Assessing the Influence of Confinement on the Stability of Polyoxometalate-Functionalized Surfaces: A Soft Sequential Immobilization Approach for Electrochromic Devices

A functionalization process has been developed and the experimental conditions optimized allowing the immobilization of first-row transition metal (Mn+) containing polyoxometalates (POMs) with the formula [M(H2O)P2W17O61](10–n)– on transparent indium–tin oxide (ITO) electrodes for electrochromic applications. Both flat ITO grafted with 4-sulfophenyl moieties and sulfonate-functionalized vertically oriented silica films on ITO have been used as electrode supports to evaluate possible confinement effects provided by the mesoporous matrix on the stability of the modified surfaces and their electrochromic properties. Functionalization involved a two-step sequential process: (i) the immobilization of hexaaqua metallic ions, such as Fe(H2O)63+, onto the sulfonate-functionalized materials achieved through hydrogen bonding interactions between the sulfonate functions and aqua ligands (water molecules) coordinated to the metallic ions facilitating and stabilizing the attachment of the metallic ions to the sulfonated surfaces; (ii) their coordination to [P2W17O61]10– species to generate "in situ" the target [Fe(H2O)P2W17O61]7– moieties. Comparison of the characterized surfaces clearly evidenced a significant improvement in the long-term stability of the nanostructured [Fe(H2O)P2W17O61]7–-functionalized silica films compared to the less constrained flat [Fe(H2O)P2W17O61]7–-modified ITO electrodes for which a rapid loss of [P2W17O61]10– species was observed. Concordantly, the [Fe(H2O)P2W17O61]7– POM confined in the mesoporous films coated on ITO gave rise to much better and stable electrochromic properties, with a transmittance modulation of 40% at 515 nm.