Kinetic effects of alcohol addition on the anodic behavior of silicon in acid fluoride media

The anodic dissolution of silicon in acid solutions of ammonium fluoride is investigated in aqueous and water-alcohol media by rotating disk voltammetry and impedance spectroscopy. The voltammograms recorded in water-alcohol media show, in comparison to water, the following effects: an increase of the dissolution currents measured at pH > 2 and an opposite effect at pH < 2; a distortion of the curve, with a relative increase of the characteristic currents in the region of low applied potentials, indicating easier dissolution of the "wet" oxide forming under those conditions; a shift of the maximum of the current-pH curves from about pH 3 in water to about pH 4 in 50% ethanol v/v. Analysis of impedance data at pH values 4-5 in water and water-ethanol media show similar dependence on potential for the parameters C-hf (high-frequency capacitance) and RhfI (R-hf is a high-frequency resistance and I is the steady state current), suggesting the formation of oxide films with similar properties. The marked differences of anodic behavior in the two types of media are discussed and explained in terms of the effects of alcohol addition on the speciation equilibria of hydrofluoric acid and on the kinetic rate constants of dissolution of the surface oxide layer.