Marigold-flower-like MoS₂ nanosheet assemblies for enhanced alkaline hydrogen evolution

Abstract

The development of efficient, cost-effective, and noble-metal-free electrocatalysts is critical for sustainable hydrogen production via water splitting. Herein, hierarchical MoS₂ nanosheets were synthesized through a facile hydrothermal method, producing a marigold flower-like morphology with abundant exposed edge sites. Structural and compositional analyses using XRD, Raman, XPS, SEM, and BET confirmed the formation of crystalline, layered, and mesoporous MoS₂. Electrochemical evaluation in 1.0 M KOH revealed excellent hydrogen evolution reaction (HER) activity, with a low overpotential of 180 mV at 10 mA cm⁻², a Tafel slope of 122.3 mV dec⁻¹, and reduced charge-transfer resistance, highlighting efficient electron transport. Chronoamperometric measurements demonstrated outstanding long-term stability over 10 h. The combination of tailored morphology, high surface area, and favorable electronic structure establishes hydrothermally synthesized MoS₂ as a promising and practical electrocatalyst for sustainable hydrogen generation.

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1 Introduction

The increasing depletion of fossil fuel reserves and the severe environmental consequences associated with their consumption have intensified global efforts toward the development of sustainable and carbon-neutral energy technologies.^1–7 Among various clean energy vectors, hydrogen has emerged as a highly promising fuel due to its exceptionally high gravimetric energy density and environmentally benign combustion, which produces only water as a byproduct. In this context, electrochemical and photoelectrochemical water splitting have attracted considerable attention as a green route for hydrogen production. The overall efficiency of this process, however, is largely limited by the sluggish kinetics of the hydrogen evolution reaction (HER), which necessitates the development of highly active, stable, and cost-effective electrocatalysts to replace noble metals such as Pt.^8–15 Transition metal-based catalysts, particularly earth-abundant nanostructured materials, have gained significant interest as viable alternatives for HER applications. Among them, transition metal dichalcogenides (TMDs) have emerged as a distinctive class of layered materials with tunable electronic structures and excellent catalytic properties.^14,16–18 Molybdenum disulfide (MoS₂), a prototypical TMD, has been extensively explored owing to its suitable hydrogen adsorption free energy, high chemical stability, and layered structure composed of S–Mo–S units held together by van der Waals interactions. Notably, the catalytic activity of MoS₂ is predominantly associated with its edge sites, while the basal planes are relatively inert. However, recent pioneering studies have demonstrated that the basal plane can also become highly active when defects, atomic vacancies, or heteroatom substitutions are introduced. The spontaneous oxygen substitution in single-layer MoS₂ under ambient conditions has been shown to activate basal-plane sites and significantly enhance catalytic performance. Furthermore, transition-metal chalcogenide monolayers have been identified as versatile platforms for single-atom catalysis, where isolated metal atoms anchored on the basal plane act as highly efficient catalytic centers. In addition, several studies have reported that transition-metal doping—such as Pd and Ni can effectively modulate the electronic structure, create defect-rich active sites, and improve hydrogen evolution reaction activity.^19–22 Apart from these, polymorph-engineered MoS₂ can effectively modulate the electronic structure, create defect-rich active sites, and improve hydrogen evolution reaction activity.²³ These findings highlight that both edge-site engineering and basal-plane activation through defect creation or heteroatom doping are crucial strategies for enhancing the intrinsic catalytic performance of MoS₂-based electrocatalysts. Consequently, rational design strategies such as nano-structuring, defect engineering, phase modulation, and exposure of active edge sites are critical for enhancing HER performance. These attributes position MoS₂ as a compelling candidate for next-generation, noble-metal-free HER electrocatalysts.^24–27 The synthesis method plays a pivotal role in tailoring the morphology, crystallinity, defect density, and exposed active sites of MoS₂-based catalysts. In this regard, the hydrothermal method stands out as a versatile, scalable, and environmentally benign approach for synthesizing nanostructured TMDs with controlled size and morphology.^28–33 Hydrothermal synthesis enables uniform nucleation and growth under mild reaction conditions, facilitating the formation of few-layered or defect-rich MoS₂ structures that are highly favorable for HER. Furthermore, this method allows facile incorporation of dopants, heterostructure formation, and phase engineering, which collectively enhance charge transfer kinetics and catalytic efficiency. Therefore, hydrothermally synthesized MoS₂-based materials represent a promising pathway toward the development of efficient, durable, and economically viable HER electrocatalysts for sustainable hydrogen production.^34–39

Extensive efforts have been devoted to overcoming the intrinsic limitations of pristine MoS₂ for HER and advancing its performance toward practical application. State-of-the-art strategies include phase engineering from the semiconducting 2H phase to the metallic 1T or mixed 1T/2H phases, which significantly enhances electrical conductivity and accelerates charge transfer kinetics.⁴⁰ Defect engineering, such as sulfur vacancies and edge-rich architectures, has been demonstrated to modulate the hydrogen adsorption free energy (ΔG_H*) toward thermoneutral values, thereby improving HER activity. In recent years, hierarchical nanoflower-type architectures of transition-metal dichalcogenides have attracted considerable attention because they provide efficient electrolyte accessibility, high surface area, and abundant exposed edge sites, all of which are advantageous for electrochemical water splitting and energy storage applicates. In particular, MoS₂ nanosheets assembled into three-dimensional flower-like structures have been widely explored through hydrothermal routes, where controlled nucleation and anisotropic growth of marigold-like MoS₂ architectures integrated with MOF-derived N-doped carbon have been reported as high-capacity anode materials for sodium-ion batteries.⁴¹ The MoS₂/MOF architectures integrated with MOF demonstrating that hierarchical nanosheet assemblies can significantly enhance electrochemical stability and charge transport. Similarly, Re-doped MoS₂ flower-like microspheres has been synthesized by hydrothermal method in which dopant-assisted growth and defect generation improved catalytic activity for hydrogen evolution.⁴² Flower-like heteronanosheet arrays such as CoS₂/MoS₂ have also been fabricated via solution-based methods, where the assembly of ultrathin nanosheets into three-dimensional architectures resulted in enhanced active-site exposure and improved durability in alkaline hydrogen evolution reactions.⁴³ Moreover, in recent reviews of MoS₂ electrocatalysts emphasize that morphology engineering—especially the formation of nanoflower or hierarchical nanosheet assemblies—plays a key role in improving the intrinsic activity by increasing edge density and facilitating charge/mass transport.⁴⁴

Furthermore, heteroatom doping (e.g., Co, Ni, Fe) and construction of MoS₂-based heterostructures with carbon materials, metal oxides, or other transition metal compounds have shown synergistic effects, leading to reduced overpotentials, smaller Tafel slopes, and enhanced durability across a wide pH range. These advancements have enabled MoS₂-based electrocatalysts to approach or, in some cases, rival the performance of noble-metal catalysts, positioning MoS₂ as a benchmark non-precious HER catalyst and a cornerstone material in next-generation hydrogen energy technologies.^45–49 Despite these advances, controlling the assembly of two-dimensional MoS₂ nanosheets into well-defined hierarchical flower-like architectures remains challenging, and subtle variations in precursor composition, reaction environment, and growth kinetics can significantly influence morphology and catalytic performance. Therefore, developing simple and reproducible synthesis strategies to obtain hierarchical MoS₂ nanosheet assemblies with enhanced electrochemical activity is still of considerable interest.

In this work, we report a marigold-like assembly of MoS₂ nanosheets synthesized via a controlled hydrothermal route to obtain nanostructures with enhanced exposure of active edge sites and catalytic performance toward alkaline hydrogen evolution. The material was comprehensively characterized using PXRD, Raman spectroscopy, XPS, and electron microscopy to confirm phase composition, defect chemistry, and morphology. Electrochemical studies revealed excellent HER performance, delivering a low overpotential at 10 mA cm⁻², a small Tafel slope, and good long-term stability, demonstrating the effectiveness of hydrothermal engineering in activating MoS₂ as an efficient noble-metal-free HER electrocatalyst.

2 Materials and methods

2.1 Materials

All chemicals used in the present investigation were of analytical reagent (AR) grade and were used without further purification. Sodium molybdate dihydrate (Na₂MoO₄·2H₂O) and the sulfur (S) source were procured from Sigma-Aldrich. Absolute ethanol (99.5% purity, Sigma-Aldrich) was used as the solvent, while a 35% (w/w) aqueous solution of hydrochloric acid (HCl, Sigma-Aldrich) served as the catalyst. The pH of the reaction medium was adjusted using ammonium hydroxide (NH₄OH, Sigma-Aldrich). Double-distilled water (DDW) was used throughout for the preparation of all solutions.

2.2 Synthesis of MoS₂ via hydrothermal method

The MoS₂ nanoparticles were synthesized utilizing the hydrothermal method. A total of 0.4838 g (2 mmol) of sodium molybdate dihydrate (Na₂MoO₄·2H₂O) was dissolved in DDW, to which 0.7612 g (10 mmol) of thiourea was added incrementally with continuous stirring. The complete solution was then transferred into a 100 mL Teflon-lined stainless-steel autoclave and subjected to thermal treatment in an oven at 200 °C for 24 hours. Following this, the autoclave was allowed to cool to room temperature, and the solid nanoparticles of MoS₂ were subsequently washed with deionized water and anhydrous ethanol three times at 7000 rpm, respectively. The MoS₂ nanoparticles were then dried at 80 °C in a vacuum oven for 12 hours.

The controlled precursor ratio, gradual addition of thiourea, and extended reaction duration (200 °C, 24 h) play a crucial role in directing the nucleation and growth of two-dimensional MoS₂ nanosheets, which subsequently self-assemble into a three-dimensional hierarchical morphology. This subtle modification of reaction parameters results in improved nanosheet stacking, higher exposure of edge sites, and enhanced electrochemical activity.

The as-synthesized marigold-flower-like MoS₂ electrocatalyst differs from earlier studies^41–44 in several important aspects. For instance, previously reported marigold-flower-like MoS₂ structures often involve multi-step processes, dopant incorporation, required MOF templating and subsequent carbonization. Whereas, some of the marigold-like MoS₂ were fabricated through substrate-assisted growth to form heterostructured arrays rather than free-standing MoS₂ assemblies. In contrast, the present work employs a single-step hydrothermal route without dopants, and templates, enabling the direct self-assembly of ultrathin MoS₂ nanosheets into a marigold-like hierarchical architecture. The present approach represents a simplified and template-free route for obtaining marigold-like nanosheet assemblies with promising electrocatalytic performance.

2.3 Physico-chemical characterizations

X-ray diffraction (XRD) patterns were collected from a Rigaku D/Max-TTR III X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) over the 2θ range of 5–90°. The Raman spectra were measured on a Renishaw UK Sales Model Renish INVIA 0120-20 Raman spectrometer. The morphology of the material was observed by the FE-SEM using JSM-6701F (JEOL, Japan). The Brunauer–Emmett–Teller (BET) instrument is a surface area analyzer that measures the surface area and porosity of a material the Anton Paar makes the NOVA 1000e model was used instrument. The XPS instrument used for surface area measurements was performed JEOL Japan Make, Model JPS-9030. X-ray photoelectron spectroscopy, which is a compositional characterization technique used at the Institution: Common Facility Centre (CFC)-Sophisticated Analytical Instrument Facilities (SAIF), Kolhapur.

2.4 Electrochemical measurements

Electrochemical measurements were carried out using a CH Instruments CHI 660 electrochemical workstation. A conventional three-electrode configuration was employed for the HER setup in 1.0 M KOH electrolyte (N₂-saturated, 30 mL electrochemical cell). Nickel foam (1 × 1 cm²) served as the working electrode and was coated with catalyst ink prepared by ultrasonically dispersing 20 mg of ZCF-x in a mixed solvent containing 796 µL of deionized water, 200 µL of isopropyl alcohol, and 4 µL of Nafion solution. An Ag/AgCl electrode (3 M KCl) and a graphite rod were used as the reference and counter electrodes, respectively. Linear sweep voltammetry (LSV) was conducted to evaluate electrocatalytic activity, with all measured potentials converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation.⁵⁰ Tafel plots were derived from the polarization curves to elucidate reaction kinetics.⁵¹ Electrochemical impedance spectroscopy (EIS) measurements were performed with an AC perturbation of 5 mV over a frequency range of 1000 kHz to 0.1 Hz. Long-term electrocatalytic stability was assessed by chronoamperometry at a constant current density of 10 mA cm⁻², ensuring a comprehensive evaluation of catalytic activity, charge-transfer behavior, and durability.

3 Result and discussion

Fig. 1a presents the XRD pattern of the synthesized MoS₂ sample. The diffraction profile exhibits well-defined and sharp peaks, confirming the crystalline nature of the prepared material. The observed reflections at 2θ values of approximately 14.40°, 33.00°, 36.15°, 43.18°, and 58.23° can be indexed to the (002), (100), (102), (103), and (110) crystallographic planes, respectively, in good agreement with the standard hexagonal phase of MoS₂ (JCPDS reference code no. 00-037-1492). The presence of the prominent low-angle (002) reflection indicates a layered structure with preferential orientation along the c-axis, characteristic of two-dimensional MoS₂ nanosheets. Such a well-defined lamellar architecture is highly desirable, as it promotes the exposure of catalytically active edge sites and facilitates charge transport, thereby rendering the synthesized MoS₂ particularly suitable for hydrogen evolution reactions.⁵²

Fig. 1 (a) XRD pattern and (b) Raman spectrum of MoS₂, confirming its crystalline phase and characteristic vibrational modes.

The Raman spectrum of the synthesized MoS₂ (Fig. 1b) exhibits two characteristic vibrational modes at 380 cm⁻¹ and 405 cm⁻¹, corresponding to the in-plane E¹_2g and out-of-plane A¹_g phonon modes, respectively. These Raman features are the fingerprint of hexagonal 2H-MoS₂, confirming the successful formation of layered MoS₂ nanosheets.⁵³ The frequency separation (Δ ≈ 24.9 cm⁻¹) between the E¹_2g and A¹_g modes indicates the presence of few-layer MoS₂, which is known to enhance the exposure of active edge sites and improve charge transfer kinetics. Such structural characteristics are highly favorable for hydrogen evolution reaction (HER) applications.

The textural properties of the synthesized MoS₂ were evaluated using N₂ adsorption–desorption measurements (Fig. 2a and b). The Brunauer–Emmett–Teller (BET) analysis revealed a specific surface area of 18.45 m² g⁻¹, indicating the presence of accessible active sites for electrocatalytic reactions. The Barrett–Joyner–Halenda (BJH) pore size distribution showed an average pore radius of 2.89 nm, confirming the mesoporous nature of the material. The total pore volume was calculated to be 2.67 × 10⁻² cm³ g⁻¹ for pores with radii below 78.27 nm, determined at a relative pressure (P/P₀) of 0.9876. This mesoporous architecture facilitates efficient electrolyte penetration and mass transport, which is advantageous for enhancing charge transfer and catalytic activity during the hydrogen evolution reaction.

Fig. 2 (a) N₂ adsorption–desorption isotherms and (b) BJH pore size distribution of MoS₂, revealing its surface area characteristics and mesoporous nature.

The SEM images demonstrate that the as-synthesized MoS₂ exhibits a hierarchical marigold-like nanoflower morphology composed of interconnected and vertically aligned nanosheets (Fig. 3a and b), formed by the self-assembly of thin, crumpled nanosheets into three-dimensional hierarchical structures. Such three-dimensional assemblies of ultrathin MoS₂ sheets are consistent with previously reported flower-like MoS₂ architectures prepared via hydrothermal, solvothermal methods.^41–44 Earlier studies have shown similar hierarchical morphologies in doped or composite systems, where nanosheet assembly leads to increased surface area and exposure of catalytically active sites. However, unlike those systems, which often require dopants, templates, substrates, or multi-step synthesis, the present material forms a free-standing marigold-like assembly through a single-step, template-free hydrothermal route. The nanosheets in the current structure appear relatively thin, loosely stacked, and hence significantly increases the surface roughness and exposes a large number of catalytically active edge sites, which may facilitate electrolyte penetration and augment the accessibility of edge and defect sites. This structural configuration is highly advantageous is comparable to earlier reports, thus charge transport and reaction kinetics has been improved resulted enhanced HER activity.

Fig. 3 (a and b) FE-SEM images of MoS₂ showing a well-defined flower-like morphology formed by the assembly of thin nanosheets, providing abundant exposed edges and active sites.

The deconvoluted XPS spectra of the synthesized MoS₂ provide insight into the chemical composition and electronic states of the material. The Mo 3d spectrum exhibits two prominent peaks at 233.00 eV and 236.44 eV, corresponding to Mo 3d_5/2 and Mo 3d_3/2, which are characteristic of Mo⁴⁺ in the MoS₂ lattice (Fig. 4a).⁵⁴ This confirms the successful incorporation of molybdenum in its expected oxidation state. The S 2p spectrum displays two peaks at 160.51 eV and 162.57 eV, assigned to S 2p_3/2 and S 2p_1/2, respectively, verifying the presence of sulfide species in the lattice (Fig. 4b).⁵⁵ Collectively, the XPS results confirm the stoichiometric formation of MoS₂ and support the structural integrity inferred from XRD, Raman, and electron microscopy analyses. These features are crucial for exposing active sites and enhancing electrocatalytic performance in the HER.

Fig. 4 Deconvoluted XPS spectra of MoS₂ showing (a) Mo 3d and (b) S 2p core levels, confirming the chemical states of Mo⁴⁺ and S²⁻ and the formation of stoichiometric MoS₂.

The HER activity of the MoS₂ catalyst was evaluated in 1.0 M KOH electrolyte using linear sweep voltammetry (LSV) at a scan rate of 10 mV s⁻¹ (Fig. 5a). The presented LSV curves are with iR-correction and that all samples were measured under identical conditions, ensuring that the comparative electrocatalytic trends remain valid. As shown in Fig. 5a, the MoS₂-modified electrode exhibited a significantly lower overpotential of 142 mV at 10 mA cm⁻² compared to 409 mV for bare Ni foam, highlighting the enhanced catalytic activity of the MoS₂ nanosheets. The kinetics of the MoS₂ were shown by the Tafel slope as illustrated in Fig. 5b, the corresponding Tafel slope for MoS₂ was derived from iR-corrected polarization curves and it is a 101.89 mV dec⁻¹, markedly smaller than that of Ni foam (157.12 mV dec⁻¹) indicating more favorable HER kinetics. The hydrogen evolution reaction in alkaline media generally proceeds through a Volmer–Heyrovsky mechanism involving two elementary steps. The Volmer step corresponds to the electrochemical discharge of water and adsorption of hydrogen on the catalyst surface:

H₂O + M + e⁻ → M–H_ads + OH⁻ (rate-limiting step)

Fig. 5 Hydrogen evolution reaction (HER) performance of MoS₂ in alkaline electrolyte: (a) linear sweep voltammetry (LSV) polarization curves, (b) corresponding Tafel slope, (c) electrochemical impedance spectroscopy (EIS) Nyquist plot, and (d) long-term stability evaluated by chronoamperometry, demonstrating efficient activity and durable electrocatalytic performance, (e) CV measured at scan rates between 20 to 100 mV s⁻¹, (f) plot of current density at 0.2 V vs. RHE against scan rate for double layer capacitance calculation.

This step involves water dissociation and formation of adsorbed hydrogen intermediates at the active sites (M). Subsequently, hydrogen evolution occurs via the Heyrovsky step:

H₂O + M–H_ads + e⁻ → H₂ + OH⁻

which represents the electrochemical desorption of hydrogen. In this case, M stands for “working sites.” Based on the obtained Tafel slope (122.3 mV dec⁻¹), the HER over the MoS₂ electrode is suggested to follow a Volmer–Heyrovsky pathway, where the Volmer step is likely the rate-determining step. The layered structure and high surface area of MoS₂ provide abundant active sites that facilitate water dissociation and hydrogen adsorption in alkaline electrolyte, thereby enhancing HER kinetics. The observed Tafel slope is consistent with this mechanism and indicates that the catalytic process is governed by hydrogen adsorption followed by rapid electrochemical desorption. Electrochemical impedance spectroscopy (EIS) was further employed to probe the charge-transfer behavior of the electrodes. Compared with the bare Ni foam electrode, the MoS₂-modified electrode displayed a lower charge-transfer resistance (R_ct) (Fig. 5c), suggesting improved electrical conductivity and more efficient electron transport at the electrode–electrolyte interface. These features collectively demonstrate that the hydrothermally synthesized MoS₂ possesses abundant active sites, rapid charge transfer, and excellent electrocatalytic efficiency for HER in alkaline media. The long-term durability of MoS₂ was evaluated through a chronoamperometric i–t measurement at a constant current density in 1.0 M KOH for 48 hours (Fig. 5d). The catalyst exhibited negligible loss in current over the testing period, indicating excellent electrochemical stability and robust structural integrity under prolonged HER conditions. However, the long-term stability test was carried out at a current density of 10 mA cm⁻², which is widely used as a benchmark current density for evaluating electrocatalytic performance in water splitting at lab-scale and allows reliable comparison with previously reported catalysts in the literature.²² The electrochemically active surface area (ECSA) of MoS₂ was evaluated from the double-layer capacitance (C_dl) obtained by cyclic voltammetry in the non-faradaic region at different scan rates (Fig. 5e). Based on the CV curve, the graph of current density at 0.2 V vs. RHE against scan rate as shown in Fig. 5f. The slope value is 0.18334 mF cm⁻², exhibited that MoS₂ electrode has elatively large electrochemically accessible surfaces. Using a standard specific capacitance (C_s) value of 40 µF cm⁻² for metal sulfide-based electrodes, the corresponding ECSA was calculated to be approximately 4.58 cm². This enhanced ECSA suggests the presence of abundant exposed active sites, which can be attributed to the layered structure of MoS₂ with edge-rich domains and defect-induced surface roughness. The increased electrochemical surface area facilitates improved electrolyte–electrode interaction and faster charge transfer, thereby contributing to the favourable electrocatalytic performance of MoS₂.

4 Conclusion

In summary, hierarchical MoS₂ nanosheets were successfully synthesized via a facile hydrothermal method, yielding a well-defined marigold-flower-like morphology with abundant exposed edge sites. Comprehensive characterization using XRD, Raman, XPS, SEM, and BET analyses confirmed the crystalline nature, layered structure, chemical composition, and mesoporous architecture of the material. The MoS₂ catalyst exhibited excellent electrocatalytic activity for the HER in 1.0 M KOH, achieving a low overpotential of 142 mV at 10 mA cm⁻² and Tafel slope 101.89 mV dec⁻¹, and superior charge-transfer characteristics as revealed by EIS. Moreover, chronoamperometric testing demonstrated remarkable long-term stability over 48 h. These results highlight that hydrothermally engineered MoS₂ with optimized morphology and surface properties is a promising, cost-effective, and noble-metal-free electrocatalyst for sustainable hydrogen production.

Author contributions

Rajvardhan K. Chougale; methodology, writing – original draft. Prashant D. Sanadi; revision and writing – original draft. Sanket N. Yadav; validation, resources. Babasaheb D. Bhosale; supervision. Srinivaas Masimukku; resources, Guo-Ping Chang-Chien; resources, Ganesh S. Kamble; supervision, project administration, validation, investigation, conceptualization.

Conflicts of interest

There are no conflicts to declare.

Data availability

All raw data supporting BET, UV/Vis, Ramen and XRD is contained within the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra00011h.

Acknowledgements

The authors (RKC, GSK) would want to express gratitude to the CFC-SAIF-DST Center, Shivaji University Kolhapur for the instrumentation facilities. The authors (RKC, GSK) have thankful to Chhatrapati Shahu Maharaj Research, Training and Human Development Institute (SARTHI), Pune (Government of Maharashtra) for the financial support.

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