Dependence of electromagnetic wave absorption properties on the topography of Ni anchoring on reduced graphene oxide

Luyng Ling,Zhoyng Li,Zhongyi Bi,Yuezhn Feng*,Xioqin Guo,*,Jinmin M*,Chunti Liu

a Key Laboratory of Advanced Materials Processing & Mold, Ministry of Education, National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou 450002, China

b Henan Key Laboratory of Aeronautical Materials and Application Technology, School of Material Science and Engineering, Zhengzhou University of Aeronautics, Zhengzhou 450046, China

c Key Laboratory for Micro-/Nano-Optoelectronic Devices, Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410022, China

ABSTRACT Specific topographic Ni anchoring on reduced graphene oxide (rGO) composites show an astronomical potential as effective wave absorbers due to the synergistic electromagnetic loss effects.Herein,Ni/rGO composites with different topography were successfully prepared via hydrothermal in-situ reduction method.The structure and morphology characteristics revealed that particle-like, chain-like, coin-like and flower-like Ni were closely anchored onto rGO, respectively.The electromagnetic wave absorption(EMA)performance revealed that chain-like Ni/rGO exhibited the optimal reflection loss of-43.7 dB with a thickness of 1.8 mm as well as the EAB of 6.1 GHz at 2.0 mm among all samples due to the good impedance match and the synergistic dielectric and magnetic losses.Besides, one conclusion can be drawn that excellent magnetic coupling effect and impedance matching were the main reasons for significantly improving the EMA performance.Considering the systematic dependence of morphology on EMA, this work provides a perspective for designing high-performance absorbing materials.

Keywords:Ni/rGO Morphology structure Magnetic coupling effect Synergistic effect Electromagnetic wave absorption

With the rapid proliferation of advanced electronic devices and precision radar networks working in the gigahertz(GHz)band,the growing electromagnetic (EM) interference often causes serious harm to the environment and human health [1,2].As a consequence, designing and developing of lightweight and highefficiency electromagnetic absorbing(EMA)materials has become a huge challenge for researchers to deal with the EM wave pollution and the aircraft stealth problems [3,4].

Traditionally, magnetic metals, such as Fe, Co, Ni, have been widely used as EMA materials due to their desired conductive and magnetic loss for EM waves [5].However, unilateral magnetic metals with large density cannot meet the requirements of light-weight,small volume and corrosion resistance at the same time in practical applications [6].Therefore, combining magnetic metals and dielectric materials has been developed as an effective strategy for preparing high-efficient and practical EMA materials.For example, Xiao et al.obtained a lightweight magnetic hybrid composed of Co and carbon nanotubes (CNTs), and the optimal absorption effect of -49.16 dB at 2.5 mm can be detected [7].Recently, especially since many 2D lightweight dielectric materials including graphene[8],MXene[9]and MoS2[10] have been discovered and applied, hybrids of magnetic metals and 2D materials have gradually become one of designing hotspots in EMA field.Among them, reduced graphene oxide (rGO) not only exhibits a high specific surface area and excellent electrical conductivity,but also remains large number of oxygen-containing functional groups for dipole polarization,which make rGO-based composites be very promising for applications in EM wave absorbing and shielding [11–14].For instance,Zhao et al.reported an ultra-thin CoNi/rGO aerogel with a RLminvalue of -53.3 dB and an effective bandwidth of 3.5 GHz at only 0.8 mm [15].From above, one can be noticed that the hybridization of rGO and magnetic metals plays a crucial role in promoting the EM wave absorption.In addition to the inherent characteristics of materials, the micro-morphology of magnetic metals also has a crucial effect on the absorption of EM waves[16,17].There have been some reports showing different EMA effects of individual topography of Ni/rGO(e.g.,urchin[18],flower[19],particle-like[20]).Unfortunately,as far as we know,there are rare reports on exploring the topography dependence of Ni on EMA property of its rGO hybrid under the same standard.

In this work,Ni/rGO with different topographical nano-Ni were fabricated via unified hydrothermal in-situ reduction technology by changing additives and reaction conditions (see Experimental section and Table S1 in Supporting information for details).Taking the formation process of chain-like Ni/rGO as an example, the forming mechanism has been studied deeply, which is shown in Fig.S1 (Supporting information).The result revealed that chainlike nano-Ni loaded on rGO was able to exert the more absorbing potential of hybrids comparing to others with particle-like, coinlike and flower-like topography nano-Ni.More importantly, the absorbing mechanism in chain-like Ni/rGO hybrids was proposed based on EM loss theory, which may provide a reference for the design of the EMA material’s perimeter structure.

On the basis of the transmission line theory,the reflection loss value of EMA material can be evaluated by the following equations (Eqs.1 and 2) [21]:

where Zinis input impedance,Z0is the impedance of free space,f is frequency, d is thickness and c is the velocity of light.When reflection loss(RL)<10 dB,more than 90% of EM wave is absorbed,and the corresponding frequency range is called effective absorption band (EAB).

The morphologies and microstructures of Ni/rGO were characterized by SEM(Fig.1).Fig.1a reveals the particle-like Ni with size of about 200 nm is uniformly and regularly anchored on rGO surface.Fig.1b exhibits that the chain-like Ni was formed by connecting Ni nanoparticles each other with an average diameter of 0.8 mm,which closely adheres on rGO surface.Noteworthily,the unique one dimension (1D) structure showing a large long diameter and strong magnetic anisotropy is conducive to receiving more EM waves[22].By contrast,coin-like Ni seems to easily cause agglomeration and densification on rGO surface due to mutual inplane attraction (Fig.1c), which is adverse for the dispersion and interfacial polarization of the absorbing material.Fig.1d reveals the flower-like Ni embedding on the surface of rGO.In addition,rGO in all samples exhibits a crepe-like transparent gauze structure.The appearance of wrinkles is mainly due to the reduction of the carbon skeleton integrity and the residual of oxygen-containing functional groups during the reduction of GO,which is beneficial to increase the transmission path and space of EM waves and dipole polarization [23].

Fig.1.SEM images of Ni/rGO hybrids with (a) particle-like, (b) chain-like, (c) coin-like and (d) flower-like Ni; (e) XRD and (f) Raman patterns of them.

Fig.2.Frequency dependence of (a) real parts of complex permittivity; (b)imaginary parts of complex permittivity;(c)real parts of complex and(d)imaginary parts of complex permeability for Ni/rGO hybrids.

Fig.3.(a) Attenuation constant; (b) dielectric/magnetic loss tangent; (c)impedance match of Ni/rGO hybrids and(d) corresponding RL-f curves at 2.0 mm.

As shown in Fig.3a, attenuation constant α, representing the ability of converting incident EM waves into heat or other energy,can be indicated as:

Especially, 1D Ni nanochain with antenna-like structure is considered to be able to form point charge and micro-current at the tip to better accept EM waves.Contrary to attenuation ability,chain-like Ni/rGO hybrid shows the closest tangent values(tandetandm), which implies a more suitable impedance matching [29].From Eq.2, the values of |Zin/Z0| representing the impedance matching were calculated and shown in Fig.3c.It is obtained that in 1016 GHz, the |Zin/Z0| value of chain-like Ni/rGO hybrid is closer to 1 comparing to others,meaning that more EM waves can be received.Recently, on the basis of the impedance matching theory,the degree of impedance matching()can further confirm this explanation, which can be determined as follows (Eqs.4–6)[30]:

Fig.4.(a) RL-f and (b) 3D RL-f-d curves of chain-like Ni/rGO with various thicknesses; (c, d) EMA mechanisms of chain-like Ni/rGO hybrid.

According to Eq.2, d is one of the key parameters of EMA performance.Therefore, in order to better understand the EMA performance, Figs.4a and b depict the RL-f curves and 3D curves of chain-like Ni/rGO at different sample thickness.It can be seen that at 1.8 mm, a double absorption peak occurs at 15.4 and 16.2 GHz with the RLminof-25.0 and-43.7 dB,respectively.The existence of double absorption peaks is conducive to strong EM wave loss in multiple frequency bands.Besides, at this thickness, the EAB can reach to 4.5 GHz from 13.5 GHz to 18 GHz.Further increasing the thickness to 2.0 mm, the corresponding EAB can be widened to 6.1 GHz.In particular, as the thickness increases, the absorption band gradually moves towards lower frequencies (Fig.S5 in Supporting information), which can be well illustrated by the quarter-wave (/4) cancellation theory [26,31], that is, when incident wave and reflected wave areout of phase, the reflected wave can be completely eliminated at the surface of absorbers,thus resulting in the migration of the absorption band.The 3D RL-f-d curves of other samples are also reflected in Fig.S6(Supporting informaton).In consistent with the analysis results,chain-like Ni/rGO reveals the best EMA effect at different sample thickness.Compared with previous reports(Table S2 in Supporting informaton) [18,29,32–36], our chain-like Ni/rGO hybrid has the advantages in light weight,strong absorbing ability,thin thickness and absorbing frequency bandwidth.

The above results confirm that the anchored nano-Ni on rGO with different morphologies have significant effects on the EMA properties of hybrids.Chain-like Ni/rGO reveals a strong absorption ability due to the good impedance match and the synergistic loss effect between dielectric loss (e.g., conductance loss and polarization relaxation) and magnetic loss (e.g., magnetic resonance and magnetic coupling effects)as shown in Fig.4c.In order to understand the full EM loss mechanism, the possible interactions between the EM wave and Ni/rGO are summarized in Fig.4d and Fig.S7(Supporting information).First of all,when 1D chain/antenna-like Ni interacts with alternating EM field, the electrons accumulate at the tip of Ni chains,which can form a point charge, and then EM energy is converted into a micro-current by means of electronic conduction to be dissipated[37].Secondly,the chain-like features of Ni/rGO are considered to form a spatial 3D network in the paraffin sample to amplify the magnetic coupling effect and form a stable attenuation effect on the applied EM field.Thirdly,in Ni/rGO hybrid,the large aspect ratio of 1D chain-like Ni and 2D rGO prolongs the EM wave transmission path, which is beneficial to attenuate EM waves.Correspondingly, the thermal energy generated by the EM reaction can be transferred to the thermal conductive rGO on both sides in time, thereby achieving the effect of rapid heat dissipation.Fourth, under the alternating EM field,dipole polarization relaxation and interfacial polarization relaxation caused by the rearrangement of electrons between Ni/rGO, Ni/paraffin, and rGO/paraffin are also considered to be an important dissipation mechanism.

In summary, magnetic nano-Ni with diverse morphologies loading rGO hybrids were synthesized via hydrothermal in-situ reduction method.Morphology and structure characteristics of Ni/rGO hybrids were cleared through XRD, SEM, FT-IR and Raman tests.In addition, the EM parameters of Ni/rGO samples were tested and discussed between 2-18 GHz.Significantly, results revealed that chain-like Ni/rGO hybrid show an excellent impedance matching and the synergistic loss effect between dielectric loss and magnetic loss,which heavily improves its acceptance and attenuation of EM waves.When the absorber thickness is 1.8 mm,the RLmincan reach-43.7 dB,as well as EAB can come up to 6.1 GHz at 2.0 mm.In short, chain-like Ni/rGO hybrid exhibits the promising characters of wide absorbing waveband,thin thickness and strong EMA performance,which are expected to be applied in military and commercial fields.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge the financial support for this work by the National Natural Science Foundation of China (Nos.51903223 and U1704162), China Postdoctoral Science Foundation (No.2018M642781).

Appendix A.Supplementary data

Supplementarymaterialrelatedtothisarticlecanbefound,inthe online version,at doi:https://doi.org/10.1016/j.cclet.2020.06.014.