Understanding the effect of temperature on the crystallization of Eu3+:YPO4 nanophosphors prepared by MW-assisted method

The influence of the reaction temperature on the crystallization process of Eu3+:YPO4 (5 % mol of Eu3+) luminescent materials was investigated under MW-assisted heating, in aqueous media and without the use of organic templating agents. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM) analyses were employed to investigate the crystallization and morphological evolution. The syntheses were conducted via a MW-assisted hydrothermal method by heating at temperatures from 120 to 240 °C with a holding time of 20 min. The majority of the resulting powders were observed to be amorphous at 120 and 140 °C, exhibiting only minimal evidence of very small crystallites. Conversely, crystalline Eu3+:YPO4 powders with a single-phase tetragonal Xenotime-Y structure were obtained at 160 °C, in the form of nanosticks aggregating into nanobundles. It was found that a crystallization threshold for the formation of nanocrystalline Eu3+:YPO4 materials exists between 140 °C and 160 °C. As the reaction temperature was increased, a dehydration process occurred, resulting in the gradual disappearance of the residual amorphous phase. This resulted in the formation of fully crystalline nanoparticles at 200 °C. At this temperature, the coalescence of nanobundles into nanorods was observed, and this morphology became increasingly defined as the synthesis temperature was increased up to 240 °C. These findings highlight the pivotal role of temperature in the crystallization of these nanophosphors in an aqueous media at pH 7, thereby addressing a gap in the existing scientific literature.