Aerosol synthesis of TiO(2):Er(3+)/Yb(3+) submicron-sized spherical particles and upconversion optimization for application as anti-counterfeiting materials

Er(3+)/Yb(3+)-doped TiO(2) 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 Er(3+) and Yb(3+) dopants. TiO(2):Er(3+)/Yb(3+) showed green and red emissions due to the (2)H(11/2)/(4)S(3/2) → (4)I(15/2) transition and the (4)F(9/2) → (4)I(15/2) transition of Er(3+) 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 Yb(3+) concentration. The dependence of UC luminescence intensity on 980 nm IR pumping power showed that both the red and green UC luminescence of TiO(2):Er(3+)/Yb(3+) occurred through a typical two-photon process. In terms of achieving the highest red UC emission intensity, the optimal Er(3+) and Yb(3+) contents are 0.3% and 7.0%, respectively. The UC intensity of TiO(2):Er(3+)/Yb(3+) 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 TiO(2):Er(3+)/Yb(3+) occur from anatase to rutile, but also the Yb(2)Ti(2)O(7) impurity phase is formed. According to SEM and TEM/EDX analysis, the prepared TiO(2):Er(3+)/Yb(3+) 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 TiO(2):Er(3+)/Yb(3+) powder was found to have a UC emission high enough to be visually observed when irradiated with a portable 980 nm IR lamp.