Comprehensive study of the growth of thin oxide layers on Pt electrodes under well-defined temperature, potential, and time conditions

Anodic polarization of Pt electrodes in 0.5 M aqueous H2SO4 at various polarization potentials (E-p) from 0.90 to 1.50 V, for polarization times (t(p)) up to 10(4) s, and at 278 <= T <= 323 K leads to formation of sub-monolayer and monolayer oxide films. These oxide layers reveal only one feature in the oxide-reduction profiles, the OCl peak, which corresponds to the reduction of PtO to Pt. The oxide growth behavior is influenced by the experimental conditions, such as E-p, t(p), and T, and in general the higher E-p and/or longer t(p) and/or higher T, the thicker the oxide layer. An increase of E-p and/or t(p) shifts the OCl peak towards less-positive potentials. On the other hand, an increase of T does not lead to any shift of the OCl peak. Application of oxide-growth theories and theoretical data treatment indicate that the growth of PtO follows two distinct kinetic laws, each arising from a different growth mechanism: (i) the logarithmic growth for oxide whose thickness is up to 1 ML of PtO, i.e., at 0.9 <= Ep <= 1.0 V for t(p) <= 10(4) s, and (ii) the inverse-logarithmic growth for oxide whose thickness is more than 1 ML of PtO, i.e., at E-p > 1.0 V for t(p) > 10 s. The logarithmic growth law originates from the interfacial place exchange between O-chem and the top-most Pt atoms that is the rate-determining step, whereas the inverse-logarithmic growth law arises from the escape of the Pt cation, pt(2+), from the metal into the oxide at the inner metal/oxide interface. The dipole moment of the Pt delta+ - O-chem(delta-) surface species that drives the place exchange is consistently 1.30 +/- 0.10 D. The electric field, E, which drives the interfacial Pt2+ escape is found to be consistently in the 0.33-0.46 x 10(9) V m(-1) range. (c) 2006 Elsevier B.V. All rights reserved.