Two-dimensional transition metal dichalcogenides (TMDs), particularly molybdenum disulfide (MoS2) and tungsten disulfide (WS2), have emerged as highly promising electrocatalysts for the hydrogen evolution reaction (HER). Their unique layered structure, tunable electronic properties, and abundance of active edge sites make them attractive alternatives to platinum-based catalysts. This article examines the synthesis, structural engineering, catalytic performance, and stability of MoS2 and WS2 nanostructures for HER applications.
### Synthesis and Exfoliation Techniques
The catalytic activity of TMDs is strongly influenced by their synthesis method. Mechanical exfoliation produces high-quality monolayers but is limited by low yield. Liquid-phase exfoliation, using solvents like N-methyl-2-pyrrolidone (NMP) or surfactants, enables scalable production of few-layer nanosheets. Chemical vapor deposition (CVD) allows precise control over layer thickness and crystallinity, often resulting in vertically aligned nanosheets with abundant edge sites. Hydrothermal synthesis is another common approach, offering tunable morphology through temperature and precursor selection. For example, MoS2 synthesized at 200°C exhibits a higher density of edge sites compared to samples grown at lower temperatures.
### Edge-Site Engineering
The HER activity of MoS2 and WS2 primarily originates from their edge sites, as the basal planes are catalytically inert. Strategies to maximize edge exposure include creating defect-rich structures, synthesizing vertically aligned nanosheets, and designing porous architectures. Plasma treatment introduces sulfur vacancies, increasing the number of unsaturated metal sites. Nanostructuring via templating or etching enhances edge density, with some studies reporting a 10-fold improvement in HER activity compared to bulk materials. For instance, vertically aligned MoS2 nanosheets synthesized by CVD exhibit an overpotential of 140 mV at 10 mA/cm², significantly lower than that of bulk MoS2.
### Heteroatom Doping
Doping with transition metals (e.g., Co, Ni, Fe) or non-metals (e.g., P, N) modifies the electronic structure of TMDs, improving their intrinsic activity. Cobalt-doped MoS2 shows a reduced Gibbs free energy for hydrogen adsorption (ΔGH*), a key descriptor for HER performance. Nickel doping in WS2 similarly enhances conductivity and active site availability. Non-metal doping, such as phosphorus incorporation into MoS2, alters the charge distribution, facilitating proton adsorption. Doped MoS2 catalysts have achieved Tafel slopes as low as 45 mV/dec, approaching the performance of platinum.
### Hybrid Structures with Conductive Supports
Coupling TMDs with conductive materials like graphene, carbon nanotubes, or metallic substrates improves charge transfer and prevents aggregation. MoS2/graphene hybrids exhibit synergistic effects, where graphene provides electrical conductivity while MoS2 supplies active sites. WS2 grown on carbon cloth shows enhanced stability due to strong interfacial bonding. Such hybrids often demonstrate overpotentials below 100 mV at 10 mA/cm², with Tafel slopes in the range of 40–60 mV/dec. The integration of TMDs with 3D porous carbon frameworks further enhances mass transport and active site accessibility.
### Catalytic Activity Metrics
The HER performance of MoS2 and WS2 is typically evaluated using overpotential and Tafel slope. Overpotential reflects the energy required to drive the reaction, while the Tafel slope indicates the rate-determining step. Pristine MoS2 exhibits an overpotential of 200–300 mV and a Tafel slope of 90–120 mV/dec. Edge-enriched and doped variants reduce these values significantly. For example, cobalt-doped MoS2 nanosheets report an overpotential of 85 mV and a Tafel slope of 48 mV/dec. WS2-based catalysts show comparable trends, with optimized structures achieving overpotentials below 100 mV.
### Stability and Durability
Long-term stability is critical for practical applications. TMD catalysts are tested under continuous HER operation, often for over 1,000 cycles or 24 hours. MoS2 on carbon supports maintains stable performance with minimal activity loss, attributed to the robust mechanical and electrical integration. WS2 hybrids similarly demonstrate durability, with less than 10% degradation in current density after extended testing. The presence of protective carbon coatings or conductive polymers further mitigates oxidation and dissolution.
### Conclusion
Two-dimensional MoS2 and WS2 represent a versatile class of HER electrocatalysts, with performance tunable through synthesis, doping, and hybridization. Edge-site engineering and heteroatom doping optimize their intrinsic activity, while conductive supports enhance charge transfer and stability. Continued advancements in nanostructuring and material design will further bridge the gap between TMD-based catalysts and noble metal benchmarks. Future research should focus on scalable synthesis methods and mechanistic studies to unlock their full potential for hydrogen production.