Electrochemical formation of porous silica: toward an understanding of the mechanisms

Porous silica may be formed by anodization of silicon in a neutral fluoride electrolyte. Applying a high potential leads to initiation of porous layer growth, which may be pursued at lower potentials. We have tried to improve our understanding of the growth process. Initiation of the growth appears to be caused by oxygen evolution, which leads to local acidification of the medium. In the steady state, ion depletion in the electrolyte may lead to the presence of a strong space charge. Experiments indicate that such a charged layer may represent at most a small fraction of the porous layer thickness. Theoretical estimates suggest that this effect may be neglected. A model, making use of the diffusion equations of the various species (H + , F - , HF, SiF2- 6), accounts for the current/potential curves and oxide layer thickness in a semiquantitative manner.