Strain control of oxygen kinetics in the Ruddlesden-Popper oxide La1.85Sr0.15CuO4

Designing Oxide Catalysts for Oxygen Electrocatalysis: Insights from Mechanism to Application

The electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are fundamental processes in a range of energy conversion devices such as fuel cells and metal-air batteries. ORR and OER both have significant activation barriers, which severely limit the overall performance of energy conversion devices that utilize ORR/OER. Meanwhile, ORR is another very important electrochemical reaction involving oxygen that has been widely investigated. ORR occurs in aqueous solutions via two pathways: the direct 4-electron reduction or 2-electron reduction pathways from O2 to water (H2O) or from O2 to hydrogen peroxide (H2O2). Noble metal electrocatalysts are often used to catalyze OER and ORR, despite the fact that noble metal electrocatalysts have certain intrinsic limitations, such as low storage. Thus, it is urgent to develop more active and stable low-cost electrocatalysts, especially for severe environments (e.g., acidic media). Theoretically, an ideal oxygen electrocatalyst should provide adequate binding to oxygen species. Transition metals not belonging to the platinum group metal-based oxides are a low-cost substance that could give a d orbital for oxygen species binding. As a result, transition metal oxides are regarded as a substitute for typical precious metal oxygen electrocatalysts. However, the development of oxide catalysts for oxygen reduction and oxygen evolution reactions still faces significant challenges, e.g., catalytic activity, stability, cost, and reaction mechanism. We discuss the fundamental principles underlying the design of oxide catalysts, including the influence of crystal structure, and electronic structure on their performance. We also discuss the challenges associated with developing oxide catalysts and the potential strategies to overcome these challenges.

Defect Engineering in A2 BO4 Thin Films via Surface-Reconstructed LaSrAlO4 Substrates

Ruddlesden-Popper oxides (A₂ BO₄ ) have attracted significant attention regarding their potential application in novel electronic and energy devices. However, practical uses of A₂ BO₄ thin films have been limited by extended defects such as out-of-phase boundaries (OPBs). OPBs disrupt the layered structure of A₂ BO₄ , which restricts functionality. OPBs are ubiquitous in A₂ BO₄ thin films but inhomogeneous interfaces make them difficult to suppress. Here, OPBs in A₂ BO₄ thin films are suppressed using a novel method to control the substrate surface termination. To demonstrate the technique, epitaxial thin films of cuprate superconductor La_{2- x} Sr_x CuO₄ (x = 0.15) are grown on surface-reconstructed LaSrAlO₄ substrates, which are terminated with self-limited perovskite double layers. To date, La_{2- x} Sr_x CuO₄ thin films are grown on LaSrAlO₄ substrates with mixed-termination and exhibit multiple interfacial structures resulting in many OPBs. In contrast, La_{2- x} Sr_x CuO₄ thin films grown on surface-reconstructed LaSrAlO₄ substrates energetically favor only one interfacial structure, thus inhibiting OPB formation. OPB-suppressed La_{2- x} Sr_x CuO₄ thin films exhibit significantly enhanced superconducting properties compared with OPB-containing La_{2- x} Sr_x CuO₄ thin films. Defect engineering in A₂ BO₄ thin films will allow for the elimination of various types of defects in other complex oxides and facilitate next-generation quantum device applications.

Role of Sr doping and external strain on relieving bottleneck of oxygen diffusion in La2-xSrxCuO4-δ

In many complex oxides, the oxygen vacancy formation is a promising route to modify the material properties such as a superconductivity and an oxygen diffusivity. Cation substitutions and external strain have been utilized to control the concentration and diffusion of oxygen vacancies, but the mechanisms behind the controls are not fully understood. Using first-principles calculations, we find how Sr doping and external strain greatly enhances the diffusivity of oxygen vacancies in La_2−xSr_xCuO_4−δ (LSCO) in the atomic level. In hole-doped case (2x > δ), the formation energy of an apical vacancy in the LaO layer is larger than its equatorial counterpart by 0.2 eV that the bottleneck of diffusion process is for oxygen vacancies to escape equatorial sites. Such an energy difference can be reduced and even reversed by either small strain (< 1.5%) or short-range attraction between Sr and oxygen vacancy, and in turn, the oxygen diffusivity is greatly enhanced. For fully compensated hole case (2x ≦ δ), the formation energy of an apical vacancy becomes too high that most oxygen vacancies cannot move but would be trapped at equatorial sites. From our electronic structure analysis, we found that the contrasting change in the formation energy by Sr doping and external strain is originated from the different localization natures of electron carrier from both types of oxygen vacancies.

Cation-Deficiency-Dependent CO2 Electroreduction over Copper-Based Ruddlesden-Popper Perovskite Oxides

We report an effective strategy to enhance CO₂ electroreduction (CER) properties of Cu-based Ruddlesden-Popper (RP) perovskite oxides by engineering their A-site cation deficiencies. With La_2-x CuO_4-δ (L_2-x C, x=0, 0.1, 0.2, and 0.3) as proof-of-concept catalysts, we demonstrate that their CER activity and selectivity (to C₂₊ or CH₄ ) show either a volcano-type or an inverted volcano-type dependence on the x values, with the extreme point at x=0.1. Among them, at -1.4 V, the L_1.9 C delivers the optimal activity (51.3 mA cm^-2 ) and selectivity (41.5 %) for C₂₊ , comparable to or better than those of most reported Cu-based oxides, while the L_1.7 C exhibits the best activity (25.1 mA cm^-2 ) and selectivity (22.1 %) for CH₄ . Such optimized CER properties could be ascribed to the favorable merits brought by the cation-deficiency-induced oxygen vacancies and/or CuO/RP hybrids, including the facilitated adsorption/activation of key reaction species and thus the manipulated reaction pathways.

Facile Removal of Bulk Oxygen Vacancy Defects in Metal Oxides Driven by Hydrogen-Dopant Evaporation

Oxygen vacancy is a common defect in metal oxides that causes appreciable damage to material properties and performance. Removing bulk defects of oxygen vacancy (V_O) typically needs harsh conditions such as high-temperature annealing. Supported by first-principles simulations, we propose an effective strategy of removing V_O bulk defects in metal oxides by evaporating hydrogen dopants. The hydrogen dopants not only lower the migration barrier of V_O but also push V_O away due to their repulsive interaction. The coevaporation mechanism was supported by a neural networks potential-based molecular dynamics simulation, which shows that the migration of hydrogen dopants from inside to surface at 400 K promotes the migration of V_O as well. Our proof-of-concept study suggests an alternative and efficient way of modulating oxygen vacancies in metal oxides via reversible hydrogen doping.

Local-electrostatics-induced oxygen octahedral distortion in perovskite oxides and insight into the structure of Ruddlesden-Popper phases

As the physical properties of ABX₃ perovskite-based oxides strongly depend on the geometry of oxygen octahedra containing transition-metal cations, precise identification of the distortion, tilt, and rotation of the octahedra is an essential step toward understanding the structure-property correlation. Here we discover an important electrostatic origin responsible for remarkable Jahn-Teller-type tetragonal distortion of oxygen octahedra during atomic-level direct observation of two-dimensional [AX] interleaved shear faults in five different perovskite-type materials, SrTiO₃, BaCeO₃, LaCoO₃, LaNiO₃, and CsPbBr₃. When the [AX] sublayer has a net charge, for example [LaO]⁺ in LaCoO₃ and LaNiO₃, substantial tetragonal elongation of oxygen octahedra at the fault plane is observed and this screens the strong repulsion between the consecutive [LaO]⁺ layers. Moreover, our findings on the distortion induced by local charge are identified to be a general structural feature in lanthanide-based A_n + 1B_nX_3n + 1-type Ruddlesden-Popper (RP) oxides with charged [LnO]⁺ (Ln = La, Pr, Nd, Eu, and Gd) sublayers, among more than 80 RP oxides and halides with high symmetry. The present study thus demonstrates that the local uneven electrostatics is a crucial factor significantly affecting the crystal structure of complex oxides.

Strain-Induced Atomic-Scale Building Blocks for Ferromagnetism in Epitaxial LaCoO3

The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO₃ (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.

Atomic Origin for Hydrogenation Promoted Bulk Oxygen Vacancies Removal in Vanadium Dioxide

Oxygen vacancies (V_O), a common type of point defect in metal oxides materials, play important roles in the physical and chemical properties. To obtain stoichiometric oxide crystal, the pre-existing V_O is always removed via careful post-annealing treatment at high temperature in an air or oxygen atmosphere. However, the annealing conditions are difficult to control, and the removal of V_O in the bulk phase is restrained because of the high energy barrier of V_O migration. Here, we selected VO₂ crystal film as the model system and developed an alternative annealing treatment aided by controllable hydrogen doping, which can realize effective removal of V_O defects in the VO_2-δ crystal at a lower temperature. This finding is attributed to the hydrogenation accelerated oxygen vacancies recovery in the VO_2-δ crystal. Theoretical calculations revealed that the H-doping-induced electrons are prone to accumulate around the oxygen defects in the VO_2-δ film, which facilitates the diffusion of V_O and thus makes it easier to be removed. The methodology is expected to be applied to other metal oxides for oxygen-related point defects control.

Metal-insulator transition tuned by oxygen vacancy migration across TiO2/VO2 interface

Oxygen defects are essential building blocks for designing functional oxides with remarkable properties, ranging from electrical and ionic conductivity to magnetism and ferroelectricity. Oxygen defects, despite being spatially localized, can profoundly alter global properties such as the crystal symmetry and electronic structure, thereby enabling emergent phenomena. In this work, we achieved tunable metal–insulator transitions (MIT) in oxide heterostructures by inducing interfacial oxygen vacancy migration. We chose the non-stoichiometric VO_2-δ as a model system due to its near room temperature MIT temperature. We found that depositing a TiO₂ capping layer on an epitaxial VO₂ thin film can effectively reduce the resistance of the insulating phase in VO₂, yielding a significantly reduced R_OFF/R_ON ratio. We systematically studied the TiO₂/VO₂ heterostructures by structural and transport measurements, X-ray photoelectron spectroscopy, and ab initio calculations and found that oxygen vacancy migration from TiO₂ to VO₂ is responsible for the suppression of the MIT. Our findings underscore the importance of the interfacial oxygen vacancy migration and redistribution in controlling the electronic structure and emergent functionality of the heterostructure, thereby providing a new approach to designing oxide heterostructures for novel ionotronics and neuromorphic-computing devices.

Applying Configurational Complexity to the 2D Ruddlesden-Popper Crystal Structure

The layered Ruddlesden-Popper crystal structure can host a broad range of functionally important behaviors. Here we establish extraordinary configurational disorder in a layered Ruddlesden-Popper (RP) structure using entropy stabilization assisted synthesis. A protype A₂CuO₄ RP cuprate oxide with five cations on the A-site sublattice is designed and fabricated into epitaxial single crystal films using pulsed laser deposition. When grown on a near lattice matched substrate, the (La_0.2Pr_0.2Nd_0.2Sm_0.2Eu_0.2)₂CuO₄ film features a T'-type RP structure with uniform A-site cation mixing and square-planar CuO₄ units. These observations are made with a range of combined characterizations using X-ray diffraction, atomic-resolution scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray absorption spectroscopy measurements. It is further found that heteroepitaxial strain plays an important role in crystal phase formation during synthesis. Compressive strain over ∼1.5% results in the formation of a non-RP cubic phase consistent with a CuX₂O₄ spinel structure. The ability to manipulate configurational complexity and move between 2D layered RP and 3D cubic crystal structures in cuprate and related materials promises to enable flexible design strategies for a range of functionalities, such as magnetoresistance, unconventional superconductivity, ferroelectricity, catalysis, and ion transport.

Effect of Lattice Strain on the Formation of Ruddlesden-Popper Faults in Heteroepitaxial LaNiO3 for Oxygen Evolution Electrocatalysis

A great deal of research has recently been focused on Ruddlesden-Popper (RP) two-dimensional planar faults consisting of intervened [AO] monolayers in an ABO₃ perovskite framework due to the structurally peculiar shear configuration. In this work, we scrutinize the effect of elastic strain on the generation behavior of RP faults, which are electrocatalytically very active sites for the oxygen evolution reaction (OER), in (001) epitaxial LaNiO₃ thin films through by using two distinct single-crystal substrates with different cubic lattice parameters. Atomic-scale direct observations reveal that RP faults can be more favorably created when tensile misfit strain is exerted. Furthermore, we demonstrate that the controlled growth of thin films show notably enhanced OER activity by the RP faults. The findings in this study highlight the impact of symmetry-breaking defect formation for better oxygen electrocatalysis in perovskite oxides.

Film-thickness-driven superconductor to insulator transition in cuprate superconductors

The superconductor-insulator transition induced by film thickness control is investigated for the optimally doped cuprate superconductor La_1.85Sr_0.15CuO₄. Epitaxial thin films are grown on an almost exactly matched substrate LaAlO₃ (001). Despite the wide thickness range of 6 nm to 300 nm, all films are grown coherently without significant relaxation of the misfit strain. Electronic transport measurement exhibits systematic suppression of the superconducting phase by reducing the film thickness, thereby inducing a superconductor-insulator transition at a critical thickness of ~10 nm. The emergence of a resistance peak preceding the superconducting transition is discussed based on the weak localization. X-ray photoelectron spectroscopy results show the possibility that oxygen vacancies are present near the interface.

Oxygen-Deficient Ruddlesden-Popper-Type Lanthanum Strontium Cuprate Doped with Bismuth as a Cathode for Solid Oxide Fuel Cells

Ruddlesden-Popper-type strontium-doped lanthanum cuprates are unique in oxygen defects because of the oxygen-deficient composition. This work increases the oxygen vacancy concentration through bismuth-doping and thus promotes the electrochemical performance for oxygen reduction reaction (ORR) in solid oxide fuel cells. X-ray diffraction shows that up to 10% A-site elements can be doped with bismuth. The doping improves the catalytic activity through (1) increasing oxygen vacancy concentration by 87.5 and 65.5% at room temperature and 800 °C, respectively, as demonstrated by iodometric titration and thermogravimetric analysis, (2) greatly reducing the energy for oxygen vacancy formation as shown by density functional theory calculation, (3) forming additional reactive oxygen species at the near surface region as suggested with X-ray photoelectron spectroscopy, and (4) enhancing the oxygen transport properties as exhibited with electrical conductivity relaxation. In addition, bismuth doping reduces the thermal expansion coefficient to a level that could exactly match the thermal expansion behavior to the electrolytes. Consequently, the interfacial polarization resistance for ORR is decreased by 43% at 800 °C for the cuprate-based composite electrodes. The decrease is greatly attributed to the enhancement in the charge-transfer process, the rate-limiting step. Further, the peak power density for a model cell is increased from 530 to 630 mW·cm^-2 at 800 °C. Bismuth-doping is a promising strategy to modify the catalytic properties of unique cuprates toward ORR.

Controlling the Oxygen Electrocatalysis on Perovskite and Layered Oxide Thin Films for Solid Oxide Fuel Cell Cathodes

Achieving the fast oxygen reduction reaction (ORR) kinetics at the cathode of solid oxide fuel cells (SOFCs) is indispensable to enhance the efficiency of SOFCs at intermediate temperatures. Mixed ionic and electronic conducting (MIEC) oxides such as ABO3 perovskites and Ruddlesden-Popper (RP) oxides (A2BO4) have been widely used as promising cathode materials owing to their attractive physicochemical properties. In particular, oxides in forms of thin films and heterostructures have enabled significant enhancement in the ORR activity. Therefore, we aim to give a comprehensive overview on the recent development of thin film cathodes of SOFCs. We discuss important advances in ABO3 and RP oxide thin film cathodes for SOFCs. Our attention is also paid to the influence of oxide heterostructure interfaces on the ORR activity of SOFC cathodes.

Room-Temperature Ferromagnetic Insulating State in Cation-Ordered Double-Perovskite Sr2 Fe1+ x Re1- x O6 Films

Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, FMIs are innately rare to find in nature as ferromagnetism generally accompanies metallicity. Here, novel room-temperature FMI films that are epitaxially synthesized by deliberate control of the ratio between two B-site cations in the double perovskite Sr₂ Fe_{1+ x} Re_{1- x} O₆ (-0.2 ≤ x ≤ 0.2) are reported. In contrast to the known FM metallic phase in stoichiometric Sr₂ FeReO₆ , an FMI state with a high Curie temperature (T_c ≈ 400 K) and a large saturation magnetization (M_S ≈ 1.8 µ_B f.u.^-1 ) is found in highly cation-ordered Fe-rich phases. The stabilization of the FMI state is attributed to the formation of extra Fe³⁺ Fe³⁺ and Fe³⁺ Re⁶⁺ bonding states, which originate from the relatively excess Fe ions owing to the deficiency in Re ions. The emerging FMI state created by controlling cations in the oxide double perovskites opens the door to developing novel oxide quantum materials and spintronic devices.