Ni/Cu co-doping engineered TiO₂ for efficient solar-light-driven photocatalytic water treatment

Titanium dioxide (TiO₂) is a benchmark photocatalyst, but its wide bandgap (∼3.2 eV) and fast charge recombination limit its efficiency under solar irradiation. Here, we demonstrate a practical co-doping strategy wherein nickel (Ni) and copper (Cu) are incorporated into TiO₂ via a simple aqueous impregnation method to synergistically engineer its structural and electronic properties for enhanced solar-light responsiveness. X-ray diffraction and Raman spectroscopy revealed co-doping-induced lattice distortion and a partial anatase-to-rutile phase transformation, accompanied by reduced surface area and pore volume. Despite these morphological changes, the co-doped TiO₂ exhibited a narrowed bandgap (2.78 eV), suppressed photoluminescence, and improved charge separation, as evidenced by electrochemical impedance and photophysical analyses. Under simulated solar irradiation (200 W m^–2), the co-doped photocatalyst achieved 86.5 % degradation of methylene blue within 60 minutes, surpassing pristine (57.6 %) and singly doped counterparts (70.4 %). Mechanistic studies confirmed hydroxyl radicals (·OH) as the dominant reactive species, facilitated by Ni-induced electron traps and Cu-mediated hole stabilization. These results highlight the potential of transition metal co-doping as an effective design strategy for engineering visible-light-responsive TiO₂ photocatalysts, offering a scalable route for high-efficiency solar-driven environmental remediation applications.