Electron energy levels in semiconductor electrochemistry

The significance of the flat-band potential and the energetic position of the band edges at the semiconductor/electrolyte interface in semiconductor electrochemistry and photoelectrochemistry is pointed out. Different methods for determining these parameters experimentally are discussed, such as methods based on the measurement of the photovoltage or photocurrent, as well as the method for determining the flat-band potential from interfacial capacitance measurements. The capacitance-voltage relationship of the ideal semiconductor/electrolyte Schottky barrier is described. Subsequently, possible complications of the capacitance behavior are discussed, and conditions indicated under which the determination of the flat-band potential from non-ideal capacitance results is still possible. A critical survey is then given of flat-band data for some selected semiconductor electrodes (ZnO, CdS, GaP, GaAs, TiO₂, SrTiO₃), comprising a discussion of problems encountere, factors on which the flat-band potential depends and discrepancies between different results. Attempts to predict the flat-band potential and the position of the band edges from atomic electronegativity data are reviewed. The relationship between flat-band potential or band-edge position and electrochemical behaviour is considered, i.e., as far as the magnitude of the photovoltage as well as the electrochemical and photoelectrochemical reactivity are concerned.