Perfect MgO Surface Research Articles

Charge reordering of MgO (1 0 0) surface by Sn cluster deposition: Implications for heterogeneous catalysis

Catalysis with cluster-substrate complex materials has relevance for fundamental understanding and technological applications. Present work reports a systematic DFT study on Snn (n = 1–6, 10) cluster adsorbed on catalytically relevant MgO(1 0 0) substrate. Results reveal that Sn-Sn interaction is stronger than Snn-MgO interaction which helps in retaining gas phase motif during interaction. While smaller size tin clusters maximizes interaction with perfect MgO(1 0 0) surface through parallel orientation (more number of bond with surface atoms), larger cluster anchors on surface by single Sn-O bond only. Adsorption form of Sn cluster on perfect MgO surface are different from those reported in literature for gold cluster. Whilst gold cluster has been known for pulling charge from MgO, tin clusters provides electronic charge to MgO surface. Sn cluster reciprocate relatively higher reconstruction in MgO substrate due to charge transfer from adsorbate to substrate. This lead to significant reordering of electronic charge in MgO surface and induces variance in charge polarity across slab. Variance of charge polarity was reaffirmed by different adsorption behavior of water molecule across slab. Since ionic liquids has huge implications in catalyzing a solution phase reaction due to their polarity effect, likewise observed polarity variance along MgO surface endorses its implication in heterogeneous catalysis.

Tuning the charge states of CrW2O9 clusters deposited on perfect and defective MgO(001) surfaces with different color centers: A comprehensive DFT study

The structures and electronic properties of bimetallic oxide CrW2O9 clusters supported on the perfect and defective MgO(001) surfaces with three different color centers, FS (0), FS (+), and FS (2+) centers, respectively, have been investigated by density functional theory calculations. Our results show that the configurations, adsorption energies, charge transfers, and bonding modes of dispersed CrW2O9 clusters are sensitive to the charge states of the FS centers. Compared with the gas-phase configuration, the CrW2O9 clusters supported on the defective surfaces are distorted dramatically, which exhibit different chain structures. On the perfect MgO surface, the depositions of clusters do not involve obvious charge transfer, while the situation is quite different on the defective MgO(001) surfaces in which significant electron transfer occurs from the surface to the cluster. Interestingly, this effect becomes more remarkable for electron-rich oxygen vacancies (FS (0) center) than that for electron-poor oxygen vacancies (FS (+) and FS (2+) centers). Furthermore, our work reveals a progressive Brønsted acid sites where spin density preferentially localized around the Cr atoms not the W atoms for all kinds of FS-centers, indicating the better catalytic activities can be expected for CrW2O9 cluster on defective MgO(001) surfaces with respect to the W3O9 cluster.

Theoretical Study of the Adsorption of Formaldehyde on Magnesium Oxide Nanosurfaces: Size Effects and the Role of Low-Coordinated and Defect Sites

The electronic and geometrical structures of nanoclusters of MgO have been investigated by density functional calculations. The dependence of the properties on size is studied. Defect formation energies are also calculated for various ionic and atomic vacancies. A study of the adsorption of formaldehyde at various sites has shown that the lower coordination sites are preferred so much that, in cases where the nanocluster can adapt itself to expose more of the lower coordination sites, it readily does so. A strong four-membered ring involving the carbonyl group and surface MgO units is formed. This results in a weakening of the carbonyl bond to almost a single bond. The adsorbed species is formate-like in structure, the second CO bond being formed with a surface oxide. The surface Mg−O bonds in the four-center complex are also considerably weakened. The adsorption process is highly exothermic. An investigation of adsorption of formaldehyde at defect sites has revealed that many interesting adsorption products are formed depending on the nature of the vacancy. Adsorption at these positions releases more energy than at perfect MgO surfaces.

F+ laser performance and interaction of Cu, Ag and Au at the reduced oxygen coordination of MgO surface: first principles calculations

F+ laser performance and interaction of the title group IB transition metals at the reduced oxygen coordination of MgO surface were investigated using the TD and DFT methods of ab initio molecular electronic structure calculations. The considered ion clusters were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces and the nearest neighbor ions to the F+ site were allowed to relax to equilibrium in each case. The F+ laser performance fades quickly as the reduced oxygen coordination decreases from 5 to 4 to 3. The relaxed excited states (RESs) of the defect containing surfaces are compact and deep below the conduction bands of the perfect MgO surface. The probability of orientational destruction of the center in laser experiment is expected to follow the order flat>corner>edge. The excited state at the edge has higher energy than that at the flat or at the corner. F+ is easily formed at the lower oxygen coordination and the disappearance of anisotropy and 2p splitting observed in absorption of F+ at the surface follow the order corner>flat>edge. The Glasner–Tompkins relation is generalized to include the F+ bands at the reduced oxygen coordination of a metal oxide surface. As far as the adsorbate–substrate interactions are concerned, the F+ center enhances the adsorptivity of Ag, Cu and Au by ca. 1.91–3.33 eV and changes the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies follow the order Cu>Au>Ag and are explainable in terms of electrostatic potential curves, energy gaps and spin pairing. Cu and Ag act as electron donors while Au acts as electron acceptor and the MgO surface cannot be made semiconducting by F+ imperfection.

The prediction of metastable impact electronic spectra (MIES): perfect and defective MgO(001) surfaces by state-of-the-art methods

We re-examine the theory of metastable impact electron spectroscopy (MIES) in its application to insulating surfaces. This suggests a quantitative approach which takes advantage of recent developments in highly efficient many-electron computational techniques. It gives a basis to the interpretation of experimental MIES spectra for perfect and defective surfaces. Our method is based on a static approach to predicting Auger de-excitation (AD) rates of He∗(1s2s) projectiles. A key quantity is the surface density of states (DOS) projected on the 1s orbital of the He∗ atom, which is calculated along its trajectory. We use density functional theory within both supercell geometry and embedded cluster models to calculate MIES spectra for the perfect MgO surface and for an MgO surface with different concentrations of adsorbed oxygen atoms. First we calculate the Auger de-excitation rates at various positions of the projectile above the surface. To predict MIES spectra, we integrate over projectile trajectories, with a subsequent weighted averaging with respect to various lateral positions of He∗ above the MgO surface unit cell. It is important to examine final-state effects for a correct comparison between theory and experiment, especially when there are localised defect states.

Mg clusters on MgO surfaces: study of the nucleation mechanism with MIES and ab initio calculations

We combined experimental studies using ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) with abinitio calculations of metal adsorption on the perfect MgO surface and at defect sites in order to elucidate the role of surface defects in the initial stages of nucleation and growth of metal clusters at oxide surfaces. MgO films (2 nm thick) grown on Mo and W substrates were used as a prototype system. The MIES and UPS (HeI) spectra were collected insitu, and the growth of Mg clusters was observed by monitoring the dynamics of additional MIES peaks during Mg deposition. TPD experiments were made in order to monitor the surface coverage by Mg clusters and to determine the Mg desorption energies. Interpretation of the results was made on the basis of theoretical modelling using density functional theory (DFT) calculations in both periodic and embedded cluster models. The geometric and electronic structures of the surface terrace, F-centre, positively charged anion vacancy, and step edge at the MgO(001) surface were calculated, and their role in adsorption and clustering of Mg atoms on this surface was studied. The absolute position of the top of the surface valence band of MgO with respect to the vacuum was calculated and compared with the MIES results. The MIES spectra were modelled on the basis of surface density of states (SDOS). The calculated SDOS predicted the location of additional peaks in the band gap and their shift as a function of Mg concentration on the surface in agreement with the MIES data. The desorption energies of Mg atoms from small Mg clusters formed at step edges are found to be about 1.3 eV atom-1. Comparison between the theoretical results and the experimental data suggests preferential initial adsorption of Mg atoms at steps and kinks, rather than at charged and neutral vacancies. At larger exposures these Mg atoms serve as the nucleation sites.

Water monolayers on MgO(100): structural investigations by LEED experiments, tensor LEED dynamical analysis and potential calculations

LEED experiments and tensor LEED dynamical analysis of diffracted intensities are combined with semi-empirical potential calculations to determine the detailed structure of water monolayers adsorbed on MgO(100) single-crystal surfaces. The densities of the two experimentally observed solid phases are evaluated from LEED kinetic isotherms. A numerical procedure based on energy minimization at 0 K gives the positions and the orientations of the water molecules in various commensurate phases. The calculations show that the perfect MgO surface accommodates very stable flat monolayers with the oxygen atoms of the water molecules adsorbed nearly above the cation sites. These structures, which are energetically very close, differ only by the mutual orientations of the molecules in a plane parallel to the surface. The most stable configuration corresponds to a (1×1) geometry, with an adsorption energy per molecule of −945 meV, a value in fair agreement with both experiments and ab initio calculations. The other geometries including the experimental p(3×2) phase correspond to slightly higher energy values. The results of the calculations on the positions of the oxygen atoms in the p(3×2) water monolayer are used as reference structure for a dynamical tensor LEED analysis and it is shown that the agreement is good.

Acido-basic properties of specific surface sites of magnesium oxide

We report a self consistent tight binding calculation of the electronic structure of (100) and (110) perfect MgO surfaces, as well as a rough (100) surface. The local charges and electronic gap are discussed with special attention borne on the value of the Madelung field. Acido-basic properties of various sites at these surfaces are further analyzed through the adsorption energy of H+ and OH− species.