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 p hotoelectron 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 numbe r of surface and bulk oxygen-containing species, including surface Ag2O-lik e species, surface and bulk OH groups (OHads and OHbulk respectively), surf ace and bulk atomic oxygen, has been observed on the emersed electrode. The present work provides the first direct evidence of the hydroxide incorp oration 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 e V, while surface OHads manifests itself as the peak at ca. 531.6 eV, The or igin of the positive binding energy shift is discussed. Surface and bulk OH groups demonstrate substantially different thermal stability. Surface spec ies desorb below 470 K, while dissolved OH groups exhibit high stability to wards prolonged annealing in vacuum at temperatures up to 750 K; they remai n 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 decom poses after heating in vacuum at a temperature of about 470 K. The He(I) an d He(II) UP spectra of the emersed electrode along with the XPS data provid e evidence that the coverage with surface oxide is less than 1 ML. A tentat ive scheme of Ag(111) underpotential oxidation is discussed. (C) 1998 Elsev ier Science B.V. All rights reserved.