Novel method to control initial crystallization of Eu³⁺ doped ZrO₂ nanophosphors derived from a Sol–Gel route based on HNO₃ and their site-selective photoluminescence

Abstract

In this study, Eu³⁺-doped ZrO₂ nanophosphors were obtained by a sol–gel method with HNO₃ as a catalyst, which led to white powders of small ZrO₂ nanocrystals after low temperature calcination. However, exothermic reactions of NO₃⁻ were observed during the heat-treatment leading to ZrO₂ crystallization. To avoid such reactions and to understand the initial crystallization of ZrO₂ doped with Eu³⁺ ions, we developed an original route in which the xerogel powders were pre-washed in ethanol before immersing them in Eu³⁺ solution to remove NO₃⁻. The effects of heat-treatment on the Eu³⁺ photoluminescence (PL), crystallization of ZrO₂ xerogels, and Eu³⁺ localization were studied with PL spectroscopy, thermogravimetric analysis and powder X-ray diffraction. It was found that the amorphous ZrO₂ xerogels crystallized in a tetragonal structure, with a small amount of monoclinic ZrO₂ also being precipitated after longer calcination periods. Finally, the Eu³⁺ ions on the surface of the ZrO₂ xerogels were found to diffuse into higher symmetric Zr substitutional sites in the tetragonal ZrO₂ matrix after heat-treatment.