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NiCo2O4 microrod with wideband electromagnetic wave absorption capacity
Journal of Colloid and Interface Science 566 (2020) 347–356
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Filter paper templated one-dimensional NiO/NiCo2O4 microrod with wideband electromagnetic wave absorption capacity Ming Qin, Hongsheng Liang, Xiaoru Zhao, Hongjing Wu ⇑ School of Physical Science and Technology, Northwestern Polytechnical University, Xi’an 710072, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Filter paper was selected as the
template for synthesis of 1D NiO/ NiCo2O4 composite. Ultra simple synthetic procedures of only metal salts adsorption and calcination was referred. The effective bandwidth is up to 6.08 GHz at thin thickness of 1.88 mm.
a r t i c l e
i n f o
Article history: Received 17 January 2020 Revised 28 January 2020 Accepted 28 January 2020
Keywords: Filter paper One-dimensional NiO/NiCo2O4 microrod Wideband electromagnetic wave absorption
a b s t r a c t Filter paper, as a widely used and low-cost lab consumable, can serve as template for the synthesis of one-dimensional (1D) materials since it is composed of 1D cellulosic fiber. In this work, 1D NiO/ NiCo2O4 composite was simply fabricated by adsorption of Ni and Co acetate on the filter paper and subsequent calcination process. The precursors were obtained without further treatment to filter paper or chemical reaction between metal salts. Calcination process leads to the removal of template and formation of binary NiO/NiCo2O4 composite. Electromagnetic (EM) wave absorption performance of NiO/ NiCo2O4 composite could be tuned by adjusting the calcination temperatures and dosage of metal salts. For the optimized absorber (calcination temperature of 600 °C and dosage of Ni and Co acetate with 2 mmol and 4 mmol), a wide absorption bandwidth of 6.08 GHz could be achieved with thickness of 1.88 mm and minimum reflection loss value was up to 57.4 dB by changing the thickness. The facile synthetic route may also inspire the preparation of other high-performance one-dimensional EM wave absorbing materials. Ó 2020 Elsevier Inc. All rights reserved.
1. Introduction In recent years, the electromagnetic (EM) pollution has become an increasing issue that human beings have to face. This pollution originates from the excessive utilization of electronic devices that rely on the EM wave as information carrier. As a consequence, human beings’ health could be severely affected by this issue ⇑ Corresponding author. E-mail address: [email protected] (H. Wu). https://doi.org/10.1016/j.jcis.2020.01.114 0021-9797/Ó 2020 Elsevier Inc. All rights reserved.
[1-6]. Developing EM wave absorbing materials is an effective approach to remove the excessive EM wave and the energy it carries. The EM wave absorbers can consume the energy by dielectric loss and magnetic loss mechanisms, turning it into heat or other kinds of energy. To acquire high-performance EM wave absorbing materials, considerable efforts have been devoted on the synthesis of carbonaceous materials, conducting polymer and ferrites [7-14]. Though great advances have been reported, producing thinthickness, light-weight and wide-bandwidth EM wave absorbers with facile routes are still required.
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Nowadays, it is found that taking advantage of biomass carbon as matrix or template has become an effective approach to prepare high-performance EM wave absorbers [15-22]. This is ascribed to the fact the biomass carbon templates generally possess onedimensional fiber morphology in microstructure. This feature can be readily inherited during the synthesis process. Due to the shape anisotropy and high aspect ratio of this 1D structure, the charge transportation rate can be facilitated along radial direction and the improved interfacial polarization can be achieved as well [23-28]. In consequence, the EM wave absorption performance can be strengthened. For example, Ji’s group reported [22] that carbon cotton/Co@nanoporous carbon products could be fabricated by anchoring ZIF-67 on the biomass cotton and followed calcination process. The optimized sample displayed strong absorption ability of 51.2 dB and effective bandwidth of 4.4 GHz. Likewise, Che and his coworkers also reported that [28] hierarchically tubular C/Co nanoparticle prepared based on kapok fiber template exhibited wide absorption bandwidth of 5.1 GHz and the minimum reflection loss also reached up to 52.3 dB. However, current researches focus more on the synthesis of biomass carbon based EM absorbers by coupling magnetic components to prepare composites that provide both dielectric loss and magnetic loss. To harvest the final composites, tedious synthetic procedures are often required. For example, the biomass-derived carbon/MnO nanorods absorbing material [19] was obtained through hydrothermal reaction and calcination for two times, respectively. The reaction procedures were time-consuming and extra energy was required. Therefore, novel idea should be put up with to address these problems. As another research point, utilization of biomass carbon as bare template has scarcely been reported. Taking advantage of biomass carbon as bare template to directly adsorb reactant solutions and remove it and generate products in air atmosphere may provide a more facile synthetic process to prepare high-performance EM wave absorbers. Filter paper, as an extensive used material in laboratory, also shows great potential as a biomass carbon template. Impressively, due to the essential character of filter paper, the reactant solutions can be directly adsorbed on the surface of filter paper without other reaction or energy consuming synthetic procedure. The products generation reaction and template removal can be simultaneously accomplished during the calcination process. This approach can greatly reduce the synthetic process and provide a more facile route for preparing high-performance 1D EM wave absorbers. It has been demonstrated by previous works that NiCo2O4based materials show potential as effective EM wave absorbing materials [29-35]. However, in view of its practical performance, it is still urgent need to reduce its thickness when high absorption performance is required. Moreover, it has also been confirmed that the NiCo2O4 phase can be generated by the reaction of corresponding metal salts precursors during calcination process [36]. Based on above discussion, it is prospective that one-dimensional NiCo2O4 can be readily fabricated by means of filter paper template method. The metal salts solutions that contain Ni and Co can firstly adsorbed on the filter paper to obtain the precursor with onedimensional microstructure. When subject to heating treatment, the precursors will convert into NiCo2O4 with retained microstructure. Therefore, NiCo2O4 is a reasonable choice as the target product to investigate this synthetic method. To the best of our knowledge, synthesis of one-dimensional NiCo2O4-based materials by simple filter paper adsorption and subsequent heating treatment has not been reported yet. Herein, one-dimensional NiO/NiCo2O4 microrod composite is simply fabricated by facile and low-cost filter paper-template method and subsequent heating treatment. The dosage of metal salts and calcination temperatures were found to be vital parameters that influence the EM wave absorption properties of products.
The optimized absorber displayed the widest absorption bandwidth of 6.08 GHz with thickness of 1.88 mm, which is thinner than previous reported NiCo2O4-based absorbers. In addition, a strong absorption capacity of 57.4 dB could also be achieved by adjusting the thickness. This method provides a reliable template and simple route for the synthesis of one-dimensional ferrite with excellent EM wave absorption performance. 2. Experimental section 2.1. Synthesis of NiO/NiCo2O4 samples Metal acetate of tetrahydrate nickel acetate (Ni(CH3COO)2 4H2O) and tetrahydrate cobalt acetate (Co(CH3COO)24H2O) were purchased from National Reagent Corp. (Shanghai, China). The qualitative filter papers with diameter of 11 cm were obtained from Civil filter paper factory (Fushun, China). The synthesis of one-dimensional NiO/NiCo2O4 microrod EM wave absorbing materials was accomplished through filter paper-template method and followed by calcination process. The synthetic procedures were described as follow. Firstly, 2 mmol of Ni(CH3COO)24H2O and 4 mmol of Co(CH3COO)24H2O were together dissolved in certain amount of deionized water to ensure the thorough dissolution of metal salts. Then, the solution was dropped on both side of a piece of filter paper until all the solution was consumed. After being dried in the oven, the resultant filter paper was cut into small pieces and placed in combustion boat and calcined at 600 °C for 3 h at heating rate of 2 °C/min in air condition to gain the final products. For comparison, the precursors were also calcined at 400 °C, 500 °C and 700 °C to investigate the influence of calcination temperatures on the EM wave absorption performance of onedimensional NiO/NiCo2O4 microrod. In addition, the dosage of metal salts was also optimized by utilization of different amount of Ni(CH3COO)26H2O at 1 mmol and 3 mmol, in which the molar ratio between Ni and Co was still kept 1:2. The NiO/NiCo2O4 samples obtained at different calcination temperatures were labeled as NC2-400, NC2-500, NC2-600 and NC2-700 while the samples acquired at varied metal salts dosage were marked as NC1-600 and NC3-600, respectively. 2.2. Characterization The samples were measured by a series of techniques including X-ray diffraction (XRD, D2 PHASER X), Microscopic confocal Raman spectrometer (WITec Alpha 300R), vibrating sample magnetometer (VSM, Cryogenic CFMS-14T), X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) and field emission scanning electron microscopy (SEM, FEI Verios G4). The electromagnetic parameters were recorded on Agilent N5230A vector network analyzer. The samples for test were prepared by uniformly mixed powders and paraffin with mass ratio of 1:1 and pressed into a columnar ring. The EM wave absorption performance was calculated based on transmission line theory as expressed below:
Z in Z 0 RL ¼ 20log Z in þ Z 0 Z in ¼
rffiffiffiffiffi lr 2pfd pffiffiffiffiffiffiffiffiffi lr e r tanh j c er
ð1Þ
ð2Þ
In these equations, the Zin and Z0 are the input impedance of absorbing materials and the impedance of free space, d is the thickness of the absorbing materials, c is the speed of light in vacuum, f is the frequency of EM wave, lr (lr = l0 -jl00 ) and er (er = e0 -je00 ) are the complex permittivity and permeability of the absorber, respectively.
M. Qin et al. / Journal of Colloid and Interface Science 566 (2020) 347–356
3. Results and discussion The synthetic schematic diagram of one-dimensional NiO/ NiCo2O4 microrods is present in Fig. 1.When homogeneous reactants solution that contains Ni and Co was dropped on the surface of filter paper, the metal ions could be adsorbed on the abundant one-dimensional fibers of filter paper. After the evaporation of water in the filter paper, the metal salts could compactly arrange on the surface of fibers. When the metal salts were fully consumed, the original white filter paper was uniformly covered by the purple metal salts coating. Then, the obtained precursors were subjected to heating treatment in air atmosphere. During this process, the filter paper template can be removed. In addition, the metal salts can successfully convert into the final products. Due to the excellent stability of the filter paper, the one-dimensional fibrous morphology of products could be readily retained. The morphologies of the as-prepared NC2-600 sample are shown in Fig. 2. The distinctive microrods structure can be observed from field-emission SEM. This result confirms that the filter paper can serve as reliable template for the synthesis of 1D NiO/ NiCo2O4 materials. The 1D NiO/NiCo2O4 possess length of around 10 lm and diameter of 3.5–4 lm. The magnified SEM images reveal the rough surface of the microrods and pores are in the presence of the surface, which may be aroused by the release of CO2 during the calcination process. The coarse surface may lead to the multiple reflections and scattering of the incident EM wave, beneficial to the attenuation of the energy it carries. In addition, the NiO/NiCo2O4 nanocrystals are also found to compactly aggregate on the surface of rod structure. EDS mapping demonstrates the uniform distribution of the Ni, Co and O elements in the asobtained sample. Based on the atom ratio obtained from EDS measurement, we can approximately estimate that the molar ratio of NiO: NiCo2O4 in the composite is around 3: 4. The presence of multiple phases of NiO and NiCo2O4 in the sample is beneficial to improve the dielectric loss capacity by providing interfacial polarization. The XRD patterns of these samples are recorded and shown in Fig. 3a. For all the samples, the diffraction peaks in the patterns can be readily assigned to the standard XRD patterns of NiCo2O4 and NiO. This result confirms the NiO/NiCo2O4 composite is successfully prepared. One can see that as the calcination temperatures increase from 400 °C to 700 °C, these diffraction peaks become sharper, proving the better crystalline samples. In addition, the NiO phase is found to be more prominent with gradually ascending calcination temperatures. This phenomenon may be caused by the decomposition of NiCo2O4 thus more NiO phase is induced under higher calcination temperatures. Though the multi-
349
ple phases in NiO/NiCo2O4 composite are conducive to provide enhanced interfacial polarization effect, the excessive NiO phase in the composite may be adverse to the EM wave absorption performance in consideration of its poor attenuation capacity. Thus, the selection of proper calcination temperature is of significance in the synthetic process. The Raman spectra of NiO/NiCo2O4 composite are presented in Fig. 3b. There are four distinctive peaks indexed to the characteristic peaks of spinel NiCo2O4 [37,38]. The peaks ~650 cm1 are corresponding to the vibration of the octahedral oxygen ions in octahedral site. Compared with the original Co3O4 of 691 cm1, the blue shift to lower frequency is caused by the replacement of Ni ions in the octahedral sites. Moreover, a broad peak centered at 1050 cm1 is assigned to the 2 longitudinal optical (LO) combination phonon mode of NiO [39,40]. Therefore, the Raman spectra of these materials also verify the coexistence of NiO and NiCo2O4 in the products. The characteristic peaks of carbonaceous materials’ D and G bands are not detected in these samples, implying the filter paper template has been completely removed during the heating treatment. More detailed information about the chemical states and compositions in the samples was disclosed by XPS analysis. The XPS spectrum of NC2-600 is shown in Fig. 3c-f. Four elements of C, O, Co and Ni are found to coexist in the sample. By using Lorentzian-Gaussian fitting method, the high-resolution spectrum of each element in the samples can be divided into different peaks. For C 1s spectra, three kinds of carbon species CAC, CAO and C@O are detected. The deconvolution of O 1s spectra discloses that there are four O species in the composite, that is, metal–oxygen bond located at 529.6 eV (Ni-O) and 530.4 eV (Co-O), oxygen vacancy at 531.8 eV and chemical or physical adsorbed water at 533.1 eV [35]. Both of the high-resolution spectra of Co and Ni can be fitted with two spin–orbit doublets and two shakeup satellite peaks. These results verify the existence of multiple valence states of Co2+, Co3+, Ni2+, Ni3+ coexists in the as-prepared NiO/NiCo2O4 composite. The highly consistent characterization results of XRD, Raman spectroscopy and XPS demonstrate the successful synthesis of NiO/NiCo2O4 composite. The magnetic properties of these composites are measured at room temperature. It is obvious that the saturation magnetization gradually decreases with the rising calcination temperatures. The NC2-400 possesses the highest saturation magnetization of 8.72 emu/g while the NC2-700 hardly behaves magnetic hysteresis. This result may be ascribed to the fact that the elevated calcination temperatures lead to the presence of more NiO phase, which displays poorer magnetic properties in these composites. As a result, the decline of NiCo2O4 component will result in the
Fig. 1. The schematic diagram for the synthesis of 1D NiO/NiCo2O4 microrods.
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Fig. 2. (a-d) SEM images of 1D NiO/NiCo2O4 microrods, (e-g) corresponding elemental mapping images and (h) EDS pattern of (d).
decreasing saturation magnetization. With regard to the samples obtained with different metal salt dosages, the detail data can be learned from Fig. 4b, which is the magnified image of red square in Fig. 4a. The saturation magnetizations are barely changed compared with NC2-600, further confirming that the saturation magnetization is determined by different proportions of NiO and NiCo2O4 in the composite. The coercivity is also found to decrease from NC2-400 to NC2-700, which may be caused to the grain growth and consequently spin canted effect in the surface layer. As a whole, the poor magnetization values of these samples are adverse to strong magnetic loss ability [41]. Based on transmission line theory, the EM wave absorption performance of these absorbers are calculated and depicted in Fig. 5. Typically, the reflection loss (RL) value lower than 10 dB is regarded as the criterion for effective EM wave absorption. For NC2-400, the widest effective absorption bandwidth (fE) of 1.86 GHz is obtained with thickness of 4.8 mm. When calcination temperature increased to 500 °C, the fE increased to 2.96 GHz and can be accomplished at thinner thickness of 2.0 mm as well. Impressively, the EM wave absorption performance can be dramatically promoted by NC2-600. A wide fE of 6.08 GHz from 11.92 GHz to 18 GHz can be realized at thin thickness of 1.88 mm. In addition, by adjusting the thickness to 2.51 mm, minimum RL (RLmin) of 57.4 dB can be achieved at 9.36 GHz. The corresponding fE also reaches to 2.56 GHz (frequency range from 8.48 GHz to 11.04 GHz). As the temperature further increased, the EM wave attenuation behavior is deteriorated again, which fE can only reach to 2.96 GHz. The influence of dosage of metal salts on EM wave absorption behavior is investigated and the results turn out that the NC1-600 and NC3-600 still display narrow fE of 2.16 and 3.52 GHz, respectively. The detailed information about the optimized thickness, fE and RLmin are presented in Fig. 5g-h. In consideration of the wide fE, strong absorption capacity and facile synthesis process, the NC2-600 sample shows great potential as effective EM wave absorber. The EM parameters including complex permittivity and complex permeability are crucial parameters that determine the EM wave absorption capacity of the materials. Thus, the EM parameters of these absorbing materials are measured and investigated. For the real part of complex permittivity (e0 ), the NC2-600 possesses the highest value from frequency range of 2 GHz to 13 GHz and dramatically reduced from 10.0 to 2.4. Among the absorbers, NC2-700 displays the lowest e0 value, showing the poor EM wave storage ability. As for the imaginary part of complex permittivity (e00 ), the value of NC2-600 sample is still the highest almost in the investigated frequency range, revealing the strong
dielectric loss capacity. The distinguishable resonance peak located at 14.5 GHz is ascribed to the shape anisotropy of 1D NiO/NiCo2O4 composite while the other two peaks are associated to the interfacial polarization effect occurred in the heterogeneous interfaces between NiO and NiCo2O4 [25]. The corresponding dielectric loss tangent tande is calculated and plotted in Fig. 6c. The dielectric loss tangent tande values display the same trend as the e00 , that is, the value of NC2-600 is the highest among these samples. Therefore, we can deduce that the NC2-600 possesses the strongest dielectric loss capacity. Dielectric loss of EM wave absorbing materials is generally originated from dielectric relaxation polarization process and conduction loss [42]. As discussed above, due to the existence of multiple phases of NiO and NiCo2O4 in the composite, their discrepant electrical signal and internal potential will lead to the redistribution of spatial charge on their contact surface. Under the altering electromagnetic field, the charge distribution is forced to rearrange orderly. The transition between the two charge distribution states could consume the energy of EM wave. Therefore, the interfacial polarization effect arouses by the heterogeneous interfaces between NiO and NiCo2O4 is conducive to its EM wave attenuation performance. On the other hand, the dipoles can be induced due to the defects (i.e. oxygen vacancies) in the composite. In consequence, the dipole polarization also occurs under altering electromagnetic field and contributes to enhanced dielectric loss capacity. These polarization relaxation processes can also be confirmed by Cole-Cole semicircles, which is obtained based Debye’s theory and expressed below:
0
e
es þ e1 2 2
þ
0 0 2
e
¼
e e 2 0 1 2
ð3Þ
The semicircles are distorted (Fig. S1), confirming that other relaxation process, such as Maxwell-Wagner relaxation exists in the NiO/NiCo2O4 composite [23,35]. In addition, the 1D structure of NiO/NiCo2O4 can act as an antenna to receive incident EM wave and microcurrent can be induced under the altering electromagnetic field. During the transmission of microcurrent along the 1D NiO/NiCo2O4 microrods, the energy of EM wave can be converted into thermal energy due to the resistance of NiO/NiCo2O4 [43]. The real and imaginary parts of complex permeability of these samples are plotted in Fig. 6d, e. One can observe that the imaginary part of complex permeability l00 is negative in frequency range of 7–18 GHz. This is caused by the eddy current which is induced under alternating electromagnetic field can lead to the generation of extra magnetic field. As a result, the inherent magnetic field is cancelled, leading to the negative permeability
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Fig. 3. (a) XRD patterns, (b) Raman spectra of as-prepared NiO/NiCo2O4 composites and (c-f) the high-resolution XPS spectra of C 1s, O 1s, Ni 2p and Co 2p of NC2-600 sample.
[5,25]. The magnetic loss tangent tandl values of these composites are lower than that of dielectric loss tangent. Thus, we can infer that the EM wave attenuation mainly relies on dielectric loss. In general, the magnetic loss of EM wave absorbing materials originates from natural resonance, exchange resonance and eddy current loss in the investigated frequency [44,45]. If eddy current
loss is the only contribution of magnetic loss, the value of C0 calculated from equation (4) would be a constant. However, it is obvious that the C0 fluctuates over the frequency region (Fig. S2). Therefore, it can be deduced that the magnetic loss is not caused by the eddy current loss. Moreover, the resonance peak located at 4.2 GHz verifies the presence of natural resonance in the composite. Thus, the
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Fig. 4. (a) The magnetization curves of NiO/NiCo2O4 composites and (b) the coercivity (Hc) and saturation magnetizations of these samples.
Fig. 5. 2D counter images of frequency dependence of RL values at different thickness for (a) NC2-400, (b) NC2-500, (c) NC2-600, (d) NC2-700, (e) NC1-600 and (f) NC3-600, respectively. (g) and (h) the specific fE and RLmin of these absorbers. (i) and (h) frequency dependence of RL values and corresponding the 3D image of NC2-600 sample.
main cause of magnetic loss is natural resonance in the NC2-600 absorber. 00
2 1
C 0 ¼ l ðl0 Þ f
2
¼ 2pl0 d
r
ð4Þ
Except for the above factors, the well matched impedance Z and strong attenuation ability a are also required to achieve excellent EM wave dissipation. The Z and attenuation constant a are obtained based on following formulas:
Z ¼ Z in =Z 0
ð5Þ
pffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf 2 a¼ ðl00 e00 l0 e0 Þ þ ðl00 e00 l0 e0 Þ2 þ ðl0 0 e0 þ l0 e0 0 Þ c
ð6Þ
Basically, when the Z value equals to 1, the incident EM wave can entirely enter into the EM wave absorbing materials without reflection on its surface. Combined with strong attenuation ability, namely, extended attenuation constant a, the EM wave get into the absorbers can be greatly dissipated. The optimized fE at corresponding thickness and their Z values are depicted in Fig. 7a, c. The results turn out that the NC2-600 possesses suitable Z values of 0.9 and 1 with thickness of 1.88 mm and 4.2 mm, respectively.
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Fig. 6. (a) The real part and (b) imaginary part of complex permittivity, (c) the dielectric loss tangent of the samples, the (d) real part, (e) imaginary part of complex permeability and (f) the magnetic loss tangent.
Fig. 7. (a) frequency dependence of RL values at the optimized fE thickness, (b) the attenuation constant a of the absorbers, c) corresponding impedance matching characteristic to (a) and (d) the minimum RL values in different thickness regions.
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Fig. 8. The EM wave absorption mechanisms of the 1D NiO/NiCo2O4 microrods.
Table 1 Previously reported excellent spinel structure EM wave absorbers. Absorbers
RLmin (dB)
fE (GHz)
fE matching thickness (mm)
Refs.
NiCo2O4 CuCo2O4-CuO MgCo2O4/Co3O4 NiCo2O4/CoNiO2 Ni1.71Co1.29O4 NiCo2O4/Co3O4/NiO C@NiCo2O4@Fe3O4 MnO2@NiCo2O4 ZnCo2O4 NiCo2O4 NiO/NiCo2O4
42.9 –23 48.54 42.13 44.5 28.6 43 58.4 36.3 42.8 57.4
4.28 4.02 5.16 3.92 5.13 4.72 2.1 2.7 5.11 6.08 6.08
1.39 2.8 2.3 1.55 2 1.64 3.4 4 2.5 2.06 1.88
[25] [46] [47] [29] [44] [30] [31] [32] [48] [49] This work
Moreover, the attenuation constant a in Fig. 7b clearly reveals that the NC2-600 exhibits the highest value on the whole investigated frequency. Therefore, it is no doubt that the decent complex permittivity and complex permeability of the NC2-600 sample ensure the well matched impedance Z and strong attenuation ability. In consequence, excellent EM wave attenuation performance can be fulfilled. Though the Z values of other samples also close to 1, the EM wave enters into the absorbing materials is unable to be dissipated due to their poor attenuation capacity. Based on above analysis, the EM wave absorption mechanism of 1D NiO/NiCo2O4 microrod is proposed and intuitively shown in Fig. 8. Firstly, the morphology is responsible for its excellent absorption performance. The pores and uneven surface which is composed of abundant nanoparticles can induce multiple reflections and scatterings when incident EM wave encounters with the absorbers. Thus, the energy that EM wave carries can be attenuated during its propagation. In addition, the randomly distributed 1D NiO/NiCo2O4 microrods can act as anisotropic antennas and microcurrent can be induced under the altering electromagnetic
field. The microcurrent generated and transmitted along the 1D NiO/NiCo2O4 would lead to the energy consumption due to the resistance of absorbing materials. Secondly, dielectric loss contributes to the superior EM wave attenuation behavior. The heterogeneous interfaces between NiO and NiCo2O4 would lead to the interfacial polarization effect, beneficial to promote dissipation of EM wave by dielectric loss. Finally, the ferromagnetism of NiO/ NiCo2O4 composite can contribute to EM wave attenuation by natural resonance. These factors all lead to the excellent EM wave absorption performance of NC2-600 sample. To compare the EM wave absorption performance of NC2-600 sample with other NiCo2O4-based absorbing materials, their corresponding performance is collected and listed in Table. 1. The comparison diagram of the widest fE values and the RLmin values is also intuitively provided in Fig. 9. Wide fE of 6.08 GHz can be accomplished at thickness of 1.88 mm, which is thinner than our previous research. Besides, the RLmin value also reaches to 57.4 dB by regulating the thickness. More importantly, the synthetic procedures proposed in this work are significantly reduced and less energy
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Fig. 9. The comparison of EM wave absorption performance between 1D NiO/NiCo2O4 microrods and previously reported spinel absorbers.
consumption is needed. Therefore, the 1D NiO/NiCo2O4 microrods prepared by facile and low-cost filter paper-template method can be attractive candidate as EM wave absorbing material.
4. Conclusion In summary, 1D NiO/NiCo2O4 microrods composite was simply prepared by taking advantage of filter paper as template. Except for the adsorption of metal salts on the filter paper, no extra treatment including reaction between metal salts and pre-treatment to filter paper is required. The final products with 1D structure inherited from carbon fibers could be harvested by calcination of metal salts coated filter paper in air atmosphere. By manipulating the calcination and dosage of the metal salts, the NiO/NiCo2O4 microrods composite with superior EM wave absorption performance can be realized with calcination temperature of 600 °C and dosage of Ni and Co acetate with 2 mmol and 4 mmol, respectively. The effective absorption bandwidth could reach to 6.08 GHz from 11.92 GHz to 18 GHz at thin thickness of 1.88 mm. In addition, the minimum RL value is up to 57.4 dB by adjusting the thickness. This work sheds light on the synthesis of high-performance EM wave absorbers by facile and stable filter paper-template method.
CRediT authorship contribution statement Ming Qin: Writing - original draft, Visualization, Investigation. Hongsheng Liang: Validation, Methodology. Xiaoru Zhao: Supervision. Hongjing Wu: Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments Financial support was provided by the National Science Foundation of China (Grants nos. 51872238 and 21806129), the Fundamental Research Funds for the Central Universities (Nos. 3102018zy045 and 3102019AX11) and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2017JQ5116).
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Filter paper templated one-dimensional NiO/NiCo2O4 microrod with wideband electromagnetic wave absorption capacity Ming Qin, Hongsheng Liang, Xiaoru Zhao, Hongjing Wu ⇑ School of Physical Science and Technology, Northwestern Polytechnical University, Xi’an 710072, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Filter paper was selected as the
template for synthesis of 1D NiO/ NiCo2O4 composite. Ultra simple synthetic procedures of only metal salts adsorption and calcination was referred. The effective bandwidth is up to 6.08 GHz at thin thickness of 1.88 mm.
a r t i c l e
i n f o
Article history: Received 17 January 2020 Revised 28 January 2020 Accepted 28 January 2020
Keywords: Filter paper One-dimensional NiO/NiCo2O4 microrod Wideband electromagnetic wave absorption
a b s t r a c t Filter paper, as a widely used and low-cost lab consumable, can serve as template for the synthesis of one-dimensional (1D) materials since it is composed of 1D cellulosic fiber. In this work, 1D NiO/ NiCo2O4 composite was simply fabricated by adsorption of Ni and Co acetate on the filter paper and subsequent calcination process. The precursors were obtained without further treatment to filter paper or chemical reaction between metal salts. Calcination process leads to the removal of template and formation of binary NiO/NiCo2O4 composite. Electromagnetic (EM) wave absorption performance of NiO/ NiCo2O4 composite could be tuned by adjusting the calcination temperatures and dosage of metal salts. For the optimized absorber (calcination temperature of 600 °C and dosage of Ni and Co acetate with 2 mmol and 4 mmol), a wide absorption bandwidth of 6.08 GHz could be achieved with thickness of 1.88 mm and minimum reflection loss value was up to 57.4 dB by changing the thickness. The facile synthetic route may also inspire the preparation of other high-performance one-dimensional EM wave absorbing materials. Ó 2020 Elsevier Inc. All rights reserved.
1. Introduction In recent years, the electromagnetic (EM) pollution has become an increasing issue that human beings have to face. This pollution originates from the excessive utilization of electronic devices that rely on the EM wave as information carrier. As a consequence, human beings’ health could be severely affected by this issue ⇑ Corresponding author. E-mail address: [email protected] (H. Wu). https://doi.org/10.1016/j.jcis.2020.01.114 0021-9797/Ó 2020 Elsevier Inc. All rights reserved.
[1-6]. Developing EM wave absorbing materials is an effective approach to remove the excessive EM wave and the energy it carries. The EM wave absorbers can consume the energy by dielectric loss and magnetic loss mechanisms, turning it into heat or other kinds of energy. To acquire high-performance EM wave absorbing materials, considerable efforts have been devoted on the synthesis of carbonaceous materials, conducting polymer and ferrites [7-14]. Though great advances have been reported, producing thinthickness, light-weight and wide-bandwidth EM wave absorbers with facile routes are still required.
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Nowadays, it is found that taking advantage of biomass carbon as matrix or template has become an effective approach to prepare high-performance EM wave absorbers [15-22]. This is ascribed to the fact the biomass carbon templates generally possess onedimensional fiber morphology in microstructure. This feature can be readily inherited during the synthesis process. Due to the shape anisotropy and high aspect ratio of this 1D structure, the charge transportation rate can be facilitated along radial direction and the improved interfacial polarization can be achieved as well [23-28]. In consequence, the EM wave absorption performance can be strengthened. For example, Ji’s group reported [22] that carbon cotton/Co@nanoporous carbon products could be fabricated by anchoring ZIF-67 on the biomass cotton and followed calcination process. The optimized sample displayed strong absorption ability of 51.2 dB and effective bandwidth of 4.4 GHz. Likewise, Che and his coworkers also reported that [28] hierarchically tubular C/Co nanoparticle prepared based on kapok fiber template exhibited wide absorption bandwidth of 5.1 GHz and the minimum reflection loss also reached up to 52.3 dB. However, current researches focus more on the synthesis of biomass carbon based EM absorbers by coupling magnetic components to prepare composites that provide both dielectric loss and magnetic loss. To harvest the final composites, tedious synthetic procedures are often required. For example, the biomass-derived carbon/MnO nanorods absorbing material [19] was obtained through hydrothermal reaction and calcination for two times, respectively. The reaction procedures were time-consuming and extra energy was required. Therefore, novel idea should be put up with to address these problems. As another research point, utilization of biomass carbon as bare template has scarcely been reported. Taking advantage of biomass carbon as bare template to directly adsorb reactant solutions and remove it and generate products in air atmosphere may provide a more facile synthetic process to prepare high-performance EM wave absorbers. Filter paper, as an extensive used material in laboratory, also shows great potential as a biomass carbon template. Impressively, due to the essential character of filter paper, the reactant solutions can be directly adsorbed on the surface of filter paper without other reaction or energy consuming synthetic procedure. The products generation reaction and template removal can be simultaneously accomplished during the calcination process. This approach can greatly reduce the synthetic process and provide a more facile route for preparing high-performance 1D EM wave absorbers. It has been demonstrated by previous works that NiCo2O4based materials show potential as effective EM wave absorbing materials [29-35]. However, in view of its practical performance, it is still urgent need to reduce its thickness when high absorption performance is required. Moreover, it has also been confirmed that the NiCo2O4 phase can be generated by the reaction of corresponding metal salts precursors during calcination process [36]. Based on above discussion, it is prospective that one-dimensional NiCo2O4 can be readily fabricated by means of filter paper template method. The metal salts solutions that contain Ni and Co can firstly adsorbed on the filter paper to obtain the precursor with onedimensional microstructure. When subject to heating treatment, the precursors will convert into NiCo2O4 with retained microstructure. Therefore, NiCo2O4 is a reasonable choice as the target product to investigate this synthetic method. To the best of our knowledge, synthesis of one-dimensional NiCo2O4-based materials by simple filter paper adsorption and subsequent heating treatment has not been reported yet. Herein, one-dimensional NiO/NiCo2O4 microrod composite is simply fabricated by facile and low-cost filter paper-template method and subsequent heating treatment. The dosage of metal salts and calcination temperatures were found to be vital parameters that influence the EM wave absorption properties of products.
The optimized absorber displayed the widest absorption bandwidth of 6.08 GHz with thickness of 1.88 mm, which is thinner than previous reported NiCo2O4-based absorbers. In addition, a strong absorption capacity of 57.4 dB could also be achieved by adjusting the thickness. This method provides a reliable template and simple route for the synthesis of one-dimensional ferrite with excellent EM wave absorption performance. 2. Experimental section 2.1. Synthesis of NiO/NiCo2O4 samples Metal acetate of tetrahydrate nickel acetate (Ni(CH3COO)2 4H2O) and tetrahydrate cobalt acetate (Co(CH3COO)24H2O) were purchased from National Reagent Corp. (Shanghai, China). The qualitative filter papers with diameter of 11 cm were obtained from Civil filter paper factory (Fushun, China). The synthesis of one-dimensional NiO/NiCo2O4 microrod EM wave absorbing materials was accomplished through filter paper-template method and followed by calcination process. The synthetic procedures were described as follow. Firstly, 2 mmol of Ni(CH3COO)24H2O and 4 mmol of Co(CH3COO)24H2O were together dissolved in certain amount of deionized water to ensure the thorough dissolution of metal salts. Then, the solution was dropped on both side of a piece of filter paper until all the solution was consumed. After being dried in the oven, the resultant filter paper was cut into small pieces and placed in combustion boat and calcined at 600 °C for 3 h at heating rate of 2 °C/min in air condition to gain the final products. For comparison, the precursors were also calcined at 400 °C, 500 °C and 700 °C to investigate the influence of calcination temperatures on the EM wave absorption performance of onedimensional NiO/NiCo2O4 microrod. In addition, the dosage of metal salts was also optimized by utilization of different amount of Ni(CH3COO)26H2O at 1 mmol and 3 mmol, in which the molar ratio between Ni and Co was still kept 1:2. The NiO/NiCo2O4 samples obtained at different calcination temperatures were labeled as NC2-400, NC2-500, NC2-600 and NC2-700 while the samples acquired at varied metal salts dosage were marked as NC1-600 and NC3-600, respectively. 2.2. Characterization The samples were measured by a series of techniques including X-ray diffraction (XRD, D2 PHASER X), Microscopic confocal Raman spectrometer (WITec Alpha 300R), vibrating sample magnetometer (VSM, Cryogenic CFMS-14T), X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) and field emission scanning electron microscopy (SEM, FEI Verios G4). The electromagnetic parameters were recorded on Agilent N5230A vector network analyzer. The samples for test were prepared by uniformly mixed powders and paraffin with mass ratio of 1:1 and pressed into a columnar ring. The EM wave absorption performance was calculated based on transmission line theory as expressed below:
Z in Z 0 RL ¼ 20log Z in þ Z 0 Z in ¼
rffiffiffiffiffi lr 2pfd pffiffiffiffiffiffiffiffiffi lr e r tanh j c er
ð1Þ
ð2Þ
In these equations, the Zin and Z0 are the input impedance of absorbing materials and the impedance of free space, d is the thickness of the absorbing materials, c is the speed of light in vacuum, f is the frequency of EM wave, lr (lr = l0 -jl00 ) and er (er = e0 -je00 ) are the complex permittivity and permeability of the absorber, respectively.
M. Qin et al. / Journal of Colloid and Interface Science 566 (2020) 347–356
3. Results and discussion The synthetic schematic diagram of one-dimensional NiO/ NiCo2O4 microrods is present in Fig. 1.When homogeneous reactants solution that contains Ni and Co was dropped on the surface of filter paper, the metal ions could be adsorbed on the abundant one-dimensional fibers of filter paper. After the evaporation of water in the filter paper, the metal salts could compactly arrange on the surface of fibers. When the metal salts were fully consumed, the original white filter paper was uniformly covered by the purple metal salts coating. Then, the obtained precursors were subjected to heating treatment in air atmosphere. During this process, the filter paper template can be removed. In addition, the metal salts can successfully convert into the final products. Due to the excellent stability of the filter paper, the one-dimensional fibrous morphology of products could be readily retained. The morphologies of the as-prepared NC2-600 sample are shown in Fig. 2. The distinctive microrods structure can be observed from field-emission SEM. This result confirms that the filter paper can serve as reliable template for the synthesis of 1D NiO/ NiCo2O4 materials. The 1D NiO/NiCo2O4 possess length of around 10 lm and diameter of 3.5–4 lm. The magnified SEM images reveal the rough surface of the microrods and pores are in the presence of the surface, which may be aroused by the release of CO2 during the calcination process. The coarse surface may lead to the multiple reflections and scattering of the incident EM wave, beneficial to the attenuation of the energy it carries. In addition, the NiO/NiCo2O4 nanocrystals are also found to compactly aggregate on the surface of rod structure. EDS mapping demonstrates the uniform distribution of the Ni, Co and O elements in the asobtained sample. Based on the atom ratio obtained from EDS measurement, we can approximately estimate that the molar ratio of NiO: NiCo2O4 in the composite is around 3: 4. The presence of multiple phases of NiO and NiCo2O4 in the sample is beneficial to improve the dielectric loss capacity by providing interfacial polarization. The XRD patterns of these samples are recorded and shown in Fig. 3a. For all the samples, the diffraction peaks in the patterns can be readily assigned to the standard XRD patterns of NiCo2O4 and NiO. This result confirms the NiO/NiCo2O4 composite is successfully prepared. One can see that as the calcination temperatures increase from 400 °C to 700 °C, these diffraction peaks become sharper, proving the better crystalline samples. In addition, the NiO phase is found to be more prominent with gradually ascending calcination temperatures. This phenomenon may be caused by the decomposition of NiCo2O4 thus more NiO phase is induced under higher calcination temperatures. Though the multi-
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ple phases in NiO/NiCo2O4 composite are conducive to provide enhanced interfacial polarization effect, the excessive NiO phase in the composite may be adverse to the EM wave absorption performance in consideration of its poor attenuation capacity. Thus, the selection of proper calcination temperature is of significance in the synthetic process. The Raman spectra of NiO/NiCo2O4 composite are presented in Fig. 3b. There are four distinctive peaks indexed to the characteristic peaks of spinel NiCo2O4 [37,38]. The peaks ~650 cm1 are corresponding to the vibration of the octahedral oxygen ions in octahedral site. Compared with the original Co3O4 of 691 cm1, the blue shift to lower frequency is caused by the replacement of Ni ions in the octahedral sites. Moreover, a broad peak centered at 1050 cm1 is assigned to the 2 longitudinal optical (LO) combination phonon mode of NiO [39,40]. Therefore, the Raman spectra of these materials also verify the coexistence of NiO and NiCo2O4 in the products. The characteristic peaks of carbonaceous materials’ D and G bands are not detected in these samples, implying the filter paper template has been completely removed during the heating treatment. More detailed information about the chemical states and compositions in the samples was disclosed by XPS analysis. The XPS spectrum of NC2-600 is shown in Fig. 3c-f. Four elements of C, O, Co and Ni are found to coexist in the sample. By using Lorentzian-Gaussian fitting method, the high-resolution spectrum of each element in the samples can be divided into different peaks. For C 1s spectra, three kinds of carbon species CAC, CAO and C@O are detected. The deconvolution of O 1s spectra discloses that there are four O species in the composite, that is, metal–oxygen bond located at 529.6 eV (Ni-O) and 530.4 eV (Co-O), oxygen vacancy at 531.8 eV and chemical or physical adsorbed water at 533.1 eV [35]. Both of the high-resolution spectra of Co and Ni can be fitted with two spin–orbit doublets and two shakeup satellite peaks. These results verify the existence of multiple valence states of Co2+, Co3+, Ni2+, Ni3+ coexists in the as-prepared NiO/NiCo2O4 composite. The highly consistent characterization results of XRD, Raman spectroscopy and XPS demonstrate the successful synthesis of NiO/NiCo2O4 composite. The magnetic properties of these composites are measured at room temperature. It is obvious that the saturation magnetization gradually decreases with the rising calcination temperatures. The NC2-400 possesses the highest saturation magnetization of 8.72 emu/g while the NC2-700 hardly behaves magnetic hysteresis. This result may be ascribed to the fact that the elevated calcination temperatures lead to the presence of more NiO phase, which displays poorer magnetic properties in these composites. As a result, the decline of NiCo2O4 component will result in the
Fig. 1. The schematic diagram for the synthesis of 1D NiO/NiCo2O4 microrods.
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Fig. 2. (a-d) SEM images of 1D NiO/NiCo2O4 microrods, (e-g) corresponding elemental mapping images and (h) EDS pattern of (d).
decreasing saturation magnetization. With regard to the samples obtained with different metal salt dosages, the detail data can be learned from Fig. 4b, which is the magnified image of red square in Fig. 4a. The saturation magnetizations are barely changed compared with NC2-600, further confirming that the saturation magnetization is determined by different proportions of NiO and NiCo2O4 in the composite. The coercivity is also found to decrease from NC2-400 to NC2-700, which may be caused to the grain growth and consequently spin canted effect in the surface layer. As a whole, the poor magnetization values of these samples are adverse to strong magnetic loss ability [41]. Based on transmission line theory, the EM wave absorption performance of these absorbers are calculated and depicted in Fig. 5. Typically, the reflection loss (RL) value lower than 10 dB is regarded as the criterion for effective EM wave absorption. For NC2-400, the widest effective absorption bandwidth (fE) of 1.86 GHz is obtained with thickness of 4.8 mm. When calcination temperature increased to 500 °C, the fE increased to 2.96 GHz and can be accomplished at thinner thickness of 2.0 mm as well. Impressively, the EM wave absorption performance can be dramatically promoted by NC2-600. A wide fE of 6.08 GHz from 11.92 GHz to 18 GHz can be realized at thin thickness of 1.88 mm. In addition, by adjusting the thickness to 2.51 mm, minimum RL (RLmin) of 57.4 dB can be achieved at 9.36 GHz. The corresponding fE also reaches to 2.56 GHz (frequency range from 8.48 GHz to 11.04 GHz). As the temperature further increased, the EM wave attenuation behavior is deteriorated again, which fE can only reach to 2.96 GHz. The influence of dosage of metal salts on EM wave absorption behavior is investigated and the results turn out that the NC1-600 and NC3-600 still display narrow fE of 2.16 and 3.52 GHz, respectively. The detailed information about the optimized thickness, fE and RLmin are presented in Fig. 5g-h. In consideration of the wide fE, strong absorption capacity and facile synthesis process, the NC2-600 sample shows great potential as effective EM wave absorber. The EM parameters including complex permittivity and complex permeability are crucial parameters that determine the EM wave absorption capacity of the materials. Thus, the EM parameters of these absorbing materials are measured and investigated. For the real part of complex permittivity (e0 ), the NC2-600 possesses the highest value from frequency range of 2 GHz to 13 GHz and dramatically reduced from 10.0 to 2.4. Among the absorbers, NC2-700 displays the lowest e0 value, showing the poor EM wave storage ability. As for the imaginary part of complex permittivity (e00 ), the value of NC2-600 sample is still the highest almost in the investigated frequency range, revealing the strong
dielectric loss capacity. The distinguishable resonance peak located at 14.5 GHz is ascribed to the shape anisotropy of 1D NiO/NiCo2O4 composite while the other two peaks are associated to the interfacial polarization effect occurred in the heterogeneous interfaces between NiO and NiCo2O4 [25]. The corresponding dielectric loss tangent tande is calculated and plotted in Fig. 6c. The dielectric loss tangent tande values display the same trend as the e00 , that is, the value of NC2-600 is the highest among these samples. Therefore, we can deduce that the NC2-600 possesses the strongest dielectric loss capacity. Dielectric loss of EM wave absorbing materials is generally originated from dielectric relaxation polarization process and conduction loss [42]. As discussed above, due to the existence of multiple phases of NiO and NiCo2O4 in the composite, their discrepant electrical signal and internal potential will lead to the redistribution of spatial charge on their contact surface. Under the altering electromagnetic field, the charge distribution is forced to rearrange orderly. The transition between the two charge distribution states could consume the energy of EM wave. Therefore, the interfacial polarization effect arouses by the heterogeneous interfaces between NiO and NiCo2O4 is conducive to its EM wave attenuation performance. On the other hand, the dipoles can be induced due to the defects (i.e. oxygen vacancies) in the composite. In consequence, the dipole polarization also occurs under altering electromagnetic field and contributes to enhanced dielectric loss capacity. These polarization relaxation processes can also be confirmed by Cole-Cole semicircles, which is obtained based Debye’s theory and expressed below:
0
e
es þ e1 2 2
þ
0 0 2
e
¼
e e 2 0 1 2
ð3Þ
The semicircles are distorted (Fig. S1), confirming that other relaxation process, such as Maxwell-Wagner relaxation exists in the NiO/NiCo2O4 composite [23,35]. In addition, the 1D structure of NiO/NiCo2O4 can act as an antenna to receive incident EM wave and microcurrent can be induced under the altering electromagnetic field. During the transmission of microcurrent along the 1D NiO/NiCo2O4 microrods, the energy of EM wave can be converted into thermal energy due to the resistance of NiO/NiCo2O4 [43]. The real and imaginary parts of complex permeability of these samples are plotted in Fig. 6d, e. One can observe that the imaginary part of complex permeability l00 is negative in frequency range of 7–18 GHz. This is caused by the eddy current which is induced under alternating electromagnetic field can lead to the generation of extra magnetic field. As a result, the inherent magnetic field is cancelled, leading to the negative permeability
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Fig. 3. (a) XRD patterns, (b) Raman spectra of as-prepared NiO/NiCo2O4 composites and (c-f) the high-resolution XPS spectra of C 1s, O 1s, Ni 2p and Co 2p of NC2-600 sample.
[5,25]. The magnetic loss tangent tandl values of these composites are lower than that of dielectric loss tangent. Thus, we can infer that the EM wave attenuation mainly relies on dielectric loss. In general, the magnetic loss of EM wave absorbing materials originates from natural resonance, exchange resonance and eddy current loss in the investigated frequency [44,45]. If eddy current
loss is the only contribution of magnetic loss, the value of C0 calculated from equation (4) would be a constant. However, it is obvious that the C0 fluctuates over the frequency region (Fig. S2). Therefore, it can be deduced that the magnetic loss is not caused by the eddy current loss. Moreover, the resonance peak located at 4.2 GHz verifies the presence of natural resonance in the composite. Thus, the
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Fig. 4. (a) The magnetization curves of NiO/NiCo2O4 composites and (b) the coercivity (Hc) and saturation magnetizations of these samples.
Fig. 5. 2D counter images of frequency dependence of RL values at different thickness for (a) NC2-400, (b) NC2-500, (c) NC2-600, (d) NC2-700, (e) NC1-600 and (f) NC3-600, respectively. (g) and (h) the specific fE and RLmin of these absorbers. (i) and (h) frequency dependence of RL values and corresponding the 3D image of NC2-600 sample.
main cause of magnetic loss is natural resonance in the NC2-600 absorber. 00
2 1
C 0 ¼ l ðl0 Þ f
2
¼ 2pl0 d
r
ð4Þ
Except for the above factors, the well matched impedance Z and strong attenuation ability a are also required to achieve excellent EM wave dissipation. The Z and attenuation constant a are obtained based on following formulas:
Z ¼ Z in =Z 0
ð5Þ
pffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf 2 a¼ ðl00 e00 l0 e0 Þ þ ðl00 e00 l0 e0 Þ2 þ ðl0 0 e0 þ l0 e0 0 Þ c
ð6Þ
Basically, when the Z value equals to 1, the incident EM wave can entirely enter into the EM wave absorbing materials without reflection on its surface. Combined with strong attenuation ability, namely, extended attenuation constant a, the EM wave get into the absorbers can be greatly dissipated. The optimized fE at corresponding thickness and their Z values are depicted in Fig. 7a, c. The results turn out that the NC2-600 possesses suitable Z values of 0.9 and 1 with thickness of 1.88 mm and 4.2 mm, respectively.
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Fig. 6. (a) The real part and (b) imaginary part of complex permittivity, (c) the dielectric loss tangent of the samples, the (d) real part, (e) imaginary part of complex permeability and (f) the magnetic loss tangent.
Fig. 7. (a) frequency dependence of RL values at the optimized fE thickness, (b) the attenuation constant a of the absorbers, c) corresponding impedance matching characteristic to (a) and (d) the minimum RL values in different thickness regions.
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Fig. 8. The EM wave absorption mechanisms of the 1D NiO/NiCo2O4 microrods.
Table 1 Previously reported excellent spinel structure EM wave absorbers. Absorbers
RLmin (dB)
fE (GHz)
fE matching thickness (mm)
Refs.
NiCo2O4 CuCo2O4-CuO MgCo2O4/Co3O4 NiCo2O4/CoNiO2 Ni1.71Co1.29O4 NiCo2O4/Co3O4/NiO C@NiCo2O4@Fe3O4 MnO2@NiCo2O4 ZnCo2O4 NiCo2O4 NiO/NiCo2O4
42.9 –23 48.54 42.13 44.5 28.6 43 58.4 36.3 42.8 57.4
4.28 4.02 5.16 3.92 5.13 4.72 2.1 2.7 5.11 6.08 6.08
1.39 2.8 2.3 1.55 2 1.64 3.4 4 2.5 2.06 1.88
[25] [46] [47] [29] [44] [30] [31] [32] [48] [49] This work
Moreover, the attenuation constant a in Fig. 7b clearly reveals that the NC2-600 exhibits the highest value on the whole investigated frequency. Therefore, it is no doubt that the decent complex permittivity and complex permeability of the NC2-600 sample ensure the well matched impedance Z and strong attenuation ability. In consequence, excellent EM wave attenuation performance can be fulfilled. Though the Z values of other samples also close to 1, the EM wave enters into the absorbing materials is unable to be dissipated due to their poor attenuation capacity. Based on above analysis, the EM wave absorption mechanism of 1D NiO/NiCo2O4 microrod is proposed and intuitively shown in Fig. 8. Firstly, the morphology is responsible for its excellent absorption performance. The pores and uneven surface which is composed of abundant nanoparticles can induce multiple reflections and scatterings when incident EM wave encounters with the absorbers. Thus, the energy that EM wave carries can be attenuated during its propagation. In addition, the randomly distributed 1D NiO/NiCo2O4 microrods can act as anisotropic antennas and microcurrent can be induced under the altering electromagnetic
field. The microcurrent generated and transmitted along the 1D NiO/NiCo2O4 would lead to the energy consumption due to the resistance of absorbing materials. Secondly, dielectric loss contributes to the superior EM wave attenuation behavior. The heterogeneous interfaces between NiO and NiCo2O4 would lead to the interfacial polarization effect, beneficial to promote dissipation of EM wave by dielectric loss. Finally, the ferromagnetism of NiO/ NiCo2O4 composite can contribute to EM wave attenuation by natural resonance. These factors all lead to the excellent EM wave absorption performance of NC2-600 sample. To compare the EM wave absorption performance of NC2-600 sample with other NiCo2O4-based absorbing materials, their corresponding performance is collected and listed in Table. 1. The comparison diagram of the widest fE values and the RLmin values is also intuitively provided in Fig. 9. Wide fE of 6.08 GHz can be accomplished at thickness of 1.88 mm, which is thinner than our previous research. Besides, the RLmin value also reaches to 57.4 dB by regulating the thickness. More importantly, the synthetic procedures proposed in this work are significantly reduced and less energy
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Fig. 9. The comparison of EM wave absorption performance between 1D NiO/NiCo2O4 microrods and previously reported spinel absorbers.
consumption is needed. Therefore, the 1D NiO/NiCo2O4 microrods prepared by facile and low-cost filter paper-template method can be attractive candidate as EM wave absorbing material.
4. Conclusion In summary, 1D NiO/NiCo2O4 microrods composite was simply prepared by taking advantage of filter paper as template. Except for the adsorption of metal salts on the filter paper, no extra treatment including reaction between metal salts and pre-treatment to filter paper is required. The final products with 1D structure inherited from carbon fibers could be harvested by calcination of metal salts coated filter paper in air atmosphere. By manipulating the calcination and dosage of the metal salts, the NiO/NiCo2O4 microrods composite with superior EM wave absorption performance can be realized with calcination temperature of 600 °C and dosage of Ni and Co acetate with 2 mmol and 4 mmol, respectively. The effective absorption bandwidth could reach to 6.08 GHz from 11.92 GHz to 18 GHz at thin thickness of 1.88 mm. In addition, the minimum RL value is up to 57.4 dB by adjusting the thickness. This work sheds light on the synthesis of high-performance EM wave absorbers by facile and stable filter paper-template method.
CRediT authorship contribution statement Ming Qin: Writing - original draft, Visualization, Investigation. Hongsheng Liang: Validation, Methodology. Xiaoru Zhao: Supervision. Hongjing Wu: Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments Financial support was provided by the National Science Foundation of China (Grants nos. 51872238 and 21806129), the Fundamental Research Funds for the Central Universities (Nos. 3102018zy045 and 3102019AX11) and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2017JQ5116).
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