Well-Defined CoreâShell Fe3O4@Polypyrrole Composite

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Well-Defined Core-Shell Fe3O4@Polypyrrole Composite Microspheres with Tunable Shell Thickness: Synthesis and Their Superior Microwave Absorption Performance in the Ku Band Mingtao Qiao, Xingfeng Lei, Yong Ma, Lidong Tian, Ke He Su, and Qiuyu Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04814 • Publication Date (Web): 19 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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Industrial & Engineering Chemistry Research

Well-Defined Core-Shell Fe3O4@Polypyrrole Composite Microspheres with Tunable Shell Thickness: Synthesis and Their Superior Microwave Absorption Performance in the Ku Band Mingtao Qiao, Xingfeng Lei, Yong Ma, Lidong Tian, Kehe Su and Qiuyu Zhang* Department of Applied Chemistry, Key Laboratory of Space Applied Physics and Chemistry of Ministry of Education, School of Science, Northwestern Polytechnical University, Youyi Road 127#, Xi’an 710072, PR China.

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Abstract: Highly regulated core-shell Fe3O4@polypyrrole composite microspheres have been successfully prepared via chemical oxidative polymerization in the presence of polyvinyl alcohol and p-toluenesulfonic acid. The polypyrrole shell thickness can be adjusted from 20 to 80 nm with the variation of the pyrrole/Fe3O4 ratio. Investigations microwave absorbing properties indicate that the polypyrrole shell plays an important role and the maximum reflection loss of composite microspheres can reach as much as -31.5 dB (>99.9% absorption) at 15.5 GHz with a matching layer thickness of 2.5 mm. Compared the physically blending Fe3O4-PPy composites, Fe3O4@polypyrrole composite microspheres not only possess better reflection loss performance but also have a wider absorbing bandwidth of 5.2 GHz (12.8-18 GHz) in the Ku band, which may be attributed to intensive synergistic effect of dielectric loss from polypyrrole shells and magnetic loss from Fe3O4 cores. Therefore, regulated core-shell Fe3O4@polypyrrole composite microspheres are believed to be more promising in the microwave absorption applications. Keywords: core-shell, Fe3O4, polypyrrole, electromagnetic, microwave absorption.


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1. Introduction Investigations of conducting polymers are paving the way for exploring the novel functional materials in nanotechnology. Due to the special π-conjugated polymeric chains, conducting polymers (CPs) including polyaniline (PANI), polypyrrole (PPy) and polythiophene (PTh) are endowed with advantages of readily tunable bandgaps, rich electroactivity, good processibility and excellent flexibility.1-3 Moreover, these conjugated polymers can be electrical insulators, semiconductors or conductors, relying on the level of doping and nature of the dopants. Thus, they can be widely applied in many fields, resulting in promising materials in the future. Simultaneously, inorganic nanoparticles possess peculiar and fascinating properties, such as quantum size effect, small bulk effect, surface effect and macroscopic quantum tunneling effect. So the hybrid of conducting polymers and inorganic nanoparticles has received steadily growing interests. The novel conducting polymer nanocomposites have been emerging.4-7 In recent years, conducting polymer composites based on Fe3O4 magnetic nanoparticles have attracted more attention. Fe3O4/CPs composites not only display combination of the electrical and magnetic properties but also take the advantages of inorganic nanoparticles and conducting polymers. Additionally, these conducting polymers coating the magnetic nanoparticles will prevent the aggregation of Fe3O4 nanoparticles caused by the high surface activity and protect the microstructure of Fe3O4 nanoparticles from chemical corrosion. Thus, these Fe3O4/CPs composites have great potential applications in nanoelectronic devices, nonlinear optics and electromagnetic interference shielding, sensors, microwave absorbing and corrosion protection coatings.8-12 More importantly, compared with single metallic materials and ferrites, Fe3O4/CPs nanocomposites have been optimal candidates in the microwave absorption due to light weight, good stability and simple process. Up to now, a considerable progress has been achieved in the


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scientific researches about Fe3O4/CPs nanocomposites. Belaabed et al. synthesized hybrid conducting composites based on PANI/Fe3O4 fillers, which showed high microwave absorption with a maximum coefficient reflection of about -42 dB in Ku band.13 At the same time, they have testified that the incorporation of Fe3O4 contributed to the improvement of permeability and leaded to high interfacial polarization in hybrid PANI/Fe3O4/epoxy resin. Cui et al. also prepared PANI/Fe3O4 composites by a two-step oxidative polymerization and proved that embedding Fe3O4 microspheres in the polyaniline matrixes can effectively improve the impedance and reflection loss, producing the excellent microwave absorbing properties.14 Varshney et al. synthesized polypyrrole/Fe3O4 nanocomposites via in situ oxidative polymerization and revealed that these composites were lightweight and displayed good electromagnetic shielding properties with a maximum shielding effectiveness value of SEA(max) = 20.4 dB in the frequency range of 12.4-18 GHz.15 Indeed so, the cooperation of CPs and Fe3O4 particles is favorable in the application of microwave absorption. On the basis, the design of core-shell Fe3O4@CPs nanocomposites has attracted considerable attentions due to special microstructure. For example, core-shell Fe3O4@PANI hybrid microspheres have been synthesized and the samples with a shell thickness of 100 nm exhibit a maximum reflection loss of -37.4 dB at 15.4 GHz.16 Highly regulated core-shell Fe3O4-poly(3,4ethylenedioxythiophene) microspheres were prepared by a two-step method, and a maximum reflection loss of the samples reaches about -30 dB at 9.5 GHz.17 Fe3O4@PPy nanocomposites consists of Fe3O4 core with the mean diameter of 100 nm and adjacent PPy shell with a thickness of about 70 nm and their maximum reflection loss is -22.4 dB at 12.9 GHz, moreover, a broad peak with a bandwidth lower than -10 dB is about 5 GHz.18 So far, although the studies of coreshell structured and physically mixture composites are numerous, few scholars mention the


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difference between them in the microwave absorption performance. Therefore, an existing question is whether core-shell nanocomposites always keep superior microwave absorption capability than physical blending composites at the same material ratio. In the latest literature, Tian et al. have evidenced that core-shell [email protected] composites exhibited much better reflection loss property as compared to physically mixed PPy-PANI-1.2 composites due to the advantages of microstructure.19 However, with regard to the Fe3O4@CPs nanocomposites, unfortunately, there is no exact answers. Herein, a contrast between core-shell Fe3O4@PPy composite microspheres and physically mixture Fe3O4-PPy nanocompsites for microwave absorption has been carried out. Firstly, we develop a novel method to fabricate highly regulated core-shell Fe3O4@PPy composite microspheres, which exhibit good dispersion and stability. With the change of pyrrole/Fe3O4 ratios, the shell thickness of Fe3O4@PPy composite microspheres can be tuned uniformly. Then, thermogravimetric analysis (TGA) indicates the mass percentages of PPy in Fe3O4@PPy composite microspheres with different shell thicknesses. Based on the above calculated results, the corresponding physical blending Fe3O4-PPy nanocompsites have been synthesized. Subsequently, the microwave absorbing properties of all samples are investigated. Our results indicate that when the thickness of polypyrrole shell reaches a critical point, these hybrid microspheres exhibit strong reflection loss in the corresponding frequency range. As the layer thickness of absorber gradually increases, the maximum reflection loss of composite microspheres appear in lower frequency range. Additionally, the comparison of physically blending Fe3O4-PPy composites and core-shell Fe3O4@PPy composite microspheres demonstrates that the latter ones exhibit the superior microwave absorbing performance arising from the special microstructure. 2. Experiment Section


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2.1 Materials Pyrrole monomer (99.0%) was purchased from Acros Organics Co. of USA. Ferric chloride (FeCl3·6H2O, 99.0%), ethylene glycol (EG, 99.0%), sodium acetate (NaAc, 99.0%), hydrochloric acid (HCl, 37.0%), polyethylene glycol (PEG Mw = 4000 g/mol), polyvinyl alcohol (PVA 1799, alcoholysis degree of 99%), p-toluenesulfonic acid (p-TSA, 99.0%), ammonium persulfate (APS, 98.0%) and ethanol (99.7%) were purchased from Sinopharm Chemical Reagent Co. Ltd (Beijing, China). All reagents were analytical grade and used as received. The deionized water was used throughout the whole experiment. 2.2 Preparation of Fe3O4 Microspheres The Fe3O4 magnetic microspheres were prepared through a solvothermal method according to the published literature with a minor modification.20 Briefly, FeCl3·6H2O (2.7 g, 10 mmol) was dissolved in ethylene glycol (100mL) to form a clear solution with stirring, followed by the addition of NaAc (7.2 g) and polyethylene glycol (2.0 g). The mixture was stirred vigorously for 1 h at 50 °C and then transferred to a Teflon-lined stainless-steel autoclave (100 mL capacity). The autoclave was sealed and maintained at 200 °C for 8 h and then naturally cooled to room temperature. The black magnetic microspheres were separated magnetically and rinsed several times successively with ethanol and deionized water under ultrasonic conditions. Finally, the Fe3O4 magnetic microspheres were obtained by vacuum drying at 80°C overnight. 2.3 Fabrication of Fe3O4@PPy composite microspheres 0.02 g of as-prepared Fe3O4 microspheres were well dispersed in PVA (1 wt. %, 80 mL) aqueous solution with ultrasonic for 30 min. Subsequently, p-TSA (3.0 g) and pyrrole (0.2 g) monomer were successively added to the above mixture with stirring for 2 h. Then, 10 mL of


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ammonium persulfate (APS) solution (the mole ratio of APS/pyrrole = 1) was added dropwise to the above solution to begin the polymerization. After being stirred for 3 h at room temperature, the composite microspheres were collected with the help of a magnet and washed several times with a mixed solvent of deionized water/ethanol (1/1, v/v) to gain the resulting Fe3O4@PPy composite microspheres. Finally, the samples were dried under vacuum at 80°C overnight. Table 1. Dosages of reagents for preparing the Fe3O4@PPy composite microspheres Reagents Fe3O4 (g)

1 wt. % PVA aqueous solution (mL)

p-TSA (g)

pyrrole (g)

APS (g)

Shell thickness (nm)

Fe3O4@PPy-5

0.02

80.0

3.0

0.1

0.34

20

Fe3O4@PPy-10

0.02

80.0

3.0

0.2

0.68

42

Fe3O4@PPy-20

0.02

80.0

3.0

0.4

1.36

80

Samples

2.4 Characterization Morphology of as-prepared composite microspheres was observed by a field emission scanning electron microscopy (FE-SEM, ZEISS EVO 18 Research) and a transmission electron microscopy (TEM, JEOL JEM-3010). In detail, all samples dispersed in ethanol were deposited onto silicon wafers and sputtered with gold before scanning electron microscopy was employed; the samples dispersed at an appropriate concentration were placed on the copper grid before the transmission electron microscopy was employed at an accelerating voltage of 75 kV. Fourier transform infrared (FTIR) spectra in the range of 400-4000 cm-1 were obtained on KBr powder-pressed pellets with a BRUKER TENSOR 27 spectrometer. X-ray diffraction patterns (XRD) were obtained with Shimadzu XRD-7000s diffractometer with Cu Kα radiation (λ = 1.542 Å) from 20° to 80°. Themalgravimetric analyses (TGA) of the composite microspheres were carried out with a Mettler


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Toledo TGA/DSC STARe apparatus at a heating rate of 10 °C/min under air atmosphere from room temperature to 800°C. Surface compositions of the products were analyzed on a Thermo Scientific K-Alpha X-ray photoelectron spectroscopy (XPS). The magnetic properties of products were assessed using a vibrating sample magnetometer (VSM, LakeShore 7307). Conductivity of the samples was determined using a RTS-9 four-point probe instrument. The mixture of Fe3O4@PPy composite microspheres and paraffin wax at 1:1 mass ratio was prepared and then pressed into a toroidal shape with different thicknesses. Subsequently, the relative complex permeability (μr) and permittivity (εr) were carried out by a (HP8720ES) network analyzer at the frequency range of 2-18 GHz. In the end, the reflection losses of products were calculated using the measured μr and εr. 3. Results and discussion 3.1 Structure characterization

Figure 1. XRD patterns of (a) pure Fe3O4 microspheres and (b) JCPDS card No. 65-3107. Figure 1 shows the XRD pattern of pure Fe3O4 microspheres synthesized via a solvothermal method. The diffraction peaks are located at 2θ = 30.1º, 35.6º, 43.1º, 53.5º, 57.1º and 62.7º corresponding to (220), (311), (400), (422), (511) and (400) Bragg reflections, which match with


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the standard XRD pattern of Fe3O4 with face-centered cubic structure (JCPDS No. 65-3107). The grain size of the Fe3O4 can be calculated according to the Scherrer’s formula D = κλ/βcos θ, where λ is the X-ray wavelength, κ is the shape factor, D is the average diameter of the crystals in angstroms, θ is the Bragg angle in degrees and β is the line broadening measured by the half-height in radians. The value of κ depends on several factors including the Miller index of the reﬂecting plane and the shape of the crystal. If the shape of crystals is unknown, κ is often assigned a value of 0.89.21, 22 When the reflecting peak at 2θ = 35.6° is chosen to calculate the crystallite size, the average size is about 21.2 ± 0.2 nm. In addition, the chemical composition of magnetic particles is determined by the combination of Red-Ox titration, and the detailed characterization is described in the supporting information.23 Results indicate that the amount ration of Fe2+/Fe3+ is about 0.49 in the magnetic particles. It can be concluded that the chemical composition of magnetic particles is Fe2O2.67 (Fe2.99O4) which is very close to the magnetite (Fe3O4).

Figure 2. FTIR spectra (A) and XRD patterns (B) of pure Fe3O4 (a) and Fe3O4@PPy composite microspheres synthesized with different pyrrole/Fe3O4 ratios: (b) 5, (c) 10 and (d) 20. According to the FTIR spectra (Fig. 2A) of Fe3O4 and Fe3O4@PPy composite microspheres prepared with pyrrole/Fe3O4 ratios of 5, 10 and 20, we can find that the FTIR spectrum of pure Fe3O4 shows a Fe-O peak (586 cm-1, Fig. 2A a). With the introduction of polypyrrole, this peak


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shifts to 576 cm-1 in the Fe3O4@PPy composite microspheres, which is attributed to the interaction between Fe3O4 microspheres and polypyrrole coating.24 Among the spectra of composite microspheres, the peak at 1554 cm-1 can be assigned to the fundamental vibration of pyrrole ring; the peaks at 1043 cm-1 and 1307 cm-1 should be associated with the C-H deformation vibrations and C-N stretching vibrations, respectively. Moreover, the two strong peaks at 1184 cm-1 and 908 cm-1 indicate the doping state of polypyrrole and the broad band at 3200-3500 cm-1 can be assigned to the N-H stretching vibrations.25 Fig. 2B shows the XRD patterns of Fe3O4-PPy composite microspheres together with pure Fe3O4. Through a clear comparison, it is found that the diffraction peaks of Fe3O4@PPy composites and pure Fe3O4 have no significant difference and all the diffraction peaks are also located at 2θ = 30.1º, 35.6º, 43.1º, 53.5º, 57.1º and 62.7º, which correspond to (220), (311), (400), (422), (511) and (400) Bragg reflections respectively. Above all, both of FTIR spectra and XRD patterns confirm the coexistence of Fe3O4 microspheres and polypyrrole coating and suggest that the introduction of polypyrrole has negligible influence on crystallographic form of Fe3O4.26

Figure 3. XPS C 1s core-level (A) and wide scan spectra (B) of Fe3O4@PPy composite microspheres. To further identify the chemical compositions of Fe3O4@PPy composite microspheres, the XPS analysis was adopted and the results are given in Fig. 3. Apparently, the C 1s core-level spectrum


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of as-prepared composite microspheres is composed of four sub peaks at binding energies of 284.6, 286.0, 287.2 and 288.5 eV respectively, which can be attributed to C-C/C-H, C-N, C-O and OC=O (Fig. 3A).27-30 This indicates that PPy and PVA are in the final products. According to the wide scan spectra (Fig. 3B), Fe peak is not clearly observed and there is a weak peak at ca. 400 eV corresponding to the pyrrole nitrogen (-NH-) and the other two peaks of C 1s and O 1s exhibit stronger intensities.31 Therefore, it is concluded that Fe3O4 microspheres have been completely coated by PPy and PVA as stabilizers cover the surface of the Fe3O4@PPy composite microspheres.

Figure 4. N 1s core-level spectra of Fe3O4@PPy composite microspheres synthesized with different pyrrole/Fe3O4 ratio: (A) 5, (B) 10 and (C) 20. Table 2. The atomic content of three nitrogen species in the composites. N 1s / BE Samples =N– (397.3 eV)

–NH– (399.4 eV)

N+ (401.0 eV)

Fe3O4@PPy-5

3.38 %

84.47 %

12.15 %

Fe3O4@PPy-10

9.88 %

64.50 %

25.62 %

Fe3O4@PPy-20

7.09 %

66.12 %

26.79 %

Generally, the N atoms of both polypyrrole and polyaniline polymers have three different nitrogen species, namely the imine-like (=N–), amine-like (–NH–), and positively charged


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nitrogen (N+) structures.32, 33 The doping degree of polypyrrole can be evaluated by the ratio of the amount of N+ to the total of N 1s. Figure 4 shows the N 1s core-level spectra of three different Fe3O4@PPy composite microspheres. The N 1s core-level spectrum can be deconvoluted three major components with binding energies at 397.3, 399.4 and 401.0, which are ascribed to the imine-like (=N–), amine-like (–NH–), and positively charged nitrogen (N+), respectively.34 Table 2 lists the atomic content of three nitrogen species in the different Fe3O4@PPy composite microspheres. It is found that the atomic percentages of N+ are 12.15 %, 25.62 % and 26.79 %, which indicates that the doping level of Fe3O4@PPy-5 composite microspheres is very low whereas both Fe3O4@PPy-10 and Fe3O4@PPy-20 composite microspheres have the relatively high doping level. In other words, Fe3O4@PPy composite microspheres synthesized with different pyrrole/Fe3O4 ratio of 5, 10 and 20 have the doping degree of 12.15 %, 25.62 % and 26.79 % in the polypyrrole shells, respectively.

Figure 5. SEM images of pure Fe3O4 microspheres (A) and Fe3O4@PPy core-shell composite microspheres prepared with different pyrrole/Fe3O4 ratios: (B) 5, (C) 10 and (D) 20.


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Figure 6. TEM images of pure Fe3O4 microspheres (A) and Fe3O4@PPy core-shell composite microspheres prepared with different pyrrole/Fe3O4 ratios: (B) 5, (C) 10 and (D) 20. The morphology of Fe3O4 microspheres and Fe3O4@PPy composite microspheres can be observed in both Fig. 5 and Fig. 6. It is clearly indicated that Fe3O4 particles are regular spheres with relatively narrow size distribution and diameters ranging from 400-600 nm. According to above XRD results of Fe3O4, it can be speculated that the Fe3O4 microspheres are formed from the high packing of nano-sized grains on the basis of minimum energy principle. The high-resolution TEM image (Fig. S1) reveals that the interplanar spacing (d) of grains is ca. 2.583 Å, which is in accord with the (311) Bragg diffraction in the XRD pattern. This result demonstrates that the Fe3O4 microspheres is consisting of grains with average size of 21.2 ± 0.2 nm. As can be seen from the SEM images (Fig. 5B, 5C and 5D), the amount of the nanoparticles (pointed by the yellow arrows) attached to the surface of Fe3O4@PPy composite samples increases with increasing the pyrrole/Fe3O4 ratio. It is plausible that these nanoparticles are pristine polypyrrole polymers.


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Meanwhile, TEM images not only exhibit the regular smooth shape of Fe3O4@PPy composite microspheres but also reveal the core-shell morphology of the microstructures in which the outer layer is polypyrrole coatings and the inner layer is Fe3O4 microspheres. Moreover, the thickness of the outer polypyrrole shell of composite microspheres could be tuned by tailoring the pyrrole/Fe3O4 ratio, and the smaller the pyrrole/Fe3O4 ratio is, the smaller the thickness of the polypyrrole shell is. In detail, when the pyrrole/Fe3O4 ratio reaches 5, the thickness of polypyrrole shell is ca. 20 nm (Fig. 6B). Moreover, the shell thickness increases to ca. 42 and 80 nm when pyrrole/Fe3O4 ratio is up to 10 (Fig. 6C) and 20 (Fig. 6D), respectively. In addition, we try to augment the ratio to 50 and increase the PPy shell thickness as possible. Unfortunately, desirable samples have not been found and resultant products still keep the shell thickness of about 80 nm, which are shown in the Fig. S2. Therefore, the three types of Fe3O4@PPy composite microspheres is focused in the following researches.

Figure 7. SEM images of Fe3O4@PPy composite microspheres prepared at different concentration of p-TSA: (A) 0 mol/L, (B) 0.13 mol/L and (C) 0.20 mol/L. 3.2 Formation Mechanism How have highly regulated core-shell Fe3O4@PPy composite microspheres been achieved? Why does polypyrrole polymerization occur on the surface of Fe3O4 microspheres? It is supposed that during the polymerization the coexistent of PVA and p-TSA plays an important role in the formation of Fe3O4@PPy composite microspheres. To verify the above assumption, without the


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inclusion of PVA and p-TSA, it is found that the core-shell Fe3O4@PPy composite microspheres could not form. In the absence of PVA, only a mixture of Fe3O4 microspheres and polypyrrole was obtained, which can be determined by the SEM and TEM images shown in the Fig. S3. To identify the role of p-TSA on the formation of Fe3O4@PPy composite microspheres, we have changed the amount of p-TSA in the same system. Firstly, in the absence of p-TSA, Fe3O4 microspheres are merely coated by a small amount of polymer, while plenty of polymer aggregate and apart from Fe3O4 microspheres (Fig. 7A). Nevertheless, when the concentration of the p-TSA (ptoluenesulfonic acid) is 0.13 mol/L in the mixed solution (Fig. 7B), the aggregated phenomenon significantly disappears. Meanwhile, the surfaces of composite microspheres are attached to a lot of nanoparticles, which may be the pristine polypyrrole polymer. When the concentration of the p-TSA is 0.20 mol/L, the uniform and smooth Fe3O4@PPy microspheres are successfully fabricated and few nanoparticles are observed on their surfaces (Fig. 7C). When the concentration of the p-TSA is further enhanced to 0.25 mol/L, the Fe3O4 cores suffer from chemical corrosion (Fig. S4). To further identify the key role of p-TSA, hydrochloric acid of 0.20 mol/L takes the place of p-TSA in the reaction system of pyrrole/Fe3O4 ratio = 10. According to the TEM image of resultant products shown in the Fig. S5, it is found that the shell is very thin and the thickness is merely 5-7 nm. We guess that the minority PPy is grown on the surface of Fe3O4 microspheres via in situ polymerization. Above results suggest that the p-TSA probably guides the pyrrole monomer to the surface of Fe3O4 particles and then a possible mechanism scheme of core-shell Fe3O4@PPy microspheres formation is shown in Scheme 1.


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Scheme 1. Formation Mechanism of Core-Shell Fe3O4@PPy composite microspheres Since Fe3O4 particles are relatively hydrophilic on account of abundant hydroxyls on the surface,20 hydrogen bonds can be formed between Fe3O4 surfaces and PVA, which is conducive to the well dispersion of Fe3O4 particles. When p-TSA is added, the hydrophobic part of p-TSA will interact with the hydrocarbon skeleton of PVA due to the van der waals force, and the hydrophilic of p-TSA tends to the aqueous bulk solution. The existence of p-TSA contributes to improving the protonation of pyrrole. Subsequently, the pyrrole monomers with positive charge tend to be connected with the SO3- group around the PVA, as a result, pyrrole monomers would gather around the Fe3O4 microspheres. Once ammonium persulfate (APS) is introduced, the polymerization will simultaneously occur in many places. As the pyrrole monomers around Fe3O4 microspheres are polymerized, the Fe3O4@PPy composite microspheres are gradually formed. During the formation, p-TSA as the guider promotes the “oriented attachment” to join the asformed polypyrrole and gives rise to the shell, and PVA serves as good stabilizer in the end.35 In fact, this assumption can be proved by changing the content of the pyrrole monomer in the reaction. Just as shown in the TEM images, when the amount of pyrrole monomer increases, the average


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thickness of the shell also increases, which denotes that pyrrole is polymerized to make a joint with the as-existed polypyrrole. 3.3 Thermal Analysis

Figure 8. TG curves (A) of core-shell Fe3O4@PPy composite microspheres prepared with different pyrrole/Fe3O4 ratios: (a) 5, (b) 10, (c) 20; FTIR spectra (B) of the residual products. Table 3. The mass of composites before and after burning samples

m0 (mg) mt (mg) w.t. % R (mt/m0) w.t. % Fe3O4 w.t. % PPy

Fe3O4@PPy-5

9.47

8.25

87.12

84.22

15.78

Fe3O4@PPy-10

10.03

8.41

83.85

81.05

18.95

Fe3O4@PPy-20

10.05

7.33

72.94

70.51

29.49

m0— the mass of composite before burning; mt — the mass of composite after burning;

The prepared Fe3O4 microspheres were characterized by the thermal gravimetric analysis (TGA) at the nitrogen atmosphere. The corresponding curve is shown in the Fig. S6, in which it is found that the mass of Fe3O4 microspheres have a less loss about 0.03 % in the whole course with increasing the temperature. This result may be reason of pyrolysis of a small amount of PEG polymer, and the less loss can be negligible. After the polymerization is over, the formed coreshell Fe3O4@PPy composite microspheres are also characterized by the TGA at air atmosphere


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from room temperature to 800 °C at a heat ramping of 10 °C/min. As the temperature is gradually enhanced, the polymer are first carbonized and then the generated carbon are reacted with the air, leading to the much loss of mass. At the same time, the Fe3O4 can be oxidized to Fe2O3 completely. In Fig. 8A, what captures our eyes is one slight weight increase in the temperature range of ca. 135-215°C due to the transformation of Fe3O4 to Fe2O3.36 As the organic shells can be completely burned in air, the eventual remanent products are the pure Fe2O3, which can be proved by the FTIR spectra (Fig. 8B). Therefore, the amount of ferrite and PPy in these composites can be calculated by the following formulas: wt % Fe3 O4 = wt % R MFe3O4 ⁄(1.5 MFe2O3 )

(1)

wt % PPy = 1 − wt % Fe3 O4

(2)

where wt % R is the residual weight percentage after combustion, and M denotes the molecular weight of the compound. Table 3 shows the mass of composites before and after burning. The calculated results demonstrate that the mass fractions of ferrite are ca. 84.22%, 81.05% and 70.51% and the corresponding mass fractions of PPy in the composite microspheres are ca. 15.78%, 18.95% and 29.49%, when the pyrrole/Fe3O4 ratios are 5 (a), 10 (b) and 20 (c), respectively. This is well consistent with the increasing shell thickness of the composite spheres shown in Fig. 6. 3.4 Electric and Magnetic Properties The electrical properties of resultant Fe3O4@PPy composite microspheres were estimated by a four-probe analyzer at room temperature. Measured conductivity refers to a material’s ability to conduct an electric current. The resultant conductivities of composite microspheres are 1.43×10-2 S/cm, 3.57×10-2 S/cm and 7.27×10-2 S/cm when the pyrrole/Fe3O4 ratios are 5, 10 and 20, respectively. It is obvious that conductivity of composite microsphere gradually enhances with increasing the polypyrrole content. This could be attributed to the gradual growth of the PPy shells


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with a good doping degree.37 Moreover, the connectivity between polypyrrole shells will develop a conductive network, which is beneficial for improving the electrical conductivity. As the polypyrrole shells become thicker, the electrical conductivity of composites tends to be more stable. In the other aspect, the decrease of Fe3O4 content has positive influence on improving conductivity due to the insulating behavior of Fe3O4 in the core of composite microspheres.24 The magnetic properties of all samples were investigated using a vibrating sample magnetometer (VSM). Fig. 9 shows the hysteresis loops of Fe3O4 and Fe3O4@PPy microspheres in the applied magnetic field sweeping from -30 to 30 kOe at room temperature. According to the magnified hysteresis loops shown in the Fig. S7, it is found that both retentivity and coercivity of the samples are close to zero, suggesting the samples display quasi superparamagnetic characterizations. From the inset picture in the Fig. S7, one can find that the composite microspheres can be dispersed in water by vigorous shaking, resulting in a black-colored suspension. When an external magnetic field is applied, the fast aggregation of composite microspheres from their homogeneous dispersion is observed, which indicates the excellent magnetic property. In detail, the pure Fe3O4 possesses high specific saturation magnetization (σ, emu/g) and the magnetic saturation value is about 70.12 emu/g, whereas the magnetic saturation values of the Fe3O4@PPy composites decrease to ca. 65.90 emu/g, 60.94 emu/g and 52.46 emu/g with the reduction of Fe3O4 content (Fig. 9, inset).


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Figure 9. Magnetization curves applied magnetic field at room temperature of pure Fe3O4 microspheres and Fe3O4@PPy composite microspheres with different pyrrole/Fe3O4 ratios; the relationship of saturation magnetization and Fe3O4 content was shown (inset). 3.5 Microwave Absorbing Properties As we know, microwave absorbing properties of the samples can be estimated by the reflection loss (RL) curves.38-42 According to the transmission line theory, the reflection loss (RL) values of as-synthesized samples can be deduced from the following equations. RL (dB) = 20 log|(Zin − 1)⁄(Zin + 1)|

(3)

Zin = √μr ⁄εr tanh[j(2πfd⁄c)√μr εr ]

(4)

where Zin refers to the normalized input impedance of a metal-backed microwave absorption layer, c is the velocity of light in free space, f is the frequency of the wave, and d is the layer thickness. Additionally, the relative complex permittivity (𝜀r) and relative permeability (𝜇r) of the composite medium are expressed as 𝜀r = 𝜀 ′ − 𝑗𝜀 ′′ , 𝜇r = 𝜇 ′ − 𝑗𝜇 ′′ . Hence, the microwave absorption properties are dominated by the combination of the six parameters:𝜀 ′ , 𝜀 ′′ , 𝜇 ′ , 𝜇 ′′ , f and d.


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Figure 10. Electromagnetic parameters of bare Fe3O4 and Fe3O4@PPy composite microspheres with different pyrrole/Fe3O4 ratios at 50% mass fraction in the frequency range of 2-18 GHz: (A) real and (B) imaginary parts of the relative complex permittivity and (C) real and (D) imaginary parts of the relative complex permeability. The microwave absorption behavior of all samples were investigated in a frequency range of 2-18 GHz. Fig. 10 shows the frequency dependence of electromagnetic parameters of bare Fe3O4 and Fe3O4@PPy composite microspheres with different pyrrole/Fe3O4 ratios. The real permittivity (𝜀 ′ ) and the real permeability (𝜇 ′ ) stand for the storage ability of electric and magnetic energy, while the imaginary permittivity ( 𝜀 ′′ ) and the imaginary permeability ( 𝜇 ′′ ) represent the dissipation of electric energy and the magnetic loss, respectively.43 Apparently, with the introduction of the polypyrrole, both 𝜀 ′ and 𝜀 ′′ values of the composites are higher than that of bare Fe3O4. Meanwhile, the composites with higher pyrrole/Fe3O4 ratios display enhanced values of 𝜀 ′ and 𝜀 ′′ (Fig. 10A, 10B), which is related to higher electrical conductivity of polypyrrole


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shells.17 In other words, the thicker polypyrrole shells show much higher efficiency in storing and dissipating the electrical energy. In the view of the Fig. 10C, the 𝜇 ′ values of all samples abruptly decrease in the 2-5 GHz range and then gradually increase in the rest range; at that time the 𝜇 ′ values show a very minor fluctuation in the range 14-18 GHz. The 𝜇 ′′ values of all samples exhibit positive in the whole range; moreover, the 𝜇 ′′ values display a sudden decline in the frequency range of 2-11 GHz and then keep constant in the range of 11-18GHz (Fig. 10D). Based on the values of 𝜇 ′′ , it is supposed that Fe3O4@PPy composite microspheres exhibits relative strong magnetic loss in the frequency ranges of 2-10 GHz. Above results denote that magnetic loss from Fe3O4 microspheres together with dielectric loss from polypyrrole shells is present.36, 44 So it is expected that the reflection loss of incident electromagnetic waves will be enhanced as possible.

Figure 11. Microwave reflection losses (absorber thickness = 2.5 mm) of bare Fe3O4 microspheres and Fe3O4@PPy composite microspheres prepared with different pyrrole/Fe3O4 ratios at 50% mass fraction in the frequency range of 2-18 GHz. According to Eq. (3) and Eq. (4), we calculated the reflection losses of samples with a thickness of 2.5 mm in the frequency range of 2-18 GHz. Fig. 11 shows the reflection loss variation of bare Fe3O4 and Fe3O4@PPy composites with different pyrrole/Fe3O4 ratios. It is obvious that the poor reflection loss of bare Fe3O4 is about -5 dB in the whole frequency range. On the contrary,


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the maximum reflection loss of composites with the ratios of 10 is close to -10.0 dB (90% absorption) at 15.73 GHz while that of composites with the ratios of 20 can reach -31.5 dB (>99.9% absorption) at 15.42 GHz with an absorption bandwidth (＜-10.0 dB) of 5.2 GHz (12.8-18.0 GHz). That is to say, all Fe3O4@PPy composite microspheres exhibit superior absorption performance than bare Fe3O4 owing to the addition of dielectric loss. What’s more, a comparison of three reflection loss curves for composites indicates that microwave absorbing intensity of Fe3O4@PPy composites is enhanced significantly with further increase of the polypyrrole shells. In addition, according to conductivity results, we can also find that the Fe3O4@PPy composites with higher conductivities possess better microwave absorption parameters. Therefore, it is believed that the polypyrrole with the p-TSA dopants plays an essential role in the enhancement of microwave absorbing capability. Besides, we further speculated that enhanced microwave absorption mostly stems from the increase of dielectric loss.


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Figure 12. Microwave reflection losses in different thicknesses of (A) bare Fe3O4 microspheres and Fe3O4@PPy composite microspheres prepared with (B) pyrrole/Fe3O4=5, (C) 10 and (D) 20 at 50% mass fraction in the frequency range of 2-18 GHz. Inset of panels (A)-(D) are graphs of the dependence of the RL maximum and the corresponding frequency upon the sample thickness. As is known, when the phase difference between the entering wave and the emerging wave is 180°, the electromagnetic wave cancelation effect will occur on the surface of microwave absorbers. Hence, a proper layer thickness is one of the most crucial factors impacting the microwave absorption. Then, we investigated the reflection losses of samples with different thicknesses of 1.0, 2.0, 2.5, 3.0, 4.0 and 5.0 mm in the range of 2-18 GHz and the curves were shown in the Fig. 12. From the Fig. 12A, it is clearly seen that the maximum reflection loss of pure Fe3O4 just exceeds -10 dB (90% microwave absorption) in the whole frequency range. The composite with pyrrole/Fe3O4=5 exhibits better reflection loss performance than pure Fe3O4 at the thicknesses of 3.0, 4.0 and 5.0 mm (Fig. 12B). As seen in the Fig. 12C, the two reflection loss peaks are -20 dB (9.5 GHz) and -30 dB (5.4 GHz) at the thicknesses of 4.0 and 5.0 mm in the composites with pyrrole/Fe3O4=10, respectively. Moreover, what captures our eyes in the Figure 12d is two sharp and strong peaks at the frequency of 12.9 GHz (-23.5 dB) and 15.5 GHz (-31.5 dB) with the thicknesses of 3.0 and 2.5 mm, which illustrates that the composites with pyrrole/Fe3O4=20 manifest superior microwave absorption performance. According to the four inset graphs, one can find that the reflection loss peak shifts from high frequency range to low frequency range with the increase of layer thickness in each sample. Additionally, the fore three inset graphs deliver a same information that the greater the lay thickness is, the higher the maximum reflection loss is. However, the inset graph D shows a valley point (-31.5 dB, 15.5 GHz) at a thickness of 2.5 mm, which may be attributed to the matching relationship: t = nλ/4, where


24

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t, λ and n respectively stand for the layer thickness, the wavelength of entering wave and the cardinal number (n=1, 3, 5…).40 Anyway, the layer thickness has a great influence on the microwave absorption of composites. Table 4. Microwave absorption performance of representative Fe3O4-CPs composites Max RL (dB)

Frequency range (RL ＜ -10 dB)

Effective bandwidth

refs

1.5

-8.0 (9.3 GHz)

8.0-12.0

4.0

48

50

2.0

-6.5 (14.3 GHz)

—

—

47

Fe3O4 Microspheres/ PANI Composite

50

3.0

-31.3 (9.0 GHz)

7.8-10.7

2.9

14

Fe3O4/PANI core-shell microspheres

50

2.5

-37.4 (12.1 GHz)

10.1-14.3

4.2

16

Fe3O4-PEDOT hybrids

50

4.0

-31.4 (4.5 GHz)

3.8-5.2

1.4

46

Core-shell Fe3O4@PEDO T microspheres

20

4.0

-30.0 (9.5 GHz)

7.6-12.8

5.2

17

PPy– BaFe12O19/Fe3 O4 composite

50

1.5

-28.0 (11.5 GHz)

9.6-12.0

2.4

45

Fe3O4/PPy core/shell nanocomposite

50

2.3

-22.4 (12.9 GHz)

10.5-15.5

5.0

18

Core-shell Fe3O4@PPy composite microspheres

50

2.5

-31.5 (15.4 GHz)

12.8-18.0

5.2

Herein

Absorbers

Mass ratio (wt %)

Thicknes s (mm)

Fe3O4/PTh composites

50

Fe3O4– polyelectrolyte @PANI

“—”denotes the nonexistence;


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To date, various Fe3O4-CPs (conducting polymers) composites have been applied in the field of microwave absorption and the detailed performance parameters are listed in the following table 4.14, 16-18, 45-48 Generally, the RL value of -10 dB is equivalent to 90% efficiency of microwave absorption, and the RL ＜ -10 dB represents the effective microwave absorption. From the Table 4, it is found that as-synthesized core-shell Fe3O4@polypyrrole composite microspheres have an excellent reflection loss, which ranks only second to the Fe3O4/PANI core-shell microspheres. The effective absorbing frequency range is from 12.8 GHz to 18.0 GHz (medium-high frequency), which is first presented among the Fe3O4-CPs composites. Moreover, the fact that composite microspheres not only possess the reflection loss value less than -30 dB but also have the broad effective bandwidth (5.2 GHz) in the Ku band (12.0-18.0 GHz) is rarely reported in the previous literatures. Therefore, core-shell Fe3O4@polypyrrole composite microspheres will be more promising microwave absorbents in the Ku band. Impedance matching can make the electromagnetic wave enter the interior of materials as possible, which also conduces to the enhancement of microwave absorption. Then, the impedance matching degree of Fe3O4@polypyrrole composite microspheres has been analyzed. Relative input impedance of |Zin⁄Z0| can be utilized to assess the matching degree, and the corresponding values are close to one, indicating the perfect impedance matching.19, 40 The relative input impedance curves of Fe3O4@PPy composite microspheres are shown in the Fig. 13A and the pink dotted line denotes the value of |Zin⁄Z0| is equal to one. From 2.0 GHz to 12.0 GHz, three curves have no significant difference. With the frequency increases to 14.6 GHz, we can find that the red and blue lines are much closer to one than the black line. When the frequency band ranges from 14.6 GHz to 17.0 GHz, the blue and black lines approach the pink lines. The above results indicate that the Fe3O4@PPy composite microspheres with pyrrole/Fe3O4 = 20 (blue line) exhibit relatively better


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impedance matching than two other composite microspheres over the whole frequency band (2-18 GHz). Due to the skin effect, the skin depth of Fe3O4@polypyrrole composite microspheres must be taken into consideration. The depth at which the field drops to 1/e of the incident value is called the skin depth (δ), which is given by δ = 1⁄√𝜋𝑓𝜇𝜎, where f = frequency, µ = permeability, and σ = electrical conductivity. Fig. 13B shows the variation of skin depth with respect to the frequency in three different composite microspheres. One can see that three curves keep the same trend that the skin depth values gradually decline as the frequency increases. Interestingly, the Fe3O4@PPy composite microspheres with ca. 80 nm shell thickness possess the shallowest skin depth in the 218 GHz frequency range, and the value of skin depth gradually decrease from 0.82 mm to 0.33 mm with increasing the frequency. Therefore, it is supposed that the superior microwave absorbing properties may be related to the shallowest skin depth of materials to a certain extent.

Figure 13. Relative input impedance (A) and skin depth with respect to frequency (B) of Fe3O4@PPy composite microspheres with different shell thicknesses. To further testify the advantages of core-shell structure, we conduct a contrast between coreshell Fe3O4@PPy composite microspheres and physically blending Fe3O4-PPy composites in the applications of microwave absorption. In the previous part, we have researched the microwave absorbing properties of core-shell Fe3O4@PPy composite microspheres with different shell


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thicknesses. Then, on the basis of the calculated PPy content (wt. %) of different Fe3O4@PPy composite microspheres (Fig. 8), Fe3O4-PPy composites with the corresponding wt. % PPy of 15.78 %, 18.95 % and 29.49 % are prepared and the detailed preparation process is depictured in the Supporting Information (SI). Subsequently, the microwave absorbing properties of Fe3O4-PPy composites are investigated and the frequency dependence of 𝜀 ′ , 𝜀 ′′ , 𝜇 ′ , 𝜇 ′′ curves are shown in the Fig. S9. One can see that all the four parameters display decreasing tendency with the increase of frequency. The value of real permittivity (𝜀 ′ ) increases with the augment of PPy content, so does the imaginary permittivity ( 𝜀 ′′ ), which manifests the positive effect of PPy on the dielectric properties of composites. However, compared with 𝜀 ′ and 𝜀 ′′ of Fe3O4@PPy composite microspheres, the corresponding values in the Fe3O4-PPy composites are poor, which may be attributed to the insufficient contact between Fe3O4 particles and PPy polymers.19 According to the Fig. S9 C and D, one can find that the growth of PPy content gives rise to gradual decline of 𝜇 ′ and 𝜇 ′′ values, which can be attributed to the decrease of magnetization. And the two types of nanocomposites have no significant difference in the magnetic energy capability and dissipation. On the basis of above measured data of the complex permittivity and the complex permeability, the reflection loss values of Fe3O4-PPy composites can be deduced according to Eq. (3) and Eq. (4). Fig. 14A shows the reflection loss curves of Fe3O4-PPy composites with a layer thickness of 2.5 mm. It is found that these composites with 15.78 wt. % PPy exhibit a reflection loss of -7.9 dB at 15.0 GHz, which is greater than that of bare Fe3O4 with 2.5 mm (Fig. 11). As the PPy content are enhanced to 18.95 wt. % and 29.49 wt. %, the values of maximum reflection loss are -11.0 dB (14.3 GHz) and -12.5 dB (12.4GHz) respectively. In the Fe3O4@PPy composite microspheres, the corresponding values of maximum reflection loss are -5.7 dB (16.33 GHz), 10.0 dB (15.73 GHz) and -31.5 dB (15.42 GHz) at the PPy content of 15.78 wt. %, 18.95 wt. %


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and 29.49 wt. %. The resultant data indicate that both Fe3O4@PPy composite microspheres and Fe3O4-PPy composites display similar reflection losses at the PPy content of 15.78 wt. % and 18.95 wt. %. However, as the PPy content increases to 29.49 wt. %, Fe3O4@PPy composite microspheres not only possess a larger reflection loss but also have a wider absorbing bandwidth of 5.2 GHz (12.8-18 GHz), whereas Fe3O4-PPy composites just have a slight improvement in the reflection loss. The results may be reason of intensive synergistic effect of dielectric loss from PPy shells and magnetic loss from Fe3O4 cores.19, 44, 49, 50 In the research of microwave absorption, it is necessary to take into accounts the layer thickness of absorbers. Then, we take Fe3O4-PPy composites with 18.95 wt. % and 29.49 wt. % PPy content as examples and investigate their microwave absorbing properties at different thickness of 1.0, 2.0, 3.0, 4.0 and 5.0 mm. The reflection loss curves of the two nanocomposites are shown in the Fig. 14B and Fig. 14C, respectively. As we can see from the both pictures, the Fe3O4-PPy composites at the coating thickness of 1.0 and 2.0 mm display very small reflection loss (＜ -10 dB), which can be negligible. The microwave absorbing properties of Fe3O4-PPy composites with the coating thickness of 3.0, 4.0 and 5.0 mm are emphases to study. When the mass fraction of PPy is 18.95 wt. %，the maximum reflection loss (-11.8 dB) of Fe3O4-PPy composites is very close to that (-11.9 dB) of Fe3O4@PPy composite microspheres at the layer thickness of 3.0 mm. As the thickness is enhanced to 4.0 mm and even 5.0 mm, the Fe3O4-PPy composites have no striking variation in the reflection loss, whereas the peak values of reflection loss in the Fe3O4@PPy composite microspheres increase to -19.7 dB (9.5 GHz) and -30.5 dB (5.8 GHz). Above discussions indicate that Fe3O4@PPy composite microspheres have a higher potential in the enhancement of reflection loss performance than Fe3O4-PPy composites. When the PPy content is changed from 18.95 wt. % to the 29.49 wt. %, the reflection losses of Fe3O4-PPy


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composites have slight improvement from ca. -12 dB to ca. -14 dB, and the maximum values of reflection losses are barely affected by the variation of layer thicknesses. In the Fe3O4@PPy composite microspheres, the maximum values of reflection losses are -23.5 dB (12.9 GHz), -17.2 dB (8.9 GHz) and -10.1 dB (7.6 GHz) at the layer thicknesses of 3.0, 4.0 and 5.0 mm, respectively. It is found that the reflection losses of Fe3O4@PPy composite microspheres are more intensive than that of Fe3O4-PPy composites at the layer thicknesses of 3.0 and 4.0 mm, and it is worthy noted that Fe3O4@PPy composite microspheres with the thinner layer thickness possess a larger reflection loss. Indeed so, when the coating thickness is adjusted to 2.5 mm, the maximum reflection loss of Fe3O4@PPy composite microspheres can reach -31.5 dB (15.42 GHz), which have been study and shown in the Fig. 11. Above results demonstrate that Fe3O4@PPy composite microspheres are capable of more intensive absorption than Fe3O4-PPy composites. According to the factor of effective bandwidth, the widest absorbing band of Fe3O4-PPy composites is 4.9 GHz (5.6-10.2 GHz) whereas Fe3O4@PPy composite microspheres can possess the widest effective bandwidth of 5.2 GHz (12.8-18 GHz) in the Ku band. Both of Fe3O4-PPy composites and Fe3O4@PPy composite microspheres exhibit relatively wider absorbing bandwidth in the different frequency band. The comprehensive comparison based on the three factors of absorption strength, effective bandwidth and thin thickness demonstrates that Fe3O4@PPy composite microspheres are more promising to be applied in the microwave absorption than Fe3O4-PPy composites. Simultaneously, it is verified that core-shell microstructure will be beneficial to the enhancement of microwave absorbing properties to some extent.


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Figure 14. Microwave reflection losses of Fe3O4/PPy composites with (A) different PPy contents at the coating thickness of 2.5 mm, with (B) 18.95 % PPy contents and (C) 29.49 % PPy contents at various coating thicknesses. 4. Conclusions Highly regulated core-shell Fe3O4@PPy composite microspheres were successfully prepared by the method of oxidative polymerization. During the formation of these composite microspheres, p-TSA as an imperative guider of pyrrole monomer facilitates the growth of PPy shell and PVA promotes the dispersion and stability of as-synthesized composite microspheres. Through changing pyrrole/Fe3O4 ratios, the shell thickness of Fe3O4@PPy composite microspheres is tunable from 20 nm to 80 nm. The investigations of microwave absorbing properties indicate that (1) as-obtained Fe3O4@PPy composite microspheres have stronger microwave absorption capability than bare Fe3O4 microspheres; (2) when the shell thickness is 80 nm, Fe3O4@PPy composite microspheres exhibit a maximum reflection loss peak of -31.5 dB (15.42 GHz) with the layer thickness of 2.5 mm and an effective absorption bandwidth (＜10 dB) of about 5.2 GHz (12.8-18.0 GHz), which are much better than as-reported core-shell Fe3O4/PPy composites;18 moreover, the fact that composite microspheres not only possess the reflection loss value less than -30 dB but also have the broad effective bandwidth (5.2 GHz) in the Ku band is rarely reported; (3) with the increase of layer thickness, the absorbing frequency band can be adjusted from high


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frequency to low frequency in the 2-18 GHz. At last, compared with physically blending Fe3O4PPy composites, core-shell Fe3O4@PPy composite microspheres can possess superior microwave absorption performance. It is verified that core-shell microstructure will be beneficial to the enhancement of microwave absorbing properties to some extent. Supporting Information Chemical Red-Ox titration characterization of pure magnetic particles; High-resolution TEM image of pure Fe3O4 magnetic particles; SEM (A) and TEM (B) images of a mixture containing Fe3O4 microspheres and polypyrrole prepared in the absence of PVA; TEM images of core-shell Fe3O4@PPy composite microspheres prepared with the pyrrole/Fe3O4 ratio of 50, at the concentration of p-TSA (2.5 mol/L) and in the replacement of p-toluenesulfonic acid by hydrochloric acid; The thermal gravimetric curve of prepared Fe3O4 microspheres at nitrogen atmosphere from room temperature to 800 °C; Magnified hysteresis loops of samples from -500 to 500 Oe at room temperature and magnetic separation-redispersion process of Fe3O4@PPy composite microspheres (inset); Fabrication process and SEM images of pure polypyrrole polymer and Fe3O4-PPy composites; Electromagnetic parameters of Fe3O4-PPy composite microspheres with different PPy content at 50% mass fraction in the frequency range of 2-18 GHz. AUTHOR INFORMATION *Corresponding Author, E–mail: [email protected]; Tel/Fax: +86–029–88431653. ACKNOWLEDGEMENTS The authors are grateful for the financial support provided by the National Natural Science Foundation of China (Grant No. 51173146).


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