Si Quantum Dots Assist Synthesized Microﬂower-Like Si / MoS 2 Composites for Supercapacitors

: The microﬂower-like Si / MoS 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. 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.


Introduction
The progress of science and technology has brought about rapid changes to human life, but has also brought about irreversible problems, such as global warming, the near exhaustion of non-renewable energy, and environmental pollution. Humans are eager to seek new, green energy to alleviate the energy crisis and environmental crisis, and even replace the traditional non-renewable energy [1][2][3]. Clean energy storage devices have been widely researched, due to their wider and wider application in numerous electronic devices. As a new type of clean energy storage device, supercapacitors have high power density, good cyclic stability, fast charge, and high rate of discharge [4][5][6][7][8].
Nowadays, two-dimensional (2D) materials have shown many unique advantages. Firstly, because electrons are confined to the 2D plane, especially for monolayer 2D materials, their electronic properties are enhanced. Therefore, 2D materials are ideal materials for basic research in condensed matter physics and electronic/optoelectronic devices. Secondly, because 2D materials can maintain the thickness of atoms while possessing the maximum plane size, they have a large specific surface area. This property has greatly attracted research into applications concerning surface area, such as catalysis and supercapacitors. Thirdly, ultra-thin 2D nanomaterials based on liquid phase processing can be prepared by simple methods into a single high-quality film, which is very necessary for practical applications, such as supercapacitors and solar cells.
2D transition metal disulfides (TMDs) have attracted widespread attention, due to their special layer structure. They have better performance of conductivity and larger surface area than oxide, the metal element and sulfur element through the weak van der Waals force of interaction between a

Materials and Methods
A total of 0.1 g of sodium ascorbate was dissolved in 10 mL deionized (DI) water while stirring for 30 min, then, a solution could be obtained. Following this, 3.75 mL of sodium ascorbate solution, 1 mL of N-aminoethyl-γ-aminopropyltrimethoxysilane, and 12 mL DI water were mixed with magnetic stirring for 20 min to obtain a Si quantum dot solution.
Firstly, 0.7062 g (NH 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.
An electrochemical station (CHI 660E, Chenhua, Shanghai, China) was taken to test electrochemical performance in 6 M KOH. The sample was used as the working electrode, with a platinum foil (2 cm × 2 cm) and a saturated calomel electrode as the counter and the reference electrode, respectively. The specific capacitance of the single electrode was calculated using the equation: (1) where C is the specific capacitance, i is the discharge current, ∆t is the discharge time, S and m are assigned as the area and mass of the electrode material, and ∆V represents the potential window.

Results
Fourier transform infrared (FT-IR) spectroscopy (TENSOR II, Bruker, Karlsruhe, Germany) was used to investigate the bonding interaction and functional groups of samples [26]. As shown in Figure 1a, the peak appearing at 590 cm −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  1 mL of N-aminoethyl-γ-aminopropyltrimethoxysilane, and 12 mL DI water were mixed with magnetic stirring for 20 min to obtain a Si quantum dot solution.
Firstly, 0.7062 g (NH4)6Mo7O24·6H2O and 1.3049 g H2NCSNH2 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/MoS2 composite. For better comparison, the MoS2 was prepared following the same procedures, but without the Si QDs.
An electrochemical station (CHI 660E, Chenhua, Shanghai, China) was taken to test electrochemical performance in 6 M KOH. The sample was used as the working electrode, with a platinum foil (2 cm × 2 cm) and a saturated calomel electrode as the counter and the reference electrode, respectively. The specific capacitance of the single electrode was calculated using the equation: where C is the specific capacitance, i is the discharge current, Δt is the discharge time, S and m are assigned as the area and mass of the electrode material, and ΔV represents the potential window. Fourier transform infrared (FT-IR) spectroscopy (TENSOR II, Bruker, Karlsruhe, Germany) was used to investigate the bonding interaction and functional groups of samples [26]. As shown in Figure  1a, the peak appearing at 590 cm −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 MoS2, the peaks observed at 1132 and 1651 cm −1 are attributed to the C-H bending X-ray photoelectron spectroscopy (XPS, ESCA-LAB Mk II, VG Scientific Co., London, England) was used to analyze the elemental valence state and surface chemical composition. Figure 2 shows the XPS high-resolution spectra of Mo 3d and S 2p. The peaks at 230.3 eV, 233.5 eV, and 236.3 eV, observed in the pristine spectrum of Mo in Figure 2a, are identified as Mo 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 . X-ray photoelectron spectroscopy (XPS, ESCA-LAB Mk II, VG Scientific Co., London, England) was used to analyze the elemental valence state and surface chemical composition. Figure 2 shows the XPS high-resolution spectra of Mo 3d and S 2p. The peaks at 230.3 eV, 233.5 eV, and 236.3 eV, observed in the pristine spectrum of Mo in Figure 2a, are identified as Mo 4+ 3d5/2, Mo 4+ 3d3/2, and Mo 6+ 3d3/2, respectively [32]. The presence of Mo 6+ indicates that there may be a small amount of MoO3 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 2p5/2 and S 2p3/2.  In order to study the effect of Si QDs on the microstructure of the whole composite, field emission scanning electron microscope (FESEM, S-4800, Hitachi, Tokyo, Japan) analysis was used for MoS 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.

Results
A possible growth mechanism is depicted in the schematic diagram in Figure 4. Without Si QDs, MoS 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. the XPS high-resolution spectra of Mo 3d and S 2p. The peaks at 230.3 eV, 233.5 eV, and 236.3 eV, observed in the pristine spectrum of Mo in Figure 2a, are identified as Mo 4+ 3d5/2, Mo 4+ 3d3/2, and Mo 6+ 3d3/2, respectively [32]. The presence of Mo 6+ indicates that there may be a small amount of MoO3 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 2p5/2 and S 2p3/2. In order to study the effect of Si QDs on the microstructure of the whole composite, field emission scanning electron microscope (FESEM, S-4800, Hitachi, Tokyo, Japan) analysis was used for MoS2 and Si/MoS2 composite. Figure 3 shows the SEM images of MoS2 and Si/MoS2 composite. In  A possible growth mechanism is depicted in the schematic diagram in Figure 4. Without Si QDs, MoS2 tends to grow tiled in the direction with the minimum binding potential energy. With the prolongation of reaction time, the MoS2 nanosheets grew more and more, and were stacked. A small part of MoS2 nanosheets can overcome the crimping of partial binding potential energy or the semiupright growth. It may be caused by the dynamic force generated by the easy agglomeration of MoS2 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 MoS2 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.   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 .
MoS2 tends to grow tiled in the direction with the minimum binding potential energy. With the prolongation of reaction time, the MoS2 nanosheets grew more and more, and were stacked. A small part of MoS2 nanosheets can overcome the crimping of partial binding potential energy or the semiupright growth. It may be caused by the dynamic force generated by the easy agglomeration of MoS2 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 MoS2 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.   In order to compare the electrochemical properties of the MoS 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.
Nyquist plots of the MoS 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.
Crystals 2020, 13, x FOR PEER REVIEW 6 of 11 corresponding interplanar distance of 0.61 nm, which relates to the (002) plane of the MoS2 [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 MoS2. In order to compare the electrochemical properties of the MoS2 and Si/MoS2 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/MoS2 composite are bigger than those of MoS2, which suggests that the specific capacitance of Si/MoS2 composite is bigger than the MoS2. The GCD curves of the MoS2 and Si/MoS2 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/MoS2 composite possesses a longer discharge time than MoS2, which indicates that it has a large specific capacitance. The specific capacitance of Si/MoS2 composite  In order to evaluate the potential practical application of the Si/MoS 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. virtual axis show lower resistance and better capacitive behavior [39]. The Nyquist curve of the Xray intercepts the equivalent series resistance of said electrode (RS), which is 0.506 Ω. In order to evaluate the potential practical application of the Si/MoS2 composite electrode, an asymmetric capacitor (Si/MoS2//AC) was constructed, with Si/MoS2 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/MoS2 composite and AC separately tested in the three-electrode test system indicated that the optimal potential window of Si/MoS2//AC is 1.5 V, and a good matching for asymmetric supercapacitors. Figure 7b shows CV curves of Si/MoS2//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

Conclusions
In this work, microflower-like structure Si/MoS 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