Ti2C MXenes Research Articles

Exploring the structural and electronic properties of N-doped Ti2C MXenes for novel applications in advanced materials and devices: A DFT study

MXenes, a category of two-dimensional transition metal carbides and nitrides, have attracted significant interest owing to their distinctive characteristics. This study utilizes Density Functional Theory (DFT) to examine the effects of nitrogen doping on the structural and electrical properties of Ti2C MXenes. N-doping induces significant alterations in the lattice structure and electronic properties, culminating in an elevated density of states at the Fermi level, indicating improved conductivity. Furthermore, optical characteristics such as reflectivity and loss function are affected by N-doping, resulting in a shift in the intensity of the reflectivity peak. The alterations provide N-doped Ti2C MXenes viable candidates for advanced applications in energy storage, catalysis, and electronic devices, facilitating future empirical and theoretical investigations in MXene-based materials.

DFT based investigations on inherent CO2 adsorption and direct dissociation capability of Ti2C and Nb2C mxenes

Transition metal carbide MXenes, specifically Ti2C and Nb2C, have been investigated for their ability to efficiently adsorb and dissociate CO2 using First Principles Calculations based on Density Functional Theory (FPS-DFT). Pure Ti2C and Nb2C sheets were studied with the adsorption of 1 to 4 CO2 molecules, and the results were analyzed for each case. For 4 CO2 molecules adsorbed on Ti2C and Nb2C sheets, the weight percentage (wt%) ratio was found to be 21.39% and 14.33%, respectively. The Density of States (DOS) analysis revealed that, upon CO2 adsorption, the CO2 molecule dissociates into CO and O atoms on the MXene surface. The CO molecule showed no significant hybridization with the host sheet atoms, whereas the O atom exhibited strong hybridization with the MXene surface. Transition State (TS) profiles illustrated the dissociation steps on both Ti2C and Nb2C surfaces. Phonon calculations confirmed the dynamic stability of both pure and CO2 adsorbed MXenes. Molecular Dynamics (MD) simulations conducted at 900 K indicated uniform temperature and Mean Square displacement (MSD) profiles, suggesting the stability of both pure and CO2 adsorbed MXenes at elevated temperatures, as well as uniform CO2 adsorption behavior. The adsorption energies for Ti2C and Nb2C sheets were calculated to be in the range of -1.89 to -2.77eV, suggesting that the adsorption process is favorable, spontaneous, and predominantly involves chemisorption. These findings indicate that Ti2C and Nb2C MXenes, in their intrinsic forms, are promising candidates for CO2 adsorption and dissociation technologies.

Removal of erythrosine B dye from wastewater using Ca2C and Ti2C MXenes: A theoretical study

In this work, the Ca2C and Ti2C MXenes have been investigated for the adsorption of erythrosine B (EB) dye from wastewater using density functional theory (DFT) calculations. The thermodynamic studies were carried out to examine the feasibility of EB adsorption on the Ca2C and Ti2C MXenes. Our results revealed that the EB adsorption is endothermic and spontaneous in nature with the negative Gibbs free energy values. Moreover, the obtained binding energies for the EB/Ca2C and EB/Ti2C systems are −2.24 eV and −2.96 eV, respectively. Adsorption procedure and binding energies obtained for the EB/Ca2C and EB/Ti2C systems revealed that our findings support the hard and soft acid-base principle. We found that both Ca2C and Ti2C MXenes are effective adsorbents for the removal of the EB from wastewater. Our findings might be helpful for future studies toward the application of carbide MXene materials for removing wastewater pollutants.

The underlying mechanism of the strain-induced reversible magnetic state transition in MXene nanosheets

MXenes have emerged as one of the most promising members among the two-dimensional (2D) material family owing to their unique physical-chemical properties and outstanding application prospects. Here, we investigated the unique magnetic properties of Ti2C MXenes monolayer by means of the first-principles calculations combining with the Heisenberg spin Hamiltonian approach. A strain-induced reversible magnetic transition for the Ti2C monolayer could be found in our study. When the strain is applied to the Ti2C monolayer, the magnetic configuration of the Ti2C monolayer transforms from the A-type antiferromagnetic (A-AFM) phase to the ferromagnetic (FM) phase with the biaxial tensile strain larger than 6%. Furthermore, the transition from the A-AFM phase to the FM phase was found to be closely related with the change of the Ti and C atomic magnetic moments. The direct magnetic coupling between Ti and C atoms with the increased Ti and C magnetic moments is able to stabilize the structure due to the weakened superexchange interaction between Ti and C atoms under large tensile strain. This is the underlying driving force for the transition from the A-AFM phase to the FM phase. This work provides a promising pathway to better understand the existence of magnetic reversal behavior in the Ti2C MXenes monolayer and opens the door to future magnetic applications of MXenes.

Prediction of induced magnetism in 2D Ti2C based MXenes by manipulating the mixed surface functionalization and metal substitution computed by xTB model Hamiltonian of the DFTB method.

We employed the recently developed density functional tight binding (DFTB) method's Hamiltonian, GFN1-xTB, for modeling the mixed termination in Ti2C MXenes, namely three types of termination by combining -O and -OH, -O and -F, and -F and -OH. We demonstrated that the approach yields reliable predictions for the electronic and magnetic properties of such MXenes. The first highlighted result is that the mixed surface functionalization in Ti2CAxBy MXenes induces spin polarization with diverse magnetic alignments, including ferromagnetism and two types of antiferromagnetism. We further identified the magnetic alignment for the investigated MXene in terms of the compositions of the terminal groups. Moreover, the effect of the transition metal (Ti) substituted by the Sc atom on the electronic and magnetic properties was also investigated. We found that the studied systems maintain the magnetism and the metallic characteristics. A magnetic transition from antiferromagnetic (AFM) to ferrimagnetic (FiM) ordering was found for ScTi15C8F8(OH)8 and ScTi15C8F12(OH)4 compounds. Finally, we proved that incorporating the Sc atom into the lattice of Ti2CO2 and the mixed surface termination in Ti2CAxBy is an effective strategy to induce magnetism. Our study may provide a new potential application for designing MXene-based spintronics.

Halogenated and chalcogenated Ti2C MXenes: High capacity electrode materials for lithium-ion batteries

Inspired by the recent successful growth of Ti2C monolayers, we investigated the effects of Ti2CT2 (T = F, Cl, Br, O, S and Se) anode in lithium-ion batteries (LIBs) by first-principles calculations. The results revealed that Ti2CT2 MXenes exhibit metallic conductivity with higher structural stability and mechanical strength than Ti2C. Compared with other Ti2CT2 monolayers, Ti2CS2 monolayer display lower diffusion barrier, larger elastic modulus, lower open circuit voltage and higher storage capacity. Moreover, surface sulfuration yields the expanded interlayer spacing, which can improve Li-ion accessibility in double-layer Ti2CS2.

How does thickness affect magnetic coupling in Ti-based MXenes.

The magnetic nature of Ti2C, Ti3C2, and Ti4C3 MXenes is determined from periodic calculations within density functional theory and using the generalized gradient approximation based PBE functional, the PBE0 and HSE06 hybrids, and the on-site Hubbard corrected PBE+U one, in all cases using a very tight numerical setup. The results show that all functionals consistently predict a magnetic ground state for all MXenes, with spin densities mainly located at the Ti surface atoms. The analysis of solutions corresponding to different spin orderings consistently show that all functionals predict an antiferromagnetic conducting ground state with the two ferromagnetic outer (surface) Ti layers being antiferromagnetically coupled. A physically meaningful spin model is proposed, consistent with the analysis of the chemical bond, with closed shell, diamagnetic, Ti2+ like ions in inner layers and surface paramagnetic Ti+ like centers with one unpaired electron per magnetic center. From a Heisenberg spin model, the relevant isotropic magnetic coupling constants are extracted from an appropriate mapping of total energy differences per formula unit to the expected energy values of the spin Hamiltonian. While the numerical values of the magnetic coupling constants largely depend on the used functional, the nearest neighbor intralayer coupling is found to be always ferromagnetic, and constitutes the dominant interaction, although two other non-negligible interlayer antiferromagnetic terms are involved, implying that the spin description cannot be reduced to NN interaction only. The influence of the MXene thickness is noticeable for the dominant ferromagnetic interaction, increasing its value with the MXene width. However, the interlayer interactions are essentially due to the covalency effects observed in all metallic solutions which, as expected, decay with distance. Within the PBE+U approach, a U value of 5 eV is found to closely simulate the results from hybrid functionals for Ti2C and less accurately for Ti3C2 and Ti4C3.

MXene-based Ti2C/Ta2C lateral heterostructure: an intrinsic room temperature ferromagnetic material with large magnetic anisotropy.

Two-dimensional (2D) lateral heterostructures (LH) combining Ti2C and Ta2C MXenes were investigated by means of first-principles calculations. Our structural and elastic properties calculations show that the lateral Ti2C/Ta2C heterostructure results in a 2D material that is stronger than the original isolated MXenes and other 2D monolayers such as germanene or MoS2. The analysis of the evolution of the charge distribution with the size of the LH shows that, for small systems, it tends to distribute homogeneously between the two monolayers, whereas for larger systems electrons tend to accumulate in a region of ∼6 Å around the interface. The work function of the heterostructure, one crucial parameter in the design of electronic nanodevices, is found to be lower than that of some conventional 2D LH. Remarkably, all the heterostructures studied show a very high Curie temperature (between 696 K and 1082 K), high magnetic moments and high magnetic anisotropy energies. These features make (Ti2C)/(Ta2C) lateral heterostructures very suitable candidates for spintronic, photocatalysis, and data storage applications based upon 2D magnetic materials.

Theoretical identification of the superior anchoring effect and electrochemical performance of Ti2CS2 by single atom Zn doping for lithium-sulfur batteries.

As one of the promising next-generation energy storage systems, lithium-sulfur (Li-S) batteries have been the subject of much recent attention. However, the polysulfide shuttle effect remains problematic owing to the dissolution of intermediate polysulfide species in the electrolyte and the sluggish reaction dynamics in Li-S batteries. To overcome these issues, this work reports an effective strategy for enhancing the electrochemical performance of Li-S batteries using single atom Zn doping on the S-terminated Ti2C MXenes (Ti2-xZnxCS2). Spin-polarized density functional theory (DFT) calculations were performed to elucidate the interactions of lithium polysulfides (LiPSs) and the Ti2-xZnxCS2 surface in terms of geometric and electronic properties, as well as the delithiation process of Li2S on the Ti2-xZnxCS2 surface. It is found that doping single atom Zn could induce a new Lewis acid-based sites, which could provide proper affinity toward LiPSs. Combined with the metallic character, a low Li diffusion barrier and high catalytic activity for the delithiation process of Li2S, makes Ti2-xZnxCS2 a promising cathode material for Li-S batteries. The results demonstrate the importance of surface chemistry and the electronic structure of MXenes in LiPSs' adsorption and catalysis capability. We believe that our findings provide insights into the recent experimental results and guidance for the preparation and practical application of MXenes in Li-S batteries.

Design of single-atom catalysts on S-functionalized Mxenes for enhanced activity and selectivity in N2 electroreduction

Design of high-performance catalysts is one of the key research focuses in electrochemical nitrogen reduction reaction (NRR). Single atom catalysts (SACs) hosted by a suitable template can be a potential candidate for NRR. Recently, two-dimensional (2D) S-functionalized Ti2C (Ti2CS2) and Nb2C (Nb2CS2) MXenes have been successfully synthesized. Here, we screen 18 different single metal atoms on these S-functionalized MXenes by examining their NRR activity and selectivity using the state-of-the-art density functional theory calculations. The calculated free energy change between the first H+/e− pair reduction step and the last H+/e− pair reduction step suggests that Mo@MXene, Nb@MXene and V@MXene are the suitable candidates. The calculation results further reveal that V and Nb on Ti2CS2 and Nb2CS2 weaken the *N adsorption, leading to an enhanced performance in NRR. Our analyses on the electronic properties, structural stability as well as HER indicate that Mo@Nb2CS2 system overall is a highly promising NRR catalyst. The present work provides an interesting route to design SACs based on the S-functionalized MXenes.

Computational Pourbaix Diagrams for MXenes: A Key Ingredient toward Proper Theoretical Electrocatalytic Studies

AbstractMXenes, a rather new family of 2D carbides and nitrides, have shown to be promising materials in many technological applications, particularly in electrocatalysis. The as‐synthesized MXenes exhibit a variety of surface terminations involving mixtures of O, OH, H, or F surface groups. These terminations play a crucial role in the electrocatalytic performance of these materials as these may change depending on the reaction conditions. The Pourbaix diagrams have long being used to provide the thermodynamically stable surface under certain conditions of pH and potential, U. However, experimental determination of Pourbaix diagrams may be quite challenging while first‐principles studies, considering the most likely terminations, allow deriving reliable insights. Here, Pourbaix diagrams for a series of representative MXenes are provided; the Ti2C, Ti3C2, V2C, and Mo2C MXenes, with the novelty of considering single and several double mixed terminations. The possible implications of the obtained results are discussed, especially for a proper choice of models in theoretical electrocatalysis studies, including the water splitting related hydrogen evolution reaction (HER), or the oxygen reduction reaction (ORR), but also serving as a guide to any further computational studies and also to electrocatalytic experiments.

Study of Quantum Capacitance of Pure and Functionalized Nb2c and Ti2c Mxenes for Supercapacitor Applications

Supercapacitors in combination with renewable energy sources can provide environment friendly and sustainable energy devices. But due to their lower energy density, they are not able to replace batteries for all commercial applications. Their energy density can be increased by using high quantum capacitance electrodes. MXenes have potential to be used for such electrodes. In present work, density of states, band structure and quantum capacitance (QC) of bare and functionalized niobium carbide and titanium carbide MXenes are studied using DFT simulations. The calculated quantum capacitances of Nb2C and Ti2C show significant values of 324.1 and 246.2 µF/cm2 respectively. Nb2C shows higher value for QC in comparison to Ti2C for both positive as well as negative electrodes. These calculations are also performed for functionalized MXenes using =O, -F and –OH as functional group. Functionalization of MXenes with these groups reduces the QC of Nb2C and Ti2C MXenes at Fermi level.

Electronic and magnetic properties in nitrogen-doped 2D titanium carbides (MXenes): Insight from first-principles calculations

First-principles computations were performed to investigate the electronic and magnetic properties of pristine and nitrogen (N)-doped Ti2C and Ti3C2 MXenes. The effects of the concentrations and the distributions of nitrogen were considered. Our results show that N doping can enhance the conductivity of Ti2C and Ti3C2 MXenes, which both present metallic character under different N concentration. The metallicity of N-doped Ti2C and Ti3C2 MXenes indicates potential in metal-ion batteries. Also, N doping leads to an antiferromagnetic (AFM)-ferromagnetic (FM) transition for Ti3C2. The critical concentration from AFM to FM configurations is between 25% and 50%. Importantly, the magnetic state of the N-doped Ti3C2 monolayers is related with the N distribution. However, Ti2C systems maintain an AFM state regardless of N concentrations or N distributions. Thus, based on the basic understanding of the N-doped Ti2C and Ti3C2 monolayer, we believe that our findings would provide theoretical support for the multifaceted application of the C-based MXenes.

In-situ SEM compression of accordion-like multilayer MXenes

A relatively new and large class of materials, two-dimensional (2D) early transition metal carbides (MXenes), produced by etching of A elements from their parent MAX phases, have demonstrated outstanding performance in many applications. However, the mechanical behavior of MXenes, which is critical for device fabrication and performance, has not been sufficiently investigated. In this work we report the results from in-situ scanning electron microscopy (SEM) uniaxial compression experiments conducted on two types of multilayer accordion-like MXene particles, Ti2CTx and Ti3C2Tx, along both in-plane and out-of-plane directions. Significant anisotropic behavior was observed associated with progressive failure mechanisms. Molecular dynamics (MD) modeling was conducted to analyze the behavior of intact and defected monolayer Ti2C and Ti3C2 MXenes in tension and compression. Structural analysis for both in- and out-of-plane uniaxial compression is presented and compared with experimental data, revealing that the overall mechanism is controlled by the intrinsic elastic and plastic properties of MXenes, as well as their multilayer structures. The intrinsic material properties were estimated based on structural analysis from experimental results.

Understanding Chemistry of Two-Dimensional Transition Metal Carbides and Carbonitrides (MXenes) with Gas Analysis.

MXenes, a large family of two-dimensional materials that are intensely investigated for a broad range of applications, are unstable in water, spontaneously forming TiO2. Several hypotheses have been proposed recently to explain the transformations of MXenes in aqueous environments based on characterization of solid products and measurements of solution pH. However, no studies of the gaseous products of these reactions have been reported. In this work, we demonstrate the use of Raman spectroscopy and gas chromatography techniques to study the gaseous reaction products of Ti2C, Ti3C2, Ti3CN, and Nb2C MXenes in aqueous environments. Based on the analysis of gases, the reactivities of MXenes with different monolayer thickness and chemical composition have been analyzed. We demonstrate the analysis of gases produced during MXene transformations as a powerful technique that can be used for better understanding of their nontrivial chemistry.

A simple, low-cost and green method for controlling the cytotoxicity of MXenes.

MXene phases are a member of the intriguing 2D material family, beyond graphene. They are good candidates for many applications, however, their potential toxicity is of crucial importance for future development. Herein, we present a simple, low-cost and fully green approach for controlling the potential cytotoxicity of 2D MXenes after delamination by harnessing the interactions that occur between the surface of MXene phases and natural biomacromolecule - collagen. We also demonstrate that the step-by-step adsorption and desorption of collagen from the surface of 2D MXenes is easily controlled using in situ zeta potential measurements coupled with dynamic light scattering (DLS) method. The obtained results demonstrated that the electrostatically driven unprecedented susceptibility of the MXenes' surfaces to collagen. Surface-modification reduces toxicity of MXenes in vitro i.e. adjust cells' viabilities as well as reduce their oxidative stress. This indicates enhanced biocompatibility of 2D Ti3C2 and Ti2C MXenes surface-modified with collagen, which is involved in many bio-interactions as important building blocks in the human body. The presented study opens new avenues for designing MXenes with defined surface properties and paves the way for their future successful management in nano-medicinal applications.

Theoretical Prediction of Catalytic Activity of Ti2C MXene as Cathode for Li–O2 Batteries

First-principles calculations were performed to investigate the catalytic activity of Ti2C MXene material as a potential cathode in Li–O2 batteries. Stable non-, O-, F-, OH-terminated Ti2C MXenes are constructed to show the real state of Ti2C MXene monolayer in the experiment. Then, the interfacial models of LixO2 (x = 4, 2, and 1) molecules adsorbed on Ti2C, Ti2CO2, Ti2CF2, and Ti2C(OH)2 MXenes were used to simulate the structural evolution during discharging (oxygen reduction reaction, ORR) and charging (oxygen evolution reaction, OER) processes. The catalytic activity was quantitatively assessed by calculating the ORR and OER overpotentials. Among them, Ti2CO2 MXene displays the best catalytic activity with the lowest ORR/OER/TOT overpotential, suggesting that O-terminated Ti2C MXene is more favorable for Li–O2 batteries. This is because the Ti 3d orbital of the Ti2CO2 surface is completely polarized near the Fermi level, showing strong oxidation capability toward O22–. These findings give a theoretical guidance for Ti2C MXene used in Li–O2 batteries and widen the applications of MXene-based materials.

Exfoliation of Ti2C and Ti3C2 Mxenes from bulk trigonal phases of titanium carbide: A theoretical prediction

MXenes, a novel class of two dimensional materials with several interesting properties, are prepared from their MAX phases by etching out the A element using strong chemical reagents. This results in passivation of the surfaces of MXene with different functional groups like O, –OH, –F, etc., which in many cases tend to degrade their properties. In this work, using first principle density functional theory based calculations, we propose a novel method that might open up possibility of synthesizing pristine Ti2C and Ti3C2 from the bulk titanium carbides with corresponding stoichiometry. Based on the values of cleavage energy obtained from our calculations, we envisage that pristine Ti2C and Ti3C2 MXenes can be prepared, using mechanical or sonication-assisted liquid-phase exfoliation techniques, from their bulk phases.

The potential application of 2D Ti2CT2 (T = C, O and S) monolayer MXenes as anodes for Na-ion batteries: A theoretical study

First-principle calculations are performed to study the electronic properties and Na-ion storage capabilities of two-dimensional Ti2C MXene monolayers. The surface derivatives, namely Ti2CC2, Ti2CO2, and Ti2CS2, are also considered. The results demonstrate that the bare and terminated monolayers are metallic in character with high electrical conductivity. We find that these monolayers are stable, with Ti2C and Ti2CO2 offering the most stable configurations overall. We also consider functionalized Ti2C MXenes terminated with -C and -O and find that, compared to S-terminated surfaces, these display better activation adsorption. Theoretical results underscore the excellent adsorption properties of Ti2CC2 and Ti2CO2 and reveal that they have lower energy barriers for sodium diffusion than Ti2C. In addition, the results showed that sodium ion adsorption on Ti2C monolayers achieved a high adsorption energy (−2.5 to −0.4 eV) during sodiation, which could enhance the motion capabilities of bare and oxide systems. The structural transformation behaviors of bare and terminated Ti2C monolayers were confirmed during sodiation by ab initio molecular dynamic simulations. In view of its outstanding capacity, electrical conductivity and high Na-ion diffusivity, we conclude that Ti2C and its surface termination group-based monolayers have excellent potential for use as anodes in Na-ion batteries.

Thermal stability of two-dimensional titanium carbides Tin+1Cn (MXenes) from classical molecular dynamics simulations

Abstract