MoS2 Nanosheets Vertically Grown on Graphene Sheets for Lithium

Aug 24, 2016 - The interfacial interaction of the C–O–Mo bonds can enhance electron transport rate and structural stability of the MoS2/G electrod...
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MoS2 Nanosheets Vertically Grown on Graphene Sheets for Lithium-Ion Battery Anodes Yongqiang Teng,† Hailei Zhao,*,†,‡ Zijia Zhang,† Zhaolin Li,† Qing Xia,† Yang Zhang,† Lina Zhao,† § ́ Xuefei Du,† Zhihong Du,† Pengpeng Lv,† and Konrad Swierczek †

School of Materials Science and Engineering and ‡The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 100083, China § Faculty of Energy and Fuels, Department of Hydrogen Energy, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Krakow, Poland S Supporting Information *

ABSTRACT: A designed nanostructure with MoS2 nanosheets (NSs) perpendicularly grown on graphene sheets (MoS2/G) is achieved by a facile and scalable hydrothermal method, which involves adsorption of Mo7O246− on a graphene oxide (GO) surface, due to the electrostatic attraction, followed by in situ growth of MoS2. These results give an explicit proof that the presence of oxygen-containing groups and pH of the solution are crucial factors enabling formation of a lamellar structure with MoS2 NSs uniformly decorated on graphene sheets. The direct coupling of edge Mo of MoS2 with the oxygen from functional groups on GO (C−O−Mo bond) is proposed. The interfacial interaction of the C−O−Mo bonds can enhance electron transport rate and structural stability of the MoS2/G electrode, which is beneficial for the improvement of rate performance and long cycle life. The graphene sheets improve the electrical conductivity of the composite and, at the same time, act not only as a substrate to disperse active MoS2 NSs homogeneously but also as a buffer to accommodate the volume changes during cycling. As an anode material for lithium-ion batteries, the manufactured MoS2/G electrode manifests a stable cycling performance (1077 mAh g−1 at 100 mA g−1 after 150 cycles), excellent rate capability, and a long cycle life (907 mAh g−1 at 1000 mA g−1 after 400 cycles). KEYWORDS: molybdenum disulfide, oxygen-containing groups on GO, C−O−Mo bond, long cycle life, Li-ion batteries

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its practical application. In attempts to ameliorate the electrical conductivity and the structural stability of MoS2 electrode material during cycling, carbonaceous materials, such as amorphous carbon,7 carbon fibers,8 porous carbon,9 carbon spheres,10 carbon nanotubes,11,12 or graphene,13−16 have been employed to form a composite with MoS2. A carbon component can enhance the electronic conductivity and concurrently buffer the volume changes of MoS2 during the lithiation/delithiation reaction, leading to fast electrode kinetics and stable cycling performance. Among various kinds of carbonaceous materials, graphene has attracted great attention, due to its high electrical conductivity, excellent mechanical properties, and a large specific surface area. Many MoS2/ graphene composites with different morphologies were already prepared. Chang et al.13 developed an L-cysteine-assisted solution-phase method to prepare a three-dimensionally (3-

ithium-ion batteries (LIBs) have been developed as leading energy storage devices for many applications, such as consumer electronics, artificial satellites, military equipment, renewable energy storage for smart grids, and electric vehicles. The latter two are of a great importance concerning alleviation of environmental pollution and global warming issues.1−3 Graphite, the currently commercialized anode material with a theoretical capacity of 372 mAh g−1, cannot cater to the increasing demand of LIBs, especially for high-energy density and excellent rate capability usage. Therefore, enormous efforts have been devoted to explore new anode materials delivering a high performance.4−6 Recently, molybdenum disulfide (MoS2) has gained widespread interest in view of its analogous layered structure with graphite, high lithium storage capacity (ca. 670 mAh g−1), and low cost. However, it suffers from structural deterioration, due to a large volume change upon charge/discharge, and low intrinsic electrical conductivity between two adjacent S−Mo−S sheets (c-direction), which together lead to a poor cycling performance and inferior rate capability and, therefore, hamper © 2016 American Chemical Society

Received: June 4, 2016 Accepted: August 24, 2016 Published: August 24, 2016 8526

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large volume changes of MoS2 during lithiation/delithiation processes. (4) The nanoscale MoS2 nanosheets, with a larger specific surface area than the sheet structure, can promote the active material/electrolyte interface reaction to take place and provide a shorter diffusion distance for both lithium ions and electrons. As a consequence to all of the above reasoning, it is not unexpected that the prepared MoS2/G manifests an outstanding reversible capacity of 1077 mAh g−1 at 100 mA g−1 and an excellent cycling performance of 907 mAh g−1 at 1000 mA g−1 rate for 400 cycles. The as-prepared hierarchical MoS2/G composite is expected to be a promising candidate as anode material for high-performance, next-generation lithiumion batteries.

D) architectured MoS2/graphene composite with MoS2 layers growing parallel on the graphene sheet surface, which exhibited a specific capacity of ∼1100 mAh g−1. A two-step strategy was proposed by Wang et al.14 to grow honeycomb-like MoS2 nanosheets anchored into 3-D graphene foam, which displayed a discharge capacity of 1050 mAh g−1 at a current density of 200 mA g−1 after 60 cycles. Liu et al.15 synthesized a better kind of MoS2/graphene nanocomposite by hydrolyzing the lithiated MoS2, delivering a reversible capacity of 1300−1400 mAh g−1 but also showing unsatisfactory rate performance, ∼500 mAh g−1, at a current density of 1000 mA g−1. MoS2 possesses a layered structure, composed of three-atom stacked layers (S−Mo−S), held together by van der Waals interactions. The layered structure feature makes it easy for the lithium ions to be inserted/removed along the direction parallel to the a−b plane. In this regard, the perpendicular growth of MoS2 particles on the graphene sheet with the (xy0) plane exposure will offer an increased number of active sites for electrochemical reaction, provide open channels for lithium-ion intercalation, and, therefore, facilitate the electrode reaction kinetics. Additionally, tight coupling between MoS2 and graphene is expected to enable fast charge transfer kinetics and good structural stability of the electrode. Rational design of a synthesis process is vital to realize the favorable growth and direct linkage of MoS2 to graphene and, therefore, to improve the electrochemical performance of the MoS2/graphene electrode. Due to the ionization of the carboxylic acid and phenolic hydroxyl groups, the surface of graphene oxide (GO) is usually negatively charged in aqueous solution.17 From this point of view, Mo7O246− or MoO42− anions acting as the Mo source are difficult to couple with the GO surface, which is due to the electrostatic repulsion. To achieve a favorable electrostatic assembly, many researchers have tried to modify the surface charge of GO by introducing cationic surfactants18−21 or ionic liquids,22,23 which act as positively charged intermediates to bridge GO and negatively charged Mo sources. However, a large intermediate group may weaken the direct charge transfer and the synergistic effect between graphene and MoS2. It is known that the surface charge of GO sheets is strongly dependent on the pH value.17 This gives an opportunity to tune the surface charge feature by adjusting the pH value of solution without any other additives used. Herein, we demonstrate a facile approach to prepare a hierarchical nanostructure of MoS2/graphene (MoS2/G) composite. By controlling pH of a solution, MoS2 grains were selectively grown on GO under hydrothermal conditions, forming a designed nanostructure with MoS2 sheets perpendicularly connecting with graphene through a direct coupling of edge Mo of MoS2 nanosheets (NSs) with the oxygen from functional groups present on GO (Mo−O−C bonds). This structural characteristic endows the MoS2/graphene composite anode with many advantages: (1) MoS2 NSs distributed vertically on a large-area graphene can not only prevent the restacking of graphene sheets and the aggregation of MoS2 sheets during charge/discharge processes but also provide abundant, active edge sites for lithium reaction. (2) Good adhesion of MoS2 NSs on graphene sheets via the C−O−Mo bonds ensures a good structural stability and thus is favorable for the long-term cycling performance. The Mo−O−C bond also provides an effective electron transfer path between MoS2 and graphene. (3) The graphene sheet acts as a high-speed channel for electron transfer and a flexible basis for buffering

RESULTS AND DISCUSSION X-ray diffraction (XRD) examination was performed to identify the presence of phases and the crystal structure of MoS2 and the MoS2/G composite. As illustrated in Figure 1, the observed

Figure 1. XRD patterns of MoS2 and MoS2/G powders.

diffraction peaks for both samples can be indexed well to the hexagonal phase of MoS2 (JCPDS No. 75-1539), without any other phases being visible. Notably, the absence of the (002) reflection peak for the MoS2/G composite, which indicates a stacked nature of layered MoS2, suggests formation of a fewlayer MoS2 structure.24 No characteristic peaks of restacked reduced graphene oxide (rGO) are observed for MoS2/G, implying a high dispersion state of rGO in this composite. The tuning of the surface charge feature of GO by H2C2O4 plays an important role in the synthesis of a well-dispersed MoS2/G composite. A proton from H2C2O4 acts as an intermediate to connect the negatively charged GO and Mo7O246− and makes MoS2 easily grow on the GO sheets and prevents the aggregation of GO sheets. Without H2C2O4 addition, the MoS2 is difficult to grow on GO sheets and the GO sheets are prone to restack, forming graphite-like carbon bulk, which is evidenced by the XRD examination (Figure S1) and field emission scanning electron microscope (FESEM) observation (Figure S2). The carbon content estimated for MoS2/G is 7.4 wt % by the thermogravimetric (TG) analysis result (Figures S3−S5), which is a suitable amount concerning application. A morphology difference between the synthesized MoS2 and MoS2/G composite is shown in Figure 2. A lamellar structure is observed for MoS2/G (Figure 2a), while pristine MoS2 exhibits lumps with aggregated granular particles (Figure 2c,d). More detailed observation of MoS2/G (Figure 2b) reveals that the sheet surface of graphene is rough with nanosheets uniformly decorated on it. The sheets can be inferred as being MoS2. 8527

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Figure 3. TEM (a,b) and HRTEM (c,d) images of the MoS2/G sample; inset is the energy-dispersive X-ray spectroscopy profile of the red square region in (a).

Figure 2. SEM images of MoS2/G (a,b) and MoS2 (c,d) samples.

Moreover, the sheet size of MoS2 in the MoS2/G composite is much smaller than that of pristine MoS2 (∼100 nm). The ultrasmall size can greatly shorten the diffusion length of lithium ions during charge/discharge processes and also promote the active material/electrolyte interface reaction.25−28 The difference in morphology between the considered two samples implies that graphene sheets are favorable for tailoring the crystal size and dispersing the active material during the synthesis procedure. Besides, heat treatment was found to make no difference on the morphology (Figure S6a,b). Furthermore, after successful removal of the MoS2 NSs by reaction with heated HNO3, smooth graphene sheets could be observed again, confirming formation of the MoS2/G heterostructure and that the structure of graphene is not destroyed during the hydrothermal process (Figure S6c,d). To further investigate the structure characteristics of the prepared MoS2/G composite, transmission electron microscopy (TEM) observations were performed. The MoS2/G composite was sonicated in ethanol for 2 h, and the suspension was then dropwise put onto a TEM grid and dried at ambient conditions. As observed, the sheet-like structure was preserved during the ultrasonication, indicating a rather strong interfacial interaction between the MoS2 and graphene. The lowmagnification TEM images (Figure 3a,b) reveal that MoS2 NSs are uniformly scattered on transparent graphene sheets. High-resolution TEM (HRTEM) images confirm a crucial feature that most of MoS2 NSs stand vertically on the surface rather than lie flat. As labeled in the HRTEM images (Figure 3c), it can be clearly seen that the graphene sheets act as a bed for the growth of MoS2. The lattice plane spacings of MoS2 are 0.63 and 0.27 nm (Figure 3d), corresponding to the (002) and (100) crystal planes, respectively. Significant amounts of (002) planes are observed, and in some places, they distribute randomly on the graphene sheet (Figure S7) and show a homogeneous elemental distribution in a relatively low resolution (Figure S8). The presence of a (100) plane can be attributed to a curl up of MoS2 nanosheets to decrease the

system’s surface energy and make the hierarchical nanostructure more stable.29,30 The elemental composition of the MoS2/G composite was determined by the energy-dispersive X-ray spectroscopy (EDX) analysis (inset in Figure 3a). The elements of C, Mo, S, and O are detected in the checked region. A small amount of element O derives from the residual oxygen-containing functional group of GO. The peaks assignable to Cu come from the Cu foil substrate. The atomic ratio of S and Mo is approximately 2:1 (2.13:1) (Table S1), which agrees well with the expected one and the XRD results (Figure 1). In previous reports, GO was usually used as a matrix to prepare layer-by-layer MoS2/graphene composites, in which MoS2 nanosheets generally grew parallel on the graphene surface.13−16,31 Chang et al.31 suggested that a co-shared electron cloud could be formed between the electrons of the S atom layer and its adjacent carbon layer. However, as mentioned above, our results indicate that MoS2 NSs stand on the graphene surface. It is interesting to consider what controls the observed vertical growth of MoS2 NSs on the graphene surface. Based on the experimental conditions, the formation mechanism of lamellar structured MoS2/G is considered to be related to the charge state of GO sheet surface. As shown in Figure 4, the suspension of GO at pH 6 reveals a negatively charged surface with a ζ-potential value of −25.1 mV, due to the ionization of its oxygen-containing groups. However, the surface charge of GO suspension turns to positive (+8.3 mV) in a solution at pH 1. Based on the aforementioned characteristics, we adopted a facile strategy to synthesize a hierarchical MoS2/ G composite via a one-pot hydrothermal route, as depicted in Scheme 1. Without any other additives, the introduction of abundant hydrogen ions endows GO sheets with positive electrostatic charge, with which Mo7O246− (as Mo source) prefers to couple, due to the electrostatic attraction. During the primary reaction period, MoOx crystal nuclei are in situ formed 8528

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231.9 eV, which can be attributed to the Mo 3d5/2 and Mo 3d3/2 binding energies, respectively,12,35 and are ascribed to the characteristic peaks of Mo4+ in MoS2. The peaks at 232.1 and 235.3 eV are related to the Mo 3d5/2 and 3d3/2 of Mo6+ (typical of the Mo−O bond).35 Another small peak at 225.8 eV corresponds to the S 2s component. The peaks at 162.5 and 161.3 eV (Figure 5c) are attributed to the coexistence of S 2p1/2 and S 2p3/2.12 As illustrated in Figure 5d, the C 1s spectra can be decomposed into a dominant peak at 284.6 eV for CC/ C−C, while other three weak peaks of oxygen-containing functional groups are at 286.8 eV for C−O, 287.9 eV for CO, and 288.9 eV for O−CO.36 The relatively weak peaks of the C(O)−O suggest that the GO sheets have been almost reduced to graphene. Notably, the high-resolution spectrum of O 1s in Figure 5e can be deconvoluted into three peaks, in which a new peak at 532.4 eV is present, apart from the one at 531.2 eV for CO and another at 533.3 eV for C−OH. It is reported that the O 1s peak in the C−O−metal bond is located at ca. 530− 533 eV.36−39 Therefore, we can conclude that the C−O−Mo bond is formed in the MoS2/G composite. From the above data, it is easy to conclude that C−O−Mo bonds can be formed by bonding Mo atoms and oxygen-containing functional groups in GO in the considered composite. The model of interactions resulting in a successful synthesis process is schematically demonstrated in Scheme 1. Similarly to the above discussion, a covalent C−O−M bonding (M = Co, Ni, Fe, and Sn) between MxOy and GO has been validated, and the interaction was found to enhance the electron transport rate and structural stability of the MxOy/graphene electrode.36−39 Hence, the interfacial interaction of the C−O−Mo bond between MoS2 and graphene should be beneficial for the improvement of electrochemical performance of the designed and synthesized MoS2/G as anode material for LIBs. In order to evaluate the electrochemical activity of the MoS2/ G electrode, cyclic voltammetry (CV) measurements were carried out at a scan rate of 0.1 mV s−1, and the recorded curves are plotted in Figure 6a. Upon the first discharge, three cathodic peaks appear at 0.94, 0.51, and 0.30 V. In detail, the broad shoulder appearing in the vicinity of the 0.94 V peak can be related to the Li+ intercalation into MoS2 interlayer space, forming LixMoS2, which is accompanied by a phase transformation from the 2H to the 1T structure of LixMoS2.14,15,40 With a proceeding electrode reaction, the 0.51 V peak can be ascribed to the conversion reaction of LixMoS2 into Li2S and metallic Mo. Consequently, the 0.30 V peak may be attributed to the ongoing reaction with the electrolyte solution to form

Figure 4. ζ-Potentials of 0.2 mg mL−1 GO in aqueous solution at different pH.

on the GO surface and react with H2S released from CH 3CSNH 2 , which acts as S resource and reductant simultaneously, to obtain MoS2 nanocrystals.32 Meanwhile, the initial GO sheets are reduced to rGO. Thus, the graphene sheets serve as substrate for the nucleation and growth of MoS2 during the hydrothermal process. Considering the layered structural feature of MoS2, the (100) plane is composed of a Mo layer sandwiched between two S layers, while the Mo atoms at the end-face of MoS2 are bare.33,34 Consequently, the possibility for formation of the C−O−Mo bonds between bare Mo atoms and the oxygen-containing groups is proposed. It can also be concluded that MoS2 NSs are more apt to grow vertically on graphene, influenced by presence of C−O−Mo bonds. In comparison, to verify the above plausible mechanism of the formation process, rGO was adopted as a substitute of GO to prepare MoS2/graphene mixture via the same synthesis procedures. As presented in Figure S9a,b, and what is of a great importance, MoS2 NSs and graphene sheets were found to aggregate separately, and the sheet surfaces of graphene remained smooth. Therefore, it is reasonable to state that the oxygenated functional groups on GO play a crucial role in the selective growth of MoS2 on the graphene surface. To clarify the interfacial nature between MoS2 and graphene, X-ray photoelectron spectroscopy (XPS) measurements were also carried out. Overall, XPS spectra in Figure 5a show that the MoS2/G composite is composed of C, O, Mo, and S elements. As shown in Figure 5b, two peaks are observed at 228.6 and

Scheme 1. Schematic Illustration of the Synthesis Procedure of MoS2/G

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Figure 5. (a) XPS survey spectrum, high-resolution (b) Mo 3d spectrum, (c) S 2p spectrum, (d) C 1s spectrum, and (e) O 1s spectrum of MoS2/G composite.

Figure 6. (a) CV curves at a scanning rate of 0.1 mV s−1 of the first five cycles for the MoS2/G electrode and (b) charge−discharge curves at 100 mA g−1 of the 1st, 10th, 20th, 30th, and 40th cycles for the MoS2/G electrode.

SEI film.14,15,40 In the following discharge curves, the cathodic peaks shift to 1.90, 1.20, and 0.45 V, signifying different lithiation mechanisms after the first cycle. In the anodic scans, two peaks at 1.54 and 2.25 V are observed. The weak and broad peak at 1.54 V may be assigned to a partial oxidation of Mo to form MoS2. The pronounced oxidation peak at 2.25 V indicates formation of sulfur. All recorded peaks are in accordance with

the results previously shown in the literature.14,15,40 In fact, it can be stated that after the first cycle the electrode material is mainly composed of sulfur, Mo, and few MoS2, instead of the initial MoS2, and such composition remains stable in the following cycles. Apart from the first discharge curve, the remaining profiles are almost identical, demonstrating an excellent stability of the MoS2/G electrode in the electro8530

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Figure 7. (a) Cycling performance of MoS2/G, MoS2, and rGO electrodes at a current density of 100 mA g−1 for 150 cycles. (b) Rate capability of MoS2 and MoS2/G. (c) Cycling performance of the MoS2/G electrode at a current density of 1000 mA g−1.

during the initial cycles, which is probably related to the increasing number of electrochemically active surfaces during cycling, which is due to an appearance of gradual cracks on the (002) basal planes of MoS241,42 or the formation of a gel-like polymeric layer.43 After 20 cycles, the capacity of MoS2 electrode shows a rapid decline to about 200 mAh g−1, probably due to its low conductivity and severe damage of the structure by volume changes, leading to pulverization of the active material. On the contrary, the MoS2/G electrode delivers a stable, reversible capacity of 1077 mAh g−1 with a negligible capacity loss after 150 cycles. Although the prepared rGO electrode also shows good stability, the delivered capacity is only 377 mAh g−1. These results indicate that the greatly enhanced lithium storage capacity of MoS2/G might also arise from the synergetic effect, such as interfacial charge storage via weakly bound Li at the maximized junction areas between graphene and MoS2 nanocrystals.21,44−46 Since the high rate capability is beneficial to the design of high-power-type LIBs, excellent rate performance of the electrode is also an important aspect. Figure 7b shows a superior rate capability of the MoS2/G electrode compared to that of the pristine MoS2. The measured reversible charge capacities of the MoS2/G electrode are 1035, 1012, 986, and 890 mAh g−1 at current densities of 200, 300, 500, and 1000 mA g−1, respectively. In contrast, the pristine MoS2 electrode shows a fast capacity fading as the C rate increases. Notably, the MoS2/G electrode exhibits an excellent long-term cycling performance (Figure 7c) when charged/discharged at a high current density of 1000 mA g−1. A capacity of ∼900 mAh g−1, which is much higher than the capacity of graphite cycled at low current densities,47 can be retained after 400 cycles even at the current density of 1000 mA g−1, and apparently, it is much higher than that of the pristine MoS2 and MoS2/graphene mixture electrodes (Figure S12). Comparing the reported

chemical processes. It should be also noticed that the intensity of the reduction peak at 1.90 V slightly increases with cycling, indicating an ongoing activation process, which is consistent with the observed capacity rise, as discussed below. Figures 6b and S10 present the galvanostatic charge− discharge profiles of the MoS2/G composite recorded for different cycles at a current density of 100 mA g−1. It is evident that the plateaus on the charge−discharge curves are consistent with the respective peaks on the CV curves. Similar charge and discharge curves can be also observed in the case of the pristine MoS2 electrode (Figure S11). The discharge plateau of the MoS2 electrode diminishes remarkably with an increasing cycle number, while the MoS2/G electrode displays overlapped charge/discharge curves, except for the initial cycle, revealing the excellent electrochemical stability and reversibility of the composite. These results confirm that the highly conductive graphene network and stable sheet-like structure play a crucial role in improving structural stability of the MoS2/G electrode. The cycling performance of the MoS2/G electrode was evaluated, and the result is shown in Figure 7a. For comparison, the respective performance data for MoS2 and rGO electrodes were also included. The initial charge and discharge capacities of the three electrodes at 100 mA g−1 current density are 896/ 1160, 693/945, and 554/2786 mAh g−1 , respectively, corresponding to the Coulombic efficiencies of 77.2, 73.3, and 19.9%. Such improved efficiency and increased capacity of the MoS2/G electrode can be related to MoS2 NSs uniformly anchored on the conductive graphene network. Obviously, the irreversible capacity loss during the first cycle is inevitable and is attributed to the formation of the SEI layer.14 In the subsequent cycles, the Coulombic efficiency of the MoS2/G electrode gradually increases to ∼99%, superior to that of pristine MoS2 electrode (∼96%). Interestingly, both MoS2 and MoS2/G electrodes exhibit a capacity climbing phenomenon 8531

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Figure 8. (a) Cycling performance and (b) capacity retention of the MoS2/G electrode at a current density of 100 mA g−1 at different temperatures.

relevant studies (gathered in Table S2), the hierarchical MoS2/ G electrode shows a much better long-term cycling stability. The outstanding rate performance of the MoS2/G composite can be ascribed to the 3-D conductive network of graphene and the interfacial interaction of the C−O−Mo bonds between MoS2 and graphene. The electrochemical performance of the MoS2/G electrode at harsh temperature conditions was also evaluated, as shown in Figure 8. The specific capacity of MoS2/G electrode still maintains ∼700 mAh g−1 even at a low temperature of −20 °C at 100 mA g−1 current density, indicating an outstanding cold resistance (Figure 8a). The recorded specific capacity of MoS2/ G electrode varies with changes of the temperature in the expected way (Figure 8b). When the temperature increases back to 20 °C, the specific capacity of ∼800 mAh g−1 is recovered at 500 mA g−1, suggesting a wide working temperature range of the MoS2/G electrode, on account of the good structural stability of the MoS2/G composite. Electrochemical impedance spectra studies were also conducted on MoS2 and MoS2/G electrodes, to have further insight into their distinct electrochemical properties. The Nyquist plots of MoS2 and MoS2/G electrodes, recorded after 130 cycles at a current density of 100 mA g−1, are plotted in Figure 9a. Data for the two electrodes consist of two semicircles present in the high-frequency region and inclined straight lines in the low-frequency range. Generally, the first semicircle is associated with the resistance of lithium ion migration through the SEI films (Rf), and the second semicircle corresponds to the charge-transfer resistance (Rct). Re denotes electrolyte resistance; CPE is a constant phase element of the electrode/electrolyte interface, and Zw is the impedance related to the diffusion of lithium ions within the electrode. Using ZView software, the respective resistance values for MoS2 and MoS2/G electrodes were fitted. As shown in Figure 9b, Rct of the MoS2/G electrode (13.5 Ω) is clearly smaller than that of pristine MoS2 (57.5 Ω), implying that the incorporation of rGO can greatly boost the charge transfer process during electrochemical reactions at the electrode and thereby lead to a better rate performance. To confirm the stability of the structure and morphology, FESEM observations were performed on the MoS2 and MoS2/ G electrodes after 130 cycles performed at the current density of 100 mA g−1. As shown in Figure S13a, the MoS2/G electrode maintains good structural integrity without any cracks and breaks present, which is an indication of the excellent stability of the electrode. The red circle region of Figure S13b reveals that many active nanoparticles are still uniformly and closely attached on the surface of the graphene sheets after repeated

Figure 9. (a) Nyquist plots of MoS2 and MoS2/G electrodes at fully charged state after 130 cycles at 100 mA g−1, and (b) values of Re, Rf, and Rct obtained by fitting data according to the equivalent circuit model presented in (a).

charge/discharge processes, whereas a pristine MoS2 electrode (Figure S13c,d) suffers from severe cracks, due to the huge volume expansion and aggregation of particles during cycling. The result proves that graphene sheets can efficiently cushion the volume expansion/shrinkage and prevent the detachment of MoS2 sheets during cycling through the presence of the C− O−Mo bonds. From the above discussion, the outstanding electrochemical performance can be attributed to the designed structure of the MoS2/G composite. As illustrated in Scheme 2, the advantages of such an electrode are three-fold. First, the plethora of O atoms among oxygen-containing functional groups act as “connectors” and “controllers”, causing MoS2 NSs to be distributed vertically on graphene sheets, which provides abundant active edge sites for reaction with lithium and a shorter diffusion distance for both lithium ions and electrons. Second, the formed C−O−Mo bonds endow the MoS2/G 8532

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stainless steel autoclave. After being sealed tightly, the autoclave was heated and the temperature was maintained at 180 °C for 12 h. After being cooled rapidly, the black product was collected by centrifugation and washed several times with deionized water and ethanol, followed by drying in a vacuum oven at 80 °C for 24 h. The as-prepared precursor was then annealed in a tube furnace at 800 °C for 1 h in an argon atmosphere, with a ramp rate of 5 °C min−1, yielding the desired MoS2/G composite. The synthesis procedure of a pristine MoS2 was similar to that of MoS2/G composite but was performed in an absence of GO. The rGO sample was derived from GO powder after a heat treatment process at 800 °C for 1 h in argon atmosphere. Material Characterizations. Crucial for the successful synthesis, ζ-potential of the samples was determined by a Zetasizer Nano ZS (Malvern, England) at room temperature (25 °C) to characterize the surface charge properties of the particles. The crystalline phase of all samples was identified by powder XRD using a D/MAX-A diffractometer (Rigaku, Japan) equipped with Cu Kα radiation source (λ = 1.54056 Å) in the 2θ range of 10−80°. Thermogravimetric measurements were performed on STA 449F3 apparatus (NETZSCH, Germany) between 50 and 700 °C in air, with a heating rate of 10 °C min−1. Morphology of the samples was observed by FESEM (SUPRA55, Germany) operated at 10 kV. The exact phase composition, and the lattice structures of the composite were characterized by using HRTEM (FEI-F20, America) operated at an accelerating voltage of 200 kV, combined with HRTEM and EDX measurements. XPS was used to identify Mo-related bond features in the synthesized MoS2/G on a RBD upgraded PHI-5000C ESCA system (PerkinElmer, USA) with Mg Kα radiation (hν = 1253.6 eV). Electrochemical Measurements. The electrochemical characteristics of the synthesized samples were evaluated by using half cells. The working electrodes were prepared by mixing 70 wt % of active material (MoS2/G, MoS2, or rGO), 15 wt % of acetylene black as conducting agent, and 15 wt % of polyvinylidene fluoride as binder in Nmethylpyrrolidinone. The obtained slurries were then spread uniformly on a copper foil. After being dried in a vacuum environment at 70 °C for 6 h, the copper foil with respective electrode material was punched into circular discs. The discs were dried again at 120 °C in vacuum for 24 h. CR2032-type coin cells were assembled in an argonfilled glovebox with metal lithium foil as a counter electrode, Celgard 2400 microporous membrane as separator, and 1 M LiPF6 in a mixture of ethylene carbonate, dimethylcarbonate, and ethyl methyl carbonate with a volume ratio of 1:1:1 used as the electrolyte. Galvanostatic cycling was conducted on a computer-controlled Land CT2001A (Wuhan, China) battery test system at different current densities in a potential range of 0.01−3.0 V. The CV tests were carried out to examine the electrode reaction under the scan rate of 0.1 mV s−1 with a voltage range of 0.01−3.0 V. Electrochemical impedance spectra of pristine MoS2 and MoS2/G were recorded in a frequency range from 106 to 0.1 Hz, while the disturbance amplitude was 5 mV.

Scheme 2. Schematic Illustration Showing Paths for LithiumIon Diffusion in the MoS2/G Composite Electrode

composite with a highly stable structure for long-term cycling in LIB applications. The C−O−Mo bond also provides a good electron transfer path between MoS2 and graphene. Third, the robust structure of graphene sheets can effectively limit the excessive volume expansion during cycling, while the conductive matrix of graphene sheet networks can ensure fast transport of electrons in the whole electrode. Thus, MoS2/G anode material displays a superior rate performance, high reversible capacity, and long-term cycle life.

CONCLUSIONS In summary, a facile approach was proposed and successfully performed to prepare a hierarchical MoS2/graphene composite. By controlling the pH value of the GO suspension, MoS2 grains were made to perpendicularly grow on the graphene sheets through the direct coupling of edge Mo of MoS2 with the oxygen from the functional groups present on GO (creation of Mo−O−C bonds). This leads to the formation of the MoS2/G composite nanostructure. The designed architecture prevents the restacking of graphene sheets and the agglomeration of MoS2 nanosheets during charge/discharge processes. When evaluated as an anode of LIBs, the MoS2/G composite presents a high reversible capacity, a superior rate capability, and a long cycle life. The electrode manifests an outstanding reversible capacity of 1077 mAh g−1 at 100 mA g−1 after 150 cycles, retaining 92.8% of the initial reversible capacity, and a long, stable cycle life (907 mAh g−1 at 1000 mA g−1) after 400 cycles. The obtained excellent performance is attributed to the presence of a sufficient number of electrochemically active sites, due to the vertical growth of MoS2 nanosheets on graphene sheets. The nanosize of MoS2 active material ensures facile penetration of the electrolyte and shortens the diffusion distance of lithium ions. In addition, the C−O−Mo bonds facilitate fast electron hopping from graphene to MoS2, allowing high reversible capacity and excellent rate performance to be achieved. The hierarchical MoS2/G composite can be considered as a promising anode material candidate for highperformance LIBs.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03683. XRD pattern and FESEM images of MoS2/G without H2C2O4 added, TG curve of MoS2/G composite, FESEM images of MoS2/G composite before heat treatment and after heated HNO3 treatment, FESEM images of MoS2/graphene mixture, FESEM images of MoS2/G electrode and bare MoS2 electrode after cycles, and summary of electrochemical performance data of the reported relevant MoS2 electrode materials (PDF)

MATERIALS AND METHODS Material Preparation. The hierarchical MoS2/G composite was fabricated by an acid-assisted hydrothermal route. All the chemicals were of analytical grade and used without further purification. First, the colloidal suspension of GO was prepared by the oxidation of natural graphite powder using a modified Hummers method. The detailed process was already described in the literature.48 Then, in a typical batch, the as-prepared GO-dispersed suspension (14 mL, 4.6 mg mL−1) was added into deionized water (46 mL), with ultrasonication for an hour, to form a homogeneous solution, which was followed by an addition of PVP (K30, 0.1 g), thioacetamide (1.315 g), ammonium paramolybdate (0.620 g), and oxalate dihydrate (1 g). Then, the mixture suspension was transferred into a 100 mL Teflon-lined

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 8533

DOI: 10.1021/acsnano.6b03683 ACS Nano 2016, 10, 8526−8535

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ACS Nano Notes

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The authors declare no competing financial interest.

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