Mechanistic Insight into the Precursor Chemistry of ZrO2 and HfO2 Nanocrystals; 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 ZrCl₄ 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_N1 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.

Scheme 1. Proposed Reaction Scheme of Zirconium Chloride and Zirconium Isopropoxide Isopropanol Complex, Reacting towards ZrO₂ Nanocrystals, According to Joo et al.

Figure 1

(A) Scheme for the reaction of ZrCl₄·2THF with TOPO. (B) ¹H NMR of the titration of a solution of 0.05 M ZrCl₄·2THF in CDCl₃ with increasing equivalents of TOPO. The latter is added as a 0.5 M solution, gradually diluting the zirconium complex. (C) ³¹P NMR of the same titration. The spectrum for 0.2 equiv was amplified twofold to observe the resonances more clearly. The spectra have a relative x-offset of 1 ppm with respect to each other. (D) The different TOPO species over the course of the titration. The total amount of Zr in the sample was 25 μmol.

Figure 2

(A) Scheme showing the exchange reaction between the ZrCl₄·2THF and tri-n-propylphosphine oxide ligands, comparing both cis and trans structures (R = propyl chain). (B) ΔH of the exchange reactions for the trans complexes and their corresponding optimized structures at the B3LYP/ aug-cc-pVDZ level of theory.

Figure 3

(A) Scheme for the reaction of Zr(OiPr)₄·iPrOH with TOPO. (B) ¹H NMR of the titration of a solution of 0.05 M Zr(OiPr)₄·iPrOH in CDCl₃ with increasing equivalents of TOPO relative to Zr. TOPO is added as a 0.5 M solution, gradually diluting the zirconium complex. The total amount of Zr in the sample was 25 μmol.

Figure 4

(A) Reaction scheme for the titration of a 1:1 mixture of ZrCl₄:Zr(OiPr)₄·iPrOH with TOPO in C₆D₆ at room temperature. (B) The ³¹P NMR spectra of the titration. The ratio of Zr to TOPO is indicated in the figure.

Figure 5

¹H and ³¹P NMR of the reaction mixture with 1 equiv of ZrCl₄ in C₆D₆. Aliquots were taken at different temperatures during the ramp and at different times at the final reaction temperature of 340 °C.

Figure 6

(A) X-Ray PDFs (acquired at 80 °C to melt TOPO) of reaction mixtures heated to different temperatures as indicated. The range of distances as determined from the DFT optimized structures of (5) and (6) are indicated by the grey zones. (B) PDF refinement for the reaction product at 340 °C after 90 min, using a dual-phase model with the tetragonal zirconia (P42/nmc) and the DFT optimized ZrCl₄·2TPPO complex (2). The refined values are shown in Table S3.

Figure 7

(A) Concentration of various intermediates at different times at the final reaction temperature of 340 °C for a 1:1 mixture of ZrCl₄:Zr(OiPr)₄·iPrOH. The amount of different species is calculated corresponding to the integrals of TOPO bound to the Zr-centers in ³¹P NMR. (B) Normalized, relative concentration of propene and isopropyl chloride in the reaction headspace at different temperature/time points.

Scheme 2. Our Alternative Pathway for the Formation of Zirconia Nanocrystals Is Based on E1 Elimination, Ligand Redistribution, and Condensation Reactions

Figure 8

(A) Scheme showing the Zr(OiPr)₄·iPrOH injection strategy to increase particle size and yield. (B) TEM and histogram of particles before and after each injection. The average size is indicated. (C) PDF fit for the purified product after three injections with the tetragonal zirconia (P42/nmc) model. The refined crystallite diameter is 5.33 nm. The other refined values are shown in Table S4. The PDF fit for purified particles before injection is shown in Figure S25.