Influence of ionic liquid and oleic acid assisted methods on the spectroscopic properties of Nd³⁺-doped GdPO₄ nano-particles

We report a detailed analysis of both the structural and spectroscopic properties of Nd3+-doped GdPO4 nano-powders obtained via various wet synthesis methods. This type of material may be useful in optoelectronics, bioimaging and biomedicine. Three series of Nd3+-doped nano-materials with different concentrations of the optical activator (1-10 mol%) were obtained via ionic liquid assisted classical hydrothermal (IL HT) and microwave (IL MW) methods as well as via an oleic acid assisted hydrothermal (OA HT) method. A powder X-ray diffraction (PXRD) technique confirmed that the obtained GdPO4 nano-powders crystallize in a monoclinic system (P2(1)/n) with the average grain size ranging from 40 to 100 nm; the smallest grains are obtained via the ionic liquid assisted microwave method, and no other phase has been detected for any of the materials obtained. Electron microscopy techniques i.e. SEM and TEM were used to demonstrate the differences in the morphology and grain size, which resulted in the spectroscopic properties of nano-materials. The nine-fold coordinated Gd3+ ion in GdPO4 is easily substituted by the Nd3+ one with the C-1 symmetry due to their similar ionic radii, so that high-resolution low-temperature absorption and emission spectra do not show any structural distortion. The F-4(3/2) excited state dynamics studied at 77 K showed abnormal behavior, similar to that already observed for Nd3+ ions embedded in the YPO4 and LuPO4 tetragonal orthophosphates crystallizing in a xenotime-type crystal system.