Characterization of electrodeposited undoped and doped thin ZnO passive films on zinc metal in alkaline HCO3 /CO3 2 buffer solution

Abstract

Electrochemical characterization of anodically grown thin ZnO films on pure zincmetal 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 (Va) 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 Va and the value of either total resistance (Rf) of undoped passive film or its thickness (df). It is evident that anodization can afford better conditions for forming thicker compact passive films with more advanced barrier properties. On the other hand, Rf decreases with increasing Li-doping level in the oxide film, and increases in case of In-doping. Interestingly, Rf 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 (ND) and the passive film thickness (df), where the trend of their values are to decrease for Li+-doped and to increase for In3+-doped films.