Eco-friendly enhancement of optical and structural properties in polyvinyl alcohol films via eggplant peel dye doping

Abstract

This study presents an eco-friendly strategy to enhance the optical and structural properties of polyvinyl alcohol (PVA) films through doping with eggplant peel dye (EPPD), a natural pigment extracted from agricultural waste via a green aqueous synthesis (~ 33% yield from 30 g of peel). EPPD was uniformly dispersed in PVA films (PVA-D1, PVA-D2, PVA-D3) using an ultrasonic-assisted solution casting technique, with chitosan (CS) added to prevent fungal growth. Comprehensive characterization (Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), ultraviolet-visible spectroscopy (UV-Vis), and differential scanning calorimetry (DSC)) confirmed successful integration of EPPD, revealing its polyhydric alcohol content, amorphous nature, and uniform distribution within the polymer matrix. Doping with EPPD significantly reduced the optical band gap from 6.314 eV (pure PVA) to 1.8 eV (PVA-D3), introducing localized states that enhanced light absorption (peaking at 554 nm in PVA-D3), as supported by Tauc's model (transition type: direct allowed → direct forbidden) and dielectric loss analysis. Additionally, the refractive index increased from 1.165 to 1.27, while the optical dielectric constant (ε₁) improved from 1.366 to 1.609 due to enhanced charge carrier density. XRD analysis revealed a decrease in crystallinity from 30.50% (pure PVA) to 18.11% (PVA-D3), leading to a reduction in the glass transition temperature (from 30.5 °C to 25 °C) and melting temperature (from 240 °C to 194 °C). The Urbach energy (Eu), an indicator of structural disorder, increased from 0.43 eV (pure PVA) to 0.62 eV (PVA-D3), reflecting a higher density of localized states in the amorphous matrix and broader tail states in the band structure. These tunable optoelectronic properties position EPPD-doped PVA films as promising candidates for various applications, including UV-protective textiles, smart packaging, biomedical dressings, and energy-efficient optoelectronic devices.

Keywords: Eco-friendly optoelectronic materials; Eggplant peel dye (EPPD); Natural dye-doped polymers; Polyvinyl alcohol (PVA); Tunable band gap materials.

Scheme 1

Synthesis of EPPD-doped PVA films. (a) Extraction and purification of EPPD dye from eggplant peel. (b) Synthesis of EPPD-doped PVA films. (c) Photographs of pure PVA, PVA-C, PVA-D1, PVA-D2, and PVA-D3.

Fig. 1

FTIR spectra for (a) pure PVA, (b) chitosan, (c) EPPD, (d) PVA-C, (e) PVA-D1, (f) PVA-D2, and (g) PVA-D3.

Scheme 2

Proposed mechanism for hydrogen bond interactions in EPPD-doped PVA films.

Fig. 2

XRD patterns of (a) EPPD, (b) pure PVA, (c) PVA-C, (d) PVA-D1, (e) PVA-D2, and (f) PVA-D3.

Fig. 3

FESEM images of composite films (a) Pure-PVA, (b) PVA-C, (c) PVA-D1, (d) PVA-D2, and (e) PVA-D3.

Fig. 4

The absorption spectra of all the films show a clear shift to higher wavelengths with increasing dye concentration in the PVA.

Fig. 5

(a) Absorption coefficient vs. photon energy for all films, showing a red shift in the absorption edge with increasing EPPD concentration. (b) Refractive index spectra indicating an enhancement from 1.165 (Pure-PVA) to 1.270 with EPPD doping. (c) Optical dielectric constant (ε₁) increases proportionally with EPPD concentration, highlighting improved polarizability. (d) Urbach energy analysis for all films, reflecting changes in structural disorder with doping.

Fig. 6

Plot of (αhν)^1/γ as a function of photon energy (hν) is presented for all samples. (a) (αhν)² vs. (hν) direct Allowed (b) (αhν)^2/3 vs. (hν) direct Forbidden (c) (αhν)^2/3 vs. (hν) direct Allowed (d) (αhν)^1/2 vs. (hν) indirect forbidden.

Fig. 7

The variation of optical dielectric loss with photon energy (hv) is presented for all samples. Notably, distinct linear regions are observed at higher photon energy levels, which can serve as a basis for determining the optical band gap.

Fig. 8

Comparison of the band gap energy of various PVA-based films.

Fig. 9

Differential Scanning Calorimetry (DSC) Analysis of PVA and EPPD-doped PVA Films: Effect of EPPD on Glass Transition (Tg) and Melting Temperature (Tm).