Hexaniobate Cluster Anion Monolayers on Gold Nanoparticles: A New Structural Role for Alkali Metal Countercations

Abstract

Monolayer shells of polyoxotungstate cluster anions on gold nanoparticles in water were electrostatically stabilized by structurally integrated countercations, with formation constants, K, increasing in the order: Li + < Na + < K + < TMA + < Cs + (TMA + = tetramethylammonium). We now report that for hexaniobate cluster anions, K values increase in the same order, with the notable exception of TMA + , which is effectively unable to induce monolayer formation. These findings point to a new structural model in which hexaniobate anions form a spherical coordination polymer at the gold surface with alkali metal countercations serving as single-atom structural building units between hexaniobate linkers.

Directional Bonding in Decaniobate Inorganic Frameworks

“…Because Cs + is the only cation that will exchange into the Nb 10 ‐frameworks, and it selectively replaces the alkalis before it replaces the TMA, we believe Cs + has high bonding affinity for PONbs. This has been previously noted for hexaniobate (as well as hexatantalate) even in solution [46–50] . This was prior correlated to the high anionic charge‐density of [Nb 6 O 19 ] 8− , but [Nb 10 O 28 ] 6− is considerably less charge‐dense, (i.e.…”

“…Instead, the affinity may be attributed to the high basicity of Nb‐bound oxos, regardless of the charge of the cluster. Additionally, some covalent character of the (Nb−O)−Cs bond has been described as a contributing factor [48] . Interestingly, Nb substitution into a titanosilicate ion‐exchanger prior led to high Cs‐selectivity, exploited in removal of radioactive Cs from nuclear wastes and contaminated seawater [51–53] …”

“…[45] Because Cs + is the only cation that will exchange into the Nb 10 -frameworks,and it selectively replaces the alkalis before it replaces the TMA, we believe Cs + has high bonding affinity for PONbs.T his has been previously noted for hexaniobate (as well as hexatantalate) even in solution. [46][47][48][49][50] This was prior correlated to the high anionic charge-density of [Nb 6 O 19 ] 8À , but [Nb 10 O 28 ] 6À is considerably less charge-dense,(i.e.charge/ atoms is 0.32 for hexaniobate and 0.16 for decaniobate). Instead, the affinity may be attributed to the high basicity of Nb-bound oxos,r egardless of the charge of the cluster.…”

“…Additionally,s ome covalent character of the (Nb À O) À Cs bond has been described as acontributing factor. [48] Interestingly,Nbsubstitution into atitanosilicate ion-exchanger prior led to high Cs-selectivity,exploited in removal of radioactive Cs from nuclear wastes and contaminated seawater. [51][52][53] Prototype transition metal-Nb 10 framework…”

Adsorption of Keggin-Type Polyoxometalates on Rh Metal Particles under Reductive Conditions

“…Some structure changes might be induced by the electron radiation during the TEM observation. Literature studies for POM-decorated noble metal particles frequently used cryo-TEM instruments for direct observation; ,, such instruments may give clearer images of adsorbed POM at a molecular level.…”