Characteristic Plasmon Energies for 2D In2Se3 Phase Identification at Nanoscale.

Two-dimensional (2D) materials with competing polymorphs offer remarkable potential to switch the associated 2D functionalities for novel device applications. Probing their phase transition and competition mechanisms requires nanoscale characterization techniques that can sensitively detect the nucleation of secondary phases down to single-layer thickness. Here we demonstrate nanoscale phase identification on 2D In2Se3 polymorphs, utilizing their distinct plasmon energies that can be distinguished by electron energy-loss spectroscopy (EELS). The characteristic plasmon energies of In2Se3 polymorphs have been validated by first-principles calculations, and also been successfully applied to reveal phase transitions using in situ EELS. Correlating with in situ X-ray diffraction, we further derive a subtle difference in the valence electron density of In2Se3 polymorphs, consistent with their disparate electronic properties. The nanometer resolution and independence of orientation make plasmon-energy mapping a versatile technique for nanoscale phase identification on 2D materials.

Boosting Cycling Stability of Polymer Sodium Battery by "Rigid-Flexible" Coupled Interfacial Stress Modulation.

The discontinuous interfacial contact of solid-state polymer metal batteries is due to the stress changes in the electrode structure during cycling, resulting in poor ion transport. Herein, a rigid-flexible coupled interface stress modulation strategy is developed to solve the above issues, which is to design a rigid cathode with enhanced solid-solution behavior to guide the uniform distribution of ions and electric field. Meanwhile, the polymer components are optimized to build an organic-inorganic blended flexible interfacial film to relieve the change of interfacial stress and ensure rapid ion transmission. The fabricated battery comprising a Co-modulated P2-type layered cathode (Na0.67Mn2/3Co1/3O2) and a high ion conductive polymer could deliver good cycling stability without distinct capacity fading (72.8 mAh g-1 over 350 cycles at 1 C), outperforming those without Co modulation or interfacial film construction. This work demonstrates a promising rigid-flexible coupled interfacial stress modulation strategy for polymer-metal batteries with excellent cycling stability.

Non-Fluorinated Ethers to Mitigate Electrode Surface Reactivity in High-Voltage NCM811-Li Batteries.

Lithium (Li) metal batteries (LMBs) with nickel (Ni)-rich layered oxide cathodes exhibit twice the energy density of conventional Li-ion batteries. However, their lifespan is limited by severe side reactions caused by high electrode reactivity. Fluorinated solvent-based electrolytes can address this challenge, but they pose environmental and biological hazards. This work reports on the molecular engineering of fluorine (F)-free ethers to mitigate electrode surface reactivity in high-voltage Ni-rich LMBs. By merely extending the alkyl chains of traditional ethers, we effectively reduce the catalytic reactivity of the cathode towards the electrolyte at high voltages, which suppresses the oxidation decomposition of the electrolyte, microstructural defects and rock-salt phase formation in the cathode, and gas release issues. The high-voltage Ni-rich NCM811-Li battery delivers capacity retention of 80 % after 250 cycles with a high Coulombic efficiency of 99.85 %, even superior to that in carbonate electrolytes. Additionally, this strategy facilitates passivation of the Li anode by forming a robust solid-electrolyte interphase, boosting the Li reversibility to 99.11 % with a cycling life of 350 cycles, which outperforms conventional F-free ether electrolytes. Consequently, the lifespan of practical LMBs has been prolonged by over 100 % and 500 % compared to those in conventional carbonate- and ether-based electrolytes, respectively.

Phase Instability in van der Waals In2 Se3 Determined by Surface Coordination.

van der Waals In2 Se3 has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In2 Se3 have not yet been adequately addressed. Using a combination of experimental and theoretical approaches, we here unravel the phase instability in both α- and ß'-In2 Se3 originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In2 Se3 in air to form amorphous In2 Se3-3x O3x layers and Se hemisphere particles. Both O2 and H2 O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In2 Se3-3x O3x layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In2 Se3 performance for device applications.

Rational Crystal Structure Design and Nonlinear-Optical Properties of Noncentrosymmetric RbCu2NbS4.

Crystal structure design based on known materials is an efficient strategy for exploring new compounds with evident second-harmonic-generation (SHG) effects. By the introduction of Rb+ ions into the 3D framework of Cu3NbS4, the new compound RbCu2NbS4 (space group Ama2) composed of [Cu22∞NbS4]- layers is obtained. The band gap of RbCu2NbS4 is 2.1 eV, indicating that this compound is a semiconductor. Band-structure calculations indicate that the electronic transition mainly occurs from the S 3p/Cu 3d to Nb 4d states. The intercalation of Rb+ ions induces a high degree of local distortion and symmetry reduction, which results in a dipole moment of 11.7 Debye for RbCu2NbS4. RbCu2NbS4 shows a moderate SHG response of 0.33 × AgGaS2 (particle size of 20-41 µm).

Superconductivity in the Electron-Doped Chevrel Phase Compound Mo6S6.8Te1.2.

Chevrel-phase superconducting compounds AyMo6Q8 (A = Pb, Mg, etc. and Q = S, Se, Te) have attracted much attention due to their ultrahigh upper critical magnetic fields (Hc2). However, the binary compounds Mo6S8 and Mo6Te8 show no superconducting properties above 2 K. In this work, the new Chevrel-phase pseudobinary compound Mo6S6.8Te1.2 with a superconducting transition temperature (Tc) of 3.6 K was successfully synthesized by a two-step process. A detailed investigation implies that the superconductivity in Mo6S6.8Te1.2 may derive from the increased electronic density near the Fermi level in comparison to Mo6S8. Our work provides a feasible scheme to search for new superconducting compounds.

Dismutation of Titanium Sub-oxide into TiO and TiO2 with Structural Hierarchy Assisted by Ammonium Halides.

Black TiO2-x attracts enormous attention due to its large solar absorption and improved conductivity. In this work, a novel structure of TiO2-x with conductive TiO layer, performing full-spectrum absorption, was synthesized in one step by the unforeseen dismutation reaction of titanium sub-oxides (Tin O2n-1 ) in ammonium halide atmosphere. For this new reaction, a possible mechanism of decomposition-etching-disproportionation-rehydrolysis process was proposed. The vital intermediate reactant TiCl4 , which verifies the assumption, has been captured in the form of (NH4 )2 TiCl6 , especially where Ti2 O3 is the reactant. Furthermore, this work not only can nominate TiO as an alternative for noble metals or carbon materials in the aim to improve the electron conductivity and solar absorption of black TiO2-x , which are important in electrochemistry and optoelectronics fields, but also can be a new route to synthesize special structures for other multivalent transition metals.

Metastable MoS2 : Crystal Structure, Electronic Band Structure, Synthetic Approach and Intriguing Physical Properties.

The 2H molybdenum disulfide (MoS2 ), as a stable hexagonal phase, has been one of the most studied transition metal dichalcogenides over the past decades. In the last five years, the metastable phases of MoS2 (1T, 1T', 1T'', and 1T''') have seen a revival of interests. Different from the edge-sharing [MoS6 ] trigonal prisms in the 2H MoS2 phase, these metastable phases are composed of the edge-sharing [MoS6 ] octahedra, in which the neighboring Mo-Mo distances differ. Due to the various crystal structures and different electronic configurations of the building [MoS6 ] motifs, these metastable polytypes are endowed with intriguing physical properties and potential applications in diverse fields. In this Review, the recent research progress on metastable MoS2 is summarized, especially with an emphasis on the diverse synthetic approaches and the newly uncovered physical properties. The phase structures and electronic band structures are also outlined. In the end, a perspective of the future investigation on metastable MoS2 is discussed.

Structure Re-determination and Superconductivity Observation of Bulk 1T MoS2.

2H MoS2 has been intensively studied because of its layer-dependent electronic structures and novel physical properties. Though the metastable 1T MoS2 with a [MoS6 ] octahedron was observed over the microscopic area, the true crystal structure of 1T phase has not been strictly determined. Moreover, the true physical properties have not been demonstrated from experiments owing to the challenge for the preparation of pure 1T MoS2 crystals. 1T MoS2 single crystals were successfully synthesized and the crystal structure of 1T MoS2 re-determined from single-crystal X-ray diffraction. 1T MoS2 crystallizes in the space group P3‾ m1 with a cell of a=b=3.190(3) Šand c=5.945(6) Å. The individual MoS2 layer consists of MoS6 octahedra sharing edges with each other. More surprisingly, the bulk 1T MoS2 crystals undergo a superconducting transition of Tc =4 K, which is the first observation of superconductivity in pure 1T MoS2 phase.

Efficient and durable seawater electrolysis with a V2O3-protected catalyst.

The ocean, a vast hydrogen reservoir, holds potential for sustainable energy and water development. Developing high-performance electrocatalysts for hydrogen production under harsh seawater conditions is challenging. Here, we propose incorporating a protective V2O3 layer to modulate the microcatalytic environment and create in situ dual-active sites consisting of low-loaded Pt and Ni3N. This catalyst demonstrates an ultralow overpotential of 80 mV at 500 mA cm-2, a mass activity 30.86 times higher than Pt-C and maintains at least 500 hours in seawater. Moreover, the assembled anion exchange membrane water electrolyzers (AEMWE) demonstrate superior activity and durability even under demanding industrial conditions. In situ localized pH analysis elucidates the microcatalytic environmental regulation mechanism of the V2O3 layer. Its role as a Lewis acid layer enables the sequestration of excess OH- ions, mitigate Cl- corrosion, and alkaline earth salt precipitation. Our catalyst protection strategy by using V2O3 presents a promising and cost-effective approach for large-scale sustainable green hydrogen production.

P-type doping in 2M-WS2 for a complete phase diagram.

2M-WS2 as a new phase of transition metal dichalcogenides possesses many novel physical properties, such as superconductivity and topological surface states. The effect of n-type doping on the superconductivity of this material has been studied. However, p-type doping has not been studied, because it is difficult to implement p-type doping in metastable 2M-WS2. In this paper, p-type doping was achieved in 2M-WS2 for the first time by using Mo. With the increase of the Mo content, the carrier concentration rises slightly from 1.42 × 1021 cm-1 to 1.56 × 1021 cm-1. Meanwhile, the superconducting transition temperature decreases monotonously with the increase of Mo doping and reaches a minimum value of 4.37 K at the doping limit of x = 0.6 in W1-xMoxS2. Combining the data of n-type doped 2M-WS2 from our previous research, we summarize the carrier concentration and superconducting transition temperature in a phase diagram, which shows a typical dome-like shape. These results uncover the relationship between the carrier concentration and electronic state of 2M-WS2.

Nb2Se2C: a new compound as a combination of transition metal dichalcogenide and MXene for oxygen evolution reaction.

Transition metal dichalcogenides and carbonitrides (TMDs and MXenes) have attracted great attention in electrochemistry due to their tunable electronic structures. Herein, a new compound of Nb2Se2C is designed as a "TMD-MXene"-like material. It exhibits better oxygen evolution reaction performance than most other reported TMDs, MXenes, and commercial electrocatalysts due to the enriched active sites and excellent conductivity.

Crystal structure design and multiband physical properties of quaternary sulfide Ba5Bi2Co2S10 for optoelectronic conversion.

Multiband materials have received increasing attention due to their superior solar absorption properties. Here we design a new multiband compound, namely Ba5Bi2Co2S10, which crystallizes in the space group C22h-P21/m (No. 11) of the monoclinic system. Ba5Bi2Co2S10 is composed of one-dimensional 1∞[Bi2Co2S10]10- chains along the a axis. The adjacent chains are separated by Ba2+ ions. The optical band gap of the compound is 1.05 eV and 0.74 eV, presenting typical multi-absorption characteristics. First-principles calculations, which are perfectly consistent with the experimental results, show that the hybrid coupling effect between Co and S gives rise to multiband characteristics. Evident optoelectronic conversion properties were observed under visible light illumination with a photocurrent density of 4.0 mA cm-2 at 1 V.

Enhanced Superconductivity in Rock-Salt TiO.

Oxygen stoichiometry is critical for physical properties, but it is hard to precisely control in many multivalent transition metal oxides, for example, cuprate superconductors, magnetoresistive manganite oxides, and TiO x (x < 2). We have developed a new method to synthesize rock-salt TiO in a sealed and evacuated quartz tube by using KClO4 as the only oxygen source to react with elemental Ti (in a Ti/O molar ratio of 1:1). The stoichiometric titanium monoxide (TiO) exhibits an enhanced superconductivity transition temperature (T c) of 5.5 K, which is superior to the reported results of 0.5-2.3 K. The new synthetic method provides an excellent way to prepare stoichiometric oxides, and the enhanced superconductivity of TiO may initialize the restudy of the transport properties of Ti-containing oxides.

Ultrathin LiCoO2 Nanosheets: An Efficient Water-Oxidation Catalyst.

Ultrathin cation-exchanged layered metal oxides are promising for many applications, while such substances are barely successfully synthesized to show several atomic layer thickness, owing to the strong electrostatic force between the adjacent layers. Herein, we took LiCoO2, a prototype cation-exchanged layered metal oxide, as an example to study. By developing a simple synthetic route, we synthesized LiCoO2 nanosheets with 5-6 cobalt oxide layers, which are the thinnest ever reported. Ultrathin nanosheets thus prepared showed a surprising coexistence of increased oxidation state of cobalt ions and oxygen vacancy, as demonstrated by magnetic susceptibility, X-ray photoelectron, electron paramagnetic resonance, and X-ray absorption fine spectra. This unique feature enables a higher electronic conduction and electrophilicity to the adsorbed oxygen than the bulk. Consequently ultrathin LiCoO2 nanosheets provided a current density of 10 mA cm-2 at a small overpotential of a mere 0.41 V and a small Tafel slope of ∼88 mV/decade, which is strikingly followed by an excellent cycle life. The findings reported in this work suggest that ultrathin cation-exchanged layered metal oxides could be a next generation of advanced catalysts for oxygen evolution reaction.

Impact of hole doping on spin transition in perovskite-type cobalt oxides.

Series of perovskite PrCo1-xNixO3-δ (x = 0-0.4) were prepared and carefully investigated to understand the spin state transition driven by hole doping and further to reveal the effect of spin state transition on electronic conduction. It is shown that with increasing doping level, the transition temperature Ts for Co(3+) ions from low-spin (LS) to intermediate-spin (IS) reduces from 211.9 K for x = 0 to 190.5 K for x = 0.4. XPS and FT-IR spectra demonstrate that hole doping promoted this transition due to a larger Jahn-Teller distortion. Moreover, a thermal activation of spin disorder caused by thermal population of the spin states for Co ions has a great impact on the electrical transport of these perovskite samples. This work may shed light on the comprehension of spin transition in cobalt oxides through hole doping, which is promising for finding new strategies of enhancing electronic conduction, especially for energy and catalysis applications.