Fe Single-Atoms Combined with Chitosan for Enhanced Electromagnetic Wave Absorption

On account of that the single carbon material possess exceeding electrical conductivity, which will cause impedance mismatch and thus seriously affects the electromagnetic wave (EMW) absorption performance of the material. Therefore, it is an effective method to combine carbon material with magnetic material to make the material have dielectric loss and magnetic loss at the same time. In this paper, magnetic Fe nanospheres were loaded onto the conductive network of chitosan and the composite 800Fe-Chitosan was obtained through freeze drying and calcination treatment. As a result, the composite shows superior EMW absorption performance with a reflection loss (RL) of -40.22 dB and a broad effective absorption bandwidth (EAB) of 4.24 GHz.


Introduction
With the continuous progress of the times, science and technology are also developing rapidly.Nowadays, more and more electronic devices come into people's life, such as cell phones, airplanes, radar, satellites and so on 1 .However, the electromagnetic radiation produced by these electronic devices has caused serious interference and inconvenience to people's life.In order to reduce and avoid electromagnetic radiation, scientists have designed and developed a series of electromagnetic wave absorption materials [2][3][4] .Among them, carbon material has attracted wide attention of scientists because of its excellent electrical conductivity, light weight, porous structure, large specific surface area and other advantages.These carbon materials include graphene, MXene, carbon nanotubes, carbon fiber, carbon quantum dots, and so on [5][6][7] .However, the excessive electrical conductivity of these carbon materials leads to poor impedance matching.Therefore, it is necessary to introduce other materials to compound the carbon material to adjust its impedance matching.The introduction of magnetic materials can not only adjust the excessive conductivity of carbon materials, but also introduce magnetic loss, so the introduction of magnetic materials soon attracted the attention of scientists.Magnetic materials include metal iron cobalt nickel and its corresponding metal oxides, but because metal iron cobalt nickel has a large density, it can not meet the characteristics of light absorbing materials.Later, scientists found that by carbonizing the metal-organic framework (MOF) at high temperatures, they could obtain very lightweight magnetic metal or metal oxide nanospheres.Therefore, it is a good strategy to prepare microwave absorbing materials with both dielectric and magnetic losses by composite and calcination of MOF materials with carbon materials [8][9][10] .Inspired by this, in this work, we use chitosan as carbon source, skip the synthesis of MOF, directly use iron ion as magnetic source, through freeze drying and calcination, and then get 800Fe-chitosan composite material.As a result, the composite shows superior EMW absorption performance with a reflection loss (RL) of -40.22 dB and a broad effective absorption bandwidth (EAB) of 4.24 GHz.

Synthesis of 800Fe-Chitosan composite
1 g chitosan and 0.5 g Fe(NO 3 ) 3 • 9H 2 O were dissolved in 50 ml acetic acid solution in a beaker and stirred 24 h.Subsequently, the mixture was transferred to the refrigerator for pre-freezing and then transferred to the freeze-dryer.Afterward, the dried samples were calcinated at a temperature of 800 °C for 2 h under Ar atmosphere with a heating rate of 2 °C/min.

Characterizations
The crystalline structure of materials were investigated via X-ray diffraction (XRD, Japanese Rigaku Ultima IV).The morphological characteristics of the materials were studied by Scanning Electron Microscopy (SEM, Thermo Fly Apreo S).

Performance tests
In order to explore the electromagnetic wave absorption properties of materials, the prepared aerogels were mixed with paraffin by heating at 70 ℃ to obtain the mixture.Subsequently, the mixture was made into a cylindrical ring with a thickness of 2.0 mm through a mould with an inner diameter of 3.04 mm and an outer diameter of 7.0 mm.Where the filling content of aerogel is 6 wt% for each ring.The vector network analyzer (Keysight E5063A) was performed to test the electromagnetic parameters of these rings.The test frequency range is from 2 to 18 GHz.

Morphology investigations
In order to investigate the morphology of 800Fe-Chitosan, Scanning Electron Microscope (SEM) measurement was conducted and the results are shown in Figure 2. According to Figure 2a, multiple stacked layers can be seen, which are corresponding to the chitosan.Besides, a mass of small lumps can be observed in Figure 2b, which are assigned to metal Fe.These observations were further confirmed by Energy Dispersive Spectrometer (EDS).Figure 2c

Electromagnetic wave absorption performance of material.
The electromagnetic wave (EMW) absorbing properties of material can be evaluated through reflection loss (RL) and effective absorption bandwidth (EAB).Accordingly, vector network analyzer was carried out to gain the original electromagnetic parameters of material.Subsequently, the RL value can be calculated through the following equations 11 : Where Z in , Z 0 , f, c and d denote input impedance, free space impedance, frequency, velocity of light and thickness of material, respectively.According to Figure 3a, the 800Fe-Chitosan shows superior EMW absorption properties with a strong RL of -40.22 dB and a broad EAB of 4.24 GHz at a thickness of 1.5 mm.Furthermore, it represents outstanding impedance matching performance which can be observed in Figure 3d.The closer the value of Z in /Z 0 is to 1, the better the impedance matching performance is.The superior EMW absorption properties and outstanding impedance matching performance of 800Fe-Chitosan are ascribed to the synergistic effect of the abundant three-dimensional conductive network of Chitosan and the magnetic loss caused by the magnetic nano Fe lumps.In order to further figure out the mechanism of EMW absorption, the electromagnetic parameters of material including the real part (ε′ and μ′) and the imaginary part (ε″ and μ″), dielectric loss tangent (tan   ) and magnetic loss tangent (tan   ) are explored and the results are shown in Figure 4.As for the complex permittivity ε′ and μ′, which was shown in Figure 4a, denoting the storage capability and dissipation of electric energy, respectively.It is obvious that the values descend with the frequency increasing, which was ascribed to the dispersion phenomenon.Furthermore, several peaks can be seen at the range of 2-18 GHz, which are attributed to the polarization relaxation.Notably, according to Figure 4c and 4d, it can be seen that the tan   value (0.45) are about ten times than the tan   value (0.045), indicating the main loss is ascribed to the conduction loss.Besides, it is worth noting that there are several peaks appearing in Figure 4d which corresponding to the natural resonance and exchange resonance peaks, indicating the magnetic loss of material.In addition, the Cole-Cole curve which based on the Debye relaxation theory, was conducted to ulteriorly investigate the multiple loss mechanisms of material.It can be seen that the whole curve can be divided into two parts.One is the semicircles which assigned to the polarization loss, and the other is the straight line at the tail part which referring to conduction loss.These observations are in good accordance with the results of Figure 4a.

Conclusion
In this work, three-dimensional stacked chitosan loaded with Fe nanoblocks was prepared by simple solution blending, freeze-drying and high-temperature calcination.The crystal structure and morphology of the materials were analyzed by XRD, SEM and EDS.The electromagnetic parameters of the material were measured by vector network, and the loss mechanism was analyzed.The 800Fe-Chitosan shows superior EMW absorption properties with a strong RL of -40.22 dB and a broad EAB of 4.24 GHz at a thickness of 1.5 mm and outstanding impedance matching performance.The excellent EMW absorption and impedance matching performance of the material can be attributed to the interface polarization loss, dipole polarization loss and conduction loss caused by the rich three-dimensional conductive network of chitosan as well as the magnetic loss caused by the magnetic nano Fe lumps.This provides guidance and help for the design of monatomic metallic carbon-based absorbing materials.

3. 1
Structure characterizationsThe crystal structure of materials were analyzed by X-Ray Diffraction (XRD) and the corresponding results are shown as Figure1.As for Fe(NO 3 ) 3 • 9H 2 O, several peaks appear at 22.4°,25.8°,38.7°and 52.7°are assigned to the characteristic peaks of Fe(NO 3 ) 3 • 9H 2 O.As for chitosan, a broad peak presents at 20°is corresponded to the amorphous carbon peak.As for 800Fe-Chitosan, the amorphous carbon peak shift to the right direction (22.6°).Moreover, three peaks arise at 44.5°, 65.0°and 82.2°are ascribed to the (111), (200) and (220) lattice planes of metal Fe.These above results indicate that the successful fabrication of 800Fe-Chitosan.
displays the element content and atom distribution of C, N ,O and Fe.And Figure 2d shows the element mapping distribution of C, N ,O and Fe.