Interaction Of Oxygen Vacancies Research Articles

Strategic Defect Engineering Enabled Efficient Oxygen Evolution Reaction in Reconstructed Metal‐Organic Frameworks

AbstractMetal‐organic frameworks (MOFs) have emerged as promising pre‐catalysts for oxygen evolution reaction (OER) due to their marvelous structural reconstruction process in strongly alkaline media. However, targeting design MOF structures to achieve excellent OER performance of reconstructed products is a challenge. Here, a strategic defect engineering is used to promote the OER performance of reconstructed products. Briefly, modified linkers with monocarboxylic acids (ferrocene carboxylic acid, FcCA) are incorporated into MOF (NiBDC‐FcCA), leading to its stepwise reconstruction into Fe‐doped Ni(OH)2 and NiOOH during the OER process, with the oxygen vacancy and strategic doping of metal Fe persisting throughout the multi‐step reconstruction. Benefiting from the synergistic interaction of oxygen vacancies and Fe doping, NiBDC‐FcCA delivers the extremely enhanced current density at 1.6 V versus reversible hydrogen electrode by ≈9 times compared with that of NiBDC. Moreover, the optimized NiBDC‐FcCA/Fe foam exhibits excellent OER catalytic activity and stability with a low overpotential of 250 mV at 200 mA cm−2 and negligible activity decay after 1200 h at 1 A cm−2. Density function theory calculations reveal that Fe doping weakens the interaction of oxygen intermediate with Ni sites, favoring the formation of OOH* to accelerate the OER process.

Aeration-Free In Situ Fenton-like Reaction: Specific Adsorption and Activation of Oxygen on Heterophase Oxygen Vacancies.

Aeration accounts for 35-51% of the overall energy consumption in wastewater treatment processes and results in an annual energy consumption of 5-7.5 billion kWh. Herein, a solar-powered continuous-flow device was designed for aeration-free in situ Fenton-like reactions to treat wastewater. This system is based on the combination of TiO2-x/W18O49 featuring heterophase oxygen vacancy interactions with floating reduced graphene/polyurethane foam, which produces hydrogen peroxide in situ at the rates of up to 4.2 ppm h-1 with degradation rates of more than 90% for various antibiotics. The heterophase oxygen vacancies play an important role in the stretching of the O-O bond by regulating the d-band center of TiO2-x/W18O49, promoting the hydrogenation of *·O2- or *OOH by H+ enrichment, and accelerating the production of reactive oxygen species by spontaneous adsorption of hydrogen peroxide. Furthermore, the degradation mechanisms of antibiotics and the treatment of actual wastewater were thoroughly investigated. In short, the study provides a meaningful reference for potentially undertaking the "aeration-free" in situ Fenton reaction, which can help reduce or even completely eradicate the aeration costs and energy requirements during the treatment of wastewater.

Porous spherical indium oxide-loaded BiOIO3 nanosheets to construct Z-scheme heterojunction to enhance photocatalytic activity

In this work, In2O3/BiOIO3 Z-scheme heterojunction composites enriched with oxygen vacancies were prepared by hydrothermal and calcination methods. Combining the experimental results with DFT calculations, it is found that the photogenerated electrons of In2O3-loaded BiOIO3 are transferred along the Z-scheme path, which proves that the In2O3/BiOIO3 Z-scheme heterojunction is successfully synthesized. The introduction of In2O3 has resulted in abundant oxygen vacancies on the surface of the composites. Oxygen vacancies construct new carrier transfer channels to accelerate the separation and transfer efficiency of electron-hole pairs. The photocatalytic activity was evaluated by the removal efficiency of the heavy metal mercury in the gas phase. The best sample had a Hg removal efficiency of 98.5 %. Finally, we propose that the synergistic interaction of oxygen vacancies and Z-scheme heterojunction promotes the activation of molecular oxygen and the generation of more ROS (·O2- and·OH) with high redox reactivity and analyze the mechanism in detail.

Efficient heterogeneous activation of peroxymonosulfate by Ag-doped CoFe2O4 nanoparticles for sulfamethoxazole degradation

In this work, silver-doped cobalt ferrite catalysts Agx-CoFe2O4 (x = n(Ag+): n(Fe3+); x = 0, 0.02, 0.04, 0.06, and 0.08) were synthesized through the reverse co-precipitation method and employed to activate peroxymonosulfate (PMS) for sulfamethoxazole (SMX) degradation. Ag0.06-CoFe2O4 exhibited the best catalytic activity and PMS utilization efficiency, indicating the optimal silver doping ratio. The characterization results revealed that Ag doping led to the exposure of more active sites and the increase of the concentration of oxygen vacancies (OV). The effects of Ag0.06-CoFe2O4 dosage, PMS concentration, initial pH, inorganic ions, and humic acid on SMX degradation by Ag0.06-CoFe2O4/PMS system were investigated. The utilization of 0.1 g/L Ag0.06-CoFe2O4 and 0.1 mM PMS resulted in a remarkable 95.1 % degradation efficiency of 10 μM SMX in 30 min at initial pH of 5.0. Quenching experiments and electron paramagnetic resonance (EPR) analysis revealed the presence of both radical species (HO•, SO4•− and O2•−) and non-radical species (1O2) in Ag0.06-CoFe2O4/PMS system, among which SO4•− and 1O2 played a predominant role in SMX degradation. The catalytic cycle between≡CoOH+/≡CoO+ and PMS and the interaction of OV with adsorbed oxygen should contribute to PMS activation. The cycling experiments and X-ray diffraction (XRD) patterns before and after the reaction showed that Ag0.06-CoFe2O4 exhibited remarkable stability and reusability. The transformation products (TPs) of SMX were identified, and three possible degradation pathways were proposed. Most of these TPs demonstrated lower toxicity compared to SMX, as predicted by the Ecological Structure−Activity Relationship Model.

In-situ construction of Schottky junctions with synergistic interaction of oxygen vacancies in Mo@MoO3 nanosheets for efficient N2 photoreduction

Defect engineering is a promising technique that can activate nitrogen by injecting electrons into the anti-bonding molecular orbital of nitrogen through anion vacancies. Moreover, compared to a single component, Schottky junctions can effectively overcome rapid electron recombination, thereby significantly enhancing photoelectron utilization efficiency. In this work, we prepared Mo modified MoO3 nanosheets with abundant oxygen vacancies by a solvothermal method and investigated the effect of synergistic interactions between Schottky junctions and oxygen vacancies on the photocatalytic performance of N2 reduction reaction (NRR). The photocatalytic nitrogen fixation performance of Mo@MoO3 nanosheets (MoO3-6) reached 50.78 μmol·g−1·h−1 without any scavenger, which was about 3 times that of commercial MoO3. The Schottky barrier creates a built-in electric field generating charge transfer channels during the photocatalytic reaction, while the oxygen vacancies trap electrons to activate N2, and together with Mo, broaden the light absorption range of the catalyst, facilitating more efficient transfer of excited electrons to the active site. The synergetic advantages of Schottky junctions and oxygen vacancies are exploited in advancing the photocatalytic effect, providing new opportunities and challenges for the development of metal oxide-based materials.

Lattice oxygen diffusion in YFeO3-δ perovskite: DFT study

The motion of oxygen in YFeO3 through oxygen vacancies both in the bulk and on its surface was studied using the electron density functional theory and the pseudopotential method. The movement of oxygen through vacancies of two types (O1 and O2) made it possible to recognize two paths with nigh barrier values of 1.0 and 1.1 eV. For the (100) surface, we demonstrate the barrier-free formation of an oxygen surface complex upon the surface oxygen vacancy interaction with an oxygen molecule. The transfer of oxygen from the (100) surface to near-surface oxygen vacancies was considered since this process can be a limiting stage at the oxygen complex formation on the YFeO3 surface. The two most probable paths are determined, with a barrier value of about 1 eV, associated with the movement of an oxygen atom into the crystal through oxygen vacancies, from the surface position of the O2 type.

Ab Initio Study of HfO2/Ti Interface VO/Oi Frenkel Pair Formation Barrier and VO Interaction With Filament

Frenkel pair (FP) formation at HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ti interface and oxygen vacancy interaction with the filament in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ti-based resistive random access memory (ReRAM) are the important mechanisms in the ReRAM operations. In this article, the formation barriers, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{A}$ </tex-math></inline-formula> , and formation energies of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{O}/{O}_{i}$ </tex-math></inline-formula> FP across the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ti interface with and without preexisting vacancy are calculated using the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab initio</i> tool under zero external electric field. It is found that both the formation barrier and energy can be increased or decreased due to a preexisting oxygen vacancy. The spread of the effect increases inversely with the distance between the FP and the preexisting vacancy. A compact model is then proposed. The parallel migration barrier of an oxygen vacancy at the nearest and the second nearest neighbors (NNs) of a filament is also studied. It is found that the second NN has the lowest barrier due to its ability to retain a +2 charge state. The merging and dissolution of an oxygen vacancy with a filament (equivalent to perpendicular migration) is found to have a similar behavior that +2 states enhance the interaction. It is found that the fourfold coordinated oxygens (4C) and threefold coordinated oxygens (3C) filaments behave similarly during dissolution. However, 4C and 3C filaments prefer merging with 4C and 3C vacancy, respectively. The barrier of merging a +2 4C vacancy with a 4C filament is only 0.56 eV. The data and model obtained from this study may be used to implement device-level simulators, such as kinetic Monte Carlo (KMC) simulators, for ReRAM.

Phase-field simulation on the interaction of oxygen vacancies with charged and neutral domain walls in hexagonal YMnO3

A three-dimensional model of the interaction between the charged or neutral domain walls and oxygen vacancies in the hexagonal manganite YMnO3 was proposed, and simulated using Landau–Ginzburg–Devonshire (LGD) theory, dynamic diffusion equation and Maxwell’s equation. The calculation proves that stiffness anisotropic factors can adjust the domain wall state and ultimately affect the distribution of oxygen vacancies. The head-to-head domain wall corresponds to low oxygen vacancy density, and the tail-to-tail domain wall corresponds to high oxygen vacancy density. The electrostatic field generated by the bound charge is the key factor leading to the change of oxygen vacancy distribution. Finally, e-index law N d = ae b*dP/dz can fit the relationship between the oxygen vacancy concentration and the polarization gradient along z direction. Our theory provides a new way to modulate the distribution of oxygen vacancies through domain wall morphology in hexagonal YMnO3.

Negatively Charged In-Plane and Out-Of-Plane Domain Walls with Oxygen-Vacancy Agglomerations in a Ca-Doped Bismuth-Ferrite Thin Film.

The interaction of oxygen vacancies and ferroelectric domain walls is of great scientific interest because it leads to different domain-structure behaviors. Here, we use high-resolution scanning transmission electron microscopy to study the ferroelectric domain structure and oxygen-vacancy ordering in a compressively strained Bi0.9Ca0.1FeO3−δ thin film. It was found that atomic plates, in which agglomerated oxygen vacancies are ordered, appear without any periodicity between the plates in out-of-plane and in-plane orientation. The oxygen non-stoichiometry with δ ≈ 1 in FeO2−δ planes is identical in both orientations and shows no preference. Within the plates, the oxygen vacancies form 1D channels in a pseudocubic [010] direction with a high number of vacancies that alternate with oxygen columns with few vacancies. These plates of oxygen vacancies always coincide with charged domain walls in a tail-to-tail configuration. Defects such as ordered oxygen vacancies are thereby known to lead to a pinning effect of the ferroelectric domain walls (causing application-critical aspects, such as fatigue mechanisms and countering of retention failure) and to have a critical influence on the domain-wall conductivity. Thus, intentional oxygen vacancy defect engineering could be useful for the design of multiferroic devices with advanced functionality.

Proton-Assisted Reconstruction of Perovskite Oxides: Toward Improved Electrocatalytic Activity.

Electrocatalysis is indispensable to various emerging energy conversion and storage devices such as fuel cells and water electrolyzers. Owing to their unique physicochemical properties, perovskite oxide materials are one of the most promising water oxidation (OER) catalysts solely comprising earth-abundant elements. Nonetheless, many perovskite oxide catalysts suffer from a number of inherent problems such as the A-site cation segregation on the surface, coarse particles due to agglomeration/sintering, and surface decomposition during catalytic reactions. Besides, the catalytic activity is often incomparable with those of the state-of-the-art catalysts. In this work, we developed a proton-assisted approach to mitigate these common challenges. The protonation via the interaction of oxygen vacancies and water molecules induced the formation of protonic defects and the lattice expansion of the perovskite, leading to the fracture of big particles to yield small nanoparticles. This hydration in an acidic solution also selectively removed the A-site cation segregates and generated a spinel/perovskite heterostructure on the surface. We verified this approach using three typical perovskite OER catalysts including Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), La0.6Sr0.4Co0.8Fe0.2O3 (LSCF), and La0.75Sr0.25MnO3 (LSM). The processed catalysts showed much improved activity while maintaining their excellent stability, surpassing most of today's OER catalysts based on complex oxides.

Zn Metal Atom Doping on the Surface Plane of One-Dimesional NiMoO4 Nanorods with Improved Redox Chemistry.

The effect of zinc (Zn) doping and defect formation on the surface of nickel molybdate (NiMoO4) structures with varying Zn content has been studied to produce one-dimensional electrodes and catalysts for electrochemical energy storage and ethanol oxidation, respectively. Zn-doped nickel molybdate (Ni1-xZnxMoO4, where x = 0.1, 0.2, 0.4, and 0.6) nanorods were synthesized by a simple wet chemical route. The optimal amount of Zn is found to be around 0.25 above which the NiMoO4 becomes unstable, resulting in poor electrochemical activity. This result agrees with our density functional theory calculations in which the thermodynamic stability reveals that Ni1-xZnxMoO4 crystallized in the β-NiMoO4 phase and is found to be stable for x≤0.25. Analytical techniques show direct evidence of the presence of Zn in the NiMoO4 nanorods, which subtly alter the electrocatalytic activity. Compared with pristine NiMoO4, Zn-doped NiMoO4 with the optimized Zn content was tested as an electrode for an asymmetric supercapacitor and demonstrated an enhanced specific capacitance of 122 F g-1 with a high specific energy density of 43 W h kg-1 at a high power density of 384 W kg-1. Our calculations suggest that the good conductivity from Zn doping is attributed to the formation of excess oxygen vacancies and dopants play an important role in enhancing the charge transfer between the surface and OH- ions from the electrolyte. We report electrochemical testing, material characterization, and computational insights and demonstrate that the appropriate amount of Zn in NiMoO4 can improve the storage capacity (∼15%) due to oxygen vacancy interactions.

A Simulation Study on Minimizing Threshold Voltage Variability by Optimizing Oxygen Vacancy Concentration Under Metal Gate Granularity

In this work, we discuss the mitigation of threshold voltage ( ${V}_{{TH}}$ ) variability in nanoscale FinFET by exploiting the interaction of oxygen vacancies (OV) against metal gate granularity (MGG). Deposition of a metal gate on high- $\kappa $ dielectric HfO2 is known to generate OV in the dielectric which induces variability in surface potential. A higher workfunction of metal grain increases the probability of OV formation. Interestingly, while a higher workfunction metal grain decreases the surface potential of the channel underneath, the positive charge of OV increases the potential. Therefore, the concentration of OV can be controlled to negate the surface potential variability induced by MGG. We discuss the law of mass action-based concentration models of OV under MGG to analyze this. We also present TCAD simulations supporting the above hypothesis. In our simulations, for 10-nm channel FinFET, MGG-induced $\sigma $ ( ${V}_{{TH}}$ ) can be reduced to ~20 mV from ~35 mV for four-sided planar grains with 8 nm average edge length under the optimal OV concentration.

Composition, crystallography, and oxygen vacancy ordering impacts on the oxygen ion conductivity of lanthanum strontium ferrite.

This work presents a comprehensive computational study showing how aliovalent doping, crystal structure, and oxygen vacancy interactions impact the oxygen vacancy conductivity of lanthanum strontium ferrite (LSF) as a function of temperature in air. First, density functional theory (DFT) calculations were performed to obtain the oxygen vacancy migration barriers and understand the oxidation state changes on neighboring Fe atoms during oxygen vacancy migration. The oxygen migration barrier energy and the corresponding diffusion coefficient were then combined with previously determined mobile oxygen vacancy concentrations to predict the overall oxygen vacancy conductivity and compare it with experimentally measured values. More importantly, the impact of phase changes, the La/Sr ratio, and the oxygen non-stoichiometry on the mobile oxygen vacancy concentration, diffusivity, and conductivity were analyzed. It was found that stabilizing rhombohedral LSF or cubic SFO (through doping or other means), such that oxygen-vacancy-ordering-induced phase transitions are prevented, leads to high oxygen conductivity under solid oxide fuel cell operating conditions.

Comprehensive investigation of evanescent wave optical fiber refractive index sensor coated with ZnO nanoparticles

Abstract Combination of optical fiber and semiconductor metal oxide nanostructure provide a label-free refractive index (RI) sensor with an excellent limit of detection. ZnO NPs (nanoparticles) coated evanescent based multimode optical fiber sensor is established to detect different crude oil concentrations blended in diethyl ether. The efficacy of fabricated sensors with respect to wavelength shift and intensity changes is tested with different crude oil concentration from 0% (RI: 1.35) to 100% (RI: 1.47). By increasing the crude oil concentration, the maximum shift of ~10 nm towards lower wavelength is occurred. The signal in visible area originated from the electron–hole recombination at a deep level via singly ionized oxygen vacancies (VO+). Increasing the crude oil concentration leads to decreasing the transmission intensity up to 72% of its maximum value. Changes in refractive index of ZnO nanostructure from 2.671 to 2.726 with the change in oil concentration from 0% to 100% respectively, leads to different quantity of evanescent wave absorption taken placed. Different absorption rate and reflection angle in the cladding-ZnO interface are responsible for intensity modulation. The interaction of oxygen vacancies on ZnO surface, electron migration from donor media to the valance band and lowering the band-gap with decreasing the crude oil concentration are responsible for RI modification of ZnO. Therefore, an economic, high sensitive and multipurpose dual sensing scale has been proposed for crude oil detection.

Understanding the influence of defects and surface chemistry on ferroelectric switching: a ReaxFF investigation of BaTiO3.

Ferroelectric materials such as barium titanate (BaTiO3) have a wide range of applications in nano scale electronic devices due to their outstanding properties. In this study, we developed an easily extendable atomistic ReaxFF reactive force field for BaTiO3 that can capture both its field- and temperature-induced ferroelectric hysteresis and corresponding changes due to surface chemistry and bulk defects. Using our force field, we were able to reproduce and explain a number of experimental observations: (1) the existence of a critical thickness of 4.8 nm below which ferroelectricity vanishes in BaTiO3; (2) migration and clustering of oxygen vacancies (OVs) in BaTiO3 and a reduction in the polarization and the Curie temperature due to the OVs; (3) domain wall interaction with the surface chemistry to influence the ferroelectric switching and polarization magnitude. This new computational tool opens up a wide range of possibilities for making predictions for realistic ferroelectric interfaces in energy-conversion, electronic and neuromorphic systems.

Oxygen Vacancy-Modified B-/N-Codoped ZnGa2O4 Nanospheres with Enhanced Photocatalytic Hydrogen Evolution Performance in the Absence of a Pt Cocatalyst

Here, we report oxygen vacancy (VO)-modified B/N-codoped ZnGa2O4 (VO-B/N-ZGO) nanospheres, showing excellent photocatalytic H2 production even without a Pt cocatalyst, which is better than that obtained with VO-modified B-doped ZnGa2O4 (VO-B-ZGO) or N-doped ZnGa2O4 (N-ZGO) and as high as about three times of that achieved with the undoped ZnGa2O4 (ZGO) photocatalyst. The dramatically enhanced photocatalytic activity of VO-B/N-ZGO predominately originates from the improvement of charge separation and surface activation. Experimental characterization combined with the theoretical calculation method demonstrates that VO-B/N-ZGO can show effective charge compensation more easily through the interaction of oxygen vacancies, interstitial boron, and substitutional nitrogen; especially for the formation of a B–N bond, it avoids the presence of semioccupied states as new recombination centers in a doped photocatalyst. In addition, VO-B/N-ZGO rich in reactive sites is generated by oxygen vacancy-modified B/N-codopi...

Predicting Oxygen Vacancy Polaron Size from D-Orbital Splitting in ABO3 (A=La, Sr; B=Mn, Fe, Co) Perovskites

Mixed Ionic Electronic Conducting (MIEC) transition metal oxide (ABO3) perovskites are utilized for catalytic,1 energy conversion,2,3 and other applications. The oxygen ionic conductivity of these MIECs depends on their oxygen vacancy concentration and ionic mobility. In turn, the oxygen ion concentration and mobility both depend on the charge of the B-site atom,4 which itself depends on factors such as the A site La/Sr ratio, the crystal structure, etc. Previous calculations demonstrated that the Fe atom charge state distribution observed in strontium ferrite (SrFeO3-δ) is caused by differences in the d-orbital splitting of octahedral (Oh) versus square pyramidal (SP) Fe-O coordination (Figure 1).5 This d-orbital splitting ensures that the two electrons left behind when a charge-neutral oxygen vacancy forms in cubic SrFeO3-δ are transferred to the second nearest Fe neighbors,5 resulting in a large oxygen vacancy polaron size that produces strong oxygen vacancy interactions at high oxygen nonstoichiometry. As a result, the oxygen vacancy formation energy increases with oxygen nonstoichiometry in cubic SrFeO3-δ.5 Previous calculations also demonstrated that in orthorhombic lanthanum ferrite (LaFeO3-δ) the formation of a neutral oxygen vacancy produces a polaron that is localized to the oxygen-vacancy-adjacent Fe atoms. Hence, in orthorhombic LaFeO3-δ the oxygen vacancies do not interact with increasing oxygen nonstoichiometry.6 The objective of the present work was to examine whether d-orbital splitting could also explain the oxygen vacancy behavior of other MIEC perovskites. Hence, the impact of d-orbital splitting on oxygen vacancy polaron size and formation energies in lanthanum strontium manganite and lanthanum strontium cobaltite were modeled for the first time. Here, the charge redistribution caused by oxygen vacancy formation was studied for six different compositions: LaFeO3-δ, SrFeO3-δ, LaMnO3-δ, SrMnO3-δ, LaCoO3-δ, and SrCoO3-δ. The GGA+U method with PAW potentials implemented in VASP was utilized for all the structural energy calculations. First, the Hubbard-U parameter was calibrated to describe the charge on the B-site atom (U = 3 was selected for Fe, as done previously).6 The oxygen vacancy formation energy as a function of oxygen non-stoichiometry (δ) was then calculated at 0 K in vacuum by varying the size of the supercell, with oxygen vacancy interactions being enabled by the periodic boundary conditions. The calculated polaron size for neutral-oxygen-vacancy–containing ABO3 structures are summarized in Table 1.In LaMnO3-δ, Mn3+ has an [Ar] 3d4 electronic configuration in Oh coordination. After the formation of a neutral oxygen vacancy, the two Mn atoms adjacent to the oxygen vacancy go from Oh to SP coordination and the excess electrons left behind by the removed oxygen are pushed to the a1g level of the second nearest neighboring Oh-Mn (instead of remaining localized to the b1g level of the SP-Fe). This long range charge transfer results in large polaron size. This behavior is in contrast with that of LaFeO3-δ. However, LaMnO3-δ and SrFeO3-δ exhibit comparable polaron sizes since Fe4+ has an [Ar] 3d4 electronic- configuration. In SrMnO3-δ, Mn4+ has a [Ar] 3d3 electronic configuration in Oh coordination. Therefore, the formation of a neutral oxygen vacancy localizes the electrons to the oxygen vacancy adjacent Mn atoms, producing a small polaron, as explained by the d-orgbial splitting in Figure 1. This d-orbital splitting analysis indicates that the polaron size in LSM > LSF > LSC. Additional work aimed at calculating the oxygen vacancy formation energies of other ABO3 compositions are in progress.

First-Principles Studies of Oxygen Vacancy Interactions and Their Impact on Oxygen Migration in Lanthanum Strontium Ferrite

Due to their mixed ionic-electronic conduction (MIEC) properties, advanced functional materials like La1-xSrxFeO3-δ (LSF) are being explored for a variety of fuel cell, catalyst, and other electrochemical applications.1–3 The ionic conductivity of MIEC materials depends on their oxygen vacancy concentration and ionic mobility; quantities which both depend on the Fe charge state and hence vary with the LSF La/Sr ratio. Past modeling work has shown that a distribution in the Fe charge state, originated by the different d-orbital splitting in octahedral (Oh) and square pyramidal (SP) Fe-O polyhedra, exists in cubic SrFeO3.4 Further, this d-orbital splitting ensures that the two electrons left behind when a charge-neutral oxygen vacancy forms do not transfer to the Fe atoms directly connected to the oxygen vacancy (as one might expect), but instead are transferred to the second nearest neighbors in cubic SrFeO3.4 This new “long-range charge transfer” mechanism results in large oxygen vacancy polaron sizes that cause strong oxygen vacancy interactions and an increase in the oxygen vacancy formation energy with increasing oxygen vacancy site fraction. The objective of the present work was to examine the impact of lanthanum doping on 1) this oxygen vacancy formation behavior and 2) the oxygen vacancy migration behavior of a variety of LSF compositions. Here, the GGA+U method with PAW potentials implemented in VASP was utilized for all the structural energy calculations. First, the Hubbard-U parameter was calibrated to describe Fe from 2+ to 4+ (U = 3 was selected, as done previously).4 The charge state of Fe in different LSF phases was then determined by interpreting the DFT-predicted Fe magnetic moment. The oxygen vacancy formation energy as a function of oxygen non-stoichiometry (δ) was then calculated at 0K in vacuum by varying the size of the supercell, with oxygen vacancy interactions being enabled by the periodic boundary conditions. The oxygen vacancy concentration (c) at elevated temperatures at an oxygen partial pressure of 0.21 atm was then determined using a previously developed thermodynamic model.4 Next, the oxygen migration barriers between all the possible oxygen vacancy sites in each material were calculated and compared at 0K. The oxygen vacancy diffusivity (D) at elevated temperature and an oxygen partial pressure of 0.21 atm was then determined by assuming Arrhenius behavior of the diffusion coefficient and a temperature and oxygen partial pressure independent migration energy. Lastly, the ionic conductivity was determined from the oxygen vacancy concentration and the diffusivity by applying Einstein’s relation and the definition of the ionic conductivity. For all modeled LSF compositions (i.e. cubic SrFeO3, rhombohedral La0.5Sr0.5FeO3, cubic La0.5Sr0.5FeO3, and orthorhombic LaFeO3), the oxygen vacancies remained dilute (i.e. they did not interact) when the oxygen nonstoichiometry (δ) was < 0.1. When δ > 0.1, the oxygen vacancy formation energy was observed to increase for SrFeO3 and La0.5Sr0.5FeO3-δ with increasing δ. However, the oxygen vacancy formation energy in LaFeO3 remained constant for δ = 0 to 0.25. The calculated oxygen migration barriers for cubic SrFeO3 were 0.58 and 0.62 eV at δ = 0.04 and 0.13, respectively. Due to the presence of multiple oxygen vacancy migration paths in tetragonal and orthorhombic strontium ferrite, the minimum energy path was considered (the migration paths in Figure 1 are designated by different Wyckoff positions traversed by the oxygen atoms). For tetragonal and orthorhombic strontium ferrites the migration barriers were 0.77 and 1.17 eV, respectively. In LaFeO3, the calculated oxygen vacancy migration barrier was 0.88 eV which is comparable to the 0.77 eV observed in oxygen tracer diffusion experiments.5 Additional work aimed at calculating the oxygen vacancy formation energies, migration barriers, and ionic conductivities of other LSF compositions are in progress.

First-Principles Studies of Oxygen Vacancy Interactions and Their Impact on Oxygen Migration in Lanthanum Strontium Ferrite

DFT with a Hubbard-U method was utilized to elucidate oxygen vacancy formation and migration in lanthanum strontium ferrite (LSF) at different La:Sr ratios (∞, 1, and 0) and oxygen non-stoichiometries (0-0.5). With alteration of the La:Sr ratio, it was found that the oxygen vacancy formation energy varies by ~ 3 eV but the oxygen vacancy migration barrier only varies by ~ 0.5 eV. The Fe oxidation state controls the ease of oxygen vacancy formation, as well as the size and shape of the oxygen vacancy polarons; which, in turn, play the principal role in determining the amount of oxygen vacancy interactions in LSF. Even though these oxygen vacancy interactions are responsible for phase changes which dramatically alter the oxygen vacancy formation energy, the oxygen vacancy polarons (and hence the amount of oxygen vacancy interactions) and the LSF crystal structure have little effect on the oxygen migration barrier.

Polaron size and shape effects on oxygen vacancy interactions in lanthanum strontium ferrite

A change in the oxygen vacancy polaron size with La/Sr ratio in La1−xSrxFeO3−δ, leads to a change in oxygen vacancy interactions at a higher vacancy concentration.