Nickel Carbon Composite With Carbon Nanotubes for Ecient Electromagnetic Wave Absorption

Nickel carbon composite with carbon nanotubes (Ni-C/CNTs) has been fabricated by pyrolysis the mixture of nickel-based metal organic framework (Ni-MOF) and melamine. The resultant Ni-C/CNTs composite is assembled from one-dimensional (1D) CNTs and three dimensional (3D) spherical Ni-C composite. Besides, the Ni-C/CNTs composite contains abundant nitrogen(N) dopants that contributes to defect dipole polarization. The diameter and length of CNTscan beeffected by changing the mass ratio of Ni-MOF and melamine. The optimizedNi-C/CNTs composite exhibits reection loss of -55.1 dB at 10.56 GHz, and the effective absorbing bandwidth (reection loss < -10 dB) is 11.2 GHz (6.0-17.2 GHz)with the thickness range of 1.5-4.0 mm, when the mass ratio of Ni-MOF: melamine is 1: 2. These results indicate that the mixed 1D-3D hierarchical architecture synergistically improves electromagnetic wave absorption properties. This strategy willcontribute for fabricating the carbon hybrid network consisting of metal organic frameworks derived metal/carbon hybrid and CNTs for electromagnetic wave absorption.


Introduction
Recently, with the popularity of local area networks, radar systems, as well as the extensive application of computers, mobile phones and other electronic equipment, the problem of electromagnetic (EM) interference tends to be serious. Therefore, it is very important to effectively suppress and reduce EM radiation. EM wave absorbing materials can absorb and attenuate the incident EM wave, which provides an effective solution for the EM wave pollution [1,2]. With the continuous development of research and application of EM absorbing materials, the types of materials tend to be diversi ed and complicated [3,4]. The new EM absorbents need to to satisfy the characteristics of low thickness, lightweight, wide absorption frequency bandwidth and strong absorption performance [5,6].
Metal organic frameworks (MOFs) consisting of metal ions and organic ligand possess the advantages of tunable porosity and controllable microstructure [7]. Over the past decade, MOFs and their derivatives (metal oxide and metal carbon composites) have received considerable attention in many applications [8][9][10]. Magnetic metal/carbon composites derived from MOFs inherit the morphology and the pore structure of original MOFs. Besides, magnetic particles are wrapped by carbon and uniformly dispersed in carbon matrix [11][12][13]. Especially in the eld of EM absorption, the MOFs containing magnetic metal ions (Fe 3+ , Co 2+ and Ni 2+ ) derived metal/carbon composites have both magnetic loss and electric loss, which provide multiple ways for the attenuation of EM waves. For example, Peng et al. [14] fabricated ower-like Fe 3 C/C and Fe 3 C/Fe/C composites by carbonization of MIL-101 with a maximum re ection loss (RL max ) of -39.43 dB and − 20.31 dB at 2 mm, respectively. Wang and co-workers [15] used Co-MOF-74 as the precursor to fabricate porous Co-C core-shell nanocomposites with RL max of -62.12 dB at 11.85 GHz. Ni/C composite prepared by pyrolyzed nickel based MOF at high temperatures showed a strong RL of -86.8 dB at the thickness of 2.7 mm [16]. Nevertheless, for MOFs derived metal/carbon composites, it is still di cult to achieve excellent EM wave absorption performances with low-thickness regions and broad effective absorption bandwidth (EAB).
1D CNTs have unique hollow tubular structure, large aspect ratio, prominent electrical and mechanical properties [17,18], which make them have a good application prospect in EM wave absorbing materials.
However, the weak impedance matching limits the practical application of CNTs as EM wave absorbent [19,20]. Therefore, combining CNTs with magnetic materials is one of the effective strategies to get desirable EM wave absorbent. However, the CNTs require being pretreatment by complicated chemical process before they can be combined with other materials [21,22]. Studies on utilizing facile synthesis methods to prepare the composites of CNTs and magnetic materials are rarely reported.
The structure of EM absorbing material is the key of absorbing EM wave. Many studies have been proved that the mixed hierarchical architecture could integrate the advantages from different structural units [23,24]. Herein, in this work, 3D Ni-MOF microsphere is used as the precursor, and melamine is used as initiator for the growth of 1D CNTs. After pyrolyzing the composite of melamine/Ni-MOF at argon atmosphere, Ni-C/CNTs composite is obtained. The Ni-MOF microspheres are transformed into Ni-C composite and the generated Ni nanoparticles can also be used as the catalyst for CNTs growth. 2.2 Synthesis of Ni-MOF. Ni-MOF was synthesized by solvothermal method similar to the previous report [25]. 864 mg of Ni(NO 3 ) 2 ·6H 2 O, 300 mg H 3 BTC and 3 g PVP-K30 were mixed with 60 mL DMF and stirred until the solution became light green transparent. Then it was sealed into a 100 mL Te on-lined stainlesssteel autoclave and maintained at 150 o C for 10 h. The obtained green precipitate was washed with ethanol and dried at 60 o C for 12 h.
2.3 Synthesis of Ni-C/CNTs composite. 400 mg of as-prepared Ni-MOF was mixed with various amounts of melamine (400 mg, 800 mg and 1200 mg) in 100 mL ethanol and stirred for 12 h at 25 o C. After that, the suspension was dried at 60 o C in air until the solvent completely evaporated. Afterward, it was rstly pyrolyzed at 550 o C for 3 h with heating rate of 2 o C min − 1 and then subsequently at 700 o C for 3 h with a heating rate of 3 o C min − 1 . The obtained Ni-C/CNTs composites were recorded as Ni-C/CNTs-1, Ni-C/CNTs-2 and Ni-C/CNTs-3, respectively. As a contrast, the pure Ni-MOF was also pyrolyzed at the same conditions, and it was named as Ni/C composite. 2.4 Characterization. The crystal structure of samples was investigated by X-ray diffraction (XRD, D8 Advance, Germany). The morphology of products was characterized by eld-emission scanning electron microscope (FE-SEM, Hitachi, S-4800) and transmission electron microscope (TEM, JEM-2100). Raman spectra (Raman, DXRxi) was used to study the graphitization degree of samples. The surface composition of composites was performed using X-ray photoelectron speatra (XPS, AXIS SUPRA). 2.5 Performance Measurements. The complex permittivity ε r (ε r = ε' -jε") and permeability µ r (µ r = µ' -jµ") of samples were measured by vector network analyzer (VNA, HP8720ES, Agilent, USA) in the range of 2-18 GHz using the coaxial method. Before testing, the mixture containing 30 wt% the as-prepared materials (Ni/C, Ni-C/CNTs-1, Ni-C/CNTs-2 and Ni-C/CNTs-3) and 70 wt% wax was uniformly mixed at 80 o C and then pressed into a toroidal ring (outer diameter: 7.0 mm, inner diameter: 3.04 mm and thickness: 3.0 mm).

Results And Discussion
The preparation process of Ni-C/CNTs composite is schematically illustrated in Scheme 1. The microspherical Ni-MOF is prepared by solvothermal reaction which is used as precursor to prepare Ni-C/CNTs composites. The precursor is mixed with melamine and pyrolysed by two-stage programmed heating process. The melamine is transformed to g-C 3 N 4 at 550 o C for 3 h under argon atmosphere through its poly-addition and thermo-polymerization [26]. Then it permeates into the Ni-MOF microspheres. Finally, metal cations are reduced to metal nanoparticles and can be used as catalysts to facilitate the formation of CNTs after further increasing the temperature to 700 o C for 3 h. As a result, the Ni-C/CNTs composite is obtained.
The morphology of Ni/C and Ni-C/CNTs composites are observed by FE-SEM. The Ni/C composite inherits the spherical structure of Ni-MOF precursor, as exposed in Fig. 1(a). Without the melamine participating in the reaction, CNTs cannot be generated. SEM images of Ni-C/CNTs composites are displayed in Fig. 1(b-d). Apparently, the diameter and length of CNTs vary with the amount of melamine. Ni-C/CNTs composites are composed of microspheres linked with plenty of exible nanotubes.
As observed in TEM images of Fig. 2(a-d), Ni-MOF has evolved into Ni/C composite with the initial spherical structure maintained, and nickel ions in Ni-MOF are transformed into nickel nanoparticles, which are embedded in carbon shell. Under pyrolysis process under argon atmosphere, PVP and organic ligands of H 3 BTC are converted into carbon. The TEM images for representative sample of Ni-C/CNTs-2 composite as shown in Fig. 2(e-g). it consists of Ni-C composite and CNTs with Ni nanoparticles encapsulated in nanotubes. The generated Ni nanoparticles could be regarded as the catalyst to promote the conversion of melamine into CNTs. HR-TEM images (Fig. 2(h) [29]. After melamine participates in the reaction, a broad peak at approximately 2θ = 25° is the (002) peak of graphitic carbon materials [30]. Diffraction peaks of metallic nickel for Ni-C/CNTs composites are bigger than those of Ni/C composite, implying the better crystallinity of cubic metallic nickel.
The relative graphitization degree of carbon and carbon nanotubes in these composites are discerned by Raman spectra. Two typical peaks located at ~ 1350 and ~ 1590 cm − 1 present the so-called D band and G band that reveals the characteristic of disordered and graphitic carbon, respectively [31]. The relative graphitization degree of carbon materials is evaluated by the intensity ratio of D and G band (I D /I G ) [32].
XPS measurement for the representative Ni-C/CNTs-2 composite is conducted to investigate the element valence state of material. The wide scan XPS spectrum (Fig. 5(a)) veri es the existence of C, N, O, and Ni elements. Figure 5 Both ε r and µ r are the basis for judging the absorbing performance of the wave absorber. The real parts (ε' and µ') represent the storage capability and the imaginary parts (ε″ and µ″) are related the dissipation ability of electric and magnetic energy [38]. As discovered in Fig. 6, the content of melamine plays an important role in modifying the EM parameters of Ni/C composite. From Fig. 6(a), the ε' values reduce as the frequency increases, roughly obeying the frequency dispersion behavior [39][40][41][42]. The ε' values of Ni/C composite are enhanced by doping CNTs, which may be caused by the polarization of interfacial and dipole. The ε' values of Ni-C/CNTs-3 are highest among the samples, which suggests highest energy storage and polarization action. The ε″ curves (Fig. 6(b)) show similar trend with ε′ curves and the ε″ values for Ni-C/CNTs composites (Ni-C/CNTs-1, Ni-C/CNTs-2 and Ni-C/CNTs-3) are bigger than those of Ni/C composite at 2-11 GHz. Therefore, CNTs can signi cantly change the dielectric constant of Ni-C composite. Besides, the downward tendency for Ni-C/CNTs composites are more obvious than that of Ni/C composite indicating that the former can produce more electric dipoles [43]. In Fig. 6(d), the µ' values of all composites exhibit decrease (2-6.2 GHz) at rst, then increase, and last decrease with increasing frequency. It can be observed from Fig. 6(e) that the µ″ values for Ni-C/CNTs composites are higher than those of Ni/C composite at the frequencies of 9-18 GHz. The tanδ ε = ε″/ε′ and tanδ µ = µ″/µ′ values are used to characterize the dielectric loss and magnetic loss ability of absorbers. Besides, it should be noted that in Fig. 6(c, f) the values of tanδ ε in 2-11 GHz are larger than tanδ µ values, while in high frequency region this phenomenon is contrast, suggesting that the dielectric loss and magnetic loss are the dominated electromagnetic attenuation loss mechanism for the obtained Ni/C and Ni-C/CNTs composites at low frequency and high frequency range, respectively.
The polarization relaxation process of materials has a great in uence on dielectric loss. The Cole-Cole curves of simples are presented in Fig. 7. The relationship of ε' and ε″ satis es the following Formula [44]: where ε s refers to the static permittivity. ε ∞ presents relative dielectric permittivity at the high-frequency limit. One semicircle appears in the ε'-ε″ diagram corresponds to a polarization relaxation process, suggesting a Debye relaxation process. The multiple semicircles in Cole-Cole plot manifests the existence of multiple relaxation processes under microwave irradiation. This phenomenon is caused by dipole polarization relaxation and interfacial polarization relaxation [44]. Defects such as doped elements (N and O) in carbon nanotubes and carbon skeletons act as jumping centers to endow electrical dipoles in composites, resulting in stable dipole polarization relaxation. The accumulation of interfacial charges between Ni nanoparticles, C and CNTs lead to interfacial polarization. The straight line at the end of Cole-Cole curves is related to the conduction loss that mainly comes from C, CNTs and Ni particles.
The magnetic loss is also important for absorbing the EM waves. Generally, the magnetic loss mainly comes from domain wall resonance, hysteresis loss, and eddy current loss [45]. Domain wall resonance loss and hysteresis loss always occur at low frequency [46]. If the values of C 0 = µ″(µ′) −2 f − 1 keep constant with the increase of frequency, the eddy current loss contributes to magnetic loss. Contrarily, if the values of C 0 change a lot, the magnetic loss mainly originates from natural resonance and exchange resonance [47]. The values of C 0 for all composites are investigated in Fig. 8. Obviously, C 0 curves of composites uctuate over the entire frequency range, manifesting that natural resonance and the exchange resonance dominate the magnetic loss.
The RL values are de ned by the following equations [22]: where Z in denotes the normalized input impedance and Z 0 is the impedance of free space, respectively, f is the EM frequency, d is the thickness of the absorber, c is 3 × 10 8 m/s. Particularly, when the RL≤ -10 dB, 90% of incident EM wave will be attenuation. Figure 8 exhibits three dimension and two dimension curves of RL for simples at 1.5-4.0 mm. The RL max value of Ni/C composite is -21.7 dB at 8.6 GHz when the matching thickness is 4.0 mm ( Fig. 9(a, e, i)). For the Ni-C/CNTs-1 composite ( Fig. 9(b, f, j)), the RL max value of -24.1 dB at 2.5 mm is obtained at 10.9 GHz. Besides, when the thickness is 1.7 mm, the superior EAB is 5.7 GHz (12.3-18 GHz) at the corresponding RL max value of -21.3 dB (Fig. 9(f)). As for Ni-C/CNTs-2 composite, the RL max value is -55.1 dB at 10.56 GHz at 2.5 mm (Fig. 9(c, g, k)). Moreover, by changing the thickness from 1.5 mm to 4.0 mm, the EAB reaches 11.2 GHz (6.0-17.2 GHz). However, the RL max value of Ni-C/CNTs-3 composite is only − 17.5 dB at 13.6 GHz with a thickness of 1.5 mm ( Fig. 9(d, h, l)).
Overall, CNTs can effectively improve the EM wave absorption properties of Ni-C composites.
Impedance matching is one of important parameters to determine the EM absorption properties. When the surface resistance of absorber is close to the resistance of free space (|Z in /Z 0 | = 1), the external EM wave penetrate the absorber. Figure 10 compares the |Z in /Z 0 | values of Ni/C and Ni-C/CNTs composites with a constant sample thickness of 2.5 mm. Apparently, the |Z in /Z 0 | values of Ni-C/CNTs-2 composite is closest to 1, which indicates that the Ni-C/CNTs-2 composite has best impedance matching compared with other composites. Accordingly, the RL max of -55.1 dB is achieved for Ni-C/CNTs-2 composite at the same frequency when |Z in /Z 0 | = 1.0, which satis es the near zero re ection condition. According to the above analysis, the feasible EM wave absorption mechanism of Ni/CNTs composite could be attributed to the following aspects, as shown in Scheme 2. Firstly, suitable impedance matching is a prerequisite for EM waves to enter the material and then be attenuated. Secondly, the accumulation of space charges at multiple interfaces among para n, Ni nanoparticles, carbon, and CNTs cause interface polarization. Besides, these heterogeneous interfaces also facilitate multiple scattering and re ection of incident EM waves. Thirdly, Ni nanoparticles and carbon core-shell structure derived from Ni-MOF provides magnetic loss and dielectric loss. And the CNTs catalyzed by Ni particles are bene cial to the improvement of the conduction loss and provide an effective way for electronic hopping and migration. Finally, plentiful functional groups N doped defects in Ni-C/CNTs composites generate amount of dipoles, which act as polarized centers to attenuate incident EM waves. Therefore, the excellent microwave absorption ability of the Ni-C/CNTs composite is ascribed to the good impedance matching as well as the synergistic effect of magnetic and dielectric loss.

Conclusions
In summary, Ni-C/CNTs composites were prepared via calcining the mixture of Ni-MOF microspheres and melamine through two-stage programmed heating process, which does not require complicated chemical treatment to combine Ni/C composite with CNTs. Meanwhile, each CNT tip is capped with Ni@C coreshell particle, and the spherical Ni/C composite is composed of many Ni@C core-shell particles, which reduce the oxidation or corrosion by air or acid. The as-synthesized Ni-C/CNTs composite exhibits strong EM wave absorption of -55.1 dB with thickness of 2.5 mm and wide EAB 11.2 GHz (6.0-17.2 GHz) as the thickness varies from 1.5 mm to 4.0 mm. This work is expected to pave a way to construct novel materials for EM wave absorption.

Con ict of interest
The authors declare that they have no con ict of interest.