Theoretical investigations on the potential-induced formation of Pt-oxide surfaces

Using the extended ab initio atomistic thermodynamics approach together with density functional theory calculations the interfacial structure and composition of Pt-electrodes in electrochemical environments at elevated electrode potentials was studied. Focusing on the electrode potential region, at which the oxide-formation occurs, the bulk systems and all low-index surfaces of α-PtO2, β-PtO2, and PtO bulk-oxides were calculated. On the basis of the bulk-oxide formation energies we first deduced the stability ranges at which the bulk-oxides are the thermodynamically stable phases. In agreement with experimental observations, we find that at experimental temperature and pressure conditions α-PtO2 and β-PtO2 bulk-oxides are stable above 1.2 V, while PtO requires Δϕ > 1.3 V. Afterwards the corresponding (p, T, ϕ)-phase diagrams of surface structures were obtained, showing a preference for α-PtO2(0 0 1), β-PtO2(1 1 0), and PtO(1 0 0), respectively, having bulk-like compositions even on their surfaces. However, in case of thin oxide layers a PtO composition might also be present.