Cu 2O(1 1 0) formation on Co 3O 4(1 1 0) induced by copper impurity segregation

Abstract

The surface crystal structure of the Co 3O 4(1 1 0) 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 Co 3O 4(1 1 0) 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 Å ± 0.2 Å and 5.50 Å ± 0.2 Å in the 〈0 0 1〉 and 〈 1 ¯ 10 〉 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 Co 3O 4(1 1 0) structure or stoichiometry detectably in the submonolayer levels at which they were observed. However, the copper impurity resulted in the formation of a Cu 2O(1 1 0) 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 Co 3O 4(1 1 0) stoichiometric surface and with periodic spacings of 3.86 Å ± 0.2 Å in the 〈0 0 1〉 and 4.10 Å ± 0.2 Å in the 〈 1 ¯ 10 〉 directions in agreement with Cu 2O(1 1 0) bulk termination. The Co 3O 4(1 1 0) substrate could not be fully re-oxidized until all detectable copper had been removed from the surface.