Solar-driven Direct Blue 71 dye decontamination with metal- and non-metal-doped titanium dioxide photocatalysts

Abstract

The presence of azo dyes in aqueous environments poses numerous environmental threats that require effective and sustainable decontamination methods. Herein, commercial TiO2 photocatalysts were co-doped with N and one metal dopant (including Al, Zn, Cr, Fe, Ni, or Cu) for the advanced degradation of Direct Blue 71 (DB71), a typical representative of azo dye, under simulated solar conditions. Nitrogen is hypothesized to trap photogenerated holes, while the metal dopant may serve as a sink for excited electrons. All co-doped TiO2 samples exhibited an improvement in DB71 adsorption capability compared to pristine TiO2 owing to the increased surface area. Meanwhile, their photocatalytic performances were promoted in different manners, which highlighted the importance of the dopant content and the alignment between the impurity energy level and the band structure of TiO2. Among synthesized samples, Fe/N-doped TiO2 displayed the superior photodegradation efficiency of DB71, with approximately 93% of dye being removed in 2 h, due to its highest visible-light absorption capacity and suppressed recombination rate of electron-hole pairs. The DB71 photodegradation of Fe,N-TiO2 sample was primarily attributed to the photogenerated holes, followed by singlet oxygen radicals. Photogenerated holes and singlet oxygen radicals played pivotal roles in degrading DB71 by Fe,N-TiO2 under solar-irradiated conditions. The optimal photocatalytic removal of DB71 of Fe,N-TiO2 occurred at neutral pH, highlighting the unnecessary chemical additions for pH adjustment in actual wastewater systems.