Modification of a Z-scheme ZnO-CuO nanocomposite by Ag loading as a highly efficient visible light photocatalyst

Abstract

This paper has conducted a comparative study between ZnO-CuO and ZnO-CuO-Ag nanocomposites from a characterization and photodegradation mechanism point of view. The photocatalysts with different CuO contents and Ag loadings were synthesized through a quick and convenient co-precipitation method. The microstructural analysis showed the ZnO nanorods and CuO quasi-spherical formation with uniform distribution and proper contact, while Ag induced morphological changes to aggregation of spherical nanoparticles. X-ray photoelectron spectroscopy (XPS) proved the presence of metallic Ag (Ag0) in the Ag-loaded samples. 5 mol.% Ag loading in the ZnO-7.5 mol.%CuO nanocomposite increased band gap energy from 2.85 to 2.94 eV. Significant PL (photoluminescence) intensity reduction was observed by compositing 7.5 mol.% CuO, whereas Ag loading did not cause notable change. Among various prepared nanocomposites, the ZnO-7.5 mol.%CuO with 5 mol.% Ag loading was found to be the most influential with almost complete photodegradation efficiency toward methylene blue (MB) after 90 min under visible light irradiation. Both ZnO-CuO and ZnO-CuO-Ag nanocomposites exhibited a Z-scheme mechanism for charge transfer proposed based on the radical scavenging experiments. Superoxide radicals were the main active species participating in photocatalytic reactions. The loaded Ag acted as an electron mediator and improved the charge separation efficiency in the ZnO-CuO-Ag nanocomposite. The recycling of the nanocomposite was evaluated and the results showed a reusable photocatalyst with high stability for environmental wastewater purification.