The Structure of Self-Assembled Multilayers with Polyoxometalate Nanoclusters

Abstract

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.

Lanthanopolyoxotungstates in silica nanoparticles: multi-wavelength photoluminescent core/shell materials

“…15,16 Lanthanopolyoxometalate-based materials have been investigated in particular due to their photoluminescent properties. Lanthanopolyoxometalates have been assembled in Langmuir-Blodgett films 17,18 and in layer-by-layer deposition films, 19,20 or incorporated in double layer hydroxides 21 and in liquid crystals, 22 envisaging new optically active materials. Polyoxometalates (POMs) have also been used in the fabrication of siliceous sol-gel hybrid materials.…”

Metallo-supramolecular modules as a paradigm for materials science

“…This is demonstrated in figure 24 for the deposition of [Eu(III)(H 2 O)P 5 W 30 O 110 ] 12− and PAH [121]. If no salt is added to the POM solution, residual electrostatic and dipolar repulsion within the interfacial layer keep the surfaceconfined POM anions separated, thereby resulting in submonolayer coverage, compared to the packing density of the POMs in the crystalline solid.…”

Redox, surface and electrocatalytic properties of layer-by-layer films based upon Fe(III)-substituted crown polyoxometalate [P8W48O184Fe16(OH)28(H2O)4]20-

“…The LBL deposition of 10 and 11 layers led to partial suppression of cationic probe's response, whereas thicker films revealed a complete loss of permeability for the cationic probe. The plateau shape of the probe response at low layer numbers indicates the presence of pin hole or hindered diffusion within the LBL assembly [27]. Thus both the thickness of the LBL films and the nature of the terminal layer have an effect upon the film's permeability.…”

“…A variety of charged moieties have be employed for the construction of LBL assemblies, including polycations, such as, poly(diallyldimethylammonium chloride) (PDDA), poly(ethyleimine) (PEI) and poly(allylaminehydrochloride) (PAH) [21][22][23][24][25][26][27][28][29], transition metal complexes [16], dye molecules such as Methylene Blue, Thionin, Azure A, Brilliant Cresyl Blue and Nile Blue chloride [30], metallodendrimers [31], dendrimers [18], metalloporphyrins [32] and nanoparticles [33]. The growth of such films on solid surfaces has been monitored by cyclic voltammetry [16,20,22,31,[34][35][36], UV -Vis absorption spectroscopy [16,20,22,30,35,36] or impedance spectroscopy [20].…”