1,721,057 research outputs found
Bottom-up synthesis in UHV of a metal oxide as supported nanoparticles from molecular precursors, investigated by photoemission Dragone, G. Granozzi, G. A. Rizzi, M. Sambi, P. J. Møller
No full textWatching atoms at work during reactions
Full text linkThe development of new technologies for the current energy and environmental challenges requires the acquisition of a very fundamental knowledge about the structure and activity of catalytic materials at the nanometric scale. As a consequence, in situ and operando methodologies are blossoming, but only a fraction of them really aims at a local vision that would allow watching atoms at work during reactions. In this short report, we want to outline the merits of a new technique based on scanning tunnelling microscopy (Current-roughness electrochemical scanning tunneling microscopy, cr-EC-STM) which can visualize electrocatalytic reactions down at the single atom level. Results of two case studies in the field of hydrogen evolution reaction (HER) are briefly summarized, witnessing the capability of cr-EC-STM to provide critical information about the structure and catalytic performance of the active sites with atomic resolution
Reversible adsorption of oxygen as superoxide ion on cerium doped zirconium titanate
Full text linkZirconium titanate (ZrTiO4) modified via cerium ions insertion in the oxide lattice has been successfully prepared via Sol-Gel synthesis forming solid solutions in the molar range between 0 and 10%. Cerium ions are hosted both as Ce4+ and Ce3+ ions whose ratio depends on the treatment undergone by the solid, with trivalent cerium always abundantly present even in strong oxidative conditions. Surface exposed Ce3+ ions are capable to adsorb O2 in the range of temperature between 273 K and 77 K, as superoxide anion (O2[rad]−), having side-on structure and characterized by complete surface-to-molecule electron transfer (about 98% of spin density on O2 molecule). Surprisingly such abundant adsorption is pressure dependent and fully reversible opening the perspective of applications of this system in the field of oxygen separation from gas mixtures
UV photoelectron and theoretical studies of organometal carbonyl clusters of ruthenium and osmium. μ-hydrido-μ3-alkynyl triangulo cluster compounds
No full textGrowth and the structure of epitaxial VO2 at the TiO2(110) surface
No full textEpitaxial VO2 layers have been grown on the TiO2(110) rutile surface up to thicknesses of 5 ML. These ultrathin films have been characterized by means of x-ray photoelectron spectroscopy (XPS), x-ray photoelectron diffraction (XPD), low-energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS) measurements. LEED and XPD structural data demonstrate that the layer is both short- and long-range ordered, and that it has a rutile structure. The success in preparation of a single-crystalline epitaxial VO2 layer opens possibilities for studying the properties and the surface chemistry of this interesting oxide, so far complicated by the difficulties in growing macroscopic crystals. From the He I spectra it turns out that the shape, position, and width of the 3d band closely resemble the UPS data of the bulk monoclinic semiconductive phase. The reported results could add new clues to a better understanding of the metal-to-semiconductor phase-transition phenomenon in VO2
Porphyrin bi-layer formation induced by a surface confined reduction on an iodine-modified Au(100) electrode surface
No full textA controlled electron transfer can be employed as a tool to drive the crafting of long-range self-assembled nanostructures of electro-active species on surfaces. In this work, we show this approach with the formation of an ordered bi-layer of tetra(N-methyl-4-pyridyl)-porphyrin molecules (TMPyP) on an iodine-modified Au(100) electrode studied by means of Cyclic Voltammetry (CV) and in situ Electrochemical Scanning Tunneling Microscopy (EC-STM). This bi-layer exists only towards negative electrode potentials between the first reduction potential of the adsorbed molecules and the desorption potential of the iodine. Starting a cathodic potential sweep at positive potentials, where the surface is covered with an adsorbed monolayer of TMPyP molecules which did not undergo any previous electrochemical reaction yet (Ref. Surfaces 2018, 1, 3–17), the formation of the ordered bi-layer coincides with the first reduction step of the adsorbed molecules; and it disappears again in a joint desorption process with the iodine. Reversal of the potential sweep leads to the opposite observations. Depending on the actual potential value as well as the scan direction up to five different competing ordered bi-layer phases can be identified. Synergistic effects between the modified substrate and the adsorbate are determined to be responsible of the corresponding structures, and at the same time surface-mediated catalytic effects shifting the first reduction step of the adsorbed molecules to a more positive potential in comparison to the reduction potential of the species from the bulk solution are observed. Moreover, based on detailed potentiodynamic STM measurements, a mechanism is also proposed to explain these observations
X-RAY PHOTOELECTRON-SPECTROSCOPY AND SCANNING ELECTRON-MICROSCOPY OF BETA-FESI2 FILMS GROWN BY ION-BEAM-ASSISTED DEPOSITION
No full textThis paper reports the investigation of polycrystalline beta-FeSi2 films grown by Ion Beam Assisted Deposition (IBAD), performed by using a broad Ar+ beam bombarding (001) Si substrates during the evaporation of Fe atoms. Several energies (200-650 eV) and current densities (10-70 muA cm-2) have been used for the Ar+ beam, keeping the Fe evaporation rate at about 0.08 nm s-1. The formation of the silicide was achieved by in situ thermal annealing at T = 600-degrees-C, performed during or after the deposition process. Rutherford Backscattering Spectroscopy (RBS), Scanning Electron Microscopy (SEM) and X-Ray Photoelectron Spectroscopy (XPS) have been used to study the stoichiometry, the morphology and the chemical status of several samples obtained using different ion beam parameters. Strong morphological improvements, such as smoothing and pinhole closing, are observed for the IBAD films grown during thermal annealing. The XPS depth profiling technique has been used to investigate the morphology of the silicide/Si interface
Preparation and electronic structure of the WSe2/graphene/NiSex/Ni(111) heterostructure
No full textIn this work, a stacked heterostructure made up of single-layer WSe2 and graphene was created through a scalable and efficient way. Graphene was grown on a Ni (111) single crystal, producing an ordered and well-defined carbon overlayer that is strongly hybridized with the support, disrupting its peculiar conductive properties. A monolayer WSe2 was deposited on top of graphene by the simultaneous evaporation of metal W and elemental Se. We demonstrate here that the chalcogen can efficiently intercalate between graphene and the Ni surface, decoupling the two materials and forming a buffering NiSex layer. The concurrent selenization of both W and Ni effectively eliminates the need for an additional decoupling step in the synthesis of a free-standing graphene/Ni heterostructure. The formation process of the complex WSe2/Graphene/NiSex/Ni(111) heterostructure was studied by means of low-energy electron diffraction, x-ray photoelectron spectroscopy, angle-resolved ultraviolet photoelectron spectroscopy, and ex situ Raman spectroscopy. These analyses confirm the presence of single-layer WSe2 on top of a free-standing graphene
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