Chromium Oxide Thin Films on Pt(111): An STM and DFT Excursion through the Phase Diagram

Abstract

The formation of chromium oxide thin films on Pt(111) was investigated by electron diffraction, X-ray photoelectron spectroscopy, and lowtemperature scanning tunneling microscopy. Depending on the nominal Cr coverage and the oxygen chemical potential during preparation, two distinct oxide phases were identified. A (√3 × √3)R30°phase emerges at submonolayer Cr exposure and Orich oxidation conditions, while a (2 × 2) phase develops at higher coverage and after a vacuum-annealing step. For both phases, the atomic nature of cationic and anionic sublattices and the chemical composition were determined in detail. Guided by this experimental input, a comprehensive global structure optimization was performed by genetic algorithms, and key configurations were refined by density functional theory calculations in a second step. For the (√3 × √3)R30°phase, good agreement was revealed for a Cr 3 O 6 trilayer, comprising a mixture of Cr 3+ and Cr 4+ ions in the central cationic plane. The (2 × 2) phase matches well with a Cr 6 O 11 film, in which an O−Cr−O trilayer is capped by a Cr 2 O 3 honeycomb plane. The identified configurations not only reproduce the structure and symmetry deduced from experiments but also have the most favorable energetics at the employed oxygen chemical potential. Our work delivers reliable insights into the atomic nature of the two main CrO x phases on Pt(111) and clears up the conflicting models found in the literature.