XPS observation of OH groups incorporated in an Ag(111) electrode

An Ag(lll) single crystal electrode emersed from NaF + NaOH electrolyte (pH 11) under anodic polarisation has been studied ex situ by means of X-ray photoelectron spectroscopy (XPS). "Underpotential" oxidation has been found at + 0.2 and 0.0 V vs. Hg/HgO, that is by 0.2-0.4 V more negative than the reversible potential of the Ag2O phase formation. The generation of a number of surface and bulk oxygen-containing species, including surface Ag2O-like species, surface and bulk OH groups (OHads and OHbulk respectively), surface and bulk atomic oxygen, has been observed on the emersed electrode. The present work provides the first direct evidence of the hydroxide incorporation in the bulk of an Ag(111) electrode in the course of underpotential oxidation. OHbulk is characterised by a O 1s peak at approximately 532.8 eV, while surface OHads manifests itself as the peak at ca. 531.6 eV, The origin of the positive binding energy shift is discussed. Surface and bulk OH groups demonstrate substantially different thermal stability. Surface species desorb below 470 K, while dissolved OH groups exhibit high stability towards prolonged annealing in vacuum at temperatures up to 750 K; they remain in the near-surface region even after sputtering by Ar+ and He+ ions. The oxide-like species is characterised by the O 1s peak at 529.5 eV and decomposes after heating in vacuum at a temperature of about 470 K. The He(I) and He(II) UP spectra of the emersed electrode along with the XPS data provide evidence that the coverage with surface oxide is less than 1 ML. A tentative scheme of Ag(111) underpotential oxidation is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.