Adsorption of ions on semiconductor surfaces. I. Silver and halide ions on silver halides

Interaction potentials of silver and halide ions adsorbed on the surfaces of silver halides crystals are calculated. The adsorption of such ions leads to a redistribution of charges at the interface between the semiconductor and an electrolyte (emulsion) containing such ions. This has important implications for such processes as photography and liquid-junction solar cells. We find that the adsorption of silver and halide ions occurs all over the crystal surface [(100) plane], with a pronounced tendency of Ag⁺ to be adsorbed onto X ^- sites, the X^- onto Ag⁺ sites, and both to be adsorbed onto the midpoint site. The adsorption of Ag⁺ ions is found to be weaker than that of the X^- ions, in accordance with experiments. The adsorption of ions when interstitial Ag⁺ ion and Ag⁺ vacancy (Frenkel defect) are present close to the surface is also considered. We find that adsorbed X^- ions enrich the space-charge layer of the AgX crystal with extra positive charge, shifting the band edges towards a more cathodic potential. The reverse trend is found when Ag ⁺ ions are adsorbed. In a forthcoming publication, we extend the present model to include model of charge transfer processes at these interfaces. From these calculations we obtain some guidelines for the construction of systems with high charge transfer probability. Surprisingly, these guidelines are satisfied by most photoelectrochemical cells as well as by other systems.