2D-Mn2C12: An Optimal Electrocatalyst with Nonbonding Multiple Single Centers for CO2-to-CH4 Conversion.

The CO2 reduction reaction (CO2RR) is a promising method that can both mitigate the greenhouse effect and generate valuable chemicals. The 2D-M2C12 with high-density transition metal single atoms is a potential catalyst for various catalytic reactions. Using an effective strategy, we screened 1s-Mn2C12 as the most promising electrocatalyst for the CO2RR in the newly reported 2D-M2C12 family. A low applied potential of -0.17 V was reported for the CO2-to-CH4 conversion. The relative weak adsorption of H atom and H2O in the potential range of -0.2 to -0.8 V, ensures the preferential adsorption of CO2 and the following production of CH4. The different loading amounts of Mn atoms on γ-graphyne (GY) were also investigated. The Mn atoms prefer doping in the nonadjacent triangular pores instead of the adjacent ones due to the repulsive forces between d-orbitals when the Mn loading is less than 32.3 wt % (5Mn). As the Mn concentration further increases, adjacent Mn atoms begin to appear, and the Mn@GY becomes metallic or half-metallic. The presence of four adjacent Mn atoms increases the d-band center of Mn@GY, particularly the dz2 center involved in CO2 adsorption, thereby enhancing the adsorption capacity for CO2. These findings indicate that 1s-Mn2C12 with high Mn atomic loadings is an excellent CO2RR electrocatalyst, and it provides new insights for designing efficient CO2RR electrocatalyst.

Influence of Zr aggregation on Li-ion conductivity of amorphous solid-state electrolyte Li-La-Zr-O.

In our study, we investigated the influence of the local structure of amorphous Li-La-Zr-O (a-LLZO) on Li-ion conductivity using ab initio molecular dynamics (AIMD). A-LLZO has shown promising properties in inhibiting the growth of lithium dendrites, making it a potential candidate for solid electrolytes in all-solid-state lithium batteries. The low Li-ion conductivity of a-LLZO is currently limiting its practical applications. Our findings revealed that the homogeneous distribution of Zr-O polyhedra within the pristine structure of a-LLZO contributes to enhanced Li-ion conductivity. By reducing the interconnections among Zr-O polyhedra, the AIMD-simulated a-LLZO sample achieved a Li-ion conductivity of 5.78 × 10-4 S/cm at room temperature, which is slightly lower than that of cubic LLZO (c-LLZO) with a Li-ion conductivity of 1.63 × 10-3 S/cm. Furthermore, we discovered that Li-ion conductivity can be influenced by adjusting the elemental ratios within a-LLZO. This suggests that fine-tuning the composition of a-LLZO can potentially further enhance its Li-ion conductivity and optimize its performance as a solid electrolyte in lithium batteries.

In Situ Induced Lattice-Matched Interfacial Oxygen-Passivation-Layer Endowing Li-Rich and Mn-Based Cathodes with Ultralong Life.

The high capacity of Li-rich and Mn-based (LRM) cathode materials is originally due to the unique hybrid anion- and cation redox, which also induces detrimental oxygen escape. Furthermore, the counter diffusion of released oxygen (into electrolyte) and induced oxygen vacancies (into the interior bulk phase) that occurs at the interface will cause uncontrolled phase collapse and other issues. Therefore, due to its higher working voltage (>4.7 V) than the activation voltage of lattice oxygen in LRM (≈4.5 V), the anion-redox-free and structurally consistent cobalt-free LiNi0.5 Mn1.5 O4 (LNMO) is selected to in situ construct a robust, crystal-dense and lattice-matched oxygen-passivation-layer (OPL) on the surface of LRM particles by the electrochemical delithiation to protect the core layered components. As expected, the modified sample displays continuously decreasing interfacial impedance and high specific capacity of 135.5 mAh g-1 with a very small voltage decay of 0.67 mV per cycle after 1000 cycles at 2 C rate. Moreover, the stress accumulation during cycling is mitigated effectively. This semicoherent OPL strengthens the surface stability and interrupts the counter diffusion of oxygen and oxygen vacancies in LRM cathode materials, which would provide guidance for designing high-energy-density layered cathode materials.

First-Principles Observation of Bonded 2D B4C3 Bilayers.

Two-dimensional (2D) B-C compounds possess rich allotropic structures with many applications. Obtaining new 2D B4C3 structures is highly desirable due to the novel applications of three-dimensional (3D) B4C3 in protections. In this work, we proposed a new family of 2D B4C3 from the first-principles calculations. Distinct from previous observations, this family of 2D B4C3 consists of bonded 2D B4C3 bilayers. Six different types of bilayers with distinct bonded structures are found. The phonon spectrum calculations and ab initio molecular dynamics simulations at room temperature demonstrate their dynamic and thermal stabilities. Low formation energies suggest the high possibility of realizing such structures in experiments. Rich electronic structures are found, and the predicted Young's moduli are even higher than those of the previous ones. It is revealed that the unique electronic and mechanical properties are rooted in the bonding structures, indicating the prompting applications of this family of 2D B4C3 materials in photovoltaics, nanoelectronics, and nanomechanics.

Significant second-harmonic generation and bulk photovoltaic effect in trigonal selenium and tellurium chains.

One-dimensional (1D) selenium and tellurium crystalize in helical chainlike structures and thus exhibit fascinating properties. By performing first-principles calculations, we have researched the linear and nonlinear optical (NLO) properties of 1D Se and Te, and find that both systems exhibit pronounced NLO responses. In particular, 1D Se is found to possess a large second-harmonic generation coefficient with the χ value being up to 7 times larger than that of GaN, and is even several times larger than that of the bulk counterpart. On the other hand, 1D Te also produces significant NLO susceptibility χ which exceeds that of bulk GaN by 5 times. Furthermore, 1D Te is shown to possess a prominent linear electro-optic coefficient rxxx(0). In particular, the Te chain exhibits a large shift current response and the maximum is twice as large as the maximal photovoltaic responses obtained from BaTiO3. Therefore, 1D Se and Te may find potential applications in solar energy conversion, electro-optical switches, and so on. Finally, the much stronger NLO effects of 1D Se and Te are attributed to their one-dimensional structures with high anisotropy, strong covalent bonding and lone-pair electrons. These findings will contribute further to experimental studies and the search for excellent materials with large NLO effects.

HOT Graphene and HOT Graphene Nanotubes: New Low Dimensional Semimetals and Semiconductors.

We report a new graphene allotrope named HOT graphene containing carbon hexagons, octagons, and tetragons. A corresponding series of nanotubes are also constructed by rolling up the HOT graphene sheet. Ab initio calculations are performed on geometric and electronic structures of the HOT graphene and the HOT graphene nanotubes. Dirac cone and high Fermi velocity are achieved in a non-hexagonal structure of HOT graphene, implying that the honeycomb structure is not an indispensable condition for Dirac fermions to exist. HOT graphene nanotubes show distinctive electronic structures depending on their topology. The (0,1) n (n ≥ 3) HOT graphene nanotubes reveal the characteristics of semimetals, while the other set of nanotubes (1,0) n shows continuously adjustable band gaps (0~ 0.51 eV) with tube size. A competition between the curvature effect and the zone-folding approximation determines the band gaps of the (1,0) n nanotubes. Novel conversion between semimetallicity and semiconductivity arises in ultra-small tubes (radius < 4 Å, i.e., n < 3).

Manipulating External Electric Field and Tensile Strain toward High Energy Density Stability in Fast-Charging Li-Rich Cathode Materials.

Li-rich layered oxides (LLOs) are promising cathodes for lithium-ion batteries because of their high energy density provided by anionic redox. Although great improvements have been achieved in electrochemical performance, little attention has been paid to the energy density stability during fast charging. Indeed, LLOs have severe capacity fading and voltage decay especially at a high state of charge (SOC), disabling the application of the frequently used constant-current-constant-voltage mode for fast charging. Herein, we address this problem by manipulating the external electric field and tensile strain induced by lattice expansion effect in nanomaterials under the guidance of theoretical calculations, which indicate that LLOs at high SOC have almost a zero band gap and a low oxygen formation energy. This strategy will weaken polarization, stabilize lattice oxygen, and restrict phase transition simultaneously. Thus, the energy density during fast charging can be highly stabilized. Therefore, it may be of great value for the practical application of layered cathodes.

Anionic Redox Processes in Maricite- and Triphylite-NaFePO4 of Sodium-Ion Batteries.

In recent years, NaFePO4 has been regarded as one of the most promising cathode materials for next-generation rechargeable sodium-ion batteries. There is significant interest in the redox processes of rechargeable batteries for high capacity applications. In this paper, the redox processes of triphylite-NaFePO4 and maricite-NaFePO4 materials have been analyzed based on first-principles calculations and analysis of Bader charges. Different from LiFePO4, anionic (O2-) redox reactions are evidently visible in NaFePO4. Electronic structures and density of states are calculated to elaborate the charge transfer and redox reactions during the desodiation processes. Furthermore, we also calculate the formation energies of sodium extraction, convex hull, average voltage plateaus, and volume changes of Na1-x/12FePO4 with different sodium compositions. Deformation charge density plots and magnetization for NaFePO4 are also calculated to help understand the redox reaction processes.

Dependence of Electronic and Optical Properties of MoS2 Multilayers on the Interlayer Coupling and Van Hove Singularity.

In this paper, the structural, electronic, and optical properties of MoS2 multilayers are investigated by employing the first-principles method. Up to six-layers of MoS2 have been comparatively studied. The covalency and ionicity in the MoS2 monolayer are shown to be stronger than those in the bulk. As the layer number is increased to two or above two, band splitting is significant due to the interlayer coupling. We found that long plateaus emerged in the imaginary parts of the dielectric function [Formula: see text] and the joint density of states (JDOS) of MoS2 multilayers, due to the Van Hove singularities in a two-dimensional material. One, two and three small steps appear at the thresholds of both the long plateau of [Formula: see text] and JDOS, for monolayer, bilayer, and trilayer, respectively. As the number of layers further increased, the number of small steps increases and the width of the small steps decreases accordingly. Due to interlayer coupling, the longest plateau and shortest plateau of JDOS are from the monolayer and bulk, respectively.

Lithium Deficiencies Engineering in Li-Rich Layered Oxide Li1.098Mn0.533Ni0.113Co0.138O2 for High-Stability Cathode.

Li-rich layered oxides have been in focus because of their high specific capacity. However, they usually suffer from poor kinetics, severe voltage decay, and capacity fading. Herein, a long-neglected Li-deficient method is demonstrated to address these problems by simply reducing the lithium content. Appropriate lithium vacancies can improve dynamics features and induce in situ surface spinel coating and nickel doping in the bulk. Therefore, the elaborately designed Li1.098Mn0.533Ni0.113Co0.138O2 cathode possesses improved initial Coulombic efficiency, excellent rate capability, largely suppressed voltage decay, and outstanding long-term cycling stability. Specifically, it shows a superior capacity retention of 93.1% after 500 cycles at 1 C (250 mA g-1) with respect to the initial discharge capacity (193.9 mA h g-1), and the average voltage still exceeds 3.1 V. In addition, the discharge capacity at 10 C can be as high as 132.9 mA h g-1. More importantly, a Li-deficient cathode can also serve as a prototype for further performance enhancement, as there are plenty of vacancies.

A Guideline for Tailoring Lattice Oxygen Activity in Lithium-Rich Layered Cathodes by Strain.

Lattice oxygen activity plays a dominant role in balancing discharge capacity and performance decay of lithium-rich layered oxide cathodes (LLOs). On the basis of density functional theory (DFT) and tight-binding theory, the activity of lattice oxygen can be improved by tensile strain and suppressed by compressive strain. To verify this conclusion, LLOs with large lattice parameters (L-LLOs) were synthesized taking advantage of the lattice expansion effect in nanomaterials. Compared with conventional LLOs with small lattice parameters (S-LLOs), particles in L-LLOs are imposed by tensile strain. L-LLOs show a larger initial discharge capacity and decay faster in the prolonged cycles than S-LLOs. Actually, most of the modified methods in LLOs can come down to strain-induced changes in lattice parameters. We believe this conclusion is a useful guideline to understand and tailor the lattice oxygen activity and may be generalized to other layered oxide cathodes involving anionic redox.

Electronic Properties of Vanadium Atoms Adsorption on Clean and Graphene-Covered Cu(111) Surface.

The electronic properties of vanadium atoms adsorbed on clean and graphene-covered Cu(111) surface have been systematically studied using ab initio theoretical method. Two coverages (1/9 ML and 1 ML) of vanadium adsorption are considered in this work. Our calculations indicate that V staying underneath the Cu surface is found to be the most stable adsorption site at the aforementioned two coverages for V/Cu(111). However, such adsorption may lead to undesired properties. Therefore, we introduce graphene as a buffer layer to effectively alleviate the direct interaction between V and Cu surface. The calculations show that electronic properties of the original graphene layer are significantly affected by the interactions of C atoms with the V adatoms; the Dirac point of graphene is "destroyed" as a consequence at both coverages. In the V/Gra/Cu(111) system, the interaction between graphene layer and the substrate Cu atoms remains weak as in the Gra/Cu(111) system. Moreover, a relatively low coverage of 1/9 ML gives rise to a spin-polarized system while a non-spin-polarized system is observed at the coverage of 1 ML. This finding offers a new way for the application of vanadium-based materials in reality.

Chemically initiated liquid-like behavior and fabrication of periodic wavy Cu/CuAu nanocables with enhanced catalytic properties.

Solid crystalline materials have long range order in their atomic arrangement while liquids have short range order, and the transition between them is usually caused by heat and/or pressure. Herein, we report the finding that chemical processes may play a similar role as heat and initiate liquid-like behavior of crystalline nanomaterials at a temperature far below their melting points. When the straight Cu/CuAu crystalline nanocables are dispersed in organic amine at 80 °C under ambient conditions, the continuous oxidation of Cu atoms on the surface and diffusion of Cu atoms from the core to the surface would break up the long-range ordered arrangement of atoms and lead to the transformation of an anisotropic crystal into an isotropic liquid-like state, which resulted in the evolution of the straight morphology of the nanocables into periodic wavy structures following the Rayleigh instability. It was also demonstrated that periodic wavy Cu@CuAu nanocables exhibit much better catalytic activity than straight Cu@CuAu nanocables towards the reduction of p-nitrophenol into p-aminophenol by NaBH4. Our results not only provide new insights into the transition between a solid crystal and a liquid-like state at the nanoscale, but also facilitate the development of new strategies for the synthesis of functional nanomaterials.

Recent advances in the chemical transformations of functionalized alkylidenecyclopropanes (FACPs).

During the past several years, functionalized alkylidenecyclopropanes (FACPs) have attracted intensive attention in synthetic chemistry. Many interesting transformations of FACPs have been developed to synthesize a lot of structurally diverse and valuable polycyclic and heterocyclic compounds. This review will classify FACPs into aryl-FACPs, alkyl-FACPs and ring-FACPs for the first time, and recent interesting chemical transformations in these research fields will be included, respectively, from 2011. Moreover, we will pay more attention to the clarification of the reaction mechanism, in which the C-C bond cleavage of alkylidenecyclopropanes (ACPs) will be emphasized.

Cr0.5Nb24.5O62 Nanowires with High Electronic Conductivity for High-Rate and Long-Life Lithium-Ion Storage.

Intercalation-type TiNbxO2+2.5x (x = 2, 5, and 24) anode materials have recently become more interesting for lithium-ion batteries (LIBs) due to their large theoretical capacities of 388-402 mAh g-1. However, the Ti4+/Nb5+ ions in TiNbxO2+2.5x with empty 3d/4d orbitals usually lead to extremely low electronic conductivity of <10-9 S cm-1, greatly restricting their practical capacity and rate capability. Herein, we report a class of highly conductive Cr0.5Nb24.5O62 nanowires as an intercalation-type anode material for high-performance LIBs. The as-made Cr0.5Nb24.5O62 nanowires show an open shear ReO3 crystal structure (C2 space group) with 4% tetrahedra and a conducting characteristic with ultrahigh electronic conductivity of 3.6 × 10-2 S cm-1 and a large Li+-ion diffusion coefficient of 2.19 × 10-13 cm2 s-1. These important characteristics make them deliver outstanding electrochemical properties in term of the largest reversible capacity (344 mAh g-1 at 0.1 C) in all the known niobium- and titanium-based anode materials, safe working potential (∼1.65 V vs Li/Li+), high first-cycle Coulombic efficiency (90.8%), superior rate capability (209 mAh g-1 at 30 C), and excellent cycling stability, making them among the best for LIBs in niobium- and titanium-based anode materials.

Superlight and Superflexible Three-Dimensional Semiconductor Frameworks A(X≡Y)4 (A=Si, Ge; X/Y=C, B, N) with Tunable Optoelectronic and Mechanical Properties from First-Principles.

Silicon carbide materials, as leading wide band gap semiconductors, hold significant importance in semiconductor technologies. Herein, diamond-like 3D materials with low density, but high elasticity properties, have been designed from first-principles calculations. They are porous single-crystalline materials composed of sp3 -hybridized silicon (or germanium) and sp-type C≡C (or B≡N) linear moieties; their stabilities are comparable to those of recently prepared SiC4 materials. Moreover, such wide band gap semiconductors have strong absorption over a wide UV range and exhibit superlight, superflexible, and incompressible mechanical properties, and their optoelectronic and mechanical properties can be well tuned through structural modifications. Such features provide high potential for practicable application under extreme conditions, and suggest promising applications for the design of UV optoelectronic devices.

Electronic halocyclization and radical haloazidation of benzene-linked 1,7-dienes for the synthesis of functionalized 3,1-benzoxazines.

A novel electronic halocyclization and radical haloazidation of benzene-linked 1,7-dienes for the formation of functionalized 3,1-benzoxazines has been achieved by using TMSN3 as an azido source and NBS as a halogen source. This methodology is highlighted by its mild conditions and wide substrate scope, which concomitantly introduces one C-N and two C-halogen bonds into one molecule.

Palladium-catalyzed cascade cyclization of allylamine-tethered alkylidenecyclopropanes: facile access to iodine/difluoromethylene- and perfluoroalkyl-containing 1-benzazepine scaffolds.

The unprecedented palladium-catalyzed cascade cyclization of allylamine-tethered alkylidenecyclopropanes with an ethyl difluoroiodoacetate or perfluoroalkylated reagent is developed, providing facile access to a variety of synthetically and medicinally valuable iodine/difluoromethylene- and perfluoroalkyl-containing 1-benzazepine frameworks. These reactions exhibited good yields and functional group tolerance via a radical mechanism.

Atomic-scale insights into structural and thermodynamic stability of Pd-Ni bimetallic nanoparticles.

Atomic-scale understanding of structures and thermodynamic stability of core-shell nanoparticles is important for both their synthesis and application. In this study, we systematically investigated the structural stability and thermodynamic evolution of core-shell structured Pd-Ni nanoparticles by molecular dynamics simulations. It has been revealed that dislocations and stacking faults occur in the shell and their amounts are strongly dependent on the core/shell ratio. The presence of these defects lowers the structural and thermal stability of these nanoparticles, resulting in even lower melting points than both Pd and Ni monometallic nanoparticles. Furthermore, different melting behaviors have been disclosed in Pd-core/Ni-shell and Ni-core/Pd-shell nanoparticles. These diverse behaviors cause different relationships between the melting temperature and the amount of stacking faults. Our results display direct evidence for the tunable stability of bimetallic nanoparticles. This study provides a fundamental perspective on core-shell structured nanoparticles and has important implications for further tailoring their structural and thermodynamic stability by core/shell ratio or composition controlling.

Substrate-controlled Rh(II)-catalyzed single-electron-transfer (SET): divergent synthesis of fused indoles.

Rh(II)-catalyzed diversified ring expansions controlled by single-electron-transfer (SET) have been disclosed in this communication, producing a series of indole-fused azetidines and 1H-carbazoles or related derivatives in moderate to good yields via Rh2(III,II) nitrene radical intermediates. The direction of ring expansion branches according to different ring sizes of methylenecycloalkanes.