Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.

The dynamics of hydrogen sensing with nanoporous Pt/TiO2 Schottky barriers is studied by time-resolved electronic transport measurements. The development with time of the doping density, the average Schottky barrier height, and the built-in voltage are determined from current-voltage and capacitance-voltage characteristics. The current-voltage characteristics change from exponential towards Ohmic behavior as time proceeds. This is interpreted in terms of local areas that switch from diodic to Ohmic response due to a combination of increasing doping density and decreasing Schottky barrier height.

Angle-scanned X-ray photoelectron diffraction (XPD) and scanning tunneling microscopy (STM) are used to characterise the structure of TiO2 nanoparticles grown on a Pt(111) single crystal surface. The nanoparticles grow over a well-ordered oxide interfacial layer that displays a (square root 43 x square root 43) - R7.6 degrees superstructure with a unit cell (18.2 x 18.2 A), as demonstrated by STM and low-energy electron diffraction (LEED). Our XPS Ti 2p core level spectra suggest a significant contribution from reduced titanium ions within the interfacial layer. On the contrary, according to XPS binding energy data, the nanoparticles are mostly composed of Ti(IV) ions. During the initial stage of the growth, the nanoparticles are on the average 2 nm high and about some tens of nm wide, and show a flat on-top surface, while the interparticle region show the structure of the ordered interfacial layer. During later stages of the deposition, the particles become larger and they show a more irregular, globular-like shape as well as coalescence. But, even at this stage of the growth, large interparticle regions are present. Moreover, the nanoparticles produce a distinct XPD pattern which demonstrates that they grow with a preferential azimuthal orientation with respect to the substrate surface. A simple geometrical analysis of the XPD data in terms of forward scattering events suggests that the particles crystallize in the rutile TiO2 structure and expose the (100) surface. This hypothesis is supported by means of multiple scattering simulations of the XPD patterns.

1 2 ͑⌽ tip + ⌽ sample ͒ ͑for small tunneling voltages V Ӷ ͒ and is related with the inverse decay length PHYSICAL REVIEW B 71, 235416 ͑2005͒