DOI: 10.1002/chem.201301406

Size-Dependent Enhancement of Electrocatalytic Oxygen-Reduction andHydrogen-Evolution Performance of MoS2 Particles

Tanyuan Wang,[a] Dongliang Gao,[a] Junqiao Zhuo,[a] Zhiwei Zhu,[a]

Pagona Papakonstantinou,[b] Yan Li,[a] and Meixian Li*[a]

Introduction

The energy demands in our society have increased greatlyduring the recent years and will continue to increase due toimprovements in economic growth and our living standards.Therefore, energy conversion and storage technologies thatwould possibly offer us clean and sustainable energy tomeet our energy demands have been one of the most attrac-tive research fields.[1] In this respect, the hydrogen-evolutionreaction (HER) and oxygen-reduction reaction (ORR) aretwo important reactions in the field of energy conversion.They are of key significance in the transformation of solarenergy or electricity to chemical energy and the conversionof the chemical energy to electricity.[2] For example, they arecentral to the operation of direct solar and electrolyticwater-splitting devices as well as to fuel cells and air batter-ies.[3]

Both ORR and HER reactions are sluggish in nature andtraditionally require the use of precious metal catalysts suchas platinum.[4] The high cost and scarcity of the preciousmetals greatly hinder the large-scale implementation ofthese materials. Thus it is of great importance to develop

ORR and HER catalysts based on non-noble metals. Agreat many materials have been tested for the ORR orHER, including various transition-metal complexes,[5] metaloxides,[6] carbon nanomaterials doped with nonmetallic ele-ments[7] or polymers.[8] Although these materials have showninteresting variations in catalytic behaviour for either ORRor HER, none of them has shown high catalytic activity forboth reactions as is the case for precious metals. If a materi-al is both ORR and HER active, it would not only turnclean hydrogen energy into electricity efficiently, but wouldalso play a key role in the production of hydrogen, andtherefore it would be of central importance to the acquisi-tion of clean and sustainable energy.

MoS2 is a kind of layered material that is similar to gra-phene. Earlier studies have indicated that its edges are pre-ferred sites for the chemisorption of O2.

[9] More significantly,recent work has proven that it could be active for HEReven though the bulk MoS2 is inert.[10] All of the aboveimply that it might be a potential catalyst for both ORRand HER. Here we demonstrate an extremely easy and con-venient way to prepare different sizes of MoS2 particlesfrom inert and easily available bulk MoS2 by a combinationof sonication and centrifugation. The influence of the sizeeffect on ORR and HER has been investigated systemati-cally. It was found that the relatively small MoS2 nanoparti-cles show a four-electron reaction route for ORR and a lowoverpotential for HER, which could have potential applica-tions in energy conversion and storage fields.

Abstract: MoS2 particles with differentsize distributions were prepared bysimple ultrasonication of bulk MoS2

followed by gradient centrifugation.Relative to the inert microscale MoS2,nanoscale MoS2 showed significantlyimproved catalytic activity toward theoxygen-reduction reaction (ORR) andhydrogen-evolution reaction (HER).The decrease in particle size was ac-companied by an increase in catalyticactivity. Particles with a size of around2 nm exhibited the best dual ORR and

HER performance with a four-electronORR process and an HER onset po-tential of �0.16 V versus the standardhydrogen electrode (SHE). This is thefirst investigation on the size-depend-ent effect of the ORR activity of MoS2,and a four-electron transfer route wasfound. The exposed abundant Mo

edges of the MoS2 nanoparticles wereproven to be responsible for the highORR catalytic activity, whereas theorigin of the improved HER activity ofthe nanoparticles was attributed to theplentiful exposed S edges. This newlydiscovered process provides a simpleprotocol to produce inexpensive highlyactive MoS2 catalysts that could easilybe scaled up. Hence, it opens up possi-bilities for wide applications of MoS2

nanoparticles in the fields of energyconversion and storage.

Keywords: electrocatalysis · hydro-gen evolution · molybdenum ·nanoparticles · reduction

[a] T. Wang, D. Gao, J. Zhuo, Prof. Z. Zhu, Prof. Y. Li, Prof. M. LiCollege of Chemistry and Molecular EngineeringPeking University, Beijing 100871 (P.R. China)E-mail : lmwx@pku.edu.cn

[b] Prof. P. PapakonstantinouSchool of Engineering, Engineering Research InstituteUniversity of Ulster, Newtownabbey, BT37 0QB (UK)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201301406.

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Results and Discussion

It has been reported that ultrasonication can be used to ex-foliate MoS2 or some other layered materials into nano-sheets.[11] In addition to nanosheets, however, our previouswork proved that MoS2 nanoparticles/nanodots could alsobe obtained by this method.[12] So far no research has beencarried out to investigate the intrinsic catalytic properties ofthese particles in ORR and HER systematically. In thisstudy, long-term ultrasonication was applied to bulk MoS2 toexfoliate it into nanosheets and break it into nanoparticleswith N,N-dimethylformamide (DMF) as the solvent. Thenthe mixture was centrifuged at 1000, 3000, 6000 and12 000 rpm successively, and the precipitates were collected.Transmission electron microscopy (TEM) was used to char-acterise the size and structure of these products. We couldobserve that the mixture consisted of particles with differentsizes after sonication (Figure S1 in the Supporting Informa-tion). Gradient centrifugation was able to separate the parti-cles according to their sizes (Figure 1). Figure 1a shows that

the size of the precipitates centrifuged at 1000 rpm waslarger than 500 nm. The precipitates centrifuged at 3000 rpmwere mainly multilayer nanosheets and particles with diame-ters smaller than 200 nm (Figure 1b). Centrifugation at6000 rpm resulted in a collection of nanoparticles with diam-eters of about 10 nm (Figure 1c). After centrifugation at12 000 rpm, nanoparticles with a diameter of around 2 nmwere obtained (Figure 1d). It is important to note that MoS2

nanosheets mainly existed in the 3000 rpm precipitate inthese experiments. There were hardly any sheets in the

6000 rpm precipitate and absolutely no sheets in the12 000 rpm precipitate. These MoS2 nanoparticles formed astable solution for more than one month after being redis-persed into DMF. The dispersions of these precipitates wereinvestigated by UV/Vis absorption spectroscopy (Figure 2).

All the dispersions in DMF from precipitates collected at3000, 6000 and 12 000 rpm exhibited absorption peaks atabout 300, 400 and 600–700 nm, which were similar to theabsorption bands of MoS2 particles prepared by other meth-ods.[13] Notably, a number of distinct differences were ob-served among the dispersions. The UV/Vis spectrum of the3000 rpm dispersion showed rather redshifted peaks relativeto those of 6000 and 12 000 rpm dispersions, thus indicatingan increase in the size of the particles. The absorption peaksof 2 nm MoS2 particles (12 000 rpm) at about 300 and400 nm were more intense and exhibited a small blueshiftrelative to those of 10 nm MoS2 particles (6000 rpm). Thesephenomena were consistent with the size distribution of thedifferent nanoparticles since the blueshift in the absorptionspectra is associated with a decrease in the nanoparticlesize.[14]

To investigate the effect of the size on the catalytic activi-ties of MoS2 particles toward ORR and HER, DMF disper-sions from MoS2 precipitates obtained at different centrifu-gation speeds were used to modify the surfaces of glassycarbon (GC) electrodes with a loading of 0.2 mg cm�2.Figure 3 shows the scanning electron microscopy (SEM)images of the modified GC electrodes. MoS2 platelets withdiameters of several micrometers could be observed fromthe 1000 rpm precipitates (Figure 3a). Such morphology wassimilar to the electrode modified with bulk MoS2 (Figure S2in the Supporting Information). In contrast, for the elec-trode modified with 3000 rpm precipitates, large plateletscould barely be observed since the precipitates mainly con-sisted of nanosheets and smaller particles with diameters inthe range of 100 to 200 nm (Figure 3b). The surface mor-phology became much smoother when the size of the modi-

Figure 1. TEM images of the MoS2 particles collected at successive cen-trifugation speeds of a) 1000, b) 3000, c) 6000 and d) 12000 rpm, respec-tively.

Figure 2. UV/Vis spectra of dispersions in DMF from MoS2 precipitatesobtained at 3000, 6000 and 12000 rpm centrifugation speeds, respectively.

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fied particles decreased to the nanoscale (approximately 10and 2 nm), as shown in Figure 3c and d, which suggests thatthe ultrasmall nanoparticles easily aggregate to form com-pact films. The possible reason for this is related to the highself-cohesion of the ultrasmall nanoparticles.

X-ray photoelectron spectroscopy (XPS) was used tocharacterise the elemental composition and bonding config-uration of the modified electrodes. Intense S and Mo peakswere present in the XPS survey spectra in addition to thepeaks of C and O that originated from adventitious pollu-tion (Figures S3–S7 in the Supporting Information). The Sto Mo ratios of all the modified electrodes were calculatedto be 2.0:1–2.1:1, which indicated that the average composi-tion of the modified films was MoS2. Figures 4 and 5 showthe S 2p region and Mo 3d region of electrodes modifiedwith MoS2 that result from various centrifugation speeds.The S 2p3/2 and Mo 3d5/2 binding energies of the particlescollected at 1000 rpm were 162.2 and 229.4 eV, respectively.These binding energies were nearly the same as those ofbulk MoS2. However, slightly positive shifts of the S and Mobinding energies could be observed for the nanoparticle-modified electrodes. The 3000 and 6000 rpm precipitates ex-hibited an S 2p3/2 binding energy at 162.6 eV and an Mo 3d5/2

binding energy at 229.7 eV. For the 12 000 rpm precipitates,the binding energies were 162.7 and 229.7 eV, respectively.Clearly, the significant decrease in the MoS2 particle size re-sulted in an increase of the S and Mo binding energies,which was similar to the observed trend reported for thegold nanoparticles in the literature.[15] The full width at half-maximum (fwhm) of the S 2p peaks and Mo 3d peaks forprecipitates collected at 3000, 6000 and 12 000 rpm werelarger than those of bulk MoS2 and the 1000 rpm precipitate.The fwhm of S 2p peaks increased from 0.86 to 0.91 whenthe size of the modified MoS2 particle decreased from themicroscale to the nanoscale, and the fwhm of Mo 3d peaks

increased from 0.90 to 0.97. These trends are in accordancewith reported results in which the fwhm of the smaller parti-cles is usually larger,[15b] which further verifies the observedsize effect. Nevertheless, the XPS spectra of precipitates

Figure 3. SEM images of the electrodes modified with MoS2 nanoparti-cles collected at a centrifugation speed of a) 1000, b) 3000, c) 6000 andd) 12000 rpm, respectively.

Figure 4. XPS spectra of the S 2p region for the electrodes modified withMoS2.

Figure 5. XPS spectra of the Mo 3d region for the electrodes modifiedwith MoS2.

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from higher speeds (3000, 6000 and 12 000 rpm) did notshow any distinct differences even though their sizes werequite different according to the TEM characterisation. Apossible explanation can be found on the agglomeration ofnanoparticles when dried on the electrode, especially for thesmaller ones collected at 6000 and 12 000 rpm centrifugationspeeds, as revealed by the SEM images. Since these particlesare no longer individual nanoparticles, they would not showany notable size effects in the fwhm of the XPS peaks,except when compared with the rough precipitates of1000 rpm and the bulk material.

X-ray diffraction patterns of the MoS2 samples with differ-ent sizes are shown in Figure 6. The microscale MoS2 (bulkMoS2 and 1000 rpm precipitate) demonstrated an intense(002) diffraction peak as well as some weaker (103), (006),

(105) and (110) peaks that correspond to reflections ofnanostructured 2 H�MoS2. With the increase of the centrifu-gation speed, the intensities of all the diffraction peaks de-creased, whereas their fwhm increased and even some ofthem disappeared. These trends suggest a decrease in theparticle size and a crystal-to-amorphous structural transfor-mation, which further confirm the XPS observations. Theprecipitate collected at 12 000 rpm still showed weak andbroad (002), (006) and (110) diffraction peaks even thoughit did not exhibit a selected-area electron diffraction patternin our early research.[12] The possible reason for this discrep-ancy is that even though the majority of the particles collect-ed at 12 000 rpm were amorphous, a minute amount of themmight still have an ordered lattice structure since the centri-fugation method used cannot control precisely the structureof the nanoparticles. With the existence of some crystallineorder in the MoS2 nanoparticles, the collected powderwould display weak diffraction peaks.

On the basis of earlier reports, which claimed that “theedge sites of MoS2 crystal would selectively adsorb O2 inpreference to basal sites”[9] and that the surface state and

orientation of MoS2 had a significant influence on the redoxreaction,[16] we decided to explore the ORR activity of thesize-controlled MoS2 material for the first time. Clearoxygen-reduction peaks could be observed for various elec-trodes in O2-saturated 0.1 m KOH solution (Figures S8–S13in the Supporting Information). Figure 7 displays cyclic vol-

tammograms of the bare GC and modified electrodes withdifferent MoS2 sizes. Two reduction peaks located at�0.40 V and approximately �0.90 V versus SCE were ob-served at the bare GC electrode, which correspond to two-step two-electron reduction processes of O2. After beingmodified with bulk MoS2, the first oxygen-reduction peakshifted to �0.44 V versus SCE, thereby revealing deteriora-tion in the activity relative to the bare GCE. Upon replacingthe bulk MoS2 with the MoS2 particles collected at 1000 rpmcentrifugation speed, the second reduction peak shifted posi-tively, thus demonstrating a small improvement in the ORRactivity of the electrode. However, the first oxygen-reduc-tion peak appeared at �0.42 V versus SCE, which was stillslightly more negative than the corresponding peak of thebare GC electrode. Overall, these phenomena indicate thatthe ORR activity of microscale MoS2 particles was quitelow. The large particles were rather inactive and hinderedthe ORR activity underneath the GC electrode. When theseMoS2 particles became nanosized, the cyclic voltammetry(CV) responses of the electrode changed significantly. Onepair of redox peaks appeared in the solution without O2 forthe electrodes modified with the 3000, 6000 and 12 000 rpmprecipitates (Figures S11–S13 in the Supporting Informa-tion), which was ascribed to the existence of surface defectson the nanoparticles, thus the MoS2 nanoparticles were notcompletely chemically inert.[17] The area of the CV loop in-creased with a drop in the particle size. In particular, theelectrodes modified with the smallest particles from thehighest centrifugation speed of 12 000 rpm displayed a con-

Figure 6. XRD patterns of a) bulk MoS2 and MoS2 precipitates collectedat b) 1000, c) 3000, d) 6000 and e) 12 000 rpm centrifugation speeds.

Figure 7. CV curves of oxygen reduction on the bare GC electrode andGC electrodes modified with bulk MoS2 and MoS2 particles collected at1000, 3000, 6000 and 12000 rpm centrifugation speeds in O2-saturated0.1m KOH at a scan rate of 50 mV s�1 with a loading of 0.2 mg cm�2.

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siderable increase, which indicated an increase in the effec-tive surface area of the modified electrodes. A new reduc-tion peak at around �0.65 V versus SCE appeared in theO2-saturated solution for the MoS2-nanoparticle-modifiedelectrodes, the intensity of which increased with a decreasein the MoS2 particle size, becoming more distinguishable forthe smallest nanoparticles. One explanation for this size de-pendence is related to the fact that the second reductionpeak shifts gradually toward more positive potentials as thesize decreases and tends to merge with the new peak ataround �0.65 V versus SCE, thereby enhancing its intensity.The potential of the first reduction peak for these electrodesalso showed a tendency toward a positive shift with the de-crease in the MoS2 particle size. For particles collected at3000 and 6000 rpm the peak potential was around �0.37 Vversus SCE, and for the 12 000 rpm precipitate the reductionpeak shifted to �0.35 V versus SCE. All these nanosizedMoS2 electrodes exhibited improved ORR activity relativeto the bare and microsized MoS2-modified GC electrodes.

The ORR performance of the electrodes was further in-vestigated through linear sweep voltammetry (LSV) on a ro-tating-disk electrode (RDE) in an O2-saturated 0.1 m KOHsolution. Owing to the existence of the inherent reactioncurrent from MoS2, it was rather hard to judge the exactonset potential for oxygen reduction of the modified elec-trode. However, it was clear that the onset potential shiftednegatively with increasing particle size (Figure 8). The elec-

trode modified with bulk MoS2 or the 1000 rpm MoS2 pre-cipitate showed inferior catalytic activity relative to the bareGC, at potentials more negative than �0.5 V versus SCE.The electrode modified with MoS2 nanoparticles collected at12 000 rpm centrifugation speed showed the largest limitingcurrent density. Its limiting current density was3.91 mA cm�2 at �1.0 V versus SCE at the rotating rate of900 rpm, which was similar to the limiting current density of

a Pt/C-modified electrode (3.95 mAcm�2), even though itsonset potential for ORR was more negative.

RDE voltammetry was further used to evaluate theoxygen-reduction kinetics of all the electrodes. Figure 9ashows rotating disk voltammograms of the 12 000 rpm-MoS2-precipitate-modified electrode at different rotation rates. Anincrease in the limiting current density could be observedwith increasing rotation rate. Relative to the bare GC andthe other MoS2-modified electrodes (Figures S14–S18 in theSupporting Information), the 12 000 rpm MoS2 precipitateexhibited the best diffusion-controlled region. The ORR ki-netics of the electrodes was further analysed by the Kou-tecky–Levich equation [Eqs. (1), (2) and (3)]:

j�1 ¼ j�1k þ j�1

l ¼ j�1k þ B�1w�0:5 ð1Þ

B ¼ 0:2nFCOD2=3O n�1=6 ð2Þ

jk ¼ nFkCO ð3Þ

Figure 8. LSV curves of oxygen reduction on the GC electrode, GC elec-trodes modified with bulk MoS2 and MoS2 particles collected at 1000,3000, 6000 and 12 000 rpm centrifugation speeds in O2-saturated 0.1m

KOH at a scan rate of 10 mV s�1 with a rotation speed of 900 rpm.

Figure 9. a) LSV curves at various rotation rates for the electrode modi-fied with MoS2 nanoparticles collected at 12 000 rpm centrifugation inO2-saturated 0.1m KOH at a scan rate of 10 mV s�1. b) Koutecky–Levichplots of the 12 000 rpm-MoS2-precipitate-modified electrode at differentpotentials.

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in which j is the measured current density, jk is the kineticlimiting current density and jl is the diffusion limiting cur-rent density. The variable w represents the rotation rate ofthe electrode [rpm], n is the number of electrons transferredper O2 molecule, F is the Faraday constant (96485 C mol�1),CO is the bulk concentration of O2 (1.2 �10�6 molcm�3), DO

is the diffusion coefficient of O2 in 0.1 m KOH (1.9 �10�5 cm2 s�1), n is the kinetic viscosity of the electrolyte(0.01 cm2 s�1) and k is the electron-transfer rate constant.The transferred electron number (n) and the kinetic limitingcurrent density (jk) of the electrodes can be determinedfrom the slope and intercept of the Koutecky–Levich plots,respectively, according to the above equations. Figure 9bdemonstrates the corresponding Koutecky–Levich plots ofthe electrode modified with MoS2 nanoparticles collected at12 000 rpm centrifugation at various potentials. A goodlinear relationship between j�1 and w�0.5 was observed. Theslopes remained almost constant within the potential rangefrom �0.4 to �1.2 V versus SCE for the 12 000 rpm-precipi-tate-modified electrodes, in contrast to the slopes of the GCelectrode and the electrode modified with larger MoS2 parti-cles (Figures S19–S23 in the Supporting Information). Theelectron-transfer number of the 12 000 rpm-precipitate-modified electrode was calculated to be 3.6 at �0.5 V versusSCE and ranged from 3.6 to 4.1 at different potentials(Figure 10), which suggests that the oxygen-reduction reac-

tion proceeded by means of a four-electron reaction path-way for this electrode. However, the electrodes modifiedwith 6000 and 3000 rpm precipitates exhibited an electron-transfer number of only 2.7 at �0.5 V versus SCE. For thebare GC electrode and electrodes modified with microscaleMoS2 (bulk MoS2 or 1000 rpm precipitate), the electron-transfer number at �0.5 V versus SCE decreased to 2.3 oreven less. All of these results indicate that a two-electron re-

action route is favoured for the GC substrate and GC modi-fied with MoS2 particles of sizes equal to or larger than10 nm at a potential of �0.5 V versus SCE. The four-elec-tron reaction route could only be observed when the reduc-tion potential turned to �0.7 V versus SCE or even morenegative. Since the CV test (Figure 7) demonstrated that theMoS2 nanoparticles exhibited a new oxygen-reduction peakat around �0.65 V versus SCE in stark contrast to the MoS2

microparticles, we believe that the positive shift in thesecond step for ORR on the MoS2 nanoparticles should con-tribute to the divergence in the ORR activity for these dif-ferent sizes of MoS2 particles. Therefore, this propensity fora positive shift of the second two-electron oxygen-reductionprocess would make its combination possible with the firsttwo-electron oxygen-reduction process, thereby resulting inan apparent four-electron reaction route. The 12 000 rpmMoS2 precipitate showed the most intense peak at �0.65 Vversus SCE and nearly no peak at �0.9 V versus SCE,which proved that its second oxygen-reduction step had themost significant positive shift. Therefore it showed a notablefour-electron oxygen-reduction route. The kinetic limitingcurrent density of the modified electrodes demonstrated ageneral tendency to increase with the decrease in the MoS2

particle size. The kinetic limiting current density of the bulkMoS2 was 1.3 mA cm�2 at the potential of �0.5 V versusSCE. This was increased to values of 3.4, 4.3 and5.0 mA cm�2 for the 1000, 3000 and 6000 rpm precipitates,respectively. For the 12 000 rpm precipitate, the kinetic limit-ing current density decreased slightly to 4.8 mA cm�2. Theelectron-transfer rate constants were calculated to be 5.6 �10�3, 1.2 � 10�2, 1.4 � 10�2, 1.6 � 10�2 and 1.2 � 10�2 cm s�1 forbulk MoS2, 1000, 3000, 6000 and 12 000 rpm precipitates, re-spectively.

It is of particular importance to identify the active sitesthat are responsible for the improved ORR performance. Itseems that the larger amount of edge sites of the MoS2

nanoparticles contributed significantly to their enhancedORR activity relative to the bulk materials. However, amore insightful understanding is still needed. Since it hasbeen reported that Mo-based alloys or compounds areactive for ORR,[18] we thought that the Mo edge sites mightbe responsible for the four-electron ORR process of MoS2

particles. To prove this hypothesis, we used a common che-lating agent, ethylenediaminetetraacetic acid (EDTA), to in-teract with the MoS2 nanoparticles. We adopted an approachsimilar to that used by Dai and co-workers to determine therole of Fe in forming active ORR sites on the N-dopedcarbon-nanotube–graphene complex.[19] Since EDTA couldeasily react with metal ions, it would block the Mo edge siteon the nanoparticles and hence reduce the amount of Mosites that would possibly be responsible for the ORR activi-ty of the materials. XPS characterisation proved that therewas an interaction between EDTA and the MoS2-nanoparti-cle-modified electrode (Figure 11). The pristine EDTAshowed N 1s double peaks at 401.9 and 399.4 eV, which cor-respond to the protonated N and nonprotonated N, respec-tively. However, its N 1s binding energy shifted to 400.0 eV

Figure 10. Electron-transfer numbers for ORR at different potentials, cal-culated for the bare GC electrode and GC electrodes modified with bulkMoS2 and MoS2 particles collected at 1000, 3000, 6000 and 12000 rpmcentrifugation speeds, respectively.

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upon the presence of MoS2 nanoparticles. This change wasattributed to the complexation of N.

The CV response of the MoS2-nanoparticle-modified elec-trode showed evident change after being treated withEDTA (Figure 12). The electrode modified by MoS2 nano-particles would usually demonstrate a reduction peak ataround �0.65 V versus SCE that was not observed for thebulk-MoS2-modified electrode in 0.1 m KOH saturated withO2. However, after the electrode was incubated in the solu-tion of EDTA, the intensity of the peak at �0.65 V versusSCE that was possibly responsible for the four-electronORR route decreased greatly. These results suggest that theMo edge sites are important to the oxygen-reduction proc-ess, and we attribute this peak to them. The plethora of Moedges on the 2 nm MoS2 particles has contributed to thefour-electron oxygen-reduction process of this modifiedelectrode. After interaction with EDTA, the exposure ofMo edge sites declines, which leads to a decrease in the re-duction current at the same potential. These results provethat the abundance of Mo edges on the smaller MoS2 nano-particles improves the ORR reduction peak at �0.65 Vversus SCE (Figure 7). Following the interaction with

EDTA, the electrode was then immersed in sulfuric acid sol-ution or water and its ORR property was examined. Wecould see that its reduction peak recovered partly. This indi-cates that the interaction between EDTA and MoS2 nano-particles was not as strong as that of EDTA–metal complexcompounds. Dissociation would take place for the MoS2–EDTA mixture, which partly reinstated the Mo edges on theMoS2 nanoparticles. A control experiment was also carriedout; however, no clear change could be observed in the CVresponses without the existence of EDTA (Figure S24 in theSupporting Information).

Since we have provided clear evidence that an electrodemodified with 2 nm MoS2 particles (12 000 rpm precipitate)showed the best ORR activity with a four-electron reactionprocedure relative to larger MoS2-particle-modified electro-des, it was deemed necessary to study its resistance to cross-over effects and stability to provide a more complete evalu-ation of this catalyst. Figure 13a shows the chronoampero-metric responses of the Pt/C- and MoS2- (12 000 rpm precipi-tate) modified electrodes in O2-saturated 0.1 m KOH solu-tion before and after the addition of methanol. After theaddition of methanol (2.4 mL), the reaction current de-creased greatly for the Pt/C catalyst, whereas only a smalldecrease could be observed for the 12 000 rpm-MoS2-precipi-tate-modified electrode. The durability of both catalysts forORR was evaluated by chronoamperometry at �0.3 Vversus SCE in 0.1 m KOH solution saturated with O2 at a ro-tation rate of 1000 rpm (Figure 13b). The two catalystsshowed similar stability with a persistent relative current ofabout 72 % after 20 000 s.

Although the catalytic performance of MoS2 nanomateri-als on HER has been reported,[2e, 3b, 10b, c,17, 20] the influence ofthe size effect of MoS2 particles on HER has not been sys-tematically investigated. Here we have studied the HERcatalytic activity of the MoS2-particle-modified electrodes in

Figure 11. XPS spectra of the N 1s region for a) EDTA and b) MoS2-EDTA.

Figure 12. CV curves of i) the MoS2-nanoparticle-modified electrode,ii) the MoS2-nanoparticle-modified electrode incubated in EDTA andiii) the restored MoS2-nanoparticle-modified electrode after the dissocia-tion of EDTA in 0.1 m KOH saturated with O2 at a scan rate of50 mV s�1.

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0.5 m H2SO4 with a three-electrode system. Relative to thebulk MoS2 and microscale MoS2 products collected at1000 rpm, the nanoscale MoS2 particles exhibited significant-ly improved HER activity (Figure 14). Particles collected at12 000 rpm showed the best HER catalytic property. Itsonset potential was about �0.16 V versus SHE and the cur-rent density was 0.4 mA cm�2 at h=0.2 V. The 6000 rpm pre-cipitate also showed an onset potential of �0.16 V versusSHE, but its current density was a little smaller than that ofthe 12 000 rpm precipitate. At h=0.2 V its current densitywas about 0.3 mA cm�2. The precipitate collected at3000 rpm displayed inferior HER activity with an onset po-tential of about �0.20 V versus SHE. Both the 1000 rpmprecipitate and bulk MoS2 did not show any HER activity,even at the overpotential of 0.4 V, which was similar to thebare GC electrode. Since it has been reported that bulkMoS2 is inert in HER and the S edge is active forHER,[10a,21] we conjecture that the nanosized particles withsufficient surface S sites contributed to the enhanced HERactivity of the modified electrode. Therefore it is not surpris-ing that the electrode modified with the smallest MoS2 parti-cles demonstrated the highest HER activity.

Tafel plots of the GC electrode together with those of allmodified electrodes are presented in Figure 15. The Tafelslopes of the modified electrodes showed a decreasing trendwith the diminution of the MoS2 particle size. The Tafelslope of the bulk-MoS2-modified electrode was 166 mV perdecade. For the 1000 and 3000 rpm precipitates, the Tafel

slope decreased to 120 and 115 mV per decade, respectively.The electrode modified with nanoparticles obtained at6000 rpm (10 nm in size) exhibited a Tafel slope of 89 mVper decade. When the size of the nanoparticle decreased toabout 2 nm (12 000 rpm), the Tafel slope turned to 82 mVper decade. The decrease of the Tafel slope might representthe increase in the adsorbed hydrogen coverage on the sur-face of the electrode when the size of the modified MoS2

particle decreases. Generally, the mechanism of the HER

Figure 13. Chronoamperometric responses of commercial Pt/C and12000 rpm-MoS2-precipitate-modified electrode a) with 3m methanoladded at around 200 s and b) at �0.3 V versus SCE in the O2-saturated0.1m KOH at a rotation rate of 1000 rpm.

Figure 14. Polarisation curves of the bare GC electrode and GC electro-des modified with bulk MoS2 and MoS2 particles collected at 1000, 3000,6000 and 12000 rpm centrifugation speeds, respectively, in 0.5 m H2SO4 ata scan rate of 2 mV s�1. The loading was 0.2 mg cm�2.

Figure 15. Tafel plots of the GC electrode, GC electrodes modified withbulk MoS2 and MoS2 particles collected at 1000, 3000, 6000 and12000 rpm centrifugation speeds, respectively, in 0.5 m H2SO4 at a scanrate of 2 mV s�1.

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would be a Volmer–Heyrovsky or a Volmer–Tafel mecha-nism. When the adsorbed hydrogen coverage on the elec-trode was low, the Volmer reaction would possibly be therate-determined step, which resulted in a theoretical Tafelslope of 120 mV per decade. For highly adsorbed hydrogencoverage, a Heyrovsky reaction or Tafel reaction would usu-ally be the rate-determined step, which led to a theoreticalTafel slope of 40 or 30 mV per decade, respectively. The ad-sorbed hydrogen coverage on 1000 and 3000 rpm precipi-tates might be low since the sizes of the modified particlewere large and the basal plane of MoS2 was inert for HER.Thus they displayed a Tafel slope of about 120 mV perdecade that corresponded to the Volmer reaction. The bulkMoS2 showed a slightly larger Tafel slope relative to the the-oretical value, which was possibly caused by the internal re-sistance of the modified film. When the size of the MoS2

particle decreased to 10 nm or less, its specific surface areawould expand and the proportion of HER active S edgeswould increase. Therefore the adsorbed hydrogen coverageof the 6000 and 12 000 rpm-precipitate-modified electrodeswould be higher, even though the nanoparticles would ag-glomerate during the modification, which led us to the con-clusion that the Volmer reaction was no longer the onlyroute that affected the HER rate of the electrode. Since ithas been proven that electrodeposited MoSx film[22] and gra-pheme-supported MoS2

[23] would show a Volmer–HeyrovskyHER mechanism with the Heyrovsky reaction as the rate-determined reaction, we suggest that the HER of the 6000and 12 000 rpm-precipitate-modified electrodes might alsofollow the Volmer–Heyrovsky mechanism and both of thetwo reactions might have influenced the HER rate of themodified electrodes. The exchange current density of the12 000 rpm-precipitate-modified electrode was 1.3 �10�6 A cm�2, which was similar to the exchange current den-sity of the earlier reported MoS2 nanoparticles prepared bychemical vapour deposition.[17]

The stability of the 12 000 rpm-precipitate-modified elec-trode was tested by applying a cyclic potential scan from�0.4 to +0.6 V versus SHE at a scan rate of 100 mV s�1.Figure 16 shows the typical polarisation curves of the MoS2-nanoparticle-modified electrode before and after 1000cycles. No clear change could be observed, which provedthat the MoS2-nanoparticle-modified electrode was verystable for HER.

Conclusion

In summary, we have developed a very easy and convenientmethod by combination of sonication and centrifugation toobtain MoS2 particles with different size distributions, start-ing from bulk MoS2. Among the products, particles with thesmallest size (�2 nm) showed outstanding activity not onlyfor HER, but also for ORR, in contrast to the inert bulkMoS2. A four-electron ORR process was demonstrated, andan HER onset potential as low as �0.16 V versus SHE wasobtained for the approximately 2 nm-MoS2-particle-modi-

fied electrode. This work describes the first investigation onthe ORR activity of MoS2 and its particle size effect. We be-lieve that the Mo edges on the extremely small MoS2 nano-particles (�2 nm) should contribute to the four-electron re-action process of ORR on the modified electrode, whereasthe HER activity of the MoS2-nanoparticle-modified elec-trode is attributed to the abundant S edges on the nanopar-ticles. This kind of MoS2 nanomaterial might have potentialapplications in water splitting and fuel cells due to its activi-ty for HER and ORR. It could be used not only to obtainhydrogen energy from solar energy or electricity, but alsofor the efficient conversion of hydrogen energy or otherkinds of chemical energy to electrical energy. Importantly,we have demonstrated an extremely simple method for ob-taining catalytically active MoS2 nanoparticles, starting fromeasily obtainable bulk material. As a result, it has the poten-tial to be implemented on an industrial scale not only forMoS2 but also for other inorganic compounds.

Experimental Section

Preparation of MoS2 particles with different sizes : MoS2 (99 %, 2 mm insize, Aldrich) was mixed with DMF (99.9 %, Aldrich) at a concentrationof 1 mg mL�1. This mixture was ultrasonicated using an SB-2200 sonifier(Shanghai Branson, China) at room temperature ((22�2) 8C) for 4 h toform a black suspension. After this process, the suspension was centri-fuged at 1000 rpm for 15 min. Then the precipitate and the supernatantwere collected separately, and the supernatant was centrifuged at3000 rpm for 15 min. The precipitate and the supernatant were also col-lected separately. After that the supernatant was centrifuged at 6000 rpmfor 30 min to obtain MoS2 particles with a size of around 10 nm in theprecipitate and a light yellow supernatant. Finally this supernatant wascentrifuged at 12000 rpm for 30 min and the precipitation was gathered.The GS-15R centrifugation system (Beckman, America) was used forthese procedures. These precipitates were re-dispersed into DMF for thecharacterisation of their UV/Vis absorption spectra.

Fabrication of the MoS2-modified electrode : MoS2 particles with differentsizes were dispersed into DMF with a concentration of approximately1 mg mL�1. Then the solutions were dropped on the GC electrodes and

Figure 16. Stability test of the 12000 rpm-MoS2-precipitate-modified elec-trode by cyclic potential scanning from �0.4 to +0.6 V versus SHE in0.5m H2SO4 at a scan rate of 100 mV s�1.

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FULL PAPERMoS2 Particles

dried in the air. The MoS2 nanoparticles would form very stable filmseven without the protection of Nafion, whereas the microparticle filmswere not so stable.

Characterisation : All electrochemical measurements were carried outusing a CHI 660D instrument (Chenhua, China) at room temperature. APt electrode was used as the counter electrode, and a GC or MoS2-modi-fied GC electrode was used as the working electrode. A saturated calo-mel electrode (SCE) was used as the reference electrode for all the elec-trochemical tests, and the potential was calibrated to the SHE for thetests of HER (the potential of the SCE was 0.248 V versus SHE at20 8C). The rotation speed of the electrode for ORR was controlled by amodulated speed rotator (Pine, America). UV/Vis absorption spectrawere recorded using a U-4100 UV/Vis-NIR spectrophotometer (Hitach,Japan). TEM images were acquired using a JEM-2100F electron micro-scope (JOEL, Japan). SEM images were obtained using a S-4800 electronmicroscope (Hitach, Japan). XPS data were collected using an Axis Ultraspectrometer (Kratos Analytical Ltd., Japan), and the binding energywas calibrated by the C 1s hydrocarbon peak at 284.8 eV. XRD patternswere acquired using a DMAX-2400X-Ray diffractometer (Rigaku,Japan).

Acknowledgements

This work was financially supported by the National Natural ScienceFoundation of China (grant nos. 20875005, 21075003 and 21275010).

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Received: April 13, 2013Published online: July 19, 2013

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M. Li et al.