2D Transition Metal Carbides Research Articles

MXene-based electrochemical sensors for detection of environmental pollutants: A comprehensive review

Since the discovery of MXenes at Drexel University in the United States in 2011, there has been extensive research regarding various applications of MXenes including environmental remediation. MXenes with a general formula of Mn+1XnTx are a class of two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides with unique chemical and physical characteristics as nanomaterials. MXenes feature characteristics such as high conductivity, hydrophobicity, and large specific surface areas that are attracting attention from researchers in many fields including environmental water engineering such as desalination and wastewater treatment as well as designing and building efficient sensors to detect hazardous pollutants in water. In this study, we review recent developments in MXene-based nanocomposites for electrochemical (bio) sensing with a particular focus on the detection of hazardous pollutants, such as organic components, pesticides, nitrite, and heavy metals. Integration of these 2D materials in electrochemical enzyme-based and affinity-based biosensors for environmental pollutants is also discussed. In addition, a summary of the key challenges and future remarks are presented. Although this field is relatively new, future research on biosensors of MXene-based nanocomposites need to exploit the remarkable properties of these 2D materials.

Solvent‐Assisted Anisotropic Cleavage of Transition Metal Carbide into 2D Nanoflakes

2D Transition Metal Carbides Nanoflakes In article number 2100039, Yu-Long Tang, Qiang Wang, Hao-Li Zhang, and co-workers fabricate 2D nanoflakes from nonlayered transition metal carbides by a solvent-assisted anisotropic cleavage method utilizing the intrinsic anisotropic hardness properties. The nonlayered 2D nanoflakes are successfully applied in ultrafast photonic as saturable absorbers. This method paves a new way for the fabrication of novel 2D nanomaterials.

2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air.

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo2 CTx flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 101 -104 ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H2 O down to 10ppm. Moreover, humidity suppresses the response of Mo2 CTx to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10-2 -106 Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo2 CTx chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2eV. Density functional theory calculations, elucidating the Mo2 C surface interaction with organic analytes and H2 O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

Printed 2D materials make high-quality electrodes

Using ink made of 2D materials called MXenes , researchers have made soft electrode arrays that can be used on the skin or implanted in the body ( Sci. Transl. Med. 2021, DOI: 10.1126/scitranslmed.abf8629 ). They give better-quality signals than the metal electrodes used today, without the need for sticky conductive gels. The electrodes can be printed at low cost and could be customized for individual patients, says Flavia Vitale, a chemical engineer and neurologist at the University of Pennsylvania. MXenes, a family of 2D transition-metal carbides and nitrides, are highly conductive. Vitale and her colleagues etched electrode array patterns into a cellulose-polymer substrate, infused it with a water-based Ti 3 C 2 MXene ink, and coated the device with an insulating polymer. They used electrode arrays to record electrical activity of the hearts and brains of volunteers through the skin. The MXene electrodes produced signals with a higher signal-to-noise

Ti3C2Tx solid lubricant coatings in rolling bearings with remarkable performance beyond state-of-the-art materials

Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, are a growing class of 2D materials, which offer great solid lubrication ability for low friction applications due to their weakly bonded multi-layer structure and tribo-layer formation with self-lubricating characteristics. To date, most studies have assessed their tribological response in basic laboratory tests. However, these tests do not adequately reflect the complex geometries, kinematics, and stresses present in machine components. Here, we aim at bridging this gap through assessment of the friction and wear performance of multi-layer Ti3C2Tx MXene solid lubricant coatings used in rolling bearings. MXenes’ tribological response is compared with state-of-the art solid lubricant coatings, which include molybdenum disulfide (MoS2), tungsten-doped hydrogenated amorphous carbon (a-C:H:W), and hydrogen-free, more graphite-like amorphous carbon (a–C). Multi-layer Ti3C2Tx MXene coatings reduce wear on the bearing washers by up to 94%, which can be attributed to the transfer of the lubricious MXene nano-sheets to secondary tribo-contacts of the bearing. While the frictional torque of all solid lubricant coatings is similar during steady-operation, the MXene-coated bearings extend the service life by 30% and 55% compared to MoS2 and DLC, respectively. This contribution demonstrates the ability of MXene solid lubricant coatings to outperform state-of-the-art solid lubricants in dry-running machine components such as rolling bearings.

Enhancing Hydrogen Storage Kinetics and Cycling Properties of NaMgH3 by 2D Transition Metal Carbide MXene Ti3C2

Metal hydrides have recently been proposed for not only hydrogen storage materials but also high-efficiency thermal storage materials. NaMgH3 contains a considerable theoretical thermal storage density of 2881 kJ/kg. However, its sluggish de/re-hydrogenation reaction kinetics and poor cycling stability exhibit unavailable energy efficiency. Doping with active catalyst into NaMgH3 is deemed to be a potential strategy to mitigate these disadvantages. In this work, the enhancement of de/re-hydrogenation kinetics and cycling properties of NaMgH3 is investigated by doping with lamellar-structure 2D carbon-based MXene, Ti3C2. Results shows that introducing 7 wt.% Ti3C2 is proved to perform excellent catalytic efficiency for NaMgH3, dramatically reducing the two-step hydrogen desorption peak temperatures (324.8 and 345.3 °C) and enhancing the de/re-hydrogenation kinetic properties with the hydrogen desorption capacity of 4.8 wt.% H2 within 15 min at 365 °C and absorption capacity of 3.5 wt.% H2 within 6 s. Further microstructure analyses reveal that the unique lamellar-structure of Ti3C2 can separate the agglomerated NaMgH3 particles homogeneously and decrease the energy barriers of two-step reaction of NaMgH3 (114.08 and 139.40 kJ/mol). Especially, lamellar-structure Ti3C2 can improve the reversibility of hydrogen storage of NaMgH3, rendering 4.6 wt.% H2 capacity remained after five cycles. The thermal storage density of the composite is determined to be 2562 kJ/kg through DSC profiles, which is suitable for thermal energy storage application.

Correlating structure and orbital occupation with the stability and mechanical properties of 3d transition metal carbides

The development of novel transition metal carbides for improved hard coating technologies requires a detailed understanding of the factors influencing their stability and mechanical performance. To this end, we carried out first principles calculations based on density functional theory to tabulate the electronic structures, formation energies, and phonon dispersion curves of 3d transition metal carbides adopting zincblende, rocksalt, and cesium chloride structures. By analyzing the corresponding results, we outline a theoretical framework that describes how valence electron concentration and bonding configuration control the stability of these compounds. Many early transition metal carbides are predicted to be stable in the rocksalt and zincblende structures, enabled by filled bonding states, whereas the cesium chloride structure shows persistent instability. For compounds that are predicted to be stable, mechanical properties were investigated through calculation of elastic tensors, from which observable properties including Vicker’s hardness and ductility were derived. A robust mechanical performance is shown to be correlated with complete filling of bonding orbitals as illustrated for rocksalt TiC and VC, which have calculated hardnesses of 25.66 and 22.63 GPa respectively. However, enhanced ductility and toughness can be achieved by allowing partial occupation of the antibonding states as in CrC, which has a relatively low Pugh’s ratio of 0.51.

Magnetic Doping Induced Superconductivity-to-Incommensurate Density Waves Transition in a 2D Ultrathin Cr-Doped Mo2C Crystal.

In the vicinity of a competing electronic order, superconductivity emerges within a superconducting dome in the phase diagram, which has been demonstrated in unconventional superconductors and transition-metal dichalcogenides (TMDs), suggesting a scenario where fluctuations or a partial melting of a parent order are essential for inducing superconductivity. Here, we present a contrary example, the two-dimensional (2D) superconductivity in transition-metal carbide can be readily turned into charge density wave (CDW) phases via dilute magnetic doping. Low temperature scanning tunneling microscopy/spectroscopy (STM/STS), transport measurements, and density functional theory (DFT) calculations were employed to investigate Cr-doped superconducting Mo2C crystals in the 2D limit. With ultralow Cr doping (2.7 atom %), the superconductivity of Mo2C is heavily suppressed. Strikingly, an incommensurate density wave (IDW) and a related partially opened gap are observed at a temperature above the superconducting regime. The wave vector of IDW agrees well with the calculated Fermi surface nesting vectors. By further increasing the Cr doping level to 9.4 atom %, a stronger IDW with a smaller periodicity and a larger partial gap appear concurrently. The resistance anomaly implies the onset of the CDW phase. Spatial-resolved and temperature-dependent spectroscopy reveals that such CDW phases exist only in a nonsuperconducting regime and could form long-range orders uniformly. The results provide the understanding for the interplay between charge ordered states and superconductivity in 2D transition-metal carbide.

Elucidating the electrochemical mechanism for enhanced corrosion of Ti3C2Tx-coated mild steel

Two-dimensional (2D) transition metal carbides, carbonitrides and nitrides (MXenes) represent a burgeoning group of 2D layered materials with widespread applications due to unique performance characteristics, in which titanium carbide (Ti3C2Tx) as corrosion inhibitor provides good physical barrier effect. Given that, as-prepared Ti3C2Tx MXene via chemical etching of titanium aluminium carbide (MAX) phase using HF/oxidants have good corrosion resistance of their own, but it is of inevitable importance to understand the effect of their hydrophilic surfaces and high electrical conductivities on corrosion of alloy substrate. Herein Ti3C2Tx MXene coating was prepared on mild steel substrate (Q345) to explore its corrosion behaviors in different conditions. The corrosion current density (i corr) of MXene-coated Q345 was 6.39 × 10–6 A · cm−2, which was higher than that of uncoated Q345 sample (5.66 × 10–6 A · cm−2). After 3 days of storage in wet environment of 40% and 90% relative humidity (RH) at 30 °C, MXene-coated Q345 had suffered from a higher degree of spatially inhomogeneous corrosion, while the surface of uncoated substrate was still neat and glossy in spite of undergoing 1 month of storage under the same condition. The results demonstrate that Ti3C2Tx MXene accelerates the corrosion of Q345. Conductive and hydrophilic Ti3C2Tx could act as an electrode to propagate the charge generated by the oxidation of Fe, which forms a micro corrosion cell with substrate. Thus, the electrochemical corrosion process of substrate is accelerated.

Tuning of electronic structure, magnetic phase, and transition temperature in two-dimensional Cr-based Janus MXenes

Two-dimensional (2D) transition metal carbides and nitrides, called as MXenes, displaying astonishing properties are emerged as a new class of 2D layered materials. In this study, we have studied the structural, electronic, and magnetic properties of 2D bare ${M}_{2}\mathrm{C}$ and Janus Cr-based ${MM}^{\ensuremath{'}}\mathrm{C}$ metal carbides $(M\ensuremath{\ne}{M}^{\ensuremath{'}}=\mathrm{Cr}, \mathrm{Ti}, \mathrm{Sc}, \mathrm{V})$ based on density functional theory. We found that 2D Janus ${MM}^{\ensuremath{'}}\mathrm{C}$ MXenes are dynamically and thermally stable and show unique electronic and magnetic properties which are not seen in their individual bare cases. Our calculated electronic band structures indicate that the Janus ${MM}^{\ensuremath{'}}\mathrm{C}$ MXenes have Dirac-type band dispersion as seen in stanene with tiny band gaps. In addition, all considered bare ${M}_{2}\mathrm{C}$ MXenes show metallic property. However, Janus CrScC has dilute magnetic semiconducting property, CrVC shows half-metallicity, and CrTiC shows metallic property which can easily turn to half-metallicity by external effects. Our extensive density functional theory and Monte Carlo calculations indicate that Janus ${MM}^{\ensuremath{'}}\mathrm{C}$ MXenes also show different magnetic properties than their bare cases. While ${\mathrm{Cr}}_{2}\mathrm{C}$ and ${\mathrm{Sc}}_{2}\mathrm{C}$ have ferromagnetic spin orientations with ${T}_{C}=2000$ and 226 K Curie temperatures, respectively, CrScC shows ferrimagnetic character with antiferromagnetic N\'eel spin orientation and has ${T}_{N}=1120$ K magnetic phase transition temperature between ferrimagnetic and paramagnetic phases. Finally, external magnetic field effects on each crystal have also been elucidated in detail. The numerical outcomes reveal that Janus Cr-based ${MM}^{\ensuremath{'}}\mathrm{C}$ metal carbides have some unusual and interesting hysteresis characters. Our investigations presented here are not only interesting from a theoretical perspective, but also they show that Janus ${MM}^{\ensuremath{'}}\mathrm{C}$ MXenes are promising candidates for future spintronic applications, which should encourage their synthesis.

Advances in the Synthesis of 2D MXenes

2D transition metal carbides, nitrides, and carbonitrides, also known as MXenes, are versatile materials due to their adjustable structure and rich surface chemistry. The physical and chemical diversity has recognized MXenes as a potential 2D material with a wide spectrum of application domains. Since the discovery of MXenes in 2011, a wide variety of synthetic routes has been proposed with advancement toward large-scale preparing methods for MXene nanosheets and derivative products. Herein, the critical synthesis aspects and the operating conditions that influence the physical and chemical characteristics of MXenes are discussed in detail. The emerging etching methods including HF etching methods, in situ HF-forming etching methods, electrochemical etching methods, alkali etching methods, and molten salt etching methods, as well as delamination strategies are discussed. Considering the future developments and practical applications, the large-scale synthesis routes and the antioxidation strategies of MXenes are also summarized. In summary, a generalized overview of MXenes synthesis protocols with an outlook for the current challenges and promising technologies for large-scale preparation and stable storage is provided.

Ten Years of Progress in the Synthesis and Development of MXenes.

Since their discovery in 2011, the number of 2D transition metal carbides and nitrides (MXenes) has steadily increased. Currently more than 40 MXene compositions exist. The ultimate number is far greater and in time they may develop into the largest family of 2D materials known. MXenes' unique properties, such as their metal-like electrical conductivity reaching ≈20000 S cm-1 , render them quite useful in a large number of applications, including energy storage, optoelectronic, biomedical, communications, and environmental. The number of MXene papers and patents published has been growing quickly. The first MXene generation is synthesized using selective etching of metal layers from the MAX phases, layered transition metal carbides and carbonitrides using hydrofluoric acid. Since then, multiple synthesis approaches have been developed, including selective etching in a mixture of fluoride salts and various acids, non-aqueous etchants, halogens, and molten salts, allowing for the synthesis of new MXenes with better control over their surface chemistries. Herein, a brief historical overview of the first 10 years of MXene research and a perspective on their synthesis and future development are provided. The fact that their production is readily scalable in aqueous environments, with high yields bodes well for their commercialization.

Recent advances in MXene-based nanomaterials for desalination at water interfaces

The best exceptional Physico-chemical attributes of MXenes including high conductivity, high surface area, high functionalization, hydroxide site, and other interesting properties have attracted recently the attention of scientists in the applications of MXene (Mn+1XnTx)-based nanomaterials for water treatment. To provide a full and comprehensive vision of the current state of the art, and improve the treatment performance, and motivate new researches in this area, this review focused on the uses of these novel 2D transition metal carbides for desalination of water and the general methods of fabrication of MXenes; thus, MXene-based nanomaterials are very efficient candidates in water desalination processes, in this review, the main properties of previous and current works about MXenes applications in this area were properly investigated. Moreover, a particular overview about the different properties of MXenes in desalination such as etching method, hydrophobicity, structural modification, and chemical modification has been performed; meanwhile, the investigation of MXenes and MXenes-based composites would be an excellent candidate in the future of water purification and environmental remediation fields, since they have several good properties compared to the other 2D materials.

MXene Reinforced Thermosetting Composite for Lightning Strike Protection of Carbon Fiber Reinforced Polymer

AbstractTi3C2 – a member of the MXenes (2D transition metal carbides and nitrides) family, is investigated as an effective filler to improve the electrical, mechanical, and thermal properties of divinylbenzene (DVB) thermosetting resin. Consequently, its performance as a lightning strike protection (LSP) coating for carbon fiber reinforced polymer (CFRP) is evaluated. Polyaniline (PANI) – dodecylbenzene sulfonic acid (DBSA) complex is used to cure the DVB resin. The synergic effect of MXenes (with surface that is negatively charged) with polyaniline (positive charge) shows electrostatic bonding and improved electrical conductivity in the composite. The addition of MXenes at 2 wt% into the PANI‐DVB composite shows ≈139%, 10%, and 9% improvement in electrical conductivity, flexural strength, and flexural modulus, respectively, compared to the neat PANI‐DVB composite. The composites are investigated using various material characterization techniques including Fourier transforms infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. Furthermore, MXenes‐DVB is utilized to create a conductive thermosetting coating on top of a CFRP substrate and tested against a lightning strike of 100 kA. CFRP with MXenes‐DVB coating reduced the surface damage from 40.61 cm2 (reference CFRP panel) to 13.29 cm2 (CFRP coated with MXenes‐DVB).

Recent advances in the rational design of 2D MXenes in energy conversion and storage systems

The most encouraging prospect of energy storage gadgets is batteries and supercapacitors to overcome the need for energy. The environmental problems presently captivate a huge interest in scientific awareness. The improvement of two-dimensional (2D) profoundly effective cathode materials is the way to new research in the fields of energy transformation as well as energy storage devices. MXenes, another fascinating group of 2D transition metal carbides, nitrides, and carbo-nitrides has gained impressive consideration because of their phenomenal properties like high electrical conductivity, hydrophilic nature, magnificent thermal strength, huge interlayer separation, effectively tunable construction, and high surface area. In this review, various MXenes synthesis courses utilizing distinctive materials for example, hydrofluoric acid, ammonium hydrazine, lithium fluoride, and hydrochloric acid have been discussed. Additionally, different combinations of MXenes with the leading polymers are still there and should have been investigated. Potential applications of MXenes have also been discussed in perspective of energy harvesting. MXenes have been proved as promising candidates to resolve energy crisis because of their pre-eminent applications in batteries, supercapacitors, and cells. MXenes have also been used for carbon dioxide reduction and hydrogen production.

Facile synthesis of cobalt modified 2D titanium carbide with enhanced hydrogen evolution performance in alkaline media

2D transition metal carbides, nitrides and carbonitrides, namely the MXenes, attract more and more attentions due to their unique properties. Here, we report a simple one-step molten salt etching method to prepare Co modified MXene hybrid (Ti3C2Tx:Co) by the reaction of Ti3AlC2 with Lewis acid CoCl2 at 750 °C. Most of Co atoms aggregates in the interlayered space of Ti3C2Tx. Benefitting from the improved electron charge transfer efficiency and increased active sites, the sulfuric acid treated Ti3C2Tx:Co-12h hybrid exhibits excellent electrocatalytical activity for hydrogen evolution reaction in alkaline media, delivering a current density of 10 mA cm−2 at an overpotential of 103.6 mV, which is lower than most noble metal free MXene based electrocatalysts. The results illustrate that the proposed method is very facile and useful to incorporate mid-to-late transition metals into the MXene phase to prepare MXene based HER electrocatalysts.

Flexible GO/Nb2CT x hybrid films for high-performance piezoresistive sensors

High-performance flexible piezoresistive sensors have attracted more and more scientific attention due to their promising applications in robotics and portable electronic devices. However, most flexible piezoresistive sensors require a fine structure design at the nanoscale, which makes the manufacturing process very complex and expensive. Two-dimensional (2D) transition metal carbides named MXene have been regarded as an important candidate material for piezoresistive sensors due to their good conductivity, large interlayer spacing, and the inherent nature of 2D materials. The interlayer spacing between MXene nanosheets would be varied under an external pressure, resulting in the conductivity modulation. To improve the sensitivity of MXene-based piezoresistive sensors, the interlayer spacing of MXene-based films should be further tuned and optimized. In this contribution, flexible Nb2CT x -based films were developed using a simple filtration method and evaluated for piezoresistive sensors. 2D GO nanosheets were introduced between the Nb2CT x layers to tune the interlayer spacing to enhance piezoresistive performance. The fabricated flexible piezoresistive sensors based on GO/Nb2CT x films possess high sensitivity up to 228 kPa−1 and rapid response time as fast as 101 ms. This study would promote the application of MXenes-based 2D nanomaterials in the field of piezoresistive sensors.

Atomic-resolution in-situ cooling study of functionally terminated 2D transition metal carbides.

Atomic-resolution in-situ cooling study of functionally terminated 2D transition metal carbides.

Atomic-scale investigation of electronic properties and Na storage performance of Ti3C2Tx-MXene bilayers with various terminations

2D transition metal carbides MXenes stand out on energy storage application due to superior electrochemical performance and encouraging capacitances, and the energy storage properties strongly depend on the nature of terminations on the surface. Here, five different terminal ratios of Ti3C2Tx (O: F: OH = 9:0:0, 6:2:1, 4:4:1, 3:5:1, 0:9:0, respectively) are designed to explore the influence of different terminations on electronic properties and Na storage capability using density functional theory, moreover the Ti3C2Tx with O: F: OH = 3:5:1 has been synthesized to verify some calculation results. The results show that the Ti3C2Tx MXene, with terminal ratios of 6:2:1, is thermally stable when 36Na atoms (26.67%) intercalate. Meanwhile, it also exhibits superior conductivity, lower open circuit voltages, especially the higher theoretical capacity. The theoretical capacity is more than 6 times than that of pure F termination’s structure and 3 times than that of the system with the O: F: OH = 3:5:1. The presence of F can ensure good conductivity and O provide larger capacity. The finding is one to establish interlayer models of Ti3C2Tx-MXene with different terminal ratios and provides a basic understanding about the influence of terminates on electronic properties and Na storage performance of Ti3C2Tx (T = O, F and OH).

Oxidized-co-crumpled multiscale porous architectures of MXene for high performance supercapacitors

MXene, an example of two-dimensional (2D) transition metal carbides, is a promising electrode material for batteries and supercapacitors because of its excellent electrochemical properties derived from transition metal carbide/nitride constituents and its high electrical conductivity. However, similar to other 2D nanomaterials, MXene-based electrodes have a notable shortcoming of reduced specific surface area and reduced ion diffusion due to the self-restacking of MXene nanosheets. Here, to overcome these limitations, we present a novel method to create oxidized-co-crumpled multiscale porous architectures of MXene by a sequential process consisting of partial oxidation by hydrogen peroxide and then flocculation in acid. These treatments result in simultaneous generation of mesopores by oxidation and macropores caused by structural crumpling. This oxidized-co-crumpled MXene has a five times larger specific surface area (72 m2/g) than pristine MXene. In addition, the capacitance performance (307 F/g at 20 mV/s under 1 M H2SO4) and the capacitance at a higher scan rate (225 F/g at 100 mV/s) are improved. Additionally, after 6000 cycles, 87.6% of the capacitance is retained. Such an improvement in the properties is attributed to the structural advantages of the multiscale porous oxidized-co-crumpled MXene, wherein ion diffusion through mesopores is facilitated, while the crumpled macropores prevent re-stacking. Overall, this study demonstrates a promising strategy to create MXene-based high performance electrode materials for a variety of electrochemical applications.