Sol–Gel synthesis of Yb-Er Co-doped TiO2-ZnO heterostructures for enhanced photocatalytic degradation of methylene blue: performance and mechanism

To overcome the intrinsic limitations of TiO 2 , specifically its wide band gap and rapid photogenerated carrier recombination, for dyeing wastewater treatment, a Yb 3+ /Er 3+ co-doped TiO 2 -ZnO heterostructure was fabricated by sol–gel method. X-ray diffraction revealed inhibited anatase-to-rutile phase transition. X-ray photoelectron spectroscopy confirmed Ti 2p binding energy shifts, indicating strong electronic interactions between dopants and the host lattice. Ultraviolet–visible diffuse reflectance spectroscopy showed band gap narrowing to 2.927 eV, with upconversion effect extending light absorption to the near-infrared region. Photoluminescence measurements demonstrated suppressed carrier recombination, and XPS confirmed abundant oxygen vacancies. The optimal sample achieved 74.31% methylene blue degradation under visible light for 6 h and 98.58% under ultraviolet light for 2 h, outperforming commercial P25. The Yb 3+ /Er 3+ co-dopants serve dual functions: they act as upconversion centers to harvest low-energy photons and function as electronic modulators that induce Ti 2p binding energy shifts and create oxygen vacancies, thereby enhancing charge separation and surface reactivity. This work integrates rare-earth co-doping with heterojunction engineering to boost TiO 2 photocatalysis.