Preparation and Electrochemical Properties of Molybdenum Disulfide Nanomaterials ^†

Abstract

As a transition metal chalcogenide, molybdenum disulfide is an important two-dimensional material. Due to its structural anisotropy, its different morphological structures impact performance. Therefore, improving existing preparation methods enhances its applications. Single-layer molybdenum disulfide is a direct bandgap semiconductor with excellent mechanical properties and chemical stability. We chose ammonium molybdate as the molybdenum source and L-cysteine as the sulfur source. By changing the pH and the reaction time in the environment, the hydrothermal method is used to synthesize the precursor and molybdenum disulfide with different morphologies to control its morphology. Electrochemical test results showed that the specific capacity of molybdenum disulfide synthesized at a current density of 0.6 A reaches 187.79 F/g at a reaction time of 24 h and a pH of 0.6. Its microstructure is in the shape of a flower ball, with a single piece size of about 50 nm and a thickness of about 5 nm. Its specific surface area reaches 36.88 m²/g, which provides enough active sites.

1. Introduction

Sustainable development has been emphasized. With economic progress, fossil fuels have been overexploited and excessively used. In the future, fossil fuels may become scarce, and the environmental problems could become worse. Therefore, new green alternative energy sources are necessary to avoid over-reliance on fossil energy, and energy storage is important.

Supercapacitors are promising energy storage devices due to their long cycle life [1], high power density [2], and fast charge and discharge [3,4]. They are widely used in hybrid vehicles and memory backup systems [5]. The morphology, porosity, and size of electrode materials influence the performance of supercapacitors. Two-dimensional metal sulfides are electrochemically active [6], making them suitable for use in supercapacitors due to their high conductivity and specific surface area. Among metal sulfides, molybdenum disulfide (MoS₂) has a graphene-like layered structure. Its particular form and unique properties attract research interest [7].

In this study, molybdenum disulfide was prepared using the hydrothermal method, and the relationship between its morphological structure and capacitance was studied by controlling the pH and reaction time.

2. Experiment

2.1. MoS₂ Nanomaterials

1 mmol amine molybdate is dissolved in 20 mL of deionized water and added 9 mL of ethylenediamine. Next, 3 N hydrochloric acid is added with magnetic stirring, and the pH of the solution is adjusted to 2. At the same time, 2.2726 g of cysteine is dissolved in 120 mL of deionized water. The solution is then mixed into two solutions. The prepared solution is transferred to a 200 mL polytetrafluoroethylene-lined reactor for reaction. The reaction temperature is set to 240 °C, and the reaction times are 6, 12, 18, and 24 h. The obtained product is washed thoroughly with deionized water and absolute ethanol, centrifuged, and dried at 60 °C for 24 h. Finally, the dried powder is heated at 700 °C for 2 h in nitrogen gas to obtain MoS₂ powder. The experimental methods corresponding to each sample number are shown in

Table 1. Reaction time and pH for samples.

Sample	LMo-1	LMo-2	LMo-3	LMo-4	LMo-5	LMo-6
Time (hr)	6	12	18	24	24	24
pH	2	2	2	2	0.6	9.5

below.

2.2. Characterization

The structure of the MoS₂ sample was characterized using an X-ray diffractometer and analyzed using a Raman spectrometer. The morphology of the MoS₂ samples was characterized using a scanning electron microscope, and the specific surface area and pore size distribution of the material were measured using a nitrogen adsorption- and desorption-specific surface area analyzer. The electrochemical performance of the samples was tested under a conventional three-electrode system using CHI6081C. The prepared samples, conductive carbon black, and binder polyvinylidene fluoride (PVDF) were mixed evenly in the mass ratio of 80:10:10, drop-cast onto nickel foam, dried at 60 °C for 12 h, and used as the working electrode. The platinum sheet was used as the auxiliary electrode, Ag/AgCl was used as the reference electrode, and 3 M KOH was used as the electrolyte. The cyclic voltammetry curve was drawn at a potential range of 0−0.5 V with scan speeds of 10, 20, 50, 100, and 200 mV/s. Galvanostatic charge–discharge tests were conducted at a current density of 0.6 A/g to explore charge and discharge performance.

3. Results and Discussion

Figure 1 shows the XRD patterns of the products prepared at different reaction times when the hydrothermal reaction temperature is 240 °C, the pH is 2, and the molar ratio of molybdenum to sulfur is 1:2.4. The crystallinity of the MoS₂ crystal is poor. When the reaction time is increased to 24 h, the diffraction peak intensity of the sample LMo-4 becomes low.

Due to the growth of the nanosheet structure on the rod, its surface area increases, while defects in the surface structure cause the intensity of the diffraction peak to decrease [8,9]. After annealing at 700 °C for 2 h in the nitrogen atmosphere, the crystallinity increases. Figure 2 shows that the diffraction peaks at 14.1°, 33.2°, 39.7°, and 58.8°, correspond to the crystal planes of (002), (100), (103), and (110), which are consistent with the standard spectrum (JCPDS No.37- 1492). They show the characteristic peaks of hexagonal MoS₂ [10].

To understand the hydrothermal synthesis process and growth mechanism of MoS₂, the changes in morphology at different reaction heating times and pH values are studied. Figure 3a–d show MoS₂ prepared at pH value = 2. When the reaction time is 6 h, irregular structures are agglomerated, and the surface shows flakes attached. As the reaction time increases to 12 h, the original irregular structure gradually develops into a spherical shape, and the agglomeration phenomenon still exists. However, the surface flake boundaries become obvious (Figure 3b). When the reaction time reaches 18 h, spherical particles form in a spherical chain-like structure (Figure 3c). At the reaction time of 24 h, a rod-like structure appears (Figure 3d). The lamellar structure grows out of the rod-like crystal.

To explore the impact of the pH value on the morphology of the obtained product MoS₂, the microstructure of the precursor MoO₃ is observed. It is a rod-shaped structure with varying lengths and a diameter of 3−10 μm, a smooth surface, and no pattern structure (Figure 3g). Figure 3e shows MoS₂ prepared at pH = 0.6, where flower-like structures are formed by the self-assembly of many lamellae. The particle size is approximately 100 nm. Figure 3f shows no flaky structure on the particle surface. The surface is smooth compared to other samples. Figure 3h shows a commercial MoS₂ powder with a platelet structure of approximately 3 μm and a thickness of 100 nm. In the reaction process, the dissolution rate of MoO₃ increases as the pH value decreases. At pH = 2, the surrounding rod-like MoO₃ is observed due to less dissolved MoO₃. The concentration of molybdenum ions is high, so MoS₂ grows from the surface layer based on MoO₃. When the pH continues to decrease, the dissolution rate of MoO₃ accelerates, and the diffused molybdenum ions react more with the sulfur ions in the solution, forming a flower-ball-like structure composed of flakes of MoS₂. In acidic conditions, H+ accumulates on the surface and edge of the MoS₂ nanostructure. Due to the H+ charge shielding effect, many combinations occur between the edge and the surface and between the surface and the surface, preventing the crystal synthesis from becoming larger to form spherical MoS₂ [11].

To confirm the composition of the material, Raman testing is performed on the synthesized material. Figure 4a shows the Raman spectrum excited at 532 nm wavelength. Figure 4b shows the vibration modes of two energy levels of MoS₂. MoS₂ prepared and the commercial MoS₂ show two characteristic peaks near 380 and 400 cm⁻¹. They correspond to the ($E_{2g}^1$) and ($A_{1g}$) energy levels of MoS₂, where $E_{2g}^1$ corresponds to the vibration mode of the two S atoms in the layer opposite to the Mo atom in the middle [12,13]. A_1 g reacts to the S atoms in a single out-of-plane vibration mode in the cell [14]. As the number of MoS₂ layers changes, the peaks corresponding to $E_{2g}^1$ and $A_{1g}$ shift relatively. As the number of MoS₂ layers decreases, the $A_{1g}$ vibration mode weakens. The peak position is red-shifted. Compared with the position of the $A_{1g}$ characteristic peak of commercial MoS₂ (412 cm⁻¹), the $A_{1g}$ characteristic peak appears at 405 cm⁻¹ and is red-shifted.

Figure 5 shows the cyclic voltammogram curves of the samples in 1 mol/L KOH solution at a scan rate of 50 mV/s and a voltage of 0−0.5 V. Usually, transition metal oxides or hydroxides undergo redox reactions. A redox peak is observed in Figure 6, indicating a Faradaic capacitance behavior. Under the same scan rate conditions, the capacitance of the sample is proportional to the scan area of the cyclic voltammetry (CV) curve. The order of the scanning area is LMo-5 > LMo-4 > LMo-3 > LMo-6 > LMo-2 > LMo-1, which shows that the spherical sample LMo-5 has the largest specific capacitance due to the high specific surface area and excellent conductivity of the flower ball structure at the same scanning rate. To study the capacitive behavior of MoS₂ with different morphologies, a galvanostatic charge and discharge test (GCD) is performed (Figure 5). Figure 6 shows the galvanostatic charge and discharge curves of MoS₂ electrodes with different morphologies at a current density of 0.6 A/g. The discharge time of LMo-5 (144 s) is the longest. According to the specific capacitance C_sp = I∆t/(m∆V), the specific capacity of the MoS₂ electrode is calculated. The capacities of samples LMo-1 to LMo-6 are 28.19, 37.12, 73.17, 120.23, 187.79, and 42.56 F/g. There is an obvious charge–discharge plateau in the charge–discharge curve, indicating that it has a Faradaic capacitance behavior, which is consistent with the test results of the cyclic voltammetry curve.

4. Conclusions

To produce MoS₂, ammonium molybdate is used as the molybdenum source, and L-cysteine is used as the sulfur source. By hydrothermal heating at 240 °C and holding for 24 h, a self-assembly of many sheet-like structures is obtained. Spherical MoS₂ particle size is about 100 nm. The capacitance of MoS₂ with different morphologies is high enough to be used as supercapacitor electrode material. When the current density is 0.6 A/g, the discharge-specific capacity of the sample LMo-5 with a flower ball structure is 187.79 F/g. The structure with a highly specific surface area has a higher capacitance and conductivity as an excellent supercapacitor electrode material.