Facts and Challenges in the Electrochemistry and Wet Surface Chemistry of Silicon

Silica being insoluble in non-fluoride medium, anodic oxide films are readily formed on silicon. Dissolution of any oxide film in HF leaves a surface covered with hydrogen. Anodization in HF leads to the formation of porous silicon, the hydrogen coating staying present during the process. At higher potentials, electropolishing is observed, with the presence of a thin oxide layer on the surface. At still higher potentials, mesoporous oxide films are obtained. In non-aqueous media, trace amounts of water unavoidably lead to irreversible formation of a silica layer. In methanol a more stable behavior is obtained, due to chemical surface modification by grafting of methoxy groups. Other modifications of the silicon surface include the formation of covalent Si-C bonds, which holds promises for applications, especially sensors. In the near future, challenges in silicon electrochemistry include a better understanding of several fundamental issues, and also improved preservation of the surface against oxidation.