Cu2O(110) formation on Co3O4(110) induced by copper impurity segregation

The surface crystal structure of the Co3O4(110) spinel was characterized by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Well-defined LEED diffraction patterns showed an unreconstructed Co3O4(110) Surface in Type A termination, and XPS and Auger indicated the surface to be stoichiometric with octahedral and tetrahedral cation sites occupied by 3+ and 2+ cations, respectively. The experimental lattice parameters of 8.22 angstrom +/- 0.2 angstrom and 5.50 angstrom +/- 0.2 angstrom in the < 001 > and < 110 > directions, respectively, are in agreement with a bulk-terminated unit cell. The impurities: K, Ca, Na, and Cu segregated to the surface after prolonged heating to 630 K. K, Ca and Na could easily be removed by routine cleaning procedures and did not affect the Co3O4(110) structure or stoichiometry detectably in the submonolayer levels at which they were observed. However, the copper impurity resulted in the formation of a Cu2O(110) overlayer, with the accompanying reduction of the spinel surface to a rocksalt metal monoxide-like surface. The copper oxide formed a distorted hexagonal overlayer incommensurate with that of the Co3O4(110) stoichiometric surface and with periodic spacings of 3.86 angstrom +/- 0.2 angstrom in the < 001 > and 4.10 angstrom +/- 0.2 angstrom in the < 110 > directions in agreement with Cu2O(110) bulk termination. The Co3O4(110) substrate could not be fully re-oxidized until all detectable copper had been removed from the surface. (c) 2005 Elsevier B.V. All rights reserved.