Growth and complex characterization of nanoporous oxide layers on metallic tin during one-step anodic oxidation in oxalic acid at room temperature

Abstract

Nanoporous tin oxide layers were synthesized by anodic oxidation of low purity Sn foil (98.8%) in oxalic acid electrolytes at various operating conditions including anodizing potential (3–10V), concentration of the electrolyte (0.1, 0.3 and 0.5M), and duration of the process (300s, 600s and 1800s). A detailed quantitative inspection of the effect of anodizing conditions on the structural features of anodic oxide layers was performed. A special emphasis was put on the correlation of the sample morphology with the current vs. time curves, especially in terms of differences between the surface morphology and the inner oxide structure. When the potential of 3V was applied during anodization, micron-sized particles of SnC2O4 were obtained on the surface of metallic tin, independently of the electrolyte concentration. On the contrary, the anodic oxidation of Sn foil at potentials between 4 and 8V resulted in the formation of nanoporous tin oxides with two distinguishable layers: a less regular outer layer with smaller nanopores formed at the beginning of anodization, and an inner layer with well-defined and larger nanochannels. A dense, compact barrier layer was also observed at the bottom side of oxide. The morphology of an outer layer was found to be strongly dependent on anodizing conditions. For instance, a significant increase in the average pore diameter in the outer layer with increasing potential and concentration of the electrolyte, being a result of more effective field-assisted etching of anodic oxide and more vigorous oxygen evolution under the severe anodizing conditions, was observed. Contrary to this, no significant effect of anodizing conditions on the structure of the inner oxide layer was found. A strong linear relationship between the average steady-state current density and anodizing potential was observed for the potentials in the range of 5–8V, what suggests that the reaction is limited by the mass transfer in the electrolyte. Finally, when potentials of 9 and 10V were applied, an initial formation of the dense, passive outer layer, its further breakdown, and formation of the well-defined, nanoporous inner oxide layer was observed.