An amorphous phase precedes crystallization; unraveling the colloidal synthesis of zirconium oxide nanocrystals

One can nowadays readily generate monodisperse colloidal nanocrystals, but the underlying mechanism of nucleation and growth is still a matter of intense debate. Here, we combine X-ray pair distribution function (PDF) analysis, small angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM) to investigate the nucleation and growth of zirconia nanocrystals from zirconium chloride and zirconium isopropoxide at 340 °C, in the presence of surfactant. We find that initially, many amorphous particles are formed. Over time, the total particle concentration decreases while the amorphous particles recrystallize into ZrO2 nanocrystals. After a sudden increase, the concentration of nanocrystals stays constant over the course of the reaction. Both findings stand in contrast to reports of continuous nucleation in other surfactant-assisted nanocrystal syntheses. The non-classical nucleation is likely related to the precursor decomposition rate that is an order of magnitude higher than the observed crystallization rate. Comparing different zirconium precursors, we observe higher smaller particles for reactions with ZrBr4 or with Zr(OtBu)4, which we could correlate with a higher precursor decomposition rate.