Boridene: Two-dimensional Mo4/3B2-x with ordered metal vacancies obtained by chemical exfoliation

Two-dimensional molybdenum boride coordinating with ruthenium nanoparticles to boost hydrogen generation from hydrolytic dehydrogenation of ammonia borane

The coordination between carrier and active metal is critical to the catalytic efficiency of ammonia borane (AB) hydrolysis reaction. In the present study, we report a new type of catalytic support based on molybdenum boride (MBene) MoAl1-xB and demonstrate that the effective combination of MoAl1-xB with Ru nanoparticles can realize the significantly enhanced performance for hydrogen generation. Owing to the efficient activation and dissociation of reactants, the optimal Ru/MoAl1-xB catalyst achieves the large turnover frequency of 494 molH2 molRu-1 min-1, high hydrogen generation rate of 119817 mL min-1 gRu-1 and favorable apparent activation energy of 39.2 kJ mol-1 for the catalytic hydrolysis of AB under alkaline-free condition. The isotopic test suggests the cleavage of OH bond in H2O is the rate-determining step for hydrolysis reaction, while the fracture of B-H bond in AB is also well revealed by attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy. Significantly, the flexible on-demand hydrogen generation is achieved by using chemical switches for on-off AB hydrolysis. This study provides a new support platform based on two-dimensional MBene to exploit efficient catalysts to boost AB dehydrogenation.

Density Functional Theory Investigation on the Nitrogen Reduction Mechanism in Two-Dimensional Transition-Metal Boride with Ordered Metal Vacancies

The creation of ordered collective vacancies in experiment proves challenging within a two-dimensional lattice, resulting in a limited understanding of their impact on catalyst performance. Motivated by the successful experimental synthesis of monolayer molybdenum borides with precisely ordered metal vacancies [Zhou et al. Science 2021, 373, 801-805] through dealloying, the nitrogen reduction reaction (NRR) in monolayer borides was systematically investigated to elucidate the influence of such ordered metal vacancies on catalytic reactions and the underlying mechanisms. The results reveal that the N-containing intermediates tend to dissociate, facilitating the NRR process with reduced UL. The emergence of ordered metal vacancies modulates the electronic properties of the catalyst and partially facilitates the decomposition of N-containing intermediates. However, the UL for NRR in Mo4/3B2 and W4/3B2 exhibits a significant increase. The compromised electrochemical performance is explained through the development of a simple electronic descriptor of the d-p band center (ΔdM-pB). Among these materials, Mo4/3Sc2/3B2 exhibits the most superior catalytic activity with a UL of -0.5 V and favorable NRR selectivity over the HER. Our results provide mechanistic insights into the role of ordered metal vacancies in transition-metal boride for the NRR and highlight a novel avenue toward the rational design of superior NRR catalysts.

An MBene Modulating the Buried SnO2/Perovskite Interface in Perovskite Solar Cells

The interface of perovskite solar cells (PSCs) plays an important role in transferring and collecting charges. Interface defects are important factors affecting the efficiency and stability of PSCs. Here, the buried interface between SnO2 and the perovskite layer is bridged by two-dimensional (2D) MBene, which improves charge transfer. MBene can deposit additional electrons on the surface of SnO2, passivate its surface defects and facilitate the charge collection. Moreover, the dipole moment formed at the interface increases the electron transfer ability in the PSCs. MBene also regulates the growth of perovskite crystals, improves the quality of perovskite films, and reduces its grain boundary defects. As a result, PSCs based on FA0.2MA0.8PbI3 and (FAPbI3)0.95(MAPbBr3)0.05 get the enhanced efficiencies of 22.34 % and 24.32 % with negligible hysteresis. Furthermore, the optimized device exhibits better stability. This work opens up the application of MBene materials in PSCs, reveals a deeper understanding of the mechanism behind using 2D materials as an interface modification layer, and shows opportunities for using MBene as potential material in photoelectric devices.

Multilayer Fluorine-Free MoBTx MBene with Hydrophilic Structural-Modulating for the Fabrication of a Low-Resistance and High-Resolution Humidity Sensor

2D transition metal borides (MBenes) with abundant surface terminals hold great promise in molecular sensing applications. However, MBenes from etching with fluorine-containing reagents present inert -fluorine groups on the surface, which hinders their sensing capability. Herein, the multilayer fluorine-free MoBTx MBene (where Tx represents O, OH, and Cl) with hydrophilic structure is prepared by a hydrothermal-assisted hydrochloric acid etching strategy based on guidance from the first-principle calculations. Significantly, the fluorine-free MoBTx-based humidity sensor is fabricated and demonstrates low resistance and excellent humidity performance, achieving a response of 90% to 98%RH and a high resolution of 1%RH at room temperature. By combining the experimental results with the first-principles calculations, the interactions between MoBTx and H2O, including the adsorption and intercalation of H2O, are understood first in depth. Finally, the portable humidity early warning system for real-time monitoring and early warning of infant enuresis and back sweating illustrates its potential for humidity sensing applications. This work not only provides guidance for preparation of fluorine-free MBenes, but also contributes to advancing their exploration in sensing applications.

Nanometabolomics Elucidated Biological Prospective of Mo4/3B2-x Nanosheets: Toward Metabolic Reprogramming of Amino Acid Metabolism

Mo4/3B2-x nanosheets are newly developed, and 2D transition metal borides (MBene) were reported in 2021, but there is no report on their further applications and modification; hence, this article sheds light on the significance of potential biological prospects for future biomedical applications. Therefore, elucidation of the biocompatibility, biotoxicology, and bioactivity of Mo4/3B2-x nanosheets has been an urgent need to be fulfilled. Nanometabolomics (also referred as nanomaterials-based metabolomics) was first proposed and utilized in our previous work, which specialized in interpreting nanomaterials-induced metabolic reprogramming through aqueous metabolomics and lipidomics approach. Hence, nanometabolomics could be considered as a novel concept combining nanoscience and metabolomics to provide bioinformation on nanomaterials' biomedical applications. In this work, the safe range of concentration (<50 mg/L) with good biosafety toward human umbilical vein endothelial cells (HUVECs) was discovered. The low concentration (5 mg/L) and high concentration (50 mg/L) of Mo4/3B2-x nanosheets were utilized for the in vitro Mo4/3B2-x-cell interaction. Nanometabolomics has elucidated the biological prospective of Mo4/3B2-x nanosheets via monitoring its biocompatibility and metabolic shift of HUVECs. The results revealed that 50 mg/L Mo4/3B2-x nanosheets could lead to a stronger alteration of amino acid metabolism with disturbance of the corresponding amino acid-related pathways (including amino acid metabolism, amino acid degradation, fatty acid biosynthesis, and lipid biosynthesis and metabolism). These interesting results were closely involved with the oxidative stress and production of excess ROS. This work could be regarded as a pathbreaking study on Mo4/3B2-x nanosheets at a biological level, which also designates their further biochemical, medical, and industrial application and development based on nanometabolomics bioinformation.

Recent Developments and Future Perspectives of Molybdenum Borides and MBenes

Metal borides have received a lot of attention recently as a potentially useful material for a wide range of applications. In particular, molybdenum-based borides and MBenes are of great significance, due to their remarkable properties like good electronic conductivity, considerable stability, high surface area, and environmental harmlessness. Therefore, in this article, the progress made in molybdenum-based borides and MBenes in recent years is reviewed. The first step in understanding these materials is to begin with an overview of their structural and electronic properties. Then synthetic technologies for the production of molybdenum borides, such as high-temperature/pressure methods, physical vapor deposition (PVD), chemical vapor deposition (CVD), element reaction route, molten salt-assisted, and selective etching methods are surveyed. Then, the critical performance of these materials in numerous applications like energy storage, catalysis, biosensors, biomedical devices, surface-enhanced Raman spectroscopy (SERS), and tribology and lubrication are summarized. The review concludes with an analysis of the current progress of these materials and provides perspectives for future research. Overall, this review will offer an insightful reference for the understanding molybdenum-based borides and their development in the future.

Defect Engineering of Hexagonal MAB Phase Ti2InB2 as Anode of Lithium-Ion Battery with Excellent Cycling Stability

Hexagonal MAB phases （h-MAB） have attracted attention due to their potential to exfoliate into MBenes, similar to MXenes, which are predicted to be promising for Li-ion battery applications. However, the high cost of synthesizing MBenes poses challenges for their use in batteries. This study presents a novel approach where a simple ball-milling treatment is employed to enhance the purity of the h-MAB phase Ti2InB2 and introduce significant indium defects, resulting in improved conductivity and the creation of abundant active sites. The synthesized Ti2InB2 with indium defects (VIn-Ti2InB2) exhibits excellent electrochemical properties, particularly exceptional long-cycle stability at current densities of 5 A g-1 (5000 cycles, average capacity decay of 0.0018%) and 10 A g-1 (15 000 cycles, average capacity decay of 0.093%). The charge storage mechanism of VIn-Ti2InB2, involving a dual redox reaction, is proposed, where defects promote the In-Li alloy reaction and a redox reaction with Li in the TiB layer. Finally, a Li-ion full cell demonstrates cycling stability at 0.5 A g-1 after 350 cycles. This work presents the first accessible and scalable application of VIn-Ti2InB2 as a Li-ion anode, unlocking a wealth of possibilities for sustainable electrochemical applications of h-MAB phases.

Two-dimensional Mo1-xB2 with ordered metal vacancies obtained for advanced thermoelectric applications based on first-principles calculations

The study and development of high thermoelectric properties is crucial for the next generation of microelectronic and wearable electronics. Derived from the recent experimental realization of layers of transition metal molybdenum and boride, we report the theoretical realization of advanced thermoelectric properties in two-dimensional (2D) transition metal boride Mo1-xB2 (x = 0, 0.05, 0.10, 0.125, 0.15)-based defect sheets. The introduction of metal vacancies results in stronger d-p exchange interactions and hybridization between the Mo-d and B-p atoms. Meanwhile, the ordered metal vacancies enabled transition metal borides (n-type Mo0.9B2) to widen the d-bandwidth and raise the d-band center, leading to a relatively high carrier mobility of 3262 cm2 V-1 s-1 and conductivity twice that of a bug-free n-type MoB2 layer, which indicates that it presents good electronic and thermal transport properties. Furthermore, investigations of the thermoelectric performance exhibit a maximum ZT of up to 3.29, which is superior to those of currently reported 2D materials. Modulation by defect engineering suggests that 2D transition metal boride sheets with ordered metal vacancies have promising applications in microelectronics, wearable electronics and thermoelectric devices.

MBenes for Energy Conversion: Advances, Bottlenecks, and Prospects

The advent of two-dimensional layered materials has bolstered the development of catalytic endeavors for energy conversion and storage. MXenes (transition metal carbides/nitrides) have already consolidated their candidature in the past decade due to their enhanced compositional and structural tunabilities through surface modifications. Perseverant research in engineering MXene based materials has led to the inception of MBenes (transition metal borides) as promising catalytic systems for energy-driven operations. Physicochemical superiorities of MBenes such as escalated conductivity and hydrophilicity, unique surface and geometrical domains, and higher stability and modulus of elasticity provide the reaction-friendly milieu to exploit these materials. Nevertheless, the research on MBenes is embryonic and requires the thorough realization of their scientific significance. Herein, we aim to discuss the advancements, challenges, and outlooks of MBenes with respect to their energy conversion HER, CO2RR, and NRR applications.

2D MoB MBene: An Efficient Co-Catalyst for Photocatalytic Hydrogen Production under Visible Light

Highly active and low-cost co-catalysts have a positive effect on the enhancement of solar H2 production. Here, we employ two-dimensional (2D) MBene as a noble-metal-free co-catalyst to boost semiconductor for photocatalytic H2 production. MoB MBene is a 2D nanoboride, which is directly made from MoAlB by a facile hydrothermal etching and manual scraping off process. The as-synthesized MoB MBene with purity >95 wt % is treated by ultrasonic cell pulverization to obtain ultrathin 2D MoB MBene nanosheets (∼0.61 nm) and integrated with CdS via an electrostatic interaction strategy. The CdS/MoB composites exhibit an ultrahigh photocatalytic H2 production activity of 16,892 μmol g-1 h-1 under visible light, surpassing that of pure CdS by an exciting factor of ≈1135%. Theoretical calculations and various measurements account for the high performance in terms of Gibbs free energy, work functions, and photoelectrochemical properties. This work discovers the huge potential of these promising 2D MBene family materials as high-efficiency and low-cost co-catalysts for photocatalytic H2 production.

In-Plane Chemical Ordering (Mo2/3R1/3)2AlB2 (R = Tb, Dy, Ho, Er, Tm, and Lu) i-MAB Phases and their Two-Dimensional Derivatives (MBene): Synthesis, Structure, Magnetic, and Supercapacitor Performance

A family of hexagonal in-plane chemical ordering (Mo2/3R1/3)2AlB2 (R = Tb, Dy, Ho, Er, Tm, and Lu) i-MAB phases are synthesized with R-3m hexagonal structure. The i-MAB phases with R = Tb to Tm are considered to have a nonlinear ferromagnetic-like coupling magnetic ground state with gradually weakened magnetocrystalline anisotropy due to variant R-R distances and 4f electrons. Their 2D derivatives (2D-MBene) with rare-earth (R) atom vacancies are obtained by chemical etching. The delamination solvent, surface functional terminations, and chemical bond of 2D-MBene can be modified by one-step nitridation in environment-friendly nitrogen instead of ammonia. A phase conversion is caused by nitridation at 973 K from 2D-MBene to Mo2N, leading to the optimized specific capacitance of 229 F g-1. Besides exploring more rare-earth-containing laminated boride systems, this work also demonstrates the promising application of their 2D derivatives with R vacancies in supercapacitors.

Functionalized MBenes as promising anode materials for high-performance alkali-ion batteries: a first-principles study

The discovery of novel electrode materials based on two-dimensional (2D) structures is critical for alkali metal-ion batteries. Herein, we performed first-principles computations to investigate functionalized MXenes, Mo2BT2(T = O, S), which are also regarded as B-based MXenes, or named as MBenes, as potential anode materials for Li-ion batteries and beyond. The pristine and T-terminated Mo2BT2(T = O, S) monolayers reveal metallic character with higher electronic conductivity and are thermodynamically stable with an intrinsic dipole moment. Both Mo2BO2and Mo2BS2monolayers exhibit high theoretical Li/Na/K storage capacity and low ion diffusion barriers. These findings suggest that functionalized Mo2BT2(T = O, S) monolayers are promising for designing viable anode materials for high-performance alkali-ion batteries.

Construction of MoB@LDH heterojunction and its derivates through phase and interface engineering for advanced supercapacitor applications

Layered double hydroxide (LDH) has been attracted widespread attention in supercapacitor due to their unique layered structure and associated advantages. However, the inherent limitations of low electrical conductivity and reaction kinetics rate of LDH restrict its widespread application. Various modification techniques, such as heterojunction formation, phosphorization and introduction of phosphorus vacancies, are employed to modify LDH with the goal of improving the electrochemical performance. Preparation of composite materials using MoB MBene as conductive template and phosphorization are the effective ways for enhancing the electrical conductivity of electrode materials. MoB MBene is prepared using a modified method that combines NaOH etching and a high-temperature hydrothermal process. The presence of phosphorus vacancy is beneficial for enhancing the kinetics rate during electrode reactions. Through the synergistic effect of various modification methods, MP2 demonstrates an optimal electrochemical performance with a superior specific capacitance of 1731.19F/g (238.28 mAh g-1) at 1 A/g. It also demonstrates an impressive rate capacity of 81.28 % at 10 A/g and maintains a satisfactory capacitance retention of 88.14 % after 5000 cycles. In addition, a fabricated MP2//AC ASC device achieves an impressive energy density of 39.91 Wh kg-1 at the power density of 948.25 W kg-1 and demonstrates satisfactory cycling stability of 78.76 % after 5000 cycles. This work presents a comprehensive framework for analyzing the impact of material structure, components, and crystal phases on energy storage performance. It also examines the regulatory impact of different modification methods on energy storage mechanisms.

Nitrogen-Anchored Boridene Enables Mg-CO2 Batteries with High Reversibility

Nanoscale defect engineering plays a crucial role in incorporating extraordinary catalytic properties in two-dimensional materials by varying the surface groups or site interactions. Herein, we synthesized high-loaded nitrogen-doped Boridene (N-Boridene (Mo4/3(BnN1-n)2-mTz), N-doped concentration up to 26.78 at %) nanosheets by chemical exfoliation followed by cyanamide intercalation. Three different nitrogen sites are observed in N-Boridene, wherein the site of boron vacancy substitution mainly accounts for its high chemical activity. Attractively, as a cathode for Mg-CO2 batteries, it delivers a long-term lifetime (305 cycles), high-energy efficiency (93.6%), and ultralow overpotential (∼0.09 V) at a high current of 200 mA g-1, which overwhelms all Mg-CO2 batteries reported so far. Experimental and computational studies suggest that N-Boridene can remarkably change the adsorption energy of the reaction products and lower the energy barrier of the rate-determining step (*MgCO2 → *MgCO3·xH2O), resulting in the rapid reversible formation/decomposition of new MgCO3·5H2O products. The surging Boridene materials with defects provide substantial opportunities to develop other heterogeneous catalysts for efficient capture and converting of CO2.

MBene with Redox-Active Terminal Groups for an Energy-Dense Cascade Aqueous Battery

Two-dimensional (2D) transition metal borides (MBenes), new members of the 2D materials family, hold great promise for use in the electrocatalytic and energy storage fields because of their high specific area, high chemical activity, and fast charge carrier mobility. Although various types of MBenes are reported, layered MBenes featuring redox-active terminal groups for high energy output are not yet produced. A facile and energy-efficient method for synthesizing MBenes equipped with redox-active terminal groups for cascade Zn||I2 batteries is presented. Layered MBenes have ordered metal vacancies and ─Br terminal groups, enabling the sequential reactions of I-/I0 and Br-/Br0. The I2-hosting MBene-Br cathode results in a specific energy as high as 485.8 Wh kg-1 at 899.7 W kg-1 and a specific power as high as 6007.7 W kg-1 at 180.2 Wh kg-1, far exceeding the best records for Zn||I2 batteries. The results of this study demonstrate that the challenges of MBene synthesis can be overcome and reveal an efficient path for producing high-performance redox-active electrode materials for energy-dense cascade aqueous batteries.

Two-Dimensional Molybdenum Boride (MBene) Mo4/3B2Tx with Broadband and Termination-Dependent Ultrafast Nonlinear Optical Response

Two-dimensional molybdenum borides (MBenes) comprise a new class of 2D transition metal borides that exhibit potential photonics applications. Recently, the synthesis of individual single-layer Mo4/3B2Tx (T = O, F, OH) MBene sheets has been realized, which attracted considerable attention in optoelectronics. However, there is still a lack of understanding and regulation of the photophysical processes of Mo4/3B2Tx MBene. Here, we demonstrate that Mo4/3B2Tx MBene exhibits a surface termination-dependent electronic structure, carrier dynamics, and nonlinear optical response over a wide wavelength range (500-1550 nm). As prepared 2D Mo4/3B2F2 MBene possesses a semimetal material property that exhibits a shorter intraband scattering process (<100 ps) and a considerable nonlinear optical response at a broadband cover optical communication C band at 1550 nm. These thrilling results are confirmed theoretically and experimentally. The analysis of these results adds to the regulating and understanding of the basic photophysical processes, which is anticipated to be beneficial for the further design of MBene-based photonics and nanoelectronics devices.

Prediction of M3 B4 -type MBenes as Promising Catalysts for CO2 Capture and Reduction

The rational design of novel catalysts with high activity and selectivity for carbon dioxide reduction reaction (CO2 RR) is highly desired. In this work, we have extensive investigations on the properties of two-dimensional transition metal borides (MBenes) to achieve efficient CO2 capture and reduction through first-principles calculations. The results show that all the investigated M3 B4 -type MBene exhibit remarkable CO2 capture and activation abilities, which proved to be derived from the lone pair of electrons on the MBene surface. Then, we emphasize that the investigated MBenes can further selectively reduce activated CO2 to CH4 . Moreover, a new linear scaling relationship of the adsorption energies of potential-determining intermediates (*OCH2 O and *HOCH2 O) versus ΔG(*OCHO) has been established, where the CO2 RR limiting potentials on MBenes are determined by the different fitting slopes of ΔG(*OCH2 O) and ΔG(*HOCHO), allowing significantly lower limiting potentials to be achieved compared to transition metals. Especially, two promising CO2 RR catalysts (Mo3 B4 and Cr3 B4 MBene) exist quite low limiting potentials of -0.48 V and -0.66 V, as well as competitive selectivity concerning hydrogen evolution reactions have been identified. Our research results make future advances in CO2 capture by MBenes easier and exploit the applications of Mo3 B4 and Cr3 B4 MBenes as novel CO2 RR catalysts.

Recent advancement and key opportunities of MXenes for electrocatalysis

MXenes are promising materials for electrocatalysis due to their excellent metallic conductivity, hydrophilicity, high specific surface area, and excellent electrochemical properties. Herein, we summarize the recent advancement of MXene-based materials for electrocatalysis and highlight their key challenges and opportunities. In particular, this review emphasizes on the major design principles of MXene-based electrocatalysts, including (1) coupling MXene with active materials or heteroatomic doping to create highly active synergistic catalyst sites; (2) construction of 3D MXene structure or introducing interlayer spacers to increase active areas and form fast mass-charge transfer channel; and (3) protecting edge of MXene or in situ transforming the surface of MXene to stable active substance that inhibits the oxidation of MXene and then enhances the stability. Consequently, MXene-based materials exhibit outstanding performance for a variety of electrocatalytic reactions. Finally, the key challenges and promising prospects of the practical applications of MXene-based electrocatalysts are briefly proposed.

h-MBenes: Promising Two-Dimensional Material Family for Room-Temperature Antiferromagnetic and Hydrogen Evolution Reaction Applications

Recently, a new class of two-dimensional (2D) hexagonal transition-metal borides (h-MBenes) was discovered through a combination of ab initio predictions and experimental studies. These h-MBenes are derived from ternary hexagonal MAB (h-MAB) phases and have demonstrated promising potential for practical applications. In this study, we conducted first-principles calculations on 15 h-MBenes and identified four antiferromagnetic metals and 11 electrocatalysts for the hydrogen evolution reaction (HER). Notably, the h-MnB material exhibited a remarkable Néel temperature of 340 K and a high magnetic anisotropy energy of 154 μeV/atom. Additionally, the hydrogen adsorption Gibbs free energies (ΔGH*) for h-ZrBO, h-MoBO, and h-Nb2BO2 are close to the ideal value of 0 eV, indicating their potential as electrochemical catalysts for HER. Further investigations revealed that the electronic structure, Néel temperature, and HER activity of the studied h-MBenes can be tuned by applying biaxial strains. These findings suggest that h-MBenes have wide-ranging applicability in areas such as antiferromagnetic spintronics, flexible electronic devices, and electrocatalysis, thereby expanding the potential applications of 2D transition-metal borides.

Two-Dimensional Transition Metal Boron Cluster Compounds (MBnenes) with Strain-Independent Room-Temperature Magnetism

Two-dimensional (2D) metal borides (MBenes) with unique electronic structures and physicochemical properties hold great promise for various applications. Given the abundance of boron clusters, we proposed employing them as structural motifs to design 2D transition metal boron cluster compounds (MBnenes), an extension of MBenes. Herein, we have designed three stable MBnenes (M4(B12)2, M = Mn, Fe, Co) based on B12 clusters and investigated their electronic and magnetic properties using first-principles calculations. Mn4(B12)2 and Co4(B12)2 are semiconductors, while Fe4(B12)2 exhibits metallic behavior. The unique structure in MBnenes allows the coexistence of direct exchange interactions between adjacent metal atoms and indirect exchange interactions mediated by the clusters, endowing them with a Néel temperature (TN) up to 772 K. Moreover, both Mn4(B12)2 and Fe4(B12)2 showcase strain-independent room-temperature magnetism, making them potential candidates for spintronics applications. The MBnenes family provides a fresh avenue for the design of 2D materials featuring unique structures and excellent physicochemical properties.

Villin headpiece unfolding upon binding to boridene mediated by the "anchoring-perturbation" mechanism

We employ molecular dynamics (MD) simulations to investigate the influence of boridene on the behavior of a protein model, HP35, with the aim of assessing the potential biotoxicity of boridene. Our MD results reveal that HP35 can undergo unfolding via an "anchoring-perturbation" mechanism upon adsorption onto the boridene surface. Specifically, the third helix of HP35 becomes tightly anchored to the boridene surface through strong electrostatic interactions between the abundant molybdenum atoms on the boridene surface and the oxygen atoms on the HP35 backbone. Meanwhile, the first helix, experiencing continuous perturbation from the surrounding water solution over an extended period, suffers from potential breakage of hydrogen bonds, ultimately resulting in its unfolding. Our findings not only propose, for the first time to our knowledge, the "anchoring-perturbation" mechanism as a guiding principle for protein unfolding but also reveal the potential toxicity of boridene on protein structures.

Co3X8 (X = Cl and Br): multiple phases and magnetic properties of the Kagome lattice

The unique magnetic properties of two-dimensional (2D) materials have demonstrated huge potential for applications in nanodevices and spintronics. In this work, we propose a new Kagome lattice, Co3X8 (X = Cl and Br), based on density functional theory (DFT) calculation. We find that Co/X in Co3X8 has spontaneous movement in the lattice, resulting in 156- and 12-phases of Co3X8 and diverse magnetic and electronic properties. We show that the magnetic and electronic properties of Co3X8 can be engineered by strain, and the magnetic properties of Co3X8 are highly related to the spontaneous movement of X. Moreover, the transmission property of 12-Co3X8 shows clear angle-dependent features due to the symmetry breaking as caused by the spontaneous movement of X. Our findings may provide not only a possible Kagome lattice with unique properties, but also a strategy for designing nanodevices and for spintronics.

Improved ion adsorption capacities and diffusion dynamics in surface anchored MoS2⊥Mo4/3B2 and MoS2⊥Mo4/3B2O2 heterostructures as anodes for alkaline metal-ion batteries

First-principles calculations were performed to analyze the atomic structures and electrochemical energy storage properties of novel MoS2⊥boridene heterostructures by anchoring MoS2 nanoflakes on Mo4/3B2 and Mo4/3B2O2 monolayers. Both thermodynamic and thermal stabilities of each heterostructure were thoroughly evaluated from the obtained binding energies and through first-principles molecular dynamics simulations at room temperature, confirming the high formability of the heterostructures. The electrochemical properties of MoS2⊥Mo4/3B2 and MoS2⊥Mo4/3B2O2 heterostructures were investigated for their potential use as anodes for alkaline metal ion batteries (Li+, Na+ and K+). It was revealed that Li+ and Na+ can form multiple stable full adsorption layers on both heterostructures, while K+ forms only a single full adsorption layer. The presence of a negative electron cloud (NEC) contributes to the stabilization of a multi-layer adsorption mechanism. For all investigated alkaline metal ions, the predicted ion diffusion dynamics are relatively sluggish for the adsorbates in the first full adsorption layer on MoS2⊥boridene heterostructures due the relatively large migration energies (>0.50 eV), compared to those of second or third full adsorption layers (<0.30 eV). MoS2⊥Mo4/3B2O2 exhibited higher onset and mean open circuit voltages as anodes for alkaline metal-ion batteries than MoS2⊥Mo4/3B2 hybrids because of enhanced interactions between the adsorbate and the Mo4/3B2O2 monolayer with the presence of O-terminations. Tailoring the size and horizontal spacing between two neighboring MoS2 nano-flakes in heterostructures led to high theoretical capacities for LIBs (531 mA h g-1), SIBs (300 mA h g-1) and PIBs (131 mA h g-1) in the current study.

Theoretical prediction on the stability, elastic, electronic and optical properties of MAB-phase M4AlB4 compounds (M = Cr, Mo, W)

This research employs first-principles calculations to address the challenges presented by processing complexity and low damage tolerance in transition metal borides. The study focuses on designing and investigating MAB phase compounds of M4AlB4 (M = Cr, Mo, W). We conduct a comprehensive assessment of the stability, phononic, electronic, elastic, and optical properties of Cr4AlB4, Mo4AlB4, and W4AlB4. The calculated results reveal formation enthalpies of -0.516, -0.490, and -0.336 eV per atom for Cr4AlB4, Mo4AlB4, and W4AlB4, respectively. Notably, W4AlB4 emerges as a promising precursor material for MABene synthesis, demonstrating exceptional thermal shock resistance. The dielectric constants ε1(0) were determined as 126.466, 80.277, and 136.267 for Cr4AlB4, Mo4AlB4, and W4AlB4, respectively. Significantly, W4AlB4 exhibits remarkably high reflectivity (>80%) within the wavelength range of 19.84-23.6 nm, making it an ideal candidate for extreme ultraviolet (EUV) reflective coatings. The insights gleaned from this study provide a strong research framework and theoretical guidance for advancing the synthesis of innovative MAB-phase compounds.

Breaking New Ground: MBene Route toward Selective Vinyl Double Bond Hydrogenation in Nitroarenes

Doping, or incremental substitution of one element for another, is an effective way to tailor a compound's structure as well as its physical and chemical properties. Herein, we replaced up to 30% of Ni with Co in members of the family of layered LiNiB compounds, stabilizing the high-temperature polymorph of LiNiB while the room-temperature polymorph does not form. By studying this layered boride with in situ high-temperature powder diffraction, we obtained a distorted variant of LiNi0.7Co0.3B featuring a perfect interlayer placement of [Ni0.7Co0.3B] layers on top of each other─a structural motif not seen before in other borides. Because of the Co doping, LiNi0.7Co0.3B can undergo a nearly complete topochemical Li deintercalation under ambient conditions, resulting in a metastable boride with the formula Li0.04Ni0.7Co0.3B. Heating of Li0.04Ni0.7Co0.3B in anaerobic conditions led to yet another metastable boride, Li0.01Ni0.7Co0.3B, with a CoB-type crystal structure that cannot be obtained by simple annealing of Ni, Co, and B. No significant alterations of magnetic properties were detected upon Co-doping in the temperature-independent paramagnet LiNi0.7Co0.3B or its Li-deintercalated counterparts. Finally, Li0.01Ni0.7Co0.3B stands out as an exceptional catalyst for the selective hydrogenation of the vinyl C═C bond in 3-nitrostyrene, even in the presence of other competing functional groups. This research showcases an innovative approach to heterogeneous catalyst design by meticulously synthesizing metastable compounds.

Establishing theoretical landscapes for identifying basal plane active sites in MBene toward multifunctional HER, OER, and ORR catalysts

Exploring multifunctional electrocatalysts to realize efficient hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) is urgently desired for developing novel renewable energy storage and conversion technologies. However, integrating these three merits in one single catalyst remains a big challenge due to the difficulty in balancing the adsorption strengths of multiple reaction intermediates. Herein, through first-principles calculations, we systematically investigated the electrocatalytic activity of M2B2, M3B4, and M4B6 type MBenes (M = Cr, Mn, Fe, Co, and Ni) for multifunctional HER, OER, and ORR. The results indicate that most of the investigated MBenes show outstanding catalytic activity for HER with hydrogen adsorption Gibbs free energy close to the optimal value (0 eV). Thereinto, Ni2B2 and Co3B4 MBenes can be promising multifunctional HER/OER/ORR electrocatalysts, and Fe3B4 MBene is expected to be a promising bifunctional electrocatalyst for HER/ORR. Especially, Ni2B2 MBene is even better than the benchmark RuO2 catalyst with ultralow low overpotentials of 0.26 and 0.30 V for OER and ORR, respectively. Then, we proposed that the overpotentials of OER/ORR can be well described by the varied ΔGOH* on MBene, which has been further illuminated through the d-band center and charge transfer analysis. Importantly, new scaling relations between the adsorption energies of OOH* and O* on MBenes have been established, where ΔGOOH* and ΔGO* possess different slopes versus ΔGOH*, allowing the significantly lower overpotentials of OER and ORR to be achieved. This work provides not only promising multifunctional HER/OER/ORR electrocatalysts but also new scaling relations to achieve the rational design of MBene-based electrocatalysts.

Atomic Scale Design of MXenes and Their Parent Materials─From Theoretical and Experimental Perspectives

More than a decade after the discovery of MXene, there has been a remarkable increase in research on synthesis, characterization, and applications of this growing family of two-dimensional (2D) carbides and nitrides. Today, these materials include one, two, or more transition metals arranged in chemically ordered or disordered structures of three, five, seven, or nine atomic layers, with a surface chemistry characterized by surface terminations. By combining M, X, and various surface terminations, it appears that a virtually endless number of MXenes is possible. However, for the design and discovery of structures and compositions beyond current MXenes, one needs suitable (stable) precursors, an assessment of viable pathways for 3D to 2D conversion, and utilization or development of corresponding synthesis techniques. Here, we present a critical and forward-looking review of the field of atomic scale design and synthesis of MXenes and their parent materials. We discuss theoretical methods for predicting MXene precursors and for assessing whether they are chemically exfoliable. We also summarize current experimental methods for realizing the predicted materials, listing all verified MXenes to date, and outline research directions that will improve the fundamental understanding of MXene processing, enabling atomic scale design of future 2D materials, for emerging technologies.

Soft Chemistry of Hard Materials: Low-Temperature Pathways to Bulk and Nanostructured Layered Metal Borides

Conspectus"Synthesis by design" is often considered to be the primary goal of chemists who make molecules and materials. Synthetic chemists usually have in mind a target they want to make, and they want to be able to design a pathway that can get them to that target as quickly and efficiently as possible. Chemists who synthesize refractory solids, which have melting points above 1000 °C and are often chemically inert at these high temperatures, have access to only a small number of synthetic strategies due to the need to overcome solid-state diffusion, which is the rate-limiting step in such reactions. The use of extremely high temperatures to facilitate diffusion among two or more refractory solids, which precedes any chemical reaction that must occur, generally drives the system to form only the product that is the most thermodynamically stable-the global minimum on an energy landscape-for a certain combination of elements. When trying to target a different product in the same system, one generally cannot rely on thermally driven reactions. Lower-temperature reactions that side step this diffusion limitation can succeed where high temperatures fail by providing access to local minima on an energy landscape. These local minima represent metastable phases that are primed for synthesis, but only if an appropriate pathway and set of reactions can be identified. It is therefore important to develop and understand low-temperature, or "soft", chemical reactions in "hard" refractory systems. These reactions allow us to apply the retrosynthetic framework that molecular chemists rely on to systems where chemists have not previously had such control over reactions, reactivities, and metastable product formation.In this Account, we discuss the development of soft chemical reactions of hard materials in the context of a class of layered, refractory metal borides that are precursors to an emerging family of two-dimensional nanomaterials. Layered ternary metal boride phases such as MoAlB have layers of metal borides, which are chemically unreactive, interleaved with layers of aluminum, which are reactive. Some of the interlayer aluminum can be deintercalated at room temperature in dilute aqueous sodium hydroxide, transforming stable MoAlB into destabilized MoAl1-xB. Mild thermal treatment of submicrometer grains of this destabilized MoAl1-xB sample allows it to traverse the energy landscape and crystallize as Mo2AlB2, a metastable compound. Further thermal treatment transforms Mo2AlB2 into a Mo2AlB2-alumina nanolaminate and ultimately mesoporous MoB, all through continued traversing of the energy landscape using mild chemical and thermal treatments. Similar topochemical manipulations, which maintain structure but change composition, are emerging for other MAB phases and are opening the door to new types of metastable compounds and nanostructured materials in traditionally refractory systems.

Stability and superconductivity of freestanding two-dimensional transition metal boridene: M4/3B2

The small atomic mass of boron indicates strong electron-phonon coupling (EPC), so it may have a brilliant performance in superconductivity. Recently, a new 2D boride sheet with ordered metal vacancies and surface terminals (Mo4/3B2-x) was realized in experiments (Zhouet al2021Science373801). Here, the 2D monolayer freestanding Mo4/3B2is evidenced to be thermodynamically stable. Through electronic structure, phonon spectrum and EPC, monolayer Mo4/3B2is found to be an intrinsic phonon-mediated superconductor. The superconducting transition temperature (Tc) is determined to be 4.06 K by the McMillian-Allen-Dynes formula. Remarkably, theTcof monolayer Mo4/3B2can be increased to 6.78 K with an appropriate biaxial tensile strain (+5%). Moreover, we predict that other transition metal replacing Mo atoms is also stable and retaining the superconductivity. Such as monolayer W4/3B2is also a superconductor with theTcof 2.37 K. Our research results enrich the database of 2D monolayer superconductors and boron-related formed materials science.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Optically Active MXenes in Van der Waals Heterostructures

The vertical integration of distinct 2D materials in van der Waals (vdW) heterostructures provides the opportunity for interface engineering and modulation of electronic as well as optical properties. However, scarce experimental studies reveal many challenges for vdW heterostructures, hampering the fine-tuning of their electronic and optical functionalities. Optically active MXenes, the most recent member of the 2D family, with excellent hydrophilicity, rich surface chemistry, and intriguing optical properties, are a novel 2D platform for optoelectronics applications. Coupling MXenes with various 2D materials into vdW heterostructures can open new avenues for the exploration of physical phenomena of novel quantum-confined nanostructures and devices. Therefore, the fundamental basis and recent findings in vertical vdW heterostructures composed of MXenes as a primary component and other 2D materials as secondary components are examined. Their robust designs and synthesis approaches that can push the boundaries of light-harvesting, transition, and utilization are discussed, since MXenes provide a unique playground for pursuing an extraordinary optical response or unusual light conversion features/functionalities. The recent findings are finally summarized, and a perspective for the future development of next-generation vdW multifunctional materials enriched by MXenes is provided.

Discovery of Two-dimensional Hexagonal MBene HfBO and Exploration on its Potential for Lithium-Ion Storage

The practical applications of two-dimensional (2D) transition-metal borides (MBenes) have been severely hindered by the lack of accessible MBenes because of the difficulties in the selective etching of traditional ternary MAB phases with orthorhombic symmetry (ort-MAB). Here, we discover a family of ternary hexagonal MAB (h-MAB) phases and 2D hexagonal MBenes (h-MBenes) by ab initio predictions and experiments. Calculations suggest that the ternary h-MAB phases are more suitable precursors for MBenes than the ort-MAB phases. Based on the prediction, we report the experimental synthesis of h-MBene HfBO by selective removal of In from h-MAB Hf2 InB2 . The synthesized 2D HfBO delivered a specific capacity of 420 mAh g-1 as an anode material in lithium-ion batteries, demonstrating the potential for energy-storage applications. The discovery of this h-MBene HfBO added a new member to the growing family of 2D materials and provided opportunities for a wide range of novel applications.

Extending the Chemistry of Layered Solids and Nanosheets: Chemistry and Structure of MAX Phases, MAB Phases and MXenes

MAX phases are layered solids with unique properties combining characteristics of ceramics and metals. MXenes are their two-dimensional siblings that can be synthesized as van der Waals-stacked and multi-/single-layer nanosheets, which possess chemical and physical properties that make them interesting for a plethora of applications. Both families of materials are highly versatile in terms of their chemical composition and theoretical studies suggest that many more members are stable and can be synthesized. This is very intriguing because new combinations of elements, and potentially new structures, can lead to further (tunable) properties. In this review, we focus on the synthesis science (including non-conventional approaches) and structure of members less investigated, namely compounds with more exotic M-, A-, and X-elements, for example nitrides and (carbo)nitrides, and the related family of MAB phases.

Toward MBenes Battery Electrode Materials: Layered Molybdenum Borides for Li-Ion Batteries

Lithium-ion and sodium-ion batteries (LIBs and SIBs) are crucial in our shift toward sustainable technologies. In this work, the potential of layered boride materials (MoAlB and Mo2 AlB2 ) as novel, high-performance electrode materials for LIBs and SIBs, is explored. It is discovered that Mo2 AlB2 shows a higher specific capacity than MoAlB when used as an electrode material for LIBs, with a specific capacity of 593 mAh g-1 achieved after 500 cycles at 200 mA g-1 . It is also found that surface redox reactions are responsible for Li storage in Mo2 AlB2 , instead of intercalation or conversion. Moreover, the sodium hydroxide treatment of MoAlB leads to a porous morphology and higher specific capacities exceeding that of pristine MoAlB. When tested in SIBs, Mo2 AlB2 exhibits a specific capacity of 150 mAh g-1 at 20 mA g-1 . These findings suggest that layered borides have potential as electrode materials for both LIBs and SIBs, and highlight the importance of surface redox reactions in Li storage mechanisms.

The single-atom catalytic activity of the hydrogen evolution reaction of the experimentally synthesized boridene 2D material: a density functional theory study

CONTEXT

Previous theoretical studies have suggested that two-dimensional (2D) MBene materials might display adequate monatomic catalytic activity for the hydrogen evolution reaction (HER). Recently, a study reported the experimental synthesis of a 2D MBene (Mo_4/3B₂), re-defined as boridene, albeit no effort has been devoted to explore the single-atom catalytic activity for HER of this experimentally synthesized 2D material. Therefore, we herein investigate the single-atom HER performance of the boridene. Interestingly, with Mo defects mixed with single Au and Zn atoms shows excellent hydrogen evolution performance, and the change in the Gibbs free energy ([Formula: see text]) value is close to 0 eV, which can even match the performance of Pt-based materials. Through analysis of the charge density difference and density of states, the mechanism affecting the HER performance is explained at the electronic level. This work provides a new direction for the use of the Mo_4/3B₂ monolayer two-dimensional materials in the field of single-atom catalysis for HER.

METHODS

This study used the DFT calculations in Vienna ab initio simulation package. The GGA-Perdew-Burke-Ernzerhof functional with DFT-D2 correction is used to describe the exchange-correlation interactions. The projection augmented wave is used with the plane wave cutoff of 500 eV. The convergences of energy and force are 10^-5 eV and 0.01 eV/Å, respectively. A vacuum layer with a height of 20 Å is set in the Z direction. For geometry optimization, self-consistent, and DOS calculations, the k-point grids sampled in Brillouin zones are 3 × 3 × 1, 9 × 9 × 1, and 9 × 9 × 1, respectively. The AIMD simulation is performed in the canonical ensemble (NVT), and the temperature was maintained at 300 K by Nosé-Hoover thermostats with a time step of 2.0 fs.

Controllable Synthesis of 2D Materials by Electrochemical Exfoliation for Energy Storage and Conversion Application

2D materials have captured much recent research interest in a broad range of areas, including electronics, biology, sensors, energy storage, and others. In particular, preparing 2D nanosheets with high quality and high yield is crucial for the important applications in energy storage and conversion. Compared with other prevailing synthetic strategies, the electrochemical exfoliation of layered starting materials is regarded as one of the most promising and convenient methods for the large-scale production of uniform 2D nanosheets. Here, recent developments in electrochemical delamination are reviewed, including protocols, categories, principles, and operating conditions. State-of-the-art methods for obtaining 2D materials with small numbers of layers-including graphene, black phosphorene, transition metal dichalcogenides and MXene-are also summarized and discussed in detail. The applications of electrochemically exfoliated 2D materials in energy storage and conversion are systematically reviewed. Drawing upon current progress, perspectives on emerging trends, existing challenges, and future research directions of electrochemical delamination are also offered.

Two-dimensional MBenes with ordered metal vacancies for surface-enhanced Raman scattering

As an emerging class of two-dimensional (2D) materials, MBenes show enormous potential for optoelectronic applications. However, their use in molecular sensing as surface-enhanced Raman scattering (SERS)-active material is unknown. Herein, for the first time, we develop a brand-new high-performance MBene SERS platform. Ordered vacancy-triggered highly sensitive SERS platform with outstanding signal uniformity based on a 2D Mo_4/3B₂ MBene material was designed. The 2D Mo_4/3B₂ MBene presented superior SERS activity to most of the semiconductor SERS substrates, showing a remarkable Raman enhancement factor of 3.88 × 10⁶ and an ultralow detection limit of 1 × 10^-9 M. The underlying SERS mechanism is revealed from systematic experiments and density functional theory calculations that the ultrahigh SERS sensitivity of 2D Mo_4/3B₂ MBene is derived from the efficient photoinduced charge transfer process between MBene substrates and adsorbed molecules. The abundant electronic density of states near the Fermi level of 2D Mo_4/3B₂ MBene enables its Raman enhancement by a factor of 100 000 times higher than that of the bulk MoB. Consequently, the 2D Mo_4/3B₂ MBene could accurately detect various trace chemical analytes. Moreover, with ordered metal vacancies in the 2D Mo_4/3B₂ MBene, uniform charge transfer sites are formed, resulting in an outstanding signal uniformity with a relative standard deviation down to 6.0%. This work opens up a new horizon for the high-performance SERS platform based on MBene materials, which holds great promise in the field of chemical sensing.

Revealing the Elusive Structure and Reactivity of Iron Boride α-FeB

Crystal structures can strongly deviate from bulk states when confined into nanodomains. These deviations may deeply affect properties and reactivity and then call for a close examination. In this work, we address the case where extended crystal defects spread through a whole solid and then yield an aperiodic structure and specific reactivity. We focus on iron boride, α-FeB, whose structure has not been elucidated yet, thus hindering the understanding of its properties. We synthesize the two known phases, α-FeB and β-FeB, in molten salts at 600 and 1100 °C, respectively. The experimental X-ray diffraction (XRD) data cannot be satisfactorily accounted for by a periodic crystal structure. We then model the compound as a stochastic assembly of layers of two structure types. Refinement of the powder XRD pattern by considering the explicit scattering interference of the different layers allows quantitative evaluation of the size of these domains and of the stacking faults between them. We, therefore, demonstrate that α-FeB is an intergrowth of nanometer-thick slabs of two structure types, β-FeB and CrB-type structures, in similar proportions. We finally discuss the implications of this novel structure on the reactivity of the material and its ability to perform insertion reactions by comparing the reactivities of α-FeB and β-FeB as reagents in the synthesis of a model layered material: Fe₂AlB₂. Using synchrotron-based in situ X-ray diffraction, we elucidate the mechanisms of the formation of Fe₂AlB₂. We highlight the higher reactivity of the intergrowth α-FeB in agreement with structural relationships.

S-functionalized 2D V2B as a promising anode material for rechargeable lithium ion batteries

The development of novel high specific capacity anode materials is urgently needed for rechargeable metal ion batteries. Herein, S-functionalized V₂B as the electrode material for Li/Na/K ion batteries are comprehensively investigated using first-principles calculations. Specifically, V₂BS₂ was verified with good electrical conductivity via band structure and density of states calculations. Phonon dispersion and ab initio molecular dynamic simulations were performed and confirmed the dynamic and thermal stability of V₂BS₂. The use of V₂BS₂ with a high theoretical specific capacity of 606 mA h g^-1 for lithium ion batteries (LIBs) due to the bilayer adsorption of Li atoms is encouraging, which is attributed to the double empty orbitals of the S atoms and small lattice mismatch (1.5%) between the Li layers and substrate. Furthermore, dendrite formation would be well prohibited and safety issues for battery operation would be ensured for V₂BS₂ as electrode materials because of the low open circuit voltage with 0.37 V. The high charge/discharge rate for LIBs is also achievable owing to the high mobility of adatoms on the surface of V₂BS₂. Our work not only finds use as a promising material for the field of energy storage, but also provides constructive design strategies for developing high performance anode materials for rechargeable metal ion batteries.

Chemical Potential-Modulated Ultrahigh-Phase-Purity Growth of Ultrathin Transition-Metal Boride Single Crystals

Two-dimensional (2D) transition-metal borides (TMBs) are especially expected to exhibit excellent performance in various fields among electricity, superconductivity, magnetism, mechanics, biotechnology, battery, and catalysis. However, the synthesis of ultrathin TMB single crystals with ultrahigh phase purity was deemed extremely challenging and has not been realized till date. That is because TMBs have the most kinds of crystal structures among inorganic compounds, which possess generous phase structures with similar formation energies compared with other transition-metal compounds, attributing to the metalloid and electron-deficient characteristics of boron. Herein, for the first time, we demonstrate a chemical potential-modulated strategy to realize the precise synthesis of various ultrahigh-phase-purity (approximately 100%) ultrathin TMB single crystals, and the precision in the phase formation energy can reach as low as 0.01 eV per atom. The ultrathin MoB2 single crystals exhibit an ultrahigh Young's modulus of 517 GPa compared to other 2D materials. Our work establishes a chemical potential-modulated strategy to synthesize ultrathin single crystals with ultrahigh phase purity, especially those with similar formation energies, and undoubtedly provides excellent platforms for their extensive research and applications.

Active Pd Catalyst for the Selective Synthesis of Methylated Amines with Methanol

Selective N-methylation of amines with methanol is an important reaction in the synthesis of high-value-added fine chemicals, including dyes, surfactants, pharmaceuticals, agrochemicals, and materials. However, N-methylated amines possess higher reactivities and are prone to further transform into N,N-dimethylated amines. Therefore, it is still a challenge to controllably regulate the selectivity of N-methylation using heterogeneous catalysts without the use of base. Herein, we developed a series of Pd/Zn(Al)O catalysts with abundant basic sites, and the selectivity of N-methylation was controlled by a heterogeneous Pd/Zn(Al)O catalyst with a Zn/Al ratio of 10 and a Pd loading of 0.4 wt % in the pressure of H₂. The experimental results showed that the appropriate basic properties of the catalyst were beneficial to form the desired N-methylated amine. The low loading of Pd in the catalyst was highly dispersed on the support, providing sufficient active sites. These were attributed to the Zn vacancies formed by Al-doped Zn, which were beneficial to form the highly active and stable Pd sites. Furthermore, a series of amines and nitrobenzenes with different functional groups were well tolerated for the selective synthesis of N-methylated amines in the absence of base.

New horizons of MBenes: highly active catalysts for the CO oxidation reaction

The search for materials with high intrinsic carbon monoxide oxidation reaction (COOR) catalytic activity is critical for enhancing the efficiency of reducing CO contamination. COOR catalysts, however, have long relied heavily on noble metals and CeO₂. Herein, in order to search for non-noble COOR catalysts that are more active than CeO₂, 18 oxygen-functionalized MBenes with orthorhombic and hexagonal crystal structures, denoted as orth-M₂B₂O₂ and hex-M₂B₂O₂ (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W), were investigated in terms of their COOR catalytic activity by high-throughput first-principles calculations. Hex-Mo₂B₂O₂, orth-Mo₂B₂O₂, hex-V₂B₂O₂ and hex-Cr₂B₂O₂ were found to be more active than CeO₂ and possess structural stability below 1000 K, showing the potential to replace CeO₂ as the substrates of COOR catalysts. Moreover, orth-Mo₂B₂O₂, hex-V₂B₂O₂ and hex-Cr₂B₂O₂ exhibit even higher COOR catalytic activity than Pt-CeO₂ and Au-CeO₂, and are expected to be applied as COOR catalysts directly. Further investigations showed that the formation energy of oxygen vacancies could be used as the descriptor of COOR catalytic activity, which would help to reduce the amount of calculations significantly during the catalyst screening process. This work not only reports a series of 2D materials with high COOR catalytic activity and opens up a new application area for MBenes, but also provides a reliable strategy for highly efficient screening for COOR catalysts.

Prediction of boridenes as high-performance anodes for alkaline metal and alkaline Earth metal ion batteries

We conducted a comprehensive density functional theory investigation using the r²SCAN-rVV10 functional on the structural stability and electrochemical properties of boridenes for their use as anode materials in rechargeable alkaline (earth) metal-ion batteries (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺). According to first-principles molecular dynamics simulations and reaction thermodynamic calculations, Mo_4/3B₂(OH)₂ and Mo_4/3B₂F₂ are unstable in the presence of alkaline (earth) metal ions due to the surface-conversion reactions between the surface terminations and adsorbates. Meanwhile, the bare Mo_4/3B₂ and Mo_4/3B₂O₂ monolayers not only can accommodate alkaline (earth) metal ions, but also form stable multi-layer adsorption structures for most of the studied metal ions (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺). The predicted gravimetric capacities of the bare Mo_4/3B₂ monolayer (Mo_4/3B₂O₂) are 625.9 mA h g^-1 (357.3 mA h g^-1), 247.20 mA h g^-1 (392.1 mA h g^-1), 101.8 mA h g^-1 (206.4 mA h g^-1), 667.0 mA h g^-1, and 413.0 mA h g^-1 (485.4 mA h g^-1) for Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺ ions, respectively. The bare Mo_4/3B₂ exhibits lower onset charging open circuit voltages for alkaline (earth) metal ions than that of Mo_4/3B₂O₂. The diffusivities of the metal ions were revealed to be high on the boridene monolayer especially for the outer fully stable adsorption layers, where the migration energy barriers were found to be less than 0.10 eV. Similar to that of MXenes, the negative electron cloud (NEC) also plays a vital role in stabilizing the observed multi-layer adsorption structures for various metal ions on either the bare Mo_4/3B₂ or Mo_4/3B₂O₂ monolayer.

A Rising 2D Star: Novel MBenes with Excellent Performance in Energy Conversion and Storage

As a flourishing member of the two-dimensional (2D) nanomaterial family, MXenes have shown great potential in various research areas. In recent years, the continued growth of interest in MXene derivatives, 2D transition metal borides (MBenes), has contributed to the emergence of this 2D material as a latecomer. Due to the excellent electrical conductivity, mechanical properties and electrical properties, thus MBenes attract more researchers' interest. Extensive experimental and theoretical studies have shown that they have exciting energy conversion and electrochemical storage potential. However, a comprehensive and systematic review of MBenes applications has not been available so far. For this reason, we present a comprehensive summary of recent advances in MBenes research. We started by summarizing the latest fabrication routes and excellent properties of MBenes. The focus will then turn to their exciting potential for energy storage and conversion. Finally, a brief summary of the challenges and opportunities for MBenes in future practical applications is presented.

Novel boron nitride MXenes as promising energy storage materials

MXenes are promising materials for rechargeable metal ion batteries and supercapacitors due to their high energy storage capacities, high electrical and ionic conductivities, and ease of synthesis. In this study, we predict the structure and properties of hitherto unexplored Ti-boron nitride MXenes (Ti₃BN and Ti₃BNT₂ where T = F, O, OH) using high-throughput density functional theory calculations. We identify multiple stable structures exhibiting high thermodynamic and mechanical stability with B and N atoms evenly dispersed in the lattice sites. The predicted properties of the BN MXenes show remarkable similarities to their carbide counterparts, including in their metallicity, elastic constants, and cation absorption properties. Significantly, these novel MXene compounds display high lithium storage capacities (>250 mA h g^-1), as well as suitability for non-lithium ion storage (Na, K, Ca, Mg), making them attractive candidates for both batteries and supercapacitors. This class of MXenes therefore merits further theoretical and experimental investigation.

M4 B6 X6 as a New Family of High-Efficient Electrocatalysts: The Role of Surface Reconstruction in Water Oxidization

Searching for highly-efficient electrocatalysts for water splitting has been greatly endowed due to the huge demand for green energy sources. Two-dimensional (2D) materials are widely explored for the purpose because of their unique physical and chemical properties, abundant active sites, and easy fabrication. Here, we present a new family of 2D M₄ B₆ X₆ (2D Boridenes) and investigate their physical and chemical properties for their potential applications into electrocatalysis based on first-principles calculations. We demonstrate that 2D M₄ B₆ X₆ (M=Cr, Mo, and W; X=O and F) are dynamically, thermodynamically, and mechanically stable, and show intriguing electronic and catalytic properties. Importantly, we find that M₄ B₆ O₆ are intrinsically active for oxygen evolution reaction (OER). Our results demonstrate that: (1) the adsorbate-escape mechanism dominates the OER process with a low overpotential of 0.652 V on Cr₄ B₆ O₆ ; (2) the partial surface-oxidization can improve the catalytic performance of M₄ B₆ F₆ dramatically; and (3) the surface reconstruction greatly affects the OER performance of M₄ B₆ X₆ . Our findings illustrate that the surface reconstruction is critical to the OER activity, which may provide a new strategy on the design of 2D materials for electrocatalysis and offer theoretical insight into the catalytic mechanism.

2D MBenes: A Novel Member in the Flatland

2D MBenes, early transition metal borides, are a very recent derivative of ternary or quaternary transition metal boride (MAB) phases and represent a new member in the flatland. Although holding great potential toward various applications, mainly theoretical knowledge about their potential properties is available. Theoretical calculations and preliminary experimental attempts demonstrate their rich chemistry, excellent reactivity, mechanical strength/stability, electrical conductivity, transition properties, and energy harvesting possibility. Compared to MXenes, MBenes' structure appears to be more complex due to multiple crystallographic arrangements, polymorphism, and structural transformations. This makes their synthesis and subsequent delamination into single flakes challenging. Overcoming this bottleneck will enable a rational control over MBenes' material-structure-property relationship. Innovations in MBenes' postprocessing approaches will allow for the design of new functional systems and devices with multipurpose functionalities thus opening a promising paradigm for the conscious design of high-performance 2D materials.