Ti2CO2 MXene Research Articles

Pressure-Induced Modulation of Electronic and Optical Properties of Surface O-Functionalized Ti2C MXene.

Functionalized MXenes have gained increasing interest in the fields of thermoelectric materials, hydrogen storage, and so forth. In this work, pressure-induced band modulation and optical properties of the Ti2CO2 monolayer are investigated by using density functional theory with the hybrid (HSE06) functional. The calculation reveals that Ti2CO2 MXenes under pressure are stable because of the positive Ecoh. Ti2CO2 undergoes a semiconductor-to-metal phase transition at about 7 GPa. The metallization of Ti2CO2 mainly results from the Ti-d state. Research indicates that there exist strong interactions between Ti-d and C-p, and Ti-d and O-p states, which are further confirmed by the charge analysis. In addition, the absorption is enhanced in the visible region with increasing pressure. We also observed some new absorption peaks in the visible region.

First-Principles Calculations on the Adsorption Behavior of Amino Acids on a Titanium Carbide MXene.

Due to their vast range of promising biomedical and electronic applications, there is a growing interest in bioinorganic lamellar nanomaterials. MXenes are one such class of materials, which stand out by virtue of their demonstrated biocompatibility, pharmacological applicability, energy storage performance, and feasibility as single-molecule sensors. Here, we report on first-principles predictions, based on density functional theory, of the binding energies and ground-state configurations of six selected amino acids (AAs) adsorbed on O-terminated two-dimensional titanium carbide, Ti2CO2. We find that most AAs (aspartic acid, cysteine, glycine, and phenylalanine) prefer to adsorb via their nitrogen atom, which forms a weak bond with a surface Ti atom, with bond lengths of around 2.35 Å. In contrast, histidine and serine tend to adsorb parallel to the MXene surface, with their α carbon about 3 Å away from it. In both adsorption configurations, the adsorption energies are on the order of the tenths of an electronvolt. In addition, we find a positive, nearly linear correlation between the binding energy of each studied AA and its van der Waals volume, which suggests an adsorption dominated by van der Waals forces. This relationship allowed us to predict the adsorption energies for all of the proteinogenic AAs on the same Ti2CO2 MXene. Our analysis additionally shows that in the parallel adsorption mode there is a negligible transfer of charge density from the AA to the surface but noticeable in the N-bonded adsorption mode. In the latter, the isosurfaces of charge density differences show accumulation of shared electrons in the region between N and Ti, confirming the predicted N-Ti bond. The moderate adsorption energy values calculated, as well as the preservation of the integrity of both the AAs and the surface upon adsorption, reinforce the capability of Ti2CO2 as a promising reusable biosensor for amino acids.

Strain-Induced Band Modulation, Work Function, and QTAIM Analysis of Surface O-Functionalized Ti2C MXene.

Functionalized MXenes have wide applications in the fields of gas sensors, thermoelectric materials, and hydrogen storage. Strain-induced band engineering and the work function (WF) of Ti2CO2 MXene are investigated theoretically. The calculations reveal that Ti2CO2 MXenes are stable because of the negative Ecoh, and all the strains considered are within the elastic limit. For Ti2CO2 MXene, strain-induced blue shift of the Ti d state results in the transformation from a semiconductor to a metal. At about 4%, Ti2CO2 MXene transforms from an indirect band gap to a direct band gap. The decreased WF induced by the strain improves the power efficiency of Ti2CO2 MXene.

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.

Strain engineering and electric field tunable electronic properties of Ti2CO2 MXene monolayer

In this work, we investigate theoretically the structural and electronic properties of the monolayer Ti2CO2 MXene using first-principles calculations. At the ground state, the monolayer Ti2CO2 MXene structure is dynamically stable and is a semiconductor with an indirect band gap of 0.75 eV, which can be controlled by strain engineering and electric field. We find that the indirect band gap of the monolayer Ti2CO2 MXene could be transformed into a direct one. Whereas, a semiconductor–metal transition can be achieved by applying the compressive biaxial strain of −10%. Our results demonstrate that the monolayer Ti2CO2 MXene can be a potential material for designing optical nanodevices with the strain engineering and electric field tunable of the band gap.

Effect of vacancies on the structural and electronic properties of Ti2CO2.

Ti2CO2 MXene is widely considered as a potential candidate material for sensors and optical devices. In this paper, first-principles calculations are performed to investigate the structural and electronic properties of pristine and vacancy defect Ti2CO2 monolayer. The results indicate that C-vacancy is energetically more favorable than Ti-vacancy and O-vacancy because of the smaller formation energy of C vacancy. The introduction of vacancy defects results in the transition from semiconductor to metal, and improves the electronic conductivities of Ti2CO2 monolayer. The introduction of C and O vacancies causes the Ti-d state to split into several peaks in the energy range of 0 to 2 eV, while the introduction of Ti vacancy makes the Ti-d state weaker and the C-p state stronger. Furthermore, the work function can be effectively engineered by vacancy defects.

Stabilization of 2D graphene, functionalized graphene, and Ti2CO2 (MXene) in super-critical CO2: a molecular dynamics study.

The stabilization of nanoparticles is a main concern to produce an efficient nanofluid. Here, we carry out Molecular Dynamics (MD) modeling to evaluate the stabilization of 2D graphene, oxygen- and hydrogen-functionalized graphene, and Ti2CO2 MXene nano-sheets in carbon dioxide (CO2) liquid under supercritical conditions. The rheological properties (i.e., viscosity and self-diffusion) of the nanofluids with the corresponding nano-sheets are also calculated. The results show that pristine graphene aggregates in the liquid and the adsorbed CO2 molecules on the graphene surfaces could not provide enough repulsive forces to avoid their aggregation. The stabilization of graphene is improved by surface functionalization of graphene with oxygen and hydrogen functional groups. This is because, first, the presence of the corresponding surface groups can alter the electrostatic forces of the graphene nano-sheets and, second, a larger number of CO2 molecules attracted by the functionalized nano-sheets can provide additional repulsive forces between the nano-sheets. The results demonstrate that Ti2CO2 MXene nano-sheets aggregate severely although they are covered by oxygen surface terminations and attract more CO2 molecules than the pristine graphene. This is attributed to the strong interlayer coupling between the MXene nano-sheets so that the electrostatic repulsion of the MXene surfaces could not overcome the interlayer coupling. Based on our results, the stabilization of the corresponding 2D nano-sheets in CO2 liquid is in the order: oxygen-functionalized graphene > hydrogen-functionalized graphene > pristine graphene > Ti2CO2 MXene. The calculation of the rheological properties of the nanofluids with different nano-sheets demonstrates that immersing the nano-sheets in the CO2 liquid affords a superior viscosity enhancement as much as possible to be dispersed in the liquid. This study expands the potential applications of these 2D materials in producing SC-CO2 based nanofluids.

Single atom-supported MXene: how single-atomic-site catalysts tune the high activity and selectivity of electrochemical nitrogen fixation

A series of single atom supported on Ti2CO2 and Mo2CO2 MXenes were systematically explored as efficient electrocatalysts for electro-catalytic N2 reduction. We demonstrate that Ru and Mo atoms anchored MXenes are highly activity.

High TMR in MXene-Based Mn2CF2/Ti2CO2/Mn2CF2 Magnetic Tunneling Junction.

We suggest an MXene-based magnetic tunnel junction (MTJ) design. The device characteristics of the MTJ were investigated by nonequilibrium Green's function formalism within the density functional theory. Inspired by the first synthesized magnetic MAX crystal of Mn2GaC, its two-dimensional (2D) counterpart of the half-metallic Mn2CF2 MXene layer was selected as the magnetic electrode. The tunneling barrier was chosen as Ti2CO2 MXene, which is one of the most studied MXenes in experimental and theoretical works. It is beneficial that both the electrodes and the tunneling barrier are 2D materials from the same material family and have similar structures. The common device problem of lattice mismatch does not occur in our MTJ design because the lattice parameters are compatible. In addition, the band gap of Ti2CO2 tunneling barrier is almost the same as the half-metallic gap of Mn2CF2 electrodes. Both the barrier and the electrodes have a common C layer that contributes the most to the transmission. Our MTJ design consists of structurally and electronically well-matched components. We find that the tunneling magnetoresistance ratio has a peak value of ≈106 and stays higher than ≈103 under the bias voltages up to 1 V. Since the applied bias voltages are within the energy gap of the tunneling barrier, the half-metallic character of the conduction is preserved up to 1 V. The tunneling-based transmission was observed in all of the three devices with varying tunneling barrier widths, and the current decreases with increasing width. The MXene-based MTJ has promising device characteristics.

Exploring adsorption behavior and oxidation mechanism of mercury on monolayer Ti2CO2 (MXenes) from first principles

Developing low-cost, high adsorption capacity, and recyclable mercury sorbents is an important step for control of mercury in coal-fired flue gas. However, few sorbents meet all the requirements. Herein, we explore the adsorption behavior and oxidation mechanisms of mercury on 2D Ti2CO2 (MXenes) monolayer and defective Ti2CO2 monolayer with an Ov (Ov-Ti2CO2) through first-principles computations based on density functional theory (DFT). Our calculation results show that Hg0 adsorption on Ti2CO2 and Ov-Ti2CO2 monolayers are mainly physisorption and chemisorption mechanisms, respectively. Moreover, the presence of a single oxygen vacancy on Ti2CO2 monolayer can improve the interaction between the mercury and Ov-Ti2CO2 monolayer, thereby enhancing the adsorption energy of mercury by 66 kJ/mol. Hg0 has a strong interaction with Ti atom on Ov-Ti2CO2 monolayer by the atomic orbital hybridization and overlap. The adsorption of HgO molecule on Ti2CO2 and Ov-Ti2CO2 monolayers are chemisorption processes. Furthermore, electron density difference analysis indicates that the remarkable charge accumulation across the interface is closely related to the strong interaction between HgO molecule and Ti2CO2 or Ov-Ti2CO2 monolayers. The three-step reaction processes (Hg0 → Hg(ads) → HgO(ads) → HgO) are taken place on Ti2CO2 and Ov-Ti2CO2 monolayers, resulting in the formation of gaseous HgO molecule. The energy barrier for the Hg0 oxidation reaction step on Ov-Ti2CO2 monolayer (54.85 kJ/mol) is about half of that on Ti2CO2 monolayer (104.73 kJ/mol). The desorption steps of the resultant HgO molecule from Ti2CO2 and Ov-Ti2CO2 monolayers are the rate-determining steps, which need external energies of about 119.3 and 183.2 kJ/mol, respectively.

Structures and Mechanical and Electronic Properties of the Ti2CO2 MXene Incorporated with Neighboring Elements (Sc, V, B and N)

Ti2CO2, as the representative MXene with semiconducting characteristics and ultrahigh carrier mobility, has attracted increasing attention in material science. Herein, various Ti2CO2 alloys with Ti displaced by neighboring elements Sc and V, or C by B and N are investigated in this paper based on the first-principles density functional calculations. The structures and mechanical and electronic properties are thoroughly studied for the configurations with varying alloying atom concentrations. The choices of alloying elements play a critical role in determining the lattice parameters and layer thickness. The Sc substitutions generally increase the lattice parameter but decrease the layer thickness. In contrast, the introduction of N presents slight influence on the structural parameters. The mechanical strength shows remarkable variations by introducing the alloying elements. The maximum elastic constant c11 is determined to be 425 GPa in (Ti0.25V0.75)2CO2, and the corresponding minimum value is only 104 GPa found in (Ti0.125Sc0.875)2CO2. With respect to the electronic properties, although B and Sc both present one less valance electron compared to their replaced elements C and Ti, it is easier to realize the p-type semiconductor in the configurations containing Sc. Both the V and N substitutions are capable of generating n-type semiconductors, but their optimal stoichiometric compositions are quite different. Among all the configurations investigated, only (Ti0.5V0.5)2CO2 and (Ti0.375V0.625)2CO2 are magnetic, with their magnetism determined to be 2.61 uB/cell and 1.52 uB/cell, respectively. Thus, the method of alloying neighboring elements provides an effective approach in manipulating the physical properties of the Ti2CO2, which might widen the possible applications of MXene materials.