DOI:10.1002/chem.201503356 Full Paper

& Lithium-IonBatteries

Mesoporous CarbonNanofibersEmbedded with MoS2 Nanocrystals for Extraordinary Li-Ion Storage Shan Hu,[a, b] Wen Chen,*[a] Evan Uchaker,[b] Jing Zhou,[a] and Guozhong Cao*[b, c]

1 Abstract: MoS2 nanocrystals embedded in mesoporous rent density of 100 mAgÀ ,aswell as an outstanding rate ca- carbon nanofibers are synthesized through an electrospin- pability. The greatly improved kinetics and cycling stability

ning process followed by calcination. The resultant nanofib- of the mesoporous MoS2@C nanofiberscan be attributed to

ers are 100–150nmindiameter and constructedfrom MoS2 the crosslinked conductive carbon nanofibers, the large spe-

nanocrystals with alateral diameter of around 7nmwith cific surface area, the good crystallinity of MoS2,and the 2 1 specificsurface areas of 135.9 m gÀ .The MoS2@C nanofib- robust mesoporousmicrostructure. The resulting nanofiber

ers are treated at 4508CinH2 and comparison samples an- electrodes, with short mass- and charge-transport pathways,

nealed at 8008CinN2.The heat treatments are designed to improved electrical conductivity,and large contact area ex- achievegood crystallinity and desiredmesoporous micro- posedtoelectrolyte,permitting fast diffusional flux of Li structure, resulting in enhanced electrochemical per- ions, explainsthe improved kinetics of the interfacial charge-

formance. The small amount of oxygeninthe nanofibersan- transfer reactionand the diffusivity of the MoS2@C mesopo-

nealed in H2 contributes to obtaining alower internalresist- rous nanofibers. It is believedthat the integration of MoS2 ance, and thus, improving the conductivity.The resultsshow nanocrystals and mesoporouscarbon nanofibers may have

that the nanofibers obtained at 4508CinH2 deliver an extra- asynergistic effect, giving apromisinganode,and widening 1 ordinary capacity of 1022 mAhgÀ and improvedcyclic sta- the applicability range into high performanceand mass pro- bility,with only 2.3 %capacity loss after 165 cycles at acur- ductioninthe Li-ion battery market.

1. Introduction Amongvarious nanostructured materials, considerable interest has been directed to 1D nanostructures such as nanowires, There is increasing demand for the development of clean nanotubes, nanorods,and nanofibers as promising electrode energy-storage devices for applications in portable electronics, materials for LIBs. In general,the advantages of 1D nanomate- electric vehicles (EVs), and hybrid electric vehicles (HEVs), rials are:1)their large specific surfacearea between the active which must have the advantages of high powerdensity,high materials and electrolyte, which increases surface/interface energy density,and high safety.[1] Lithium-ion batteries(LIBs) storage; 2) ashort diffusion length for Li ions, which gives rise have dominated energy-storage technologiesand power- to ahigh-ratecharge/discharge capability;and 3) their ability source markets because of their highenergy density,high volt- to absorb alarge volumeexpansion, which leads to stable age, and long lifespan, which are desirable features for large- cycle performance.[3] However,there also are some disadvan- scale energy applications.[2] Currently,numerous research ef- tages of 1D nanomaterials, such as low energy density and forts are aimed at designing electrode materials with nano- self-aggregation, which may block the electron/Li-ion trans- structuredarchitectures to achieve high-performanceLIBs. port.[4]Specially designed nanostructures should be considered to address these problems. Mesoporous nanostructured mate- [a] Dr.S.Hu, Prof. W. Chen, Prof. J. Zhou rials, which possessalarge specific surfacearea and exhibit State Key Laboratory of Advanced Technology for Materials Synthesisand the intrinsic nanomaterial nature of self-aggregation, have Processing, SchoolofMaterialsScience and Engineering been desirable structures allowing efficient contact with the Wuhan University of Technology, Wuhan, 430070 (P.R.China) electrolyte and fast Li-ion transport, leadingtoimproved LIB Fax:(+ 86)27-87760129 E-mail:[email protected] electrochemical performance. For example, mesoporous V2O5 1 [b] Dr.S.Hu, Dr.E.Uchaker,Prof. G. Cao nanofibersexhibited ahigh discharge capacity of 370 mA hgÀ , 1 Department of Materials Science and Engineering and ahigh rate capability at 800 mAgÀ with little cyclic degra- University of Washington,Seattle, Washington, 98195 (USA) [5] dation. Mesoporousgraphene-based TiO2/SnO2 nanosheets E-mail:[email protected] 2 1 with alarge surfacearea of 138 m gÀ and large pore volume [c] Prof. G. Cao 3 1 (0.133 cm gÀ ), synthesized through afacile and scalable step- BeijingInstituteofNanoenergyand Nanosystems Chinese Academy of Sciences, Beijing100083 (P.R.China) wise method, could effectively accommodate the volume Supportinginformation for this article is available on the WWW under changeand shorten the transport lengths for Li ions and elec- 1 http://dx.doi.org/10.1002/chem.201503356. trons,and presented areversible capacity of 600 mAhgÀ for

Chem. Eur.J.2015, 21,18248 –18257 18248 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper over 300 cycles.[6] Therefore, mesoporousnanostructured ma- nanoflakes embedded in carbon nanofibers synthesized terials are of particular interestaselectrodes owing to their through ahydrothermalmethod combined with electrospin- large surfaceareas, structuralintegrity,and effectivemechani- ning and carbonization exhibited areversible capacity of 1 cal support and void space to mitigatevolume expansion, approximately 350 mAhgÀ at ahighcurrent density of 1 [14] which make them arational nanoarchitecture for LIB anode/ 1000 mA gÀ . In our work, electrospinning has been adopted cathodematerials. for the controllable fabrication of MoS2 nanocrystals embed- In recent years, avariety of preparation methods have been ded in mesoporous carbon nanofiberstogenerate functional developed to obtain mesoporous nanostructured materials. materialmats as anodesfor LIBs. Furthermore, among carbo- The hard-template approach is very common for the fabrica- naceousmaterials, carbon nanofibers have agood diffusion co- 7 2 1 [15] tion of mesoporous nanostructured materials. Shen et al. suc- efficient ( 2”10À cm sÀ ), whichisacritical parameter de-  cessfully prepared mesoporous Li4Ti5O12/C nanocomposites termining the powerdensity of the Li-ion battery.Compared throughananocasting technique, using CMK-3 as the hard with our previous studies on flower-like C@MoS2 composites template.[7] However, this process requires severe and costly re- obtained through the hydrothermalmethod,[10a] we have fabri- action conditions. Subsequently,solution-based routes and cated MoS2@C mesoporous nanofibers successfully through electrodeposition methods for the synthesis of mesoporous electrospinning with low-costand accessible raw materials, nanostructured materials were introduced.[8] These methods which may be of benefit for the application of LIBs in industrial are easily accessible, but scale-up for mass production is still production.Inaddition, our results demonstrate that MoS2@C challenging. Electrospinning, aversatile, facile, and economical mesoporousnanofibers with improved interfacial charge-trans- method, can be appliedtothe large-scale synthesis of nano- fer kinetics and arobustmicrostructure exhibit highreversible structured materials.[9] In addition, electrospinning can gener- capacities and excellent cycling stabilityupon evaluation as ate mass-controllable 1D nanostructures and sophisticated ar- anode materials for LIBs. chitectures, and does not require complex technology.

Atypical layered transition metal sulfide, MoS2,which con- sists of Mo atoms sandwiched betweentwo layers of closely 2. Experimental Section packed Satoms, is the focus of much attention as an anode 2.1 Materials synthesis materialfor applicationinLIBs. It is an inorganic analogueof the graphene layered structure, in whichthe 2D molecular Poly(vinyl alcohol) (PVA, Mw =67000 Sigma-Aldrich Co.,Ltd.,USA), layers are weakly linked by van der Waals interactions. The ammonium tetrathiomolybdate (ATTM, Alfa Aesar Co.,Ltd.,USA), and dimethyl sulfoxide (DMSO, Alfa Aesar Co.,Ltd.,USA) were weak van der Waalsforces allow afast diffusion path for the used without further purification. Aqueous PVAsolution (15 wt%) movement of Li ions without asignificant increase in was first prepared by dissolving the PVAindeionized water at [10] volume. The specialS-Mo-S layered structure enables expe- 908C, and stirring for 3h.The concentration of ATTM in DMSO was 1 ditious Li-ion insertion/extractionduring the discharge/charge set to be 0.33 gmLÀ and the solution was stirred at room temper- process.Theoretically,MoS2 can store four moles of Li ions on ature for 30 min. The above PVAwas added into the precursor so- the basis of aredox conversion reaction, in which the MoS2 is lution and stirred until ahomogeneous reddish-brown solution was obtained. The weight ratio of PVAtoATTM was 1.05. The pre- reducedtoMonanocrystals dispersed in aLi2Smatrix upon lithiation, and is then reversibly restored to the corresponding cursor solution was then transferred into a3mL plastic syringe with astainless steel needle and driven by asyringe pump at acon- disulfideafter delithiation, leading to acapacity 3.5 times 1 stant flow rate of 0.15 mLhÀ .The needle was connected to greater than that of commercial graphite anodes ahigh-voltage power supply (ES40P-5W,Gamma High Voltage, 1 (372 mA hgÀ ). However,akey limiting factor in employing Inc.), with the voltage set to be approximately 20 kV.Apiece of MoS2-based anodesistheir low energy density,which is attrib- aluminum foil collector was grounded, with adistance of around uted to the relativelynarrow voltage window.Various nano- 15 cm between the collector and the tip of the needle. The ATTM/ structured MoS2 materials have been fabricated, which is an ef- PVAnanofibers were generated and formed flexible mats, which fective way to shorten the Li-ion diffusion pathway,and thus, were then dried at 808Ctoevaporate the solvent. Afterwards, the to improve the rate performance compared with their “micro” nanofibers were annealed in aconventional tube furnace at 4508C [11] in H2 for the reduction of ATTM to obtain good crystallinity with MoS2 counterparts. There is furtherinterest in the synthesis subsequent heat treatment under N2 at 8008Cfor 3hto carbonize of MoS2-carbonhybrids because carbon modification has the the PVA; the product was designated as MNF450. For comparison, potentialtoenhance the electricalconductivity of the whole the ATTM/PVAnanofibers were annealed at 8008CinN2 for 3h, system,thereby giving ahigh reversiblecapacity andgood cy- with the resulting material designated as MNF800. cling stability.[12] In terms of integration of the above possible improving 2.2 Materials characterization routes, the MoS2-carbon nanostructured materials synthesized through aconvenient and versatile electrospinning method The crystal structure of the obtained samples was characterized by X-ray diffraction (XRD, D8 Bruker X-ray diffractometer with CuK ra- could be one of the most favorable architectures as the anode a diation (l=1.5418 Š)). The morphology and composition of the of high-performance LIBs. So far,there are two publications sample were examined by using scanning electron microscopy concerning this field. Single-layered nanoplates of MoS2 em- (SEM, JEOL JSM-7000F) and energy-dispersive X-ray spectroscopy bedded in thin carbon nanowires by electrospinning obtained (EDX). Transmission electron microscopy (TEM) was performed with 1 [13] ahigh capacity of 661 mAhgÀ after 1000 cycles. Thin MoS2 an FEI, Tecnai G2 F20 transmission electron microscope operating

Chem. Eur.J.2015, 21,18248 –18257 www.chemeurj.org 18249 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper at 200 kV accelerating voltage. The Brunauer–Emmett–Teller (BET) This electrospinning method is conceptually straightforward, specific surface areas and pore size distributions were measured simple,and generic,asused in the fabrication of CoO-carbon with aQuantaChrome NOVA4200e analyzer.Thermogravimetric [5,16] and mesoporousV2O5 nanofibers. analysis (TGA) was performed on aTGinstrument (NET ZSCH STA Figure 2a shows SEM images of the ATTM/PVA nanofibers; 409C). X-ray photoelectron spectroscopy (XPS) analysis was per- they are randomly oriented with smooth surfaces, and their formed with aVGMultilab 2000 with AlK as the X-ray source. a lengths are more than tens of micrometers. The high-magnifi- cation image in Figure 2b demonstrates that the diameters of 2.3 Electrochemical measurement the nanofibers are distributed in the range 150–220 nm. With For evaluation of the electrochemical performance, coin-type half- cells were assembled using the as-prepared nanofibers as anode materials for Li-ion batteries. The as-prepared active material was prepared by mixing the as-synthesized MoS2@C nanofibers, Super Pconductive carbon (TIMCAL Graphite &Carbon), and poly(vinyl- idene fluoride) (PVDF,Sigma-Aldrich) binder dispersed in N-methyl- 2-pyrrolidone (NMP,Alfa Aesar) solution in aweight ratio of 80:10:10, respectively.The as-prepared active material slurry was spread uniformly onto Cu foil and dried in avacuum oven at 808C overnight prior to coin-cell assembly.The electrochemical experi- ments were performed using CR2016-type coin cells, which were assembled in an argon-filled dry glovebox (Innovative Technology,

IL-2GB) with MoS2@C nanofibers as the working electrode, pure Li foil as the counter and reference electrodes, polypropylene mem- brane film as the separator,and 1m LiPF6 in ethylene carbon (EC)/ dimethyl carbonate (DMC) as the electrolyte. Galvanostatic dis- charge and charge measurements were performed under different current densities in the voltage range 0.01–3.0 Vatroom tempera- ture. Cyclic voltammetry (CV) was conducted on an electrochemical analyzer (CH Instruments, model 605C) in the voltage range 0.01– + 1 3.0 V(vs. Li/Li )atascan rate of 0.2 mVsÀ .Electrochemical impe- dance spectroscopy (EIS) was recorded by applying the Solartron Figure 2. a,b) SEM imagesofas-prepared ATTM/PVAnanofiber;c,d) SEM 1287A in conjunction with aSolartron 1260FRA/impedance ana- images of MNF450; e,f) SEM images of MNF 800;g)EDX spectrum of lyzer with an amplitude of 5.0 mV in the frequency range 100 kHz MNF450; h) XRD patterns of MNF450nanofibers before and after heat treat- to 0.01 Hz. The half-cells were tested at various current rates on ment. the basis of the weight of the active material alone.

the different calcination processes, however,new features 3. Results and Discussion startedtoevolve on the surface;the resulting morphologyof the nanofibers was dependent on the calcination conditions. Figure 1shows aschematic illustration of the fabrication of After heat treatment at 4508CinH andat8008CinN,orange mesoporousMoS @C nanofibers. First, ATTM and PVApowders 2 2 2 nanofiberswere transformed to black ones with aclear reduc- were dissolved in DMSO with continuous stirring to generate tion in diameter from approximately 220 to around 130 nm (as ATTM/PVAgel. Then, the gel was transferred to asyringe with 1 showninFigure 2c and2d). No fracture of the nanofibers was the flow rate of the gel precursor set to about 0.15 mL hÀ to observed after the heat treatment. The MoS @C nanofibers form ATTM/PVAfibers. After electrospinning and ashort post- 2 (namedMNF450) were uniform, smooth, and consistent in annealing procedure, the ATTM/PVA was transformed into toughness. The decreased diameter of the nanofibers could be MoS nanocrystals to form mesoporousMoS @C nanofibers. 2 2 attributed to pyrolysis, reduction, and carbonization of PVA

and ATTM. The images of the MoS2@C nanofibers (named MNF800) in Figure2edemonstrate that the nanofiber mor- phologywas conserved. Figure 2f,however,shows that the surfaceofthe nanofibers became rough and their diameter was around 150 nm. It was also found that there were many pores in the fibers,which were composed of interconnected

MoS2 nanoplates.EDX was used to characterize the elemental compositions of the nanofibers forMNF450, as shown in Fig- ure 2g.The EDX measurements confirmed the coexistence of Mo, S, C, and Oelements with no detectable impurities, indi-

cating that the nanofibers were made mainly of MoS2.The ele- mental compositions of MNF450 and MNF800 are shown in Table 1. Thecalculated atomic ratios of StoMofor MNF450 Figure 1. Schematic illustration of the fabrication of mesoporous MoS2@C nanofibers. and MNF800 are 2.16 and 2.18, respectively,which are close to

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Figure 3a displaysaTEM image of the mesoporous MoS @C Table 1. Elemental composition of the MoS @C nanofibers. 2 2 nanofibers(MNF450). The fibers are about 150 nm in diameter,

Elemental composition of the MoS2@C nanofibersdetermined usingEDX and corn-like MoS2 nanocrystals are embedded in the mesopo- CompositionMo S C O MoS S/Mo 2 rous carbon fibers. It is clearthat the corn-like MoS2 nanocrys- [wt%] [wt%] [wt%] [wt%] [wt %] ratio tals are very small and are distributed homogeneously in the MNF450 22.01 10.1564.26 3.58 32.16 2.16 fibers. Upon closer observation, as shown in Figure 3b,itis MNF800 16.86 12.2959.52 10.90 29.15 2.18

the stoichiometry of MoS2.The carbon component emanates from the carbonization of PVA, and asmall amount of oxygen is attributedtothe incomplete decomposition of PVAand par- tially oxidized Mo. More importantly,itisshown clearly that the oxygen mass fractioninMNF450 is 3.58 wt%, which is three times smaller than that of MNF800 (10.90 wt%). It can be explained that H2 heat treatment is helpful to remove the re- sidual oxygen.XRD analysiswas performed to determine the phase information and crystallinity of the MNF450 sample before and after heattreatment. As shown in Figure 1h,there is aclear structuralchange from hydrous ammonium tetrathio- molybdate to hexagonal MoS2.Indetail, the XRD pattern of Figure 3. a) TEM image;b)HRTEM image of the MoS2 nanocrystals embed- the as-electrospun MoS2 nanofibers before annealing has ded in the carbon nanofiber;c)correspondingHRTEM images from the abroad peak around 178,which could be assigned to the hy- markedregionin(b);d)corresponding FFT pattern. drous ammonium tetrathiomolybdate, as reported in the litera- ture.[13,17] There is clearly alarge area of noise peaks, indicating the amorphous phase of the nanofibers before annealing. The lower intensities of diffractionpeaks also confirmed the low found that the surfaceofthe nanofibers is rough and uneven, crystallinity.After annealing, the nanofibers showedadifferent and the MoS2 nanocrystals are interconnected within the XRD pattern. All the diffraction peaks of the nanofibers can be fibers.The morphologyand size of the sample obtained from indexedtothe hexagonal MoS2 phase with lattice parameters the TEM results match well with those observed from the SEM a= 3.16, b =3.16, c= 12.30 Š, and b =90.008,which are in images. The uniform dispersion of MoS2 nanocrystals in the good agreement with the literature values (JCPDS:00-037- carbon fibers (highlightedbythe white square is further con- 1492). The interlayer space and grain size of the hexagonal firmedbyHRTEM (shown in Figure 3c), which provides more

MoS2 were estimated using the Bragg equation and Scherrer detailedstructural information.Itisclear that the typicalmeso- equation. The interlayer spacing of the phase was calculated porousMoS2@C nanofibersare composed of ultrafinenano- from the (002) diffraction peak to be 6.6 Š, indicating that the crystalsembedded in overlaying amorphous carbon as well as interlayer distance of the nanocrystals is expanded. According uniformly dispersed within the nanofibers. The nanocrystals to the Scherrerequation,[18] the average crystallite thickness is are approximately 7nminthickness.Auniform thin carbon approximately 7nm, which equates to approximately ten layer of about 2nmisformed from the carbonization. The ob- layers of MoS2.These findings corroborate the HRTEM results servedlattice spacingofthe planesis0.66 nm, which corre- that the MoS2@C nanofibers are composed of nanosheets. The sponds to the (002) plane of hexagonal MoS2,slightly larger expandedlayerscan afford more space for Li-ion intercalation, than the reported data (0.615 nm), and is consistentwith the thus leading to an enhanced storagecapacity.Inaddition, all XRD results(JCPDS: 00-037-1492). The diffractionring pattern the diffraction peaks are strong and narrow,and the noise of SAED (shown in Figure3d) indicates thepolycrystalline peaks decrease after the annealing process, which attests to nature of the MoS2@C nanofibers. For determination of the the good crystallinity of MNF450. Ramanspectroscopy was uti- weightratio of MoS2 to carbon, TGA was performed on the lized to identify the formation of MNF450 and MNF800. As nanofibersfrom room temperature to 7008Catarate of 1 shown in Figure S1 (Supporting Information), there are two 10 8CminÀ in air (Figure S2, Supporting Information). The 1 peaks of MNF450, located at 378and 405 cmÀ ,which can be large weightloss at 4008Cisattributedtothe oxidation of attributed to the vibrational mode of the S Mo Sbond. How- MoS to MoO and the combustion of the carbon nanofibers.[19] À À 2 3 ever,the same vibrational mode of the S Mo Sbond is shifted The weight fractionofMoS in the MoS @C nanofibers was es- À À 2 2 to aslightly lower value for MNF800. The narrow and strong timatedtobeabout 70%byassuming that the remaining peaks of MNF450 may be attributed to its bettercrystallinity product after TGA was pure MoO3. and fewer defects, whichisingood agreement with the XRD The surfaceelectronic state and chemical composition of results. Consequently,MoS2 nanofibers treatedinareduction mesoporousMoS2@C nanofibers(MNF450) was analyzedusing atmosphere with asubsequentannealing process have good XPS. Figure 4a shows four characteristic peaks located at crystallinity and an improved electrochemical performance. 531.9, 284.6, 162.5, and 229.4 eV,corresponding to O1s, C1s,

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Figure 4. a) Wide-survey XPS spectrum of MNF450 nanofibers;the fine XPS of spectrab)Mo3d; c) S2p; d) C1s; e) O1s.

S2pand Mo3d. These resultsindicate that the sample contains 3.7 nm. The annealing process is necessary to remove unde-

Mo, S, C, and Oelements. The locationsofMo3d5/2,Mo3d3/2, sired functional groups and to open up the porous structure. and S2pinthe survey spectrum can be indexed as MoS2, The results confirm that the controlled heat treatment is akey which is highly consistentwith the EDX results. The high-reso- factor affecting the microstructure. The nanofibers with porous lution scans shown in Figure 4b emphasized the spectrum of feature would have more activeopenings that provide alarge Mo3d, in which the peaks located at 233.1 and 229.5 eV are in- contactarea between electrode and electrolyte, allowing fast dicativeofdoublet Mo3d3/2 andMo3d5/2.The chemical valence electrolyte transport. 4 + in MoS2 is confirmed as Mo .The peak binding energies of The electrochemical properties of the mesoporous MoS2@C S2pshown in Figure 4c are162.3and 163.5 eV,corresponding nanofiberswere evaluated as the anode electrode for Li-ion 2 to the S2p3/2 and S2p1/2 orbitals, which can be indexed as S À. batteries assembled in ahalf-cell configuration. Figure 5shows These surveys are in good agreementwith literature reports the 1st,2nd,and 5th cyclic voltammetry (CV) curvesofthe [20] for MoS2. TheXPS spectrum of C1sshows astrong peak at MNF800 and MNF450, respectively,obtained at ascan rate of 1 284.6 eV (Figure 4d), which is assigned to the C Cbonds, indi- 0.2 mVsÀ between 0.01and3V. Both samples appear to have À cating the formation of carbonnanofibers, whereas the weak similarCVcurves, which explain their identical electrochemical ones located at 285.2, 285.9, and 286.6 eV correspond to reaction characteristics. For MNF800, in the initial cathodic carbon in phenolicgroups, carbon in sp3 groups such as C O sweep,there are two clear peaks located at 1.12 and 0.45 V À À HorC O C, and carbon in absorbed CO or CO2,respective- (Figure 5a). The first peak can be assigned to the formation of [21] À À [23] ly. These resultsconfirm that the MoS2 nanocrystals are LixMoS2 accompanied by Li-ion intercalation into MoS2. The formed after pyrolysis and the PVAistotally carbonized, dem- second peak is very intense, and corresponds to the conver- [24] onstrating that the MoS2 nanocrystals are encapsulated suc- sion of MoS2 to Mo nanoparticles embedded in aLi2Smatrix. cessfully in the carbon nanofibers. The strong O1speak at Agel-likeSEI layer is formed owing to the electrochemical re- 532.3 eV indicates thepresence of oxygen-containing groups action occurring at the interface between the electrode and such as C O HorC O Cinthe carbon nanofibers. electrolyte. In the reverseanodic sweep, there is apronounced À À À À The N2 adsorption/desorption isotherms for the mesoporous peak at 2.25 V, which can be attributedtothe extraction of

MoS2@C nanofibers are shown in Figure S3 (Supporting Infor- Li2Sand oxidation of Mo to MoS2.During the subsequent mation) and the corresponding pore size distribution of the cycles, it is worth noting that the 0.45 Vpeak disappears, and mesoporousmaterials was calculated using the BJH method the 1.12 Vpeak shifts to 1.22 V. In addition, anew reduction (showninFigure S4). The nitrogen sorption data for MNF800 peak appears at 1.91 V. The electrode can be considered as and MNF450 are summarized in TableS1, including the surface amixture of Mo and Srather than the original MoS2.Accord- area, and pore size distribution of mesopores and micropores. ingly,the two reduction peaks at 1.22 and 1.91 Vcan be attrib- 2 1 The BET-derived surface area of MNF450 is 135.9 m gÀ ,which uted to the stepwise conversion reaction from StoLi2S, which 2 1 is much larger than that of MNF800 (82.1 m gÀ ). Also, the sur- includes the reduction of sulfur to intermediate lithium poly- face area is superior to other MoS2 nanomaterials obtained sulfides, and the reduction of higher-order polysulfides to [22] [10b,25] throughhard-template or hydrothermal methods. Further- Li2S. In the fifth cycle, the anodicpeak moves gradually to more, the average size of the mesopores increases from 3.3 to highervoltage, which indicates agap between the redox

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Figure 6. 1st,2nd,and 50th dischargeand charge curves of the samples Figure 5. CV curves of a) MNF800,b)MNF450 measured in the voltage range 1 a) MNF800 and b) MNF450atacurrentdensity of 100 mAgÀ . 1 0.01–3.0 Vatascan rate of 0.2 mV sÀ .

1 couple. The decrease in peak intensityofthe redox couple in- of 100 mA gÀ between 0.01 and3.0 V. MNF800 exhibits initial 1 dicatesthat an irreversible reactiontakes place during the discharge and charge capacities of 710 and 573 mA hgÀ ,re- charge/discharge process. Overall, the CV curvesofMNF800 spectively.The discharge and charge capacity loss of 19.3%is overlap well except for aslightdegradation, which means aca- attributed mainly to the formation of an SEI film and the irre- pacity loss from the second cycle. Upon close examination,the versible side reactionbetween the electrolyte andelectrode. CV curvesofMNF450 (Figure 5b)were found to be slightly dif- There are two clear potentialplateaus at 1.1 and 0.5 Vinthe ferent from those of MNF800. According to the calculations, discharging process and one potentialplateau at 2.1 Vinthe the potential difference of the redox couple a/a’ of MNF450 is chargingprocess, which are in good agreement with those ob- clearly smaller than that of MNF800 after five cycles. Specifical- served in the CV profiles. In the 2nd cycle, the discharge and 1 ly,the potential differences of the redox couples at a/a’ of chargecapacities decrease to 591 and 568 mAhgÀ ,respec- MNF450 and MNF800 are 458 and 512 mV,respectively.The tively,and the reduction potential plateau movesto1.97 V smaller the potential differenceinthe redox couple,the more from 1.1 Vinthe initial cycles. The capacity experiences reversible the reactiontaking place.[26] The first cathodic peak asharp decay after 50 cycles and retains 64.8 %ofthe initial becameambiguous and narrow.This result may indicatethat discharge capacity.These resultsindicatethat the capacity loss the Li ions not only intercalate into the MoS2 layers, but are is related to the formation of apassivation film, which may also located in the interface of the electrode because of the block some active openings and reduce the effective contact porousstructure of the nanofibers. The cathodic and anodic area betweenthe electrode and electrolyte. In comparison peaks located at around 0.45 and 2.25 Vcorrespond to the with MNF800, the MNF450 electrode delivers high initial dis- + 1 conversion reactionMoS +4Li +4eÀ Mo+2LiS. Similarly charge/charge capacities of 953 and968 mA hgÀ with only 2 ! 2 to MNF800, there is asmall narrow peak located at 1.48 V, 1.5%capacity loss. After the initial cycle, MNF450 exhibits mo- which can be attributed to the partial oxidation of Mo.[27] In notonouschargeand discharge profiles with apotential pla- the subsequentcycles,the cathodic peak at 0.45 Visabsent, teau at 1.81 V, indicating that Li ions are intercalated randomly and two peaks at 1.01 and 1.86 Vare observed. These peaks between nanofibers without the formation of astage structure remain almost unchanged and steady,suggesting ahighly re- like in the MNF800 electrodes. The chargeand discharge po- versible reactionand excellent stabilityofthe MNF450 during tential curves overlap very well, explaining the good reversible the Li-ion insertion/extraction process. Accordingly,MNF450 reactionduring Li-ioninsertion/extraction. The discharge/ 1 shows better kinetics for Li-ion insertion/extraction. charge capacities of 961 and 965 mAhgÀ are achieved, and Figure 6shows the discharge and charge curvesofMNF800 the potential plateaus of the redox couple are closer after 50 and MNF450 for the 1st,2nd,and 50th cycles at acurrentdensity cycles, which confirms the results obtained from the CV pro-

Chem.Eur.J.2015, 21,18248 –18257 www.chemeurj.org 18253 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper files. The capacity retention can be as high as 99.5%ofthat in Figure 7b shows the AC impedance spectroscopy measure- the first cycle, demonstrating excellent stability,which is as- ments on anewly assembled cell of MNF450 and the data col- cribed to the robustskeleton and mesoporous structure of the lected after 200 cycles. It is clear from the inset for Rct (charge nanofibers. transfer resistance) that the MNF450electrode exhibits amuch Figure 7compares the electrochemical performances of lower charge-transferresistance(148 W)than that of the fresh MNF800 and MNF450. The cycling performances of MNF800 cell (385 W). Theresistance of the MNF450electrode after 200 1 and MNF450 at 0.5 AgÀ are shown in Figure 7a.Itisevident cycles decreased by more than ahalf, indicating that the po- that the two electrodes can obtain good capacity retention rosity of the nanofibers may facilitate charge transfer between and highly reversible Li-ion storagecapability after 200 cycles. the electrode and electrolyte interface, which may be helpful Specifically,the MNF450 electrode has ahigher specific capaci- in reducingpolarization.Hence, the interfacial charge-transfer 1 ty of 460 mA hgÀ with 13 %capacity loss, whereas the corre- reactionmay be the rate-determining step for the high diffu- 1 [15] sponding values for MNF800 are 342 mAhgÀ with 35%ca- sional flux of Li ions in the host nanofibers. pacity loss. At the same time, the two electrodes were tested The rate capabilities at various current densities and cycling 1 under acurrent density of 1AgÀ for 200 cycles (shown in the performances of the mesoporous MoS2@C nanofiber electrodes 1 inset of the Figure 7a). The MNF450 electrode retains 71% were measured after 50 cycles at 0.1 AgÀ ,and the resultsfor more of the 10th capacity than the MNF800 electrode, indicat- MNF450 and MNF800 were compared.Figure 7c shows acom- 1 ing the good capacity retention and excellent Li-ion storage parisonofthe rate capability behavior from 0.1 to 5AgÀ in capability of the MNF450 electrode. These results are relatively the voltage range 0.01–3.0V.Notably,these two electrodes competitive compared with previous reports.[28] The improved have much closer specific capacities with increasing current cycling stability under relativelyhighcurrent density shows rates. Unlike the data collected at the highercurrent rates, that the nanofibers with mesoporous microstructure and good both the electrodes can recover to their originalcapacity 1 crystallinity can achieve agood diffusion rate, which dominates values after 50 cyclesatacurrent density of 0.1 AgÀ .Specifi- the overall reaction rate, leading to fast charge-transfer kinet- cally,the MNF450 electrode can deliver discharge capacities of [15] 1 ics. These findings are further confirmedbythe EIS results. 1330, 1035, 788, 584, 396, and 196 mAhgÀ at rates of 0.1, 0.2,

1 1 Figure 7. Comparison of electrochemicalproperties of MNF800 and MNF450:a)Cycling performances at scan rates of 0.5 AgÀ and 1AgÀ ;b)Nyquist plots; c) rate and cycling performances at variouscurrentdensities;d)rate capacity retention.

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1 0.5, 1, 2, and 5AgÀ ,respectively.The MNF800 electrode main- tains discharge capacities of only 807, 640, 520, 424, 320, and 1 160 mA hgÀ at the same rates. After the current rates changed 1 back to 0.1 AgÀ ,the discharge capacity of the MNF450 elec- 1 trode could recover to 1006 mA hgÀ with only 7.7 %capacity loss throughout the cycling process, which increased to the ends of the cycling. However,the MNF800 electrode experi- enced adrop in capacity after the current rates changed back 1 to normal, with the capacity then stabilizing at 493 mAhgÀ after 50 cycles. The MNF450 electrode has ahigh rate capacity, double that of its counterpart, which can be attributed to the followingreasons. First, the porosity of MNF450, which leads to alargesurface area of the nanofibers, indicates an increased Figure 8. Cyclingstability and Coulombic efficiency of MNF450 at acurrent contact area between the electrode and electrolyte. Secondly, 1 density of 100 mAgÀ compared with the theoreticalcapacity of graphite. the annealing conditions also play akey role in determining the crystallinity and mesoporousmicrostructure of the nanofib- ers, which may affect the Li-ion intercalation within the nano- fiber anode. In summary,the disparitybetween MNF450 and MNF800 in terms of rate capability is associated with the diffu- sion rate in the nanofiber electrodes, which may decidethe overall reaction rate. Acomparison of rate capacity retention of the MNF450 and MNF800 electrodes is shown in Figure 7d. It is clear that the MNF450 results are all greater than those for MNF800. At the lower current rates in particular, the capacity of MNF450 is nearly twice that of MNF800. Despite the small Figure 9. Schematic diagram of the mesoporous MoS2@C nanofiberelec- differences between these two electrodes, the MNF450 elec- trode. 1 trode can deliver 204 mAhgÀ at ahigh current rate of 1 10 AgÀ .The good rate retention can be ascribed to the better crystallinity of the nanofibers, which affects the diffusion be- stood by considering asimple model made to depict how the havior of Li ions, and the fast interfacialcharge-transferreac- mesoporousMoS2@C nanofibers favor the Li-ion storage tion of the carbon nanofibers is improved by embeddingMoS2 (showninFigure 9). First, the nanofibers provide arobust and within the mesoporousnanofibers. More importantly,the free-standingframework, which retains the structuralintegrity oxygen mass fraction in MNF450 is smaller than that in to circumvent the volume variation of the electrode. This fea- MNF800, which is akey factor influencing the electrochemical ture guarantees areversible reaction upon Li-ion insertion/ex- performance. The small amount of oxygen leads to better crys- traction during cycling. Secondly, the carbon nanofibers act as tallinity, which brings aboutalower internal resistance, thus an adaptable overcoat, which is able to deform synchronously improving the conductivity of MNF450. during the volume change of the nanofibers rather than be- The cycling stabilityand Coulombic efficiency of MNF450 comingcracked. In addition, the carbon nanofibers improve 1 were evaluated at acurrent density of 100 mA gÀ (Figure 8). the conductivity of the overall electrode, thus contributing to

The initial chargeand discharge capacities are 1005 and the total capacity.Thirdly,the MoS2@C nanofibers possesssu- 1 1022 mA hgÀ ,respectively.The MNF450 electrode exhibits ex- perior crystallinity and mesoporous characteristics, which allow cellent cycling stabilityinspite of acapacity loss during the sufficient penetration of the electrolyte into the electrode, and midway charge/dischargeprocess, which may be attributed to thus, facilitate the diffusionalflux of Li ions and the interfacial the decreased ionic diffusion or the passive filmformed at the charge-transfer reactionathigh current rates. In addition, the interface of the electrode. After 165 cycles, areversible capaci- increased surface areas provide efficient contact areasbetween 1 ty of 995 mAhgÀ can be retained, corresponding to 97.4%of the electrodes and electrolyte. Furthermore, the more active the initial discharge capacity.The Coulombic efficiency of the openings reduce the diffusion barrierfor the Li ions to cross, MNF450 electrode approaches nearly 100%, indicating excep- leadingtoahigh rate capability.The ultrasmall nanosized tional reversible capacity retention.Toour knowledge,the the- MoS2 serves as ashortened lateral contact between the nano- 1 oreticalcapacity of graphite is 372 mA hgÀ .The capacity of crystalsand carbon nanofibers, whichisbeneficial for the fast the MNF450 electrode is almost twice that of graphite anodes, diffusion of Li ions andelectrons. Finally,the small amount of indicating an alternative for the high-capacity anodes in practi- oxygen leads to abetter crystallinity,which brings about cal cells. alower internal resistance,and thus, improves the conductivi-

As expected, the mesoporousMoS2@C nanofibers exhibit an ty.These advantageous features of thenanofibers can contrib- extraordinary reversible capacity and long-term cycling stabili- ute to their enhanced electrochemical performance for high- ty.This can be explained by the following factors. The im- performance Li-ion battery applications. provement in the electrochemical performance can be under-

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