Photocatalytic degradation of Rhodamine B dye over Ni–Cd doped and co-doped ZnO nanoparticles

This study demonstrates the photocatalytic degradation efficiency of doped NiZnO and co-doped CdNiZnO NPs. Initially, ZnO NPs with a unique mesoporous ellipsoidal morphology were synthesized by simple precipitation and calcination. Powder X-ray diffraction revealed the formation of a hexagonal phase of the wurtzite structure. The average crystallite size of pristine ZnO NPs is 48 nm. The NPs possess higher thermal stability with the surface area, pore volume, and pore size of 9.1302 m²/g, 0.028299 cm³/g, and 12.39819 nm, respectively. Furthermore, different mesoporous doped NiZnO and co-doped CdNiZnO NPs in the range of 34 and 29 nm were synthesized by co-precipitation method and characterized by XRD, EDX, SEM, TEM, PL, Raman, BET analyses and UV-Vis spectroscopy. The calculated optical bandgaps for pure ZnO, doped NiZnO and co-doped CdNiZnO NPs were found to be 3.1, 2.62 and 2.33 eV, respectively. The photocatalytic activity of co-doped CdNiZnO was significantly higher than that of pure ZnO. After 50 min of irradiation, approximately 98% of rhodamine B was degraded by CdNiZnO, compared to 65% with pure ZnO. This enhancement is attributed to the synergistic effects of Ni and Cd, which trap electrons and holes, reducing recombination and extending charge carrier lifetimes. The photocatalytic efficiency of the synthesized materials to decompose the RhB dye (30 mgL^-1) in aqueous media was tested via Langmuir-Hinshelwood model under UV-visible light. The degradation process followed pseudo-first order kinetic model. The synthesized NPs were re-used for five cycles without any significant decrease in the photodegradation ability. The mechanistic concept of generating reactive oxygen species by electron and hole charge (e‾/h⁺ ) carriers seems to be responsible for the photocatalytic degradation of the dye by CdNiZnO NPs. Zeta potential analysis revealed positive surface charges for all catalysts, with co-doping significantly increasing charge and colloidal stability, thereby supporting the pH-dependent photocatalytic performance.