An asymmetric electrically conducting self aligned graphene/polymer composite thin film for efficient electromagnetic interference shielding An asymmetric electrically conducting self aligned graphene[.]
An asymmetric electrically conducting self-aligned graphene/polymer composite thin film for efficient electromagnetic interference shielding Pradip Kumar, Asheesh Kumar, Kie Yong Cho, Tapas Kumar Das, and V Sudarsan Citation: AIP Advances 7, 015103 (2017); doi: 10.1063/1.4973535 View online: http://dx.doi.org/10.1063/1.4973535 View Table of Contents: http://aip.scitation.org/toc/adv/7/1 Published by the American Institute of Physics AIP ADVANCES 7, 015103 (2017) An asymmetric electrically conducting self-aligned graphene/polymer composite thin film for efficient electromagnetic interference shielding Pradip Kumar,1,a Asheesh Kumar,1 Kie Yong Cho,2 Tapas Kumar Das,1 and V Sudarsan1 Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India of Materials Science and Engineering, Korea University, Seoul 02841, South Korea Department (Received 27 September 2016; accepted 13 December 2016; published online 12 January 2017) Here, we study the self-aligned asymmetric electrically conductive composite thin film prepared via casting of graphene oxide (GO)/poly (vinylidene-hexafluoropropylene) (PVDF-HFP) dispersion, followed by low temperature hydriodic acid reduction The results showed that composite thin film revealed the high orientation of graphene sheets along the direction of film surface However, graphene sheets are asymmetrically distributed along the film thickness direction in the composite film Both sides of as prepared composite film showed different surface characteristics The asymmetric surface properties of composite film induced distinction of surface resistivity response; top surface resistivity (21 Ohm) is ∼ times higher than bottom surface resistivity (5 Ohm) This asymmetric highly electrically conducting composite film revealed efficient electromagnetic interference (EMI) shielding effectiveness of ∼ 30 dB This study could be crucial for achieving aligned asymmetric composite thin film for high-performance EMI shielding radiation © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4973535] I INTRODUCTION The recent development in information technology and fast portable devices packed with highly integrated circuits generates severe electromagnetic radiation These undesirable electromagnetic radiations have harmful effects on highly sensitive electronic equipments as well as on living environment for human beings To overcome the problem of undesirable electromagnetic radiation, materials with the high electrical conductivity and excellent electromagnetic interference (EMI) shielding efficiency are urgently required Traditionally, copper was used as EMI shielding material because of its excellent electrical conductivity However, copper is heavy and bears low flexibility, which limits its use in modern portable devices Polymer composite materials with enhanced electrical conductivity can be used as an alternative to conventional EMI shielding materials because of their lightweight, resistance to corrosion, flexibility, good processability and low cost compared to the metal based materials.1,2 On the other hand, ceramics, ferrites, metallic magnets, CNTs and their hybrids are widely utilized as conducting fillers into polymer matrix to achieve high electrical conducting composites for efficient EMI shielding application.3–8 Cao et al reported the composite loading with vol.% CdS-MWCNTs shows the best absorption of - 47 dB at 473 K with a thickness of 2.6 mm in the temperature range of 323–573 K and X band.6 CNT/Silica composite loading with 10 wt% MWCNTs can show EMI SE of ∼ 24.5 dB in X-band at temperature range from 100 to 500◦ C.7 Further, carbon fiber/silica composites can show shielding effectiveness greater than 10 dB in X-band region.8 Despite good EMI shielding performance in some cases, their a Author to whom correspondence should be addressed Electronic mail: pradipk@barc.gov.in 2158-3226/2017/7(1)/015103/7 7, 015103-1 © Author(s) 2017 015103-2 Kumar et al AIP Advances 7, 015103 (2017) drawbacks are high loading content, thickness, high density and poor stability, which severely hinder their applications In recent years, significant effort has been devoted to the development of shielding materials with efficient microwave attenuation performance.9–18 However, there is still lack of flexible and light weight shielding materials, which is required for current electronics/devices Graphene-based composite materials have been attracted a significant research interest due to their remarkable properties including low density, high surface area, large aspect ratios, versatile processing, ultrahigh electrical conductivity and excellent mechanical stiffness.16 The unique properties of graphene make its composites best choice for EMI shielding materials Great progress has been made in the development of graphene/polymer composite including sandwich, foam, segregated and porous structured materials for shielding application, but further progress is still needed for the implementation of these materials and their viable and cheap manufacturing.12,17–19 Wen and co-workers reported lightweight composites of 20 wt% rGO reaches EMI shielding of -38 dB.20 In further study, ultrathin graphene composites with absorption value of ∼ -15 dB and reflection loss of -30 dB has been reported.21 In another study by Cao et al., magnetic nanoparticles decorated reduced graphene oxide (NiFe2 O4 /r-GO) composite would have good tuneable EMI attenuation capacity.22 Improvements are needed in the fabrication process to develop new lightweight and flexible composite materials such as film, which can give very high EMI shielding efficiency due to their high electrical conductivity Thus far, various approach including layer-by-layer assembly, vacuum-assisted, solution casting have been reported for fabrication of graphene-reinforced composite thin films.23–25 However, it was difficult to control the distribution and orientation of graphene sheets in the composite films Moreover, it is also challenge to get high electrical conductivity in composite thin films due to either insufficient in situ reduction process or poor dispersion and interconnectivity of rGO sheets within the polymer matrix Graphene with poor dispersiblity in matrix produced agglomerates or multilayer of graphene sheets instead of single layer without any preferential molecular arrangement It is critically important to fabricate the polymer composite films with preferred orientation of nanofillers in matrix because of their much improved performance along oriented direction Further, the internal morphology of the composite film is highly depends on the interaction between graphene and matrix and also on graphene content Low graphene content exhibits random distribution and low network connectivity inside matrix, while layered structure formed above critical graphene composition In this work, first time we report dispersion casting approach for aligned asymmetric conducting rGO/PVDFHFP composite thin film with high-performance EMI shielding The results indicate that asymmetric composite film exhibited EMI shielding effectiveness of ∼ 30 dB, which can shield up to 99.9% of incident radiation This study could be crucial in the field of flexible composite thin films for high-performance EMI shielding application II EXPERIMENTAL DETAIL Graphene oxide is prepared from oxidation of natural graphite flakes as described in our previous work.13,26–29 Then, obtained aqueous GO dispersion was dispersed in highly polar organic solvents, dimethylformamide (DMF) by using solvent exchange procedure.30 Further, the PVDF-HFP pellets were individually dissolved into DMF solvent GO can uniformly dispersed into PVDF-HFP matrix because of the strong specific interaction between carbonyl groups of GO and fluorine group of PVDF-HFP matrix.31 As prepared, GO/DMF dispersion (40 wt %) was mixed into PVDF-HFP/DMF solution and continue stirring for 4-5 hours at room temperature Finally, GO/PVDF-HFP/DMF dispersion was used for composite film fabrication Figure illustrates the fabrication procedure for self-assembled asymmetrically conducting composite thin film by simple casting of GO/PVDF-HFP/DMF dispersion on Teflon petri dish and followed by HI reduction.32 First, as prepared GO/PVDF-HFP/DMF dispersion was poured into a Teflon petri dish and then allowed to dry overnight at 60◦ C in a conventional oven The resulting free-standing composite film (dark brown) was completely dried under vacuum Once, the solvent is completely removed from the composite film, GO sheets become immobilized in the rigid polymer matrix Then, reduction process was carried out by immersing the composite film into HI solution 015103-3 Kumar et al AIP Advances 7, 015103 (2017) FIG Systematic illustration for self-assembled asymmetrically conducting composite thin film (a) graphene oxide/PVDFHFP dispersion in DMF, (b) casted GO/PVDF-HFP dispersion on Teflon petridish, (c) reduced rGO/PVDF-HFP composite thin film, and (d) cross-sectional view of composite film Inset of Figure 1d shows optical image of flexible composite thin film (a-c) Reproduced with permission from Kumar et al., Carbon 101, 120 (2016) Copyright 2016 Elsevier Ltd in a sealed cuvette and placed the cuvette in warm oil bath at 80◦ C for h The reduction mechanism for GO to rGO using HI acid has been reported elsewhere.33 The reduced composite film (black) was washed by water several times to remove the excessive HI, yielded a highly aligned but asymmetric and flexible film as shown in Figure 1d Inset of Figure 1d shows flexibility of the composite film High resolution transmission electron microscopy (HRTEM) images was recorded by Zeiss TEM at 200 kV Raman spectrum of the composite film was recorded using Renishaw with 532 nm Nd: Yag laser by fixing the film on glass slide The X-ray diffraction (XRD) pattern was recorded by X-ray diffractometer using Cu Kα radiation (1.54 Å) on µm depth from both sides of films Further, X-ray photoelectron spectroscopy with a Sigma Probe and monochromatic X-ray source (XPS, K-Alpha, Thermo Scientific) was used to analyze the elemental composition and the degree of reduction produced by the simple HI treatment The internal microstructure and surface morphology of composite film was examined by scanning electron microscopy (SEM) The SEM-EDX spectrum were recorded by Oxford Instrumentation, UK (model No INCAE350), using highly polished Co metal as reference The DC electrical conductivity was measured using four probe method The EMI shielding effectiveness of asymmetric composite film was measured in X-band frequency ranging from 8.2 – 12 GHz III RESULTS AND DISCUSSION The morphology and structural analysis of asymmetric composite film is evaluated by TEM and SEM measurements The GO sheets were homogenously dispersed in polymer solution Homogenous dispersion of polymer molecules and GO sheets is clearly observed in HRTEM measurement as shown in Figure 2a This uniform distribution of GO sheets in the composite is ascribed to the strong interaction between GO and PVDF-HFP polymer, which is supported by FTIR measurements.32 Figure 2b demonstrates the cross-sectional view of as fabricated composite film It clearly shows the aligned rGO/PVDF-HFP composite film and exhibits that graphene sheets are highly aligned throughout the matrix along film surface This aligned orientation of rGO sheets in polymer matrix 015103-4 Kumar et al AIP Advances 7, 015103 (2017) FIG (a) HRTEM image of GO/PVDF-HFP composite, (b) Fracture edge topography of rGO/PVDF-HFP composite film (c) Top and (d) bottom side EDX spectra of composite film along with high resolution SEM surface view (Inset) of both top and bottom surface, respectively may be due to good dispersion of GO in matrix and strong specific interaction between GO sheets and matrix However, graphene sheets are not equally distributed in matrix along film thickness direction We can clearly see that graphene sheets distribution in matrix is asymmetric; top side have less number of graphene sheets in matrix than bottom side, which is also supported by further SEMEDX measurements Figure 2c–d shows the SEM-EDX pattern with their surface SEM micrographs of both side of composite film Top surface (inset of Figure 2c) clearly shows the different surface morphology than bottom surface (inset of Figure 2d), which indicating the asymmetric distribution of graphene sheets in PVDF-HFP matrix Top surface is polymer rich and rough, while bottom surface is graphene rich and smooth, which is clearly revealed from EDX spectra and SEM surface images In other words, we can say that in top surface, aligned graphene sheets in polymer matrix is less than the bottom surface or polymer molecules are more in top surface than bottom surface This self-aligned ordered layered composite structure can be driven by several mechanisms including specific interaction between GO and PVDF-HFP, steric hindrance, gravitational force, excluded volume interactions, π−π interaction and van der waals forces between adjacent amphiphilic GO sheets At above a critical concentration, GO/polymer dispersion can form a self-aligned ordered structure as a result of configurational-entropy driven excluded volume interactions between the GO sheets and the high affinity between the polymer wrapped amphiphilic GO sheets.34 Especially, the extremely high aspect ratio (∼ 15000) of GO sheets induced a highly anisotropic (aligned) composite film during self-assembly process Further, it is noted that density of GO is ∼ 2.24 g/cm3 , while PVDFHFP possesses density of 1.78 g/cm3 Therefore, gravity sedimentation can also play a significant role during self-assembly process and induced an asymmetric composite film Our results indicate that anisotropic nature of graphene sheets and large aspect ratio are the key factors for producing asymmetric GO/PVDF-HFP composite film Asymmetric response of composite film was further confirmed by XRD and XPS spectroscopy Figure 3a,b shows the full-scale XPS spectrum of GO/PVDF-HFP and rGO/PVDF-HFP composites films XPS spectrum was measured from both side of the film Both top and bottom side of GO/PVDF-HFP sample (Figure 3a) shows C1S , F1S and O1S transitions, including FKLL and OKLL auger electron emission In contrast, O1S peak intensity in rGO/PVDF-HFP sample is negligible (Figure 3b), which strongly demonstrate that oxygen containing functional groups from GO surface were successfully removed by HI treatment The atomic ratio of rGO/PVDF-HFP (C/O = ∼7) film significantly increased than that of GO/PVDF-HFP (C/O ∼ 1.7) film, confirming the effective 015103-5 Kumar et al AIP Advances 7, 015103 (2017) FIG Full scale XPS spectra of both (a) GO and (b) rGO composite films taken from both sides (c) Top and bottom side XRD patterns of composite film obtained from both sides Inset shows the magnified view of top and bottom side XRD patterns of composite film (d) Raman spectra of GO and rGO composite films reduction of GO to rGO.13,32,33 Interestingly, in both GO and rGO composite films, top side shows very high F1S peak than bottom side, which again confirmed the asymmetric distribution of graphene in matrix and confirmed the formation of asymmetric graphene/PVDF-HFP composite film Figure 3c show the recorded the XRD patterns of the GO and rGO composite films from both surface sides It should be noted that incident X-ray beam was impinged on composite film into µm deep from surface As prepared, GO/PVDF-HFP composite film exhibited 2θ peak at ∼ 9.6o (d-spacing of 9.21 Ao ) and ∼ 9.8o (d-spacing of 9.01 Ao ), corresponding to top and bottom surface In contrast, rGO/PVDF-HFP composite film showed 2θ peak at ∼ 24.0o (d-spacing of 3.70) and ∼ 24.15o (d-spacing of 3.67) corresponding to top and bottom surface The 2θ peak shift from 9.6o to 24.0o or 9.8o to 24.2o confirms that the GO sheets were well reduced.13 It would be interesting to see the recorded XRD patterns from both surface of composite film Inset of Figure 3c clearly shows that XRD patterns recorded from top and bottom surface have distinguishable difference in their patterns For GO/PVDF-HFP composite film, top and bottom surface exhibited d-spacing of 9.21 and 9.01 Ao , respectively This higher d-spacing recorded from top surface than bottom surface, indicates that distribution of graphene sheets is not symmetric in the matrix In top side, more number of polymer molecules intercalated between graphene sheets and continuously decrease to bottom side In contrast, bottom side has less number of polymer molecules or high content of graphene sheets than top side, which decreases the d-spacing in graphene sheets in composite film Therefore, the difference between d-spacing values of composite film recoded from both top and bottom surface side is due to asymmetric distribution of graphene sheets, which is similar to SEM and XPS observations (Figure and Figure 3a,b) Thus, our XRD results also confirm that GO was effectively reduced to rGO by using HI treatment and composite film exhibited asymmetric surface properties Chemical reduction of composite film was also confirmed by Raman spectroscopy Figure 3d presents the composite film Raman spectrum and showed well known Raman peaks at ∼ 1350 (D-band) and ∼ 1590 (G-band) cm-1 It has been well reported that the D band at 1350 cm-1 corresponds to the breathing mode of A1g symmetry, while the G band at 1590 cm-1 is due to the stretching mode of E2g phonons of sp2 bonded carbon atoms In this study, the I D /I G ratio of rGO composite after HI treatment significantly increased than GO composite film This increase in 015103-6 Kumar et al AIP Advances 7, 015103 (2017) FIG EMI shielding effectiveness of rGO/PVDF-HFP composite thin film D-band intensity is attributed to the increased defect and decrease in the average size of sp2 domains upon reduction of GO The increased intensity ratio I D /I G after chemical reduction is commonly reported in literatures.33 Furthermore, the intensity of the 2D peak at ∼ 2690 cm-1 and S3 peak at ∼ 2930 cm-1 increased, showing better graphitization and no charge transfer due to the absence of impurities.35 The asymmetric response of the opposite sides for the asymmetric composite film was finally confirmed by surface resistance measurements The top surface resistivity is much higher than bottom surface resistivity Top surface showed ∼ times higher surface resistance (21 ohm) than bottom surface (5 ohm), which revealed that bottom side is highly conducting than top side This asymmetric electrically conductive composite film can have potential application in electrochemistry, flexible conducting composite materials and electromagnetic interference shielding applications We examined the EMI shielding performance of this asymmetric composite thin film The EMI shielding effectiveness is defined as the logarithmic ratio of incident (Pi) to transmitted power (Pt) of an electromagnetic radiation and generally expressed in decibel (dB) Higher the dB level of EMI shielding effectiveness, the less energy is transmitted through the shielding material For example, a material with 20 dB EMI shielding efficiency (required for commercial applications) can block 99% of incident EM radiation Theoretically, when an EM wave is incident on a shielding material, the incident power is divided into reflected, absorbed, and transmitted power and the corresponding power coefficients of absorbance (A), reflectance (R), and transmittance (T) are such that A + R + T = In a vector network analyzer, we measured scattering parameters S 11 (or S 22 ) and S 21 (or S 12 ) to calculate the reflectance and transmittance power coefficient such as R = |S11 | and T = |S21 | , and the absorbance can be indirectly derived from A = − R − T The shielding effectiveness could be expressed by equation; SER = −10 log10 (1 − R), SEA = −10 log10 T (1 − R) , and SET = SEA + SER 13 Figure shows the recorded EMI SE of asymmetric conducting composite film in X-band (8.2 to 12 GHz) frequency The composite film revealed EMI shielding of SETotal , SER , and SEA to be ∼ 30, 10, and 20 dB, respectively The Total EM SET ∼ 30 dB suggests that composite film can shield up to 99.9% of incident EMI radiation It is worth noting that the contribution of absorption to the total EMI SE is much greater than that of reflection These results suggest that asymmetric conducting rGO/PVDF-HFP composite thin film have both reflective and absorptive characters to electromagnetic radiation with absorption as the dominant shielding efficiency IV CONCLUSION In conclusion, we present a facile fabrication of flexible and conducting asymmetric rGO/PVDFHFP composite thin film The composite film has very aligned graphene/polymer structure with asymmetric distribution of graphene sheets The concentration of graphene sheets increased from top to bottom along the film thickness direction This asymmetric distribution of graphene sheets induced 015103-7 Kumar et al AIP Advances 7, 015103 (2017) different surface characteristics of the composite film The asymmetric composite film is highly electrically conducting However, bottom surface is times more conductive than top surface Due to this highly conducting nature, asymmetric composite film showed efficient shielding effectiveness EMI shielding of ∼ 30 dB was achieved using this asymmetric composite thin film Our findings would be useful for design and fabrication of flexible composite thin films for high-performance EMI shielding applications ACKNOWLEDGMENTS This work was supported by DST Inspire Faculty grant funded by Department of Science and Technology (DST), New Delhi, India Y Yang, M C Gupta, K L Dudley, and R W Lawrence, Nano Lett 5, 2131 (2005) Thomassin, C 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Nat Nanotechnol 4, 25 (2008) J.-M ...AIP ADVANCES 7, 015103 (2017) An asymmetric electrically conducting self- aligned graphene/polymer composite thin film for efficient electromagnetic interference shielding Pradip Kumar,1,a... casting approach for aligned asymmetric conducting rGO/PVDFHFP composite thin film with high-performance EMI shielding The results indicate that asymmetric composite film exhibited EMI shielding effectiveness... flexible conducting composite materials and electromagnetic interference shielding applications We examined the EMI shielding performance of this asymmetric composite thin film The EMI shielding