Range Of Oxygen Chemical Potentials Research Articles

Structures and stability of GaN/Ga2O3 interfaces: a first-principles study

The structures and stability of the interface between GaN and Ga2O3 are theoretically investigated on the basis of first-principles calculations. We determine the stable structures of the interfaces with α-phase and β-phase Ga2O3 (GaN/α-Ga2O3 and GaN/β-Ga2O3 interfaces, respectively) using absolute interface energies. The calculated absolute interface energies for various atomic configurations demonstrate that the stable structure strongly depends on the oxygen chemical potential. For the GaN/α-Ga2O3 interface, the structures with Ga vacancy are stabilized over the wide range of oxygen chemical potential. On the other hand, the interface with Ga–O bonds are favorable over the wide range of oxygen chemical potential for the GaN/β-Ga2O3 interface. Furthermore, we clarify the interface structures of GaN/Ga2O3 by considering the experimental thermal oxidation conditions. The calculated results provide firm theoretical frameworks to understand the formation mechanism of oxide films by thermal oxidation of GaN substrate.

Oxygen-rich tetrahedral surface phase on high-temperature rutile VO2(110)T single crystals

Vanadium dioxide undergoes a metal-insulator transition (MIT) from an insulating (monoclinic) to a metallic (tetragonal) phase close to room temperature, which makes it a promising functional material for many applications, e.g. as chemical sensors. Not much is known about its surface and interface properties, although these are critical in many of its applications. This work presents an atomic-scale investigation of the tetragonal rutile VO$_2$(110)$_\text{T}$ single-crystal surface and reports results obtained with scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS), supported by density-functional theory-based (DFT) calculations. The surface reconstructs into an oxygen-rich (2$\times$2) superstructure that coexists with small patches of the underlying, unreconstructed (110)-(1$\times$1) surface. The best structural model for the (2$\times$2) surface termination, conceptually derived from a vanadium pentoxide (001) monolayer, consists of rings of corner-shared tetrahedra. Over a wide range of oxygen chemical potentials this reconstruction is more stable than the unreconstructed (110) surface as well as models proposed in the literature.

Surface termination and stoichiometry of LaAlO3(001) surface studied by HRTEM

As an important topic of condensed matter physics, metal oxide surfaces often exhibit exotic properties such as high catalytic activity, enhanced ferroelectricity and electronic phase transition, originating from the different local symmetry with respect to the bulk. As the structure determination of oxide surfaces presents challenges to conventional surface science techniques like scanning tunneling microscopy, aberration-corrected transmission electron microscopy (TEM) has been increasingly used to solve structures of oxide surfaces. In this work, the (001) surface of LaAlO3, one of the most used components of oxide heterostructures, has been investigated. Our TEM experiments and extensive image simulations show that the La-O terminated LaAlO3(001) surface undergoes significant reconstructions, forming La vacancies on the surface layer. Energetically, the LaAlO3(001) surface is stable with the reconstructed La-O termination in a wide range of oxygen chemical potentials. Polarity compensation, reduced density of states at the Fermi level and bond enhancement of subsurface oxygen anions all contribute to the stabilization of the reconstructed surface.

Structures and thermodynamic stability of cobalt molybdenum oxide (CoMoO4-II)

This contribution reports density functional theory (DFT) calculations on structural and electronic properties of bulk and surfaces of cobalt molybdenum oxide CoMoO4-II; i.e., a material that enjoys a wide array of chemical catalytic and optical applications. Estimated lattice constants and atomic charges for bulk CoMoO4-II reproduce limited analogous experimental measurements. Bader's charges confirm the ionic nature for metal-O bonds in bulk and surfaces of CoMoO4-II. Plotted partial density of states reveal a narrow band gap of 1.8 eV for bulk CoMoO4-II. We found that cleaving bulk of CoMoO4-II along the low-Miller indices afford twelve distinct surfaces. Upward displacement of oxygen atom becomes evident when contrasting bulk positioning of atoms with relaxed surfaces. The two mixed Mo/O- and Co/O–terminated surfaces dominate the thermodynamic stability diagram at 1 atm and 300 – 1400 K, and across a wide range of oxygen chemical potential. The presence of surface oxygen atoms in these stable surfaces is expected to facilitate the occurrence of oxygen reduction reactions as experimentally demonstrated. Likewise, the adjacent surface cations (Mo4+/Co2+) and anions (O2−) serve as Lewis-acid pairs; i.e., very potent active sites in prominent catalysis reactions.

Electronic, magnetic and structural properties of Co3O4 (100) surface: a DFT+U study

Abstract The three most stable (100), (110), and (111) surfaces exposed by Co 3 O 4 are effective catalysts for various oxidation reactions. Among these surfaces, (100) has not yet received ample attention. In this study, we investigated the structural, electronic and magnetic properties of Co 3 O 4 (100) surface using density functional theory calculations. By considering both stoichiometric and nonstoichiometric surface structures of the two possible terminations, A and B. Besides the greater stability of the newly proposed stoichiometric models compared to nonstoichiometric models reported in previous studies, the results show that the B termination is energetically preferred over the entire range of oxygen chemical potentials. Unlike the bulk, Co 3+ octahedral ions become magnetic at the surface, which leads to interesting surface magnetic properties. Density of states (DOS) indicate a small band gap of 1.15 eV for the B-stoichiometric model, due to the presence of surface states in the bulk band gap. More polar surface with a very narrow band gap is found in the A-nonstoichiometric model. These surface states may play an important role in the magnetism and metallicity observed experimentally in several Co 3 O 4 systems.

Synthesis and properties of fuel cell anodes based on (La0.5 + x Sr0.5 − x )1 − y Mn0.5Ti0.5O3 − δ (x = 0–0.25, y = 0–0.03)

Results are presented of studying electrochemical properties of perovskite-like solid solutions (La0.5 + x Sr0.5 − x )1 − y Mn0.5Ti0.5O3 − δ (x = 0–0.25, y = 0–0.03) synthesized using the citrate technique and studied as oxide anodic materials for solid oxide fuel cells (SOFC). X-ray diffraction (XRD) analysis is used to establish that the materials are stable in a wide range of oxygen chemical potential, stable in the presence of 5 ppm H2S in the range of intermediate temperatures, and also chemically compatible with the solid electrolyte of La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 − δ (LSGMC). It is shown that transition to a reducing atmosphere results in a decrease in electron conductivity that produced a significant effect on the electrochemical activity of porous electrodes. Model cells of planar SOFC on a supporting solid-electrolyte membrane (LSGMC) with anodes based on (La0.6Sr0.4)0.97Mn0.5Ti0.5O3 − δ and (La0.75Sr0.25)0.97Mn0.5Ti0.5O3 − δ and a cathode of Sm0.5Sr0.5CoO3 − δ are manufactured and tested using the voltammetry technique.

Defective α-Fe2O3(0001): an ab initio study.

By using density functional theory calculations at the PBE+U level, we investigated the properties of hematite (0001) surfaces decorated with adatoms/vacancies/substituents. For the most stable surface termination over a large range of oxygen chemical potentials (muO), the vacancy formation and adsorption energies were determined as a function of muO. Under oxygen-rich conditions, all defects are metastable with respect to the ideal surface. Under oxygen-poor conditions, O vacancies and Fe adatoms become stable. Under ambient conditions, all defects are metastable; in the bulk, O vacancies form more easily than Fe vacancies, whereas at the surface the opposite is true. All defects, that is, O and Fe vacancies, Fe and Al adatoms, and Al substituents, induce important modifications to the geometry of the surface in their vicinity. Dissociative adsorption of molecular oxygen is likely to be exothermic on surfaces with Fe/Al adatoms or O vacancies.

Effect of surface reconstruction on the electronic structure of ZnO(0001)

Density functional calculations are presented for a (4$\ifmmode\times\else\texttimes\fi{}$4) reconstruction of the polar ZnO(0001) surface which is, in agreement with previous knowledge, characterized by one ZnO double-layer high triangular islands or pits. Its structure and electronic properties are compared to those of other reconstructions, the (1$\ifmmode\times\else\texttimes\fi{}$1) and the (2$\ifmmode\times\else\texttimes\fi{}$2) reconstructions with a Zn vacancy, and a (4$\ifmmode\times\else\texttimes\fi{}$4) reconstruction with triangular islands and pits that satisfies the electron counting rule. Among the four calculated reconstructions, the model surface is verified to be energetically preferable for a large range of oxygen chemical potential. This justifies its further exploration. Its layer and wave-vector-resolved projected density of states (PDOS) of the O atoms reveals highly surface-localized valence states well below the Fermi level. The PDOS around the Fermi level, however, is very small, which is compatible with the states not being observed by photoemission. Qualitatively experimental surface core-level shifts by Lahiri et al. [Phys. Rev. B 78, 155414 (2008)] compare favorably with the calculated electrostatic potential variations close to the ZnO(0001) surface. An ionic model with everywhere equally charged Zn and O ions turns out not to explain the potential variation obtained from density functional theory. This points towards an additional charge transfer at the surface not covered by the ionic model.

Electronic states and magnetic structure at the Co3O4(110) surface: A first-principles study

Tricobalt tetraoxide (Co${}_{3}$O${}_{4}$) is an important catalyst and Co${}_{3}$O${}_{4}$(110) is a frequently exposed surface in Co${}_{3}$O${}_{4}$ nanomaterials. We employed density-functional theory with the on-site Coulomb repulsion U term to study the atomic structures, energetics, and magnetic and electronic properties of the two possible terminations, $A$ and $B$, of this surface. These calculations predict $A$ as the stable termination in a wide range of oxygen chemical potentials, consistent with recent experimental observations. The Co${}^{3+}$ ions do not have a magnetic moment in the bulk, but become magnetic at the surface, which leads to surface magnetic orderings different from that in the bulk. Surface electronic states are present in the lower half of the bulk band gap and cause partial metallization of both surface terminations. These states are responsible for the charge compensation mechanism stabilizing both polar terminations. The computed critical thickness for polarity compensation is four layers.

Tuning electronic structure and photocatalytic properties by Ag incorporated on (0 0 1) surface of anatase TiO2

The incorporation of Ag on (001) surface of anatase TiO2 has been systematically investigated by means of density functional theory to understand the Ag effects on the electronic structure and photocatalytic properties in Ag/TiO2 composites. Several possible adsorptional, substitutional and interstitial sites with two different Ag concentrations at surface and subsurface layers are examined. Our results about stability of various Ag-incorporated (001) surfaces indicate that the adsorption site is favorable neglect of the oxygen conditions and the substitution site becomes more stable under the oxygen-rich condition, but it becomes difficult to incorporate Ag onto the surface with high concentration, especially for substitution sites in the limited range of oxygen chemical potential. The adsorption of Ag introduces gap states near or below the conduction band minimum (CBM) and the Fermi level locates near or in the conduction band, which can act as photo-generated electron trap centers and inhibit the recombination of electron–hole pairs. The electron transitions from the impurity level to the level above the Fermi level may be responsible for the small visible light absorption peak in experiment. Substitution Ag introduces some localized gap states, while the Fermi level is pinned near the top of valence band, and the impurity states can trap the hole to suppress the recombination of photo-generated carriers. For the interstitial Ag in surface, the Fermi level locates at the bottom of conduction band, and the partial occupied states may also act as electron trap centers which can improve the photocatalytic efficiency.

Growth and Surface Structure of Zinc Oxide Layers on a Pd(111) Surface

The growth and geometric structure of ultrathin zinc oxide films on Pd(111) has been studied by scanning tunneling microscopy, low-energy electron diffraction, and density functional theory calculations. For sub-monolayer coverages, depending on the oxygen pressure, two well-ordered zinc oxide phases with (4 × 4) and (6 × 6) coincidence structures form, which are attributed to H-terminated Zn6O5 and graphite-like Zn6O6 layers, respectively. The (6 × 6) phase exhibits a pronounced oxygen pressure dependence: at low p(O2) a well-ordered (6 × 6) two-dimensional array of O vacancies develops, yielding a layer with a formal Zn25O24 stoichiometry, while at high p(O2) the Zn6O6 monolayer transforms into bilayer islands. For oxide coverages up to 4 monolayers the graphite-like Zn6O6 structure is thermodynamically the most stable phase over a large range of oxygen chemical potentials, before it converges to the bulk-type wurtzite structure. Under oxygen-poor conditions a compressed overlayer of Zn adatoms can be stabilized on top of the Zn6O6 structure.

Intrinsic vacancies in cubic-zirconia bulk and surface

Formation of charged (neutral) vacancies of bulk (surface) cubic-ZrO 2 is calculated from density functional theory, and compared with available experiments. Relationships among vacancy formation energy, electron and element chemical potentials are established within a wide range of oxygen chemical potentials. The +2 (−2) oxygen (zirconium) vacancy is predicted stable near the valence (conduction) band edge of the bulk, and the neutral oxygen vacancy has the lowest formation energy on (1 0 0) O-terminated surface without exhibiting a defect state within the band gap. Therefore, manipulation of oxygen vacancy concentration becomes possible by optimizing growth surface and oxygen chemical potentials.

Surface oxides of the oxygen–copper system: Precursors to the bulk oxide phase?

To gain an initial understanding of the copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water–gas shift reaction, we performed density-functional theory calculations, investigating the interaction of oxygen and copper, focusing on the relative stability of surface oxides and oxide surfaces of the O/Cu system. By employing the technique of “ ab initio atomistic thermodynamics”, we show that surface oxides are only metastable at relevant pressures and temperatures of technical catalysis, with no stable chemisorption phase observed even at very low coverage. Although exhibiting only metastability, these surface oxides resemble the bulk oxide material both geometrically and electronically, and may serve as a precursor phase before onset of the bulk oxide phase. Having identified the bulk oxide as the most stable phase under realistic catalytic conditions, we show that a Cu 2O(1 1 1) surface with Cu vacancies has a lower free energy than the stoichiometric surface for the considered range of oxygen chemical potential and could be catalytically relevant.

Thermodynamic Equilibrium Compositions, Structures, and Reaction Energies of PtxOy(x= 1−3) Clusters Predicted from First Principles

As synthetic nanocatalysis strives to create and apply well-defined catalytic centers containing as few as a handful of active metal atoms, it becomes particularly important to understand the structures, compositions, and reactivity of small metal clusters as a function of size and chemical environment. As a part of our effort to better understand the oxidation chemistry of Pt clusters, we present here a comprehensive set of density functional theory simulations combined with thermodynamic modeling that allow us to map out the T-p(O)2 phase diagrams and predict the oxygen affinity of Pt(x)O(y) clusters, x = 1-3. We find that the Pt clusters have a much stronger tendency to form oxides than does the bulk metal, that these oxides persist over a wide range of oxygen chemical potentials, and that the most stable cluster stoichiometry varies with size and may differ from the stoichiometry of the stable bulk oxide in the same environment. Further, the facility with which the clusters are reduced depends both on size and on composition. These models provide a systematic framework for understanding the compositions and energies of redox reactions of discrete metal clusters of interest in supported and gas-phase nanocatalysis.

Combined STM, LEED and DFT study of Ag(1 0 0) exposed to oxygen near atmospheric pressures

We have investigated the interaction of molecular oxygen with the Ag(1 0 0) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O 2, the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O 2 at room temperature, small, p(2 × 2) reconstructed patches are formed on the surface. After oxidation at ≈470 K and 10 mbar O 2 pressure the surface undergoes a c(4 × 6) reconstruction coexisting with a (6 × 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 × 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential.