Investigation of optical band gap in PEO-based polymer composites doped with green-synthesized metal complexes using various models

Abstract

In this study, PEO-based polymer composites with enhanced optical properties were fabricated by employing a green chemistry approach. A Mn(II) metal complex was synthesized by combining dissolved manganese acetate with an extract of black tea (BT). Then, polymer composite films were prepared using a casting method, incorporating different concentrations of the Mn–polyphenol complex into the PEO matrix. Characterization of the Mn(II) complex was carried out using XRD, FTIR, and UV-vis spectroscopy. The results demonstrated that black tea extract is an effective medium for synthesizing the Mn(II)–polyphenol complex. FTIR analysis confirmed the formation of Mn-PPHNL complexes and their interaction with BT. XRD patterns indicated the amorphous nature of the Mn–polyphenol complex and showed that increasing the complex concentration led to a more amorphous PEO matrix, which was further analyzed using Urbach energy. Additionally, morphological analysis using an optical microscope (OM) image demonstrates that the size of the spherulites attributed to the crystalline phase drastically reduces as the concentration of Mn–polyphenol in a PEO polymer composite increases. UV-vis spectroscopy revealed key optical features, including optical density (n), Urbach energy (E_u), band gap (E_g), and localized state density (N/m*). Various models, including Tauc's model, absorption edge, ASF, optical dielectric loss, Cody representation and differentiation (dT/dλ, dR/dλ, dn/dλ) approaches, were used to identify the optical band gap of the films. Tauc's plots confirmed the nature of electronic transitions. Dielectric loss measurements indicated a reduction in the PEO bandgap from 5.5 eV to 1.4 eV upon metal complex incorporation. The ASF method further corroborated bandgap changes using only absorbance data. Metallization criteria were applied to classify the polymer's behavior as insulating or metallic. A redshift in absorption edge with increasing complex concentration, from 5.3 eV to 1.42 eV, confirmed the successful interaction between PEO and the metal complex, as validated by UV-vis spectroscopy.