Paper SS-TuP6
Structural Growth and Oxidation of TbO_x Thin Films on Pt(111)

Tuesday, October 31, 2017, 6:30 pm, Room Central Hall

Within the family of rare earth oxides (REOs), the terbium oxides exhibit favorable properties in selective oxidation catalysis due to the flexibility in the storage and release of oxygen within the lattice, specifically through structural rearrangement into well-ordered intermediates between the Tb₂O₃ and TbO₂ stoichiometries. We investigated the growth and structures of TbO_x films grown on Pt(111) in ultra-high vacuum (UHV) as well as the oxidation of the films by plasma-generated gaseous atomic oxygen. LEED and STM show that the deposition produces crystalline Tb₂O₃ films that adopt an oxygen deficient cubic fluorite structure where the film conforms to the hexagonal registry of the Pt(111) substrate. This is characterized by initial surface wetting up to 2 ML of Tb₂O₃ followed by 3D Stranski-Krastanov island growth at higher coverages.

We also find that the terbia film undergoes isomeric reorganization into the longer-order bixbyite Tb₂O₃ conformation when subject to a combination of atomic oxygen exposure along with subsequent annealing at 1000 K. LEED and TPD show that coexisting, ordered intermediates between Tb₂O₃ and TbO₂ may then be created by further oxidizing the bixbyite Tb₂O₃ film via atomic oxygen beam exposure. In particular, two distinct O₂ desorption peaks in TPD spectra provide evidence of the sequential phase stabilization of Tb₇O₁₂ (ι-phase) and Tb₁₁O₂₀ (δ-phase) along with lower temperature peaks corresponding with more weakly bound surface oxygen. The rapid reorganization of oxygen and oxygen vacancies within this intermediate regime is promising in that it suggests that specific structural arrangements of the terbia lattice may readily adjust to accommodate dissimilar metal cations into the metallic lattice to stabilize ordered, substitutionally doped films. The future outlook is to characterize structure and promoted surface chemistry of doped terbium oxide films, particularly the changes in oxidation induced by the incorporation of high valence dopants and low valence dopants into the metallic framework of the oxide. The larger implication is that the substitutional doping of terbium oxides can provide fundamental insight into doped oxide catalysis, which can provide the additional degree of tuneability towards increased reactivity or selectivity towards partial oxidation pathways required for an effective oxidative coupling of methane (OCM) catalyst.