Quantum chemical study of oxygen adsorption on graphite: I. Molecular orbital study of adsorption and migration of molecular oxygen on graphite

Possible stable Van der Waals complexes of molecular oxygen on the surface of graphite modelled by 16 C and 10 H atoms were investigated by the method based on the Buckingham potential. The CNDO/2 calculations of potential curves of chemisorption of oxygen molecule on graphite were used to determine the adsorption heat on different surface sites. The migration of molecular oxygen on the surface of graphite was examined by means of energy contour maps. Also, the influence of preadsorbed oxygen on the chemisorption of the oxygen molecule was studied.     

First principles study of oxygen adsorption on nickel-doped graphite

Density functional theory is used in a spin-polarized plane wave pseudopotential implementation to investigate molecular oxygen adsorption and dissociation on graphite and nickel-doped graphite surfaces. Molecular oxygen physisorbs on graphite surface retaining its magnetic property. The calculated adsorption energy is consistent with the experimental value of ?0.1?eV. It is found that substituting a carbon atom of the graphite surface by a single doping nickel atom (2.8% content) makes the surface active for oxygen chemisorption. It is found that the molecular oxygen never adsorbs on doping nickel atom while it adsorbs and dissociates spontaneously into atomic oxygens on the carbon atoms which are bound to the nickel. The adsorption energy of ?1.4?eV and zero activation energy barrier indicate that O₂ dissociative adsorption is both thermodynamically and kinetically favoured over the surface. The large electric field near the doping nickel atom along with the excess electrons on the neighbouring carbon atoms, which are bound to the nickel induce molecular oxygen to adsorb and dissociate favourably.     

X-ray study of molecular oxygen adsorbed on graphite

We have performed a detailed X-ray diffraction study of O₂ adsorbed on UCAR-ZYX and Le Carbon Lorraine vermicular exfoliated graphite between 15 and 50 K. At least four phases of physisorbed oxygen are found. The monolayer δ phase consists of a centered parallelogram lattice, with the molecular axes parallel to the graphite surface. The data are consistent with a triple point at 26 K. The melting transition at a coverage of one monolayer appears to be first order. At higher coverages the molecules undergo a lying-down to standing-up transition; the higher coverage ζ phase froms an approximately triangular lattice with the molecular axes perpendicular to the graphite surface. Satellite peaks around the (1, 0) Bragg peak indicate, however, that this cannot be a simple triangular lattice; possible explanations include successively incommensurate layers or a sinusoidal density modulation. For coverages in the two-layer region the ζ phase modulation peaks disappear at 37 K, and at 40 K the adsorbed oxygen appears to undergo a first order melting transition into a fluid phase. With increasing coverage, the 2D X-ray diffraction profiles and phase boundaries do not connect smoothly onto those of the 3D α and β phases. At low temperatures (T < 30 K) the ζ phase always coexists with bulk crystallites; for temperatures near the 2D melting transition the 3D peaks are not observable. These data, taken together with the heat capacity results, suggest a wetting transition with only the bilayer lamellar phase or bulk O₂ being stable at low temperatures.     

Molecular mechanics modeling of the adsorption of methionine on graphite

In this study we were modeling the adsorption of the amino acid methionine on a graphite surface using molecular mechanics calculations. We were employing two different force fields, namely MM+ and AMBER, and considering the molecule in its non-ionic and zwitterionic form. The surface was modeled as a single sheet of graphite. We found that each of the force fields delivers qualitative consistency with experimental results, but the AMBER force field with the parameter set of AMBER3 leads to the best quantitative agreement regarding adsorption energy, bonding energies and distances.     

Electron spectroscopic characterization of oxygen adsorption on gold surfaces: I. Substrate impurity effects on molecular oxygen adsorption in ultra high vacuum

Tentative adsorption on clean gold (110) and (111) crystals of molecular oxygen in the pressure range 10 ^?10 to 10 ^?5 Torr, at a temperature varying between 100 and 800 K is reported together with the subsequent characterization of the surfaces by High Resolution Electron Energy Loss, Auger and X-ray Photoelectron Spectroscopies. It is found that oxygen does not adsorb in these UHV conditions, except when a contaminant is present on the surface. Such an interaction with a low level silicon impurity is described.     

The adsorption of oxygen on silicon (111) surfaces. II

At a wavelength of 546 nm the change of the ellipsometric angles δΔ (relative phase change) and δψ (relative amplitude change) has been studied on clean cleaved silicon (111) surfaces during adsorption of oxygen. δψ increases linearly with exposure up to a saturation value. The saturation dose, i.e. also the sticking coefficient for the corresponding process depends exponentially on the mean step density of the surface. δΔ is nearly independent of step density. The interpretation in terms of a macroscopic theory (Drude model) gives evidence of two adsorption processes, the formation of a monolayer of oxygen controlled by step density and a subsequent further oxidation process.     

The direct molecular orbital dynamics study on the hydrogen species adsorbed on the surface of planar graphite cluster model

In order to investigate thermal behaviors of the hydrogen species, H⁺ ion and H atom, adsorbed on the surface of planar graphite, the direct molecular orbital dynamics (MO) calculations at AM1 level are applied to the hydrogen terminated planar cluster models included by the species, C₅₄H₁₈·H⁺ and C₅₄H₁₈·H, respectively. This is the first trial to describe the thermal behaviors of these species in terms of MO dynamics. Both hydrogen species form the covalent bonds with the surface carbon atoms through the sp³ hybrid orbitals which inhibit their dissociation at high temperature up to 2000 K. It was found that the reduction of band gap is introduced by the adsorption of H⁺ ion.     

取代硝基苯炸药的理论研究

在133LYP/6.31 G*理论水平上优化了一系列取代硝基苯类化舍物的几何构型,计算了它们的电子结构和Wiberg键级,结果表明,随着氨基的引入,起爆引发键C-NO2键的强度有可能得到加强,炸药的撞击感度减小.讨论了分子内氢键与其撞击感度的关系,结果表明氢键的形成可能对分子的撞击感度起钝化作用.讨论了C-NO2键的强弱与其位置的关系.     

Orientation dependence of oxygen adsorption on a cylindrical GaAs sample: II. Photoemission measurements

The orientation dependence of oxygen adsorption has been investigated by XPS and UPS on the surface of a cylindrically shaped GaAs single crystal with [11&#x0304;0] as its axis. This sample exposes the main low-index orientations (001), (111)Ga, (110), (111&#x0304;)As, etc. and all intermediate orientations. It was prepared by ion bombardment and annealing [see part I, Surface Sci. 120 (1982) 67). XPS was used to separate the known two forms of adsorbed oxygen quantitatively. The less tightly chemisorbed a-oxygen has an O 1s binding energy 1.50 ± 0.05 eV larger than the β-oxygen for all surface orientations. Comparison with UPS shows that for hv =40.8 eV the excitation probability of the O 2p orbitals for α-oxygen is four times larger than for β-oxygen. The Ga 3d peak shows the known chemical shift of 1.1 ± 0.1 eV induced by β-oxygen whereas α-oxygen causes no Ga 3d shift. Most of the α-oxygen desorbs thermally starting near room temperature, only a portion smaller than 20% is converted to β-oxygen; β-oxygen desorbs starting at 300–350°C slightly depending on orientation. These results confirm the interpretation of β-oxygen as dissociated oxygen. However, the nature of α-oxygen still is not clear. The strong orientation dependence of the total amount of oxygen is consistent with the Auger results published recently (see part I). The nature of α- and β-oxygen is found to be the same for all orientations. Also the ratio of adsorbed α- to β-oxygen depends surprisingly weakly on the orientation and on the exposure varying only between 0.35 and 0.7 as the extreme values. This suggests that the adsorption of α- and β-oxygen is connected. A possible model is that adsorption, probably of β-oxygen, starts from edges or other energetically less favourable sites and that this disturbance creates stresses or defects which serve as adsorption site for the α-oxygen.     

Platinum nanoclusters on graphite substrates: a molecular dynamics study

Molecular dynamics simulations are used to investigate the shape and structure evolution of single platinum clusters of cubic and spherical shape containing 256 and 260 atoms, respectively, deposited on a static graphite substrate. The evolution is monitored at variable temperature, and as a function of metal-substrate interactions at constant temperature. The Pt-Pt interactions are modelled with the many-body Sutton-Chen potential, whereas a Lennard-Jones potential is used to describe the Pt-C interactions. Heating and cooling curves calculated between 200 K and 1800 K are used to determine solid-solid and solid-liquid transitions. Structural changes are detected through analyses of density profiles and diffusion coefficients. A clear analogy is observed between temperature-induced wetting phenomena and those resulting from enhancement of the metal-substrate interactions.     

Clusters and bulk beryllium: A molecular orbital versus crystal orbital study

The quasirelativistic CNDO/1 and EHT methods have been applied to investigate the electronic structure of {Be}₁₂₅, {Be}₂₁₆ and {Be}₃₄₃ clusters generated by a multiplication of the unit cell. The DOS profiles and their projections were calculated. These data were correlated with the periodic crystal orbitals of the EHT and ab-initio quality, respectively, as well as with the ASW method.We acknowledge C. Roetti for making the New version of CRYSTAL 92 available and thank P. C. Schmidt for ASW computer code. This work is in part supported by the Slovak Grant Agency for Science (research grant No. 4340).     

Methane adsorption on graphite（0001） films： A first-principles study

Using first-principles methods, we have systematically investigated the electronic density of states, work function, and adsorption energy of the methane molecule adsorbed on graphite(0001) films. The surface energy and the interlayer relaxation of the clean graphite(0001) as a function of the thickness of the film were also studied. The results show that the interlayer relaxation is small due to the weak interaction between the neighboring layers. The one-fold top site is found most favourable on substrate for methane with the adsorption energy of 133 meV. For the adsorption with different adsorption heights above the graphite film with four layers, the methane is found to prefer to appear at about 3.21 A above the graphite. We also noted that the adsorption energy does not dependent much on the thickness of the graphite films. The work function is enhanced slightly by adsorption of methane due to the slight charge transfer from the graphite surface to the methane molecule.     

The effect of non-orthogonality in the study of adsorption: Hydrogen on graphite

The effect of non-orthogonality between the localized orbitals of an adsorbed atom (or impurity atom) and those of the surrounding atoms of the substrate solid has been studied. The overlap matrix S takes in this case a simple form which enables us to calculate local densities of states and other local effects such as charges on the atoms by the continued fraction expansion. The method has been applied to a simplified model for the adsorption of hydrogen on graphite, and the results show an improvement with respect to calculations where the overlap is neglected.     

The adsorption of iron phthalocyanine on graphite: A scanning tunnelling microscopy study

Different adsorption phases of iron phthalocyanine (FePc) on highly oriented pyrolitic graphite (HOPG) have been characterized by scanning tunnelling microscopy (STM). Evaporation of FePc onto the graphite (0 0 0 1) surface, kept at room temperature, results in the formation of three-dimensional molecular islands.After annealing to 400 °C different two-dimensional features are identified, depending on the initial coverage. At low doses, domains with well defined boundaries have been observed, within which molecules tend to organise in chains. At higher coverage, islands exhibiting well-ordered densely-packed square or hexagonal molecular arrangement have been resolved. For the adsorption structures corresponding to one monolayer islands our results show that the molecules adsorb with the molecular plane parallel to the surface. The high resolution STM images allow us to resolve the orientation of single molecules and subsequently we suggest that the molecular monolayer is stabilized by van der Waals interactions. The characterization of the observed Moiré contrast and a comparison with other similar systems underlines the importance of the central metal in the molecule-molecule and molecule-substrate interactions, which govern the molecular adsorption geometry.     

Thin layers of aluminium: A molecular orbital study

Molecular beam epitaxy has been used to prepare thin layers (200 nm thickness) of aluminium grown either on aluminium or gallium arsenide substrates; their He(I) and He(II) photoelectron spectra have been recorded. The quasirelativistic CNDO/1 method has been applied to investigate the band structure of {Al}₁₇₂, {Al}₂₈₄ and {Al}₄₂₄ clusters obtained by a duplication of the unit cell: the DOS profiles and their projections were generated. These data were correlated with the periodic crystal orbitals of the EHT quality. The first excitation energy serves as a better estimate of the vanishing energy gap showing thus a metallic character of aluminium.     

Thin layers of gallium-arsenide: A molecular orbital study

The quasirelativistic INDO/1 method has been used to generate molecular orbitals for some {GaAs}_n clusters up to 160 atoms. On the basis of these one-electron energy levels the density of states (DOS) and density of hole functions have been calculated. Various projections of DOS functions are discussed. The calculations are compared with those generated by periodic crystal orbitals of the EHT quality and with experimental ESCA spectra on thin layers of GaAs.