Carbonized wood with ordered channels decorated by NiCo2O4 for lightweight and high-performance microwave absorber

Wood-derived carbon has a 3D porous framework composed of through channels along the growth direction, which is a suitable matrix for preparing electromagnetic wave (EMW) absorbing materials with low-cost, light-weight, environmentally friendly and excellent MA performance. Herein, the carbonized wood decorated by short cone-like NiCo2O4 (CW@NiCo2O4) with highly ordered straightway channels architecture were successfully manufactured through a facile calcination procedure. Finite Element Analysis (FEA) simulation is carried out to detect the interaction between the prepared material and EMW when the ordered channels are arranged in different directions. Simultaneously, the microwave absorption properties of all samples are investigated in terms of complex permittivity and permeability. The horizontal arrangement of the through channels of CW@NiCo2O4 (H-CW@NiCo2O4) exhibits a strong reflection loss value of -64.0 dB at 10.72 GHz with a thickness of 3.62 mm and a low filling ratio of 26wt% (with the density of 0.98g·cm), and the effective absorption bandwidth (EAB) is 8.08 GHz (9.92-18.0 GHz) at the thickness of 3.2 mm. The extremely advantageous structure of H-CW@NiCo2O4 is the key to achieving excellent MA property, which enables multiple EMW loss mechanisms to be effectively realized. What’s more, the introduction of NiCo2O4 increases the values of ε and ε, resulting in enhanced dielectric loss. This research provides a lowcost, sustainable and environmentally friendly strategy for using carbonized wood to fabricate microwave absorbers with strong attenuation capabilities and lightweight.


Introduction
Due to the serious problem of electromagnetic pollution and the urgent need of military anti-reconnaissance capabilities, a variety of electromagnetic wave (EMW) absorbing materials with excellent performance have been developed, which attenuate EMW by converting them into other forms of energy [1,2]. Among various microwave absorbing materials, the carbonaceous materials have been paid high attention due to their chemical stability, low density and tunable electromagnetic parameters. In past reports, commonly used carbon materials for microwave absorption (MA), such as carbon fiber [3], carbon nanotube (CNT) [4,5] and graphene [6] , exhibited good MA performance. Carbon materials have greatly promoted the development of absorbing materials.
Simultaneously, the concept of sustainable development and environmental friendliness is also being valued more and more. Traditional carbon materials often rely on modern industrial techniques, which are complex and costly, and the by-products of the preparation process can damage the natural environment. Fortunately, the biomassderived carbon overcome the shortcomings of man-made carbon materials with many characteristics such as light weight, large natural reserves and low cost. Nowadays, the natural wood-derived carbon materials with through channels parallel to the growth direction have been successfully applied in many fields, for example, energy storage materials, catalysts and sensors [7][8][9]. The three-dimensional (3D) skeleton composed of orderly-arrangement and through channels existing in wood-derived carbon can also be applied to MA materials, which not only is favor of microwave scattering and multireflections to prolong the propagation path of EMW [10] and forms an efficient conductive network to enhance the conductive loss [11], but also provides a load position for magnetic materials to improve impedance matching and cause magnetic loss [12]. Hence, this 3D porous structure can be used as an ideal carbon substrate for electromagnetic interference (EMI) shielding and MA materials.
Xi et al. reported that wood-based straightway channel structure, which was analogous to a waveguide, exhibited excellent MA performance with maximum reflection loss (RL) of -68.3 dB at 4.28 mm and effective absorption bandwidth (RL < -10 dB) up to 7.63 GHz at 3.73 mm [13]. This unique structure has provided inspiration for the subsequent design of wood-derived carbon MA materials. Zheng et al. have successfully fabricated the wood-derived magnetic porous carbon composites with a highly ordered anisotropic porous architecture on the foundation of the 3D orderlychannel skeleton, this Ni/porous carbon composite exhibited an exceptional EMI shielding effectiveness of 50.8 dB at the whole X band (8.2-12.4 GHz) with the thickness of 2 mm [11].
In order to make full use of the 3D ordered porous framework derived from woodderived carbon, our research introduces NiCo2O4 mixture on the wall of highly ordered straightway channels inherited from wood. The NiCo2O4, a spinel-structure bimetal oxide, owns high electrochemical activity, was mainly used in the electrochemistry field [14,15]. Therefore, from the point of its high electronic conductivity, NiCo2O4 has great potential value in the field of absorber with superior dielectric loss [12,16]. It can be foreseen that various loss mechanisms makes as-prepared samples obtain excellent absorbing performance due to the natural favorable structure, improved impedance matching and optimized conductivity. Furthermore, the finite element simulation of CW@NiCo2O4 testifies that horizontal arrangement of the through channels leads to the excellent EMW attenuation ability. Our study provides an alternative strategy to utilize low-priced, environmentally friendly natural wood and simple and efficient preparation procedure to design microwave absorbers with high absorption efficiency and wide absorption frequency. We hope that this work lays a significant foundation for the design and application of wood-derived carbon absorbers to fulfill the ever-growing demands.

Preparation of carbonized wood (CW)
All reagents used in our experiments were used as received without further purification.
Poplar wood were purchased from plantation (Hubei China).
To prepare the lignin removal solution, 4g NaClO2 (80%, Shanghai Macklin Biochemical CO., Ltd) and 1.35ml CH3COOH (99.5% AR, Tianjin Fuyu Fine Chemical CO., Ltd) were dissolved in 400ml DI water. Then a slice of poplar wood slice cut into 10 mm×10 mm×10 mm was immersed in the as prepared solution in the beaker at 80 °C for 12 h. The lignin-removed wood (LFW) was prepared. Finally, the LFW was placed in a tubular furnace and carbonized at 400 ℃ under flowing nitrogen for 1.5 h with a heating rate of 3 ℃·min -1 . The CW was placed in a vacuum drying oven for further use.

Synthesis of CW@NiCo2O4 composites
The CW was immersed in the solution of 1 mol·L -1 of Ni(NO3)2·6H2O and 2 mol·L -1 of Co(NO3)2·6H2O (98% AR, Tianjin Fengchuan Chemical Reagent Technologies CO.,Ltd) in the beaker, then, the beaker was placed in ultrasonic environment for 30 minutes, then allowed to stand for 12 h with -0.1 MPa environment, and finally heated at 50 ℃ for 6 hours. The CW was fully saturated with Ni(NO3)2·6H2O and Co(NO3)2·6H2O solution. Subsequently, the CW/Ni(NO3)2·6H2O/ Co(NO3)2·6H2O composite was placed in a drying oven for 12 h with 60 ℃. Finally, the as-obtained material was placed in a tubular furnace for heating to 300 ℃ for 1 h in air. After the temperature dropped to room temperature, the CW@NiCo2O4 hybrid was obtained. For comparison, the CW was also post treated by the same calcination conditions, the post-treated CW was prepared.

Characterization
Powder XRD pattern was recorded on BRUKER diffractometer using Cu Kα radiation. The morphology and microstructure were observed using SEM images on SUPRA™55 (ZEISS, United Kingdom) at high current of 20 kV. The elemental composition was detected by energy dispersive spectrometer (EDS) spectrum deriving from the SEM, coupled with copper grids. Raman spectra were obtained by a Renishaw Raman microscope using an Ar ion laser (532 nm). Thermogravimetric analysis (TGA) measurements for the composite were carried out by a DTG-60H thermal analyzer under flowing oxygen atmosphere and with a heating rate of 10 ℃ min −1 . The X-ray Photoelectron Spectroscopy (XPS) measurements were recorded on KRATOS Axis Ultra DLD equipped with a monochromatic X-ray source (Al Ka, hν = 1486.6 eV). The room temperature hysteresis loop was performed on vibrating sample magnetometer (VSM, JDAW-2000C&D, Changchun Yingpu CO.,Ltd).

Electromagnetic parameters measurements
The EM parameters were obtained on vector network analyzer (Agilent, N5230A) by the T/R coaxial line method at 2-18 GHz band. Testing samples of microwave absorption were prepared by cutting method, which can retain the micron-level structure inside the material. Two pieces of CW@NiCo2O4 material were respectively tailored into cylindrical toroidal specimens along parallel to the wood growth direction and perpendicular to the wood growth direction (called vertical and horizontal) with an outer diameter of 7.00 mm and inner diameter of 3.00 mm and then were immersed in liquid paraffin wax, after solidification, the sample was prepared, named V-CW@NiCo2O4 and H-CW@NiCo2O4 respectively. In the same way, the CW material was cut into cylindrical toroidal specimens, named V-CW (vertical) and H-CW (horizontal) respectively. The average filling ratio of coaxial sample was 26 wt% (with the density of 0.98g·cm -3 ) and 21 wt%, respectively, obtained by weighing several times.
In addition, the disordered samples (named CW and CW@NiCo2O4) were prepared by mixing the powders with paraffin matrix and pressed into a toroidal ring with an outer diameter of 7.00 mm and inner diameter of 3.00 mm with the same filling ratio of ordered-channels samples. The MA performances reflection loss (RL) values were obtained by EM parameters on the basis of transmission line theory, which can be calculated by the following equations based on the metal back-panel model [17,18].
Here, Z in is the input impedance of absorber, 0 is the impedance of free space, ε r and μ r represent the complex permittivity (ε r = ′ − ′′ ) and the complex permeability ( = ′ − ′′ ), respectively, indicates the frequency of the incident microwaves, means the thickness of the microwave absorber, and is the velocity of the microwave. RL is smaller than -10 dB, it is implied that more than 90% of the microwave is absorbed by MA, and the frequency range can be considered as effective absorption bandwidth (EAB) [19].

Results and discussion
The preparation procedure of CW@NiCo2O4 with the structure of oriented channels and standard coaxial samples was schematically illustrated in Fig. 1. The mass fraction of carbon in CW@NiCo2O4 composites is obtained by TGA, described in Fig. S1.
Raman spectrum was applied to demonstrate the chemical environment of carbon atoms in the CW and CW@NiCo2O4 powder, which is an important factor for electron transportation and make a significant impact on the electromagnetic parameters [20].
In addition, Raman spectrum was also used to characterize the existence of cobalt and nickel atoms. Two obvious peaks located at 1320 cm -1 (D band) and 1580 cm -1 (G band) can be demonstrated in Fig. 2b. The intensity of the D band peak represents the number of defects or the degree of disorder in the sp 2 -hybridized carbon atoms or amorphous carbon deposits, while the G band peak is associated with the in-plane vibrations of sp 2 atoms in a 2D hexagonal graphitic lattice. The degree of disorder of carbon in the material can be characterized by the ratio of D band to G band (ID/IG) [21]. As shown in Fig. 2b, the intensity ratio value of ID/IG was 0.85 for CW, and the ID/IG value of CW@NiCo2O4 was 0.78, which suggested abundant defects exist in both CW and CW@NiCo2O4, and the existence of NiCo2O4 reduced the degree of disorder of carbon in the material, increased the graphitization degree. The Raman spectrum of spinel Co3O4 has a high-frequency peak at 693 cm -1 ，determined by the octahedral/Co 3+ [22].
As Co 3+ was replaced by Ni 2+ , the octahedral/Co 3+ peak weakens and shifts to lower frequency (Fig. 2b, the band at 650 cm -1 of CW@NiCo2O4), simultaneously, the octahedral/Ni 2+ peak was observed at 502 cm -1 result from Ni 2+ substitution at octahedral sites. The peak at 460 cm -1 was caused by tetrahedral/Co 2+ . It can be inferred that the Ni 2+ substitution at octahedral sites would induce the formation of dipole and dipole polarization [22], which is a benefit for EM wave attenuation and thus results in enhanced microwave absorption properties.  . 2e). Similarly, the Co 2p spectrum given in Fig.   2f consists of one spin-orbit doublet characteristic of Co 2+ (binding energies at 781.9 and 796.8 eV) and Co 3+ (binding energies at 779.9 and 795.0 eV) and two shakeup satellites (binding energies at around 787.9 and 803.7 eV, identified as ''Sat.''). The high-resolution C 1s spectrum (Fig. 2d) consisted of three carbon peaks, which represent C-C/C=C (284.7 eV), C-O (285.8 eV) and C=O (288.7 eV), respectively [23].
As shown in Fig. S2, the O 1s spectrum shows three peaks located at 529.5, 531.2, and 532.8 eV, which could be ascribed to the metal-oxygen bonds, defects in oxygen and absorbed water on the surface, respectively [24].  SEM images of CW and CW@NiCo2O4 were displayed in Fig. 3. Vertical direction of CW (Fig. 3a) and horizontal direction of CW (Fig. 3b) show that the carbonized wood has through, directional and ordered channels, and the shape of the channels presents an irregular ellipse with a pore size of 10~30 μm and channel wall thickness of 1~3 μm. Vertical direction of CW@NiCo2O4 (Fig. 3c) and horizontal direction of CW@ NiCo2O4 (Fig. 3d) demonstrate that the high-density short cone-like NiCo2O4 composites uniformly distributed in the through channels in CW. The thickness of carbon channels wall is obviously thinner than that of CW, because a large number of carbon atoms are dispersed in the NiCo2O4 hybrid in the heat treatment process, which can be further confirmed by energy dispersive X-ray spectroscopy (EDS) mapping images (Fig. S4). An image in the inset of Fig. 3c reveals that the magnified cone-like C/NiCo2O4. Simultaneously, the through, directional and ordered channels were retained in the CW@NiCo2O4, this unique ordered pore microstructure can enable abundant multiple reflections of electromagnetic waves and interfacial polarization, which are favorable for the microwave absorption performance. Generally, the hybrid magnetic/dielectric composites present excellent EMW absorption capabilities due to electromagnetic complementation and impedance matching effects. Our ordered-channel CW@NiCo2O4 composites with unique microstructure perform excellent microwave absorption abilities. Impedance matching ratio (| ∕ 0 |) is also an important parameter for EMW absorption. The impedance characteristic of the absorber close to that of free space (the value of | ∕ 0 | close to 1), which means that the incident EMW can enter the absorber to the greatest extent [16].   (3)  represent dielectric and magnetic loss capacities of the absorber, respectively [29].
From Fig. 5a and 5b, it is observed that the ′ and ′′ values of the H-CW@NiCo2O4 greater than that of the other three samples, and possess a decreased (4) trend as the frequency increases, which is attributed to the increased polarization hysteresis versus the higher frequency electric-field variation [30]. As for V-CW@NiCo2O4 sample, its ′ and ′′ values are slightly higher than that of V-CW  Generally speaking, the dielectric loss is closely associated with conductive loss and polarization loss, and the conductive loss lies on the electrical conductivity [31].
The introduction of NiCo2O4 could effectively enhance the conductive loss result from the high electrical conductivity. In detail, the NiCo2O4 hybrid uniformly distributed in the through channels formed by amorphous carbon, which allows the CW@NiCo2O4 composites to form an efficient conductive network, which could be confirmed by the ′′ curves of as-prepared samples, the ′′ values of CW@NiCo2O4 composites are higher than that of single CW. Simultaneously, the H-CW@NiCo2O4 sample displays the greatest conductive loss capacity. The polarization loss comes from atomic polarization, electron polarization, dipole polarization and interfacial polarization [32]. The atomic polarization and electron polarization could be eliminated firstly, because these two polarizations could only appear at higher frequency range. The dielectric loss for as-prepared samples should be mostly ascribed to the dipole polarization, which can be explained by the Cole-Cole model [33][34][35]: Here, , ∞ , and stands for the static dielectric constant, the dielectric constant at infinite frequency, the frequency and the polarization relaxation time, respectively. And then the ′ and ′′ can be expressed as: 2 2 Consequently, the relationship between ′ and ′′ can be depicted as: Therefore, the relationship between ′ and ′′ can be determined as a single semicircle, called the Cole-Cole semicircle, each Cole-Cole semicircle represents a Debye dipolar relaxation process. Fig. 6c, 6d and Fig. S7 demonstrates the Cole-Cole curves of all tested samples in the 2-18 GHz frequency range. As mentioned above, enhancement of the Debye dipolar relaxation is definitely reflected in the increasing number of semicircles [36,37]. As for V-CW and H-CW (Fig. S7), the points on the curves are concentrated in a small range, which will lead to the existence of a large number of Cole-Cole semicircles. It can be inferred that the numerous defects within the CW were served as the polarization centers to promote the formation of Debye   loss tangent tan of these two samples were depicted in Fig. 5f, the values are almost below 0.2, and there is no obvious resonance peak appears on the curves, which means that the magnetic loss does not contribute much to the MA performance.
In general, magnetic loss mainly result from natural resonance, exchange resonance and eddy current loss in the microwave frequency bands [38].
Here, the low-frequency resonance is mainly assigned to the natural resonance, while the exchange resonance almost takes place at higher frequencies [39]. The eddy current loss effect could be demonstrated by the 0 curve [40]: If the eddy current loss is the dominated factor for the magnetic loss, the value of 0 should be a constant, which makes the 0 value appear as a horizontal line in 0 − curve. As shown in Fig. 6b, in 2-7 GHz frequency range, the noticeable resonance peaks can be detected, which is assigned to natural resonance. The 0 curve of H-CW@NiCo2O4 is nearly never change in the frequency range of 7-18 GHz, while a tiny resonance peak was found in the 0 curve of V-CW@NiCo2O4 at 10.5 GHz. The great difference of MA performance between H-CW@NiCo2O4 and V-CW@NiCo2O4 samples is discovered in Fig. 4 and Fig. S5c, while the magnetic parameters of them behave basically similar, which implies that the dielectric loss controlled by microstructure and composition is the main influence factor on absorbing electromagnetic wave.
In order to explore the influence of the arrangement direction of ordered channels on the performance of MA, the two channel arrangement directions of CW@NiCo2O4 composite were constructed as simplified models for finite element analysis (FEA) simulation. The simplified model of V-CW@NiCo2O4 is demonstrated in Fig. 7, with the channel diameter and channel wall thickness set to 10 μm and 3 μm, respectively, which is derived from the SEM pictures of actual structure shown in Fig. 3c and 3d. In addition, the inside of the channels is filled with paraffin wax, and the section to be explored is marked in the model diagram. The frequency parameter (8) of the simulation calculation process was set to 10.72GHz. The time-average power flow ( ) of section 1 and section 2 are shown in Fig. 7b and Fig. 7c, respectively.
The can be regarded as the propagation path and energy distribution of EMW inside the material, which is reflected in the direction and size of arrow.
Obviously, the vertical channel is less obstructive to the EMW, and the EMW can easily pass through the channel without sufficient interaction with the material.
Furthermore, Fig. 7d, 7e and 7f proves that V-CW@NiCo2O4 composite has little attenuation ability to EMW. As for H-CW@NiCo2O4, the transmission direction and intensity of EMW change significantly near the channels wall (Fig. 8b), which indicates that the material has an effective interaction with microwave, due to multiple reflection and scattering caused by ordered porous structure. Simultaneously, as shown in Fig. 8c, 8d and 8e, strong dielectric loss occurs near the channels wall, result from the high-efficiency conductive network formed by carbonized wood decorated with NiCo2O4 hybrids and polarization relaxation. It is worth noting that the intensity distributions of total loss power density (Fig. 8c) and dielectric loss power density ( Fig. 8d) are very similar and the intensity of magnetic loss power density (Fig. 8e) is extremely low compared to dielectric loss power density. In other words, the contribution of magnetic loss to MA is extremely small compared to dielectric loss, this result is consistent with the previous analysis of EM parameters.  In summary, the possible MA mechanism of the H-CW@NiCo2O4 hybrid composites is described in Fig. 9. Firstly, the NiCo2O4 hybrids infiltrated into a large number of carbon atoms are uniformly distributed on the wall of the ordered channels, result in the conductivity of the material is effectively enhanced, so that the 3D conductive skeleton derived from natural wood would promote the electron transportation and improve the dielectric loss ability. Secondly, the interface between carbon and NiCo2O4 would form non-uniform charge distribution and interfacial polarization loss, because of the difference of electrical conductivity. Thirdly, the presence of defects within the amorphous carbon and NiCo2O4 hybrids would serve as the polarization centers to induce the multiple reflection and scattering processes.
Fourthly, the magnetic loss including natural resonance and eddy current loss should also contribute to the microwave absorption property to a slight extent. Therefore, benefiting from the matched impedance and the enhanced attenuation constant, the carbonized wood with horizontally ordered channels decorated by cone-like NiCo2O4 hybrids (H-CW@NiCo2O4) cause the outstanding MA performance.

Conclusion
This work reported a wood-derived carbon-based material with ordered channels decorated by NiCo2O4 hybrids, which is manufactured through a facile, low-cost and sustainable method with outstanding EMW absorption performance. The combination of 3D carbon skeleton with horizontal ordered channels derived from natural wood and NiCo2O4 hybrids together enable multiple EMW loss mechanisms to be