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.

From Gel to Crystal: Mechanism of HfO2 and ZrO2 Nanocrystal Synthesis in Benzyl Alcohol

“…It is also often postulated that a nanocrystal precursor (P) converts into a monomer (M). , Here, we observe the conversion of the precursor into a gel. While the exact mechanism by which the gel crystallizes into the nanocrystals remains unclear, we can exclude a LaMer mechanism, and consequently, the derived mass balance by Sugimoto is not valid in this case. , Our results agree with other reports of nonclassical pathways to (oxide) nanocrystals, featuring disordered intermediates. − However, such previous reports did not report a macroscopic gel phase. The amorphous intermediates were identified as nanoparticles, and the reaction mixture remains liquid.…”

Tailoring of Colloidal HfO2 Nanocrystals with Unique Morphologies and New Self‐Assembly Features

“…This is consistent with our observations of amorphous‐like material at the tip ends of the nanorods. [ 38 ]…”

“…Yet, reproducing T phase nanocrystals was somewhat contradictory in follow‐up researches involving this specific sol–gel method, and stabilization of the T phase involved reactions with aliovalent metal species. [ 36‐40 ] The proneness of hafnia to undergo a phase transformation from T to M phase is believed to result from the greater difference of these two phases in bulk free energies Δ G bulk compared to zirconia. While Δ G bulk for HfO 2 is 194 meV and just 140 meV for ZrO 2 , the difference in surface free energies ( γ ) for both systems can be neglected and by that the critical size for the stabilization of the T phase in hafnia is lower than for zirconia.…”

The Chemistry of Spinel Ferrite Nanoparticle Nucleation, Crystallization, and Growth

“… 37 Another example is the study of the hydrothermal formation of CeO 2 nanoparticles from a Ce(NH 4 ) 2 (NO 3 ) 6 aqueous solution, which showed the presence of a larger dimeric precursor complex, which upon heating is converted into CeO 2 likely by clustering followed by fast internal restructuring prior to growth. 47 Additional examples of nucleation mechanisms of solvothermal nanoparticle systems that have been studied include different oxide nanocrystallites, such as HfO 2 , 48 Nb 2 O 5 , 49 WO 3 , 50 TiO 2 , 51 ZrO 2 , 52 , 53 ZnWO 4 , 54 as well as metal halide nanoparticles, e.g., Ir x Cl y , 55 metallic nanoparticles, e.g., Pt/Pt 3 Gd, 56 Pd–Pt, 40 , 57 PdIn, 58 FeSb 2 /FeSb 3 , 59 and high entropy alloy nanoparticles. 60 In several of the mentioned metal oxide systems, the precursors were observed to contain dimeric or trimeric metal oxide/hydroxide polyhedra, which “polymerize” and/or reconfigure to form larger clusters during nucleation.…”