Effect of the surface stoichiometry on the interaction of Mo with TiO2 (110)

Molydenum has been deposited at room temperature on (110) TiO2 surfaces wit h different stoichiometries, roughnesses and crystallinities. Whatever the substrate preparation is, in-situ Auger electron spectroscopy (AES) and X-r ay photoelectron spectroscopy (XPS) studies as well as ex-situ atomic force microscopy (AFM) and reflexion high-energy electron diffraction (RHEED) st udies reveal a Stranski-Krastanov growth mode: the completion of three mono layers followed by islands growth is observed in every case. The three mono layers are always composed of amorphous molybdenum oxide with an oxidation state of molybdenum less than IV. The oxidation of the molybdenum layers ge nerates Ti3+ and Ti2+ in the substrate, and induces a reconstruction of the surface: during the layer formation, the roughness of the material strongl y decreases. Moreover. if the substrate is prereduced prior to the depositi on, the reduction induced by molybdenum oxidation can easily migrate in the TiO2 bulk. In the case of an initial stoichiometric surface, the reduction is more located in the interfacial layers. After the growth of three layer s, metallic BC islands without preferential orientation appear. (C) 2000 El sevier Science B.V. All rights reserved.