RESUMO
The electrochemical and semiconductive properties of spontaneously formed passive films on pure Zn were investigated in alkaline carbonate/bicarbonate buffer solutions as functions of pH and temperature. The study was performed in 0.1 M (CO3 2- + HCO3 -) mixtures over the pH range 9.2 to 9.8 using open circuit potential, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and Mott-Schottky analysis techniques. Generally, zinc passivation is enhanced with either increasing pH or decreasing the ambient temperature. The steady state potential (E ss) value reveals that in pH 9.8 buffer the propensity of Zn for passivation is superior when compared with those in the other tested buffer solutions. The total surface film resistance (R t) derived from the impedance data proves this result, which is likely attributed to changes in composition and/or microstructure of the film. In pH 9.8 buffer solution the passivation tendency always decreases with temperature increase. However, in pH 9.2 the system behaves similarly up to 25 °C; afterwards zinc passivation trend was found to re-increase once more. The apparent activation energy for the corrosion process was evaluated and discussed. Analysis of Mott-Schottky plots was found to be suitable for characterizing the semiconductor properties of the naturally deposited barrier layers which are all consistent with the well-known n-type character of the oxide film on zinc. The absence of any evidences for the p-type semiconductive behavior indicates a preponderance of oxygen vacancies and zinc interstitials over metal vacancies. Moreover, Mott-Schottky results demonstrate that the donor concentration increases with either increasing pH or deceasing temperature commensurate with the increasing trends in the passive film thickness.
RESUMO
Electrochemical characterization of anodically grown thin ZnO films on pure zinc metal was studied in pH 9.2 bicarbonate/carbonate buffer solution. The different undoped passive films were formed potentiostatically in pH 9.2 borate buffer solution at processing anodic voltage (V a) of -1.04, -1.02, -1.0 and -0.99 V (vs. Ag/AgCl). While, various doped ZnO films were fabricated by anodizing the metal at a fixed potential of -1.00 V in the same borate buffer solution containing different amounts of LiCl or InCl3. The electrochemical and semiconducting properties of all formed films were investigated using chronoamperometric measurements, EIS and Mott-Schottky analysis supported by scanning electron microscopy. The impedance results showed a direct correlation between V a and the value of either total resistance (R f) of undoped passive film or its thickness (δ f). It is evident that anodization can afford better conditions for forming thicker compact passive films with more advanced barrier properties. On the other hand, R f decreases with increasing Li-doping level in the oxide film, and increases in case of In-doping. Interestingly, R f values of the doped films are always lower when compared to its value for the undoped film grown at -1.00 V, likely due to possible change in the film microstructure upon doping. For both undoped and doped ZnO films, Mott-Schottky plots reveals unintentional n-type conductivity with high electron density. Moreover, with increasing dopant level in ZnO host materials, Mott-Schottky analysis revealed a parallel correlation between charge carrier donor concentration (N D) and the passive film thickness (δ f), where the trend of their values are to decrease for Li+-doped and to increase for In3+-doped films.
