Monolayer MoS₂ film supported metal electrocatalysts: a DFT study

Abstract

The structures and electrocatalytic performance of metal clusters (Pd, Pt and Ag) supported on monolayer MoS₂ film were investigated using DFT calculation and compared with those supported on graphene. The calculation revealed the clusters supported on MoS₂ were more stable than those on graphene because of the much larger binding energy and smaller distance between the cluster and support surface. Furthermore, metal clusters on MoS₂ had improved electrocatalytic activity compared to those on graphene due to the more continuous d states of PDOS and larger negative charge, which was further confirmed by the free energy of hydrogen evolution. This study revealed that the monolayer MoS₂ film could be a better support for metal electrocatalysts than graphene.

---

Introduction

Two-dimensional (2D) nanomaterials have received great attention in the last decade since the successful preparation of single-layer graphene.^1–4 Because of the outstanding mechanical, thermal and electrical properties, graphene has been applied as a support for metal electro-catalysts, which exhibited superior performance than those supported on other carbon materials, for example, Pd/graphene (Pd/G) towards formic acid oxidation reaction,⁵ Pt/G towards methanol oxidation,⁶ Ag/G or Au/G towards oxygen reduction reaction.^7,8 Recently, layered transition metal dichalcogenides (TMDs) as novel 2D materials, for example monolayer MoS₂, have drawn significant attention because some of them are semiconductors with tunable bandgap and are naturally abundant.^9–11 MoS₂ has been studied as support for metal electrocatalysts in various applications.¹² For example, Zhang group prepared MoS₂ supported Pt nanoparticles using wet-chemical methods and obtained much higher electrocatalytic activity towards hydrogen evolution reaction (HER).¹³ Kim and co-workers reported Au nanoparticles decorated MoS₂ by a spontaneous redox reaction in water which showed significantly enhanced electrocatalytic performance towards HER.¹⁴ Huang et al. developed a facile method to prepare Pd/MoS₂ by using carboxymethyl cellulose as a stabilizer in aqueous solution and studied the electrocatalytic activity for methanol oxidation reaction.¹⁵ Though varied metal electrocatalysts supported on monolayer MoS₂ have been reported and superior electrocatalytic activity has been achieved, there is lack information of the mechanism for the good electrocatalytic performance of MoS₂ supported metal nanocatalysts.

Herein, we carried out density functional theory (DFT) calculation to investigate monolayer MoS₂ supported metal nanoclusters (Pd, Pt, Ag) as electrocatalysts for HER and compared with those supported on graphene respectively. The calculation showed metal clusters had larger binding energy on MoS₂ than those on graphene which made MoS₂ supported metal nanoparticles more stable. Meanwhile the more continuous d states of the partial density of states (PDOS) resulted in MoS₂ supported metal nanoparticles more active as electrocatalyst, which was further clarified using the free energy of hydrogen evolution of metal clusters on different supports. This work provides a detailed mechanism of monolayer MoS₂ film supported metal nanoparticles as effective electrocatalysts.

Methods

The geometries and electronic property of metal clusters were studied by DFT calculations using VASP program^16–19 in a supercell of 12.3 × 12.3 × 15 ų for graphene support and 9.83 × 9.83 × 15 ų for MoS₂. The supercell was large enough to reduce the interaction between clusters in adjacent cells. During the calculation, the projector-augmented wave (PAW) formalism implementing the generalized gradient approximation as parameterized by Perdew et al. (PBE)²⁰ was used. The plan-wave kinetic energy cutoff was 300 eV, and the k-points meshes were set as 12 × 12 × 1. All structures were fully relaxed until the convergence in energy and force reached 1.0 × 10⁻⁵ eV and 1.0 × 10⁻⁴ eV Å⁻¹ and ensured the configures were global minimization.^21,22 The convergence tests revealed all these parameters were sufficient. The distance between the metal cluster and support was defined as the distance between the center of mass of metal cluster and support respectively. The interaction between metal cluster and support was evaluated by the binding energy (E_b) as

E_b = E_m–s − E_m − E_s

where E_m, E_s, and E_m–s are the energy of metal cluster, support, and metal cluster–support system respectively. The binding energy was corrected using D3 correction method.²³

The catalytic activity of metal clusters towards hydrogen evolution reaction was described by the free energy of hydrogen at zero pH and zero potential²⁴ as

ΔG_H^* = ΔE_H + ΔE_ZPE − TΔS_H

where ΔE_H is the adsorption energy of hydrogen, ΔE_ZPE and ΔS_H are the difference in zero point energy and entropy between hydrogen at the adsorbed state and the gas phase, respectively. ΔE_ZPE is used as 0.04 eV as reported by Nørskov's work.²⁵ Meanwhile, where is the entropy of H₂ in the gas phase at standard conditions. As a result, ΔG_H^* = ΔE_H +0.24 eV.

Results and discussion

Fig. 1 showed the relaxed structures of Pd, Pt and Ag clusters on graphene and MoS₂ respectively. The binding energy and distance between the center of mass of metal clusters and support were summarized in Table 1. The Pd cluster on graphene or MoS₂ exhibited a much ordered structure with three atoms on the bottom and four atoms on the top. The distance and binding energy between Pd cluster and graphene was 3.44 Å and −1.87 eV respectively. The distance between Pd cluster and MoS₂ reduced to 2.47 Å, meanwhile the binding energy significantly increased to −5.68 eV. The reduced distance and increased binding energy indicate Pd cluster was more stable on MoS₂ than that on graphene. Same phenomenon appeared for Pt and Ag clusters. For Pt, the distance reduced from 3.59 Å to 2.40 Å for Pt/G and Pt/MoS₂ respectively, and the binding energy increased from −2.77 eV to −7.11 eV. For Ag cluster, the distance reduced from 3.60 Å to 2.95 Å and the binding energy increased from −0.88 eV to −3.12 eV when the support varied from graphene to MoS₂. Therefore, all three metal clusters supported on MoS₂ had smaller distance and larger binding energy than those on graphene respectively, which indicates MoS₂ was a more stable support for these metal clusters.

Fig. 1 Relaxed structures of different metal clusters supported on graphene or MoS₂.

Table 1 The distance, binding energy (E_b), and Bader charge of different metal clusters on graphene and MoS₂ respectively

	Distance/Å	Eb/eV	Charge/e
Pd/G	3.44	−1.87	+0.259
Pd/MoS2	2.47	−5.68	+0.255
Pt/G	3.59	−2.77	+0.056
Pt/MoS2	2.40	−7.11	−0.032
Ag/G	3.60	−0.88	+0.361
Ag/MoS2	2.95	−3.12	+0.657

---

The partial density of states (PDOS) and Bader charge were calculated to better understand the electronic structures of metal clusters supported on graphene or MoS₂. Fig. 2 showed the d states of PDOS of Pd, Pt and Ag clusters on graphene and MoS₂ respectively. The d states of PDOS of Pd cluster on MoS₂ were more continuous than that on graphene as shown in Fig. 2a. The trend for Pt or Ag was quite similar. According to Zhuang's work,²⁶ more continuous d states of PDOS would contribute to a higher electrocatalytic activity.

Fig. 2 The d states of PDOS of (a) Pd, (b) Pt, and (c) Ag clusters supported on graphene in comparison on those on MoS₂ respectively.

The Bader charges of metal clusters on graphene or MoS₂ were shown in Table 1 to investigate the charge transfer between metal clusters and support. For Pd cluster, it held a charge of +0.259 on graphene and +0.254 on MoS₂ respectively. For Pt cluster, it held a positive charge of +0.056 on graphene and a negative charge of −0.032 on MoS₂. However, the trend of Ag cluster is opposite where it held a positive charge of +0.361 on graphene and a larger positive charge of +0.657 on MoS₂. A metal nanoparticle with more negative charge would have better ability for charge transfer towards HER, therefore, MoS₂ supported Pd or Pt nanoparticles would be good electrocatalysts towards HER.

The electronic structure of Pt cluster on graphene or MoS₂ was further investigated by the difference charge density as shown in Fig. 3. Fig. 3a showed the difference charge density of Pt/G. The negative charge mainly appears between Pt cluster and graphene, and positive charge appears around Pt atoms, which indicates there was charge transfer from Pt atoms to graphene. However, Pt atoms held more negative charges on MoS₂ as shown in Fig. 3b. The difference charge density revealed that Pt cluster on MoS₂ have more negative charge than that on graphene, which is in consistent with the Bader charge calculation.

Fig. 3 Difference charge density of Pt cluster supported on (a) graphene and (b) MoS₂. Green: positive; blue: negative.

The free energy of hydrogen evolution (ΔG_H^*) at zero pH value and zero potential could be a good descriptor to evaluate the electrocatalytic activity of a catalyst towards HER, where the good catalyst should have a free energy close to zero.^24,27 ΔG_H^* of Pd, Pt and Ag clusters on graphene or MoS₂ were calculated and illustrated in Fig. 4. ΔG_H^* was −0.614 eV and −0.598 eV for Pd clusters on graphene and MoS₂ respectively, which indicated the electrocatalytic activity of Pd/G or Pd/MoS₂ had only slight difference. For Pt/G and Pt/MoS₂, ΔG_H^* was −0.272 eV and −0.156 eV respectively which showed Pt/MoS₂ had improved activity towards HER catalysis. The ΔG_H^* for Ag/G and Ag/MoS₂ was −0.403 and 0.588 eV respectively, which indicates Ag/MoS₂ has worse activity than Ag/G towards HER. The trend of activity of the catalysts indicated by ΔG_H^* agreed well with the Bader charge analysis as shown in Table 1, where a metal cluster with more negative charge exhibits improved catalytic activity towards HER. The results revealed Mo-based materials would be a good candidate for promising HER catalysts.^28–31

Fig. 4 The free energy diagram of hydrogen evolution at zero pH and zero potential for Pd, Pt, and Ag clusters supported on graphene and MoS₂ respectively.

Conclusions

In summary, the structures and electrocatalytic performance towards hydrogen evolution reaction of Pd, Pt and Ag clusters supported on monolayer MoS₂ film were evaluated using DFT calculation and compared to those on graphene respectively. The clusters supported on MoS₂ exhibited much smaller distance and larger binding energy, which indicated these metal nanoparticles were more stable on MoS₂ than those on graphene. For example, the binding energy of Pt/MoS₂ (−7.11 eV) was twice higher than that of Pt/G (−2.77 eV). The more continuous d states of PDOS and larger negative charge resulted in improved electrocatalytic activity of Pt cluster on MoS₂ than that on graphene, which was further confirmed by the free energy of hydrogen evolution reaction. This study revealed MoS₂ could be a better support for metal electrocatalysts than graphene.

Acknowledgements

This study was financially supported by grants from the National Natural Science Foundation of China (21503010), International Science & Technology Cooperation Program of China (2015DFG52700) and the Fundamental Research Funds for the Central Universities.

Notes and references

1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666.
2. K. S. Novoselov, V. I. Falko, L. Colombo, P. R. Gellert, M. G. Schwab and K. Kim, Nature, 2012, 490, 192.
3. J. Molina, RSC Adv., 2016, 6, 68261.
4. F. Liu and D. Xue, Sci. China: Technol. Sci., 2015, 58, 1841.
5. H. Wang, S. Lu, Y. Zhang, F. Lan, X. Lu and Y. Xiang, J. Mater. Chem. A, 2015, 3, 6282.
6. Y. Li, L. Tang and J. Li, Electrochem. Commun., 2009, 11, 846.
7. R. Liu, X. Yu, G. Zhang, S. Zhang, H. Cao, A. Dolbecq, P. Mialane, B. Keita and L. Zhi, J. Mater. Chem. A, 2013, 1, 11961.
8. H. Yin, H. Tang, D. Wang, Y. Gao and Z. Tang, ACS Nano, 2012, 6, 8288.
9. K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, Phys. Rev. Lett., 2010, 105, 136805.
10. I. Song, C. Park and H. C. Choi, RSC Adv., 2015, 5, 7495.
11. M. Bosi, RSC Adv., 2015, 5, 75500.
12. C. Tan and H. Zhang, Chem. Soc. Rev., 2015, 44, 2713.
13. X. Huang, Z. Zeng, S. Bao, M. Wang, X. Qi, Z. Fan and H. Zhang, Nat Commun., 2013, 4, 1444.
14. J. Kim, S. Byun, A. J. Smith, J. Yu and J. Huang, J. Phys. Chem. Lett., 2013, 4, 1227.
15. L. Yuwen, F. Xu, B. Xue, Z. Luo, Q. Zhang, B. Bao, S. Su, L. Weng, W. Huang and L. Wang, Nanoscale, 2014, 6, 5762.
16. G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 47, 558.
17. G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 49, 14251.
18. G. Kresse and J. Furthmüller, Comput. Mater. Sci., 1996, 6, 15.
19. G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169.
20. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865.
21. S. J. Li, X. Zhou and W. Q. Tian, J. Phys. Chem. A, 2012, 116, 11745.
22. H. Wang, S. Lu, Y. Zhang, F. Lan, J. Shang and Y. Xiang, ChemPlusChem, 2016, 81, 172.
23. S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104.
24. B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Nørskov, J. Am. Chem. Soc., 2005, 127, 5308.
25. J. K. Nørskov, T. Bligaard, A. Logadottir, J. R. Kitchin, J. G. Chen, S. Pandelov and U. Stimming, J. Electrochem. Soc., 2005, 152, J23.
26. Y. Sun, L. Zhuang, J. Lu, X. Hong and P. Liu, J. Am. Chem. Soc., 2007, 129, 15465.
27. J. K. Nørskov, T. Bligaard, J. Rossmeisl and C. H. Christensen, Nat. Chem., 2009, 1, 37.
28. Z. Xing, Q. Liu, A. M. Asiri and X. Sun, Adv. Mater., 2014, 26, 5702.
29. W. Cui, N. Cheng, Q. Liu, C. Ge, A. M. Asiri and X. Sun, ACS Catal., 2014, 4, 2658.
30. W. Zhu, C. Tang, D. Liu, J. Wang, A. M. Asiri and X. Sun, J. Mater. Chem. A, 2016, 4, 7169.
31. C. Tang, L. Gan, R. Zhang, W. Lu, X. Jiang, A. M. Asiri, X. Sun, J. Wang and L. Chen, Nano Lett., 2016, 16, 6617.

---