Structural and optical properties of Mn-doped anatase-rutile TiO₂ nanoparticles

Undoped and Mn-doped TiO₂ nanoparticles (NPs) are synthesized by a sol–gel method and calcined at 500 °C for 2 h. The shift in XPS analyses confirmed the incorporation of Mn in the TiO₂ lattice, which induces surface-related oxygen vacancies accompanied by the formation of Ti³⁺ and Mn⁴⁺ oxidation states. The XRD analysis reveals that a pure anatase phase is observed for undoped and 0.05 mol% Mn-doped, whereas anatase–rutile mixed phase with a dominant anatase phase is formed at 0.1–0.2 mol% Mn doping. The crystallite size calculated using the Scherrer model decreases from 25 to 17 nm, while the specific surface area increases from 65.6 to 77.7 m²/g with increasing Mn content (0–0.2 mol%). The HRTEM image reveals that the lattice fringe spacing is 0.35 nm, which is in line with the (101) plane of the TiO₂ anatase phase. The SAED pattern further supports the polycrystalline nature of the observed structure. The optical band gap for undoped TiO₂ NPs was found to be 3.2 eV, and this value is increased to 3.3 eV for 0.2 mol% Mn-doped TiO₂ NPs. The highest PL emission intensity was obtained for 0.2 mol% Mn, and this intensity was significantly reduced at a higher Mn doping level. The oxygen defects and Mn related impurities act as luminescence centers and increase the PL emission intensity. The composition-optimized 0.2 mol% Mn-doped TiO₂ exhibited strong PL emission, a high value of quantum efficiency, and a prolonged lifetime. The results of this work suggest that Mn-doped TiO₂ nanoparticles may be a good option for making light-emitting devices.