Preparation of Few-Layered MoS2 by One-Pot Hydrothermal Method for High Supercapacitor Performance

Molybdenum disulfide (MoS2), a typical layered material, has important applications in various fields, such as optoelectronics, catalysis, electronic devices, sensors, and supercapacitors. Extensive research has been carried out on few-layered MoS2 in the field of electrochemistry due to its large specific surface area, abundant active sites and short electron transport path. However, the preparation of few-layered MoS2 is a significant challenge. This work presents a simple one-pot hydrothermal method for synthesizing few-layered MoS2. Furthermore, it investigates the exfoliation effect of different amounts of sodium borohydride (NaBH4) as a stripping agent on the layer number of MoS2. Na+ ions, as alkali metal ions, can intercalate between layers to achieve the purpose of exfoliating MoS2. Additionally, NaBH4 exhibits reducibility, which can effectively promote the formation of the metallic phase of MoS2. Few-layered MoS2, as an electrode for supercapacitor, possesses a wide potential window of 0.9 V, and a high specific capacitance of 150 F g−1 at 1 A g−1. This work provides a facile method to prepare few-layered two-dimensional materials for high electrochemical performance.


Introduction
In recent decades, it has been essential to develop a new generation of energy storage devices that are eco-friendly, sustainable, and capable of meeting the evolving energy demands [1][2][3].Supercapacitors have attracted significant attention due to their superior power density and energy density, excellent cycle performance, and fast charge-discharge characteristics [4][5][6].Supercapacitors can be divided into two categories (electrostatic double layer capacitors (EDLCs) and pseudocapacitors) based on the energy storage mechanisms [7].Regarding EDLCs, the double-layer capacitor on the surface of the electrode material is formed through the effect of electrostatic force [8].Pseudocapacitors store energy through rapid and reversible redox reactions or intercalation in the electrode material [9].Thus, the selection of electrode materials is crucial for the performance of supercapacitors.
Molybdenum disulfide (MoS 2 ) is a two-dimensional material composed of molybdenum and sulfur atoms, belonging to the family of transition metal dichalcogenides [10,11].In the structure of MoS 2 , a Mo atomic layer is sandwiched between two sulfur atomic layers, forming a two-dimensional layered structure similar to graphene [12][13][14].This layer structure imparts unique properties to MoS 2 , such as excellent electrical, optical, and mechanical properties [15,16].Within the interlayer space of molybdenum disulfide, electrolyte ions and molecules can diffuse, providing convenience for its electrochemical and catalytic applications [17][18][19].The atomic-level thickness and two-dimensional structure of MoS 2 make it an ideal choice for researching and developing advanced materials and devices [20].Specifically, the central Mo atoms exhibit oxidation states ranging from +2 to +6, creating an interlayer space that allows for the diffusion of electrolyte ions into the interlayers for Faraday reactions [21].These distinctive features play a critical role in enhancing charge storage capabilities, ultimately enabling MoS 2 to achieve an impressive theoretical specific capacitance of approximately 1000 F g −1 [22].The few-layered treatment of MoS 2 is able to expose more active sites, increase contact area with the electrolyte, and shorten the ion diffusion distance to further effectively enhance its electrochemical performance [23].However, due to the challenges in preparing few-layered MoS 2 and the high technical requirements involved, the synthesis of few-layered MoS 2 faces significant hurdles.
In this work, few-layered MoS 2 is successfully prepared by the one-pot hydrothermal method with the effect of NaBH 4 .Furthermore, the influence of different NaBH 4 dosages on few-layered MoS 2 is studied.The presence of few-layered MoS 2 after the NaBH 4 treatment is obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).MoS 2 -0.3894 exhibits the excellent supercapacitor performance, including a wide potential window of 0.9 V and a high specific capacitance of 150 g −1 at 1 A g −1 .This work provides a simple way to achieve few-layered MoS 2 , which has a significant impact on the development of two-dimensional layered materials.

Preparation of MoS 2 Nanosheets
First, 0.8225 g of sodium molybdate dihydrate (Na 2 MoO 4 •2H 2 O) and 0.7370 g of thioacetamide (C 2 H 5 NS) were dissolved in deionized water (25 mL) and stirred for 30 min.After the mixture was transferred to a Teflon-lined stainless-steel autoclave (100 mL), it was heated to 200 • C and kept for 20 h.The resulting product was washed several times with deionized water and ethanol, and then the sample was dried.Finally, the dried sample was ground into powder in a mortar to obtain MoS 2 .

Preparation of MoS 2 with Different Amounts of NaBH4
First, 0.2595, 0.3894 and 0.5192 g of sodium borohydride (NaBH 4 ) was dissolved in homogeneous mixture composed of Na 2 MoO 4 •2H 2 O and C 2 H 5 NS, and then stirred for 30 min.The rest of the operation process was the same as for the preparation of MoS 2 nanosheets.According to the various dosages of NaBH 4 , the product samples are named MoS 2 , MoS 2 -0.2595,MoS 2 -0.3894, and MoS 2 -0.5192.

Results and Discussion
As shown in Figure 1, the layered-material MoS 2 can be prepared into few-layered MoS 2 with the effect of NaBH 4 .NaBH 4 is an excellent exfoliating and reducing agent.The alkali metal Na + ions have a significant exfoliating effect during synthesis processing.The group of BH 4 − exhibits strong reducing properties.In addition, various NaBH 4 have different effects on the exfoliation of MoS 2 layers [24,25].The products are named MoS 2 , MoS 2 -0.2595,MoS 2 -0.3894, and MoS 2 -0.5192.group of BH4 − exhibits strong reducing properties.In addition, various NaBH4 have different effects on the exfoliation of MoS2 layers [24,25].The products are named MoS2, MoS2-0.2595,MoS2-0.3894, and MoS2-0.5192.The microscopic morphologies of all the samples obtained using scanning electron microscopy (SEM) are illustrated in Figure 2. As shown in the SEM images for MoS2, MoS2-0.2595,MoS2-0.3894, and MoS2-0.5192, it is clear that the nanoflowers of MoS2 gradually disintegrated and agglomerated with an increase in the NaBH4 dosage, which reflects the significant effect of NaBH4 on MoS2 morphology.In order to clearly demonstrate this result, the morphology of MoS2 treated with NaBH4 was also observed using transmission electron microscopy (TEM).As shown in Figure 3a, the nanoflowers of MoS2 are composed of a large number of nanosheets.Interestingly, for MoS2-0.2595and MoS2-0.3894, the diameters of the nanoflowers are reduced, and their thickness is noticeably decreased.However, for MoS2-0.5192, the nanosheets are much stacked, which resulted from the nanosheets of MoS2 re-agglomerating after severe few-layered exfoliation.The layered structure of MoS2 was observed using high-resolution transmission electron microscopy (HRTEM), and the characteristics of the multiple layers are shown in Figure 3e.As depicted in Figure 3f-h, MoS2-0.2595consists of approximately 5-6 layers, MoS2-0.3894contains around 3-4 layers, and MoS2-0.5192only exhibits 2 layers, indicating that NaBH4 significantly influences the exfoliation process.Alkali metal Na + ions can intercalate into the interlayers of MoS2 and achieve exfoliation of the layered materials during the hydrothermal process.The TEM results indicate that the amount of NaBH4 is crucially related to the exfoliation effect.A significant amount of NaBH4 can lead to evident fragmentation of MoS2, ultimately resulting in its re-aggregation.The microstructure of materials plays a critical role in determining its electrochemical group of BH4 − exhibits strong reducing properties.In addition, various NaBH4 have different effects on the exfoliation of MoS2 layers [24,25].The products are named MoS2, MoS2-0.2595,MoS2-0.3894, and MoS2-0.5192.The microscopic morphologies of all the samples obtained using scanning electron microscopy (SEM) are illustrated in Figure 2. As shown in the SEM images for MoS2, MoS2-0.2595,MoS2-0.3894, and MoS2-0.5192, it is clear that the nanoflowers of MoS2 gradually disintegrated and agglomerated with an increase in the NaBH4 dosage, which reflects the significant effect of NaBH4 on MoS2 morphology.In order to clearly demonstrate this result, the morphology of MoS2 treated with NaBH4 was also observed using transmission electron microscopy (TEM).As shown in Figure 3a, the nanoflowers of MoS2 are composed of a large number of nanosheets.Interestingly, for MoS2-0.2595and MoS2-0.3894, the diameters of the nanoflowers are reduced, and their thickness is noticeably decreased.However, for MoS2-0.5192, the nanosheets are much stacked, which resulted from the nanosheets of MoS2 re-agglomerating after severe few-layered exfoliation.The layered structure of MoS2 was observed using high-resolution transmission electron microscopy (HRTEM), and the characteristics of the multiple layers are shown in Figure 3e.As depicted in Figure 3f-h, MoS2-0.2595consists of approximately 5-6 layers, MoS2-0.3894contains around 3-4 layers, and MoS2-0.5192only exhibits 2 layers, indicating that NaBH4 significantly influences the exfoliation process.Alkali metal Na + ions can intercalate into the interlayers of MoS2 and achieve exfoliation of the layered materials during the hydrothermal process.The TEM results indicate that the amount of NaBH4 is crucially related to the exfoliation effect.A significant amount of NaBH4 can lead to evident fragmentation of MoS2, ultimately resulting in its re-aggregation.The microstructure of materials plays a critical role in determining its electrochemical In order to clearly demonstrate this result, the morphology of MoS 2 treated with NaBH 4 was also observed using transmission electron microscopy (TEM).As shown in Figure 3a, the nanoflowers of MoS 2 are composed of a large number of nanosheets.Interestingly, for MoS 2 -0.2595 and MoS 2 -0.3894, the diameters of the nanoflowers are reduced, and their thickness is noticeably decreased.However, for MoS 2 -0.5192, the nanosheets are much stacked, which resulted from the nanosheets of MoS 2 re-agglomerating after severe few-layered exfoliation.The layered structure of MoS 2 was observed using high-resolution transmission electron microscopy (HRTEM), and the characteristics of the multiple layers are shown in Figure 3e.As depicted in Figure 3f-h, MoS 2 -0.2595 consists of approximately 5-6 layers, MoS 2 -0.3894 contains around 3-4 layers, and MoS 2 -0.5192only exhibits 2 layers, indicating that NaBH 4 significantly influences the exfoliation process.Alkali metal Na + ions can intercalate into the interlayers of MoS 2 and achieve exfoliation of the layered materials during the hydrothermal process.The TEM results indicate that the amount of NaBH 4 is crucially related to the exfoliation effect.A significant amount of NaBH 4 can lead to evident fragmentation of MoS 2 , ultimately resulting in its re-aggregation.The microstructure of materials plays a critical role in determining its electrochemical performance.Upon exfoliation of MoS 2 , its increased interlayer spacing allows for improved ion transport and accessibility, leading to enhanced electrochemical reactions.Furthermore, the expanded specific surface area provides more active sites for electrochemical processes, ultimately boosting the material's performance in energy storage applications.Additionally, the presence of more defects can promote electrolyte penetration and enhance charge transfer kinetics, further optimizing the electrochemical properties of electrode materials.However, the re-aggregation of few-layered MoS 2 can inhibit its electrochemical performance.
performance.Upon exfoliation of MoS2, its increased interlayer spacing allows for improved ion transport and accessibility, leading to enhanced electrochemical reactions.Furthermore, the expanded specific surface area provides more active sites for electrochemical processes, ultimately boosting the material's performance in energy storage applications.Additionally, the presence of more defects can promote electrolyte penetration and enhance charge transfer kinetics, further optimizing the electrochemical properties of electrode materials.However, the re-aggregation of few-layered MoS2 can inhibit its electrochemical performance.To investigate the crystal structure of all samples, X-ray diffraction (XRD) was applied.As shown in Figure 4a, there are three characteristic peaks located at 9.3°, 32.2°, and 56.9°, which correspond to the (002), (100), and (110) planes of MoS2 (JCPDS no.37-1492), respectively [26].The interlayer spacing was about 0.94 nm after calculation using the Bragg formula [27].The wide interlayer spacing results from the intercalation of alkali metal Na + ions.On the one hand, the (002) peak of MoS2 shifts from 9.4° to 8.2°, indicating the exfoliation of MoS2 into few layers with the increase in the amount of NaBH4.On the other hand, the (002) peak intensity of MoS2 weakens after exfoliation, which resulted in defects that affected its X-ray diffraction and weaken the intensity of the characteristic peak.Large interlayer spacing and numerous defects can have a significant impact on the electrochemical performance of electrode materials.A large interlayer spacing helps to improve ion diffusion within the material, enhances electrolyte permeability, and facilitates more effective charge-discharge processes.An increased number of defects can provide additional active sites, thereby promoting ion and electron transfer, and enhancing the reactivity and electrochemical performance of the electrode [28].Therefore, the fewlayered treatment is often considered key technology for improving electrochemical performance.To investigate the crystal structure of all samples, X-ray diffraction (XRD) was applied.As shown in Figure 4a, there are three characteristic peaks located at 9.3 • , 32.2 • , and 56.9 • , which correspond to the (002), (100), and (110) planes of MoS 2 (JCPDS no.37-1492), respectively [26].The interlayer spacing was about 0.94 nm after calculation using the Bragg formula [27].The wide interlayer spacing results from the intercalation of alkali metal Na + ions.On the one hand, the (002) peak of MoS 2 shifts from 9.4 • to 8.2 • , indicating the exfoliation of MoS 2 into few layers with the increase in the amount of NaBH 4 .On the other hand, the (002) peak intensity of MoS 2 weakens after exfoliation, which resulted in defects that affected its X-ray diffraction and weaken the intensity of the characteristic peak.Large interlayer spacing and numerous defects can have a significant impact on the electrochemical performance of electrode materials.A large interlayer spacing helps to improve ion diffusion within the material, enhances electrolyte permeability, and facilitates more effective charge-discharge processes.An increased number of defects can provide additional active sites, thereby promoting ion and electron transfer, and enhancing the reactivity and electrochemical performance of the electrode [28].Therefore, the few-layered treatment is often considered key technology for improving electrochemical performance.Further phase analysis of MoS 2 , MoS 2 -0.2595,MoS 2 -0.3894, and MoS 2 -0.5192 was conducted using Raman spectroscopy, and the results are shown in Figure 4b, revealing characteristic peaks of 1T-MoS 2 and 2H-MoS 2 [29].Two typical vibration modes of 2H-MoS 2 , E 2g 1 and A 1g , can be observed at 377.0 cm −1 and 401.8 cm −1 , respectively.Interestingly, the characteristic peaks corresponding to 1T exhibited blue shifts toward the shortwave region.Peaks such as j 1 , j 2 , j 3 , and E 1g shifted, accompanied by the appearance of two new peaks, N 1 at 195.2 cm −1 and N 2 at 352.0 cm −1 .The blue shift phenomenon often coincides with the generation of new peaks, highlighting structural changes within the material.According to the Raman spectroscopy results, the sample contained part of 1T-MoS 2 .NaBH 4 provided additional electrons for the formation of 1T-MoS 2 due to its certain reduction properties during the exfoliation process of MoS 2 .1T-MoS 2 possesses high electrical conductivity, facilitating electron and ion transport to improve the response rate and charge-discharge efficiency of the electrode.
To display the supercapacitor performance of all samples, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were evaluated using the three-electrode configuration.As shown in Figure 5a, the potential windows for MoS 2 , MoS 2 -0.2595,MoS 2 -0.3894, and MoS 2 -0.5192 are at 0.7, 0.8 and 0.9 V. Furthermore, as shown in Figure S1, the potential window for MoS 2 -0.3894 is from −0.8 to 0.1 V, which implies that MoS 2 -0.3894 has the largest potential window.Expanding the potential window not only enhances the capacitance capacity, contributing to the improved energy density and power density of supercapacitors, but also boosts response rates, enhancing the electrochemical kinetics performance of electrode materials.The expanded potential window was derived from the formation of the partial metallic phase of MoS 2 during the exfoliation process, thereby enhancing the conductivity, and contributing to the properties of electrode materials.Additionally, the areas surrounded by the CV curves represent the capacitance of the electrode materials.It is clear that MoS 2 -0.3894 has the largest area of the CV curve compared to the other electrode materials, indicating the highest specific capacitance.
In order to more accurately reflect the specific capacitance, the GCD curves of each electrode are displayed in Figure 5b.There were some specific capacitances of 105.9, 148.2, 150 and 129.8 F g −1 for MoS 2 , MoS 2 -0.2595,MoS 2 -0.3894 and MoS 2 -0.5192, respectively, at a current density of 1 A g −1 .Obviously, the specific capacitance of MoS 2 was improved with the assistance of NaBH 4 .As shown in Figure 5c, the rate performances of MoS 2 , MoS 2 -0.2595,MoS 2 -0.3894, and MoS 2 -0.5192 are displayed as 50.1%, 56.9%, 60.8%, and 49.3%, respectively, with a current density from 1 to 20 A g −1 .As the amount of NaBH 4 increased, the exfoliation of few-layered MoS 2 becomes more pronounced, leading to the formation of a larger specific surface area and an increased presence of metallic phase MoS 2 , which enhanced the rate performance.However, in the case of MoS 2 -0.5192, the re-stacking of exfoliated few-layered nanosheets resulted in a reduction in the specific surface area, impeding the diffusion of ions.As depicted in Table 1, following treatment with NaBH 4 , the voltage window of MoS 2 was expanded, the specific capacitance of MoS 2 was enhanced, and the rate performance of MoS 2 was improved.These results adequately demonstrate that few-layered MoS 2 displays outstanding electrochemical performance.The EIS of each sample was measured, and the results are shown in Figure 5d.The EIS consists of high frequency and low frequency regions.In the high-frequency range, it typically represents the double-layer capacitance of the electrolyte at the electrode surface and the charge transfer process.In the low-frequency range, it usually represents the pseudocapacitance effect of the electrode material and the charge transfer process between the electrode and the electrolyte.MoS2-0.3894had the smallest radius, indicating the lowest charge transfer resistance in the high-frequency region.Furthermore, the slope of MoS2-0.3894 was the biggest in the low frequency range compared to the other electrode materials, implying its capacitive-like behavior.Therefore, this indicates that MoS2-0.3894has low equivalent series resistance (ESR) and high conductivity.
As shown in Figure S2, the CV and GCD curves of MoS2, MoS2-0.2595,MoS2-0.3894 and MoS2-0.5192are shown as a series of scan rates and current densities.It is clear that the CV curves are all rectangle-like, while the GCD curves are all triangle-like, which indicates the capacitive-like behavior of all electrodes.To determine the dynamics of each electrode, the b-value fitting of each electrode is shown.The CV curves of MoS2-0.3894electrode at various scan rates of 5, 10, 20, 30, 50, 70 and 100 mV s −1 were selected.The b- The EIS of each sample was measured, and the results are shown in Figure 5d.The EIS consists of high frequency and low frequency regions.In the high-frequency range, it typically represents the double-layer capacitance of the electrolyte at the electrode surface and the charge transfer process.In the low-frequency range, it usually represents the pseudocapacitance effect of the electrode material and the charge transfer process between the electrode and the electrolyte.MoS 2 -0.3894 had the smallest radius, indicating the lowest charge transfer resistance in the high-frequency region.Furthermore, the slope of MoS 2 -0.3894 was the biggest in the low frequency range compared to the other electrode materials, implying its capacitive-like behavior.Therefore, this indicates that MoS 2 -0.3894 has low equivalent series resistance (ESR) and high conductivity.
As shown in Figure S2, the CV and GCD curves of MoS 2 , MoS 2 -0.2595,MoS 2 -0.3894 and MoS 2 -0.5192 are shown as a series of scan rates and current densities.It is clear that the CV curves are all rectangle-like, while the GCD curves are all triangle-like, which indicates the capacitive-like behavior of all electrodes.To determine the dynamics of each electrode, the b-value fitting of each electrode is shown.The CV curves of MoS 2 -0.3894 electrode at various scan rates of 5, 10, 20, 30, 50, 70 and 100 mV s −1 were selected.The b-value fitting was calculated with the formula i(v) = av b , where i represents the current density (A g −1 ), v represents the scan rate (V s −1 ), a and b are both constant, respectively [30].When the bvalue is close to 0.5, it indicates semi-diffusion-controlled behavior, whereas a b-value close to 1 indicates capacitance-controlled behavior [31].According to Figure S3, the b-value had a range from 0.75 to 1, suggesting capacitive-like behavior for the MoS 2 -0.3894 electrode.To clarify the contribution of capacitance behavior, the formula of i(v) = k 1 v + k 2 v 1/2 was used for fitting, where k 1 v represents the contribution of capacitance behavior, while k 2 v 1/2 represents the contribution of semi-diffusion behavior [32].Figure S4 shows the fitting calculations of the capacitance contribution at different scan rates of 10, 20, 30, 50, 70, and 100 mV s −1 .Moreover, the capacitance contributions increased from 77.1% to 90.3% at scan rate of 10, 20, 30, 50, 70 and 100 mV s −1 , indicating high capacitance behavior.

Conclusions
In summary, few-layered MoS 2 was successfully prepared using the one-pot hydrothermal method with the assistance of NaBH 4 .The exfoliation effects of different dosages of NaBH 4 were also demonstrated in the results of the SEM and TEM images.Furthermore, a part of metallic phase MoS 2 was obtained in the process of exfoliation.In terms of the electrochemical performance, the optimal sample of MoS 2 -0.3894 had a wide potential window of 0.9 V, the specific capacitance 150 F g −1 at 1 A g −1 , and a high rate performance of 60.8%.NaBH 4 plays an important role in the preparation of few-layered MoS 2 .This work provides a simple and effective solution for the preparation of few-layered two-dimensional materials.

Figure 1 .
Figure 1.Schematic diagram of few-layered MoS 2 preparation.The microscopic morphologies of all the samples obtained using scanning electron microscopy (SEM) are illustrated in Figure 2. As shown in the SEM images for MoS 2 , MoS 2 -0.2595,MoS 2 -0.3894, and MoS 2 -0.5192, it is clear that the nanoflowers of MoS 2 gradually disintegrated and agglomerated with an increase in the NaBH 4 dosage, which reflects the significant effect of NaBH 4 on MoS 2 morphology.