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International audience; With the help of a DFT+U approach, we have analyzed the characteristics of a series of pure and mixed 3d transition metal oxide honeycomb monolayers of M2O3 and MM′O3 stoichiometries (M, M′ = Ti, V, Cr, and Fe) deposited on a metal substrate (Me = Ag, Au, Pt). We show that the substrate-induced structural polarization, interfacial electron transfer, and oxide-metal interaction strength display general trends which are governed by the offsets between the oxide band structure and the metal Fermi level. They are the strongest for the least electronegative cations (Ti and V) deposited on supports with the largest work functions (Au, Pt), where the depletion of purely 3d Ti and V states provokes an increase of the cation oxidation state. Mixing generally induces electron transfers from the least (Ti and V) to the most (Cr and Fe) electronegative cations. However, the systematic delocalization of the Ti2O3 d valence electrons to the metal substrates limits any significant mixing-induced electronic rearrangements to the V-based compounds only. We show that these electronic effects directly impact the energetics of cationic mixing and are responsible for a dramatic destabilization of the mixed TiFeO3 monolayers compared to the bulk ilmenite phase, while they stabilize ordered mixed VFeO3 films which have no bulk equivalent. Our findings give general guidelines on how oxide electronic, magnetic, and reactivity characteristics can be efficiently engineered by tuning the oxide stoichiometry and the metal substrate, of direct interest for modern technologies

Considering the importance of sub-monolayer transition metal oxides supported on another oxide in many industrial processes, with the help of a DFT + U approach, we provide information on the structural and electronic properties of pure M 2O3 and mixed MM′O3 oxide monolayers (M, M′ = Ti, V, Cr, Fe) supported on an α-Al2O3(0001) support. With their structure in the prolongation of the alumina corundum lattice, the monolayers have non-equivalent surface and interface cations, which leads to two different cation configurations in the mixed oxides. In all cases, the interfacial charge transfer is weak, but strong cation–cation electron redistributions may take place as in TiVO3, TiFeO3, VFeO3, and TiCrO3 in which actual redox processes lead to cation oxidation states different from the expected +3 value. We show that the tendency to mixing relies on the interplay between two very different driving forces. Cation–cation redox reactions, in most cases, strongly stabilise mixed configurations, but preference for a given cation position in the monolayer, because of surface energy reasons, may strengthen, weaken or even block the mixing tendency. By comparison with results obtained in bulk ilmenite, in free-standing monolayers and in MLs deposited on transition metal substrates, we evidence the flexibility of their electronic structure as a function of size, dimensionality and nature of support, as a lever to tune their properties for specific applications.

International audience; The Cr/α-Al2O3(0001) interface has been explored by X-ray photoemission spectroscopy, X-ray absorption spectroscopy (XAS) and ab initio first-principles calculations of core level shifts including final state effects. After an initial oxidation via a reaction with residual surface OH but no reduction of the alumina substrate, Cr grows in a metallic form without any chemical effect on the initially oxidized Cr. However, Cr metal lacks crystallinity. Long-range (reflection high energy electron diffraction) and short-range (XAS) order are hardly observed. Thus photoemission combined with atomistic simulations becomes a unique tool to explore the chemistry and environment at the Cr/alumina interface. Cr 2p, O 1s and Al 2s shifted components are all explained by the formation of moieties involving Cr3+ and/or Cr4+ and of metallic Cr0, which supports the previously found Cr buffer mechanism for poorly adhesive metals. Beyond the situation under study, the present data demonstrate the ability of a combined experimental and theoretical approach of core-level shifts to exhaustively describe the general case of disordered metal/oxide interfaces.

Transition metal oxide layers on metal surfaces often show superior catalytic performance as compared to conventional, i.e., metal-on-oxide, systems. In this work, we studied the CO oxidation reaction over FeO supported by Pt(1 1 1) and observed strong size effects on the reactivity. The monolayer islands below 5 nm in size showed a light-off temperature for CO2 production 200 K lower than for the 30 nm islands. In an attempt to rationalize the size effect, we have performed oxygen desorption experiments and have combined them with an extended DFT analysis to provide insight into the bonding of various oxygen species in such systems. The theoretical results on isolated FeO2 islands indicate a substantially lower stability of the boundary oxygen atoms as compared with the same islands embedded into an FeO layer in qualitative agreement with experiment showing that the smaller the island, the lower the desorption temperature. To the best of our knowledge, the results demonstrate the first example of a size effect in oxidation catalysis on two-dimensional systems.

International audience; Despite the importance of cuprous oxide for industrial applications, the precise structure of its most stable reconstructed surface remains unknown. Here, by a combination of highresolution STM measurements and simulations based on a DFT-HSE approach, we challenge the previous interpretations of the Cu 2 O(111) (√ 3× √ 3)R30 • surface reconstruction, based on the removal of 1/3 of the under-coordinated surface oxygen atoms. We show that these models do not satisfy the shamrock-type features found by STM, i.e. their position in the Cu-O ring centers, the orientation towards the surface O cus atoms and the topographic height in the STM images. Moreover, they are thermodynamically unstable with respect to moving the vacancy subsurface, a property not recognized before which seems specific to the Cu 2 O (111) surface. We propose a nanopyramidal model of the (√ 3× √ 3)R30 • surface reconstruction which is free from all these shortcomings. Here, the shamrock protrusions seen in STM are formed by three copper adatoms located at the center of the Cu-O rings and capped by an oxygen atom. This structure profoundly differs from existing models of the Cu 2 O(111) surface, and would change common perceptions on its reactivity.

The formation of an ultra-thin aluminum oxide film at Fe0.85Al0.15(110) surface (A2 random alloy) has been studied by a variety of surface sensitive techniques (X-ray photoemission, low-energy electron diffraction, surface X-ray diffraction and scanning tunneling microscopy) supplemented by ab initio atomistic simulations. Since iron is not oxidized in the used conditions, the study focused on the coupling between aluminum oxidation and segregation processes. Compared to the bare surface, whose average composition (Fe0.6Al0.4) is closer to the B2-CsCl structure over a ∼ 3 nm depth, the oxidation hardly affects the subsurface segregation of aluminum. All the structural and chemical fingerprints point to an oxide film similar to that found on NiAl(110). It is a bilayer (∼ 7.5A thick) with a composition close to Al10O13 and a large (18.8 × 10.7)A 2 nearly rectangular unit cell; an almost perfect match between substrate periodicity and the (1 × 2) oxide supercell is found. Nevertheless, microscopy reveals the presence of anti-phase domain boundaries. Measured Al 2p and O 1s core level shifts match calculated ones; their origin and the relative contributions of initial/final state effects are discussed. The ubiquity of the present oxide on different supports asks for the origin of its stability.

International audience; Formation of mixed compounds at interfaces between materials often has significant impact on the properties of the junction. With the goal of assessing if interfacial oxide mixing improves the adhesion of anti-corrosive zinc coatings on silica, we report an ab initio study of the structural, electronic, and adhesion characteristics of interfaces between zinc and zinc silicate. We show that, regardless the precise thermodynamic conditions, ZnO-rich zinc sili-cate interfaces are more stable than SiO 2-rich ones because their formation does not require breaking strong Si-O bonds. Moreover, we find that zinc adhesion at such ZnO-rich interfaces is by at least a factor two larger than that of the zinc/silica contact. Due to a dense network of interfacial Zn-O bonds, this improvement does not produce any unfavorable decohesion effects in the zinc deposit. The formation of an interfacial silicate layer enables the suppression of the weak zinc/silica interface, which is responsible for the poor zinc adhesion. The cohesive cleavage within zinc predicted for the complex zinc/silicate/silica junction offers a promising improvement of performances of anti-corrosive zinc coatings of Si-rich steel grades, of crucial importance in steel-making and automotive industries.

International audience; The understanding of interfacial effects and adhesion at oxide-metal contacts is of key importance in modern technology. Metal-silica interfaces specifically are relevant in electronics, catalysis and nanotechnology. However, adhesion at these interfaces is hindered by a formation of siloxane rings on the silica surface which saturate the dangling bonds at stoichiometric terminations. In this context, we report a thorough density functional theory study of the interaction between β-cristobalite and selected 3d transition metals under different oxygen conditions. For any given interface stoichiometry, we find a progressive decrease of the metal/silica interaction along the series, following the increase of metal electronegativity. Crucially, in presence of early transition metals (Ti or Cr) the surface siloxane rings are spontaneously broken, allowing for strong adhesion. Late transition metals interact only weakly with reconstructed surfaces, similarly to what was found for zinc. In absence of reconstruction, stoichiometric silica/metal contacts behave similarly to alumina/metal contacts, but display larger interactions across the interface. Based on these results, we show that early transition metal or stainless steel buffers can significantly improve the weak adhesion between silica and zinc, responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels.

International audience; The formation of ultrathin copper oxide layers on an Au(111) surface is explored with scanning tunneling microscopy and density functional theory. Depending on the thermal treatment of as-grown Cu-O samples, a variety of thin-film morphologies is observed. Whereas 1D oxide stripes with Au[11 % 2] and Au[1 % 10] orientation emerge at 450 and 550 K annealing, respectively, a planar (2 2) Cu-O network with specific domain structure develops at higher temperature. The latter is ascribed to a Cu 3 O 2 honeycomb lattice with oxygen ions alternatingly located in surface and interface positions. Strain minimization and a thermodynamic preference for Cu-rich edges lead to the formation of structurally well-defined boundaries, delimiting either triangular, elongated or stripe-like Cu 3 O 2 domains. The low-temperature phases compirse complex arrangements of hexagonal and square Cu-O units, similar to those found in Cu 2 O(111) and (100) surfaces, respectively. The transitions between different thin-film phases are driven by Cu dissolution in the gold crystal and O 2 evaporation and therefore accompanied by a thinning of the oxide layer with increasing temperature.

International audience; The weak interaction between zinc and silica is responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels which are fundamental in the automotive industry, and important for rail transport, shipbuilding, or aerospace. With the goal of identifying possible levers for its improvement, we report an ab initio study of the effect of surface hydroxylation on the adhesion characteristics of model zinc/β-cristobalite interfaces, representative for various surface hydroxylation/hydrogenation conditions. We show that surface silanols resulting from dissociative water adsorption at the most stable stoichiometric (001) and (111) surfaces prevent strong zinc-silica interaction. However, dehydrogenation of such interfaces produces oxygen-rich zinc/silica contacts with excellent adhesion characteristics. They are due to a partial zinc oxidation and the formation of strong iono-covalent Zn-O bonds between zinc atoms and the under-coordinated excess anions, remnant of the hydroxylation layer. Interestingly, these interfaces appear as the most thermodynamically stable in a wide range of realistic oxygen-rich and hydrogen-lean environments. We also point out that the partial oxidation of zinc atoms in direct contact with the oxide substrate may somewhat weaken the cohesion in the zinc deposit itself. This fundamental analysis of the microscopic mechanisms responsible for the improved zinc wetting on pre-hydroxylated silica substrates furnishes useful guidelines towards practical attempts to improve adhesion.

International audience; The control of adhesion at metal/oxide interfaces is of key importance in modern applications, whenever three-dimensional metal clusters or two-dimensional metal overlayers are to be synthesized on an oxide support. By focusing on the zinc/alumina system, we address here one of the long-standing issues in this context, which is the poor wetting of wide band-gap oxides by noble and post-transition metals. It has recently been recognized to have detrimental industrial consequences for the adhesion of anti-corrosive zinc coatings to new high strength steels grades. We have combined photoemission, thermal desorption and plasmonics with atomistic simulation to describe the energetics of zinc deposits on dry and hydroxylated α-Al2O3(0001) surfaces. Both experimental and computational results show that an activated reaction of the metal with the OH-covered surface, followed by hydrogen desorption, produces dispersed interfacial moieties involving both oxidized Zn species and under-coordinated oxygen ions, that lead to a significant improvement of adsorption/adhesion characteristics on the hydroxylated surface. In particular, the key role of interfacial under-coordinated anions, remnants of the hydroxylation layer, is highlighted for the first time, pointing to a general mechanism by which surface hydroxylation appears as a promising route towards a systematic improvement of wide band gap oxide wetting by metals.

We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1–4 ML thickness range.

International audience; We address herein the question of the termination of the Al-terminated α-Al 2 O 3 (0001) surface. Over decades, various analyses made by different groups repeatedly suggested the presence of a residual coverage of surface OH groups on Al 2 O 3 crystals and powders after annealing in vacuum. However, other authors came to contrary conclusions, thus maintaining a persistent blur on the issue. The present work examines the Cr/alumina interface via Cr K absorption edge analysis (Extended X-Ray Absorption Fine Structure and X-ray Absorption Near Edge Structure) and photoemission with the support of density functional (DFT) calculations. Experiments support the presence of surface OH groups to account for the observed environment as well as the oxidation state of Cr adatoms (Cr 3+). Following a comprehensive DFT-based analysis of Cr z+-O n H m configurations (z=0 to 6, n and m=0 to 3), the Cr 3+-O 2 H alumina-supported surface moieties are found to successfully fit the XAS edge calculations. Most importantly, the combination of experiment and theory that is developed unambiguously demonstrates the presence of surface OH groups on α-Al 2 O 3 (0001) after annealing in vacuum.

International audience; Silicate compounds are ubiquitous in nature and display a vast variety of structures and properties. Thin silicate films may also form under specific conditions at interfaces between metals and silica. In the present study, we focus on zinc silicate and present a thorough density functional theory-based study of polar and non-polar (001) surfaces of various stoichiometry of its tetrago-nal polymorph t-Zn 2 SiO 4. At the non-polar surfaces, the main features are the existence of the chain reconstruction at the ZnO termination, and the presence of unsaturated surface silanols at the SiO 2 termination. Stabilization of polar surfaces is provided by the formation of O 2− 2 peroxo groups, reduction of the surface or subsurface Si atoms or formation of Zn 2+ 2 groups, depending upon the surface stoichiometry. While the non-polar stoichiometric and ZnO rich terminations are the most stable in a large part of the accessible phase diagram, the SiO 2 termination is less stable due to the absence of siloxane group formation. We show that, while bulk Zn 2 SiO 4 is stable with respect to decomposition into the ZnO and SiO 2 oxides, the same is not true for ultra-thin films due to the fundamental difference of silicate and silica surface energies. Preliminary results show that a similar conclusion could be drawn Fe 2 SiO 4. This study opens towards a deeper understanding and possible improvement of zinc adhesion at silica surfaces, of crucial industrial importance.

International audience; PACS numbers : 61.46.-w, 68.55.-a, 71.15.Mb, 78.70.Dm The growth mode of strained epitaxial films relies on the interaction strength, the lattice matching and the mechanical response of the system. The present work focuses on the basic physics of supported nano-islands by examining the characteristics of MgO/Ag(100) taken as a case study. The combination of experiments and simulations highlights the existence of a small size regime in which, despite the largest adhesion and the smallest mismatch, the islands are the least distorted by the substrate. We assign this unexpected behavior to the enhanced island stiffness which makes the cost of elastic distortion prohibitive compared to the associated gain of MgO-Ag interaction energy. The analysis provides a general framework to predict and/or understand nanoscale effects on interfacial pseudomorphism. These are likely to hold whatever the nature of the deposit and substrate under consideration. It may have far reaching consequences on many properties of supported nano-objects.

International audience; Two-dimensional materials with a honeycomb lattice, such as graphene and hexagonal boron nitride, often contain local defects in which the hexagonal elements are replaced by four, five, seven, and eight-membered rings. An example is the Stone-Wales (S-W) defect, where a bond rotation causes four hexagons to be transformed into a cluster of two pentagons and two heptagons. A further series of similar defects incorporating divacancies results in larger structures of non-hexagonal elements. In this paper, we use scanning tunneling microscopy (STM) and density functional theory (DFT) modeling to investigate the structure and energetics of S-W and divacancy defects in a honeycomb (2 × 2) Ti 2 O 3 monolayer grown on an Au(111) substrate. The epitaxial rumpled Ti 2 O 3 monolayer is pseudomorphic and in a state of elastic compression. As a consequence, divacancy defects, which induce tension in freestanding films, relieve the compression in the epitaxial Ti 2 O 3 monolayer and therefore have significantly 2 lower energies when compared with their freestanding counterparts. We find that at the divacancy defect sites there is a local reduction of the charge transfer between the film and the substrate, the rumpling is reduced, and the film has an increased separation from the substrate. Our results demonstrate the capacity of the substrate to significantly influence the energetics, and hence favor vacancy-type defects, in compressively strained 2D materials. This approach could be applied more broadly, for example to tensile monolayers, where vacancy-type defects would be rare and interstitial-type defects might be favored.

International audience; The structural quality of oxide/oxide interfaces, their chemistry, and the nature of their electronic states are critical properties for new promising applications and require continuous advances in their fundamental understanding. To this goal, we have performed a theoretical study of a series of (0001) M2O3/M’2O3 interfaces (M, M’ = Al, Ti, V, Cr, Fe) between simple and transition metal oxides crystallizing in the corundum structure. Our DFT+U results reveal two qualitatively different mechanisms of interface charge redistribution: an interfacial oxidation–reduction reaction occurring at Ti2O3/V2O3 and Ti2O3/Fe2O3 contacts, and a much weaker electron transfer along anion-cation bonds which follows the difference of ionicity between the two constituting oxides at all interfaces. At variance with interfaces between sp semiconductors, the band bending does not propagate beyond one or two atomic layers. More generally, our results show that the nature of the interface electronic structure and the energetics of its formation can be tightly linked to the properties of the corresponding bulk oxides, thus providing a precious tool for designing oxide/oxide interfaces of required characteristics.

International audience; We have used a DFT + U approach to study the characteristics of a series of 3d transition-metal oxide monolayers (MLs) of M 2 O 3 stoichiometry (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) in freestanding and Au(111)-supported honeycomb structures. They include), and purely charge-transfer (Sc 2 O 3) oxides. We have found that besides a structural polarization, the interaction with the Au substrate induces an interfacial electron transfer and considerably modifies the electronic characteristics of the monolayers. In particular, in Ti 2 O 3 , V 2 O 3 , and Ni 2 O 3 , due to the purely d character of the relevant gap edges, these modifications may be rationalized in terms of a change of the cation oxidation state with respect to the corresponding bulk and freestanding ML reference. We propose a general conceptual framework that links the properties of the separate systems (band offsets and character of the states at the gap edges), the value of the interfacial electron transfer, and the electronic and energetic characteristics of the supported MLs. Our results and analysis furnish guidelines toward the tuning of electronic and magnetic characteristics beyond those imposed by the oxide stoichiometry, of direct interest for modern technologies.

International audience; With the advent of new steel grades, the galvanic protection by zinc coating faces a new paradigm. Indeed, enrichment in strengthening elements prone to oxidation such as Al, Mn, and Si, leads to the formation of oxide films that are poorly wet by zinc. We study herein routes for improvement of adhesion at the model Zn/α-Al 2 O 3 interface by metallic additions. As a first step, with help of ab initio results on adsorption characteristics of transition metal adatoms at α-alumina surfaces, we establish and rationalize clear trends in the behavior of both metal-alumina interaction strength and in the relative thermodynamic stability of configurations with weakly and strongly bound metal adatoms. The reasons of enhanced binding strength of transition metals, such as Cr, maintained regardless the precise alumina termination and surface charge state are pointed out. On these grounds, possible improvements of adhesion in realistic conditions are discussed. It is predicted that an enrichment in transition metals, such as Cr, may produce strongly adhesive interfaces that lead to cohesive cleavage.

International audience; The growing interest of modern technologies in oxide materials and the advances in the fabrication and control of ultra-thin oxide films raise new questions on how to use cation doping or mixing to engineer the properties of such two dimensional objects. With a special focus on their intrinsic properties in the absence of interaction with a substrate, we have modeled a series of free-standing pure M 2 O 3 and mixed MM'O 3 compounds (M, M' = Ti, V, Cr, Fe) in two-dimensional honeycomb structures within DFT+U and hybrid approaches. A systematic comparison with their bulk counterparts enabled the identification and discussion of their properties which are driven by their low dimensionality and peculiar atomic structure. On the one hand, it concerns the mixing-induced changes of cation oxidation states, which are controlled by the structure-dependent characteristics of their band edges. On the other hand, an overall increase of the energetic tendency for cationic mixing in low dimensions is highlighted. This comparison shows that low dimensionality enables stabilization of ternary oxides of compositions and properties with no bulk equivalents, thus of a direct interest for modern technologies.

The growing interest of modern technologies in oxide materials is in part due to the possibility of their doping and/or mixing, which often leads to artificial compounds with tunable properties, such as the gap width or the band edge positions. However, engineering of mixed oxide materials requires a better fundamental understanding of their structural, electronic, and ordering properties, well beyond low doping levels, and with reference to their parent materials. In the present work, a series of mixed $M{M}^{\ensuremath{'}}{\mathrm{O}}_{3}$ oxides $(M,{M}^{\ensuremath{'}}=\text{Al},\text{Ti},\text{V},\text{Cr},\text{Fe})$ in three corundum-type structures---ilmenite, ${\mathrm{LiNbO}}_{3}$, and C---is studied by means of first-principles DFT+$U$ simulations. We find that regardless of the precise atomic structure, the local structural and electronic characteristics of most of the compounds are very close to those of their parent corundum ${M}_{2}{\mathrm{O}}_{3}$ oxides. The two noticeable exceptions are ${\mathrm{TiVO}}_{3}$ and ${\mathrm{TiFeO}}_{3}$, for which the structural, electronic, and magnetic characteristics are consistent with a mixing-induced change of the cation oxidation states. We show that this actual oxidoreduction process can be rationalized by analyzing the relative band edge positions of the parent oxides. The formation energies of these mixed oxides correlate well with the experimental evidence and, within a mean-field approximation, allow predicting the thermodynamics of solid solutions ${M}_{2\ensuremath{-}2x}{M}_{2x}^{\ensuremath{'}}{\mathrm{O}}_{3}$ at finite temperature.

International audience; The weak interaction between zinc and silica is responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels. With the goal of identifying its microscopic origin, we report an extensive ab initio study of the structural, electronic, and adhesion characteristics of a variety of model zinc|β-cristobalite interfaces, representative for different oxidation conditions. We show that the weakness of the zinc-silica interaction at non polar interfaces is driven by the presence of surface siloxane rings. These latter are drastically detrimental to interface adhesion when intact and their breaking is impeded by a large energy barrier. Conversely, the characteristics of polar interfaces are principally driven by the capacity of zinc to screen the surface compensating charges and to form O-Zn bonds. This screening is especially efficient in oxygen-rich environment where the substrate-induced partial oxidation of the zinc deposit produces a considerable enhancement of interface adhesion. The identified microscopic mechanisms of interface interaction furnish precious guidelines towards practical attempts to improve adhesion. In particular, processes which enable breaking the surface siloxane rings are expected to noticeably reinforce the interaction at non-polar interfaces.

International audience; Despite the application potential of nickel tungstate (NiWO4) in heterogeneous catalysis, humidity and gas sensing, etc, its surfaces have essentially remained unexplored. In this work, NiWO4 nanoparticles and films with the wolframite structure have been grown via a solid-state reaction of (WO3)3 clusters and a NiO(100) film on a Ni(110) crystal surface and characterized by a variety of experimental techniques, including x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and x-ray diffraction (XRD), combined with ab-initio density functional theory (DFT) calculations. NiWO4 grows initially as three-dimensional (3D) crystalline nanoparticles displaying mainly two crystalline facets vicinal to the (100) surface, which merge with increasing the (WO3)3 coverage into a quasi-continuous epitaxial film. The DFT results provide an account of the energetics of NiWO4 low index surfaces and highlight the role of faceting in the stabilization of extended polar (100) terraces. These combined experimental and theoretical results show that interaction with a metal substrate and vertical confinement may stabilize oxide nano-objects with high energy facets, able to enhance their reactivity.

International audience; We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1-4 ML thickness range.

International audience; Scanning tunneling microscopy (STM) and hybrid density functional theory (DFT) have been used to study the stability and electronic characteristics of the Cu 2 O(111) surface. We challenge previous interpretations of its structure and composition and show that only appropriate (hybrid) calculations can correctly account for the relative thermodynamic stability of stoichiometric versus Cu-deficient terminations. Our theoretical finding of the stoichiometric surface to be most stable at oxygen-lean conditions is confirmed by an excellent matching between STM spectros-copy data and the calculated surface electronic structure. Beyond the specific case of the Cu 2 O(111) surface, and beyond the known deficiencies of GGA-based approaches in the description of oxide electronic structures, our work highlights the risk of an erroneous evaluation of the surface stability, in cases where the energetics and electronic characteristics are strongly coupled, as in a wide class of polar and/or non-stoichiometric oxide surfaces.

International audience; We review the manifestations of electrostatic interactions, and in particular polarity effects, in oxide ultra-thin films and two-dimensional nano-objects. We discuss the efficiency of various compensation mechanisms such as overall structural transformations, strong lattice relaxations, inhomogeneous charge redistributions, support effects, and/or the formation of 2D or 1D electron/hole gases.

International audience; In view of improving the performance of anti-corrosive zinc coatings, we report a first principles DFT study on the effect of a metal buffer on the adhesion characteristics at the weakly interacting alumina/zinc interface. With results obtained for metals across the first transition series (M = Cr, Fe, Ni) we show that such buffers may enhance considerably the interface strength, the effect being particularly well pronounced for Cr. Moreover, relying on a series of model MO x oxide buffers (x= 1, 3/2, 2), we demonstrate that buffer oxidation is in most cases detrimental to adhesion. Systematic analysis of the interfacial bonds enables to ascribe the predicted beneficial effect of metallic buffers to the formation of strong interfacial metal-oxygen and metal-zinc bonds. Reduction of the number of such bonds upon buffer oxidation drives the decrease of interface adhesion. If the oxidation of buffers composed of more reactive metals cannot be avoided, late transition elements may be more promising candidates for practical applications.

Thickness matters: Ultrathin oxide films on metals can greatly enhance catalytic activity, for example, in CO oxidation on an FeO(111) film grown on a Pt(111) substrate. Under the reaction conditions, the bilayer FeO film restructures to form a trilayer OFeO film (see picture). Experimental evidence for the structure/morphology of the film and theoretical modeling of the mechanism of its formation and CO oxidation on its surface are presented. (Figure Presented) © 2010 Wiley-VCH Verlag GmbH & Co. KGaA Weinhelm.

The structural and magnetic properties of ultrathin FeO(111) films on Pt(111) with thicknesses from 1 to 16 monolayers (ML) were studied using the nuclear inelastic scattering (NIS) of synchrotron radiation. Distinct evolution of vibrational characteristics with thickness that is revealed in the phonon density of states (PDOS) witnesses a textbook transition from 2D to 3D lattice dynamics. For the thinnest films of 1 and 2 ML, the low energy part of the PDOS followed a linear dependence in energy that is characteristic for 2-dimensional systems. This dependence gradually transforms with thickness to the bulk ~E-square relation. Density functional theory phonon calculations perfectly reproduced the measured 1 ML PDOS within a simple model of a pseudomorphic FeO/Pt(111) interface. The calculations show that the 2D PDOS character is due to a weak coupling of the FeO film to the Pt(111) substrate. The evolution of the vibrational properties with an increasing thickness is closely related to a transient long range magnetic order and stabilization of an unusual structural phase.

We studied CO oxidation on FeO(1 1 1) films on Pt(1 1 1) at submonolayer oxide coverages at ultrahigh vacuum and near-atmospheric pressure conditions. The FeO(1 1 1) bilayer islands are inert towards CO2 formation. In contrast, the FeO2-x trilayer structure shows substantial CO2 production that reaches a maximum at ≈40 % coverage at both pressure conditions. The results provide compelling evidence that the FeO2-x/Pt(1 1 1) interface is the most active in CO oxidation. Although FeO2-x boundaries possesses weakly bound oxygen species, strong binding of CO to Pt favors the reaction at the FeO2-x/Pt interface as compared to the FeO2-x/FeO one, thus giving a rationale to the reactivity enhancement observed in systems exposing metal/oxide boundaries. In addition, oxygen diffusion from the interior of an FeO2-x island to the active edge sites may be effective for the oxygen replenishment in the CO oxidation catalytic cycle.

Relying on HSE03, ${\mathrm{G}}_{0}{\mathrm{W}}_{0}$, and random phase $\mathrm{approximation}+{\mathrm{G}}_{0}{\mathrm{W}}_{0}$ approaches, this study provides a complete characterization of the electronic properties of six ZnO bulk polymorphs, including quasiparticle spectra, absorption, and electron energy-loss spectra. It focuses on the series zinc blende, wurtzite, body-centered tetragonal, sodalite, and cubane structures, along which atoms have similar local tetrahedral environments and decreasing atomic density, to which the hexagonal boron nitride structure was added. All trends in the electronic properties---gap opening, blueshift of low-energy interband transitions, redshift of the main absorption peak, the frequency at which the real part of the dielectric tensor vanishes, and the plasmon energy---were interpreted in terms of the two main effects resulting from the decrease of the atomic density along the series: a bandwidth narrowing close to the gap edge and a decrease (in absolute value) of the electrostatic potential acting on the zinc and oxygen ions. This work gives firm grounds to help and foster spectroscopic experiments on ZnO polymorphs. It represents a preliminary step towards the study of thin-film properties.

International audience; Compact CeO2(111) films grown on Ru(0001) can be transformed into well-shaped nanoparticles by annealing them in an oxygen-poor environment. With increasing temperature, the particles undergo a distinct shape evolution that finally leads to crystallites exposing wide (100) facets. The atomic structure of the (100) termination is determined with a combination of high-resolution scanning tunneling microscopy and density functional theory. Two surface reconstructions are identified that are compatible with the need to compensate for the intrinsic dipole of the (100) plane and with a substantial reduction of the oxide material. Our study provides insights into the rarely explored (100) surface of ceria, which can be considered as model system for studying chemical processes on the polar termination of reducible oxides.

International audience; We report DFT+U results on oxygen adsorption and extraction characteristics obtained for a realistic model of a strong metal-support interaction catalyst, exemplified by a Pt(111)-supported FeOx ultra-thin film. Derived from recent experimental results, the model enables a coherent analysis of the activity towards oxygen at a large variety of sites, which range from FeO and FeO2 oxide terraces to different oxide-metal and oxide-oxide boundaries. We show that the trends in calculated adsorption and desorption energetics are to a large extent specific to the small thickness of the supported oxides and are mainly driven by electron exchange with the underlying metal substrate. The thorough mapping of the activity of different local surface environments enables the identification of the sites that are most likely responsible for the observed low-temperature activity of FeOx/Pt films in CO oxidation. Beyond the oxide-metal boundaries known for their higher activity than the separate materials, we find that the sites at boundaries between the coexisting oxide phases of different compositions can easily adsorb and release oxygen.

International audience; We report a first principles study on the properties of Pt(111)-supported FeO(111) monolayer. We confront results issued from PBE+U and HSE06 approximations, and analyze the impact of the more accurate hybrid description of the electronic structure of the metal/oxide interface on a large variety of calculated characteristics of this system. In particular, we analyze the behavior of its work function and its consequences on the spontaneous charging of adsorbed Au adatoms. We also consider the FeO2 nano-oxide phase and its peculiar oxygen storage characteristics, responsible for the unusual catalytic properties of FeOx/Pt system. We show that while the hybrid approximation does indeed substantially improve the electronic characteristics of iron oxide, of individual Au adatoms, or oxygen molecules, its overall impact on the calculated properties of the composed FeO/Pt system is very small. We assign this to the relatively small effect of the hybrid approximation on the band structure alignment. This shows that the less computationally demanding DFT+U approximation remains a fully adequate tool in theoretical studies on this kind of systems. This is particularly important for calculations on realistic systems, with large-size reconstructions induced by the lattice mismatch at the interface between the two materials. (C) 2014 AIP Publishing LLC.

International audience; Strong electrostatic coupling between the local structure of metal-supported oxide ultra-thin films and the charge state of adsorbed species (such as Au atoms and O-2 molecules) is at the origin of a co-existence of alternative adsorption configurations, with drastically different structural and electronic characteristics. This effect may contribute also to the stabilization of oxide phases of unusual stoichiometries, where the ion valences are compensated by an electron exchange with the metal substrate. We report an analysis of the spectroscopic signatures of such differently charged adsorbates and unusual oxide phases, including valence and core electrons spectroscopies and vibrational characteristics. We show that the adsorbate charge states can be identified by their signature in the electronic states around the Fermi level and by the modification of the vibrational frequencies of adsorbed probe molecules. Moreover, core level analysis may contribute to the identification of the oxygen-rich phase, formed at high oxygen exposure. The detailed theoretical investigation of the spectroscopic signatures of alternative adsorption configurations and adsorbate charge states supplies a set of data, which may help the experimental identification of such species.

International audience; Relying on Frenkel Kontorova (FK) models of diatomic chains of increasing levels of complexity, this study presents an overall view of the diversity of structural effects that a compound (oxide) chain supported on a metal may display and helps assigning them to precise microscopic mechanisms. At each stage, the models are solved numerically, in order to provide phase diagrams as a function of chain-substrate interaction and misfit. Analytic derivations of transition lines are also provided within the continuum approximation. Their predictions are shown to quantitatively account for the numerical results, thus showing the validity of the continuum approximation in the misfit range under consideration. The present study thus extends our knowledge of the FK model by specifically focusing on diatomic chains and brings new information on a potentially interesting system which experimentalists just start being able to synthesize-oxide chains on metal substrates. (C) 2013 AIP Publishing LLC.

International audience; no abstract

International audience; Presently, density functional computational studies of nanostructures in heterogeneous catalysts consider either sufficiently big (''scalable with size'') unsupported metal nanoparticles (NPs) or small supported metal clusters. Both models may not be sufficiently representative of a few nm in size supported transition metal NPs dealt with in experiment. As a first step in closing the gap between theoretical models and prepared systems, we investigate the effect of a rather chemically inert oxide support, MgO(100), on relative energies and various properties of Pd and Pt NPs that consist of 49-155 atoms (1.2-1.6 nm in size) and exhibit bulk-like fcc structural arrangements. Shapes and interface configurations of metal NPs on MgO were obtained as a result of thorough optimization within the fcc motif using interatomic potentials. Then the stability and properties of the NPs were studied with a density functional method. We comprehensively characterize interaction between the NPs and MgO(100) support, their interface and effect of the support on NP properties. While the effect of MgO on relative stabilities of NPs with different shapes is found to be significant, other properties of the NPs such as electronic structure and interatomic distances within NP do not notably change upon deposition. This work paves the way to large-scale first-principles computational studies of more realistic models of oxide-supported metal catalysts. (C) 2013 AIP Publishing LLC.

International audience; We present a computational study on MgO(100) and MgO(111) nanoribbons with polar edges as a generic example of polar two-dimensional irreducible oxide nanostructures. Four inequivalent polarity compensation mechanisms-edge metallization, nonstoichiometry, hydroxylation, and screening by a metal substrate-are analyzed, giving basis for a discussion on their relative efficiency, and enabling a comparison with known polarity characteristics of three-dimensional objects such as bulk surfaces or thin films. Aside from the similarities, we point out that polarity in 2D objects may require smaller compensating charges and thus may be stabilized more easily than in 3D objects. Moreover, compensating charges can be provided more efficiently by the metal substrate and/or humid environment. This suggests that such polar objects may be fabricated and tuned more easily and in a more controlled manner than their 3D counterparts. DOI: 10.1103/PhysRevB.87.035405

A self-consistent electronic structure calculation, in the slab geometry, is performed to model the dissociative adsorption of water on several oxide surfaces in the limit of complete saturation. We discuss the adsorption characteristics along a series of oxides presenting a growing acidity: BaO, SrO, CaO, MgO, TiO2, and SiO2, and on three MgO surfaces: (100), (110), and (211) on which the atoms have different environments. Special emphasis is borne on the charge transfers, the densities of states and the oxide gap modification upon hydroxylation. We show that these quantities are dependent upon the coverage and that unstable MgO surfaces are more reactive towards water dissociation. Finally, by optimizing the geometry on the three MgO surfaces, we discuss the link between the electronic and structural degrees of freedom on hydroxylated surfaces.

We associate a self-consistent electronic structure calculation with a conjugate gradient technique for geometry optimization, to study structural distortions on stoichiometric oxide surfaces. We discuss the relaxation and rumpling effects and their consequences on the surface electronic structure in a series of three MgO surfaces: (100), (110), (211) characterized by an increasing number of broken bonds, on the (110) faces of rocksalt oxides presenting various ionic characters: MgO, CaO, SrO and BaO, and on two non-polar rutile TiO 2 faces: (110) and (001). The numerical results are interpreted in the framework of an analytical model and the competition between covalent and electrostatic effects is investigated. A new mechanism of rumpling on oxide surfaces is proposed.

Systematic trends in the surface electronic structure of simple insulating oxides are studied thanks to a quantum self-consistent numerical approach. Four oxides of various ionicity have been chosen: SrO, MgO, rutile TiO2 and α-quartz SiO2, and for each of them, several surfaces, involving atoms with different coordination numbers, have been modeled: the (100), (110) and (211) faces of the rocksalt oxides, the (110), (100) and (001) faces of rutile and the (0001) and (1010) faces of quartz. We discuss the competition between electrostatic and covalent processes, through an analysis of the density of states characteristics, the gap width, the ionic charges and the surface energy.

International audience; Relying on first-principles calculations within density functional theory and on an analytical model for the electronic structure, we present an overview of specific electronic and structural features of polar ultrathin films. MgO( 111) unsupported films of finite thickness are chosen as a generic system, in order to extract general concepts associated with polarity at the nanoscale, relate them to the well-known semi-infinite case, and unravel specific scenarios of polarity compensation which are not present for the latter. Size dependent behavior of the compensating charge and formation energy, changes in crystallographic structure, and the possibility of substantial lattice distortions throughout entire films are analyzed and discussed.

Thin oxide films grown on metal single crystals are used in many “surface science” research groups in attempts to understand the surface chemistry of metal oxides. In addition, these films are employed as suitable supports for modeling highly dispersed metal catalysts (for reviews, see Refs. [1]–[4]). However, in the case of ultrathin films that are only a few angstroms in thickness, the metal substrate underneath the film often affects the properties of metal clusters by charge transfer through the film. These observations can, in principle, be traced back to the so-called “electronic theory of catalysis” developed in the 1950s and 1960s, and are primarily based on a Schottky barrier model, which predicts, in particular, that by varying the thickness of oxide films, the reactivity of heterogeneous catalysts can be controlled. However, these ideas faded away, primarily because of a lack of successful examples of the promotional effects of thin oxide films on catalytic activity and/or selectivity. Recently, it has been demonstrated that a thin oxide film grown on a metal may exhibit higher catalytic activity than the metal substrate under the same reaction conditions. Indeed, a thin FeO(111) film grown on Pt(111) is active for CO oxidation at 450 K, a temperature far below that at which Pt(111) itself is active. Furthermore, the rate enhancement was observed on Fe3O4-supported Pt nanoparticles, [13] which underwent encapsulation by an FeO(111) film as a result of the strong metal– support interaction. It has been suggested that, in the millibar pressure range (1 mbar=100 Pa) of O2, the bilayer Fe–O film on Pt(111) transforms into a trilayer O–Fe–O film that catalyzes CO oxidation according to a Mars–van Krevelen-type mechanism. A density functional theory (DFT) study corroborated this scenario. The DFT results showed that, by overcoming a small energy barrier of about 0.3 eV, O2 is chemisorbed on the Fe atom, which is pulled out of the pristine FeO film. In the chemisorbed state, electrons are transferred from the oxide/metal interface to oxygen, resulting in a O2 2 species, which then dissociates, thus forming a local O–Fe–O trilayer structure. Further DFT studies revealed that the reaction is site-specific within the large Moir unit cell formed due to an approximately 10% mismatch between the FeO(111) and Pt(111) lattices. This mismatch explains scanning tunneling microscopy (STM) results that showed the formation of close-packed O–Fe–O islands rather than a continuous FeO2 film. [15]

We model and compare the electronic characteristics of clean and hydroxylated simple oxide surfaces (BaO(001), SrO(001), CaO(001), MgO(001), TiO 2 (110) rutile and SiO 2 (0001) α-quartz): ionic charges, densities of states, gap widths and band widths. Special emphasis is put on the competition between band and electrostatic effects in determining these characteristics and on the relationship between the creation of the dipolar layer on the hydroxylated surfaces with the known acid or basic character of the surfaces

International audience; We present a theoretical density-functional theory study of the effect of deposition on a metal substrate (Mg, Pt, Ag, and Au) on the phase diagram of ultrathin MgO(111) polar films. By considering various crystallographic structures (rocksalt, zinc-blende, and graphiticlike), layer stackings and lattice registries at the interface, we identify the two principal mechanisms by which oxide films interact with the metal substrate. On one hand, metal provides the charges necessary for polarity compensation at the interface, enables the oxide ions at the interface to recover their usual valence, and induces a strong stabilization of polar films. On the other hand, due to interfacial bonding and band alignment (function of the metal electronegativity), a second type of electron exchange occurs at the interface, dominant in films which are either nonpolar or uncompensated polar. It induces a structural distortion of the oxide film in the interface region and yields a nonvanishing film polarization. The stabilizing effect of the metal substrate is considerably weaker in this case.

International audience; Relying on an ab initio approach to model MgO/metal interfaces, we unravel the specificity of polarization effects in the oxide monolayer limit. We show that rumpling of the supported oxide film is a structural response to the interfacial charge transfer. Surface characteristics of such complex supports can thus be directly tuned by an adequate choice of the metal and the oxide. This mechanism equally affects films of nonpolar and polar orientations. Contrary to thicker films, there is thus no electrostatic signature distinguishing the two types of orientation.

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.

International audience; We present a combined experimental (STM/scanning tunneling spectroscopy) and theoretical (density functional theory) study on the deposition of Au and Pd metal atoms on FeO/Pt(111) ultrathin films. We show that while the Pd atoms are only slightly oxidized, the Au atoms form positive ions upon deposition, at variance to a charge transfer into the Au atoms as observed for MgO/Ag(100). The modulation of the adsorption properties within the surface Moire cell and the charging induce the formation a self-assembled array of gold adatoms on FeO/Pt(111), whereas Pd atoms are randomly distributed.

We have investigated the magnetic and electronic properties of a FeO film grown on Pt(111). Coupling first-principles density-functional theory calculations with scanning tunneling microscopy (STM) measurements we have identified the principal mechanisms for the structural and electronic characteristics observed in the Moire unit cell formed between oxide film and metal support. We show that both free and supported FeO(111) monolayers present an antiparallel alignment of the magnetic moments. Due to the lattice mismatch between Pt(111) and the FeO film, the interface properties are different in different regions of the supercell. The resulting modulation of the structural and electronic properties of the film is determined by the strength of the film-substrate interaction in various regions of the supercell. In particular, for the Fe-top site, where this interaction is weak, there is a small rumpling of the FeO layer which increases the interface separation. In correspondence of this structural modification, we observe a change in the work function, coherent with the most recent experimental findings. Our results show that a good agreement with the experimental interpretation of the work function modulation and of the STM images can be obtained only within the more robust, nonpseudomorphic computational models.