Microwave-Assisted Sol−Gel Synthesis and Photoluminescence Characterization of LaPO₄:Eu³⁺,Li⁺ Nanophosphors

Abstract

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 Li0.05Eu0.05La0.9PO4 nanophosphor showed the relatively most promising PL performance with the most intense emission.