Growth and thermal properties of ultrathin cerium oxide layers on Rh(111)

Ultrathin layers of cerium oxide have been deposited on a Rh(111) surface and their growth morphology, structure, and thermal stability have been investigated by LEED, STM, XPS, and valence band resonant,photoemission. STM and LEED indicate that the ceria grows epitaxially in form of ordered CeO2 islands at elevated substrate temperature (250-300 degreesC), with (111) faces parallel and orientationally aligned to the main azimuthal directions of the substrate. The ultrathin ceria films contain significant amounts of reduced Ce3+ species, which appear to be located predominantly at the ceria Rh interface. For thicker films (>6 equivalent monolayers) stoichiometric CeO2 is detected in XPS. Vacuum annealing produces morphologically well-defined hexagonal islands, accompanied by partial reduction and the formation of oxygen vacancies at the ceria surface. The thermal stability and the degree of reduction is a function of the oxide layer thickness, with thinner layers being thermally less stable. At temperatures >800 degreesC, the ceria decomposes and Ce-Rh alloy phases are identified. (C) 2002 Elsevier Science B.V. All rights reserved.