A Bi2WO6/Ag2S/ZnS Z-scheme heterojunction photocatalyst with enhanced visible-light photoactivity towards the degradation of multiple dye pollutants

A novel visible-light-driven Z-scheme heterojunction, Bi2WO6/Ag2S/ZnS, was synthesized and its photocatalytic activity was evaluated for the treatment of a binary mixture of dyes, and its physicochemical properties were characterized using FT-IR, XRD, DRS and FE-SEM techniques. The Bi2WO6/Ag2S/ZnS Z-scheme heterojunctions not only facilitate the charge separation and transfer, but also maintain the redox ability of their components. The superior photocatalytic activity demonstrated by the Z-scheme Bi2WO6/Ag2S/ZnS attributes its unique properties such as the rapid generation of electron-hole pairs, slow recombination rate, and narrow bandgap. The performance of the Bi2WO6/Ag2S/ZnS was evaluated for the simultaneous degradation of methyl green (MG) and auramine-O (AO) dyes, while the influences of the initial MG concentration (4-12 mg L-1), initial AO concentration (2-6 mg L-1), pH (3-9), irradiation time (60-120 min) and photocatalyst dosage (0.008-0.016 g L-1) were investigated through the response surface methodology. The desirability function approach was applied to optimize the process and results revealed that maximum photocatalytic degradation efficiency was obtained at optimum conditions including 6.08 mg L-1 of initial MG concentration, 4.04 mg L-1 of initial AO concentration, 7.25 of pH, 90.58 min of irradiation time and 0.013 g L-1 of photocatalyst dosage. In addition, a possible photocatalytic mechanism of the Bi2WO6/Ag2S/ZnS heterojunction was proposed based on the photoinduced charge carriers.