Abstract
Abstract An extensive electrochemical and photoelectrochemical investigation has been carried out with very thin anodic oxide layers grown on niobium metal in sulphuric acid solutions. From the experiments the presence of an initial suboxide layer on the metal surface was inferred. Upon anodic polarization a strongly non-stoichiometric pentoxide film is formed, which is subsequently oxidized to the normal amorphous pentoxide phase (a-Nb 2 O 5 ). The use of photocurrent spectroscopy (PCS) allowed this oxidation process to be followed at various potentials and polarization times. Moreover, from the experimental results we obtain information both on the composition and the solid-state structure of the different phases grown on the metal surface. It is shown that the strong non-stoichiometry introduces a continuous defect band near to the conduction band edge of the initial film. Evidence for the existence of localized states inside the mobility gap of the films is also reported. The nature of the defects in anodic a-Nb 2 O 5 is briefly discussed.
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