CNRS Brittany Magnetism Laboratory

Brest, France

CNRS Brittany Magnetism Laboratory

Brest, France
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Lebon A.,CNRS Brittany Magnetism Laboratory | Garcia-Fuente A.,University of Valladolid | Vega A.,University of Valladolid | Aguilera-Granja F.,Autonomous University of San Luis Potosi
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We report a theoretical density functional theory study of the absorption and adsorption of hydrogen in Pd core/Pt shell cubo-octahedral nanoparticles of 55 and 147 atoms. We have explored all the possible sites available for hydrogen loading. The relative stability of H in the different sites is calculated and discussed in terms of the local geometrical and chemical environments. As a general trend, the most stable absorption sites are pyramidal interstices at the interface offering a volume for H around 2×2.60 3, with a moderate atomic rearrangement upon H insertion and in which the H atom maximizes the number of Pd neighbors. Our theoretical findings give support to the recent hydrogen pressure-composition isotherms and NMR measurements carried out in this type of bimetallic nanoparticle. © 2011 American Physical Society.


Lebon A.,CNRS Brittany Magnetism Laboratory | Garcia-Fuente A.,University of Valladolid | Vega A.,CNRS Brittany Magnetism Laboratory | Vega A.,University of Valladolid | Aguilera-Granja F.,Autonomous University of San Luis Potosi
Journal of Physical Chemistry C | Year: 2012

A theoretical study of hydrogen deposition and insertion in bimetallic Pd-Pt nanoalloys has been carried out in the framework of the density functional theory. Our model systems are 147-atom clusters of cuboctahedral shape and stoichiometries of Pd 1.00, Pd 0.91Pt 0.09, Pd 0.82Pt 0.18, and Pd 0.54Pt 0.46, resembling those recently produced and characterized in the context of H insertion (Kobayashi, H. et al. J. Am. Chem. Soc.2010, 132, 5576.). Adsorption and absorption energies have been computed for H in all stable positions for the different compositions. Absorption is found to improve for nanoalloys with a small Pt concentration of 8-20%. However, when the Pt content approaches 50%, the absorption capability worsens, even as compared with the pure Pd nanoparticles. These trends are fully consistent with the experimental data. The local geometrical and electronic environments of hydrogen in these alloy nanoparticles are explored in detail so as to understand the observed behavior. Concerning the H deposition, our results indicate that Pd-Pt nanoalloys with high Pt content might be efficient in regard to preferential oxidation reaction mediated by hydrogen. © 2011 American Chemical Society.


Rioual S.,CNRS Brittany Magnetism Laboratory | Lescop B.,CNRS Brittany Magnetism Laboratory | Quentel F.,University of Western Brittany | Gloaguen F.,University of Western Brittany
Physical Chemistry Chemical Physics | Year: 2015

An electrocatalytic material for the H2 evolution reaction (HER) in acidic aqueous solution has been prepared by electropolymerization of Co(ii) dibenzotetraaza[14] annulene (CoTAA). Chemical analysis by X-ray photoelectron spectroscopy (XPS) confirms that the structural integrity of the [CoII-N4] motif is preserved in the poly-CoTAA film. In acetate buffer solution at pH 4.6, an overpotential η = -0.57 V is required to attain a catalytic current density -ik = 1 mA cmgeom-2. The faradaic efficiency of poly-CoTAA for the HER is 90% over a period of one hour of electrolysis, but there is a decrease of the apparent concentration of Co sites after prolonged H2 production, which we ascribe to partial demetallation of the poly-CoTAA film at negative potentials. This journal is © the Owner Societies 2015.


Lebon A.,CNRS Brittany Magnetism Laboratory | Aguado A.,University of Valladolid | Vega A.,University of Valladolid
Physical Chemistry Chemical Physics | Year: 2015

We demonstrate, by means of fully unconstrained density functional theory calculations, that cluster Zn17 endohedrally doped with a Cr impurity can be qualified as a magnetic superalkali cluster. We explain the origin of its high stability, its low vertical ionization potential and its high total spin magnetic moment which amounts to 6 μB, exactly the same value as that of the isolated Cr atom. With the aim of exploring the possibility of designing a bistable magnetic nanoparticle, with a corresponding inter-unit exchange coupling, we also consider the assembling of two such units through different contact regions and in different magnetic configurations. Furthermore, we analyze up to which extent is the Zn shell able to preserve the electronic properties of the embedded Cr atom, both against coalescence of the two superatoms forming the magnetically bistable nanoparticle, and upon the adsorption of an O2 molecule or even under an oversaturated O2 atmosphere. Our results are discussed not only emphasizing the fundamental physical and chemical aspects, but also with an eye on the new prospects that those Cr@Zn17 magnetic superalkali clusters (and others of similar kind) may open in spintronics-, molecular electronics- or biomedical-applications. © 2015 Owner Societies.


Lebon A.,CNRS Brittany Magnetism Laboratory | Carrete J.,University of Santiago de Compostela | Longo R.C.,University of Santiago de Compostela | Vega A.,University of Valladolid | Gallego L.J.,University of Santiago de Compostela
International Journal of Hydrogen Energy | Year: 2013

We performed ab initio density-functional calculations to investigate the structural, electronic and magnetic properties of nanostructures comprising single-adatoms of Sc, Ti or V adsorbed on a hydrogen-passivated zigzag graphene nanoribbon (GNR). We also investigated the affinity of the resulting doped nanostructures for molecular hydrogen. In all cases, the most stable structures featured the adatom at positions near one of the edges of the GNR. However, whereas in the most stable structures of the systems Sc/GNR and V/GNR the adatom was located above a bay of the zigzag edge, Ti/GNR was found to be most stable when the adatom was at a first-row hole site. Adsorption at sites near one of the ribbon edges reduced drastically the average magnetic moment of the carbon atoms at that edge. On the other hand, the magnetic moments of the adatoms on the GNR, as the electronic character of the doped nanostructures, depended on the adsorption site and on the adatom species, but their absolute values were in all cases, except when Sc was at an edge bay site, greater than those of the corresponding free atoms. Our results showed that, of the three systems investigated in this paper, Ti/GNR (except when Ti is adsorbed at an edge bay site) and V/GNR appear to satisfy the criterion specified by the U. S. Department of Energy for efficient H2 storage, as far as binding energy is concerned. We discussed in detail the differences between the adsorption of H2 on the system Ti/GNR and the adsorption of H 2 on Ti-adsorbed carbon nanotubes, which have been proposed as a high-capacity hydrogen storage media. Copyright © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.


Lebon A.,CNRS Brittany Magnetism Laboratory | Aguado A.,University of Valladolid | Vega A.,University of Valladolid
Journal of Physical Chemistry C | Year: 2015

A systematic theoretical study of the doping of Zn17 nanoparticle with 3d transition-metal (TM) impurities is presented. Calculations are conducted within the density functional framework as implemented in the VASP code at the generalized gradient approximation for the exchange and correlation effects. The ground state of Zn17 has been recently shown to be a dihedral superatom hollow cage in a singlet state [ Angew. Chem., Int. Ed. 2015, 54, 2111 ]. We show that, among the 3d elements, endohedral doping of a quasi-undeformed cage results favorable only for Cr and Cu, which as free atoms display a very stable (half-filled and fully occupied, respectively) d shell electron configuration. This structural configuration allows to maximize the spin magnetic moment of the chromium-doped cage. Co and Ni also adopt an endohedral configuration, but with a strong deformation of the cage and a rather compact structure due to the large electron hybridization between the TM states and those of the Zn host. Doping with the other TM impurities results in an exohedral arrangement, also concomitant with a marked electron hybridization. Most of the TM impurities retain a large part of the spin magnetic moment that they have in a vacuum due to localization of part of their 3d states. The Zn atomic environment is found to be spin-polarized and modulates the total spin moment of these molecular magnets depending on the induced moments and magnetic couplings. © 2015 American Chemical Society.


Diler E.,CNRS Brittany Magnetism Laboratory | Diler E.,French Corrosion Institute | Rouvellou B.,CNRS Brittany Magnetism Laboratory | Rioual S.,CNRS Brittany Magnetism Laboratory | And 3 more authors.
Corrosion Science | Year: 2014

The corrosion behaviour of pure zinc and zinc-magnesium-aluminium alloy (ZMA) has been studied during 6months of exposure in marine environment (Brest, France). The composition of corrosion products is analysed using infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). An improved corrosion resistance of ZMA is observed. This improvement is found to be connected to Mg2+ and Al3+ induced quenching of corrosion activity and to the enhancement of NaZn4Cl(OH)6SO4·6H2O in the formed corrosion product. © 2014 Elsevier Ltd.


Diler E.,CNRS Brittany Magnetism Laboratory | Diler E.,French Corrosion Institute | Rioual S.,CNRS Brittany Magnetism Laboratory | Lescop B.,CNRS Brittany Magnetism Laboratory | And 2 more authors.
Thin Solid Films | Year: 2012

Zinc oxide (ZnO) is a chemical compound of great interest used, for example, as photocatalyst in the purification of wastewater or polluted air. However, neither dissolution, nor photo-dissolution of ZnO is negligible: indeed, both processes reduce significantly the efficiency of photocatalysis and then lead to a secondary pollution by free Zn 2 +. In the present study, the stability of ZnMgO thin films in weak alkaline solution is investigated. We demonstrate that the replacement of Zn 2 + ion with Mg 2 + ion results in the production of a Zn 0.84Mg 0.16O solid solution, whose stability is higher than that of the ZnO sample. This alloy, thus, constitutes an alternative to the use of ZnO in photocatalysis applications. To gain more insights into the higher resistance of such alloys to the dissolution process, X-Ray photoelectron spectroscopy measurements were performed. They highlighted the role of OH group adsorption in the experimentally observed enhancement of ZnMgO stability. © 2011 Elsevier B.V. All rights reserved.


Diler E.,CNRS Brittany Magnetism Laboratory | Diler E.,French Corrosion Institute | Rioual S.,CNRS Brittany Magnetism Laboratory | Lescop B.,CNRS Brittany Magnetism Laboratory | And 2 more authors.
Corrosion Science | Year: 2012

We present a corrosion behaviour study of pure phases of zinc and zinc-magnesium contaminated with NaCl and exposed to humid air for 30days: Zn, Mg 2Zn 11 and MgZn 2. The composition of corrosion products is analysed using infrared spectroscopy (FTIR), X-ray diffraction (XRD), ion chromatography (IC), and X-ray photoelectron spectroscopy (XPS). The improved corrosion stability of MgZn 2 is found to be connected to changes in the surface pH and to the nature of the formed corrosion products. The presence of magnesium modifies the proportion of the OH and CO 3 bonds in the corroded products. This explains the improvement in corrosion resistance. © 2012 Elsevier Ltd.


Diler E.,CNRS Brittany Magnetism Laboratory | Diler E.,French Corrosion Institute | Lescop B.,CNRS Brittany Magnetism Laboratory | Rioual S.,CNRS Brittany Magnetism Laboratory | And 3 more authors.
Corrosion Science | Year: 2014

A corrosion behaviour study of pure phases of zinc and zinc-magnesium (MgZn2) exposed to humid air at 35°C for 4days is presented. For zinc, hydroxide formation at the surface and zinc oxide in the first innerlayers is observed. The corrosion of the MgZn2 leads to the segregation of magnesium at the surface to mainly form magnesium hydroxycarbonate: the presence of magnesium modifies the corrosion products. At larger depths, metallic zinc coexists with magnesium oxide and hydrozincite. The higher reactivity of MgZn2 alloy can be attributed to the interaction with carbonate ions. © 2013 Elsevier Ltd.

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