Applications Of MXenes Research Articles

Surface chemistry engineering and the applications of MXenes

MXenes have captured extensive attention in various fields by virtue of unique hydrophilicity, high conductivity and tunable surface terminations. In this review, the state-of-the-art progresses of designing functional MXenes have been summarized. Firstly, the synthesis methods of MXenes are summarized and classified into HF etching, in-situ HF etching and fluoride-free etching approaches based on the effect on the surface chemistry of MXenes. Secondly, the factors that affect the surface termination groups are discussed, including synthesis methods, heat treatment temperature and atmosphere. Thirdly, this review gives an overview of the synthetic routes of functional MXenes including termination modification by synthesis methods and heat treatment, heteroatom (N, S, or P) doping, cation and organic molecule intercalation and hybridization with polymer, which inhibit restacking and increase active sites for intrinsically enhancing the inherent physical and chemical properties of MXenes. Finally, the applications with respect to energy storage and conversion, catalysis, sensors, electromagnetic interference shielding and microwave absorption of functional MXenes are introduced. Additionally, the critical challenges and development prospects of functional MXenes are also highlighted.

Preparation of larger MXene layers and research progress in the field of gas adsorption and separation

Recently, MXene, a novel two-dimensional (2D) material, shows promising potential in many fields. Among these fields, gas separation and adsorption have emerged as new application areas for MXene. The number of studies relevant to the application of MXene in gas separation and adsorption is gradually increasing, making it a hot spot in scientific research. Therefore, timely and systematic reviews of the strengths and weaknesses of MXene are crucial to its further development. This article focused on the application of MXene in gas separation and adsorption. In this review paper, we presented some preparation methods of large MXene layers and practicable strategies for utilizing them to enhance the operational stability of MXene membranes, and the latest research advancements in the application of MXene in gas separation and adsorption. In terms of gas separation, we provided a systematic summary of the research progress based on the techniques used to prepare MXene membranes. As for gas adsorption, we discussed the research progress from both experimental and theoretical perspectives. This discussion was divided into three parts: (1) exploration of the impact of the surface groups on gas adsorption process; (2) analysis on the effect of gas adsorption in gas sensing; and (3) study on theoretical calculation pertaining to gas adsorption and separation. Finally, we evaluated the advantages and disadvantages of using MXene in gas separation and adsorption and then outlined the potential avenues for the future development of MXene. It is expected that this review may provide valuable insights and guidance for the researchers engaged in promoting the intensive study and the broad application of MXene.

CTAB intercalation into two-dimensional transition-metal carbides: Enhancing adsorption performance of Cr (VI)

Nowadays, adsorption is the effective and widely used technology for removing toxic and harmful heavy metals from wastewater. As the most widely used star material in MXenes based materials, Ti3C2 exhibits great brilliance in the field of heavy metal adsorption. Due to the inefficient adsorption performance of Ti3C2 for Cr (VI), the intercalation method is used to treat Ti3C2 to obtain an efficient adsorbent. Therefore, 1CTAB-Ti3C2 material was constructed by intercalating Ti3C2 with Cetyl-trimethyl Ammonium Bromide (CTAB) and used to remove Cr (VI). The CTAB ultrasonic intercalation could make the positively charged CAT+ ionized from the solution attract to the negatively charged Ti3C2 surface through static electricity, and further insert between the negatively charged Ti3C2 layers. Therefore, the layer spacing of Ti3C2 became larger to improve its layer spacing, so that it exposed more adsorption sites and improved the adsorption performance of MXenes. The maximum adsorption capacity of 1CTAB-Ti3C2 for Cr (VI) was approximately 206.8 mg/g. The 1CTAB-Ti3C2 could absorb Cr (VI) at various temperatures and maintain a high adsorption efficiency. This study resoundingly prepared efficient adsorbents using a low-cost intercalation method, and it provided a new idea for the application of MXenes based materials in the field of heavy metal adsorption.

Lubrication mechanism of polyimide/V2CTx MXene composites based on surface chemistry

AbstractA series of polyimide (PI)/V2CTx MXene composites was synthesized via polyamide acids (PAA) and two‐dimensional (2D) V2CTx MXene obtained by etching precursor V2AlC. PI/V2CTx MXene composites with different mass ratios were obtained by blending and heating with PAA. The results showed that V2CTx MXene has excellent dispersion in PI. In particular, V2CTx MXene exhibited excellent friction reduction and anti‐wear properties as a solid lubricant additive, and the wear rate (WR) was reduced by 43.2% by incorporating only 0.8 wt% of V2CTx MXene in the pristine PI and 71.9% by incorporating 1.2 wt%. Scanning electron microscopy (SEM) and x‐ray photoelectron spectroscopy (XPS) analyses showed that the continuous transfer film consisting of a high proportion of tribo‐chemical products formed on the contact surface is significant to improve the lubrication performances of the PI composites. This work broadens the application of MXene in the field of tribology and contributes to energy conservation and sustainable development of human society.Highlights The tribological properties could be tuned by changing the ratio of PI to V2CTx MXene. The WR of PI composites was reduced by 71.9% with incorporating 1.2 wt% of V2CTx MXene. The formation of transfer films on the contact surface is significant for improving anti‐wear of PI.

Dual Localized Surface Plasmon Resonance effect enhances Nb2AlC/Nb2C MXene thermally coupled photocatalytic reduction of CO2 hydrogenation activity

Nb2AlC/Nb2C MXene (NAC/NC) heterojunction photocatalysts with Schottky junctions were obtained by selective etching of the Al layer, resulting in 146.25 μmol·g−1 electrons and 15.28 μmol·g−1 holes stored in the heterojunction. The average conversion of NAC/NC thermally coupled photocatalytic reduction of CO2 under the simulated solar irradiation reached 110.15 μmol⋅g−1⋅h−1, and the CO selectivity reached over 92%, which was 1.49 and 1.74 times higher than that of pure Nb2AlC and Nb2C MXene, respectively. After light excitation, the localized surface plasmon resonance (LSPR) effect of holes distributed on the surface of Nb2C MXene crystals in the heterojunction will form high-energy thermal holes to dissociate H2 to H+ and reduce CO2 to form H2O at the same time. The high-energy electrons formed by the LSPR effect of Nb2C MXene and the conduction band electrons generated by the photoexcitation of Nb2C MXene can be migrated to Nb2AlC under the action of the interfacial Schottky junction to supplement the electrons needed for the LSPR effect of Nb2AlC, which continuously forms high-energy hot electrons to convert the adsorbed CO2 into *CO2−, b-HCO3, and HCOO. Subsequently, HCOO releases ⋅OH in a cyclic reaction to continuously reduce to form CO. The dual LSPR effect of Nb2AlC and Nb2C MXene is used to enhance the hydrogenation activity of thermally coupled photocatalytic reduction of CO2, which provides a new research idea for the application of MXene in thermally coupled photoreduction of CO2.

Mo-MXene-filled gel polymer electrolyte for high-performance quasi-solid-state zinc metal batteries

The production of MXenes from the MAX phase typically requires hydrofluoric acid etching, a process with significant toxicity and severe environmental implications. Given the wide-ranging applications of MXenes, developing a large-scale, environmentally benign chemical etching process is highly desirable. The goal of this study was to establish selective electrochemical aluminum etching (E-etching) from the Mo3AlC2 MAX phase using an innovative 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (EMITFSI) and zinc trifluoromethane sulfonate (Zn(OTF)2) battery electrolyte formulation. The E-etching technique effectively removes the “A” layer from the Mo3AlC2 MAX phase at 60 °C using a TFSI−-based electrolyte. This method demonstrates that the resulting MXene terminal (Tx) contains O and OH groups. Subsequently, the resulting Mo2CTx MXene was embedded into a gel polymer electrolyte (GPE) as an inorganic filler. The Mo2CTx/EMITFSI/Zn(OTF)2/poly(vinylidene fluoride-co-hexafluoropropylene) (PVHF) GPE membrane displayed excellent ionic conductivity, Zn2+ transference number, and galvanostatic Zn plating/stripping compatibility with Zn anodes. Finally, the performance of a zinc-metal battery using a belt-like CaV6O16⋅3H2O cathode was evaluated as a full-cell assembly of CaV6O16⋅3H2O//Mo2CTx/EMITFSI/Zn(OTF)2/PVHF//Zn. This assembly showcased an impressive capacity of 155 mA h g−1, excellent durability (99%), coulombic efficiency approaching 100%, and excellent self-life over 200 h.

Control of electrical properties of Ti3C2Tx mediated by facile alkalization

The surface functional groups of MXenes make the material group special and play a critical role in determining their electrical properties. In this work, Ti3C2Tx MXene's surface functional groups were adjusted by a facile alkalization method, using NaOH and KOH. The alkalization was effective even at very low alkaline concentrations below 0.1 M. It was disclosed that the interlayer spacing became wider and the crystal quality was improved after alkalization. Molecular characterizations showed a clear transition from F-rich terminations to OH/O-rich terminations as a consequence of alkalization. Hall measurements, which can trace multiple electrical parameters elaborately, were utilized to examine the electrical property change induced by alkalization. An abrupt rise of sheet resistance and a sharp drop of carrier concentration were simultaneously observed for a MXene alkalized with a very low alkali concentration. Temperature-dependent Hall measurements and UV–Vis absorption tests revealed that the alkalized MXene did not behave like a semiconductor. Such electrical property change was explained in relation to the modulation of surface functional groups. The results of this work demonstrate that the electrical properties of Ti3C2Tx MXene can be widely tuned within a metal domain by a simple alkalization method, offering a practical property control technique for the expanded application of MXenes.

Simultaneously addressing self-stacking and oxidative degradation issues of Ti3C2Tx MXene through biothermochemistry induced 3D crosslinking

As a recent rising star 2D material, MXenes have attracted great attention in many fields. However, the weak interaction induced self-stacking phenomenon and surface oxidation caused structure failure seriously hinder the practical application of MXenes. Herein we develop a biothermochemistry method to construct 3D crosslinked Ti3C2Tx (3D-MX). This novel structured 3D-MX can be prepared in five different kinds of biological reagents, showing good university. After crosslinking, 3D-MX not only resists layer stacking but also exhibits extreme on-shelf stability up to 550 days. We reveal that the formation mechanism originates from the electrostatic/chemical interaction between surface functional groups and the crosslinking agents. Interestingly, the biothermochemical treated Ti3C2Tx MXenes exhibit an extremely-rapid film-processed capability (only 3 min), addressing the long-standing issue of low-efficient and tedious solution-vacuum filtrated 2D films. To further verify its practical application, we assemble 3D-MX based supercapacitors that show a high specific capacitance of 265 Fg−1 at 1 A g−1, and moreover, no supercapacitance attenuation within 720 h. This proposed biothermochemistry method to simply and rapidly build 3D Ti3C2Tx crosslinked network provides a new perspective to promote the practical application of MXenes through simultaneously addressing their self-stacking and oxidative degradation issues.

Suppressing the self-discharge of MXene-based supercapacitors by liquid crystal additive

MXenes are widely used as electrodes for supercapacitors (SCs) due to their excellent conductivity and abundant redox active sites. However, the severe self-discharge of MXene-based SCs results in significant energy loss, limiting their integration with various energy-harvesting devices. Here we introduced the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) molecules into Ti3C2Tx films and LiCl aqueous electrolytes to suppress the self-discharge rate. The nematic liquid crystal molecules would arrange parallel to each other when the Ti3C2Tx films were charged, while the electrolyte viscosity would increase due to the electrorheological effect, causing the slow cross-diffusion of active ions and reducing the self-discharge. As a result, the MXene-based SCs had a low voltage drop of 35% after 4600 s and a low leakage current of 3.0 µA, respectively. The MXene-based SCs could store energy more effectively, improving their practical application potential in integrated energy harvesting devices, for example, storing the electrical energy harvested by the triboelectric nanogenerators. This work proposed a simple strategy to suppress the self-discharge of Ti3C2Tx-based SCs by liquid crystal additive, which can be extended to other MXenes, promoting the applications of MXenes in energy storage devices and integrated electronics.

Application of MXene in the Electrochemical Detection of Neurotransmitters: A Review

Neurotransmitters are small chemical signaling molecules crucial for the proper function of the nervous system. The dysregulation of neurotransmitters results in several mental disorders like Parkinson’s and Alzheimer’s diseases, schizophrenia, and conditions such as depression and addiction. These signaling molecules are present at low concentrations, and obtaining information about these molecules’ levels is vital. Moreover, neurotransmitter monitoring in the nervous system remains challenging due to its low concentrations and rapid response. Electrochemical detection continues to garner significant attention as an attractive technique due to its facile nature, high sensitivity, and cost-effectiveness. The electroactive materials of electrochemical sensors are at the heart of this sensing technology. Although multiple nanomaterials have been explored as active components in electrochemical sensors for detecting neurotransmitters, MXenes are gaining attention in the electrochemical sensing of neurotransmitters. This review aims to discuss the use of MXenes and their composites for the electrochemical detection of neurotransmitters, describe the various MXene composites based on the nature of the composite viz pristine and chemical functionalized, carbon nanomaterial, polymer, metal nanoparticle, and transition metal dichalcogenide composites, and define the future directions in leveraging the properties of MXene composites for early-stage electrochemical detection of neurological diseases originating from an imbalance in neurotransmitters.

Comprehensive utilization and biomedical application of MXenes - A systematic review of cytotoxicity and biocompatibility

MXenes are the two-dimensional single-layer atomic nanosheets derived from MAX-phase ceramics. Currently, 2D MXene nanomaterials are extensively studied owing to their multifunctional properties. In a systematic review, we found several studies on the cytotoxicity and biocompatibility of surface-modified or functionalized MXene materials for drug delivery against cancer, tissue engineering, antimicrobial activity, bioimaging, photothermal therapy, advanced sensing, and patient monitoring. Further, more than 10 MXenes have been recently studied as in vitro and in vivo models for biomedical applications are reviewed. Recently, the toxicology and biodegradability of unmodified MXene materials have been well studied, possibly with the existing state-of-the-art research as MXenes can reach clinical trials. Relevant concerns and issues arising from the cytotoxicity and drug clearance data available for MXene materials are highlighted, with an emphasis on the regulatory concerns required for the commercial utilization of MXenes in medical devices or drug delivery.

MXene-Embedded Electrospun Polymeric Nanofibers for Biomedical Applications: Recent Advances.

Recently MXenes has gained immense attention as a new and exciting class of two-dimensional material. Due to their unique layered microstructure, the presence of various functional groups at the surface, earth abundance, and attractive electrical, optical, and thermal properties, MXenes are considered promising candidates for various applications such as energy, environmental, and biomedical. The ease of dispersibility and metallic conductivity of MXene render them promising candidates for use as fillers in polymer nanocomposites. MXene-polymer nanocomposites simultaneously benefit from the attractive properties of MXenes and the flexibility and facile processability of polymers. However, the potentiality of MXene to modify the electrospun nanofibers has been less studied. Understanding the interactions between polymeric nanofibers and MXenes is important to widen their role in biomedical applications. This review explores diverse methods of MXene synthesis, discusses our current knowledge of the various biological characteristics of MXene, and the synthesis of MXene incorporated polymeric nanofibers and their utilization in biomedical applications. The information discussed in this review serves to guide the future development and application of MXene-polymer nanofibers in biomedical fields.

Emerging two-dimensional (2D) MXene-based nanostructured materials: Synthesis strategies, properties, and applications as efficient pseudo-supercapacitors

Supercapacitors, which are essential for addressing modern energy challenges, possess outstanding energy storage capabilities without requiring manual maintenance. However, their low energy density and capacity limit their applicability for electrode materials. Recently, two-dimensional materials (2D), and particularly 2D transition metal carbonitrides (MXenes) have attracted considerable interest for developing energy storage devices because of their large specific surface area and fast ion-transport pathways. MXenes mainly comprise a transition metal (M) layer, carbon and/or nitrogen (X) layer, and active functional groups exterior to the layer. Metallic properties of the M layer offer excellent pseudocapacitance characteristics, while X layer is characterized by abundant active sites and conductivity similar to that of graphene. Compared with conventional materials, the existence of various functional groups on the surface of MXenes endows them with stronger hydrophilicity and higher wettability in aqueous electrolytes, which, combined with their diverse frame structures and functional group combinations, significantly augment their applicability for energy storage devices. Therefore, it is necessary to summarize and analyze the research results on MXenses. This review addresses the applications of MXenes in energy storage supercapacitors along with current challenges and solutions including intercalation, doping, surface modification methods, and heterogeneous structures. Finally, the prominent contemporary research trends are comparatively discussed to promote research and development in this field in the near future.

Recent advances in the application of MXenes for neural tissue engineering and regeneration.

Transition metal carbides and nitrides (MXenes) are crystal nanomaterials with a number of surface functional groups such as fluorine, hydroxyl, and oxygen, which can be used as carriers for proteins and drugs. MXenes have excellent biocompatibility, electrical conductivity, surface hydrophilicity, mechanical properties and easy surface modification. However, at present, the stability of most MXenes needs to be improved, and more synthesis methods need to be explored. MXenes are good substrates for nerve cell regeneration and nerve reconstruction, which have broad application prospects in the repair of nervous system injury. Regarding the application of MXenes in neuroscience, mainly at the cellular level, the long-term in vivo biosafety and effects also need to be further explored. This review focuses on the progress of using MXenes in nerve regeneration over the last few years; discussing preparation of MXenes and their biocompatibility with different cells as well as the regulation by MXenes of nerve cell regeneration in two-dimensional and three-dimensional environments in vitro. MXenes have great potential in regulating the proliferation, differentiation, and maturation of nerve cells and in promoting regeneration and recovery after nerve injury. In addition, this review also presents the main challenges during optimization processes, such as the preparation of stable MXenes and long-term in vivo biosafety, and further discusses future directions in neural tissue engineering.

Two‐dimensional MXenes as Emerging Materials: A Comprehensive Review

AbstractBecause of their peculiar two‐dimensional layered microstructure, the existence of numerous functionalities on the surface and excellent electrical, thermal and optical features, MXenes are regarded as promising candidates for solving energy and environment related problems. It is noted that energy conversion and storge capability of MXenes could be raised by altering their dimensions, structure, surface chemistry and chemical composition. Therefore, it is critical to recognize how one can boost the relationship between structure and property from applied viewpoint. In the present study, we reviewed the synthesis, properties and potential applications of MXenes. Furthermore, several properties of MXenes including structural, chemical, optical, mechanical and thermal have been explored. In addition, the potential applications of MXenes in various areas such as photocatalysis, gas sensing, supercapacitors, electrocatalysis and environmental remediation have also been discussed. Based on reported works, it can clearly be noticed that features and potential applications of MXenes may be further improved by applying many alteration and functionalization strategies. This study also focuses on the current progresses and future prospective relating to MXene based composites, which will surely assist the scientists who are doing work in areas of academia and material sciences.

The formation potential of disinfection by-products of Ti3C2Tx MXene

Owing to the outstanding properties, including non-toxicity, large surface area, bactericidal ability, and versatile surface chemistry, Ti3C2Tx MXene-based adsorbents and membranes have been widely explored in drinking water and wastewater treatment. At the same time, with the rapid increase in MXene applications, it is inevitable that MXene will find its way into water and wastewater treatment systems. It is therefore imperative to assess its potential for disinfection by-products (DBPs) formation and physicochemical transformation by chlorination. This study demonstrated that Ti3C2Tx Mxene has a high formation potential of DBPs including trichloromethane (TCM), dichloroacetonitrile (DCAN) and dichloroacetic acid (DCAA) during the chlorination and chloramination procedures. Few-layered Ti3C2Tx with a larger specific surface area produced more DBPs. The concentrations of TCM and DCAN increased monotonically with the prolongation of chlorine contact time, while the amount of DCAA tended to be stable after 10 h of chlorination. Moreover, the effects of chlorine dose were also considered to better understand the pattern of DBPs formation by Ti3C2Tx MXene. The possible mechanism of DBPs formation was deduced by analyzing the results of FTIR, Raman and XPS before and after chlorination. The DBPs were primarily formed with the destruction of TiC bond by chlorine with the concurrent formation of TiO2. The findings of this research add to the understanding of the potential for DBPs formation from carbon-containing nanomaterials in water treatment systems and shed light on the environmental potential risk of MXene.

Mild synthesis of flower-like CuS modified MXene nanocomposites for high-performance supercapacitors and efficient microwave absorption

Benefiting from rich surface functional groups, two-dimensional transition metal carbides (MXene) exhibit excellent metallic conductivity, hydrophilicity and film-forming properties and are widely used in energy storage and microwave absorption. However, the agglomerate and stacking of MXene nanosheets during preparation are largely detrimental to the full realization of their merits at two-dimensional scale. Therefore, we developed an ultrasonic assisted and vacuum filtration method to obtain MXene/CuS thin film electrode material with a "sandwich" structure without destroying its inherent properties, which still maintains a good lamellar structure and exhibits excellent tensile properties. Due to the synergistic effect of MXene and CuS and their redox reaction, the MXene/CuS thin film electrode exhibits a specific capacitance of 491 F g−1 at a current density of 1 A g−1. It can be used directly as positive and negative electrodes to assemble a flexible supercapacitor with a capacitance retention of 89 % at current density of 2 A g−1 after 5000 cycles. In addition, the composite can achieve an effective absorption band width of 3.3 GHz (14.7–18 GHZ) at a thickness of 1 mm. The study shows that layer spacing modulation offers the possibility of large-scale applications of MXene in the field of flexible supercapacitors and microwave absorption, and provides new ideas for the development of multi-purpose materials to meet the needs of different applications.

Stabilizing MXene suspension with polyhydric alcohols

MXenes have promises in myriad applications by virtue of two-dimensional nature and adjustable functional groups. To achieve the applications, MXenes are always first prepared in the form of aqueous suspension. However, fast degradation caused by the attack of dissolved oxygen and water molecules is the main obstacle to the application of MXenes. It has come to light that the degradation preferentially takes place at defective sites and edges where defects enrich. To tackle this problem and increase the stability, herein, using Ti3C2Tx MXene as a model material, we report a simple yet efficient strategy for long term storage of MXene suspension by introducing glycerol, a typical polyhydric alcohol. The effectiveness of the strategy is evidenced by structural compositional and morphological investigations. Glycerol protects the defective sites of MXene flakes through restricting water and/or oxygen molecules from reactive sites. This is supported by ab initio molecular dynamics simulations that form hydrogen bonds between MXene and glycerol molecules just over defective sites. Following this mechanism, other polyhydric alcohols, such as ethylene glycol and propylene glycol, are also effective in stabilizing Ti3C2Tx MXene suspension. The strategy based on polyhydric alcohols has the potential to be extended to other MXenes, solving the most urgent challenge in the field of MXene engineering.

Microstructure and Mechanical Property of MXene-Added Ti–6Al–4V Alloy Fabricated by Laser Powder Bed Fusion

Laser powder bed fusion (L-PBF) has been utilized to prepare a high-strength titanium alloy builds using 0.3 mass% MXene-decorated Ti–6Al–4V alloy particles produced by an electrostatic self-assembly. The MXene/Ti–6Al–4V powder mixture had similar spherical morphology and powder size, while showing higher laser absorptivity as compared with the Ti–6Al–4V powders. Under high-energy laser irradiation, MXene was fully decomposed and uniformly dissolved into the Ti matrix. Microstructure observations illustrated that the MXene/Ti–6Al–4V build was completely consisted of ultrafine carbon-saturated martensite structures. Consequently, the MXene/Ti–6Al–4V build exhibited a high tensile strength of ∼1.4 GPa, attributing to the refinement of martensitic structure and the solid solution strengthening of carbon. These findings of this study may broaden the potential application of MXene and pave up the way towards the fabrication of advanced titanium parts via L-PBF.

Synthesis of Low-Crystalline MnO2/MXene Composites for Capacitive Deionization with Efficient Desalination Capacity

MXene has drawn widespread attention as a potential material for electrode use in capacitive deionization (CDI). However, the applications of MXene are limited by its property of low electrical capacity. Herein, a MnO2/MXene composite was firstly evaluated in a capacitive deionization system, in which the MnO2 acts as intercalation-type pseudocapacitive electrodes to enhance the electrical capacity, and MXene provides an electron conduction highway network that improves the charge transfer of the MnO2. The result showed that the low-crystallinity MnO2 with irregular particles was well-distributed on the surface of the MXene. The desalination capacity of 30.5 mg·g−1 is achieved at a voltage window of 1.2 V, which was higher than that of the reported pure MXene and MnO2. The electrical double-layer (EDL) capacitive and the diffusion-controlled processes are the main charge storage mechanisms, and the EDL contribution provides 50.3% to the total capacitance. This result suggests a promising direction for further applying a MnO2/MXene composite in CDI.