Chemisorption on (001), (110) and (111) nickel surfaces: A correlated study using LEED spectra, Auger spectra and work function change measurements

Abstract

An experimental study of the chemisorption of a variety of closely related absorbates (H, O, C, CO, S, Se and Te) on clean single crystal nickel surfaces is presented. The nature of chemisorption is investigated through a correlated study of the modifications in low energy electron diffraction (LEED) patterns and intensity-energy spectra, Auger electron energy distributions, and work function change measurements which occur as a function of coverage and heat treatment. These experimental features are used to categorize the observed chemisorption phenomena into three distinct groups: (1) Molecular-like adsorption associated with relatively weak bonding, characteristic of carbon monoxide on (001), (110) and (111) nickel and oxygen and sulfur on (111) nickel. The adsorption is associated with attenuation of the LEED spectra intensities where only relatively minor changes occur in the structure and position of peaks in the integral order beams from that of the corresponding clean surfaces. The occurrence of relatively larger work function changes (∼ 1 eV) is characteristic of this group. (2) Atomic-like adsorption associated with strong bonding, but negligible substrate disruption, characteristic of oxygen, sulfur, selenium, tellurium and carbon on (001) Ni and for relatively low coverages (≲ 0.5 monolayers) of oxygen on (110) Ni. Here strong modifications of the specular beam LEED intensity-energy features occur with only slight modification of the other integral order beams from those of the clean surface. The surface dipole is smaller than in case (1) and produces a relatively small work function change (≲ 0.5 eV). (3) Disruptive adsorption associated with possible distortion and/or penetration of the lattice. This is characteristic of adsorption temperatures above room temperature and/or higher coverages (≳ 0.5 monolayers) of oxygen on both (001) and (110) surfaces, for hydrogen exposure to (001) and to (110) nickel, and to a lesser extent, possibly sulfur on (001) and (110) nickel. The LEED intensity-energy spectra show strong modifications in the integral order beam lineshapes from that of the clean surface. Associated work function changes again are small (∼ 0.50 eV). In summary, chemisorption of reactive gases on nickel is dependent on specific factors including the adsorbate, the surface crystallography, the adsorption temperature and the intensity of exposure. Specific variations among these factors correlate with large variations in the adsorption process from superficial bonding to the surface at one extreme to significant surface penetration and lattice disruption at the other.