Encapsulated-polyoxometalates in surfactant/silica gel hybrid films: Electrochemical behavior and main characteristics

Immobilization of various polyoxometalates (POMs) of different sizes and charges in surfactant/silica gel hybrid films is investigated. A change in the electrochemical behavior is noticed between dissolved and immobilized POMs. This change is attributed to the microenvironment effect that POM clusters experience inside the films. The presence of surfactant Triton X-405 in the matrix is essential for these observations. Results from SEM showed rough and porous morphology for the films prepared with surfactant, while smooth and fractured film structure is observed in absence of surfactant. For POMs such as [P2W18O62]6- and [SiW12O40]4-, the first two one-electron redox waves merged into one two-electron redox wave. However, for other POMs such as [PW12O40]3-, [H4AsW18O62]7-, [P2W15Mo2VIVO62]7-, α1-[P2W17O61FeIII]7-, α2-[P2W17VO62]8- and [P8W48O184]40-, the microenvironment effect results only in a shift of the potentials towards the negative values. The magnitude of the shift in potential is shown to decrease with increasing the POM's charge. Results from Particle size analysis showed that Triton X-405 influences the POM's particle size and with increasing the POM's charge, the POM's particle size in presence of Triton X-405 decreases. The latter is attributed to formation of micelles POM-Triton X-405, which consequently create the microenvironment effect for the immobilized POMs. The main characteristics of the surfactant/silica gel immobilized POM films are explored. The influence of the scan rate demonstrates that the redox behavior of the surfactant/silica gel immobilized composites is fast and involves surface-confined electron transfer processes. The study of these films in different proton concentrations showed that they are stable at low pH, and to obtain a better electron transfer, they have to be stored in the reference medium of 0.5 M H2SO4.