Unraveling the interaction of Ta atoms with Pt(111)

This study investigates the interaction between Ta and Pt by following the evolution of Ta atoms upon their evaporation in sub-monolayer quantities (similar to 2 % ML) on a Pt(111) single crystal in vacuo at 40 K. Adsorption and bonding configurations are studied employing high-resolution X-ray photoelectron spectroscopy (HR-XPS). This is combined with density functional theory (DFT) calculations and reference measurements on atomically precise supported clusters that allow disentangling the contributions of distinct surface species to the complex Ta 4f spectra. The transient Ta atom mobility upon evaporation allows - besides atomic adsorption on the terraces - for atom migration to the Pt(111) step edges and the formation of small monolayer ad-islands even at 40 K. Interestingly, the Ta species bonded with Pt can be easily oxidized using a photon-induced O-2 dissociation process, showing the extremely high oxygen affinity of Ta even under the applied ultra-high vacuum (UHV) conditions. Annealing to 900 K leads to the sub-surface migration of Ta and the formation of a sub-surface alloy with Pt. Here, the extremely sharp Ta core-level components are observed at binding energies corresponding to a nominal oxidation state of +2, as confirmed by calculations of the projected density of states (pDOS). This sub-surface alloy is resistant to facile oxidation by atomic oxygen, unlike the Ta surface species that remain susceptible to oxidation. The oxide species and the sub-surface alloy are, however, perfectly interconvertible upon annealing to 900 K in vacuum, respectively in an oxygen environment, and thereby reveal an intriguing interplay between reductive intermixing of Ta and Pt and oxidative Ta segregation.