C fiber derived from cotton and metal-organic-framework

Journal Pre-proof Dramatically enhanced electromagnetic wave absorption of hierarchical CNT/Co/C fiber derived from cotton and metal-organic-framework Minglong Yang, Ye Yuan, Ying Li, Xianxian Sun, Shasha Wang, Lei Liang, Yuanhao Ning, Jianjun Li, Weilong Yin, Renchao Che, Yibin Li PII:

S0008-6223(20)30080-4

DOI:

https://doi.org/10.1016/j.carbon.2020.01.073

Reference:

CARBON 15008

To appear in:

Carbon

Received Date: 13 December 2019 Revised Date:

20 January 2020

Accepted Date: 21 January 2020

Please cite this article as: M. Yang, Y. Yuan, Y. Li, X. Sun, S. Wang, L. Liang, Y. Ning, J. Li, W. Yin, R. Che, Y. Li, Dramatically enhanced electromagnetic wave absorption of hierarchical CNT/Co/C fiber derived from cotton and metal-organic-framework, Carbon (2020), doi: https://doi.org/10.1016/ j.carbon.2020.01.073. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Graphical Abstract

A hierarchical carbon fiber coated with dodecahedral Co/C nanoparticles and villus-like CNTs was fabricated using cotton and ZIF-67(HCF@CZ-CNTs), which shows ultra-low apparent density (0.0198 g/cm3), minimum -53.5 dB microwave reflection loss at 7.8 GHz as well as broad effective absorption bandwidth of 8.02 GHz.

Dramatically enhanced electromagnetic wave absorption of hierarchical CNT/Co/C fiber derived from cotton and metal-organic-framework Minglong Yang1, Ye Yuan2, Ying Li3, Xianxian Sun1,4, Shasha Wang1, Lei Liang1, Yuanhao Ning1, Jianjun Li1, Weilong Yin*1,4, Renchao Che*5, Yibin Li*1,4 1

National Key Laboratory of Science and Technology on Advanced Composites in

Special Environments, Harbin Institute of Technology, Harbin 150080, P. R. China. 2

School of Materials Science and Technology, Tianjin Key Laboratory of Materials

Laminating Fabrication and Interface Control Technology, Hebei University of Technology, Tianjin, 300401, P. R. China. 3

School of Civil Engineering, Qingdao University of Technology, Qingdao, 266033, P.

R. China. 4

Shenzhen STRONG Advanced Materials Institute Ltd. Corp, Shenzhen 518000, P. R.

China. 5

Department of Materials Science and Laboratory of Advanced Materials, Fudan

University, Shanghai 200433, P. R. China. *Corresponding authors. E-mail: [email protected] (Weilong Yin); [email protected] (Renchao Che); [email protected] (Yibin Li).

Abstract Lightweight, low-cost electromagnetic (EM) wave absorption materials are in urgent need, as EM interference pollution is increasingly serious. Carbon material as EM absorber has attracted ever increasing attention. However, their absorption property is still limited. Herein, we propose a very simple strategy to prepare hierarchical carbon fiber coated with Co/C nano-dodecahedron particles where CNTs were anchored (HCF@CZ-CNTs), using cotton and metal-organic-framework (MOF) as raw materials. One of MOF, ZIF-67 particles were in-situ grown onto the surface of cotton fibers. Lightweight HCF@CZ-CNTs samples were obtained by direct carbonization, the apparent density of which is only 0.0198 g/cm3. The minimum reflection loss (RLmin) of hierarchical HCF@CZ-CNTs was dramatically enhanced to -53.5 dB at 7.8 GHz, compared with pure carbonized cotton fiber (RLmin = -18.9 dB at 16 GHz). Meanwhile the effectively absorption bandwidth was also remarkably enlarged to 8.02 GHz. The excellent EM wave absorption performance is attributed to the micro-to-nano hierarchical hollow fibrous structure, as well as the synergetic effect of polarization relaxation and magnetic loss. These results illuminate a low-cost and sustainable strategy to prepare ultra-lightweight microwave absorbers with excellent EM wave absorbing ability utilizing biomass precursor and MOF.

1. Introduction Electromagnetic interference (EMI) pollution caused by the excessive use of wireless communication devices, high-power signal base stations and even household WIFI transmitters has become one of the top concerns for daily life.[1-3] On the other hand, military planes are strongly hindered by the full-time and Omni-directional radar detection. Therefore high performance EM wave absorbents are in desperate need both for civilian and military purposes. In the past few years, biomass materials have been drawing increasing research attention as low-cost sustainable raw materials for fabricating various lightweight functional materials.[4-12] Especially, biomass-derived carbon materials, which can be obtained by directly carbonization, provide a new strategy to fabricate ultra-lightweight high performance EM wave absorbers.[13-16] These carbon materials maintained the favorable natural morphology such as well-organized porous structure, hollow tube-like structure and hierarchical structure,[17-19] which are very helpful for improving the EM wave absorption ability and controlling composites weight. Meanwhile defects and heteroatoms doping can be introduced to the carbon frame during carbonization, which is benefit to the boosted EM wave dissipation capability by inducing abundant dipole polarization sites. So far, carbonized spinach stem,[20] wood,[21] walnut shell,[22] Loofah Sponge,[23] rice,[24] eggshell membrane[25] and waxberry[26] were reported for lightweight EM wave absorption materials. However, since this kind of material are usually nonmagnetic, the EM wave absorption performance are strongly hindered by the unicity of EM attenuation

mechanism and mismatching of EM wave input impedance. For the further improvement of EM wave absorption capability, various ferromagnetic nanoparticles are introduced into the carbon-based absorbers for improving both impedance matching and magnetic loss ability. Among them Fe and Co based nanoparticles prepare by hydrothermal or coprecipitation method are suffering from larger density, agglomeration and poor anchoring on carbon matrix. On contrary,

metal-organic

framework

(MOF)

derived

magnetic

metal/carbon

nanocomposites show effectively reduced density, good dispersity and superior EM wave absorption ability.[27-29] MOF is usually synthesized by coordinative bonding between metal ions or clusters and organic linkers.[30-33] Magnetic metal/carbon nanocomposites with various nanostructures can be fabricated by directly pyrolysis of Fe/Co/Ni containing MOF particles, such as zeolitic imidazolate framework-67 (ZIF-67) particles. These magnetic nanocomposites show porous structure, high specific surface area and ferromagnetism. Synergistic EM dissipation effect between carbon matrix and magnetic metal nanoparticles makes them promising candidates for lightweight broadband EM wave absorbers. Furthermore, when pyrolysised in reductive atmosphere, the catalyzed growth of carbon nanotubrs (CNTs) can be carried out simultaneously, building up various hierarchical structures for both weight control and EM wave dissipation. Herein, we propose a feasible method to fabricate hierarchical hollow carbon fiber coated with Co/C particles and villus-like CNTs (HCF@CZ-CNTs) for enhanced EM wave absorption. Dodecahedron ZIF-67 particles were grown onto the surface of

cotton fibers uniformly, followed by annealing in H2/Ar for the carbonization of cotton fibers, pyrolysis of ZIF-67 and in-situ catalytic growth of CNTs simultaneously. The ultra-lightweight (0.0198 g/cm3) HCF@CZ-CNTs shows excellent EM wave absorption ability, including a -53.5 dB minimum reflection loss at 7.8 GHz with the thickness of 2.9 mm and an 8.02 GHz effective absorption bandwidth at 2 mm, which was dramatically enhanced compared with pure carbonization cotton (RLmin -18.9 dB and bandwidth 2.26 GHz). The excellent broadband EM wave absorption performance is attributed to the hierarchical fibrous structure and the synergetic effect of polarization relaxation and magnetic loss. The combination of biomass and MOF illuminates a low-cost and sustainable strategy to prepare ultra-lightweight EM wave absorbents with excellent absorption performance.

2. Experimental 2.1 Materials Cotton fiber was purchased from local market. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), methanol, 2-methylimidazole (2-mIM) were purchased from Aladdin Industrial Corporation. All chemical reagents were used without further purification. 2.2 Synthesis of cotton fiber coated with ZIF-67 particles precursor In a typical synthesis process, 4 g 2-mIM was dissolved in 120 mL absolute methanol. Then 2 g cotton fiber, which was pre-dried in a vacuum drying oven at 60 ℃ overnight, was immersed in the 2-mIM solution in a culture dish and kept at ~100 Pa

for 10 min in a vacuum drying oven to make sure the cotton fiber was fully infiltrated. 3.87 g Co(NO3)2·6H2O was dissolved in another 120 mL absolute methanol, then poured into the culture dish slowly under constant shake. The reaction was aged at room temperature for 24 h before taken out. Then the ZIF-67 coated cotton fiber (CF@ZIF-67) was washed with ethanol for several times and dried at 60 ℃ for 12 h before further use. 2.3 Synthesis of hollow carbon fiber coated with Co/C nano-dodecahedron and villus-like CNTs (HCF@CZ-CNTs) The CF@ZIF-67 precursor was loaded in a combustion boat and annealed in a tube furnace. It was first heated at 480 °C for 2 h under Ar flow with a ramp rate of 2 °C/min, and then heated to 900 °C with a ramp rate of 5 °C/min and kept for another 2 h under H2/Ar mixed atmosphere containing 30 vol% H2. The carbonized HCF@CZ-CNTs sample was cooled to room temperature before taking out. As references, cotton fiber carbonized in Ar (HCF), and CF@ZIF-67 annealed in Ar flow (HCF@CZ) were prepared following the same heating procedure. 2.4 Characterization Scanning electron microscope (SEM, Hitachi S4800) and transmission electron microscope (TEM, JEOL 2010F) were employed to observe the micro-morphologies. Powder X-ray diffraction patterns (XRD) were acquired on a Shimadzu XRD-7000s diffraction instrument. Raman spectra were collected on a Renishaw inVia Raman microscope with an excitation wavelength of 532 nm. The magnetic hysteresis loops

was

measured

with

a vibration

sample

magnetometer

(VSM

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 characterization, HCF, HCF@CZ and HCF@CZ-CNTs were uniformly mixed into paraffin with a mass ratio of 10%. Then the mixture was compressed into toroidal-shaped specimens with inner diameter of 3.04 mm, outer diameter of 7 mm and thickness of 3 mm. Relative complex permittivity ( ε r = ε ′ − jε ′′ ) and relative complex permeability ( µr = µ ′ − j µ ′′ ) were measured with a network analyzer (Agilent Technologies N5222A) in 2-18 GHz. Reflection loss curves were calculated with the measured ε r and µr according to the transmission line theory.

Details of the measurement method and derivation for the reflection loss

calculation equations can be found in the Supporting Information.

3. Results and discussion Cotton fiber exhibits a natural micron-sized hollow fibrous structure, which makes it a perfect raw material for fabricating low-coat lightweight EM wave absorption carbon materials.[34] Meanwhile, as a kind of Co-containing zeolitic imidazolate framework, ZIF-67 has a high porosity, therefor very appropriate for fabricating light-weight porous Co/C particles with in-situ grown CNTs by directly pyrolysis.[35] Hierarchical hollow carbon fibers coated with Co/C nano-dodecahedron particles and villus-like CNTs (HCF@CZ-CNTs) was prepared by in-situ growth of ZIF-67

particles on the surface of cotton fibers, followed by annealing at 900 °C in H2/Ar flow. The schematic illustration of the synthesis process was shown in Fig. 1. Cotton fiber was pre-dried to remove the absorbed water, then immersed and fully infiltrated in a methanol solution of 2-mIM, making sure this organic linker being uniformly attached to the surface. After that, cobalt nitrate hexahydrate solution was added into the reaction as a donator of Co2+. The reaction system was then aged for 24 h to ensure the fully growth of ZIF-67 particles on the surface of cotton fibers before taken out. After washed with ethanol to remove the residual reactant and ZIF-67 particles not anchored, ZIF-67 coated cotton fiber precursor was dried and carbonized in a tube furnace under H2/Ar mixed flow for the fabrication of HCF@CZ-CNTs. During annealing, cotton fiber was carbonized into hollow carbon fiber, meanwhile Co2+ ions in ZIF-67 were reduced into Co nanoparticles, which simultaneously promote the graphitization of carbon and the in-situ growth of CNTs as a catalyst, in this way building up the hierarchical carbon fiber coated with Co/C nano-dodecahedron villus-like CNTs. It is worth mention that an obvious volume shrinkage was observed after annealing, which is attributed to the significant mass loss and much twisted structure of the hollow fibers after carbonization. Furthermore, HCF@CZ-CNTs sample shows a 0.0198 g/cm3 ultra-low apparent density being very favorable for lightweight EM wave absorption materials.

Fig. 1. Schematic illustration of the synthesis procedure for hollow carbon fiber coated with Co/C nano-dodecahedron particles and villus-like CNTs. Fig. 2a-h exhibit the scanning electron microscope (SEM) images of cotton fibers, CF@ZIF-67 precursor, HCF@CZ and HCF@CZ-CNTs. It can be clearly observed in Fig. 2a and e that cotton fibers have a natural hollow tube-like structure with the diameter of approximately 10-20 mm. After treated with 2-mIM and Co2+, the surfaces of cotton fibers are coated with crystalline ZIF-67 particles uniformly, as shown in Fig. 2b and f. The size of these ZIF-67 particles range from ~500 nm to ~1 µm. After carbonization, both HCF@CZ and HCF@CZ-CNTs maintained the hollow fibrous morphology, while there diameter obviously shrank to 5-8 µm and the surface of fiber becomes more wrinkled. This is owing to the significant mass loss and the much curved and twisted structure of the fibers as mentioned above. As shown in Fig. 2g, the Co/C nanoparticles in HCF@CZ inherited the dodecahedron morphology.

However, compared with ZIF-67 particles, they process a much rough and porous surface, which may be coursed by the decomposition and gasification of organic ligands during pyrolysis. The size of these Co/C nanoparticles particles was also obviously decreased to 200-500 nm. As presented in Fig. 2h, after annealed in H2/Ar mixed flow multiple CNTs were observed on the surface of HCF@CZ-CNTs with diameter of 50 mm and typical length of ~500 nm. The CNTs displays both straight and curly morphology similar to the villus on the vine of a plant such as pumpkin vine. Carbonized cotton fiber and those villus-like CNTs built up a micro-to-nano size hierarchical structure. Higher resolution SEM image clearly exhibited the villus-like CNTs (Fig. S3 Supporting Information). What’s more, the dodecahedron morphology of ZIF-67 becomes more damaged but can still be distinguished. This is attributed to the depletion of carbon source during the catalytic growth of CNTs.

Fig. 2. SEM images for cotton fiber (a, e), ZIF-67 coated cotton fiber precursor (b, f), HCF@CZ (c, g) and HCF@CZ-CNTs (d, h), TEM images for HCF@CZ (i-k) and HCF@CZ-CNTs (l-n). TEM images give a clear view of the carbonized ZIF-67 particles. As shown in Fig. 2i, j and Fig. 2l, m, after pyrolysis the ZIF-67 crystal retains the rhombic dodecahedron structure. Co2+ ions were reduced in to Co particles (black area) with typical size of 40-100 nm, which were embedded in the rhombic dodecahedron

carbon frameworks both for HCF@CZ and HCF@CZ-CNTs. Curved CNTs grow from Co particles can be clearly observed in Fig. 2m confirming the catalytic growth of CNTs during the annealing in reductive atmosphere. Microstructures of HCF@CZ and HCF@CZ-CNTs were further analyzed by high-resolution TEM (HRTEM). As shown in Fig. 2k and Fig. 2n, Co nanoparticles are highly crystalline showing distinct lattice fringes with spacing of 0.20 nm, which can be assigned to the (111) lattice plane of metallic Co. Lattice fringes with spacing of 0.24 nm were also observed at the edge of Co nanoparticles indexed to the (111) plane of CoO (Fig. 2k). The absence of these lattice fringes in HRTEM of HCF@CZ-CNTs reveals the higher reduction degree of Co2+. What’s more, distinct lattice fringes with spacing of 0.34 nm demonstrates that the Co nanoparticles are covered with graphitized carbon shell in HCF@CZ and HCF@CZ-CNTs. As shown in Fig. 3, XRD patterns, Raman spectra and XPS spectra were carried out for the chemical composition analysis. In Fig. 3a, XRD pattern for HCF presents two obvious broad peaks at 21.1 ° and 44.0 °,[36] corresponding to amorphous carbon and (101) lattice plane of graphite respectively, indicating the partial graphitization of cotton fibers. XRD patterns for HCF@CZ and HCF@CZ-CNTs were presented in Fig. 3b, sharp peaks at 2θ = 44.4 °, 51.6 ° and 76.0 ° can be indexed to the (111), (200), (220) lattice planes of fcc Co nanoparticles.[37] Broad peaks at 21.1 ° and 44.0 ° were also observed for HCF@CZ and HCF@CZ-CNTs, meanwhile a distinguishable peak at 25.8 ° appeared, which belongs to the (002) plane of graphitized carbon. Compared to HCF, this additional peak indicates that carbon matrix of Co/C particles in

HCF@CZ and HCF@CZ-CNTs show a higher degree of graphitization, owing to the catalyzed graphitization by Co nanoparticles and the present of CNTs in HCF@CZ-CNTs.

Fig. 3. XRD patterns (a) and (b), Raman spectra (c) of HCF, HCF@CZ and HCF@CZ-CNTs and XPS spectra (d), (e) and (f) of HCF@CZ-CNTs. Fig. 3c presents the Raman spectra for the as prepared HCF, HCF@CZ and HCF@CZ-CNTs. Typical D band and G band were observed at 1350 cm-1 and 1590

cm-1 respectively, together with a sharp D’ band at 1736 cm-1. In general, D band indicates

in-plane

defects

for

carbon

material

including

heteroatoms,

oxygen-containing functional groups, and non-hexatomic rings, while G band is attributed to the E2g mode vibration of sp2 hybridized carbon atoms. Therefor relative intensity ratio of D and G band (ID/IG) is widely used to evaluate the graphitization level of carbon materials. In other words, larger ID/IG indicates more defects in carbon materials. HCF, HCF@CZ and HCF@CZ-CNTs have decreasing ID/IG ratios, 0.92, 0.86, and 0.85 respectively, implying increase in graphitization level for both cotton fiber and ZIF-67 particles. Compared with HCF@CZ, the slightly smaller ID/IG ratio of HCF@CZ-CNTs can be owed to the in-situ grown CNTs. 2D band at 2700 cm-1 was also observed for HCF@CZ and HCF@CZ-CNTs, which is also attributed to both higher graphitization level and CNTs in HCF@CZ-CNTs. X-ray photoelectron spectroscopy (XPS) was applied to investigate the valence state of surface chemical compositions of HCF@CZ-CNTs. As can be seen in Fig. 3d, full survey XPS spectra of the sample indicates the existence of Co, O and C. The C 1s spectrum in Fig. 3e can be resolved into four peaks, the peaks located at 284.8 eV is assigned to C 1s, and peaks at 285.9, 287.3 and 290.6 eV are attributed to C=O, C-OR and COOR oxygen-containing groups respectively. Co 2p spectrum was shown in Fig. 3f, which was resolved into Co 2p3/2 and 2p1/2 located at 781.5 and 797.2 eV, two corresponding satellite peaks at 787.1 and 803.5 eV, which were characteristic signifier for oxidation state. Distinguishable peaks at 778.8 eV and 793.4 eV were assigned to Co elementary substance, indicating the partial reduction of Co2+ during pyrolysis.[37,

38] The magnetic hysteresis loops of HCF@CZ and HCF@CZ-CNTs were measured by a VSM at room temperature. Saturation magnetization ( M s ) and coercivity ( H c ) are two essential factors to define the magnetic loss in magnetization relaxation. The initial permeability ( µi ) usually implies the magnetic loss ability, which can be expressed as follow:[39]

µi =

M s2 akH c M s + bλξ

(1)

Where a, b are constants determined by the material, k is a proportionality coefficient, λ and ξ are magnetostriction constant and elastic strain parameter respectively. The above equation shows that higher M s and smaller H c are favorable for higher µi and magnetic loss ability.[40] As can be seen in Fig. 4, HCF@CZ shows a 26.4 emu/g

Ms

and a 198.3 Oe coercivity, while

HCF@CZ-CNTs displays a much higher 42.2 emu/g M s but a lower H c (103.5 Oe), which indicate that HCF@CZ-CNTs exhibits an enhanced magnetic loss ability.

Fig. 4. Hysteresis loops of HCF@CZ and HCF@CZ-CNTs. Complex permittivity ( ε r = ε ′ − jε ′′ ) and permeability ( µr = µ ′ − j µ ′′ ) are crucial parameters for evaluating the EM wave absorption ability of microwave absorbers. It is well accepted that the real part of permittivity ( ε ′ ) and permeability ( µ ′ ) correspond to the storing ability of electric and magnetic energy, while the imaginary parts ( ε ′′ and µ ′′ ) are related to the dissipation ability within the medium. Therefor the ratio of imaginary part to real part of permittivity ( tan δ e = ( tan δ m =

ε ′′ ) and permeability ε′

µ ′′ ) are widely used to characterize the dielectric and magnetic loss µ′

ability.[41] The frequency dependence of complex permittivity and permeability for the as prepare HCF-A, HCF@CZ and HCF@CZ-CNTs samples are recorded from a network analyzer in 2-18 GHz. As shown in Fig. 5a, the ε ′ value of HCF sample is almost constant, which shows a slight decrease from 8.67 to 6.51 in 2-18 GHz. The

ε ′′ of HCF has a small value and decreases from 1.73 to almost 0. As a result, the HCF sample demonstrates relatively low dielectric loss tangent (< 0.2) in the whole frequency range. As contrasts, the ε ′ and ε ′′ of HCF@CZ and HCF@CZ-CNTs possess an obvious increase in the whole frequency range. With increasing frequency,

ε ′ of HCF@CZ monotonously decrease from 12.73 to 7.21, and the ε ′′ decreases from 5.14 to 2.25 with multiple fluctuation peaks. Increased ε ′′ lead to enhanced dielectric loss tangent, which first increase from 0.41 to 0.58 at 2-7.1 GHz than decrease to 0.32 with multiple fluctuation peaks. The fluctuation dielectric loss peaks

were attributed to multiple dielectric loss mechanisms caused by the hierarchical structure and complex chemical state of components. The real part of permittivity for HCF@CZ-CNTs is very close to HCF@CZ, except for the obvious increase in the frequency range of 6.7-12.4 GHz, which monotonously decrease from 12.74 to 5.03. As for the ε ′′ of HCF@CZ-CNTs, it has a typical value of 4-5.3, which is smaller than that of the HCF@CZ in 2.0-8.1 GHz, but possesses increased value than HCF@CZ in 8.1-18.0 GHz as a result of a broad peak at about 9.8-16.7 GHz. Similar to ε ′′ , tan δ e of HCF@CZ-CNTs is obviously advanced in higher frequency range (10.7-18 GHz). As shown in Fig. 5c, an increase tendency was observed for tan δ e of HCF@CZ-CNTs in 2.0-16.6 GHz, and a peak value of 0.78 at 16.6 GHz was obtained. Two fluctuation peaks were distinguished at 9.78 and 14.08 GHz for tan δ e of HCF@CZ-CNTs too. Increase in ε ′

ε ′′ and tan δ e =

ε ′′ of HCF@CZ and ε′

HCF@CZ-CNTs indicate that dielectric loss ability of HCF was dramatically enhanced after the attachment of Co/C nanoparticles derived from ZIF-67, and the villus-like CNTs further increase dielectric loss in higher frequency range.

Fig. 5. Complex permittivity and permeability for HCF, HCF@CZ and HCF@CZ-CNTs. (a) real part and (b) imaginary part of permittivity, (c) dielectric loss tangent, (d) real part and (e) imaginary part of permeability, (f) magnetic loss tangent. It is well known that polarization relaxation is very favorable for enhancement of EM wave absorption. According to the Debye dipolar relaxation theory, the relationship of ε ′ and ε ′′ can be described by the following equation.[42] εs − ε∞ 1 + ω 2τ 2 ε −ε ε ′′ = s 2 ∞2 ωτ 1+ ω τ

ε ′ = ε∞ +

εs + ε∞  2   εs − ε∞   ε ′ − 2  + ( ε ′′ ) =  2      2

(2) (3) 2

(4)

Where τ is the polarization relaxation time, ω represents the angular frequency,

ε s is the static permittivity and ε ∞ is the relative dielectric permittivity at high frequency limit. Therefor plot of ε ′′ versus ε ′ should be a semicircle, which is generally denoted as Cole–Cole semicircle, and each semicircle in the ε ′′ - ε ′ plot

corresponds to a Debye dipolar relaxation process. Fig. S4 (Supporting Information) shows the Cole-Cole semicircles plots of the as prepared HCF, HCF@CZ and HCF@CZ-CNTs. As can be seen in Fig. S4a-c, five distinguishable semicircles can be observed for HCF in the ε ′′ - ε ′ plot in 2-18 GHz, meanwhile HCF@CZ and HCF@CZ-CNTs show six semicircles, indicating that there are multiple Debye dipolar relaxation processes within the carbonized cotton fibers, Co/C dodecahedron and CNTs under external EM field. These dipolar relaxation processes can be attributed to the surface/interface polarization relaxation and displacement polarization, which are induced by the alternating electric field in the partly graphitized carbon with defects and oxygen-containing groups (C=O, C-OR and COOR) and Co/C heterojunctions nano-dodecahedron as well as villus-like CNTs. These dipolar relaxation processes play crucial roles in EM wave absorption capability. The magnetic loss ability was evaluated with the complex permeability, as shown in Fig. 5d-f. The µ ′ of non-magnetic HCF sample is almost constant, approximately 1 in the whole frequency range. The imaginary part of permeability has a very low value. After annealing, µ ′ of the HCF@CZ and HCF@CZ-CNTs samples increase obviously, especially in higher frequency (>10 GHz) owing to the existence of magnetic Co/C nanoparticles. Compared to HCF, HCF@CZ and HCF@CZ-CNTs has a slightly increased µ ′′ , as shown in Fig. 5f. HCF@CZ has slightly larger µ ′′ in 5.8-11.1 GHz, and HCF@CZ-CNTs has the largest µ ′′ in higher frequency (11.2-18.0 GHz) due to a broad peak in this frequency range. Strong fluctuation was

also observed for the µ ′′ curves. Fig. 5f shows the magnetic loss tangent for three kinds of samples. All of them demonstrate low tan δ m value, while HCF@CZ and HCF@CZ-CNTs show slightly increase similar to µ ′′ . Considering the loading mass ratio of the as prepared hollow fibers (HCF@CZ and HCF@CZ-CNTs) in the EM absorption test samples is only 10%, and the amount of ZIF-67 particles attached to the cotton fiber is limited too, the low µ ′′ and tan δ m value of the sample can be attributed the small amount of magnetic Co/C nanoparticles in HCF@CZ and HCF@CZ-CNTs samples. Generally speaking, magnetic loss in EM wave absorbers mainly include magnetic hysteresis, domain wall resonance, eddy current, natural and exchange resonance. Among them, magnetic hysteresis originates from the hysteretic deflection of magnetic moments following the alternating magnetic field. As discussed above, HCF@CZ-CNTs shows higher M s and lower H c , which are favorable for higher permeability and higher magnetic Loss. Domain wall resonance only occurs at low frequency range < 2 GHz. Eddy current loss and natural resonance can be expressed by[43]

C0 = µ′′(µ′)2 f −1

(5)

Constant C0 with changing frequency means magnetic loss arises from eddy current, otherwise natural and exchange resonance is in domination. Fig. S5 (Supporting Information) shows the C0-f results of HCF@CZ and HCF@CZ-CNTs. C0 of HCF@CZ shows a fluctuation in 2-3 GHz, and keeps almost constant in the rest

frequency range indicating that eddy current dominates. However, except for the fluctuate peak in 2-3 GHz, C0 of HCF@CZ-CNTs shows a strong and broad peak in 10-18 GHz, implying magnetic loss mainly stems from natural resonance and exchange resonance. According to the transmission line theory, reflection loss (RL) curves of HCF, HCF@CZ and HCF@CZ-CNTs were calculated from the measured complex ε r and

µr at 2-18 GHz with the following equations.[43] RL (dB) = 20 log

Z in =

Z in − 1 Z in + 1

µr  2π  tanh  j ( ) fd µ r ε r  εr c  

(6)

(7)

Where Zin represents the normalized input impedance, ε r and µr are the complex relative permittivity and permeability respectively, c represents the free-space light velocity, f is the frequency of the electromagnetic wave, and d refers to the thickness of the absorbers. For EM wave absorption materials, minimum reflection loss and effective absorption bandwidth are the most important parameters for EM wave absorption ability evaluation. When RL ≤ -10 dB, more than 90% of the EM energy can be dissipated, therefor RL ≤ -10 dB is usually termed as the effective EM wave absorption. Frequency dependence of reflection loss properties in 2-18 GHz and the corresponding two-dimensional (2D) contour plots of HCF, HCF@CZ and HCF@CZ-CNTs are shown in Fig. 6. It can be clearly observed that with increasing

thickness the RL peaks shift to lower frequency range for all the three kinds of samples, indicating thickness of the sample is a crucial parameters for determining the matching frequency and intensity of the RLmin peaks. According to the 1/4 wavelength destructive interference model, the minimal reflection loss can achieved at certain frequency if the EM wave absorber thickness equals to an odd number of times of the 1/4 wavelength in the medium, which can be was described as follows.[44-46]

tm =

nc 4f

ε r µr

( n = 1,3,5...)

(8)

Where tm represents for the matching thickness of the RLmin, c represents light velocity, ε r and µr are the relative permittivity and permeability as declared above. Hence, with increasing thickness the RL peaks will automatically move to lower frequency range. Frequency dependence of tm for HCF, HCF@CZ and HCF@CZ-CNTs are shown in Fig. S6 and all the corresponding RLmin peaks agree well with 1/4 wavelength destructive interference model. As displayed in Fig. 6a, HCF demonstrates a -18.9 dB minimum reflection loss value at 16.1 GHz with the thickness of 5 mm together with a 2.26 GHz (15.04-17.30 GHz) maximum effective absorption bandwidth. Moreover, absorption peaks were observed at 13.9 GHz and 9.4 GHz for HCF at the thickness of 2 and 3 mm respectively, indicating that the carbonized cotton fibers has the potential for low thickness broadband EM wave absorber. This potential is further proved by the effectively enhanced RL curves of HCF@CZ and HCF@CZ-CNTs. When the

absorber thickness varies from 1.0 to 5.0 mm, RL values of HCF@CZ below -10 dB are achieved in the frequency range of 3.86-17.56 GHz. A -45.3 dB minimum RL value is achieved at 14.7 GHz as well as a corresponding 5.52 GHz (12.04-17.56 GHz) effective absorption bandwidth at the absorber thickness of only 1.8 mm. When the annealing process was carried out in a hydrogen and argon mixture atmosphere, the 2D contour plot of HCF@CZ-CNTs shows two absorption islands. But, as a contrast, 2D RL contour plots of HCF and HCF@CZ show just one absorption island, indicating that the in-situ grown villus-like CNTs has an obvious effective and positive effect on the EM wave absorption properties. It is well supported by the related result demonstrated in Fig. 6c. HCF@CZ-CNTs possesses a further improved 8.02 GHz (9.98-18 GHz) maximum effective absorption bandwidth at 2 mm. Even the absorber thickness is just 1.6 mm, effective absorption is also achieved in 12.86-18 GHz (5.14 GHz), and the minimum RL reaches -41.89 dB at the same time. When the thickness is increased to 2.9 mm, HCF@CZ-CNTs shows a -53.5 dB maximum RL and a 2.64 GHz (6.82-9.46 GHz) absorption bandwidth are observed. Compared with HCF,

HCF@CZ and

HCF@CZ-CNTs

show dramatically enhanced

RLmin

(HCF@CZ-CNTs > HCF@CZ > HCF) and maximum effective absorption bandwidth at thin thickness. It is very favorable in practical application to achieve a broad absorption bandwidth with a small absorber thickness, in this case, HCF@CZ and HCF@CZ-CNTs is very promising for this kind of high performance broadband EM wave absorber.

Fig. 6. Reflection loss curves and the corresponding 2D contour plots of (a) HCF, (b) HCF@CZ, (c) HCF@CZ-CNTs samples at different thicknesses in 2-18 GHz. Table 1 present the EM wave absorption properties of previously reported biomass derived carbon materials obtained by carbonization. It can be clearly seen that the enhanced EM wave absorption properties of them rely on relatively larger absorber thickness. When the sample thickness is less than 2 mm their effective absorption bandwidth decrease obviously to less than 5 GHz. Compared with these previous works, HCF@CZ and HCF@CZ-CNTs shows obvious advantage in effective EM wave absorption bandwidth (RL<-10 dB) at low absorber thickness, which has important significance in engineering applications. It is also worth mention that the filler content of HCF@CZ and HCF@CZ-CNTs in the testing samples is just 10 wt%, while earlier reports are ≥ 15 wt%. The improved broadband EM wave absorption property and low filler content also confirms the excellent EM wave dissipation ability of HCF@CZ and HCF@CZ-CNTs.

Table 1. EM wave absorption properties of some earlier reported biomass derived carbon materials Absorption bandwidth

RLmin

Thickness

Filler content

(GHz)

(dB)

(mm)

(wt%)

Materials

Ref.

PBPC-690 (wood)

7.63 (9.83–17.46)

-16.3

3.73

>50

[21]

PC-600 (walnut shell)

2.24 (10.48–12.72)

-19.4

1.5

30

[22]

MPC600 (loofah sponge)

5 (13-18)

-49.6

2

30

[23]

HPMC (rice)

5 (13-18)

-30.5

1.65

15

[24]

~2.8 (8-10.8)

-49.6

2.5

30

[25]

~1.9 (14.8-16.7)

-23.6

6

50

[47]

HCF@CZ

5.52 (12.04-17.56)

-45.3

1.8

10

This work

HCF@CZ-CNTs

8.02 (9.98-18)

-14.4

2.0

10

This work

C@CoFe2O4 (eggshell membrane) Ni(OH)2/AC (jackfruit peel)

The dramatically enhanced microwave absorption performance of HCF@CZ and HCF@CZ-CNTs at such low absorber thicknesses can be attributed to the hierarchical fibrous structure and synergistic effect of dielectric and magnetic loss. It is well accepted that the input impedance matching condition and attenuation ability are the two essential properties for high performance EM wave absorption materials. The frequency dependence of normalized input impedance ( Zin ) were calculated following equation (6). When the normalized input impedance is close to 1, EM wave can transmit into the absorber without surface reflection. As revealed in Fig. S7 a-c, HCF shows strong input impedance peaks with typical peak value of 2.2-2.5, which is too large to obtain strong absorption property. As a contrast, input impedance of HCF@CZ and HCF@CZ-CNTs are obviously improved, HCF@CZ has typical input

impedance peak value of 0.81-0.95 and input impedance peak value of HCF@CZ-CNTs located at 0.81-1.35. All of those input impedance peak values are much close to 1. What’s more, those input impedance peak are broad, benefiting the broadband EM wave absorption. Attenuation constants ( α ) of were also calculated as[40]

α=

2π f × (µ′′ε ′′ − µ ′ε ′) + (µ′′ε ′′ − µ ′ε ′)2 + (µ′ε ′′ − µ′′ε ′)2 c

(9)

which indicating the EM wave dissipation capability. The results are shown in Fig. S7 d, Attenuation constant of HCF has a relatively low value, 0.3-91.9 approximately. After the addition of Co/C nanoparticle and CNTs (HCF@CZ-CNTs), α of HCF@CZ and HCF@CZ-CNTs possess much larger value, 28.3-177.7 for HCF@CZ and 17.8-299.3 for HCF@CZ-CNTs. This is another proof for the ZIF-67 derived Co/C nano-dodecahedrons are very helpful for the enhancement of attenuation capability and the in-situ grown CNTs also improves the attenuation capability by building up the hierarchical structure with the carbonized cotton fiber. Therefor HCF@CZ-CNTs with villus-like CNTs shows higher EM wave attenuation ability in 10.2-18 GHz.

Fig. 7. Possible EM wave attenuation mechanisms of hollow carbon fiber coated with Co/C nano-dodecahedron particles and CNTs. A schematic diagram of the possible EM wave absorption mechanisms for the as prepared hierarchical hollow carbon fiber coated with Co/C nano-dodecahedron particles and villus-like CNTs is demonstrated in Fig. 7. Firstly, the tangled fibrous structure with villus-like CNTs give rise to multiple reflection for EM wave in this micron scale magnetic-dielectric absorber. Because of the existence of abundant Co/C nanoparticles and villus-like CNTs in the HCF@CZ-CNTs absorber, EM wave scattering can be greatly promoted, which dramatically increases the transmission path length of EM wave. As a result, the EM wave attenuation ability of HCF@CZ-CNTs can be remarkably enhanced compare with pure carbonized cotton fiber. Secondly, unique HCF@CZ-CNTs composites assembled by carbon fibers Co/C nanoparticles provide strong dielectric and magnetic dissipation effect for electromagnetic energy. The dielectric loss is mainly stem from the interface and

dipole polarization, which contribute to the enhanced conductivity and energy conversion behaviors. The highly porous feature and abundant interfaces cause strong space and interface polarization. EM wave energy was attenuated by polarization and displacement of dipoles. The abundant lattice defects and crystal boundary inside the Co/C particles and CNTs generated during annealing can act as polarization centers under extra electromagnetic field, further boost the EM wave dissipation. As a magnetic EM wave absorber, possess eddy current in 3-10 GHz and exchange resonance make more contribution in 2-3 GHz and natural resonance.10-18 GHz, constructing multi-frequency magnetic loss behaviors. The excellent EM wave absorption performance for the HCF@CZ-CNTs is attributed to the micro-to-nano hierarchical hollow fibrous structure, as well as the synergetic effect of dielectric and magnetic loss. Above-mentioned results confirm the low-cost and sustainable strategy to fabricate an ultra-lightweight microwave absorber with excellent EM wave absorbing capacity utilizing biomass template and MOF precursor.

4. Conclusions In conclusion, a hierarchical carbon fiber coated with Co/C nano-dodecahedron particles and villus-like CNTs was fabricated using cotton and ZIF-67. The as-prepared HCF@CZ-CNTs with ultra-low apparent density (0.0198 g/cm3) shows a minimum -53.5 dB reflection loss at 7.8 GHz. Meanwhile, we have achieved the broad effective EM wave absorption bandwidth of 8.02 GHz with the sample thickness of only 2 mm. Compared with pure carbonized cotton fibers, both the minimum RL and maximum effective absorption bandwidth are dramatically

optimized. The excellent broadband EM wave absorption performance is attributed to the micro-to-nano hierarchical structure and synergetic effect of polarization relaxation and magnetic loss including eddy current, natural resonance and exchange resonance. The combination of biomass and MOF illuminates a low-cost and sustainable strategy to prepare ultra-lightweight EM wave absorber with excellent electromagnetic wave absorption performance. Appendix A. Supplementary data Supplementary data to this article can be found online. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgements This manuscript was financially sponsored by National Natural Science Foundation of China (NSFC, Grant No. 51772063) and Shenzhen Science and Technology Program (Grant No. KQTD2016112814303055).

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CRediT author statement Minglong Yang: Investigation, Methodology, Writing - Original Draft; Ye Yuan: Investigation, Methodology, Writing - Review & Editing; Ying Li: Investigation, Methodology, Writing - Review & Editing; Xianxian Sun: Visualization, Writing Review & Editing; Shasha Wang: Visualization, Writing - Review & Editing; Lei Liang: Visualization, Writing - Review & Editing; Yuanhao Ning: Visualization; Jianjun Li: Formal analysis, Writing - Review & Editing; Weilong Yin: Visualization, Writing - Review & Editing; Renchao Che: Conceptualization, Supervision, Writing - Review & Editing; Yibin Li: Conceptualization, Supervision, Funding acquisition, Writing - Review & Editing.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: