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ABSTRACT: The ability of Mo (Cr) impurities in a CaO (MgO) matrix to act as charge donors to adsorbed gold has been investigated by means of scanning tunneling microscopy and density functional theory. Whereas CaO(Mo) features a robust donor characteristic, as deduced from a charge-transfer-driven crossover in the Au particles' geometry in the presence of dopants, MgO(Cr) is electrically inactive. The superior performance of the CaO(Mo) system is explained by the ability of the Mo ions to evolve from a +2 oxidation state in ideal CaO to a +5 state by transferring up to three electrons to the Au adislands. Cr ions in MgO, on the other hand, are stable only in the +2 and +3 charge states and can provide a single electron at best. Since this electron is likely to be captured by cationic vacancies or morphological defects in the real oxide, no charge transfer to Au particles takes place in this case. On the basis of our findings, we have developed general rules on how to optimize the electron donor characteristics of doped oxide materials.
Journal of the American Chemical Society 06/2012; 134(28):11380-3. · 9.91 Impact Factor
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ABSTRACT: Oxides surfaces and thin films are finding continuous new technological applications and represent an important class of systems
in materials science. Today we assist to a considerable effort to characterize the surfaces and the interfaces of oxide materials
at an atomistic level. The intense experimental activity in this field has stimulated a parallel computational activity based
on high-quality first principle calculations. In this review we focus our attention on the properties of oxide surfaces, and
we describe the main factors that contribute to determine their behaviour: (1) nature of the bonding and electronic structure
of the oxide; (2) surface morphology and defectivity; (3) doping and functionalization; (4) redox properties; (5) nano-dimensionality
(e.g. in ultra-thin films). We also show how each of these parameters can affect the properties of supported metal atoms and
nano-particles.
Theoretical Chemistry Accounts 04/2012; 117(5):827-845. · 2.16 Impact Factor
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ABSTRACT: The properties of MgO/Mo(100) ultrathin films where the Mg2+ cation is substituted with Li, Al, or Ni impurities have been investigated by means of density functional theory calculations. While on bare MgO the dopants affect the oxide electronic structure by creating holes or excess electrons, for supported ultrathin films the metallic support provides a reservoir to compensate charges via electron transfer through the oxide layer. The charge transfer has direct consequences on the work function (phi) of the combined oxide/metal system. As MgO/Mo(100) films are characterized by a very low work function, doping with monovalent Li atoms results in an increase of phi. On the contrary, a small reduction of phi can be obtained with low concentration of trivalent Al dopants. A more sizable reduction of phi is obtained for Al-doped MgO/Ag(100) films, where the work function of the undoped system is higher. In the case of Ni, the impurity atoms can assume different oxidation states, Ni2+, Ni+, and Ni-0, depending on the position within the film. For all configurations considered, substitutional Ni leads to an increase of the work function.
The Journal of Physical Chemistry C 01/2012; 116(9):5781-5786. · 4.80 Impact Factor
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Angewandte Chemie International Edition 11/2011; 50(48):11525-7. · 13.45 Impact Factor
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ABSTRACT: The structural and chemical properties of a monolayer FeO(111)/Pt(111) exposed to air, liquid water, and controlled atmospheres of water vapor and water vapor/oxygen mixtures have been studied by a combination of infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning tunneling microscopy experiments together with density functional calculations using ab initio thermodynamics. The FeO(111)/Pt(111) film is inert toward pure water vapor up to mbar pressure. Coadsorption of oxygen and water, however, transforms the film into a hydroxyl terminated trilayer with a (Pt−)O–Fe–OH structural motif. The trilayer film forms spontaneously upon contact of FeO(111)/Pt(111) with air and preserves long-range order even in liquid water. The calculated phase diagram in the relevant range of oxygen and water chemical potentials is in agreement with the experimental results and shows that the oxygen chemical potential is the main driving force for the formation of the trilayer film. Results for Au nucleation on and CO oxidation over the FeO(OH) film are presented and compared to previous studies on nonhydroxylated films.
09/2011;
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ABSTRACT: We all make use of oxide ultrathin films, even if we are unaware of doing so. They are essential components of many common devices, such as mobile phones and laptops. The films in these ubiquitous electronics are composed of silicon dioxide, an unsurpassed material in the design of transistors. But oxide films at the nanoscale (typically just 10 nm or less in thickness) are integral to many other applications. In some cases, they form under normal reactive conditions and confer new properties to a material: one example is the corrosion protection of stainless steel, which is the result of a passive film. A new generation of devices for energy production and communications technology, such as ferroelectric ultrathin film capacitors, tunneling magnetoresistance sensors, solar energy materials, solid oxide fuel cells, and many others, are being specifically designed to exploit the unusual properties afforded by reduced oxide thickness. Oxide ultrathin films also have tremendous potential in chemistry, representing a rich new source of catalytic materials. About 20 years ago, researchers began to prepare model systems of truly heterogeneous catalysts based on thin oxide layers grown on single crystals of metal. Only recently, however, was it realized that these systems may behave quite differently from their corresponding bulk oxides. One of the phenomena uncovered is the occurrence of a spontaneous charge transfer from the metal support to an adsorbed species through the thin insulating layer (or vice versa). The importance of this property is clear: conceptually, the activation and bond breaking of adsorbed molecules begin with precisely the same process, electron transfer into an antibonding orbital. But electron transfer can also be harnessed to make a supported metal particle more chemically active, increase its adhesion energy, or change its shape. Most importantly, the basic principles underlying electron transfer and other phenomena (such as structural flexibility, electronic modifications, and nanoporosity) are now largely understood, thus paving the way for the rational design of new catalytic systems based on oxide ultrathin films. Many of the mechanisms involved (electron tunneling, work function changes, defects engineering, and so forth) are typical of semiconductor physics and allow a direct link between the two fields. A related conceptual framework, the "electronic theory of catalysis", was proposed a long time ago but has been largely neglected by the catalytic community. A renewed appreciation of this catalytic framework, together with spectacular advances in modeling and electronic structure methods, now makes it possible to combine theory with advanced experimental setups and meet the challenge of designing new materials with tailored properties. In this Account, we discuss some of the recent advances with nanoscale oxide films, highlighting contributions from our laboratory. Once mastered, ultrathin oxide films on metals will provide vast and unforeseen opportunities in heterogeneous catalysis as well as in other fields of science and technology.
Accounts of Chemical Research 08/2011; 44(11):1244-52. · 21.64 Impact Factor
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Yu Lei,
Mikolaj Lewandowski,
Ying-Na Sun,
Yuichi Fujimori,
Yulija Martynova,
Irene M N Groot,
Randall J Meyer, Livia Giordano,
Gianfranco Pacchioni,
Jacek Goniakowski,
Claudine Noguera,
Shamil Shaikhutdinov,
Hans-Joachim Freund
ChemCatChem 04/2011; 3(4):671-674. · 5.21 Impact Factor
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Angewandte Chemie International Edition 03/2011; 50(11):2635-8. · 13.45 Impact Factor
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ABSTRACT: The nature of Ag(111)/TiO2 rutile and anatase interfaces, of interest for the design of memristors, has been studied by means of density functional theory calculations using various computational approaches. We have considered interfaces where the lattice mismatch of the oxide crystalline phase and the metal electrode does not result in excessive strain. The bonding at the interface is very weak, and the charge transfer is negligible for stoichiometric oxides. The formation of O vacancies has a lower cost at the interface with Ag than on the bare titiania surface and results in stronger adhesion between the Ag electrode and the reduced TiO2-x oxide. The diffusion of Ag and O atoms or ions across the interface is a thermodynamically unfavorable process which can occur only at high temperatures or under the effect of an external electric field. Once Ag atoms are incorporated in the bulk of TiO2 they can be stabilized in an interstitial site (more favorable) or a position substitutional to Ti. In both cases Ag is ionized and transfers the valence electron to the host crystal with formation of Ti3+ states. The Ag atoms remain positively charged, even when extended Ag chains are formed (nanofilaments). For an Ag filament inside TiO2 to exhibit conductive behavior, a higher density of Ag atoms is required, but this is hardly possible in the regular bulk crystalline lattice of TiO2 without inducing a structural breakdown.
Physical Review B 01/2011; 83(24). · 3.69 Impact Factor
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ABSTRACT: By means of scanning tunneling microscopy, Auger spectroscopy, and density-functional theory, we have identified a different strain-relaxation mechanism taking place in ultrathin CaO films grown on Mo(001). Whereas CaO films are amorphous at low growth temperature due to the substantial lattice mismatch with the support, a crystalline phase develops upon annealing to 1000 K. This phase is characterized by a sharp (2 x 2) pattern in low-energy electron diffraction and displays wide, atomically flat terraces in scanning tunneling microscopy images. The phase transition is initiated by the diffusion of Mo from the support into the film, where it replaces 25% of the Ca ions. The resulting rocksalt-type Ca(3)MoO(4) structure has a negligible lattice mismatch with the Mo(001), enabling the growth of extended, defect-free oxide patches. The oxidation of Mo atoms from the support provides the thermodynamic stimulus for the phase transition. For MgO films grown on the same Mo(001) surface, no relevant intermixing is revealed at the interface, most likely because of the smaller lattice mismatch between both systems.
Physical Review B 01/2011; 83(24). · 3.69 Impact Factor
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Angewandte Chemie International Edition 06/2010; 49(26):4418-21. · 13.45 Impact Factor
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ABSTRACT: Defect sites on oxide surfaces play a dominant role in surface chemistry. The direct atomistic study of these sites is important but very difficult. We have mimicked the adsorbate-defect interaction by a dynamic force microscope tip measuring the interaction with a color center (F(0)) on the MgO(001) surface. The experimental findings, complemented by density functional theory calculations, show a highly attractive adsorbate-defect interaction and a charge transfer at a critical distance.
ACS Nano 05/2010; 4(5):2510-4. · 10.77 Impact Factor
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ABSTRACT: The stabilization of single Fe atoms in the nanopores of an ultrathin silica film grown on Mo(112) is demonstrated with scanning tunneling microscopy (STM) and density functional theory (DFT). The Fe atoms are able to penetrate the openings in the oxide surface and adsorb in two different binding configurations at the metal-oxide interface. In the energetically preferred site, the Fe stays monomeric even at temperatures above 300 K. In the second configuration that is adopted in 10% of the cases, surface atoms can be attached to the subsurface species, resulting in the formation of Fe surface clusters. The interfacial Fe atoms preserve their magnetic moment, as shown by a distinct Kondo-like response in STM conductance spectra and DFT calculations.
ACS Nano 02/2010; 4(2):863-8. · 10.77 Impact Factor
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ABSTRACT: We present the results of supercell DFT calculations on the adsorption properties of Au atoms and small clusters (Au(n), n < or = 5) on a SiO(2)/Mo(112) thin film and on the same system modified by doping with Li atoms. The adsorbed Li atoms penetrate into the pores of the silica film and become stabilized at the interface where they donate one electron to the Mo metal. As a consequence, the work function of the Li-doped SiO(2)/Mo(112) film is reduced and results in modified adsorption properties. In fact, while on the undoped SiO(2)/Mo(112) film Au interacts only very weakly, on the Li-doped surface Au atoms and clusters bind with significant bond strengths. The calculations show that this is due to the occurrence of an electron transfer from the SiO(2)/Mo(112) interface to the adsorbed gold. The occurrence of the charge transfer is related to the work function of the support but also to the possibility for the silica film to undergo a strong polaronic distortion.
ChemPhysChem 12/2009; 11(2):412-8. · 3.41 Impact Factor
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ABSTRACT: The adsorption properties of Ni, Pd, Pt, Cu, Ag, and Au atoms on MgO ultrathin films deposited on Mo(100) or Ag(100) substrates are determined from first principle DFT calculations and compared with the corresponding adsorption characteristics on the bare MgO(100) surface. The three supports exhibit different behaviors. On MgO/Mo(100) thin films there is an electron transfer from the metal/oxide interface to all considered adatoms. In some cases, like for Pd, the charge transfer is small and the species remains atomic-like, while for other atoms like Cu, Ag, Au or Pt there is a net transfer of one electron with formation of full anions. This has dramatic consequences on the preferred adsorption sites, on the bond strength, and on the magnetic state of the adatoms: all these quantities are totally different from what is found for the same atoms on the bare MgO(100) surface. MgO/Ag(100) films have an intermediate behavior between that of MgO/Mo(100) thin films and of the MgO(100) surface: the charge transfer is found only for Au and, to a smaller extent, for Pt atoms. All other atoms considered keep essentially the same electronic structure and preferred adsorption site shown on MgO(100). The reasons for the different behavior are rationalized in terms of global properties of the metal/oxide interface, and in particular of its work function.
09/2009;
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ABSTRACT: We present a theoretical density-functional theory study on the deposition of metal atoms (Ir, Pd, Pt, Ag, and Au) on FeO(111) and MgO(111) monolayers supported on Pt(111). We show the existence of a strong coupling between the charge state of the adsorbed adatom and the local polaroniclike distortion of the oxide film, and we identify two qualitatively different adsorption modes in which the distortion either reinforces the rumpling of the supported oxide film (positively charged adsorbates) or reduces or even reverses the cation-anion stacking (negatively charged adsorbates). Thus, the adsorption mode is a response to the charge state of the adsorbate and is driven mainly by the capacity of adatoms to exchange electrons with the support.
Physical Review B 09/2009; 80(12). · 3.69 Impact Factor
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ABSTRACT: The possibility to modify the adsorption properties of a porous silica/Mo(112) film by controlling its work function has been studied by a combined STM and density-functional theory approach. While the original film is inert towards metal adsorption, Au atoms and clusters can be stabilized on the surface after Li doping. The Li atoms penetrate the topmost silica layer and bind as Li+ cations at the metal-oxide interface, thereby reducing the oxide work function. This induces a charge transfer into Au adatoms, which in turn enables strong Au-silica interaction mediated by a polaronic distortion of the oxide lattice.
Physical Review Letters 07/2009; 103(5):056801. · 7.37 Impact Factor
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ABSTRACT: The adsorption properties of CO molecules adsorbed on free and MgO supported Au clusters have been investigated by means of a first principles density functional theory (DFT) approach. We have focused our attention on the vibrational properties of CO adsorbed on gas-phase Au3−7, Au12, and Au22 cluster anions, and on Au3−5, Au12, and Au22 clusters deposited on MgO/Ag(001) films or on the bare MgO(001) surface. We also considered a full gold monolayer deposited on MgO/Ag(001). CO does not bind to the high-coordinated Au atoms of one- or two-dimensional (1D or 2D) gold clusters or islands but only to the low-coordinated atoms at the periphery of these structures. A red-shift of about 50−60 cm−1 is found in ω(CO) for gold clusters deposited on MgO/Ag(001) thin films compared to the same clusters deposited on bare MgO(001) or to the neutral gas-phase counterparts. This shift, due to the occurrence of a charge transfer from the metal support to the deposited gold cluster, is found only for small 1D or 2D clusters or, for 3D clusters, when the CO molecule is bound at the MgO/Au interface. On 3D gold clusters, CO adsorbs on the top layers with binding energies and vibrational frequencies typical of neutral supported gold particles. This shows that the charging effect observed on ultrathin MgO films is largely restricted to the gold layer at the interface with the oxide support.
06/2009;
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ABSTRACT: The adsorption properties of thin silica films on Mo(112) have been tailored by embedding single Pd atoms into the nanopores of the oxide material. The embedded Pd is able to anchor metal adatoms that would not bind to the inert silica surface otherwise. Several adsorption structures, e.g., Pd-Pd, Ag-Pd, and Au-Pd complexes, have been prepared in this way and analyzed with the STM and density functional theory. The binding strength of the different adatoms to the surface is determined by the number of electrons in their frontier orbitals, which introduce a repulsive interaction with the oxide electronic states and weaken the covalent bond to the Pd anchor.
Physical Review Letters 02/2009; 102(1):016102. · 7.37 Impact Factor
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Violeta Simic-Milosevic,
Markus Heyde,
Xiao Lin,
Thomas König,
Hans-Peter Rust,
Martin Sterrer,
Thomas Risse,
Niklas Nilius,
Hans-Joachim Freund, Livia Giordano,
Gianfranco Pacchioni
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ABSTRACT: Gold deposition onto ultrathin MgO films on Ag(001) results in the formation of linear Au clusters, as revealed from a combined scanning tunneling microscopy and density-functional theory study. The equilibrium structure of small Au clusters containing three to seven atoms is therefore different on thin film and bulk MgO(001) but also deviates from the shape of the respective gas-phase clusters. The peculiar one-dimensional growth regime of gold is stimulated by an electron transfer from the MgO/Ag interface to the deposited Au clusters, resulting in singly and doubly charged cluster anions. Only for larger atom numbers, the formation of compact two-dimensional clusters prevails on the MgO thin films.
Phys. Rev. B. 12/2008; 78(23).
