Chemisorption of OCN on Cu (100) surface: a density functional study

The chemisorption of cyanato radial (OCN) on Cu (100) surface is studied by using density functional theory (DFT) and the cluster model method. Cu₁₄ cluster is used to simulate the surface. Vertical bonding geometries with the nitrogen or oxygen atom down, and a parallel bonding geometry are considered, respectively. The present calculations show that cyanato-N species absorbed on the surface is more favorable than the other configurations. It indicates that OCN species is linearly bonded to the Cu (100) surface via the nitrogen atom, and is in good agreement with the experimental result. The cyanato-N species at the bridge site is most stable. For cyanato-N, the calculated symmetric and asymmetric OCN stretch frequencies are all blue-shifted compared with the calculated gaseous values, which is consistent with the experiment result. The charge transfer from the surface to OCN causes a work function increase on the surface. Bonding of OCN to the metal surface is largely ionic.     

Ab initio density functional study of MgO (001) surface with topological defects

Magnesium oxide (MgO) is a useful material because of its catalytical properties. Moreover, surface defects have been found to play a crucial role in determining the activity and selectivity of catalytic surfaces. We present the results of a theoretical investigation of the electronic structure of the MgO (001) surface with three different topological defects. Our calculations are based on density functional theory (DFT) and the pseudopotential method. Slab geometry and periodic boundary conditions have been included with occupied orbitals expanded in plane waves. We compare the obtained results of the (001) MgO clean surface with those containing topological defects. The density of states DOS is analyzed in each case, as well as the chemical nature of the atoms belonging to the defects. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Thermodynamic and structural properties of water adsorbed film on MgO (100) ionic surface

We have investigated by adsorption isotherms and neutron diffraction measurements, respectively the thermodynamic and structural properties of water physisorbed film on MgO (100) powder. Thanks to a high temperature thermal treatment, under vacuum, our MgO powder samples are characterized by a highly homogeneous (100) MgO surface. We have determined the structure of the (2D) water film physisorbed on such an ionic surface. This one is a commensurate P(2×3) structure which is very similar to the (110) planes of ice-VII. Recall that ice VII, which is stable at very high pressure, is characterized by a quite large density (d = 1.6).     

Au atoms and dimers on the MgO(100) surface: a DFT study of nucleation at defects

The adsorption of Au atoms at the surface of MgO and the formation of Au dimers have been studied by means of first principles DFT supercell calculations. Au atoms have been adsorbed on flat MgO terraces and monatomic steps but also at point defects such as oxygen vacancies (F centers) or divacancies. Very low barriers for diffusion of Au atoms on the MgO(100) terraces have been found. Atom diffusion is stopped only at strong binding sites such as the F and F+ centers (adsorption energy E(a) = 3-4 eV), divacancies (E(a) = 2.3 eV), or, to less extent, steps (E(a) = 1.3 eV). The combination of two Au adatoms with formation of a dimer is accompanied by an energy gain, the dimer binding energy, E(b), between 2 and 2.4 eV for all sites considered, with the exception of the paramagnetic F+ center where the gain is negligible (0.3 eV). The dimerization energy on the surface is not too different from the bond strength of Au2 in the gas phase (2.32 eV). Thus, defects sites on MgO do not have a special role in promoting or demoting Au dimerization, while they are essential to trap the diffusing Au atoms or clusters. Calculations on Au3 formed on an F center show that the cluster is fluxional.     

Theoretical study of Sn adsorbed on the MgO(100) surface with defects

In this study, the adsorption of Sn atom at various sites on the MgO(100) surface was characterized using a theoretical approach based on density functional theory calculations. Both regular adsorption centers (O^2? and Mg²⁺) and defects (such as neutral and charged O and Mg vacancies) were considered. Several key parameters for these sites with the adsorbed Sn atom were determined to provide its geometric, energetic, and electronic characterization. The interaction between Sn and the Mg vacancy sites is very strong and is associated with a relatively small distance of the adsorbed Sn atom from the surface and with a large electronic charge transfer from Sn to the surface. A much smaller strength of Sn atom adsorption is observed for the O vacancies and regular sites. Among them, the ${{\text{F}}_{\text{s}}}^{\text{0}}$ center binds the Sn atom strongest and, in consequence, this atom acquires a significant amount of electronic charge.     

Electronic and magnetic properties of substituted BN sheets: a density functional theory study

Using density functional calculations, we investigate the geometries, electronic structures and magnetic properties of hexagonal BN sheets with 3d transition metal (TM) and nonmetal atoms embedded in three types of vacancies: V(B), V(N), and V(B+N). We show that some embedded configurations, except TM atoms in V(N) vacancy, are stable in BN sheets and yield interesting phenomena. For instance, the band gaps and magnetic moments of BN sheets can be tuned depending on the embedded dopant species and vacancy type. In particular, embedment such as Cr in V(B+N), Co in V(B), and Ni in V(B) leads to half-metallic BN sheets interesting for spin filter applications. From the investigation of Mn-chain (C(Mn)) embedments, a regular 1D structure can be formed in BN sheets as an electron waveguide, a metal nanometre wire with a single atom thickness.     

Adsorption of atomic oxygen and nitrogen at beta-cristobalite (100): a density functional theory study

The adsorption of atomic oxygen and nitrogen on the beta-cristobalite (100) surface is investigated from first principles density functional calculations within the generalized gradient approximation. A periodic SiO2 slab model (6 layers relaxing 4 or 6) ended with a layer of Si or O atoms is employed throughout the study. Several adsorption minima and diffusion transition states have been characterized for the two lowest spin states of both systems. A strong chemisorption is found for either O or N in several sites with both slab endings (e.g., it is found an average adsorption energy of 5.89 eV for O (singlet state) and 4.12 eV for N (doublet state) over the Si face). The approach of O or N on top O gives place to the O2 and NO abstraction reactions without energy barriers. Atomic sticking coefficients and desorption rate constants have been estimated (300-1900 K) by using the standard transition state theory. The high adsorption energies found for O and N over silica point out that the atomic recombination processes (i.e., Eley-Rideal and Langmuir-Hinshelwood mechanisms) will play a more important role in the atomic detachment processes than the thermal desorption processes. Furthermore, the different behavior observed for the O and N thermal desorption processes suggests that the published kinetic models for atomic O and N recombination reactions on SiO2 surfaces, based on low adsorption energies (e.g., 3.5 eV for both O and N), should probably be revised.     

Optical surface properties and morphology of MgO and CaO nanocrystals

Optical absorption and photoluminescence emission properties of dehydroxylated MgO and CaO nanocrystals are discussed with respect to particle morphology and size. On MgO nanocubes with pronounced corner and edge features two emission bands at 3.4 and 3.3 eV result from the excitation of 4-coordinated surface O(4C)(2-) anions in edges at 5.4 eV and of regular oxygen-terminated corners at 4.6 eV, respectively. Morphologically ill-defined CaO particles are a factor of 5 larger, do not display regular corner features, and show only one photoluminescence emission band at 3.0 eV. The associated excitation spectrum indicates electronic excitations above the energy required to excite regular oxygen-terminated CaO corners. It is concluded that in the case of morphologically well-defined MgO nanocubes variations in the next coordination of oxygen-terminated corners can effectively be probed by photoluminescence spectroscopy and thus allows for discrimination between 3-coordinated surface O(2-) in regular corner sites and kinks.     

Optical properties of Cu nanoclusters supported on MgO(100)

The vertical transitions of Cu atoms, dimers, and tetramers deposited on the MgO surface have been investigated by means of ab initio calculations based either on complete active space second-order perturbation theory or on time-dependent density functional theory. Three adsorption sites have been considered as representative of the complexity of the MgO surface: regular sites at flat (100) terraces, extended defects such as monoatomic steps, and point defects such as neutral oxygen vacancies (F or color centers). The optical properties of the supported Cu clusters have been compared with those of the corresponding gas-phase units. Upon deposition a substantial modification of the energy levels of the supported cluster is induced by the Pauli repulsion with the substrate. This causes shifts in the optical transitions going from free to supported clusters. The changes in cluster geometry induced by the substrate have a much smaller effect on the optical absorption bands. On F centers the presence of filled impurity levels in the band gap of MgO results in a strong mixing with the empty levels of the Cu atoms and clusters with consequent deep changes in the optical properties of the color centers. The results allow to interpret electron energy loss spectra of Cu atoms deposited on MgO thin films.     

Geometrical,spectroscopic, and magnetic properties of an oxygen atom adsorbed on the Ni(100) surface

Spin-polarized linear combinations of Gaussian-type orbital–model core potential–local spin density (LCGTO –MCP –LSD ) computations have been performed for oxygen chemisorption on a Ni(100) surface simulated by four different clusters. Results show that the oxygen atom chemisorbs preferentially on the fourfold hollow site with an equilibrium distance of 1.931 Å and a vertical vibrational frequency of 401 cm^?1. The corresponding experimental values are 1.960 Å and 423 or 430 cm^?1. A satisfactory agreement with experiment is also found for the adsorption energy (6.7 vs. 5.6 eV). The bridge position lies at only 0.4 eV above the fourfold hollow one. It is found that oxygen adsorption leaves the bare cluster total spin magnetic moment unchanged, but induces appreciable reductions of the local atomic moment on the surface nickel atoms. © 1993 John Wiley & Sons, Inc.     

Pt adsorption on the PbTiO3(110) polar surface: a density functional theory study

The monolayer (ML) and submonolayer Pt on both terminations of PbTiO₃(110) polar surface have been studied by using density functional theory (DFT) with projector‐augmented wave(PAW) potential and a supercell approach. The most favored ML Pt arrangements on PbTiO and O₂ terminations are the hollow site and the short‐bridge site, respectively. By examining the geometries of different ML arrangements, we know that the dominant impetus for stability of the favored adsorption site for PbTiO termination is the Pt–Ti interaction (mainly from covalent bonding), while that for O₂ termination is the Pt–O interaction (mainly from ionic bonding). In addition, the appearance of the gap electronic states in the outermost layers of each termination indicates that a channel for charge transfer between adsorbed layer and substrate is formed. Moreover, the interface hybridization between Pt 5d and O 2p orbitals is also observed, especially for ML Pt on O₂ termination. The stability sequences for various arrangements of 1/2 ML Pt adsorption conform well with those of ML Pt adsorption, and the most stable arrangement is energetically more favorable than the corresponding ML coverage in the view of adsorption energy maximization. The behavior, i.e. the increase in adsorption energy with decrease in coverage, indicates that Pt? Pt interactions weaken those between Pt and the substrate. Copyright © 2009 John Wiley & Sons, Ltd.     

Infrared characterization of hydroxyl groups on MgO: a periodic and cluster density functional theory study

The infrared OH stretching frequencies of the various types of hydroxyl groups on MgO surfaces have been calculated by periodic (VASP) and cluster (Gaussian) DFT simulations. Surface irregularities (mono and diatomic steps, corners, step divacancies, and kinks) have been considered to model the IR spectra of hydroxylated MgO powders. A good correspondence between calculated and experimental frequencies is obtained with the B3LYP functional. Hydrogen-bonding is the parameter which influences most the IR frequency of OH groups, followed by location of OH groups in concave or convex areas of the surface and then oxygen coordination. The evolution of experimental IR spectra upon evacuation at increasing temperature can be rationalized on the basis of calculated thermal stabilities of each kind of OH groups. A new model is finally proposed to help assign the experimental bands, in terms of hydrogen-bonding, local topology of the hydroxylated sites, and coordination of oxygen.     

DOPA醌在Cu(100)表面吸附的密度泛函理论研究

采用广义梯度密度泛函理论(GGA)的BLYP方法结合周期性平板模型,以原子簇Cu41为模拟表面,对DOPA醌分子在Cu(100)表面不同位置的吸附模型进行了构型优化、能量计算以及Mulliken布居分析,结果表明通过相邻的羰基垂直吸附在表面的桥位是其最佳吸附方式,吸附能为247.2310kJ/mol;其次为顶位、顶位R45和穴位,吸附能分别为227.7162kJ/mol、220.7305kJ/mol和217.8456kJ/mol。Mulliken布居分析结果表明整个吸附体系发生了由Cu原子向DOPA醌分子的电荷转移。     

X-ray emission spectroscopy and density functional study of CO/Fe(100)

We report x-ray emission and absorption spectroscopy studies of the electronic structure of the predissociative α(3) phase of CO bound at hollow sites of Fe(100) as well as of the on-top bound species in the high-coverage α(1) phase. The analysis is supported by density functional calculations of structures and spectra. The bonding of "lying down" CO in the hollow site is well described in terms of π to π? charge transfer made possible through bonding interaction also at the oxygen in the minority spin-channel. The on-top CO in the mixed, high-coverage α(1) phase is found to be tilted due to adsorbate-adsorbate interaction, but still with bonding mainly characteristic of "vertical" on-top adsorbed CO similar to other transition-metal surfaces.     

Structure, bonding nature, and binding energy of alkanethiolate on As-rich GaAs (001) surface: a density functional theory study

Chemisorption of alkanethiols on As-rich GaAs (001) surface under a low coverage condition was studied using first principles density functional calculations in a periodic supercell approach. The thiolate adsorption site, tilt angle and its direction are dictated by the high directionality of As dangling bond and sulfur 3p orbital participating in bonding and steric repulsion of the first three CH2 units from the surface. Small charge transfer between thiolate and surface, strong dependence of total energy on tilt angle, and a relatively short length of 2.28 A of the S-As bond indicate the highly covalent nature of the bonding. Calculated binding energy of 2.1 eV is consistent with the available experimental data.     

Ab initio study of epitaxial growth on stepped Si(100) surface

Diffusion of a Si adatom over the reconstructed Si(100) surface with a single-height step on it is studied using the pseudopotential total energy method. The S_B rebonded step is shown to act as a good sink for adatoms descending onto the lower ledge. This is due to the presence of deep traps on the lower terrace and to the negative Ehrlich-Schwoebel barrier (the activation barrier for descent from the edge is 0.23 eV lower than for the motion on a flat surface). The diffusion characteristics of the adatom on both terraces are virtually unaffected by the presence of the step. However, the dimer buckling sequence on a lower terrace is strongly dependent on the position of the adatom along the diffusion path. © 1997 John Wiley & Sons, Inc.     

Adsorption of N2O on Cu(100): a combined scanning tunneling microscopy and density functional theory study

The adsorption of N(2)O on Cu(100) has been studied by using scanning tunneling microscopy (STM). In the first molecular layer N(2)O forms a densely packed c(3 x 2) structure, in which the molecules occupy two different adsorption sites. The bonding strength of this layer is found to be very weak as revealed by a low desorption temperature and the formation of misalignments and defects. Density functional theory (DFT) finds a stable c(3 x 2) structure in which the molecules are considerably bent due to charge transfer. In model calculations for a 2 x 2 hollow phase we show that in order to reach the chemisorbed, bent configuration, the molecules have to pass an activation barrier. In the experimentally accessible range, this is apparently not possible and the molecules remain in a stable physisorbed state.     

Adsorption and migration growth of the Ce,O, and F adatoms on the CeO2 (111) surface: A density functional theory study

In order to investigate the microscopic behavior of the crystal surface growth of the fluorinated cerium dioxide polishing powder, the adsorption and migration of the Ce, O, and F atoms on the CeO₂ (111) surface were studied by using density functional theory with Hubbard correction +U. The adsorption energies of three single atoms at five high-symmetry sites and the migration activation energies along the migration pathway on the CeO₂ (111) surface were calculated. Results show that the most stable adsorption sites of the Ce, O, and F atoms were the O_h, Ce_bri, and Ce_t sites, respectively. The Ce atom migrated from the O_h to the O_t site. The O atom migrated from the Ce_bri to the O_bri site. The F atom migrated from the Ce_t to the O_h site. The migration activation energies of the Ce, O, and F atoms along the migration pathways were 1.526, 0.597, and 0.263 eV, respectively. The F adatom does not change the spatial configuration of the Ce and the O atoms. When the O vacancy occurs on the CeO₂ (111) surface, the F adatom can make up for the O vacancy defect.     

CO adsorption on Ag(100) and Ag/MgO(100)

Theoretical studies of CO adsorption on a two-layer Ag(100) film and on a two-layer Ag film on a MgO(100) support are reported. Ab initio calculations are carried at the configuration interaction level of theory using embedding methods to treat the metal-adsorbate region and the extended ionic solid. The metal overlayer is considered in two different structures: where Ag-Ag distances are equal to the value in the bulk solid, and for a slightly expanded lattice in which the Ag-Ag distances are equal to the O-O distance on the MgO(100) surface. The calculated adsorption energy of Ag(100) on MgO(100) is 0.58 eV per Ag interfacial atom; the Ag-O distance is 2.28 A. A small transfer of electrons from MgO to Ag occurs on deposition of the silver overlayer. CO adsorption at an atop Ag site is found to be the most stable for adsorption on the two-layer Ag film and also for adsorption on Ag deposited on the oxide; CO adsorption energies range from 0.12 to 0.19 eV. The CO adsorption energy is reduced for the Ag/MgO system compared to adsorption on the unsupported metal film thereby providing evidence for a direct electronic effect of the oxide support at the metal overlayer surface. Expansion of the Ag-Ag distance in the two-layer system also reduces the adsorption energy.