Effect of oxygen vacancies on adhesion at the Nb/Al2O3 and Ni/ZrO2 interfaces

The atomic and electronic structures of the Nb/Al₂O₃(0001) and Ni/ZrO₂(001) interfaces are calculated using density-functional theory. The formation energy of oxygen vacancies is estimated in bulk materials and in surface layers and interfaces for different uppermost atomic layers of oxide surfaces. The work of separation of metal films from oxide surfaces is determined. The effect of oxygen vacancies on the bonding of transition metals to atoms of a substrate determining adhesion at the metal-oxide interfaces is discussed. It is shown that the Nb(Ni)-O interaction at the interfaces weakens in the presence of surface oxygen vacancies.     

Oxygen reduction reactions in the SOFC cathode of Ag/CeO2

The interactions between oxygen molecules and a silver surface or a CeO₂(111) supported atomic layer of silver are predicted using first-principles calculations based on spin polarized DFT with PAW method. The juncture between the CeO₂(111), the atomic layer of silver, and O₂ represents a triple-phase boundary (TPB) whereas the interface between silver surfaces and O₂ corresponds to a 2-phase boundary (2PB) in a solid oxide fuel cell (SOFC). Results suggest that the O₂ dissociation process on a monolayer of silver supported by CeO₂(111) surfaces (or TPB) with oxygen vacancies has lower reaction barrier than on silver surfaces (or 2PB), and the dissociated oxygen ions can quickly bond with subsurface Ce atom via a barrierless and highly exothermic reaction. The oxygen vacancies at TPB are found to be responsible for the lower energy barrier and high exothermicity because of the strong interaction between subsurface Ce and adspecies, implying that oxygen molecules prefer being reduced at TPB than on silver surfaces (2PB). The results suggest that, for a silver-based cathode in a SOFC, the adsorption and dissociation of oxygen occur rapidly and the most stable surface oxygen species would be the dissociated oxygen ion with − 0.78|e| Bader charges; the rate of oxygen reduction is most likely limited by subsequent processes such as diffusion or incorporation of the oxygen ions into the electrolyte.     

Atomic oxygen adsorption on Pb(1 0 0)

We study atomic oxygen adsorption on a Pb(1 0 0) surface using density functional theory. The structures, binding energies, work function, and charge transfer of on-surface and subsurface adsorption are investigated at a range of coverages from 0.06 to 1.00 ML. The energetically favored adsorption site for on-surface adsorption is found to be a distorted hollow site for the whole coverage range studied. The distorted structures are stabilized by mixing of 6s and 6p states of lead mediated by the 2p states of oxygen. For subsurface adsorption, the sub-bridge site is found to be preferred to the sub-hollow site at low coverages, the two being nearly equal in energy at monolayer coverage. At 0.11 ML coverage, diffusion from an on-surface hollow site to a sub-bridge site is found to be barrierless, suggesting facile subsurface oxidation at low coverages. Combined on-surface and subsurface adsorption leads to the formation of a two-layer oxide structure resembling β-PbO.     

Adsorption of oxygen on Ag(110) studied by high resolution ELS and TPD

Adsorption of oxygen on Ag(110) has been studied by high resolution electron energy loss spectroscopy (ELS) and temperature programmed desorption (TPD) in the temperature range from ? 160°C to 310°C. At ? 160°C oxygen is absorbed as a diatomic species. The low vibrational frequency of the O-O stretch vibration is explained in terms of charge transfer from the metal into the π¹ antibonding orbital and donation from the π bonding orbital to the metal. A tentative model is presented, according to which the molecule is adsorbed in the grooves of the (110) surface with its axis parallel to the surface. It is explicitly shown that this diatomic species is the precursor for dissociative adsorption of oxygen at temperatures above ? 100°C. Upon dissociation part of the diatomic species is desorbed. Between ? 100°C and + 310°C a single type of adsorbed atomic oxygen is observed which is desorbed at 310°C. Above 150°C adsorbed atomic oxygen also diffuses to subsurface sites. Below 450°C subsurface oxygen neither desorbs nor diffuses into the bulk, although it does exchange with adsorbed atomic oxygen at a temperature below 310°C. Therefore, both forms of atomic oxygen coexist at temperatures at which ethylene epoxidation occurs.     

Structure and interconversion of oxygen-vacancy-related defects on amorphous silica

Atomic structure and structural stability of neutral oxygen vacancies on amorphous silica are investigated using combined Monte Carlo and density functional calculations. We find that, unlike their bulk counterparts, the Si-Si dimer configuration of surface oxygen vacancies is likely to be unstable due to the high tensile strains induced, thereby undergoing thermally activated transformation with a moderate barrier into other stable configurations including dicoordinated silicon, silanone, or a subsurface Si-Si dimer, depending on the local surface structure. Pathways for the interconversion between these oxygen-vacancy-related defects are presented with a discussion of their viability.     

Pattern formation during the CO-oxidation involving subsurface oxygen

This paper focuses on subsurface oxygen and its influence on pattern formation during CO-oxidation on platinum surfaces. For the observation of spatiotemporal pattern formation during catalytic reactions the photoelectron emission microscope (PEEM) has proven to be an excellent real-time imaging instrument, capable of tracking local work function changes. The existence of subsurface oxygen on platinumlike surfaces has been extensively discussed and for palladium its presence has been clearly established during rate oscillations. Subsurface oxygen is defined at this point as an atomic O species located directly underneath the uppermost metal crystal layer; its dipole moment therefore considerably lowers the work function of the surface. Here we review some of the investigations involving subsurface oxygen, focusing on the role subsurface oxygen might play in pattern formation during CO-oxidation on platinum. We will also present some new results, where this species clearly interacts with chemisorbed oxygen under restrictions by boundary conditions on the Pt(110) single crystal. These previously (through microlithography) constructed domain boundaries on the surface are made out of Rh or Pd, and they are acting as an additional source of CO molecules for the Pt surface. (c) 2002 American Institute of Physics.     

Reversible transition between thermodynamically stable phases with low density of oxygen vacancies on the SrTiO3(110) surface

The surface reconstruction of SrTiO3(110) is studied with scanning tunneling microscopy and density functional theory (DFT) calculations. The reversible phase transition between (4×1) and (5×1) is controlled by adjusting the surface metal concentration [Sr] or [Ti]. Resolving the atomic structures of the surface, DFT calculations verify that the phase stability changes upon the chemical potential of Sr or Ti. In particular, the density of oxygen vacancies is low on the thermodynamically stabilized SrTiO3(110) surface.     

Correlation between green luminescence and morphology evolution of ZnO films

Photoluminescence and atomic force microscopy have been used to characterize ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD) at varied growth pressures. The surface morphology with different grain structures has strong influence on the green photoluminescence of ZnO. When large discrete islands or structureless overgrowth cover the rough surface, broad green emissions around 500 nm go beyond the ultraviolet (UV) emission band; whereas, when the surface is packed closely with small grains, only weak green emission is observed with a red-shift to 528 nm. This variation of green emissions is ascribed to changes in the charge states of oxygen vacancies, which is strongly dependent on the surface morphology and grain structures. Based on the grain boundary defect model, two possible recombination processes for the green emission are proposed and discussed in detail. PACS 68.55.Jk; 78.55.Et; 81.05.Dz     

An Ab Initio Study of the Structural and Electronic Properties of the Low-Defect TiC(110) Surface Simulating Oxygen Adsorption after Exposure to Laser Plasma

An ab initio simulation of the adsorption of atomic oxygen on the low-defect titanium carbide (110) surface reconstructed by laser radiation was performed. The relaxed atomic structures of the (110) surface of the O/Ti _x C _y system with Ti and C vacancies observed during the thermal treatment were studied in terms of the density functional theory. DFT calculations of their structural, thermodynamic, and electronic properties were performed. The bond lengths and adsorption energies were determined for various reconstructions of the atomic structure of the O/Ti _x C _y (110) surface. The effects of the oxygen adatom on the band and electronic spectra of the O/Ti _x C _y (110) surface were studied. The effective charges on the titanium and carbon atoms surrounding the oxygen atom in various reconstructions were determined. The charge transfer from titanium to oxygen and carbon atoms was found, which is determined by the reconstruction of the local atomic and electronic structures and correlates with chemisorption processes. The potential mechanisms of laser nanostructuring of the titanium carbide surface were suggested.     

Theoretical study of oxygen adsorption at the Fe(1 1 0) and (1 0 0) surfaces

Electronic, magnetic and structural properties of atomic oxygen adsorbed in on-surface and subsurface sites at the two most densely packed iron surfaces are investigated using density functional theory combined with a thermodynamics formalism. Oxygen coverages varying from a quarter to two monolayers (MLs) are considered. At a 1/4 ML coverage, the most stable on-surface adsorption sites are the twofold long bridge sites on the (1 1 0), and the fourfold-hollow sites on the (1 0 0) surface. The presence of on-surface oxygen atoms enhances the magnetic moments of the atoms of the two topmost Fe layers. Detailed results on the surface magnetic properties, due to O incorporation, are presented as well. Subsurface adsorption is found unfavored. The most stable subsurface O, in tetrahedral positions at the (1 0 0) and octahedral ones at the (1 1 0) surface, are characterized by substantially lower binding than that in the on-surface sites. Subsurface oxygen increases the interplanar distance between the uppermost Fe layers. The preadsorbed oxygen overlayer enhances binding of subsurface O atoms, particularly for tetrahedral sites beneath the (1 1 0) surface.     

Atomic and electronic origins of a type- C defect on Si(001)

We present an extensive set of ab initio calculations for a type- C defect on Si(001). Various models belonging to subsurface defects are studied. A substitutional B in the second surface layer is predicted as a possible atomic origin of this defect. However, H and O coupled with second-layer vacancies and a substitutional C are not responsible for a type- C defect. We also discuss how the electronic structure of a type- C defect contributes to its specific scanning tunneling microscopy images.     

The influence of oxygen vacancies on the electronic and magnetic properties of perovskite-like SrFeO3-x

The electronic, magnetic properties and lattice relaxations of oxygen-deficient cubic strontium ferrite, SrFeO_2.875, in ferromagnetic configuration are studied by means of the density functional theory using LCAO basis (SIESTA code) calculations. It is shown that Fe and Sr atoms are displaced from oxygen vacancies while oxygen anions are attracted to the vacancies. The DOS distributions, magnetic moments and atomic effective charges are analyzed in comparison with vacancy free SrFeO₃; these parameters are found to change weakly with appearance of oxygen vacancies, in contrast to conventional ionic picture. Some strengthening of Fe-O covalent bonds in the vicinity of the oxygen vacancy is found. The formation energy of oxygen vacancies and divacancies are evaluated.     

A reactive force-field (ReaxFF) Monte Carlo study of surface enrichment and step structure on yttria-stabilized zirconia

To investigate surface segregation in yttria-stabilized zirconia (YSZ), DFT energies describing surface energy as a function of yttrium lattice position were used to parameterize a reactive-force field (ReaxFF). We used ReaxFF to perform Monte Carlo (MC) simulated annealing to sample structural configurations of flat YSZ (111) and vicinal YSZ (111) stepped surfaces. We evaluated yttrium surface segregation, oxygen vacancy position, and surface step composition for flat and stepped YSZ surfaces. It is thermodynamically favorable for yttrium atoms to segregate to the surface of YSZ, and specifically to step edge sites. Surface saturation of yttrium occurs at approximately 40% (40:60 Y:Zr ratio) while yttrium concentration at the step edge does not approach a saturation value, suggesting that steps on the YSZ surface are mainly yttria-terminated. We found that it is thermodynamically favorable for oxygen vacancies to occupy positions in the subsurface layer of YSZ, and a higher fraction of vacancies occupy positions NN to Y than NN to Zr. Yttrium segregation to step edges on the YSZ surface does not lower the surface formation energy of the stepped surface below that of the flat (111) termination, suggesting that the stability of YSZ surface steps observed experimentally is due to kinetic barriers for surface re-ordering.     

Structure and local-equilibrium thermodynamics of the c(2x2) reconstruction of rutile TiO2 (100)

We resolve the structure of a c(2x2) reconstruction of the rutile TiO2 (100) surface using a combination of transmission electron diffraction, direct methods analysis, and density functional theory. The surface structure contains an ordered array of subsurface oxygen vacancies and is in local thermodynamic equilibrium with bulk TiO2, but not the with oxygen gas-phase environment. The transition into a bulklike (1x1) reconstruction offers insights into the time-dependent local thermodynamics of TiO2 surface reconstruction under global nonequilibrium conditions.     

Effects of O defects on adsorption of small Ag clusterson a MgO(001) surface

The interaction of Ag atoms with a defective MgO(001) surface is systematically studied based on density functional theory. The Ag clusters are deposited on neutral and charged oxygen vacancies of the MgO(001) surface. The structures of Ag clusters take the shape of simple models of two- or three-dimensional (2D and 3D) metal particles deposited on the MgO surface. When the nucleation of the metal clusters occurs in the F_s (missing neutral O) centre, the interaction with the substrate is considerably stronger than that in the F_s⁺ (missing O^- ) centre. The results show that the adsorption of Ag atoms on the MgO surface with oxygen vacancy is stronger than on a clear MgO surface, thereby attracting more Ag atoms to cluster together, and forming atomic islands.     

Effect of preheating temperatures on microstructure and optical properties of Na-doped ZnO thin films by sol–gel process

The chemical composition, crystalline structure, surface morphology and photoluminescence spectra of Na-doped ZnO thin films with different heat treatment process were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and a fluorescence spectrometer. The results show that preferred orientation, residual stress, average crystal size and surface morphology of the thin films are strongly determined by the preheating temperature. The effects of preheating temperature on microstructure and surface morphology have been discussed in detail. The photoluminescence spectra show that there are strong violet & UV emission, blue emission and green emission bands. The violet & UV emission is ascribed to the electron transition from the localized level below the conduction band to the valence band. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band, and the electron transition from the shallow donor level of interstitial zinc to the valence band. The green emission is assigned to the electron transition from the level of ionized oxygen vacancies to the valence band.     

Ab-initio investigation of Ni(Fe)/ZrO2(0 0 1) and Ni-Fe/ZrO2(0 0 1) interfaces

The atomic and electronic structures of Me/ZrO₂(0 0 1) interfaces, where Me is Ni, Fe or a Ni-Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO₂(0 0 1) interfaces is calculated. High adhesion at both Me/(ZrO₂)_O and Me/(ZrO₂)_Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal-ceramic interfaces is also investigated. It is shown that Ni(Fe)-O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni-Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.     

Surface structural evolution during the thermal decomposition of a PdO(101) thin film

We used scanning tunneling microscopy (STM) to characterize PdO(101) thin films grown on Pd(111), and the structural changes that occur during isothermal decomposition. We find that the PdO(101) thin films have high-quality surface structures that are characterized by large, crystalline terraces with low concentrations of point defects. Small domains of single-layer oxide are also present on the top layer of relatively thick PdO(101) films grown at 500 K. The thinner PdO(101) films exhibit negligible quantities of such domains, apparently because new domains agglomerate rapidly as the film thickness decreases. We find that the isothermal decomposition rate of a PdO(101) film at 720 K exhibits an autocatalytic regime in which the rate of oxygen desorption increases as the oxide decomposes. Our STM results demonstrate that reduced sites created during oxide decomposition catalyze further PdO decomposition, and reveal strong kinetic anisotropies in the decomposition. The kinetic anisotropies produce one-dimensional reaction fronts that propagate preferentially along the atomic rows of the PdO(101) surface, resulting in the formation of long chains of reduced sites. We also find that reduced sites promote oxygen recombination in neighboring rows of the Pd(101) structure, causing loops and larger aggregates of reduced sites to form. The promotion of decomposition across the atomic rows can qualitatively explain the autocatalytic desorption kinetics. Finally, the STM images provide evidence that underlying PdO(101) layers transfer oxygen to reduced surface domains, thus producing large domains of PdO(101) islands that coexist with reduced domains as well as the larger PdO(101) terraces of the initial surface. Re-oxidation of the surface acts to sustain the autocatalytic decomposition kinetics, and provides a mechanism for oxygen atoms to ultimately evolve from the subsurface of the PdO(101) film.     

On the origin of the substrate-induced oxidation of Ni/NiO(001) studied by X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulations

Metallic Ni, vapor-deposited on NiO(001) near room temperature, could be gradually oxidised upon annealing between 800 K and 940 K in Ultra High Vacuum (UHV), as evidenced by X-ray Photoelectron Spectroscopy for initial Ni coverage of 1.6, 3.8 and 7.5 equivalent monolayers (ML). The time dependence of the oxidation process was consistent with a diffusion mechanism, supplying oxygen via the NiO crystal to a coalesced particulate deposit and resulting in an oxide shell, which grew over the entire surface and enclosed a shrinking metallic core. Similar to the well known behaviour upon gas phase oxidation, the process was fast within a depth of two atomic layers of Ni, limited by the diffusive supply of oxygen from the substrate. Molecular Dynamics Simulations for 0.06, 0.11 and 0.22 ML of Ni ions deposited on a model NiO(001) substrate indicated the formation of NiO islands via oxygen ions transferred from the surface and near-surface layers of the crystal. A significant atomic concentration of oxygen vacancies of the order of 10 to 20% could be created in each underneath layer, before the next one started donating lattice anions. This suggests a possible explanation for the aforementioned NiO-substrate-induced oxidation of deposited Ni, whereby the formation of oxygen vacancies inside the crystal supplies the necessary oxygen.     

La1−xCaxCoO3 perovskite-type oxides: Identification of the surface oxygen species by XPS

A detailed study of the La_1−xCa_xCoO₃ perovskites surface by XPS was carried out since this is a potentially useful tool to identify the oxygen species involved in the catalytic reaction and discriminate them. Mainly, the concentration of surface oxygen vacancies (λ′) can be estimated from the XPS atomic ratio.