Growth and structure of ultrathin cerium oxide films on Rh(111)

Growth of a quasicrystal-related structure and superstructure for ultrathin Ce-Ti-O films on Pt(111)

Herein, oxide quasicrystal-related (OQC-R) structure and Ce-Ti-O-(3 × 3) superstructure ultrathin films were prepared on Pt(111) and characterized using scanning tunneling microscopy (STM) and low-energy electron diffraction. The OQC-R structure with dodecagonal clusters consisting of triangles, squares, and rhombuses was observed in STM images. The first discovery of the OQC-R structure with a magnetic rare earth metal expands the possibility of discovering new oxide quasicrystals with novel magnetism or superconductivity. By depositing Ti on an OQC-R ultrathin film and post-annealing, a honeycomb lattice of the Ce-Ti-O-(3 × 3) superstructure was prepared. From X-ray photoelectron spectroscopy (XPS) and resonant-photoelectron spectroscopy, the chemical states of the Ce and Ti atoms in the OQC-R structure corresponded to the Ce3+ and Ti2+ states, while those for the Ce-Ti-O-(3 × 3) superstructure corresponded to the Ce3+, Ti3+, and Ti2+ states. The phase transformation from the OQC-R structure to the Ce-Ti-O-(3 × 3) honeycomb superstructure likely occurred when the amount of Ti increased and was more oxidized. The elemental atomic density was also calibrated using XPS and Rutherford backscattering spectroscopy. These results propose tentative structural models of the OQC-R structure as Ce18Ti14O41 and the Ce-Ti-O-(3 × 3) superstructure as CeTi6O9.

Redox-mediated transformation of a Tb2O3(111) thin film from the cubic fluorite to bixbyite structure

We used temperature programmed desorption (TPD) and low energy electron diffraction (LEED) to investigate the isomeric structural transformation of a Tb2O3 thin film grown on Pt(111). We find that repeated oxidation and thermal reduction to 1000 K transforms an oxygen-deficient, cubic fluorite (CF) Tb2O3(111) thin film to the well-defined bixbyite, or c-Tb2O3(111) structure, whereas annealing the CF-Tb2O3(111) film in UHV is ineffective in causing this structural transformation. We estimate that the final stabilized film consists of about ten layers of c-Tb2O3(111) in the surface region plus about eight layers of CF-Tb2O3(111) located between the c-Tb2O3(111) and the Pt(111) substrate. Our measurements reveal the development of two distinct O2 TPD peaks during the CF to bixbyite transformation that arise from oxidation of c-Tb2O3 domains to the stoichiometrically-invariant ι-Tb7O12 and δ-Tb11O20 phases and demonstrate that the c-Tb2O3 phase oxidizes more facilely than CF-Tb2O3. We present evidence that nucleation and growth of c-Tb2O3 domains occurs at the buried TbOx/CF-Tb2O3 interface, and that conversion of the interfacial CF-Tb2O3 to bixbyite takes place mainly during thermal reduction of TbOx above ∼900 K and causes newly-formed c-Tb2O3 to advance deeper into the film. The avoidance of low Tb oxidation states may facilitate the CF to bixbyite transformation via this redox mechanism.

Atomic structures and local electronic properties of K- and Rh-modified ceria/Pt(111) inverse model catalysts

Ceria has been widely applied as a support in heterogeneous catalysis due to its unique capability to store and release oxygen. As a typical inverse model catalyst, a ceria/Pt(111) system has attracted much attention due to its strong metal-oxide interaction. The structural and electronic properties of the ceria/Pt(111) system can be effectively modified by the introduction of alien K and Rh atoms. Here, the K- and Rh-modified ceria/Pt(111) inverse model catalysts have been investigated with high resolution scanning tunneling microscopy and apparent local work function measurement. The experimental results indicate that the K atoms prefer to occupy the top sites of the stoichiometric ceria, while the Rh atoms are prone to stay at the electron-rich ceria island edges. The K and Rh atoms act as an electron donor and acceptor on ceria/Pt(111), respectively. Such a study on the modification of the ceria-based catalysts should help understand strong metal-oxide interaction in heterogeneous catalysis at the atomic level.

STM Images of Anionic Defects at CeO2(111)-A Theoretical Perspective

We present a theoretically oriented analysis of the appearance and properties of plausible candidates for the anionic defects observed in scanning tunneling microscopy (STM) experiments on CeO₂(111). The simulations are based on density functional theory (DFT) and cover oxygen vacancies, fluorine impurities and hydroxyl groups in the surface and sub-surface layers. In the surface layer, all three appear as missing spots in the oxygen sublattice in filled state simulated STM images, but they are distinguishable in empty state images, where surface oxygen vacancies and hydroxyls appear as, respectively, diffuse and sharp bright features at oxygen sites, while fluorine defects appear as triangles of darkened Ce ions. In the sub-surface layer, all three defects present more complex patterns, with different combinations of brightened oxygen ion triangles and/or darkened Ce ion triangles, so we provide image maps to support experimental identification. We also discuss other properties that could be used to distinguish the defects, namely their diffusion rates and distributions.

Use and mis-use of x-ray photoemission spectroscopy Ce3d spectra of Ce2O3 and CeO2

X-ray photoemission spectroscopy (XPS) work on Ce₂O₃ and CeO₂ oxides has been an active topic of research over the past four decades or so. Such research aimed to find the reasons for the unusual complexity of Ce3d spectra of the two oxides, and it studied catalytic properties of materials that contained them. I discuss how theoretical and experimental studies exploited the diagnostic potential of XPS to reach our current knowledge of the electronic properties of the two oxides. A part of these studies provided peak-fitting guidelines to resolve Ce3d spectra produced by the co-existence of both oxides into the individual spectral components arising from Ce³⁺ and Ce⁴⁺ ions. Basing myself on the analysis of several peak-fittings of Ce3d spectra carried out in studies of the catalytic applications of CeO₂-based materials, I show that more often than not they largely ignore the findings of theoretical, experimental, and methodological XPS work. I discuss typical problems that flaw Ce3d peak-fittings, and how they affect their accuracy. I argue that, although several XPS studies do list primary literature of Ce3d spectra in their bibliography, they often do so for decorative purposes, rather than practical purposes.

Thermal reduction of ceria nanostructures on rhodium(111) and re-oxidation by CO2

The thermal reduction of cerium oxide nanostructures deposited on a rhodium(111) single crystal surface and the re-oxidation of the structures by exposure to CO2 were investigated. Two samples are compared: a rhodium surface covered to ≈60% by one to two O-Ce-O trilayer high islands and a surface covered to ≈65% by islands of four O-Ce-O trilayer thickness. Two main results stand out: (1) the thin islands reduce at a lower temperature (870-890 K) and very close to Ce2O3, while the thicker islands need higher temperature for reduction and only reduce to about CeO1.63 at a maximum temperature of 920 K. (2) Ceria is re-oxidized by CO2. The rhodium surface promotes the re-oxidation by splitting the CO2 and thus providing atomic oxygen. The process shows a clear temperature dependence. The maximum oxidation state of the oxide reached by re-oxidation with CO2 differs for the two samples, showing that the thinner structures require a higher temperature for re-oxidation with CO2. Adsorbed carbon species, potentially blocking reactive sites, desorb from both samples at the same temperature and cannot be the sole origin for the observed differences. Instead, an intrinsic property of the differently sized CeOx islands must be at the origin of the observed temperature dependence of the re-oxidation by CO2.

Morphology of size-selected Ptn clusters on CeO2(111)

Supported Pt catalysts and ceria are well known for their application in automotive exhaust catalysts. Size-selected Pt clusters supported on a CeO₂(111) surface exhibit distinct physical and chemical properties. We investigated the morphology of the size-selected Pt_n (n = 5-13) clusters on a CeO₂(111) surface using scanning tunneling microscopy at room temperature. Pt_n clusters prefer a two-dimensional morphology for n = 5 and a three-dimensional (3D) morphology for n ≥ 6. We further observed the preference for a 3D tri-layer structure when n ≥ 10. For each cluster size, we quantitatively estimated the relative fraction of the clusters for each type of morphology. Size-dependent morphology of the Pt_n clusters on the CeO₂(111) surface was attributed to the Pt-Pt interaction in the cluster and the Pt-O interaction between the cluster and CeO₂(111) surface. The results obtained herein provide a clear understanding of the size-dependent morphology of the Pt_n clusters on a CeO₂(111) surface.