A study of thin anodic WO3 films by electrochemical impedance spectroscopy

The properties of thin anodic passive films potentiostatically formed (1 V less than or equal to E(f) less than or equal to 5 V vs sce) on polycrystalline tungsten in aqueous H3PO4, H2SO4, HNO3 and HClO4 solutions (pH approximate to 1.3) were studied using electrochemical impedance spectroscopy at room temperature. The data were analysed with a transfer function using a non-linear fitting routine, assuming that the resistance of the film is coupled in series with the faradaic impedance of the W(0)-->W(VI) reaction, with these in parallel with the capacitance of the metal/passive film/electrolyte system. The relative permittivity (epsilon(r)) of the films was found to depend on the acid solution used. Except for the case of films grown in HNO3, the values of epsilon(r) are smaller than those of thicker films grown galvanostatically. The film grown in H3PO4 solutions presented the lowest value of epsilon(r), which could be due to the incorporation of phosphate ions into the film. The number concentration of donors in the films (N-D) was found to decrease with E(f) (ie increasing film thickness) and to be quite dependent on the electrolyte used: N-D(H3PO4) < N-D(HClO4) < N-D(HNO3) < N-D(H2SO4). The values for N-D(H3PO4) are, nevertheless, 10(3) times larger than the ones for much thicker WO3 films and are of the same order of magnitude as the ones for annealed WO3 films. The flat band potential of the films grown in H3PO4 was found to be more than 0.1 V smaller than those for films grown in the other solutions. (C) 1997 Elsevier Science Ltd.