The impact of film thickness on the properties of ZnO/PVA nanocomposite film

Polymer inorganic nanocomposites are attracting a considerable amount of interest due to their enhanced electrical and optical properties. The inclusion of inorganic nanoparticles into the polymer matrix results in a significant change in the nanocomposite’s properties. With this in mind, we have developed a nanocomposite film based on zinc oxide (ZnO) and polyvinyl alcohol (PVA) using a solution casting method with varying concentrations of ZnO nano powder in the PVA matrix. The ZnO / PVA film surface morphology was observed by the scanning electron microscope (SEM). The micrographs indicate that ZnO nanoparticles in the PVA matrix are homogeneously distributed. XRD results indicated that the crystallinity of the film was influenced by the interaction of ZnO nanoparticles and the PVA main chain. Crystallinity is also affected by the doping of ZnO nanoparticles in the PVA matrix and it increases when the concentration of ZnO is low and then decreases when the excess concentration of ZnO is present in the PVA matrix. The FTIR transmission spectra confirmed that significant interaction took place between the ZnO nanoparticles and the PVA main chain over the wave number range of 400–4000 cm−1. The UV–vis spectra reveal that the increase in concentration of ZnO nanoparticles in the polymer matrix results in the movement of the absorption edge in the direction of higher wavelength or lower energy associated with the blue/green portion of the visible spectrum. A decrease in the optical energy bandgap is observed with the increase in nano ZnO concentration in the matrix. Thickness has a signifcant affect on the properties of the ZnO/PVA nanocomposite and the morphology, particle size, degree of crystallinity and bandgap of the ZnO/PVA nanocomposite samples were influenced by the thickness of the sample. The optimal thickess of 0.03 mm with a weight percentage of 16.6% (ZnO) and 83.3% (PVA matrix) was selected due to its higher bandgap of 4.22 eV, reduced agglomeration/aggregation and smaller ZnO particle size of 14.23 nm in the matrix. The optimal film can be used in photovoltaic research.


Introduction
When composites have at least one phase at nanoscale, nanocomposites are made. The power of nanoscale materials maximizes the composite characteristics and can lead to new characteristics. In recent times, great attention has been paid to nanocomposites that that contain organic and inorganic phases. The properties of nanoparticles depend not only on the properties of the various components, but also on whether the attractions or compatibility between the two phases are significant and essential [1,2]. An important application of nanocomposite is in electronics, where thin-film nanocomposites are useful in improving and enhancing the electrical characteristics. Polymer nanocomposites focuses on the final morphology, which depends on interaction between polymer nanoparticles that promote good dispersion and distribution of nanoparticles in the polymer matrix [3,4].
Zinc oxide is an inorganic compound which has a chemical formula of ZnO. It is an almost insoluble white powder in water. It crystallizes in two primary forms, the hexagonal wurtzite and the cubic zinc mixture [5]. This Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. is a group II-VI semiconductor with approximately 3.33 eV broad band gap. The strong and broad band difference in the near-UV spectral field and the great free exciton binding energy make it a promising functional semiconductor with a wide range of new applications [6]. ZnO has recently been doped with nanoparticles of other materials. By monitoring the dopants and concentrating them, it was determined that the particle's physical properties, such as optical, electrical, and magnetic, could be engineered. That modification and enhancement was obtained because of the electronic structure change and the band gap. Studies are very important in this field for developing applications that can be used in optical devices [7]. ZnO is a technologically important material due to its diverse properties.
PVA is whitish, odorless, non-toxic, biocompatible, thermostable, semi-crystalline or linear synthetic polymer [8,9]. Poly (vinyl alcohol) (PVA) is also a synthetic water-soluble and biodegradable polymer. Its degradability can be optimized by hydrolysis due to the inclusion of hydroxyl groups. PVA can be dissolved in water for 30 min, with a minimum of~100°C solvent temperature. The characteristics of polyvinyl acetate depend on the degree or extent of hydrolysis, whether whole or in part. It has a double impact on its classification: partly hydrolyzed and completely hydrolyzed [10]. For completely hydrolyzed and partially hydrolyzed grades, PVA has a melting point of 230°C and 170°C-190°C. It breaks down rapidly above 200°C because, at high temperatures, it can undergo pyrolysis. It has excellent optical characteristics. Nanofillers with doping can easily change its mechanical, optical, and electrical characteristics. PVA is the perfect matrix for the production of versatile devices in the fields of electronics, optoelectronics, bioengineering, and other areas due to its lack of toxicity, biocompatibility, high hydrophilicity and easy processability. It is therefore very important to adjust the PVA properties for various targeted applications [11].
Various physical and chemical techniques have been used to create, manufacture and obtain the desired ZnO/PVA nanocomposite architecture, such as solvent or solution casting, vapor-liquid-solid growth phase, melt processing, chemical vapor deposition, and in situ methods [12]. Different researchers have seen enhanced optical, electrical, thermal, and dielectric properties in ZnO-based PVA nanocomposites prepared by various methods [13][14][15]. However, any stand-alone paper on the effect of film thickness on the properties of ZnO/ PVA nanocomposite film has not been reported. Therefore, the efforts have been made in this study to find out the optimal thickness of ZnO/PVA nanocomposite film for its possible use in photovoltaic research.

Zinc Oxide (ZnO)
Zinc Oxide (ZnO) was purchased from R&M Chemicals . It was used as a filler in polymer/ZnO nanocomposites. The size of the nanoparticles was<100 nm (nano-meters and 10×10 -3 mm ) of diameter.

Solvent
Deionized water (H 2 O) has no charge (ion free and does not conduct electricity) was used as a solvent for PVA supplied by Sigma-Aldrich. It is a common solvent for this type of polymer. It is a polar protic solvent which structurally has a long hydrocarbon chain.

Synthesis of ZnO/PVA nanocomposite
The Solution Casting Method (SCM) is an easy and versatile process to develop Nano-composite laboratory scale thin films. In this study, the SCM wasused to prepare a laboratory scale thin film of ZnO/PVA Nanocomposites. A template (petri dish) has been made with ceramic based on the desired shape (laboratory scale) of the Nano-composite thin film for this study to characterize its composition.
The procedure of preparing the ZnO/PVA nanocomposite is briefly explained in figure 1. The Composition of the total solution of ZnO/PVA mixture and the weight percentages of the blended mixture of ZnO/PVA with deionized water are mentioned in tables 1 and 2, respectively. A total of 10 samples were prepared with different weight percentages of ZnO and PVA.
Different samples of ZnO / PVA Nano-composites with different weight percentages of ZnO and PVA were prepared in order to optimize the sample to obtain a better and more efficient film in terms of thickness, as shown in table 3. The laboratory scale ZnO-PVA Nano-composite thin film has been made based on the size of length × width (L × W) of 130 mm×60 mm. The thickness of the nanocomposite sample was measured using a digital micrometer. The thickness of different composite films varies from 0.03 mm to 0.10 mm.

Results and discussion
A total of 10 samples were selected for characterization. Each sample is characterized using the SEM, FTIR, XRD, and UV-vis Spectrophotometer. The following are the characterization results of each sample.  Observed the presence of functional groups present in ZnO/PVA Nanocomposite X-ray diffraction (XRD) Examine the diffraction pattern of the ZnO/PVA Nanocomposite UV-visible Spectrophotometer (UV-vis) Find out the absorption and bandgap of ZnO/PVA Nanocomposite

SEM analysis
The SEM image in figure 2 (a) shows that nano-size crystal aggregates have formed as a result of the high surface energy. However, the micrograph contains several small particles with an average size of 14 nm. The surface morphology study of the ZnO/PVA nano-composite film shows a variety of aggregates or chunks spread randomly on the upper surface of the film. The results indicate that some nano-sized ZnO particles tend to form small aggregates when dispersed in the PVA polymer matrix [16]. The micrograph obtained from prepared film as shown in figure 2 Figure 3 shows the FTIR spectra of ten samples ranging from ZnO  Figure 4 depicts the XRD patterns of 9 samples of ZnO / PVA Nanocomposite from ZnO (1) [19]. The formation of the ZnO/PVA nanocomposite has been confirmed by these diffraction peaks. By employing the Debye-Scherrer formula, the crystallite size of 9 samples of PVA / ZnO nanocomposite film was measured as shown in table 5. Doping of ZnO nanoparticles results in an increase in crystallinity, but excessive doping of ZnO nanoparticles results in a decrease in crystallinity due to variability in crystalline size and aggregate formation. Contractions in peak width and the sharpness of peaks show the growth in grain size. There has been a rise in strain in the nanocomposite films due to PVA's matrix effect on the ZnO nanocrystals. The XRD pattern of ZnO(6)PVA(10) sample peaks did not match with the PVA diffraction peak and the regular ZnO wurtzite hexagonal crystal structure PDF database (JCPDS 36-1451). There is no sharp peak observed in the pattern due to which crystallite size of ZnO/PVA nanocomposite films cannot be calculated using Debye-Scherrer formula. Figure 5 shows the absorption spectra of 10 samples of ZnO / PVA Nanocomposite from ZnO(1)PVA(5) to ZnO The performance of sample ZnO(1)PVA(5) comparable to other samples shows less agglomeration/ aggregation and smaller ZnO particle size of 14.23 nm. It also shows a high energy bandgap of 4.22 eV by converting the spectra into Tauc's Plot, which is higher than the other samples made in this study using the

Conclusion
In summary, different samples of ZnO/PVA nanocomposite were successfully prepared by the solution casting method. Each sample was characterized by SEM, XRD, FTIR and UV-vis and the results of each sample were briefly discussed. The findings revealed that the morphology, particle size, degree of crystallinity and bandgap of the ZnO/PVA nanocomposite film samples were affected by the thickness of the sample. The sample ZnO(1) PVA(5) has an optimal thickness of 0.03 mm because of its optimum value of 4.22 eV bandgap and less agglomeration/aggregation of 14.23 nm particles. A thin film of ZnO/PVA nanocomposite with optimal thickness can be applied in photovoltaic research due to its fascinating properties. However, future study on   fabrication aspects of ZnO/PVA nanocomposites could be carried out for further improvement and investigation into their long-lasting use in photovoltaic research.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).