Structural and optical properties of polymer blend nanocomposites based on PVP/PVA incorporated AgNO3

In this study, polymer nanocomposite based on polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and silver nitrate (AgNO3) has been prepared through chemical reduction rate and casting method for varying concentrations of AgNO3. The PVP/PVA blend consisted of 0.6 wt% PVP and 0.4 wt% PVA. Following that, polymer nanocomposites were prepared by incorporating different concentrations of AgNO3 (0, 10, 20, 30, 40, and 50 wt%) into the polymer blend. The effects of different concentrations of AgNO3 on the structural and optical properties of the PVP/PVA blend were investigated using x-ray diffraction (XRD) and UV–vis absorption spectroscopy. The XRD analysis demonstrated that increasing the concentration of AgNO3 results in a decrease in the degree of crystallinity from 53.73 in the PVP/PVA blend to 15.77 in the PVP/PVA nanocomposite containing 50 wt% AgNO3. UV–vis absorbance spectra were examined to determine optical properties such as the absorption coefficient, absorption edge, optical band gap, and tails of localized states. The results revealed that the increase in AgNO3 concentrations caused a reduction in the absorption edge and optical band gap, alongside an increase in Urbach energy.


Introduction
Polymeric materials have garnered significant attention from scientists in recent decades owing to their multifaceted applications.They find utility across a spectrum of fields, including high-energy rechargeable batteries, supercapacitors, photoelectrochemical systems, fuel cells, and electrochromic displays [1][2][3][4].Blending different polymers has the potential to improve the performance of polymeric materials.Such blends often manifest properties distinct from their individual constituents [5,6].Moreover, polymer-based nanocomposite materials have witnessed substantial growth, emerging as key players in diverse scientific realms, particularly in electronic applications [7][8][9].Polymer blends have become integral across industries, leveraging their versatile and advantageous properties.The superiority of polymer blends in such applications was established through the discovery of their improved optical, electrical, structural, thermal, and mechanical properties [10,11].Polymers are commonly employed as particle stabilizers and capping agents in the irradiation synthesis of metal nanoparticles.This is because they effectively prevent the nanoparticles from clumping together and settling out of the solution.The incorporation of these particles into a polymer matrix is also beneficial in terms of film casting.Polyvinyl alcohol (PVA) is recognized for its effective role as a stabilizer for metal nanoparticles [8,12].Moreover, it is the preferred polymer in numerous industries due to its semi-crystalline structure and desirable physical and chemical characteristics, such as its water solubility, high transparency, non-toxicity, and high flexibility [13].Due to its hydrophilic nature and the abundance of hydroxyl groups linked to the methane carbons on its backbone, PVA possesses a unique chemical potential for electrolyte creation [14][15][16].Furthermore, the remarkable solubility of Polyvinylpyrrolidone (PVP) polymer in water has demonstrated its benefits as a casting solution during the production of polymeric blends using PVA.The PVA/PVP blends piqued the interest of researchers worldwide because they aimed to optimize these appealing characteristics [17,18].Researchers examined blends of both polymers in varying quantities [19][20][21][22][23] to monitor changes in physical, optical, and electrical properties and identify which was most advantageous for a particular application.PVA/PVP blends have been created with increasing PVP concentrations to further enhance the permeability and adhesion properties of the polymer blend and lessen its crystalline nature for use as a cell culture biomaterial [24,25].In contrast, Abou Taleb [26] observed changes in the crystalline structure, optical properties, and IR absorption band broadening when he combined PVA with PVP.These modifications were explained by the interaction between the two polymers' hydroxyl and carbonyl groups.Furthermore, adding certain nanometal composites, such as gold (Au) or silver (Ag), has improved the optical properties of the blends [27,28].Silver nanoparticles (AgNPs), with their exceptional antibacterial, electrical, and optical properties, find applications in photonics and electronic devices [29][30][31][32][33][34][35][36].
This study investigates the structural and optical properties of polymer nanocomposites based on PVP/PVA blends incorporating silver nitrate (AgNO 3 ) in various ratios.The research aims to improve the impact of integrating AgNPs on both the structure and optical characteristics of the PVP/PVA host matrix through XRD and UV-vis spectroscopy analyses.

Materials and experimental work 2.1. Materials
The host materials were PVA with a molecular weight (M.W.) of 31,000 g mol −1 and PVP with an M.W. of 24,000 g mol −1 in powder form were obtained from Carl Roth GmbH + Co. KG-Schoemperlenstr, 3-5D-76185 Karlsruhe.AgNO 3 (>99.5%)purity was obtained from BIOCHEM Chemopharma, ZA Cosnen sur liore, 58200 France.Distilled water (D.W.) was used to dissolve the PVP, PVA polymers, and AgNO 3 .These materials were sourced from the Physics Research Laboratory, UoH, Halabja, Iraq.

Synthesis of polymer nanocomposites
Nanocomposite samples comprising PVP/PVA polymer blends have been produced by the casting technique.At first, 1g of PVP was dissolved in 25 ml of D.W., followed by dissolving 1g of PVA in 25 ml of D.W. at 90 ℃.The solutions were thoroughly stirred separately with a magnetic stirrer for 24 h resulting in a uniform liquid.The PVP/PVA blend was prepared in the second step by mixing 0.6 wt% PVP and 0.4 wt% PVA with continually stirring for 1 h.The third step involved dissolving the appropriate amount of AgNO 3 (10,20,30,40, and 50 wt%) in 5 ml of D.W., then heating the PVP/PVA blend to 90 ℃. and stirring continuously.The AgNO 3 solution was then added dropwise to the blend and stirred continuously; The colorless solution turned brown after 30 min, signifying that the nanoparticles had formed [37].The resultant solutions were poured into clean 8 cm-diameter Petri dishes and left to dry at room temperature for about two weeks to create films.The achieved samples were coded according to the concentration of AgNO 3 , as shown in table 1.The thickness of the prepared samples ranged between 70-190 nm.The absorbance spectra of the samples were recorded across the wavelength range of 190-1100 nm using a double beam UV-Vis spectrophotometer (Model: Lambda 25) from Perkin Elmer.Using an x-ray diffractometer (X'PERT-PRO) with a Cu Kα radiation source with a wavelength of 1.5406 Å and glancing angles ranging from 5°to 80°, the XRD patterns of the produced films were recorded.

XRD analysis
The x-ray diffraction patterns of the PVP/PVA blend and PVP/PVA-AgNO 3 nanocomposites are shown in figure 1.The strongest diffraction peak, which is roughly at 2θ = 20.19°,shows that the PVP/PVA composite has a semicrystalline structure with both amorphous and crystalline areas.Our findings are in line with previous studies [10,30,[38][39][40][41][42].The PVP/PVA-AgNO 3 samples also exhibit a semi-crystalline structure.The 2 graph shows that the peak intensity of the PVP/PVA blend at 2θ = 21.58°isdecreased by incorporating AgNPs.It also demonstrates how the crystalline peaks, which are extremely apparent in the PVP/PVA blend, decrease and eventually disappear at maximum loading when the AgNO 3 salt concentration rises.This validates the previous study's discovery that the addition of AgNPs reduced the crystallinity of the materials under study [28,43,44].This broadening and decrease in intensity is caused by the disruption of the hydrogen bond between the hydroxyl and amino groups in PVP/PVA nanocomposite [43,45].The diffraction peak of the nanocomposites switched from 2θ = 20.19°to2θ = 22.17°, indicating reduced inter-planar spacing.This results from the interlayer spacing of the matrix, which contains molecules of nanofiller [46,47].These shifts could result from the AgNPs and PVP/PVA composite interaction, leading to an enhancement of the PVP/PVA-AgNO 3 nanocomposite samples' amorphous phase, a similar result obtained by Yang and Ragab [40,48].To investigate the impact of varying AgNO 3 concentrations on variations in the polymer blend's crystal structure, the crystallinity degree (Xc) for the PVP/PVA blend and PVP/PVA-AgNO 3 nanocomposite films was calculated using the Hermans and Weidinger equation [49,50].
Where the crystalline area is represented by A C and both the crystalline and amorphous area is represented by A .
T Table 2 shows that increasing the AgNO3 concentration causes the host polymer's crystallinity to decrease continuously.The significant change in crystalline degree from 53.73% for the PVP/PVA blend to 15.77% for PVP/PVA-AgNO3 (50 wt%) indicates a high reactivity of the AgNPs with the host polymer matrix.The crystallinity results were consistent with previous studies on nanoparticles added to polymer matrices [3,4,40,49].In figure 1 small peaks were observed between 35°− 40°which indicates that AgNPs exist in the samples with AgNO 3 concentration.The existence of these peaks, associated with the (111) plane, indicates that the AgNPs possess a face-centered cubic (FCC) crystalline structure.This information aligns with prior research that references the (JCPDS file number 04-0783) [37,43,44,51,52].When transition metal salt is added to polar polymers, charge transfer complexes are formed in the host lattice, which reduces the band gap energy [43,53,54].Below, the UV-vis absorption analysis was performed to find the optical band gap energy and the Urbach energy.

Optical studies
In optics, the absorption spectrum is a direct, simple, and important method for investigating the band structure of polymer nanocomposite films.Using this analysis method, one can determine the band gap and band structures of the materials most directly [55,56].Figure 2 shows the UV-vis-NIR absorbance spectra that have been recorded for each sample.Unsaturated bonds, especially (C=O), at 220 nm, induce the shoulder or hump, This is caused by the n → π * electronic transitions, this is also shown by Ragab [40].The absorption intensity of the peak at 220 nm shifts to longer wavelengths and widens when the concentration of AgNPs increases, as a result of the interactions between the AgNPs and the PVP/PVA composite.The PVP/PVA blend had no absorption peak, but the PVP/PVA-AgNO 3 nanocomposite had strong absorption peaks at 450 nm, 480 nm, 515 nm, 549 nm, and 545 nm for AgNO 3 concentrations of (10, 20, 30, 40, and 50 wt%), which corresponded to the wavelength of the surface plasmon resonance (SPR) AgNPs.SPR occurs when the electrons in AgNPs collectively oscillate in resonance with the light wave.In contrast to the much heavier ionic cores of AgNPs, incident light waves usually generate electron polarization through the electric field.Particle size, dielectric medium and chemical surroundings significantly influence this absorption.A blue shift reveals a decrease in particle size, whereas a red shift refers to an increase in particle size.This trend is consistent with previous studies [57 -61].As the incident photon's wavelength increases, absorbance decreases and eventually soothes at higher wavelengths.A sudden increase in absorption, commonly known as the fundamental absorption edge, can be utilized for determining the band gap energy and type of transition [62,63].The absorption coefficient α is the measure of the rate at which light intensity decreases relative to its initial value.The value of α can be obtained using the optical absorbance spectra A, applying the Beer-Lambert formula [62,64,65].
Where d is the sample thickness.Figure 3 represents the optical absorption coefficient versus photon energy for PVP/PVA blend and PVP/PVA doped nanocomposite solid films.The addition of AgNPs shifts the PVP/PVA composite's absorption edge to lower photon energy, indicating that the optical band gap for the doped samples has decreased in the energy range [16,66,67].The absorption edge of the PVP/PVA blend is 4.48 eV which decreased to 0.81 eV of the 50 wt % addition of AgNO 3 .This decrease was accompanied by changes in the number of holes and electrons in the valence and conduction bands [68].
The shift in the optical absorption edge shows that electrons are moving between the filler and the blend matrix, and the composites are becoming less ordered [69].The Tauc model [9,70,71] describes the relationship between the absorption coefficient (α) and photon energy (hv): The constant b in the equation depends on the structure of the specimen, E g represents the energy of the optical band gap, and h refers to Planck's constant.The exponent g is an index that identifies the specific type of electronic transition responsible for absorption.It can have values of 1/2, 3/2, 2, and 3, which correspond to direct-allowed, direct-forbidden, indirect-allowed, and indirect-forbidden band gap transitions, respectively [4, 40,66,72,73].Figure 4 illustrates an extrapolation of the linear graph of (αhv) 2 versus (hv), providing an estimate of the value of (E gd ) for the direct transition.For the PVP/PVA blend, the E gd value is 4.56 eV.As the AgNO 3 solution is added to the blend, the E gd value decreases with increasing AgNO 3 concentrations, which decreases to 1.54 eV for 50 wt% of AgNO 3 .Similarly, as illustrated in figure 5, extrapolating the linear graph of the square root of (αhv) versus (hv) provides an estimated value for the indirect transition energy (E gi ).As the direct transition band gap energy decreases with increasing the AgNO 3 ratio to the PVP/PVA blend, the indirect  hv for all samples.The inset represents PASN0.transition band gap energy also decreases.For the blend, the E gi value is 4.08, eV while for the addition of 50 wt% of AgNO 3 , the E gi decreases to 0.69 eV.A decrease in the energy band gap of polymer nanocomposites with the addition of salts was also reported in previous papers [12,16,40,65,67,74].Table 3 contains E g 's experimental results for the direct and indirect transitions, as well as the Urbach energy and absorption edge.The presence of both direct and indirect energy band gaps in the samples indicates that certain electrons interact with both photons and phonons [75].
The Urbach relationship is a mathematical expression that describes the exponential relationship between the absorption coefficient (α) and the photon energy (hv) in the area of the absorption edge, was used to calculate the width of the localized state (band tail) represented by Urbach energy.The width of the Urbach tail indicates that there are defects in the forbidden band gap [76].The Urbach energy (E u ) is calculated by using the following formula [72,77]:  Where b is constant.The value of E u can be calculated using the reciprocal slope of the graph between ln(α) and (hv), as evident in figure 6.The E u values for each sample are listed in table 3, it has been noticed that the value of E u increased from 0.432 eV for the PVP/PVA blend to 1.059 eV for the PVP/PVA nanocomposite with 50 wt% AgNO 3 .Figure 7 shows the relation between absorption edge, direct and indirect band gap energy, and the urbach energy with the AgNO 3 concentration.These values vary in inverse proportion to the transition band gap.This result can be explained as a change in the disorder of the PVP/PVA blend and the PVP/PVA-AgNO 3 nanocomposite.As a result of the inclusion of AgNPs, the blend matrix's structure changed [78].It has been demonstrated that the increase in E u is associated with the material's disordered nature; this results in the formation of the tail in the valence and conduction bands.The preceding publications [4,42,65,67,72,79] reported an increase in the value of Urbach energy.This outcome corroborates the XRD data analysis, which revealed that the characteristic peak of PVP/PVA nanocomposites decreased as the concentration of AgNO 3 nanoparticles increased, eventually dissipating at high doping levels.When comparing the results in table 3 to the results of H. Elhosiny Ali et al [67] our results demonstrate significant improvement.The disparities between our study and the work conducted by Elhosiny Ali et al lie in the utilization of different ratios for the PVP/PVA blend.We employed a ratio of 0.6/0.4,whereas they employed a ratio of 0.5/0.5.Additionally, there are variations in the AgNO 3 concentrations employed by both studies.They utilized concentrations of 0, 0.037, 0.37, and 3.7 wt%, while we employed concentrations of 0, 10, 20, 30, 40, and 50 wt%.Table 4 displays the results of Elhosiny Ali et al [67].

Conclusion
In the present work, PVP/PVA:AgNO 3 nanocomposite thin films were prepared with the casting technique at room temperature.The PVP/PVA blend was doped with various concentrations of AgNO 3 to investigate its structural and optical properties.The XRD analysis confirms the successful fabrication of PVP/PVA:AgNO 3

Figure 6 .
Figure 6.Urbach plot of the natural logarithm of the absorption coefficient (ln(α)) against photon energy (hv) for calculating the Urbach energy of all PVP/PVA-AgNPs samples.

Figure 7 .
Figure 7. Variation absorption edge, direct band gap, indirect band gap, and Urbach energy with different AgNO 3 wt% concentrations.

Table 3 .
Absorption edge, direct and indirect optical band gap energy, and Urbach energy.

Table 4 .
[67]lts of Elhosiny Ali et al[67]for Absorption edge, direct and indirect optical band gap energy, and Urbach energy.indicating that increasing the rate of AgNO 3 reduces the degree of crystallinity from 53.73 for a pure PVP/PVA blend to 15.77 for PVP/PVA loaded with 50 wt% AgNO 3 .It indicates that the polymeric matrix-amorphous regions were improved.UV-vis absorbance spectra were analyzed to determine optical quantities such as the absorption coefficient, absorption edge, optical band gap, and tails of localized states.The energy gap was reduced from 4.56 eV for PVP/PVA to 1.54 eV for PVP/PVA loaded with 50 wt% AgNO 3 for direct transitions, as well as from 4.08 eV for PVP/PVA to 0.69 eV for PVP/PVA loaded with 50 wt% AgNO 3 for indirect transitions, owing to the incorporation of extra energy levels.Moreover, the Urbach energy was enhanced from 0.432 eV for the PVP/PVA blend to 1.059 eV for the PVP/PVA nanocomposite with 50 wt% AgNO 3 .The current system is suitable for use in optoelectronics and optical devices (such as solar cells) due to its extremely high absorption coefficient and the compositional dependence of the optical parameters.