Synthesis, Characterization, and Photocatalytic Performance of Rare-Earth-Modified ZnO Nanoflowers for Degradation of 2,5-Diphenyl-1,3-oxazole and 2-(4-Biphenyl)-5-phenyl-1,3,4-oxadiazole

The photocatalytic degradation of the 2,5-diphenyl-1,3-oxazole (PPO) and 2-(4-biphenyl)-5-phenyl-1,3,4-oxadiazole (PBD), which are laser dyes and scintillator compounds, was investigated under ultraviolet (UV) light irradiation using pure and rare-earth (Sm3+, Eu3+, and Gd3+)-modified zinc oxide (ZnO) hierarchical flower-like microstructures. The synthesized photocatalysts (powder) were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), confirming the formation of flower-like ZnO structures and successful modification by the oxides Sm2O3, Eu2O3, and Gd2O3. Residual concentrations of Zn2+, Gd3+, Sm3+, and Eu3+ in the treated aqueous solutions were determined by ICP-MS to evaluate catalyst stability, while chemical oxygen demand (COD) analysis was used to assess mineralization efficiency. For both PPO and PBD, the photocatalytic activity followed the order ZnO < ZnO/Gd2O3 < ZnO/Sm2O3 < ZnO/Eu2O3, which can be attributed to the enhanced charge separation and reduced electron–hole recombination caused by rare-earth ions, with Eu3+ providing the most effective electron trapping. PPO showed faster degradation than PBD, mainly due to the structure of the PBD molecule, which is more rigid and conjugated, owing to its higher resistance to oxidative degradation.