Influence of pH and europium concentration on the luminescent and morphological properties of Y₂O₃ powders

This work reports on the synthesis and characterization of Y₂O₃:Eu³⁺ powders obtained by the hydrothermal method. We studied the influence of different pH values (7-12) and Eu³⁺ concentrations (2.5-25 mol%) on the structural, morphological and luminescent characteristics of Y₂O₃:Eu³⁺ powders. The hydrothermal synthesis was performed at 200 °C for 12 h by employing Y₂O_3, HNO₃, H₂O and Eu (NO₃)₃ as precursors, in order to obtain two sets of samples. The first set of powders was obtained with different pH values and named Eu5PHx (x = 7, 8, 9, 10, 11, and 12), and the second set was obtained by using a constant pH = 7 with different Eu concentrations, named EuxPH7 (x = 2.5, 5, 8, 15, 20 and 25). The XRD spectra showed that the Y₂O₃:Eu³⁺ powders exhibited a cubic phase, regardless of the pH values and Eu³⁺ concentrations. The SEM observations indicated that pH influenced the morphology and size of phosphors; for instance, for pH = 7, hexagonal microplatelets were obtained, and microrods at pH values from 8 to 12. Doping Y₂O₃ with various Eu³⁺ concentrations (in mol%) also produced changes in morphology, in these cases, hexagonal microplatelets were obtained in the range of 2.5-5 mol%, and non uniform plates were observed at higher doping concentrations ranging from 8 to 25 mol%. According to our results, the microplatelets synthesized with a pH of 7 and an 8 mol% Eu³⁺ concentration presented the highest luminescence under excitation at 254 nm. All of these results indicate that our phosphors could be useful for applications of controlled drug delivery, photocatalysis and biolabeling.