Graphene Superlattice for Efficient Sodium

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A Unilamellar Metallic MoS2/Graphene Superlattice for Efficient Sodium Storage and Hydrogen Evolution Pan Xiong, Renzhi Ma, Nobuyuki Sakai, Leanddas NURDIWIJAYANTO, and Takayoshi Sasaki ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00110 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Energy Letters

A Unilamellar Metallic MoS2/Graphene Superlattice for Efficient Sodium Storage and Hydrogen Evolution Pan Xiong, Renzhi Ma, Nobuyuki Sakai, Leanddas Nurdiwijayanto and Takayoshi Sasaki* International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan AUTHOR INFORMATION Corresponding Author *[email protected]

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ABSTRACT

Unilamellar metallic nanosheets possess superiority for electrochemical energy storage and conversion applications compared to the few-layered bulks and semiconducting counterparts. Here, we report the utilization of unilamellar metallic 1T phase MoS2 nanosheets for efficient sodium storage and hydrogen evolution through a MoS2/graphene superlattice. The superlatticelike assembly composed of alternately restacked unilamellar MoS2 and modified reduced graphene oxide nanosheets was prepared by a facile solution-phase direct restacking method. As an anode for sodium storage, the MoS2/graphene superlattice anode exhibited an excellent rate capability of ~240 mA h g−1 at 51.2 A g−1 and a stable reversible capacity of ~380 mA h g−1 after 1000 cycles at 10 A g−1. In addition, a low onset potential of ~88 mV and a small Tafel slope of 48.7 mV decade−1 were attained for the hydrogen evolution reaction. Our findings are important for further developing the potential of 2D nanosheets for energy storage and conversion.

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ACS Energy Letters

Two-dimensional (2D) layered materials have inspired great research interest in broad applications owing to a set of unprecedented physical and chemical properties. To fulfill their potential, intensive studies have been devoted to controllable exfoliation to produce ultrathin nanosheets, especially molecularly thin unilamellar nanosheets.1,2 Molybdenum disulfide (MoS2), a well-defined layered structure, has been widely employed as a functional material for a number of applications in energy storage and conversion.3 For example, MoS2 has shown great promise as an anode material for both lithium ion batteries (LIBs) and sodium-ion batteries (SIBs) because its interlayer space is capable of reversibly storing Li+ and Na+ ions. Additionally, both theoretical and experimental studies have demonstrated that MoS2 has catalytic active sites for the hydrogen evolution reaction (HER).4 Remarkable advances in structural engineering. such as morphology control,5,6 doping,7,8 defect engineering,9,10 vertically aligned structures,11,12 interlayer expansion,13-16 have led to the optimization of the capability and stability of MoS2based materials in energy storage and electrocatalysis. On the other hand, due to the strong inplane covalent Mo-S bonds and weak interlayer van der Waals interactions, it is possible to exfoliate bulk crystals into MoS2 monolayers. It has been shown that chemical exfoliation of bulk MoS2 (2H-phase) via Li intercalation is an attractive route to the large-scale synthesis of unilamellar MoS2 nanosheets.17-19 Inspired by the unique properties of graphene, it is of great interest to isolate and employ unilamellar MoS2 nanosheets which are expected to trigger some attractive properties, including abundant active sites and largely shorten diffusion lengths for improved properties in energy storage and conversion applications.17,20-23 Another important superiority of chemically exfoliated unilamellar MoS2 nanosheets is the emergence of metallic properties. MoS2 usually exists in the thermodynamically favored 2H phase with semiconducting properties. The limited conductivity is a common obstacle that impedes the electrochemical


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performance of MoS2-based materials. Chemical exfoliation via Li intercalation induces modifications of the crystal structure due to electron transfer between the lithium compound and MoS2, resulting in the evolution of the metallic 1T phase within the unilamellar nanosheets.18 The metallic 1T phase not only improves the charge transfer kinetics of MoS2, which facilitates both electron transport and ion diffusion for electrochemical energy storage technologies,24,25 but also renders the basal plane catalytically active, thereby greatly enhancing the HER performance.26,27 Despite these advantages, in practice, exfoliated metallic unilamellar MoS2 nanosheets are unstable and inevitably tend to restack/aggregate. Some groups have proposed bottom-up approaches, such as hydrothermal methods, to prepare stable metallic 1T phase MoS2. However, the obtained MoS2 consisted of thicker nanoplatelets or smaller flakes with a thickness of ∼100 nm.28-30 To retain the single layer morphology from restacking/aggregation, one possible solution is the construction of unilamellar MoS2-based composites. Lerner’s group reported a MoS2poly(ethylene oxide) (MoS2-PEO) composite through an exfoliation-restacking method.31 All the chemically exfoliated metallic MoS2 nanosheets were separated by intercalated PEO molecules. Furthermore, Yao’s group demonstrated that the MoS2-PEO composites showed improved diffusion kinetics for various metal ions.15,16 However, notably, the intercalation of nonconductive species between the interlayers may decrease the overall conductivity to some extent, especially the conductivity between the adjacent layers of MoS2. Several groups have reported the stabilization/separation of single-layered MoS2 by embedding them in carbon nanofibers23 or intercalating various forms of carbon nanosheets (more precisely carbonized organic layers) between the interlayers.32-35 However, these composites were prepared by subsequent high temperature annealing processes, which usually leads to the formation of the


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ACS Energy Letters

semiconducting 2H phase. Stabilization of the metallic 1T phase and the simultaneous preservation of the unilamellar morphology of chemically exfoliated MoS2 nanosheets still remains a big challenge. The fabrication of the 2D heterostructure or superlattice by alternate stacking of different nanosheets on top of each other has been explored to combine the advantages of different 2D nanosheets to obtain new properties via the molecular-scale integration.36-40 The structural similarity of MoS2 and graphene motivates us to combine them at the molecular scale. If chemically exfoliated MoS2 nanosheets could be directly and alternately restacked with graphene monolayers, a MoS2/graphene superlattice with a maintained metallic 1T phase, unilamellar morphology and high overall conductivity could be produced to fully bring out the potential of MoS2. MoS2/graphene van der Waals heterostructures have already been explored by mechanical transfer stacking or sequentially growth of each layer on substrates, showing great potential for wide applications in nanoelectronic and optoelectronic devices.41-44 However, these reported 2D heterostructures in ultrathin film form fabricated by time-consuming and complicated processes are difficult to be scaled up for large-scale synthesis and utilization in practical applications. Our group reported a facile synthesis of superlattice-like composites by solution-phase spontaneous flocculation of different types of unilamellar nanosheets, including oxides, hydroxides, and graphene oxide/reduced graphene oxide. Stabilization of conversion-based unilamellar oxide nanosheets, such as MnO2, between graphene bilayers could maintain the overall lamellar morphology, resulting high cycling stability for Li and Na storage.45-50 This provides an important route for the bulk-scale synthesis of the superlattice-like structure composed of unilamellar metallic MoS2 and graphene nanosheets for fundamental studies and practical applications in energy storage and conversion.


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Figure 1. Schematic representation of the synthesis of the MoS2/graphene superlattice and control superlattice. Here, we report the synthesis of a MoS2/graphene superlattice via a facile solution-phase direct assembly of chemically exfoliated unilamellar MoS2 and modified reduced graphene oxide nanosheets (Figure 1). In contrast to the reported MoS2/graphene composites,51-55 the MoS2/graphene superlattice is composed of alternately restacked unilamellar MoS2 and modified


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ACS Energy Letters

reduced graphene oxide nanosheets. As every MoS2 nanosheet is directly hybridized and stabilized by modified reduced graphene oxide at the molecular scale, the favorable metallic 1T phase, unique unilamellar morphology and high overall conductivity may arise simultaneously in one structure for ultrafast Na storage and electrocatalytic reactions. In addition, one should note that the loosely and limited number of restacked nanosheets with highly expanded interlayer spacing compared with the bulk layered MoS2 may offer more accessible active sites for enhanced performance. For comparison, a control superlattice composed of unilamellar 2H phase MoS2 and reduced graphene oxide nanosheets (Figure 1) was also designed to illustrate the superiority of metallic 1T phase MoS2. Consequently, the MoS2/graphene superlattice exhibits outstanding rate capability and long-term cycling stability for Na storage and highly efficient and durable activities for HER catalysis, superior to those of the control superlattice and previously reported MoS2-based materials. The MoS2/graphene superlattice demonstrates great potential for high-performance energy storage and conversion applications.


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Figure 2. Typical AFM images and height profiles for (a) the exfoliated metallic MoS2 nanosheets and (b) PDDA-graphene nanosheets. (c and d) SEM images of the MoS2/graphene superlattice with different magnifications. (e) SEM image and corresponding elemental mapping images of the MoS2/graphene superlattice. A stable suspension of unilamellar metallic MoS2 nanosheets was prepared through the Li intercalation and chemical exfoliation procedures with modification based on our previous work.56 A transmission electron microscopy (TEM) image showed a flat and nearly transparent sheet-like morphology, demonstrating the ultrathin thickness of the as-prepared MoS2 (Figure S1). An atomic force microscopy (AFM) image further revealed a unique thickness of ∼1.0 nm, suggesting the monolayer geometry (Figure 2a). The negatively charged nature of the MoS2 nanosheets was confirmed by zeta-potential measurements (Figure S2). Graphene oxide nanosheets are negatively charged (Figure S2) due to some oxygen-containing functional groups. Conventional direct assembly of MoS2 and graphene oxide nanosheets resulted in randomly restacked 3D architectures.57 The alternate restacking into a superlattice-like structure is hampered by the statistic restacking between the two negatively charged nanosheets with cations. In this regard, the graphene oxide nanosheets were modified with a cationic polymer, poly(diallyldimethylammonium chloride) (PDDA), resulting in positively charged PDDAmodified reduced graphene oxide (PDDA-graphene) nanosheets.48 As shown in Figure 2b, the PDDA-graphene nanosheets showed an estimated thickness of ~1.5 nm. Compared to the reduced graphene oxide nanosheets without PDDA modification (~0.6 nm) (Figure S3), the increased thickness suggests the successful modification with PDDA. In addition, the uniform distribution of N, C and O was observed for PDDA-graphene (Figure S4). The zeta-potential


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ACS Energy Letters

measurements also showed the change from anionic graphene oxide to cationic PDDA-graphene after the modification with PDDA (Figure S2). Before the solution-phase direct assembly of these two unilamellar nanosheets, a hypothesized area-matching model in the lateral dimensions of the corresponding unilamellar nanosheets was considered for the full hybridization via face-to-face restacking.45-48 As shown in Figure S5, the mass ratio between MoS2 and PDDA-graphene nanosheets was estimated to be ~4.1 based on the area-matching model using the in-plane unit cells of both nanosheets. Flocculation was induced immediately when these two nanosheet suspensions were mixed, during which the unilamellar MoS2 and PDDA-graphene nanosheets electrostatically interact in a homogeneous alternate stacking order and further assemble into a 3D architecture (Figure 2c). The real mass ratio between MoS2 and PDDA-graphene was estimated to be ~4.2 from the thermogravimetric (TG) analysis (Figure S6), which is close to the calculated value. Figure 2d shows the high-resolution scanning electron microscopy (SEM) images of the resulting MoS2/graphene, in which a 3D porous network composed of crumpled thin nanosheets was observed. In contrast, the restacked MoS2 nanosheets, or PDDA-graphene only showed a densely thick plate without pores (Figure S7a and b). The 3D porous structure allows for easy electrolyte penetration and rapid ion diffusion, which contribute to fast Na storage and electrocatalytic reactions.51,57 In addition, the uniform dispersion of C, N, O, Mo and S elements (Figure 2e) verifies the homogeneous mixing of the MoS2 and PDDA-graphene nanosheets.


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Figure 3. (a) XRD data of the as-prepared MoS2/graphene and simulated superlattice intensities of the 00l basal series. (b) TEM image of the MoS2/graphene superlattice. (c) SAED pattern showing the in-plane reflections of the MoS2 and graphene nanosheets. HRTEM images of (d) the MoS2/graphene superlattice and (e) control superlattice. (f) Mo 3d XPS spectra of the MoS2 nanosheets, MoS2/graphene superlattice and control superlattice. (g) Schematic illustration of the formation of the 2H phase and decreased interlayer spacing of MoS2 during the annealing process for the control superlattice.


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ACS Energy Letters

The X-ray diffraction (XRD) pattern of as-prepared MoS2/graphene (Figure 3a) showed two broad peaks at low angles with a basal spacing of ca. 1.18−1.26 nm. The other two peaks at ~33° and ~57° can be ascribed to the in-plane 10 and 11 reflections of the unilamellar MoS2 nanosheets. A repeating periodicity of ~2.5 nm, as the sum of thicknesses of the PDDA-graphene nanosheets (thickness: 1.5 nm) and MoS2 nanosheets (thickness: 1.0 nm), was expected to be obtained. However, the observed d-spacing is close to half of the expected value, which suggests that the observed broad peak may be identified as the second-order peak of the superlattice.46,48 The XRD simulation result based on the superlattice-like structure (Figure S8) indicates that the second-order diffraction peak shows a much stronger intensity than that of the first-order peak, matching with the measured XRD pattern of as-prepared MoS2/graphene. The thickness along the restacking direction could be roughly calculated from the Scherer formula based on the second-order peak of the MoS2/graphene superlattice and was estimated to be ~6.5 nm. This suggests a rather limited number of restacked layers in the as-obtained superlattice, which is much smaller than that of the bulk layered materials. The XRD data strongly verifies the successful formation of the superlattice-like structure; although, the limited number of restacked layers suggests that the stacking order is not well-developed. Figure 3b shows the transmission electron microscopy (TEM) image of the as-prepared MoS2/graphene superlattice. Semitransparent thin nanosheets with crumples were observed, consistent with the SEM results (Figure 2). The selected area electron diffraction (SAED) pattern (Figure 3c) showed the in-plane diffraction rings of both the MoS2 and graphene nanosheets, suggesting their intimate stacking. Moreover, the high-resolution TEM image shown in Figure 3d exhibits a distinct multilayered structure with a repeating periodicity of ~2.4 nm, which is twice the observed spacing in XRD pattern (Figure 3a). In a control experiment, the MoS2/graphene superlattice was further annealed


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at 300 °C in Ar, and the obtained sample was denoted as the control superlattice. XRD patterns of the MoS2/graphene superlattice and control superlattice are compared in Figure S9. After thermal treatment, the organic PDDA species was removed, and a new broad peak with a contracted basal spacing of ~1.1 nm was observed. This may correspond to the first-order peak of the control superlattice. The high-resolution TEM image of the control superlattice (Figure 3e) exhibited a total repeated spacing of ~1.2 nm, which is close to the observed d-spacing in XRD pattern (Figure S9). These results also support the formation of the superlattice-like structure. Figure 3f shows the Mo 3d X-ray photoelectron spectroscopy (XPS) spectra of the MoS2 nanosheets, MoS2/graphene superlattice and control superlattice. The spectrum of the MoS2 nanosheets shows a high concentration of the metallic 1T phase, being compatible with previous reports of chemically exfoliated MoS2 nanosheets.18,24,26,27,56 After hybridization with the PDDAgraphene nanosheets, the domination of metallic 1T phase MoS2 was maintained in the MoS2/graphene superlattice. However, the Mo 3d spectrum of the control superlattice showed two prominent peaks of 2H phase MoS2, suggesting the formation of the 2H phase during the annealing process (Figure 3g).32-35 The maintained metallic 1T phase in the MoS2/graphene superlattice is expected to not only improve the overall charge transfer kinetics for high-rate energy storage but also provide the additional catalytically active basal plane for enhanced HER activity. Moreover, one should notice the rather limited number of restacked layers in the asobtained MoS2/graphene superlattice compared to the bulk layered materials. In addition, compared with bulk MoS2, which shows an interlayer spacing of 0.615 nm, the MoS2/graphene superlattice has an obviously expanded interlayer spacing of ~2.5 nm. The loosely restacked structure with a highly expanded interlayer spacing may offer more accessible active sites for enhanced performance. These features suggest that the MoS2/graphene superlattice could be


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ACS Energy Letters

considered as a promising candidate for electrochemical energy storage and conversion applications.

Figure 4. (a) Initial charge/discharge profiles of the MoS2/graphene superlattice at 0.1 A g–1. (b) Cycling performances of the MoS2/graphene superlattice, control superlattice and MoS2 nanosheets at 0.1 A g–1 for 100 cycles. (c) Rate capability of the MoS2/graphene superlattice, control superlattice and MoS2 nanosheets. (d) Comparison of the rate capability of our MoS2/graphene superlattice with recently reported MoS2-based anode materials. (e) Long-term cycling stability of the MoS2/graphene superlattice at 10 A g–1 for 1000 cycles. (f) Comparison


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of the cycling performance of our MoS2/graphene superlattice with recently reported MoS2based anode materials. The electrochemical performance of the MoS2/graphene superlattice as an anode for SIBs was first investigated. Figure S10 shows the cyclic voltammetry (CV) curves of the MoS2/graphene superlattice anode for the initial five cycles. During the first cathodic sweep, a sharp reduction peak at 1.1 V is ascribed to the intercalation of Na ions into the superlattice galleries of the MoS2/graphene and the irreversible formation of a solid-electrolyte-interface (SEI) layer.58,59 The broad peak at ~0.55 V is related to the conversion reaction to form Na2S and Mo nanoparticles.51,60 In the first anodic scan, two pronounced oxidation peaks at 1.0 and 1.8 V were observed, corresponding to the partial oxidation of Mo to MoS2 and the oxidation of Na2S to S, respectively.51,60 The original reduction peak at 1.1 V disappeared in the following cycles, suggesting that there is an irreversible reaction during the first discharge process. From the second cycle, the CV curves were almost overlapped, suggesting good reversibility. Figure 4a shows the discharge/charge profiles of the MoS2/graphene superlattice anode for the first three cycles at 0.1 A g–1. The initial discharge and charge capacities were found to be 2220 and 1100 mA h g–1, respectively, which correspond to a Coulombic efficiency (CE) of ~50%. The low initial CE may be attributed to the formation of the SEI layer, which is commonly observed for ultrathin nanosheet-based anodes.23,51,60 The Coulombic efficiency increased to 90% during the 3rd cycle and further increased to 98% after 10 cycles (Figure S11). Moreover, the discharge capacity of the MoS2/graphene superlattice anode remained at ~960 mA h g–1 after 100 cycles at a current density of 0.1 A g–1 (Figure 4b). In contrast, the pure MoS2 nanosheets delivered an initial reversible capacity of 550 mA h g–1 at a current density of 0.1 A g–1 (Figure S12a). Additionally, the capacity decreased rapidly, and only a low capacity of ~50 mA h g–1 was


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ACS Energy Letters

obtained after 50 cycles (Figure 4b). The PDDA-graphene nanosheets delivered an initial reversible capacity of ~60 mA h g–1 at the same current density (Figure S13). Based on the mass ratio between the MoS2 and PDDA-graphene nanosheets, a nominal capacity of ~456 mA h g–1 (550 × 0.808 + 60 × 0.192) can be calculated for the simply mixed hybrid of unilamellar MoS2 and PDDA-graphene nanosheets. However, the MoS2/graphene superlattice anode achieved a much higher reversible capacity than the nominal value. This phenomenon clearly indicates a possible synergistic effect in the alternately stacked MoS2/graphene superlattice-like structure, which is far beyond the simple mixing of these two nanosheets. For comparison, the control superlattice was also examined as an anode for SIBs. An initial reversible capacity of ~ 1010 mA h g−1 was obtained for the control superlattice, comparable to that of the MoS2/graphene superlattice (Figure S14a). However, the capacity gradually decreased, and a reversible capacity of ~410 mA h g−1 was obtained at the end of the 100th cycle (Figure 4b), resulting in obviously poorer cycling stability than that of the MoS2/graphene superlattice. The MoS2/graphene superlattice anodes were further discharged/charged at various current densities ranging from 0.1 to 51.2 A g−1 (Figure S15). Remarkable rate performances with high reversible capacities of ~1100, 1040, 980, 900, 800, 685, 550, 455, and 355 mA h g−1 were exhibited at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, and 25.6 A g−1, respectively. Even at a high current density of 51.2 A g−1, a specific capacity of ~240 mA h g−1 was still obtained (Figure 4c). However, the MoS2 nanosheet anodes lost almost all their capacity when the current density increased to 6.4 A g−1, and the capacity could not recover to the initial value when the current density returned to 0.1 A g−1 (Figure 4c and Figure S12b). The control superlattice exhibited improved rate performance compared to that of the MoS2 nanosheets but was still much worse than that of the MoS2/graphene superlattice, especially at large current


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densities (Figure 4c and Figure S14b). The remarkably improved rate capability can be attributed to the high overall conductivity due to the intimate molecular-scale hybridization between metallic MoS2 and graphene (Figure S16). Notably, very few MoS2-based anodes for SIBs can be charged/discharged at such a high current density of 51.2 A g−1 (Figure 4d and Table S1).23,33,51,61,62 The long-term cycling performance of the MoS2/graphene superlattice anode is also noteworthy. After 1000 cycles, a reversible capacity of ~380 mA h g−1 was still obtained at a high current density of 10 A g−1 (Figure 4e), outperforming most previously reported MoS2based anodes for SIBs (Figure 4f and Table S1).23,33,51,63 It is known that the conversion-type reactions (below 0.4 V) of the MoS2-based anode in SIBs often lead to a collapse and pulverization of the initial structure, resulting in capacity decay. Chen’s group improved the cycling performance of MoS2 nanoflower anodes by controlling the terminal voltage to 0.4 V. However, the specific capacity was only 350 mAh g–1.13 Zhu et al. utilized the conversion-type reaction to obtain large specific capacities by embedded ultrasmall MoS2 nanoplates in carbon nanofibers.23 However, the capacity decreased to ~63% after 100 cycles. In this work, the MoS2/graphene superlattice exhibited both high specific capacities and excellent cycling stability, which can be attributed to its unique superlattice-like structure, in which every unilamellar MoS2 nanosheet was confined between the graphene bilayers. The initial unilamellar MoS2 nanosheets are possibly decomposed after the initial several cycles due to the conversion-based reactions, but the resulting species are still confined and stabilized between the graphene interlayers. Figure S17 shows the TEM images and corresponding SAED pattern of the MoS2/graphene superlattice after 1000 cycles, which suggest that the overall morphology is preserved by the robust graphene backbone. Based on this comprehensive summary (Figure 4d, 4f and Table S1), to the best of our knowledge, this is the best rate performance and cycling stability for sodium storage among


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ACS Energy Letters

MoS2-based materials. In addition, Figure S18 and S19 show the kinetics analysis based on the CV curves for the MoS2/graphene superlattice. The large b-values close to 1 and dominating capacitive contribution suggest unique intercalation pseudocapacitive characteristics, which also contribute to the excellent rate capability and cycling stability of the MoS2/graphene superlattice. In our previous study, the unilamellar MnO2 nanosheets themselves showed a poor capacity of only 15 mA h g−1 for sodium storage,50 while the MoS2 nanosheets showed a much larger capacity of ~400 mA h g−1 (Figure S12). This may suggest the superior performance of the MoS2/graphene superlattice compared to the MnO2/graphene. Besides, we suspected a possible reversible Na-S reaction may involve in addition to the general conversion mechanism (reduction/oxidation of Mo),64 which may also contribute to the high specific capacity of MoS2/graphene superlattice. Detailed investigation of this is still under study.

Figure 5. (a) Linear sweep voltammetry (LSV) polarization curves of the MoS2/graphene superlattice, control superlattice, MoS2 nanosheets, PDDA-graphene, and Pt/C. (b) Tafel plots of the MoS2/graphene superlattice, control superlattice, MoS2 nanosheets, and Pt/C. (c) EIS Nyquist


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plots of the MoS2/graphene superlattice, control superlattice, and MoS2 nanosheets. (d) The polarization curves of the MoS2/graphene superlattice before and after the 105 s test. (e) Longterm stability measurement of the MoS2/graphene superlattice at a current density of 10 mA cm−2 for 105 s. We also examined the HER electrocatalytic activity of the MoS2/graphene superlattice. Figure 5a shows the cathodic polarization curves of the MoS2/graphene superlattice, control superlattice, MoS2 nanosheets, PDDA-graphene nanosheets and benchmark Pt/C. PDDA-graphene showed almost no electrochemical activity. The MoS2/graphene superlattice exhibited a much lower onset overpotential of 88 mV versus the reversible hydrogen electrode (RHE) compared with those of the pure MoS2 nanosheets (258 mV) and control superlattice (222 mV) and is also superior compared to other reported MoS2-based HER electrocatalysts (Table S2).5,9,14 Moreover, the MoS2/graphene superlattice required a low overpotential of 137 mV versus RHE to achieve significant hydrogen evolution (j = 10 mA cm−2). In contrast, the MoS2 nanosheets and control superlattice required higher overpotentials of 435 and 361 mV, respectively. The improved HER activity for the MoS2/graphene superlattice is further validated by Tafel plots. The MoS2/graphene superlattice possessed a Tafel slope of 48.7 mV decade−1 (Figure 5b), which is remarkably lower than those of the MoS2 nanosheets (156 mV decade−1) and control superlattice (103 mV decade−1). Although the Tafel slope with a value of 48.7 mV decade−1 is still higher than 30 mV decade−1 for the Pt/C electrocatalyst, the MoS2/graphene superlattice compares favorably to recently reported high-performance MoS2-based HER electrocatalysts (Table S2).9,14,65 These results confirm the greatly improved electrocatalytic activity of the MoS2/graphene superlattice, which can be attributed to improved charge transfer in the metallic MoS2/graphene superlattice-like structure, as revealed by electrochemical impedance


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spectroscopy (EIS) studies (Figure 5c). Compared with the MoS2 nanosheets and control superlattice, the EIS Nyquist plot of the MoS2/graphene superlattice revealed the smallest charge transfer resistance (Rct), suggesting rapid charge transfer processes and hence a rapid reaction rate in electrocatalytic kinetics. The metallic 1T phase MoS2 nanosheets in the MoS2/graphene superlattice could offer the catalytically active basal plane in addition to edges for enhanced HER performance,26,27 which is likely the reason for the higher activity of the MoS2/graphene superlattice than that of the control superlattice of 2H phase MoS2. In addition, due to the intercalation of the PDDA-graphene layers, the MoS2 layers in the MoS2/graphene superlattice show an increased interlayer spacing of ~2.5 nm. Such a highly expanded interlayer spacing compared to that of the restacked MoS2 nanosheets can largely reduce the Gibbs free energy of hydrogen adsorption for improved HER catalytic activity.14,15 The catalytic stability is another significant criterion for an HER catalyst. The long-term stability of the MoS2/graphene superlattice was evaluated via chronopotentiometric measurements at a current density of 10 mA cm−2. As shown in Figure 5d, the overpotentials of the MoS2/graphene superlattice remained almost unchanged. Figure 5e also records the electrocatalytic overpotential of the superlattice electrode during 105 seconds of continuous operation, indicating the ultrahigh electrochemical stability for HER. Figure S20 shows the TEM images of the MoS2/graphene superlattice after the HER stability test. The overall morphology composed of restacked nanosheets is preserved, suggesting the high structural stability of the as-obtained MoS2/graphene superlattice. In summary, the MoS2/graphene superlattice composed of alternately restacked unilamellar metallic MoS2 and modified graphene nanosheets was produced by a facile solution-phase direct assembly method for superior performance for SIBs and HER. As an anode for SIBs, the MoS2/graphene superlattice anode delivered a large specific capacity of 240 mA h g−1 at 51.2 A


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g−1 and more than 380 mA h g−1 at 10 A g−1 over 1000 cycles, while the control superlattice and MoS2 nanosheets could not be charged/discharged at such high current densities. The MoS2/graphene superlattice also showed a low onset potential of 88 mV vs RHE and a small Tafel slope of 48.7 mV decade−1 for HER, which is much superior to that of control superlattice (222 mV and 103 mV decade−1) and MoS2 nanosheets (258 mV and 156 mV decade−1), respectively. In addition, we note that this synthesis method could be applied to other 2D nanosheets, and thus, this method provides a facile route for the versatile manufacturing of specially designed structures and devices, in which different 2D molecularly thin building blocks with required properties can be layer-by-layer hybridized with each other by choice. Our findings will open new possibilities to exploit 2D nanosheets for fundamental studies and practical applications in energy storage and conversion.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental sections, TEM, SEM, AFM and zeta potential characterizations of unilamellar nanosheets, additional characterizations and electrochemical measurements of MoS2/graphene superlattice and control samples, and comparison of the electrochemical performance. AUTHOR INFORMATION Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was partly supported by the World Premier International Research Center Initiative on Materials Nanoarchitectonics (WPI-MANA), MEXT, Japan. P. X. thanks the support of the JSPS research fellowship.

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Graphene Superlattice for Efficient Sodium

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