2-Dimensional layered molybdenum disulfide nanosheets and CTAB-assisted molybdenum disulfide nanoflower for high performance supercapacitor application

Abstract

In this study, the supercapacitor performance of the hydrothermal synthesized molybdenum disulfide (MoS₂) nanosheets and the cetyltrimethylammonium bromide (CTAB)-assisted MoS₂ nanoflower morphology have been investigated. The as-synthesized MoS₂ nanoflower and nanosheet morphology structures were investigated via field emission scanning electron microscopy (FESEM), and the internal microstructure was examined via high resolution-transmission electron microscopy (HR-TEM) technique. The Fourier transform infrared (FT-IR) spectra were obtained to identify the chemical interaction and the functional groups present in the material. The shifting of the binding energy, oxidation states, and elemental identification were conducted by X-ray photon spectroscopy (XPS). The MoS₂ nanoflower possesses surface defects, which produce numerous active sites. The MoS₂ nanoflower and nanosheet electrodes demonstrate the high specific capacitance (C_sp) values of 516 F g⁻¹ and 438 F g⁻¹, respectively, at a current density of 1 A g⁻¹. However, the MoS₂ nanoflower shows high C_sp due to the large surface area with active edges, making them store more energy in the electrode.

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1. Introduction

In the modern world, energy plays a predominant role in various applications, like energy storage and conversion, electronics devices, optical portable devices, laptops, computers, mobiles, and routine human daily domestic needs. The electrochemical energy storage technology is one of the emerging fields that provides solid-state batteries, fuel cells, and supercapacitors.^1–5 Among these, supercapacitors have attracted more attention because of their higher power density than the batteries, quick charging time, slow discharging time, more storage capacity, and longer durability than a conventional capacitor. Unfortunately, the practical application of the supercapacitor device could not achieve full success in large industrial production due to drawbacks, like poor cyclic stability, limited operating voltage, and chemical leakage. Therefore, the researchers are more focused on developing a new electrode material with high C_sp, good cyclic retention, wide operating voltage, and chemical stability. The strategy has been made to overcome these issues by adopting the well-controlled synthesis of nanomaterials with large surface area nanosheets, nanoflowers, and optimized morphology. The most significant morphology of the material may enhance the large surface area, chemical stability, and electrical conductivity; these are helpful in electrochemical energy storage applications. Recently, the development of an energy storage system as an effective clean energy carrier system was achieved, and replaced the non-renewable fossil fuels. Therefore, researchers traced out a variety of 2-D nanomaterials for energy storage and conversion applications, like fuel cells, batteries, and supercapacitors. Presently, the supercapacitor is a highly demanded device in energy storage and conversion systems.^6–8 The supercapacitor is categorized into two types, the pseudocapacitor and electrical double layer capacitor.^9,10 The pseudocapacitor is a faradaic reversible redox reaction process. An electrical double layer capacitor is a collection of charge separation and an interface between the electrode and electrolytes.^11,12 Transition metal sulfides (TMSs), such as CuS/Cu₂S,^13,14 NiS,¹⁵ MoSe₂¹⁶ SnS₂,¹⁷ ZnS,¹⁸ CoS,¹⁹ FeS,²⁰ MoS₂, and WS₂,^21,22 have been exhaustively studied as an electrochemically active material for supercapacitor performance. MoS₂ is a highly studied TMSs compared to other metal sulfides, as well as the most important material scientifically and for industrially applications because of the different oxidation state with polymorphism, distinctive surface morphology structure, atomic structure, tremendous physical, chemical stability, electroactive properties, direct band gap, and small thickness, unique layered structure, wide specific surface area, good specific capacitance, and reported high electrochemical properties.^23,24 This material is assembled by weak van der Waals force and connected with a covalent bond sandwich between the S–Mo–S tri-layers closely linked with Mo and S atoms. This layered material can be categorized into three types: 1T MoS₂ trigonal, 2H MoS₂ hexagonal, and 3R MoS₂ rhombohedral shapes. Among these derivatives, 2H MoS₂ is a stable state, while 1T MoS₂ and 3R MoS₂ are metastable in nature.²⁵ The MoS₂ material has received special attention in the field of hydrogen evolution,^26,27 solar cells,²⁸ supercapacitors,²⁹ chemical sensors,³⁰ and gas sensors.³¹ Recently, several methods were adapted to synthesize the MoS₂ material, such as chemical exfoliation method,³² top-down and bottom-up processes like the soft template method,³³ chemical vapor deposition method,³⁴ electrospinning,³⁵ liquid phase sonication exfoliation,³⁶ hydrothermal/solvothermal method, and microwave synthesis.³⁷ Among these methods, the hydrothermally synthesized MoS₂ material has several advantages like eco-friendliness, less expensive, non-toxicity, good dispersion of the solution, and it can easily control the various morphology structures.³⁸

In general, a supercapacitor electrode material highly depends on the surface morphology structure, electrical conductivity, and cyclic stability. Frangping Wang et al.³⁹ reported a flower-like MoS₂ microsphere morphology, which gives the maximum C_sp of 518.7 F g⁻¹ at a current density of 1 A g⁻¹, and the cyclic stability retention is 88.2% after 2500 long cycles. Nikhitha Joseph et al.⁴⁰ reported a high electrical conductivity for the 1-T MoS₂ nanostructure with rich active edges, and achieved a maximum C_sp of 379 F g⁻¹ at 1 A g⁻¹. Xiuhuawang et al.⁴¹ demonstrated a 3-D structured MoS₂ nanoflower for the electrochemical properties, and observed a C_sp of 168 F g⁻¹ at a current density of 1 A g⁻¹. The novelty of the present work is the synthesized MoS₂ nanoflower morphology with layered edges and nanosheet, in which the nanoflower MoS₂ shows the highest C_sp of 436.25 F g⁻¹ at 10 mV s⁻¹. Furthermore, the GCD curve exhibits a maximum C_sp of 516 F g⁻¹ at a current density of 1 A g⁻¹. It is a high C_sp compared to the previously reported MoS₂ material.⁴¹

In this present work, we reported the MoS₂ nanosheets and nanoflower synthesized by using the hydrothermal route in the presence of a soft template CTAB, which controls the surface morphology, shape, and size of the morphology structure. The CV curve shows the electrochemical double layer nature with a large loop. The GCD analysis reveals that the MoS₂ nanosheets and MoS₂ nanoflower electrode material is effective in supercapacitor device performance. The obtained results suggest that the MoS₂ nanoflower electrode material is a prominent candidate for energy storage and other applications.

2. Experimental methods

2.1 Materials used

Cetyltrimethylammonium bromide (CTAB), ammonium molybdate tetrahydrate (AMTH) thiourea, polyvinyl alcohol (PVA), potassium hydroxide (KOH), activated charcoal, and N-methyl-2 pyrrolidone (NMP) were obtained from Merck India, and polyvinylidene difluoride (PVDF) was purchased from Sigma Aldrich. Throughout the experiment, double-distilled water was used.

2.2 Synthesis of MoS₂ nanosheets and nanoflower morphology

The 2D-layered MoS₂ nanosheets were prepared by one-pot hydrothermal route. In this process, the precursor material ammonium molybdate tetrahydrate (AMTH) 16.184 (mmol) and 328.435 (mmol) thiourea were added to 80 ml of deionized water. Then, the mixed solution was sonicated about 30 minutes, and magnetically stirred for 30 minutes. The solution was further transferred to 100 ml of a Teflon-containing autoclave kit treated at 200 °C for about 48 hours. The collected MoS₂ black mixture product was centrifugated at 8000 rpm for about 30 minutes, and it was washed with water and ethanol, then vacuum dried at 50 °C for 8 hours.

Similarly, the MoS₂ nanoflower was synthesized by following the same procedure. In addition, 6 mmol of cation CTAB was added, which acted as a surfactant. It changed the nanosheet into nanoflower morphology due to the effect of the surface modifier. It is clearly represented in Scheme 1.

Scheme 1 Schematic illustration of the MoS₂ nanosheets and nanoflower morphology.

2.3 Characterization techniques

Characterization of the synthesized MoS₂ nanosheets and MoS₂ nanoflower active material morphology was carried out by using FESEM (Sigma-Zeiss), and HR-TEM measured through Titan (Themis 300 kV FEI), and the crystalline structure was examined by X-ray diffraction (XRD) techniques. The atomic percentage of the sample was analyzed through energy-dispersive X-ray spectroscopy (EDAX) and X-ray photoelectron spectroscopy (XPS) analyzed by Kartos axis ultra DLD (the used excitation source was monochromatic Al Kα and the emission angle was 90°; the spot size was 300 μm × 700 μm with 10⁻⁹ torr, Ultra-high vacuum). The chemical functional group of the material was characterized by ATR-FT-IR (alpha Bruker). The Brunauer Emmett Teller (BET) and Barrett Joyner Halenda (BJH) measurements were done by using a Quanta Chrome Instrument with a nitrogen adsorption/desorption environment.

2.4 Electrochemical characterization

Electrochemical techniques evaluated from cyclic voltammetry (CV), electrochemical Nyquist plot (EIS) and galvanostatic charging–discharging performance were carried out by using a computerized interface CHI 660E workstation.

3. Results and discussion

3.1 XRD pattern and FT-IR spectra analysis

Fig. 1(a) the XRD pattern was used to analyze the crystal structure of the synthesized MoS₂ nanosheets, and nanoflower diffraction peaks were observed at 14.28°, 33.56°, 40.0° and 59.01° and 14.04°, 33.34°, 39.53°, and 59.01°, and correspondingly indexed with the crystal planes (002), (101), (103), and (110), respectively.⁴²

Fig. 1 (a) XRD pattern of the MoS₂ nanosheets and MoS₂ nanoflower. (b) FT-IR spectra of the MoS₂ nanosheets and MoS₂ nanoflower.

These characteristic peaks represent the presence of the hexagonal structure of MoS₂, and the highly intense peak observed at (002) reveals the good formation of a layered structure in the material. This crystal plane structure was well-defined with the previously reported results.⁴³Fig. 1(b) shows the FT-IR spectra of the MoS₂ nanosheets, and MoS₂ nanoflower were carried out in the range of 4000 to 500 cm⁻¹ to identify the chemical interaction, and functional groups are present in the electroactive material. The characteristic peaks observed at 601 cm⁻¹ and 930 cm⁻¹ are related to the Mo–S and S–S elements. The peaks shown at 1412 and 1617 cm⁻¹ were assigned to the Mo–O functional group.¹¹ The broad peak observed at 1114 cm⁻¹ is due to the aromatic ring C–H stretching and bending mode. The peak appeared at 715 cm⁻¹ is due to bending out of the plane. The MoS₂ nanosheet peaks slightly shift towards lower intensity. These observed results clearly confirmed the proper chemical interaction and presence of the chemical functional groups and the change in the structural phase.

3.2 Surface morphology and elemental analysis of MoS₂ nanosheets and MoS₂ nanoflower

The surface morphology of the CTAB-assisted MoS₂ nanoflower and MoS₂ nanosheets (without CTAB) is shown in Fig. 2. It can be observed from the surface morphology of the MoS₂ nanosheets that surface is roughly covered by thin branched-like multi-layers and a sharp edge, as shown in Fig. 2(a). Fig. 2(b) displays the presence of CTAB in MoS₂ and shows that the flower (rose flower)-like structure with plenty of edges and nanoflower are arranged by the layers of petals, which enlarges the electroactive surface area mainly due to the presence of the CTAB surfactant. Obviously, defects appeared on the surface of MoS₂. They are responsible for the production of a greater number of active sites. Therefore, the active sites allow for assembling the holes and electron. They easily transport via the electrolytes during the device performance, which could allow for the large surface area. Furthermore, the created defects should help in easily transferring electrons as a result of the high electrical conductivity. It can be interpreted that the nanoflower is uniformly dispersed on the surface of MoS₂, and the surfactant CTAB is impacted on the morphology structure.

Fig. 2 (a and b) FESEM images of the MoS₂ nanosheets and MoS₂ nanoflower. (c and d) EDAX spectrum of the MoS₂ nanosheets and MoS₂ nanoflower.

However, MoO_x and the thiourea mixture underwent a chemical reaction, and then nucleated in the synthesis process. Thiourea then releases the sulfur source under elevated temperature, and Mo is converted into a 2-D layered MoS₂ material. The formation of the MoS₂ chemical reaction is mentioned below.

CH₄N₂S + 2H₂O = 2NH₃ + CO₂ + H₂S²⁻

(NH₄)₆Mo₇O₂₄ or H₃₂Mo₇O₂₄ = 6NH₃ + 7MoO₃ + 3H₂O

MoO₂ + 2H₂S = MoS₂ + 2H₂O

Fig. 2(c and d) shows the EDAX spectrum of the elemental study of the synthesized MoS₂ nanosheets, and the MoS₂ nanoflower clearly shows the presence of molybdenum (Mo) and sulphur (S) elements. The atomic percentages of S and Mo present in the nanosheet and nanoflower were indexed to 64.68% and 35.32%, and 65.49% and 34.51%, respectively. These results reveal that sulphur is more dominant than molybdenum.

3.3 HR-TEM image analysis of MoS₂ nanosheets and MoS₂ nanoflower

Fig. 3(a) shows the HR-TEM images of the MoS₂ nanosheets. It can be seen that the noticeable contrast between the sheet and few layers covered with each other. In Fig. 3(b), the MoS₂ nanoflower reveals the tightly packed multi-layers and thin nanosheet layers connected branch-like rose flower structure. Because of this, the natural rose flower demonstrated in Scheme 1 for comparison. In Fig. 3(c and d), the high resolutions HR-TEM images show the few layered structures and hexagonal shape defects presented on the surface of the MoS₂ layers. The d-spacing is 0.64 nm and ascribed to the (002) plane.⁴⁰Fig. 3(e and f) SAED pattern of the MoS₂ nanosheets and MoS₂ nanoflower confirmed the hexagonal structure formation on MoS₂ and the d-spacing of 0.64 nm is consistent with the XRD indexed plane (002).

Fig. 3 HR-TEM images of the (a) MoS₂ nanosheets, (b) MoS₂ nanoflower, and (c and d) HR-TEM high resolutions images, (e and f) SAED pattern of MoS₂ nanosheets and MoS₂ nanoflower.

Obviously, the defects were generated at the edge sites and instantly influenced the reduction of the charge transfer resistance in the electroactive material, which may be helpful to improve the charge storage capacity. It was confirmed that the CTAB-assisted MoS₂ nanoflower morphology was more significant due to the CTAB surfactant effect, which established that the high active edges improves the surface area as a result.

3.4 X-ray photon spectroscopy of the MoS₂ nanoflower

The XPS spectra give the chemical nature and electronic structure of the composition, and the oxidation valence state of the MoS₂ nanoflower. Fig. 4(a) reveals the two broad Mo 3d distinguished peaks observed at 228.7 eV and 231.9 eV, which have been indexed to Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2, respectively.

Fig. 4 XPS spectra of the MoS₂ nanoflower: (a) Mo 3d, (b) S 2p.

The deconvoluted peaks show the existence of 1T MoS₂ at 228.82 eV and 231.94 eV, while 2H-MoS₂ indicates the presence of two small peaks with corresponding binding energies at 229.8 eV and 233 eV. The 1T and 2H-phase peaks are consistent with the Mo 3d spectrum. The 1T-phase enhances the electrical conductivity due to the enhanced charge density or surface area, and the 2H-phase gives the stable state in nature. Here, it is observed that the two peaks at 234.8 eV and 236.06 eV are related to Mo⁶⁺ 3d_5/2 and Mo⁶⁺ 3d_3/2, respectively, and the S 2s attributed the binding energy at 225.90 eV.⁴⁴Fig. 4(b) shows the S 2p spectrum. Strong peaks positioned at binding energies of 161.5 eV and 162.6 eV belong to S 2p_3/2 and S 2p_1/2, respectively.^45–47 All of these results confirm the oxidation state of the MoS₂ material.

3.5 BET surface area analysis of the MoS₂ nanosheets and MoS₂ nanoflower

The BET method was performed to investigate the specific surface area, pore size and volume distribution of the porous electroactive material in the nitrogen (N₂) adsorption/desorption environment.

Fig. 5(a) shows that the N₂ adsorption/desorption isotherms of the MoS₂ nanosheets and MoS₂ nanoflower exhibit type IV characteristics with a hysteresis loop, which suggests the presence of a mesoporous structure and large pores. The specific surface area of the MoS₂ nanoflower (20.419 m² g⁻¹) is much higher than that of the MoS₂ nanosheets (16.334 m² g⁻¹). Fig. 5(b) shows the BJH pore diameter size and volume distribution curves. The observed pore size is about 4.829 nm and 11.852 nm for the MoS₂ nanosheets and MoS₂ nanoflower, respectively. The large specific surface area obtained in the MoS₂ nanoflower is mainly due to the petals of the layered edges and more mesoporous pores. This can offer a greater number of active sites and enhances the surface area, which is beneficial for the supercapacitor application.

Fig. 5 (a) N₂ adsorption and desorption isotherms. (b) BJH Pore size distribution curves of the MoS₂ nanosheets and MoS₂ nanoflower.

4. Supercapacitor property of the MoS₂ nanosheets and MoS₂ nanoflower

To fabricate the working electrode on copper foil, the first copper foil was cleaned with acetone and dried at ambient temperature. The material composition is chosen in weight ratios of MoS₂ active material/activated charcoal/PVDF (80 : 15 : 5), which were mixed with a NMP solution to form a slurry. It was coated on the copper foil electrode surface using a doctor blade method, then dried in a vacuum oven at 60 °C about 12 hours. The prepared electrode is used as a working electrode in a two-electrode configuration. The CV curve performance was done in the range between −0.8 to 0.6 V applied potential window with various scan rates. The galvanostatic charging–discharging (GCD) curve is measured at different current densities. The electrochemical behaviour of the MoS₂ nanosheet and MoS₂ nanoflower electrode materials was investigated by using a two-electrode system.

The Fig. 6(a) CV plots show the symmetric shape of the oxidation and redox reaction curve at a scan rate of 30 mV s⁻¹. The MoS₂ nanoflower electrode shows a large surface area CV curve and higher current rate than the MoS₂ nanosheets because it undergoes a more faradaic site reaction. The Fig. 6(b and c) CV profile was recorded at different scan rates from 10 mV s⁻¹ to 50 mV s⁻¹. The observed CV curves simultaneously shift towards the positive and negative current rate. It is clearly observed from the CV curve that it maintains a similar shape with a wide area, signifying the strong electrochemical double-layer capacitor properties. It can be interpreted that the MoS₂ nanoflower morphology electrode active material is more stable. The PVA/KOH gel electrolyte ion reacts with the MoS₂ electrode active material, and quickly creates a fast electron transfer due to the low resistance of the active material and kinetic reaction dynamics. Obviously, the MoS₂ material at a low scan rate provides a faradaic behaviour capacitance, which plays a crucial role in increasing the charge storage capacitive nature.⁴⁸ The MoS₂ nanoflower has a higher current density than the MoS₂ nanosheets with an applied potential from −0.8 to 0.6 V.

Fig. 6 CV images of the (a) MoS₂ nanosheets and MoS₂ nanoflower at a scan rate of 30 mV s⁻¹. (b and c) MoS₂ nanosheets and MoS₂ nanoflower at various scan rates. (d and e) The log scan rate against the log current density, and (f) the scan rate proportional to the capacitance.

Figure 6(d and e) shows the log current density (I_P) versus log scan rate (anodic and cathodic peak), which is the linear affinity between the log V (scan rate). As per the Power-law (I_P = av^b) the “b” value is determined from the slope of log(V) versus log(I_P) “a” and is a constant. The parameter of the “b” value is 0.5 and represents the controlled diffusion properties, and 1 is the capacitive behaviour.⁴⁹ The obtained slopes for the MoS₂ nanosheets and MoS₂ nanoflower of the anodic and cathodic peaks expressed the “b” values of 0.43, 0.56, and 0.81, 0.82, respectively. It can be emphasized that the MoS₂ nanosheets indicate the controlled diffusion process, and the MoS₂ nanoflower non-controlled diffusion process or capacitive behaviour due to the cause of polarization, and the high current response, as well as rapid reversible redox reaction.⁵⁰

Fig. 6(f) shows the capacitance of the MoS₂ nanosheets and MoS₂ nanoflower electrode material. The wide area of the CV curve was calculated by using the following relation:

where I(V)dV is the integral area of the CV plot, (V₂ − V₁) is the applied potential range, ν is the scan rate, and m is the mass of the electroactive material. It was observed that the C_sp was 218.88 F g⁻¹ and 436.26 F g⁻¹ for the MoS₂ nanosheets and nanoflower electrode material, respectively, at a slow scan rate of 10 mV s⁻¹. The MoS₂ nanoflower exhibits excellent capacitive storage capacity due to the high surface area.

GCD analysis of the MoS₂ nanosheets and MoS₂ nanoflower

In Fig. 7(a and b), the GCD curve of the MoS₂ nanosheets and nanoflower electrode material shows a capacitive nature and slightly deviated from the triangular shape. The MoS₂ electrode material is highly active for the PVA/KOH gel electrolyte due to the high intrinsic ion fast diffusion process, and the obtained results clearly showed the outstanding synergetic effect of the electrode capacity performance.

Fig. 7 GCD curve of the (a) MoS₂ nanosheets, (b) MoS₂ nanoflower. (c) MoS₂ nanosheets and MoS₂ nanoflower current density versus C_sp, and (d) Nyquist plot of the MoS₂ nanosheets and MoS₂ nanoflower.

The C_sp of the electroactive material was determined from the GCD curve at various current densities using the following expression:⁵¹

C_sp = (IΔt)/mΔV

where I is the applied current, Δt is the discharging time, m is the mass of the electrode active material, and ΔV is the total potential variation of the discharging curve.

Fig. 7(c) shows the MoS₂ nanosheets and nanoflower electrode C_spversus the current density. We can observe that at the current density of 1 A g⁻¹ to 2.5 A g⁻¹, the C_sp correspondingly found decreases from 438 F g⁻¹ to 159.23 F g⁻¹, and 516 F g⁻¹ to 200.69 F g⁻¹, which are clearly displayed from the downward tendency specific capacitance curve. The MoS₂ nanoflower exhibits a high C_sp of 516 F g⁻¹ at 1 A g⁻¹ current density compared with the MoS₂ nanosheets.

The observed C_sp value of the nanoflower clearly signifies the improvement in the energy storage capacity due to the presence of defects, plenty of edge sites, stable state of the electrode material, high intrinsic ion conductivity, and interface relation between the electrode material surface and electrolyte. The obtained data agreed well with the previously reported values, as mentioned in Table 1.

Table 1 The specific capacitance (C_sp) of different MoS₂ electrode materials at the current density of 1 A g⁻¹

Electrode material	Electrolytes	C sp F g^−1	References
Metallic 1T-MoS2	1 M KOH	379	40
MoS2/graphene	1 M Na2So4	243	48
Flower-like MoS2 nanostructure	1 M KCl	168	41
MoS2/PPy	1 M H2So4	400	56
MoS2/PANI	0.5 M H2So4	450	57
BCN/MoS2	1 M H2So4	283	58
MoS2 nanosheets	PVA/KOH	438	This work
MoS2 nanoflower	PVA/KOH	516	This work

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Fig. 7(d) is the graph of the electrochemical impedance spectroscopy (EIS) results measured to investigate the interface resistance contact between the electrode active material and electrolyte. According to the Nyquist plot, the quasi-semicircle with the charge transfer resistance (R_ct) value shows the MoS₂ nanosheets is 4.86 Ohm and the MoS₂ nanoflower is 3.40 Ohm. The MoS₂ nanoflower material possesses a small R_ct value. It may offer high electrical conductivity, and also reveals the faster electron transfer and ion diffusion process. Overall, the MoS₂ electrode contributed high electrochemical performance in accordance with the supercapacitor device application. CTAB is a cationic surfactant used in the synthesis, and it is responsible for changing the surface morphology of the MoS₂ nanosheets into a MoS₂ nanoflower. The CTAB-assisted MoS₂ nanoflower established more edge layer structure and atomic defects on the surface. It can also be interpreted that the layered edges easily possess the quick transfer of electrons in the electrochemical mechanism.

The cyclic stability of the MoS₂ nanosheets and MoS₂ nanoflower electrode was performed by GCD technique at a current density of 2.5 A g⁻¹ for 2000 cycles, as shown in Fig. 8 (inset). The MoS₂ nanosheets and MoS₂ nanoflower electrode possess 83.3% and 87.42% capacitance retention after 2000 cycles. The MoS₂ nanoflower exhibits higher capacitance retention than those previously reported MoS₂-based results. For example, the MoS₂/rGO/PPy/ITO⁵² supercapacitor electrode shows 76.68% of the capacitance retention, the MoS₂/PANI⁵³ core shell composite electrode exhibits 86% capacitance retention after 1000 cycles, the MoS₂ sponge electrode shows 80% capacitance retention,⁵⁴ and the MoS₂/PANI-38 (ref. 55) composite electrode shows 86% capacitance retention. The observed results clearly reveal that the MoS₂ nanoflower electrode materials are highly favorable for supercapacitor application.

Fig. 8 The cyclic stability analysis of the MoS₂ nanosheets and the MoS₂ nanoflower (inset figure: first 5 cycles and last 10 cycles).

Conclusions

The MoS₂ nanosheets and CTAB-assisted nanoflower were successfully synthesized by using the hydrothermal route, and the MoS₂ material confirmed the presence of the chemical constituents S–S and Mo–S by FT-IR spectra. The XRD indexed plane (002) intense peak reveals the hexagonal structure formation of the MoS₂ material. The FESEM and HR-TEM images confirm the presence of nanosheets and the nanoflower morphology. The EDAX spectrum of the elemental study suggested that sulphur is more dominant than molybdenum, and the XPS graph demonstrates the presence of Mo and S elements. The MoS₂ nanoflower CV curve exhibits the electrical double layer capacitance nature with a large loop. The maximum capacitance achieved by 436.26 F g⁻¹ at 10 mV s⁻¹ and C_sp from GCD found at 516 F g⁻¹ at the current density of 1 A g⁻¹, and the MoS₂ nanosheets electrode shows the C_sp from CV and GCD of 218.88 F g⁻¹ at 10 mV s⁻¹ and 438 F g⁻¹ at the current density of 1 A g⁻¹. These results confirmed that the MoS₂ nanoflower had a greater capacitance magnitude than the MoS₂ nanosheets electrode active material due to the nanoflower having a large surface area, and these materials are suitable for the high potential energy storage applications of the supercapacitor.

Conflicts of interest

There is no conflicts of interest.

Acknowledgements

The authors acknowledge Mangalore University for providing a research fellowship, the PURSE LAB, Mangalore University, and the IISc, Department of Centre for Nanoscience, Bangalore for the characterization facility.

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