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.
[Show abstract][Hide abstract] 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. DOI:10.1080/01442350903172453 · 7.03 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
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