Role of Strain and Conductivity in Oxygen Electrocatalysis on LaCoO₃ Thin Films

The slow kinetics of the oxygen reduction and evolution reactions (ORR, OER) hinder energy conversion and storage in alkaline fuel cells and electrolyzers employing abundant transition metal oxide catalysts. Systematic studies linking material properties to catalytic activity are lacking, in part due to the heterogeneous nature of powder-based electrodes. We demonstrate, for the first time, that epitaxial strain can tune the activity of oxygen electrocatalysis in alkaline solutions, focusing on the model chemistry of LaCoO₃, where moderate tensile strain can further induce changes in the electronic structure leading to increased activity. The resultant decrease in charge transfer resistance to the electrolyte reduces the overpotential in the ORR more notably than the OER and suggests a different dependence of the respective rate-limiting steps on electron transfer. This provides new insight into the reaction mechanism applicable to a range of perovskite chemistries, key to the rational design of highly active catalysts

Enhancing Perovskite Electrocatalysis through Strain Tuning of the Oxygen Deficiency

Oxygen vacancies in transition-metal oxides facilitate catalysis critical for energy storage and generation. However, promoting vacancies at the lower temperatures required for operation in devices such as metal–air batteries and portable fuel cells has proven elusive. Here we used thin films of perovskite-based strontium cobaltite (SrCoO_<i>x</i>) to show that epitaxial strain is a powerful tool for manipulating the oxygen content under conditions consistent with the oxygen evolution reaction, yielding increasingly oxygen-deficient states in an environment where the cobaltite would normally be fully oxidized. The additional oxygen vacancies created through tensile strain enhance the cobaltite’s catalytic activity toward this important reaction by over an order of magnitude, equaling that of precious-metal catalysts, including IrO₂. Our findings demonstrate that strain in these oxides can dictate the oxygen stoichiometry independent of ambient conditions, allowing unprecedented control over oxygen vacancies essential in catalysis near room temperature

Fabrication and Thermoelectric Properties of Freestanding Ba_1/3CoO₂ Single-Crystalline Films

Thermoelectric energy conversion has attracted attention as an energy-harvesting technology for converting waste heat into electricity via the Seebeck effect. Conducting oxide-based thermoelectric materials that exhibit a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Among many conducting oxides, Ba1/3CoO2 epitaxial films exhibit the highest figures of merit. For the practical use of Ba1/3CoO2, bulk ceramics or single-crystalline Ba1/3CoO2 is necessary. Here, we show that freestanding Ba1/3CoO2 single-crystalline films can be fabricated by peeling Ba1/3CoO2 epitaxial films from the substrate. We fabricated Ba1/3CoO2 epitaxial films and immersed them in 40 °C hot water for several tens of minutes. Subsequently, the Ba1/3CoO2 epitaxial film spontaneously peeled off and floated on the surface of the water like seaweed. We measured and analyzed the crystal structure, chemical composition, and thermoelectric properties before and after peeling and realized that there was no significant difference. The present results provide a useful method for fabricating freestanding single-crystalline oxide films for thermoelectrics

Solid-State Electrochemical Thermal Transistors with Strontium Cobaltite–Strontium Ferrite Solid Solutions as the Active Layers

Thermal transistors have potential as thermal management devices because they can electrically control the thermal conductivity (κ) of the active layer. Recently, we realized solid-state electrochemical thermal transistors by utilizing the electrochemical redox reaction of SrCoOy (2 ≤ y ≤ 3). However, the guiding principle to improve the on/off κ ratio has yet to be clarified because the κ modulation mechanism is unclear. This study systematically modulates κ of SrCo1–xFexOy (0 ≤ x ≤ 1, 2 ≤ y ≤ 3) solid solutions used as the active layers in solid-state electrochemical thermal transistors. When y = 3, the lattice κ of SrCo1–xFexOy is ∼2.8 W m–1 K–1 and insensitive to x. When x = 0 and y = 3, κ increases to ∼3.8 W m–1 K–1 due to the contribution of the electron κ. When y = 2, κ slightly depends on the ordered atomic arrangement. Materials that are high electrical conductors with highly ordered lattices when the transistor is on but are electrical insulators with disordered lattices when the transistor is off should be well-suited for the active layers of solid-state electrochemical thermal transistors

Low Temperature Nanoscale Oxygen-Ion Intercalation into Epitaxial MoO₂ Thin Films

In transition metal oxides (TMOs), lots of physical phenomena such as metal–insulator transitions (MIT), magnetism, and ferroelectricity are closely related to the amounts of oxygen contents. Thus, understanding surface oxidation process in TMOs and its effect are important for enhancing performances of modern electronic and electrochemical devices due to miniaturization of those devices. In this regard, MoO_2+<i>x</i> (0 ≤ <i>x</i> ≤ 1) is an interesting TMO, which shows MIT driven by the change of its oxygen content, i.e. metallic MoO₂ and insulating MoO₃. Hence, understanding thermally driven oxygen intercalation into MoO₂ is very important. In this work, we conducted <i>in situ</i> postannealing of as-grown epitaxial MoO₂ thin films at different temperatures in oxidative condition to investigate the thermal effect on oxygen ion intercalation and resultant MIT in MoO_2+<i>x</i>. Through the spectroscopic techniques such as spectroscopic ellipsometry and X-ray absorption spectroscopy, we observed that oxygen ions can intercalate into MoO₂ and trigger a phase transition in nanoscale at surprisingly low-temperature as low as 250 °C. In addition, after oxygen annealing at 350 °C, we find that both hybridization and interband transition energy between O 2p and Mo 4d t_2g are significantly shifted to low energy nearly 0.2 eV, which clearly supports that the electronic transition of MoO_2+<i>x</i> is predominantly driven by change of oxygen contents

Tunneling Electroresistance Induced by Interfacial Phase Transitions in Ultrathin Oxide Heterostructures

The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics

Effect of Ceramic-Target Crystallinity on Metal-to-Insulator Transition of Epitaxial Rare-Earth Nickelate Films Grown by Pulsed Laser Deposition

We demonstrate the effect of the crystallinity of ceramic targets on the electronic properties of LaNiO3 (001) thin films epitaxially grown by pulsed laser deposition (PLD). We prepared two kinds of LaNiO3 targets with different crystallinity by manipulating calcination temperature (i.e., 300 and 1000 °C) in the solid state reaction for ceramic synthesis. X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS) experiments of the as-sintered LaNiO3 ceramic targets clearly show that the LaNiO3 target sintered after high-temperature (1000 °C, high crystallinity) calcination is more oxidized to Ni3+ with better crystallinity than the LaNiO3 target sintered after low-temperature (300 °C, poor crystallinity) calcination. Using these two LaNiO3 ceramics as PLD targets, we fabricated epitaxial LaNiO3/LaAlO3 (001) thin-film heterostructures to examine how target crystallinity affects the physical properties of LaNiO3 films. Intriguingly, the electrical transport properties of the as-grown LaNiO3 thin films are quite different depending on crystallinity of the LaNiO3 ceramic target used for film deposition. In conjunction with subsequent XPS analyses of our LaNiO3 thin films, it appears that LaNiO3 (001) films deposited from the high-temperature-calcined target with better crystallinity are less disproportionate in Ni charge valency with more Ni3+ oxidation states compared with LaNiO3 (001) films deposited from the low-temperature-calcined target with poor crystallinity. This difference in degree of charge disproportionation can induce a discrepancy in the metal-to-insulator transition temperature of ultrathin LaNiO3 (001) films and in their electrical conductance

Strain-Induced Spin States in Atomically Ordered Cobaltites

Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is recognized as a powerful tool for identifying new properties and exploring the vast potential of materials performance. A particular example is LaCoO₃, a zero spin, nonmagnetic material in the bulk, whose strong ferromagnetism in a thin film remains enigmatic despite a decade of intense research. Here, we use scanning transmission electron microscopy complemented by X-ray and optical spectroscopy to study LaCoO₃ epitaxial thin films under different strain states. We observed an unconventional strain relaxation behavior resulting in stripe-like, lattice modulated patterns, which did not involve uncontrolled misfit dislocations or other defects. The modulation entails the formation of ferromagnetically ordered sheets comprising intermediate or high spin Co³⁺, thus offering an unambiguous description for the exotic magnetism found in epitaxially strained LaCoO₃ films. This observation provides a novel route to tailoring the electronic and magnetic properties of functional oxide heterostructures