Magnesium Oxide Surface Research Articles

Cyclic Ozone Identified in Magnesium Oxide (111) Surface Reconstructions

We report the first experimental evidence of the cyclic form of ozone, found in three air stable surface reconstructions of MgO (111) annealed above 1450 \ifmmode^\circ\else\textdegree\fi{}C. The $\mathrm{MgO}(111)\ensuremath{-}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ surface consists of equilateral oxygen trimers while the $\mathrm{MgO}(111)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}2)$ and $\mathrm{MgO}(111)\ensuremath{-}(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ surfaces are periodic arrangements of trimers and single oxygen atoms. The oxygen trimers appear to be centered over underlying Mg atoms. The structures fit transmission electron diffraction data better than neutral plane faceting based models proposed for the polar MgO (111) surface.

Generation of atomically flat MgO(100) surfaces: Influence of ambient gas composition during high temperature anneals

Atomic force microscopy, reflection high energy electron diffraction (RHEED), and x-ray photoelectron spectroscopy (XPS) have been used to follow the regeneration of atomically flat magnesium oxide (100) surfaces from samples initially etched in phosphoric acid. This regeneration process has been carried out by annealing samples at high temperatures in a range of gas compositions. The gas compositions investigated include argon, nitrogen, carbon dioxide, high partial pressures of water in nitrogen, and a range of partial pressures of oxygen. While acid etching produces initially rough surfaces, high temperature anneals (1000 °C) are found to significantly reduce the surface roughness independent of gas composition. In contrast, only annealing cycles in the presence of oxygen (O2) produced atomically flat stoichiometric MgO surfaces exhibiting good crystalline character, evidenced by distinct square-shaped terraces with well-defined step edges, a 1×1 RHEED pattern, and a 1:1 ratio between magnesium and oxygen determined by XPS analysis.

Atomic process of epitaxial growth of gold on magnesium oxide studied by cross-sectional time-resolved high-resolution electron microscopy

Gold was vacuum deposited on a (001) surface of magnesium oxide inside a high-resolution electron microscope. The atomic process of epitaxial growth was observed cross sectionally in situ with time-resolved high-resolution transmission-electron microscopy. Various types of growth phenomena, such as nucleation center formation, structural fluctuations, shape modulation, secondary nucleation, and coalescence, were analyzed in real space at a spatial resolution of 0.2 nm and a time resolution of 1/60 s. It was found that a central corner of a cluster is truncated and constructed repeatedly during the growth.

The Dynamics of Mgo Surface Faceting

Abstract The {001} surface of magnesium oxide (MgO) has been the focus of numerous studies, which were prompted by the importance of MgO for its use as a substrate for thin film growth and also as a chemical catalyst. In the present work, atomic force microscopy (AFM) was used for studying the dynamics of surface processes of MgO which occur at elevated temperatures. AFM was chosen, in part, because it allows for imaging of topographical details at the atomic level with minimal sample preparation. Additionally, because the surface morphology of the same area was traced through a series of heat treatments, scanning electron microscopy analysis would be difficult because no conductive coating could be used (such a coating may have altered the surface between subsequent heat treatments). AFM images were recorded in contact mode, in air, on a Nanoscope III (Digital instruments, Santa Barbara, CA) using Si3N4 cantilevers (Ultralevers, Park Inst., Sunnyvale, CA) with a nominal applied force of 10-15 nN.

Models of Magnesium Oxide (111) Surface Reconstructions Obtained from Direct Phasing of Transmission Electron Diffraction Data

Abstract It has long been thought that (111) surfaces of rock salt oxides microfacet to neutral surfaces upon annealing because of the very large energies involved in bulk terminating a layer of like ions. However in a recent reflection electron microscopy (REM) study Gajdardziska-Josifovska et al. found that MgO(lll) surfaces annealed in flowing oxygen furnaces at 1500°C not only did not microfacet, but displayed a √3×√3R30° surface periodicity that was stable in air. To determine the structure of this unusually stable surface MgO (111) transmission electron microscopy (TEM) samples were annealed in a vacuum furnace in the present study and their transmission electron diffraction (TED) patterns were analyzed with direct phasing methods. The TEM samples were prepared by orienting a MgO single crystal and sawing lmm wafers along a (111) plane. Disk samples were then ultrasonically drilled, dimpled, mechanically polished and/or hot nitric acid etched, and milled with 5 KeV Ar+ ions.

Preparation and characterization of MgO(100) surfaces

The preparation of crystalline magnesium oxide surfaces by means of mechanical polishing, acid etching, oxidative annealing and vacuum annealing has been studied on an atomic scale using low energy electron diffraction (LEED) and ultrahigh vacuum (UHV) atomic force microscopy (AFM). Mechanical polishing results in a microscopically smooth and an atomically disordered surface. Although acid etching of the magnesium oxide surface results in LEED patterns indicating a crystalline nature, AFM characterization of these surfaces reveals that surface layers are removed in an anisotropic fashion resulting in a surface roughness on the order of 100 Å (which is not probed by the LEED studies). Annealing this same surface in vacuum only modestly reduces the surface roughness while high temperature anneals of an acid etched surface in a 1 atm oxygen environment produces a well ordered surface characterized by 1000 Å terraces separated by single and double layer step heights. Further annealing these terraced surfaces to 1000°C under vacuum produced well-ordered and crystallographic surfaces on an atomic scale. Topographic images of the MgO(100) surface obtained under UHV conditions reveal rows of oxygen atoms oriented in a 〈110〉 direction with a spacing of 3.0 Å and indicate that this surface preparation results in an unreconstructed, bulk termination of the rock salt structure.

Catalytic properties of F-centres at the magnesium oxide surface: hydrogen abstraction from methane

Surface defects on MgO are known to be involved, as active sites, in the catalytic abstraction of hydrogen from methane. The energetics for the formation at the (001) surface of some of these defects, as well as their electronic structure and reactivity, are explored. In particular, F-centres (anion vacancies with two electrons trapped), Li + F + couples (a vacancy with one electron coupled to a substitutional Li ion) and the step (where cations and anions are four-fold coordinated) are examined. This study is complementary to a previous investigation, where the activity of Li + O − ionic couples has been considered. In all the cases examined the reaction results to be endothermic at 0 K and F-centres are less reactive than the O − sites in the doped substrate.

Embedded-cluster study of core-level binding energies of magnesium and alkali impurities at the surface of MgO

We present the results of a theoretical study concerning the shifts in the binding energy of core electrons at the (001) surface of magnesium oxide. The core levels considered are the 1s, 2s, 2p z ones of the magnesium ion, the 1s of substitutional lithium, and the 1s, 2s, 2p z of substitutional sodium. The shifts are evaluated by means of ab initio embedded-cluster unrestricted Hartree-Fock ΔSCF calculations. The host perfect crystal is modeled by a two-dimensional MgO slab; within the slab, a cluster is considered which contains at its center the ion where the core hole is created. This technique allows the effects of the surroundings to be taken into account accurately and self-consistently. The dependence of the binding energy shift on the location of the ion with respect to the surface is analysed and discussed. Surface effects are evident but well localized, since they are negligible below the second layer from the surface. The entity of the shift is dictated by the local electrostatic potential, the polarizability of neighboring anions and their number; for the formation of the substitutional impurities, the repulsion with neighboring oxygens also comes into play. A semi-quantitative explanation of the whole of the calculated data has been provided in terms of three parameters. For the case of lithium in the surface layer, the effects of relaxation around the impurity have also been studied.

Methodology for measuring contact pressure to cause failure of ceramics and coatings using softer blunt cones

This paper presents results of experiments on the ability of ceramic surfaces to withstand the pressures developed beneath circular contacts which are significantly lower than the indentation hardness of those surfaces. First, a simulated fatigue curve for magnesium oxide crystals is discussed, which establishes a limiting mean pressure below which the surface will not fail by fatigue. This pressure is 0·41 GPa for (001) surfaces of magnesium oxide and compares with the indentation hardness of around 6 GPa. It is shown that the conventional mechanism of fatigue of crystalline solids operates, i.e. dislocation multiplication, work hardening, and crack initiation by dislocation interaction on specific crystallographic planes. Second, it is shown that the same experimental approach can be taken to evaluate the integrity of diamond coatings on single crystal (001) silicon substrates. In addition, it is shown that the mean contact pressure to induce failure is reduced under contact fatigue conditions.

Car-Parrinello simulation of NH 3 adsorbed on the MgO(100) surface

The Car-Parrinello method has been applied the adsorption of ammonia on regular (100) surfaces of solid magnesium oxide. One NH 3 molecule was positioned on a surface consisting of three layers with four Mg and four O atoms in each. The samples were first subjected to simulated annealing up to temperatures in the range of 50–300 K and then to a free molecular dynamics run covering time intervals of 2–3 ps. NH 3 is physisorbed on the surface. The N atom is bound to the Mg but not centered above it. The dynamics of the NH 3 molecule are discussed in terms of three modes, a stochastic reorientation of the molecule around its figure axis, a nutation around an axis vertical to the figure axis, and the NH stretching modes. The calculation confirms recent inelastic neutron scattering results, which indicate rotation-translation coupling and a low barrier to the rotational motion.

FTIR studies of the adsorption of CO on supported metallic clusters [formula omitted

Recently, we have evidenced a strong size effect in the adsorption energy of CO on small palladium clusters supported on single crystalline magnesium oxide surfaces. One possible interpretation is the presence of several types of adsorption sites on the metallic clusters. In order to investigate more precisely the adsorption of CO, a new experiment has been developed. The main technique is the fast Fourier transform infrared spectroscopy (FTIR) used to measure the stretching vibration frequencies of the adsorbed CO molecules. New results demonstrating the availability of FTIR to determine adsorption sites of CO on Pd MgO(100) model catalysts are presented. Actually, two distinct absorption bands are observed, tentatively assigned one to the facets of the cluster and the other to the edges.

DYNAMIC ELECTRON MICROSCOPY OF SURFACE DIFFUSION OF TUNGSTEN CLUSTERS ON VACUUM-DEPOSITED MgO SINGLE-CRYSTALLINE FILMS

The surface-diffusion process of tungsten atoms and clusters on magnesium oxide (001) surfaces is observed directly by high-resolution electron microscopy (HREM) with time resolution of 1/60 sec. It is found from the data reproduced from video-tape images that the diffusion process is classified into a short-jump process to neighboring sites and a long-jump process over a 1-nm distance. The present observation indicates a new potential for the study of the surface-diffusion process with HREM.

TIME-RESOLVED ATOMIC OBSERVATION OF FRAGMENTATION PROCESS IN GOLD CLUSTERS BY HIGH-RESOLUTION TRANSMISSION ELECTRON MICROSCOPY

Fragmentation in gold clusters was for the first time observed by the time-resolved high-resolution transmission electron microscopy at 0.2-nm spatial resolution and 1/60 s time resolution. A gold cluster of about 1.5-nm diameter made asymmetrical fragmentation on a magnesium oxide (001) surface. It was suggested that the present fragmentation process was similar to the Coulomb explosion process.

Rate Enhancing Effect of Carbon Dioxide in the Reaction of Acetonitrile with Methanol to Acrylonitrile over Magnesium Oxide Catalyst

Abstract Carbon dioxide greatly enhanced the formation rate of acrylonitrile in the gas-phase reaction of acetonitrile with methanol over magnesium oxide catalyst. The reaction in the presence of carbon dioxide was accompanied by the reaction of methanol with carbon dioxide to give carbon monoxide and water. An adsorbed carbon dioxide species on the basic surface of magnesium oxide seems to afford an active methanol-derived species for the reaction with acetonitrile.

Cluster quantum-chemical study of the chemisorption of methane on a lithium-promoted magnesium oxide doped by zinc oxide

The reaction of a methane molecule with a lithium-doped magnesium oxide catalyst ( Li MgO ) containing small amounts of Zn 2+ cations (Zn/Li/MgO) was theoretically studied using a modified MINDO/3 method and applying a supermolecular approach. The surface of magnesium oxide (MgO) was modelled by a Mg 32O 32 four-layer molecular cluster containing all types of structural defects i.e., low-coordinated magnesium and oxygen ions (Mg 2+ LC and O 2− LC) of various faces, edges, corners etc. Molecular clusters of lithium-promoted magnesia ( Li MgO ) were simulated by isomorphic substitution of Mg 2+ LC by Li + LC; the excess negative charge of the cluster was compensated by a proton connected to an O 2− 3C site. For Zn-doped Li MgO or MgO an isomorphic substitution of Mg 2+ LC by Zn 2+ LC was assumed. The calculations indicate that for Zn/Li/MgO or Zn MgO the substitution of a threefold coordinated magnesium cation by zinc is more favourable by energetics than for four- and five-fold coordinated Mg cations. The computational results are used to interpret the experimentally observed increase of C 2+ hydrocarbons selectivity in the oxidative coupling of methane when doping a NaOH-promoted CaO catalyst with minor amounts of Zn 2+ cations.

Neutron spectroscopy and Car-Parrinello simulation at adsorbates on magnesium oxide surfaces.

The spectra of dry MgO and of MgO covered with hydrogen bonded hydroxyl groups have been recorded by inelastic neutron scattering in an energy range up to 1000 meV. In contrast to IR absorption spectra the scattering function consists of a broad intensity extending from 10 to 800 meV. At the position of the O-H stretching vibration no distinct peaks are seen. The neutron spectrum of the hydrogen bond is dominated by multiphonon transitions of the proton which vibrates in phase with the MgO lattice phonons. This experimental results confirms a model emerging from recent Car-Parrinello simulations. There the characteristic shape of the O-H stretching band had been derived from adiabatic coupling of the hydrogen bond to the substrate phonons.

Interaction of CO2 with Magnesium Oxide Surfaces: a TPD, FTIR, and Cluster-Model Calculation Study

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTInteraction of CO2 with Magnesium Oxide Surfaces: a TPD, FTIR, and Cluster-Model Calculation StudyY. Yanagisawa, K. Takaoka, S. Yamabe, and T. ItoCite this: J. Phys. Chem. 1995, 99, 11, 3704–3710Publication Date (Print):March 1, 1995Publication History Published online1 May 2002Published inissue 1 March 1995https://doi.org/10.1021/j100011a043RIGHTS & PERMISSIONSArticle Views1604Altmetric-Citations49LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts

Coordination behavior of the core and valence levels of low-coordinated oxygen ions on the surface of magnesium oxide

For low-coordinated (N=2,3,4,5) oxygen ions on the surface and in the bulk of magnesium oxide, the behavior of the core O1s- and valence O2s-, O2p-levels is considered. The MgO610−, OMg610+, Mg13O142−, O13Mg142+, Mg4O76−, and OMg22+ clusters are calculated by the SCF-Xα-SW method. The binding energies of oxygen 1s-, 2s-, and 2p-states decrease with coordination. This coordination dependence is explained by the greater change of the Madelung potential as compared to variations of the purely electronic terms of the binding energies of oxygen ions. The dependence of oxygen atomic charges and relaxation energies on coordination is also discussed.

Comment on ‘Influence of surface composition on initial hydration of aluminium in boiling water’ by A. Strålin and T. Hjertberg

A polemic on the recent paper entitled “Influence of surface composition on initial hydration of aluminium in boiling water”, by A. Strålin and T. Hjertberg [Appl. Surf. Sci. 74 (1994) 263]. Attention is drawn to the misinterpretation of a feature that can appear in the Al2p XPS spectra of Al-Mg alloys when using a Mg Kα X-ray source. Strå and Hjertberg suggested that this feature was related to the formation of MgAl 2O 4 on the metal surface. This feature, however, was most likely the Mg Kα X-ray-induced Mg KLL Auger line from MgO, which can have a significant intensity if the magnesium oxide surface concentration exceeds that of aluminum oxide. This feature would be absent in the Al2p XPS spectra of Al-Mg alloys when using an Al Kα X-ray source, because of the apparent binding energy shift of the Mg KLL Auger line caused by the (233 eV) difference in kinetic energy between the two X-ray sources.

Self-Condensation of n-Butyraldehyde over Solid Base Catalysts

The catalytic properties of various solid bases for self-condensation of n-butyraldehyde in liquid phase were studied to elucidate the factors governing the activity and selectivity. For alkaline earth oxide catalysts and γ-alumina catalyst, aldol condensation occurred, followed by Tishchenko-type cross-esterification of n-butyraldehyde with the dimer produced by the aldol condensation to form trimeric glycol ester. Alkali ion-modified alumina catalysts exhibited a high selectivity for the aldol condensation dimer, the trimeric glycol ester being formed tittle. Both basic and acidic sites on the surfaces of the alkaline earth oxides and γ-alumina were assumed to contribute to Tishchenko-type cross-esterification. The suppression of Tishchenko-type cross-esterification for alkali ion-modified alumina catalysts is due to the absence of acidic sites on the surfaces. The catalytic performances of alumina-supported magnesium oxide and alkali zeolite X were also examined. Alumina-supported magnesium oxide exhibited lower activity but higher selectivity to trimeric glycol ester than MgO. This catalytic feature was caused by the lower basicity and higher acidity on the surface of alumina-supported magnesium oxide as compared with MgO. The activity of alkali ion-exchanged zeolites was lowest among the catalysts examined in this study. The modification of zeolites with excess alkali ions improved the activity.