Engineering a Double S-Scheme Heterojunction Using Nd-Based <i>N</i>-GQDs/ZnO for Superior Photocatalytic Performance

The development of highly efficient photocatalysts with enhanced charge separation and reactive species generation is critical for advanced oxidation processes. In this study, a double S-scheme heterojunction of N-CQDs/Nd(OH)3/ZnO was successfully synthesized, exhibiting a significantly increased surface area that promotes abundant active sites and improved light absorption. The optimization studies were carried out by changing the amount of each pristine material to find out the point where they couple best synergistically, and the optimized ternary heterojunction exhibited exceptional catalytic activity, achieving a 92.14% removal efficiency for the target contaminant. The kinetic analysis revealed a rate constant that was 5.16, 4.32, and 2.11 times higher than that of pristine and binary structure Nd(OH)3, ZnO, and Nd(OH)3/ZnO, respectively. The significantly enhanced photocatalytic activity can be attributed to two key factors. The formation of a double S-scheme charge-transfer pathway effectively promotes the separation of photogenerated electron–hole pairs, suppresses recombination, and preserves the strongest redox potential of the system. The incorporation of N-CQDs not only enhances visible-light absorption but also provides additional active sites due to their high surface area and electron reservoir properties. Furthermore, radical trapping experiments confirmed that •O2– and •OH were the dominant reactive species driving the degradation process. GC–MS analysis confirmed that N-CQDs/Nd(OH)3/ZnO degrades TC into mostly harmless intermediates through oxidative bond cleavage. This work provides a strategic design for high-performance double S-scheme photocatalysts with broad applicability in environmental remediation.