- Email: [email protected]
functionalized graphene sheets nanocomposites
Journal of Alloys and Compounds 570 (2013) 65–69
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Hydrothermal synthesis of CdS/functionalized graphene sheets nanocomposites Shancheng Yan a,b,⇑, Yi Shi b, Bin Zhao b, Tao Lu b, Dong Hu a, Xin Xu a, Jiansheng Wu a, Jingsong Chen a a b
School of Geography and Biological Information, Nanjing University of Posts and Telecommunications, Nanjing 210046, PR China National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 10 March 2013 Accepted 19 March 2013 Available online 27 March 2013 Keywords: Functionalized graphene sheets Cadmium sulfide Nanocomposites Nanoparticles
a b s t r a c t CdS/Functionalized Graphene Sheets (CdS/FGSs) nanocomposites were successfully prepared in a one-step hydrothermal synthesis route. The morphology, structure characterization, and crystal structure of the asprepared nanocomposites were performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Through controlled experimental conditions, such as cadmium source and sulfide source, the different morphologies and dimensions of the final products have been controlled. The synthetic approach was simple and fast, and it may be extended for the synthesis of other FGS–metal-sulfide nanocomposites. By combining the unique properties of FGS and CdS nanoparticles, the strategy presented in this study is expected to be useful to prepare highly efficient graphene-based hybrids for potential applications in various fields. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the assembly of semiconductor nanoparticles (NPs) on matrices has been extensively studied for their promising optoelectronic applications [1–3]. Graphene, as a new two-dimensional carbon nanomaterial, has received increasing attention during recent years by virtue of its outstanding physical, chemical properties and excellent electrocatalytic ability [4–9]. It has been revealed that when incorporated semiconductor nanoparticles into matrices, graphene can remarkably improve the properties of these host materials [10,11]. All these above indicate that the graphenebased nanocomposites exhibit more excellent performance than pure graphene in optoelectronic fabrication, and as a new kind of nanocomposite materials, they have been widely used for optoelectronic application. Traditionally, CdS semiconductor nanoparticles have been regarded as attractive candidates for photoelectric applications due to their size-tunable optical and electronic properties, as well as their efficient multiple charge carrier generations. To enhance the photocurrent generated by the semiconductor materials, it is essential to retard the recombination of electron– hole species in the semiconductors by molecular electron-relay semiconductor structures or efficient electron-transport matrices, such as conductive polymer films, carbon nanotubes or graphene [2,12–14].
Motivated by these investigations, the aim of this work was to develop a rapid, simple and highly effective method for synthesis of CdS/Functionalized Graphene Sheets (CdS/FGS) nanocomposites. It is well known that glutathione, a thiol-containing tripeptide, (cL-glutamyl-cysteinyl-glycine), as the most abundant non-protein, low molecular-weight thiol source in most mammalian tissues, plays an important role in many biological functions involving antioxidant defense, signal transduction, and cell proliferation [15]. In this study, glutathione is used to guide the growth of CdS nanocrystals in an oriented approach as capping agent due to biomolecules with naturally defined chemical compositions and structures [16]. As a capping agent, glutathione prevented the aggregation of CdS/FGS nanocomposites [17]. The advantage of this approach is that it does not need much time and organic solvent when compared to the other reported methods [18–20]. Moreover, CdS NPs were directly decorated on the FG sheets, and no molecular linkers were used to bridge CdS and the FG nanosheets. Even more important, different cadmium precursors and sulfide precursors affected the size and crystalline structure of the CdS nanocrystals. This rapid and simple route in this study can be used to synthesize the other grapheme-based nanocomposites in the photoelectric applications.
2. Experimental section
⇑ Corresponding author at: School of Geography and Biological Information, Nanjing University of Posts and Telecommunications, Nanjing 210046, PR China. Tel./fax: +86 25 85866634. E-mail address: [email protected] (S. Yan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.173
Cadmium nitrate Cd(NO3)24H2O and Cadmium chloride CdCl22.5H2O were obtained from Shanghai Jinshan Tingxin Chemical Reagent Factory. Thiourea was from Chengdu Kelong Chemical Reagent Factory. Cadmium acetate dihydrate C4H6CdO42H2O, Sodium sulfide Na2S9H2O, and Sodium thiosulfate pentahydrate Na2S2O3-
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5H2O were purchased from Nanjing Chemical Reagent Co., Ltd. Glutathione were purchased from Sigma Co., Ltd. All the chemicals were used without further treatments. Ultrapure water used in the experiment was purified with the Millipore water-purification system (Milli-Q Academic). Graphite oxide (GO) was produced from natural graphite powders (universal grade, 99.985%) according to Hummers method. The dried GO was thermally exfoliated at 300 °C for 3 min under air atmosphere, and then it was subsequently treated at 900 °C in Ar for 3 h with a heating rate of 2 °C/min. The obtained samples were denoted as FGS. A certain amount of FGS were used as the matrix materials [21,22]. In a typical reaction, the solution used for the preparation of CdS/FGS nanocomposites was first composed of 0.5 mmol L 1 of cadmium nitrate Cd(NO3)24H2O, 1.5 mmol L 1 of Sodium sulfide, and 0.03 mmol L 1 of glutathione. Then, the 0.0090 g FGS was placed in the above Teflon-lined stainless-autoclave, which was finally then sealed and maintained at 180 °C for 3.5 h. After the reaction completed, the autoclave was cooled to room temperature naturally, the resulting solid products were centrifuged, washed with distilled water several times, and then dried in vacuum at 60 °C for 4 h.
3. Results and discussion X-ray diffraction (XRD) measurements were generally employed for the investigation of the phase and structure of the product. In this study, XRD patterns were recorded for the CdS nanocrystals and the typical CdS/FGS nanocomposites powder to investigate the influence of graphene on the crystallinity of CdS NPs. Fig. 1 displays the XRD patterns of the CdS and the typical CdS/FGS nanocomposites. The characteristic peaks at 26.60, 43.85, and 51.98 respectively correspond to (0 0 2), (1 1 0), and (1 1 2) planes of hexagonal-phase CdS crystals (JCPDS Card No. 41-1049) [23–28]. The results suggest that CdS NPs decorated on graphene sheets are in hexagonal form. The diffraction peaks are broad because the crystallite sizes of CdS NPs in the samples are relatively small [29]. The crystallinity of the CdS/FGS nanocomposites is close to that of CdS, indicating that the FGS supplies a platform in which the CdS NPs can nucleate and grow. The XRD results also clearly suggest that the addition of functionalized graphene sheets did not influence the crystal structure of phase CdS because of the relatively low amount of functionalized graphene sheets. The morphology of anchored CdS NPs onto FGS was evaluated by using SEM and TEM analysis. The CdS NPs are assembled and well distributed on the sheet of FG, as shown in Fig. 2a. Further evidence for the attachment of CdS NPs onto the FGS is provided by TEM, as shown in Fig. 2b. Many CdS NPs are present on the FG nanosheet, which has wrinkles and folded regions on the edge. The magnified high-resolution TEM (HRTEM) image of a distinct region revealed that the CdS NPs were successfully assembled onto the FG nanosheet, as shown in Fig. 2c. The size of the CdS NPs in CdS/FGS nanocomposites is around 20 nm as shown in Fig. 2c by
Fig. 2. (a) Typical field-emission SEM image of the as prepared CdS/FGS nanocomposites. (b) TEM image of the as prepared CdS/FGS nanocomposites. (c) The corresponding HRTEM image of the CdS/FGS nanocomposites.
Fig. 1. XRD patterns of the as-prepared CdS nanocrystals and the typical CdS/FGS nanocomposites samples.
the HRTEM image. HRTEM analysis also showed that the lattice fringe had an interplanar distance of 0.33 nm, which was assigned to the (0 0 2) plane of hexagonal CdS, which is fully consistent with the above XRD results. There is no apparent aggregation of CdS NPs on the grapheme sheets, nor large areas of the graphene sheets without CdS decoration. The good distribution and coverage of CdS NP on the FG sheets will guarantee efficient optoelectronic performance of the CdS/FGS nanocomposites [30]. As has been reported previously, nanoparticles may interact with graphene sheets through physisorption, electrostatic binding, or charge transfer interaction [31], and the exact mechanism is still under investigation.
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Fig. 3. (a) Full XPS spectrum of the CdS/FGS nanocomposites synthesized for 3.5 h at 180 °C. (b, c and d) High resolution XPS spectra of the CdS/FGS nanocomposites in the C 1s, Cd 3d, and S 2p.
Fig. 4. The products obtained with different cadmium source. (a) FESEM image of the product with cadmium chloride as cadmium source. (b) Its corresponding TEM image. (c and d) FESEM images of the products with cadmium acetate dehydrate as cadmium source.
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Fig. 5. The products obtained with different sulfide source. (a and b) FESEM image of the product with thiourea as sulfide source. (c) Its corresponding TEM image. (d) FESEM images of the products with sodium thiosulfate pentahydrate as sulfide source.
XPS analysis, as given in Fig. 3, was also employed to determine the chemical composition. Fig. 3a–d displays the full XPS spectrum and high-resolution spectra of the C(1s), Cd(3d), and S(2p) regions, respectively. Fig. 3a is the typical survey spectrum of the CdS/FGS nanocomposites, which indicates the presence of C, Cd, and S, as well as absorbed O on the sample surface. From the full XPS spectrum, the Cd/S ratio in the CdS/FGS nanocomposites can be estimated as 1.2:1. The high-resolution XPS in Fig. 2b gives the binding energies of C 1s to be 284.03 eV, which is close to the previous reported C–C group value for grapheme [3]. From Fig. 2c, the peaks at 404.62 and 411.44 eV are attributed to the 3d level of Cd atoms and indicate that cadmium was present as cadmium sulfur. Similarly, the S 2p peak is found at 161.16 eV, which implies that sulfur exists as sulfides from Fig. 2d. Our XPS results are in good agreement with reported values for CdS [17]. Different cadmium source were tested to reveal the cadmium source effect the final morphology of the products. In the experiment, when the cadmium chloride was dissolved into 40 ml water as cadmium source, the CdS are chrysanthemum-shaped (Fig. 4a). From Fig. 4b, it is found that the chrysanthemum-shaped CdS was composed of many small CdS nanocrystals by the TEM image. When the cadmium acetate dehydrate was selective as cadmium source, the product is similar with the one obtained from the typical experiment (Fig. 2a). Many CdS NPs are attached on the sheet of FG, as shown in Fig. 4c. The CdS NPs have diameter of 100 nm, which is larger than that of the typical experiment. In general, the solubility product constant (Ksp) for CdS particles is quite small, leading to fast nucleation and agglomeration of CdS nanocrystals [32]. When the cadmium acetate dehydrate was selective as cadmium source, the cadmium acetate dehydrate may regulate the nucleation rate of CdS particles by slowly releasing Cd2+ ions into solution, resulting in a much smaller crystallite size.
In our experiments, we also find that the sulfide source also plays an important role in the formation of the CdS/FGS nanocomposites. The thiourea is selective as sulfide source, the CdS are chrysanthemum-shaped (Fig. 5a), which is similar with the experiment result in Fig. 4a. Moreover, it is found that the chrysanthemum-shaped CdS was composed of many small CdS nanocrystals from the magnified FESEM image (Fig. 5b). The HRTEM image (Fig. 5c) shows that the lattice fringes of individual CdS nanocrystal with d spacing of ca. 0.33 can be assigned to the (0 0 2), lattice planes of the hexagonal CdS. When the sodium thiosulfate pentahydrate is uses as sulfide source, the product is CdS nanospheres. The all above results clearly indicate that the denseness and crystallinity of CdS nanocrystals decorated on FGS can be altered by controlling the reaction parameters, meaning that the solvothermal method is very effective to prepare and deposit nanoparticles on the surface of FGS. Of course, the photoelectric applications of the CdS/FGS nanocomposites are still under study.
4. Conclusions In conclusion, we have demonstrated a convenient hydrothermal method for the controllable preparation of the CdS/FGS nanocomposites. Characterizations reveal that tiny CdS NPs with an average diameter of 10 nm were homogeneously distributed on the FGS. Such a noncovalent functionalization not only gives rise to a graphene material decorated with small-sized NPs but also restrains the structural destruction of FGS. It was also found that the cadmium source and sulfide source played the important role in determining the final morphology. This simple but efficient strategy should allow us to prepare highly efficient graphene-based hybrids for potential applications in various fields.
S. Yan et al. / Journal of Alloys and Compounds 570 (2013) 65–69
Acknowledgments This work was financially supported by the National Basic Research Program of China (973 Program: 2013CB932903), the National Science Foundations of China (Nos. 61205057 and 61204050), China Postdoctoral Science special Foundation (2012T50488), China Postdoctoral Science Foundation (2011M500896), Jiangsu Planned Projects for Postdoctoral Research Funds (1102015C), Natural Science Foundation of Education Bureau of Jiangsu Province (12KJB180009), the Scientific Research Foundation of Nanjing University of Posts and Telecommunications (NY210083), the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University, and the open research fund of Key Laboratory of MEMS of Ministry of Education, Southeast University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2013.03.173. References [1] C. Colliex, Science 336 (2012) 44–45. [2] Y.Y. Huang, Y.J. Chen, C.L. Hu, B. Zhang, T. Shen, X.D. Chen, M.Q. Zhang, Journal of Materials Chemistry 22 (2012) 10999–11002. [3] T.Y. Peng, K. Li, P. Zeng, Q.G. Zhang, X.G. Zhang, Journal of Physical Chemistry C 116 (2012) 22720–22726. [4] K. Wang, Q. Liu, L. Dai, J.J. Yan, C. Ju, B.J. Qiu, X.Y. Wu, Analytica Chimica Acta 695 (2011) 84–88. [5] L. Britnell, R.V. Gorbachev, R. Jalil, B.D. Belle, F. Schedin, A. Mishchenko, T. Georgiou, M.I. Katsnelson, L. Eaves, S.V. Morozov, N.M.R. Peres, J. Leist, A.K. Geim, K.S. Novoselov, L.A. Ponomarenko, Science 335 (2012) 947–950. [6] M.F. El-Kady, V. Strong, S. Dubin, R.B. Kaner, Science 335 (2012) 1326–1330. [7] N.N. Klimov, S. Jung, S.Z. Zhu, T. Li, C.A. Wright, S.D. Solares, D.B. Newell, N.B. Zhitenev, J.A. Stroscio, Science 336 (2012) 1557–1561. [8] H. Yang, J. Heo, S. Park, H.J. Song, D.H. Seo, K.E. Byun, P. Kim, I. Yoo, H.J. Chung, K. Kim, Science 336 (2012) 1140–1143.
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[9] J.M. Yuk, J. Park, P. Ercius, K. Kim, D.J. Hellebusch, M.F. Crommie, J.Y. Lee, A. Zettl, A.P. Alivisatos, Science 336 (2012) 61–64. [10] J.L. Wu, S. Bai, X.P. Shen, L. Jiang, Applied Surface Science 257 (2010) 747–751. [11] K.J. Zhang, X.H. Liu, Applied Surface Science 257 (2011) 10379–10383. [12] J.H. Bang, P.V. Kamat, Advanced Functional Materials 20 (2010) 1970–1976. [13] N. Zhang, Y.H. Zhang, X.Y. Pan, M.Q. Yang, Y.J. Xu, Journal of Physical Chemistry C 116 (2012) 18023–18031. [14] E.Z. Shi, J.Q. Nie, X.J. Qin, Z.J. Li, L.H. Zhang, Z. Li, P.X. Li, Y. Jia, C.Y. Ji, J.Q. Wei, K.L. Wang, H.M. Zhu, D.H. Wu, Y. Li, Y. Fang, W.Z. Qian, F. Wei, A.Y. Cao, Energy & Environmental Science 5 (2012) 6119–6125. [15] X.M. Zhao, S.W. Zhou, Q.M. Shen, L.P. Jiang, J.J. Zhu, Analyst 137 (2012) 3697– 3703. [16] F. Chen, R.J. Zhou, L.G. Yang, M.M. Shi, G. Wu, M. Wang, H.Z. Chen, Journal of Physical Chemistry C 112 (2008) 13457–13462. [17] X.M. Zhao, S.W. Zhou, L.P. Jiang, W.H. Hou, Q.M. Shen, J.J. Zhu, Chemistry – A European Journal 18 (2012) 4974–4981. [18] N. Zhang, Y.H. Zhang, X.Y. Pan, X.Z. Pu, S.Q. Liu, Y.J. Xu, Journal of Physical Chemistry C 116 (2012) 4347. [19] X.W. Wang, H.W. Tian, Y. Yang, H. Wang, S.M. Wang, W.T. Zheng, Y.C. Liu, Journal of Alloys and Compounds 524 (2012) 5–12. [20] Q. Li, B.D. Guo, J.G. Yu, J.R. Ran, B.H. Zhang, H.J. Yan, J.R. Gong, Journal of the American Chemical Society 133 (2011) 10878–10884. [21] Q.L. Du, M.B. Zheng, L.F. Zhang, Y.W. Wang, J.H. Chen, L.P. Xue, W.J. Dai, G.B. Ji, J.M. Cao, Electrochimica Acta 55 (2010) 3897–3903. [22] P.V. Barr, E.J. Osinski, J.K. Brimacombe, M.A. Khan, P.J. Readyhough, Ironmaking & Steelmaking 21 (1994) 44–55. [23] J.Q. Qian, S.C. Yan, Z.D. Xiao, Journal of Colloid and Interface Science 366 (2012) 130–134. [24] S.C. Yan, D. Hu, F.H. Hu, J.S. Wu, N.P. Huang, Z.D. Xiao, CrystEngComm 13 (2011) 4580–4585. [25] S.C. Yan, D. Hu, J.S. Wu, X. Xu, J. Wang, Z.D. Xiao, Journal of Alloys and Compounds 509 (2011) L239–L243. [26] S.C. Yan, K. Shen, X. Xu, Y. Shi, J.S. Wu, Z.D. Xiao, Synthetic Metals 161 (2011) 1646–1650. [27] S.C. Yan, L.T. Sun, Y. Sheng, N.P. Huang, Z.D. Xiao, New Journal of Chemistry 35 (2011) 299–302. [28] J. Chu, X. Li, J.Y. Qi, CrystEngComm 14 (2012) 1881–1884. [29] Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan, J.R. Gong, Journal of the American Chemical Society 133 (2011) 10878–10884. [30] A.N. Cao, Z. Liu, S.S. Chu, M.H. Wu, Z.M. Ye, Z.W. Cai, Y.L. Chang, S.F. Wang, Q.H. Gong, Y.F. Liu, Advanced Materials 22 (2010). 103. [31] G. Williams, B. Seger, P.V. Kamat, ACS Nano 2 (2008) 1487–1491. [32] N. Bao, L. Shen, T. Takata, K. Domen, Chemistry of Materials 20 (2008) 110– 117.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Hydrothermal synthesis of CdS/functionalized graphene sheets nanocomposites Shancheng Yan a,b,⇑, Yi Shi b, Bin Zhao b, Tao Lu b, Dong Hu a, Xin Xu a, Jiansheng Wu a, Jingsong Chen a a b
School of Geography and Biological Information, Nanjing University of Posts and Telecommunications, Nanjing 210046, PR China National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 10 March 2013 Accepted 19 March 2013 Available online 27 March 2013 Keywords: Functionalized graphene sheets Cadmium sulfide Nanocomposites Nanoparticles
a b s t r a c t CdS/Functionalized Graphene Sheets (CdS/FGSs) nanocomposites were successfully prepared in a one-step hydrothermal synthesis route. The morphology, structure characterization, and crystal structure of the asprepared nanocomposites were performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Through controlled experimental conditions, such as cadmium source and sulfide source, the different morphologies and dimensions of the final products have been controlled. The synthetic approach was simple and fast, and it may be extended for the synthesis of other FGS–metal-sulfide nanocomposites. By combining the unique properties of FGS and CdS nanoparticles, the strategy presented in this study is expected to be useful to prepare highly efficient graphene-based hybrids for potential applications in various fields. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the assembly of semiconductor nanoparticles (NPs) on matrices has been extensively studied for their promising optoelectronic applications [1–3]. Graphene, as a new two-dimensional carbon nanomaterial, has received increasing attention during recent years by virtue of its outstanding physical, chemical properties and excellent electrocatalytic ability [4–9]. It has been revealed that when incorporated semiconductor nanoparticles into matrices, graphene can remarkably improve the properties of these host materials [10,11]. All these above indicate that the graphenebased nanocomposites exhibit more excellent performance than pure graphene in optoelectronic fabrication, and as a new kind of nanocomposite materials, they have been widely used for optoelectronic application. Traditionally, CdS semiconductor nanoparticles have been regarded as attractive candidates for photoelectric applications due to their size-tunable optical and electronic properties, as well as their efficient multiple charge carrier generations. To enhance the photocurrent generated by the semiconductor materials, it is essential to retard the recombination of electron– hole species in the semiconductors by molecular electron-relay semiconductor structures or efficient electron-transport matrices, such as conductive polymer films, carbon nanotubes or graphene [2,12–14].
Motivated by these investigations, the aim of this work was to develop a rapid, simple and highly effective method for synthesis of CdS/Functionalized Graphene Sheets (CdS/FGS) nanocomposites. It is well known that glutathione, a thiol-containing tripeptide, (cL-glutamyl-cysteinyl-glycine), as the most abundant non-protein, low molecular-weight thiol source in most mammalian tissues, plays an important role in many biological functions involving antioxidant defense, signal transduction, and cell proliferation [15]. In this study, glutathione is used to guide the growth of CdS nanocrystals in an oriented approach as capping agent due to biomolecules with naturally defined chemical compositions and structures [16]. As a capping agent, glutathione prevented the aggregation of CdS/FGS nanocomposites [17]. The advantage of this approach is that it does not need much time and organic solvent when compared to the other reported methods [18–20]. Moreover, CdS NPs were directly decorated on the FG sheets, and no molecular linkers were used to bridge CdS and the FG nanosheets. Even more important, different cadmium precursors and sulfide precursors affected the size and crystalline structure of the CdS nanocrystals. This rapid and simple route in this study can be used to synthesize the other grapheme-based nanocomposites in the photoelectric applications.
2. Experimental section
⇑ Corresponding author at: School of Geography and Biological Information, Nanjing University of Posts and Telecommunications, Nanjing 210046, PR China. Tel./fax: +86 25 85866634. E-mail address: [email protected] (S. Yan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.173
Cadmium nitrate Cd(NO3)24H2O and Cadmium chloride CdCl22.5H2O were obtained from Shanghai Jinshan Tingxin Chemical Reagent Factory. Thiourea was from Chengdu Kelong Chemical Reagent Factory. Cadmium acetate dihydrate C4H6CdO42H2O, Sodium sulfide Na2S9H2O, and Sodium thiosulfate pentahydrate Na2S2O3-
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5H2O were purchased from Nanjing Chemical Reagent Co., Ltd. Glutathione were purchased from Sigma Co., Ltd. All the chemicals were used without further treatments. Ultrapure water used in the experiment was purified with the Millipore water-purification system (Milli-Q Academic). Graphite oxide (GO) was produced from natural graphite powders (universal grade, 99.985%) according to Hummers method. The dried GO was thermally exfoliated at 300 °C for 3 min under air atmosphere, and then it was subsequently treated at 900 °C in Ar for 3 h with a heating rate of 2 °C/min. The obtained samples were denoted as FGS. A certain amount of FGS were used as the matrix materials [21,22]. In a typical reaction, the solution used for the preparation of CdS/FGS nanocomposites was first composed of 0.5 mmol L 1 of cadmium nitrate Cd(NO3)24H2O, 1.5 mmol L 1 of Sodium sulfide, and 0.03 mmol L 1 of glutathione. Then, the 0.0090 g FGS was placed in the above Teflon-lined stainless-autoclave, which was finally then sealed and maintained at 180 °C for 3.5 h. After the reaction completed, the autoclave was cooled to room temperature naturally, the resulting solid products were centrifuged, washed with distilled water several times, and then dried in vacuum at 60 °C for 4 h.
3. Results and discussion X-ray diffraction (XRD) measurements were generally employed for the investigation of the phase and structure of the product. In this study, XRD patterns were recorded for the CdS nanocrystals and the typical CdS/FGS nanocomposites powder to investigate the influence of graphene on the crystallinity of CdS NPs. Fig. 1 displays the XRD patterns of the CdS and the typical CdS/FGS nanocomposites. The characteristic peaks at 26.60, 43.85, and 51.98 respectively correspond to (0 0 2), (1 1 0), and (1 1 2) planes of hexagonal-phase CdS crystals (JCPDS Card No. 41-1049) [23–28]. The results suggest that CdS NPs decorated on graphene sheets are in hexagonal form. The diffraction peaks are broad because the crystallite sizes of CdS NPs in the samples are relatively small [29]. The crystallinity of the CdS/FGS nanocomposites is close to that of CdS, indicating that the FGS supplies a platform in which the CdS NPs can nucleate and grow. The XRD results also clearly suggest that the addition of functionalized graphene sheets did not influence the crystal structure of phase CdS because of the relatively low amount of functionalized graphene sheets. The morphology of anchored CdS NPs onto FGS was evaluated by using SEM and TEM analysis. The CdS NPs are assembled and well distributed on the sheet of FG, as shown in Fig. 2a. Further evidence for the attachment of CdS NPs onto the FGS is provided by TEM, as shown in Fig. 2b. Many CdS NPs are present on the FG nanosheet, which has wrinkles and folded regions on the edge. The magnified high-resolution TEM (HRTEM) image of a distinct region revealed that the CdS NPs were successfully assembled onto the FG nanosheet, as shown in Fig. 2c. The size of the CdS NPs in CdS/FGS nanocomposites is around 20 nm as shown in Fig. 2c by
Fig. 2. (a) Typical field-emission SEM image of the as prepared CdS/FGS nanocomposites. (b) TEM image of the as prepared CdS/FGS nanocomposites. (c) The corresponding HRTEM image of the CdS/FGS nanocomposites.
Fig. 1. XRD patterns of the as-prepared CdS nanocrystals and the typical CdS/FGS nanocomposites samples.
the HRTEM image. HRTEM analysis also showed that the lattice fringe had an interplanar distance of 0.33 nm, which was assigned to the (0 0 2) plane of hexagonal CdS, which is fully consistent with the above XRD results. There is no apparent aggregation of CdS NPs on the grapheme sheets, nor large areas of the graphene sheets without CdS decoration. The good distribution and coverage of CdS NP on the FG sheets will guarantee efficient optoelectronic performance of the CdS/FGS nanocomposites [30]. As has been reported previously, nanoparticles may interact with graphene sheets through physisorption, electrostatic binding, or charge transfer interaction [31], and the exact mechanism is still under investigation.
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Fig. 3. (a) Full XPS spectrum of the CdS/FGS nanocomposites synthesized for 3.5 h at 180 °C. (b, c and d) High resolution XPS spectra of the CdS/FGS nanocomposites in the C 1s, Cd 3d, and S 2p.
Fig. 4. The products obtained with different cadmium source. (a) FESEM image of the product with cadmium chloride as cadmium source. (b) Its corresponding TEM image. (c and d) FESEM images of the products with cadmium acetate dehydrate as cadmium source.
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Fig. 5. The products obtained with different sulfide source. (a and b) FESEM image of the product with thiourea as sulfide source. (c) Its corresponding TEM image. (d) FESEM images of the products with sodium thiosulfate pentahydrate as sulfide source.
XPS analysis, as given in Fig. 3, was also employed to determine the chemical composition. Fig. 3a–d displays the full XPS spectrum and high-resolution spectra of the C(1s), Cd(3d), and S(2p) regions, respectively. Fig. 3a is the typical survey spectrum of the CdS/FGS nanocomposites, which indicates the presence of C, Cd, and S, as well as absorbed O on the sample surface. From the full XPS spectrum, the Cd/S ratio in the CdS/FGS nanocomposites can be estimated as 1.2:1. The high-resolution XPS in Fig. 2b gives the binding energies of C 1s to be 284.03 eV, which is close to the previous reported C–C group value for grapheme [3]. From Fig. 2c, the peaks at 404.62 and 411.44 eV are attributed to the 3d level of Cd atoms and indicate that cadmium was present as cadmium sulfur. Similarly, the S 2p peak is found at 161.16 eV, which implies that sulfur exists as sulfides from Fig. 2d. Our XPS results are in good agreement with reported values for CdS [17]. Different cadmium source were tested to reveal the cadmium source effect the final morphology of the products. In the experiment, when the cadmium chloride was dissolved into 40 ml water as cadmium source, the CdS are chrysanthemum-shaped (Fig. 4a). From Fig. 4b, it is found that the chrysanthemum-shaped CdS was composed of many small CdS nanocrystals by the TEM image. When the cadmium acetate dehydrate was selective as cadmium source, the product is similar with the one obtained from the typical experiment (Fig. 2a). Many CdS NPs are attached on the sheet of FG, as shown in Fig. 4c. The CdS NPs have diameter of 100 nm, which is larger than that of the typical experiment. In general, the solubility product constant (Ksp) for CdS particles is quite small, leading to fast nucleation and agglomeration of CdS nanocrystals [32]. When the cadmium acetate dehydrate was selective as cadmium source, the cadmium acetate dehydrate may regulate the nucleation rate of CdS particles by slowly releasing Cd2+ ions into solution, resulting in a much smaller crystallite size.
In our experiments, we also find that the sulfide source also plays an important role in the formation of the CdS/FGS nanocomposites. The thiourea is selective as sulfide source, the CdS are chrysanthemum-shaped (Fig. 5a), which is similar with the experiment result in Fig. 4a. Moreover, it is found that the chrysanthemum-shaped CdS was composed of many small CdS nanocrystals from the magnified FESEM image (Fig. 5b). The HRTEM image (Fig. 5c) shows that the lattice fringes of individual CdS nanocrystal with d spacing of ca. 0.33 can be assigned to the (0 0 2), lattice planes of the hexagonal CdS. When the sodium thiosulfate pentahydrate is uses as sulfide source, the product is CdS nanospheres. The all above results clearly indicate that the denseness and crystallinity of CdS nanocrystals decorated on FGS can be altered by controlling the reaction parameters, meaning that the solvothermal method is very effective to prepare and deposit nanoparticles on the surface of FGS. Of course, the photoelectric applications of the CdS/FGS nanocomposites are still under study.
4. Conclusions In conclusion, we have demonstrated a convenient hydrothermal method for the controllable preparation of the CdS/FGS nanocomposites. Characterizations reveal that tiny CdS NPs with an average diameter of 10 nm were homogeneously distributed on the FGS. Such a noncovalent functionalization not only gives rise to a graphene material decorated with small-sized NPs but also restrains the structural destruction of FGS. It was also found that the cadmium source and sulfide source played the important role in determining the final morphology. This simple but efficient strategy should allow us to prepare highly efficient graphene-based hybrids for potential applications in various fields.
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Acknowledgments This work was financially supported by the National Basic Research Program of China (973 Program: 2013CB932903), the National Science Foundations of China (Nos. 61205057 and 61204050), China Postdoctoral Science special Foundation (2012T50488), China Postdoctoral Science Foundation (2011M500896), Jiangsu Planned Projects for Postdoctoral Research Funds (1102015C), Natural Science Foundation of Education Bureau of Jiangsu Province (12KJB180009), the Scientific Research Foundation of Nanjing University of Posts and Telecommunications (NY210083), the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University, and the open research fund of Key Laboratory of MEMS of Ministry of Education, Southeast University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2013.03.173. References [1] C. Colliex, Science 336 (2012) 44–45. [2] Y.Y. Huang, Y.J. Chen, C.L. Hu, B. Zhang, T. Shen, X.D. Chen, M.Q. Zhang, Journal of Materials Chemistry 22 (2012) 10999–11002. [3] T.Y. Peng, K. Li, P. Zeng, Q.G. Zhang, X.G. Zhang, Journal of Physical Chemistry C 116 (2012) 22720–22726. [4] K. Wang, Q. Liu, L. Dai, J.J. Yan, C. Ju, B.J. Qiu, X.Y. Wu, Analytica Chimica Acta 695 (2011) 84–88. [5] L. Britnell, R.V. Gorbachev, R. Jalil, B.D. Belle, F. Schedin, A. Mishchenko, T. Georgiou, M.I. Katsnelson, L. Eaves, S.V. Morozov, N.M.R. Peres, J. Leist, A.K. Geim, K.S. Novoselov, L.A. Ponomarenko, Science 335 (2012) 947–950. [6] M.F. El-Kady, V. Strong, S. Dubin, R.B. Kaner, Science 335 (2012) 1326–1330. [7] N.N. Klimov, S. Jung, S.Z. Zhu, T. Li, C.A. Wright, S.D. Solares, D.B. Newell, N.B. Zhitenev, J.A. Stroscio, Science 336 (2012) 1557–1561. [8] H. Yang, J. Heo, S. Park, H.J. Song, D.H. Seo, K.E. Byun, P. Kim, I. Yoo, H.J. Chung, K. Kim, Science 336 (2012) 1140–1143.
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