The growth kinetics of thin anodic WO3 films investigated by electrochemical impedance spectroscopy

The growth kinetics and structural properties of the passive film potentiostatically formed (1 V≤Ef≤6 V) on tungsten in deaerated 1.0 M hydrochloric acid solution, at 25 °C, were studied using cyclic voltammetry and electrochemical impedance spectroscopy. Impedance spectra (30 mHz≤f≤50 kHz) were fitted with the electric equivalent circuit model, and the corresponding transfer function, using the complex non-linear least-square method. The capacitive response of the impedance spectrum in the high frequency region was related to the thickness and dielectric properties of the barrier oxide film. Pseudoinductive impedance behavior of the tungsten ∣ oxide film ∣ electrolyte system at low frequencies was explained by the transport of oxygen vacancies (formally having a positive charge) across the oxide film and their consumption at the film ∣ solution interface. A negative surface charge built up at this interface via accumulation of metal vacancies (minority charge carriers in the anodic WO3 film) accelerated the oxygen vacancy transport as well as served as capture centers for the ‘positive’ charge. The relaxation phenomenon was ascribed to a time variation of the surface charge at the film ∣ solution interface. The main kinetic and structural characteristics of the tungsten ∣ anodic oxide film ∣ electrolyte system were determined: the effective electric field strength (Hef), the steady-state electric field strength (Hss), the atomic half jump distance (a), the polarizability of the film ∣ solution interface (α), the cross-capture section for positive defects (oxygen vacancies) by the negative surface charge (S), the current efficiency for film formation (λ), the diffusion coefficient of oxygen vacancies (D0), and the zero field activation energy for vacancy motion (A).