Synthesis, Characterization, and Comparative Study of Novel Ternary PANI/rGO-Cr2O3, PANI/rGO-NiO, PANI/NiO-Cr2O3 Nanocompsites for Dielectric and Thermal Application

INTRODUCTION
Advancements in materials science have greatly influenced the electrical and industrial properties of materials used in energy storage [1]. Due to superior electrochemical properties, low cost, environmentally friendly nature, high stability and simplicity of synthesis, polyaniline and its derivatives are crucial materials developed for energy storage applications [2, 3]. Nevertheless, this particular polymer exhibits subpar mechanical qualities and limited process ability [1, 4]. However, it is possible to overcome these limitations and enhance the electrical, mechanical, and thermal properties of conductive polymers by employing carbon nanostructures like graphene, graphene oxide (GO), carbon nanotubes, and metal oxides [5-8]. Recently, graphene has attracted a lot of attention because of its extraordinary mechanical, thermal, and electrical capabilities, which allow it to significantly improve the properties of polymer-based nanocomposites. The attributes stem from its honeycomb-like configuration, which enhances its distinctive properties. The properties arise from its honeycomb-like structure, characterized by the thickness of a single carbon atom and a high density of structural components [9, 10]. Graphene oxide is a complex hydrocarbon network with a multicyclic layer that is partially aromatic. Furthermore, it lies in the production of nano-electronic devices and sensors. Metal oxides nanoparticles demonstrate characteristics of semiconductors. The NiO, and Cr2O3 nanoparticle are solid powders that are insoluble in water (H2O) but can slowly dissolve in NH4Cl, dilute acid, and ammonia solution [11]. The current investigation involved the preparation of a ternary polymeric nanocomposite consisting of PANI/rGO-Cr2O3, PANI/rGO-NiO, and PANI/NiO-Cr2O3. The products were characterized using techniques such as FTIR, XRD, FESEM. Finally, the electrical characteristics and thermal conductivity of these materials were examined.

MATERIALS AND METHODS
Synthesis of graphene oxide
GO was produced by the modified Hummer technique. The graphite (Alpha, 95%) was mixed with concentrated H2SO4 (MERCK, 95%) and KMnO4 (Alpha, 95.5%) in a beaker. The mixture was stirred continuously for 20 min, and then heated to 98 οC. Non-ionic water was added, followed by H2O2 (HIMEDIA, 30%). The mixture turned yellow, washed with HCl (THOMAS BAKER, 35%) and non-ionic water, and dried at 70 οC for 4 h [12, 13]. 

Synthesis of reduced graphene oxide (rGO)
rGO was prepared by chemical reduction of an aqueous solution of GO in 250mL of deionized water using an ultrasonic device for 30 min. After that, 5mL of hydrazine hydrate was added to the flask and left for the sublimation process reverse reflux for 24 h at 90 οC. The mixture was filtered and the precipitate was washed with deionized water several times. Finally, it was dried for 24 h at 90 οC  [13, 14]. 

Synthesis of PANI
1.5 mL of purified aniline (LOBALA Chemie/India, 99%) was placed in a beaker with a capacity of 50 mL. The beaker was then placed in an ice bath at a temperature of 0°C for a duration of 10 min. Next, 20 mL of HCl 1 M was slowly added into above solution while continuously stirring for 20 min. This was followed by the addition of ammonium persulphate APS (MERCK, 99.5%) at 0°C. The solution was thereafter agitated for duration of 2 h in an ice bath, and subsequently stored overnight in the refrigerator. The precipitate underwent filtration and was thereafter washed 4 times with distilled water and ammonium hydroxide NH4OH (LOBALA Chemie/India, 25%). This was followed by further washing with distilled water until the pH reached neutrality. Ultimately, the solid residue was subjected to a drying process at a temperature of 80°C for a duration of 2 h [15-17].

Synthesis of extract
1 g of both extracts (ground ginger and ground cumin) were taken and each of them was placed in 50 mL of water on the thermal heater while continuously stirring for 15 min. Then, the temperature was raised for 1 h. The sediment was then separated from the solution.

Synthesis of NiO and Cr2O3
1 g of aqueous nickel nitrate and chromium (III) nitrate were separately dissolved in 25 mL of water. Then, 10 mL of extracts were added in a burette, followed by the addition of NH4OH until PH reached 10 for NiO and PH reached 8 for Cr2O3. Then the precipitate was washed 4 times, dried and then placed in an oven at 500°C for 3 h.

Synthesis of rGO-NiO, rGO-Cr2O3, Cr2O3-NiO
The rGO-NiO was synthesized by dispersing 1 g of rGO in 200 mL of water using an ultrasonic water bath at 25°C for 1 h, resulting in a solution of rGO. Subsequently, 0.7g of NiO was added into the rGO solution and stirred for 1 h at 25°C. The precipitate was separated using centrifugation. Finally, the precipitate was dried at 90°C for 4 h [18]. The similar conditions were employed for fabrication of rGO-Cr2O3 and Cr2O3-NiO.

Synthesis of PANI/rGO-Cr2O3, PANI/rGO-NiO, and PANI/NiO-Cr2O3
The ternary nanocomposite was synthesized by dispersing 0.5 g of the binary composite rGO-NiO, rGO-Cr2O3 and Cr2O3-NiO in 50 mL of non-ionic water using an ultrasonic bath at a temperature of 25°C for 1 h. Subsequently, the resulting mixture was combined with 1 g of PANI dissolved in 100 mL of 0.1M HCL. Then, a solution of an oxidizing agent by dissolving 6 g of APS in 100 mL of 0.1M HCI was fabricated. This solution was introduced into the main mixture while vigorously stirring for 12 h at 25°C. Next, the sediment was washed and then dried in the oven at 80°C for 4 h.

Synthesis of PANI/rGO-Cr2O3, PANI/rGO-NiO, and PANI/NiO-Cr2O3 nanocomposite membranes
The nanocomposite membranes were fabricated by incorporating diverse weight percentages of the PANI/rGO-Cr2O3, PANI/rGO-NiO, and PANI/NiO-Cr2O3 nanocomposite. A solution containing a specified amount of PVA in 20 mL of distilled water was combined with (2-10%) of ternary nanocomposite in 3 mL of distilled water. The combination was agitated at a temperature of 60 °C until a noticeable alteration occurred in the mixture. The sample was subjected to sonication (405 Power China) for 15 min at a temperature of 25 °C, followed by the process of casting onto a glass mold. 

Dielectric constant measurement
In order to determine the dielectric properties of the produced films, a circular piece with a diameter of 3 cm was cut from the film. This was done to match the diameter of the electrodes in the LCR meter (Impedance Analyze Agilent 50Hz-5mHz). Dielectric investigations were conducted at several frequencies ranging from 1 to 5 KHz at a temperature of 25 °C. the dielectric constant (ε′), dielectric loss factor (ε′′) were calculated according the Eq. 1 [13, 19]:

Where, ε0 is permittivity of the vacuum (F/m), which is equal to 8.85 x 10-12. d (m) denotes the distance between the two conducting plates. A (m2) is conductive plate’s surface area. C (F) denotes the capacity in the presence of a vacuum (Eq. 2).

Where tan δ is dissipation factor. The K value can be computed using the Eq. 3 

Where, TA, TB, TC indicates the temperature of disk A, B, and C, correspondingly. d (m) is disc’s thickness. r (m) is disk’s radius. I (A) show the current passed via heating coils. V (v) is the heating coil’s potential difference on both ends. 

RESULTS AND DISCUSSION
FTIR analysis
Fig. 1 presents the FTIR spectrum of rGO. The distinct bands observed at 3433.95, 1641.1, and 1115.19 cm−1 are associated with vibrational frequencies of the O–H, C=C, and C–O groups in the carbon-hydroxyl groups [20].
Fig. 2 shows the FTIR spectrum of PANI. The peak at 3434.13cm-1 is attributed to N-H stretching vibrations. The peaks at 1558.40cm-1 and 1475.47cm-1 are attributed to the stretching vibrations of quinoid and benzenoid ring of PANI, respectively. Moreover, the characteristic of N=Q=N and the stretching vibration of C=C were appeared at 1104.55cm-1 and 1293.88cm-1. Band at 797.17 cm-1 is attributed to C-H aromatic ring [21].
To understand the dominant functional group present in the catalyst, FTIR spectrum was recorded at room temperature. The FTIR spectrum of NiO is shown in Fig. 3. Peak around the 418.80 cm−1 is because of the Ni-O bond stretching vibrations. The absorption band at 1637.87 cm−1 is due to the symmetric and the asymmetric stretching mode of vibrations of the CO2 molecule absorbed from the air [22]. Also, the broad peak at 3437.01 cm−1 may be because of stretching and bending vibrations of –OH group absorbed on catalyst surface from the atmosphere when FTIR analysis was carried out [23].
FTIR spectrum of rGO-NiO nanocomposite is shown in Fig. 4. The main peaks detected at 3434.25, 1629.04, 1125.26, 423.02, and 630.89 cm− 1 are recognized to the O–H, C –C, C–OH, C–C, and C–O, respectively. Because of the partially reduction of oxygenated functional groups during the reaction, the intensities of these peaks reduced dramatically following the formation of NiO nanoparticles on rGO sheets. Indeed, after reducing GO with NH4OH, the peaks caused by O–H, C–O, and C–O stretching reduced considerably.
Fig. 5 illustrates the existence of numerous distinct bands for the hybrid composite, with absorption bands in the range of 3364.25cm-1 attributed to the NH2 group, and prominent bands in the range of 1508.15-1582.05cm-1 corresponding to the C=C group, associated with the extension of cyclic compounds and selective vibrations. The absorption band at 1218.89cm-1 corresponds to the C-N in the quinoid and benzoid groups, while the band at 1046.57cm-1 is associated with the alkoxy and epoxy groups. Furthermore, the band located at 422.12cm-1 corresponds to the Cr-O group. 
The FTIR spectrum of NiO-Cr2O3 nanocomposites is shown in Fig. 6. A broad band at 3420 cm-1 corresponds to the stretching modes of surface OH groups. Metal oxide generally reveals absorption bands below 1000 cm-1 due to inter-atomic vibrations. Two sharp peaks displayed at 652 and 562 cm-1 attributed to Cr-O stretching modes, which are clear evidence for the presence of the crystalline Cr2O3  [24]. Interestingly, it was observed  that intensity of the higher frequency decrease with enhancement in NiO concentration.

 
FESEM analysis
The FESEM image of rGO in Fig. 7 displayed some distortions on the layer’s surface. This is due to the reduction by hydrazine hydrate.
SEM micrograph of Cr2O3 is shown in Fig. 8. It can be seen that the sample is spherical and particles are agglomerated at the surface. Spherical shaped formations were aggregated in the form of clusters.
SEM investigation of PANI in Fig. 9 identified plate-like structures in the composition. The generated material displayed a porous structure [25]. 
According to the Fig. 10, NiO particles were distributed randomly throughout rGO sheet. On the curled morphology of the reduced graphene oxide, NiO particles are dispersed. SEM image show the NiO particles have nanosheet-based irregular structures. 
The FESEM micrograph of Cr2O3-NiO nanocomposites is shown in Fig. 11. FESEM analysis provides the information about the shape and size. The results of FESEM showed that the average diameter 30-50nm. It can also be seen that the shape of synthesized Cr2O3 nanoparticals were homogenous and spherical. 

XRD analysis
XRD analysis of rGO is depicted in Fig. 12. The distance between the interlayers was determined to be relatively small compared to graphene oxide, which can be attributed to the removal of a majority of the functional oxygen groups through the process of reduction  [26].
Fig. 13 illustrates XRD pattern of synthesized Cr2O3  nanoparticles. The diffraction peaks obtained at 2θ =24.5, 33.6, 36.2, 39.7, 41.5, 44.2, 50.2, 54.8, 58.3, 63.4, 65, 72.9, 76.7, and 79.1 degrees, can be related to the Cr2O3 standard JCPDS card No. 201103. The standard JCPDS card shows that the alignment of the diffraction peaks perfectly matches the rhombohedral structure of Cr2O3 nanoparticles.  The results showed that the synthesized material is completely crystalline [27, 28]. The crystallite size of the Cr2O3 was calculated from the Debye-Scherrer formula, and the value is 28 nm.  

Dielectric properties
The dielectric properties of the pure PVA polymer film and its hybrid nanocomposite films were determined at room temperature. The results in Figs. 14-16 illustrates that the real permittivity (ε′), imaginary permittivity (ε′′), and loss factor (tanδ=ε′′/ε′ ) are used as the amount of energy lost or dissipated for any dielectric material and alternating conductivity (σa.c). From Table 1, it can be notice both ε′ and ε′′ for all synthesized films were gradually decreased as the frequency increases  [29]. Electronic polarization exhibits a brief duration, yet it surpasses ionic polarization in terms of length. On the other hand, dipolar polarization necessitates a comparatively longer time when compared to the other forms of polarization. Thus, the dielectric constant of nonpolar polymers remains rather stable at high frequencies. Consequently, the values of ε′ exhibit a substantial and rapid decline as the frequency increases in the low-frequency areas [30]. While frequency ratio of PVA (1MHz) was found to be the dielectric constant’s greatest value, while PANI/rGO-NiO was low (5MHz). All of these dielectric constant values exceed the ε′ value for pure PVA. Typically, the rise in the dielectric constant is ascribed to the augmentation in polarity and the growth in charge carriers. Fig. 14 demonstrates a clear inverse relationship between the dielectric loss factor (ε′′) and frequency for all samples. Additionally, we see that the dielectric loss is elevated at lower frequencies and then diminishes as the applied frequency increases, owing to the reduction in space charge polarization. Fig. 15 illustrates that the dielectric loss factor diminishes as frequency increases for synthesized hybrid compounds. This drop is commonly attributed to increasing the concentration of conductive fillers (nanoparticles) which leads to the development of more conductive routes and hence leads to increased current leakage and loss of electrical insulation [21]. Calculations were made to determine the AC electrical conductivity of the pure PVA polymer sheet and all generated hybrid composites. We can see from Fig. 16 that all prepared hybrid composites have an enhancement in isotropic electrical conductivity when nanoparticle is added. This show that the structure contains various conduction mechanisms, which is the (π-π*) interaction between the nanoparticle surface and the quinoid ring of the polymer chain (PANI). This interaction involves electrons passing through the (SP2-bond), which leads to coupling and electrical conduction. Tables 1-3 lists the values of the real (ε′), imaginary (ε′′) dielectric constant, and loss factor (tanδ) for the nano hybrid composite.

Thermal conductivity coefficient
Fig. 17 displays the heat conductivity coefficient of the pure PVA polymer membrane and the nano hybrid composite membranes. By employing nano metal oxides, the thermal conductivity coefficient (k) increases as the reinforcement ratio increases (Table 4). The heat absorption affect the material’s ability to conduct heat, thereby increasing its thermal conductivity over time. The elevated temperature of these granules induces their vibration as the temperature increases. The vibration facilitates heat flow across the composite material, enhancing its thermal conductivity. As the proportions of metal oxide, GO, and rGO in the composite material increase, their thermal absorption also escalates, thereby enhancing conductivity. The incorporation of nano oxides may enhance the polymer’s crystallization rate, leading to a superior thermal conductivity coefficient compared to a pure PVA polymer film [31]. 

CONCLUSION 
In this work, a ternary polymeric nanocomposite consisting of PANI/rGO-Cr2O3, PANI/rGO-NiO, and PANI/NiO-Cr2O3 were prepared. The products were characterized using techniques such as FTIR, XRD, FESEM. Finally, the electrical characteristics and thermal conductivity of these materials were examined.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.