Luminescence properties and aqueous degradation of Eu2+/3+-activated (Zn,Sr)Al2O4 phosphors

In this research, Eu2+ and Eu3+-activated (Zn,Sr)Al2O4 phosphors are synthesized by the solid-state reaction method. The (Zn,Sr)Al2O4 host lattice is synthesized at three different compositions: Zn1-ySryAl2O4 (y = 0.25, 0.50, 0.75), and the lattice is doped with 1 mol% Eu2+/3+. The Eu oxidation state (Eu3+ or Eu2+) is determined by sintering in either an oxidizing (Eu3+) or reducing (Eu2+) atmosphere. XRD reveals that all phosphors regardless of composition or sintering conditions are composed of two phases, ZnAl2O4 and SrAl2O4. However, no residual Eu2O3 phase is detected, indicating that the Eu ions are substituted into the host lattices. PL spectroscopy reveals a peak phosphorescent emission at 517 nm (green) for the Eu2+ doping chemistry, while peak emission shifts to 614 nm (red) for the Eu3+ doping chemistry. Aqueous degradation studies reveal that the SrAl2O4 phase is susceptible to rapid hydration and phosphor degradation, while the ZnAl2O4 phase remains stable in water. Interestingly, the green phosphorescence is nearly eliminated within 2 h of water immersion, suggesting that the Eu2+ activator is mostly dissolved in the SrAl2O4 phase. In contrast, the red phosphor shows more stability with water immersion, particularly for cases of higher concentrations of ZnAl2O4, suggesting the Eu3+ activator is at least partially incorporated into the ZnAl2O4 phase.