55 research outputs found
CO chemisorption at vacancies of supported graphene films: a candidate for a sensor?
We investigate CO adsorption at single vacancies of graphene supported on Ni(111) and polycrystalline Cu. The borders of the vacancies are chemically inert but, on the reactive Ni(111) substrate, CO intercalation occurs. Adsorbed CO dissociates at 380 K, leading to carbide formation and mending of the vacancies, thus preventing their effectiveness in sensor applications
Influence of Defects and Heteroatoms on the Chemical Properties of Supported Graphene Layers
A large and growing number of theoretical papers report the possible role of defects and heteroatoms on the chemical properties of single-layer graphene. Indeed, they are expected to modify the electronic structure of the graphene film, allow for chemisorption of different species, and enable more effective functionalisation. Therefore, from theoretical studies, we get the suggestion that single and double vacancies, Stone–Wales defects and heteroatoms are suitable candidates to turn nearly chemically inert graphene into an active player in chemistry, catalysis, and sensoristics. Despite these encouraging premises, experimental proofs of an enhanced reactivity of defected/doped graphene are limited because experimental studies addressing adsorption on well-defined defects and heteroatoms in graphene layers are much less abundant than theoretical ones. In this paper, we review the state of the art of experimental findings on adsorption on graphene defects and heteroatoms, covering different topics such as the role of vacancies on adsorption of oxygen and carbon monoxide, the effect of the presence of N heteroatoms on adsorption and intercalation underneath graphene monolayers, and the role of defects in covalent functionalisation and defect-induced gas adsorption on graphene transistors
Ethylene adsorption on clean and oxygen covered flat and stepped Ag(001)
In the present paper we review our findings on ethylene adsorption on clean and oxygen covered Ag(001) surfaces investigated by dosing the gas with a Supersonic Molecular Beam and analysing the adsorption state either by High Resolution Electron Energy Loss Spectroscopy or by High Resolution X Rays Photoemission Spectroscopy. The final adsorption state depends on the translational and on the internal energy of the gas-phase molecules and on the presence of defects. At low translational energy ethylene either physisorbs or very weakly chemisorbs at flat terrace sites. The physisorption probability is thereby hindered by rotational excitation. A more strongly bound, pi bonded, state forms at higher translational energy, the activation barrier being related to the energy needed to form the relevant defect at which chemisorption takes place. A further even more strongly bound state forms only when dosing vibrationally excited molecules from the gas phase
Phonons in Thin Oxide Films
Thin oxide films have physical and chemical properties which may be significantly different from those of the corresponding bulk materials. For their complete characterization the information on the lattice dynamics which can be retrieved by vibrational spectroscopy is mandatory. Here we show that the number of observed phonon modes and their frequencies can indeed provide relevant information about stoichiometry, structure and thickness of the film
Coverage dependence of the sticking probability of ethylene on Ag(410)
The interaction of ethylene with Ag(410), a vicinal surface of Ag(100) characterised by a high density of open steps, is investigated. Energy and angle resolved measurements of the sticking coefficient are performed with the retarded reflector method of King and Wells using a supersonic molecular beam. We find that open steps remove the translational barrier for adsorption into the pi-bonded state present for Ag(100). Steering removes the angle dependence of the initial sticking probability. The coverage dependence of the adsorption probability indicates that a precursor mechanism is present and that adsorbate assisted adsorption is important at hyperthermal energies. With increasing coverage steering becomes ineffective and rotations inhibit adsorption. Admolecules with second nearest neighbours are destabilised and these sites can be populated only by gas-phase molecules impinging along trajectories for which the step heights are exposed
Influence of Defects on Adsorption\u2014Model Studies With Stepped Surfaces
In this article, we review the latest experimental results on the influence of surface defect in the interaction of different kinds of molecules with metal surfaces, using stepped surfaces as a model system. We will discuss both the adsorption of \u201csimple\u201d molecules of catalytic interest, such as H2, O2, CO, and NO at metal surfaces, and the organic\u2013inorganic interface. The latter is represented, on the one side, by polyaromatic molecules, relevant for nanoelectronics, and on the other side, by amino acids as representative of biological molecules. The substrates are chosen among \u201cnonreactive\u201d noble metals as well as among \u201creactive\u201d transition metals
Interaction of ethylene and oxygen with stepped Ag surfaces
The active role of defects in some catalytic reactions was predicted in the very early days of surface science. Most of the studies on gas adsorption at solid surfaces dealt, however, so far with nearly perfect low Miller index surfaces, which are rather unlike the active powders employed as catalysts in industrial reactors. The structure gap between the systems studied by surface scientists and the surface structure of real catalyst powders was therefore often indicated as the reason for the failure in reproducing some chemical reactions, which occur readily in industrial reactors, also in controlled conditions. Overcoming this limit without losing control over the experiment at the nanoscopic level is therefore an issue of pivotal importance, which could be addressed only now that the understanding of adsorption processes at flat surfaces is reasonably established. In this paper we shall review our most recent results on O-2 and C2H4 interaction with Ag(410) and Ag(210), which are vicinal surfaces of Ag(100) characterised by open (110)-like steps and narrow (100) terraces. The gases were dosed with a supersonic molecular beam, allowing to perform experiments at selected and well defined angles of incidence of the gas-phase particles. For both gases we find that the open steps affect gas-surface interaction considerably, changing the energy barriers to adsorption as well as the final chemisorption state
Surface plasmon dispersion on sputtered and nanostructured Ag(001)
The surface plasmon dispersion of ion bombarded Ag(001) is investigated by angle resolved high resolution electron energy loss spectroscopy. We find it to be parabolic and dominated by the quadratic term, contrary to the case of flat Ag(001) where the linear term is dominant. The case is reminiscent of surface plasmon dispersion on the oxygen covered, missing row reconstructed Ag(001) surface, for which case the change of the dispersion from nearly linear to quadratic was attributed to the removal of a surface plasmon decay channel associated to the surface interband transition at (X) over bar. We show moreover, that for the sputtered surface, slope and curvature of the dispersion depend on surface morphology at the nanoscale. When sputtering is performed at low crystal temperature and no ordered superstructure forms, the quadratic term coincides with the isotropic value dictated by bulk properties, reported for flat Ag(110) and Ag(111) and for reconstructed Ag(001). When on the contrary sputtering is performed at room temperature and the checkerboard superstructure develops, the quadratic term becomes twice as large
Dynamics of propene adsorption on Ag(001)
The interaction of propene with Ag(001) is investigated by high resolution electron energy loss spectroscopy and supersonic molecular beam methods under ultra high vacuum conditions. Propene adsorbs molecularly at 110 K and desorbs intact leaving a clean surface after annealing to 160 K. Two adsorption sites, characterized by slightly different vibrational modes, exist. The low frequency species is observed already at low coverage for molecules impinging at strongly hyperthermal energies while at lower translational energy it appears only at high coverage. The initial sticking probability S-0 decreases with increasing translational energy, as appropriate for nonactivated adsorption systems. The angle and energy dependence of S-0 indicate that scaling is intermediate between total and normal energy. From the coverage dependence of the sticking probability we infer that both a nonthermal intrinsic and a thermal extrinsic precursor exist
Supersonic Molecular Beams Studies of Surfaces
Supersonic Molecular Beams allow to select energy, angle of incidence and, to some extent, vibrational and rotational state of gas phase molecules. They are therefore employed for state resolved studies of the adsorption dynamics and of chemical reactions. Most recent developments regard the possibility to select the adsorption state as well as to affect chemical reactions by the rotational alignment of the reactants and to improve the quality of organic films, built by relatively large molecules, by using supersonic beams for the deposition process
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