Phase-transition-induced defect and valence engineering in Sr/S co-doped W18O49 oxysulfide for efficient NaBH4-assisted dark reduction of pollutants.

Conventional photocatalytic systems generally rely on light irradiation to initiate catalytic reactions, which substantially limits their practical applicability under realistic environmental conditions. Herein, a defect- and valence-engineered Sr/S co-doped W18O49 oxysulfide (Sr/S-W18O49) was rationally constructed via a hydrothermal method for efficient NaBH4-assisted dark catalytic reduction of pollutants. XRD analysis reveals that Sr incorporation induces the phase transformation from hexagonal WO3 to oxygen-deficient monoclinic W18O49, whereas S doping primarily regulates the local electronic structure. XPS and EPR characterizations confirm the simultaneous enrichment of heterovalent W4+/W6+ redox couples and abundant oxygen vacancies (Ov). SEM observations demonstrate that Sr/S co-doping transforms the compact bulk morphology into loosely packed nanostructures, while UPS, photoluminescence, and electrochemical analyses demonstrate enhanced charge-transfer capability and accelerated interfacial electron migration. Benefiting from synergistic defect-valence coupling effect, the optimized Sr/S-W18O49-3, featuring a n(W4+)/[n(W4+) + n(W6+)] ratio of 18.26% and a Vo/(Vo + OL) ratio of 25.38%, exhibits the largest electrochemically active surface area (7.88 μF cm-2) and outstanding catalytic activity. Complete reduction of 100 mL of 20 ppm 4-nitrophenol, methylene blue, new carmine, and Cr(VI) is achieved within 6, 4, 14, and 12 min, respectively, with corresponding apparent rate constants of 0.36, 0.39, 0.17, and 0.20 min-1. Moreover, Sr/S-W18O49-3 exhibits excellent catalytic durability, broad pH adaptability, and efficient reduction performance toward mixed-pollutant systems in the presence of coexisting ions. This work provides new mechanistic insight into defect-assisted dark catalytic reduction and offers a rational strategy for designing advanced oxysulfide catalysts for environmental remediation.