Effect of sample thickness on the structural and optical properties of Polyvinyl Chloride/Zinc Oxide Nanocomposite Films for photocatalysis application

Considered Technique spin coating of the most important techniques used to prepare thin films because it is easy to use and inexpensive since it is closely dependent on the number of layers deposited. This work aims to study the effect of samples thickness for nanocomposites of polyvinyl chloride doped with zinc oxide nanoparticles on the structural, optical properties, and photocatalytic activities. Nanocomposite films of polyvinyl chloride (PVC) - zinc oxide (ZnO) with a different number of the deposited layers (15, 20, 25, and 30 layers) were synthesized by a sol-gel method (spin coating) using tetrahydrofuran as a solvent, and investigated by various techniques. X-ray diffraction measurements indicated in the case of 15, 20, and 25 layers do not clearly show the presence of diffraction peaks. On the other hand, in the case of 30 layers, the presence of several peaks is observed, which testify to the presence of ZnO crystallites of wurtzite structure in PVC films. The size of the crystallites is almost equal to 32 nm depending on the working conditions. Raman and infrared spectra confirmed the result of X-ray diffraction on the incorporation of ZnO crystallites in the films produced by showing peaks corresponding to the modes of vibration of the crystal lattice of the ZnO doping semiconductor. Optical transmittance spectra have shown that The layers obtained have an optical transmission varying from 75 to 86% in the visible region of the spectrum. The values of the band gap energies, determined from the transmission spectra for the films deposited on glass, vary between 3.45 and 3.94 eV. The Photoluminescence spectra of ZnO/PVC nanocomposites studied revealed a strong ultraviolet and green luminescence, attributed to structural defects in the zinc oxide. The photocatalytic reaction has been shown using MB in the UV irradiation action of films stacked in an MB solution. The result showed that the 30-layer (PVC / ZnO) sample gave an efficiency to remove MB of 79% at 60 min, Similar to other samples which gave a lower efficiency.


Introduction
The study of matter at the nanoscale has been the subject of an increasing number of studies since the second half of the 20th century, due to technological advances in the elaboration and characterization of nanomaterials. Nanocomposites are materials that have a dispersed phase with particles of at least one of the three dimensions on the order of a nanometer (10 -9 meters) or a few tens of nanometers at most [1,2]. Thanks to the surface and/or size effects generated by the miniaturization of the particles, nanocomposites can exhibit improved properties compared to conventional composites such as electrical conductivity [3], mechanical strength [4], as well as various optical properties [4]. The size of the nanoparticles also allows the material to retain its original optical properties (e.g. transparency) as well as a good surface finish. Nanocomposites thus offer numerous perspectives for new applications in fields as varied as sensors [5], microelectronics, photocatalysis, photovoltaic cells [6], nonlinear optics, and photonics [7].
Important investigations are focused on II-VI semiconductor nanocrystals due to their large gap which makes them preferred materials for many applications in nanotechnology: highperformance light-emitting diodes, new types of lasers for quantum computing, photodetectors, biological markers, optical memories, photoluminescent devices [8][9][10]. Due to their promising applications especially in optoelectronics, ZnO semiconductor is the most studied semiconductor of this family [11][12][13][14]. ZnO is a potential candidate for short wavelength (UV) emitting systems because it has interesting characteristics: a wide band gap (3.37 eV), a large exciton binding energy (60 meV), and a low optical pumping threshold at room temperature [15,16]. These interesting optical properties of semiconductor nanomaterials are mainly due to the effect of charge carrier confinement, which is responsible for the optical emission shift towards short wavelengths [17][18][19][20][21][22]. It is therefore possible to fabricate optical systems emitting excitations of increasingly smaller wavelengths by further decreasing the size of the semiconductor nanocrystals.
The methods followed for the elaboration of nanomaterials are generally expensive and require sophisticated equipment [23]. With the development of chemical synthesis techniques in the 1990s, the elaboration of nanocrystals has progressed greatly by fabricating nanocrystals with variable geometries and low size dispersions [24]. The use of nanocrystals often requires material supports such as substrates of different shapes or matrices of varying chemical composition [25][26][27][28][29].
To master these new chemical methods of nanomaterials elaboration and to establish the optimal experimental conditions to obtain nanocomposites possessing intense optical activity in the ultraviolet-visible range, we have undertaken the present work which consists of elaborating and characterizing nanocomposites based on the large gap semiconductors ZnO.
In this work, the soft chemistry method was used to enhance the optoelectronic properties of PVC/ ZnO nanocomposites thin films. The coating solutions were prepared using ZnO nanoparticles as doping and PVC as the host's matrix. PVC / ZnO nanocomposites thin films were deposited on cleaned glass substrates by the spin coating technique. Structural, optical properties, and photocatalytic activities of ZnO thin films were investigated. The effect of nature and the number of layers deposited on optical properties were too studied.

Experimental Details
 A known amount (1 g) of polyvinyl chloride was dissolved in 20 ml tetrahydrofuran (THF) as a solvent and stirred by a magnetic stirrer for about 6 h at 50 °C to form a homogeneous solution. The part of this solution is used for the preparation of the undoped thin films (pure polymer) which will serve as control samples.

X-ray analysis of ZnO/PVC nanocomposites
X-ray diffraction measurements were conducted to examine the nature of crystallinity of the polymer films with respect to pure PVC and to investigate the occurrence of complexation between the polymer and the filler. Figure 1 represents respectively the XRD for pure ZnO nanoparticles, pure polyvinyl chloride, and doped with 6% ZnO nanocomposites at depositing different layers (15,20,25, and 30 layers), which were obtained by the spin coating method.
In Figure 1a, we observe a weak broadening of the diffraction peaks, which is due to the nanoscale, grains size of ZnO powder.  The X-ray diffraction results can be used to determine the average crystal size by applying the Scherrer relation to the individual diffraction peaks, given the first approximation that the width of the diffraction lines is primarily due to the crystal size [31].
As for the samples: the first (15 layers), the second (20 layers), and the third (25 layers), we cannot calculate the grain size because the diffraction peaks are not available, but for the fourth sample (30 layers) the average values found are reported in Table 1. Table 1: the estimated sizes for the ZnO particles dispersed in PVC

Raman Spectroscopy Analysis
Raman spectroscopy was performed at room temperature with laser radiation excitation (λ = 632.8 nm).

FTIR analysis
In Figure 3 we can observe the infrared spectra obtained from the pure and doped PVC samples doped with 6% ZnO crystallites, for the different layers deposited (15,20,25, and 30 layers).

UV-Vis Analysis
To determine the optical properties of ZnO/PVC nanocomposite thin film UV-Vis Spectroscopy was employed. The UV-VIS absorbance spectra in the region 200-900 nm for doped and undoped films are shown in Figure 4. Although the general appearance of the spectra is identical; they are composed of two regions:  A region of high transparency located between 400 and 900 nm, the pure PVC is fully transparent (85٪) and has a steep absorption edge at 300 nm. The transmission value is around 75 to 85%. In this region, we notice a decrease in transmittance with an increase in the number of layers.
 A region of high absorption corresponds to the fundamental absorption (λ <400 nm). This absorption is probably due to electron transition from the valence band to the conduction band.

 Determination of optical band and thickness
Tauc' plot method was employed to determine the optical band gap of Pure PVC and PVC/ZnO nanocomposites films. Figure 5 shows the Curve (αhυ) 2 as a function of (hυ) for a pure The estimated values of Eg listed in Table 2   The thickness of the thin layers was determined from the spectrum of the transmittance, using software of "Fit" [42] which allows to vary a certain number of parameters, such as the thickness, the refractive index, and optical gap, and use the least-squares method to adjust a simulated transmittance curve to that measured. Table 3 gives the results of the thickness of our thin layers.

Photoluminescence Analysis
PL spectroscopy is a powerful tool for characterizing the optical quality of semiconductor materials. The intensity of the PL peak corresponds directly to the density of defects in the material.
Typically the PL spectrum of ZnO includes bands in the UV-visible range that correspond to exciton emission. Figure 6 represents the spectrum of the photoluminescence of a pure PVC film and doped with ZnO nanocomposites obtained by the spin-coating method, where we notice:  0 3,1 3,2 3,3 3,4 3,5 3,6 3,7 3,8 3,9 4,0 4,1 4,2  )  For pure PVC: One observes, on this spectrum, a weak luminescence centered around 2.86 eV. PVC is therefore a blue light emitter [43][44][45]. The latter is due to the recombination of the charge carriers after being trapped by gaps or defects of the structure.
 For the PVC/ZnO nanocomposite spectra, we observe well-defined peaks, for the PVC samples doped by ZnO crystallites, whose positions are: 3.24 eV, 2.72 eV, and 2.47 eV.
The first peak (3.24 eV) is due to phonon emissions from replicas and/or band-to-band

Photocatalytic Tests
The photocatalytic activity was studied by the degradation of the methylene blue dye in the presence of PVC / ZnO nanocomposites. Figure 7 shows