The synthesis and fluorescent properties of nanoparticulate ZrO2 doped with Eu using continuous hydrothermal synthesis

Abstract

Fluorescent nanoparticles of zirconia doped with europium (ZrO₂:Eu³⁺) have been produced via the co-precipitation of simple metal salt precursors using continuous supercritical water hydrothermal synthesis. The effect of reaction temperature and residence time on both particle size and conversion were investigated. Increasing the reaction temperature from 200 to 400 °C increases the size of the particles: smaller diameters (<5 nm) were produced at lower temperatures and larger diameters (>20 nm) at higher temperatures. Varying residence time at constant temperatures (between 3.7 and 7 seconds) did not significantly affect the particle size. The structure of the nanoparticles synthesised at 400 °C was revealed to be cubic by X-ray diffraction. Conversion of the metal precursors to ZrO₂:Eu³⁺ was improved from 4 to 99% on increasing the temperature between 250 and 350 °C. The photoluminescent properties of the nanoparticles were also explored with respect to synthesis temperature. Fluorescence upon UV excitation at 255 nm, originating from Eu³⁺ doped in ZrO₂, was not significantly affected by the synthesis temperature. The emission spectra demonstrated characteristic f → f transitions of Eu³⁺ ions and can be explained through an effective dispersion of these ions within the ZrO₂nanoparticles. Initial cell toxicity studies suggest that ZrO₂:Eu³⁺ nanoparticles do not cause any significant increase in cell death in Chinese hamster ovary cells following an 18 hour incubation at 37 °C. This paper describes a ‘green’ and controlled method for the continuous production of research scale quantities of nanomaterials.