Theoretical study of CO and Pb adsorption on the Ni(1 1 1) and Ni3Al(1 1 1) surfaces

Adsorption of CO molecules and Pb atoms on the Ni(1 1 1) and Ni₃Al(1 1 1) substrates is studied theoretically within an ab initio density-functional-theory approach. Stable adsorption sites and the corresponding adsorption energies are first determined for stoichiometric surfaces. The three-fold hollow sites (fcc for Pb and hcp for CO) are found most favourable on both substrates. Next, the effect of surface alloying by a substitution of selected topmost substrate atoms by Pb or Ni atoms on the adsorption characteristics is investigated. When the surface Al atoms of the Ni₃Al(1 1 1) substrate are replaced by Ni atoms, the Pb and CO adsorption energies approach those for a pure Ni(1 1 1) substrate. The Pb alloying has a more substantial effect. On the Ni₃Al(1 1 1) substrate, it reduces considerably adsorption energy of CO. On the Ni(1 1 1) substrate, CO binding strengthens slightly upon the formation of the Ni(1 1 1)p(2×2)-Pb surface alloy, whereas it weakens drastically when the Ni(1 1 1)-Pb surface alloy is formed.     

DFT study of Pt adsorption on low index SrTiO3 surfaces: SrTiO3(1 0 0), SrTiO3(1 1 1) and SrTiO3(1 1 0)

Density Functional Theory has been used to determine the energetically preferred structures of submonolayer, monolayer, and multilayer Pt films on both ideal terminations of SrTiO₃(1 0 0), SrTiO₃(1 1 1), and SrTiO₃(1 1 0). The strength of the resulting metal/metal oxide interfaces was characterized by the adsorption energy of the film and the film’s work of separation. The two polar surfaces, SrTiO₃(1 1 1) and SrTiO₃(1 1 0), form significantly stronger interfaces than the non-polar SrTiO₃(1 0 0) surface. Approximate criteria were applied to predict the growth mode of Pt on each surface.     

Density functional study of hypophosphite adsorption on Ni (1 1 1) and Cu (1 1 1) surfaces

Surface structures and electronic properties of hypophosphite, H₂PO₂⁻, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces are investigated in this work by density functional theory at B3LYP/6-31++g(d, p) level. We employ a four-metal-atom cluster as the simplified model for the surface and have fully optimized the geometry and orientation of H₂PO₂⁻ on the metal cluster. Six stable orientations have been discovered on both Ni (1 1 1) and Cu (1 1 1) surfaces. The most stable orientation of H₂PO₂⁻ was found to have its two oxygen atoms interact the surface with two PO bonds pointing downward. Results of the Mulliken population analysis showed that the back donation from 3d orbitals of the transition metal substrate to the unfilled 3d orbital of the phosphorus atom in H₂PO₂⁻ and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H₂PO₂⁻ play very important roles in the H₂PO₂⁻ adsorption on the transition metals. The averaged electron configuration of Ni in Ni₄ cluster is 4s^0.634p^0.023d^9.35 and that of Cu in Cu₄ cluster is 4s^1.004p^0.033d^9.97. Because of this subtle difference of electron configuration, the adsorption energy is larger on the Ni surface than on the Cu surface. The amount of charge transfers due to above two donations is larger from H₂PO₂⁻ to the Ni surface than to the Cu surface, leading to a more positively charged P atom in Ni_nH₂PO₂⁻ than in Cu_nH₂PO₂⁻. These results indicate that the phosphorus atom in Ni_nH₂PO₂⁻ complex is easier to be attacked by a nucleophile such as OH⁻ and subsequent oxidation of H₂PO₂⁻ can take place more favorably on Ni substrate than on Cu substrate.     

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S_A and rebonded S_B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and directions.     

Adsorption of methanol and methoxy on NiAl(1 1 0) and Ni3Al(1 1 1): A DFT study

The adsorption of methanol and methoxy on NiAl(1 1 0) and Ni₃Al(1 1 1) has been investigated using density functional theory (DFT). Optimised adsorption geometries and core level shifts are presented. On both surfaces we find that methanol binds to the Al on-top site via its oxygen atom and with the C–O axis tilted away from the surface normal. Methoxy also shows a preference for Al-dominated sites. On NiAl(1 1 0), we predict that methoxy adsorbs with its oxygen atom in the Al–Al bridge site, while it is determined to be adsorbed with its oxygen atom in a 2Ni + Al hollow site on Ni₃Al(1 1 1), closer to Al than Ni. Surface and adsorbate induced binding energy shifts in the Al 2p states are calculated and found to be in good agreement with experimental high resolution photoelectron spectroscopy results.     

A density functional theory study of H2S decomposition on the (1 1 1) surfaces of model Pd-alloys

In this work, we report density functional theory calculations exploring H₂S dissociation on the (1 1 1) surfaces of Pd, Cu, Ag, Au, and various bimetallic surfaces consisting of those metals. To understand the contributions of lattice strain and electronic ligand effects, the thermodynamics of each elementary dissociation step were explored on model bimetallic surfaces, including PdMPd sandwiches and Pd pseudomorphic overlayers, as well as strained Pd(1 1 1) surfaces and homogeneous Pd₃M alloys. Sulfuric (H₂S, SH, and S) adsorption energies were found to correlate very well with lattice constant, which can be explained by the strong correlation of the lattice constant with d-band center, Fermi energy, and density of states at the Fermi level for strained Pd(1 1 1) surfaces. Compressing the Pd lattice shifts the d-band center away from the Fermi level, lowers the Fermi energy, and reduces the density of d-states at the Fermi level. All three effects likely contribute to the destabilization of sulfuric adsorption on Pd alloys. Introducing ligand effects was found to alter the distribution of the d-states and shift the Fermi level, which eliminates the correlation of the d-band center with the density of states at the Fermi level and the Fermi energy. As a result, the d-band center by itself is a poor metric of the H₂S reaction energetics for bimetallic surfaces. Furthermore, combining strain with ligand effects was found to lead to unpredictable alterations of the d-band. Therefore, adsorption of H₂S, SH, and S on PdMPd surfaces do not accurately predict adsorption on Pd₃M surfaces.     

Density functional study of oxygen adsorption on the Mg3Nd (0 0 1) surface

The adsorption of oxygen atoms on Mg₃Nd (0 0 1) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the generalized gradient approximation (GGA) in the Perdew and Wang (PW91). The most preferred adsorption position was at the top-hollow site. Upon the optimization on top-hollow site with different coverage, it was found that the adsorption energy decreased with oxygen coverage. The density of states analysis showed that obvious charge transfer took place between O atom and the nearest Nd atom and chemical bond formed between O atom and the nearest Nd atom after O adsorption. The result of surface energy as a function of chemical potential change of oxygen indicated the clean Mg₃Nd (0 0 1) surface was easy to adsorb oxygen and form 1.00 ML surface.     

Adsorption and reactions of HN3 on Si(1 0 0)-2 × 1: A computational study

We have studied the adsorption and decomposition of HN₃ on Si(1 0 0)-2 × 1 surface using the hybrid density functional B3LYP method and effective core potential basis, LanL2DZ, with Si₁₅H₁₆ as a double dimer surface model for cluster calculations. The result shows that the barriers for the dissociative adsorption of HN₃ forming HN(a) + N₂(g) are quite low by stepwise dissociation processes occurring either on a dimer or across the dimers. The low activation energies are consistent with previous experimental observations that the molecularly adsorbed HN₃(a) can undergo decomposition producing HN(a) at low surface temperatures. On the other hand, the predicted activation energies for the N₃(a) + H(a) formation processes are all relatively higher. These results also explain the absence of the N₃(a) species in HREELS measurements following each annealing experiment. Several selected reaction paths were also confirmed with slab model calculations using an optimization approach coupling the energy and gradient calculations by the slab model with the geometrical optimization using Berny algorithm.In addition, the adsorbate effect was examined for the end-on and side-on molecular configurations. For the side-on adsorption configuration, all possible combinations with 1-4 adsorbates can exist on the four surface Si sites of the double dimers, with adsorption energies lying closely to the multiples of that of a single side-on adsorbate (LM2); i.e., adsorption energies are nearly additive. Interestingly, for the end-on adsorption, only 1 and 2 HN₃ molecules can adsorb on a dimer due to the presence of the negative charges on the remaining Si sites in the neighboring dimer. For the two end-on adsorbates on the same dimer, the total adsorption energy is close to two times that of HN₃(a) or LM1. For the mixed end-on/side-on configurations, only one of each type can co-exist on a single dimer pair (Si₁-Si₂ or Si₃-Si₄) sites with adsorption energy close to the sum of those of one end-on and one side-on adsorbate. Finally, the predicted reaction routes and vibrational frequencies showed good agreement with previous experimental results. The stabilities of many ad-species involved in these reactions with end-on and/or side-on configurations have been predicted together with the transition states connecting those species.     

O2 dissociation on Pd(2 1 1) and Cu(2 1 1) surfaces

We have performed ab initio Density Functional Theory (DFT) based calculations to observe the reactivity of the Pd(2 1 1) and Cu(2 1 1) surfaces towards O₂. In order to properly address the adsorption dynamics, the static potential energy surface calculations have been complemented with first principles molecular dynamics calculations, which reveal interesting steering effects that complicate the dissociation dynamics. We have found that on both surfaces the step microfacets are very reactive and the dissociation of the O₂ molecule at room temperature occurs mostly on those sites.     

A density functional study of atomic oxygen and water molecule adsorption on Ni(1 1 1) and chromium-substituted Ni(1 1 1) surfaces

Density functional theory (DFT) for generalized gradient approximation calculations has been used to study the adsorption of atomic oxygen and water molecules on Ni(1 1 1) and different kind of Ni-Cr(1 1 1) surfaces. The fcc hollow site is energetically the most favorable for atomic oxygen adsorption and on top site is favorable for water adsorption. The Ni-Cr surface has the highest absorption energy for oxygen at 6.86 eV, followed by the hcp site, whereas the absorption energy is 5.56 eV for the Ni surface. The Ni-O bond distance is 1.85 Å for the Ni surface. On the other hand, the result concerning the Ni-Cr surface implies that the bond distances are 1.93-1.95 Å and 1.75 Å for Ni-O and Cr-O, respectively. The surface adsorption energy for water on top site for two Cr atom substituted Ni-Cr surface is 0.85 eV. Oxygen atoms prefer to bond with Cr rather than Ni atoms. Atomic charge analysis demonstrates that charge transfer increases due to the addition of Cr. Moreover, a local density of states (LDOS) study examines the hybridization occurring between the metal d orbital and the oxygen p orbital; the bonding is mainly ionic, and water bonds weakly in both cases.     

A theoretical study of CH4 dissociation on Pt(1 0 0) surface

Density functional theory calculations have been performed on the adsorption of H and CH₃, and the dissociation of CH₄ on Pt(1 0 0) surface. It was found that H was adsorbed on the top and bridge sites, while CH₃ was adsorbed only on the top site. The coadsorption of methyl and hydrogen which has also been investigated shows that the interaction between the two adsorbates is stabilising. In addition, two distinct pathways were explored, differing by the initial adsorbed state of CH₄. They converge readily to the same transition state corresponding to an activation energy value of 0.53 eV. These results compare favourably with existing data in the literature for Pt(1 1 1) and Pt(1 1 0).     

First-principles studies of the oxygen adsorption on unreconstructed and reconstructed Ni(1 1 0) surfaces

First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, (n×1)(n=2,3) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p(n×1) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably.     

Adsorption of Si on Ag(0 0 1) from ab initio study

We have systematically investigated the adsorption of Si on the Ag(0 0 1) surface employing density-functional theory. Various adsorption geometries have been considered for Si coverages up to 2.0 monolayers. Our results show that the behaviors of Si at the early stages of growth on the Ag(0 0 1) surface are governed by a competition between the Si-Si and Si-Ag interactions. From the calculated results, we presented alternative models for the observed 3 × 3 and 4 × 7 structures. Our results provide a reasonable explanation for the experimental findings in a previous work.     

First-principles study on field evaporation of surface atoms from W(0 1 1) and Mo(0 1 1) surfaces

The simulations of field-evaporation processes for surface atoms on W(0 1 1) and Mo(0 1 1) surfaces are implemented using first-principles calculations based on the real-space finite-difference method. The threshold values of the external electric field for evaporation of the surface atoms, which are ∼6 V/Å for tungsten and ∼5 V/Å for molybdenum, are in agreement with the experimental results. While the threshold value of the electric field and the local-field enhancement around the evaporating atoms agree with those expected from the conclusion of the previous study using structureless jellium, the induced charge around the surface atom has a significant difference from that obtained by the jellium model.     

Growth and oxidation of a Ni3Al alloy on Ni(1 0 0)

The growth and oxidation of a thin film of Ni₃Al grown on Ni(1 0 0) were studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (EELS). At 300 K, a 12 Å thick layer of aluminium was deposited on a Ni(1 0 0) surface and subsequently annealed to 1150 K resulting in a thin film of Ni₃Al which grows with the (1 0 0) plane parallel to the (1 0 0) surface of the substrate. Oxidation at 300 K of Ni₃Al/Ni(1 0 0) until saturation leads to the growth of an aluminium oxide layer consisting of different alumina phases. By annealing up to 1000 K, a well ordered film of the Al₂O₃ film is formed which exhibits in the EEL spectra Fuchs-Kliewer phonons at 420, 640 and 880 cm⁻¹. The LEED pattern of the oxide shows a twelvefold ring structure. This LEED pattern is explained by two domains with hexagonal structure which are rotated by 90° with respect to each other. The lattice constant of the hexagonal structure amounts to ∼2.87 Å. The EELS data and the LEED pattern suggest that the γ^′-Al₂O₃ phase is formed which grows with the (1 1 1) plane parallel to the Ni(1 0 0) surface.     

Structure and dynamics of the missing-row reconstruction on O/Cu(0 0 1) and O/Ag(0 0 1)

The geometry and the vibrational properties of missing row reconstructed O/Cu(0 0 1) and O/Ag(0 0 1) surfaces are investigated by means of density functional theory and density functional perturbation theory, using the local density and the generalized-gradient approximations. Our results predict very similar structural and vibrational properties for the two reconstructed surfaces. In the case of copper our calculations reproduce quite accurately the experimental results, while for the missing row reconstructed O/Ag(0 0 1) surface the agreement between theory and experiment is less satisfactory.     

The interaction of carbon with Si(1 1 1):As and Si(1 1 1):H surfaces, a theoretical study

Density functional theory calculations have been performed to determine the adsorption site of carbon at the Si(1 1 1):As and Si(1 1 1):H surfaces at different coverages. The As- and H-passivated surfaces were simulated by replacing the topmost Si layer by As or by saturating the Si dangling bonds with hydrogen atoms, respectively. Different high symmetry sites were considered. Carbon was placed successively in the fourfold (T₄) or threefold coordinated (H₃), the ontop (T₁) sites or substituted for a Si atom in the S₅ position located underneath the Si adatom in the T₄ site. We found that the preferred carbon adsorption site depends on the coverage of the passivated surfaces. At low coverages i.e. at 1/16 ML and 1/3 ML, it prefers a distorted T₄ position whereas at 1 ML, it occupies an H₃ site. This contrasts with the clean surface where the most energetically favored site is the S₅ at all coverages. Carbon adsorption induces a significant change in the structural geometry of the surface atoms, leading to a charge re-arrangement in the surface layers.     

Comparison of O adsorption on Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfaces through first-principles calculations

First-principles calculations were performed to study the properties of O adsorption on Ni₃Al (0 0 1), (0 1 1), and (1 1 1) surfaces using the Cambridge serial total package (CASTEP) code. Stable adsorption sites are identified. The atomic and electronic structures and adsorption energies are predicted. The adsorption sites for O on the Ni₃Al (0 0 1) surface are at the 2Ni–2Al fourfold hollow site, whereas O prefers to adsorb at the Ni–Al bridge site on (0 1 1) surface and 2Ni–Al threefold hollow site on (1 1 1) surface. It is found that O shows the strongest affinity for Al and the state of O is the most stabilized when O adsorbs on (0 0 1) surface, while the affinity of O for Al on (0 1 1) surface is weaker than (0 0 1) surface, and (1 1 1) surface is the weakest. The stronger O and Al affinity indicates more stable Al₂O₃ when oxidized. The experiment has shown that the oxidation resistance of single crystal superalloy in different orientations improves in the order of (1 1 1), (0 1 1), and (0 0 1) surface, suggesting that the oxidation in different crystallographic orientations may be related to the affinity of O for Al in the surface.     

A DFT study of the chemisorption of methoxy on clean and low oxygen precovered Ru(0 0 0 1) surfaces

Adsorption of the methoxy radical on clean and on low oxygen precovered Ru(0 0 0 1) metallic surfaces has been studied by density-functional theory cluster calculations. Methoxy is predicted to be preferentially chemisorbed on both hollow sites (hcp and fcc) of such surfaces, and adopts an upright orientation (C_3ν local symmetry). Such prediction is supported by the good agreement found between the calculated vibrational frequencies and the experimentally observed RAIRS spectra. Regarding the charge transfer process between the adsorbate and the surface, our results suggest that the bonding is dominantly polar covalent which arises from a charge donation from the ruthenium surface to the radical, and the co-adsorbed electronegative oxygens deplete slightly the surface electron density. However, the major difference is not induced through much O-Ru bonding, but indirectly, by lowering the valence d-band center of the system. This results in a lower adsorption energy for methoxy than on the clean Ru(0 0 0 1) surface, in accordance with experimental data. Further, accordingly to the present calculations, the radical is expected to dissociate or desorb more easily on the modified surface but with no participation from the co-adsorbed oxygen atoms.     

First principles study of H2S adsorption and dissociation on Fe(1 1 0)

We report first principles density functional theory (DFT) results of H₂S and HS adsorption and dissociation on the Fe(1 1 0) surface. We investigate the site preference of H₂S, HS, and S on Fe(1 1 0). H₂S is found to weakly adsorb on either the short bridge (SB) or long bridge (LB) site of Fe(1 1 0), with a binding energy of no more than 0.50 eV. The diffusion barrier from the LB site to the SB site is found to be small (∼0.10 eV). By contrast to H₂S, HS is predicted to be strongly chemisorbed on Fe(1 1 0), with the S atom in the LB site and the HS bond oriented perpendicular to the surface. Isolated S atoms also are predicted to bind strongly to the LB sites of Fe(1 1 0), where the SB is found to be a transition state for S surface hopping between neighboring LB sites. The minimum energy paths for H₂S and HS dehydrogenation involve rotating an H atom towards a nearby surface Fe atom, with the S-H bonds breaking on the top of one Fe atom. The barrier to break the first S-H bond in H₂S is low at 0.10 eV, and breaking the second S-H bond is barrierless, suggesting deposition of S on Fe(1 1 0) via H₂S is kinetically and thermodynamically facile.