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 degrees 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 degrees 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.