Reactivity between molybdenum and TiO2(110) surfaces : evidence of a sub-monolayer mode and a multilayer mode

Small amounts of molybdenum (from 0.03 to 1.3 eqML) were deposited on non-stoichiometric TiO2(1 1 0) surface. The deposits were investigated by means of LEED and X-ray/UV photoemission using synchrotron radiation. For the smallest coverage ( < 0.2 eqML), deposition leads to oxidation of molybdenum into species close to Mo4+. In such a case, states appearing in TiO2 band gap are mainly due to reduced titanium. For higher coverages, metallic behaviour of molybdenum is observed. This phenomenon was explained, thanks to first principle calculations, as a decrease of the Mo-O interactions for the benefit of the Mo-Mo interactions as the surface molybdenum atom density increases.