Controlled preparation of Gd2O2SO4:Eu3+ monospheres via hydrothermal precursor engineering for enhanced photoluminescence

Rare-earth oxysulfates (RE2O2SO4) attracted attention for large-capacity oxygen storage, low-temperature magnetism and luminescence, whose preparation mostly involves toxic SOx gases and/or complicated procedures. In the phosphor field, monospheres are preferred for a number of applications due to their better luminescence and high packing density. With the simple reactants of RE(NO3)3, Na2CO3 and (NH4)2SO4 for hydrothermal reaction, dispersed monospheres and nanoplates were selectively synthesized for the recently discovered RE2(OH)2CO3SO4·nH2O (RE = Gd0.95Eu0.05, REOCSH) layered compound precursor, from which RE2O2SO4 phosphors mostly retaining their precursor morphologies were facilely derived via calcination in air at 800 °C, without involving SOx. Solution pH was found to decisively determine the chemical composition and crystallization kinetics of the initial precipitate, and the possible mechanisms of phase/morphology evolution during hydrothermal treatment were proposed by comparatively investigating the temperature-course and time-course of REOCSH formation under the typical pH values of 6 and 7. It was revealed that (Gd0.95Eu0.05)2O2SO4 monospheres emit ∼1.35 times as strong as nanoplates (λex = 257 nm, λem = 617 nm) and may retain as high as ∼81% of the room-temperature intensity at 150 °C. The phosphor monospheres were also analyzed to have a fluorescence lifetime of ∼1.32 ms at room temperature and an activation energy of ∼0.19 eV for the thermal quenching of luminescence.