Enhanced dielectric properties of green synthesized Nickel Sulphide (NiS) nanoparticles integrated polyvinylalcohol nanocomposites

A green synthesis approach has been adopted to prepare nickel sulphide nanoparticles (NiS NPs) using banana peel extract (BPE) as a reducing and capping agent. Polyvinyl alcohol (PVA)/NiS nanocomposite films were fabricated using a cost-effective solution casting technique by dispersing different contents of NiS NPs (0–3 wt%) in the PVA matrix. Various characterization techniques were employed to analyze the structural, thermal and morphological properties of the PVA/NiS nanocomposite films. Further, the dielectric behaviour of these nanocomposite films was investigated at frequency range 50 Hz–20 MHz and in the temperature range 40 °C–140 °C. Also, there exists a significant interaction between the polymer matrix and the nanofiller as evident from the notable improvement in the dielectric properties of the nanocomposites. The dielectric constant (ε) value of PVA/NiS nanocomposite film with 3 wt % NiS NPs loading was found to be 154.55 at 50 Hz and at 140 °C which is 22 times greater than the dielectric constant value of neat PVA (6.90). These results suggest that NiS NPs were dispersed homogeneously in the PVA matrix.


Introduction
In recent times, tremendous research interest has been devoted to the development of novel polymer nanocomposites (PNCs) because of their enhanced properties [1][2][3]. The PNCs reinforced with inorganic nanoparticles (NPs) have potential applications such as optical power limiters, membranes for gas separation, flexible light emitters etc [4,5]. Polymer/filler interface, the geometry of the dispersed phase (orientation, size, shape etc), their relative contents, and also volume fraction can alter or modify the final properties of PNCs [6]. The unavailability of low-cost techniques to control the distribution of NPs into the host polymer matrix is the main obstruction to the large-scale fabrication of PNCs [7]. For the PNCs, the major limitation in the application side is that the aggregation of NPs or their distribution in the polymer matrix. So, there is a necessity that the NPs have to be dispersed homogeneously within the polymer matrix because it causes an improvement in the chemical and physical properties of the PNCs [8,9]. Therefore, to control the functioning of PNCs, two important parameters are required i.e., the spatial distribution of NPs and their interactions with the host polymer matrix [10][11][12]. For the past few years, it has been considered that the improved dielectric properties of the PNCs loaded with inorganic NPs is believed to be a strong candidate for energy storage applications [13,14]. Some of the reports revealed that the dielectric properties of PNCs filled with the semiconductor NPs were enhanced when compared with pure polymers [15,16].

Preparation of BPE
First, banana peels were cleaned using deionized (DI) water to take away unwanted organic impurities and dust present in it. Subsequently, these peels were dried on paper towel. Then, appropriate quantity of peel was heated at 90°C in a beaker containing DI water and subsequently filtered. This obtained filtrate was utilized as reducing or capping agent to synthesize NiS NPs.

Synthesis of NiS NPs
Green synthesis approach was employed to prepare the NiS NPs. Ni(NO 3 ) 2 · 6H 2 O was used as nickel ions source and Na 2 S was used for obtaining the sulphide ions. 1M Ni(NO 3 ) 2 · 6H 2 O and 1M Na 2 S were dissolved in DI water separately and stirred using magnetic stirrer for one hour at room temperature (RT) to obtain clear solution. Subsequently, both the solutions were mixed and stirred for additional one hour to get homogeneous solution. In this solution, an appropriate quantity of BPE was added and stirred for 6 h at 70°C. After that as obtained precipitate was washed several times using DI water and ethanol to eliminate the unwanted impurities or residues. The final material was dried in hot air oven at 95°C for 24 h and subsequently in muffle furnace at 300°C for 2 h [40]. Finally, the black powder was obtained which was grinded using mortar and pestle and utilised for the synthesis of PVA/NiS nanocomposites. Figure 1 schematically represents the preparation procedure of PVA/NiS nanocomposite films using solution casting technique. The desired amount of PVA solution was obtained by dissolving PVA powder in DI water by heating at 65°C in hot air oven for 3 h. The NiS NPs were dispersed in DMF, sonicated for two hours and then mixed with PVA solution. Further, PVA/NiS mixture was constantly stirred for 9 h until the uniform distribution of NiS NPs in PVA is ensured. This resultant dispersion was poured onto Petri dish, afterwards kept at 70°C in hot air oven for 5 h to obtain nanocomposite films. These PVA/NiS nanocomposite films having 70-80 μm thickness was peeled off from Petri dish and used for further characterizations.
X-ray diffraction experiments were performed usingan x-ray diffractometer (Germany, Bruker, Advanced D8) with a wavelength of 1.54 Å and 1°min −1 scanning speed using Cu Kα radiation. The recorded data was obtained in 2θ ranging from 10-90°.
Thermogravimetric analysis (TGA) of the prepared samples was performed from RT to 800°C using TGA thermal analyzer (USA, TA Instruments, Q500 model) and differential scanning calorimeter (DSC) (TA Instruments, Q200 model) from RT to 400°C at the heating rate of 10°C min −1 in a N 2 atmosphere.
Crossed polarizing optical microscope (POM) (Singapore, BX-53, Olympus) with a magnification 10X was employed to assess the dispersion state of NiS NPs in the PVA matrix.
Scanning electron microscopy (SEM) (UK, Carl Zeiss EVO/18SH) was used to investigate the surface morphology of the samples. An accelerating voltage of 15 kV was applied for obtaining SEM images. Energy dispersive analysis of x-ray diffraction (EDAX) was coupled with the SEM to analyze the chemical compositions present in the synthesized NiS NPs.
Precision impedance analyzer (UK, Chichester, West Sussex, Wayne Kerr 6500B) was used to test the dielectric behaviour of PVA and PVA/NiS nanocomposite films in the frequency range 50 Hz-20 MHz and the temperature range 40°C-140°C.

Results and discussions
3.1. FTIR study Figure 2(a) displays the FTIR spectrum of NiS NPs. The bands appeared at 3421 and 1631 cm −1 are assigned to stretching and bending vibrations respectively of hydroxyl groups present on the surface of the sample [41]. The peak at 623 cm −1 could be assigned to bending vibration mode in Ni-S-Ni [42]. Figure 2 demonstrates the FTIR spectra of PVA and PVA/NiS nanocomposite films. The pure PVA (figure 2(b) (i)) depicts absorption and strong band at 3306 cm −1 assigned to the vibration of -OH group (stretching) [43]. The bands observed at 2916 and 1726 cm −1 may be attributed to C-H asymmetric stretching vibration (alkyl groups) and of C=O (vinyl acetate group) stretching vibration respectively [44]. The CH 2 bending vibration in the PVA backbone chain is represented by the band that appeared at 1431 cm −1 [45]. The CH 2 and C-H wagging vibrations are represented by the bands at 1363 and 1235 cm −1 respectively [17]. The bands at 1083 and 838 cm −1 are attributed to the stretching vibration of C-O in the backbone of PVA (acetyl group) and skeletal vibration in PVA respectively [46,47]. Moreover, almost the same peaks were noticed in the FTIR spectra of PVA/NiS nanocomposites (figure 2(b) (ii)-(vii)) along with some additional peaks below 660 cm −1 which may be attributed to bending vibration of Ni-S-Ni [48]. However, a small shift in the peak position was noticed in the FTIR spectra of PVA/NiS nanocomposites representing good compatibility between the NiS and hydroxyl groups of PVA. Figure 3(a) depicts XRD pattern of NiS NPs indicating major diffraction peaks at 2θ=30.25°, 34.53°, 45.72°a nd 53.57°which corresponds to the reflections of crystal planes (100), (101), (102) and (110) match with NiS (αphase) with space group P63/mmc (hexagonal phase, JCPDS: 075-0613) [40,49]. Normally, peak broadening in the x-ray diffraction peaks consists of two components namely physical broadening and instrumental broadening which happens due to the effect of crystallite size and and the intrinsic strain respectively. The corrected instrumental broadening (b hkl ) for the peaks of NiS NPs can be written as,

XRD analysis
And the crystallite size (D) can be calculated using Debye-Scherrer formula, Where k is the shape factor ( = k 0.9), λ is the wavelenth of the x-rays (λ=0.154056 nm), b hkl is the broadening of the diffraction peak measured at half of its maximum intensity and θ is the angle of diffraction [50].
From Scherrer's formula, the average crystallite size is found to be around 19 nm. But, Scherrer's formula provides only the effect of crystallite size on the peak broadening and it is just a part of the instrumental broadening function. Also, Scherrer formula doesnot give the information about the intrinsic strain which is developed in the nanocrystals because of stacking faults, triple junction, grain boundary and point defects. Here, in order to calculate two independent factors i.e. the crystallite size and intrinsic strain, uniform deformation In general, the instrumental broadening function can be written as, The expression for strain induced peak brodening can be given by, b e q = 4 . tan 4 strain ( ) The total brodening owing to size and strain for a particular peak having hkl value can be given as, On rearranging equation (6), (7) is an equation of straight line and is called UDM equation and this is considerable for isotropic nature of crystals [50,51]. A plot is drawn by taking q 4 sin values along X-axis and b q . cos hkl values along Y-axis for most of the intensed peaks as depicted in figure 3(b). In W-H-UDM model, the intercept made by the plot represents the average crystallite size, whereas the slope of the straight line corresponds to the value of the intrinsic strain. From the W-H-UDM method, the average crystallite size and the intrinsic strain are calculated to be around 26 nm and 0.121×10 -3 respectively. Also, from the figure 3(b), it has been noticed that the slope of the W-H-UDM plot is positive which represents the lattice expansion [51]. Figure 3(c) displays HRTEM image of NiS NPs which shows a quasi-spherical shape with very small aggregations [28]. The particle size histogram based on HRTEM is depicted in figure 3(d). The total numbers of NPs accounted for particle size histogram are 509 per field of view. From this, it can be observed that the average particle size ranges from 20-30 nm which is in good agreement with XRD results. The XRD patterns of pure PVA and PVA/NiS nanocomposite films are represented in figure 4. Figure 4(a) depicts a diffraction peak around 2θ=19.67°which is ascribed to the semi-crystalline nature of neat PVA ascribing to (101) plane reflections [52][53][54][55][56][57]. The PVA/NiS nanocomposites (figures 4(b)-(g)) also show the broad diffraction peak at 2θ=19.82°representing the existence of PVA with several additional small peaks at 2θ=30.38°, 35.11°, 45.99°and 54.09°which strongly corroborate the existence of NiS in the nanocomposites. Moreover, it has been noticed that the intensity of the diffraction peak corresponding to PVA is decreased with an increase in the NiS NPs loading in the nanocomposites. This may be due to the possible intermolecular interactions between the nanofiller and the host polymer matrix which tends to decrease within the polymer chains and therefore the degree of crystallinity [58][59][60].  figure 5(a)) depict two weight loss stages (overall weight loss is about 36.06%). The first decomposition stage is noticed between the temperatures 39°C and 350°C (weight loss about 8.82%) which could be due to the elimination of hydroxide group and residual moisture. The second stage of weight loss about 22.92% occurs in the temperature range 350-540°C which may be attributed to the gradual decomposition of the sulfur during structural collapse process [61]. The residual mass of NiS NPs is observed to be 63.73%. The neat PVA ( figure 5(b)) exhibits three degradation steps (overall weight loss about 99.83%). The first step of degradation (about 4.21% weight loss) is identified in the temperature range 52°C-130°C which is because of elimination of hydrolyzed water in the film [62]. The second step of degradation (weight loss about 62.04%) appears between  the temperatures 150°C and 390°C owing to chain scission, splitting of monomer unit and degradation in the polymer's backbone [63,64]. The third step of degradation exhibits weight loss about 33.58% above 420°C which may because of the cleavage of C-C (residual carbon) bond of PVA and complete decomposition of its backbone [63][64][65][66]. The PVA/NiS nanocomposite films also show three stages of decomposition (figures 5(c)-(h). The overall weight loss of these nanocomposite films are identified to be about in the range 99.53-93.7%. The first decomposition stage appears between the temperatures 48°C and 134°C (weight loss is in the range 3.06%-3.65%) which is ascribed to the removal of adsorbed water present in the nanocomposites. The second stage of decomposition is located in the temperature range 147°C-403°C which could be due to the bond scission and splitting of the monomer unit in the backbone of PVA [58]. The weight loss in the second decomposition stage is observed to be in the range 52.99%-67.40%. The third decomposition stage appears above 415°C (weight loss is in the range 27.68%-37.49%) which is attributed to the further structural decomposition of PVA resulting in the formation of carbonaceous residue consisting of macromolecular fragmentation of both the NiS NPs and PVA [58,67,68]. From figures 5(c)-(h), it has been noticed that the final residual mass of the PVA/NiS nanocomposite films isincreased with increase in NiS content (wt %) in the PVA matrix and found to be in the range 1.19%-6.02% whereas for neat PVA, the residual mass is observed to be 0.73% only. This residual mass demonstrates non-degraded polymer chains, alkenes and other organic compounds that existed in the nanocomposites [69]. These results suggest that the PVA/NiS nanocomposite films exhibit better thermal stability as compared with neat PVA. The cause for showing better thermal stability of nanocomposites is the successful inclusion of NiS NPs in the PVA matrix and also good interaction between them. Hence, the incorporation of NiS NPs into PVA restricts the motion of a polymeric chain and consequently decreases the weight loss and results in a sluggish decomposition process [70].

Thermal analysis
Furthermore, the thermal properties of PVA/NiS nanocomposites were carried out using DSC which gives the information about the effect of nanofiller concentration on glass transition temperature (T g ), melting temperature (T m ), and degradation temperature (T d ) etc on the host polymers [71][72][73]. Figures 6(a)-(g) displays the DSC thermographs of pure PVA and PVA/NiS nanocomposite films. For all the samples, three endothermic peaks were identified. The first peak ascribed to T g was identified at 103.29°C for PVA and in the temperature   interactions between the nanofiller and host polymer matrix [72,76]. The third endothermic peak appears at the temperature 320.82°C and in the temperature range 307.58°C-319.69°C which is ascribed to pyrolysis of PVA and PVA/NiS nanocomposites respectively. Finally, an additional exothermic peak occurs at 385.09°C which is attributed to T d for neat PVA and its value lies in the temperature range 373.58°C-384.95°C for PVA/NiS nanocomposites. The degree of crystallinity (C) of the polymer can be calculated from DSC endothermic curves using the following equation by assuming a linear relationship between the endothermal peak area and crystallinity [77].
Where C is the crystallinity of a semicrystalline polymer, DH is the heat of fusion of a semicrystalline polymer, and DH o is the heat required for melting of 100 % PVA (138.6 J g −1 ). The endothermic melting transition and the calculated crystallinity of the samples are presented in table 1.   content. Finally, the morphology of the PVA/NiS nanocomposite films was investigated using HRTEM and the results are shown in figure 11. The HRTEM images of PVA/NiS nanocomposite films filled with 3 wt% of NiS NPs (figures 11(a)-(d)) revealed that the NiS NPs have been uniformly dispersed in the PVA matrix revealing good compatibility between the polymer matrix and the nanofiller [12].

Dielectric studies
The PNCs having higher dielectric constant and lower dielectric loss are essential for energy storage applications [79][80][81]. As the energy storage capacity of these materials is the key for utilization in energy generating devices, significant interest has been shown in the analysis of dielectric properties of various PNCs [82]. Also, the chain interactions and the polarity of the host polymer may affect the interface between the polymer matrix and the NPs     defined as the accumulation of charge carriers at the interface between the polymer matrix and the nanofiller [84,85]. At higher frequency, the interfacial dipoles do not have enough time to align themselves in the direction of an applied electric field [53,86,87]. Also, the MWS effect in polarization is typified by the dependence of frequency and the value of ε in the low-frequency region. At higher frequencies (10 KHz-20 MHz), no significant change in dielectric constant was observed which demonstrating the good frequency stability of samples. Generally, it happens because the dipole orientation is difficult at the higher frequencies [53]. The maximum dielectric constant value was found to be 154.55 (at 50 Hz and at140°C) for PVA/NiS nanocomposite films reinforced with 3 wt% NiS NPs which is 22 times greater than the dielectric constant value of pure PVA (6.90). This indicates an excellent interfacial adhesion between the nanofiller and the host polymer matrix. Furthermore, it has been noticed that the ε values have increased with an increment in the nanofiller loading in the host polymer matrix. For a dielectric material, tanδ is also one of the significant parameters which give information about the dissipation of energy (loss) of an electromagnetic field. Figures 13(a)-(g) displays the tanδ values of PVA and PVA/NiS nanocomposite films. These nanocomposite films show high values of tanδ at lower frequencies and low tanδ values at higher frequencies which could be due to the interfacial polarization [44,58]. Moreover, the tanδ values have increased with an increase in NiS loading in the PVA matrix (figures 13(b)-(g)). However, as compared with the ε values, the tanδ values are very low which is very attractive for energy storage device applications [44,88].

Conclusions
In this work, NiS NPs and PVA/NiS nanocomposite films have been successfully prepared via a green synthesis approach and solution casting method respectively. The PVA/NiS nanocomposites were analyzed using various characterization techniques. The FTIR and XRD results revealed that the NiS NPs and PVA have significant interaction with each other. TGA and DSC results evidenced an improvement in the thermal stability of PVA/ NiS nanocomposite films as compared with pure PVA film. The POM, SEM and HRTEM results revealed that the morphology and microstructure of PVA/NiS nanocomposites were significantly modified with the addition of NiS NPs in the PVA matrix. Moreover, high ε and low tanδ values obtained for the PVA/NiS nanocomposite films suggesting their potential applications in energy storage devices.