Bandgap tailoring of ZnO using metallic sulphides for enhanced visible-light-active photocatalytic water treatment

Abstract

The combination of a wide bandgap Zinc oxide (ZnO ~3.4 eV) semiconductor photocatalyst with typical metallic sulphides, namely, silver sulphide (Ag2S), ferric sulphide (FeS) and nickel sulphide (NiS), all of a similar narrow bandgap (~0.90 eV), resulted in the red-shifting of photocatalytic activity towards the visible-light region. In this work, Ag2S/ZnO, FeS/ZnO and NiS/ZnO nanocomposites were synthesised via a facile combustion method. The physicochemical properties of the synthesised nanocomposites were characterised using X-ray diffractometer (XRD), scanning electron microscopy (SEM), and Ultraviolet diffuse reflectance spectroscopy (UV-vis DRS) techniques. Data obtained from the UV-vis DRS results were interpolated to estimate the bandgap energy of each nanocomposite using Tauc plots. The bandgap energy of ZnO was tailored from 3.30 eV to 2.15 eV, 1.91 eV and 1.85 eV for Ag2S/ZnO, FeS/ZnO and NiS/ZnO. The photocatalytic efficiency of the assynthesised nanocomposites at was investigated on the removal of 10 mgL-1 of methylene blue (MB) dye at 1 gL-1 catalyst loading. Results showed that Ag2S/ZnO, FeS/ZnO and NiS/ZnO achieved the removal of 99 %, 88 % and 81 % of MB dye after 120 min and a complete removal after 150 min of simulated visible light irradiation. The slope of the linear regression curve was calculated and found to follow a pseudo-first-order reaction kinetics with R2 values higher than 0.96. Finally, the results obtained from this study point to a way forward in the concept of bandgap engineering in photocatalysis as well as the efficient treatment method of dye-polluted effluent from the textile industry.