Experimental and theoretical evidence for substitutional molybdenum atoms in the TiO2(110) subsurface

Molybdenum was deposited at room temperature on the TiO2(110) surface in the 0–1.3 equivalent monolayer (eqML) range and was then annealed at 400 °C in order to reach a kind of equilibrium state. A threshold was found in the behavior of the deposit: below 0.2 eqML, substitutional molybdenum occurs in titanium sites located under the bridging oxygen atoms of the TiO2(110) surface. In this position, molybdenum atoms are in a structural and chemical MoO2-like environment. Density-functional theory calculations show that this molybdenum site is actually the most stable one in the case of isolated molybdenum atoms. Angle-scanned photoelectron diffraction data are in perfect agreement with such a hypothesis. For deposits higher than 0.2 eqML, the increased amount of molybdenum atoms raises the probability of Mo-Mo interactions during the annealing at 400 °C, taking to a reduction process of the deposit. However, such annealing does not allow the deposit to become fully metallic: molybdenum clusters formed during the annealing are in strong interaction with the substrate, and metallic molybdenum can be obtained only depositing a film thicker than 1 ML.