Walnut-Lignin-Derived Boron-Doped Carbon Quantum Dots on Zinc Oxide for Congo Red Photocatalytic Degradation

In this study, a sustainable strategy was utilized to synthesize boron-doped carbon quantum dots from deep eutectic solvent-extracted lignin derived from walnut shell biomass. These dots were subsequently anchored onto zinc oxide surfaces to engineer highly active heterojunctions. Comprehensive characterizations confirmed uniform decoration without disrupting the wurtzite zinc oxide lattice. Interfacial analyses revealed strong electronic interactions, whereas Brunauer-Emmett-Teller measurements demonstrated a significant enhancement in specific surface area (17.65 m²/g) and porosity, providing abundant active sites. Optically, dot integration extended the light absorption edge into the visible region and narrowed the band gap from 3.08 to 3.02 eV. The interfacial contact suppressed electron-hole recombination, corroborated by photoluminescence quenching. Consequently, the nanocomposite exhibited exceptional photocatalytic degradation of Congo Red, achieving a pseudo-first-order rate constant of 0.0513 per minute under ultraviolet irradiation, approximately 1.41 times higher than pristine zinc oxide. Mechanistic studies identified superoxide radicals as the primary active species driving the degradation process. Exhibiting robust stability over four consecutive cycles, this work highlights the potential of strategically engineered, biomass-derived carbon quantum dot and metal oxide interfaces as eco-friendly, high-performance photocatalysts for wastewater treatment.