Tuning the optical band gap of polyvinyl alcohol via aqueous nitrogen-doped carbon dots: a sustainable nanocomposite approach

Polyvinyl alcohol (PVA) is a transparent, biocompatible, low-cost polymer with excellent film-forming capability, yet its wide intrinsic optical band gap restricts its use in optoelectronic systems. Here, a simple and sustainable method is reported to tailor the optical properties of PVA by incorporating aqueous nitrogen-doped carbon dots (ANCDs). These ANCDs were produced via a one-step hydrothermal route using 4-aminoantipyrine as the sole precursor. The prepared ANCDs exhibited strong blue photoluminescence, an average diameter of approximately 5 nm, and a quantum yield of 18%. PVA/ANCD nanocomposite films were prepared through a straightforward solution-casting technique with varying ANCD loadings. FT-IR and XRD analyses revealed robust intermolecular interactions between PVA chains and surface functional groups of ANCDs, along with a progressive decrease in polymer crystallinity. UV-Vis optical measurements showed a substantial red shift in the absorption edge and a marked narrowing of the optical band gap from 5.42 eV (pure PVA) down to 2.9 eV at the highest ANCD concentration. Additionally, the refractive index rose from 1.18 to 1.24 at λ ≈ 800 nm, while the real part of the dielectric constant increased from 1.3 to 1.8 at λ ≈ 800 nm, attributed to enhanced electronic polarization and the generation of localized electronic states within the polymer matrix. These findings demonstrate that ANCDs effectively modify the electronic and optical structure of PVA, producing substantial optical bandgap narrowing and lower-band-gap characteristics relevant to future optoelectronic and photonic applications. The distinctive contribution of this work is the correlation of the concentration-dependent structural changes induced by 4AA-derived ANCDs with the resulting modification of the optical-transition behavior of PVA. The developed nanocomposite films offer a straightforward, eco-friendly, and potentially scalable platform for future polymer-based optoelectronic and photonic applications.