CHEMICAL DISSOLUTION OF ZINC-OXIDE CRYSTALS IN AQUEOUS-ELECTROLYTES - AN ANALYSIS OF THE KINETICS

The chemical dissolution of zinc oxide (ZnO) single crystals with three different surface orientations-the two polar faces terminated by zinc or oxygen, respectively, and the non-polar side face of the prismatic crystal-has been investigated for dependence on the electrolyte composition. Parameters of the electrolyte being varied were the pH, the anions of the acids, including a weak acid, and the ammonia concentration at constant basic pH. The crystals were strongly n-type and the rate of dissolution could be instantaneously measured at anodic bias as an electric current. The interpretation of the results assumes that the rate is controlled by the hydrolytic breaking of Zn-O bonds at kink sites which is catalysed by acids and by ligands which can form complexes with Zn2+ ions. The structure of the kink site at these three faces is not very different, as a crystal model suggests. In accordance with such models, no significant difference in the activation energy of the dissolution rate was found for the three orientations. It is concluded that the forces of the electrical double layer at the interface play a minor role in the dissolution kinetics of such an oxide.