Electrochemical behavior and multilayer assembly films with fine functional activities of the sandwich-type polyoxometallate [Sb₂W ₂₀Fe₂O₇₀(H₂O)₆] ^8-

The iron-substituted sandwich-type polyoxometalate (POM), comprised of the main group element Sb(III) as the central heteroatom, [Sb₂W ₂₀Fe₂O₇₀(H₂O)₆] ^8- (Sb₂W₂₀Fe₂), is one of the Krebs-type derivatives. For the first time, the POMs' redox electrochemistry has been elucidated under acidic conditions employing cyclic voltammetry. It exhibited what is believed to be a bielectronic redox couple associated with the two Fe(III) centers followed by four-electron and two-electron redox processes, respectively, with these being attributed to redox processes of the tungsten-oxo framework. The oxidized form of this POM was found to be stable from pH 1.5 to pH 6. Release of the iron centers from the complex, namely demetallization, was observed upon reduction of the Fe(III) sites at room temperature, with an influence of the solution pH being observed. Through the technique of layer-by-layer (LBL) assembly, the POM was successfully immobilized on both quartz and glassy carbon electrode (GCE) surfaces by alternate deposition with the polyelectrolyte poly(diallydimethylammonium chloride) (PDDA). Thus-prepared multilayer films have been characterized by cyclic voltammetry (CV), UV-vis spectroscopy (UV-vis) and XPS. The electrocatalytic activities of the multilayer films containing Sb₂W ₂₀Fe₂ have been investigated towards the reduction of NO₃^- and IO₃^-. With an increase in the number of Sb₂W₂₀Fe₂ monolayers within the assembly, the catalytic current towards the reduction of IO₃ ^- was enhanced.