Enhancing the supercapacitor performance of MoS2 nanostructures through metallic phase enrichment and morphology control

The rapid growth of renewable energy technologies has intensified the demand for efficient, durable, and environmentally sustainable energy storage systems. Among electrochemical energy storage devices, supercapacitors have attracted significant attention due to their high power density, fast charge–discharge capability, and long cycle life, making them suitable for applications ranging from portable electronics to hybrid energy systems. Supercapacitors are generally classified into electric double-layer capacitors (EDLCs), which store charge through electrostatic ion adsorption, and pseudocapacitors, which rely on fast and reversible Faradaic redox reactions [1,2]. While EDLCs exhibit excellent cycling stability, their relatively low energy density limits broader applications, thereby motivating the development of advanced pseudocapacitive electrode materials.

The performance of supercapacitor electrodes is strongly governed by the physicochemical properties of the active materials, including electrical conductivity, accessible surface area, ion diffusion pathways, and the density of electrochemically active sites. Carbon-based materials are widely used in EDLCs [3,4]; however, their limited specific capacitance has prompted increasing interest in pseudocapacitive materials such as transition metal oxides, hydroxides, sulfides, and conducting polymers [3,5]. In this context, layered transition metal dichalcogenides (TMDs) have emerged as promising candidates owing to their two-dimensional crystal structure, tunable electronic properties, and abundance of exposed edge sites that facilitate redox reactions. Molybdenum disulfide (MoS₂) is among the most extensively investigated TMDs for energy storage applications due to its layered structure, mechanical flexibility, chemical stability, and multiple oxidation states of molybdenum [4,6]. MoS₂ exists primarily in two crystallographic phases: the thermodynamically stable semiconducting 2H phase and the metastable metallic 1T phase. These phases exhibit markedly different electrical conductivities and electrochemical behaviors. The 2H phase generally suffers from limited electrical conductivity, whereas the metallic 1T phase offers enhanced charge transport and faster redox kinetics, which are highly desirable for supercapacitor electrodes. For example, hydrothermally prepared 2H-MoS₂ nanoflowers provide a specific capacitance of 255.65 F g⁻¹ at 0.25 A g⁻¹ [7], while 1T-MoS₂ microspheres yield 228 F g⁻¹ [8]. Interestingly, materials containing both phases display exceptional performance, with specific capacitance values as high as 742 F g⁻¹ [9]. Consequently, strategies aimed at phase engineering and morphology control have been widely explored to improve the electrochemical performance of MoS₂-based materials.

Structural modification through doping has also proven effective; for instance, Ni-doped MoS₂ on carbon cloth electrodes achieves 305.9 F g⁻¹, compared to 170.2 F g⁻¹ for pristine MoS₂ [10]. Another strategy to enhance the conductivity of MoS₂ involves the incorporation of carbon-based composites. For example, MoS₂ nanobelts/carbon hybrids exhibit 77.5 F g⁻¹ at 0.5 A g⁻¹ [11], while carbon nanotube/MoS₂ composites reach 150 F g⁻¹ at 0.2 A g⁻¹ [12]. Mixed-phase MoS₂ nanoflakes/carbon nanofiber composites show a remarkable specific capacitance of 626.08 F g⁻¹ at 1 A g⁻¹, compared to only 159.35 F g for pristine MoS₂ [13].

Previous studies have demonstrated that the synthesis method plays a decisive role in determining the phase composition, morphology, and electrochemical properties of MoS₂. Hydrothermal synthesis often yields nanostructured MoS₂ with reduced particle size, abundant defects, and partial transformation from the 2H to the metallic 1T phase, leading to improved pseudocapacitive performance. In contrast, chemical vapor deposition (CVD) typically produces highly crystalline, large-area MoS₂ nanosheets dominated by the 2H phase, which are advantageous for electronic and optoelectronic applications but may exhibit limited capacitance due to a lower density of active sites and reduced ionic accessibility. Despite extensive studies on MoS₂-based supercapacitor electrodes, a direct and systematic comparison between MoS₂ synthesized by fundamentally different routes, namely, CVD and hydrothermal methods, remains relatively unexplored. Such a comparison is crucial for establishing clear correlations between synthesis strategy, structural features, phase composition, and electrochemical performance. Understanding these relationships is essential for rational electrode design and for selecting appropriate synthesis routes tailored to high-performance energy storage applications.

In this work, we present a comparative study of MoS₂ nanostructures synthesized via CVD and a hydrothermal (HT) method employing a polymeric surfactant additive. The CVD approach yields few-layer, highly crystalline MoS₂ nanosheets, whereas the hydrothermal route produces nanospherical MoS₂ with reduced particle size and a mixed 1T/2H phase composition. Comprehensive structural and morphological characterization was carried out using X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The supercapacitive performance of both materials was systematically evaluated using cyclic voltammetry (CV), galvanostatic charge–discharge measurements, and cycling stability tests. The results demonstrate that hydrothermally synthesized MoS₂ exhibits superior electrochemical performance, which is attributed to the synergistic effects of metallic phase enrichment and nanoscale morphology. This study provides valuable insights into the role of synthesis-driven phase and morphology control in optimizing MoS₂-based electrodes for next-generation supercapacitor applications.

Ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) and thioacetamide (CH₃CSNH₂, ≥99.0%) were purchased from Sigma-Aldrich and used as molybdenum and sulfur precursors, respectively. Tergitol NP-9, a nonionic surfactant, was also obtained from Sigma-Aldrich and used as a morphology- and phase-regulating agent during hydrothermal synthesis. Molybdenum trioxide (MoO₃, 99.5%) was supplied by Alfa Aesar, and sulfur powder (99.9%) was purchased from VWR Chemicals. All reagents were used without further purification, and deionized water was employed throughout the experiments.

HT-MoS₂ was synthesized via a hydrothermal chemical route. In a typical synthesis, 1.41 g of ammonium molybdate tetrahydrate and 0.26 g of thioacetamide were dissolved in 20 mL of deionized water under continuous magnetic stirring to obtain a clear and homogeneous solution. Subsequently, 36 µL of Tergitol NP-9 was added, and the mixture was stirred for an additional 30 min to ensure uniform dispersion of the surfactant molecules. The homogeneous solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave, sealed, and maintained at 200°C for 24 h without a controlled heating rate. Upon completion of the reaction, the autoclave was allowed to cool naturally to room temperature. The resulting black precipitate was collected by centrifugation and washed several times with deionized water and ethanol to remove unreacted species and residual surfactant. The final product was dried at 60°C, yielding HT-MoS₂ powder enriched in the metallic phase [14]. The addition of Tergitol NP-9 plays a critical role in controlling particle growth and morphology during hydrothermal synthesis. The surfactant induces lattice distortion and structural strain, which promotes partial phase transformation from the semiconducting 2H phase to the metallic 1T phase while simultaneously stabilizing the metastable 1T structure. This results in nanosized MoS₂ particles with enhanced electrochemical activity.

CVD-MoS₂ nanosheets were synthesized using a horizontal quartz-tube chemical vapor deposition system. A mixture of MoO₃ and NaCl (40 wt%) was placed in a quartz boat at the center of the furnace, serving as the molybdenum source. A clean quartz substrate was positioned downstream to collect the grown MoS₂ film. Prior to deposition, the substrate was sequentially cleaned with acetone and methanol, followed by sonication in deionized water and drying under nitrogen flow. The furnace temperature was ramped to 780°C at a heating rate of 20°C min⁻¹ under a constant nitrogen flow of 20 sccm. Sulfur powder (200 mg), placed outside the main heating zone, was heated separately to 250°C using an external heater once the furnace reached the target temperature. The sulfur vapor was transported into the reaction zone by the carrier gas, and the growth was maintained for 20 min. After synthesis, the furnace was allowed to cool naturally to room temperature under a nitrogen atmosphere.

The synthesis procedure followed the method reported by Aleithan et al. [15], in which NaCl was employed as a growth promoter to reduce the effective melting point of MoO₃. The as-grown MoS₂ film was mechanically scraped from the substrate, dispersed in deionized water, and sonicated for 3 h to obtain CVD-MoS₂ powder for electrochemical studies.

Phase composition and crystallinity were examined using XRD with Cu Kα radiation (λ = 0.154 nm) (Malvern, GH Eindhoven, Netherlands). Raman spectroscopy (HORIBA, LabRAM HR, excitation wavelength 445 nm) was employed to identify vibrational modes associated with the 2H and 1T phases of MoS₂. FTIR spectroscopy was used to investigate chemical bonding and functional groups using a Bruker spectrometer (Ettlingen 76275, Germany). Surface morphology and elemental composition were analyzed using SEM equipped with energy-dispersive X-ray spectroscopy (EDX) (Tescan Vega 3 SBU, Czech Republic). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) (JEOL JEM-2100, Tokyo, Japan) were used to determine particle size, interlayer spacing, and crystallographic features.

Electrochemical performance was evaluated using a three-electrode configuration with a potentiostat/galvanostat. The working electrodes were prepared by mixing 80 wt% active material (HT-MoS₂ or CVD-MoS₂), 10 wt% carbon black, and 10 wt% polytetrafluoroethylene (PTFE) binder to form a uniform slurry, following our previously reported method [16]. The slurry was coated onto nickel foam substrates (1 × 1 cm²), which served as conductive current collectors due to their high electrical conductivity and porous three-dimensional structure. The mass loading of active material was approximately 1 mg for all electrodes. The coated electrodes were dried at 60°C before electrochemical testing.

Electrochemical measurements were performed in 2 M KOH aqueous electrolyte using an Ag/AgCl reference electrode and a platinum plate counter electrode. CV and galvanostatic charge–discharge (GCD) tests were conducted within a potential window of 0.0–0.45 V (Nova Auto Lab, Metrohm). The specific capacitance (C, F g⁻¹) was calculated from GCD curves according to the following equations: C = I Δ t m Δ V C=\frac{I\Delta t}{m\Delta V} where I is the discharge current, Δt is the discharge time, m is the mass of the active material, and ΔV is the potential window.

To ensure a fair comparison, all capacitance values were normalized to the mass of the active material. The nickel foam substrate was used solely as a current collector, and its contribution to the measured capacitance was consistent for both electrodes.

Raman spectroscopy was employed to further investigate the phase composition and structural disorder of the samples. As shown in Figure 2(a), the CVD-MoS₂ sample exhibits two prominent Raman-active modes at approximately 385 cm⁻¹ (E _2g, in-plane vibration) and 405 cm⁻¹ (A _1g, out-of-plane vibration). The separation between these modes of about 20 cm⁻¹ is indicative of few-layer MoS₂ with high crystallinity and confirms the dominance of the 2H phase [19,20]. Weak and broadened Raman features associated with MoO₃ indicate its low concentration and poor crystallinity, in good agreement with the XRD results. No Raman signatures of quartz are observed, suggesting that quartz is not uniformly distributed within the sample and is more likely present as isolated fragments introduced during mechanical removal of the film from the substrate.

Figure 2

(a) Raman spectra for CVD MoS₂ and (b) Raman spectra for HT MoS₂.

In contrast, the Raman spectrum of HT-MoS₂ (Figure 2(b)) shows broadened and weakened E _2g (376.5 cm⁻¹) and A _1g modes, reflecting reduced crystallinity and increased structural disorder. Notably, additional peaks appear at approximately 145, 238, and 336 cm⁻¹, corresponding to the J ₁, J ₂, and J ₃ vibrational modes, which are characteristic fingerprints of the metallic 1T-MoS₂ phase [21,22,23,24]. A semi-quantitative assessment of the phase composition was performed by comparing the relative intensity of the J modes to that of the E _2g mode. The pronounced intensity of the J ₁–J ₃ bands in HT-MoS₂ indicates significant enrichment of the metallic 1T phase compared to the purely 2H-dominated CVD-MoS₂ sample. The Raman-based 1T/2H intensity ratio exceeds 1, indicating dominant 1T spectral contributions when considering relative intensity ratios rather than absolute phase fractions.

Although precise quantification requires X-ray photoelectron spectroscopy, the Raman analysis provides reliable evidence of phase transformation induced by the hydrothermal synthesis route. Moreover, sharp Raman peaks at 111, 123, 194.3, and 220 cm⁻¹ are attributed to crystalline MoO₃ nanoparticles, as shown in Figure 2(b) and consistent with previous reports [23,25,26]. Combined Raman and XRD analyses confirm that both samples predominantly consist of hexagonal 2H-MoS₂ with minor MoO₃ impurities [27]. In contrast, the HT-MoS₂ sample additionally exhibits a pronounced partial phase transformation from semiconducting 2H-MoS₂ to metallic 1T-MoS₂, consistent with the observed J-mode features and structural distortions.

The FTIR spectra of both samples (Figure 3(a) and (b)) exhibit characteristic absorption bands corresponding to Mo–S bonding, confirming the formation of MoS₂. The band near 1,400 cm⁻¹ is attributed to S–Mo–S stretching vibrations, while peaks in the range of 590–620 cm⁻¹ correspond to Mo–S stretching modes. Additional bands observed between 860 and 1,100 cm⁻¹ are assigned to Mo–O and O–Mo–O vibrations, indicating the presence of minor MoO₃ species. Broad absorption features around 1,600 and 3,000 cm⁻¹ arise from hydroxyl groups associated with adsorbed moisture or surface-bound species.

Figure 3

(a) FTIR spectra for CVD MoS₂ and (b) FTIR spectra for HT MoS₂.

SEM images of CVD-MoS₂ (Figure 4(a) and (b)) reveal large-area, layered flakes with lateral dimensions exceeding 5 µm, consistent with the high crystallinity observed in XRD and Raman analyses. The smooth and continuous morphology suggests limited edge exposure and reduced ion-accessible surface area, which may restrict electrochemical activity despite excellent structural quality.

Figure 4

(a and b) SEM images of CVD-MoS₂. (c and d) SEM images of HT-MoS₂. (e) EDX spectra of CVD-MoS₂. (f) EDX spectra of HT-MoS₂.

In contrast, HT-MoS₂ exhibits a nanoparticulate morphology, as shown in Figure 4(c) and (d). The particles are uniformly distributed and form loosely agglomerated nanospheres with sizes below 100 nm. TEM analysis (Figure 5) reveals that the HT-MoS₂ nanoparticles have an average diameter of approximately 50 nm and possess sharp edges and multigrain structures. The expanded interlayer spacing observed in HRTEM images (∼0.75 nm) further confirms the presence of the metallic 1T phase and structural strain. The SAED pattern exhibits continuous diffraction rings (Figure 5(e)), confirming the polycrystalline nature of the HT-MoS₂ nanocrystals. Such nanoscale features significantly increase the accessible surface area and provide abundant electrochemically active edge sites. The EDX spectra confirm the presence of Mo, S, and O in both samples; however, precise quantitative determination of their weight ratios is limited due to overlap between the S Kα (2.31 keV) and Mo Lα (2.29 keV) emission lines (Figure 4(e) and (f)). The detected oxygen signal further supports the presence of minor MoO₃ species, consistent with the XRD and Raman analyses.

Figure 5

(a–c) Different magnification HRTEM images for the HT MoS₂ sample. (d) HRTEM shows layer structures with an average layer interspacing of 0.755 nm. (e) SAED pattern for the HT-MoS₂ sample.

The electrochemical performance of HT-MoS₂ and CVD-MoS₂ electrodes was evaluated using CV and GCD measurements in a three-electrode configuration with 2 M KOH electrolyte. Figure 6(a) shows the CV curves recorded at a scan rate of 30 mV s⁻¹. Both electrodes exhibit distinct redox peaks, indicating pseudocapacitive behavior governed by reversible Faradaic reactions. Notably, the CV curve of HT-MoS₂ encloses a significantly larger area than that of CVD-MoS₂, reflecting its higher charge-storage capability. CV measurements performed at various scan rates (Figure 7(a) and (b)) demonstrate that HT-MoS₂ maintains well-defined redox features even at higher scan rates, indicating improved charge-transfer kinetics and ion diffusion. This enhanced performance is attributed to the metallic 1T phase, which provides higher electrical conductivity, and the nanoscale morphology, which shortens ion diffusion pathways.

Figure 6

Comparative (a) CV, (b) CD, and (c) calculated specific capacitance of the HT-MoS₂ and CVD-MoS₂ electrodes at fixed current loads.

Figure 7

CV profile of the (a) CVD-MoS₂ and (b) HT-MoS₂, and GCD profile of the (c) CVD-MoS₂ and (d) HT-MoS₂ electrodes.

GCD curves recorded at a current density of 1 A g⁻¹ (Figure 7(c) and (d)) further confirm the superior performance of HT-MoS₂. The specific capacitance of HT-MoS₂ reaches 466.66 F g⁻¹, significantly higher than the 371.10 F g⁻¹ obtained for CVD-MoS₂. At increasing current densities, HT-MoS₂ consistently outperforms CVD-MoS₂, demonstrating superior rate capability. The HT-MoS₂ electrode delivered specific capacitance values of 466.66, 444.40, 432.50, 280.00, 176.00, 113.50, and 89.00 F g⁻¹ at 1, 1.25, 1.5, 1.75, 2, 3, and 5 A g⁻¹, respectively. By contrast, the CVD-MoS₂ electrode exhibited capacitances of 371.10, 269.50, 156.60, 81.70, 75.55, 53.40, and 44.10 F g⁻¹ at the same current densities. This behavior highlights the synergistic effect of metallic phase enrichment and nanoscale morphology on electrochemical performance.

The long-term cycling stability of the HT-MoS₂ electrode was evaluated over 2,000 charge–discharge cycles (Figure 8). The electrode retained approximately 89.23% of its initial capacitance, indicating excellent structural and electrochemical stability. The robust cycling performance can be attributed to the stable coexistence of 1T and 2H phases and the mechanically resilient nanostructure. It is important to note that nickel foam was employed solely as a conductive current collector due to its three-dimensional porous structure. All capacitance values were normalized to the mass of the active material, and identical substrates and mass loadings were used for both electrodes. Therefore, the observed differences in electrochemical performance primarily reflect the intrinsic properties of the synthesized MoS₂ materials rather than contributions from the substrate.

Figure 8

Percent retention profile of the HT-MoS₂ electrode.

While the 1T phase enhances electrical conductivity and charge transfer, the amorphous domains introduce structural disorder, provide abundant active sites, and facilitate ion diffusion. Together, these features are responsible for the remarkable capacitance and cycling stability observed.

The comparative analysis clearly demonstrates that the hydrothermal synthesis route is more effective than CVD for producing MoS₂ with enhanced supercapacitive performance. While CVD yields highly crystalline, large-area nanosheets dominated by the 2H phase, the hydrothermal method enables phase engineering and morphology control, resulting in metallic phase enrichment and nanoscale particles with abundant active sites. These structural advantages translate directly into higher specific capacitance, improved rate capability, and excellent cycling stability, highlighting the critical role of synthesis-driven phase and morphology optimization in MoS₂-based supercapacitor electrodes.

These results surpass those reported in recent studies on MoS₂ nanostructures modified via plasma treatment or heteroatom doping to enhance electrical conductivity and defect density for energy storage applications [28,29,30].

The enrichment of the metallic 1T phase in HT-MoS₂ plays a critical role in enhancing electrochemical performance. The distorted octahedral coordination of Mo atoms in the 1T phase results in a higher electronic density of states at the Fermi level, leading to improved electrical conductivity compared to the semiconducting 2H phase. In addition, the structural disorder and reduced crystallite size associated with the hydrothermal synthesis introduce a higher density of edge sites and defects, which serve as electrochemically active centers for Faradaic charge storage. These combined effects facilitate faster charge transfer and ion diffusion during electrochemical cycling, consistent with the superior capacitance and rate capability observed for the HT-MoS₂ electrode [31,32,33,34].

In this work, MoS₂ nanostructures were synthesized via CVD and a hydrothermal (HT) method to elucidate the influence of synthesis strategy on phase composition, morphology, and supercapacitor performance. The CVD-grown MoS₂ consisted of highly crystalline, few-layer nanosheets dominated by the semiconducting 2H phase, whereas the hydrothermal route produced nanoscale MoS₂ particles with expanded interlayer spacing and significant enrichment of the metallic 1T phase. Structural and spectroscopic analyses confirmed the coexistence of 1T and 2H phases in HT-MoS₂, accompanied by reduced crystallite size and increased defect density.

Electrochemical measurements revealed that HT-MoS₂ exhibits superior specific capacitance, enhanced rate capability, and excellent cycling stability compared to its CVD counterpart. These improvements are attributed to the synergistic effects of metallic phase enrichment, nanoscale morphology, and increased density of electrochemically active sites, which collectively enhance electrical conductivity and ion transport kinetics. This study highlights the critical role of synthesis-driven phase and morphology engineering in optimizing MoS₂-based electrode materials and provides valuable insights for the rational design of high-performance supercapacitors.

This work was supported by the Annual Funding track by the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia [Project No. KFU252468].

Shrouq Aleithan: investigation, funding acquisition; Shroq Laradhi and Shrouq Aleithan: sample preparation; Shrouq Aleithan, Sajid Ansari and Khan Alam: characterization, data curation, and formal analysis; Shrouq Aleithan and Sajid Ansari: writing – original draft, review & editing.

Authors state no conflict of interest.