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Silica particles wrapped with poly(aniline-co-pyrrole) and reduced graphene oxide for advanced microwave absorption
Materials Chemistry and Physics 244 (2020) 122691
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Silica particles wrapped with poly(aniline-co-pyrrole) and reduced graphene oxide for advanced microwave absorption Quyen Vu Thi a, Sungho Lim a, Eunsuk Jang a, Jeongrae Kim a, Nguyen Van Khoi b, Ngo Trinh Tung b, Daewon Sohn a, * a b
Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, Seoul, South Korea Institute of Chemistry, Vietnam Academy of Science and Technology, 18- Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam
H I G H L I G H T S
� In-situ pre-polymerization used to prepare P(ANi-co-Py)/RGO@SiO2 nanocomposite. � The minimum reflection loss reached to 40.6 dB at 15.8 GHz. � The effective absorption bandwidth was 5.5 GHz from 12.3 to 17.8 GHz in Ku band. � It is expected to be applied in wide range frequency EM attenuation materials. A R T I C L E I N F O
A B S T R A C T
Keywords: Silica particles Poly(aniline-co-pyrrole) Reduced graphene oxide Pre-polymerization Microwave absorption
In this work, a novel nanocomposite composed of silica nanoparticles wrapped with a poly(aniline-co-pyrrole) copolymer and reduced graphene oxide (P(ANi-co-Py)/RGO@SiO2) was prepared by an in situ prepolymerization method and then characterized. The aniline:pyrrole molar ratio was 1:3 and the prepolymerization time was 30 min before the in situ polymerization of the copolymer nanocomposites occurred. The results revealed that the microwave absorbing performance of the copolymer nanocomposites was signifi cantly improved compared to that of the component homopolymer nanocomposites. The minimum reflection loss of P(ANi-co-Py)/RGO@SiO2, PANi/RGO@SiO2, and PPy/RGO@SiO2 was 40.6, 38.9, and 13.9 dB, respectively. The effective absorption bandwidth of P(ANi-co-Py)/RGO@SiO2 was 5.5 GHz from 12.3 to 17.8 GHz. In contrast, the effective absorption bandwidth of a component homopolymer nanocomposite, such as PANi/RGO@SiO2, was 2.2 GHz (15.8–18 GHz), and for PPy/RGO@SiO2, was only 1.5 GHz (12.4–13 GHz and 16.7–17.6 GHz) in Ku band. The enhanced microwave absorption was mainly contributed to the interfacial polarization in which the interfacial interaction between SiO2 particles, rGO sheets and conducting polymer chains played critical roles.
1. Introduction Electromagnetic interference (EMI) phenomena have become a serious problem with the rapid development and application of elec tronic devices as well as the widespread use of transient power sources. To overcome this problem, electromagnetic (EM) absorbing materials with highly efficient absorption, wide dissipation bandwidth, low cost, and light weight have been urgently pursued [1–3]. In general, EMI shielding materials should possess a balanced combination of electrical conductivity, dielectric permittivity, magnetic permeability, and
physical geometry. So far, intrinsically conducting polymers (ICP) such as polyaniline (PANi) and polypyrrole (PPy) have attracted much research interest as EMI shielding materials because of their moderate electrical conductivity, facile processing, low density, corrosion resis tance, and compatibility with other polymeric matrices [4]. Recently, graphene has been considered as a powerful candidate for EMI shielding materials due to its high specific surface area, ultra-low density, thermal stability, and high electrical conductivity. However, perfect graphene, with an extraordinarily high conductivity, performed poorly for EM dissipation because of badly mismatched impedance.
* Corresponding author. E-mail addresses: [email protected] (Q.V. Thi), [email protected] (S. Lim), [email protected] (E. Jang), [email protected] (J. Kim), [email protected] (N. Van Khoi), [email protected] (N.T. Tung), [email protected] (D. Sohn). https://doi.org/10.1016/j.matchemphys.2020.122691 Received 26 November 2019; Received in revised form 15 January 2020; Accepted 19 January 2020 Available online 20 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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Surface modification of graphene with an oxygen-functionalized group called graphene oxide was reported to significantly enhance the mi crowave absorption properties compared to that of pure graphene [5]. To yield high EM attenuated performance, polymer nanocomposites based on ICPs and graphene have been investigated. Hailong et al. re ported that a composite matrix of paraffin with only 20 wt% loading of graphene/polyaniline composites achieved a maximum reflection loss (RL) of 45.1 dB for an absorber with a thickness of only 2.5 mm [6]. Prerana et al. synthesized a PANi matrix with 5 wt% of graphene in an in situ chemical oxidation method which had a maximum shielding effec tiveness between 51 and 52 dB in the frequency range of 2–12 GHz [7]. Similarly, Ankit et al. reported that a PPy/multiple layer graphene/TiO2 composite reached a maximum total shielding effectiveness of 53 dB in the frequency range of the Ku band (12.4–18 GHz) with only 5 wt% composites [8]. Sujuan et al. fabricated a composite of PPy with nickel particles based on reduced graphene oxide (RGO) that had a peak RL of 47.32 dB at 5.76 GHz and a thickness of 4 mm [9]. Unfortunately, the combination of polymeric chains with graphene sheets caused a signif icant reduction of the inner space of the resultant composites, as they were at risk of being constrained by the polymer chains. Polymer chains anchored and blocked the porous network or constrained the space between the graphene layer causing a decrease of the exposed surface area, which was unfavorable for EM attenuation due to a reduction of interfacial polarization [10]. This problem could be potentially over come by the intercalation of silica particles into the layer of graphene. Because of their unique characteristics such as very high specific surface area, low density, high thermal stability, and numerous possibilities for functionalization, silica particles do not act as an exfoliation agent of graphene sheets and also reinforce the composite so that they can endure harsh environments [11,12]. In recent years, many studies on the preparation of ICPs such as poly (aniline-co-pyrrole) (P(ANi-co-Py)) have been performed to combine the unique properties of both homopolymers. The electrical properties of the copolymer depend strongly on the component ratio and the synthesis method. The combination of these two homopolymers in P(ANi-co-Py) also revealed other outstanding properties, such as high thermal sta bility and better electrochemical capacitive performance compared to PANi or PPy homopolymers [13]. To date, research on P(ANi-co-Py) mainly focused on applications as a supercapacitor [14] or the removal of anti-inflammatory drugs [15]. In this work, novel polymer nanocomposite based on copolymer P(ANi-co-Py) and RGO-coated silica for EM attenuation materials were investigated. To the best of our knowledge, this is the first report of this kind of EMI shielding materials. The EMI shielding effectiveness of the polymer nanocomposites was examined over the entire frequency range of the X and Ku bands (8.2–18 GHz).
2.2. Synthesis of amino-functionalized silica particles (SiO2–NH2) For better anchoring of silica particles on RGO, the silica particles were modified with an amino-functional group by APTS. The aminofunctionalized silica particles (SiO2–NH2) were prepared by the modi €ber method [16]. Typically, 4 ml of TEOS was added dropwise fied Sto (0.5 ml/min) to a mixture of 100 ml of ethanol and 9 ml of ammonium hydroxide while stirring and the reaction was continued for 12 h. Af terward, 6 ml of APTS was quickly added to the reaction solution and allowed to react for another 12 h at room temperature. The white pre cipitates of SiO2–NH2 particles were collected by centrifugation at 8000 rpm for 10 min. The product was rinsed several times with ethanol and DI water to remove excess reagent. The final products were dried in a freezing dryer and stored for further steps. 2.3. Preparation of reduced graphene oxide (RGO) Thermally-reduced graphene oxide was synthesized following our previous publication using a strong acid treatment [17]. In this method, a mixture of concentrated sulfuric acid and phosphoric acid in a 9:1 vol ratio (240 ml:27 ml) was prepared while stirring for 30 min. Next, 2 g of natural graphite flake was dispersed into the mixture, then 12 g of po tassium permanganate was added to the solution, which was chilled in an ice bath for 30 min. The reaction temperature was then raised to 90 � C. After 12 h of reaction, the mixture was cooled to room temperature followed by dilution with 400 ml of DI water. To remove the unreacted residual, 20 ml of hydrogen peroxide was added, and the color of mixture bubbled and turned to black. The product was washed with 0.1 M HCl and DI water several times. To remove unreacted graphite flakes and collect the uniform RGO sheets, the products were centrifuged 3 times at 1000 rpm for 2 min. The final RGO products were collected by centrifugation at 8000 rpm for 15 min and dried in a vacuum oven at ambient temperature. 2.4. Synthesis of RGO-coated amino functionalized silica particles (RGO@SiO2) The preparation of composite RGO@SiO2 was carried out with an in situ method with an RGO:SiO2–NH2 weight ratio of 1:1. Briefly, 0.2 g of RGO was dispersed in 100 ml of DI water. The mixture was sonicated in an ultrasonicator for 30 min for better exfoliation of the RGO sheets. Separately, 0.2 g of SiO2–NH2 particles was dispersed in an ethanol: water solution (2:1 vol ratio). The solution of SiO2–NH2 particles was slowly added into the mixture of RGO (1 ml/min). The reaction was continued for 24 h to complete the amidation reaction between the carboxyl functionality on the surface of the RGO and the amino group on the silica particles. The resultant products were collected by centrifu gation and filtered 3 times with ethanol and water. The final product, RGO@SiO2, was dried in vacuum at 60 � C for 24 h.
2. Experimental 2.1. Materials
2.5. Synthesis of the copolymer nanocomposite P(ANi-co-Py)/ RGO@SiO2
Natural graphite flake, aminopropyltriethoxysilane (APTS, 99%), and aniline (99.5%) were obtained from Sigma Aldrich. Pyrrole (98%) and tetraethylorthosilicate (TEOS, 99.9%) were purchased from Alfa Aesar. Sufuric acid (98% purity), anhydrous ethanol (� 99.9%), potas sium permanganate (99.3% purity), and ammonium hydroxide solution (28.0–30.0% ammonia) were supplied by Daejung Chemical. Phos phoric acid (Duksan Pure Chemical, 85% purity), hydrogen peroxide (Samchun Chemical, 35% purity), hydrochloric acid (Samchun Chemi cal, 35.0–37.0%), ammonium persulfate (APS, Samchun Chemical 98%), EpoFix® resin, and EpoFix® hardener (Struer) were used as received. Copper plate was supplied by Shapiro Metal Supply (USA). Deionized (DI) water used in the reaction was purified with a Mili-Q water purification system (Milipore).
In this work, the copolymer nanocomposites were prepared by an in situ chemical oxidative polymerization method at low temperature (5 � C). According to previous work [11,18] polymer synthesis with an RGO@SiO2:monomer weight ratio of 1:5 formed polymer nano composites with the best electrical conductivity. P(ANi-co-Py), with its highly improved electrical conductivity, was prepared by a pre-polymerization method as reported by Xianghui et al. [19]. The aniline:pyrrole molar ratio was 1:3. The procedure for the synthesis of the P(ANi-co-Py)/RGO@SiO2 copolymer nanocomposite follows. Briefly, 0.3 ml aniline monomer (3.3 mmol) and 0.7 ml pyrrole monomer (0.01 mol) were dispersed in 20 ml (solution A) and 60 ml (solution B), respectively. Solutions A and B were poured into separate round-bottom flask and immediately cooled in an 2
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0.05 cm3 piece of copper. Then the cured epoxy composites were held at room temperature for 12 h. The absorption (A) was calculated with A ¼ 1 - jS11 j2 - jS12 j2 , where S11 and S12 are the reflection and the trans mission parameters, respectively. In this study, the boundary back layer was the perfect electric conductor of copper, so S12 ¼ 0 and therefore, A
ice bath. For the pre-polymerization process, 0.8 and 2.0 g of APS dis solved in an HCl solution were added dropwise to solutions A and B, respectively, for 30 min. Meanwhile, 0.2 g RGO@SiO2 was dispersed in 100 ml of DI water and 10 ml of 1 M HCl under ultra-sonication. After 30 min of the pre-polymerization process, solutions A and B were mixed and poured into the RGO@SiO2 solution and allowed to react for 12 h. For the sake of comparison, PANi/RGO@SiO2 and PPy/RGO@SiO2 polymer nanocomposites were also prepared with the same procedure, as described below and shown in Scheme 1.
¼ 1 - jS11 j2 . The RL could be expressed by RL ¼ 20log10jS11 j. 3. Result and discussion 3.1. Morphology and SEM images
2.5.1. PANi/RGO@SiO2 1 ml of aniline monomer was added dropwise to a solution of 100 ml of DI water and 10 ml of 1 M HCl and the mixture was stirred for 1 h to obtain a homogeneous solution. After that, 0.2 g of RGO@SiO2 was dispersed in the solution and sonicated for another 1 h. Then, 2.4 g of APS dissolved in 10 ml of HCl was added and the polymerization solu tion was held at 5 � C for 12 h.
Fig. 1 shows the SEM images of silica particles, RGO@SiO2, PANi/ RGO@SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2. The asprepared SiO2–NH2 had a spherical structure with an average diam eter of 200 nm, as shown in Fig. 1a. Fig. 1b presents the morphology of RGO@SiO2 in the wrinkled and folded region of the RGO sheets (RGO), with partially anchored spherical silica particles. The remaining wrin kled space of the RGO could be considered as a good template for the attachment of monomer for better polymeric chain formation in the final ICP/RGO@SiO2 composites. The polymerization of aniline on RGO@ SiO2 resulted in a worm-like networks with diameters of 40–60 nm and lengths 120–180 nm, which were uniformly covered with the RGO@ SiO2 composites (Fig. 1c and d). The combination of pyrrole with the quasi-spherical particles on the surface of RGO@SiO2 provided a rougher surface for the PPy/RGO@SiO2 composites (Fig. 1e and f). In the case of P(ANi-co-Py)/RGO@SiO2, the quasi-spherical particles were halved in diameter, from 250 to 520 nm in PPy/RGO@SiO2 to 135–270 nm in P(ANi-co-Py)/RGO@SiO2. This observation agreed with a previ ous report [20], which confirmed the successful formation of P (ANi-co-Py). However, there were some worn-like pieces attached on the copolymer chains that could be considered as the formation of PANi chains from the pre-polymerization method.
2.5.2. PPy/RGO@SiO2 Similarly, PPy/RGO@SiO2 nanocomposites were synthesized with the same procedure. Typically, 1.2 ml of pyrrole was added slowly to a solution of 100 ml of DI water and 10 ml of 1 M HCl while stirring. Subsequently, 0.2 g of RGO@SiO2 was dispersed into the solution and sonicated for 1 h. After that, 3.3 g of APS in 10 ml of HCl was slowly added and the polymerization solution was kept at 5 � C for 12 h. 2.6. Characterization Scanning electron microscopy (SEM) images were collected on a NOVA SEM operating at 10 kV. Raman spectra were collected using a dispersive Raman microscope equipped with a diode laser (λ ¼ 785 nm, power ¼ 40 mW) and a CCD detector (Kaiser Optical Inc., USA). Fourier transform infrared (FTIR) spectra were obtained with a FTLA 2000 (ABB Ltd., Switzerland) using a KBr pellet mixed with the given samples. FTIR analysis was carried out with 16 scans over the range of 500–4000 cm 1 with a resolution of 2 cm 1. Brunauer-Emmett-Teller (BET, BEL JAPAN Inc., Japan) analysis with absorption/desorption isotherms of N2 at 77 K was used to determine the surface area of the particles. The samples were pretreated at 100 � C for 3 h prior to the BET measurement. For electrical conductivity characterization, composite powders were pressed at high pressure (9 tons/cm2 for 30 s) into a pellet with a thickness of 0.01 cm. The electrical resistivity of the polymer nano composites was measured using a four-probe measuring device (HP 4155C, Agilent Technologies). A Hewlett-Packard E8362B vector network analyzer was used to study the absorption properties of the polymer nanocomposites in the X band region (8.2–12.4 GHz) and Ku band region (12.4–18 GHz). Samples for EM measurements were pre pared with an epoxy resin/hardener cured with a resin:hardener weight ratio of 25:3, with 9.0 wt% of the composite (PANi/RGO@SiO2, PPy/ RGO@SiO2, or P(ANi-co-Py)/RGO@SiO2). The mixture was vigorously stirred for 30 min before it was fabricated on the surface of a 15 � 15 �
3.2. Raman shift and FTIR To confirm the structure of the composites with RGO, the Raman shift of each sample was measured, as shown in Fig. 2. RGO had a Raman spectrum with two typical peaks at around 1606 and 1332 cm 1, which corresponded to the G and D bands, respectively. The G band was pro duced from the sp2 hybridized carbon, while the D band was activated both by carbon impurities and by defects participating in double reso nance Raman scattering near the K point of the Brillouin zone. The in tensity ratio of the D/G bands (ID/IG) generally has been used as a useful index to define the defect level of the carbon structure materials [21–23]. The G and D bands were obtained in the RGO@SiO2 compos ites without additional peaks because the SiO2 was amorphous in structure. The G and D bands were shifted to 1604 and 1331 cm 1, respectively. Moreover, the ID/IG in RGO was 1.28 while its value in RGO@SiO2 was 1.29, which indicated the successful anchoring of SiO2–NH2 particles on the RGO sheets. In the Raman spectra of the ICP/RGO@SiO2 composites, we also
Scheme 1. Schematic of the preparation procedure of for the composites of intrinsically conducting polymers (ICP, polyaniline, polypyrrole, or poly(aniline-copyrrole)) and RGO@SiO2. 3
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Fig. 1. SEM images of (a) SiO2–NH2 particles, (b) RGO@SiO2, (c, d) PANi/RGO@SiO2, (e, f) PPy/RGO@SiO2 and (g, h) P(ANi-co-Py)/RGO@SiO2.
aniline units, which was reasonable because of the higher ratio of the pyrrole monomer. Compared to RGO and RGO@SiO2, PAN i/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2 exhibited a Raman shift in G and D peaks and a broadening of the D band. Addi tionally, the calculated values of ID/IG were 0.75, 0.78, and 0.96 for PANi/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2, respectively. These ID/IG values provided evidence of the pi-pi interac tion of the polymer chain with the RGO, confirming the successful coating of polymer chains on the RGO@SiO2 nanocomposites. FTIR measurements gave complementary information about the chemical interactions of each component inside the structural nanocomposites, as shown in Figs. 3–4. In the FTIR spectrum of RGO the broad peaks at 3395 and 1720 cm 1 –O showed the carboxyl O–H carboxylic acid stretching and the C– stretch of carboxyl group, respectively. The absorption peak at 1218 cm 1 was assigned to the C–OH stretching of the alcohol group [28]. Hence, the RGO was not completely reduced by the thermal reduction synthesis method. The oxygen-containing functional group proved the presence of carboxylic groups on the RGO, which was beneficial for the attachment of SiO2–NH2 particles on the surface of the RGO sheets through the chemical interaction formed by the amidation reactions. The absorption peaks of the Si–O–Si asymmetric stretching vibration and the Si–OH asymmetric bending appeared at 1103 and 951 cm 1, respectively. The peak at 802 cm 1 was the stretching vibration of Si–OH. Additionally, the peak at 1632 cm 1 was due to the N–H bond that resulted from the amine-functionalization of silica surface after
Fig. 2. Raman shift of RGO, RGO@SiO2, PANi/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2.
observed prominent characteristic peaks of PANi and PPy in addition to the contributions of the G and D peaks of RGO. In the case of PANi/ RGO@SiO2, the peak at 1506 cm 1 was assigned to the N–H deforma tion. The intense peak at 1373 cm 1 was associated with the C–N vi bration of polarons. The peak at 1170 cm 1 originated from the C–H vibration of the aromatic ring. In addition, the observed peaks at 417, 517, 804, and 972 cm 1 provided information about the deformation vibration of the benzene rings. The Raman peaks at 1596 and 1333 cm 1, apart from the G and D band of RGO, were caused by the C–C stretching vibration of the benzene ring and the semiquinone radical cation structure in PANi, respectively [14,24,25]. In the spectra of PPy/RGO@SiO2, the peaks at 1487 and 1250 cm 1 were assigned to the C–C stretching vibration and the C–H in-plane bending vibration, respectively. Two additional peaks were observed at 1074 and 934 cm 1 that corresponded to the bipolaron ring deformation and polaron sym metric C–H in plane vibration, respectively. The peaks at 1590 and 1329 cm 1, half of which were formed by the G and D bands of RGO, came – C stretching vibration and ring stretching mode of the from the C– polymer backbone, respectively [14,26,27]. For P(ANi-co-Py)/RGO@ SiO2, all of the characteristic peaks of PANi and PPy were observed at the low frequency of 400–1000 cm 1, which proved the co-existence of PANi and PPy in the copolymer composites. Interestingly, the Raman pattern of P(ANi-co-Py)/RGO@SiO2 displayed more pyrrole units than
Fig. 3. FTIR spectra of RGO, SiO2–NH2, and RGO@SiO2. 4
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transmission into the absorber [31,32]. Therefore, the conductivity of absorbing materials is the key for excellent EM absorption performance. In this work, the electrical conductivity of the resultant composite was carefully measured to verify the conductivity of the composites was in the moderate region, thus preventing impedance mismatch while balancing the attenuation properties. The conductivity was calculated based on the following equations: � � πτ V Resistivityðρ; ΩcmÞ ¼ ln2 I � � 1 Conductivity σ; S=cm ¼ ;
ρ
where τ is the thickness of samples, V is the measured voltage, and I is the applied current [32,33]. Each sample was tested at least ten times and the average results were used. The electrical conductivity of three types of prepared composites, PANi/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-coPy)/RGO@SiO2, are presented in Table 1. The conductivities of RGO and RGO@SiO2 were also measured for comparison. The electrical conductivity of RGO was 0.08 S/cm while its value in RGO@SiO2 was remarkably lower, 3.2 � 10 4 S/cm. It is reasonable that the addition of the semiconducting material of silica particles would result in a decrease in the electrical conductivity of RGO@SiO2. After the polymerization of the ICP chains on the RGO@SiO2, the conductivity of both prepared composites increased significantly. As listed in Table 1, the value of electrical conductivities of PANi/RGO@ SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2 were 0.54, 2.21, and 0.44 S/cm, respectively. The notable enhancement in conductivity of the ICP/RGO@SiO2 nanocomposites was assigned to the presence of the ICP chains. The presence of polyaniline and polypyrrole chains on the surface of the RGOs sheets in the RGO@SiO2 composites enabled electrons to travel through the graphene oxide sheets along the poly meric chains and inside the inner structure of the composites, which led to better delocalization of electrons and reduced the forbidden bands present in the nanocomposites [32]. The formation of charge carrier polarons and bipolarons on the chain of each polymer also supported the mobility of photo-generated charge carriers resulting in enhanced con ductivity [4,34,35]. However, whenever aniline was polymerized with pyrrole, the conductivity of the copolymer was reduced compared to the individual polymer because the incorporation of the second co-monomer could interrupt the continuity of the polymer chain, thereby hindering the carrier transport between the different molecular chains of PANi and PPy [14,20]. To prevent a sharp reduction in elec trical conductivity of the resultant copolymer-nanocomposites, a pre-polymerization synthesis method was applied to polymerize indi vidual homopolymer first before they were copolymerized to produce the final P(ANi-co-Py)/RGO@SiO2 nanocomposites [19]. The ratio of aniline:pyrrole was set to 1:3 to limit the disruption of the conjugated system of polypyrrole chains by forming short polyaniline chains after the optimized pre-polymerization time of 30 min. Hence, the conduc tivity of P(ANi-co-Py)/RGO@SiO2 could reach 0.44 S/cm, which pro vided an advantage for better EM attenuation with P
Fig. 4. FTIR spectra of PANi/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-coPy)/RGO@SiO2.
adding APTES [16,29]. In the case of RGO@SiO2, the characteristic peak of Si–O–Si/Si–OH was obtained at 1099 cm 1. The peak at 1190 cm 1 indicated the presence of the C–N alkyl chain of APTES on the RGO. The – O bond of RGO (1720 cm 1) also appeared, peaks assigned to the C– with a shift to higher wavelength in RGO@SiO2 (1725 cm 1). The accompanying peak of N–H at 1589 cm 1 confirmed the formation of an amino linkage between the carboxyl functionality of the RGO and the amine functionality of the SiO2–NH2 particles [30]. For PANi/RGO@SiO2, the peaks at 1480 and 1567 cm 1 were – C stretching vibration of the bezenoid attributed to the distinctive C– and quinoid, respectively (Fig. 4). The signals appearing at 1241 and – N and C–N stretching vibration modes, 1296 cm 1 matched the C– respectively [11,18]. In addition, the peak at 1103 cm 1 was associated with the C–H in-plane bending of the 1,2,4-substituted benzene of PANi, which was accompanied by the Si–O–Si asymmetric vibration resulting from the higher intensity of the signal. In the FTIR spectrum of PPy/R GO@SiO2, the bands at 1302, 1097, and 1041 cm 1 corresponded to the C–N in plane deformation vibration, the C–H in-plane deformation vi bration, and the N–H in-plane deformation vibration, respectively. The absorption peaks at 1180 and 964 cm 1 were consistent with the vi bration of the C–N bond and the out-of-plane vibration of ¼ C–H on the pyrrole ring [9,12]. The peaks in the range of 1550–1473 cm 1 were assigned to the vibrations of the pyrrole ring, which were also obtained in the spectrum of P(ANi-co-Py)/RGO@SiO2, although clearly shifted to 1549-1477 cm 1. The spectrum of P(ANi-co-Py)/RGO@SiO2 showed a series of peaks characteristic of homopolymer nanocomposites such as at 1549, 1173, 1305, and 963 cm 1 [14,19,20]. The FTIR patterns of P (ANi-co-Py)/RGO@SiO2 and PPy/RGO@SiO2 were similar due to the 1:3 vol ratio of aniline:pyrrole. Moreover, as observed in all three kind of nanocomposites, the peak at ~795 cm 1 was attributed to the bending vibration of Si–O bond, which indicated that all of the polymers were successfully fabricated on the RGO@SiO2 composites [11].
Table 1 The electrical conductivity of RGO, RGO@SiO2, and ICP/RGO@SiO2 composites.
3.3. Electrical conductivity The EM wave can be attenuated quickly in a good conductor because of the induced current created in the conductor [3]. However, a very high electrical conductivity is not favorable for dissipation of an EM microwave because of the skin effect, which can cause a mismatch in impedance. It will cause more reflection from the surface of absorber materials. In contrast, very low conductive materials are also not effective for consumption of EM energy, as they will be transparent under the penetration of EM radiation, causing more microwave 5
Sample
Thickness (cm)
Resistivity (Ω⋅cm)
Conductivity (S/ cm)
RGO RGO@SiO2
0.01 0.01
264 69 � 103
0.08 3.2 � 10
PANi/RGO@SiO2 PPy/RGO@SiO2 P(ANi-co-Py)/ RGO@SiO2
0.01 0.01 0.01
41 10 50
0.54 2.21 0.44
4
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(ANi-co-Py)/RGO@SiO2.
Table 2 Specific surface area and total pore volume of PANi/RGO@SiO2, PPy/RGO@ SiO2 and P(ANi-co-Py)/RGO@SiO2
3.4. EM absorbance measurement Composites with a high interface area increase the multiple reflec tion of EM waves, and if the matrix is conductive, the EM wave gradually gets absorbed and decays. If the matrix is nonconductive, multiple re flections are slightly detrimental to the absorbing performance of the shielding materials [36]. However, if the matrix has a very high con ductivity it is also not suitable for attenuation performance of epox y/ICP/RGO@SiO2 nanocomposites due to the impedance mismatch phenomenon mentioned above. Therefore, it is very important to control the filler content to ensure that the matrix is conductive to balance both requirements. As discussed by Ye et al., the essential content of carbon filler necessary to form an effective conductive network inside a matrix is about 10 wt% [3]. Additionally, based on the moderate conductivity values of the three as-prepared ICP/RGO@SiO2 nanocomposites, we decided to set the filler content of the ICP/RGO@SiO2 to 9 wt% of the total weight of epoxy/ICP/RGO@SiO2 composites to not only ensure sufficient conductivity in the network of final epoxy composites, but also to avoid the impedance mismatch of highly conducting materials. It is worth to note that the matching thickness is essential factor to meet the demand application at broaden bandwidth frequency. However, in this study, the design of ICP/RGO@SiO2 composite was also considered in the technological structure and mechanical properties to reach the maximum EM attenuated performance. Therefore, the absorbers of ICP/RGO@SiO2 composites in epoxy matrix was fabricated at a proper thickness of 5 mm, and their EM performance at whole X and Ku bands (8.2–18 GHz) were compared in Fig. 5. The epoxy/PPy/RGO@SiO2 composite had the poorest electromag netic absorbing capacity. Its minimum reflection loss (RLmin) value was 18 dB at 12.5 GHz and its effective absorption bandwidth had only short ranges, from 12.4 to 13 GHz and from 16.7 to 17.6 GHz in Ku band (Fig. 5a). Here, the effective absorption range was defined as when the RL value was less than 10 dB, which corresponded to 90% absorption of incident microwave energy. Its high electrical conductivity accom panied with its low specific surface area were the main reasons for the poor EM performance of PPy/RGO@SiO2. The high conductivity of PPy/ RGO@SiO2 might lead to the impedance mismatch, resulting in more reflected waves on the surface of the EM absorber, which is unfavorable for microwave dissipation. In addition, the coverage of the polypyrrole chains on the surface of RGO@SiO2 resulted in the bunching of the RGO sheets, which in turn caused a decrease in the composite’s inner space. As listed in Table 2, the specific surface area of PPy/RGO@SiO2 was only 6.3 m2/g, which was 3� less than in PANi/RGO@SiO2 (22.5 m2/g) and 2� less than its value in P(ANi-co-Py)/RGO@SiO2 (12.9 m2/g). The limited specific surface area led to weaker interfacial polarization by reducing the opportunity for contact between the guest (incident wave) on its moving pathway to the host (the matrix of composites) [10]. As a result, the EM absorption ability was limited. Epoxy/PANi/RGO@SiO2 composites exhibited better EM attenua tion properties. The RLmin reached to 38.9 dB at 17.1 GHz and 25.6 dB at 12.4 GHz in the Ku band and X band, respectively (Fig. 5b). The
Sample names
Specific surface area (m2/ g)
Total pore volume (cm3/ g)
PANi/RGO@SiO2 PPy/RGO@SiO2 P(ANi-co-Py)/ RGO@SiO2
22.5 6.3 12.9
0.113 0.014 0.046
epoxy/P(ANi-co-Py)/RGO@SiO2 composite displayed the best EM wave damping capacity. Its RLmin was 40.6 dB at 15.8 GHz in the Ku band and 19.8 dB at 9.8 GHz in the X band (Fig. 5c). Additionally, the effective absorption ranges of epoxy/P(ANi-co-Py)/RGO@SiO2 covered a wider frequency range of 5.5 GHz bandwidth, from 12.3 to 17.8 GHz, compared to the 2.2 GHz effective bandwidth of epoxy/PANi/RGO@ SiO2, from 15.8 to 18 GHz in the Ku band. The better absorbing per formance of PANi/RGO@SiO2 and P(ANi-co-Py)/RGO@SiO2 could be explained by the well-matched impedance because these two materials had only a moderate electrical conductivity. The matched impedance avoided reflected light at the surface and allowed the incident light to come into the interior of absorber where it could be dissipated. More over, the moderate specific surface area of PANi/RGO@SiO2 and P(ANico-Py)/RGO@SiO2 composites gave a rise to interfacial polarization under alternating EM field which benefitted the EM attenuation. In addition, the co-existence of the PANi and PPy chains in the epoxy composites of P(ANi-co-Py)/RGO@SiO2 particles could provide more defect centers, leading to dipole polarization and accompanying inter facial polarization generated from the heterogeneous components, resulting in P(ANi-co-Py)/RGO@SiO2 having the best EM attenuation performance [32]. The anticipated EM capability of copolymer composites was further studied at different thickness. The RL values of P(ANi-co-Py)@RGO@ SiO2 at filler content of 9 wt% in epoxy matrix as a function of frequency with varying thickness was conducted and shown in Fig. 6. The P(ANico-Py)@RGO@SiO2 absorber displayed the poor EM suppressing ability at 3 mm thickness when the RL value was lower than 7.5 dB in whole range frequency from 8.2 GHz to 18 GHz. By increasing thickness of absorber, the RLmin reached to 40.6 dB at 15.8 GHz, 23.5 dB at 12.5 GHz and 26.3 dB at 9.8 GHz at thickness of 5 mm, 6 mm and 7 mm. The minimum absorption value shifted to lower frequency when the thick ness of absorber increased. This observation was in good agreement to the quarter-wavelength equation: ! nλ nc pffiffiffiffiffiffiffiffiffiffiffiffiffi n ¼ 1; 3; 5; …: tm ¼ ¼ 4 4fm jμr jjεr j where tm is matching thickness, λ is EM wavelength, fm is frequency of the RLmin, μr is the complex permittivity, εr is the complex permeability and c is the velocity of the light [9]. The influence of filler content on EM performance of copolymer composite was also investigated. As the 5 mm thick absorber of P(ANi-
Fig. 5. The reflection loss (RL) curves of (a) PPy/RGO@SiO2, (b) PANi/RGO@SiO2, and (c) P(ANi-co-Py)/RGO@SiO2 in epoxy matrix at 5-mm-thick absorbers. 6
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Fig. 6. The RL curves of 9 wt% P(ANi-co-Py)/RGO@SiO2 in epoxy matrix at thickness of 3 mm, 5 mm, 6 mm and 7 mm.
co-Py)/RGO@SiO2 exhibited the best microwave dissipated ability, we fabricated the 5-mm-thick P(ANi-co-Py)/RGO@SiO2 absorbing material at loading contents of 3, 9 and 15 wt% and the EM dissipated ability of each sample was illustrated in Fig. 7a–c. The RLmin value was remark ably increased from 6.8 dB at 9.2 GHz to 40.6 dB at 15.8 GHz and 39.2 dB at 13.9 GHz when the P(ANi-co-Py)/RGO@SiO2 loading contents increased from 3 wt% to 9 wt% and 15 wt%, respectively. The average shielding efficiency of 3 wt% absorber was only 30% while it reached over 91% for both cases of 9 wt% and 15 wt% in a whole range frequency of X and Ku band. The results demonstrated that the content of P(ANi-co-Py)/RGO@SiO2 played an important role to maximum the wastage of EM wave energy. At low loading content of 3 wt% was not enough to create a conductive network inside composites, resulting the poor in EM absorbing performance. However, by increasing the P(ANi-
co-Py)/RGO@SiO2 content from 9 wt% to 15 wt%, the RLmin value was slightly declined by value of 1.4 dB. This could be explained that the higher filler content generates a higher conductive network to the composites matrix. As the result, it might cause to the skin effect where more reflection could occur on the surface of composites and lower the minimum absorption value. For a sake of comparison, the EM attenuated efficiency of P(ANi-co-Py)/RGO@SiO2 was compared with literatures where the other typical graphene-based composites were also reported (Table 3). The comparative results showed that the relative low loading content accompanying to the high RLmin value made P(ANi-co-Py)/ RGO@SiO2 become a potential material in the field application of EM shielding. The mechanism of the wave absorption properties of the three nanocomposites was generated from dielectric loss, interfacial polari zation, dipole polarization, multiple reflection, and scattering. First, the introduction of polymeric chains on the RGO@SiO2 composites not only combined all components in a unique structure of composites but also raised their electrical conductivities significantly. As a result, it pro moted the movement of free electrons and charge carriers between the RGO sheets and the polymer chains, which contributed to the dielectric loss. Second, there were at least three components in each composite, such as RGO sheets, SiO2 particles, and conducting polymer chains, which meant that there was a greater possibility for the creation of de fects at the interfaces. For instance, the interface between RGO sheets and SiO2 particles, SiO2 and polymeric chains, and polymeric chains and RGO sheets could each have interfacial polarization when the composite was under EM irradiation. These interfaces also created some internal space inside the composite along with the layered structure of the RGO sheets, which was a benefit in enabling multiple internal reflections and scattering. Third, the different electron catching ability of carbon, oxy gen, silicon, and nitrogen atoms played roles as defect points and induced dipoles, resulting in additional polarization relaxation pro cesses that further enhanced the microwave absorption properties of the composites. The detail mechanism can be obtained in Fig. 7d.
Fig. 7. The RL values of 5-mm-thick P(ANi-co-Py)/RGO@SiO2 at filler content of (a) 3 wt%, (b) 9 wt%, (c) 15 wt% and (d) proposal mechanism of EM attenuation. 7
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Table 3 EM absorbing efficiency of recent reported on graphene-based composites. Samples
Filler content (wt%)
PANi nanorods/graphene PPy nanosphere/RGO Graphene-pFe3O4@ZnO Graphene@Fe3O4@SiO2@PANi P(ANi-o-Py)@RGO/SiO2
50 30 30 25 9
RLmin (dB) 51.1 19.0 40.0 19.4 40.6
Thickness (mm)
Effective absorption bandwidth (range, GHz)
Refs.
3 1.46 5 2 5
12.2 (5.8–18.0) 2.8 (15.2–18) 2 (10–12) 4.4 (10.4–18) 5.5 (12.3–17.8)
[5] [37] [38] [39] This work
4. Conclusion [5]
The copolymer nanocomposite P(ANi-co-Py)/RGO@SiO2 was suc cessfully prepared through an oxidative pre-polymerization method. The electrical conductivity of the resultant composite was 0.44 S/cm, lower than that of the component homopolymer nanocomposites. Interestingly, the EM attenuation effectiveness of the copolymer nano composites was much better in comparison with homopolymer component nanocomposites of either PANi/RGO@SiO2 or PPy/RGO@ SiO2. The RLmin of P(ANi-co-Py)/RGO@SiO2, PANi/RGO@SiO2, and PPy/RGO@SiO2 in the Ku band were 40.6 dB, 38.9 dB, and 13.9 dB, respectively. The effective absorption bandwidth of the P(ANi-coPy)/RGO@SiO2 was 5.5 GHz, from 12.3 to 17.8 GHz. For comparison, the absorption bandwidth was about 2.2 GHz, from 15.8 to 18 GHz, for PANi/RGO@SiO2 and discretely in two short ranges of 0.6 GHz from 12.4 to 13 GHz and 0.9 GHz from 16.7.1–17.6 GHz for PPy/RGO@SiO2. However, this study has investigated neither the morphology nor the conductivity of the copolymer nanocomposite P(ANi-co-Py)/RGO@SiO2 as a function of the molar ratio of the two monomers, aniline and pyr role. These remaining problems require more investigation to determine the EMI performance of P(ANi-co-Py)/RGO@SiO2 for further applications.
[6] [7] [8] [9] [10] [11] [12] [13] [14]
Prime novelty statement
[15]
By pre-polymerization method, the electrical conductivity of the copolymer nanocomposites of P(ANi-co-Py)/RGO@SiO2 was significantly improved in comparison with traditional in situ polymerization method. The as-prepared nanocomposites displayed the minimum reflection loss (RL) of 40.6 dB at 15.8 GHz with only 9 wt% filler content in epoxy matrix. The relative low loading content (9 wt%) accompanying with the broaden effective bandwidth of 5.5 GHz (12.3–17.8 GHz) made P (ANi-co-Py)/RGO@SiO2 becomes a better candidate for future micro wave absorption material.
[16] [17] [18] [19]
Acknowledgements
[20]
This research was supported by to the Creative Materials Discovery Program (2015M3D1A1068061) administered by the National Research Foundation of Korea (NRF) and funded by the Ministry of Science and ICT, Republic of Korea. Tung and Quyen thanks to the financial support from VAST (Grant No. NCVCC 06.08/19-19 to NCVCC 06.14/20-20), Vietnam.
[21] [22]
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Silica particles wrapped with poly(aniline-co-pyrrole) and reduced graphene oxide for advanced microwave absorption Quyen Vu Thi a, Sungho Lim a, Eunsuk Jang a, Jeongrae Kim a, Nguyen Van Khoi b, Ngo Trinh Tung b, Daewon Sohn a, * a b
Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, Seoul, South Korea Institute of Chemistry, Vietnam Academy of Science and Technology, 18- Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam
H I G H L I G H T S
� In-situ pre-polymerization used to prepare P(ANi-co-Py)/RGO@SiO2 nanocomposite. � The minimum reflection loss reached to 40.6 dB at 15.8 GHz. � The effective absorption bandwidth was 5.5 GHz from 12.3 to 17.8 GHz in Ku band. � It is expected to be applied in wide range frequency EM attenuation materials. A R T I C L E I N F O
A B S T R A C T
Keywords: Silica particles Poly(aniline-co-pyrrole) Reduced graphene oxide Pre-polymerization Microwave absorption
In this work, a novel nanocomposite composed of silica nanoparticles wrapped with a poly(aniline-co-pyrrole) copolymer and reduced graphene oxide (P(ANi-co-Py)/RGO@SiO2) was prepared by an in situ prepolymerization method and then characterized. The aniline:pyrrole molar ratio was 1:3 and the prepolymerization time was 30 min before the in situ polymerization of the copolymer nanocomposites occurred. The results revealed that the microwave absorbing performance of the copolymer nanocomposites was signifi cantly improved compared to that of the component homopolymer nanocomposites. The minimum reflection loss of P(ANi-co-Py)/RGO@SiO2, PANi/RGO@SiO2, and PPy/RGO@SiO2 was 40.6, 38.9, and 13.9 dB, respectively. The effective absorption bandwidth of P(ANi-co-Py)/RGO@SiO2 was 5.5 GHz from 12.3 to 17.8 GHz. In contrast, the effective absorption bandwidth of a component homopolymer nanocomposite, such as PANi/RGO@SiO2, was 2.2 GHz (15.8–18 GHz), and for PPy/RGO@SiO2, was only 1.5 GHz (12.4–13 GHz and 16.7–17.6 GHz) in Ku band. The enhanced microwave absorption was mainly contributed to the interfacial polarization in which the interfacial interaction between SiO2 particles, rGO sheets and conducting polymer chains played critical roles.
1. Introduction Electromagnetic interference (EMI) phenomena have become a serious problem with the rapid development and application of elec tronic devices as well as the widespread use of transient power sources. To overcome this problem, electromagnetic (EM) absorbing materials with highly efficient absorption, wide dissipation bandwidth, low cost, and light weight have been urgently pursued [1–3]. In general, EMI shielding materials should possess a balanced combination of electrical conductivity, dielectric permittivity, magnetic permeability, and
physical geometry. So far, intrinsically conducting polymers (ICP) such as polyaniline (PANi) and polypyrrole (PPy) have attracted much research interest as EMI shielding materials because of their moderate electrical conductivity, facile processing, low density, corrosion resis tance, and compatibility with other polymeric matrices [4]. Recently, graphene has been considered as a powerful candidate for EMI shielding materials due to its high specific surface area, ultra-low density, thermal stability, and high electrical conductivity. However, perfect graphene, with an extraordinarily high conductivity, performed poorly for EM dissipation because of badly mismatched impedance.
* Corresponding author. E-mail addresses: [email protected] (Q.V. Thi), [email protected] (S. Lim), [email protected] (E. Jang), [email protected] (J. Kim), [email protected] (N. Van Khoi), [email protected] (N.T. Tung), [email protected] (D. Sohn). https://doi.org/10.1016/j.matchemphys.2020.122691 Received 26 November 2019; Received in revised form 15 January 2020; Accepted 19 January 2020 Available online 20 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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Surface modification of graphene with an oxygen-functionalized group called graphene oxide was reported to significantly enhance the mi crowave absorption properties compared to that of pure graphene [5]. To yield high EM attenuated performance, polymer nanocomposites based on ICPs and graphene have been investigated. Hailong et al. re ported that a composite matrix of paraffin with only 20 wt% loading of graphene/polyaniline composites achieved a maximum reflection loss (RL) of 45.1 dB for an absorber with a thickness of only 2.5 mm [6]. Prerana et al. synthesized a PANi matrix with 5 wt% of graphene in an in situ chemical oxidation method which had a maximum shielding effec tiveness between 51 and 52 dB in the frequency range of 2–12 GHz [7]. Similarly, Ankit et al. reported that a PPy/multiple layer graphene/TiO2 composite reached a maximum total shielding effectiveness of 53 dB in the frequency range of the Ku band (12.4–18 GHz) with only 5 wt% composites [8]. Sujuan et al. fabricated a composite of PPy with nickel particles based on reduced graphene oxide (RGO) that had a peak RL of 47.32 dB at 5.76 GHz and a thickness of 4 mm [9]. Unfortunately, the combination of polymeric chains with graphene sheets caused a signif icant reduction of the inner space of the resultant composites, as they were at risk of being constrained by the polymer chains. Polymer chains anchored and blocked the porous network or constrained the space between the graphene layer causing a decrease of the exposed surface area, which was unfavorable for EM attenuation due to a reduction of interfacial polarization [10]. This problem could be potentially over come by the intercalation of silica particles into the layer of graphene. Because of their unique characteristics such as very high specific surface area, low density, high thermal stability, and numerous possibilities for functionalization, silica particles do not act as an exfoliation agent of graphene sheets and also reinforce the composite so that they can endure harsh environments [11,12]. In recent years, many studies on the preparation of ICPs such as poly (aniline-co-pyrrole) (P(ANi-co-Py)) have been performed to combine the unique properties of both homopolymers. The electrical properties of the copolymer depend strongly on the component ratio and the synthesis method. The combination of these two homopolymers in P(ANi-co-Py) also revealed other outstanding properties, such as high thermal sta bility and better electrochemical capacitive performance compared to PANi or PPy homopolymers [13]. To date, research on P(ANi-co-Py) mainly focused on applications as a supercapacitor [14] or the removal of anti-inflammatory drugs [15]. In this work, novel polymer nanocomposite based on copolymer P(ANi-co-Py) and RGO-coated silica for EM attenuation materials were investigated. To the best of our knowledge, this is the first report of this kind of EMI shielding materials. The EMI shielding effectiveness of the polymer nanocomposites was examined over the entire frequency range of the X and Ku bands (8.2–18 GHz).
2.2. Synthesis of amino-functionalized silica particles (SiO2–NH2) For better anchoring of silica particles on RGO, the silica particles were modified with an amino-functional group by APTS. The aminofunctionalized silica particles (SiO2–NH2) were prepared by the modi €ber method [16]. Typically, 4 ml of TEOS was added dropwise fied Sto (0.5 ml/min) to a mixture of 100 ml of ethanol and 9 ml of ammonium hydroxide while stirring and the reaction was continued for 12 h. Af terward, 6 ml of APTS was quickly added to the reaction solution and allowed to react for another 12 h at room temperature. The white pre cipitates of SiO2–NH2 particles were collected by centrifugation at 8000 rpm for 10 min. The product was rinsed several times with ethanol and DI water to remove excess reagent. The final products were dried in a freezing dryer and stored for further steps. 2.3. Preparation of reduced graphene oxide (RGO) Thermally-reduced graphene oxide was synthesized following our previous publication using a strong acid treatment [17]. In this method, a mixture of concentrated sulfuric acid and phosphoric acid in a 9:1 vol ratio (240 ml:27 ml) was prepared while stirring for 30 min. Next, 2 g of natural graphite flake was dispersed into the mixture, then 12 g of po tassium permanganate was added to the solution, which was chilled in an ice bath for 30 min. The reaction temperature was then raised to 90 � C. After 12 h of reaction, the mixture was cooled to room temperature followed by dilution with 400 ml of DI water. To remove the unreacted residual, 20 ml of hydrogen peroxide was added, and the color of mixture bubbled and turned to black. The product was washed with 0.1 M HCl and DI water several times. To remove unreacted graphite flakes and collect the uniform RGO sheets, the products were centrifuged 3 times at 1000 rpm for 2 min. The final RGO products were collected by centrifugation at 8000 rpm for 15 min and dried in a vacuum oven at ambient temperature. 2.4. Synthesis of RGO-coated amino functionalized silica particles (RGO@SiO2) The preparation of composite RGO@SiO2 was carried out with an in situ method with an RGO:SiO2–NH2 weight ratio of 1:1. Briefly, 0.2 g of RGO was dispersed in 100 ml of DI water. The mixture was sonicated in an ultrasonicator for 30 min for better exfoliation of the RGO sheets. Separately, 0.2 g of SiO2–NH2 particles was dispersed in an ethanol: water solution (2:1 vol ratio). The solution of SiO2–NH2 particles was slowly added into the mixture of RGO (1 ml/min). The reaction was continued for 24 h to complete the amidation reaction between the carboxyl functionality on the surface of the RGO and the amino group on the silica particles. The resultant products were collected by centrifu gation and filtered 3 times with ethanol and water. The final product, RGO@SiO2, was dried in vacuum at 60 � C for 24 h.
2. Experimental 2.1. Materials
2.5. Synthesis of the copolymer nanocomposite P(ANi-co-Py)/ RGO@SiO2
Natural graphite flake, aminopropyltriethoxysilane (APTS, 99%), and aniline (99.5%) were obtained from Sigma Aldrich. Pyrrole (98%) and tetraethylorthosilicate (TEOS, 99.9%) were purchased from Alfa Aesar. Sufuric acid (98% purity), anhydrous ethanol (� 99.9%), potas sium permanganate (99.3% purity), and ammonium hydroxide solution (28.0–30.0% ammonia) were supplied by Daejung Chemical. Phos phoric acid (Duksan Pure Chemical, 85% purity), hydrogen peroxide (Samchun Chemical, 35% purity), hydrochloric acid (Samchun Chemi cal, 35.0–37.0%), ammonium persulfate (APS, Samchun Chemical 98%), EpoFix® resin, and EpoFix® hardener (Struer) were used as received. Copper plate was supplied by Shapiro Metal Supply (USA). Deionized (DI) water used in the reaction was purified with a Mili-Q water purification system (Milipore).
In this work, the copolymer nanocomposites were prepared by an in situ chemical oxidative polymerization method at low temperature (5 � C). According to previous work [11,18] polymer synthesis with an RGO@SiO2:monomer weight ratio of 1:5 formed polymer nano composites with the best electrical conductivity. P(ANi-co-Py), with its highly improved electrical conductivity, was prepared by a pre-polymerization method as reported by Xianghui et al. [19]. The aniline:pyrrole molar ratio was 1:3. The procedure for the synthesis of the P(ANi-co-Py)/RGO@SiO2 copolymer nanocomposite follows. Briefly, 0.3 ml aniline monomer (3.3 mmol) and 0.7 ml pyrrole monomer (0.01 mol) were dispersed in 20 ml (solution A) and 60 ml (solution B), respectively. Solutions A and B were poured into separate round-bottom flask and immediately cooled in an 2
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0.05 cm3 piece of copper. Then the cured epoxy composites were held at room temperature for 12 h. The absorption (A) was calculated with A ¼ 1 - jS11 j2 - jS12 j2 , where S11 and S12 are the reflection and the trans mission parameters, respectively. In this study, the boundary back layer was the perfect electric conductor of copper, so S12 ¼ 0 and therefore, A
ice bath. For the pre-polymerization process, 0.8 and 2.0 g of APS dis solved in an HCl solution were added dropwise to solutions A and B, respectively, for 30 min. Meanwhile, 0.2 g RGO@SiO2 was dispersed in 100 ml of DI water and 10 ml of 1 M HCl under ultra-sonication. After 30 min of the pre-polymerization process, solutions A and B were mixed and poured into the RGO@SiO2 solution and allowed to react for 12 h. For the sake of comparison, PANi/RGO@SiO2 and PPy/RGO@SiO2 polymer nanocomposites were also prepared with the same procedure, as described below and shown in Scheme 1.
¼ 1 - jS11 j2 . The RL could be expressed by RL ¼ 20log10jS11 j. 3. Result and discussion 3.1. Morphology and SEM images
2.5.1. PANi/RGO@SiO2 1 ml of aniline monomer was added dropwise to a solution of 100 ml of DI water and 10 ml of 1 M HCl and the mixture was stirred for 1 h to obtain a homogeneous solution. After that, 0.2 g of RGO@SiO2 was dispersed in the solution and sonicated for another 1 h. Then, 2.4 g of APS dissolved in 10 ml of HCl was added and the polymerization solu tion was held at 5 � C for 12 h.
Fig. 1 shows the SEM images of silica particles, RGO@SiO2, PANi/ RGO@SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2. The asprepared SiO2–NH2 had a spherical structure with an average diam eter of 200 nm, as shown in Fig. 1a. Fig. 1b presents the morphology of RGO@SiO2 in the wrinkled and folded region of the RGO sheets (RGO), with partially anchored spherical silica particles. The remaining wrin kled space of the RGO could be considered as a good template for the attachment of monomer for better polymeric chain formation in the final ICP/RGO@SiO2 composites. The polymerization of aniline on RGO@ SiO2 resulted in a worm-like networks with diameters of 40–60 nm and lengths 120–180 nm, which were uniformly covered with the RGO@ SiO2 composites (Fig. 1c and d). The combination of pyrrole with the quasi-spherical particles on the surface of RGO@SiO2 provided a rougher surface for the PPy/RGO@SiO2 composites (Fig. 1e and f). In the case of P(ANi-co-Py)/RGO@SiO2, the quasi-spherical particles were halved in diameter, from 250 to 520 nm in PPy/RGO@SiO2 to 135–270 nm in P(ANi-co-Py)/RGO@SiO2. This observation agreed with a previ ous report [20], which confirmed the successful formation of P (ANi-co-Py). However, there were some worn-like pieces attached on the copolymer chains that could be considered as the formation of PANi chains from the pre-polymerization method.
2.5.2. PPy/RGO@SiO2 Similarly, PPy/RGO@SiO2 nanocomposites were synthesized with the same procedure. Typically, 1.2 ml of pyrrole was added slowly to a solution of 100 ml of DI water and 10 ml of 1 M HCl while stirring. Subsequently, 0.2 g of RGO@SiO2 was dispersed into the solution and sonicated for 1 h. After that, 3.3 g of APS in 10 ml of HCl was slowly added and the polymerization solution was kept at 5 � C for 12 h. 2.6. Characterization Scanning electron microscopy (SEM) images were collected on a NOVA SEM operating at 10 kV. Raman spectra were collected using a dispersive Raman microscope equipped with a diode laser (λ ¼ 785 nm, power ¼ 40 mW) and a CCD detector (Kaiser Optical Inc., USA). Fourier transform infrared (FTIR) spectra were obtained with a FTLA 2000 (ABB Ltd., Switzerland) using a KBr pellet mixed with the given samples. FTIR analysis was carried out with 16 scans over the range of 500–4000 cm 1 with a resolution of 2 cm 1. Brunauer-Emmett-Teller (BET, BEL JAPAN Inc., Japan) analysis with absorption/desorption isotherms of N2 at 77 K was used to determine the surface area of the particles. The samples were pretreated at 100 � C for 3 h prior to the BET measurement. For electrical conductivity characterization, composite powders were pressed at high pressure (9 tons/cm2 for 30 s) into a pellet with a thickness of 0.01 cm. The electrical resistivity of the polymer nano composites was measured using a four-probe measuring device (HP 4155C, Agilent Technologies). A Hewlett-Packard E8362B vector network analyzer was used to study the absorption properties of the polymer nanocomposites in the X band region (8.2–12.4 GHz) and Ku band region (12.4–18 GHz). Samples for EM measurements were pre pared with an epoxy resin/hardener cured with a resin:hardener weight ratio of 25:3, with 9.0 wt% of the composite (PANi/RGO@SiO2, PPy/ RGO@SiO2, or P(ANi-co-Py)/RGO@SiO2). The mixture was vigorously stirred for 30 min before it was fabricated on the surface of a 15 � 15 �
3.2. Raman shift and FTIR To confirm the structure of the composites with RGO, the Raman shift of each sample was measured, as shown in Fig. 2. RGO had a Raman spectrum with two typical peaks at around 1606 and 1332 cm 1, which corresponded to the G and D bands, respectively. The G band was pro duced from the sp2 hybridized carbon, while the D band was activated both by carbon impurities and by defects participating in double reso nance Raman scattering near the K point of the Brillouin zone. The in tensity ratio of the D/G bands (ID/IG) generally has been used as a useful index to define the defect level of the carbon structure materials [21–23]. The G and D bands were obtained in the RGO@SiO2 compos ites without additional peaks because the SiO2 was amorphous in structure. The G and D bands were shifted to 1604 and 1331 cm 1, respectively. Moreover, the ID/IG in RGO was 1.28 while its value in RGO@SiO2 was 1.29, which indicated the successful anchoring of SiO2–NH2 particles on the RGO sheets. In the Raman spectra of the ICP/RGO@SiO2 composites, we also
Scheme 1. Schematic of the preparation procedure of for the composites of intrinsically conducting polymers (ICP, polyaniline, polypyrrole, or poly(aniline-copyrrole)) and RGO@SiO2. 3
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Fig. 1. SEM images of (a) SiO2–NH2 particles, (b) RGO@SiO2, (c, d) PANi/RGO@SiO2, (e, f) PPy/RGO@SiO2 and (g, h) P(ANi-co-Py)/RGO@SiO2.
aniline units, which was reasonable because of the higher ratio of the pyrrole monomer. Compared to RGO and RGO@SiO2, PAN i/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2 exhibited a Raman shift in G and D peaks and a broadening of the D band. Addi tionally, the calculated values of ID/IG were 0.75, 0.78, and 0.96 for PANi/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2, respectively. These ID/IG values provided evidence of the pi-pi interac tion of the polymer chain with the RGO, confirming the successful coating of polymer chains on the RGO@SiO2 nanocomposites. FTIR measurements gave complementary information about the chemical interactions of each component inside the structural nanocomposites, as shown in Figs. 3–4. In the FTIR spectrum of RGO the broad peaks at 3395 and 1720 cm 1 –O showed the carboxyl O–H carboxylic acid stretching and the C– stretch of carboxyl group, respectively. The absorption peak at 1218 cm 1 was assigned to the C–OH stretching of the alcohol group [28]. Hence, the RGO was not completely reduced by the thermal reduction synthesis method. The oxygen-containing functional group proved the presence of carboxylic groups on the RGO, which was beneficial for the attachment of SiO2–NH2 particles on the surface of the RGO sheets through the chemical interaction formed by the amidation reactions. The absorption peaks of the Si–O–Si asymmetric stretching vibration and the Si–OH asymmetric bending appeared at 1103 and 951 cm 1, respectively. The peak at 802 cm 1 was the stretching vibration of Si–OH. Additionally, the peak at 1632 cm 1 was due to the N–H bond that resulted from the amine-functionalization of silica surface after
Fig. 2. Raman shift of RGO, RGO@SiO2, PANi/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2.
observed prominent characteristic peaks of PANi and PPy in addition to the contributions of the G and D peaks of RGO. In the case of PANi/ RGO@SiO2, the peak at 1506 cm 1 was assigned to the N–H deforma tion. The intense peak at 1373 cm 1 was associated with the C–N vi bration of polarons. The peak at 1170 cm 1 originated from the C–H vibration of the aromatic ring. In addition, the observed peaks at 417, 517, 804, and 972 cm 1 provided information about the deformation vibration of the benzene rings. The Raman peaks at 1596 and 1333 cm 1, apart from the G and D band of RGO, were caused by the C–C stretching vibration of the benzene ring and the semiquinone radical cation structure in PANi, respectively [14,24,25]. In the spectra of PPy/RGO@SiO2, the peaks at 1487 and 1250 cm 1 were assigned to the C–C stretching vibration and the C–H in-plane bending vibration, respectively. Two additional peaks were observed at 1074 and 934 cm 1 that corresponded to the bipolaron ring deformation and polaron sym metric C–H in plane vibration, respectively. The peaks at 1590 and 1329 cm 1, half of which were formed by the G and D bands of RGO, came – C stretching vibration and ring stretching mode of the from the C– polymer backbone, respectively [14,26,27]. For P(ANi-co-Py)/RGO@ SiO2, all of the characteristic peaks of PANi and PPy were observed at the low frequency of 400–1000 cm 1, which proved the co-existence of PANi and PPy in the copolymer composites. Interestingly, the Raman pattern of P(ANi-co-Py)/RGO@SiO2 displayed more pyrrole units than
Fig. 3. FTIR spectra of RGO, SiO2–NH2, and RGO@SiO2. 4
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transmission into the absorber [31,32]. Therefore, the conductivity of absorbing materials is the key for excellent EM absorption performance. In this work, the electrical conductivity of the resultant composite was carefully measured to verify the conductivity of the composites was in the moderate region, thus preventing impedance mismatch while balancing the attenuation properties. The conductivity was calculated based on the following equations: � � πτ V Resistivityðρ; ΩcmÞ ¼ ln2 I � � 1 Conductivity σ; S=cm ¼ ;
ρ
where τ is the thickness of samples, V is the measured voltage, and I is the applied current [32,33]. Each sample was tested at least ten times and the average results were used. The electrical conductivity of three types of prepared composites, PANi/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-coPy)/RGO@SiO2, are presented in Table 1. The conductivities of RGO and RGO@SiO2 were also measured for comparison. The electrical conductivity of RGO was 0.08 S/cm while its value in RGO@SiO2 was remarkably lower, 3.2 � 10 4 S/cm. It is reasonable that the addition of the semiconducting material of silica particles would result in a decrease in the electrical conductivity of RGO@SiO2. After the polymerization of the ICP chains on the RGO@SiO2, the conductivity of both prepared composites increased significantly. As listed in Table 1, the value of electrical conductivities of PANi/RGO@ SiO2, PPy/RGO@SiO2, and P(ANi-co-Py)/RGO@SiO2 were 0.54, 2.21, and 0.44 S/cm, respectively. The notable enhancement in conductivity of the ICP/RGO@SiO2 nanocomposites was assigned to the presence of the ICP chains. The presence of polyaniline and polypyrrole chains on the surface of the RGOs sheets in the RGO@SiO2 composites enabled electrons to travel through the graphene oxide sheets along the poly meric chains and inside the inner structure of the composites, which led to better delocalization of electrons and reduced the forbidden bands present in the nanocomposites [32]. The formation of charge carrier polarons and bipolarons on the chain of each polymer also supported the mobility of photo-generated charge carriers resulting in enhanced con ductivity [4,34,35]. However, whenever aniline was polymerized with pyrrole, the conductivity of the copolymer was reduced compared to the individual polymer because the incorporation of the second co-monomer could interrupt the continuity of the polymer chain, thereby hindering the carrier transport between the different molecular chains of PANi and PPy [14,20]. To prevent a sharp reduction in elec trical conductivity of the resultant copolymer-nanocomposites, a pre-polymerization synthesis method was applied to polymerize indi vidual homopolymer first before they were copolymerized to produce the final P(ANi-co-Py)/RGO@SiO2 nanocomposites [19]. The ratio of aniline:pyrrole was set to 1:3 to limit the disruption of the conjugated system of polypyrrole chains by forming short polyaniline chains after the optimized pre-polymerization time of 30 min. Hence, the conduc tivity of P(ANi-co-Py)/RGO@SiO2 could reach 0.44 S/cm, which pro vided an advantage for better EM attenuation with P
Fig. 4. FTIR spectra of PANi/RGO@SiO2, PPy/RGO@SiO2, and P(ANi-coPy)/RGO@SiO2.
adding APTES [16,29]. In the case of RGO@SiO2, the characteristic peak of Si–O–Si/Si–OH was obtained at 1099 cm 1. The peak at 1190 cm 1 indicated the presence of the C–N alkyl chain of APTES on the RGO. The – O bond of RGO (1720 cm 1) also appeared, peaks assigned to the C– with a shift to higher wavelength in RGO@SiO2 (1725 cm 1). The accompanying peak of N–H at 1589 cm 1 confirmed the formation of an amino linkage between the carboxyl functionality of the RGO and the amine functionality of the SiO2–NH2 particles [30]. For PANi/RGO@SiO2, the peaks at 1480 and 1567 cm 1 were – C stretching vibration of the bezenoid attributed to the distinctive C– and quinoid, respectively (Fig. 4). The signals appearing at 1241 and – N and C–N stretching vibration modes, 1296 cm 1 matched the C– respectively [11,18]. In addition, the peak at 1103 cm 1 was associated with the C–H in-plane bending of the 1,2,4-substituted benzene of PANi, which was accompanied by the Si–O–Si asymmetric vibration resulting from the higher intensity of the signal. In the FTIR spectrum of PPy/R GO@SiO2, the bands at 1302, 1097, and 1041 cm 1 corresponded to the C–N in plane deformation vibration, the C–H in-plane deformation vi bration, and the N–H in-plane deformation vibration, respectively. The absorption peaks at 1180 and 964 cm 1 were consistent with the vi bration of the C–N bond and the out-of-plane vibration of ¼ C–H on the pyrrole ring [9,12]. The peaks in the range of 1550–1473 cm 1 were assigned to the vibrations of the pyrrole ring, which were also obtained in the spectrum of P(ANi-co-Py)/RGO@SiO2, although clearly shifted to 1549-1477 cm 1. The spectrum of P(ANi-co-Py)/RGO@SiO2 showed a series of peaks characteristic of homopolymer nanocomposites such as at 1549, 1173, 1305, and 963 cm 1 [14,19,20]. The FTIR patterns of P (ANi-co-Py)/RGO@SiO2 and PPy/RGO@SiO2 were similar due to the 1:3 vol ratio of aniline:pyrrole. Moreover, as observed in all three kind of nanocomposites, the peak at ~795 cm 1 was attributed to the bending vibration of Si–O bond, which indicated that all of the polymers were successfully fabricated on the RGO@SiO2 composites [11].
Table 1 The electrical conductivity of RGO, RGO@SiO2, and ICP/RGO@SiO2 composites.
3.3. Electrical conductivity The EM wave can be attenuated quickly in a good conductor because of the induced current created in the conductor [3]. However, a very high electrical conductivity is not favorable for dissipation of an EM microwave because of the skin effect, which can cause a mismatch in impedance. It will cause more reflection from the surface of absorber materials. In contrast, very low conductive materials are also not effective for consumption of EM energy, as they will be transparent under the penetration of EM radiation, causing more microwave 5
Sample
Thickness (cm)
Resistivity (Ω⋅cm)
Conductivity (S/ cm)
RGO RGO@SiO2
0.01 0.01
264 69 � 103
0.08 3.2 � 10
PANi/RGO@SiO2 PPy/RGO@SiO2 P(ANi-co-Py)/ RGO@SiO2
0.01 0.01 0.01
41 10 50
0.54 2.21 0.44
4
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(ANi-co-Py)/RGO@SiO2.
Table 2 Specific surface area and total pore volume of PANi/RGO@SiO2, PPy/RGO@ SiO2 and P(ANi-co-Py)/RGO@SiO2
3.4. EM absorbance measurement Composites with a high interface area increase the multiple reflec tion of EM waves, and if the matrix is conductive, the EM wave gradually gets absorbed and decays. If the matrix is nonconductive, multiple re flections are slightly detrimental to the absorbing performance of the shielding materials [36]. However, if the matrix has a very high con ductivity it is also not suitable for attenuation performance of epox y/ICP/RGO@SiO2 nanocomposites due to the impedance mismatch phenomenon mentioned above. Therefore, it is very important to control the filler content to ensure that the matrix is conductive to balance both requirements. As discussed by Ye et al., the essential content of carbon filler necessary to form an effective conductive network inside a matrix is about 10 wt% [3]. Additionally, based on the moderate conductivity values of the three as-prepared ICP/RGO@SiO2 nanocomposites, we decided to set the filler content of the ICP/RGO@SiO2 to 9 wt% of the total weight of epoxy/ICP/RGO@SiO2 composites to not only ensure sufficient conductivity in the network of final epoxy composites, but also to avoid the impedance mismatch of highly conducting materials. It is worth to note that the matching thickness is essential factor to meet the demand application at broaden bandwidth frequency. However, in this study, the design of ICP/RGO@SiO2 composite was also considered in the technological structure and mechanical properties to reach the maximum EM attenuated performance. Therefore, the absorbers of ICP/RGO@SiO2 composites in epoxy matrix was fabricated at a proper thickness of 5 mm, and their EM performance at whole X and Ku bands (8.2–18 GHz) were compared in Fig. 5. The epoxy/PPy/RGO@SiO2 composite had the poorest electromag netic absorbing capacity. Its minimum reflection loss (RLmin) value was 18 dB at 12.5 GHz and its effective absorption bandwidth had only short ranges, from 12.4 to 13 GHz and from 16.7 to 17.6 GHz in Ku band (Fig. 5a). Here, the effective absorption range was defined as when the RL value was less than 10 dB, which corresponded to 90% absorption of incident microwave energy. Its high electrical conductivity accom panied with its low specific surface area were the main reasons for the poor EM performance of PPy/RGO@SiO2. The high conductivity of PPy/ RGO@SiO2 might lead to the impedance mismatch, resulting in more reflected waves on the surface of the EM absorber, which is unfavorable for microwave dissipation. In addition, the coverage of the polypyrrole chains on the surface of RGO@SiO2 resulted in the bunching of the RGO sheets, which in turn caused a decrease in the composite’s inner space. As listed in Table 2, the specific surface area of PPy/RGO@SiO2 was only 6.3 m2/g, which was 3� less than in PANi/RGO@SiO2 (22.5 m2/g) and 2� less than its value in P(ANi-co-Py)/RGO@SiO2 (12.9 m2/g). The limited specific surface area led to weaker interfacial polarization by reducing the opportunity for contact between the guest (incident wave) on its moving pathway to the host (the matrix of composites) [10]. As a result, the EM absorption ability was limited. Epoxy/PANi/RGO@SiO2 composites exhibited better EM attenua tion properties. The RLmin reached to 38.9 dB at 17.1 GHz and 25.6 dB at 12.4 GHz in the Ku band and X band, respectively (Fig. 5b). The
Sample names
Specific surface area (m2/ g)
Total pore volume (cm3/ g)
PANi/RGO@SiO2 PPy/RGO@SiO2 P(ANi-co-Py)/ RGO@SiO2
22.5 6.3 12.9
0.113 0.014 0.046
epoxy/P(ANi-co-Py)/RGO@SiO2 composite displayed the best EM wave damping capacity. Its RLmin was 40.6 dB at 15.8 GHz in the Ku band and 19.8 dB at 9.8 GHz in the X band (Fig. 5c). Additionally, the effective absorption ranges of epoxy/P(ANi-co-Py)/RGO@SiO2 covered a wider frequency range of 5.5 GHz bandwidth, from 12.3 to 17.8 GHz, compared to the 2.2 GHz effective bandwidth of epoxy/PANi/RGO@ SiO2, from 15.8 to 18 GHz in the Ku band. The better absorbing per formance of PANi/RGO@SiO2 and P(ANi-co-Py)/RGO@SiO2 could be explained by the well-matched impedance because these two materials had only a moderate electrical conductivity. The matched impedance avoided reflected light at the surface and allowed the incident light to come into the interior of absorber where it could be dissipated. More over, the moderate specific surface area of PANi/RGO@SiO2 and P(ANico-Py)/RGO@SiO2 composites gave a rise to interfacial polarization under alternating EM field which benefitted the EM attenuation. In addition, the co-existence of the PANi and PPy chains in the epoxy composites of P(ANi-co-Py)/RGO@SiO2 particles could provide more defect centers, leading to dipole polarization and accompanying inter facial polarization generated from the heterogeneous components, resulting in P(ANi-co-Py)/RGO@SiO2 having the best EM attenuation performance [32]. The anticipated EM capability of copolymer composites was further studied at different thickness. The RL values of P(ANi-co-Py)@RGO@ SiO2 at filler content of 9 wt% in epoxy matrix as a function of frequency with varying thickness was conducted and shown in Fig. 6. The P(ANico-Py)@RGO@SiO2 absorber displayed the poor EM suppressing ability at 3 mm thickness when the RL value was lower than 7.5 dB in whole range frequency from 8.2 GHz to 18 GHz. By increasing thickness of absorber, the RLmin reached to 40.6 dB at 15.8 GHz, 23.5 dB at 12.5 GHz and 26.3 dB at 9.8 GHz at thickness of 5 mm, 6 mm and 7 mm. The minimum absorption value shifted to lower frequency when the thick ness of absorber increased. This observation was in good agreement to the quarter-wavelength equation: ! nλ nc pffiffiffiffiffiffiffiffiffiffiffiffiffi n ¼ 1; 3; 5; …: tm ¼ ¼ 4 4fm jμr jjεr j where tm is matching thickness, λ is EM wavelength, fm is frequency of the RLmin, μr is the complex permittivity, εr is the complex permeability and c is the velocity of the light [9]. The influence of filler content on EM performance of copolymer composite was also investigated. As the 5 mm thick absorber of P(ANi-
Fig. 5. The reflection loss (RL) curves of (a) PPy/RGO@SiO2, (b) PANi/RGO@SiO2, and (c) P(ANi-co-Py)/RGO@SiO2 in epoxy matrix at 5-mm-thick absorbers. 6
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Materials Chemistry and Physics 244 (2020) 122691
Fig. 6. The RL curves of 9 wt% P(ANi-co-Py)/RGO@SiO2 in epoxy matrix at thickness of 3 mm, 5 mm, 6 mm and 7 mm.
co-Py)/RGO@SiO2 exhibited the best microwave dissipated ability, we fabricated the 5-mm-thick P(ANi-co-Py)/RGO@SiO2 absorbing material at loading contents of 3, 9 and 15 wt% and the EM dissipated ability of each sample was illustrated in Fig. 7a–c. The RLmin value was remark ably increased from 6.8 dB at 9.2 GHz to 40.6 dB at 15.8 GHz and 39.2 dB at 13.9 GHz when the P(ANi-co-Py)/RGO@SiO2 loading contents increased from 3 wt% to 9 wt% and 15 wt%, respectively. The average shielding efficiency of 3 wt% absorber was only 30% while it reached over 91% for both cases of 9 wt% and 15 wt% in a whole range frequency of X and Ku band. The results demonstrated that the content of P(ANi-co-Py)/RGO@SiO2 played an important role to maximum the wastage of EM wave energy. At low loading content of 3 wt% was not enough to create a conductive network inside composites, resulting the poor in EM absorbing performance. However, by increasing the P(ANi-
co-Py)/RGO@SiO2 content from 9 wt% to 15 wt%, the RLmin value was slightly declined by value of 1.4 dB. This could be explained that the higher filler content generates a higher conductive network to the composites matrix. As the result, it might cause to the skin effect where more reflection could occur on the surface of composites and lower the minimum absorption value. For a sake of comparison, the EM attenuated efficiency of P(ANi-co-Py)/RGO@SiO2 was compared with literatures where the other typical graphene-based composites were also reported (Table 3). The comparative results showed that the relative low loading content accompanying to the high RLmin value made P(ANi-co-Py)/ RGO@SiO2 become a potential material in the field application of EM shielding. The mechanism of the wave absorption properties of the three nanocomposites was generated from dielectric loss, interfacial polari zation, dipole polarization, multiple reflection, and scattering. First, the introduction of polymeric chains on the RGO@SiO2 composites not only combined all components in a unique structure of composites but also raised their electrical conductivities significantly. As a result, it pro moted the movement of free electrons and charge carriers between the RGO sheets and the polymer chains, which contributed to the dielectric loss. Second, there were at least three components in each composite, such as RGO sheets, SiO2 particles, and conducting polymer chains, which meant that there was a greater possibility for the creation of de fects at the interfaces. For instance, the interface between RGO sheets and SiO2 particles, SiO2 and polymeric chains, and polymeric chains and RGO sheets could each have interfacial polarization when the composite was under EM irradiation. These interfaces also created some internal space inside the composite along with the layered structure of the RGO sheets, which was a benefit in enabling multiple internal reflections and scattering. Third, the different electron catching ability of carbon, oxy gen, silicon, and nitrogen atoms played roles as defect points and induced dipoles, resulting in additional polarization relaxation pro cesses that further enhanced the microwave absorption properties of the composites. The detail mechanism can be obtained in Fig. 7d.
Fig. 7. The RL values of 5-mm-thick P(ANi-co-Py)/RGO@SiO2 at filler content of (a) 3 wt%, (b) 9 wt%, (c) 15 wt% and (d) proposal mechanism of EM attenuation. 7
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Materials Chemistry and Physics 244 (2020) 122691
Table 3 EM absorbing efficiency of recent reported on graphene-based composites. Samples
Filler content (wt%)
PANi nanorods/graphene PPy nanosphere/RGO Graphene-pFe3O4@ZnO Graphene@Fe3O4@SiO2@PANi P(ANi-o-Py)@RGO/SiO2
50 30 30 25 9
RLmin (dB) 51.1 19.0 40.0 19.4 40.6
Thickness (mm)
Effective absorption bandwidth (range, GHz)
Refs.
3 1.46 5 2 5
12.2 (5.8–18.0) 2.8 (15.2–18) 2 (10–12) 4.4 (10.4–18) 5.5 (12.3–17.8)
[5] [37] [38] [39] This work
4. Conclusion [5]
The copolymer nanocomposite P(ANi-co-Py)/RGO@SiO2 was suc cessfully prepared through an oxidative pre-polymerization method. The electrical conductivity of the resultant composite was 0.44 S/cm, lower than that of the component homopolymer nanocomposites. Interestingly, the EM attenuation effectiveness of the copolymer nano composites was much better in comparison with homopolymer component nanocomposites of either PANi/RGO@SiO2 or PPy/RGO@ SiO2. The RLmin of P(ANi-co-Py)/RGO@SiO2, PANi/RGO@SiO2, and PPy/RGO@SiO2 in the Ku band were 40.6 dB, 38.9 dB, and 13.9 dB, respectively. The effective absorption bandwidth of the P(ANi-coPy)/RGO@SiO2 was 5.5 GHz, from 12.3 to 17.8 GHz. For comparison, the absorption bandwidth was about 2.2 GHz, from 15.8 to 18 GHz, for PANi/RGO@SiO2 and discretely in two short ranges of 0.6 GHz from 12.4 to 13 GHz and 0.9 GHz from 16.7.1–17.6 GHz for PPy/RGO@SiO2. However, this study has investigated neither the morphology nor the conductivity of the copolymer nanocomposite P(ANi-co-Py)/RGO@SiO2 as a function of the molar ratio of the two monomers, aniline and pyr role. These remaining problems require more investigation to determine the EMI performance of P(ANi-co-Py)/RGO@SiO2 for further applications.
[6] [7] [8] [9] [10] [11] [12] [13] [14]
Prime novelty statement
[15]
By pre-polymerization method, the electrical conductivity of the copolymer nanocomposites of P(ANi-co-Py)/RGO@SiO2 was significantly improved in comparison with traditional in situ polymerization method. The as-prepared nanocomposites displayed the minimum reflection loss (RL) of 40.6 dB at 15.8 GHz with only 9 wt% filler content in epoxy matrix. The relative low loading content (9 wt%) accompanying with the broaden effective bandwidth of 5.5 GHz (12.3–17.8 GHz) made P (ANi-co-Py)/RGO@SiO2 becomes a better candidate for future micro wave absorption material.
[16] [17] [18] [19]
Acknowledgements
[20]
This research was supported by to the Creative Materials Discovery Program (2015M3D1A1068061) administered by the National Research Foundation of Korea (NRF) and funded by the Ministry of Science and ICT, Republic of Korea. Tung and Quyen thanks to the financial support from VAST (Grant No. NCVCC 06.08/19-19 to NCVCC 06.14/20-20), Vietnam.
[21] [22]
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