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Abstract:
X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and ion scattering spectroscopy (ISS) have been used to study the Ag(111) single-crystal surface after exposure to O2 at high temperature and at atmospheric pressure. The activated formation of a strongly bound surface layer has been observed, as identified by an asymmetry of the Ag 3d5/2 core-level peak at 367.3 eV and an O 1s peak at 529.0 eV (Oγ). In addition, oxygen was found to be dissolved in the bulk (Oβ), exhibiting an O 1s binding energy between 531 and 530 eV depending on its abundance. X-ray-excited oxygen KVV Auger electron spectroscopy revealed the presence of Oγ by additional peaks at 514.8 and 494.7 eV. UPS displayed oxygen-derived bands located above the emission from the Ag 4d band at 3.2 and 2.5 eV. Oxygen-related peaks below the Ag 4d band were identified as resulting from OH groups formed by reaction of surface oxygen (Oα) with residual hydrogen. The incorporated oxygen caused a pronounced charge separation as reflected by a 1 eV increase in the work function. ISS measurements revealed that Oγ is incorporated in the topmost surface layer, shielding underlying Ag atoms from the He+ beam. All spectroscopic data point to the presence of one monolayer of silver-embedded oxygen, which is in dynamic equilibrium with surface atomic oxygen segregated from the bulk at high temperature. The oxygen embedded in the topmost silver layer is strongly bound to the metal, with its interaction being different from adsorbed atomic oxygen and bulk Ag2O. It is stable up to 900 K, in contrast to the binary silver oxides, and relevant for high-temperature oxidation reactions catalyzed by Ag. A qualitative analysis is presented of the chemical bonding of the different surface species in comparison to the situation of a complex silver oxide reference.
