Influence of steps on the H 2O adsorption on Ni(s)(111)

Abstract

To expand previous investigations about the adsorption of H 2O on a stepped Ni(s)(111) surface with nominal (221) orientation, the adsorption of H 2O on a stepped Ni(s)(111) with nominal (11 11 9) orientation was studied in the present paper. The surfaces exhibit the same (111)-terrace and (11 1 )-step orientation but they differ in the terrace width, which is about three times greater for Ni(11 11 9) than for Ni(221). The adsorption was studied with thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) as well as work function change measurements ( Δφ). The LEED data from the clean Ni(s)(111) surface are consistent with the atomic arrangement expected for the “ideal” non-reconstructed (11 11 9) surface. In TDS experiments five desorption states denoted as A ( T M = 155–160 K), B ( T M = 174–177 K), C ( T M = 225 K), D and E ( T M = 260 K) were found. A and B exist at similar temperatures on flat Ni(111) as well as Ni(221) and are assigned to adsorption of H 2O clusters at terrace sites (B) or adsorption of ice multilayers (A). The step-induced states C, D and E, which were found at higher temperatures, are related to the adsorption of H 2O monomers at steps (C) and the recombination of dissociation products (D and E). As expected, the amount of H 2O molecules adsorbed at step sites as well as the contribution by step molecules to the work function change is about three times smaller than for Ni(221), corresponding to the lower step density of Ni(11 11 9). At higher coverages terrace sites are occupied by H 2O molecules developing a modified bilayer structure. Between 120 and 150 K a (√3 × √3)R30° structure, which has also been found on other flat surfaces with hexagonal symmetry, could be observed. Upon heating to 160 K the LEED pattern changed into a (2 × 2) structure. This behaviour is believed to be due to a temperature- and coverage-induced reordering of state B.