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Monolayer ruthenate nanosheets obtained by exfoliating layered ruthenium oxide exhibit excellent electrical conductivity, redox activity, and catalytic activity, which render them suitable for advanced electronic and energy devices. However, to fully exploit the benefits, we require further structural insights into a complex polymorphic nature and diversity in relevant electronic states of two-dimensional (2D) ruthenate systems. In this study, the 2D structures, stability, and electronic states of 2D ruthenate are investigated on the basis of thermal and chemical phase engineering approaches. We reveal that contrary to a previous report, exfoliation of an oblique 1T phase precursor leads to nanosheets having an identical phase without exfoliation-induced phase transition to a 1H phase. The oblique 1T phase in the nanosheets is found to be metastable and, thus, transforms successively to a rectangular 1T phase upon heating. A phase-controllable synthesis via Co doping affords nanosheets with metastable rectangular and thermally stable hexagonal 1T phases at a Co content of 5-10 and 20 at%, respectively. The 1T phases show metallic electronic states, where the d-d optical transitions between the Ru 4d (t 2g ) orbital depend on the symmetry of the Ru framework. The Co doping in ruthenate nanosheets unexpectedly suppresses the redox and catalytic activities under acidic conditions. In contrast, the Co 2+/3+ redox pair is activated and produces conductive nanosheets with high electrochemical capacitance in an alkaline condition.

Mechanical nonreciprocity, or the asymmetric transmission of mechanical quantities between two points in space, is crucial for developing systems that can guide, damp, and control mechanical energy. We report a uniform composite hydrogel that displays substantial mechanical nonreciprocity, owing to direction-dependent buckling of embedded nanofillers. This material exhibits an elastic modulus more than 60 times higher when sheared in one direction compared with the opposite direction. Consequently, it can transform symmetric vibrations into asymmetric ones that are applicable for mass transport and energy harvest. Furthermore, it exhibits an asymmetric deformation when subjected to local interactions, which can induce directional motion of various objects, including macroscopic objects and even small living creatures. This material could promote the development of nonreciprocal systems for practical applications such as energy conversion and biological manipulation.

Neat (dense and nonoverlapped) monolayer tiling of 2D nanosheets on a substrate surface is very important because we can conduct artificial lattice-engineering by repeating the tiling process in a designed sequence to tailor various hierarchical nanostructures, leading to a range of sophisticated functions. It is recently reported that a facile spin-coating technique realizes the neat monolayer tiling of various 2D nanosheets. Establishing universal guidelines to neatly tile 2D nanosheets on substrates of various materials and size/shape is of essential importance to fully apply this technique, but the mechanism of how the nanosheets are tiled has not been clarified yet. In the present study, we have systematically examined the nanosheet deposition process at various rotation speeds by microscopic observations and found that the neat monolayer tiling of nanosheets is attained on the solvent surface during the spin-coating, and then the monolayer film is deposited onto the substrate surface from its center toward the edges upon evaporation of the solvent. Furthermore, we have clarified how the rotation speed and the nanosheet concentration govern the deposition behaviors in terms of neat tiling, overlap, or noncoverage in a such process. On the basis of the guidelines, we can predict the optimum spin-coating conditions for attaining the neat monolayer tiling of various nanosheets over an entire surface of the substrate.

Two-dimensional (2D) metal oxide nanosheets can exhibit exceptional electrochemical performance owing to their shortened ion diffusion distances, abundant active sites, and various valence states. Especially, genuine unilamellar nanosheets with an atomic-scale thickness are expected to exhibit the ultimate energy storage capability but have not yet achieved their potential. Here, we demonstrate the utilization of genuine unilamellar MnO 2 nanosheets for high-performance Li and Na storage using an alternately stacked MnO 2 /graphene superlattice-like structure. Different from previous reports, all unilamellar MnO 2 nanosheets are separated and stabilized between the graphene monolayers, resulting in highly reversible 2D-confined conversion processes. As a consequence, large specific capacities of 1325 and 795 mA h g -1 at 0.1 A g -1 , high-rate capacities of 370 and 245 mA h g -1 at 12.8 A g -1 , and excellent cycling stabilities after 5000 cycles with ∼0.004% and 0.0078% capacity decay per cycle were obtained for Li and Na storage, respectively, presenting the best reported performance to date.