Single-Metal Atoms Supported on MBenes for Robust Electrochemical Hydrogen Evolution

Environmental and biomedical applications of 2D transition metal borides (MBenes): recent advancements

Recently, interest has surged in the environmental and biomedical applications of two-dimensional transition metal borides, commonly referred to as MBenes. These materials have emerged as promising candidates for energy storage devices, such as batteries and supercapacitors. Additionally, MBenes have shown remarkable catalytic activity due to their high surface area and tunable electronic properties. They exhibit significant promise in various catalytic applications, particularly in nitrogen reduction reactions (NRRs), electrocatalytic conversion of nitrogen oxides, and several electrochemical reactions such as the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). Notably, MBenes have shown great potential in water treatment and pollutant removal applications, such as desalination and water purification. Their high water permeability, ion selectivity, and excellent stability make them suitable for efficient water treatment processes. On the other hand, MBenes are emerging as versatile materials with significant potential in various biomedical applications, particularly in biosensing, cancer therapy, and the treatment of neurodegenerative diseases. However, several challenges hinder their practical implementation in biomedical and environmental fields. One significant issue is the scalability of synthesis methods; producing MBenes in large quantities while maintaining high purity and uniformity is often complex and costly. Moreover, the stability of MBenes and their composites under different environmental and biological conditions raises concerns, as they may undergo degradation or lose their functional properties over time, which could limit their long-term effectiveness. Additionally, there is a need for comprehensive toxicity assessments to ensure the safety of MBenes in biomedical applications, particularly when interacting with human tissues or biological systems. This review aims to systematically investigate the environmental and biomedical applications of MBenes and their composites, emphasizing their unique characteristics and potential roles in addressing pressing global challenges. Furthermore, the review will identify and discuss the existing challenges and limitations in the operational performance of MBenes and their composites, providing a critical assessment of their current state in various applications.

Novel 2D Material of MBenes: Structures, Synthesis, Properties, and Applications in Energy Conversion and Storage

2D transition metal borides (MBenes) have garnered significant attention from researchers due to their exceptional electrical conductivity, strong mechanical rigidity, excellent dynamic and thermodynamic stability, which stimulates the enthusiasm of researchers for the study of MBenes. Over the past few years, extensive research efforts have been dedicated to the study of MBenes, resulting in a growing number of synthesis methods being developed. However, there remains a scarcity of comprehensive reviews on MBenes, particularly in relation to the synthesis techniques employed. To address this gap, this review aims to provide a comprehensive summary of the latest research findings on MBenes. An exhaustive exploration of the crystal structure types of MBenes is presented, highlighting the greater structural diversity compared to MXenes. Furthermore, a comprehensive review of the recent advancements in MBenes synthesis methodologies is provided. The review also delves into the physical and chemical properties of MBenes, while elucidating their applications in the realms of energy conversion and energy storage. Lastly, this review concludes by summarizing and offering insights on MBenes from three angles: synthesis, structure-property relationships, and application prospects.

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.

Electrochemical Nitrate Reduction Catalyzed by Two-Dimensional Transition Metal Borides

We investigated 2D transition metal borides (MBenes) for the efficient conversion of nitrate to ammonia. MBenes have been previously shown to bind oxygen with extraordinary strength, which should translate toward selective adsorption of nitrate in aqueous media. Using Density Functional Theory, we screened MBenes by computing their nitrate and water adsorption energies, seeking materials with strong nitrate binding and weak water binding. We identified MnB, CrB, and VB as the best materials for selective nitrate adsorption and proceeded by computing their free energies for generating ammonia. Of the three candidates, CrB requires the lowest overpotential, making it the best candidate. To further decrease the overpotential, we doped the CrB MBene with secondary transition metals and found the addition of Mn to the active site further reduced the overpotential. We then computed the reaction mechanism grand canonically to observe the effect of applied potential on the free energy landscape.

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.

Working mechanism of MXene as the anode protection layer of aqueous zinc-ion batteries

In recent years, the research on intrinsically safe aqueous zinc-ion batteries (AZIBs) has gained significant attention. However, the commercialization of AZIBs is hindered because of the formation of dendrites in them and undesired hydrogen evolution reaction (HER) at their anode. MXene is a promising two-dimensional material that can inhibit dendrite growth and undesired HER at the anode when used as a protective layer for the anode in AZIBs. MXene's surface functional groups play a crucial role in this protective function. However, the working mechanisms of these surface functional groups have not been thoroughly understood. Based on first-principles calculations and molecular dynamics simulation, we investigated the mechanisms of MXene with nine surface functional groups, including oxygen and halogen elements, as an anode protection layer. We checked their structural stability, electronic structure, adsorption energy, HER reaction free energy, Zn2+ diffusion energy barriers, coordination number of Zn2+- H2O and diffusion coefficients of Zn2+. The MXene species with -S and -O functional groups exhibit good electrical conductivity and greatly adsorb Zn2+. Conversely, MXene species with halogen-functional groups significantly inhibit HER reactions. MXene materials with -Se functional group have the best desolvation effect (ΔCN = 0.31), while those with -I end group have the fastest ability to diffuse zinc ion. This research provides a theoretical guidance for the design of MXene based anode protection layers, which can help to develop dendrite-free and low side-reaction AZIBs.

Rational design of 2D MBene-based bifunctional OER/ORR dual-metal atom catalysts: a DFT study

Designing highly active, low-cost, and bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts is urgent for the development of metal-air batteries. Herein, by density functional theory (DFT) calculations, we systematically reported a series of dual-metal atom adsorbed novel two-dimensional (2D) MBenes as efficient bifunctional catalysts for the OER/ORR (namely 2TM/TM1TM2-Mo2B2O2, TM = Mn, Fe, Co, Ni). Our theoretical results show that 2Ni-Mo2B2O2, FeCo-Mo2B2O2 and CoNi-Mo2B2O2 exhibit outstanding OER/ORR catalytic activity with overpotentials of 0.49/0.27 V, 0.38/0.50 V and 0.25/0.51 V, respectively, exceeding those of IrO2(110) for the OER and Pt(111) for the ORR. Additionally, these highly active bifunctional catalysts can effectively suppress the hydrogen evolution reaction (HER), ensuring the absolute preference for the OER/ORR. More importantly, the Bader charge (QTM) of adsorbed dual-metal atoms is used as a descriptor of OER/ORR catalytic activity, which is linearly related to ηORR and volcanically related to -ηOER. Our work not only provides new theoretical guidance for developing noble metal-free bifunctional electrocatalysts but also enriches the application of MBenes in electrocatalysis.

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.

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.

Doping of the Mn vacancy of Mn2B2 with a single different transition metal atom as the dual-function electrocatalyst

The design of efficient electrocatalysts is essential to enhance the performance of rechargeable metal-air cells, renewable fuel cells and overall water splitting. Based on this, how to improve the catalytic activity of oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) becomes self-evident. Currently, single atom catalysts (SACs) are widely used as structural design models for the OER, ORR and HER because of the single active site and maximum metal atom utilization, but significant challenges remain. Herein, the catalytic properties of the OER, ORR and HER with a single metal atom as the active site are discussed through first-principles calculations by introducing a single metal atom in the Mn vacancy of Mn₂B₂ (TM@Mn₂B₂, TM = Au, Ag, Co, Cd, Cu, Ir, Pd, Ni, Rh, Ru and Pt). The results show that Ni@Mn₂B₂ is suitable as a dual-function electrocatalyst for the OER/ORR with overpotentials of 0.38 V and 0.37 V, which are lower than those of the OER overpotential of RuO₂/IrO₂ (0.42 V/0.56 V) and the ORR overpotential of Pt (0.45 V). Meanwhile, Pt@Mn₂B₂ is available as an OER/HER dual-function electrocatalyst for overall water splitting with a lower overpotential of OER (0.45 V) and lower |ΔG_H| (-0.15eV) under 1/4 hydrogen coverage for the HER. This work proposes a practical strategy for developing single metal atom doped MBene as a dual-function electrocatalyst.

Ir Single Atom Catalyst Loaded on Amorphous Carbon Materials with High HER Activity

The research of high efficiency water splitting catalyst is important for the development of renewable energy economy. Here, the progress in the preparation of high efficiency hydrogen evolution reaction (HER) catalyst is reported. The support material is based on a polyhexaphenylbenzene material with intrinsic holes, which heals into carbon materials upon heating. The healing process is found to be useful for anchoring various transition metal atoms, among which the supported Ir Single-atom catalyst (SAC) catalyst shows much higher electrocatalytic activity and stability than the commercial Pt/C and Ir/C in HER. There is only 17 mV overpotential at 10 mA cm-2 , which is significantly lower than that of commercial Pt/C and Ir/C catalysts respectively by 26 and 3 mV, and the catalyst has an ultra-high mass activity (MA) of 51.6 A mg Ir - 1 ${\text{ A mg}}_{{\rm{Ir}}}^{ - 1}$ at 70 mV potential and turn over frequencies (TOF) of 171.61 s-1 at the potential of 100 mV. The density functional theory (DFT) calculation reveals the significant role of carbon coordination around the Ir center. A series of monatomic PBN-300-M are synthesized by using of designed carbon materials. The findings provide an enabling and versatile platform for facile accessing SACs toward many industrial important reactions.

Theoretical Investigation on the Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction Reactions Performances of Two-Dimensional Metal-Organic Frameworks Fe3(C2X)12 (X = NH, O, S)

Two-dimensional metal-organic frameworks (2D MOFs) inherently consisting of metal entities and ligands are promising single-atom catalysts (SACs) for electrocatalytic chemical reactions. Three 2D Fe-MOFs with NH, O, and S ligands were designed using density functional theory calculations, and their feasibility as SACs for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) was investigated. The NH, O, and S ligands can be used to control electronic structures and catalysis performance in 2D Fe-MOF monolayers by tuning charge redistribution. The results confirm the Sabatier principle, which states that an ideal catalyst should provide reasonable adsorption energies for all reaction species. The 2D Fe-MOF nanomaterials may render highly-efficient HER, OER, and ORR by tuning the ligands. Therefore, we believe that this study will serve as a guide for developing of 2D MOF-based SACs for water splitting, fuel cells, and metal-air batteries.

Hematite/selenium disulfide hybrid catalyst for enhanced Fe(III)/Fe(II) redox cycling in advanced oxidation processes

Regeneration of Fe(II) is a key issue for heterogeneous advanced oxidation processes (AOPs) using iron-based catalysts. Herein, a hybrid catalyst was developed from α-Fe₂O₃ and SeS₂ to enhance the Fe(III)/Fe(II) redox cycling in both hydrogen peroxide (H₂O₂) system and persulfate (PS) system. The regeneration of Fe(II) was evidenced by the increased Fe(II)/Fe(III) ratio in the used catalyst (205.8% in the H₂O₂ system or 125.4% in the PS system), compared to 68.4% in the fresh hybrid catalyst Fe/Se-3. Methyl orange was used as a model pollutant to evaluate the degradation performance of the hybrid catalyst. Owing to the promotion of Fe(II) regeneration, Fe/Se-3 achieved a pollutant removal efficiency of 100.0% in 12 min in both systems, significantly higher than that with pure α-Fe₂O₃ (33.9 ± 3.6% in the H₂O₂ system or 30.7 ± 2.8% in the PS system). The dominant active species were identified as hydroxyl radicals in the H₂O₂ system and sulfate radicals in the PS system. In the proposed mechanism, soluble and surface-bound Fe species are provided by α-Fe₂O₃ to activate H₂O₂ or PS to radicals, and SeS₂ participates in the reactions via Se(IV) reducing Fe(III) to Fe(II) and S atoms being released through protonation to expose more active Se sites.

Rational Design of Graphene-Supported Single-Atom Catalysts for Electroreduction of Nitrogen

Critically, the central metal atoms along with their coordination environment play a significant role in the catalytic performance of single-atom catalysts (SACs). Herein, 12 single Fe, Mo, and Ru atoms supported on defective graphene are theoretically deigned for investigation of their structural and electronic properties and catalytic nitrogen reduction reaction (NRR) performance using first-principles calculations. Our results reveal that graphene with vacancies can be an ideal anchoring site for stabilizing isolated metal atoms owing to the strong metal-support interaction, forming stable TMC_x or TMN_x active centers (x = 3 or 4). Six SACs are screened as promising NRR catalyst candidates with excellent activity and selectivity during NRR, and RuN₃ is identified as the optimal one with an overpotential of ≥0.10 V via the distal mechanism.

Hexagonal MBene (Hf2BO2): A Promising Platform for the Electrocatalysis of Hydrogen Evolution Reaction

Hexagonal MAB (h-MAB) phases and their two-dimensional (2-D) derivatives (h-MBenes) have emerged as promising materials since the discovery of Ti₂InB₂. Herein, we identified that a possible h-MBene, 2-D Hf₂BO₂, can be an excellent platform for the electrocatalysis of hydrogen evolution reaction (HER) by density functional theory calculations. We proposed two approaches of transition metal (TM) modifications by atom deposition and implanting to optimize the HER performance of 2-D Hf₂BO₂. It is revealed that a moderate charge reduction of surface O, which is induced by the introduction of TM atoms, is conductive to a higher catalytic performance. The synergistic effect between implanted TM atoms and Hf₂BO₂ matrix can efficiently activate the surface by broadening O-p orbitals and shifting up p-band center, especially for V, Cr, and Mo as dopants, which can reduce the Gibbs free energy (ΔG_H*) from 0.939 to -0.04, 0.05 and -0.04 eV, respectively. Interestingly, this effect works within a local region and the activity can also be evaluated by bond length of Hf-O, in addition to ΔG_H*. This work suggests that due to its excellent electrocatalysis properties, h-MBenes can open up a new area for 2-D materials and will stimulate researchers to explore the synthesis of h-MAB phases and the stripping of h-MBenes.

Two-dimensional transition metal borides as high activity and selectivity catalysts for ammonia synthesis

In comparison to defect/doping induced activity in materials, transition metal borides with exposed metal atoms, large specific surface area, and high active site density show advantages as durable and efficient catalysts for specific electrochemical reactions. In this work, ReB₂ for N₂ reduction reaction (NRR) for ammonia (NH₃) with a record-low limiting potential of U_L = -0.05 V and high Faraday efficiency (FE) of 100% is screened out from a new class of TMB₂. It is concluded that high pressure/temperature is favorable to N₂ adsorption and kinetic barrier minimization; the maximal turnover frequency (TOF) at 700 K and 100 bar is 1.24 × 10^-2 per s per site, which is comparable to that of the benchmark Fe₃/Al₂O₃ catalysts, achieving an extremely fast reaction rate. In addition, crystal orbital Hamilton population (COHP) of *N₂ reveals the intrinsic origin of N₂ activation by analyzing the d-2π* interactions, and integrated COHP could be a quantitative descriptor to describe the N₂ activation degree. It is evident that our results not only identify an efficient NRR electrocatalyst in particular, paving the way for sustainable NH₃ production, but also explain the chemical and physical origin of the activity, advancing the design principle for catalysts for various reactions in general.

Rhodanine-based fluorometric sequential monitoring of silver (I) and iodide ions: Experiment, DFT calculation and multifarious applications

A simple rhodanine derived fluorophoric unit has been designed for selective detection of Ag⁺ and I^- ions in DMSO-H₂O medium. The sensor R1 showed an obvious "turn-on" fluorescence response toward Ag⁺ due to the inhibition of both C-N single bond free rotation, internal charge transfer (ICT) and the formation of chelation enhanced fluorescence (CHEF) effects. The fluorescence quantum yield (Φ) was increased from 0.0013 to 0.032 for receptor R1 upon the Ag⁺-complexation. In addition, the 1:1 complexing stoichiometry was employed based on Job's plot analysis with detection limit of 24.23 × 10^-7 M. Conversely, receptor R1+Ag⁺ particularly detects I^- ion over other co-existing anions by the "turn-off" fluorescence response due to the formation of AgI, displacing the receptor R1 with the quantum yield of 0.0014. The detection limit was calculated to be 22.83 × 10^-7 M. The sensing behaviour of receptor R1 toward Ag⁺ was also supported by density functional theory (DFT) calculations. Moreover, the sensing ability of reported receptor R1 could be exercised in multifarious applications like paper strip, silica-supported analysis, staining test for latent finger print, logical behaviour, smartphone-assisted quantitative detection and real water samples studies.

Computational screening of MBene monolayers with high electrocatalytic activity for the nitrogen reduction reaction

As an emerging family of two-dimensional (2D) materials, transition metal borides (MBenes) have attracted increasing interest due to their potential applications in electrochemistry, especially electrocatalysis. In this work, we addressed six MB (M = Sc, Ti, V, Cr, Mo and W) monolayers as catalysts to explore their electrocatalytic activity for the nitrogen reduction reaction (NRR) using first-principles calculations. Our results demonstrated that N₂ molecules could be strongly adsorbed on these MB monolayers to provoke the NRR process. Furthermore, we examined five possible catalytic reaction pathways of the NRR, i.e., the alternating, distal, and three mixed pathways, on the MB monolayers with N₂ adsorption (both side-on and end-on) configurations, and screened out three highly efficient NRR catalysts: VB, CrB, and MoB monolayers with the onset potential of -0.396, -0.277, and -0.403 V, respectively. By comparison of the limiting potentials, the most effective reaction pathways of the NRR were ascertained to be the alternating pathway on the VB monolayer with the end-on configuration and the mixed I pathway on the CrB monolayer with the end-on configuration and on the MoB monolayer with the side-on configuration. Our work sheds light on the electrocatalytic mechanisms of the NRR on 2D MBenes, and provides a theoretical foundation for developing highly efficient MBene electrocatalysts for the NRR.

Theoretical exploration of quaternary hexagonal MAB phases and two-dimensional derivatives

The newly discovered hexagonal MAB (h-MAB) phases have shown great promise in synthesizing 2-D borides, but wide application is hindered by the limited kinds of transition metals that can be involved. In this work, an extensive structure search was performed to identify stable quaternary h-MAB phases by alloying ternary h-MAB phases with a fourth component. The predicted 22 stable quaternary h-MAB phases reveal that there is plenty of room for further exploration of new materials by element alloying. Moreover, we theoretically proved the possibility of exfoliating h-MBenes from the predicted quaternary phases through the selective removal of A components. The simulations for the hydrogen evolution reaction (HER) revealed that the bi-metal combination demonstrates a great advantage to enrich the application perspectives of h-MBenes. The discovery of quaternary h-MAB phases and two-dimensional derivatives offers new insights and understanding of boride-based materials.

Recent Advances in Immunosafety and Nanoinformatics of Two-Dimensional Materials Applied to Nano-imaging

Two-dimensional (2D) materials have emerged as an important class of nanomaterials for technological innovation due to their remarkable physicochemical properties, including sheet-like morphology and minimal thickness, high surface area, tuneable chemical composition, and surface functionalization. These materials are being proposed for new applications in energy, health, and the environment; these are all strategic society sectors toward sustainable development. Specifically, 2D materials for nano-imaging have shown exciting opportunities in in vitro and in vivo models, providing novel molecular imaging techniques such as computed tomography, magnetic resonance imaging, fluorescence and luminescence optical imaging and others. Therefore, given the growing interest in 2D materials, it is mandatory to evaluate their impact on the immune system in a broader sense, because it is responsible for detecting and eliminating foreign agents in living organisms. This mini-review presents an overview on the frontier of research involving 2D materials applications, nano-imaging and their immunosafety aspects. Finally, we highlight the importance of nanoinformatics approaches and computational modeling for a deeper understanding of the links between nanomaterial physicochemical properties and biological responses (immunotoxicity/biocompatibility) towards enabling immunosafety-by-design 2D materials.

Discovery of intrinsic two-dimensional antiferromagnets from transition-metal borides

Intrinsic two-dimensional (2D) magnets are promising materials for developing advanced spintronic devices. A few have already been synthesized from the exfoliation of van der Waals magnetic materials. In this work, by using ab initio calculations and Monte Carlo simulation, a series of 2D MBs (M = Cr, Mn or Fe; B = boron) are predicted possessing robust magnetism, sizeable magnetic anisotropy energy, and excellent structural stability. These 2D MBs can be respectively synthesized from non-van der Waals compounds with low separation energies such as Cr2AlB2, Mn2AlB2, and Fe2AlB2. 2D CrB is a ferromagnetic (FM) metal with a weak in-plane magnetic anisotropy energy of 23.6 μeV per atom. Metallic 2D MnB and FeB are Ising antiferromagnets with an out-of-plane magnetic easy axis and robust magnetic anisotropy energies up to 222.7 and 482.2 μeV per atom, respectively. By using Monte Carlo simulation, the critical temperatures of 2D CrB, MnB, and FeB were calculated to be 440 K, 300 K, and 320 K, respectively. Our study found that the super-exchange interaction plays the dominant role in determining the long-range magnetic ordering of 2D MBs. Moreover, most functionalized 2D MBTs (T = O, OH or F) are predicted to have AFM ground states. Alternating transition metals or functional groups can significantly modulate the magnetic ground state and critical temperature of 2D MBTs. This study suggests that the 2D MBs and MBTs are promising metallic 2D magnets for spintronic applications.

Hydroxyl-Boosted Nitrogen Reduction Reaction: The Essential Role of Surface Hydrogen in Functionalized MXenes

MXenes, an emerging family of two-dimensional (2D) metal carbides and nitrides, have been demonstrated to be effective nitrogen reduction reaction (NRR) catalysts. So far, most of the theoretical studies toward NRR are based on bare MXenes; however, the structural stabilities are questionable. In this work, we studied the NRR process on several synthesized MXenes (Ti₂C, V₂C, Cr₂C, Zr₂C, Nb₂C, Mo₂C, Hf₂C, and Ta₂C) with hydroxyl (OH) termination since the structures are preferred under NRR operating conditions as per Pourbaix stability diagrams. It is found that OH plays an essential role in tuning the NRR chemistry, as a new surface-hydroxylation mechanism. Different from the widely accepted NRR mechanism where only protons are involved in the reaction, hydrogen (H) atoms from surface hydroxyl could be captured by the intermediate and participate into the NRR, while the remaining H vacancy can subsequently be self-repaired by the protons under the applied potential. The cooperative effect of surface hydroxylation can effectively boost the NRR, while Mo₂C(OH)₂ stands out with the most favorable limiting potential of -0.62 V and highest selectivity. Moreover, new scaling relationships based on the H vacancy energy are established, elucidating the possibility for structure-activity tuning. This study not only elaborates the essential role of surface OH functionalization in evaluating NRR performance but also affords new insights into advance sustainable NH₃ production.

A systematic computational investigation of the water splitting and N2 reduction reaction performances of monolayer MBenes

Recently, transition metal borides (MBenes, analogous to MXenes) have attracted interest due to their potential applications in energy conversion and storage. In this work, we performed density functional theory calculations to systematically explore the exfoliation properties of 14 MAlB phases and their water splitting and N₂ reduction reaction (NRR) performances. Results showed a linear relationship between the binding energy and exfoliation energy with the coefficient (R²) of 0.95, indicating that the lower the binding energy of element Al in MAlB (M₂AlB₂), the higher the exfoliation energy required to synthesize monolayer MB from MAlB (M₂AlB₂). NiB (B site) was predicted to possess the best electrocatalytic activity for water splitting, hydrogen evolution reaction (HER), and oxygen evolution reaction (OER) among the studied MBenes, and overpotentials on the NiB surface were calculated to be 0.08 V (for HER) and 0.37 V (for OER), respectively. The electronic properties and dynamic simulations indicated that NiB is the best candidate catalyst for water splitting. Conversely, the Fe site on FeB (FeB-Fe) was predicated to have the highest nitrogen reduction reaction (NRR) activity among the studied MBenes, with the overpotential η_NRR of 0.11 V. Furthermore, the B site of TaB (TaB-B) was identified as the best NRR catalyst against HER among the studied MBenes considering the HER side reaction.

Ferromagnetic TM2BC (TM = Cr, Mn) monolayers for spintronic devices with high Curie temperature

Transition metal boro-carbide (TM2BC) structures crystallize in the layered orthorhombic structure in their bulk phases. In this study, however, we find that TM2BC (TM = Cr, Mn) prefer a tetragonal (t) crystal structure in their monolayer phases due to the occurrence of strong sp2 bonds between the metal and B/C atoms instead of sp3 + sp2 bonds which exist in the orthorhombic phase. The calculated energy difference between the orthorhombic and the tetragonal structures based on density functional theory (DFT) is more than 1 eV per unit cell. In addition, t-Cr2BC and t-Mn2BC monolayers are dynamically and thermally stable with their magnetic metal electronic structures. For further investigations, we combine our DFT calculations with the Monte Carlo simulations and find that both t-TM2BC monolayers show ferromagnetic properties. The calculated Curie temperatures are 846 K and 128 K for t-Cr2BC and t-MnBC sheets, respectively. In addition, we examine the magnetic anisotropy energies (MAE) of t-TM2BC monolayers and find that both structures prefer out-of-plane as the easy axis magnetization direction and the applied electric field can easily modulate the MAE of the monolayers. Our theoretical calculations reveal that t-TM2BC (TM = Cr, Mn) sheets have great potential for the future design of controllable spintronic devices with their tunable MAE properties.

Determination of cesium ions in environmental water samples with a magnetic multi-walled carbon nanotube imprinted potentiometric sensor

A potentiometric sensor, based on the glassy carbon electrode (GCE) modified with a magnetic multi-walled carbon nanotubes/cesium ion-imprinted polymer composite (MMWCNTs@Cs-IIP), is introduced for the detection of cesium(i). The IIP was synthesized using cesium ions as the template ions, chitosan as the functional monomer and glutaraldehyde as the cross-linking agent. The membrane, which was coated on the surface of the GCE, was prepared using MMWCNTs@Cs(i)-IIP as the modifier, PVC as the neutral carrier, 2-nitrophenyloctyl ether as the plasticizer and sodium tetraphenylborate as the lipophilic salt. The proposed sensor exhibited a Nernstian slope of 0.05954 V dec⁻¹ in a working concentration range of 1 × 10⁻⁷ to 1 × 10⁻⁴ M (mol L⁻¹) with a detection limit of 4 × 10⁻⁸ M. The sensor exhibited high selectivity for cesium ions and was successfully applied for the determination of Cs(i) in real samples.

A Cs(i)-selective potentiometric microsensor based on the glassy carbon electrode (GCE) modified with a magnetic multi-walled carbon nanotubes/cesium ion-imprinted polymer has been developed.

Direct Visualization of Atomic-Scale Graphene Growth on Cu through Environmental Transmission Electron Microscopy

The functionalities of two-dimensional (2D) materials are solely determined by their perfect single-layer lattice or precisely stacking of multiple lattice planes, which is predominately determined during their growth process. Although the growth of graphene has been successfully achieved on different substrates with a large area up to millimeters, direct visualization of atomic-scale graphene growth in real time still lacks, which is vital to decipher atomistic mechanisms of graphene growth. Here, we employ aberration-corrected environmental transmission electron microscopy (AC-ETEM) to visualize the nucleation and growth of graphene at the atomic scale in real time. We find a unique lateral epitaxial growth process of graphene on Cu edges under the CO₂ atmosphere with a ledge-flow process. The nucleation of graphene nuclei from amorphous carbon atoms also has been found to proceed with a gradual ordering of in-plane carbon atoms. The coalescence of smaller graphene nanoislands to form large ones is thermodynamically favored, and the evolution of atomic structures at grain boundaries is also revealed in great details. These atomic insights obtained from real-time observations can provide direct evidence for the growth mechanisms of graphene, which can be extended to other 2D materials.

Platinum Single Atoms Supported on Nanoarray-Structured Nitrogen-Doped Graphite Foil with Enhanced Catalytic Performance for Hydrogen Evolution Reaction

Platinum-based single-atom catalysts (SACs) are among the most promising candidates for the practical applications of electrochemical hydrogen evolution reaction (HER), but their catalytic efficiency remains to be further enhanced. Herein, a well-designed nanoarray-structured nitrogen-doped graphite foil (NNGF) substrate is introduced to support Pt SACs in Pt-N₄ construction (Pt₁/NNGF) for HER. Within NNGF, the constructed nanoarray-structured surficial layer for supporting Pt SACs could enhance the exposure of active sites to the electrolyte and improve the reaction and diffusion kinetics; meanwhile, the retained graphite structures in bulk NNGF provide not only the required electrical conductivity but also the mechanical stability and flexibility. Because of such double-layer structures of NNGF, stable Pt-N₄ construction, and binder-free advantages, the Pt₁/NNGF electrode exhibits a low overpotential of 0.023 V at 10 mA cm^-2 and a small Tafel slope of 29.1 mV dec^-1 as well as an excellent long-term durability.