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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Stoichiometric two-dimensional non-van der Waals AgCrS2 with superionic behaviour at room temperature.

Layered materials have attracted tremendous interest for accessing two-dimensional structures. Materials such as graphite or transition metal dichalcogenides, in which the layers are held together by van der Waals interactions, can be exfoliated through a variety of processes in a manner that retains the structure and composition of the monolayers, but this has proven difficult for solids with stronger interlayer interactions. Here, we demonstrate the exfoliation of AgCrS2, a member of the AMX2 family (where A is a monovalent metal, M is a trivalent metal and X is a chalcogen), through intercalation with tetraalkylammonium cations, chosen for their suitable redox potential. The as-exfoliated nanosheets consist of Ag layers sandwiched between two CrS2 layers, similar to their structure in the bulk. They show superionic behaviour at room temperature, with an ionic conductivity of 33.2 mS cm-1 at 298 K that originates from Ag+ ions rapidly hopping between neighbouring tetrahedral interstices; in the bulk, this behaviour is only observed above 673 K.

Single-Layer Zirconium Dihalides ZrX2 (X = Cl, Br, and I) with Abnormal Ferroelastic Behavior and Strong Anisotropic Light Absorption Ability.

Recently, two-dimensional (2D) metal halides have brought out an intensive interest for their unique mechanical, electronic, magnetic, and topological properties. Here, we theoretically report the existence of the single-layer (SL) zirconium dihalide materials ZrX2 (X = Cl, Br, and I) using first-principles calculations. SL ZrX2, which can be obtained from its bulk phase through simple mechanical exfoliation, shows the dynamic, thermodynamic, and mechanical stability. Halogen atoms can effectively tune the electronic structure, dipole moment transition, band alignment, and light absorption. Specifically, ZrX2 monolayers intrinsically exhibit a ferroelasticity with an abnormal 120° orientation rotation, possessing a moderate switching barrier of 24-39 meV/atom. Importantly, we observe superior anisotropic light absorption responses on SL ZrX2 in the visible region. Besides, a series of ZrX2-based excitonic solar cells have been proposed, which hold a large power conversion efficiency limit of 12.4-18.7%.

Orbital Design of Two-Dimensional Transition-Metal Peroxide Kagome Crystals with Anionogenic Dirac Half-Metallicity.

Assembling p orbital ferromagnetic half-metallicity and a topological element, such as a Dirac point at the Fermi level, in a single nanomaterial is of particular interest for long-distance, high-speed, and spin-coherent transportation in nanoscale spintronic devices. On the basis of the tight-binding model, we present an orbital design of a two-dimensional (2D) anionogenic Dirac half-metal (ADHM) by patterning cations with empty d orbitals and anions with partially filled p-type orbitals into a kagome lattice. Our first-principles calculations show that 2D transition-metal peroxides h-TM2(O2)3 (TMO3, TM = Ti, Zr, Hf), containing group IVB transition-metal cations [TM]4+ bridged with dioxygen anions [O2]8/3- in a kagome structure, are stable ADHMs with a Curie temperature over 103 K. The 2/3 filled π* orbitals of dioxygen anions are ferromagnetically coupled, leading to p orbital ferromagnetism and a half-metallic Dirac point right at the Fermi level with a Fermi velocity reaching 2.84 × 105 m/s. We proposed that 2D h-TM2(O2)3 crystals may be extracted from ABO3 bulk materials containing 2D TMO3 layers.

Two-Dimensional Cr2X3S3 (X = Br, I) Janus Semiconductor with Intrinsic Room-Temperature Magnetism.

The exploration of two-dimensional (2D) semiconductors with intrinsic room-temperature magnetism for use in nanoscale spintronic devices is of particular interest. Recently, the ferromagnetic CrX3 monolayer (X = Br, I) has received growing attention, but low critical temperature hinders its practical applications in spintronics. Here, using first-principles calculations, we report 2D Cr2X3S3 (X = Br, I) Janus semiconductors with room-temperature magnetism by replacing one layer of halogon atoms with sulfur atoms in CrX3 monolayer. Our results demonstrate that Cr2Br3S3 and Cr2I3S3 Janus crystals are ferrimagnetic semiconductors, that maintain their magnetic order, with a direct bandgap of 1.19 and 0.61 eV and high critical temperature of 387 and 447 K, respectively. The residual unpaired p electrons on the S anions lead to a strong direct-exchange interaction between the Cr and S atoms. Moreover, their room-temperature magnetism is robust under biaxial strain, while the bandgap can be remarkably modulated with strain. The novel magnetic properties in 2D Cr2X3S3 Janus magnetic semiconductors give them promising applications in spintronics.

Spatial Confinement of a Carbon Nanocone for an Efficient Oxygen Evolution Reaction.

A major bottleneck of large-scale water splitting for hydrogen production is the lack of catalysts for the oxygen evolution reaction (OER) with low cost and high efficiency. In this work, we proposed an electrocatalyst of a curved carbon nanocone embedded with two TMN4 active sites (TM = transition metal) and used first-principles calculations to investigate their OER mechanisms and catalytic activities. In the particular spatial confinement of a curved nanocone, we found that the distance between intermediates adsorbed on two active sites is shorter than the distance between these two active sites. This finding can be used to enhance OER activity by distance-dependent interaction between intermediates through two different mechanisms. The first mechanism in which an O2 molecule is generated from two neighboring *O intermediates exhibits a linear activity trend, and the lowest overpotential is 0.27 V for the FeN4 system. In the second mechanism, selective stabilization of the *OOH intermediate is realized, leading to a new scaling relationship (ΔG*OOH = ΔG*OH + 3.04 eV) associated with a modified OER activity volcano (theoretical volcano apex at 0.29 V). The studied mechanisms of the spatial confinement of a carbon nanocone provide a new perspective for designing efficient OER catalysts.