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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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The Etching, Exfoliation Properties of CrAlC into CrCO and Electrocatalytic Performances of 2D CrCO MXene 2

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Yuwen Cheng, Lijuan Wang, Yue Li, Yan Song, and Yumin Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03120 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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The Journal of Physical Chemistry

The Etching, Exfoliation Properties of Cr2AlC into Cr2CO2 and Electrocatalytic Performances of 2D Cr2CO2 MXene

Yuwen Cheng,†,‡ Lijuan Wang, ‡ Yue Li,‡ Yan Song,‡,* Yumin Zhang†,* †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

ABSTRACT: Recent years, two-dimensional 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, i.e., the Cr2CO2, and the Cr2CF2 and Cr2C(OH)2 will translate into Cr2CO2 even if they were generated firstly during 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 to be generated during etching and exfoliation reaction, and the formation energy of carbon vacancy (1.71 eV 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 with considering 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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INTRODUCTION Two-dimensional (2D) materials have large surface area and extreme thinness making them are relatively easily 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. Recent years, the 2D MXenes with 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 surface functional groups of OH, O or F) have captured more attentions due to their excellent mechanical and electronic properties.3-9 The presence of surface functional group Tx can enhance the interaction of hydrogen with MXenes, create new active sites, and expand the reaction area further,10-11 resulting MXenes can be as 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 MAX phase into multilayer MXenes firstly by HF solutions, then exfoliating multilayer MXenes into monolayer MXene by intercalated metal cation solutions, and finally centrifuging and drying the monolayer MXene. But till now, only a small part of MXenes has been synthesized successfully, such as Ti3C216 and Mo2C.10 Thus there are lack of knowledge about the large family of MXenes, especially for their electrochemical properties, which are significantly important for energy conversation and storage applications. Moreover, the electrochemical properties of Ti3C2 and Mo2C are poor comparing with that of MoS2 or Pt. Therefore, more works are needed for clarifying electrochemical properties of MXenes. Compositing with other materials is the promising ways for enhance electrochemical performances of Ti3C2Tx.17-23 The composited Ti3C2Tx shows the enhanced electrochemical properties (capacitance or cycle number) than that of pure Ti3C2Tx. However, the compositing approach is complex and time consuming, which hindering the large scale applications of 2D MXenes. Therefore, it is important to insight the synthetic process, such as how the three functional groups, O, F, and 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


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functional theory 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, i.e., Cr2CO2. The exfoliation energy of Cr2C MXenes with different terminations is estimated, results indicated that exfoliation energy for delaminating 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 multilayer Cr2CO2 MXenes. Moreover, the formation of carbon vacancy in monolayer Cr2CO2 is calculated. Carbon vacancies are easily to be generated during etching and exfoliating reaction, 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, 6.2 eV for forming Mo vacancy). HER performances of Cr2CO2 were further studied with considering solvent effect. 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.

COMPUTATIONAL DETAILS All of calculations were performed with Vienna ab initio simulation package (VASP) code24,25

under

the

framework

of

density

functional

theory

(DFT)

with

the

Perdew-Burke-Ernzerhof (PBE)26 form of generalized gradient approximation (GGA) potential27 The projector augmented wave (PAW)28 method was applied to accurate the electronic structures, and the van der Waals interaction was considered by using the empirical correction in Grimme’s scheme, i.e., DFT+D3,29 in all calculations except 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, respectively. A 2×2×1 supercell of Cr2CO2 was employed to investigate the adsorption of hydrogen and the termination properties of Cr2CO2. 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 each atom and energy on 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 interaction caused by the periodic boundary condition. The reaction



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Gibbs free energy of hydrogen adsorption (∆GH*) is evaluated via eq (1) ∆GH * = ∆EH +∆EZPE -T∆SH

(1)

where ∆EH, ∆EZPE and T∆SH are the differences of hydrogen adsorption energy, zero-point energy and the entropy between adsorbed hydrogen and hydrogen in gas phase, respectively.14,30,31 The binding energy of surface oxygen with Mn+1Cn substrates is defined as eq (2) Eb1={E(Cr2CO2) –E(Cr2C) –E(O2)}/2

(2)

where E(Cr2CO2), E(Cr2C), and E(O2) are total energies of oxygen terminal MXene and pristine MXene, and oxygen molecule, respectively.32 The binding energy of Al in Cr2AlC is obtained through eq (3) Eb2={E(Cr2AlC)–E(Cr2C)–E(Al)}

(3)

where E(Cr2AlC), E(Cr2C), and E(Al) are the total energies of 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 2×2×1 Cr2CO2 substrates without and with an carbon vacancy, respectively, and E(C) is the total energy per carbon atom derived from the graphene state.33

RESULTS AND DISSCUSSION The Etching and Exfoliation Properties of Cr2AlC to Cr2C MXene The bare Cr2C MXene is packed in face center cubic arrangement with two exposed metal layers and each C atoms bond with six Cr atoms as well as other 2D MXenes displaying the P3m1 symmetry, as shown in Figure 1. Till now, information about synthesis of Cr2C MXene is lack. Based on the studies of Ti3C2 MXene,

34-36

a possible approach to synthesize Cr2C MXene is

etching the Cr2AlC in hydrofluoric (HF) acid solution, removing Al layers, and leaving multilayer Cr2C. Then monolayer Cr2C MXene is synthesized by exfoliating the Cr2C multilayer. The schematic of etching Cr2AlC to the Cr2C multilayer is shown in Figure 2. Based on the literature3, we propose a possible synthetic route to etch Cr2AlC through following reactions (eqs. (5) to (7)) R1: Cr2AlC(s)+5HF(aq)→Cr2CF2(s)+AlF3(aq)+5/2H2(g) R2: Cr2AlC(s)+3HF(aq)+2H2O(aq)→Cr2CO2(s)+AlF3(aq)+7/2H2(g)


(5) (6)

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R3: Cr2AlC(s)+3HF(aq)+2H2O(aq)→Cr2C(OH)2(s)+AlF3(aq)+5/2H2(g)

(7)

which will generate Cr2CF2, Cr2CO2 and Cr2C(OH)2 from R1 to R3, respectively. The HF and H2O are referred as gas phase, and AlF3 is referred as solid phase for calculation, respectively. The calculated reaction energies (∆E) of R1 to R3 are -6.73 eV, -7.19 eV and -0.47 eV, respectively, via definition of ∆E=Eproducts–Ereactions. It can be seen that three reactions are exothermic, and ∆E of R2 is much lower than that of R1 and R3, suggesting that the O* terminal Cr2CO2 is more favor to form during etching Cr2AlC phase. In addition, we also considered alternative reactions that is Cr2CF2 generated firstly, and then translates into Cr2CO2 or Cr2C(OH)2 via reactions 4 to 6 as below. R4: Cr2CF2(s)+2H2O(aq)→Cr2C(OH)2+2HF(aq)

(8)

R5: Cr2CF2(s)+2H2O(aq)→Cr2CO2+2HF(aq)+H2(g)

(9)

R6: Cr2C(OH)2→Cr2CO2(s)+H2(g)

(10)

The calculated ∆E of R4 to R6 are 0.19 eV, -0.16 eV, and -0.35 eV, respectively. The R4 is endoergic, while R5 and R6 are exoergic, meaning that Cr2CO2 will be the final product even that the Cr2CF2 or Cr2C(OH)2 was generated firstly during etching reactions. The reaction barriers of R1 to R3 are calculated by climbing nudged elastic band method and results are presented in Figure S1. It shows that Cr2CO2 is energetically generated among three terminal structures. Details discussions are presented in Supporting Information (SI).

Figure 1. Atomic arrangement of Cr2CTx MXene structure with (a) three possible functional



group sites (top view) and (b) surface termination (side view).

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

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 hcp sites, for functional groups (O*, F* or OH*) to be occupied. Schematic of etching Cr2AlC into multilayer Cr2CTx is shown in Figure S2. The binding energies of 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 three possible sites, which is consistent with latest experimental findings that it reaches the lowest binding energy when a Tx was adsorbed at the hcp site for Cr2C with three different termiantions.10 The calculated binding energy (via eq (2)) are -3.64, -4.23, and -1.01 eV for F*, O* and OH* terminal Cr2C, respectively, suggesting that system with O* termination is more stable than that of the F* and OH* terminal. There is another possibility that the reactions are carried out in solution, the relevant O (F) chemical potential would be like 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 above hypothesis. The O*


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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 are performed and results are shown in Table 1. It show that Cr atoms will lose electrons during the etching with 0.14 e valance electrons in Cr2AlC changing to 0.87, 1.53, 1.48, and 1.38 e in 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 to R3 and R4 to R6 are redox types. Furthermore, the calculated binding energy (via eq (3)) of Al in Cr2AlC system is 1.24 eV, meaning that the etching reactions of Cr2AlC could be easily achieved in experimental synthetics. For the surface terminal functional groups, the O atoms in Cr2CO2 receive 0.91 e electrons, while F atoms in Cr2CF2 receive 0.69 e electrons. The O and H atoms in Cr2C(OH)2 receive and lose 1.67 e and 0.98 e electrons, respectively. It also illustrates that the net electrons captured by O are 0.69 e (∆e=1.67 e–0.98 e). Obviously, the binding strength between O* and Cr2C is stronger (O received more electrons) than that of F* or OH* functional groups. Above results indicate that the etching reactions of Cr2AlC to Cr2C could easily occurr, and the produced Cr2C tends to form O* termination, i.e., Cr2CO2. Table 1. Valence electrons of Cr, Al, C, O, F and H atoms in Cr2AlC, Cr2CO2, Cr2CF2 and Cr2C(OH)2 system based on the Bader charge analysis. Systems

Cr (e)

C (e)

Cr2C Cr2AlC Cr2CO2 Cr2CF2 Cr2C(OH)2

+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*)

To further study the reactions process of Cr2AlC etching in HF acid solutions, we calculated the layer distance, defined as the distance of two adjacent Cr metal layers of Cr2AlC system and the bond lengths of Al-F and Cr-Al bonds at different F concentrations. The results are shown in Figures 3a and b, and Tables S2 and S3. In this study, in a 4×1 Cr2AlC supercell with surface area of 0.29 nm2, one F atom bonds to Al atom and form Al-F bond, thus the HF (F-) concentrations can be evaluated as 3/nm2 in terms of the number of F atoms contained in the supercell. With the HF concentrations increasing, the binding energy of F with Al layers is decreased (Figure 3a),



which means that high HF is in favor to weaken the bonding strength between Al and F, and promotes the etching reaction. The layer distances of Cr2AlC are 4.21 Å and 4.26~5.71 Å before and after etching (Figure 3b), respectively. It shows that the higher concentrations of HF solutions, the larger layer distance of Cr2AlC system.

Figure 3. (a) Binding energy of Al with F and (b) layer distance (distance of two Cr metal layers) against the F concentrations, (c) the exfoliation energies of Cr2CO2, Cr2CF2, and Cr2C(OH)2 without intercalated LiF and (d) the variation of exfoliation energies of Cr2CO2 against numbers of LiF. The schematic of 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. Before etching dAl-F is 1.72 Å but elongates to 1.83 to 2.09 Å after etching. While 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 of 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 Cr-Al bonds are larger than 3.4 Å the typical dCr-Al value in the HF coverage more than 6/nm2. Thus Cr-Al bonds will be broken when the HF coverage is over 6 per nm2. The optimal dAl-F in AlF3 is about 1.83 Å, one can see that when HF coverage is in 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 favorite for Al-F bonding (dAl-F is higher than the normal values).


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For 2D materials, delamination is the necessary step to explore their properties. The multilayer MXenes have two six-fold 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 aqueous solutions of ionic compound,39,40 When etching Ti3AlC2 with a mixture of fluoride salt and acid (for example, HCl and LiF), the multilayer Ti3AlC2 can be exfoliated into monolayer Ti3C2.41,42 According to these previous findings, we selected Li as metal cation sources, and studied the exfoliation properties of Cr2CO2 with and without Li intercalating by calculating the exfoliation energy of Cr2CO2 (with 2×2 supercells). The exfoliation energy of Li intercalated Cr2CO2 is defined as Ex = {ECr2CTx - Li - 2ECr2CTx - ELi}/4S

(11)

where ECr2CTx - Li, ECr2CTx and ELi are the total energies of multilayer Cr2CTx with and without Li intercalated, and the pure Li metal, respectively.43 The surface area of S= 3a2/2, and where a is the lattice parameter of Cr2CTx. Here, each unit of Cr2CTx will be exfoliated into two MXenes monolayer generating 4 surfaces. The results are shown in Figures 3c and 3d. For C2C MXenes, the calculated exfoliation energy is lowest for T=F* surfaces (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, above analysis indicates that Cr2CF2 is unstable and will translate into Cr2CO2 in acid conditions, meaning the exfoliation of Cr2CF2 may not be occurred or there is a competition between the exfoliation and translation. For the O* terminal Cr2CO2, the larger radius of oxygen ions than that of fluorine leads to the higher polarization ability and a stronger interaction between Cr2CO2 layers. The exfoliation energy of Cr2CO2 is larger than that of graphite (about 1 eV/nm2)44, which exactly explain that the approach, such as metal cation intercalation is required to isolate multilayer Cr2CO2 MXenes. With increasing of the concentration of the intercalated Li, the exfoliation energy decreases first and then increases (Figure 3d). The lowest exfoliation energy is -3.01 eV/nm2, appearing at the numbers of Li ions is equal to 5 for 2×2 Cr2CO2 supercells, and the corresponding LiF/Cr2CO2 ratio is 1.25, the optimal value of LiF/Cr2CO2 in term of obtaining an isolate 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 exfoliation energy of Cr2CO2 without intercalated LiF by DFT-D2 and compared these results with those calculated by DFT-D3, and the results and the discussions are presented in SI.

Carbon Defect in Cr2CO2 MXene Atomic defects, such as carbon vacancy, are inevitable during etching the Mn+1AlCn and intercalated multilayer Mn+1CnO2, which have been observed in Ti3C2 MXene in experimental synthetics.40 Thus, we further calculated formation energy of carbon vacancy (via eq (4)) in Cr2CTx MXene and compared with other 2D materials. Results are shown in Figure 4a. The schematic of Cr2CO2 with carbon vacancy (denoted as Cr2CO2-VC) is shown in Figures 4b to 4d. Owning to 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 carbon vacancy in Cr2C, Cr2CO2, Cr2CF2 and Cr2C(OH)2 are 3.56, 3.01, 1.59 and 2.66 eV, respectively, with referring energy per carbon atom in grapheme 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 formation of vacancy in Cr2CT2 MXene is energetically favorite. The Ef of carbon vacancies in Cr2C, and Cr2CT2 is estimated by referring energy per carbon atom in graphite state and the Ef of some 2D materials under the same calculation conditions was also calculated. Results and discussion are provided in SI. Results show that Ef of carbon vacancy in Cr2CT2 MXene is energetically favorite. 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 Ef of carbon vacancy in Cr2CO2 is lowest (1.71 eV) among the studied 2D materials. Based on these evaluations, we further calculated the densities of states (DOSs) of Cr2C-VC, Cr2CO2-VC, Cr2CF2-VC and Cr2C(OH)2-VC (Figures 4e and f). From Figure 4e, one can see that the DOSs of carbon vacancy contained systems shift to lower energy slightly with respect to the systems without carbon vacancy while the DOSs lied 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


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effect on the hybridization between Cr d orbitals and C p orbitals. The valance electrons of Cr (1.50 and 1.56 e), C(-1.46 and -1.45 e) and O (-0.94 and -1.06 e) in Cr2CO2-VC system are listed in Table S4, respectively, while their corresponding values in pure Cr2CO2 are +1.53, -1.24, and -0.91 e, 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, results show that the Ef of Cr vacancy is unstable with larger formation energy than that of C vacancy. More details are given in SI.

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 Cr2CO2-VC structure are illustrated via 6×6 supercells, (e) densities of states of Cr2C and Cr2CT2 and (f) partial density of states of p-Cr and h-Cr in Cr2C and Cr2CO2.



Property of Hydrogen Evolution Reaction Recently, the electrochemical properties of some 2D MXenes have been reported in literatures,54-57 illustrating that they are potential candidates for applying in many fields, especially in energy conversation applications, such as electrocatalysts for HER. Therefore, the HER performances of Cr2CO2 MXene is further investigated. HER usually occurs on the surface of catalysts, i.e., forming a water-solid interface, therefore the solvent effect should be considerated properly. In this paper, we adopted the approach previously used in study the reaction mechanism of HER on Pt (111)58 to model the water-solid interface under electrochemical potentials. The Hemholtz layer (interface region, see in Figures 6a and b) is often approximated by about 3 Å thickness 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 water-solid interface. Model A, the H atom (proton) of each H2O molecule (H3O+) points down toward the Cr2CO2 surface (Figure 5a), model B, the H atom (proton) of each H2O molecules (H3O+) point away from the Cr2CO2 catalyst surface (Figure 5b), and model C, the molecular plane of H2O molecules parallels to the catalyst surface (Figure 5c). After relaxation, model A has the relatively lowest energy (-1.7 eV) comparing that with model B (-0.9 eV) and model C (0 eV), thus subsequent studies were carried on model A (the representative structure is shown in Figure 5d).


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

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

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 was represented by the number of protons and the hydrogen coverage was simulated by the numbers of hydrogen atoms adsorbed on the surface of catalyst. The ratio of proton over H2O molecule (RH + /H2O) is set as 1, 0.5, 0.33, and 0.25, respectively. For example, for RH + /H2O=0.5 represents that the 3×3 supercells includes one proton and two H2O molecules. It shows that the water and proton have greatly 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 implicit model59 that the ΔGH* increases monotonously with increase of θ. For implicit model of Cr2CO2, the binding strength



between adsorbed hydrogen and surface O* is strong at low θ, and weak at high θ, while for the water-solid model of Cr2CO2, different in the binding strength between adsorbed hydrogen and surface O* occurs in the medium hydrogen coverage θ (0.25~0.375), the strength of H-O* bonding reduces firstly and then increases suggesting that HER performance of Cr2CO2 is affected by solvent. The concentration of protons also affects the HER performances, for example, at θ=0.125, ΔGH* are 0.45, -0.59, -0.33 and 0.11 eV at RH + /H2O equals to 0.25, 0.33, 0.5 and 1, respectively. With increasing 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 found that with increasing of θ from 0.125 to 0.5, values of the difference between the smallest and the largest ΔGH* are 1.10, 1.60, 1.28, 0.85 eV at RH + /H2O=0.25, 0.33, 0.5 and 1, respectively, suggesting that 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 performances of Cr2CO2 is stable at higher proton concentration. In addition, the optimal ΔGH* for above HERs is -0.06 eV at θ=0.375 and RH + /H2O=1. The HER performances of Cr2CO2 with 25% carbon vacancy is studied and results are presented in Figure 6d. One can found that the ΔGH* is slightly different with 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 the ∆GH* variation with the hydrogen coverage and proton concentration is same in the perfect Cr2CO2 and the Cr2CO2 with carbon vacancy. Above results show that the HER performances of Cr2CO2 MXenes are slightly different between interface-solid model (solvent model) and implicit model, and the HER performances is stable at high proton concentration, which is benefit to the HER. We also calculated the reaction energy of hydrogen desorption from Cr2CO2 surface at RH + /H2O= 0.25. Results show that hydrogen desorption from Cr2CO2 surface will easily occur at middle hydrogen coverage (0.25 and 0.375) with reaction energies are lower than -0.4 eV. Details discussions are given in SI.

CONCLUSIONS In summary, the etching and exfoliation properties of Cr2AlC to Cr2CO2 and electrocatalytic properties of Cr2CO2 without and with carbon defect are studied by performing density functional theory calculations. Results show that O* terminal Cr2CO2 is favor to be formed through etching


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the pristine Cr2AlC by HF solutions, while Cr2CF2 and Cr2C(OH)2 will translate into Cr2CO2 even they were generated firstly during the etching reactions. The exfoliation energy of multilayer Cr2CO2 into monolayer layer Cr2CO2 MXenes is large (2.78 eV/nm2), and the delamination process requires association of metal cations, such as Li+ (LiF). Carbon vacancies are easily to be generated during 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 HER catalyst for water splitting in the future.

ASSOCIATION CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DIO: Reaction energies of R1 to R3 by CI-NEB; Schematic of etching Cr2AlC to Cr2CTx; Schematic of Al-F (dAl-F) and Cr-Al (dCr-Al) bond lengths in Cr2AlC structure; Schematic of exfoliation of multilayer Cr2CO2 into monolayer Cr2CO2; The 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; The reaction energy of the second step (Volmer−Heyrovsky) of HER on Cr2CO2 surface ORCID Yan Song: 0000-0002-9081-6518 AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y.S). *E-mail: [email protected] (Y.M.Z).

Notes The authors declare no competing financial interest



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

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