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Template-Grown MoS2 Nanowires Catalyze Hydrogen Evolution Reaction: Ultra-low Kinetic Barriers with High Active Site Density Chongyi Ling, Yixin Ouyang, Li Shi, Shijun Yuan, Qian Chen, and Jinlan Wang ACS Catal., Just Accepted Manuscript • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 14, 2017

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Template-Grown MoS2 Nanowires Catalyze Hydrogen Evolution Reaction: Ultra-low Kinetic Barriers with High Active Site Density Chongyi Ling1, Yixin Ouyang1, Li Shi1, Shijun Yuan1, Qian Chen1 and Jinlan Wang*1,2 1

School of Physics, Southeast University, Nanjing 211189, China

2

Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal

University, Changsha 410081, China

KEYWORDS: MoS2 nanowires; Volmer-Tafel mechanism; low barriers; hydrogen evolution reaction; electrochemical water splitting

ABSTRACT: Molybdenum disulfide (MoS2) is considered as one of the most promising lowcost catalyst for hydrogen evolution reaction (HER). So far, the limited active sites and high kinetic barriers for H2 evolution still impede its practical application in electrochemical water splitting. In this work, on basis of comprehensive first-principles calculations, we predict that the recently produced template-grown MoS2 nanowires (NWs) on Au(755) surfaces hold both ultralow kinetic barriers for H2 evolution and ultra-high active site density simultaneously. The calculated kinetic barrier of H2 evolution through the Tafel mechanism is only 0.49 eV on the Mo edges, making the Volmer-Tafel mechanism operative and Tafel slope can be as low as 30

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mV/decade. Through substituting Au(755) substrate with non-noble metals, such as Ni(755) and Cu(755), the activity will be maintained. This work provides a possible way to achieve the ultrahigh HER activity of MoS2 based catalysts.

Introduction Molybdenum disulfide (MoS2), as a promising low-cost catalyst for hydrogen evolution reaction (HER), has attracted enormous attention since 2005.1 However, because of the limited active site density and high kinetic barriers for H2 evolution, the catalytic performance of MoS2 is still far from the requirements to commercial application. Generally, there are two strategies to enhance the activity of a catalyst: i) increasing the active site density and ii) improving the intrinsic activity of each active site. As the active sites of MoS2 are limited at their open edges,2 increasing the active site density has become a widely used strategy, including exposing more edge sites through preparing defect-rich, mesoporous or vertically aligned MoS23-7, and activating the inert basal planes by introducing defects or doping8-10. On the contrary, enhancing the intrinsic activity has gained little attention. In fact, the available Tafel slope (b) limit for 2.303RT

MoS2 based catalysts is as high as 40 mV/decade ( b = (1+α)F ≈40 mV/dec), suggesting the Volmer-Heyrovsky mechanism is operative.11-19 Theoretical studies have also shown that the evolution of H2 through Tafel reaction on MoS2 based catalysts needs to conquer a relatively high kinetic barrier (~1.50 eV).20-22 In comparison, the so-far best HER catalyst, platinum (Pt), can catalyze HER through Volmer-Tafel mechanism with an energy barrier as low as ~0.80 eV, leading to the Tafel slope only 30 mV/decade ( b =

2.303RT 2F

≈30 mV/dec).23,

24

The high H2

evolution barriers of MoS2 based catalysts are believed to be the reason why they present poorer catalytic activity than Pt.25 Therefore, designing MoS2 based HER catalysts with low kinetic


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barrier for Tafel reaction to improve the intrinsic activity is highly desirable, which has not yet been arrived. Actually, the high kinetic barrier for H2 evolution through Tafel reaction (Ea) on MoS2 based catalysts originates from the large H-H distance between two neighboring adsorbed H atoms (dH-H).21 Decreasing the dH-H is thus expected to reduce the Ea and improve the intrinsic activity of MoS2. Very recently, MoS2 nanowire (NW) arrays have been successfully grown on an Au(755) surface template,26 where the produced NWs were aligned well along the substrate steps with high density. Moreover, through a careful analysis on the bonding characteristics, we found this structure contains a very short dH-H at its Mo edges. Meanwhile, the rather narrow width of ~0.67 nm signifies the ultra-high density of exposed edge sites of MoS2. Therefore, such a MoS2 nanostructure may present extremely low Ea for HER and have ultra-high active site density simultaneously. In this work, we systematically evaluate the HER performance of MoS2 NWs@Au(755), within the framework of density functional theory (DFT). It is found the kinetic barrier for H2 evolution on their Mo edges via the Tafel reaction is indeed greatly reduced as compared with other MoS2 based catalysts, which is as low as 0.49 eV. The ultra-low Ea endow MoS2 NWs@Au(755) with H2 evolution rates about 16 orders faster than other MoS2 based catalysts. The HER performance of MoS2 NWs@Cu(755) and Ni(755) are further investigated considering the high cost of Au, which also present ultra-high HER activity. Therefore, these ultra-narrow MoS2 NWs are expected to present the smallest Tafel slope and achieve greatest HER performance among MoS2 based catalysts ever reported. Computational Details


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The DFT calculations were performed through the projector augmented wave (PAW) method27 as implemented in the Vienna ab initio simulation package (VASP).28,

29

The

generalized gradient approximation in the Perdew-Burke-Ernzerhof (PBE) form30, 31 and a cutoff energy of 350 eV for plane-wave basis set were adopted. The convergence threshold was 10-5 eV and 0.03 eV/Å for energy and force, respectively. To avoid the interaction between two periodic units, a vacuum space at least 10 Å was used. The model of MoS2 NW@Au(755) was composed of a three layer Au(755) substrate with (1 × 10) surface unit cells and a MoS2 NW with 9 times of unit cell along the growth direction. For MoS2 NW@Ni(755) or MoS2 NW@Cu(755), the model was constructed by a three layer Ni(755) or Cu(755) substrate with (1 × 5) surface unit cells and a MoS2 NW with 4 times of unit cell along the growth direction (more details are included in Figure S1 in Supporting Information). To reduce the computational burden, the bottom two layers were fixed for all the three systems. DFT-D232 calculations were used to describe the weak interaction. The minimum energy pathway for H2 evolution is determined by using the climbing nudged elastic band method,33, 34 and each transition state is further confirmed by vibrational analysis. The HER performance was evaluated by calculating the reaction free energy for hydrogen adsorption (∆GH), which can be obtained by35, 36: ∆GH = ∆EH + ∆EZPE – T∆S. The ∆EH is the hydrogen adsorption energy, while the ∆EZPE and ∆S are the zero-point energy difference and the entropy difference between the adsorbed and gas phase, respectively. For the case of MoS2 NWs, the ∆EZPE – T∆S is calculated to be 0.29 eV. Results and Discussions To uncover how to reduce the dH-H of MoS2 based catalysts, analysis on bonding characters of various catalysts are carried out. Generally, the S atoms bond to other atoms by forming sp3 hybrid orbitals in MoS2. These four sp3 hybrid orbitals form a tetrahedral structure. On the


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basal planes of both 2H and 1T-MoS2, S atoms bond to three neighboring Mo atoms. Due to the directional properties of covalent bonds, the orientation of S-H bonds (OS-H) have only one direction, which is perpendicular to the S-S orientation (OS-S) as shown in Figure 1a. As a result, the shortest dH-H is equal to the distance between two adjacent S atoms, which exceeds 3.0 Å. For the case of Mo edge in MoS2 sheet, each S atom interacts with two Mo atoms. Thereby, the adsorbed H atoms have two orientations, both of which are perpendicular to the OS-S, resulting in two distinct structures as displayed in Figure 1b and 1c. Therefore, the shortest dH-H is still equal to the distance between two neighboring S atoms, larger than 3.0 Å. Under these circumstances, significant distortions of the surface geometry are needed to allow the dH-H to be small enough for a bond formation.21 The corresponding barriers are thus high (~1.50 eV) and the Tafel reaction is inhibited.


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Figure 1. Models of S atoms bond to other atoms in MoS2 (a) basal plane, (b) & (c) Mo edge as well as (d), (e) & (f) NWs grown on Au(755) surfaces. The cyan, yellow, golden and white balls represent the Mo, S, Au and H atoms, respectively. The cyan spindle structures describe the sp3 hybrid orbitals of S atoms. The red and blue arrows represent the orientations of S-S and S-H, respectively. For the newly synthesized MoS2 NW@Au(755), each S atom of Mo-edge interacts with a Mo atom and an Au atom when a H atom adsorbs. Thus, the adsorbed H atoms also have two orientations, resulting in three distinct structures (Figure 1d to 1f). However, unlike the former two cases where the OS-H is perpendicular to the OS-S, the OS-H in the Mo edge of NWs is nearly parallel to the OS-S. Accordingly, when two H atoms adsorb along different OS-H (Figure 1d), the H-H distance will be significantly reduced, which is about half of that in 2H-MoS2 Mo edge or 1T-MoS2 basal plane. Therefore, such a MoS2 nanostructure may possess a very low energy barrier for H2 evolution via Tafel reaction. Moreover, this template-grown NW presents ultranarrow width of 0.67nm, which provides abundant exposed edge sites as well. Therefore, we systematically study the HER performance of MoS2 NW@Au(755).

Figure 2. (a) Structure of MoS2 nanowire grown on Au(755) surface and labels of different sites for hydrogen adsorption. (b) Calculated free energy diagram for hydrogen evolution on different kinds of S sites. (c) Minimum-energy pathways of the Tafel reaction on SMo-edge sites and the


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insets are the initial state (IS), transition state (TS) and final state (FS). The golded, yellow and cyan balls represent the Au, S and Mo atoms, respectively. We first perform a qualitative description on the HER performance of MoS2 NW@Au(755). The equilibrium structure is presented in Figure 2a, and there are four kinds of sites for hydrogen adsorption, labeled as SMo-edge, Ssurf, SS-edge and SS-edge(Au), respectively. The calculated ∆GH of different sites are illustrated in Figure 2b, where the SMo-edge and SS-edge(Au) sites are highly active with ∆GH of 0.15 and 0.20 eV, respectively. Besides, the ∆GH of Ssurf or SS-edge site is either positive (0.66 eV) or relatively negative (-0.30 eV), indicating that both of them are not suitable for HER. These results are in accordance with that of 2H-MoS2 sheet, whose Mo edge, S edge and basal plane own modulate, relatively negative and too positive ∆GH, respectively.21 Besides, S atoms on both SMo-edge and SS-edge(Au) sites present metallic nature (Figure S2a and S2b), However, a question remains unresolved: can the intrinsic activity of MoS2 NW@Au(755) be largely improved as we expected? To answer this question, kinetic energy barriers (Ea) for H2 evolution on SMo-edge site of MoS2 NW@Au(755) is further explored. As shown Figure 2c, the Ea of Tafel reaction on SMo-edge of MoS2 NW@Au(755) is as low as 0.49 eV, which is not only greatly reduced as compared with that of other MoS2 based catalysts (~1.5 eV), but also much lower than that of Pt(111) surface (~0.8 eV). According to the Arrhenius formula (k = A·exp(Ea/RT)), the rate of H2 production on MoS2 NW@Au(755) SMo-edge site is estimated to be amazingly increased by about 9 orders faster than that of 1T MoS2 (catalyze HER through Volmer-Heyrovsky mechanism with a Heyrovsky reaction barrier of ~1.0 eV)20 at room temperature. It is even about 5 orders faster than that of Pt(111) with hydrogen coverage lower than one monolayer, whose barrier is about 0.8 eV.


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Figure 3. Structures of (a) MoS2 NW@Ni(755) and (b) MoS2 NW@Cu(755) as well as the labels of different sites for hydrogen adsorption. (c) Calculated free energy diagram for hydrogen evolution on different S sites of MoS2 NW@Ni(755) and MoS2 NW@Cu(755), respectively represented by solid and dash lines. (d) Calculated ∆GH as a function of number of electron S atom gains (Ne) for MoS2 NW@Ni(755) (blue dots and line) and MoS2 NW@Cu(755) (red quadrangles and line), respectively. The R2 is respectively 0.99 and 0.98, indicating the high linear correlation between the Ne and ∆GH. The blue, bronze, yellow, cyan and white balls represent the Ni, Cu, S, Mo and H atoms, respectively. Despite the prominent HER performance, the high expense of Au makes such a catalyst commercially impracticable, alternative non-noble metal substrates for the growth of MoS2 NWs are very necessary. For the template growth of MoS2 NWs, the atomic staircases of Au(755) surface is crucial.26 Furthermore, the special Au-S (edge) bonds lead to the greatly reduced H-H distance between two adjacent adsorbed H on S edge, which finally results in the ultra-low energy barrier of Tafel reaction. As non-noble metals, Ni and Cu have similar crystal structure to Au (face-centered-cubic structure). Naturally, the (755) surfaces of Ni and Cu also contain periodical atomic staircases which are analogous to that of Au(755) surface. Moreover, Ni(755) and Cu(755) surfaces have been used as templates to prepare nanomaterials experimentally.37, 38 Moreover, we computed the binding energies of MoS2 NWs on different substrates (Details can be found in SI). As shown in Table S1, Eb for MoS2 NW@Au(755), Cu(755) and Ni(755) are -


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0.92, -2.55 and -3.97 eV, respectively, indicating that MoS2 NWs on Ni(755) and Cu(755) is more stable than that on Au(755). Therefore, the experimental synthesis of MoS2 NWs on Ni(755) and Cu(755) surfaces and their excellent performance for HER can be expected.

Figure 4. Minimum-energy pathways of the Tafel reaction on different active sites: (a) or (b) SSedge

and (c) or (d) SMo-edge sites of MoS2 NW@Cu(755) or MoS2 NW@Ni(755), respectively. The

insets are the corresponding structures of initial state (IS), transition state (TS) and final state (FS). The blue, bronze, yellow and cyan balls represent the Ni, Cu, S and Mo atoms, respectively. Similar to MoS2 NW@Au(755), both MoS2 NW@Ni(755) and MoS2 NW@Cu(755) contain four different kinds of S sites for hydrogen adsorption: SMo-edge, Ssurf, SS-edge and SSedge(Ni/Cu)

as shown in Figure 3a and 3b. For MoS2 NW@Ni(755), both the edge sites, SMo-edge and

SS-edge present ultra-high HER activity with the calculated ∆GH of 0.19 and -0.12 eV, respectively. Other two kinds of sites (Ssurf and SS-edge(Ni)) have relatively positive ∆GH (0.83 and


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0.54 eV, respectively), therefore they are not active sites for HER (Figure 3c). Similar tendency has also been observed for MoS2 NW@Cu(755): SMo-edge and SS-edge are highly active, while Ssurf and SS-edge(Cu) are inert for HER. The calculated ∆GH for these four kinds of sites are 0.01, -0.16, 0.69 and 0.42 eV, respectively (see Figure 3c). The different HER activities of different sites can be ascribed to the different number of electrons that S atom gains (Ne). As shown in Figure 3d, the calculated ∆GH present nearly perfect linear correlation with the Ne of corresponding S atoms and a larger Ne will lead to a more positive ∆GH, which is in accordance with our previous studies.39, 40 Similar to MoS2 NW@Au(755), MoS2 NW@Ni(755) and Cu(755) can also ensure efficient charge transfer due to the metallic nature of S atoms on the active sites (Figure S2c to S2f). The kinetic energy barriers for H2 evolution on both kinds of edge sites of MoS2 NW@Ni(755) and MoS2 NW@Cu(755) are further explored. We first calculate the free energy diagrams of the second H* adjacent to the first one. As displayed in Figure S3, ∆GH tends to increase upon the adsorption of the second H, consistent with previous studies.35, 40 Except for SMo-edge site of NW@Ni(755) whose ∆GH of the second H adsorption is a little high, the rest three kinds of sites still possess nearly zero ∆GH. We then compute the Tafel reaction barriers on these active sites. As shown in Figure 4a and 4b, the Ea of Tafel reaction for SS-edge of both NWs keep costly, 1.80 and 1.74 eV for MoS2 NW@Cu(755) and NW@Ni(755), respectively, which is similar to other MoS2 based catalysts and suggesting that the Tafel-Heyrovsky is operative. Nevertheless, similar to that of MoS2 NW@Au(755), the Ea of Tafel reaction on SMo-edge of NW@Cu(755) and Ni(755) are very low, just 0.64 and 0.48 eV, respectively, as shown in Figure 4c and 4d. To determine the underlying mechanism of HER on these two kinds of sites, the energy barriers of Volmer reaction and Heyrovsky reaction are also calculated referring to


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previous works.20, 24 20 H2O molecules are used to simulate the solution effect and the results are presented in Figure 5. Similar to the cases of Pt(111) and 1T MoS2,20,

24

the Ea of Volmer

reaction are very small for both NW@Ni(755) and Cu(755), just 0.17 and 0.06 eV, respectively (Figure 5a and 5b). However, for Heyrovsky reaction, the Ea are 1.29 and 1.31 eV for NWs@Ni(755) and Cu(755) respectively, demonstrating that Volmer-Tafel mechanism is operative and Tafel reaction is the rate-determining step. Correspondingly, the Tafel slope can be as low as 30 mV/decade according to the equation (b =

2.303RT 2F

≈30 mV/dec), which will be the

lowest record for MoS2 based HER catalysts. Therefore, not only the cost will be greatly reduced, but also the ultra-high HER performance can be maintained by substituting Au(755) with Ni(755) or Cu(755) as the templates for the growth of MoS2 NWs. The experimental realization of MoS2 NW@Cu(755) and NW@Ni(755) is thus desirable. Closer examination of the structures reveals that the ultra-low Ea of Tafel reaction for these NWs indeed originates from the greatly reduced H-H distance between two neighboring adsorbed H atoms on SMo-edge sites. As clearly displayed in Figure 6, the barrier increases linearly with the enlargement of H-H distance. For 2H-MoS2 Mo edge or 1T-MoS2 basal plane, the shortest dH-H is normally larger than 3.0 Å. To form the H-H bond, a significant distortion of the surface geometry is thus needed, leading to a high kinetic barrier for H2 evolution, e.g., ~1.5eV. For Mo edges of MoS2 NWs, the dH-H between two neighboring H atoms is only ~1.50 Å, respectively, nearly half of that on 2H-MoS2 Mo edge or 1T-MoS2 basal plane. Correspondingly, the Ea is dramatically reduced by nearly 1.0 eV as compared with that of other MoS2 based catalysts, making the H2 evolution through the Volmer-Tafel route possible, which eventually improves the HER activity. It should be noted that Ea and dH-H of different systems present close


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linear correlation, suggesting that dH-H may be served as a simple descriptor for intrinsic HER activity of catalysts.

Figure 5. Minimum-energy pathways of Volmer and Heyrovsky reaction on SMo-edge sites of MoS2 NW@Ni(755) (a and c, respectively) and NW@Cu(755) (b and d, respectively). The inserts are the corresponding initial state (IS), transition state (TS) and final state (FS). The blue, bronze, yellow, cyan, red and white balls and lines represent the Ni, Cu, S, Mo, O and H atoms, respectively.


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Figure 6. Calculated activation barriers (Ea) for H2 desorption through the Tafel mechanism as a function of the H-H distance (dH-H), where the R2 is 0.95, indicating the high linear correlation between the Ea and dH-H. The purple pentagon, red stars, blue triangles, orange dot and green quadrangle represent the cases of MoS2 NWs@Ni(755), NWs@Cu(755), Mo edge and Pt(111) surface, respectively. The detailed information of two middle points is presented in Figure S4. Conclusions In summary, we have predicted the newly template-grown MoS2 nanowire on Au(755) and its analogous MoS2 NW@Ni(755) and MoS2 NW@Cu(755), owns both abundant active sites and greatly improved intrinsic HER activity as compared to other MoS2 based catalysts. The free energy for hydrogen adsorption is close to ~ 0 eV on both edges and more importantly, the kinetic barrier of H2 evolution through the Tafel mechanism can be as low as 0.48 eV on the Mo edges. The ultra-low kinetic barrier of MoS2 NW@Au(755) and MoS2 NW@Ni(755) suggests that the H2 evolution rate is at least 16 orders faster than other MoS2 based catalysts and even 5 orders faster than that on the Pt(111) surface. The Volmer-Tafel route is thus possible and a Tafel slope as low as 30 mV/decade is expected for MoS2 based catalysts. The ultra-low kinetic


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barriers are originated from the greatly reduced H-H distance between two neighboring adsorbed hydrogen atoms, suggesting that H-H distance may be served as a simple descriptor for intrinsic HER activity of the electrochemical catalysts. This work suggests that template-grown of MoS2 nanowire catalysts provide a promising way to achieve the greatest HER performance of MoS2 based catalysts, which is even better than the noble metal catalyst Pt. ASSOCIATED CONTENT Supporting Information. The detail information of the models we used, the partial density of states of S atom p orbital at different sites of MoS2@Au(755), Cu(755) and Ni(755), free energy diagram for the adsorption of the second H adjacent to the first one and the minimum-energy pathways of the Tafel reaction on S edges of MoS2 NW@Cu(755) Ni(755) as well as the binding energies of MoS2 NWs with different substrates are included in the Supporting Information. AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT This work is supported by the National Science Funding (Grants 21525311, 21373045), the Ministry of Science and Technology (Grant 2017YFA0204800), Jiangsu 333 project (BRA2016353), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1670) in China. The authors thank the computational resources at the SEU and National Supercomputing Center in Tianjin.


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Template-Grown MoS2 Nanowires Catalyze the ... - ACS Publications

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