Limit to extent of formation of the quasi-two-dimensional oxide state on Au electrodes

Potentiostatic polarization of Au electrodes in 0.5 M aqueous H2SO4 and 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 formation of thick oxide films which comprise up to four oxide states, designated OC1, OC2, OC3 and OC4, as resolved from linear sweep voltammetry oxide reduction profiles. The oxide growth proceeds by inverse logarithmic kinetics with two distinguishable kinetic regions leading to linear 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 (OC1) 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 the OC3 and OC4 states is significantly faster. The oxide growth kinetics 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 layers within the oxide films, the inner layer designated alpha and the outer one designated beta. The alpha film corresponds to the OC1 state whereas the beta film corresponds to the OC2, OC3 and OC4 states. The OC1 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 aqueous KOH. The beta state which resides on top of the a one grows without reaching any limiting thickness and up to 100equiv, monolayers of Au2O3 or Au(OH)(3), depending on the electrolyte composition, can be formed under the conditions described in the present paper.