Enhanced Electromagnetic Interference Shielding in a Au–MWCNT

Article pubs.acs.org/JPCC

Enhanced Electromagnetic Interference Shielding in a Au−MWCNT Composite Nanostructure Dispersed PVDF Thin Films R. Kumaran,† S. Dinesh kumar,‡ N. Balasubramanian,† M. Alagar,† V. Subramanian,‡ and K. Dinakaran*,§ †

Department of Chemical Engineering, Anna University, Chennai 600 025, India Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India § Department of Chemistry, Thiruvalluvar University, Serkkadu, Vellore 632115, India ‡

ABSTRACT: Electromagnetic interference (EMI) shielding effectiveness and conductivities in a flexible conducting polymer composite film, prepared via a simple solvent cast method, of PVDF incorporated with Au−MWCNT are studied. The scanning electron microscopic analysis showed that the Au-loaded MWCNT uniformly dispersed in PVDF. The HRTEM images show that Au nanoparticles having a particle size of 20−30 nm have been deposited on the nodes of the MWCNT. The values of dielectric constant were found to be 12.11 and 13.89 at 1 MHz upon the incorporation of 1 and 3 wt % MWCNT in PVDF. The impedance and electromagnetic interference shielding effectiveness studies reveal that the polymer nanocomposites possess enhanced conductivity of 1.12 × 10−4 S/cm at 1 MHz, minimum return loss of 4.4 dB, and effective electromagnetic shielding of 26.7 dB at 12 GHz for 3 wt % Au NPs in 3 wt % MWCNT/PVDF thin film.

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INTRODUCTION The rapid developments of commercial electronic appliances have experienced an important issue called electromagnetic interference (EMI). The electromagnetic radiations emitted from mobile phones, radar, and various electronic appliances are responsible for interfering with electronic devices, which affects the lifetime and functionality of such electronic instruments. To address this problem, flexible, cost-efficient, and lightweight polymer composite shielding materials have been employed in fabricating the electronic devices which attenuates the undesirable electromagnetic radiation interference.1−7Among the various fillers used, MWCNTs attracted researchers due to their beneficial properties such as crystalline, good electronic conductivity, ultra light weight, chemical stability, ease of processing, compatibility, and effective microwave absorption.8−16 The acid-functionalized MWCNT, which is given credit for the dipole moment,17 and reinforced polymer flexible conducting films are suitable to be applied in electronic devices. Ning et al.18report the EMI shielding of SWCNT−epoxy composites with 15−20 dB at 1.5 GHz. Saini et al.19 describe the EMI shielding of −71 dB for highly conductive composites with BaTiO3 and polyaniline fillers with 2 mm sample thickness as in ku band regions (12−18 GHz). Ling et al.20 report the EMI shielding of lightweight PEI/ graphene nanocomposite foams with 44 dB, which is due to remarkable electrical conductivities through facile synthesis. Yang21 has studied the EMI shielding effectiveness for a 7 wt % carbon nanotube−polystyrene foam composite, and the values are found to be 20 dB in the x-band region. The EMI shielding © 2016 American Chemical Society

of polyaniline-coated 7 wt % MWCNT/polystyrene composites with −18 dB at 12 GHz has been reported.22 Liang et al.23 report the EMI shielding for 15 wt % graphene in epoxy polymer at about 21 dB at 8−12 GHz, which in turn was assigned to high aspect ratio and homogeneous dispersion of the filler in a polymer matrix. Besides carbon nanotubes, metal nanoparticles such as gold also have been used in EMI shielding because of their excellent conductivity resulting from their impedance match between materials and free space.24,25 Among the various polymer matrices, poly(vinylidene fluoride) (PVDF), an excellent piezoelectric polymer, has attracted great interest among the chemists due to its good flexibility, large intrinsic polarization, dielectric properties, ferroelectric behavior, low cost, and ease of processing, and PVDF has been used in wide device fabrications such as sensors, actuators, transducers, and memory devices.26−30 Saini et al.31,32 reported that the simple combination of nanofillers in polymers is not reliable for controlling EMI issues. The perfect reinforcements of different combinations of conducting particles are required to prepare composite materials for shielding the electromagnetic radiations. They also prepared MWCNT−PVDF by a melt-mixing method at 200 °C which exhibited EMI shielding efficiency of 47 dB at the x-band frequency.33 In the present work, we have studied the Received: February 10, 2016 Revised: June 6, 2016 Published: June 8, 2016 13771

DOI: 10.1021/acs.jpcc.6b01333 J. Phys. Chem. C 2016, 120, 13771−13778

Article

The Journal of Physical Chemistry C EMI shielding of 1 and 3 wt % of Au nanoparticle incorporated MWCNT (3 wt % w.r.t, PVDF) reinforced PVDF nanocomposites prepared by a simple solvent casting method. The incorporation of Au NPs into MWCNT-filled PVDF for total EMI SE has not been reported so far, and we also observed that the lower percentage of Au in MWCNT resulted in accumulation of Au NPs at the edges of the MWCNT. We anticipate that the uniform dispersion of the heterogeneous nanostructure of Au−MWCNT in PVDF will enhance the conductivity that related to a number of mobile charge carriers within the nanocomposite film.

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EXPERIMENTAL SECTION Initially, 1 g of MWCNT (MWCNT−TMC 100−20) was treated with 3:1 conc. H2SO4 and conc. HNO3 for 24 h to generate COOH functional groups by oxidation as reported elsewhere.34,35 The COOH−MWCNT was treated to 1.0 × 10−3 mol dm−3 HAuCl4 solution and sonicated for 5 h. After adding 5 drops of citric acid to the suspension, the content was heated to 100 °C, and then drops of NaBH4 were added and stirred for 15 min. Then the solution was washed and centrifuged with DD H2O, and the Au NP−MWCNT powder was dried at 60 °C for 24 h. Preparation of Polymer Nanocomposites. Au NP− MWCNT powder was mixed in dimethylformamide (DMF) solution and sonicated well for 3 h, and it was added to PVDF/ DMF solution. Then the mixed solution was sonicated for 2 h and stirred well to obtain a homogeneous solution, and it was cast in films at 60 °C for 8 h. The thickness of the prepared composites is 0.5 mm. Characterization. X-ray diffraction (XRD) patterns were recorded on a Cu Kα radiation XRD-RIGAKU MINIFLEX II− C XRD system. Raman spectra were determined by a confocal micro-Raman microscope (Renishaw in Via Reflex) with Ar ion laser source of 0.6 mW power and 514.5 nm. A high-resolution image and the selected-area electron diffraction (SAED) pattern were observed with LA D6 source in a TECNAI T30 high-resolution transmission electron microscope (HRTEM). XPS analyses were carried out in a Thermofisher spectrometer with Al Kα surface analyzer. The dielectric constant, conductivity, and dielectric loss were calculated using BDS novocontrol-concept 80 instrument. A N5230A Vector Network Analyzer with an S-parameter set was used to analyze the frequency dependence of EMI SE. Sample thickness is 0.5 mm for the EMI SE measurements. The power level of incident microwave radiation is −10 dBm (0.1 mW).

Figure 1. XRD patterns of pristine PVDF, MWCNT−PVDF, and Au NP−MWCNT−PVDF.

nanocomposites are shown in Figure 2. The α and β phase of pristine PVDF are well characterized as strong bands at 803

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RESULTS AND DISCUSSION Figure 1 explains the crystallization behavior of Au NP incorporated MWCNT/PVDF nanocomposites. The diffraction peak at 2θ = 20.4° which is assigned to the (110) reflection (JCPDS File No: 38−1638) shows pristine PVDF is in a monoclinic structure, and the diffraction peak of MWCNT/ PVDF also appears at 2θ = 20.6° assigned to (110) of PVDF. The reason is attributed to better dispersion of MWCNT in PVDF by sonication.36 The diffraction peaks observed at 38.2°, 44.4°, 64.8°, and 77.7° that are assigned to (111), (200), (220), and (311) reflection of gold nanoparticles (JCPDS File No: 04−0784) clearly account for the incorporation of Au NPs in MWCNT/PVDF nanocomposites.37 The Raman spectra of pristine PVDF, COOH−MWCNT/ PVDF, and Au NPs decorated COOH−MWCNT/PVDF

Figure 2. Raman spectrum of pristine PVDF, MWCNT−PVDF, and Au NP−MWCNT−PVDF.

and 832 cm−1, respectively.38−40 The characteristic bands observed at 1589 cm−1 (D-band) and 1315 cm−1 (G-band) signify the COOH−MWCNT reinforcing in PVDF, and the defect peak showing higher intensity than the graphitic peak clearly interprets the acid functionalization of MWCNTs.41 The lower wavenumber shifts from 1589 to 1582 cm−1 and 1315 to 1309 cm−1 expound the incorporation of Au NPs in MWCNT/ PVDF composites. The chemical interaction and charge transfer process between MWCNT and Au NPs is accountable for the wavenumber shifts in Au NPs/MWCNT/PVDF composites.42 13772

DOI: 10.1021/acs.jpcc.6b01333 J. Phys. Chem. C 2016, 120, 13771−13778

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The Journal of Physical Chemistry C

Figure 3. SEM images of pristine (a) PVDF, (b) 1 wt % MWCNT−PVDF, (c) 3 wt % MWCNT−PVDF, (d) 1 wt % Au NP−MWCNT/PVDF, and (e) 3 wt % Au NP−MWCNT/PVDF composites.

The surface morphologies of pristine PVDF, MWCNT/ PVDF, and Au NPs/MWCNT/PVDF composite are clearly shown in Figure 3. The SEM image of pristine PVDF is shown in Figure 3(a) and clearly exhibits the rodlike morphology. The reinforcement of MWCNTs in PVDF appeared like a fracture in the composites which is well dispersed by sonication, and it is obvious in Figure 3(b and c).43 The decoration of Au nanoparticles in the MWCNT/PVDF composite is clearly shown in Figure 3(d and e). The typical HRTEM image of Au NPs/MWCNT and surface area enhanced diffraction (SAED) pattern is shown in Figure 4(a and b). The surface image of the MWCNT sample is

Figure 5. XPS results for Au NPs/MWCNT/PVDF nanocomposites.

found at 288.8 and 531.2 eV, respectively, and the peak encountered at 292.1 eV corresponds to carbon bonded as in the −CF2− peak of PVDF. The two peaks formed at 288.5 and 288.8 eV are attributed to sp2 and sp3 carbons of COOH− MWCNT. At 293.8 eV, a new peak was found, which emphasizes the CF3 group.44 The doublet peaks formed at 88.1 and 89.9 eV ascribed to Au 4f5/2 proposes Au NPs in the Au0 state.45 The reduced oxidation state of gold is anticipated to increase the local dielectric constant as reported elsewhere.46 Figure 6a exhibits the dielectric constants of Au NPincorporated MWCNT-reinforced PVDF composites. The graphs clearly exhibit that the dielectric values are independent of frequencies. The dielectric constant value for pristine PVDF is found to be 5.09 at 1 MHz. The lower value of the dielectric constant for PVDF is assigned to the case that there will not be any interface for storing charges in insulating materials. On reinforcing PVDF with 1 and 3 wt % MWCNT, the dielectric constant value is enhanced up to 12.11 and 13.89 at 1 MHz. The reason is ascribed to the increase in the interface between

Figure 4. High-resolution transmission electron microscope image of (a) Au NP−MWCNT−PVDF and (b) SAED pattern.

well observed in the HRTEM image, and some of the blurred surface is attributed to the acid functionalization of MWCNTs. That Au nanoparticles are well decorated on the edges of MWCNTs is ascertained with 30−40 nm. The SAED pattern distinctly proves the crystalline behavior of the composites. Figure 5 shows the XPS results for Au NPs/MWCNT/ PVDF nanocomposites, which imparts the elemental constitution and nature of bonding in the composites. C, F, and O signals were detected in the wide scan spectrum of composites. The neutral C 1s and O 1s peaks for COOH−MWCNT were 13773

DOI: 10.1021/acs.jpcc.6b01333 J. Phys. Chem. C 2016, 120, 13771−13778

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The Journal of Physical Chemistry C

increasing trend of dielectric values after Au NP incorporation is not so high which is attributed to the existence of a saturated point of optimum concentration of Au NPs in the composites. The electrical conductivity of Au NP-decorated MWCNT/ PVDF composites is presented in Figure 6b. The conductivity of pristine PVDF is found to be 1.38 × 10−7 S/cm at 1 MHz. The value of conductivity of 1 and 3 wt % MWCNT-reinforced PVDF is 2.92 × 10−7 S/cm and 3.23 × 10−7 S/cm, respectively. The reinforcement of MWCNT to the insulating PVDF matrix enhances the electrical conductivity slightly. The reason is assigned to that the MWCNT approaches the critical value which accounts for the increase in conductivity of composites. Another noteworthy cause for comparative enhancment of electrical conductivity is the polymer encapsulation in MWCNT filler in the composites. In general, on lower reinforcement of MWCNT (1 wt %), the interparticle distance of MWCNT is higher in the PVDF matrix due to effective dispersion on sonication. The conductivities of fucntionalized MWCNTs on the reinforcement in PVDF insulators are found to be decreased. There is a formation of defects on the surface of MWCNTs by the acid functionalization, and the enhancement of the conductivities is not possible on lower concentration of MWCNTs in the PVDF matrix.51,52 To overcome this challenge, the higher concentration of MWCNT (3 wt %) in the PVDF matrix is reinforced to enhance the conductivity of the composites. On increasing the MWCNT concentration, the neighboring distance of particles in the MWCNT filler will get reduced, and subsequently the electrical conductivity of MWCNT/PVDF is enhanced.53−55 For 1 and 3 wt % of Au NPs in MWCNT filler, the conductivity values are increased to a value of 5.41 × 10−5 S/cm and 7.51 × 10−5 S/cm at 1 MHz, respectively. The incorporation of Au NPs in the MWCNT is creditworthy for charge distribution and synergy effect in the composites, which plays a significant role in enhancing the conductivity. The Au NPs in MWCNTs are trustworthy at increasing the surface area of the composites, and they promote the electron transfer by forming the conducting junction bridge between MWCNT fillers and enhance the electrical conductivities of Au NP/MWCNT/ PVDF composites.56−58 The dielectric loss (tan δ) for Au NPs/MWCNT/PVDF composites is elucidated in Figure 6c. The tan δ of pristine PVDF is 0.07. On incorporation of 1 and 3 wt % of MWCNT in PVDF, the tan δ is found to be 0.059 and 0.051, respectively, and the incorporation of 1 and 3 wt % of Au NPs in MWCNTs lowered the tan δ values of the composites. The acid functionalization of MWCNTs is accountable for good dispersion in the polymer. The conductor insulator bridge formed in MWCNT−PVDF composites at the percolation threshold plays a credible role for high tan δ. The COOH− MWCNT is tightly packed in PVDF polymer, and the free volume is reduced. Also it is feasible for energy dissipation through its walls. The cohesive forces formed between the interparticles in electrically conductive MWCNT fillers propose the enhancement of compactness of crystals in the composites which accounts for high tan δ. After Au NP loading, the tan δ values are 0.038 and 0.022. The creditworthy factors for minimum tan δ are free charge carriers (charge distributions), dipole loss, and DC conduction in the composites.59−61 The EMI shielding effectiveness of Au NPs/MWCNT/ PVDF composites is depicted in Figure 7 and reported in Table 1. The effective EMI SE can be calculated by the formula

Figure 6. Dielectric constant (a), conductivity (b), and dielectric loss (c) for pristine PVDF (A), 1 wt % MWCNT−PVDF (B), 3 wt % MWCNT−PVDF (C), 1 wt % Au NP−MWCNT−PVDF (D), and 3 wt % Au NP−MWCNT−PVDF.

the MWCNT and PVDF. A percolation threshold which is close to the MWCNT in polymer also features the enhancement of dielectric values in the composites.47,48 The enhanced values of dielectric constants of the PVDF matrix after reinforcing MWCNTs are attributed to the fact that the mobile charge carriers accumulated at the interfaces between the MWCNT filler and PVDF insulator.49,50 For 1 and 3 wt % of Au NPs incorporated in MWCNT, the dielectric constant values are found to be 16.3 and 20.22, respectively. With increasing Au NP concentration, the interaction between MWCNT and PVDF gets stronger, and dipole moments are formed from this interaction which in turn is creditworthy for enhancing the dielectric constants of composites. Also, the 13774

DOI: 10.1021/acs.jpcc.6b01333 J. Phys. Chem. C 2016, 120, 13771−13778

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The Journal of Physical Chemistry C

Figure 7. Return loss, reflection, absorption, and total EMI SE at 12 GHz, respectively, for (A) pristine PVDF, (B) 1 wt % MWCNT−PVDF, (C) 3 wt % MWCNT−PVDF, (D) 1 wt % Au NP−MWCNT−PVDF, and (E) 3 wt % Au NP−MWCNT−PVDF.

effective EMI SE = SE R + SEA

The effective EMI shielding effectiveness of pristine PVDF is 1.5 dB at 12 GHz. On incorporation of COOH−MWCNT to PVDF, the EMI shielding value increases to 16.4 dB at 12 GHz. The electromagnetic interference shielding effectiveness is widespread correlated to electrical conductivities of polymer composites. The peripheral enhancement of conductivity value is notable which is attributed to lower loading (1%) of MWCNT in PVDF. This minimum increase of conductivities in polymer composites is percolation threshold. The higher loading (3 wt %) of MWCNT in PVDF will lead to tremendous enhancement of electrical conductivities of MWCNT/PVDF composites which is credibly attributed to strong conductive networks formed in composites This conductive network path crosses the percolation threshold,63 which is reliable for enhancing shielding value of 21.6 dB. The conductive networks formed in the polymer composites mainly account for interacting EM waves which in turn are responsible for ionic conduction and dipole polarization relaxation. The higher conducting networks formed in the polymer composites due to MWCNT which is accountable for interaction with incoming electromagnetic radiations and results in the reflection of EM waves. We report the total EMI shielding values which are calculated from the reflection and absorption property of the materials at 12 GHz frequency. There are some disturbances, and high peaks are observed, in absorption graphs in MWCNTreinforced PVDF, at a particular frequency; however, the

Table 1. Return Loss, Reflection, Absorption, and Total EMI Shielding Values for Different Composites sample designations pristine PVDF 1 wt % MWCNT/ PVDF 3 wt % MWCNT/ PVDF 1 wt % Au NPs (w.r.t. CNT)/3 wt % MWCNT/PVDF 3 wt % Au NPs (w.r.t. CNT)/3 wt % MWCNT/PVDF

return loss reflection (dB) at 12 (dB) at 12 GHz GHz

absorption (dB) at 12 GHz

total EMI SE (dB) at 12 GHz

33.5 14.9

1.23 10.17

0.42 5.97

1.54 16.4

11.6

19.98

1.04

21.6

7.7

21.52

1.48

23.08

4.4

24.8

2.49

26.71

Here SE R (dB) = −10 log(1 − S112 ) SEA (dB) = −10 log(S212 /1 − S112 )

SER is shielding in terms of reflection, and SEA is shielding in terms of absorption.62 13775

DOI: 10.1021/acs.jpcc.6b01333 J. Phys. Chem. C 2016, 120, 13771−13778

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The Journal of Physical Chemistry C reflection peaks are found to be consistent in the x-band region. Jia et al.64 reported that the minimum loading of 5% CNT in polyethylene, by compression molding techniques with the sample thickness of 2.1 mm, resulted in a segregated network structure which exhibited an EMI shielding value of 46.4 dB. However, in our investigation, the EMI shielding effectiveness reaches 21.6 dB at 12 GHz on 3 wt % reinforcement of COOH−MWCNT to the insulating PVDF matrix. This enhancement is imputed to the acid functionalization of MWCNT which is creditworthy in patterning good conductive networks by excellent dispersion in the PVDF matrix and prevents the agglomeration of CNTs in the composites. High aspect ratio plays a crucial role in ascertaining electrical conductivity in polymer composites. The van der Waals’ forces in MWCNTs which are credent for the agglomeration in the composites will generally reduce the aspect ratio.65,66 To overcome this difficulty, the MWCNT is acid funtionalized, and CNTs are well interconnected with the polymer and increase the electrical conductivities. This enhancement of electrical conductivity is in turn a good factor for higher EMI shielding values. Maya et al.67 report the attenuation of EM waves in the PVDF matrix by grafting of barium titanate (BT) nanoparticles and cobalt nanowires (Co NWs) by interconnected networks that form after the concentration of MWCNTs, and the forms of interaction between the materials and EM radiations are responsible for the reflection mechanism rather than absorption in EMI shielding. Chen et al.68 report the minimum reflection loss for 20% loading of 3D Fe3O4−MWCNT composites, and 6.8 mm thickness is found to possess −23 dB and −52 dB at 4 and 12 GHz, respectively. Our investigation proves that the minimum incorporation of the fillers and metal nanoparticles enhances the shielding values of the prepared composites by reflection mechanisms. The incorporation of Au nanoparticles on MWCNT in a PVDF matrix was found to enhance the EMI shielding values. The Au NPs integrated with MWCNTs in a PVDF matrix Are mainly reliable for the free charge carriers in the composites which in turn are the cause of the enhancing shielding values. The highly conducting Au NP improves the interaction between the conducting fillers and PVDF in the composites which is the main cause for increasing EMI shielding values up to 23.08 dB. The 3 wt % loading of Au NPs enhances the shielding value, 26.71 dB, which is attributed to the better dispersion of Au NPs, interfacial polarization, and larger aspect ratio of the composites. Another cause for enhancing EMI shielding value is the impedance match between the materials and air, which in turn reflects the EM radiations from the surface.69 The further higher concentrations of Au NPs are not focused in the composites since the inferior electrical conductivity might be possible which is attributed to the fact that higher loading may cross the percolation paths in the composites. Liu et al.69 report the EM absorption of Core− multishell MWCNT/Fe3O4/polyaniline (PANI)/Au hybrid nanotubes by a facile layer by layer technique. The introduction of Au nanoparticles enhances the EM absorption since the charge redistribution occurs and the formation of multiple interfacial polarizations is responsible for the attenuation of incident EM radiation. The large number of carbon nanotube bundles that exist in the composites decreases the tube−tube junctions which are ascribed to resistivity in electrical conductivities. The higher loading of Au NPs also exhibits resistivity in the composites which may lower the EMI shielding values. To overcome this challenge, the lower concentration of

Au NPs is incorporated in a MWCNT-reinforced PVDF matrix to enhance the electromagnetic interference shielding values effectively. Another fascinating and credible property for shielding is chiefly return loss. Return loss is the transmission of electromagnetic radiation through the materials. So, the effective EMI shielding values predominantly depend on return loss values.70 On decreasing the penetration of EM radiations through effective conducting materials, the shielding properties will get enhanced which is attributed to the conductive networks in the shielding materials. In this study, with an increase in the loading of conducting materials, the values of return loss are lowered due to the increase in impedance match between Au NPs and MWCNTs in PVDF composites. This accounts for the satisfactory enhancement of effective EMI shielding values in the composites. Our composites reach an EMI shielding value of 26.71 dB at 12 GHz, which meets the commercial value of 20 dB and evidently proposes that it is feasible for protecting electronic devices from harmful electromagnetic radiations.71

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CONCLUSION In summary, we have successfully prepared a flexible conducting thin film by a simple solvent casting method. Xray diffraction patterns and morphology studies clearly evidenced the incorporation of Au NPs in MWCNT/PVDF composites. The composites exhibit a good conductivity of 1.12 × 10−4 S/cm and minimum dielectric loss up to 0.02. The return loss value decreases to 4.7 dB at 12 GHz for 3 wt % Au NP in 3 wt % MWCNT/PVDF which is correlated in meliorating the total EMI shielding up to 26.71 dB at 12 GHz. The efficient value clearly proves the prepared composites are promising materials which are applicable for high-tech fields such as defense, satellite broadcasting, and gigahertz electronic systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the University Grants Commission, New Delhi, India, through grant UGC F.No: 39-784/2010. The authors also acknowledge the financial support of Council of Scientific and Industrial Research, India, through Grant CSIR No. 01(2706)/13/EMRII.

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DOI: 10.1021/acs.jpcc.6b01333 J. Phys. Chem. C 2016, 120, 13771−13778

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The Journal of Physical Chemistry C

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