Low temperature solution synthesis of reduced two dimensional Ti3C2 MXenes with paramagnetic behaviour

Maximizing Potential Applications of MAX Phases: Sustainable Synthesis of Multielement Ti3AlC2

This study employs the molten-salt-shielded method to dope the Ti3AlC2 MAX phase with Nb and Mo, aiming to expand the intrinsic potential of the material. X-ray diffraction confirms the preservation of the hexagonal lattice structure of Ti3AlC2, while Raman and X-ray photoelectron spectroscopic analyses reveal the successful incorporation of dopants with subtle yet significant alterations in the vibrational modes and chemical environment. Scanning electron microscopy with energy-dispersive X-ray spectroscopy characterizations illustrate the characteristic layered morphology and uniform dopant distribution. Density functional theory simulations provide insights into the modified electronic structure, displaying changes in carrier transport mechanisms and potential increases in metallic conductivity, particularly when doping occurs at both the M and A sites. The computational findings are corroborated by the experimental results, suggesting that the enhanced material may possess improved properties for electronic applications. This comprehensive approach not only expands the MAX phase family but also tailors its functionality, which could allow for the production of hybrid materials with novel functionalities not present in the pristine form.

Insights on updates in sodium alginate/MXenes composites as the designer matrix for various applications: A review

Advancements in two-dimensional materials, particularly MXenes, have spurred the development of innovative composites through their integration with natural polymers such as sodium alginate (SA). Mxenes exhibit a broad specific surface area, excellent electrical conductivity, and an abundance of surface terminations, which can be combined with SA to maximize the synergistic effect of the materials. This article provides a comprehensive review of state-of-the-art techniques in the fabrication of SA/MXene composites, analyzing the resulting structural and functional enhancements with a specific focus on advancing the design of these composites for practical applications. A detailed exploration of SA/MXene composites is provided, highlighting their utility in various sectors, such as wearable electronics, wastewater treatment, biomedical applications, and electromagnetic interference (EMI) shielding. The review identifies the unique advantages conferred by incorporating MXene in these composites, examines the current challenges, and proposes future research directions to understand and optimize these promising materials thoroughly. The remarkable properties of MXenes are emphasized as crucial for advancing the performance of SA-based composites, indicating significant potential for developing high-performance composite materials.

Temperature Evolution of Composition, Thermal, Electrical and Magnetic Properties of Ti3C2Tx-MXene

MXenes are a family of two-dimensional nanomaterials. Titanium carbide MXene (Ti3C2Tx-MXene), reported in 2011, is the first inorganic compound reported among the MXene family. In the present work, we report on the study of the composition and various physical properties of Ti3C2Tx-MXene nanomaterial, as well as their temperature evolution, to consider MXenes for space applications. X-ray diffraction, thermal analysis and mass spectroscopy measurements confirmed the structure and terminating groups of the MXene surface, revealing a predominant single OH layer character. The temperature dependence of the specific heat shows a Debye-like character in the measured range of 2 K-300 K with a linear part below 10 K, characteristic of conduction electrons of metallic materials. The electron density of states (DOS) calculations for Ti3C2OH-MXene reveal a significant DOS value at the Fermi level, with a large slope, confirming its metallic character, which is consistent with the experimental findings. The temperature dependence of electrical resistivity of the MXene samples was tested for a wide temperature range (3 K-350 K) and shows a decrease on lowering temperature with an upturn at low temperatures, where negative magnetoresistance is observed. The magnetoresistance versus field is approximately linear and increases its magnitude with decreasing temperature. The magnetization curves are straight lines with temperature-independent positive slopes, indicating Pauli paramagnetism due to conduction electrons.

Progress of 2D MXene as an Electrode Architecture for Advanced Supercapacitors: A Comprehensive Review

Supercapacitors, designed to store more energy and be proficient in accumulating more energy than conventional batteries with numerous charge-discharge cycles, have been developed in response to the growing demand for energy. Transition metal carbides/nitrides called MXenes have been the focus of researchers' cutting-edge research in energy storage. The 2D-layered MXenes are a hopeful contender for the electrode material due to their unique properties, such as high conductivity, hydrophilicity, tunable surface functional groups, better mechanical properties, and outstanding electrochemical performance. This newly developed pseudocapacitive substance benefits electrochemical energy storage because it is rich in interlayer ion diffusion pathways and ion storage sites. Making MXene involves etching the MAX phase precursor with suitable etchants, but different etching methods have distinct effects on the morphology and electrochemical properties. It is an overview of the recent progress of MXene and its structure, synthesis, and unique properties. There is a strong emphasis on the effects of shape, size, electrode design, electrolyte behavior, and other variables on the charge storage mechanism and electrochemical performance of MXene-based supercapacitors. The electrochemical application of MXene and the remarkable research achievements in MXene-based composites are an intense focus. Finally, in light of further research and potential applications, the challenges and future perspectives that MXenes face and the prospects that MXenes present have been highlighted.

Recent progress in energy, environment, and electronic applications of MXene nanomaterials

Since the discovery of graphene, two-dimensional (2D) materials have gained widespread attention, owing to their appealing properties for various technological applications. Etched from their parent MAX phases, MXene is a newly emerged 2D material that was first reported in 2011. Since then, a lot of theoretical and experimental work has been done on more than 30 MXene structures for various applications. Given this, in the present review, we have tried to cover the multidisciplinary aspects of MXene including its structures, synthesis methods, and electronic, mechanical, optoelectronic, and magnetic properties. From an application point of view, we explore MXene-based supercapacitors, gas sensors, strain sensors, biosensors, electromagnetic interference shielding, microwave absorption, memristors, and artificial synaptic devices. Also, the impact of MXene-based materials on the characteristics of respective applications is systematically explored. This review provides the current status of MXene nanomaterials for various applications and possible future developments in this field.

High-Capacity Ti3C2Tx MXene Electrodes Achieved by Eliminating Intercalated Water Molecules Using a Co-solvent System

Synthesizing layered transition-metal carbides, MXenes, with a mesoporous structure remains challenging but is highly useful because it converts the laminated two-dimensional structures into versatile porous materials. Hydrogen bonds between intercalated H₂O molecules and oxygen terminal groups on the surface are formed in aqueous solution processes, and this is a determining factor of surface area. We developed an extraction method to remove intercalated water molecules based on a simple intermolecular force attraction strategy in a co-solvent system using a combination of polar-protic/-aprotic and non-polar solvents. As a result, self-aggregated mesoporous Ti₃C₂T_x was realized without any additives. The dipole-dipole interaction between H₂O and CHCl₃ molecules under non-polar solvent conditions assists the extraction of intercalated H₂O from the MXene suspension, which can form a self-aggregated morphology (not re-stacked horizontally). The process yields Ti₃C₂T_x with a layered structure of embedded mesopores and a specific surface area that is 13-fold higher than that of standard MXene. Electrodes made with the resulting MXene exhibited a larger specific capacitance of 224 F/g (1 A/g), with an improved cyclic retention of 96.4%@10,000 cycles. This intermolecular attraction-induced approach, involving the manipulation of morphology, is simple to mass-produce and can be used for MXene-based electrochemical applications.

Thermal Properties of MXenes and Relevant Applications

The properties and applications of MXenes (a family of layered transition metal carbides, nitrides, and carbonitrides) have aroused enormous research interests for a decade since the successful synthesis of few-layer transition metal carbides in 2011. Though MXenes, as the building blocks, have already been applied in various fields (such as wearable electronics) owing to the distinctive optical, mechanical and electrical properties, their thermal stability and intrinsic thermal properties were less thoroughly investigated compared to other characteristics in early reports. The pioneering theoretical prediction of the thermoelectric nature of MXenes was performed in 2013 while the first experiment-based report concerning the degradation behavior of the 2D structure at elevated temperatures in a controlled atmosphere was published in 2015, followed by numerous discoveries regarding the thermal properties of MXenes. Herein, after a brief description of the synthesis, this Review summarized the latest insights into the thermal stability and thermophysical properties of MXenes, and further associated these unique properties with relevant applications by multiple examples. Finally, current hurdles and challenges in this field were provided along with some advices on potential research directions in the future.

Biomedical engineering of two-dimensional MXenes

The emergence of two-dimensional (2D) transition metal carbides, carbonitrides and nitrides, referred to MXenes, with a general chemical formula of M_n+1X_nT_x have aroused considerable interest and shown remarkable potential applications in diverse fields. The unique ultrathin lamellar structure accompanied with charming electronic, optical, magnetic, mechanical and biological properties make MXenes as a kind of promising alternative biomaterials for versatile biomedical applications, as well as uncovering many new fundamental scientific discoveries. Herein, the current state-of-the-art advances of MXenes-related biomaterials are systematically summarized in this comprehensive review, especially focusing on the synthetic methodologies, design and surface engineering strategies, unique properties, biological effects, and particularly the property-activity-effect relationship of MXenes at the nano-bio interface. Furthermore, the elaborated MXenes for varied biomedical applications, such as biosensors and biodevices, antibacteria, bioimaging, therapeutics, theranostics, tissue engineering and regenerative medicine, are illustrated in detail. Finally, we discuss the current challenges and opportunities for future advancement of MXene-based biomaterials in-depth on the basis of the present situation, aiming to facilitate their early realization of practical biomedical applications.

MXene (Ti3C2Tx)-Embedded Nanocomposite Hydrogels for Biomedical Applications: A Review

Polymeric nanocomposites have been outstanding functional materials and have garnered immense attention as sustainable materials to address multi-disciplinary problems. MXenes have emerged as a newer class of 2D materials that produce metallic conductivity upon interaction with hydrophilic species, and their delamination affords monolayer nanoplatelets of a thickness of about one nm and a side size in the micrometer range. Delaminated MXene has a high aspect ratio, making it an alluring nanofiller for multifunctional polymer nanocomposites. Herein, we have classified and discussed the structure, properties and application of major polysaccharide-based electroactive hydrogels (hyaluronic acid (HA), alginate sodium (SA), chitosan (CS) and cellulose) in biomedical applications, starting with the brief historical account of MXene’s development followed by successive discussions on the synthesis methods, structures and properties of nanocomposites encompassing polysaccharides and MXenes, including their biomedical applications, cytotoxicity and biocompatibility aspects. Finally, the MXenes and their utility in the biomedical arena is deliberated with an eye on potential opportunities and challenges anticipated for them in the future, thus promoting their multifaceted applications.

First observation on emergence of strong room-temperature ferroelectricity and multiferroicity in 2D-Ti3C2Tx free-standing MXene film

Two-dimensional (2D) multiferroics are key candidate materials towards advancement of smart technology. Here, we employed a simple synthesis approach to address the long-awaited dream of developing ferroelectric and multiferroic 2D materials, especially in the new class of materials called MXenes. The etched Ti₃C₂T_x MXene was first synthesized after HF-treatment followed by a delamination process for successful synthesis of free-standing Ti₃C₂T_x film. The free-standing film was then exposed to air at room-temperature and heated at different temperatures to form a TiO₂ layer derived from the Ti₃C₂T_x MXene itself. The ferroelectric measurement showed a clear polarization hysteresis loop at room-temperature. Also, due to the reported ferromagnetic behavior of Ti₃C₂T_x MXene, our composite could show multiferroic properties at room-temperature. The magnetoelectric coupling test was also performed that showed a clear, switchable spontaneous polarization under applied magnetic field. TiO₂ is reported to be an incipient ferroelectric that assumes a ferroelectric phase in composite form. The structural and morphological analysis confirmed successful synthesis of free-standing film and the Raman spectroscopy revealed the formation of different phases of TiO₂ and the observed ferroelectricity could be due to structural deformation as a result of the formation of this new phase. The measured value of remanent polarization is 0.5 μC cm⁻². This is the first report on the existence of a ferroelectric phase and multiferroic coupling in 2D free-standing MXene film at room-temperature which opens-up the possibility of 2D material-based electric and magnetic data storage applications at room-temperature.

Two-dimensional (2D) multiferroics are key candidate materials towards advancement of smart technology.

Plasmon-enabled N2 photofixation on partially reduced Ti3C2 MXene

Benefiting from the superior conductivity, rich surface chemistry and tunable bandgap, Ti₃C₂ MXene has become a frontier cocatalyst material for boosting the efficiency of semiconductor photocatalysts. It has been theoretically predicted to be an ideal material for N₂ fixation. However, the realization of N₂ photofixation with Ti₃C₂ as a host photocatalyst has so far remained experimentally challenging. Herein, we report on a sandwich-like plasmon- and an MXene-based photocatalyst made of Au nanospheres and layered Ti₃C₂, and demonstrate its efficient N₂ photofixation in pure water under ambient conditions. The abundant low-valence Ti (Ti^(4-x)+) sites in partially reduced Ti₃C₂ (r-Ti₃C₂) produced by surface engineering through H₂ thermal reduction effectively capture and activate N₂, while Au nanospheres offer plasmonic hot electrons to reduce the activated N₂ into NH₃. The Ti^(4-x)+ active sites and plasmon-generated hot electrons work in tandem to endow r-Ti₃C₂/Au with remarkably enhanced N₂ photofixation activity. Importantly, r-Ti₃C₂/Au exhibits ultrahigh selectivity without the occurrence of competing H₂ evolution. This work opens up a promising route for the rational design of efficient MXene-based photocatalysts.

Tuning the Magnetic Properties of Two-Dimensional MXenes by Chemical Etching

Two-dimensional materials based on transition metal carbides have been intensively studied due to their unique properties including metallic conductivity, hydrophilicity and structural diversity and have shown a great potential in several applications, for example, energy storage, sensing and optoelectronics. While MXenes based on magnetic transition elements show interesting magnetic properties, not much is known about the magnetic properties of titanium-based MXenes. Here, we measured the magnetic properties of Ti3C2Tx MXenes synthesized by different chemical etching conditions such as etching temperature and time. Our magnetic measurements were performed in a superconducting quantum interference device (SQUID) vibrating sample. These data suggest that there is a paramagnetic-antiferromagnetic (PM-AFM) phase transition and the transition temperature depends on the synthesis procedure of MXenes. Our observation indicates that the magnetic properties of these MXenes can be tuned by the extent of chemical etching, which can be beneficial for the design of MXenes-based spintronic devices.

Evidence of a magnetic transition in atomically thin Cr2TiC2Tx MXene

Two-dimensional (2D) transition metal carbides and nitrides known as MXenes have shown attractive functionalities such as high electronic conductivity, a wide range of optical properties, versatile transition metal and surface chemistry, and solution processability. Although extensively studied computationally, the magnetic properties of this large family of 2D materials await experimental exploration. 2D magnetic materials have recently attracted significant interest as model systems to understand low-dimensional magnetism and for potential spintronic applications. Here, we report on synthesis of Cr₂TiC₂T_x MXene and a detailed study of its magnetic as well as electronic properties. Using a combination of magnetometry, synchrotron X-ray linear dichroism, and field- and angular-dependent magnetoresistance measurements, we find clear evidence of a magnetic transition in Cr₂TiC₂T_x at approximately 30 K, which is not present in its bulk layered carbide counterpart (Cr₂TiAlC₂ MAX phase). This work presents the first experimental evidence of a magnetic transition in a MXene material and provides an exciting opportunity to explore magnetism in this large family of 2D materials.

Porosity-Engineering of MXene as a Support Material for a Highly Efficient Electrocatalyst toward Overall Water Splitting

The use of 2 D transition metal carbide MXenes as support materials to incorporate catalytically active compounds is of interest because of their unique properties. However, the preparation of well-dispersed catalytic phases on the inter-connected porous MXene network is challenging and has been rarely explored. This work focuses on the synthesis of basal-plane-porous titanium carbide MXene (ac-Ti₃ C₂ ) that is used subsequently as an effective host for the incorporation of a known catalytically active phase (IrCo) as an effective bifunctional electrocatalyst toward water splitting. The porous ac-Ti₃ C₂ with abundant macro/meso/micropores is prepared by a wet chemical method at room temperature and provides ideal anchor sites for intimate chemical bonding with alien compounds. The resulting IrCo@ac-Ti₃ C₂ electrocatalyst exhibits an excellent reactivity (220 mV at 10 mA cm^-2 ) towards the oxygen evolution reaction in 1.0 m KOH, which surpasses that of the benchmark RuO₂ , a low voltage cell of 1.57 V (@ 10 mA cm^-2 ) and good long-term durability. Our work demonstrates the effectiveness of porosity engineering in MXene nanosheets as a support material to shorten ion migration pathways, to increase electrolyte accessibility between inter-sheets and to overcome inherited re-stacking and aggregation issues.

Structural transformation and nature of defects in titanium carbide treated in different redox atmospheres

The defect structure and phase formation processes occurring in the bulk and on the surface of titanium carbide (TiC) under thermal treatment in different ambient atmospheres (air, hydrogen, vacuum) were characterized using XRD, SEM, EPR, XPS and IR-spectroscopy. The oxidized states of both titanium (TiO_2−x) and carbon, in the form of carbonate–carboxylate structures (O–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O), were found on the surface of untreated TiC. Carbon vacancies were detected as paramagnetic defects in the crystalline lattice of TiC. The heat treatment of TiC in reducing conditions leads to an increase of the electrical conductivity associated with the formation of structural defects. Annealing of TiC in air causes its oxidation with the formation of an anatase-type TiO₂ phase. Paramagnetic defects typical of both TiC and TiO_2−x were revealed.

The state of the surface and the formation of paramagnetic defects (carbon vacancies, Ti³⁺ and O⁻ centers) in titanium carbide after thermal treatment under different ambient conditions have been studied.

Edge Capping of 2D-MXene Sheets with Polyanionic Salts To Mitigate Oxidation in Aqueous Colloidal Suspensions

MXenes have shown promise in myriad applications, such as energy storage, catalysis, EMI shielding, among many others. However, MXene oxidation in aqueous colloidal suspensions when stored in water at ambient conditions remains a challenge. It is now shown that by simply capping the edges of individual MXene flakes, Ti₃ C₂ T_z and V₂ CT_z , by polyanions such as polyphosphates, polysilicates or polyborates, it is possible to quite significantly reduce their propensity for oxidation even when held in aerated water for weeks. This breakthrough resulted from the realization that the edges of MXene sheets are positively charged. It is thus an example of selectively functionalizing the edges differently from the MXene sheet surfaces.

Shape-Adaptable 2D Titanium Carbide (MXene) Heater

Prior to the advent of the next-generation heater for wearable/on-body electronic devices, various properties are required, including conductivity, transparency, mechanical reliability, and conformability. Expansion to two-dimensional (2D) structure of metallic nanowires based on network- and mesh-type geometries has been widely exploited for realizing these heaters. However, the routes led to many drawbacks such as the low-density cross-bar linking, self-aggregation of wire, and high junction resistance. Although 2D carbon nanomaterials such as graphene and reduced graphene oxide (rGO) have shown their potentials for the purpose, CVD-grown graphene with sufficiently high conductivity was limited due to its poor processability for large-area applications, while rGO fabricated with a complex reduction process involving the use of toxic chemicals suffered from a low electrical conductivity. In this study, we demonstrate a simple and robust process, utilizing electrostatic assembling of negatively charged MXene flakes on a positively treated surface of substrate, for fabricating a metal-like 2D MXene thin film heater (TFH). Our TFH showed a high optical property (>65%), low sheet resistance (215 Ω/sq), fast electrothermal response (within dozens of seconds) with an intrinsically high electrical conductivity, and mechanical flexibility (up to 180° bending). Its capability for forming a firm and stable ionic-type interface with a counterpart surface allows us to develop a shape-adaptable and patchable thread heater (TH) that can be shaped on diverse substrates even under harsh conditions of conventional sewing or weaving processes. This work suggests that our shape-adaptable MXene heaters are potentially suitable not only for wearable devices for local heating and defrosting but also for a variety of emerging applications of soft actuators and wearable/flexible healthcare monitoring and thermotherapy.

Surface-Engineered MXenes: Electric Field Control of Magnetism and Enhanced Magnetic Anisotropy

Controlling magnetism in two-dimensional (2D) materials via electric fields and doping enables robust long-range order by providing an external mechanism to modulate magnetic exchange interactions and anisotropy. In this report, we predict that transition metal carbide and nitride MXenes are promising candidates for controllable magnetic 2D materials. The surface terminations introduced during synthesis act as chemical dopants that influence the electronic structure, enabling controllable magnetic order. We show ground-state magnetic ordering in Janus M₂XO _xF_{2- x} (M is an early transition metal, X is carbon or nitrogen, and x = 0.5, 1, or 1.5) with asymmetric surface functionalization, where local structural and chemical disorder induces magnetic ordering in some systems that are nonmagnetic or weakly magnetic in their pristine form. The resulting magnetic states of these noncentrosymmetric structures can be robustly switched and stabilized by tuning the interlayer exchange couplings with small applied electric fields. Furthermore, bond directionality is enhanced by Janus functionalization, resulting in improved magnetic anisotropy, which is essential to stable 2D magnetic ordering. The mixed termination-induced anisotropy leads to robust Ising ferromagnetism with an out-of-plane easy axis over the full range of relevant termination compositions for Janus Mn₂N. Janus Cr₂C, V₂C, and Ti₂C were found to be robustly antiferromagnetic. Our results provide a strategy for exploiting asymmetric surface functionalization to achieve room-temperature nanoscale magnetism under ambient conditions in MXenes with currently available synthesis techniques.