Atomic structure of adsorbed sulfide on Cu(1 1 1) in acidic solution: In situ STM studies

Combined cyclovoltammetry (CV) and electrochemical scanning tunneling microscopy (ECSTM) studies on the electrochemical behavior of a Cu(1 1 1) single crystal electrode in a mixed solution of equal volumes of 0.5 mM H2SO4 and 1 mM Na2S are presented. After an overview over all structural surface phases observed at different electrode potentials E, i.e. a chain-structure (E = +200 to −150 mV versus RHE), a Moiré-structure (E = −50 to −300 mV versus RHE) as well as a (√7 × √7)R19.1°-S-(E = −100 to −450 mV versus RHE) and a (2√7 × 2√7)R19.1°-S-structure (E = −300 to −650 mV versus RHE), this paper focuses on a detailed description of the latter two. All phases are regular adsorbate structures on the surface and reversibly transform into each other. Except step alignment no reconstruction of the copper surface is observed. The unit cell of the √7-structure is rotated by ±19.1° with respect to that of the substrate lattice and contains, in contrast to a publication by Wang et al. (D. Wang, Q.M. Xu, L.J. Wan, C. Wang, C.L. Bai, Surf. Sci. 499 (2002) L159.), only one extra S-atom and therefore corresponds to a coverage of 0.286 (2/7) ML. Detailed atomistic models are proposed for the structure of the rotational domain boundaries in perfect agreement with the experimental observation. With decreasing electrode potential defects develop within the √7-structure due to partial sulfide desorption until a perfect 2√7-structure is formed. At given potential the creation and annihilation of defects occurs by an exchange process with the solution and not by intralayer hopping of adsorbed sulfur atoms.