Nanostructured graphene Fe3O4 incorporated.pdf

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Feb 11, 2013 - Swati Varshney,c Jinhee Jang,a Seung Hyun Hur,a Won Mook Choi,a Mukesh Kumar ...... 27 M. G. Han, S. K. Cho, S. G. Oh and S. S. Im, Synth.

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Nanostructured graphene/Fe3O4 incorporated polyaniline as a high performance shield against electromagnetic pollution Kuldeep Singh,a Anil Ohlan,b Viet Hung Pham,a Balasubramaniyan R.,a Swati Varshney,c Jinhee Jang,a Seung Hyun Hur,a Won Mook Choi,a Mukesh Kumar,a S. K. Dhawan,c Byung-Seon Kongd and Jin Suk Chung*a The development of high-performance shielding materials against electromagnetic pollution requires mobile charge carriers and magnetic dipoles. Herein, we meet the challenge by building a threedimensional (3D) nanostructure consisting of chemically modified graphene/Fe3O4(GF) incorporated polyaniline. Intercalated GF was synthesized by the in situ generation of Fe3O4 nanoparticles in a graphene oxide suspension followed by hydrazine reduction, and further in situ polymerization with aniline to form a polyaniline composite. Spectroscopic analysis demonstrates that the presence of GF hybrid structures facilitates strong polarization due to the formation of a solid-state charge-transfer complex between graphene and polyaniline. This provides proper impedance matching and higher dipole interaction, which leads to the high microwave absorption properties. The higher dielectric loss

Received 5th December 2012 Accepted 22nd January 2013

(300 ¼ 30) and magnetic loss (m00 ¼ 0.2) contribute to the microwave absorption value of 26 dB (>99.7% attenuation), which was found to depend on the concentration of GF in the polyaniline matrix. Moreover, the interactions between Fe3O4, graphene and polyaniline are responsible for superior

DOI: 10.1039/c3nr33962a

material characteristics, such as excellent environmental (chemical and thermal) degradation stability

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and good electric conductivity (as high as 260 S m1).

1

Introduction

Graphene, a new crystalline form of carbon consisting of a twodimensional sp2 bonded sheet, has drawn much attention owing to its exotic in-plane properties such as high electric conductivity, exibility, and mechanical strength.1 These unique features offer great promise for many potential applications in many technological elds such as nanoelectronics,2 sensors,3 batteries,4 supercapacitors,5,6 liquid crystal devices,7 and polymer composites.8 Graphene not only possesses a stable structure but also high specic surface area and excellent electronic conductivity. These properties make graphene or graphene-based materials very promising for electromagnetic interference (EMI) shielding, which is designed to absorb and dissipate incident electromagnetic waves by converting them to thermal energy. The residual defects and functional groups

a

School of Chemical Engineering and Bioengineering, University of Ulsan, 93 Daehakro, Namgu, Ulsan 680-749, Republic of Korea. E-mail: [email protected] ac.kr

b

Department of Physics, Maharshi Dayanand University, Rohtak 124001, India

c

Polymeric and So Materials Section, National Physical Laboratory (CSIR), Dr K. S. Krishnan Road, New Delhi 110012, India d

KCC Central Research Institute, Mabookdong 83, Yongin, Gyunggido 446-716, Republic of Korea

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present in chemically reduced graphene are helpful in impedance matching, defect polarization relaxation and electronic dipole relaxation, which all serve to improve electromagnetic wave absorption.9 Thus, chemically reduced graphene oxide shows enhanced microwave absorption compared with graphite and carbon nanotubes, and can be expected to display better absorption than high quality graphene, making it a promising prospect as a microwave absorbing material. Most microwave absorbers are composed of magnetic materials like ferrites10 and dielectric materials such as barium titanates,11 carbon nanotubes (CNTs)12 and conducting polymers.13,14 Metallic magnetic materials show high permeability, but they have to be insulated to prevent eddy currents due to a drop in the complex permeability (mr) in the gigahertz range. Yang et al.15 reported a CNT polystyrene foam structure composite with an EMI shielding effectiveness (SE) of 19 dB. Che et al.16 reported the synthesis of CNTs encapsulated with Fe nanoparticles, showing good absorption behavior; however, the complex synthesis of CNTs lled with magnetic nanoparticles is not favorable for practical applications. Only a few articles have reported on the electromagnetic wave absorbing properties of chemically reduced graphene oxide composites. Liang et al. reported an EMI SE up to 21 dB at 8.2 GHz for solution-processable functionalized graphene (SPGF) into an epoxy matrix containing 15 wt% (8.8 vol%) SPFG.17 In another report, Xu et al. investigated

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Nanoscale the electromagnetic properties of Fe3O4 hollow spheres/graphene oxide, and they reported a maximum absorption of 24 dB at 12.9 GHz by 30 wt% as-synthesized hollow Fe3O4/r-GO,18 whereas the quaternary and ternary nanocomposites of G/[email protected] and G/[email protected]/ZnO respectively have shown minimal reection loss values of less than 30 dB for the quaternary nanocomposite thicknesses of 2.5–5 mm.19 Sun and coworkers report a facile solvothermal route to synthesize laminated magnetic graphene. The results show that there have been signicant changes in the electromagnetic properties of magnetic graphene when compared with pure graphene. With a coating layer 4.0 mm thick, the minimum value of reection loss is 26.4 dB at 5.3 GHz of the corresponding RGO–Fe3O4 composite.20 Hexagonal Ni nanocrystals with uniform size and high dispersion were assembled on graphene nanosheets (GN) via a facile one-step solution-phase strategy and exhibit a maximum absorption of 17.8 dB.21 The reduced graphene oxide (r-GO) coated with Fe3O4 composite synthesized by the decomposition of Fe(OH)3 in argon atmosphere and reduction in a mixed hydrogen and argon atmosphere demonstrates the maximum absorption of 22.2 dB at 17.3 GHz.22 The shielding effectiveness (SE) of a composite material mainly depends on the ller's intrinsic conductivity, dielectric constant and aspect ratio.23 Absorption loss in the material is caused by the electric dipole and/or magnetic dipole in the shielding material and the electromagnetic eld; so, the absorption loss is a function of the conductivity and the magnetic permeability of the material. The electromagnetic (EM) wave absorption behavior of graphene in the gigahertz frequency range, particularly in composites with dielectric loss and magnetic-loss components, remains unexplored to date. Therefore, the incorporation of graphene decorated with Fe3O4 into a conducting polymer (dielectric loss material) constitutes a new kind of ternary hetero-structured carbon materials having great potential for EMI shielding applications. Here, in the present report, we developed a 3D nano-architecture in polyaniline by incorporating a hybrid structure of graphene/Fe3O4. The resulting composite possesses good magnetic permeability and high dielectric properties with moderate conductivity, making it a next-generation material for use in shielding against electromagnetic radiation. The designed ternary polyaniline is lightweight and favorable for practical EMI shielding applications in the areas of aircra, spacecra, and automobiles, due to material and energy savings.

2

Experimental section

2.1

Materials

Expandable graphite (Grade 1721) by Asbury Carbon, potassium permanganate (KMnO4), hydrochloric acid (HCl), hydrogen peroxide (H2O2), aniline (An), ammonium peroxydisulphate (NH4)2S2O8 (APS), dodecylbenzenesulfonic acid (DBSA), isopropyl alcohol, FeCl3$6H2O, FeCl2$4H2O, H2SO4, and an aqueous ammonia solution were purchased from SigmaAldrich, South Korea. The aniline monomer was puried by

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Paper distillation in vacuum before use. The other chemicals were of reagent grade and were used as received. 2.2

Synthesis of nanostructured graphene/Fe3O4(GF) hybrid

The synthesis of GF was carried out by the in situ generation of Fe3O4 nanoparticles in a graphene oxide suspension by chemical co-precipitation of Fe2+ and Fe3+ ions followed by hydrazine reduction. Prior to GF synthesis, an aqueous suspension of graphene oxide (GO) was synthesized from natural graphite powders using a modied Hummers method.24,25 In a typical process, 7 g of graphite was charged into a 1 L beaker and was heated for 10 s in a microwave oven; this expanded the graphite to 150–200 times its original volume. The resulting graphite was dispersed in concentrated H2SO4 (1000 mL) at 0  C with continuous stirring. Then, 42 g of KMnO4 was slowly added so that the temperature did not exceed 20  C. The temperature was then elevated to 35  C, and the suspension was stirred for 2 h. The ask was then chilled again in the ice bath, and distilled water (ice) was slowly added, maintaining the temperature below 70  C. The mixture was stirred for 1 h and subsequently diluted with 5 L of deionized water. Subsequently, 50 mL of H2O2 (30 wt%) was added, and vigorous bubbles appeared as the color of the suspension changed from dark brown to yellow. The suspension was centrifuged and washed with 10% HCl solution four times, followed by centrifuging at 10 000 rpm and washing with deionized water to completely remove the acid until the pH of the GO dispersion reached 6. At this point, the as-synthesized GO dispersion was a paste. The concentration of GO was 1.0 wt%, as determined aer drying the GO dispersion at 80  C under vacuum for 24 h. Next, 100 mL of the GO suspension was diluted to 200 mL and was ultrasonicated for 30 min. A solution of 1.0 M FeCl2$4H2O and 2.0 M FeCl3 was slowly added to the GO solution and was precipitated with a 1 M NH4OH solution slowly with continuous stirring, maintaining the pH at 10. Aer 3 h of stirring, 10 mL of hydrazine was added to the reaction solution, and the temperature was raised to 80  C with further stirring for 5 h. The resulting solution was cooled to room temperature, ltered out and washed thoroughly with distilled water. The Fe3O4 decorated graphene nanoparticles (GF) thus obtained were dried at 80  C  5  C in a vacuum oven. Pristine Fe3O4 nanoparticles were also formed in a similar manner without the addition of GO. The formation of Fe3O4 nanoparticles in graphene was conrmed by X-ray diffraction (XRD). 2.3 Synthesis of graphene/Fe3O4 incorporated polyaniline (PGF) composite The GF nanoparticles were ultra-homogenized in a 0.3 M aqueous solution of DBSA with an appropriate amount of aniline (0.1 M) for 3 h to form a homogeneous dispersion. The micellar solution thus formed was polymerized at 2  C through chemical oxidization polymerization by using ammonium peroxydisulphate (0.1 M) as an oxidant. The resulting reaction solution was treated with an equal amount of isopropyl alcohol. The resulting precipitates were ltered out, washed with alcohol and dried at 60–65  C. Polymer composites having

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Paper different weight ratios of monomer to GF have been prepared, namely, An : GF: 1 : 1 (PGF1), 1 : 2 (PGF 2). Beside these, polyaniline doped with DBSA (PD13), and polyaniline with Fe3O4 (Aniline/Fe3O4 in wt ratio 1 : 2) (PF12) were also synthesized in a similar manner without GF.

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2.4

Measurements

The morphology of the GF and ternary PGF composite were examined using FESEM (eld emission scanning electron microscopy), JEOL JSM-6500FE. The distribution of constituents in the PGF composite has been analyzed using a Hitachi H8100 transmission electron microscope (TEM). The presence of Fe3O4 in the polymer composites was conrmed by XRD studies, carried out on a D8 Advance X-ray diffractometer ˚ in the scattering (Bruker) using CuKa radiation (l ¼ 1.540598 A)   1 range (2q) of 10–70 with a scan rate of 0.02 s and slit width of 0.1 mm. Electromagnetic shielding and dielectric measurements were carried out on an Agilent E8362B vector network analyzer in the frequency range of 12.4–18 GHz (Ku-band). Powder samples were compressed into rectangular pellets (15.8  7.9 mm2) of various thicknesses and were inserted into a copper sample holder connected between the waveguide anges of the network analyzer. The conductivity of the composites was measured using a four-probe method and a Keithley programmable current source and nanovoltmeter. Raman spectra were measured using a confocal Raman microscope (Alpha300S, WITec) at a 633 nm wavelength incident laser light. Fourier transform infrared spectra (FTIR) were recorded on a Nicolet IR 200 FT-IR spectrometer (Thermo Scientic) in transmission mode in the wavenumber range of 400–4000 cm1. Spectroscopic grade KBr discs were used for collecting the spectra with a resolution of 4 cm1, performing 32 scans. Thermal gravimetric analysis was performed under a nitrogen atmosphere at a heating rate of 10  C min1 (Q50, TA Instruments) from 25– 700  C.

3

Results and discussion

Coating of polyaniline over the GF was carried out using emulsion polymerization (oil in water type). The monomer aniline (oil) was emulsied with surfactant DBSA containing GF particles in a continuous phase of water. When aniline is added to the solution containing GF, it diffuses through water to the GF. The addition of oxidant APS to the solution leads to oxidative polymerization, where aniline is oxidized to anilinium radical cations, which subsequently combine with another unit to form a neutral dimer. Further oxidation of this dimer leads to the formation of a trimer, and nally to polyaniline. As shown in Fig. 1, the in situ polymerization of aniline in the presence of well-dispersed GF produces a coating of polyaniline in the emeraldine salt state on the GF sheets, which acts as an efficient template for aniline nucleation and polymerization. SEM (Fig. 2) also reveals a planar morphology of reduced graphene oxide (Fig. 2b) and PGF (Fig. 2d), indicating the formation of a thin layer coating of polyaniline on the surface of the GF. Further support for the presence of polyaniline is provided by

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Nanoscale characteristic peaks observed in the FTIR and Raman spectra of the PGF composite. The coating of polyaniline over the GF is a direct consequence of the template effect, as aniline polymerized in the DBSA medium under similar conditions in the absence of GF shows a globular or nanobrilar morphology depending on the reaction conditions.26,27 Valles et al. reported that aniline polymerization in the presence of GO in a feed ratio of 1 : 1 leads to the formation of a coating on the graphene, where GO acts as a template.28 The morphologies of Fe3O4, hydrazine reduced graphene oxide (RGO), GF and nanostructured PGF composites obtained were investigated by SEM. Fig. 2a illustrates the highly aggregated nanoparticles of as synthesized pristine Fe3O4. In general, the reduction of the aqueous graphene oxide suspension results in agglomerated graphene-based nanosheets. Similar to the previous observations,29 Fig. 2b shows the hydrazine reduced graphene oxide (RGO) consisted of folded and wrinkled sheets that were randomly aggregated in a disordered solid. When in situ synthesis of Fe3O4 was carried out by chemical coprecipitation, the formation of Fe3O4 takes place on graphene sheets having homogenous distribution in bunches, as shown in Fig. 2c. A higher magnication SEM image (Fig. 2d) shows that these Fe3O4 nanoparticles are not single, but decorated in bunches on the curled and thin wrinkled sheets, having a uniform distribution on the surface of RGO. When aniline monomer was polymerized with GF in a DBSA medium, a thin coating of polyaniline formed over the GF, as shown in Fig. 2d. The TEM images of Fe3O4, GF and PGF2 composite have been shown in Fig. 3. As Fig. 3a shows, large-scale Fe3O4 nanoparticles with a relatively uniform size of 10–20 nm were obtained. It is evident that two-dimensional graphene's surface was decorated by a large quantity of Fe3O4 nanoparticles (shown in Fig. 3b), and both the outline of graphene and Fe3O4 nanoparticles can be clearly observed. It is further demonstrated that Fe3O4 nanoparticles have grown on the graphene sheets and were distributed over the graphene's surface as compared to the agglomerated morphology of pristine Fe3O4 nanoparticles. Fe3O4 nanoparticles can deposit on both sides of these sheets in an orderly, dense and even manner. Furthermore, there are no large areas of the graphene that are not decorated with Fe3O4 nanoparticles. Besides, these Fe3O4 particles are rmly attached to the graphene sheets: even sonication was applied during the preparation of TEM specimens, indicating that an excellent adhesion of the composites between graphene and Fe3O4 particles was obtained. Fig. 3c and d show the TEM images of the PGF2 composite, the Fe3O4 nanoparticles distributed on the graphene and polyaniline. Fig. 4 demonstrates the X-ray diffraction patterns of graphene oxide (GO), PD13, PF12, PGF1, and PGF2 composites. Main peaks of Fe3O4 were observed at 2q values of 30.16 (d ¼ 2.96), 35.53 (d ¼ 2.53), 43.18 (d ¼ 2.09), 57.11 (d ¼ 1.61) and ˚ corresponding to the (2 2 0), (3 1 1), (4 0 0), (5 62.71 (d ¼ 1.48 A) 1 1) and (4 4 0) reections. All the observed peaks of Fe3O4 have been matched with the standard XRD pattern (JCPDS no. 880315). The peaks of GF observed in PGF1 and PGF2 conrm the presence of a GF hybrid in the polymer matrix. The crystallite size (D) of Fe3O4 has been calculated using Scherer's formula30

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Fig. 1

Paper

Schematic representation of the formation of PGF composites and polaronic and bipolaronic solid state charge transformation in PGF composites.

Fig. 3 TEM images of (a) Fe3O4, (b) GF, (c) PGF2 at 30 K and (d) PGF2 composite at 60 K resolution respectively. Fig. 2 SEM images of (a) Fe3O4, (b) reduced graphene oxide, (c) graphene/ Fe3O4 at 50 K, (d) ternary PGF composite at 50 K resolution.

and is estimated to be 10.1 and 14.2 nm for the GF and PGF12 samples, respectively. The inset shows the XRD pattern of graphene oxide, with a characteristic reection plan (002) at 2q ¼ 10.5 , indicating the d-spacing of 0.84 nm. This is attributed to intercalation of water molecules and generation of oxygenated functional groups such as epoxy and hydroxyl groups between the inter-galleries of the graphite sheets during severe oxidation. Disappearance of the reection plane at (002) and merging of the planes of Fe3O4 and polyaniline show the good interfacial interaction between the planes, and conrm the complete reduction of GO. Polyaniline doped DBSA is semi-crystalline in nature, as conrmed by the broad peaks at 2q ¼ 19.795 (d ¼ ˚ and 25.154 (d ¼ 3.537 A). ˚ 30 As shown, the intensity of 4.481 A)

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these peaks decreases with increasing GF concentration. The Fe3O4 peaks at 2q values of 30.28 (d ¼ 2.949) and 35.69 (d ¼ 2.513) clearly dominated in PGF composites, showing the formation of composites having separate phases of both compounds properly dispersed in the polymer matrix. In order to assess the reduction of GO and the formation of PGF composites, FT-IR spectra of various samples have been recorded (Fig. 5). The spectrum of GO shows the presence of various oxygen-containing functional groups; characteristic peaks at 1725 and 1105 cm1 have been assigned to carboxyl (COOH) and epoxide (C–O–C) groups, respectively, and the peak at 1630 cm1 arises due to the contributions from the skeletal vibrations of unoxidized graphitic domains or the remaining sp2 carbon character of graphite. However, in the case of GF and

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Paper

Fig. 4 X-ray diffraction patterns of (a) Fe3O4, (b) PF, (c) PGF2, (d) PGF1, (e) PD13. Inset shows the XRD graph of graphene oxide.

Fig. 5 Comparison of FTIR spectra of (a) polyaniline doped with DBSA (PD13), (b) GF, (c) PGF1, (d) PGF2, and (e) GO.

PGF, most oxygen-containing groups decrease or disappear, which demonstrates the presence of reduced GO. Simultaneously, the reduction also established a solid state charge (SSC) transfer relationship between the Fe3O4, GF and polyaniline layers. The SSC transfer can be supported by the fact that graphene is an excellent electron acceptor, while, on the other hand, aniline is a very good electron donor. As such, there is a donor–acceptor interaction, establishing the ground state charge-transfer complex between28 graphene and aniline, as depicted in Fig. 1. In this sense, the charge separated state of polyaniline takes the form of the positively charged emeraldine salt state, whereas RGO becomes negatively charged and functions as an anionic counter ion to the emeraldine salt. Equilibrium between both species is established through charge transfer along the interface of polyaniline and graphene, resulting in partially charged species typical of charge-transfer complexes; this easily explains the polaronic/bipolaronic polarization observed in the dielectric measurement. In the spectra of polyaniline (PD13), bands near 1460 and 1570 cm1 are assigned to the C]C stretching of the benzenoid

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Nanoscale and quinoid rings, respectively.31 There is blueshiing of 10 and 14 cm1 in PGF2 and PGF1 to 1560 and 1556 cm1, respectively, clearly supporting the SSC transfer phenomenon. The peak at 1296 cm1 is assigned to the C–N stretching of the secondary amine, a characteristic band of the conducting emeraldine salt form of polyaniline originating from a bipolaron structure, related to the C–N stretching vibration. The band at 1107 cm1 can be assigned to an in plane bending vibration of the C–H (mode of N]Q]N, Q]N+H–B, and B–N+H]B), which is formed during protonation. Hence, these FT-IR spectra correspond to a well-doped emeraldine salt. Contrary to our expectation, the intensity of the peaks of PGF2 is rather higher than that of PD13, although the same amount of DBSA (0.3 M) was used in the polymerization process. In addition, a decrease in the intensity of the quinoid band relative to the benzenoid band in PGF2 is more dominant than in PGF1, which indicates more pronounced SSC relationships between polyaniline and graphene sheets due to the enhanced synergistic effect with Fe3O4. The shiing and merging of the band at 675 cm1 (Fe–O starching) conrms the formation of Fe3O4 on the graphene sheets. The signicant structural changes from GO to PGF composites are also reected in the Raman spectra, which conrms the interaction between polyaniline and GF (Fig. 6). Raman spectra of GF exhibit two regular peaks, corresponding to the D-band line (about 1323 cm1) and the G-band line (about 1588 cm1). Here, the G-band corresponds to the rstorder scattering of the E2g mode observed for sp2 carbon domains, while the pronounced D band is caused by structural effects or edges that can break the symmetry and selection rule. The intensity ratio of D band to G band (ID/IG) is usually used to measure the graphitization degree of carbon materials. The increase in D/G ratio from 0.96 (GO) to 1.57 (GF) suggests that GO is reduced, and more defects have been generated due to the interfacial interaction between the graphitic plan and Fe3O4 in the hybrid GF. Also, the decrease in the average size of the sp2 domains upon reduction of the exfoliated GO can be explained

Fig. 6 Raman spectra of Fe3O4, graphene oxide (GO), RGO, PD13 and PGF composites.

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Nanoscale by the creation of new graphitic domains that are smaller in size than those present in GO before reduction. The shiing in the peak position of the G band has previously been used to reveal the interaction between nanoparticles and graphene or carbon nanotubes.32,33 For GO and GF, the G band arises at 1575 and 1589 cm1, respectively. Generally, a shi in the G peak in the Raman spectra of carbon-based composites with nanoparticles is indicative of charge transfer between carbon materials and nanoparticles. The Fe3O4 nanoparticles induce a blue shi in the G band by 14 cm1 in GF, suggesting the charge transfer from graphene to Fe3O4. When polyaniline comes in the eld forming a PGF composite, there is no shi in the G band, but the D band shis from 1326 to 1336 cm1, indicating a p–p interaction along with charge transfer from graphene to polyaniline. In the case of PGF1, the shiing is slightly less (shi to 1330 cm1) than PGF2, which is basically due to the lower GF concentration. Pristine PD13 shows a peak at 1326 cm1, which is assigned to the C–C stretching mode of the quinoid ring. The wavenumber of the C–C stretching vibration varies due to differences in the conformation of the polyaniline chain and the extent of doping. It is known that low doping levels support the existence of polaronic structures, while bipolarons exist at high doping levels. So, a low concentration of GF favors high doping levels of DBSA in the polyaniline chain; this strengthens C–C bonds in the protonation-induced polaronic lattice. Further, it is also possible to have intense overlapping of bands at 1326 and 1337 cm1, corresponding to C–N+_ stretching modes of delocalized polaronic charge carriers, which is characteristic of the protonated imine form of polyaniline. In both PGF composites, C–H bending of the quinoid ring at 1167 cm1, C– N+_ stretching at 1334 cm1, and C–N stretching vibration at 1487 cm1 are observed, revealing the presence of the polyaniline structures. Compared with polyaniline, the PGF composite presents a shi of the C–N+_ stretching peak towards lower wavenumbers resulting from the p–p* electron interaction between GF and aniline monomer. The p–p* electronic interaction between GF and polyaniline along with SSC transfer may meliorate the disordered structure of graphene, leading to the better microwave absorption properties of PGF, as depicted in the dielectric measurements. Thermogravimetric analysis plots of Fe3O4, GF PGF composites and polyaniline (PD13) are shown in Fig. 7. The effect of GF content on the thermal stability of the composites has been studied. Thermograms of polyaniline (PD13) show three major weight losses. The rst is at about 100  C and indicates the loss of water. The second is in the range of 230– 380  C and is due to a loss of the dopant, i.e. DBSA, from the polymer matrix. Finally, the third major loss from 380–700  C has been attributed to the degradation of the polymeric backbone. At 330  C, the bounded dopant responsible for conductivity of the polyaniline starts degrading from the polymer chain. In the case of PGF composite samples, the rst major loss is observed at 120  C due to the moisture entrapped inside the polymer moiety. A second loss of about 3% at 280  C is due to the loss of the unbound dopant. From 370  C onwards, the weight loss is due to the loss of the bound dopant and the

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Paper

Fig. 7 TGA graphs of Fe3O4, GF, PGF1, PGF2 and polyaniline doped with DBSA (PD13).

degradation of the polymer backbone. From the TGA curves of the as-prepared samples, it is concluded that the thermal stability of PGF polymer composites increases to 370  C by the incorporation of a GF hybrid structure into the polyaniline system. This enhancement in the thermal stability is due to some ionic interaction between graphene–Fe3O4 and the amine group of the aniline ring. These may form a coordinate bond between Fe and N, as Fe can facilitate back bonding between the d-orbital and –NH2 group of the aniline ring. When an EM wave impinges on the shielding material, a scattered or induced eld is created inside the material by electrons and other charged particles in response to the incident wave. This induced eld affects the total eld inside the material and modies the charge motion.34,35 When an incident wave strikes the shield, a portion of the wave is reected back via interaction with surface charges (Fig. 8). Therefore, the material used as the EMI shield tends to be conductive, due to mobile charge carriers. However, this does not mean that conductivity is the absolute criterion for EMI shielding, as conduction needs connectivity between the llers. The SE is a measure of the material's ability to attenuate the intensity of EM waves and can be expressed as36 SEðdBÞ ¼ 10 log

Pt ¼ SER þ SEA þ SEM P0

(1)

where Pt and P0 are the transmitted and incident electromagnetic power, respectively. SER and SEA are the shielding effectiveness due to reection and absorption, respectively. SEM is multiple reection effectiveness inside the material, which can be negligible when SE > 10 dB. It has been observed that a minimum SE of 20 dB (indicates 99% of the EM radiation has been blocked by the shield) is required for commercial applications in electronic appliances like laptops and desktop computers.15 For military applications, the required SE is more than 30 dB.37 Therefore, the demand to develop lightweight, thinner electromagnetic (EM) wave absorbers with wider bandwidths is ever increasing. Graphene, being an extremely thin material, has shown great potential to shield against the

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Fig. 8 Dependence of shielding effectiveness (SEA and SER) of PF12, PGF1 and PGF2 composites as a function of frequency for sample thickness d  2.5 mm and EMI shielding representation. The inset illustrates the variation of microwave conductivity and skin depth with frequency.

EM waves. Graphene based binary composites having organized structures with ferrite and insulating polymers and epoxies have been exploited as EM wave absorbing and shielding materials due to their multifunctional electrical and magnetic properties and good shielding effectiveness for various electromagnetic sources. Liang et al.17 reported a total shielding of 21 dB shielding efficiency at 15 wt% (8.8 vol%) graphene/epoxy at 8.2–12.4 GHz (X-band). Pure graphene is non-magnetic and contributes to microwave absorption mostly because of its dielectric loss. So, compositing with a Fe3O4/graphene nanocomposite exhibits both enhanced dielectric losses and magnetic losses in a wide frequency range, resulting from the interfacial polarizations between the Fe3O4 nanoparticles and the graphene.18 However, ferrites and their composites have been restricted in practical applications due to their higher density. In contrast, conducting polymers are light in weight, and graphene can easily be added to form composites. In Fig. 8 the variation of the shielding effectiveness with frequency for the different ratios of the [aniline]/[GF] is shown. It is interesting to note that, in PGF composites, the contribution to SE values mainly comes from the absorption rather than reection, as observed in metals. PGF composites have shown excellent frequency stability in the measured frequency range, which was found to increase with increasing GF content. PGF2 has a higher SEA of 22–26 dB (le black scale) with a SER of 4.7–6.3 dB (right blue scale) as compared to PGF1 (SEA  21 dB and SER  4.5 dB) in the 12.4–18 GHz range, while polyaniline–Fe3O4 (PF12) has a lower value shielding effectiveness (SEA  7–9 dB and SER  1.5–2.5 dB) in comparison to PGF composites in the same frequency range having a thickness of 2.5 mm. While comparing the microwave absorption properties with the similar composites previously reported, PGF composites have improved properties for the wide band application.20–22 The increase in the absorption with the addition of GF hybrid nanoparticles mainly arises due to the synergistic effect of graphene and Fe3O4 with polyaniline; this contributes to higher dielectric and magnetic losses, which are responsible for the higher SE. The dependence of SE on complex permittivity and permeability can be expressed as38

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d SEA ðdBÞ ¼ 20 log e ¼ 20d d

rffiffiffiffiffiffiffiffiffiffiffiffiffi mr usS log e 2

  sS SER ðdBÞ ¼ 10 log 16umr 30

(2)

(3)

where d is the thickness of the shield, mr is the relative magnetic permeability, d is the skin depth, sS ¼ u30300 is the frequency dependent conductivity,36 300 is the imaginary part of permittivity (dielectric loss factor), u is the angular frequency (u ¼ 2pf) and 30 is the permittivity of the free space. From eqn (2) and (3), it is observed that, with an increase in frequency, the SEA values increase while the contribution of the reection decreases. As reported earlier, EMI SE is improved by increasing the conductivity.39,40 Graphene has a very large surface area, resulting in its potential applications in lightweight EM absorbers. However, its high conductivity may degrade its EM absorption ability.41,42 According to the free-electron theory, 300 ¼ s/u30, where s is the electrical conductivity. Thus, the strong dielectric loss of graphene is related to the electron polarization process owing to its very high carrier mobility.43 However, the high conductivity of graphene can also result in the occurrence of a signicant skin effect as its surface is irradiated by an EM wave. By the incorporation of graphene and Fe3O4 into the polyaniline matrix the skin effect due to the high conductivity of the graphene was considerably reduced. Thereby, coating of polyaniline on Fe3O4 and graphene may improve the impedance match characteristics, decreasing the surface reection and benetting the absorption. The dependence of SEA and SER on conductivity and permeability reveal that a material having higher conductivity and magnetic permeability can achieve better absorption properties. To relate sS with the shielding parameters of the material, sS and skin depth were plotted against the frequency for the composites. The skin depth of the samples was calculated using the relapffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi tionship d ¼ 2=umsS , and its variation with frequency is shown in Fig. 8 (inset). From the plot, it was observed that skin depth decreases with frequency for PF and PGF1, which

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Nanoscale demonstrates that mainly surface conduction exists at the higher frequencies. In the case of PGF2, some anomalous behavior exists, which may be due to the high concentration of GF in the composite, leading to more polarization due to pronounced SSC transfer. Eqn (2) and (3) show that the SE depends on the thickness, so the variation of SEA and SER is shown in Fig. 9. The SE of the composites increases with increasing thickness of the absorber, for d ¼ 1.73 mm, SEA (max) has been ca. 16–20 dB in 12.4 to 18 GHz and for d ¼ 3.34 mm, SEA (max) has been ca. 28 dB at 18 GHz, respectively. With an increase in composite thickness, there is a shi in the reection loss peak toward lower frequency (Fig. 9 inset). This kind of behavior is important because it reveals that the range of absorption frequency can be tuned via a change in nanocomposite thickness. Moreover, in the 12.4– 18 GHz frequency range, the SEA is higher than 16 dB for all the samples at various thicknesses, which suggests that these nanocomposites can be used as broadband absorbers. Complex permittivity and the permeability of composites were calculated using scattering parameters (S11 and S21) based on the theoretical calculations given by Nicholson, Ross and Weir.44,45 The real part (30 ) is mainly associated with the amount of polarization occurring in the material, and the imaginary part (300 ) is related to the dissipation of energy. The dielectric performance of the material depends on ionic, electronic, orientational (arising due to the presence of bound charges) and space charge polarization (due to the heterogeneity in the system). In a heterogeneous system, the accumulation of virtual charges at the interface of two media having different dielectric constants, 310 and 320 , and conductivities s1 and s2, respectively, leads to interfacial polarization and is known as Maxwell– Wagner polarization.46 Here, graphene along with the Fe3O4 nanoparticles were incorporated in the polyaniline matrix having different conductivities and dielectric constants and therefore show interfacial polarization in the applied frequency range. The presence of insulating Fe3O4 in the conducting matrix results in the formation of an increased interface and a

Fig. 9 Comparison of SEA of the PGF2 composite calculated for the different sample thicknesses. Inset shows the variation of SER with frequencies for different sample thicknesses.

Nanoscale

Paper heterogeneous system, wherein some space charge accumulates at the interface, contributing to the higher microwave absorption. Guan et al.47 have shown theoretical simulations considering a hybrid assembly system where electromagnetic loss properties originated from the coupling between Fe3O4 nanoparticles and graphene. For conjugated polymers, two types of charged species are present. The rst includes polarons (radical cations) and bipolarons (biradical cations) that are free to move along the chain. A second type is bound charges (dipoles), which have restricted mobility and account for strong polarization in the system. Therefore, orientation, or dipolar polarization, is expected to contribute to the dielectric permittivity. Of these polarization mechanisms, dipolar and interfacial (space charge) are more frequency-dependent. It has been shown that polyaniline coated Fe3O4 and graphene contribute to more conducting regions in the system, leading to an increase in space charge polarization which depends more on the frequency of the applied eld. The observed decrease in 30 & 300 with increasing frequency may be attributed to the decrease in space charge polarization with increasing frequency.48,49 In addition, the enhanced electromagnetic absorption properties can also be attributed to the special structural characteristics and the charge transfer from polyaniline to graphene sheets as shown in the Raman spectra. Fig. 10 shows the real and imaginary parts of the permittivity in the 12–18 GHz frequency range. Higher values of dielectric constant and dielectric loss (30  23.7–6.8 and 300  30–22) have been observed in PGF2 than PGF1 (30  24–14 and 300  27.8–21.3), which are both much higher in comparison with PF12 (30  13–5 and 300  7.3–7). The synergistic effect of graphene along with Fe3O4 leads to a difference in the relative dielectric constant of PF and PGF, leading to improved values of microwave absorption. Fig. 11 shows the variation in the real (m0 ) and imaginary (m00 ) parts of the permeability with frequency for the composites. The real part of the permeability remains constant with a little uctuation in the measured frequency range as compared to PF, whereas the magnetic loss slightly decreases with frequency. Higher values of m00 have been observed for PGF composites as compared to PF, which conrms the existence of greater

Fig. 10 Behavior of real and imaginary parts of the permittivity of PF, PGF1 and PGF2 composites as a function of frequency over 12–18 GHz.

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Paper

Fig. 11 Behavior of real and imaginary parts of the permeability of PF, PGF1 and PGF2 composites as a function of frequency over 12–18 GHz.

magnetic losses in PGF nanocomposites. The dielectric and magnetic loss over the whole frequency range proves the balance of properties due to EM matching in the composite, suggesting that the enhanced microwave absorption properties unambiguously result from the cooperative effect of polyaniline, graphene and Fe3O4.

4

Conclusions

In conclusion, three-dimensional (3D) nanostructures consisting of GF incorporated polyaniline were successfully synthesized in DBSA medium. The in situ emulsion polymerization resulted in a coating of polyaniline over the GF hybrid due to the template effect. This leads to enhanced interfacial polarization due to solid state charge transformation between the polyaniline and graphene decorated with Fe3O4, and contributes to the higher microwave absorption. PGF nanocomposites demonstrate strong microwave absorption properties over 12.4–18 GHz having a SEA value of 26 dB at 15 GHz with a reection loss of 6 dB. The high absorption properties of PGF composites mainly arise from the high dielectric and magnetic losses. As a result, polyaniline nanocomposites with graphene/Fe3O4 hybrids provide promising properties as a new type of sheath material for electromagnetic pollution in the high frequency range, maintaining strong absorption.

Acknowledgements This work was supported by the 2013 Research Fund of University of Ulsan.

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