Structure of a TiOx Zigzag-Like Monolayer on Pt(111)

Abstract

The structure of a monolayer phase of TiO x on Pt(111) has been investigated by low-energy electron diffraction (LEED), atomic resolved scanning tunneling microscopy (STM), and density functional (DF) calculations. According to LEED, the rectangular unit cell (6.8 x 8.6 A 2 ) is incommensurate with respect to the Pt(111) substrate unit cell. The STM data show a clear zigzag-like motif and dimensions in perfect agreement with the LEED data. A structural model, which is in tune with the whole set of experimental data, has been obtained by a DF geometry optimization starting from a guessed structure proposed on the basis of chemical considerations and the comparison with literature data. The stoichiometry of the monolayer is Ti 6 O 8 and the Ti atoms are formally in the +2.7 oxidation state, in agreement with previously reported photoemission data. However, two different types of Ti atoms have been found, that is, Ti atoms coordinated by four oxygen atoms, which give rise to the brighter bumps in the zigzag-like STM motif, and Ti atoms coordinated by only three oxygen atoms, which appear darker in the STM images. Analogously, two different types of oxygen atoms can be distinguished, with those lying in the throughs of the STM images (bridge oxygens) being less coordinated and in a lower oxidation state. The energetics of the interaction of the oxide monolayer with the Pt substrate has been computationally evaluated. Even if the oxide/metal interaction is important in determining the high stability and the structure of the oxide film, it conveys that this interaction is only weakly directional, thus, justifying the incommensurate nature of the film.