Influence of Potentiostatic Aging on Nb and W Oxides Formed in 0.1 M HClO[sub 4]

The evolution of the electrical properties of Nb 2 O 5 and WO 3 films, at different formation potentials ( E f ) , in 0.1 M HClO 4 , were followed by electrochemical impedance spectroscopy (EIS). During the growth of the oxide films, EIS spectra were acquired every hour, maintaining the potentiostatic pulse during 20 h . The Nb 2 O 5 and WO 3 impedance spectra show variations vs potentiostatic aging. However, the Mott–Schottky analysis showed that the density of donors ( N D ) does not depend on the potentiostatic aging but depends on the E f , while, the flatband potential ( E FB ) is almost constant with the potentiostatic aging and E f , for both oxides. The relationship between the invariability of the semiconductor properties ( N D and E FB ) with the potentiostatic aging and the variation of the impedance spectra, in the case of the W, can be explained by the formation of an external layer of WO 3 ∙ ( H 2 O ) x , which must be very thin; the detachment of this layer causes a very small difference in the thickness, not modifying the semiconductor properties ( N D and E FB ), but greatly changing the electric properties of the film, since WO 3 ∙ ( H 2 O ) x is more resistive. A similar hydration process may occur in the Nb 2 O 5 films to a lesser extent.