The photocatalytic efficiency of low cost TiO2 thin layers for discoloration of methylene blue

• Low-cost TiO 2 thin films prepared via sol–gel spin coating and annealing. • Optimized annealing at 400 °C improves crystallinity, band gap, and surface hydrophilicity. • Photocatalytic degradation of elevated concentration 40 mg·L −1 of methylene blue is significantly enhanced at 50 °C. • Pseudo-first-order kinetics demonstrate high efficiency under short irradiation at elevated temperature. TiO 2 thin films were fabricated on glass substrates via a low-cost sol–gel spin-coating route and annealed between room temperature and 400 °C to elucidate the influence of thermal treatment on structure–property–photocatalytic performance relationships. X-ray diffraction and Raman analyses confirmed the formation of phase-pure anatase TiO 2 , with crystallite size increasing from ∼ 11 to ∼ 15 nm as the annealing temperature increased. SEM observations revealed uniform and granular film morphology, while EDX analysis confirmed the presence of Ti and O without detectable impurities. The films exhibited high optical transparency (∼97%) and a tunable optical band gap reduced from 3.48 to 3.22 eV. Surface wettability was significantly enhanced, with the water contact angle decreasing from 96.4° to 41.0°, indicating improved surface activity. Photocatalytic performance was evaluated using methylene blue degradation under UV irradiation at a relatively high dye concentration (50 mg·L −1 ). The TiO 2 film annealed at 400 °C showed the highest activity, achieving degradation efficiencies of ∼ 25% at 25 °C and ∼ 35% at 50 °C within only 50 min. Despite the short irradiation time, these results compare favorably with many reported thin film systems operating under less demanding conditions. This study demonstrates that efficient early-stage photocatalytic performance can be achieved through controlled low-temperature annealing, providing a scalable and application relevant strategy for functional TiO 2 coatings in environmental remediation.