EDTA-assisted phase conversion synthesis of (GdRE)PO nanowires (RE = Eu, Tb) and investigation of photoluminescence.

Hexagonal (GdRE)PO·HO nanowires ~300 nm in length and ~10 nm in diameter have been converted from (GdRE)(OH)NO·HO nanosheets (RE = Eu, Tb) in the presence of monoammonium phosphate (NHHPO) and ethylene diamine tetraacetic acid (EDTA). They were characterized by X-ray diffraction, thermogravimetry, electron microscopy, and Fourier transform infrared and photoluminescence spectroscopies. It is shown that EDTA played an essential role in the morphology development of the nanowires. The hydrothermal products obtained up to 180 °C are of a pure hexagonal phase, while monoclinic phosphate evolved as an impurity at 200 °C. The nanowires undergo hexagonal→monoclinic phase transformation upon calcination at ≥600 °C to yield a pure monoclinic phase at ~900 °C. The effects of calcination on morphology, excitation/emission, and fluorescence decay kinetics were investigated in detail with (GdEu)PO as example. The abnormally strong D→F electric dipole Eu emission in the hexagonal phosphates was ascribed to site distortion. The process of energy migration was also discussed for the optically active Gd and Eu/Tb ions.