Structural Characterization of Mn₃O₄(001) Thin Films and Their Interaction with CO

In this study, we investigated the atomic-level interaction between CO molecules and a well-ordered Mn3O4(001) thin film. The oxide film, initially grown on a single-crystal Au(111) substrate, was characterized using X-ray photoelectron spectroscopy (XPS) to determine its oxidation state. Further examination of the film structure through scanning transmission electron microscopy (STEM) and low-energy electron diffraction (LEED) revealed its growth orientation along the [001] direction, with three domains rotated by 60° and aligned with the ⟨110⟩Au directions. Notably, the termination of the film features a layer of Mn3+ cations covered with oxygen atoms. The study then focused on the interaction of the film with CO, employing infrared reflection absorption spectroscopy (IRAS) and temperature-programmed IRAS (TP-IRAS). The results unveiled two distinct IR peaks related to CO adsorption: one at 2140 cm–1, signifying that CO molecules weakly adsorbed on the oxygen-terminated surface of Mn3+ cations, and another at 2105 cm–1, indicative of CO molecules bound to Mn2+ cations at oxygen vacancies.