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

Abstract

Anodic polarization of Pt electrodes in 0.5 M aqueous H2SO4 at various polarization potentials (Ep) from 0.90 to 1.50 V, for polarization times (tp) up to 104 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 OC1 peak, which corresponds to the reduction of PtO to Pt. The oxide growth behavior is influenced by the experimental conditions, such as Ep, tp, and T, and in general the higher Ep and/or longer tp and/or higher T, the thicker the oxide layer. An increase of Ep and/or tp shifts the OC1 peak towards less-positive potentials. On the other hand, an increase of T does not lead to any shift of the OC1 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 tp ⩽ 104 s, and (ii) the inverse-logarithmic growth for oxide whose thickness is more than 1 ML of PtO, i.e., at Ep > 1.0 V for tp > 10 s. The logarithmic growth law originates from the interfacial place exchange between Ochem 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, Pt2+, from the metal into the oxide at the inner metal/oxide interface. The dipole moment of the Ptδ+-Ochemδ- 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 × 109 V m−1 range.