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 (tri-n-octylphosphine oxide). Through E1 elimination, precursor conversion leads to the formation of small amorphous particles (less than 2 nm in diameter). Over the course of the reaction, the total particle concentration decreases while the concentration of nanocrystals stays constant after a sudden increase (nucleation). Kinetic modeling suggests that amorphous particles nucleate into nanocrystals through a second order process and they are also the source of nanocrystal growth. There is no evidence for a soluble monomer. The nonclassical nucleation is related to a precursor decomposition rate that is an order of magnitude higher than the observed crystallization rate. Using different zirconium precursors (e.g., ZrBr4 or Zr(OtBu)4), we can tune the precursor decomposition rate and thus control the nanocrystal size. We expect these findings to help researchers in the further development of colloidal syntheses.