Experimental and theoretical evidence for substitutional molybdenum atoms in theTiO2(110)subsurface

Abstract

Molybdenum was deposited at room temperature on the ${\mathrm{TiO}}_{2}(110)$ surface in the 0--1.3 equivalent monolayer (eqML) range and was then annealed at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ 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 ${\mathrm{TiO}}_{2}(110)$ surface. In this position, molybdenum atoms are in a structural and chemical ${\mathrm{MoO}}_{2}$-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 $\mathrm{Mo}\text{\ensuremath{-}}\mathrm{Mo}$ interactions during the annealing at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, 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 $1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$.