Nitrogen-Plasma-Treated Continuous Monolayer MoS2 for Improving Hydrogen Evolution Reaction.

Theoretically, the edges of a MoS2 flake and S-vacancy within the lattice have nearly zero Gibbs free energy for hydrogen adsorption, which is essentially correlated to the exchange currents in hydrogen evolution reaction (HER). However, MoS2 possesses insufficient active sites (edges and S-vacancies) in pristine form. Interestingly, active sites can be effectively engineered within the continuous MoS2 sheets by treating it with plasma in a controlled manner. Here, we employed N2 plasma on a large-area continuous-monolayer MoS2 synthesized via metal-organic chemical vapor deposition to acquire maximum active sites that are indeed required for an efficient HER performance. The MoS2 samples with maximum active sites were acquired by optimizing the plasma exposure time. The newly induced edges and S-vacancies were directly verified by high-resolution transmission electron microscopy. The 20 min treated MoS2 sample showed maximum active sites and thereby maximum HER activity, onset overpotential of ∼-210 mV vs reversible hydrogen electrode (RHE), and Tafel slope of ∼89 mV/dec. Clearly, the above results show that this approach can be employed for improving the HER efficiency of large-scale MoS2-based electrocatalysts.

Introduction

Hydrogen (H2) gas is an excellent source of clean energy and perceived as a suitable alternative for hydrocarbon-based fuels.[1−4] Hydrogen is abundant in diverse natural compounds such as water, hydrocarbons, and so on. Hydrogen evolution reactions (HERs) from water-splitting reaction that breaks the H–O bond and releases oxygen and hydrogen in gaseous form can be achieved through an electrochemical or a photo-electrochemical process.[5−8] These processes are essentially facilitated by the introduction of a catalytic substance. The best catalytic substances known for efficient HER are the Pt metal and/or Pt-based materials,[9] which are not viable economically.[2,10] Therefore, finding new cost-effective materials with comparable or even better HER performance is highly desirable.

Two-dimensional (2D) materials, such as MoS2, have been widely investigated for electronics and optoelectronic applications.[11−13] MoS2 also possesses great potential in terms of HER owing to its structural stability, high electrocatalytic activity, and earth abundance.[14−16] The electrochemical reaction that produces H2 gas mainly occurs at the outmost layer and/or at the edges of MoS2 where the active sites are located.[14,15,17−20] The inner layers do not take part in the reaction. Therefore, monolayer MoS2 (1L-MoS2) is more suitable for HER compared to bulk counterpart. Another advantage of 1L-MoS2 is the relatively easy transfer of an electron from the active sites to the electrode due to maximum hopping of electrons in the vertical direction.[17,21] This phenomenon essentially reduces the contact resistance required for an efficient reaction. However, crystalline 1L-MoS2 has a low density of active sites in a pristine form that limits the HER performance. Interestingly, active sites in MoS2 can be engineered by various strategies, such as, doping,[20,22] forming cracks on the surface,[23,24] and creating S-vacancies.[25,26] The 1L-MoS2 with a high edge density and defects could be an excellent catalytic material for an efficient HER.

Pertinently, MoS2 grown over an Au foil via conventional chemical vapor deposition (CVD) showed excellent HER performance and Tafel slope of 61 mV/dec attributed to the excellent coupling between the Au substrate and MoS2.[27] On the other hand, a relatively cost-effective approach is the enhancement of HER performance by engineering defects in CVD-grown MoS2 on SiO2 substrate using plasma treatment or hydrogen annealing.[24] However, 2D MoS2 samples grown by conventional CVD also could not break the limitation in terms of cost-effectiveness, as the sample area is typically limited to ∼1 cm2 or below per synthesis. Such a drawback arises from the utilization of a solid-phase inorganic precursor (mostly MoO3 and sulfur powder). In contrast, the metal–organic CVD (MOCVD) technique uses gas-phase precursors and therefore offers a wafer-scale area of 1L-MoS2 on arbitrary substrates. Moreover, the low reaction rate, low reaction temperature, and precisely controllable flow rate of precursors make MOCVD suitable to yield uniform, conformal, and wafer-scale 2D transition-metal dichalcogenides (TMDs).[28−33] These properties fulfill the essential requirements for economical and scalable HER electrocatalytic application.

In this article, we synthesized a large-scale 1L-MoS2 via MOCVD and employed N2-plasma treatment to induce more active sites that are required for an efficient HER. The creation of active sites by N2 plasma was confirmed by inspecting the surface morphology and the crystal structure by field emission scanning electron microscopy (FE-SEM) and scanning transmission electron microscopy (STEM). Micro-photoluminescence (μ-PL) and Raman spectroscopies were employed to study the effect of N2 plasma on the optical characteristics of MoS2. Both microscopic and spectroscopic characterization tools clearly showed that increasing the treatment time results in the formation of more active sites (edges and S-vacancies). The optimum treatment time is about 20 min that yielded the best HER activity. We believe that our simple post-treatment technique can be widely employed for improving the HER efficiency of transition-metal dichalcogenide (TMD)-based electrocatalysts in general.

Results and Discussion

Morphological and Structural Analysis

Initially, a large-area crystalline continuous 1L-MoS2 film was grown by MOCVD, as discussed in Section . The crystalline MoS2 shows a poor HER performance because of the low density of active sites, which are essential for HER activity. Nevertheless, the active sites including edges and S-vacancy sites can be generated by using N2-plasma treatment. We employed N2 plasma on 1L-MoS2 to induce active sites, as schematically depicted in Figure b,c. The creation of active sites was comparatively studied by inspecting the surface morphologies of 1L-MoS2 treated with N2 plasma for various times. Evidently, the untreated sample shows a perfectly clean surface with no apparent cracks or deformation (Figure S2a). However, a network of cracks, which work as active sites, begins to appear in the plasma-treated films (Figure S2b,c). The density of cracks was found to be increased with the treatment time. The FE-SEM image of 20 min treated sample clearly shows more cracks compared to 10 min treated sample, as shown in Figure S2b,c, respectively. Further treatment destroys the film (Figure S2d). Although the FE-SEM image of untreated 1L-MoS2 does not show any apparent structural deformation/distortion, some distortions were observed at the nanoscopic level resolution, as shown in the high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image in Figure d. These structural distortions essentially happen at the grain boundaries, which arise from the lateral junction of two flakes. Here, it is worth mentioning that the MOCVD technique produces continuous polycrystalline 1L-MoS2 by laterally connecting the neighboring flakes and eventually form a large-area continuous film. The HADDF-STEM images that were taken within the grain exhibited a uniform hexagonal configuration, as shown in Figure S3a,b. Moreover, the as-grown film was a dominantly single layer; however, some negligibly small bilayer regions were also observed (Figure S3c).

Figure 1

Effect of N2-plasma treatment on the morphology of MOCVD-grown continuous 1L-MoS2. (a) Schematic illustration of the MOCVD system. (b) Schematic illustration of the development of cracks in a pristine continuous 1L-MoS2 when exposed to N2 plasma. (c) A structural model for N2-plasma-treated 1L-MoS2. (d) HAADF-STEM image of pristine 1L-MoS2 across the grain boundary (GB) with the insets corresponding to the diffraction patterns of each grain. HAADF-STEM images of N2-plasma-treated MoS2 observed at (e, f) cracked grain boundary, (g) edge sites, and (h) S-vacancies.

The atomic-scale resolution images of N2-plasma-treated 1L-MoS2 film are depicted in Figures e–h and S3d–f, respectively. The high-resolution image taken at the cracked region clearly shows the formation of new edge sites. The edge seems to be terminated at the Mo edge, as depicted in Figure g. Another important aspect of plasma treatment is the creation of more S-vacancies in the MoS2 lattice. The HADDF-STEM image taken away from the crack boundary revealed the formation of S-vacancies (Figures h and S3f), which were absent in the untreated sample. Although it is well known that S-vacancies ubiquitously exist in CVD-grown MoS2, the density of these intrinsic S-vacancies is not sufficient to ensure a good HER performance. The S-vacancies should be at least up to ∼14% for the optimized Gibbs free energy for hydrogen adsorption (ΔGH* ≈ 0).[25] These edges and S-vacancies work as active sites and thereby enhance the HER performance of MoS2.

Optical Characterization

The effect of N2 plasma on the optical characteristics of 1L-MoS2 was investigated by micro-Raman scattering and PL spectroscopy, as shown in Figure a. The wavenumber separation between the two dominant Raman modes, namely, E2g1 and A1g of MoS2, was found to be ∼20.7 cm–1 for the pristine sample, which is the typical Raman signature of a single-layer MoS2. After plasma treatment, the peak positions of both modes did not show a significant shift; however, the peak intensities rapidly drop accompanied by peak broadening, indicating the lattice distortion and defects generation such as cracks and S-vacancies. A similar trend was also observed in the PL spectra. The untreated sample shows a strong PL peak at ∼667 nm corresponding to the radiative recombination of A-excitons across the direct band gap at the K and K′ points in reciprocal space. This again confirms that the as-grown film was essentially a monolayer. The PL intensity also rapidly drops with increasing treatment time, which can be also attributed to enhanced defects sites, created by N2 plasma (Figure b).[24,34] Moreover, the PL intensity mapping taken over the selected area of 20 μm2 of the MoS2 film before and after 20 min treatment further supports the above results (Figure S4).

Figure 2

Evolution of Raman modes and PL characteristics of 1L-MoS2 with plasma treatment. (a) Raman spectra shows the evolution of typical Raman modes, namely, E2g1 and A1g of MoS2, and (b) PL spectra as a function of treatment time. The Raman spectra were calibrated by the Si Raman peak. All of the spectroscopic measurements were carried out at the same position.

Chemical Compositional Analysis

The effect of N2-plasma treatment on elemental bonding states of MoS2 was investigated by X-ray photoelectron spectroscopy (XPS), as shown in Figure . A comparative XPS analysis of pristine and 20 min plasma-treated MoS2 sample reveals a slight red shift in S2– 2p1/2 (163.2 eV) and S2– 2p3/2 (161.9 eV) peaks in the treated sample, which is consistent with the previous report,[35] as illustrated in Figure a. The shift in the peak positions in the treated sample essentially indicates a change in the electronic band structure. Most likely, the Fermi level has been shifted down toward the valence band due to the formation of S-vacancies.[36] The S-vacancies might transform the MoS2 from n- to p-type.[35,37] Moreover, decrease in the S/Mo ratio from 1.93 of the as-grown MoS2 to 1.71 of the plasma-treated MoS2 further confirmed the generation of S-vacancies by plasma treatment. In addition, the peak located at 397.5 eV (green fitted line in Figure b) clearly increased in the plasma-treated sample. This peak can be attributed to the Mo–N bond.[22,38,39] However, the quantitative analysis shows that the Mo–N bond position can be easily confusable with the peak position of CN and CN–H species, which might be formed during N2-plasma treatment of MoS2 due to the excessive availability of carbon species in ambient environment.[35] To exactly confirm that the peak at 397.5 eV indeed corresponds to Mo–N bonding, an annealing process would be needed to completely desorb the carbon species and quickly load to in situ XPS characterization.[35] Nevertheless, it was beyond the scope of this work, even though the N-doped MoS2 can be a practical approach to form even more active sites at the interior of the MoS2 lattice. Overall, the XPS results suggest that N2-plasma treatment is a feasible way to cleave the surface of MoS2 and increase the number of active sites in 1L-MoS2 for enhancing HER activity.

Figure 3

Chemical composition analysis on the effect of plasma treatment. XPS spectra at (a) S 2p and (b) Mo 3p of 1L-MoS2 before and after 20 min treatment.

HER Performance

Finally, the HER performance of N2-plasma-treated MoS2 samples was evaluated. It is evident that the untreated MoS2 sample exhibited relatively weak HER activities with an onset overpotential at −460 mV vs the reversible hydrogen electrode (RHE), as shown in the iR-corrected linear sweep voltammograms (LSVs) (Figure a). Although the ideal MoS2 crystal has very limited active sites available for HER, CVD- or MOCVD-grown MoS2 possesses some S-vacancies.[26] These vacancies along with the grain boundaries are indeed responsible for the limited HER activities of untreated MoS2.[40] On the other hand, the samples treated with N2 plasma for 10–30 min showed considerable decrease in the onset overpotential (Figure a). The smallest value of the onset overpotential of −210 mV was obtained for the 20 min treated sample. The above results can simply be understood in terms of more active sites generated by N2-plasma treatment. As discussed earlier, the plasma treatment induced active sites (edges and S-vacancies) in MoS2 lattice that are beneficial for HER performance. However, the treatment time should be carefully chosen to acquire optimum active sites. In our case, the optimum time was found to be 20 min. Employing plasma for a longer time involves the risk of destroying the film. Nevertheless, the results show that N2 plasma irradiation is an effective way to engineer the active sites in a controlled manner, and thereby improving the HER performance of MoS2. The corresponding iR-corrected Tafel plot was also characterized to manifest the enhancement of the HER performance in plasma-treated MoS2 (Figure b). The slope of 89 mV/dec was observed for the optimally treated sample (20 min), which is considerably smaller than 118 mV/dec of the untreated sample. The overpotentials of the samples of overly treated samples (>30 min) were even larger compared to those of the as-grown MoS2 because most of the active area in MoS2 was destroyed. This observation is also consistent with the morphological and optical characterizations. For a comparative analysis, the HER performance of N2-plasma-treated MoS2 grown via MOCVD and recently reported CVD methods is tabulated in Table .

Figure 4

Comparative analysis of HER activities of the pristine and plasma-treated MoS2 samples. (a, b) iR-corrected linear sweep voltammograms and Tafel plots, respectively. (c) Impedance Nyquist plot. (d) CV curves of the 20 min treated sample at various scan rates. (e) Linear fitting of the average capacitive current density vs the scan rate for the as-grown and N2-plasma-treated samples at different treatment times. The slop represents CDL of each sample, which is proportional to the ECSA and the roughness factor. (f) Stable HER performance of N2-plasma-treated samples using the potential vs time plot at −1.5 mA/cm2.

Table 1

Comparison of HER Performances of 2D-MoS2

catalyst	electrode	growth method (substrate)	post-treatment method	onset overpotential (mV)	Tafel slope (mV/dec)	ref
MoS2 1L	glassy carbon	CVD (SiO2)	O2 plasma	–400	162	(24)
MoS2 1L	glassy carbon	CVD (SiO2)	H2 annealing	–300	147	(24)
MoS2 1L	Au (111)	CVD (SiO2)	Ar plasma	∼−110	82	(25)
MoS2 1L	Au	LPCVD (Au)		∼−100	61	(27)
MoS2 1L flakes	high oriented pyrolytic graphite	CVD		–286	70	(47)
MoS2, 10 nm thick	FTO/glass	MOCVD (FTO)		∼−300	109	(31)
continuous 1L MoS2	graphite foil	MOCVD (SiO2)	N2 plasma	–210	89	this work

Moreover, the charge-transfer resistance (Rct) of the plasma-treated MoS2 was estimated by the electrochemical impedance spectroscopy (EIS) measurement conducted at −0.4 V vs RHE (Figure c). The 20 min treated sample clearly shows a smaller semicircle diameter than the rest of the samples, indicating the lowest Rct (18 Ω) by fitting the corresponding EIS spectrum using a Randles circuit model (Figure S5). For the overly treated samples (40 and 50 min), considerably higher Rct values of 202–280 Ω were found, respectively, which is consistent with the linear sweep measurements shown in Figure a.

The electrochemically active surface area (ECSA) of catalysts can be evaluated from the value of the double-layer capacitance, CDL, which is known to be proportional to the ECSA and the roughness factor.[41−44] The cyclic voltammetry (CV) curves were characterized for all of our samples in non-Faradic potential regions at various scan rates from 10 to 70 mV/s. This is shown in Figure d for the optimal plasma treatment time of 20 min. The values of CDL were calculated from CV characterizations in Figure e, where a 20 min treatment yielded the maximum CDL of 0.74 mF/cm2, which is 4.3 times larger than that of the as-grown sample (0.17 mF/cm2). Our CDL results are also consistent with LSV scan and impedance measurement in Figure a,c, respectively, confirming the effect of N2-plasma treatment on the electrochemical performance of MoS2 samples.

Furthermore, the stability of the HER activity was evaluateded using the transient Chrono potentiometric study for 12 h at 1.5 mA/cm2, as shown in Figure f. The samples exhibited inappreciable degradation in HER and maintained the overpotential, except those samples that were treated for more than 30 min.

Conclusions

In summary, we employed a simple N2-plasma treatment mechanism to generate sufficient active sites in terms of S-vacancies and edges in a large-area 1L-MoS2. This essentially allowed exploiting the large-area 1L-MoS2 film grown via MOCVD for efficient HER activities. The density of active sites could be effectively controlled simply by exposing the sample to N2 plasma for various times. In our case, the sample treated for 20 min showed the best HER performance with the onset overpotential of −210 mV vs RHE and the corresponding Tafel slope of 89 mV/dec. Our proposed method is simple and controllable to maximize the HER efficiency of the wafer-scale working medium, which may also be applied to other members of the TMD family for even further improving the HER efficiency.

Experimental Section

Synthesis of 1L-MoS2

A large-area 1L-MoS2 film was prepared by MOCVD, as schematically illustrated in Figure a. In brief, the synthesis was carried out in a sealed quartz tube of 1 inch diameter. A SiO2/Si substrate was placed at the center of the tube. The metal–organic compounds of molybdenum hexacarbonyl (MHC, Mo(CO)6) and diethyl sulfide (DES, (C2H5)2S) were used as gas-phase precursors. These precursors have a high equilibrium vapor pressure at room temperature. Initially, the base pressure (∼1 mTorr) was created in the chamber. The temperatures of MHC and DES holder were kept at 27 and 40 °C, respectively. The temperatures of the precursor’s carrier-line and reaction zone were set to 50 and 500 °C, respectively. 30 sccm of Ar and 3 sccm of H2 was flowed into the reaction chamber. The reaction pressure was set to 60 Torr. The reaction process was started by introducing 1 sccm of DES and MHC. The reaction time was set to 10 h. The detailed schematic description is given in Figure S1a and our previous report.[29]

N2-Plasma Treatment

The as-synthesized MoS2 (2 × 3 cm2) was cut into several pieces, and few samples were selected to study the impact of plasma treatment as a function of treatment time (Figure b). The electric potential of 20 kV was applied with 100 sccm of N2 flow to generate plasma (Figure S1b). The distance between the sample and the high potential tip was adjusted and the corresponding electrical current was about 0.2 mA. For a comparative study of HER activity, six samples were prepared with different treatment times of 0, 10, 20, 30, 40, and 50 min.

Characterization

The morphologies of the as-grown and N2-plasma-treated 1L-MoS2 samples were characterized by FE-SEM (JEOL JSM-6500F), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). STEM samples were prepared without using any polymer backing layer. Quantifoil holey carbon TEM grids were placed onto flakes on SiO2/Si substrate and a drop of isopropyl alcohol was dropped and dried. The SiO2 layer was etched by 30% KOH solution for 4 h and the TEM grids were detached from the substrate. Then, TEM grids were rinsed several times in deionized (DI) water and dried in ambient condition. STEM imaging was performed by a JEOL ARM 200F equipped with image and probe aberration correctors operated at 80 kV. The convergence semiangle was set at ∼25 mrad. HAADF-STEM images were acquired from 80 to 200 mrad range. Optical properties were characterized by micro-Raman and PL spectroscopies using 473 nm excitation source under ambient conditions. The chemical composition analysis was studied by X-ray photoelectron spectroscopy (XPS; Theta Probe AR-XPS System, Thermo Fisher Scientific).

HER Measurements

The MoS2 samples on SiO2/Si were transferred to a graphite foil that is a working electrode for studying the HER activity (Figure S1c). The films were transferred by the wet transfer method.[24,37,45] First, MoS2 (1 × 1 cm2) on SiO2/Si was syringed by 0.3 mL poly methyl methacrylate (PMMA, 5% toluene), spin-coated at 5000 rpm for 30 s, and dried at 90 °C for 30 min. The PMMA/MoS2/SiO2/Si was then dipped in 10 mL of KOH solution (1 M) at 90 °C to etch the SiO2 layer. The PMMA/MoS2 was floated on the solution. After several washings with DI water, the PMMA/MoS2 layer was transferred onto the target electrode (graphite foil). Finally, the PMMA layer was removed by 99.99% warm acetone.

The HER measurements including linear sweep and electrochemical impedance spectroscopy (EIS) were conducted in an electrochemical workstation (IviumStat, Ivium Tech) with three electrodes including a Pt wire as the counter electrode, an Ag/AgCl as the reference electrode, and the MoS2/graphite foil as the working electrode (Figure S1d). For HER stability study, the graphite rod was used as the counter electrode instead of Pt wire to avoid the Pt dissolution and deposition on the working electrode during the HER stability experiment.[46] All of the three electrodes were placed in 0.5 M H2SO4 electrolyte to observe the HER activity. The EIS result was fitted using the equivalent circuit evaluator (Ivium Tech) modeled with two constant-phase elements (CPEs).