Surface Characterization Of 2d Transition Metal Carbides Mxenes

Research on two-dimensional (2D) materials is a rapidly growing field owing to the wide range of new interesting properties found in 2D structures that are vastly different from their three-dimensional (3D) analogues. In addition, 2D materials embodies a significant surface area that facilitates a high degree of surface reactions per unit volume or mass, that is imperative in many applications such as catalysis, energy storage, energy conversion, filtration, and single molecule sensing. MXenes constitute a family of 2D materials consisting of transition metal carbides and/or nitrides, which are typically formed after selective etching of their 3D parent MAX phases. The latter, are a family of nanolaminated compounds that typically follow the formula Mn+1AXn (n=1-3), where M is a transition metal, A is a group 13 or 14 element, and X is C and or N. Selective etching by aqueous F- containing acids removes the A layer leaving 2D Mn+1Xn slabs instantly terminated by a mix of O-, OH- and F-groups. The first and most investigated MXene is Ti3C2TX, where TX stands for surface termination, which has shown record properties in a range of applications (eg. electrode in Li-batteries, supercapacitors, sieving membrane, electromagnetic interference shielding, and carbon capture). Adding to that, over 30 different MXenes have been discovered since 2011, exhibiting alternative or superior properties. Most importantly, elegant routes for property design in the MXene family has been demonstrated, by means of either varying the chemistry in the Mn+1Xn compound, by alloying two M elements, or by changing the structure of the MXene by introducing vacancies. The present work has a led to an additional route for post synthesis property tuning in MXenes by manipulation of surface termination elements. This enables a unique toolbox for property tuning which is not available to other 2D materials and is highly beneficial for applications that is dependent on surface reactions. Furthermore, chemical and structural characterization of terminations on single sheets is essential to rule out the influence of intercalants or contamination that is typically present in multilayer MXene samples or thin films. For that purpose, a method for preparing isolated contamination free single sheets of MXene samples for transmission electron microscopy (TEM) characterization was established. In order to determine vacancy and termination sites, atomically resolved scanning (S)TEM imaging and image simulations was carried out. Two main processes were employed to substitute the termination elements. 1) An initial thermal treatment in vacuum facilitates F desorption and it was shown that O-terminations rearranges on the evacuated sites. H2 gas exposure in a controlled environment demonstrated a removal of the remaining O-terminations. As a result, termination-free MXene is possible to realize under vacuum conditions. 2) CO2 was introduced as a first non-inherent termination on MXene by in situ CO2 gas exposure at low temperatures. That was a first demonstration of Ti3C2TX as promising material for carbon capture. Additionally, O-saturated surfaces were demonstrated after introduction of O2 gas on the F-depleted Ti3C2TX MXene, which is highly relevant for hydrogen evolution reactions where fully O-terminated Ti3C2TX are predicted to improve efficiency. A Lewis acid melt synthesis method was used to realize the first MXene exclusively terminated with Cl. Moreover, this was the first report of a MXene directly synthesised with terminations other than O, OH, and F. Furthermore, we have expanded the space of property tuning by introduction of chemical ordering, by selective etching of Y in an alloyed (Mo2/3Y1/3)2CTX MXene. This either produced chemical ordering with one M (Mo) element and vacancies, or ordering between two M (Mo and Y) elements. This was further reported to significantly increase volumetric capacitance because of the increased number of active sites around vacancies, leading to an increasing charge density. As a final note, the stability of Nb2CTX MXene under ambient conditions was investigated. It was found that the surface Nb adatoms, present after etching, got oxidized over time which resulted in local clustering and effectively degraded the MXene. This work has demonstrated reproducible surface characterization methods for determining termination elements and sites in 2D MXenes, that is ultimately governing MXene properties. Most importantly, we report on a new approach for MXene property tuning as well as contributing to several existing property tuning approaches.

Since the isolation and characterization of graphene, there has been a growing interest in 2D materials owing to their unique properties compared to their 3D counterparts. Recently, a family of 2D materials of early transition metal carbides and nitrides, labelled MXenes, has been discovered (Ti2CTz, Ti3C2Tz, Mo2TiC2Tz, Ti3CNTz, Ta4C3Tz, Ti4N3Tz among many others), where T stands for surface-terminating groups (O, OH, and F). MXenes are mostly produced by selectively etching A layers (where A stands for group A elements, mostly groups 13 and 14) from the MAX phases. The latter are a family of layered ternary carbides and/or nitrides and have a general formula of Mn+1AXn (n = 1-3), where M is a transition metal and X is carbon and/or nitrogen. The produced MXenes have a conductive carbide core and a non-conductive O-, OH- and/or F-terminated surface, which allows them to work as electrodes for energy storage applications, such as Li-ion batteries and supercapacitors. Prior to this work, MXenes were produced in the form of flakes of lateral dimension of about 1 to 2 microns; such dimensions and form are not suitable for electronic characterization and applications. I have synthesized various MXenes (Ti3C2Tz, Ti2CTz and Nb2CTz) as epitaxial thin films, a more suitable form for electronic and photonic applications. These films were produced by HF, NH4HF2 or LiF + HCl etching of magnetron sputtered epitaxial Ti3AlC2, Ti2AlC, and Nb2AlC thin films. For transport properties of the Ti-based MXenes, Ti2CTz and Ti3C2Tz, changing n from 1 to 2 resulted in an increase in conductivity but had no effect on the transport mechanism (i.e. both Ti3C2Tx and Ti2CTx were metallic). In order to examine whether the electronic properties of MXenes differ when going from a few layers to a single flake, similar to graphene, the electrical characterization of a single Ti3C2Tz flake with a lateral size of about 10 μm was performed. These measurements, the first for MXene, demonstrated its metallic nature, along with determining the nature of the charge carriers and their mobility. This indicates that Ti3C2Tz is inherently of 2D nature independent of the number of stacked layers, unlike graphene, where the electronic properties change based on the number of stacked layers. Changing the transition metal from Ti to Nb, viz. comparing Ti2CTz and Nb2CTz thin films, the electronic properties and electronic conduction mechanism differ. Ti2CTz showed metallic-like behavior (resistivity increases with increasing temperature) unlike Nb2CTz where the conduction occurs via variable range hopping mechanism (VRH) - where resistivity decreases with increasing temperature. Furthermore, these studies show the synthesis of pure Mo2CTz in the form of single flakes and freestanding films made by filtering Mo2CTz colloidal suspensions. Electronic characterization of free-standing films made from delaminated Mo2CTz flakes was investigated, showing that a VRH mechanism prevails at low temperatures (7 to ≈ 60 K). Upon vacuum annealing, the room temperature, RT, conductivity of Mo2CTx increased by two orders of magnitude. The conduction mechanism was concluded to be VRH most likely dominated by hopping within each flake. Other Mo-based MXenes, Mo2TiC2Tz and Mo2Ti2C3Tz, showed VRH mechanism at low temperature. However, at higher temperatures up to RT, the transport mechanism was not clearly understood. Therefore, a part of this thesis was dedicated to further investigating the transport properties of Mo-based MXenes. This includes Mo2CTz, out-of-plane ordered Mo2TiC2Tz and Mo2Ti2C3Tz, and vacancy ordered Mo1.33CTz. Magneto-transport of free-standing thin films of the Mo-based MXenes were studied, showing that all Mo-based MXenes have two transport regimes: a VRH mechanism at lower temperatures and a thermally activated process at higher temperatures. All Mo-based MXenes except Mo1.33CTz show that the electrical transport is dominated by inter-flake transfer. As for Mo1.33CTz, the primary electrical transport mechanism is more likely to be intra-flake. The synthesis of vacancy ordered MXenes (Mo1.33CTz and W1.33CTz) raised the question of possible introduction of vacancies in all MXenes. Vacancy ordered MXenes are produced by selective etching of Al and (Sc or Y) atoms from the parent 3D MAX phases, such as (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering of Mo and Sc. However, not all quaternary parent MAX phases form the in-plane chemical ordering of the two M metals; thus the synthesis of the vacancy-ordered MXenes is restricted to a very limited number of MAX phases. I present a new method to obtain MXene flakes with disordered vacancies that may be generalized to all quaternary MAX phases. As proof of concept, I chose Nb-C MXene, as this 2D material has shown promise in several applications, including energy storage, photothermal cell ablation and photocatalysts for hydrogen evolution. Starting from synthetizing (Nb2/3Sc1/3)2AlC quaternary solid solution and etching both the Sc and Al atoms resulted in Nb1.33C material with a large number of vacancies and vacancy clusters. This method may be applicable to other quaternary or higher MAX phases wherein one of the transition metals is more reactive than the other, and it could be of vital importance in applications such as catalysis and energy storage.

This book describes the rapidly expanding field of two-dimensional (2D) transition metal carbides and nitrides (MXenes). It covers fundamental knowledge on synthesis, structure, and properties of these new materials, and a description of their processing, scale-up and emerging applications. The ways in which the quickly expanding family of MXenes can outperform other novel nanomaterials in a variety of applications, spanning from energy storage and conversion to electronics; from water science to transportation; and in defense and medical applications, are discussed in detail.

This book presents the fundamental aspects, recent developments in fabrication and characterization techniques, structure, properties, and emerging applications of MXenes. It shows the advancement in scale-up, challenges, and their futuristic perspectives. An overview of all the latest developments in energy storage and conversion applications, catalysis, environmental remediation, and radiation shielding, etc is reported.

MXenes and their Composites: Synthesis, Properties and Potential Applications presents a state of the art overview of the recent developments on the synthesis, functionalization, properties and emerging applications of two-dimensional (2D) MXenes and their composites.The book systematically describes the state-of-the-art knowledge and fundamentals of MXene synthesis, structure, surface chemistry and functionalization. The book also discusses the unique electronic, optical, mechanical and topological properties of MXenes. Besides, this book covers the various emerging applications of MXenes and their composites across different fields such as energy storage and conversion, gas sensing and biosensing, rechargeable lithium and sodium-ion batteries, lithium-sulphur and multivalent batteries, electromagnetic interference shielding, hybrid capacitors and supercapacitors, hydrogen storage, catalysis and photoelectrocatalysis, gas separation and water desalination, environmental remediation and medical and biomedical applications. All these applications have been efficiently discussed in the specific chapters and in each case, the processing of MXene composites has also been discussed.This book will be an excellent reference for scientists and engineers across various disciplines and industries working in the field of highly promising 2D MXenes and their composites. The book will also act as a guide for academic researchers, material scientists, and advanced students in investigating the new applications of 2D MXenes based materials. Covers fundamentals of technologically important MAX phases, MXene derivatives, MXene synthesis methods, intercalation and delamination strategies, surface functionalization, fundamental characteristics and properties Demonstrates major application areas of MXenes, including catalytic, energy storage and energy generation, flexible electronics, EMI shielding, sensors and biosensors, medical and biomedical, gas separation and water desalination Presents a detailed discussion on the processing and performance of various MXenes towards different applications

This book comprises select proceedings of the International Conference on VLSI, Communication and Signal processing (VCAS 2020). The contents are broadly divided into three topics – VLSI, Communication, and Signal Processing. The book focuses on the latest innovations, trends, and challenges encountered in the different areas of electronics and communication, especially in the area of microelectronics and VLSI design, communication systems and networks, and image and signal processing. It also offers potential solutions and provides an insight into various emerging areas such as Internet of Things (IoT), System on a Chip (SoC), Sensor Networks, underwater and underground communication networks etc. This book will be useful for academicians and professionals alike.

This is the very first book on the highly promising topic of MXenes; focusing on their fundamental characteristics and properties, fabrication techniques and applications. MXenes are two-dimensional materials consisting of few atoms thick layers of transition metal carbides or nitrides. These are characterized by high electrical conductivity, good hydrophilicity, chemical stability, and ultrathin 2D sheet-like morphology. Applications in the energy, environmental, biomedical and electronic industries include catalysis, membrane separation, supercapacitors, hybrid-ion capacitors, batteries, flexible electronics, hydrogen storage, nanoelectronics, and sensors.

Emerging Nanomaterials for Recovery of Toxic and Radioactive Metal Ions from Environmental Media covers nanomaterials used in the environmental remediation of sites contaminated by toxic or radioactive heavy metals. The book comprehensively covers the use of MOF-based nanomaterials, COF-based nanomaterials, MXene-based nanomaterials, nZVI-based nanomaterials and carbon-based nanomaterials in remediation techniques and details the main interaction mechanisms between toxic/radioactive metal ions and the described novel nanomaterials through kinetic analysis, thermodynamic analysis, spectroscopic techniques and theoretical calculations. It provides a thorough reference on the use of the described novel nanomaterials for academics, researchers and advanced postgraduates in the environmental sciences and environmental chemistry. Provides a comprehensive and systematic reference on various novel nanomaterials that are available for use in the treatment of heavy metal ions and radioactive wastes Presents the latest knowledge on the interaction of toxic and radioactive metal ions with novel nanomaterials, including how to choose different materials for specific uses Covers the principles and functionalization of nanomaterials in environmental remediation, enabling an understanding of methodologies and best choice in nanomaterials