Structure of electrochemically deposited sulfide layers on Cu(100)

In-situ EC-STM has been used to examine distinct phases of electrochemically deposited sulfide layers on Cu(100). The observed structures can be clearly assigned to specific ranges of the electrode potential and the sulfide concentration in solution. The scope of the present contribution is to report the various structural phases and their potential dependence. A full description of their concentration dependence will be given in a forthcoming paper. All incipient adsorption processes have been performed exclusively at negative electrode potentials close to the on-set of the hydrogen evolution reaction in order to avoid heavy surface reactions of the copper substrate with sulfide as seen at positive electrode potentials. In a very early stage of the sulfidation first A metastable, transient p(2 x 2)-S layer with a nominal coverage of Theta = 0.25 ML is formed which gets irreversibly replaced by an energetically more stable c(6 x 2)-S phase with Theta = 0.33 ML. This process at constant potential is kinetically hindered and depends on the sulfide supply from the electrolyte of given concentration. All further phase transitions induced by a positive going potential sweep are influenced by the sulfide supply from the electrolyte phase as well. A low sulfide supply from a low concentration electrolyte during a positive going potential sweep provokes a uniaxial compression of the preexisting c(6 x 2)-S phase ending up with, a striped c(4 x 2)-S phase. Conversely a high sulfide supply from a high concentration. solution during a positive going potential sweep causes a massive reorganization of both the atomic structure and the surface morphology on a mesoscopic scale. This latter process involves a massive transport of copper material out of the topmost copper layer and is, hence, identified as an adsorbate induced reconstruction of the copper substrate. The symmetry of the resultant sulfide structure on-top of the reconstructed copper substrate is similar to a square c(2 x 2) arrangement but on a copper surface whose lattice constants are expanded by 9% with respect to the copper bulk structure. Hence, the atomic surface structure is denoted as a pseudo-c(2 x 2)-S reconstruction.