Xiuling Li

Metallic Nickel Nitride Nanosheets Realizing Enhanced Electrochemical Water Oxidation

Exploring efficient and inexpensive oxygen evolution reaction (OER) electrocatalysts is of great importance for various electrochemical energy storage and conversion technologies. Ni-based electrocatalysts have been actively pursued because of their promising activity and earth abundance. However, the OER efficiency for most of the developed Ni-based electrocatalysts has been intrinsically limited due to their low electrical conductivity and poor active site exposure yield. Herein, we report metallic Ni3N nanosheets as an efficient OER electrocatalyst for the first time. The first-principles calculations and electrical transport property measurements unravel that the Ni3N is intrinsically metallic, and the carrier concentration can be remarkably improved with dimensional confinement. The EXAFS spectra provide solid evidence that the Ni3N nanosheets have disordered structure resultant of dimensional reduction, which then could provide more active sites for OER. Benefiting from enhanced electrical conductivity with metallic behavior and atomically disordered structure, the Ni3N nanosheets realize intrinsically improved OER activity compared with bulk Ni3N and NiO nanosheets. Our finding suggests that metallic nitride nanosheets could serve as a new group of OER electrocatalysts with excellent property.

Gram‐Scale Aqueous Synthesis of Stable Few‐Layered 1T‐MoS2: Applications for Visible‐Light‐Driven Photocatalytic Hydrogen Evolution

Most recently, much attention has been devoted to 1T phase MoS2 because of its distinctive phase-engineering nature and promising applications in catalysts, electronics, and energy storage devices. While alkali metal intercalation and exfoliation methods have been well developed to realize unstable 1T-MoS2 , but the aqueous synthesis for producing stable metallic phase remains big challenging. Herein, a new synthetic protocol is developed to mass-produce colloidal metallic 1T-MoS2 layers highly stabilized by intercalated ammonium ions (abbreviated as N-MoS2). In combination with density functional calculations, the X-ray diffraction pattern and Raman spectra elucidate the excellent stability of metallic phase. As clearly depicted by high-angle annular dark-field imaging in an aberration-corrected scanning transmission electron microscope and extended X-ray absorption fine structure, the N-MoS2 exhibits a distorted octahedral structure with a 2a0 × a0 basal plane superlattice and 2.72 Å Mo-Mo bond length. In a proof-of-concept demonstration for the obtained materials applications, highly efficient photocatalytic activity is achieved by simply hybridizing metallic N-MoS2 with semiconducting CdS nanorods due to the synergistic effect. As a direct outcome, this CdS:N-MoS2 hybrid shows giant enhancement of hydrogen evolution rate, which is almost 21-fold higher than pure CdS and threefold higher than corresponding annealed CdS:2H-MoS2.

Ultrathin Nanosheets of Vanadium Diselenide: A Metallic Two-Dimensional Material with Ferromagnetic Charge-Density-Wave Behavior†

A new metallic 2D material with high electrical conductivity (1×10(3) S m(-1)) consists of VSe2 ultrathin nanosheets with 4-8 Se-V-Se atomic layers. This is the first 2D transition-metal dichalcogenide with intrinsic room-temperature ferromagnetism. The nanosheets increase the charge-density-wave transition temperature to 135 K by dimensional reduction.

Band-gap engineering via tailored line defects in boron-nitride nanoribbons, sheets, and nanotubes

We perform a comprehensive study of the effects of line defects on electronic and magnetic properties of monolayer boron-nitride (BN) sheets, nanoribbons, and single-walled BN nanotubes using first-principles calculations and Born-Oppenheimer quantum molecular dynamic simulation. Although line defects divide the BN sheet (or nanotube) into domains, we show that certain line defects can lead to tailor-made edges on BN sheets (or imperfect nanotube) that can significantly reduce the band gap of the BN sheet or nanotube. In particular, we find that the line-defect-embedded zigzag BN nanoribbons (LD-zBNNRs) with chemically homogeneous edges such as B- or N-terminated edges can be realized by introducing a B(2), N(2), or C(2) pentagon-octagon-pentagon (5-8-5) line defect or through the creation of the antisite line defect. The LD-zBNNRs with only B-terminated edges are predicted to be antiferromagnetic semiconductors at the ground state, whereas the LD-zBNNRs with only N-terminated edges are metallic with degenerated antiferromagnetic and ferromagnetic states. In addition, we find that the hydrogen-passivated LD-zBNNRs as well as line-defect-embedded BN sheets (and nanotubes) are nonmagnetic semiconductors with markedly reduced band gap. The band gap reduction is attributed to the line-defect-induced impurity states. Potential applications of line-defect-embedded BN nanomaterials include nanoelectronic and spintronic devices.

Stable Metallic 1T‐WS2 Nanoribbons Intercalated with Ammonia Ions: The Correlation between Structure and Electrical/Optical Properties

Stable metallic 1T-WS2 nanoribbons with zigzag chain superlattices, highly stabilized by ammonia-ion intercalation, are produced using a facile bottom-up process. The atomic structure of the nanoribbons, including W-W reconstruction and W-S distorted octahedral coordination, results in distinctive electrical transport and optical Raman scattering properties that are very different from semiconducting 2H-WS2 . The correlations between structure and properties are further confirmed by theory calculations.

Cobalt nitrides as a class of metallic electrocatalysts for the oxygen evolution reaction

The development of highly-efficient, stable and cost-effective electrocatalysts for the oxygen evolution reaction (OER) is critical for a range of renewable-energy technologies, including metal–air batteries, fuel cells and water-splitting reactions. However, most of the well-developed electrocatalysts are semiconductors or insulators with poor conductivity, which has profoundly inhibited their overall OER efficiency. In this study, metallic cobalt nitrides (Co2N, Co3N and Co4N) arising from electron delocalization modulation have been investigated for OER electrocatalysts in alkaline solution for the first time. Benefiting from the synergistical engineering of the electrical conductivity and nitrogen content, the simple metallic Co4N catalyst without modifications exhibits a stable current density of 10 mA cm−2 at a small overpotential of 330 mV for OER with a Tafel slope as low as 58 mV dec−1 in alkaline medium, which is superior to most of the unmodified metal oxide electrocatalysts reported to date. Our finding introduces new possibilities for the design of highly active electrocatalysts using synergistical electrical conductivity regulation and composition modulation.

Ultrathin nanosheets of feroxyhyte: a new two-dimensional material with robust ferromagnetic behavior

Two dimensional (2D) nanosheets have shown great potential for their applications in next-generation nanoelectronic devices. However, developing 2D nanosheets for next-generation spintronics has been blocked due to their lack of robust intrinsic ferromagnetic behavior. Here, we highlight the robust ferromagnetic behavior in a 2D inorganic graphene-like structure. δ-FeOOH ultrathin nanosheets, as a new 2D material, exhibit an unprecedented high saturation magnetization value of 7.5 emu g−1 at room temperature, which is the highest value among graphene and graphene analogues. Atomic-scale topochemical transformation ensures the formation of 2D δ-FeOOH ultrathin nanosheets from intermediate Fe(OH)2 nanosheets. Moreover, the δ-FeOOH ultrathin nanosheet is found to be a semiconductor with a direct band gap of 2.2 eV. Owing to the advantages of robust ferromagnetism in semiconductors, δ-FeOOH ultrathin nanosheets are promising candidates for the construction of next-generation spintronics.

Two‐dimensional monolayer designs for spintronics applications

The continuous reduction in the size of spintronic devices highly requires new low‐dimensional magnetic materials to mimic the traditional structures of spintronics in nanoscale. Since the discovery of graphene, two‐dimensional (2D) crystalline materials with atomic thickness have attracted extraordinary interests, partly due to their novel properties and potential applications in spintronics. In the past decades, many theoretical understandings and designs of 2D materials have been proposed for spintronics, indicating that the combination of spintronics and two‐dimensional crystals electronics should be an ideal evolution toward nanoscale spintronics devices and make for the bottom‐up spintronics nanoengineering. WIREs Comput Mol Sci 2016, 6:441–455. doi: 10.1002/wcms.1259

Modulation of Metal and Insulator States in 2D Ferromagnetic VS2 by van der Waals Interaction Engineering

2D transition-metal dichalcogenides (TMDCs) are currently the key to the development of nanoelectronics. However, TMDCs are predominantly nonmagnetic, greatly hindering the advancement of their spintronic applications. Here, an experimental realization of intrinsic magnetic ordering in a pristine TMDC lattice is reported, bringing a new class of ferromagnetic semiconductors among TMDCs. Through van der Waals (vdW) interaction engineering of 2D vanadium disulfide (VS2 ), dual regulation of spin properties and bandgap brings about intrinsic ferromagnetism along with a small bandgap, unravelling the decisive role of vdW gaps in determining the electronic states in 2D VS2 . An overall control of the electronic states of VS2 is also demonstrated: bond-enlarging triggering a metal-to-semiconductor electronic transition and bond-compression inducing metallization in 2D VS2 . The pristine VS2 lattice thus provides a new platform for precise manipulation of both charge and spin degrees of freedom in 2D TMDCs availing spintronic applications.

Half-Metallicity in One-Dimensional Metal Trihydride Molecular Nanowires

The development of one-dimensional (1D) molecular nanowires with high spin-polarization is important for both fundamental research and practical applications in nanoscale spintronics. Herein, we report new 1D metal trihydride molecular nanowires MH3 (M = Sc, Cr, Mn, and Co) with versatile magnetic properties on the basis of first-principles calculations and molecular assembly of their molecular motifs. Among the 1D nanowires considered, CrH3, MnH3, and CoH3 are either antiferromagnetic or ferromagnetic in their ground states. In particular, CoH3 nanowire is a half-metal, which ideally could provide 100% spin-polarized currents. Moreover, carrier doping in MnH3 nanowire can induce a transition from ferromagnetic metal to half-metal. Strong metal-metal interaction in 1D MH3 nanowires is responsible to versatile magnetic properties and high Curie temperature. Born-Oppenheimer molecular dynamics simulation indicates that these nanowires are stable at elevated temperature. In particular, the ScH3 nanowire appears to have the highest structural stability at temperature 1200 K. These novel properties of 1D MH3 nanowires render their potential applications in nanoscale spintronics.