Lactic-acid enhanced solvothermal crystallization, color-tunable photoluminescence, and thermal stability of h-LaPO4:Ce3+, Tb3+, Sm3+ nanocrystals

Abstract

Lactic acid (LA) was originally employed as a cosolvent for solvothermal synthesis of LaPO₄:Ce³⁺, Tb³⁺, Sm³⁺ nanocrystal phosphors, whose effect on crystal growth was revealed by the combined techniques XRD, FE-SEM and TEM. It was shown that LA not only remarkably restricted the intrinsic one-dimensional (1D) growth of hexagonal LaPO₄ crystals along the [001] direction, resulting in a shape variation from microwires to nanoparticles, but also promoted crystallite growth. With more efficient Ce³⁺ → Tb³⁺ energy transfer resulting from larger crystallite size, the h-(La_0.90Ce_0.05Tb_0.05)PO₄ nanoparticles presented more intense and thermally stable green emission of Tb³⁺ as compared to the nanowires. Moreover, a series of (La_0.93−xCe_0.05Tb_xSm_0.02)PO₄ (x = 0–0.40) nanocrystals, with the emission color varying from blue to green and eventually to yellow, were successfully prepared through adjusting the concentration of Tb³⁺, in which the Tb³⁺ ion acted as an energy transfer bridge between Ce³⁺ and Sm³⁺. The mechanism of Ce³⁺ → Tb³⁺ → Sm³⁺ energy transfer was proposed, and the optimal Tb³⁺ content for Sm³⁺ emission was determined to be x = 0.20. The optimized (La_0.73Ce_0.05Tb_0.20Sm_0.02)PO₄ phosphor exhibited good thermal stability for Sm³⁺ emission, whose activation energy for thermal quenching was found to be ∼0.33 eV.