A double-Z-scheme ZnO/AgI/WO3 photocatalyst with high visible light activity: Experimental design and mechanism pathway in the degradation of methylene blue

A double Z-scheme AgI/ZnO/WO3 photocatalyst was constructed and characterized by various characterization techniques. The crystallite size of about 25 nm was estimated by the Williamson-Hall method. The PL intensity of the ternary system was decreased showing a lower electron/hole recombination that favors the photocatalytic activity. The band gap values of 2.71, 2.84. 3.15, and 2.79 eV were obtained for the direct electronic transitions in AgI, WO3, ZnO, and the ternary system, respectively. Simultaneous interactions between the influencing variables on the degradation of methylene blue (MB) were investigated by RSM with an optimal run, including pH 4, catalyst dose of 3.8 g/L, C-MB 7 mg/L, and irradiation time of 48.5 min. The goodness of the model was confirmed by the model F-value of 116.9 > F-0.05(,)14(,)14 = 2.48, and the LOF F-value of 0.75 < F-0.05, (10).4 = 5.96. The effects of various scavengers were investigated that the results confirmed that the photogenerated holes and hydroxyl radicals have the most essential roles in MB photodegradation by the composite. The composite can be considered as a double Z-scheme system, including WO3-ZnO and ZnO-AgI systems. The overall composite accumulates electrons in AgI-CB and the holes in WO3-VB. The Agl-CB and WO3-VB positions are more substantial reducing and oxidizing centers than the WO3-CB and AgI-VB positions, respectively, which later cases were accumulated by the electrons and holes in a cascade mechanism. Based on the potential positions, the Z-scheme mechanism is a better pathway to illustrate the MB photodegradation by the composite. This mechanism also can use for the reduction of protons in hydrogen production and water splitting to evolve oxygen molecules. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

A double Z-scheme AgI/ZnO/WO3 photocatalyst was constructed and investigated for methylene blue (MB) degradation, revealing that photogenerated holes and hydroxyl radicals play crucial roles in MB photodegradation. The composite is considered to be a double Z-scheme system with interactions between WO3-ZnO and ZnO-AgI systems, accumulating electrons and holes in AgI-CB and WO3-VB, respectively, through a cascade mechanism.