Anthraquinone-rich Rheum ribes L. as a source of nitrogen-doped carbon quantum dots for ZnO-based S-scheme heterojunction photocatalysts in tetracycline degradation

Photocatalysis has emerged as a promising, sustainable solution to address environmental pollution caused by industrial growth and increasing societal demands. An ideal photocatalyst requires both strong redox capabilities and environmentally benign materials. In this study, nitrogen-doped carbon quantum dots (N-CQDs) were synthesized from anthraquinone-rich Rheum ribes L., using an eco-friendly approach that leverages natural sources. These N-CQDs were then combined with ZnO to form a heterojunction photocatalyst for the degradation of tetracycline (TC) as a model pollutant under UV light irradiation. The N-CQDs/ZnO composite exhibited significantly enhanced photocatalytic activity compared to its individual components, attributed to the robust interactions between the polar functional groups of N-CQDs and ZnO, as verified through advanced characterization techniques. The photocatalytic performance of the composite was systematically assessed by varying operational parameters (pH, temperature, catalyst/pollutant amount), and optimization of the N-CQDs content within the heterojunction produced a synergistic effect, increasing light absorption and promoting efficient charge separation through an S-scheme heterojunction mechanism. With the highly efficient catalyst in hand, the scope of pollutants was expanded to include dyes, heavy metals, and various antibiotics, demonstrating the catalyst's broad applicability. Radical trapping experiments further suggested a photooxidation mechanism while GC-MS results showed the intermediates transition products during the degradation of TC. These findings indicate that the N-CQDs/ZnO heterojunction is a highly effective photocatalyst for antibiotic contaminant removal, offering promising potential for advanced water treatment applications.