Microwave-assisted sol-gel synthesis and photoluminescence characterization of LaPO₄:Eu³⁺,Li⁺ nanophosphors

Here, we describe the fast and mass fabrication of monazite lanthanum orthophosphate (LaPO₄) nanoparticles via a simple sol-gel method under the assistance of microwave irradiation. The procedure involves formation of homogeneous, transparent, metal-citrate-EDTA gel precursors using both citric acid (CA) and ethylenediamine tetraacetic acid (EDTA) as the complexing agent followed by microwave irradiation, which promotes prompt thermal decomposition of the metal-citrate-EDTA gel precursors to yield the final nanoparticles. Thermogravimetric/differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) were used to characterize the as-synthesized nanoparticles. About 23 g of single monoclinic phase, approximately 100 nm diameter, LaPO₄ spherical nanoparticles were readily obtained at 800 °C within 0.5 h, and the nanospheres were themselves composed of Ultrafine nanocrystals of a few nanometers in diameter. Furthermore, photoluminescence (PL) characterization of the Li⁺- and Eu³⁺-codoped LaPO₄ nanocrystals was carried out. The effects of microwave irradiation temperature and Eu³⁺ active center concentration, especially the doping concentration of Li⁺ on the PL properties, were elaborated in detail. Room-temperature photoluminescence (PL) characterization revealed that the optical brightness as well as the intensity ratio of ⁵D₀-⁷F₁ to ⁵D₀-⁷F₂ is highly dependent on the Li⁺ ions concentration. Introduction of 5 mol % Li⁺ into the crystal structure enhanced the PL emission brightness more than 2-fold, and the Li_0.05Eu_0.05La_0.9PO₄ nanophosphor showed the relatively most promising PL performance with the most intense emission.