Materials Research Bulletinnem.smse-njust.com/uploads/PDF/107.pdfin Fig. 3, six Raman bands located at 129, 226, 251, 420, 610, and 965cm 1 can be observed, agreeing well with the

Materials Research Bulletin xxx (2017) xxx–xxx

G ModelMRB 9366 No. of Pages 7

Bi2S3 nanoparticles anchored on graphene nanosheets with superiorelectrochemical performance for supercapacitors

Haochen Lua,b,1, Qiubo Guoa,b,1, Feng Zana,b,*, Hui Xiaa,b,*a School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, ChinabHerbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing 210094, China

A R T I C L E I N F O

Article history:Received 2 April 2017Received in revised form 5 May 2017Accepted 25 May 2017Available online xxx

Keywords:SupercapacitorsCompositesBi2S3GrapheneNanoparticles

A B S T R A C T

Great progress has been made in developing nanostructured metal oxides and metal sulfides as electrodematerials for supercapacitors. Poor electrical conductivities of these materials, however, limit theirsupercapacitive performance. In this work, a facile synthesis strategy is developed to prepare Bi2S3/graphene composites with Bi2S3 nanoparticles anchored on graphene nanosheets. The Bi2S3/graphenecomposite electrodes exhibit promising electrochemical performance in a negative potential window of�0.9–0 V (vs. Ag/AgCl) in 1 M Na2SO4 electrolyte. A large specific capacitance of about 400 F/g can beachieved by the Bi2S3/graphene composite electrode with good rate capability and cycle performance.The present work indicates that the Bi2S3/graphene composites are promising anode materials fordeveloping high-performance asymmetric supercapacitors.

© 2017 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

journal homepage: www.else vie r .com/ locat e/mat resbu

1. Introduction

As a bridge connecting traditional capacitors and batteries,supercapacitors are becoming a new type of energy storagedevices, which are getting more and more attention [1]. In order tomeet the power requirements of fast-developing electrical devices,advanced supercapacitors with large capacitance, high powerdensity, high energy density, and long cycle life are imminentlyneeded. As the key component for supercapacitors, electrodematerials play an important role in determining electrochemicalperformance. Among the electrode materials, transition metaloxides and sulfides have attracted more and more attention due totheir large pseudocapacitance from reversible faradic redoxreactions. As a member of metal oxides, Bi2O3 is a promisingelectrode material for supercapacitors owing to its large capaci-tance, appropriate potential window, good electrochemicalstability, and simple preparation process [2–4]. Compared withBi2O3, Bi2S3 could be more attractive as electrode material forsupercapcitors because of its improved electronic conductivity [5].Many metal sulfides exhibit enhanced electronic conductivitiesthan their oxide counterparts because the electronegativity of

* Corresponding authors at: School of Materials Science and Engineering, NanjingUniversity of Science and Technology, Nanjing 210094, China

E-mail addresses: [email protected] (F. Zan), [email protected] (H. Xia).1 Both authors contributed equally to this work.

http://dx.doi.org/10.1016/j.materresbull.2017.05.0470025-5408/© 2017 Elsevier Ltd. All rights reserved.

Please cite this article in press as: H. Lu, et al., Bi2S3 nanoparticles

performance for supercapacitors, Mater. Res. Bull. (2017), http://dx.doi.

sulfur is lower than that of oxygen, making it easier for electrons totransport in the structure. The study of Bi2S3 as electrode materialfor supercapcitors, however, is very rare in literature. Therefore, itis imperative to investigate the electrochemical performance ofBi2S3 as electrode materials for supercapacitors.

As charge storage occurs at the electrode surface for super-capacitors, it is a rational way to develop the electrode materialswith large surface areas. Therefore, various nanostructures,including nanoparticles, nanowires, nanosheets, and etc., havebeen developed for improving the supercapacitive performance[6]. Although nanoparticles with large surface area are promisingto obtain large specific capacitance, they tend to aggregate duringthe charge and discharge process due to the large surface energy,thus losing their advantage during cycling. To solve this problem,these metal oxide or sulfide nanoparticles are usually mixed withconductive additives to make composites, which can suppressaggregation of nanoparticles and further improve their electricalconductivity [7,8]. Specifically, decorating metal oxide or sulfidenanoparticles on graphene nanosheets has been demonstrated tobe an effective strategy to improve the supercapacitive perfor-mance because graphene can provide a large conductive matrix,offering large surface area and fast electron transport [9–11].

In this work, a facile hydrothermal method was developed toprepare Bi2S3/functionalized graphene nanosheets (FGS) compo-sites. In the composite, Bi2S3 nanoparticles of 20–50 nm in sizewere well distributed and tightly anchored on the FGS withoutsevere aggregation. The Bi2S3/FGS composite electrode exhibited a

anchored on graphene nanosheets with superior electrochemicalorg/10.1016/j.materresbull.2017.05.047

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http://www.sciencedirect.com/science/journal/00255408

www.elsevier.com/locate/matresbu

Fig. 2. XRD patterns of the Bi2S3/FGS composites with different mass ratios: (a) 1: 1,(b) 2: 1, and (c) 3: 1.

2 H. Lu et al. / Materials Research Bulletin xxx (2017) xxx–xxx


large specific capacitance of about 396 F/g when tested as anode in1 M Na2SO4 electrolyte between �0.9–0 V (vs. Ag/AgCl) forsupercapcitors. In addition to large specific capacitance, theBi2S3/FGS composite electrode also presented good rate perfor-mance and good cycling stability. The present study indicated thatthe Bi2S3/FGS composite could be promising anode material fordeveloping high-performance asymmetric supercapcitors.

2. Experimental

2.1. Preparation of FGS

Graphene oxide (GO) was synthesized by Hummers methodfrom purified natural graphite. After that, the dried GO wasthermally exfoliated at 300 �C for 5 min in a tube furnace in air andthen heated at 900 �C in Ar for 3 h with a heating rate of 2 �C/min toobtain the FGS.

2.2. Preparation of Bi2S3/FGS composites

As illustrated in Fig. 1, the Bi2S3/FGS composites weresynthesized by a simple hydrothermal method. In a typicalsynthesis, 0.05 g FGS and 0.185 g thioacetamide (TAA) was addedto 20 mL deionized water with ultrasonication and magneticstirring for 30 min, respectively, and then 2 mL 0.4 M HNO3 and0.2530 g Bi(NO3)3�5H2O were added into the solution. After stirringfor 1 h, 20 mL of dimethylformamide was added into the abovesolution and the well mixed solution was transferred into a 50 mLTeflon-lined stainless steel autoclave and heated at 150 �C for 2 h.After cooling down to room temperature, the products werecollected by centrifugation and washed several times with distilledwater and ethanol. After that, the samples were dried at 60 �C in avacuum oven for further characterization. By changing thequantities of TAA and Bi(NO3)3�5H2O, the mass ratios of Bi2S3/FGS in the composites can be tuned as 1:1, 2:1, and 3:1. In thefollowing text, the samples with different Bi2S3 contents arereferred as 1:1, 2:1, and 3:1 samples.

2.3. Characterizations and electrochemical measurements

The X-ray diffraction (XRD, Bruker-AXS D8 Advance withmonochromatized Cu Ka radiation), Raman spectroscopy (Jobin-Yvon T6400 Micro-Raman system), and X-ray photoelectronspectroscopy (XPS, PhiQuantera SXM spectromenter using AlKaX-ray as the excitation source) were performed to characterizestructural features and phase purity of the samples. Themorphology and microstructure of the samples were investigatedby using field emission scanning electron microscope (FESEM,Quant 250 FEG) and transmission electron microscope (TEM, FEITecnai 20). Electrochemical properties of the samples wereinvestigated by using three-electrode cells with a Pt foil as counter

Fig. 1. Schematic diagram of the preparatio



electrode, Ag/AgCl as reference electrode, and Bi2S3/FGS compositeas working electrode. To prepare the working electrode, activematerial, conductive agent (Super P), and polyvinylidene fluoride(PVDF) binder with a mass ratio of 8:1:1 were dissolved in N-Methyl-2-pyrrolidone (NMP) to form a slurry, and then the slurrywas pasted on a Ti foil and dried in a vacuum oven at 80 �C for 12 h.CHI760C electrochemical workstation (Chenhua, Shanghai) andbattery test system (5 V 1 mA LANDCT2001A) were used to carryout cyclic voltammetry (CV), electrochemical impedance spec-troscopy (EIS), and galvanostatic charge-discharge measurementsin 1 M Na2SO4 aqueous electrolyte.

3. Results and discussions

The XRD patterns of the Bi2S3/FGS composites with differentBi2S3 contents (1:1, 2:1, and 3:1) are shown in Fig. 2. Standard XRDpatterns of Bi2S3 and graphene are also presented at the bottom inFig. 2. Fig. 2a shows the XRD pattern of the 1:1 Bi2S3/FGS sample.Except for the diffraction peak of graphene at 26.6� (JCPDS card No.26-1079), another two diffraction peaks at 24.9� and 28.6� can beobserved, which correspond to the (130) and (211) planes of Bi2S3,respectively. As shown in Fig. 2b and c, all diffraction peakscorresponding to Bi2S3 can be clearly observed for the 2:1 and 3:1samples due to the large Bi2S3 contents in the composites. Based onthe XRD analysis, it is clear to see that the Bi2S3/FGS composites can

n process of the Bi2S3/FGS composites.



Fig. 3. Raman spectra of the Bi2S3/FGS composites with different mass ratios of (a) 3:1, (b) 2:1, and (c) 1:1, (d) pristine Bi2S3, and (e) pristine FGS.

Fig. 4. (a) Full survey scan XPS spectrum, (b) core-level C 1s XPS spectrum, (c) core-level Bi 4f XPS spectrum, and (d) core-level S 2 s XPS spectrum of the Bi2S3/FGS composites(2:1 sample).

H. Lu et al. / Materials Research Bulletin xxx (2017) xxx–xxx 3


Please cite this article in press as: H. Lu, et al., Bi2S3 nanoparticles anchored on graphene nanosheets with superior electrochemicalperformance for supercapacitors, Mater. Res. Bull. (2017), http://dx.doi.org/10.1016/j.materresbull.2017.05.047




be successfully synthesized without the trace of impurity phase bythe facile hydrothermal method [12,13].

Fig. 3 shows the Raman spectra of the pristine Bi2S3, FGS, andthe Bi2S3/FGS composites with different Bi2S3 contents. The D bandat 1325 cm�1 and the G band at 1598 cm�1 of graphene can beclearly observed in the Raman spectra of the FGS and Bi2S3/FGScomposites [14]. As shown in Fig. 3d and the enlarged imageinserted in Fig. 3, six Raman bands located at 129, 226, 251, 420,610, and 965 cm�1 can be observed, agreeing well with the Raman

Fig. 5. The morphology and microstructure of the Bi2S3/FGS composites characterized bBi2S3 contents: 1: 1 sample, (c, d) 2:1 sample, and (e, f) 3: 1 sample. (g–i) TEM images



feature of Bi2S3 in literature [15]. These Raman bands attributed toBi2S3 can also be seen in Fig. 3a–c. Agreeing well with XRD results,there is no impurity phase detected from the Raman spectra,further confirming the successful preparation of the Bi2S3/FGScomposites.

To investigate the chemical states of various bonded elementsat the surface of the Bi2S3/FGS composite, XPS measurements werecarried out on the 2:1 sample. As shown in Fig. 4a, the full surveyscan XPS spectrum demonstrates the presence of C, Bi, S, and O

y FESEM and TEM. (a, b) FESEM images of the Bi2S3/FGS composites with different of the Bi2S3 /FGS composite at different magnifications (2:1 sample).





elements in the composite. Fig. 4b shows the C 1 s core-level XPSspectrum, which can be deconvoluted into four components. Thepeak at 284.6 eV corresponds to graphitic carbon in graphene(C��C), while other three peaks can be attributed to C��OH(285.5 eV), C��O (286.7 eV), and C¼O (289 eV), respectively,because of the existence of functional groups on the graphenesurface. The Bi 4f core-level XPS spectrum (Fig. 4c) shows twodistinct peaks located at 158 eV for Bi 4f7/2 and 163 eV for Bi 4f5/2[16,17], agreeing well with literature reported for Bi2S3. Fig. 4dshows the S 2 s core-level XPS spectrum, and the peak at 225 eV canbe assigned to S 2s of Bi2S3 [18,19]. The XRD, Raman, and XPSresults agree well with each other, confirming the successfulsynthesis of Bi2S3/FGS composites.

Fig. 6. (a) CV curves of the 2:1 Bi2S3/FGS composite electrode at different scan rates froelectrode at different current densities from 1 to 40 A/g. (c) The specific capacitances asplots and (e) charge/discharge cycle performances of different Bi2S3/FGS composite ele



Fig. 5 shows the morphology and microstructure of the Bi2S3/FGS composites characterized by FESEM and TEM. As shown inFig. 5a and b, the 1:1 Bi2S3/FGS composite well resembles themorphology of graphene with crumpled nanosheets beingobserved. No obvious Bi2S3 nanoparticles can be seen from theFESEM images, which can be attributed to the small particles sizeand low Bi2S3 content in the 1:1 sample. As the Bi2S3 content in thecomposite increases, the Bi2S3 nanoparticles can be clearlyobserved in the FESEM images in Fig. 5c–f for the 2:1 and 3:1samples. The composites still retain the two-dimensional nano-sheet morphology with Bi2S3 nanoparticles uniformly distributedand anchored on the FGS surface. To further investigate themicrostructure of the Bi2S3/FGS composite, TEM measurements

m 5 to 1600 mV/s. (b) Charge and discharge curves of the 2:1 Bi2S3/FGS composite a function of the scan rate of different Bi2S3/FGS composite electrodes. (d) Nyquistctrodes.





were carried out on the 2:1 sample. Fig. 5g–i shows the TEMimages of the Bi2S3/FGS composite at different magnifications. Itcan be seen that Bi2S3 nanoparticles of 20–50 nm in size are welldistributed on the transparent graphene nanosheets, agreeing wellwith the FESEM images. The selected area electron diffraction(SAED) pattern (inset in Fig. 5i) of the Bi2S3/FGS compositepresents three diffraction rings, corresponding to (130), (221), and(002) crystal planes of Bi2S3, respectively.

The electrochemical properties of the Bi2S3/FGS compositeswere investigated by CV and galvanostatic charge-dischargemeasurements using three-electrode cells with 1 M Na2SO4

aqueous electrolyte. Fig. 6a shows the CV curves of the Bi2S3/FGS composite (2:1 sample) electrode at different scan rates from 5to 1600 mV/s in a potential window of �0.9–0 V (vs. Ag/AgCl). TheCV curves retain rectangular shape even at high scan rate of1600 mV/s, indicating ideal capacitive behavior and high revers-ibility. The specific capacitance of the Bi2S3/FGS compositeelectrode can reach 396 F/g at a scan rate of 5 mV/s. Obviousredox peaks can be observed in the CV curves, indicatingpseudocapacitance contribution from the Bi2S3/FGS compositeelectrode due to faradic redox reactions at the surface. The redoxpeaks can be attributed to Na+ intercalation and deintercalation,and the reaction can be expressed by the following equation:

Bi2S3 + xNa+ + xe�! NaxBi2S3 (1)

Fig. 6b shows the charge-discharge curves of the Bi2S3/FGScomposite (2:1 sample) electrode between �0.9 and 0 V (vs. Ag/AgCl) at different current densities from 1 to 40 A/g. The Bi2S3/FGScomposite electrode can deliver a large specific capacitance ofabout 292 F/g at a current density of 1 A/g. Small voltage plateauscan be observed in the charge-discharge curves, corresponding tothe redox peaks observed in the CV curves. The specificcapacitances as a function of scan rate of different Bi2S3/FGScomposite (1:1, 2:1, and 3:1) electrodes are compared in Fig. 6c.The Bi2S3/FGS composite (2:1) electrode can still deliver a largespecific capacitance of about 100 F/g even at a high scan rate of1600 mV/s, indicating good rate capability. To further understandthe electrochemical behavior for the Bi2S3/FGS composites, EISmeasurements were carried out on different composite electrodesand the obtained Nyquist plots are shown in Fig. 6d. It can be seenthat the graphene content in the composite plays an important rolein determining the resistance of the electrodes. The 1:1 sampleexhibits the smallest ohmic resistance and charge transferresistance due to its large graphene content, favoring fast electrontransport and fast faradic redox reactions. Fig. 6e compares thecycle performances of the different Bi2S3/FGScomposite electrodesat the current density of 5 A g-1. Among the three electrodes, the2:1 sample exhibits the best cycle performance with a capacitanceretention of 75% after 5000 cycles. The superior cycle performanceof the 2:1 sample can be attributed to its appropriate graphenecontent, which can effectively suppress the volume change of Bi2S3during charge and discharge, providing improved structuralstability.

The promising supercapacitive performance of the Bi2S3/FGScomposite electrodes can be attributed to its unique hetero-structure with Bi2S3 nanoparticles anchored on the FGS surface[20]. First, the FGS has a large specific surface area and excellentelectrical conductivity, offering a stubborn conductive matrix forfast electron and ion transports. Second, Bi2S3 nanoparticles arewell separated and dispersed on the FGS, guaranteeing largecharge storage sites and high utilization of Bi2S3 with largepseudocapacitance contribution. Finally, the soft FGS can alsofunction as buffer layer, which can reduce the stain associated withvolume change of Bi2S3 during charge and discharge, thus resultingin improved structural stability and good cycle performance.



4. Conclusions

In summary, a facile hydrothermal method was developed toprepare the Bi2S3/FGS composites with various Bi2S3 contents. TheBi2S3/FGS composites retain the nanosheet morphology ofgraphene with Bi2S3 nanoparticles uniformly anchored on thesurface of FGS. The Bi2S3/FGS composites were first timeinvestigated as electrode materials for supercapacitors in neutralaqueous electrolyte, and they exhibit promising supercapacitiveperformance. The specific capacitance of the 2:1 Bi2S3/FGScomposite electrode can reach 396 F/g at a scan rate of 5 mV/sbetween �0.9 and 0 V (vs. Ag/AgCl). In addition to large specificcapacitance, the Bi2S3/FGS composite electrode also exhibits goodrate capability and cycle performance, owing to the synergisticeffect between Bi2S3 and FGS. The present results indicate that theBi2S3/FGS composites could be promising anode materials fordeveloping high-performance asymmetric supercapacitors.

Acknowledgements

This work was supported by National Natural Science Founda-tion of China (No. 51572129 and U1407106), International S&TCooperation Program of China (No. 2016YFE0111500), QingLanProject of Jiangsu Province, A Project Funded by the PriorityAcademic Program Development of Jiangsu Higher EducationInstitutions (PAPD), and the Fundamental Research Funds for theCentral Universities (No. 30915011204).

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Materials Research Bulletinnem.smse-njust.com/uploads/PDF/107.pdfin Fig. 3, six Raman bands located at 129, 226, 251, 420, 610, and 965cm 1 can be observed, agreeing well with the