A Z-scheme Cobalt(II) oxide-silver tungstate nano photocatalyst: Experimental design and mechanism study for the degradation of methylene blue

Abstract

Ag2WO4 and CoO nanoparticles (NPs) and their binary coupled CoO-Ag2WO4 system (CS) were prepared and characterized via powder UV-Vis diffuse reflectance spectroscopy (DRS) and scanning electron microscopy equipped with an energy dispersed x-ray system (SEM-EDX). The band gap energies for CoO, Ag2WO4, and CoO/Ag2WO4 were obtained as 3.07, 3.11, and 2.67 eV by DRS data, while their pHpzc values were 7.9, 7.4, and 7.4. In the initial methylene blue (MB) removal steps, about 22, 23, and 35% MB molecules were removed by CoO, Ag2WO4, and CoO/Ag2WO4 photocatalysts. The CoO: Ag2WO4 mole ratio of 1:2 had the best photocatalytic efficiency. In the RSM study, the model goodness was confirmed by the model F-value of 491.13 > F0.05, 13, 12 = 2.4, and the lack-of-fit F value of 4.19 < F0.05, 10, 3 = 8.79. The optimal runs’ conditions were pH: 11, catalyst dosage: 0.3 g/L, irradiation time: 60 min, and CMB: 3 mg/L. A scavenging agent study confirmed all four reactive species have critical roles in the MB photodegradation, while the main critical effects belong to the photogenerated holes and superoxide radicals. Finally, the results confirmed that the photodegradation pathway follows the direct Z-scheme mechanism to degrade MB molecules because this pathway accumulates the photoinduced electrons in the CB-CoO and the holes in the VB- Ag2WO4 as the critical reducing and oxidizing centers, respectively.