Tuning the Electromagnetic Synergistic Effects for Enhanced

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Tuning the Electromagnetic Synergistic Effects for Enhanced Microwave Absorption Via Magnetic Nickel Core Encapsulated in Hydrogenated Anatase TiO2 Shell Jianle Xu, Xiaosi Qi, Yuan Sun, Zhongchi Wang, Yong Liu, Chengzhi Luo, Bingjie Li, Wei Zhong, Qiang Fu, and Chunxu Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02350 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Sustainable Chemistry & Engineering

Tuning the Electromagnetic Synergistic Effects for Enhanced Microwave Absorption Via Magnetic Nickel Core Encapsulated in Hydrogenated Anatase TiO2 Shell Jianle Xu,† Xiaosi Qi,‡* Yuan Sun,§ Zhongchi Wang,† Yong Liu ,† Chengzhi Luo,† Bingjie Li,† Wei Zhong,§ Qiang Fu,†,ǁ and Chunxu Pan†,ǁ* †

School of Physics and Technology, and MOE Key Laboratory of Artificial Micro-

and Nano-structures, Wuhan University, No. 299 Bayi Road, Wuchang District, Wuhan, Hubei Province, China 430072 ‡

College of Physics, Guizhou University, Huaxi District, Guiyang, Guizhou Province,

No. 2708 Huaxidadao Road, China 550025. §

College of Physics, Nanjing University, Gulou District, Nanjing, Jiangsu Province,

No. 22 Hankou Road, China 210093. ǁ

Center for Electron Microscopy, Wuhan University, No. 299 Bayi Road, Wuchang

District, Wuhan, Hubei Province, China 430072

*Author to whom correspondence should be addressed. Phone: +86-27-68752481 ext. E-mail: [email protected] (C. Pan) E-mail: [email protected] (X. Qi)

ACS Paragon Plus Environment

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ABSTRACT: Owing to tremendous potential applications, hydrogenated TiO2 (H-TiO2) nanomaterial has been considered as an important member in the field of microwave absorption. In this paper, a kind of core-shell structured microspheres (MPs) consisting of outer-shell hydrogenated anatase TiO2 (H-A-TiO2) and inner-core magnetic Ni MPs were prepared successfully using a simple method. Experimental results revealed the as-prepared hydrogenated anatase TiO2@Ni (H-A-TiO2@Ni) composite exhibited enhanced microwave absorption property, when compared to pristine anatase TiO2@Ni (A-TiO2@Ni). 1) The minimum reflection loss (RL) of the H-A-TiO2@Ni/wax composite reached to -64.2 dB at a thin thickness of 2 mm, indicating a large absorption efficiency over 99.99 %. 2) The H-A-TiO2@Ni showed an effective microwave absorption bandwidth (RL< -20dB) in a range from 2.2 to 11GHz. Due to the introduction of core-shell media, coating Ni microspheres with H-A-TiO2 shells will be not only beneficial for impedance matching behavior, but also improving the electromagnetic synergistic effects. This study provides a new application area of hydrogenated TiO2 materials as an efficiency energy absorber and opens up a new pathway for design of efficiency microwave absorbing materials.

KEYWORDS:

Hydrogenated

TiO2,

microwave

absorbing,

electromagnetic synergistic effects, electromagnetic properties.


core-shell;

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INTRODUCTION Currently, due to the fast-growing electromagnetic (EM) pollution arising from the EM wave technology, the radiation of EM wave has become a potential threat to information safety, operation of electronic devices and biological systems .1-3 In reality, efficient EM wave absorption plays an important role to keep away from the detection of aircrafts by radar, and reduce information leakage in various electronic equipment.4-6 Up to now, many efforts have been devoted to explore high-efficiency microwave absorption materials (MAMs) with an enhanced microwave absorption property, lightweight, and wide microwave absorption bandwidth.7-9 Past research indicated that the conventional single metal absorbers are usually associated with large loading ratio and high density, which restrict their practical applications in EM wave field.10,11 In addition, these MAMs ineluctably share some defects, such as low minimum reflection loss (RL) value and narrow bandwidth. One of the pathways to exceed the above limitations is to combine dielectric materials with magnetic materials.12-17 Compared to a single type of absorber, the core-shell structured magnetic composites, in which dielectric materials are designed as a shell and the magnetic materials are as a core, have received increasing attentions, owing to their unique interfacial and electromagnetic synergistic effects.18,19 The double attenuation mechanism and well impedance matching of core-shell structured magnetic composites result in the enhanced EM wave absorption performance. Hence, different kinds of dielectric materials have been researched to synthesize magnetic materials @ dielectric materials over the past years, such as Fe3O4@Graphene,20 Fe@CNTs,21



Co@CNTs,22 Ni@ZnO,23 CoNi@TiO2,24 Ni@TiO2,35 etc. Thus, designing magnetic materials @ dielectric materials absorbers have been considered as a valid pathway to improve the EM wave absorption property. Among above dielectric materials, H-TiO2 has widely potential applications in desalination of sea water, super capacitor, photocatalysis, biomedicine, etc.25-28 Compared to pristine TiO2 and other single dielectric materials,29-31 H-TiO2 has been regarded as a candidate for a microwave absorbent.32-34 For a single H-TiO2 dielectric material, the efficient absorption property is mainly originated from the enhanced dipole polarization.33 Actually, the magnetic material @dielectric material absorbers have multiple attenuation ways in the dielectric loss part. So far, core-shell structured magnetic hybrids have rarely been reported, which is designed with strong dielectric H-TiO2 shells and magnetic Ni cores for microwave absorption. Herein, we report a systematic work in constructing a kind of the core-shell structure of inner-core Ni MPs and outer-shell H-A-TiO2 for microwave absorption. The EM wave absorption properties of the core-shell structured hybrids are accurately studied. The experimental results indicated a high-performance and light-weight absorber. The present work shed insight on the development of effective microwave absorbent with high reflection loss and broaden absorption bandwidth.


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RESULTS AND DISCUSSION Figure 1(a) schematically provides the main steps in synthesizing the H-A-TiO2@Ni by a three-step method (the specific experimental method are shown in the Supporting Information). Firstly, Ni MPs were synthesized based on a previous work. 36 Secondly, the Ni MPs were enwrapped by anatase TiO2 by using a sol-gel method. Thirdly, the H-A-TiO2@Ni was fabricated from the A-TiO2@Ni by means of sealing-transfer reduction method at 450 oC. Figure 1(b) presents XRD patters of the Ni MPs. All peaks at 44.5°, 51.9° and 76.4° were well indexed to the characteristic reflections of Ni (JCPDS card No. 65-2865). The phase of as-synthesized pristine A-TiO2@Ni and the H-A-TiO2@Ni were presented in Figure 1(c). For pristine A-TiO2@Ni, a typical XRD pattern of anatase TiO2 and Ni were obtained, and the diffraction peaks at 25.2°, 37.8°, 48°, 53.9°, 62.6° were indexed to characteristic (101), (004), (200), (105), and (204) crystal face of anatase TiO2 (JCPDS card No.21-1272). Meanwhile, three peaks at 44.5°, 51.9° and 76.4° were indexed to the (111), (200) and (220) crystal faces of Ni. These results demonstrated that the as-prepared A-TiO2@Ni was composed of anatase TiO2 and cubic crystal structure of nickel. However, the XRD peaks of the H-A-TiO2@Ni showed a much broader and lower intensity after hydrogenated treatment, which should be attributed to the amorphous nature of the H-TiO2 shell.37 Similar results of H-TiO2 were previously reported.34, 38 In addition, the PL emission was measured, it was useful to research the holes in TiO2.39,40 The RT PL spectra for pristine A-TiO2@Ni and the H-A-TiO2@Ni with excitation wavelength at 300 nm is illustrated in Figure S1. Clearly, the PL intensity



of the as-prepared H-A-TiO2@Ni exhibited a significant decrease, compared to pristine TiO2@Ni, which mainly resulted from the formation of new defects and oxygen vacancy by hydrogenation.41 Figure 2 illustrates the microstructures of the as-prepared samples. Obviously, Ni MPs have an average diameter of 850 nm. However, the diameter of the A-TiO2 was about 900-950 nm. Further TEM observations confirmed that Ni MPs were coated by the A-TiO2, and the HRTEM image revealed the fringe spacing of 0.35 nm corresponded to (101) planes of A-TiO2. It also could be confirmed that the A-TiO2 shells of the pristine A-TiO2@Ni were highly crystallized with clear lattice fringes. In contrast, the fringes were blur everywhere for the H-A-TiO2@Ni, as shown in Figure 2(f), which indicated that lattices structures of the A-H-TiO2@Ni were mostly disordered. Furthermore, comparing to the related fast Fourier transform (FFT) analysis images as displayed in Figure 2, pristine A-TiO2@Ni MPs exhibited a sharp FFT image. However, after hydrogenation, the FFT photograph of the H-TiO2@Ni became much blurred, which demonstrated the distortion of the A-TiO2 lattices structures after hydrogenation. These results were well matched with the XRD and PL analysis, which was also consistent with our previous studies on the H-TiO2 materials. 34, 42

For the purpose of investigate the elemental distribution, corresponding EDS mappings of the A-TiO2@Ni were carried out, as displayed in Figure S2. Clearly, the elements of Ti, O and Ni could be detected over the pristine A-TiO2@Ni, which was consistent with aforementioned analysis. In addition, the distribution of the three element was homogeneous, and the distribution of Ti was slightly wider than that of Ni, suggesting the MPs was a structure of core-shell with Ni as the core (thickness ranging from 830-850nm) and A-TiO2 as the shell (thickness ranging from 50-100nm)


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To investigate the EM wave absorption performance of as-prepared samples, the corresponding three dimensional reflection loss (RL) values, and attenuation constant (α) of the H-TiO2@Ni and the pristine A-TiO2@Ni with filler loading of 50wt o/o were calculated, according to the following equations:

[

Z in = Z0

µr tanh j (2πfd/c ) µr εr εr

RL = 20lg

Z in − Z0 Z in + Z0

 2π f   α =    C 

(µ ε

'' ''

]

(1)

(2)

) (µ ε

− µ' ε' +

'' ''

− µ' ε'

) + (ε µ 2

'

''

+ ε'' µ'

)

2

(3)

Where Zin is the input impedance, f is the frequency, c is the velocity, Z0 is the impedance, µr is the relative complex permeability, εr is the complex permittivity, d is the layer thickness of absorber. Figure 3 illustrates the microwave absorption performance of the A-TiO2@Ni/wax and the H-A-TiO2@Ni/wax (filler loading ratios of 50wt %). According to equation (1), the thickness (d) is one of the key parameters for microwave absorption performance. Thus, to better understand the microwave absorption performance, as shown in Figure 3(b) and 3(d), we further investigated the thickness-dependent RL performance with different thickness of 1-5 mm, respectively. Obviously, the absorption performance of the A-TiO2@Ni was so weak that the minimum RL value reached to -16.7 dB. Compared to pristine A-TiO2@Ni, the absorption ability was obviously improved in the H-A-TiO2@Ni, and the optimal minimum RL value could reach to -64.2 dB at a thin layer thickness of 2 mm. And with increase of the thickness in a range of 1-5 mm, the peaks shifted toward lower frequency. And the reflection loss peak was observed with minimal values between – 30 and – 64.2 dB in the thickness range from 1 to 5.0 mm. The RL is lower than – 20 dB that corresponding to more than 99.9% EM wave absorption, and



value of – 30 and – 64.2 dB meant that the absorption was 99.9 and 99.99%. And an effective microwave absorption bandwidth (RL< – 20dB) was obtained range from 2.2 to 11GHz. Surprisingly, a superior RL of – 40.2 dB was enhanced at 14.1 GHz with a thin thickness of 1mm, which was attribute to the improved synergistic effects. Especially, double absorption bands of 5.8 GHz (RL< – 10dB) were obtained in the H-A-TiO2@Ni at the thickness of 3.5 mm, which ranged from 3 to 5 GHz and 10.2 to 14 GHz. These results were comparable to other related literature (Table 1), and the H-A-TiO2@Ni achieved an improved microwave absorption performance. In general, the magnetic loss and dielectric loss are the main mechanisms for microwave energy attenuation. As for as we were concerned, the improved absorption property of the H-A-TiO2@Ni originated from following aspects: well impedance matching, strong dielectrics and magnetic loss. In order to study the absorption mechanisms, their relative complex permittivity and permeability (Figure 4). The ε' and µ' were connected with the stored EM energy.50 The µ" and ε" were connected with the dissipation EM energy.51 The microwave-band dielectric (ε' and ε") of the samples were given in Figure 4(a) and (b). It could be seen that the ε' of the pristine A-TiO2@Ni showed a constant value in the whole frequency range, while the H-A-TiO2@Ni displayed a higher value. The ε" were connected with the dielectric loss, which consisted of relaxation and conductivity loss. As displayed in Figure 4(b), the ε" values of the pristine A-TiO2@Ni were also relatively low in the whole frequency, which meant that the dielectric loss was weak compared to H-A-TiO2@Ni. However, the strong relaxation peaks were also found in the H-A-TiO2@Ni, indicating that interfacial polarization were attributed to the relaxation loss.24,52 The


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dielectric tangent loss was an important factor contributing to the EM wave absorption ability.53 As show in Figure S5(a), it revealed that higher tanδE (tan δE = ε' ' /ε' ) values were showed in the H-A-TiO2@Ni, indicating a enhanced energy conversion ability when compared to the pristine A-TiO2@Ni. Considering the enhanced dielectric performance was scarcely ever found in ordinary materials, we considered this phenomenon to the composites having core-shell structure of the hybrids. This kind of behavior was also attributed to the H-A-TiO2, resulting in the formation of polarization after hydrogenation. According to these facts, the interfacial polarization and relaxation loss were considered to the key factor for the dielectric performance of the H-A-TiO2@Ni. Therefore, it could be understandable that the application of hydrogenation technology to shell of the A-TiO2@Ni resulted in an enhancement in εr and tanδE, compared to the A-TiO2@Ni. The measured µ' and µ" values of the complex permeability of samples are illustrated the Figure 4(c) and (d). Which were related storage and attenuation EM wave energy arising from magnetic contribution.54 The µ' values of the pristine A-TiO2@Ni and H-A-TiO2@Ni exhibited great fluctuation over the all frequency range. Obviously, the µ' value of the H-A-TiO2@Ni was higher than that of the A-TiO2@Ni in the range of 11-15 and 15.5-18 GHz. Meanwhile, the µ" value of the H-A-TiO2@Ni was also larger than that of the A-TiO2@Ni in the range of 2-14.8 and 15.5-18 GHz (Figure 4d). As shown in Figure S5b, the magnetic loss tangents ( tanδM = µ' ' /µ' ) of the A-TiO2@Ni and the H-A-TiO2@Ni followed the similar trend as the value of µ". The smaller fluctuations of the permeability value of the A-TiO2@Ni was similar to that of



regular TiO2 as previously reported.33-34 It can been found that the values of tanδE changed greatly when compared to that of tanδM. The aforementioned results indicated that the application of hydrogenation method to the A-TiO2@Ni is useful to optimize the dielectric performance. To understudy the effects of the reduction temperature, the samples treated at different reduction temperatures of 350, 400 oC. As shown in Figure S6, it was found that the color of the sample turned from light blue to dark blue and finally into black with the increase of the reaction temperature. The extent of hydrogenation gradually increased with increase of the reaction temperature, which was consistent with the variations of sample color. Similar results of H-TiO2 have been previously reported.55 Figure S7 illustrates the XRD analysis of pristine and hydrogenated samples. The intensity became much broader and the intensity was low with increase of the reaction temperature. These features were also observed in the earlier case of the reported hydrogenated TiO2, where increased defects states attributed with the structure.34, 38 Figure 5 illustrates the microwave absorption performances of the H-A-TiO2@Ni at different reduction temperatures of 350, 400

o

C. As demonstrated for the

H-A-TiO2@Ni at 350 oC, the reflection loss peaks were of the minimal values in between -10 and -26 dB. However, when the reduction temperature rose to 400 oC, the reflection loss peaks had the minimal values in between -20 and -28 dB (99% absorption). For the sample H-A-TiO2, the similar measurements as above at the reduction temperature of 450 oC still exhibited an superior microwave absorption, as shown in Figure 3d, compared to that of 350 and 400 oC.


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Figure S8 shows the complex permittivity of the H-A-TiO2@Ni sample at different reduction temperature of 350, 400, 450 oC. the ε" values at 450 oC was even larger than that of at 300 oC and 450 oC. According to the literature,56 the polarization effects such as interface polarization and dipolar mostly be able contribute to the enhanced ε".36,58

This increasing appearance may caused by effects of the dipolar polarization

and interface. Because the extent of hydrogenation increased with the reduction temperature, which increase the vacancy of defects in the hydrogenated TiO2, the interfacial polarization in the hydrogenation TiO2 shell is greatly enhanced. As displayed in Figure S8c, with increase of the reduction temperature, the H-A-TiO2@Ni composite improved dielectric performance, and the results highly conflicted with the influence of the dielectric factors, which meant the dielectric loss played an key role in EM wave absorption of the H-A-TiO2@Ni composites. To further reveal the EM absorption property of H-A-TiO2, According to equation (3), the attenuation ability was also discussed in details. Figure 6(a) gives the attenuation constant (α) values of the pristine A-TiO2@Ni and the H-A-TiO2@Ni. It was found that the α values of samples exhibited the similar influence and the H-A-TiO2@Ni had the much higher values than those of the pristine A-TiO2@Ni, which further confirmed the enhanced microwave absorption of the H-A-TiO2@Ni. It is widely-recognized that impedance matching is a important factor to the microwave absorption ability. The EM wave performance will be optimized when the the magnetic contribution matches dielectric contribution according to transmission line theory.57 Thus, we applied the impedance matching ration to reveal the degree of



impedance matching of the H-A-TiO2@Ni composite, which could be expressed by the equations (4-7): Z r = Z / Z0

(4)

Z = Z0 µ r / ε r

(5)

ε r = ε ' − jε ''

(6)

µ r = µ ' − jµ ''

(7)

As displayed in Figure S9, the impedance values of the samples without nickel were much lower than that of the A-TiO2@Ni and the H-A-TiO2 composites. Compare to the magnetic loss tangents, the values of magnetic loss tangents of samples without the presence of nickel were also smaller than that of Ni composites at the most of frequency range, the single TiO2 and H-TiO2 showed a lower permeability because of the lack of magnetic factor. By encapsulating the nickel core, the better impedance values of regular A-TiO2 and the H-A-TiO2@Ni were acquired. In addition, the better impedance matching properties of H-A-TiO2@Ni indicated that more microwave entered to the interior of materials and had enhanced microwave propagation attenuation. The above experimental results and discussion further demonstrated the excellent EM absorption property of the H-A-TiO2@Ni. Based on the above analysis, Figure 7 gives the possible attenuation schematic. The strong dielectric loss in H-A-TiO2@Ni was attribution to the enhanced relaxation loss arisen from strong interface polarization. Firstly, consistent with the XPS results and details in Supporting Information (SI-5),it could found the existence of Ti3+ and oxygen vacancies in H-TiO2@Ni, the dipole rotation (from -OH, Ti3+, and oxygen


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vacancy) was expected to occur on the surface of MPs, constituting some modes of plasmon resonance. Secondly, benefiting better EM wave penetration arisen from the interfacial polarization on/between the H-A-TiO2 and Ni, the H-A-TiO2@Ni MPs exhibited an improved electromagnetic synergistic effects. Thirdly, compared to H-A-TiO2 and the A-TiO2@Ni, the core-shell structure contained the dielectric matter (H-A-TiO2) and magnetic components (Ni), which was valid to regulate well impedance matching and the details were provided in Supporting Information (SI-9). CONCLUSIONS We have developed an effective controllable synthesis pathway for the preparation of H-A-TiO2@Ni MPs. Due to the hydrogenation process, H-A-TiO2@Ni MPs exhibited the improved electromagnetic synergistic effects. Compared to pristine A-TiO2@Ni, the H-A-TiO2@Ni MPs demonstrated a dramatic improvement of the microwave absorption performances. An optimal RL value reached to -64.2 dB at a thin thickness of 2 mm, whereas the maximum absorption bandwidth (RL below -10 dB) was at the thickness of 2.5 mm, which ranged from 4.5 to 8 GHz and 15.5 to 18 GHz. Meanwhile, an effective absorption bandwidth (RL< -20dB) was obtained range from 2.2 to 11GHz in thickness from 1 to 5.0 mm. In summary, the present hydrogenated synthesis media could be enlighten for developing other kind of enhanced performance MAMs. And this application of hydrogenation technology holded promising prospects in many fields, such as photocatalysis, radar dodging, information protection, IR sensing, etc.



ASSOCIATED CONTENT Supporting Information. Detailed methods; PL spectra of the A-TiO2@Ni and the H-A-TiO2@Ni. Magnetic hysteresis of curve of the A-TiO2@Ni and the H-A-TiO2@Ni. Frequency dependence of real permittivity, imaginary permittivity, dielectric loss tangents of the H-A-TiO2@Ni with the different reduction temperatures of 350 oC, 400 oC, 450 oC. The sample H-A-TiO2@Ni with different reduction temperature of 350 oC, 400 oC, 450 oC. XRD patters of the A-TiO2@Ni and the H-A-TiO2@Ni with different reduction temperature of 350 oC, 400 oC, 450 oC. Two dimensional representation RL values of samples/paraffin composites

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (C. Pan) *E-mail: [email protected] (X. Qi) ORCID Chunxu Pan: 0000-0001-9007-8562. Jianle Xu: 0000-0003-0832-3528. Notes There are no conflicts of interest to declare.

ACKNOWLEDGEMENTS This work was supported by the National Basic Research Program of China (973 Program) (No. 2009CB939705), the Platform of Science and Technology and Talent


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Table and figure captions: Table 1 EM wave absorption properties of some TiO2 composites reported in recent papers. Figure 1. (a) Schematic diagram of the process for preparing the H-A-TiO2@Ni MPs; (b) XRD patter of the as-synthesized Ni MPs; (c) XRD patters of the A-TiO2@Ni and the H-A-TiO2@Ni. Figure 2. SEM morphologies, TEM and HRTEM micrographs of the samples. (a) Ni MPs; (b)H-A-TiO2; (c) and (d) A-TiO2@Ni (inset FFT pattern); (e) and (f) H-A-TiO2@Ni (inset FFT pattern). Figure 3. Two dimensional representation RL values of the samples/paraffin composites with filler loading ratio of 50 wt %: (a)A-TiO2@Ni; (c) H-A-TiO2@Ni; RL curves of the samples with different absorber thicknesses: (b) A-TiO2@Ni; (d) H-A-TiO2@Ni. Figure 4. Frequency dependence of the A-TiO2@Ni, H-A-TiO2@Ni/paraffin composite (50 wt%). (a) real permittivity; (b) imaginary permittivity; (c) real permeability; (d) imaginary permeability. Figure 5. RL curves of the H-A-TiO2@Ni at different reduction temperature with different absorber thicknesses. (a) 350 oC; (b) 400 oC. Figure 6. (a) Impedance matching; (b) Attenuation loss of the A-TiO2@Ni and the H-A-TiO2@Ni. Figure 7. Schematic illustration of the microwave absorption mechanism of the H-A-TiO2@Ni.



Graphical abstracts: Synopsis Synthesis of the H-A-TiO2@Ni composites, which were designed to deal with electromagnetic pollution through the electromagnetic synergistic assisted microwave absorption.


Page 26 of 48

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Table 1. EM wave absorption properties of some TiO2 composites reported in recent papers. Loading ratio

Minimum RL

Frequency range

(wt %)

(dB)

(RL< -10dB)

TiO2@Co

unknown

-32 (4 mm)

5.1 GHz

43

TiO2@Fe3O4

50

-33.4 (2 mm)

7.8

45

TiO2/Al2O3

40

-11 (2.8 mm)

0.7

46

TiO2-C

70

-25.4 (3 mm)

6.6

47

Ni

70

-5.2 (N/A)

N/A

44

TiO2

50

-6.5 (N/A)

N/A

34

H-TiO2

50

-40.8(7.16mm)

2

34

ZnO@Ni

50

-48.6(2mm)

6

49

Rutile TiO2@Ni

70

-38 (1.8mm)

6.2

35

Amorphous TiO2@Ni

70

-35.4 (4mm)

1.5

36

Anatase TiO2@Ni

70

-15.4 (2.5mm)

2.4

48

A-TiO2@Ni

50

-16.7 (4.5mm)

1.5

This work

H-A-TiO2@Ni

50

-64.2 (2 mm)

9.5

This work

Reference

Absorber



Figure 1. (a) Schematic diagram of the process for preparing the H-A-TiO2@Ni MPs; (b) XRD patter of the as-synthesized Ni MPs; (c) XRD patters of the A-TiO2@Ni and the H-A-TiO2@Ni.


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Figure 2. SEM morphologies, TEM and HRTEM micrographs of the samples. (a) Ni MPs; (b) H-A-TiO2; (c) and (d) A-TiO2@Ni (inset FFT pattern); (e) and (f) H-A-TiO2@Ni (inset FFT pattern).



Figure 3. Two dimensional representation RL values of the samples/paraffin composites with filler loading ratio of 50 wt %: (a)A-TiO2@Ni; (c) H-A-TiO2@Ni; RL curves of the samples with different absorber thicknesses: (b) A-TiO2@Ni; (d) H-A-TiO2@Ni.


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Figure 4. Frequency dependence of the A-TiO2@Ni, H-A-TiO2@Ni/paraffin composite (50wt %). (a) real permittivity; (b) imaginary permittivity; (c) real permeability; (d) imaginary permeability.

Figure 5. RL curves of the H-A-TiO2@Ni at different reduction temperature with different absorber thicknesses. (a) 350 oC; (b) 400 oC.



Figure 6. (a) Impedance matching; (b) Attenuation loss of the A-TiO2@Ni and the H-A-TiO2@Ni.

Figure 7. Schematic illustration of the microwave absorption mechanism of the H-A-TiO2@Ni.


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Tuning the Electromagnetic Synergistic Effects for Enhanced

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