Fabrication P-N Heterostructure Of Polyaniline – Polyvinyl Alcohol – Tio2 Nanocomposite for Effective Electromagnetic Shielding Application

The demand for advanced materials in electronics and communication systems continues to grow, making the need for effective electromagnetic interference (EMI) shielding solutions paramount. Among the various materials investigated for this purpose, conducting polymers have garnered significant interest due to their lightweight nature, flexibility, and tunable electrical properties [1]. Electromagnetic interference (EMI) can negatively impact the performance and reliability of electronic circuits, resulting in operational failures and signal degradation. As a result, there is a high demand for effective shielding materials that can mitigate EMI [2]. Among various conductive polymers, polyaniline (PANI) is particularly notable due to its tunable conductivity, environmental friendliness, and ease of synthesis. However, its effectiveness as an EMI shielding material can be significantly enhanced when combined with high-conductivity materials such as carbon nanotubes (CNTs), graphene, and metal oxide nanoparticles. The development of PANI-based composites, especially the PVA-ZnO blend, offers a promising approach to improving electromagnetic interference (EMI) shielding [3]. Polyaniline's intrinsic electrical conductivity and environmental stability make it a prime candidate for electromagnetic interference (EMI) applications. When combined with polyvinyl alcohol (PVA) and zinc oxide (ZnO), the resulting composite can potentially harness the unique properties of each component to enhance shielding effectiveness and improve mechanical properties [4]. Research indicates that polyaniline composites exhibit improved EMI shielding when combined with dielectric materials such as PVA, which enhances the dielectric properties crucial for attenuating electromagnetic waves. One of the primary considerations for any EMI shielding material is its effectiveness in reducing electromagnetic wave transmission [5]. The incorporation of zinc oxide (ZnO) not only enhances the conductivity of the composite but also improves its capacitive and inductive properties, resulting in superior electromagnetic interference (EMI) shielding efficiency. The mechanism of EMI shielding in these composites typically involves the absorption, reflection, and multiple scattering of electromagnetic waves, with enhanced absorption attributed to the conductive nature of polyaniline (PANI) and the dielectric loss associated with titanium dioxide (TiO₂). In addition to EMI shielding performance, the mechanical and thermal properties of the PANI-PVA-TiO₂ nanocomposite are of significant interest. The blend of polyvinyl alcohol (PVA), known for its excellent film-forming capabilities and flexibility, provides good mechanical stability to the composite, making it suitable for practical applications [6 -8].  Kanavi et al., [9] reported the in-situ polymerization of polyvinyl alcohol-polyaniline composite films with varying concentrations of zinc oxide (ZnO) (0.2%, 0.4%, 0.6%, and 1%), which were then cast and dried. Several methods were used to characterize the ZnO and PPZ film samples. It was discovered that the AC conductivity of PPZ1 was 20.06 S/m at 150 °C and 1% ZnO content. The PPZ films' usefulness is demonstrated by such conductivity behaviour samples. Moreover, the thermal stability of the composite tends to improve with the inclusion of ZnO, which aids in heat dissipation, an essential feature for materials used in electronics exposed to heat. Shakir et al., [10] reported the electromagnetic interference (EMI) shielding properties of PS/PANI blend films in the microwave and near-infrared (NIR) spectrums were examined. A shielding effectiveness of 45 dB was found in the frequency range of 9 GHz to 18 GHz. The NIR analysis of CPBs revealed a transmittance of less than 1%. In the presence of a homogeneous magnetic field of 0.5 T, Ge et al. created a hollow spherical nanostructure of PANI/CoS/CDs; the nanostructure under a 0.5 T magnetic field shown improved results [11]. The core-shell structural MnFe₃O₄@SiO₂ nanocomposite was developed by Zhu et al. [12] and in order to create a ternary composite and analyze the microwave absorption, they combined them with polyaniline and polyvinylidene fluoride. They explained that the dielectric and magnetic loss are significantly impacted by the ternary compound of MnFe₃O₄@SiO₂ with PANI and PVDF. Luo et al. [13] investigated the microwave absorption characteristics of polyaniline (PANI) and erbium (Er) doped strontium ferrites. A polyurethane (PU)-PANI composite sheet, measuring 1.9 mm in thickness, exhibits an average shielding effectiveness (SE) of approximately 10 dB in the X-band, achieving a maximum electromagnetic interference (EMI) SE of about 26.7 dB at 8.8 GHz. According to Saini et al. [14], the PANI-multiwall CNT dispersed polystyrene (30 weight percent) composite, with a thickness of 1 mm, exhibits an electromagnetic interference shielding effectiveness (EMI SE) of approximately 24 dB in the Ku-band (12.4–18 GHz). Furthermore, it was reported that an EMI SE of 40 to 45 dB could be attained in the same frequency band by increasing the thickness to 2 mm. Hong et al. [15] investigated the electromagnetic interference (EMI) shielding effectiveness (SE) of polyaniline emeraldine salt (PANI ES) films over a wide frequency range of 50 MHz to 13.5 GHz. They observed that the PANI ES film, with a thickness of 90 µm, exhibits an EMI SE of approximately 18 dB, which includes an EMI SE from absorption (SEA) of about 12 dB and an EMI SE from reflection (SER) of around 6 dB [16-18].  In the present study, we have reported the preparation of PANI – PVA – TiO₂ nanocomposite by solvent casting method. The prepared nanocomposite employed for FTIR and SEM analysis.

MATERIALS AND METHOD

All the chemicals used for the preparation of PANI – PVA – TiO₂ nanocomposite are analytical grade. The aniline monomer (C₆H₅NH₂) with mol.wt 93.13 and 99.5 % purity, ammonium persulfate (APS) with 99.9 % purity, titanium nitrate (Ti(NO₃)₄) with 99 % purity, and 99 % purity isopropyl alcohol (C₃H₈O), sodium dodecylbenzene sulfonate (SDBS) with  99 % purity, and hydrochloric acid (HCl) were procured from Merck. All the obtained chemicals were used directly in experiments except aniline, which was first distilled, and then it was put in a cool and dark place before usage.

2.1 Synthesis of Titanium dioxide

Titanium dioxide nanoparticles were synthesized through aqueous sol–gel route in acidic medium. 14.64 g of titanium nitrate (Ti(NO₃)₄) were added to a mixture of solvents i.e. isopropyl alcohol and water in 1:4 ratio. 10 mL of glycerin were added and pH of the aqueous solution was maintained at 1 using nitric acid. The mixture was stirred constantly for two hours at 70 ?. The sol formed was also dried at 70 ? for 24 h. The gel thus obtained was ground and calcined for 2 hrs at 500 ? to obtain titanium dioxide which is further washed with distilled water followed by acetone several times until get colourless aliquant [19, 20].

2.2 Synthesis of Polyaniline (PANI)

Synthesis 0f PANI was carried out by in-situ chemical oxidation polymerization Technique. Aniline (0.1M) was mixed in 1M hydrochloric acid with 1M sodium dodecylbenzene sulfonate (SDBS) and stirred for 15 to 20 min to form aniline hydrochloride. To this solution, add 0.1M of ammonium persulphate, which acts as an oxidizer was slowly added drop-wise with continuous stirring at 0-5⁰C for 4 hrs to get polymerized. The precipitate was filtered, washed with distilled water, acetone and finally dried in an oven at 60⁰C for 24 hrs to achieve a constant mass. In this way, polyaniline (PANI) is synthesized [21].

2.3 Synthesis of Polyaniline – Polyvinyl alcohol (PVA) – TiO₂ nanocomposite

Nanocomposite were prepared by doping 3 wt % of TiO₂ in PANI via in-situ polymerization method [22]. The prepared nanocomposite blend was prepared by 0.3 wt % of polyvinyl alcohol (PVA) is mixed with PANI by mechanical grinding in acetone for 1 hr. 1.82 gm of PANI – PVA – TiO₂ nanocomposite is dissolve in N-Methyl-2-pyrrolidone (NMP) stirred for 6 hrs and poured onto the glass plate to form a thin film [23].  

3. Characterization

A Nicolet 750 spectrometer with a wavenumber range of 4000-400 cm^-1 was utilized for Fourier Transform Infrared Spectroscopy (FTIR) measurements. For the examination of morphology, a scanning electron microscope (SEM) with specifications - JEOL JAPAN, model JSM-6610 LV, operated in high vacuum mode at 20 kV, with adjustable voltage (1-30 kV), magnification (× 5 to × 300,000), and resolution (3 nm) was utilised. The SEM images of the powder of the fabricated composites of PANI and PANI-PVA-TiO₂ nanocomposite were captured at different resolutions. A two-port vector network analyzer (VNA) (Make- Agilent N5230C, 10 MHz – 40GHz PNA-L) was employed to study the dielectric properties and shielding effectiveness of the composites. Using the imaginary part of complex permittivity, the AC conductivity (σ_ac = 2πfε_o ε”) of composites was calculated.

RESULTS AND DISCUSSION

4.1 Fourier Transform Infrared Spectroscopy

Figure 1 (a – d) shows the FTIR spectra of TiO₂, PVA, PANI and PANI – PVA – TiO₂ nanocomposite. Figure 1 (a) curve indicates that the characteristics peaks at 409 cm^-1 is corresponds to Ti – O bending out of the plane, 483 cm^-1 Ti – O stretching vibration in symmetrical plane, 612 cm^-1 for Ti – O – Ti stretching vibration, and 3364 cm^-1 for bending of hydroxyl (- OH) group of moisture molecules [24]. Figure 1 (b) curve shows the characteristics peaks of PVA at 1093 cm^-1 is due to the C – O stretching of vinyl group, 1419 cm^-1 for C – H bending of ether, 2862 cm^-1 for C – H stretching in symmetry and 3225 cm^-1 for hydroxyl (- OH) group of moisture molecules. Figure 1 (c) curve the FTIR spectra of PANI, which indicates the characteristic peaks at 834 cm^-1 for aromatic C –H vibration in-plan, 1061 cm^-1 for C – N stretching of aromatic ring, 1150 cm^-1 for B–(NH⁺) = Q structure stretching, 1301 cm^-1 is due to the long chain of aromatic ring used to determine the delocalization of charge carriers, 1454 cm^-1 is attributed to the -C=C benzenoid ring stretching vibration, 1581 cm^-1 for  N=Q=N quinoid ring stretching vibration, and 3820 cm^-1 for hydroxyl ion H(–OH) of water molecules [25]. Figure 1 (d) curve shows the PANI – PVA – TiO₂ nanocomposite which indicates the following peaks at 409 cm^-1, 483 cm^-1, 612 cm^-1, 913 cm^-1, 1093 cm^-1, 1180 cm^-1, 1301 cm^-1, 1419 cm^-1, 1496 cm^-1, 1636 cm^-1, 2862 cm^-1 and 3395 cm^-1. It is observed that the PVA and TiO₂ peaks appeared in PANI nanocomposite which confirms the formation of PANI – PVA – TiO₂ nanocomposite [26].

Figure 1 shows the FTIR spectra of TiO2, PVA, PANI and PANI – PVA – TiO2 nanocomposite

4.2 Scanning Electron Microscopy

Surface morphology plays a critical role in the effectiveness of electromagnetic shielding materials. The surface texture can influence how electromagnetic waves are reflected or absorbed. A rough surface may scatter incoming waves, leading to greater absorption, while smooth surfaces might reflect them more effectively [27]. The interaction of electromagnetic waves with the material also depends on the surface morphology. For certain frequencies, the skin effect causes electromagnetic fields to penetrate only a limited depth. The morphology can alter how deep the waves penetrate and impact the overall shielding effectiveness. The quality of the interface between polyaniline- PVA thin films and TiO₂ nanoparticles is affected by surface morphology. A well-prepared surface can lead to better adhesion and improved mechanical and electrical properties, enhancing shielding performance [28].

Figure 2 shows the SEM image of TiO2, PANI and PANI-PVA-TiO2 nanocomposite

In PANI-PVA- TiO₂ nanocomposite, the size and distribution of TiO₂ nanoparticles or fillers can affect the overall shield's effectiveness. Optimizing these features through controlled surface morphology can enhance conductivity and thus improve shielding performance. The presence of roughness or features on the surface can provide additional scattering mechanisms for electromagnetic waves, further contributing to the shielding effectiveness [29]. Figure 2 shows the SEM image of (a) TiO₂ (b) PANI and (c) PANI-PVA- TiO₂ nanocomposite. It is observed that the TiO₂ nanoparticles were prepared sol – gel method is spherical shape, indusial particle with varying average particles size from 143 nm to 242 nm. The significant role that particle size plays in optimizing microwave absorption properties, which are inherently linked to electromagnetic shielding capabilities [30]. The challenges faced in developing lightweight and efficient microwave absorbers underscore the necessity for fine-tuning particle dimensions. It is expected that the homogeneous and identical particle size may help in the enhancement of microwave absorption properties, indicating that optimizing particle size can enhance the overall performance of composites used for EMI shielding [31]. The variations in particle size significantly influence the dielectric constant and initial permeability of composites, which are essential parameters for effective electromagnetic interference (EMI) shielding. PANI fibers observed in figure 2 (b) indicates that the polyaniline is irregular, agglomerated fibers which has granular averages size is about 480 nm. However, when the film was prepared in 0.3 wt % of PVA in PANI-TiO2 nanocomposite forms a smooth surface without any cracks or agglomerated hips on nanocomposite as shown in figure 2 (c) [32]. 

5. DC Conductivity

This paper explores the fundamental principles governing the DC conductivity in polymer semiconductors, discussing the mechanisms of charge transport, the factors influencing conductivity, and the common measurement techniques. The impact of molecular structure, processing conditions, and doping on the electrical behaviour of these materials will be highlighted. DC conductivity elucidates a material's ability to conduct a direct electrical current [34]. It essentially takes the ratio of the current density (how much electric current flows per unit cross-sectional area) to the electric field applied to a material. This ratio defines the ability of a material to transport this current. This 'ability', denoted by s_dc and expressed as s_dc = J/E, essentially represents DC conductivity. DC conductivity revolves around determining how effectively an object conducts direct electric current [35]. However, numerous factors affect this conductivity. The DC conductivity of a polyaniline (PANI)–TiO? nanocomposite depends on various factors, including the composition, morphology, doping level, and method of synthesis of the composite. DC conductivity varies with temperature, mainly owing to changes in free charge mobility. For instance, as temperature increases, the free electrons in a conductor gain more kinetic energy and “jump” more frequently, which can affect the conductivity. Another complexity is that DC conductivity is intrinsically linked to the specific material. Different materials exhibit starkly different conductivity depending on their atomic structure and the availability of free charge carriers. Metals typically possess a high number of free charges, translating to high conductivity. Conversely, insulators have a low number of free charges and thus, exhibit minimal conductivity [36].

Figure 3 shows the ?dc conductivity of PVA, PANI and PANI-PVA-TiO2 nanocomposite as a function of temperature

Figure 3 illustrates the DC conductivity of PVA, PANI, and PANI-PVA-TiO? nanocomposites, which typically exhibit thermally activated behavior. The trend observed in PANI and its nanocomposites aligns with Mott’s variable-range hopping (VRH) model, which depends on temperature and the composition of TiO?. The ratio of PANI to TiO? significantly influences the overall DC conductivity. An excess of TiO? may reduce conductivity due to its insulating properties [37]. The figure indicates that DC conductivity increases with rising temperature, ranging from 25 to 140 °C. It is observed that conductivity increases in three distinct stages. In the first stage, the conductivity of PANI and its nanocomposites is nearly linear, likely due to insufficient activation energy for polarons and bipolarons to hop from low-energy states to higher energy levels. In the second stage, it is observed that conductivity gradually increases due to the motion of polyaniline fibers and the hopping of charge carriers, both of which significantly affect band energy [38]. Additionally, the uniform dispersion of TiO? nanoparticles within the PANI matrix ensures better interaction and can enhance the electronic transport network. In the third stage, it is noted that conductivity increases exponentially, likely due to high activation energy. However, morphology also plays a vital role, as we can observe that the absence of cracks in the films facilitates a smooth and steady flow of charge carriers in the polyaniline nanocomposite [39].

6. Dielectric property and EMI sheilding

The dielectric permittivity of polyaniline (PANI) is a crucial property that indicates its capacity to store electrical energy in the presence of an electric field. This property is influenced by various factors, including the doping level, frequency, morphology, and environmental conditions. Doping increases the charge carrier density in PANI, which enhances polarization and, consequently, the dielectric permittivity [40]. The dielectric permittivity of PANI is frequency-dependent. At low frequencies, multiple polarization mechanisms—such as electronic, ionic, dipolar, and interfacial (Maxwell-Wagner)—contribute to a high dielectric permittivity. Conversely, at high frequencies, only electronic polarization remains, leading to a decrease in dielectric permittivity [41]. Dielectric properties can vary with temperature; typically, an increase in temperature results in a decrease in permittivity due to enhanced thermal motion that disrupts dipole alignments. Polyaniline (PANI) is frequently combined with inorganic fillers, such as titanium dioxide (TiO?), barium titanate (BaTiO?), and silicon dioxide (SiO?), to improve dielectric properties. In these composites, interfacial polarization at the polymer-filler boundary can significantly enhance dielectric permittivity [42].

Figure 4 shows the variation of dielectric perimittivity as function of applied frequency

The real part of permittivity (ε?) as a function of X-band frequency is illustrated in Figure 4. The graphs for PVA, PANi, and PANI–TiO₂ nanocomposites indicate that the permittivity values decrease with increasing frequency. In these composites, strong polarization occurs due to the presence of polarons, bipolarons, and other bound charges, resulting in high ε? values at lower frequencies [43]. As the frequency increases, the dipoles within the system are unable to reorient themselves in alignment with the applied electric field, leading to a decrease in the dielectric constant. Additionally, the effective anisotropy of oxide particles, specifically the shape anisotropy and rotation of domains, may become more challenging [44]. Furthermore, the particle size of the composites is in the nanometer range in one dimension. The surface area, number of dangling bond atoms, and unsaturated coordination on the surface are all enhanced. These variations contribute to interface polarization and multiple scattering, which are beneficial for the absorption of a large number of microwaves. Among PVA and pure PANI, the PANI–PVA–TiO₂ nanocomposites exhibit the lowest real permittivity values, specifically 489 at 8.5 GHz [45]. The behaviour of the loss factor (tan δ) for PVA, PANI, and PANI–PVA–TiO₂ nanocomposites in the frequency range of 8.0–12.0 GHz is illustrated in Fig. 5. The loss factor value of the PANI–PVA–TiO₂ nanocomposites decreases with increasing frequency. The energy loss in the dielectric material is attributed to the viscous forces acting on the movement of dipoles. The lead titanate particles interact with the one-dimensional PANI fibers, and the motion of the dipoles becomes restricted as the doping levels in PANI increase. Consequently, a decrease in dielectric loss is anticipated due to the interaction between TiO₂ and the PANI fibers, as observed in the current experiment. At a concentration of 3 wt% TiO₂ in the PANI/PVA matrix, a low tangent loss value of 0.34 is recorded, which decreases to 0.084 with increasing frequency [46].

Figure 5 shows the tangent losses as a function of applied frequency

Figure 6 illustrates the frequency dependence of the reflection loss (RL) of the PANI-PVA-TiO₂ nanocomposites as the weight percentage of lead titanate in the PANI matrix increases, within the frequency range of 8–12 GHz. Among all the composites, the 3 wt% PANI-PVA-TiO₂ nanocomposite exhibits the highest microwave absorption, registering below 10 dB, along with broad absorption peaks across the frequency range of 8 to 12 GHz, followed by the other composites. Notably, the maximum RL peak for the 3 wt% TiO₂ in the PANI-PVA nanocomposites is observed at 16.4 dB at 57.1 GHz, which may be attributed to minimal reflection loss of microwave energy [47]. The percentage of dopant in the polymer matrix is a crucial parameter that influences both the intensity and the frequency position at which the maximum RL occurs. Moreover, PANI-PVA-TiO₂ nanocomposites with a composition of 3 wt% exhibit a very smooth surface morphology, as revealed by the SEM images. TiO₂ is a non-magnetic material; however, it enhances the real part of both permittivity and permeability of the composites. As a result, the gap between the two particles in the conducting matrix increases, which can reduce eddy current loss by increasing electrical resistivity. This enhancement is a significant factor contributing to the excellent microwave absorption properties of the material [48].

Figure 6 shows the reflection coefficient as a function of applied frequency

The variation in the absorption coefficient of PANI-PVA-TiO₂ nanocomposites at different weight percentages, measured at X-band frequency, is illustrated in Figure 7. The absorption coefficient increases with the addition of 3 wt% TiO₂ in PANI. The behaviour of electromagnetic absorption is critically dependent on the dielectric properties of the materials, which are represented by relative permittivity and permeability [49]. PANI-PVA-TiO₂ nanocomposites consist of non-magnetic, particle-doped organic compounds, without the inclusion of magnetic fillers such as ferrite or carbonyl iron. Consequently, the microwave absorption of these composites is primarily attributed to their dielectric constant. Among the composites, the PANI-PVA-TiO₂ nanocomposite with 3 wt% TiO₂ exhibits the highest absorption coefficient value of 67.4, surpassing that of the other composites. The low dielectric properties of pure PANI and PVA may be attributed to the Maxwell–Wagner–Sillars (MWS) effect. The energy loss, manifested as heat, occurs due to the electric dipoles of the PANI-PVA-TiO₂ nanocomposite oscillating in response to an applied microwave frequency. This phenomenon can be considered one of the significant absorption-dominated characteristics of the material. Dielectric and non-magnetic losses often arise from the enhanced electrical conductivity of PANI nanocomposites, while while additional losses could may generated as due to the increased permeability the PANI-PVA-TiO₂ nanocomposite [50].

Figure 7 shows the absorption coefficient as a function of applied frequency

Electromagnetic interference (EMI) shielding is defined as the attenuation caused by irradiated microwaves through various processes, including absorption, reflection, and transmission. When electromagnetic (EM) waves with incident power (Pt) penetrate a shielding device, a small fraction of these waves is converted into reflected power (Pr), absorbed power (Pa), and transmitted power (Ptr). The findings indicate that a composite containing 3% nanoparticles and 2% microparticles achieved superior linear attenuation coefficients across gamma-ray energies compared to composites composed solely of microparticles [51]. This disparity highlights how nano-structuring materials can lead to improved shielding outcomes, thereby reinforcing the idea that finer particles may provide enhanced protective characteristics against various forms of radiation. The electromagnetic interference (EMI) shielding properties of the device are associated with two key physical processes: absorption and reflection mechanisms [52]. When electromagnetic waves interact with the surface of the shielding material, a portion of these waves is reflected due to the interaction of charge carriers, while another portion penetrates the material and dissipates energy through absorption. The interaction process of higher-power electromagnetic (EM) waves with the shielding material can be represented by the following formula [53];

         (P_t) = (P_r) + (P_a) + (P_tr)

The term electromagnetic shielding effectiveness (EMI-SE) refers to the attenuation of electromagnetic waves generated inside the shielding material. A formula could be used to represent the EMI-SE parameter is [54];

          SE (dB) = − 10 log (P_in?P_tr)

In the formula, Ptr is the transmitted electromagnetic power in order, and Pin is the irradiated electromagnetic power. The formula is used to determine the overall EMI-SE is given below;

             SE_T = SE_R + SE_A + SE_M

Thus, in the case of shielding materials, the reflection (SE_R), absorption (SE_A), and multiple reflection (SE_M) inside the shielding materials might be added to determine the total shielding effectiveness (SE_T) [55].

Figure 8 shows the EMI shielding effect as a function of applied frequency