Evidence for the degradation of an emerging pollutant by a mechanism involving iso-energetic charge transfer under visible light

This study presents the first reports for ZnO/CuxO sequentially sputtered leading to stable and uniform films effective accelerated degradation of the emerging pollutant Norfloxacin (NFX) under visible light. The most effective ZnO film was sputtered from a Zn -target for 4 min followed by Cu-sputtering for 30 s on non-thermal resistant polystyrene (PS). The NFX-degradation proceeded more readily on ZnO/CuxO compared to ZnO-films although the conduction band potential energy position of the two semiconductors were iso-energetic. By X-ray photoelectron spectroscopy (XPS), Cu2O was identified as the predominant Cu-oxidation state in the sputtered film. A mechanism for the NFX degradation is suggested in which the holes generated by Cu2O transfer to ZnO in the ZnO(n)/CuO(p) in the hetero-junction. This transfer is favored by the electrostatic interaction between both semiconductors. The catalyst optical properties and surface potential were monitored during NFX degradation. Stable repetitive NFX degradation kinetics was observed under visible light on the sputtered ZnO/CuxO films. The use of appropriate scavengers allowed the identification of the intermediate radical species (ROS). XPS analysis provided the proof for redox catalysis taking place within the time of NFX abatement. The application of ZnO/CuxO films in a photo-reactor for the degradation of emerging pollutants seems feasible due to the low cost of Zn and Cu and the fast kinetics found for NFX degradation.