Etching and Exfoliation Properties of Cr2AlC into Cr2CO2 and the

May 31, 2019 - A common approach to synthesize MXenes is etching the MAX phase into ... the large family of MXenes, especially for their electrochemic...
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Etching and Exfoliation Properties of Cr2AlC into Cr2CO2 and the Electrocatalytic Performances of 2D Cr2CO2 MXene Yuwen Cheng,†,‡ Lijuan Wang,‡ Yue Li,‡ Yan Song,*,‡ and Yumin Zhang*,† †

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National Key Laboratory of Science and Technology for National Defence on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150001, PR China ‡ School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, 2 West Wenhua Road, Weihai 264209, PR China S Supporting Information *

ABSTRACT: In the recent years, two-dimensional (2D) metal carbide and nitride (MXenes) materials have shown characteristics of promising electrocatalytic properties for hydrogen evolution reaction (HER). Here, by using density functional theory calculations, the etching and exfoliating properties of Cr2AlC to Cr2CO2 and electrocatalytic (for HER) properties with and without carbon defect were studied. Results show that etching the pristine Cr2AlC by HF solutions could intend to generate Cr2C MXenes with O* termination, that is, Cr2CO2, and Cr2CF2 and Cr2C(OH)2 will translate into Cr2CO2 even if they were generated first during the etching reactions. The exfoliation energy of multilayer Cr2CO2 into monolayer Cr2CO2 MXene is large (2.78 eV/nm2), and the delamination process requires Li+ (LiF) cation. Carbon vacancy is easily generated during the etching and exfoliation reactions, and the formation energy of carbon vacancy (1.71 and 1.59 eV with and without considering charge correction) of Cr2CO2 is lower than that of common 2D materials, such as graphene (7.4 eV) and MoS2 (2.12 eV for forming S vacancy and 6.2 eV for forming Mo vacancy). HER performances of Cr2CO2 were further studied considering the solvent effect. The studies indicated that the solvent can affect the performance of HER of Cr2CO2 at medium hydrogen coverage. Gibbs free energy of hydrogen adsorption (ΔGH*) increases at low hydrogen coverage, then reduces at medium hydrogen coverage, and finally increases at high hydrogen coverage. These results provide a guideline for experimentally synthesizing the 2D MXene materials and developing new promising HER catalysts for water splitting.

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intercalated metal cation solutions, and finally centrifuging and drying the monolayer MXene. However, till now, only a small part of MXenes has been synthesized successfully, such as Ti3C216 and Mo2C.10 Thus, there is a lack of knowledge about the large family of MXenes, especially for their electrochemical properties, which are significantly important for energy conversion and storage applications. Moreover, the electrochemical properties of Ti3C2 and Mo2C are poor compared with that of MoS2 or Pt. Therefore, more works are needed for clarifying the electrochemical properties of MXenes. Compositing with other materials is a promising way to enhance the electrochemical performances of Ti3C2Tx.17−23 The composited Ti3C2Tx shows enhanced electrochemical properties (capacitance or cycle number) than pure Ti3C2Tx. However, the compositing approach is complex and timeconsuming, which hinders the large-scale applications of 2D MXenes. Therefore, it is important to understand the synthetic process, such as how the three functional groups, O, F, and

INTRODUCTION Two-dimensional (2D) materials have large surface area and extreme thinness, making them relatively easy to tune their surface properties, and thus show vast potential applications as functional and structural materials with novel electronic and excellent mechanical properties.1 Especially, the large specific surface area of 2D materials can afford a great number of active sites for hydrogen evolution reaction (HER),2 opening a large window to design promising HER electrocatalysts. In the recent years, 2D MXenes with the general formula of Mn+1XnTx (n = 1−3; M = early transition metal, e.g., Ti, Cr, and Mo; X = C or N; and Tx is the surface functional group of OH, O, or F) have captured more attention because of their excellent mechanical and electronic properties.3−9 The presence of the surface functional group Tx can enhance the interaction of hydrogen with MXenes, create new active sites, expand the reaction area further,10,11 and result in MXenes that can be promising catalysts for HER. Since the discovery of MXenes, the HER activities of different O-terminal 2D MXenes have been studied substantially.12−15 A common approach to synthesize MXenes is etching the MAX phase into multilayer MXenes first by HF solutions, then exfoliating the multilayer MXenes into monolayer MXene by © 2019 American Chemical Society

Received: April 3, 2019 Revised: May 31, 2019 Published: May 31, 2019 15629

DOI: 10.1021/acs.jpcc.9b03120 J. Phys. Chem. C 2019, 123, 15629−15636

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The Journal of Physical Chemistry C

molecule, respectively.32 The binding energy of Al in Cr2AlC is obtained through eq 3

OH, exist under different HF solutions and how the solvent affects the HER performances, and so on. Therefore, in this work, taking Cr2AlC and Cr2CO2 as examples, we utilized density functional theory (DFT) calculations to predict the etching and exfoliation properties of Cr2AlC to Cr2CO2, formation and influence of the carbon vacancy, and its influences on the electrocatalytic properties of Cr2CO2. It shows that etching the pristine Cr2AlC by HF solutions could intend to generate Cr2C MXenes with O* termination, that is, Cr2CO2. The exfoliation energy of Cr2C MXenes with different terminations is estimated, and the results indicated that the exfoliation energy for delaminating the multilayer Cr2CO2 into monolayer Cr2CO2 MXenes is large (2.78 eV/nm2), and the delamination process requires intercalation of metal cations, such as Li+ (LiF) to isolate the multilayer Cr2CO2 MXenes. Moreover, the formation of carbon vacancy in the monolayer Cr2CO2 is calculated. Carbon vacancies are easily generated during the etching and exfoliating reactions, and the formation energy of carbon vacancy is lower than that of common 2D materials, such as graphene (7.4 eV) and MoS2 (2.12 eV for forming S vacancy and 6.2 eV for forming Mo vacancy). HER performances of Cr2CO2 were further studied considering the solvent effect. The results show that the solvent can affect the HER performances of Cr2CO2 at medium hydrogen coverage. Gibbs free energy of hydrogen adsorption (ΔGH*) increases at low hydrogen coverage, then reduces at medium hydrogen coverage, and finally increases at higher hydrogen coverage.

E b2 = {E(Cr2AlC) − E(Cr2C) − E(Al)}

(3)

where E(Cr2AlC), E(Cr2C), and E(Al) are the total energies of the pristine Cr2AlC MAX phase, Cr2C MXene, and metal Al in its ground state, respectively. For a 2 × 2 × 1 supercell, the formation energy of carbon vacancy is evaluated via eq 4 Ef = E(Cr8C3O8) + E(C) − E(Cr8C4O8)

(4)

where E(Cr8C4O8) and E(Cr8C3O8) are the total energies of the 2 × 2 × 1 Cr2CO2 substrates without and with a carbon vacancy, respectively, and E(C) is the total energy per carbon atom derived from the graphene state.33

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RESULTS AND DISCUSSION Etching and Exfoliation Properties of Cr2AlC to Cr2C MXene. The bare Cr2C MXene is packed in face-centered cubic (fcc) arrangement with two exposed metal layers, and each C atom bonds with six Cr atoms as well as other 2D MXenes, displaying the P3̅m1 symmetry, as shown in Figure 1.

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COMPUTATIONAL DETAILS All calculations were performed with the Vienna ab initio Simulation Package code24,25 under the framework of DFT with the Perdew−Burke−Ernzerhof26 form of generalized gradient approximation potential.27 The projector augmentedwave28 method was applied to accurately determine the electronic structures, and the van der Waals interaction was considered by using the empirical correction in Grimme’s scheme, that is, DFT + D3,29 in all calculations except the Cr2AlC system. The electronic configurations are [Ar]3p63d54s1 for Cr, [Ne]3s23p1 for Al, 1s22s1 for Li, 1s22s22p5 for O, 2s22p2 for F, 2p4s2 for C, and 1s1 for H. A 2 × 2 × 1 supercell of Cr2CO2 was employed to investigate the adsorption of hydrogen and the termination properties of Cr2CO2. The 2 × 1 × 1 supercell of Cr2AlC is used to clarify the exfoliation property of Cr2AlC. The energy cutoff is set to 500 eV for all cases. The convergence tolerance for the residual force of each atom and the energy during structural relaxation are set to 0.01 eV/Å and 10−5 eV, respectively. The vacuum space in the z-direction is greater than 20 Å to avoid the interaction caused by the periodic boundary condition. The Gibbs free energy of hydrogen adsorption (ΔGH*) is evaluated via eq 1 ΔG H * = ΔE H + ΔEZPE − T ΔSH

Figure 1. Atomic arrangement of the Cr2CTx MXene structure with (a) three possible functional group sites (top view) and (b) surface termination (side view).

Till now, information about the synthesis of Cr2C MXene is lacking. On the basis of the studies of Ti3C2 MXene,34−36 a possible approach to synthesize Cr2C MXene is etching Cr2AlC in hydrofluoric (HF) acid solution, removing the Al layers and leaving the multilayer Cr2C. Then, the monolayer Cr2C MXene is synthesized by exfoliating the Cr2C multilayer. The schematic of etching Cr2AlC to the Cr2C multilayer is shown in Figure 2. On the basis of the literature,3 we propose a possible synthetic route to etch Cr2AlC through the following reactions (eqs 5−7)

(1)

where ΔEH, ΔEZPE, and TΔSH are the differences of the hydrogen adsorption energy, zero-point energy, and the entropy between the adsorbed hydrogen and hydrogen in gas phase, respectively.14,30,31 The binding energy of surface oxygen with Mn+1Cn substrates is defined by eq 2 E b1 = {E(Cr2CO2 ) − E(Cr2C) − E(O2 )}/2

R1: Cr2AlC(s) + 5HF(aq) → Cr2CF2(s) + AlF3(aq) + 5/2H 2(g)

(2)

(5)

R2: Cr2AlC(s) + 3HF(aq) + 2H 2O(aq)

where E(Cr2CO2), E(Cr2C), and E(O2) are the total energies of the oxygen-terminal MXene, pristine MXene, and oxygen

→ Cr2CO2 (s) + AlF3(aq) + 7/2H 2(g) 15630

(6)

DOI: 10.1021/acs.jpcc.9b03120 J. Phys. Chem. C 2019, 123, 15629−15636

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OH*) to be occupied. The schematic of etching Cr2AlC into multilayer Cr2CTx is shown in Figure S2. The binding energies of the three functional groups (O*, F*, and OH*) on Cr2C MXene are calculated and shown in Table S1. It shows that the hcp site owns the lowest binding energy among the three possible sites, which is consistent with the latest experimental findings that it reaches the lowest binding energy when Tx was adsorbed at the hcp site for Cr2C with three different terminations.10 The calculated binding energies (via eq 2) are −3.64, −4.23, and −1.01 eV for the F*, O*, and OH* terminal Cr2C, respectively, suggesting that the system with O* termination is more stable than that of the F* and OH* terminals. There is another possibility that the reactions are carried out in solution, and the relevant O (F) chemical potential would be like that of H2O−H2 (HF−H). The calculated binding energies are −0.88, −1.35, and −1.01 eV, respectively, for F*, O*, and OH* terminal Cr2C based on the above hypothesis. The O* terminal system is still the most stable one than that of the F* and OH* terminal systems. The Bader charge37 analysis for Cr, Al, C, O, and H in Cr2AlC, Cr2CO2, and Cr2C(OH)2 is performed and the results are shown in Table 1. It shows that Cr atoms will lose electrons

Figure 2. Schematic of etching Cr2AlC phase into the Cr2C multilayer. The light blue, brown, red, pink, silver gray, and light pink balls represent Cr, C, O, Al, F, and H atoms, respectively.

Table 1. Valence Electrons of Cr, Al, C, O, F, and H Atoms in the Cr2AlC, Cr2CO2, Cr2CF2, and Cr2C(OH)2 Systems Based on the Bader Charge Analysis

R3: Cr2AlC(s) + 3HF(aq) + 2H 2O(aq) → Cr2C(OH)2 (s) + AlF3(aq) + 5/2H 2(g)

(7)

which will generate Cr2CF2, Cr2CO2, and Cr2C(OH)2 from R1−R3, respectively. HF and H2O are referred as the gas phases, and AlF3 is referred as the solid phase for calculation. The calculated reaction energies (ΔE) of R1, R2, and R3 are −6.73, −7.19, and −0.47 eV, respectively, via the definition of ΔE = Eproducts − Ereactions. It can be seen that the three reactions are exothermic, and ΔE of R2 is much lower than that of R1 and R3, suggesting that the O* terminal Cr2CO2 is more favorably formed during the etching Cr2AlC phase. In addition, we also considered the alternative reactions, that is, Cr2CF2 is generated first and then translates into Cr2CO2 or Cr2C(OH)2 via reactions 4−6 as below. (8)

R5: Cr2CF2(s) + 2H 2O(aq)

R6: Cr2C(OH)2 → Cr2CO2 (s) + H 2(g)

Cr (e)

C (e)

+0.87 +0.14 +1.53 +1.48 +1.38

−1.74 −1.63 −1.24 −1.58 −1.39

Al (e)

group functional (e)

+1.49 −0.91 (O*) −0.69 (F*) −1.67 (O*), +0.98 (H*)

during etching with 0.14e valence electrons in Cr2AlC, changing to 0.87, 1.53, 1.48, and 1.38e in the Cr2C, Cr2CO2, Cr2CF2, and Cr2C(OH)2 systems, respectively, and the corresponding oxidation states are about +0.1, +0.9, and +1.5 in the pristine Cr2AlC, Cr2C, and Cr2CO2 MXene systems, respectively, indicating that the reactions of R1−R3 and R4−R6 are of redox type. Furthermore, the calculated binding energy (via eq 3) of Al in the Cr2AlC system is 1.24 eV, meaning that the etching reaction of Cr2AlC could be easily achieved in experimental synthetics. For the surface terminal functional groups, the O atoms in Cr2CO2 receive 0.91e electrons, whereas the F atoms in Cr2CF2 receive 0.69e electrons. The O and H atoms in Cr2C(OH)2 receive and lose 1.67e and 0.98e electrons, respectively. It also illustrates that the net electrons captured by O are 0.69e (Δe = 1.67e − 0.98e). Obviously, the binding strength between O* and Cr2C is stronger (O received more electrons) than that of the F* or OH* functional groups. The above results indicate that the etching reaction of Cr2AlC to Cr2C could easily occur, and the produced Cr2C tends to form O* termination, that is, Cr2CO2. To further study the reaction process of Cr2AlC etching in HF acid solution, we calculated the layer distance, defined as the distance between two adjacent Cr metal layers of the Cr2AlC system and the bond lengths of Al−F and Cr−Al bonds at different F concentrations. The results are shown in Figure 3a,b and Tables S2 and S3. In this study, in a 4 × 1 Cr2AlC supercell with the surface area of 0.29 nm2, one F atom bonds to Al atom and forms Al−F bond; thus, the HF (F−) concentration can be evaluated as 3/nm2 in terms of the

R4: Cr2CF2(s) + 2H 2O(aq) → Cr2C(OH)2 + 2HF(aq)

→ Cr2CO2 + 2HF(aq) + H 2(g)

systems Cr2C Cr2AlC Cr2CO2 Cr2CF2 Cr2C(OH)2

(9) (10)

The calculated ΔE of R4, R5, and R6 are 0.19, −0.16, and −0.35 eV, respectively. R4 is endoergic, whereas R5 and R6 are exoergic, meaning that Cr2CO2 will be the final product even if Cr2CF2 or Cr2C(OH)2 was generated first during the etching reactions. The reaction barriers of R1−R3 are calculated by the climbing-image nudged elastic band (CI-NEB) method, and the results are presented in Figure S1. It shows that Cr2CO2 is energetically generated among the three terminal structures. A detailed discussion is presented in the Supporting Information. The multilayer 2D Cr2C MXenes will form after etching Cr2AlC by HF solution. The exposed metal layers are electron donors and could attract functional groups during etching. There are three potential sites, top, fcc, and hexagonal closepacked (hcp) sites, for the functional groups (O*, F*, or 15631

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exfoliation energy of Cr2CO2 (with 2 × 2 supercells). The exfoliation energy of Li-intercalated Cr2CO2 is defined as Ex = {ECr2CTx − Li − 2ECr2CTx − E Li}/4S

(11)

where ECr2CTx−Li, ECr2CTx, and ELi are the total energies of multilayer Cr2CTx with and without Li intercalation, and the pure Li metal, respectively.43 The surface area of S = 3a2/2, where a is the lattice parameter of Cr2CTx. Here, each unit of Cr2CTx will be exfoliated into two MXene monolayers generating four surfaces. The results are shown in Figure 3c,d. For C2C MXenes, the calculated exfoliation energy is the lowest for T = F* surface (1.47 eV/nm2) and highest for T = O* (2.78 eV/nm2), indicating that the F* terminal surface (Cr2CF2) can facilitate its exfoliation. However, the above analysis indicates that Cr2CF2 is unstable and will translate into Cr2CO2 in acid conditions, meaning the exfoliation of Cr2CF2 may not have occurred or there is a competition between exfoliation and translation. For the O* terminal Cr2CO2, the larger radius of oxygen ions than that of fluorine leads to higher polarization ability and a stronger interaction between the Cr2CO2 layers. The exfoliation energy of Cr2CO2 is larger than that of graphite (about 1 eV/nm2),44 which exactly explains that the approach such as metal cation intercalation is required to isolate multilayer Cr2CO2 MXenes. With an increasing concentration of intercalated Li, the exfoliation energy decreases first and then increases (Figure 3d). The lowest exfoliation energy is −3.01 eV/nm2, appearing at the number of Li ions equal to 5 for 2 × 2 Cr2CO2 supercells, and the corresponding LiF/Cr2CO2 ratio is 1.25, the optimal value of LiF/Cr2CO2 in terms of obtaining an isolated monolayer Cr2CO2 MXene through intercalating Li cations. The schematic of exfoliation of mutilayer Cr2CO2 into monolayer Cr2CO2 MXene is shown in Figure S4. The DFT-D245 method is also widely used to calculate the van der Waals interactions of 2D materials; we have calculated the exfoliation energy of Cr2CO2 without intercalated LiF by DFT-D2 and compared these results with those calculated by DFT-D3, and the results and discussions are presented in the Supporting Information. Carbon Defect in Cr2CO2 MXene. Atomic defects, such as carbon vacancy, are inevitable during the etching of Mn+1AlCn and the intercalated multilayer Mn+1CnO2, which have been observed in Ti3C2 MXene in experimental synthetics.40 Thus, we further calculated the formation energy of carbon vacancy (via eq 4) in Cr2CTx MXene and compared with the other 2D materials. The results are shown in Figure 4a. The schematic of Cr2CO2 with carbon vacancy (denoted as Cr2CO2-VC) is shown in Figure 4b−d. Owning to the distortion of the symmetry of Cr2CO2 by the presence of carbon vacancy, Cr atoms in Cr2CO2-VC can be categorized into two different types, the hexagonal-coordinated Cr (h-Cr, in dark cyan) and the pentagonal-coordinated Cr (p-Cr, in green). The calculated formation energies (Ef) of the carbon vacancy in Cr2C, Cr2CO2, Cr2CF2, and Cr2C(OH)2 are 3.56, 3.01, 1.59, and 2.66 eV, respectively, which refer to the energy per carbon atom in graphene state. It can be seen that the formation energy of carbon vacancy in the studied systems is lower than that in graphene (7.4 eV) and other 2D materials (Figure 4a),46−51 indicating that the formation of vacancy in Cr2CT2 MXene is energetically favorable. The Ef of carbon vacancies in Cr2C and Cr2CT2 is estimated by referring to the energy per carbon atom in graphite state, and the Ef of some 2D materials under the same calculation conditions was also

Figure 3. (a) Binding energy of Al with F and (b) layer distance (distance between two Cr metal layers) against the F concentration; (c) exfoliation energies of Cr2CO2, Cr2CF2, and Cr2C(OH)2 without intercalated LiF and (d) variation of exfoliation energies of Cr2CO2 against the number of LiF.

number of F atoms contained in the supercell. With the HF concentration increasing, the binding energy of F with the Al layer is decreased (Figure 3a), which means that high HF is in favor of weakening the bonding strength between Al and F and promotes the etching reaction. The layer distances of Cr2AlC are 4.21 and 4.26−5.71 Å before and after etching (Figure 3b), respectively. It shows that the higher the concentration of HF solution, the larger is the layer distance of the Cr2AlC system. The schematic of the bond lengths of Al−F bond, dAl−F, and Cr−Al bond, dCr−Al, is displayed in Figure S3, and the values of dAl−F and dCr−Al at different HF coverages are presented in Tables S2 and S3, respectively. Before etching, dAl−F is 1.72 Å, but it elongates to 1.83−2.09 Å after etching, whereas dCr−Al is 2.67 Å before etching but altered to range from 2.53 to 2.87 Å and 2.61 to 3.95 Å after etching. Both dAl−F and dCr−Al are increased with increasing HF coverage, and the amplitude of increase of dCr−Al is larger than that of dAl−F, indicating that dCr−Al is more sensitive to the HF coverage than that of dAl−F. Moreover, over half of the Cr−Al bonds are larger than 3.4 Å, and the typical dCr−Al value in the HF coverage is more than 6/ nm2. Thus, the Cr−Al bonds will be broken when the HF coverage is over 6/nm2. The optimal dAl−F in AlF3 is about 1.83 Å; one can see that when the HF coverage is in the range from 3 to 6/nm2, the bond lengths of most Al−F bonds are around 1.83 Å. Too higher HF coverage is not favorable for Al−F bonding (dAl−F is higher than the normal values). For 2D materials, delamination is the necessary step to explore their properties. The multilayer MXenes have two sixfold stronger interlayer interactions than those in MoS2 and graphite; thus, the mechanical exfoliation of multilayer MXene into monolayer MXene is not effective.38 Previous works showed that MXenes, such as Ti3C2Tx, can be intercalated with different metal cations using the aqueous solutions of ionic compounds.39,40 When etching Ti3AlC2 with a mixture of fluoride salt and acid (e.g., HCl and LiF), the multilayer Ti3 AlC2 can be exfoliated into monolayer Ti 3 C2 .41,42 According to these previous findings, we selected Li as the metal cation source and studied the exfoliation properties of Cr2CO2 with and without Li intercalation by calculating the 15632

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for application in many fields, especially in energy conversion applications, such as electrocatalysts for HER. Therefore, the HER performances of Cr2CO2 MXene is further investigated. HER usually occurs on the surface of catalysts, that is, forming a water−solid interface; therefore, the solvent effect should be considered properly. In this paper, we adopted the approach previously used to study the reaction mechanism of HER on Pt(111)58 to model the water−solid interface under electrochemical potentials. The Helmholtz layer (interface region; see in Figure 6a,b) is often approximated by about 3 Å thick electrical double layers and can be replaced by one water layer. The acid concentrations of the water−solid interface can be modeled by adding protons into water. There are three different initial structures of the water−solid interface. In model A, the H atom (proton) of each H2O molecule (H3O+) points down toward the Cr2CO2 surface (Figure 5a); in model

Figure 4. (a) Formation energies of carbon vacancy in Cr2CT2 (T = O, F, and OH) and other 2D materials; (b) top view, (c) enlarged top view, and (d) side view of the Cr2CO2-VC structure are illustrated via the 6 × 6 supercells; (e) DOSs of Cr2C and Cr2CT2 and (f) PDOSs of p-Cr and h-Cr in Cr2C and Cr2CO2.

calculated. The results and discussion are provided in the Supporting Information. The results show that Ef of carbon vacancy in Cr2CT2 MXene is energetically favorable. The Ef of carbon vacancy in Cr2C, and Cr2CT2 are also calculated by considering the charge correction52,53 of carbon vacancy, and the results are presented in Figure S9. It also shows that the Ef of carbon vacancy in Cr2CO2 is the lowest (1.71 eV) among the studied 2D materials. On the basis of these evaluations, we further calculated the densities of states (DOSs) of Cr2C-VC, Cr2CO2-VC, Cr2CF2-VC, and Cr2C(OH)2-VC (Figure 4e,f). From Figure 4e, one can see that the DOSs of carbon vacancycontained systems shift to lower energy slightly with respect to the systems without carbon vacancy, whereas the DOSs lying in the conduction band are almost unchanged. The partial densities of states (PDOSs) of Cr in Cr2C-VC and Cr2CO2-VC are shown in Figure 4f. It can be found that the PDOSs of p-Cr and h-Cr are identical, indicating that the emerging carbon vacancy has little effect on the hybridization between Cr d orbitals and C p orbitals. The valence electrons of Cr (1.50 and 1.56e), C(−1.46 and −1.45e), and O (−0.94 and −1.06e) in the Cr2CO2-VC system are listed in Table S4, whereas their corresponding values in pure Cr2CO2 are +1.53, −1.24, and −0.91e, respectively. Therefore, the presence of carbon vacancy in Cr2CO2 will rarely alter the electronic properties of Cr2CO2 MXene. Besides, we also calculated the formation energy of Cr vacancy in Cr2C and Cr2CO2, and the results show that the Ef of Cr vacancy is unstable with larger formation energy than that of C vacancy. More details are given in the Supporting Information. Properties of HER. Recently, the electrochemical properties of some 2D MXenes have been reported in the literature,54−57 illustrating that they are potential candidates

Figure 5. Structure of water−solid interface models: (a) model A, (b) model B, (c) model C, and (d) representative structure used in calculating ΔGH*.

B, the H atom (proton) of each H2O molecule (H3O+) points away from the Cr2CO2 catalyst surface (Figure 5b), and in model C, the molecular plane of H2O molecules is parallel to the catalyst surface (Figure 5c). After relaxation, model A has the relatively lowest energy (−1.7 eV) compared with that of model B (−0.9 eV) and model C (0 eV); thus, subsequent studies were carried out on model A (the representative structure is shown in Figure 5d). The calculated Gibbs free energies of hydrogen adsorption (ΔGH*) of the water−solid interface model are shown in Figure 6c. The concentration of hydrogen ions is represented by the number of protons and the hydrogen coverage is simulated by the number of hydrogen atoms adsorbed on the surface of the catalyst. The ratio of proton over H2O molecule (RH+/H2O) is set as 1, 0.5, 0.33, and 0.25, respectively. For example, RH+/H2O = 0.5 represents that the 3 × 3 supercells include one proton and two H2O molecules. It shows that water and proton have great impact on the HER performances of Cr2CO2. At different proton concentrations, ΔGH* varies against the hydrogen coverage, θ, in the manner of increase (θ = 0.25) and decrease (θ = 0.375), and increase again (θ = 0.5), which is different from the results obtained with the implicit model59 that ΔGH* increases monotonously with an increase of θ. For the implicit model of Cr2CO2, the binding strength 15633

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the reaction energy of hydrogen desorption from the Cr2CO2 surface at RH+/H2O = 0.25. The results show that hydrogen desorption from the Cr2CO2 surface will easily occur at medium hydrogen coverage (0.25 and 0.375) with the reaction energies lower than −0.4 eV. Detailed discussions are given in the Supporting Information.

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CONCLUSIONS In summary, the etching and exfoliation properties of Cr2AlC to Cr2CO2 and the electrocatalytic properties of Cr2CO2 without and with carbon defect are studied by performing DFT calculations. The results show that O* terminal Cr2CO2 is favorably formed through etching the pristine Cr2AlC by HF solutions, whereas Cr2CF2 and Cr2C(OH)2 will translate into Cr2CO2 even if they were generated first during the etching reactions. The exfoliation energy of multilayer Cr2CO2 into monolayer Cr2CO2 MXenes is large (2.78 eV/nm2), and the delamination process requires the association of metal cations, such as Li+ (LiF). Carbon vacancies are easily generated during the etching and exfoliation reactions, and the formation energy of carbon vacancy in Cr2CO2 is 1.59 eV without considering the charge correction (1.71 eV by considering the charge correction). The solvent can affect the HER performance of Cr2CO2 at medium hydrogen coverage. These results provide a guideline for experimentally synthesizing the 2D materials and expanding the scope of the HER catalyst for water splitting in the future.

Figure 6. (a) Side and (b) top views of the Helmholtz layer (interface region) in the studied interface model; Gibbs free energy of hydrogen adsorption (ΔGH*) of the Cr2CO2 surface (c) without and (d) with carbon vacancy at different hydrogen coverages, considering the solvent effect. The black, red, blue, and dark green symbols stand for the ratio of proton over H2O molecules.

between the adsorbed hydrogen and surface O* is strong at low θ and weak at high θ, whereas for the water−solid model of Cr2CO2, a different binding strength between the adsorbed hydrogen and surface O* occurs at medium hydrogen coverage θ (0.25−0.375); the strength of the H−O* bonding reduces first and then increases, suggesting that the HER performance of Cr2CO2 is affected by the solvent. The concentration of protons also affects the HER performance, for example, at θ = 0.125, ΔGH* are 0.45, −0.59, −0.33, and 0.11 eV at RH+/H2O = 0.25, 0.33, 0.5, and 1, respectively. With an increase of θ to 0.375, ΔGH* are −0.22, 0.43, 0.32, and −0.06 eV at RH+/H2O = 0.25, 0.33, 0.5, and 1, respectively. Obviously, the HER performances of Cr2CO2 are altered by the concentration of protons. Moreover, one can find that with an increase of θ from 0.125 to 0.5, the values of the difference between the smallest and the largest ΔGH* are 1.10, 1.60, 1.28, and 0.85 eV at RH+/H2O = 0.25, 0.33, 0.5, and 1, respectively, suggesting that the change of ΔGH* is larger at low proton concentration (RH+/H2O = 0.25, 0.33, and 0.5) and smaller at higher proton concentration (RH+/H2O = 1). This indicates that the HER performance of Cr2CO2 is stable at higher proton concentration. In addition, the optimal ΔGH* above HERs is −0.06 eV at θ = 0.375 and RH+/H2O = 1. The HER performance of Cr2CO2 with 25% carbon vacancy is studied, and the results are presented in Figure 6d. One can find that ΔGH* is slightly different from that of perfect Cr2CO2. For example, ΔGH* are 0.11 and 0.17 eV at 0.125 hydrogen coverage and proton concentration of RH+/H2O = 1. However, the tendency of ΔGH* variation with the hydrogen coverage and proton concentration is the same in the perfect Cr2CO2 and Cr2CO2 with carbon vacancy. The above results show that the HER performances of Cr2CO2 MXenes are slightly different from the interface solid model (solvent model) and the implicit model, and the HER performances are stable at high proton concentration, which is beneficial to HER. We also calculated

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03120.

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Reaction energies of R1−R3 by the CI-NEB method; schematic of etching Cr2AlC to Cr2CTx; schematic of Al−F (dAl−F) and Cr−Al (dCr−Al) bond lengths in the Cr2AlC structure; schematic of exfoliation of multilayer Cr2CO2 into monolayer Cr2CO2; exfoliation energies of Cr2CO2, Cr2CF2, and Cr2C(OH)2 without intercalated LiF calculated via DFT-D2 and DFT-D3; formation energy of Cr and C vacancies in Cr2C and Cr2CT2; formation energies of carbon vacancy in Cr2CT2 (T = O, F, and OH) and other 2D materials; and reaction energy of the second step (Volmer−Heyrovsky) of HER on the Cr2CO2 surface (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.S). *E-mail: [email protected] (Y.Z). ORCID

Yan Song: 0000-0002-9081-6518 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Shandong, China, grant no. ZR2014EMM013; the Natural Science Foundation of Shandong, China, grant no. ZR2014EMQ009; and the Fundamental Research Funds for the Central Universities, grant no. HIT.KITP.2014030. This 15634

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work was carried out at LvLiang Cloud Computing Center of China, and the calculations were performed on TianHe-2.

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