Rapid microwave hydrothermal synthesis of rare Earth-modified ZnO photocatalysts: Enhanced activity and comprehensive structural analysis

This study demonstrates that, under the specific synthesis conditions applied, the addition of rare earth ions (Ce³⁺, Y³⁺, Eu³⁺) to ZnO does not lead to their incorporation into the lattice as dopants but instead results in their surface decoration, as revealed by advanced nanoscale characterization. ZnO and rare earth-modified ZnO photocatalysts (ZnO:RE = ZnO:Eu, ZnO:Y, ZnO:Ce) with a rare earth (RE) concentration of 2 at.% were synthesized via a rapid and environmentally friendly microwave-assisted hydrothermal method. The effect of adding different RE elements on the structural, morphological, and photocatalytic properties of the samples was systematically investigated. A thorough characterization was conducted using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), and photoluminescence spectroscopy (PL) at various excitation wavelengths and temperatures. XRD analysis confirmed that all ZnO:RE samples retained the hexagonal wurtzite crystal structure of ZnO. FESEM images revealed that pure ZnO consisted of randomly distributed smooth nanosheets, while the addition of RE elements led to the formation of small particles dispersed over the nanosheet surfaces. A detailed structural analysis using STEM revealed that the rare earth elements formed structures decorating the surface of ZnO nanosheets rather than being fully incorporated into the ZnO lattice, indicating a dispersion of RE species over the ZnO matrix. This unique distribution significantly influenced the material's properties. The photocatalytic performance of the ZnO:RE samples was evaluated through the degradation of methylene blue (MB), demonstrating superior activity compared to pure ZnO and TiO 2 -P25. Among the modified samples, the cerium-modified ZnO (ZnO:Ce) exhibited the highest MB degradation efficiency. Furthermore, PL spectroscopy combined with TEM analysis provided critical insights into the relationship between defect characteristics and photocatalytic activity, offering a deeper understanding of the mechanisms driving performance enhancement. These findings highlight the potential of rare earth surface structures-ZnO nanosheets heterojunctions as a strategy for optimizing the photocatalytic properties of ZnO-based materials.