Hydrothermal synthesis of flower-like molybdenum disulfide microspheres and their application in electrochemical supercapacitors

Three-dimensional flower-like molybdenum disulfide microspheres composed of nanosheets were prepared by a hydrothermal method using ammonium molybdate as the molybdenum source and thiourea as the sulfur source. Structural and morphological characterizations were performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrochemical properties of MoS2 electrode were studied by performing cyclic voltammetry (CV), galvanostatic charge–discharge analysis and electrochemical impedance spectroscopy (EIS). When used as an electrode material for supercapacitor, the hybrid MoS2 showed a high specific capacity of 518.7 F g−1 at a current density of 1 A g−1 and 275 F g−1 at a high discharge current density of 10 A g−1. In addition, a symmetric supercapacitor composed of MoS2 as positive and negative electrodes was prepared, which exhibited a high energy density of 12.46 W h kg−1 at a power density of 70 W kg−1 and still maintains an impressive energy density of 6.42 W h kg−1 at a large power density of 7000 W kg−1. The outstanding performance of the MoS2 electrode material indicates its great potential for applications in high-performance energy storage systems.


Introduction
Supercapacitors, also known as electrochemical capacitors, have gathered growing interest of researchers in the era of miniaturization of devices. 1,2 These present fascinating properties of higher energy density, higher power density, longer life, lower toxicity than batteries, and so on, compared with those of traditional capacitors. [3][4][5][6] According to the charge-discharge mechanisms, SCs can be divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors. 7,8 Pseudocapacitance arises from reversible faradaic reactions of redox active materials, such as transition metal oxides, hydroxides, and suldes. Among those materials, ruthenium oxide (RuO 2 ) has exhibited excellent pseudocapacitive performance, but the toxicity and high cost of RuO 2 restrict its widespread commercial application. 9,10 The low cost active material MnO 2 can also achieve a high specic capacitance; however, MnO 2 -based pseudocapacitors suffer from poor electrical conductivity and cyclic stability. 11 Application of nanometal suldes in the energy storage devices, such as fuel cells, solar energy pools, lithiumion batteries, and supercapacitors, have aroused widespread interest among researchers. At present, carbon materials (such as activated carbon), transition metal oxides (nickel oxide, etc.), and conductive polymers are oen used as electrode materials for supercapacitors. 12,13 However, the growing demand for energy storage devices has prompted researchers to develop new types of electrode materials. Therefore, the research of nanometer-scale metal sulde as the material of supercapacitor electrode has become a new eld. For example, cobalt sulde (CoS, CoS 2 ), nickel sulde (NiS, NiS 2 , Ni 3 S 2 ), molybdenum sulde (MoS 2 ), copper sulde (CuS, Cu 2 S), and vanadium sulde (VS, VS 2 ) have been used as supercapacitors electrode materials. [14][15][16] In particular, MoS 2 has aroused interest among other transition metal suldes due to its layered structure and inherent conductivity, 17 and it is considered to be a suitable replacement for graphene and carbon nanotubes in energy storage applications. In addition, molybdenum-based materials (such as MoO 3 , MoO 2 , and MoS 2 ) exhibit various valences and rich chemical properties, making them viable candidate materials for electrochemical applications. 18 MoS 2 is a transition metal sulde with a layered structure, where a metal molybdenum layer is sandwiched between two sulfur layers; the layers are connected by weak van der Waals forces and the interlayer S-Mo-S atoms are strongly covalently linked. [19][20][21] MoS 2 possesses unique physicochemical properties due to its unique atomic and electronic structure. It is mainly used in the solid lubricants, catalysts, supercapacitors and lithium-ion batteries. [22][23][24] Among these, the research on the application of MoS 2 as a supercapacitor electrode material is the most extensive. For example, Soon et al. 25 found that the MoS 2 nano-lm presented an electric double layer capacitance behavior. Ma et al. 26 reported that nano-MoS 2 intercalated in polypyrrole could improve its capacitance performance. Cao et al. 27 fabricated micro-supercapacitors using coated MoS 2 nanolms, and showed that MoS 2 has excellent electrochemical performance in aqueous electrolytes.
In particular, the structure of the electrode directly affects its electrochemical properties. Generally, the electrochemical electrode is 2-dimensional and suffers from inadequate contact with electrolyte and low surface-area-utilization efficiency. Numerous efforts have been made to design three-dimensional (3D) electrodes, such as MoS 2 /mesoporous carbon spheres. Recently, there have been some reports related to NiCo 2 S 4 and graphene oxide composites applied in supercapacitors. Krishnamoorthy et al. 15 reported 92.85 F g À1 specic capacitance of chemically prepared MoS 2 nanostructure. Huang et al. 28 reported polyaniline/MoS 2 composites as supercapacitor electrodes with the specic capacitance of 575 F g À1 .
In this paper, the morphologically regular ower-like molybdenum disulde microspheres were successfully synthesized by a hydrothermal method (Fig. 1). The as-prepared MoS 2 was directly used as a supercapacitor electrode and exhibited high specic capacitance (518.7 F g À1 at current density of 1 A g À1 ) and excellent cycling performance (88.2% retention aer 2500 cycles). In addition, a high performance symmetric supercapacitor was successfully fabricated by using MoS 2 as both positive electrode and negative electrode, which exhibited a high energy density of 12.46 W h kg À1 at power density of 70 W kg À1 .

Materials
Ammonium molybdate ((NH 4 ) 6 Mo 7 O 24 $4H 2 O) and thiourea (CH 4 N 2 S) were obtained from Tianjin Kaixin Chemical Industry Co. Ltd. All the chemical reagents were of analytical purity and used without any further purication.

Synthesis of MoS 2
In a typical process, 0.8 g of ammonium molybdate and 5.12 g thiourea were dissolved into 80 mL deionized water and stirred until the solution was clear and transparent. The solution was transferred into 100 mL PTFE-lined stainless steel autoclave and heated at 200 C for different time periods (8 h, 16 h, and 24 h). The obtained MoS 2 was ushed with water and ethanol, in sequence, and then dried at 70 C for 12 h. The MoS 2 electrode materials were denoted as MoS 2 -8, MoS 2 -16, and MoS 2 -24, according to the hydrothermal treatment time.

Material characterization
The morphology and microstructure of the samples were characterized by eld-emission scanning electron microscopy (FESEM JSM-6701F, Japan), transmission electron microscopy (TEM; JEOL, JEM-2010, Japan), and X-ray diffraction (XRD, D/ Max-2400, Japan) with Cu Ka radiation (l ¼ 1.5418Å) operating at 40 kV, 100 mA. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI 5702 spectrometer using a standard Al Ka X-ray source of 300 W and an analyser pass energy of 29.35 eV.

Electrode preparation and electrochemical characterization
The electrochemical properties of the MoS 2 nanostructures were investigated in 1 M Na 2 SO 4 solution using a threeelectrode system in an electrochemical work station (CHI660E, Shanghai). Initially, 8 mg of MoS 2 -16 was dispersed in 400 mL of 0.5 wt% Naon solution by ultrasonication to obtain a well dispersed suspension. Then, 6 mL of the suspension was drop-casted onto the pre-treated glassy carbon electrode (GCE) and le to dry at room temperature. Saturated calomel electrode, platinum wire, and a loadable glassy carbon electrode were respectively the reference, the counter, and the working electrodes. 29 Cyclic voltammetry (CV) in the range À0.3 to 0.5 V was performed at different scan rates. Galvanostatic charge-discharge curves were recorded in the potential range of À0.3 to 0.5 V at different constant current density. The cycle life tests were performed by galvanostatic charge-discharge measurements with a constant current density of 4 A g À1 for 2500 cycles. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 0.01 Hz to 100 kHz with 5 mV amplitude at current open circuit voltage.
A two-electrode symmetric supercapacitor cell was assembled to measure the device performances. MoS 2 was used as the positive electrode and negative electrode. The negative electrode was prepared by the traditional slurry coating method. The mass loading of electroactive material in symmetric supercapacitor was 0.3 mg. The specic capacitances (C m ) were calculated according to the following equations: 30-32 Fig. 1 Schematic of the MoS 2 synthesized by hydrothermal method.
where C m is the specic capacitance, I is the current of the charge-discharge, Dt (s) is the discharge time, DV is the voltage window, and m is the mass of active materials.
In the symmetrical supercapacitors, the corresponding power density (P) and energy density (E) were calculated according to the following equations. 8 Aer a 24 h long hydrothermal process, the nanosheets of MoS 2 -24 arranged regularly but too tightly, and some collapsed, resulting in a decrease or disappearance of the pore size in the material, which could degrade the electrochemical performance of the electrode material. Fig. 3 shows the TEM images for MoS 2 -16. As shown in Fig. 3a, the interconnected nanoakes consist of nano-owers. As seen in the magnied image (Fig. 3b) . 75-1539). Energydispersive X-ray (EDX) spectroscopy (Fig. 4b) demonstrates the existence of Mo and S elements. The Raman spectrum of the asprepared MoS 2 nanoowers was recorded in this study, as shown in Fig. 4c. At low wave numbers the Raman spectrum of the MoS 2 sample showed peaks at 145, 227, 283, 371 and 403 cm À1 , related to the characteristic vibrations of pure metallic phase MoS 2 . The main peak associated with Mo-Mo metallic vibration is located at 145 cm À1 . Two characteristic peaks are observed at 371 and 403 cm À1 , which correspond to the E 2g 1 and A g 1 modes of hexagonal MoS 2 , and are attributed to the out-of-plane Mo-S phonon mode and the in-plane Mo-S phonon mode, respectively. The chemical and surface states of the Mo and S elements in the as-prepared MoS 2 -16 electrodes have been investigated via X-ray photoelectron spectroscopy. The XPS survey spectrum of the MoS 2 electrodes is shown in Fig. 5a, which revealed the presence of Mo 3d, Mo 3p, S 2p, C 1s and O 1s states. 33 The C and O signals originated from the CO 2 and H 2 O impurities, as seen in many XPS analyses. The ne tted spectrum of Mo 3d is shown in Fig. 5b, which revealed the presence of two major peaks at around 228.5 and 232 eV, corresponding to the Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 states, respectively. Small peaks belonging to S 2s in the vicinity of 226 eV are also observed. 34 The ne tted spectrum of S 2p (Fig. 5c) indicated the presence of two major peaks at around 161.5 and 162.9 eV, which corresponds to the S 2p 3/2 and S 2p 1/2 states, respectively. 35 These studies conrm the formation of MoS 2 by the hydrothermal method.  Fig. 6b shows the cyclic voltammetry (CV) curves of MoS 2 -16 at different scan rates. On increasing the scanning speed from 10 mV s À1 to 100 mV s À1 , the shape of the CV curve did not change signicantly, indicating that MoS 2 -16 presented better rate performance and small polarization. 36,37 Galvanostatic chargingdischarging (GCD) technique was also applied to study the electrochemical capacitive properties of MoS 2 -8, MoS 2 -16 and MoS 2 -24 at a current density of 1 A g À1 , as shown in Fig. 6c. The longer discharge time of MoS 2 -16 electrode again conrmed its enhanced capacitance. Fig. 6d shows the galvanostatic chargedischarge curve (GCD) of MoS 2 -16 at various current densities varying from 1 to 10 A g À1 , with a potential window range from À0.3 V to 0.5 V. Based on eqn (2), for MoS 2 -16 electrode, at a discharge current of 1 A g À1 , the specic capacitance reached 518.7 F g À1 , while at a high discharge current of 10 A g À1 , the specic capacitance was as high as 275 F g À1 . Using these GCD curves, the specic capacitances of ve electrodes at various current densities were calculated and depicted in Fig. 6e. The calculated specic capacitances of MoS 2 -16 electrode were calculated to be 518.7, 415, 363.7, 335, 318.7, and 275 F g À1 at discharge current densities of 1, 2, 3, 4, 5, and 10 A g À1 , respectively, which are much higher than those for MoS 2 -8 and MoS 2 -24 at the same current densities. Table 1 compares the electrochemical performance of the MoS 2 electrode material prepared in this study with that of the MoS 2 electrode material reported in the literature. It can be seen that the electrochemical performance of the MoS 2 electrode material prepared  in this experiment is superior. The superior electrochemical behaviors of MoS 2 -16 nanoower observed in this study should be partially attributed to its ultrathin and porous features, which can offer even richer electroactive sites, and more efficient and convenient electronic transport.

Electrochemical and energy storage performance
Electrochemical impedance spectroscopy (EIS) analysis is an important tool to examine the interface resistance of electrode materials for supercapacitors. For an ideal supercapacitor, the Nyquist plot comprises a vertical line, which can be simulated by an equivalent circuit. The semicircle at high frequency region is indicative of interfacial charge transfer resistance. In the equivalent circuit, the series resistance (R) depends on electrolyte resistance and electrode electronic resistance. Nyquist plots based on the radius of the high frequency arc on the real axis are shown in Fig. 6f. Clearly, the semicircle over the high frequency range of the MoS 2 -16 electrode is smaller than that of others, indicating the smaller charge-transfer resistance. Furthermore, the slope of the line for MoS 2 -16 was larger than that of MoS 2 -8 and MoS 2 -24, implying a better capacitive behavior and a lower diffusion resistance of ions in the MoS 2 -16 electrode material. The differences in the electrochemical properties of MoS 2 material are mainly due to disparity in the material electrolyte interface properties and electrolyte ion diffusion rates during the charge-discharge processes, which are in good accordance with its abovementioned electrochemical performance.  The cyclic stability of the electrode material is very important for practical supercapacitor applications. The cycling performance of MoS 2 -16 electrode was tested by 2500 cycles of continuous galvanostatic charge/discharge at the current density of 4 A g À1 (Fig. 6g). Although the specic capacitance gradually decreases with the increase of cycle number, there is still 88.2% retention of the initial capacitance.   To further evaluate the practical application potential of MoS 2 -16 electrode, an aqueous SC was rst assembled using the MoS 2 -16 electrode as both positive electrode and negative electrode. Fig. 7a shows a series of CV curves collected at 30 mV s À1 with an operating SC voltage ranging from 0.8 to 1.6 V to obtain the best operating potential of MoS 2 -16//MoS 2 -16. Fig. 7b shows typical CV curves for the SC device corresponding to different sweep rates. With the increment of sweep rate from 20 to 120 mV s À1 , all the curves presented similar shapes, revealing the splendid high-rate charge-discharge performance of the device. [46][47][48] Fig. 7c shows the typical GCD curves of the cells at various current densities with a potential window of 0-1.4 V.
During the charge and discharge processes, the charge curve of MoS 2 -16//MoS 2 -16 (SSC) and its corresponding discharge curve are observed to be symmetrical, conrming that it has excellent electrochemical reversibility. 49 The calculated specic capacitance values based on the discharge curves are plotted in Fig. 7d . Previously reported literature also reported energy storage tests on these similar electrode materials. The as-fabricated selfcharging supercapacitor power cell (SCSPC) delivered a specic capacitance of 18.93 mF cm À2 with a specic energy of 37.90 mJ cm À2 at a specic power density of 268.91 mW cm À2 , which were obtained at a constant discharge current of 0.5 mA. 50 For the s-MoS 2 /CNS-based symmetric pseudocapacitor, the equivalent values were 108 F g À1 , 7.4 W h kg À1 and 3700 W kg À1 . 51 The MoS 2 -based wire-type solid state supercapacitors (WSCs) device delivered a specic capacitance of 119 mF cm À1 , and energy density of 8.1 nW h cm À1 . 52 Furthermore, the cycling stability of the as-fabricated SC was performed by repeating the GCD test at a current density of 1.6 A g À1 . The specic capacitance retention of MoS 2 -16//MoS 2 -16 was about 73.5% aer 1100 cycles, revealing that this symmetric supercapacitor has eminent cycling stability.

Conclusions
In summary, we have designed and successfully fabricated ower-shaped MoS 2 microspheres assembled from many nanoakes. The capacitive properties of ower-shaped molybdenum disulde microspheres as the material of the supercapacitor electrode were studied. The obtained MoS 2 -16 nanoower delivered a high specic capacitance of 518.7 F g À1 at a current density of 1 A g À1 , with capacitance retention of 88.2% aer 2500 cycles in alkaline system in a three-electrode cell. To further conrm its practicability, an symmetric supercapacitor was assembled using the MoS 2 -16 nanoower as both positive electrode and negative electrode. This supercapacitor delivered a maximum energy density of 12.46 W h kg À1 at a power density of 70 W kg À1 . Even at the highest power density of 7000 W kg À1 , the MoS 2 -16//MoS 2 -16 device still maintained an energy density of 6.42 W h kg À1 . Such outstanding capacitive behaviors imply the MoS 2 -16 nanoower as a promising material for energy storage devices.

Conflicts of interest
There are no conicts to declare.