Enhanced photocatalytic activity of g-C₃N₄-ZnO/HNT composite heterostructure photocatalysts for degradation of tetracycline under visible light irradiation

A novel graphitic carbon nitride (g-C₃N₄)-ZnO/halloysite nanotube (HNT) nanocomposite photocatalyst was synthesized via a facile calcination method in order to enhance the visible-light photocatalytic activity and stability of pure ZnO photocatalysts for degradation of tetracycline. The network-layered structure of g-C₃N₄ was formed after compositing with previously prepared ZnO/HNTs and the g-C₃N₄-ZnO heterojunction has been formed during the coupling process. Furthermore, the HNTs can efficiently extend the surface area of g-C₃N₄, which leads to strengthening of the pathways of charge transfer and prolonging the lifetimes of photoexcited carriers. Electrochemical impedance spectroscopy (EIS) and incident-photon-to-current conversion efficiency (IPCE) measurements showed the improvement of the as-obtained g-C₃N₄-ZnO/HNT photocatalysts' performance which can be attributed to enhanced charge transfer as a result of more effective separation of photogenerated electron-hole pairs. Such a notable enhancement of photocatalytic performance was mainly ascribed to the improved charge transfer and separation rate of photogenerated electron-hole pairs by the heterostructure of the g-C₃N₄-ZnO/HNT catalyst. The mechanism of photodegradation was systematically analysed by active species trapping test and electron spin resonance (ESR) spin-trap technique with dimethyl pyridine N-oxide (DMPO), which conclude that OH and O₂^- radicals are the major reactive species during the photocatalytic reaction for g-C₃N₄-ZnO/HNT composite photocatalysts.