NaAc were dissolved in 60 mL of ethylene glycol, stirred, and sonicated for 10 min until the suspension spread evenly. Next, the EG dispersion of GO/MWCNT and the EG dispersion of FeCl3·6H2O and NaAc were mixed, and the mixture was stirred and sonicated for about 10 min until the suspension spread evenly. The mixture was poured into a 150 mL Teflon-lined stainless steel autoclave and the temperature maintained at 180 °C for 24 h. The black products were washed five times with ethanol and ultrapure water and freeze-dried for 48 h. The raw material ratio and the corresponding sample abbreviations are shown in the Supporting Information (Table S1).

Scheme 1

Schematic of the Synthesis Procedure of the GMFs

Characterization

The X-ray diffraction diffractometer (Rigaku MiniFlex 600) was used to investigate the phase structures of the products, using Cu Kα radiation from 10 to 90° with the scan rate of 5°/min. A scanning electron microscope (HITACHI S-4800(II)) and a transmission electron microscope (JEOL JEM-2100) were applied to examine the microstructures and morphologies of the products. A Raman spectroscopy system (HJY LABRAM) was used to investigate the structure and ID/IG of the products. An X-ray photoelectron spectroscopy (Thermo ESCALAB 250XI) was used to investigate the surface states of GMFs.

A vibration sample magnetometer (LakeShore 7404) was employed to measure the hysteresis loops of the products, which was applied at a maximum magnetic field of 18 kOe at room temperature. An Agilent E5071C network analyzer was applied to measure the microwave absorption properties of the products at 2–18 GHz according to the coaxial-line method. The toroidal samples (inner diameter Φin = 3.04 mm, outer diameter Φout = 7.00 mm, and sample thickness d = 2.00–3.50 mm) for microwave-absorbing measurements were fabricated by homogeneously mixing the paraffin wax with products (mass ratio = 4:6). The samples had the same name as their corresponding GMFs. The test software was Keysight Technologies N1500A, and the test mode was Nicholson–Ross–Well, and the measurement frequency range was 2–18 GHz. Next, the values of ε′, ε″, μ′, and μ′′ can be calculated via the test software that has been installed by Agilent E5071C network analyzer. Finally, the RL values can be calculated based on the transmission line theory.[75,76]where Zin is the input impedance at the absorber surface, Z0 is the impedance of the air, f is the microwave frequency, d is the thickness of the absorber, and c is the velocity of light in free space. εr (εr = ε′ – jε″) and μr (μr = μ′ – jμ″) are the complex permittivity and permeability of the absorber, respectively.