Water dispersible ligand-free rare earth fluoride nanoparticles: water transfer versus NaREF4-to-REF3 phase transformation

Abstract

The chemical stability of oleate-capped sub-10 nm α- and β-NaREF₄ NPs (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu for α- and RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy for β-phase NPs) was evaluated under the acidic conditions used for ligand removal towards water dispersibility. It was found that for such small NPs, a pH lower than 3 was necessary for the water transfer to be efficient and to yield well-dispersed ligand-free NPs. In stark contrast to the generally considered good chemical stability of NaREF₄, these conditions were observed to pose a risk to phase transformation of the NaREF₄ NPs into much larger, hexagonal- or orthorhombic-phase REF₃, depending on the NP composition. A correlation between the thermodynamic stability of the α/β-NaREF₄ and the hexagonal/orthorhombic REF₃ phases – dictated by the RE ion choice – and the chemical stability of the NPs was found. For instance, β-NaGdF₄ NPs remained stable, while α-NaGdF₄ NPs underwent phase transformation into hexagonal GdF₃. More general, NaREF₄ NPs based on lighter RE ions were more prone towards phase transformation, while those based on heavier RE ions exhibited stability. Herein, within the RE series, the borderline for phase transformation was identified as Tb/Dy for α-NaREF₄ NPs and Sm/Eu for β-NaREF₄ NPs, respectively. Also, given the large interest in luminescent NPs for, e.g. biomedical applications, optically active Ln³⁺ ions (Ln = Nd, Eu, Tb, Er/Yb) were doped into α/β-NaGdF₄ host NPs, and the dopant influence on the chemical stability was evaluated. Steady state and time-resolved spectroscopy unveiled spectral features characteristic for Ln³⁺ f–f transitions, i.e. downshifting and upconversion, before and after ligand removal. Overall, the results herein described emphasise the importance of minding the chemical procedure used for ligand removal of NaREF₄ NPs of different crystalline phases and RE compositions.