Oxygen-Incorporated MoS2 Nanosheets with Expanded Interlayers for

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Oxygen-Incorporated MoS Nanosheets with Expanded Interlayers for Hydrogen Evolution Reaction and Pseudocapacitor Applications Jiang Zhou, Guozhao Fang, Anqiang Pan, and Shuquan Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11811 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016

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ACS Applied Materials & Interfaces

Oxygen-Incorporated MoS2 Nanosheets with Expanded Interlayers for Hydrogen Evolution Reaction and Pseudocapacitor Applications Jiang Zhou*a, b, Guozhao Fang a, Anqiang Pan a, b, and Shuquan Liang*a,b a

School of Materials Science and Engineering, Central South University, Changsha 410083,

Hunan, China b

Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, Hunan, China

Abstract Two-dimensional (2D) transition metal dichalcogenides (TMDs) nanosheets have attracted tremendous research interests. Engineering the structure of MoS2 may result in desirable performance for energy applications. In this work, oxygen-incorporated MoS2 nanosheets with expanded interlayers have been synthesized by a solvothermal reaction. The oxygen-incorporated MoS2 nanosheets with rich defects demonstrate excellent hydrogen evolution reaction activity with a small Tafel slope of 42 mV decade-1 as well as excellent long-term stability. Interestingly, large expanded interlayer of (002) faces with ~8.40 Å can be achieved by controlling the reaction time. This material also shows excellent long-term cycling stability (up to 20000 cycles) as well as high specific capacitance for pseudocapacitors. We believe the structural modified strategy can be applied for other TMDs to futher optimize the performance for various applications.

Keywords: MoS2 nanosheets, oxygen-incorporated, expanded interlayer, hydrogen evolution reaction, pseudocapacitors

*

Corresponding author: Tel.: +86 0731-88836069. Fax: +86 0731-88876692. E-mail address: [email protected] (J. Zhou), [email protected] (S. Liang) 1 ACS Paragon Plus Environment


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1. Introduction The increasing demand of energy has stimulated the intensive research on sustainable and renewable resources.1-2 Among them, hydrogen (H2) is considered as one of the ideal energy carriers in the future. Although Pt-group metals have proved to be the best catalysts for the hydrogen evolution reaction (HER) in acidic media,3 the high cost and limited amount of Pt restrict its wide applications. Therefore, it is an urgent requirement to find a low-cost, highly efficient alternative to replace Pt-group catalysts. Recently, two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets have attracted tremendous research interest,4-6 because of their unique physical and chemical properties and wide applications in electronics,7 sensors,8 catalysis,9 memory devices,10 energy storage devices,11-12 etc. As a typical TMD nanomaterials, MoS2 has been extensively explored as an HER catalyst. Theoretical and experimental studies have identified more edge sites and high electric conductivity of MoS2 catalyst may enhance its HER activity.13-18 Therefore, lots of researches have focused on the preparation of various morphologies of MoS2 catalysts, e.g. nanoparticles,13 nanosheets,19-20 nanofilms,21-22 nanoflowers,23 and microboxes,24 to expose more active sites, or incorporation of conductive addictives, such as carbon materials,25-27 and noble metal materials,9, 28 to improve its conductivity. Although some reported MoS2-based catalysts have showed good catalytic activity, they suffer from difficult preparation, or high cost of the conductive addictive. Furthermore, previous studies have demonstrated that the oxygen-incorporated MoS2 may improve the intrinsic conductivity,17, 29-30 and more defects may lead to more catalytically active sites.31 However, few attempts focus on the optimization of both active sites and conductivity of MoS2 catalysts.17 Recent study has demonstrated that moderate oxygen-incorporation into MoS2 may result in more edge sites and enhance the intrinsic conductivity, which is beneficial for

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improving the HER catalytic activity.30 Thus, we aim to propose the oxygen-incorporated defect-rich MoS2 nanosheets for highly efficient HER catalyst. As known, supercapacitor is one of the major devices for the electric energy storage and conversion, which has been widely used in portable electronics, hybrid electric vehicles (HEVs).32 Although high capacitance and high-rate capability have been achieved in pseudocapacitors, their poor cycling stability hampers the commercial use. Therefore, development of new electrode materials with long cycle life as well as high energy and power densities is urgent for supercapacitors. MoS2 can store more charges via the faradaic charge transfer process due to the several oxidation states of Mo atom, making it a promising pseudocapacitive material for high-performance pseudocapacitors.33-34 Actually, high specific capacitance and good rate performance have been reported in some MoS2-based materials, the long-term cycling performance is unsatisfactory and needs to be further improved. Herein, we develop a solvothermal reaction to engineer the interlayer spacing of oxygenincorporated MoS2 nanosheets, referred to as MoS2-xOx. The obtained MoS2-xOx nanosheets with rich defects provide high intrinsic conductivity and abundant active sites, making it a superior catalyst for HER. As expected, the MoS2-xOx catalyst possesses a small HER onset potential of -148 mV, a small Tafel slope of 42 mV decade-1, and excellent long-term stability. In addition, by controlling the reaction time, large expanded interlayer of (002) faces with ~8.40 Å is achieved, which is beneficial to the intercalation of foreign ions (OH-), leading to the enhanced electrochemical performance of pseudocapacitors. Moreover, the MoS2-xOx nanosheets demonstrate the excellent long-term cycling stability (up to 20,000 cycles) and high specific capacitance. 2. Experimental section Chemicals. All chemicals used were of analytical purity and used as received without any further purification. Potassium thiocyanate (KSCN), Molybdenum oxide (MoO3) and 3 ACS Paragon Plus Environment


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bulk Molybdenum disulfide (bulk MoS2) were purchased from Sigma-Aldrich. Anhydrous ethanol was purchased from Merck chemical. Milli-Q water (18.2MΩ cm, Milli-Q System, Millipore, USA) was used in all experiments. Materials synthesis. The oxygen-incorporated MoS2 nanosheets were synthesized through a solvothermal reaction by using KSCN as reducing reagent as well as vulcanizer. Typically, 0.0720 g MoO3, and 0.1215 g KSCN were added into a mixture of 11 ml of MilliQ water and 22 ml of anhydrous ethanol. After actively magnetic stirring at room temperature for 20 min, the as-obtained suspension was transferred to a 50 ml Teflon autoclave and kept in an electrical oven at 200 oC for different reaction time (5 h, 13 h, 26 h, 39 h). After cooling down naturally, the black precipitates were washed with Milli-Q water and anhydrous ethanol several times. Then the products were collected by drying at 60 oC in vacuum oven overnight. The oxygen-incorporated MoS2 nanosheets synthesized at different time are designated as MoS2-xOx-5h, MoS2-xOx-13h, MoS2-xOx-26h, MoS2-xOx-39h, respectively. For comparison, the oxygen-free MoS2 nanosheets were prepared using the same synthetic procedure with a reaction time of 39 h, but only 33 ml Milli-Q water and no anhydrous ethanol was used as the solvent. The oxygen-free MoS2 nanosheets is designated as MoS2-39h. Materials characterization. The composition of as-prepared samples were characterized by X-ray power diffraction (XRD, Rigaku D/max2500) using Cu Kα radiation (λ=1.54178 Å). X-Ray photoelectron spectroscopy (XPS, VG ESCALAB 220i-XL instrument) with monochromatized Al-Kα radiation (1486.7 eV) was used to characterize the elemental composition of as-prepared samples. Prior to XPS measurement, the samples were first sonicated in ethanol for about 10 min, and then dropped on a silicon substrate and dried naturally. The morphologies of the samples were characterized by a field emission scanning electron microscopy (FESEM, Model JSM-7600F, JEOL Ltd., Tokyo, Japan). Transmission electron microscopy (TEM) and high-resolution trans-mission electron microscope (HRTEM)


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analysis were performed on a transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Tokyo) with energy dispersive X-ray spectroscopy (EDX). Electrochemical measurements. Hydrogen evolution reaction (HER): The HER measurements were carried out in a three-electrode system on an Autolab PGSTAT302 potentiostat (Eco Chemie BV, Netherlands). A catalyst suspension was obtained by dispersing the catalysts in water (1 mg ml-1), followed by sonicating for 30 min to guarantee good dispersion of the catalysts. To increase the binding strength, 10 ul Nafion solution (5 wt%) was added into the catalysts suspension. Then 20 µl of the suspension was loaded onto a glassy carbon electrode (diameter of 3 mm) and naturally dried overnight. The loading of the catalysts is 0.282 mg cm-2. Linear sweep voltammetry with a scan rate of 2 mV s-1 was conducted in 0.5 M H2SO4 using a Pt wire as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode. All the potentials were converted to values with reference to a reversible hydrogen electrode (RHE). The stability was evaluated by using cyclic voltammetry (CV) at a scan rate of 100 mV s-1 between -0.45 and 0 V vs RHE. The Nyquist plots were performed with frequencies ranging from 100 kHz to 0.1 Hz at an potential of 250 mV vs RHE . Supercapacitors: The supercapacitor tests were performed on Solartron analytical equipment (Model 1470E, AMETEK, UK) with a standard three-electrode testing system. The working electrodes were prepared by dispersing the as-prepared MoS2-xOx samples, acetylene black, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 80: 10: 10 in a N-methyl-2-pyrrolidone (NMP) solution, which were pasted onto the nickel foam electrode and dried in a vacuum oven at 50 oC for ~30 h. The electrolyte was an aqueous solution of 6 M KOH. A Pt wire and saturated Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. The potential range for cyclic voltammetry (CV) test is from 0 to 0.5 V (vs. standard Ag/AgCl electrode), while for galvanostatic charge/discharge test is from 0 to 0.4 V (vs. standard Ag/AgCl electrode). The specific capacitance and current 5 ACS Paragon Plus Environment


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density are based on the weight of active materials. The mass loading of the active materials for electrochemical test is about 1.6 mg and a geometric surface area of about 1 cm-2. The specific capacitances are obtained from the formula as follow: ‫=ܥ‬

‫ݐ∆ × ܫ‬ ݉ × ∆ܸ

where C (F/g) is the specific capacitance, I (mA) is the discharge current, ∆‫( ݐ‬s) is the discharge time, ߂ܸ (V) is the potential drop during discharge process after internal resistance (IR) drop, and m (g) is the mass of active materials only.

3. Results and discussion The time-dependent phase and morphology evolution of solvothermally prepared samples are characterized. In order to understand the morphology evolution of MoS2-xOx nanosheets, the samples prepared by different reaction time via the simple one-pot solvothermal reaction are characterized by SEM and the result is showed in Figure 1. The microsized bulk MoO3 purchased from Sigma-Aldrich is displayed for comparison (Figure 1a). After solvothermal reaction for 5 h at 200 oC, microsized bulk materials mixed with some nanosheets assembled nanoflowers are clearly presented. When the reaction time increased to 13 h, the dominate morphology is nanoflowers, even though bulk materials can also be observed. With further increasing the reaction time to 26 h, a little bulk materials are still observed, but they are constructed by nanosheets (Figure 1d). For MoS2-xOx-39h, as showed in Figure 1e, no bulk materials are observed, the sample is only composed of nanosheets assembled nanoflowers. Moreover, the nanosheets look very thin, and the surface is smooth. Besides, the SEM image of MoS2 -39h is displayed (Figure 1f), and the thin nanosheets assembled nanoflowers morphology is observed, indicating the similar morphology to that of MoS2-xOx-39h. Therefore, we think that the MoS2-xOx nanosheets can be prepared at 200 oC above 26 h via the solvothermal reaction, which will be further proved by XRD and TEM 6 ACS Paragon Plus Environment

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results. Figure 2 shows the TEM images of MoS2-xOx-26h, MoS2-xOx-39h, and MoS2-39h, respectively. As showed in Figure 2a, a mixture of nanotubes and nanoflowers structures is observed. The high magnification TEM image (Figure 2b) demonstrates that the nanotube is assembled by thin nanosheets, which is consistent with the SEM images (Figure 1d). No nanotubes are observed in MoS2-xOx-39h, instead of thin graphene-like nanosheet structure is presented (Figure 2c and d). Moreover, MoS2-39h exhibits the similar nanosheet structure with that of MoS2-xOx-39h (Figure 2e, f). The as-prepared MoS2 materials demonstrate a graphene-like nanosheet structures, which may be helpful for HER and supercapacitor applications. Compared to the XRD pattern of raw MoO3 (Figure S1 in supporting information (SI)), the diffraction peaks of MoO3 are disappeared after solvothermal reaction for 13 h (Figure 3a). However, no obvious diffraction peaks of MoS2 are observed for MoS2-xOx-13h, which is consistent with the SEM image that large quantity of bulk materials are exhibited (Figure 1c). Interestingly, three diffraction peaks centered at 2θ = ~11.9 o, ~ 32.7o, and ~58.4o are clearly observed for MoS2-xOx-26h. When the reaction time increased to 39 h, a similar XRD pattern is obtained except that the diffraction peak shift from 2θ = ~11.9 o to ~10.5o. For MoS2-39h, its XRD pattern is clearly presented and match well with the standard hexagonal MoS2 phase (space group: P63/mmc (194), JCPDS card No. 37-1492).35-37 As showed in XRD patterns, it is clearly that all the difference among these samples is the 2θ degree of (002) diffraction peak. The interlayer spacing (d(002)) of as-prepared MoS2 samples can be calculated to be 7.4 Å for MoS2-xOx-26h, 8.4 Å for MoS2-xOx-39h, and 6.4 Å for MoS2-39h. The HRTEM images of MoS2-xOx-26h, MoS2-xOx-39h, and MoS2-39h are displayed in Figure 3b-d. The interlayer spacing of (002) plane measured from HRTEM images are ~7.4 Å, ~8.4 Å, and 6.4 Å for MoS2-xOx-26h, MoS2-xOx-39h, and MoS2-39h, respectively, which coincides with the calculation from XRD analysis. The expanded interlayers of MoS2-xOx-26h and MoS2-xOx-39h are ascribed to the oxygen incorporation into MoS2 structure during the solvothermal 7 ACS Paragon Plus Environment


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process.17,

30

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As shown in Figure 3b-d, MoS2-xOx-39h and MoS2-39h show distorted

structures, while MoS2-xOx-26h exhibits a relatively well-crystallized structure. It suggests that long time hydrothermal reaction may induce the high degree structure disorder of samples. Moreover, many defects are observed for MoS2-xOx-39h and MoS2-39h as indicating by white arrows in Figure 3c and d and Figure S2 (SI). To obtain the elemental composition of the as-prepared samples, we performed XPS analysis of MoS2-xOx-26h, MoS2-xOx-39h, MoS2-39h and the high-resolution Mo 3d spectra are showed in Figure 4a, b, c. For MoS2-xOx-26h and MoS2-xOx-39h, the Mo 3d spectra can be fitted with two spin-orbit doublets characteristic of MoVI and MoIV (Figure 4a and b), which is similar to the recent reports about oxygen-containing MoS2 hybrid.33, 38-40 Specifically, the fitting peaks at 229.3 (MoIV 3d5/2) and 232.2 eV (MoIV 3d3/2) are from the MoS2, while the peaks at 235.8 (MoVI 3d3/2) and 233 eV (MoVI 3d5/2) are ascribed to MoVI-O bonds.39,

41

However, only MoIV can be detected for MoS2-39h, (Figure 4c), suggesting the pure MoS2 is obtained. The XPS results indicate that the oxygen is incorporated into MoS2, rather than the surface oxidation, which is consistent with the XRD results. Moreover, the element mapping analyses displayed in Figure 4d demonstrate that Mo, S, O are homogeneous distributedin the nanosheets, indicating the successful synthesis of MoS2-xOx nanosheets. The XRD of calcined samples of MoS2-xOx-39h and MoS2-39h further confirmed that the oxygen is incorporated in MoS2-xOx-39h clearly (Figure S3, SI). Based on the above analyses, we perform the structural models of MoS2-xOx-39h with enlarged interlayer spacing of 8.4 Å, and the MoS2-39h with interlayer spacing of 6.4 Å (Figure 5). The electrocatalytic HER activity of as-prepared samples (catalyst loading: 0.282 mg cm2

) was examined using a standard three-electrode system in 0.5 M H2SO4 with a scan rate of 2

mV s-1. Bulk MoS2 and commercial hydrogenation Pt catalyst (10% Pt on activated charcoal, referred to as Pt-C) were also examined for comparison. The polarization curves after iR correction revealing the normalized current density versus voltage (vs RHE) of all samples are 8 ACS Paragon Plus Environment

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showed in Figure 6a. MoS2-xOx-39h demonstrates a small onset potential of -148 mV, which is much smaller than that of hierarchical MoS2 microboxes,24 MoS2 nanoporous films,42 MoO3-MoS2 nanowires,43 suggesting a good catalytic activity. Moreover, MoS2-xOx-39h exhibits large cathodic current density of 40 mA cm-2 at an overpotential of 300 mV, which is higher than that of MoS2-xOx-26h (11.2 mA cm-2), MoS2-39h (22.5 mA cm-2), bulk MoS2 (0.16 mA cm-2) and also MoS2-xOx-52h and MoS2-xOx-39h-800 (Figure S4 and Figure S5, SI). Note that MoS2-39h has the similar nanosheet morphology and high defects to that of MoS2xOx-39h,

but there is no oxygen incorporation. MoS2-xOx-26h with oxygen incorporation but

lacks of defects. This phenomenon suggests that defect-rich structure and oxygen incorporation may lead to the enhanced HER activity. To obtain further insight into the defect-rich and oxygen-incorporated MoS2-xOx nanosheets, the comparison Tafel slopes derived from the polarization curves were demonstrated. As showed in Figure 6b, a smaller Tafel slope of ~42 mV dec-1 was observed for MoS2-xOx-39h, which is much smaller than that of MoS2-xOx-26h (~51 mV dec-1), MoS2-39h (~56 mV dec-1) , and bulk MoS2 (~125 mV dec-1). Importantly, the small Tafel slope of ~42 mV dec-1 is comparable to the MoS2/RGO hybrid,44 and almost smaller than that of all the single-component MoS2 electrocatalysts,17, 22, 31, 45-46 even better than those of many MoS2 hybrid electrocatalysts, such as MoS2 on carbon fiber paper,25 MoS2 on graphene-coated Ni foam,47 MoS2 film on 3D nanoporous gold.48 In order to investigate the electrode kinetics under HER process, we preformed the electrochemical impedance spectroscopy (EIS) analysis and the Nyquist plots are showed in Figure 6c. The EIS simulation parameters of the samples are listed in Table S1 (SI). MoS2-xOx-39h possesses the smallest Rct value of 55.6 Ω among all the samples, indicating the high interdomain conductivity, which apparently benefits from synergistically defect-rich and oxygen incorporation structure. The charge-transfer resistance Rct is related to the electrocatalytic kinetics, and a lower value corresponds to a faster reaction rate.49 As expected, by construction of structural and electronic modulations for HER catalysis, MoS2-xOx-39h 9 ACS Paragon Plus Environment


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achieves the best HER activity. Besides the high electrocatalytic activity, stability is another critical aspect in the development of electrocatalysts. Long-term cycling stability of MoS2xOx-39h

is investigated by taking continuous cyclic voltammograms at a scan rate of 100 mV

s-1 for 2000 cycles. As showed in Figure 6d, the polarization curve after 2000 cycles overlaps almost exactly with the initial one, suggesting the excellent durability of MoS2-xOx-39h. Besides, the current density for the MoS2-xOx-39h electrode remains stable with the continuous sweeps for 30000s, indicating the good stability for HER (Figure S6, SI). In all, the high HER activity as well as good stability of MoS2-xOx-39h catalyst may be due to three aspects as follows: (1) oxygen-incorporated MoS2-xOx nanosheets lead to enhanced intrinsic conductivity;17 (2) defect-rich structure increase the exposure of active edge sites for HER catalysis;31 (3) nanosheet structure of MoS2-xOx provides higher specific surface area and more active sites.50 Materials with expanded interlayers have demonstrated the superiority for their application in Li-ion batteries and Na-ion batteries.51-53 It is believed that oxygen-incorporated MoS2-xOx nanosheets with expanded interlayers can be used for high performance pseudocapacitors. As evidence of its multifunctional applications, we further employed MoS2xOx-39h

as electrode material for supercapacitors by using a standard three-electrode testing

system in 6M KOH solution. MoS2-39h was also investigated for comparison. Figure 7a displays the CV curve of MoS2-xOx-39h at different scanning rates from 5 to 100 mV s-1. A pair of remarkable redox peaks related to the Faradic redox reactions are observed, which is consistent with the previous reports.54-55 No obvious distortion of CV curve shape is observed and the peak current increases linearly with the increment of the scan rate from 5 to 100 mV s1

, suggesting the good pseudocapacitive behavior. Well-defined discharge voltage plateaus at

around 0.2-0.1 V are displayed in the galvanostatic discharge curves (Figure 7b), which is in accordance with the CV results. Moreover, the charge/discharge curves of the MoS2-xOx-39h are symmetrical, with no obvious iR drop at low current densities, indicating the good 10 ACS Paragon Plus Environment

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electrochemical reversibility. The specific capacitances of MoS2-xOx-39h and MoS2-39h as a function of current densities calculated from the discharge curves are showed in Figure 7c. The specific capacitances of MoS2-xOx-39h are 246, 227, 208, 183, 174, 158, and 144 F g-1 at current densities of 0.5, 1, 2, 4, 6, 8, and 10 A g-1, respectively, highlighting the good rate capability. As for MoS2-39h electrode, 145 F g-1 is observed at 0.5 A g-1, and only 98 F g-1 can be remained at 10 A g-1. Obviously, the specific capacitances of MoS2-xOx-39h are higher than that of MoS2-39h, and the capacitance values are also superior to some reported results.56-58 Beside the high capacitance, the long-term cycling stability is a very important parameter for supercapacitors. We also evaluated the cycling performance of MoS2-xOx-39h up to 20000 cycles using galvanostatic charge-discharge measurement at a current density of 4 A g-1. As showed in Figure 7d, the specific capacitance of MoS2-xOx-39h decreases quickly to 126 F g-1 after 300 cycles, and then it becomes stable with 110 F g-1 remained after 20000 cycles. The good electrochemical performance may be due to the high intrinsic conductivity through oxygen incorporation and the easier intercalation of foreign ions (OH-) via expanded interlayers.

4. Conclusion In conclusion, we have prepared the oxygen-incorporated defect-rich MoS2 nanosheets with expanded interlayers via a simple solvothermal reaction. Due to high intrinsic conductivity through oxygen-incorporated, and more active edge sites from defect-rich structure, the MoS2-xOx nanosheets demonstrate an excellent HER activity with a small Tafel slope of 42 mV decade-1, as well as excellent long-term stability. In addition, MoS2-xOx nanosheets with expanded interlayers of (002) faces demonstrate the excellent long-term cycling stability (up to 20000 cycles) as well as high specific capacitance for high



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performance pseudocapacitors. We believe the MoS2-xOx nanosheets with expanded interlayers may find more promising applications like Li-ion batteries and Na-ion batteries.

ASSOCIATED CONTENT * S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.----. XRD pattern of bulk MoO3 and the calcined samples, more HRTEM images, linear sweep voltammetry curves of long reaction time sample and calcined sample, i-t curve, EIS simulation parameters of different samples (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by National High Technology Research and Development Program of China (863 Program) (Grant no. 2013AA110106), National Natural Science Foundation of China (Grant no. 51374255 and 51572299) and the Fundamental Research Funds for the Central Universities of Central South University (Grant no. 2015zzts174).


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Figures and captions.

Figure 1. SEM images of (a) MoO3, (b) MoS2-xOx-5h, (c) MoS2-xOx-13h, (d) MoS2-xOx-26h, (e) MoS2-xOx-39h, and (f) MoS2-39h. Insets in (e) and (f) are the high-magnification SEM images of MoS2-xOx-39h and MoS2-39h, respectively.



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Figure 2. TEM images of (a, b) MoS2-xOx-26h, (c, d) MoS2-xOx-39h, and (e, f) MoS2 -39h with different magnifications.


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Figure 3. (a) XRD patterns of the as-prepared samples. HRTEM images of (b) MoS2-xOx-26h, (c) MoS2-xOx-39h, (d) MoS2-39h.



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Figure 4. High-resolution Mo XPS spectra of (a) MoS2-xOx-26h, (b) MoS2-xOx-39h, and (c) MoS2-39h. (d) TEM image and the corresponding EDS mapping images of MoS2-xOx-39h.


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Figure 5. Structural models of (a) MoS2-xOx-39h and (b) MoS2-39h.



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Figure 6. (a) Linear sweep voltammetry curves measured at 2 mV s-1, (b) corresponding Tafel slopes, and (c) Nyquist plots of the bulk MoS2, MoS2-xOx-26h, MoS2-xOx-39h, MoS2-39h, respectively. Inset shows the equivalent circuit model for the impedance spectra. Commercial Pt-C was examined for comparison. (d) Stability test for the MoS2-xOx-39h electrode.


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Figure 7. (a) Cyclic voltammograms (CVs) of MoS2-xOx-39h electrode at different scan rates (from 5 to 100 mV s-1). The potential range of CV test is from 0 to 0.5 V (vs. standard Ag/AgCl electrode). (b) The typical galvanostatic charge/discharge curves of MoS2-xOx-39h electrode at various current densities. (c) The calculated specific capacitance of MoS2-xOx-39h and MoS2-39h electrode as a function of current densities. (d) Cycling performance of MoS2xOx-39h

and MoS2-39h electrode at the current density of 4 A g-1. The potential range of

galvanostatic charge/discharge test is from 0 to 0.4 V (vs. standard Ag/AgCl electrode).



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Oxygen-incorporated MoS2 nanosheets with expanded interlayers have been synthesized by a solvothermal reaction. The oxygen-incorporated MoS2 nanosheets with rich defects demonstrate excellent hydrogen evolution reaction activity with a small Tafel slope of 42 mV decade-1 as well as excellent long-term stability. This material also shows excellent long-term cycling stability (up to 20000 cycles) as well as high specific capacitance for pseudocapacitors. 140x59mm (220 x 220 DPI)

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Oxygen-Incorporated MoS2 Nanosheets with Expanded Interlayers for

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