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Carbon 99 (2016) 35e42

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Carbon

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Hydrothermal synthesis of layered molybdenum sulfide/N-dopedgraphene hybrid with enhanced supercapacitor performance

Bingqiao Xie a, Ying Chen a, *, Mengying Yu a, Tu Sun a, Luhua Lu a, Ting Xie c,Yong Zhang b, c, Yucheng Wu b, c

a Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 388 Lumo RD, Wuhan 430074,Chinab School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, Chinac Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei, 230009, China

a r t i c l e i n f o

Article history:Received 9 June 2015Received in revised form28 November 2015Accepted 30 November 2015Available online 10 December 2015

* Corresponding author.E-mail address: chenying@cug.edu.cn (Y. Chen).

http://dx.doi.org/10.1016/j.carbon.2015.11.0770008-6223/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Graphene-based composites have been deemed as promising materials in renewable energy-storageapplications. Herein, we report a hybrid architecture consisting of layered molybdenum sulfidenanosheets/N-doped graphene (MoS2/NG) synthesized by one-pot hydrothermal method. By adjustingprecursor ratios, flower-like MoS2/NG hybrid with nitrogen content of 3.5 at.% on the graphene layers canbe obtained. Electrochemical characterizations indicate that the maximum specific capacitance of theMoS2/NG electrodes reaches up to 245F/g at 0.25A/g (and 146F/g at 20A/g). In addition, the electrodeexhibits superior cyclic stability with 91.3% capacitance retention after 1000 cycles at 2A/g. Theoutstanding performance of the MoS2/NG hybrid benefits from the synergistic effect between the layeredMoS2 and N-doped graphene.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

To meet the ever-increasing energy and power demands,exploration of advanced electrode materials has been triggered fordeveloping novel energy storage devices. Among various energystorage devices, supercapacitors (SCs) have attracted tremendousattention due to their superior advantages such as high powerdensity, excellent cycling stability and fast charge/discharge capa-bility [1].

Two-dimensional (2D) nanosheets feature unique structure aswell as unusual chemical and physical properties, which promisegreat applications in catalysts [2], solid lubricants [3] and renew-able energy storage [4,5]. While a wide range of graphene-relatedachievements have been harvested, researchers are now begin-ning to turn to other two-dimensional crystals such as monolayerand few-layer crystals of hexagonal boron nitride (hBN), and layer-structured transition-metal sulfides [6e9]. Among various 2Dnanosheets, MoS2 nanosheet is especially attractive due to itsintrinsic fast ionic conductivity [10](than zeolite-like oxides) and

higher theoretical capacity of alkaline metal cations (than graphite)[11], rendering it potentially suitable for supercapacitor applica-tions. MoS2 is consisted of covalently bonded SeMoeS sheets heldtogether by van der Waals force and the weak interlayer couplingfacilitates the extraction of ultra-thin layers by exfoliation. How-ever, this material has low electrical conductivity and the tendencyto form fullerene-like nanoparticles or nanotube structures duringthe fabrication process which may lead to inferior electric/ionictransmission rate and lower effective contact areas between elec-trode and electrolyte. By introducing graphene layers into thesynthesis of MoS2, high surface area and superior conductivity ofgraphene would provide an ideal platform for the growth of MoS2nanostructures. Moreover, complex morphology of the MoS2/gra-phene hybrid is expected by doping heteroatoms nitrogen, boronand phosphorous, which endows it with enhanced charge storagecapabilities, electronic conductivity and structural stability.

Although MoS2/graphene (or MoS2/N-doped graphene) hybridshave been investigated for lithium ion batteries and hydrogenevolution reaction applications [5,12e15], their potential for SCshas seldom been explored [16e19]. Firmiano fabricated layeredMoS2/graphene hybrids by microwave heating method, discussingthe effect of covalent chemical bonds MoeOeC for electrochemicalsupercapacitor applications [18]. Huang synthesized MoS2/

B. Xie et al. / Carbon 99 (2016) 35e4236

graphene hybrid by L-cysteine-assisted solution-phasemethod, andthe maximum specific capacitance of the hybrid electrode reachedup to 243 F/g [19]. Patil [17] demonstrated the layer-by-layer (LBL)assembling technique (together with the intercalating of MoS2 byH2SO4) to obtain the MoS2-GNs hybrid film, the obtained MoS2nanosheets are large-area, mono-layered/few-layered and well-crystallized, which favorably contribute to the high Csp value(255F/g at 2A/g) and the excellent cycling performance (only 7%decays after 1000 cycles at 2A/g) of the hybrid. However, thepreparation methods involved in this article are relatively compli-cated and low-yield. To the best of our knowledge, the hybrid ofMoS2 nanosheets based on N-doped graphene for SCs applicationshas not been reported in previous literature.

In the present work, we report a facile way of constructing anovel 3D architecture of layered MoS2/N-doped graphene hybridusing sodium molybdate, L-cysteine, graphene oxide and urea asstarting materials by a hydrothermal method. The layered MoS2 isface-to-face assembled on the graphene sheet followed by N atomdoping. Compared with the MoS2/graphene hybrid and N-gra-phene, this hybrid demonstrates superior electrochemical perfor-mance as the supercapacitor electrode, such as high specificcapacitance at high current value and good cycling stability. Theremarkable supercapacitor performance of the hybrid could beattributed to the synergistic effect between layered MoS2 and N-doped graphene.

2. Experimental

2.1. Sample preparation

2.1.1. Synthesis of graphene oxide (GO)Natural graphite powder (Qingdao Ruisheng graphite Co., Ltd.)

was oxidized to graphene oxide by a modified Hummers method.The detailed procedure was as follows: 3 g graphite powder wasput into a 1000 mL flask, with 360 mL H2SO4 (98%) and 43 mLH3PO4 (85%) added. The mixture was stirred under an ice waterbath, then 18 g KMnO4 was slowly added in the flask undervigorous agitation, keeping the reaction temperature lower than5 �C for 0.5 h. Next, the mixture reacted at room temperature for1 h, and subsequently the mixture was heated to 50 �C and allowedto stir for 5 days. Afterward, the reaction system was removed tothe icewater bath followed by adding a volume of 180mLH2O, afterthat the mixture was continuously stirred below 20 �C for 0.5 h,followed by a slow addition of 30 mL H2O2 (30%) and 10 mLconcentrated HCl (38%). Then the obtained suspension wascentrifuged by rising with large amounts of water repeatedly untilthe pH reached ~7, and graphene oxide powder (~4.5 g with 3 ggraphite powder) was obtained after freeze drying.

2.1.2. Synthesis of MoS2/graphene sheets (GNs) hybridsThe hybrids were prepared by a routine hydrothermal method.

First, 0.1 g GO was transferred into a 200 mL flask with adding60mL H2O, followed by the addition of 0.5 g of Na2MoO4$2H2O, and1 g L-cysteine. Second, 0.1 M NaOH was added to the mixture toadjust the pH to 6.5. After sonication for 30min, the above mixturewas stirred continuously for 20 min. After that, the hybrid wastransferred into a 80 mL Teflon-lined stainless steel autoclave,sealed tightly and heated at 180 �C for 36 h followed by naturalcooling to room temperature. The obtained product was washedand filtrated with DI water until the pH of the solution approached7. Finally, the filter cake was dried at 80 �C for 12 h to obtain MoS2/GNs hybrid.

2.1.3. Synthesis of N-doped graphene (NG) and MoS2/NG hybridswith different precursor ratios

Synthesis of MoS2/NG(5) was conducted following the sameprocess as that of growing MoS2/GNs, except that 10 g urea wasintroduced during the sonication process. MoS2/NG(5) representsthe hybrid with a precursor mass ratio of GO:Na2MoO4:L-cysteine:Urea ¼ 1:5:10:100, analogically, MoS2/NG(3), MoS2/NG(1.5) and MoS2/GNs represent the precursor mass ratio of1:3:6:100, 1:1.5:3:100 and 1:5:10:0, respectively. Accordingly, theyield of these hybrids also varies with the adding amount of theprecursors and appears to be 395, 327, 243 and 374 mg for MoS2/NG(5), MoS2/NG(3), MoS2/NG(1.5) and MoS2/GNs, respectively. Forcomparison, NG and pure MoS2 was prepared with a precursormass ratio of GO:Na2MoO4:L-cysteine:Urea ¼ 1:0:0:100 andGO:Na2MoO4:L-cysteine:Urea¼ 0:1.5:3:0 in the same experimentalcondition.

2.2. Characterization of GO, NG, MoS2/NG and MoS2/GNs hybrids

X-ray diffraction (XRD) patterns of the samples were recordedby a Bruker D8 ADVANCE X-ray diffractometer with Cu Ka radiation(l ¼ 0.154187 nm). High resolution transmission electron micro-scopy (HRTEM) images were obtained by a JEOL JEM-200CX mi-croscope operating at 200 kV. Morphology analysis was performedwith scanning electron microscope (SEM). The elemental compo-sition of the samples was analyzed by X-ray photoelectron spec-troscopy (XPS). Raman spectrawere collected on a Jobin Yvon LaborRaman HR-800spectrometer with an argon ion laser (l ¼ 514 nm)in ambient atmosphere.

2.3. Electrochemical measurements in a three-electrodes system

A mixture containing 80 wt% active materials (3 mg), 10 wt%carbon black, and 10 wt% poly-tetrafluoroethylene (PTFE) was wellmixed in N,N-dimethylformamide (DMF) until they formed a slurrywith the proper viscosity, and then the slurry was uniformly laid ona piece of Ni foam about 1 cm2 that was used as a current collectorand then dried at 80 �C for 2 h. The Ni foam coated with thecomposite was pressed for 1 min under 8.0 MPa and dried at 120 �Cfor another 12 h. A Pt electrode and Hg/HgO electrode filled with1 M KOH aqueous solution were used as the counter electrode andreference electrode respectively. Cyclic voltammetry (CV), galva-nostatic charge/discharge (GCD) and electrochemical impedancespectroscopy (EIS) were measured on a CHI760E electrochemicalworkstation in a three-electrode system in 6 M KOH electrolyte.The specific capacitance is calculated according to equation C ¼ It/V, where I is the mass normalized current (A/g), t is the dischargetime (t), and V is the voltage (1 V). The reported specific capacitanceis all normalized to the weight of sample.

3. Results and discussion

3.1. Structure and morphology characterization of the hybrids

The synthesized freestanding graphene oxide (GO) is trans-parent and displays gauze-like morphology with slight folds andcrimped edges which might originate from the bonded oxygenicfunctional groups. Thickness of the typical GO sheet was measuredto be 1.07 nm by AFM (Fig. S1c). The formation process of MoS2/GNsand MoS2/NG hybrids is illustrated in Fig. 1. First, the Mo precursorwas fully dispersed on active sites of the GO under the ultrasonictreatment. Subsequently, small-sized MoS2 crystals were in-situformed on the surface of graphene nanosheets, and finally MoS2/-graphene sheets grew and crossed into 3D flower-like structure.Formation of this 3D architecture was attributed to self-assembly

Fig. 1. The formation process of MoS2/NG and MoS2/GNs hybrids.

B. Xie et al. / Carbon 99 (2016) 35e42 37

(jointing or coalescing) of the flexible graphene during the hydro-thermal process [20]. When urea was added, N atoms were alsoinduced into the graphene surface and the formed MoS2 nano-sheets stacked severely on the graphene or N-doped graphenenanosheets.

In order to tune the microstructure of MoS2/NG which plays acrucial role in dominating the performance of SCs, control experi-ments with different [Na2MoO4]/[GO] ratios were conducted. SEMimages (Fig. 2aec) suggest that the morphology of MoS2/NG isgreatly influenced by the precursor ratios. In general, the hybridnanosheets tend to stack together or self-assemble into 3D flower-like architecture due to high surface energy of 2D materials. Bycarefully controlling the ratio of MoS2 to graphene, aggregation ofthe nanosheets was restrained. Herein, MoS2/NG(1.5) hybrid withpetal-like morphology was obtained at 180 �C as shown in the TEMimages (Fig. 2d and Fig. S1b). Chang et al. [13] ever synthesizedMoS2/graphene hybrids with similar morphology with 1:2 M ratioof MoS2 to graphene at higher hydrothermal temperature (240 �C).

As indicated in the energy dispersive X-ray (EDX) spectra(Fig. 2d), main elements of the composites include Mo, S, C, N andO(Cu is from substrate). Fig. 2e gives the high-resolution TEM(HR-TEM) image of the MoS2/NG. It shows that the layered MoS2 with alattice spacing of 0.62 nm is dispersed on the surface of NG

Fig. 2. Microstructure of MoS2/NG hybrid: (aec) SEM images, [Na2MoO4]/[GO] ratio is (1

nanosheets. The discontinuous MoS2 lattice fringes imply that thegrowth of layered MoS2 is restrained by the graphene substrate,typically revealed by the growth of MoS2 (002) plane [13,14].

XRD patterns of the GO, MoS2/GNs and MoS2/NG are shown inFig. 3a. The peaks at 14.3ο, 33.8ο, and 57.5ο are attributed to the(002), (100), and (110) plane of MoS2 crystal (2HeMoS2, JCPDS 37-1492) [14]. The relatively high intensity peak of (002) plane at2q ¼ 14.3ο, corresponding to a d-spacing of 0.62 nm, indicates astacked layered structure of MoS2 crystals. A weak diffraction peakat 2q ¼ 25.2ο can be indexed to the (002) plane of graphene.Compared with the pristine MoS2 crystal, the patterns of MoS2/GNsand MoS2/NG(1.5) hybrids are weak and broad because ofconfinement growth of MoS2 crystal on the graphene. Fig. 3b dis-plays Raman spectra of above samples. All samples exhibit twodominant Raman peaks at 1349 and 1580 cm�1, which match wellwith the D and G bands of graphene, respectively [20]. The ID/IGvalue for NG(1.02) is smaller than that for GO(1.12), revealing thatthe delocalized p-conjugation is partially restored during the hy-drothermal process. The increase of the ID/IG value for MoS2/NG(1.5)indicates that the introduction of MoS2 gives rise to higher disorderof the carbon lattice. The inset of Fig. 3b illustrates Raman char-acteristic peaks of MoS2, the two peaks at 376 and 406 cm�1 areassociatedwith the in-plane E12g and out-of-plane A1gmodes of the

.5:1), (3:1) and (5:1), respectively, (d) TEM and (e) HRTEM images of MoS2/NG(1.5).

Fig. 3. (a) XRD patterns of GO, MoS2/GNs and MoS2/NG(1.5) hybrids. (b) Raman spectra of GO, NG and MoS2/NG(1.5).

B. Xie et al. / Carbon 99 (2016) 35e4238

hexagonal MoS2 [15].Composition and chemical bonding configuration of the NG and

MoS2/NG were characterized by XPS. XPS survey spectra of allsamples are shown in Fig. 4a and Fig. S4, which confirm the pres-ence of C, O, N, S and Mo elements with the Mo/S atomic ratio ofz1:2 (see Table 1), which is in good agreement with the stoi-chiometric ratio of MoS2. It is believed that the better dispersity ofMoS2/NG (compared to pure MoS2) in water (Fig. S2) is tightlyconnected with the proper content of oxygen atoms. According tothe data in Table 1, the amount of MoS2 obviously increases withincreasing concentration of sodium molybdate accompanied bydeclining nitrogen doping content, implying a competition be-tween the nucleation of the MoS2 and nitrogen doping on occu-pying the active sites on the graphene oxide surface [14,21], whichin turn accounts for lower N content of all hybrids than that of pureNG(6.0 at.%). The high-resolution N 1s spectra of the threeMoS2/NG(Fig. 4d) can be fitted into five peaks centered at about 398eV,399.5eV, 401.5eV, 404eV, and 405.8eV, which are assigned to pyr-idine N(N-6), pyrrolic N or pyridone N(N-5), quaternary N(N-Q),and oxidized pyridine N(N-X) [22], respectively. The high-resolution Mo3d spectra of the MoS2/NG(1.5) are shown inFig. 4b. It can be divided into five peaks, and the one centered at226.3eV actually corresponds to S2s of MoS2. The two intense Mo3d5/2 (229.3eV) and Mo 3d3/2 (232.4eV) components are charac-teristic peaks of MoS2, while the peaks centered at 230.2 and235.9eV confirm the presence of MoeO(3d5/2) and Mo5þ [14].Likewise, S species are determined from the high-resolution XPSS2p spectrum (Fig. 4c). The main doublet located at binding en-ergies of 162.0 and 163.1 eV corresponds to the S 2p3/2 and S 2p1/2lines of MoS2 [14], respectively. Meanwhile, the high-energycomponent at 169.2eV can be assigned to S4þ species in sulfategroups (SO3

2�) [14,23], and these groups could locate at the edges ofMoS2 layers, which were still detected even after removing solublesalts by thorough washing.

The O 1s spectra of MoS2/NG can be deconvoluted [24,25] intoquinone(OeI) or MoeO, phenolic hydroxyl or ether(O-II), andcarboxyl(O-III) peaks centered at 531.5, 532.8, and 534.7eV,respectively (Fig. 4e). Decreasing the amount of sodium molybdatecauses an obvious increase in the percentage of O-II þ O-III andpeaks at 93% (MoS2/NG(1.5)). It is worthy of note that reduction ofO-II and deprotonation of O-III exhibit quasi-reversible pseudoca-pacitances [24]. The Fourier transform infrared (FTIR) spectra(Fig. S3) of MoS2/GNs and MoS2/NG contain a band at 950 cm�1

which confirms the presence of MoeO stretching vibrations. Theseresults support the assumption that MoS2 layer and graphene areconnected via CeOeMo, which agrees well with the assumption

proposed by Firmiano [18].Fig. 4f also gives the increasing tendency of N-6þN-5, N-Q, O-

IIþO-III (calculated from Table S1), which are generally regarded asfavored groups for SC electrode materials to improve the conduc-tivity (N-Q) and pseudocapacitance. And the comprehensive effectof these N-and O- groups for the electrochemical performance ofMoS2/NG samples as shown in Table 1.

3.2. Formation mechanism

Comparing the microstructure and chemical composition of thehybrids (Fig. 1 and Table 1) of MoS2/GNs and MoS2/NG(5), it can beseen that the N-doped hybrid exhibit the stacking layered struc-ture. Although the precursor ratio ([Na2MoO4]/[GO] ¼ 5:1) forgrowing the two hybrids was the same, nitrogen doping affectedthe morphology of the hybrid significantly while brought littledifference to the MoS2 loading amount on the graphene. Highprecursor ratio would promote the formation of MoS2 nanosheetsthat would decrease the contact area between the graphene andthe electrolyte and hinder the formation of the interconnectedconducting network as well, which restricts the ionic transport inthe charge/discharge process [18,26]. And decrease of the Moprecursor amount would make the hybrid retrieve the novel 3Darchitecture morphology. These findings confirm that NG actuallyacts as a movable flat (substrate) (see Fig. S1a) for the nucleationand growth of MoS2, and the morphology of the hybrid is closelylinked to the loading of MoS2. The low crystallinity of MoS2 shownin XRD (Fig. 3a) and HRTEM (Fig. 2e) is attributed to the involve-ment of the graphene in restraining the growth of layered MoS2crystals, especially the (002) plane of MoS2. Significantly, oxygenicfunctional groups on the GO surface can not only facilitate thenucleation of MoS2 but also accelerate the N-doping process.

The growthmechanism ofMoS2/graphene has been discussed inmany reports [13,19,26]. In this process, H2S, which is released fromNH2CSNH2 under hydrothermal conditions, reduces MoO4

2� and GOto MoS2 and reduced GO (rGO), respectively. By adding urea, thenegative charged functional groups (eO�, eCOO�) were partiallydepleted by ammonia released from urea, which efficiently reducedthe electrostatic repulsion between GO and MoO4

2�, that means theN-doped behavior have no obviously effect to formed layeredMoS2,inspite of the they both consume the active sites on the surface ofgraphene oxide.

3.3. Electrochemical performance

To evaluate the electrochemical properties of MoS2/GNs and

Fig. 4. (a) XPS survey spectra of three MoS2/NG samples, (b) Mo 3d and (c) S 2p of MoS2/NG(1.5), (d) N 1s and (e) O 1s of MoS2/NG, (f) oxygen and nitrogen types statistics of MoS2/NG, (g) schematic illustration of nitrogen and oxygenic functional groups in the carbon lattice.

Table 1XPS analysis and electrochemical properties of the samples.

Sample XPS analysis (atom%) Cg (F/g)

C/at.% O/at.% N/at.%a S/at.% Mo/at.% 0.25A/g 20A/g

GO 61.4 38.6NG 81.5 12.5 6.0 176 65MoS2/GNs 42.3 9.8 32.1 15.8 204 92MoS2/NG(5) 38.4 7.1 2.5 34.7 17.3 210 110MoS2/NG(3) 46.9 7.5 3.1 28.4 14.1 231 118MoS2/NG(1.5) 52.3 7.3 3.5 24.9 12.0 245 146

a Notes: the N atom percentage compared to graphene (C þ N þ O) of MoS2/NG(5), MoS2/NG(3), and MoS2/NG(1.5) is 5.2%, 5.4%, and 5.5%, respectively.

B. Xie et al. / Carbon 99 (2016) 35e42 39

MoS2/NG hybrids, cyclic voltammetry (CV) and galvanostaticcharge/discharge (GCD) measurements were conducted in a three-

electrode system. A platinum wire and saturated electrode wereused as the counter and the reference electrode, respectively.

CV curves of MoS2/NG(1.5) with the scan rate of 5e100 mV/s arepresented in Fig. 5a. All CV curves exhibit shapes with the quasi-rectangular feature even at a high scan rate of 100 mV/s, whichsuggests a good electric double-layer capacitance (EDLC) behavior.The appearance of humps in the CV curves can be ascribed to extrapseudocapacitance due to the redox reactions of Mo(Mo(IV)/Mo(V)) [18] and nitrogen(N-5/N-6) [21,27] active atoms on thesurface of the electrode. Fig. 5b shows CV curves of MoS2/NG andMoS2/GNs at a scan rate of 20mV/s, inwhich the area of the curve iscommonly used to calculate the value of the specific capacitance.Clearly, MoS2/NG(1.5) reflects the highest specific capacitanceamong the four samples. Fig. 5c shows the galvanostatic charge/discharge (GCD) curves of MoS2/NG(1.5) at different current

Fig. 5. Electrochemical characterizations of samples in a three-electrode configuration (6 M KOH): (a) cyclic voltammetry (CV) of MoS2/NG(1.5) composite at different scan rates, (b)CV curves of the three MoS2/NG and MoS2/GNs composites at a scan rate of 20 mV/s, (c) Galvanostatic charge/discharge (GCD) curves of MoS2/NG(1.5) at different current densities,(d) Specific capacitance of the three MoS2/NG and MoS2/GNs composites from 0.25 to 20A/g, the inset shows the GCD curves of all samples at 1A/g. (e) Nyquist plots of the threeMoS2/NG and MoS2/GNs composites, (f) Cyclic performance of MoS2/NG(1.5) at 2A/g, the inset shows the GCD curves of the last four cycles.

B. Xie et al. / Carbon 99 (2016) 35e4240

densities. It can be seen that the charge curves of MoS2/NG(1.5) isalmost linear and symmetrical to its corresponding dischargecounterpart, which indicates the excellent reversibility of MoS2/NG(1.5).

The specific capacitance of the electrode can be calculated fromthe equation: Csp ¼ It/△Em. Where Csp, I, t, △E, and m are specificcapacitance (F/g), constant current (A), discharge time (s), potentialwindow (V) andmass of the active material (g), respectively. Fig. 5dshows the rate performance of the sample electrodes at variouscurrent densities. The specific capacitance at 0.25, 1, 5, 20A/g isshown in Table 1. The capacitive performance of three MoS2/NG

samples is inversely proportional to the amount of MoS2, but pro-portional to the nitrogen content (with a highest value of 245F/g(0.25A/g). Furthermore, all MoS2/NG samples are superior to MoS2/GNs, indicating a significant contribution of nitrogen functionalgroups to the increase of Csp. Remarkably, MoS2/NG(1.5) maintainshigh value of Csp even at high current value (20A/g, 146F/g). Theinset of Fig. 5d also shows the GCD curves of all samples at 1A/g,which are in accordance with the results obtained from Fig. 5b.Although the maximum Csp value of MoS2/NG(1.5) is not the bestone reported to date [Table S2], it reaches the same level or evenbetter than that reported in the literature and shows an preferable

B. Xie et al. / Carbon 99 (2016) 35e42 41

capacitance retention especially at a high current value (>5A/g).To understand the enhanced supercapacitor performance of

MoS2/NG(1.5) better, we also synthesized pure MoS2 and NG andmade a comparisonwith them on the electrochemical performance(Fig. S5). CV curves of NG and pure MoS2 both exhibit a deformedrectangular shape which indicating the existence of Faraday pro-cesses in the samples. The GCD curves (Fig. S5b) at 1A/g imply thatthe Csp value of MoS2/NG(1.5) (227F/g) is much higher than that ofNG(155F/g) and MoS2(70F/g), together with the fact that the NGand MoS2 are covalent bonded (via CeOeMo, instead of physicalmixing) which has been proven in the previous section. Thus, webelieve that there do exist some extra contributions to the capac-itance performance in MoS2/NG hybrid benefiting from the syner-gistic effect between layered MoS2 and N-doped graphene. Theclaim of “synergistic effect” herein is supported by many reports[16,17,19], in which two aspects of contributions have beenmentioned mostly: 1) the flexible N-doped graphene lamella isspatially crosslinked by petal-like MoS2 nanosheets to construct a3D architecture which can effectively increase its effective specificsurface areas and enhance the stability of the composite due to highstrength of graphene; 2) considering the fact that the energystorage process of MoS2(in SCs) is completed by the intercalation ofalkaline ion (Liþ, Naþ and Kþ) into the MoS2 layers along with theelectron transfer for charge compensation [18], N-doped graphenecan behave as a highly conductive current collector therein. Addi-tionally, the favored nitrogen doped structure (which can improvethe wettability of the electrode (N-Q) [28] as well as contributing topseudocapacitance (N-6/N-5)) and the existence of sufficient elec-trochemically active oxygenic functional groups (O-II and O-III) onthe graphene surface play a really significant role for the increasingof the electrochemical performance of MoS2/NG(1.5) hybrid.

As one of the principal methods in evaluating fundamentalbehaviors of electrode materials, the Electrochemical ImpedanceSpectroscope (EIS) analysis is conducted on the samples in thefrequency range of 100kHz-0.01 Hz at the open circuit potential(see Fig. 5e). A partial semicircle in high frequency region and avertical linear feature in the mid-to low-frequency region areobserved in the plot. The high-frequency semicircle corresponds tothe charge transfer resistance (Rct) which can be measured as thediameter [19,29], it indicates that the value of Rct for MoS2/NG issmaller than that of MoS2/GNs. The intersection of the curves at thex-axis represent the internal resistance or equivalent series resis-tance (ESR) of the electrodes, determining the rate at which thesupercapacitor can be charged/discharged [30]. It can be seen thatESR of MoS2/NG is also smaller than that of MoS2/GNs. The MoS2/NG(1.5) has the lowest ESR which could be explained by its com-bination of high electrical conductivity (due to a high content of N-Q) and abundant pore structures between clusters.

The cycling stability measurement of MoS2/NG is performed at2A/g (see Fig. 5f). Although the capacitance of MoS2/NG(1.5) grad-ually decays with increasing cycle number, the capacitance onlyreduces by about 8.7% of the initial capacitance after 1000cycles,indicating a good cycling life which can be attributed to thestructural stability of the 3D hybrid as well as convenient iondiffusion channels.

4. Conclusions

In this work, we have successful fabricated a novel 3D flower-like MoS2/NG hybrids based on the one-pot hydrothermalmethod. Investigation on the growth mechanism of the hybridsreveals that the presence of nitrogen in graphene surface is a keyfactor in dominating final morphology and specific capacitance ofthe hybrids. Benefiting from the 3D flower-like structure and the Nand O species (such as N-5, N-6, O-II), the MoS2/NG(1.5) hybrid

delivers a high specific capacitance (245F/g at 0.25A/g) and excel-lent cycling stability (preserving 91.3% of the initial capacitanceafter 1000 cycles at 2A/g). The present study of the MoS2/NG hy-brids may provide further insight into the design and the con-struction of high-performance electrode materials forsupercapacitors.

Acknowledgments

1. The National Natural Science Foundation of China (Grant No.41202022, 21303129, 51372063). 2. The Fundamental ResearchFunds for National University (CUG150413, 130403, 1410491B03)China University of Geosciences (Wuhan). 3. National BasicResearch Program of China (973 Program, GrantNo.2014CB660815). 4. Anhui International Cooperation Project(Grant No. 1303063014).

Appendix. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.carbon.2015.11.077.

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