Well-Constructed Single-Layer Molybdenum Disulfide Nanorose Cross-Linked by Three Dimensional-Reduced Graphene Oxide Network for Superior Water Splitting and Lithium Storage Property

A facile one-step solution reaction route for growth of novel MoS2 nanorose cross-linked by 3D rGO network, in which the MoS2 nanorose is constructed by single-layered or few-layered MoS2 nanosheets, is presented. Due to the 3D assembled hierarchical architecture of the ultrathin MoS2 nanosheets and the interconnection of 3D rGO network, as well as the synergetic effects of MoS2 and rGO, the as-prepared MoS2-NR/rGO nanohybrids delivered high specific capacity, excellent cycling and good rate performance when evaluated as an anode material for lithium-ion batteries. Moreover, the nanohybrids also show excellent hydrogen-evolution catalytic activity and durability in an acidic medium, which is superior to MoS2 nanorose and their nanoparticles counterparts.

A facile one-step solution reaction route for growth of novel MoS 2 nanorose cross-linked by 3D rGO network, in which the MoS 2 nanorose is constructed by single-layered or few-layered MoS 2 nanosheets, is presented. Due to the 3D assembled hierarchical architecture of the ultrathin MoS 2 nanosheets and the interconnection of 3D rGO network, as well as the synergetic effects of MoS 2 and rGO, the as-prepared MoS 2 -NR/rGO nanohybrids delivered high specific capacity, excellent cycling and good rate performance when evaluated as an anode material for lithium-ion batteries. Moreover, the nanohybrids also show excellent hydrogen-evolution catalytic activity and durability in an acidic medium, which is superior to MoS 2 nanorose and their nanoparticles counterparts.
T he ability to yield graphene and its hybrids opened up new opportunities because of its exceptional electronic, optical and mechanical properties 1,2 . In analogy with graphene, other kinds of single-layered or fewlayered inorganic two-dimensional (2D) materials and their heterostructures such as hexagonal BN and transition metal dichalcogenides (TMDs) derived from their layered bulk counterparts have been attached much attention due to their promising properties and a broad range of applications including electronics, optoelectronics, catalysis, energy storage and conversion devices [3][4][5][6][7][8] .
Molybdenum disulfide (MoS 2 ) as a typical layer-structured TMDs where the Mo layer is sandwiched between two sulfur layers by covalent bonds [8][9][10][11][12] has been intensely studied for electrochemical energy storage and conversion, including as an electrocatalyst for the hydrogen evolution reaction (HER) [13][14][15][16][17][18][19][20][21][22][23][24][25] , for electrode materials in lithium-ion batteries (LIBs) [26][27][28][29][30][31][32] , and as supercapacitors 33,34 , due to its good anti-corrosion, catalytic abilities and electrochemical activities. Theoretical 35 and experimental 36 results have demonstrated that the edges of 2D MoS 2 layers are coordinative unsaturated and thermodynamically unfavorable, and the basal surfaces are chemically inert. Consequently, the excellent electrochemical properties of 2D MoS 2 layers are closely related to the active sites located along the edges of the material 15,17,26,27 . On the other hand, due to the high surface energy and strong interlayer p-p interactions, the MoS 2 layered nanostructures tend to re-stack and condense which results in poor stability and the loss of active sites and unusual properties during practical applications 20,21,37 . Moreover, the intrinsic poor conductivity and structural pulverization of MoS 2 usually limit the energy storage and conversion process. Up till now many approaches have been adopted to overcome the limitations and improve the electrochemical performance. One feasible approach is the design and synthesis of MoS 2 based materials with reasonable composition, morphology, microstructure, and architecture on the nanoscale [17][18][19][20][21][22][29][30][31][32] . In particular, hierarchical hybrid structures, which are assembled by 2D MoS 2 layers and composited with electronically conductive agents for example, carbon nanofibers, carbon nanotubes, graphene, reduced graphene oxide (rGO) sheets and so on [23][24][25][38][39][40][41][42][43][44][45][46][47] , have drawn special interest. In the respect of hierarchical structures, three-dimensional (3D) architecture assembled with 2D MoS 2 layers not only well inherits the advantages from the single 2D layer but also arises novel properties due to the synergistic interactions between the layers 21,22,37 . The 3D architecture is also favored for preventing the aggregation of these 2D layers and thus retaining active sites 37,[48][49][50] . The nano-building blocks, 2D MoS 2 layers, can maximize the number of exposed active sites and provide extra active sites for ion storage. In addition, the 2D MoS 2 layers subunits can reduce effective distance for ions diffusion, enhance fast mass transport of reactants and products, and provide large electrodeelectrolyte contact area. Furthermore, when the 3D architecture is composited with carbonaceous matrix, the hybrids possess good electrical conductivity to facilitate electronic transfer and decrease the inner resistance of the electrochemical system [48][49][50][51] . All of the above are crucial for improving the HER and LIB performances.
Herein, we report a facile method for the assembly of 3D MoS 2 nanorose cross-linked by 3D rGO network (MoS 2 -NR/rGO) through a solution reaction route. In this system (As shown in Figure 1a), the 3D MoS 2 nanorose is constructed by single-layered or few-layered MoS 2 which is interconnected by 3D graphene network. MoS 2 -NR/ rGO exhibits high reversible capacity, excellent rate capability and significantly enhanced cycling performance making it promising for application in high-power LIBs. They also show excellent hydrogenevolution catalytic activity and durability in an acidic medium, which is superior to MoS 2 nanorose and their nanoparticles counterparts. As depicted in Figure 1b, the excellent electrochemical properties are attributed to the 3D assemble architecture and the enhanced electrical conductivity, and this may facilitate the transport and storage of lithium ion or mass to better withstand the volume change on cycling and expose more contact sites with active materials. In addi-tion, the strong electronic coupling between single-layers of MoS 2 -NR and ultrathin layer of 3D graphene network makes the electrons filled in the whole system quickly. 3D graphene network works as branches which transport the electrons from the electrode root to the MoS 2 -NR quickly which bring a superior electrochemical performance than the single component MoS 2 .

Results
The synthesis was achieved in a mixed solution of ethanol and octylamine at 200uC, in which the spontaneous self-assembly of MoS 2 layers and reduction of graphene oxide occured in the solution. The crystal structures of the yielding nanohybrids were characterized by XRD as shown in Figure 2. The results for the sample without addition of rGO, MoS 2 3D assembled tubes, rGO and standard pattern of MoS 2 are also displayed for comparison. It can be seen that the diffraction patterns of MoS 2 related samples are similar, and the identified peaks of the three samples can be indexed to hexagonal MoS 2 (JCPDS No. . The broadening of the diffraction peaks indicates the fine size of the samples. Moreover, the absent (002) diffraction peak that was observed in the case of well-stacked layered MoS 2   indicates that stacking of the single layers has not taken place in the samples, which suggests that the MoS 2 in the as-prepared samples should have single or few layers. Those results are in good agreement with our previous reports 37 . For the nanohybrid sample, in addition to the peaks from MoS 2 phase, there is additional diffraction peaks located at 2h 5 23.4u (marked with red star) which can be attributed to the (002) reflection of reduced GO sheets (JCPDS No. 75-1621).
The structure and morphology of the as prepared pristine MoS 2 and nanohybrids with addition of rGO were investigated by fieldemission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Typical FESEM images for the two kinds of samples are shown in Figure 3. For pristine MoS 2 sample, low magnification SEM image (Figure 3a) reveals that product consists of microspheres with diameters in the range of several hundred nanometers. From the high magnification SEM image (Figure 3b) it can be clearly seen that the MoS 2 nanoparticles present rose-like characteristic (termed as MoS 2 -NR in the following text), which are assembled with ultrathin MoS 2 nanosheets (Figure 3c,d). The corresponding Energy Dispersive Spectrometer (EDS) pattern (see Figure S1 in the Supplementary   hybrids can form an interconnected conducting network and act as an buffer zone to accommodate the volume change of MoS 2 -NR, which are important to improve the cycling stability and rate performance. Finally, the 3D architecture assembled with MoS 2 -NR and rGO nanosheets is favored for preventing the aggregation of these nano/microcrystals and graphene sheets, which is also essential for the cycling stability 52 . Similar to the case of Li ion battery, the 3D rGO nanosheets skeleton would play an important role in rapidly delivering electrons to the active MoS 2 sites for proton reduction and H 2 evolution. The 3D rGO electrons highways will reduce the potential drop which is induced by the intrinsically poor electric conduction of MoS 2 .  Figure S3 in the Supplementary Information) as we reported previously. However, from the peak positions it is observed that the Mo binding energy peak in MoS 2 -NR/rGO nanohybrids is negatively shifted as compared to MoS 2 -NR sample (DE 5 0.2-0.3 eV) (the inset in Figure 5b) indicating electron transfer process that may exist between MoS 2 -NR and rGO network, which is beneficial for decreasing the inner resistance of batteries and are favorable for stabilizing the electronic and ionic conductivity. High resolution spectrum of S 2p shows the peak located at 162.4 eV, which corresponds to the sulfur species in the MoS 2 ( Figure 5c). The high-resolution XPS spectra of the N 1s region for MoS 2 -NR/rGO nanohybrids and MoS 2 -NR are shown in Figure 5d. The peak shapes for the both samples are similar, and N element may come from nitrogen contained precursor in the solution (octylamine, ammonium molybdate).
The MoS 2 -NR/rGO nanohybrids and MoS 2 -NR samples are further examined by Raman spectroscopy as displayed in Figure 5e. The Raman bands located between ,100 cm 21 to , 1000 cm 21 can be assigned as vibration modes for MoS 2 . For MoS 2 -NR/rGO nanohybrids, apart from the MoS 2 Raman feature, there were another two Raman peaks centered at ,1358 and ,1575 cm 21 . The peak at 1575 cm 21 (G band) is attributed to the vibration of sp 2 hybridized C-C bond of in-plane hexagonal lattice. The peak at 1358 cm 21 (D band) is associated with the vibrations of carbon atoms with dangling bonds in plane terminations of the disordered graphite from the defects and disorders of structures in carbon materials 23,24,31 . In addition, a broader 2D peak appeared at around 2686 cm 21 , which is consistent with that of the few-layer graphene. Furthermore, compared with GO and rGO, the downshift of the G band in MoS 2 -NR/ rGO nanohybrids was observeed, which may be attributed to the incorporation of N heteroatoms. The increase in the I D /I G ratios from GO (0.78) to rGO (0.86), and the MoS 2 -NR/rGO nanohybrids (1.01) also confirms the conversion of GO to rGO with more disorderly stacked graphene sheets (see Figure S4 in the Supplementary Information). In our reaction system, the octylamine served as solvent and surface ligands, which is confirmed by FT-IR spectroscopy as shown in Figure 5f. The CH 2 and CH 3 stretching vibrations at 2800,3000 cm 21 and N-H modes at 1650,1450 cm 21 in the FT-IR spectrum indicate that the MoS 2 -NR/rGO nanohybrids and MoS 2 -NR samples were capped with octylamine 37 . In addition, the FT-IR spectrum of GO is also given for comparison. It exhibits C5O stretching at 1724 cm 21 , skeletal vibration of unoxidized graphitic domains at 1624 cm 21 , carboxyl O-H deformation at 1402 cm 21 , C-OH stretching at 1224 cm 21 and C-O stretching at 1057 cm 21 , which are all the characteristic functional groups of GO. Those peaks become weak or absent in the spectrum of MoS 2 -NR/rGO nanohybrids, further indicating that the GO sheets have been reduced to graphene 53 . H 2 production from electrochemical water splitting is an efficient approach to store those sustainable but intermittent energy such as wind energy, solar energy and so forth 54,55 . Few layered MoS 2 has been confirmed as one of excellent candidates as cathode material 15,17,18 . In this work, the electrochemical HER tests are performed using three-electrode system in the acidic condition of 0.5 M H 2 SO 4 solution (see Experiment section for details). As a reference, we also performed measurements using a commercial Pt/C catalyst which exhibits high HER catalytic performance. Typical linear sweep voltammetry (LSV) curve (j-V plot) exihites that MoS 2 -NR/rGO electrode presents a low onset overpotential (g) of ,115 mV (versus RHE) for taking off HER activity (Figure 6a). Further negative poten-tial induces rapid rise of cathodic current. The HER performances of commercial Pt/C catalyst, MoS 2 -NT, MoS 2 -NR and MoS 2 nanoparticles (MoS 2 -NP) are compared in the same experimental conditions. Commercial Pt/C (with 20 wt.% Pt loading, YiBang/RuiBang New Power Sources Technology Co. LTD.) catalyst shows the highest HER activity with negligible onset overpotential. MoS 2 -NP exhibits negligible HER activity during the studied electrochemical window. The characters of other samples are among them, the MoS 2 -NR/rGO hybrid catalysts and MoS 2 -NT catalysts exhibited best HER activity. As a typical reference metric for electrochemical catalytic performance, the overpotential value for 10 mA/cm 2 current density is frequently employed 18 . Interestingly, MoS 2 -NR/rGO hybrid catalysts require , 210 mV to achieve 10 mA/cm 2 , which is far better than free MoS 2 -NR. Because the latter one is limited by the less exposed sites for proton reduction and low electrical conductivity the HER performance particularly in terms of current density. As shown in Figure 1, when the rGO nanosheets were inserted into the MoS 2 -NR, electrons can be rapidly delivered to the active MoS 2 sites for proton reduction and H 2 evolution, which is especially important for the high overpotential polarization region. Tafel plots based on polarization curves are acquired to calculate their electrochemical dynamic parameter of Tafel slope, as shown in Figure 6b. The linear regions of Tafel plots were fit to Tafel equation (g 5 a 1 blogj, where j is the current density and b is the Tafel slope) to obtain slope 35,56 , which yields Tafel slopes of ,38, ,46, ,60 and 175 mV/decade for Pt/C, MoS 2 -NR/rGO, MoS 2 -NR and MoS 2 -NP, respectively. Obviously, MoS 2 -NR/rGO hybrid catalysts possess lower Tafel slope than free MoS 2 -NR and MoS 2 -NP. Lower Tafel slope gives rise to less overpotential demand toward high current density acquired. Moreover, we can deduce the HER mechanism based on Tafel slope. In general, the following three principal steps can be involved in a HER 23    Any HER mechanism consists of the discharge step and at least one desorption step. If the Volmer step associated with proton adsorption is rate-determining, a slope of ,120 mV/decade should be obtained, while Heyrovsky and Tafel steps should give ,40 and ,30 mV/decade, respectively 23 . Similar to many other HER catalysts such as Ni 2 P (46 mV/decade) 55 , the observed Tafel slope of ,46 m V/decade for MoS 2 -NR/rGO hybrid catalysts in the current work is close to that of Heyrovsky reaction with 39 mV/decade, so we can assign HER mechanism as quasi-Volmer-Heyrovsky mechanism that electrochemical desorption is the rate-limiting step, although the observed Tafel slope of ,46 m V/decade does not absolutely match any value of the above discussed three steps. While, Tafel slope of 60 mV/decade of free MoS 2 -NR catalysts may indicate that the reaction is to some extent to be determined by the discharge step with higher Tafel slope. This catalytic process mainly occurs either on the surface or at the exposed edges of the MoS 2 layers. However, the freshly prepared MoS 2 layers have a tendency to aggregate during practical application even in the drying process, resulting in the loss of active sites of ultrathin 2D nanostructures. This rose structure assembled by single-layer will expose many edges, which is closely related to the large surface area of the layers and decrement the agglomeration efficiently. In addition, this 3D graphene network composed of interconnected ultrathin graphene layer penetrates the whole system and supports these MoS 2 -NR from different direction, which facilitates the electron transportation from the electrode to the surfaces and edges of the single-layer of MoS 2 . The strong electronic coupling between single-layers of MoS 2 -NR and 3D graphene network make the electrons filled with whole system quickly. 3D graphene network works as branches which transport the nutrient (electron) from the root (electrode) to the roses (MoS 2 -NR).
As shown in the inset of Figure 6c, the electrochemical interface electrode and solution for HER can be modeled by a equivalent circuit, which consists of Ohm resistance (R V ), double layered capacitance (C d ) and charge transfer resistance (R ct , Faradic resistance). We ignore the Warburg resistance (R W ) due to the low overpotential polarization. It is well known that R ct is highly associated with the electrochemical dynamics of HER. The characterizations of the above circuit elements including fluent charge transport could be identified by electrochemical AC impedance spectroscopy (EIS) (Figure 6c). Under the same bias of -0.40 V (vs. Ag/AgCl), MoS 2 -NR/rGO shows much lower charge transfer resistance than other contrast samples. According to the Nyquist plots, the electron transfer resistance R ct of MoS 2 -NR/rGO is only 170 V, which is far less than that of MoS 2 -NT (230 V) and MoS 2 -NR(450 V). Moreover, the stable-state method of chronopotentiometry curves (with current density of 1 mA/cm 2 ) was carried out to further investigate the HER performance. For the MoS 2 -NR/rGO, MoS 2 -NR and MoS 2 -NT, they all reached stable state quickly (Figure 6d). Well consistent with the LSV and EIS study, the MoS 2 -NR/rGO hybrid catalysts demand the lowest overpotential to acquire the current density of 1 mA/cm 2 . To evaluate the electrochemical performance of the MoS 2 -NR/ rGO nanohybrids and MoS 2 NRs for LIB applications, the galvanostatic charge and discharge measurements of the assembled cells are performed at a rate of 0.1 Ag 21 in the voltage range of 0.01-3 V (versus Li 1 /Li) at room temperature. Figure 7a and b show the charge-discharge voltage profiles of MoS 2 -NR and MoS 2 -NR/rGO nanohybrids cells for the first three cycles. The shape of the first discharge curves is not significantly altered indicating the stability of the nanostructures as anode. Specifically, as shown in Figure 7a for MoS 2 -NR, in the initial discharge process, a voltage plateau at ,0.7 V followed by tail at a lower voltage is observed, which is attributed to the irreversible reaction between the electrolyte and MoS 2 . In the second and third discharge curves, the potential plateau at ,0.7 V in the first discharge disappears. In the first three charge curves, the MoS 2 NRs electrodes display an inconspicuous potential plateau at ,2.3 V due to the lower crystallinity and defect sites of the graphene-like MoS 2 . For MoS 2 -NR/rGO nanohybrids electrodes as shown in Figure 7b, the charge-discharge voltage profiles possess similar characters with that of MoS 2 -NR electrodes. It can also be seen that the initial discharge and charge capacities are 1010 and 792 mAhg 21 for MoS 2 -NR, 1196 and 997 mAhg 21 for MoS 2 -NR/ rGO nanohybrid electrodes, yielding irreversible capacity losses of 22% and 17%, respectively. The values of initial discharge and charge capacities for MoS 2 -NR/rGO nanohybrid electrodes are much larger than the capacity of MoS 2 -NT, MoS 2 -NP. The subsequent Coulombic efficiency (the ratio of charge capacity to discharge capacity) quickly increases to 97%, 99.6% and 97.9%, 99.8% in the second, third cycle for MoS 2 -NR and MoS 2 -NR/rGO nanohybrid electrodes, respectively. It should be mentioned here that the capacity of the MoS 2 -NR/ rGO nanohybrid s is calculated based on the total weight including rGO and MoS 2 . The mass fraction of carbon in the hybrids can be determined to be ,33.3% by employing thermogravimetric analysis (see Figure S5 in the Supplementary Information for details).
The lithium storage behavior of the MoS 2 -NR/rGO nanohybrid cell is investigated by cyclic voltammetry (CV) experiments between 0.01 and 3 V at a scan rate of 0.5 mVs 21 . Figure 7c displays the representative CV graph of the first three cycles for MoS 2 -NR/rGO nanohybrids cell. In the first cathodic sweep, the peak at ,0.82 V is attributed to the intercalation of lithium ions into the MoS 2 lattice which transforms the triangular prism into an octahedral structure, i.e., intercalation of lithium-ion on different defect sites of MoS 2 to form Li x MoS 2 . The other peak at ,0.46 V is assigned to the complete reduction of MoS 2 to Mo nanoparticles embedded into a Li 2 S matrix. In the reverse anodic scan, a very small oxidation peak at ,1.88 V is found, corresponding to the partial oxidation of Mo. Another peak at ,2.39 V can be attributed to the oxidation of Li 2 S into S. The present electrochemical details are consistent with the previous results 21, 40 . In the subsequent cycles, the anodic peaks intensity decreased sharply, suggesting an irreversible conversion reaction during the lithium-ion insertion/extraction process. The reversible capacity loss arising is due to the incomplete conversion reaction and the formation of SEI layer due to the irreversible degradation of electrolyte and other secondary reactions. Similar phenomena have also been reported by others [37][38][39][40][41][42] .
The involved reactions can be described as follows:  Figure 7f shows the Nyquist plots of the AC impedance analysis for MoS 2 -NR/rGO and MoS 2 -NR cells. In the impedance spectrum, the high frequency semicircle is attributed to the contact resistance occurring because of the SEI film, the medium-frequency semicircle is related to the charge-transfer resistance on electrolyte and the electrode interface, and the inclined lines correspond to the Li diffusion process inside the electrode material 49 . The Nyquist plots in the frequency range from100 kHz to 0.01 Hz clearly show that the diameter of the semicircle of MoS 2 -NR/rGO nanohybrid cell is much smaller than that of the MoS 2 -NR cell, indicating that the addition of rGO enhanced the charge transfer process compared to the bare MoS 2 -NR, which is beneficial for improving rate capability. The detail kinetic parameters of the cells are further investigated by modeling AC impendence spectra using the standard equivalent circuit as shown in the inset of Figure 7f. The values of the electrolyte resistance R e , charge-transfer resistance R ct , and SEI resistance R SEI of MoS 2 -NR/ rGO cell are 2.2, 203.1, and 140.5 V, respectively, which are significantly lower than those of MoS 2 -NR (2.8, 252.2, and 173.2 V). The smaller charge-transfer impedance value can lead to highly utilization of the cell even under high rate discharge conditions as reported.
The EIS results show that the addition of rGO not only preserved the high conductivity of the composite electrode, but also largely enhanced the electrochemical activity of MoS 2 -NR during the cycling processes. After the rate capability testing (50 cycles), the morphology and structure of the MoS 2 -NR/rGO nanohybrid electrodes were checked by FESEM observations. The sample still maintain the initial morphology after the cycling test (see Figure S6 in the Supplementary Information), which reveals the good stabilities of the nanohybrid structures during charge/discharge cycling.

Discussion
The excellent lithium storage performance of the MoS 2 -NR/rGO can be attributed to the rational design of the unique MoS 2 nanostructure and the synergistic effect between MoS 2 NRs and rGO. (1) 3D assembly of single-layer MoS 2 nanosheets into flowers. Firstly, the singlelayer MoS 2 nanosheets as the nanobuilding blocks can provide extra active sites for the storage of lithium ions, which is beneficial for enhancing the specific capacity. Secondly, the single-layer MoS 2 nanosheets subunits can reduce effective diffusion distance for Li ions diffusion and large electrode-electrolyte contact area for high Li ions flux across the SEI layer, resulting in enhanced rate capability. Thirdly, the MoS 2 -NR can accommodate the local volume change upon charge/discharge cycling and is able to alleviate the problem of pulverization and aggregation of the electrode material, hence improving the cycling performance. Fourthly, The 3D nanoflowers architecture assembled with MoS 2 -NR and rGO nanosheets is favored for preventing the aggregation of these nano/microcrystals and graphene sheets, which is also essential for the cycling stability. (2) The addition of rGO nanosheets. Firstly, rGO nanosheets in the obtained nanohybrids can act as an buffer zone to accommodate the volume change of MoS 2 -NR during charge/discharge cycling processes. Secondly, the rGO sheets have a good electrical conductivity and serve as the conductive channels between MoS 2 -NR, which decrease the inner resistance of LIBs. Finally, The electron transfer induced by the interaction between MoS 2 -NR and rGO is also beneficial for decreasing the inner resistance of batteries and are favorable for stabilizing the electronic and ionic conductivity, therefore leading to a higher reversible capacity.
In summary, we have successfully constructed a 3D MoS 2 nanorose cross-linked by rGO network by a facile route, in which the 3D MoS 2 nanorose is constructed by single-layered or few-layered MoS 2 . When used as the anode materials of LIBs and catalyst for HER, the as-prepared MoS 2 -NR/rGO nanohybrids delivered high performance as compared to rGO free MoS 2 -NR, MoS 2 -NP and MoS 2 -NT. It is suggested that the good electrochemical performance can be attributed to the 3D assembled hierarchical architecture and the interconnection of 3D rGO network. We believe that the properties of the electrode materials and catalyst can be further optimized by carfully tailoring the microstructures, including morphology, composition, crystal plane structure, and assembled fashion of the building blocks. min 21 under an air flow), and X-ray photospectroscopy (XPS; Escalab 250, Al Ka, binding energies are referenced to the C 1s of carbon contaminants at 284.6 eV). Crystallographic information for the samples was collected using a Bruker Model D8 Advance X-ray powder diffractometer (XRD) Cu-Ka irradiation (l51.5418 Å ). Raman spectra were collected by using Raman microscopes (Renishaw, UK) under a 488 nm excitation. Fourier transform infrared spectra (FT-IR) spectra were recorded with a Nicolet 205 FTIR spectrometer using the KBr pellet technique.

Synthesis of MoS
Electrochemical water splitting measurements. The electrocatalytic performance was measured in N 2 purified 0.5 M H 2 SO 4 solution (pH < 0.31) with CHI 660C electrochemical workstation by three electrode system. The catalysts ink was prepared by dispersing 4 mg catalyst powder into 1 mL of N, N-dimethylfomamide (DMF) with the assistence of untrasonic. 3.5 mL of the catalyst ink was dropped onto a glassy carbon electrode (3 mm diameter) and dried naturally as work electrode. The catalyst loading was about 0.2 mg/cm 2 . A graphite rod electrode was used as counter electrode, and a KCl-saturated Ag/AgCl electrode was used as reference electrode. All the measurements were performed at room temperature (about 18uC). The stable linear scanning voltammograms (LSV) were recorded at a scanning rate of 5 mV/s with a quiet time of 5 seconds. The electrochemical AC impedence measurements were performed under the bias of -0.40 V (vs. Ag/AgCl electrode) from 100 kHz to 0.1 Hz with an AC voltage amplitude of 2 mV and a quiet time of 2 seconds. The chronopotentiometry curves were recorded at the current density of 1 mA/cm 2 . The current density was normalized by geometric electrode area (0.07 cm 2 ), and the potential was iR-drop corrected and normalized to reversible hydrogen electrode (RHE) potential as the following equation: E RHE 5 E SHE 1 0.0591 pH -iR V 5 E App 1 Q Ag/AgCl 1 0.0591pH -iR V . Herein, E SHE is the potential versus standard hydrogen electrode (SHE) potential, E App is the applied potential vs. Ag/AgCl reference, Q Ag/AgCl is the electrode potential of KCl-saturated Ag/AgCl reference (0.197 V vs. SHE) and R V is the Ohm resistance containing solution resistance and electric curve resistance.
Cell assembly and lithium storage performance measurements. To measure the lithium storage performance, electrodes were constructed by mixing the active materials, acetylene black (AB) and poly(vinylidene fluoride) (PVDF), in a weight ratio of 70520510. The mixture was mixed with n-methyl pyrrolidone (NMP) to form slurry and spread onto copper foil. The electrode was dried under vacuum at 120uC for 5 h to remove the solvent before pressing. Then the electrodes were cut into disks (12 mm in diameter) and dried at 100uC for 24 h in vacuum. The cells were assembled inside an Ar-filled glove box by using a Li metal foil as the counter electrode and the reference electrode and microporous polypropylene as the separator. A solution of 1 M LiPF6 in a 15151 weight ratio of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) was used as the electrolyte. Assembled cells were allowed to soak overnight, and then electrochemical tests on a LAND battery testing unit were performed. The cells were galvanostatically charged and discharged in a current density range of 0.1 Ag 21 within the voltage range of 0.01-3.0 V for 80 cycles. For the high rate testing, the discharge current gradually increased from 0.1 Ag 21 to 0.5, 1.0, and 5.0 Ag 21 , then decreased to 0.1 Ag 21 . Electrochemical impedance spectroscopy (IM6, Zahner) was carried out by applying an AC voltage of 5 mV in the frequency range of 100 kHz to 0.01 Hz.