Photoelectrochemical studies of passive films on zirconium and amorphous iron-zirconium alloys

Abstract

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.

Influences of structure and composition on the photoelectrochemical behaviour of anodic films on Zr and Zr–20at.%Ti

“…In this case we can attribute the photocurrent generated under illumination with high photon energy (λ < 260 nm) to optical transitions occurring from occupied electronic states of the valence band to the empty states of the conduction band of the oxide. According to this we estimate a band gap of 4.8 ± 0.05 eV for polycrystalline m-ZrO 2 , which is very close to those estimated by previous photoelectrochemical measurements [35][36][37][38][39] and within the range (actually very wide) of those theoretically predicted [14,18,26,31]. Moreover, this value is in agreement with that experimentally determined by EELS [27,28,31,33] or by VUV [26,44] for monoclinic zirconia.…”

“…Newmark and Stimming [35] measured the photocurrent response of thin anodic ZrO 2 films estimating an indirect band gap value of 4.8 eV, even if they also observed a photocurrent tailing down to 3 eV. The same behaviour was observed by Goossens et al [36], who attributed the sub-band gap photocurrent to optical transitions involving localized states inside the gap, related to the presence of oxygen vacancies.…”

Photoelectrochemical study of hydrogen in Zircalloy-2 oxide films

“…This result shows that the Zry-2 oxide films have two impurity levels in the original band gap as shown in Fig. 4, because the value of 4.9 eV agrees well with previously reported band gap energy for ZrO 2 films, 4.8 eV [1,5,6]. The photo-response for lower photon energy was increased with the hydrogen content in the oxide film shown in Fig.…”

Physicochemical Characterization of Passive Films and Corrosion Layers by Differential Admittance and Photocurrent Spectroscopy

“…Further evidence in favour of the proposed corre-lation for anodic films grown on Mo-Ta, Ti-Zr, Hf-W metallic alloys can be found in Refs. [144,[146][147][148]. We have to mention that apart the nature of the oxide other experimental parameters can affect the lattice disorder degree of passive films and then the ΔE am value in anodic films.…”