Hydrogen physisorption based on the dissociative hydrogen chemisorption at the sulphur vacancy of MoS2 surface

We provide a new insight that the sulphur-depleted MoS2 surface can store hydrogen gas at room temperature. Our findings reveal that the sulphur-vacancy defects preferentially serve as active sites for both hydrogen chemisorption and physisorption. Unexpectedly the sulphur vacancy instantly dissociates the H2 molecules and strongly binds the split hydrogen at the exposed Mo atoms. Thereon the additional H2 molecule is adsorbed with enabling more hydrogen physisorption on the top sites around the sulphur vacancy. Furthermore, the increase of the sulphur vacancy on the MoS2 surface further activates the dissociative hydrogen chemisorption than the H2 physisorption.

eV, and valence band, obtained at hν = 50 eV. The cleaved MoS2 surface (solid lines) was exposed to the H2 gas for 2100 L (dashed lines), and then post-annealed at 300 o C for 30 min., together with exposing to the H2 gas.
Another sample was cleaved in the UHV chamber. The cleaved MoS2 surface shows the existence of the carbon impurity in the inset of Fig. S3a. In contrast to the cleaved surface of main text, this surface was inert to the H2 exposure of 2100 L. When the sample was directly annealed, the carbon impurity was disappeared, together with exposing to H2 gas. After annealing 1 , all the spectra were shifted toward EF.
For comparison, they were aligned to those before annealing; +0.52 eV for both Mo 3d (Fig. S3b) and S 2p ( Fig. S3c) peaks, and +0.60 eV for valence band (Fig. S3d). Interestingly, the C4-like feature of Fig.   3a is appeared at the higher binding energy side of both Mo 3d and S 2p core-level spectra (indicated by 4 red arrows) with the relative intensity ratio of 5 % to the main peaks. This means that the hydrogenation, i.e., the hydrogen bonding to the Mo and S atoms, is well activated at the elevated temperature. Notably, although the VBM was shifted from 1.35 eV to 0.75 eV after the H2 annealing, it is still far away from EF, to be remained in the n-type conductivity. This a clear difference of the current surface in comparison with that of the main text (Fig. 2e).

Fig. S4
(a)-(d) Photoemission spectra of C 1s, Mo 3d and S 2p core levels, and valence band, obtained at hν = 340 eV. The air-exposed MoS2 surface was exposed to the H2 gas for 3600 L, and then postannealed at 300 o C for 1h, together with exposing to the H2 gas. Figure S4a shows that the binding energy of the adsorbed carbon on the air-exposed MoS2 surface moves from 284.60 eV to 285.00 eV with a reduced intensity after the initial H2 exposure (30 L), resulting in the reaction of hydrocarbon on the surface. And then, although the binding energies of all 5 spectra are slightly shifted toward EF at the further exposure of H2 molecules or even after the hydrogenation, the intensities are remained constant. The strong hydrocarbon seems to make the MoS2 surface catalytically inactive.
It has been reported that the mechanical exfoliation of the MoS2 sample occasionally creates the VS defect 2 , the increase of the VS density reduces the bandgap and enhance the adsorption strength on the 6 VS sites 3 . Thus, in order to clarify the intrinsic effect of the increased VS density on the cleaved MoS2 surface, we repeatedly performed the PES measurements by cleaving the several MoS2 samples in the UHV chamber. Figures S5a-c show the changes of the C 1s, Mo 3d and S 2p core-level spectra, which were obtained at the air-exposed MoS2surface (i), cleaved surface in the UHV chamber (ii), and then exposed to the hydrogen gas (iii). All PES measurements were performed at room temperature. The influence of the hydrogen interaction on the air-exposed MoS2 surface (i) is explained in Fig. S4. In Fig.   4d, the Mo 3d and S 2p core levels and valence-band spectra of the cleaved MoS2 surface (ii) moved toward the high binding energy side by ~0.4 eV after exposing to the H2 gas (iii). It is likely the electron doping. Their intensities were also decreased as shown in Fig. S5e. In addition, the full width at half maximum of both Mo 3d and S 2p was slightly increased due to the hydrogen bonds (see Fig. S3).
Notably, the change of these spectra is in contrast to that of the C 1s spectrum. After exposing to the H2 gas at this condition, the spectrum of the carbon impurity moved to the low binding energy side ( Fig.   S5d) with increasing intensity (Fig. S5e). Although the carbon impurity can reduce the intensities of the MoS2 surface-related spectra, it is difficult to understand the contrasting binding energy shift of those spectra with the C 1s spectrum (see Fig. S4).
On the other hand, we note that the VBM (0.22 eV) of the cleaved surface (Fig. S6a) is very closer to the EF than that of Fig. 2e (0.45 eV). This is consistent with that the more defective MoS2 surface exhibits the p-type feature 4 . However, after the H2 exposure, the VBM moved to be ~0.69 eV. This is in contrast with the H2 physisorption showing no (binding) energy shifts in the calculations of the one VS defect (Fig. 1) and PES spectra (Fig. 2). In order to understand these conflicting results, we performed the DFT calculations. Similar to the line defect 5 , in the case of the two VS defects on the 3 × 3 monolayer, the adsorption energy is dramatically reduced to -1.143 eV for the dissociative chemisorption of H2 molecule. The bandgap of the DOSs is of 1.45 eV, which is smaller than that (1.61 eV) of the system having one Vs defect (Fig. 1f). On the other hand, when two H atoms are bonded with the exposed Mo atoms around two VS defects, the DOSs are shifted toward low (high binding) energy side by -0.78 eV. These results elucidated that the dissociative hydrogen chemisorption is more activated than the H2 physisorption as the VS density increases. These results explain the reason of the dissociative hydrogen chemisorption at the substoichiometric MoSx surface 6 . The DFT calculations based on the optB86b-vdW functional 7 result in much higher adsorption energies (Figs. S7) than those of the PBEsol functional (Fig. 1). For example, the adsorption energy of the dissociative hydrogen chemisorption is of -0.212 eV (-0.223 eV) for monolayer (bilayer) system (Fig.  S7g). Additionally, the dissociative chemisorption of H2 molecule in the case of the two VS defects on the 3 × 3 monolayer results in the adsorption energy of -0.925 eV. More remarkably, the adsorption energy of the TS position is higher than those of the other sites and remains almost unchanged even after the creation of the VS. Especially, the TMo and TH sites in the bilayer system (Fig. S1h) have more reduced adsorption energies than that of the T(VS) site. These results including vdW interaction imply that the Ts site is unfavorable for the H2 physisorption. However, it is in contrast with the PES results showing the reduction of the S 3s and 2p core-level spectra (Figs. 2 and 3). This difference is supposed to be related to the increased lattices in the vdW calculations.