Tandem photocatalytic oxidation of Rhodamine B over surface fluorinated bismuth vanadate crystals

Abstract

BiVO₄ crystals with monoclinic-phase and controllable morphologies were synthesized by NaF-mediated hydrothermal processes using Bi(NO₃)₃ and V₂O₅ as precursors. The NaF added as a structural controller not only affected the crystal evolution processes of BiVO₄ crystals, but also enabled the in situ surface fluorination of the as-synthesized BiVO₄ crystals. Interestingly, the photocatalytic oxidation reactions of RhB occurred in a stepwise manner over fluorinated BiVO₄ photocatalyst, that is, a faster de-ethylation process (conversion of RhB into rhodamine) followed by a relatively slower mineralization process, involving the destruction of the conjugated structure in rhodamine. Surface fluorination favored the RhB adsorption and hole transfer between RhB molecules and BiVO₄ photocatalyst, thus progressively enhancing the initial direct hole transfer mediated de-ethylation process. In contrast, surface fluorination exerts compromised effects on the ·O₂ ⁻ mediated mineralization process, enhancing surface RhB adsorption versus retarding electron transfer from BiVO₄ photocatalyst to O₂ giving rise to ·O₂ ⁻, and consequently, moderate surface fluorine coverage is required to balance the aforementioned conflicting effects and achieve the higher mineralization rate. The present study not only demonstrates that the photocatalytic efficiency can be modified by tuning photogenerated active species and photocatalytic reaction processes, but also provides new insights into the fluorination effects on the semiconductor photocatalysis.