Characterization of electrodeposited undoped and doped thin ZnO passive films on zinc metal in alkaline HCO3−/CO32− buffer solution

Abstract

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.