SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2006; 38: 510–513 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.2269 First stages of growth of cerium oxide deposited on alumina and reduced titania surfaces C. Mansilla, 1 F. Yubero, 1* M. Zier, 2 R. Reiche, 2 S. Oswald, 2 J. P. Holgado, 1 J. P. Espin ´ os 1 and A. R. Gonz ´ alez-Elipe 1 1 ICMSE (CSIC-USE) Am ´ erico Vespucio s/n. E-41092 Sevilla, Spain 2 Leibniz-Institut f ¨ ur Festk ¨ orper-und Werkstofforschung Dresden, Postfach 270116, D-01171 Dresden, Germany Received 12 July 2005; Revised 22 November 2005; Accepted 29 November 2005 We have studied the first nucleation steps and the chemical interactions of small amounts of cerium oxide deposited on two surfaces with different chemical nature, alumina and a partially reduced titania. Cerium oxide was deposited by electron bombardment of CeO 2 pellets in ultra-high vacuum conditions. All experiments were done in situ in order to avoid contamination related to exposure to air. The relative amounts of Ce 3+ and Ce 4+ species present at the successive deposition steps have been quantified. It is found that for initial steps of deposition, high amounts of Ce 3+ are stabilised at the CeO x /TiO x interface, while an approximately even mixture of Ce 3+ and Ce 4+ species appear at the CeO x /Al 2 O 3 interface. Thicker deposits are characterised by the presence of only Ce 4+ species independently of the substrate. It is found that cerium oxide grows in the form of islands on both substrates. These islands are taller in the case of deposition on reduced titania than on alumina. This result is confirmed by both angle resolved X-ray photoelectron spectroscopy (XPS) and inelastic XPS peak shape analysis. The correlation between the growing microstructure and the chemical interaction at the interfaces is discussed. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: X-ray photoelectron spectroscopy (XPS); growth mechanism; nucleation; physical vapour deposition (PVD); cerium oxide INTRODUCTION The study of the first stages of nucleation of thin films can help to understand the performance of materials and devices where interfaces are formed. 1 Two different aspects are noteworthy here: the chemistry of the formed interfaces (i.e. the possible chemical reactions taking place due to the fact that two materials are put in contact), and the growth mechanism of the deposited layer on the substrate (i.e. the evolution of the first nucleation steps). X-ray photoelectron spectroscopy (XPS) is specially suited to the characterisation of both phenomena. Thus, the ability to distinguish different chemical states, 2 and the possibility of differentiating between growth mechanisms by either angle resolved XPS 3 and inelastic XPS peak shape analysis 4,5 yields this technique as a very powerful tool for the characterisation of this kind of systems at the nanometre scale. Cerium oxide is a material of interest in different technological fields, as an electrolyte in solid oxide fuel cells in which it acts as ionic conductor, 6 or as an additive in the so-called ‘three ways catalyst’ used in the automobile L Correspondence to: F. Yubero, ICMSE (CSIC-USE) Am´ erico Vespucio s/n. E-41092 Sevilla, Spain. E-mail: yubero@icmse.csic.es Contract/grant sponsor: Ministry Education and Science; Contract/grant number: MAT2004-01558. Contract/grant sponsor: Acciones Integradas; Contract/grant number: HA2002-0028. industry. 7 The ability to participate in redox reactions in which the oxidation state of Ce changes between Ce 3C and Ce 4C is a well-known characteristic for this material. 8 XPS is well suited to distinguishing between these two oxidation states, because their photoemission spectra are quite different in shape. 9 In this paper we present a basic XPS study, correlating the chemical interactions and the initial nucleation steps for the deposition of cerium oxide on alumina and partially reduced titania surfaces. We have found that cerium oxide deposited in these two substrates behaves quite differently from both the point of view of the chemical interactions at the interface and from that of the first stages of the nucleation mechanism. EXPERIMENTAL Cerium oxide was deposited by thermal evaporation of CeO 2 pellets using electron beam heating (min e-beam evaporator from TECTRA). The pellets were previously sintered at 1370 K for a duration of 9 h. Deposition rates of a few tenths of nanometre per minute were used as evaluated by inelastic XPS peak shape analysis using QUASES software 10 (see following text). After each deposition essay, the samples were exposed to 10 2 mbar of oxygen for ¾60 s. The substrates were a sapphire crystal and a titanium dioxide thin film (¾100-nm thick) prepared by ion beam induced chemical vapour deposition on polished Si(100) wafer. 11 The surface Copyright  2006 John Wiley & Sons, Ltd.