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Synthesis and characterization of ZnS nanowires by AOT micelle-template inducing reaction
Materials Research Bulletin 39 (2004) 1517–1524
Synthesis and characterization of ZnS nanowires by AOT micelle-template inducing reaction Ruitao Lv, Chuanbao Cao*, Hesun Zhu Material Science Research Center, Beijing Institute of Technology, No. 5, Zhongguancun South Street, Beijing 100081, PR China Received 16 May 2003; received in revised form 6 April 2004; accepted 12 April 2004
Abstract ZnS nanowires, with diameters around 30 nm and lengths up to 2.5 mm, had been successfully synthesized from solutions containing an anionic surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT). Powder X-ray diffraction (XRD) pattern, energy-dispersive X-ray spectroscopy (EDS) and selected-area electron diffraction (SAED) pattern indicated that the product was pure polycrystalline cubic-phase b-ZnS. The morphology and size of the as-synthesized product were determined by the transmission electron microscopy (TEM). The effects of some of the key reaction parameters (such as the ratio of surfactant to water, the reactant concentration and reaction temperature, etc.) had been explored in this paper. A growth mechanism of ZnS nanowires by micelletemplate inducing reaction was also proposed. # 2004 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductor; A. Nanostructures; B. Chemical synthesis; C. X-ray diffraction
1. Introduction Quasi one-dimensional (1D) nanoscale materials, such as nanowires (nanorods) and nanotubes, have attracted much attention due to their intriguing properties [1–6]. These new nanoscale materials are expected to have many potential applications in both mesoscopic research and development of nanodevice [7]. During the last decade, a considerable effort has been spent in the preparation and investigation of the group of wide-bandgap II–VI group nanoscale semiconductors due to their important optoelectronic application for laser light-emitting diodes and optical devices based on electronic and optical properties [8]. For instance, Xu et al. and Routkevitch et al. [9,10] have fabricated CdS nanowires in porous anodic aluminum membranes (AAM) by electrochemical *
Corresponding author. Tel.: þ86-106-891-3792; fax: þ86-106-891-5023. E-mail address: [email protected] (C. Cao).
0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2004.04.019
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R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
deposition. Wang et al. [11] prepared ZnS nanowires by thermal evaporation of ZnS powders onto a silicon substrate with the presence of Au catalyst. Meanwhile, Qian’s group established a new kind of chemical techniques to synthesize CdS and ZnS nanorods via solvothermal route and polymercontrolled growth [12,13]. However, it was difficult to separate CdS or ZnS nanowires (nanorods) from the products synthesized by the above mentioned methods, and moreover their reaction conditions are usually rigorous, such as high temperature and high pressure, etc. Herein we report a new, simple and low cost process to synthesize ZnS nanowires based on AOT micelle-template inducing reaction. Surfactant micelle-template inducing reaction is a particularly efficient method to prepare 1D nanoscale materials. Surfactant micellar solutions are isotropic, thermodynamically stable system in which the oil phase is dispersed as nano-sized droplets surrounded by a monolayer of surfactant molecules in the continuous water phase. The morphologies of the micelle are different with the variance of surfactant concentration. When the concentration of the surfactant is 10 times above the critical micelle concentration (CMC), the surfactant will form both hydrophobic and hydrophilic rodlike micelle [14]. The rod-like micelle plays important roles of microreactors and templates to induce the growth of nanowires. We have successfully synthesized ZnS nanowires by this method. It could be used to synthesize other II–VI group semiconductor nanowires.
2. Experimental The reactants and solvents used in our reaction system are all analytical grade and used without any further purification. In a typical preparation of ZnS nanowires, the procedure was as follows: To a 250 ml flask containing solution of AOT (2.0005 g, 4.5 mmol) in 25 ml cyclohexane, zinc acetate (Zn(AC)2, 2.2172 g, 10 mmol) was added and stirred rigorously to form a good suspension A. To a 100 ml beaker containing solution of thiourea (0.7689 g, 10 mmol) with 40 ml distilled water, identical weight of NaOH was added to form a solution B. Then, solution B was introduced dropwise under constant stirring, to the suspension A at 40 8C. The flask was heated from 40 to 75 8C, and refluxed at 75 8C for 24 h and left overnight. The precipitate was filtered off, washed with distilled water and absolute ethanol for several times, then dried in vacuum at 60 8C for 4 h. The final white powder was collected for characterizations. XRD data were got from a Japan Rigaku D/max RB X-ray diffractometer (Cu Ka radiation, l ¼ 0:15418 nm). The morphology was observed by transmission electron microscopy. The TEM images, EDS and SAED were taken by a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. The samples used for TEM observations were prepared by dispersing some products in absolute ethanol followed by ultrasonic vibration for 10 min, then placing a drop of dispersion onto a copper grid coated with a layer of amorphous carbon.
3. Results and discussion Fig. 1 shows the XRD pattern of ZnS nanowires synthesized by micelle-template inducing reaction in the presence of an anionic surfactant, AOT. The data were in good agreement with that of pure cubic-phase b-ZnS (JCPDS No.: 01-0792). The three strong peaks with 2y values of 28.72, 47.84, 56.688 corresponded to the three crystal planes of
R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
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Fig. 1. The XRD pattern of as-synthesized ZnS nanowires.
(1 1 1), (2 2 0), (3 1 1) of cubic-phase b-ZnS, respectively. The broadening of the diffraction peaks was due to the small diameter size (about 30 nm) of the as-synthesized nanowires. As mentioned above, the morphologies of the micelle are different with the variance of surfactant concentration. We defined R as the ratio of the molar number of AOT to the volume of water. Therefore, it is necessary to find an appropriate R value. In our synthetic system, the choice of R ¼ 0:11 mmol/ml was proved to be able to obtain uniform nanowires with diameters of about 30 nm and lengths up to 2.5 mm, as shown in Fig. 2a. The inset is the SAED pattern of the square part in Fig. 2a. The rings in the ˚ and d(2 2 0) ¼ 1.9012 A ˚, inset from the innermost to the outside corresponded to d(1 1 1) ¼ 3.1082 A respectively. Fig. 2b showed an individual nanowire with length about 1.5 mm. However, according to Marchand et al.’s report [15], the mean diameter of AOT micelles is around 20 nm, which is smaller than that of as-obtained nanowires. We conjecture that the diameters of nanowires are not confined strictly by the inner diameters of AOT rod-like micelles. Due to the flexibility of micelle interface, ZnS nanowires will grow continually along the radial and axial direction at the basis of initially formed ZnS cores. As a result, ZnS nanowires will become longer and thicker to some extent than the initial AOT rod-like micelles. Fig. 2c was a magnified image of the rectangular part in Fig. 2b. The nanowire appeared to be polycrystalline as shown by the SAED pattern in the inset of Fig. 2c. The EDS of the nanowires in Fig. 2a was shown in Fig. 2d. Quantitative analysis indicated that the atomic ratio of Zn:S 1:1, which is close to the stoichiometric ratio of ZnS. In the following experiments, R value would be remained 0.11 mmol/ml unless specified. Other R values varying from 0.70 to 0 mmol/ml were also tested in order to investigate the shape and size evolution of ZnS nanocrystals in AOT micelle. Irregular large floccules were observed at R ¼ 0:70 mmol/ml. With less surfactant content addition to R ¼ 0:25 mmol/ml, two kinds of morphologies would be observed by TEM (Fig. 3c). Some nanorods with diameters ranging in 22–44 nm and lengths up to 440 nm were found coexisting with amorphous product. By decreasing surfactant content to R ¼ 0:08 mmol/ml, more thicker nanorods with diameters ranging from 86 to
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Fig. 2. The TEM images of as-synthesized ZnS nanowires with R ¼ 0:11 mmol/ml, at 75 8C. (a) Typical morphology of ZnS nanowires, the inset is the SAED pattern of the square part in (a). (b) An individual nanowire with length about 1.5 mm. (c) A magnified image of the rectangular part in (b), the inset is the SAED pattern of as-obtained nanowire. (d) The EDS of the nanowires in (a).
R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
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Fig. 3. The TEM images of as-synthesized ZnS nanowires with R ¼ 0:03 (a), 0.08 (b) and 0.25 mmol/ml (c), respectively.
114 nm would be synthesized (Fig. 3b). But their lengths were shorter than that synthesized at R ¼ 0:11 mmol/ml (Fig. 1a). This phenomenon could be attributed to the shape evolution of AOT micelle with the variance of AOT content. With the increase of surfactant content to some extent, longer and thinner rod-like micelle might form. At R ¼ 0:03 mmol/ml, product with floccule-like morphology would be observed (Fig. 3a). Under further reduction of AOT to R ¼ 0 mmol/ml, only floccules deposited in large amounts and no nanorods or nanowires were observed. From the experimental facts in the context, we could find out that no systematic shape and size evolution tendency was captured with the variation of R value. This indicates the diversity of the nanostructure and the phase transition of the microemulsion solution when the water content is adjusted. However, R could be optimized to obtain uniform nanowires, suggesting the existence of relatively stable rod-like nanochannels from spherical micelles. The contribution of reactant concentration to the morphology of final ZnS product was also investigated, by selecting [Zn2þ] ¼ [S2] ¼ 0.15 mol/l (Fig. 2a), 0.05 and 0.40 mol/l. Other reaction parameters were all fixed unless specified. When the reactant concentrations were lowered to 0.05 mol/ l, amorphous product was observed coexisting with a very small quantity of nanorods(Fig. 4a). When the concentrations were increased to 0.40 mol/l, quasi-spherical particles were the dominant morphology, and no nanorods or nanowires were found on the Cu grid (Fig. 4b). Therefore, appropriate reactant concentration, e.g. 0.15 mol/l, is favorable to the formation of uniform nanowires.
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Fig. 4. The TEM images of as-synthesized ZnS nanowires with [Zn2þ] ¼ [S2] ¼ 0.05 (a) and 0.40 mol/l (b), respectively.
Lower of higher concentration seems to go against the nucleation and growth of regular products, as pointed out by Xu and Li [16]. Reaction temperature is also one of the key reaction parameters. We performed similar experiment at room temperature and kept other conditions the same as in Fig. 2a. As shown in Fig. 5 and its inset, shorter and thinner polycrystalline nanorods would be got at room temperature, which was different from that synthesized at 75 8C (Fig. 2a). This might be attributed to the shape transition of the micelle, but further experimental data are needed. According to above experimental results, the proposed mechanism of synthesis of ZnS nanowires by micelle-template inducing reaction might be as follows: At appropriate surfactant concentration and moderate temperature, AOT rod-like micelle might form. Thiourea in the water phase might be enwrapped in the rod-like micelle with constant stirring, and Zn2þ will transfer to the surface of thiourea through the interface of micelle because of the concentration difference. Thiourea may act as a bidentate ligand to form relatively stable Zn–thiourea complexes with
Fig. 5. The TEM images of as-synthesized ZnS nanowires at room temperature, the inset is the corresponding SAED pattern.
R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
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Zn2þ, which prevent the production of more free S2 ions in the reaction system, and will be favorable for the formation of the nanowires. The stability of the Zn–thiourea complexes is expected to decrease with the increase of the temperature. At some relatively high temperature (e.g. above 70 8C) and enough long aging time, the Zn–thiourea complexes may undergo a slow decomposition process to produce long ZnS nanowire. In this process, the AOT micelle may act as templates to induce the growth of ZnS nanowires. The formation of ZnS nanowires can be expressed as the following [13]:
Therefore, the formation of uniform nanowires needs appropriate temperature and suitable surfactant concentration. With regard to the coexistence of different morphologies with different sizes, it could be proposed that the growth might take place in different reaction environments [16]. The mechanisms proposed above need to be tested by further experiments.
4. Conclusions In conclusion, ZnS nanowires have been successfully synthesized by a simple, low-cost, efficient method. XRD, EDS and SAED pattern shows that as-synthesized products are pure cubic-phase b-ZnS. TEM pattern shows that as-synthesized ZnS nanowires have diameters around 30 nm and lengths up to 2.5 mm. Investigations show that suitable ratio of surfactant molar number to the volume of water (e.g. 0.11 mmol/ml), moderate reactant concentration (e.g. [Zn2þ] ¼ [S2] ¼ 0.15 mol/l) and appropriate temperature (e.g. 75 8C) are necessary for the growth of long and uniform ZnS nanowires. Considering the simplicity of the procedure, micelle-template inducing reaction here is likely to be of a good method to synthesize other II–VI group semiconductor nanowires.
Acknowledgements This work was supported by the research fund for the Doctoral Program of Higher Education of China (Grant No.: 20020007029) and BIT Basic Research Fund. References [1] W. Wong, E. Sheehan, M. Lieber, Science 277 (1997) 1971. [2] J.Y. Li, X.L. Chen, H. Li, M. He, Z.Y. Qiao, J. Cryst. Growth 233 (2001) 5.
1524 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59. M. Li, H. Schnablegger, S. Mann, Nature 402 (1999) 393. G. Chopra, R.J. Luyken, K. Cherrey, H. Crespi, L. Cohen, G. Louie, A. Zettl, Science 269 (1995) 966. B.A. Korgel, D. Fitzmaurice, Adv. Mater. 10 (1998) 661. P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Science 278 (1997) 100. Y.-T. Chen, J.-B. Ding, Y. Guo, L.-B. Kong, H.-L. Li, Mater. Chem. Phys. 77 (2002) 734. D. Xu, Y. Xu, D. Chen, G. Guo, L. Gui, Y. Tang, Adv. Mater. 12 (2000) 520. D. Routkevitch, T. Bigioni, M. Moskovits, M. Xu, J. Phys. Chem. 100 (1996) 14037. Y. Wang, L. Zhang, C. Liang, G. Wang, X. Peng, Chem. Phys. Lett. 357 (2002) 314. J. Yang, J.-H. Zeng, S.-H. Yu, L. Yang, G. Zhou, Y. Qian, Chem. Mater. 12 (2000) 3259. Y.-D. Li, H.-W. Liao, Y. Ding, Y.-T. Qian, L. Yang, G.-E. Zhou, Chem. Mater. 10 (1998) 2301. Y. Xiong, Y. Xie, J. Yang, R. Zhang, C. Wu, G. Du, J. Mater. Chem. 12 (2002) 3712. K.E. Marchand, M. Tarret, J.P. Lechaire, L. Normand, S. Kasztelan, T. Cseri, Coll. Surf. A Physicochem. Eng. Aspects 214 (2003) 239. [16] J. Xu, L. Yadong, J. Coll. Interf. Sci. 259 (2003) 275.
Synthesis and characterization of ZnS nanowires by AOT micelle-template inducing reaction Ruitao Lv, Chuanbao Cao*, Hesun Zhu Material Science Research Center, Beijing Institute of Technology, No. 5, Zhongguancun South Street, Beijing 100081, PR China Received 16 May 2003; received in revised form 6 April 2004; accepted 12 April 2004
Abstract ZnS nanowires, with diameters around 30 nm and lengths up to 2.5 mm, had been successfully synthesized from solutions containing an anionic surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT). Powder X-ray diffraction (XRD) pattern, energy-dispersive X-ray spectroscopy (EDS) and selected-area electron diffraction (SAED) pattern indicated that the product was pure polycrystalline cubic-phase b-ZnS. The morphology and size of the as-synthesized product were determined by the transmission electron microscopy (TEM). The effects of some of the key reaction parameters (such as the ratio of surfactant to water, the reactant concentration and reaction temperature, etc.) had been explored in this paper. A growth mechanism of ZnS nanowires by micelletemplate inducing reaction was also proposed. # 2004 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductor; A. Nanostructures; B. Chemical synthesis; C. X-ray diffraction
1. Introduction Quasi one-dimensional (1D) nanoscale materials, such as nanowires (nanorods) and nanotubes, have attracted much attention due to their intriguing properties [1–6]. These new nanoscale materials are expected to have many potential applications in both mesoscopic research and development of nanodevice [7]. During the last decade, a considerable effort has been spent in the preparation and investigation of the group of wide-bandgap II–VI group nanoscale semiconductors due to their important optoelectronic application for laser light-emitting diodes and optical devices based on electronic and optical properties [8]. For instance, Xu et al. and Routkevitch et al. [9,10] have fabricated CdS nanowires in porous anodic aluminum membranes (AAM) by electrochemical *
Corresponding author. Tel.: þ86-106-891-3792; fax: þ86-106-891-5023. E-mail address: [email protected] (C. Cao).
0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2004.04.019
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R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
deposition. Wang et al. [11] prepared ZnS nanowires by thermal evaporation of ZnS powders onto a silicon substrate with the presence of Au catalyst. Meanwhile, Qian’s group established a new kind of chemical techniques to synthesize CdS and ZnS nanorods via solvothermal route and polymercontrolled growth [12,13]. However, it was difficult to separate CdS or ZnS nanowires (nanorods) from the products synthesized by the above mentioned methods, and moreover their reaction conditions are usually rigorous, such as high temperature and high pressure, etc. Herein we report a new, simple and low cost process to synthesize ZnS nanowires based on AOT micelle-template inducing reaction. Surfactant micelle-template inducing reaction is a particularly efficient method to prepare 1D nanoscale materials. Surfactant micellar solutions are isotropic, thermodynamically stable system in which the oil phase is dispersed as nano-sized droplets surrounded by a monolayer of surfactant molecules in the continuous water phase. The morphologies of the micelle are different with the variance of surfactant concentration. When the concentration of the surfactant is 10 times above the critical micelle concentration (CMC), the surfactant will form both hydrophobic and hydrophilic rodlike micelle [14]. The rod-like micelle plays important roles of microreactors and templates to induce the growth of nanowires. We have successfully synthesized ZnS nanowires by this method. It could be used to synthesize other II–VI group semiconductor nanowires.
2. Experimental The reactants and solvents used in our reaction system are all analytical grade and used without any further purification. In a typical preparation of ZnS nanowires, the procedure was as follows: To a 250 ml flask containing solution of AOT (2.0005 g, 4.5 mmol) in 25 ml cyclohexane, zinc acetate (Zn(AC)2, 2.2172 g, 10 mmol) was added and stirred rigorously to form a good suspension A. To a 100 ml beaker containing solution of thiourea (0.7689 g, 10 mmol) with 40 ml distilled water, identical weight of NaOH was added to form a solution B. Then, solution B was introduced dropwise under constant stirring, to the suspension A at 40 8C. The flask was heated from 40 to 75 8C, and refluxed at 75 8C for 24 h and left overnight. The precipitate was filtered off, washed with distilled water and absolute ethanol for several times, then dried in vacuum at 60 8C for 4 h. The final white powder was collected for characterizations. XRD data were got from a Japan Rigaku D/max RB X-ray diffractometer (Cu Ka radiation, l ¼ 0:15418 nm). The morphology was observed by transmission electron microscopy. The TEM images, EDS and SAED were taken by a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. The samples used for TEM observations were prepared by dispersing some products in absolute ethanol followed by ultrasonic vibration for 10 min, then placing a drop of dispersion onto a copper grid coated with a layer of amorphous carbon.
3. Results and discussion Fig. 1 shows the XRD pattern of ZnS nanowires synthesized by micelle-template inducing reaction in the presence of an anionic surfactant, AOT. The data were in good agreement with that of pure cubic-phase b-ZnS (JCPDS No.: 01-0792). The three strong peaks with 2y values of 28.72, 47.84, 56.688 corresponded to the three crystal planes of
R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
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Fig. 1. The XRD pattern of as-synthesized ZnS nanowires.
(1 1 1), (2 2 0), (3 1 1) of cubic-phase b-ZnS, respectively. The broadening of the diffraction peaks was due to the small diameter size (about 30 nm) of the as-synthesized nanowires. As mentioned above, the morphologies of the micelle are different with the variance of surfactant concentration. We defined R as the ratio of the molar number of AOT to the volume of water. Therefore, it is necessary to find an appropriate R value. In our synthetic system, the choice of R ¼ 0:11 mmol/ml was proved to be able to obtain uniform nanowires with diameters of about 30 nm and lengths up to 2.5 mm, as shown in Fig. 2a. The inset is the SAED pattern of the square part in Fig. 2a. The rings in the ˚ and d(2 2 0) ¼ 1.9012 A ˚, inset from the innermost to the outside corresponded to d(1 1 1) ¼ 3.1082 A respectively. Fig. 2b showed an individual nanowire with length about 1.5 mm. However, according to Marchand et al.’s report [15], the mean diameter of AOT micelles is around 20 nm, which is smaller than that of as-obtained nanowires. We conjecture that the diameters of nanowires are not confined strictly by the inner diameters of AOT rod-like micelles. Due to the flexibility of micelle interface, ZnS nanowires will grow continually along the radial and axial direction at the basis of initially formed ZnS cores. As a result, ZnS nanowires will become longer and thicker to some extent than the initial AOT rod-like micelles. Fig. 2c was a magnified image of the rectangular part in Fig. 2b. The nanowire appeared to be polycrystalline as shown by the SAED pattern in the inset of Fig. 2c. The EDS of the nanowires in Fig. 2a was shown in Fig. 2d. Quantitative analysis indicated that the atomic ratio of Zn:S 1:1, which is close to the stoichiometric ratio of ZnS. In the following experiments, R value would be remained 0.11 mmol/ml unless specified. Other R values varying from 0.70 to 0 mmol/ml were also tested in order to investigate the shape and size evolution of ZnS nanocrystals in AOT micelle. Irregular large floccules were observed at R ¼ 0:70 mmol/ml. With less surfactant content addition to R ¼ 0:25 mmol/ml, two kinds of morphologies would be observed by TEM (Fig. 3c). Some nanorods with diameters ranging in 22–44 nm and lengths up to 440 nm were found coexisting with amorphous product. By decreasing surfactant content to R ¼ 0:08 mmol/ml, more thicker nanorods with diameters ranging from 86 to
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Fig. 2. The TEM images of as-synthesized ZnS nanowires with R ¼ 0:11 mmol/ml, at 75 8C. (a) Typical morphology of ZnS nanowires, the inset is the SAED pattern of the square part in (a). (b) An individual nanowire with length about 1.5 mm. (c) A magnified image of the rectangular part in (b), the inset is the SAED pattern of as-obtained nanowire. (d) The EDS of the nanowires in (a).
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Fig. 3. The TEM images of as-synthesized ZnS nanowires with R ¼ 0:03 (a), 0.08 (b) and 0.25 mmol/ml (c), respectively.
114 nm would be synthesized (Fig. 3b). But their lengths were shorter than that synthesized at R ¼ 0:11 mmol/ml (Fig. 1a). This phenomenon could be attributed to the shape evolution of AOT micelle with the variance of AOT content. With the increase of surfactant content to some extent, longer and thinner rod-like micelle might form. At R ¼ 0:03 mmol/ml, product with floccule-like morphology would be observed (Fig. 3a). Under further reduction of AOT to R ¼ 0 mmol/ml, only floccules deposited in large amounts and no nanorods or nanowires were observed. From the experimental facts in the context, we could find out that no systematic shape and size evolution tendency was captured with the variation of R value. This indicates the diversity of the nanostructure and the phase transition of the microemulsion solution when the water content is adjusted. However, R could be optimized to obtain uniform nanowires, suggesting the existence of relatively stable rod-like nanochannels from spherical micelles. The contribution of reactant concentration to the morphology of final ZnS product was also investigated, by selecting [Zn2þ] ¼ [S2] ¼ 0.15 mol/l (Fig. 2a), 0.05 and 0.40 mol/l. Other reaction parameters were all fixed unless specified. When the reactant concentrations were lowered to 0.05 mol/ l, amorphous product was observed coexisting with a very small quantity of nanorods(Fig. 4a). When the concentrations were increased to 0.40 mol/l, quasi-spherical particles were the dominant morphology, and no nanorods or nanowires were found on the Cu grid (Fig. 4b). Therefore, appropriate reactant concentration, e.g. 0.15 mol/l, is favorable to the formation of uniform nanowires.
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Fig. 4. The TEM images of as-synthesized ZnS nanowires with [Zn2þ] ¼ [S2] ¼ 0.05 (a) and 0.40 mol/l (b), respectively.
Lower of higher concentration seems to go against the nucleation and growth of regular products, as pointed out by Xu and Li [16]. Reaction temperature is also one of the key reaction parameters. We performed similar experiment at room temperature and kept other conditions the same as in Fig. 2a. As shown in Fig. 5 and its inset, shorter and thinner polycrystalline nanorods would be got at room temperature, which was different from that synthesized at 75 8C (Fig. 2a). This might be attributed to the shape transition of the micelle, but further experimental data are needed. According to above experimental results, the proposed mechanism of synthesis of ZnS nanowires by micelle-template inducing reaction might be as follows: At appropriate surfactant concentration and moderate temperature, AOT rod-like micelle might form. Thiourea in the water phase might be enwrapped in the rod-like micelle with constant stirring, and Zn2þ will transfer to the surface of thiourea through the interface of micelle because of the concentration difference. Thiourea may act as a bidentate ligand to form relatively stable Zn–thiourea complexes with
Fig. 5. The TEM images of as-synthesized ZnS nanowires at room temperature, the inset is the corresponding SAED pattern.
R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
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Zn2þ, which prevent the production of more free S2 ions in the reaction system, and will be favorable for the formation of the nanowires. The stability of the Zn–thiourea complexes is expected to decrease with the increase of the temperature. At some relatively high temperature (e.g. above 70 8C) and enough long aging time, the Zn–thiourea complexes may undergo a slow decomposition process to produce long ZnS nanowire. In this process, the AOT micelle may act as templates to induce the growth of ZnS nanowires. The formation of ZnS nanowires can be expressed as the following [13]:
Therefore, the formation of uniform nanowires needs appropriate temperature and suitable surfactant concentration. With regard to the coexistence of different morphologies with different sizes, it could be proposed that the growth might take place in different reaction environments [16]. The mechanisms proposed above need to be tested by further experiments.
4. Conclusions In conclusion, ZnS nanowires have been successfully synthesized by a simple, low-cost, efficient method. XRD, EDS and SAED pattern shows that as-synthesized products are pure cubic-phase b-ZnS. TEM pattern shows that as-synthesized ZnS nanowires have diameters around 30 nm and lengths up to 2.5 mm. Investigations show that suitable ratio of surfactant molar number to the volume of water (e.g. 0.11 mmol/ml), moderate reactant concentration (e.g. [Zn2þ] ¼ [S2] ¼ 0.15 mol/l) and appropriate temperature (e.g. 75 8C) are necessary for the growth of long and uniform ZnS nanowires. Considering the simplicity of the procedure, micelle-template inducing reaction here is likely to be of a good method to synthesize other II–VI group semiconductor nanowires.
Acknowledgements This work was supported by the research fund for the Doctoral Program of Higher Education of China (Grant No.: 20020007029) and BIT Basic Research Fund. References [1] W. Wong, E. Sheehan, M. Lieber, Science 277 (1997) 1971. [2] J.Y. Li, X.L. Chen, H. Li, M. He, Z.Y. Qiao, J. Cryst. Growth 233 (2001) 5.
1524 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
R. Lv et al. / Materials Research Bulletin 39 (2004) 1517–1524
X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59. M. Li, H. Schnablegger, S. Mann, Nature 402 (1999) 393. G. Chopra, R.J. Luyken, K. Cherrey, H. Crespi, L. Cohen, G. Louie, A. Zettl, Science 269 (1995) 966. B.A. Korgel, D. Fitzmaurice, Adv. Mater. 10 (1998) 661. P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Science 278 (1997) 100. Y.-T. Chen, J.-B. Ding, Y. Guo, L.-B. Kong, H.-L. Li, Mater. Chem. Phys. 77 (2002) 734. D. Xu, Y. Xu, D. Chen, G. Guo, L. Gui, Y. Tang, Adv. Mater. 12 (2000) 520. D. Routkevitch, T. Bigioni, M. Moskovits, M. Xu, J. Phys. Chem. 100 (1996) 14037. Y. Wang, L. Zhang, C. Liang, G. Wang, X. Peng, Chem. Phys. Lett. 357 (2002) 314. J. Yang, J.-H. Zeng, S.-H. Yu, L. Yang, G. Zhou, Y. Qian, Chem. Mater. 12 (2000) 3259. Y.-D. Li, H.-W. Liao, Y. Ding, Y.-T. Qian, L. Yang, G.-E. Zhou, Chem. Mater. 10 (1998) 2301. Y. Xiong, Y. Xie, J. Yang, R. Zhang, C. Wu, G. Du, J. Mater. Chem. 12 (2002) 3712. K.E. Marchand, M. Tarret, J.P. Lechaire, L. Normand, S. Kasztelan, T. Cseri, Coll. Surf. A Physicochem. Eng. Aspects 214 (2003) 239. [16] J. Xu, L. Yadong, J. Coll. Interf. Sci. 259 (2003) 275.