Path-dependent hydrogen evolution reaction via selective etching of bilayer MoS₂ catalysts

Abstract

Molybdenum disulfide (MoS₂) is being considered as a promising candidate for replacing noble metal catalysts in the hydrogen evolution reaction (HER). However, low active site density, semiconducting properties, and electrochemically inert basal planes limit the catalytic performance of MoS₂ catalysts. Therefore, various strategies have been developed for activating basal planes in MoS₂ catalysts. Here, we propose an efficient laser technology for activating basal planes and creating controlled artificial patterns. The precisely controlled laser etching technique allows for etching only the top layer of the bilayer MoS₂ catalyst, and using this technique, the appropriately etched area increases the active sites without any degradation of catalytic performance, enabling efficient carrier injection. The catalytic behavior of line patterned MoS₂ exhibited line density dependent catalytic performance. Compared with pristine MoS₂ catalysts, the etched catalysts with 5 lines in a specific area (L5-MoS₂) showed a decreased overpotential and Tafel slope up to 65.1% and 73.7%, respectively. Moreover, in the case of L5-MoS₂ devices, the vertical contact between the etched line and electrode exhibited improved catalytic performance with a low Tafel slope and charge transfer resistance, which is mainly attributed to the facilitated charge transfer induced rapid charge exchange process at the electrolyte/catalyst interface. This effective strategy proposes a controllable defect engineering technique for improving the catalytic performance in 2D TMDC catalysts.

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1 Introduction

Recently, two-dimensional transition metal dichalcogenides (2D TMDCs) have drawn tremendous attention due to their catalytically active properties in the hydrogen evolution reaction (HER).^1–6 Molybdenum disulfide (MoS₂), among these 2D TMDCs, is known as a promising substitute for Pt due to its reasonable cost, elemental abundance in nature, and thermodynamically neutral Gibbs free energy (ΔG_H*) at the active Mo sites. However, unlike Pt, MoS₂ has a disadvantage of low catalytically active site density, which is restricted to the unsaturated Mo sites (edge and S-vacancy sites) within the MoS₂ crystals.^7–9 Additionally, the semiconducting nature of MoS₂ crystals showed poor electrical conductivity and a large band gap, which gave rise to sluggish kinetics of the HER. Therefore, various strategies have been employed to increase the active sites of MoS₂ through defect engineering, doping, fabricating heterostructures, etc.^10–13

Defect engineering has emerged as a promising technology for activating basal planes in MoS₂ catalysts. Among them, thermal etching can effectively generate atomic vacancies through a chemical vapor deposition (CVD) method. However, in the case of thermal etching, it is hard to specifically control the density of defects, which leads to crystal deformation. The chemical etching method offers simple and efficient strategies for creating atomic defect sites in MoS₂ catalysts through a chemical reaction. However, excessive chemical treatment leads to unwanted crystal structure degradation and chemical doping, which modifies the intrinsic material properties. Therefore, simple and controllable defect engineering strategies are needed for improving the catalytic activity of the basal planes of MoS₂ catalysts.

In this study, we propose controlled laser etching techniques to precisely control the density of active sites on the basal plane of bilayer MoS₂, which can significantly enhance the hydrogen evolution reaction efficiency of MoS₂. Through carefully controlled laser intensity, we can selectively etch only the top layer of bilayer MoS₂ catalysts, thereby enabling efficient charge injection from the maintained bottom layer to active sites. We confirmed that the electrochemical hydrogen evolution performance improved as the line density increased in the same area. Laser etching increases the number of exposed Mo-edge sites in the MoS₂ crystal, which possesses ideal proton adsorption energy. Thus, the catalytic activity was improved with a lowered Tafel slope and overpotential in the etched MoS₂ catalysts. In addition, the differences in catalytic performance were further confirmed by modifying the current path of the line pattern. The perpendicularly contacted bilayer MoS₂ catalysts exhibited optimized catalytic performance with a low overpotential (329 mV at 100 mA cm⁻²) and a low Tafel slope of (72 mV dec⁻¹), which is attributed to bottom-layer-induced facilitated charge transport in the catalytically active top layer in bilayer MoS₂ catalysts. This effective strategy provides a controllable defect engineering technology for improving the catalytic performance in semiconducting 2D TMDC catalysts.

2 Results and discussion

Fig. 1a illustrates a schematic of the laser etching process of bilayer MoS₂ catalysts. In previous studies, laser etching in 2D materials was attributed to the oxidation mediated thermal sublimation of crystals. Therefore, when the bilayer MoS₂ was irradiated with a 532 nm laser with moderate intensity (∼12 mW μm⁻²) under ambient conditions, we were able to selectively etch only the top layer in the bilayer MoS₂ catalysts.^14,15 As a result, a line-pattern was formed in the top layer of the bilayer MoS₂ through line scanning of the laser, and this line-pattern can expose the active Mo-sites at the top layer of the bilayer MoS₂, as shown in Fig. 1b.¹⁶ Therefore, as shown in Fig. 1b, by adjusting the line density, the active site density at the basal plane can be modulated. In addition, the remaining bottom layer contributes to transporting the charge carriers to active sites, preventing catalytic performance degradation due to retarded electron transport. Increasing the supply of electrons also improves the overall hydrogen generation performance of MoS₂.^17,18 Through this controllable laser etching technique, catalytically active Mo-edge sites were created on the top layer of bilayer MoS₂, while leaving the bottom layer intact and unaltered.

Fig. 1 (a) Schematic illustration of the bilayer MoS₂ etching technique with a 532 nm laser. (b) A schematic diagram of the process of generating hydrogen from the catalytically active etched area in the MoS₂ bilayer.

To investigate the formation of the line-pattern in the bilayer MoS₂ crystal, we utilized the Raman, photoluminescence (PL) and atomic force microscopy (AFM) analyses. At first, before laser etching, we confirmed the characteristics of chemical vapor deposition (CVD) grown bilayer MoS₂ catalysts by using various spectroscopic analyses, as shown in Fig. S1.† Line-patterned bilayer MoS₂ was transferred on the target substrate, and the optical image of the sample is shown in Fig. 2a. The height profile between the top layer and etched area in the AFM mapping image (Fig. 2b) exhibited a thickness of 0.8 nm, which corresponds to the thickness of the MoS₂ monolayer.^19–23 Therefore, it was confirmed that only top layer MoS₂ was etched through the precisely controlled laser etching technique.

Fig. 2 (a) Optical microscopy image of etched bilayer MoS₂ and (b) atomic force microscopy (AFM) mapping image. Inset exhibits the corresponding height profile. (c) Raman and (d) photoluminescence mapping image of etched bilayer MoS₂. Layer-dependent (e) Raman and (f) PL spectra from a top layer, etched site, and the bottom layer of etched bilayer MoS₂. The scale bar for all figures is 10 μm.

In this etching process, the laser induced deformation of the crystal structure was caused by thermal degradation.^14,24,25 Therefore, under these harsh conditions, MoS₂ may experience lattice deformation, additional chemical contamination, and excessive layer etching. To investigate the chemical state changes and residual strain effects in the bilayer MoS₂ crystal during laser etching, we utilized Raman and PL spectroscopic analyses. Increasing the number of layers in MoS₂ resulted in notable spectral shifts for both the E¹_2g and A_1g Raman modes (Fig. 2e). In Fig. 2e, there was no discernible difference observed in the A_1g and E¹_2g peaks between the etched site and the bottom layer, which implies that the only top layer was etched through a precisely controlled laser etching technique. However, in contrast to the peaks seen in the etched site and the bottom layer, each individual peak in the top layer showed an increased separation and intensity. This phenomenon can be attributed to several factors. The A_1g peak's shift provided a clear indication of the enhanced stiffness in van der Waals interlayer interactions, attributed to the strengthening of restoring forces with increasing layer thickness.^26,27 Meanwhile, the E¹_2g peak's shift underscored the softening of the wavelength due to intensified Coulomb interaction effects. Additionally, as shown in Fig. 2c, an observable increase in Raman spectra intensity was correlated with the increasing number of layers, a phenomenon attributed to the optical interference that occurs as the laser penetrates the MoS₂ structure.^26,28,29 In addition, there was no difference in the full width at half maximum (FWHM) of the two A_1g peaks, and there was no satellite peak, confirming that there was no chemical contamination or lattice strain in the etched MoS₂. In the case of the PL mapping image, the dark image indicates the bilayer MoS₂ crystal, which was attributed to indirect band gap nature induced non-radiative recombination in the bilayer MoS₂ crystal (Fig. 2f).^26,30 As shown in Fig. 2d and f, the etched area also showed similar intensity to the bottom layer MoS₂ crystal, which indicates that the laser etching technique was successfully conducted for peeling only the top layer of the MoS₂ crystal.³¹ Through the above processes, it was confirmed that selective etching was successfully performed without chemical damage and strain effect on the etched MoS₂.^19,32–38

To investigate the line density dependent catalytic behavior in line-patterned bilayer MoS₂ catalysts, we examined the hydrogen evolution catalytic performance with a three-electrode system.⁷ In Fig. S2a,† the electrochemical measurement setup was composed of a graphite rod (counter electrode), Ag/AgCl electrode (reference electrode), and line-patterned MoS₂ bilayer (working electrode). To compare the catalytic activity of line-patterned bilayer MoS₂ catalysts, we open specific areas (59.1 μm²) of the MoS₂ catalysts by using photolithography, as shown in Fig. S2b.† In the electrochemical measurements, as shown in Fig. 3a, the line density dependent catalytic activity was clearly observed in the linear sweep voltammetry (LSV) curve. In the case of pristine MoS₂ (L0), the overpotential at 100 mA cm⁻² was measured at 942 mV, which corresponds to the poor catalytic activity of the basal plane of MoS₂ catalysts. However, after the line pattern was created, the overpotential at 100 mA cm⁻² gradually decreased to 703 mV, 504 mV and 329 mV for 2-line (L2), 3-line (L3) and 5-line (L5) patterned bilayer MoS₂ catalysts, respectively. The Tafel slope also gradually decreased with increasing line density in the Tafel plot (Fig. 3b), achieving its lowest value (72 mV dec⁻¹) with L5-MoS₂ catalysts. These results indicate that the catalytic activity in bilayer MoS₂ catalysts was gradually improved depending on the number of lines at the basal plane, created through laser etching.

Fig. 3 Electrochemical hydrogen evolution performance of line-patterned bilayer MoS₂ catalysts. (a) Polarization curve, (b) Tafel plot and (c) Nyquist plot when the number of etched lines is 1 to 5. (d) Etched line dependent overpotential and Tafel slope.

The line density dependent charge exchange process at the catalyst/electrolyte interface was further investigated through electrochemical impedance spectroscopy (EIS). The Nyquist plot in Fig. 3c shows a remarkable change of charge transfer resistance (R_ct) between pristine (L0) and line patterned (L2, L3, L5) bilayer MoS₂ catalysts. The pristine MoS₂ catalysts measured high R_ct at 46.7 Ω cm², which indicates poor electrochemical activity in the basal plane of MoS₂ catalysts. In contrast, the R_ct gradually decreased when the line density increased. Compared with pristine MoS₂ (L0-MoS₂), L5-MoS₂ showed the lowest R_ct with 4.84 Ω cm². These results indicate that the laser etching-induced activated MoS₂ basal plane effectively exchanges the charge carriers with the ions in the electrolyte, resulting in reduced charge transfer resistance.^11,39–41

In respect of etched MoS₂ catalysts, excessive etching leads to degradation of the catalytic performance and electrical conductivity, despite increased catalytically active site density.^40,42,43 Therefore, effective charge carrier injection into catalytically active sites is crucial for activating etched sites. Thus, we utilized bilayer MoS₂ catalysts for investigating the current path dependent catalytic behavior in line-patterned bilayer MoS₂ catalysts. As shown in Fig. 4a and b, we fabricated two types of devices with different current paths: (1) parallel-contact: the etched lines are parallel to the electrode. Therefore, the charge carriers were injected from the bottom layer to the line-patterned top layer of the MoS₂ catalysts and (2) perpendicular-contact: the etched lines are perpendicular to the electrode (Fig. S3†). Therefore, the charge carriers were injected directly into both the bottom and top layers of MoS₂ catalysts. In electrochemical measurements, 5-line-patterned MoS₂ (5L-MoS₂) catalysts exhibited contact direction dependent catalytic behavior. The perpendicularly contacted MoS₂ catalysts exhibited better catalytic performance compared to parallel contacted MoS₂ catalysts. In Fig. 4c, the overpotential was measured at 136 mV and 326 mV at 10 mA cm⁻² for perpendicularly and parallel contacted MoS₂ catalysts, respectively. Furthermore, the Tafel slope also exhibited consistent behavior in both catalysts (Fig. 4d). The perpendicularly contacted MoS₂ catalysts exhibited a decreased Tafel slope with 72 mV dec⁻¹ compared to parallel contacted MoS₂ catalysts (134 mV dec⁻¹). EIS measurements also showed lowered R_ct in perpendicularly contacted MoS₂ catalysts (4.84 Ω cm²). In parallel contacted catalysts, the charge carriers are only injected through bottom layer MoS₂. In this case, the charge carriers transport through interlayer gaps between bottom layer and top layer MoS₂ catalysts. Therefore, the interlayer gap induced high potential barrier leads to hopping transport from the bottom layer to the etched MoS₂ site, which corresponds to high resistance and sluggish charge transport.⁴⁴ The sluggish supply of charge carriers limits the activation of catalytically active etched sites in bilayer MoS₂ catalysts, which was related to the high overpotential and large charge transfer resistance in electrochemical measurements. Moreover, the etched line, which was far from the electrode, was hard to supply sufficient charge carriers from the electrode.

Fig. 4 The schematic illustration of contact direction dependent device architecture. (a) Parallel contacted and (b) perpendicularly contacted MoS₂ electrochemical cells. (c) Polarization curve, (d) Tafel plot and (e) Nyquist plot dependent when the etching direction and electrode direction are vertical and horizontal.

In contrast, in the case of perpendicularly contacted catalysts, the charge carriers transport from the electrode to both layers (bottom layer and etched layer) in bilayer MoS₂ catalysts. Therefore, the supply of sufficient charge carriers in perpendicularly contacted bilayer MoS₂ accelerates the electrochemical reaction at the catalytically active etched sites in bilayer MoS₂ catalysts. The facilitated charge transport induced rapid electrochemical reaction at the catalyst/electrolyte interface gave rise to low charge transfer resistance, and this catalytic behavior was attributed to low in-plane resistance in the MoS₂ crystal. Moreover, the increased active edge sites in the basal plane leads to a facilitated hydrogen adsorption/desorption process, which corresponds to a lowered Tafel slope. As a result, we confirmed that the optimization of catalytic activity at the etched active sites was realized through efficient charge transport through the line pattern in the top layer of bilayer MoS₂ catalysts.⁷

3 Conclusions

In this research, we verified that the catalytically inert MoS₂ basal plane can be activated through a simple laser etching technique under ambient conditions. Successful etching of only the top layer of the bilayer MoS₂ crystal was realized through moderate layer intensity, which was verified by using AFM, Raman, and PL analyses. As a result, it was confirmed that a clean etching process without any change in the chemical state was successfully implemented. Exposing the catalytically active Mo sites at the basal plane through a laser etching technique showed an improvement in catalytic performance as the line density increased. Moreover, the optimized catalytic performance was realized by a vertically contacted electrode, which effectively injected charge carriers through the catalytically active line-pattern in bilayer MoS₂ catalysts. In summary, our research proposed an efficient method to improve the catalytic behavior of 2D TMDC materials, while analyzing the effect of charge injection characteristics at the active sites on catalytic performance.

4 Experimental section

4.1 MoS₂ synthesis

To precisely control the growth layers of MoS₂ crystals, we employed a solution-processed synthesis method through a chemical vapor deposition (CVD) process. Molybdenum oxide (MoO₃) powder was dissolved in ammonium hydroxide (NH₄OH) solution. We utilized a two-zone furnace CVD system. Sulfur powder (Sigma-Aldrich) was placed upstream and heated to 200 °C. The MoO₃ solution, along with a Si/SiO₂ substrate, was placed downstream. The growth process was carried out at 800 °C under a continuous flow of Ar + H₂ gas at 65 sccm for 20 minutes.^20,22,34

4.2 MoS₂ characterization and line-patterning method

The atomic force microscopy (AFM) images were obtained using a commercial AFM setup (XE7, Park Systems). The Raman and PL measurements were carried out using a 532 nm laser on a Witec Confocal Raman spectrometer. Optical spectroscopic measurements were carried out using a 532 nm wavelength laser at an intensity of 2 mW with an irradiation time of 60 s. Laser etching was conducted with a laser intensity of 14 mW and an irradiation time of 0.1 s μm⁻¹ at room temperature and air ambient.

4.3 Electrochemical measurements

The fabricated microcell was measured in a three-electrode system. Photolithography was performed so that only the etched part of MoS₂ reached the electrolyte. Thereafter, 0.5 M H₂SO₄ solution was dropped on the sample to perform measurements. The polarization curve was measured at a scan rate of 5 mV s⁻¹ when it was 0 to −1 V compared to the reference electrode.

E_RHE = E_(Ag/AgCl) + 218 mV

With the above equation, the reversible hydrogen electrode (E_RHE) was corrected in consideration of the potential of 0.5 M H₂SO₄, which is a reference electrode and an acid electrolyte.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Research Foundation (NRF) of Korea (RS-2023-00220471 and RS-2024-00355424).

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Footnotes

† Electronic supplementary information (ESI) available: Characterization section, AFM mapping image of bilayer MoS₂, Raman and PL spectra mapping of CVD grown MoS₂ bilayers, image of the setup for electrochemical analysis, optical image of the etched MoS₂ catalyst. See DOI: https://doi.org/10.1039/d4ta03197k

‡ These authors contributed equally.

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