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Highly Efficient Microwave Absorption of Magnetic NanospindlesConductive Polymer Hybrids by Molecular Layer Deposition Lili Yan, Xixi Wang, Shichao Zhao, Yunqin Li, Zhe Gao, Bin Zhang, Maosheng Cao, and Yong Qin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16864 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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ACS Applied Materials & Interfaces

Highly Efficient Microwave Absorption of Magnetic NanospindlesConductive Polymer Hybrids by Molecular Layer Deposition Lili Yan,†,‡ Xixi Wang,§ Shichao Zhao,†,‡ Yunqin Li,†,‡ Zhe Gao,† Bin Zhang,† Maosheng Cao§* and Yong Qin†* †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

Sciences, Taiyuan 030001, China. E-mail: [email protected]. ‡

University of Chinese Academy of Sciences, Beijing 100039, China

§

School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081,

China. E-mail: [email protected] KEYWORDS: oxidative molecular layer deposition, electromagnetic parameters, microwave absorption, conductive polymer, magnetic materials

ABSTRACT Oxidative molecular layer deposition (oMLD) is applied to fabricate conductive polymer-magnetic materials core-shell microwave absorbers in this work. One dimensional Fe3O4-poly(3,4-ethylenedioxythiophene) (PEDOT) nanospindles with controllable PEDOT thickness are successfully synthesized. Their absorption performance was evaluated in 2-18 GHz frequency range. With the advantage of oMLD, PEDOT shell thicknesses can be controlled precisely. Because the permittivity of Fe3O4-PEDOT nanospindles increase obviously while their permeability decrease slightly with PEDOT cycle, the properties can be tuned effectively by only adjusting PEDOT cycle number. With a proper PEDOT shell thickness, excellent reflection

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characteristics can be obtained. Remarkably high absorption strength (-55.0 dB at 16.2 GHz) and good absorption bandwidth (4.34 GHz less than -10 dB) was realized. Such excellent performance is better than most magnetic material-based absorbers reported previously. Considering the precise controllability and excellent absorption performance of the prepared microwave absorbers, we believe that oMLD is a facile synthetic route for microwave absorbers.

INTRODUCTION Conductive polymers and their composites have attracted extensive attention and found numerous applications such as energy storage1,2, sensors3,4, metal corrosion protection5,6, electromagnetic shielding7,8 and microwave absorption9,10. In recent years, a number of researches have successfully been focused on microwave absorption properties of conductive polymers. As one of the dielectric microwave absorption materials (MAMs), conductive polymers have a lot of advantages compared with traditional MAMs, such as light weight, resistance to corrosion, ease of processing and with wide conductivity range.11,12 The research of conductive polymer MAMs was concentrated on polypyrrole13,14, polyaniline15,16 and polythiophene17,18, etc. Among conductive polymers, Poly(3,4-ethylenedioxythiophene) (PEDOT), a polythiophene derivative, is a widely studied conductive polymer having high electrical conductivity, visible transparency, electrochemical activity, moderate band gap, and excellent environmental stability.19-22 Some researchers have studied the microwave absorption performance of PEDOT and its composites.23 These studies usually composite PEDOT with magnetic particles to synergy their dielectric properties and magnetic properties, and gain excellent PEDOT-based MAMs by adjusting the proportion of the dielectric to magnetic components.24-26 For example, Zhou et al. prepared core-shell Fe3O4-PEDOT composites first by a solvothermal process for Fe3O4


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synthesis, and then a polymerization process for PEDOT coating.27 Anil et al. synthesized coreshell barium ferrite-PEDOT nanoparticles by in situ emulsion polymerization.28 Zhou et al. also synthesized iron oxides-PEDOT composites using a one-step method.29 These studies found that the microwave absorption performance of the composites highly relied on the PEDOT content, the interfacial polarization and the anisotropic energy. However, although the composites consisted of magnetic particles and PEDOT have an attractive prospect, the preparation of PEDOT-magnetic particles MAMs with highly controllable PEDOT shells is still a challenge. Therefore, it is really difficult to tune the microwave absorption properties precisely by traditional PEDOT coating methods for broad band, multi-band absorption compatibility, and efficient absorption as well as tuning flexibility. Oxidative molecular layer deposition (oMLD), a newly chemical route similar to atomic layer deposition (ALD) first developed by Sarah in 2014, opens a new route to design and prepare MAMs.30-34 oMLD is a subset of ALD. It is a high-level film deposition method for polymer on base of sequential, self-limiting surface reactions. Monomer and oxidant are introduced to the substrate alternately to repeat the process of adsorption and oxidation, respectively. Nanometresized polymer layer with constant thickness was deposited on the substrate surface for each cycle. The monolayer growth mode of oMLD leads to the uniform and precisely tunable thickness of the PEDOT films, benefiting for control of core-shell structure and enhanced microwave absorption properties.35-38 Furthermore, one dimensional (1D) magnetic structures usually exhibit high magnetic loss because of their large shape anisotropy.39-42 Therefore, 1D magnetic MAMs with PEDOT shells may exhibit enhanced microwave absorption performance. Unfortunately, there have been no such reports till now.



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Herein, we present oMLD fabrication of 1D core-shell Fe3O4-PEDOT nanospindles, and evaluate their microwave absorption properties. Thanks to the controllability and conformality of PEDOT deposition, we can tune microwave absorption properties effectively by simply adjusting the oMLD cycle number.

RESULTS AND DISCUSSION Scheme 1 illustrates a process for preparation of Fe3O4-xPEDOT nanospindles. Fe2O3 nanospindle was first synthesized via a hydrothermal process (Scheme 1a). Fe3O4 nanospindle was then obtained by a procedure of hydrogen reduction (Scheme 1b). The final step is PEDOT oMLD deposition on the surface of Fe3O4 nanospindle (Scheme 1c). The EDOT vapour was first introduced and EDOT molecules were adhered to functional groups of Fe3O4 via strong physical adsorption, such as hydrogen bonding and van der waals.43-45 A purge step then removed unadsorbed EDOT (Scheme 1ci). Afterwards a dose of MoCl5 vapor is introduced, then the adsorbed EDOT reacts with MoCl5 and forms a stable PEDOT film which does not desorb from Fe3O4 surface. The residual gases were purged out by nitrogen (Scheme 1cii). PEDOT oMLD was carried out by repeating EDOT and MoCl5 dose in alternating sequence for a desired cycle number (Scheme 1ciii).


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Scheme 1. Schematic preparation process for the Fe3O4-xPEDOT nanospindles. (a) Hydrothemal synthesis for Fe2O3 nanospindle. (b) Reduction for Fe3O4 nanospindle. (c) Oxidative polymerization of PEDOT. The morphology and element composition of the Fe3O4 and Fe3O4-xPEDOT nanospindles were investigated by scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS). The Fe3O4 sample consists of monodispersed nanospindles which are 60–80 nm in diameter and 300–500 nm in length (Figure S1a). Figure 1a, b, c show the morphology of the obtained Fe3O4-20PEDOT, Fe3O4-40PEDOT and Fe3O4-60PEDOT core-shell nanospindles prepared by applying 20, 40 and 60 oMLD cycles of PEDOT on Fe3O4 nanospindles. The morphology of the samples did not change significantly, excepting for some changes in size and surface roughness caused by the introduction of PEDOT. Figure 1d shows the corresponding concentration ratio between C and Fe calculated by EDS results in Figure S1d, f, and h. The



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concentration ratio increase with the oMLD cycle, proving that PEDOT content also becomes higher with the oMLD cycle.

Figure 1. SEM images of (a) Fe3O4-20PEDOT, (b) Fe3O4-40PEDOT, (c) Fe3O4-60PEDOT nanospindles and (d) corresponding C/Fe concentration ratio. Figure 2a shows the transmission electron microscopy (TEM) image of a Fe3O4 nanospindle, whose outline is consistent with the SEM image in Figure S1a, while some holes emerge in the middle of the spindle due to the oxygen atom losing caused by H2 reduction. Figure 2b, c and d show the morphology of Fe3O4-20PEDOT, Fe3O4-40PEDOT and Fe3O4-60PEDOT nanospindle, respectively. The Fe3O4 nanospindles are fully covered by a PEDOT layer, which indicates that PEDOT films were successfully grown via oMLD process, using MoCl5 and EDOT. The PEDOT film thicknesses are 8.0, 16.4 and 22.8 nm for 20, 40 and 60 cycles, respectively. This suggests that the thicknesses of PEDOT films increase linearly with PEDOT cycle numbers (Figure 2e).


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Figure 2. TEM images of (a) Fe3O4 nanospindle, (b) Fe3O4-20PEDOT nanospindle, (c) Fe3O440PEDOT nanospindle, (d) Fe3O4-60PEDOT nanospindle. (e) Relationship between PEDOT cycle numbers and PEDOT layer thicknesses. Figure 3 shows the TEM images and SAED pattern of the Fe3O4-20PEDOT nanospindle. All the Fe3O4 nanospindles are fully coated by PEDOT and the identical thickness can be observed clearly, demonstrating that PEDOT oMLD is a controllable method for the preparation of coreshell structures (Figure 3a, b). Representative high-resolution transmission electron microscopy (HRTEM) image reveals the high crystallinity of the Fe3O4 core (Figure 3c). The interplanar



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spacing of 0.484, 0.417, 0.296 nm correspond to the (111), (002) and (220) plane of the Fe3O4, respectively. The SAED pattern of Fe3O4-20PEDOT reveals that the Fe3O4 nanospindles retain single crystal structure after hydrogen reduction of hematite nanospindles (Figure 3d).

Figure 3. (a, b) TEM images, (c) HRTEM image, and (d) SAED pattern of Fe3O4-20PEDOT nanospindles. The crystalline structures of the nanospindles were investigated by X-ray diffraction (XRD) characterization. Figure S3a presents the XRD patterns of the as-synthesized α-Fe2O3 sample, and all peaks match well with a rhombohedral structure of hematite (JCPDS no. 89-0598). After reduction, the XRD pattern shows new characteristic of Fe3O4 (Figure S3b). The peaks of the reduced Fe3O4 nanospindles can be indexed to magnetic Fe3O4 (JCPDS no.75-0033). The XRD


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patterns of Fe3O4-20PEDOT, Fe3O4-40PEDOT, Fe3O4-60PEDOT (Figure S3c, d, e) exhibit similar diffraction peaks to pure Fe3O4. No PEDOT diffraction peaks exist in the XRD patterns, indicative of its amorphous state. The diffraction patterns indicate that Fe3O4-xPEDOT composites do not contain impurity. To further confirm a PEDOT film was deposited on as prepared nanospindle, Fourier transform infrared (FTIR) spectroscopy analyses were performed to investigate the chemical bonding. Figure 4 shows the FTIR spectra of the samples with different PEDOT cycles. A characteristic peak at 565 cm-1 is presented in the FTIR spectrum of Fe3O4 (Figure 4a), which is due to the stretching of Fe-O bond. Figure 4b shows the FTIR pattern of Fe3O4-20PEDOT. The peaks of δ(C-S) at 697, 841, 929, and 982 cm-1, ν(C-O-C) at 1092, 1140, and 1211 cm-1, δ(C-C) at 1329 cm-1, and δ(C=C) at 1473 and 1523 cm-1 are clearly observed in the IR spectra. Figure 4c, d show the FTIR patterns of Fe3O4-40PEDOT and Fe3O4-60PEDOT nanospindles. The peaks of Fe3O4 and PEDOT shift to lower wavenumber by increasing the PEDOT oMLD cycles, which is resulted from the interaction between magnetic particles and conductive polymer.27,28,46 The bulk features of PEDOT increase with by increasing the oMLD cycles, while the peak corresponding to Fe3O4 becomes weaker. The FTIR spectra confirm that Fe3O4 and PEDOT coexist in the composite.



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Figure 4. FTIR spectra of (a) Fe3O4, (b) Fe3O4-20PEDOT, (c) Fe3O4-40PEDOT, and (d) Fe3O460PEDOT nanospindles. Thermogravimetric (TG) analysis was applied to investigate the thermal decomposition behaviors of Fe3O4 and Fe3O4-xPEDOT nanospindles. As shown in Figure S4, an obvious weight increase emerges in 150−320 °C range because of the transformation of Fe3O4 to Fe2O3, and two weight losses occurs at about 100 °C and 220 °C because of the loss of water and pyrolysis of PEDOT, respectively. The PEDOT components are completely combusted and Fe3O4 components are oxidized to Fe2O3 in air, so the content of PEDOT can be calculated with Equation 1: wt %PEDOT 1 wt % R

.

(1)

where wt%R is the residual mass ratio. The calculated PEDOT contents after 20, 40, 60 oMLD cycles are 5.12, 11.55, and 15.08 wt%, respectively. The amounts of PEDOT increase with oMLD cycle number. Therefore, it is possible to conveniently control the amount of PEDOT


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shells by simply changing the oMLD cycle number, which is difficult to realize by traditional coating methods. The static magnetism of the Fe3O4 and Fe3O4-xPEDOT nanospindles was investigated by a vibrating sample magnetometer (VSM) at room temperature. The magnetic hysteresis curves reveal the saturation magnetization (Ms) and corresponding coercivity (Hc) values are 66.20, 61.08, 51.89, 45.13 emu·g−1 and 38.96, 41.53, 42.64, 44.36 Oe for Fe3O4, Fe3O4-20PEDOT, Fe3O4-40PEDOT, and Fe3O4-60PEDOT, respectively (Figure 5a). The high Ms of Fe3O4 is due to its intrinsic strong magnetism. The decrease of Ms with the increase of PEDOT oMLD cycle can be easily explained by the increased content of nonmagnetic PEDOT. Figure 5b shows the relationship between the Ms values, Hc values and the PEDOT cycle numbers. The Ms values decrease while Hc values increase quasi-linearly with the increase of PEDOT cycle number, further revealing that the magnetic properties of the composites can be tailored by adjusting PEDOT thickness.



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Figure 5. The magnetic properties of Fe3O4, Fe3O4-20PEDOT, Fe3O4-40PEDOT, and Fe3O460PEDOT nanospindles. (a) Hysteresis loops of the samples. (b) Relationships between saturation magnetization, coercivity and the PEDOT cycle. On the base of the transmission line theory, electromagnetic absorption performance of MAMs can be calculated with their relative complex permittivity (εr) and permeability (µr) at a given thickness and frequency. Figure 6 shows the complex permittivity and the complex permeability of the Fe3O4-xPEDOT samples at the same mass fraction of 50 wt% mixed with paraffin within 2–18 GHz. The electromagnetic parameters of Fe3O4 sample with the same filler content (50 wt%) are presented in Figure S5 for comparison. The Fe3O4 has lager ε' and ε" values than Fe3O4-xPEDOT and the ε' and ε" values fluctuate in the range of 11.7-21.9 and 4.7-7.8, respectively. The ε' values of Fe3O4-xPEDOT samples reduce with the increased frequency, consistent with the general rule of dielectric materials (Figure


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6a).47 It was found that the ε' values of Fe3O4-xPEDOT samples gradually rise with the increase of PEDOT cycle. This enhancement can be attributed to higher polarization ability introduced by PEDOT shell.7

Figure 6. Electromagnetic parameters of the Fe3O4-xPEDOT nanospindles mixed with paraffin: (a) Real permittivity, (b) Imaginary permittivity, (c) Dielectric loss tangent, (d) Real permeability, (e) Imaginary permeability, (f) Magnetic loss tangent. The insets are partially enlarged views. The ε" curves of Fe3O4-xPEDOT samples exhibit obvious relaxation peaks in the frequency band of 6-10 GHz and 12-16 GHz (Figure 6b). According to the reported literatures48, the relaxation peak I may probably arise from defect dipole polarization in the Fe3O4 crystals. Furthermore, the HRTEM images present obvious interfaces between Fe3O4 crystals and PEDOT,



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indicating the relaxation peak II may attribute to dipole polarization from the interfaces.49 These two kinds of relaxations play important roles in dielectric losses.50 Furthermore, the conductivity increases with the PEDOT shell thickness, which also contributes to the increased imaginary permittivity. Therefore, when PEDOT cycle increases, the volume fraction of Fe3O4-xPEDOT gradually arises, leading to the enhancement of ε". In addition, the tan δe values of Fe3O4xPEDOT samples increase with increasing PEDOT cycles, also indicating the samples have the strongest dielectric loss, as shown in Figure 6c.51-53 The Cole-Cole curves of the Fe3O4-xPEDOT samples were also used to analyze the polarization behaviors (Figure S6). Two Cole−Cole semicircles are displayed on each ε"-ε' curves, indicating there are two kinds of polarization mechanism. The semicircle at the frequency of ~14.32 GHz implies one kind of dielectric relaxation process, which is most probably due to interfacial polarization of Fe3O4-PEDOT interfaces. Another semicircle at the frequency of ~9.12 GHz may arise from defect dipole polarization in the Fe3O4 crystals, which can be observed easily in the HRTEM images (Figure 2, 3). Figure 6d and e show the µ' and µ" values of the Fe3O4-xPEDOT nanospindles. The µ' curves of Fe3O4-xPEDOT nanospindles show decline tendency in 2-7 GHz and almost constant in 7-18 GHz. Meanwhile, the decline rates of µ' curves reduce gradually by increasing PEDOT cycle number, resulted from the reduce of Ms because of the introduction of PEDOT.54 Resonance of the µ" can be obviously observed in 2-7 GHz and 10-18 GHz, as shown in Figure 6e. The peaks are induced by natural and exchange resonance of Fe3O4 in the electromagnetic field.55 Moreover, µ"(µ')-1f-1 values are almost constant in 8-18 GHz (Figure S7), which indicates that the Fe3O4-xPEDOT nanospindles probably generate eddy current at higher frequency of electromagnetic field, also contributing to µ" values.56 On the other hand, the


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volume fraction of Fe3O4 slightly decrease with the increase of PEDOT cycle, and then the magnetization of Fe3O4-xPEDOT is weakened gradually, which results in the decrease of µ" values.57 Furthermore, the tan δm values also present decline tendency with increasing PEDOT cycle (Figure 6f). The reflection loss (RL) values of the Fe3O4 and Fe3O4-xPEDOT nanospindles were obtained on base of the measured complex permittivity and permeability using the transmission line theory: Z Z ε tan h [j % µ

&π'(

* +µ, ε, ]

)

5 85

RL 20 log | 567 :59 | 67

9

(2) (3)

where εr and µr are the relative complex permittivity and permeability, respectively, c is the velocity of light, f the microwave frequency, d the absorber thickness , Z0 the impedance of free space, and Zin the input impedance of the absorber. Figures 7 a, b, c, d show the three-dimensional projection charts versus frequency for the Fe3O4 and Fe3O4-xPEDOT nanospindles with a content of 50 wt% in paraffin. Compared with Fe3O4 nanospindles, the minimum RL of Fe3O4 covered with only 60 cycles of PEDOT increases ~1.4 times, reaching -55.0 dB. Meanwhile, the matching thickness of Fe3O4-60PEDOT nanospindles reduces to less than one fifth of the original Fe3O4 nanospindles, which is only 1.3 mm and much beneficial to wide applications. Furthermore, with increasing the PEDOT cycle number of Fe3O4-xPEDOT samples, the minimum RL value enhances, as shown in Figure 7e. Moreover, the strongest absorption bands (RL ≤ -30 dB) of Fe3O4-xPEDOT samples shift to higher frequency and lower thickness, as shown in Figure 7f. The result demonstrates that deposition of PEDOT by oMLD is an efficient strategy for significantly enhancing microwave absorption, decreasing matching thickness and tailoring strong absorption bands. 15 Environment ACS Paragon Plus


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Figure 7. Absorption performance of Fe3O4-xPEDOT nanospindles in 2–18 GHz: (a) Fe3O4, (b) Fe3O4-20PEDOT, (c) Fe3O4-40PEDOT, and (d) Fe3O4-60PEDOT nanospindles. (e) The minimum RL values versus the PEDOT cycle numbers. (f) The matching thicknesses versus the PEDOT cycle numbers. Figure 8a depicts the microwave absorption properties of samples at 1.4 mm thickness. The bandwidths with RL values less than -10 dB get much broader with the increase of PEDOT cycles, achieving 4.34 GHz. Moreover, the absorption peaks move to lower frequencies by


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increasing the PEDOT cycle and sample thickness (Figure 8b), covering Ku, X and C three microwave bands. It indicates that deposition of PEDOT by oMLD also broadens absorption bandwidth and achieves selective-frequency absorption. The relationship between peak frequencies and sample thicknesses were also analyzed. According to the quarter-wavelength matching model, the incident wave encounter reflected wave at the air-absorber interface, when their phase difference is 180°, they extinct each other and zero reflection takes place. The peak frequency (;< ) are associated with the absorber thickness (=< ) by Equation 4: =<

>?

@AB CDE CCFE C

(n=1, 3, 5…)

(4)

where c is the light velocity in vacuum, Cεγ C and Gµγ G are the modulus of the measured εγ and µγ at fI . Figure S8 shows variations of the peak frequency versus absorber thickness of Fe3O420PEDOT nanospindles (black line) according to the above-mentioned Equation 4. The experimental data (red dots) are directly obtained from the data in Figure 8b. The experimental data fits well with the simulations, indicating the good microwave absorption properties can be reasonably interpreted with this model.



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Figure 8. (a) RL curves of Fe3O4-xPEDOT nanospindles with a 1.4 mm thickness. (b) RL curves of Fe3O4-20PEDOT nanospindles with different matching thicknesses. The Fe3O4-xPEDOT nanospindles, especially which have thicker PEDOT shell, exhibit excellent microwave absorption ability; the Fe3O4-60PEDOT nanospindles have strong absorption, wide bandwidth, and small thickness. Thus, the effective introduction of PEDOT can significantly enhance their microwave absorption performance. On the one hand, the volume fractions increase with PEDOT cycles for the same mass fraction, thus form a better conductive network, which lead to increase of conductance loss (Scheme2a, c).58 On the other hand, the induction of PEDOT will also increase relaxation loss. Meanwhile, the Fe3O4-xPEDOT nanospindles with thicker PEDOT film have stronger internal reflection and scattering loss, resulting in significantly improved microwave absorption performance (Scheme 2b, d).59 More importantly, the deposition of PEDOT can effectively control electromagnetic parameters of the


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composites. The permittivity of the samples increase obviously while the permeability of them decline slightly as PEDOT shells thicken, which make the samples denser, that is, the effective thickness gets thicker while the geometric thickness maintains, so the strongest absorption shifts to lower frequency. Meanwhile, compared with Fe3O4 nanospindles, the impedance matching of Fe3O4–xPEDOT is improved by introduction of dielectric components and interface polarization. In addition, the thickness control of PEDOT oMLD is also in favour of electromagnetic impedance matching, realizing resonance absorption.12,60,61

Scheme 2. Schematic illustration of electromagnetic loss. (a,c) Formation of conductive network. (b,d) Internal reflection and scattering loss. CONCLUSIONS We successfully developed a facile route to synthesize core-shell 1D Fe3O4-PEDOT nanospindles by using oMLD for the first time. The thicknesses of PEDOT shells can be well controlled by changing the PEDOT cycles. The Fe3O4-PEDOT nanospindles show more excellent microwave absorption properties than many Fe3O4 based composites ever reported. Excellent microwave absorption properties arise from more microwave internal reflection,



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scattering and the formation of conductive network. More importantly, the microwave absorption properties are highly dependent on the thicknesses of PEDOT shells, which means that the absorption ability can be simply modulated by only adjusting the PEDOT oMLD cycles. We believe this study provides a new route to design and optimize advanced microwave absorbents with uniform microstructure and enhanced functional property. EXPERIMENTAL SECTION Synthesis of Fe3O4 nanospindles. α-Fe2O3 nanospindles were first produced by a solvothermal method. Briefly, 260mg FeCl3 solution (0.02M) and 6.6mg NH4H2PO4 (7.2×10-4M) were dissolved into 80 mL deionized water. The above homogeneous solution was then transferred into a Teflon-lined stainless steel autoclave for solvothermal reaction (220°C, 2 h). The resultant precipitates were washed with deionized water and ethanol for several times, and vacuum dried in an oven (40°C, 10 h). Afterwards, Fe3O4 nanospindles were obtained by reduction of the a-Fe2O3 nanospindles in a 5% H2/Ar atmosphere (430°C, 3h). Synthesis of the Fe3O4-PEDOT core-shell nanospindles. PEDOT oMLD approach was performed in a home-made ALD reactor. Before oMLD, the Fe3O4 nanospindles were put in ethanol and dispersed under ultrasonic agitation for 10 min and subsequently dropped on a quartz substrate. After being dried in air, PEDOT was coated by sequential exposure of EDOT monomer and MoCl5 oxidant to Fe3O4 nanospindles. The precursor MoCl5 and EDOT were heated to 80°C and 60°C, respectively, to obtain reasonable vapor pressures for both precursors. One cycle was composed of an exposure time of 7 s for MoCl5 oxidant followed by a 60 s N2 purge, 10 s EDOT dose, and then 60 s N2 purge. The depositions of PEDOT were conducted at 115°C. The samples were denoted as Fe3O4-xPEDOT nanospindles, where x refers to the PEDOT cycle number.


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Characterization. The morphology of the Fe3O4 and Fe3O4-xPEDOT nanospindles was investigated using SEM (JEOL, JSM-7100F), TEM and HRTEM (JEOL-2100F). The element composition was examined by EDS (JEOL, JSM-7100F). The structural characterization was performed by XRD (BrukerD8 Advance system). FTIR spectra was recorded using an infrared spectrophotometer (Bruker, Tensor II)in the range of 500-2000 cm-1. The thermal decomposition properties of the samples were investigated by TG analysis on a SETSYS EVOLUTION TGA 16/18 instrument. The hysteresis loops were obtained on a Lake Shore 7410 VSM. The Fe3O4xPEDOT samples were mixed with paraffin at a mass ratio of 50 wt% and then pressed into toroidal whose outer diameter is 7.00 mm and inner diameter is 3.04 mm. The complex permittivity and permeability were measured using an Agilent N5422A vector network analyzer in 2–18GHz. ASSOCIATED CONTENT Supporting Information Representative SEM images of as-synthesized Fe3O4 and Fe3O4-xPEDOT (x=20, 40 and 60) nanospindles and the corresponding EDS results; TEM images, TG and Eddy-current loss curves of Fe3O4 and Fe3O4-xPEDOT (x=20, 40 and 60) nanospindles; X-ray diffraction patterns of αFe2O3, Fe3O4 and Fe3O4-xPEDOT (x=20, 40 and 60) nanospindles; Electromagnetic parameters of the Fe3O4 nanospindles; Cole−Cole semicircles of Fe3O4-xPEDOT (x=20, 40 and 60) nanospindles; Simulation of the absorber thickness versus peak frequency for Fe3O4-20PEDOT nanospindles. AUTHOR INFORMATION Corresponding Authors



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*E-mail: [email protected].*E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We appreciate the financial support from the National Nature Science Foundation of China ( 21376256, 21673269 and 51132002), the Hundred Talent Program of the Chinese Academy of Sciences, the Hundred Talent Program of ShanXi Province, Youth Innovation Promotion Association of the Chinese Academy of Sciences, the ShanXi Science and Technology Department, Department of Human Resource and Social Security of ShanXi Province, and Innovation Fund of Science and Technology for Graduate Students of BIT (Nos. 2016CX06004.)

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