Systematic Synthesis and Characterization of Single-Crystal Lanthanide Orthophosphate Nanowires

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Abstract
A simple hydrothermal method has been developed for the systematic synthesis of lanthanide orthophosphate crystals with different crystalline phases and morphologies. It has been shown that pure LnPO4 compounds change structure with decreasing Ln ionic radius: i.e., the orthophosphates from Ho to Lu as well as Y exist only in the tetragonal zircon (xenotime) structure, while the orthophosphates from La to Dy exist in the hexagonal structure under hydrothermal treatment. The obtained hexagonal structured lanthanide orthophosphate LnPO4 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) products have a wirelike morphology. In contrast, tetragonal LnPO4 (Ln = Ho, Er, Tm, Yb, Lu, Y) samples prepared under the same experimental conditions consist of nanoparticles. The obtained hexagonal LnPO4 (Ln = La → Tb) can convert to the monoclinic monazite structured products, and their morphologies remained the same after calcination at 900 °C in air (Hexagonal DyPO4 is an exceptional case, it transformed to tetragonal DyPO4 by calcination), while the tetragonal structure for (Ho→ Lu, Y)PO4 remains unchanged by calcination. The resulting LnPO4 (Ln = La → Dy) products consist almost entirely of nanowires/nanorods with diameters of 5−120 nm and lengths ranging from several hundreds of nanometers to several micrometers. Europium doped LaPO4 nanowires were also prepared, and their photoluminescent properties were reported. The optical absorption spectrum of CePO4 nanowires was measured and showed some differences from that of bulk CePO4 materials. The possible growth mechanism of lanthanide phosphate nanowires was explored in detail. X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, electron diffraction, infrared absorption spectra, X-ray photoelectron spectroscopy, optical absorption spectra, and photoluminescence spectra have been employed to characterize these materials.