Morphological Effect of Graphene Nanosheets on Ultrathin CoS

Oct 17, 2014 - ... using a conductivity detection meter (Shanghai Fortune Instrument, FZ-2010). .... The specific Brunauer–Emmett–Teller (BET) sur...
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Morphological Effect of Graphene Nanosheets on Ultrathin CoS Nanosheets and Their Applications for High-Performance Li-Ion Batteries and Photocatalysis Shaofeng Kong, Zhitong Jin, Hong Liu,* and Yong Wang* Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shangda Road 99, Shanghai, P. R. China, 200444 S Supporting Information *

ABSTRACT: A unique CoS-graphene sheet-on-sheet nanocomposite has been successfully prepared by anchoring CoS nanosheets on the surface of graphene nanosheets (GNS) with the assistance of the structure-directing agent of ethylenediamine. The shape and size of the introduced CoS nanosheets can be further adjusted by varying the amount of GNS. The unprecedented sheet-like CoS structure is believed to be matched well with GNS basically due to their similar two-dimensional structure with maximum contact areas between two components. The strong interaction between CoS and the underlying highly conductive graphene can facilitate fast electron and ion transport and improve structure stability of the composite. The composite with 26.2% GNS displays excellent electrochemical performance when evaluated as an anode for rechargeable lithium-ion battery. A larger-than-theoretical reversible capacity of 898 mAh/g can be delivered after 80 cycles at 0.1 C along with excellent highrate cycling performance. The CoS-graphene sheet-on-sheet composite is also used for the first time as a photocatalyst with promising properties for the degradation of methylene blue.

1. INTRODUCTION Recently, transition-metal chalcogenide semiconductor compounds are increasingly applied in various areas.1−8 Cobalt sulfides, a significant class of semiconductors with different chemical formulas, such as CoS2, CoS, Co9S8, Co3S4, and Co1−xS, have been investigated for lithium ion batteries,9−27 catalysis,28−34 dye-sensitized solar cell,35−40 and supercapacitors.41−46 Their thermal stability and electronic conductivity are usually better than other metal sulfides. In particular, CoS, one of the important cobalt sulfides, has attracted significant interest for lithium ion batteries13−15 and photocatalysis.28−30 As a promising anode material for lithium ion batteries, CoS has a theoretical capacity of ∼589 mAh/g, which is larger than that of the commercial graphite anode (372 mAh/g).13−15 However, similar to other high-capacity anodes (e.g., Sn-based anodes47−49 and transitional metal oxides50,51), capacity fading is pronounced during repetitive charge−discharge cycling.13−15 This is ascribed to the electrode pulverization and a severe loss of electrical contact between active materials and the current collector during the process of repetitive lithium insertion and extraction. One effective way to circumvent the drawback is to fabricate hybrid nanostructures by the introduction of carbonaceous materials to form carbon-supported cobalt sulfide nanocomposite.20−22 Graphene, with its unique flexible and robust mechanical structure, is such a good matrix to buffer the volume change induced by the Li+ insertion and extraction during charge and discharge process.23−27,49−51 Pioneering graphene supported CoS223 and CoS24 nanoparticles have © 2014 American Chemical Society

been prepared and both exhibit higher reversible capacities and better rate performance compared with their counterparts in the absence of graphene. Hou and co-workers also report an interesting graphene supported Co3S4 nanotube composite with substantially improved electrochemical performance.25 In the 21st century, environmental pollution is ringing the alarm bells for modern society. Photocatalyst has been widely used to solve the problems of organic pollutants because of its potential for environmental amelioration during the past decades.28−32 However, the rapid recombination of electron− hole pairs significantly restricts its photocatalytic application. Graphene exhibits superior electrical conductivity, large surface area, and excellent charge carrier mobility.52−55 These unique properties have stimulated extensive research on the preparation and modification graphene-based semiconductor photocatalysts to achieve an enhanced charge separation in charge transport and photocatalytic activity.52−55 For instance, Feng et al. fabricated ZnS-reduced graphene oxide composites, which exhibit better photocatalytic properties compared with pristine ZnS for the degradation of methylene blue (MB) under UV light irradiation.52 Liang and Wang synthesized TiO2/graphene nanocrystal hybrid photocatalysts with superior photocatalytic activity in degrading organic dye rhodamine B.53 The enhancement of the graphene-based semiconductor photocatalysts is Received: August 28, 2014 Revised: October 13, 2014 Published: October 17, 2014 25355

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measured by an elemental analyzer (VARIO EL111). UV−vis absorption spectra of the samples were recorded on a UV−vis spectrophotometer (Hitachi U-3010) with a wavelength range of 200−800 nm. Photoluminescence (PL) spectra were measured using a Hitachi F-7000 fluorescence spectrophotometer at room temperature. 2.3. Electrochemical Measurement. Electrochemical performances were measured using two-electrode cells at a temperature of 20 °C. These cells were assembled in an argonfilled glovebox. The working electrode was composed of 80 wt % active materials, 10 wt % each of the conductive agent (acetylene black) and the binder (poly vinylidene difluoride, PVDF, Aldrich). The loading amount of the electrode on copper foil was around 1.5 mg cm−2. Lithium foil (China Energy Lithium Co., Ltd.) was used as counter and reference electrode and a polypropylene film (Celgard-2300) was used as a separator. The electrolyte was 1 M LiPF6 in a 50:50 w/w mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Electrochemical measurements were performed on a LAND CT2001 test system. The Swagelok-type cells were discharged (lithium insertion) and charged (lithium extraction) at a constant current (58.9 mA/g, 0.1 C, 1 C = 589 mA/g) in the fixed voltage range 5 mV to 3.0 V. Higher rates (0.5, 2, and 3 C) were also used and the first cycle discharging was kept at 0.1 C. Cyclic voltammetry (CV) was carried out on CH Instruments electrochemical workstation (model 660D) at a scan rate of 0.1 mV/s. Electrochemical impendence spectra were obtained on the same instrument with the frequency range from 0.01 Hz and 100 kHz. 2.4. Photocatalytic Measurement. The photocatalytic activities of the samples were conducted by the photocatalytic degradation of methylene blue (MB) in an aqueous solution under visible light irradiation. A 500 W Xe lamp with a λ < 420 nm cutoff filter was used as visible light source. A water circulation system and a ventilating fan were used to prevent any thermal catalytic effects. All experiments were conducted at room temperature in air. In a typical photocatalytic experiment, 10 mg photocatalyst (pristine CoS or CoS-GNS composites) was added into 50 mL MB aqueous solution (10 mg/L) in a reaction cell with a Pyrex jacket. Prior to irradiation, the suspension was magnetically stirred in the dark for 30 min to reach an adsorption−desorption equilibrium of the pollutant on the catalyst surface. These suspensions were then exposed to visible light irradiation under magnetic stirring. At given time intervals, about 5 mL suspensions were collected and centrifuged (13000 rpm, 20 min) to remove the photocatalyst particles. The MB concentration of the obtained solution was analyzed by a UV−vis spectrophotometer (Hitachi, U-3310) by checking the absorbance at 664 nm.

mainly attributed to the presence of graphene, which is able to accept and shuttle photogenerated electrons from semiconductors. In consequence, the charge recombination of photoinduced electron−hole pairs is effectively suppressed. In particular, CoS nanoparticles have been loaded on mesoporous zeolite28,29 and polyacrylonitrile nanofiber,30 and the obtained composites achieved good photocatalytic properties. However, to the best of our knowledge, there is no report about the CoSgraphene composite used as photocatalyst in the degradation of organic pollutants. Herein, this work reports a new CoS/graphene composite structure, namely, CoS-graphene sheet-on-sheet composite with its applications as an anode for lithium ion batteries and a photocatalyst to decompose methylene blue (MB) under visible light irradiation. The structure matching between ultrathin 2D CoS nanosheets and graphene nanosheets enables the synergetic effect during applications in the CoS-graphene sheet-on-sheet composite. It exhibits a highly reversible large capacity of 898 mAh/g after 80 cycles of discharge and charge at a current density of 58.9 mA/g (0.1 C) and excellent high-rate performance. The photoactivity of the sheet-on-sheet composite is also substantially improved for the sheet-on-sheet composite. These property enhancements are found to be strongly dependent on the weight ratio of graphene in the composite and the optimum value of graphene is determined to be ∼26.2 wt % in the composite.

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. Graphene oxide (GO) was synthesized by a modified Hummers method as reported elsewhere previously.50 Graphite nanopowders (XF NANO, 40 nm in thickness) was used as the starting graphite to be exfoliated. Then GO was heated in a tube furnace under N2 atmosphere at 300 °C for 2 h to obtain graphene nanosheets (GNS). To synthesize CoS-GNS composites, CoCl2·6H20 (0.8 mmol) and CH4N2S (1.6 mmol) were dissolved in 14 mL of absolute ethanol and formed a transparent blue solution after vigorous agitation for 30 min. A calculated amount of graphene nanosheets was ultrasonically dispersed in 10 mL absolute ethanol and then mixed with 16 mL ethylenediamine (En) and the above blue solution (theoretical graphene contents were 5, 15, 25, and 35 wt % in the composites). The mixture suspension was transferred to a Teflon-lined autoclave (50 mL) immediately and heated at 180 °C for 12 h, and cooled to room temperature. The obtained product was collected by centrifugation, washing with absolute ethanol, and drying. As a benchmarked sample, pristine CoS nanosheets were also synthesized by the similar solvothermal preparation in the absence of GNS. 2.2. Materials Characterization. The products were characterized by X-ray diffraction (XRD, Rigaku D/max-2550 V, Cu Kα radiation), field-emission scanning electron microscopy (FE-SEM, JSM-6700F) with an energy-dispersive X-ray spectrometer (EDS), and transmission electron microscopy/ selected area electron diffraction (TEM/SAED, JEOL JEM200CX, and JEM-2010F). The specific surface area and porous structures were characterized by an accelerated surface area and porosimetry analyzer (Micromeritics Instrument Corp, ASAP 2020 M+C, analysis adsorptive: N2). Raman spectroscopy was recorded on Renishaw in plus laser Raman spectrometer (spot size ≈ 1.2 μm, wavelength = 785 nm, power = 3 mW). The electrical conductivity was measured by a four-electrode method using a conductivity detection meter (Shanghai Fortune Instrument, FZ-2010). The carbon and sulfur elements were

3. RESULTS AND DISCUSSION 3.1. Materials Characterization. The XRD patterns of pristine CoS and various CoS-GNS nanocomposites are shown in Figure 1. There are four main diffraction peaks corresponding to the (100), (101), (102), and (110) planes, which are in good agreement with standard hexagonal CoS (PDF 65−3418). No other phases of cobalt sulfide impurities such as Co3S4, CoS2 are detected, indicating the phase purity of CoS in the product. The (002) reflection peak of graphitic carbon is not obvious, possibly because the disordered graphene structure is obtained. The morphology and microstructure of the as-prepared CoS nanosheets and CoS-graphene composites were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A large number of bare CoS products with 25356

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and Figure S2, and TEM images in Figure 2e,f). The obtained CoS hexagon-like nanosheets (∼200−300 nm for each side and ∼10−20 nm in thickness) are uniformly dispersed on the graphene surface and form a sheet-on-sheet composite structure. The presence of several elements (S, Co, and C) can be confirmed by the EDS spectra in Figure 2d. The practical carbon and sulfur contents were measured by the elemental analyzer. The carbon content is determined to be ∼26.2 wt % in the CoSgraphene composites for the sample shown in Figure 2. The effect of graphene on the morphology and size of CoS-graphene composites was also investigated. Figure 3 shows the SEM and TEM images of various CoS-graphene composites with ∼5.2, 15.5, and 33.0 wt % of graphene. In general, CoS nanosheet has irregular morphology and large size in the presence of a small amount of graphene (5.2 wt %) as shown in Figure 3a,b, which is similar to pristine CoS nanosheets as shown in Figure 2a,b. With the increase of graphene contents (15.5 wt % as shown in Figure 3c,d and 33.0 wt % as shown in Figure 3e,f) in the composites, the size of the CoS nanosheet tends to be smaller and most products exhibit regular polygon sheet-like morphologies. Based on these observations, the morphology and size change should be ascribed to the effect of surface groups of graphene, which can affect the crystal growth process of CoS nanosheets and hinder their size growth.23,24 As investigated by XPS spectrum in our previous publication,50 the reduced graphene oxide still displays oxygen-

Figure 1. XRD patterns of CoS and CoS-graphene composites.

irregular sheet-like morphology are shown in SEM image of Figure 2a and TEM image of Figure 2b. In the presence of the theoretical 25 wt % graphene in the composite, the morphology of CoS nanosheets is changed to regular hexagon-like nanosheets (SEM images in Figure 2c, Supporting Information, Figure S1

Figure 2. (a) SEM image of pristine CoS nanosheets, (b) TEM image of pristine CoS nanosheets, (c) SEM image of CoS-26.2 wt % GNS sheet-on-sheet composite, (d) the EDS of CoS-26.2 wt % GNS composite, (e,f) TEM images of CoS-26.2 wt % GNS sheet-on-sheet composite. 25357

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Figure 3. CoS-5.2 wt % GNS composite: (a) SEM image and (b)TEM image, CoS-15.5 wt % GNS composite: (c) SEM image and (d)TEM image, and CoS-33.0 wt % GNS composite: (e)SEM image, (f)TEM image.

Figure 4. (a) TEM image, (b) SAED pattern showing the single crystalline structure and HRTEM images of CoS-26.2 wt % GNS sheet-on-sheet composite: (c) the lattice of CoS, (d) the few-layer graphene.

containing groups such as −COOH and −OH. There is electrostatic interaction between these negatively charged surface groups and positively charged cobalt ions, thus affecting the crystal growth process of CoS product.

HRTEM measurement with electron diffraction was further used to characterize the obtained CoS-graphene composite. Figure 4a demonstrates that a CoS nanosheet is embedded into the wrinkled GNS network. The selected area electron diffraction 25358

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Figure 5. Elemental mapping images of CoS-26.2 wt % GNS sheet-on-sheet composite: (a) SEM image, (b) carbon element, (c) cobalt element, (d) sulfur element.

(SAED) pattern (Figure 4b) reveals the single crystalline structure of a CoS nanosheet. The distinct fringe space of 0.25 nm can be determined and ascribed to the (101) planar spacing of hexagonal CoS (PDF 65−3418) as shown in Figure 4c. These results agree well with the previous XRD patterns. The cross section of few-layer graphene can be also observed on the crystalline CoS nanosheet as shown in Figure 4d, indicating that the CoS nanosheet is in tight contact with GNS. The intimate interfacial contact could improve the lithium diffusion and transfer of photogenerated charge carriers from CoS. It is worth noting that the sheet-on-sheet structure offers a large contact area between CoS nanosheets and GNS; therefore, the self-assembled agglomeration of few-layer graphene nanosheets into graphite platelets can be largely prevented. To further confirm the favorable structure of CoS-26.2 wt % GNS and uniform distribution of CoS on the surface of the graphene scaffold, an elemental mapping technique was employed. Figure 5a shows a SEM image of the composite and the corresponding elemental mapping images (carbon element in Figure 5b, cobalt element in Figure 5c, and sulfur element in Figure 5d). These elements are all uniformly distributed in the CoS-graphene composite. Raman spectrum is an essential tool to characterize the structural change of graphene and CoS-graphene materials as presented in Figure 6a. For the CoS-(26.2 wt %) graphene composite, the intensity ratio ID:IG of the D band (at 1334 cm−1) to G band (at 1600 cm−1) is calculated to be 1.35, which is larger than that of bare graphene nanosheets (1.18). This change reveals the structural features of CoS-graphene composite and further indicates the presence of more defects in the disordered graphene. Figure 6b shows the N2 adsorption/ desorption isotherm curves of CoS and CoS-graphene composite. The specific Brunauer−Emmett−Teller (BET) surface area of CoS-(26.2 wt %) graphene composite is 96.2 m2/g, which is substantially larger than pristine CoS nanosheets (34.6 m2/g). Both CoS-graphene composite and pristine CoS nanosheets show a similar average pore size of ∼3.1−3.4 nm (the inset of Figure 6b). The enhanced specific surface area of the composite can not only increase the surface active sites, but also facilitate the transport of charge carriers and lithium diffusion.

Figure 6. (a) Raman spectra of GNS and CoS-26.2 wt % GNS composite; (b) N2 adsorption -desorption isotherm of CoS and CoS26.2 wt % GNS composite. The inset shows the pore size distribution.

3.2. Lithium-Ion Storage Properties. To evaluate the Liion storage electrochemical reactions of CoS-GNS composite, CV curves of the initial three cycles were performed (Figure 7a). A sweep rate of 0.1 mV/s and a voltage range from 0 to 3.0 V were used. In the first cathodic scan, a small peak at ∼1.57 V can 25359

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Figure 7. Representative cyclic voltammograms (CVs) at a scan rate of 0.1 mV/s: (a) CoS-26.2 wt % graphene, (b) pristine CoS. The first-cycle discharge and charge curves: (c) CoS-26.2 wt % graphene, (d) pristine CoS.

Figure 8. (a) Cycling performance of pristine CoS, GNS, CoS-26.2 wt % GNS, and CoS-33.0 wt % GNS at 0.1 C, (b) First-cycle discharge−charge curves at large currents, (c) High-rate cycling performances of CoS-26.2 wt % GNS.

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sulfide-based anodes including cobalt sulfides/graphene anodes.9−27 To further understand the outstanding electrochemical performance and of CoS-GNS composite, an electrochemical impedance spectra (EIS) test was carried out in the frequency range from 0.01 Hz to 100 kHz at room temperature (Figure 9a)

be assigned to an initial process of Li insertion reaction: CoS + xLi+ + xe− → LixCoS. A sharp peak at ∼1.06 V is related to the electrochemical reduction reaction: (2 − x)Li+ + LixCoS + (2 − x)e− → Co + Li2S. The peak at ∼0.64 V is attributed to the formation of the solid electrolyte interphase (SEI) film. It is found that the cathodic peaks at ∼1.06 and 1.57 V are positively shifted to ∼1.30 and 1.74 V in the second and third cycles. For the first anodic process, a very weak peak at ∼1.44 V may be ascribed to the partial decomposition of SEI film. This peak is disappeared in the subsequent second and third cycles. There is a strong oxidation peak at ∼2.15 V, which can be assigned to the oxidation reaction of Co metal: Co + Li2S → 2Li + CoS. The second cycle of CoS-graphene composite is largely overlapped with the third cycle, but there is substantial decrease of the peak intensity for pristine CoS nanosheet (Figure 7b). It is indicated that the lithium insertion and extraction reactions between lithium ions and CoS-GNS composite are more reversible compared to pristine CoS anode. The first-cycle discharge (lithium insertion) and charge (lithium extraction) curves of the CoS-graphene composite (26.2 wt % graphene) are shown in Figure 7c. There is a long voltage plateau at ∼1.3 V in the discharge curve, which agrees well with the corresponding CV performance. The composite can deliver a high discharge (lithium insertion) capacity of 1766 mAh/g. The subsequent charge curve exhibits a voltage slope at ∼2.1 V in accordance with the previous CV curve and the charge (lithium extraction) capacity is 1126 mAh/g with a Coulombic efficiency of 63.8%. In comparison, the pristine CoS shows two longer voltage plateaus at the same positions (∼1.3 and 2.1 V) in the discharge and charge curve (Figure 7d). Figure 8a shows the cycling performances of CoS, GNS, CoS26.2 wt % GNS, and CoS-33.0 wt % GNS composite electrodes at 0.1 C. Two CoS-GNS composites display better cycling stability than that of pristine CoS or bare GNS electrode. The CoS-26.2 wt % graphene delivers a high reversible capacity of 898 mAh/g after 80 cycles, which is larger than the reversible capacities of the CoS-33.0 wt % graphene (685 mAh/g), bare GNS (483 mAh/g), and pristine CoS nanosheets (359 mAh/g) after the same cycle number. The capacity is also substantially larger than graphene supported cobalt sulfide nanoparticles23,24 and nanotubes.25 It is indicated that graphene plays a significant effect in the composite. The highly stable capacity can be attributed to the matched sheet-on-sheet structure of CoS-GNS composite, which can deliver a synergetic effect compared to two individual components. The theoretical capacities of the composite based on the weighted sum of two individual components are compared with the observed capacities in Supporting Information, Figure S3. It is clear that the sheet-on-sheet composite shows enhanced reversible capacities and better cyclability than the theoretical values. On the one hand, graphene acts as a buffer to alleviate the volume changes of the CoS anode, and the latter’s electrical contact can also be enhanced by graphene. On the other hand, CoS nanosheet has a large contact area with graphene nanosheets and can be used as an effective spacer to keep few-layer graphene separated; therefore, the promising properties of graphene can be preserved during cycling and ensure a stable cycling performance. High-rate cycling performance of CoS-26.2 wt % graphene anode was also tested at large current rates of 0.5, 2, and 3 C (Figure 8b,c). Even at a large current rate of 3 C, the composite can still deliver a reversible capacity of 391 mAh/g after repetitive 200 discharge and charge cycles. This large reversible capacity after 200 cycles at 3 C has not been witnessed for previous cobalt

Figure 9. (a) Nyquist plots of pristine CoS and CoS-26.2 wt % GNS composite, (b) the simulation model with the estimated values.

with the fitting simulation model presented in Figure 9b. In the EIS equivalent circuit, the semicircle in the medium frequency is corresponded to the Rct (charge transfer resistance). The diameter of the semicircle for the composite is smaller than pristine CoS before cycling. It is worth noting that there is only a small change of the curve for the composite after 80 cycles of discharge and charge; however, there is a large diameter increase for pristine CoS nanosheets after 80 cycles. It is indicated that the sheet-on-sheet structures preserve stable structure integrity during repetitive cycling, which can be confirmed by the TEM image of the electrode after cycling (Supporting Information, Figure S4). The electrical conductivities of the composite and pristine CoS were also measured by a four electrode method. The composite exhibits an increased electrical conductivity of 2.01 s/ cm compared to pristine CoS (0.39 s/cm), which agrees well with the EIS results. 3.3. Photocatalytic Performance. The photocatalytic activities of as-prepared samples were evaluated by measuring the degradation of MB under visible light irradiation. Figure 10a shows time profiles of [C/C0]; here C is the concentration of MB at the irradiation time t and C0 is the initial concentration. After 30 min, the samples and dye molecules reached the adsorption equilibrium before the start of irradiation. The amount of 25361

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Table 1. Pseudo-First-Order Rate Constants of the Catalytic Photodecomposition of MB catalyst

K (h−1)

R2

CoS CoS-5.2 wt % GNS CoS-15.5 wt % GNS CoS-26.2 wt % GNS CoS-33.0 wt % GNS

0.1235 0.1504 0.2328 0.4351 0.2778

0.9856 0.9976 0.9936 0.9955 0.9993

removal rate of over 90% after 5 cycles (Figure 10b), indicating the good stability of the CoS-GNS composites during photocatalytic degradation of model pollutant. It is known that the photocatalytic activity of a semiconductor is mainly related to the photoabsorption ability in the region of available light energy, the adsorptivity for pollutants, and the separation and transport rate of the photogenerated carriers in the catalyst. To find out the reason for enhanced photocatalytic activity of the CoS-GNS composites under visible light, the UV− vis diffuse reflectance spectrometer was used to determine the optical response of the synthesized samples. As indicated by Figure 11a, pristine CoS exhibits a continuous absorption band in

Figure 10. (a) Photocatalytic activities of as-prepared samples for the degradation of MB under visible-light irradiation and (b) the cyclic test of CoS-26.2 wt % GNS.

adsorbed MB for CoS-GNS composites is obviously higher than pristine CoS, which is attributed to the strong adsorptive ability and high surface area of graphene. Under visible light irradiation, CoS-5.2 wt % GNS, CoS-15.5 wt % GNS, CoS-26.2 wt % GNS, and CoS-33.0 wt % GNS can remove 60.0%, 78.6%, 91.9%, and 89.5% of MB, respectively, after 180 min irradiation, whereas only 54.4% of MB is removed with bare CoS. This comparison suggests that the introduction of graphene can efficiently enhance the photocatalytic performance of CoS. The photoactivity of the CoS-GNS composites was found to be dependent on the ratio of CoS and graphene. The CoS-26.2 wt % GNS sample shows the highest MB removal efficiency (91.9%). To analyze the photocatalysis kinetics of the MB degradation, the pseudo-first-order model ln(C0/C) = kt was applied, which is generally used for photocatalytic degradation if the initial concentration of the pollutant is low. C0 and C are the concentrations of MB at adsorption−desorption equilibrium and the irradiation time t, respectively. The pseudo-first-order rate constants were evaluated from the data plotted in Figure 10a and are summarized in Table 1. The rate constant for CoS-26.2 wt % GNS to remove MB is ∼0.4351 h−1, which is ∼3.5 times as large as that of pristine CoS (0.1235 h−1). The stability of photocatalysts is another important issue for their practical applications. To confirm the stability of the CoSGNS composites, the circulating runs in the photocatalytic degradation of MB in the presence of CoS-26.2 wt % GNS under visible-light was examined (Figure 10b). In this work, CoS-26.2 wt % GNS was recycled five times for the same photocatalytic reactions. After 180 min irradiation in each cycle, the photocatalyst was separated from the aqueous suspension by filtration, washed with deionized water and ethanol, and dried. It can be seen that the photodegradation of MB still retains a

Figure 11. (a) UV−vis absorption spectra of various products and (b) PL spectra of various products (λex = 400 nm). The inset figures are the enlarged selected sections of the corresponding curves.

the range of 200−800 nm. The addition of graphene induces the increased light absorption intensity in both the UV and visible light regions compared to pristine CoS. To investigate the efficiency of charge transfer and separation in CoS and the graphene coupling system, the photoluminescence (PL) emission spectra of these samples were measured. Figure 11b shows that the spectrum of pristine CoS exhibits an emitting peak centered at 600 nm, which could be attributed to the radioactive recombination process of self25362

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The authors also thank Lab for Microstructure, Instrumental Analysis and Research Center, Shanghai University, for materials characterizations.

trapped excitations. The CoS-GNS composites all exhibit much lower emission intensity than pristine CoS, implying that the recombination of photogenerated electrons and holes is inhibited greatly in the CoS-GNS composites. Graphene has superior electrical conductivity, making it an excellent electrontransport material in the photocatalytic process. The photogenerated electrons of excited CoS can transfer instantly from the conduction band of CoS to graphene sheets, resulting in a minimized charge recombination and offering enhanced photocatalytic activity. In addition, the order of PL intensity is CoS > CoS-5.2 wt % GNS > CoS-15.5 wt % GNS > CoS-33.0 wt % GNS > CoS-26.2 wt % GNS, which agrees well with the observed results of their photocatalytic activity mentioned above. It is worth noting that the excessive graphene (33.0 wt %) can act as a kind of recombination center, which leads to decreased photocatalytic activity compared with the CoS-26.2 wt % GNS. The adsorption of pollutant molecules is also a crucial factor to influence the photocatalytic activity of materials. As shown in Figure 10a, the adsorption ability of CoS is significantly enhanced by the introduction of graphene, which is a prerequisite for good photocatalytic activity.

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4. CONCLUSION In summary, CoS/graphene composites with unique sheet-onsheet nanostructure have been successfully synthesized by a facile ethylenediamine-mediated solvothermal method. The morphology and size of CoS nanosheets can be varied by adjusting the amount of graphene nanosheets in the composites. The obtained CoS-graphene sheet-on-sheet composite with 26.2% graphene displays the best performance for both lithium-ion storage and photocatalysis. Reversible large capacities are achieved at both small and large currents rates (898 mAh/g after 80 cycles at 0.1 C and 391 mAh/g after 200 cycles at 3 C). The sheet-on-sheet composite also exhibits good photocatalytic properties to decompose methylene blue dyes. The superior performance in Li-ion batteries and photocatalysis has been mainly attributed to the synergetic effect induced by the perfect structure matching between two-dimensional CoS nanosheets and graphene nanosheets.

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ASSOCIATED CONTENT

S Supporting Information *

SEM and TEM images of CoS-graphene composite. The cycling performance comparison of the composite with its theoretical calculation. TEM image of the cycled CoS-graphene electrode. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Telephone: +86-21-66137723. Fax: +86-21-66137725. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the follow-up Program for Professor of Special Appointment in Shanghai (Eastern Scholar), the National Natural Science Foundation of China (51271105), Shanghai Municipal Government (12ZR1410300, 11SG38,) and Innovative Research Team (IRT13078) for financial support. 25363

dx.doi.org/10.1021/jp508698q | J. Phys. Chem. C 2014, 118, 25355−25364

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