SINGLE-CRYSTAL STUDY OF THE INITIAL-STAGES OF SILVER SULFIDATION - CHEMISORPTION AND REACTIVITY OF MOLECULAR SULFUR (S2) ON AG(111)

Molecular sulphur undergoes rapid dissociative chemisorption on Ag(111) with an essentially constant sticking probability of unity up to the completion of the first layer of S atoms. At this stage a (√39 R 16.1° × √39 R ̄ 16.1°) structure is formed in which the S atom arrangement and spacing is similar to that in the (100) plane of γ-Ag2S (the high temperature form of silver sulphide). Further dosing with S2 leads to continued rapid uptake of sulphur and the appearance of a (√7 × √7) R 10.9° structure, the Auger, Δφ and thermal desorption data all indicate that fast formation of Ag2S now occurs. Very well-ordered growth of γ-Ag2S(111) is now observed, and low-temperature S2 desorption spectra appear which show that the activation energy for S2 desorption is ~175 kJ mol-1 ; this value is in excellent agreement with that observed for the enthalpy of decomposition of bulk Ag2S (2 Ag2S(s) → 4 Ag(s) + S2(g), ΔH = +179 kJ mol-1). All the properties of the Ag(111)-S system imply that the material characterised by the √39 structure (i.e. the first adsorbed layer of S) is very different from bulk Ag2S. This is discussed and compared with the results of other studies on metal-sulphur systems. © 1979.