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 p olycrystalline tungsten in aqueous H3PO4, H2SO4, HNO3 and HClO4 soluti ons (pH approximate to 1.3) were studied using electrochemical impedan ce spectroscopy at room temperature. The data were analysed with a tra nsfer function using a non-linear fitting routine, assuming that the r esistance of the film is coupled in series with the faradaic impedance of the W(0)-->W(VI) reaction, with these in parallel with the capacit ance of the metal/passive film/electrolyte system. The relative permit tivity (epsilon(r)) of the films was found to depend on the acid solut ion used. Except for the case of films grown in HNO3, the values of ep silon(r) are smaller than those of thicker films grown galvanostatical ly. The film grown in H3PO4 solutions presented the lowest value of ep silon(r), which could be due to the incorporation of phosphate ions in to 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) 19 97 Elsevier Science Ltd.