Nanoflower-like ZnO–carbon quantum dot heterostructures for solar-driven degradation of methylene blue: a high-performance and recyclable photocatalyst for sustainable wastewater treatment

Developing robust, high-performance photocatalysts for environmental remediation remains a critical scientific pursuit. This work successfully synthesized novel ZnO–carbon quantum dot (CQD) heterostructured nanocomposites with distinct nanoflower-like morphology by incorporating 5%, 10%, and 15% CQDs onto ZnO surfaces. The strategic integration of CQDs not only enhanced solar light harvesting and facilitated superior charge carrier separation, but also improved the recyclability and stability of the composites, surpassing the limitations of conventional photocatalytic systems. Comprehensive characterization using XRD, FTIR, XPS, BET, PL, UV-Vis-DRS, FE-SEM, EDS, and HR-TEM analyses confirmed the synthesized composites' high crystallinity, enlarged surface area, morphology, and light response. Photocatalytic studies conducted under natural sunlight irradiation demonstrated an impressive 97.7% degradation of methylene blue (MB) in merely 60 minutes, following pseudo-first-order kinetics and achieving a rate constant of about 0.047 min⁻¹, which is 5.7 times greater than that of the benchmark TiO₂–P25 catalyst. Systematic investigations of solution pH, photocatalyst dosage, light sources, and radical scavenging further validated the robustness and versatility of the composite. Impressively, the ZnO/CQD nanocomposite retained 85% of its photocatalytic efficiency after six consecutive cycles, demonstrating exceptional reusability and operational stability. The photocatalytic degradation pathway was determined using LC-MS analysis, showing notable decreases of 65% in TOC (total organic carbon) and 58% in COD (chemical oxygen demand), which confirmed effective mineralization. Given the persistence, toxicity, and carcinogenic potential of MB in aquatic ecosystems, this study introduces a sustainable, scalable, and highly effective solar-driven photocatalyst. These outstanding results position the ZnO/CQD nanocomposite as a leading candidate for next-generation wastewater remediation technologies and offer a compelling advancement warranting publication in high-impact scientific forums.