Green Chemistry-Assisted Synthesis of Metal Nanoparticles and Fabrication of Microstructurally Engineered Conductive and Endurable M⁰@PEO Functional Films

The present research reports the synthesis of poly-[ethylene oxide]-based composite films (500 μm) containing metal nanoparticles (NPs) [Ag⁰ (<i>d</i> _p ∼ 6 nm), Cu⁰ (<i>d</i> _p ∼ 25 nm), and Fe⁰ (<i>d</i> _p ∼ 35 nm)] as the mobile phase. The novelty of the study is in the corroboration of a plausible mechanism for the generation of metal NPs through green synthesis using herbal extracts of <i>Camellia sinensis</i> (Tea) and <i>Azadirachta indica</i> (Neem). Density functional theory (DFT) is used to optimize the phytoreductants present in both biosources, wherein the reducing and/or stabilizing functional entities are primarily hydroxyl groups (-OH). The transition energy (band gap, Δ<i>E</i> _|LUMO-HOMO|) is found to be minimum for Epicatechin gallate (1.05 eV, tea) and Sitosterol (0.58 eV, neem), which could act as potent phytoreductants for initiating a redox reaction, followed by subsequent capping through secondary bond formation. Upon increasing the loading of metal NPs from 1 to 7 wt %, the ionic conductivity of a PEO composite increases (0.1 S·cm^-1) for PAg_N (N: neem). With a subsequent increase in loading (10 wt %), the crystalline region within PEO is enhanced (≥83% using DSC), which restricts the ion migration and lowers the charge storage capacity, as studied using dielectric constants and complex relaxation processes (EIS and DRT). Among all of the compositions, PCu_N is observed to exhibit negligible performance deterioration (Δtan δ for 18,500 h → 0). However, PAg system(s) are good ion conductors with significant dielectric nature, but they suffer from particle ripening. Hence, metal NPs, which functionalize PEO films, could be effectively synthesized using a green synthesis route and applied as a solid electrolyte for device application.