Opposite effect of Li⁺ codoping on the upconversion emissions of Er³⁺-doped TiO₂ powders

The Er³⁺-Li⁺ codoped TiO₂ powders have been prepared by the non-aqueous sol–gel method. The green and red upconversion emissions centered at about 526, 550 and 663 nm were observed by the ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively. Li⁺ codoping has opposite effect on the upconversion emissions intensities for Er³⁺-doped TiO₂ at sintering temperatures of 1,073 and 1,273 K. At 1,073 K, the Er³⁺-doped TiO₂ phase transition from anatase to rutile was accelerated with increasing Li⁺ codoping concentration, leading to the increase of crystal field symmetry of Er³⁺, thus the upconversion emissions intensities decreased. At 1,273 K, Li⁺ codoping had no effect on the phase structure of Er³⁺-doped TiO₂ and only increased the Er–O bond length, it indicated that the upconversion emissions intensities greatly enhanced because of the decrease of crystal field symmetry of Er³⁺.