Rational construction of a direct Z-scheme g-C3N4/CdS photocatalyst with enhanced visible light photocatalytic activity and degradation of erythromycin and tetracycline

The composite photocatalysts consisting of cadmium sulfide (CdS) and graphitic carbon nitride (g-C3N4) with a different mass ratio of CdS were successfully prepared. X-ray diffraction (XRD), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were exploited to characterize the structure, micro-topography and composition of g-C3N4/CdS (CNCS) composite photocatalyst. The optical properties of the as-prepared samples were characterized by ultraviolet-visible diffuse reflection spectroscopy (DRS). The photocatalytic performance of the CNCS composite photocatalysts was confirmed by estimating the photocatalytic activity for the degradation of organic pollutants under the irradiation of simulated solar. The photocatalytic activity of the as-synthesized CNCS composite for the degradation of erythromycin (Ery) and tetracycline (TC) was remarkably enhanced relative to that of pure g-C3N4 and CdS. The CNCS composite with a g-C3N4/CdS mass ratio of 3:1 exhibited the best photocatalytic activity and the photodegradation rate was 2.55 times and 2.14 times higher than that of pure g-C3N4 for Ery and TC, respectively, which was 2.15 times and 4.43 times higher than that of pure CdS for Ery and TC, respectively. The main active species in the photocatalytic reaction process were accurately identified by adding a series of free radical scavenger under the same experimental condition. No apparent deactivation of the CNCS = 3:1 composite photocatalyst was observed in the experiment after degrading Ery and TC three consecutive runs, indicating that the addition of the g-C3N4 substantially improved the photocatalytic stability of CdS nanorods. The concentration of Cd2+ ions dissolved in the supernatant of CNCS-3:1 photocatalysts was lower than that of pure CdS, suggesting that the photocorrosion of CdS could be effectively inhibited via forming heterostructure with g-C3N4. The synergistic effect between CdS and g-C3N4 increases the separation efficiency of the photogenerated carriers, enhancing the photocatalytic activity of CNCS composite samples. Furthermore, a possible photocatalytic mechanism for the enhanced photocatalytic activity of the CNCS composite photocatalyst was proposed based on the band alignments of g-C3N4 and CdS.