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. 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.
Alkali, alkaline earth, transition metal, and alkylammonium AA cations are reviewed in-depth. The first three groups lead to co-intercalation with H 2 O 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. 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.
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 Ti 3 C 2 T 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. 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. 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.
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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. 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.
Chemical exfoliation of layered MAX phase compounds into novel two-dimensional transition metal carbides and nitrides, the so-called MXenes, has opened new opportunities in materials science and technology. In recent extensive theoretical studies, it has been demonstrated that MXenes containing transition metals with open d orbital shells exhibit a multitude of interesting properties because of different oxidation and spin states and a relatively large spin-orbit coupling of the transition metals.
Hence, they provide an excellent platform for exploring and exploiting the internal degrees of freedom of electrons — charge, orbital, and spin — and their interplay for fundamental research and device applications. In this book chapter, we provide an insight into possibilities regarding the exfoliation of MAX phases into 2D MXenes. We then highlight the computational attempts that have been made to understand the physics and chemistry of the MXene family and to exploit their novel and unique properties for electronic and energy harvesting applications. 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.
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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.
Nathan C. Frey, Christopher C. 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.
These results show the importance of single-flake measurements for revealing the intrinsic properties of various MXene materials and their comparison with each other.
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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 Ti 3 C 2 T x. 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 Ti 3 C 2 T x flakes in humid air.
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 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.
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. 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.
Two-Dimensional Carbides and Nitrides (MXenes) Challenge Graphene | Department of Chemistry
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. Instead of intercalation chemistry, these batteries rely on conversion chemistry, which yields a high theoretical capacity. Carbide Bur Catalog. Metal Carbides. Fischer Tropsch Synthesis.
Union carbide. Log in Get Started. Download for free Report this document. Embed Size px x x x x The activation temperature causes these catalysts to exhibit significant differences in their catalytic activity for CO hydrogenation. The solid-state phase transformations from the oxide phase hematite to the iron carbide are studied by high resolution transmission electron microscopy HRTEM , X-ray diffraction and elemental analysis.
The differing extents of formation of the carbide phase correlate well with the catalytic activity of iron. HRTEM shows that as magnetite transforms into carbide, the crystals of magnetite break down into smaller crystallites of the carbide phase. Deposition of carbon on the surface causes the carbide crystallites to further separate from each other.
Morphological transformations such as these determine the attrition of the catalyst and help provide clues to understanding the deactivation of iron Fischer-Tropsch catalysts. Interest in the use of iron for FT synthesis stems from its water-gas shift activity as well as its easy availability and low cost. Iron catalysts can be used in a fixed bed, or in a slurry phase fluidized bed depending on the desired product mix and operating mix and operating pressure and reaction conditions.
A serious problem in the use of iron catalysts in the slurry process is their tendency to undergo attrition2 during use. This can cause fouling of downstream equipment as well as make separation of the catalyst from product wax virtually impossible. Oyama ed. These catalysts must typically be pretreated before use in Ff synthesis. The common pretreatment conditions employed in the case of iron catalysts are H2 reduction, CO reduction, or reduction in the reactant gas induction.
A calcination step may precede the reduction of the catalyst. However, the role of each of these phases during the reaction has not been resolved. This assertion was questioned in by Dictor and Bell,l1 whose results suggest that the active phase is a mixture of x- and 10'-carbides and some metallic a-Fe. The debate over the identity of the active phase has been revived in recent years by Kuivila et ai. In a series of review articles, Butt13 ,14 asserts that magnetite may be active for reaction in the absence of carbide phases.