6 research outputs found

    Atomic Structure and Special Reactivity Toward Methanol Oxidation of Vanadia Nanoclusters on TiO<sub>2</sub>(110)

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    We have grown highly controlled VO<sub><i>x</i></sub> nanoclusters on rutile TiO<sub>2</sub>(110). The combination of photoemission and photoelectron diffraction techniques based on synchrotron radiation with DFT calculations has allowed identifying these nanostructures as exotic V<sub>4</sub>O<sub>6</sub> nanoclusters, which hold vanadyl groups, even if vanadium oxidation state is formally +3. Our theoretical investigation also indicates that on the surface of titania, vanadia mononuclear species, with oxidation states ranging from +2 to +4, can be strongly stabilized by aggregation into tetramers that are characterized by a charge transfer to the titania substrate and a consequent decrease of the electron density in the vanadium 3d levels. We then performed temperature programmed desorption experiments using methanol as probe molecule to understand the impact of these unusual electronic and structural properties on the chemical reactivity, obtaining that the V<sub>4</sub>O<sub>6</sub> nanoclusters can selectively convert methanol to formaldehyde at an unprecedented low temperature (300 K)

    Computational Study of Amino Mediated Molecular Interaction Evidenced in N 1s NEXAFS: 1,4-Diaminobenzene on Au (111)

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    Primary amines can interact with neighbor molecules or with a metal substrate via weak bonds involving the electron lone pair of their amino functional group. Near edge X-ray absorption spectra (NEXAFS) on the N 1s edge show that the structure of the empty molecular orbitals localized on the nitrogen atom is very sensitive to these interactions. Here we investigate the origin of these changes by means of theoretical calculations. NEXAFS spectra are simulated for the 1,4-benzenediamine (BDA) molecule in its free, crystalline, and monolayer on Au(111) forms. We identify the electronic states which are affected by these amino-based interactions. In the case of the molecular layer grown on the gold substrate, we show how the results of the calculations can be used to identify intermolecular interactions influencing adsorption geometries in molecular monolayers

    Chemistry of the Methylamine Termination at a Gold Surface: From Autorecognition to Condensation

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    13The self-assembly of the naphthylmethylamine molecules (NMA) on the Au(111) surface is investigated by a combined experimental and theoretical approach. Three well-defined phases are observed upon different thermal treatments at the monolayer stage. The role played by the methylamine termination is evidenced in both the molecule–molecule and molecule–substrate interactions. The autorecognition process of the amino groups is identified as the driving factor for the formation of a complex hydrogen bonding scheme in small molecular clusters, possibly acting also as a precursor of a denitrogenation condensation process induced by thermal annealing.reservedmixedDri, Carlo; Fronzoni, Giovanna; Balducci, Gabriele; Furlan, Sara; Stener, Mauro; Feng, Zhijing; Comelli, Giovanni; Castellarin-Cudia, Carla; Cvetko, Dean; Kladnik, Gregor; Verdini, Alberto; Floreano, Luca; Cossaro, AlbanoDri, Carlo; Fronzoni, Giovanna; Balducci, Gabriele; Furlan, Sara; Stener, Mauro; Feng, Zhijing; Comelli, Giovanni; Castellarin Cudia, Carla; Cvetko, Dean; Kladnik, Gregor; Verdini, Alberto; Floreano, Luca; Cossaro, Alban

    Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization

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    We provide insight into surface-catalyzed dehalogenative polymerization, analyzing the organometallic intermediate and its evolution into planar polymeric structures. A combined study using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and first-principles calculations unveils the structural conformation of substrate-bound phenylene intermediates generated from 1,4-dibromobenzene precursors on Cu(110), showing the stabilizing role of the halogen. The appearance of covalently bonded conjugated structures is followed in real time by fast-XPS measurements (with an acquisition time of 2 s per spectrum and heating rate of 2 K/s), showing that the detaching of phenylene units from the copper substrate and subsequent polymerization occur upon annealing above 460 ± 10 K

    Intermolecular Hydrogen Bonding and Molecular Orbital Distortion in 4‑Hydroxycyanobenzene Investigated by X‑ray Spectroscopy

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    Electronic structure of 4-hydroxycyanobenzene in the gas phase, thick films, and single crystals has been investigated by X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). We have used resonant photoemission spectroscopy (RESPES) to identify the symmetry and atomic localization of the occupied and unoccupied molecular orbitals for the free molecule. Upon condensation into a thick film, we find XPS energy shifts in opposite directions for the oxygen and nitrogen core levels, consistent with the formation of an intermolecular hydrogen bond. This interaction is also accompanied by a significant spatial distortion of the lowest unoccupied molecular orbital that is displaced from the nitrogen atom, as indicated by the RESPES measurements. Thick films and single crystals display the same dichroism in polarization dependent NEXAFS, indicating that the intermolecular hydrogen bonding also steers the molecular assembly into a preferred molecular orientation

    Tailored Formation of N‑Doped Nanoarchitectures by Diffusion-Controlled on-Surface (Cyclo)Dehydrogenation of Heteroaromatics

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    Surface-assisted cyclodehydrogenation and dehydrogenative polymerization of polycyclic (hetero)aromatic hydrocarbons (PAH) are among the most important strategies for bottom-up assembly of new nanostructures from their molecular building blocks. Although diverse compounds have been formed in recent years using this methodology, a limited knowledge on the molecular machinery operating at the nanoscale has prevented a rational control of the reaction outcome. We show that the strength of the PAH–substrate interaction rules the competitive reaction pathways (cyclodehydrogenation <i>versus</i> dehydrogenative polymerization). By controlling the diffusion of N-heteroaromatic precursors, the on-surface dehydrogenation can lead to monomolecular triazafullerenes and diazahexabenzocoronenes (N-doped nanographene), to N-doped oligomeric or polymeric networks, or to carbonaceous monolayers. Governing the on-surface dehydrogenation process is a step forward toward the tailored fabrication of molecular 2D nanoarchitectures distinct from graphene and exhibiting new properties of fundamental and technological interest
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