Polyaniline/graphene hybrid film as an effective broadband electromagnetic shield

Abstract

Conducting polyaniline (PANi)–graphene (G) free standing, hybrid films were obtained by a solution intercalation method suitable for electromagnetic interference (EMI) shielding applications. The films were characterized structurally using Fourier transform infrared spectroscopy, micro-Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The electrical characterization was done using Hall measurement setup to estimate the dc electrical conductivity and establish the charge carrier type and mobility and investigate the possibility of using the hybrid films in other applications. The thermal stability of the films was investigated using thermo gravimetric analysis. The EMI shielding effectiveness (SE) of the composite films was tested over a broad microwave frequency range covering 4–12 GHz (C and X bands) by waveguide transmission line technique. The films exhibit very high values of reflected power (P_r) in the range of 85–90% in the C-band and 75–80% in the X-band, respectively. A maximum total shielding effectiveness value of SE_T ∼ 42 dB could be observed in the frequency range of 4–8 GHz and SE_T ∼ 32 dB could be observed in the frequency range of 8–12 GHz, corresponding to more than 99.99% microwave attenuation in both the C and X bands. In the entire frequency range of analysis, contributions from reflection to the total EMI SE is very high compared to that of absorption. The results suggest that the hybrid film may be used as effective, lightweight and flexible, reflection dominated EMI shielding material in a broad range of electromagnetic spectrum.

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1. Introduction

Electromagnetic interference (EMI) can occur when electrical devices receive electromagnetic radiations emitted by other electric or electronic devices and this has become one of the unfortunate by-products of the rapid proliferation of electronic devices. This silent, invisible pollution has increased many fold due to the increasing complexity of electronic devices and higher packing density to attain miniaturization.^1–5 As this interference effects can generate disastrous results in communication systems, including computers and many electronic devices, there is a huge demand for materials that can act as shields by the reflection and/or absorption of electromagnetic waves. Due to the increase in the use of high operating frequencies and wider bands in electronic systems, EMI shielding in the C and X bands (4 to 12 GHz) are more important for commercial and military applications. There are two methods of achieving EMI shielding, which include complete reflection or total absorption of the electromagnetic energy. Typically metals are employed for reflection based electromagnetic shielding owing to their high values of electrical conductivity and high reflection coefficient for electromagnetic radiation.^6–9 However, metallic shields have many disadvantages because of their heavy weight, physical rigidity, susceptibility to corrosion and poor and complex processability. There is a greater demand for light weight, flexible, non-corrosive and processable shielding materials in applications where reflection is the prime requisite.

Since the discovery of intrinsically conducting polymers (ICPs), there has been intense focus on applied research in this field for the optimization of these materials for technologically important applications. Conducting polymer composites have gained immense popularity mainly because of the interesting and tuneable electrical properties, light weight, resistance to corrosion, flexibility and processability.^6,10–12 These composites are expected to exhibit combined functions and characteristics of individual component materials. Polyaniline (PANi) is unique among the family of conjugated polymers due to its diverse electronic, optical, redox and acid–base properties as well as numerous potential applications in a variety of technological fields. Polyaniline films have been reported to be used as effective absorber of electromagnetic radiations.^13,14 Although single layer PANi films show poor shielding performance, multi-layer films of camphorsulphonic acid doped polyaniline films of 1–30 μm thickness possessing conductivity values in the order of 10–100 S cm⁻¹ are found to exhibit better microwave absorption.¹⁴ In another article, self-supported bacterial cellulose polyaniline conducting membrane has been used as electromagnetic interference shielding material. The membrane exhibits absorption properties in the X-band region of the microwave. But the absorption values are far below when compared to other shields prepared using pure polyaniline films of appreciable electrical conductivity.¹⁵ In order to improve and extend its potential applications, research attention has been extensively diverted to investigations on PANi composites.^16–18 The electrical conductivity and electromagnetic response of PANi-based composites can be suitably modified by incorporating conductive fillers, such as carbon nanotubes, carbon black, and carbon fibres.^19–23 These composites are characterized by a notable synergistic behaviour between each component. The discovery of graphene and its unique and outstanding mechanical, electrical and optical properties have resulted in the subsequent development of graphene-based polymer nanocomposites for various techno-commercial applications.^16–29 Graphene is a good reflector of electromagnetic radiations owing to its remarkable electrical properties whereas PANi is an inherent absorber capable of forming good quality thin films over a large area. Composites with high electromagnetic interference shielding effectiveness (EMI SE) dominated by reflection can be realized when graphene is introduced into a conductive PANi matrix. The presence of graphene facilitates better electron transport in the conducting PANi, which increases the overall electrical conductivity of the PANi composite, and enhances its reflectivity and thereby leads to enhanced shielding effectiveness. A very recent study on the electromagnetic shielding properties of conducting polymer composites presents multiphase composites of PANi with graphene and multi-walled carbon nanotubes as effective absorbers in the Ku-band and the total SE value goes up to −98 dB in a composite of 2.5 mm thickness loaded with 10% MWNTs.³⁰ Graphene/polyaniline nanorod arrays in the form of toroidal pressed pellets of 2.5 mm thickness have been reported to exhibit high reflection loss values approaching −45 dB at its peak.³¹ In all these studies, the composites were thick samples with low electrical conductivity values.

The data on graphene influence on the broadband EMI shielding properties of the conducting polymer composites are very scarce.^20,23 There are no available reports on the investigations on effective broadband EMI shielding properties of thinner conducting PANi–graphene hybrid films carried out in a wider frequency range, encompassing both C and X bands. The aim of the present work is to study the influence of graphene nanoflakes on the electromagnetic properties (reflection and absorption of the electromagnetic waves) of PANi–graphene composite films in a broad microwave frequency range of practical relevance.

In the present work, PANi–graphene hybrid films prepared by the simple solution intercalation method are investigated for the prospects of being used as effective EMI shielding materials. The films are characterized thermally, structurally and electrically using TGA, FTIR, micro-Raman, XRD, SEM and Hall measurements. The EMI shielding properties of the composite films are systematically investigated in the frequency range 4–12 GHz and the results suggest that the PANi–graphene composite is a promising material for broadband shielding applications.

2. Experimental section

2.1 Materials and sample preparation

Aniline, ammonium peroxydisulphate (APS), HCl, m-cresol, chloroform and D-10-camphorsulphonic acid (CSA) were purchased from SD-fine chemicals, India. Graphene powder (8 nm nanoflakes) was purchased from Graphene Laboratories Inc., NewYork. Aniline and m-cresol were vacuum distilled prior to usage.

2.2 Preparation of PANi–graphene hybrid film

Polyaniline was synthesized by the self-stabilized dispersion polymerization of aniline monomer in the presence of HCl and chloroform (1 : 2 v/v) as reported elsewhere.²⁷ APS was used as the initiator. The reaction was carried out, at −30 °C using dry ice–acetone mixture, with stirring for 4 hours. The resultant solution was filtered, washed and dried to obtain PANi in the powder form. The self-stabilized dispersion polymerization is known to produce high molecular weight, homogeneous PANi chains with better conjugation lengths compared to those produced via chemical oxidative polymerization process.^32,33 The dispersion polymerization process, adopted in the present work could yield PANi samples with better electrical conductivity. Polyaniline–graphene hybrid solution was prepared by solution intercalation method³⁴ in which the graphene nanoflakes (0.05 g) dispersed in m-cresol (5 mL) was added to the deprotonated PANi powder (0.25 g) and CSA (0.25 g) dissolved in m-cresol (15 mL) via ultrasonication (SONICS Vibra Cell CV33) for 2 hours in an ice bath. Good quality films of ∼5 μm thickness were cast on clean glass substrates which could be peeled off the substrates to get free standing films.

2.3 Sample characterization

The Fourier transform infrared spectrum of the free standing PANi–graphene hybrid film was recorded using JASCO FT/IR 4100 spectrophotometer to verify the formation of the PANi–graphene hybrid and to analyze the interaction between the PANi chains and graphene nanoflakes. The formation of PANi–graphene hybrid was again confirmed using X-ray diffraction (Rigaku Cu-Kα diffractometer, λ = 1.5418 Å, 5° min⁻¹) and micro-Raman analysis (Horiba Jobin Yvon LabRAM HR Spectrophotometer, argon-ion laser – 514.4 nm) of the film. SEM images were recorded using Hitachi SU6600 FESEM. The thermo gravimetric analysis of the sample was carried out using TA Q500 thermo gravimetric analyser in nitrogen atmosphere at a rate of 10° min⁻¹. The electrical characterization was performed at room temperature using the Hall measurement unit Ecopia HMS-5300. FTIR, Raman and XRD spectra and the EMI shielding effectiveness data of the pure PANi film are provided as ESI.†

The EMI shielding measurements of PANi–graphene hybrid films were carried out using wave guide transmission line technique in a Vector Network Analyser^10,35,36 (Agilent-model PNA E8362B; 10 MHz to 20 GHz). Thin free standing hybrid films of ∼5 μm thickness were inserted between standard C and X band rectangular waveguide adapter of the network analyser. The scattering parameters (S-parameters) corresponding to the transmission (S₁₂/S₂₁) and reflection (S₁₁/S₂₂) of the electromagnetic waves from the film samples were recorded over both the C and X bands. These S-parameters are used to compute the shielding by reflection, shielding by absorption and the total shielding effectiveness in the entire 4–12 GHz microwave spectrum. Reflected power (P_r), absorbed power (P_a) and transmitted power (P_t) of the incident microwaves can also be calculated from scattering parameters. A schematic of the electromagnetic interference shielding test setup is shown in Fig. 1.

Fig. 1 Schematic of the EMI-SE measurement test set-up.

3. Results and discussion

3.1 FTIR analysis

The FTIR spectrum of PANi–graphene film (Fig. 2) was analysed to confirm the formation of the hybrid and to study the interaction of PANi with graphene. The spectrum shows all the signature vibrations of doped PANi. The bands at 1582 and 1477 cm⁻¹ are the characteristic vibrations of the emeraldine form of PANi showing the presence of C=C stretching of the quinoid and benzenoid rings, respectively.³⁷ It can be seen that the intensities of these bands have similar values showing a good percentage of the quinoid rings in the PANi chains. The bands seen at 1299 cm⁻¹, 1232 cm⁻¹ and 1124 cm⁻¹ are due to the C–N stretching of the secondary aromatic amines and the C–H bending of the benzenoid and quinoid rings, respectively.³⁷ The presence of the most intense ‘electronic-like’ band in the spectrum located at 1124 cm⁻¹ can be confirmed to be the reason for the observed higher electrical conductivity of the hybrid film which shows a strong π–π interaction between the conjugated PANi chains and graphene sheets.^37–39 The intense band at 3437 cm⁻¹, a signature of the N–H stretching, can also be attributed to the ‘charge transfer’ effect between the graphene sheets and conducting PANi chains.³⁷

Fig. 2 FTIR spectrum of PANi–graphene hybrid film.

3.2 XRD analysis

The detailed analysis of the X-ray diffraction pattern (Fig. 3) confirms the presence of PANi and graphene in the hybrid film. The intense peak at 26° is characteristic of graphene and the semicrystalline peaks around 12° and 24° confirm the presence of PANi in the film. Another peak of graphene generally seen around 42.8° has almost disappeared indicating considerable PANi–graphene interaction.⁴⁰

Fig. 3 XRD pattern of PANi–graphene hybrid film.

3.3 Micro-Raman spectral analysis

The micro-Raman spectrum of PANi–graphene hybrid film is shown in Fig. 4. The presence of the slightly shifted D and G-bands of graphene, seen at 1350 and 1571 cm⁻¹ can be ascribed to the disordered A_1g phonons due to the impurities or defects in the graphene and E_2g phonons resulting from the in-plane bond stretching of the ordered sp² hybridized carbon atoms. The slight shift in the D and G bands of graphene could be due to the strong π–π interaction in the PANi–graphene film.^40,41 The vibrations of the semiquinone radical and C=C stretching of the quinoid rings of PANi at 1350 and 1571 cm⁻¹ respectively have merged with the D and G vibrations of graphene showing a tight interaction between PANi and graphene. The peak at 610 cm⁻¹ and the small projection at 1185 cm⁻¹, which are signatures of PANi in the film, show C–N–C torsion and C–H bending of the quinoid ring respectively.⁴² It can also be seen from the spectrum that the graphene sheets have low defect levels as the intensity of G-band is greater than that of the D-band.^42,43

Fig. 4 Micro-Raman spectrum of PANi–graphene hybrid film.

3.4 Hall measurements

The dc electrical conductivity and the mobility of the majority charge carriers have been evaluated using the Hall measurement technique. The majority charge carriers are found to be the holes and hence the hybrid can be classified as a p-type material. The film possesses a high electrical conductivity value of ∼116 S cm⁻¹ all along the surface without the application of any stretching process to orient the PANi chains. The charge carrier mobility has been found to be 18.9 cm² V⁻¹ s⁻¹ which is one to three orders higher than that observed in conductive polymer films. Higher conductivity values are favourable for exhibiting better shielding effectiveness and the appreciable mobility of the charge carriers makes it a promising material for use in organic field effect transistors as well.

3.5 Thermo gravimetric analysis

The thermal stability of the hybrid film was studied using a thermo gravimetric analyser. From Fig. 5, it can be seen that the thermal stability is quite high (∼200 °C) for the hybrid film sample. The dopant evolution begins when the temperature crosses 200 °C and the functional groups are completely removed in the 300–600 °C range and thereafter the polymer backbone degrades. Graphene being highly stable, degrades only at higher temperatures. The higher thermal stability of the sample makes it suitable for high temperature EMI shielding applications.

Fig. 5 Thermal analysis of PANi–graphene hybrid film.

3.6 Scanning electron microscopy

The scanning electron micrographs of the graphene nanoflakes and PANi–graphene hybrid film are shown in Fig. 6a–c. The figures b and c, clearly demonstrate that PANi has been adsorbed on the surfaces of graphene sheets which supports the inferences from the FTIR and Raman spectral analysis. PANi chains form thin layers over the graphene surface and facilitate the formation of good quality thin films. It can also be seen that the surface area of the film has increased as a result of the hybrid formation which could possibly help in enhancing the shielding effectiveness.

Fig. 6 FESEM images of (a) graphene nanoflakes, (b & c) PANi–graphene hybrid film (at different magnifications).

3.7 EMI shielding properties

EMI shielding effectiveness (SE) represents the attenuation of the propagating electromagnetic waves produced by the shielding material. The total EMI shielding effectiveness of a shielding material is the measure of the loss of electromagnetic (EM) energy in transmission through the material compared to direct delivery of energy in the absence of shield.^44,45 EMI SE is an important electromagnetic compatibility (EMC) requirement for protecting susceptible electronic systems from the electromagnetic pollution.⁴⁶ A good EMI shielding material exhibits maximum attenuation (by reflection and/or absorption mechanism) of the EM wave with lowest possible or negligible transmission.⁴⁷ An EMI SE of 20 dB is adopted to be a standard for typical electromagnetic wave shielding materials.^48–50 It means that 99% of the total energy of electromagnetic wave incident on it is attenuated. The total EMI SE is measured in dB and can be expressed as,^45,51–53

SE_T (dB) = −10 log (P_t/P_i) = SE_R + SE_A (1)

where, P_i and P_t are the powers of the incident and transmitted electromagnetic waves and SE_R and SE_A the reflection shielding effectiveness and absorption shielding effectiveness, respectively. Using the scattering parameters S₁₁ and S₂₁ of vector network analyser (obtained by the waveguide transmission line technique), the reflection coefficient, R, and the transmission coefficient, T, can be expressed as R = |S₁₁|² and T = |S₂₁|². The absorption coefficient, A, can be calculated from the power balance relation A + R + T = 1.^54–56 SE_R and SE_A can be expressed in terms of ‘R’ and ‘T’ as SE_R = −10 log(1 − R) and SE_A = −10 log[T/(1 − R)].⁵⁷ The power values are calculated using the scattering parameters as, P_r = P_i (S₁₁)², P_t = P_i (S₂₁)², and P_a = P_i − (P_r + P_t),^58,59 where P_r is the reflected power and P_a, the absorbed power.

The variation of SE_R, SE_A, and SE_T with frequency for PANi–graphene composites in the 4–12 GHz range (C and X bands) is shown in Fig. 7a and b. From the figures, it can be seen that PANi–graphene composite shows reflection dominated shielding effectiveness rather than absorption. In the C-band, PANi–graphene hybrid films with thickness 5 μm, show SE_R in the range of 34 to 36 dB and in the X-band, it lies in the range of 23.2 to 24.2 dB. Considering the absorption, the SE_A of PANi–graphene composite film is in the range of 5.7 to 6.3 dB in the C-band and 5.8 to 8.4 dB in the X-band, respectively. This clarifies the unique reflection dominant EMI shielding effectiveness of PANi–graphene hybrid in the broad microwave frequency band. The observed results are very significant in optimizing the dominant parameters of this composite to serve as an effective replacement to conventional heavy weight and corrosive metallic shields. Since conductivity is an important factor determining the shielding effectiveness of a shielding material,⁵⁶ the observed reflection dominant shielding characteristics of PANi–graphene composite can be attributed to the high electrical conductivity and larger surface area of the hybrid film. When an electromagnetic wave is incident on a highly conducting surface, the electron cloud near the surface gets distorted and generates an electric field opposite to the applied electric field. This combined opposite electric fields cause reflection of the electromagnetic wave from the surface rather than penetration through the material, as a result of the impedance mismatch. The presence of graphene nanoflakes in the hybrid film provides large surface area and connectivity in the PANi matrix and hence contributes towards the improved electrical conductivity, subsequently leading to better shielding effectiveness.^60,61 The EMI shielding effectiveness of the composite shows almost stabilized behaviour over the measured frequency with better SE_R in the C-band, which indicates that this composite has a broad bandwidth of shielding. Most of the EMI shielding studies based on conducting polymers and their composites reported in the literature deals with thicker films or millimeter thick pellets and the shielding limits are much below 30 dB in most cases. The highlight of the present study is that, compared to the previous reports, much thinner films, which can be coated on any type of substrates useful for practical applications have been used for the EMI shielding studies. The hybrid film of the present work exhibits SE_T values ranging from 32–42 dB in a very broad microwave frequency band in which many devices are generally found to operate and hence can be classified as an excellent material suitable for practical applications.

Fig. 7 Dependence of shielding effectiveness (SE_R and SE_A) of PANi–graphene composite as a function of (a) C-band and (b) X-band frequencies.

The power values of the PANi–graphene hybrid film in both the C and X bands as functions of frequency are shown in Fig. 8a and b. The results are in accordance with the observed shielding characteristics and reveal that the sample is reflective towards incident microwave radiation and provides negligible transmission. Reflected power is observed in the range of 85–90% in the 4–8 GHz frequency band and 75–80% in the 8–12 GHz frequency band, respectively. Highly conducting PANi–graphene composite with modified interfaces brings about the interception of the incident EM energy which affects both reflection and absorption, thus contributing to the overall shielding effectiveness. The values of P_t in the range of 0.01% in the 4–8 GHz frequency range and 0.06 to 0.1% in the 8–12 GHz frequency range indicate that the PANi–graphene hybrids are effective microwave attenuators and are more reflective and less absorptive to electromagnetic radiation in both C and X-band frequencies.

Fig. 8 Reflected power (P_r), absorbed power (P_a) and transmitted power (P_t) vs. (a) C-band and (b) X-band frequencies of PANi–graphene composite.

4. Conclusions

In summary, conducting, free standing films of PANi–graphene hybrids of approximately 5 microns thickness with commendable EMI shielding effectiveness could be realized from self-stabilized dispersion polymerized PANi powder and graphene nanoflakes via the simple solution intercalation method. The reflection dominated SE_T reaches up to 42 dB with an SE_R of 36 dB in the C-band (4–8 GHz) frequency and upto 32 dB with an SE_R of 24.2 dB in the X-band (8–12 GHz) frequency. These values are among the best reported values for polymer based conducting films in the broadband microwave range. These thermally stable hybrid films showing excellent electromagnetic shielding effectiveness mainly due to reflection, are ideal for EMI shielding applications in a broad microwave frequency band, in which many devices are generally found to operate.

Acknowledgements

The authors would like to acknowledge Professor Mohanan, Department of Electronics, Cochin University of Science and Technology(CUSAT), Kerala, India and Dr Soney Varghese, Assistant Professor, School of Nanoscience and Nanotechnology, National Institute of Technology, Calicut, India, for their help in characterizing the samples and Professor M. K. Jayaraj, Nanophotonic & Optoelectronic Devices laboratory, Department of Physics, CUSAT for Raman measurements under DST Nano-mission initiative programme, Government of India.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13704c

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