XPS investigation of surface oxidation layers on a platinum electrode in alkaline solution

A thick oxidation layer on a platinum electrode has been grown in 1 N NaOH at 3 V vs Ag/AgCl reference electrode. After transferring the Pt electrode into an ultrahigh vacuum chamber the surface layer was analysed by X-ray photoelectron spectroscopy. Pt4f5/2 amd O1s electron binding energies of 74.3, 77.6 and 530.9 eV respectively, as well as the broad peak shape of the O1s signal and the oxygen-to-platinum intensity ratio of 3.08 point towards a platinum—oxyhydroxide PtO(OH)2. This formula is in good agreement with cyclic voltammetry curves, measured for the same electrode, that revealed two cathodic reduction peaks for oxygen surface coverages equivalent to more than two hydrogen monolayers. These two peaks were assigned to PtOH and PtO. XPS analysis at elevated temperatures showed that the thick (5 nm) oxidation layer decomposes at 400 K to a mixture of several oxides and hydroxides of Pt4+ and Pt2+ and Pt metal with a ratio of O-to-Pt of 1. This mixture further gradually decomposes to only a monolayer of oxygen at 770–870 K. Sodium cations were found to be present in trace amounts in the adlayer and to strongly shift the O1s binding energy to lower values.