Composition and Structure Design of Co3O4 Nanowires Network by

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Composition and Structure Design of Co3O4 Nanowires Network by Nickel Foam with Effective Electromagnetic Performance in C and X Band Weihua Gu, Bin Quan, Xiaohui Liang, Wei Liu, Guangbin Ji, and Youwei Du ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00017 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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ACS Sustainable Chemistry & Engineering

Composition and Structure Design of Co3O4 Nanowires Network by Nickel Foam with Effective Electromagnetic Performance in C and X Band Weihua Gua, Bin Quana, Xiaohui Lianga, Wei Liua, Guangbin Ji*a, Youwei Dub a College

of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, P. R. China.

b Laboratory

of Solid State Microstructures, Nanjing University, Nanjing 210093, China.

*Corresponding Author: Prof. Dr. Guangbin Ji. Tel: +86-25-52112902; Fax: +86-25-52112626 E-mail: [email protected] Address: 29# Yudao Street, Nanjing 210016, P.R China

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ABSTRACT Effectively attenuating electromagnetic waves in C and X band through composition and structure design remains formidable challenges for most absorbing materials. To achieve tunable electromagnetic properties, in this study, one-dimensional (1D) Co3O4 nanowires were successfully grew onto the 3D porous nickel foam (NF) through a facile liquid synthesis. Herein, electromagnetic parameters and micro-perspective structure have been controlled via changing the hydrothermal temperature, more importantly, the as-prepared composites at 100 oC exhibited prominent microwave dissipation performance in gigahertz. The minimum reflection loss (RL) value reached -41.1 dB at a relatively small matching thickness of 2.1 mm, and the optimal effective bandwidth (RL < -10 dB) of 3.46 GHz at 2.3 mm was also achieved. It should be noted that the RL values of the obtained NF@Co3O4 samples appeared two and three sharp peaks at the thickness of 2.3 mm, 2.5 mm, respectively. Good impedance matching, efficient magnetic loss, dielectric loss and suitable interfacial polarization should be indispensable for ideal microwave absorption. The porous binary NF@Co3O4 composites not only employ cost-effective raw materials for microwave-absorbing materials in many applications, but also dissipate incident microwave which is in favor of reducing severe electromagnetic pollution all over the world. Key words: Co3O4 nanowires; nickel foam; three-dimensional network; multiple interfaces; microwave absorption.


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INTRODUCTION Nickel foam (NF) is an attractive material for its wide range of applications, such as electrocatalysts,1, 2 electrode materials3, 4 and supercapacitors5. As compared to traditional materials, NF has distinguished features such as low cost, remarkable electrical conductivity, high thermal conductivity as well as open-pore structures.6, 7, 8 Owing to excellent chemical stability, magnetic characteristic and low-density9, NF has great potential to be promising microwave absorbers. However, few studies have been performed on NF because of its poor impedance matching performance, thus, in this study, we ingeniously design a novel structure of 1D Co3O4 nanowires with prominent dielectric properties attaching on 3D nickel porous network to achieve good interfacial polarization, which can lead to the improved electromagnetic properties.10 In addition, the 3D nickel foam with high porosity and large specific surface area11 may be able to act as the physical support for Co3O4 nanowires, which is beneficial to the maximum distribution of Co3O4 on NF.12 In our daily life, severe electromagnetic pollution, electric safety, information counterwork, environmental defense and disease morbidity, cause by a wide range of electronic, radar and wireless devices, have resulted in serious issues everywhere.13 Take sustainable development of the human beings into consideration, more close attention has been paid on electromagnetic shielding and microwave absorption since it not only reduces electromagnetic pollutions but also protects human health.14,

15

ceramics18-21, carbon materials8,

So far, many microwave absorbers, including ferrites16, 22,

17,

etc., have been employed. It is well known that micro-

morphology and sizes of microwave absorbers are important for microwave attenuation abilities.23 Moreover, the components of absorbers are known to have a significant effect on electromagnetic parameters.24 Materials with different structures, such as spindle25, core-shell26, yolk-shell27, hollow28, three-dimensional29,

30

and one-dimensional31, have been extensively studied. In

recent years, one-dimensional (1D) materials have aroused intensive interest due to their ACS Paragon Plus Environment


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superior aspect ratios, large specific surface area, outstanding thermal stability, excellent mechanical properties, and great electron transport properties.31, 32 The Co3O4 materials, one class of transition metal oxide material, have been studied to possess tunable morphologies, such as nanosheet (NS), nanowire (NW) and nanocluster (NC).7 For example, Zhao et al. fabricated one-dimensional Co3O4, and obtained the minimal reflection loss of -23.8 dB with a thickness of 2.0 mm.33 Based on the ideas, the design of 1D Co3O4 nanowires on 3D nickel foam is worth to be investigated to obtain multiple interfaces for proper impedance matching and strong microwave absorption. However, several problems like large-scale preparation and low loading capacity still exist, thus, efforts should be devoted to in the future. An ideal absorber, according to electromagnetic theories, should transform electromagnetic energies into other types of energy like heat as much as possible.24, 34, 35 Besides, the value of intrinsic impedance matching (Z = |Zin/Z0|) should be close to 1, which means the value of materials impedance is approximately equal to the value of free space impedance, lead to less reflection of incident microwave at the surface of absorbers.36-38 Unfortunately, there is scare report on the impedance matching and microwave absorption of the 3D network NF@Co3O4 composites. Therefore, in this work, a novel 3D network with 1D wire shaped NF@Co3O4 composites were obtained, more importantly, superior impedance matching performance was achieved. In addition, 1D nature and 3D network surface has an important effect on interfacial polarization, thus, long microwave travel distance due to multiple reflections and scattering can contribute to the microwave absorption. Consequently, this study not only offers facile synthesis method to obtain 3D network with 1D wire shaped NF@Co3O4 composites, but also provides new rational design insight into the microwave absorber working in gigahertz.

EXPERIMENTAL SECTION Chemicals ACS Paragon Plus Environment

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Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was bought from Nanjing chemical reagent Co. Ltd (Nanjing, China). Urea was purchased from Shanghai Titan Scientific Co. Ltd (Shanghai, China). Nickel foam was purchased from Kunshan Jiayesheng Electronics Co. Ltd (Suzhou, China). Preparation of Porous NF@Co3O4 composites As shown in Figure 1, NF@Co3O4 composites was prepared by simple hydrothermal and calcinations process. The nickel foam (2.5 cm × 5 cm × 1.0 mm) was firstly etched in 6 mmol·L-1 hydrochloric acid for 20 min in order to remove oxide film. Then, it was cleaned in ethanol for 20 min to get rid of organic materials on surface and the sheet was rinsed with deionized water for 20 min and then dried in vacuum for 2 h. After that, 0.005 mol Co(NO3)2·6H2O and a certain amount of CO(NH2)2 were dissolved in 30 mL deionized water using ultrasonic stirring method for 10 min to generate a uniform pink solution. The mixture and the pretreated NF were transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 70 oC (S1)，100 oC (S2)，130 oC (S3) for 12 h to tune the electromagnetic parameters (Table S1). Figure 2 illustrates the photographs of nickel foams and reaction solution. As the nickel foam was treated at 70 oC, we can scarcely see any pink powder loading on the foam, meanwhile, the reaction solution after the hydrothermal process still presents pink. When the temperature increased to 100 oC, the transparent solution proves the complete reaction and the dried nickel foam shows structural perfection of nickel network and high uniformity of pink powders. However, the reaction solution appears blue after hydrothermal treatment at 130 oC and the prepared foam has a crack in it. The obtained precursor was washed by deionized water and anhydrous ethanol for several times under ultrasonic. Finally, the composite was annealed under 400 oC for 2 h in air, the black product was collected after thoroughly cleaning by deionized water and anhydrous ethanol, then dried at 60 oC for 5 h. For comparison, NF substrate was washed by hydrochloric acid, ethanol and deionized water. Pure NF was obtained by the same method except the addition of Co(NO3)2·6H2O and urea. Pure Co3O4-100 was achieved by the same method except the addition of nickel foam. ACS Paragon Plus Environment


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Materials characterization In order to obtain more information about the crystal morphology properties, X-ray diffraction (XRD, Bruker D8 ADVANCE) in the range of a 2θ=10-90º at 40 kV and 40 mA using Cu-Kα as the irradiation source were conducted. X-ray photoelectron spectroscopy technique (XPS) with an Al Kα X-ray source at 150 W uses a PHI 5000 VersaProbe to detect the elemental compositions and chemical states. The microscopic characterization was determined by field emission scanning electron microscopy (FE-SEM, Hitachi S4800). The S parameters (S11 and S22) of complex permittivity (εr) and complex permeability (μr) in the specific frequency range of 2-18 GHz were tested using an Agilent PNA N5244A vector network analyzer. The samples were first ground in an agate mortar, then homogeneously mixed with 40 wt % paraffin wax matrix in a ceramic crucible at 90 °C. The obtained mixture was then pressed into toroidal-shaped samples (φout: 7.00 mm, φin: 3.04 mm) using the mould and measured by vernier caliper. To further investigate electromagnetic wave absorption property influenced by the loading mass of Co3O4 on NF, the mass before and after loading were measured by electronic analytical balance (Table S2).

Figure 1. Schematic diagram of the synthesis process of NF@Co3O4.

RESULTS AND DISCUSION Figure 2d exhibits the XRD pattern of the as-prepared NF@Co3O4. Prominent peaks at 44.832 o, 52.228 o and 76.807 o were found to match well with the (111), (200) and (220) peaks ACS Paragon Plus Environment

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(JCPDS No.70-0989), that is to say the crystal structure of Ni still maintain after hydrothermal and annealing treatment. The detected (311) plane at 36.845 º is consistent with Co3O4 (JCPDS No.74-2120), but the weak diffraction peaks of samples indicated that the products were mainly nickel foam with a small amount of Co3O4. In addition, the color conversion from pink to black is well identified with the materials change from Co2(OH)2CO3 to a small amount cubic phase Co3O4 (Figure 2a-c). From Figure 2e and Figure 2f, there are shift of binding energy for both Co and O elements. For an atom with a given configuration and a structural environment, the binding energy of the inner electrons shifts almost the same. However, owing to the differences in electronegativity, the outermost electrons go through migration and detachment. Therefore, we can obtain additional information about the electronic interaction. For S1 sample, more tight contact between nickel foam and Co3O4 would make more electron transfer, which may lead to an evident shift to higher binding energy. Figure 2e exists two main peaks of Co 2p1/2 and Co 2p3/2 spin-orbital photoelectrons, corresponding to the binding energy of 794.3 eV and 779.9 eV, respectively. Other peaks at higher binding energy of 797.0 eV and 781.6 eV were related to the shake-up satellite of Co2+ and Co3+.39 Comparing to S2 and S3 samples, the main peaks of Co 2p1/2 and Co 2p3/2 on S1 sample shift to the higher binding energy, resulting from not only the increase of Co3+ species, but also the presence of Co2+ species which caused by the spin-orbital splitting between the Co 2p3/2 and Co 2p1/2 peaks, as well as the shake-up satellite peaks of Co2+. With respect to the O1s XPS spectra of the samples, one peak at around 529.3 eV should be lattice oxygen species like O2– (Oi), another peak at 531.6 eV is highly oxidative oxygen species which can be ascribed to O2 −,

O − or OH− (Oii).7 The energy dispersive X-ray spectrometry (EDS) image of S2 illustrates that the samples are

composed of three elements, including Ni ， Co and O. The EDS element mapping of S2 ACS Paragon Plus Environment


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sample (Figure 3b) is well consistent with XRD, XPS and SEM images. The atom ratio of Co to O in Figure 3b is detected to be 0.69 (Figure 3b), which is closed to the value of the stoichiometric ratio of Co3O4 crystal, indicating the fact that Co3O4 particles were loaded on nickel foam separately. As we can see from the mapping images (Figure 3c-d), Ni and Co component are homogeneously distributed, but oxygen tends to distribute more on one side, which result from asymmetrical may anneal in air.

Figure 2. Photographs of nickel foams after hydrothermal (the first row) and reaction solutions after hydrothermal (the second row) and nickel foams after annealing (the third row) at (a) 70 oC, (b) 100 oC

and (c) 130 oC, and (d) XRD patterns, (e) Co 2p and (c) O 1s XPS profiles of S1, S2 and S3. ACS Paragon Plus Environment

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Figure 3. (a) SEM images of pure NF, (b) EDS result and (c) Ni, (d) Co, (e) O element EDS mapping of S2 sample. The morphology features of NF substrate, pure Co3O4-100, pure NF, S1, S2 and S3 samples were obtained through SEM, as shown in Figure S2, Figure S3, Figure 3a and Figure 4. It can be seen that NF substrate (Figure S2) presents a three-dimensional porous network microarchitecture with smooth surface. Figure S3 presents the SEM image of flower-like pure Co3O4-100 nanowires. From Figure 3a, one can find that the surfaces of pure NF are very rough. With the changes of temperature, the loading dense of Co3O4 on nickel foam is also changed. From Figure 4, we can observe that one-dimensional hedgehog shaped Co3O4 grow on the 3D pure nickel foam. Obviously, Figure 4a shows that the S1 composite is a fractured NF network with fibrous particles. When the reaction temperature increased to 100 oC (Figure 4b), the S2 sample not only emerged with longer and thicker nanoneedles, but also arranged more flowerlike nanostructures with dense aggregation on the backbones of porous nickel foam. These



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structures may result in large contact interfacial area and superior aspect ratios as well as great electron transport properties. However, the S3 sample (Figure 4c) represents small snowflake figure loading on NF network, which may cause deterioration on mechanical properties. The average nanowire sizes of S1, S2 and S3 are 650 nm, 930 nm and 720 nm, respectively. Therefore, we can conclude that only at the appropriate temperature Co3O4 nanowires could be better attached on nickel foam. Given the results mentioned above, the possible synthesis mechanism of Co3O4 nanowires attached on nickel network can be illustrated by equations as follows (1-5).40-42 Through facile hydrothermal process, the hydrolysis reaction of urea CO(NH2)2 leads to the generation of NH4+ and NCO-, and then NCO- dissolved into water to form HCO3-, NH4+ and OH-. Moreover, HCO3- decomposed into CO32- and OH- due to its instability in an aqueous solution, which could trigger the precipitation of Co 2p ions in the reaction solution to form Co2CO3(OH)2 precursor. During annealing treatment, the precursor transformed into Co3O4 crystal growing on nickel network surface with vigorous binding force. More significantly, the process of Co3O4 crystal growth could be well controlled by the slow generation of OH- during the hydrothermal approach. When heated at 70 oC, the rate of the thermal decomposition of urea CO(NH2)2 was slow. As can be seen from Figure 2a2, the pink solution can prove that there was still large amount of Co(NO3)2·6H2O that were not involved in the reaction. As a result, the precursor can load on nickel foam under a certain limitation. However, when the temperature increased to 100 oC, OH-, CO32- and Co2+ can form the precursor in force, the transparent solution after the hydrothermal process demonstrate that all raw materials completely are used to produce Co2(OH)2CO3. When temperature further increased to 130 oC, the color of the solution changed to blue, which may result from the production of biuret, triuret and ammonia gas owing to the side reactions, as can be illustrated by equations (6-7).12, 41 ACS Paragon Plus Environment

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Besides, the formation of NH3 may weaken the adhesion force between Co2(OH)2CO3 and nickel network, thus pink particles distributed on the NF surface in homogeneously. Consequently, the precursor obtained at 100 oC is the most appropriate candidate for further annealing. CO(NH2)2 + hydrolysis  NH4+ + NCO-

(1)

NCO- + 3H2O  HCO3- + NH4+ + OH-

(2)

HCO3-  CO32- + OH-

(3)

2Co2+ + 2OH- + CO32- Co2(OH)2CO3

(4)

Co2(OH)2CO3 + O2 + Cacination  2Co3O4 + 3H2O +3CO2

(5)

2CO(NH2)2  NH2CONHCONH2 + NH3

(6)

NH2CONHCONH2 + CO(NH2)2  NH2(CONH)2CONH2 +NH3

(7)

Figure 4. SEM images of (a) S1 (b) S2 (c) S3 samples.



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For purpose of better understanding the microwave absorption abilities of the as-prepared samples, the real (ε') and imaginary (ε'') parts of permittivity and the real (μ') and imaginary (μ'') parts of permeability of pure nickel foam, S1, S2 and S3 from 2 to 18 GHz are shown in Figure 5a-d. The real parts (ε' and μ') signify the storage ability of electrical and magnetic energy in the medium, while the imaginary parts (ε'' and μ'') represent the dissipation performance of electrical and magnetic energy.43,

44

Pure NF shows the highest relative

complex permittivity and relative complex permeability compared to NF@Co3O4 composites with the same filling ratio. The ε' value of the NF sample decreased from 22.27 to 11.79 with the increasing frequency from 2 GHz to 18 GHz and the ε'' value declined from 11.03 to 3.89 with the increasing frequency in the range 2–18 GHz. In addition, the average values of μ' and μ'' are 0.98 and 0.24, respectively. This result suggests that dielectric loss contributes to the electromagnetic absorption of the pure NF over the full tested frequency region. The μ' and μ'' of S1, S2 and S3 samples (Figure 5b-d) maintain almost constant without any decay at 2-18 GHz range, indicating that the magnetic dissipation capacity of the samples is quite stable. It can be found that the ε' values of the three samples S1, S2 and S3 (Figure 5b-d) all accompanied by some fluctuations, while the values of ε'' increase a little in the whole testing frequency range. Moreover, in the ε'' curve of Figure 5b-d, obvious peaks indicate the dielectric resonance behavior and the multiple peaks represent dual resonances behavior, which can be ascribed to interfacial polarization caused by the multi-interfaces between one-dimensional Co3O4 nanowires and three-dimensional porous nickel foam.45 With the increase of hydrothermal temperature, the permittivity of samples decreased a little, the binding force between Co3O4 nanowires and nickel foam have also changed, which may be a significant influence on interfacial polarization. However, high permittivity sometimes leads to insufficient microwave absorption performance because of poor impedance matching.46 According to the free electron ACS Paragon Plus Environment

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theory, there shows an equation ε''=1/πε0ρf, where ρ represents the electrical resistivity.47 The low imaginary part (ε'') of the complex permittivity indicates a high electrical resistivity of the materials, but high conductivity implies better reflection of electromagnetic waves. Thus, proper values of real and imaginary parts of permittivity have a contribution to impedance matching behavior. As shown in Figure 5e and Figure 5f, values of tan δε are much higher than that of tan δm, indicating that the dielectric loss plays a dominant role in the microwave absorption behavior of as-prepared samples. In addition, with the increasing hydrothermal temperature, the numbers of the evident relaxation peaks reduce obviously. For the S1, S2 and S3, the average values of tan δε are 0.26, 0.13 and 0.10, and those of tan δm are 0.07, 0.13 and 0.13, respectively. As detected in Figure 5e, tan δε values of NF trend to be stronger than S2, manifesting that complete and smooth 3D conductive network structure is beneficial for dielectric loss because of its conductive path. Compared with S1 and S3, the S2 sample exhibits higher tan δm values, as shown in Figure 5f, revealing magnetic loss contributes to the microwave absorption of the NF@Co3O4 over the full tested frequency region.14 This result suggests that thicker and longer 1D Co3O4 nanowires structure can contribute a lot to microwave attenuation. Consequently, through hydrothermal process at appropriate temperature (100 oC), 3D nickel network firm combined with 1D Co3O4 wires shows enhanced microwave abilities in magnetic and dielectric loss.



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Figure 5. Frequency dependence of the electromagnetic parameters of (a) pure NF (b) S1 (c) S2 and (d) S3; the dielectric loss tangents (e) and the magnetic loss tangents (f) of pure NF, S1, S2 and S3 composites in the range of 2-18 GHz. On the basis of previous literature, the electromagnetic absorption performance is intuitive criterion to show the quality of microwave absorbers.27, 48 According to the transmission line theory, the reflection loss values can be clearly calculated by the following equations: 36, 49, 50 Zin  Z0 μ r ε r tanh j2 πfd c  μ r ε r

(8)

(9) Where μr and εr represent the relative complex permeability and permittivity of absorber, f is the microwave frequency, d signifies the thickness of absorber, c means the velocity of light, ACS Paragon Plus Environment

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Zin stands for the input characteristic impedance and Z0 represents the wave impedance of free space. Generally, a RL value below -10 dB indicates that ninety percent of microwave has been effectively absorbed.34, 36, 46 Figure S4 and Figure 6 illustrate that the relationship between the RL values and frequency at different thickness from 1.9 mm to 2.9 mm. Based on previous literature36, we can find that single metal nickel shows poor microwave absorption, the reflection loss values of single metal nickel cannot reach -10 dB because of the impedance mismatching. Moreover, NF substrate presents better electromagnetic performance than single metal nickel, but it cannot achieve -10 dB (Figure S4a). Clearly, pure NF (Figure 6a) has great electrical conductivity which may cause efficient conductive loss, but it cannot reach excellent microwave reflection due to the impedance mismatch. In conclusion, 3D structural nickel foam in this work not only act as structure support but also presents more multiple internal scattering loss, which leads to stronger absorption abilities than Ni elementary substance, thus certifies the advantage of three-dimension architecture in microwave absorbing. In addition, Figure S4b shows poor electromagnetic dissipation of pure Co3O4-100, but Co3O4 nanowires attaching on NF may harvest multiple interfaces in favor of excellent microwave absorption. As described in Figure 6a, the pure nickel foam after 100 oC hydrothermal process can obtain reflection loss which is below -10 dB in view of the 3D network structure with high electrical conductivity, but it still cannot reach excellent microwave reflection. However, for the sample S2 (Figure 6c and 6g), the minimum RL value is -41.1 dB at 11.20 GHz with the matching thickness of only 2.1 mm. It should be noted that multiple interfaces caused by the formation of Co3O4 crystal on NF and optimized hydrothermal temperature improved the microwave absorption properties in C and X band. For 2.3 mm (Figure 6h) and 2.5 mm (Figure 6i) samples, the S2 composites exhibit satisfactory microwave absorption with multiple reflection loss peaks, which may result from great electron transport properties of 1D wires and conductive 3D nickel network. ACS Paragon Plus Environment


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Obviously, for 2.3 mm (Figure 6h), three effective peaks represent the reflection loss value of -17.3 dB, -20.1 dB and -13.2 dB, respectively, with the effective bandwidth of 3.46 GHz. Equally important, for 2.5 mm, one peak of the RL value reaches -24.7 dB at the frequency of 8.84 GHz and another peak of the RL value attains -27.4 dB at 10.08 GHz, and the reflection loss values less than -10 dB are observed with a bandwidth of 2.82 GHz (7.88-10.70 GHz). Based on the above hypothetical synthesis mechanism, we can conclude that S3 (Figure 6d) do not show excellent microwave absorption properties which may result from the negative effects of side reactions during the hydrothermal process.12, 41 When temperature increased to 130 oC, the reaction may produce biuret, triuret, ammonia gas and other adverse by-products which may have negative effect on electromagnetic properties. It is known to all that an ideal electromagnetic microwave absorber should possess the characteristics like thin thickness, strong absorption, wide bandwidth and low density.51 For the sample S1(Figure 6b), the minimum reflection loss value of S1 is -33.7 dB at 7.92 GHz with microwave absorption bandwidth of 2.72 GHz at the thickness of 2.9 mm. The multiple absorption peaks of S2 composites may result from several reasons. First, the formation of these peaks could be ascribed to the resonant absorption which caused by the quarter-wavelength attenuation, fm=nc/4tm (εrμr)1/2, where tm shows thickness and fm is peak frequency.52, 53 As shown in Figure 6c and Figure 6e, several RL peaks shift to low-frequency direction with increased layer thickness, which can be explained by λ/4 curve. As can be seen from Figure 6e, the black curves reveal the simulation thickness according to quarter wavelength theory, and the magenta dots stand for the practical matching thickness at peak frequency. It can be observed that practical thickness is in good agreement with the simulated thickness, indicating the influential role of resonant absorption in the electromagnetic wave dissipation process. Besides, some obvious peaks in the curves of ε'' can be seen from Figure ACS Paragon Plus Environment

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5c, which have a relationship with the strong dipole polarization and interfacial polarization.38 In addition, the corresponding position of dielectric resonant peaks in Figure 6c reveals their generation because of strong resonant behaviors due to optimizing the interfaces between nickel foam and Co3O4 wire.54 As we can see from Figure 6f, with the thickness decreases, the reflection loss values become weaker then stronger at a relatively low thickness range (2.1-2.5 mm) with effective absorption bandwidth. In conclusion, compared with the sample pure NF, S1 and S3, the as-prepared sample S2 possesses preferable absorption performance. Hence, the S2 sample is of bright future for application to be broadband microwave absorbers.



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Figure 6. Reflection loss values versus frequency of (a) pure NF (b) S1 (c) S2 (d) S3; (e) comparison of practical RL values for S2 and these positions correspond to the simulation thickness under λ/4 conditions; (f) development tendency of RL and effective frequency bandwidth of S2 at a small thickness range and (g-i) RL curves of S2 at various thickness. It is acknowledged that the variation tendency of the electromagnetic properties can be explained by not only suitable impedance matching characteristics but also remarkable attenuation performance of the samples. The impedance matching properties are determined as the following formulas: 55-57 ACS Paragon Plus Environment

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(10)

(11) where Zin signifies the input impedance of the absorber, Z0 represents the impedance of free space, εr stands for complex permittivity and μr shows complex permeability. If the value of Z is close to 1, that is to say εr equals μr and then Zin equals Z0. In other words, there is no reflection between air and the absorbent, and the microwave could enter into materials entirely by further dissipation. For comparison, Figure 7a-d exhibits the impedance matching values of the as-prepared samples with the same paraffin wax filler ratio. The values which are near 1 are marked by purple bold lines. Clearly, pure nickel foam shows the poorest impedance matching performance as can be seen from Figure 7a. All the Z values are less than 0.8, thus 3D nickel network should combine with 1D Co3O4 nanowires of superior dielectric properties in order to optimize impedance matching. Evidently, for the S1, S2 and S3 samples (Figure 7b-d), purple bold lines cover broader frequency and wider range of thickness, thus, they possess preferable impedance matching properties. When the thickness is 2.1 mm, |Zin/Z0| values of 0.7-1.3 cover the frequency range 8.44-13.52 GHz (Figure 7f), and in this frequency range, the RL values of S2 are more negative than -10 dB, manifesting that more superior impedance matching is available for better microwave absorption performance. However, for S1 and S3, the values which are close to 1 have narrower range and impedance matching is even worse at almost the whole band. Thus, S1 or S3 cannot meet the promising RL performance requirements. Apart from the aforementioned factor, another factor that should be taken into consideration is attenuation constant α, which can be defined as the integral attenuation characteristics of the absorber, is given in the following equation: 58, 59

(12) ACS Paragon Plus Environment


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It is obvious that a higher value of ε'' and a higher value of μ'' will cause a higher attenuation constant α. As presented in Figure 7e, the attenuation constant α of pure NF is higher than that of other composites, which may be caused by the high value of ε'' and μ''. However, pure NF exhibits poor microwave absorption abilities because of indecent impedance matching performance (Figure 7a). In this work, Co3O4 with excellent dielectric properties growing on NF at the suitable temperature can achieve proper impedance matching for efficient electromagnetic absorption abilities. There is no doubt that superior impedance matching properties and attenuation constant work together to make NF@Co3O4 products excellent microwave absorbers.

Figure 7. 3D representation of Z values for (a) pure NF (b) S1 (c) S2 and (d) S3, and (e) attenuation constants and (f) comparison of impedance matching at 2.1mm of pure NF, S1, S2 and S3. ACS Paragon Plus Environment

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Figure 8 shows the possible absorption mechanism of the NF@Co3O4 composite. The enhanced microwave absorption abilities of the three-dimensional with one-dimensional shaped NF@Co3O4 composite can be explained using the following points. First, the magnetic loss is beneficial for the attenuation of electromagnetic microwaves because of porous nickel magnetic network.60 In addition, multiple contact interfaces between NF and Co3O4 wires act as polarization centers inducing dielectric loss.61 Conductive nickel network who provide conductive paths for the transportation to active hopping electrons also contribute to the dielectric loss.62 Furthermore, three-dimensional network structure is useful in the absorption and exhaustion of microwave, due to its multi-scatter propagation and multiple internal absorption performance.

Figure 8. Schematic of the microwave absorption mechanism of 3D network with 1D wire shaped NF@Co3O4.

CONCLUSIONS In summary, a novel microwave absorbing composite of the 3D cross-linking network with 1D nanowires have been successfully synthesized by a facile hydrothermal treatment, accompanied by subsequent calcination process. The electromagnetic parameters can be ACS Paragon Plus Environment


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effectively controlled by changing the hydrothermal temperature. The composites gained at different temperatures exhibit diverse morphology and microwave absorption performance in C and X band. Thus, we traced the root of the formation mechanism and attenuation principles of these obtained composites. An optimal reflection loss of -41.1 dB was achieved at 11.2 GHz with a relatively thin matching thickness of 2.1 mm. It is worth noting that the RL curve of the S2 sample appeared dual peaks at 2.3 mm and triple peaks at 2.5 mm, which can be potentially applied in broadband electromagnetic materials. ASSOCIATED CONTENT Supporting Information The Table of the synthesis conditions, the Table of loading mass of Co3O4 on NF, the photos of the mould and toroidal-shaped samples, SEM images of NF substrate, SEM images of pure Co3O4-100 sample, reflection loss values versus frequency of NF substrate and pure Co3O4-100 sample. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors approved the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial supports from the National Nature Science Foundation of China (No. 11575085), the Aeronautics Science Foundation of China (No. 2017ZF52066), the Qing Lan Project, Six talent peaks project in Jiangsu Province (No. XCL-035), Jiangsu 333 Talent Project, the Open ACS Paragon Plus Environment

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The rationally designed NF@Co3O4 composites via facial synthesis process carry out effective microwave absorbing performance, which is beneficial to protect human from serious electromagnetic pollution.


Composition and Structure Design of Co3O4 Nanowires Network by

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