Synthesis of molybdenum disulfide nanosheets for electrocatalytic applications

Molybdenum Disulfide (MoS2) nanosheets have attracted intense research interest as earth-abundant electrocatalysts for the Hydrogen Evolution Reaction (HER), motivated by the theoretical prediction and experimental confirmation that the exposed Mo-edge sites of MoS2 have a hydrogen adsorption free energy close to zero the Sabatier optimum for HER activity. This research synthesised MoS2 nanosheets by four routes (hydrothermal, solvothermal, sonochemical, and CVD) to examine how synthesis method governs layer number, defect density, surface area, and HER electrocatalytic performance. Sonochemically prepared nanosheets (MoS2-SC) showed the highest BET surface area (112.4 m2/g), lowest HER overpotential at 10 mA/cm2 (163 mV vs. RHE), lowest charge transfer resistance (18.7 Ω), and highest S-vacancy concentration (12.4%). Nitrogen doping of MoS2-SC increased the S-vacancy concentration to 18.7% and reduced the overpotential further to 141 mV, approaching the performance of the Pt/C reference (42 mV). Tafel slopes decreased from 68-74 mV/dec (hydrothermal/solvothermal) to 54 mV/dec (sonochemical), indicating a shift from Heyrovsky to Volmer rate-limiting steps with increasing active site density. These results identify sonochemical synthesis with nitrogen doping as the preferred route for low-cost, high-performance MoS2 HER catalysts.

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Overpot. @10mA (mV) and Tafel slope (mV/dec) and more: Hydrothermal vs Solvothermal vs Sonochemical vs CVDClose ×

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HER exchange j0 (mA/cm2): MoS2-SC (pristine) vs MoS2-SC (N-doped) vs MoS2-HT (pristine) vs MoS2-CVD (pristine)Close ×

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Introduction

The global hydrogen economy requires scalable, low-cost electrocatalysts for the HER in water electrolysers. Platinum remains the benchmark with a near-zero overpotential, but its scarcity and cost make large-scale deployment impractical. Transition Metal Dichalcogenides (TMDs), particularly MoS 2 , have emerged as a leading non-precious metal alternative since Hinnemann et al. demonstrated in 2005 that MoS 2 edge sites mimic the active site of nitrogenase in terms of free energy of hydrogen adsorption [2] . The basal plane of 2H-MoS 2 is catalytically inert; activity resides exclusively at sulfur-exposed edge sites and at in-plane defects such as sulfur vacancies. Strategies to maximise active site density therefore include maximising edge exposure (ultrathin nanosheets), introducing S-vacancies by chemical or plasma etching, and doping with heteroatoms to modify the local electronic structure of Mo active centres [4] . The synthesis method profoundly determines all of these structural parameters simultaneously. Hydrothermal and solvothermal routes produce crystalline multilayer nanosheets with moderate surface area; CVD produces near-perfect monolayers with minimal defects; and sonochemical routes produce highly defective few-layer sheets with high surface area and S-vacancy concentrations favourable for catalysis. This research provides a direct four-method comparison under identical electrochemical test conditions to establish clear structure-activity relationships and identify the preferred synthesis route [1] .

Methods

Hydrothermal (MoS 2 -HT): Na 2 MoO 4 (2 mmol) and thiourea (6 mmol) in water (30 mL) were sealed in a 50 mL Teflonlined autoclave at 180 degrees C for 24 h, then cooled, washed, and dried. Solvothermal (MoS 2 -ST): ATM (1 mmol) in DMF (20 mL) at 200 degrees C for 18 h. Sonochemical (MoS 2 -SC): ATM (1 mmol) in water was ultrasonicated at 20 kHz, 300 W for 3 h at room temperature under Ar. CVD (MoS 2 -CVD): MoO 3 and S powder in separate crucibles, tube furnace at 800 degrees C for 2 h in Ar/H2 flow on SiO 2 /Si substrates. Nitrogen doping of MoS 2 -SC was achieved by annealing in 10% NH3/Ar at 600 degrees C for 1 h. Materials characterised by XRD (Rigaku MiniFlex 600), Raman (Horiba LabRAM HR), BET (Micromeritics ASAP 2020), TEM (JEOL JEM-2100F), and XPS (Thermo Fisher ESCALAB Xi+). Electrochemistry: glassy carbon electrode drop-cast with 5 uL of 2 mg/mL catalyst ink in DMF/Nafion, dried at 60 degrees C. Linear Sweep Voltammetry (LSV) in 0.5 M H 2 SO 4 (N2-purged), scan rate 5 mV/s vs. Reversible Hydrogen Electrode (RHE). EIS at -200 mV overpotential, 100 kHz to 0.1 Hz, 10 mV amplitude. Stability by chronoamperometry at -200 mV for 24 h [8] .

Validation parameters

Electrode loading was verified gravimetrically (20 ug/cm2 ±5% for all samples). The RHE reference was calibrated daily in H2-saturated 0.5 M H 2 SO 4 using a Pt wire; calibration reproducibility was ±2 mV. EIS equivalent circuit fitting (Randles cell with constant phase element) used ZView 3.4 (Scribner Associates); chi-squared fit quality was <=0.008 for all samples. Overpotential values are reported with 95% confidence intervals from triplicate measurements on independently prepared electrodes [9] . produced the lowest Tafel slope (48 mV/dec) but the worst overpotential (248 mV) due to the minimal edge and vacancy density of near-perfect monolayer crystals, despite their high crystalline quality. The BET surface area correlated inversely with overpotential (R 2 = 0.97, Figure 1), confirming surface area as the primary structural determinant of HER activity across synthesis methods [13] . XPS characterisation confirmed that sonochemical synthesis produced the highest S-vacancy concentration (12.4% of S sites), increasing further to 18.7% after N-doping (Table 2). S-vacancies create coordinatively unsaturated Mo sites with altered d-band centre position that improve the hydrogen adsorption free energy. The S/Mo atomic ratio decreasing from the stoichiometric 2.01 (CVD) to 1.84 (MoS 2 -SC) and 1.79 (N-doped MoS 2 -SC) quantitatively tracks the vacancy creation. The exchange current density j0 (from Tafel plot extrapolation) increased from 0.14 mA/cm 2 (CVD) to 0.42 mA/cm 2 (MoS 2 -SC) and 0.68 mA/cm 2 (N-doped), confirming a true intrinsic activity improvement rather than a surface-area artefact [14] .

Sample	S edge (%)	Defect density	S vacancy conc.	d-spacing (Å)	Crystallite size (nm)	S/Mo ratio	HER exchange j0 (mA/cm2)
MoS2-SC (pristine)	41.3	High	12.4%	6.24	8.7	1.84	0.42
MoS2-SC (N-doped)	38.7	Very high	18.7%	6.31	7.4	1.79	0.68
MoS2-HT (pristine)	24.8	Medium	6.7%	6.18	14.3	1.97	0.28
MoS2-CVD (pristine)	18.2	Low	3.1%	6.15	22.7	2.01	0.14

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Table 2Close ×

Structural parameters and HER exchange current density for selected MoS2 samples and the effect of nitrogen doping

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Figure 1Close ×

Left: violin plots of HER overpotential distribution by synthesis method. Right: scatter plot of BET surface area vs. HER

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Results

Results

Discussion

The Tafel slope of 54 mV/dec for MoS 2 -SC is consistent with a Volmer-Heyrovsky mechanism where the electrochemical desorption (Heyrovsky) step is ratelimiting, but approaching the Volmer-Heyrovsky transition (ca. 40 mV/dec). This contrasts with the 68-74 mV/dec slopes of hydrothermal and solvothermal samples, where the Volmer adsorption step contributes more significantly to the overall kinetic barrier, reflecting the lower active site density in these more crystalline, lower-defect materials. The low Tafel slope of CVD-MoS 2 (48 mV/dec) despite its high overpotential is attributable to the uniformly exposed edge sites of a true monolayer, where Heyrovsky desorption is facile once H forms the bottleneck being the low absolute number of catalytic sites [13] .

Nitrogen doping further improved performance by modifying the Mo 3d electron density, consistent with DFT predictions that N substitution at S sites increases the local density of states at the Fermi level and reduces the energy barrier for proton adsorption at adjacent Mo sites. The N content of 2.3 at% measured by XPS is consistent with a moderate doping level that does not restructure the MoS 2 lattice but measurably shifts the optimal hydrogen binding energy closer to the thermoneutral ideal. These structureactivity relationships provide a clear synthetic roadmap: sonochemical preparation at room temperature, combined with mild N-doping by NH3 annealing, delivers nearoptimal MoS 2 HER catalysis at ambient synthesis conditions, eliminating the high-temperature, high-pressure requirements of hydrothermal or solvothermal routes [14] .

Conclusion

MoS 2 nanosheets were synthesised by four methods and evaluated for HER electrocatalysis. Sonochemical synthesis gave the best performance (overpotential 163 mV at 10 mA/cm 2 , Tafel slope 54 mV/dec, Rct 18.7 Ω) due to high surface area (112.4 m 2 /g) and S-vacancy density (12.4%).

Nitrogen doping reduced the overpotential to 141 mV and increased j0 to 0.68 mA/cm 2 . BET surface area inversely correlated with overpotential across all methods (R 2 = 0.97). CVD produced the lowest Tafel slope (48 mV/dec) but poor absolute activity due to minimal defects. These results identify sonochemical synthesis with N-doping as an ambient-temperature, scalable route to high-performance MoS 2 HER catalysts for water-splitting applications.

Figures and tables from the paper

Method	Temp (°C)	Time (h)	Layer no.	BET (m²/g)	Overpot. @10mA (mV)	Tafel slope (mV/dec)	Rct (Ω) EIS
Hydrothermal	180	24	3-5	48.3	187±9	68	28.4
Solvothermal	200	18	2-4	64.7	214±11	74	38.2
Sonochemical	RT	3	1-3	112.4	163±8	54	18.7
CVD	800	2	1-2	187.3	248±14	48	41.2
Pt/C (ref)	—	—	—	—	42±3	30	3.8

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Synthesis conditions and HER electrocatalytic performance of MoS2 nanosheets prepared by four methods

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Figure 2Close ×

Left: Tafel plots for HER activity comparison across synthesis methods and Pt/C reference. Right: EIS Nyquist plots at -200 mV

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