Defect‐Mediated Photocatalytic Activity of Thermally Evaporated ZnO Thin Films: Correlating PL and EPR Signatures With Methylene Blue Degradation

Zinc oxide (ZnO) thin films are promising immobilized photocatalysts,but their performance is strongly influenced by native point defects introduced during deposition and post‐annealing. In this work, Zn thin films were deposited on glass substrates by thermal evaporation and converted to ZnO by annealing at 600 °C for 2 h in either air or an oxygen‐poor argon atmosphere. The films were characterized by X‐ray diffraction, scanning electron microscopy, optical transmittance, photoluminescence (PL), electron paramagnetic resonance (EPR) spectroscopy, and methylene blue (MB) photodegradation under UV–visible irradiation. The apparent optical bandgap increased from 2.96 eV for the as‐deposited film to 3.16 and 3.32 eV for the air‐ and argon‐annealed films, respectively, mainly due to improved crystallinity and reduced disorder‐related sub‐bandgap absorption. PL deconvolution and EPR analysis revealed enhanced defect‐related signatures after argon annealing, including visible deep‐level emission and an EPR signal near g  ≈ 1.96. The argon‐annealed film exhibited the highest MB degradation efficiency, reaching 93% after 180 min, with an apparent first‐order rate constant of 0.01604 min ⁻¹ . These results support a strong structure–defect–function correlation in defect‐mediated ZnO photocatalysis.