The Structure of Self-Assembled Multilayers with Polyoxometalate Nanoclusters

Using electrostatic layer-by-layer self-assembly (ELSA), the formation of multilayers with
polyelectrolytes and nanoscopic polyoxometalate (POM) clusters of different sizes and charges is
investigated. The multilayers are characterized by UV−vis absorption spectroscopy, optical ellipsometry,
cyclic voltammetry, and atomic force microscopy. In all cases, it is possible to find experimental conditions
to achieve irreversible adsorption and regular multilayer deposition. Most importantly, the surface coverage
is directly related to the total charge of the POM anion and can be controlled from submonolayer to multilayer
coverage by adjusting the ionic strength of the dipping solutions. Imaging the interfaces after POM deposition
by atomic force microscopy reveals a granular surface texture with nanometer-sized features. The average
interfacial roughness amounts to approximately 1 nm. Cyclic voltammetry indicates that the electrochemical
properties of the POM clusters are fully maintained in the polyelectrolyte matrix, which opens a route toward
practical applications such as sensors or heterogeneous catalysts. Moreover, the permeability toward
electrochemically active probe molecules can be tailored through the multilayer architecture and deposition
conditions. Finally, we note that despite the low total charge and comparably small size of the discrete
POM anions, the multilayers are remarkably stable. This work provides basic guidelines for the assembly
of POM-containing ELSA multilayers and provides detailed insight into characteristic surface coverage,
permeability, and electrochemical properties.