Modulating photoinduced chlorine activation pathways and reactive species via facet engineering of bismuth vanadate

Coupling heterogeneous photocatalysis with free chlorine (HOCl/ClO^-) emerges as an effective strategy to enhance the yield of reactive species, while the chlorine activation mechanism is yet to be clear. In this study, facet- and morphology-engineered BiVO₄ was synthesized and employed to activate HOCl/ClO^- under visible light irradiation, termed as Vis/BiVO₄/chlorine process. The HOCl/ClO^- activation mechanisms in the Vis/BiVO₄/chlorine process was investigated through use of of oxalate (hole (h_VB⁺) quencher) or Cu²⁺ (electron (e_CB^-) shuttle). e_CB^- and superoxide radicals (O₂^•-) activates HOCl to form hydroxyl radicals (HO•) (one-electron transfer pathways) while only reduces ClO^- to Cl^- (two-electron transfer pathways). O₂ not only promotes HO• production, but also enables more HO• to reach target compound without being scavenged. The reactions between h_VB⁺ and chlorine are both chlorine species- and valance band (VB) potential-dependent. h_VB⁺ activates both HOCl and ClO^- to form ClO•. While at pH 5.0, the more positive VB potential of BiVO₄ than the E°(Cl⁺/HOCl) enables h_VB⁺ to oxidize HOCl to HO• and Cl⁺, which reacts rapidly with H₂O to regenerate HOCl. Using truncated bipyramid-like BiVO₄ at larger exposed area of {110} facet (h_VB⁺-dominated) and plate-like BiVO₄ at larger exposed area of {010} facet (e_CB^-/O₂^•--dominated) favors the h_VB⁺- and e_CB^-/O₂^•--induced HOCl/ClO^- activation pathway, respectively. These findings provide novel insights into the chlorine activation mechanism and the modulation of reaction pathways that HOCl and ClO^- undergo and the corresponding radicals/ions.