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Graphene–ZnS quantum dot nanocomposites produced by solvothermal route
Materials Letters 65 (2011) 2518–2521
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
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 b
Key laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China Graduate University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e
i n f o
Article history: Received 27 January 2011 Accepted 11 May 2011 Available online 15 May 2011 Keywords: Graphene ZnS Solvothermal Luminescence Nanocomposites
a b s t r a c t Graphene–ZnS (G–ZnS) quantum dots nanocomposites were prepared by one pot solvothermal route. Graphite oxide (GO) was well dispersed in dimethyl sulfoxide (DMSO) solution of ions Zn 2+, where DMSO acts as a sulphide source as well as reducing agent, resulting in the formation of graphene–ZnS quantum dot nanocomposites and simultaneous reduction of GO to graphene nanosheets. The size of the ZnS QDs in G–ZnS is around 10 nm, and the large 2D flexible atom-thin layer of graphene makes it easier to control the distribution of ZnS. The resulting G–ZnS QDs display a well-defined excitonic emission feature by luminescence analysis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing number of potential applications for materials with dimensions in the nanometer range. Such systems show improved or even new properties emerging as a result of electronic and optical properties confinement [1,2]. For various applications, for instance, quantum dots (ODs), the band gap of nanoparticles can be tuned either by controlling the particle size or the interactions of the particles with the matrix [3–5]. Nano-ZnS particles have been studied extensively due to their excellent optical and catalytic properties. Furthermore, there is a strong interest to attach nanoparticles to carbon materials such as carbon nanotubes [6] and graphene nanosheets (GNs) [4]. Compared with one-dimensional carbon nanotubes, the superior electrical conductivity and the large twodimensional flexible atom-thin layer of GNs would make it an excellent electron-transport matrix and fabricate future optoelectronic devices owing to its high specific surface area for a large interface, high mobility up to 10,000 cm 2 V −1 s −1, and tunable band gap. Currently, ZnS [7], CdS [7,8], TiO2 [9,10] have been conjugated with GNs. The main hurdle in the synthesis of semiconductor QDs on the surface of GNs is that introducing the metal ions in an aqueous solution of GNs causes their immediate aggregation. Herein, a graphene– ZnS quantum dot nanocomposites (G–ZnS QDs) were synthesized by one pot approach. GO was dispersed well in dimethyl sulfoxide (DMSO) solution of ions Zn 2+, resulting in the formation of graphene– ZnS quantum dot nanocomposites and simultaneous reduction of GO to GNs. The resulting G–ZnS QDs display a well-defined excitonic
⁎ Corresponding author. Tel./fax: + 86 351 4049061. E-mail address: [email protected] (Y. Yang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.05.055
emission feature by luminescence analysis, and hold great promise to become prime candidates for future applications in nanoscale photosensors. 2. Experimental GO was prepared by modified Hummers method [11]. In a typical synthesis, the graphite oxide (50 mg) and Zn(CH3COO)2 (0.082 g≥99.0%) were dispersed in DMSO (50 mL) solution by vigorous stirring to form a stable suspension. Then the resulting suspension was transferred into a Teflon-lined stainless steel autoclave (50 mL), and treated at 180 °C for 10 h. The solid products were obtained after washing with acetone and absolute ethanol, then dried in a vacuum oven at 60 °C (Fig. 1a). All the samples were characterized by powder X-ray diffraction (pXRD, Rigagu D/MAX 2500 V/PC diffractometer), transmission electron microscopy (TEM, JEOL JEM-2010), X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250), Fourier-transform infrared spectroscopy (FTIR, Nicolet IS10), UV–vis absorption spectroscopy (JASCO, Corporation V-570), and photoluminescence spectroscopy (Hitachi 850 fluorescence spectrometer), 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 nm which 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) planes of sphalerite ZnS, respectively, and no peak due to GO or graphite is observed demonstrating complete exfoliation and reduction of GO as in the case of G–ZnS.
Y. Li et al. / Materials Letters 65 (2011) 2518–2521
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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.
Transmission electron microscopy (TEM) image (Fig. 2a) displays the image of the as-prepared GOs dispersed in DMSO. As shown in Fig. 2b and c, the G–ZnS consisted of single-layer 2D graphene sheets are coated with ZnS QDs. Besides, wrinkles of G–ZnS are observed which consistent with the recent reports [7,8], demonstrating the morphology of GNs. Both Fig. 2b and c show that the individual ZnS nanoparticles are well separated from each other and distributed uniformly on the GNs. The good distribution of ZnS QDs on GNs guarantees the efficient optoelectronic properties of G–ZnS. The size of the ZnS QDs in G–ZnS is around 10 nm as shown by the highresolution TEM image in Fig. 3c, which is nearly agree well with the XRD 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-
matic C C (1624 cm−1), epoxy C–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 interaction between C C double-bond and sulfur. The GO related stretching bands of C–O and carboxyl groups are not observed in the case of G–ZnS composites, indicating the reduction of GO to GNs. The C/O ratio and information regarding any functional groups of the GO and G–ZnS QDs were investigated by XPS. O1s peak of G–ZnS QDs is obviously decreased as compared to that of GO in the wide region (Fig. 3b), suggesting that the C/O ratio of G–ZnS composites increase remarkably after solvothermal process. The percentage of surface oxygen groups in GO is estimated to be 34.06%, and decreases to 15.97% by atomic composition after solvothermal process, suggesting that the most of the oxygen functional groups
Fig. 2. (a) TEM image of GNs in DMSO, (b,c) TEM image of a G–ZnS sheet sparsely coated with ZnS QDs, showing natural wrinkles of a single graphene sheet.
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Y. Li et al. / Materials Letters 65 (2011) 2518–2521
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.
are successfully removed. This result can also be confirmed by the C1s region of GO and G–ZnS (Fig. 3c and d). In addition, the increase of peak for G–ZnS at 284.8 eV (C–C) in the C1s region also suggests the formation of graphene. The XPS results are corroborated by FTIR spectra analysis. Although there are still some oxygenated carbons in the G–ZnS composite, our control experiments with pristine graphene oxide show that the oxygenated groups that remain are not necessary for the binding of ZnS particles to the graphene sheets. The diffuse reflectance UV–visible spectra of pure ZnS and G–ZnS QDs are shown in Fig 4a. It can be found a shoulder at 219.5 nm,
indicating that the bandgap energy (Eg) of G–ZnS QDs is about 5.65 eV, which is much larger than that of bulk ZnS (3.66 eV). This blue shift was obviously caused by the strong quantum confinement effect of the attached ZnS nanocrystals on the GNs. In addition, a further weaker shoulder appeared at 249.5 nm (4.97 eV) which might be caused by the sulfur vacancy defects [13]. PL spectra of the free ZnS QDs and G–ZnS QDs at the excitation wavelength of 280 nm are shown in Fig 4b. Compared to the emission spectra of ZnS QDs (361 nm), the band-edge emission spectra of G–ZnS QDs are blueshifted to 327.5 nm due to quantum confinement effect and a high
Fig. 4. (a) UV–vis absorption spectrum of free ZnS nanoparticles and G–ZnS QDs, (b) PL spectra of free ZnS nanoparticles and G–ZnS QDs excited at 280 nm.
Y. Li et al. / Materials Letters 65 (2011) 2518–2521
concentration of point defects in the lattice. On the other hand, the low intensity surface defect related emissions of G–ZnS QDs are completely quenched due to interactions of the surface of ZnS QDs with graphene. 4. Conclusions In conclusion, G–ZnS QDs with good structure have been successfully synthesized from GO by solvothermal method. GO has been simultaneously reduced to graphene nanosheets during the deposition of ZnS QDs. The size of the ZnS QDs in G–ZnS is around 10 nm, and the large 2D flexible atom-thin layer of graphene makes it easier to control the distribution of ZnS on it and fabricate future optoelectronic devices. Acknowledgments The authors acknowledge Dr Zhijie Li (Southwest University of Science and Technology) for his UV–visible spectra and PL characterization.
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References [1] Xue FL, Chen JY, Guo J, Wang CC, Yang WL, Wang PN, et al. J Fluoresc 2007;17: 149–54. [2] Zhang YB, Tan YW, Stormer HL, Kim P. Nature 2005;438:201–4. [3] Yu JH, Joo J, Park HM, Baik SI, Kim YW, Kim SC, et al. J Am Chem Soc 2005;127: 5662–70. [4] Yan X, Cui X, Li LS. J Am Chem Soc 2005;132:5944–5. [5] Juarez BH, Klinke C, Kornowski A, Weller H. Nano Lett 2007;7:3564–8. [6] Zhang MN, Su L, Mao LQ. Carbon 2006;44:276–83. [7] Nethravathi C, Nisha T, Ravishankar N, Shivakumara C, Rajamathi M. Carbon 2009;47:2054–9. [8] Cao AN, Liu Z, Chu SS, Wu MH, Ye ZM, Cai ZW, et al. Adv Mater 2010;22:103–6. [9] Williams G, Seger B, Kamat PV. ACS Nano 2008;2:1487–91. [10] Wang DH, Choi DW, Li J, Yang ZG, Nie ZM, Kou R, et al. ACS Nano 2009;3:907–14. [11] Kovtyukhova NI, Ollivier PJ, Martin BR, Mallouk TE, Chizhik SA, Buzaneva EV, et al. Chem Mater 1999;11:771–8. [12] Liu YZ, Li YF, Yang YG, Wen YF, Wang MZ. New Carbon Mater 2011;26:41–5. [13] Tong H, Zhu YJ, Yang LX, Li L, Zhang L, Chang J, et al. J Phys Chem C 2007;111: 3893–900.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
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 b
Key laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China Graduate University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e
i n f o
Article history: Received 27 January 2011 Accepted 11 May 2011 Available online 15 May 2011 Keywords: Graphene ZnS Solvothermal Luminescence Nanocomposites
a b s t r a c t Graphene–ZnS (G–ZnS) quantum dots nanocomposites were prepared by one pot solvothermal route. Graphite oxide (GO) was well dispersed in dimethyl sulfoxide (DMSO) solution of ions Zn 2+, where DMSO acts as a sulphide source as well as reducing agent, resulting in the formation of graphene–ZnS quantum dot nanocomposites and simultaneous reduction of GO to graphene nanosheets. The size of the ZnS QDs in G–ZnS is around 10 nm, and the large 2D flexible atom-thin layer of graphene makes it easier to control the distribution of ZnS. The resulting G–ZnS QDs display a well-defined excitonic emission feature by luminescence analysis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing number of potential applications for materials with dimensions in the nanometer range. Such systems show improved or even new properties emerging as a result of electronic and optical properties confinement [1,2]. For various applications, for instance, quantum dots (ODs), the band gap of nanoparticles can be tuned either by controlling the particle size or the interactions of the particles with the matrix [3–5]. Nano-ZnS particles have been studied extensively due to their excellent optical and catalytic properties. Furthermore, there is a strong interest to attach nanoparticles to carbon materials such as carbon nanotubes [6] and graphene nanosheets (GNs) [4]. Compared with one-dimensional carbon nanotubes, the superior electrical conductivity and the large twodimensional flexible atom-thin layer of GNs would make it an excellent electron-transport matrix and fabricate future optoelectronic devices owing to its high specific surface area for a large interface, high mobility up to 10,000 cm 2 V −1 s −1, and tunable band gap. Currently, ZnS [7], CdS [7,8], TiO2 [9,10] have been conjugated with GNs. The main hurdle in the synthesis of semiconductor QDs on the surface of GNs is that introducing the metal ions in an aqueous solution of GNs causes their immediate aggregation. Herein, a graphene– ZnS quantum dot nanocomposites (G–ZnS QDs) were synthesized by one pot approach. GO was dispersed well in dimethyl sulfoxide (DMSO) solution of ions Zn 2+, resulting in the formation of graphene– ZnS quantum dot nanocomposites and simultaneous reduction of GO to GNs. The resulting G–ZnS QDs display a well-defined excitonic
⁎ Corresponding author. Tel./fax: + 86 351 4049061. E-mail address: [email protected] (Y. Yang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.05.055
emission feature by luminescence analysis, and hold great promise to become prime candidates for future applications in nanoscale photosensors. 2. Experimental GO was prepared by modified Hummers method [11]. In a typical synthesis, the graphite oxide (50 mg) and Zn(CH3COO)2 (0.082 g≥99.0%) were dispersed in DMSO (50 mL) solution by vigorous stirring to form a stable suspension. Then the resulting suspension was transferred into a Teflon-lined stainless steel autoclave (50 mL), and treated at 180 °C for 10 h. The solid products were obtained after washing with acetone and absolute ethanol, then dried in a vacuum oven at 60 °C (Fig. 1a). All the samples were characterized by powder X-ray diffraction (pXRD, Rigagu D/MAX 2500 V/PC diffractometer), transmission electron microscopy (TEM, JEOL JEM-2010), X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250), Fourier-transform infrared spectroscopy (FTIR, Nicolet IS10), UV–vis absorption spectroscopy (JASCO, Corporation V-570), and photoluminescence spectroscopy (Hitachi 850 fluorescence spectrometer), 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 nm which 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) planes of sphalerite ZnS, respectively, and no peak due to GO or graphite is observed demonstrating complete exfoliation and reduction of GO as in the case of G–ZnS.
Y. Li et al. / Materials Letters 65 (2011) 2518–2521
2519
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.
Transmission electron microscopy (TEM) image (Fig. 2a) displays the image of the as-prepared GOs dispersed in DMSO. As shown in Fig. 2b and c, the G–ZnS consisted of single-layer 2D graphene sheets are coated with ZnS QDs. Besides, wrinkles of G–ZnS are observed which consistent with the recent reports [7,8], demonstrating the morphology of GNs. Both Fig. 2b and c show that the individual ZnS nanoparticles are well separated from each other and distributed uniformly on the GNs. The good distribution of ZnS QDs on GNs guarantees the efficient optoelectronic properties of G–ZnS. The size of the ZnS QDs in G–ZnS is around 10 nm as shown by the highresolution TEM image in Fig. 3c, which is nearly agree well with the XRD 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-
matic C C (1624 cm−1), epoxy C–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 interaction between C C double-bond and sulfur. The GO related stretching bands of C–O and carboxyl groups are not observed in the case of G–ZnS composites, indicating the reduction of GO to GNs. The C/O ratio and information regarding any functional groups of the GO and G–ZnS QDs were investigated by XPS. O1s peak of G–ZnS QDs is obviously decreased as compared to that of GO in the wide region (Fig. 3b), suggesting that the C/O ratio of G–ZnS composites increase remarkably after solvothermal process. The percentage of surface oxygen groups in GO is estimated to be 34.06%, and decreases to 15.97% by atomic composition after solvothermal process, suggesting that the most of the oxygen functional groups
Fig. 2. (a) TEM image of GNs in DMSO, (b,c) TEM image of a G–ZnS sheet sparsely coated with ZnS QDs, showing natural wrinkles of a single graphene sheet.
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Y. Li et al. / Materials Letters 65 (2011) 2518–2521
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.
are successfully removed. This result can also be confirmed by the C1s region of GO and G–ZnS (Fig. 3c and d). In addition, the increase of peak for G–ZnS at 284.8 eV (C–C) in the C1s region also suggests the formation of graphene. The XPS results are corroborated by FTIR spectra analysis. Although there are still some oxygenated carbons in the G–ZnS composite, our control experiments with pristine graphene oxide show that the oxygenated groups that remain are not necessary for the binding of ZnS particles to the graphene sheets. The diffuse reflectance UV–visible spectra of pure ZnS and G–ZnS QDs are shown in Fig 4a. It can be found a shoulder at 219.5 nm,
indicating that the bandgap energy (Eg) of G–ZnS QDs is about 5.65 eV, which is much larger than that of bulk ZnS (3.66 eV). This blue shift was obviously caused by the strong quantum confinement effect of the attached ZnS nanocrystals on the GNs. In addition, a further weaker shoulder appeared at 249.5 nm (4.97 eV) which might be caused by the sulfur vacancy defects [13]. PL spectra of the free ZnS QDs and G–ZnS QDs at the excitation wavelength of 280 nm are shown in Fig 4b. Compared to the emission spectra of ZnS QDs (361 nm), the band-edge emission spectra of G–ZnS QDs are blueshifted to 327.5 nm due to quantum confinement effect and a high
Fig. 4. (a) UV–vis absorption spectrum of free ZnS nanoparticles and G–ZnS QDs, (b) PL spectra of free ZnS nanoparticles and G–ZnS QDs excited at 280 nm.
Y. Li et al. / Materials Letters 65 (2011) 2518–2521
concentration of point defects in the lattice. On the other hand, the low intensity surface defect related emissions of G–ZnS QDs are completely quenched due to interactions of the surface of ZnS QDs with graphene. 4. Conclusions In conclusion, G–ZnS QDs with good structure have been successfully synthesized from GO by solvothermal method. GO has been simultaneously reduced to graphene nanosheets during the deposition of ZnS QDs. The size of the ZnS QDs in G–ZnS is around 10 nm, and the large 2D flexible atom-thin layer of graphene makes it easier to control the distribution of ZnS on it and fabricate future optoelectronic devices. Acknowledgments The authors acknowledge Dr Zhijie Li (Southwest University of Science and Technology) for his UV–visible spectra and PL characterization.
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References [1] Xue FL, Chen JY, Guo J, Wang CC, Yang WL, Wang PN, et al. J Fluoresc 2007;17: 149–54. [2] Zhang YB, Tan YW, Stormer HL, Kim P. Nature 2005;438:201–4. [3] Yu JH, Joo J, Park HM, Baik SI, Kim YW, Kim SC, et al. J Am Chem Soc 2005;127: 5662–70. [4] Yan X, Cui X, Li LS. J Am Chem Soc 2005;132:5944–5. [5] Juarez BH, Klinke C, Kornowski A, Weller H. Nano Lett 2007;7:3564–8. [6] Zhang MN, Su L, Mao LQ. Carbon 2006;44:276–83. [7] Nethravathi C, Nisha T, Ravishankar N, Shivakumara C, Rajamathi M. Carbon 2009;47:2054–9. [8] Cao AN, Liu Z, Chu SS, Wu MH, Ye ZM, Cai ZW, et al. Adv Mater 2010;22:103–6. [9] Williams G, Seger B, Kamat PV. ACS Nano 2008;2:1487–91. [10] Wang DH, Choi DW, Li J, Yang ZG, Nie ZM, Kou R, et al. ACS Nano 2009;3:907–14. [11] Kovtyukhova NI, Ollivier PJ, Martin BR, Mallouk TE, Chizhik SA, Buzaneva EV, et al. Chem Mater 1999;11:771–8. [12] Liu YZ, Li YF, Yang YG, Wen YF, Wang MZ. New Carbon Mater 2011;26:41–5. [13] Tong H, Zhu YJ, Yang LX, Li L, Zhang L, Chang J, et al. J Phys Chem C 2007;111: 3893–900.