Investigation of Structural and Optical Characteristics of Biopolymer Composites Based on Polyvinyl Alcohol Inserted with PbS Nanoparticles

The film casting method is implemented to synthesize a series of films consisting of polyvinyl alcohol (PVA) films and 4 wt%, 8 wt%, and 12 wt% lead sulfide (PbS) nanoparticles (NPs). X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Ultraviolet-visible (UV-vis) spectroscopy were used to ensure the impact of PbS loading on PVA properties, particularly optical ones. The FTIR results show a decrease in the intensity for all the bands and the XRD results show different features for the composites from that of the pure PVA. The optical properties, including optical transmission, surface reflection, and absorption, were investigated. Additionally, the significant optical parameters, for instance, the dielectric nature and refractive index of the films, were analyzed. The findings have shown that PbS NPs in the PVA films decrease UV and visible transmission through polymer composites and increase their ability to reflect the incident light. Tauc’s equation is applied to determine the optical bandgap energy (Eg) and verify whether it is direct or indirect. Using the relation between photon energy and optical dielectric loss, the Eg was measured and the type of electron transition was measured, which eases the exponent value (𝜸) specification from Tauc’s method. The Eg decreases from 6.3 eV to 5.25 eV when PbS NPs are added. The refractive index is improved from 1.27 to 2.16 for the polymer nanocomposite (NCPs) film with optimum PbS NPs. Then, the Wemple–DiDomenico model corresponding to a single oscillator is applied to the dispersive medium to determine the refractive index dispersion. Both the dispersive energy (Ed) and single-oscillator energy (Eo) are evaluated accurately. Moreover, the variation of both real and imaginary parts of the dielectric constant of polymer films were studied. Finally the optical parameters such as charge density, dielectric constant at high frequencies, optical mobility (µ), angular frequency (ωp), optical resistivity (ρ), and relaxation time (τ) of electrons are shown quantitatively. The ωp of the electron is increased from 1.06 × 1029 to 81.5 × 1029 Hz when the PbS NPs is added. The µ of the electrons is also increased from 4.85 to 6.22 cm2/(V·s) by adding the PbS NP.