Yolk–Shell Polystyrene@Microporous Organic Network: A Smart

Nov 7, 2017 - MoS2/C-58 showed the optimized performance, which can be attributed to its high surface area, the microporosity of materials, and an opt...
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Article Cite This: ACS Omega 2017, 2, 7658-7665

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Yolk−Shell Polystyrene@Microporous Organic Network: A Smart Template with Thermally Disassemblable Yolk To Engineer Hollow MoS2/C Composites for High-Performance Supercapacitors Hyunjae Lee,† Jaewon Choi,† Yoon Myung,‡ Sang Moon Lee,§ Hae Jin Kim,§ Yoon-Joo Ko,∥ MinHo Yang,*,⊥ and Seung Uk Son*,† †

Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, Korea § Division of Electron Microscopic Research, Korea Basic Science Institute, Daejeon 34133, Korea ∥ Laboratory of Nuclear Magnetic Resonance, The National Center for Inter-University Research Facilities (NCIRF), Seoul National University, Seoul 08826, Korea ⊥ Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States ‡

S Supporting Information *

ABSTRACT: Yolk−shell-type polystyrene@microporous organic network (Y-PS@MON) materials were prepared by the Sonogashira coupling of tetra(4-ethynylphenyl)methane and 1,4-diiodobenzene on the surface of PS@SiO2 and by the etching of SiO2. The diameter of PS yolk spheres and the thickness of MON shells were 150 and ∼10 nm, respectively. The thickness of the void space between the PS yolk and the MON shell was ∼30 nm. Y-PS@MONs were used as templates for the synthesis of MoS2/C composite materials. Because of the microporosity of the MON shells and the void space between the yolk and the shell, MoS2 precursor compounds were efficiently incorporated into Y-PS@MONs. The heat treatment under argon resulted in the formation of hollow MoS2/C composites. The contents of MoS2 in the composites were systematically controlled by changing the amounts of precursor. MoS2/C with 58 wt % of MoS2 showed the best energy storage performance with a capacitance of 418 F/g at a 0.5 A/ g current density as an electrode material of a coin cell supercapacitor, which is attributable to its hollow structure, high surface area, and the good distribution of the sliced MoS2 in the carbon matrix. Also, the MoS2/C-58 composite showed excellent retention of capacitances during 5000 cycles.

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INTRODUCTION Template synthesis is one of the efficient methods to prepare tailored functional materials.1,2 The performance of the functional materials is critically dependent on their morphologies and structures. For example, hollow materials have been extensively engineered1,2 and applied as electrical energy storage materials.2 The thin thickness of shells can maximize the utilization of active materials and can induce fast energy storage kinetics because of the facile diffusion of electrolyte solutions. The morphology and structure of functional materials can be controlled through the engineering of template materials. Inorganic materials such as silica have been extensively engineered and used as templates.3−5 Compared to inorganic materials, the engineering of organic templates has been relatively less explored;6−9 especially, porous organic templates are rare,6,7 whereas nonporous polymer beads have been used as templates.8,9 The porosity of organic templates is beneficial to the incorporation of precursors into the templates. © 2017 American Chemical Society

Moreover, the organic templates can be converted to carbon materials through heating under inert gases.10,11 Thus, porous organic templates can be versatile for the synthesis of inorganic material/carbon composites which have been applied as energy storage materials in supercapacitors or batteries. Microporous organic networks (MONs) have been synthesized by various synthetic approaches.12−17 On the basis of their microporosities and high surface areas, MONs have been applied for various purposes, including as adsorbents18 and catalysts.19 The beneficial properties of MONs make them promising materials as templates.6,7 However, the engineering of efficient templates based on MON chemistry was less explored.6,7 For example, the void space in yolk−shell-type materials can be utilized for the incorporation of precursors to tailor functional materials.20 Although yolk−shell-type inorReceived: September 25, 2017 Accepted: October 27, 2017 Published: November 7, 2017 7658

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ganic materials have been engineered,20 as far as we are aware, organic ones with microporous shells have not been reported. During our exploration on template synthesis using polystyrene (PS) beads, we found that PS gradually disappeared through heat treatment even under argon. Similar results were reported in the literature.21,22 By contrast, conventional MONs were converted to carbon materials under the same conditions. Thus, we have designed a synthesis scheme for yolk−shell-type PS@ MON (Y-PS@MON) materials to engineer hollow inorganic material/carbon composites such as hollow MoS2/C materials. MoS2 materials with a unique layered structural motif have been applied as energy-related materials including electrocatalysts and electrode materials.23 Recently, the pseudocapacitive property of MoS2,24−32 which is comparable to that of expensive RuO2,33 has attracted significant attention from scientists. Mo species on the surface of MoS2 plate join in energy storage events through changing oxidation states in a range of +2 to +6. Although MoS2 alone showed poor performance in supercapacitors because of its intrinsic low conductivity and facile stacking behavior, its carbon composites showed synergistic enhancement of electrochemical performance for supercapacitors, compared to carbon- or MoS2-only materials.34−42 It can be reasoned that MoS2 with a high theoretical capacitance (>1000 F/g) 37,38 enhances the capacitance and carbon enhances the conductivity and cycling stability of composites. To achieve the high performance of MoS2/C composites, the sliced MoS2 with a few layers should be well-distributed in the carbon matrix. Moreover, a hollow structure is beneficial to the utilization of active sites. Although there have been efforts to engineer efficient MoS2/C composites,34−42 more exploration is required to improve capacitances and stabilities. To achieve this, the development of efficient organic templates is required. In this work, we report the preparation of Y-PS@MONs, their application as templates for the engineering of MoS2/C composites, and electrochemical performances as energy storage materials in coin cell supercapacitors.

Figure 1. (a) Synthesis scheme for Y-PS@MON, control hollow carbon material, and MoS2/C composites and (b) thermogravimetric analysis (TGA) curves of PS and Y-PS@MON.

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RESULTS AND DISCUSSION Figure 1a,b shows the synthesis scheme for Y-PS@MON and hollow MoS2/C composites and the thermal decomposition properties of PS yolk and Y-PS@MON. Nanosized PS spheres with an average diameter of 150 nm were prepared by radical polymerization in the presence of a polyvinylpyrrolidone (PVP) surfactant (Figure 2a). The PS spheres were coated with silica layers through the hydrolysis of tetraethylorthosilicate (TEOS) in the presence of an ammonia catalyst. The silica layers showed an undulated surface and a thickness of ∼30 nm (Figure 2b). For the successful coating of silica on the surface of PS, the use of 2,2′-azobis(2methylpropionamidine)·2HCl (AIBA) as an initiator was critical because the surface of PS spheres generated by AIBA has positive charges, matching well with the negative characteristics of silica precursors.43 The MON networks were formed on the surface of PS@SiO2 through the Sonogashira coupling of tetra(4-ethynylphenyl)methane44 with 1,4-diiodobenzene. The thickness of the MON layers on PS@SiO2 was ∼10 nm (Figure 2c). The silica layer in PS@SiO2@MON was etched by the treatment of HF solution to form yolk−shell-type Y-PS@MON materials (Figure 2d). The materials were investigated by TEM. As shown in Figure 2d, the void space between the PS yolk and the MON shell was clearly observed. The thickness of the void

Figure 2. Transmission electron microscopy (TEM) images of (a) PS spheres, (b) PS@SiO2, (c) PS@SiO2@MON, (d) Y−PS@MON, (e) PS@(NH4)2MoS4@MON, and (f) MoS2/C-68.

space layers was ∼30 nm, replicating the domains of silica layers (Figure S1 in the Supporting Information). 7659

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and 45 ppm were assigned to alkyl moieties. The aromatic 13C peaks appeared at 126 and 145 ppm. The additional small 13C peak at 165 ppm (indicated by asterisks in Figure 3c) is attributable to the initiator which exists at the end of polymer chains. Y-PS@MON showed clear 13C peaks of MON shells. The 13C peak of benzyl carbon of tetra(4-ethynylphenyl)methane building blocks appeared at 64 ppm. The 13C peaks of internal alkyne of MON shells were observed at 85−95 ppm, matching well with the spectra in the literature.49 The aromatic 13 C peaks of MON shells overlapped with those of PS yolks. These observations support the expected chemical structure of Y-PS@MON materials. The contents of MoS2 in the MoS2/C composites were controlled by screening the amount of precursors and analyzed by combustion elemental analysis. We increased the amount of (NH4)2MoS4 with a fixed quantity of Y-PS@MON (50 mg) from 57 mg to 86, 114, and 142 mg to form MoS2/C with 49, 58, 68, and 84 wt % MoS2, respectively. The corresponding composites were denoted as MoS2/C-49, MoS2/C-58, MoS2/ C-68, and MoS2/C-84, respectively. As the contents of MoS2 increased, the overall contrast of the materials in the TEM images became darker (Figures 2f and 4a−d). In a control experiment, the heat treatment of Y-PS@MON at 700 °C for 3 h under argon without the loading of precursor compounds resulted in the formation of a hollow carbon material with a surface area of 817 m2/g (Figures 4f,g and S3 in

Considering the microporosity of the MON shells and the void space in Y-PS@MONs, we incorporated a MoS2 precursor compound (NH4)2MoS446 into Y-PS@MONs. As shown in Figure 2e, the precursors were added to the void space and micropores of the MON shells. The heat treatment of PS@(NH 4 ) 2 MoS 4 @MON under argon resulted in the formation of hollow MoS2/C composites (Figure 2f). The diameter and the shell thickness of the hollow MoS2/C were 235 and 30 nm, respectively. The physical and chemical properties of the materials were further characterized by the analysis of N2 isotherm curves, powder X-ray diffraction studies (PXRD), and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy (Figure 3).

Figure 3. (a) N2 adsorption−desorption isotherm curves and (b) pore size distribution diagrams [based on the density functional theory (DFT) method] of Y-PS@MON, PS@(NH4)2MoS4@MON, and MoS2/C-68. (c) Solid-state 13C spectra of PS and Y-PS@MON.

The surface areas of PS and PS@SiO2 were measured as 33 and 37 m2/g, respectively. Through MON coating, the surface area of PS@SiO2@MON increased to 250 m2/g. After SiO2 etching from PS@SiO2@MON, the surface area of Y-PS@ MON further increased to 300 m2/g (Figure 3a). The micropores and mesopores of Y-PS@MON were clearly observed in the pore size analysis based on the DFT method (Figure 3b). The micropores and mesopores disappeared in PS@(NH4)2MoS4@MON, and the surface area dropped to 7 m2/g, indicating the successful incorporation of (NH4)2MoS4 into Y-PS@MONs. The surface area of MoS2/C-68 (with 68 wt % MoS2) was measured as 23 m2/g. The PXRD study showed the amorphous character of Y-PS@MON, matching with the conventional properties of MON materials prepared by the Sonogashira coupling of organic building blocks in the literature46−48 (Figure S2 in the Supporting Information). Solid-state 13C NMR spectroscopy on Y-PS@MON showed the existence of PS yolk and MON shells (Figure 3c). In the control 13C NMR spectrum of PS particles, the 13C peaks at 40

Figure 4. TEM images of (a) MoS2/C-49, (b,d) MoS2/C-58, and (c) MoS2/C-84, and (e) high-resolution (HR) TEM image of MoS2/C-58. See Figure 2f for the TEM image of MoS2/C-68. (f) Normal and (g) TEM images of microtomed control hollow carbon materials obtained through the heat treatment of Y-PS@MON. 7660

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14.1, 33.1, 39.8, and 58.9°, corresponding to the (002), (100), (103), and (110) diffraction peaks of known MoS2 (JCPDS# 37-1492). As the contents of MoS2 increased, the (002) peaks became sharper because of an increased number of layers (Figures 5d and S5 in the Supporting Information). Considering the sliced and well-distributed MoS2 material in the hollow MoS2/C composites, we studied the electrochemical performance of the MoS2/C composites as energy storage materials in symmetrical coin cell (CR2032-type) supercapacitors. (Refer to Experimental Section for detailed cell fabrication and the inset of Figure 6f.) The results of electrochemical studies are summarized in Figure 6. We conducted cyclic voltammetry (CV) and galvanostatic charge/discharge cycles of symmetrical coin cell supercapacitors in 0.5 M aqueous Na2SO4 electrolyte in a voltage range of −0.5 to +0.5 V. As shown in Figures 6a,b and S7 in the Supporting Information, MoS2/C-58 showed the best performance among the MoS2/C composites, with a maximum capacitance of 418 F/g at a current density of 0.5 A/g. MoS2/C-84 showed very poor capacitances (13 F/g at a current density of 0.5 A/g) because of the overloaded MoS2 in the composites and resultant poor conductivity, matching with the conventional results of MoS2-only materials in the literature.24−32,34−42 Compared to MoS2/C-84, MoS2/C-68 showed increased capacitances (304 F/g at a current density of 0.5 A/g). However, it showed poor rate performance at high current densities. MoS2/C-58 showed the optimized performance, which can be attributed to its high surface area, the microporosity of materials, and an optimal distribution of MoS2 in the composites. When the content of MoS2 was further reduced to MoS2/C-49, the capacitance decreased to 320 F/g at a current density of 0.5 A/g, compared to MoS2/C-58. In the CV curves of MoS2/C, the reversible redox peaks corresponding to the redox contribution of MoS2 were observed in the range −0.2 to +0.2 V besides the typical electrostatic doublelayer capacitive behavior of carbon materials (Figure 6a). In comparison, the control hollow carbon material without MoS2 showed conventional rectangular CV curves and a capacitance of 135 F/g at a current density of 0.5 A/g (Figure 6a,b). MoS 2 /C-58 showed a good rate performance with capacitances of 418, 333, 290, 262, 250, 239, and 235 F/g at current densities of 0.5, 1, 2, 4, 6, 8, and 10 A/g, respectively (Figure 6b). When the scan rates increased from 10 to 100 mV/s, the CV curves of MoS2/C-58 retained the mixed contribution of pseudocapacitive property of MoS2 and the typical double-layer capacitive behavior of carbons (Figure 6c). The charge−discharge curves of MoS2/C-58 showed slight curvatures, matching with the redox behaviors observed in its CV curves (Figures 6d and S7 in the Supporting Information). Electrochemical impedance spectroscopy and Nyquist plots of the MoS2/C composites and hollow carbon materials showed that as the contents of carbon increased, the charge-transfer resistances increased gradually from 20 Ω (hollow carbon material) to 23 (MoS2/C-49), 37 (MoS2/C-58), 65 (MoS2/C68), and 325 Ω (MoS2/C-84) (Figure 6e). It has been reported that the pseudocapacitive materials including MoS2 often suffer from poor cycling stability and thus should be assisted by secondary composite materials.24−32,34−42 Figure 6f shows the cycling performance of MoS2/C-58, revealing the gradual increase of specific capacitances from 300 to 384 F/g at a current density of 2 A/g after 5000 cycles. The Nyquist plots of MoS2/C-58 before and after 5000 cycles showed charge-transfer resistances of 37 and 29 Ω, respectively

the Supporting Information). The inner PS yolks did not maintain their original spherical morphology when heating at 700 °C but were disassembled, matching well with the known properties of PS materials in the literature.21,22 (Refer to the TGA curves of PS and Y-PS@MON in Figure 1b.) Thus, the heating of PS@(NH4)2MoS4@MON at 700 °C resulted in hollow MoS2/C composites. The hollow structure of MoS2/C58 was further confirmed by microtoming studies, indicating the inner void space of the MoS2/C materials (Figure S4 in the Supporting Information). HRTEM studies showed the unique layered structure of MoS2 with an ∼0.65 nm interlayer distance (Figures 4e and S5 in the Supporting Information). The disordered MoS2 domains with 3−5 layers were distributed over the MoS2/C-58 materials (Figure 4e). The analysis of N2 sorption isotherm curves showed that as the MoS2 contents in MoS2/C increased, the surface areas of MoS2/C-49, MoS2/C-58, MoS2/C-68, and MoS2/C-84 decreased from 330 m2/g to 304, 23, and 14 m2/g, respectively (Figure 5a,b). Interestingly, while MoS2/C-49 and MoS2/C-58

Figure 5. (a) N2 adsorption−desorption isotherm curves obtained at 77 K, (b) pore size distribution diagrams (based on the DFT method), (c) X-ray photoelectron spectroscopy (XPS) spectra, and (d) PXRD patterns of the MoS2/C composites.

significantly maintained micro- and mesoporosities, the degree of porosity decreased in the cases of MoS2/C-68 and MoS2/C84 because of the increased amount of MoS2. The electronic surroundings of Mo and S in the MoS2/C composites were characterized by XPS. As shown in Figure 5c, the 3d5/2 and 3d3/2 orbital peaks of Mo appeared at 228.8 and 232.1 eV, respectively. The 2p3/2 and 2p1/2 orbital peaks of S were observed at 161.7 and 162.9 eV, respectively. These observations match well with those of MoS2 in the literature.23 According to Raman spectroscopy, as the amount of MoS2 in the MoS2/C composites increased, ID/IG ratios increased from 0.769 (MoS2/C-49) to 0.817 (MoS2/C-58), 0.824 (MoS2/C68), and 0.838 (MoS2/C-84) (Figure S6 in the Supporting Information). The PXRD studies showed main peaks at 2θ of 7661

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Figure 6. Electrochemical performance of symmetrical coin cell (CR2032) supercapacitors of MoS2/C composites and model carbon hollow material. (a) Cyclic voltammograms (scan rate: 50 mV/s), (b) rate performance, and (e) Nyquist plots of the MoS2/C electrodes and model hollow carbon material. (c) CV curves at different scan rates, (d) charge−discharge profiles at different current densities, and (f) cycling performance (current density: 2 A/g) of the MoS2/C-58 electrode. Refer to Figure S7 in the Supporting Information for the charge−discharge profiles of the MoS2/C electrodes. Photograph of the coin cell supercapacitor of the MoS2/C-58 electrode and Nyquist plots of the supercapacitor before and after 5000 cycles are given in the inset of (f).

showed a capacitance of 333 F/g at a current density of 1 A/g. (Refer to Table S1 in the Supporting Information.) The good electrochemical performance of MoS2/C-58 can be ascribed to the following points: First, the sliced MoS2 materials with 3−5 layers were distributed over the carbon materials to maximize the utilization of active sites. Second, MoS2/C in our work has a hollow structure to induce efficient electrochemical events over materials. Third, MoS2/C-58 maintained the high surface area and microporosity because of the harmonized relative amounts of MoS2 and carbon. Fourth, the pseudocapacitive property of MoS2 and the doublelayer capacitive behavior of carbon materials co-contributed to the energy storage events, in addition to the stabilization effect of carbon materials toward the redox behavior of MoS2.

(inset of Figure 6f). The gradual increase of the specific capacitance of MoS2/C-58 during cycling is attributable to the increased conductivity of materials, which is a common observation in the performance of supercapacitors based on the MoS2/C composite materials24−32,34−42 (Figure S8 in the Supporting Information). According to scanning electron microscopy (SEM) and TEM analyses, MoS2/C-58 retrieved after the cycling test showed complete retention of its original hollow morphology (Figure S9 in the Supporting Information). The capacitance, rate performance, and cycling stability of MoS2/C-58 are superior or comparable to those of the recent MoS2/C composite materials.35−42 In our surveys, the capacitances of MoS2/C in the recent literature35−42 were mostly reported in a range of 189−276 F/g at a current density of 1 A/g. Recently, Ji et al. reported a capacitance of 472 F/g at a current density of 1 A/g.38 In comparison, MoS2/C-58 7662

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Synthesis Procedure for Y-PS@MON. PS@SiO2 (0.20 g), (PPh3)2PdCl2 (8.4 mg, 0.012 mmol), and CuI (2.3 mg, 0.012 mmol) were added to a flame-dried 50 mL Schlenk flask. After triethylamine (TEA, 30 mL) was added, the solution was sonicated at room temperature for 2 h. After tetra(4ethynylphenyl)methane44 (48 mg, 0.12 mmol) and 1,4diiodobenzene (79 mg, 0.24 mmol) in TEA (10 mL) were added, the reaction mixture was heated at 90 °C with stirring for 48 h. After the solution being cooled to room temperature, the resultant PS@SiO2@MON particles were retrieved by centrifugation, washed with methanol, methylene chloride, and acetone, and dried under vacuum. The obtained PS@SiO2@ MON (0.35 g) was added to a 50 mL Falcon tube. After the diluted HF solution [25 mL, a mixture of 48−51% HF solution (5 mL), water (10 mL), and methanol (10 mL)] was added, the reaction mixture was stirred at room temperature for 1 h. Caution: HF solution is extremely toxic and should be very carefully handled with special gloves. The resultant Y-PS@ MON particles were retrieved by centrifugation, washed with water, methanol, and acetone, and dried under vacuum. Synthesis Procedure for the MoS2/C Composites. A MoS2 precursor (NH4)2MoS4 was prepared by the synthesis procedures in the literature.45 In our work, the following procedures were applied. After Na2MoO4·2H2O (5.0 g, 21 mmol), ammonia solution (28−30%, 30 mL), and water (10 mL) were added to a 100 mL flask, the reaction mixture was stirred for 30 min. FeS (20 g, 0.23 mol) was added to a second flask. In a third flask, CuSO4·5H2O (10 g, 40 mmol) was dissolved in water (150 mL). The outlet of the flask with FeS was connected to the flask with Na2MoO4 which was connected to the flask with CuSO4. HCl (1 M) was slowly added to the flask with FeS to generate H2S gas. The temperature of the Na2MoO4 solution was gradually increased to 60 °C. After the reaction mixture was stirred for 3 h, it was cooled to 0 °C using an ice bath, inducing the growth of crystals. The crystals of (NH4)2MoS4 were collected, washed with ether, and dried under vacuum. For the preparation of MoS2/C-68, Y-PS@MON (50 mg) and methanol (15 mL) were added to a 50 mL Schlenk flask. The reaction mixture was refluxed for 1 h. After the solution being cooled to room temperature, (NH4)2MoS4 (114 mg, 0.438 mmol) in methanol (5 mL) was added. The reaction mixture was refluxed overnight. After methanol was removed by an evaporator, the solid was dried under vacuum, washed with distilled water (100 mL), and then dried under vacuum. In a furnace, PS@(NH4)2MoS4@MON was heated at 700 °C for 3 h (increasing temperature for 2 h and maintaining the temperature for 3 h). After being cooled to room temperature, MoS2/C-68 was collected. For the preparation of MoS2/C-49, MoS2/C-58, and MoS2/C-84, 57 (0.21 mmol), 86 (0.33 mmol), and 142 mg (0.547 mmol) of (NH4)2MoS4 were used, respectively. Other synthesis procedures were same as those for MoS2/C-68. Procedure of Electrochemical Tests for Coin Cell Supercapacitors. For the engineering of two working electrodes, a mixture of MoS2/C (80 mg), carbon black (10 mg), N-methyl-2-pyrrolidone (NMP, 100 mg), and polyvinylidene fluoride (PVDF, 10 mg, 77 mg of 13 wt % PVDF in NMP) was ground in a mortar. The resultant slurry was loaded on one side of two titanium foils (0.94 cm × 0.74 cm, 0.127 mm thickness, Alfa Aesar Co.) using a spatula. The two electrodes were dried at 80 °C in an oven for 1 h and at 110 °C in a vacuum oven overnight. The loading amount was

CONCLUSIONS This work shows that versatile organic templates can be engineered based on MON chemistry and can be applied to develop high-performance composite materials. Yolk−shelltype PS@MONs were prepared by the formation of MON layers on PS@SiO2 and successive silica etching. Because of the existence of void space in Y-PS@MON and the microporosity of the MON shells, the MoS2 precursors were easily incorporated into Y-PS@MON. For the incorporation of MoS2 precursors into Y-PS@MON, the MON layers were critical because of their microporosities. PS particles without MON layers were not efficient in the engineering of MoS2/C composites (Figure S10 in the Supporting Information). The heating of PS@(NH4)2MoS4@MON resulted in hollow MoS2/ C composites. As energy storage materials for supercapacitors, MoS2/C-58 with the optimized contents of MoS2 showed high capacitances up to 418 F/g at a 0.5 A/g current density, excellent rate performances with 235 F/g at a 10 A/g current density, and cycling stabilities during 5000 cycles. We believe that the yolk−shell-type organic templates with MON shells can be further applied to engineer more various inorganic material/carbon composite materials.

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EXPERIMENTAL SECTION General Information. TEM and HRTEM studies were performed by a JEOL 2100F unit. PXRD patterns were recorded by a Rigaku MAX-2200. N2 isotherm curves were obtained at 77 K by a BELSORP II-mini equipment. Pore size distribution was analyzed by the DFT method. Solid-state 13C NMR spectroscopy was performed by a Bruker 500 MHz AVANCE II NMR spectrometer at the NCIRF of Seoul National University, utilizing a 4 mm magic-angle spinning probe (spinning rate: 5 kHz). TGA curves were obtained by Seiko an EXSTAR 7300. Infrared (IR) absorption spectroscopy was conducted by a Bruker VERTEX 70 FT-IR spectrometer. XPS spectra were obtained using a Thermo VG instrument and monochromatic Al Kα radiation. Raman spectroscopy was conducted by a Renishaw inVia Raman instrument. Elemental analysis was performed by a CE EA1110 instrument. Synthesis Procedure for PS@SiO2. Colloidal PS nanospheres with positive surface charges and PS@SiO2 were prepared by the synthesis procedures in the literature.43 In our work, the following procedures were applied. Distilled water (200 mL) was added to a 250 mL Schlenk flask, and the solution was bubbled with argon for 30 min. PVP (Mw: 40 000, Aldrich Chem. Co., 25 mg), styrene (1.5 mL, 13 mmol), and divinylbenzene (0.030 mL, 0.27 mmol) were added. The reaction mixture was heated at 40 °C for 30 min. After AIBA (25 mg, 0.092 mmol) was added, the mixture was then heated at 70 °C with stirring for 24 h. After being cooled to room temperature, a white solution of colloidal PS spheres was obtained. The colloidal PS sphere solution (50 mL) was added to a 500 mL round-bottom flask. After ethanol (350 mL) was added, the solution was stirred for 10 min. After ammonia solution (28− 30%, 10 mL) was added, the solution was stirred for 30 min. TEOS (0.50 mL, 2.2 mmol) was added dropwise for 40 min. The reaction mixture was stirred for 24 h at room temperature. After a 1:5 mixture of methylene chloride and hexane (900 mL) was added, the resultant PS@SiO2 particles were retrieved by centrifugation, washed with hexane and acetone, and dried under vacuum. 7663

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calculated at 1.57 mg/cm2. The cells were assembled using CR2032-type components. After one working electrode plate was located on the cap of CR2032 cell, 0.5 M Na2SO4 (80 mg) was loaded on the electrode plate. The separator (filter paper, No. 20 Hyundai Micro, HD20 MN.020. 5−8 μm) was placed on the electrode plate. The other working electrode plate was located on the separator. The active materials on both Ti electrodes were directed to the separator. Na2SO4 (0.5 M, 80 mg) was loaded on the electrode plate. After a space disc and a spring were located on the electrode plate, the cell was assembled using the other cap of the CR2032 cell and a crimper. The next day the electrochemical tests were conducted using an electrochemical workstation (WonATech, ZIVE SP). Specific capacitances were calculated using the following equation: Ccell = I/[(ΔV/Δt)m], where I is the applied current (A), ΔV/Δt is the slope of the discharge curve after an IR drop at the beginning of the discharge curve, and m is the total mass of the electrode materials (g). The specific capacitance of the single electrode was obtained by the equation Cs = 4Ccell.50

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01426. HRTEM image and PXRD pattern of Y-PS@MON, characterization data of model hollow carbon materials, TEM image of microtomed MoS2/C-58 and HRTEM images of of MoS2/C composites, Raman spectra, charge−discharge profiles, extended cycling test of the MoS2/C composites, and SEM and TEM images of MoS2/C-58 retrieved after the cycling test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.Y.). *E-mail: [email protected] (S.U.S.). ORCID

Yoon Myung: 0000-0002-5774-6183 Seung Uk Son: 0000-0002-4779-9302 Author Contributions

H.L. and J.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program (2016R1E1A1A01941074) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning and the grants CAP-15-02KBSI (R&D Convergence Program) of National Research Council of Science & Technology (NST) of Korea.

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REFERENCES

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DOI: 10.1021/acsomega.7b01426 ACS Omega 2017, 2, 7658−7665