Halogenation effect on physicochemical properties of Ti3C2 MXenes

Halogenated MXenes have been experimentally demonstrated to be promising two-dimensional materials for a wide range of applicability. However, their physicochemical properties are largely unknown at the atomic level. In this study, we applied density functional theory (DFT) to theoretically investigate the halogenation effects on the structural, electronic, and mechanical characteristics of Ti3C2, which is the most studied MXene material. Three atomic configurations with different adsorption sites for four kinds of halogen terminals (fluorine, chlorine, bromine, and iodine) were considered. Our DFT results reveal that the adsorption site of terminals has a considerable impact on the properties of MXene. This can be ascribed to the different coordination environments of the surface Ti atoms, which change d-orbital splitting configurations of surface Ti atoms and the stabilities of systems. According to the density of states, crystal orbital Hamilton population, and charge analyses, all the considered halogenated MXenes are metallic. The electronic and mechanical properties of the halogenated MXenes are strongly dependent on the electronegativity of the halogen terminal group. The Ti–F bond has more ionic characteristics, which causes Ti3C2F2 mechanically behave in a more ductile manner. Our DFT results, therefore, suggest that the physicochemical properties of MXenes can be tuned for practical applications by selecting specific halogen terminal groups.


Introduction
MXene is one of the emerging two-dimensional (2D) materials with a chemical structure of M n+1 X n T 2, where M is an early transition metal (e.g. Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), X is carbon or nitrogen, and T represents the surface terminal group [1]. The chemical composition can easily be engineered to manipulate their structural, electronic, mechanical, and chemical properties. The diverse physicochemical properties of MXenes enable a wide range of applications including solar cells [2], supercapacitors [3], batteries [4], catalysis [5,6], electronics [7], sensing [8], and medicine [9]. The most important characteristics of MXene materials are dependent on their surface functionality [10]. This is due to the etching techniques by using HF, LiF, and HCl to synthesize MXenes, which lead to surface functionality mainly by O, OH, and F terminal groups. As a result, most previous investigations have concentrated on MXenes with -F, -O, and -OH functional groups. Previous studies demonstrate that the electronic and mechanical properties of MXenes can be manipulated by the choice of surface termination for specific applications [11][12][13]. For instance, oxygen terminals can change Cr 2 C, Mo 2 C, and W 2 C to topological insulators [7]. Oxygen-terminated MXenes are mechanically stronger than the F-or OH-terminated counterparts [14,15]. Furthermore, the work function of MXene has been theoretically predicted to be adjustable by the use of various terminal groups [7]. To this end, recent synthesis processes have been developed to synthesize MXenes terminated by a unitary group to better control the surface properties for their practical uses [7,14,15].
Different from the widely studied MXenes terminated by F, O or OH, some novel halogenated MXenes terminated by Cl, Br, or I have recently been designed. In 2019, the Cl-terminated MXenes were made by etching in Lewis acidic chloride melts at a high temperature [12,13]. Soon, the Br-terminated Ti 3 C 2 and Ti 2 C were synthesized using lower melting point alkali metal halides as the solvent [16]. Moreover, the halogenated MXenes have been demonstrated to be good candidates for a variety of applications [13,[16][17][18]. The fluorine terminated MXenes have been utilized as an anode for a lithium-ion battery, which can efficiently optimize the electromigration of lithium ions to improve the cycle stability and capacities [19]. F-terminated Ti 3 C 2 has also been reported for application in sodium-ion batteries [20]. Cl-terminated Ti 3 C 2 has been used for photocatalytic degradation of contaminants, antibacterial, soil remediation and optical precision processing due to its high stability, and oxidation activity and excellent biocompatibility [17]. A recent study investigated the cathode performance of different halogenated Ti 3 C 2 MXenes in Zinc-ion batteries [18]. Ti 3 C 2 Br 2 and Ti 3 C 2 I 2 exhibit a considerably high electrochemical performance, which makes them highly competitive among most of the currently used cathodes with discharge platforms capacities of 97.6 and 135 mAh·g −1 respectively. As a comparison, Ti 3 C 2 Cl 2 and Ti 3 C 2 (OF) exhibit no discharge platforms with only half of the capacities and energy densities of Ti 3 C 2 Br 2 [18]. Moreover, Br-terminated MXenes can be engaged in a new type of surface reaction with the exchange of halide ions for other functional groups for the production of Ti 3 C 2 S, Ti 3 C 2 Se, Ti 3 C 2 Te, Ti 3 C 2 (NH) MXenes [16].
Despite the great potential of the halogenated MXenes, the systematic study on the structural, electronic, and mechanical properties of the MXenes functionalized by -F, -Cl, -Br and -I is still rare. Especially, the mechanical properties remain unknown while these properties are essential to their practical applications. The in-plane mechanical properties of MXenes are directly relevant to their applications in biology, sensors, membranes, pollutant degradation, catalysis, and energy storage [21,22]. Additionally, the high mechanical strength of MXenes as electrodes enable them to handle large volumetric expansions during charge and discharge cycling [23][24][25]. The studies on the mechanical properties can also enable the MXene-based catalysts design through strain engineering [26][27][28]. A combination of flexibility and remarkable high durability under high strains of MXenes are the key features in supercapacitor applications [29][30][31]. Nevertheless, it is difficult to conduct a direct measurement of the in-plane mechanical properties of MXene. At this point, a promising alternative is to conduct first-principles density functional theory (DFT) computations to numerically predict the mechanical properties [22].
In this study, the physicochemical properties of halogenated Ti 3 C 2 were computationally investigated by virtue of the DFT with the consideration of different adsorption sites for halogen terminals. Ti 3 C 2 was selected as a model MXene material because it is the most widely used MXenes for halogenation in the reported applications. Our computational results reveal that the electronic and mechanical properties of the halogenated MXenes are strongly dependent on the adsorption sites and electronegativity of the halogen terminal group.

Computational method
All DFT calculations were performed by using the Vienna ab initio simulation package (VASP) [32,33]. Electron-ion interactions were described by standard projector augmented wave pseudopotentials [34], with valence configurations of 3s 2 3p 6 4s 2 3d 2 for Ti (Ti_sv), 2s 2 2p 2 for C (C), 2s 2 2p 5 for F (F), 3s 2 3p 5 for Cl (Cl), 4s 2 4p 5 for Br (Br) and 5s 2 5p 5 for I (I). The exchange-correlation functional was applied with Perdew-Burke-Ernzehof framework at the generalized gradient approximation level [35]. The van der Waals force adjustment was considered via the DFT-D3 approach [36]. For the structural optimization, gamma-cantered (12 × 12 × 1) k-point meshes with a reciprocal space resolution of 2π × 0.04 Å −1 were employed with the cut-off kinetic energy of 520 eV. All atoms were allowed to relax until the energy convergence was set to 1 × 10 −8 eV and the Hellmann-Feynman forces were less than 1 × 10 −5 eV Å −1 . For both their electrical and mechanical properties calculations, a denser gamma-cantered (21 × 21 × 1) k-point mesh with a reciprocal space resolution of 2π × 0.02 Å −1 was used. The vacuum was given a thickness of 25 Å to guarantee that it was thick enough to prevent contact between the 2D MXene monolayer and its periodic images.
The density of states (DOS), Mulliken charge, crystal orbital Hamiltonian population (COHP), and work function of the systems have all been examined for an in-depth understanding of the electronic properties and bonding mechanisms. The mechanical properties were investigated based on the elastic constants (C ij ) of the MXenes, which were calculated according to generalized Hooke's law [22]. First, the total energies as a function of strain (ε) in the strain range −2.0% ⩽ ε ⩽ 2.0% with an increment of 0.5% were calculated.
Then, the elastic constants C ij were obtained by fitting a second-order polynomial to the change in the total energy versus applied strain by using post-processing VASPKIT code [37]. The Young's and shear moduli and Poisson's ratio were computed using the same method explained in our previous studies in detail [14]. To understand the thermodynamic stability, the cohesive energies of the investigated MXenes (E coh ) were calculated according to the following equation: where E MXenes is the total energy of the MXene, E Ti , E C and E T are the energies of the single Ti, C, and halogen atoms from the spin-polarization computations, respectively. In each unit cell, there are seven atoms including three Ti atoms, two C atoms and two halogen atoms. The work function (Φ) was calculated as below [38,39]: where E vacuum and E Fermi represent the vacuum energy and Fermi energy, respectively.

Structural and energetic properties
To systematically study and compare the properties of the Ti 3 C 2 T 2 (T = F, Cl, Br, I) MXenes, three configurations with different adsorption sites for halogen terminals were considered. Figure 1(a) and (b) show the side and top views of these configurations, which are identified as types I, II, and III. In type I configuration, all halogen atoms point to the Ti atoms in the middle layer on both sides of the MXenes. The halogen atoms are on the top of the C atoms on both sides in type II structure. And type III is a mixture of types I and II. All the Ti atoms in the central layer are associated with 6 C atoms with an octahedral symmetry. However, the coordination environment of surface Ti atoms vary significantly. In type I, surface Ti atoms are bonded with three T and three C atoms still with the octahedral coordination environment. In contrast, surface Ti atoms are connected with three T and three C atoms with a trigonal prismatic structure in type II configurations. As a result, the 3d orbitals of all Ti atoms in type I split into two groups according to the crystal field theory. Due to the identical octahedral structure, the central Ti atoms in type II have the same electronic configuration as those in type I MXene. However, 3d orbitals for surface Ti atoms are divided into three groups in type II structure due to the trigonal prismatic symmetry (see figure 1©). The calculated lattice constants, Ti-C and Ti-T bond lengths, and the thickness of the MXenes are listed in table 1. The thickness is the height difference between the atoms at the top and the bottom. It shows that, the lattice constants, Ti-T bond lengths, and thickness increase as the atomic number of halogens increases. This matches the expectation due to the rise in the atomic radius of halogens along the periodic table. It is worth noting that the Ti-C bond lengths are slightly enlarged when the atomic number of halogens increases, implying that there is a considerable impact of the terminal groups on the Ti-C bond strength. Generally, the lattice constants of type I Ti 3 C 2 T 2 are larger than that of type II. This can be ascribed to the change of the symmetry of surface atoms as shown in figure 1(a). Due to the small size of the trigonal prismatic [TiC 3 X 3 ] unit, the lattice constants of type II Ti 3 C 2 T 2 are lowered. The lengths of the Ti-C and Ti-T bonds, however, are nearly identical. Thus, type II is about 0.1-0.3 Å thicker than type I. The lattice constants and thickness of type III fall midway between types I and II. This is expected given that type III is created when types I and II hybridize.
The thermodynamic stability of Ti 3 C 2 T 2 can be evaluated based on their cohesive energies obtained from equation (1), which are also listed in table 1. The more negative cohesive energy value suggests a higher stability. The calculated cohesive energies of all the halogenated MXenes are lower than that of some stable 2D materials, including silicene (−4.57 eV) [40], α-phosphorene (−3.58 eV) and β-phosphorene (−3.56 eV) [41]. It indicates that all the Ti 3 C 2 T 2 considered here are stable. The computed E coh values follow the order: Ti 3 C 2 F 2 < Ti 3 C 2 Cl 2 < Ti 3 C 2 Br 2 < Ti 3 C 2 I 2, indicating that the thermodynamic stabilities of halogenated MXenes diminish along the periodic table. This can be ascribed to the decreased electronegativity values of halogen down the Group 17. The stability of type I is higher than type II, type III Ti 3 C 2 T 2 stability is midway between type I and type II stabilities. This is consistent with the trend of the related structural Ti 3 C 2 T 2 characteristics.

Electronic properties
The band structures and partial DOS (PDOS) of halogenated Ti 3 C 2 were examined to comprehend their electronic characteristics. The band structures of Ti 3 C 2 T 2 with Types I and II configurations are shown in   To further understand the interactions between the surface atoms, the PDOS of Ti 3 C 2 T 2 (T = F or Cl) with types I and II configurations are presented in figure 3. The DOS images confirm that all Ti 3 C 2 T 2 materials are metallic. Ti 3d states mostly contribute to the evolution of the DOS at the Fermi level. It is also discovered that Ti 3d states are primarily responsible for the non-bonding and antibonding states found  figure 1(c). Additionally, the d-splitting diagram indicates that the d xy and d x2-y2 of type I Ti 3 C 2 T 2 belong to the anti-bonding states. Consequently, the fact that all the electrons in this [TiC 3 T 3 ] octahedra structure are in the bonding d xy , d xz , and d yz orbitals explains why type I Ti 3 C 2 T 2 are more stable.
The Mulliken charge of atoms was estimated to support the PDOS analysis findings. The type I configuration is selected here as model systems due to its higher stability compared to other configurations. It is suggested that the majority of experimentally synthesized halogenated Ti 3 C 2 MXenes exhibit this configuration. Therefore, in our analysis, we focused on the type I configuration to investigate the influence of halogens on Mulliken charge and work function. The results are shown in table 2. Due to a decrease in their electronegativity, the charges of surface Ti atoms (termed as Ti(s) in table 2) fall as the atomic number  of T increases. Notably, as compared to Ti 3 C 2 Cl 2 , Ti 3 C 2 Br 2 , and Ti 3 C 2 I 2 , respectively, the charge of Ti(s) in Ti 3 C 2 F 2 is roughly 55%, 72%, and 108% greater, respectively. This confirms that the Ti-F bond has a more ionic feature. Moreover, the charges of central Ti of MXene (termed as Ti (c) in table 2) rise with the increase of the atomic number of T while the charge difference of Ti(c) is small in terms of that of Ti(s). The calculated work function (Φ) values of the functionalized Ti 3 C 2 by F, Cl, Br and I are valued at 4.77, 4.49, 4.13, and 3.47 eV, respectively. The work function is defined as the minimum energy needed to remove an electron from the Fermi level and place it in the vacuum. As such, the relatively small work function values indicate that I-and Br-terminated MXenes are more reactive than Cl-and F-terminated MXenes. It matches the experimental observation that MXenes with novel terminals, such as S, Se, Te, and NH, have been synthesized using Br-terminated MXenes [16]. The COHP analysis was further performed to comprehend the covalent Ti (s)-C and Ti (s)-T bonding strengths (see figure 4) [42][43][44][45][46]. The -pCOHP is a theoretical tool to understand the covalent bonding strength between a pair of adjacent atoms, which partitions the band-structure energy into orbital-pair interactions. The -IpCOHP values, which are obtained by integrating the -pCOHP up to the Fermi energy level, can be used to quantitatively evaluate the covalent bonding strength. A stronger covalent bond is represented by a lower -IpCOHP value. The bonding and antibonding peaks of Ti-T are significantly smaller than those of Ti-C, according to the -pCOHP curves for all halogenated MXenes. The -IpCOHP values of Ti-X bonds are around −2 eV. Ti-C bonds have -IpCOHP values that are about 1.5 eV lower that of Ti-X bond. It demonstrates that a weak covalent Ti-T connection in terms of the Ti-C. Furthermore, the -IpCOHP values of Ti-C bond increase with the increase of the atomic number of halogen terminal groups, which indicates that the adsorption of halogen can considerably affect the bonding strength between surface Ti and C atoms. Along the periodic table, the Ti-C covalent bonding strength is weakened due to the prolonged Ti-C bond lengths (see table 1). A similar trend is also observed for the Ti-X bonding strength as suggested by the -IpCOHP values. The only exception is the Ti-F bond, which has a higher -IpCOHP values than Ti-Cl. Given the -IpCOHP is used to evaluate the covalent bonding strength, it supports that a Ti-F bond has more ionic nature revealed by the PDOS and charge analyses. Figure 4 also shows that the -IpCOHP values of Ti-C and Ti-X with type I configurations are smaller than that with type II configurations. It suggests that both Ti-C and Ti-X bonds in type I are stronger than that in type II. This matches the trend of the cohesive energies, which further confirms that the [TiC 3 T 3 ] octahedra structure is more stable. Figure 5 shows the Young's moduli (E), Shear moduli (G), Poisson's ratios (σ) and ratios between bulk moduli and shear moduli (K/G) of halogenated MXenes. Our DFT results indicate that all models under investigation are mechanically stable. type I systems have lower Young's moduli and shear moduli in comparison with those of type II systems. In type I, all the halogenated Ti 3 C 2 MXene have a similar Young's moduli. As a comparison, Young's moduli, and Shear moduli of type II halogenated MXenes decreases with the increase of the atomic number of the halogen with a larger variation. It suggests that the symmetry of the surface [TiC 3 X 3 ] unit also has a considerable impact on mechanical strength. Specifically, the trigonal prismatic configuration in type II configurations can greatly enhance the C 11 values, which leads to larger Young's modulus, shear modulus and bulk modulus values in this configuration. Among all the considered systems, type II Ti 3 C 2 F 2 possesses the largest E and G values at 339 N m −1 and 138 N m −1 , respectively. However, it is worth noting that all the halogenated MXenes have a lower mechanical strength compared with the oxygen-terminated MXenes [14,15].

Mechanical properties
Poisson's ratio reflects mechanical ductility and flexibility. The higher Poisson's ratio value suggests better ductility and flexibility [22]. All the halogenated Ti 3 C 2 MXene have a similar Poisson's ratio values in terms of atomic configurations, which demonstrates that the symmetry of the surface [TiC 3 X 3 ] unit has negligible influences on the ductility and flexibility. On the other hand, the terminal group has a larger impact. The I-terminated MXenes have smaller Poisson's ratio values, which indicates that Ti 3 C 2 I 2 is less flexible than other halogenated MXenes. Pugh once suggested using the ratio of K/G to distinguish the ductility and brittleness of a material [47]. The material with a K/G ratio lower than 1.75 behaves in a brittle manner. From figure 5(d), type I Ti 3 C 2 T 2 is more ductile than type II. The brittleness of halogenated Ti 3 C 2 increases with the increase of the atomic number of halogen terminal groups. The K/G values of type I Ti 3 C 2 F 2 is the highest at 1.80, which suggests that Ti 3 C 2 F 2 has the most ductile characteristics among all the considered MXenes. This can be explained given that the Ti-F bonds in Ti 3 C 2 F 2 possess more ionic bonding features. The ionic bond is non-directional, which enable Ti 3 C 2 F 2 to have a more ductile behavior.

Conclusion
In summary, the DFT calculations were performed to investigate the structural, electronic, and mechanical characteristics of the halogen group (Fluorine, chlorine, Bromine, and Iodine) terminated Ti 3 C 2 MXenes. Three atomic configurations with different adsorption sites of the terminal group were considered. Our results reveal that type I halogenated Ti 3 C 2 MXenes with octahedral surface [TiC 3 X 3 ] unit are generally more thermodynamically stable that type II with trigonal prismatic surface [TiC 3 X 3 ] unit as evidenced by the lower cohesive energies. This can be ascribed to the different d-orbital splitting configurations of surface Ti atoms caused by the symmetry of the surface [TiC 3 T 3 ] unit. The different d-orbital splitting of surface Ti atoms was further confirmed through the PDOS analysis. The symmetry of the surface [TiC 3 T 3 ] also greatly affects the mechanical properties. Type II Ti 3 C 2 T 2 possess a higher mechanical strength. In addition, the size and electronegativity of halogen atoms can also significantly affect the physiochemical properties of MXenes.
The COHP and charge distribution analyses demonstrate that the Ti-F bond is more ionic. Moreover, the relative thermal stability decreases with the increase in the atomic number of halogens. The covalent bonding components increase following the trend of the periodic table. As a result, I-terminated MXenes with the smallest Poison's ratio values are less flexible. The F-terminated MXenes behave in a ductile manner. The theoretical findings of this comparative study, therefore, offer a knowledge base for the advance of recently discovered halogenated MXenes for the specific application.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).