cobalt co-doped carbonaceous materials with electromagnetic waves absorption promoting nanostructures

Journal Pre-proof Hierarchical nitrogen/cobalt co-doped carbonaceous materials with electromagnetic waves absorption promoting nanostructures Xin Zhao, Dong-Mei Guo, Qing-Da An, Shu-Feng Bo, Zuo-Yi Xiao, Wei-Jie Cai, HaiSong Wang, Shang-Ru Zhai, Zhong-Cheng Li PII:

S0925-8388(20)30029-3

DOI:

https://doi.org/10.1016/j.jallcom.2020.153666

Reference:

JALCOM 153666

To appear in:

Journal of Alloys and Compounds

Received Date: 23 October 2019 Revised Date:

2 January 2020

Accepted Date: 3 January 2020

Please cite this article as: X. Zhao, D.-M. Guo, Q.-D. An, S.-F. Bo, Z.-Y. Xiao, W.-J. Cai, H.-S. Wang, S.-R. Zhai, Z.-C. Li, Hierarchical nitrogen/cobalt co-doped carbonaceous materials with electromagnetic waves absorption promoting nanostructures, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153666. 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 B.V.

Author Contribution List Term Conceptualization

Name Xin Zhao,

Shang-Ru

Zhai,

Qing-Da An

Methodology

Xin Zhao, Shang-Ru Zhai

Software

Dong-Mei Guo, Shu-Feng Bo, Zuo-Yi Xiao

Validation

Qing-Da

An,

Wei-Jie

Cai,

Hai-Song Wang Formal analysis

Xin

Zhao,

Shang-Ru

Zhai,

Dong-Mei

Guo,

Zuo-Yi

Xiao,

Zhong-Cheng Li Investigation

Xin

Zhao,

Qing-Da

An,

Shang-Ru Zhai

Resources

Qing-Da An, Zuo-Yi Xiao, Wei-Jie Cai, Hai-Song Wang, Shang-Ru Zhai, Zhong-Cheng Li

Data Curation

Shu-Feng

Bo,

Zuo-Yi

Xiao,

Shang-Ru Zhai, Zhong-Cheng Lib

Writing - Original Draft

Xin Zhao

Writing - Review & Editing

Xin

Zhao,

Dong-Mei

Guo,

Qing-Da An, Shu-Feng Bo, Zuo-Yi Xiao,

Wei-Jie

Cai,

Hai-Song

Wang,

Shang-Ru

Zhai,

Zhong-Cheng Li

Visualization

Xin

Zhao,

Dong-Mei

Guo,

Qing-Da An, Shu-Feng Bo, Zuo-Yi Xiao,

Wei-Jie

Wang,

Cai,

Hai-Song

Shang-Ru

Zhai,

Zhong-Cheng Li

Supervision

Qing-Da An, Zuo-Yi Xiao, Wei-Jie Cai, Hai-Song Wang, Shang-Ru Zhai, Zhong-Cheng Li

Project administration

Qing-Da

An,

Zuo-Yi

Xiao,

Shang-Ru Zhai Funding acquisition

Qing-Da An, Zuo-Yi Xiao, Wei-Jie Cai, Hai-Song Wang, Shang-Ru Zhai, Zhong-Cheng Li

Hierarchical nitrogen/cobalt co-doped carbonaceous materials with electromagnetic waves absorption promoting nanostructures Xin Zhaoa, Dong-Mei Guoa, Qing-Da Ana,*, Shu-Feng Boa, Zuo-Yi Xiaoa, Wei-Jie Caia, Hai-Song Wanga, Shang-Ru Zhai a,*, Zhong-Cheng Lib a

Faculty of Light Industry and Chemical Engineering, Dalian Polytechnic University,

Dalian 116034 b

Key Laboratory of Optic-electric Sensing and Analytical Chemistry for Life Science,

MOE, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042 Graphical Abstract:

-0.2000 -6.100

-6

-12.00 -17.90

-12 -18

Reflection Los

-23.80 -29.70 -35.60

-24

-41.50

-30 -36

-47.40 -5

-42

-4

6

18

14

-1

16

10

12

8

-2

Thic k

4

-3

Fre que ncy (GH z)

ness (mm )

Electromagnetic Waves

s (dB)

(d)

0

Hierarchical nitrogen/cobalt co-doped carbonaceous materials with electromagnetic waves absorption promoting nanostructures were controllably prepared via a seaweed-based hydrogels strategy.

Hierarchical nitrogen/cobalt co-doped carbonaceous materials with electromagnetic waves absorption promoting nanostructures Xin Zhaoa, Dong-Mei Guoa, Qing-Da Ana,*, Shu-Feng Boa, Zuo-Yi Xiaoa, Wei-Jie Caia, Hai-Song Wanga, Shang-Ru Zhai a,*, Zhong-Cheng Lib a

Faculty of Light Industry and Chemical Engineering, Dalian Polytechnic University,

Dalian 116034 b

Key Laboratory of Optic-electric Sensing and Analytical Chemistry for Life Science,

MOE, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042

*Corresponding authors. E-mail: [email protected] (Q.-D. An).Tel: 0411-86322638 [email protected] (S.-R. Zhai). Tel: 13516058041

Abstract In order to solve the pollution of electromagnetic interference (EMI), hierarchical Co/N-co-doped carbonaceous compounds with absorption promoting nanostructures were prepared as absorbing materials via a seaweed-based hydrogel strategy. XRD, Raman, SEM, TEM, VSM and XPS were used to characterize the resultant samples. All the tests indicated that the absorber carbonized at 700oC (SA/N-CoxOy-700, among, SA represents sodium alginate and 700 means that the carbonization temperature was 700oC) has an excellent performance, which minimal RL can reach to -47.31 dB at 16.16 GHz and the effective absorption bandwidth (RL<-10 dB) is 4.56 GHz from 13.44 to 18 GHz. The relative superior performance is mainly attributed to the cooperative effect of nitrogen-doped carbon skeleton, expanded surface, multiple defects, various phase interface and hierarchal pore structures, thus leading to the improvement in multiple reflections and scattering, nature resonance, eddy-current effect, the consumption of thermal energy, dipoles and interfacial polarization.

Keywords:

Co/N-co-doped,

three-dimensional

hierarchical

pores,

promoting nanostructures, cooperative effect, microwave absorption

absorption

1. Introduction The rapid development of electronic equipment makes our life more and more convenient. However, electromagnetic interference (EMI) pollution cannot be overlooked, because it has bad effects on operation electronic devices[1], and especially does harm to human beings[2]. Therefore, it is high time to explore some feasible and effective ways or methods to solve such serious problem! Over the past decades, increasing electromagnetic wave (EMW) absorption materials have been used, which can absorb EMW and convert the energy into thermal energy or dissipate through dielectric loss, magnetic loss or interference[3, 4]. There are a lot of materials which widely used on practice as electromagnetic absorbing material[5, 6]. Generally, the absorbents with high-performance are usually required to have a wide absorption frequency range , low density, good thermal stability, strong absorption properties , and antioxidant capability[7]. Given that, scholars found that carbon materials and ferromagnetic metals or their compounds have an excellent performance in EMW absorption[8, 9]. For example, both conductive polymers, carbon nanotubes and nanofibers, as well as graphene have been widely used, mainly due to their strong dielectric loss, low density. Moreover, their high temperature resistance and outstanding chemical stability also are the most important factors[10]. Nevertheless, carbon materials are usually limited by their relative lower conductive, lower saturation magnetization and lower permeability[11]. In consideration of these issues,

some attention has been paid to the use of ferromagnetic metal particles and nanoscale magnetic materials[12]. The reason can be explained as follows: compared with carbon-based materials, the interface activation and polarization of the nanoparticles will be highly improved by the high saturation magnetization, small coercive forces, low magnetostriction and high Curie temperatures of magnetic metals[13]. Although it possesses plenty merits, the nanoparticles are easily oxidized by oxidants in air, and always agglomerate in preparation process. Meanwhile, their narrow frequency also restricted their potential applications[14]. Therefore, there is high requirement for the development of novel materials with all the above advantages. As a result, taking into account the individual advantages of carbon and magnetic materials respectively, remarkable efforts have been devoted to the combination of carbon materials with ferromagnetic nanoparticles[15]. Amazingly, tremendous related works have obtained satisfying achievements. For example, Lü etal successfully synthesized porous Co/C composites absorbent by the thermal treatment of Co-based MOFs (Co-MOF, ZIF-67), which maximum reflection loss (RL) was -35.3 dB, and the effective absorption bandwidth (RL≤-10 dB) reached 5.80 GHz (8.40 GHz–14.20 GHz)[16]. More recently, nitrogen/ferromagnetic nanoparticles co-doped biomass derived carbon-materials also enjoy a great popular in the filed of EMW absorption. It is because the unique semiconductor-like structure of this kinds of materials can lead to an easy transfer of electrons[17]. Moreover, the introduction of nitrogen can alter the local electronic structure of carbon skeleton and generate C-N, N-M or C-M (M represents various ferromagnetic elements) active sites. This

step would lead to the formation of heterojunction texture and defects[18, 19], hence result in the enhancement of polarization and the increase in microwave attenuation or impedance

matching.

As

was

demonstrated,

Xu

successfully

prepared

honeycomb-like carbon microwave absorbers using natural walnut shell as precursor, and manifested that three crystal structures and various morphologies of metallic cobalt played an important role in the absorption[20]. There is no doubt that all of the above researches are indelible and fascinating. Nevertheless, the design of 3D porous structure materials still faces great challenges. It is because such kinds of materials can promote the effects of microwaves absorption[21], such as the interaction promotion between magnetic properties and dielectric properties[22], the improvement in electromagnetic waves reflection and multi-polarization at the hetero-interface[23]. Recently, researchers pointed out that sodium alginate, as an available and low-cost seaweed byproduct, can be tailored to design controllable assembly process by binding metal ions to form a cross-linking network and an egg-box structure. Building such kinds of absorption promoting structures is favorable for the unlimited synergy between the reflection reduction, multi-polarization, cancellation of each other, magnetic loss and dielectric loss of microwaves[24]. It was shown that the developed interior porosity, enlarged surface area as well as 3D core-shell structure play an important role in efficiency enhancement[25, 26]. Herein, a new type of seaweed-derived hierarchical nitrogen/cobalt co-doped carbonaceous materials (MCB, SA/N-CoxOy-X, SA represents sodium alginate and X means the carbonization temperature)) with

polarization and dielectric/magnetic loss promoting nanostructures were prepared. Among, sodium alginate, melamine and cobalt salt were used as the precursors of carbonaceous network, nitrogen source and metal species, respectively. The as-prepared composites were characterized by various techniques. The resultants showed that the absorbent not only has extraordinary EMW absorbing performance but also displays a broadened bandwidth due to the synergetic cooperation of cobalt oxides and nitrogen-doped 3D “core-shell” absorption promoting carbon skeleton. 2. Experiment 2.1 Materials and chemicals Sodium alginate was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Polyethyleneimine (PEI, 99%) and cobalt salt (Co(NO3)2·6H2O) were obtained from Aladdin Chemistry Co., Ltd., China. Melamine was provided by Tianjin Guangfu Fine Chemical Industry Research Institute Co., Ltd. All above mentioned chemicals were of analytical grade and used directly without any further purification. 2.2 Fabrication of SA/N-CoxOy-X and SA/PEI-CoxOy-700 Nitrogen/cobalt co-doped carbonaceous composites were fabricated through sol-gel strategy with carbonization process. Firstly, 1 g of melamine was dissolved in 100 mL distilled water with 30min stirring. Afterwards, 2 g of sodium alginate was gradually poured into melamine solution and continuously stirred for one night. Thirdly, the above solution was dropped into 100 ml 5% Co(NO3)2 (w/v) solution via peristaltic pump. In this step, sodium alginate/melamine would chelate with cobalt ions to form SA/melamine-Co(II) hydrogel beads undergoing 6 h cross-linking. Then,

the hydrogels were washed by deionized water and ethanol/water commixture (1:1), respectively. The next step was to put the spherules into vacuum freezing for anhydration and then put into oven at 60 oC for 2 h. Finally, the dried beads were placed into tube furnace and heated with different temperatures (600, 700 and 800 oC) for carbonization in N2 atmosphere with a rate of 5 oC min

-1

and kept for 2 h. Then,

they were denoted as SA/N-CoxOy-X, in which X represents different carbonization temperature and x,y mean the ratio between cobalt and oxide. The last, the as-prepared samples were grinded into powder. Meanwhile, SA-CoxOy-700 was prepared in the same way but without nitrogen-doping. The preparation process of SA/PEI-CoxOy-700 is as follows. 2 g of sodium alginate and 0.25g of PEI were dissolved in 100 mL deionized water at the same time, and continuous stirred about 2h. Then, 1g glutaraldehyde solution (50wt%) was added for cross-linking reaction. After being stirred about 24 h, the above mixture was dripped into 100 ml 5% Co(NO3)2 (w/v) solution by peristaltic pump for chelating with cobalt ions to form SA/PEI-Co(II) hydrogel with another 6h stirring. The other steps, such as washing and carbonization proceeding, were similar to the fabrication process of SA/N-CoxOy-700. 2.3 Characterization The composition, structure or morphology of the atoms or molecules can be obtained through X-ray diffraction with the use of Bragg's Law[27, 28]. Herein, the crystal structure was observed by a Rigaku model D/max-2700 diffractometer at Cu Kα radiation (λ = 1.5406 Å) at 40 kV and was analyzed by the software of Jade. The

morphology and structure were analyzed using scanning electron microscope (SEM, JEM JEOL 2100) and transmission electron microcopy (TEM, Hitachi H9000NAR), respectively. Carbon skeleton was observed by Raman spectra (RM2000, Renishaw, UK). The surface properties were characterized by the X-ray photoelectron spectroscopy (XPS, PHI 5000). The surface area and pore size distribution of as-prepared samples were calculated by Brunauer-Emmett-Teller (BET, 3H2000, China). The magnetic properties were tested by vector network analyzer (Agilent PNA N5244A) at 2–18 GHz , which can obtain the complex permeability (µr = µ′-jµ'') and complex permittivity (εr = ε′-jε''). The tests used paraffin as the matrix material and then crushed paraffin/samples into an annulus with an outer diameter of 7 mm and an inner diameter of 3 mm for EMW measurement. The RL (dB) of the EMW absorber followed the rule of the transmission line theory[29, 30]. 3. Result and discussion 3.1 Assembly mechanism of nitrogen/cobalt-co-doped carbonaceous materials As shown in scheme 1, the formation of SA/N-CoxOy-X is discussed. As was demonstrated, when sodium alginate was combined with melamine, they had excellent compatibility, such as physical co-mixing performance and chemical binding capacity, because there are abundant functional groups in their molecules respectively.[31] For example, exactly as the acylation mechanism illuminated, a few of nitrogen may be acylated by carboxyls, leading to the enhancement in the binding force. For cobalt ions, they were firmly fixed on the network through ion-exchange

between cobalt and sodium ions as well as the electronic attraction between carboxyl and positive ions.[32] After cheating with cobalt ions, the nitrogen/cobalt-co-doped hydrogels beads were generated. Among, the two pairs of sodium alginate not only cheated with cobalt ions but also bonded with each other through the link of ether bond. Such kind of formation proceeding can effectively anchor cobalt and firmly encapsulate melamine simultaneously. At last, with temperature increasing and the crystallization process, most cobalt ions were turned into its oxides nanoparticles, and the others were changed to cobalt nanoparticles. Among, the Co oxide were produced from the reaction between cobalt and the oxygen of sodium alginate. Almost all of the oxygen is released before it reacted with the carbon, because the carbonization process was heated up gradually. Moreover, due to the protective of N2 atmosphere, it can prevent the carbon oxidation. Additionally, cobalt oxides can be reduced by carbon species at high temperature, and the reduced zero-valent cobalt can induce the formation of graphite.[33] Meanwhile, melamine could release ammonia gas when temperature reached to 300 o

C.[31]All above processes led to the generation of nitrogen/cobalt-co-doped

carbonaceous beads with three-dimensional porous framework. Finally, it is only need to grind the beads into powder for further use. 3.2 XRD The crystalline structure of the absorbers is depicted in Fig. 1. Among, the locations at 43.08, 47.62, and 50.58o belong to Co (PDF # 15-0806),[34] while the peaks near 35.94, 39.88, 42.2, 62.18o are in line with cobalt monoxide (JCPDS NO

43-1004) [15] . The others about 30.48, 48.94, 54.32, 57.58o are indexed to Co (II,III) oxide (JCPDS NO 89-1093)[35]. In addition, near 25.46o, there appears one weak peak which is due to the formation of graphene. It is because cobalt oxides can be reduced by carbon species at high temperature, and the reduced zero-valent cobalt can lead to the formation of graphite.[36]These peaks manifests that cobalt and its oxides nanoparticles, and a small amount of graphene were generated and were successfully encapsulated in nitrogen-doped carbon skeleton. As can be seen, the XRD of SA-CoxOy-700 is slightly different from SA/N-CoxOy-700, which is perhaps because nitrogen could change the particles arrangement and crystal texture of original cobalt oxides. Besides, the spectrum of SA/N-CoxOy-600 is also different. It is the result of the insufficient carbonization of precursor and deficient energy of the crystal lattice formation of cobalt oxides at lower carbonization temperature. Finally, after being analyzed, there perhaps are both hcp and fcc-phases of Co in cobalt and its oxides, which most of them are multidomain-particles[3]. 3.3 Raman Raman spectroscopy is employed to detect the rotational level and lattice vibration of carbon skeleton. As depicted in Fig. 2, D-band and G-band locates at 1334.2cm-1 and 1590 cm-1, respectively. Among, the former one is attributed to the atomic lattice deficiency and sp3 hybridization of disordered carbon[24]. Oppositely, the latter band belongs to the graphene layer structure and sp2 hybridization of ordered carbon texture[37] . The formed two peaks demonstrate that irregular amorphous and ordered structure carbon were generated at the same time in all the absorbers. Generally, these two kinds of carbon are beneficial to the enhancement of electromagnetic microwaves

absorption. At first, the regular graphene texture can promote material conductivity and enhance dielectric loss efficiency[38]. While the disordered curly carbon plays an important role in the surface area expansion of carbon materials, and in nanoparticles dispersion,

thus

leading

to

the

improvement

of

magnetic

loss

and

multi-polarization[18]. Moreover, the intensity ratios of ID/IG ratios of SA-CoxOy-700, SA/PEI-CoxOy-700, SA/N-CoxOy-X (600, 700, and 800oC) are 0.801, 0.843, 0.924, 0.963 and 0.993, respectively. The intensity ratios of SA/N-CoxOy-700 is higher than SA-CoxOy-700, which is mainly ascribed to the changed carbon atoms lattice and expand carbon structure caused by nitrogen. It would further increase the generation of amorphous carbon. The distinction between different carbonization temperature is mainly due to the increasing formation of defects with temperature increasing. Clearly, after being treated with the same temperature, the value of SA/PEI-CoxOy-700 is lower than SA/N-CoxOy-700. The reason can be explained as follows: the polymerization of PEI and the strong binding force with sodium alginate have confinement effect on the nanoparticle growth. 3.4 SEM The

internal

morphology

and

microstructure

of

SA-CoxOy-700,

SA/PEI-CoxOy-700 and SA/N-CoxOy-700 were characterized by SEM, and corresponding results are shown in Fig. 3. Fig. 3a shows that the material has a complete framework structure but without expanded spongy tissue, which reduced the surface polarization and internal reflection. Compared with SA-CoxOy-700, the carbon framework of SA/PEI-CoxOy-700 was dilated, increasing the surface area and bore diameter, which contributed to the microwave absorption. However, the frameworks around particles stack up due to the polymerization of PEI[39], limiting the size of nanoparticles (Fig. 3b). Fig. 3c-f are the interior structure of

SA/N-CoxOy-700 which was prepared through introducing melamine. Similar to the framework structure of SA/PEI-CoxOy-700, SA/N-CoxOy-700 has expanded hexagonal framework and developed channel structures, which would realize a good matching of dielectric loss and magnetic loss[40, 41]. The cross profile of SA/N-CoxOy-700 is consist of many cell-like textures with core-shell structure, which are assembled by cell wall-like interconnection. It is conducive to the consumption of internal multiple reflection and surface polarization[29]. Moreover, the carbon framework is ductile silk texture that like gauze, reducing the incident reflection loss of electromagnetic wave. Above all, cobalt oxides nanoparticles were evenly fixed in the nitrogen-doped carbon framework through chelation, which greatly improved the efficiency of magnetic loss (Fig. 3f). In addition, EDS mappings of SA/N-CoxOy-700 is shown in bottom of Fig. 3. The C, N, O, Co elements were all uniformly dispersed on the carbon framework, demonstrating that melamine had been introduced to SA/N-CoxOy-700, and also manifesting that the melamine and cobalt oxides nanoparticles were uniformly distributed in the material. 3.5 TEM Fig. 4 shows the TEM images. As can be seen from Fig. 4(a), cobalt oxides nanoparticles were successfully formed in the fabrication process, but with some aggregation. Besides, the carbon skeleton was stacked together to a certain degree. It is mainly due to the lack of nitrogen-doping, result in insufficient expanding of carbon skeleton. SA/PEI-CoxOy-700 image indicates that the polymerization of PEI and the strong binding force with sodium alginate made them bond to cobalt surrounding, which had confinement effect on the nanoparticle growth[37]. It is also why the skeleton appears to agglomeration. The temperature effect on nanoparticles and skeleton display in Fig. 4(c)-(e). Material treated at 600oC has a poor morphology

with uneven particles of different size, which is because there was not enough energy for nanoparticles growth. Thus, it can be found that the oxides size of SA/N-CoxOy-800 is bigger than SA/N-CoxOy-600. Meanwhile, the growth of nanoparticles was influenced by each other, appearing serious aggregation. By contrast, SA/N-CoxOy-700 has a better morphology. Namely, the particles size gradually increased with the enhancement of carbonization temperature without obvious agglomeration. The oxides were uniformly anchored on the carbon skeleton, but nowhere else. As shown in Fig. 4(f), cobalt oxides were in sphere shape with the diameter of 30-40nm. Besides, graphene was also generated in the preparation proceeding, which formation mechanism was described in XRD. Noting that, the formation of graphene would enhance materials conductivity, hence leading to the improvement in dielectric loss and energy conversion[7, 17]. 3.6 XPS Since the electron configuration of atom changes with the change of chemical bond, the type and existence state of elements could be determined through XPS characterization. The XPS spectra of SA/N-CoxOy-700 are shown in Fig. S1. Fig. S1(a) displays the C1s spectrum, in which binding energies at 281.18, 281.83 and 282.26 eV relate to C-C/C=C, C-N, and C-O/C=N, respectively[42]. The high-resolution spectrum of O1s is consist of two peaks locating at 530.37 and 532.10 eV, which are assigned to Co-O and C-O respectively[22]. In the N1s spectrum, bending energies at 396.35, 398.28, 399.68 and 401.18eV are in conformity with Co-N, pyridinic N, pyrrolic N and graphitic N, respectively[31]. Moreover, the high resolution Co2p spectrum is analyzed (Fig. S1d). Binding energies at 775.23 and 790.41 eV are for metal Co, which were generated through the reduction reactions between cobalt oxides and carbon under high temperature. The peaks at 778.26 and 799.27 eV are

assigned to tetrahedron CoII and octahedron CoII, respectively[3]. It is worth noting that a peak appearing at 782.91 eV is characteristic peak of CoII-shake-up, which further confirms the existence of CoII. In addition, the peak at 794.31 eV is ascribed to CoIII, stating that cobalt of many valence states coexists on the SA/N-CoxOy-700. 3.7 VSM VSM

is

used

to

detect

the

magnetic

properties

of

SA-CoxOy-700,

SA/N-CoxOy-700 and SA/PEI-CoxOy-700, which results exhibits in Fig. S2. It is not difficult to find that the VSM curves of the three kinds of materials are all in “S” shapes. This is due to the existence of magnetic cobalt oxide particles[43]. Among, the three magnetic parameters, namely saturation magnetization (Ms), coercivity (Hc), and remnant magnetization (Mr), are 50.05, 43.23, 38.79 emu/g (Ms), and 256.87, 228.95,

214.69 Oe (Hc), and 6.45,

4.89, 3.51emu/g

(Mr) corresponding to

SA-CoxOy-700, SA/N-CoxOy-700 and SA/PEI-CoxOy-700, respectively. Obviously, the parameters of SA-CoxOy-700 are higher than the two others, which is attributed to that whatever melamine or PEI is non-magnetic, leading to the inactivation of magnetization proceeding. In addition, SA/PEI-CoxOy-700 has the lowest indexes because PEI would accumulate around the nanoparticles, thus result in the covering and burying of magnetic cobalt oxides. It is reported that reduction on magnetization of particles due to the oxidation can affect the absorption performance[35]. 3.8 EM Absorption Property Reflection losses (RL) of SA/N-CoxOy-X (X=600, 700, 800oC), SA-CoxOy-700 and SA/PEI-CoxOy-700 are evaluated through vector network analyzer. As shown in Fig. 5, SA/N-CoxOy-600 and SA/N-CoxOy-800 nearly have no absorbent performance. As for

SA/N-CoxOy-600, it is mainly due to the insufficient growth of nanoparticles and carbon skeleton[15]. However, the reason of relative worse performance for SA/N-CoxOy-800 is the aggravation and stacking of cobalt oxides and carbon framework. Accordingly, 700 oC was the best carbonization temperature, thus SA/N-CoxOy-700 has a good performance which minimal RL can reach to 47.31 dB at 16.16 GHz coating with 2 mm, and effective absorption (<10 dB) bandwidth is 4.56 GHz from 13.44 to 18 GHz. Compared with SA/N-CoxOy-700, the efficiency of nitrogen-free material (SA-CoxOy-700) reduces to some extent. With the same thickness, its minimal RL decreases to 25.25 dB, but the effective absorption bandwidth increases to 8.13 GHz (9.87-18 GHz). The decrease in performance may be because the electronic distribution and original structure of carbon skeleton was not modified by nitrogen, resulting in the reduction in multi-polarization and the decrease in heterojunction formation[5]. Meanwhile, due to the existence of amino, SA/PEI-CoxOy-700 also possesses excellent performance. Coating with a thin thickness of 2.5 mm, the minimal RL of SA/PEI-CoxOy-700 is 42.24 dB. Moreover, it has a wide effective absorption bandwidth about 7.68 GHz from 8.88 to 16.56 GHz. Undoubtedly,

the

relative

superior

efficiency

of

SA/N-CoxOy-700

and

SA/PEI-CoxOy-700 are ascribed to the formation of absorption promoting structures and their developed “core-shell” texture. It will lead to the improvement of internal multiple reflection, original scattering, interfacial polarization, and multiple reflections[10, 18, 44]. To our best knowledge, the relative complex permittivity (εr=ε'-jε'') and

permeability (µr=µ'-jµ'') are the indictor of microwave absorption performance[16]. In which, the real parts of permittivity (ε') and permeability (µ') are in line with the storage capability of absorber, however, the imaginary parts (ε'' and µ'') are related to its dissipation capability[8]. As demonstrated in Fig. 6, carbonization temperature has a great influence on materials permittivity, which is mainly because the carbon skeleton became rough and nubby[17]. It is beneficial to the improvement of interfacial polarization and dielectric loss. Additionally, the permittivity seems to reduce and emerge fluctuation with frequency ascending. It is ascribed to frequency dispersion, eddy-current effect and polarization relaxation, resulting in good impedance matching of incident microwaves[5, 19]. Moreover, it is not difficult to find that almost the real permeability of all the samples appear fluctuation corresponding to the positions of the imaginary parts of permeability. For example, from 3.5 to 4.5 GHz, the real permeability of SA/N-CoxOy-700 reduces to 0.58, while the imaginary part increases to 1.18. Besides, at the frequency region of 10-14 GHz, the permeability of other samples also changes a lot in high frequency. It is attributed to the effects of dimensional resonance, domain wall resonance and natural resonance, which are caused by magneto-crystalline anisotropic field[45]. Generally, dielectric tangent loss and magnetic tangent loss can clearly reflect the absorption performance. As displayed in Fig. 6(e) and Fig. 6(f), the dielectric loss tangent gradually increased with the carbonization temperature enhanced. As Debye theory explained, dielectric loss is associated with polarization and conductive loss, which will be carefully analyzed in the next sections. Additionally, it is not difficult to find that the change

trend of magnetic loss tangent is similar with the imaginary permeability. Notably, domain-wall resonance, natural and exchange resonances, hysteresis loss, especially eddy current effect are regarded as the main factors of magnetic loss[18]. As we all know, the performance of microwave absorption can be indicated by impedance matching ratio and attenuation constant (α) because both of them can evaluate the synergistic effect of relative complex permittivity and permeability[46]. Among, the impedance matching ratio can be expressed as eqn (1) and (2)[8, 13], where Zin is the input characteristic impedance and Z0 means the impedance of free space. Furthermore, d represents absorber thickness but c symbolizes microwave velocity in free space. Zin = Z0 (µ r/ε r)1/2 tan h [ j(2πƒd/c) (µ r ε r)1/2]

(1)

RL(dB)=20log | (Zin – Z0)/(Zin + Z0) |

(2)

Similarly, the attenuation constant (α) can be formulated as following[16, 47]. In which, its various parameters have been explained in the former: α = (√2πƒ/c)×√[(µ'' ε'' - µ' ε')+√[(µ'' ε'' - µ' ε')2 +(µ'' ε'' + µ' ε')2]]

(3)

Generally, the normalized characteristic impedance (Z = |Zin/Z0|) is used to detected absorber performance. Apparently, the impedance ratios of SA/N-CoxOy-700, SA-CoxOy-700 and SA/PEI-CoxOy-700 are in a relative high level in comparation with SA/N-CoxOy-600 and SA/N-CoxOy-800. It is attributed to the complete formation of nitrogen/cobalt co-doped carbon framework, and due to the optimal carbonization

process.

Significantly,

the

impendence

matching

ratio

of

SA/N-CoxOy-700 even reaches to 1, which means that the microwave was more efficiently absorbed by SA/N-CoxOy-700 than reflected on the air/absorber interface. This phenomenon is ascribed to the synergistic effect of the formed “core-shell”

structure, various defects, N-M (M represents cobalt metal, cobalt oxides nanoparticles) bonds, and C-N bonds. In additional, attenuation constant is the symbol of the attenuation ability and energy conversion capacity of the microwave absorbing materials. As depicted in Fig. 7b, all the samples have high attenuation constant spanning from 50 to 250, illuminating the efficient energy conversion ability and outstanding absorbed performance of the sodium alginate-based materials. Obviously, the tendency and fluctuation of attenuation constants curves are identical with the electromagnetic parameters (ε and µ) of these electromagnetic wave absorbers, again manifesting the absorption mechanism. As demonstrated as eqn (4), dielectric relaxation, one of the most important mechanisms, can be expressed via Debye relaxation theory[48]. (εs– ε∞)2= (ε' – ε∞)2+ (ε'')2

(4)

Among, εs is the static dielectric constant and ε͚ means the dielectric constant at extremely-high frequency. Thus, if use ε' and ε'' as abscissa and ordinate, respectively, namely, exploring the relationship between ε' and ε'' would attain some Cole-Cole semicircles which represent Debye relaxation process. It is not difficult to find that there are some Cole-Cole semicircles in SA/N-CoxOy-700, showing in Fig. 8a. It comes from the dipole polarization from the intrinsic defects of absorbing materials as well as comes from cobalt oxides/nitrogen-doped carbon skeleton interfacial polarization. Moreover, the conductive loss also play an important role in dielectric loss corresponding to the formation of conductive network.[23] However, these semicircles are not obvious, which means polarization loss was hidden by the conductivity loss.[49, 50] These relaxations are mainly attributed to Maxwell-Wagner relaxation, dielectric relaxation, and electron polarization, conductive loss. They are the results of the existence of heterogeneous interfaces between structure defects,

cobalt oxides, nitrogen and carbon skeleton as well as void and solid with different dielectric constants and conductivities. These would lead to the generation of polarization centers and dipoles, hence result in the enhancement of electron polarization relaxation. Similarly, as displayed in Fig. 8b, the magnetic loss mechanism is also researched. The eddy-current effect (Co) can be summarized as following[18, 29]. Co = µ''(µ')-2ƒ-1

(5)

Arguably, as illumined as eqn (5) and in Fig. 8b, the electromagnetic parameters of all the samples are in a high level which can be up to 0.57 for SA/N-CoxOy-700 and 0.083 for others, manifesting the relative excellent performance of magnetic loss. However, all curves are not straight lines, and the values of Co have been changed in some positions. This is because both nature resonance and eddy-current effect are involved in the absorption of EMW. Especially, dimensional resonance, domain wall resonance and hysteresis loss may also contribute some efforts in the improvement of absorption efficiency. Additionally, it can be found that the fluctuations positions of different absorbers agree with their permeability curves which are exhibited in Fig. 6c and Fig. 6d. As electromagnetic wave has waves properties, besides the dielectric loss and magnetic loss, the microwave also can be attenuated with each other by interference effect, leading to the improvement of attenuation. Nevertheless, only under certain conditions, can the interference effect occur. The mathematic equation can be described by eqn (6)[4, 35]. dm = nλ/4 = nϲ/[4ƒm√(|εr||µr|)]

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

(6)

The eqn (6) is also called l/4 λ matching model, in which, dm means the absorber thickness, and λ represents the microwave wavelength, meanwhile, c symbols the

lightspeed, and fm is the microwave frequency. As can be seen from Fig. 8c, the curve of experimental thickness (

) corresponds to l/4 λ model, and the optimal

absorption positions basically locate at the corresponding thickness. This is ascribed to the changed phase between the incident and reflected microwave caused by the “core-shell” structure, result in the interference loss of the two kinds of microwaves. Commonly, the usual frequency of used electromagnetic waves can be divided into S band (2-4 GHz), C band (4-8 GHz), X band (8–12 GHz) and Ku band (12–18 GHz), corresponding to military radar systems, satellite and weather communications. Fig. 8d is the 3D RL image of SA/N-CoxOy-700. Clearly, almost all the electromagnetic waves with different frequency can be absorbed and attenuated by SA/N-CoxOy-700 with different thickness, respectively. meaning the widely scope applications of such kinds of materials. 3.9 Absorption mechanism To obtain a clear absorption mechanism, the whole system was modeled with the result demonstrating in Scheme 2. As mentioned in the above, due to the existence of expanded surface, multiple defects, various phase interface and hierarchal pore structures, the absorption efficiency is significantly improved. Firstly, the most waves will be absorbed in SA/N-CoxOy-700 rather than be reflected because the absorption promoting structures leads to the excellent synergy between permittivity and permeability, thus further result in a high impendence matching ratio. Afterwards, these absorbed waves will be attenuated through dipoles and interfacial polarization as well as thermal conversion. Besides, the formation of nitrogen/cobalt co-doped heterojunction and the generation of the absorption promoting nanostructures

markedly enhance the efficiency of internal multiple reflection, conductive loss and original scattering. Additionally, nature resonance, eddy-current effect, dimensional resonance, domain wall resonance and hysteresis loss may also contribute remarkable efforts in the absorption of EMW. By the way, besides the dielectric loss and magnetic loss, the microwave also can be attenuated with each other by interference effect. 4 Conclusion Herein, nitrogen/cobalt co-doped carbonaceous materials SA/N-CoxOy-X with polarization and dielectric/magnetic loss promoting nanostructures were prepared. Relative to other works on electromagnetic waves, SA/N-CoxOy-700 has a better absorption performance due to the existence of expanded surface, multiple defects, various phase interface and hierarchal pore structures. Among, its minimal RL can reach to 47.31 dB at 16.16 GHz with a thin thickness of 2 mm, and effective absorption (<10 dB) bandwidth is 4.56 GHz from 13.44 to 18 GHz. The relative superior performance is mainly associated to the cooperative effect of 3D “core-shell” nitrogen-doped carbon skeleton, and magnetic cobalt oxides, thus result in the enhancement in nature resonance, eddy-current effect, thermal conversion, dipoles and interfacial polarization. Meanwhile, SA/PEI-CoxOy-700 employing PEI as nitrogen resource also shows excellent performance, which minimal RL is 42.24 dB and has a wide effective absorption bandwidth about 7.68 GHz from 8.88 to 16.56 GHz. Given that these kinds of materials have been used to the degradation of many typical

organic

pollutants

in

advanced

oxidation

process

(AOPs),

these

semiconductor-like heterojunctions would be multifunction materials on other related fields such as supercapacitor, energy storage, and oxygen reduction electrocatalysts.

Acknowledgments Financial support from the National Natural Science Foundation of China (21676039), State Key Laboratory of Bio-Fibers and Eco-Textiles (No. 2017kfkt12), and Innovative talents in Liaoning universities and colleges (LR2017045) are kindly acknowledged. References: [1] J. Feng, Y. Hou, Y. Wang, L. Li, Synthesis of Hierarchical ZnFe2O4@SiO2@RGO Core-Shell Microspheres for Enhanced Electromagnetic Wave Absorption, ACS Appl Mater Interfaces, 9 (2017) 14103-14111. [2] W. Liu, H. Li, Q. Zeng, H. Duan, Y. Guo, X. Liu, C. Sun, H. Liu, Fabrication of ultralight three-dimensional graphene networks with strong electromagnetic wave absorption properties, Journal of Materials Chemistry A, 3 (2015) 3739-3747. [3] M.M. Rajeev Kumar, A.V. Anupama, K.P. Ramesh, Balaram Sahooa,, Synthesis, composition and spin-dynamics of FCC and HCP phases of pyrolysis derived Co-nanoparticles embedded in amorphous carbon matrix, Ceramics International, (2019) 19879–19887. [4] V.G.K. A.V. Trukhanov, L.V. Panina, V.V. Korovushkin, V.A. Turchenko, P. Thakur, A. Thakur, Y. Yang, D.A. Vinnik, E.S. Yakovenko, L. Yu Macuy, E.L. Trukhanova, S.V. Trukhanov, Control of electromagnetic properties in substituted M-type hexagonal ferrites, Journal of Alloys and Compounds, 18 (2018). [5] A.V. Trukhanov, L.V. Panina, S.V. Trukhanov, V.G. Kostishyn, V.A. Turchenko, D.A. Vinnik, T.I. Zubar, E.S. Yakovenko, L.Y. Macuy, E.L. Trukhanova, Critical influence of different diamagnetic ions on electromagnetic properties of BaFe 12 O 19, Ceramics International, 44 (2018) 13520-13529. [6] D.S.K. D.A. Vinnik, V.E. Zhivulin, A.I. Malkin, M.G. Vakhitov, S.A. Gudkova, D.M. Galimov, D.A. Zherebtsov, E.A. Trofimov, N.S. Knyazev, V.V. Atuchin, S.V. Trukhanov, A.V. Trukhanov, Electromagnetic properties of BaFe12O19:Tiat centimeter wavelengths, Journal of Alloys and Compounds, 18 (2018). [7] H.K. Choudhary, M. Manjunatha, R. Damle, K.P. Ramesh, B. Sahoo, Solvent dependent morphology and (59)Co internal field NMR study of Co-aggregates synthesized by a wet chemical method, Phys Chem Chem Phys, 20 (2018) 17739-17750. [8] H.K. Choudhary, R. Kumar, S.P. Pawar, S. Bose, B. Sahoo, Effect of Microstructure and Magnetic Properties of Ba-Pb-Hexaferrite Particles on EMI Shielding Behavior of Ba-Pb-Hexaferrite-Polyaniline-Wax Nanocomposites, Journal of Electronic Materials, (2019). [9] H.K. Choudhary, R. Kumar, S.P. Pawar, U. Sundararaj, B. Sahoo, Enhancing absorption dominated microwave shielding in Co@C-PVDF nanocomposites through improved magnetization and

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5000

Intensity(a.u.)

4000 3000

o

Co3O4

o

CoO

SA/N-CoxOy-800 C SA/N-CoxOy-700 C o

2000

SA/N-CoxOy-600 C

Co o

1000

SA/PEI-CoxOy-700 C o

0 10

SA-CoxOy-700 C 20

30

40

50

60

2Theta (degree) Fig. 1 XRD of SA-CoxOy-700, SA/PEI-CoxOy-700 and SA/N-CoxOy-X

D 8000

G SA/N-CoxOy-800oC

ID/IG=0.993

SA/N-CoxOy-700oC

Intensity(a.u.)

6000 ID/IG=0.963 4000

SA/N-CoxOy-600oC

ID/IG=0.924

2000

SA/PEI-CoxOy-700oC ID/IG=0.843

0 SA-CoxOy-700oC

ID/IG=0.801 -2000 500

1000

1500

2000

2500

Raman shift(cm-1)

Fig. 2 Raman spectra of different samples

3000

Fig. 3 SEM of (a) SA-CoxOy-700; (b) SA/PEI-CoxOy-700; (c)-(f) SA/N-CoxOy-700; and EDS mapping.

Fig. 4 TEM morphology of (a) SA-CoxOy-700; (b) SA/PEI-CoxOy-700; (c)-(e) SA/N-CoxOy-X (600, 700, 800oC) and (f) 20nm plotting scale of SA/N-CoxOy-700

5

(a)

Reflection Loss (dB)

Reflection Loss (dB)

-5 -10 -15 -20 -25 -30 -35 -40 2

1mm 1.5mm 2mm 2.5mm 3mm 3.5mm

4mm 4.5mm 5mm 5.5mm

4

8

6

10

(b)

0

0

-10 -20 1mm 1.5mm 2mm 2.5mm 3mm 3.5mm

-30 -40 -50

12

14

16

2

18

4

6

4mm 4.5mm 5mm 5.5mm 8

10

12

14

16

18

Frequency (GHz)

Frequency (GHz)

5

(c)

(d)

0

0 Reflection Loss (dB)

Reflection Loss (dB)

-5

-5 1mm 1.5mm 2mm 2.5mm 3mm 3.5mm

-10

4mm 4.5mm 5mm 5.5mm

-10 -15 -20 -25 1mm 1.5mm 2mm 2.5mm

-30 -35

-15

-40

2

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4mm 4.5mm 5mm 5.5mm

3mm 3.5mm 8

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5

(e)

0 Reflection Loss (dB)

-5 -10 -15 -20 -25 -30 1mm 1.5mm 2mm 2.5mm

-35 -40 -45 2

4

6

3mm 3.5mm 4mm 4.5mm 5mm 5.5mm 8

10

12

14

16

18

Frequency (GHz)

Fig. 5 Reflection losses of (a) SA/N-CoxOy-600; (b) SA/N-CoxOy-700; (c) SA/N-CoxOy-800; (d) SA-CoxOy-700; (e) SA/PEI-CoxOy-700

35 (a)

SA/N-CoxOy-600oC SA/PEI-CoxOy-700oC

25

SA/N-CoxOy-700oC SA-CoxOy-700oC

20

SA/N-CoxOy-800oC

SA/N-CoxOy-600oC SA-CoxOy-700oC

30 Imaginary permittivity

Real permittivity

30

(b)

35

15 10 5

25

SA/PEI-CoxOy-700oC SA/N-CoxOy-700oC

20

SA/N-CoxOy-800oC

15 10 5

0

0

2

4

6

8

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12

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18

2

4

6

8

Frequency (GHz)

SA-CoxOy-700oC SA/N-CoxOy-600oC

2.0

SA/N-CoxOy-800oC

1.5

SA/PEI-CoxOy-700oC SA/N-CoxOy-700oC

1.5 1.0 0.5

14

16

18

(d) SA/N-CoxOy-800oC SA-CoxOy-700oC

1.0

SA/PEI-CoxOy-700oC SA/N-CoxOy-700oC

SA/N-CoxOy-600oC 0.5

0.0

-0.5

2

4

6

8

10

12

14

16

18

2

4

6

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4.0

(e) SA/N-CoxOy-600oC SA-CoxOy-700oC

3.5 3.0

8

10

12

14

16

18

Frequency (GHz)

SA/N-CoxOy-700oC SA/N-CoxOy-800oC

1.6

SA/N-CoxOy-800oC SA/N-CoxOy-600oC

(f)

SA-CoxOy-700oC

1.2

SA/PEI-CoxOy-700oC tan δµ

2.5 tan δε

12

Frequency (GHz)

Imaginary permeability

Real permeability

(c) 2.5

10

2.0 1.5

0.8

SA/PEI-CoxOy-700oC SA/N-CoxOy-700oC

0.4

1.0 0.0

0.5 0.0

-0.4

2

4

6

8

10

12

Frequency (GHz)

14

16

18

2

4

6

8

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12

14

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Frequency (GHz)

Fig. 6 Electromagnetic parameters (permittivity (ε) and permeability (µ)) of (a) real permittivity (ε'); (b) imaginary permittivity(ε''); (c) real permeability (µ'); (d) real permeability (µ'') ; (e) dielectric loss tangent (tan δε); (f) magnetic loss tangent (tan δµ).

0.8

SA/N-CoxOy-800oC

0.4

0.0

(b)

SA-CoxOy-700oC

700

SA/PEI-CoxOy-700oC SA/N-CoxOy-600oC SA/N-CoxOy-700oC

Attenuation constant

Impedance matching ratio

1.2

800

(a)

SA-CoxOy-700oC

SA/PEI-CoxOy-700oC SA/N-CoxOy-600oC

600

SA/N-CoxOy-700oC SA/N-CoxOy-800oC

500 400 300 200 100 0

2

4

6

8

10

12

Frequency (GHz)

14

16

18

2

4

6

8

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Frequency (GHz)

Fig. 7 (a) the impedance matching ratio of various materials; (b) the attenuation constant of the

0.6

(a)

SA/N-CoxOy-700oC

5.5

SA-CoxOy-700oC 0.5

5.0

0.4

SA/PEI-CoxOy-700oC SA/N-CoxOy-600oC

nature resonance

4.5

SA/N-CoxOy-700oC SA/N-CoxOy-800oC

0.3

ε''

C0

4.0

0.2

3.5

eddy current effect 0.1

3.0

0.0

2.5

2

2.0 6

7

8 ε'

9

10

4

6

8

11

10

12

14

0

(d)

0 (c)

-30

1mm 1.5mm 2mm

-40 6

2.5mm 3mm 3.5mm

-17.90 -23.80 -29.70 -35.60

-24 -30

1/4λ

4

-12.00

-12 -18

dexp m

5

-6.100

-6

5mm 5.5mm

4mm 4.5mm

3 2

-41.50 -47.40

-36 -42

-5

-4

4

16

18

Fre que nc

Frequency (GHz)

-2

y (G Hz)

-1

18

14

16

12

12

10

14

8

8

6

10

4

6

2

Thic k

-3

1

-0.2000

mm)

-20

18

ness (

-10

16

Frequency (GHz)

ss (dB) Reflection Lo

Reflection Loss (dB)

5

Thickness (mm)

(b)

Fig. 8 SA/N-CoxOy-700 of (a) Cole–Cole semicircles; (b) µ''(µ')-2ƒ-1; (c) the λ/4 model; (d) 3D image of RL

Cross-Linking Scheme 1. The formation progress of SA/N-CoxOy-X.

Scheme. 2 The possible absorption mechanism of electromagnetic waves

Highlights:

SA/N-CoxOy-X (SA represents sodium alginate and X means the carbonization temperature) was fabricated via the processes of sol-gel assembly and carbonization. Selected starting materials furnished nucleation sites for cobalt oxides particles in-situ growth. The 3D core-shell structure leads to the enhancement in nature resonance, thermal conversion, and multiple reflections. The heterojunctions of semiconductor-like texture are beneficial to electron transfer and eddy-current effect, dipoles and interfacial polarization.

Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.