Improved impedance matching by multi- componential metal-hybridized rGO toward high performance of microwave absorption

Microwave-absorbing materials with good microwave absorption performance are of great interest for military applications and human health, which is threatened by electromagnetic radiation pollution. Herein, the design and synthesis of multi-componential metal-hybridized graphene composites via freeze drying and pyrolysis of ferrocene hydrazone complex precursor are reported. Various magnetic nanoparticles are loaded on reduced graphene oxide (rGO) via controlling their pyrolysis temperature. The complex electromagnetic parameters of these hybrids are therefore regulated by the hybrid components. Among them, rGO hybridized by the sea-island-like Fe2O3/Fe3O4/ FeNi3 multi-componential metals shows a good balance of dielectric and magnetic constants. Thus, the improved impedance matching with free space brings about a superior electromagnetic wave absorption performance, especially on the effective absorption bandwidth. The minimum reflection loss (RL) of the hybrids is as low as −40.3 dB at 11 GHz with the RL bandwidth of −10 dB being 4.55 GHz (from 9.25 to 13.8 GHz).


Introduction
The rapid growth of modern technologies promotes the applications of electronic devices, providing convenience to human lives or military equipment. However, electronic equipment leads to serious electromagnetic wave pollution [1][2][3][4][5]. Therefore, the development of absorbing materials seems to be necessary. Generally, the microwave absorption (MA) performances of materials are determined by their impedance matching and attenuation behaviors. The former decides the incident electromagnetic wave into the interior of materials that are subsequently consumed by dielectric and magnetic loss as described by the latter [6][7][8][9]. But the mismatched dielectric constant and permeability, namely, impedance mismatching that exists in most materials, always lead to a narrow absorbing bandwidth. In this regard, the componential and structural design of materials has aroused the enthusiasm toward good MA performance with broad absorption bandwidth.
Among these research studies, constructing pores in carbon-based materials has been proved to be a meaningful strategy for improving the impedance matching. The air existing in the porous ensures the impedance value of materials close to that of free space. Thus, the porous structure allows a broad absorption bandwidth [10][11][12][13][14][15]. However, porous materials always possess poor strength [16,17] and are easy to be saturated by adsorbed molecules or clusters due to their high specific surface area [18]. Incorporation of magnetic metal particles with dielectric carbon materials, such as graphene, is another important method for designing absorbing materials. The balanced dielectric constant and permeability achieve a good impedance matching. Graphene layers hybridized with a series of single magnetic metal particles, including Fe, Ni, and their oxides, are synthesized. For example, Co/rGO is simply synthesized by hydrothermal method [19], and Fe-hybridized rGO is achieved through primitive chemical reduction [20]. However, these hybrids exhibit improved but uncontrollable MA performances due to the single component of magnetic particles [21,22]. Subsequently, multi-component metal (McM) compounds are used for constructing graphene-based nanostructures, achieving a tunable MA performance [23,24]. But the limited methods for the componential and structural regulations of McM compounds restrict the development of related graphene nanostructures.
In this work, ferrocene hydrazone condensation bimetallic complex is captured by graphene oxide (GO) sheets. The following freeze drying and pyrolysis of the complex precursor form varieties of magnetic particles depended on the pyrolysis temperature. Compared with the existing methods, pyrolyzing bimetallic precursor is able to achieve a controllable McM@rGO hybrid by simply tuning the annealing temperature. The relationships between their MA performances and structure are systematically studied, which provides a meaningful perspective for the componential and structural design of McM-hybridized graphene toward high MA performances.

Materials
GO was synthesized by a modified Hummers method [25]. Nitrilotriacetic acid, hydrazine hydrate, 1,1′-diacetylferrocen, and nickel acetate tetrahydrate were purchased from KeLong Chemistry Company. All the reagents involved in the experiment are analytical reagent without any further purification.

Synthesis of ferrocene hydrazone condensates
A volume of 100 mL of ethanol was mixed with 100 mL of deionized (DI) water, followed by the addition of nitrilotriacetic acid (3.9 g) and hydrazine hydrate (6 mL). The solution was heated to 85°C and stirred for 0.5 h. Then, 8.1 g of 1,1′-diacetylferrocen was added into the solution together with 10 mL of acetic acid. The mixture was maintained for another 2 h. The separated red precipitate was filtered, washed with ethanol, and then dried at 60°C under vacuum for 12 h to obtain the ferrocene hydrazone condensates (Fc).

Synthesis of Fc-Ni derivatives
A total of 5.6 g of Fc and 5.6 g of nickel acetate tetrahydrate were added to N,N-dimethylformamide (150 mL). The mixture was heated to 160°C and kept stirring for 1 h. The precipitate was filtered, washed with ethanol, and then dried at 60°C under vacuum for 12 h to obtain the brown Fc-Ni derivatives.

Synthesis of multi-componential metal-hybridized rGO
The preparation of McM@rGO is diagrammatically illustrated in Figure S1. In brief, 100 mg of GO was first dissolved in 100 mL of DI water, and the mixture was mildly sonicated (70 W) for 0.5 h to obtain a GO dispersion (1 mg/mL). A total of 180 mg of Fc-Ni derivatives was then added into 20 mL of GO dispersion, followed by the demulsification for 3 min (2,000 rpm) using a demulsified machine. The suspensions were freeze-dried to get the precursors and then annealed at different temperatures for 2 h under the argon atmosphere. The final product was referred to as McM@rGO.

Characterization
The SEM images were characterized by a field-emission scanning electron microscope (JSM-7001F; JEOL). TEM and high-resolution TEM (HR-TEM) were performed on a transmission electron microscope (Zeiss Libra200). The element distribution was discriminated by the energydispersive spectrometer (EDS) mapping (Oxford 8118). XRD patterns were obtained on an X-ray diffractometer (D8 ADVANCE; Bruker) with a Cu Kα radiation (λ = 1.54056 Å). The composition of samples was characterized by the XPS (Escalab Xi + ; Thermo Fisher Scientific). Hysteresis loops were tested on a vibrating sample magnetometer (PPMS-9; Quantum Design). The electromagnetic parameters of samples were measured using a vector network analyzer (E5071C, Agilent) in the frequency range of 2-18 GHz. The samples were mixed with wax with a ratio of 20 wt% and then pressed into cyclic annular with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm for further test.

Results and discussion
The annealing process of the precursors always involves the reduction of graphene layers as well as the decomposition of Fc-Ni.  Figure 1. At high temperature, the Fc-Ni attached on the surface of GO decomposes into nanoparticles and annealing temperatures remarkably influence the structures of decomposed particles. It is clear that a higher annealing temperature benefits the uniformity of particle size. But the annealing temperature of 800°C results in submicron particles as shown in Figure 1(c). We notice that an appropriate annealing temperature is important for controlling the particle size. At 600°C, decomposed nanoparticles disperse on the graphene layers uniformly with a size of about 10 nm. As the annealing process may influence the decomposition of precursors, the XRD patterns of McM@rGO synthesized at different annealing temperatures are tested to recognize their crystalline structures. As shown in Figure [28,29].
TEM characterizations are applied for further investigations of the nanoparticle-hybridized rGO. As shown in Figure 3 Figure 3(b) reveals the uniform distribution of nanoparticles on the surfaces of graphene layers. The size of these particles is about 10 nm, coinciding with the results of SEM characterizations in Figure 1(b). The EDS mapping in Figure S3 identifies the elementary composition of nanoparticles, majorly involving Fe, Ni, and O. This result is in accordance with the XPS as shown in Figure S4. The   Figure 3(d) clearly demonstrates the multi-componential metals in McM@rGO-600. The well-resolved lattice of 0.364 nm is attributed to the interplanar spacing of (012) in Fe 2 O 3 [30]. The particle is surrounded by Fe 3 O 4 and FeNi 3 as recognized by the marked lattices in the image [31,32]. Thus, the HR-TEM, along with the aforementioned XRD patterns, proves convincing proofs about the multi-componential sea-island structure comprising the island-like Fe 2 O 3 nanoparticle as well as the sea-like Fe 3 O 4 and FeNi 3 alloy as illustrated in Figure 3( Figure 4. Generally, saturation magnetization and coercivity are regarded as the most important parameters for a magnetic material [33]. As shown in the inset, the magnified curves demonstrate the ferromagnetic of all McM@rGO. Normally, the saturation magnetic intensity is sequenced as follows: Fe 3 O 4 [34], FeNi 3 [35], NiFe 2 O 4 [36], and α-Fe 2 O 3 [37]. The saturation magnetization (Ms) of McM@rGO-400 is only 18.91 emu/g due to the relatively weak ferromagnetism of NiFe 2 O 4 . But the Ms of McM@rGO-800 is much larger than that of McM@rGO-600, perhaps due to the increased ratio of FeNi 3 to Fe 3 O 4 as proved by the structural characterization. We notice that McM@rGO-600 possesses the largest remanent magnetization (Mr) and coercivity (Hc) among these samples, which might owe to its refined grains and good dispersion among all samples [38]. Thus, the area enclosed by the hysteresis loop, which represents the magnetic loss capability, is thought to be higher for McM@rGO-600.

HR-TEM images exhibited in
The cooperation between dielectric rGO and magnetic McM leads to the changes in complex permittivity (ε r = ε′ -jε′′) and permeability (μ r = μ′ -jμ′′), which is closely related to the MA [39][40][41]. To investigate the MA performance of the McM@rGO hybrids, frequency-dependent electromagnetic parameters are measured. The real parts of permittivity (ε′) and permeability (μ′) represent the storage ability of field energy, whereas the imaginary parts of permittivity (ε″) and permeability (μ″) indicate the dissipation capacity [42][43][44][45]. The complex permittivities of these synthesized hybrids are given in Figure 5(a  and b). Both the real and imaginary parts decrease with an increase in frequency, indicating a frequency dispersion behavior induced by the enhanced polarization lagging in the high frequency [46,47]. The dielectric behaviors of McM@rGO majorly originate from the interfaces between nanoparticles and graphene layers as well as the polarization of residual functional groups [48]. Benefitting from the uniform hybridization between sea-islandlike McM and rGO layers, McM@rGO-600 displays an improvement in dielectric dissipation in the range of 9-14 GHz as described by dielectrical dissipation factors (tan δε) in Figure 5(c). For the situation of complex permeabilities (Figure 5d and e), McM@rGO-400 presents a comparable permeability but few abilities in magnetic loss due to the lossless NiFe 2 O 4 [49]. The magnetic loss factor (tan δμ) in Figure 5(f) demonstrates a strong magnetic loss in the range of 11-14 GHz, especially for McM@rGO-600. The relative low magnetic loss peak for McM@rGO-800 might be due to the decreased contents of Fe 3 O 4 and Fe 2 O 3 as proved by XRD patterns.
Reflection loss (RL) is an important index that directly reflects the microwave absorption performance  of materials. According to the transmission line theory, the RL is calculated by the following equations (1) and (2) [50,51].
where d, f, and c are the thickness of absorber, frequency of incident microwave, and velocity of light. In Figure 6(a), McM@rGO-400 demonstrates a weak RL peak, majorly owing to its poor magnetic loss. We notice that the McM@rGO-600 holds a superior MA ability even better than McM@rGO-800. The minimum RL value is down to −40.3 dB at 11 GHz with the effective absorption bandwidth (RL < −10 dB) of 4.55 GHz (from 9.25 to 13.8 GHz) at 2.7 mm. Moreover, the widest effective absorption bandwidth is calculated to be 5.25 GHz with a thickness of only 2.0 mm.
The superior MA performance of McM@rGO-600 is closely related to the improved impedance matching and increased dissipation. The impedance matching values (|Z in /Z 0 |) of McM@rGO are derived from electromagnetic parameters and shown in Figure 7. |Z in /Z 0 | illustrates the proximity between the input impedance (Z in ) of the absorber and the impedance (Z 0 ) of the free space. The   |Z in /Z 0 | value of hybrids is expected to be 1 to achieve the maximum incident wave into the interior of the absorber [52]. The blue area represents a good impedance matching (0.8-1.2) as the other colors indicate a mismatching. Benefitting from the matching between permittivity and permeability, McM@rGO-600 possesses the best impedance matching as recognized by the maximum blue area in Figure 7, profiting the incident of microwave for further dissipation.
On the other hand, the attenuation value (α) is used to evaluate the dissipation of incident wave in absorbers [53,54]. The α values of samples are calculated using equation (3), and the results are shown in Figure 8. The increased α, especially in the range of 10-14 GHz for McM@rGO-600, originates from the synergistic enhancements of dielectric and magnetic loss. Moreover, the tiny but uniformly dispersed nanoparticles may also benefit to their MA performance [55].

Conclusions
In conclusion, this work achieves the compositional and structural regulations of McM-hybridized rGO by controlling the decomposition and reduction of Fc-Ni precursors. The rGO hybridized by the sea-island-like Fe 2 O 3 / Fe 3 O 4 /FeNi 3 McM displays a good balance of dielectric and magnetic constants, significantly improving the impedance matching with free space. Therefore, a superior MA performance is realized for the McM@rGO hybrids. The minimum RL of the hybrids is as low as −40.3 dB at 11 GHz with the RL bandwidth of −10 dB being 4.55 GHz (from 9.25 to 13.8 GHz), which seems to be an ideal candidate for high-performance EM wave absorption. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

Conflict of interest:
The authors state no conflict of interest.