Surface-oxide growth at platinum electrodes in aqueous H{sub}2SO{sub}4 Reexamination of its mechanism through combined cyclic-voltammetry, electrochemical quartz-crystal nanobalance, and Auger electron spectroscopy measurements

摘要

The mechanism of platinum surface electro-oxidation is examined by combined cyclic-voltammetry (CV), in situ electrochemical quartz-crystal nanobalance (EQCN) and ex situ Auger electron spectroscopy (AES) measurements. The CV, EQCN and AES data show that the charge density, interfacial mass variation and intensity of the O-to-Pt AES signal ratio increase in a continuous, almost linear manner as the potential is raised from 0.85 to 1.40V. In addition, the charge density, mass variation and O-to-Pt signal ratio profiles follow each other, thus indicating that the surface oxidation proceeds by a progressive coordination of O-containing species to the Pt substrate. The coupled CV and EQCN measurements lead to in situ determination of the molecular weight of the interfacial species; these were identified as chemisorbed O (O{sub}(chem)) at 0.85 ≤ E ≤ 1.10 V and as O{sup}(2-) in the form of a surface PtO at 1.20 ≤ E ≤ 1.40V. The AES results reveal that the first half-monolayer of O{sub}(chem) is formed through discharge of H{sub}2O molecules and such formed O{sub}(chem) resides on the Pt surface. Subsequent discharge of H{sub}2O molecules leads to formation of the second half-monolayer of O{sub}(chem) that is accompanied by the interfacial place exchange of O{sub}(chem) and surface Pt atoms; this process results in the development of a quasi-3D surface PtO lattice comprising Pt{sup}(2+) and O{sup}(2-). AES data demonstrate that the place-exchange process occurs in the 1.10-1.20 V potential range. The experimentally determined molecular weight of the species added to the surface is 15.8 g mol{sup}(-1), which points to O and to anhydrous PtO as the surface oxide formed.