Ultrathin Bismuth-Based Composite Nanosheets with Synergistic [Bi²⁺-V_O]/V_N Dual-Defect Engineering and g-C₃N₄ Modification for Solar-Driven Photocatalytic Antibiotic Degradation

Synergistically coupling complementary vacancy pairs with an ultrathin framework provides a promising yet underexplored route to robust photocatalysts for water purification. Herein, we report the synthesis of ultrathin SBBC (Bi₁₂SiO₂₀-Bi₂O₃-BiOCl-Bi₂SiO₅)/g-C₃N₄ (CN) nanosheets (SBCN-6) in which surface [Bi²⁺–V_O] defect centers are deliberately paired with nitrogen vacancies (V_N), thereby uniting dual-defect chemistry with an ultrathin framework. Electron-sequestering [Bi²⁺–V_O] clusters introduce deep trap states, which robustly capture photoexcited electrons generated in SBBC. V_N sites in CN serve as efficient electron-trapping centers that significantly suppress internal electron-hole recombination within CN, and this combined action accelerates visible-light redox reactions. This synergistic dual-defect network, comprising [Bi²⁺–V_O] in SBBC and V_N sites in CN, significantly enhances charge separation. Density Functional Theory (DFT) and X-ray Photoelectron Spectroscopy (XPS) unveil a type-Z/type-II hybrid transfer cascade: SBBC (V_B) → [Bi²⁺–V_O] → SBBC (C_B) → CN (C_B) ← [V_N] ← CN (V_B). Under visible-light irradiation, the photocatalyst exhibits exceptional activity in degrading refractory antibiotics, achieving a high apparent rate constant (k) of 0.129 min^-1 for tetracycline (93% degradation in 120 min) and removing 91% of ciprofloxacin in 120 min. Moreover, it rapidly degrades the Rhodamine B dye, with over 99% removal within just 30 min. It also retains excellent recyclability over multiple cycles and outperforms most reported Bi-based photocatalysts. Coupling dual-defect engineering with morphology control, this study advances the mechanistic exploration of defect-mediated catalysis and furnishes a valuable reference for constructing visible-light photocatalysts that simultaneously degrade dyes and antibiotics.