Facile Preparation of CoreâShell Fe3O4@Polypyrrole Composites

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Facile Preparation of Core-Shell Fe3O4@Polypyrrole Composites with Superior Electromagnetic Wave Absorption Properties Zhengchen Wu, Donggui Tan, Ke Tian, Wei Hu, Jingjing Wang, Mengxing Su, and Lei Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04230 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 9, 2017

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The Journal of Physical Chemistry

Facile Preparation of Core−Shell Fe3O4@Polypyrrole Composites with Superior Electromagnetic Wave Absorption Properties Zhengchen Wu, a Donggui Tan, a Ke Tian, a Wei Hu, a Jingjing Wang, b ,c Mengxing Su, b, c

a

b

and Lei Li a,*

College of Materials, Xiamen University, Xiamen 361005, China

Advanced Materials Academy, Luoyang Ship Material Research Institute, Xiamen 361001, China

c

State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266101

ABSTRACT: Core–shell Fe3O4@polypyrrole (PPy) composites with excellent electromagnetic wave absorption properties have been prepared by a sequential process of etching, polymerization and replication. Templating from pre-prepared Fe3O4 microspheres, ferric ions were released from the skin layer of the microspheres by acid etching and initiated the oxidative polymerization of pyrrole in suit. The morphological and textural evolution of core−shell Fe3O4@PPy composites depending on etching time was investigated by scanning and transmission electron microscope. A maximum reflection loss of as much as −41.9 dB (> 99.99% absorption) at 13.3 GHz with a matching layer thickness of 2.0 mm was achieved 1

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when the etching time was 5 min. In comparison with other conductive polymer-based

core−shell composites

reported

previously,

the

Fe3O4@PPy

composites in this study not only possess better reflection loss performance but also demonstrate a wider effective absorption bandwidth (< −10.0 dB) over the entire Ku band (12.0−18.0 GHz). The excellent electromagnetic wave absorption properties of the core-shell Fe3O4@PPy composites are mainly attributed to the enhanced dielectric loss from the PPy shell.

INTRODUCTION

Electromagnetic (EM) wave absorbents have attracted enormous attention due to their significant contribution to solving EM pollution and developing stealth technology.1-4 In the past decades, magnetic materials (Ni, Co, etc.), ferrites (Fe3O4, α-Fe2O3, etc.), and carbon materials (carbon nanotube, carbon fiber, etc.) have been widely utilized as fillers in matrices to fabricate EM wave absorbents.5-11 However, it is difficult with single-component EM wave absorbents to achieve the requirements of practical application.12-14 Based on classical EM theory, the EM wave absorption properties are strongly dependent on dielectric loss and magnetic loss, as well as impedance

characteristic.15

The

performance

of

delicately

designed

multiple-component EM wave absorbents, including embedding,16 sandwich,17 and core-shell composites,18 can be further improved by multiple attenuation ways and satisfied impedance matching. Particularly, the core−shell structure exhibits clear advantages, such as interfacial polarization, confinement effect, complementary 2


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behavior, and core-corrosion protection.12 Therefore, constructing composites with a designed core−shell structure can circumvent the intractably mismatched impedance and poor attenuation ability. As a result, a variety of core−shell composites with satisfactory EM wave absorption, e.g., Fe3O4@C,19 C@C,20 Co@CoO,21 and Fe3O4@TiO2,22 have been extensively developed. Aside from the structure, the compositions of absorbents also play an important role in the enhancement of the absorption performance. Fe3O4 is believed to be an ideal core material for core−shell EM wave absorbents due to its remarkable superparamagnetism.18, 23 Conductive polymers can act as the shell because of their excellent dielectric loss and good processability.24,

25

Taking the advantages of

multiple components and core−shell structure, uniform Fe3O4 in conjunction with conductive polymer matrices have already demonstrated dramatically enhanced EM wave absorption in the resulting composites. For example, Zhou et al. prepared highly regulated core−shell hollow Fe3O4@poly(3,4-ethylenedioxythiophene) microspheres with a maximum reflection loss (RL) of −30 dB at 9.5 GHz.26 Core−shell Fe3O4@polyaniline hybrid microspheres have been synthesized by Han et al. and the sample with a shell thickness of 100 nm exhibited a maximum RL of −37.4 dB at 15.4 GHz.27 Among the conductive polymers, polypyrrole (PPy) is considered a qualified shell material due to its high conductivity, low density, facile preparation, and favorable physiochemical properties.28, 29 According to the free electron theory, there is an optimal conductivity and a trade-off between attenuation capacity and impedance matching for the conductive polymer: high conductivity brings strong 3



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attenuation capacity while low conductivity results in good impedance matching.30 Thus, a best EM wave absorption performance is achieved when the employed conductive polymer exhibits conductivity in a wide range. The conductivity of PPy is mainly tuned by the polymerization degree and the doping level, both of which are related to the polymerization process.31, 32 One of the most common approaches for the preparation of PPy in composites is chemical method.33, 34 All the PPy coatings in core−shell Fe3O4@PPy composites used for EM wave absorption are prepared using ammonium persulfate (APS) as oxidant, HCl as dopant, and surfactant for uniform coating.35,

36

However, these composites cannot possess a satisfactory EM wave

absorption performance because of the following drawbacks. First, it is difficult to control their dielectric behavior owing to the complicated synthetic method involved. Second, the increase in dielectric loss of these composites always co-occurs with the decrease in magnetic loss, because increasing the content of conductive polymer diminishes the magnetic properties of the composites. Last, all these composites possess low complex permittivity, resulting in low dielectric loss. Additionally, there are

also

disadvantages

in

these

reported

preparation

methods,

including

time-consuming process lasting 24 h or longer; harsh conditions such as sub-zero reaction temperature; complex posting process for removing surfactant, modifier, and stabilizer. Therefore, it is desirable to develop a novel and facile preparation strategy of a high performance core−shell Fe3O4@PPy composite with tuned conductivity and without surfactant.

4


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Herein, we present alternative preparation of Fe3O4@PPy composites with core-shell structure initiated by Fe3+ dissociated from Fe3O4 microsphere template. HCl was employed to etch the pre-prepared Fe3O4 microspheres to produce Fe3+ in situ and initiate the rapid polymerization of pyrrole encapsulating the un-etched parts. Compared with Fe3O4/PPy composites reported previously, the composites in this work possess following merits: i) facile preparation process that can be achieved in a few minutes without the aid of surfactants; ii) easily adjustable dielectric behavior and strong dielectric loss; iii) coexistence of enhanced dielectric loss and large magnetic loss. The time-dependent evolution of the core−shell composites is characterized to elucidate the relationship between the etching time and the EM wave absorption performance. When the etching time is 5 min, the obtained core−shell Fe3O4@PPy composites exhibit a maximum reflection loss of −41.9 dB and a wide effective absorption bandwidth (< −10.0 dB) of all the Ku band (12.0−18.0 GHz). Such an excellent EM wave performance paves the way for the exploitation of the core−shell Fe3O4@PPy composite as an ideal EM wave absorbent. EXPERIMENTAL SECTION Preparation of Fe3O4 Microspheres. Fe3O4 microspheres were prepared according to a previous literature.37 Briefly, FeCl3·6H2O (2.7 g) was dissolved in 80 ml of ethylene glycol, followed by the addition of sodium acetate (3.6 g) and polyethylene glycol (1.0 g ). Then the obtained homogeneous yellow solution was transferred into a Teflon-lined stainless-steel autoclave and sealed to heat at 200 °C for 8 h. The

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obtained black magnetite particles were washed with distilled water and absolute ethanol and finally dried at 60 °C under vacuum. Preparation of Core−Shell Fe3O4@PPy Composites. The as-prepared Fe3O4 microspheres were immersed in 30 ml of distilled water under sonication. Subsequently, pyrrole monomer (1 ml) that dissolved in absolute ethanol (5 ml) and 6 M HCl (10 ml) were added into the above solution in turn under sonication. Finally, the black product was collected by centrifugation, washed with absolute ethanol and distilled water, and dried at 60 °C under vacuum. The obtained samples were named Fe3O4@PPy-x, where x represented the time that HCl etched Fe3O4. For example, Fe3O4@PPy-5 meant that the core-shell composite was prepared under the etching time of 5 min. In addition, the pure PPy was prepared by extending the etching time to 1 h. Characterization. The morphologies and microstructures of the core−shell Fe3O4@PPy composites were characterized by scanning electron microscope (SEM, Hitachi SU-70) and transmission electron microscope (TEM, JEM-2100), respectively. The hysteresis loops were achieved on vibrating sample magnetometer (VSM, LakeShore 7404) at room temperature. The components of obtained composites were measured by power X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectrometer. The XRD patterns were carried out on Bruker-Axe x-ray diffractometer with Cu Kα radiation source (40.0 kV, 40.0 mA). FT-IR spectra were obtained on KBr powder-pressed pellets with a NICOLET iS10 spectrometer. Thermogravimetric analysis (TGA) was carried out with a TA 6


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SDTQ-600 at a heating rate of 10 °C/min in air from room temperature to 800 °C. The relative complex permittivity and relative complex permeability were obtained from a network analyzer (Agilent Technologies, N5222A) at the frequency ranging from 2.0 to 18.0 GHz for the calculation of reflection loss (RL). In the test, samples were prepared from uniformly mixing the powders (50 wt %) in paraffin and pressed into coaxial rings with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm. The RL of an absorber can be deduced from the transmission line theory, in

dB 20lg

in

, (1)

in refers to the normalized input impedance of a metal-backed EM wave absorbing layer and is given by

tanh ! √#$ %$ &, (2) where %$ %$ %$( ) %$(( and #$ #$ #$( ) #$(( are the complex permittivity and permeability, respectively, of the composite medium, c is the velocity of EM waves in free space, f represents the frequency of EM wave, and d means the thickness of absorbent. RESULTS AND DISCUSSION

Scheme 1. Schematic illustration of preparing core−shell Fe3O4@PPy composites.

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The preparation process of core−shell Fe3O4@PPy composites is schematically shown in Scheme 1. Firstly, Fe3+ ions are produced around the pre-prepared Fe3O4 microspheres by HCl etching to initiate the rapid oxidation polymerization of pyrrole monomer in situ. Then, the Fe3O4 microspheres are gradually encapsulated by the formed conductive polymer to produce a core−shell structure. With increasing of reaction time, the Fe3O4 cores are further etched and more Fe3+ ions are dissociated from the cores to initiate the polymerization inside the shells. Eventually, the core−shell Fe3O4@PPy composites with time-dependent internal structure and contents of components can be obtained.

Figure 1. TEM and SEM (insert) images of bare Fe3O4 microspheres (A), Fe3O4@PPy-5 (B), Fe3O4@PPy-15 (C), Fe3O4@PPy-30 (D), and pure PPy (E). Scale bars: 200 nm; scale bars in the inserts: 500 nm. 8


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The morphologies and microstructures of Fe3O4 microspheres, pure PPy, and core−shell Fe3O4@PPy composites for different etching time are investigated by SEM and TEM respectively (Figure 1). It is found that pristine Fe3O4 microspheres are compact and have a smooth surface, though their size distribution is not uniform (Figure 1A). After etching for 5 min, the surface becomes rough. TEM image reveals the core-shell microstructure in which the outer layer is PPy coating and the inner core is Fe3O4 microsphere. Some voids and cracks formed by etching are found in the solid Fe3O4 cores, where HCl and pyrrole monomer can initiate the polymerization (Figure 1B). With further increasing etching time, the solid cores are continuously corroded and split into pieces (Figure 1C). After 30 min etching, most of the solid cores have been replaced by PPy while the surface morphology remains unchanged. Moreover, the composites exhibit a loose internal structure with large internal cavities (Figure 1D). Eventually, the solid Fe3O4 pieces are consumed completely while the loose internal structure is retained (Figure 1E). The structure dissimilarity between shell and inside may be the result of different polymerization degree of PPy, because the diffusion obstacle can cause changes in concentrations of pyrrole and ferric ion. The SEM-energy dispersive spectrometer (SEM-EDS) mapping images of as-prepared composites are shown in Figure S1. It is evident that PPy shells cover Fe3O4 cores completely after 5 min. When the etching time is 15 min, the mapping images demonstrate a heterogeneous distribution of Fe element, suggesting the consumption and splitting of cores. Further increasing etching time, the Fe3O4 content is too low to present a clear distribution of Fe element. Combined with the former 9



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SEM and TEM characterization, these results definitely reveal that the core-shell structure has been successfully constructed via our synthesis routine.

Figure 2. FTIR spectra of bare Fe3O4 microspheres (A), Fe3O4@PPy-5 (B), Fe3O4@PPy-15 (C) Fe3O4@PPy-30 (D), and pure PPy (E). The FTIR spectra of Fe3O4 microspheres, pure PPy, and core−shell Fe3O4@PPy composites are displayed in Figure 2. For Fe3O4 microspheres, a characteristic peak at 587 cm-1 indexed to the Fe−O bond stretching can be found. With the introduction of PPy, this peak shifts to 579 cm-1 owing to the interaction between the Fe3O4 cores and the PPy shells.38 Some characteristic peaks of the pure PPy, such as the fundamental vibrations of the pyrrole ring (1560 cm-1), the C−N deformation vibrations in the ring (1478 cm-1), the C−H and N−H in-plane deformation vibrations (1213 cm-1), and the C−H out-of-plane deformation vibrations of the ring (930 cm-1), can be observed in the core−shell composites, indicating the formation of PPy shells.39

10


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Figure 3. XRD patterns of bare Fe3O4 microspheres (A), Fe3O4@PPy-5 (B), Fe3O4@PPy-15 (C), Fe3O4@PPy-30, and pure PPy (D). The XRD patterns of Fe3O4 microspheres, pure PPy and core−shell Fe3O4@PPy composites are shown in Figure 3. The obvious peaks in the pattern of Fe3O4 microspheres at 30.1°, 35.4°, 43.1°, 53.4°, 56.9° and 62.5° correspond to the (220), (311), (400), (422), (333) and (440) lattice planes, respectively, which are typically indexed to the crystal phase of Fe3O4 (JCPDS 89-4319). From Scherrer’s equation, the average particle size of Fe3O4 is estimated to be about 25 nm.40, 41 Accordingly, these Fe3O4 microspheres are actually assembled by the nanoparticles and can split into nano-sized pieces (Figure 1C). A broad peak around 24.7° in the patterns of the pure PPy and the core-shell composites is attributed to the amorphous structure of the polymer. The peak positions of the core−shell composites remain unchanged, indicating that both the etching and the formation of PPy shell have a negligible influence on the crystal form of Fe3O4 core.

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Figure 4. EM parameters of bare Fe3O4 microspheres, pure PPy, and core−shell Fe3O4@PPy composites: Real (A) and imaginary parts of complex permittivity (B); real (C) and imaginary parts (D) of complex permeability.

According to the EM energy conversion principle, the EM wave absorption property is mostly associated with the electromagnetic parameters, i.e., relative complex permittivity (%$ % ( ) % (( ) and relative complex permeability (#$ # ( ) #% (( ), where the real parts of the complex permittivity ( %′ ) and the complex permeability (#′) symbolize the storage ability of the electrical and magnetic energy, while the imaginary parts (% (( and #′′) stand for the dissipation or loss of energy.42-45 Figure 4A and B show the complex permittivity of Fe3O4 microspheres, pure PPy and core−shell Fe3O4@PPy composites in the frequency range of 2.0−18.0 GHz. Although Fe3O4 is usually used as a kind of EM wave absorbent with moderate 12


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complex permittivity, in our case, the Fe3O4 microspheres exhibit low %′′ throughout the whole frequency range, indicating poor dielectric loss. Apparently, the % ( values are higher for Fe3O4@PPy-5 than for Fe3O4 microspheres, suggesting that the enhanced dielectric loss is induced by coating highly conductive PPy shells. With increasing etching time, both %′ and %′′ are obviously decreased, which is related to the decreased conductivity. As reported in previous works, the conductivity of composite is positively correlated with the content of conductive polymer with a good doping degree.26,

35

Based on the TGA curves, the mass percentage of PPy is

calculated as 11.9 %, 67.1 %, 93.6 %, and 100.0% when the etching time is 5, 15, 30 and 60 min, respectively (Figure S2 and Table S1). The higher the content of PPy in composite is, the lower complex permittivity the composite presents, indicating that later-formed PPy exhibits lower conductivity. It is widely accepted that the conductivity of conductive polymers is mainly affected by dopants and conjugation length.31,

32

Thus, the reason for the decrease in conductivity of the core-shell

composites can be explained as follows. On one hand, the polymer inside the core−shell composites possesses lower polymerization degree owing to the decreased concentrations of Fe3+ and HCl, as demonstrated in the TEM figures (Figure 1D and E). On the other hand, the later-formed PPy may exhibit low dopant percentage due to the short immersion time in the dopant solution. In order to prove this, Fe3O4@PPy-15 and Fe3O4@PPy-30 were dispersed in the FeCl3 solution for 24 h, and both %′ and %′′ are much higher for the obtained samples than for the core−shell composites prior to immersion (Figure S3). That is, the dopant percentage of PPy in 13



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the core−shell composites increases considerably after long-time immersion in the dopant solution. Moreover, the complex permittivity of PPy microspheres also increases conspicuously (Figure S4) when extending etching time to 24 h, because of the increased dopant percentage and the formed conductive network (Figure S5). Based on these results, it can be concluded that both low polymerization degree and dopant percentage are responsible for the low complex permittivity of the core−shell composite with high content of PPy. The dielectric loss tangent (tan +, % (( ⁄% ( ) is used to further evaluate the dielectric loss of the core−shell composites.46 As shown in Figure S6, Fe3O4@PPy-5 exhibits the highest tan +, , indicating the strongest dielectric loss among the samples. In addition, it can also be found that the increase in etching time results in the decrease in dielectric loss of the core−shell composites. The dependences of the complex permeability of Fe3O4 microspheres, pure PPy, and core−shell Fe3O4@PPy composites on frequency are shown in Figure 4C and D. It is clear that Fe3O4@PPy-5, Fe3O4@PPy-15, and Fe3O4 microspheres demonstrate similar #′, indicating comparable store abilities for magnetic energy. The #′ values of these three samples largely rise in high-frequency region where their %′ values decrease sharply, owing to the capacitance lead or behind an angle of 90° than the inductance.47, 48 Moreover, the increase in # ( always co-occurs with the decrease in negative #′′, as reported in previous works.23, 49, 50 Between 2.0 and 10.0 GHz, #′′ of the core−shell composites decrease with increasing etching time. It is well known that the complex permeability is mainly dependent on magnetic properties.51 The magnetization curves (Figure S7) and the calculated saturation magnetization (./ , 14


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Table S1) values of the core−shell composites reduce from 70.7 to 4.6 emu/g when increasing etching time from 5 to 30 min. Moreover, it is noteworthy that the coercivity 0 values of the core-shell composites show the same trend as ./ , which may result from the enhanced magnetocrystalline anisotropy constant K. According to Stoner-Wohlfarth theory: 0

1

2 34

, the changes in shape and size of

the magnetic core caused by etching may lead to an increase in K, and finally enlarge

0 .52, 53 All the samples, except for pure PPy, possess negative #(( values in the range of 10.0−18.0 GHz, which can be attributed to the eddy current. According to Maxwell equations, a strong magnetic field induced by a large eddy current can be radiated out to cancel or dominate the external magnetic field, which can greatly increase the dielectric loss and then result in a negative # (( .47 As it is well known, in addition to eddy current loss, natural resonance in the microwave region can also contribute to magnetic loss. Thus, the dependence of #(( # (

on the frequency

is used to study the main mechanisms behind magnetic loss of the core−shell composites. If the values of #(( # (

keep constant when the frequency is

changed, magnetic loss should result from eddy current loss; if not, magnetic loss is ascribed to natural resonance.54 In Figure S8, the values of #(( #(

of the

core-shell composites decline in the range of 2.0−10.0 GHz while they are almost constant in the remainder of the range, indicating that magnetic loss of the core−shell composites originates from natural resonance and eddy current loss. In addition, the enhanced dielectric loss can be observed in tan +, of the core−shell composites (Figure S6), which further confirms the existence of eddy current loss. It can be found 15



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that pure PPy cannot generate magnetic loss owing to the absence of Fe3O4, where the values of # ( and #′′ are almost constant and close to 1 and 0, respectively. The comparisons between tan +, and the magnetic loss tangent (tan +3 # (( /# ( ) of the core-shell composites are shown in Figure S9. In the entire frequency range, the values of tan +, are obviously larger than those of tan +3 , indicating that dielectric loss makes major contribution on EM wave absorption. Thus, the main influence factor on EM wave absorption performance is the content of PPy, because the dielectric loss can be enhanced by introducing PPy and weakened by increasing PPy content in the composites.

Figure 5. RL curves of bare Fe3O4 microspheres (A), Fe3O4@PPy-5 (B), Fe3O4@PPy-15 (C), Fe3O4@PPy-30 (D), and pure PPy (E). Table 1. EM Wave Absorption Properties of the Conductive Polymer/Fe3O4 Composites in Previous References and This Work

absorbents Fe3O4@PANI50

thickness (mm)

maximum RL (−dB)

effective absorption bandwidth (GHz)

refs

3.0

−31.3

3.0

57 16


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Fe3O4@PANI

2.0

−37.4

4.2

27

Fe3O4−polyelectrolyte @PANI

2.0

−6.5

0

56

Fe3O4@PEDOT

4.0

−30.0

4.2

26

Fe3O4/PEDOT hybrids

4.0

−34.4

1.4

55

Fe3O4@PPy

2.0

−16.9

4.2

35

Fe3O4@PPy

2.5

−31.5

5.2

36

Fe3O4@PPy-5

2.0

−41.9

6.0

Herein

Fe3O4@PPy-15

2.0

−41.6

4.2

Herein

On the basis of the complex permittivity and complex permeability, the RL properties of Fe3O4 microspheres, core−shell Fe3O4@PPy composites, and pure PPy can be deduced according to eqs 1 and 2. As shown in Figure 5, the maximum RL values of Fe3O4 microspheres and PPy microspheres are −10.5 dB and -6.5 dB, respectively, which are much lower than those of core−shell Fe3O4@PPy composites. Fe3O4@PPy-5 remarkably reaches an optimal RL value of −41.9 dB at 13.3 GHz with a broad effective absorption bandwidth of 6.0 GHz (12.0−18.0 GHz) while the thickness is 2.0 mm, owing to its strongest dielectric loss and magnetic loss among the samples (Figure S9). Although both the dielectric loss and the magnetic loss of Fe3O4@PPy-15 are lower than that of Fe3O4@PPy-5, Fe3O4@PPy-15 can also exhibit a high RL value of −41.6 dB. This phenomenon may be attributed to the synergetic effect between high conductive PPy and low conductive PPy, where the high conductive parts increase the dielectric loss of the low conductive parts, and the low conductive parts optimize the matched characteristic impedance of the high conductive parts.12 When the etching time is 30 min, the maximum RL value of the 17



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core−shell composite decreases to −31.3 dB at 9.2 GHz. Moreover, the obtained composites not only exhibit a large RL in the Ku band, but also behave well in other frequency ranges. For example, at a thickness of 3.0 mm, the maximum RL values of Fe3O4@PPy-5 and Fe3O4@PPy-15 are −35.8 dB at 9.4 GHz and −41.6 dB at 7.4 GHz, respectively. The EM wave damping capacity of some typical conductive polymer/Fe3O4 composites reported in recent years is listed in Table 1. It is evident that the EM wave absorption properties of Fe3O4@PPy-5 and Fe3O4@PPy-15 are indeed superior to those of the other composites. More importantly, to our knowledge, the effective absorption bandwidth that covers all of the Ku band is rare among the reported EM wave absorbents. These results substantially indicate that the as-prepared core−shell Fe3O4@PPy composites can serve as ideal EM wave absorbents owing to their features of lightweight, thinness, strong absorption ability, and wide absorption bandwidth.

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Figure 6. (A) Typical Cole-Cole semicircles for bare Fe3O4 microspheres and core−shell Fe3O4@PPy composites in the frequency range of 2.0–18.0 GHz. (B) Schematic illustration of the possible EM wave absorption mechanisms.

The processes of Debye dipolar relaxation of core-shell Fe3O4@PPy composites are investigated to explain the possible mechanisms behind the impressive EM wave absorption performance. The complex permittivity (%$ ) can be written as:

% ( )

4 6

7 % ((

4 6

, (3)

19



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where %/ and %8 are the static permittivity and the relative dielectric permittivity, respectively. Thus, the plot of % (( versus % ( will be a single semicircle, generally regarded as the Core-Core semicircle.58 Each semicircle represents one Debye relaxation process. As shown in Figure 6A, compared with the single semicircle of Fe3O4 microspheres, all the core−shell composites show two superimposed Cole-Cole semicircles, which suggests that the introduction of PPy induces a new relaxation process. This relaxation process is shown in Figure 6B. Under a sufficient EM field, electrons that reside on the nitrogen atoms of the PPy can gain enough energy to surmount the interfaces between the PPy shells and the Fe3O4 cores and then move onto the Fe3O4 particles. Thus, an interfacial behavior of charge accumulation occurs while the energy of the EM wave is dissipated greatly.59 The shape of the HCl-etched Fe3O4 core also influences the relaxation behavior. The cracks in the cores can increase the contact areas between PPy and Fe3O4, which is beneficial for interfacial relaxation. Apparently, the magnetic loss from the Fe3O4 core and the enhanced dielectric loss induced by the eddy current contribute significantly to the EM wave absorption ability. Moreover, void spaces and multiple interfaces that exist in the samples/paraffin can effectively interrupt the spread of microwaves and generate dissipation due to the existing impendence difference.60, 61 CONCLUSION

In summary, core−shell Fe3O4@PPy composites with excellent EM wave absorption abilities have been successfully prepared through a facile method. During the formation of these composites, HCl plays an important role because it can etch 20


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Fe3O4 and subsequently, the ferric ions act as an oxidizer in the in situ polymerization of the pyrrole monomers. The core−shell Fe3O4@PPy composites exhibit a maximum RL of −41.9 dB at 13.3 GHz with a matching layer thickness of 2.0 mm and a broad absorption bandwidth that covers all Ku band, which represents a considerable improvement compared to the Fe3O4 microspheres. Studies on the mechanism of the EM wave absorption suggest that interfacial relaxation contributes significantly to the dielectric loss, which is beneficial for producing an enhanced EM wave absorption ability. Moreover, the EM wave absorption performances are highly dependent on the etching time. Thus the EM wave absorption can be simply tuned not only by the matching thickness, but also by the etching time to satisfy applications in different frequency bands. Accordingly, we believe that these core−shell Fe3O4@PPy composites can serve as ideal EM wave absorbents owing to their features of light weight, thinness, strong absorption ability, and wide absorption bandwidth. ASSOCIATED CONTENT

Supporting Information

SEM-EDS mapping images, Magnetic properties, component contents, dielectric loss tangent, magnetic loss tangent, and curves of # (( #(

of core−shell

Fe3O4@PPy composites. Complex permittivity of Fe3O4@PPy-15 and Fe3O4@PPy-30 after immersing in FeCl3 solution for 24 h and pure PPy prepared at etching time of 24 h. These materials are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *Tel: +86-592-2186296. Fax: +86-592-2183937. E-mail: [email protected].

Author Contributions All authors contributed to the experimental design and data analyses. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (No. 51373143 and 21674087). REFERENCES (1)

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