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 SN1 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.

Fatty acid capped, metal oxo clusters as the smallest conceivable nanocrystal prototypes

“…This result stands in stark contrast to the regular practice of ignoring the ligands in the PDF renement of nanocrystals and larger clusters. 7,[50][51][52][53][54][55][56][57][58][59] The ligand effect is further demonstrated by tting the Zr12-acetate PDF with various models, see Fig. 4.…”

“…In this case, the organic ligands have usually been neglected in the structural models because the organic ligands scatter very little compared to the inorganic core. [50][51][52][53][54][55][56] In the case of larger clusters (35-144 metal atoms) PDF could conrm the structure of the core, 7,57-59 again by largely neglecting the organic ligands. However, to our knowledge, PDF has not yet been applied to small discrete oxo clusters capped with organic ligands, as discussed here.…”

“…Atomically precise clusters are an exciting material class. 1,2 They span a broad range from oxo clusters, over metal clusters to semiconducting clusters, e.g., Zr 6 O 4 (OH) 4 (OMc) 12 (Zr6, OMc = methacrylate), 3 [Hf 6 O 4 (OH) 4 (OOCMe) 12 ] 2 (Hf12), 4 Ti 8 O 8 -(OOCPh) 16 , 5 Au 55 (PPh 3 ) 12 Cl 6 , 6 Cd 84 Se 56 (O 2 CPh) 56 (RNH 2 ) 56 , 7 or In 37 P 20 (O 2 CCH 2 Ph) 51 . 8 Clusters are used to build stimuliresponsive materials, 9 or hierarchical (porous) structures.…”

Chemical Considerations for Colloidal Nanocrystal Synthesis

“…We have recently shown that the reaction mixture contains mixed chloroalkoxide species: ZrCl 3 (OiPr) and ZrCl 2 (OiPr) 2 . 38 These species are unreactive toward isopropanol, and thus only a stoichiometric amount of zirconium chloride is required in the reaction. We also provided evidence for an alternative mechanism based on E1 elimination; see Scheme 4.…”

“…As a result, the nanocrystals grow to larger sizes, and the yield increases significantly. 38 The cases of zirconia and InP clearly show that mechanistic understanding directly enables the nanochemist to go further than the state-of-the-art. Either new materials could be formed, or the size was tuned.…”