Cryogenic transmission electron microscopy
(cryo-TEM) was recently used to directly image the intact “solution-state”
structures of POM-protected silver (Ag) and
gold (Au) nanoparticles (NPs) in
water. Those findings represented a significant contribution to the
establishment of structure/reactivity relationships for POM-protected metal(0)
NPs, a growing class of
catalytically active nanostructures. Data obtained using 14-nm-diameter Au NPs
led to a structural model involving the extensive incorporation of
countercations into the POM monolayer itself. Accordingly,
POM-protected metal NPs might be viewed as pivotal members in a continuumof
electrostatically stabilized structures ranging from two-dimensional arrays of
POMs on planar surfaces to
the spherical single walls of hollow POM vesicles. If this electrostatic model is
correct, the stabilities of the POM monolayers on Au NPs should vary with the
nature of the integrated countercation, just as do lattice enthalpies of
crystalline POM salts and energies of cation association with POMs in solution.
Polyoxometalate (POM)-monolayer stability constants,
K, for three POM anions vary with the cation size, in the same order as that
for increasing ion-pair formation with α-SiW11O398–in the early nucleation
phase of monolayer self-assembly: Li+ < Na+ < K+ < Cs+. Cryo-TEM
images demonstrating the use of the cation size to rationally control monolayer
formation provide definitive evidence that the POM monolayers are
electrostatically stabilized (ionic) shells, analogous in that respect to the
monolayer walls of “hollow” POM-macroanion vesicles
Yifeng Wang, (et al.). Role of the Alkali-Metal Cation Size in the Self-Assembly of
Polyoxometalate-Monolayer Shells on Gold Nanoparticles. Inorganic Chemistry. American Chemical Society. Apr 1, 2012.
http://pubs.acs.org/doi/full/10.1021/ic300431a
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