engleski

In situ investigation of duplex semiconducting films on tin

The passive film on tin formed under dynamic conditions represents a duplex layer of the same oxidation state, with a stepwise concentration profile of two semiconducting or almost insulating oxide films in series. Their electrochemical properties are limited by their electronic properties. The electrode capacity (C) and photopotential (Eph) were measured in situ during oxide formation and reduction on tin in slightly alkaline solution. Analysis showed that the changing electronic properties of the oxide formed have a great impact on Eph and C. The formation of the inner SnO2 layer takes place homogeneously all over the surface with constant thickness. Schottky-Mott type of behaviour was found, indicating a surface donor concentration of 10 exp 15 cm-1 and a flat band potential vs. SCE, Efb, of -0.95 V. For the outer layer the changes of electronic properties with increasing film thickness, as well as ageing processes, limit the validity of all electronic models with constant parameters. The oxide/hydroxide ratio increases with anodic polarization. The reaction is favoured by a free-energy decrease. At higher potentials, especially under illumination, the electrode is covered with a very stable SnO2 species which is difficult to reduce.

Interfaces ; passivity ; photoelectrochemistry ; semiconductors ; tin

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