Micro-nanospheres assembled with helically coiled nitrogen-doped carbon nanotubes: Fabrication and microwave absorption properties

Materials & Design 186 (2020) 108290

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Micro-nanospheres assembled with helically coiled nitrogen-doped carbon nanotubes: Fabrication and microwave absorption properties Kaiyue Li a, Hao Sun a, b, Xiao Zhang a, b, Shen Zhang a, b, Hongwei Dong c, Chunling Zhu b, **, Yujin Chen a, b, * a b c

Key Laboratory of In-Fiber Integrated Optics, Ministry of Education and College of Science, Harbin Engineering University, Harbin, 150001, China College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, 150001, China Department of Electronic Information Engineering, Heilongjiang College of Business and Technology, Harbin, 150025, 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


Micro-nanospheres assembled with helically coiled carbon nanotube are designed.
Both cobalt and zinc are important to produce the coiled carbon nanotubes.
Nitrogen-doping, single atom sites and defects are formed in carbon nanotubes.
The micro-nanospheres have excellent microwave absorption property.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 September 2019 Received in revised form 17 October 2019 Accepted 17 October 2019 Available online 23 October 2019

Micro-nanospheres assembled with helically coiled nitrogen-doped carbon nanotube (HNCNT) are successfully fabricated using CoZn double hydroxide nanosheets as substrates and catalytic precursors and dicyandiamide as carbon and nitrogen sources. The micro-nanospheres have diameters of 3e5 mm, composed of helically coiled NCNTs with diameters of 40e100 nm. The metallic Co NPs covered with graphene shell were encapsulated in HNCNTs. Experimental results demonstrate that both Co and Zn are important to the formation of the specially coiled NCNTs. The optimized micro-nanostructures exhibit excellent microwave absorption property with a minimal reflection loss of 44.2 dB at the thickness of absorber film (d) of only 1.7 mm, and efficient absorption bandwidth (reflection loss less than 10 dB) of 4.8 GHz at d of 2.0 mm. Furthermore, even at a thin thickness (1.5e2.0 mm) the minimal reflection losses of the optimized micro-nanostructures can reach 10 dB. The enhanced microwave absorption property can be explained by increased dielectric loss caused by defects and N dopants, better impedance matching characteristic, and the unique helically coiled structure. © 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Microwave absorption Carbon nanotube Coil-like structure N doping Metal nanoparticle

1. Introduction

* Corresponding author. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, 150001, China. ** Corresponding author. E-mail addresses: [email protected] (C. Zhu), [email protected] (Y. Chen).

Microwave absorbing materials (MAMs) have attracted great attention due to their abilities to protect human health and normal operation of precise instruments when being exposed to electromagnetic radiation [1e5]. For an ideal MAM, the following features such as light weight, thin thickness, strong absorption and wide efficient absorption bandwidth are required [6e8]. Carbon

https://doi.org/10.1016/j.matdes.2019.108290 0264-1275/© 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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nanotubes (CNTs) have unique physicochemical properties like low density, excellent electrical conductivity and mechanical property, affording their potential applications in various fields such as clean energy technology and microwave absorption, etc [9e12]. Microwave absorption mechanism of CNTs is mainly based on the single dielectric loss, and thus large filler loading of CNTs to the matrix is required to achieve high microwave absorption property. To improve the microwave absorption property, transition metal nanoparticles (NPs) were coupled with CNTs [12e17]. For example, Peng's group reported the enhanced microwave absorption property of Fe encapsulated within carbon nanotubes [12]. Zheng et al. found that the microwave absorption property of CNTs was greatly increased by embedding Co NPs within CNTs [15]. Cao's group synthesized Fe(Co or Ni)-NPs encapsulated within CNTs, and found that Fe-encapsulated-CNTs showed the best microwave absorption property with a minimal reflection loss of 30.43 dB and an effective absorption bandwidth of 5.7 GHz at thickness of absorbing film of 3.2 mm [16]. On the other hand, the morphologies of the carbon nanomaterials have important effect on their physicochemical properties. Typically, helically coiled CNTs and carbon nanofibers (CNFs) have different mechanical, electromagnetic and optical properties from those of aligned CNTs and CNFs due to their special morphologies and the presence of pentagons and heptagons in the structure of CNTs and CNFs [17e25]. In particular, the special electromagnetic properties leaded to potential applications of these helically coiled CNTs (HCNTs) and CNFs (HCNFs) in microwave absorption [26e32]. For example, Tang et al. fabricated HCNFs using Ni NPs as catalysts and acetylene as carbon sources, of which minimal reflection loss toward microwave was 36.09 dB at 17.29 GHz with a matching thickness of 2.0 mm [26]. Liu et al. grown HCNFs on carbon microfibers at a flow of the mixture of hydrogen and ethylene using NieSneO as catalysts and found that the minimal reflection loss of as-fabricated HCNFs exceeded 30 dB with a matching thickness of 2.5 mm [22]. To further improve the microwave absorption property of HCNFS, Qin's group deposited magnetic Fe3O4 or Ni on the HCNFs and they found that combination of dielectric-magnetic loss multiple mechanisms could improve microwave absorption properties of absorbers [30,31]. Therefore, these results demonstrated that the HCNFs with unique morphologies could be applied in microwave absorption. Due to hollow features, the HCNTs have lower density than those of HCNFs, and thus, the HCNTs may be more suitable for lightweight microwave absorbers. However, the microwave absorption properties of HCNTs have been investigated seldom. In this paper, we develop a facile method to fabricate micronanospheres assembled with helically coiled nitrogen-doped CNTs (CoxZny@HNCNTs) using hollow CoZn double hydroxide spheres as substrate and dicyandiamide (DCD) as carbon and nitrogen sources. We find that both Co and Zn are important to obtained HNCNTs. The optimized micro-nanostructures exhibited greatly improved microwave absorption property with a minimal reflection loss of 44.2 dB at d ¼ 1.7 mm and effective absorption bandwidths (RL＜e10 dB, EAB10) of 4.8 GHz (12.50e17.50 GHz) at d ¼ 2 mm. 2. Experimental sections 2.1. Preparation of the samples Fabrication of CoxZny@HNCNTS. CoxZny glyceride spheres were systhesized through a solvothermal method (x and y represent molar numbers of Co and Zn elements in glyceride spheres) [33,34]. CoxZny hyroxide (CoxZny-DH) spheres were then fabricated using CoxZny glyceride spheres as precursors through a hydrothermal method [33,34]. The microspheres assembled HNCNTs containing

Co and Zn were obtained using dicyandiamide (DCD) as carbon and nitrogen sources and CoZn-DH spheres as catalysts through an annealing process at 800  C at Ar atmosphere. In this work, Co2Zn1@HNCNTS, Co1Zn1@HNCNTS, and Co1Zn2@HNCNTS were synthesized. For comparison, the samples using cobalt and zinc hydroxides separately as catalysts were also fabricated at 800  C, and denoted as Co@NCNTS and ZnCN, respectively. 2.2. Structural characterization Scanning electron microscopy (SEM) images were obtained by a HITACHI SU8000 with acceleration voltage of 10 keV under vacuum. Transmission electron microscope (TEM) images were performed using a FEI Tecnai-F20 transmission electron microscope with a field emission gun operating at 200 kV. X-ray diffraction (XRD) patterns are recorded on a Panalytical X-pert diffractometer with Cu Ka radiation (l ¼ 1.5418 Å), XPS analyses were using X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher Scientic Company) with Al Ka radiation. Raman spectra were recorded on a Raman spectrometer (Lab RAMA ramis, Horiba Jobin Yvon) using a 488 nm HeNe laser. BrunauerEmmettTeller (BET) surface area and pore distribution employed on a Tristar II 3020 gas adsorption analyzer at 77 K. The ratio of the elements was determined by inductively coupled plasma optical emission spectrometer (ICPOES). The magnetic properties were investigated by a vibrating sample magnetometer (Lakeshore 7410) at room temperature. 2.3. Measurements of electromagnetic parameters The microwave absorption properties of the absorbing materials were carried out using a vector network analyzer (Anritsu MS4644A Vectorstar) in the 2e18 GHz range. The cylindrical samples (with the inner diameter and outer diameter are 3 mm and 7 mm respectively, and 3.0 mm thickness) were prepared by mixing absorbing materials with paraffin matrix with a filler loading of 20 wt%. 3. Results and discussion Scheme 1 illustrates the preparation process of CoxZny@NCNTSs. First, CoxZny glyceride spheres were synthesized through a solvothermal method [33,34]. The spheres have smooth surfaces with diameters of 1e2 mm, consistent with the previous report (Fig. S1). The pure Co glycerides exhibit similar morphology to those of CoxZny glyceride (Fig. S2), while the pure zinc glycerides show irregular shapes (Fig. S3). After hydrothermal treatments, CoxZnyDH spheres were obtained (Fig. S4). Scanning electron microscopic (SEM) images indicate that the surfaces of CoxZny-DH spheres become rough. The CoxZny-DH and cobalt hydroxide spheres were assembled with nanosheets, and their diameters were approximately 1.5e3 mm (Fig. S5). The pure zinc hydroxides exhibit irregular shapes, like those of pure zinc glycerides (Fig. S6). Finally, CoxZny@HNCNTS were obtained using DCD as carbon and nitrogen sources and CoxZny-DH spheres as catalysts through an annealing process at 800  C at Ar atmosphere. Fig. 1 shows SEM image of Co1Zn1@HNCNTS-800. The Co1Zn1@HNCNTS-800 has a diameter of 3e5 mm, assembled with HNCNTs. A magnified image indicates that most HNCNTs exhibit helically coiled shapes with a diameter of 40e100 nm. However, using sole cobalt and zinc hydroxides as catalysts, the uniform HNCNTs could not obtained. Fig. S7 shows SEM images of Co@NCNTs obtained using cobalt hydroxides as catalysts. It can be found that the formed spheres are composed of imperfect NCNTs with uneven diameter and length. Only small amount of HNCNTs are observed in the spheres. If sole zinc hydroxides were served as catalysts, sheet-like materials without NCNTs was produced, as shown in Fig. S8. The results above

K. Li et al. / Materials & Design 186 (2020) 108290

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Scheme 1. Schematic presentation of the synthesis process of CoxZny@HNCNTS spheres consist of coiled NCNTs.

demonstrate that both Co and Zn are important to form HNCNTs in the products. To investigate the effect of Co/Zn ratio on the formation of HNCNTs, we prepared Co2Zn1@HNCNTS and Co1Zn2@HNCNTS using Co2Zn1-DH and Co1Zn2-DH spheres as catalysts. SEM images show that the HNCNTs are formed in both Co2Zn1@HNCNTS and Co1Zn2@HNCNTS (Fig. S9 and S10). However, the contents of HNCNTs in these samples are lower than that in Co1Zn1@HNCNTS-800, suggesting the optimal Co/Zn ratio is 1: 1. In addition, we study the effect of the annealing temperature on the formation of HNCNTs. At a lower temperature (700  C), the obtained spheres (Co1Zn1@HNCNTS-700) are also assembled with HNCNTs, but the content of perfect HNCNTs is lower than that of the sample obtained at 800  C (Fig. S11). This may be due to low catalytic activity of metals at low temperature, which is insufficient to catalytically grow perfect HNCNTs. At higher temperature (900  C), the obtained sample (Co1Zn1@HNCNTS-900) has similar helical structure to that of Co1Zn1@HNCNTS-800 (Fig. S12) [35]. Based on the experimental results above, the ideal temperature for growth of perfect HNCNTs is higher than 800  C. The previous reports have been explained why the HCNTs could be formed. Zhang et al. proposed a concept of a spatial-velocity hodograph to elucidate the formation mechanism of HCNTs, and pointed out that the stresses of the HCNTs could be eliminated through introduction of pentagonal/heptagonal carbon ring pairs [36]. Gao et al. experimentally demonstrated that the orientation of the pentagonal/ heptagonal carbon ring pairs along the growth direction of CNTs was a key factor to produce a helical structure [37]. Zhang et al. reported the synthesis of CNT arrays in a double helix using Fe/Mg/ Al or Co/Mg/Al or Fe/Co/Mg/Al layered double hydroxide (LDH) flakes as both substrates and catalytic precursors [38]. They

Fig. 1. SEM images of Co1Zn1@HNCNTS-800 (a,b), TEM and HRTEM images of Co1Zn1@HNCNTS-800 (cef) and the corresponding EDX elemental mappings of Co1Zn1@HNCNTS-800 g).

believed that the catalytic metal NPs were formed first at reducing atmosphere on the both sides of LDH flakes, then promoted the initial growth of aligned CNTs, and finally the double helices were produced due to spatial confinements from adjacent flakes and the initially formed CNTs. In our case, we used CoxZny-DH spheres as catalytic precursors, which were assembled with nanoflakes and had similar structures to those of LDH flakes. Thus, our CoxZny@HNCNTSs have similar formation mechanism reported by Zhang et al. [38]. In addition, due to the usage of DCD as carbon source, the nitrogen was simultaneously doped into HCNTs. The microstructure of Co1Zn1@HNCNTS-800 was further characterized by transmission electron microscopy (TEM). The HNCNTs exhibit clearly helical shapes with some small encapsulated metal NPs (Fig. 1c). The magnified TEM image displays that the diameter of the HNCNT and the wall thickness are 65 and 16.4 nm, respectively (Fig. 1d). The encapsulated metal NP has irregular shape with a size of approximately 20 nm (Fig. 1d). High-resolution TEM (HRTEM) image indicates that the metal NP is coated with graphene shell with interlay spacing of 0.401 nm (Fig. 1e). The interlay spacing is greatly larger than that of bulk graphitic carbon (d002 ¼ 0.338 nm, JCPDS: 41-1487), which is due to the nitrogen doping in the graphited carbon during annealing process [39]. Moreover, the evaporation of Zn at high temperature may also make contribution to the expansion of graphene layers. The adjacent lattice distance in the NP region is 0.204 nm, which corresponds to (111) plane of metallic Co (JCPDS: 15-0806). Many defects such as lattice distortion and broken lattice can be clearly observed in the walls of HNCNTs, which is caused by the coiled structure (Fig. 1f). The lattice distance of the short-range ordered region is 0.34 nm, slightly larger than the value of bulk graphitic carbon (Fig. 1f). This result suggests that the interlayer spacing in the wall regions is slightly expanded. Elemental mappings indicate that the C and N elements are uniformly distributed throughout the walls of HNCNTs, suggesting the successful N doping into carbon (Fig. 1g). In the NP regions, only Co signals are detected, revealing that the NPs encapsulated in HNCNTs are metallic Co. Besides, the Co and Zn signals are examined in the walls of HNCNTs, indicating the existence of Co and Zn atomic sites in the walls. Notably, the Zn signals are weak relevant to Co signals, which is due to evaporation of Zn at a high temperature. Furthermore, the Zn signals become weaker and weaker with the increase of the annealing temperature, as shown in Fig. S13 and S14. In addition, TEM images reveal that Co@NCNTS, Co1Zn1@HNCNTS-700 and Co1Zn1@HNCNTS-900 are composed of metallic Co and NCNTs (Fig. S15 e 17). ICP-OES measurements indicate that the Co contents in the Co@NCNTS, Co1Zn1@HNCNTS-700, Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are 7.5%, 11.6%, 4.8%, 3.0%, respectively. Because the high temperature can facilitate the growth of HNCNTs, the Co content is decreased with the increase of annealing temperature. Fig. 2 shows X-ray diffraction (XRD) patterns of Co@NCNTS and Co1Zn1@HNCNTSs. The peaks at 44.2 and 51.5 can be ascribed to (111) and (200) planes of metallic Co (JCPDS: 150806) while the peak at about 26.3 is indexed to (002) plane of graphitic carbon (JCPDS: 41-1487). The diffraction peaks from the expanded interlayer spacing of graphene shell were not detected,

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Fig. 2. XRD patterns of (a) Co1Zn1@HNCNTS-900, (b) Co1Zn1@HNCNTS-800, (c) Co1Zn1@HNCNTS-700 and (d) Co@NCNTS.

which is attributed to its small amount in the products. The XRD pattern reveals that the ZnCN is composed of ZnCN (JCPDS: 010788) and C3N4 (JCPDS: 50-1512) (Fig. S18). Xeray photoelectron spectroscopy (XPS) spectra were performed to investigate the surface compositions of the Co@NCNTS and Co1Zn1@HNCNTSs. Fig. 3a shows the Co 2p sptectra of Co@NCNTS and Co1Zn1@HNCNTSs. The signals of metallic (777.9 eV, 793.5 eV), CoNx (781.1 eV, 795.5 eV) and Co2þ species (779.3 eV, 794.9 eV) were detected. The existence of small amount of Co2þ species is due to the surface oxidation of Co upon exposure of the samples to air. The weaker signals of Co at a high temperature is due to the decrease of the Co content, consistent with ICP-OES results. N 1s XPS spectra of Co@NCNTS, Co1Zn1@HNCNTS-700, Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 can be deconvoluted into four peaks at 398.3, 399.1, 400.8 and 404.5 eV, which correspond to pyridinic N, metal-N, graphitic N, and oxidized N, respectively (Fig. 3b). The peaks at 284.6, 285.2 and 286.5 eV in the C 1s XPS spectra can be assigned to CeC, CeN, and C]O bonds, respectively (Fig. 3c). The binding energies at 1021.2 and 1044.5 eV correspond to Zn 2p1/2

and Zn 2p3/2 in the Zn 2p XPS spectra of Co1Zn1@HNCNTS-700 and Co1Zn1@HNCNTS-800, respectively (Fig. 3d). These peaks become weak at the annealing temperature of 900  C, which is due to almost complete evaporation of Zn [2,40e42]. Two characteristic peaks at 1347 cm1 and 1581 cm1 are observed in the Raman spectra of Co@NCNTS and Co1Zn1@HNCNTSs, which correspond to D and G bands, respectively (Fig. 4a). The intensity ratios (ID/IG) of Co@NCNTS, Co1Zn1@HNCNTS-700, Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are 0.86, 0.84, 0.93 and 0.92, respectively, suggesting the rich defects in these samples. Nitrogen adsorption/ desorption isotherms were measured to get the information on the surface area and pore size distribution of samples (Fig. S19). The BrunauereEmmetteTeller (BET) and pore volume of Co1Zn1@HNCNTS-800 were determined to be 51.48 m2/g and 0.17 cm3/g, comparable to those of Co1Zn1@HNCNTS-900 (50.36 m2/g and 0.15 cm3/g), but greatly higher than those of Co@NCNTS (27.45 m2/g and 0.07 cm3/g) and Co1Zn1@HNCNTS-700 (37.62 m2/g and 0.12 cm3/g). Micro- and mesopores are found in these samples, which may facilitate their microwave absorption property (the insets in Fig. S19). The magnetic properties of the Co@NCNTS and Co1Zn1@HNCNTSs were measured, as shown in Fig. 4b. The saturation magnetization values (Ms) of Co@NCNTS, Co1Zn1@HNCNTS-700, Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are 17.91, 18.83, 11.31, 8.68 respectively. The corresponding coercivities (Hc) are 470, 577, 612 and 594 Oe, while the retentivity magnetizations (Mr) are 3.99, 4.91, 3.26 and 3.16 emu g1 respectively. The smaller Ms of Co1Zn1@HNCNTS-900 is due to lower Co content in this sample. The obvious ferromagnetic performance may be in favor of the microwave absorption property. The electromagnetic parameters including complex permittivity (εr ¼ ε0 e jε00 ) and complex permeability (mr ¼ m0 e jm00 ) were measured by vector network analyzer to analyze the dielectric and magnetic loss properties of Co@NCNTS and Co1Zn1@HCNTSs. Fig. 5a shows the ε0 e f plots of these samples. The ε0 values of the Co1Zn1@HNCNTS-800 is in range of 7.5e12.5, larger than those of Co@NCNTS (6.0e8.5), Co1Zn1@HNCNTS-700 (5.5e7.5) and Co1Zn1@HNCNTS-900 (6.5e10.5). CoZn@HNCNTS-800 has larger ε00 values than Co1Zn1@HNCNT900 at 2e10 GHz, but they have almost same ε00 values at a high frequency region (10e18 GHz), as shown in Fig. 5b. The ε00 values of both Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are much larger than those of Co1Zn1@HNCNTS-700 (1.2e2.4) and

Fig. 3. (aec) Co 2p, N 1s and C 1s XPS spectra of Co1Zn1@HNCNTS-900 (ⅰ), Co1Zn1@HNCNTS-800 (ⅱ), Co1Zn1@HNCNTS-700 (ⅲ) and Co@NCNTS (ⅳ), (d) Zn 2p XPS spectra of Co1Zn1@HNCNTS-900 (ⅰ), Co1Zn1@HNCNTS-800 (ⅱ) and Co1Zn1@HNCNTS-700 (ⅲ).

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Fig. 4. (a) Raman spectra and (b) magnetization hysteresis loops of Co1Zn1@HNCNTS-900 (i), Co1Zn1@HNCNTS-800 (ii), Co1Zn1@HNCNTS-700 (iii) and Co@NCNTS (iv).

Fig. 5. (a) Real parts of permittivity, (b) imaginary parts of permittivity, (c) dielectric loss tangents, (d) real parts of permeability, (e) imaginary parts of permeability and (f) magnetic loss tangents of Co1Zn1@HNCNTS-900 (purple lines), Co1Zn1@HNCNTS-800 (green lines), Co1Zn1@HNCNTS-700 (blue lines) and Co@NCNTS (gray lines).

Co@NCNTS (0.5e2.5), suggesting their better dielectric loss properties (Fig. 5b). Notably, the ε0 and ε00 values of ZnCN are close to 2 and 0 respectively, and kept constants over 2e18 GHz, which indicates that the ZnCN has negligible dielectric loss toward microwave (Fig. S20). Fig. 5c shows the dielectric loss tangent (tande) values of Co@NCNTs and Co1Zn1@HNCNTSs. It can be found that the tande values have similar changing tendency to the ε00 values. The tande values of Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are in range of 0.6e0.35 and 0.4e0.51, respectively, larger than Co@NCNTS (0.07e0.26) and Co1Zn1@HNCNTS-700 (0.22e0.32), which further confirming their better dielectric loss properties. Fig. 5c and d shows the changes of m0 , m00 and tandm of Co@NCNTs and Co1Zn1@HNCNTSs with the frequency over 2e18 GHz. The m0 , m00 and tandm values of all samples are lower than 1.20, 0.13 and 0.17, suggesting that they have relatively weaker magnetic loss properties in comparison to their dielectric loss properties. Notably, the m00 values of ZnCN fluctuate around zero, indicating its bad magnetic loss performance (Fig. S20). The reflection loss (RL) of an absorber can be calculated by the following equation [43],

Zin ¼ Z0

 . pffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi  mr =εr tan h j2pfd c  mr εr

RL ¼ 20 logjðZin  Z0 Þ = ðZin þ Z0 Þj

(1)

(2)

where Zin is the normalized input impedance of the absorber, Z0 is the impedance of free space, d is the thickness of the absorber, and c is the velocity of microwave in free space. Obviously, ZnCN

has negligible microwave absorption property with all RL values of approximately zero at d in range of 1.5e5.0 mm (Fig. 6a). The minimal RL (RL,min) values of the Co@NCNTS are below e 10 dB only when the thickness is larger than 3.0 mm and the RL,min value is only 12.5 dB at d ¼ 5.0 mm, suggesting its weak microwave absorption property (Fig. 6b). In contrast, upon introduction of Zn, the microwave absorption property is increased significantly. RL,min values of Co1Zn1@HNCNTS-700, Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 can reach to 12.8 dB at d ¼ 5 mm, 44.2 dB at d ¼ 1.7 mm, 39.0 dB at d ¼ 3.5 mm, respectively (Fig. 6cef). Furthermore, the effective absorption bandwidths (RL＜-10 dB, EAB10) of Co1Zn1@HNCNTS-700, Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are up to 2.6 GHz (10.20e12.80 GHz) at d ¼ 3 mm, 4.8 GHz (12.50e17.30 GHz) at d ¼ 2 mm and 4.1 GHz (13.90e18 GHz) at d ¼ 2 mm respectively, while the EAB10 of Co@NCNTS is only 1.22 GHz (8.39e9.61 GHz) at d ¼ 3.5 mm. Besides, the effective absorption bandwidths with RL,min below 20 dB (EAB20) of Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are 1.35 GHz (13.95e15.30 GHz) at d ¼ 2 mm and 1.18 GHz (15.40e16.58 GHz) at d ¼ 2 mm respectively. In the frequency region, 99.9% microwave energy can be attenuated, suggesting excellent microwave absorption properties of Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900. Moreover, the Co1Zn1@HNCNTS-800 exhibits exciting microwave absorption property even at a thin thickness. As shown in Fig. 6f, the RL,min values of Co1Zn1@HNCNTS-800 can reach to 10 dB at d in range of 1.5e2.0 mm. Compared with other carbon-based and transition

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Fig. 6. RL e f curves of (a) ZnCN, (b) Co@NCNTS, (c) Co1Zn1@HNCNTS-700, (d) Co1Zn1@HNCNTS-900 and (e, f) Co1Zn1@HNCNTS-800.

frequency limit, and t is the relaxation time. Fig. S22a shows that Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 have higher conductive losses than those of Co1Zn1@HNCNTS-700 and Co@NCNTS, especially at low frequency range. On the other hand, the polarization relaxation losses of Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are much higher than those of Co1Zn1@HNCNTS-700 and Co@NCNTS over 2e18 GHz (Fig. S22b). However, Co1Zn1@HNCNTS-700 has only higher dielectric relaxa-

metal-based composites like CoNi@C [44], Co/C [45], (Fe, Co, Ni)/ MWCNTs [46], M/NCNTs [16], Cu/CF [47], Ni/MWCNT [48], Fe/G [43], and Co filled CNT [14], Co1Zn1@HNCNTS-800 shows superior microwave absorption property in view of reflection loss and the filler mass loading in the paraffin matrix (Table S1). In addition, attenuation constant (a) is also an important index which represent the ability of attenuation ability for the microwave, and a can be written as,

pffiffiffiffiffiffiffiffiffiffiffi a ¼ 2m0 ε0 pf

, sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi c

m00 ε00

m0 ε0  1 þ

ðm ε =m ε Þ2 þ ðε =ε Þ2 þ ðm =m Þ2 þ 1 00

00

0

0

00

0

00

Fig. S21 shows a e f plots of the Co@NCNTS and Co1Zn1@HNCNTSs over 2e18 GHz a values of Co1Zn1@HNCNTS-800 are increased from 60 to 287.9 with an increase of frequency, higher than those of Co@NCNTS (25e100), Co1Zn1@HNCNTS-700 (25e160) and Co1Zn1@HNCNTS-900 (56e244). Therefore, our designed helically coiled NCNTs have potential application in microwave absorption field. In view of RL,min, EAB10, d, and a data, the microwave absorption ability of our designed samples follows the order: Co1Zn1@HNCNTS-800 ~ Co1Zn1@HNCNTS-900>Co1Zn1@HNCNTS700>Co@NCNTS>ZnCN. The order of microwave absorption properties of these samples is consistent with their order of dielectric loss. Thus, the increased dielectric loss properties of Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 contribute to their superior microwave absorption properties. In 2e18 GHz range, the dielectric loss generally comes from conductive loss (εc00 ) and dielectric relaxation loss (εp00 ),

i . . h 00 00 00 1 þ u2 t2 ut þ s uε0 ¼ εp þ εc ε ¼ ðεs  ε∞ Þ

(4)

where u is angular frequency, s is electrical conductivity, εs is static permittivity, ε∞ is the relative dielectric permittivity at high

0

(3)

tion loss than that of Co@NCNTS. Thus, the introduction of Zn leads to an increase of dielectric relaxation loss of the sample synthesized at a low temperature, but improvement of both conductive and dielectric relaxation losses of sample obtained at a high temperature. TEM image shows that our designed absorbers have abundant defects, resulted from N doping, existence of single metal atom sites and the helically coiled carbon nanotubes (Fig. 1f). The defects can serve as polarized centers and thus cause polarization relaxation. Raman spectra indicate that Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 have larger ID/IG values than those of Co1Zn1@HNCNTS-700 and Co@NCNTS, suggesting that they have more abundant defects (Fig. 4a). Thus, the increased polarization relaxation caused by defects contributes to their enhanced dielectric losses. The interfacial polarization relaxation is related to the interfaces formed among heterostructures in the absorber. In our samples, the Co NPs are covered with graphene shell, and the coreshell NPs are further embedded within HNCNTs. The existence of numerous interfaces can lead to interfacial polarization relaxation. In addition, the dielectric relaxation can be elucidated by Debye relaxation equation [49],

K. Li et al. / Materials & Design 186 (2020) 108290

0

00

½ε  ðεs þ ε∞ Þ=22 þ ðε Þ2 ¼ ½ðεs þ ε∞ Þ=22

(5)

in the ε00 eε0 plots, one semicircle means one relaxation process [49]. Fig. S23 displays the ε00 eε0 plots of Co1Zn1@HNCNTSs and Co@NCNTS. More than one semicircle are observed in these samples, revealing that multiple relaxation processes occur when the samples are exposed to microwave radiation. The result also demonstrates that both dipole polarization and interfacial polarization contribute to dielectric losses of our designed samples [50e52]. Besides the dielectric and magnetic losses, the impedance matching between the impedance of free space (Z0) and the input impedance of absorber (Zin) plays an important role in the microwave absorption property of the absorber. The impedance matching can be evaluated by following equation based on transmission line theory.

Z ¼ jZin = Z0 j

(6)

When the value of Z is 1, the reflection of microwave on the surface of absorber is greatly limited. Z e f curves in Fig. S24 demonstrate that the Z values of Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are closer to 1 than those of Co1Zn1@HNCNTS-700 and Co@NCNTS. Therefore, the better microwave absorption properties of Co1Zn1@HNCNTS-800 and Co1Zn1@HNCNTS-900 are attributed to their impedance matching characteristics. In addition, the helically coiled structures of our samples facilitate the microwave absorption properties. As reported by Zhang et al., when the voltage was added on the HCNTs, the current would produce along HCNTs, and then the magnetic field was induced [38]. Therefore, the induced magnetic field by the helically coiled structure could attenuate the magnetic energy of the microwave, and then boosted the microwave absorption properties [53]. Such enhanced microwave absorption induced by microstructures were also found in other absorbers with special morphologies such as hollow carbon spheres and two-dimensional graphene etc. [50e52,54e59]. Notably, as reported previously, the length of single-wall carbon nanotube (SWNT) had important effect on the electromagnetic responses of individual SWNT and SWNT composites [52]. For example, Shuba et al. found that the frequency of conductivity peak of the SWNTs was decreased with the increase of their length [52]. The NCNTs in Co2Zn1@HNCNTS-800, Co1Zn1@HNCNTS-800 and Co1Zn2@HNCNTS-800 have similar lengths, however, they exhibit different microwave absorption properties. Thus, the length of NCNTs in our designed samples does not play main role in the microwave absorption properties. On the other hand, our micro-nanospheres were dispersed into paraffin matrix to form cylindrical sample (with 3.00 mm inner diameter, 7.00 mm outer diameter and 3.00 mm thickness) for electromagnetic parameter measurements. Thus, the scattering effect of the monolithic network in the cylindrical sample toward microwave may also contribute to the microwave absorption.

4. Conclusions In summary, we successfully synthesized micro-nanospheres assembled with helically coiled NCNTs through a facile method. We found that both Co and Zn were important to the formation of the specially coiled NCNTs at a moderate annealing temperature. The HNCNTs possessed abundant defects that caused by coiled structure, N-doping and single metal atom sites in the walls of HNCNTs, and numerous interfaces as well as their specially coiled structure, affording the micro-nanospheres to have excellent absorption property including strong absorption performance, wide absorption bandwidth and lightweight feature. Our results

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demonstrate that development of NCNTs with a special structure such as coil-like shape is an efficient way for high-performance absorber for microwave absorption. Conflicts of interest There are no conflicts to declare. CRediT authorship contribution statement Kaiyue Li: Data curation, Formal analysis, Writing - original draft, Writing - review & editing. Hao Sun: Data curation. Xiao Zhang: Formal analysis. Shen Zhang: Methodology. Hongwei Dong: Formal analysis. Chunling Zhu: Conceptualization, Methodology, Supervision, Writing - original draft, Writing - review & editing. Yujin Chen: Conceptualization, Methodology, Supervision, Writing - original draft, Writing - review & editing. Acknowledgements This work is supported by the NNSF of China (Grant Numbers: 51972077 and 51572051), the Fundamental Research Funds for the Central Universities (HEUCF201708, 3072019CFQ2501 and 3072019CFQ2502), the Open Project Program of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education of the People’s Republic of China (PEBM201703 and PEBM201703), and also Heilongjiang Touyan Innovation Team Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matdes.2019.108290. References [1] Q. Liu, Q. Cao, H. Bi, C. Liang, K. Yuan, W. She, Y. Yang, R. Che, CoNi@SiO2@TiO2 and CoNi@air@TiO2 microspheres with strong wideband microwave absorption, Adv. Mater. 28 (2016) 486e490, https://doi.org/10.1002/ adma.201503149. [2] W. Feng, Y. Wang, J. Chen, B. Li, L. Guo, J. Ouyang, D. Jia, Y. Zhou, Metal organic framework-derived CoZn alloy/N-doped porous carbon nanocomposites: tunable surface area and electromagnetic wave absorption properties, J. Mater. Chem. C 6 (2018) 10e18, https://doi.org/10.1039/c7tc03784h. [3] O. Balci, E.O. Polat, N. Kakenov, C. Kocabas, Graphene-enabled electrically switchable radar-absorbing surfaces, Nat. Commun. 6 (2015) 6628e6637, https://doi.org/10.1038/ncomms7628. [4] M. Zhang, X.-X. Wang, W.-Q. Cao, J. Yuan, M.-S. Cao, Electromagnetic functions of patterned 2D materials for microenano devices covering GHz, THz, and optical frequency, Adv. Optical Mater. (2019) 1900689, https://doi.org/ 10.1002/adom.201900689. [5] M.S. Cao, X.X. Wang, M. Zhang, J.C. Shu, W.Q. Cao, H.J. Yang, X.Y. Fang, J. Yuan, Electromagnetic response and energy conversion for functions and devices in low-dimensional materials, Adv. Funct. Mater. 29 (2019) 1807398, https:// doi.org/10.1002/adfm.201807398. [6] X.G. Liu, B. Li, D.Y. Geng, W.B. Cui, F. Yang, Z.G. Xie, D.J. Kang, Z.D. Zhang, (Fe, Ni)/C nanocapsules for electromagnetic-wave-absorber in the whole Ku-band, Carbon 47 (2009) 470e474, https://doi.org/10.1016/j.carbon.2008.10.028. [7] Z. Wu, K. Pei, L. Xing, X. Yu, W. You, R. Che, Enhanced microwave absorption performance from magnetic coupling of magnetic nanoparticles suspended within hierarchically tubular composite, Adv. Funct. Mater. (2019) 1901448, https://doi.org/10.1002/adfm.201901448. [8] J. Xiang, J. Li, X. Zhang, Q. Ye, J. Xu, X. Shen, Magnetic carbon nanofibers containing uniformly dispersed Fe/Co/Ni nanoparticles as stable and highperformance electromagnetic wave absorbers, J. Mater. Chem. A 2 (2014) 16905e16914, https://doi.org/10.1039/c4ta03732d. [9] N. Li, G.W. Huang, Y.-Q. Li, H.M. Xiao, Q.P. Feng, N. Hu, S.Y. Fu, Enhanced microwave absorption performance of coated carbon nanotubes by optimizing the Fe3O4 nanocoating structure, ACS Appl. Mater. Interfaces 9 (2017) 2973e2983, https://doi.org/10.1021/acsami.6b13142. [10] H. Sun, R. Che, X. You, Y. Jiang, Z. Yang, J. Deng, L. Qiu, H. Peng, Cross-stacking aligned carbon-nanotube films to tune microwave absorption frequencies and increase absorption intensities, Adv. Mater. 26 (2014) 8120e8125, https:// doi.org/10.1002/adma.201403735. [11] T. Wang, Z. Kou, S. Mu, J. Liu, D. He, I.S. Amiinu, W. Meng, K. Zhou, Z. Luo, S. Chaemchuen, F. Verpoort, 2D dual-metal zeolitic-imidazolate-framework-

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