Importance Of Oxygen Vacancies Research Articles

The impact of oxygen deficiency on giant exchange bias in perovskite oxide: SrFe0.5Co0.5O3−δ (δ = 0.37)

Perovskite and double perovskite oxides exhibit various interesting phenomena depending on the presence of magnetic and non-magnetic ions, degree of disorder, oxygen vacancies, etc. In this study, the effect of oxygen deficiency on the giant exchange bias of a cubic perovskite SrFe0.5Co0.5O3−δ (δ = 0.37) is investigated. X-ray diffraction and scanning electron microscopy were used to confirm the crystal structure and grain size, while magnetic relaxation and memory effects suggest a spin-glass nature. This work sheds light on the importance of oxygen vacancies and defects in perovskites, which continue to be an active area of research in materials science.

CO oxidation over CuOx/TiO2 catalyst: The importance of oxygen vacancies and Cu+ species

It is well documented that the existence of oxygen vacancies and active Cu species can improve CO oxidation activity over TiO2 substrate. Herein, TiO2-supported copper catalysts (CuOx/TiO2) with different ratio of oxygen vacancies (Ov) and Cu+ has been synthesized. The existence and amount of Ov and Cu+ species were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectrum (EPR). The synergetic effect between Cu+ and oxygen vacancies was investigated in terms of O2 temperature-programmed desorption (O2–TPD), H2 temperature-programmed reduction (H2–TPR) and in situ DRIFTS spectra. Cu+ is the adsorption site for CO molecules, the abundant Ov can significantly activate molecular oxygen, which effectively oxidize the adsorbed CO molecules. The most suitable ratio between Cu+ and Ov has been confirmed. The CuOx/TiO2(Cu+ rich) catalyst (51% Cu+, 26.3% Ov) shows superior CO oxidation activity (T90 = 136 °C) compared to CuOx/TiO2(Ov rich) catalyst (39% Cu+, 28.5% Ov, T90 > 350 °C). This work provides comprehensive investigation of the role for abundant oxygen vacancies and Cu+ in CO oxidation reactions.

A Review of CeO2 Supported Catalysts for CO2 Reduction to CO through the Reverse Water Gas Shift Reaction

The catalytic conversion of CO2 to CO by the reverse water gas shift (RWGS) reaction followed by well-established synthesis gas conversion technologies could be a practical technique to convert CO2 to valuable chemicals and fuels in industrial settings. For catalyst developers, prevention of side reactions like methanation, low-temperature activity, and selectivity enhancements for the RWGS reaction are crucial concerns. Cerium oxide (ceria, CeO2) has received considerable attention in recent years due to its exceptional physical and chemical properties. This study reviews the use of ceria-supported active metal catalysts in RWGS reaction along with discussing some basic and fundamental features of ceria. The RWGS reaction mechanism, reaction kinetics on supported catalysts, as well as the importance of oxygen vacancies are also explored. Besides, recent advances in CeO2 supported metal catalyst design strategies for increasing CO2 conversion activity and selectivity towards CO are systematically identified, summarized, and assessed to understand the impacts of physicochemical parameters on catalytic performance such as morphologies, nanosize effects, compositions, promotional abilities, metal-support interactions (MSI) and the role of selected synthesis procedures for forming distinct structural morphologies. This brief review may help with future RWGS catalyst design and optimization.

Effect of oxygen vacancy and A-site-deficiency on the dielectric performance of BNT-BT-BST relaxors

There is an urgent need for dielectrics to suit multitudinous burgeoning applications with high-temperature dielectric stability. Here the intrinsic defect and polarization mechanism are investigated in A-site-deficient 0.66(Bi0.5Na0.5)TiO3-0.06BaTiO3-0.28(BixSr1–3x/2▯x/2)TiO3 (BNT-BT-BST) relaxors where strontium vacancies (VSr′′) are designed to compensate the negative charge shortage when Sr2+ is substituted by Bi3+ (BiSr•). Incremental Bi3+ is conducive to the polarization in nano-regions at high temperatures resulting in an enhancement of the temperature stability of dielectric permittivity. It reveals a strong defect induced disorder accompanied with slightly increased dielectric loss due to the partial reduction of Ti4+. Furthermore, the existing oxygen vacancies confirmed by X-ray photoelectron and atmosphere dependent ac impedance spectroscopies are suppressed effectively when sintering at O2 for BNT-BT-BST to the benefit of maintaining stable dielectric properties. Remarkably high energy storage density and efficiency are achieved simultaneously under relatively low electric fields. This work not only provides promising candidates in Bi0.5Na0.5TiO3 (BNT) based relaxors but also demonstrates an effective route considering the importance of oxygen vacancies in the optimization for dielectric applications.

Oxygen Vacancy Engineering in Titanium Dioxide for Sodium Storage.

Titanium dioxide (TiO2 ) is a promising anode material for sodium-ion batteries (SIBs) due to its low cost, natural abundance, nontoxicity, and excellent electrochemical stability. Oxygen vacancies, the most common point defects in TiO2 , can dramatically influence the physical and chemical properties of TiO2 , including band structure, crystal structure and adsorption properties. Recent studies have demonstrated that oxygen-deficient TiO2 can significantly enhance sodium storage performance. Considering the importance of oxygen vacancies in modifying the properties of TiO2 , the structural properties, common synthesis strategies, characterization techniques, as well as the contribution of oxygen-deficient TiO2 on initial Coulombic efficiency, cyclic stability, rate performance for sodium storage are comprehensively described in this review. Finally, some perspectives on the challenge and future opportunities for the development of oxygen-deficient TiO2 are proposed.

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

The source of n -type conductivity in undoped transparent conducting oxides has been a topic of debate for several decades. The point defect of most interest in this respect is the oxygen vacancy, but there are many conflicting reports on the shallow versus deep nature of its related electronic states. Here, using a hybrid quantum mechanical/molecular mechanical embedded cluster approach, we have computed formation and ionization energies of oxygen vacancies in three representative transparent conducting oxides: In 2 O 3 , SnO 2 , and ZnO. We find that, in all three systems, oxygen vacancies form well-localized, compact donors. We demonstrate, however, that such compactness does not preclude the possibility of these states being shallow in nature, by considering the energetic balance between the vacancy binding electrons that are in localized orbitals or in effective-mass-like diffuse orbitals. Our results show that, thermodynamically, oxygen vacancies in bulk In 2 O 3 introduce states above the conduction band minimum that contribute significantly to the observed conductivity properties of undoped samples. For ZnO and SnO 2 , the states are deep, and our calculated ionization energies agree well with thermochemical and optical experiments. Our computed equilibrium defect and carrier concentrations, however, demonstrate that these deep states may nevertheless lead to significant intrinsic n -type conductivity under reducing conditions at elevated temperatures. Our study indicates the importance of oxygen vacancies in relation to intrinsic carrier concentrations not only in In 2 O 3 , but also in SnO 2 and ZnO.

Phase transformation and chemical decomposition of nanocrystalline SnO2 under heavy ion irradiation

A crystalline-to-crystalline phase transformation, including chemical decomposition, has been observed in SnO2 nanopowder irradiated by 2.2GeV 197Au ions. X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the transformation from tetragonal SnO2 (P42/mnm) to tetragonal SnO (P4/nmm), with trace quantities of β-Sn (I41/amd). At a fluence of approximately 2.0×1012ions/cm2, diffraction maxima corresponding to SnO became clearly evident and increased in intensity as fluence increased. The proportion of SnO, as determined by Rietveld refinement of XRD data, reached 23.1±0.8% at the maximum fluence investigated of 2.4×1013ions/cm2. Raman spectra show high photoluminescence (PL) intensity before and during initial SnO formation, indicating the importance of oxygen vacancies in the transformation process. Small-angle X-ray scattering (SAXS) analysis provided evidence of ion tracks, but no tracks were observed using high-resolution TEM (HRTEM). The transformation likely occurs through a multiple-impact mechanism, based on the accumulation of O vacancies, defect ordering, and partially localized Sn reduction.

A Reactive Force Field (ReaxFF) for Molecular Dynamics Simulations of NiO Reduction in H2 Environments

Ni/YSZ cermet is a widely used SOFC anode material, particularly due to its combination of good electric conductivity, high catalytic activity and low cost. Nonetheless, Ni/YSZ cermet anodes are facing unique challenges such as nickel agglomeration and reoxidation, eventually resulting in anode degradation. A solution to these problems might be obtained via a better comprehension of the underlying manufacturing processes, particularly the final H2-induced reduction reaction of NiO to Ni. Experimental approaches have contributed immensely toward a better understanding of these phenomena, yet are generally limited in terms of spatial and temporal resolution, thus motivating us to investigate the problem from a multiscale simulation viewpoint. In this study, we developed a reactive force field (ReaxFF) for nickel/oxygen interactions, which was specifically tailored toward molecular dynamics (MD) simulations of NiO reduction in H2 environments. The force field parameters were derived by fitting the ReaxFF energies and forces of various nickel/oxygen structures to the respective values of first-principles calculations. The structures considered here include bulk NiO, Ni2O3 and Ni3O4 at various densities, NiO under shear, the NiO(100) surface, oxygen vacancies in bulk and surface NiO as well as oxygen adsorbed onto the NiO(100) surface. The developed force field was subsequently used to predict the reduction kinetics of nano-scale NiO crystals in a H2 atmosphere at different temperatures and hydrogen partial pressures. Here, we observed a significantly improved structural stability of bulk NiO at temperatures over 700 K, compared to our previous ReaxFF nickel/oxygen description [1] and similar ReaxFF parameter sets published by other authors [2]. Besides quantifying the reduction rate, further core phenomena such as the induction period before the onset of a steady state reduction process, the pore formation in NiO at lower temperatures, as well as the importance of oxygen vacancies during the reduction are discussed and compared with experimental results. Fig. 1 NiO nanocrystal after 0.2 ns of MD reduction simulation at a global temperature of 850 K and hydrogen partial pressure of 10 bars, using our previous ReaxFF parameter set (left) and the parameter set introduced in this publication (right).

Occurrence of spintronics behaviour (half-metallicity, spin gapless semiconductor and bipolar magnetic semiconductor) depending on the location of oxygen vacancies in BiFe0.83Ni0.17O3.

The current communication signifies the effect of oxygen vacancies (OVs) both qualitatively and quantitatively in multiferroic BiFe0.83Ni0.17O3 by an in-depth atomic-level investigation of its electronic structure and magnetization properties, and these materials have a variety of applications in spintronics, optoelectronics, sensors and solar energy devices. Depending on the precise location of OVs, all the three types of spintronic material namely half-metallic, spin gapless semiconductor and bipolar magnetic conductor have been established in a single material for the first time and both super-exchange and double-exchange interactions are possible in accordance with the precise location of OVs. We have also calculated the vacancy formation energies to predict their thermodynamic stabilities. These results can highlight the impact and importance of OVs that can alter the multiferroic properties of materials.

Largely enhanced electrochemical performance in MoO3-x nanobelts formed by a “sauna reaction”: Importance of oxygen vacancies

MoO3–x nanobelts (MoO3-x NBs) were synthesized via a novel “sauna reaction”, providing us an opportunity for illustrating the effect of oxygen vacancies (OVs) on its largely enhanced electrochemical performance. Very exciting, the MoO3-x NBs exhibits much superior performance with 400 and 267mAhg−1, which is about three times larger than that of the pristine MoO3 NBs (123 and 84mAhg−1), at the current densities of 100 and 200mAg−1, respectively. Notably, the OVs and the resulting improved electrical conductivity could be used to account for their largely enhanced specific capacity and rate properties. On one hand, OVs lowered the energy barrier of Li+ into the intralyer of MoO6 octahedron sheet layer and the interlayer of MoO3-x, and provided extra lithium storage sites of MoO3 during discharge-charge process. On the other hand, the improved conductivity of MoO3-x provided efficient electron transport pathways. The results show that OVs control can be a promising strategy for enhancing high-performance Mo-based electrodes by taking advantage of structural characteristics of multiple oxide sates of Mo elements. This study not only provides a new method for the preparation of MoO3-x with rich OVs using a simple, environmentally friendly “sauna reaction”, but also indicates the importance of OVs in the largely enhanced electrochemical performance in MoO3-x NBs, which may shed some light on the future development of electrode materials of lithium-ion batteries (LIBs).

Charge compensation and mixed valency inLaAlO3∕SrTiO3heterointerfaces studied by the FLAPW method

The electronic structures and properties of heterointerfaces in the perovskite oxide superlattices ${\mathrm{LaAlO}}_{3}∕{\mathrm{SrTiO}}_{3}\phantom{\rule{0.3em}{0ex}}[001]$ are presented using the first-principles all-electron full-potential linearized augmented plane-wave (FLAPW) method. Superlattices with three types of interfaces, (i) electron-doped, (ii) hole-doped, and (iii) both electron- and hole-doped, are studied and compared. For the electron-doped interface, the mixed valency of Ti along with the Jahn-Teller effect are found to explain the metallicity, in agreement with experiment, whereas for the hole-doped interface metallicity is found, in contrast to experiment. Oxygen vacancies introduce an additional $n$-type carrier to compensate the holes present at the interface, which now becomes insulating and agrees well with experiment. For a system with both types of interfaces, the metallicity found for the unrelaxed structure is changed to insulating after relaxation is included. For all cases, the relaxation results in a complicated buckled geometry, which is a good indication of the adjustment due to polarity discontinuity. Our findings support recent experimental results, namely, (i) the mixed-valence character of Ti at the electron-doped interface, and (ii) the existence and importance of oxygen vacancies at the hole-doped interface.