Single Oxygen Vacancies Research Articles

Self-Assembled Catalyst Growth and Optical Properties of Single-Crystalline ZnGa2O4 Nanowires

Uniform zinc gallate (ZnGa2O4) nanowires are successfully synthesized by a one-step simple thermal evaporation of a mixture of ZnO and Ga2O3 powders under controlled conditions. Its morphology and microstructures were determined by X-ray powder diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, and photoluminescence spectroscopy. The observations reveal that the as-synthesized ZnGa2O4 nanowires are single-crystalline with a cubic spinel structure, usually several tens of microns in length. The typical diameters of ZnGa2O4 nanowires range from 100 to 200 nm. The room-temperature photoluminescence spectrum of ZnGa2O4 nanowires features four luminescence peaks around 400, 503, and 686 nm, which are attributed to self-activation center of the octahedral Ga−O, electronic transitions of localized Ga3+ ion in the octahedral Ga−O, and single oxygen vacancies (VO*), respectively. A self-assembled Ga catalytic vapor−liquid−solid (VLS) f...

Electronic and magnetic structure of cuprous oxideCu2Odoped with Mn, Fe, Co, and Ni: A density-functional theory study

We investigate the effect of transition metal (TM) substitution in cuprous oxide ${\mathrm{Cu}}_{2}\mathrm{O}$ on the basis of ab initio calculations employing density-functional theory $(\mathrm{GGA}+\mathrm{U})$. By using the supercell approach, we study the effect of substituting Cu by Mn, Fe, Co, and Ni, assuming both low TM concentrations (3.2%) in a cubic geometry and higher TM concentrations (9.1%) in a trigonal setup. For the elements Mn and Co, magnetic exchange constants up to the fifth nearest neighbor are calculated, assuming both cases, perfect $\mathrm{Mn}∕\mathrm{Co}:{\mathrm{Cu}}_{2}\mathrm{O}$ as well as defects in the host such as single copper and oxygen vacancies. Our results clearly show the importance of defects in these materials and thus offer an explanation for various, seemingly opposed, experimental results.

Polaron-like Charge Trapping in Oxygen Deficient and Disordered HfO2: Theoretical Insight

We suggest that the polaronic trapping of electrons is possible in the high-k dielectrics alongside with the commonly discussed Poole-Frankel resonant tunneling process or recapture of the electrons from the conduction band onto pre- existed defect states. The main feature of this mechanism is the charge self-trapping, i.e. formation of a localized defect state as a result of the electron-lattice interaction. We consider the charge trapping properties of single oxygen vacancies and di-vacancies in the m-HfO2 as well as the various models for oxygen deficient and stoichiometric disordered hafnia. We find evidence of polaronic trapping in all these systems, which may indicate its universality in high- k materials.

The deoxygenation of benzoic acid as a probe reaction to determine the impact of superficial oxygen vacancies (isolated or twin) on the oxidation performances of Mo-based oxide catalysts

The deoxygenation of benzoic acid (DBA) is a probe reaction of the organisation of oxygen vacancies at the surface of molybdenum based oxide catalysts. In the present work, the DBA was used in coupling with the selective oxidation of propylene so that it could offer an insight into the nature of oxygen vacancies involved in the later reaction. By correlating the superficial oxidation state of the catalytic elements measured by X-ray photoelectron spectroscopy with the information provided by the DBA probe reaction, it was possible to progress in the description of the atomic scale architecture of the oxidation sites at work. Sites bringing boosted performances in the oxidation of propylene are those which are slightly reduced thanks to the presence of single (isolated) oxygen vacancies. Conversely, twin oxygen vacancies do not participate in the oxidation reaction.

Justification of the Schottky emission model at the interface of a precious metal and a perovskite oxide with dilute oxygen vacancies

The properties of perovskite (i.e., SrTiO3) precious-metal interfaces and particular causes of leakage currents are well studied in microelectronics capacitor applications. The Schottky emission model based on oxygen vacancies is a promising theory; however, in technically interesting cases, vacancy density is too small to describe a continuous band diagram. There is a need to explain why the model seems promising while its assumption of the band diagram is not justified. Thus, we calculated a leakage current induced by a single oxygen vacancy and found that linear approximation is applicable; the linearity justifying the use of the Schottky emission model results.

First-principles calculations of the atomic and electronic structure ofFcenters in the bulk and on the (001) surface ofSrTiO3

The atomic and electronic structure, formation energy, and the energy barriers for migration have been calculated for the neutral O vacancy point defect F center in cubic SrTiO3 employing various implementations of density functional theory DFT. Both bulk and TiO2-terminated 001 surface F centers have been considered. Supercells of different shapes containing up to 320 atoms have been employed. The limit of an isolated single oxygen vacancy in the bulk corresponds to a 270-atom supercell, in contrast to commonly used supercells containing 40– 80 atoms. Calculations carried out with the hybrid B3PW functional show that the F center level approaches the conduction band bottom to within 0.5 eV, as the supercell size increases up to 320 atoms. The analysis of the electronic density maps indicates, however, that this remains a small-radius center with the two electrons left by the missing O ion being redistributed mainly between the vacancy and the 3dz 2 atomic orbitals of the two nearest Ti ions. As for the dynamical properties, the calculated migration energy barrier in the low oxygen depletion regime is predicted to be 0.4 eV. In contrast, the surface F center exhibits a more delocalized character, which leads to significantly reduced ionization and migration energies. Results obtained are compared with available experimental data.

Coupling the deoxygenation of benzoic acid with the oxidation of propylene on a Co molybdate catalyst

An innovating coupling between the deoxygenation of benzoic acid and the oxidation of propylene was set up and gave new information about the mechanism involved in the oxidation of propylene on a Co–Mo based oxide catalyst. The production of CO 2 during the catalytic reaction is bound to the formation of benzene and benzaldehyde. The first case corresponds to the removal of the carboxyl function of the benzoic acid. The second case is the evidence that benzaldehyde and products coming from the oxidation of propylene are formed on the same catalytic sites during the Mars and van Krevelen cycle. In this cycle, the oxygen atoms used for the oxidation come from the benzoic acid by the intermediate of the oxide lattice. In particular, it has been demonstrated that the oxidation of propylene involves lattice oxygen atoms far from each other in such a way that the reaction leaves single oxygen vacancies at the surface of the catalyst. Such coupling between a deoxygenation reaction and an oxidation one is reported for the first time.

A positron lifetime study of lanthanum and niobium doped Pb(Zr0.6Ti0.4)O3

A study of vacancy-related defects in lanthanum and niobium doped PbZr0.6Ti0.4O3 with dopant concentrations of 0–6 and 0–4mol%, respectively has been performed using positron annihilation spectroscopy X-ray diffraction, and photoelectron spectroscopy. Positron lifetime as well as coincidence annihilation radiation Doppler line broadening measurements were carried out. It was found that the samples exhibit vacancylike defects that act as positron traps. Two main defect lifetime components were found in both sample sets one at ≈150ps and one at ≈300ps. These defect trapping sites can be attributed to single oxygen vacancies and A-site vacancies, respectively. Doppler line broadening measurements, however, do not show significant changes as a function of dopant concentrations in terms of shape S and wing W parameters.

First‐principles periodic and semiempirical cyclic cluster calculations for single oxygen vacancies in crystalline Al2O3

AbstractSingle neutral and positively charged oxygen vacancies (F and F+ centers) in α‐Al2O3 are investigated using the supercell model (SCM) at the Hartree‐Fock (HF) and density functional theory (DFT) level and the cyclic cluster model (CCM) implemented in the semiempirical MSINDO method. Results of supercell and cyclic cluster calculations for the cohesive energy of the perfect Al2O3 crystal are compared with the experimental value for the heat of atomization. The defect formation energy of the unrelaxed F center is calculated with the SCM. The CCM is used to calculate the defect formation energy of the F and the F+ centers. Relaxation effects for the nearest neighbors (NN) and the next‐nearest neighbors (NNN) of the F and F+ centers are investigated. The obtained values are compared to other theoretical literature data. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Color variation of ZnGa2O4 phosphor by reduction-oxidation processes

The color of the emission of zinc gallate (ZnGa2O4) oscillates between ultraviolet and blue by hydrogen ambient reduction and air ambient oxidation heat treatments. The photoluminescence spectra and electron paramagnetic resonance signals show that ultraviolet emission of reduced ZnGa2O4 always accompanies 680 nm emission originating from single oxygen vacancies (VO*). The increasing difference in binding energy between Ga3+ and O2− in reduced ZnGa2O4 indicates that the configuration of octahedral sites is distorted due to VO* generation and it becomes more ionic which shifts the emission band from 430 to 360 nm. The x-ray diffraction patterns and Raman scattering spectra show that β-Ga2O3 from ZnGa2O4 is formed in both reduction and oxidation processes which suggests the vaporization of Zn ions. We propose a model in which the origin of 360 nm emission is the Ga–O transition at distorted octahedral sites with VO* in ZnGa2O4, whereas 430 nm emission originates from the Ga–O transition of regular octahedral sites without VO* in ZnGa2O4.

Ionic conduction due to oxygen diffusion in La0.8Sr0.2GaO3−δ electrolyte

Dielectric and dc measurements have been carried out on La0.8Sr0.2GaO3−δ, which has a high potential for the electrolyte material in a solid oxide fuel cell, so as to elucidate the correlation between O2− diffusion and electric conduction. A dielectric relaxation shows up in loss tangent. The relative relation of activation energies as to dielectric relaxation and dc conduction suggests the ionic conduction due to O2− diffusion assisted by free single oxygen vacancies taking place. The change of dc conductive behavior associated with attenuation of dielectric relaxation around 950 K indicates that another type of O2− diffusion dominates the ionic conduction at high temperature.

Bonding of gold nanoclusters to oxygen vacancies on rutile TiO2(110).

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile TiO2(110) surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the TiO2(110) system involving vacancy-cluster complex diffusion is presented.

First-principles modeling of paramagnetic Si dangling-bond defects in amorphousSiO2

We modeled paramagnetic Si dangling-bond defects in amorphous SiO2 using a generalized-gradient density-functional approach. By creating single oxygen vacancies in a periodic model of amorphous SiO2, we first generated several model structures in which the core of the defect consists of a threefold coordinated Si atom carrying a dangling bond. These model structures were then fully relaxed and the hyperfine parameters calculated. We found that the hyperfine parameters of such model defects, in both the neutral and positive charge states, reproduced those characteristic of the E', in accord with experimental observations for amorphous SiO2. By eliminating a second O atom in the nearest-neighbor shell of these defect centers, we then generated model defects in which the Si atom carrying the dangling bond forms bonds with two O atoms and one Si atom. In this defect, the spin density is found to delocalize over the Si-Si dimer bond, giving rise to two important hyperfine interactions. These properties match the characteristics of the hyperfine spectrum measured for the S center. Our results are complemented by the calculation of hyperfine interactions for small cluster models which serve the threefold purpose of comparing different electronic-structure schemes for the calculation of hyperfine interactions, estimating the size of core-polarization effects, and determining the reliability of cluster approximations used in the literature.

Effect of neutron irradiation on hardening in MgO crystals

Using a nanoindentation technique, hardness and Young's modulus were determined in both nominally pure MgO and lithium-doped MgO crystals, before and after neutron irradiation in the dose range ${10}^{15}\char21{}{10}^{19} {\mathrm{n}/\mathrm{c}\mathrm{m}}^{2}.$ The resulting defects were monitored by optical-absorption spectroscopy. The concentrations of single oxygen vacancies and higher-order point defects involving oxygen vacancies increase with neutron dose. A constant value of $(290\ifmmode\pm\else\textpm\fi{}15)$ GPa for the Young's modulus was measured in all the crystals, indicating that the elastic properties are not influenced by either impurities or defects produced by irradiation. Hardness increases with neutron dose and is independent of the presence of lithium in the crystal. Neutron-irradiated crystals contain oxygen vacancies, higher-order point defects, and interstitials, whereas thermochemically reduced (TCR) crystals contain oxygen vacancies. Comparison between a neutron irradiated and a TCR MgO crystal containing similar concentrations of oxygen vacancies, shows that 70% of the hardening by neutron irradiation is produced by interstitials, 30% by oxygen vacancies, and a negligible amount by higher-order point defects.

Ab initio study of the oxygen vacancy in SrTiO3

The electrical properties of SrTiO3 are strongly dependent on the oxygenvacancy concentration. We have studied the atomic and electronic properties ofa single oxygen vacancy using a local spin density approximation-plane-wavepseudopotential method. The total energies, relaxed geometries, valence chargeand spin densities, and densities of electron states are calculated fordifferent charge states of the vacancy. The doubly positively charged state isfound to be the most stable. With some charge states, a spin-polarized defectstate is observed. Furthermore, we have evaluated the formation energy of thevacancy in the OO→VO + ½O2 process.

QUANTUM CHEMISTRY OF OXIDE SURFACES: FROM CO CHEMISORPTION TO THE IDENTIFICATION OF THE STRUCTURE AND NATURE OF POINT DEFECTS ON MgO

The electronic structure and chemisorption properties of the surface of ionic crystals are reviewed, with emphasis on two topics: a critical overview of the experimental and theoretical studies of the adsorption of CO on single crystal and polycrystalline MgO, and a discussion on the most important defect centers at the MgO surface — low-coordinated sites, single oxygen and magnesium vacancies, divacancies, and impurity or substitutional atoms. The two subjects are to some extent interconnected. From the detailed theoretical and experimental study of the adsorption of a nonreactive molecule like CO and from the comparison of experiments done on single crystal or thin films and on powder samples, one can learn about the nature and concentration of the defects at the surface. A more precise characterization of defects requires, however, a careful spectroscopic investigation and a direct comparison with quantum-chemical calculations of both geometric structure and observable properties. The combined theoretical–experimental approach offers new opportunities for a better understanding of the complexity of oxide surfaces.

Quantum Chemistry of Oxide Surfaces From CO Chemisorption to the Identification of the Structure and Nature of Point Defects on MgO

The electronic structure and chemisorption properties of the surface of ionic crystals are reviewed, with emphasis on two topics: a critical overview of the experimental and theoretical studies of the adsorption of CO on single crystal and polycrystalline MgO, and a discussion on the most important defect centers at the MgO surface — low-coordinated sites, single oxygen and magnesium vacancies, divacancies, and impurity or substitutional atoms. The two subjects are to some extent interconnected. From the detailed theoretical and experimental study of the adsorption of a nonreactive molecule like CO and from the comparison of experiments done on single crystal or thin films and on powder samples, one can learn about the nature and concentration of the defects at the surface. A more precise characterization of defects requires, however, a careful spectroscopic investigation and a direct comparison with quantum-chemical calculations of both geometric structure and observable properties. The combined theoretical–experimental approach offers new opportunities for a better understanding of the complexity of oxide surfaces.

Segregation of Fission Products to Surfaces of UO2

AbstractIn order to predict the release rates of fission products from UO2 nuclear fuel, it is necessary to determine the energetics associated with their segregation from the bulk to surfaces. Here segregation is determined by calculating the total energy of a simulation repeat unit which includes a fission product, as a function of the distance of the fission product from the surface. We have investigated the possibility that specific fission products segregate preferentially to certain surfaces. In particular, Ce4+ and Zr4+ segregate to the (100) surface, while Ba2+ and Sr2+ segregate to the (111) surface. Two issues make these calculations more complex. First, charged defects need to be compensated by oxygen vacancies. For example, divalent Ba2+ and Sr2+ substituting on uranium sites are compensated by a single oxygen vacancy. Second, certain surfaces, such as the (100), are inherently unstable, but can be stabilized by oxygen vacancies. Arrangements of these surface defects lead to complex surface structures that affect segregation energetics.

Experimental evidence of different contributions to the photoluminescence at 4.4 eV in synthetic silica

Photoluminescence activity in high-purity synthetic silica samples, both as grown and -irradiated, was investigated by exciting in the vacuum-ultraviolet region. An emission band centred at 4.4 eV, excited within the absorption band at 7.6 eV and exhibiting a strong temperature dependence, was unequivocally evident. All of these features, together with its very fast decay time (2.3 ns at T = 10 K) make this emission distinguishable from the well known isoenergetic bands detected in oxygen-deficient or in -irradiated silica. Our results are discussed in the light of structural models reported in the literature and seem to be consistent with the occurrence of a change in the structure of a point defect, from the single oxygen vacancy, responsible for the absorption at 7.6 eV, to the twofold-coordinated silicon, giving rise to the photoluminescence at 4.4 eV.

Ab initioformation energies of point defects in pure and Ge-dopedSiO2

We have studied by means of ab initio calculations including electron correlation and cluster models the formation energies of a series of defects in bulk ${\mathrm{SiO}}_{2}$ and in silica glasses containing Ge impurities. The defects considered include oxygen vacancies, \ensuremath{\equiv}Si---Si\ensuremath{\equiv}, peroxyl linkages, \ensuremath{\equiv}Si---O---Si\ensuremath{\equiv}, peroxyl radicals, ${\mathrm{\ensuremath{\equiv}}\mathrm{Si}\mathrm{---}\mathrm{O}\mathrm{---}\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}},$ other radicals, ${\mathrm{\ensuremath{\equiv}}\mathrm{Si}\mathrm{---}\mathrm{O}}^{\mathrm{\ifmmode\bullet\else\textbullet\fi{}}}$ and ${\mathrm{\ensuremath{\equiv}}\mathrm{Si}}^{\mathrm{\ifmmode\bullet\else\textbullet\fi{}}},$ ${E}^{\ensuremath{'}}$ centers, $\ensuremath{\equiv}{\mathrm{Si}}^{\mathrm{\ifmmode\bullet\else\textbullet\fi{}}}{\mathrm{}}^{+}\mathrm{Si}\mathrm{\ensuremath{\equiv}},$ double oxygen vacancies, \ensuremath{\equiv}Si---Si---Si\ensuremath{\equiv}, Frenkel pairs, \ensuremath{\equiv}Si---Si---O---O---Si\ensuremath{\equiv}, etc. The corresponding analogs with Ge atoms replacing the network Si atoms have also been considered. We found that the formation of a single oxygen vacancy, defined as the energy required to remove and bring to infinity a neutral O atom, is 8.5 eV. This is consistent with the most recent thermodynamic estimates. The stability as well as the geometry of the other defects is discussed. It is shown that in the presence of Ge impurities the formation of oxygen deficient centers occurs at a lower energy cost.