The photoelectrochemical properties of passive films on zirconium of varying thicknesses and on amorphous iron-zirconium alloys of varying compositions have been investigated. Photocurrents were measured as a function of wavelength and potential. A tailing of the photocurrent is observed in photocurrent spectra down to photon energies well below the band-gap energy. From photocurrent spectra, band-gap energies of 4.8 and 3.3 eV for the passive films on Zr and Fe-Zr alloys, respectively, have been determined. The spectra together with the potential dependence of the photocurrent imply that localized states play an important role in the photoexcitation process of these films. Cathodic photocurrents at potentials above the flat-band potential are observed on the thinnest passive films formed on zirconium. Reverse tunneling is discussed as a possible explanation for this, which emphasizes the importance of localized states in thin passive films.
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