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

• Published in: RSC advances 2018-11, Vol.8 (69), p.39321-39333
• Main Authors: El-Taib Heakal, F, Abd-Ellatif, W. R, Tantawy, N. S, Taha, A. A
• Format: Article
• Language: English
• Subjects: Bicarbonates; Buffer solutions; Carrier density; Chemistry; Correlation analysis; Current carriers; Doped films; Doping; Electrochemical analysis; Electron density; Film thickness; Lithium chloride; Oxide coatings; Scanning electron microscopy; Silver chloride; Thin films; Zinc oxide
• ISSN: 2046-2069
• DOI: 10.1039/c8ra06899b

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 InCl 3 . 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 In 3+ -doped films. The trend of charge carrier density ( N D ) and film thickness ( δ f ) dependence on the parameter is indicated on each arrow for undoped, Li-doped, and In-doped ZnO semiconductor materials.