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 smallamorphousparticles (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.,
ZrBr4or 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.