<p>In this study, the impact of doping yttrium (Y), samarium (Sm), and terbium (Tb) at 0.5 mol% on the structural and photocatalytic properties of biphasic (anatase-brookite) TiO₂ nanoparticles was investigated. The materials were synthesized via the sol-gel method and characterized using X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, photoluminescence (PL) spectroscopy, high-resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray analysis (EDX) and X-ray photoelectrons Spectroscopy (XPS). Photocatalytic activity was evaluated through methylene blue (MB) degradation under UV, sunlight, and dark conditions. Appropriate doping was found to enhance the light absorption, shifting it into the visible region by creating impurity energy levels within the TiO₂ bandgap. The improved photocatalytic performance is attributed to increased dye adsorption, a red shift in the absorption edge, and accelerated interfacial electron transfer. Among the doped samples, Sm-modified TiO₂ achieved the highest rate constant (0.0152 min⁻¹) and 96% MB degradation under sunlight. Further investigations on the optimized Sm-based catalyst including kinetics, stability, scavenger effect, degradation mechanism and reusability were also done. Electrochemical studies, including cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), demonstrated that Sm-doped TiO₂ nanoparticles possess lower charge transfer resistance compared to pristine TiO₂ and Y and Tb doped TiO₂ nanoparticles. Also, these Sm doped TiO₂ nanoparticles showed higher current responses and excellent electrochemical stability, highlighting their promise as efficient materials for photocatalytic and environmental remediation applications.</p> Graphical Abstract <p></p>