Electrochemical and XPS measurements on thin oxide films on zirconium |
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Affiliation: | 1. Department of Chemistry, Faculty of Science, Saitama University, Saitama 338-8570, Japan;2. Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India |
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Abstract: | The potentiodynamic growth of thin oxide films on zirconium electrodes was investigated by coulometric and simultaneous impedance measurements, as a function of the electrode potential (0 V ⩽ E ⩽ 9 V), the pH (0 ⩽ pH ⩽ 14) and the surface preparation (electropolishing, etching and mechanical polishing). The initial film thickness d0 is at least 4–6 nm; with increasing potential, the oxide grows irreversibly by 2.6 nm/V (pH 0.3) up to 3.2 nm/V (pH 14). In Cl−- and ClO−4-containing solutions the oxide growth is limited by localized corrosion. The oxide behaves like a typical insulator with a donor concentration ND < 1019 cm−3 and a dielectric constant D = 31. Below −0.5 V (vs. SHE) only, th film behaves like an n-type semiconductor with ND ≈ 3 × 1019 cm−3. From photoelectrochemical measurements a direct and an indirect transition with band gap energies of Eg = 5 eV and Eg = 2.8 eV could be derived. Anodic electron-transfer reactions (ETRs) are blocked at the homogeneous oxide surface, but cathodic ETRs are possible at larger overvoltages. Near the flatband potential Efb ≈ −1.3 ± 0.2 V (vs. SHE) hydrogen evolution takes place with a simultaneous increase of the capacity which may be attributed to hydrogen incorporation. With XPS measurements the stoichiometry of the oxide film was determined as ZrO2 at all the pH values examined, but a thin outer layer contained some hydroxide. Components of the forming electrolyte could not be detected (sulphate, borate and perchlorate < 1%), but etching in HF caused accumulation of F− at the inner boundary. |
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