Colloidal
multi-metal oxide nanocrystals that contain the element
vanadium are attractive materials for enabling diverse technologies.
Few nanocrystal systems are synthetically well developed, however,
which is attributable to a scarcity of molecular precursors that enable
facile incorporation of vanadium. Here, we demonstrate the viability
of polyoxovanadates as precursors for colloidal nanocrystal synthesis,
which is compelling because they contain polyhedral VO
n
(n = 4, 5, 6) units that are often
also present in the multi-metal oxides of interest. First, we discovered
that a commercial tetraalkylammonium halide phase transfer reagent
extracts aqueous metavanadate ions into organic solvent, conferring
solubility to the resulting polyoxovanadates required for nanocrystal
synthesis. We then studied the thermal decomposition of an iron oleate
complex mixed with a phase-transferred polyoxovanadate precursor,
which yielded previously inaccessible colloidal ternary oxide nanocrystals
in high yield, including triclinic FeVO4 and spinel-type
vanadium-doped magnetite (Fe3–x
V
x
O4) phases. The crystal
structure, composition, and morphology of the nanocrystals, which
were characterized using X-ray diffraction, Raman scattering, X-ray
photoelectron spectroscopy, and transmission electron microscopy,
were found to depend strongly on the iron-to-vanadium precursor ratio.
The magnetic properties of these colloidal nanocrystals correlated
strongly to the phase of vanadium iron oxide, as FeVO4 displayed
antiferromagnetic behavior and a Néel temperature of 21 K,
while Fe3–x
V
x
O4 showed a superparamagnetic-like response with
a ferrimagnetic blocking temperature at 162 K. The emergence of polyoxovanadates
as precursors to ternary metal oxide nanocrystals that contain vanadium
is a significant synthetic advance that may be extensible to other
important multi-metal oxide systems.