Z-scheme CdS/CQDs/g-C3N4 composites with visible-near-infrared light response for efficient photocatalytic organic pollutant degradation

We have successfully synthesized novel Z-scheme CdS/CQDs/g-C3N4 composites with visible-near-infrared light response for the photocatalytic degradation of rhodamine B (RhB), methylene Blue (MB) and phenol. Based on the energy band matching theory, CdS was coupled with g-C3N4 using carbon quantum dots (CQDs) as the electron mediator to form the Z-scheme heterojunctions through a simple calcination process. Compared with the single-phase and binary composites, the Z-scheme CdS/CQDs/g-C3N4 composites not only exhibited enhanced photocatalytic activity and photostability but also realized near-infrared light response. CQDs, as the electron mediator, can shuttle the electrons in the CdS/CQDs/g-C3N4 interface via the Z-scheme electron transfer pathway, which lead to improvements in charge separation and oxidizability of the composites. The Z-scheme electron transfer was verified using various techniques, including PL, EIS, EPR and transient photocurrent response. The mechanism of Z-scheme charge transfer was also proposed for the improved photocatalytic RhB degradation activity. In addition, CQDs can capture near-infrared light through the upconversion fluorescence property, ameliorating the broad- spectrum photocatalytic competence. Therefore, the Z-scheme heterojunction with visible-near-infrared light response was utilized to improve charge separation, oxidizability and solar energy utilization, as well as to provide new insights for the construction of CQDs-based Z-scheme composites for photocatalysis applications.