Physicochemical characterization of passive films on niobium by admittance and electrochemical impedance spectroscopy studies

An analysis of the electronic properties of amorphous semiconductor–electrolyte junction is reported for thin (Dox < 20 nm) passive film grown on Nb in acidic electrolyte. It will be shown that the theory of amorphous semiconductor–electrolyte junction (a-SC/El) both in the low band-bending and high band-bending regime is able to explain the admittance data of a-Nb2O5/El interface in a large range (10 Hz–10 kHz) of frequency and electrode potential values. A modelling of experimental EIS data at different potentials and in the frequency range of 0.1 Hz–100 kHz is presented based on the theory of amorphous semiconductor and compared with the results of the fitting of the admittance data obtained in a different experiment. Some preliminary insights on the possible dependence of the density of state (DOS) distribution on the mobile defects concentration and mechanism of growth of anodic film on valve metals are suggested.