Unlocking the effect of Zn2+ on crystal structure, optical properties, and photocatalytic degradation of perfluoroalkyl substances (PFAS) of Bi2WO6

Abstract

Bismuth tungstate (Bi₂WO₆) with a layered structure and visible light response exhibits excellent photocatalytic activity. To enhance its photocatalytic activity for the degradation of perfluoroalkyl substances (PFAS), Zn²⁺ is partially substituted for Bi³⁺ in the Bi₂WO₆ lattice in this study. Particularly, the effect of Zn²⁺ content (0–22.5 at%) on the crystal structure, optical property, and photocatalytic activity for the photodegradation of PFAS of Bi₂WO₆ is investigated. According to the Le Bail fits, the unit-cell volume is slightly reduced from 487.7 ų to 480.8 ų by the partial substitution of smaller Zn²⁺ (0.74 Å for CN = 6) for larger Bi³⁺ (1.03 Å for CN = 6) in the Bi₂WO₆ crystal lattice, and the solubility of Zn²⁺ in the Bi₂WO₆ lattice is found to be below 17.5 at%. The partial substitution of Zn²⁺ influences the self-aggregation of nanoparticles, Ostwald ripening, and self-organization of nanoplates, resulting in different morphologies. Although the optical bandgap energy of Bi₂WO₆ is not significantly altered upon the partial substitution of Zn²⁺, the conduction and valence bands simultaneously shift upward. Among the Bi_2−xZn_xWO_6+δ photocatalysts, 2.5 at% Zn²⁺-substituted Bi₂WO₆ exhibits larger water oxidation photocurrent density (0.316 mA cm⁻² at 1.23 V_RHE) and the highest photocatalytic activity for the photodegradation of PFH_xA (k₁ = 0.012 min⁻¹). The trapping experiments confirm that the photo-excited holes (h⁺) and superoxide radicals (O₂˙⁻) are the major reactive species involved in the photodegradation of PFH_xA. Liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS) reveals that decarboxylation and defluorination are the main possible routes for the photodegradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts. Our findings suggest that the partial Zn²⁺-to-Bi²⁺ substitution can enhance the photocatalytic activity of Bi₂WO₆ for the degradation of PFAS.