Sol–gel supercritical synthesis of Eu-doped ZnO nanostructures: Defect engineering and ROS generation for efficient methylene blue degradation

Photocatalytic degradation is a promising sustainable strategy for removing dyes contaminants from water; however, achieving high degradation efficiency in an environmentally friendly manner remains challenging. In this study, the Hexagonal wurtzite-structured ZnO nanostructures, both pure and doped with europium (Eu) at concentrations ranging from 0.5 to 5 at. %, were synthesized via a sol-gel method under supercritical isopropanol conditions and evaluated as an efficient photocatalyst for the degradation of methylene blue (MB) under UV- light irradiation. The photocatalytic performance of ZnO is significantly limited by intrinsic Zn 2+ defects, which act as recombination centers for photogenerated charge carriers. To address this issue, Eu 2+ doping is employed to modulate the defect structure of Eu-doped ZnO. Electron paramagnetic resonance (EPR) spectroscopy was employed to evaluate the photoactivity of the materials under UV irradiation, both in the solid state to investigate charge carrier generation, and in solution to assess the formation of radical species. In particular, the production of OH • and HOO • /O 2 •− radicals was monitored using DMPO (5, 5-dimethyl-1-pyrroline-N-oxide) as a spin-trapping agent, while singlet oxygen ( 1 O 2 ) was detected using 2,2,6,6-tetramethylpiperidine (TEMP).This work, demonstrates, on the one hand, that Eu is successfully incorporated into the Zn sites of the Eu-doped ZnO lattice, predominantly in the form of Eu 2+ , which effectively suppresses the formation of Zn 2+ defects and, on the other hand, the optimized Eu-doped ZnO sample (ZnO: Eu (0.5%)) is obtained. Consequently, remarkable photocatalytic activity, achieving degradation rates of 95% during only 60 min. The Eu- doping at appropriate concentrations is a highly effective strategy for enhancing the photocatalytic performance of Eu-doped ZnO, offering new insights for the design of advanced photocatalysts intended for depollution in aqueous environments.