Abstract <p>ZnO, La-doped ZnO, and La–Cd co-doped ZnO nanostructures were synthesized via a sol–gel route to examine the role of rare-earth and transition-metal incorporation on their structural, optical, dielectric, and photo-catalytic properties. X-Ray Diffraction confirmed phase-pure wurtzite ZnO in all samples, with reduced crystallite size upon doping due to lattice distortion from larger La³⁺ and Cd²⁺ ions. UV–Vis spectroscopy revealed enhanced UV absorption and maximum transmittance for La–Cd co-doped ZnO, and a systematic bandgap narrowing from 3.29&#xa0;eV (undoped) to 3.18&#xa0;eV (co-doped), accredited to defect-induced mid-gap states and Cd-driven conduction band modification. Photoluminescence spectra showed dominant green emission in all cases; La doping enhanced emission via defect level creation, while Cd co-doping quenched intensity due to increased non-radiative pathways. Photocatalytic degradation of methylene blue was highest for La–Cd co-doped ZnO, likely due to suppressed electron–hole recombination from synergistic defect engineering, with La-doped ZnO also outperforming undoped ZnO. Higher pH and catalyst loading further improved degradation efficiency. AC conductivity, dielectric constant, and dielectric loss followed the trend co-doped &gt; La-doped &gt; undoped, consistent with increased carrier concentration and space-charge polarization from oxygen vacancies. These findings demonstrate that controlled La³⁺/Cd²⁺ co-doping effectively tailors ZnO’s optoelectronic and photocatalytic performance for environmental remediation and energy-related applications.</p> Graphical Abstract <p></p>