Materials Letters 65 (2011) 2518–2521

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Materials Letters

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Graphene–ZnS quantum dot nanocomposites produced by solvothermal route

Yongfeng Li a,b, Yanzhen Liu a,b, Wenzhong Shen a, Yonggang Yang a,⁎, Yuefang Wen a, Maozhang Wang a

a Key laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Chinab Graduate University of Chinese Academy of Sciences, Beijing, 100049, China

⁎ Corresponding author. Tel./fax: +86 351 4049061.E-mail address: yangyg@sxicc.ac.cn (Y. Yang).

0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.05.055

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 January 2011Accepted 11 May 2011Available online 15 May 2011

Keywords:GrapheneZnSSolvothermalLuminescenceNanocomposites

Graphene–ZnS (G–ZnS) quantum dots nanocomposites were prepared by one pot solvothermal route.Graphite oxide (GO)waswell dispersed in dimethyl sulfoxide (DMSO) solution of ions Zn2+,where DMSO actsas a sulphide source as well as reducing agent, resulting in the formation of graphene–ZnS quantum dotnanocomposites and simultaneous reduction of GO to graphenenanosheets. The size of the ZnSQDs inG–ZnS isaround 10 nm, and the large 2D flexible atom-thin layer of graphenemakes it easier to control the distributionof ZnS. The resulting G–ZnS QDs display a well-defined excitonic emission feature by luminescence analysis.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

There is an increasing number of potential applications formaterials with dimensions in the nanometer range. Such systemsshow improved or even new properties emerging as a result of elec-tronic and optical properties confinement [1,2]. For various applica-tions, for instance, quantum dots (ODs), the band gap of nanoparticlescan be tuned either by controlling the particle size or the interactionsof the particles with the matrix [3–5]. Nano-ZnS particles havebeen studied extensively due to their excellent optical and catalyticproperties. Furthermore, there is a strong interest to attach nanopar-ticles to carbon materials such as carbon nanotubes [6] and graphenenanosheets (GNs) [4]. Compared with one-dimensional carbonnanotubes, the superior electrical conductivity and the large two-dimensional flexible atom-thin layer of GNs would make it an ex-cellent electron-transport matrix and fabricate future optoelectronicdevices owing to its high specific surface area for a large interface, highmobility up to 10,000 cm2 V−1 s−1, and tunable band gap.

Currently, ZnS [7], CdS [7,8], TiO2 [9,10] have been conjugated withGNs. The main hurdle in the synthesis of semiconductor QDs on thesurface of GNs is that introducing the metal ions in an aqueous solu-tion of GNs causes their immediate aggregation. Herein, a graphene–ZnS quantum dot nanocomposites (G–ZnS QDs) were synthesizedby one pot approach. GO was dispersed well in dimethyl sulfoxide(DMSO) solution of ions Zn2+, resulting in the formation of graphene–ZnS quantum dot nanocomposites and simultaneous reduction ofGO to GNs. The resulting G–ZnS QDs display a well-defined excitonic

emission feature by luminescence analysis, and hold great promiseto become prime candidates for future applications in nanoscalephotosensors.

2. Experimental

GO was prepared by modified Hummers method [11]. In a typicalsynthesis, the graphiteoxide (50 mg)andZn(CH3COO)2 (0.082 g≥99.0%)were dispersed in DMSO (50 mL) solution by vigorous stirring to form astable suspension. Then the resulting suspension was transferred intoa Teflon-lined stainless steel autoclave (50 mL), and treated at 180 °Cfor 10 h. The solid products were obtained after washing with acetoneand absolute ethanol, then dried in a vacuum oven at 60 °C (Fig. 1a). Allthe samples were characterized by powder X-ray diffraction (pXRD,Rigagu D/MAX 2500 V/PC diffractometer), transmission electron micros-copy (TEM, JEOL JEM-2010), X-ray photoelectron spectroscopy (XPS,Thermo ESCALAB 250), Fourier-transform infrared spectroscopy (FTIR,Nicolet IS10), UV–vis absorption spectroscopy (JASCO, CorporationV-570), and photoluminescence spectroscopy (Hitachi 850 fluorescencespectrometer), respectively.

3. Results and discussion

Fig. 1b shows the pXRD patterns of the pristine GO and G–ZnS QDs.The basal spacing of GO calculated from the 001 reflection is 0.747 nmwhich matches well with the values reported in the literatures [12].Interestingly, the pattern of G–ZnS QDs displays peaks only at 28.6°,47.8°, and 56.6°, corresponding to the (111), (220), and (311) planesof sphalerite ZnS, respectively, and no peak due to GO or graphite isobserved demonstrating complete exfoliation and reduction of GO asin the case of G–ZnS.

Fig. 1. (a) Schematic illustration of coating of graphene with ZnS QDs, (b) The pXRD patterns of the as-prepared GO and G–ZnS QDs.

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Transmission electron microscopy (TEM) image (Fig. 2a) displaysthe image of the as-prepared GOs dispersed in DMSO. As shown inFig. 2b and c, the G–ZnS consisted of single-layer 2D graphene sheetsare coated with ZnS QDs. Besides, wrinkles of G–ZnS are observedwhich consistent with the recent reports [7,8], demonstrating themorphology of GNs. Both Fig. 2b and c show that the individual ZnSnanoparticles are well separated from each other and distributeduniformly on the GNs. The good distribution of ZnS QDs on GNsguarantees the efficient optoelectronic properties of G–ZnS. The sizeof the ZnS QDs in G–ZnS is around 10 nm as shown by the high-resolution TEM image in Fig. 3c, which is nearly agree well with theXRD results above.

Fig. 3a shows the FTIR spectra of GO and G–ZnS nanocomposites.The stretching vibrations of C O (1739 cm−1), O–H (3421 cm−1), aro-

Fig. 2. (a) TEM image of GNs in DMSO, (b,c) TEM image of a G–ZnS sheet sparsel

matic C C (1624 cm−1), epoxyC–O (1226 cm−1), and C–O (1066 cm−1)are observed. For G–ZnS, the decrease of frequency of C C (1624 cm−1)and increase of its intensity were taken placed due to interactionbetween C C double-bond and sulfur. The GO related stretching bandsof C–O and carboxyl groups are not observed in the case of G–ZnScomposites, indicating the reduction of GO to GNs.

The C/O ratio and information regarding any functional groupsof the GO and G–ZnS QDs were investigated by XPS. O1s peak ofG–ZnS QDs is obviously decreased as compared to that of GO in thewide region (Fig. 3b), suggesting that the C/O ratio of G–ZnS com-posites increase remarkably after solvothermal process. The percent-age of surface oxygen groups in GO is estimated to be 34.06%, anddecreases to 15.97% by atomic composition after solvothermalprocess, suggesting that the most of the oxygen functional groups

y coated with ZnS QDs, showing natural wrinkles of a single graphene sheet.

Fig. 3. (a) The FTIR spectra of the as prepared GO and G–ZnS QDs, (b) the XPS spectra of GO and G–ZnS, C1s region of (c) GO and (d) G–ZnS.

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are successfully removed. This result can also be confirmed by the C1sregion of GO and G–ZnS (Fig. 3c and d). In addition, the increase ofpeak for G–ZnS at 284.8 eV (C–C) in the C1s region also suggests theformation of graphene. The XPS results are corroborated by FTIRspectra analysis. Although there are still some oxygenated carbons inthe G–ZnS composite, our control experiments with pristine grapheneoxide show that the oxygenated groups that remain are not necessaryfor the binding of ZnS particles to the graphene sheets.

The diffuse reflectance UV–visible spectra of pure ZnS and G–ZnSQDs are shown in Fig 4a. It can be found a shoulder at 219.5 nm,

Fig. 4. (a) UV–vis absorption spectrum of free ZnS nanoparticles and G–ZnS QDs

indicating that the bandgap energy (Eg) of G–ZnS QDs is about5.65 eV, which is much larger than that of bulk ZnS (3.66 eV). Thisblue shift was obviously caused by the strong quantum confinementeffect of the attached ZnS nanocrystals on the GNs. In addition, afurther weaker shoulder appeared at 249.5 nm (4.97 eV) whichmightbe caused by the sulfur vacancy defects [13]. PL spectra of the freeZnS QDs and G–ZnS QDs at the excitation wavelength of 280 nmare shown in Fig 4b. Compared to the emission spectra of ZnS QDs(361 nm), the band-edge emission spectra of G–ZnS QDs are blue-shifted to 327.5 nm due to quantum confinement effect and a high

, (b) PL spectra of free ZnS nanoparticles and G–ZnS QDs excited at 280 nm.

2521Y. Li et al. / Materials Letters 65 (2011) 2518–2521

concentration of point defects in the lattice. On the other hand, thelow intensity surface defect related emissions of G–ZnS QDs arecompletely quenched due to interactions of the surface of ZnS QDswith graphene.

4. Conclusions

In conclusion, G–ZnS QDs with good structure have been suc-cessfully synthesized from GO by solvothermal method. GO has beensimultaneously reduced to graphene nanosheets during the deposi-tion of ZnS QDs. The size of the ZnS QDs in G–ZnS is around 10 nm, andthe large 2D flexible atom-thin layer of graphene makes it easier tocontrol the distribution of ZnS on it and fabricate future optoelec-tronic devices.

Acknowledgments

The authors acknowledge Dr Zhijie Li (Southwest Universityof Science and Technology) for his UV–visible spectra and PLcharacterization.

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