We review the nonaqueous precursor chemistry of the group 4 metals to gain insight into the formation of their oxo clusters and colloidal oxide nanocrystals. We first describe the properties and structures of titanium, zirconium, and hafnium oxides. Second, we introduce the different precursors that are used in the synthesis of oxo clusters and oxide nanocrystals. We review the structures of group 4 metal halides and alkoxides and their reactivity toward alcohols, carboxylic acids, etc. Third, we discuss fully condensed and atomically precise metal oxo clusters that could serve as nanocrystal models. By comparing the reaction conditions and reagents, we provide insight into the relationship between the cluster structure and the nature of the carboxylate capping ligands. We also briefly discuss the use of oxo clusters. Finally, we review the nonaqueous synthesis of group 4 oxide nanocrystals, including both surfactant-free and surfactant-assisted syntheses. We focus on their precursor chemistry and surface chemistry. By putting these results together, we connect the dots and obtain more insight into the fascinating chemistry of the group 4 metals. At the same time, we also identify gaps in our knowledge and thus areas for future research.
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.
Ligands are a fundamental
part of nanocrystals. They control and
direct nanocrystal syntheses and provide colloidal stability. Bound
ligands also affect the nanocrystals’ chemical reactivity and
electronic structure. Surface chemistry is thus crucial to understand
nanocrystal properties and functionality. Here, we investigate the
synthesis of metal oxide nanocrystals (CeO
2-
x
, ZnO, and NiO) from metal nitrate precursors, in the presence
of oleylamine ligands. Surprisingly, the nanocrystals are capped exclusively
with a fatty acid instead of oleylamine. Analysis of the reaction
mixtures with nuclear magnetic resonance spectroscopy revealed several
reaction byproducts and intermediates that are common to the decomposition
of Ce, Zn, Ni, and Zr nitrate precursors. Our evidence supports the
oxidation of alkylamine and formation of a carboxylic acid, thus unraveling
this counterintuitive surface chemistry.
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