Electrochemical Study of Gold Electrodes with Anodic Oxide Films-ii. Inhibition of Electrochemical Redox Reactions by Monolayers of Surface Oxides

The anodically first formed oxide on gold (oxide I) reaches two complete monolayers, ie 1.5 rnCcm_’ oxide charge, before a ssond type of oxide (oxide II) starts to form. Electrochemical redox reactions with redox couples such as the tris(o-phenantroline) ferrous/ferric ion couple and others which are well known to behave reversibly on bare gold are inhibited completely by gold covered with this critical coverage of two monolayers of oxide I. Up to nearly completion of this two monolayer coverap, the redox reactions behave reversibly. In the transition to the completion of two monolayers the reaction appears to be increasingly irreversible. This behaviour is explained on the basis ofditTusion layer thickness, heterogeneous rate constants and the size and distribution of the active bare gold sites as well as of the inhibiting oxide covered sites on the surface. The inhibition by two complete monolayers of gold oxide seems to be of general character for redox couples independent of pH and electrolyte.