Aerosol synthesis of TiO2:Er3+/Yb3+ submicron-sized spherical particles and upconversion optimization for application as anti-counterfeiting materials

Abstract

Er³⁺/Yb³⁺-doped TiO₂ up-conversion (UC) phosphors were prepared by spray pyrolysis, and the UC luminescence properties were optimized by changing the calcination temperature and the concentration of Er³⁺ and Yb³⁺ dopants. TiO₂:Er³⁺/Yb³⁺ showed green and red emissions due to the ²H_11/2/⁴S_3/2 → ⁴I_15/2 transition and the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ ions, respectively. The R/G ratio between red (R) and green (G) emissions does not change significantly with Er concentration but increases linearly with increasing Yb³⁺ concentration. The dependence of UC luminescence intensity on 980 nm IR pumping power showed that both the red and green UC luminescence of TiO₂:Er³⁺/Yb³⁺ occurred through a typical two-photon process. In terms of achieving the highest red UC emission intensity, the optimal Er³⁺ and Yb³⁺ contents are 0.3% and 7.0%, respectively. The UC intensity of TiO₂:Er³⁺/Yb³⁺ particles increases until they are calcined at temperatures up to 600 °C and then decreases rapidly above 800 °C. This is because when the calcination temperature is 800 °C and higher, not only does the phase transition of TiO₂:Er³⁺/Yb³⁺ occur from anatase to rutile, but also the Yb₂Ti₂O₇ impurity phase is formed. According to SEM and TEM/EDX analysis, the prepared TiO₂:Er³⁺/Yb³⁺ UC powders have an average particle size of 680 nm, a spherical shape with a dense structure, and Er and Yb are uniformly dispersed throughout the particles without local separation. A mark prepared using TiO₂:Er³⁺/Yb³⁺ powder was found to have a UC emission high enough to be visually observed when irradiated with a portable 980 nm IR lamp.