Impact of Heterovalent Cu2+ and Ag+ Doping on the Structural, Optoelectronic, and Photocatalytic Properties of ZnO for Enhanced Solar-Driven Hydrogen Evolution and Organic Pollutant Degradation

The preparation of photocatalysts capable of harnessing solar light energy as a sustainable and renewable source for treating wastewater containing organic pollutants and for H₂ production from water splitting represents a significant direction in the photocatalyst field. In this study, S-scheme CuO/Ag₂O–ZnO heterostructures with different CuO contents were prepared by the sol–gel method. XRD results confirmed the doping of ZnO by Ag⁺ and Cu²⁺. Also, the intensity of peaks decreased obviously after the addition of Ag⁺, while the addition of Cu²⁺ was accompanied by an increase in the peak intensity. UV–visible analysis results displayed a significant red shift of the absorption edge to the visible region, and a remarkable reduction in the band gap of ZnO was detected after the addition of Ag⁺ and Cu²⁺. The photocatalytic performance was evaluated by photodegradation of MB, TC, and H₂ evolution under sunlight illumination. The results revealed that the addition of Ag⁺ and Cu²⁺ enhanced the photocatalytic activity of ZnO significantly due to the formation of the S-scheme (p–n)-heterojunction heterostructure, which efficiently improved the separation and increased the lifetime of the photogenerated charge carriers compared with pure ZnO nanoparticles. The ESR and radical quenching results revealed that ^•O₂^– and ^•OH are the active radicals in the photodegradation of methylene blue (MB) and tetracycline (TC). The photodegradation mechanism, mineralization (TOC), and kinetic degradation were studied. The CuO/Ag₂O–ZnO heterostructure exhibited excellent photocatalytic activity, reusability, and stability.