Electrolytic decomposition and photodecomposition of compound semiconductors in contact with electrolytes

Electrons and holes affect the bond strength of surface atoms. Therefore, in most systems their surface concentration controls the rate of electrolytic decomposition reactions. The thermodynamics of such reactions are characterized by their redox potentials which are equivalent to the Fermi energies of electrons or holes. It is shown that the energy positions of the redox Fermi levels for decomposition with respect to the position of the band edges and the Fermi levels of competing redox reactions, give an immediate indication for the susceptibility of a semiconductor to electrolytic decomposition. This concept is especially useful for the discussion of photodecomposition where the electronic free energy can be described by individual quasi-Fermi levels for electrons and holes. Data are given for the semiconductors ZnO, TiO2, Cu2O, CdS, MoS2, GaP, and GaAs. A model for bond breaking by holes at a kink site of a compound semiconductor is discussed to demonstrate what role the surface bond character plays for the height of activation barriers and how kinetics modify the thermodynamic conclusions on stability.