INORGANIC CHEMISTRYF R O N T I E R S

Volume 3 | Number 9 | September 2016

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INORGANIC CHEMISTRYFRONTIERS

RESEARCH ARTICLE

Cite this: Inorg. Chem. Front., 2016,3, 1160

Received 22nd June 2016,Accepted 12th July 2016

DOI: 10.1039/c6qi00198j

rsc.li/frontiers-inorganic

Vertically aligned oxygen-doped molybdenumdisulfide nanosheets grown on carbon clothrealizing robust hydrogen evolution reaction

Junfeng Xie,† Jianping Xin,† Guanwei Cui, Xinxia Zhang, Lijie Zhou, Yunlong Wang,Weiwei Liu, Caihua Wang, Mei Ning, Xinyuan Xia, Yingqiang Zhao and Bo Tang*

The catalytic activity of an electrocatalyst is determined by the density of active sites and the electric con-

ductivity, namely, the density of electrically connected active sites. In this work, elemental incorporation,

disorder engineering and material hybridization were applied to molybdenum disulfide (MoS2) simul-

taneously to realize a high-level synergistic optimization for both active sites and electric conductivity,

achieving highly efficient hydrogen-evolving performance finally. Benefitting from the synergistic optimi-

zation, the vertically aligned oxygen-doped MoS2/carbon cloth catalyst shows an ultralow onset overpoten-

tial of 90 mV to initiate the HER process, and an extremely high catalytic current of 225 mA cm−2 was

measured at an overpotential of 300 mV. Not only that, superior stability was also achieved, making this

novel catalyst promising for practical applications such as electrolytic water splitting and a co-catalyst for

photocatalytic/photoelectrochemical hydrogen production. The synergistic optimization strategy reported

in this work would shed light on the systematic design of highly efficient electrocatalysts in the future.

Introduction

Exploring efficient catalysts for the electrochemical hydrogenevolution reaction (HER) has become more and more impera-tive owing to the urgent demand for clean energy to face theemerging energy crisis and subsequent environmentalissues.1–4 To date, increasing the density of active sites andimproving the electric conductivity of hydrogen-evolving cata-lysts have been considered as two efficient pathways to opti-mize the electrocatalytic HER activity, which can also beregarded integratedly as increasing the electrically connectedactive sites that are effective to the HER.5–9 Thus, developing asystematic strategy for synergistic structural and electronicmodulations is in urgent demand to achieve excellent perform-ance towards hydrogen-evolving electrocatalysis, which stimu-lates the research passion of chemists and materials scientists.Among a large variety of as-explored HER catalysts, molyb-denum disulfide (MoS2) has been considered as one of themost classical candidates to replace highly efficient noblemetal catalysts due to its low price, earth abundance, high

efficiency and definite catalytic mechanism, for which plentyof strategies have been readily involved to improve its catalyticactivity, including elemental doping, crystal facet engineering,thickness control, interface engineering and hybridizationwith highly conductive materials.10–28 While for furtherimproving the HER activity of MoS2-based catalysts, synergisticoptimization with beneficial active site density as well as fastelectron transport is urgently required. Unfortunately, theexposure of highly active edge sites of semiconducting MoS2often results in the damage or degradation of the two-dimen-sional electron conjugation system that guarantees the intra-layered electron transport, thus leading to a lower density of“effective” active sites, namely, the electrically connected activesites, that can catalyze the HER process. Owing to such a con-tradiction between the crystal structure and the electronicstructure of MoS2, optimizing the relationship between theactive sites and the electric conductivity synergistically stillremains a challenge, and would be a key route to achieve theenhancement of HER catalytic efficiency.

During the past few years, highly conductive carbonaceousmaterials have been considered as a kind of effective supportto load nanosized catalysts with high exposure of activesites.29–34 Benefitting from the freestanding skeleton with highelectric conductivity, rich active sites can be effectively involvedin catalytic processes, finally leading to a significant enhance-ment of the catalytic efficiency. Following this idea, lots ofefforts have been made to realize the enhancement of the elec-†These authors contributed equally to this work.

College of Chemistry, Chemical Engineering and Materials Science, Collaborative

Innovation Center of Functionalized Probes for Chemical Imaging in Universities of

Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education,

Shandong Normal University, Jinan, Shandong, 250014, People’s Republic of China.

E-mail: tangb@sdnu.edu.cn

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tric conductivity of catalysts by introducing carbonaceous sup-ports, and thus boost the catalytic reactions.29–34 However, forthese hybrid catalysts, simultaneously enriching the thermo-dynamically unstable active edge sites and improving theintrinsic conductivity of the catalytically active MoS2 com-ponent are rarely reported. Recently, the authors have exploredthe oxygen-incorporated MoS2 ultrathin nanosheets (OMS)with controllable disorder engineering, in which the oxygendopants and the unique disordered structure can lead to theeffective synergistic optimization of both active sites and elec-tric conductivity.22 Unfortunately, the semiconducting natureof the MoS2 nanosheet catalyst still significantly impedes theelectron transport during the HER process. To overcome thisobstacle, we propose that highly conductive carbon cloth (CC)could be involved as a freestanding support to act as an elec-tron “superhighway” to facilitate the electron transport andfurther boost HER catalysis. In this work, a highly efficient andfreestanding MoS2-based electrode was prepared by growingstructurally optimized oxygen-incorporated MoS2 ultrathinnanosheets on highly conductive carbon cloth. The optimizedOMS possesses balanced active sites as well as intralayeredelectric conductivity, while the carbon cloth can efficientlytransfer the electrons during the HER catalysis. Benefittingfrom the synergistic optimization strategy, the oxygen-incor-porated MoS2 ultrathin nanosheets/carbon cloth (OMS/CC)hybrid catalyst shows tremendous enhancement in HER per-formance, which achieves an ultralow onset overpotential of90 mV and a small Tafel slope of 58 mV per decade,accompanied by a high cathodic current density (225 mA cm−2

at an overpotential of 300 mV). Furthermore, a superior stabi-lity was also achieved, for which negligible degradation of cata-lytic current was observed even after 2000 cyclic voltammetry(CV) cycles or 12-hour continuous HER operation, making thehybridized catalyst a promising alternative for noble metals inpractical water splitting. The synergistic optimization strategyin this work will shed light on the design of highly active cata-lysts for electrocatalysis in energy-related fields.

ExperimentalSynthesis of OMS/CC

Typically, 1/7 mmol (NH4)6Mo7O24·4H2O (177 mg, 1 mmol Mo)and 5 mmol thiourea (381 mg) were dissolved in 40 mL dis-tilled water under vigorous stirring to form a homogeneoussolution. After stirring for 30 min, the solution was transferredinto a 50 mL Teflon-lined stainless steel autoclave, a piece ofhydrophilic carbon cloth with a size of 1 × 3 cm was placed in,and maintained at 180 °C for 18 h. Then the reaction systemwas cooled down to room temperature naturally. The as-obtained products were rinsed with distilled water and ethanolrepeatedly, and dried at 60 °C under vacuum overnight. TheOMS/CC catalysts with different disorder degrees were syn-thesized by controlling the synthesis temperature rangingfrom 140 °C to 200 °C.

Characterization

X-ray diffraction (XRD) was performed on a Philips X’Pert ProSuper diffractometer with Cu Kα radiation (λ = 1.54178 Å). Thescanning electron microscopy (SEM) images were taken on aJEOL JSM-6700F SEM. Transmission electron microscopy(TEM) was carried out on a JEM-2100F field emission electronmicroscope at an acceleration voltage of 200 kV. The high-resolution TEM (HRTEM), high-angle annular dark-field scan-ning transmission electron microscopy (HAADF-STEM) andcorresponding energy-dispersive spectroscopy (EDS) mappinganalyses were performed on a JEOL JEM-ARF200F TEM/STEMwith a spherical aberration corrector. X-ray photoelectronspectra (XPS) were acquired on an ESCALAB MK II with Mg Kα

as the excitation source.

Electrochemical measurements

All the electrochemical measurements were performed in athree-electrode system on an electrochemical workstation(CHI660D). Typically, linear sweep voltammetry with a scanrate of 5 mV s−1 was conducted in 0.5 M H2SO4 (sparged withpure H2) by using the OMS/CC freestanding catalyst as theworking electrode, the Ag/AgCl (in 3 M KCl solution) electrodeas the reference electrode, and a graphite rod (Alfa Aesar,99.9995%) as the counter electrode. For the measurements ofpowdery catalyst, 4 mg OMS and 30 μL Nafion solution (SigmaAldrich, 5 wt%) were dispersed in 1 mL water–isopropanolsolution with a volume ratio of 3 : 1 by sonicating for 1 h toform a homogeneous ink. Then 5 μL of the ink (containing20 μg of catalyst) was loaded onto a glassy carbon electrodewith 3 mm diameter (loading 0.285 mg cm−2) as the workingelectrode. Cyclic voltammetry (CV) was conducted between−0.3 and 0.2 V vs. RHE at 50 mV s−1 to investigate the cyclingstability. The Nyquist plots were obtained with frequenciesranging from 100 kHz to 0.1 Hz at an overpotential of 250 mV.All the potentials were calibrated to a reversible hydrogen elec-trode (RHE).

Results and discussion

The synergistically optimized catalyst was prepared via a one-step route of in situ growth of oxygen-incorporated MoS2 ultra-thin nanosheets on carbon cloth with hydrophilic character-istics (see the Experimental section for details). Of note is thatunlike the synthesis of freestanding oxygen-incorporated MoS2nanosheets, the concentration of reactants in synthesizingOMS/CC nanohybrids is much lower to avoid the accumulationof OMS and obtain a vertically aligned morphology. At an opti-mized temperature of 180 °C, OMS with optimized disorderengineering can be intimately grown on a CC support. Asshown in the inset of Fig. 1A, the as-obtained OMS/CC catalystis of freestanding nature, which can be directly used as anelectrode for electrochemical water splitting. The coadjacentcarbon fibers offer a robust and porous support for the OMS,and the high electric conductivity of the carbon cloth wouldprovide an ideal electron transport pathway for the electro-

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catalytic HER process. In order to investigate the structuralinformation of the product, X-ray diffraction (XRD) analysiswas applied, from which the as-obtained XRD pattern indi-cates that the product is of high crystallinity (Fig. 1A). Notably,the XRD pattern is different from the standard pattern of2H-MoS2 (JCPDS card no. 73-1508), of which two new peaksemerge in the low-angle region with the corresponding d spa-cings of 9.50 Å and 4.75 Å, respectively. The diploid relation-ship of the d spacings confirms the formation of a lamellarstructure with an enlarged interlayer spacing of 9.5 Å, whichmatches well with the value of disorder-engineered MoS2nanosheets.22 Furthermore, two broadened peaks in the high-angle region (32° and 57°) can be well indexed to the (100) and(110) planes of the pristine 2H-MoS2, indicating a similaratomic arrangement along the basal planes. It is noteworthythat the absence of high-index diffraction peaks reveals theshort-range disordering characteristics of the hybrid catalyst,which can provide more active sites for HER catalysis. Scanningelectron microscopy (SEM) was utilized to investigate the mor-phological information of the hybridized product. As can beseen from Fig. 1B, the product is a robust intertexture wove bycarbon fibers with interconnected microstructure. The intercon-nected carbon-based skeleton can ensure a robust and highlyconductive feature, which enables the product to be used as afreestanding hydrogen-evolving electrode without binders. Highresolution SEM images revealed that the oxygen incorporatedMoS2 nanosheets are of high density, and in particular, arevertically grown on the carbon fibers (Fig. 1B and C). With the

consideration of the fact that the active sites of MoS2 are locatedat the edges, abundant active edge sites can be preferentiallyexposed benefitting from this unique morphology, thus facilitat-ing the electrochemical hydrogen evolution reaction.35–37

Transmission electron microscopy (TEM) was utilized tofurther investigate the morphological information of the ultra-thin nanosheets grown on a CC support. As depicted inFig. 2A, the nanosheets stripped from the carbon cloth are in atypical nanosheet morphology, with a uniform size of approxi-mately 200 nm. Of note is that the TEM image highlights theultrathin nature of the nanosheets, which is beneficial to theelectrochemical processes due to the feasible ion permeationor gas release.38–41 The high-resolution TEM (HRTEM) imageof an individual ultrathin nanosheet highlights the disorderedstructure in the basal plane (Fig. 2B). Detailed analyses suggestthat the disordered structure of the nanosheets is built by tinyhexagonal MoS2 nanodomains with a quasi-periodic align-ment. This unique alignment can ensure the relatively highelectron transport along the nanosheets due to the partialmaintenance of the two-dimensional (2D) electron conjugationsystem. The inset of Fig. 2B shows the fast Fourier transform(FFT) pattern transformed from the HRTEM image, furtherconfirming the quasi-periodic hexagonal alignment of nano-domains with the disordered structure. The disordered structurewith quasi-periodic features can not only provide abundantunsaturated sulfur atoms to catalyze the HER, but also facili-tates the electron transport on the nanoscale to furtherenhance the HER activity. A cross-sectional HRTEM image wasobtained to investigate the thickness of the OMS on thecarbon cloth. As shown in Fig. 2C, a typical layered structurecan be clearly observed from the crystal fringe of OMS, fromwhich a uniform interplanar spacing of 0.95 nm can be identi-

Fig. 1 (A) XRD pattern of the OMS/CC freestanding catalystaccompanied by the standard pattern of 2H-MoS2. Inset shows a digitalphotograph of the OMS/CC freestanding catalyst. (B) Low-resolutionSEM image showing the cross-linked feature of the hybrid catalyst.(C–D) SEM images in high resolution clearly revealed the surface struc-ture of OMS/CC with vertically aligned ultrathin nanosheets.

Fig. 2 (A) TEM image of the oxygen-incorporated MoS2 ultrathinnanosheets stripped from the carbon cloth support. (B) HRTEM image ofthe OMS, verifying the disordered structure. Inset depicts the FFTpattern with the feature of six individual diffraction arcs, confirming thequasi-periodic alignment of the MoS2 nanodomains. (C) Cross-sectionalHRTEM image of the curled fringe of an OMS nanosheet giving theinterlayer spacing of 0.95 nm. (D) HAADF-STEM image of an individualoxygen-incorporated MoS2 ultrathin nanosheet grown on carbon cloth.(E–G) Corresponding EDS mapping images indicate that molybdenum,sulfur and oxygen are homogeneously distributed on the wholenanosheet.

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fied, which agrees well with the result from XRD analysis. Theenlarged interplanar spacing compared with the traditional2H-MoS2 may arise from the unique disordered structurewhich significantly reduces the van der Waals interactionbetween layers. In addition, the thickness of the nanosheet onthe CC skeleton is ∼5 nm, which corresponds to 5 MoS2 layers,indicating the ultrathin feature of the nanosheets.

Of note is that low-temperature synthesis is not only proneto lead to the formation of the disordered structure, but is alsoresponsible for the possible elemental incorporation owing tothe incomplete reaction between reactants. In order to surveythe chemical composition of the product, high-angle annulardark-field scanning transmission electron microscopy(HAADF-STEM) and corresponding energy-dispersive spectro-scopy (EDS) mapping analyses were performed (Fig. 2D–G). Ascan be seen, the nanosheets are comprised of molybdenum,sulfur and oxygen, and the uniform distribution of these threeelements clearly confirms the oxygen incorporation in MoS2nanosheets. The atomic ratio of Mo : S is identified to be1 : 2.06, which can be attributed to the highly disordered struc-ture that can expose additional unsaturated sulfur atoms. Theunsaturated sulfur atoms can act as the active sites for HERcatalysis. Furthermore, the oxygen incorporation in this hybridcatalyst also benefits the HER performance. As demonstratedin previous literature, both theoretical and experimentalresults indicated that oxygen incorporation in MoS2 can leadto the decease of bandgap and achieve a fast electron transportto boost HER catalysis.22 Hence, oxygen incorporation in theOMS/CC hybrid catalyst can facilitate the intralayered electrontransport. From an universal viewpoint, when combining thebeneficial elemental incorporation with the high electric con-ductivity gained from the carbonaceous electron transport“superhighway”, the overall electric conductivity can be maxi-mally optimized, thus leading to the highest density of“effective” active sites for the HER. Furthermore, the dis-ordered structure provides abundant active sites that can beelectrically connected, finally achieving a maximum optimi-zation of active sites and electric conductivity synergistically.

X-ray photoelectron spectroscopy (XPS) was performed tofurther investigate the chemical state of the hybrid catalyst. Asshown in Fig. 3A, two characteristic peaks located at 229.1 eVand 232.2 eV arising from Mo 3d5/2 and Mo 3d3/2 orbitals canbe identified, suggesting the dominance of MoIV in theOMS/CC hybrid catalyst.20 Besides, the S 2p region (Fig. 3B)exhibits a single doublet with the 2p3/2 peak at 161.7 eV, whichis consistent with the −2 oxidation state of sulfur,20 thus con-firming the MoS2 structure. Detailed compositional analysisreveals that the atomic ratio of Mo : S is 1 : 2.05, which is con-sistent with the result from EDS analysis. It is worth notingthat, although the oxygen atoms doped in the MoS2 latticeseem to replace the sulfur atoms which would cause theincrease of Mo : S ratio, the highly disordered structure canoffer enough edge sites that can expose additional sulfuratoms, leading to rich sulfur atoms making the Mo : S ratioexceeding the stoichiometric atom ratio of 1 : 2 and thus pro-viding abundant active sulfur sites for the HER. In addition,

the oxygen incorporation in MoS2 was also identified. Asshown in Fig. 3C, the signal of oxygen can be divided into twoindependent peaks. In detail, the O 1s peak located at 530.1 eVcorresponds to the binding energy of oxygen in MoIV–Obonds,42 thus verifying the successful oxygen incorporationrather than surface oxidation, while the peak located at 531.9eV can be attributed to the adsorbed water molecules.22

Hence, the oxygen-incorporated MoS2 nanosheets on thecarbon cloth were identified.

In order to verify our hypothesis on the synergistic optimi-zation strategy, electrochemical measurements were carried outin 0.5 M H2SO4 solution. As shown in Fig. 4A, the polarizationcurve of OMS/CC suggests an extremely high HER activity,from which an ultralow onset overpotential of 90 mV can bedetermined, which is the best record for MoS2-based HER cata-lysts up to now47,51,56–62 and also close to the theoreticalvalue.19 As a sharp comparison, the OMS without hybridi-zation of carbon cloth shows an onset overpotential of 120 mV,confirming the significant role of the conductive carbon skele-ton in HER catalysis. Of note is that the ultralow onset over-potential also takes precedence over a variety of MoS2-based HERcatalysts,32,34,43–55 including many newly reported hybrid cata-lysts coupled with carbonaceous materials (Fig. 5), suggestingthat the ultrahigh HER activity of OMS/CC is not only deter-mined by the hybridization with highly conductive materials,but also benefits from the excellent intrinsic catalytic behavior

Fig. 3 XPS data of the OMS/CC hybrid catalyst. (A) The binding energiesof molybdenum show that no oxidation of MoIV occurs. (B) The spectraof sulfur indicate that the chemical state is −2. (C) The binding energiesof oxygen suggest that two kinds of chemical states exist in the sample,which correspond to the oxygen in MoIV–O bonds and adsorbed watermolecules.

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of the OMS that guarantees the fast intralayered conductivityas well as the abundant active sites.

Besides, after the hybridization with carbon cloth, thecathodic current density is remarkably improved. An ultrahighcurrent density of 225 mA cm−2 was detected at an overpoten-tial of 300 mV, whereas only 124 mA cm−2 of catalytic currentwas gained for bare OMS without hybridization, showing an1.8 fold enhancement in HER activity. The overpotentialrequired to drive a 10 mA cm−2 catalytic current is an impor-tant criterion to evaluate an advanced HER electrocatalyst,

which could be regarded as overpotential to trigger an obviousHER catalysis. As shown in the inset of Fig. 4A, the enlargedLSV curves near the onset region demonstrate that theOMS/CC hybrid catalyst requires as low as 120 mV to drive a10 mA cm−2 cathodic current (with extraction of the back-ground current); while for the bare OMS catalyst, the over-potential required to achieve the same current density is 193 mV,confirming the significant improvement of HER activity viamaterial hybridization. Of note is that the current density ofthe OMS/CC hybrid catalyst before reaching the HER onset islarger than those materials without hybridization, which canbe attributed to the large double-layer capacitance of thecarbon cloth. Moreover, the catalytic current density at a givenpotential of the OMS/CC hybrid catalyst is also sound whencomparing with recent reported results (Fig. 5). At an overpo-tential of 200 mV, the current density of the OMS/CC hybridcatalyst shows an ultrahigh current density of 78 mA cm−2,which is 1.8–52 times higher than a large variety of MoS2-based HER catalysts, demonstrating the superior HER activityof the optimized hybrid catalyst. Tafel plots were obtained toinvestigate the kinetic information during HER catalysis. Ascan be seen from Fig. 4B, the Tafel slope of OMS/CC of 58 mVper decade is comparable to that of the bare OMS, suggestingthe same catalytic mechanism of the HER process. Of note isthat this value is among the best records of MoS2-based cata-lysts, demonstrating the facile kinetics during HERcatalysis.47,51,56–62

The remarkable enhancement of cathodic current can beattributed to the high electric conductivity and abundantactive sites, which guarantees the fast electron transport, andfinally, enriches the “effective” active sites. In order to under-stand the role of the MoS2 structure in HER activity in theOMS/CC hybrid system, the catalysts synthesized at varioustemperatures are tested, which possess different effectiveactive site densities, that is, the lower the temperature con-ditions, the more the number of unsaturated sulfur atoms(active sites). As demonstrated in Fig. 6A, the catalyst preparedat 180 °C displays the highest HER activity, which may beattributed to its optimized active sites and intrinsic conduc-tivity. Nyquist plots were applied to survey the electric conduc-tivity of the OMS/CC hybrid catalyst and bare OMS (Fig. 6B),

Fig. 4 (A) Polarization curves of OMS/CC, OMS, and bare carbon cloth.Inset: Polarization curves near the HER onset. A low overpotential(120 mV) is observed to drive a 10 mA cm−2 catalytic current for theOMS/CC, which is much lower than that of the OMS catalyst (193 mV).(B) Corresponding Tafel plots of the OMS/CC hybrid catalyst, OMS cata-lyst, the bare carbon cloth as well as the 5% Pt/C benchmark catalyst.

Fig. 5 Performance comparison of various MoS2-based HER catalysts,including the OMS/CC hybrid catalyst in this work, MoS2/carbon nanofi-bers (CNF),43 amorphous MoSx/carbon paper (CP),44 MoS2 nanosheets/carbon fiber cloth (CFC),45 MoS2/MoO2 composites,46 1T-MoS2nanosheets,47 MoS2 supported on reduced graphene oxide-modifiedcarbon nanotube/polyimide (PI/CNT-rGO) film,48 MoS2 nanoparticles/graphene,32 amorphous MoSx/graphene/Ni foam,34 MoSSe alloynanoflakes,49 MoS2/graphite hybrids,50 core–shell MoO3/MoS2 nano-wires,51 monolayer MoS2 quantum dots,52 MoS2 nanosheets/CC,

53 MoS2nanosheets/rGO,54 and MoS2 nanodots.

55

Fig. 6 (A) Polarization curves of OMS/CC synthesized at various temp-eratures. (B) Nyquist plots indicating the enhanced conductivity afterhybridization with carbon cloth.

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from which one can observe that after hybridizing with CC,the resistivity of the catalyst decreases obviously, giving directand solid evidence of the fact that the interconnected carbonfibers can facilitate the electron transport, enrich the densityof the electrically connected active sites, and finally enhancethe HER activity.

Except for the HER activity, long-period stability is also akey criterion to evaluate an electrocatalyst. In order to investi-gate the stability of OMS/CC, a long-term CV test was con-ducted. As shown in Fig. 7A, only slight degradation of thecatalytic current density can be revealed even after 2000 CVcycles. A high retention of 97% in the current density at anoverpotential of 300 mV can be achieved, suggesting the excel-lent operational stability of the as-prepared OMS/CC hybridcatalyst. For commercial utilization in electrolytic water split-ting, continuous operation under a static overpotential is moremeaningful. As demonstrated in Fig. 7B, the cathodic currentdensity under a static overpotential of 200 mV shows negligiblechanges even after 12-hour continuous operation, giving solidevidence of the superior electrochemical stability of theOMS/CC hybrid catalyst. The good operational stability ofOMS/CC may arise from the intimate hybridization betweenOMS and CC. The superior HER activity combined with theexcellent stability and freestanding feature makes the novelOMS/CC hybrid catalyst a promising candidate of noble metalsfor practical water splitting.

Conclusions

In this work, oxygen-doped MoS2 ultrathin nanosheets with ahigh density of active sites were vertically grown on a highlyconductive carbon cloth skeleton, and a synergistic optimi-zation of active sites and electric conductivity was successfullyachieved, realizing remarkably improved HER performance. Inorder to enrich the active sites, disorder engineering of MoS2nanosheets was performed, while for enhancing the electricconductivity, oxygen incorporation as well as hybridizationwith carbonaceous materials were carried out, finally realizingthe maximum optimization of the HER catalyst. After the

synergistic optimization, the vertically aligned oxygen-dopedMoS2/carbon cloth catalyst exhibits an ultralow onset over-potential and an ultrahigh catalytic current density accompaniedby superior operational stability, making it the best among theMoS2-based HER catalysts to date. The unique synergisticoptimization strategy reported in this work can not only beused to improve the performance of various electrocatalysts,but can also provide the opportunity for future design of novelcatalysts.

Acknowledgements

This work was financially supported by the 973 Program(2013CB933800), the National Natural Science Foundation ofChina (21501112, 21535004, 21227005, 21390411), and theNatural Science Foundation of Shandong Province(ZR2014BQ007).

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