Morphology of nanoporous anodic films formed on tin during anodic oxidation in less commonly used acidic and alkaline electrolytes

Abstract

Porous anodic tin oxide layers were obtained by simple one-step potentiostatic anodic oxidation (anodization) of Sn foil in aqueous ammonia and citric acid in a transpassive region. Different anodizing conditions including electrolyte concentration, applied potential, and duration of the electrolysis have been examined in order to find the effect of process parameters on the morphology of anodic films. Layers with completely clogged surface and porous interior have been obtained at all studied conditions. Moreover, the synthesized samples exhibit a typical stacked morphology with a lot of internal cracks and discontinuities within the anodic film being a result of local accumulation and trapping of gaseous oxygen inside the pores. Diameters of nanochannels were found to be almost independent on the applied potential, and their formation was also attributed to the O2 co-evolution at the positive electrode during anodization. Increasing electrolyte concentration and anodizing potential usually result in higher rates of the oxide formation. However, no significant effect of anodizing conditions on the nanochannel diameter, and morphology of the compact outer layer was found. Anodic films with the almost identical morphology have been successfully obtained in other electrolytes such as tartaric acid and phosphoric acid solutions. It was proven that the as-obtained layers were amorphous in nature and composed mainly of tin and oxygen with a surface O/Sn ratio close to stoichiometric SnO2. Incorporation of phosphorous species to the anodic film during anodization in H3PO4 was also confirmed. Band gaps of all materials were found to be significantly lower (2.94–3.05 eV) than that typically observed for pure SnO2 (3.6 eV).