Synthesis and photoluminescence properties of Eu³⁺-doped ZrO₂ hollow spheres

ZrO₂:Eu³⁺ hollow spheres were successfully fabricated with the resin microspheres as the template. The sample characterizations were carried out by means of x-ray diffraction (XRD), scanning electron microscope (SEM), and photoluminescence spectra. XRD results revealed that Eu³⁺-doped samples were pure t-ZrO₂ phase after being calcined at 873 K. SEM results exhibited that this Eu³⁺ doped ZrO₂ was hollow spheres; the diameter and thickness of which were about 450 and 50 nm, respectively. Upon excitation at 394 nm, the orange-red emission bands at the wave length longer than 570 nm were from ⁵D₀ → ⁷F_J (J = 1, 2) transitions. The asymmetry ratio of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) intensity is about 1.61, 1.26, 1.42, 1.42, 1.40, and 1.38 for the Eu³⁺ concentration 0.4, 0.7, 1.0, 1.5, 2.0, and 2.5 mol%, respectively. These values suggest that the asymmetry ratio of Eu³⁺ ions is independent of the doping concentration. The optimal doping concentration of Eu³⁺ ions in ZrO₂ is 1.5 mol%. According to Dexter's theory, the critical distance between Eu³⁺ ions for energy transfer was determined to be 16 Å.