Highly efficient sunlight-driven photo-adsorptive degradation of organic pollutants by green synthesized Z-scheme heterojunction CeO2@ZnO nanocomposite†

Creating a high-efficiency heterojunction with enhanced photocatalytic properties is considered a promising approach to wastewater decontamination. Herein, Sapindus mukorossi seed extract was used to act as capping and reducing agent due to the presence of saponins and polyphenols during the synthesis of ZnO and CeO₂ nanoparticles. Sharp PXRD peaks confirmed that the spherical nanocomposite had great crystallinity and purity. The CeO₂@ZnO nanocomposite efficiently removes eriochrome black T (EBT) dye (98%) and endosulfan (ES) pesticide (96%). In addition to improved redox capacity, the heterojunction system exhibits quick transfer, long lifetime of photoinduced charge carriers, high-efficiency separation, and long-lived charge carriers. The band gap of ZnO observed was 3.1 eV and that of CeO₂ was 2.8 eV which decreased after doping to 2.6 eV which showed the Z-scheme of CeO₂@ZnO nanoparticles. The flow of electrons and holes followed the unique Z-scheme heterojunction mechanism between hierarchical ZnO and CeO₂ which produced active radical species. First-order kinetics followed by initial Langmuir adsorption constituted the degradation process. From experiments using different radical quenchers (t-BuOH, p-BZQ, Na₂EDTA), it was concluded that peroxide radical plays a significant role in the degradation of toxic EBT and ES. The green-fabricated nanocomposite also showed excellent efficiency in the degradation of ES and EBT pollutants in actual wastewater samples. LC-MS analysis confirmed the formation of safer metabolites after the degradation of both pollutants. This study offers a fresh and green methodology for building Z-scheme heterojunctions of modified ZnO in photocatalysis application.