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 E coh. 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.

Introduction

Lithium-ion batteries (LIBs) have been the promising energy storage devices in portable electronic devices and smart grids.[1] However, the safety issues and the high cost seriously hindered the development of LIBs. Recently, two-dimensional (2D) materials have received great interest because of their wide applications[2−6] in LIBs, catalysis (ECs), and so forth.

2D materials designated as MX (MXenes) have outstanding electrochemical and mechanical properties and are extensively used in battery electrode materials.[7−9] MXenes are obtained by etching the A layer from MAX phases (MAX). Here, M is an early transition metal, n = 1, 2, or 3, and X is C or N.[10] In the etching process, some functional groups are left on the MXene. Therefore, the formula of functionalized MXenes is MXT (T = F, O, and OH and x is the termination number).[11] Recent studies have presented the wide applications of MXenes[12,13] as the anode material, optical device, and so forth.

Previous research indicated that MXenes with O or OH surface groups have a more stable structure,[14] and the OH group can be changed to an O group at high temperature.[13] For these reasons, O-functionalized Ti2C MXenes are extensively investigated. Among all potential MXene phases, Ti2C is one of the thinnest MXenes and has the biggest superficial area per weight, which makes it become one of the most potential electrode materials.[15−18]

Guo et al.[19] thought that the O termination on the Ti2C surface can enhance the mechanical properties. Gao et al.[20] reported the adsorption behavior of mercury on the Ti2CO2 monolayer. Theoretical investigation indicated that the calculated band gaps of the Ti2CO2 monolayer with the Perdew–Burk–Ernzerhof (PBE) and Heyd–Scuseria–Ernzerhof (HSE06) functionals are 0.24 and 0.88 eV, respectively,[21] and exhibits superhigh carrier mobility[22−25] and thermal stability.[26] In addition, Ti2CO2 has the highest hole mobility among MXenes.[27] Nayak et al.[28] investigated the electronic structure and lattice vibrational dynamics of MoS2 under pressure in experiment and theory. Pressure is an important parameter to effectively alter the distance and interaction between atoms. Experimental research indicates that a high-pressure technique can improve the conductivity of some LIBs.[29] However, up to now, the related properties under pressure are still unavailable for the Ti2CO2 monolayer.

Structural variation has a great effect on 2D materials[30] and can make 2D materials show great diversity.[31−34] The diversity makes it possible to develop next-generation electronics with specific functionalities. Ti2CO2 has a moderate indirect band gap, and using it in optical devices can interrupt efficient light emission. Hence, indirect-to-direct band engineering can improve the light emission in optical devices. We investigated the band modulation of Ti2CO2 under pressure in the present work. The semiconductor-to-metal transition occurs at 7 GPa. The optical properties under pressure are also explored.

Results and Discussion

Structural Properties

Figure a presents the Ti2AlC MAX phase. Etching Al atoms from Ti2AlC can produce the Ti2C monolayer. Figure b,c presents the O-terminated Ti2C MXene (Ti2CO2), from the top and side views. Previous studies indicate that the position of the etched-away Al atoms is energetically more favorable for the O-site.[12,35]Figure d presents the calculated HSE06 band structure of the Ti2CO2 MXene. The band gap calculated with the HSE06 hybrid functional is 0.9012 eV, which is consistent with the theoretical result of Xie et al.[21]

Figure 1

Atomic structure. (a) Ti2AlC MAX phase; (b) Ti2CO2 monolayer top view; (c) Ti2CO2 monolayer side view; and (d) band structure of the Ti2CO2 monolayer.

Cohesive energy (Ecoh) is defined as the energy required to separate a solid to isolated atoms. For the Ti2CO2 monolayer, Ecoh is calculated using the equationwhere Etot is the electronic total energy of the Ti2CO2 monolayer and Eatm is the energy of isolated atoms Ti, C, and O, respectively. The larger the Ecoh, the more stable the crystal structure.

Table lists the calculated Ecoh for the Ti2CO2 monolayer under pressure. From Table , positive Ecoh indicates the stability of Ti2CO2 MXenes under pressure. In order to have a comparison, we further calculated the Ecoh of Ti2C MXenes at zero pressure, which is 1.441 eV/atom. This shows that Ti2CO2 MXenes are more stable than Ti2C MXenes. Previous studies reported the Ecoh of FeB6 (5.56 eV/atom),[36] Al2C (4.49 eV/atom),[37] and Be2C monolayers (4.86 eV/atom).[38] The Ecoh of Ti2CO2 MXenes under pressure is smaller when compared with that of FeB6, Al2C, and Be2C. This indicates that the chemical bonds between Ti and O atoms are not so stronger than those in FeB6, Al2C, and Be2C.

Table 1

Cohesive Energy Ecoh of the Ti2CO2 Monolayer under Pressure

pressure (GPa)	Ecoh (eV/atom)
0	3.572
1	3.566
2	3.552
3	3.531
4	3.501
5	3.463
6	3.415
7	3.358

Band Gap Modulation

Figure presents the band structures of Ti2CO2 under pressure. No magnetism is found for Ti2CO2 MXenes under pressure. From Figure d, there is a 0.9012 eV band gap at zero pressure. The band gap decreases gradually with the increase in pressure and becomes zero when the pressure increases to 7 GPa. This indicates that Ti2CO2 under pressure undergoes a semiconductor-to-metal transition.

Figure 2

Band gap of Ti2CO2 under pressure.

We further examine the energy per atom to investigate the electronic properties under pressure. The energy under pressure Ep = (Epressure – Enonpressure)/n, n is the number of atoms in the unit cell. Figure presents band gap and energy under pressure. With the increase in pressure, the energy increases and the band gap decreases gradually. At about 7 GPa, Ti2CO2 has a semiconductor-to-metal transition.

Figure 3

Band gap and energy of the Ti2CO2 monolayer under pressure.

Figure presents the partial density of states (PDOS) under pressure. The dashed vertical lines indicate the Fermi level (EF). From Figure , we can see that Ti2CO2 below 7 GPa exhibits a semiconductor nature with a small band gap. The Ti-3d state provides the main contribution to the conduction band under pressure, while C-p and O-p states have little contribution. The mixture of Ti-d, C-p, and O-p states provides the main contribution to the valence band under pressure, indicating the strong interaction between Ti-d and C-p, and Ti-d and O-p states. In the energy range of −7 to −5 eV, the main contribution is from the O-p state, with little from Ti-d, Ti-p, and C-p states.

Figure 4

PDOS of Ti2CO2 under pressure.

The PDOS of Ti-d, Ti-p, C-p, and O-p states is presented in Figure . We found that (1) Ti-p and O-p states have little contribution at EF, while Ti-d and C-p states have the main contribution. (2) PDOS shapes of Ti-d, Ti-p, O-p, and C-p under pressure changed distinctly. First, the Ti-d state has two peaks at about 1.5 eV (labeled in Figure ). The two peaks under pressure split into several peaks. Second, DOS values of the Ti-d state under pressure at EF increase, while DOS values of the C-p state are nearly constant. This implies that the Ti-d state results in the metallization of Ti2CO2. Third, the peak of the C-p state at about 3.5 eV splits into two visible peaks, and the obvious changes of Ti-p and O-p states make several peaks converged in the energy range of −2 to −7 eV and −4.5 to −5.5 eV, respectively.

Figure 5

PDOS of Ti-p, Ti-d, O-p, and C-p states under pressure.

Optical Properties

We know that Ti2CO2 has the band gap under the pressure from 0 to 6 GPa with the indirect band gap range of 0.0958–0.9012 eV. For the materials with zero band gap, the photoactivated electrons and holes within materials can recombine easily and accomplish photocatalytic reactions,[39] which shows that the material cannot be utilized for photocatalytic applications. Hence, we further investigated the optical properties under pressure from 0 to 6 GPa.

For the complex dielectric function ε(ω) = ε1(ω) + iε2(ω), we can obtain the real part ε1(ω) from the imaginary part ε2(ω). The absorption coefficient α(ω) can be obtained from the equation.[40]

Figure presents the ε2(ω) for Ti2CO2 under pressure. There is a very strong peak in the visible region (about 2.5 eV). The strong peak is considered as the charge transfer between Ti-d and C-p, and Ti-d and O-p in Figure . The peak of ε2(ω) at about 2.5 eV gradually increases with increasing pressure (2.6053, 2.7767, 2.8967, 2.9696, 3.1770, 3.3985, and 3.6652 for 0, 1, 2, 3, 4, 5, and 6 GPa, respectively), which corresponds to electron excitation. Hence, we think that the charge transfer under pressure between Ti-d and C-p, and Ti-d and O-p states is strengthened in the visible region. The second peak is not so strong and appears at about 5.5 eV, which mainly derived from the interaction of Ti-d and C-p states.

Figure 6

Imaginary part ε2(ω) of the dielectric function of Ti2CO2 under pressure.

Figure presents the optical absorption of Ti2CO2 under pressure. The strongest absorption zone between 0 and 3 eV is at 2.5 eV. The maximum peaks of the absorption spectrum in this range increase with increasing pressure, and the absorption is enhanced in the visible region. A valley occurs at 4 eV under zero pressure, and the valley (labeled in Figure ) gradually declines and becomes steep with increasing pressure. This indicates that the absorptions under pressure at 4 eV are the lowest, with the maximum and minimum absorptions for 0 and 6 GPa, respectively. Then, the absorption fluctuantly increases with increasing pressure and reaches the maximum at about 7 eV, with the maximum and minimum absorption for 6 and 0 GPa, respectively.

Figure 7

Absorption coefficient spectrum of Ti2CO2 under pressure.

Charge Analysis

In order to gain deep insight into electronic distribution, the atomic charge and bond length under pressure are investigated and listed in Table .

Table 2

Atomic Charge (|e|) and Bond Length (Å) of Ti2CO2 under Pressure

	atomic charge			bong length
pressure (GPa)	C	O	Ti	rC–Ti	rO–Ti
0	–1.504	–0.985	1.735	2.188	1.980
1	–1.504	–0.975	1.724	2.171	1.968
2	–1.503	–0.965	1.715	2.160	1.960
3	–1.500	–0.956	1.706	2.151	1.954
4	–1.497	–0.947	1.697	2.140	1.947
5	–1.490	–0.937	1.682	2.130	1.940
6	–1.481	–0.927	1.666	2.121	1.934
7	–1.471	–0.918	1.654	2.111	1.929

It is noted that O and C atoms gain more electrons from Ti atoms for Ti2CO2 under pressure. With the increasing pressure, the charge donated by Ti atom decreases, while the accepted charge by C and O atoms also decreases. The bond lengths of O–Ti and C–Ti bonds become shorter with increasing pressure, so the covalent characteristics of Ti–O and Ti–C bonds are stronger, which is consistent with the analysis of DOS.

Conclusions

In summary, we investigated the electronic and optical properties of Ti2CO2 under pressure theoretically. The negative Ecoh confirms the stability of Ti2CO2 under pressure. Ti2CO2 undergoes the semiconductor-to-metal transition at about 7 GPa. The metallization of Ti2CO2 mainly results from the Ti-d state. The Ti-3d state provides the main contribution to the conduction band, and the valence band is contributed from Ti-d, C-p, and O-p states. The absorption under pressure is enhanced, and some new absorption peaks occur in the low-energy region.

Computational Details

All calculations were performed using density functional theory[41] implemented in the plane-wave VASP code.[42] The HSE06[43,44] hybrid functional is used. The generalized gradient approximation of the PBE scheme[45] is used to optimize the structure through relaxing the lattice parameters a and b and the position of all the atoms until the convergence tolerance of force on each atom is less than 1.0 × 10–6 eV/Å. The orbits of Ti (3d34s1), C (2s22p2), and O (2s22p4) for Ti, C, and O are treated as valence electrons, respectively. The pressure was imposed in the direction parallel to the Ti2CO2 plane. The plane wave cutoff energy is 700 eV. A large vacuum layer of 25 Å is imposed on both sides to simulate the isolated monolayer. A 25 × 25 × 1 k-point mesh is used during optimization and a 103 k-points grid is utilized for obtaining the reliable energy band. The spin polarization and vdW interaction are taken into consideration.[46]