Nanoparticles seize the scientific world and have long found their fields of application. However, synthesis and handling of these small particles remain a big challenge: They tend to agglomerate, not only in dispersions but also during the production process.

In this work, various factors governing agglomeration were examined. The influences of pH, surfactants and the application of high power ultrasound (HPUS) have been investigated for nanoscaled praseodymium doped zirconia. In order to devise efficient strategies for stabilising a dispersion of zirconia nanoparticles, the topological distribution of dopants within the particles is important to know.

Nanoscaled ZrO₂:Pr³⁺ powders were obtained via a hydrothermal microwave driven process. [1] The Pr³⁺ content varied between 0.05 and 10 mol%. The particle size distribution as a function of pH and dopant quantity was measured to unravel the impact of the isoelectric point (IEP). Pure zirconia has an IEP at pH5, whereas pure Pr₂O₃ has its IEP at pH9. Due to missing electrostatic interactions, the agglomeration tendency is strong around the IEP of the sample. Measurements of the particle size distribution reveal a strong variation of the maximum average particle size depending on Pr³⁺ content and pH. At low Pr³⁺ doping levels, particle agglomeration occurs close to the IEP of zirconia, whereas at higher amounts of Pr³⁺, strong agglomeration appears at pH values close to the IEP of pure Pr₂O₃. Due to the significant changes, it is assumed that the Pr³⁺ ions are predominantly located in the particle surface region.

In order to obtain stable water-based dispersions of Pr³⁺ doped zirconia, it is thus important to set the pH according to the content of praseodymium. The addition of Sodium Dodecyl Sulphate (SDS) as a surfactant was examined and resulted in dispersions with a small tendency to form agglomerates. Another efficient way to obtain dispersions with small particles is the application of HPUS for a limited time. It was shown that the application of HPUS can improve the luminescence properties of ZrO₂:Pr³⁺, especially for the samples with a small quantity of praseodymium. Both, excitation and emission are affected by the application of HPUS.

[1] Bondioli et al: Structure, morphology and luminescence properties of Pr-doped nanocrystalline ZrO₂ obtained by hydrothermal method. Solid State Phenomena Vol. 94 (2003), pp 141-144