N-doped ZnO nanoparticles supported on ZnS based blue phosphors in the photocatalytic removal of eriochrome black-T dye

Photocatalysis appears one of the most viable solutions to remove organic pollutants from wastewater. One of the limitations of commercial application of photocatalysis is the low photoreactivity. It has been reported that ZnO has better photocatalytic activity compared to TiO2; however, due to the value of its band-gap energy, about 3.4 eV, it is effective only under irradiation of UV light. In order to increase the photocatalytic performances of ZnO, the doping of its crystalline structure with nitrogen could make possible the exploiting of the visible light. Moving from our finding that N-doped TiO2 coupled with another semiconductor, like ZnS based phosphors (ZSP), leads to enhanced photocatalytic activity, the aim of this work was to verify the effect of coupling N-doped ZnO (N-ZnO) with ZnS-based phosphors (ZSP) in the photocatalytic removal of eriochrome black-T (EBT), used as model dye, since the latter represents more than a half of the global dye production. To get ZSP supported N-ZnO nanoparticles (N-ZnO/ZSP), a modified sol-gel method was used. The nominal content of N-doped ZnO was varied from 15 to 70 wt%. The photocatalytic tests were carried out with a pyrex cylindrical photoreactor equipped with an air distributor device, a magnetic stirrer to maintain the photocatalyst suspended in the aqueous solution, and four UV lamps (nominal power: 32 W) with wavelength emission centred at 365 nm. ZSP phosphors emit in visible region when activated with UV light. The visible light emitted by phosphors particles is able to photoexcite N-ZnO nanoparticles supported on their surface. Photocatalytic results showed that, on unsupported N-doped ZnO nanoparticles, total EBT discoloration was reached after 2h of irradiation, however total organic carbon (TOC) removal was only 20 %. Also ZSP particles showed ability in EBT photodiscoloration, moreover leading to a higher TOC removal (about 47 % for both cases). A very interesting result has been found with N-ZnO/ZSP catalyst with a nominal content of N-ZnO equal to 30 wt %, for which the highest photoactivity was observed, allowing to obtain both the complete EBT discoloration and the total TOC removal within the same time of UV irradiation.