[email protected] nanocomposites with a hierarchical bowknot-like nanostructure for high performance broadband electromagnetic wave absorption

Accepted Manuscript Full Length Article Co/CoO@C nanocomposites with a hierarchical bowknot-like nanostructure for high performance broadband electromagnetic wave absorption Minglong Yang, Ye Yuan, Weilong Yin, Shuang Yang, Qingyu Peng, Jianjun Li, Yibin Li, Xiaodong He PII: DOI: Reference:

S0169-4332(18)32742-9 https://doi.org/10.1016/j.apsusc.2018.10.045 APSUSC 40613

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

20 July 2018 21 September 2018 4 October 2018

Please cite this article as: M. Yang, Y. Yuan, W. Yin, S. Yang, Q. Peng, J. Li, Y. Li, X. He, Co/CoO@C nanocomposites with a hierarchical bowknot-like nanostructure for high performance broadband electromagnetic wave absorption, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.10.045

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Co/CoO@C nanocomposites with a hierarchical bowknot-like nanostructure for high performance broadband electromagnetic wave absorption Minglong Yang, Ye Yuan, Weilong Yin, Shuang Yang, Qingyu Peng, Jianjun Li, Yibin Li*, Xiaodong He Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China *Corresponding author E-mail address: [email protected] (Yibin. Li) , Tel & Fax: +86-451-86402326.

Abstract High performance broadband microwave absorption material is in urgent demand as the radar detection frequency range has been greatly broadened and electromagnetic radiation pollution has become increasingly serious. In this work, we fabricated a hierarchical bowknot-like Co/CoO@C nanocomposite with a two-step hydrothermal method, followed by high-temperature pyrolysis under argon atmosphere, which is composed of amorphous carbon and uniformly dispersed irregular Co/CoO nanoparticles. Complex permittivity and complex permeability were modulated by varying the calcination temperature (500-700 °C). Reflection loss (RL) curves of the Co/CoO@C nanocomposites show superior broadband electromagnetic wave absorption performance. Especially, the effective absorption bandwidth was maximized to 13.6 GHz (4.4-18 GHz) at the thickness of 6mm, and a -45.0 dB minimum RL appears at 6 GHz. Such superior broadband electromagnetic wave absorption properties are attributed to the synergistic effect of the hierarchical bowknot-like structure and strong surface/interface polarization, as well as the eddy current and magnetic natural resonance. These results indicate the bowknot-like Co/CoO@C nanocomposite is promising in dealing with broadband electromagnetic radiation pollution and broadband military radar detection. Key words: bowknot-like nanocomposite, Co/CoO@C nanocomposite, broadband microwave absorption, hierarchical nanostructure

1. Introduction

Rapid development of electronic technology, leads to the widely use of electromagnetic (EM) wave in both civil and military fields such as wireless communication

and

radar

detection

systems.

However,

these

high-power

electromagnetic radiation sources have caused serious EM radiation pollution simultaneously, which has become a major concern for both human health and electronic devices stability [1-5]. On the other hand, the detection frequency range of military radars has been greatly broadened. In order to increasing the survival rate of air force aircrafts, broadband radar EM wave absorption materials have now become an urgent demand for the military. Therefore, EM wave absorption materials with broad absorption bandwidth, high absorption capacity and thin thickness have drawn extensive research attention. Significant efforts have been made to explore various high performance broadband EM wave absorption materials in the past few decades, but it remain challenges. EM absorbing materials such as ferrites, carbon-based materials have been extensively used as high performance EM wave absorbent on the aircrafts. Ferromagnetic metal nanoparticles including iron, cobalt, nickel and some of their magnetic oxides have been explored as EM wave absorption materials for decades due to their large saturation magnetization and large Snoek limit at higher frequency range [6-11]. But this kind of EM wave absorption material usually has a narrow effective absorption bandwidth, which cannot meet the requirement of engineering applications. Carbon based materials such as carbon fibers (CFs), carbon nanotubes (CNTs), mesoporous carbon and graphene have also been attractive candidates for

EM wave absorption materials due to their facile tunable EM properties [12-16]. However, since carbon materials are nonmagnetic, there is only dielectric loss mechanism for the dissipation of EM energy, which results in a weak EM wave absorptive capacity in the radar frequency range. Also, the absorption performance of these kinds of carbonaceous EM wave absorption materials usually rely on a relatively large thickness, which is surly unacceptable in practical application. In recent years, more strategies have been employed to achieve the maximal absorption of EM energy in a wider frequency range [17-25]. The decoration of ferromagnetic nanoparticles into carbon based materials turns out to be a feasible way to enlarge the absorption frequency bandwidth and attain a superior absorption capacity at the same time, due to the incorporation of dielectric loss and magnetic loss mechanism [26-30]. Therefor, ferromagnetic nanoparticles/carbon nanocomposites with special nanostructures have drawn growing research interest. In this work, a Co/CoO@C nanocomposite with a bowknot-like morphology was successfully fabricated with a two-step hydrothermal method, followed by high-temperature pyrolysis under argon atmosphere. The EM wave absorption performance of the bowknot-like Co/CoO@C composites was surveyed. Porous bowknot-like nanostructure endows the composite a favorable impedance matching condition. Incorporating with the synergistic effect of the strong surface/interface polarization as well as the eddy current and magnetic natural resonance, the as prepared bowknot-like Co/CoO@C composite exhibited an enhanced superior broadband EM wave absorption performance. With the superior broadband EM wave

absorption performance, the as-prepared bowknot-like Co/CoO@C composite will be a very promising candidate for high performance EM wave absorption material.

2. Experimental 2.1 Materials Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, analytical grade), urea (CO(NH2)2, analytical grade) and gelatin (photographic grade) were purchased from Aladdin Industrial Corporation. All reagents were used as received without further purification. 2.2 Synthesis of bowknot-like precursor The bowknot-like precursor were synthesized via a two-step hydrothermal method according to a previous reported literature [31]. Briefly, 2.5 g of gelatin was dissolved in 22.5 mL deionized water with the assist of magnetic stirring at 50 °C. Then 1.46 g cobalt nitrate hexahydrate and 1.46 g urea were dissolved in 22 mL deionized water to obtain a pink transparent solution. After that, the two solutions were mixed and transferred into a 100 mL flask keep stirring at 75 °C for 1 hour. Then, the pink solution was poured into a Teflon-lined stainless steel autoclave (50 mL). After sealed the autoclave was heated at 100 °C for another 6-10 hours (6 h, 7 h, 8 h and 10 h in particular) in a thermostatic oven. After that, it was cooled to room temperature naturally. The pink precipitate were collected by centrifugation and washed with deionized water and pure ethanol for a few times. Finally, the precipitate was dried in a thermostatic oven at 60 °C for 24 h.

2.3 Synthesis of bowknot-like Co/CoO@C nanocomposite The porous bowknot-like Co/CoO@C nanocomposite was prepared though a calcination process. The as prepare pink precursor was loaded into a combustion boat, calcined with a heating rate of 10 °C·min−1 and maintained at the required temperature (500°C, 600 °C and 700 °C) for 2 h under argon. After that, the black powder was collected and stored at 60 °C in a thermostatic oven. The as synthesized products were denoted as Co/CoO@C-500, Co/CoO@C-600, and Co/CoO@C-700 respectively according to the applied calcination temperatures. Comparative experiments were carried out by calcining the precursor in air and H2/Ar mixed atmosphere (1:3 volume ratio) at 600 °C for 2 h with a same heating rate of 10 °C·min−1. The products were denoted as Co3O4-600 and Co@C-600 respectively. 2.4 Characterization Scanning electron microscope (SEM, Hitachi S4800) and transmission electron microscope (TEM, JEOL 2010F) were employed to observe the morphologies of the as prepared precursor and the Co/CoO@C nanocomposites. Powder X-ray diffraction patterns (XRD) were acquired on a Shimadzu XRD-7000s diffraction instrument. Nitrogen adsorption-desorption isotherms were measured with a Quadrasorb-SI instrument at 77K. The magnetic hysteresis loops was measured with a vibration sample magnetometer (Tamakawa TM-VSM2014-MHR). Surface elemental analyses were performed by X-ray photoelectron spectroscopy (XPS) (VG Scientific ESCALAB Mark II spectrometer).

For EM wave absorption performances characterization, the bowknot-like Co/CoO@C nanocomposites as well as Co3O4-600 and Co@C-600 were uniformly mixed into paraffin with a mass ratio of 1:1. Then the mixture was compressed into toroidal-shaped specimens with an inner diameter of 3.04 mm, outer diameter of 7 mm and thickness of 3 mm. Relative complex permittivity (  r     j  ) and relative complex permeability ( r     j   ) were measured with a network analyzer (Agilent Technologies N5222A) in the frequency range of 2-18 GHz. Reflection Loss curves were calculated with the measured  r and  r according to the transmission line theory.

3. Results and discussion Fig. 1 shows the SEM images of the as-prepared Co2+@gelatin metal ion-polymer frameworks precursors with different hydrothermal reaction times. Fig. 1(a)-(d) corresponding to a hydrothermal reaction time of 6 h, 7 h, 8 h and 10 h respectively. It can be clearly observed that the hierarchical precursor exhibits a bowknot-like morphology made up of a large number of nanosheets with thickness between a few nanometers to ten nanometers approximately. These nanosheets overlapped with each other making up the bowknot-like nanostructure. the possible growth mechanism was investigated [31]. Briefly, when the cobalt nitrate solution was added into the gelatin solution and keep stirring at 75 °C, Co2+ coordinate with the amino, amide and carboxyl groups on the gelatin molecules though a coordination reaction to form metal ion-polymer frameworks. During the hydrothermal reaction the CO(NH2)2 slowly hydrolyzed into CO32- and OH-, these anions reacted with Co2+ to

generate precipitate, meanwhile the metal ion-polymer frameworks overlapped with each other because of intermolecular forces to form nanosheets. As the hydrothermal reaction time increases, more and more nanosheets appeared and overlapped with each other to build up the hierarchical bowknot-like nanostructure. As show in Fig. 1(a), after hydrothermal reacted in the autoclave for 6h, just a few nanosheets were assembled to the collected precursor, and the size of the precursor was 1-1.2 μm. When the reaction time was extended to 7 h and 8 h (Fig. 1(b) and (c)), more nanosheets appeared and the number of nanosheets assembled onto the bowknot increased rapidly, the size of the bowknot-like nanostructure grew to 2.5-3 μm. It is worth notice that compared to precursor collected after a 8 h hydrothermal reaction, there is only a slight change in the size and the number of nanosheets assembled to the bowknot-like precursor collected after 10 h. For the consideration of timesaving, bowknot-like precursor collected after 8 h was chosen for the subsequent calcination process and all the characterizations. After high temperature calcination the non-magnetic pink bowknot-like precursor powder turns into black magnetic powder. Low and high-magnification SEM images of the product obtained at 600 °C were shown in Fig. 1(e) and f. It can be clearly observed that both the bowknot-like morphology and the size of the nanostructure were well maintained. However, after calcination, the nanosheets with smooth surfaces evolved into numerous irregular nanoparticles, which still piled up to preserve the nanosheet configuration. The size of these particles ranges from 10 nm to 100 nm, and the thicknesses of the nanosheets range between 30-50 nm.

Typical TEM images with different magnifications of the bowknot-like precursor and Co/CoO@C-600 nanocomposite were shown in Fig. 2. It can be clearly observed in Fig. 2(a)-(b) that the precursor collected after 8 h hydrothermal reaction exhibits a symmetrical bowknot-like nanostructure, which consists of a large number of nanosheets. Fig. 2(c) provides a clear view of the profile of these nanosheets. It can be clearly observed that the typical thickness of these nanosheets ranges from 5 nm to 8 nm. As shown in Fig. 2(d) and Fig. 2(e), after calcined at 600 °C for 2 hours under argon, the nanosheets evolved into nanoparticles. The dimension of them varies from 10 nm to 100 nm. But, the nanosheets configuration and the bowknot-like nanostructure maintain undamaged. It can be attributed to the carbonaceous matrix generated after the gelatin was carbonized. The carbonaceous matrix sticks those irregular nanoparticles together to maintain the bowknot-like morphology. Fig. 2(f) shows the HRTEM images of Co/CoO@C-600. Carbon layers at the surface of the nanoparticles are amorphous. Distinguishable lattice spacing of the crystalline nanoparticles are 0.257 nm and 0.219 nm, corresponding to the (111) and (200) lattice planes of CoO [32]. Inset in Fig. 2(f) shows the selected area electron diffraction (SAED) pattern of Co/CoO@C-600. The presents of both bright spots and diffraction rings in the SAED pattern reveals the polycrystalline feature of the as prepared bowknot-like nanocomposites [33, 34]. The constituents of the final products were analyzed by powder XRD. Fig. 3(a) shows the XRD pattern of the Co/CoO@C nanocomposites. All the diffraction peaks

can be precisely indexed to the standard patterns of CoO (JCPDS 48-1719) and Co (JCPDS 15-0806). In specific, diffraction peaks located at 2theta = 36.51°, 42.33°, 61.53°, 73.71° and 77.58° corresponding to the (111), (200), (220), (311) and (222) lattice planes of the cubic cobalt oxide respectively [32]. Other resolved diffraction peaks located at 2theta = 44.30°, 51.59° and 75.90° can be well assigned to the (111), (200) and (220) lattice planes of cubic cobalt [35]. Meanwhile, with increasing calcination temperature, these diffraction peaks become increasingly sharp and strong, which indicate that higher calcination temperature results in better crystallinity. It is noteworthy that no obvious graphitic carbon diffraction peak was observed. This is unexpected, because pyrolysis of the gelatin ligands is supposed during the calcination, remain only residual carbon. The absence of graphitic carbon diffraction peak may be caused by poor graphitization or low carbon content. For quantitative component analysis and confirming the existent of carbon matrix in the products, XPS spectrum was employed. Full spectrum of Co/CoO@C-600 is shown in Fig. 3(b). Sharp peaks were observed at 282.7 eV, 528.7 eV and 778.7 eV corresponding to the characteristic peaks of C1s, O1s and Co2p respectively [31]. Other distinguishable peaks can also be assigned to other chemical state of Co, C and O. Characteristic peaks with binding energies of 58.7 eV, 100.7 eV and 925.7 eV are corresponding to Co 3p, Co 3s and Co 2s peaks respectively. Peaks located at 713.7 eV and 836.7 eV are both Co Auger peaks. And peaks with binding energies of 972.7 eV and 1222.7 eV are assigned to the O Auger and C Auger peaks respectively. These results reveal the present of cobalt, oxygen and carbon at the surface of the products. The high

resolution Co 2p and C 1s XPS spectra are illustrated in Fig. 3(c) and Fig. 3(d) respectively. Co 2p spectrum comprised of two main peaks at 780.1 eV and 794.8 eV which corresponding to Co 2p1/2 and 2p3/2 peaks respectively. Meanwhile two satellite peaks at 784.5 eV and 801.0 eV were observed, which are characteristic of cobalt oxides [36, 37]. To further determine the oxidation state of cobalt, peak fitting of the Co 2p3/2 peak was performed. The Co 2p3/2 peak can be fitted into two peaks, Co metal at 779.5 eV and Co2+ at 778.9 eV [37]. Quantified ratio of the two chemical states for Co was determined by the peak area ratio Co/ Co2+ ≈ 0.25. High resolution C1s spectrum was shown in Fig. 3(d). It comprised of a main peak at 284.8 eV and a distinguishable weak peak at 288.9 eV. Peak fitting shows the main peak comprise of strong C-C band peak at 284.8 eV and weak C-O band peak at 285.4 eV, and the broad weak peak at 288.9 eV was attributed to O-C=O band [38, 39]. These peaks indicate residual carbon in the product，which also imply some of the residual carbon in the Co/CoO@C nanocomposite remains in oxygen-containing groups after high temperature calcination. Fig. 4 shows the typical SEM image and in situ cobalt, oxygen and carbon elemental distribution mapping images of Co/CoO@C-600. It can be clearly observed that cobalt, oxygen and carbon present a highly homogeneous dispersion all over the bowknot-like nanocomposite. It also reveals the presents of a carbon matrix. And confirms the bowknot-like nanocomposite is made up of CoO/Co nanoparticles piled up by a carbonaceous matrix. The bowknot-like morphology endows the Co/CoO@C nanocomposite a porous

nanostructure. Specific surface area and porosity feature of the products were determined by nitrogen absorption-desorption isotherms. As shown in Fig. S1, it exhibit a Type Ⅳ isotherm with a hysteresis loop, which is corresponding to a typical mesoporous structure [31]. The specific surface area was calculated to be 10.2 m2g-1 with the BET method. Inset in Fig. S1 shows the pore size distribution of the Co/CoO@C nanocomposite powder, which reveals the pore size of Co/CoO@C nanocomposite mainly concentrated at 3 nm to 9 nm, while a broad peak is observed at 30-40 nm. The wide range pore size distribution is attributed to the hierarchical bowknot-like morphology. The irregular nanoparticles were stacked together, leaving out pores between each other with the size of a few nanometers. And the bigger pores corresponding to those located at the edge and the surface. Previous reports have demonstrated that porous structures are very favorable for microwave absorption due to the multi-reflection process and surface polarization loss [40-44]. The magnetic property of the Co/CoO@C nanocomposites is investigated with a vibrating sample magnetometer. Fig. 5 displays the magnetic hysteresis loops for the Co/CoO@C nanocomposites. All the measure samples exhibit typical ferromagnetic hysteresis

loops.

The saturation magnetization (MS)

for

Co/CoO@C-500,

Co/CoO@C-600 and Co/CoO@C-700 is 21.2, 35.1 and 46.9 emu/g respectively. It is very close to previously reported Co ferromagnetic nanocomposite [45, 46]. The low MS were attributed to the polycrystalline feature of the Co/CoO@C nanocomposite and the nanoscale of the irregular nanoparticles. Magnetic domain is naturally limited to nanoscale by the polycrystalline and nanosize of the magnetic nanoparticles.

Meanwhile, large surfaces and interfaces among these nanoparticles induce the enhancement of spin disorder. The rising trend of MS with elevating calcination temperature is mainly owing to the enhanced crystallinity and increased size for the nanoparticles. Different from MS, the coercivity (HC) decreases with the increasing calcination temperature. As shown in the inset of Fig. 5, the HC for Co/CoO@C-500, Co/CoO@C-600 and Co/CoO@C-700 is 230, 160 and 60 Oe respectively. Spin canted effect theory shows higher annealing temperature induces growth of magnetic nanoparticles and courses increased MS but decreased HC [3]. The results in this work are in good consistent with this theoretical prediction. As it is well known, microwave absorption property of EM wave absorbers are determined by their complex relative permittivity (  r     j  ) and complex relative permeability ( r     j   ). According to previous reports,   and   are corresponding to the EM energy stored within the material, while   and   are related to the dissipation effect of EM energy. Dielectric and magnetic loss tangent,

tan       and tan        respectively, are very useful factors to evaluate the EM energy dissipation ability of the absorber [6]. Fig. 6 shows the frequency dependency of electromagnetic parameters for the wax composites containing 50 wt% bowknot-like Co/CoO@C nanocomposites. As shown in Fig. 6(a), the real part of relative permittivity (   ) for all the Co/CoO@C composites decrease monotonically with increasing frequency. In specific,   of Co/CoO@C-500 decreases from 3.5 to 1.9 as frequency increase from 2 GHz to 18 GHz. Co/CoO@C-600 and Co/CoO@C-700 shows a similar decrease trend, the value

of   drops from 3.9 to 1.9 (Co/CoO@C-600) and from 3.1 to 2.0 (Co/CoO@C-700) respectively. It is noteworthy that when the frequency is less than 10GHz, as the calcination temperature increases from 500 °C to 600 °C the   value increases, but when the calcination temperature was further increased to 700 °C,   of the composite decreases to less than the value of Co/CoO@C-500. In 10-18GHz the values of   are very close for all the three kinds of nanocomposites, which monotonically decease from 2.5 to 2 approximately. This unexpected trend can be attributed to integrated effect of both crystallization level and kinetic growth of the nanoparticles. Fig. 6(b) shows the   value of composites. An increase trend with fluctuation peaks can be clearly observed for all of the composites. The   value of Co/CoO@C-500 increases from 0.11 to 0.76. As calcination temperature was elevated to 600 °C, the   value increases significantly, which increases from 0.14 to 0.91. However, the   of Co/CoO@C-700 increases from 0.06 to 0.78 in 2 - 18GHz, it is smaller than both Co/CoO@C-600 and Co/CoO@C-500. Multiple fluctuation peaks in the whole frequency range indicate that bowknot-like Co/CoO@C nanocomposites show multiple dielectric loss effects. And it is very helpful for broadband microwave dissipation. Since Co/CoO@C-600 has the largest   value and   values are very close to each other, Co/CoO@C-600 demonstrates the largest tan   value among all the investigated composites (as shown in Fig. 6(c)). It also indicates that Co/CoO@C-600 composite exhibits higher dissipative capacity for EM energy. According to the Debye dipolar relaxation theory, the real and imaginary part of the complex permittivity can be described by the following formulas [49].

s   1  j 0

(1)

 0 ( s    ) 1  ( 0 ) 2

(2)

       

And the relationship between   and   can be deduced as [27, 49]

s    2   s                2    2  2

2

(3)

 0 is the relaxation time,  s is the static permittivity and   is relative dielectric permittivity at high frequency limit. According to this equation the plot of   versus

  should be a semicircle, which is generally denoted as Cole–Cole semicircle. In other words, each semicircle in the   -   plot corresponds to a Debye dipolar relaxation. Fig. S2 displays the typical Cole-Cole semicircles plots of the as prepared Co/CoO@C nanocomposites. As it can be seen, there are 4 semicircles in the   -   plot in 2-18GHz, indicating that the Co/CoO@C nanocomposites presents multiple dielectric relaxation processes which account for the presence of higher EM wave absorption.. These multiple Debye dipolar relaxation processes are attribute to the hierarchical bowknot-like nanostructure, including dipolar relaxation of the irregular pleomorphic nanoparticles and the surface/interfacial dipolar relaxation among there nanoparticles. The nanoparticles act as polarized centers for dipolar relaxation, while the nanosheets and bowknot-like structure endow the Co/CoO@C nanocomposite surface/interfacial dipolar relaxation. Magnetic Co/CoO nanoparticles in the bowknot-like nanocomposite introduce magnetic loss in the whole frequency range. As shown in Fig. 6(d),   for the three

kinds of composites first decreases with increasing frequency, then increases with fluctuation peaks to a maximum value in the rest frequency range. In particular,   of Co/CoO@C-500 composite decreases from 1.15 to 1.00 in 2-8.6 GHz, then increases to 1.35 at 18 GHz. With increasing annealing temperature, the   value of Co/CoO@C-600 decreases from 1.11 to 0.80 in 2-8.8 GHz. With frequency continuously increases, it increases to 1.30 at 18 GHz. Similar to Co/CoO@C-500 and Co/CoO@C-600, the   value of Co/CoO@C-700 first decrease from 1.13 to 0.98, then shows a slight increase trend. Fig. 6(e) shows   of three kinds of composites containing 50 wt% Co/CoO@C nanocomposites. All of them exhibit a monotonically increasing trend in 2-18 GHz. The   value for Co/CoO@C-500 increases from 0.23 to 0.87. With increasing calcination temperature,   for Co/CoO@C-600 increases obviously, which increases from 0.32 to 1.01. While, similar to the case for   , the   for Co/CoO@C-700

decreases

obviously

compared

to

Co/CoO@C-500

and

Co/CoO@C-600. As shown in Fig. 6(f), tan   for three kinds of composite follows Co/CoO@C-600 > Co/CoO@C-500 > Co/CoO@C-700. Multiple fluctuation peak values can be distinguished at 4.8 GHz, 8.8 GHz, 11.4 GHz, 13.4 GHz and 15.4 GHz for   and tan   . This indicates multiple ferromagnetic resonances mechanism of the Co/C nanocomposites [47]. It is worth noticing that tan   value of Co/CoO@C composites is much bigger than tan   , which reveals magnetic loss of the as prepared Co/CoO@C nanocomposites contribute more in microwave dissipation. To explore why the composite containing Co/CoO@C-700 has the unexpected

electromagnetic parameters (lower   and   value than both Co/CoO@C-500 and Co/CoO@C-600). The morphology of Co/CoO@C-500, Co/CoO@C-600 and Co/CoO@C-700 was further investigated. As shown in Fig. S3, with the increasing calcination temperature, the dimension of the nanoparticles increase obviously. When calcined at 500 °C, the nanoparticles only have a size of 10-40 nm. As calcination temperature increases, the size of the nanoparticle increases to 30-100 nm for Co/CoO@C-600 and 50-400 nm or even larger for Co/CoO@C-700. At the same time, a higher crystallinity level was achieved, which can be clearly observed in the X-ray diffraction pattern results. The special bowknot-like morphology made up of multiple nanoparticles is very favorable for microwave absorption. Because when radiated by EM wave, these nanoparticles play the role of scattering sites, the dissipation effect for EM energy can be greatly enhanced. When the calcination temperature increases, a better crystallinity was achieved which increases the  r and  r

of the

Co/CoO@C nanocomposites. But, the dynamic growth of nanoparticles leads to a significant decrease in the number of the nanoparticles, which decreases the apparent

 r and  r of the Co/CoO@C nanocomposite significantly. The final value of  r and  r after calcination at different temperature is a synergistic effect result of both better crystallinity and scattering sites (nanoparticles) number decrease. It explains the abnormal decrease of the electromagnetic parameters for Co/CoO@C-700. When the calcination temperature is relatively low, Co/CoO@C-500 shows a poor crystallinity, which greatly limited the value of  r and  r . With calcination temperature increased to 600 °C, crystallinity level was well promoted yet the nanoparticle

number decrease is not obvious. The superior crystallinity and large enough scattering site number ensure Co/CoO@C-600 has a promoted electromagnetic parameters. As the calcination temperature was increased to 700 °C, the geometric size of the nanoparticles was dramatically enlarged (Fig. S3(c)). The scattering site number decreases sharply and the multiple reflection dissipation effect was dramatically weakened. As a result, the electromagnetic parameters decrease to the level for Co/CoO@C-500 or even less. To evaluate the EM wave absorption property, reflection loss (RL) curves of the bowknot-like Co/CoO@C nanocomposites were calculated from the measured complex  r and  r at 2-18 GHz according to the transmission line theory equations [48-51].

R(dB)  20 log

Zin 

Zin  1 Zin  1

r  2  tanh  j ( ) fd r  r  r  c 

(4)

(5)

Where Z in represents the normalized input impedance of the microwave absorbing layers,  r

and  r

are the complex relative permittivity and permeability

respectively. And c is the free-space light velocity, f represents the frequency of the electromagnetic wave, d refers to the thickness of the absorbers. Minimum reflection loss and response bandwidth are two essential aspects for EM wave absorption ability evaluation. When RL value is less than -10 dB, more than 90% of the EM energy was absorbed, therefor RL ≤ -10 dB is usually donated as

effective EM wave absorption [27]. Fig. 7(a)-(c) show the frequency dependence of RL curves for composites containing 50% Co/CoO@C-500, Co/CoO@C-600 and Co/CoO@C-700 respectively. Fig. 7(d)-(f) are the corresponding 2D contour plots. All the composites with different pyrolysis temperature show a superior effective EM wave absorption bandwidth. With increasing sample thickness the minimum RL value obviously move towards the lower frequency range. As shown in Fig. 7(a), with the thickness increases from 1 mm to 6 mm, effective absorption bandwidth for Co/CoO@C-500 is enlarged from 0 GHz to 12.8 GHz (5.2-18 GHz). A -23.3 dB minimum reflection loss at 10 GHz is observed with a matching thickness of only 4 mm. At the same time, a 10.9 GHz effective absorption bandwidth (7.1-18 GHz) is obtained. As shown in Fig. 7(b), Co/CoO@C-600 reveals an even better broadband microwave absorption performance. A 9.9 GHz effective absorption bandwidth (8.1-18 GHz) is achieved with a -33.6 dB minimum RL at the thickness of only 3 mm. As the thickness increases, the minimum RL move towards lower frequency. As a result an optimized effective absorption bandwidth was observed. The effective absorption bandwidth of Co/CoO@C-600 composite is enormously enlarged to 13.6 GHz (4.4-18 GHz) at the thickness of 6 mm, which covers 85% of the investigated 2-18 GHz frequency range. More excitingly, a -45.0 dB minimum RL is observed at 6 GHz, which means more than 99.99% of the incident EM energy is dissipated. This super strong absorption at relatively low frequency reveals the bowknot-like nanocomposite has the potential for a high performance low frequency EM wave absorber. However, when the calcination temperature was further increased to 700 °C,

the microwave absorption performance starts to deteriorate. Compared to Co/CoO@C-600 both effective absorption bandwidth and absolute RL value of Co/CoO@C-700 decrease obviously (Fig. 7(c)). Effective absorption bandwidth for Co/CoO@C-700 covers 10.6-18 GHz (7.4 GHz in total) with a thickness of 3mm. Meanwhile Co/CoO@C-700 attains a -20 dB minimum RL peak value. It is worth emphasizing that all the effective absorption bandwidth mentioned above was achieved at a single declared absorber thickness, instead of the superposed result of all investigated thicknesses as some previous reports demonstrated. The corresponding graphically frequency and absorber thickness dependent 2D contour plots of the RL date were shown in Fig. 7(d)-(f). It can be clearly observed that the minimum absorption peaks shift towards lower frequency range as the absorber thickness increases from 1mm to 6mm, which can be interpreted by the quarter-wavelength cancellation model theory [26]. As a result, the effective absorption bandwidth ( RL < -10dB ) of the Co/CoO@C nanocomposite extends towards lower frequency range. Among three kinds of Co/CoO@C nanocomposites Co/CoO@C-600 shows a better broadband microwave absorption performance, which has both larger effective absorption bandwidth and enhanced absorption intensity. Consistent with the evolution tendency of EM parameters, with increasing calcination temperature the microwave absorption performance was first optimized then start to deteriorate. This phenomenon was caused by the synergistic effect of both crystallinity level increase and nanoparticle number decrease as explained above. It is well known that good impedance matching condition and strong EM energy

dissipation ability are the two most essential factors for EM absorption materials to achieve high EM wave absorbing performance. There for we calculated the frequency dependencies of normalized characteristic input impedance (Zin) for the bowknot-like Co/CoO@C nanocomposites according to equation (5) [52]. As shown in Fig. S4 (a)-(c), the samples show good impedance matching condition in a broad frequency range at the thickness of 3-6mm.

Especially for Co/CoO@C-600, Zin fluctuated

between 0.68 and 1 in the frequency range of 3.7-18GHz at the thickness of 6mm. which is due of the low   and the similar electric and magnetic loss factors (   and

  ) value. Attenuation constants (  ) of were also calculated as[53-55] 

2 f  (       )2  (       )2 c

(6)

which indicated the EM wave dissipation capability. Fig. S4(d) shows the attenuation constants for the bowknot-like Co/CoO@C nanocomposites, which follows Co/CoO@C-600 ＞ Co/CoO@C-500 ＞ Co/CoO@C-700. Effect of components on EM wave absorption properties was investigated. As displayed in Fig. S6, XRD patterns of Co3O4-600 and Co@C-600 indicate after calcined in air the bowknot-like precursor was totally oxidized into Co 3O4, and those calcined in H2/Ar mixed atmosphere turns into Co@C nanocomposite. As shown in Fig. S7, compared to Co/CoO@C-600, calcination in reductive H2/Ar atmosphere endows Co@C-600 larger   and   , while due to the absence of residual carbon matrix after calcination in air, Co3O4-600 shows obvious decrease in   and   . But Co/CoO@C-600 shows larger   and   , which indicate

Co/CoO@C-600 has a stronger dielectric and magnetic loss ability. Fig. S8 thickness dependent RL curves of Co 3O4-600 and Co@C-600. Compared to Co/CoO@C-600, both effective absorption band width and absorption intensity of Co3O4-600 and Co@C-600 decrease obviously, which follow the order Co/CoO@C-600> Co@C-600 > Co3O4-600. More details can be found in the supporting information. Table 1 shows the EM wave absorption properties of some earlier reported magnetic carbon nanocomposites [1-4, 17, 24, 45, 47, 48, 56-58]. It can be seen clearly that compared to those magnetic carbon nanocomposites, the bowknot-like Co/CoO@C-600 nanocomposite with the thickness of 6mm possesses broader absorption bandwidth and stronger EM wave absorbing ability at the same time. Even if with the similar absorber thickness (3mm), the effective absorption bandwidth of Co/CoO@C-600 is still larger than all the previously reported results we can found, meanwhile the EM wave absorbing ability is stronger than most of them. In order to illuminate the contribution of the bowknot-like structure more demonstrably. Some earlier reported Co/C nanocomposites with other typical nano/microstructures were listed in Table S1. Compared to those previously reported literatures, the Co/CoO@C nanocomposite has an obviously improved effective absorption bandwidth. Since those earlier reported Co/C nanocomposites have similar compositions to the Co/CoO@C nanocomposite, this enhanced broadband EM wave absorption performance is obviously attributed to the hierarchical bowknot-like nanostructure, which improves both the impedance matching and EM wave dissipation ability.

Table 1 EM wave absorption properties of some earlier reported magnetic carbon nanocomposites.

Absorption bandwidth

Minimum

Absorber Reference

Sample

(RL<-10dB) (GHz)

RL (dB)

thickness (mm)

(Fe, Ni)/C nanocapsules

5.6 (12.4-18)

-26.9

2

56

Fe3O4/C nanocoils

3.5 (10.5-14.0)

-24.0

1.85

24

Co/porous C composites

4.93 (8.31-13.24)

-30.31

3

48

MnFe2O4/rGO

4.88 (8.00-12.88)

-29.0

3

17

Fe3O4/C nanorings

3.6 (12.4-15.8)

-55.43

1.9

1

rGO/α-Fe2O3 composite

6.4 (10.8-17.2)

-33.5

3

4

Fe3O4/C nanorods

5.2 (12.8-18)

-27.9

2

57

Ni/C nanofibers

7.8 (10.2-18)

-61

1.7

45

Ni/C microspheres

4.5 (13.0-17.5)

-28.4

1.8

2

Fe/C nanocubes

7.2 (10.8-18.0)

-20.3

2

3

Co/C nanocomposites

5.8 (8.4-14.2)

-35.3

2.5

47

Co3O4/C nanoparticles

3.4 (10-13.4)

-32.3

2.3

58

Co/CoO@C nanobowknot

9.9 (8.1-18.0)

-33.6

3

This work

Co/CoO@C nanobowknot

13.6 (4.4-18.0)

-45.0

6

This work

The EM wave absorption mechanism of the as prepared bowknot-like Co/CoO@C nanocomposite was explored. As demonstrated in Fig. 8(a), when the EM

wave incident into the toroidal-shaped Co/CoO@C and paraffin composite, it is strongly scattered by the uniformly dispersed nano-bowknots. In every scattering process, the EM wave energy was dissipated by the individual bowknot-like Co/CoO@C nanocomposite. As shown in Fig. 8(b) and (c), the EM wave energy was dissipated mainly by three kinds of dissipation effects: I. the bowknot-like Co/CoO@C nanocomposite has a typical hierarchical structure, which results in a hierarchical multiple reflection dissipation. When the EM wave incident into this Co/CoO@C nanocomposite, it is reflected between the two main petals. At the same time, the EM wave is reflected among the nanosheets, and more microscopically among the nanoparticles. This multi-reflection effect will greatly promote the intrinsic dielectric and magnetic loss of the Co/CoO@C nanocomposite. II. The alternating electric field of the EM wave results in dipole polarization and depolarization at the surface and the interface of the Co/CoO@C core-shell nanoparticles in the bowknot-like structure. Due to the polarization damping coursed by the phase delay in the dipole polarization relaxation, the EM energy is dissipated effectively. This dipole polarization dissipation is embodied in dielectric loss. III. Eddy current in the carbon shells coursed by the alternating magnetization of the Co/CoO nanoparticles and magnetic natural resonance are responsible for the magnetic loss of the bowknot-like Co/CoO@C nanocomposites. These magnetic loss effects can be expressed as follows[53, 54]. C0   ( )2 f 1

(7)

Constant C0 with changing frequency mains magnetic loss arises from eddy current,

otherwise natural resonance is in domination. Fig. S5 shows the C0-f results of the bowknot-like Co/CoO@C nanocomposites. It can be clearly observed that C0 decrease sharply in 2-6GHz and show a fluctuation peak at 4.6GHz, when f > 6GHz C0 keeps almost constant, which indicate natural resonance is the main magnetic loss effect in 2-6GHz, and eddy current contributes more in 6-18GHz. All in all, synergistic effect

of the

hierarchical

bowknot-like

structure

and strong

surface/interface polarization, as well as the eddy current and magnetic natural resonance leads to the excellent broadband microwave absorption performance.

4. Conclusions In conclusion, a bowknot-like hierarchical Co/CoO@C nanocomposite was successfully prepared with a two-step hydrothermal method followed by high-temperature calcination in argon atmosphere. The hierarchical Co/CoO@C nanocomposites were made up of nanosheets composed of amorphous carbon and uniformly dispersed irregular Co/CoO nanoparticles. These nanosheets stacked up with each other making up a bowknot-like geometry. Complex permittivity and complex permeability were well modulated by varying the calcination temperature. RL curves of the Co/CoO@C nanocomposites show excellent broadband EM wave absorption performance. Especially for the nanocomposite obtained at 600 °C, the effective absorption bandwidth was dramatically extended to 13.6 GHz (4.4-18 GHz) at the thickness of only 6mm, and a -45.0 dB minimum RL appears at 6 GHz. Such excellent broadband EM wave absorption properties are attributed to the synergistic effect of the hierarchical bowknot-like structure and strong surface/interface

polarization, as well as eddy current and magnetic natural resonance of the bowknot-like Co/CoO@C nanocomposite. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgements This work is supported by the National Natural Science Foundation of China (NSFC, Grant No. 51503052) and the Funding Project of National Key Laboratory of Science and Technology on Advanced Composites in Special Environments (KL.PYJH.2017.005).

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Figure captions

Fig. 1. SEM images of Co 2+@gelatin metal ion-polymer frameworks precursors with different hydrothermal reaction times: (a) 6 h, (b) 7 h, (c) 8 h, (d) 10 h. (e) Low-magnification and (f) high-magnification SEM images of the bowknot-like Co/CoO@C-600 nanocomposite.

Fig. 2. TEM images of the bowknot-like precursor (a)-(c) and Co/CoO@C-600 (d)-(f) with different magnifications.

Fig. 3. (a) XRD pattern of bowknot-like Co/CoO@C nanocomposite, (b) XPS spectrum of Co/CoO@C-600, (c) and (d) high resolution Co 2p and C 1s XPS spectra.

Fig. 4. (a) SEM image of Co/CoO@C-600, and (b)-(d) cobalt, oxygen and carbon elemental distribution mapping of Co/CoO@C-600.

Fig. 5. Magnetic hysteresis loops of Co/CoO@C-500, Co/CoO@C-600, and Co/CoO@C-700 at 300 K. Inset shows the corresponding magnified hysteresis loops at low magnetic fields.

Fig. 6. (a) Real part and (b) imaginary part of complex permittivity, (d) real part and (e) imaginary part of complex permeability, (c) and (f) dielectric and magnetic loss tangent of Co/CoO@C nanocomposite.

Fig. 7. Reflection loss curves of (a) Co/CoO@C-500, (b) Co/CoO@C-600, (c) Co/CoO@C-700 samples at different thicknesses in the frequency range of 2-18 GHz.

Fig. 8. EM wave dissipation mechanism for the bowknot-like Co/CoO@C nanocomposite.

Graphical Abstract

Highlights 1. Hierarchical bowknot-like Co/CoO@C nanocomposites were successfully fabricated. 2. The bowknot-like Co/CoO@C nanocomposite shows a superior effective absorption (< -10dB) bandwidth (13.6 GHz) at the thickness of 6mm, and a -45.0 dB minimum RL was observed at 6 GHz at the same time. 3. The electromagnetic wave absorption mechanisms were discussed.