Properties Of MXenes Research Articles

Reversible Stacking and Delamination-Regulation of MXene via Controlled Freezing.

MXene's configuration, whether it is aggregated or dispersed in a monolayer, determines the specific application areas and even greatly influences the intrinsic properties of MXene. However, how to desirably control MXene's configuration is challenging. Here, a simple, additive-free, chemical reaction-free, and scalable strategy to optionally and reversibly regulate MXene's ordered stacking and delamination of MXene aggregates (AM) is reported. Just by controlled freezing of MXene aqueous dispersions, the aggregation percentage, delamination percentage, and interlayer spacing of AM can be finely tuned. Experimental results reveal that the freezing-induced aggregation and delamination effects can be explained by the squeezing action of growing ice grains on the MXene excluded/concentrated between ice grains and the expanding action caused by the ice formation between AM lamellae, respectively. The dominance between them depends on the freezing parameter-influenced ice nucleation sites, numbers, and ice grain sizes. This work not only contributes to the preparation, storage, and practical applications of MXene, but also opens a new and green avenue for controlling materials' assembly structures.

Machine Learning-Assisted Survey on Charge Storage of MXenes in Aqueous Electrolytes.

Pseudocapacitance is capable of both high power and energy densities owing to its fast chemical adsorption with substantial charge transfer. 2D transition-metal carbides/nitrides (MXenes) are an emerging class of pseudocapacitive electrode materials. However, the factors that dominate the physical and chemical properties of MXenes are intercorrelated with each other, giving rise to challenges in the quantitative assessment of their discriminating importance. In this perspective, literature data on the specific capacitance of MXene electrodes in aqueous electrolytes is comprehensively surveyed and analyzed using machine-learning techniques. The specific capacitance of MXene electrodes shows strong dependency on their interlayer spacing, where confined H2O in the interlayer space should play a key role in the charge storage mechanism. The electrochemical behavior of MXene electrodes is overviewed based on atomistic insights obtained from data-driven approaches.

Tuning MXene Properties through Cu Intercalation: Coupled Guest/Host Redox and Pseudocapacitance.

MXenes are 2D transition metal carbides, nitrides, and/or carbonitrides that can be intercalated with cations through chemical or electrochemical pathways. While the insertion of alkali and alkaline earth cations into Ti3C2Tx MXenes is well studied, understanding of the intercalation of redox-active transition metal ions into MXenes and its impact on their electronic and electrochemical properties is lacking. In this work, we investigate the intercalation of Cu ions into Ti3C2Tx MXene and its effect on its electronic and electrochemical properties. Using X-ray absorption spectroscopy (XAS) and ab initio molecular dynamics (AIMD), we observe an unusual phenomenon whereby Cu2+ ions undergo partial reduction upon intercalation from the solution into the MXene. Furthermore, using in situ XAS, we reveal changes in the oxidation states of intercalated Cu ions and Ti atoms during charging. We show that the pseudocapacitive response of Cu-MXene originates from the redox of both the Cu intercalant and Ti3C2Tx host. Despite highly reducing potentials, Cu ions inside the MXene show an excellent stability against full reduction upon charging. Our findings demonstrate how electronic coupling between Cu ions and Ti3C2Tx modifies electrochemical and electronic properties of the latter, providing the framework for the rational design and utilization of transition metal intercalants for tuning the properties of MXenes for various electrochemical systems.

Two-dimensional Janus transition-metal carbide for flexible anode through surface engineering

Two-dimensional transition metal carbides (MXenes) have received tremendous attention because of their great promise in flexible energy storage. Although surface engineering plays an important role in regulating the properties of MXenes with the development of synthesis technology, an atomistic design of surface diversity for desired functionalities is generally limited by purely unified surface termination. Herein, the Janus MXenes consisting of different surface terminations are proposed for flexible anodes through first-principles calculations. Taking Ti2CTT' (T, T' = O, F, or OH) as an illustration of Janus MXenes, critical strains, ideal strengths, Li-ion diffusion barriers, equilibrium voltages, and theoretical capacities are determined and used as descriptors to evaluate the properties of flexible anodes. These Janus MXenes exhibit high critical strains and ideal strengths, indicating their mechanical flexibility. The strain-independent Li-ion diffusion barrier of Ti2COF suggests its fast Li-ion transport even suffering from mechanical deformation, endowing its application potential in flexible anodes. These results establish a theoretical framework for the comprehensive evaluation of flexible anode materials, providing a theoretical scheme for designing flexible anode materials.

MXene on Textile Substrates: A Review

MXenes have emerged as a promising 2D material on account of their diverse properties and applications, especially in portable, wearable, and flexible smart textiles. This review presents the different properties of MXene, the development of electroconductive MXene‐based fibers, different types of MXene‐based fibers, and fabrics suitable for treatment with MXene. Interactions of natural and man‐made fibers with MXenes have been presented. Functional fibers have been incorporated into textiles, promoting the creation of strain sensors, pressure sensors, antennas, and heaters. Textiles with wearable multifunctional properties, such as humidity sensors, actuators, and electromagnetic interference shielding, are very promising for use in MXene‐based intelligent garments.

Bioinspired Dynamic Matrix Based on Developable Structure of MXene‐Cellulose Nanofibers (CNF) Soft Actuators

AbstractInspired by natural organisms, soft actuators can convert environmental stimuli into mechanical deformation, making them indispensable for applications in a variety of fields such as soft robotics. MXene, displaying exceptional attributes in conductivity, thermal efficiency, good dispersibility etc., has emerged as a preferred material for high‐performance thermal actuators. However, single actuators struggle to achieve complex tasks realized by intelligent robotic systems. Herein, a bioinspired dynamic matrix is presented utilizing the developable structure of MXene‐cellulose nanofibers (CNF) soft actuators. The inclusion of CNF considerably bolsters the mechanical properties of MXene. The MXene‐CNF film possessed a higher working strain range (≈14%) than pure MXene film (≈2%). The designed developable structures complete the actuating movements. The sensing layer integration with the actuating layer led to an extremely low touch detection limit (0.3 kPa) and expedited actuating‐feedback due to the interaction between contact charging and electrostatic induction. A responsive dynamic matrix containing 3 × 3 soft actuators, completed with a close‐looped sensing‐feedback function, is developed similar to the behavior of the mimosa plant. This mimosa‐inspired dynamic matrix is capable of identifying the source of touch stimuli and providing immediate feedback. This research broadens the potential for enhancing adaptability and intelligence of soft robotic system.

MXene-Mediated Catalytic Redox Reactions for Biomedical Applications.

Reactive oxygen species (ROS) play a crucial role in orchestrating a myriad of physiological processes within living systems. With the advent of materdicine, an array of nanomaterials has been intricately engineered to influence the redox equilibrium in biological milieus, thereby pioneering a distinctive therapeutic paradigm predicated on ROS-centric biochemistry. Among these, two-dimensional carbides, nitrides, and carbonitrides, collectively known as MXenes, stand out due to their multi-valent and multi-elemental compositions, large surface area, high conductivity, and pronounced local surface plasmon resonance effects, positioning them as prominent contributors in ROS modulation. This review aims to provide an overview of the advancements in harnessing MXenes for catalytic redox reactions in various biological applications, including tumor, anti-infective, and anti-inflammatory therapies. The emphasis lies on elucidating the therapeutic mechanism of MXenes, involving both pro-oxidation and anti-oxidation processes, underscoring the redox-related therapeutic applications facilitated by self-catalysis, photo-excitation, and sono-excitation properties of MXenes. Furthermore, this review highlights the existing challenges and outlines future development trends in leveraging MXenes for ROS-involving disease treatments, marking a significant step towards the integration of these nanomaterials into clinical practice.

Double-layered solar evaporator based on polymeric ionic liquid grafted attapulgite and MXene/PPy for efficient desalination and photodegradation

Solar-driven interfacial evaporation (SIE) is a promising fresh water acquisition technology to address the global water resource shortage. Therefore, solar evaporators with efficient photothermal conversion efficiency and multifunctional purification capabilities are still in high demand. At present, a variety of photothermal conversion materials have been explored, however, there are few studies on integrating interfacial evaporation and photocatalytic capabilities into one solar steam generation system collaboratively. In this work, a new double-layered solar interfacial evaporator (MXene/PPy-ACA) was fabricated by using polymeric ionic liquid grafted attapulgite (ATP-PIL) as the matrix and modified the upper surface by MXene and polypyrrole (PPy). The evaporator can not only carry out seawater desalination efficiently, but also has good photocatalytic degradation effect on organic dyes. The MXene/PPy-ACA achieves an evaporation rate of 1.53 kg m−2 h−1 and corresponding to an efficiency of 92.17 % under irradiation of 1 sun. In addition, duing to the ionic repulsion effect of ATP-PIL, the MXene/PPy-ACA possesses good salt resistance. When tested in 20 wt% high brine, the evaporation rate is measured to be 1.40 kg m−2 h−1. Moreover, owing to the superb photocatalytic properties of MXene and PPy, the MXene/PPy-ACA exhibits remarkable photocatalytic efficiency towards organic dyes, achieving degradation rates of 94.9 %, 67.49 %, and 65.11 % for methyl orange (MO), rhodamine B (RhB), and methylene blue (MB), respectively. The prepared interfacial evaporation system can also run stably in acid and alkali solution, and has a good practical application prospect. This study may offer an alternative approach in achieving clean water via SIE of seawater and complex wastewater.

Molten salt-modified Ti3C2Tx MXene with tunable oxygen-functionalized surfaces for effective detection of NO2 at room temperature

The abundant tunable functional groups and room-temperature sensing properties of MXenes have garnered increasing interest in the field of gas sensing. However, the majority of research has exclusively focused on the combined impact of external substances with MXenes functional groups, with limited exploration into the modulation of MXenes’ intrinsic functional groups. In this work, we successfully produced molten salt-modified Ti3C2Tx MXene (M-Ti3C2Tx) with tunable oxygen-functionalized surfaces using the molten salt immersion method and employed it as a chemical resistive gas sensor to detect NO2 at room temperature (RT). Using diverse analytical techniques, we propose the O−addition effect on the surface titanium atoms and the O−substitution effect on the edge carbon atoms, refining the existence form of –O functional groups as well as realizing the effective regulation of the intrinsic functional groups. With a high response (∼25.87%−100 ppm NO2, whose response value is five times higher than that of pristine Ti3C2Tx), relatively rapid response speed (∼99 s−25 ppm NO2), high sensitivity (∼0.29 ppm−1), low detection limits (theoretical LoD is 3.3 ppm), good selectivity and long-term stability (maintain stable within 28 days) at RT, the sensor based on M-Ti3C2Tx demonstrates excellent NO2 sensing performance. To understand the sensing mechanism, density functional theory (DFT) calculations were conducted to explore adsorption behaviors. Theoretical analysis validates that M-Ti3C2Tx has a stronger adsorption capacity for NO2 (Ead = −1.149 eV) owing to the formation of covalent bond between the –O functional group and the nitrogen atom of NO2. The application of the molten salt can effectively modulate the intrinsic functional groups of MXenes, expanding the scope of MXenes surface modification engineering and creating new opportunities for enhancing the gas sensing capabilities of MXenes.

The first-principles study on the magnetic and electronic properties of Janus Cr-based MXene

Using the first-principles calculations, the geometric, magnetic and electronic properties of two-dimensional Janus MXenes Cr2CAB (A/B = F, O, OH) were investigated. The results show that Cr2COF, Cr2CO(OH) and Cr2CF(OH) all exhibit A-type antiferromagnetic. The Janus MXene Cr2CO(OH) and Cr2COF are half metals with the atomic orbital of Cr-d and O-p states contributing to the metal states at the Fermi level in spin up channel. However, Cr2CF(OH) is a semiconductor. Bader charge and geometric structure analyses show that the electron transfer number difference between the O2- and F-/OH- groups and the bond length difference between Cr-O and Cr-OH/F lead to an unequal interaction at the two ends, resulting in a transition from semiconductor properties of Cr2CT2 (T = F, OH) to half metal of Cr2CAB (A = O; B = F, OH). These findings highlight the importance of surface groups on tuning the magnetic and electronic properties of MXene, which benefits to the applications of two-dimensional Janus MXenes in spintronics.

Recent Advances in Diverse MXenes‐Based Structures for Photocatalytic CO2 Reduction into Renewable Hydrocarbon Fuels

AbstractPhotocatalytic CO2 reduction into renewable hydrocarbon fuels is a green solution to address CO2 emission and energy issues simultaneously. However, the fast recombination of photogenerated charge carriers and the sluggish surface reaction kinetics restrict the efficiency of photocatalytic CO2 reduction. The emergence of 2D MXenes has potential in improving the efficiency of photocatalytic CO2 reduction, owing to their high electrical conductivity, flexible structural properties, and abundant active sites. Hence, this review will concisely summarize and highlight recent advances of MXenes‐based photocatalysts used in photocatalytic CO2 reduction. First, the synthesis and properties of MXenes is briefly introduced. Second, the mechanism of CO2 photoreduction along with the roles of MXenes in photocatalytic CO2 reduction are summarized, including promoting the adsorption of CO2, enhancing the separation of photo‐induced charge carriers, acting as a robust support, and photothermal effect. Third, different kinds of MXenes‐based photocatalysts such as MXenes/metal oxides, MXenes/nitrides, MXenes/LDH, MXenes/perovskite, and MXene‐derived photocatalysts for CO2 reduction are classified via the type of semiconductors. Finally, the challenges and perspectives are also presented, including exploring suitable MXenes via machine learning, uncovering the structure‐activity relationship by in situ, time‐ and space‐resolved characterization techniques, improving anti‐oxidization ability, and scale‐up applications.

Application of MXene-Based Materials for Cathode in Lithium-Sulfur Batteries.

The lithium-sulfur (Li-S) batteries have a high theoretical specific capacity of 1675 mAh ⋅ g-1 and have become the most promising high-energy storage system for the next generation batteries technology. However, their applications are hindered by insulated feature and volume expansion of sulfur, as well as the "shuttle effect" of polysulfides. MXenes own metallic conductivity and strong ability of polysulfides adsorption. Besides, their unique two-dimensional (2D) structure, large specific surface area, abundant functional groups, and adjustability are beneficial to overcome the drawbacks of the sulfur cathode. In this review, different mainstream preparation methods and excellent properties of MXenes are summarized. Significant achievements and recent progress of MXene-based cathodes and interlayers applied to Li-S cathodes are concluded later. Finally, the challenges, possible solutions and potential applications of MXenes for Li-S batteries are also presented.

Construction of fully coated polypyrrole oxygen-barrier film based on MXene nanosheets for high reliability capacitive deionization

MXene has emerged as a promising capacitive deionization (CDI) faradic material for the advantages of excellent hydrophilicity, outstanding specific capacitance, and highly reversible ions intercalation/de-intercalation. However, its practical applications are severely restricted by the bottleneck problems of severe self-stacking and susceptibility to be oxidized by dissolved oxygen in aqueous solution. Herein, polypyrrole (PPy) oxygen-barrier film was fabricated and fully coated on the surface of MXene by the in-situ polymerization method to construct the composite electrode of PPy@MXene. In this unique architecture, the PPy oxygen-barrier film both enhances the antioxidant properties of MXene and weakens the strong interfacial interactions between MXene nanosheets to reduce the self-aggregation. Meanwhile, by doping different ions (Cl– and dodecyl benzene sulfonate, DBS–), PPy films were provided with excellent conductivity, abundant hole sites, and good ion exchange properties, which significantly promoted the charge priority adsorption capability of PPy-Cl@MXene and PPy-DBS@MXene electrodes. Consequently, compared with MXene//MXene cell, the PPy-Cl@MXene//PPy-DBS@MXene CDI cell presents dominant deionization capacity (55 mg g−1) and outstanding cycling stability (3 % degradation after 40 cycles). Most importantly, the morphology, structure and surface charge characteristics of the electrodes after long-term desalination confirmed that the MXene with fully coated PPy film was not destroyed by dissolved oxygen and maintained good structural integrity, which is conducive to improving the reliability of the MXene-based electrodes in CDI. Additionally, ion doped PPy@MXene electrodes highlight preferential adsorption to divalent cations. The work proposes a new strategy to advance the practical application of MXene by simultaneously solving its self-stacking and oxidative degradation issues.

Thickness-dependent optoelectronic properties of titanium carbide MXene

Due to its tunable interlayer chemistry, MXene shows great potential in various sensor applications, but the optoelectronic properties of MXene are greatly hindered by the slow photoresponsivity of the intercalation chemistry. The main challenge to MXene in photoelectric sensor applications arises from its composite resistive properties. This paper analyzes the photoelectric responses separately by constructing singlelayer and multilayer structures. Under light, the resistance of a monolayer of MXene increases to 100.1% and recovers in 1 s. When the number of layers increases, the resistance of MXene quickly drops to 99.6% under light and recovers slowly within 15 s. In summary, this study reveals how MXene's layered structure affects its photoresponsivity, offering insights for advancing MXene-based optoelectronic sensors.

First-principles study of MXene properties with varying hydrofluoric acid concentration

With varying hydrofluoric acid (HF) concentrations under three etching conditions, we presented a comparative study of the effects of both the ordered and randomly ternary mixed terminated Ti3C2Tx surfaces with a wide variation of O/OH/F stoichiometry on the thermodynamic stability and electronic properties. Regardless of the HF concentration, an OH-rich surface is found to be thermodynamically stable and the electrical conductivity of Ti3C2Tx is substantially affected by the OH concentration. The charge density difference and electron localization function demonstrated a significant electron localization at the hydroxyl group on the O/OH/F mixed terminated surface, which could yield a locally induced dipole on the surface that renders favorable reaction sites on the functionalized surface. In addition, a large tunability in the work function (ΔΦ ∼ 3.5 eV) is predicted for Ti3C2Tx. These findings provide a pathway for strategically tuning the electronic and structural properties of Ti3C2 MXenes etched with HF.

Capacitance breakthroughs in free-standing electrodes through MXene functionalization

The discovery of the large group of 2D materials called MXenes has gathered much attention due to their potential applications in various fields as well as the evergrowing discoveries about their nature. One of the most interesting applications is energy storage devices. Herein, we demonstrated the impact of the functionalization with two different silanes on the electrochemical and structural properties of Ti3C2Tx MXene. Materials are prepared and characterized as free-standing films obtained by a simple vacuum filtration method. The structure of the modified MXene electrodes was confirmed via SEM, FTIR, and XPS analysis. At the same time, the electrochemical properties were determined by cyclic voltammetry and galvanostatic charge-discharge in a two-electrode setup in aqueous electrolyte. Capacitance values at lower scanning speeds revealed the benefits of PEG silane functionalization (25.38 F/g), showing higher values compared to pure MXene (23.95 F/g) and MEMO silane (23.37 F/g). Capacitance retention decreased with the addition of silanes, but there was a significant increase in capacitance in the first 100 cycles. This study clarifies the benefits of adding different silanes to the structural and electrochemical properties of MXene for energy storage applications.

Phase transition and electrochemical properties of S-functionalized MXene anodes for Li-ion batteries: a first-principles investigation.

The advancement of anode materials for achieving high energy storage is a crucial topic for high-performance Li-ion batteries (LIBs). Here, first-principles calculations were used to conduct a thorough and systematic investigation into lithium storage properties of MXenes with new S functional groups as LIB anode materials. Density of states, diffusion energy barriers, open circuit voltages and storage capacities were calculated to comprehensively evaluate the lithium storage properties of S-functionalized MXenes. Based on the computational results, Ti2CS2 and V2CS2 were selected as excellent candidates from ten M2CS2 MXenes. The diffusion energy barriers of M2CS2 within the range of 0.26-0.32 eV are lower than those of M2CO2 and M2CF2, indicating that M2CS2 anodes exhibit faster charge/discharge rates. By examining the stable crystal structures and comparing atomic positions before and after Li adsorptions, structural phase transitions during Li-ion adsorptions could happen for nearly all M2CS2 MXenes. The phase transitions predicted were directly observed using ab initio molecular dynamic simulations. The cycle stability, storage capacity and other lithium storage properties were enhanced by the reversible structural phase transition.

Exploring the large chemical space in search of thermodynamically stable and mechanically robust MXenes via machine learning.

To effectively utilize MXenes, a family of two-dimensional materials, in various applications that include thermoelectric devices, semiconductors, and transistors, their thermodynamic and mechanical properties, which are closely related to their stability, must be understood. However, exploring the large chemical space of MXenes and verifying their stability using first-principles calculations are computationally expensive and inefficient. Therefore, this study proposes a machine learning (ML)-based high-throughput MXene screening framework to identify thermodynamically stable MXenes and determine their mechanical properties. A dataset of 23 857 MXenes with various compositions was used to validate this framework, and 48 MXenes were predicted to be stable by ML models in terms of heat of formation and energy above the convex hull. Among them, 45 MXenes were validated using density functional theory calculations, of which 23 MXenes, including Ti2CClBr and Zr2NCl2, have not been previously known for their stability, confirming the effectiveness of this framework. The in-plane stiffness, shear moduli, and Poisson's ratio of the 45 MXenes were observed to vary widely according to their constituent elements, ranging from 90.11 to 198.02 N m-1, 64.00 to 163.40 N m-1, and 0.19 to 0.58, respectively. MXenes with Group-4 transition metals and halogen surface terminations were shown to be both thermodynamically stable and mechanically robust, highlighting the importance of electronegativity difference between constituent elements. Structurally, a smaller volume per atom and minimum bond length were determined to be preferable for obtaining mechanically robust MXenes. The proposed framework, along with an analysis of these two properties of MXenes, demonstrates immense potential for expediting the discovery of stable and robust MXenes.

Theoretical insights and design of MXene for aqueous batteries and supercapacitors: status, challenges, and perspectives.

Aqueous batteries and supercapacitors are promising electrochemical energy storage systems (EESSs) due to their low cost, environmental friendliness, and high safety. However, aqueous EESS development faces challenges like narrow electrochemical windows, irreversible dendrite growth, corrosion, and low energy density. Recently, two-dimensional (2D) transition metal carbide and nitride (MXene) have attracted more attention due to their excellent physicochemical properties and potential applications in aqueous EESSs. Understanding the atomic-level working mechanism of MXene in energy storage through theoretical calculations is necessary to advance aqueous EESS development. This review comprehensively summarizes the theoretical insights into MXene in aqueous batteries and supercapacitors. First, the basic properties of MXene, including structural composition, experimental and theoretical synthesis, and advantages in EESSs are introduced. Then, the energy storage mechanism of MXene in aqueous batteries and supercapacitors is summarized from a theoretical calculation perspective. Additionally, the theoretical insights into the side reactions and stability issues of MXene in aqueous EESSs are emphasized. Finally, the prospects of designing MXene for aqueous EESSs through computational methods are given.

The dawn of MXene duo: revolutionizing perovskite solar cells with MXenes through computational and experimental methods.

Integrating MXene into perovskite solar cells (PSCs) has heralded a new era of efficient and stable photovoltaic devices owing to their supreme electrical conductivity, excellent carrier mobility, adjustable surface functional groups, excellent transparency and superior mechanical properties. This review provides a comprehensive overview of the experimental and computational techniques employed in the synthesis, characterization, coating techniques and performance optimization of MXene additive in electrodes, hole transport layer (HTL), electron transport layer (ETL) and perovskite photoactive layer of the perovskite solar cells (PSCs). Experimentally, the synthesis of MXene involves various methods, such as selective etching of MAX phases and subsequent delamination. At the same time, characterization techniques encompass X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy, which elucidate the structural and chemical properties of MXene. Experimental strategies for fabricating PSCs involving MXene include interfacial engineering, charge transport enhancement, and stability improvement. On the computational front, density functional theory calculations, drift-diffusion modelling, and finite element analysis are utilized to understand MXene's electronic structure, its interface with perovskite, and the transport mechanisms within the devices. This review serves as a roadmap for researchers to leverage a diverse array of experimental and computational methods in harnessing the potential of MXene for advanced PSCs.