A facile route for preparing rhabdophane rare earth phosphate nanorods

Abstract

Lanthanum orthophosphate nanorods with typical dimensions of 8 nm in diameter and 80 nm in length were prepared simply by mixing NaH₂PO₄ and LaCl₃ aqueous solutions in an oil bath of 100 °C. Eu³⁺- and Ce³⁺-doped nanorods were also be prepared in a similar way by introducing Eu³⁺ and Ce³⁺ ions, respectively, into the reaction systems. TEM (transmission electron microscope) and XRD (X-ray diffraction) results revealed that the above-mentioned lanthanide orthophosphate nanorods possessed a rhabdophane-type structure. Compositional analysis by X-ray photoelectron spectroscopy demonstrated that LaPO₄ nanorods had an anion-rich surface, which might be the main reason for their colloidal stabilities. The appearance of radiating aggregates during the early stage of the precipitation reactions and their pH dependent morphologies indicated that the formation of lanthanide orthophosphate nanorods was mainly governed by the growth nature of hexagonal LaPO₄. The optical measurements indicated that LaPO₄ ∶ Ce³⁺ with the rhabdophane structure has stronger electron–phonon coupling than monazite-type. The fluorescence quantum yield of LaPO₄ ∶ Ce³⁺ nanorods was estimated to be 35%. In comparison with rhabdophane nanoparticles, the strongest emission of the Eu³⁺-doped nanorods originates from the ⁵D₀ to ⁷F₁ transition rather than ⁵D₀ to ⁷F₂. This may be attributed to the nature of the 1D structure as well as the lattice distortions in the nanorods.