CATHODIC PHOTOEMISSION PROCESSES AT THE AL-ELECTROLYTE INTERFACE IN THE INITIAL-STAGES OF PASSIVE FILM FORMATION

The photocurrent behavior of the aluminum/aluminum oxide/electrolyte j unction under cathodic polarization has been investigated at different thicknesses of the surf ace oxide film, ranging between few to hundre ds nm. Regardless of the investigated solution, for oxide thicknesses higher than about 6 nm, experiments point to a mechanism of photocurre nt generation due to internal electron emission from the metal into th e oxide conduction band. With increasing the oxide thickness the catho dic photocurrent measured at constant potential decreases. For very th in oxide layers (1-2 nm thick) the threshold energy for the cathodic p hotocurrent generation changes with the bias, suggesting a direct elec tron injection into the electrolytic solution. The variation of the ph otocurrent with the electrode potential and the energy of the incident photons is discussed in the light of existing theories, and the resul ts are compared with previous ones regarding the anodic behavior of th e junction in order to get a physical picture of the interface under i nvestigation. The shape of the cathodic threshold energy vs. formation potential plot is explained assuming increasing quantum-size effects at decreasing oxide thickness.