LIMIT TO EXTENT OF FORMATION OF THE QUASI-2-DIMENSIONAL OXIDE STATE ON AU ELECTRODES

Potentiostatic polarization of Au electrodes in 0.5 M aqueous H2SO4 an d KOH solutions at polarization potentials E-p between 2.00 and 2.43 V (RHE) for polarization times t(p) up to 10(4) s leads to the formatio n of thick oxide films which comprise up to four oxide states, designa ted OC1, OC2, OC3 and OC4, as resolved from linear sweep voltammetry o xide reduction profiles. The oxide growth proceeds by inverse logarith mic kinetics with two distinguishable kinetic regions leading to linea r relations between 1/q(OX), and log t(p). The first 1/q(OX) vs. log t (p) linear region corresponds to development of the quasi-2D state (OC 1) and the very initial growth of the OC2 state, and the oxide growth in this t(p) domain is slow. Further growth of OC2 and formation of th e OC3 and OC4 states is significantly faster. The oxide growth kinetic s change when 1/q(OX) reaches a value of ca. 0.10mC(-1) cm(2). In situ ellipsometry measurements indicate the presence of two distinct layer s within the oxide films, the inner layer designated alpha and the out er one designated beta. The alpha film corresponds to the OC1 state wh ereas the beta film corresponds to the OC2, OC3 and OC4 states. The OC 1 state reaches a limiting thickness of 3 equiv. monolayers of AuO or Au(OH)(2) in aqueous H2SO4 and 1 monolayer of AuO or Au(OH)(2) in aque ous KOH. The beta state which resides on top of the a one grows withou t reaching any limiting thickness and up to 100equiv, monolayers of Au 2O3 or Au(OH)(3), depending on the electrolyte composition, can be for med under the conditions described in the present paper.