Composites Part B 178 (2019) 107518

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Composites Part B
journal homepage: www.elsevier.com/locate/compositesb

Fabrication of nitrogen-doped cobalt oxide/cobalt/carbon nanocomposites

derived from heterobimetallic zeolitic imidazolate frameworks with
superior microwave absorption properties
Ruiwen Shu a, b, *, Weijie Li a, Yue Wu a, Jiabin Zhang a, Gengyuan Zhang a, Mingdong Zheng a, **
a
School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China
b
School of Earth and Environment, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China

A R T I C L E I N F O A B S T R A C T

Keywords: Nowadays, developing novel microwave absorbers with thin matching thickness, strong absorption intensity,
Zeolitic imidazolate frameworks broad absorption bandwidth and low filler loading is of great importance for solving the increasingly serious
Nanocomposites problem of electromagnetic pollution. Herein, nitrogen-doped cobalt oxide/cobalt/carbon (CoO/Co/C) nano­
Nitrogen doping
composites were fabricated by high-temperature pyrolysis of heterobimetallic zeolitic imidazolate frameworks
Microwave absorption
High-temperature pyrolysis
(Co/Zn-ZIFs). Results demonstrated that the micromorphology and magnetic properties of as-prepared nano­
composites could be regulated by changing the molar ratios of Co to Zn in the ZIFs precursors. Moreover, the
effects of molar ratios of Co to Zn and filler loadings on the microwave absorption properties of obtained
nanocomposites were systematically investigated in the frequency range of 2–18 GHz. Remarkably, the as-
prepared nanocomposites with a molar ratio of Co to Zn of 1:1 exhibited the best microwave absorption prop­
erties. The optimal minimum reflection loss reached 66.7 dB at 7.2 GHz with a thickness of 3.3 mm and
effective absorption bandwidth (EAB) achieved 5.1 GHz (12.6–17.7 GHz) covering most of Ku-band for an ul­
trathin thickness of 1.8 mm and a low filler loading of 25 wt%. Furthermore, the EAB could reach 14.16 GHz
(88.5% of 2–18 GHz) by facilely modulating the thicknesses from 1.5 to 5 mm, which spanned the whole C, X and
Ku bands. In addition, the underlying microwave absorption mechanisms were carefully investigated and further
proposed. Therefore, our results could be helpful for designing and fabricating the magnetic nanoparticles/
carbon nanocomposites derived from ZIFs as lightweight and high-efficient microwave absorbers.

1. Introduction facile preparation, low density, good chemical stability and strong
dielectric loss [13–27]. However, single RGO or MWCNTs used as MAMs
With the increasingly serious problem of electromagnetic pollution suffers from inferior impedance matching and poor microwave attenu­
derived from the wide usage of electronic equipment, microwave ation [19–27]. Thus, it is very urgent to improve impedance matching
absorbing materials (MAMs) have gained great attentions in the field of and enhance microwave absorption performance of carbon materials for
functional materials [1–6]. Generally, strong absorption intensity, broad dealing with the growing problem of electromagnetic pollution.
absorption bandwidth, thin matching thickness and light weight are four Recently, carbon-based composites derived from high-temperature
key factors that need to be considered for designing ideal microwave pyrolysis of metal–organic frameworks (MOFs) have shown great po­
absorbers [7–12]. Therefore, tremendous efforts have been focused on tentials as the lightweight and high-efficient microwave absorbers
designing and fabricating the high-performance MAMs in the past few [28–47]. Especially, magnetic metals/carbon composites derived from
decades [1–12]. MOFs exhibited superior microwave absorption performance owing to
Among these materials, carbon materials such as reduced graphene the dual attenuation loss mechanisms (magnetic loss and dielectric loss)
oxide (RGO) and multi-walled carbon nanotubes (MWCNTs) have drawn and enhanced interfacial polarization inducing by multiply heteroge­
significant attentions due to the advantages such as abundant sources, neous interfaces among magnetic metals, carbon and paraffin wax

* Corresponding author. School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China.
** Corresponding author.
E-mail addresses: rwshu@aust.edu.cn (R. Shu), mdzheng@aust.edu.cn (M. Zheng).

https://doi.org/10.1016/j.compositesb.2019.107518
Received 4 May 2019; Received in revised form 4 October 2019; Accepted 6 October 2019
Available online 7 October 2019
1359-8368/© 2019 Elsevier Ltd. All rights reserved.
R. Shu et al. Composites Part B 178 (2019) 107518

Fig. 1. Schematic illustration of the preparation procedures of CoO/Co/C nanocomposites.

Fig. 2. (a) XRD patterns of the samples of S1, S2, S3 and S4; XPS spectra: (b) wide scan, (c) C 1s, (d) N 1s, (e) Co 2p and (f) Zn 2p of the sample of S2.

matrix [28–36,38–47]. For instance, Qiang et al. fabricated Fe/C composites showed the RLmin of 35.3 dB and effective absorption
nanocubes through an in situ high-temperature pyrolysis route from bandwidth (EAB, RL � 10 dB) of 5.8 GHz with a d of 2.5 mm and φw of
Prussian blue (PB) precursor. The obtained Fe/C nanocubes/paraffin 60 wt% [30]. Liu et al. synthesized the rod-like Ni/C composites through
wax composites exhibited enhanced microwave absorption performance high-temperature pyrolysis of Ni-based MOFs precursors. The obtained
with the minimum reflection loss (RLmin) of 22.6 dB at 15 GHz with a Ni/C/paraffin wax composites exhibited the RLmin of 51.8 dB and EAB
thickness (d) of 2 mm and filler loading (φw) of 40 wt% [29]. Lü et al. of 3.48 GHz with a d of 2.6 mm and φw of 40 wt% [31]. Yin et al. re­
prepared Co/C nanocomposites by high-temperature pyrolysis of ported the synthesis and microwave absorption properties of magneti­
Co-based MOFs precursors. The as-prepared Co/C/paraffin wax cally aligned Co–C/MWCNTs composite derived from

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R. Shu et al. Composites Part B 178 (2019) 107518

Fig. 3. SEM images with different magnifications: (a) (c) S1, (d) (f) S2, (g) (i) S3 and (j) (l) S4.

Fig. 4. (a) SEM image and the corresponding EDS mapping images of (b) C, (c) N, (d) O, (e) Co and (f) Zn for the sample of S2.

MWCNT-Interconnected zeolitic imidazolate frameworks (ZIFs). The meaningful for clarifying the influence of filler loadings on the micro­
obtained composite showed strong microwave absorption with the wave absorption performance of magnetic metals/carbon composites
RLmin of 48.9 dB at 9.12 GHz for a d of 2.99 mm and φw of 15 wt% [33]. derived from MOFs.
However, the design and development of magnetic metals/carbon It was well-documented that the bimetallic MOFs displayed diversely
composites derived from MOFs with comprehensively superior micro­ structural topologies owing to variable metal centers and ligand struc­
wave absorption performance including strong absorption intensity ture, and adjustable electromagnetic parameters compared with that of
(RLmin < 60 dB), broad absorption bandwidth (EAB > 5 GHz) and thin monometallic MOFs [34,35,38,42,45]. In our recent work, we fabricated
matching thickness (d < 2 mm) at a low filler loading (φw < 30 wt%) still the nitrogen-doped Co–C/MWCNTs composites derived from Co/Zn
remain a great challenge. Furthermore, recent investigations revealed bimetallic ZIFs by a facile high-temperature pyrolysis strategy [45]. The
that the filler loadings could significantly influence the electromagnetic as-prepared Co–C/MWCNTs/paraffin wax composites exhibited excel­
parameters (ε0 , ε’’, μ0 , μ’’) and microwave absorption properties of lent microwave absorption properties with the RLmin of 50 dB and EAB
MAMs according to the percolation theory [32,48–50]. Therefore, it is of 4.3 GHz with a filler loading of 25 wt%. Furthermore, the results

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R. Shu et al. Composites Part B 178 (2019) 107518

Fig. 5. TEM images with different magnifications: (a)–(c), HRTEM image (d), and particle size distribution histogram (e) of the sample of S2.

revealed that Co/Zn bimetallic ZIFs derived nanocomposites showed microwave absorption properties with strong absorption, broad band­
remarkably enhanced microwave absorption properties than that of the width, thin thickness and low filler loading, which could be used as
Co monometallic ZIFs (ZIF-67) and Zn monometallic ZIFs (ZIF-8) potential candidates in the field of electromagnetic absorption. Besides,
derived nanocomposites [45]. the underlying microwave absorption mechanisms of obtained nano­
Herein, we fabricated nitrogen-doped CoO/Co/C nanocomposites composites were clarified.
derived from Co/Zn heterobimetallic ZIFs by a facile high-temperature
pyrolysis strategy. Various technologies were adopted to explore the 2. Experimental
relationship among structure, compositions, magnetic properties and
microwave absorption properties of as-prepared nanocomposites. The detailed preparation and characterization sections of nitrogen-
Moreover, the influence of molar ratios of Co to Zn in the ZIFs precursors doped CoO/Co/C nanocomposites could be found in the electronic
and filler loadings on the microwave absorption properties of obtained supplementary materials. For simplicity, the obtained nanocomposites
nanocomposites was systematically investigated. Results demonstrated with different molar ratios of Co to Zn in the precursors were labeled as
that the as-prepared nanocomposites showed comprehensively superior S1 (12:0), S2 (1:1), S3 (1:2) and S4 (0:12), respectively.

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R. Shu et al. Composites Part B 178 (2019) 107518

(S1) exhibits a shrinkage of rhombic dodecahedral morphology with

concave surfaces derived from the thermal decomposition of 2-methyli­
midazole during the high-temperature pyrolysis process [33,45]. As
shown in Fig. 3(d)–(f), Co/Zn bimetallic ZIFs derived carbon nano­
composite (S2 with a molar ratio of Co to Zn of 1:1 in the precursor)
presents similar rhombic dodecahedral morphology as the S1. However,
numerously short carbon nanotubes (CNTs) grown in situ on the rough
surfaces of carbon frameworks can be clearly observed. This phenome­
non could be explained as follows: Firstly, the decomposition of 2-meth­
ylimidazole during the high-temperature pyrolysis process leads to the
formation of carbon matrix. Subsequently, some of the amorphous car­
bon can be transformed into the graphitization carbon, i.e. CNTs by the
Co catalysis graphitization process. Similar results could be found in our
recent work and other literatures [34,45,47]. Therefore, the growth of
conductive CNTs is helpful for enhancing the electric conductivity and
thus conduction loss of as-prepared nanocomposite under the alter­
Fig. 6. Magnetic hysteresis loops of the samples of S1, S2, S3 and S4 at room
temperature. Inset: the magnified magnetization curves at the low field. nating electromagnetic fields. With further decrease the molar ratio of
Co to Zn (1:2), the sample of S3 shows a dodecahedral shape with
obvious aggregation, and the amounts of in situ formed CNTs obviously
Fig. 1 describes the schematic preparation procedures of CoO/Co/C
decrease in comparison with S2 (Fig. 3(g)–(i)). From Fig. 3(j)‒(l), it is
nanocomposites. Firstly, the Co/Zn bimetallic ZIFs precursors were
clear that the Zn ZIFs derived carbon nanocomposite (S4) displays a
synthesized by a room temperature coordination reaction among Co2þ/
dodecahedral morphology with smooth surfaces. Therefore, the molar
Zn2þ and 2-methylimidazole in a mixed solution of methanol/ethanol.
ratios of Co to Zn in the ZIFs precursors can notably influence the
Subsequently, the as-prepared heterobimetallic ZIFs precursors were
micromorphology and microwave absorption properties of obtained
converted into CoO/Co/C nanocomposites by the high-temperature
nanocomposites. As displayed in Fig. S1, the EDS pattern reveals that the
pyrolysis reactions under argon gas atmosphere.
existence of Co, Zn, N, O and C elements in the sample of S2, which is in
good accordance with the results of XPS analysis.
3. Results and discussion
As shown in Table S2, the weight percentage of N elements in the
sample of S2 is 13.91 wt%, which suggests that a large number of N
3.1. Structural analysis
elements derived from the decomposition of 2-methylimidazole are
doped into the carbon matrix. Previous investigations demonstrated that
X-ray diffraction (XRD) was used to reveal the crystalline structure of
the N elements could be easily doped into the carbon matrix under high-
as-prepared nanocomposites. As shown in Fig. 2(a), it can be seen a
temperature environment [32,33].
broad diffraction peak appearing at around 23.6� in the sample of S4,
Fig. 4 shows the SEM image and the corresponding EDS mapping
which is the characteristic of amorphous carbon [34]. The diffraction
images, which illustrate the existence of C, N, O, Co and Zn elements
peaks appearing at 2θ ¼ 61.4, 42.4 and 36.5� are in good accordance
with a fine homogeneous distribution in the sample of S2. The uniform
with the (220), (200) and (111) crystal planes of CoO (JCPDS No.
dispersion of elements is helpful for improving the microwave absorp­
65-2902), respectively [51]. The other three diffraction peaks appearing
tion properties of as-prepared nitrogen-doped CoO/Co/C
at 2θ ¼ 76.0, 51.6 and 44.3� can be ascribed to the (220), (200) and
nanocomposite.
(111) crystal planes of Co (JCPDS No. 15-0806), respectively [46,51].
The micromorphology and structure of the sample of S2 were further
However, it is difficult to distinguish the diffraction peaks associated
characterized by transmission electron microscopy (TEM), as depicted in
with Zn species in the samples of S2, S3 and S4. Recent investigations
Fig. 5. From Fig. 5(a) (c), the S2 shows a porous structure with
revealed that most of Zn species could evaporate during the
numerous nanoparticles uniformly encapsulated in the carbon matrix.
high-temperature pyrolysis process as the pyrolysis temperature was
Notably, some short CNTs can also be clearly observed, which is
above 600 � C [34,45]. Thus, the absence of diffraction peaks associated
consistent with the results of SEM observations in Fig. 3(f) and (i). As
with Zn species in this work could be attributed to the evaporation of Zn
shown in Fig. 5(d), the high-resolution transmission electron microscopy
species during the high-temperature pyrolysis process at 700 � C.
(HRTEM) image manifests that the inter-plane distances are 0.20, 0.21
X-ray photoelectron spectroscopy (XPS) was used to analyze the
and 0.33 nm, which correspond to the (111), (200) and (002) crystal
surface chemical compositions and valence states of the sample of S2.
planes of Co, CoO and graphitization carbon, respectively. Fig. 5(e)
From Fig. 2(b), the total spectrum reveals that the S2 contains Co, Zn, N,
demonstrates that the obtained nanoparticles show a small statistical
O and C elements. As depicted in Fig. 2(c), the C 1s spectra are fitted into
average size of 7.79 nm.
C–OH, C– – O, C–N and C–C/C– – C bonds, respectively [45,47]. As shown
in Fig. 2(d), the N 1s spectra could be split into four kinds of N (graphitic
3.3. Magnetic properties
N, pyridinic N, pyrrolic N and oxidized N) [45,47], which suggests that
the N atoms have been successfully doped into the carbon matrix. From
The magnetic hysteresis loops of the samples of S1, S2, S3 and S4
Fig. 2(e), the binding energy values of Co 2p3/2 are located at 780.7 and
were acquired by a vibrating sample magnetometer (VSM) at room
778.9 eV, which could be corresponded to CoO and Co respectively [51].
temperature. As displayed in the inset of Fig. 6, the magnified magne­
As described in Fig. 2(f), Zn 2p1/2 and Zn 2p3/2 spectra correspond to the
tization curves clearly reveal that the S1, S2 and S3 present typically
peaks of 1045.1 and 1022.0 eV, respectively [45]. Thus, it should be
ferromagnetic behaviors, which could generate notable magnetic loss in
noted the XPS results reveal the existence of Zn as Zn2þ in the S2.
the gigahertz frequency region [32,33]. Specifically, the saturation
magnetization (Ms) of S1, S2 and S3 are 40.8, 22.7 and 8.3 emu/g,
3.2. Morphological analysis respectively. However, the magnetization of S4 almost approximates to
0 emu/g, which indicates its essentially non-magnetic characteristic.
Scanning electron microscopy (SEM) was used to observe the Therefore, the as-prepared nanocomposites exhibit remarkably
micromorphology of the samples of S1, S2, S3 and S4. From Fig. 3(a)– decreased Ms with the decreasing of the molar ratios of Co to Zn. It
(c), it can be observed that the Co ZIFs derived carbon nanocomposite should be noted that the ferromagnetism of S1, S2 and S3 mainly

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R. Shu et al. Composites Part B 178 (2019) 107518

Fig. 7. Frequency dependence of reflection loss with different thicknesses and 3D plots of reflection loss for various filler loadings: (a) and (a’) of 5 wt%, (b) and (b’)
of 10 wt%, (c) and (c’) of 15 wt%, (d) and (d’) of 25 wt%; (e) Reflection loss as a function of frequency with different filler loadings at a fixed thickness of 2.5 mm for
the sample of S2.

originates from the Co nanoparticles [52]. The CoO constituent shows 14.16 GHz (88.5% of 2–18 GHz) by facilely modulating the matching
the antiferromagnetic behavior, which contributes to little magnetic loss thicknesses from 1.5 to 5 mm, which spans the whole C, X and Ku bands.
for microwave absorption [52,53]. Fig. 7(a’)‒(d’) display the three-dimensional (3D) plots of reflection loss
for S2 with different filler loadings. Remarkably, the RLmin corre­
sponding to the maximum microwave absorption could locate at various
3.4. Microwave absorption properties frequencies by modulating the thicknesses of absorbers [25,45]. To
intuitively compare the microwave absorption performance of S2 with
As mentioned above, the filler loading has great influence on the different filler loadings, the frequency-dependent reflection loss curves
microwave absorption properties of MAMs [32,48–50]. Therefore, we were plotted, as shown in Fig. 7(e). It is obvious that the microwave
have investigated the effect of filler loadings on the microwave ab­ absorption intensity of S2 notably enhances with the increasing of filler
sorption properties of the sample of S2. As described in Fig. 7(a)–(d), the loadings.
values of RLmin for S2 achieve 4.0, 8.6, 12.6 and 66.7 dB as the For the sake of exploring the influence of molar ratios of Co to Zn on
filler loadings are 5, 10, 15 and 25 wt%, respectively. Significantly, the the microwave absorption properties of as-prepared nanocomposites,
S2 exhibits the RLmin of 66.7 dB at 7.2 GHz with a matching thickness the fill loading was fixed at 25 wt%. As shown in Fig. 8, the values of
of 3.3 mm and EAB of 5.1 GHz (12.6–17.7 GHz) with a thin thickness of RLmin are 11.0, 43.4, 18.4 and 4.3 dB for the samples of S1, S2, S3
merely 1.8 mm and filler loading of 25 wt%. Furthermore, the RLmin less and S4, respectively. It can be found that the |RLmin| firstly increases and
than 20 dB could be achieved almost in the whole investigated then decreases with the decreasing of molar ratios of Co to Zn. Thus, the
thicknesses range (from 1.5 to 5.0 mm). Besides, the EAB can reach

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R. Shu et al. Composites Part B 178 (2019) 107518

Fig. 8. Frequency dependence of reflection loss with different thicknesses for the samples of S1, S2, S3 and S4.

molar ratios of Co to Zn should be carefully regulated for achieving increasing. Besides, some resonance peaks of μ’’ [32,45] could be
superior microwave absorption properties. observed in the range of 2.5–5.0 GHz for all the samples.
Generally, the electromagnetic parameters (ε0 , ε’’, μ0 , μ’’) are vitally Attenuation loss includes dielectric loss and magnetic loss, which
important to determine the microwave absorption properties of ab­ plays a vital role for microwave attenuation [32,45]. Thus, the
sorbers [24,26,45]. The real permittivity (ε0 ) and real permeability (μ0 ) frequency-dependent magnetic loss tangent (tanδm ¼ μ’’/μ0 ) and
represent the storage ability of electric and magnetic field energy, dielectric loss tangent (tanδe ¼ ε’’/ε0 ) for the four samples were inves­
whereas the imaginary permittivity (ε’’) and imaginary permeability tigated. From Fig. 9(e), the tanδe firstly enhances and then decreases
(μ’’) indicate the dissipation capacity of electric and magnetic field with the decreasing of molar ratios of Co to Zn. Moreover, the S2
energy, respectively [32]. Thus, the frequency dependence of electro­ generally presents the biggest tanδe among all the samples, which sug­
magnetic parameters for the samples of S1, S2, S3 and S4 was carefully gests the strongest dielectric loss against the incident microwaves. It
investigated, as depicted in Fig. 9(a)–(d). From Fig. 9(a), the ε0 of S1, S2 should be noted that the obviously enhanced dielectric loss could be
and S3 shows a declining trend as the increasing of frequency and re­ attributed to the notably increased ε0 and ε’’ of S2. As described in Fig. 9
veals a frequency dispersion effect, which is beneficial to the microwave (f), the tanδm displays a similar tendency as μ’’ with the increasing of
absorption [54]. Specifically, the ε0 decreases from 5.0 to 3.7, 12.2 to frequency for all the samples. Especially, the S2 presents the biggest
7.9, 9.0 to 7.0 for S1, S2 and S3, respectively. However, the ε0 of S4 tanδm of 0.20 at 2.6 GHz, which indicates the strongest magnetic loss
almost keep a constant value around 3.0 with the increasing of fre­ capacity. Besides, combined Fig. 9(e) with Fig. 9(f), it can be found that
quency. As shown in Fig. 9(b), the ε’’ of S1 and S3 presents a similar both dielectric loss and magnetic loss play important roles for the mi­
decline trend as the ε0 with the increasing of frequency. Furthermore, the crowave attenuation.
ε’’ of S2 shows obvious fluctuations with the increasing of frequency, � 0 ε ε∞ �2 �ε ε∞ �2
while the ε’’ of S4 almost keep constant in the whole frequency range. It (1)
s s
ε þ ðε00 Þ2 ¼
2 2
should be noted that both ε0 and ε’’ firstly enhance and then decrease
with the decreasing of molar ratios of Co to Zn. On the basis of free Herein εs, ε∞, ε0 and ε’’ are the static permittivity, relative dielectric
electron theory, the ε’’ enhances with the increasing of electric con­ permittivity at high-frequency limit, real part and imaginary part of
ductivity [45,55,56]. The in situ formed short CNTs with good electric permittivity, respectively [32,33,35]. Based on equation (1), the curve
conductivity on the surfaces of carbon frameworks in the samples of S2 of (ε00 ~ε0 ) should be a single semicircle, which is known as Cole-Cole
and S3 (as shown in Fig. 3(f) and (i)), which lead to the enhancement of semicircle [32,33,35]. Each semicircle represents a Debye relaxation
ε’’ in comparison with S1 and S4. Besides, the S2 shows the largest process [32,33,35].
amounts of in situ formed CNTs, which causes the strongest electric Fig. 10(a)–(d) describe the Cole-Cole plots of the samples of S1, S2,
conductivity. As a result, the S2 displays the biggest ε’’ among all the S3 and S4. It can be seen that the S4 exhibits an obvious Cole-Cole
samples. Numerous investigations demonstrated that ε0 kept pace with semicircle, which manifests only one Debye relaxation process. How­
the variation in electric conductivity and there existed identical change ever, the semicircles in the S1, S2 and S3 are distorted in some degree,
rules about ε0 and ε’’ [54,57,58]. Therefore, the S2 exhibits the biggest ε0 which suggests that other mechanism such as conductive loss make more
and ε’’, which manifests the enhanced storage capability of electric contribution to the overall dielectric loss than Debye polarization [32].
energy and dielectric loss [54,59]. From Fig. 9(c), the μ0 displays obvious Generally, eddy current loss and natural resonance are the main
fluctuations with the increasing of frequency and the μ0 values are in the reasons for magnetic loss in the microwave frequency range [32,35,45].
range of 1.0–1.3 for all the samples. As described in Fig. 9(d), the μ’’ of The eddy current loss can be evaluated by the following equations [32,
all the samples generally presents a decreasing trend as the frequency 35,45]:

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R. Shu et al. Composites Part B 178 (2019) 107518

Fig. 9. Frequency dependence of (a) ε0 , (b) ε’’, (c) μ0 , (d) μ’’, (e) tanδe and (f) tanδm for the samples of S1, S2, S3 and S4. According to Debye theory, the ε0 and ε’’
follow the equation [32,33,35].

.
μ’’ � 2πμ0 ðμ’ Þ2 σd2 f 3 (2) � �
�Zin �
�rffiffiffiffi
� μr
�� � ��
2πfd pffiffiffiffiffiffiffi ��
Z ¼ �� �� ¼ ​ �� tanh j μr εr � (4)
Z0 εr c
’’
C0 ​ ¼ ​ μ ðμ Þ f’ 2 1
(3)
Herein Zin and Z0 are the input impedance and free space, respec­
Herein C0 is eddy current coefficient, σ is the electric conductivity, tively. When the value of Z is equal to 1, the optimal impedance
μ0 is the vacuum permeability and d is the thickness of absorber. If matching can be achieved, which indicates that the incident microwaves
magnetic loss exclusively results from eddy current effect, the values of can totally enter into the inner of the absorber [35,45]. As to a practical
C0 should keep constant as the frequency increasing [32,35,45]. How­ absorber, the value of Z cannot reach 1. Thus, the value of Z is closer to 1,
ever, the curves of C0 ~ f for all the samples in Fig. 10(e) display notable suggesting the better impedance matching [35,45].
fluctuations with the increasing of frequency, thus excluding the eddy Electromagnetic attenuation capacity is often reflected by the
current loss [32]. For all the samples, an obvious change of C0 with the attenuation constant (α), which can be expressed as follows [24–26,45,
increasing of frequency can be observed in the frequency range of 60,61]:
2–6 GHz, which suggests that the natural resonance should be respon­ pffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiq
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
sible for magnetic loss under this situation [32,35,45]. α¼
2π f
� ðμ’’ε’’ μ’ε’Þ þ ðμ’’ε’’ μ’ε’Þ2 þ ðε’μ’’ þ ε’’μ’Þ2 (5)
In general, an ideal microwave absorber should satisfy the two re­ c
quirements of impedance matching and maximum attenuation [35,45]. For the sake of revealing the underlying mechanisms of microwave
Normalized impedance matching (Z) is often described as follows [35, absorption, the impedance matching and attenuation loss characteristics
45]: were investigated. Fig. 11 shows the Z and α of the samples of S1, S2, S3
and S4. As shown in Fig. 11(a), the values of Z for the S4 obviously
deviate from 1, which indicates inferior impedance matching.

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R. Shu et al. Composites Part B 178 (2019) 107518

Fig. 10. Cole-Cole semicircle (ε’’ versus ε0 ) curves: (a) S1, (b) S2, (c) S3 and (d) S4; Frequency dependence of C0 for the samples of S1, S2, S3 and S4.

Fig. 11. Frequency dependence of (a) normalized impedance matching (Z) at a thickness of 3.3 mm and (b) attenuation constant (α) for the samples of S1, S2, S3
and S4.

Furthermore, the values of Z for the samples of S1, S2 and S3 are much of molar ratios of Co to Zn. Remarkably, the S2 shows the largest value of
closer to 1 in comparison with S4, suggesting the improved impedance α (215.4), suggesting the strongest microwave attenuation capacity. As
matching. Notably, the values of Z for S2 is most close to 1, which re­ the optimal impedance matching is achieved, most of the incident mi­
flects the best impedance matching among all the samples. From Fig. 11 crowaves can enter into the specimen; meanwhile, the strongest mi­
(b), the values of α firstly enhance and then decrease with the decreasing crowave attenuation capacity could effectively transform the

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R. Shu et al. Composites Part B 178 (2019) 107518

45]. Therefore, it is meaningful to design the thickness of absorber ac­

cording to the quarter-wavelength matching theory. Besides, the stron­
gest RL peak (at 7.2 GHz and 3.3 mm) corresponds well with the optimal
impedance matching of Z ¼ 1 (Fig. 12(c)).
Fig. 13 describes the possible microwave absorption mechanisms of
as-prepared nitrogen-doped CoO/Co/C nanocomposites. Firstly, the
generated defects in the carbon matrix induced by nitrogen doping could
be considered as dipoles and served as polarization centers that generate
the dipole polarization relaxations under the alternating electromag­
netic field [33]. Meanwhile, the C–N bonds could also act as dipoles
owing to the different electronegativity between C and N atoms and
produce the polarization relaxations [33]. Secondly, abundantly het­
erogeneous interfaces among CNTs, porous carbon, Co and CoO nano­
particles can be considered as the capacitor-like structure [45,62]. Cao’s
model reveals that the capacitor-like structure at the heterogeneous
interfaces can damp the incident microwaves by aligning the charges or
polar bonds under the alternating electromagnetic fields [62]. Thirdly,
the CNTs grown in situ on the rough surfaces of carbon frameworks in
the as-prepared nanocomposites could notably enhance the conduction
loss, which are helpful for attenuating the incident microwaves and
transforming the electromagnetic energies into heat energies [33,34,45,
47,48]. Lastly, uniform distribution of ferromagnetic Co and antiferro­
magnetic CoO nanoparticles in the carbon matrix could notably enhance
the magnetic loss [33,34,45] and dielectric loss capacity [53],
Fig. 12. (a) Frequency dependence of reflection loss, (b) simulations of the tm respectively.
versus fm under the λ/4 model and (c) normalized impedance matching (Z) as a As shown in Table 1, we have summarized the typical carbon-based
function of frequency for the sample of S2 with a filler loading of 25 wt%. composites derived from MOFs used as microwave absorbers reported in
this work and recent literatures. It is clear that the as-prepared nitrogen-
electromagnetic energies into thermal energies [45]. As a consequence, doped CoO/Co/C nanocomposite (S2, molar ratio of Co to Zn of 1:1)
the S2 demonstrates the best microwave absorption performance among exhibits comprehensively superior microwave absorption performance
all the samples. with strong absorption, broad bandwidth and thin thickness at a low
Generally, quarter-wavelength (λ/4) matching theory is used to filler loading. Remarkably, the S2 demonstrates the optimal RLmin of
clarify the relationship between absorption peak frequency (fm) and 66.7 dB for a thickness of 3.3 mm and EAB of 5.1 GHz for an ultrathin
matching thickness (tm), which can be described by the following thickness of merely 1.8 mm at a low filler loading of 25 wt%. Therefore,
equation [24–26,45]: it can be concluded that the as-prepared nitrogen-doped CoO/Co/C
nanocomposites in this work could be used as lightweight and high-
nλ nc
tm ¼ ¼ pffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 3; 5; :::Þ (6) efficiency microwave absorbers.
4 4fm jεr μr j

If tm and fm meets the above equation, a phase cancellation effect 4. Conclusions

could effectively attenuate the incident microwaves [24–26,45].
As depicted in Fig. 12(a), it can be observed that the reflection loss In conclusion, nitrogen-doped CoO/Co/C nanocomposites were
peaks shift to lower frequency as the tm increases. Fig. 12(b) shows the successfully prepared by high-temperature pyrolysis of heterobimetallic
simulations of tm versus fm under the λ/4 model. The pentagram signifies ZIFs precursors. Results revealed that the micromorphology and mag­
the experimental tm (denoted as texp exp
m ). Significantly, all the tm are
netic properties of as-prepared nanocomposites could be regulated by
exactly located at the λ/4 curve. This finding indicates the λ/4 matching changing the molar ratios of Co to Zn in the ZIFs precursors. Further­
theory essentially determines the relationship between tm and fm [26, more, it was found that both the molar ratios of Co to Zn and filler

Fig. 13. Schematic illustration of the possible microwave absorption mechanisms of nitrogen-doped CoO/Co/C nanocomposites.

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R. Shu et al. Composites Part B 178 (2019) 107518

Table 1 Research Foundation of Anhui University of Science and Technology

Typical carbon-based composites derived from MOFs used as microwave ab­ (Grant No. ZY537).
sorbers reported in this work and recent literatures.
Samples Matrix Loading RLmin EAB Ref. Appendix A. Supplementary data
(wt%) (dB) (thickness/
mm)
Supplementary data to this article can be found online at https://doi.
S2 Paraffin 25 66.7 5.1 GHz This org/10.1016/j.compositesb.2019.107518.
(1.8 mm) work
Fe–Co/NPC Paraffin 50 21.7 5.8 GHz [28]
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