Annealing BaAl2O4:Tb3+/Tb4+ in air and a reducing atmosphere: a strategy to enhance luminescence by eliminating Tb4+

Abstract

The enhancement of green luminescence in terbium (Tb)-doped BaAl2O4 phosphors was significantly influenced by the annealing environment, primarily due to the reduction of Tb4+ to Tb3+ - a key factor for solid-state lighting applications. In this research, BaAl2O4:Tb3+/Tb4+ luminescent materials were synthesized through a coprecipitation method. The sample underwent annealing at 1200 °C for 2 h in an alumina crucible in air and under varying pressures of a H2 (5%)/Ar atmosphere to investigate the influence of annealing at different atmospheres on the luminescent characteristics of BaAl2O4:Tb3+. X-ray powder diffraction analysis of Tb3+-doped BaAl2O4 revealed a hexagonal crystal structure, corresponding to the space group P63. X-ray photoelectron spectroscopy (XPS) showed that Ba atoms occupied two distinct sites in BaAl2O4. Additionally, the XPS analysis confirmed the presence of both Tb3+ and Tb4+ oxidation states in the BaAl2O4:Tb sample annealed in air, which is further supported by electron paramagnetic resonance analysis, substantiating the presence of Tb4+. The samples annealed in a H2 (5%)/Ar atmosphere demonstrated superior photoluminescence (PL) relative to those annealed in air, attributable to the absence of Tb4+ in the samples. PL emission in blue, green, and red was observed under excitation from the interconfigurational 4f8 → 4f7 5d1 transitions of Tb3+ (228 nm). UV-Vis diffuse reflectance studies indicated the nonexistence of Tb4+ in samples annealed in a H2 (5%)/Ar environment at varying pressures. These investigations suggest that the presence of Tb4+ in BaAl2O4 functions as a luminescence quencher for Tb3+, presumably due to charge compensation and electron acceptor defects that inhibit the luminescence of Tb3+