No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Chemical bonding in carbide MXene nanosheets
Abstract
The chemical bonding in the carbide core and the surface chemistry in a new group of transition-metal carbides Tin+1Cn-Tx (n=1,2) called MXenes have been investigated by surface-sensitive valence band X-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thicknesses are analyzed in connection to known hybridization regions of TiC and TiO2 that affect elastic and transport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s - Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, the O 2p and F 2p bands strongly depend both on the bond lengths to the surface groups and the adsorption sites. The effect of surface oxidation and Ar⁺ sputtering on the electronic structure is also discussed.
... Even though there are numerous publications proposing OH as a termination species, there is no solid proof that the detected OH is bonded to the Ti 3 C 2 T x -surface. Different techniques, such as Raman spectroscopy [4,13], nuclear magnetic resonance (NMR) [14], electron energy loss spectroscopy (EELS) [15,16], X-ray absorption spectroscopy (XAS) [17,18], and ultraviolet photoelectron spectroscopy (UPS) [19,20], have been employed to provide confirmation of OH as a termination species on Ti 3 C 2 T x . Identification of specific species on a surface is, however, a challenge. ...
... Very few Ti 3 C 2 T x samples are well defined and almost all Ti 3 C 2 T x samples contain contaminations and impurities. For example, the parent material Ti 3 AlC 2 is prone to oxidation [21] and Ti 3 C 2 T x obtained from Ti 3 AlC 2 powders prepared in an alumina tube furnace is incorporated with small amounts of non-reacted TiC, oxidized components (TiO 2 and Al 2 O 3 ), and components from other side reactions [19]. These impurities can pass through the MXene-forming process and become embedded in the Ti 3 C 2 T x sample [19] and especially TiO 2 is known to adsorb H 2 O that dissociates into OH [22][23][24][25][26][27]. ...
... For example, the parent material Ti 3 AlC 2 is prone to oxidation [21] and Ti 3 C 2 T x obtained from Ti 3 AlC 2 powders prepared in an alumina tube furnace is incorporated with small amounts of non-reacted TiC, oxidized components (TiO 2 and Al 2 O 3 ), and components from other side reactions [19]. These impurities can pass through the MXene-forming process and become embedded in the Ti 3 C 2 T x sample [19] and especially TiO 2 is known to adsorb H 2 O that dissociates into OH [22][23][24][25][26][27]. The challenge of probing the termination species can be exemplified through Raman spectroscopy studies of Ti 3 C 2 T x performed by Sarycheva et al. [13] and Benchakar et al. [4]. ...
One of the most explored MXenes is Ti3C2Tx, where Tx is designated to inherently form termination species. Among many applications, Ti3C2Tx is a promising material for energy storage, energy conversion, and CO2-capturing devices. However, active sites for adsorption and surface reactions on the Ti3C2Tx-surface are still open questions to explore, which have implications for preparation methods when to obtain correct and optimized
surface requirements. Here we use X-ray photoelectron spectroscopy (XPS) to study the adsorption of common gas molecules such as H2, CO2, and H2O, which all may be present in energy storage, energy converting, and CO2-capturing devices based on Ti3C2Tx. The study shows that H2O, with a strong bonding to the Ti-Ti bridge-sites, can be considered as a termination species. An O and H2O terminated Ti3C2Tx-surface restricts the CO2 adsorption to the Ti on-top sites and may reduce the ability to store positive ions, such as Li+ and Na+. On the
other hand, an O and H2O terminated Ti3C2Tx-surface shows the capability to split water. The results from this study have implications for the correct selection of MXene preparations and the environment around the MXene in different implementations, such as energy storage, CO2-capturing, energy conversion, gas sensing, and catalysts.
... Between MXene and VCuS, chemical interactions such as covalent or ionic bonding are possible. The formation of these interactions at the interface of the two materials has the potential to improve the overall stability and integrity of the composite [46]. The formation of electrostatic forces on MXene and VCuS surfaces can be accredited to the presence of charged species because the electrochemical characteristics are influenced by the charge distribution [47]. ...
MXene, a two-dimensional (2D) material composed of transition metal carbides (TMCs) and nitrides, have fascinated substantial scientific interest. This increased interest results from their exceptional properties, which include extraordinary conductivity, transparency, outstanding absorbing capacity, and significant charge storage capacities. In this work, the MXene-doped vanadium copper sulfide (VCuS) was synthesized through the hydrothermal method. In three electrode measurement system, the VCuS/MXene composite electrode showed exhibited a specific capacity (Qs) of 1620 Cg⁻¹. As application point of view, the hybrid device is designed and measured the electrochemical properties. The hybrid device showed the remarkable Qs of 1528 C.g⁻¹, power density (Pd) of 2347 Wkg⁻¹ and an energy density (Ed) of 34.99 Whkg⁻¹. Further, the VCuS/MXene//AC device is measured up to 6000 cycles to check the stability and durability. The device showed the capacity retention (CR) of 88.5% and a high Coulombic efficiency of 82.6%. Additionally, the VCuS/MXene electrode material is utilized as an electrochemical glucose sensor for the precise detection of H2O2 down to a minimal concentration of H2O2/mm, exhibiting exceptional precision. The use of multifunctional VCuS/MXene nanocomposite electrode material presents novel possibilities for the construction of hybrid energy harvesting systems.
... The AFM analysis of the obtained dispersion of delaminated Ti 3 C 2 T z nanosheets showed thickness (Figures 6d and S3) ranging from 1.5 to 2 nm, confirming the presence of a few-layer Ti 3 C 2 T z . 5,9,46 The addition of −OH groups on the annealed ML-Ti 3 C 2 T z was further confirmed by the FTIR spectra shown in Figure 7a. The FTIR peak at 3334 cm −1 can be attributed to the stretching vibrations of −OH, confirming the addition of the −OH group to delaminated Ti 3 C 2 T z . ...
... However, some non-equilibrium Ti x C y phases can be formed (Gusev 2002;Knyazeva and Korosteleva 2020;Krinitcyn et al. 2020), the number of which varies depending on the synthesis conditions. According to published data, the appearance of the TiC 2 , Ti 3 C 2 and Ti 2 C phases can be expected (Enyashin and Ivanovskii 2013; Magnuson et al. 2018). However, the Ti 3 C 2 one is unstable, so it is not taken into account in the model (Anisimova and Knyazeva 2020). ...
In this work, the homogenization theory is applied within the framework of three-dimensional linear micropolar media. The fundamental results derived by the asymptotic homogenization method to compute the effective engineering moduli for a laminated micropolar elastic composite with centro-symmetric constituents are summarized, in which the interface between the layer phases is considered imperfect spring type. The layers are considered with isotropic symmetry. Non-uniform and, as a particular case, uniform imperfections are assumed, where different imperfec- tion parameters and cell lengths in the y3-direction are assigned for the analysis. The analytical expressions of the engineering constants related to the stiffness and torque are given as functions of the imperfection parameters. The behavior of the engineering coefficients depending on the imperfection is studied. The influence of the imperfection and the cell length in the direction of the imperfection is observed. The present study allows validating other models and experimental results, as well as the investigation of fracture prediction in laminated composite materials.
... They also found that TiÀ C bond in Ti 3 C 2 -T X is shorter than in Ti 2 C-T X , which can influence the elastic features of the material. [149] Zha et al. discovered that O-terminated MXenes should be used preferably for supercapacitors and structure materials due to high mechanical strength. [150] MXenes confess higher ion adsorption abilities in various findings, rendering the researchers to their possible use in flexible electronics and sensors. ...
Because of their peculiar two‐dimensional layered microstructure, the existence of numerous functionalities on the surface and excellent electrical, thermal and optical features, MXenes are regarded as promising candidates for solving energy and environment related problems. It is noted that energy conversion and storge capability of MXenes could be raised by altering their dimensions, structure, surface chemistry and chemical composition. Therefore, it is critical to recognize how one can boost the relationship between structure and property from applied viewpoint. In the present study, we reviewed the synthesis, properties and potential applications of MXenes. Furthermore, several properties of MXenes including structural, chemical, optical, mechanical and thermal have been explored. In addition, the potential applications of MXenes in various areas such as photocatalysis, gas sensing, supercapacitors, electrocatalysis and environmental remediation have also been discussed. Based on reported works, it can clearly be noticed that features and potential applications of MXenes may be further improved by applying many alteration and functionalization strategies. This study also focuses on the current progresses and future prospective relating to MXene based composites, which will surely assist the scientists who are doing work in areas of academia and material sciences.
... Transition metal atoms' mass and surface terminal groups influence MXenes' mechanical properties. Magnuson and Halim [88] demonstrated that surface terminal groups reduce the bond of Ti-C strength by omitting its charge. It was discovered that the Ti-C bond lengths in Ti 2 C-T x are longer than those in Ti 3 C 2 T x , which can affect the elasticity of the material. ...
The increase in pollutants such as hazardous refractory contaminants, organic dyes, pharmaceuticals, and pesticides entering water resources on a large scale due to global population growth and industrialization has become a significant health concern worldwide. The two-dimensional (2D) MXene material is a new type of transition metal carbide or carbonitride material, which has demonstrated the capability to adsorb various heavy contaminants, particularly metals such as chromium, copper, lead, and mercury. In addition, MXenes have a tunable band gap (0.92–1.75 eV) and exhibit good thermal stability and considerable damage resistance, which means that they are well suited as adsorbents for waste removal. In this review article, MXene nanocomposites are introduced for the removal of pollutants from water. The idea of water remediation, the applications of MXene-based nanocomposites, and the effects on the degradation of water and wastewater contaminants are reviewed. Future trends in MXene-based nanocomposites for water treatment and environmental applications will also be discussed.
... The intended bond dimensions of OeH, TieO and TieF are found to be almost 1.9 Å, 0.97 Å and 2.1 Å individually. The oxygen termination (-O) exhibits the highest adsorption energy of 7.7 eV, in the order of eO, eF, eOH, eCl and eH [279]. That is yet incredible to achieve MXenes with no or pure surface termination because of the random distribution of the termination's points, according to the information from the theoretical calculation regarding the considerable impact of specific surface terminations [280]. ...
MXenes are regarded as a type of two-dimensional (2D) inorganic material, mainly comprising a number of transition metal carbides, nitrides, or carbonitrides atomic planes. Nevertheless, the scientific community is continuously interested in exploring and structuring the engineered-based multifunctional material for numerous applications. The MXenes-based materials in this context, have emerged as highly active compounds owing to their superior surface area, substantial interlayer spacing, highly reactive surface-active sites and surface functional group, even though, recent studies have shown significant scientific and theoretical progress related to enormous prospects in MXenes, chemical nature, robust electrochemistry and high hydrophilicity of MXenes. The role of MXenes in all kinds of strategies is still in an upgrading phase for their further improvement, and is not sufficiently summarized in the literature now. To begin with this, herein, present review article is intended to critically discuss the diversity of MXenes with respect to different composition, formulation, plasmonic, complexation, and numerous geometric and morphological aspects, along with novel construction strategies to improve their surface characteristics in all aforesaid multidimensional applications. Following that, in terms of broadening the application, this review article is envisaged to endorse the use of MXenes and their hybrid configuration in a series of emerging environmental decontamination via adsorption, photodegradation, photocatalytic fuel production via hydrogen evolution, CO2 reduction, electrocatalytic sensing, along with membrane distillation and energy storage. In addition, comprehensive information about existing obstacles and future perspectives have been addressed. Finally, an overview is succinctly summarized and discussed regarding the emerging prospects of MXenes for their potential uses in numerous research fields. At the end, it is anticipated that this review article will pave the way for the effective use of MXenes in different fields of environmental remediation, energy conversion, storage and biomedical applications as an innovative, reliable, and multifunctional material.
... Additionally, Magnuson et al. discovered that the surface groups weaken the Ti-C bonds by drawing charge from them. They reported that the Ti-C bond is longer in Ti 2 C-T x than in Ti 3 C 2 -T x , which was believed to impact the materials' elastic characteristics (Magnuson et al., 2018). The study also added that the bonding strength can be altered to enhance elasticity. ...
The detection of toxins that contaminate food needs highly sensitive and selective techniques to prevent substantial monitory loss. In this regard, various nanostructured material-enabled biosensors, have recently been developed to improve the detection of food toxins among them aflatoxin is the prevalent one. The biosensor-based detection of aflatoxin is quick, cheaper, and needs less skilled personnel, therefore overcoming the shortcomings of conventional techniques such as LC/MS-MS, HPLC, and ELISA assays. 2D MXenes manifest as an efficient material for biosensing due to their desirable biocompatibility, magnificent mechanical strength, easiness of surface functionalization, and tuneable optical and electronic features. Contrary to this, aptamers as biorecognition elements (BREs) possess high selectivity, sensitivity, and ease of synthesis when compared to conventional BREs. In this review, we explored the most cutting-edge aptamer-based MXene-enabled biosensing technologies for the detection of the most poisonous mycotoxins (i.e., Aflatoxins) in food and environmental matrices. The discussion begins with the synthesis processes and surface functionalization/modification of MXenes. Computational approaches for designing aptasensors and advanced data analysis based on artificial intelligence and machine learning with special emphasis over Internet-of-Thing integrated biosensing devices has been presented. Besides, the advantages of aptasensors over conventional methods along with their limitations have been briefed. Their benefits, drawbacks, and future potential are discussed concerning their analytical performance, utility, and on-site adaptability. Additionally, next-generation MXene-enabled biosensing technologies that provide end users with simple handling and improved sensitivity and selectivity have been emphasized. Owing to massive applicability, economic/commercial potential of MXene in current and future perspective have been highlighted. Finally, the existing difficulties are scrutinized and a roadmap for developing sophisticated biosensing technologies to detect toxins in various samples in the future is projected.
... Also, because the bonding strength between Ti-O is higher than in Ti-OH and Ti-F cases, the Oterminated MXenes's stiffness is very high. Magnuson et al. also determined that elastic properties of Ti 2 C-Tx and Ti 3 C 2 -T x are different because of the different Ti-C bond in them that in Ti 2 C-T x is longer than in Ti 3 C 2 -T x [25]. It was recommended by the authors that to optimize the elasticity, you can modify the bond strength. ...
Solar energy as a clean energy plays an important role in producing sufficient energy for both home and industry usage. It’s an alternative for fossil fuels which damaging the environment by emission lots of greenhouse gases. Water splitting is a highly potential method to produce “green” hydrogen without using fossil fuels. It is a chemical reaction that converts solar energy into chemical energy and is also called an artificial photosynthesis process. There have been different materials (e.g., TiO2, GaN and Silicon) with photoelectrochemical (PEC) characteristics to be used in water splitting. As novel 2D materials, MXenes are attractive for use in green hydrogen production due to their outstanding properties. Ti3C2Tx is one type of representative MXene materials that can be the photoanodes for water splitting. This review focuses on the electrical, magnetic and optical properties of Ti3C2Tx MXenes, and summarizes their preparation methods (top-down and bottom-up). Moreover, it discusses the important applications of MXenes in various fields including solar absorber, biology, catalysis, energy storages, and membrane separation. In addition, the challenges and perspectives have been provided.
... and so on) have been determined and predicted [2], as shown in Figure 1. Owing to the unusual electronic [3], mechanical [4] and optical properties, numerous functions were explored in batteries [5], supercapacitors [6], photocatalysts [7], catalysts [8], transparent conducting films [9], electromagnetic interference shielding [10], sensors [11], adsorption agents [12], and flexible high-strength composites [13]. The emerging MXene family and its structure. ...
MXene, 2D transition metal carbides, nitrides, and carbonitrides with a unique 2D structure, inspired a series of function applications related to energy storage and conversion, biometrics and sensing, lighting, purification, and separation. Its surface terminations are confined by the adjacent MXene layers, and form the 2D planar space with symmetrical surfaces, which is similar to a 2D nanoreactor that can be utilized and determined MXene’s function. Based on the working principle, surface and interface play critical roles in the ion intercalation, physical/chemical adsorption, and chemical reaction process, and show significant effects on MXene’s properties and functions. Although there have been some reviews on MXene, less attention has been paid to the underlying principle of the involved surface chemistry, controllable design, and resultant properties. Herein, the regulation methods, characterization techniques, and the effects on properties of MXene surface terminations were summarized to understand the surface effects, and the relationship between the terminations and properties. We expected this review can offer the route for a series of ongoing studies to address the MXene surface environment and the guidelines for MXene’s application.
... The mechanical property of MXenes depends significantly on the surface terminal groups and mass of the transition metal atom. Magnuson and Halim (2018) showed that the surface terminal groups decrease the strength of the Ti-C bond by removing its charge. They found that in Ti 2 C-T x , the Ti-C bond is longer than that in Ti 3 C 2 T x , which may impact the material's elasticity. ...
MXenes two-dimensional materials have recently excited researchers’ curiosity for various industrial applications. MXenes are promising materials for environmental remediation technologies to sense and mitigate various intractable hazardous pollutants from the atmosphere due to their inherent mechanical and physicochemical properties, such as high surface area, increased hydrophilicity, high conductivity, changing band gaps, and robust electrochemistry. This review discusses the versatile applications of MXenes and MXene-based nanocomposites in various environmental remediation processes. A brief description of synthetic procedures of MXenes nanocomposites and their different properties are highlighted. Afterward, the photocatalytic abilities of MXene-based nanocomposites for degrading organic pollutants, removal of heavy metals, and inactivation of microorganisms are discussed. In addition, the role of MXenes anti-corrosion support in the lifetime of some semiconductors was addressed. Current challenges and future perspectives toward the application of MXene materials for environmental remediation and energy production are summarized for plausible real-world use.
... The mechanical characteristics of MXenes can vary significantly based on the surface terminations. Magnuson et al. [89] discovered that the surface terminal groups weaken the Ti-C bond by removing the charge from them. They discovered that the Ti-C bond in Ti 2 C-T x is longer than in Ti 3 C 2 -T x , which might affect the elastic properties of the materials. ...
Due to their unique layered microstructure, the presence of various functional groups at the surface, earth abundance, and attractive electrical, optical, and thermal properties, MXenes are considered promising candidates for the solution of energy- and environmental-related problems. It is seen that the energy conversion and storage capacity of MXenes can be enhanced by changing the material dimensions, chemical composition, structure, and surface chemistry. Hence, it is also essential to understand how one can easily improve the structure–property relationship from an applied point of view. In the current review, we reviewed the fabrication, properties, and potential applications of MXenes. In addition, various properties of MXenes such as structural, optical, electrical, thermal, chemical, and mechanical have been discussed. Furthermore, the potential applications of MXenes in the areas of photocatalysis, electrocatalysis, nitrogen fixation, gas sensing, cancer therapy, and supercapacitors have also been outlooked. Based on the reported works, it could easily be observed that the properties and applications of MXenes can be further enhanced by applying various modification and functionalization approaches. This review also emphasizes the recent developments and future perspectives of MXenes-based composite materials, which will greatly help scientists working in the fields of academia and material science.
... 322,325 It has been widely acknowledged that the concentration and composition of terminating functional groups have profound influence on the characteristics of MXenes. 321,327 Typically, the MXenes tend to form a more stable compound with O or OH terminations as compared to F groups; 328 however, considerably high uncertainty would be expected in the proportion of the different species of terminating functional groups in experimental research. 329 From a molecular structural perspective, the MXenes demonstrate a similar molecular structure to TMDCs, where X atoms are sandwiched by layers of M atoms (Figure 24a,b). ...
The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.
... In the case of ZnO TPs samples, it consists of two components/bands at about 10 eV and 5 eV corresponding to Zn 3d and O 2p orbitals, respectively (Iatsunskyi et al., 2017). The broad VB XPS peak at 8-3 eV for pristine Ti 3 C 2 T x MXene may be associated with the C 2s -Ti 3d hybridization region and the terminated groups on the surface (Magnuson et al., 2018). The VB maximum (VBM) for those samples was evaluated using the standard method as previously described . ...
Continuous painless glucose monitoring is the greatest desire of more than 422 million diabetics worldwide. Therefore, new non-invasive and convenient approaches to glucose monitoring are more in demand than other tests for microanalytical diagnostic tools. Besides, blood glucose detection can be replaced by continuous glucose monitoring of other human biological fluids (e.g. sweat) collected non-invasively. In this study, a skin-attachable and stretchable electrochemical enzymatic sensor based on ZnO tetrapods (TPs) and a new class of 2D materials - transition metal carbides, known as MXene, was developed and their electroanalytical behavior was tailored for continuous detection glucose in sweat. The high specific area of ZnO TPs and superior electrical conductivity of MXene (Ti3C2Tx) nanoflakes enabled to produce enzymatic electrochemical glucose biosensor with enhanced sensitivity in sweat sample (29 μA mM-1 cm-2), low limit of detection (LOD ≈ 17 μM), broad linear detection range (LDR = 0.05–0.7 mM) that satisfices glucose detection application in human sweat, and advanced mechanical stability (up to 30% stretching) of the template. The developed skin-attachable stretchable electrochemical electrodes allowed to monitor the level of glucose in sweat while sugar uptake and during physical activity. Continuous in vivo monitoring of glucose in sweat obtained during 60 min correlated well with data collected by a conventional amperometric blood glucometer in vitro mode. Our findings demonstrate the high potential of developed ZnO/MXene skin-attachable stretchable sensors for biomedical applications on a daily basis.
... The stacked nanosheet with a variety of thicknesses changed the band structure, analyzed in connection with known hybridization region TiC and TiO2 which influenced the elastic properties of MXene. The result demonstrated that the surface functional groups withdraw charges from the Ti-C bond and weaken them.The T-C bond length was larger for Ti2CTx than Ti3C2Tx which directly affects the elastic properties of the MXene[115]. ...
The transition metal carbides/nitrides referred to as MXenes has emerged as a wonder material presenting newer opportunities owing to their unique properties such as high thermal and electrical conductivity, high negative zeta-potential and mechanical properties similar to the parent transition metal carbides/nitrides. These properties of MXenes can be utilized in various societal applications including for energy storage and energy conversion. In this focused review, we provide a ready glance into the evolutionary development of the MXene family and various efforts that are made globally towards property improvement and performance enhancement. Particular attention in this review is made to direct the attention of readers to the bright prospects of MXene in the energy storage and energy conversion process-which is extremely timely to tackle the current concern on climate change. The review concludes by offering fresh insights into the future research needs and challenges that need to be addressed to develop resilient energy solutions.
... High-resolution spectra for the Ti 2p, C 1s and Sn 3d binding energies are shown in Figures 5B-5D, respectively. The high-resolution XPS spectra of Ti 2p ( Figure 5B) region were deconvoluted into Ti-C, Ti 2+ , Ti 3+ , TI-F, and TiO 2 , which matches with prior studies (Halim et al., 2016;Magnuson et al., 2018;Natu et al., 2021;Naguib et al., 2011). The presence of the Ti-C peak at binding energy of 455 eV in Ti 2p spectra indicates the removal of aluminum from MAX phase and formation of Ti 3 C 2 T z MXene nanosheets. ...
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.
Zinc‐ion capacitors (ZICs) are promising next‐generation energy storage systems (ESS) owing to high safety, material abundance, environmental friendliness, and low cost; however, the energy density of ZICs must be improved to compete with lithium‐ion batteries (LIBs). Here, the study implements three strategies to enhance the electrochemical performance and manage dendritic growth on Zn anodes, including crafting a highly efficient redox electroactive niobium pyrophosphate (NbP2O7)/Ti3C2TX‐MXene binder‐free cathode, incorporating a NaClO4 additive electrolyte, and applying a protective Ti3C2TX‐MXene layer on Zn anode. The cathode facilitates rapid Zn²⁺ ion diffusion and a stable host structure. An electrostatic protection layer formed in additive electrolyte and MXene layers regulates the uniform distribution of the electric fields and supports the equalization of nucleation sites. These results are supported by density functional theory (DFT) calculations. The ZICs display an excellent specific capacitance (113.3 F g⁻¹ at 1.5 A g⁻¹) in aqueous additive electrolytes. The flexible solid‐state ZICs exhibits a volumetric capacitance of 865.05 mF cm⁻³, and an energy density of 0.347 mWh cm⁻³ at 2.29 mW cm⁻³ along with capacitance retention of >100% over 38 000 charge‐discharge cycles.
The proliferation of Internet of Things (IoT) applications necessitates the deployment of high-performance ammonia (NH3) sensors. In recent studies, Ti3C2Tx, a novel two dimensional (2D) material, has been extensively investigated for its potential as a room temperature NH3 sensor. However, pristine MXene-based sensors have exhibited deficiencies in long-term stability and reproducibility. In response, an MXene/γ-WO3 composite has been synthesized using an ultra-sonication method to address these challenges. The resulting composite
sensor has demonstrated remarkable performance, attributable to, increased active sites from defects induced by WO3, and the presence of a p-n heterojunction. Notably, electron transfer from WO3 to Ti3C2Tx has been identified as a critical contributing factor to the enhanced characteristics of these nanohybrids, as evidenced by observed alterations in binding energies for the Ti 2p and W 4 f core levels. These findings have expanded our
understanding of the electrical interactions in Ti3C2/WO3 and their potential applications in diverse domains. The pristine MXene sensor exhibited a response of 59.39 % at 300 ppm at room temperature, while the composite MXene/γ-WO3 displayed a response of 92.3 %. Additionally, the response recovery time was recorded at 22 seconds and 9 seconds, respectively. The chemiresistive ammonia sensor’s performance was evaluated across a range of concentrations (25–300 ppm) and relative humidity levels (11–94 % RH). The composite sensor has demonstrated reproducibility and long-term stability, coupled with high sensitivity to NH3. This manuscript aims to highlight the enhanced ammonia sensing properties and selectivity of MXene/γ-WO3 composite materials.
Transition metal carbide MXenes, specifically Ti2C and Nb2C, have been investigated for their ability to efficiently adsorb and dissociate CO2 using First Principles Calculations based on Density Functional Theory (FPS-DFT). Pure Ti2C and Nb2C sheets were studied with the adsorption of 1 to 4 CO2 molecules, and the results were analyzed for each case. For 4 CO2 molecules adsorbed on Ti2C and Nb2C sheets, the weight percentage (wt%) ratio was found to be 21.39% and 14.33%, respectively. The Density of States (DOS) analysis revealed that, upon CO2 adsorption, the CO2 molecule dissociates into CO and O atoms on the MXene surface. The CO molecule showed no significant hybridization with the host sheet atoms, whereas the O atom exhibited strong hybridization with the MXene surface. Transition State (TS) profiles illustrated the dissociation steps on both Ti2C and Nb2C surfaces. Phonon calculations confirmed the dynamic stability of both pure and CO2 adsorbed MXenes. Molecular Dynamics (MD) simulations conducted at 900 K indicated uniform temperature and Mean Square displacement (MSD) profiles, suggesting the stability of both pure and CO2 adsorbed MXenes at elevated temperatures, as well as uniform CO2 adsorption behavior. The adsorption energies for Ti2C and Nb2C sheets were calculated to be in the range of -1.89 to -2.77 eV, suggesting that the adsorption process is favorable, spontaneous, and predominantly involves chemisorption. These findings indicate that Ti2C and Nb2C MXenes, in their intrinsic forms, are promising candidates for CO2 adsorption and dissociation technologies.
Transparent flexible energy storage devices are limited by the trade‐off among flexibility, transparency, and charge storage capability of their electrode materials. Conductive polymers are intrinsically flexible, but limited by small capacitance. Pseudocapacitive MXene provides high capacitance, yet their opaque and brittle nature hinders their flexibility and transparency. Herein, the development of synergistically interacting conductive polymer Ti3C2Tx MXene/PEDOT:PSS composites is reported for transparent flexible all‐solid‐state supercapacitors, with an outstanding areal capacitance of 3.1 mF cm⁻², a high optical transparency of 61.6%, and excellent flexibility and durability. The high capacitance and high transparency of the devices stem from the uniform and thorough blending of PEDOT:PSS and Ti3C2Tx, which is associated with the formation of O─H…O H‐bonds in the composites. The conductive MXene/polymer composite electrodes demonstrate a rational means to achieve high‐capacity, transparent and flexible supercapacitors in an easy and scalable manner.
MXenes have obtained noticeable interest cause of their unique features (physical and chemical features). MXenes are considered auspicious applicants considering the resolution of ecological and energy issues because of their distinctive stacked nanostructure, multiple functionalities at the surface, the excess of these compounds on earth, and their appealing optical, electrical, and thermal properties. Due to its large area, flexible chemical composition, and readily modifiable compositions of elements, MXenes need to become a viable choice to enhance photocatalytic efficiency in renewable energy and ecological treatment applications. Cause of their layered nanostructure with an abundance of functionality, they are with outstanding adjustable performance and are simple to mix with other materials, like metallic oxides, polymers, organic hybrids, and carbonaceous materials, to satisfy the demands of high-performance applications. MXenes are excellent catalysts because of their multiple interlayer groups, surface group activities, and adaptable layer spacing. The MXenes family contains more than 30 distinct members, all of which have been investigated and effectively used as catalysts. The fabrication, mechanism at the surface, and uses of MXenes with associated nanocomposites are covered in this chapter. We also discuss MXenes principles and their respective manufacturing methods, such as exfoliation delamination, HF etching, hydrothermal, polymerization, etc., to better understand. MXenes have excelled as photocatalysts for photochemical degradation, carbon dioxide reduction, hydrogen evolution, and nitrogen fixation. Moreover, surface flaws of MXenes offer lots of CO2 adsorption sites. Also, these materials’ superior 2D-nanomaterial structure and fast electron transport pathways contribute to their extremely effective oxidation reaction activity. The effectiveness of heterostructures based on MXene and their nanocomposite photocatalysts for removing organic pollutants is also thoroughly analyzed in this chapter. Lastly, a future direction for energy and ecological sciences research is suggested.
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.
Electrochemical capacitors and hydrogen production through water splitting into constituent hydrogen (H2) and oxygen (O2) molecules are promising methods for long-lasting energy storage. This work presents a simple synthesis of a cobalt ion-doped vanadium carbide MXene (Co@V2CTx) nanocomposite, utilized as an effective, durable, and stable electrode for supercapacitors and as an electrocatalyst for water electrolysis. In 1 M KOH, the Co@V2CTx nanocomposite produced a capacitance of 1071 Fg-1 at 5 mVs−1 (more than 8 times that of pristine V2CTx MXene) and an energy and power density of 26.7 Whkg−1 and 325 Wkg-1 respectively at 1 Ag-1. In addition, the Co@V2CTx nanocomposite showed good hydrogen evolution reaction (HER) catalytic activity, exhibiting a minimal overpotential of 103 mV at 10 mAcm−2 and Tafel slope of 83 mVdec−1. The excellent performance of the Co@V2CTx nanocomposite is due to the synergistic effects produced when cobalt ions are intercalated into the vanadium carbide MXene, preventing restacking of sheets and boosting ion transfer. This work presents an easy and effective method for synthesizing MXene-based nanocomposites for energy storage and conversion applications.
Two-dimensional (2D) transition-metal carbides (Ti3C2Tx) MXene have received much attention due to their
exciting prospects for application in gas sensing. Large surface area, metallic conductivity, and the functional-
ization potential all are key characteristics of MXene. These materials have optimum gas selectivity and
responsiveness among 2D materials. By using HF etching, we developed exfoliated MXene from MAX phase of
Ti3AlC2. The William-Hall (W-H) plot estimated the size of the crystallite to be 7.95 nm. The calculated optical
band gap was found 1.53 eV. The almost disappearance of aluminum (Al) layers was confirmed by the XRD(X-ray
diffraction) peak shift, EDS and XPS analysis and the accordion-like morphology confirmed by FESEM images.
The specific surface area of synthesized MXene was found to be 39.00 m2
g-1, while the pore volume of MXene
was estimated as 0.0755852 cm3
g-1 and the pore diameter was evaluated as 3.42782 nm. The experimental
findings demonstrate the fact that Ti3C2Tx incorporation improves the sensitivity, the repeatability, and the
selectivity of chemo-resistive sensors that are sensitive to VOCs (volatile organic compounds) such as acetone,
ethanol, methanol, propanol, and hexane. The fabricated MXene based sensor was tested under exposure of
aforementioned VOCs at 600 ppm. The initial finding illustrates that among these, ethanol is the most sensitive
VOC. The fabricated Ti3C2Tx sensor yielded response (%) at the concentrations of 50, 100, 200, 300, 400 and
500 ppm as 1.78, 2.63, 3.42, 4.28, 5.05, and 5.47%, respectively under purging ethanol. The maximum response
at room temperature (RT) operable ethanol sensor was calculated as 5.65% at 600 ppm under 40% of relative
humidity (RH). The outcomes demonstrate rapid responses/recovery of 5s and 8s, respectively.
MXenes are a family of two-dimensional (2D) materials that have drawn a lot of interest recently because of their distinctive characteristics and possible uses in a variety of industries. This review emphasizes the bright future prospects of MXene materials in the realm of FETs. Their remarkable properties, coupled with their tunability and compatibility, position MXenes as a promising candidate for the development of high-performance electronic devices. As research in this field continues to evolve, the potential for MXenes to drive innovation in electronics becomes increasingly evident, fostering excitement for their role in shaping the future of electronic technology. The advent of two-dimensional (2D) materials has revolutionized the landscape of electronic devices, offering unique properties and diverse applications. Among these emerging materials, MXenes have garnered significant attention for their exceptional properties and versatile potential in various technological domains. This paper presents a comprehensive overview of MXene materials, focusing on their synthesis methods, functionalization strategies, intrinsic properties, and their promising application in Field Effect Transistors (FETs).
MXenes, the largest and most diverse group of emerging two-dimensional materials, have potentials across multiple applications. The increasing attention is driven by the fascinating tunable surface properties, and synergetic chemistry facilitated by the presence of multiple chemical bonds dominated by covalent and metallic bonds between transition metals (M) and non-metals, X (such as carbon, nitrogen, or both). Although the various available synthesis approaches offer opportunities for tuning MXenes for specific applications, their inherent environmental and toxicity risks as well as poor scale-up potential are currently hampering research and commercialization progress. Therefore, ongoing efforts are focused on developing less hazardous, scalable methodologies to limit the barriers. This review comprehensively surveys the literature from the seminal MXene paper to the present, offering insights into the factors, latest advancements, limitations, trends, and existing gaps in MXene synthesis, properties, and applications in the areas of environment and energy storage. Special emphasis is placed on the need to address environmental concerns associated with fluoride-based synthesis while an overview of safer, non-fluoride alternatives is provided. Furthermore, the most recent breakthroughs in scalable top-down dry selective etching (DSE), bottom-up solid-state direct synthesis, and structural editing protocol using chemical scissors are presented. In particular, this review critically examines the current state of knowledge and identifies key research progresses and areas that deserve intensive attention to facilitate safe and industrial-scale synthesis and application of MXenes for catalysis, environmental remediation, and energy storage. Addressing the identified gaps will accelerate the transformation of MXenes and their composites into the components of our everyday appliances.
Functional 2D materials are interesting for a wide range of applications. The rapid growth of the MXene family is due to its compositional diversity, which, in turn, allows significant tuning of the properties, and hence their applicability. The properties are to a large extent dictated by surface terminations. In the present work, we demonstrate the influence of termination species (O, NH, N, S, F, Cl, Br, I) on the changes in electronic structure, work function, dynamical stability, and atomic charges and distances of MXenes (Ti2C, Nb2C, V2C, Mo2C, Ti3C2, and Nb4C3). Among these systems, the work function values were not previously reported for ∼60% of the systems, and most of the previously reported MXenes with semiconducting nature are here proven to be dynamically unstable. The results show that the work function generally decreases with a reduced electronegativity of the terminating species, which in turn is correlated to a reduced charge of both the metal and terminating species and an increased metal-termination distance. An exception to this trend is NH terminations, which display a significantly reduced work function due to an intrinsic dipole moment within the termination. Furthermore, the results suggest that halogen terminations improve the electrical conductivity of the materials.
The thermal and crystallization behavior of polymeric materials remains an important parameter that determines to a large extent, their specific engineering end use. The thermal and crystallization behavior is often linked to the handling and the usage of polymeric materials, degradation, and reuse. With MXene and their nanocomposites, the thermal and crystallization behavior is determined by the surface-terminating groups, the method of storage, the composites preparation technique, and the subsequent treatment procedures, such as annealing. In this chapter, the various methods of evaluating the thermal properties of MXene and nanocomposites have been discussed with specific emphasis on the thermal behavior of MXene-based polymer nanocomposites. The thermodynamics of crystallization and the melting of polymers and polymer nanocomposites, the non-isothermal melt, and the cold crystallization of MXene-based polymer nanocomposites are covered. Finally, the effect of MXene on the crystallization of polymer nanocomposites is discussed.
The two-dimensional (2D) MXenes are considered efficient electrochemical capacitors (EC) having high energy and power densities with high-rate capabilities as their layered structure provides redox-active surface sites and enhanced electrolytic ion transport simultaneously. V2CTx MXene, one of the lightest members, is theoretically predicted as high performing electrode material for supercapacitors however, restacking of delaminated MXene limits its effective use as EC. We have reported ZrO2-MXene composite where ZrO2 nano-particles are grown on V2CTX MXene sheets. The interconnected structure boosts up the conductivity and suppresses the restacking. The ZrO2-V2CTx composite showed an outstanding capacitance of ≈ 1200F/g @ 5mVs⁻¹ in 3 M H2SO4 which is more than twice the capacitance of pristine MXene. The composite was subjected to cyclic stability up to 10,000 cycles that showed retention of 97 % capacitance. The nonlinear galvanometric charge-discharge (GCD) curves represented the pseudocapacitive nature of composite electrodes with an energy density and power density of 15.39 Wkg⁻¹ and 4000Whkg⁻¹, respectively.
Use of MXenes (Ti3C2Tx), which belongs to the family of two‐dimensional transition metal nitrides and carbides by encompassing unique combination of metallic conductivity and hydrophilicity, is receiving tremendous attention, since its discovery as energy material in 2011. Owing to its precursor selective chemical etching, and unique intrinsic characteristics, the MXene surface properties are further classified into highly chemically active compound, which further produced different surface functional groups i. e., oxygen, fluorine or hydroxyl groups. However, the role of surface functional groups doesn't not only have a significant impact onto its electrochemical and hydrophilic characteristics (i. e., ion adsorption/diffusion), but also imparting a noteworthy effect onto its conductivity, work function, electronic structure and properties. Henceforth, such kind of inherent chemical nature, robust electrochemistry and high hydrophilicity ultimately increasing the MXene application as a most propitious material for overall environment‐remediation, electrocatalytic sensors, energy conversion and storage application. Moreover, it is well documented that the role of MXenes in all kinds of research fields is still on a progress stage for their further improvement, which is not sufficiently summarized in literature till now. The present review article is intended to critically discuss the different chemical aptitudes and the diversity of MXenes and its derivates (i. e., hybrid composites) in all aforesaid application with special emphasis onto the improvement of its surface characteristics for the multidimensional application. However, this review article is anticipated to endorse MXenes and its derivates hybrid configuration, which is discussed in detail for emerging environmental decontamination, electrochemical use, and pollutant detection via electrocatalytic sensors, photocatalysis, along with membrane distillation and the adsorption application. Finally, it is expected, that this review article will open up new window for the effective use of MXene in a broad range of environmental remediation, energy conversion and storage application as a novel, robust, multidimensional and more proficient materials. The two‐dimensional MXenes were described and classified according to the synthesis techniques used, mechanical mixing, self‐assembly, in situ decorating, oxidation, properties and application. Herein, the MXenes and its derivates are emphasized onto the improvement of its surface characteristics for multidimensional application. This review article anticipating MXenes for emerging into environmental decontamination, photocatalysis, electrochemical use, and pollutant detection via electrocatalytic sensors, photocatalysis, along with membrane distillation and the adsorption application.
2D transition metal carbides or nitrides (MXenes) have attracted considerable attention from materials scientists and engineers owing to their physicochemical properties. Currently, MXenes are synthesized from MAX‐phase precursors using aqueous HF. Here, in order to enhance the production of MXenes, an anhydrous etching solution is proposed, consisting of dimethylsulfoxide as solvent with its high boiling point, NH4HF2 as an etchant, CH3SO3H as an acid, and NH4PF6 as an intercalant. The reaction temperature can be increased up to 100 °C to accelerate the etching and delamination of Ti3AlC2 MAX crystals; in addition, the destructive side reaction of the produced Ti3C2Tx MXene is suppressed in the etchant. Consequently, the etching reaction is completed in 4 h at 100 °C and produces high‐quality monolayer Ti3C2Tx with an electrical conductivity of 8200 S cm⁻¹ and yield of over 70%. The Ti3C2Tx MXene fabricated via this modified synthesis exhibits different surface structures and properties arising from more F‐terminations than those of Ti3C2Tx synthesized in aqueous HF2T. The atypical surface structure of Ti3C2Tx MXene results in an exceptionally high ultimate tensile strength (167 ± 8 MPa), which is five times larger than those of Ti3C2Tx MXenes synthesized in aqueous HF solution (31.7 ± 7.8 MPa).
Two-dimensional (2D) materials have been studied extensively for the past 15 years, sparking a new wave of research on well-known 2D materials. Due to their specific configuration and noteworthy physiochemical features, intensive, multifaceted research attempts have been focused on the medical and clinical applications of 2D materials. In this context, 2D MXenes, a new class of ultra-thin atomic nanosheet materials produced from MAX phase ceramics, are gaining popularity as inorganic nanosystems, especially for biomedical applications. The 2D MXenes can meet the stringent biomedical standards due to their high conductivity, hydrophilicity, and other interesting physicochemical properties. Based on these characteristics, 2D MXenes have been used in wound dressing management and there are many studies on the development of nanofibers and nanosheets. Herein, we present an overview of MXenes, and their synthesis using various processes and properties. The review further focuses on the mechanism and importance of MXenes for wound dressing applications. Additionally, we summarize the toxicity and bio-safety issues of MXene-based materials. In the last section, we present the conclusions, challenges and future outlook.
We study Li and, for the first time, K, Mg and Zn ion intercalation on the surface of the Zr2CS2 MXene monolayer, taking advantage of the fact that the S terminations lower the diffusion barrier of the ions. We find that the Zr2CS2-Li, Zr2CS2-K and Zr2CS2-Mg structures are identical, with only Zr2CS2-Zn differing as to the position of the ion and Zn detaching from the MXene’s surface during migration. Regarding the use of Zr2CS2 as anode material in ion batteries, we examine as criteria the adsorption energy, diffusion barrier energy and open-circuit voltage for each of the ions considered. We show that the K ion has higher mobility, as well as lower open-circuit voltage. These results lead to the fact that KIB have fastest charge/discharge rates and higher energy density than LIB, MIB, and ZIB when it comes to the use of S-terminated, Zr-based materials as negative (anode) electrodes. KIB, therefore, seem the best alternative to LIB, especially after taking under consideration K’s low cost and abundance of resources.
For nearly 15 years, researchers have been using liquid‐phase exfoliation (LPE) to produce 2D nanosheets from layered crystals. This has yielded multiple 2D materials in a solution‐processable form whose utility has been demonstrated in multiple applications. It was believed that the exfoliation of such materials was enabled by the very large bonding anisotropy of layered materials where the strength of intra‐layer chemical bonds is very much larger than that of inter‐layer van der Waals bonds. However, over the last five years, a number of papers have raised questions about our understanding of exfoliation by describing the LPE of non‐layered materials. These results are extremely surprising because, as no van der Waals gap is present to provide an easily cleaved direction, the exfoliation of such compounds requires the breaking of only chemical bonds. Here we examine progress in this unexpected new research area. We review the structure and properties of platelets produced by LPE of non‐layered materials. We find a number of unexplained trends, not least the preponderance of isotropic materials which have been exfoliated to give high aspect‐ratio nanoplatelets. Finally, we consider the applications potential of this new class of 2D materials. This article is protected by copyright. All rights reserved
MXene‐transition metal dichalcogenide (TMD) heterostructures are synthesized through a one‐step heat treatment of Nb2C and Nb4C3. These MXenes are used without delamination or any pre‐treatment. Heat treatments accomplish the sacrificial transformation of these MXenes into TMD (NbS2) at 700 and 900 °C under H2S. This work investigates, for the first time, the role of starting MXene phase in the derivative morphology. It is shown that while treatment of Nb2C at 700 °C leads to the formation of pillar‐like structures on the parent MXene, Nb4C3 produces nano‐mosaic layered NbS2. At 900 °C, both MXene phases, of the same transition metal, fully convert into nano‐mosaic layered NbS2 preserving the parent MXene's layered morphology. When tested as electrodes for hydrogen evolution reaction, Nb4C3‐derived hybrids show better performance than Nb2C derivatives. The Nb4C3‐derived heterostructure exhibits a low overpotential of 198 mV at 10 mA cm−2 and a Tafel slope of 122 mV dec−1, with good cycling stability in an acidic electrolyte. The conversion of niobium carbide MXenes into niobium sulfide/carbide hybrids by sulfidation can be controlled to obtain specific nanoarchitecture. The obtained electrode materials show promising performance for hydrogen evolution reactions, with important differences between Nb4C3 and Nb2C derivatives.
Since the discovery of Ti3C2Tx in early 2011, a newly emerging family of post-graphene two-dimensional transition metal carbides and nitrides (MXenes) has been rigorously investigated owing to their high conductivity....
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M\ \C bonds in high-density MC slabs, and relatively weak M\ \A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity , elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
In this work, a detailed high resolution X-ray photoelectron spectroscopy (XPS) analysis is presented for select MXenes—a recently discovered family of two-dimensional (2D) carbides and carbonitrides. Given their 2D nature, understanding their surface chemistry is paramount. Herein we identify and quantify the surface groups present before, and after, sputter-cleaning as well as freshly prepared vs. aged multi-layered cold pressed discs. The nominal compositions of the MXenes studied here are Ti3C2Tx, Ti2CTx, Ti3CNTx, Nb2CTx and Nb4C3Tx, where T represents surface groups that this work attempts to quantify. In all the cases, the presence of three surface terminations, O, OH and F, in addition to OH-terminations relatively strongly bonded to H2O molecules, was confirmed. From XPS peak fits, it was possible to establish the average sum of the negative charges of the terminations for the aforementioned MXenes. Based on this work, it is now possible to quantify the nature of the surface terminations. This information can, in turn, be used to better design and tailor these novel 2D materials for various applications.
Density functional theory simulations with conventional (PBE) and hybrid
(HSE06) functionals were performed to investigate the structural and electronic
properties of MXene monolayers, \ce{Ti_{n+1}C_n} and \ce{Ti_{n+1}N_n} (n =
1--9) with surfaces terminated by O, F, H, and OH groups. We find that PBE and
HSE06 give similar results. Without functional groups, MXenes have magnetically
ordered ground states. All the studied materials are metallic except for
\ce{Ti_{2}CO_{2}}, which we predict to be semiconducting. The calculated
density of states at the Fermi level of the thicker MXenes (n 5)
is much higher than for thin MXenes, indicating that properties such as
electronic conductivity and surface chemistry will be different. In general,
the carbides and nitrides behave differently with the same functional groups.
The electronic structures of epitaxially grown films of Ti(3)AlC(2), Ti(3)SiC(2), and Ti(3)GeC(2) have been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured high-resolution Ti L, C K, Al L, Si L, and Ge M emission spectra are compared with ab initio density-functional theory including core-to-valence dipole matrix elements. A qualitative agreement between experiment and theory is obtained. A weak covalent Ti-Al bond is manifested by a pronounced shoulder in the Ti L emission of Ti(3)AlC(2). As Al is replaced with Si or Ge, the shoulder disappears. For the buried Al and Si layers, strongly hybridized spectral shapes are detected in Ti(3)AlC(2) and Ti(3)SiC(2), respectively. As a result of relaxation of the crystal structure and the increased charge-transfer from Ti to C, the Ti-C bonding is strengthened. The differences between the electronic structures are discussed in relation to the bonding in the nanolaminates and the corresponding change of materials properties.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
The graphene-like transition metal carbide (Ti3C2X2(X = OH, F)) which was synthesized from etching the layered Ti3AlC2 material was applied as a carrier for depositing Ru nanoparticles (Ru/Ti3C2X2). The as-prepared nanocomposites were characterized by SEM, TEM, XRD, XPS and FTIR. During the hydrolysis process, Ru nanoparticles were uniformly generated on the surface of the carrier and acted as catalysts for the hydrogen generation from hydrolysis of NaBH4 at room temperature. It was found that the catalyst Ru/Ti3C2X2 exhibited excellent catalytic activity toward the hydrolysis of sodium borohydride with a hydrogen generation rate of 59.04 L H-2/g(Ru).min and an activation energy of 22.1 kJ/mol. Copyright
Substituting Al for Ti in TiN(001), TiN(011), and N- and Ti-terminated TiN(111) surfaces has significant effects on adatom surface energetics which vary strongly with the adatom species and surface orientation. Here, we investigate Ti, Al, and N adatom surface dynamics using density functional methods. We calculate adatom binding and diffusion energies with both a nudged elastic band and grid-probing techniques. The adatom diffusivities are analyzed within a transition-state theory approximation. We determine the stable and metastable Ti, Al, and N binding sites on all three surfaces as well as the lowest energy migration paths. In general, adatom mobilities are fastest on TiN(001), slower on TiN(111), and slowest on TiN(011). The introduction of Al has two major effects on the surface diffusivity of Ti and Al adatoms. First, Ti adatom diffusivity on TiN(001) is significantly reduced near substituted Al surface atoms; we observe a 200% increase in Ti adatom diffusion barriers out of fourfold hollow sites adjacent to Al surface atoms, while Al adatom diffusivity between bulk sites is largely unaffected. Secondly, on TiN(111), the effect is opposite; Al adatoms are slowed near the substituted Al surface atom, while Ti adatom diffusivity is largely unaffected. In addition, we note the importance of magnetic spin polarization on Ti adatom binding energies and diffusion path. These results are of relevance for the atomistic understanding of Ti1−xAlxN alloy and Ti1−xAlxN/TiN multilayer thin-film growth processes.
Recently a new, large family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. MXenes are produced by selective etching of the A element from the MAX phases, which are metallically conductive, layered solids connected by strong metallic, ionic, and covalent bonds, such as Ti2AlC, Ti3AlC2, and Ta4AlC3. MXenes combine the metallic conductivity of transition metal carbides with the hydrophilic nature of their hydroxyl or oxygen terminated surfaces. In essence, they behave as conductive clays. This article reviews progressboth experimental and theoreticalon their synthesis, structure, properties, intercalation, delamination, and potential applications. MXenes are expected to be good candidates for a host of applications. They have already shown promising performance in electrochemical energy storage systems. A detailed outlook for future research on MXenes is also presented.
The electronic structure of the early transition metal carbides and nitrides is linked to their remarkable physical properties and their surface chemistry. In this review, we focus on experimental studies of the electronic structure and surface adsorption properties of these rock-salt structured materials. A straightforward molecular orbital framework is used to understand the surface chemical interactions of the materials, primarily the stoichiometric (0 0 1) surfaces of TiC and VC, with small molecule adsorbates. This framework is then expanded to include more comprehensive theoretical treatments of the surface adsorption, with a particular emphasis on recent density functional theory results. The adsorbates reviewed include CO, NH3, O2, H2O, SO2, methanol, methyl formate, and ethanol. This overview reveals that the properties of these materials are heavily influenced by two factors: highly covalent bonding interactions between the metal and carbon species and the total number of electrons present, as the added electron per formula unit in either VC or TiN relative to TiC, populates low lying metal 3d levels that are formally unoccupied in TiC. This results in materials that appear to be d0 (TiC), d1 (VC, TiN) or d2 (VN), with actual charges on the atoms that are close to ±1. These influences are apparent in valence band photoemission data obtained on the (0 0 1) surfaces. The surface chemistry trends with probe molecules such as CO and NH3 can be predicted based on coordination chemistry principles, with the σ-donor ammonia molecule, for example, have very similar interactions with TiC and VC, while CO adsorption is measurably stronger on the VC surface due to π-backbonding interactions. More comprehensive surface models are needed to probe surface reactions as they are heavily influenced by neighboring carbon and metal atoms on the (1 0 0) surface, with the added d-electron density on the metal in VC or TiN enabling stronger surface bonding with reaction intermediates than is found on TiC.
Layered MAX phases are exfoliated into 2D single layers and multilayers, so-called MXenes. Using first-principles calculations, the formation and electronic properties of various MXene systems, M2C (M = Sc, Ti, V, Cr, Zr, Nb, Ta) and M2N (M = Ti, Cr, Zr) with surfaces chemically functionalized by F, OH, and O groups, are examined. Upon appropriate surface functionalization, Sc2C, Ti2C, Zr2C, and Hf2C MXenes are expected to become semiconductors. It is also derived theoretically that functionalized Cr2C and Cr2N MXenes are magnetic. Thermoelectric calculations based on the Boltzmann theory imply that semiconducting MXenes attain very large Seebeck coefficients at low temperatures.
Titanium carbide (TiC) and titanium nitride (TiN) possess remarkable physical properties, such as extremely high hardness and melting point, that promote their use as antiwear materials under harsh tribological conditions. These physical properties must arise from chemical bonding phenomena that result from the inclusion of the non-metal atom within the metallic matrix, and these bonding phenomena should be apparent in measurements of the valence-band electronic structures of TiC and TiN. This paper explores the surface electronic structure and bonding in TiC(100) and TiN(110) with core and valence level photoelectron spectroscopies (PES's) using X-rays (1486.6 eV) and synchrotron radiation in the range 28-180 eV. Intensity changes in the valence-band features are followed as a function of incident photon energy; these changes are then compared to theoretical atomic photoionization cross sections to determine the atomic origins of these features. Resonant PES at the Ti 3p absorption edge is used to determine titanium 3d contributions to the valence band and to show differences in the electronic structures in TiC and TiN. A new resonance phenomenon near the Ti 3s edge in TiC was observed and its possible assignment is discussed. The electronic structure and bonding in these materials is well described by molecular orbital theory, where the Ti and non-metal ions in their formal oxidation states (e.g., Ti[sup 4+] and C[sup 4[minus]] in TiC) undergo covalent bonding interactions. Overall, the PES results indicate greater covalent mixing for TiC as compared to TiN, consistent with the differences in the electronegativities of the atoms. Specifically, stronger covalent interactions between the C 2s, 2p and the Ti 3d, 4s, 4p levels must occur to explain the spectroscopic differences between TiC and TiN.
Density functional theory (DFT) computations were performed to investigate the electronic properties and Li storage capability of Ti(3)C(2), one representative MXene (M represents transition metals, and X is either C or/and N) material, and its fluorinated and hydroxylated derivatives. The Ti(3)C(2) monolayer acts as a magnetic metal, while its derived Ti(3)C(2)F(2) and Ti(3)C(2)(OH)(2) in their stable conformations are semiconductors with small band gaps. Li adsorption forms a strong Coulomb interaction with Ti(3)C(2)-based hosts but well preserves its structural integrity. The bare Ti(3)C(2) monolayer exhibits a low barrier for Li diffusion and high Li storage capacity (up to Ti(3)C(2)Li(2) stoichiometry). The surface functionalization of F and OH blocks Li transport and decreases Li storage capacity, which should be avoided in experiments. The exceptional properties, including good electronic conductivity, fast Li diffusion, low operating voltage, and high theoretical Li storage capacity, make Ti(3)C(2) MXene a promising anode material for Li ion batteries.
The application of density functional theory to calculate adsorption properties, reaction pathways, and activation energies for surface chemical reactions is reviewed. Particular emphasis is placed on developing concepts that can be used to understand and predict variations in reactivity from one transition metal to the next or the effects of alloying, surface structure, and adsorbate-adsorbate interactions on the reactivity. Most examples discussed are concerned with the catalytic properties of transition metal surfaces, but it is shown that the calculational approach and the concepts developed to understand trends in reactivity for metals can also be used for sulfide and oxide catalysts.
Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti(2)AlC, Ta(4)AlC(3), (Ti(0.5),Nb(0.5))(2)AlC, (V(0.5),Cr(0.5))(3)AlC(2), and Ti(3)AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.
2D Ti 3C 2 nanosheets, multilayer structures, and conical scrolls produced by room temperature exfoliation of Ti 3AlC 2 in HF are reported. Since Ti 3AlC 2 is a member of a 60+ group of layered ternary carbides and nitrides, this discovery opens a door to the synthesis of a large number of other 2D crystals.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
The surface electronic structure and bonding properties of TiC, VC, and TiN are probed through density functional theory calculations on small, symmetric clusters and comparison to experimental studies. We find very strong M−C covalent mixing in the carbides and electronic structure differences that clearly impact (100) surface bonding with oxygen, carbon monoxide, and ammonia. The critical differences are the additional dπ-electron present on VC and TiN and greater charge separation on TiN adding to Coulombic effects.
- S Novoselov
- A K Geim
- S V Morozov
- D Jiang
- Y Zhang
- S V Dubonos
- I V Grigorieva
- A A Firsov
S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V.
Grigorieva, A.A. Firsov, Science 306 (2004) 666.
- M Naguib
- M Kurtoglu
- V Presser
- J Lu
- J Niu
- M Heon
- L Hultman
- Y Gogotsi
- M W Barsoum
M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y.
Gogotsi, M.W. Barsoum, Adv. Mater. 23 (2011) 4248.
- M Naguib
- O Mashtalir
- J Carle
- V Presser
- J Lu
- L Hultman
- Y Gogotsi
- M W Barsoum
M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi, M.W.
Barsoum, ACS Nano 6 (2012) 1322.
- M Naguib
- V N Mochalin
- M W Barsoum
- Y Gogotsi
M. Naguib, V.N. Mochalin, M.W. Barsoum, Y. Gogotsi, Adv. Mater. 26 (2014)
992.
- M Magnuson
- O Wilhelmsson
- J.-P Palmquist
- U Jansson
- M Mattesini
- S Li
- R Ahuja
- O Eriksson
M. Magnuson, O. Wilhelmsson, J.-P. Palmquist, U. Jansson, M. Mattesini, S. Li,
R. Ahuja, O. Eriksson, Phys. Rev. B 74 (2006) 195108.
- M Magnuson
- M Mattesini
- O Wilhelmsson
- J Emmerlich
- J.-P Palmquist
- S Li
- R Ahuja
- L Hultman
- O Eriksson
- U Jansson
M. Magnuson, M. Mattesini, O. Wilhelmsson, J. Emmerlich, J.-P. Palmquist, S.
Li, R. Ahuja, L. Hultman, O. Eriksson, U. Jansson, Phys. Rev. B 74 (2006) 205102.
- M Magnuson
- M Mattesini
- S Li
- C Höglund
- M Beckers
- L Hultman
- O Eriksson
M. Magnuson, M. Mattesini, S. Li, C. Höglund, M. Beckers, L. Hultman, O.
Eriksson, Phys. Rev. B 76 (2007) 195127.
- M Magnuson
- O Wilhelmsson
- M Mattesini
- S Li
- R Ahuja
- O Eriksson
- H Högberg
- L Hultman
- U Jansson
M. Magnuson, O. Wilhelmsson, M. Mattesini, S. Li, R. Ahuja, O. Eriksson, H.
Högberg, L. Hultman, U. Jansson, Phys. Rev. B 78 (2008) 035117.
- M Khazaei
- M Arai
- T Sasaki
- C.-Y Chung
- N S Venkataramanan
- M Estili
- Y Sakka
- Y Kawazoe
M. Khazaei, M. Arai, T. Sasaki, C.-Y. Chung, N.S. Venkataramanan, M. Estili, Y.
Sakka, Y. Kawazoe, Adv. Func. Mater. 23 (2013) 2185.
- Y Xie
- P R C Kent
Y. Xie, P.R.C. Kent, Phys. Rev. B 87 (2013) 235441.
- Q Tang
- Z Zhou
- P Shen
Q. Tang, Z. Zhou, P. Shen, J. Am. Chem. Soc. 134 (2012) 16909.
- J Halim
- K M Cook
- Michael Naguib
- Per Eklund
- Yury Gogotsi
- Johanna Rosen
- M W Barsoum
J. Halim, K.M. Cook, Michael Naguib, Per Eklund, Yury Gogotsi, Johanna Rosen,
M.W. Barsoum, Appl. Surf. Sci. 362 (2016) 406.
- P E Blöchl
P.E. Blöchl, Phys. Rev. B 50 (1994) 17953.
- S V Didziulis
- K D Butcher
- S S Perry
S.V. Didziulis, K.D. Butcher, S.S. Perry, Inorg. Chem. 42 (2003) 7766.
- S V Didziulis
- K D Butcher
S.V. Didziulis, K.D. Butcher, Coord. Chem. Rev. 257 (2013) 93.
- C Tholander
- B Alling
- F Tasnádi
- J E Greene
- L Hultman
C. Tholander, B. Alling, F. Tasnádi, J.E. Greene, L. Hultman, Surf. Sci. 630 (2014)
28.
- X Li
- G Fan
- C Zeng
- J Int
X. Li, G. Fan, C. Zeng, J. Int, Hydrogen Energy 39 (2014) 14927.
- M W Barsoum
M.W. Barsoum, Prog. Solid State Chem. 28 (2000) 201.