EMI shielding performance of PPy/Fe-WS₂ nanocomposites in the Ku band

Abstract

Electromagnetic interference (EMI) poses critical challenges to human health and the performance of electronic devices, particularly with the proliferation of advanced technologies such as next-generation telecommunication networks, smart wearables, and electric vehicles. The growing reliance on wireless technologies necessitates ultrahigh-performance, cost-effective EMI shielding materials, focusing on absorption-based mechanisms. In this study, PPy/Fe-WS₂ nanocomposites were synthesized via in situ chemical oxidative polymerization, resulting in an agglomerated structure with well-integrated Fe-WS₂ within the polypyrrole matrix. These nanocomposites demonstrated exceptional microwave shielding performance within the Ku-band frequency range (12.4–18 GHz). The PPy/Fe-WS₂ nanocomposites with a 10 wt% Fe-WS₂ content demonstrated a remarkable electromagnetic wave attenuation of 99.7%, achieving a total electromagnetic interference shielding effectiveness (EMI SE) of 36.12 dB at 18 GHz. This shielding was primarily attributed to absorption (28.80 dB), with a smaller contribution from reflection (7.32 dB) with dielectric and magnetic loss values of 1.53 and 0.48 at 18 GHz respectively. The incorporation of Fe-WS₂ enhanced dielectric loss through the formation of conductive pathways, interfacial polarization, and relaxation processes, while Fe doping improved magnetic permeability, further augmenting the shielding performance. Additionally, the nanocomposite exhibited high AC conductivity (45.2 S m⁻¹), a reduced skin depth (0.82 mm), and a stable attenuation coefficient (1267.4), underscoring its effectiveness in EMI mitigation. These attributes establish the PPy/Fe-WS₂ nanocomposite as a promising candidate for satellite communication systems and other advanced applications requiring efficient and reliable EMI attenuation.

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1. Introduction

Electromagnetic radiation, emitted or absorbed by charged particles, has increased due to the widespread use of electronic devices. Sources such as electric appliances, mobile phones, and broadcasting infrastructure contribute significantly to this phenomenon.¹ This artificially induced radiation, known as electromagnetic interference (EMI), is now recognized as a major environmental concern, ranking as the fourth most significant pollution source after water, air, and noise pollution.² The high-frequency electromagnetic waves interfere with electronic equipment, reducing efficiency and causing malfunctions. Beyond technological disruptions, prolonged exposure poses health risks, including fatigue, anxiety, headaches, and sleep disturbances, potentially leading to more severe conditions.^3,4 As a result, the demand for effective EMI shielding has surged, driving research into innovative materials and technologies. Traditionally, metals have been the preferred choice for EMI shielding due to their superior electrical conductivity. However, their high density,⁵ rigidity,⁶ corrosion susceptibility,⁷ and complex fabrication processes⁸ limit their applicability in modern technologies. These challenges have spurred interest in alternative shielding materials, particularly polymer-based composites.^9,10 These materials offer advantages such as lightweight nature,¹¹ tunable conductivity,¹² broad-spectrum frequency response,¹³ adjustable dielectric characteristics,¹⁴ and flexibility,¹⁵ making them highly suitable for electromagnetic wave absorption applications. The appeal of polymer-based EMI shielding materials stems from three fundamental advantages.¹⁶ Firstly, their low density supports miniaturization and portability in modern electronics. Secondly, their tunable electrical conductivity allows customization for specific shielding and absorption needs, enhancing versatility. Lastly, their thermal stability and resistance to environmental degradation ensure reliable performance in industrial and consumer applications, making them durable and efficient shielding solutions.¹⁷ Among the diverse array of conductive polymers, including polyaniline, polythiophene, polyacetylene, polyfuran, and poly(3,4-ethylene dioxythiophene), polypyrrole (PPy) stands out for its high electrical conductivity, environmental stability, and mechanical flexibility. A key advantage of PPy is its easy synthesis and ability to undergo structural modifications for diverse applications. Unlike many conducting polymers, it maintains electrical properties over time and under varying conditions. PPy is highly effective in electromagnetic interference (EMI) shielding, offering strong conductivity and seamless composite integration. Its nanotube and layered morphologies enhance conductivity pathways and absorption efficiency. Beyond EMI shielding, PPy is widely used in energy storage, sensors, and biomedical devices due to its cost-effectiveness, superior conductivity, and compatibility with various fabrication techniques. However, PPy faces challenges such as low mechanical strength, poor solubility, and limited processability, which can hinder large-scale applications. These issues affect the structural integrity of EMI shielding materials, their durability under mechanical stress, and the uniform dispersion of conductive networks necessary for optimal shielding efficiency. To overcome these challenges, efforts have been made to enhance the functional and structural properties of PPy composites by incorporating fillers such as carbon-based materials, polymers, and inorganic compounds. Inorganic fillers, in particular, significantly improve the stability, stiffness, and toughness of PPy composites. When combined with advanced synthesis techniques, they help mitigate PPy's limitations, broadening its use in EMI shielding. Recently, two-dimensional (2D) fillers such as graphene, MXenes, and transition metal dichalcogenides (TMDCs) have gained significant attention for their superior electromagnetic absorption properties. Their high surface-to-volume ratio, exposed reactive sites, and layered structures enhance interactions with electromagnetic waves, leading to improved absorption efficiency. Several studies have demonstrated the effectiveness of these materials in EMI shielding applications. Das et al. developed MWCNT-based XNBR nanocomposite sheets with outstanding EMI shielding and thermal management properties.¹⁸ In a separate study, a ZnO-XNBR/RGO variant was developed by them, achieving an electrical conductivity of 0.02 S cm⁻¹, a thermal conductivity of 0.75 W mK⁻¹, 54.7% self-healing capability, and 100% recyclability, along with an EMI shielding effectiveness (SE) of −34.2 dB in the X-band.¹⁹ Among various 2D fillers, TMDCs stand out due to their tunable bandgaps and excellent physicochemical properties, making them strong candidates for next-generation EMI shielding materials.^20,21 Within the realm of transition metal dichalcogenides, metal sulfides have emerged as highly promising candidates for enhancing the thermal stability, cycling durability, and electrical conductivity²² of polypyrrole composites. Their effectiveness is largely attributed to their distinctive two-dimensional structures, tunable bandgaps spanning the visible to near-infrared spectrum, and exceptional physicochemical properties. By interacting with the PPy matrix, metal sulfides facilitate electron transfer by donating or withdrawing electrons at the reaction transition states. The partially filled d-orbitals of the transition metals further enable the generation of free-moving electrons within the composite, thereby significantly improving its overall electrical conductivity.²³ Integration of metal sulfides such as molybdenum disulfide (MoS₂) with PPy demonstrates considerable promise for EMI shielding applications. Su et al. synthesized MoS₂@PPy composites via in situ polymerization and a microwave-assisted hydrothermal method, achieving a shielding efficiency of −61.1 dB due to their 3D flower-like morphology and optimized permittivity.²⁴ Liu et al. developed a MoS₂/PPy/rGO composite using a hydrothermal approach, attaining −53.5 dB shielding effectiveness with Auricularia-like MoS₂ nanosheets and reduced GO-modified PPy, enhancing impedance matching and interface polarization.²⁵ Gai et al. introduced ultrathin MoS₂ nanosheets on hollow PPy nanotubes, optimizing attenuation and impedance matching.²⁶ These studies highlight the potential of PPy/MoS₂ nanocomposites for advanced EMI shielding applications. The existing body of research underscores the significant potential of PPy/MoS₂ nanocomposites for effective EMI shielding. However, there is a notable gap in exploring the EMI shielding properties of polypyrrole-based tungsten disulfide (WS₂) nanocomposites. WS₂ offers distinct advantages due to its superior charge carrier mobility, high mechanical strength, and excellent thermal and oxidative stability. The graphite-like layered structure of WS₂ enhances electromagnetic wave absorption and scattering, optimizing attenuation mechanisms for improved shielding performance. Its tunable bandgap, particularly in monolayer or few-layer forms, broadens its shielding effectiveness across different frequency ranges, while its transition to a direct bandgap at reduced dimensions further enhances conductivity.²⁷ These properties contribute to enhanced impedance matching and interface polarization when combined with PPy, which are critical for effective EMI shielding. Additionally, WS₂'s flexibility and robustness ensure resilience in demanding environments, making PPy/WS₂ composites suitable for a wide range of applications. While bulk WS₂ exhibits relatively low conductivity due to its indirect bandgap, doping with transition metals such as iron (Fe) modifies its electronic structure, introducing additional charge carriers and significantly enhancing its electrical conductivity.²⁸ The incorporation of Fe also improves charge transfer, reduces interfacial resistance, and activates crystal surfaces, optimizing WS₂ for EMI shielding applications. Beyond conductivity enhancement, Fe doping imparts magnetic properties to WS₂, improving EMI shielding through magnetic loss mechanisms. Ling-Yun et al. used spin-polarized calculations to show that Fe doping induces magnetism in WS₂, supporting its potential for spintronic technologies.²⁹ Poornimadevi et al. applied density functional theory to confirm that higher concentrations of Fe improve the electronic properties of WS₂, highlighting the significant impact of dopants on material performance.³⁰ Driven by these insights, the PPy/Fe-WS₂ nanocomposites were synthesized by in situ oxidative polymerization, with PPy acting as a matrix and Fe-doped WS₂ as a filler. The PPy/Fe-WS₂ (10 wt%) nanocomposite demonstrated a shielding effectiveness (SE_A) of 28.36 dB in the Ku-band frequency range. So far, no studies have explored the EMI shielding performance of PPy/Fe-WS₂ nanocomposites made with this method. This work aims to fill that gap by thoroughly investigating the structural, microstructural, and electromagnetic properties of these nanocomposites, advancing their potential in EMI shielding applications.

2. Experimental section

2.1. Materials

Tungsten disulfide (WS₂ 98% metal basis), pyrrole (C₄H₄NH 98% purity) as a monomer, and ferric chloride (FeCl₃) as a dopant and an oxidant were procured from Sigma-Aldrich and SD Fine Chemicals, Mumbai, respectively. The high purity of these reagents is a fundamental requirement for maintaining the integrity of our research and obtaining reliable results.

2.2. Preparation of Fe-doped WS₂

One gram of tungsten disulfide (WS₂) was dispersed in 80 mL of deionized water and subjected to ultrasonication for 30 minutes to ensure uniform dispersion. Following this procedure, four separate samples were prepared, each containing 1 g of WS₂. Ferric chloride (FeCl₃) solutions with varying weight percentages (1, 3, 5, and 10%) were prepared by dissolving FeCl₃ in 10 mL of deionized water and subsequently added to the respective WS₂ suspensions. The resulting mixtures underwent an additional 60 minutes of ultrasonication to facilitate proper interaction between the components. Thereafter, the suspensions were subjected to a reflux process using a magnetic stirrer, maintained at a temperature range of 100–120 °C, with a stirring speed of 800–900° revolutions per minute, for a duration of ten hours. This controlled synthesis process resulted in the formation of a dark greyish product. The obtained material was thoroughly washed with acetone and deionized water to remove any residual impurities and subsequently dried in an oven to eliminate moisture, as illustrated in Fig. 1. Finally, the dried samples were finely ground and denoted as Fe-WS₂-1 wt%, Fe-WS₂-3 wt%, Fe-WS₂-5 wt%, and Fe-WS₂-10 wt%, respectively.

Fig. 1 Schematic illustration of the synthesis process of PPy/Fe-WS₂ nanocomposites.

2.3. Preparation of PPy/Fe doped WS₂ nanocomposites

The synthesis of polypyrrole/iron-doped tungsten disulfide (PPy/Fe-WS₂) nanocomposites was conducted via the in situ oxidative polymerization method. Initially, iron-doped WS₂ powder with varying weight percentages (1, 3, 5 and 10%) was individually dispersed in 10 mL of deionized water, followed by ultrasonication for 30 minutes to ensure uniform dispersion. Maintaining a monomer-to-oxidant ratio of 1 : 2, a pyrrole solution (0.03 M) was subsequently introduced into the dispersed powder, followed by an additional ultrasonication step. Ferric chloride (FeCl₃, 0.06 M) was then incrementally added dropwise to the mixture while stirring continuously in a controlled temperature range of 0–5 °C, serving as the oxidant in the polymerization reaction. This systematic approach facilitated the formation of black PPy/Fe-WS₂ nanocomposites, as illustrated in Fig. 1. The final products, exhibiting a characteristic flaky black appearance, were subjected to oven drying to eliminate residual water content. Thereafter, the dried composites were finely ground using a mortar and pestle and denoted as PPy/Fe-WS₂-1 wt%, PPy/Fe-WS₂-3 wt%, PPy/Fe-WS₂-5 wt% and PPy/Fe-WS₂-10 wt%, respectively.

2.4. Characterization

To characterize the nanocomposites, several techniques were employed, including X-ray powder diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDS), Fourier transform infrared (FT-IR) spectroscopy, and vector network analysis (VNA). Structural geometry and crystallite size of the samples were examined using a Bruker D-8 X-ray diffractometer with Cu Kα (λ = 1.5406 Å) radiation at 30 kV and 50 mA. The scanning rate employed was 2° per minute to effectively capture the diffraction patterns of the finely powdered samples. Spectroscopic analysis was conducted using a PerkinElmer FT-IR spectrometer, covering the wavenumber range of 4000–400 cm⁻¹. The spectra were obtained by collecting data from KBr discs with a resolution of 2 cm⁻¹, using 16 scans. Pellet samples were prepared by applying 12 tonnes of pressure with a hydraulic press to anhydrous KBr. The microstructures and morphological characteristics of the samples were analyzed using transmission electron microscopy (JEOL TEM) and a scanning electron microscope (JEOL SEM) equipped with an energy dispersive X-ray analyzer (EDS), which operated at an accelerating voltage range of 5–30 kV. For scanning, a small quantity of powdered sample was carefully placed onto double-sided carbon tape on a stub. Subsequently, the samples were coated with a thin layer of gold to prevent charge accumulation during imaging. Thermogravimetric analysis (TGA) was conducted using a thermal analyzer (PerkinElmer TGA4000) under a nitrogen atmosphere, covering a temperature range from room temperature to 800 °C at a heating rate of 10 °C per minute. A vibrating sample magnetometer (VSM) (Lakeshore Cryotronics Inc., USA, Model 7404) was employed to examine the magnetic characteristics of samples, while the electromagnetic interference shielding and dielectric properties of the nanocomposites were assessed using a vector network analyzer (Agilent E8362B) in the Ku-band, which operates within a 12.4–18 GHz frequency range. Rectangular pellets with dimensions of 15.8 × 7.9 mm² were prepared from the compressed powder samples using a hydraulic press. Subsequently, these pellets were meticulously positioned in a sample holder and connected between the waveguide flanges of the network analyzer for thorough analysis. To ensure accurate results, a two-port calibration was conducted in conjunction with the sample holder to eliminate any potential loss or power redistribution caused by the sample holder.

3. Results and discussion

PPy/Fe-WS₂ nanocomposites were successfully synthesized through an in situ chemical oxidative polymerization process, utilizing FeCl₃ as the oxidizing agent and Fe-WS₂ as both a dopant and functional filler as shown in Fig. 1. This synthesis method resulted in the formation of an agglomerated nanostructure, wherein Fe-WS₂ was uniformly distributed and well-integrated within the polypyrrole matrix, ensuring strong interfacial interactions. The incorporation of Fe-WS₂ significantly enhanced the dielectric loss characteristics by facilitating the development of conductive pathways, promoting interfacial polarization, and enabling effective relaxation processes. Notably the introduction of Fe as a dopant contributed to an increase in magnetic permeability, thereby further improving the overall electromagnetic shielding performance of the nanocomposite.

3.1. X-ray diffraction studies

The XRD diffraction patterns offer valuable insights into the structural properties of pure PPy, Fe-WS₂, and PPy/Fe-WS₂ nanocomposites with varying weight percentages (1 wt%, 3 wt%, 5 wt% and 10 wt%). The observed Bragg angles and corresponding (hkl) indices in the bare WS₂ sample validate its layered structure and the orientation of its crystal planes. The prominent (002) peak at 2θ = 14.37° indicates the interlayer spacing between sulfur layers, serving as a defining characteristic of WS₂. Additionally, the identification of peaks 28.89°, 32.91°, 33.79°, 35.56°, 39.89°, 43.63°, 49.79°, 35.56°, 58.47°, 58.52° and 60.86° corresponding to the (004), (100), (100), (101), (103), (006), (102), (105), (110), (008), and (112) planes aligns with previously reported findings.^31,32 The obtained data have been corroborated using JCPDS file no. 08-237.

With the increase in Fe content as shown in Fig. 2(a), the intensity of the peaks decreases suggesting the integration of Fe(II) into the WS₂ lattice. The X-ray diffraction analysis in Fig. 2(b) illustrates the crystallite sizes of PPy/Fe-WS₂ composites with varying Fe-WS₂ concentrations (1 wt%, 3 wt%, 5 wt% and 10 wt%). The presence of a broad peak at 2θ = 28° indicates the amorphous nature of PPy, suggesting a highly disordered and coiled structure. By employing Scherrer's method, the crystallite sizes were determined from the X-ray line broadening.³³ Eqn (1) is used to represent the crystallite size (D):

Here, the shape factor is denoted by (K), the diffraction angle is denoted by (θ), and FWHM (full width at half maximum) is denoted by β. Upon application of this equation to the sharp peaks, the typical crystallite sizes were calculated. The analysis of the prominent peak (002 plane) of WS₂ revealed distinct crystallite sizes for each sample, with average sizes calculated to be 44.8 nm (pure WS₂), 32.2 nm (1 wt%), 23.1 nm (3 wt%), 21.9 nm (5 wt%) and 20.4 nm (10 wt%). These findings imply that the deposition of PPy onto Fe-doped WS₂ successfully suppressed the broad peak of PPy and resulted in the prevalence of crystalline Fe-doped WS₂, thereby enhancing the crystallinity of the nanocomposite.

Fig. 2 (a) XRD spectra of pure WS₂, Fe-WS₂ 1 wt%, Fe-WS₂ 3 wt%, Fe-WS₂ 5 wt%, and Fe-WS₂ 10 wt% (b) XRD spectra of pure PPy, PPy/Fe-WS₂ 1 wt%, PPy/Fe-WS₂ 3 wt%, PPy/Fe-WS₂ 5 wt%, and PPy/Fe-WS₂ 10 wt%.

3.2. Morphological analysis

The morphological characterization conducted using Field Emission Scanning Electron Microscopy (FESEM) and High-Resolution Transmission Electron Microscopy (HRTEM) provided valuable insights, as presented in Fig. 3. Fig. 3(a) presents the FE-SEM image of pure PPy, highlighting its granular morphology. Fig. 3(b) illustrates the two-dimensional layered structure of WS₂ in the form of sheets, which appear as bundles of stacked nanosheets with areas spanning a few micrometres, consistent with previously reported literature. The PPy/Fe-WS₂ (10 wt%) nanocomposite, shown in Fig. 3(c), exhibits a highly agglomerated structure with clear integration of Fe-WS₂ within the polypyrrole matrix. This incorporation leads to significant morphological transformations, enhancing the chain alignment in PPy and promoting the formation of crystalline domains within its otherwise amorphous polymer matrix. Energy Dispersive X-ray Spectroscopy (EDS) analyses of the PPy/Fe-WS₂ (10 wt%) nanocomposite, shown in Fig. 3(d) and (g)–(n), verify successful incorporation of Fe doping. The spectra confirm its elemental composition, identifying carbon, nitrogen, and oxygen from PPy; tungsten and sulphur from WS₂; and Fe and Cl from FeCl₃, which aligns with the material's synthesis process. The TEM analysis of the PPy/Fe-WS₂ (10 wt%) nanocomposite confirms the successful incorporation of the two-dimensional Fe-WS₂ structure within the PPy matrix. The sheet-like morphology of Fe-WS₂, with iron exhibiting a characteristic hexagonal geometry, is illustrated in Fig. 3(e). Fig. 3(f) further illustrates the coexistence of PPy and Fe-WS₂, with the inset displaying a magnified view of the hexagonal configuration of iron. The nanocomposite exhibits an irregular, porous morphology, attributed to the coating of Fe-WS₂ sheets within the PPy matrix. This structural feature likely enhances surface area and interaction potential. Additionally, the presence of distinct darker regions, corresponding to Fe particles, confirms their retention within the nanocomposite structure. These morphological and elemental analyses substantiate the successful synthesis of the PPy/Fe-WS₂ nanocomposite and the structural transformations achieved through the incorporation of Fe-WS₂.

Fig. 3 FE-SEM images of (a) pure PPy, (b) pure WS₂, and (c) PPy/Fe-WS₂ 10 wt%, (d) EDX spectrum, (e) TEM image of FE-WS₂, (f) TEM image of the PPy/Fe-WS₂ 10 wt% nanocomposite where the inset shows the magnified image of Fe and (g–n) EDS spectrum of PPy/Fe-WS₂ 10 wt% showing the distribution of different constituent elements in the nanocomposite.

3.3. FT-IR spectra

Fourier Transform Infrared (FTIR) spectra are the fingerprints of any material. Therefore we carried out FTIR spectroscopy of pure PPy, pure WS₂, and PPy/Fe-WS₂ nanocomposites at varying weight percentages (1, 3, 5 and 10 wt%) as depicted in Fig. 4. The FTIR analysis of the prepared samples has identified prominent transmittance peaks, signaling a successful interaction between PPy and Fe-doped WS₂. The antisymmetric and symmetric C=C stretching vibrations of the pyrrole ring are distinctly identified at 1712 cm⁻¹ and 1622 cm⁻¹, respectively. The presence of a band at 1421 cm⁻¹ is attributed to the N–H bending vibration, further validating the polymer's structural integrity. Additionally, the band observed at 1144 cm⁻¹ corresponds to the C–N stretching vibrations, indicating the presence of primary amine functionalities within the polymer matrix. The vibrational band at 1284 cm⁻¹ suggests the presence of =C–H in-plane vibrations, signifying an extended conjugation system within the polymer backbone. Furthermore, the band at 1052 cm⁻¹ is associated with the in-plane deformation of the C–H bond in the pyrrole ring, reinforcing the structural stability of the polymer. The presence of minor peaks below 1000 cm⁻¹ serves as an additional confirmation of the polymerization process, indicating complete polymerization and ensuring the uniformity of the synthesized PPy material.^34–36

Fig. 4 FT-IR spectra of pure PPy, pure WS₂, PPy/Fe-WS₂ 1 wt%, PPy/Fe-WS₂ 3 wt%, PPy/Fe-WS₂ 5 wt% and PPy/Fe-WS₂ 10 wt%.

For WS₂, significant absorption bands at 567 cm⁻¹ and 875 cm⁻¹ correspond to W–S and S–S stretching vibrations, characteristic of the layered structure of WS₂ and bonding interactions within the tungsten sulfide lattice.³¹ The band observed at 586 cm⁻¹ confirms the presence of iron.³⁷ The FTIR spectra of the nanocomposites reveal subtle variations in the W–S and S–S stretching vibration peaks, indicating that Fe doping alters the lattice structure and modifies the local bonding characteristics of WS₂. A notable frequency shift from 1622 cm⁻¹ in pure PPy to 1635 cm⁻¹ in the PPy/Fe-WS₂ 10 wt% system is observed, followed by a similar change in subsequent systems. This shift indicates a modification in the vibrational energy of the C=C bond. This shift can be described by the equation for the bond's natural vibrational frequency, derived from Hooke's law,³⁸ and is expressed as:

where the masses of the two atoms are represented by m₁ and m₂, the force constant of the bond by k, the velocity of light by c and the reduced mass of the system by μ, expressed as

The molecule's mass decreases when the peak shifts to a higher wavenumber, as demonstrated in eqn (2) and (3). There is an inverse relationship between the vibrational frequency and the mass of the vibrating molecule; therefore, lighter molecules show higher vibrational frequencies and larger wavenumbers. The observed shift of the C=C stretching band to a higher frequency indicates either stronger bond interactions or a reduced molecular mass, likely due to Fe doping in WS₂. Applying these equations to the PPy/Fe-WS₂ nanocomposite with 10 wt% Fe-WS₂ doping, the experimental values align with the theoretical data shown in Table 1, validating the theoretical model's accuracy in interpreting the observed spectral shifts.

Table 1 Comparison between the experimental and theoretical vibrational frequency values of various bands observed in the pure PPy, pure WS₂, and PPy/Fe-WS₂ (10 wt%) nanocomposite

Vibrational frequency	Experimental value			Theoretical value (cm^−1)
	Pure PPy (cm^−1)	Pure WS2 (cm^−1)	PPy/Fe-WS2 (10 wt%) (cm^−1)
C–N stretching	1144	—	1170	1196
C=C stretching	1622	—	1635	1682
W–S stretching	—	567	584	557

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As seen in the FTIR spectra, with the increase in Fe-WS₂ content, the particles distribute more broadly across the nanocomposites, resulting in a wider range of particle sizes. A slight shift in the peaks of PPy/Fe-WS₂ towards a higher wavenumber, compared to pure PPy, points to interfacial interactions between the matrix and the filler. The preservation of these distinct PPy/Fe-WS₂ peaks in nanocomposites confirms a successful integration, maintaining the original chemical structure of PPy. The shifts in absorption bands are likely due to molecular-level interactions, including synergistic or electronic effects, between PPy chains and Fe-WS₂.

3.4. Thermogravimetric analysis

TGA investigations have been made to examine the thermal stability of the nanocomposites. In Fig. 5(a), the TGA plots of the PPy/Fe-WS₂ nanocomposites are shown. As seen in the figure, with an increase in temperature, PPy/Fe-WS₂ exhibited weight loss in three stages as reported in the previous literature.^39,40 During the initial phase, at around 100 °C, the reduction in weight can be linked to the evaporation of moisture or water molecules contained within the polymer. In the next phase, occurring at around 220 °C, the observed weight decline is due to the release of the dopant, which neutralized the positive charge of PPy chains. In the final phase, beyond 400 °C, the weight reduction results from the structural breakdown of PPy chains following the removal of dopant ions. As the temperature increases, PPy/Fe-WS₂ nanocomposites demonstrate a consistent pattern of weight loss. The total weight loss by PPy/Fe-WS₂ 1 wt%, PPy/Fe-WS₂ 3 wt%, PPy/Fe-WS₂ 5 wt%, and PPy/Fe-WS₂ 10 wt% are 4, 8, 9, and 10% at 800 °C, respectively, due to different filler amounts of Fe-WS₂.

Fig. 5 (a) TGA curves of PPy/Fe-WS₂ 1 wt%, PPy/Fe-WS₂ 3 wt%, PPy/Fe-WS₂ 5 wt%, and PPy/Fe-WS₂ 10 wt%. (b) VSM plots of Fe-WS₂ 5 wt%, Fe-WS₂ 10 wt%, PPy/Fe-WS₂ 5 wt%, and PPy/Fe-WS₂ 10 wt%.

3.5. VSM studies

The magnetic properties of the nanocomposites, in terms of magnetization (M) as a function of the applied magnetic field (H), are illustrated in Fig. 5(b) for Fe-WS₂ 5 wt%, Fe-WS₂ 10 wt%, PPy/Fe-WS₂ 5 wt%, and PPy/Fe-WS₂ 10 wt%. As observed, the magnetization curve for Fe-WS₂ 5 wt% is nearly negligible, but with an increase in Fe content to 10 wt%, the curve becomes more pronounced. Following polymerization, the magnetization further increases, with PPy/Fe-WS₂ 5 wt% showing a visible curve, which becomes most prominent in the case of the PPy/Fe-WS₂ 10 wt% nanocomposite and a saturation magnetization (M_s) value of 2.8 emu g⁻¹ has been observed. The nanocomposites exhibit ferromagnetic hysteresis behavior with a small coercive field around 300 gauss, suggesting slight ferromagnetic interactions due to the presence of nano-sized magnetic particles. As the Fe content increases, so does the magnetization.^18,19 Among all the samples, PPy/Fe-WS₂ 10 wt% demonstrates the highest saturation magnetization as seen in the figure. The observed increase in saturation magnetization for the nanocomposites may be attributed to oxidation occurring during the polymerization process, where iron, being a metal, potentially oxidizes to iron oxide, thereby contributing to the overall magnetization.

3.6. Dielectric studies

To evaluate the potential of PPy/Fe-WS₂ nanocomposites as effective electromagnetic wave absorbers, a comprehensive investigation of their complex permittivity was carried out within the Ku-band frequency range (12.4–18 GHz). The real part of the relative permittivity (ε′) quantifies the material's ability to store electrical energy by generating polarization. In contrast, the imaginary part (ε′′) represents the dissipation of this energy, primarily by electrical conduction. The effectiveness of these nanocomposites in absorbing microwave radiation is significantly influenced by their dielectric loss properties and the ability to achieve impedance matching with the surrounding medium. The established Nicholson–Ross–Weir method⁴¹ determined the complex permittivity values, providing a detailed insight into the material's electromagnetic response across the Ku-band spectrum. As shown in Fig. 6(a) and (b) the real permittivity of all nanocomposites decreases with increasing frequency, while the imaginary permittivity increases with the incorporation of Fe-WS₂. This behavior can be attributed to the delayed response of dipoles to the applied AC field. The measured (ε′) values for PPy/Fe-WS₂ 1 wt%, PPy/Fe-WS₂ 3 wt%, PPy/Fe-WS₂ 7 wt%, and PPy/Fe-WS₂ 10 wt% are 20.2848, 25.662, 31.8908, and 37.5266 while the measured (ε′′) values are 22.5704, 29.6237, 36.4358, and 58.8172 respectively at 12.4 GHz. The relative decrease in (ε′) values compared to (ε′′) values with increasing Fe-WS₂ content is attributed to the formation of microcapacitors due to interfacial dipole moments caused by the electronegativity difference between W in WS₂ and N in PPy. Moreover, incorporating Fe-WS₂ into the PPy matrix introduces multiple interfaces, which likely enhance losses through interfacial polarization. Furthermore, the imaginary component of permittivity exceeds the real part, a characteristic typically necessary for effective EM wave attenuation. Therefore, the resulting nanocomposite is likely to enhance dielectric loss and EM wave absorption, owing to its conductive network structure and the multi-interface arrangement between Fe-WS₂ and the PPy matrix, which improves interfacial polarization.

Fig. 6 (a) Real permittivity, (b) imaginary permittivity, (c) change in tangent loss of PPy/Fe-WS₂ nanocomposites in the Ku-band and (d) Cole–Cole plot of the PPy/Fe-WS₂ 1 wt% nanocomposite.

To further explore the loss mechanism, the tangent loss (tan δ_ε) has been plotted which increases systematically as the weight percent of Fe-WS₂ increases in the nanocomposites with PPy/Fe-WS₂ 10 wt% achieving a maximum value of 1.56 at 12.4 GHz as shown in Fig. 6(c). The tangent loss represents the dielectric loss contributed by interfacial and surface charge polarization due to the incorporation of Fe-WS₂ in the conjugated polymer. Generation of the interfacial site and accumulation of charge carriers result in an increase in overall polarization and losses in the composites which contributes to the enhancement of their absorption-dominated electromagnetic shielding properties, making them well-suited for industrial usage. In addition to the dielectric properties of PPy/Fe-WS₂ nanocomposites, the relaxation behavior of dipoles plays a crucial role in determining the overall performance of the material. Understanding this relaxation behavior requires examining it through the lens of the Debye theory.⁴² This theory provides a framework to describe the relationship between real and imaginary permittivity. According to this theory, the real permittivity (ε′) and imaginary permittivity (ε′′) are interconnected and can be described as

where ε_∞, ε_s, ω and τ represent the optical permittivity, static permittivity, angular frequency, and relaxation time, respectively, these parameters are essential in describing the dielectric response of a material. Optical permittivity refers to the permittivity at high frequencies where the dipoles do not have time to orient themselves, while static permittivity represents the permittivity when the field frequency approaches zero, allowing full dipole alignment. Using eqn (2) and (3), the real part of permittivity and the imaginary part of permittivity can be mathematically connected as

(ε′ − ε_∞)² + (ε′′)² = (ε_s − ε_∞)² (7)

The Cole–Cole plot, which depicts the real part of permittivity (ε′) on the abscissa and the imaginary part (ε′′) on the ordinate, illustrates the relationship between these parameters. It is characterized by semicircular arcs, each representing a specific Debye relaxation process. These varying semicircles arise due to the asymmetry in relaxation times (τ). Fig. 6(d) illustrates the Cole–Cole plot for the PPy/Fe-WS₂ (1 wt%) nanocomposite, showing the presence of multiple semicircles. These semicircles indicate multiple relaxation processes linked to enhanced microwave absorption. While the specific cause of multiple semicircles has not been explicitly explained, the results suggest a significant role of interfacial dynamics. The formation of numerous interfaces within the nanocomposite is a key factor. PPy/Fe-WS₂ nanocomposite interfaces, PPy–PPy, Fe/WS₂–Fe/WS₂, and PPy–Fe/WS₂–PPy/Fe/WS₂, are likely to generate interfacial charges. These charges, coupled with the interactions at these interfaces, induce a lag in molecular polarization when responding to changes in the frequency of an applied field. This lag results in multiple relaxation processes arising from interfaces' diversity and associated charges. These distinct relaxation dynamics significantly enhance the nanocomposite's ability to absorb microwave energy, establishing its potential for advanced electromagnetic applications. The intricate balance between polarization and relaxation mechanisms offers valuable insight for tailoring the dielectric properties of the nanocomposite. Consequently, dielectric loss becomes a key contributor to the attenuation of electromagnetic waves, with its origins linked to phenomena such as interfacial polarization, dipole relaxation, and electrical conductivity. The real and imaginary permeability along with magnetic loss were analyzed as shown in Fig. 7(a)–(c), showing that the tangent loss exceeds the magnetic loss. The data reveal a resonance pattern similar to the complex permittivity. Although the real and imaginary permeability values are low, the magnetic loss for the PPy/Fe-WS₂ 10 wt% nanocomposite is significantly higher.

Fig. 7 Variation of (a) real permeability and (b) imaginary permeability, (c) magnetic loss and (d) eddy current constant (C₀) with frequency for PPy/Fe-WS₂ nanocomposites at varying weight percentages (1 wt%, 3 wt%, 5 wt%, and 10 wt%) in the Ku-band frequency range.

This is noteworthy because WS₂ itself is nonmagnetic, but the doping of iron may impart some magnetic properties. Furthermore, the inclusion of Fe-WS₂ in PPy/Fe-WS₂ nanocomposites may form interfacial magnetic domains, potentially increasing magnetic permeability by modifying the edge states of the Fe-WS₂ sheets at the interface. This behavior indicates strong impedance matching, as both dielectric and magnetic losses seem to align in phase, thereby enhancing electromagnetic wave absorption. This synergy between the magnetic and dielectric properties enhances the overall performance of the material, making it an efficient candidate for electromagnetic shielding applications. To further elucidate the influence of permeability, the eddy current constant (C₀) is analyzed as a metric for understanding its role in magnetic loss.² The constant is mathematically expressed as:

As shown in Fig. 7(d), C₀ exhibits significant variations within the Ku-band frequency range (12.4–18 GHz) across all PPy/Fe-WS₂ nanocomposites. These fluctuations underscore the intricate interplay of multiple factors, including eddy current loss, ferromagnetic resonance, exchange resonance, and anisotropic loss, as contributors to magnetic dissipation. This detailed analysis highlights the complex mechanisms contributing to magnetic dissipation in these nanocomposites thereby highlighting the multifaceted nature of their magnetic behavior.

3.7. Electromagnetic interference shielding studies

Electromagnetic (EM) shielding refers to the process of blocking EM waves through a shield, preventing them from interfering with the operation of protected equipment. A shield typically consists of a coating or thin layer of an absorbing and/or a conductive material, which mitigates the impact of EM waves by generating an induced field within the material that interacts with the incident field. This shielding effect is driven by three primary mechanisms: SE_R (scattering or reflection), SE_A (absorption), and SE_MR (multiple reflections). Reflection occurs due to the movement of charge carriers within the shielding material, while absorption results from interactions involving mobile charge carriers, electric or magnetic dipoles, and polarization effects. Multiple reflections arise from repeated internal wave interactions within the shield, which can at times reduce overall shielding effectiveness (SE). For practical implementations, a shielding effectiveness within the range of 10–30 dB is regarded as the minimum threshold for efficient EMI attenuation. Materials with a total SE of 30 dB can effectively block up to 99.9% of incident electromagnetic radiation, making them well-suited for various technological and engineering applications. Similarly, shielding materials exhibiting SE values of at least 20 dB are capable of attenuating approximately 99% of incoming electromagnetic waves.

Notably, when the absorption component surpasses 10 dB, the influence of multiple reflections becomes insignificant. Consequently, the total shielding effectiveness (SE_T) is largely dictated by the combined effects of SE_R and SE_A,¹⁴ mathematically expressed as:

SE_T = SE_R + SE_A (9)

The shielding effectiveness components can be calculated as follows:

SE_R = −10 log(1 − R) (10)

SE_A = −10 log(T/1 − R) (11)

In addition, scattering parameters, including the forward reflection coefficient (S₁₁), reverse reflection coefficient (S₂₂), forward transmission coefficient (S₁₂), and backward transmission coefficient (S₂₁), were measured using the vector network analyzer to analyse the reflection and transmission of incident radiations. These S-parameters provide the necessary data to compute the reflection coefficient (R), absorption coefficient (A), and transmission coefficient (T). Specifically, the formulae for R and T are

R = |S₁₁|² = |S₂₂|² (12)

T = |S₁₂|² = |S₂₁|² (13)

Based on these values, the SE_R and SE_A can be expressed as:

SE_R = −10 log(1 − |S₁₁|²) (14)

Fig. 8 presents the shielding efficiency of PPy/Fe-WS₂ nanocomposites with varying Fe-WS₂ loadings (1 wt%, 3 wt%, 5 wt%, and 10 wt%) across the Ku-band frequency range. As depicted in Fig. 8(a), the SE_R values for PPy/Fe-WS₂ at 1, 3, 5 and 10 wt% are 4.22 dB, 4.73 dB, 5.23 dB, and 7.32 dB, respectively. Meanwhile, Fig. 8(b) shows the corresponding SE_A values 15.41, 15.92, 23.88 and 28.80 dB at 18 GHz. The PPy/Fe-WS₂ nanocomposite with 10 wt% Fe-WS₂ loading demonstrates the highest total shielding effectiveness (SE_T) of 35.80 dB, as illustrated in Fig. 8(c). Thus, the synthesized nanocomposite has surpassed the required threshold for effective shielding, demonstrating its ability to attenuate almost 99.7%, i.e. 28.8 dB, of incoming electromagnetic waves. This finding aligns with previously reported literature.^41,43 The significant difference between SE_A and SE_R values highlights that absorption is the primary contributor to the nanocomposite's total shielding efficiency. This dominance is largely due to the material's substantial magnetic and dielectric losses. Absorption of electromagnetic waves within the nanocomposite arises from a combination of polarization and relaxation processes, including electronic, atomic, and space charge polarization, each playing a role in enhancing absorption efficiency. Electronic and atomic dipoles respond swiftly to the alternating electromagnetic field in the GHz range, aligning with the wave and preventing energy loss. Conversely, charge carriers accumulate at the interfaces of the PPy/Fe-WS₂ structure, causing space charge polarization. This results in energy dissipation as the space charges cannot dynamically adapt to the incoming EM field, contributing to EM radiation loss. Furthermore, the integration of layered Fe-WS₂ into the PPy matrix results in a heterogeneous structure with a higher density of interfaces, driven by charge delocalization between Fe-WS₂ and the PPy matrix. This architectural modification greatly improves the absorption properties of the nanocomposite. Additionally, the random propagation of electromagnetic waves within the material extends their transmission path, further enhancing the overall absorption efficiency. The equations suggest that the shielding effectiveness from reflection and absorption is influenced by the nanocomposite's intrinsic properties, including permittivity, permeability, and electrical conductivity. This synergy of mechanisms underscores the material's advanced performance in electromagnetic shielding applications.

Here, σ_ac represents the AC conductivity (σ_ac = ωε₀ε′′), ω is the angular frequency, ε₀ denotes the permittivity of free space, and t signifies the thickness of the nanocomposite. As the frequency increases, the skin effect becomes more pronounced. This phenomenon occurs when electromagnetic radiation at high frequencies can only penetrate the near-surface regions of conductive materials, limiting deeper penetration. The increase in SE_A observed within the nanocomposite can be attributed to the skin depth (δ), which becomes significant at higher frequencies. Skin depth is defined as the distance at which the electric field intensity decays to 1/e (approximately 0.37), where ‘e’ represents Euler's number.¹⁴ This relationship is mathematically expressed as:

Fig. 8 Variation of (a) SE_A, (b) SE_R and (c) SE_T of the PPy/Fe-WS₂ nanocomposites with the frequency range 12.4–18 GHz.

According to this equation, skin depth is inversely proportional to the square root of both conductivity and frequency. As frequency increases, skin depth decreases, thereby amplifying the skin effect and indicating that surface conduction predominantly occurs at higher frequencies. This trend is evident in Fig. 9(a) and (b), where the data demonstrate that PPy/Fe-WS₂ nanocomposites show a reduction in skin depth and an increase in conductivity with increasing frequency. Moreover, it is crucial to note that skin depth is inversely related to absorption loss in shielding materials, meaning that a lower skin depth results in improved absorption efficiency.

Fig. 9 Variation of (a) skin depth , (b) conductivity [σ_ac = ωε₀ε′′] and (c) attenuation constant of the PPy/Fe-WS₂ nanocomposites with the frequency range (12.4–18 GHz).

As shown in Fig. 9(a), the skin depth index of the PPy/Fe-WS₂ nanocomposite remains relatively stable across the Ku band. This consistency indicates that the synthesized PPy/Fe-WS₂ nanocomposite demonstrates strong potential for electromagnetic interference shielding applications. The observed skin depth is primarily attributed to enhanced interfacial polarization effects, further underscoring its suitability for such applications. Meanwhile, the electrical conductivity is analyzed using the Davis–Mott framework,⁹ as described in the following equation:

Here, e denotes the electronic charge and N(E_C) presents the density of states at (E_C), while W, E_C, and E_f correspond to the activation energy, conduction band edge energy, and Fermi energy, respectively. Constants C and S and Boltzmann's constant (k) also factor into the equation.⁹ The initial term in the equation reflects the contribution of conductivity arising from electron movement within the PPy matrix, while the second term accounts for the electron hopping between the layers of Fe-WS₂ in the nanocomposite. This indicates that electron transport within the PPy/Fe-WS₂ nanocomposite occurs through two primary mechanisms: electron mobility within the PPy matrix and electron hopping across the Fe-doped WS₂ layers. Together, these mechanisms combine to govern the overall conductivity of the nanocomposite. This dual mode of electron movement comprising intralayer migration and interlayer hopping forms an interconnected conductive framework within the composite. The integrity of this conductive network is instrumental in enhancing the nanocomposite's electrical conductivity, which directly contributes to conduction losses and thus improves absorption efficiency, as shown in Fig. 9(b). To further evaluate the nanocomposite's ability to attenuate electromagnetic waves, the attenuation constant (α)² is studied, as defined using the following equation:where c represents the propagation speed of electromagnetic radiation in a vacuum and f stands for the frequency. A stronger absorption ability is associated with a higher α value. As depicted in Fig. 9(c), α increases with frequency, reaching its peak for the PPy/Fe-WS₂ (10 wt%) nanocomposite. This indicates that the PPy/Fe-WS₂ (10 wt%) composite possesses the highest absorption capacity, which aligns with its superior SE_A value, as discussed earlier. Fig. 10 illustrates the EMI shielding mechanism of the nanocomposite, showcasing its remarkable ability to attenuate electromagnetic waves predominantly through absorption rather than reflection. This phenomenon is attributed to the synergistic effects of magnetic and dielectric losses. The composite's internal voids act as EM wave traps, enabling multiple scattering events that boost shielding efficiency by converting EM energy into heat. The incorporation of Fe-WS₂ within the PPy matrix establishes conductive pathways through the interconnected carbon framework. This framework supports efficient electron migration and hopping, leading to EM energy dissipation and contributing to relaxation losses. The electronegativity difference between the nitrogen of PPy and tungsten of WS₂ forms heterogeneous interfaces, enhancing interfacial polarization due to permittivity variations. Distinct interfacial zones, such as PPy–PPy, Fe/WS₂–Fe/WS₂, and PPy–Fe/WS₂, generate interfacial charges, resulting in multiple dielectric relaxations.

Fig. 10 Schematic illustration of the mechanism of the PPy/Fe-WS₂ nanocomposite.

This behavior, corroborated by dielectric analysis and Cole–Cole plots, significantly improves microwave absorption by creating a micro-capacitive resistance network. This network, driven by the electrical disparities between PPy and Fe-WS₂ facilitates further EM wave dissipation through microcurrent generation. The nanocomposite's ability to combine interfacial polarization, relaxation losses, and conductivity arising from its hetero-structured design yields pronounced dielectric losses. These synergistic effects, coupled with the interconnected architecture, optimize the nanocomposite's skin depth and permittivity, thereby enhancing its EM wave absorption efficiency. Notably, the C₀ curve fluctuations signify anisotropic energy losses, including eddy current losses induced by Fe-WS₂ within the PPy matrix, which further amplify the shielding performance. Experimental evaluations reveal that the PPy/Fe-WS₂ (10 wt%) nanocomposite achieves maximum specific shielding effectiveness, primarily attributed to absorption processes. Table 2 presents a comparative analysis of the shielding efficiency of this nanocomposite with those of previously reported PPy-based materials, emphasizing its superior performance.

Table 2 Electromagnetic interference shielding effectiveness in various PPy-based materials

Sl no.	Materials	EMI SE (dB)	Frequency (GHz)	Reference
1	Fe3O4@PPy (with HCl) nanocomposite	33.8	5–12	44
2	PPy/graphene (0.2 wt%) nanocomposite	33	8.2–12.4	45
3	PPy/MnFe2O4 nanocomposite	21	8.2–12.4	46
4	PPy/BaFe12O19 composite	23–27	8.2–12.4	47
5	PPy/PF/Fe3O4 (20 wt%) foam	31	8.2–12.4	48
6	MXene/PPy/polyester composite	27.96	8.2–12.4	49
7	PPy/SiCnw/GA (43 wt%) composite	33	12.4–18	50
8	PPy/Fe-WS 2 (10 wt%) nanocomposite	36.12	12.4–18	Proposed work

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4. Conclusions

PPy/Fe-WS₂ nanocomposites were successfully synthesized via in situ chemical oxidative polymerization, exhibiting outstanding microwave shielding performance in the Ku-band (12.4–18 GHz) with approximately 99.7% attenuation of electromagnetic waves. Integration of an Fe-WS₂ dopant into a PPy matrix significantly enhanced the total shielding effectiveness, with absorption as the primary mechanism. The shielding effectiveness by absorption (SE_A) for the PPy/Fe-WS₂ (10 wt%) nanocomposite reached 28.80 dB, while reflection contributed 7.32 dB. The superior absorption capability was attributed to enhanced dielectric loss, facilitated by conducting pathways, interfacial polarization, and relaxation processes. The inclusion of Fe-WS₂ generated multiple interfaces, further amplifying dielectric loss and improving EM wave attenuation. Fe doping introduced magnetic domains and modified the edge states of the Fe-WS₂ sheets, enhancing magnetic permeability. Additionally, the nanocomposite exhibited increased AC conductivity, a high attenuation constant, and reduced skin depth, all of which contributed to improved microwave absorption and reduced reflection. The exceptional EMI shielding performance of PPy/Fe-WS₂ nanocomposites stems from the synergistic effects of PPy's conductivity and flexibility, charge mobility and dielectric loss of WS₂, and enhanced conductivity and magnetic properties of Fe. This combination optimizes attenuation pathways, improves impedance matching, and maximizes wave absorption, making these composites highly effective for EMI shielding. These findings underscore the potential of PPy/Fe-WS₂ nanocomposites as advanced microwave absorbers, suitable for applications in satellite communication and other technologies requiring superior electromagnetic wave attenuation.

Data availability

The data supporting this article have been included in the main text. Additional raw data are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors, Raeesah Islam and Anil Kumar, sincerely appreciate Dr Akash Katoch, Assistant Professor at the Centre for Nanoscience and Nanotechnology, Panjab University, Chandigarh, for his assistance with TEM characterization. They also extend their gratitude to Dr Sumit Bhardwaj, Associate Professor in the Department of Physics, Chandigarh University, for his support in XRD characterization. Additionally, they thank Dr Dwijendra Pratap Singh, Assistant Professor at the School of Physics & Materials Science (SPMS), Thapar Institute of Engineering & Technology, Patiala, for facilitating VSM characterization.

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