Creation of two-dimensional layered Zintl phase by dimensional manipulation of crystal structure

The discovery of new families, beyond graphene, of two-dimensional (2D) layered materials has always attracted great attention. However, it has been challenging to artificially develop layered materials with honeycomb atomic lattice structure composed of multicomponents such as hexagonal boron nitride. Here, through the dimensional manipulation of a crystal structure from sp(3)-hybridized 3D-ZnSb, we create an unprecedented layered structure of Zintl phase, which is constructed by the staking of sp(2)-hybridized honeycomb ZnSb layers. Using structural analysis combined with theoretical calculation, it is found that the 2D-ZnSb has a stable and robust layered structure. The bidimensional polymorphism is a previously unobserved phenomenon at ambient pressure in Zintl families and can be a common feature of transition metal pnictides. This dimensional manipulation of a crystal structure thus provides a rational design strategy to search for new 2D layered materials in various compounds, enabling unlimited expansion of 2D libraries and corresponding physical properties. © The Author

Catalytic Synergy on PtNi Bimetal Catalysts Driven by Interfacial Intermediate Structures

© American Chemical Societysignificantly impacts catalysis. A molecular-level understanding of intermediate structures created during catalysis is essential for developing high-performance bimetal catalysts. We show that intermediate Pt-NiO1-x interfacial structures drive the catalytic synergistic effect observed on Pt3Ni nanocrystals. Real-time microscopic observations at ambient pressure show the formation of oxygen-driven Ni oxide clusters on the surface and provide direct evidence of Pt-NiO1-x interfacial structure formation. Spectroscopic analysis, including ambient-pressure X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier-transform spectroscopy, and catalytic measurements elucidate the role of Pt-NiO1-x interfacial structures and the catalytic reaction mechanism in CO oxidation. Our results indicate that metal-oxide interfacial intermediate structures in bimetal catalysts relate to the catalytic enhancement of the strong metal-support interaction (SMSI) effect11Nsciescopu

Robust Excitonic‐Insulating States in Cu‐Substituted Ta2NiSe5

Abstract Excitonic insulators exhibit intriguing quantum phases that further attract numerous interests in engineering the electrical and optical properties of Ta2NiSe5. However, tuning the electronic properties such as spin‐orbit coupling strength and orbital repulsion via pressure in Ta2NiSe5 are always accompanied with electron‐hole pair breaking, which is a bottleneck for further applications. Here, the robust excitonic‐insulating states invariant with electron‐doping concentrations in Ta2NiSe5 are demonstrated. The electron doping is conducted by substituting Cu into Ni site (Ta2Ni1‐xCuxSe5). The majority carrier of pristine sample is a hole‐type and is converted to electron‐type with a doping concentration over x = 0.01, whose carrier density can be controlled by varying the Cu concentration. The excitonic transition temperature (Tc) does not significantly alter with electron‐doping concentrations, which is stark contrast with the declining Tc as the hole‐type dopant of Fe or Co increases. The optical conductivity data also demonstrate the invariant excitonic‐insulating states in Cu‐doped Ta2NiSe5. The findings of invariant excitonic‐insulating states in n‐type Cu‐substituted Ta2NiSe5 can be utilized for further electronic device applications by using excitons

An electrochemical approach to graphene oxide coated sulfur for long cycle life

Owing to the possibilities of achieving high theoretical energy density and gravimetric capacity, sulfur has been considered as a promising cathode material for rechargeable lithium batteries. However, sulfur shows rapid capacity fading due to the irreversible loss of soluble polysulfides and the decrease in active sites needed for conducting agents. Furthermore, the low electrical conductivity of sulfur hampers the full utilization of active materials. Here we report that graphene oxide coated sulfur composites (GO-S/CB) exhibit improved electrochemical stability as well as enhanced rate performance, evidenced by various electrochemical analyses. The cyclic voltammetry and the galvanostatic cycling analysis revealed that the GO plays key roles in homogenizing the nanocomposite structures of the electrodes, in improving the electrochemical contact, and in minimizing the loss of soluble polysulfide intermediates. An electrochemical impedance spectroscopy analysis also confirms the enhanced structural stability of the GO-S/CB composites after battery operation. As a result, the GO-S/CB exhibited excellent cycle stability and specific capacity as high as similar to 723.7 mA h g(-1) even after 100 cycles at 0.5 C

N-doped monolayer graphene catalyst on silicon photocathode for hydrogen production

Carbon-based catalysts have been attracting attention in renewable energy technologies due to the low cost and high stability, but their insufficient activity is still a challenging issue. Here, we suggest that monolayer graphene can be used as a catalyst for solar-driven hydrogen evolution reaction on Si-photocathodes, and its catalytic activity is boosted by plasma treatment in N-2-ambient. The plasma treatment induces abundant defects and the incorporation of nitrogen atoms in the graphene structure, which can act as catalytic sites on graphene. The monolayer graphene containing nitrogen impurities exhibits a remarkable increase in the exchange current density and leads to a significant anodic shift of the onset of photocurrent from the Si-photocathode. Additionally, monolayer graphene shows the passivation effect that suppresses the surface oxidation of Si, thus enabling the operation of the Si-photocathode in neutral water. This study shows that graphene itself can be applied to a photoelectrochemical system as a catalyst with high activity and chemical stability

One-Step Synthesis of N-doped Graphene Quantum Sheets from Monolayer Graphene by Nitrogen Plasma

High-quality N-doped graphene quantum sheets are successfully fabricated from as-grown monolayer graphene on Cu using nitrogen plasma, which can be transferred as a film-like layer or easily dispersed in an organic solvent for further optoelectronic or photoelectrochemical applications

Strain Relaxation of Graphene Layers by Cu Surface Roughening

The surface morphology of copper (Cu) often changes after the synthesis of graphene by chemical vapor deposition (CVD) on a Cu foil, which affects the electrical properties of graphene, as the Cu step bunches induce the periodic ripples on graphene that significantly disturb electrical conduction. However, the origin of the Cu surface reconstruction has not been completely understood yet. Here, we show that the compressive strain on graphene induced by the mismatch of thermal expansion coefficient with Cu surface can be released by forming periodic Cu step bunching that depends on graphene layers. Atomic force microscopy (AFM) images and the Raman analysis show the noticeably longer and higher step bunching of Cu surface under multilayer graphene and the weaker biaxial compressive strain on multilayer graphene compared to monolayer. We found that the surface areas of Cu step bunches under multilayer and monolayer graphene are increased by ∼1.41% and ∼0.77% compared to a flat surface, respectively, indicating that the compressive strain on multilayer graphene can be more effectively released by forming the Cu step bunching with larger area and longer periodicity. We believe that our finding on the strain relaxation of graphene layers by Cu step bunching formation would provide a crucial idea to enhance the electrical performance of graphene electrodes by controlling the ripple density of graphene