Interfacial reaction between deposited molybdenum and TiO2(1 1 0) surface: role of the substrate bulk stoichiometry

The interfacial reaction between deposited molybdenum and three different TiO2(1 1 0) substrates (a bulk and surface stoichiometric TiO2; a bulk stoichiometric and surface reduced crystal; a bulk and surface slightly reduced crystal) was investigated by means of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). While the interfacial reaction is not a function of the substrate at room temperature (in any case, molybdenum layers grow with oxygen dissolved in), this study clearly reveals a strong effect of the substrate bulk stoichiometry on the chemical state of the deposit after annealing up to 750 °C whereas the substrate surface stoichiometry plays a minor role. Indeed, annealing carried out on deposits performed on bulk stoichiometric TiO2 (whatever the initial surface stoichiometry is) leads to MoO3 formation whereas annealing carried out on layers which grew on bulk non-stoichiometric TiO2 involves metallic molybdenum overlayers. These results, as well as others previously published on the same system, are discussed from a new model based on thermodynamical considerations including entropic effect.