CO adsorption on stepped Ni(111) surfaces

Abstract

The CO adsorption on Ni(331) and Ni(221) [referred to as Ni(s)(111)] was studied with low-energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and work function change measurements. LEED from the clean Ni(s)(111) crystals give evidence for nonreconstructed surfaces with atomic arrangements as expected from the ideal (331) and (221) orientations; these surfaces can be interpreted as stepped Ni(111) surfaces with (111) terraces and (111) steps. CO desorption shows on both surfaces several desorption states, which are sequentially filled with increasing CO exposure. The desorption state with the highest peak temperature [Tm=455 K for Θ(CO)≊0] is identified on both surfaces and related to CO desorption from (111) terraces. For higher coverages, due to the presence of steps, new desorption states fill in with lower desorption temperature in the range of 200–350 K. These desorption states are not present on flat Ni(111). With LEED in a definite regime of CO coverages half and one fourth-order streaks in [1̄10] directions of Ni(s)(111) are observed and related to a one-dimensional ordering of CO in directions parallel to the step directions. Ordered LEED overlayers with c(4×2) [Ni(331) and Ni(221)] and (2×1) [Ni(221)] are established on both surfaces near the CO saturation coverage at 120 K. These overlayers demonstrate that the barrier for lateral interaction introduced by the steps is overcome at higher coverages.