Oxygen defects-mediated Z-scheme charge separation in g-C₃N₄/ZnO photocatalysts for enhanced visible-light degradation of 4-chlorophenol and hydrogen evolution

g-C₃N₄nanosheets were coupled with oxygen-defective ZnO nanorods (OD-ZnO) to form a heterojunction photocatalyst with a core-shell structure. Multiple optical and electrochemical analysis including electrochemical impedance spectroscopy, photocurrent response and steady/transient photoluminescence spectroscopy revealed that the g-C₃N₄/OD-ZnO heterojunction exhibited increased visible-light absorption, improved charge generation/separation efficiency as well as prolonged lifetime, leading to the enhanced photocatalytic activities for the degradation of 4-chlorophenol under visible-light illumination (λ > 420 nm). An oxygen defects-mediated Z-scheme mechanism was proposed for the charge separation in the heterojunction, which involved the recombining of photoinduced electrons that were trapped in the oxygen defects-level of OD-ZnO directly with the holes in the valence band of g-C₃N₄at the heterojunction interface. The detection of surface generated reactive species including [rad]O₂⁻and [rad]OH clearly supported the Z-scheme mechanism. Moreover, the g-C₃N₄/OD-ZnO photocatalysts also exhibited enhanced visible-light Z-scheme H₂evolution activity, with an optimal H₂evolution rate of about 5 times than that of pure g-C₃N₄. The present work not only provided an alternative strategy for construction of novel visible-light-driven Z-scheme photocatalysts, but also gained some new insights into the role of oxygen-defects of semiconductors in mediating the Z-scheme charge separation.