Mechanistic Insight into the Precursor Chemistry of ZrO₂and HfO₂Nanocrystals; towards Size-Tunable Syntheses

Abstract

One can nowadays readily generate monodisperse colloidal nanocrystals, but a retrosynthetic analysis is still not possible since the underlying chemistry is often poorly understood. Here, we provide insight into the reaction mechanism of colloidal zirconia and hafnia nanocrystals synthesized from metal chloride and metal isopropoxide. We identify the active precursor species in the reaction mixture through a combination of nuclear magnetic resonance spectroscopy (NMR), density functional theory (DFT) calculations, and pair distribution function (PDF) analysis. We gain insight into the interaction of the surfactant, tri- n -octylphosphine oxide (TOPO), and the different precursors. Interestingly, we identify a peculiar X-type ligand redistribution mechanism that can be steered by the relative amount of Lewis base (L-type). We further monitor how the reaction mixture decomposes using solution NMR and gas chromatography, and we find that ZrCl 4 is formed as a by-product of the reaction, limiting the reaction yield. The reaction proceeds via two competing mechanisms: E1 elimination (dominating) and S N 1 substitution (minor). Using this new mechanistic insight, we adapted the synthesis to optimize the yield and gain control over nanocrystal size. These insights will allow the rational design and synthesis of complex oxide nanocrystals.

Chemical Considerations for Colloidal Nanocrystal Synthesis

“…Therefore, the first step in colloidal synthesis is usually the conversion of the precursors into the intended product. Given its importance, precursor conversion has been studied in many nanocrystal systems: metals, ,− metal sulfides and selenides, ,− metal oxides, − lead halides, and metal pnictides. − In such efforts, nanochemists aim to write down the complete and balanced chemical equation, after determining the correct stoichiometry of the reaction and identifying reaction by-products. Ideally, this is complemented with a rigorous kinetics study to obtain further details about the mechanism.…”

“… While the above equation captures the zirconia formation, it is based on two assumptions: (1) the coordinated isopropanol participates in the reaction, and (2) the reaction proceeds through an S N 1 substitution mechanism, producing isopropyl chloride. We have recently shown that the reaction mixture contains mixed chloroalkoxide species: ZrCl 3 (O i Pr) and ZrCl 2 (O i Pr) 2 . These species are unreactive toward isopropanol, and thus only a stoichiometric amount of zirconium chloride is required in the reaction.…”

“…By repeatedly injecting zirconium isopropoxide in the reaction mixture, the ZrCl 4 by-product is consumed, and the reaction is extended. As a result, the nanocrystals grow to larger sizes, and the yield increases significantly …”

“… a Reproduced from ref under the terms of the Creative Commons Attribution 4.0 International License (CC BY; ). …”

“…When studying the chemistry of nucleation, we should thus not reduce the problem to an abstract monomer, but one needs to take into account the specific chemical nature of the species. This requires detailed investigation of the coordination chemistry of precursors and their exact decomposition mechanism. ,,,,− Such studies benefit greatly from X-ray based (synchrotron) techniques, e.g., small-angle X-ray scattering (SAXS), extended X-ray absorption fine structure (EXAFS) spectroscopy, and pair distribution function (PDF) analysis. ,, …”