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: The structure of two ordered stoichiometric TiO(2) nanophases supported on Pt(111) and (1x2)-Pt(110) substrates, prepared by reactive evaporation of Ti in a high-oxygen background, is compared by discussing experimental data (i.e. low-energy electron diffraction, scanning tunneling microscopy) and density functional theory calculations. Two rectangular phases, called rect-TiO(2) and rect'-TiO(2) were obtained on both the hexagonal Pt(111) and the rectangular (1x2)-Pt(110) substrates, generally suggesting that they are weakly interacting with the substrates. The rect-TiO(2) phase is actually confined to a TiO(2) double layer, while the rect'-TiO(2) can extend up to a thickness of several layers and is obtained when higher Ti doses are evaporated. While the rect-TiO(2) is best described as a thickness-limited lepidocrocite-like nanosheet, growing as a single-domain-commensurate (14x4) phase on (1x2)-Pt(110) and as a six-domains-incommensurate phase on Pt(111), the thicker rect'-TiO(2) phase can be best described as a TiO(2)(B) supported nanolayer (NL). This represents the first example of the TiO(2)(B) phase in the form of a supported NL, whose properties are still largely unexplored. The important point is that, because of the weak interaction between the oxide NLs and the Pt surfaces, the substrate does not play a role in stabilizing the 2D nanostructures. Rather, it acts as a sort of lab bench where sub-nanosized titania crystallites self-assemble, so that the final NLs are representative of 2D confined titania at the bottom of the nanoscale.ChemPhysChem 05/2010; 11(7):1550-7. · 3.35 Impact Factor
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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