Growth and Partial Reduction of Sm₂O₃(111) Thin Films on Pt(111): Evidence for the Formation of SmO(100)

Abstract

We investigated the growth and partial reduction of Sm 2 O 3 (111) thin films on Pt(111) using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Sm 2 O 3 (111) films are high quality and grow in a defective fluorite structure wherein the Sm cations adopt a hexagonal (1.37 × 1.37) lattice in registry with the Pt(111) surface, while oxygen vacancies are randomly distributed within the film. STM measurements show that Sm 2 O 3 (111) film growth on Pt(111) occurs by the Stranski-Krastanov mechanism, in which a single O−Sm−O trilayer initially forms, followed by the growth of well-defined, multilayer islands. The Sm 2 O 3 (111) films undergo partial reduction during annealing at 1000 K in ultrahigh vacuum. LEED and STM provide evidence that a fraction of the Sm 2 O 3 in the first layer, closest to the Pt(111) substrate, decomposes to produce well-ordered domains of rocksalt SmO(100) during reduction, and that Sm 2 O 3 from the third and higher layers concurrently spreads onto the first layer to form a more contiguous second layer of Sm 2 O 3 (111). We show that the SmO(100) and Sm 2 O 3 (111) lattices can form a coincidence structure with minimal strain to the Sm-atom sublattices, and that satellite features observed in the LEED patterns are consistent with the coexistence of SmO(100) and Sm 2 O 3 (111) domains as well as the proposed Sm 2 O 3 (111)/SmO(100) coincidence structure. Lastly, we find that reoxidation of the partially reduced films restores the original Sm 2 O 3 (111) crystal structure, and significantly improves the film quality, as reflected by a flatter film morphology and better connectivity among oxide domains. An implication from this study is that the formation of (100)-oriented monoxide structures is a general characteristic of the reduction of rare-earth oxide thin films on hexagonally close-packed metal surfaces.

Methanol Adsorption and Reaction on Samaria Thin Films on Pt(111)

“…Therefore the 0.9 ML thin film, wetting the Pt surface after reduction and exhibiting only surface sites, would be restored more easily than the thicker (2.8 ML) reduced film. The Pt-related H 2 and CO peaks have the same intensity as observed before the reduction-reoxidation cycle, indicating that the reoxidized Sm 2 O 3 thin film dewets the Pt(111) substrate after the treatment, which is consistent with our finding in the previous STM work [13]. In that work, we demonstrated that the Sm 2 O 3 (111) crystal structure is completely restored by the reoxidation treatment.…”

“…As shown in our previous study, the reduced SmO x film can be reoxidized by thermal annealing in molecular oxygen [13]. Here reoxidation of the 2.8 ML reduced SmO x film was performed by annealing the film in an O 2 background (P = 5 × 10 −7 mbar) at 1000 K for 10 min.…”

“…[13], samaria grows in a defective fluorite structure on Pt(111), in which the Sm atoms form a well-ordered (1.37 × 1.37) hexagonal sub-lattice in registry with Pt(111), whereas oxygen vacancies are distributed randomly within the film. We found that the Sm 2 O 3 films can be reduced thermally, forming domains of SmO(001), and re-oxidized by annealing in molecular oxygen.…”

Model catalysis studies of the oxidation of propane over VO -based catalysts

“…Above 1 ML, LEED pattern becomes unclear, suggesting the formation of a not well-ordered film. Jhang et al [37] reported that SmO x was grown on Pt(1 1 1) with a Stranski-Krastanov mode up to 3 ML with a quasi-(3 × 3) superstructure, where a higher O 2 pressure of 5 × 10 −7 Torr and a higher oxidation temperature of 1000 K were used to improve the crystalline quality of the SmO x film thicker than 1 ML. For model catalysis study, complete cover of the substrate surface is a key issue to avoid the direct contact of the substrate surface with the reactants, thus a low oxidation temperature was used for preparing the SmO x films.…”

High-Temperature Desulfurization of Coal Gas over Sm Doped Mn-based/MSU-S Sorbents

“…Therefore, the control of operating temperatures and the optimization of regenerating conditions are very important in the desulfurization process. The samarium oxide, as one of the rare earth oxides (REOs), has received considerable attention because of high application potentials in microelectronics 11 and heterogeneous catalysis. 12 It is possible to combine the transition metal oxides with REOs in order to improve the stability and the total capacity of the sorbents.…”