Vanadium oxide nanostructures on Rh(1 1 1): Promotion effect of CO adsorption and oxidation

The adsorption of CO and the reaction of CO with pre-adsorbed oxygen at room temperature has been studied on the (2 × 1)ORh(1 1 1) surface and on vanadium oxideRh(1 1 1) “inverse model catalyst” surfaces using scanning tunnelling microscopy (STM) and core-level photoemission with synchrotron radiation. Two types of structurally well-defined model catalyst V₃O₉Rh(1 1 1) surfaces have been prepared, which consist of large (mean size of 50 nm, type I model catalyst) and small (mean size <15 nm, type II model catalyst) two-dimensional oxide islands and bare Rh areas in between; the latter are covered by chemisorbed oxygen. Adsorption of CO on the oxygen pre-covered (2 × 1)ORh(1 1 1) surface leads to fast CO uptake in on-top sites and to the removal of half (0.25 ML) of the initial oxygen coverage by an oxidation clean-off reaction and as a result to the formation of a coadsorbed (2 × 2)O + CO phase. Further removal of the adsorbed O with CO is kinetically hindered at room temperature. A similar kinetic behaviour has been found also for the CO adsorption and oxidation reaction on the type I “inverse model catalyst” surface. In contrast, on the type II inverse catalyst surface, containing small V-oxide islands, the rate of removal of the chemisorbed oxygen is significantly enhanced. In addition, a reduction of the V-oxide islands at their perimeter by CO has been observed, which is suggested to be the reason for the promotion of the CO oxidation reaction near the metal-oxide phase boundary.     

The self-assembly of metallic nanowires

We have demonstrated that nickel adatoms self-assemble into quasi one-dimensional nanowires on vicinal Rh(1 1 1) surfaces by decorating their regular monoatomic step arrays, while V adatoms do not. The step decoration process has been followed experimentally by variable-temperature scanning tunnelling microscopy and high-resolution X-ray photoelectron spectroscopy. The physical origin of the different step-assisted self-assembly behaviour of Ni and V adatoms has been elucidated theoretically and is ascribed to different diffusion barriers and trapping capability of Ni and V at Rh steps.     

Sodium adsorption on a Ge(100)-(2 × 1) surface

The adsorption of Na on a Ge(100)-(2 × 1) surface has been studied by means of AES, LEED, EELS, TPD and work-function measurements. In the submonolayer coverage region the coverage dependencies of the desorption activation energy E(Θ) and desorption frequency v(Θ) have been determined using the threshold TPD method. Our experimental data show that after the completion of the first Na layer, 3D crystallites develop on the Na/Ge(100) surface (Stranski-Krastanov growth mode). For Θ > 1 ML, formation, followed by decomposition of a certain Na---Ge surface compound occurs in the temperature range 410–550 K.     

Oxygen adsorption on Ge(111) surface: I. Atomic clean surface

Oxygen adsorption on a clean Ge(111) surface has been studied in the temperature range 300–560 K by means of Auger electron spectroscopy (AES), thermal desorption (TD), work function (WF) measurements, and electron energy loss spectroscopy (ELS). The adsorption and WF kinetics at 300 K exhibit a shape different from those observed at higher adsorption temperatures. At 300 K oxygen only removes the empty dangling bond surface state, whereas at higher temperature new loss transitions involving chemically shifted Ge 3d core levels appear. The findings imply that at 300 K only a chemisorption oxygen state exists on the Ge(111) surface whereas the formation of an oxide phase requires higher temperatures. The shapes of the TD curves show that the desorption of GeO follows $\text{1}\text{2}$ order desorption kinetics.     

Els study of alkali metal adsorption on Ni(100)

Electron energy-loss spectra on clean and alkali-covered Ni(100) surfaces have been measured. The changes in the characteristic Ni(100) loss spectra in the presence of Na, K and Cs are interpreted as a result of large charge transfer to Ni. Alkali metal losses due to core-level excitation and plasmon excitations are also discussed.     

A TEM and XRD study of the SbNbSe3 misfit layer structures

Two new misfit layer structures have been synthesized within the Sb-Nb-Se system. Powder X-ray diffraction and electron microscopy techniques (electron diffraction, HREM, XEDS) have been used to determine the nature of their structure. According to TEM and XEDS data (for more than 15 crystals studied) both phases are monolayer type, i.e. (SbSe)1+delta (NbSe2). Electron microscopy reveals a composite modulated structure that consists of the periodical intergrowth of a pseudotetragonal SbSe layer, denominated as Q, and a pseudohexagonal layer NbSe2, denominated as H. Both layers fit along b, stack along c and do not fit along a (misfit) giving rise to an incommensurate modulation along this direction. The two phases differ in the symmetry of the Q layers being in one case orthorhombic (for delta = 0.17) and monoclinic in the other (for delta = 0.19). After the characterization of the sample by electron microscopy the unit cells of the basic layers could be refined for both phases by powder X-ray diffraction: aQ = 5.824(2) A, bQ = 5.962(5) A, cQ = 23.927(6) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees and aH = 3.415(5) A, bH = 5.962(6) A,, cH = 11.962(1) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees for the orthorhombic phase; aQ = 5.844(2) A, bQ = 5.981(1) A, cQ = 23.919(5) A, alpha = 90 degrees, beta = 90 degrees and gamma = 96.00(3)degrees and aH = 3.439(1) A, bH = 5.994(2) A, cH = 11.956(3) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees for the monoclinic phase. The phase with the monoclinic Q-sublattice often appears as twinned crystals. The more abundant crystals are disordered intergrowths of both monolayer phases.     

Scanning tunneling microscopy imaging of NiO(100)(1 × 1) islands embedded in Ag(100)

The imaging of NiO(100)(1 × 1) islands embedded in Ag(100) by scanning tunneling microscopy is addressed. As a function of tunneling conditions and tip termination it is possible to resolve the NiO–vacuum interface, the second oxide layer as well as the NiO-substrate interface with atomic contrast. We find that for sub-monolayer coverages of NiO the oxide islands consist of an essentially defect-free surface layer at the vacuum interface with a number of NiO second layer patches incorporated into the Ag substrate underneath. The oxide layer is surrounded by a rim of a NiO bilayer of monoatomic width. A reduction of the density of states between a NiO monolayer and local NiO bilayer stackings is suggested to be responsible for the observed appearance of mosaic patches at the island surface.     

Electron energy loss spectroscopy of sodium covered Ge(111) surfaces

The ELS spectra of sodium covered Ge(111) surfaces are studied. It was found that sodium coverage upto 1 ML leads to a extinction of the ELS features of the clean Ge surface. With increasing sodium coverage a metal induced peak at 3.4 eV and a peak at 31.6 eV due to a sodium 2p-core level transitions appear. ELS spectra of a heat-treated surface covered with sodium imply a formation of a surface Na-Ge “compound” as a result of Na-Ge interdiffusion.