MoS2 nanobelts-carbon hybrid material for supercapacitor applications

The MoS2 nanobelts/Carbon hybrid nanostructure was synthesized by the simple hydrothermal method. The MoS2 nanobelts were distributed in the interlayers of Lemon grass-derived carbon (LG-C), provides the active sites and avoid restacking of the sheets. The structural and morphological characterization of MoS2/LG-C and LG-C were performed by Raman spectroscopy, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The electrochemical measurements were studied with cyclic voltammetry, the galvanostatic charge-discharge method, and electrochemical impedance spectroscopy. The specific capacitance of MoS2/LG-C and LG-C exhibits 77.5 F g−1 and 30.1 F g−1 at a current density of 0.5 A g−1. The MoS2/LG-C-based supercapacitor provided the maximum power density and energy density of 273.2 W kg−1 and 2.1 Wh kg−1, respectively. Furthermore, the cyclic stability of MoS2/LG-C was tested using charging-discharging up to 3,000 cycles, confirming only a 71.6% capacitance retention at a current density of 3 A g−1. The result showed that MoS2/LG-C is a superior low-cost electrode material that delivered a high electrochemical performance for the next generation of electrochemical energy storage.


Introduction
The development of energy storage devices, like rechargeable batteries, is important to produce environmentally friendly products (Huang et al., 2019;Shaqsi et al., 2020;Li et al., 2021).Nowadays, electrochemical energy storage devices such as supercapacitors and photovoltaic cells are commonly used (Kandasamy et al., 2021;Agrawal et al., 2022).Supercapacitors exhibit a quick charge storage property, which may contribute to their reduced charging times, greater cyclability, and therefore higher specific capacitance (Purkait et al., 2018;Narthana et al., 2021;Rajagopal et al., 2022;Rawat et al., 2022).The ongoing research effects focus on the production of a low-cost, long-life cycle, and high-specific capacitance material for the development of supercapacitors (Iro et al., 2016;Tomy et al., 2021).The selection of appropriate electrode material is important to improve the supercapacitor performance.Numerous materials have been used to store charges for electrochemical energy storage devices (Mohammed et al., 2019;Santoro et al., 2019).The store charges in supercapacitor devices are classified into three types, electrostatic double-layer capacitors, pseudocapacitors, and hybrid supercapacitors (Mathis et al., 2019;Atta and Fahim, 2021;Volfkovich, 2021).The double layer formed at the electrode surface is used for the storage of charges in electrostatic double-layer capacitors.In pseudocapacitors, charge storage occurs through a redox reaction (Fleischmann et al., 2022;Kour et al., 2022).
Biomass-derived carbon sources have natural renewable resources and are widely used because they are affordable, readily available, simple to prepare, and ecologically beneficial.Some examples of biomass that may be used as a resource for activated carbon for electrochemical energy storage include coconut oil, bamboo fiber, bean dregs, mango leaves, peanut shells, etc., (Madhu et al., 2014;Ruan et al., 2014;Gaddam et al., 2016;Ji et al., 2018;Bai et al., 2020) In contrast, activated carbons frequently exhibit less capacitive characteristics, such as low conductivity and restricted charge flow rates.Activated carbon exhibits poor conductivity and limited charge storage capacity (Gomes Ferreira de Paula et al., 2019;Mohammed et al., 2019;Santoro et al., 2019;Luo et al., 2021).To overcome these difficulties, it is necessary to develop structural modifications as well as hybrid materials (Khandare and Late, 2017;Tomboc et al., 2020).
The Lemon Grass (Cymbopogon citratus) plant is known for its long leaves, which are specially used for making oil and spices.In Asian countries, Lemon Grass (LG) is used to provide taste and flavor to drinks (including tea, coffee, etc.).Among them, it has been utilized as a biofertilizer and feedstock.LG consists mostly of cellulose, which is considered to be a carbon source material with high potential (Liakos et al., 2016;Thota et al., 2018).The production of carbon and inorganic-based composites is a successful method for enhancing electrochemical energy storage by combining various inorganic materials, including transition metal oxides, and transition metal dichalcogenides, with carbonbased materials (Wang et al., 2020;Kour et al., 2022).
Molybdenum (Mo) has a variable oxidation state that can vary from +2 to +6.Molybdenum disulfide (MoS 2 ) has a layered structure that shows pseudocapacitive behaviour to obtain high specific capacitance (Cook et al., 2017).Mo has been stacked in between two sulfur atoms (S-Mo-S) by weak van der Waals forces, which allow the electrolyte ions to intercalate in MoS 2 (Kukkar et al., 2016).The ion diffusion works with MoS 2 to store the charge in its faradic capacitive nature, which helps to improve the chance of storing the charge (Mahmood et al., 2016).
In this work, we report the low-cost synthesis of MoS 2 / Lemon Grass-derived carbon (MoS 2 /LG-C) hybrid material for enhancing the electrochemical performance.In situ growth of MoS 2 nanobelts on surfaces of the LG-C through a facile redox reaction between ammonium molybdate and thiourea with carbon.The morphology and structural properties of LG-C and MoS 2 /LG-C hybrid materials are examined.The electrochemical measurements performed with cyclic voltammetry, galvanostatic charge-discharge testing, and cyclic stability to obtain the superior charge storage capacity in supercapacitors are discussed.

Preparation of LG-C
The lemon grass (LG) leaves were collected from the local market based in Pune, India.Collected LG leaves were washed with DD water and dried in the sunlight for at least 2 days.The dried leaves were crushed finely and subsequently treated in KOH (1: 1 weight ratio of LG leaf powder to KOH) for impregnation.After impregnation, the pH of the powder has been adjusted to 7 (neutral) by using a 1M HCl solution.After adjusting the pH, the LG powder was dried for 12 h at 80 °C.Finally, the powder was kept in tubular furnaces in an argon atmosphere at 800 °C for 3 h.The temperature of the tubular furnace was increased slowly by 5 °C per minute and cooled naturally to room temperature (RT).

Preparation of MoS 2 /LG-C
The preparation was initiated by dissolving 100 mg of LG-C in 50 mL of DD water.The ammonium heptamolybdate tetrahydride and thiourea in quantities of 151 mg and 200 mg were added to the above reaction mixture, and the mixture was stirred at room temperature for 2 h.The obtained mixture was poured into the Teflon link autoclave and treated hydrothermally at 220 °C for 24 h before being allowed to cool down naturally to RT.The suspension was centrifuged and washed in deionized water and ethanol.The final product was dried in the oven for 12 h at 80 °C.

Material characterization
The Raman spectra were recorded at a 532 nm He-Ne laser source using a Renishaw InVia Raman microscope.The crystalline phase and crystal structure of the pristine and hybrid materials were examined using X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer using Cu Kα (= 1.5405 Å) irradiation.FESEM images were captured using the FEI Nova NANOSEM 450.X-ray photoelectron spectroscopy (XPS) was recorded on the PHI Versaprobe III by using Al Ka X-rays.Transmission electron microscopy (TEM), high resolution (HRTEM), and energy dispersive X-ray spectroscopy (EDS) were recorded using the FEI Talos F200S instrument at 200 kV.

Electrode preparation
The electrodes were prepared on carbon paper as a collector.A 1 × 1 cm 2 area of carbon paper was coated with a slurry of active electrode material, carbon acetylene, and polyvinylidene fluoride (PVDF) in ratio of 85:10:5, respectively.PVDF is used as a binder in dimethylformamide (DMF).The electrodes were dried in a hot air oven for 12 h at 120 °C.The active material in the quantity of 1 mg was used for the supercapacitor electrode fabrication.

Electrochemical measurement
All electrochemical measurements were carried out at the Biologic SP-300 potentiostat/galvanostat using a three-electrode set-up and symmetric system in 1M Li 2 SO 4 electrolyte.Ag/AgCl, platinum and LG-C, and MoS 2 /LG-C were used as reference, counter, and working electrodes, respectively.Cyclic voltammetry (CV), the galvanostatic charge-discharge (GCD) test, electrochemical impedance spectroscopy (EIS), and cyclic stability using the GCD test were examined to study the various supercapacitor parameters.

Results and discussion
The one-step hydrothermal method was used to produce the MoS 2 /LG-C hybrid material.Figure 1         and G vibrational modes of LG-C.The D band is associated with defects and disorders induced in sp 2 carbon (Medhat et al., 2021).The Raman spectra of MoS 2 /LG-C show the E 1 2g and A 1g vibrational modes, which conform to the layered 2H-MoS 2 structure (Dinh et al., 2021).The peak observed at 376.2 and 404.4 cm −1 shown in the inset of Figure 2A corresponds to the E 1 2g and A 1g modes of vibration containing in-plane and the planesymmetric modes of Mo and S. The MoS 2 /LG-C Raman spectra revealed a shift in D and G vibrating modes occurring at 1,337.9 and 1,594.7 cm −1 which can be related to the stress caused in LG-C by the growth of MoS 2 nanobelts.The intensity ratio of I D to I G is widely recognised to characterise the graphitization and amorphous nature of LG-C.In MoS 2 /LG-C, it was observed that the I D/IG ratio was approximately similar to 1:1 (Kishore et al., 2014).This indicates that higher disorder and more active sites in MoS 2 allowed for the formation of amorphous carbon and defective sites in MoS 2 /LG-C (Yu et al., 2020).
Figure 2B shows the XRD patterns of the as-synthesized LG-C and MoS 2 /LG-C hybrid materials.The diffraction peaks observed in MoS 2 /LG-C at 2θ values of 14.4 °, 39.2 °, and 49.0 °have indexed the planes (002), (103), and (105) respectively, corresponding to the 2H hexagonal phase of MoS 2 (JCPDS no. 37-1492).The pristine LG-C XRD pattern revealed two peaks at 26.0 °and 42.7 °.The broad XRD peak was obtained for the pristine LG-C, while in MoS 2 /LG-C, these peaks were found to be slightly shifted to 25.8 °and 41.9 °due to the presence of MoS 2 on the surface of LG-C (Shapira and Zucker, 2022).
FESEM images were taken at different magnifications, consisting of the micro-as well as nano-size surface morphology of pristine LG-C before and after the formation of MoS 2 .FESEM images of pristine LG-C, (Figures 3A-C) showed a thick sheet with a scattered layer appearance.LG-C sheets revealed a rough surface with the ordered stacking of the sheets.MoS 2 nanobelts distributed in the interlayer of the LG-C materials (Figures 3D-F) produce nanobelts-like morphology.MoS 2 has several precise nanobelts of sizes ranging from 2 to 3 μm in length with smooth and even surfaces.
Further, the surface morphology was investigated by TEM and HR-TEM images.The TEM images shown in Figures 4A-E further conform the size of nanobelts is not uniform, which varies from 90 to 190 nm.A clear dispersion of the MoS 2 nanobelts in the LG-C matrix can be seen.The HR-TEM image (Figure 4F) shows a lattice spacing of ~0.18 nm, which corresponds to the (105) plane of the hexagonal crystal structure of MoS 2, which matches with the XRD results.The selected-area electron diffraction (SAED) pattern of the MoS 2 /LG-C is shown in the inset of Figure 4F.SAED pattern exhibited the polycrystalline nature of MoS 2 /LG-C hybrid material is revealed.Some of the prominent reflections (105) are shown in SAED.The elemental composition determined with the help of EDS analysis is given in (Supplementary Figure S1).The  Frontiers in Chemistry frontiersin.org06 observed composition of Mo and S was found to close to the ideal conditions (1:2).
The XPS spectrum of the MoS 2 /LG-C hybrid material is depicted in Figure 5. Figure 5A shows the survey scan for four peaks conforming to Mo 3d, S 2p, C 1s, and O 1s, suggesting that the MoS 2 /LG-C hybrid material was successfully formed.Apart from them, the peaks were obtained at 186.0 and 253.4 eV called plasmon loss peaks of S and Mo respectively.These peaks occur may be a higher probability of losing a specified amount of energy as a result of the photoelectron's interaction with other electrons (Stevie and Donley, 2020).The doublet obtained 394.8 and 412.6 eV corresponds to Mo 3p 3/2 and Mo 3p ½ respectively.The Mo 3d XPS resolution scan (Figure 5B) shows the two primary peaks were obtained at 228.6 and 231.7 eV, corresponding to Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 , respectively (Feng et al., 2019).The peaks obtained at 232.6 eV and 236.2 eV correspond to Mo 6+ 3d 5/2 and Mo 6+ 3d 3/2, respectively.The existence of sulphur atom level 2s in the MoS 2 /LG-C was linked to the peak at 225.7 eV (Xiong et al., 2015).Figure 5C shows the high-resolution S 2p XPS spectra, indicating the presence of peaks conforming to the S 2p 3/2 and S 2p 1/2 situated at 161.5 and 162.5 eV.The peak attributed to 168.9 eV could be the presence of the sulphate group (Gnanasekar et al., 2020).The C1s XPS spectra in Figure 5D revealed three major peaks with binding energies of about 284.3, 284.8, and 286.1, eV, corresponding to the functional groups C-C, C-C, and C-O, respectively (Ji et al., 2018).
In Figure 6, all CV and GCD measurements were carried out using a 3-electrode system.Figures 6A, B shows the CV curves of LG-C and MoS 2 /LG-C in 1 M Li 2 SO 4 at scan rates of 5, 10, 20, 50, and, 100 mV s −1 .The CV curve of the LG-C electrode (Figure 6A), shows an almost rectangular nature with no identifiable redox peaks in the operating voltage range of 0.0-1.0V.The rectangular nature of the CV curve of LG-C exhibits ideal behavior, showing the electric double-layer capacitor (Luo et al., 2015).MoS 2 /LG-C hybrid material in Figure 6B has a dome-like quasi-rectangular shape that exhibits both electric double-layer capacitor behaviour as well as Faradic pseudocapacitance behaviour (Xu et al., 2018).MoS 2 nanobelts distributed in the interlayer of the LG-C sheets increased the entire area of the CV as well as peak current density with improved conductivity, which helps facilitate ion transport towards the electrochemical charge storage (Gopalakrishnan et al., 2020;Mahajan et al., 2022).
Figures 6C, D show the GCD curves of LG-C and MoS 2 /LG-C at current densities of 0.5, 1, 2, and 3 A g −1 with the applied potential voltage between 0.0 and 1.0 V.The specific capacitance (Cs in F g −1 ) was calculated at various current densities using the following Eq. 1 (Lin and Zhang, 2015): where I is the current density, ΔV is the applied potential window, m is the active mass of the electrode, and Δt is discharge time.
The Cs of LG-C and MoS 2 /LG-C calculate to be 30.5 F g −1 and 77.5 F g −1 at a current density of 0.5 A g −1 , respectively.MoS 2 /LG-C helps to increase the specific capacitance value of the hybrid electrode material due to the synergistic effects of MoS 2 as a conducting material with LG-C.Mo 4+ in MoS 2 supplied the more active sites for charge transfer in MoS 2 /LG-C, which provided a more interactive area between MoS 2 /LG-C and the electrolyte for ion diffusion and increased the discharge time (Feng et al., 2019).The comparative CV and GCD curves of LG-C and MoS2/LG-C are shown in Supplementary Figures S2A, B at a scan rate of 20 mV s −1 and a current density of 0.5 A g −1 .Electrochemical charge storage is obtained due to ions present in an electrolyte (Li + ) interaction through MoS 2 and carbon sheets.
Figure 7A illustrates the plot of C s versus the different current densities.At higher current densities, C s were found to decrease, which could be due to the reduction in various activities at the electrode surface.The capacitance improvement occurs due to the favourable synergistic effect of MoS 2 as an active-site hybrid material, which helps to avoid the stacking of LG-C sheets and increases the surface area of MoS 2 /LG-C.The increased surface area provided the path for electrochemical charge transformation and helped store the energy in the supercapacitor (Bi et al., 2019).The comparative data of different composite nanostructure materials with MoS 2 /LG-C in the literature related to Cs and various electrolytes are shown in Table 1.(Brousse et al., 2007;Aradilla et al., 2014;Ho et al., 2014;Masikhwa et al., 2017;Choi et al., 2020;Bello et al., 2022).The EIS measurements of LG-C and MoS 2 /LG-C were performed to study the transportation of ions in electrolytes with their electrochemical properties, equivalent series resistance (R s ), and charge transfer resistance (R ct ) (Dulyaseree et al., 2017).Figure 7B shows the Nyquist plot of LG-C and MoS 2 /LG-C hybrid materials with an imaginary axis performed at a frequency range of 0.1 MHz-100 mHz.LG-C and MoS 2 /LG-C achieved similar series resistance (R s ) for supercapacitors of about 12.09 Ω and 6.26 Ω which shows the MoS 2 /LG-C has good electrical conductivity.In the high-frequency region, a clear semicircle was not observed, which indicates it may have a low R ct (Niaz et al., 2020).MoS 2 /LG-C has increased the conductivity of hybrid material because MoS 2 provided a more active site for LG-C, which helps improve electrochemical properties.The cyclic stability of LG-C and MoS 2 /LG-C (Figure 7C was carried out using a GCD test in 1M Li 2 SO 4 electrolyte at the current density of 3 A g −1 .MoS 2 /LG-C exhibited superior cyclic stability and, 71.6% capacitance retaliation, and only 28.4% capacity degradation occurs up to 3,000 cycles at a current density of 3 A g −1 .In LG-C, 46.2% of the capacitance was retained up to 3,000 cycles.In MoS 2 /LGC hybrid materials, sulphur bonded with MoS 2 forms a layer that is collected on the carbon surface and connected to the carbon structure covalently, which helps to provide excellent cyclic stability.The abrupt decrease in capacity observed in cyclic stability plot due to the measurement might be related to active material dissolving in the electrolyte, as well as difficulties with electrodes which include expansion and active material loss owing to poor binding. The electrochemical behaviour of the LG-C and MoS 2 /LG-C hybrid nanomaterials were also studied by symmetric supercapacitor devices in 1M Li 2 SO 4 electrolyte.The CV curves of LG-C and MoS 2 /LG-C (Figures 8A, B) were measured at the potential window of 0.0-0.5 V at scan rates between 5 and 100 mVs −1 .All CV curves of LG-C have a quasi-rectangular shape, showing a good electric double-layer capacitor for fast ion transfer.While CV curves variations from an ideal rectangle with increasing the current density.The GCD measurements of LG-C and MoS 2 /LG-C exhibit minor MoS 2 /LG-C were carried out at different current densities are shown in Figures 8C, D, respectively.The charge-discharge durations of the MoS 2 /LG-C hybrid material has longer than those of pure LG-C.The MoS 2 /LG-C hybrid material has higher specific capacitance than pure LG-C.The specific capacitance values were calculated using Eq. 2 for symmetric supercapacitor devices.
The MoS 2 /LG-C hybrid material and LG-C show a specific capacitance of 59.2 F g −1 and 44.4 F g −1 , respectively at a current density of 1 A g −1 .
Furthermore, EIS measurements of MoS 2 /LG-C hybrid material and LG-C (Figure 8E) were carried out in the frequency range of 100 kHz to 10 mHz.The Nyquist plot corresponding series resistance (R s ) and charge transfer resistance (R ct ) values of LG-C are 8.5 and 3.4 Ω respectively.In the MoS 2 /LG-C Nyquist plot, Rs and R ct were determined to be 6.6 and 2.2 Ω, respectively.The lower R s value implies the internal resistance, electrolyte ionic resistance, and contact resistance of active electrode materials of the MoS 2 /LG-C electrode is more conductive than the LG-C electrode.The R ct may be ascribed to the MoS 2 /LG-C electrode's improved electron transportation and high surface area, which allows for quick redox reactions at the electrode/electrolyte interface.
The Ragone plot of LG-C and MoS 2 /LG-C obtained at various current densities is depicted in Figure 8F.The relation between energy density (E in Wh kg −1 ) and power density (P in W kg −1 ) is an important parameter to examine energy devices.Energy density and power density are calculated from the following equations: 3 and 4, respectively (Raghu et al., 2018).
The MoS 2 /LG-C-based supercapacitor performance measured 273.2 W kg −1 power density and 2.1 Wh kg −1 energy density, whereas the LG-C sheets-based supercapacitor exhibited 234.7 W kg −1 power density and 1.5 Wh kg −1 energy density at a current density of 1 A g −1 .P and E were found to increase with increasing current density, and the maximum power density, 1,366.1 W kg −1 , and energy density, 1.3 Wh kg −1 were obtained at a current density of 5 A g −1 for MoS 2 /LG-C symmetric device.The inset of Figure 8F shows the operating red light-emitting diode for symmetric supercapacitor devices.

Conclusion
In conclusion, we successfully synthesized the MoS 2 Nanobelts/ LG-C nanohybrid structures by simple hydrothermal method.The low-cost and biodegradable LG leaves were used to form the nanocarbon by the pyrolysis method.The MoS 2 nanobelts help to provide pathways for ion transportation due to the interaction between active electrode materials and electrolytes.The specific capacitances of MoS 2 /LG-C and LG-C were found to be 77.5 and 30.1 F g −1 , respectively, at a current density of 0.5 A g −1 .The cyclic stability of MoS 2 /LG-C and LG-C was carried out by the GCD test, and 71.6% capacitance retention occurs up to 3,000 cycles at the current density of 3 A g −1 .The symmetric supercapacitor (MoS 2 /LG-C//MoS 2 /LG-C), the operating red light-emitting diode is illuminated.This method is favourable for the large-scale production of MoS 2 /LGC as active electrode hybrid materials for electrochemical energy storage devices.
illustrates a schematic representation of the synthesis process of LG-C and MoS 2 /LG-C.LG-C nanosheets were associated with the formation of MoS 2 nanobelts on the surfaces of LG-C.The structural information of MoS 2 and the LG carbon was carried out using Raman spectroscopy and XRD analysis.
Figure 2A shows the Raman spectra of LG-C and MoS 2 -LG-C.The two strong peaks appeared at about 1,344.3 and 1,592.8cm −1 identified to the D

FIGURE 1
FIGURE 1 Schematic illustration of synthesis procedures for LG-C and MoS 2 /LG-C hybrid material.
FIGURE 2 (A) Raman spectra (B) XRD pattern of LG-C and MoS 2 /LG-C hybrid material.

FIGURE 4
FIGURE 4MoS 2 /LG-C hybrid material: (A-E) TEM images (F) HR-TEM image, inset of (F) showing SAED pattern and selected portion of lattice fringes.
FIGURE 7 (A) Comparative plot of Cs at different current densities, (B) Nyquist plot of LG-C and MoS 2 /LG-C at the frequency range from 0.1 MHz to 100 mHz, (C) cyclic stability plot of LG-C and MoS 2 /LG-C hybrid material at current density 3 A g −1 .

FIGURE 8
FIGURE 8 Symmetric supercapacitor device electrochemical measurements: (A) CV curves of LG-C, (B) CV curves of MoS 2 /LG-C, (C) GCD curves of LG-C, (D) GCD curves of MoS 2 /LG-C, (E) Nyquist plot of LG-C and MoS 2 /LG-C, (F) Ragone plot of LG-C and MoS 2 /LG-C, Inset of (F) shows the operating red light emitting diode for symmetric supercapacitor devices.

TABLE 1
Comparative electrochemical performance of various nanostructured materials.