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

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

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