2D Metal Carbides and Nitrides (MXenes)

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are a large family of 2D materials. Although the first MXene was discovered in 2011 without any prior prediction of their existence, the family has grown significantly, both from the chemistry and application perspectives. There are about thirty stoichiometric MXene compositions reported and many more are waiting to be discovered. MXenes reported to date are hydrophilic, typically with high metallic conductivity. MXenes have a variety of applications including energy generation and storage, electromagnetic interference shielding, water purification, catalysis, optoelectronics, gas- and biosensors, reinforcement for composites, and biomedical ones. This chapter gives an overview of MXenes and describes how each chapter of this book covers different properties and applications of MXenes.

This chapter gives an overview of the class of layered, machinable, M_n+1AX_n or MAX, phases (where M = early transition metal; A = A-group element, e.g., Al or Si; and X = C or N) precursor to MXene. These materials exhibit an unusual combination of metallic and ceramic properties: good electrical and thermal conductivity, machinability, and resistance to thermal shock. Some are also quite oxidation and creep resistant and plastic at high temperatures. These remarkable properties originate from their layered structure and the mixed metallic-covalent nature of the strong M-X bonds together with M-A bonds that are relatively weak. They are also the common precursors for the class of 2D materials known as MXenes, which are typically formed by selectively etching, mostly Al, from the MAX phases. Within a couple of years of the first report, MXenes have established themselves as a fascinating class of 2D materials with remarkable possibilities for composition variations and property tuning.

The family of MXenes has expanded since the discovery of chemical order in parent quaternary MAX phases, displaying either out-of-plane (o-MAX) or in-plane (i-MAX) order upon alloying. Through selective chemical etching of these materials, corresponding MXenes can be derived, with out-of-plane and in-plane ordering of elements, as well as with ordering of vacancies. Both o-MAX and i-MAX phases have increased the number of metals that can be incorporated in these laminated carbides and nitrides. Examples of realized MXenes with out-of-plane order are Mo₂Ti₂C₃ and Mo₂ScC₂, and for in-plane ordering of vacancies, there are Mo_1.33C and W_1.33C. Their versatile chemistry shows a high promise for a range of applications, including energy storage and catalysis.

MXenes, as the new family of two-dimensional materials, have attracted extensive attention due to their widespread potential applications. To enrich the MXene family becomes an important research goal in recent years. Generally, MXenes are fabricated from selective etching of the Al-containing MAX phases. Here, in contrast to the traditional approach, the syntheses of MXenes by selective etching of non-MAX precursors are summarized in this chapter. In addition, the existing non-MAX layered carbides (MC)_n[Al(A)]_mC_m−1 (n is generally 2~4, m is 3 or 4, A is Si and/or Ge) possible for MXene precursors are reviewed. Specially, Zr₃C₂T_x is firstly synthesized by selective etching of Zr₃Al₃C₅, where an Al₃C₃ unit instead of an Al atomic layer is etched out. The obtained configuration shows relatively high thermal stability with its structure is stable even under 1200 °C. Hf₃C₂T_x is further synthesized from selective etching of a solid solution Hf₃(Al,Si)₄C₆. Si is determined to weaken the layer adhesion and facilitate the etching process. Additionally, with a mild organic base as etchant, a semiconducting MXene member ScC_xOH is realized by selective etching of ScAl₃C₃. Different from the Al-containing layered carbides, the Mo₂Ga₂C phase wherein two Ga layers stacked between Mo₂C layers is also adopted for the synthesis of Mo₂CT_x. Both Hf₃C₂T_x and Mo₂CT_x are found to show promising applications in energy storage. For instance, the volumetric capacity for Hf₃C₂T_x is measured as high as 1567 mAh cm⁻³ for lithium-ion batteries at a current density of 200 mA g⁻¹ after 200 cycles. Based on these non-MAX precursors, we look forward to more promising MXene configurations that could be realized in the near future.

Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, commonly known as MXenes, have continually gained interest since the discovery of the first MXene in 2011, due to their tunable structures, wide variety of properties, and performance in an extensive set of applications. However, the resulting structures, properties, and performances have been directly related to how the material is synthesized or processed. This chapter serves as an introduction to the current top-down chemical etching methods used to synthesize MXenes, such as fluoride-containing aqueous etchant solutions, molten-salt etching procedures, and alkaline or hydrothermal treatments. Emphasis is given to selective etching from aluminum-containing layered precursors; however, insight into selective etching of other A elements (gallium, silicon, etc.) is discussed. The latter half of the chapter is devoted to reviewing large-scale production and processing methods, general processing and properties which should be expected after each synthesis or deposition route, an overview of current challenges, and a perspective for the future of 2D MXenes.

In this chapter, three typical bottom-up methods for the synthesis of 2D transition metal carbides (including their heterostructures with graphene) and nitrides, including chemical vapor deposition (CVD), template method, and plasma-enhanced pulsed laser deposition (PEPLD), are introduced. The obtained samples show high crystalline quality and various structures. Especially, the CVD and PEPLD methods provide ideal samples for the study of high crystalline 2D superconductors and their heterostructures with graphene. Moreover, the samples fabricated by the bottom-up approaches also show excellent performances in many applications, such as electrocatalysts, Li-ion batteries, and supercapacitors.

Since the discovery of 2D transition metal carbides, nitrides, or carbonitride MXenes, methods have been developed to optimize and control synthesis. Two-dimensional MXenes are generally synthesized through the selective etching of the parent MAX phase using etchants based on acidic fluoride solution. However, the specific etchant and etching parameters can directly influence the structure and chemistry of the resultant MXene, which will ultimately affect functional properties. The etchant type, concentration, temperature, and starting particle size of MAX phase precursor have been shown to affect the required etching times and even the processing of MXenes can influence the atomic structure and surface functional chemistry. Therefore, optimizing synthesis methods requires a deep understanding of how these parameters influence the micro- and atomic structure of the resulting MXenes. In this chapter, we review advances from recent literature on the influence of synthesis methods on the crystal structure, nanopores, atomic defects, and surface functional group and discuss how the structure and defects influence properties of MXenes.

During the MAX to MXene etching process, the freshly exposed and highly reactive M element surfaces are immediately functionalized by surface terminating species that originate from the etchant. Complementary to the structure and composition of the MXene, the surface terminations constitute a powerful tool for tailoring the mechanical, optical, electronic, and magnetic properties of the emerging MXene. The present chapter describes the current understanding of the origin, structure, composition, and routes for tailoring the surface terminating species.

In this chapter, we summarized molecular dynamics (MD) simulation methods with ab initio, reactive, and non-reactive empirical force fields. We reviewed various MXene applications such as energy storage, adsorption, intercalation, catalysis, exfoliation, and photocatalytic water splitting which have been investigated with MD simulations. Non-reactive MD simulations provide high computational efficiency in simulations of large-scale systems and slow dynamics of electrode charging. Reactive force fields can accurately describe chemistry of the MXene systems to provide insights to the ion intercalation and water diffusion in MXene sheets as well as measuring the friction coefficient of these structures. Ab initio MD method is often used to predict the final structure and various properties of the system and confirm the stability of the structure. We briefly presented essential work in the literature to provide an insight on how MD simulations are incorporated in efforts to investigate MXenes.

MXenes have hydrophilic interlayer spaces that can accommodate a large variety of intercalants. These are typically molecules such as H₂O or dimethylsulfoxide (DMSO) or ions such as metal or ammonium cations. This chapter summarizes the body of literature that has explored exactly what compositions of intercalants have been studied, the nature and extent of the intercalation process, and the effects on structure of the MXenes and resulting changes to properties. This is of special interest due to MXene’s use in applications and devices involving electrochemical ion intercalation.Alkali, alkaline earth, transition metal, and alkylammonium (AA) cations are reviewed in-depth. The first three groups lead to co-intercalation with H₂O molecules dependent upon environmental relative humidity, leading to reversible expansion of the basal spacing. The latter leads to a wide range of changes in basal spacing as a function of the structure and packing of the alkylammonium cations. Both chemical and electrochemical intercalation is discussed, and the material property changes that result are highlighted, ranging from electrical conductivity to mechanical properties.

Similar to other two-dimensional (2D) materials, single-layer and few-layer MXene flakes have significantly different physiochemical properties compared to their multilayered counterparts. 2D MXene flakes offer high surface area to volume ratio, tunable functional surfaces, high electrical conductivity, and excellent mechanical properties. These properties have rendered different MXene compositions as promising materials for a wide variety of applications, such as electrochemical energy storage devices, electromagnetic interference shielding, water purification, and sensors, to name a few. MXenes are usually produced through a selective etching and liquid exfoliation process in which “A” layer atoms of MAX phases, a large group of ternary carbides and nitrides, is selectively removed in fluoride containing acidic solutions. This process, in most cases, results in multilayered MXene particles (stacks of many single-layer MXene flakes) that need to be delaminated to produce single/few layer flakes. Delamination of multilayered MXenes usually involves chemical intercalation of MXenes with large organic molecules to increase their interlayer spacing, and therefore, significantly reducing the attraction between individual MXene layers. For some MXenes, intercalated particles can be readily delaminated to individual flakes by rigorous shaking or weak sonication of their water dispersions. For some MXenes, such as Ti₃C₂T_x, the synthesis process has evolved over the past few years and through modification of the etchants, the etching and delamination steps are combined into a single process, and the exfoliated MXenes can be directly delaminated into single-layer flakes. This chapter provides a comprehensive account of various MXene exfoliation and delamination techniques reported in the literature so far. At the end of this chapter, we have briefly discussed the current challenges and potential future directions in delamination of different MXenes into their single-layer flakes.

Interest in two-dimensional (2D) transition metal carbides and/or nitrides (known as MXenes) is growing exponentially across various scientific and engineering disciplines owing to their excellent mechanical properties, rich surface chemistries, metallic conductivity, and tunable transition metal chemistries. A variety of methods have been recently developed to process MXenes into films or coatings for specific applications such as energy storage, optics, electronics, catalysts, and medical devices. This chapter reflects recent progress and outlines future prospects of these MXene processing methods. We provide a holistic overview of the state-of-the-art MXene processing methods for obtaining extremely thin films (e.g., spin coating and layer-by-layer (LbL) assembly) to bulk processing (e.g., 3D printing and stamping). In the following chapter, we describe the mechanisms of such processing techniques in detail when used with 2D MXenes. We also comment on the challenges and future directions associated with these MXene processing methods.

Organic–inorganic hybrid materials are important class of materials which find applications in electrochemical energy conversion and storage, electronics, optics, biomedical applications, and many other areas of our daily life. Material properties of hybrid nanomaterials can be improved by changing either organic or inorganic component in a given hybrid matrix resulting in nearly unlimited combinations of innovative materials. MXenes are 2D inorganic sheets which are known for their metallic conductivity, high mechanical strength, hydrophilicity, and structural diversity. These properties are much needed in an inorganic component of a hybrid material. While the potential of MXenes in their pristine form is well documented, their applications in manufacturing organic–inorganic hybrid nanomaterials are relatively less explored. In this chapter, we have reported recent advances in MXene–organic hybrid materials. We summarized various MXene–organic hybrid synthesis approaches such as oxidant-free polymerization, self-assembly, diazonium chemistry, and others. With the help of computational methods, we have explained the host–guest interaction mechanisms, charge transfer mechanisms, and propagation of monomers into polymers. The role of polarity in organic molecules/polymers is discussed which may guide the design of new MXene–organic hybrid materials with well-defined properties for a variety of applications. We have also summarized the properties and various applications of MXene–organic hybrids. This chapter concludes with the remaining challenges and outlook to our readers.

Recent success in observing long-range magnetic ordering in two-dimensional (2D) materials has fueled interest in identifying promising material platforms for fundamental investigations of magnetic phases and development of nanoscale magnetic devices. Here, we review theoretical progress on understanding and predicting magnetic properties of MXenes. Predictions of intrinsic ground state ferromagnetic and antiferromagnetic ordering, high predicted Curie temperatures, strong magnetic anisotropy, and robustness to oxygen and moisture suggest that MXenes are an ideal family of 2D materials for spintronics and quantum information applications. Moreover, magnetic MXenes are predicted to exhibit semi-metallic, semi-conducting, metallic, and half-metallic transport properties. The magnetic and transport properties are tunable via applied strain, doping, and defect engineering. Exciting challenges and opportunities remain in investigating heterostructures of magnetic MXenes and other 2D materials to realize novel device architectures and magnetic control of quantum phenomena.

MXenes are a large class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides that show a great promise for a broad spectrum of applications. More than 25 different MXenes have been already experimentally demonstrated, and many others have been studied theoretically; however, their intrinsic physical properties remain largely unexplored. Here we review the electrical measurements of bulk assemblies of MXene sheets and demonstrate that the results strongly depend not only on the MXene’s chemical composition and structure but also on the form of assembly (a filtered, spin-casted, or sprayed film, a pressed disc, a particle, etc.), as well as on the preparation and postprocessing methods and measurement conditions, which dictate the stacking of individual MXene sheets and the interflake resistances. These results show the importance of single-flake measurements for revealing the intrinsic properties of various MXene materials and their comparison with each other. Single-flake measurements have been shown imperative for a variety of other 2D materials but remain scarce for MXene monolayers and so far have been limited only to Ti₃C₂T_x. Electrical measurements of individual Ti₃C₂T_x monolayers showed their high conductivity of 4600 ± 1100 S/cm and field-effect electron mobility of 2.6 ± 0.7 cm²/ V · s. These measurements were further proved to be useful for comparing the efficiencies of different synthetic methods for preparing high-quality MXene materials and investigating the environmental stability and kinetics of oxidation of Ti₃C₂T_x flakes in humid air. Mechanical measurements of Ti₃C₂T_x monolayers revealed their high Young’s modulus of 0.33 ± 0.03 TPa, establishing their enormous potential for mechanically reinforced composites, protective coatings, and nanoresonators. These examples demonstrate the importance of single-flake physical measurements, which expand our understanding of MXenes and broaden the already impressive range of their potential applications.

In the past decade, two-dimensional (2D) materials have had a significant impact on the physics and optics research community as they are observed to interact with light in a large variety of unique ways. MXenes have been added to this class of 2D in 2011. Ever since their discovery, they have been explored by a growing number of different fields of research, including optics and nanophotonics. In relation to optics, in the past few years, researchers have demonstrated a number of widely useful and interesting features of the MXenes, for example, optical transparency, plasmonic behavior, optical nonlinearity, efficient photothermal conversion, tunability of optical response, etc. These have led to application of the MXenes in functional metamaterial devices, mode-locked lasers, surface-enhanced Raman spectroscopy (SERS), photothermal therapy (PTT), and so on. In this chapter, we start by reviewing the theoretical and experimental approaches in studying the optical properties of the MXenes and then discuss the impactful optical device demonstrations.

Being one of the key applications of MXenes, MXene-based supercapacitors attracted huge attention for their superior electrochemical performance. In this section, an overview of MXenes as supercapacitor electrodes in various electrolytes is discussed as well as strategies for improving their performance. In aqueous electrolytes, MXenes exhibit capacitive behavior in neutral and alkaline electrolytes, while a pseudocapacitive behavior was observed in acidic electrolytes, resulting in ultrahigh capacitance values up to 370 F g⁻¹ (1500 F cm⁻³). Studies were extended to nonaqueous electrolytes to achieve large voltage windows (up to 3 V), but were limited by low capacitance values. The effects of surface chemistry on energy storage in MXenes are also discussed. In addition, composite MXene electrodes have been developed to increase the electrical conductivity, the mechanical robustness, or surface accessibility of MXenes. Lastly, MXene-based supercapacitor devices including hybrid, all-solid-state, and micro-supercapacitors are introduced.

Development of advanced electrochemical energy storage devices is crucial to foster a sustainable power grid. At present, lithium-ion batteries do not have satisfactory performance for large-scale applications, and one of major challenges is to find electrode materials with better specific capacity, operating potential, rate capability, and cycle stability. MXenes have attracted attention as the potential electrode materials of various batteries such as lithium-ion, sodium-ion, potassium-ion, or magnesium-ion batteries. In this chapter, after describing the electrochemical properties of MXenes, we will summarize recent progress in their applications to batteries.

Exploration of new electrochemistries that go “beyond lithium-ion” to boost energy density and reduce cost is rapidly gaining momentum. In this pursuit, lithium-sulfur (Li-S) batteries that couple sulfur-positive electrodes (or “cathodes”) with lithium-negative electrodes (or “anodes”) are considered particularly promising candidates. The Li-S battery has received enormous attention in the past decade, due to the high theoretical specific energy (Wh kg⁻¹) and earth abundance of sulfur, which is coupled with a high-energy density Li metal anode in the cell. Instead of intercalation chemistry, these batteries rely on conversion chemistry, which yields a high theoretical capacity. MXenes can provide a vital role. MXenes have been used in Li-S batteries. Delaminated MXenes are capable of high electronic conductivity and exhibit rich surface properties, which synergistically improves the electron transport properties of the sulfur electrode and provides chemical interactions with lithium polysulfides. Another advantageous aspect is MXenes denser structure compared to most “fluffy” carbonaceous materials, which benefits the volumetric energy density of the battery. This chapter provides a brief overview of the recent development of MXenes for Li-S batteries, from material aspects on tuning the physical and electrochemical properties of the sulfur cathode to their performance in prototype cells.

The problems associated with electromagnetic interference (EMI) have grown up rapidly due to the large usage of electronic gadgets and telecommunication devices. Thus, it is extremely important to understand the EMI shielding mechanisms and develop such materials that can mitigate the harmful effects of EMI arising due to advancement of telecommunication and electronics industry. This chapter throws some light on the latest addition to EMI shielding materials, in particular, the recently discovered two-dimensional transition metal carbides or nitrides (MXenes) and their composites. First, a brief overview of EMI shielding mechanisms is presented followed by discussion of MXene-based EMI shielding materials. The literature reports are summarized in a way to give the readers an idea about the utility and progress of MXenes and their polymer composites in EMI shielding. Since the microwave absorption (MWA) properties are as important as EMI shielding, the discussion on MXene-based materials as microwave absorbers is presented in detail. Further to the end, future perspectives of MXene as EMI shielding and MWA are discussed along with suggestions to enhance the performance by controlling the surface characteristics, architectural control, physical properties, and quality of MXenes.

Since the first discovery of two-dimensional (2D) MXenes, about 30 different structures of this group have been synthesized to date. Owing to their unique mechanical, chemical, and electrical properties, many successful attempts have been focused on using MXenes in water treatment and environmental remediation applications. However, more efforts are still needed to address the stability, biocompatibility, and reusability of MXenes in aqueous media. This chapter discusses the latest research progress in the application of MXenes in pollutants adsorption/remediation, photodegradation, and membrane separation. An overview is given on recent experimental/computational attempts to explore the potential of MXenes in water treatment applications and highlight the challenges and opportunities of these advanced 2D architectures. This chapter highlights new avenues for more innovative developments of MXene materials in environmental applications.

This chapter discusses chemical properties of MXenes focusing on the potential catalytic properties of these materials that can enable chemical transformations of relevance for achieving a sustainable energy future. First, we give an overview of the status of this new field providing a summary of where MXenes have been studied both experimentally and theoretically as catalyst materials as well as where discovery has benefited from a combined computational-experimental approach. We exemplify the combined computational-experimental approach and the crucial impact of a feedback loop between the two by using the hydrogen evolution reaction. When it comes to modeling, we describe how we can use high-throughput computational screening approaches to calculate reactivity and activity properties based on fundamental insight and understanding established prior to the screening process which in turn can be used to identify MXene candidate materials for specific chemical transformations. The chapter is concluded by providing some directions on how we could proceed discovery of new multicomponent MXene materials for chemical transformations.

Active intercommunication between two different objects, whether it is person-to-person, person-to-robot, or person-to-environment, all begins with obtaining necessary information at the interface through sensing activities. Such information can be of various forms, being either mechanical forces such as pressure or strain, chemical interactions from gases or humidity, or various other physical and optical interactions. In order to obtain accurate information and properly respond to the surrounding environment, the precise and sensitive sensing is of paramount importance. The choice of material and engineering its structure play a large role in achieving this goal, due to the large variation between materials’ properties. Here, we will discuss the technical requirements in fabricating high-performance sensors and introduce several materials and their properties that have been utilized as sensing channels. Moreover, we will discuss the merits of MXenes over other materials and their potential to be used in various types of sensors.

The rapid development of portable smart electronics demands advanced components including displays and power sources. Central to these components is the quest of novel materials that can perform well as both transparent conductive electrodes (TCEs) and transparent energy storage devices. This is quite challenging, as the sheet resistance dramatically increases when thinning down the film thickness, leading to poor optoelectronic and electrochemical charge storage properties, known as percolation effects. Producing TCEs without percolation problems is quite crucial for the development of high-performance touch screens, displays, etc. This chapter briefly introduces some typical TCEs, outlines the fundamentals, and focuses on MXenes as potential TCE material. Based on that, this chapter also discusses the fabrication of transparent energy storage devices, specifically transparent solid-state supercapacitors. The excellent optoelectronic properties coupled with impressively performance on capacitive charge storage indicate that MXenes are great candidates for producing state-of-the-art TCEs and transparent energy storage devices.

The rapid pace of innovation in the fields of biomedicine and nanotechnology has generated a wide variety of novel nanomaterial-based platforms for biomedical applications. Based on the nanometer-thin two-dimensional planar morphology, the high effective surface area, the abundant surface chemistry, and the favorable physicochemical properties, there is a growing interest on exploring the potentials of MXenes at the interface with living systems. In this chapter, we review the state of the art and discuss the promise of MXenes for biomedical applications. Specifically, we first describe the properties of MXenes that make them particularly attractive and review the recent developments, with specific focus on the areas of biosensing, cancer theranostics, and antimicrobial treatments. Finally, we discuss the biocompatibility of MXenes based on the findings that have been reported so far.