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Surface Terminations and Surface Functionalization Strategies of MXenes
Abstract
MXenes, a novel group of two-dimensional (2D) transition metal nitrides and carbides, have attained substantial attention in research field. The exceptional physicochemical characteristics and diverse chemical compositions of MXenes make them attractive materials for manifold applications; catalysis, energy conversion and storage, biomedicine, sensing etc. Modifications in MXenes via surface termination, surface functionalization, and interlayer engineering can colossally influence the properties of MXenes by increasing the number of electrochemical reactive sites and enhancement in the electronic structure. The surface engineering of MXene surfaces using imido group, halogen, selenium, tellurium, oxygen, sulphur etc. have shown noteworthy electronic and configurational properties. Over the last few years, various surface functionalization and termination strategies were developed to broaden the functional applications of MXenes in various fields. This chapter throws light on various surface termination and surface functionalization strategies involved in developing modified MXenes and their employment in diverse fields.
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Ti3C2TX, as the most explored MXene, is a rising star among 2D materials due to their astonishing physicochemical properties. However, its practical applications remain extremely challenging because of chemical degradation...
Two-dimensional metal carbides and nitrides–MXenes─represent a group of materials which have attained growing attention over the last decade due to their chemical versatility, making them highly promising in areas such as energy storage, superconductivity, and heterogenous catalysis. Surface terminations are a natural consequence of the MXene synthesis, conventionally consisting of O, OH, and F. However, recent studies have extended the chemical domain of the surface terminations to other elements, and they should be considered as an additional parameter governing the MXene properties. There is a shortfall in the understanding of how various chemical species could act as terminations on different MXenes. In particular, there is limited comprehension in which chemical environments different terminations are stable. Here, we present an extensive theoretical study of the surface terminations of MXenes in different atmospheres by considering in total six experimentally achieved MXenes (Ti2C, Nb2C, V2C, Mo2C, Ti3C2, and Nb4C3) and twelve surface terminations (O, OH, N, NH, NH2, S, SH, H, F, Cl, Br, and I). We consider fully terminated (single termination) MXenes and also the impact of substituting individual terminations. Our study provides insights into what terminations are stable on which MXenes in different chemical environments, with predictions of how to obtain single-termination MXenes and which MXenes are resilient under ambient conditions. In addition, we propose synthesis protocols of MXenes which have not yet been realized in experiments. It is anticipated that alongside the development of new synthesis routes, our study will provide design rules for how to tailor the surface terminations of MXenes.
Molten-salt etching of Ti3AlC2 MAX phase offers a promising route to produce 2D Ti3C2Tz (MXene) nanosheets without hazardous HF. However, molten-salt etching results in MXene clays that are not water-dispersible, thus preventing further processing. This occurs because molten-salt etching results in a lack of -OH terminal groups rendering the MXene clay hydrophobic. Here, we demonstrate a method that produces water-dispersible Ti3C2Tz nanosheets using molten salt (SnF2) to etch. In molten salt etching, SnF2 diffuses between the layers to form AlF3 and Sn as byproducts, separating the layers. The stable, aqueous Ti3C2Tz dispersion yields a ζ potential of -31.7 mV, because of -OH terminal groups introduced by KOH washing. X-ray diffraction and electron microscopy confirm the formation of Ti3C2Tz etched clay with substantial d-spacing as compared to clay etched with HF. This work is the first to use molten salt etching to successfully prepare colloidally stable aqueous dispersions of Ti3C2Tz nanosheets.
MXenes, a new family of two-dimensional (2D) transition metal carbides and nitrides, have received great research attention in diverse applications, including biomedicine, sensing, catalysis, energy storage and conversion, adsorption, and membrane-based separation, owing to their extraordinary physicochemical properties and various chemical compositions. In recent years, many surface functionalization strategies, such as surface-initiated polymerization and single heteroatom doping, have been cleverly developed to expand the potential of MXenes in different fields. This article reviews and puts into perspective the recent advances in MXene surface functionalization and various applications of the modified MXenes. The effect of surface functionalization on MXene properties (such as electronic, magnetic, mechanical, optical, and hydrophilicity/hydrophobicity) is discussed. Potential applications of pristine and functionalized MXenes in different fields are discussed. Finally, challenges and future directions in MXene surface modification are examined.
MXenes, an emerging two-dimensional (2D) transition metal carbides, nitrides and carbonitrides, have exhibited great potential as electrocatalysts for hydrogen evolution reaction (HER) due to the excellent characters, including excellent structural and chemical stability, superior electrical conductivity, and large active surface area. In this comprehensive study, firstly, the preparation advances of MXenes are systematically summarized. Then, the representative applications of MXenes-based HER electrocatalysts are introduced, from experimental and theoretical aspects. Thirdly, the strategies for improving HER catalytic activity of MXenes are demonstrated, such as optimizing active sites by termination modification and metal-atom doping, increasing active sites by fabricating various nanostructures. Finally, the existing challenges and new opportunities for MXenes-based electrocatalysts are also elucidated. This paper provides reference for the future development of new and efficient MXenes-based electrocatalysts for hydrogen production through water-splitting technology.
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 ≈20 000 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. The transition metal carbides and nitrides (MXenes) are among the largest 2D material families. MXenes’ unique properties, such as their metal‐like electrical conductivity, render them quite useful in a large number of applications including energy storage, optoelectronic, biomedical, communications, and environmental. A brief historical overview of the first 10 years of MXene research and a perspective on their synthesis and future development are provided.
MXenes are a young family of two-dimensional transition metal carbides, nitrides, and carbonitrides with highly controllable structure, composition, and surface chemistry to adjust for target applications. Here, we demonstrate the modifications of two-dimensional MXenes by low-energy ion implantation, leading to the incorporation of Mn ions in Ti3C2T
x
(where T
x
is a surface termination) thin films. Damage and structural defects caused by the implantation process are characterized at different depths by XPS on Ti 2p core-level spectra, by ToF-SIMS, and with electron energy loss spectroscopy analyses. Results show that the ion-induced alteration of the damage tolerant Ti3C2T
x
layer is due to defect formation at both Ti and C sites, thereby promoting the functionalization of these sites with oxygen groups. This work contributes to the inspiring approach of tailoring 2D MXene structure and properties through doping and defect formation by low-energy ion implantation to expand their practical applications.
Since their discovery in 2011, MXenes (abbreviation for transition metal carbides, nitrides, and carbonitrides) have emerged as a rising star in the family of 2D materials owing to their unique properties. Although the primary research interest is still focused on pristine MXenes and their composites, much attention has in recent years been paid also to MXenes with diverse compositions. To this end, this work offers a comprehensive overview of the progress on compositional engineering of MXenes in terms of doping and substituting from theoretical predictions to experimental investigations. Synthesis and properties are briefly introduced for pristine MXenes and then reviewed for hetero‐MXenes. Theoretical calculations regarding the doping/substituting at M, X, and T sites in MXenes and the role of vacancies are summarized. After discussing the synthesis of hetero‐MXenes with metal/nonmetal (N, S, P) elements by in situ and ex situ strategies, the focus turns to their emerging applications in various fields such as energy storage, electrocatalysts, and sensors. Finally, challenges and prospects of hetero‐MXenes are addressed. It is anticipated that this review will be beneficial to bridge the gap between predictions and experiments as well as to guide the future design of hetero‐MXenes with high performance.
Lithium–sulfur (Li–S) batteries are regarded as one of the most promising candidates for next-generation energy storage systems due to their advantages of its high specific capacity and low material cost. Regrettably, the electrical insulation nature of sulfur, the volume expansion during lithiation, and the internal shuttle effect aroused by the polysulfides dissolution are the central issues for sulfur cathodes. Here, CoFe2O4@C nanocages derived from Prussian blue analogues and polydopamine were utilized as a sulfur host to address these problems. The polar Co–Fe oxide afforded chemical adsorption sites for anchoring polysulfides to reduce the shuttling effect. The hollow core structure of the nanocages could withstand the large volume change during lithiation and delithiation processes. The conductive carbon layer on the nanocages could ameliorate the extremely low electrical conductivity of S and Li2S. The S/CoFe2O4@C cathode demonstrated high capacity and excellent cyclability (821 mA h g⁻¹ after 100 cycles at 0.5C). More importantly, the combinational strategies of good conductivity, physical space encapsulation, and chemical adsorption provide a means of resolving the current cathode issues in Li–S batteries.
Similar to graphene and black phosphorus (BP), 2D transition metal carbides and nitrides (MXenes) are of great interest in a variety of fields, such as energy storage and conversion, sensors, electromagnetic interference (EMI) shielding, and photothermal therapy due to their excellent conductivity and hydrophilicity, large specific capacitance, high photothermal effect, and superior electrochemical performance. To further broaden applicable ranges beyond their existing boundaries and fully exploit these potentials, functional 2D MXene nanostructures in recent years have been rationally designed and developed by various approaches, such as doping strategies, surface functionalization, and hybridization, for next‐generation devices with the merits of low power consumption, intelligence, and high‐integration chips. This review provides an overview of the synthetic routes and fundamental properties of functional 2D MXene nanostructures, including surface‐modified 2D MXenes and mixed‐dimensional MXene‐based heterostructures, highlights the state‐of‐the‐art progress in the applications of functional 2D MXene nanostructures with regard to energy storage and conversion, catalysis, sensors, photodetectors, EMI shielding, degradation, and biomedical applications, and presents the challenges and perspectives in these burgeoning fields. It is hoped that this review will inspire more efforts toward fundamental research on new functional 2D MXene‐based devices to satisfy the growing requirements for next‐generation systems.
Versatile chemical transformations of surface functional groups in 2D transition-metal carbides (MXenes) open up a new design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with O, NH, S, Cl, Se, Br, and Te surface terminations, as well as bare MXenes (no surface termination) was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Ti
n
+1C
n
(n = 1, 2) MXenes terminated with Te2- ligands show a giant, (>18%) in-plane lattice expansion compared to the bulk TiC lattice. Nb2C MXenes exhibited surface-group-dependent superconductivity.
The 2019 Nobel Prize in Chemistry for lithium‐ion batteries is a powerful confirmation of the importance of portable energy storage devices, which will further promote collaborative innovation in the field of new energy storage. Non‐lithium rechargeable energy storage technologies are attracting attention due to their low cost and high energy densities. However, electrochemical performance depends upon the inherent properties of the electrodes. In recent decades, 2D materials have been extensively investigated owing to their unique physical and chemical properties. One of the typical representatives is the MXenes with good electrochemical properties, which have become popular material in the field of energy storage in recent years. The discovery of MXene with metal solution, expanded interlayer‐spacing and tamperable surface termination offers a valuable strategy to discover MXenes with new structures. These flexible properties of MXene allow the tuning of properties for energy storage technologies. Here, the synthesis, structure, properties and applications of MXenes in non‐lithium energy storage technologies are reviewed, and a comprehensive outlook and personal perspective on the future development of MXene in the energy storage system are also presented.
Although lithium–sulfur (Li–S) batteries are one of the most promising energy storage devices owing to their high energy densities, the sluggish reaction kinetics and severe shuttle effect of the sulfur cathodes hinder their practical applications. Here, single atom zinc implanted MXene is introduced into a sulfur cathode, which can not only catalyze the conversion reactions of polysulfides by decreasing the energy barriers from Li2S4 to Li2S2/Li2S but also achieve strong interaction with polysulfides due to the high electronegativity of atomic zinc on MXene. Moreover, it is found that the homogenously dispersed zinc atoms can also accelerate the nucleation of Li2S2/Li2S on MXene layers during the redox reactions. As a result, the sulfur cathode with single atom zinc implanted MXene exhibits a high reversible capacity of 1136 mAh g−1. After electrode optimization, a high areal capacity of 5.3 mAh cm−2, high rate capability of 640 mAh g−1 at 6 C, and good cycle stability (80% capacity retention after 200 cycles at 4 C) can be achieved. Single zinc atom implanted on MXene (Ti3C2Clx) layers not only possesses efficient electrocatalytic activity for polysulfides but also has a strong interaction with polysulfides owing to the high electronegativity of zinc atoms on MXene, greatly facilitating the nucleation and deposition of Li2S2/Li2S on MXene layers. Thus, a stable sulfur cathode with high areal capacity and high rate capabilities is achieved.
2D transition metal carbides, carbonitrides, and nitrides known as MXenes possess high electrical conductivity, large redox active surface area, rich surface chemistry, and tunable structures. Benefiting from these exceptional chemical and physical properties, the applications of MXenes for electrochemical energy storage and conversion have attracted increasing research interests around the world. Notably, the electrochemical performances of MXenes are directly dependent on their synthesis conditions, interfacial chemistries and structural configurations. In this review, we summarize the synthesis techniques of MXenes, as well as the recent advances in the interfacial structure design of MXene‐based nanomaterials for electrochemical energy storage and conversion applications. Additionally, we provide an in‐depth discussion on the relationship between interfacial structure and electrochemical performance from the perspectives of energy storage and electrocatalysis mechanisms. Finally, the challenges and insights for the future research of interfacial structure design of MXenes are outlined. In this review, we summarize the synthesis techniques of MXenes, as well as the recent advances in the interfacial structure design of MXene‐based nanomaterials for electrochemical energy storage and conversion applications. Additionally, we provide an in‐depth discussion on the relationship between interfacial structure and electrochemical performance from the perspectives of energy storage and electrocatalysis mechanisms. Finally, the challenges and perspectives for the future research of interfacial structure design of MXenes are outlined.
Nitrogen doping has been proven to be a facile modification strategy to improve the electrochemical performance of 2D MXenes, a group of promising candidates for energy storage applications. However, the underlying mechanisms, especially the positions of nitrogen dopants, and its effect on the electrical properties of MXenes, are still largely unexplored. Herein, a comprehensive study is carried out to disclose the nitrogen doping mechanism in Ti3C2 MXene, by employing theoretical simulation and experimental characterization. Three possible sites are found in Ti3C2Tx (T = F, OH, and O) to accommodate the nitrogen dopants: lattice substitution (for carbon), function substitution (for –OH), and surface absorption (on –O). Moreover, electrochemical test results confirm that all the three kinds of nitrogen dopants are favorable for improving the specific capacitance of the Ti3C2 electrode, and the underlying factors are successfully distinguished. By revealing the nitrogen doping mechanisms in Ti3C2 MXene, this work provides theoretical guidelines for modulating the electrochemical properties of MXene materials for energy storage applications.
Recently, owing to excellent structural and transport properties, two-dimensional membranes have attracted tremendous interests for their potential application in molecular separation. In this work, a new type of functionalized Ti2CTx MXene membrane was proposed by introducing different kinds of polyelectrolytes to regulate the membrane physicochemical structure for pervaporation dehydration of isopropanol dehydration. The morphology, physical and chemical properties of powders and MXene-based membranes were studied by SEM, AFM, FTIR, XPS and contact angle test. Meanwhile, the effect of polyelectrolytes functionalization on the MXene membrane performance for isopropanol dehydration was also studied. The results showed that poly (diallyldimethylammonium chloride) (PDDA) functionalized MXene membrane possesses highest separation performance owing to optimized electrostatic effect and hydrophilicity. Besides, other influencing factor including PDDA concentration, MXene loading and operating conditions on the MXene membrane performance were also systematically investigated. The developed PDDA functionalized MXene membrane showed excellent separation performance for dehydration of 90 wt% isopropanol/water at 50 oC with total flux of 1237 g/m2·h and separation factor of 1932, showing a great potential in application of solvent dehydration.
The development of non-noble metal electrocatalysts with high performance for the oxygen evolution reaction (OER) is highly desirable but still faces many challenges. Herein, we report a facile and controllable strategy to fabricate N-doped titanium carbide flakes (Ti3C1.8N0.2 and Ti3C1.6N0.4) using an in situ nitrogen solid solution, followed by an etching process. The introduction of nitrogen is beneficial to the Ti3C1.6N0.4 flakes for more exposed active sites, accelerated charge transfer upon an electrochemical reaction, and improved wettability for more accessible sites. As a result, the as-obtained Ti3C1.6N0.4 catalyst exhibits enhanced electrocatalytic properties for OER, which include a small ηonset of 245.8 mV, low Tafel slope of 216.4 mV dec⁻¹, and relatively good catalytic stability. The present work not only deepens the understanding of in situ N-doped MXene electrocatalysts, but also provides a guideline for the preparation of other N-doped MXene-based hybrid materials for other renewable energy applications.
2D MXene‐based nanomaterials have attracted tremendous attention because of their unique physical/chemical properties and wide range of applications in energy storage, catalysis, electronics, optoelectronics, and photonics. However, MXenes and their derivatives have many inherent limitations in terms of energy storage applications. In order to further improve their performance for practical application, the nanoengineering of these 2D materials is extensively investigated. In this Review, the latest research and progress on 2D MXene‐based nanostructures is introduced and discussed, focusing on their preparation methods, properties, and applications for energy storage such as lithium‐ion batteries, sodium‐ion batteries, lithium–sulfur batteries, and supercapacitors. Finally, the critical challenges and perspectives required to be addressed for the future development of these 2D MXene‐based materials for energy storage applications are presented.
MXenes as anode materials for Li-ion batteries (LIBs) have shown excellent electrochemical performance, and they still have great potential. To explore the effect of chemical composition on their electrochemical performance, herein we report an approach for preparation of (Vx, Ti1−x)2C MXenes (x = 1, 0.7, 0.5, 0.3, 0) via etching their precursor solid solutions of (Vx, Ti1−x)2AlC. The exfoliated (Vx, Ti1−x)2C MXenes with good multilayered morphology exhibit good electrochemical properties when used as anodes of LIBs, and the electrochemical performance is optimized for the solid solution of (V0.5, Ti0.5)2C which possesses the highest reversible capacity of 204.9 mAh g⁻¹ and a coulombic efficiency of nearly 100% at 1 A g⁻¹. The heteroatoms in M site of MXene are responsible for the enhanced lithium storage performance.
On the basis of first‐principles calculations, Fe, Co, Ni, Cu, Zn, Ru, Rh, Ag, Ir, Pt, and Au decorated Mo2CO2−δ monolayers are investigated as potential single‐atom catalyst (SAC) candidates for low‐temperature CO oxidation reaction. From a first screening based on intuitive criteria concerning metal sintering, CO poisoning, and O2 adsorption strength, the Zn/Mo2CO2−δ system is selected for further scrutiny by means of reactivity calculations for different CO concentrations. A lower barrier is found for Eley–Rideal reaction mechanism than for the Langmuir–Hinshelwood mechanism. The low Eley–Rideal barrier (0.15 eV) is attributed to the fact that the Zn atom weakens the O‐O bond considerably and the electrophilic attack of CO weakens it further. The main conclusion is that this system is a promising low‐temperature SAC candidate with a lower energy barrier for CO oxidation than noble metal and other 2D SAC systems investigated.
Two-dimensional, 2D, materials, such as graphene, possess a unique morphology that endows them with interesting and novel properties. Currently, the number of non-oxide materials that have been exfoliated is limited to two fairly small groups, viz. hexagonal, van der Waals bonded structures ( e.g. graphene and BN) and layered transition metal chalcogenides.
The MAX phases are a well established family of machinable layered ternary carbides and/or nitrides, with a composition of M n +1 AX n , where M is an early transition metal, A is one of A group elements, X is C and/or N; with n = 1, 2, or 3. Selective etching of the A layers from the MAX phases results in exfoliating of the latter producing 2D layers of transition metals carbides and/or nitrides. We labeled the resulting 2D M n +1 X n layers " MXenes " to emphasize the loss of the A group element from the MAX phases and the suffix " ene " to emphasize their 2D nature and their similarity to graphene.
The etching process is typically carried out using aqueous hydrofluoric acid at room temperature. Nine different MXenes have been reported to date, viz., Ti 2 C, Nb 2 C, V 2 C, (Ti 0.5 ,Nb 0.5 ) 2 C, Ti 3 C 2 , (V 0.5 ,Cr 0.5 ) 3 C 2 , Ti 3 CN, Ta 4 C 3 , and Nb 4 C 3 . The as-synthesized MXenes are terminated with a mixture of OH, O, and/or F groups.
When select MXenes were tested as electrode materials for Li-ion batteries (LIBs), each phase showed unique electrochemical characteristics. In principle, some could be used as cathodes while others as anodes for LIBs. All the tested MXenes showed excellent capability to handle quite high cycling rates (as high as 36 C) as electrode materials for LIBs, which suggest they can be used in Li-ion capacitors as well. The lithiation and delithiation mechanisms were found to be due to redox intercalation/deintercalation reactions.
As synthesized MXenes can be pressed, at room temperature, into freestanding discs, > 300 µm thick. Some of these discs can be used as electrodes for LIBs with high areal capacities exceeding 5 mAh/cm ² . In addition to LIBs, some MXene also showed good capacity as electrode material for other ion batteries such as Na-ion batteries.
Herein the progress in the synthesis of MXenes, in addition to their performance as electrode materials in LIBs and Na-ion batteries will be discussed, with special emphasis on the lithiation and delithiation mechanisms for Ti 3 C 2 .
Vanadium nitride has displayed many interesting characteristics for its use as a pseudocapacitive [1] electrode in an electrochemical capacitor, such as good electronic conductivity, good thermal stability, high density and high specific capacitance. Thin films of VN were prepared by D.C. reactive magnetron sputtering. The electrochemical stability of the films as well as the influence of dissolved oxygen in 1 M KOH electrolyte were investigated. In order to avoid material as well as electrolyte degradation, it was concluded that vanadium nitride should only be cycled between -0.4 and -1.0 V vs. Hg/HgO. After a 24 hours stabilization period, the prepared VN thin film showed an initial capacitance of 19 mF.cm ⁻² and a capacity retention of 96% after 10000 cycles. Furthermore, dissolved oxygen in the electrolyte was demonstrated to cause self-discharge up to a potential above -0.4 V vs. Hg/HgO, where VN was shown to be unstable. Additionally, the presence of oxygen was shown to shift the open circuit potential of a VN electrode to about 0 V through self-discharge processes [2].
Since the performances reached on thin film electrodes are difficult to translate to VN powder, the use of VN thin films in hybrid microdevices has been investigated [3]. Microdevices were designed using VN thin films as negative electrode and electrodeposited Co 3 O 4 as positive electrode in order to combine the high capacity of the Faradaic type cobalt based electrode and the high capacitance and good cycling ability of VN electrode when used in an optimized electrochemical window. The performance of such devices will be reported and discussed with regards to existing literature on the field.
References
[1] Brousse, T.; Belanger, D.; Long, J. W. J. Electrochem. Soc. 2015, 162, A5185–A5189.
[2] Morel, A.; Borjon-Piron, Y.; Lucio Porto, R.; Brousse, T.; Bélanger, D.; J. Electrochem. Soc. 2016, 163, A1077-A1082.
[3] Eustache, E.; Frappier, R.; Porto, R. L.; Bouhtiyya, S.; Pierson, J.-F.; Brousse, T. Electrochem. Commun. 2013, 28, 104–106.
Two-dimensional (2D) transition metal carbide/nitride (MXene) has been a hot topic of research since it was proposed in 2011. Etching methods to obtain MXene from MAX phase have been continuously investigated. The excellent hydrophilicity, conductivity and tunability make MXene widely used in electrochemistry. In addition, the development of wearable sensors has promoted the research of conductive hydrogels, and improving the performance of conductive hydrogels is still an area of effort for researchers. Therefore, the incorporation of MXene into hydrogel systems will result in significant improvements in performance. This paper reviews the etching methods of MXene, the synthesis strategies and applications of MXene conductive hydrogels, and provides an outlook on the future development of MXene and MXene conductive hydrogels.
Ultrafast flashlight sintering (FLS) has become an important green manufacturing technology for the structural reformation of various nanomaterials. Nickel oxide (NiO) has been extensively studied as a promising electrode material for electrochemical energy storage owing to its high theoretical specific capacitance, low cost, and appropriate chemical compatibility. This study reports the fabrication of mesoporous ultrathin NiO nanosheets on carbon cloth (CC) as green electrodes for flexible supercapacitors (SCs) through an exceptional ultrafast millisecond FLS process at room temperature. The optimized [email protected]i12J electrode exhibited a remarkable specific capacity of 202.3 mA h g⁻¹ (1215 F g⁻¹) at a current density of 2 A g⁻¹. Strikingly, the as-fabricated FLS-NiO electrodes outperformed the time and energy-consuming conventional thermally annealed (CTA) NiO electrodes. Furthermore, the flexible [email protected]i12J//rGO asymmetric SCs deliver a remarkable energy density of 47.18 Wh kg⁻¹ at a power density of 758.37 W Kg⁻¹ and extraordinary cycling stability performance after 15000 cycles. In addition, the [email protected]i12J electrodes offer unique mesoporous structures, high surface areas, and numerous open-pore channels of ultrathin NiO nanosheets that facilitate fast transport of ions and rapid redox reactions. Thus, the present approach is promising for designing advanced electrode materials for flexible energy storage applications.
Surface terminations of two‐dimensional MXene (Ti 3 C 2 T x ) considerably impact its physicochemical properties. Commonly used etching methods usually introduce –F surface terminations or metallic impurities in MXene. We present a new molten‐salt‐assisted electrochemical etching method to synthesize fluorine‐free Ti 3 C 2 Cl 2 . Using electrons as reaction agents, cathode reduction and anode etching can be spatially isolated; thus, no metallics are present in the Ti 3 C 2 Cl 2 product. The surface terminations can be in situ modified from –Cl to –O and/or –S, which considerably shortens the modification steps and enriches the the variety of surface terminations. The obtained –O‐terminated Ti 3 C 2 T x are excellent electrode materials for supercapacitors, exhibiting capacitances of 225 F g ‐1 at 1.0 A g ‐1 , good rate performance (91.1% at 10 A g ‐1 ) and excellent capacitance retention (100 % after 10000 charging‐discharging cycles at 10 A g ‐1 ), which is superior to multi‐layered Ti 3 C 2 T x prepared by other etching methods.
Lithium-sulfur (Li–S) batteries are regarded as potential alternatives to lithium-ion batteries due to their extremely high theoretical energy density. Nevertheless, Li–S batteries still suffer from low coulombic efficiency, low sulfur utilization, and poor cycling life, which hinder their further applications. To obtain ideal Li–S cells, intensive work has been dedicated to improve the conductivity of the electrode, inhibiting the shuttle of lithium polysulfides, and promoting the redox of sulfur. Two-dimensional transition metal carbides, nitrides or carbonitrides, also known as MXenes, attracting significant research interest in Li–S batteries due to their high conductivity, abundant active sites, layered structure and adjustable surface chemistry. In this review, we summarize the partial etching methods of MXenes with different surface terminations, the role of MXenes in Li–S batteries and MXene-based composites designed for Li–S batteries based on interfacial chemistry and interlayer structure. In the end, we also propose the perspectives of MXenes for Li–S batteries.
Ti3C2Tx MXene has attracted remarkable attention due to its promising applications in energy storage and sensors. However, traditional MXene preparation methods used HF as etchant, which was highly toxic and harmful to human and environment. Moreover, the aqueous etchants will also result in the combination of OH, O and F groups on the surfaces, making it difficult to control the varieties and contents of the surface terminations. In this paper, a green and mild electrochemical exfoliation method was proposed to synthesize Ti3C2Fx and synchronously control its fluorination degree on the surface. A non-aqueous ionic liquid, [BMIM][PF6]-based solution was used as electrolyte. The as-prepared Ti3C2Fx was fluorinated with the CF and TiF3 groups, which were electrochemically active and contributed to the excellent cycling stability of the MXene anode-based Li-ion batteries. These findings provided a facile strategy to prepare MXene materials and dope MXene with tailored property for MXene-based energy devices applications.
Precise tailoring of two-dimensional nanosheets with organic molecules is critical to passivate the surface and control the reactivity, which is essential for a wide range of applications. Herein, we introduce catechols to functionalize exfoliated MXenes (Ti3C2T
x
) in a colloidal suspension. Catechols react spontaneously with Ti3C2T
x
surfaces, where binding is initiated from a charge-transfer complex as confirmed by density functional theory (DFT) and UV-vis. Ti3C2T
x
sheet interlayer spacing is increased by catechol functionalization, as confirmed by X-ray diffraction (XRD), while Raman and atomic force microscopy-infrared spectroscopy (AFM-IR) measurements indicate binding of catechols at the Ti3C2T
x
surface occurs through metal-oxygen bonds, which is supported by DFT calculations. Finally, we demonstrate immobilization of a fluorescent dye on the surface of MXene. Our results establish a strategy for tailoring MXene surfaces via aqueous functionalization with catechols, whereby colloidal stability can be modified and further functionality can be introduced, which could provide excellent anchoring points to grow polymer brushes and tune specific properties.
Sensors are becoming increasingly significant in our daily life because of the rapid development in electronic and information technologies, including Internet of Things, wearable electronics, home automation, intelligent industry, etc. There is no doubt that their performances are primarily determined by the sensing materials. Among all potential candidates, layered nanomaterials with two-dimensional (2D) planar structure have numerous superior properties to their bulk counterparts which are suitable for building various high-performance sensors. As an emerging 2D material, MXenes possess several advantageous features of adjustable surface properties, tunable bandgap, and excellent mechanical strength, making them attractive in various applications. Herein, we particularly focus on the recent research progress in MXene-based sensors, discuss the merits of MXenes and their derivatives as sensing materials for collecting various signals, and try to elucidate the design principles and working mechanisms of the corresponding MXene-based sensors, including strain/stress sensors, gas sensors, electrochemical sensors, optical sensors, and humidity sensors. In the end, we analyze the main challenges and future outlook of MXene-based materials in sensor applications.
MXenes are a group of recently discovered 2D materials and have attracted extensive attention since their first report in 2011; they have shown excellent prospects for energy storage applications owing to their unique layered microstructure and tunable electrical properties. One major feature of MXenes is their tailorable surface terminations (e.g., −F, −O, −OH). Numerous studies have indicated that the composition of the surface terminations can significantly impact the electrochemical properties of MXenes. Nonetheless, the underlying mechanisms are still poorly understood, mainly because of the difficulties in quantitative analysis and characterization. This review summarizes the latest research progress on MXene terminations. First, a systematic introduction to the approaches for preparing MXenes is presented, which generally dominates the surface terminations. Then, theoretical and experimental efforts regarding the surface terminations are discussed, and the influence of surface terminations on the electronic and electrochemical properties of MXenes are generalized. Finally, we present the significance and research prospects of MXene terminations. We expect this review to encourage research on MXenes and provide guidance for usingthese materials for batteries and supercapacitors.
Two-dimensional (2D) transition metal carbides, nitrides and carbonitrides (MXenes) have been extensively studied in catalysis due to their high specific surface area, favorable electrochemical properties, excellent corrosion resistance, high mechanical strength, superior electrical and thermal conductivity. Additionally, the unique 2D features with adjustable surface properties of MXene enable its structure to be engineered as catalyst towards a range of the electro-, photo- and thermal-catalytic reactions. In this review, we summarize the advanced engineering strategies that have been developed for MXenes to achieve desirable catalytic performance, including termination engineering, atomic engineering, interfacial engineering and hybrid engineering. We focus on disclosing the origin of enhanced catalytic activity realized by different engineering strategies from the mechanism perspective. Then the limitations and challenges of MXenes engineering under various conditions are discussed. Furthermore, some advanced in situ techniques are recommended to elucidate the active sites in MXenes. Finally, we point out the future directions in MXenes engineering towards highly active catalysts at industrial standard.
Two demensional nanomaterials have attracted increasing research interest for various applications ranging from environmental adsorption, biomedical applications, catalytic degradation and energy storage owing to their high surface areas, facile surface functionalization. In this work, we demonstrated the preparation and environmental adsorption applications of 2D material MXenes through extracting Al layer from Ti3AlC2 in a simpler and safer way. The surface of Ti3C2 was modified with sulfonic groups via aromatic coupling-diazotization. The adsorption behavior and capacity of the Ti3C2 and functionalized Ti3C2 towards methylene blue (MB) were also examined and compared. The successful preparation of Ti3C2 and modified Ti3C2 was characterized by a number of techniques, including scanning electron microscopy (SEM), transmission electron microscope (TEM), Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The influence of different experiment conditions such as contact time, solution temperature, pH and initial MB concentrations on the adsorption behavior towards MB were also examined. The results demonstrated that sulfonic groups functionalized Ti3C2 (named as Ti3C2–SO3H) exhibited superior removal efficiency for MB. The maximum adsorption capacity of Ti3C2–SO3H towards MB is over four folds that of raw materials. The results of kinetics and isotherms studies demonstrated that experimental data were better described by pseudo-first-order model and Langmuir isotherm adsorption model, respectively. The process of MB onto surface of adsorbents was endothermic and spontaneous. Experimental details about pH indicated that the dye adsorption capability is favorable when the aqueous solution is alkaline. Based on the above results, cationic dye molecules were removed effectively by using Ti3C2–SO3H as adsorbent, which promotes the development of Ti3C2–SO3H for wastewater treatment.
Two-dimensional transition-metal carbides and nitrides, known as MXenes, have garnered increasing attention for nearly a decade by virtue of their versatile composition and structure, stability under certain conditions of interest in (electro)catalysis, and numerous appealing properties. Given the abundance of their components, large surface area and high laboratory-scale activities, MXenes are promising catalysts and supports for several heterogeneous catalytic and electrocatalytic reactions. This Perspective briefly summarizes the most relevant (electro)catalytic processes where MXenes hold promise as competitive alternatives to traditional Pt-group catalysts. We discuss how the interplay of metal elements with carbon, nitrogen, and various surface terminations modulates the catalytic activity of MXenes. In particular, we analyze the connection between experimental and simulated MXene structures and discuss the differences with bulk carbide extended surfaces. The great advances in synthesis routes and upscaling, in combination with realistic computational models, may give MXenes a leading role in the current quest for efficient catalytic processes.
Two-dimensional (2D) transition-metal carbides (Ti3C2T
x
MXene) have received a great deal of attention for potential use in gas sensing showing the highest sensitivity among 2D materials and good gas selectivity. However, one of the long-standing challenges of the MXenes is their poor stability against hydration and oxidation in a humid environment, limiting their long-term storage and applications. Integration of an effective protection layer with MXenes shows promise for overcoming this major drawback. Herein, we demonstrate a surface functionalization strategy for Ti3C2T
x
with fluoroalkylsilane (FOTS) molecules through surface treatment, providing not only a superhydrophobic surface, mechanical/environmental stability but also enhanced sensing performance. The experimental results show that high sensitivity, good repeatability, long-term stability, and selectivity and faster response/recovery property were achieved by the FOTS-functionalized when Ti3C2T
x
was integrated into chemoresistive sensors sensitive to oxygen-containing volatile organic compounds (ethanol, acetone). FOTS functionalization provided protection to sensing response when the dynamic response of the Ti3C2T
x
-F sensor to 30 ppm of ethanol was measured over in the 5 to 80% relative humidity range. Density functional theory simulations suggested that the strong adsorption energy of ethanol on Ti3C2T
x
-F and the local structure deformation induced by ethanol adsorption, contributing to the gas-sensing enhancement. This study offers a facile and practical solution for developing highly reliable MXene based gas-sensing devices with response that is stable in air and in the presence of water.
2D materials consist of a single or a few layers, each layer being one or several atoms thickness. The use of the 2D substrate facilitates the design and discovery of 2D based advanced functional materials with tuneable and superior properties from the parent 2D material through hydrothermal approaches. The utilisation of creative synthetic protocols, such as batch and continuous hydrothermal flow syntheses, create diverse and unique opportunities to engineer and deliver 2D derivatives with new or enhanced characteristics, and even unexpected new phenomena. For more details see the Minireview by S. Kellici et al. on page 6447 ff.
Since MXenes (a new family of two-dimensional materials) were first produced in 2011, they have become very attractive nanomaterials due to their unique properties and the range of potential industrial applications. Numerous recent studies have discussed the environmental applications of different MXenes in adsorption, catalysis, and membranes. Only a limited number of MXene-based membrane studies have been published to date, and most have discussed only specific MXenes (i.e., Ti3C2Tx), a small number of solutes (e.g., dyes and inorganic salts), and laboratory-scale short-term experiments under limited water-quality and operational conditions. In addition, to our knowledge, there has been no review of MXene-membrane studies. It is therefore essential to assess the current status of understanding of the performance of these membranes in liquid separation and water purification. Here, a comprehensive literature review is conducted to summarize the current preparation techniques for MXene-based membranes and their applications, particularly in terms of environmental and industrial applications (e.g., water treatment and organic solvent filtration), and to direct future research by identifying gaps in our present understanding. In particular, this review focuses on several key factors, including the effects of preparation techniques on membrane properties, operational conditions, and compound properties that influence liquid separation during MXene-based membrane filtration.
MXenes are emerging two-dimensional (2D) materials for energy-storage applications and supercapacitors. Their surface chemistry, which determines critical properties, varies due to different synthesis conditions. In this work, we synthesized TiVC solid-solution MXenes by two different synthesis methods and investigated their surface functional groups. We performed etching of the TiVAlC MAX phase using two different solutions, a highly concentrated HF (50 wt % ≈ 29 M) and a mixture of LiF and HCl (1.9 M LiF/12 M HCl). Large-scale delamination of TiVCT
x
to produce single-flake suspension was achieved by further intercalation of the resultant MXene from LiF/HCl with tetrabutylammonium hydroxide (TBAOH). X-ray diffraction indicates a large interlayer spacing of 2.18 nm for TiVCT
x
MXene flakes. To investigate the structural stability and adsorption energy of different functional groups on TiVC MXenes, density functional theory (DFT) calculations were performed and supported with X-ray photoelectron spectroscopy (XPS) measurements. A higher concentration of ═O and a lower concentration of -F were achieved on the TiVC synthesized by LiF/HCl, both of which provide a more favorable surface chemistry for energy-storage applications. Our results provide the first systematic study on the effect of synthesis conditions on the surface chemistry of solid-solution TiVC MXenes.
2D transition metal carbides and/or nitrides, known as MXenes, are a new family of 2D materials with close to 30 members experimentally synthesized and dozens more theoretically investigated. Due to the abundant surface terminations, MXenes have been compounded with various materials by multi-interactions. In addition to the prevented aggregation and oxidation of MXene flakes, the MXene/polymer membranes exhibit outstanding mechanical, thermal and electrical properties due to the synergistic effects. However, relatively little is currently known about the MXene/polymer membranes and a special review on the progress of the synthesis, properties and applications of MXene/polymer membranes has not been reported to date. In this review, we begin by reviewing the synthesis and properties of MXenes, and further elaborate the development of MXene/polymer membranes. We aim to summarize various approaches of fabricating MXene/polymer membranes and their fascinating properties. The focus then turns to their exciting potential applications in various fields such as filtration, electromagnetic interference (EMI) shielding, energy storage devices and wearable electronics, etc. Finally, we provide our outlook and perspectives for the future challenges and prospects of MXene/polymer membranes.
MXenes are a new type of two-dimensional (2D) transition metal carbide or carbonitride material with a 2D structure similar to graphene. The general formula of MXenes is Mn+1XnTx, in which M is an early transition metal element, X represents carbon, nitrogen and boron, and T is a surface oxygen-containing or fluorine-containing group. These novel 2D materials possess a unique 2D layered structure, large specific surface area, good conductivity, stability, and mechanical properties. Benefitting from these properties, MXenes have received increasing attention and emerged as new substrate materials for exploration of various applications including, energy storage and conversion, photothermal treatment, drug delivery, environmental adsorption and catalytic degradation. The progress on various applications of MXene-based materials has been reviewed; while only a few of them covered environmental remediation, surface modification of MXenes has never been highlighted. In this review, we highlight recent advances and achievements in surface modification and environmental applications (such as environmental adsorption and catalytic degradation) of MXene-based materials. The current studies on the biocompatibility and toxicity of MXenes and related materials are summarized in the following sections. The challenges and future directions of the environmental applications of MXene-based materials are also discussed and highlighted.
Metal-free black phosphorus nanosheets (BP) have emerged as a promising photocatalyst. Herein, the 2D early transition-metal carbides and nitrides (MXenes) decorated BP (Ti3C2Tx/TiO2-BP 2D-2D) nanohybrids were constructed by hydrothermal method, in which the TiO2 was produced in hydrothermal processes. The optimized Ti3C2Tx/TiO2-BP nanohybrids exhibited a higher visible-light photodegradation efficiency of rhodamine B (99.09%) and tetracycline hydrochloride (92.70%) pollutants than that of pristine BP (12.75% and 9.35%, respectively). Diversified characterization techniques and density functional theory calculations have revealed that such enhanced photocatalytic performance was due to the synergistic effect of BP and Ti3C2Tx/TiO2, which could markedly improve the stability of BP, increase visible light absorption, prolong photoexcited electron lifetime, accelerate the photoinduced electron transfer and hinder the electron-hole (e--h+) pairs recombination. Meanwhile, the mechanism analysis indicated that •O2− radicals played a leading role in the photocatalytic process. This study will motivate great interest in using 2D MXenes as co-catalysts to enhance the activity of BP for its applications.
2D materials have brought about significant technological advancements in the field of biomaterials. ‘MXene’, a ceramic-based 2D nanomaterial, is comprised of transition metal carbides, nitrides, and carbonitrides having a planar structure educed from a ceramic ‘MAX’ phase by etching out ‘A’ from it, has emerged to surpass drawbacks of conventional biomaterials. In spite of their substantial properties like large surface area, biocompatibility, hydrophilicity, metallic conductivity, and size tunability, the use of MXene is restricted in biomedical applications due of its poor stability in physiological environments, lack of sustained and controlled drug release, and low biodegradability, and these limitations lead to the notion of adopting MXene/Polymer nanocomposites. The availability of functional groups on the surface of MXenes enables polymer functionalization. These polymers functionalized MXene nanocomposites exhibit high photothermal conversion efficiency, selectivity, and stimuli-responsiveness towards malignant cells, electron sensitivity, higher antibacterial properties, and the like. This review emphasizes the innovative exemplars of polymer functionalized MXene composites for the burgeoning biomedical applications, which include controlled and sustained drug delivery, antibacterial activity, photothermal cancer therapy, unambiguous biosensing, contrast-enhanced diagnostic imaging, and bone regeneration.
MXenes, as a novel kind of two-dimensional (2D) materials, were first discovered by Gogotsi et al. in 2011. Owing to their multifarious chemical compositions and outstanding physicochemical properties, the novel types of 2D materials have attracted intensive research interest for potential applications in various fields such as energy storage and conversion, environmental remediation, catalysis, and biomedicine. Although many achievements have been made in recent years, there still remains a lack of reviews to summarize these recent advances of MXenes, especially in biomedical fields. Understanding the current status of surface modification, biomedical applications and toxicity of MXenes and related materials will give some inspiration to the development of novel methods for the preparation of multifunctional MXene-based materials and promote the practical biomedical applications of MXenes and related materials. In this review, we present the recent developments in the surface modification of MXenes and the biomedical applications of MXene-based materials. In the first section, some typical surface modification strategies were introduced and the related issues were also discussed. Then, the potential biomedical applications (such as biosensor, biological imaging, photothermal therapy, drug delivery, theranostic nanoplatforms, and antibacterial agents) of MXenes and related materials were summarized and highlighted in the following sections. In the last section, the toxicity and biocompatibility of MXenes in vitro were mentioned. Finally, the development, future directions and challenges about the surface modification of MXene-based materials for biomedical applications were discussed. We believe that this review article will attract great interest from the scientists in materials, chemistry, biomedicine and related fields and promote the development of MXenes and related materials for biomedical applications.
Herein, we demonstrate the selective sensing of Dopamine (DA), an important neurotransmitter, by using Nafion-stabilized two-dimensional transition metal carbides (Ti3C2Tx MXenes). Ti3C2Tx was deposited on a glassy carbon electrode followed by Nafion coating to achieve a robust sensitivity (∼3 nM), good selectivity, wide detection window (0.015–10 μM), high stability, reproducibility, and outstanding recoveries for DA detection in real samples. MXene-based biosensor exhibited much better electrochemical performance when compared with reduced graphene oxide-based biosensor under similar experimental conditions due to the MXenes good electrical conductivity and negatively charged surface which assisted in the selective and sensitive detection of DA.
MXenes are a new group of 2D nanomaterials with fascinating properties including high electrical conductivity, hydrophilic nature, easily tunable structure and high surface area. This is why MXene modified interfaces are extremely promising for the preparation of sensitive electrochemical biosensors. While there are numerous reports on MXene‐based enzymatic biosensors for detection of a wide range of analytes, application of MXene for construction of affinity biosensors is in its infancy. The review article summarizes current state‐of the‐art in the field with a focus on MXene modifications needed for construction of robust and high performance MXene electrochemical biosensors.
MXene, a recently discovered family of 2D materials, is well known for its application in energy storage areas due to its excellent properties. However, the low capacity of pristine MXene limits its application in energy storage devices such as lithium ion batteries. To solve this problem, we synthesized a new type of N-doped two-dimensional Nb2CTx MXene. The nitrogen content of N-doped MXene was 4.5 at% after thermal reaction with urea. The introduction of nitrogen into MXene nanosheets increases c-lattice parameter of Nb2CTx MXene from 22.32 Å to 34.78 Å. And the incorporation of nitrogen into MXene nanosheets leads to superior electrochemical performance. For example, the N-doped Nb2CTx shows an increased reversible capacity of 360 mAh
/g at 0.2C, which is much higher than that of the un-doped Nb2CTx MXene (190 mAh/g at 0.2C). And the N-doped Nb2CTx MXene also shows excellent cycling stability (288 mAh/g at 0.5C after 1500 cycles). Our results suggest N-doped Nb2CTx MXene is promising for energy storage applications.