A CQD/CdS/g-C3N4 photocatalyst for dye and antibiotic degradation: Dual carrier driving force and tunable electron transfer pathway

Photogenerated transfer and catalytic reaction mechanism in CQD/CdS/g-C3N4. [Display omitted] •A novel quasi-ternary Z-scheme heterojunction was constructed;•The catalyst has dual driving forces such as built-in electric field and energy level barrier to realize the induced charge multi-gradient transfer;•Reasonable degradation paths were proposed based on the point charge theory and characterization of LC-MS. The path and mechanism of charge transfers in composite catalyst are important for degrading organic pollutants effectively. A quasi-ternary Z-scheme heterojunction photocatalyst with carbon quantum dots (CQD), CdS, and graphitic carbon nitride (g-C3N4) (CQD/CdS/g-C3N4) has been successfully constructed in this work, and its electron transfer path is elucidated by contrast CdS/CQD/g-C3N4 photocatalyst. The built-in electric field at the interface between CdS and g-C3N4 induces exciton separations. The energy barrier between CQD and CdS accelerated charge transfers and further prevents exciton annihilation. As a result, CQD/CdS/g-C3N4 exhibits the greatest photocatalytic degradation, and the removal rates of Rhodamine B and sulfadiazine are 94.7 % and 76.0 %, which are 2.46 times and 1.47 times of CdS/CQD/g-C3N4, 7.02 times and 7.92 times of g-C3N4, respectively. It is further proved that CQD as the active site can promote the generation of H2O2 and effectively inhibit the formation of •O2– in the photocatalytic reaction. In addition, the degradation pathway and products of sulfadiazine are explored by means of HPLC-MS and calculation of Gaussian software. This work demonstrates the relationship between photocatalytic activity and electron transfer path, meanwhile, provides a reference for the design of efficient heterogeneous photocatalysts for environmental remediation.