Functional Group Effects on the Photoelectronic Properties of MXene (Sc₂CT₂, T = O, F, OH) and Their Possible Photocatalytic Activities

Abstract

In view of the diverse functional groups left on the MXene during the etching process, we computationally investigated the effects of surface-group types on the structural, electronic and optical properties of Sc₂CT₂ (T = -O, -OH, -F) MXenes. For all geometries of the Sc₂CT₂ MXenes, the geometry I of Sc₂CT₂, which has the functional groups locating above the opposite-side Sc atoms, are lowest-energy structure. Accordingly, the energetically favorable Sc₂CF₂-I, Sc₂CO₂-I and Sc₂C(OH)₂-I were selected for further evaluation of the photocatalytic activities. We found that the Sc₂CO₂-I is metallic, while Sc₂CF₂-I and Sc₂C(OH)₂ are semiconductors with visible-light absorptions and promising carrier mobilities. Compared with the Sc₂C(OH)₂-I, the Sc₂CF₂-I has not only more suitable band gap (1.91 eV), but also the higher redox capability of photo-activated carriers, which should have better photocatalytic performance.

Introduction

Since the discovery of graphene by Novoselov et al.¹, two-dimensional (2D) materials have garnered tremendous interest in experimental and theoretical studies due to the unique properties of these 2D free-standing crystals. Several families of 2D materials have been recently identified and investigated, such as transition metal dichalcogenides (TMDs)², hexagonal boron nitrides³, few-layer metal oxides⁴, metal chalcogenides (e.g. MoS₂ ⁵,WS₂ ⁶). Remarkably, the constellation of 2D materials has been augmented by a potentially quite large group of early transition metal carbides and carbonitrides (MXenes)⁷. MXenes was generally fabricated by selectively etching “A” layers from M_n+1AX_n phases through acid solution etching^8–12. Here, “M” is an early transition metal, “A” is mainly a group IIIA or IVA element (e.g. Al, Si), X is C and/or N, n = 1, 2, or 3.

During the etching process, “A” atoms were removed and functional groups (-O, -OH, or -F) were left on the surface of MXene to passivate the outer-layer metal atoms. MXene phases present a number of configurations which have variety of novel properties according to the species of functional groups. Many theoretical studies show that bare MXene are metallic and the majority of MXene are passivated by -OH, -F and -O groups still remain metallic. Exceptionally, some of the M₂C (M = Ti, Zr, Hf, Sc,) MXenes turn from metallic to semiconducting by surface functionalization (-O, -OH, or -F), which have band gaps between 0.24 ∼ 1.8 eV¹³. The suitable band gaps of surface functionalized MXenes strongly indicate their visible-light absorption and possible photocatalytic applications.

Compared with other 2D materials, MXenes possess many exceptional properties for photocatalytic application. For example, the promising electric conductivity and carrier mobility could promote the migration of photo-induced carriers, resulting in more effective participation of e^--h⁺ pairs during the photocatalytic process. The surface hydroxy group may serve as the anchor site for the reactants, resulting in more efficient adsorption and oxidation of the organic molecules. The band gaps of the functionalized MXenes are expected to be 0 ~ 2 eV, which reach the requirements of efficient visible-light absorption. Additionally, experimental studies of MXenes have mainly focused on their practical application of lithium-ion batteries, supercapacitors, and adsorbents, few results have been achieved on their photocatalytic utilization. In view of the above reasons, it is quite indispensable to evaluate the possibilities of photocatalytic application, in order to further explore the potential utilizations of these novel graphene-like materials.

In our previous work¹⁴, we theoretically investigated the electronic and optical properties of functionalized MXenes M₂CT₂ (M = Ti, Zr, Hf, T = -O, -OH, -F) with different functionalization geometries. Among the variously functionalized MXenes, only M₂CO₂ (M = Ti, Zr, Hf) with geometry I, which have the functional group located above the opposite-side metal atoms, have appropriate band gaps (0.92 ∼ 1.75 eV) for photocatalytic applications. Regarding to the Sc₂CT₂ (T = -O, -OH, -F) MXenes, first-principle calculations¹³ by using the Generalized Gradient Approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) functional found that the Sc₂C(OH)₂ has a direct band gap of 0.45 eV, while the Sc₂CF₂ and Sc₂CO₂ have indirect band gaps of 1.03 and 1.80 eV, respectively. As we known, the band gaps of these MXenes are underestimated due to the PBE functional. The hybrid functional of Heyd-Scuseria-Ernzerhof (HSE06) method was proven a reliable method in the calculation of electronic structures^15,16. Moreover, the effects of surface group on the electronic and optical properties of Sc₂CT₂ (T = -O, -OH, -F) MXenes and the possibilities of their photoelectronic applications are yet to be addressed.

In this paper, the first-principle calculations were performed to explore the structural, electronic and optical properties of Sc₂CT₂ (T = -O, -OH, -F) and to further discuss their probability for photocatalytic applications. The crucial characteristics of these Sc₂CT₂ were summarized, including the thermodynamic stabilities, band structures, optical absorptions, redox potentials and carrier mobility. The computational results suggest that Sc₂CF₂-I with -F group locating at the opposite position of outer metal atoms have appropriate band gaps of 1.91 eV, visible-light absorption and strong redox capability of photo-induced excitons. Based on our calculation results, the fluorinated Sc₂C MXene could be a promising candidate for high-performance photocatalysts.

Results

Thermodynamic stabilities of Sc₂CT₂ (T = -O, -OH, -F) MXenes

We firstly check the relative stabilities of the optimized MXenes with different geometries. The total energies (E _tot) of the optimized MXenes are listed in Table 1. In general, our results are consistent with previous report¹³. In three geometries, geometry-I structures of Sc₂CO₂, Sc₂C(OH)₂, Sc₂CF₂ usually have the lowest total energy, which demonstrates that geometry I is energetically more favorable than geometry II and III.

Table 1.

Total energy (E _tot in eV/unit cell) and Cohesive energies (E _coh in eV/atom) of the functionalized Sc₂CT₂ (T = -O, -OH, -F) MXenes.

MXenes	Total energy E tot			Cohesive energies E coh
geometry	Sc2CO2	Sc2C(OH)2	Sc2CF2	Sc2CO2	Sc2C(OH)2	Sc2CF2
I	−40.58	−49.13	−37.88	6.38	5.46	6.25
II	−39.24	−48.36	−36.67	6.11	5.35	6.01
III	−39.90	−48.77	−37.33	6.24	5.41	6.14

The most stable geometry is highlighted in bold.

We further calculated the cohesive energy to evaluate the stabilities of the investigated MXenes. The cohesive energy (E _coh) were defined as:

$$E_{coh}=\left(2n{E}_{Sc}+n{E}_C+2n{E}_T-n{E}_{S{c}_{2}C{T}_2}\right)/N$$ 1

where E _Sc, E _C, are the total energies of a single Sc atom and C atom, E _T is that of a single O, F atom or one O atom plus H atom, $E_{S{c}_{2}C{T}_2}$ denotes the total energy of one unit cell of Sc₂CT₂ monolayer. N is the number of atoms in the supercell. The results of cohesive energy calculations are presented in Table 1. According to our results, the cohesive energies of the investigated Sc₂CT₂ MXene range from 5.35 to 6.38 eV/atom, which are comparable to those of FeB₆ monolayer (5.56 ∼ 5.79 eV/atom)¹⁷, Al₂C monolayer has (4.49 eV/atom)¹⁸, and Be₂C monolayer (4.86 eV/atom)¹⁹. This indicates that the formation of strong chemical bonds between the Sc atom and T (T = -O, -OH, -F) groups. In view of the comparable E _coh values of Sc₂CT ₂ MXenes (5.35 to 6.38 eV) and FeB₆ monolayers (5.56 ~ 5.79 eV), it is proposed that the Sc₂CT₂ have the same type of chemical bonds with the FeB₆ monolayer, in which there is obvious electron transition from the Fe atom to B framework, and thus the ionic bonding between the Fe and B atoms. Therefore, the high E _coh values of Sc₂CT₂ MXenes also suggest the strong ionic bonds within them.

Generally, the structure with lower total energy usually imply the higher cohesive energy of the corresponding structure. In other word, the highest-cohesive-energy geometry should have the lowest total energy, and should be the energetically preferable structure. As shown in Table 1, The E _coh values increase with functional group in the order of Sc₂CO₂ > Sc₂CF₂ > Sc₂C(OH)₂. The largest E _coh values for O functionalized Sc₂C may due to the stronger interaction between the O and Sc atom, which results from the shorter bond length of Sc-O (2.00 Å) than those of Sc-F (2.21 Å) and Sc-OH (2.27 Å). The same trend holds for most of other transition metal carbides (e.g Ti, V, Cr, Mo, Hf)²⁰ Moreover, E _coh values are also affected by the geometric structure. The cohesive energies of Sc₂CT₂ MXenes are in the decreasing order of Geometry-I > Geometry-III > Geometry-II. The highest cohesive energy of Geometry-I indicates that Geometry-I is the most beneficial to the formation of strong chemical bonds in the Sc₂CT₂. With a definite functional group, the preference of Geometry-I rather than Geometry-II and Geometry-III is affected by several factors such as the number of electrons demanded by the functional groups, the hybridization strength between the functional groups and transition metal¹³.

Electronic structures and optical properties

According to the above results of cohesive energies, Geometry-I of Sc₂CO₂, Sc₂C(OH)₂, Sc₂CF₂ is the most stable structure. Therefore, we only calculated the electronic structures and optical properties of Sc₂CO₂-I, Sc₂C(OH)₂-I and Sc₂CF₂-I in order to find out whether they have appropriate characteristics for photocatalytic applications. Based on the HSE06 hybrid functional, the band structures of three Sc₂CT₂ (T = -O, -OH, -F) MXenes with geometry I are computed and presented in Fig. 1. We found that Sc₂CO₂-I is metallic, while the Sc₂C(OH)₂-I and Sc₂CF₂-I materials are semiconductors. Accordingly, the latter two MXene were selected for further investigation. The Sc₂C(OH)₂ presents a direct band gap of 0.81 eV with the valence band maximum (VBM) and the conduction band minimum (CBM) both located at Γ point, while Sc₂CF₂-I has an indirect band gap of 1.91 eV with the VBM located at Γ point and CBM at M point, which agree well with the previous computational results^20,21. The indirect band gap is beneficial to the restraint of electron-hole recombination in the photocatalytic reaction^22,23.

Figure 1.

Band structures of the functionalized MXenes with geometry I. The Fermi level is at 0 eV (the red dashed lines).

HSE06 hybrid functional combines one-quarter of the exact Hartree–Fock exchange energy with three-quarters of an approximate exchange-correlation energy. This combination in general produces more reliable band gaps than GGA-PBE. As expected, the values of band gaps obtained by our HSE06 computations are greater than those of Sc₂C(OH)₂ (0.45 eV) and Sc₂CF₂ (1.03 eV) in other calculation based on PBE functionals¹³. For Sc₂CO₂ materials, the Geometry-I structure with the lowest energy was selected for the comparison in our work, while the band structure of Geometry-III with the second lowest energy was computed in ref.¹³. Accordingly, the metallic property of Sc₂CO₂ in our work is not in accordance with the results in ref.¹³, in which the Sc₂CO₂ is semiconductor. The lowest energy of the Geometry-I indicates its best thermodynamic stability and the highest experimental feasibility, and accordingly, should be selected for the further evaluation of the photocatalytic activities.

We also calculated the dielectric constants $\epsilon \left(\omega \right)={\epsilon }_1\left(\omega \right)+i{\epsilon }_2\left(\omega \right)$ of Sc₂C(OH)₂-I and Sc₂CF₂-I to further confirm their optical absorption properties. The imaginary parts ${\epsilon }_2\left(\omega \right)$ of the dielectric constants consists of ${\epsilon }_{xy}\left(\omega \right)$ and ${\epsilon }_{zz}\left(\omega \right)$, which are the components perpendicular and parallel to the z direction, respectively. We can see from Fig. 2 that the Sc₂C(OH)₂-I has three obvious absorption peaks at 1.04 eV, 2.34 eV and 2.99 eV in the visible-light region (<3.0 eV). The imaginary part of Sc₂CF₂-I has one peak at 2.76 eV. The Sc₂CF₂-I also has obvious absorption in the visible-light region (1.99 eV to 2.76 eV), implying the harvest of the major portion of visible light. These evidences of the absorption spectra indicate their potential application in visible light driven photocatalysis and other photoelectronic process.

Figure 2.

Imaginary parts of dielectric constants of the Sc₂C(OH)₂-I and Sc₂CF₂-I.

From the viewpoint of photoelectrochemistry, the photocatalytic activity of the materials is also affected by the band edge position. The band edge positions were obtained by calculating the CBM and VBM energies relative to the vacuum level at 0 eV in our computations. Additionally, in order to evaluate the oxidation/reduction ability of the investigated materials, a normal hydrogen electrode (NHE) is selected as a reference, which potential is equal to −4.5 V with respect to the vacuum level (Fig. 3 and Table S1). Generally, the photocatalytic material should have photo-induced electrons with the stronger reduction capabilities if the CBM level is more negative and photo-activated holes in it should have the greater oxidation capabilities if the VBM energy is more positive.

Figure 3.

Band edge position of the Sc₂C(OH)₂-I and Sc₂CF₂-I. The redox potentials of H⁺/H₂ and H₂O/O₂ at PH = 0 are also provided as a reference (the green dashed lines).

As represented in Fig. 3, Sc₂C(OH)₂-I has a narrow band gap of 0.81 eV, which has absorption peak at the energies of 1.04 eV, 2.34 eV and 2.99 eV. However, the tiny band gap of it may result in the easier transition of the activated electrons from CBM to VBM, which lead to the fast recombination of photo-induced e^- -h⁺ pairs¹⁶. The Sc₂C(OH)₂-I with narrow bandgap may be utilized to form composites with other wide gap semiconductors, just like the graphene/TiO₂ ²⁴ and graphene/ZnO nanocomposites²⁵. These composites with well-matched band structures can broaden light absorption and enhance the separation of photo-generated electrons and holes. Compared to other composite structure^26–28, in which the great achievements have been obtained, the Sc₂C(OH)₂-I MXene may have its unique superiority. For instance, the -OH group at the surface of Sc₂C(OH)₂ MXene may be promising active sites for the effective adsorption of reactants, such as the organic molecules and heavy metal ions. The high carrier mobility and good electric conductivity of Sc₂C(OH)₂-I could lead to the efficient migration of the photo-activated electron and holes, which would effectively participate in the photoreaction on the surface of composite-structure photocatalysts.

The CBM position of the Sc₂CF₂-I is around −1.038 V (vs. NHE at PH = 0), which is more negative than the reduction potential of carbon dioxide (CO₂) into hydrocarbons, such as methane (CH₄), methanol (CH₃OH), formic acid (HCOOH). It is also more negative than the potential of H₂ evolution (0 V vs. NHE at PH = 0)¹⁶. Therefore, the Sc₂CF₂-I can be used for photocatalytic reduction of CO₂ or photocatalytic generation of hydrogen.

In semiconductors, the valence-band holes that have chemical potential of +1.0 to +3.5 V (vs. NHE) are powerful oxidants, while the conduction-band electrons with chemical potential of +0.5 to −1.5 V (vs. NHE) are good reductants²⁹. The VBM (vs. NHE) of the Sc₂CF₂-I MXene is 0.87 V. So the Sc₂CF₂-I MXene also has oxidation capacity at a certain extent. Furthermore, the band edge position can be effectively modified by approaches of Doping^30,31, tensile strain³² and heterojunctions³³ to reach the requirement of the photocatalytic process.

Migration mechanism of the carrier

It is necessary to calculate the spatial charge distribution because most photo-induced electrons and holes will transfer to spatial locations of CBM and VBM, respectively. Figure 4a shows the spatial charge distribution of the Sc₂C(OH)₂-I. the CBM predominantly distribute around OH layer, while the VBM are located at the carbon layer. Regarding to the spatial charge distribution of the Sc₂CF₂-I in Fig. 4b, CBM mostly distribute around Sc atoms, the distribution of VBM is similar to that of the Sc₂C(OH)₂-I. These different locations of CBM and VBM could lead to spatial separation of photo-activated electron and holes and inhibit the recombination of electron-hole pairs.

Figure 4.

Spatial charge distributions for (a) Sc₂C(OH)₂-I (b) Sc₂CF₂-I.

We also computed the electron localization function (ELF)³⁴ of the Sc₂C(OH)₂-I and Sc₂CF₂-I to analyze their electron distributions. Since the electron localization function (ELF)³⁴ of the Sc₂C(OH)₂-I and Sc₂CF₂-I is similar, we only list the result of the Sc₂CF₂-I for analysis. From ELF map in Fig. S2, the red region around the C atoms indicates that the electrons are mainly located around the C atoms and there is remarkable electron transfer from Sc to C atoms. As a consequence, C atoms can act as the negative charge center and efficiently capture the photo-activated holes in Sc₂CF₂-I material, which results in the efficient separation of photo-generated e^- -h⁺ pairs.

The carrier mobility is another important factor affecting photocatalytic activity. We calculated the carrier mobility of the Sc₂C(OH)₂-I and Sc₂CF₂-I along x-direction and y-direction on the basis of the hexagonal cell (Fig. S3). The results of the carrier mobility are shown in Fig. 5 and the related calculation details are presented in Fig. S4 and Table S2. As shown in Fig. 5, we found that the electron mobilities of Sc₂C(OH)₂-I and Sc₂CF₂-I are 1558 cm² V⁻¹ s⁻¹ and 1288 cm² V⁻¹ s⁻¹ along x direction, as well as 1230 cm² V⁻¹ s⁻¹, 520 cm² V⁻¹ s⁻¹ along y direction, respectively, which are comparable to those of monolayer phosphorene (1140 cm² V⁻¹ s⁻¹ along x direction and 80 cm² V⁻¹ s⁻¹ along y direction)²⁷, Ti₂CO₂ monolayer MXene (611 cm² V⁻¹ s⁻¹)³⁵, and MoS₂ (about 200 cm² V⁻¹ s⁻¹)³⁶. These results reveal the photo-induced electron can quickly migrate to the active site of photocatalytic reaction on the investigated MXenes.

Figure 5.

The carrier mobility of the Sc₂C(OH)₂-I and Sc₂CF₂-I along x-direction and y-direction.

Furthermore, in both the Sc₂C(OH)₂-I and Sc₂CF₂-I, the electron mobility is much higher than the hole mobility in x or y direction, which can be ascribed to the faster migration of electrons within the Sc₂C(OH)₂-I and Sc₂CF₂-I. The great difference between the mobility of electron and holes should result in different transport efficiency of photo-activated electron and holes within these materials. For this reason, when the photo-induced electrons reach the surface, the holes should still stay in the inner layer. This property should be valuable for the separation of electrons and holes during the photocatalytic process.

Discussion

By using first-principle computation, we have systematically investigated the functional-group-dependent structural, electronic and optical properties of Sc₂CT₂ (T = -O, -OH, -F) MXenes. We found that geometry-I of Sc₂CT₂ is the most stable one in all three geometries, which is selected to the further studies on the evaluation of their photocatalytic activity. The Sc₂CO₂-I is metallic, while the Sc₂C(OH)₂-I and Sc₂CF₂-I are semiconductors with band gaps of 0.81 eV and 1.91 eV, respectively. Both the Sc₂C(OH)₂-I and Sc₂CF₂-I exhibit very good optical absorption in the visible-light region. The presence of absorption peaks in the visible light region indicates their potential application as photocatalysts.

Additionally, the Sc₂CF₂-I MXene has an excellent reduction potential of −1.038 V (vs. NHE) and could be used for photocatalytic reduction of CO₂ or for photocatalytic hydrogen generation. The photo-induced holes within it is also a good oxidant with redox potential of 0.87 V (vs. NHE). Through further studying the migration mechanism of the carrier, we confirmed that both Sc₂C(OH)₂-I and Sc₂CF₂-I have the efficiently separated electron-hole pairs and good carrier mobility. Relatively, the Sc₂CF₂-I can be more conducive to the visible-light photocatalysis due to its more suitable band gap and band edge alignment. Our results provide some fundamental data on the possible MXene photocatalysts, which is helpful in further exploring the practical applications of MXenes.

Computational Method

For the first-principles calculations, the Vienna Ab-initio Simulation Package (VASP) were employed³⁷. The GGA with the PBE³⁸ functional was adopted for the exchange-correlation functional. The ion-electron interaction was treated by the projector augmented wave (PAW) method³⁹. The energy cutoff was set to 600 eV and the energy precision of the computations was 10⁻⁵ eV. The atomic positions were fully relaxed until the maximum force on each atom was less than 10⁻³ eV/Å. In the geometry optimization and self-consistent computations, a 9 × 9 × 1 Monkhorst-Pack k-point grid centered on the à point was used for the Brillouin zone. All of the band structures were calculated with the HSE06 hybrid functional⁴⁰. The electronic structures were visualized using the VESTA code⁴¹.

The optical absorption properties of the Sc₂CT₂ were investigated by computing the complex dielectric constants (ε) at a given frequency (ω) by using the HSE06 hybrid functional with a 21 × 21 × 1 k-point mesh. The dielectric constants can be defined as $\epsilon \left(\omega \right)={\epsilon }_1\left(\omega \right)+i{\epsilon }_2\left(\omega \right)$. The absorption coefficient I(ω) can be calculated through following equation⁴²:

$$I\left(\omega \right)=\sqrt{2}\omega {\left[\sqrt{{\epsilon }_1{\left(\omega \right)}^2+{\epsilon }_2{\left(\omega \right)}^2}+{\epsilon }_1\left(\omega \right)\right]}^{1/2}$$ 2

As shown in the equation (1), only if the imaginary part ${\epsilon }_2\left(\omega \right)>0$, $I\left(\omega \right)$ will be above zero. Therefore, the imaginary part ${\epsilon }_2\left(\omega \right)$ reflects the light absorption at a given frequency. The imaginary part can be determined as⁴³:

$$\begin{array}{ccc}\hfill {\epsilon }_{\alpha \beta }^{\left(2\right)}\left(\omega \right)& \hfill =\hfill & \frac{4{\pi }^2{e}^2}{\mathrm{\Omega }}\underset{q\to 0}{\lim }\frac{1}{q^2}\sum _{c,\nu ,\stackrel{\rightharpoonup }{k}}2{\omega }_{\stackrel{\rightharpoonup }{k}}\delta \left({\epsilon }_{c\stackrel{\rightharpoonup }{k}}-{\epsilon }_{\nu \stackrel{\rightharpoonup }{k}}-\omega \right)\hfill \\ \hfill & \hfill \hfill & \times ⟨u_{c\stackrel{\rightharpoonup }{k}+e_{\alpha }\stackrel{\rightharpoonup }{q}}\mid u_{\nu \stackrel{\rightharpoonup }{k}}⟩{⟨u_{c\stackrel{\rightharpoonup }{k}+e_{\beta }\stackrel{\rightharpoonup }{q}}\mid u_{\nu \stackrel{\rightharpoonup }{k}}⟩}^{*}\hfill \end{array}$$ 3

here the indices c and í are restricted to the conduction and valence band states and $u_{c\stackrel{\rightharpoonup }{k}}$ is the cell periodic part of the wavefunctions at the k-point.

The carrier mobility of the Sc₂CT₂ was calculated which is based on deformation potential theory⁴⁴ with the following Equation^45–47:

$${\mu }_{2D}=\frac{2e{\hslash }^{3}C}{3k_{B}T{\mid m^{*}\mid }^2{E}_1^2}$$ 4

here the temperature of T = 300 K was adopted in this study; e, $\hslash$ and k _B are electron charge, the reduced Planck constant and the Boltzmann constant, respectively. C is the elastic modulus under the uniaxial strain along the transport direction, given by $C=({\partial }^2{E}_{total}/\partial {\epsilon }^2)/S_0$, where $E_{total}$ is the total energy of a unit cell under different uniaxial strain å and S ₀ is the area of the unit cell in the xy plane; m ^* is the effective mass of the carrier along the transport direction, which is determined by $m^{*}={\hslash }^2/({\partial }^{2}E\left(k\right)/\partial k^2)$; E ₁ is the deformation potential (DP) constant of VBM for holes and CBM for electrons, calculated by using $E_1=\partial E_{edge}/\partial \epsilon$. E _edge is the energy of VBM or CBM along the transport direction.

It was well identified in previous works^13,48 that each functionalized Sc₂C MXene has three different geometries on its surface. We take the Sc₂CO₂ as an example to illustrate them. The top view and side view of Sc₂CO₂ structure are shown in Fig. 6, Sc₂C(OH)₂ and Sc₂CF₂ are shown in Fig. S1 (Supporting Information). As seen in Fig. 1a, Bare Sc₂C MXene has one carbon layer sandwiched by two scandium layers. The functionalized Sc₂C MXenes have three configurations: the geometry I with surface O atoms located above the opposite-side Sc atoms (Fig. 6b, Sc₂CO₂-I), the geometry II with two terminated O atoms located on the middle C atoms (Fig. 6c, Sc₂CO₂-II), as well as the geometry III with one terminated O atom lies on the top of the opposite Sc atom and another one lies on the top of the middle C atom (Fig. 6d, Sc₂CO₂-III). A vacuum space of 20 Å was inserted to avoid any interaction between MXene layers along the z axis.

Figure 6.

Top view (upper) and side view (lower) of the geometries for (a) bare Sc₂C; (b) Sc₂CO₂-I; (c) Sc₂CO₂-II; (d) Sc₂CO₂-III.

Author Contributions

Haijun Zhang conceived the idea, designed the study and improved the manuscript. Kuangwei Xiong, Zhongfei Liu and Xin Xu performed the calculation process and wrote the paper. Guang Yang provided and compiled the Vienna Ab-initio Simulation Package. Shaowei Jin and Peihong Wang prepared and improved the Figures and Tables. All authors reviewed the manuscript.

Competing Interests

The authors declare that they have no competing interests.