Characterization, Optical and Conductivity Study of Nickel Oxide Based Nanocomposites of Polystyrene

Different concentrations of nickel oxide (NiO) nanoparticles were successfully inserted into polystyrene (PS) polymer matrix using the solution casting method. X-ray diffraction (XRD), Fourier transform infrared (FT-IR), High-resolution scanning electron microscope (HRSEM), and UV–Vis techniques are used to characterize the nanocomposites. XRD, HRSEM, and FT-IR results indicate that PS and NiO nanoparticles have a good interaction and a high degree of complexity. Using UV–Vis technique, the absorbance of nanocomposites is improved by increasing the nanofiller content. Both the Urbach energy (EU) and the optical energy gap (Eg) were calculated using the Urbach relation and the Tauc model. Refractive indices values of nanocomposites display raising trend with NiO content which promote PS/NiO nanocomposites for use in photonic applications and designing optical devices. Electrical conductivity of nanocomposites were also investigated. The electrical conductivity of the nanocomposites has increased significantly due to the uniform dispersion of NiO nanoparticles and by increasing its content. Conduction is achieved by correlated barrier hopping (CBH).


Introduction
A polymer nanocomposite (NCs) is a composite material consisting of a polymer matrix and at least one nanometric dimension inorganic material. As a result of their applications in electronic and photonic devices, rechargeable batteries, and supercapacitors, the resulting hybrid materials are promising [1][2][3][4][5][6][7][8][9]. Inorganic nanoparticles are applied to polymeric materials to strengthen their physical properties. The high ratio of surface area to volume of nanoparticles is one of their most significant properties, implying that the atoms on the surface make up a large proportion of all the atoms in the nanoparticle. As a result, in comparison to bulk particles, this ratio, when combined with the scale and relatively uniform shapes, produces novel properties [10,11]. Adding small amounts of these nanofillers to polymers can enhance their mechanical, barrier, thermal, and flammability properties without affecting their processability [12]. Polymer/inorganic NCs combine the properties of the two components. This means that polymers with strong optical properties, durability, and hardness will help inorganic materials become less brittle. Inorganic materials also increase polymer strength [13,14]. To create smart composite materials with a high fraction of inorganic NPs and their associated properties while maintaining the flexibility and fluidity of the polymer matrix (host), keeping the inorganic NPs homogeneously dispersed (isolated) is crucial. Nanocomposites of polymer/inorganic fillers combine the fascinating electrical and optical properties of nanoparticles of inorganic with the processability of a polymer. Thus, in the fields of solar cells, magnetic recording materials, light emitting diodes, sensors, transistors, optoelectronic packaging and rechargeable batteries, these nanocomposite materials have a large range of applications [15][16][17][18][19][20][21][22]. Polystyrene (PS) is a thermoplastic polymer with excellent optical clarity and chemical tolerance to diluted acids and bases due to its lack of crystallinity. It has a wide range of uses due to its low cost and ease of manufacturing, including disposable consumer plastic products and parts for optical, electronic/electrical, and medical applications [23]. Nickel oxide (NiO) is a promising source in the fabrication of engineering nanomaterials 1 3 for multifunctional applications due to its strong dispersibility in polymeric systems [24]. It has attracted a lot of interest due to their low cost, strong sensing properties, high resistivity, environmental stability [25]. Various research groups have documented structural, thermal, magnetic, and other physical properties of PS/NiO, Hybrid silica/PS, PS/SiO 2 , PS/Fe 3 O 4 , PS/HgS, and CaCO 3 /PS [23,[26][27][28][29][30]. Polymer/ NiO nanocomposites in various polymer matrices have been prepared for applications such as magnetic materials [31], gas sensors [32], light emitting and electronic devices [33]. Nickel oxide nanoparticles (NiO) NP were inserted into a polystyrene matrix in this work with different mass fractions. PS/NiO NCs is compared to pure PS in terms of structural, optical, and conductivity investigation.
Experimental Part

Chemicals
All the chemicals used in this study were not purified further. Polystyrene (PS) [average M W = 350,000, purity ≥ 95%] and nickel (II) oxide nanopowder, < 50 nm particle size (TEM), 99.8% trace metals basis were purchased from Sigma-Aldrich co. The solvent used in this study is tetrahydrofurane, was purchased from Fisher scientific for chemicals, UK.

Synthesis of PS/NiO Nanocomposites
The synthesis procedures used for preparation of PS/NiO nanocomposite films using solution casting method are shown in Fig. 1  the ultrasonication method is used to achieve these objectives and exploit the unique properties of NiO NPs. The nanocomposite solutions were slowly poured into washed glass Petri dishes and held at 40 0 C in a vacuum oven. until the solvent completely evaporated. Pure PS film has also been prepared for comparison under the same conditions. In vacuum desiccator the final films were stored.

Measurements
X-ray diffraction (XRD) analysis was conducted on X'Pert-PRO-PANalytical channel control using Cu-Kα target (λ = 1.5406 Å, scans were collected over a 2θ range of 5º-60º). Scherrer's formula was used to measure the average crystallite size of NiO nanoparticles based on line broadening. Fourier transform infrared (FT-IR) measurements were recorded using JASCO, FT/IR-6100. FT-IR measurements were taken in the spectral range of 4000-400 cm −1 . High-resolution scanning electron microscope (HRSEM) was performed using SEM Model Quanta 250 FEG. The optical measurements of the prepared films were investigated using UV-630 (Shimadzu) UV-VIS-NIR spectrophotometer. Dielectric measurements were performed using Broadband Dielectric Spectroscopy (BDS) type Novocontrol concept 40.

X-ray Diffraction Analysis
The XRD of NiO NPs, PS, and PS doped with various NiO NP concentrations are investigated. Figure 2 shows XRD pattern of NiO NPs, it has characteristic diffraction peaks at 2θ = 37.2º, 43.2º. 62.8º, 75.4º and 79.4º which corresponded to (111), (200), (220), (311) and (222) lattice planes respectively, that related to face-centered cubic crystal structure [34,35]. The position of the peak and the relative strength of the characteristic peaks are accorded with standard spectrum (JCPDS, No. 04-0835). NiO NPs have a single phase XRD pattern, and there are no other impurities peaks. The mean crystallite size of NiO NPs was calculated from the line broadening of the diffraction peaks using Scherrer's formula [36] where D is the crystallite size, λ = 0.154 nm is the X-ray wavelength, β is the full width at half maximum (FWHM) of the diffraction peak (in radian) and θ is the Bragg's diffraction angle (in degree) of the peak maximum. The average crystallite size of NiO NPs was found to be nearly 18 nm. XRD scans of PS homopolymer and PS filled with different ratios of NiO nanoparticles are illustrated in Fig. 3. XRD pattern of PS shows a broad hump at 2θ = 20.3º that indicate the amorphous structure of PS [37]. Figures 3b-3f show XRD patterns of PS/ NiO NCs, it can be seen that with increasing NiO NPs the characteristic hump of PS is shifted to 2θ = 19.7º. Also, the peak broadening increased with increasing NiO NPs. The characteristic diffraction peaks of NiO NPs at 2θ = 75.4º and 79.4º disappeared. As well the peak position of NiO NPs at 2θ = 62.8º was shifted to 2θ = 63.2º. Diffraction peaks at 2θ = 37.2º, 43.2ºand 63.2º are increasing with an increase in the concentration of NiO NPs. Furthermore, these results suggest that the NiO NPs

Fourier Transform Infrared Analysis
FT-IR absorption spectra for the PS and PS/NiO NCs were performed at room temperature within a range 4000-400 cm −1 as shown in Fig. 4. FT-IR spectrum of PS (Fig. 4a) shows absorption band at 3025 cm −1 which attributed to axial deformation of aromatic C-H. The asymmetric and symmetric stretching vibrations of CH and CH 2 groups are seen at 2920 and 2844 cm −1 , respectively. The bands at 1600 and 1492 cm −1 are related to the stretching vibration of C = C in aromatic ring. The angular deformation of CH 2 symmetric and asymmetric is shown at 1451 cm −1 . The stretching bands at 1067 and 1027 cm −1 are related to C-C stretching vibration. The out-of-plane deformation of C-H aromatic is seen at 748 cm −1 , 695 cm −1 and 537 cm −1 [38][39][40]

High-Resolution Scanning Electron Microscope Analysis (HRSEM)
The morphology of pure PS and PS/NiO NCs as determined by HRSEM is depicted in Fig. 5a-f. As seen in Fig. 5a, which represents pure PS with Χ 3000 magnification micrograph, the surface is homogeneous and smooth. Figure 5b-f shows the distribution of NiO nanoparticles inside the PS matrix with Χ 800 magnification. NiO nanoparticles appeared as tiny white particles, which dispersed uniformly on the polymer matrix surface. As nanofiller content in the nanocomposites increases, the inertia for these nanofillers to form agglomerations is also increasing and nanoclusters are forming. The attractive van der Waals force causes nanoparticles to clump together between neighboring particles. The size of the nanoclusters depends on the amount and size of nanoparticles inside the polymer matrix. There is an absorption band around the wavelength of 260 nm for pure PS [42]. In the case of PS doped with NiO nanoparticles, a new shoulder like peak appeared at about 286 nm. The absorbance of nanocomposites is increased by increasing percentages of nanofiller weight relative to pure PS. This phenomenon indicates that NiO nanoparticles were successfully integrated and interacted with the polymer matrix [43]. Furthermore, no peak was observed in the visible region of the absorption spectra of nanocomposites. This finding indicates that synthesized nanocomposite can be used as UV-filter and UV-shielding block [44].

Optical Absorption Edge and Urbach Energy
Using the following equation, the optical absorption coefficient (α) was determined from absorbance. [45]     where 0 is a constant and E U (Urbach energy) is the width of the tail of the localized states within the band gap. Urbach energy values of PS/NiO NCs were determined by taking reciprocal of slopes of the linear parts of the graphs shown in Fig. 8. As the nanofiller ratio in the polymer matrix increases, the Urbach energy values [see Table 1] increase, suggesting an increase in the number of charge trapping centers.

Optical Energy Gap
The absorption coefficient helps to determine the nature of electron transition. In general, direct and indirect transitions can occur at the absorption edge of materials. The absorption coefficient (α) is related to the optical band gap (E g ) by Tauc relation [47] where β is constant and m is the index with the value determined by the type of possible electronic transitions. m = 2 or 1/2 for indirect and direct allowed transitions respectively. In order to obtain the values of E g for studied samples, we plot relation between (αhʋ) 2 and (αhʋ) 1/2 as a function of photon energy as shown in Fig. 9. Direct and indirect optical band gaps for pure PS and PS/NiO NCs were determined from extrapolations of the liner portions of (αhʋ) 2 and (αhʋ) 1/2 to zero absorption on hʋ axis. The direct and indirect optical band gap values of PS decreased as the ratio of NiO nanoparticles increased, according to the findings. [see Table 1], this behavior may be caused by the creation of the localized states within the polymer matrix.

Refractive Index
Based on the following relation [48,49], the Refractive indices (n) of pure PS and PS/NiO NCs were determined Figure 10 shows the relation between refractive index of nanocomposites and the ratio of NiO nanoparticles. Refractive indices display raising trend with NiO content. The packing density and number of charge carriers in the samples under review increased as the NiO ratio increased, refractive indices are therefore increasing. The obtained values of the refractive indices indicate that PS/NiO NCs can be used for photonic applications and designing optical devices.

Conductivity Measurements
In many cases, conductivity may be due to hopping, depending on different parameters, for example temperature, frequency and filler concentration.

Temperature Dependence Conductivity
The variation of conductivity ln (σ ac ) of pure PS and PS/ NiO nanocomposite samples with inverse temperature at frequency of 1 kHz is shown in Fig. 11. The plot behavior shows that the conductivity of the studied samples is thermally activated following Arrhenius law: where A 0 is the pre-exponential factor, ΔE is the activation energy, T and k are temperature and Boltzmann constant, respectively. As shown in Fig. 11, there are two different regions: I (403-343 K) and II (343-293 K). Region II has very low activation energy values and the conductivity is    Table 2 shows the values of activation energy for PS/NiO nanocomposite films in region I, values ranging from 0.65 eV for pure PS to 0.15 eV for PS/8wt% NiO nanocomposite. These values can be associated with ion hopping among vacant sites [50]. Increasing the percentage of NiO nanoparticles reduced the activation energy, which then increased at a percentage of 10% wt%. This could be due to the difference in dispersion of NiO nanoparticles in PS sample. The formation of aggregates or clusters in the polymeric matrix resists the hopping of electron from one particle to another particle leading increase in the activation energy. In general, the conductivity increases by increasing the temperature and the percentage of NiO nanoparticles which improving the conduction of the nanocomposites. The elongation of polymer chain length and polaron hopping are responsible for the increase in conductivity. It was also discovered that pure NiO has a larger surface area than pure Polystyrene. As a result, these composites are promising materials for use in transformers, energy storage systems, and rechargeable batteries as a soft electromagnetic material.

Frequency Dependence Conductivity
The fitted data of AC conductivity for pure PS and PS/NiO nanocomposites as a function of frequency at selected temperatures 20 °C, 50 °C, 70 °C and 100 °C are shown in Fig. 12. The dots are experimental data points and the lines are the fitting. The figure shows that the electrical conductivity of nanocomposites is increasing with frequency. The increase in electrical conductivity with frequency is due to the polarization of the space charge that occurs at low frequencies, as well as to the hopping of charge carriers. The frequency dependent conductivity is generally represented by the universal dispersion relaxation (UDR) or Jonscher's power law given by [51] where σ dc is the dc conductivity of the sample, ω is the angular frequency, A is a temperature dependent constant and s is the power law exponent. The exponent s represents the (7) ac ( ) = dc + A(T) s degree of interaction between mobile ions and the environments surrounding them. Many manifestations of the hopping models and experiments gave value of s in the range of 0<s<1. The thermally activated hopping process between two sites separated by an energy barrier explains the transport mechanism of mobile ions. [52,53]. When trying to fit the obtained data to the above single power term expression, the data did not give a good fit except for PS/10wt% NiO composite sample. However, an excellent fit was obtained using the two terms power law [see Fig. 12a-d], known as the super-linear power law (SLPL) given by the following equation [54][55][56] The fitting parameters s 1 , s 2 , ln A 1 and ln A 2 and ln (σ dc ) at room temperature are shown in Table 3. Figure 13 and Fig. 14 represent the fitted data at different temperatures for the studied nanocomposites. With increasing temperature, the general pattern of decreasing s 1 and s 2 could be attributed to correlated barrier hopping [57,58].

Conclusion
PS/NiO nanocomposite films were prepared by using solution casting method. The prepared samples were characterized using XRD, FTIR, HRSEM, UV-Vis techniques. Electrical conductivity of prepared samples was investigated. Both XRD and FTIR measurements confirmed the interaction and incorporation of NiO into PS. From XRD results, addition of NiO NPs into PS polymer decreases the crystallinity of the nanocomposites. HRSEM images showed well distribution of NiO NPs on the surface of PS and with increasing the concentration of the nanofiller ≥ 8wt% agglomeration appeared. The visible region of the absorption spectra of nanocomposites had no peak, according to UV-Vis study. This discovery suggests that nanocomposite can be used as a UV filter and UV shielding block. The absorption edge values for the analyzed samples decrease as the nanofiller content increases due to changes in PS crystallinity. Urbach energy values of PS/NiO NCs increases due to increasing in number of charge trapping centers, the direct and indirect optical band gap values decreases due to the creation of the localized states within the polymer matrix. Refractive indices values display raising trend with increasing NiO content because the increasing in the packing density and the number of charge carriers in the samples, so PS/NiO nanocomposites suitable for photonic applications and designing optical devices. The electrical conductivity increased with increasing the percentage of NiO NP. The activation energy reduced by increasing the percentage of NiO nanoparticles ranging from 0.65 eV for pure PS to (8)    0.15 eV these values can be associated with ion hopping among vacant sites. By increasing temperature, the general pattern of decreasing s 1 and s 2 could be attributed to correlated barrier hopping (CBH).