The Interaction of Sub-Monolayer Ta Adatoms and Clusters with Oxygen at the Pt(111) Interface

The interaction of submonolayer quantities of size-selected and soft-landed Ta_n (n = 4, 5, 6, 8, 13) clusters with Pt(111) is investigated employing high-resolution X-ray photoelectron spectroscopy (HR-XPS), scanning tunneling microscopy (STM), and density functional theory (DFT) simulations. The deposited clusters are monodispersed and stable under ultrahigh vacuum (UHV) conditions at 40 K. They display a size-specific trend in photoemission spectra, which is reasoned in terms of the distinct in plane coordination of Ta atoms in the clusters. Both the Ta coordination number and distance from the Pt surface influence its Bader charge and, accordingly, the oxidation state of the atoms in the Ta cluster. They already fragment in the presence of low amounts of oxygen and form a common oxidation product observed for all cluster sizes. Based on our observations, we propose an oxidation mechanism in the example of Ta₈ clusters, which is closely comparable to the one discussed in gas-phase studies on the oxidation of cationic Ta clusters of similar size. Concomitant to oxidation-induced fragmentation, the agglomeration into Ta-oxide islands with Ta in an oxidation state of +5 is observed. However, the strong interaction with the Pt surface leads to Ta 4f orbital photoemission features that differ from those commonly observed for Ta₂O₅. Computational insights concerning the structure of the Ta-oxide islands indicate flat agglomerates that agree with STM observations. They suggest distinct Ta 4f photoemission contributions from interfacial and surface-related Ta configurations. The respective HR-XPS spectra display specific core-level shifts as a function of bonding configuration and vicinity to the Pt surface. By annealing at 900 K in UHV, we observe oxygen loss and concomitant intermixing of Ta atoms with the Pt subsurface lattice to which results in the formation of a Ta-Pt alloy. These species, Ta-oxide islands, and Ta-Pt alloy, can reversibly interconvert by oxidative surface segregation and reductive intermixing.