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The effects of impurities and defects on transition metal oxide surfaces, cobalt oxide(110) and nickel oxide(100)
Understanding how impurities and defects modify surface chemistry and structure of transition metal oxides is important from a fundamental science perspective and for oxides continued use in industry. Transition metal oxide surfaces Co3O4(110) and NiO(100) were studied with complementary surface sensitive techniques, including low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD), to determine the effects of impurities and defects on their chemical, physical, and electronic properties. Co3O4(110) spinel AES and XPS spectra are consistent with previous stoichiometric powder and thin film Co3O4 spinel surfaces and with other cobalt containing spinels. The first published LEED patterns are reported here in, and reveal Type A bulk termination in which octahedral and tetrahedral cation sites are occupied by 3+ and 2+ cobalt cations, respectively. Extended annealing allows the elemental impurities Ca2+, K+, Na+, and Cu 1+ to segregate to the surface, and the copper impurity in particular reduces the spinel surface to a rocksalt CoO-like surface. The copper impurity induces a Cu2O(110) phase-separation overlayer on the cobalt oxide spinel surface. The Co3O4(110) substrate cannot be fully re-oxidized to the spinel stoichiometry until all detectable copper is removed from the surface. Low-coordinate transition metal oxide defects were modeled using two different vicinally-cut NiO(100) that resulted in periodic monoatomic stepped substrates with six-atom and seven-atom terrace widths. The AES and XPS spectra are consistent with previous stoichiometric single crystals, powders, and thin film surfaces. LEED shows a characteristic (1 x 1) diffraction pattern for the non-stepped surface and a characteristic spot-splitting diffraction feature for the stepped surfaces correlating to the appropriate step terrace width. TPD probed the surface reactivity of the NiO-bromobenzene (C6H 5Br) adsorbate. A molecularly adsorbed monolayer species desorbs at 169 K for the nonstepped surface and 180 K for the stepped surfaces. The multilayer adsorbate state desorbs from 174 K to 188 K depending upon coverage. A final surface species, observed only on the stepped surfaces, desorbs at 145 K by two pathways. At low coverages dehalogenation occurs resulting in adsorbed bromine, and when these adsorbate sites are saturated, molecular desorption of bromobenzene is then observed. Bromine that remains on the non-stepped and stepped NiO(100) surfaces from dehalogenation appears as nickel bromide determined by the Br 3p XPS peak with the concentration of bromine higher on the stepped surfaces than on the non-stepped surface. On the non-stepped surface bromobenzene is adsorbed “flat” with the benzene ring parallel to the surface to ensure maximum contact between the molecule and the NiO substrate. The step terraces, however, are too short to easily accommodate this geometry easily and the molecule tilts to fit the step geometry, allowing more bromobenzene to adsorb. Some interaction with the step edge is also most likely important in determining the new adsorbate geometry.
Petitto, Sarah Chapman, "The effects of impurities and defects on transition metal oxide surfaces, cobalt oxide(110) and nickel oxide(100)" (2005). ETD collection for University of Nebraska - Lincoln. AAI3186875.