The oxygen evolution reaction (OER) is a critical step for sustainable fuel production through electrochemistry process. Maximizing active sites of nanocatalyst with enhanced intrinsic activity, especially the activation of lattice oxygen, is gradually recognized as the primary incentive. Since the surface reconfiguration to oxyhydroxide is unavoidable for oxygen-activated transition metal oxides, developing a surface termination like oxyhydroxide in oxides is highly desirable. In this work, we demonstrate an unusual surface termination of (111)-facet Co₃O₄ nanosheet that is exclusively containing edge-sharing octahedral Co³⁺ similar to CoOOH that can perform at approximately 40 times higher current density at 1.63 V (vs RHE) than commercial RuO₂. It is found that this surface termination has an oxidized oxygen state in contrast to standard Co-O systems, which can serve as active site independently, breaking the scaling relationship limit. This work forwards the applications of oxide electrocatalysts in the energy conversion field by surface termination engineering.

Ferromagnetic insulator plays a pivotal role in various emerging spintronic effects and can be integrated as an essential component into future spintronic devices. However, ferromagnetic insulating perovskite oxides are rather scarce due to the strong coupling between ferromagnetism and metallicity via double exchange and superexchange mechanisms, severely restricting the development for oxide-based spintronic devices. Here, the disorder of manganite La2/3Sr1/3MnO3 (LSMO) films grown on MgAl2O4 substrates is utilized to decouple ferromagnetism and metallicity, thereby inducing a ferromagnetic insulating state over a rather broad temperature and thickness range. These films are non-crystalline with enormous disorder. The films with thicknesses less than 16 nm are in completely insulating state, and the films with thicknesses larger than 16 nm are in essentially insulating state with high resistivity. This insulating state is ascribed to the disorder in LSMO films and can be fitted by a localization model. In contrast, all LSMO films exhibit ferromagnetic transitions with considerable saturated magnetic moments and the Curie temperatures near room temperature, leading to the ferromagnetic insulator phase in our manganite heterostructures over a large temperature and thickness interval. Our results broaden the mind for inducing new ferromagnetic insulating state for potential applications in spintronic devices.

Most observed phase separation phenomena in complex oxides occur in systems with chemical dopants or structural defects, and theories have established the strong connection between phase separation and the random distribution of chemical dopants. Recent experiments on fabricated high-quality oxide superlattices also confirmed that the phase separation is suppressed in the clean systems without chemical disorders. Thus far, phase separation in strongly correlated oxides without the need of chemical dopants or structural defects has not been fully demonstrated. Here, we have built chemically ordered hybrid superlattices using prototypical SrRuO3 and SrTiO3 perovskite oxides. Contrary to previous understandings, we observe phase separation of two magnetic phases with different spin easy axes. We elucidate this phenomenon through first-principles calculations that the hybrid superlattices have a spontaneous structural instability, leading to a coexistence of ferromagnetic and antiferromagnetic phases. Our findings provide an alternative pathway other than chemical doping to introduce phase separation in correlated oxides and imply that phase separation can exist in clean systems without the need of chemical disorders.

Ferromagnetic insulators are required for many new magnetic devices, such as dissipationless quantum-spintronic devices, magnetic tunneling junctions, etc. Ferromagnetic insulators with a high Curie temperature and a high-symmetry structure are critical integration with common single-crystalline oxide films or substrates. So far, the commonly used ferromagnetic insulators mostly possess low-symmetry structures associated with a poor growth quality and widespread properties. The few known high-symmetry materials either have extremely low Curie temperatures (≤16 K), or require chemical doping of an otherwise antiferromagnetic matrix. Here we present compelling evidence that the LaCoO₃ single-crystalline thin film under tensile strain is a rare undoped perovskite ferromagnetic insulator with a remarkably high T_C of up to 90 K. Both experiments and first-principles calculations demonstrate tensile-strain-induced ferromagnetism which does not exist in bulk LaCoO₃ The ferromagnetism is strongest within a nearly stoichiometric structure, disappearing when the Co²⁺ defect concentration reaches about 10%. Significant impact of the research includes demonstration of a strain-induced high-temperature ferromagnetic insulator, successful elevation of the transition over the liquid-nitrogen temperature, and high potential for integration into large-area device fabrication processes.