Selective binding and periodic arrangement of magic boron clusters on monolayer borophene.

The synthesis and characterization of small boron clusters with unique size and regular arrangement are crucial for boron chemistry and two-dimensional borophene materials. In this study, together with theoretical calculations, the joint molecular beam epitaxy and scanning tunneling microscopy experiments achieve the formation of unique B5 clusters on monolayer borophene (MLB) on a Cu(111) surface. The B5 clusters tend to selectively bind to specific sites of MLB with covalent boron-boron bonds in the periodic arrangement, which can be ascribed to the charge distribution and electron delocalization character of MLB and also prohibits nearby co-adsorption of B5 clusters. Furthermore, the close-packed adsorption of B5 clusters would facilitate the synthesis of bilayer borophene, exhibiting domino effect-like growth mode. The successful growth and characterization of uniform boron clusters on a surface enrich the boron-based nanomaterials and reveal the essential role of small clusters during the growth of borophene.

Interlayer Delocalized Electrons Activate Basal Plane for Ultrahigh-Current-Density Hydrogen Evolution.

The basal plane of transition metal dichalcogenides (TMDCs) is inert for the hydrogen evolution reaction (HER) due to its low-efficiency charge transfer kinetics. We propose a strategy of filling the van der Waals (vdW) layer with delocalized electrons to enable vertical penetration of electrons from the collector to the adsorption intermediate vertically. Guided by density functional theory, we achieve this concept by incorporating Cu atoms into the interlayers of tantalum disulfide (TaS2). The delocalized electrons of d-orbitals of the interlayered Cu can constitute the charge transfer pathways in the vertical direction, thus overcoming the hopping migration through vdW gaps. The vertical conductivity of TaS2 increased by 2 orders of magnitude. The TaS2 basal plane HER activity was extracted with an on-chip microcell. Modified by the delocalized electrons, the current density increased by 20 times, reaching an ultrahigh value of 800 mA cm-2 at -0.4 V without iR compensation.

Ultrahigh Potassium Storage Capacity of Ca2Si Monolayer with Orderly Multilayered Growth Mechanism.

As the rising renewable energy demands and lithium scarcity, developing high-capacity anode materials to improve the energy density of potassium-based batteries (PBBs) is increasingly crucial. In this work, a unique orderly multilayered growth (OMLG) mechanism on a 2D-Ca2Si monolayer is theoretically demonstrated for potassium storage by first-principles calculations. The global-energy-minimum Ca2Si monolayer is a semiconductor with isotropic mechanical properties and remarkable electrochemical properties, such as a low potassium ion migration energy barrier of 0.07 eV and a low open circuit voltage ranging from 0.224 to 0.003 V. Most notably, 2D-Ca2Si demonstrates an ultrahigh theoretical specific capacity of 5459 mAh g-1 and a total specific capacity of 610 mAh g-1, reaching up to 89% of the capacity of a potassium metal anode. Remarkably, the OMLG mechanism facilitates stable, dendrite-free deposition of hcp-K metal layers on the 2D-Ca2Si surface, where the ultrahigh and gradually converging lattice match as the layers increase is the key to achieving theoretically near-infinite growth. The study theoretically demonstrates the Ca2Si monolayer a highly promising anode material, and offers a novel potassium storage strategy for designing 2D anode materials with high specific capacity, rapid potassium-ion migration, and good safety.

Asymmetrically Coordinated Calcium Single Atom for High-Performance Oxygen Reduction Reaction.

S-block single atoms represent an ideal catalyst for the oxygen reduction reaction (ORR) as they can suppress the Fenton reaction. However, the symmetry of the s/p orbitals tends to generate either an excessively strong or weak interaction with intermediates. Herein, Ca single atoms coordinated with -S, -OP, and three N atoms (Ca/NPS-HC) were fabricated to modulate the adsorption of intermediates and promote the efficiency of s-block ORR catalysts. The experimental results from ORR demonstrated that the Ca/NPS-HC catalyst exhibited outstanding catalytic capability with a half-wave potential of 0.89 V, a kinetic current density of 56.6 mA cm-2 at 0.85 V, and a Tafel slope of 42 mV dec-1, outperforming commercial Pt/C. The detailed mechanistic studies revealed that the asymmetric coordination of Ca single atoms led to the symmetry-breaking of electron distribution in Ca single atoms, attenuating the s-p hybridization from the intermediate adsorption process, and thereby minimizing the energy barrier of the whole ORR.

Anisotropic electrene T'-Ca2P with electron gas magnetic coupling as anode material for Na/K ion batteries.

There is an urgent need for high-performance rechargeable electrical storage devices as a supplement or a substitution for lithium ion batteries (LIBs) due to the shortage of lithium in nature. Herein we propose a stable 2D electrene T'-Ca2P as an anode material for Na/K ion batteries developed using first principles calculations. Our calculated results show that the T'-Ca2P monolayer is an antiferromagnetic semiconducting electrene with a spin-polarized electron gas. It exhibits suitable adsorption for both Na and K atoms, and its anisotropic migration energy barriers are 0.050/0.101 eV and 0.037/0.091 eV in the b/a direction, respectively. The theoretical capacities for Na and K are both 482 MA h g-1, whereas the average working voltage platforms are 0.171-0.226 V and 0.013-0.267 V, respectively. All the results reveal that the T'-Ca2P monolayer has promising prospects for application as an anode material for Na/K ion batteries.

Back-Gated van der Waals Heterojunction Manipulates Local Charges toward Fine-Tuning Hydrogen Evolution.

Charge redistribution plays a prominent role in interpreting the intrinsic electrocatalytic mechanism. Establishing a quantitative relationship between the local charges and electrochemical performance can fundamentally update the design philosophies beyond conventional methods. We describe exertion of an external electric field in the cobalt phthalocyanine (CoPc)/MoS2 heterojunction to finely manipulate intermolecular charge transfer. The injected charges (e- ) from CoPc to MoS2 migrate to natural S vacancies and enhance Mo-H bonding. Moreover, the band gap of MoS2 and CoPc can be readily tuned by the electric field, verifying band engineering at the heterointerface. In situ photoluminescence spectra and gate-dependent electrochemical measurement reveal a linear correlation between the charge accumulation and hydrogen evolution reaction (HER) activity. This approach provides a new strategy for the design of catalysts, enabling precise regulation of the electronic configuration to improve catalytic activity.

Dual-Regulation of Defect Sites and Vertical Conduction by Spiral Domain for Electrocatalytic Hydrogen Evolution.

Insufficient active sites and weak vertical conduction are the intrinsic factors that restrict the electrocatalytic HER for transition-metal dichalcogenides. As a prototype, we proposed a model of spiral MoTe2 to optimize collectively the above issues. The conductive atomic force microscopy of an individual spiral reveals that the retentive vertical conduction irrespective of layer thickness benefits from the connected screw dislocation lines between interlayers. Theoretical calculations uncover that the regions near the edge step of the spiral structures more easily form Te vacancies and have lower ΔGH * as extra active sites. A single spiral MoTe2 -based on-chip microcell was fabricated to extract HER activity and achieved an ultrahigh current density of 3000 mA cm-2 at an overpotential of 0.4 V, which is about two orders of magnitude higher than the exfoliated counterpart. Profoundly, this unusual spiral model will initiate a new pathway for triggering other inert catalytic reactions.

Computational Dissection of Two-Dimensional Rectangular Titanium Mononitride TiN: Auxetics and Promises for Photocatalysis.

Recently, two-dimensional (2D) transition-metal nitrides have triggered an enormous interest for their tunable mechanical, optoelectronic, and magnetic properties, significantly enriching the family of 2D materials. Here, by using a broad range of first-principles calculations, we report a systematic study of 2D rectangular materials of titanium mononitride (TiN), exhibiting high energetic and thermal stability due to in-plane d-p orbital hybridization and synergetic out-of-plane electronic delocalization. The rectangular TiN monolayer also possesses enhanced auxeticity and ferroelasticity with an alternating order of Possion's Ratios, stemming from the competitive interactions of intra- and inter- Ti-N chains. Such TiN nanosystem is a n-type metallic conductor with specific tunable pseudogaps. Halogenation of TiN monolayer downshifts the Fermi level, achieving the optical energy gap up to 1.85 eV for TiNCl(Br) sheet. Overall, observed electronic features suggest that the two materials are potential photocatalysts for water splitting application. These results extend emerging phenomena in a rich family 2D transition-metal-based materials and hint for a new platform for the next-generation functional nanomaterials.

Room-Temperature Ferromagnetism in Two-Dimensional Fe2Si Nanosheet with Enhanced Spin-Polarization Ratio.

Searching experimental feasible two-dimensional (2D) ferromagnetic crystals with large spin-polarization ratio, high Curie temperature and large magnetic anisotropic energy is one key to develop next-generation spintronic nanodevices. Here, 2D Fe2Si nanosheet, one counterpart of Hapkeite mineral discovered in meteorite with novel magnetism is proposed on the basis of first-principles calculations. The 2D Fe2Si crystal has a slightly buckled triangular lattice with planar hexacoordinated Si and Fe atoms. The spin-polarized calculations with hybrid HSE06 function method indicate that 2D Fe2Si is a ferromagnetic half metal at its ground state with 100% spin-polarization ratio at Fermi energy level. The phonon spectrum calculation and ab initio molecular dynamic simulation shows that 2D Fe2Si crystal has a high thermodynamic stability and its 2D lattice can be retained at the temperature up to 1200 K. Monte Carlo simulations based on the Ising model predict a Curie temperature over 780 K in 2D Fe2Si crystal, which can be further tuned by applying a biaxial strain. Moreover, 2D structure and strong in-plane Fe-Fe interaction endow Fe2Si nanosheet sizable magnetocrystalline anisotropy energy with the magnitude of at least two orders larger than those of Fe, Co and Ni bulks. These novel magnetic properties render the 2D Fe2Si crystal a very promising material for developing practical spintronic nanodevices.

Two-Dimensional Phosphorus Porous Polymorphs with Tunable Band Gaps.

Exploring stable two-dimensional (2D) crystalline structures of phosphorus with tunable properties is of considerable importance partly due to the novel anisotropic behavior in phosphorene and potential applications in high-performance devices. Here, 21 new 2D phosphorus allotropes with porous structure are reported based on topological modeling method and first-principles calculations. We establish that stable 2D phosphorus crystals can be obtained by topologically assembling selected phosphorus monomer, dimer, trimer, tetramer, and hexamer. Nine of reported structures are predicted to be more stable than white phosphorus. Their dynamic and thermal stabilities are confirmed by the calculated vibration spectra and Born-Oppenheimer molecular dynamic simulation at temperatures up to 1500 K. These phosphorus porous polymorphs have isotropic mechanic properties that are significantly softer than phosphorene. The electronic band structures calculated with the HSE06 method indicate that new structures are semiconductors with band gaps ranging widely from 0.15 to 3.42 eV, which are tuned by the basic units assembled in the network. Of particular importance is that the position of both conduction and valence band edges of some allotropes matches well with the chemical reaction potential of H2/H(+) and O2/H2O, which can be used as element photocatalysts for visible-light-driven water splitting.

Rupturing C60 Molecules into Graphene-Oxide-like Quantum Dots: Structure, Photoluminescence, and Catalytic Application.

The large-scale synthesis of graphene-oxide-like quantum dots (GOLQDs) is reported by oxidizing C(60) molecules using a modified Hummers method with a yield of ≈25 wt% readily achieved. The GOLQDs are highly soluble in water and in addition to hexagons have other carbon rings in the structure. They have an average height of ≈1.2 nm and a diameter distribution of 0.6-2.2 nm after drying on substrates. First-principle calculations indicate that a possible rupturing route may include the insertion of oxygen atoms to CC bonds in the C(60) molecule, followed by rupture of that CC bonds. The GOLQD suspension has a strong photoluminescence (PL) with peak position dependent on excitation wavelength. The PL is related to the size and emissive traps caused by oxygen-containing groups. The GOLQDs also catalyze the oxidation of benzyl alcohol with a high selectivity.

Prediction of Halogenated MXenes as Electrode Materials for Halide-Ion Batteries.

Exploring and developing new rechargeable halide-ion batteries plays an important role in the advancement and growth of the ion battery family. Here, we systematically explored the feasibility of single-layer MXenes and their hydrogenated derivatives as electrode materials for halide-ion batteries via first-principles theory. The calculated results indicate that halide ions (T ions) can be stably and efficiently adsorbed on the surfaces of M2X and M2XH2, with theoretical specific capacities ranging from 227 to 497 mAh g-1. The diffusion barriers of the T ion on MXenes are from 0.55 to 0.10 eV, comparable to those of the Li ion in graphite and LiCoO2. The electronegativity of halide anions displays significant impacts on their discharge voltage plateaus on M2X, with the highest voltage up to 5.60 V for the F ion. As a comparison, the hydrogenation of M2XH2 with less surface activity raises a 2-3 V voltage reduction. All MXene-based full cells of TxTi2C|TyTi2CH2 (where x = 0-2 and y = 2-0) and TxTi2N|TyTi2NH2 (where x = 0-2 and y = 2-0) demonstrated high full battery specific energies for F-, Cl-, and Br-ion batteries, up to 462 Wh kg-1. These results demonstrate the potential of new halide-ion battery designs, paving the way for future research and innovation in battery technology.

Two-Dimensional Biphenylene-Based Carbon Allotrope Family with High Potassium Storage Ability.

The development of new carbon materials with novel properties and excellent applications is essential and urgent in many fields, such as potassium-ion batteries (PIBs). In this study, a family of 30 two-dimensional biphenylene carbon allotropes (2D-BCAs) have been systematically extended in theory. The energies of these allotropes are slightly higher than that of graphene, which can be well described by a quantitative energy equation. The 2D-BCAs show high synthesizability consistent with the experimental biphenylene network via "HF-zipping" reactions. The 2D-BCAs are metallic or semimetallic. Six representative 2D-BCAs exhibit good lattice dynamical and thermal stability, excellent anisotropic mechanical properties, and ORR catalytic activity. Moreover, the selected 2D-BCAs demonstrate ultrahigh theoretical potassium-storage capacities of 1116-1489 mAh·g-1, low migration barriers of 0.03-0.22 eV, and low open-circuit voltages of 1.10-0.02 V. The remarkable properties render 2D-BCAs as promising anode materials in PIBs, electrocatalysts, and conductors in electronics and iontronics.

Biphenylite as Anode Materials for Alkali Metal Ion Batteries with Self-Enhanced Storage Mechanism.

In this work, we theoretically investigate the feasibility of biphenylite, the van der Waals layered bulk structure from experimental biphenylene network monolayers, as an anode material for alkali metal ions. The results indicate that the theoretical properties of Li, Na, and K in biphenylite are generally beyond those in graphite. Li-biphenylite exhibits a high specific capacity of 744 mAh·g-1, with a corresponding voltage range of 0.90-0.36 V, low diffusion barrier (<0.30 eV), and small volume change (∼9.9%), far exceeding those of Li-graphite. Moreover, a novel self-enhanced storage mechanism is observed and unveiled, in which the heavy intercalation of Li atoms (i.e., electron doping) induces puckered distortion of the nonhoneycomb carbon frameworks to enhance the intercalation ability and capacity of Li ion via a chemical activation of carbon frameworks. Possessing excellent anode performance beyond graphite, biphenylite is a promising "all-around" anode material candidate for alkali metal ion batteries, especially for lithium ion batteries.

Two-dimensional superhard silicon nitrides with widely tunable bandgap, high carrier mobility and hole-doping-induced robust magnetism.

The search for new forms of the traditional bulk materials to enrich their interactions and properties is an attractive subject in two-dimensional (2D) materials. In this work, novel tetra-hexa-mixed coordinated 2D silicon nitrides (Si3N4) and their analogues are systematically investigated via density functional theory. The results show the global minimum 2D structure, Si3N4 (T-aa), is a highly chemically and thermally stable superhard semiconductor with a wide indirect bandgap (about 6.0 eV), which is widely adjustable under both biaxial strain and vertical electric field. It also possesses anisotropic high carrier mobility, up to 5490 cm2 V-1 s-1 at room temperature. Besides, its nitride analogues of group IVA (Si, Ge, Sn, and Pb) exhibit diverse electronic structures with regular bandgap distribution. Remarkably, some nitride analogues display linearly increasing robust magnetism with hole doping. The theoretical Curie temperatures of Si3N4 and Sn3N4 with hole doping (1h+ per unit cell) are 298 and 180 K, respectively. The Si3N4 (T-aa) and its analogues have a variety of excellent properties to be potentially applied in various fields, e.g., semiconductor electronics, spintronics, high-temperature structural materials, and superhard materials.

Axial Phosphate Coordination in Co Single Atoms Boosts Electrochemical Oxygen Evolution.

Cobalt single atoms coordinated with planar four nitrogen atoms (Co1 N4 ) represent an efficient electrocatalyst for oxygen evolution reaction (OER), whereas the large energy barrier of CoOH dehydrogenation limits the OER activity. Herein, axial phosphate (PO4 ) coordination is incorporated in Co1 N4 single atoms of cobalt phthalocyanine@carbon nanotubes (P-CoPc@CNT), so as to boost the intrinsic OER performance through manipulating the reaction pathway. With a relative low mass loading of Co (2.7%), the P-CoPc@CNT shows remarkable alkaline OER activity with the overpotential of 300 mV and Tafel slope of 41.7 mV dec-1 , which dramatically outperforms the CoPc@CNT without axial PO4 coordination. Based on mechanistic analysis, the axial PO4 coordination directly participates in the OER cycle by the transformation of axial ligand. Specially, the CoOH dehydrogenation process is replaced by the dehydrogenation of HPO4 -Co1 N4 intermediate, which largely decreases the energy barrier and thus benefits the whole OER process.

Green Diamond: A Superhard Boron Carbonitride with Bandgap in Green-Light Region and Anisotropic High Carrier Mobilities.

The development of new multifunctional superhard materials beyond diamond is a great challenge for materials science and industry application. A new diamond-like boron carbonitride material (BC6N) formed by covalently alternated stacking of two-dimensional BC3 and C3N monolayers is systemically investigated through first-principles method. The electronic structure calculations show that the new structure is a direct bandgap semiconductor with a bandgap of 2.404 eV (HSE06). It exhibits anisotropic high carrier mobility (µLh = 1.88 × 104 cm2 V-1 s-1), varied absorbance in visible light and different regions of UV light, and theoretical Vickers hardness of 81.34 GPa, close to that of diamond. Furthermore, it is easily synthesizable due to its exothermic nature when reacted from the interlayer fusion of the BC3 and C3N monolayers in a bottom-up synthesis strategy. In addition, the properties of 3D-BC6N-I can be tuned by applying strain, changing stacking patterns, and 2D-nanolization. The excellent mechanical, electronic, and optical properties and good synthesizability suggest that the new structure (named as "green diamond") may find broad applications as a superhard and high-temperature material as well as a semiconductor and optical devices beyond diamond.

Highly anisotropic and ultra-diffusive vacancies in α-antimonene.

α-Antimonene has recently been successfully fabricated in experiment; hence, it is timely to examine how various types of point defects in α-antimonene can affect its novel electronic properties. Herein, we present a comprehensive investigation of a total of nine possible types of point defects in α-antimonene via first-principles calculations. Particular attention is placed on the structural stability of the point defects and the effects of point defects on the electronic properties of α-antimonene. Compared with its structural analogs, such as phosphorene, graphene, and silicene, we find that most defects in α-antimonene can be more easily generated, and that among the nine types of point defects, the single vacancy SV-(5|9) is likely the most stable one while its presence can be orders of magnitude higher in concentration than that in phosphorene. Moreover, we find that the vacancy exhibits anisotropic and low diffusion barriers, of merely 0.10/0.30 eV in the zigzag/armchair direction. Notably, at room temperature, the migration of SV-(5|9) in the zigzag direction of α-antimonene is estimated to be three orders faster than that along the armchair direction, and also three orders faster than that of phosphorene in the same direction. Overall, the point defects in α-antimonene can significantly affect the electronic properties of the host two-dimensional (2D) semiconductor and thus the light absorption capability. The anisotropic, ultra-diffusive, and charge tunable single vacancies, along with the high oxidation resistance, render the α-antimonene sheet a unique 2D semiconductor (beyond the phosphorene) for developing vacancy-enabled nanoelectronics.

Ultrahigh Lithium Storage Capacity of Al2C Monolayer in a Restricted Multilayered Growth Mechanism.

Designing anode materials with high lithium specific capacity is crucial to the development of high energy density lithium (ion) batteries. Herein, a distinctive lithium growth mechanism, namely, the restricted multilayered growth for lithium, and a strategy for lithium storage are proposed to achieve a balance between ultrahigh specific capacity and the need to avert uncontrolled dendritic growth of lithium. In particular, based on first-principles computation, we show that the Al2C monolayer with a planar tetracoordinate carbon structure can be an ideal platform for realizing the restricted multilayered growth mechanism as a two-dimensional (2D) anode material. Furthermore, the Al2C monolayer exhibits the ultrahigh specific capacity of lithium of 4059 mAh/g, yet with a low diffusion barrier of 0.039-0.17 eV and low open circuit voltage in the range of 0.002-0.34 V. These novel properties render the Al2C monolayer a promising anode material for future lithium (ion) batteries. Our study also offers a design of promising 2D anode materials with a high specific capacity, fast lithium-ion diffusion, and safe lithium storage.

Synthesis of bilayer borophene.

As the nearest-neighbour element to carbon, boron is theoretically predicted to have a planar two-dimensional form, named borophene, with novel properties, such as Dirac fermions and superconductivity. Several polymorphs of monolayer borophene have been grown on metal surfaces, yet thicker bilayer and few-layer nanosheets remain elusive. Here we report the synthesis of large-size, single-crystalline bilayer borophene on the Cu(111) surface by molecular beam epitaxy. Combining scanning tunnelling microscopy and first-principles calculations, we show that bilayer borophene consists of two stacked monolayers that are held together by covalent interlayer boron-boron bonding, and each monolayer has ß12-like structures with zigzag rows. The formation of a bilayer is associated with a large transfer and redistribution of charge in the first boron layer on Cu(111), which provides additional electrons for the bonding of additional boron atoms, enabling the growth of the second layer. The bilayer borophene is shown to possess metallic character, and be less prone to being oxidized than its monolayer counterparts.