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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.
... An earlier work has shown that oxidized sections of MXene samples are detrimental for Ti3C2Tx valence band studies as the O 2p states of oxidized Ti dominate the central part of the valence band binding energy region, i.e., 5-10 eV below the Fermi level (EF) [22]. It is therefore a necessity to produce Ti3C2Tx samples that are free from oxide formation. ...
... The variation in the dominating contribution follows the calculated photoionization cross-sections shown in figure 4. Other noteworthy findings are the enhanced C 2s states and an increased intensity around 5 eV when using Al-Kα radiation compared to the valence band UPS spectra 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A c c e p t e d M a n u s c r i p t Considering the simple model used in the study the computed valence band UPS/XPS of the bare Ti3C2-layer correlate to a certain extent with the Ti-C features A-C, and G in the experimentally recorded valence band UPS/XPS spectra shown in figures 5-8, except the enhanced intensity at 0.8 eV in the computed spectra. Hence, the Ti-C distances obtained from the recent Ti K-edge XAS study [22] are appropriate in the Ti3C2Tx structure model. Figure 12 shows the computed valence band UPS/XPS spectra where the simple model calculations included the interactions between the Ti3C2-surface and the termination species F on the fcc-site (Ffcc), O on the fcc-site (Ofcc), and O on the bridge site (Ob), see figure 1 and the structure models above the UPS/XPS spectra panels in figure 12(a), (b), and (c), respectively. ...
... The NMR-study could not provide evidences of that the detected OH was bonded to the Ti3C2-surface, although it was found that the OH was located not far away from C. Even so, Ti3C2Tx is prone to oxidation [22] and it is established that OH bonds strongly to TiO2 surfaces with O-vacancy sites [40][41][42][43][44][45][46], which argues that the obtained OH signal rather originates from OH adsorbed on oxidized material. If so, the OH would still be in close proximity of C because Ti3C2Tx is a 2D material and oxidized sections would therefore be only a few Ångström thick. ...
MXenes are technologically interesting 2D materials that show potential in numerous applications. The properties of the MXenes depend at large extent on the selection of elements that build the 2D MX-layer. Another key parameter for tuning the attractive material properties is the species that terminate the surfaces of the MX-layers. Although being an important parameter, experimental studies on the bonding between the MX-layers and the termination species are few and thus an interesting subject of investigation. Here we show that the termination species fluorine (F) bonds to the Ti 3 C 2 -surface mainly through Ti 3 p —F 2 p hybridization and that oxygen (O) bonds through Ti 3 p —O 2 p hybridization with a significant contribution of Ti 3 d and Ti 4 p . The study further shows that the Ti 3 C 2 -surface is not only terminated by F and O on the threefold hollow face-centered-cubic site. A significant amount of O sits on a bridge site bonded to two Ti surface atoms on the Ti 3 C 2 -surface. In addition, the results provide no support for hydroxide (OH) termination on the Ti 3 C 2 -surface. On the contrary, the comparison of the valence band intensity distribution obtained through ultraviolet- and x-ray photoelectron spectroscopy with computed spectra by density of states, weighed by matrix elements and sensitivity factors, reveals that OH cannot be considered as an inherent termination species in Ti 3 C 2 T x . The results from this study have implications for correct modeling of the structure of MXenes and the corresponding materials properties. Especially in applications where surface composition and charge are important, such as supercapacitors, Li-ion batteries, electrocatalysis, and fuel- and solar cells, where intercalation processes are essential.
... MXene is new emerging layered materials in 2D family, their molecular arrangement formula of M n+1 X n T x , where, M and X are transition metal, nitrogen or carbon respectively, n = 1, 2, 3, where T x corresponds to the hydrophilic surface functional groups including -OH, -O, and -F [12]. Few layers MXene were predicted to be metallic since electron density of monolayer MXene near to the Fermi level [13,14]. Because of outstanding electrical conductivity, abundant functional groups, unique dielectric properties, and high polarization anisotropy by the rich elemental composition of MXene and its related family groups could be used as an additive to fabricate good shielding materials [15]. ...
... Systematic experimental protocol planned to demonstrate stacked multilayers structure in MXene, enlargement in conductivity by introducing metallic silver nanoparticle and oxidative formation of semiconducting metal oxide particles as hybrid nanostructures synergistically expected to contribute scavenging action of EM waves. Fabrication of ternary MXene hybrid nanostructure exhibited advantageous properties for the attenuation of microwaves due to large surface to volume ratio at the nanoscale, unique delocalized d-electrons has higher electrical conductivity and hence can be used as unique composite nanostructures for high attenuation of EM radiation where a high conductivity is required [14,22]. ...
... However, the surface anchored functional sites specifically -OH groups extract charges from the metal-carbide (Ti-C and Nb-C) bonds of the conducting carbide layer. This could weaken the metalcarbide (C 2s -Ti 3d ; C 2s -Nb 4d ) hybridization process, and it will make a low valance state of transition metal species [14]. As well, according to the theoretical calculation, because of the lower work function (~2-3 eV) of -OH functional groups attached with MXene, the electronic Fermi level, which could lie below the redox energy of hydrogen ions [18]. ...
The development of smart, structured hybrid materials with superior electrical conductivity are vastly desired for the depletion of unwanted electromagnetic interference (EMI). Herein, novel multilayered two-dimensional MXene/Metal Oxides-Ag ternary hybrid nanostructures with various heterogeneous assemblies were constructed by self-reduction and oxidation of MXene (Nb2CTx and Ti3C2Tx) in the presence of metallic salt (AgNO3), and its EMI shielding performance in X- and Ku- band are demonstrated. The effect of different silver nitrate addition into MXene on their structural evaluation reveals that the successful formation of MXene/Metal Oxides-Ag (Nb2CTx/Nb2O5-Ag; Ti3C2Tx/TiO2-Ag) ternary hybrid nanostructures derived from MXene (Nb2CTx; Ti3C2Tx). Remarkably, the Nb2CTx/Nb2O5-Ag hybrid assembly attains reflection dominated shielding performance with a maximum total shielding effectiveness of 68.76 dB and 72.04 dB in the X- and Ku- band region respectively at a thickness of 1 mm compared to other nanostructures. The large enhancement in electromagnetic shielding capability of ternary nanostructure attributed to the strong electrical conductivity and increased interface polarization and multiple reflection loss between the ternary interfaces. The synergistic effect of metallic silver nanoparticles, semiconducting metal oxides and conducting stacked multilayer causes to conduction loss, polarization effect (dipole, and interfacial polarization), and multiple reflections of incoming radiation favor the complete dissipation of EM waves. Consequently, the demonstrated simple, cost-effective, and large scale production of highly conductive composites could pave for the potential prospect materials in advanced EM wave shielding applications.
... Recently, PES has been used to investigate the chemical composition and chemical bonding of Ti 3 C 2 T x 's surface terminations. [18][19][20] DFT calculations have revealed drastic difference of the electronic structures between Ti 3 C 2 T x and Ti 3 CNT x MXenes, despite their very similar structures. 16,21,22 For example, Ti 3 CNT x has the hybridization of additional Ti-N bonding that contributes to DOS near the fermi level (E F ). ...
... Ti 3p 20min 10min 5min 2min 0min 21 20 19 18 Binding energy (eV) NEC for a MXene, whereas NEC has been experimentally observed in other 2D materials, such as WSe 2 by in situ angle-resolved photoemission measurements. 28 The negative shift to a lower binding energy of Ti 3p core-level of $250 meV at 0.51e per unit cell observed in Ti 3 CNT x is larger than that observed for the Sr 3d and Ir 4f core-levels in Sr 3 Ir 2 O 7 , as it shows the negative shift to a lower binding energy of $100 meV upon electron doping. ...
Two-dimensional transition metal carbides, carbonitrides, and nitrides, called MXenes, exhibit high metallic conductivity, ion intercalation capability, and reversible redox activity, prompting their applications in energy storage and conversion, electromagnetic interference (EMI) shielding, and electronics, among many other fields. It has been shown that replacement of ∼50% of carbon atoms in the most popular MXene family member, titanium carbide (Ti3C2Tx), by nitrogen atoms, forming titanium carbonitride (Ti3CNTx), leads to drastically different properties. Such properties include very high negative charge in solution and extreme EMI shielding effectiveness, exceeding all known materials, even metals at comparable thicknesses. Here, by using ultraviolet photoemission spectroscopy (UPS), the electronic structures of Ti3CNTx and Ti3C2Tx are systematically investigated and compared as a function of charge carrier density. We observe that, in contrast to Ti3C2Tx, the Ti 3p core-level of Ti3CNTx exhibits a counterintuitive shift to a lower binding energy of up to ∼250 meV upon increasing the electron density, which is a spectroscopic signature of negative electronic compressibility (NEC). These experimentally measured chemical potential shifts are well captured by the density functional theory (DFT) calculation. The DFT results also further suggest that the hybridization of titanium–nitrogen bonding in Ti3CNTx helps to promote the available states of Ti atoms for receiving more electrons above the fermi level and leads to the observed NEC. Our findings explain the differences in electronic properties between the two very important and widely studied MXenes and also suggest a new strategy to apply the NEC effect of Ti3CNTx in energy and charge storage applications.
... 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.
... 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.
... The surface chemical structure of MXene was characterized by XPS in Figure 2b. The results showed that MXene is mainly composed of C, O, Ti, and F. In Figure 2c, Ti 2p is equipped with two pairs of dipole moments (Ti 2p 3/2 -Ti 2p 1/2 ), which is consistent with Martin's report [28]. The main peaks of Ti 2p 3/2 were concentrated at 455.1 and 456.2 eV, and the diffraction peak at 455.1 eV corresponds to Ti 2+ , indicating the existence of the Ti-C bond; meanwhile, the diffraction peak at 456.2 eV corresponds to the Ti-X binding site [29]. ...
Electrochromic materials and devices are attracting intense attention because of their low energy consumption and open-circuit memory effect. Considering the difficult processing characteristics of electrochromic conductive polymers, we developed a facile and scalable strategy to prepare solution processable polyaniline (PANI)-based nanocomposites by introducing two-dimensional titanium carbon nanosheets (MXene) through a self-assembly approach. The PANI/MXene nanocomposite can be fabricated into porous films via spray-coating process, which show an obvious synergetic effect of both materials, leading to superior electrochromic properties. The optical contrast of the optimized PANI/MXene film reached as high as 55% at =700 nm, and its response times were 1.3 s for coloration and 2.0 s for bleaching, respectively. In addition, the composite film also showed excellent cycle stability (after 500 cycles, the ΔT retention was above 87%). The improved electrochromic properties are owed to the high conductivity of MXene and the formation of the porous composite film structure, which promote the electronic/ionic transfer and migration efficiency. This research suggests that the self-assembly method and the conductive polymer/MXene nanocomposites have a potential application in the fields of electronic functional films and devices.
... The calculated bond length of the surface terminal groups of MXenes (such as -OH, -F, = O) is approximately 0.97 Å, Ti-F is 2.1 Å, and Ti-O is 1.9 Å (Harris et al. 2015;Hu et al. 2015). = O has the largest adsorption energy compared with -OH, -H, -F, and -Cl (Fredrickson et al. 2016;Fu et al. 2016;Caffrey 2018;Magnuson et al. 2018). These characteristics will affect the features of MXenes, which we will discuss in the next section. ...
Environmental pollution requires advanced methods and materials to clean drinking water. In particular, 2D transition metal carbides, named MXenes, display unique properties such as specific surface area, surface functional groups, conductivity, and antibacterial effects. Here, we review the applications of MXenes for water purification, and the factors controlling the efficiency of MXenes. We present the history of MXenes and focus on applications for adsorption, capacitive deionization, membranes, and catalysis. © 2021, The Author(s), under exclusive licence to Springer Nature Switzerland AG.
... In fact, manipulating the terminations has led to materials with large mechanical strength, high conductivity, intrinsic magnetism, strong absorption, and large reflectivity. 8,[16][17][18][19][20][21][22][23][24] The surface terminations we considered in the present study have been experimentally verified regarding their formation on the surface of Ti 3 C 2 . These are oxygen 25 (O), sulfur 26 (S), selenium 26 (Se)-fluorine 27 (F), chlorine 28 (Cl), and bromine 26 (Br), and we chose them in order to examine whether elements belonging in the same group of the periodic table exhibit similar behavior. ...
After obtaining Ti3C2 MXene structures terminated with O, S, Se, F, Cl, and Br, we calculate the energy barrier for Li-ion diffusion on the surface of each MXene, being the first to report on the Li-ion diffusivity in Cl and Br terminated Ti3C2. We find that the Ti3C2Cl2 MXene has the lowest diffusion barrier, substituting the Ti3C2S2 reported in the literature so far. In addition, a study on the adsorption energies indicates that the top binding position is the most stable adsorption position for the Li-ion. Furthermore, it is shown that the adsorption energy depends on the electronegativity of the termination atoms, as well as the distance between the terminations, the Li, and the surface Ti-atoms. Finally, we show that the bond valence sum method provides an indication of the transition state of the Li-ion and can serve as a comparison tool for the diffusion barriers of different structures.
... The bond length of Ti−F is reported to be 21 Å, with Ti−O as 1.9 Å and O−H as 0.97 Å. 67 The order of adsorption energy of terminations follows a decreasing trend of −O, −F, −OH, −Cl, and −H. 68 For instance, in a recent study, O-terminated Ti 3 C 2 has been reported to depict a surface with exposed O terminations as active sites for the electrochemical process. 69 In another study, Ti 3 C 2 T x was studied to generate −O, −OH, and −F termination with hydrofluoric acid and lithium fluoride− hydrochloric acid. ...
... 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. ...
... The Ti 3 C 2 T z spectra in Figures 3b and 6b show, on the other hand, no indication of TiO 2 or OH, which suggests that the Ti 3 C 2 T z sample, to some extent, is protected from TiO 2 formation by the termination species O and F, although probably only for a limited exposure time. 56 The Al 2 O 3 components in the Ti 3 AlC 2 spectra shown in Figures 5a and 6a are also because of the exposure to the atmosphere prior to the XPS investigation. In addition, the TiC sample shows some small Al 2 O 3 contribution, which is because of the sapphire substrate that was used when the Ti 3 AlC 2 and TiC samples were produced. ...
The inherently nanolaminated Ti3AlC2 is one of the most studied MAX-phase materials. MAX-phases consists of two-dimensional Mn+1Xn-layers (e.g., T3C2-layers) with strong internal covalent bonds separated by weakly interacting A-layers (e.g., Al-layers), where the repetitive stacking of the Mn+1Xn-layers and the A-layers suggests being the foundation for the unusual but attractive material properties of the MAX-phases. Although being an important parameter, the nature of the bonding between the Mn+1Xn-layers and the A-layers has not yet been established in detail. The X-ray photoelectron spectroscopy data presented in this paper suggest that the weak interaction between the Ti3C2-layers and the Al-layers in Ti3AlC2 is through electrostatic attraction facilitated by a charge redistribution of the delocalized electrons from the Ti3C2-layers to the Al-layers. This charge redistribution is of the same size and direction as between Ti atoms and Al atoms in TiAl alloy. This finding opens up a pathway to predict and improve MAX-phase materials properties through A-layer alloying, as well as to predict new and practically feasible MXene compounds.
... Additional inconsistencies can also originate from instability of the -F termination groups, which manifests in different surface compositions of aged samples compared to freshly prepared MXenes [206]. Furthermore, sample sputtering, which is often used to clean the surface and improve quality of the spectra, was demonstrated to modify the MXene elemental composition [206,243]. As a result, XPS determination of the MXene composition is susceptible to multiple factors and may not clearly represent the surface composition in the ambient environment. ...
The MXene field continues to grow and expand as more research groups begin to study this fascinating and very large class of 2D materials. While synthesis and applications of MXenes have been widely discussed in literature, characterization is often overlooked. Due to the large variety of MXene structures and compositions, it is often necessary to use multiple advanced characterization techniques within a single study, and each characterization technique has its own quirks, pitfalls, and benefits when applied to MXenes. This review focuses on the utilization of X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, electron microscopy/spectroscopy and a number of other techniques to understand if the precursor (MAX phase) is suitable for MXene synthesis, confirm successful synthesis of MXene, and finally determine its composition, structure and properties. Researchers should look to this review article as a guide to help them understand which techniques to use for characterization of their MXene samples and leverage the best characterization practices developed to date.
... 1−5 Affordable surface morphology, elastic layer thickness, tunable transition metal, and controllable surface functional groups either by etching or post-treatment of MXene demonstrate superior performance in various application fields. 6,7 Majority of 2D MXene layers are extracted from a three-dimensional (3D) MAX (M, transition metal; A, metal; and X, carbon or nitrogen) phase by chemical exfoliation. 2,6,8−11 As far as we understand from the detailed literature report, some critical parameters during chemical exfoliation using an etchant (HF, LiF + HCl, and NH 4 HF 2 ) to remove a metal Al layer cannot be controlled even the reaction is performed under predefined conditions due to different wet solution chemistries. ...
Defect-controlled exfoliation of few-layer transition-metal carbide (f-Ti3C2T
x
) MXene was demonstrated by optimizing chemical etching conditions, and electromagnetic interference (EMI) shielding coatings were explored. The structural features such as layer morphology, lateral size, layer thickness, defect density, and mechanical stability of the exfoliated f-Ti3C2T
x
were strongly dependent on exfoliation conditions. By selecting appropriate exfoliation conditions, moderate etching time leads to the formation of quality f-Ti3C2T
x
with lesser defects, whereas longer etching time can break the layer structure and increase defect density, structural misalignment, and oxidative products of f-Ti3C2T
x
. The resultant fabricated free-standing flexible f-Ti3C2T
x
films exhibited electrical conductivity and electromagnetic interference (EMI) shielding effectiveness (SE) in the X-band of about 3669 ± 33 S/m and 31.97 dB, respectively, at a thickness of 6 μm. The large discrepancy in EMI SE performance between quality (31.97 dB) and defected (3.164 dB) f-Ti3C2T
x
sheets is attributed to interconnections between f-Ti3C2T
x
nanolaminates interrupted by defects and oxidative products, influencing EMI attenuation ability. Furthermore, the demonstrated solution-processable high-quality f-Ti3C2T
x
inks are compatible and, when applied for EM barrier coating on various substrates, including paper, cellulose fabric, and PTFE membranes, exhibited significant EMI shielding performance. Moreover, controlling defects in f-Ti3C2T
x
and assembly of heterogeneous disordered carbon-loaded TiO2-Ti3C2T
x
ternary hybrid nanostructures from f-Ti3C2T
x
by tuning etching conditions could play an enormous role in energy and environmental applications.
... The obtained Ti3C2Tx foil is in detail described in Ref. [7]. Moreover, to observe XANES and EXAFS features originating from termination species, the surfaces must be free from oxidized material, mainly TiO2 [31]. The new-made Ti3C2Tx foils were therefore stored in argon (Ar) atmosphere and mounted on the sample holder in a glove-bag filled with nitrogen gas (N2). ...
The chemical bonding within the transition-metal carbide materials MAX phase Ti3AlC2 and MXene Ti3C2Tx is investigated by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layers of Mn+1Xn and monolayers of an A-element from the IIIA or IVA group in the periodic table, where M is a transition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination species Tx will separate the Mn+1Xn-layers forming two-dimensional (2D) flakes of Mn+1XnTx. For Ti3C2Tx the Tx corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D Mn+1Xn-flake. The Ti K-edge (1s) XANES of both Ti3AlC2 and Ti3C2Tx exhibit characteristic pre-edge absorption regions of C 2p - Ti 3d hybridization with clear crystal-field splitting's while the main-edge absorption features originate from the Ti 1s -> 4p excitation, where only the latter shows sensitivity towards the fcc-site occupation of the termination species. The coordination numbers obtained from EXAFS show that Ti3AlC2 and Ti3C2Tx are highly anisotropic with a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayers and termination species, respectively. As shown in the temperature-dependent measurements, the O contribution shifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750 {\deg}C.
... Moreover, to observe XANES and EXAFS features originating from termination species, the surfaces must be free from oxidized material, mainly TiO 2 [31]. The newly made Ti 3 C 2 T x foils were therefore stored in an argon (Ar) atmosphere and mounted on the sample holder in a glove-bag filled with nitrogen gas (N 2 ). ...
The chemical bonding within the transition-metal carbide materials MAX phase Ti 3 AlC 2 and MXene Ti 3 C 2 T x is investigated by x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine-structure (EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layers of M n+1 X n and monolayers of an A-element from the IIIA or IVA group in the Periodic Table, where M is a transition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination species T x will separate the M n+1 X n-layers forming two-dimensional (2D) flakes of M n+1 X n T x. For Ti 3 C 2 T x the T x corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D M n+1 X n-flake. The Ti K-edge (1s) XANES of both Ti 3 AlC 2 and Ti 3 C 2 T x exhibit characteristic preedge absorption regions of C 2p-Ti 3d hybridization with clear crystal-field splitting while the main-edge absorption features originate from the Ti 1s → 4p excitation, where only the latter shows sensitivity toward the fcc-site occupation of the termination species. The coordination numbers obtained from EXAFS show that Ti 3 AlC 2 and Ti 3 C 2 T x are highly anisotropic with a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayers and termination species, respectively. As shown in the temperature-dependent measurements, the O contribution shifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750°C.
... Here T x represents the terminal groups (also referred to as functional groups) inherently coming from exfoliation process commonly as -O, -F, and/or -OH and usually exhibits a mixture of terminations [27,28]. Further experimental studies by electron energy-loss spectroscopy in transmission electron microscopy [29,30], x-ray photoelectron spectroscopy [31], neutron scattering [32] and NMR [40] spectroscopy [33,34] also confirm there is a random distribution of terminations on MXene surfaces, rather than regions terminated by a certain kind of terminal group. Table 1 introduces electrochemical battery performances of MXene in Li-ion batteries obtained from experimental studies [18,19,27,[35][36][37][38][39][40][41][42][43][44]. ...
Two-dimensional materials have been attracting increasing interests because of their outstanding properties for Lithium-ion battery applications. In particular, a material family called MXenes (M n+1 C n , where n = 1, 2, 3) have been recently attracted immense interest in this respect due to their incomparable fast-charging properties and high capacity promises. In this article, we review the state-of-the-art computational progress on Li-ion battery applications of MXene materials in accordance with our systematical DFT calculations. Structural, mechanical, dynamical, and electrical properties of 20 distinct MXene (M: Sc, Ti, V, Cr, Nb, Mo, Hf, Ta, W, and Zr) have been discussed. The battery performances of these MXene monolayers are further investigated by Li-ion binding energies, open circuit voltage values, and Li migration energy barriers. The experimental and theoretical progress up to date demonstrates particularly the potential of non-terminated or pristine MXene materials in Li ion-storage applications. Stability analyses show most of the pristine MXenes should be achievable, however susceptible to the development progress on the experimental growth procedures. Among pristine MXenes, Ti 2 C, V 2 C, Sc 2 C, and Zr 2 C compounds excel with their high charge/discharge rate prospect due to their extremely low Li diffusion energy barriers. Considering also their higher predicted gravimetric capacities, Sc, Ti, V, and Zr containing MXenes are more promising for their utilization in energy storage applications.
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.
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.
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). An effective anhydrous synthetic pathway for Ti3C2Tx is developed. The etching reaction is completed within 4 h and monolayer Ti3C2Tx is produced in yields greater than 70%. Ti3C2Tx possesses an atypical surface structure, and the freestanding films prepared using these materials exhibit the highest ultimate tensile strength (>167 MPa) among the reported pristine MXene films, without losing their electrical conductivity.
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....
MXenes, a new family of two-dimensional (2D) transition metal carbides and nitrides, have received great research attention in diverse applications, including biomedicine, sensing, catalysis, energy storage and conversion, adsorption, and membrane-based separation, owing to their extraordinary physicochemical properties and various chemical compositions. In recent years, many surface functionalization strategies, such as surface-initiated polymerization and single heteroatom doping, have been cleverly developed to expand the potential of MXenes in different fields. This article reviews and puts into perspective the recent advances in MXene surface functionalization and various applications of the modified MXenes. The effect of surface functionalization on MXene properties (such as electronic, magnetic, mechanical, optical, and hydrophilicity/hydrophobicity) is discussed. Potential applications of pristine and functionalized MXenes in different fields are discussed. Finally, challenges and future directions in MXene surface modification are examined.
The two-dimensional titanium carbide MXene (Ti3C2Tx) acts as a promising pseudocapacitive material for supercapacitor electrodes. In this paper, the properties of vanadium-doped titanium carbide MXene (Ti3C2Tx) are tuned using a simple hydrothermal method to intercalate the alkali metal adsorbates (K+) into the electrode material. The synthesis of the supercapacitor device is carried on glass substrate as well as on a flexible graphite sheet. The X-ray diffraction and scanning electron microscopy are conducted to observe the change in structural properties of vanadium-doped MXene. The cyclic voltammetry and galvanostatic charge–discharge are carried out on Metrohm autolab workstation. The ratio of ammonium vanadate and MXene has been varied from 0.025:0.1 to 0.1:0.1 with a step size of 0.025 to obtain the capacitance results. The results depict that the ratio of 0.025:1 shows the highest capacitance of 258.07 mF/cm2 and 1107 mF/cm2 in 6 M KOH (20 mV/s) on glass and graphite substrate, respectively. This is mainly because the ratio of 0.025:1 provides the maximum exfoliation which allows electrolyte ions to penetrate in the active material and thus, facilitates fast electron transport resulting in high-performance supercapacitors. Further, this paper also discusses the successful fabrication of the supercapacitor devices on a flexible graphite sheet for the first time. The results show that the capacitance value on flexible substrate is at par with that of the glass substrate. To further understand the increased capacitive properties of vanadium-doped MXene, the processes involving charge transfer and mass transport are investigated by performing electrochemical impedance spectroscopy (EIS). The radius on the EIS plot of vanadium-doped MXene is smaller than that of the undoped DMSO MXene, which indicates that the vanadium doping made the charge transfer easier. Moreover, the capacitance retention of 92.7% and 82.2% is achieved on graphite as well as glass substrate after 3000 cycles.
Since 2011, MXenes, the family of two‐dimensional transition metal carbides, nitrides and carbonitrides, have been investigated as the electrodes, additives, separators and hosts for energy storage devices (ESDs, including supercapacitors and metal‐ion batteries) due to their unique properties. This report focuses on the present technical issues in the field of energy storage and the corresponding solutions involving MXenes. It begins with a series of synthesis approaches (including top‐down and bottom‐up strategies) with a brief description of the structural properties, covering crystal lattice, structural defects, and surface chemistries. In addition, the impact of surface functional groups, composition of transition metals temperature and external pressure on the electrical properties of MXenes is discussed. Then, current issues of ESDs are listed with several MXene‐based solutions, including intercalation, modification of the terminating groups, chemical doping, vacancy engineering and the design of nanocomposites. Finally, the challenges in MXene‐based ESDs are summarised with some potential research directions in the future.
MXenes are realized as an innovative family of two-dimensional (2D) structured materials resembling the structure like graphene and molybdenum disulfide. The extensive research has been explored in this novel family of MXene materials from the discovery of Ti3C2 in 2011. Around 20 variants of MXenes have been synthesized, and the structural properties of more than dozens of MXene materials have been theoretically predicted. Unlike other 2D ceramics, MXenes have excellent electrical conductivity and exceptional efficiency since they are molecular sheets composed of carbide and transition nitride metals such as titanium. MXenes are formed by etching a coating from MAX phases and adding the suffix “ene” to highlight their resemblance to graphene. MAX phases are a family of hexagonal faceted ternary transition metal carbides, carbonitrides, and nitrides composed of Mn+1AXn, where M represents for transition metals (such as Cr, Nb, Ti, V), A indicates group of A elements (such as In, Al, Si, Sn), X means carbon and/or nitrogen, and n=1, 2, or 3. MXenes have already established numerous applications such as energy storage, modular electronics, optoelectronics, medicine, and nano-biosensors. In this review article, the synthesis technique, configuration, and electronic properties of MXenes are emphasized and extensively discussed. MXenes and MXene-based nanocomposites for electrical energy storage applications are mainly highlighted and outlined. Finally, MXene-based hybrid supercapacitors as next-generation energy storage devices are summarized and briefly discussed.
The relationship between core level binding energy shifts (CLBEs), that can be experimentally determined by X-Ray Photoelectron Spectroscopy, and chemical bonding is analyzed for a series of MXenes, a new family of two-dimensional materials with a broad number of applications in nanotechnology. Based on first-principles calculations, the atomic and electronic structure of clean and O-terminated carbide MXene with M2C and M2CO2 (M= Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) stoichiometries are investigated with a focus on trends in the C(1s) and O(1s) CLBEs along the series. A rather good linear correlation between the available experimental and calculated C(1s) and O(1s) CLBEs exists that validates the conclusions from the present computational approach. The present study shows that CLBEs of clean MXenes are governed by the initial state effects and directly correlate with the net charge on the C atoms. However, for the case of O-terminated MXenes, C(1s) and O(1s) CLBEs exhibit a much less significant correlation with the net charge of either C or O atoms which is attributed to the structural changes induced on the M2C moiety by the presence of the O layers and the different stacking sequence observed depending on the MXene composition. The present study shows how and when XPS can be used to extract information regarding the nature of the chemical bond in clean or functionalized MXenes.
MXenes are a young family of two-dimensional transition metal carbides, nitrides, and carbonitrides with highly controllable structure, composition, and surface chemistry to adjust for target applications. Here, we demonstrate the modifications of two-dimensional MXenes by low-energy ion implantation, leading to the incorporation of Mn ions in Ti3C2T
x
(where T
x
is a surface termination) thin films. Damage and structural defects caused by the implantation process are characterized at different depths by XPS on Ti 2p core-level spectra, by ToF-SIMS, and with electron energy loss spectroscopy analyses. Results show that the ion-induced alteration of the damage tolerant Ti3C2T
x
layer is due to defect formation at both Ti and C sites, thereby promoting the functionalization of these sites with oxygen groups. This work contributes to the inspiring approach of tailoring 2D MXene structure and properties through doping and defect formation by low-energy ion implantation to expand their practical applications.
Since 2011, after the discovery of new ceramic two-dimensional materials called MXenes, the attention has been focused on their unique properties and various applications, from energy storage to nanomedicine. We present a brief perspective article of the properties of MXenes, alongside the most recent studies regarding their applications on energy, environment, wireless communications, and biotechnology. Future needs regarding the current knowledge about MXenes are also discussed in order to fully understand their nature and overcome the challenges that have restricted their use.
Mn, Ti substituted barium ferrite (BaFe12−x(Mn0.5Ti0.5)xO19) was fabricated by solid state reaction, which shows high efficiency and wide band microwave absorption. The minimum reflection loss (RL) reaches up to −31 dB and the bandwidth below −10 dB almost covers the whole X band. The enhancement of microwave absorption is attributed to the improvement of dielectric loss and magnetic loss. The imaginary permeability possesses a perfect resonance peak due to the response frequency shifting towards lower frequency by Mn, Ti substitution. The imaginary permittivity reaches up to 0.5. The results show a promising feasible route to tune the electromagnetic properties and develop high efficient absorbers.
Advanced electrodes with excellent rate performance and cycling stability are in demand for the fast development of sodium storage. Two-dimensional (2D) materials have emerged as one of the most investigated subcategories of sodium storage related anodes due to their superior electron transfer capability, mechanical flexibility, and large specific surface areas. Recently, 2D metal carbides and nitrides (MXenes), one type of the new 2D materials, are known to have competitive advantages in terms of high electroconductivity, terminal functional groups, large specific surface areas, tunable interlayer spacing, and remarkable safety. These advances endow MXenes and MXene-based materials with superior electrochemical performance when they are used as electrodes for sodium-ion storage. MXenes, however, share similar defects with other 2D materials, such as serious restacking and aggregation, which need to be improved in consideration of their further applications. In this review, we present the big family of MXenes and their synthetic methods. Furthermore, recent research reports related to progress on MXene-based materials for sodium storage are compiled, including materials design and reaction mechanisms in sodium-ion batteries and sodium metal batteries. Significantly, we discuss the challenges for existing MXene-based structures with respect to their future use as electrodes, such as low capacitance, aggregation, untenable termination groups, and unclear mechanisms, thereby providing guidance for future research on MXene-based materials for sodium-ion storage. MXenes and MXene-based materials for sodium storages, including sodium-ion batteries, and high-energy sodium metal batteries, are summarized in this review. Structural advances and optimization strategies toward future applications for various Na-ion storages are systematically discussed.
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 ($n$ $\geqslant$ 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.
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