An Amorphous Phase Precedes Crystallization: Unraveling the Colloidal Synthesis of Zirconium Oxide Nanocrystals

Abstract

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.

Keywords: ZrO2; growth; nanoparticle; nucleation; small-angle X-ray scattering; total scattering.

Figure 1

Synthesis of zirconia from zirconium chloride and zirconium isopropoxide isopropanol complex at 340 °C in trioctylphosphine oxide (TOPO). The chemical mechanism involves first the formation of mixed chloro-alkoxide species, which undergo E1 elimination, disproportionation, and condensation reactions.

Figure 2

Ex situ SAXS analysis on reaction aliquots. Normalized SAXS pattern (a,b) and corresponding fits (c,d) for reaction mixtures with either ZrCl₄ or ZrBr₄ as halide source. The different patterns are shifted with respect to one another for better visualization. Time evolution at 340 °C for the particle diameter (e), the polydispersity (f), and the particle concentration (g) obtained after fitting the experimental data.

Figure 3

Ex situ PDF analysis on reaction aliquots. The experimental (blue circles) and calculated (red line) PDF for the reaction aliquots with either ZrCl₄ (a) or ZrBr₄ (b) as halide source. The goodness of fit is indicated by Rw. The reaction temperature of 340 °C is reached at t = 0 min. (c) The refined nanocrystal size by PDF and corresponding Rw values (inset) of the reaction aliquots. The refined parameters are shown in Table S1–S2. (d) Extraction of PDF pattern of the amorphous intermediate phase for the 9 min aliquot (ZrCl₄ reaction). The blue circles represent the measured PDF of the aliquot, and the red line corresponds to the normalized PDF of the reaction crude product (90 min aliquot). After subtraction of the two, the data in the green circles are obtained. The experimental PDF of the ZrCl₄·2TOPO byproduct (brown line) is scaled and subtracted from the green data, obtaining the PDF of the amorphous intermediate (violet circles).

Figure 4

(a) The disappearance of zirconium isopropoxide moieties determined with ¹H NMR. The inset shows more detail for the first 9 min at 340 °C. The corresponding ¹H spectra are shown in Figures S11–S12. The yield of the reaction, from the perspective of precursor conversion, the total particles formed (from SAXS analysis), the crystalline fraction (from reciprocal total scattering analysis), and the amorphous fraction (total particles minus crystalline fraction) for reaction mixtures with either ZrCl₄ (b) or ZrBr₄ (c) as halide source. The dotted lines indicate the fit obtained using the simple two-step mechanism (eq 3). (d) The number of nanocrystals as a function of time, calculated from the crystalline yield and the crystal size (PDF).

Figure 5

Schematic representation of the formation mechanism of zirconia nanocrystals. Precursor conversion leads to amorphous intermediates, from which crystals nucleate and grow.