Bright red upconversion luminescence from Er3+ and Yb3+ co-doped ZnO-TiO2 composite phosphor powder

ZnO-TiO2 composites co-doped with Er3+ and Yb3+ ions were successfully synthesized by powder-solution mixing method and their upconversion (UC) luminescence was evaluated. The effect of firing temperature, ZnO/TiO2 mixing ratio, and dopant concentration ranges on structural and UC luminescence properties was investigated. The crystal structure of the product was studied and calculated in detail by means of X-ray diffraction (XRD). Also, the site preference of Er3+ and Yb3+ ions in the host material was considered and analyzed based on XRD results and UC luminescence characteristics. Brightest UC luminescence was observed in the ZnO-TiO2:Er3+,Yb3+ phosphor fired at 1300 °C in which the system consisted of mixed phases; Zn2TiO4, TiO2, RE2Ti2O7 and RE2TiO5 (RE = Er3+ and/or Yb3+). Under the excitation of a 980 nm laser, the two emission bands were detected in the UC emission spectrum, weak green band centered at 544 and 559 nm, and strong red band centered at 657 and 675 nm wavelengths in accordance with 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ ion, respectively. The simple chemical formula equations, for explaining the site preference of Er3+ and Yb3+ ions in host crystal matrix, were generated by considering the Zn2TiO4 crystal structure, its crystal properties, and the effect of Er3+ and Yb3+ ions to the host crystal matrix. The UC emission intensity of the products was changed by varying ZnO/TiO2 mixing ratios, and Er3+ and Yb3+ concentrations. The best suitable condition for emitting the brightest UC emission was 1ZnO:1TiO2 doped with 3 mol% Er3+, 9 mol% Yb3+ fired at 1300 °C for 1 h.