3D Fast Sodium Transport Network of MoS2 Endowed by Coupling of Sulfur Vacancies and Sn Doping for Outstanding Sodium Storage.

A sulfur vacancy-rich, Sn-doped as well as carbon-coated MoS2 composite (Vs-SMS@C) is rationally synthesized via a simple hydrothermal method combined with ball-milling reduction, which enhances the sodium storage performance. Benefiting from the 3D fast Na+ transport network composed of the defective carbon coating, Mo─S─C bonds, enlarged interlayer spacing, S-vacancies, and lattice distortion in the composite, the Na+ storage kinetics is significantly accelerated. As expected, Vs-SMS@C releases an ultrahigh reversible capacity of 1089 mAh g-1 at 0.1 A g-1, higher than the theoretical capacity. It delivers a satisfactory capacity of 463 mAh g-1 at a high current density of 10 A g-1, which is the state-of-the-art rate capability compared to other MoS2 based sodium ion battery anodes to the knowledge. Moreover, a super long-term cycle stability is achieved by Vs-SMS@C, which keeps 91.6% of the initial capacity after 3000 cycles under the current density of 5 A g-1 in the voltage of 0.3-3.0 V. The sodium storage mechanism of Vs-SMS@C is investigated by employing electrochemical methods and ex situ techniques. The synergistic effect between S-vacancies and doped-Sn is evidenced by DFT calculations. This work opens new ideas for seeking excellent metal sulfide anodes.

Cu- and Al-Decorated Monolayer TiSe2 for Enhanced Gas Detection Sensitivity: A DFT Study.

The rapid industrial development has contributed to worsening global pollution, necessitating the urgent development of highly sensitive, cost-effective, and portable gas sensors. In this work, the adsorption of CO, CO2, H2S, NH3, NO, NO2, O2, and SO2 gas molecules on pristine and Cu- and Al-decorated monolayer TiSe2 has been investigated based on first-principles calculations. First, the results of the phonon spectrum and ab initio molecular dynamics simulations demonstrated that TiSe2 is dynamically stable. In addition, compared to pristine TiSe2 (-0.029 to -0.154 eV), the adsorption energy of gas molecules (excluding CO2) significantly decreased after decorated with Cu or Al (-0.212 to -0.977 eV in Cu-decorated TiSe2, -0.438 to -2.896 eV in Al-decorated TiSe2). Among them, NH3 and NO2 have the lowest adsorption energies in Cu and Al-decorated TiSe2, respectively. Further research has shown that the decrease in adsorption energy of gas molecules is mainly due to orbital hybridization and charge transfer between decorated Cu and Al atoms and gas molecules. These findings suggest that TiSe2 decorated with Cu and Al can effectively improve its sensitivity to NH3 and NO2, respectively, making it promising in gas sensing applications.

Building up a view and understanding of the multifunctional activity of black phosphorous nanosheet modified with the metal atom.

Constructing metal-semiconductor interfaces by loading metal atoms onto two-dimensional material to build atomically dispersed single-atom catalysts (SACs) has emerged as a new frontier for improving atom utilization and designing multifunctional electrocatalysts. Nowadays, studies on black phosphorus nanosheets in electrocatalysis have received much attention and the successful preparation of metal nanoparticle/black phosphorus (BP) hybrid electrocatalysts indicates BP nanosheets can serve as a potential support platform for SACs. Herein, by using large-scale ab initio calculations, we explored a large composition space of SACs with transition metal atoms supported on BP monolayer (M-BP) and built a comprehensive picture of activity trend, stability, and electronic origin towards oxygen reduction and evolution reaction (ORR and OER) and hydrogen evolution reaction (HER). The results show that the catalytic activity can be widely tuned by reasonable regulation of metal atoms. Ni-, Pd-, and Pt-BP could effectively balance the binding strength of the target intermediates, thus achieving efficient bifunctional activity for OER and ORR. Favorable bifunctional catalytic performance for OER and HER can be realized on Rh-BP. Especially, Pt-BP exhibits promising trifunctional activity towards OER, ORR, and HER. Multiple-level corrections among overpotential, Gibbs free energy, orbital population, and d-band center reveal that the trend and origin of catalytic activity are intrinsically determined by the d-band center of metal sites. The thermodynamic and dynamic stability simulations demonstrate that the active metal centers are firmly anchored on BP substrate with intact M-P bonds. These findings provide a theoretical basis for the rational design of BP-based SACs toward promising multifunctional activity.

Janus-functionalization induced magnetism and improved optoelectronic properties in two-dimension silicene and germanene: insights from first-principles calculations.

In this paper, the structural stability, optoelectronic and magnetic properties of silicene and germanene monolayers Janus-functionalized simultaneously with hydrogen and alkali metal atoms (Li and Na) are investigated systematically by using first-principles calculations. The calculated results of theab initiomolecular dynamics simulations and cohesive energies indicate that all functionalized cases have good stability. Meanwhile, the calculated band structures show that all functionalized cases retain the Dirac cone. Particularly, the cases of HSiLi and HGeLi show metallic nature but retain semiconducting characteristics. Besides, the above two cases can present obvious magnetic behavior and their magnetic moments are mainly originated by the p states of Li atom. The metallic property and weak magnetic character can also be found in the case of HGeNa. While the case of HSiNa exhibits the nonmagnetic semiconducting property with a indirect band gap of 0.42 eV calculated by the HSE06 hybrid functional. It is also found that the optical absorption in the visible region of silicene and germanene can be effectively improved by Janus-functionalization. Specifically, a high optical absorption of visible light in an order of 4.5 × 105cm-1can be observed in the case of HSiNa. Furthermore, in the visible region, the reflection coefficients of all functionalized cases can also be enhanced. These results demonstrate the feasibility of the Janus-functionalization method to modulate the optoelectronic and magnetic properties of silicene and germanene, expanding their potential applications in the fields of spintronics and optoelectronics.

Inversion Symmetry and Exotic Interlayer Exciton Behavior in Twisted Trilayer MoS2 Produced by Vapor Deposition.

Two-dimensional materials (2DMs) that are stacked vertically with a certain twist angle provide new degrees of freedom for designing novel physical properties. Twist-related properties of homogeneous bilayer and heterogeneous bilayer 2DMs, such as excitons and phonons, have been described in many pioneering works. However, twist-related properties of homogeneous trilayer 2DMs have been rarely reported. In this work, trilayer MoS2 with the twisted angle of 12° by optimized vapor deposition rather than the conventional mechanical stacking method was successfully fabricated. The inversion symmetry of trilayer MoS2 is changed by twist. Phonons and excitons produced by twist have an enormous influence on the interlayer interaction of trilayer MoS2, making trilayer MoS2 appear to have exotic optical properties. Compared with monolayer MoS2, the phonon vibration and photoluminescence intensity of trilayer MoS2 with one-interlayer-twisted are significantly improved, and the second harmonic generation response in the non-twist region of trilayer MoS2 is ∼3 times that of monolayer MoS2. In addition, interlayer coupling, inversion symmetry, and exciton behavior of the twist region show regional differences. This work provides a new way for designing twist and exploring the influence of twist on the structures of 2DMs with few layers.

Rational design of a C3N/C3B p-n heterostructure as a promising anode material in Li-ion batteries.

It is urgent to develop high-performance anode materials for lithium-ion batteries. In this work, a C3N/C3B p-n heterostructure was systematically investigated by first-principles calculations. The bonding strength of Li in C3N is relatively low (-0.53 eV), whereas the C3N/C3B heterostructure (-1.64 eV to -2.84 eV) can greatly improve the bonding strength without compromising the Li migration capability. The good bonding strength and Li mobility in the C3N/C3B heterostructure are mainly caused by the synergy effect and internal electric field of the p-n heterostructure. Moreover, the electronic structures indicate that the C3N/C3B heterostructure has good conductivity with a tiny bandgap of 0.09 eV. Compared to pristine C3N, the stiffness of the C3N/C3B heterostructure improved significantly (549.35 N m-1). Besides, the C3N/C3B heterostructure presents a high lithium-ion storage capacity (986.61 mA h g-1). The ultrahigh stiffness, good conductivities of electrons and ions, high bonding strength of Li, and high capacity show that the C3N/C3B heterostructure is a prospective anode material for lithium-ion batteries.

Stack-dependent ion diffusion behavior in two-dimensional bilayer C3B.

In recent years, two-dimensional (2D) C-based materials have been intensively studied due to their excellent physicochemical properties. Meanwhile, extensive research has revealed that the electrical properties of layered materials can be tuned by changing the stacking pattern. However, the tuning of ion diffusion properties through stacking remains to be explored. In this work, bilayer C3B with different stackings as a lithium-ion battery anode material is systematically investigated by first-principles calculations. The calculated results show that bilayer C3B has better electronic properties (with a band gap of 0.44 eV to 0.54 eV) and enhanced bonding strength of Li (-2.82 to -3.27 eV) compared to monolayer C3B. Moreover, the intralayer migration barrier of Li can be regulated by stacking. Interestingly, the AB stacked configuration has the lowest migration barrier of 0.100 eV, which is significantly lower than those of other stacking configurations and monolayer C3B. Further studies revealed that the formation of fast ion diffusion channels in the AB stacked configuration is due to the combined effect of layer distance and in-plane charge transfer. These results offer a new strategy for the regulation of ion diffusion properties in 2D van der Waals materials.

Large-Area Synthesis of Layered HfS2(1- x )Se2 x Alloys with Fully Tunable Chemical Compositions and Bandgaps.

Alloying transition metal dichalcogenides (TMDs) with different compositions is demonstrated as an effective way to acquire 2D semiconductors with widely tunable bandgaps. Herein, for the first time, the large-area synthesis of layered HfS2(1- x )Se2 x alloys with fully tunable chemical compositions on sapphire by chemical vapor deposition is reported, greatly expanding and enriching the family of 2D TMDs semiconductors. The configuration and high quality of their crystal structure are confirmed by various characterization techniques, and the bandgap of these alloys can be continually modulated from 2.64 to 1.94 eV with composition variations. Furthermore, prototype HfS2(1- x )Se2 x photodetectors with different Se compositions are fabricated, and the HfSe2 photodetector manifests the best performance among all the tested HfS2(1- x )Se2 x devices. Remarkably, by introducing a hexagonal boron nitride layer, the performance of the HfSe2 photodetector is greatly improved, exhibiting a high on/off ratio exceeding 105, an ultrafast response time of about 190 µs, and a high detectivity of 1012 Jones. This simple and controllable approach opens up a new way to produce high-quality 2D HfS2(1- x )Se2 x layers, which are highly qualified candidates for the next-generation application in high-performance optoelectronics.

Temperature-Sensitive Structure Evolution of Lithium-Manganese-Rich Layered Oxides for Lithium-Ion Batteries.

Cathodes of lithium-rich layered oxides for high-energy Li-ion batteries in electrically powered vehicles are attracting considerable attention by the research community. However, current research is insufficient to account for their complex reaction mechanism and application. Here, the structural evolution of lithium-manganese-rich layered oxides at different temperatures during electrochemical cycling has been investigated thoroughly, and their structural stability has been designed. The results indicated structure conversion from the two structures into a core-shell structure with a single distorted-monoclinic LiTMO2 structure core and disordered-spinel/rock salt structure shell, along with lattice oxygen extraction and lattice densification, transition- metal migration, and aggregation on the crystal surface. The structural conversion behavior was found to be seriously temperature sensitive, accelerated with higher temperature, and can be effectively adjusted by structural design. This study clarifies the structural evolution mechanism of these lithium-rich layered oxides and opens the door to the design of similar high-energy materials with better cycle stability.

Fabrication and Emission Performance of Nanostructured Re-W Matrix Impregnated Cathodes.

In this study, rhenium-tungsten mixed particles with different content of rhenium have been prepared by spray-drying method followed by hydrogen reduction. Using such particles, the cathodes have been prepared by powder metallurgy followed by impregnating BaO, CaO, and Al2O3 with 4:1:1 molar ratio. After proper activation, electron emission test is performed in standard parallel-plate diode configuration. The emission results reveal that the Re-W matrix cathode containing 75% rhenium has the highest direct current emission density of 11.67 A/cm2 at 1000 °C. The work function of Re-W matrices has been investigated by density functional theory method in the frame of the generalized gradient approximation (GGA). The theoretical calculation results indicate that the work function of the matrix has limited contribution to the emission current density of Re-W matrix dispenser cathode. The in situ AES, SEM, and XRD were applied and the results reveal that the superior emission property of the 75Re cathode is owing to a plenty of nanoparticles and higher free barium concentration on the cathode surface, which is attributed to the Re3W single phase in 75Re matrix.

First-Principles Study of Phosphorene and Graphene Heterostructure as Anode Materials for Rechargeable Li Batteries.

There is a great desire to develop the high-efficient anodes materials for Li batteries, which require not only large capacity but also high stability and mobility. In this work, the phosphorene/graphene heterostructure (P/G) was carefully explored based on first-principles calculations. The binding energy of Li on the pristine phosphorene is relatively weak (within 1.9 eV), whereas the phosphorene/graphene heterostructure (P/G) can greatly improve the binding energy (2.6 eV) without affecting the high mobility of Li within the layers. The electronic structures show that the large Li adsorption energy and fast diffusion ability of the P/G origin from the interfacial synergy effect. Interestingly, the P/G also displays ultrahigh stiffness (Cac = 350 N/m, Czz = 464 N/m), which can effectively avoid the distortion of the pristine phosphorene after the insertion of lithium. Thus, P/G can greatly enhance the cycle life of the battery. Owing to the high capacity, good conductivity, excellent Li mobility, and ultrahigh stiffness, P/G is a very promising anode material in Li-ion batteries (LIBs).

Electronic and magnetism properties of two-dimensional stacked nickel hydroxides and nitrides.

Two-dimensional (2D) layered materials receive a lot of attention because of their outstanding intrinsic properties and wide applications. In this work, the structural, electronic and magnetic properties of nickel hydroxides (Ni(OH)2) and nitrides XN (X = B, Al, and Ga) heterostructures are studied by first-principles calculations. The results show that the pristine monolayer Ni(OH)2 owns no macro magnetism with antiferromagnetic (AFM) coupling between two nearest Ni atoms, the electronic structure can be modulated through the heterostructures. The Ni(OH)2-GaN and Ni(OH)2-AlN heterostructures retain the AFM coupling, while Ni(OH)2-BN heterostructure have a larger magnetic moment with ferromagnetic (FM) coupling. The complete electron-hole separation is found in the Ni(OH)2-GaN heterostructure. The tunable electronic and magnetic properties of the Ni(OH)2-XN heterostructures open a new door to design the spintronic devices in the 2D stacked nanostructures.