ZnS quantum dots implanted polyaniline-wrapped corn straw catalysts for efficient photocatalytic nitrate reduction without external addition of hole scavengers

Photocatalytic reduction to harmless nitrogen gas is considered a promising method for the removal of nitrate from water. However, achieving high efficiency of photocatalytic nitrate reduction, especially without the assistance of a hole scavenger, remains a critical issue. Herein, PANI-CS (PC) is prepared by in-situ polymerization of aniline on the surface of biomass corn straw, and then ZnS QDS is loaded to synthesize ZnS QDs-PANI-CS (ZPC) catalysts for photocatalytic reduction of nitrate. The optimized ZPC shows highly selective and efficient photocatalytic reduction of NO3– without extra sacrificial agents, with the 0.8% ZPC delivering an excellent nitrate removal rate of 98.4% and N2 selectivity of 96.8%. Outstanding catalytic performance is attributed to the ZPC having higher electron mobility and longer electron lifetime. Additionally, ZnS QDs induce the conversion of the quinone ring of PANI to the benzene ring, thereby improving the reducibility of the catalyst. Most importantly, The carbon–sulfur double bond on the ZnS quantum dots and the bicarbonate on the surface of the straw are the sources of formic acid, which provides great help for the production of CO2•- with reduction activity. The introduction of ZnS QDs also enriches the Lewis acid sites on the ZPC surface, thereby enhancing the N2 selectivity, and simultaneity, the stability of the composite is consolidated due to the formation of N-Zn covalent bonds, laying the foundation for the commercialization of the catalyst. This work introduces the characteristics of self-produced formic acid into photocatalysts through ingenious design, which provides a new idea for the photocatalytic treatment of nitrate wastewater.