Surface Defect Characterization in Oxygen-Dosed Nickel Surfaces and in NiO Thin Films by CO Adsorption−Desorption Experiments

Abstract

Oxygen-covered Ni(110) as well as thin NiO films grown by oxidation of that surface were characterized under ultrahigh vacuum by using low-energy electron diffraction (LEED), X-ray photoelectron (XPS), Auger (AES), and ion scattering (ISS) spectroscopies together with CO temperature-programmed desorption (TPD) titrations. Results from NiO(100) films with surface defects induced via Ar + ion irradiation at room temperature were compared with those from partially oxidized ordered Ni(110) surfaces. CO TPD proved to be a useful local probe for the investigation of defective NiO surfaces, since its adsorption energy varies by over 20 kcal/mol in going from a metallic Ni(110) clean surface to NiO. The CO-probing experiments also revealed that Ar + bombardment of thin NiO films leads to the formation of Ni-O phases similar to those found during the early oxidation stages of the Ni metal surface.

Thermal chemistry of diiodomethane on Ni(110) surfaces II. Effect of coadsorbed oxygen

“…It has been shown here that oxygen-treated nickel surfaces, potential models for defective nickel oxides [9,39], can promote several types of hydrocarbon steps, including hydrogenation-dehydrogenation, direct coupling, chain growth via methylene insertion, and oxygen insertion and formation of oxygenates. All these may be desirable reactions in specific catalytic processes.…”

“…The experiments reported here were conducted in an ultrahigh vacuum (UHV) system with a base pressure of 1 · 10 À10 Torr and a UTI mass quadrupole spectrometer for temperature-programmed desorption (TPD) measurements, a dual anode Xray source and a concentric hemispherical analyzer (VG 100AX) for X-ray photoelectron spectroscopy (XPS), and an ion sputtering gun for sample cleaning [39,40]. The experimental details, including the TPD setup and data deconvolution, the mounting and treatment of the single crystal, and the dosing procedures, are all described in the accompanying first part of this report [30].…”

“…Dioxygen, on the other hand, was always dosed at 400 K in order to ensure dissociation [39,41]. Fortunately, the interaction of O 2 with the Ni(1 1 0) surface has already been studied extensively [42].…”

“…It has been established that the oxidation of the Ni(1 1 0) sample above room temperature begins with the development of three sequential ordered submonolayers, starting with a low-coverage (3 · 1)O structure with a coverage of 1/3 of a monolayer (ML, as number of adsorbates per exposed surface nickel atom) after a dose of approximately 0.3 Langmuirs (L). Since oxygen adsorption displays a nearly constant sticking coefficient at those low exposures, the coverage of atomic oxygen on the surface can be easily estimated all the way from the clean surface to the formation of the 0.33 ML (3 · 1)O structure [39,43]. However, that linearity breaks down above 0.3 L, and the second (2 · 1)O ordered pattern (corresponding to a coverage of 0.5 ML) can only be effectively produced at oxygen exposures around 1.0-3.0 L [41].…”

Varied Thermal Chemistry of Hydrocarbons on Nickel Single-Crystal Surfaces

“…The experiments reported here were carried out in one of two ultrahigh vacuum (UHV) instruments described in detail previously [7,[53][54][55][56]. They both consist of stainless steel vessels pumped with turbomolecular pumps to a base pressure on the order of 1 9 10 -10 Torr.…”