Ionic-liquid-regulated Br-modified Bi2WO6 and Bi2WO6/BiOBr photocatalysts for simultaneous dye removal and Cr (VI) reduction: composition, crystal growth, and redox behavior

Heterojunction construction is widely used to enhance photocatalysis, yet improved charge separation does not necessarily increase the net rate of a target reduction when competing oxidative pathways are present. Here, Br-modified Bi 2 WO 6 and Bi 2 WO 6 /BiOBr heterostructures were prepared by a microwave-assisted hydrothermal method using ionic liquids (ILs) as bromine sources and morphology-directing media. Varying the IL cation chain length and dosage regulated phase evolution, facet exposure, crystallinity, nanosheet thickness, hierarchical assembly, and surface area. Low IL (1-ethyl-3-methylimidazolium bromide, [Emim]Br) loading produced Br-modified Bi 2 WO 6 without detectable crystalline BiOBr, whereas IL/Na 2 WO 4 ratios of at least 1 induced Bi 2 WO 6 /BiOBr formation. The optimized BWOE-0.1 sample combined exposure of Bi 2 WO 6 (001)-related facets, thin, densely assembled nanosheets, a high specific surface area (69.57 m 2 g −1 ), efficient interfacial charge transport, and the highest Cr(VI) reduction rate constant (0.139 min −1 ), which is 2.36 times that of pristine Bi 2 WO 6 . In contrast, Bi 2 WO 6 /BiOBr heterostructures showed lower net Cr(VI) reduction despite favorable carrier separation. EPR detection of a pronounced DMPO-X signal for (001)-dominant BiOBr indicates a strongly oxidative surface environment, supporting the interpretation that hole-driven reactions can counteract electron-mediated Cr(VI) reduction, potentially through Cr(III) re-oxidation. In a mixed Cr(VI)/methyl orange system, photogenerated electrons primarily drove Cr(VI) reduction, whereas •O 2 – dominated dye degradation. These results establish a structure–facet–redox relationship and demonstrate that photocatalyst design for mixed contaminants must optimize the balance between reduction and oxidation, rather than relying solely on charge separation.