Enhanced Photocatalytic and Non-Linear Optical Properties of Dual-Doped Rare Earth Ions (Gd³ ⁺ and Sm³ ⁺) in ZnO Nanostructured Materials

ZnO nanostructures have garnered substantial interest due to their multifunctional characteristics. In this work, pristine ZnO and rare-earth (RE)-doped ZnO nanostructures incorporating Gd3 + and Sm3 + metal ions at concentrations of 0.002%, 0.004%, and 0.006% were synthesized using a hydrothermal method. The influence of RE ion doping on the structural, optical, morphological, and surface characteristics was investigated through PXRD, UV–vis spectroscopy, HR-SEM, HRTEM, XPS, and BET analyses. PXRD confirmed the presence of the hexagonal wurtzite phase, with crystallite sizes ranging from 24 to 28 nm. A slight redshift in the bandgap (3.20–3.23 eV) was observed in the doped samples compared to bulk ZnO. Morphological studies revealed nanoflake structures for undoped ZnO, while doped samples exhibited a combination of flakes and rod-like morphologies. The photocatalytic performance was evaluated through the degradation of Reactive Black 5 (RB5) dye under UV light. The 0.002 wt.% doped sample demonstrated the highest degradation efficiency of 94.56% within 70 min, with all samples following pseudo-first-order kinetics. Magnetic measurements showed weak ferromagnetism in all samples, attributed to RE ion-induced lattice defects. Nonlinear optical properties were assessed using the open-aperture Z-scan technique with a Q-switched Nd:YAG laser (532 nm). The pristine ZnO sample exhibited the highest nonlinear absorption coefficient (β) and the lowest optical limiting threshold (OLT), suggesting its suitability for photonic limiter applications. These results underscore the potential of dual RE-ion-doped ZnO nanomaterials for applications in photocatalysis, environmental remediation, and nonlinear optical devices.