Carbon@Polyaniline Trilaminar CoreâShell ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10710-10721

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Fe3O4@Carbon@Polyaniline Trilaminar Core−Shell Composites as Superior Microwave Absorber in Shielding of Electromagnetic Pollution Kunal Manna and Suneel Kumar Srivastava* Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

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S Supporting Information *

ABSTRACT: The present work reports fabrication of trilaminar core−shell composites of Fe3O4@C@PANI as efficient lightweight electromagnetic wave absorber by facile hydrothermal method and subsequent high-temperature calcination followed by its encapsulation through oxidative polymerization of aniline. The prepared composite structure was characterized by FTIR, XRD, XPS, TEM, HRTEM, and SQUID. The measurement of reflection loss, complex permittivity, complex permeability, and total shielding efficiency of the composites has been carried out in the frequency range of 2−8 GHz. Our findings showed lowest reflection loss (∼33 dB) in composite comprised of Fe3O4@C:aniline (1:9 wt/wt) corresponding to shielding efficiency predominantly due to absorption (∼47 dB) than reflection (∼15 dB). Such high value of shielding efficiency could be ascribed to the presence of dual interfaces and dielectric−magnetic integration in Fe3O4@C@PANI. In all probability, higher dielectric loss through interface polarization and relaxation effects in Fe3O4@C@PANI could also contribute toward superior microwave absorption ability of Fe3O4@C@ PANI compared to Fe3O4@C and Fe3O4/PANI binary composites. This is likely to enhance the interfacial polarization, natural resonance, dielectric polarization, trapping of EM waves by internal reflection, and effective anisotropy energy in Fe3O4@C@ PANI. KEYWORDS: Ternary, Interfaces, Permittivity, Permeability, Impedance matching, Absorption

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and/or magnetic losses could be a better choice.6,14 In this regard, Fe3O4-based composites have been receiving considerable amount of attention as microwave absorber. This is mainly due to its magnetic properties, soft metallic nature, large magnetic anisotropy, good biocompatibility, and low toxicity.15−20 The available literature survey revealed that Fe3O4/C composites earned much attention owing to their tunable properties and high chemical stability of carbon materials as well as significant synergetic or complementary behavior between Fe3O4 and carbon.21 Recently, Fe3O4 (core)@C(shell) composites has also been reported as an efficient route in development of efficient absorbing material due to chemical homogeneity and effective combination of magnetic loss ability and dielectric loss ability.15,19,20 In addition, carbon-based nanotubes,22,23 carbon nanocoils,24,25 and nanofibers26,27 filled with magnetic metals have also been widely studied as lightweight microwave absorbents. Wang et al. induced Fe3O4@ZnO core−shell nanoparticle decorated carbon nanotubes to form MWCNT/Fe3O4@ZnO heterotrimers by a chemoselective route.28 C@Fe3O4 core−shell nano coils has

INTRODUCTION Electromagnetic (EM) waves contribute enormously toward diverse applications in synthetic aperture radar, satellite broadcast, telecommunication, computers, TV, radio, mobile phones, etc.1−4 The generation of electromagnetic radiations through proliferation of these electronic devices and wider ongoing instrumentation leads to serious environmental pollution; exposure to it causing potential health hazards in human beings.5,6 Further, interference of these EM waves could completely disrupt functioning and performance of many electronic devices. Additionally, it slows down the efficiency of electronic devices and degrades the reliability, lifetime, and safety of electrical equipment.5,7 In view of this, fabrication of anti-EM interference coating materials as microwave absorber remain one of the most important research attribute today.4−6,8−10 An ideal microwave absorption materials should exhibit wide bandwidth, strong absorption properties, low density, lightweight, low cost, good thermal stability, antioxidation capability, and design flexibility simultaneously.4,11,12 In addition, other crucial features that affect the performance of microwave absorption are complex permittivity/permeability, matching of electromagnetic impedance, and microstructure of the absorbent materials.4,13 Therefore, fabrication of composites consisting of polymer-inorganic material with high electric © 2017 American Chemical Society

Received: August 4, 2017 Revised: October 6, 2017 Published: October 9, 2017 10710

DOI: 10.1021/acssuschemeng.7b02682 ACS Sustainable Chem. Eng. 2017, 5, 10710−10721

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic Presentation of fabrication of Fe3O4@C@PANI ternary composite.

been synthesized using the atomic layer deposition technique.24 Fe3O4/carbon composite nanofibers were prepared by carbonizing electrospun polyacrylonitrile/magnetic Fe nanoparticles.29 Fe3O4−graphene, Fe3O4−carbon nanotube composites have also been reported as excellent shielding materials.30,31 Despite high-performance microwave absorption, synthetic process of these materials are relatively more complex.6 The presence of low content of magnetic nanoparticles in all these composites lead to lower complex permeability.28−31 This is followed by decrease in dielectric loss unfavorable for impedance matching, i.e., microwave absorption.6 Therefore, according to Ji et al.,32 it is desirable to prepare a hybrid consisting of high dielectric loss and magnetic materials. It is anticipated that inclusion of conducting polymers exhibiting high dielectric loss ability could be the better alternative in this case.6 A combination of electrically conductive and magnetic materials could be more effective in enhancing the electromagnetic wave attenuation synergistically.6 The electrical conductivity of conducting material allows the flow of eddy current induced by the magnetic field imparted by magnetic component and leads to the absorption of EM radiation.5 As far earlier reports concern, numbers of electromagnetic polymer-based composites,33,34 such as, carbon encapsulated Fe3O4/PVDF, polyaniline (PANI)/γ-Fe 2 O 3 /TiO 2, 7 Fe 3 O 4 /carbonyl iron powders (CIP)/PANI,35 nickel/hexagonal ferrite/polymer,36 γ-Fe2O3/ poly(3,4-ethylenedioxythiophene) (PEDOT),37 PANI/γ-Fe2O3 or Fe3O4 polymer,38 core−shell PEDOT, and PANI-based barium ferrite nanocomposites39 have been reported as promising electromagnetic shielding materials. The choice of PANI as conducting polymers in most of these works is guided by its high conductivity, low cost of synthesis, and great environmental stability.40,41 In contrast, strong particle−particle magnetic attraction in Fe3O4 lead to their aggregation resulting in weak interconnecting magnetic network throughout the PANI matrix. Therefore, fabrications of PANI/Fe3O4 composites taking this into consideration are a challenging task. According to available literature, uniform core−shell Fe3O4@ C@polyaniline (PANI) trilaminar composite microspheres have been fabricated from binary Fe3O4@C core−shell by subjecting ferrocene with H2O2 at 240 °C for 72 h under hydrothermal condition followed by in situ polymerization of aniline.42 This motivated us to investigate role of dual interface in superior microwave absorption and shielding performance. It is anticipated, that inclusion of PANI to create an additional interface (C@PANI) could lead to decrease in real and imaginary parts of complex permittivity. Further, the ternary combination of dielectric/magnetic materials not only upsurge the attenuation ability, but also maintains a reasonable degree of impedance matching. Moreover, aggregation of Fe3O4 in PANI matrix could be prevented by diamagnetic carbon

between the interfaces of Fe3O4 and PANI. This is likely to result higher EM absorption due to its homogeneous distribution as Fe3O4@C in PANI matrix compared to Fe3O4@PANI alone. Additionally, dual interface (Fe3O4@C and C@PANI) in Fe3O4@C@PANI trilaminar core−shell composites could lead to strong interfacial polarization and rapid conversion of the incidence EM wave into thermal energy. In view of this, we employed a tactful simple and ingenious approach to synthesize unambiguous ternary core− shell Fe3O4@C@PANI composites. For this purpose, binary Fe3O4@C core−shell were prepared from FeCl3·6H2O, glucose, and urea under hydrothermal condition (180 °C/14 h) at relatively much lower temperature than reported earlier.41 Subsequently, ternary Fe3O4@C@PANI composites were fabricated by subjecting earlier prepared different contents of Fe3O4@C to in situ aniline polymerization. Finally, trilaminar Fe3O4@C@PANI composites have been characterized by several techniques (Supporting Information, SI-1), such as XRD, XPS (SI-2), FTIR (SI-3), FESEM (SI-4), EDX (SI-4, Table S1), Raman (SI-5) TEM, HRTEM, SQUID, and its performance as microwave absorber in shielding of electromagnetic pollution.

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EXPERIMENTAL SECTION

Materials. Ferric chloride hexahydrate (FeCl3·6H2O), glucose (C6H12O6), urea, (CO(NH2)2), ammonium persulfate, (NH4)2S2O8 (APS), and aniline were purchased from Merck, India. Ethanol (C2H5OH) was procured from SRL Pvt. Mumbai. Preparation of Fe3O4@C core−shell composites. Fe3O4@C composites were synthesized according to the reported method41 following certain minor modifications. In a typical procedure, glucose (0.02 mol), FeCl3·6H2O (3.6 × 10−3 mole), and urea (0.2 mol) were dissolved in 80 mL of water. Subsequently, entire solution was transferred in a 100 mL Teflon-lined stainless steel capacity autoclave (Berghof High Pressure Reactor, Model BR-100) and heated at 180 °C for 14 h duration. The magnetically separated product (Fe3O4@C) was washed with deionized water/ethanol several times and kept for drying at 40 °C in vacuum oven for 12 h. The product obtained in this manner was subjected to calcination in a vertical tubular furnace at 600 °C for 4 h in argon atmosphere. In another experiment, Fe3O4 was prepared following hydrothermal method in absence of glucose. Preparation of Fe3O4@C@PANI ternary composites. Initially, Fe3O4@C was dispersed in a clear 0.1 (N) HCl solution of aniline monomer under ultrasonication for 1h. Next, the brownish black dispersion was cooled down to 0−5 °C under stirring. In parallel, an equimolar amount of ammonium peroxo-disulfate (APS) (aniline:APS = 1:1) was dissolved in 5 mL 0.1 (N) HCl solution and cooled to 0−5 °C. The polymerization reaction was carried out by the rapid addition of the precooled oxidant solution (APS solution) into the suspension, and the mixture was stirring for 24 h at 0−5 °C under ice bath. The dark green precipitate (Fe3O4@C@PANI ternary composite) so obtained was washed with deionized water and ethanol several times, and then dried at 40 °C. For comparison, Fe3O4@C@PANI ternary 10711


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Figure 2. XRD patterns of (a) Fe3O4 and Fe3O4@C and (b) Fe3O4@C@PANI composites.

XPS Analysis. Figure S1 (SI-2) shows typical XPS survey scan spectra and Fe 2p peaks of Fe3O4 and Fe3O4@C. The survey scans indicated presence of peaks due to C 1s, O 1s, 2p3/2 (Fe), and 2p1/2 (Fe). XPS spectra in the binding energy range of 735−705 eV showed presence of peaks, 2p1/2:Fe2+ (724.2 eV), 2p1/2:Fe3+ (724.5 eV), 2p3/2:Fe2+ (710.5 eV), 2p3/2:Fe3+ (710.4 eV) in Fe3O4 and Fe3O4@C. The broadening of 2p3/2 and 2p1/2 peaks in XPS spectra of Fe3O4 and Fe3O4@C could be ascribed to the cohabitation of the Fe3+ and Fe2+.13,21 The absence of shakeup satellites between the two Fe 2p peaks (718.3 eV) excluded the presence of γ-Fe 2 O 3 . The deconvoluted spectra of C 1s and O 1s peak of Fe3O4@C are also displayed in Figure S2a (SI-2). The asymmetric C 1s peak could be deconvoluted to CC (∼284 eV), C−H (∼285 eV), C−O (285.7 eV), and CO (287.5 eV) peaks. The peaks corresponding to C−H and CC could be attributed to the presence of disordered carbon frameworks.47 In addition, C−O and CO peaks indicated the presence of an oxygencontaining functionality on the surface of synthesized [email protected] Figure S2b (SI-2) also shows that deconvoluted O 1s spectra of Fe3O4@C is comprised of peaks corresponding to Fe−O (529.3 eV), Fe−O−C (529.75 eV), C(O)OH (530.61 eV), CO (531.39 eV), and C−OH (532.83 eV) functionality.47,48 The appearance of Fe−O−C peak in the spectra could be ascribed to formation of covalent bonding between Fe3O4 and carbon.49 This also accounted for the stability of the Fe3O4@C47 and indication of Fe3O4 (magnetite) core stability under the protection of carbon shell through Fe−O−C bond formation.21,50 FTIR Analysis. Room-temperature FTIR spectra of Fe3O4 and Fe3O4@C have been displayed in Figure S3 (SI-3). The appearance of three signature peaks (560, 1421, 1627 cm−1) could be assigned to Fe−O bond stretching and bending.51 A band at 1615 cm−1 in Fe3O4@C corresponds to CC vibration of carbon shell.52,53 The band at 3425 cm−1 appears indicating the presence of carboxyl group on the outer surface of carbon42 could facilitate coating of PANI on [email protected] FTIR spectra of Fe3O4@C@PANI and its comparison with pure PANI is shown in Figure S3b (SI-3). PANI shows the appearance of peaks due to CC (1561 cm−1) and C−C (1467 cm−1) stretching of the benzenoid and quinoid rings, respectively.54 Another band at 1292 cm−1 in the spectra of PANI is assigned to the C−N stretching originating from a bipolaron structure of the conducting salt of polyaniline.54 The other bands at 1117 and 806 cm−1 in PANI correspond to in-

composite was synthesized by varying the Fe3O4@C content (aniline: Fe3O4@C = 9:1, 8:2, 7:3) with aniline monomer under identical reaction conditions, and the products are designated as PFC-10, PFC20, and PFC-30, respectively. Figure: 1 represents the schematic presentation of formation of Fe3O4@C@PANI trilaminar core@shell composite.

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RESULTS AND DISCUSSION X-ray Diffraction. Figure 2a depicts XRD patterns of the synthesized Fe3O4 and Fe3O4@C. The diffraction peaks in both the compounds appeared at ∼30.0°, 35.4°, 43.0°, 53.4°, 57.0°, and 62.5° corresponding to the (220), (311), (400), (422), (511), and (440) lattice planes of fcc spinel phase of Fe3O4 (JCPDS 01-1111).40 The peak centered at ∼32° in Fe3O4@C and PFC composites correspond to (221) peak of γ-Fe2O3 as trace amount.43 In addition, appearance of relatively weak and broad peaks in Fe3O4@C could be attributed to the coating of amorphous carbon on Fe3O4.44 XRD patterns of PANI and in situ polymerized coating of PANI on different amounts of Fe3O4@C is also displayed in Figure 2b. It is noted that crystalline peaks of PANI are centered at ∼20.4° and 24.9°, corresponding to (020) and (200) reflection of emeraldine PANI salt.11,42 The appearance of a diffused peak at 2θ ≈ 15− 25° in the Fe3O4@C and Fe3O4@C@PANI could be ascribed to amorphous carbon.45 Further, intense diffraction peaks appeared at 2θ = 25°, 42.30°, 43.14°, and 46.34° in PFC-30 composite F3 corresponding to (002), (100), (101), and (004) reflections of carbon coating on Fe3O4.46 Figure 2a also shows that positions of diffraction peaks of Fe3O4 remained more or less unaltered in Fe3O4@C as well as Fe3O4@C@PANI. These findings clearly indicated absence of any chemical interactions between Fe3O4 and carbon in annealed [email protected] The crystallite size of the Fe3O4@C composites corresponding to the maximum intensity (311) peak was calculated using the Scherer’s equation (eq 1) as follows: P=

kλ β cos θ

(1)

where P is the mean size of the crystallite domains, k is the dimensionless shape factor (0.89), λ = 0.154 nm, β is the full width at half-maximum (fwhm) intensity (in radians), and θ is the Bragg angle. The crystallite size of Fe3O4@C composites was found to be ∼26.10 nm. 10712


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Figure 3. TEM images of (a) Fe3O4 and (b) Fe3O4@C. HRTEM images of (c) Fe3O4 and (d) Fe3O4@C composites.

TEM and HRTEM Analysis. Figure 3a,c shows TEM and HRTEM images of neat Fe3O4. Figure 3b,d shows TEM and HRTEM images of Fe3O4@C to establish conformal coating of the ultrathin carbon layer on Fe3O4. The formation of welldefined carbon-coated Fe3O4 core@shell is clearly inevitable due to the contrast difference of electron density.57 A close inspection of magnified HRTEM image revealed formation of a gray-colored continuous coverage of carbon shell on Fe3O4 nanoclusters without any obvious crack. This could be directly related to the subsistence of a confinement effect provided by the surrounding carbon layer.58 The formation of such Fe3O4@C composite promotes stability of the Fe3O4 magnetic core nanoclusters from attrition in acidic environments. In addition, presence of prolific multiple functional groups (i.e., −OH and −COO− groups) on the surface of the carbon shell accounts for their superior dispersibility in water.50,59 The magnified HRTEM image of the Fe3O4@C in Figure 4b clearly shows the formation of lattice fringes corresponding to the interplanar distance of 0.297 nm and in the agreement with the (220) facet of Fe3O4. The absence of fringes in the carbon layer of Fe3O4@C clearly indicated the amorphous nature of the disordered carbon shell.58,60 The contrast difference of electron density of Fe3O4 (core) and C (shell) also strengthened our contention on formation of interface between these two.57 Further, comparison of the lattice fringes in Fe3O4 and C in Figure 4b also reaffirmed our contention on the formation of interface between these

plane bending vibration of the C−H and out-of-plane deformation of C−H in 1,4-disubstituted benzene ring, respectively.55,56 FTIR peak (554 cm−1) of Fe−O bond (Fe3O4@C) in FT-IR spectra show blue shifting due to its weaker bond energy, i.e., lower frequency (ν) with respect to Fe3O4. This is inferred from the following equation: λ = C /ν = Cυ ̅

(2)

where λ is the absorption wavelength, ν is frequency, and υ̅ is the corresponding wavenumber. Most likely, such observations could be attributed to the formation of Fe−O−C bond in [email protected] FTIR spectra of Fe3O4@C@PANI in Figure S3b also indicated the presence of a broad peak at ∼3450 cm−1 due to N−H stretching53 ascertaining formation of a doped emeraldine salt.54 Additionally, 1106 (stretching of CN and out-of-plane bending of C−H in benzene ring paradisubstituted), 1460 (CC stretching mode of the benzenoid ring), and 1556 cm−1 (CN stretching deformation of quinoid ring) peaks in the spectra of Fe3O4@C@PANI are shifted to 1112, 1492, and 1581 cm−1, respectively. This is clear indication of mutual interaction between PANI and carbon shell in Fe3O4@C@PANI. In all probability, constrained growth of the PANI chains on the surface of carbon shell restricted vibrations of PANI molecule. Therefore, corresponding peaks shifted to higher wave numbers due to increased vibrational frequency.42 All these findings successfully confirm that aniline monomers are polymerized on the surface of Fe3O4@C composites. 10713


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Figure 4. Lattice fringes of (a) Fe3O4 and (b) Fe3O4@C. SAED patterns of (c) Fe3O4 and (d) Fe3O4@C composites.

Figure 5. HRTEM images of (a) PFC-30, (b) PFC-20, and (c) PFC-10.

findings clearly demonstrate that aniline:Fe3O4@C ratio has a significant influence on the thickness of trilaminar core−shell Fe3O4@C@PANI composites.42 On the basis of earlier findings, a possible formation mechanism of Fe3O4@C@PANI core−shell composite could be proposed. The aniline cations formed by reaction of aniline and H+ are readily self-assembled on to the surface of Fe3O4@ C. The carboxyl and hydroxyl groups on the carbon shell uphold the self-assembly of aniline cations on the surface of Fe3O4@C composites owing to electrostatic attraction and the formation of H-bond.42 The presence of amorphous carbon shell in Fe3O4@C primarily played an important role in

two.58,60 The selected area electron diffraction (SAED) pattern in Figure 4b apparently represents six well-resolved diffraction rings corresponding to (220), (311), (400), (422), (511), and (440) planes of Fe3O4 and in agreement with our XRD result discussed earlier.13,57,61 Figure 5a−c shows the TEM images of the as-synthesized Fe3O4@C@PANI composites prepared by selecting aniline:Fe3O4@C ratios of 7:3, 8:2, and 9:1, respectively. It is observed that polymerization of aniline monomers progressively cover the surface of Fe3O4@C composites as inevitable from increased shell thickness. For example, thickness of PANI shell is varied from ∼45 to 70 nm on increasing aniline:Fe3O4@C ratio from 7:3 to 9:1. These 10714


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Figure 6. Magnetic hysteresis loops of (a) Fe3O4 and Fe3O4@C and (b) PFC-10 and PFC-20 composites at 300 K. Insets show magnified images in the magnetic field ranging from −200 to +200 Oe.

formation of the explicit trilaminar core−shell structured Fe3O4@C@PANI composites. The carbon shell increases dispersion of Fe3O4@C composites in solution without any aggregation.42 Further, stability of carbon shell in acidic conditions provides appropriate experimental condition for the polymerization of aniline on [email protected] Magnetic Property Analysis. Figure 6a displays roomtemperature M−H curves of Fe3O4 and Fe3O4@C in the presence of the applied magnetic field ranging from −10 000 to +10 000 Oe. Table 1 provides saturation magnetization (Ms),

Ha =

2πfr = rHa

Ms (emu/g)

Hc (Oe)

Mr (emu/g)

Fe3O4 Fe3O4@C PFC-10 PFC-20

76 9.20 5 2

66.51 76 91.8 57.4

6.61 1.41 1.15 0.36

coercivity (Hc), and remnance (Mr) values obtained from magnetic hysteresis loops referring to Fe3O4, Fe3O4@C, and different amount of Fe3O4@C loaded PANI composites. Fe3O4 showed appearance of insignificant hysteresis loop.62 In contrast, Fe3O4@C composites exhibited ferromagnetic behavior as evident from its substantial hysteresis loop in the M−H curve.62 The comparatively lower coercivity of bulk Fe3O4 manifests weak ferromagnetism and lacks microwave absorbing capability due to reduced crystal size and shape anisotropy.60,63,64 The significant decrease in Ms and Mr values in carbon encapsulated Fe3O4 compared to pure Fe3O4 could be attributed to the coating of the nonmagnetic carbon on the surface of Fe3O4.60,63 Further higher value of Hc in carboncoated Fe3O4 could be ascribed to the size growth of bulk Fe3O4 during high temperature carbonization process.15,21 The higher Hc value also implies larger magnetic anisotropy leading to high-frequency resonance in terms of anisotropy constant (K), anisotropy energy (Ha), and resonance frequency ( f r) as anticipated on the basis of the following equations:65

K=

μ0 MsHc 2

(4) (5)

where μ0, r, and Ha stand for the universal value of permeability in free space (4π × 10−7 H m−1), gyromagnetic ratio, and anisotropy energy, respectively. The interrelationships of these equations suggest higher values of Hc lead to larger magnitudes of K, Ha, and f r. We have also investigated magnetic property to unfold the role of PANI coating on the surface of Fe3O4@C. Considering this aspect, Figure 6b shows room-temperature M−H curves corresponding to PFC-10 and PFC-20. The observed decreases in Ms and Mr values in both the ternary composites compared to pure Fe3O4 and Fe3O4@C could be related to the increase in its size and mass including presence of relatively much smaller contents of Fe3O4 in the composites.42 TEM and HRTEM images of Fe3O4 (∼30 nm) and Fe3O4@C (∼40 nm) clearly indicated the presence of carbon coating (thickness ∼10 nm) on the surface of Fe3O4. HRTEM image of PFC-10 earlier in Figure 5c clearly shows formation of highest outermost thickness of nonmagnetic polyaniline shell (∼30 nm) on Fe3O4@C due to presence of highest aniline:Fe3O4@C ratio (9:1). Considering all this, it is anticipated that PFC-10 composite exhibits larger Hc value due to its larger overall size (∼70 nm). As a consequence, larger magnetic anisotropy of the composite lead to a higher resonance frequency in EM field in accordance with eqs 3−5.21 Permittivity and Permeability Analysis. In general, two key factors should be considered for an excellent EM wave absorber. One is the impedance match, requiring the equality of the electromagnetic parameters, and the other is the best electromagnetic wave attenuation in the interior of the absorber. Therefore, in order to elucidate the mechanism of enhanced microwave absorbing performance of PFC composites, the frequency dependence of the complex permittivity (ε* = ε′ − iε″) and permeability (μ* = μ′ − μ″) was investigated, where ε′ and μ′ represent the storage capability of the electric and magnetic energy, and the imaginary parts (ε″ and μ″) stand for the loss capability of the electric and magnetic energy.66 Figure 7 shows dependence of complex permittivity and complex permeability of Fe3O4, Fe3O4@C (FC), and Fe3O4@

Table 1. Ms, Hc, and Mr values of Fe3O4, Fe3O4@C, PFC-10, and PFC-20 sample

4|K | 3μ0 Ms

(3) 10715


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Figure 7. Plot of (a) frequency vs ε′, (b) frequency vs ε′′, (c) frequency vs μ′, and (d) frequency vs μ′′ for PFC composites.

C@PANI composites (PFC-10, PFC-20, and PFC-30) on the frequency range of 2−8 GHz at room temperature. The corresponding real parts (ε′, μ′) and imaginary parts (ε″, μ″) in this were estimated from the S11 and S21 experimental scattering parameters using standard Nicholson−Ross and Weir theoretical calculations.67,68 For Fe3O4@C@PANI composites, the higher values of ε′ and ε″ increases from PFC-10 to PFC-30 with gradual increase in Fe3O4@C content. This indicates very high storage capability of electric energy and dielectric loss of the materials, which is attributed to the higher electrical conductivity of carbon.66 However, the μ′ and μ″ values of all the samples are almost same over the entire frequency range with a slight fluctuation of μ″. This indicates that all the PFC composites, including Fe3O4@C and Fe3O4, are weakly magnetic but high dielectric loss material.69 Therefore, the larger differences of values between ε′ and μ′ corresponding to Fe3O4 and Fe3O4@C lead to poor impedance matching. Such, lower impedance matching lead to reflection of EM waves from the surface of the samples which results lower absorption of EM wave. Figure 7 suggests that increasing addition of PANI coating on Fe3O4@C is accompanied by decrease in ε′ (and ε″) and increase in μ″. Furthermore, decreasing tendency of ε′ and ε″ with the increasing frequency could be explained in terms of Debye equations,70 i.e., ε′ = ε∞ +

ε″ =

2 2

1+ωτ

ωτ +

σ ωε0

(7)

where the decrease in ε′ and ε″ is attributed to the increase in angular frequency (ω), εs and ε∞ are the static permittivity and relative dielectric permittivity at the high-frequency limit, respectively, and τ is the polarization relaxation time. σ is the material conductivity. It is already observed that high dielectric loss property of PANI encapsulated Fe3O4@C composites could be harnessed for microwave application purposes. According to EM theory, several factors could be contributing in this phenomenon. Therefore, reduction in ε′ of trilaminar core@shell composites create a large orientation interface and space charge polarizations due to the presence of multi-interfaces, special shape anisotropy, high density of point defects, and dangling bonds in core@shell configuration.60 Additionally, presence of defects in the carbon layer and the interface between Fe3O4 and the carbon layers could act as polarizing centers and result in the dipole/electrons polarization.70,71 As a result, dipole and electron polarizations are not matched on application of EM field in high frequency.70,71 As a consequence, dielectric loss and electromagnetic energy dissipation is enhanced due to Debye relaxation.70,71 Further, much larger value of imaginary parts of complex permeability (μ″) of PFC composites compared to pure Fe3O4 and Fe3O4@C could be ascribed to the presence of C and PANI as additional interface facilitating the transportation of electron beneficial to enhance dipole

εs − ε∞ 1 + ω 2τ 2

εs − ε∞

(6) 10716


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Figure 8. (a) Plots of frequency vs SEA, (b) frequency vs SER, and (c) frequency vs EMI SE of PFC composites.

interface polarization and relaxation effects in Fe3O4@C@ PANI could also contribute toward superior microwave absorption ability of Fe3O4@C@PANI compared to Fe3O4@ C and Fe3O4/PANI binary composites as displayed in Table S2.54 tan δ and Reflection Loss Analysis. Dielectric and magnetic losses in microwave absorption can be expressed as tan δε = ε″/ε′ and tan δμ = μ″/μ′, respectively. The materials exhibiting either dielectric loss or magnetic loss could lead toward weak microwave attenuation.58 In contrary, a material exhibiting higher tan δε as well as tan δμ values show better microwave absorption.58 Therefore, high microwave absorption is strongly reliant between dielectric loss and magnetic loss in a complementary manner.21,58 Figure S14a,b (SI-7) shows variation of tan δε and tan δμ of Fe3O4@C@PANI trilaminar core@shell structures with frequency (2−8 GHz). It is noted that variation of tan δε and tan δμ exhibit weak frequency dependence over the entire range. Further, at any given frequency, magnitude of tan δε and tan δμ Follow the order: Fe3O4@C < Fe3O4 < PFC-30 < PFC-20 < PFC-10. These findings clearly suggest that desired synergy between dielectric

polarization and contribute to the dielectric loss, magnetic loss and microwave absorbing properties.60 Alternatively, inimitable polarization and coupling modes could also account for higher value of μ″ in trilaminar core@shell Fe3O4@C@PANI composites. However, contribution toward μ″ could also arise from surface charge accrued at the multi-interfaces between two different dielectric media forming a structure analogues to boundary layer capacitor. This could generate interfacial electrical dipolar polarization in composites and improve the dielectric loss as well as upsurge the Debye-like relaxation process.60 Therefore, impedance matching greatly improved in PFC composites due to their lower ε′ and the higher μ′ values compared to Fe3O4 and Fe3O4@C. Such balance between permeability and permittivity decreases in the reflection coefficient of the absorber.66 As a result, absorption play role as dominant mechanism in Fe3O4@C@PANI composites exhibiting superior shielding efficiency compared to Fe3O4/PANI.19,35,40,54 All these findings lead to conclusion that presence of an additional C@PANI interface in Fe3O4@C@PANI act as an efficient microwave absorber. In all probability, higher dielectric loss through 10717


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ficient), and S22 (reverse reflection coefficient), and details are provided in the Supporting Information (SI-10).74−76 Figure 8a,b shows variation of SEA and SER of Fe3O4, Fe3O4@C, and PFC composites prepared at its different loadings. It is noted that, PFC-10 exhibits highest shielding efficiency due to absorption (SEA ≈ 47 dB), and it falls off in PFC-20 (20 dB) and PFC-30 (12 dB) over the entire frequency range. Such EM wave absorption needs to be evaluated on the basis of ability to attenuate the power transmitted into the shielding material.77 Interestingly, variation in SER exhibited a reverse trend compared to SEA, i.e., PFC-30 (∼40 dB) > PFC20 (∼35 dB) > PFC −10 (∼15 dB) > FC (∼5−40 dB) > Fe3O4 (∼11 dB). This suggests that amount of energy blocked by reflection is greater than that of absorption. Therefore, exact nature of mechanism in shielding of microwaves could be followed by considering total shielding efficiency (SET):

loss and magnetic loss is achieved in all the composites on account of characteristic impedance matching. 21 As a consequence, Fe3O4@C@PANI core−shell composites act as excellent EM absorber.58 The reflection losses (RL ) of the Fe 3 O 4@C@PANI composites have been calculated from forward reflection coefficients (S11) provided directly from network analyzer using the following equation: RL = 20 log |S11|

(8)

Figure S14c (SI-7) shows variation of reflection loss (RL) of Fe3O4, Fe3O4@C, and Fe3O4@C@PANI composites. It is inferred that Fe3O4@C exhibits continuous increase in the RL from 31 dB (2 GHz) to 37 dB (8 GHz). In contrary, Fe3O4, PFC-10, PFC-20, and PFC-30 show respectively RL ≈ 12, 33, 36.5−35.5, and 37−36.5 dB over the entire frequency range. Fe3O4@C@PANI exhibits remarkable enhancement in the microwave absorption performance as evident from reflection loss (RL) data. Figure S14c (SI-7) shows lowest RL value (∼33 dB) corresponding to PFC-10 available throughout the entire frequency range (2- 8 GHz) compared to other PFCcomposites. This is in contrast to earlier work reported showing higher value at a particular frequency as referred to in Table S3.31,35 Such superior damping capability of PFC-10 composite could be ascribed to dielectric and polarization relaxation losses.69 The observed electrical loss could be attributed to the generation of leaking current in the conductive networks in [email protected] The polarization relaxation in a material mainly arises from dipolar and interfacial polarizations. In all probability, multi-interfaces/dual junctions (Fe3O4@C and C@PANI) create tiny dipoles. It is anticipated that these dipoles stimulate electron polarization in the presence of external EM field favoring the attenuation of EM wave.67 Alternatively, higher density of PANI in PFC composites is likely to provide more active sites for scattering of electromagnetic wave/dissipation of electromagnetic energy.54 Performance in EMI Shielding. EMI shielding is defined as the attenuation of the transmitting electromagnetic (EM) waves by the shielding material. In general the shielding efficiency (SE) of any material is expressed in decibel (dB) unit. The higher EMI SE (dB) level of a material infers less transmission of EM wave through the shield. Further, shielding phenomena is dependent on three crucial factors, i.e., the reflection (SER), absorption (SEA), and multiple reflections (SEM) of the shielding material.72 Reflection of EM wave radiation, is mainly conducted by the presence of mobile charge carriers like electrons or holes in the shield material to interact with the electromagnetic radiation. Therefore, electrical conductivity data displayed in Table S4 is an indispensable criterion for a shield material to shield the incident EM wave by reflection. However, the absorption of the incident EM wave by the shielding material is governed by the availability of electric or magnetic dipoles to interact with the electromagnetic radiation. In addition, multiple internal reflection is another alternative mechanism of shielding.73 This refers to the reflection at diverse interfaces in the shield material. Therefore, interface area and multi-interfaces being an inevitable prerequisite in the shield material for multiple reflection mechanism. The total shielding efficiency (SET) or EMI SE, SEA, and SER has been evaluated on the basis of scattering parameters S11 (forward reflection coefficient), S12 (forward transmission coefficient), S21 (backward transmission coef-

SE T = SEA + SE R

(9)

Figure 8c shows that SET follows the trend PFC-10 (∼65 dB) > PFC-20 (∼55 dB) > PFC-30 (∼52 dB) > FC (∼41−20 dB) > Fe3O4 (∼15 dB). Thus, highest SET value in PFC-10 could be explained on the basis of its contribution toward absorption (75.8%) and reflection (24.2%). A potential mechanism could be proposed on the basis of our earlier discussions. The fabricated Fe3O4@C@PANI composite consists of dual interface in addition to the external surface. When EM wave is incident on Fe3O4@C@PANI, distorted electron cloud near the surface and interfaces generate electric field opposite to the applied electric field resulting in reflection/ absorption of the microwaves.77 Simultaneously, the electromagnetic wave energy is converted in the form of microcurrent. In the present study, the prepared trilaminar Fe3O4@C@PANI core−shell structure convert more and more electromagnetic energy to microcurrent, due to the increase in impedance matching. The microwave current generated is then required to transmit along core−shell structure. During transmission, the C and PANI serve as an electrically conductive network. Therefore, a large part of the electric energy could be attenuated due to the resistance of the PANI. In absence of Fe3O4 the EM wave is hardly attracted due to poor impedance matching. Meanwhile, in absence of the additional C@PANI interface, the microcurrent was unable to be transmitted. In addition, the interface between Fe3O4@C and C@PANI also could rouse the electron polarization, favoring the attenuation of EM wave.32 However, possibility of multiple reflections and scattering of microwaves through dual interface also cannot be completely ruled out. All these factors effectively enlarge transmission routes of incident microwaves due to unique trilaminar Fe3O4@ C@PANI core−shell structure. As a consequence, absorption capacity and SET of the PFC composites is significantly intensified. Our findings established that EMI SE in terms of total shielding efficiency (SET) of as prepared trilaminar core@ shell composites is mainly dominated by absorption. Our findings also reaffirmed that remarkable microwave absorbing performance of the trilaminar core@shell composite is indorsed by impedance matching and EM wave attenuation and resilient microwave absorption due to their dielectric and magnetic loss. The ideal condition for the perfect absorber is εr = μr, the presence of diamagnetic carbon shell with magnetic Fe3O4 core in the core of the PANI matrix has diminished εr of the composites and upgraded the equality of εr and μr account for 10718


Research Article

ACS Sustainable Chemistry & Engineering impedance matching.67 In addition, residual defects67 and multiple internal reflections73 in Fe 3O 4@C@PANI are accountable to boost its microwave absorption ability.78,79

Author Contributions

The proposed work was done by K.M. under the supervision of S.K.S., and both authors made equal contributions in writing and reviewing this manuscript.

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CONCLUSION Fe3O4@C synthesized by hydrothermal method were successfully coated by PANI by in situ polymerization and controlling its thickness by changing Fe 3 O 4 @C:aniline ratio and characterized. It is concluded that formation of Fe3O4@C and C@PANI dual interfaces in Fe3O4@C@PANI trilaminar core−shell composites enhanced interfacial polarization and the effective anisotropy energy leading to significant shielding of electromagnetic interference following absorption as dominant mechanism. Further, encapsulation of Fe3O4@C by PANI is accompanied by decrease in complex permittivity and increase in complex permeability. This accounts for good synergy between dielectric loss and magnetic loss through characteristic impedance matching. As a consequence, Fe3O4@C@PANI (Fe3O4@C:aniline = 1:9 wt/wt); i.e., PFC-10 exhibited highest shielding efficiency (∼47 dB) in terms of absorption and total shielding efficiency (∼65 dB) involving ∼76% contribution due to absorption over almost entire frequency range (2−8 GHz). These findings suggest superior shielding efficiency through absorption in the entire frequency range (2−8 GHz). Our findings also showed minimum reflection loss (∼33 dB) throughout the frequency range (2−8 GHz) indicating its role as an efficient absorber in shielding of electromagnetic environmental pollutions. As a result, Fe3O4@C@PANI act as an efficient absorber for broad applications, such as FM radio (88−108 MHz), TV (54−220 MHz), mobile phones (824− 849 MHz), global positioning system (1.2−1.6 GHz), air traffic control radar (0.96−1.21 GHz), wireless cable (2.5−2.7 GHz) etc. In summary, fabrication of Fe3O4@C@PANI trilaminar composites exhibiting dual interface by new approach at relatively much lower temperature and duration than reported and its potential application in controlling the EMI for multifaceted applications never reported earlier remain its novel features. Such excellent features of fabricated Fe3O4@C@ PANI ternary composites are unique compared to Fe3O4@C and Fe3O4/PANI due to their applicability limited to a discrete frequency.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.S. is thankful to DRDO for providing a grant for the ENA Network Analyzer and IIT Kharagpur for providing necessary facilities. K.M. gratefully acknowledges IIT Kharagpur for providing financial support and also help received from Indrajit Srivastava, Anupam Barman, and Kalyan Ghosh on XPS, schematic representation, and EMI studies, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02682. Characterization techniques, XPS spectra, FTIR spectra, FESEM analysis, EDX (Table S1), mapping and line scanning analysis, Raman analysis, comparative table of electromagnetic attributes on the basis of literature survey (Table S2), plots of tan δ and reflection loss vs frequency, comparative table of shielding efficiency parameters on the basis of literature survey (Table S3), room-temperature dc conductivity (Table S4), and basics of EMI, including Figures S1−S14 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91 03222-283334. ORCID

Suneel Kumar Srivastava: 0000-0002-9297-2282 10719


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