Tuning Optical and Structural Properties by Doping TiO ₂ with Cu–Sr: Visible‐Light Sensitive Photocatalyst for Binary Dye Degradation

Abstract A novel caffeine‐assisted sol–gel method was developed to synthesize Cu–Sr co‐doped TiO 2 photocatalysts, targeting enhanced visible‐light‐driven degradation of binary dyes. Multiple dopant ratios were explored, and caffeine acted as a biogenic capping agent, yielding nanostructures with high surface area (up to 148.4 m 2 /g) and ultra‐fine crystallite sizes (down to 5.0 nm). Structural and optical properties were confirmed via XRD (anatase phase retention), TEM/SEM (spherical, minimally agglomerated particles), BET (mesoporosity), and UV–vis DRS, which revealed significant band gap narrowing to 2.57 eV. Electrochemical analyses including Mott–Schottky and photoluminescence demonstrated improved charge separation and suppressed electron‐hole recombination. Photocatalytic activity was validated with fast and efficient degradation of Eosin yellow (EY) and Malachite green (MG) under visible light, achieving up to 99% removal within 110 min at optimal pH values. The material exhibited excellent reusability and photochemical stability, maintaining performance across multiple cycles. Compared to earlier doping studies, this work highlights the unique synergy of Cu–Sr co‐modification and caffeine‐assisted synthesis for environmentally benign applications in water purification.