Structure and Electroactivity of Tungsten Oxides in Acid Medium: Enhancement of Charge Propagation Rates through Nanostructuring and Formation of Mixed-Metal Oxide Systems
Another important issue is the mobility and availability of protons within the tungsten oxide film. We have found that by controlled admixing of tungsten oxide with zirconia nanoparticles both nanostructured morphology and high mobility of protons can be adjusted. The resulting mixed (hybrid or composite) system is well-behaved: redox reactions (which are characteristics of WO3) are fast and reversible despite the presence of semiconducting zirconia (ZrO2). While ZrO2 is not electroative, WO3 undergoes redox reactions leading to formation of partially reduced hydrogen bronzes (HxWO3) in which mobility of H+ is coupled to electron transfers. The composite material is stabilized through electrostatic attraction between positively charged zirconium oxo cations (ZrO2+, ZrOH3+, [Zr3(OH)4]3+) and anionic tungstates (WO42- in WO3*H2O or H2WO4). Among important issues are: high population of hydroxyl groups and high mobility of protons at surfaces of zirconia (ZrO2) nanostructures in addition to the existence of fast electron transfers coupled to proton displacements in partially reduced tungsten oxides (HxWO3). In a different series of chronocoulometric experiments, diffusion coefficients (for charge propagation) have been determined, and they have been found typically higher for hybrid mixed metal (tungsten(VI)/zirconium(IV) oxide films (close to 10-7 cm s-1). Mechanistic considerations and the results of spectroelectrochemical and diagnostic spectroscopic (e.g. FTIR) experiments will also be provided.
We acknowledge collaboration with Christian Perruchot and Mohamed Jouini from ITODYS, Université Paris Diderot, France.