Rational design of three-dimensional MoS₂ nanoflower-rGO heterostructure for high-performance electrocatalytic hydrogen evolution reaction in acidic and alkaline media

Abstract

Developing efficient and low-cost non-noble-metal electrocatalysts for the hydrogen evolution reaction (HER) is essential for sustainable hydrogen production. Herein, we report the hydrothermal synthesis of a three-dimensional MoS₂ nanoflower/reduced graphene oxide (MoS₂ NF-rGO) hybrid heterostructure, where MoS₂ nanoflowers are uniformly anchored onto rGO nanosheets. The as-prepared MoS₂ NF-rGO exhibits superior HER activity, delivering a low overpotential of 172 mV at a cathodic current density of 10 mA cm⁻² with a remarkably low Tafel slope of 54 mV dec⁻¹ in 0.5 M H₂SO₄. In alkaline medium (1.0 M KOH), the catalyst achieves a cathodic current density of 10 mA cm⁻² at an overpotential of 406 mV with a Tafel slope of 190 mV dec⁻¹, while retaining nearly 100% activity after a continuous 50 h chronoamperometry stability test. These values significantly outperform pristine MoS₂, which requires overpotentials of 309 mV and 475 mV to achieve the cathodic current density of 10 mA cm⁻² in acidic and alkaline media, respectively. Comparative analysis further reveals that the MoS₂-rGO composite with spherical morphology exhibits inferior HER performance despite having a nearly similar composition to MoS₂ NF-rGO, highlighting the important role of MoS₂ morphology in determining the catalytic activity. Cross-referencing the microscopic, spectroscopic, and electrochemical results suggests that the enhanced HER performance of MoS₂ NF-rGO is associated with its hierarchical nanoflower architecture, which provides abundant exposed edge sites and a larger electrochemically active surface area, while EIS measurements show that MoS₂ NF-rGO exhibits the lowest charge-transfer resistance relative to bare MoS₂ nanoflowers and MoS₂-rGO, consistent with more facile electrochemical charge-transfer kinetics.

Hydrothermal synthesis of the three-dimensional MoS₂ nanoflower/reduced graphene oxide (MoS₂ NF-rGO) hybrid heterostructure and its hierarchical architecture with abundant exposed edge sites for enhanced electrocatalytic hydrogen evolution. Image created using Google Gemini via Google Cloud.

Introduction

The escalating global demand for sustainable and clean energy has intensified the search for efficient energy conversion and storage technologies capable of reducing dependence on fossil fuels and mitigating environmental concerns associated with greenhouse gas emissions.^1,2 Among the various renewable energy carriers, hydrogen has emerged as one of the most promising alternatives owing to its high gravimetric energy density, carbon-neutral combustion, and compatibility with next-generation fuel cell technologies.^3,4 Electrochemical water splitting, which enables the direct conversion of electrical energy into hydrogen fuel, has attracted considerable attention as a sustainable route for hydrogen production.⁵ However, the practical implementation of water electrolysis is largely hindered by sluggish electrode kinetics and the high overpotentials associated with the hydrogen evolution reaction (HER), necessitating the development of highly active and durable electrocatalysts.⁶

Currently, noble-metal-based catalysts, particularly platinum (Pt) and Ruthenium (Ru) represent the benchmark electrocatalyst for HER and complimentary partner oxygen evolution reaction because of their superior catalytic activity and low reaction overpotentials.^7–10 Nevertheless, their scarcity, high cost, and poor long-term economic viability significantly restrict large-scale commercialization of noble-metal-based water electrolyzers.¹¹ Consequently, extensive efforts have been devoted toward the development of earth-abundant, low-cost, and highly efficient alternative electrocatalysts capable of replacing noble metals without compromising catalytic performance.

Among the numerous earth-abundant electrocatalysts explored as alternatives to noble metals for HER, transition-metal-based compounds such as metal phosphides (e.g., Ni₂P, CoP), metal carbides/nitrides (e.g., Mo₂C, VN), transition metal oxides/hydroxides, and transition metal chalcogenides have attracted significant research attention due to their low cost, compositional tunability, and promising catalytic activity.^12–21 However, many of these materials still suffer from limitations such as poor long-term chemical stability, suboptimal hydrogen adsorption energies, sluggish reaction kinetics, or insufficient active-site exposure under practical operating conditions.^22,23 Among these emerging alternatives, molybdenum disulfide (MoS₂) has gained particular prominence as one of the most promising non-noble-metal HER electrocatalysts owing to its unique layered two-dimensional structure, near-thermoneutral hydrogen adsorption free energy at catalytically active edge sites, tunable electronic structure, and excellent electrochemical stability in both acidic and alkaline electrolytes.^24,25 In particular, the edge sulfur atoms of MoS₂ exhibit highly favorable hydrogen adsorption/desorption behavior, making the material intrinsically attractive for HER catalysis.²⁶ Nevertheless, despite these advantages, the practical catalytic efficiency of pristine MoS₂ remains substantially below its theoretical potential due to several intrinsic limitations, including low intrinsic electrical conductivity, limited exposure of catalytically active edge sites, and severe restacking/aggregation of nanosheets during synthesis.²⁷ These drawbacks impede charge transfer kinetics and hinder electrolyte diffusion to active sites, thereby significantly restricting overall electrocatalytic performance.

To address these limitations, various structural engineering strategies have been employed to improve the catalytic utilization of MoS₂. Among them, morphological tailoring into hierarchical three-dimensional nanoarchitectures has emerged as an effective approach for maximizing active-site exposure and enhancing electrolyte accessibility.²⁸ In particular, nanoflower-like MoS₂ architectures, consisting of interconnected ultrathin nanosheets assembled into hierarchical flower-like structures, have demonstrated considerable promise due to their high surface area, abundant exposed edge sites, shortened ion diffusion pathways, and enhanced electrolyte penetration.²⁹ Such morphological engineering can substantially improve active-site accessibility and mass transport kinetics during electrochemical reactions. However, despite these structural advantages, the inherently poor electrical conductivity of MoS₂ remains a major bottleneck limiting its catalytic efficiency, especially at higher current densities.²⁷ Consequently, integrating MoS₂ with highly conductive support materials has become a widely adopted strategy to overcome conductivity-related limitations. Various conductive substrates, including carbon nanotubes (CNTs), MXenes, and graphene-based materials, have been investigated as supporting matrices for MoS₂-based electrocatalysts.^30–33 For example, MXenes have shown promise due to their metallic conductivity and hydrophilic surface chemistry, while CNTs offer one-dimensional conductive pathways and excellent mechanical robustness. Nevertheless, challenges such as limited surface anchoring sites, poor interfacial compatibility, restacking tendencies, or complicated synthesis routes often restrict the practical effectiveness of these support materials.

Among the various conductive supports investigated, reduced graphene oxide (rGO) has emerged as one of the most attractive platforms for MoS₂ hybridization owing to its exceptional electrical conductivity, ultrahigh specific surface area, excellent mechanical flexibility, and abundant surface functional groups that facilitate strong interfacial anchoring of nanostructured catalysts.^34,35 The incorporation of rGO can effectively suppress aggregation/restacking of MoS₂ nanosheets, improve dispersion of catalytically active material, provide rapid electron transport pathways, and enhance the structural integrity of the resulting hybrid system.³⁶ More importantly, strong heterointerfacial electronic coupling between MoS₂ and rGO can induce interfacial charge redistribution, thereby modulating the electronic environment of catalytically active Mo centers and accelerating HER kinetics.³⁷ Despite these advantages, achieving uniform growth of hierarchically structured MoS₂ with strong interfacial coupling on graphene supports remains a significant challenge. In many previously reported MoS₂/rGO hybrids, insufficient dispersion of MoS₂ and weak interfacial contact with graphene substrates limit the extent of synergistic enhancement.³⁸ Therefore, rational design of three-dimensional hierarchical MoS₂ nanostructures intimately anchored onto conductive rGO frameworks is highly desirable to simultaneously maximize active-site exposure, improve electrical conductivity, and promote efficient interfacial charge transfer for superior electrocatalytic performance.

In this work, we report a three-dimensional MoS₂ nanoflower/reduced graphene oxide (MoS₂ NF-rGO) hybrid heterostructure, in which hierarchical MoS₂ nanoflowers are uniformly anchored onto rGO nanosheets. The as-prepared MoS₂ NF-rGO electrocatalyst exhibits outstanding hydrogen evolution performance, delivering a low overpotential of 172 mV at a cathodic current density of 10 mA cm⁻² in an acidic electrolyte (0.5 M H₂SO₄) along with a small Tafel slope of 54 mV dec⁻¹, indicating favorable HER kinetics. Furthermore, the catalyst demonstrates decent HER performance in an alkaline medium, requiring an overpotential of 406 mV to reach 10 mA cm⁻² in 1.0 M KOH, accompanied by a Tafel slope of 190 mV dec⁻¹, while maintaining nearly 100% activity after a continuous 50 h chronoamperometry stability test, highlighting its good electrochemical durability. Notably, the electrocatalytic activity of MoS₂ NF-rGO is significantly superior to that of bare MoS₂ nanoflowers, which require substantially higher overpotentials of 309 mV in 0.5 M H₂SO₄ and 475 mV in 1.0 M KOH, respectively, to achieve the same current density. To further elucidate the role of MoS₂ morphology, a conventional MoS₂ sphere/rGO composite (MoS₂-rGO) was also synthesized for comparison. Despite having a nearly comparable composition, MoS₂-rGO exhibits inferior HER performance relative to MoS₂ NF-rGO, highlighting the important role of MoS₂ morphology in determining the catalytic behavior. The superior electrocatalytic performance of MoS₂ NF-rGO is associated with the three-dimensional hierarchical nanoflower architecture, which provides abundant exposed edge sites, together with a higher electrochemically active surface area. Consistent with these structural characteristics, EIS measurements reveal that MoS₂ NF-rGO exhibits the lowest charge-transfer resistance relative to bare MoS₂ nanoflowers and MoS₂-rGO, indicating more facile electrochemical charge-transfer kinetics. Additionally, the intimate anchoring of ultrathin MoS₂ nanosheets onto the graphene support helps suppress excessive aggregation/restacking and maintain active-site accessibility during electrochemical operation. Collectively, these findings demonstrate that controlled MoS₂ morphology and its integration with rGO provide an effective strategy for developing high-performance and durable MoS₂-based HER electrocatalysts for sustainable hydrogen production.

Experimental section

Synthesis of MoS₂-rGO composites

First, graphene oxide (GO) was synthesized using a modified Hummers' method.³⁹ In a typical procedure, 1 g of graphite flakes was mixed with concentrated H₂SO₄, and sodium nitrate was slowly added while maintaining the reaction temperature between 0–4 °C. Subsequently, KMnO₄ was gradually introduced into the mixture, keeping the temperature around 20 °C. The reaction was then maintained at 40 °C for 7 hours. After completion, the mixture was allowed to reach room temperature and was carefully poured onto 250 mL of an ice-deionized water mixture, followed by the addition of 3 mL of H₂O₂. The resulting suspension was centrifuged and repeatedly washed with 0.1 M HCl and deionized water to remove residual ions and impurities.

To synthesize three-dimensional MoS₂ nanoflower/reduced graphene oxide (MoS₂ NF-rGO) hybrid composite, 0.4 g of ammonium molybdate tetrahydrate (AMT) ((NH₄)₆Mo₇O₂₄·4H₂O; Merck Chemicals) was first dissolved in 40 mL of deionized (DI) water under continuous magnetic stirring for 30 min at 600 rpm to obtain a clear homogeneous precursor solution. Subsequently, 2.8 g of thiourea (CH₄N₂S; Merck Chemicals) was added to the AMT solution under stirring, serving as the sulfur precursor for MoS₂ formation. The initial pH of the resulting solution was measured to be 5.22. Thereafter, 100 mg of as-prepared graphene oxide (GO) dispersion (4 mL) was introduced into the precursor solution, followed by the dropwise addition of 3 mL ammonium hydroxide to adjust the solution pH to 10.30, thereby facilitating nucleation and controlled growth of MoS₂ nanostructures under alkaline conditions. The resulting homogeneous suspension was then transferred into a Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 180 °C for 18 h. During the hydrothermal process, thiourea gradually decomposes to release sulfur-containing species, which react with molybdenum precursor ions to promote the nucleation and growth of MoS₂ nanosheets. Simultaneously, the elevated temperature and reducing hydrothermal environment facilitate the in situ reduction of graphene oxide into reduced graphene oxide (rGO) through the removal of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl moieties. This hydrothermal reduction process restores the conjugated sp² carbon network of GO, thereby significantly improving its electrical conductivity while enabling strong interfacial anchoring of the growing MoS₂ nanostructures onto the graphene surface. Following hydrothermal synthesis, the obtained black precipitate was collected by centrifugation and repeatedly washed with deionized water and ethanol at 4500 rpm to remove residual impurities and unreacted precursors. The purified product was then dried in an oven at 60 °C for 15 h, followed by manual grinding for 20 min to obtain a uniform fine powder. For comparison, MoS₂-rGO spherical composite was synthesized following a similar hydrothermal route using identical precursor concentrations, except that the hydrothermal treatment was carried out at 160 °C for 12 h instead of 180 °C for 18 h to suppress hierarchical nanosheet self-assembly and promote isotropic particle growth.

Physical characterization

The surface morphology and elemental distribution of the synthesized materials were examined using field-emission scanning electron microscopy (FESEM, JEOL JSM-7610F Plus) coupled with an energy-dispersive X-ray spectroscopy (EDS) detector for elemental mapping and compositional analysis. Structural crystallinity and phase identification were characterized through X-ray diffraction (XRD) using a Rigaku SmartLab diffractometer employing Cu Kα radiation (λ = 1.54 Å) operated at a maximum power of 3 kW. The diffraction data were recorded over a 2θ range of 10°–90° to assess the crystalline phase and structural purity of the prepared samples. Raman spectroscopic analysis was conducted using a Horiba LabRAM HR Evolution micro-Raman spectrometer integrated with a fully automated Czerny–Turner optical system and high-resolution optical microscope to probe the vibrational characteristics and structural ordering of the synthesized materials. In addition, Fourier transform infrared (FTIR) spectroscopy was carried out using a Bruker ALPHA II spectrometer within the spectral range of 500–4000 cm⁻¹ to investigate the surface functional groups and chemical bonding characteristics of the obtained samples.

Electrochemical measurements

Electrochemical performance measurements were conducted using a CHI 760F electrochemical workstation in a standard three-electrode setup at ambient temperature. A catalyst-coated glassy carbon electrode (GCE) with a geometric area of 0.16 cm² (0.4 × 0.4 cm²) served as the working electrode, while a graphite rod and a 3 M KCl-saturated Ag/AgCl electrode (E = +0.210 V vs. SHE) were employed as the counter and reference electrodes, respectively. All electrochemical experiments were performed in 0.5 M H₂SO₄ (pH ≈ 0) and 1.0 M KOH (pH ≈ 14) aqueous electrolytes.

To prepare the working electrode, catalyst ink was formulated by dispersing 5 mg of the synthesized catalyst in 1 mL of isopropanol (IPA) containing 50 µL of 5 wt% Nafion solution (DuPont), followed by ultrasonication for 1 h to obtain a uniform and stable suspension. Subsequently, 20 µL of the catalyst ink was drop-cast onto the surface of the glassy carbon electrode and allowed to dry naturally under ambient conditions before electrochemical testing. The corresponding catalyst loading was 0.0952 mg per electrode, equivalent to 0.595 mg cm⁻² based on the geometric surface area of the glassy carbon electrode.

HER activity was evaluated by linear sweep voltammetry (LSV) over the potential range of +0.20 to −0.60 V vs. RHE at a scan rate of 2 mV s⁻¹ under nitrogen-saturated conditions. Prior to HER measurements, the working electrode underwent several cyclic voltammetry activation cycles to ensure stabilization of the electrode surface and achieve reproducible electrochemical behavior. All measured potentials were converted to the reversible hydrogen electrode (RHE) scale viaE_RHE = E_Ag/AgCl + 0.210 + 0.059 × pHwhere E_Ag/AgCl is the experimentally measured potential versus a 3 M KCl-saturated Ag/AgCl reference electrode, and 0.210 V represents its standard potential relative to the standard hydrogen electrode (SHE) at 25 °C. All polarization curves presented in this work were corrected for 85% of the uncompensated resistance (iR) using the automatic iR-compensation function of the CHI electrochemical workstation.

Electrochemical impedance spectroscopy (EIS) measurements were performed in 1.0 M KOH electrolyte at an applied potential of −0.3 V vs. RHE, using a sinusoidal AC perturbation with an amplitude of 10 mV over a frequency range of 100 mHz to 1 MHz. The measurements were carried out using a logarithmic frequency sweep from high to low frequencies.

Results and discussion

Morphological and structural analysis

The morphology and microstructure of the synthesized MoS₂-based materials were investigated using field-emission scanning electron microscopy (FESEM), as presented in Fig. 1. The pristine MoS₂ sample, denoted as MoS₂–NF (Fig. 1a–c) exhibits a well-defined hierarchical nanoflower (NF) architecture composed of interconnected ultrathin nanosheets. These nanosheets are radially assembled to form three-dimensional flower-like structures, creating an open architecture with abundant exposed edge sites that are considered catalytically active for HER. The loosely stacked and vertically oriented nanosheets suggest enhanced exposure of edge sulfur sites, which play a crucial role in facilitating catalytic activity.⁴⁰ Upon integration with reduced graphene oxide (rGO), significant morphological changes are observed. The MoS₂ NF-rGO composite (Fig. 1d–f) retains the nanoflower morphology, uniformly dispersed over the crumpled rGO sheets. This intimate physical contact between the MoS₂ nanoflowers and rGO provides a well-integrated hybrid architecture. In contrast, the MoS₂-rGO composite prepared with nanosphere morphology (Fig. 1g–i) shows the formation of relatively compact and spherical MoS₂ particles distributed over the rGO sheets. These nanospheres appear more aggregated and exhibit a comparatively lower degree of structural openness than the nanoflower counterpart. The reduced exposure of edge sites in the nanosphere morphology may limit the availability of catalytically accessible sites, thereby affecting the overall electrochemical performance. The corresponding schematic illustrations further highlight the structural differences among the three systems. The corresponding schematic illustrations further highlight the structural differences among the three systems. The MoS₂ nanoflowers consist of layered nanosheets with a high density of exposed edges (Fig. 1c), while the MoS₂ NF-rGO composite demonstrates intimate spatial integration of the nanoflowers with the rGO sheets (Fig. 1f). In contrast, the MoS₂-rGO system is dominated by spherical particles with comparatively fewer accessible active sites (Fig. 1i).

Fig. 1. Morphological and structural analysis of MoS₂-based materials. (a and b) FESEM images of MoS₂ nanoflowers (MoS₂ NF) showing hierarchical, edge-rich nanosheet assemblies. (c) Schematic illustration of the MoS₂ nanoflower structure highlighting abundant exposed active edge sites. (d and e) FESEM images of the MoS₂ NF-rGO composite, demonstrating uniform anchoring of MoS₂ nanoflowers on the rGO sheets and improved dispersion. (f) Corresponding schematic representation of MoS₂ nanoflowers integrated with conductive rGO layers. (g and h) FESEM images of the MoS₂-rGO composite consisting of MoS₂ nanospheres distributed over rGO sheets. (i) Schematic illustration of MoS₂ nanospheres supported on rGO.

To further elucidate the compositional uniformity and interfacial integration of the synthesized composites, elemental mapping analysis was performed using energy-dispersive X-ray spectroscopy (EDS) coupled with FESEM (Fig. 2). The elemental distribution of carbon (C), oxygen (O), molybdenum (Mo), and sulfur (S) was analyzed for both MoS₂ NF-rGO and MoS₂-rGO samples. For the MoS₂ NF-rGO composite (Fig. 2a), the FESEM image confirms the presence of well-defined MoS₂ nanoflower structures anchored on the rGO sheets, consistent with the morphology discussed earlier. The corresponding elemental maps reveal a highly uniform distribution of Mo and S throughout the nanoflower regions, confirming the successful formation of MoS₂. Simultaneously, the homogeneous dispersion of C and O elements indicates the presence of rGO as a continuous conductive matrix. The spatial distribution of the elemental signals indicates intimate physical integration of the MoS₂ nanoflowers with the rGO framework. In contrast, the MoS₂-rGO composite (Fig. 2b) exhibits a nanosphere-dominated morphology, in agreement with previous FESEM observations. The elemental mapping for this sample shows a less uniform distribution, with localized clustering of Mo and S signals corresponding to aggregated MoS₂ nanospheres. Although the presence of C and O confirms the incorporation of rGO, the comparatively non-uniform spatial distribution of the MoS₂ domains suggests less homogeneous integration with the rGO framework. Thus, compared with MoS₂-rGO, the MoS₂ NF-rGO composite exhibits greater compositional homogeneity and more intimate spatial integration of the MoS₂ phase with the carbon support. The uniform distribution of active MoS₂ species over the conductive rGO framework is expected to provide more continuous pathways for electron transport and improve the accessibility of catalytically active sites. In contrast, the aggregated nature of MoS₂ nanospheres in MoS₂-rGO may limit interfacial contact and active-site accessibility. Furthermore, EDS quantitative analysis confirms that both MoS₂ NF-rGO and MoS₂-rGO composites possess nearly comparable Mo, S, and C contents (Fig. S1), indicating that the observed difference in electrocatalytic performance primarily arises from their distinct morphologies and associated structural characteristics rather than significant differences in elemental composition or Mo loading.

Fig. 2. FESEM image and corresponding elemental mapping of (a) MoS₂ NF-rGO and (b) MoS₂-rGO composites.

The crystalline structure and phase composition of the synthesized materials were investigated using X-ray diffraction (XRD). As shown in Fig. 3, the XRD pattern of pristine MoS₂ exhibits the characteristic diffraction features of its layered structure. Notably, the (002) reflection appears at approximately 9.96°, considerably lower than the ∼14.2° position of well-crystallized 2H-MoS₂, indicating an increased interlayer spacing arising from the expanded stacking of S–Mo–S layers.^41–43 This pronounced low-angle shift can be attributed to structural distortion arising from the coexistence of 1T and 2H phases, in which the distorted 1T phase disrupts the conventional stacking of MoS₂ layers and weakens interlayer interactions. In addition, the higher-angle reflections corresponding to the (100), (103), (105), and (110) planes are substantially broadened and weakened. These changes indicate reduced long-range crystallinity and distortion of the in-plane MoS₂ lattice, further supporting the formation of a structurally distorted and expanded MoS₂ framework.^41–43

Fig. 3. Comparative XRD patterns of rGO, MoS₂ NF, MoS₂-rGO and MoS₂ NF-rGO samples.

A clear difference is observed after integration with rGO. The MoS₂-rGO composite exhibits considerably weakened and broadened MoS₂ reflections compared with pristine MoS₂, suggesting a reduction in the degree of long-range structural ordering. This behavior is consistent with the compact nanosphere morphology observed by FESEM, where aggregation of MoS₂ domains can limit the development of well-oriented layered crystallites. In contrast, the MoS₂ NF-rGO composite retains more pronounced MoS₂ diffraction features, with the (002) reflection being particularly evident. The comparatively stronger and better-defined reflections indicate that the nanoflower architecture preserves the layered crystallographic framework more effectively than the spherical morphology.⁴⁴ This observation is consistent with the FESEM results, which show that the nanoflowers are composed of radially assembled ultrathin nanosheets rather than densely packed spherical aggregates. Notably, the significant suppression of graphitic (002) peak for rGO in both composites indicates its highly exfoliated and disordered nature, which is advantageous for facilitating intimate interfacial contact with MoS₂. However, the key difference lies in how effectively this interface is established: elemental mapping confirms that MoS₂ NF-rGO exhibits a more homogeneous spatial distribution of Mo and S over the carbon framework, whereas MoS₂-rGO shows localized clustering. This difference is directly reflected in the XRD profiles, where the NF-based system maintains structural integrity while avoiding excessive aggregation. From a structure–property perspective, the MoS₂ NF-rGO composite represents a favorable balance between crystallinity and defect engineering.

Fig. 4 presents the comparative Raman spectra of rGO, MoS₂ NF, MoS₂-rGO, and MoS₂ NF-rGO catalysts, highlighting the structural evolution of the hybrid materials and providing insights into phase composition, defect density, and structural changes associated with the integration of MoS₂ and rGO. Fig. 4a presents the full-range comparative Raman spectra of rGO, MoS₂ MoS₂-rGO, and MoS₂ NF-rGO catalysts, while the magnified spectral windows corresponding to 100–500 cm⁻¹, 500–1000 cm⁻¹, and 1000–2000 cm⁻¹ are separately shown in Fig. 4b–d, respectively, for detailed structural analysis. Accordingly, the Raman spectrum of pristine rGO exhibits two prominent characteristic bands located at approximately 1350 cm⁻¹ and 1590 cm⁻¹, corresponding to the D band and G band, respectively, as highlighted in Fig. 4d.⁴⁵ The D band originates from the breathing vibration of A¹_g symmetry associated with structural defects and disorder in the sp² carbon lattice, whereas the G band corresponds to the E²_g in-plane stretching vibration of graphitic sp²-bonded carbon atoms. The relatively intense D band confirms the defective nature of the reduced graphene oxide sheets, indicating the presence of residual oxygen-containing functional groups, vacancy defects, and lattice distortions generated during the oxidation and reduction processes.⁴⁶ Such defect-rich characteristics are favorable for providing potential anchoring sites for the nucleation and stabilization of MoS₂ nanostructures.

Fig. 4. (a) Comparative Raman spectra of rGO, MoS₂, MoS₂-rGO and MoS₂ NF-rGO catalysts. For fair comparison the zoom in regions from (b) 100–500 cm⁻¹, (c) 500–2000 cm⁻¹, (d) 1000–2000 cm⁻¹ are shown.

Upon incorporation of MoS₂, the D and G bands remain clearly observable in both MoS₂-rGO and MoS₂ NF-rGO composites, indicating the retention of the graphene framework after hybrid formation. Notably, both bands exhibit slight broadening and increased relative intensity in the composite samples compared to pristine rGO (Fig. 4c), suggesting increased disorder within the carbon lattice after MoS₂ deposition.⁴⁷ This behavior suggests modification of the local carbon environment following MoS₂ incorporation, which may be associated with interactions between MoS₂ and defect/functional sites on rGO.

The low-wavenumber Raman spectra of the MoS₂-based samples are magnified in Fig. 4b, where several characteristic vibrational modes associated with MoS₂ can be identified. For pristine MoS₂ NF and MoS₂ NF-rGO, two dominant peaks centered around ∼380 cm⁻¹ and ∼405 cm⁻¹ are observed, corresponding to the E¹_2g and A¹_g modes of 2H-MoS₂, respectively.⁴⁸ The E¹_2g mode arises from in-plane vibration of Mo and S atoms, while the A¹_g mode corresponds to out-of-plane vibration of sulfur atoms. These two modes are the characteristic fingerprints of the thermodynamically stable semiconducting hexagonal 2H phase of MoS₂.⁴⁸

In addition to the 2H-phase vibrations, extra Raman peaks are observed at approximately ∼150 cm⁻¹ (J₁), ∼230 cm⁻¹ (J₂), and ∼340 cm⁻¹ (J₃) in both MoS₂ NF and MoS₂ NF-rGO, as shown in Fig. 4b, confirming the presence of the metastable 1T metallic phase of MoS₂.⁴⁸ These J peaks arise from lattice distortion induced by the octahedral coordination environment of Mo atoms and serve as distinct Raman signatures of the distorted 1T phase. Importantly, the intensity of these J peaks is significantly enhanced in MoS₂ NF-rGO compared to MoS₂-rGO, indicating a more pronounced contribution of the 1T phase in the nanoflower-based composite. This difference may be related to the distinct morphology and structural environment of MoS₂ in the two composites. A comparison of Raman peak intensities further reveals that MoS₂ NF-rGO exhibits overall stronger and sharper MoS₂ vibrational peaks than MoS₂-rGO, indicating improved crystallinity and/or increased Raman scattering efficiency in the nanoflower-based hybrid. The enhanced Raman intensity may be attributed to the hierarchical nanoflower morphology, which provides well-defined nanosheet architectures and increased exposure of edge regions. Furthermore, the integration of rGO may contribute to the observed differences in Raman response through changes in the local structural environment and dispersion of MoS₂ nanoflowers. As shown in Fig. 4c, additional weak broad peaks are observed around ∼800–1000 cm⁻¹, marked by asterisks (*), particularly in the MoS₂-containing samples. These bands are attributed to second-order Raman scattering processes or overtone/combination modes of MoS₂, which arise from multiphonon interactions involving longitudinal acoustic phonons and zone-boundary vibrations.⁴⁹ Such second-order modes are typically observed in layered transition metal dichalcogenides and become more prominent in defect-rich, few-layered, or structurally distorted MoS₂ systems due to relaxation of Raman selection rules. Their appearance in the present samples is consistent with the presence of structural disorder, few-layer characteristics, and mixed-phase features of the synthesized MoS₂.

Another notable observation is the slight shift and broadening of the Raman modes in MoS₂ NF-rGO relative to pristine MoS₂ NF (Fig. 4b), indicating changes in the local vibrational environment and structural characteristics following integration with rGO. These spectral changes are consistent with interactions between MoS₂ and the rGO framework; however, Raman spectroscopy alone does not directly establish interfacial charge redistribution. The changes in peak position and broadening may also reflect local structural disorder and variations in the MoS₂ nanosheet environment within the hybrid structure.

The chemical bonding environment and interfacial interactions among the synthesized materials were further investigated using FTIR spectroscopy, and the comparative spectra of MoS₂ NF, MoS₂-rGO, and MoS₂ NF-rGO composites are presented in Fig. 5. The FTIR spectra provide important insights into the structural integrity of MoS₂, the presence of residual oxygen-containing functionalities, and changes in the local bonding environment following integration with rGO. For pristine MoS₂ NF, weak absorption bands observed at 3843, 3736, and 3623 cm⁻¹ are attributed to the stretching vibrations of surface hydroxyl groups (–OH) and physically adsorbed water molecules, arising from the surface characteristics of hydrothermally synthesized MoS₂ nanoflowers.⁵⁰ These hydroxyl-related features suggest the presence of unsaturated surface sites capable of adsorbing atmospheric moisture. In the intermediate region, weak bands located at 1526 and 1398 cm⁻¹ are assigned to defect-associated surface vibrational modes and residual heteroatom-containing surface species, which likely originate from trace sulfur-containing fragments remaining after hydrothermal precursor decomposition. The absorption bands at 1097 and 1014 cm⁻¹ are attributed to C–O/C–O–C stretching vibrations.⁵⁰ Additionally, the weak feature at 886 cm⁻¹ is associated with Mo–O/Mo–O–S surface vibrations, suggesting partial oxidation of unsaturated molybdenum sites, while the band at 730 cm⁻¹ corresponds to distorted Mo–S–Mo bridging vibrations arising from lattice deformation within the layered sulfide framework.⁵⁰ Most importantly, the intense bands centered at 572 and 522 cm⁻¹ correspond to the characteristic Mo–S stretching and S–Mo–S deformation vibrations, supporting the formation and preservation of the layered MoS₂ framework.⁵⁰

Fig. 5. Comparative FTIR spectra of MoS₂ NF, MoS₂-rGO, and MoS₂ NF-rGO composite.

Following incorporation of rGO, substantial changes are observed in the FTIR spectra of MoS₂-rGO and MoS₂ NF-rGO, particularly in the 1500–1000 cm⁻¹ region, consistent with the incorporation of graphene-derived carbon sheets. The broadening and enhancement of bands in this region arise from overlapping contributions of graphitic sp² C C skeletal vibrations and residual oxygen-containing functionalities (C–O/C–OH) of reduced graphene oxide, indicating the retention of partial oxygenated groups after reduction. Notably, compared to pristine MoS₂ NF, the composite samples exhibit distinct peak broadening and slight shifting in the intermediate-frequency region, suggesting significant modification of the local bonding environment upon rGO incorporation. The observed band broadening reflects increased structural heterogeneity and vibrational overlap associated with the intimate spatial integration of MoS₂ and rGO, whereas the peak shifts indicate perturbation of bond force constants resulting from changes in the local chemical environment of the MoS₂ and rGO components. Such spectral evolution supports the presence of interactions between MoS₂ and rGO rather than simple physical coexistence of the two components. Among the composites, MoS₂ NF-rGO exhibits the most pronounced peak broadening and attenuation, suggesting greater modification of the local bonding environment in the nanoflower-based composite. This behavior is consistent with the nanoflower morphology providing more intimate spatial contact between the MoS₂ nanosheets and the graphene substrate. Importantly, the retention of the characteristic Mo–S vibrational modes in all composite samples confirms that the crystalline structure of MoS₂ remains intact after hybridization.

Electrocatalytic HER performance in acidic medium

The electrocatalytic HER activity of the synthesized MoS₂ NF, MoS₂-rGO, and MoS₂ NF-rGO catalysts was first evaluated in 0.5 M H₂SO₄. Fig. 6a and b display the LSV polarization curves of all catalysts recorded under acidic conditions. It is evident that incorporation of rGO and subsequent nanoflower engineering significantly influence the HER activity of the MoS₂-based catalysts. Among the tested materials, MoS₂ NF-rGO exhibits the most positive polarization curve, indicating superior HER activity, followed by MoS₂-rGO, while pristine MoS₂ NF demonstrates comparatively inferior performance. To quantitatively compare the catalytic efficiencies, the overpotentials required to achieve benchmark current densities of 10 and 100 mA cm⁻² are summarized in Fig. 6c. The pristine MoS₂ NF catalyst requires the highest overpotential at both current densities, indicating sluggish HER kinetics relative to the rGO-containing composites. In contrast, both hybrid catalysts show significantly reduced overpotentials after rGO incorporation, with MoS₂ NF-rGO exhibiting the most pronounced improvement. Notably, MoS₂ NF-rGO exhibits the lowest overpotential among all samples, requiring 172 mV at 10 mA cm⁻² and maintaining the lowest overpotential of 255 mV even at the industrially relevant current density of 100 mA cm⁻², demonstrating its excellent catalytic efficiency under acidic HER conditions. The substantially enhanced performance of MoS₂ NF-rGO compared with MoS₂-rGO highlights the importance of the nanoflower morphology. The hierarchical nanoflower architecture provides an open nanosheet arrangement with greater exposure of catalytically accessible edge sites, thereby facilitating proton adsorption and hydrogen evolution.

Fig. 6. HER performance of MoS₂, MoS₂-rGO and MoS₂ NF-rGO catalysts measured in 0.5 M H₂SO₄. (a and b) LSV polarization curves, (c) corresponding overpotentials required to achieve benchmark current densities of 10 and 100 mA cm⁻², and (d) Corresponding Tafel plots derived from the LSV curves.

The HER kinetics were further evaluated through Tafel slope analysis (Fig. 6d). MoS₂ NF-rGO exhibits the smallest Tafel slope of 54 mV dec⁻¹, compared with 64 mV dec⁻¹ for MoS₂-rGO, while pristine MoS₂ NF shows the highest value (132 mV dec⁻¹). The lower Tafel slope of MoS₂ NF-rGO indicates more favorable HER kinetics than those of MoS₂ NF and MoS₂-rGO. Based on classical HER kinetic considerations, Tafel slopes in the range of 40–60 mV dec⁻¹ are generally associated with a Volmer–Heyrovsky pathway, in which the initial electrochemical proton adsorption step (Volmer step) is followed by electrochemical desorption of adsorbed hydrogen intermediates (Heyrovsky step). Accordingly, the Tafel slope of ∼54 mV dec⁻¹ obtained for MoS₂ NF-rGO is consistent with a Volmer–Heyrovsky pathway, with the electrochemical desorption step potentially contributing substantially to the overall HER kinetics. The slightly higher Tafel slope of MoS₂-rGO (∼64 mV dec⁻¹) suggests a similar reaction pathway but comparatively slower HER kinetics.

Electrocatalytic HER performance in alkaline medium

To further evaluate the practical electrocatalytic applicability of the prepared catalysts, their hydrogen evolution behavior was subsequently examined in 1.0 M KOH. The LSV polarization curves shown in Fig. 7a reveal a similar activity trend to that observed under acidic conditions, with the HER performance progressively improving upon hybridization with rGO and additional nanoflower structural modulation. Specifically, the MoS₂ NF-rGO catalyst demonstrates the lowest overpotential, signifying its superior catalytic activity toward hydrogen generation in alkaline electrolyte. In comparison, MoS₂-rGO exhibits intermediate performance, whereas pristine MoS₂ NF displays the poorest HER response. For quantitative comparison, the overpotentials required to deliver a cathodic current density of 10 mA cm⁻² are summarized in Fig. 7b, whereas overpotentials at the cathodic current density of 100 mA cm⁻² are shown in Fig. S2. Among all investigated samples, pristine MoS₂ NF requires the largest overpotential, reflecting its sluggish hydrogen evolution kinetics under alkaline conditions. Upon integration with rGO, the required overpotential decreases noticeably, indicating improved electrochemical performance of the hybrid catalysts. Most importantly, the MoS₂ NF-rGO catalyst achieves the benchmark current density at an overpotential of approximately 406 mV, representing the lowest value among the tested catalysts. This reduction in overpotential highlights the beneficial contribution of the hierarchical nanoflower morphology and rGO integration toward enhanced electrocatalytic performance.

Fig. 7. HER performance of MoS₂, MoS₂-rGO and MoS₂ NF-rGO catalysts measured in 1.0 M KOH. (a) LSV polarization curves showing the HER activity of the catalysts. (b) Overpotentials required to achieve benchmark current densities of 10 mA cm⁻² and (c) corresponding Tafel plots derived from the LSV curves. (d) Comparative LSV polarization curves before and after stability.

The reaction kinetics were further analyzed using the Tafel plots derived from the polarization data, as shown in Fig. 7c. The calculated Tafel slopes are approximately 269 mV dec⁻¹ for MoS₂ NF, 201 mV dec⁻¹ for MoS₂-rGO, and 190 mV dec⁻¹ for MoS₂ NF-rGO, confirming that the optimized hybrid catalyst possesses the fastest HER kinetics among all samples. Although these values are larger than the theoretical slopes associated with ideal HER elementary pathways, the relatively high Tafel slopes indicate substantial kinetic limitations under alkaline conditions, consistent with the involvement of the Volmer step, which includes water dissociation and formation of adsorbed hydrogen intermediates on the catalyst surface. This is consistent with the well-established alkaline HER mechanism, where the sluggish cleavage of H–OH bonds imposes an additional kinetic barrier absent in acidic systems. The comparatively lower Tafel slope observed for MoS₂ NF-rGO suggests more favorable kinetics for water activation and subsequent hydrogen-intermediate formation, thereby partially alleviating the kinetic constraints associated with the Volmer step. After hydrogen adsorption, the reaction is expected to proceed through either a Volmer–Heyrovsky or Volmer–Tafel route, depending on the local surface coverage of adsorbed hydrogen species.

The electrochemical durability of the optimized MoS₂ NF–rGO catalyst under alkaline HER conditions was assessed by repeated cyclic voltammetry testing, and the comparative polarization curves recorded before and after 1000 continuous CV cycles are shown in Fig. 7d. Notably, the post-stability polarization curve nearly overlaps with the initial measurement, exhibiting only negligible variation after prolonged electrochemical cycling. This excellent retention of catalytic activity demonstrates the good electrochemical durability of the hybrid catalyst under alkaline operating conditions. In addition, chronoamperometric (CA) measurements were performed to further evaluate the long-term operational stability of MoS₂ NF–rGO, as shown in Fig. S3. The CA measurement was conducted at a constant cathodic potential of −0.406 V vs. RHE, corresponding to the overpotential required to achieve a current density of 10 mA cm⁻² under alkaline HER conditions. The catalyst maintains a relatively stable current density close to the initial value over approximately 50 h of continuous operation, with only minor fluctuations observed during the measurement. The combined CV cycling and CA results therefore demonstrate good electrochemical durability of MoS₂ NF–rGO under the tested alkaline HER conditions. Such durability may be associated with the stable hierarchical architecture and intimate spatial integration of MoS₂ nanoflowers with the rGO framework, which can help maintain structural integrity and active-site accessibility during electrochemical operation. Post-stability FESEM images were also recorded to examine possible morphological changes after prolonged alkaline HER operation. As shown in Fig. S4, the MoS₂ NF-rGO catalyst largely retains its hierarchical nanoflower-like architecture and interconnected nanosheet structure, with no obvious collapse or severe structural degradation observed. The preservation of the overall morphology is consistent with the good electrochemical durability observed during the CV cycling and CA stability tests.

To further evaluate the electrochemical charge-transfer behavior during HER, EIS measurements were performed, and the corresponding Nyquist plots are presented in Fig. S5. Accordingly, the fitted solution resistance (R_s) values are 1.15, 0.83, and 0.95 Ω for MoS₂ NF, MoS₂-rGO, and MoS₂ NF-rGO, respectively, while the corresponding charge-transfer resistance (R_ct) values are 40.85, 11.37, and 5.05 Ω, respectively. The relatively close R_s values indicate comparable solution resistance among the three catalysts, suggesting that the differences in their electrochemical response are not primarily associated with solution resistance. In contrast, a pronounced decrease in R_ct is observed following rGO incorporation, with MoS₂-rGO showing an R_ct of 11.37 Ω compared with 40.85 Ω for MoS₂ NF. Notably, MoS₂ NF-rGO exhibits the lowest R_ct of only 5.05 Ω, substantially lower than those of both MoS₂ NF and MoS₂-rGO. This lower R_ct indicates more facile electrochemical charge transfer at the catalyst/electrolyte interface during HER. The substantially lower R_ct of MoS₂ NF-rGO is consistent with its lower overpotential and smaller Tafel slope, providing further electrochemical support for its superior HER performance.

The electrochemically accessible surface area of the catalysts was further examined using cyclic voltammetry measurements performed at different scan rates, with the corresponding CV curves presented in Fig. S6a, c, and e for MoS₂ NF, MoS₂-rGO, and MoS₂ NF-rGO, respectively. The capacitive current densities extracted from these measurements were plotted as a function of scan rate, as shown in Fig. S6b, d, and f, respectively, and the slopes of the corresponding linear fits were used to determine the double-layer capacitance (C_dl). The calculated C_dl values are summarized in Table S1. Accordingly, MoS₂ NF-rGO exhibits the highest C_dl value of 180.64 mF cm⁻², followed by MoS₂-rGO (119.95 mF cm⁻²) and MoS₂ NF (52.78 mF cm⁻²). Based on these values, the corresponding roughness factors and electrochemically active surface areas (ECSA) were calculated. Accordingly, MoS₂ NF-rGO shows the largest calculated ECSA of 886.9 cm², compared with 588.8 cm² for MoS₂-rGO and 259.1 cm² for MoS₂ NF. The progressive increase in C_dl and ECSA from MoS₂ NF to MoS₂-rGO and subsequently to MoS₂ NF-rGO indicates a greater electrochemically accessible surface area for the nanoflower-based composite. This trend is consistent with the hierarchical and open nanosheet architecture of MoS₂ NF-rGO, which provides increased accessibility of the catalyst surface to the electrolyte and may contribute to its improved HER performance under alkaline conditions.

Conclusion

In summary, a three-dimensional MoS₂ nanoflower/reduced graphene oxide (MoS₂ NF-rGO) hybrid heterostructure was successfully synthesized via a facile hydrothermal approach to develop an efficient and low-cost non-noble-metal electrocatalyst for hydrogen evolution reaction (HER). The hierarchical MoS₂ nanoflowers were uniformly anchored onto rGO nanosheets, forming an open architecture with abundant exposed edge sites and intimate spatial integration between the two components. As a result, the optimized MoS₂ NF-rGO electrocatalyst exhibited excellent HER activity in both acidic and alkaline media, requiring a low overpotential of 172 mV to achieve a cathodic current density of 10 mA cm⁻² with a small Tafel slope of 54 mV dec⁻¹ in 0.5 M H₂SO₄. In 1.0 M KOH, the catalyst delivered 10 mA cm⁻² at an overpotential of 406 mV with a Tafel slope of 190 mV dec⁻¹, while maintaining outstanding durability with 100% retention after 1000 CV cycles. The HER performance of MoS₂ NF-rGO was found to be significantly superior to conventional MoS₂-rGO spherical composite, with nearly comparable compositions, highlighting the important role of hierarchical nanoflower morphology in determining catalytic performance. Furthermore, EIS analysis revealed that MoS₂ NF-rGO exhibited the lowest charge-transfer resistance (5.05 Ω), compared with MoS₂-rGO (11.37 Ω) and MoS₂ NF (40.85 Ω), supporting its more favorable electrochemical charge-transfer behavior. The enhanced electrocatalytic activity is associated with the hierarchical nanoflower architecture, abundant exposed edge sites, intimate spatial integration of MoS₂ with the rGO framework, and favorable electrochemical charge-transfer characteristics. Overall, this study demonstrates that hierarchical morphology engineering combined with spatial integration of MoS₂ with rGO offers an effective strategy for designing advanced MoS₂-based electrocatalysts for efficient and durable hydrogen production.

Author contributions

Conceptualization, D. B.; methodology, Sajana Pooniya.; validation, A. S.; formal analysis, D. B. and L. V. R.; investigation, Saurabh Pathak; resources, D. B.; data curation, Sajana Pooniya; writing – original draft preparation, D. B.; writing–review and editing, D. B.; visualization, D. B. and Saurabh Pathak.

Conflicts of interest

The authors declare that there is no competing financial interest.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra06138a.