Two-Dimensional, Ordered, Double Transition Metals Carbides

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)# A New Family of Promising Catalysts for the Hydrogen Evolution Reaction Yuwen Cheng, Jianhong Dai, Yumin Zhang, and Yan Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08914 • Publication Date (Web): 17 Nov 2018 Downloaded from http://pubs.acs.org on November 17, 2018

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The Journal of Physical Chemistry

Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)：A New Family of Promising Catalysts for the Hydrogen Evolution Reaction

*

*

Yu-Wen Cheng1, Jian-Hong Dai2, Yu-Min Zhang1, , Yan Song2,

1 Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin 150001, PR China

2 School of Materials Science and Engineering, Harbin Institute of Technology atWeihai, 2 West Wenhua Road, Weihai, 264209, China

ABSTRACT: Generation of hydrogen by splitting water with the electrocatalytic approach could become a more sustainable way following the discovering of new materials, such as the 2D transition metal carbides. Developing eco-friendly, low cost, stable and high active nonprecious hydrogen evolution reaction (HER) catalysts is one of key factors for hydrogen energyeconomy. Two-dimensional metal carbide and nitride (MXenes) materials have shown characteristics of promising HER catalysts. Herein, we explored the conductive, thermal stability, and electrocatalysts performance of four 2Dordered double MXenes Mʹ2MʺC2, Cr2TiC2, Cr2VC2, Mo2TiC2, and Mo2VC2, and their corresponding oxygen (O*) or hydroxyls (OH*) terminated MXenes by using density functional calculations. Results indicated that all above MXenes are conductive, which are favored to charge transfer during HER. Four MXenes are fully terminated by O* under standard conditions [pH=0,p(H2)=1 bar, U=0 V]. The Gibbs free energy for the adsorption of atomic hydrogen (∆GH*) on the O* terminated Mʹ2MʺC2 (e.g, Cr2TiC2O2) is close to 0 eV (the ideal value) at suitable H coverage. The formability of oxygen vacancy in the O* fully terminated Mʹ2MʺC2, i.e., Mʹ2MʺC2O2 was studied and a linear relationship between the formation energy of oxygen vacancy (Ef) and ∆GH* was obtained. The electronic structure analysis indicates that the more electrons gained by the terminated O* from Mʹ2MʺC2, the higher occupation of the p orbitals of the terminated O* and thus resulting the weakness of the binding strength between the terminated O* and the adsorbed H. Our results indicated that O* terminated Mʹ2MʺC2 are promise HER electrocatalysts for generating hydrogen by water splitting.

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INTRODUCTION Hydrogen with a clean and sustainable producible manner is a promising alterative energy carriers to replace the carbon-based fules.1-4 Splitting of water through hydrogen evaluation reaction (HER) associating with catalysis is an effective approach to generate H2.5-7 Noble metals such as platinum (Pt), palladium (Pd) and iridium (Ir) are the most active catalysts for HER, but limited by the scarity of them for large-scale applications.8-9 Therefore, searching for nonprecious catalysts with high activity for HER is highly desirable.10-19 Recent studies have released that two-dimensional (2D) materials with large surface area and superior electrochemical properties, which affords a great number of active sites for the HER, show great potential as the catalysts for HER.22, 21 The 2D MXenes, discovered in 201122-24 with a general formula of Mn+1XnTx, where M stands for early transition metal, X stands for C or N, Tx is the surface functional groups O, OH or F, and n= 1 to 3.25-29 MXenes are expected to be extremely stable as they have survived the harsh synthesis conditions in concentrated hydrofluoric acid.7 The catalytic effect of MXenes for the HER is mainly contributed by the appearance of the functional groups Tx that can expand the reaction area further, enhance the interaction of hydrogen with MXene, and create new active sites. In 2014, Liu et al30 discovered an ordered double M3AX2 structure, Cr2TiAlC2, in which a Ti-layer is sandwiched between two outer Cr carbide layers in a M3AX2 structure. Following year, Anasori et al31 synthesized the M3AX2 phase with Mo-Al bonds, Mo2TiAlC2, Mo-layers sandwiched TiC2 layers. Their discovery is crucial for the potential expansion of MXene family since they can result in numerous MXenes with ordered layered 2D structures MXene that were not possible previously. Anasori et al32 combined DFT calculation and experiments methods studied more than 20 the ordered double MXenes Mʹ2MʺC2 and Mʹ2Mʺ2C2 (Mʺ stands for middle layer and Mʹ refers to the outer layer), finding that most of them to be ordered directly, and for Mʹ2MʺC2 and Mʹ2Mʺ2C2, the metal elements with smaller size refer to in the middle layers, which will increase the stability of MXenes by decreasing strain and stress between layers, e.g., Cr2TiC2,Cr avoids the middle layer, and Ti prefers to occur in middle layer. Tan et al33 using DFT and Monte Carlo (MC) simulations constructed a map of the structure−stability relationship of the ordered double MXenes, finding that Mo2TiC2 is the easiest to be synthesized, and Mo generally


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prefers the outer layers even at high temperatures (1400 to 1900 K).

As a new member of the MXenes family, the ordered double MXenes greatly expanding the family MXenes in particular and 2D materials in general and possess good electrochemistry. However, till now, there is still lack knowledge of the thermal stability and electrochemical properties of ordered double Mʹ2MʺC2. The purpose of this paper is to explore the dependence of stability of 2Dordered double MXenes (Mʹ2MʺC2, Cr2TiC2, Cr2VC2, Mo2TiC2, Mo2VC2) with oxygen (O*) or hydroxyls termination (OH*), and research whether the four ordered double O* terminated MXenes can act as the efficient HER catalysts. The results indicate find that all the studied MXenes with both the O* and OH* terminations are metallic characteristics. Then, the surface Pourbaix diagrams are conducted to identify the most stable surface structures of the 2D ordered double MXenes under the standard hydrogen electrode (USHE) and pH values. Under the standard conditions, the four MXenes are fully terminated by O*. The Gibbs free energies of adsorption of atomic hydrogen on the four ordered double O* terminated MXenes (∆GH*) are further calculated to evaluate their HER activity. Furthermore, the exchange current, i0 and volcano curve are studied for the bare Mʹ2MʺC2 and the O* terminated Mʹ2MʺC2O2. In addition, according recently report,34 formation energy of oxygen vacancy energy (Ef) can be as an effective parameter to screening the best HER catalysts, we calculated the formation energy of oxygen vacancy energy of the four Mʹ2MʺC2O2as a parameter coupling with ∆GH*to clarify the HER performance of the studied MXenes. Furthermore, the electronic structure analysis indicates that the more electrons gained by the terminated O* from Mʹ2MʺC2, the higher occupation of the p orbitals of the terminated O* and thus resulting the weakness of the binding strength between the terminated O* and the adsorbed H. The present results illustrate that the ordered double O* terminated Mʹ2MʺC2O2 can be as excellent HER catalysts at suitable H coverage.

COMPUTATIONAL DETAILS All of calculations were performed within the Vienna ab initio simulation package (VASP) code35,36 under the framework of density functional theory (DFT). The generalized gradient approximation (GGA)37 with Perdew-Burke-Ernzerhof (PBE) potential38 and the projector augmented wave (PAW)39 method were applied. The electronic configurations are [Ar]3d34s1, [Ar]3p63d44s1, [Ar]3p63d54s1, [Kr]5p54d54s2, 2s22p2, 2p4s2, and 1s1 for Ti, V, Cr, Mo, C, O and H,



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respectively. To describe the strong correlation of electrons within the transition metals the GGA+U

40

approach with U=4.0,41 4.0,42 3.0 43 and 6.0 eV

44

for Ti, V, Cr and Mo, respectively,

was employed. The van der Waals interaction was accounted by using the empirical correction in Grimme‟s scheme, i.e., DFT+D3, in all calculations.45 The energy cutoff is 500 eV for all cases, and k-point gamma-centered meshes of 8×8×1for the unit cell and 4×4×1,3×3×1 for supercell were used during the geometric optimization. The convergence tolerance is 0.01 eV/Å for the force on atom and the difference of energies between two continuous ions steps is less than 10-5eV. A large than 20 Å vacuum between two MXenes layers originated by the periodic boundary condition was induced. The USHE is the applied voltage on electrode referenced to the standard hydrogen electrode, and theoretically is defined in solution [pH=0, p(H2) = 1 bar]. Reaction Gibbs free energy of hydrogen adsorption Under the standard conditions, the HER catalytic activity of materials can be evaluated by the change of the Gibbs free energy of hydrogen adsorption reaction (∆GH*), defined as below ∆GH*=∆EH +∆EZPE -T∆SH

(1)

where ∆EH is the energy differences of the adsorbed hydrogen and the hydrogen in gas, which defined in eq (2).∆EZPE and T∆SH are the difference of zero-point energy and of the entropy between adsorbed hydrogen and gas hydrogen, respectively. n-1 H* 1

∆EH =EnH* DFT -EDFT n-1 H*

where EnH* DFT , EDFT

H (g)

2 , and EDFT

H (g)

2 - 2 EDFT

(2)

represent total energies of the catalyst with n adsorbed

hydrogen atoms, n-1 adsorbed hydrogen atoms, and H2 gas, respectively. The asterisk denotes the catalyst active site. ∆EZPE can be obtained by eq (3) (n-1)H 1

H

2 ∆EZPE =EnH ZPE -EZPE - 2 EZPE

(3)

Due to the fact that the vibrational entropy in the adsorbed state is small according to previous studies,46, 47 The ∆SH can be approximated as half of the entropy of H2 gas under the standard condition S0H2 as below,48 1

∆SH ≅- 2 S0H2

(4)

Using the values of ∆EZPE and T∆S in Ref 31, eq(1) can be therefore replaced by eq(5) ∆GH*=∆EH +0.3 eV


(5)

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As ∆GH* = 0 stands for the ideal HER, thus the smaller values of |ΔGH*|, the better HER performance of MXenes catalysts is. Surface Pourbaix diagrams The thermal stability of Mʹ2MʺXenes could be evaluated from the surface Pourbaix diagrams constructed via relevant USHE and pH. According to Refs 49, 50 and 51, water could split to OH* or O* on Mʹ2MʺXenes surface through following steps. H2 O+*→OH*+H+ +e- ∆G06

(6)

OH*→O* +H+ +e- ∆G07

(7)

The free energy of ∆G0OH* and ∆G0O* can be calculated by eq (8) ∆G0 =∆E+∆EZPE -T∆S

(8)

where ∆E is the energy difference between the products and reactants of reaction eq (6) or eq(7), and the EZPE and T∆S are calculated on the basis of Refs 52 and 53. At equilibrium the chemical potentials satisfy following equations52 1

μH+ +μe- = 2 μH

(9)

2 (g)

μH+ =μ0H+ +kB TlnaH+ 

(10)

μe- =μ0- -eU

(11)

e

μH

2 (g)

=μ0H

2 (g)

+kB TlnpH  2

(12)

where aH+ represents the activity of protons and eU is the shift of potential in electron under an applied extra filed. μ0H+ , μ0- , and μ0H e

2 (g)

are the chemical potentials of protons, electrons, and

hydrogen molecular at the standard conditions, and have following relation: 1

μ0H+ + μ0- =2 μ0H e

2 (g)

(13)

0 That is G0H+ +G0e- =GH 2 (g)

To considerate the effects of pH and potential USHE, and combining eqs (9) to (12) with eqs. (6) and (7), the free energies of eq (6) and eq (7) are as followings: ∆G6 =∆G06 -eUSHE +kB Tln10×pH

(14)

∆G7 =∆G07 -eUSHE +kB Tln10×pH

(15)

when the O* and OH* mixture termination is formed on the Mʹ2MʺC2 MXenes surface, the accounting changes of the Gibbs free energies can be expressed as:



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∆GOH* =∆G6

(16)

∆GO* =∆G6 +∆G7

(17)

Under different eU and pH conditions, the free energy change of Mʹ2MʺC2(O)x(OH)ywith various converges xO* and yOH* (x+y≤2) termination can be expressed as: ∆Gmix=x∆GO* +y∆GOH* = x+y ∆G06 +y∆G07 − x+2y eUSHE +kB Tln10×pH

(18)

Exchange current volcano curve To get exchange current, we adopted Nørskov’s assumption.46 When the proton transfer is exothermic (∆GH* 0) transfer, it is 1

i0 =-ek0 1+exp(∆G

/k T) H* B

(20)

where the kB is Boltzmann constant and k0 is the rate constant, which is set as 1 owing to the lack of available experimental data.51 Binding energy of functional group and formation energy of oxygen vacancy The terminated binding energy between the functional group Tx (T=O* and OH*) and Mʹ2MʺC2 substrates is defined as Eb={E(Mʹ2MʺC2Tx) –E(Mʹ2MʺC2)–E(Tx)}/2

(21)

Where Tx is the number of the functional group bonded to the Mʹ2MʺC2 substrates, E(Mʹ2MʺC2Tx) and E(Mʹ2MʺC2)are the total energies of Mʹ2MʺC2 substrates with and without functional group Tx, and E(Tx) are the total energy of Tx.34, 54. For a 2×2×1 supercell, the formation energy of oxygen vacancy is obtained as Ef =E(Mʹ8Mʺ4C8O7)+1/2E(O2) − E(Mʹ8Mʺ4C8O8)

(22)

where E(X) is the total energy of system X.34

RESULTS AND DISSCUSSION Stability of ordered double transition metal MXenes Schematics of structures of M3C2 and Mʹ2MʺC2 are shown in Figures 1(a) and 1(b). The bare M3C2 is packed in a fcc arrangement with two exposed metal layers, displays P3m1 symmetry. In the structure of bare Mʹ2MʺC2, the outer layer metal and inner layer metals Mʹ and Mʺ are two different metals, such as Ti, V, Cr or Mo. The C atoms occupy the octahedral sites between the Mʹ


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and Mʺ layers in all cases. Each Mʹ2MʺC2 have multiple surface termination groups. Previous works indicated that all MXenes except Sc based are saturated with oxygen (O*) or hydroxyls (OH*) during synthesis, thus we narrow our focus on Mʹ2MʺC2 with O* and OH* terminations

only,7, 20 as shown in Figure S1. At the hcp sites, the T groups (O* or OH*) positioned above the top of the C atoms, forming three T-M bonds with neighboring M bonds on the both surfaces. At the fcc site, T groups located at above the hollow sites of M3X2 on both sides.55

Figure 1. Schematics of Mʹ2MʺXenes. (a) and (b) Currently available MXenes (M3C2), where M can be Ti, V, or Nb, which can forms monatomic M layer or solid solutions (intermixing between two different M elements). (c) and (d)The ordered double transition metals Mʹ2MʺXenes.

The structure of differ in the fraction of Mʹ (green) and Mʺ (blue) atoms occupying Mʺ sites , and the energy differences between fully ordered Mʹ2MʺC2 (such as Cr2TiC2) and partially ordered configurations are show in Supporting information Figures S2 and Figure S3. The results show that the four ordered Mʹ2MʺC2 (Cr2TiC2, Cr2VC2, Mo2TiC2, Mo2VC2), with a Mʹ-Mʺ-Mʹ stacking layers have the lowest energy. Moreover, the total energy of four Mʹ2MʺC2 reduces linearly as the fraction of Mʺ atoms in the middle layer increases. On the basis of results, it is obvious that the four Mʹ2MʺC2 prefer to be a fully ordered configurations, while Mo and Cr atoms avoids exist in the middle layers, and Ti atom avoids exist in the outer layers. To study the stability of surface termination, we further calculated the binding energy (via eq (21)) of the Mʹ2MʺC2 surface with T (T= O* and OH*) termination, and results shown in Figure S4. One can find that:(1) the O*



terminated four ordered double Mʹ2MʺC2 MXenes have the lower binding energy than that of the OH* termination; (2) the adsorption site for O* at the fcc site generates the lowest adsorption energy on Cr2TiC2, Cr2VC2, while the lower adsorption energy occurs when O* adsorbs at the hcp site on Mo2TiC2, Mo2VC2 surface. That is, the fcc site is the stable O* binding site in Cr2TiC2 and Cr2VC2, but it turns to the hcp site in Mo2TiC2 and Mo2VC2. As it is well known that, the DFT-D2 method are also widely used to calculate the van der Waals interactions of 2D materials, thus, we have calculated the binding energy of surface functional groups and hydrogen adsorption Gibbs free energies of four MXenes by DFT-D256 and compared these results with those calculated by DFT-D3, the results and details discussion are displayed in supplementary information. Conductivity and thermal stability The calculated bandstructure and density of states (DOS) of the four bare Mʹ2MʺC2 and Mʹ2MʺC2 fully terminated with O* or OH* are presented in Figures 2 and Figures S5. The results indicate that all the studied materials are metallic. As the bare Mʹ2MʺC2 MXenes are exposed with metal atoms are electron donors, they are easily to be electronegative to functional groups (O* or OH*) during experimental synthesis. Therefore, 2D Mʹ2MʺC2 MXenes with O* or OH* functional groups can possess an excellent charge transferring performance for HER.

Figure 2. The band structure and density of states of (a) Cr2TiC2, (b) Cr2TiC2O2, (c) Cr2TiC2(OH)2, (d) Cr2VC2, (e) Cr2VC2O2 and (f) Cr2VC2(OH)2.The Fermi level was set as zero.

The surface Pourbaix diagrams of the four Mʹ2MʺC2 MXenes are constructed by plotting the


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thermodynamically most stable surface state under relevant USHE and pH, as shown in Figure 3. As mentioned above, the bare Mʹ2MʺC2 are easily accepted functional groups to formed O* or OH* termination. In strong acid (pH=0, reduction environment), potential (USHE) values as low as -1.51, -2.22, -1.33, and -2.56 V are required to protect Cr2TiC2, Cr2VC2, Mo2TiC2 and Mo2VC2 from oxidation by water, respectively. With increase of pH, larger USHE values are needed to prevent the bare Mʹ2MʺC2 from oxidation. Once the USHE is exceed the cathodic protection potential of the Mʹ2MʺC2, H2O starts to oxide, and the Mʹ2MʺC2 surface is covered by OH*.With increasing USHE, more OH* are adsorbed on the Mʹ2MʺC2 surface, the surfaces are fully covered with OH* (l ML). USHE of Mʹ2MʺC2 surface with full OH* termination [Mʹ2MʺC2(OH)2] reaches to -1.01, -0.46, -0.74, and -0.91 V for Cr2TiC2, Cr2VC2, Mo2TiC2 and Mo2VC2, respectively. Further increasing the USHE, the OH* will oxidize to O*, and the Mʹ2MʺC2 surface is then terminated with a mixture of OH* and O*. All terminated OH* will be oxidized if the potential continually increases, and then the stable O* terminated Mʹ2MʺC2 MXenes (Mʹ2MʺC2O2) are formed. At acidic condition (pH=0), the lowest potential values for the O* fully terminated Mʹ2MʺC2 (1ML O*) are -0.04, -0.11, -0.46, and -0.19 V for Cr2TiC2, Cr2VC2, Mo2TiC2 and Mo2VC2, respectively. Therefore, the four MXenes Cr2TiC2, Cr2VC2, Mo2TiC2 and Mo2VC2 are fully terminated with O* under the standard condition.



Figure 3. Surface Pourbaix diagrams of (a) Cr2TiC2, (b) Cr2VC2, (c) Mo2TiC2, and (d) Mo2VC2

As an efficient HER catalysts, it should possess high stability and HER activity at a USHE=0 V, thus we further investigated the relative stability of the four Mʹ2MʺC2 with different terminations under USHE = 0 V. Figure 4 illustrates the relative stability of the studied Mʹ2MʺC2 with different O* and OH* termination at a certain USHE. It can be seen that the MXenes fully terminated by O* or OH* possess the most stability than that of the bare or OH* partially covered MXenes when pH=0 (the acid environment), the most stable states of the four MXenes are those with full O* termination. With the increase of pH, the concentrations of hydroxyl ions increased, the hydroxyl groups (OH*) on the Mʹ2MʺC2 MXenes surface are gradually formed. The Cr2TiC2, Cr2VC2 and Mo2VC2 are fully terminated by OH* when the pH values are greater than 4.3, 3.8, and 2.9, respectively (the red line across blue line or green line). Therefore, the MXenes with complete O* termination could be an efficient catalyst for HER at the standard conditions and are worth to be studied further.

Figure 4.Relative stability of O* and OH* terminated of (a) Cr2TiC2,(b) Cr2VC2, (c) Mo2TiC2 and (d) Mo2VC2at USHE=0 V. All the surface terminated MXenes are relative the bare Mʹ2MʺC2 (set as


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0 eV)

Activity of Hydrogen Evolution Reaction

The HER activity of the O* terminated Mʹ2MʺC2 MXenes (Mʹ2MʺC2O2) under standard conditions is investigated to evaluate the HER properties of the four Mʹ2MʺC2 MXenes. The free energies of hydrogen adsorption (∆GH*) on Mʹ2MʺC2O2 surface at different coverage are estimated, and shown in Figure 5, Figure S6 and Table S1. For an ideal HER catalysts, the ∆GH*should close to 0.For an isolated Mʹ2MʺC2, there are two surfaces that intended to be fully terminated by O* forming a Mʹ2MʺC2O2 phase, which provides active sites (the O* sites) for hydrogen adsorption. Selecting 4×2×1 Mʹ2MʺC2O2 surpercell as an example, there are sixteen O* atoms that could capture H atoms. Thus, the increments H coverage on the surface can be set as 1 16ML. For 1 ML coveragedCr2TiC2O2, ∆GH*is -0.51 eV illustrating a strong interaction between the 8

adsorbed H and surface O*. However, the interaction between O* and H* is slightly weakened (∆GH*of -0.17 eV) when the hydrogen coverage reaches to 1 4ML. As the hydrogen coverage increases to 3 8 ML and 1 2ML, the interaction between H* and terminated oxygen atom is further weakened, and the ∆GH* values are 0.16 and 0.20 eV, respectively. The results shows that, the higher of the hydrogen coverage, the weaker of the interaction between the H* and O*, and thus, the increase the values of ∆GH*. For Cr2TiC2O2, the best hydrogen coverage is ranged from1 4 to 1 2ML (|∆GH*|≤0.20), indicating that Cr2TiC2O2 could be an excellent HER catalysts at range from1 4 to 1 2ML H coverage. For the Cr2VC2O2, there are similar trends of the interaction between H* and O* with Cr2TiC2O2. The ∆GH* of Cr2VC2 are -0.57, -0.21, -0.03, and 0.29 eV at H coverage are 1 8, 1 4, 3 , and 1 ML respectively. The results indicating that at low H coverage ( 1 ML), the 8 2 8

interaction between H* and O* is too strong, which will prevent the further release H from catalysts (Cr2VC2O2) surface. When the H coverage is high (1 2ML), the interaction between H* and O* is weakened, which will facilitate H release H from catalysts surface. Especially, at a H coverage of 3 8ML, the ∆GH* is -0.03 eV, which |∆GH*| is close to 0, which is comparable with Pt (111) surface (~0.09 eV)49 and 1T-MoS2 (~0.05 eV)57, indicating that at a suitable H coverage, Cr2VC2O2 can be an excellent catalysts for HER.



For Mo2Ti2CO2 and Mo2VC2O2, when at a low H coverage ((1 8ML)), the values of ∆GH* are -0.31 and -0.40 eV for Mo2TiC2O2 and Mo2VC2O2, respectively, indicating the interaction between H* and O* is strong. While the interaction between H* and O* is weakened following the increase of the H coverage. For the H coverage at ranges from 1 4to1 2 ML, the ∆GH* are 0.26 to 0.65 eV and 0.23 to 0.59 eV for Mo2TiC2O2 and Mo2VC2O2, respectively. When the H coverage is over 3 8ML, the interaction between H* and O* is too weak to capture H atoms on above three Mʹ2MʺC2O2 MXenes, which states that three Mʹ2MʺC2O2 MXenes are not favored to HER.

Figure 5. Free energy diagram of HER processing of(a) Cr2TiC2O2,(b) Cr2VC2O2, (c) Mo2TiC2O2 and (d) Mo2VC2O2under standard conditions. The line labels of panels (a)−(d) are the same and are shown in panel a only.

A comparison of the HER performances of the studiedMʹ2MʺC2O2 MXenes was illustrated in Figure 6 through the volcano curves. The values of ∆GH*on the bare Mʹ2MʺC2 with a hydrogen coverage of θ= 1 4ML or 3 8ML (Figure 6(b)) and the four O* terminated Mʹ2MʺC2O2 with a hydrogen coverage ranging from 1 8 to 1 2ML are calculated via eqs (19) and (20) and then


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applied to predict the theoretical exchange current values, i0,. The values of log (i0) climes up following the reduction of the absolute value of ∆GH* and will reach the maximum when ∆GH* is equal to zero. Thus, the closer the position of i0 to the peak of the volcano curve, the better catalytic performance of the MXenes will be. From the volcano curve, one can evaluated the catalytic properties of Mʹ2MʺC2 and Mʹ2MʺC2O2 MXenes for HER.51 Catalysts with negative and positive ∆GH* values are located around the left and right legs of the volcano, respectively, while catalysts with ∆GH* values close to zero are located at the peak of the volcano curve. Obviously, the interaction of H* with bare Mʹ2MʺC2 are strong than Mʹ2MʺC2O2 at same H converge (θ=1 4), while for specific Mʹ2MʺC2, the bare Mʹ2MʺC2 locates at the bottom of the left of the volcano curve with a low i0. For the Mʹ2MʺC2 terminated with O*, the interaction between adsorbed of H* and Mʹ2MʺC2O2 is weaker at same H converge. The weak interaction promotes hydrogen release and results a high value of i0. It is worth noting that for the Cr2VC2O2 at θ=3 8ML hydrogen coverage, and Cr2TiC2O2 at θ=1 4ML and θ=3 8ML hydrogen coverage, the log(i0)close to the peak of the volcano curve, implying a better catalytic performance for HER is expectable. Based on these results, the Cr2VC2O2 is expected to display the best HER activity at a suitable H coverage, following by Cr2TiC2O2. This provides a window to adjust catalytic performance of the MXenes owing to the factor that the H coverage can be easily realized by adjusting the ratio of reactants, react time, amount of surfactants in experiment for obtaining the best HER performance.47, 58

Figure 6. Volcano curve of exchange current (i0) as a function of the Gibbs free energy of hydrogen adsorption on the Mʹ2MʺC2O2 (∆GH*). (a) Exchange current, i0 of Mʹ2MʺC2O2 at different H coverage (from left to right (∆GH* from negative to positive) corresponding θ=1 8 ,



Page 14 of 22

1 , 3 , 1 for the specificMʹ2MʺC2O2); (b) Exchange current, i0 of Cr2VC2 at H coverage 4 8 2

θ=3 8, while the other three Mʹ2MʺC2 at θ=1 4.

Recently, Jiang et al34 found that the trend of oxygen desorption after adsorbing of hydrogen is the predominant mechanism of oxygen-terminated MXenes in HER process, and propose a competent new descriptor, formation energy of oxygen vacancy (Ef), to scale ∆GH*of oxygen terminated MXenes.

Following this research, we calculated the formation energy of an oxygen vacancy in the fully O* terminated Cr2TiC2, Cr2VC2, Mo2TiC2 and Mo2VC2, and scale the value of ∆GH* of above four

Mʹ2MʺC2O2, as shown in Figure 7. The values of Ef (via eq (22)) are 3.50, 3.58, 4.40 and 4.09 eV in Cr2TiC2O2, Cr2VC2O2, Mo2TiC2O2 and Mo2VC2O2, respectively. While the ∆GH*of above four Mʹ2MʺC2O2 are -0.17, -0.03, 0.26 and 0.23 eV, showing a linear relationship between the formation of oxygen vacancy (Ef) and the free energy of hydrogen adsorption ((∆GH*) (see Figure 7). This suggests present results are consistent with Jiang et al findings, i.e., the formation energy of oxygen vacancy can be set as descriptors for HER performances.

Figure 7.Linear relationship between formation energy of oxygen vacancy (Ef) and free energy of hydrogen adsorption ((∆GH*) of four Mʹ2MʺC2O2. Black, red, dark green and pink symbols stand for Cr2TiC2O2, Cr2VC2O2, Mo2TiC2O2 and Mo2VC2O2, respectively.

The results of Figure S6 and Table S1 suggest that the binding strength between the adsorbed H

and

the

surface

terminated

O*

obeys


following

orders:

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Cr2TiC2O2>Cr2VC2O2>Mo2TiC2O2>Mo2VC2O2, that is, the binding strength of the adsorbed H with Cr2TiC2O2 is stronger than that with Mo2TiC2O2 or Mo2VC2O2.The interaction between H and surface O* atoms is dominated by the hybriding between the H 1s orbital and O 2p orbital, which will split to bonding (σ) and anti-bonding (σ*)orbitals, especially by hybridization between the H 1s orbital and the partially filled σ*orbital. The higher energy level of the σ* orbital occupancy, the weaker bonding strength of H with O*. Thus, one can assume that the energy level occupation of terminated O* p orbital may serve as a descriptor of the binding strength between the adsorbed H and surface terminated O* of Mʹ2MʺC2O2. Thus, the partial density of states (PDOS) and the charge received of terminated O* are investigated to reveal the difference of HER performance of four Mʹ2MʺC2O2, results are shown in Figure 8 and Table 1.

Figure 8. Partial density of states of the terminated O* p orbital of Mʹ2MʺC2O2. (a)Cr2VC2O2, (b) Cr2TiC2O2, (c) Mo2VC2O2, and (d) Mo2TiC2O2. The red dash line is Fermi energy level, and the black arrow dash line indicates the shift of the PDOS peak shift of different Mʹ2MʺC2O2 .

Table 1. Surface terminated oxygen (O*) charge gain of surface terminated O*, NeT-O*, in different 2D Mʹ2MʺC2O2estimated based on the Bader charge analysis (in unit of e). System

Cr2TiCO2

Cr2VCO2

Mo2TiCO2


Mo2VCO2


NeT-O*

0.993

0.988

1.031

Page 16 of 22

1.016

Figure 8 shows the terminated O*p orbital PDOS of four Mʹ2MʺC2O2. The results show that there are plenty of densities of states (DOS) peaks varying with the energy from -6 to 4 eV. And there is a trend that the whole PDOS of the terminated O* p orbital shifts to higher energy in systems from Cr2VC2O2 to Mo2VC2O2, and the energy levels of the highest DOS peasks are 1.7,1.9, 2.6,and 2.9 eV above Fermi level, which are labeled as P1a, P1b, P1c and P1d, corresponding to Cr2VC2O2, Cr2TiC2O2, Mo2VC2O2, and Mo2TiC2O2, respectively. It means that the occupation abilities of O* p orbital are changed with the Mʹ metal from Cr to Mo in Mʹ2MʺC2. The charge gains of the surface terminated oxygen (O*) are calculated for further analysis as listed in Table 1. The values of NeT-O* are 0.988, 0.993, 1.016 and 1.031e for Cr2VC2O2, Cr2TiC2O2, Mo2VC2O2, and Mo2TiC2O2, respectively, revealing that Mo2VC2O2and Mo2TiC2O2 have higher NeT-O* than Cr2VC2O2and Cr2TiC2O2. One can make a conclusion from above analysis, that is, the more terminated O* received electrons from Mʹ2MʺC2, the higher terminated O* p orbital PDOS occupation and thus resulting the weakness the binding strength of terminated O* with adsorbed H, and the HER performance will over change accordingly. For ordered, double transition metals carbides (MXenes), Mʹ2MʺC2, there are definitely exist more Mʹ2MʺC2with O* possess the highly HER catalytic and comparable with rare Pt metal with different metals occupation in Mʹ and Mʺ sites, and the received electrons ability of terminated O* ever changing, thus, more works are needed for exploring promising noble metal-free HER electrocatalysts.

CONCLUSIONS In conclusion, the present study demonstrated that four 2D ordered double Mʹ2MʺC2 terminated with O* (Cr2TiC2O2, Cr2VC2O2, Mo2TiC2O2, Mo2VC2O2) exhibit excellent HER activity at suitable H coverage by using density functional calculations. All of these bare Mʹ2MʺC2, O* or OH* terminated Mʹ2MʺC2 MXenes are metallic, thus favoring excellent charge transfer. Under the standard condition, the surface Pourbaix diagrams clearly indicated that these four MXenes (Cr2TiC2, Cr2VC2, Mo2TiC2 and Mo2VC2) are terminated with O*. In the O* terminated Mʹ2MʺC2 MXenes, the surface oxygen atoms act as the active sites for HER with a suitable


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interaction strength between H* and the O* terminated Mʹ2MʺC2O2, and the Gibbs free energy for the adsorption of atomic hydrogen (∆GH*) on the O* terminated Mʹ2MʺC2 (e.g, Cr2TiC2O2) are close to 0 eV (the ideal value for the efficient HER) at suitable H coverage, indicating that Mʹ2MʺC2O2 can serve as a catalysis providing catalytic active sites for HER at suitable H coverage. The terminated O* PDOS and charge received analysis indicated that the more terminated O* received electrons from Mʹ2MʺC2, the higher terminated O* p orbital PDOS occupation and thus resulting the weakness the binding strength of terminated O* with adsorbed H. These results expand the family of MXenes for promising noble metal-free HER electrocatalysts for evaluating hydrogen by water splitting.

ASSOCIATION CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications websites at DIO: ORCID Yan Song:0000-0002-9081-6518 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail:[email protected] Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS This work was financially supported by the science and technology project of Shenzhen (No.JCY J20160506113431828), 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. Calculations were carried out on TianHe-2 at National Super Computer Center in LvLiang of China. Y W Cheng also thanks Dr W Yao‟s Da Xue Gong Jian project supporting to this work.



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Two-Dimensional, Ordered, Double Transition Metals Carbides

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