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