Aerosol synthesis of TiO2:Er3+/Yb3+ submicron-sized spherical particles and upconversion optimization for application as anti-counterfeiting materials
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 2H11/2/4S3/2 → 4I15/2 transition and the 4F9/2 → 4I15/2 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.