Study of passive films formed on Sn in the 7–14 pH range

The electronic, structural and morphological characteristics of passive layers formed anodically on tin in the 7–14 pH range were studied. Photocurrent spectroscopy measurements showed that Sn passive films behave as highly disordered n-type semiconductors. The measured band gap values, Eg, diminish as the formation pH is increased. Fourier transform infrared reflection–absorption spectroscopy results indicate that the ratio between SnOH and SnOSn bonds in the passive films increases with increasing formation pH. At pH >12.5 the films contained appreciable amounts of lattice water. A correlation between Eg and the hydroxylation degree of the films was proposed.