Solvent effects on the electrochemical performance of few layered MoS2 electrodes fabricated using FTO substrates

Owing to its exceptional structural, electrical, and optical features, Molybdenum disulphide (MoS2), a two-dimensional (2D) layered material with tuneable bandgap, finds its application in electrochemical supercapacitors for superior energy and power density. Because of their low toxicity and long-term energy storage, the development of MoS2-based supercapacitors is inevitable. The study of solvent effects on the electrochemical performance of a few layered MoS2 using FTO substrates is done for the first time to the best of our knowledge. Exfoliating bulk MoS2 powder in different solvents with variable surface tensions such as Ethanol, Ethylene Glycol (EG), Dimethylformamide (DMF), and Dimethyl Sulfoxide (DMSO) results in the formation of few-layered MoS2 structures. The sample’s structural, optical, and electrochemical behaviours are investigated using x-ray diffraction (XRD), atomic force microscopy (AFM), UV spectroscopy, Fourier transform infrared (FTIR), cyclic-voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). XRD confirms the formation of a 2D MoS2 film with (002) planes and the optical investigation revealed the variation of layer-dependent bandgap with solvents. We observe both faradaic and non-faradaic charge storage mechanisms in the samples and demonstrate a superior pseudocapacitive behaviour for MoS2 in DMF with a maximum specific capacitance of 34.25 F g−1 at a current density of 1 A/g.


Introduction
The American Physicist Richard Feynman first discussed the concepts that seeded nanotechnology in his 1959 talk 'There's plenty of Rooms at the Bottom'.In this talk, Feynman put across the possibilities of synthesis through the direct manipulation of atoms, which led to the development of nanotechnology research.This breakthrough paved the way to a new realm of various nano-dimensional materials and finds numerous applications in the field of water treatment, sensors, energy storage, nanomedicine, textile, electronics, food industry, cosmetics, and further [1,2].The necessity to withstand climate change, energy crisis, and public health, scientific society relies more on renewable energy sources like solar, wind, and hydropower.Since these sources are intermittent, the need for energy storage becomes increasingly important, which bridge the gap between energy supply and consumption.Numerous energy storage devices such as fuel cells, batteries, capacitors, flywheels, and thermal storage systems are commercially available but the choice of energy storage device is strongly correlated to the nature of energy requirement.Batteries are employed in the contexts where a continuous energy supply with a minimal power output is required, whereas supercapacitors are the heroes when a rapid power requirement rule over energy requirement.
Supercapacitors are a class of energy storage devices having quicker charge-discharge with longer cycle life and superior power density, it consists of two current collectors as electrodes, in which nanomaterials can be employed as the active material.Among two common charge storage mechanisms of supercapacitors such as electrostatic double-layer capacitance and electrochemical pseudocapacitance, the latter is preferred in our work on account of their higher capacitance and superior energy density over the former [3].Among various nanomaterials, metal oxides, MXene, and transition metal dichalcogenides (TMDCs) exhibit pseudocapacitive behaviour due to the presence of various oxidation states of metal ions in the compound, which can undergo redox reactions upon the charging and discharging process.Metal oxides have good cyclic stability and pseudocapacitive behaviour that will result in better specific capacitance, but because of their reduced electrical conductivity, they may perform less well.
On the other hand, MXene-based supercapacitors suffer from the problem of instability due to aggregation and restacking, moreover, their uncontrollable synthesis in terms of size can affect their surface area and performance [4].Out of different pseudocapacitive materials, Molybdenum disulphide (MoS 2 ) is chosen because of its superior specific surface area, layered structure, atomic layer thickness, easy exfoliation, electrical conductivity, and electrochemical stability [5].Since MoS 2 is a pseudocapacitive material with superior energy density, it is inevitable to enhance the power density of the material for future applications.
It has been reported that the power density is positively correlated to the pore size of the material [6], increased pore size can enhance the surface roughness as well as the effective surface area of the material.Surface roughness can have an impact on the power density of a capacitor or battery.In general, a rougher surface can increase the effective surface area of the electrode, which can lead to higher capacitance and higher power density because a larger surface area provides more sites for charge storage and can facilitate faster ion transport between the electrode and electrolyte [7].Drop casting technique will be employed to enhance the power density of the materials by increasing the surface roughness and thereby the effective surface area.However, there is a limit to the beneficial effect of increasing surface roughness.If the surface roughness becomes too high, it can lead to increased resistance to ion transport and decreased power density [8].This is because the roughness can create dead-end pores or pockets that trap electrolyte ions, reducing their availability for charge storage and transfer [9].
It has been reported that the capacitance of MoS 2 -based supercapacitors increases with decreasing thickness of the MoS 2 layers [10].This is due to the increased surface area and improved accessibility of the active sites in thinner layers and the surface redox reaction dominates in few layers of MoS 2 compared to bulk material supporting intercalation of charges [11].To realise few to monolayers of MoS 2 , it is advisable to ultrasonicate the bulk material with the appropriate solvent, the selection of the appropriate solvent in turn depends on the compatibility between the surface tension of the solvent and the surface energy of the material under study [12][13][14].N-methyl 2-pyrrolidone (NMP) was discovered by Shen et al to be the ideal organic solvent for the exfoliation of MoS 2 because its surface tension matches the surface energy of the substance [15].The specific surface energy of the highly crystalline few atomic layers of MoS 2 was reported in the literature as 46.5 mJ m −2 , which was calculated using contact angle measurement by Anand et al [16].
Solvent exfoliation is a novel branch of sonication-assisted exfoliation that Coleman presented in 2011.Using organic solvents to distribute large flakes and then ultrasonicating the mixture produced a small number of layered materials.Because organic solvents preserve MoS 2 's 2H structure and need less energy during exfoliation, they are the preferred option [17].The choice of an appropriate solvent is contingent upon many critical elements, including the surface energy of the material, the solvent's surface energy, and the solvent's surface tension.In order to achieve successful exfoliation, the material's surface energy must coincide with the solvent's [12,18].Ali et al reported on one of the processes behind solvent exfoliation, studying the function of hydroperoxides resulting from oxidation by H 2 O and O 2 in transforming solvent into redox active species, oxidising MoS 2 and promoting exfoliation.The following adsorption by products of oxidation to the layers give a strong negative charge to MoS 2 .This charging of the MoS 2 causes a Coulombic repulsion between nearby sheets, which in turn facilitates effective exfoliation [19].So far numerous solvents including hexane, chloroform, tetrahydrofuran, dichloromethane, ethanol, and NMP have been employed for the fruitful exfoliation of MoS 2 to few or monolayers [20], moreover, the role of various solvents in manipulating the properties of MoS 2 was also discussed but hardly any of them discussed the role of various solvents in altering the electrochemical performance of MoS 2 .We would like to conduct a comparative study on various solvents in altering the supercapacitor behaviour of MoS 2 [21,22], which enables us to fill this research gap in the realm of MoS 2 research.We have employed various solvents such as ethanol, ethylene glycol (EG), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) having surface tension 22.1 mN/m, 47.3 mN/m, 36.71mN/m, and 46.68 mN/m respectively.The extent of MoS 2 dispersion will be strongly correlated to the surface tension of the solvent which can affect the layer number of dispersed MoS 2 and thereby the electrochemical performance, we are expecting a maximum performance for EG, DMSO, and DMF, since their surface tension is comparable to the surface energy value of MoS 2 .
In this work, we have fabricated fluorine-doped tin oxide conducting glass substrate (FTO) based MoS 2 electrodes by employing various solvents such as ethanol, EG, DMF, and DMSO of known surface tension for the supercapacitor application.Poly (vinylidene fluoride) (PVDF) was employed as the binder for the successful adhesion of active material to the FTO current collector.A simple drop casting method followed by drying was employed for the electrode fabrication and the obtained electrode is characterised using UV-vis spectroscopy, XRD, FTIR and AFM to study the fundamental behaviour.Moreover, energy storage behaviour was studied by employing various electrochemical techniques such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) by employing an electrolyte solution of 2 M KOH in a three-electrode system.The solvent effects on the electrochemical behaviour of FTO-supported MoS 2 electrodes are being studied for the first time to the best of our knowledge.

Materials
Molybdenum disulphide (MoS 2 ) 99.99% pure obtained from Sigma Aldrich, Poly (vinylidene pyrrolidone) powder from Alfa Aesar, solvents such as ethanol, dimethyl sulfoxide, dimethyl formamide, ethylene glycol and FTO glass substrate are employed for the electrochemical study.

Synthesis
To investigate the solvent effects, four different solvents such as ethanol, DMSO, DMF, and EG are employed for the ultrasonic-assisted dispersion of MoS 2 .A fixed amount of MoS 2 and PVDF are taken with a ratio of 90%:10% in a 5 ml vial filled with respective solvents followed by ultrasonication for 6 h with regular intervals.The prepared samples were named sample 1 (ethanol), sample 2 (EG), sample 3 (DMF), and sample 4 (DMSO), and were kept in four different vials at room temperature for further studies.Simultaneously, the FTO glass substrates were cut into a dimension of 2 cm × 1 cm and were pretreated by ultrasonication in acetone followed by ethanol and double distilled (DD) water respectively for 20 min each and the treated substrates were dried in an oven at 60 degrees overnight.100 μl of known concentration of the prepared samples were drop cast into an area of 1 cm × 1 cm on the activated FTO substrate and dried in an oven for 24 h at 60 degrees Celsius for further characterisations.A basic electrolyte of 2 M potassium hydroxide (KOH) in DD water is employed in the electrochemical cell of three-electrode system in which a platinum (Pt) wire is used as the counter electrode, Ag/ AgCl as the reference and the MoS 2 coated FTO as the working electrode (see figure 1).
Basic structural, optical, and morphological studies of the MoS 2 samples were done using elementary characterisation techniques and in the application study, an electrochemical workstation was used to assess the energy storage capacity of the samples.A variety of electrochemical tests were used to assess the materials' ability to retain and release electrical energy.The study's findings may provide light on the possible applications of these materials in energy storage devices such as batteries or capacitors.Figure 2 shows the real-time image of performing an electrochemical workstation.

Characterisations
The x-ray diffraction patterns were produced using a PANalytical-X|Pert3 Powder XRD with monochromatic Cu K α as the radiation source in the 2θ range of 10°-90°and analysed with X'Pert High Score Plus software.The absorption spectra were acquired using an Analytic Jena (model -specord plus) UV-vis-NIR spectrometer in the 200nm-800nm wavelength range.AFM images with a resolution of 2 μm X 2 μm were produced in contact mode using the Nanosurf easyscan 2, and the statistical parameters were analysed using the Gwyddion programme.The Raman spectra were captured with a Horiba labram HR evolution spectrometer at a laser wavelength of 633 nm.The FTIR spectra were obtained by employing the Thermo Nicolet iS50 instrument in the range wavelength range 4000-400 cm-1.Finally, the electrochemical studies were done using a CHI 7007E electrochemical workstation by employing cyclic voltammetry (CV), chronopotentiometry (GCD), and AC impedance (EIS).

Results and discussion
At first, studied the electrochemical performance of MoS 2 samples exfoliated at various solvents without the involvement of any binders, but it was observed that the coated layer peeled off as we sweep to higher positive potentials.This may be attributed to the development of mechanical stress on the film generated due to the applied potential during electrochemical measurements [23], we have employed a negligible amount of PVDF binder during the ultrasonication to nullify this drawback by enhancing the adhesion between the FTO surface and the MoS 2 film [24].Moreover, PVDF behaves as an excellent piezoelectric material so it can generate electrical signals in response to the mechanical stresses that arise during the electrochemical measurements [25,26].The MoS 2 coating after the incorporation of PVDF was found to be strongly adhered to the FTO glass, which could enhance the electrochemical performance.
The surface tension of each solvent plays a vital role in the journey from bulk to multilayers since the surface tension should be matched with the surface energy of the MoS 2 layers for fruitful exfoliation.The correlation between this surface tension and surface energy can be explained by the existence of Van der Waals forces within monolayers of MoS 2 , the solvent should be able to overcome these cohesive forces, which can be achieved when the surface tension of liquid matches or exceeds the surface energy [27].If the surface tension of the liquid used for exfoliation is too high compared to the surface energy of MoS 2 , rather than spreading over the materials it can form droplets and the higher surface tension can exert additional mechanical forces on the material.Thus, it can impede penetration, increase cohesive forces, cause agglomeration, and potentially damage the material.It is crucial to find a balance between the surface tension of the liquid and the surface energy of MoS 2 to achieve successful exfoliation and obtain high-quality, isolated layers [28,29].

X-ray diffraction studies (XRD)
The structural properties of MoS 2 exfoliated in four various solvents are analysed using the powder XRD technique with the help of X'Pert high score software, from which the 2H 1 -hexagonal phase (Molybdenite phase) of MoS 2 is confirmed with P-6m2 space group and space group number 187 (reference code 00-024-0513).Figure 3 depicts the XRD pattern corresponding to various solvents such as ethanol, EG, DMF, and DMSO labelled as 1,2,3, and 4 respectively, which will yield the relation between the angle of diffraction and the intensity of diffraction from various (hkl) planes.From the XRD pattern, the reflection corresponding to (002) planes was found to be having the highest intensity, which implies the prosperity in the number of reflecting planes in that direction.The difference in the intensities of (002) planes corresponding to different solvents indicates a decrement in the constructive interference from the set of planes, which in turn, explain the transition into few layers.The XRD pattern of bulk MoS 2 was obtained from the reference [30], in which the reflection from few more sets of planes other than (002) was found to be of considerable intensity.Meanwhile, the XRD pattern of the exfoliated sample obtained after ultrasonication contains the (002) plane alone with the remaining set of planes having negligible intensity, which confirms the confinement of bulk into a twodimensional lattice.In other words, the materials become atomically thin along a particular direction (say vertical) consequently, there is no variation in the interatomic spacing along the vertical direction, which in turn leads to the overlapping of reflections from different planes into a single dominant peak [31].According to the XRD results, the exfoliated sample in EG and DMSO had the lowest diffraction intensity, which translates to successful exfoliation of bulk MoS 2 with a reduced number of layers of MoS 2 for those solvents with surface tension greater than that of the MoS 2 .
Using Bragg's formula and the Scherrer equation, the interplanar spacing, crystallite sizes, and lattice parameters for various solvents were computed, table S1 (supporting information) shows the calculation.The equation involved are described in (1), and (2) respectively for d-spacing (d), crystallite size (D), and cell parameters (a, b, & c).
Where λ is the wavelength of the x-ray used, θ is the angle of diffraction, k is the Scherrer constant (0.9), β is the full width at half maximum (FWHM) in radians, and (h), (k), (l) is the miller indices.
The (002) peaks exhibit a shifting tendency between 14.279 and 14.289, which is directly correlated to the changes observed for the interplanar spacing from table S1, a contraction in crystal structure is observed with a shift in 2θ value to the positive X-axis.Maximum contraction in the crystal lattice was observed for the sample exfoliated in Ethanol and the minimum for that in EG, which can be explained with the help of the difference in electronegativities of the solvent.First of all MoS 2 has a theoretical electronegativity of 5.32 eV [32] and ethanol with the lowest electronegativity can transfer more electrons to the lattice of the materials, which in turn strengthens the bond and thereby reduces the lattice parameters, which is observed as a contraction in crystal lattice compared to EG [33].Other than (002) peaks, the samples demonstrated diffraction peaks at 29.18°, 39.72°, 44.33°, 49.9°, and 60.32°corresponding to (101), (103), (104), (105), and (112) planes respectively, that further confirms 2 h -hexagonal structure of MoS 2 .

Optical characterisation
UV-vis absorption spectroscopy was employed to characterise the optical characteristics of MoS 2 originally dispersed in different solvents at a known concentration by liquid phase exfoliation (samples are in the solution phase for UV-vis spectroscopy), and the spectra are shown in figure 4. In the region of 350 nm to 800 nm, we found four absorption peaks A, B, C, and D observed at 684 nm, 626 nm, 486 nm, and 397 nm respectively.A and B correspond to direct excitonic transition between the valance band maximum and conduction band minimum at the K-point in the Brillouin zone, which is usually observed in the visible region [34].Even though the direct transition takes place at the K-point, we can observe a clear energy separation between the two absorption peaks, which can be attributed to the spin-orbital coupling of the valance band and the transition occurs between the split valance band and the conduction band [35].The extent of spin-orbit coupling can be scrutinised from the energy separation between absorption peaks A and B corresponding to exciton A and Exciton B. Spin-orbit coupling contributes significantly to the valence band splitting at K in multilayer MoS 2 whereas, interlayer coupling in double-layer MoS 2 widens the gap between the previously spin-orbit split states [36].The formation of absorption peaks in the bulk sample is owing to intralayer spin-orbital coupling and interlayer interaction, however, for few layers of MoS 2 , the effect of interlayer interaction lessens due to the reduction in the number of layers.The origin of peak C can also be explained by the spin-orbit coupling at the K-point in the momentum space, finally, the existence of peak D is due to the indirect transition between the conduction band and the valance band at the г-point of the Brillouin zone [37].We have observed a reduction in the absorbance of peak C and D for ethanol and EG, which may be attributed to the higher polarity of these solvents that can lead to changes in the electronic structure and properties of the material [38].
The separation between A and B peaks was found to be showing an increasing trend from 0.165 eV to 0.181 eV with variation in solvents.Since the energy difference is directly correlated to the extent of spin-orbit coupling, the variation in the separation can be explained by analysing the role of solvents in regulating the strength of spin-orbit coupling.It has been discovered that as the dielectric constant of the liquids decreases, the energy spacing between the peaks increases.This is because the interlayer contact between a few layers of MoS 2 expands the gap between the spin-orbit states, commonly known as the dielectric screening effect.An increase in dielectric screening can decline the excitation binding energy by weakening the coulombic interaction thereby reducing the strength of spin-orbit interaction, which could be observed as a redshift (lower energy) in the separation between the energies A and B peaks [39].Solvents with higher dielectric constants can polarise the material and create a coulombic interaction between the solvent and the layers of MoS 2 , causing the effect of interlayer coupling and consequently, the spin-orbit interaction to be disrupted.This observation confirms our notion that the separation of spin-split states declines as the dielectric constant increases.Table showing the variation of energy difference of absorption peaks with that of the dielectric constants of the solvents are given in table S5.The direct optical band gap of a few layers of MoS 2 exfoliated in different solvents such as ethanol, DMF, EG, and DMSO was calculated using Tau's plot and was found to be 1.62, 1.69, 1.72, and 1.75 eV respectively and is plotted in figure S1-S4 (supporting information).The observed increase in the energy gap of the materials sonicated in various solvents is attributable to the reduction in the number of layers caused by effective exfoliation, it has also been observed that the variation in optical bandgap was positively correlated to the dielectric constants of the solvents employed.The increase in bandgap with the increase in dielectric constant can be attributed to the dielectric screening effect, which reduces the coulombic interaction between the charge carriers and as a result, the energy requirement (bandgap) to promote electron transfer from the valance band to conduction band increases [40].

Atomic force microscopy (AFM)
AFM is used to obtain a high-resolution 2D topography and 3D mapping of the material's surface, as well as nano-dimensional data on the structure.The topographical image and height profile of various MoS 2 samples drop-casted on glass substrates were obtained using AFM with a resolution of 2 μm X 2 μm with contact mode, and the 2D images of the sample are shown in figures 5(A)-(D).The surface uniformity of the MoS 2 films varied with exfoliation in different solvents; to determine the dependence of the surface uniformity on the solvents the surface roughness values were analysed with the help of Gwyddion software.The surface roughness of MoS 2 exfoliated in several solvents, including EG, DMSO, ethanol and DMF was determined to be 0.45 nm, 0.7 nm, 3.7 nm, and 6.32 nm respectively.The solubility parameter (owing to London dispersion, hydrogen bonding, and dipole interaction), solvent polarity, and evaporation rate of the solvent have a substantial influence on the sample's surface roughness [41,42].
We have concentrated on AFM images towards the surface roughness and the height analysis, from which the morphology of the flakes was clear and the surface was found to be nearly uniform.Moreover, the RMS value of surface roughness gives a clear idea about the uniformity of the surface.The flakes were visible in the AFM images and were found to be having nearly spherical geometry with irregular boundaries.The material exfoliated in DMF has demonstrated maximum roughness, which will suggest a superior electrochemical performance for the DMF sample, still, there several factors influence the overall electrochemical performance of a sample.Further, to confirm the morphology the SEM image of MoS 2 dispersed in DMSO is taken with a resolution of 300 nm and is given in the supporting information (see figure S7).From the SEM images, MoS 2 flakes having irregular boundaries with nearly uniform surface was confirmed.

Fourier transform infrared (FTIR) spectroscopy
To further understand the nature of the species in the produced samples, FTIR spectra were obtained.FTIR spectroscopy is a useful instrument for detecting the presence of specific functional groups that absorb specific characteristic frequencies, providing structural information about the substance under investigation.Figure 6 depicts the obtained FTIR spectrum, and table S2 provides detailed functional group identifications based on the FTIR spectrum.The absorption band observed at 3651 cm −1 for ethanol, EG, and DMF can be attributed to the presence of −OH functional group, while the existence of the alcohol group in DMF may be due to water contamination or surface reactions.The peak observed at 2100 cm −1 and 1990 cm −1 confirms the existence of the alkyne group and the CH bending.Moreover, the existence of an absorption band at 1653 cm −1 confirms the CO group, significantly observed in DMSO and DMF, which can be attributed to the presence of oxide and amide groups present in those solvents.The S=O stretching observed at 1400 cm −1 for all the solvents confirms the presence of MoS 2 in the sample and the presence of the same group is confirmed alone for DMSO due to the vibration at 1040 cm −1 [43].Finally, the FTIR bands observe at 608 cm −1 and 866 −1 confirm the presence of two kinds of stretching in MoS 2 including Mo-S vibrations and S-S vibrations, with the help of FTIR spectra, we have confirmed the presence of MoS 2 in the sample without any impurities [44].
The intensity of the absorption band observed for the hydroxyl group and the CH (2100 cm −1 ) bending were almost similar, whereas the band observed for the alkane group at 1990 cm −1 has shown a maximum intensity for DMSO and DMF over the other two solvents, which can be attributed to the existence of two −CH 3 groups in DMSO and DMF.DSMO yielded a maximum intensity of −SO group stretching due to the presence of sulfoxide groups in its structure as well as the interaction between oxides and sulphides in MoS 2 .The characteristic bands of MoS 2 for Mo-S and S-S vibration have demonstrated a maximum intensity for ethylene glycol followed by DMSO and DMF, which may be attributed to the solvent-solute interaction.The polarity as well as the surface tension of the employed solvents are different, which can affect the extent of interaction between the solvent and the MoS 2 .It is well known that the solvents with a high value of surface tension compared to that of MoS 2 can effectively interact with MoS 2 and yield few layers of MoS 2 .Which in turn may increase the concentration of the absorption species and thereby enhance the intensity for EG and DMSO.The observed variation in intensity may also be explained in terms of the electronegativity of the solvents.As discussed, EG donates fewer electrons to the MoS 2 , making the lattice more expanded compared to that in ethanol.An expanded lattice can strongly influence the intensity of the spectrum by contributing a longer path length for greater interaction between the sample and the infrared radiation [45].

Raman spectroscopy
The Raman spectrum of the exfoliated sample is shown in the picture (figure 7).Raman spectroscopy is often employed as a molecular fingerprint to detect the existence of 2D layered materials.The literature reports that the bulk MoS 2 Raman active modes are 32 cm −1 (E 2 2g ), 286 cm −1 (E 1g ), 383 cm −1 (E 1 2g ), and 408 cm −1 (A 1g ), which correspond to the crystal structure of MoS 2 .The literature has revealed that the layer number of TMD materials may be obtained from the frequency separation between the E 2 1g and A 1g modes and the observed  separation for bulk and monolayer MoS 2 is 26.6 cm −1 and 19 cm −1 , respectively [46].Since our interest is on the realisation on few layers of MoS 2 , we will focus on the Raman spectra corresponding to E 1 2g and A 1g modes of vibrations, which is demonstrated in figure 7.
It was discovered that for DMSO, EG, Ethanol, and DMF, the frequency difference between the E 2 1g (in-plane opposite vibrations of sulfur/molybdenum atoms) and A 1g (out-of-plane vibrations of sulphur atoms) modes was 24.4 cm −1 , 25.3 cm −1 , 26.2 cm −1 , and 27.7 cm −1 , respectively.According to the Raman data, DMSO is producing successful exfoliation with the fewest possible layers, which supports our prediction that an exfoliating solvent with a surface tension similar to that of MoS 2 will produce good exfoliation, but the electrochemical performance will be affected by several factors as described in the analysis CV signatures..

Electrochemical analysis
An electrochemical study is employed to figure out the exact nature of chemical reactions involving the transport of electrons between electrodes and electrolyte solutions in a system of two or three electrodes with an electrolyte.These investigations typically concentrate on the behaviour of redox reactions and entail monitoring the flow of electric current as a function of the applied voltage or current using a potentiostat.Electrochemical studies have a wide range of applications, including energy storage, corrosion studies, and sensor and biosensor development.Cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy are the most used techniques in electrochemical investigations.These techniques enable researchers to investigate the underlying mechanics of electrochemical reactions and collect quantitative data on their kinetics and thermodynamics.
The electrochemical properties of MoS 2 samples exfoliated in various solvents such as sample 1, sample 2, sample 3, and sample 4 were investigated using the CHI7007E electrochemical workstation.Throughout the three-electrode configuration studies, the active material (MoS 2 ) drop cast over the FTO substrate was the working electrode, a platinum wire was the counter electrode and Ag/AgCl was the reference electrode.2 M KOH was employed as the electrolyte and a known volume of ultrasonicated sample was drop cast on the FTO electrode and dried overnight to prepare the working electrode.The electrochemical performance of the produced working electrode was evaluated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and AC Impedance (EIS) methods.

Cyclic voltammetry (CV)
CV is a widely used method for assessing the electrochemical performance of various energy storage materials [47].An electric potential is applied between the reference and working electrodes of a three-electrode configuration with sonicated MoS 2 films on FTO as the working electrode, Pt-wire as the counter, and Ag/AgCl as the reference electrode.The scan rate (k) of the potential sweep is measured in mVs −1 and the potential range in which the material performs is referred to as the potential window.The electrochemical reactions involved can be characterised by monitoring the transient current during the cathodic and anodic sweeps.There are two conversions used to report oxidation and reduction in CV data, we have considered a low-to-high potential scan in which the electrochemical peak observed in the forward scan (potential swept positively) corresponds to oxidation and that in the reverse scan (potential swept negatively) indicates the reduction process.During the forward scan, the equipment draws current away from the working electrode.The knowledge of potential windows obtained from cyclic voltammetry will be employed to establish upper and lower potential in galvanostatic charge-discharge techniques [48,49].The cyclic voltammogram corresponding to that of bare-FTO is given in figure S6.
The samples were fabricated by drop casting the solvent-dispersed sample in the current collector of the FTO glass plate with an exposed area of 1 × 1 cm 2 , and the active mass loading was also estimated.The FTO glass plate was employed because of its remarkable stability,low cost, chemical inertness, electrical conductivity, and carrier mobility [50,51].The potential window was found to be 0.4 V ( −0.6 V to −1 V) by employing a basic electrolyte of KOH with a molarity of 2 M in D.I water and the scan rates used to sweep the potential were taken as 10, 20, 30, 40, 50 mVs −1 .Figures 8(A)−(D) shows the cyclic voltammogram studies of sample 1, sample 2, sample 3, and sample 4 respectively by varying the sweep rate.The area under the CV profile was found to be increasing with an increase in scan rate simultaneously having an increase in the cyclic voltametric current (Y-axis of CV), this observation can be attributed to the reduction in time with increased scan rate because the current is defined as charge per unit time and to keep the same charge value the current should increase with reduced time and the reduction in time is observed while performing the cyclic voltammetry [52].Moreover, the increased surface area under the cyclic curve is solid proof of the capacitive behaviour of the electrode.
Since the obtained CV profiles were not properly rectangular, the MoS 2 samples studied were confirmed to be pseudocapacitive in nature without any cathodic or anodic peaks.The absence of cathodic and anodic peaks in the CV signature of MoS 2 can be attributed to the depletion of active species due to faster mass transport and higher scan rates [53].However, there are two suggested mechanisms for the charge storage phenomenon in MOS 2 with KOH electrolyte, the first one is the electrochemical double layer mechanism due to the electrostatic adsorption of cations to the material surface and the second one is due to the faradaic charge transfer due to the diffusion of cations into the MoS 2 interlayer structure.Equations (3) and (4) show the reaction mechanism [54] Where (3) shows the double-layer mechanism and the (4) shows the faradaic mechanism.We found hardly any variation in the shape of the CV profile with increasing scan rates up to 50 mV s −1 , which shows that the MoS 2 electrode has a good rate property and great capacitive behaviour.Furthermore, the broadening part of CV curves shifts towards positive potential with a rise in scan rates, which could be attributed to the increase in the electrode's internal resistance and thereby the ohmic drop [55].We are going to analyse the solvent effects on the capacitance values of drop cast Mos 2 sample, to determine the specific capacitance we will employ (5).

∆ ( )
Where C s stands for the specific capacitance (F g −1 ), A for the area under the cyclic signature, m for the mass of the active material, K is the scan rate and ΔV for the potential window (V).
The specific capacitance was discovered to decrease with an increased scan rate, which can be explained by an increase in the rate of a redox reaction, resulting in a decrease in the time available for the ions to diffuse into the electrode material.As a result, the quantity of ions that can be stored in the electrode material drops, causing the overall capacitance to decrease.Another explanation for the reduction in specific capacitance is due to the charge-resistive behaviour of the electrode material [56].The specific capacitance values at varying scan rates of MoS 2 samples exfoliated in various solvents such as ethanol, EG, DMF, and DMSO are shown in table S3, in which MoS 2 exfoliated in DMF has demonstrated maximum performance, which was in correlation with our hypothesis made from surface roughness observations.The higher surface roughness observed for DMF has been attributed to the higher capacitance observed for the same, since, a rougher surface can support faradaic reaction by allowing ion intercalation and a maximum capacitance of 30.6 F g −1 been demonstrated at a scan rate of 10 mV s −1 .On the other hand, exfoliation in ethanol resulted in a minimum capacitance of 7.78 F g −1 , which can be attributed to the difficulty of the ethanol in exfoliating MoS 2 to few layers, since, the increased separation between few layers can enhance the intercalation of ions compared to the bulk [57].Moreover, particle size, electrochemical conditions, surface activation, oxygen on the surface, lattice defects caused by material preparation, and the kind and concentration of electrolyte employed during testing are all elements that might affect a material's capacitance [58].Even though, the surface roughness of ethanol, EG, and DMSO are in the comparable range, the superior electrochemical performance exhibited by DMSO can be explained by the formation of few layers of MoS 2 , which was evident from the bandgap determination using the Tauc plot.

Galvanostatic charge discharge (GCD)
Further electrochemical performance of the samples was analysed by galvanostatic charge-discharge using the chronopotentiometry technique in the workstation, and the GCD curves of the samples at various current densities are shown in figures 9(A)-(D) respectively for MoS 2 exfoliated in ethanol, EG, DMF, and DMSO.The potential window determined by cyclic voltammetry was employed as the higher and lower potential limit for the GCD technique and the active mass loading was employed to calculate various current densities, the GCD curves were taken for varying current densities of 1 A /g , 2 A /g , 3 A /g , 4 A g −1 , and 5 A g −1 with two segments (one charging and one discharging).The GCD cycling curves are nearly symmetrical, and all of them have a moderate quasi-triangle shape, indicating that the composite's contribution is augmented by its improved pseudocapacitive property and reversible Faradic redox reaction [59].
The relation given in (6) was used to compute the specific capacitance from the discharge curve of the GCD.
where Cs is the specific capacitance, I is the discharge current (A), Δt is the discharge duration, m(g) is the active material mass, and ΔV (V) is the discharging potential difference.
From the GCD curve, the specific capacitance of various MoS 2 samples was found to be 9.66 F/ g, 15 F/ g, 34 .25 F g −1 , and 22.5 F/g respectively for ethanol, EG, DMF, and DMSO at a current density of 1 A g −1 , with a maximum performance for DMF as observed in CV.The specific capacitance values observed from GCD at 1 A/ g were comparable to that obtained from the CV profile at a scan rate of 10 mV s −1 .It should be noticed that the specific capacitance obtained from GCD deteriorated with an increase in current density, which can be attributed to the insufficient ion migration into the electrode material due to the faster charging and discharging.Thus, we have obtained an inverse correlation between the discharging specific capacitance and the current density [60].We can observe an initial voltage drop in the discharging curve, which can be attributed to the internal resistance of the electrode material [54].The most essential factors of an electrochemical supercapacitor device that influence its operational efficiency and performance are energy density (ED) and power density (PD).The ED and PD values of the active material are calculated from the GCD curves the equations ( 7) and (8).
Where C S is the specific capacitance from the GCD curve, ΔV and Δt are the discharge potential and discharge time.The calculated value of energy density and power density for MoS 2 exfoliated in various solvents with a current density of 1 A/g and a potential window of 0.4 V is given in table S5.We have obtained a maximum energy density of 0.76 Wh/Kg for MoS 2 exfoliated in DMF and an outstanding power density of 266.4 KW/Kg for that exfoliated in ethanol.The superior energy density observed for DMF can be attributed to the dominance of faradaic redox reaction due to the successful formation of few layers supporting the diffusion of ions and the superior power density of ethanol can be explained by the dominance of double layers charge storage mechanism due to the moderate exfoliation of MoS 2 [61], the Ragone plot for the obtained values of ED and PD is given in figure S4.However, we have demonstrated superior energy density compared to conventional capacitors and the power density was superior to that of a battery, which confirms the ability of MoS 2 to form a supercapacitor device [62].

Electrochemical impedance spectroscopy (EIS)
To further understand the fundamental behaviour of the supercapacitor electrode, electrochemical impedance spectroscopy (EIS) was carried out in a 2 M solution of KOH within a frequency range of 0.1 Hz to 1000 K Hz having an amplitude of 5 mV with an initial potential equal to the open circuit potential corresponding to each sample, from which the interfacial electron transfer properties and the internal resistance of the electrode can be analysed.Figure 10 depicts the Nyquist plots of all four samples with curved lines in the low-frequency zone.The charge transfer resistance (Rct) at the electrode/electrolyte interface corresponds to the resistance connected to the observed semi-circle in the high-frequency zone.We were unable to observe any semi-circle curve at the lowfrequency region of ethanol, EG, and DMSO, but, for DMF we have obtained a clear semi-circle with an R ct of 47 ohms, which further confirms the superiority of redox behaviour of MoS 2 exfoliated in The equivalent series resistance corresponds to the intercept of the curves on the real axis (Z') and was found to be 1 ohm, 12. 62 ohms, 14.8 ohms, and 16.85 ohms respectively for DMF, DMSO, ethanol, and EG.Ion diffusion between the electrolyte and the electrode surface is responsible for the curved line in the low-frequency zone, while the electrochemical system operates like a capacitor at low frequencies and like a resistor at high frequencies [48].The appearance of curved lines approximately parallel to the imaginary axis highlighted the MoS 2 electrode's optimal capacitive behaviour.
In contrast to previous research (see table 1), we were able to: (1) disperse MoS 2 in various solvents; (2) modify the FTO substrate with the composite material and observe very low contact resistance; (3) the electrode's rate capability and capacitive behaviour were suggested by the electrode's consistent cyclic voltammogram shape; and (4) the suggested material gives an excellent performance for exfoliation in DMSO.We believe that the DMSO based MoS 2 has greater potential in electrochemical application that can be realised with the help of suitable additive and highly conductive current collector.

Conclusion
We have carried out a novel work by investigating the role of various solvents in altering the electrochemical performance of few layered MoS 2 for the first time to date.We have exfoliated pristine bulk MoS 2 in various solvents including ethanol, EG, DMF, and DMSO and obtained a maximum pseudocapacitive behaviour for DMF on account of its higher surface roughness facilitating intercalation of ions that supports Faradaic reactions.The structural, morphological, optical, and electrochemical characterisation of the sample is done by XRD, UV-vis, AFM, and FTIR, further, the electrochemical behaviour was analysed using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and the electrochemical impedance spectroscopy (EIS).MoS 2 was found to be supporting both double-layer capacitance and faradaic redox reactions (pseudo) with superior energy and power density compared to that of conventional capacitors and batteries.EIS spectra provided information about the variation in charge transfer resistance upon variation in solvents, from which we have confirmed the dominance of faradaic behaviour in DMF exfoliation and that of other solvents have shown supremacy in electrostatic double layer mechanism, in which ethanol is dominated due to weaker exfoliation.From an integrated point of view, DMF was shown to demonstrate maximum faradaic behaviour then comes the DMSO followed by EG and ethanol.We have arrived at this conclusion by scrutinising the bending of EIS peaks in the higher frequency region along with the help of CV profile and energy density data.

Figure 1 .
Figure 1.Schematic illustration of the dispersion and electrochemical study of MoS 2 .

Figure 2 .
Figure 2. Schematic illustration of the real-time image showing the working apparatus.

Figure 5 .
Figure 5. Image showing the AFM images corresponding to drop casted films of MoS2 exfoliated in (A) ethanol, (B) EG, (C) DMF, and (D) DMSO.

Figure 7 .
Figure 7. Schematic illustration of Raman spectra of MoS 2 samples exfoliated in EG, Ethanol, DMSO, and DMF.

Table 1 .
Table showing the comparison of present work with recent literatures.