Structural aspects of the interaction of methyl thiol and dimethyldisulphide with Ni(111)

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Abstract
A combination of near-edge and surface extended X-ray absorption fine structure (NEXAFS and SEXAFS), normal incidence X-ray standing wavefield absorption (NIXSW) and (laboratory-based) X-ray photoelectron spectroscopy and diffraction (XPS and XPD) has been used to investigate the local phases and associated structures resulting from the interaction of methylthiol (CH3SH) and dimethyldisulphide ((CH3S)2-DMS) with Ni(111). The interaction was followed by adsorbing at low temperature and observing the effects of sequential heating cycles to up to 373 K. At the lowest temperature DMS appears to be intact on the surface, but at slightly higher temperatures, up to at least 223 K, a second molecular species is formed which is also obtained by the thiol adsorption, even at 153 K. This is identified as methylthiolate (CH3S-) and appears to be bonded (through the S atom) to the hollow sites on the surface with a S-Ni bondlength of 2.24 AA; essentially equal occupation of both (fcc and hcp) hollow sites is found. Both XAFS and XPD indicate that this species shows a substantial tilt angle of the S-C symmetry axis away from the surface normal; the situation is somewhat similar to that reported for the same species on Pt(111) on the basis of C K-edge NEXAFS and vibrational spectroscopy, and comparative S K-edge NEXAFS measurements for Pt(111) are also reported here. The tilted geometry found on both Ni and Pt surfaces contrasts with the perpendicular orientation found on Cu(111). Heating the Ni(111)surface to 373 K converts the thiolate species to a third state which we identify as atomic sulphur. Both SEXAFS and NEXAFS show that this final species has the same local geometry as that found in earlier studies of atomic S on Ni(111): in particular, while the S-Ni nearest-neighbour spacing is similar to that of the thiolate, the S in this phase appears to penetrate the top Ni layer.