Direct MoB MBene domain formation in magnetron sputtered MoAlB thin films

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.

Emerging Horizons in Gas Sensing: Exploring the Potential of MXenes and MBenes

This review article is focused on the development and application of two types of emerging two-dimensional (2D) materials, namely MXenes and MBenes, in the field of gas detection. Owing to its excellent electrical conductivity, specific surface area, and tunable surface functionality, MXenes have demonstrated high sensitivity and rapid response in detecting a variety of gases, such as volatile organic compounds (VOCs), nitrogen dioxide (NO2), and ammonia (NH3). MBenes are relatively newly discovered materials with excellent electronic stability and gas adsorption capabilities. Both of these materials demonstrate significant potential in improving sensor selectivity and stability through surface functionalization and heterostructure designs. However, despite the impressive gas detection performance of these materials, challenges remain in achieving long-term stability, cost-effectiveness, and commercialization. We summarize the prominent research insights on these materials and offer an outlook on future research directions and potential applications. With ongoing research and technological advancements, MXenes and MBenes are anticipated to have a substantial impact in critical areas such as environmental monitoring and industrial safety.

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.

Suppressing the Shuttle Effect of Aqueous Zinc-Iodine Batteries: Progress and Prospects

Aqueous zinc-iodine batteries are considered to be one of the most promising devices for future electrical energy storage due to their low cost, high safety, high theoretical specific capacity, and multivalent properties. However, the shuttle effect currently faced by zinc-iodine batteries causes the loss of cathode active material and corrosion of the zinc anodes, limiting the large-scale application of zinc-iodine batteries. In this paper, the electrochemical processes of iodine conversion and the zinc anode, as well as the induced mechanism of the shuttle effect, are introduced from the basic configuration of the aqueous zinc-iodine battery. Then, the inhibition strategy of the shuttle effect is summarized from four aspects: the design of cathode materials, electrolyte regulation, the modification of the separator, and anode protection. Finally, the current status of aqueous zinc-iodine batteries is analyzed and recommendations and perspectives are presented. This review is expected to deepen the understanding of aqueous zinc-iodide batteries and is expected to guide the design of high-performance aqueous zinc-iodide batteries.

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.

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.

Compositional defects in a MoAlB MAB phase thin film grown by high-power pulsed magnetron sputtering

Various compositional defects such as Mo3Al2B4, Mo4Al3B4, Mo6Al5B6 and Al3Mo, together with MoB MBene, are observed to be coexisting in a MoAlB MAB phase thin film grown at 800 °C by high-power pulsed magnetron sputtering. An overall film composition of Mo0.29Al0.33B0.38 is measured by time-of-flight elastic recoil detection analysis. The concurrent formation of these compositional defects in the MoAlB matrix occurs during the synthesis without using any chemical reagent, and their coexistence with the MAB phase is thermodynamically possible, as elucidated by density functional theory simulations. These defect phases are imaged at the atomic scale by aberration-corrected scanning transmission electron microscopy. A rough estimation of defect populations of 0.073, 0.037, 0.042 and 0.039 nm-1 for Mo3Al2B4, Mo4Al3B4, Mo6Al5B6 and Al3Mo compositional defects, respectively, is performed within the MoAlB matrix. The calculated energies of formation reveal that the Mo4Al3B4 and Mo6Al5B6 defect phases form spontaneously in the MoAlB host matrix, while the energy barrier towards the formation of the metastable Mo3Al2B4 phase is approx. 20 meV per atom. The small magnitude of this barrier is easily overcome during vapor phase condensation, and the surface diffusion of adatoms during deposition leads to local compositional variations and the coexistence of the defect phases in the host matrix. Additionally, at grain boundaries, the presence of MoB MBene is observed, with an interlayer spacing between two Mo2B2 units increasing up to ∼50% compared to the pristine MoAlB phase.

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.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

An ab initio investigation of the temperature-dependent energetic barriers towards CrAlB and (Mo,Cr)AlB formation in a metastable synthesis scenario

The orthorhombic CrAlB MAB phase has not been synthesized so far and was shown to be energetically unstable vs. the competing Cr₂AlB₂ phase in previous theoretical reports, which, however, did not explicitly investigate the magnitude of the energetic barrier towards CrAlB formation as a function of temperature. Temperature-dependent Gibbs energies of formation, obtained from density-functional-theory-based lattice dynamics simulations performed in this study, reveal that this barrier is very small (around 10 kJ mol^-1 ≈ 0.008 eV per atom, on average) and may readily be overcome during high-energy synthesis scenarios, likely resulting in metastable phase formation. Furthermore, the electronic structures of MoAlB, a phase synthesized experimentally both in bulk and thin film form, and CrAlB are shown to be similar in direct comparison, with MoAlB exhibiting a higher electronic stability due to a local DOS minimum in proximity to the Fermi level, and quaternary compositions lying between the ternaries. Likewise, bonding characteristics are qualitatively very similar between both phases, with the transition metal-boron bonds being the dominant interaction in the entire unit cell, even though individual B-B bonds are stronger; quantitatively, all interactions are again stronger in MoAlB compared to CrAlB. It is reasonable to assume that, considering the successful synthesis of phase-pure MoAlB and known formation of metastable phases during physical vapor deposition, direct synthesis of metastable CrAlB thin films is possible due to the aforementioned small energy barrier. Furthermore, stability is enhanced upon alloying with Mo as this lowers the energy of formation, with a Mo/Cr ratio of approx. 0.33 sufficient to stabilize the Cr-rich (Mo,Cr)AlB solid solution vs. the primary competing phases, allowing for deposition of Mo-concentration-dependent stable and metastable (Mo,Cr)AlB solid solution phases.

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.

Defects in an orthorhombic MoAlB MAB phase thin film grown at moderate synthesis temperature

Here, we report on atomic scale characterization of various defects in a MoAlB (MAB) phase thin film grown by DC sputtering at a synthesis temperature of 700 °C. Aberration-corrected scanning transmission electron microscopy reveals the formation of an intergrown metastable Mo₃Al₂B₄ phase accompanied by thermally stable 90° twist boundaries, coexisting within the pristine MoAlB matrix. The concurrent formation of these structural defects in the MoAlB matrix can be rationalized based on minute differences in formation enthalpies as shown via density functional theory calculations. The specific structural nature of both the twist boundary and compositional defect (Mo₃Al₂B₄) in a MoAlB matrix is hitherto unreported in literature. Apart from these defects, faceted grain boundaries are observed. In the vicinity of amorphous AlO_x regions, Al is deintercalated and a 2D MoB MBene phase is formed as reported before. Besides these amorphous AlO_x regions, a few nanometer-sized 3D MoB clusters are found. The advancement of aberration-corrected scanning transmission electron microscopy significantly improves characterization from 1D to 3D defects which is important for thin film materials design for the moderate synthesis temperature range. The reported defects might play an important role in the formation of 2D MoB MBenes.