Rare earth oxides (REOs) exhibit favorable catalytic performance for a diverse set of chemical transformations. I will discuss our efforts to develop thin film systems of samaria and terbia for model surface science investigations of REOs that are effectively irreducible and reducible, respectively, and which are thus expected to promote partial vs. complete oxidation chemistry of adsorbed species. We used low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to characterize the structural properties of samaria and terbia thin films grown on Pt(111) in ultrahigh vacuum (UHV) using physical vapor deposition. We find that both REOs grow as high quality Sm2O3(111) and Tb2O3(111) films, and adopt oxygen-deficient fluorite structures wherein the metal cations form a hexagonal lattice in registry with the Pt(111) substrate, while oxygen vacancies are randomly distributed within the films. The Sm2O3(111) films undergo partial reduction by annealing to 1000 K in UHV, whereas the Tb2O3(111) films are more stable. LEED and STM show that a fraction of the Sm2O3 in the first layer decomposes to produce rocksalt SmO(100) during heating, and that Sm2O3 from the third and higher layers concurrently spreads on top of the first layer to produce a more contiguous second layer of Sm2O3(111) as well as a Sm2O3(111)/SmO(100) coincidence structure. Re-oxidation of the partially reduced SmOx films completely restores the original Sm2O3(111) crystal structure and significantly improves the film flatness and connectivity among oxide domains. We have also found that Tb2O3(111) films can be oxidized in UHV by exposure to plasma-generated atomic oxygen beams. The oxidized films have an estimated TbO1.8 stoichiometry and decompose to Tb2O3 during heating, with O2 desorption starting at about 500 K. The development of high-quality samaria and terbia thin films on Pt(111) provides new opportunities to investigate oxidation chemistry on REOs that have distinct reduction and oxidation properties.

Structural properties and oxidation/reduction of samaria and terbia thin films on Pt(111)

247th National Meeting (2014)

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