Rapid and Energy-Saving Microwave-Assisted Solid-State Synthesis of Pr3+-, Eu3+-, or Tb3+-Doped Lu2O3 Persistent Luminescence Materials

Persistent luminescence materials Lu₂O₃:R³⁺,M (Pr,Hfᴵⱽ; Eu; or Tb,Ca²⁺) were successfully and rapidly (22 min) prepared by microwave-assisted solid-state synthesis (MASS) using a carbon microwave susceptor and H₃BO₃ as flux. Reaction times are reduced by up to 93% over previous synthetic methods, without special gases application and using a domestic microwave oven. All materials prepared with H₃BO₃ flux exhibit LuBO₃ impurities that were quantified by Rietveld refinement from synchrotron radiation X-ray powder diffraction patterns. The flux does not considerably affect the crystalline structure of the C-Lu₂O₃, however. Scanning electron micrographs suggest low surface area when H₃BO₃ flux is used in the materials’ synthesis, decreasing the amount of surface hydroxyl groups in Lu₂O₃ and improving the luminescence intensity of the phosphors. The carbon used as the susceptor generates CO gas, leading to complete reduction of Tbᴵⱽ to Tb³⁺ and partial conversion of Prᴵⱽ to Pr³⁺ present in the Tb₄O₇ and Pr₆O₁₁ precursors, as indicated by X-ray absorption near-edge structure data. Persistent luminescence spectra of the materials show the red/near-IR, reddish orange, and green emission colors assigned to the 4fⁿ → 4fⁿ transitions characteristics of Pr³⁺, Eu³⁺, and Tb³⁺ ions, respectively. Differences between the UV-excited and persistent luminescence spectra can be explained by the preferential persistent luminescence emission of R³⁺ ion in the S₆ site rather than R³⁺ in the C₂ site. In addition, inclusion of Hfᴵⱽ and Ca²⁺ codopants in the Lu₂O₃ host increases the emission intensity and duration of persistent luminescence due to generation of traps caused by charge compensation in the lattice. Photonic materials prepared by MASS with H₃BO₃ flux show higher persistent luminescence performance than those prepared by the ceramic method or MASS without flux. Color tuning of persistent luminescence in Lu₂O₃:R³⁺,M provides potential applications in bioimaging as well as in solar cell sensitizers.