Thermal and Chemical Phase Engineering of Two-Dimensional Ruthenate.

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 (t2g) 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 Co2+/3+ redox pair is activated and produces conductive nanosheets with high electrochemical capacitance in an alkaline condition.

Solution-Processed Two-Dimensional Metal Oxide Anticorrosion Nanocoating.

Molecularly thin two-dimensional (2D) nanomaterials are attractive building blocks for constructing anticorrosion nanocoatings as an ultimate pursuit in the metal-related industry. However, the nanocoating of prefocused graphene is far from industrial demands due to its high cost, low scalability, and insufficient quality. We propose all requirements to realize rational anticorrosion nanocoating of metal oxide nanosheets. The proof-of-concept study with Ti0.87O2 and Ca2Nb3O10 nanosheets demonstrates that the 10 and 20 nm thick coatings fabricated by a facile layer-by-layer (LbL) self-assembly on stainless steel (SUS) give perfect inhibition efficiency (IE) values of 99.92% and 99.89%, respectively. A driving test with a nanosheet-coated car-baffle demonstrated suitable corrosion resistance and mechanical and thermal robustness for industrial applications. The revealed and controlled thermal oxidation mechanisms are critical toward high-temperature application of the 2D oxide anticorrosion nanocoating. The advantages of nanosheet coating and extensible materials design will open a solid but exciting route to anticorrosion nanotechnology.

On/Off Boundary of Photocatalytic Activity between Single- and Bilayer MoS2.

Molecularly thin two-dimensional (2D) semiconductors are emerging as photocatalysts owing to their layer-number-dependent quantum effects and high charge separation efficiency. However, the correlation among the dimensionality, crystallinity, and photocatalytic activity of such 2D nanomaterials remains unclear. Herein, a Ag photoreduction technique coupled with microscopic analyses is employed to spatially resolve the photocatalytic activity of MoS2 as a model catalyst. Interestingly, we find that only monolayer (1L)-MoS2 is active for a Ag photoreduction reaction. The photocatalytic activity of 1L-MoS2 is enhanced by a built-in electrical field originated from the MoS2/SiO2 interface, instead of by the specific surface structure and quantum electronic state of 1L-MoS2. Furthermore, we observe photocatalytic active sites to be geometrically distributed on triangular 1L-MoS2 crystals, wherein the Ag particles are preferentially deposited on the outermost zigzag edges and defective inner parts of the triangular grains. The degradation of photocatalytic activity and electron mobility with the formation of Mo(VI) species indicates that the species inhibit the in-plane diffusion of the photogenerated electrons to the reductive sites. The monolayer-selectivity, activation, and inactivation mechanisms, unveiled in this work, will offer future directions in designing 2D nanophotocatalysts.

A RuO2 Nanosheet as a Novel Quencher-free Platform for the Detection of Nucleic Acids in a Homogeneous Solution.

A fluorescent dye-labeled DNA probe was adsorbed and quenched on the monolayer of RuO2 nanosheets. Significant fluorescent recovery was observed upon the addition of complementary DNA due to desorption of the probe from the surface of the RuO2 nanosheet through duplex formation. The efficiency of fluorescence recovery was higher than that for graphene oxide, which was known as a quencher-free platform for the detection of nucleic acids in a homogeneous solution.

Tunable Chemical Coupling in Two-Dimensional van der Waals Electrostatic Heterostructures.

Heterostructures of two-dimensional (2D) atomic crystals provide fascinating molecular-scale design elements for emergent physical phenomena and functional materials, as integrating distinct monolayers into vertical heterostructures can afford coupling between disparate properties. However, the available examples have been limited to either van der Waals (vdW) or electrostatic (ES) heterostructures that are solely composed of noncharged and charged monolayers, respectively. Here, we propose a "vdW-ES heterostructure" chemical design in which charge-neutral and charged monolayer-building blocks with highly disparate chemical and physical properties are conjugated vertically through asymmetrically charged interfaces. We demonstrate vdW-ES heteroassembly of semiconducting MoS2 and dielectric Ca2Nb3O10- (CNO) monolayers using an amphipathic molecular starch, resulting in the emergence of trion luminescence observed at the lowest energy among MoS2-related materials, probably due to interfacial confinement effects given by vdW-ES dual interactions. In addition, interface engineering leads to tailored exciton of the vdW/ES heterostructures owing to the pronounced dielectric proximity effects, bringing an intriguing interlayer chemistry to modify 2D materials. Furthermore, the current approach was successfully extended to create a graphene/CNO heterostructure, which verifies the versatility of the preparative method.

Insight into the structural and electronic nature of chemically exfoliated molybdenum disulfide nanosheets in aqueous dispersions.

Chemical exfoliation of molybdenum disulfide (2H-MoS2) for preparing high-yield single-layer sheets has attracted considerable attention in recent years. However, the stability and nature of the resulting nanosheets are poorly understood. Storing the dispersion in ambient air brings about the reoxidation of the nanosheets, releasing their residual negative charges into the environment. The reoxidation facilitates lateral fractures and destabilizes the dispersion. In-plane X-ray diffraction of the nanosheets indicates that they have a 1T structure with a 2D √3 × 1 rectangular cell as the intrinsic structure for chemically exfoliated MoS2. We found that the 1T structure was preserved after reoxidation upon aging the dispersions in air, suggesting the formation of metastable neutral MoS2. The changes in the chemical nature of the nanosheets can be monitored by X-ray diffraction of the restacked nanosheets. The restacked nanosheets, obtained by drying the freshly prepared dispersion, exhibited an expanded bilayer hydrate structure, accommodating Li ions. On the other hand, dried samples from the aged dispersions were substantially composed of a deintercalated phase and the bilayer hydrate. Upon prolonged aging, the former phase became predominant with total disappearance of the latter. This evolution suggests that the reoxidation occurred sheet by sheet with a direct restoration of the original oxidation states of the nanosheets, whereas the oxidation states of the nanosheets can be discrete at 4+ and (4 - δ)+.

Stability and Nature of Chemically Exfoliated MoS2 in Aqueous Suspensions.

We reveal that chemically exfoliated MoS2 nanosheets undergo lateral fracture and aggregation upon prolonged storage of the dispersion in ambient air, which was found to be associated with the reoxidation of the nanosheets. Such nanosheet degradation could be effectively prevented by storing the sample in an inert atmosphere to suppress the reoxidation process.