Efficient Hydrogen Evolution Reaction in 2H-MoS 2 Basal Planes Enhanced by Surface Electron Accumulation

: An innovative strategy has been developed to activate the basal planes in molybdenum disulfide (MoS 2 ) to improve their electrocatalytic activity by controlling surface electron accumulation (SEA) through aging, annealing, and nitrogen-plasma treatments. The optimal hydrogen evolution reaction (HER) performance was obtained on the surface treated with nitrogen-plasma for 120 s. An overpotential of 0.20 V and a Tafel slope of 120 mV dec − 1 were achieved for the optimized condition. The angle-resolved photoemission spectroscopy measurement confirmed the HER efficiency enhanced by the SEA conjugated with the sulfur vacancy active sites in the MoS 2 basal planes. This study provides new insight into optimizing MoS 2 catalysts for energy applications


Introduction
Hydrogen (H 2 ) holds great attention as a green energy carrier for future technologies due to its high energy density and being environment friendly [1,2].Electrochemical water-splitting is a promising route for sustainable H 2 production due to its advantages of abundant sources and non-pollutants [2,3].Hydrogen evolution reaction (HER) is one of the most efficient pathways to produce H 2 from water splitting using suitable catalysts.Currently, platinum (Pt) and other noble metals or alloys are the most popular electrocatalysts for HER, while their scarcity and high cost limit their widespread utilization [4,5].The new challenge for researchers is to develop earth-abundant, low-cost, and high-performance catalysts as an alternative to Pt and other noble metals for electrochemical H 2 production.
Two dimensional (2D) transition metal dichalcogenides (TMDCs) have been extensively investigated as promising alternative catalysts for HERs due to their exceptional physicochemical properties including high mechanical properties, good semiconducting ability, large surface area, and high catalytic activity [6][7][8][9][10][11][12].Among TMDCs, molybdenum disulfide (MoS 2 ) has been found to be one of the most economical alternatives to noble metal catalysts for HER due to its high electron mobility, stability, non-toxic nature, low cost, and excellent electrocatalytic properties [13][14][15].Due to its inert basal plane, the overall activity of the two-layered hexagonal phase MoS 2 (2H-phase MoS 2 ) catalyzes mainly from the active edge sites, which limits the practical application of MoS 2 for HER [9,[16][17][18].Various techniques have been developed to overcome the limited catalytic activity of the MoS 2 basal plane, such as interface electronic coupling, phase engineering, and introducing active unsaturated defects and strain [19][20][21][22][23][24].The pioneer strategy to activate the basal plane of MoS 2 that enhances HER activity was the introduction of sulfur (S)-vacancies into the basal plane [22,25,26].
In HER, the optimal H 2 adsorption free energy (∆G H ) is 0 eV, where H 2 is bound to the catalyst neither too strongly nor too weakly.For the basal plane of pristine 2H-MoS 2 , the ∆G H value is approximately 2 eV, making the basal plane inert to HER performance.
The ∆G H values can be decreased with an increase in S-vacancies on the MoS 2 basal plane.A ∆G H value between ±0.08 eV for S-vacancies in the range of 9-19% is the most favorable value for HER [22,[27][28][29].To date, a limited number of techniques have been used to create S-vacancies on 2D TMDC materials such as electrochemical reduction, H 2 annealing, and plasma bombardment [29][30][31].According to our previous work, long term air exposure (aging) spontaneously creates S-vacancies, which cause surface electron accumulation (SEA) on MoS 2 surfaces [32].
In this report, we utilize and control SEA to activate the basal planes of 2H-MoS 2 .The SEA was performed through various techniques such as aging, annealing, and nitrogen (N 2 )-plasma treatments to create S-vacancies on the basal plane of 2H-MoS 2 .The effect of SEA on HER efficiency was investigated.Angle-resolved photoemission spectroscopy (ARPES) measurements were performed to confirm the electronic structures of different MoS 2 surfaces.The results revealed that 2H-MoS 2 was converted from intrinsic (fresh) to degenerate semiconductor (using N 2 -plasma treatments) by creating S-vacancies which provided more active sites and charge carriers for HER.

Structural Characterization
Figure 1a illustrates the crystal structure of the MoS 2 monolayer formed by a hexagonal plane of Mo and S atoms.These triple planes are stacked together by weak van der Waals forces of attraction to form bulk MoS 2 .The van der Waals force allows the preparation of mono and a few layers of MoS 2 flakes through a simple mechanical exfoliation technique using dicing tape (see Figure 1b).The bulk fresh surface was created by the exfoliation of the top layers of pristine MoS 2 and the other pristine MoS 2 crystal was subjected to N 2 -plasma treatments.The X-ray diffraction (XRD) pattern of pristine, fresh, and N 2 -plasma-treated bulk samples is shown in Figure 2. The diffraction peaks, located at 14.3 • , 29.0 • , 44.1 • , 60.1 • , and 77.9 • were assigned to (002), (004), (006), (008), and (0010) planes of standard 2H-MoS 2 , respectively (JCPDS #872416) [32].The diffraction peak along the (002) plane was the most prominent compared with the other peaks.Moreover, a broad reflection around 21 • was observed for the fresh and pristine MoS 2 samples which may have been due to the glass substrate [33].A reduction in the intensity of diffraction peaks of fresh and N 2 -plasma MoS 2 was observed when compared with pristine MoS 2 , and this may have been due to the absence of constructive interference from the bulk crystal planes [34].The position of diffraction peaks did not show any shift, which indicated that there was no inner structural change in the MoS 2 samples after N 2 -plasma treatment.
may have been due to the glass substrate [33].A reduction in the intensity of diffraction peaks of fresh and N2-plasma MoS2 was observed when compared with pristine MoS2, and this may have been due to the absence of constructive interference from the bulk crystal planes [34].The position of diffraction peaks did not show any shift, which indicated that there was no inner structural change in the MoS2 samples after N2-plasma treatment.(out-of-plane) modes of bulk MoS2, respectively [35,36].The 2H-MoS2 phase contains four may have been due to the glass substrate [33].A reduction in the intensity of diffraction peaks of fresh and N2-plasma MoS2 was observed when compared with pristine MoS2, and this may have been due to the absence of constructive interference from the bulk crystal planes [34].The position of diffraction peaks did not show any shift, which indicated that there was no inner structural change in the MoS2 samples after N2-plasma treatment.Figure 3 shows the Raman spectra of pristine, fresh, and N2-plasma-treated MoS2.The observed Raman modes at 385 and 411 cm −1 belong to the  2 1 (in-plane) and  1 (out-of-plane) modes of bulk MoS2, respectively [35,36].The 2H-MoS2 phase contains four Figure 3 shows the Raman spectra of pristine, fresh, and N 2 -plasma-treated MoS 2 .The observed Raman modes at 385 and 411 cm −1 belong to the E 1 2g (in-plane) and A 1g (out-ofplane) modes of bulk MoS 2 , respectively [35,36].The 2H-MoS 2 phase contains four Raman active vibrations, namely, E 1 2g , A 1g , E 1g and E 2 2g , and the absence of the other two modes may have been due to the selection rules of the scattering geometry and limited rejection of Rayleigh scattered radiation [35,37,38].After the exfoliation and plasma treatments, the positions of the observed two modes did not show any shift.However, the peak intensity of the N 2 -plasma-treated sample showed considerable decrement when compared with the pristine MoS 2 sample.This indicated that the lattice distortion on the MoS 2 basal plane was due to the gradual increase in S-vacancies and cracks caused by the N 2 -plasma treatment [31].
rejection of Rayleigh scattered radiation [35,37,38].After the exfoliation and plasma treatments, the positions of the observed two modes did not show any shift.However, the peak intensity of the N2-plasma-treated sample showed considerable decrement when compared with the pristine MoS2 sample.This indicated that the lattice distortion on the MoS2 basal plane was due to the gradual increase in S-vacancies and cracks caused by the N2-plasma treatment [31].

SEA through Aging
To examine the effects of SEA on a MoS2 electrocatalyst for a HER, the fresh surface was exposed to air for a different number of days to create SEA on the MoS2 surface.Figure 4a shows the polarization curves of pristine and fresh MoS2 surfaces with an overpotential of 0.75 and 0.82 V, respectively.The fresh MoS2 used a high overpotential of 0.82 V (vs.RHE) to generate a 10 mA/cm 2 current density.The higher overpotential of fresh MoS2 was due to the intrinsic (insulating) nature of the cleaved fresh surface, which was confirmed by ARPES and scanning tunneling spectroscopy (STS) measurements in our previous study [32].The overpotential of the fresh surface was reduced by the aging effect, which created the S-vacancies.The aging effect of the fresh surface was examined over a different number of days, and their polarization curves are shown in Figure 4c.The overpotential reduced gradually with the increase in aging time and reached a minimum value of 0.31 V for a surface aging of 58 days.This reduction in the overpotential was evidence

Electrochemical HER Efficiency Enhanced by SEA 2.2.1. SEA through Aging
To examine the effects of SEA on a MoS 2 electrocatalyst for a HER, the fresh surface was exposed to air for a different number of days to create SEA on the MoS 2 surface.Figure 4a shows the polarization curves of pristine and fresh MoS 2 surfaces with an overpotential of 0.75 and 0.82 V, respectively.The fresh MoS 2 used a high overpotential of 0.82 V (vs.RHE) to generate a 10 mA/cm 2 current density.The higher overpotential of fresh MoS 2 was due to the intrinsic (insulating) nature of the cleaved fresh surface, which was confirmed by ARPES and scanning tunneling spectroscopy (STS) measurements in our previous study [32].The overpotential of the fresh surface was reduced by the aging effect, which created the S-vacancies.The aging effect of the fresh surface was examined over a different number of days, and their polarization curves are shown in Figure 4c.The overpotential reduced gradually with the increase in aging time and reached a minimum value of 0.31 V for a surface aging of 58 days.This reduction in the overpotential was evidence of the HER efficiency enhanced by SEA on the MoS 2 basal plane due to the aging effect.Generally, the exposure of the sulfide surface to air causes desulfurization (escape of sulfur atoms) and adsorption of foreign molecules such as oxygen and water molecules.The ARPES results of an in situ-cleaved surface and the same surface kept for 11 h excluded the possibility of the SEA being induced by the adsorption of foreign molecules, and it was concluded that the SEA was induced by long-term air exposure originating from the S-vacancy surface defects [32].The S-defects created a donor-like surface state close to the conduction band edge which increased the electron concentration at the MoS 2 surface.
to increase the current density in the HER mechanism by one order of magnitude.Hence, among these, MoS2 aged for 58 days was the most favorable catalyst for HER.Normally, HER activity depends on the number of active sites on, and electron density of, the catalyst.The major active sites for the electrocatalytic reaction were formed mainly by S-vacancies which were created on the MoS2 surface due to the spontaneous desulfurization in air [32].Tafel slope is the linear fitting region in the Tafel plot according to the Tafel equation of η = b log |j| + a, where η is the overpotential, b is the Tafel slope, and a is the current density [39,40].The Tafel plots of pristine and fresh MoS 2 surfaces are presented in Figure 4b, with corresponding Tafel slopes of 248 and 253 mV dec −1 , respectively.The Tafel plots of aging surfaces are illustrated in Figure 4d, and it was noted that the Tafel slope reduced with an increase in aging time.Finally, the lowest Tafel slope of 148 mV dec −1 was obtained for the MoS 2 surface aged for 58 days.Generally, a lower Tafel slope requires less voltage to increase the current density in the HER mechanism by one order of magnitude.Hence, among these, MoS 2 aged for 58 days was the most favorable catalyst for HER.Normally, HER activity depends on the number of active sites on, and electron density of, the catalyst.The major active sites for the electrocatalytic reaction were formed mainly by S-vacancies which were created on the MoS 2 surface due to the spontaneous desulfurization in air [32].

SEA through Annealing
To control the SEA and optimize HER efficiency, the fresh surface was annealed at 80 • C for different times (1, 2, and 3 h).The polarization curves of fresh and annealed MoS 2 surfaces for different times are shown in Figure 5a and corresponding Tafel plots can be seen in Figure 5b.The overpotential of MoS 2 decreased from 0.82 to 0.28 V when the fresh MoS 2 surface was exposed to annealing at 80 • C for 2 h, and the Tafel slope attained the minimum value of 168 mV dec −1 .Further, on increasing the heat treatment to 3 h, the overpotential and Tafel slope increased from 0.28 to 0.32 V and 168 to 199 mV dec −1 , respectively.This could have been due to the increase in S-vacancies over the optimum when the fresh MoS 2 surface was exposed to heat for more than 2 h.The catalytic activity depends on the density of S-vacancies.Li et al. observed maximal catalytic activity for MoS 2 films when the density of S-vacancies was in the range of 7−10%.Further, an increase in the density of S-vacancies reduced the catalytic activity [41].Density functional theory studies have revealed that the optimal HER activity of MoS 2 occurs below the S-vacancy concentration of 12.5% [42].The S-vacancies will disperse homogeneously like point defects up to a vacancy concentration of 12.5%, and with further increases in vacancy concentration (up to 18.75%) some of the vacancies will form agglomerate clusters.At higher vacancy concentrations, the combination of isolated point defects and clustered defects may induce structural defects in MoS 2 .The relatively strong hydrogen adsorption (∆G H = −0.278and −0.290 eV) at higher vacancy concentrations (15.63 and 18.75%) shows very high activity for proton adsorption, but desorption of *H to form H 2 would be kinetically more difficult.Hence, the optimal hydrogen adsorption (∆G H = ±0.15eV) occurs below the S-vacancy concentration of 12.5%, which corresponds to the highly active sites for hydrogen evolution.
potential and Tafel slope increased from 0.28 to 0.32 V and 168 to 199 mV dec −1 , respectively.This could have been due to the increase in S-vacancies over the optimum when the fresh MoS2 surface was exposed to heat for more than 2 h.The catalytic activity depends on the density of S-vacancies.Li et al. observed maximal catalytic activity for MoS2 films when the density of S-vacancies was in the range of 7−10%.Further, an increase in the density of S-vacancies reduced the catalytic activity [41].Density functional theory studies have revealed that the optimal HER activity of MoS2 occurs below the S-vacancy concentration of 12.5% [42].The S-vacancies will disperse homogeneously like point defects up to a vacancy concentration of 12.5%, and with further increases in vacancy concentration (up to 18.75%) some of the vacancies will form agglomerate clusters.At higher vacancy concentrations, the combination of isolated point defects and clustered defects may induce structural defects in MoS2.The relatively strong hydrogen adsorption (ΔGH = −0.278and −0.290 eV) at higher vacancy concentrations (15.63 and 18.75%) shows very high activity for proton adsorption, but desorption of *H to form H2 would be kinetically more difficult.Hence, the optimal hydrogen adsorption (ΔGH = ±0.15eV) occurs below the S-vacancy concentration of 12.5%, which corresponds to the highly active sites for hydrogen evolution.and 5d, respectively.During plasma treatment, the pristine surface was kept 1 cm away from the ICP N 2 plasma generator to avoid rapid S-vacancies and surface damage.The overpotential of the MoS 2 surface reduced from 0.75 to 0.20 V on N 2 -plasma exposure for 120 s.Further increases in N 2 -plasma exposure time resulted in increases in overpotential.Thus, it was concluded that the optimal N 2 -plasma treatment time for better HER activity was 120 s.Generally, the Tafel slopes of 120, 40, and 30 mVdec −1 are an indication of the Volmer, Heyrovsky, and Tafel reaction steps in the HER mechanism, respectively [43,44].The optimal N 2 -plasma treatment for 120 s exhibited the Tafel slope value of 120 mV dec −1 (see Figure 5d) and hence, it followed the Volmer mechanism of HER.

Evidence of Enhanced SEA Observed by ARPES
ARPES measurements were performed for the in situ-cleaved, annealed for 2 h, and N 2 -plasma treatment for 120 s -MoS 2 surfaces to confirm the existence of SEA as shown in Figure 6a.The magnified normal spectra at Γ point can be seen in Figure 6b.The valence band edge (E V ) of the in situ-cleaved surface was at a binding energy of −0.80 eV which is the indication of intrinsic Fermi level (E F ) according to the bulk MoS 2 band gap of >1.3 eV at 85 K [32].The difference between E F and E V (E F − E V = 0.80 eV) of the in situ-cleaved surface was in good agreement with the reported value of 0.80 eV for in situ exfoliated MoS 2 under UHV conditions [45].The SEA of aging surfaces was confirmed by ARPES in our previous work [32].The E V of the MoS 2 surface annealed for 2 h resulted in red shift and moved to a binding energy of −1.16 eV.This moved E F to near the conduction band edge (E C ) (E C − E F = 0.19 eV).The carrier concentration (n) of the annealing surface can be calculated using the following formula [46]: where N C is the effective density of states function in the conduction band and is given by N C = 2 2πm * e kT/h 2 The best HER activity of N2-plasma-treated MoS2 with an overpotential of 0.20 V was compared with other MoS2-based catalysts including nanoflakes [50], thin films [51][52][53], hierarchical hollow architectures [54], homostructures [55], and hybrid nanostructures [56,57], and is tabulated in Table 1.It was noticed that MoS2 bulks performed better than MoS2-based nanostructures and, hence, control of SEA can be a promising strategy to en- The E V showed a remarkable red shift and moved to −1.56 eV for the N 2 -plasmatreated MoS 2 surface.This moved the E F to beyond E C (E F − E C = 0.26 eV), which showed the n-type degenerate semiconductor nature.n can be calculated using the following formula [48]: The calculated n value was in the range of 1.50 × 10 20 -3.10 × 10 20 cm −3 .This value was around five orders of magnitude higher than the MoS 2 bulk value (n = 2 × 10 15 cm −3 ) [49].The higher n on the N 2 -plasma-treated MoS 2 surface denotes the highly conductive nature of the basal plane.This result was consistent with the electrochemical HER results in which the lowest overpotential was obtained for the N 2 -plasma treatment for 120 s on the MoS 2 surface.Hence, it was confirmed that HER activity can be enhanced by producing conjugated active sites and SEA in the 2H-MoS 2 basal plane.
The best HER activity of N 2 -plasma-treated MoS 2 with an overpotential of 0.20 V was compared with other MoS 2 -based catalysts including nanoflakes [50], thin films [51][52][53], hierarchical hollow architectures [54], homostructures [55], and hybrid nanostructures [56,57], and is tabulated in Table 1.It was noticed that MoS 2 bulks performed better than MoS 2based nanostructures and, hence, control of SEA can be a promising strategy to enhance HER activity in 2D TMDCs.In addition, the obtained overpotential of 0.20 V was higher than the overpotential of Ru doped 2H-MoS 2 (0.16 V at 10 mAcm −2 in 0.5 M H 2 SO 4 ) [58], and the MoS 2 nanosheet (0.07 V 10 mAcm −2 in 0.5 M H 2 SO 4 under back-gate voltage of 3V) [59].This suggests that HER activity in MoS 2 basal planes can be improved further by a combination of SEA with doping or Fermi level modulation.

Discussion
The schematic of SEA and its enhancement on HER in 2H-MoS 2 is shown in Figure 7.The SEA was produced by the S-vacancies which act as donor-like surface states.The accumulated free electrons were injected from the donor-like surface states and leave the positively charged state on the surface which produced the downward band bending.This moved the E C below E F according to the ARPES measurement of the N 2 -plasma-treated MoS 2 surface due to its n-type degenerate nature.S-vacancies are the origin of high surface electron concentration and, meanwhile, provide abundant active sites.The conjugated SEA and HER active sites provide the required conditions to enhance the electrochemical activity of HER.
Usually, in MoS 2 , the metallic 1T-phase basal plane is considered for the HER due to the possible source of active sites [9,23].The SEA in 2H-MoS 2 enhances HER activity and this result may be comparable with the excellent HER activity of the metallic 1T-phase MoS 2 [7,23,60].In our previous reports, we observed SEA formation in MoS 2 and MoSe 2 due to the S-and Se-vacancies, respectively [32,48].The MoS 2 catalyst has been widely investigated for HER but, unexpectedly, the effect of SEA on HER efficiency has been ignored in previous reports.This investigation gives a new insight into the tuning of basal plane active sites through S-vacancies by keeping the edge defects that can enhance HER activity.Moreover, the catalytic vacancy sites and abundant free electrons at the surface are the major areas responsible for efficient HER activity.
The SEA was produced by the S-vacancies which act as donor-like surface states.The accumulated free electrons were injected from the donor-like surface states and leave the positively charged state on the surface which produced the downward band bending.This moved the EC below EF according to the ARPES measurement of the N2-plasma-treated MoS2 surface due to its n-type degenerate nature.S-vacancies are the origin of high surface electron concentration and, meanwhile, provide abundant active sites.The conjugated SEA and HER active sites provide the required conditions to enhance the electrochemical activity of HER.Usually, in MoS2, the metallic 1T-phase basal plane is considered for the HER due to the possible source of active sites [9,23].The SEA in 2H-MoS2 enhances HER activity and this result may be comparable with the excellent HER activity of the metallic 1T-phase MoS2 [7,23,60].In our previous reports, we observed SEA formation in MoS2 and MoSe2 due to the S-and Se-vacancies, respectively [32,48].The MoS2 catalyst has been widely investigated for HER but, unexpectedly, the effect of SEA on HER efficiency has been ignored in previous reports.This investigation gives a new insight into the tuning of basal plane active sites through S-vacancies by keeping the edge defects that can enhance HER activity.Moreover, the catalytic vacancy sites and abundant free electrons at the surface are the major areas responsible for efficient HER activity.

Preparation of MoS 2 Layer Crystals
The chemical vapor transport method was used to synthesize MoS 2 single crystals.Fine powders of sulfur (99.99%) and molybdenum (99.99%) were used as source materials.Bromine (Br 2 ) was used as the transport agent and enabled an effective and faster vapor transport to produce MoS 2 single crystals.The source materials including sulfur and molybdenum powders together with Br 2 were sealed in a quartz ampoule of a length of 30 cm at a vacuum of 10 −5 Torr.The inner and outer diameters of the quartz ampoule were 1.3 and 1.6 cm, respectively.The preheated material temperatures of growth MoS 2 single crystals were kept at 950 and 1000 • C, respectively.The controlling temperatures of the source and crystallization ends were maintained at 1050 and 960 • C, respectively.After ten days of processing, the as-synthesized MoS 2 layer crystals area was in the range of a few square millimeters to centimeters.The thickness of the crystals ranged from a few nanometers to hundreds of nanometers.Typical mechanical exfoliation was used for obtaining fresh surface MoS 2 from the pristine layer crystals using dicing tape.

Characterization of MoS 2 Bulks
An X-ray diffraction (XRD-Bruker D 2 phaser diffractometer) system with Cu K α1 radiation (λ = 1.54056Å) and Raman spectroscopy (Jobin-Yvon LabRAM HR800) with a 633 nm He-Ne laser as the excitation source were used to examine the structural characterization of the MoS 2 bulks.The pristine surface (non-fresh) meant that the surface of the as-synthesized crystal was exposed to air for a prolonged period.The fresh surface was obtained by stripping the top layers of the bulk crystal with dicing tape.The N 2 -plasma treatment was performed on a different pristine MoS 2 surface.

N 2 -Plasma Treatment of Pristine MoS 2
An inductively coupled plasma (ICP) system with a commercial 13.56 MHz RF source was used to perform the N 2 -plasma treatment on the MoS 2 /glassy carbon electrode.In the quartz tube, the distance between the samples and the center of the coil was maintained at 1 cm and the samples were placed on the downstream side.After that, the vacuum in the chamber was maintained at 10 mTorr.High purity N 2 (99.999%) was allowed into the quartz tube at a flow rate of 400 sccm.The N 2 -plasma was generated under a pressure of 130 mTorr due to excited N 2 ions when powered by 100 W at ambient temperature.Finally, the pristine MoS 2 surfaces were exposed to a N 2 -plasma atmosphere for fixed different time intervals.

Electrochemical Measurements
An electrochemical test system (Biologic Bi-stat) was used to perform the HER activity measurements using a standard three electrode cell at ambient temperature.The working electrode had a rotating disk electrode (RDE, PINE AFE5T050GC) with a glassy carbon (GC) disk (diameter 5 mm) and a PTFE shroud (diameter 15 mm).The platinum foil and Ag/AgCl were utilized as counter and reference electrodes, respectively.The measured potential values were transferred to a reversible hydrogen electrode (RHE), where E RHE = E Ag/AgCl + 0.059 pH + E • Ag/AgCl .The HER polarization curves were recorded in a 0.5 M H 2 SO 4 (pH ≈ 0.3) electrolyte at a scan rate of 5 mVs −1 .The high purity N 2 gas was passed through the electrolyte to remove other gases.As-prepared MoS 2 catalyst was loaded onto the surface of the GC electrode and thread sealant (Loctite ® ) was used to cover the uncovered GC area beside the MoS 2 catalyst.For the pristine surface, the bulk MoS 2 crystal was placed on the surface of the GC electrode and fixed.For the fresh surface, first we fixed the pristine bulk on the GC electrode and removed the top surface using dicing tape; then for the annealing process, the electrode with the fresh surface was exposed to heating.For the N 2 -plasma-treated samples, first we fixed the pristine bulk on the GC electrode and then exposed it to plasma.Cyclic voltammetry (CV) was performed in the potential range of −1.0 to +0.2 V (vs.RHE) and a scan rate of 50 mVs −1 until the curves reached a stable state.Linear sweep voltammetry (LSV) was executed in the same potential range as CV with a scan rate of 5 mVs −1 .The electrochemical measurements were repeated three times to obtain concordant data.

Angle-Resolved Photoelectron Spectroscopy (ARPES) Measurement
The ARPES experiment was performed at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan, at a BL21B1 U9-CGM beamline.The photoemission spectra of MoS 2 annealed and N 2 -plasma-treated surfaces were measured in a UHV chamber equipped with a hemispherical analyzer (Scienta R4000) with a collecting angle of ±15 • at a base pressure of 5.6 × 10 −11 Torr.The polarization was invariably in the angular dispersive plane.The spectra of all MoS 2 surfaces were recorded with a photon energy of 42 eV at 85 K.The energy resolution was better than 23 meV and the angular resolution was 0.2 • .After loading samples into a load-lock vacuum chamber, the samples were left in a vacuum until the pressure and temperature reached the required steady state.

Conclusions
We improved the electrocatalytic activity of 2H-MoS 2 by activating its basal planes through SEA using aging, annealing, and nitrogen-plasma treatments.The SEA caused by the S-vacancies was catalytically active, and a high electron concentration in the order of ~10 20 cm −3 was obtained for the N 2 -plasma-treated MoS 2 surface.The optimal HER performance was obtained for the surface which underwent N 2 -plasma treatment for 120 s, with an overpotential of 0.20 V vs RHE at 10 mA cm −2 .The ARPES measurements confirmed that the HER efficiency enhanced by the SEA conjugated with the S-vacancy active sites in the 2H-MoS 2 basal planes.This work provides an efficient and low cost MoS 2 -based HER catalyst and also opens up comprehensive insights into basal planes of 2D TMDCs through SEA for energy related applications.

Figure 1 .
Figure 1.(a) The crystal structure of the MoS2 mono layer.(b) The exfoliation process for obtaining a fresh MoS2 surface and the S-vacancies were created by aging and annealing processes on a fresh surface.

Figure 3
Figure 3 shows the Raman spectra of pristine, fresh, and N2-plasma-treated MoS2.The observed Raman modes at 385 and 411 cm −1 belong to the  2 1 (in-plane) and  1

Figure 1 .
Figure 1.(a) The crystal structure of the MoS 2 mono layer.(b) The exfoliation process for obtaining a fresh MoS 2 surface and the S-vacancies were created by aging and annealing processes on a fresh surface.

Figure 1 .
Figure 1.(a) The crystal structure of the MoS2 mono layer.(b) The exfoliation process for obtaining a fresh MoS2 surface and the S-vacancies were created by aging and annealing processes on a fresh surface.

Figure 4 .
Figure 4. (a) Polarization curves and (b) corresponding Tafel plots for pristine and fresh surfaces of MoS2.(c) Polarization curves and (d) corresponding Tafel plots of fresh and aging surfaces for 8, 36, and 58 days of MoS2.

Figure 4 .
Figure 4. (a) Polarization curves and (b) corresponding Tafel plots for pristine and fresh surfaces of MoS 2 .(c) Polarization curves and (d) corresponding Tafel plots of fresh and aging surfaces for 8, 36, and 58 days of MoS 2 .

2. 2 . 3 .
SEA through N 2 -Plasma Treatment Further, N 2 -plasma treatment was conducted on pristine MoS 2 surfaces over different times to enhance HER activity.The polarization curves and corresponding Tafel plots of pristine and N 2 -plasma-treated MoS 2 surfaces over different times are shown in Figures 5c

3 2 . 14 Figure 6 .
Figure 6.ARPES characterization of MoS2 surfaces at different conditions.(a) The normal emission spectra and (b) magnified normal emission spectra of in situ-cleaved, annealed for 2 h, and N2plasma-treated for 120 s MoS2 surfaces.The black dotted lines represent the extrapolation of the linear region of the respective data points to find the EV values.The experimental data of the in situcleaved surface were taken from our previously published work [32].

Figure 6 .
Figure 6.ARPES characterization of MoS 2 surfaces at different conditions.(a) The normal emission spectra and (b) magnified normal emission spectra of in situ-cleaved, annealed for 2 h, and N 2plasma-treated for 120 s MoS 2 surfaces.The black dotted lines represent the extrapolation of the linear region of the respective data points to find the E V values.The experimental data of the in situ-cleaved surface were taken from our previously published work [32].

Figure 7 .
Figure 7. Schematic diagram of SEA and its effect on HER in MoS2.Surface band bending and SEA are produced by the donor-like surface states.

Figure 7 .
Figure 7. Schematic diagram of SEA and its effect on HER in MoS 2 .Surface band bending and SEA are produced by the donor-like surface states.

Table 1 .
The comparison of MoS 2 bulk overpotential with reported MoS 2 nanostructures.