Bandgap engineering and enhanced charge separation in Zn-modified BiW₁₁ polyoxometalate for azo dye photodegradation

Polyoxometalates (POMs) have attracted increasing attention as photoactive inorganic materials for dye-related photochemical processes. In this work, a Zn-modified BiW₁₁ polyoxometalate (Zn-BiW₁₁) was synthesized and investigated for the UV-induced photodegradation of two structurally distinct azo dyes, Congo Red (CR) and Phenol Red (PR). Structural and optical characterizations using FTIR, XRD, SEM, and UV-Vis spectroscopy confirm that Zn modification alters the microstructural features and optical response of the parent BiW₁₁ system. UV-Vis absorption analysis reveals a noticeable narrowing of the optical bandgap from 3.68 eV (BiW₁₁) to 3.45 eV (Zn-BiW₁₁), indicating enhanced photo-absorption capability under UV irradiation. Photodegradation experiments demonstrate that Zn-BiW₁₁ exhibits superior activity toward both dyes, achieving degradation efficiencies of 65.8% for CR and 76.1% for PR, following pseudo-first-order kinetics. The faster degradation of PR compared to CR is attributed to differences in molecular structure, charge distribution, and dye-surface interaction. A plausible photodegradation mechanism involving photo-induced reactive oxygen species is proposed based on optical excitation and dye-specific reactivity. The results highlight the effectiveness of Zn-modified POMs as photoactive systems for investigating the photodegradation of dyes and for elucidating structure-reactivity relationships.