Structure and Electroactivity of Tungsten Oxides in Acid Medium: Enhancement of Charge Propagation Rates through Nanostructuring and Formation of Mixed-Metal Oxide Systems

Electrochromism in tungsten oxides and related systems has been recognized for long time and has been still of interest for display applications. Redox transitions and related reversible coloration of thin WO₃ films are believed to be controlled by simultaneous injection or removal of electrons and protons. The dynamics of partial reduction (in acid medium) of transparent tungsten(VI) oxides to blue hydrogen tungsten(VI,V) oxide bronzes or lower tungsten(VI,IV) oxides is largely dependent of the oxide morphology, nanostructuring, degree of hydration, or population of colored W(VI,V) or W(VI,IV) sites. For example, by depositing nanorods of tungsten oxide, rather than conventional tungsten oxide, on the ITO conducting glass (or  the inert glassy carbon) substrate, the enhancement in the degree of the system's voltammetric reduction in acid medium (0.5 M H₂SO₄) has been observed. Tungsten oxide nanorods seem to be partially and reversibly reduced not only to nonstoichiometric hydrogen tungsten(VI,V) oxide bronzes (H_xWO₃, 0<x<1) but also substoichiometric lower tungsten (VI,IV) oxides (WO_3-y, 0<y<1).

            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 (H_xWO₃) 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 (WO₄^2- 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 (H_xWO₃). 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.