Fe-Doping Strategy for Bandgap Tuning in Sillen X1 Oxychlorides CaBiO₂Cl and PbBiO₂Cl toward Improved Visible-Light...

Sillen X1 oxychlorides, with the general formula MBiO₂Cl (M = Ca, Sr, Ba, Cd), possess unique two-dimensional layered structures featuring self-induced internal electric fields that promote charge separation; however, their wide band gaps limit their applications in the visible portion of the solar spectrum. There is no straightforward cation-doping scheme to tune the optical bandgap into the visible region in Sillen X1 oxyhalides. In this study, we tuned the bandgap of CaBiO₂Cl to the visible range by systematically incorporating Fe at the Bi site and synthesizing the samples via a modified gas-solid synthesis method. Substitution of up to 20 mol % Fe in CaBiO₂Cl was successful, and the monoclinic symmetry (S.G.: P2₁/m) was retained. The Fe inclusion caused lattice contraction and local cationic disorder. Fe existed in mixed Fe²⁺/Fe³⁺ oxidation states, which led to the oxidation of some amount of Bi³⁺ to Bi⁵⁺. A significant red shift of the band edge with signatures of extended LMCT, intervalence charge transfer, and subtle d-d transitions (due to Fe²⁺) was observed in UV-visible spectra of the Fe-containing samples. The bandgap narrowed from 3.39-3.52 (CaBiO₂Cl) to 2.02 eV (CaBi_0.80Fe_0.20O₂Cl), indicating electronic band structure modification. A similar set of changes was observed when Bi in orthorhombic PbBiO₂Cl (S.G.: Cmcm) was substituted with Fe, where the bandgap narrowing was limited (from 2.53-2.71 to 2.04 eV (PbBi_0.80Fe_0.20O₂Cl)). The Fe-substituted samples catalyzed the decoloration of crystal violet dye within 120 min under visible-light irradiation, following pseudo-first-order kinetics. The reactive oxygen species involved in the photocatalytic decoloration were identified. Both catalysts demonstrated recyclability, with their crystal structures remaining intact after use. The demonstrated strategy for tuning the band gaps of the Sillen X1 phases, together with the enhancement of the visible-light photocatalytic properties by efficient charge migration via redox shuttling, qualifies them as sustainable photocatalysts.