Effect of sulfur doping on the photocatalytic performance of BiVO₄ under visible light illumination

Porous monoclinic bismuth vanadate (BiVO_4-δ) and sulfur-doped bismuth vanadates (BiVO_4-δS_0.05, BiVO_4-δS_0.08, and BiVO_4-δS_0.12) were synthesized by a dodecylamine-assisted alcohol-hydrothermal route in the absence and presence of thiourea or Na₂S. The physicochemical properties of the materials were characterized and their photocatalytic performance for the degradation of methylene blue and formaldehyde under visible light was evaluated. The samples have a single phase monoclinic scheetlite crystal structure with a porous olive-like morphology, surface areas of 8.4-12.5 m²/g, and bandgap energies of 2.40-2.48 eV. Surface Bi⁵⁺, Bi³⁺, V⁵⁺, and V⁴⁺ species were present on the S-doped BiVO_4-δ samples. Sulfur doping influenced the surface Bi⁵⁺/Bi³⁺, V⁵⁺/V⁴⁺, and O_ads/O_latt molar ratios, and the amount of sulfur doped had an important effect on the photocatalytic performance. Under visible light, BiVO_4-δS_0.08 performed the best in the photodegradation of methylene blue and formaldehyde. A higher surface oxygen species concentration and a lower bandgap energy were responsible for the excellent visible light photocatalytic performance of BiVO_4-δS_0.08.