Surface Al Atoms Research Articles

Surface complexes of acetate on edge surfaces of 2:1 type phyllosilicate: Insights from density functional theory calculation

To explore the complexation mechanisms of carboxylate on phyllosilicate edge surfaces, we simulate acetate complexes on the (0 1 0) type edge of pyrophyllite by using density functional theory method. We take into account the intrinsic long-range order and all the possible complex sets under common environments. This study discloses that H-bonding interactions occur widely and play important roles in both inner-sphere and outer-sphere fashions. In inner-sphere complexes, one acetate C–O bond elongates to form a covalent bond with surface Al atom; the other C–O either forms a covalent bond with Al or interacts with surface hydroxyls via H-bonds. In outer-sphere complexes, the acetate can capture a proton from the surface groups to form an acid molecule. For the groups of both substrate and ligand, the variations in geometrical parameters caused by H-bonding interactions depend on the role it plays (i.e., proton donor or acceptor). By comparing the edge structures before and after interaction, we found that the carboxylate binding can modify the surface structures. In the inner-sphere complexes, the exposed Al atom can be stabilized by a single acetate ion through either monodentate or bidentate schemes, whereas the Al atoms complexing both an acetate and a hydroxyl may significantly deviate outwards from the bulk equilibrium positions. In the outer-sphere complexes, some H-bondings are strong enough to polarize the metal–oxygen bonds and therefore distort the local coordination structure of metal in the substrate, which may make the metal susceptible to release.

Atomic structure and electronic properties of Ni 3Al(0 0 1) surface

We present the results of experimental and theoretical study of the structural and electronic properties of the (0 0 1) surface of an ordered, paramagnetic Ni 3Al alloy. Experimental part of this work is based on STM measurements which enabled us to obtain, for the first time, images of the Ni 3Al(0 0 1) surface with atomic resolution. Topographies of these images indicate the presence of a superstructure with the square unit cell and the lattice parameter equal to the distance between nearest surface Al or Ni atoms. We have observed that this order of the Ni 3Al(0 0 1) surface has a long-range character. This is remarkably different from the Ni 3Al(1 1 1) surface, where a (2 × 2) order (indicated by previous STM study) appears only within small surface domains. Our theoretical study of Ni 3Al(0 0 1) is based on ab initio calculations performed in the framework of density-functional theory and the use of a plane wave basis set. The obtained results indicate that changes of the atomic positions caused by the relaxation process are small and practically limited to the three topmost atomic layers. However, in the relaxed structure, surface Al atoms have higher vertical positions than surface Ni atoms, in accordance with experimental data provided by LEED measurements. We have also analyzed the differences between results of structural calculations performed independently using LDA and GGA descriptions of the exchange-correlation effects, and found that the GGA results are in a considerably better agreement with LEED data than the corresponding LDA results or the results of earlier theoretical studies.

Stability of Silanols and Grafted Alkylsilane Monolayers on Plasma-Activated Mica Surfaces

We investigated the effect of physical and chemical modifications of mica surfaces induced by water vapor-based plasma treatments on the stability of silanols and grafted alkylsilane monolayers. The plasma-activated substrates were characterized using XPS, TOF-SIMS, and contact angle measurements. They revealed a large surface coverage of silanol groups (Si-OH) and a loss of aluminum atoms compared to freshly cleaved mica surfaces. The stability of plasma-induced silanol groups was investigated by contact angle measurements using ethylene glycol as a probe liquid. The Si-OH surface coverage decreased rapidly under vacuum or thermal treatment to give rise to hydrophobic dehydrated surfaces. The stability of end-grafted monofunctionalized n-alkylsilanes was investigated in different solvents and at different pH using water contact angle measurements. The degrafting of alkylsilanes from the activated mica was promoted in acidic aqueous solutions. This detachment was associated with the hydrolysis of covalent bonds between the alkylsilanes and the mica surface. The monolayer stability was enhanced by increasing the length of the alkyl chains that probably act as a hydrophobic protective layer against hydrolysis reactions. Stable alkylsilane monolayers in water with pH greater than 5.5 were obtained on mica surfaces activated at low plasma pressure. We attributed this stability to the loss of surface Al atoms induced by the plasma treatment.

Probing the electronic and structural properties of doped aluminum clusters: MAl12− (M=Li, Cu, and Au)

Photoelectron spectroscopy (PES) is combined with theoretical calculations to investigate the electronic and atomic structures of three doped aluminum clusters, MAl12- (M=Li, Cu, and Au). Well-resolved PES spectra have been obtained at two detachment photon energies, 266 nm (4.661 eV) and 193 nm (6.424 eV). Basin-hopping global optimization method in combination with density-functional theory calculations has been used for the structural searches. Good agreement between the measured PES spectra and theoretical simulations helps to identify the global minimum structures. It is found that LiAl12- (C(5nu)) can be viewed as replacing a surface Al atom by Li on an icosahedral Al13-, whereas Cu prefers the central site to form the encapsulated D3d-Cu@Al12-. For AuAl12- (C1), Au also prefers the central site, but severely distorts the Al12 cage due to its large size.

Enhanced Methanol Dissociation on Nanostructured 2D Al Overlayers

Three well-defined two-dimensional (2D) Al overlayers including the Al/Si(111) α-phase with Al coverage of ∼0.25 ML, the Al/Si(111) γ-phase with Al coverage of ∼0.8 ML, and the bulk Al film with Al coverage of 16 ML on Si(111) have been prepared using the molecular beam epitaxy technique. Their surface morphology and electronic structure were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). The 16 ML Al film presents the metallic Al character, while the nanostructured Al/Si(111) α-phase and γ-phase are semiconducting due to space confinement of the surface Al atoms. The interfacial Al−Si interaction results in electron deficiency in surface Al atoms which follows the order of Al/Si(111) α-phase > Al/Si(111) γ-phase > bulk Al film. Methanol dissociation reactions on the Al surfaces at room temperature were studied by XPS and high-resolution electron energy loss spectroscopy (HREELS). The Al/Si(111) α-phase presents the highest activity for the dissociation of CH3OH and CH3Oads and the lowest activity for the bulk Al film. The exceptional activity of the nanostructured 2D Al surfaces for O−H and C−O bond scission has been attributed to their unique geometric structures as well as their semiconducting and electron-deficient characters.

Interactions of methane, ethane and pentane with the (110C) surface of γ-alumina

Adsorptions of methane, ethane and pentane on the γ-alumina (110C) surface are investigated with semi-empirical (PM3) cluster calculations. It is found that the abstraction of an H atom accompanied by the formation of a C–O bond is the most favorable reaction for methane on the alumina surface. For ethane– and pentane–alumina interactions, the abstraction of two H atoms accompanied by the formation of an alkene is the most favorable reaction. The surface Al atoms help to promote the reactions, but are not directly involved in the bond formation.

Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni3Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E sp, and adatom creation, E ad, are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E sp of 6.5 eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0 eV for a perfect M surface. High values of E sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E sp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E ad imply that an electron beam could create more Al adatoms on surfaces intersected by APBs than on those without. The equilibrium, minimum energy structures for a (001) surface intersected by either Σ5[001](210) or Σ25[001](340) symmetric tilt grain boundaries are computed. E sp for surface Al atoms near these GBs increases monotonically with increasing recoil angle to the surface normal, with a minimum value, which is only about 1 eV different from that obtained for a perfect surface. Temperature up to 300 K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for E sp studied here. Possible reasons for this are discussed.

First Principles Study of Gallium Atom Adsorption on the α-Al2O3(0001) Surface

The adsorption of Ga atoms in low coverage on the Al-terminated alpha-Al(2)O(3)(0001) surface has been studied theoretically by using first principles periodic boundary condition (PBC) calculations within the framework of the generalized gradient approximation (GGA). Eight possible adsorption sites are investigated, but only two are found to correspond to stationary points. Both of these locations are characterized as hollow sites, with three surrounding surface O atoms and an Al atom in the center located deeper within the Al(2)O(3) slab. The slight difference in the stability of these two sites is due to a difference in the chemical interactions between the Ga atom and the surface O atoms. Strong interactions between the Highest Occupied Molecular Orbital (HOMO) of the Ga atom and the surface state of the Al(2)O(3) surface are observed. This interaction promotes charge transfer from the Ga to the surface Al atoms, which in turn causes the surface Al atoms to move up from the surface.

Magnetic property calculations for B2-Co/sub x/Al/sub 1-x/ structures at the interface of Co-Al multilayer

The magnetic property of B2-Co/sub x/Al/sub 1-x/ as a function of Co atomic concentration was investigated using the ab initio method. The antistructured (AS) supercell model based on B2 structure was employed to calculate the magnetic moment of the Co/sub x/Al/sub 1-x/ compounds originated from spontaneous intermixing of Co atoms with Al surface atoms. In the case of Al-rich Co/sub x/Al/sub 1-x/, no apparent ferromagnetism was represented. However, the magnetic moment was calculated to be rapidly increased with Co concentration in the range above 50 atomic % and was saturated at a level of a pure HCP Co system. Ferromagnetism in Co/sub x/Al/sub 1-x/ alloys can be attributed to the existence of local BCC-Co and local AS-Co atoms.

THE OXIDATION OF NIAL: What Can We Learn from Ab Initio Calculations?

▪ Abstract We review here the theory of the early stages of oxidation of the (110) surface of Ni1−x Alx, based on ab initio calculations using a plane-wave pseudopotential method. The clean surface and several oxidized surfaces have been investigated, with oxygen coverages up to 2ML of oxygen (1ML = 3 O atoms per 2 surface Al atoms). The theory to date is a description in terms of equilibrium thermodynamics, with a comparison of the free energies of several surfaces of different composition, implemented at the atomic scale. Three environmental parameters are singled out as control variables in this treatment, namely the alloy composition x (assumed to be near 0.5), the temperature T and the partial pressure of oxygen pO2. With certain reasonable approximations an analytic formula for the surface energy σ is derived in terms of these variables and some constants that are calculated ab initio together with others that are derived from experimental thermodynamic tables. At oxygen pressures just above the threshold for bulk oxidation of NiAl, the calculations explain the observed formation of a thin film of alumina in place of NiAl surface layers, with the consequent dissolution of Ni into the bulk. Ab initio calculations illustrate how the energetics of supplying Al to the surface depends on bulk stoichiometry, which alters the relative stability of different surface oxidation states so as to favour oxidation more if the alloy is Al-rich than if it is Ni-rich.

Formation of STM images ofNi3Al(001) and (111) surfaces

We present the results of scanning tunnel microscope (STM) measurements performed on a ${\mathrm{Ni}}_{3}\mathrm{Al}(111)$ surface, combined with a theoretical study of the formation of STM images of (001) and (111) surfaces of this alloy. The STM images of the ${\mathrm{Ni}}_{3}\mathrm{Al}(111)$ surface show a superstructure with the lattice constant corresponding to the size of the surface unit cell. An earlier interpretation of this result assumes that this superstructure represents the distribution of the surface Al atoms, while Ni atoms remain invisible in the STM images. This supposition is confirmed here by presented STM simulations. Moreover, numerical calculations show that a similar effect should also appear in the STM images of the ${\mathrm{Ni}}_{3}\mathrm{Al}(001)$ surface. A detailed theoretical analysis indicates that the domination of Al atoms in the STM images of both surfaces is mainly caused by the intra-atomic $s\ensuremath{-}{p}_{z}$ interference. This kind of intra-atomic interference reduces the s and ${p}_{z}$ current contributions tunneling through surface Ni atoms and increases considerably the corresponding contributions flowing through Al atoms. As a result, only the surface Al atoms appear in the STM images.

Surface core level shift observed on NiAl(1 1 0)

Using high resolution core level spectroscopy, a surface core level shift towards lower binding energy of −0.13 eV is determined for the 2p level of the outwardly relaxed Al surface atoms on NiAl(110). Density functional theory based calculations with inclusion of final state effects yield a value of −0.14 eV for this shift in excellent agreement with experiment. We show that the initial state approximation yields a value of +0.09 eV, i.e. the inclusion of final state relaxation effects is vital not only to obtain the correct value but even the correct sign for this shift.

超純水のみによる電気化学的加工法の研究―陰極Ａｌ（００１）表面における除去加工現象の第一原理分子動力学シミュレーション―

In order to reveal the mechanism of the electrochemical processes of cathode surfaces in ultrapure water, first-principles molecular-dynamics simulations of chemical reaction on the Al(001) surfaces interacting with H atoms, H2O molecules and OH molecule were carried out on the basis of the Kohn-Sham local-density-functional formalism. A plane-wave basis set was used, and the cut-off energy is 594eV(64Ry). A norm-conserving pseudopotential was also used. We adopt the standard molecular-dynamics method for the optimization of the ionic system and the preconditioned conjugate-gradient (CG) method for the quenching procedure of the electronic degrees of freedom. We determined the optimized atomic configurations and electronic distributions for H atom and H2O or OH molecule chemisorbed Al(001) surfaces. It was confirmed that an H atom reacts with H2O molecules on the Al(001) surface to produce an OH molecule. Chemisorption of an OH molecules and H atoms to the Al(001) surface atom breaks the back-bonds and side-bonds, and the Al(001) surface atom is etched as an AIH2OH molecule. From these simulated results, we concluded that the electrochemical etching of Al(001) cathode surface in ultrapure water is induced by H atoms and OH molecules.

Layer intermixing during metal/metal oxide adsorption: Ti/sapphire(0001)

First principles density functional calculations for adsorption of Ti on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ indicate that $\mathrm{Ti}:{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ interfaces become intermixed. Substitutional Ti replaces a surface Al atom rather than a subsurface Al, and the Al-terminated surface is unstable under Ti adsorption. Adsorbed Ti displaces the surface Al, resulting in a mixed Ti/Al interfacial layer instead of a sharp $\mathrm{Ti}:{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interface. Our results provide a coherent picture of the structural and electronic properties of this interface and are consistent with available experimental data.

Dynamics and free energy of α-alumina(0001) surfaces: I. Semi-empirical model

We report an atomistic investigation of static and dynamic properties of the (0001) surface ofα-aluminaat different temperatures. Lattice dynamics calculations are performed within thequasiharmonic approximation using a periodic slab geometry and with twoparametrizations of the shell model. One of our aims is to prepare the ground fora more precise first-principles treatment, by checking the convergence ofdynamical properties with respect to slab thickness, vacuum size andk-pointsampling. The surface systematically undergoes a large relaxation, and westudy the role of surface vibrational modes in the apparent position ofthe surface plane. An analysis of the mean square displacement of theatoms shows that the amplitudes of vibration of the atoms are 1.5 timelarger at the surface than in the bulk. We find that these amplitudesconverge slowly with slab thickness, and that the surface modes involve morethan just the surface Al atoms. Another aim is to study the surface freeenergy, which we find has two favourable properties from the point of viewof ease of computation. Firstly, we have calculated it for various coarseand fine samplings of phonon wavevectors in the Brillouin zone and findthat it can already be well approximated with a sampling of just twok-points.Secondly, the vibrational contribution to the surface free energy converges muchfaster with respect to slab thickness than the mean square amplitude of vibration.The values of the surface free energy obtained with the two shell models differ by20% but have similar temperature dependence and bracket the experimentalvalue.

Locating the Al atom in Ni14Al–Ni19Al clusters

Reactions of N2 with NinAl clusters, n=14–19, are used to determine the location of the Al atom within the cluster framework. N2 saturation levels are consistent with cluster structures in which one surface atom of the corresponding Nin+1 cluster is replaced with an Al atom. For n=14 and 17–19 it is possible to precisely locate the Al atom within the surface. In general, its placement maximizes the number of Ni–Al bonds for a surface Al atom.

Experimental study of the effect of Si/Al composition on the aluminum distribution in (Al)MCM-41

The effect of framework Al content on the number of framework Al atoms on the surface of (Al)MCM-41 channels and the distribution of these Al atoms among single Al atoms and various Al–Si–Al and Al–Si–Si–Al pairs was investigated. The number of framework Al atoms on the wall surface was characterized by the ion exchange capacity for Na+ ions, number of Al pairs on the surface by exchange capacity for Co2+ ions. Two different geometrical arrangements of Al pairs were characterized by VIS spectroscopy of bare Co2+ ions. At low framework Al content, Al atoms occupy only T sites on the surface of the channels. In aluminum rich (Al)MCM-41, Al atoms are incorporated also into the inner volume of the channel walls and at Si/Al∼11 only about 50% of Al atoms are on the channels surface. Al atoms on the wall surface preferentially form Al–Si1,2–Al pairs, enabling siting of divalent cations. Number of single Al atoms increases with framework Al content and overcome 50% of surface Al atoms for Si/Al∼11. The Al–Si1,2–Al pairs are present in two cationic sites and also the distribution of Al pairs into these sites depends on the framework Al content.

Rippled surface structure and electronic and magnetic properties ofNi3Al(001)

Structural, electronic, and magnetic properties of ${\mathrm{Ni}}_{3}\mathrm{Al}(001)$ are investigated by the all-electron thin film full-potential linearized augmented plane wave method based on the local density approximation. A stable rippled surface atomic geometry is determined by atomic force and total energy calculations. The surface Ni atoms contract down to the bulk region by 4.2% of the bulk interlayer spacing while all of the other atoms including the surface Al atoms remain close to their bulk positions. The amount of rippling found by calculation (0.06 \AA{}) is almost within experimental error (0.02\ifmmode\pm\else\textpm\fi{}0.03 \AA{}). Charge densities, calculated work functions, and densities of states for the relaxed rippled and unrelaxed surfaces are reported. The spin polarized calculation predicts that the ${\mathrm{Ni}}_{3}\mathrm{Al}(001)$ surface is ``magnetically dead,'' unlike the bulk region.

Structures, Energies, and Electrostatics for Methane Complexed with Alumina Clusters

Ab initio calculations were used to investigate properties of complexes formed from the association of CH4 with Al2O3, Al4O6, and Al8O12 alumina clusters. Methane attaches to a surface Al atom of the cluster to form a complex with an AlC separation that varies between 2.2 and 2.5 Å. The rotational motion for methane in these complexes is highly fluxional. Extrapolated G2MP2 well depths for the CH4- - -Al2O3, CH4- - -Al4O6, and CH4- - -Al8O12 complexes are 21, 14, and 17 kcal/mol, respectively. These different well depths are determined by the accessibility of the Al atom to which CH4 binds and the size of the alumina cluster. The electrostatics of the three alumina clusters are very similar, with a charge on the surface Al atom of +2.2 to 2.3. The potential energy surface for a CH4- - -Al2nO3n cluster is represented semiquantitatively by an analytic function consisting of two-body potentials. The results of this study suggest that the adsorption energy for alkane molecules binding to alumina materials depends very strongly on the structure of the binding site.

Low-Energy Ion Scattering Measurements from an Al-Pd-Mn Quasicrystal

AbstractEnergy-angle distributions of low-energy inert-gas ions scattered from surfaces provide information about surface composition and structure. We have measured energy spectra of He+ scattered from an Al71Pd20Mn9 quasicrystal, which was oriented perpendicular to the 5-fold axis, along various azimuthal directions. Strong scattering signals are seen from Al and Pd, but only a weak Mn signal is observed. From measurements made of He+ at an oblique angle of incidence scattered in the forward direction, we observe a 72° periodicity in the azimuthal dependence of the scattering signal intensity from Al surface atoms. The effect arises from shadowing effects involving neighboring surface atoms and provides direct evidence that Al surface atoms exist in a local environment with 5-fold symmetry. In addition, measuring the variation of the signal intensity with incidence angle provides information about neighboring atom distances, which compare favorably with a model of the quasicrystal surface derived from the bulk structure.