A LEEM/micro-LEED investigation of phase transformations in TiOx/Pt(111) ultrathin films.
ABSTRACT A combined use of low energy electron microscopy (LEEM) and microprobe LEED (micro-LEED) allows the in-situ observation of dynamical processes at the TiOx/Pt(111) interface. The transformations between different surface-stabilized phases are investigated in the case of room temperature TiOx reactive deposition with subsequent post-annealing. For a coverage of 0.6 MLeq, UHV annealing to 400 degrees C leads to the formation of the zigzag-like z-TiO1.33 layer. At higher temperatures a rotated z-TiO1.33 phase is observed, its lateral distribution being strongly influenced by surface morphology. Concurrently, the z-TiO1.33 layer partially transforms into a kagomé-like TiO1.5 structure. The resulting oxygen enrichment of the interface is interpreted by invoking Ti interdiffusion into the substrate. At a coverage of 0.45 MLeq, UHV annealing at 500 degrees C transforms the z-TiO1.33 layer into a different zigzag-like z'-TiO1.25 layer. Post-annealing in oxygen of the reduced phases or direct reactive deposition at high temperature both produce the rect-TiO2 stoichiometric phase, showing characteristic needle-like domains aligned according to the rect-TiO2 unit cell orientation.
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ABSTRACT: High resolution X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy (LEEM) have been used to investigate the growth of ultrathin CeOx(111) on Re(0001), a model catalyst system. Rotational domains of CeOx(111) are identified with microprobe low energy electron diffraction (LEED) and dark-field LEEM. In the regions not covered by the ceria islands, a surface rhenium-oxide layer has been observed using energy-filtered XPEEM imaging and spectroscopy. The oxidation state of the ceria is key to its catalytic activity. For this reason we have employed resonant photoelectron spectroscopy of the Ce 4f contributions to the valence band to monitor the relative Ce3+ and Ce4+ concentrations. The overall stoichiometry of the moderately reduced film was CeO1.63. Resonant energy-filtered XPEEM imaging of the Ce oxidation state allowed us to confirm the uniformity of this stoichiometry across the ceria islands that constituted the film.The Journal of Physical Chemistry C 07/2013; 117(32):16509. · 4.84 Impact Factor
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ABSTRACT: First-principles calculations have been performed to study the interface electronic structure of Pt/TiO2 and to analyze the rectifying property of the Pt/TiO2/Pt structure. For the stoichiometric interface, the metal-induced gap states (MIGS) have amplitude appreciably only at the interface TiO2. We will show that the presence of MIGS makes oxygen-vacancy formation energy small at the interface. It is therefore expected that the interfacial TiO2 layer can be easily reduced. We will then demonstrate that the Schottky barrier height is strongly affected by oxygen deficiency. According to the present calculation, the interface is of Schottky-contact type for the fully oxidized interfacial TiO2 while it becomes almost ohmic for strongly reduced one.Physical review. B, Condensed matter 11/2009; 80(19). · 3.66 Impact Factor
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ABSTRACT: The growth of ultrathin oxide films on metal substrates offers a solution to many of the experimental difficulties inherent to the studies of surfaces of bulk oxides and provides new interesting materials with unprecedented structures and properties. In this article we review the preparation and characterisation of ultrathin titanium oxide (TiOx) and aluminium oxide (AlOx) films grown on metal and metal alloy surfaces, emphasising those results that highlight new concepts and insights into metal oxide surface physics and chemistry. Different methods of preparation and characterisation are discussed and the resulting chemical compositions and surface structures are described by taking into account the results provided by computational approaches, and putting emphasis in outlining the structural novelty of interface-stabilised versus bulk-like phases and on the importance of kinetic effects in orienting the growth.International Reviews in Physical Chemistry 10/2009; 28(4):517-576. · 4.92 Impact Factor