#
Si Quantum Dots Assist Synthesized Microflower-Like Si/MoS_{2} Composites for Supercapacitors

^{1}

^{2}

^{*}

## Abstract

**:**

_{2}composites were fabricated using Si quantum dots (QDs) to assist a facile hydrothermal method. The electrochemical performance of Si/MoS

_{2}composite in symmetric and asymmetric systems was studied. Electrochemical characterization revealed that the Si/MoS

_{2}composite electrode in a three-electrode system has a high specific capacitance of 574.4 F·g

^{−1}at 5 A·g

^{−1}. Furthermore, the Si/MoS

_{2}composite electrode in a two-electrode system had the maximum energy density of 27.2 Wh·kg

^{−1}when a power density of 749.1 W·kg

^{−1}was achieved. Therefore, this investigation proves the Si/MoS

_{2}composite microflower-like structure should be a promising candidate electrode material for supercapacitors.

## 1. Introduction

_{2}[10] CoS [11], NiS [12], CuS [13], etc.) are considered as potential materials for electrodes, because of their wide range of sources, low prices, and unique physicochemical properties. As a representative transition metal sulfide, MoS

_{2}has received extensive attention in capacitor research [14]. Manuraj et al. [15] reported a specific capacitance of 972 F·g

^{−1}at 1 A·g

^{−1}for MoS

_{2}-RuO

_{2}nanocomposite. Lien et al. [16] fabricated high voltage symmetric supercapacitors of graphene/MoS

_{2}, which works at a wide operating voltage of 2.3 V and achieves the maximum energy density of 31.2 Wh kg

^{−1}. Vattikuti et al. [17] reported that the hydrothermal reaction synthesized MoS

_{2}/MoO

_{3}heterostructure electrodes have a high specific capacitance of 287.7 F·g

^{−1}at 1 A·g

^{−1}with good cycling stability. The MoS

_{2}preparation method is simple and low cost. MoS

_{2}has different morphologies, such as nanoball [18], nanoflake [19], nanoflower [20], nanofilament [21], nanoparticle [22], etc. It is found that MoS

_{2}can not only show good capacitance in the electrochemical double layer, but can also generate an extra Faraday capacitance, due to the interaction between Li

^{+}, Na

^{+}, K

^{+}, and other ions inserted into the MoS

_{2}layer. Nevertheless, the energy storage application of pure MoS

_{2}as an electrode material is still very limited, with low conductivity and easy agglomeration, leading to a relatively low capacitance of MoS

_{2}. Therefore, the current research focus is to improve the electrochemical performance and increase the capacitance of composite materials

_{2}using Si QDs to assist hydrothermal synthesis. Using the Si QDs as the center of nucleation promotes the nucleation process of MoS

_{2}, and accelerates the reaction speed. Si/MoS

_{2}composite presents a flower-like structure, with the petals crisscrossing each other. This structure makes the active substance fully contact the electrolyte, which results in the redox reaction. Moreover, to a certain extent, the structure reduces the changes in the microstructure of the material caused by the charging and discharging process. By comparing the electrochemical performance of the two kinds of electrodes (MoS

_{2}and Si/MoS

_{2}composite), the Si/MoS

_{2}composite electrode demonstrates excellent performance, including high specific capacitance and good cyclic stability. This work can arouse wide interest in the preparation of high-performance mixed-metal sulfide electrode materials by a simple method.

## 2. Materials and Methods

_{4})

_{6}Mo

_{7}O

_{24}·6H

_{2}O and 1.3049 g H

_{2}NCSNH

_{2}were diluted with 20 mL DI water. The prepared solution was mixed with 1 mL Si QD solution, and subjected to continuous magnetic stirring for 1 h. Subsequently, the reaction mixture was kept in a Teflon lined stainless steel autoclave at 180 °C for 18 h. After that, the final mixture product was filtered, washed with DI water, and dried at 60 °C in a vacuum oven to obtain the Si/MoS

_{2}composite. For better comparison, the MoS

_{2}was prepared following the same procedures, but without the Si QDs.

## 3. Results

^{−1}is assigned to the Mo–S vibration [27], the peak at 1010 cm

^{−1}corresponds to the S–O stretching [28], and the peak at 1413 cm

^{−1}is attributed to C–O stretching vibration [29]. For MoS

_{2}, the peaks observed at 1132 and 1651 cm

^{−1}are attributed to the C–H bending vibration [27] and the existence of C=O stretching vibration [30]. For Si/MoS

_{2}, the absorption peak at 1106 cm

^{-1}is ascribed to the characteristic stretching of the Si–O–Si band [31]. Figure 1b gives the XRD pattern for Si/MoS2 composite. The main diffraction peaks of MoS2 at 2θ = 33.3°, 39.2° and 48.5° correspond to (101), (103), and (105) reflections, respectively (JCPDS card no. 37-1492). The peak of Si at 2θ = 58.6° can be attributed to the (101) plane (JCPDS card no. 47-1186).

^{4+}3d

_{5/2}, Mo

^{4+}3d

_{3/2}, and Mo

^{6+}3d

_{3/2}, respectively [32]. The presence of Mo

^{6+}indicates that there may be a small amount of MoO

_{3}in the sample [33]. Moreover, there is another peak at 227.1 eV, which represents S

^{2-}2s [34]. For S 2p in Figure 2b, the peaks at 163.7 eV and 164.2 eV, observed in the pristine spectrum of S, correspond to S 2p

_{5/2}and S 2p

_{3/2}.

_{2}and Si/MoS

_{2}composite. Figure 3 shows the SEM images of MoS

_{2}and Si/MoS

_{2}composite. In Figure 3a–c, it can be seen that without Si QDs, MoS

_{2}is composed of irregular nanosheets with a diameter of 200–300 nm. Most of the nanosheets are tiled, and there are some curled-off phenomena. On the other hand, it still conforms to the rule that MoS

_{2}nanosheets tend to grow in flat under the control of MoS

_{2}. Figure 3d,f is the SEM diagram of Si/MoS

_{2}composite material. After adding a small amount of Si QDs, the Si/MoS

_{2}composite presents a microflower spherical structure composed of staggered nanosheets, which indicates that the MoS

_{2}nanosheets grow with Si QDs as nucleation centers, and finally grow into three-dimensional flower spheres. The energy dispersive X-ray (EDX) elemental maps in Figure 3g–i reveal that there are Si, Mo, and S elements in the Si/MoS

_{2}composite. It is evident that the Si QDs are uniformly distributed in Si/MoS

_{2}composite.

_{2}tends to grow tiled in the direction with the minimum binding potential energy. With the prolongation of reaction time, the MoS

_{2}nanosheets grew more and more, and were stacked. A small part of MoS

_{2}nanosheets can overcome the crimping of partial binding potential energy or the semi-upright growth. It may be caused by the dynamic force generated by the easy agglomeration of MoS

_{2}itself, which can overcome the crimp growth of certain binding potential energy, and finally form the structure in Figure 3c; when, with Si QDs, due to the small size of Si QDs, it can be used as the nucleation center in the reaction process. The generated MoS

_{2}nanosheet tends to pile up around Si QDs, overcoming the potential energy for rapid three-dimensional growth, and finally forming a micron flower ball. As a result, the structure in Figure 3f is formed with the continuation of the reaction time.

_{2.}It confirms the microflower structure further, in which many nanosheets are stacked on each other. The result is consistent with the SEM image in Figure 3f. Figure 5b is a high-resolution-TEM image of Si/MoS

_{2}, in which it is clearly observable the ordered lattice fringes with the corresponding interplanar distance of 0.61 nm, which relates to the (002) plane of the MoS

_{2}[35]. Figure 5c is the high-resolution image after the Fourier transform of the box area in Figure 5b. The crystal lattice of the sample can be observed to be 0.22 nm, which corresponds to the (100) plane of the MoS

_{2}.

_{2}and Si/MoS

_{2}composite, CV curves with a scan rate of 100 mV·s

^{−1}were given in Figure 6a,b. In Figure 6a, the integral areas of the CV curve of the Si/MoS

_{2}composite are bigger than those of MoS

_{2}, which suggests that the specific capacitance of Si/MoS

_{2}composite is bigger than the MoS

_{2}. The GCD curves of the MoS

_{2}and Si/MoS

_{2}composite at the same current density of 50 A·g

^{−1}are shown in Figure 6b. The plateaus in the GCD curves indicate that the two samples performed pseudocapacitor behaviors [36], which is consistent with the CV results. In Figure 6b, Si/MoS

_{2}composite possesses a longer discharge time than MoS

_{2,}which indicates that it has a large specific capacitance. The specific capacitance of Si/MoS

_{2}composite is higher than that of MoS

_{2}for the following three reasons: (1) after the addition of Si QDs, Si QDs as the nucleation center, play the role of supporting MoS

_{2}nanosheets, which greatly increases the ion transport channel; (2) the porous and open microflower structure of Si/MoS

_{2}composite, which can shorten the diffusion path of electrons and ions and enhance electrochemical dynamics [37]; and (3) the open space of the Si/MoS

_{2}composite microflower can act as the robust reservoir for electrolyte ions, supplying sufficient redox reactions for energy storage [38]. The CV curves for MoS

_{2}and Si/MoS

_{2}composite at different scan rates of 5–100 mV/s are shown in Figure 6c,d. With the increase of the scanning rate, the peak of current gradually increased, and the curve shape remained basically unchanged in the CV curves of MoS

_{2}and Si/MoS

_{2}composite, indicating that the two electrode materials had the potential to provide high power performance. The GCD curves of the MoS

_{2}and Si/MoS

_{2}composite at different current densities are shown in Figure 6e,f. The specific capacitances of the MoS

_{2}and Si/MoS

_{2}composite can be calculated on the basis of the GCD curves, and the results are presented in Figure 6g. The specific capacitance of Si/MoS

_{2}composite at any of the same current densities is higher than that of MoS

_{2}. With the increase of current density, the discharge time is shortened continuously, so the specific capacity is also decreased continuously. When the current density of Si/MoS

_{2}composite is at the 5 A·g

^{−1}, 10 A·g

^{−1}, 20 A·g

^{−1}, 30 A·g

^{−1}, 40 A·g

^{−1}, and 50 A·g

^{−1}, the corresponding specific capacitance of Si/MoS

_{2}composite electrode is 574.4 F·g

^{−1}, 500.0 F·g

^{−1}, 404.4 F·g

^{−1}, 284.4 F·g

^{−1}, and 233.3 F·g

^{−1}, respectively, which is consistent with the analysis results of CV curves in Figure 6c,d. Figure 6i shows cyclic stability of capacitive performance of Si/MoS

_{2}electrode tested for 1000 cycles at 1 A·g

^{−1}. The Si/MoS

_{2}composite electrode exhibits specific capacitance retention of 84.5% for 1000 cycles.

_{2}and Si/MoS

_{2}composite electrodes are shown in Figure 6h. Generally, a semicircle in the high frequency region and a straight line with slope in the low frequency region are shown. In the low frequency region, it consists of a straight line, while in the high frequency region, it consists of a semicircle. The diameter of the semicircle is related to the charge transfer resistance (Rct). The radius of the Rct from the high frequency arc is on the real axis. Additionally, the smaller the radius, the smaller the Rct. A smaller Rct for Si/MoS

_{2}and a vertical line along the virtual axis show lower resistance and better capacitive behavior [39]. The Nyquist curve of the X-ray intercepts the equivalent series resistance of said electrode (R

_{S}), which is 0.506 Ω.

_{2}composite electrode, an asymmetric capacitor (Si/MoS

_{2}//AC) was constructed, with Si/MoS

_{2}composite active material as positive electrode and AC as negative electrode, to investigate its electrochemical performance. An electrochemical workstation was used to test the asymmetric supercapacitor electrochemical properties. As shown in Figure 7a, CV curves of the Si/MoS

_{2}composite and AC separately tested in the three-electrode test system indicated that the optimal potential window of Si/MoS

_{2}//AC is 1.5 V, and a good matching for asymmetric supercapacitors. Figure 7b shows CV curves of Si/MoS

_{2}//AC at various scan rates, which show that the pseudocapacitance and double-layer capacitance are common influences on the supercapacitor. The shapes of CV curves were not deformed when the sweep speeds increased, indicating good rate performance. The C (specific capacitance (86.9 F·g

^{−1})) in the asymmetric capacitor was calculated based on the Figure 7c discharge curves when current density was at 1 A·g

^{−1}. The Ragone plot is displayed in Figure 7d. The maximum energy density of 27.2 Wh·kg

^{−1}is obtained at a power density of 749.1 Wh·kg

^{−1}, which is more than other reported systems, such as MoS

_{2}-150 mg GF//AEG (16 Wh·kg

^{−1}/758.2 W·kg

^{−1}) [6], BCN/MoS

_{2}-31 composite (8.92 Wh·kg

^{−1}/254.8 W·kg

^{−1}) [40], PANI-G-MoS

_{2}(2.65 Wh·kg

^{−1}/119.2 W·kg

^{−1}) [41], and MoS

_{2}/RCF electrode (22.5 Wh·kg

^{−1}/703 W·kg

^{−1}) [9].

## 4. Conclusions

_{2}composite was prepared by hydrothermal method. The addition of Si QDs can improve the electrochemical performance of MoS

_{2}. The capacitance of the Si/MoS

_{2}composite is 574.4 F·g

^{−1}at 5 A·g

^{−1}. When the energy density is 27.2 Wh·kg

^{−1}, the power density of the Si/MoS

_{2}//AC reaches 749.1 W·kg

^{−1}. The capacitance retention is 84.5%, even after 1000 cycles. Therefore, the Si/MoS

_{2}composite microflower will be a great potential electrode material for supercapacitors.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**FT-IR spectrum of MoS

_{2}and Si/MoS

_{2}composite electrodes (

**a**), and XRD pattern of Si/MoS

_{2}composite (

**b**).

**Figure 3.**SEM images of MoS

_{2}(

**a**–

**c**), S and Si/MoS

_{2}composite (

**d**–

**f**), and EDX elemental mapping image of Si (

**g**), Mo (

**h**), and S (

**i**) in Si/MoS2 composite.

**Figure 6.**(

**a**,

**b**) Comparison of CV and GCD curves of two electrodes; (

**c**,

**e**) CV and GCD curves of MoS

_{2}and (

**d**,

**f**) Si/MoS

_{2}composite; (

**g**) rate capacitance calculated by GCD curves; (

**h**) EIS curves of two electrodes (the inset shows the equivalent circuit model used for fitting the EIS plot); (

**i**) cyclic stability test at 1 A·g

^{−1}for Si/MoS

_{2}composite electrode.

**Figure 7.**(

**a**) CV curve of AC and Si/MoS

_{2}composite, separately tested in three-electrode system at 100 mV·s

^{−1}; (

**b**) CV curve of asymmetric supercapacitor (Si/MoS

_{2}//AC); (

**c**) GCD curve of Si/MoS

_{2}//AC; (

**d**) Ragone plots of Si/MoS

_{2}//AC.

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**MDPI and ACS Style**

Zheng, J.; Cheng, K.; Zhang, R.; Yang, Y.; Wu, Y.; Yu, P.
Si Quantum Dots Assist Synthesized Microflower-Like Si/MoS_{2} Composites for Supercapacitors. *Crystals* **2020**, *10*, 846.
https://doi.org/10.3390/cryst10090846

**AMA Style**

Zheng J, Cheng K, Zhang R, Yang Y, Wu Y, Yu P.
Si Quantum Dots Assist Synthesized Microflower-Like Si/MoS_{2} Composites for Supercapacitors. *Crystals*. 2020; 10(9):846.
https://doi.org/10.3390/cryst10090846

**Chicago/Turabian Style**

Zheng, Jiahong, Kangkang Cheng, Runmei Zhang, Yamei Yang, Yuntao Wu, and Pengfei Yu.
2020. "Si Quantum Dots Assist Synthesized Microflower-Like Si/MoS_{2} Composites for Supercapacitors" *Crystals* 10, no. 9: 846.
https://doi.org/10.3390/cryst10090846