Electrodissolution and passivation of silicon in aqueous alkaline media: A voltammetric and impedance investigation

The behavior of Si electrodes in aqueous alkaline media has been investigated by voltammetric and impedance techniques. The flatband potential has been estimated on the basis of Mott-Schottky plots and photocurrent onset as EFB = -1.15 V (SCE) for p-Si and EFB = -0.45 V(SCE) for p-Si. A dissolution/passivation mechanism is proposed, assuming: (1) irreversible electrochemical oxidation of H-terminated Si to a monohydroxylated surface species, which may undergo either (2) irreversible electrochemical oxidation to a dihydroxylated species responsible of 2D surface blocking or (3) chemical dissolution as Si(II) regenerating an H-terminated surface; furthermore, (4) the blocking species may be dissolved chemically as Si(IV); (5) a chemical reaction with water generating monohydroxilated Si (and H2) is also considered. A kinetic model is developed for the case of n-Si, assuming Helmholtz-layer control. The model reproduces the essential experimental features of both J - E curves and impedance diagrams in the active dissolution and passivation regions but is not expected to be realistic in the passive region where 3D surface blocking should be considered.