Mauro Sambi

Role and effective treatment of dispersive forces in materials: Polyethylene and graphite crystals as test cases.

A semiempirical addition of dispersive forces to conventional density functionals (DFT‐D) has been implemented into a pseudopotential plane‐wave code. Test calculations on the benzene dimer reproduced the results obtained by using localized basis set, provided that the latter are corrected for the basis set superposition error. By applying the DFT‐D/plane‐wave approach a substantial agreement with experiments is found for the structure and energetics of polyethylene and graphite, two typical solids that are badly described by standard local and semilocal density functionals.

Molecules–Oligomers–Nanowires–Graphene Nanoribbons: A Bottom-Up Stepwise On-Surface Covalent Synthesis Preserving Long-Range Order

We report on a stepwise on-surface polymerization reaction leading to oriented graphene nanoribbons on Au(111) as the final product. Starting from the precursor 4,4″-dibromo-p-terphenyl and using the Ullmann coupling reaction followed by dehydrogenation and C-C coupling, we have developed a fine-tuned, annealing-triggered on-surface polymerization that allows us to obtain an oriented nanomesh of graphene nanoribbons via two well-defined intermediate products, namely, p-phenylene oligomers with reduced length dispersion and ordered submicrometric molecular wires of poly(p-phenylene). A fine balance involving gold catalytic activity in the Ullmann coupling, appropriate on-surface molecular mobility, and favorable topochemical conditions provided by the used precursor leads to a high degree of long-range order that characterizes each step of the synthesis and is rarely observed for surface organic frameworks obtained via Ullmann coupling.

Tuning the catalytic activity of Ag(110)-supported Fe phthalocyanine in the oxygen reduction reaction.

A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule-surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.

Fullerene/Porphyrin Multicomponent Nanostructures on Ag(110): From Supramolecular Self-Assembly to Extended Copolymers

A novel two-step bottom-up approach to construct a 2D long-range ordered, covalently bonded fullerene/porphyrin binary nanostructure is presented: in the first place, reversible supramolecular interactions between C60 and 5,15-bis(4-aminophenyl)-10,20-diphenylporphyrin are exploited to obtain large domains of an ordered binary network, subsequently a reaction between fullerene molecules and the amino-groups residing on porphyrin units, triggered by thermal treatment, is used to freeze the supramolecular nanostructure with covalent bonds. The resulting nanostructure resists high temperature treatments as expected for an extended covalent network, whereas very similar fullerene/porphyrin nanostructures held together only by weak interactions are disrupted upon annealing at the same or at lower temperatures.

An angle-scanned photoelectron diffraction study on the surface relaxation of ZnO (0001)

Abstract An angle-scanned X-ray photoelectron diffraction (XPD) study of the polar Zn-terminated ZnO(0001) surface has been carried out in order to look for a hypothetical inward relaxation of the outermost Zn layer. From the comparison of the O 1s polar scan with single-scattering-cluster (SSC) simulations it is clearly evident that the surface is bulk terminated and any relaxation is ruled out. Moreover, the capability of the XPD technique in determining the surface polarity (atomic termination) by inspection of θ/φ 2D plots without the aid of theoretical simulations has been demonstrated.

Surface carboxylate species on Cu(100) studied by angle-scanned photoelectron diffraction and LCAO-LDF calculations

Chemisorption site geometries of formate and acetate species on Cu(100) have been studied by means of angle-scanned X-ray photoelectron diffraction (XPD) and first-principle quantum-mechanical calculations on a Cu60-formate cluster in the framework of local density functional theory (LDF). According to our LDF calculations, the short bridge site results to be more stable than the cross bridge one by about 20kcalmol. The reasons for the more effective interaction of the adsorbate in the short bridge site have been clearly outlined on the basis of the analysis of the LDP wavefunctions. LDF-optimized structural parameters have been used as a starting input in a series of single-scattering cluster spherical wave (SSC-SW) simulations of the O 1s XPD curves, which have experimentally confirmed the occupation of the short bridge site proposed by LDF calculations. The SSC-SW simulations have also outlined the role of the low-frequency-hindered rotational modes of the adsorbate (librations) and have furnished an estimate of the frequency (∼ 70 cm−1) of the hindered rotation on the axis perpendicular to the surface. Furthermore, as regards the formate species, the experimental data give evidence of a Cu-O distance of 1.95 A, in agreement with the LDF calculations and with angle-resolved photoemission fine structure (ARPEFS) literature results. An estimate of the OCO angle (129° ± 5°) has been obtained from polar scans for both formate and acetate species.

Multiple doping of graphene oxide foams and quantum dots: new switchable systems for oxygen reduction and water remediation

Single- and multi-boron, nitrogen, sulphur doped graphene oxide quantum dots and three-dimensional foams are synthesized by a simple and environmentally friendly electrochemical method. The electrochemical activity of these materials in the oxygen reduction reaction is investigated by cyclic voltammetry and rotating disk electrode measurements. The experimental data demonstrate that the reaction selectivity is controlled by the oxidation degree of the materials: as-prepared graphene oxide quantum dots, which present highly oxidized functional groups, follow a two-electron reduction pathway and produce hydrogen peroxide, whereas after a reduction treatment by NaBH4, the same materials favour a four-electron reduction of oxygen to water. The high selectivity and high efficiency of the graphene oxide quantum dots for the production of hydrogen peroxide can be efficiently used for water remediation applications (phenol decomposition).

Photoelectron diffraction study on the structure of a vanadium ultrathin film deposited at the TiO2(110) surface

Abstract This Letter reports an X-ray photoelectron diffraction study of ∼5 V monolayers deposited by electron beam evaporation at the TiO 2 (110) surface. Data clearly show that V grows as islands with a bcc structure having the [100] direction normal to the substrate surface. A detailed analysis of the V″2p and Ti″3p azimuthal curves indicates also an alignment of the [001] overlayer azimuth with the [1¯10] direction of the TiO 2 substrate. These results are consistent with an epitaxial growth of bcc V at the TiO 2 (110) surface where a double V surface unit cell is matched to the TiO 2 (110) rectangular cell.

Growth and structural characterisation of vanadium oxide ultrathin films on TiO2 (110)

Abstract The research activity of our group in the last few years has mainly been devoted to the study of ultrathin vanadium oxide films deposited on a (110)-oriented TiO 2 single crystal, in order to prepare systems which may be largely thought of as simplified models for the investigation of the structure/properties relationships in real world catalysts, sensing and optical devices. The main objective of our work consists of setting up reproducible synthesis routes for the deposition of vanadium oxide ultrathin films on TiO 2 (110), through strict control of the reaction parameters. The films obtained are then characterised from a chemical, electronic and structural point of view, and their properties are compared to those of their bulk-related phases. Results are presented concerning growth procedures and structural and electronic properties of vanadium oxide ultrathin films on titania, with a stoichiometry ranging from VO 2 , down to approximately VO. In particular, it will be shown that the oxidation product of metallic vanadium in an oxygen or water atmosphere (in the 10 −6 mbar range) retains the rutile lattice structure typical of stoichiometric VO 2 , despite the increasing degree of oxygen defectiveness and the electronic properties, very similar to those pertaining to bulk V 2 O 3 . The peculiar behaviour of vanadium oxide on titania demonstrates how important the epitaxial influence of the substrate is on the nature of the overlayer. These results could represent a good starting point to understand why vanadium oxides on TiO 2 show an enhanced catalytic activity and selectivity in many industrially relevant reactions.

Stereoselective Photopolymerization of Tetraphenylporphyrin Derivatives on Ag(110) at the Sub-Monolayer Level

We explore a photochemical approach to achieve an ordered polymeric structure at the sub-monolayer level on a metal substrate. In particular, a tetraphenylporphyrin derivative carrying para-amino-phenyl functional groups is used to obtain extended and highly ordered molecular wires on Ag(110). Scanning tunneling microscopy and density functional theory calculations reveal that porphyrin building blocks are joined through azo bridges, mainly as cis isomers. The observed highly stereoselective growth is the result of adsorbate/surface interactions, as indicated by X-ray photoelectron spectroscopy. At variance with previous studies, we tailor the formation of long-range ordered structures by the separate control of the surface molecular diffusion through sample heating, and of the reaction initiation through light absorption. This previously unreported approach shows that the photo-induced covalent stabilization of self-assembled molecular monolayers to obtain highly ordered surface covalent organic frameworks is viable by a careful choice of the precursors and reaction conditions.