Monazite LaPO4: Eu nanorods annealed in air atmosphere: Luminescence optimization and co-existence of Eu2+/Eu3+

Abstract

Monazite LaPO₄: 2–25 mol% Eu nanorods were synthesized using a simple homogeneous co-precipitation method followed by calcination in air at 800 °C. Their luminescence properties were optimized by annealing in the same atmosphere at 1200 °C. The effects caused by this post-synthesis treatment on the structure, morphology and luminescence properties of the nanorods were analysed using X-ray diffraction (XRD), transmission electronic microscopy (TEM), Fourier transform infrared spectroscopy (FTIRS), photoluminescence spectroscopy (PLS) and laser scanning confocal microscopy (LSCM). The quality of the monazite phase was improved at 1200 °C and the rod-shaped (100 nm) particles were converted into pseudo-oval sintered grains of 140 nm. The maximum orange-red light emission of Eu³⁺ excited by 393 nm, with high color purity (93–98 %) and a phosphorescence lifetime of 4 ms was found at 20 mol% Eu. This optimal concentration was verified by X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS). The co-existence of Eu²⁺/Eu³⁺ in the surface of the grains of LaPO₄: 20 mol% Eu was proved by analysis of the Eu 3d/4d core levels and valence band spectra. The relative fraction of Eu²⁺ was calculated and compared with that of C–Eu₂O₃, taking into account the “surface valence transition” phenomenon. This is the first report that proves the co-existence of Eu²⁺/Eu³⁺ in LaPO₄: Eu monazite treated in an oxidant environment.