MoO2 heterogeneous nanobelts for wideband electromagnetic wave absorption

Journal Pre-proof Novel synthesis of MoO3/Mo4O11/MoO2 heterogeneous nanobelts for wideband electromagnetic wave absorption Longfei Lyu, Fenglong Wang, Jing Qiao, Xiuwei Ding, Xue Zhang, Dongmei Xu, Wei Liu, Jiurong Liu PII:

S0925-8388(19)34555-4

DOI:

https://doi.org/10.1016/j.jallcom.2019.153309

Reference:

JALCOM 153309

To appear in:

Journal of Alloys and Compounds

Received Date: 26 September 2019 Revised Date:

5 December 2019

Accepted Date: 6 December 2019

Please cite this article as: L. Lyu, F. Wang, J. Qiao, X. Ding, X. Zhang, D. Xu, W. Liu, J. Liu, Novel synthesis of MoO3/Mo4O11/MoO2 heterogeneous nanobelts for wideband electromagnetic wave absorption, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153309. 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. © 2019 Published by Elsevier B.V.

Longfei

Lyu: Conceptualization,

Methodology,

Software,

Formal

Investigation, Validation, Writing - Original Draft, Fenglong Wang: Writing - Review & Editing Jing Qiao: Software Xiuwei Ding: Visualization Xue Zhang: Software Dongmei Xu: Resources Wei Liu: Supervision, Project administration Jiurong Liu: Writing - Review & Editing, Supervision, Funding acquisition

analysis,

1

Novel Synthesis of

MoO3/Mo4O11/MoO2

Heterogeneous Nanobelts for

2

Wideband Electromagnetic Wave Absorption

3

Longfei Lyua, Fenglong Wanga, Jing Qiaoa, Xiuwei Dinga, Xue Zhanga, Dongmei Xub, Wei Liu b, Jiurong

4

Liua*

5

a

School of Materials Science and Engineering, Shandong University, Jinan 250061, P. R. China

6

b

State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China

7

*

8

E-mail address: [email protected] (W. Liu); [email protected] (J.R. Liu)

9

Corresponding author.

Abstract

10

The MoO3/Mo4O11/MoO2 heterogeneous nanobelts was synthesized by hydrothermal method and

11

PH3 reduction. The reduction degree of the samples depend on the calcination temperature which

12

further influence the complex permittivity of the sample. The textural properties and electromagnetic

13

wave absorption performances of MoO3/Mo4O11/MoO2 composites with various components

14

compositions were thoroughly investigated. It is found that the belt-shaped MoO3/Mo4O11/MoO2

15

composites with length of 200 nm and width of 70 nm exhibit porous surface features.

16

Electromagnetic wave absorption evaluations show that formation of heterogeneous structure could

17

significantly

18

MoO3/Mo4O11/MoO2 can reach −59.2 dB at 10.8 GHz with the thickness of 4.6 mm. The bandwidth

19

of MoO3/Mo4O11/MoO2 below −10 dB is 9.5 GHz (7.5−17 GHz). The investigations on the

20

absorption mechanism reveal that the high dielectric loss, suitable impedance matching and

21

geometric shape effect contribute to the excellent electromagnetic wave absorption. These results

improve

the

impedance

matching

1

and

the

minimum

reflection

loss

of

1

indicate that MoO3/Mo4O11/MoO2 possess potential application prospects in electromagnetic

2

pollution abatement in the future.

3

Keywords: MoO3/Mo4O11/MoO2; Electromagnetic Wave Adsorption; Hydrothermal; Interfacial

4

Polarization.

5

Introduction

6

Although the wide-spread applications of electronic devices greatly improves the life quality of

7

human beings, the electromagnetic (EM) wave pollution from these equipment brings serious threat

8

for our health [1-6]. As such, it is highly desired to explore high performance microwave absorbers

9

to reduce the electromagnetic leakage and ensure human safety [7, 8]. Many dielectric materials own

10

excellent EM wave absorption performance, such as ZnO [9], ZrO2 [10], MnO [11] Fe3O4 [12] and

11

some others. Limited by the poor relaxation polarization and the narrow effective bandwidth of

12

single materials, expanding the absorption bandwidth of the dielectric materials is the problem to be

13

solved urgently. For dielectric materials, interfacial polarization, dipole polarization and relaxation

14

polarization are playing an important role in attenuating EM waves [13-16]. If two or more dielectric

15

materials can be combined, the proper electromagnetic parameters in the composited EM absorber

16

could reduce the reflection EM wave [17-19], and on the other hand, the multicomponent material

17

with abundant interfaces and relaxation polarizations could also facilitate the absorption and

18

scattering of EM waves, which further expands the absorption bandwidth [20-23].

19

As a typical dielectric material, MoO2 has been widely used for lithium-ion batteries [24, 25],

20

supercapacitors [26, 27], adsorbents and microwave absorptions[28, 29]. Owing to good electronic

21

conductivity, MoO2 could produce lots of dielectric loss which caused by the interaction between the

22

EM waves and electric dipoles [30]. Zhang et al synthesized the C and N co-doped MoO2 fiber and 2

1

the minimum reflection loss of -40.1 dB with a thickness of 3.5 mm [29]. Ji et al synthesized 1D

2

mesoporous MoO2/C heteronanowires through an in situ facile synthesis process with minimum

3

reflection loss of -47.6 dB at 11.1 GHz and a bandwidth of 3.8 GHz [28]. These results identified the

4

direction of utilizing MoO2 for EM wave-absorbing materials is right.

5

Based on the property of high electric conductivity of MoO2, the poor impedance matching is need

6

to be solved to achieve excellent EM wave absorption performance. Under the process of hydrogen

7

reduction of MoO3, a low dielectric constant materials, the ratio of MoO3 to MoO2 can be designed

8

by controlling the calcination temperature. Meanwhile, intermediate state always coexists with the

9

raw material and the production. This can be predicted that it will be an effective way of synthesizing

10

EM wave-absorbing materials composed of multiple composites. Not only do they offer kinds of

11

interfaces to attenuate different frequency of EM wave energy, but also improving the impedance

12

matching by adjusting the ratio of low dielectric constant materials and high conductive materials.

13

Herein, we report a facile procedure for preparation of hetero-structured MoO3/Mo4O11/MoO2 as a

14

composited dielectric loss microwave absorption agent. The complex permittivity of the samples can

15

be controlled by adjusting the calcination temperature. The phase-structure in the nanocomposites

16

can be adjusted through a deoxidization process as evidenced by X-ray diffraction characterizations.

17

It is found that the MoO3/Mo4O11/MoO2 composites with size of 200 nm exhibit a belt-like shape

18

with rough surfaces. The RLmin value is up to −59.2 dB, with a matching thickness of 4.6 mm, and

19

the effective bandwidth covers 10 GHz. The strong absorption properties make this material a

20

potential alternative for wideband EM wave-absorbing materials.

21

2. Material and Methods

22

2.1. Materials 3

1

Na2MoO4·2H2O, HNO3 and NaH2PO2 were purchased from Sinopharm Chemical Reagent Co.,

2

Ltd. All of the chemicals used in this work were analytically grade and used as received.

3

2.2. Synthesis

4

2.2.1. Synthesis of MoO3 Nanobelts

5

Fig. 1 shows the fabrication process of the MoO3/Mo4O11/MoO2 [31]. Typically, 0.02 mol of

6

Na2MoO4·2H2O and 16 mL of concentrated HNO3 were dissolved in 65 mL deionized water and

7

stirred at 298 K for 2 h. Then, the precursor solution was sealed in a 100 mL Teflon-lined stainless

8

autoclave. After being hydrothermally treated at 200 °C for 24 h, the MoO3 nanobelts were obtained

9

and dried in the vacuum at 90 °C for further use. 1 g of MoO3 and 10 g of NaH2PO2 were placed at

10

two separated positions in a sealed crucible with NaH2PO2 at the upstream section of the tube

11

furnace. The MoO3 powders were calcined at 250, 350 and 450 °C under N2 atmosphere for 4 h with

12

a ramping rate of 5 °C min-1. The samples were denoted as M250, M350, M450, respectively,

13

corresponding to different calcination temperatures. The MoO3 without calcination was described as

14

M0.

15 16 17

Fig. 1 Schematic illustration of the synthesis process of MoO3/Mo4O11/MoO2. 2.2 Characterization

18

The X-ray diffraction (XRD) patterns of the studied samples were obtained on a Bruker D8

19

venture X-ray diffractometer with Cu Kα radiation. The morphologies and surface structures of the 4

1

samples were characterized on a Nova Nano SEM 230 scanning electron microscopy. The

2

microstructure of the samples was obtained on a high-resolution JEOL-2100 transmission electron

3

microscopy (HR-TEM). X-ray photoelectron spectroscopy data were collected on an ESCALAB 250

4

system with an Al Kα source. The microwave absorption properties were tested by Agilent PNA

5

N5244A vector network analyzer. The sample was pressed into a ring specimen (Φin: 3.00 mm; Φout:

6

7.03 mm) by mixing absorbing materials with paraffin matrix, and the solid content was controlled to

7

be 60%.

8

3 Results and Discussion

9

3.1. Microstructure characterization

10

The phase structures of M0, M250, M350 and M450 is revealed in their X-ray diffraction (XRD)

11

patterns (Fig. 2). For M0, the sharp diffraction peaks from molybdite (JCPDS no.05-0508) appear

12

obviously. It can be observed that the diffraction peaks appeared at 23.344°, 25.684°, 27.244°,

13

38.946°, 45.763°, 46.238°, 58.821° and 67.538° correspond to the (110), (040), (021), (060), (200),

14

(210), (081) and (0100) planes of MoO3, which confirms that the molybdenum salts have been

15

totally converted into MoO3 respectively. The diffraction peaks of M250 are quite similar to those of

16

M0 with the temperature increasing, while the peak intensity decreases. Because the reason is that

17

there is a gaseous transport phase between MoO3 and Mo4O11. First, the intermediate gaseous

18

transport phase is deposited on a nucleus of Mo4O11 and reduced to Mo4O11. Then Mo4O11 nucleus

19

grows and forms a particle. With the proceeding of the reduction, the Mo4O11 layer is formed [32].

20

As the calcination temperature increases, it is discernible that the peak shape of MoO3 diffraction

21

peaks is inconspicuous, indicating the content of MoO3 decreases and the signal of MoO2 grows up

22

slightly. A diffraction peak around 24.545° can be seen for the M350 that is related to the (60-1) 5

1

crystal plane of Mo4O11. It can also be observed in M350 three phases co-existed, namely molybdite

2

(JCPDS no.05-0508), tugarinovite (JCPDS no.32-0671) and molybdenum oxide (JCPDS

3

no.72-0447). It should be noted that MoO3 is reduced to Mo4O11, and Mo4O11 is reduced to MoO2,

4

which two reactions occur simultaneously [32]. Based on this primise, the MoO3 ,Mo4O11 and MoO2

5

are in M350 at the same time. When the calcination temperature rises to 450 °C, all MoO3 and

6

Mo4O11 diffraction peaks disappeared, indicating the complete conversion to MoO2 phase has been

7

reached. From these observations, it is clearly shown that the components in the nanocomposites

8

could be finely tuned by adjusting the calcination temperatures.

9 10

Fig. 2 XRD patterns of M0, M250, M350 and M450

11

The scanning electron microscopy (SEM) images of M0, M250, M350 and M450 exhibit the

12

morphology evolution of the obtained samples at different calcination temperatures. It can be seen

13

that the calcination temperatures exert great effects on the morphology of the composites. Fig.3a

14

displays a typical SEM image of M0. The morphology of M0 is in shape of belt with the length of

15

size ranging from 1 µm to 50 µm and a width of about 400 nm. After calcination process at 250 °C,

16

the M250 still retains the original morphology while some short belts with a length of about 800 nm 6

1

appear in the image (Fig. 3b). Upon the M0 at 350 °C, the composites are the mixture of small

2

nanoparticles around 700 nm and short belts of 6 µm in length (Fig. 3c). After high temperature

3

calcination, the M450 is primarily comprised of small nanoparticles and short belts (Fig. 3d). The

4

HR-TEM was employed to further confirm the microstructure of M350. The M350 presents the

5

shape of rod with each width of about 100 nm. Fig. 3e shows that M350 exhibited the rod-like

6

morphology agreeing well with the SEM observation. In selected area electron diffraction (SAED)

7

image of M350 in Fig. 3f implies high crystallinity. The space distances of 0.39 nm, 0.24 nm and

8

0.19 nm are corresponding to (211) plane of Mo4O11, (200) plane of MoO2 and (001) plane of MoO3.

9

The morphology progression of MoO3 to MoO2 can explain, the belt-like MoO3 is one-dimensional

10

layer structure. The layered structure connects with two Mo-O bonds along (010) and only one Mo-O

11

bond connect with the unit cell which is perpendicular to (010) direction. During the calcination

12

reduction procedure, the one Mo-O bond is easy to break and the morphology of nanobelts is

13

transferred to rod-like with the oxygen vacancies and structure disorder [33]. Finally, the

14

morphology of the sample changes from belt-like to rod-like with the increasing calcination

15

temperatures.

7

1 2

Fig. 3 The scanning electron microscope images of (a) M0, (b) M250, (c) M350 and (d) M450. (e)

3

high-resolution transmission electron microscope image and (f) the SAED image of M350.

4

X-ray photoelectron spectroscopy (XPS) analysis is also performed to explore the chemical states

5

of the elements in the M350, and the broad survey scan of the M350. Fig. 4a shows the existence of

6

Mo, O, C and P elements. The high-resolution XPS spectra of Mo 3d (Fig. 4b) can be divided into

7

two spin-orbit doublets. The doublets at 232.9 (Mo 3d5/2) and 236.1 eV (Mo 3d3/2) shows the Mo

8

3d characteristics of MoO3, while the doublets at 231.7 (Mo 3d5/2) and 234.8 eV (Mo 3d3/2) shows

9

the Mo 3d characteristics of Mo4O11 [34]. It also can be seen that the peak of Mo with the binding

10

energy at 229.5 eV revealing the existence of MoO2 [28, 35]. The peak of P 2p with the binding

11

energy at 133.6 eV can be ascribed to the remaining NaH2PO2 (Fig. 4c), indicating there is no 8

1

phosphides in M350 [36].

2 3

Fig. 4 XPS patterns of (a) survey spectra, (b) Mo 3d spectrum, (c) P 2p spectrum and (d) O1s

4

spectrum of M350.

5

3.2. Microwave absorbing properties

6

Fig. 5 shows the EM parameters of M0, M250, M350 and M450. The relative complex permittivity

7

(εr=ε'-jε'') and the relative complex permeability (µr=µ'-jµ'') directly dominate the microwave

8

absorption property [37]. The real part of permittivity (ε') represents the polarization degrees of the

9

materials which is an indicator of the capacity of storing electric energy. The imaginary part of

10

permittivity (ε'') represents the capacity of dissipating electric energy. Meanwhile, the real part of

11

permeability (µ') and the imaginary part of permeability (µ'') represent the capacity of storing

12

magnetic energy and dissipating magnetic energy respectively [38]. Fig. 5a and b exhibit ε' and ε'' of 9

1

permittivity for the samples with the proportion of 60%. The ε' value of the four samples (Fig. 5a)

2

presents a downward trending. The ε' values are 2.47−1.99 for M0, 2.48−1.97 for M250, 7.78−2.02

3

for M350 respectively. However, the values of the ε' of M450 fluctuate significantly, ranging from

4

8.37 to 1.30. Through the addition of Mo4O11 and MoO2 in MoO3, not only has the electron been

5

accumulated at the heterogeneous interface further forming the micro capacitance, but also the

6

multi-component material (M350) interacts with EM waves producing multiple scattering [39]. Both

7

of them can effectively enhance the ε' [22]. It is reported that the high ε' is good for the storage

8

capacity of the incident EM wave energy, meanwhile, the high ε'' makes that the EM wave energy

9

can be converted into heat energy efficiently by the EM wave absorber [40, 41]. The ε'' of M0 and

10

M250 are almost constant with no obvious variation, and the low values display the weak dielectric

11

loss (Fig. 5b). At the elevated temperature of 350 °C, the ε'' of M350 vary in the range of 2.51−1.23

12

with several obvious resonant peaks which is combined by polarization relaxation and conduction

13

loss [16, 42, 43]. The phenomenon that the ε' and ε'' go down as the increasing of frequency is called

14

frequency dispersion behavior, which is good for the impedance matching[44]. There are several

15

polarization such as electronic, ionic, dipole polarization and interfacial polarization in the M350.

16

Among them, the electronic and ionic polarization often occurred at THz and PHz, which can be

17

ignored at GHz, so the interfacial polarization and dipole polarization is the dominated way of

18

attenuating the EM wave energy[45-49].The co-existence of MoO3, Mo4O11 and MoO2 provide

19

additional interfaces, which could enhance the dielectric loss through interfacial polarization. Based

20

on the Debye theory, the ε' and ε'' can be expressed by the following equations [50]:

21

=

+

(1)

22

10

1 2

=

+

(2)

From the above mentioned equations, ω and τ denote the angular frequency and relaxation time

3

respectively; εs denotes the static permittivity;

denotes the conductivity and ε∞ denotes the

4

relative dielectric permittivity. Based on the Equation (1) and (2), the introducing of MoO2 with

5

relative high conductivity and complicated interfaces of M350 have enriched the polarization

6

processes of M350 which can cause the relative high ε''. According to the free electric theory,

7

correlation is significant between

8

MoO3 is deoxidized into MoO2 with great metallic-like electric conductivity. Therefore, M450 owns

9

the maximum of the complex permittivity among the four samples [28]. Besides, few resonant peaks

10

also appear in the curve of ε' of M450, suggesting the weaken polarization effect of material-wax

11

interfaces and dangling bonds. Fig. 5c and d show the values of the complex permeability of the

12

three samples, the µ' is approximately equal to one and the µ'' is about zero because of no

13

ferromagnetic components in our samples [52]. Dielectric tangent loss (tanδε=ε''/ε') and magnetic

14

loss tangent (tanδµ=µ''/µ') are always used to evaluate the contributions of dielectric relaxation and

15

the magnetic loss [53]. Fig. 6a shows different change laws of tanδε trend of the M0, M250, M350

16

and M450. It is found that the increase of annealing temperature further enhances the dielectric loss

17

through the comparison among M250, M350 and M450. Fig. 6b shows tanδµ trend of the M0, M250,

18

M350 and M450. It is illustrated that the magnetic loss can be neglected in the studied samples [54].

and ε'' [41, 51]. As the calcination temperature arrives at 450 °C,

11

1 2

Fig. 5 Complex permittivity of M0, M250, M350 and M450: (a) real part, (b) imaginary part;

3

Complex permeability of M0, M250, M350 and M450 (c) real part (d) imaginary part.

4

5 6

Fig. 6 Dielectric loss tangent (a) and magnetic loss tangent (b) of M0, M250, M350 and M450.

7

12

1 2

Fig. 7 Reflection loss curves for (a) M0, (b) M250, (c) M350 and (d) M450 with different thickness

3

in the frequency of 2−18 GHz.

4

Fig. 7 presents the reflection loss curves for M0, M250, M350 and M450 in the 2−18 GHz

5

frequency range with different thicknesses, which are obtained from Equation 3 and 4 [55-59]:

6

Z = Z ( / ) tan h[#(2π&'/()(

7

RL = 20 log|(Z − Z )/(Z + Z )|

)]

(3) (4)

8

Wherein, Zin is the input impedance of the absorber, Z0 is the impedance of free space, µr is the

9

relative complex permeability, εr is the complex permittivity, f is the frequency of the EM waves, d is

10

the thickness of the absorber, and c is the velocity of light. If the reflection loss value is less than −10

11

dB, it means that 90% of EM wave energy is converted into heat [60, 61]. The outstanding

12

performance EM absorbers have the advantages of high RLmax and wide effective absorption

13

bandwidth. Due to the low ε' and ε'', there is no efficient polarization to attenuate the EM energy, so 13

1

M0 and M250 do not have effective absorption with the number of below −10 dB (Fig. 7a and b),

2

and the M350 displays an effective bandwidth (RL ≤10 dB) of 9.5 GHz (7.5−17 GHz) with the

3

thickness of 4.62 mm (Fig. 7c). For M450, the effective bandwidth is covered 6.1 GHz (7.9−14 GHz)

4

at a thickness of 4.5 mm (Fig. 7d). The above analyses indicate the calcination temperature plays an

5

important role in governing the EM wave absorption capacity by controlling the content of the

6

samples.

7

Fig. 8 demonstrates the 2D colormap of the samples. It is obviously that the EM performance of

8

M0 and M250 is poor, the RLmax of M0 and M250 are less than −10 dB (Fig. 8a and b). The RLmax

9

value of −20.9 dB is displayed at 10.8 GHz of M450 (Fig. 8d).The M350 exhibits excellent EM

10

wave absorption, the RL value of −59.2 dB is displayed at 10.8 GHz (Fig. 8c). Through the research

11

on the EM wave absorption for the single component of MoO3 and MoO2, the MoO2 owns the better

12

EM wave absorption, which is produced by the high conductivity [28]. By controlling the calcination

13

temperature, Mo4O11 and MoO2 are formed during the calcination process. The polarization loss

14

caused by interfaces among MoO3, Mo4O11 and MoO2, and the dangling bond produced by the

15

breaking spall are the main reason for the excellent EM wave absorption performance.

14

1 2

Fig. 8 2D colormap of (a) M0, (b) M250, (c) M350 and (d) M450.

3

To obtain the excellent absorbing property, the prerequisite condition for EM wave absorbing

4

materials is that the reflected EM wave to the free space should be as little as possible in order to

5

optimize impedance matching when EM wave irradiates the surface of the materials [62]. Fig. 9

6

illustrates the impedance matching value of Zin/Z0 with a thickness of 4.5 mm. When the value of

7

Zin/Z0 of is equal to 1, there is nearly no EM wave can be reflected to the free space [3]. The samples

8

of M0 and M250 show large Zin/Z0 values, which means lots of EM waves can be reflected to the air,

9

signifying a bad impedance matching [3]. The Zin/Z0 values exhibited by M450 are much smaller

10

than that of M0 and M250, indicating the impedance matching should be modified. It is significant

11

that the M350 has the best impedance matching among the studied samples. The frequency

12

dispersion behavior which the ε' and ε'' decrease with the increase of the frequency of EM wave, is

13

beneficial for the impedance matching of the incident electromagnetic wave[44]. Compared with M0 15

1

and M250, the tendency of M350 is obvious. As the intermediate phase of MoO3 and MoO2, Mo4O11

2

resembling the structure of MoO3 and MoO2 act as a buffer layer to improve the impedance

3

matching.

4

Fig. 9 Frequency dependence of impedance match ratio (Zin/Z0) for M0, M250, M350 and M450.

5

For further understanding the polarization of the samples, the relationship between ε' and ε'' can

6 7

be expressed as follows [63-65]:

8

2

−

4

5

5

6 +(

)5 = 2

4

5

5

6

(5)

9

Where εs is the static permittivity, ε∞ is the relative dielectric permittivity. Fig. 10 shows the

10

Cole-Cole curves of the three samples in the frequency range of 2−18 GHz, and the semicircle in the

11

Cole-Cole curves of the ε' versus ε'' is named as the Cole-Cole semicircle[66]. Each semicircle

12

corresponds to one Debye relaxation process. In detail, the small-radius semicircle appearing at high

13

frequency means the electronic relaxation polarization and the big-radius semicircle appearing at low

14

frequency means the dipole relaxation polarization [67]. In addition, the radius of semicircle of

15

M350 and M450 is larger than that of M0 and M250, which means the higher calcination

16

temperature can induce an enhanced Debye relaxation [68-70]. According to the Cole-Cole curves, 16

1

the EM wave absorption mechanism can be simply explained by Debye relaxation process. By

2

comparison, M0 and M250 display some irregular semicircles in the low ε' and ε''. Compared with

3

M450, the possible explanation is the electronic relaxation polarization occurs in the pure material at

4

the low range of ε' and ε''. One possibly obvious phenomenon is that the radius of semicircles at the

5

high range of ε' and ε'' of M350 is bigger than that of the other two samples, indicating there are

6

more dipole relaxation polarization for them [70]. This phenomenon can be explained by three

7

reasons: First, the rough surface increases the interfacial area for material-wax contact; second, the

8

belt-like rods overlap with the phase interface among MoO3, Mo4O11 and MoO2 to form more

9

interfaces, so more charge is accumulated on the interface, and then the oxygen deficiency,

10

dislocation and dangling bonds offer amounts of polarization center which could induce multiple

11

polarization processes. Finally, a certain amount of broken rods filled in the space that is built by the

12

long rod create more path to make EM wave reflexes multiply [71].

13 14

Fig. 10 Cole-cole curves of M0, M250, M350 and M450. 17

1

The EM wave absorption performance comparison among other molybdate compound-based

2

composites showed is listed in Table 1. It also can be observed from Table 1 directly. It can be

3

concluded that the M350 composite could be a broadband EM wave absorbing material.

4

Table 1 EM adsorption properties of molybdate compound-based composites in publications Loading Reflection Effective Sample Ref. fraction (wt %) loss (dB) bandwidth (GHz) Cr-doped MoS2 20 –43.00 7.20 [72] MoS2/rGO 20 –55.00 4.56 [73] Ni/MoS2 60 –55.00 4.00 [74] MoS2/C 30 –44.67 3.32 [75] C, N doped 20 –40.10 4.20 [29] MoO2 MoO2/C 25 –47.60 3.80 [28] PANI/MoO3 10 –33.70 3.00 [76] This M350 60 –59.20 9.50 work 5

Conclusions

6

In conclusion, MoO3/Mo4O11/MoO2 heterogeneous structure was synthesized successfully

7

through a solvothermal and heat treatment method. The complex permittivity of the samples is

8

improved with the increase of calcination temperature on account of the introduction of MoO2. The

9

introduction of Mo4O11 and MoO2 provides lots of interfaces to attenuate the EM wave and improves

10

the impedance matching results greatly. The produced MoO3/Mo4O11/MoO2 exhibits a belt-like

11

structure. The result shows that M350 demonstrates superior EM wave absorption abilities. The

12

minimum RL value could reach –59.2 dBwith a matching thickness of 4.6 mm, and the effective

13

bandwidth covers 10 GHz. The structure offers possible path for the multiple reflection of EM wave.

14

This work provides a potential route to design of wide-band and strong absorption EM wave 18

1

absorbers.

2

Acknowledgements

3

This work was supported by the National Natural Science Foundation of China (No. 51572157) and

4

the Natural Science Foundation of Shandong Province (ZR2016BM16). WFL is grateful for the

5

financial support from the Qilu Young Scholar Program of Shandong University (No.

6

31370088963043) and the Fundamental Research Funds of Shandong University (2018JC046).

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1) Preparing multicomponent composite from single component by controlling the calcination temperature. 2) Electromagnetic parameters is influenced by the reduction degree. 3) The buffer layer (Mo4O11) enhances the impedance matching.

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: