3.4 Results and Discussion
3.4.3 Theoretical Insight for Improved HER Performance
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Figure 3.16. HER performances and Tafel plots of all synthesized catalysts in (a, b) 0.5 M H2SO4 and (c, d) 0.1 M PBS electrolytes. (e) Comparison of HER performances of Co9S8-MoS2 in different pH of electrolytes. (f) The 𝜂10 and 𝜂50 values in each electrolyte are compared.
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3.17, Co9S8 (111) surface has the lowest surface energy. Thus, the most stable (111) surface of Co9S8
was used to model the Co9S8-MoS2 heterostructure.
Figure 3.17. The calculated surface energy for (a) (100) surface, (b) (110) surface, and (c) (111) surface of Co9S8 slab.
Besides, there are two configurations for growing Co9S8 NP on the MoS2 sheet. The binding energy for Co atoms on S-top is higher than that on Mo-top positions (Figure 3.18), hence the model 0D-2D heterostructure of Co9S8-MoS2 could be depicted as in Figure 3.19a. During reductive annealing, H2S gas is in-situ generated leaving S defects behind on the MoS2 basal plane and reacts with Co3O4 toform Co9S8. Thus, the formation energies for every single defect near the Co9S8 NP according to the distance were calculated in Figure 6d. A defect can be formed near the Co9S8 NP because the formation energy near the NP is lower than other sites. Therefore, the defect site with the lowest formation energy (–
0.20697 eV) was utilized to model the single-defected Co9S8-MoS2 as shown in Figure 6b. The formation energy of the second defect as in Figure 6c was also estimated as –0.20715 eV, which is similar to the first defect formation energy.
Figure 3.18. Two configurations for growing the Co9S8 NP on the MoS2 layer and corresponding binding energies: Co atoms bonded on (a) Mo-top and (b) S-top positions.
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Figure 3.19. (a) A side view of the optimized Co9S8-MoS2 geometries for DFT calculation. The box with dotted lines is magnified with (b) The formation energies of single S defect by the distance between the Co9S8 NP and a defect site. (c) Single S defect (red), and (d) two defects (red and purple). The S bonded with Co (S′) is colored with olive green. The atomic position of Mo in a single-defected Co9S8- MoS2 model is determined as follows: Mo1 = bonded with three S′, Mo2 = bonded with one S′ and one defect site, and Mo3 = bonded with one S′. The atomic position of Mo in a two-defected Co9S8-MoS2
model is also defined: Mo1 = bonded with no defect site, Mo2= bonded with one defect site, and Mo3
= bonded with two defect sites.
Heterostructuring Co9S8 NP on the MoS2 sheets brought the charge transfer from Co to Mo as verified by XPS and XANES/EXAFS spectra. To elucidate the change in electronic structure, the projected density of state (PDOS) of Mo 3d orbitals for each Mo atomic position and the charge density difference derived by Co9S8-MoS2 binding are shown in Figure 3.20 and Figure 3.21. For the single-defected Co9S8-MoS2 in Figure 3.20b, the Mo2 position connected to the defect site shows stronger electron localized state compared to Mo1 and Mo3 positions. For the two-defected Co9S8-MoS2 in Figure 3.21b, the Mo2 and Mo3 connected with one and two defect sites have some electronic states near the Fermi energy level (EF). The charge density difference (i.e. charge accumulation and depletion) and the amount of transferred charge (ΔQ) are also depicted in Figures 3.20c, 3.21c. The strong charge
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redistribution occurs mainly at the Co-S-Mo interface. The quantitative values of ΔQ calculated by the Bader charge analysis confirmed that the electron density was transferred from Co to Mo site in single- defected (0.2833 e) and two-defected (0.4129 e) Co9S8-MoS2. The charge transfer indicates the strong interaction between Co9S8 and MoS2 phases and the newly occurred electronic state near the Fermi level reveals the enhanced electron-doped characteristic of the inert MoS2 basal plane in the Co9S8-MoS2
heterostructure.14, 67
Figure 3.20. (a) The optimized geometries for the single-defected (red) Co9S8-MoS2. The atomic positions of Mo in a single-defected Co9S8-MoS2 model are determined as follows: Mo1 = bonded with three S′, Mo2 = bonded with one S′ and one defect site, and Mo3 = bonded with one S′, where S′
indicates the S atom bonded with Co. (b) Comparison of PDOS for each Mo site. (c) The charge density difference and the amount of transferred charge (∆Q) for Co9S8-MoS2 heterostructure along the z- direction.
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Figure 3.21. (a) The optimized geometries for the two-defected (red and purple) Co9S8-MoS2. The atomic positions of Mo in a two-defected Co9S8-MoS2 model are determined as follows: Mo1 = bonded with no defect site, Mo2= bonded with one defect site, and Mo3 = bonded with two defect sites. (b) Comparison of PDOS for each Mo site. (c) The charge density difference and the amount of transferred charge (∆Q) for Co9S8-MoS2 heterostructure along the z-direction.
According to d-band center theory, the d-band center away from the Fermi energy level usually leads to more antibonding states filled, weakening the adsorption of H*. The d-band center of single- and two- defected Co9S8-MoS2 obtained from the PDOS in Figure 3.20b, 3.21b are compared in Figure 3.22a.
The d-band center of Mo2 position in single-defected Co9S8-MoS2 is the closest to the EF, thus it has the strongest H adsorption, matching well with Figure 3.22c. The d-band center of two-defected Co9S8- MoS2 is also compared and consequent H adsorption Gibbs free energy (ΔGH) is obtained in Figure 3.22d. The Mo3 position connected with two defect sites is the closest to the EF, resulting in the strongest H adsorption. However, ΔGH near zero is ideal due to the optimal strength in H adsorption, as stated by the Sabatier principle.68 The ΔGH of Mo3 is farther from zero, thus too strong H adsorption.
Therefore, the ΔGH values of Mo2 for single- and two-defected Co9S8-MoS2 are compared to pristine Co9S8 and MoS2 in Figure 3.22b and Table 3.3. The charge density of two-defected Co9S8-MoS2 was more transferred, but the d-band center is much closer to EF causing too strong H adsorption to occur.
The stronger H adsorption on the Mo2 site of two-defected Co9S8-MoS2 profits concentrating H*, but
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the Mo2 site of single-defected Co9S8-MoS2 has near-zero ΔGH value, indicating it is the more active site for HER.36 In the DFT calculations, the synergistic effect and the HER active site of our covalent Co9S8-MoS2 heterostructure with in-situ formed defects were thoroughly revealed: (i) The electron density is transferred from Co to Mo near the interface of the heterostructure. (ii) The S defects can be formed around the Co9S8 NP. (iii) The Mo site neighboring the defect is the most active site for HER, which is considered as optimal H adsorption center having near-zero ΔGH value as well as a H concentrating center.
Figure 3.22. (a) The position of d band center for each Mo atomic position in the single and two- defected Co9S8-MoS2. (b) The Gibbs free energies of hydrogen adsorption for single and two-defected Co9S8-MoS2, MoS2, and Co9S8. The Gibbs free energies of hydrogen adsorption on each Mo site for (c) single- and (d) two-defected Co9S8-MoS2.
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Table 3.3. The Gibbs free energies of hydrogen adsorption for single and two-defected Co9S8-MoS2, MoS2, and Co9S8.
Catalyst ΔEH (eV) ΔGH (eV)
MoS2 -0.11378 0.127222
Co9S8 -0.78572 -0.54472
Single-Defect Co9S8-MoS2
Mo1 1.623887 1.864887
Mo2 -0.24763 -0.00663
Mo3 1.39496 1.63596
Two-Defects Co9S8-MoS2
Mo1 0.558802 0.799802
Mo2 -0.33621 -0.09521
Mo3 -0.4017 -0.1607