Oxygen Vacancies Research Papers - Academia.edu (original) (raw)

Oxygen vacancies at the SnO2(110) and (101) surface and subsurface sites have been studied in the framework of density functional theory by using both all-electron Gaussian and pseudopotential plane-wave methods. The all-electron calculations have been performed using the B3LYP exchange-correlation functional with accurate estimations of energy gaps and density of states. We show that bulk oxygen vacancies are responsible for the appearance of a fully occupied flat energy level lying at about 1 eV above the top valence band, and an empty level resonant with the conduction band. Surface oxygen vacancies strongly modify the surface band structures with the appearance of intragap states covering most of the forbidden energy window, or only a small part of it, depending on the vacancy depth from the surface. Oxygen vacancies can account for electron affinity variations with respect to the stoichiometric surfaces as well. A significant support to the present results is found by comparing them to the available experimental data.

- by
- •
- Materials Science, Soft Matter, Self Assembly, Nanoparticles

Ferromagnetism is observed in LiNiO3 nanocrystals exposed to a reducing atmosphere intended to create oxygen vacancies. The existence of vacancies is confirmed by measuring the oxygen depletion across the selected nanoparticles by TEM. The magnetism shows no temperature dependence in the range of 4– 300 K. The density functional theory was used to perform spin polarized electronic structure calculations for LiNiO3 with and without oxygen vacancies. The calculated magnetic data qualitatively support the observed magnetic behavior.

- by F. Espinosa-Magaña and +4
- •
- Ferroelectric materials, Magnetic Properties, Oxygen Vacancies, Lithium Niobate

Alkaline water electrolyzers promise very high purity hydrogen production but suffer from large overpotential for anodic oxygen evolution reaction (OER). Here we describe the effect of lithium (Li+)-substitution into nickel oxide on the electrocatalytic activity towards OER in alkaline electrolyte. The X-ray diffraction patterns of lithiated nickel oxides (LixNi1−xO, x = 0.00–0.50) synthesized by the solution-combustion method suggest that pure phase of lithiated nickel oxide was formed until x = 0.30; thereafter, a secondary phase of LiNiO2 was observed. Rietveld analysis showed that Li+-substitution caused a contraction in the lattice structure as shown by the decrease in lattice parameters upon Li+-substitution. Further, the weight fraction of LiNiO2 was found to be dominant for x = 0.50. Deconvolution of the high resolution X-ray photoelectron spectroscopy for O 1s and Ni 2p spectra suggested that concentration of oxygen vacancies increased linearly, whereas that of Ni3+ increased till x = 0.30 and it decreased when Li+-substitution was further increased to x = 0.40 and 0.50. Although electrical conductivity increased upon Li+-substitution, no significant effect was observed for lithiated samples with varying Li+-content (x = 0.10–0.50). The activities for OER were measured using the rotating disk electrode in 0.5 M NaOH electrolyte, and the data suggest that lithiated nickel oxide synthesized with x = 0.30 shows the highest current density at 1.70 vs. RHE (V). The decrease in OER activity for x = 0.40 and 0.50 was attributed to the decline in OER active Ni3+ sites (probably due to the presence of chemically unstable LiNiO2).

- by Ravi Sankannavar
- •
- Rietveld refinement, Transition-Metal Oxides, Deconvolution, Hydrogen Energy

Ferroelectric (FE) materials, which typically adopt the perovskite structure with non-centrosymmetry and exhibit spontaneous polarization, are promising for applications in memory, electromechanical and energy storage devices. However, these advanced applications suffer from the intrinsic limitations of perovskite FEs, including poor complementary metal oxide semiconductor (CMOS) compatibility and environmental issues associated with lead. Hafnium oxide (HfO2), with stable bulk centrosymmetric phases, possesses robust ferroelectricity in nanoscale thin films due to the formation of non-centrosymmetric phases. Owing to its high CMOS compatibility and high scalability, HfO2 has attracted significant attention. In the last decade, significant efforts have been made to explore the origin of the ferroelectricity and factors that influence the FE properties in HfO2 films, particularly regarding the role of microstructure, which is vital in clarifying these issues. Although several comprehensive reviews of HfO2 films have been published, there is currently no review focused on the relationship between microstructure and FE properties. This review focuses on the microstructure-property relationships in FE polycrystalline and epitaxial HfO2 films. The crystallographic structures and characterization methods for HfO2 polymorphs are first discussed. For polycrystalline HfO2 films, the microstructure-FE properties relationships, driving force and kinetic pathway of phase transformations under growth parameters or external stimuli are reviewed. For epitaxial films, the lattice matching relations between HfO2 films and substrates and the corresponding impact on the FE properties are discussed. The FE properties between polycrystalline and epitaxial HfO2 films are compared based on their different microstructural characteristics. Finally, a future perspective is given for further investigating FE HfO2 films.

- by Micro- structures
- •
- Phase Transformations, Transmission Electron Microscopy, HfO2, Oxygen Vacancies
- by Yaoguo Fang
- •
- Engineering, Physics, Applied Physics, Nanotechnology

We have applied the bond valence method to cerium oxides to determine the oxidation states of the Ce ion at the various site symmetries of the crystals. The crystals studied include cerium dioxide and the two sesquioxides along with some selected intermediate phases which are crystallographically well characterized. Our results indicate that cerium dioxide has a mixed-valence ground state with an f-electron population on the Ce site of 0.27 while both the A- and C-sesquioxides have a nearly pure f^1 configuration. The Ce sites in most of the intermediate oxides have non-integral valences. Furthermore, many of these valences are different from the values predicted from a naive consideration of the stoichiometric valence of the compound.

- by elvis shoko
- •
- Heterogeneous Catalysis, Ceria, Oxygen Vacancies

Introducing a charge into a solid such as a metal oxide through chemical, electrical, or optical means can dramatically change its chemical or physical properties. To minimize its free energy, a lattice will distort in a material specific way to accommodate screen the Coulomb and exchange interactions presented by the excess charge. The carrier-lattice correlation in response to these interactions defines the spatial extent of the perturbing charge and can impart extraordinary physical and chemical properties such as superconductivity and catalytic activity. Here we investigate by experiment and theory the atomically resolved distribution of the excess charge created by a single oxygen atom vacancy and a hydroxyl OH impurity defects on rutile TiO 2 110 surface. Contrary to the conventional model where the charge remains localized at the defect, scanning tunneling microscopy and density functional theory show it to be delocalized over multiple surrounding titanium atoms. The characteristic charge distribution controls the chemical, photocatalytic, and electronic properties of TiO 2 surfaces.

- by Mauro Boero
- •
- Solid State Physics, Silicon, Solid State Chemistry, Hyperfine Interactions

The functionality of solid materials is defined by the type and ordering of the constituent atoms. By introducing defects that perturb the ordered structure, new functionality is created within the solid material. Atomic defects in titanium dioxide, such as oxygen vacancies, atomic hydrogen, and interstitial Ti, typically create new functionality. However, the fundamental physical properties of atomic defects in TiO2 are not fully understood and still remain controversial. In this account, the progress and issues for debate regarding the physical properties, electronic structure, and manipulation mechanisms of atomic defects in TiO2 as well as their interaction with gold nanoclusters are described.

- by Taketoshi Minato
- •
- Surface Science, Catalysis, Photocatalysts, Semiconductors

We obtain a single cadmium oxide phase from powder synthesized by a thermal decomposition method of cadmium acetate dehydrate. The yielded powder is annealed in air, vacuum, and H 2 gas in order to create point defects. Magnetization-field curves reveal the appearance of diamagnetic behavior with a ferromagnetic component for all the powders. Powder annealing under vacuum and H 2 atmosphere leads to a saturation magnetization 1.15 memu g À1 and 1.2 memu g À1 respectively with an increase by 45% and 16% compared to the one annealed in air. We show that annealing in vacuum produces mainly oxygen vacancies while annealing in H 2 gas creates mainly Cd vacancy leading to room temperature ferromagnetic (RTFM) component together with known diamagnetic properties. Ab initio calculations performed on the CdO nanoparticles show that the magnetism is governed by polarized hybrid states of the Cd d and O p orbitals together with the vacancy.

Hydroxyapatite (HAp) has structural features that define its basic physical properties, which have an important role at the surface, and it is one of the most used materials in bone implants. In this work, we present a density functional modeling (DFT) study of HAp both as bulk and with special HAp models with various defects, especially oxygen vacancies in HAp surface layers, which can also determine photocatalytic properties, confirmed experimentally. The first-principles calculations of bulk and modified HAp were carried out using local basis (AIMPRO) and plane-wave (VASP) codes. Data obtained are analyzed using both approaches, and compared.

- by Robert C Pullar
- •
- Modeling, Hydroxyapatite, Photocatalysts, Modeling and Simulation

We report a first-principle study of the E01 defect in a-quartz and of the analogous E0g defect in amorphous SiO2. Our calculation supports the attribution of both these defects to a positively charged oxygen vacancy. The ground-state configuration of these defects is characterized by a large local
relaxation of the atomic network, which leads to a localization of the unpaired electron on a Si dangling bond. Using the calculated electronic spin densities, we fully characterize the hyperfine interactions with nearby 29Si. Our results explain well both the strong and the weak features that are observed in
the experimental spectra.

- by Mauro Boero
- •
- Solid State Physics, Semiconductor Physics, Electron Spin Resonance, Silicon

In recent years, nanotechnology has gained significant interest for applications in the medical field. In this regard, a utilization of the ZnO nanoparticles for the efficient degradation of bilirubin (BR) through photocatalysis was explored. BR is a water insoluble byproduct of the heme catabolism that can cause jaundice when its excretion is impaired. The photocatalytic degradation of BR activated by ZnO nanoparticles through a non-radiative energy transfer pathway can be influenced by the surface defect-states (mainly the oxygen vacancies) of the catalyst nanoparticles. These were modulated by applying a simple annealing in an oxygen-rich atmosphere. The mechanism of the energy transfer process between the ZnO nanoparticles and the BR molecules adsorbed at the surface was studied by using steady-state and picosecond-resolved fluorescence spectroscopy. A correlation of photocatalytic degradation and time-correlated single photon counting studies revealed that the defect-engineered ZnO nanoparticles that were obtained through post-annealing treatments led to an efficient decomposition of BR molecules that was enabled by Förster resonance energy transfer.

- by Tanujjal Bora
- •
- Phototherapy, Defects in Semiconductors, Photocatalysis, Annealing

A qualitative approach using room-temperature confocal microscopy is employed to investigate the spatial distribution of shallow and deep oxygen vacancy (Vo) concentrations on the polar (0001) and non-polar (10ī0) surfaces of zinc oxide (ZnO) nanowires (NWs). Using the spectral intensity variation of the confocal photoluminescence of the green emission at different spatial locations on the surface, the Vo concentrations of an individual ZnO NW can be obtained. The green emission at different spatial locations on the ZnO NW polar (0001) and non-polar (10ī0) surfaces is found to have maximum intensity near the NW edges, decreasing to a minimum near the NW center. First-principles calculations using simple supercell-slab (SS) models are employed to approximate/model the defects on the ZnO NW (10ī0) and (0001) surfaces. These calculations give increased insight into the physical mechanism behind the green emission spectral intensity and the characteristics of an individual ZnO NW. The highly accurate density functional theory (DFT)-based full-potential linearized augmented plane-wave plus local orbitals (FP-LAPW + lo) method is used to compute the defect formation energy (DFE) of the SSs. Previously, using these SS models, it was demonstrated through the FP-LAPW + lo method that in the presence of oxygen vacancies at the (0001) surface, the phase transformation of the SSs in the graphite-like structure to the wurtzite lattice structure will occur even if the thickness of the graphite-like SSs are equal to or less than 4 atomic graphite-like layers [Wong et al., J. Appl. Phys. 113, 014304 (2013)]. The spatial profile of the neutral Vo DFEs from the DFT calculations along the ZnO [0001] and [10ī0] directions is found to reasonably explain the spatial profile of the measured confocal luminescence intensity on these surfaces, leading to the conclusion that the green emission spectra of the NWs likely originate from neutral oxygen vacancies. Another significant result is that the variation in the calculated DFE along the ZnO [0001] and [10ī0] directions shows different behaviors owing to the non-polar and polar nature of these SSs. These results are important for tuning and understanding the variations in the optical response of ZnO NW-based devices in different geometric configurations.

- by Kin Mun Wong
- •
- Confocal Microscopy, ZnO nanorods, Defects in Semiconductors, DFT calculation