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Microemulsion-mediated solvothermal synthesis of ZnS nanowires
Materials Letters 61 (2007) 4651 – 4654 www.elsevier.com/locate/matlet
Microemulsion-mediated solvothermal synthesis of ZnS nanowires Lihong Dong a,b , Ying Chu a,⁎, Yanping Zhang a a
Department of Chemistry, Northeast Normal University, Changchun, 130024, PR China b Department of Chemistry, Tonghua Normal College, Tonghua, 134002, PR China Received 20 March 2006; accepted 1 March 2007 Available online 7 March 2007
Abstract Uniform and high-aspect-ratio ZnS nanowires with length up to several micrometers and diameter of 30–50 nm are synthesized by a facile and low-cost microemulsion-mediated solvothermal method. Moreover, ZnS nanorods and bamboo-leaf-like ZnS nanostructures were also obtained by modulating the reaction parameters. Especially, hollow bamboo-leaf-like ZnS nanostructures formed by radiating those bamboo leaves with electron beam. A reasonable mechanism to the formation of the as-prepared one-dimension zinc blend ZnS nanocrystals is also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnS; Nanowires; Nanorods; Bamboo leaves
1. Introduction Nanometer scale one-dimensional structures have attracted considerable attention due to their unique electronic, optical and mechanical properties [1,2]. In recent years, synthesis of onedimensional semiconductor materials such as nanowires, nanorods or nanotubes has been the focus of research work [3–6]. In which semiconductor nanowires may one day be employed in high-performance field-effect transistors [7,8], logic circuits [9], nonvolatile memories [10], and biosensors [11]. They are considered promising candidates to augment or even replace silicon planar technology, which is approaching fundamental scaling limits [12]. As one of the most important semiconductors, ZnS has been known for a long time as a versatile and excellent phosphor host material and it has a wide band-gap of 3.8 eVand a small Bohr radius (2.4 nm), which make it an excellent candidate for exploring the intrinsic recombination processes in dense excitonic systems. Therefore, the synthesis and physical properties of ZnS nanocrystals have been widely investigated. Up to date, ZnS nanocrystals with various morphologies, such as nanowires [13], nanobelts [14], hollow nanospheres [15], hollow nanovessels [16] and nanotubes [17], have been prepared successfully. However, most ZnS nanowires were obtained by chemical vapor deposition (CVD) method [13,18–20], the wet ⁎ Corresponding author. E-mail address: [email protected] (Y. Chu). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.001
chemical route, which is simple, inexpensive and typically scalable for industrial production, has seldom been reported, [21,22] and that uniform, straight and high-aspect-ratio ZnS nanowires with large scale production have not been reported so far to our best knowledge. So it is still a challenge for chemists and materialists to explore facile and low-cost methods to synthesize uniform and high-aspect-ratio ZnS nanowires. In the recent past, surfactant-assisted reverse micelles or microemulsions have been widely and successfully used as an ideal media to prepare inorganic nanoparticles. A reverse micelle or microemulsion is a transparent and isotropic liquid medium with nanosized water pools dispersed in a continuous phase and stabilized by surfactant and cosurfactant molecules at the water/ oil interface. Accordingly, reverse micelles or microemulsions are thermodynamically stable systems and isotropic on a molecular scale and have the ability to solubilize proper solution. As the nanosized water pools, they have been widely used as spatially constrained microreactors for controlled synthesis of nanoparticles with desired narrow size distribution. In addition, the solvothermal method has been widely used to synthesize nanomaterials because of its unique reaction environment. It has been proven that the use of solvothermal method for the synthesis of nanomaterials cannot only decrease reaction temperature of systems but also improve the crystallinity of the products. Whereas the microemulsion-mediated solvothermal method, as a combination of the two methods mentioned above, possesses all the merits of both and has already been approved as
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an effective tool to fabricate inorganic nanocrystals with uniform morphology, narrow size distribution and good crystallinity [23–26]. Therefore, in this paper, the microemulsion-mediated solvothermal method is reported to synthesize uniform and long ZnS nanowires with a diameter of 30–50 nm. Moreover, nanorods and bamboo-leaf-like ZnS nanostructures were also obtained by modulating the reaction parameters. 2. Experimental section The reaction to form ZnS in present case can be formulated as the following equation: ZnSO4 þ 3Na2 S2 O4 →ZnS↓ þ Na2 SO4 þ NaS2 O5 þ SO2 Fig. 1. XRD pattern of ZnS nanowires prepared at 160 °C for 12 h with the w0 = 20.
This reaction cannot take place till the solution is boiling, so it was chosen to synthesize ZnS nanocrystals under solvothermal condition. A quaternary microemulsion, cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-hexanol, was
Fig. 2. TEM image of a) ZnS nanowires obtained at 160 °C for 12 h with the w0 = 20; b) ZnS nanowires coexisting with bamboo-leaf-like nanostructures obtained at 160 °C for 12 h with the w0 = 10; c) ZnS round ring-like nanowires obtained at 160 °C for 12 h with the w0 = 10; d) ZnS nanorods obtained at 160 °C for 12 h with the w0 = 40.; e) SAED pattern and f) HRTEM image of a single ZnS nanowire.
L. Dong et al. / Materials Letters 61 (2007) 4651–4654
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Fig. 3. TEM image of a) ZnS nanorods obtained at 140 °C for 12 h with the w0 = 20; b) bamboo-leaf-like ZnS nanostructures obtained at 120 °C for 12 h with the w0 = 20.
selected for this study. As a typical synthesis, two identical solutions were prepared by dissolving CTAB (0.5 g) in 15 mL of cyclohexane and 0.75 mL n-hexanol. The mixing solution was stirred for 30 min until it became transparent. Next, 0.5 mL 0.5 M ZnSO4 aqueous solution or 0.5 mL 1.5 M Na2S2O4 aqueous solution was added into the above solution to make the w0 (w0 is defined as the molar ratio of water to surfactant) equal 20. After substantial stirring, the two optically transparent microemulsion solutions were mixed and stirred for another 10 min. The resulting microemulsion solution was then transferred into a 50 mL stainless Teflon-lined autoclave and heated at 160 °C for 12 h. The as-obtained suspension was naturally cooled to room temperature, and then the precipitates were collected and washed several times with absolute ethanol and distilled water. Finally, the sample was dried in a vacuum at 50 °C. Results with samples prepared with different w0 value and reaction temperature were also compared. The as-prepared powder samples were characterized by Xray powder diffraction (XRD) on a Rigaku X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å). The morphology and size of as-obtained product were observed by transmission electron microscopy (TEM), which was carried out on the Hitachi H-800 transmission electron microscope. High resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) were performed with transmission electron microscope (JEOL JEM-2010, 200 kV). UV–vis absorption was recorded on a SHIMADZU UV-2550 UV–visible spectrophotometer using 1 cm quartz absorption cell.
[011] zone axis. The spot pattern reveals that the nanowire is single crystals in nature, which is further confirmed by the HRTEM image presented in Fig. 2f. The measuring d spacings of (111) planes are 0.312 nm, which is consistent with the ideal values of zinc blende ZnS and this is the growth direction of the nanowires. When the w0 was reduced to 10, as presented in Fig. 2b, besides lots of nanowires, there were also many bamboo-leaf-like nanostructures found. It is very interesting that part of the nanowires curls into round rings as shown in Fig. 2c. An increase of w0 to 40 resulted in the formation of nanorods with wider diameter and length distribution as shown in Fig. 2d. Therefore, it could be deduced that w0 = 20 is the optimal condition to the fabrication of ZnS nanowires. Temperature also played an important role in the morphology of the product. When the w0 value was kept at 20 and the temperature was at 140 °C, only ZnS nanorods were obtained (Fig. 3a). The diameter of the nanorods is about 80 nm and the length ranges from 500 nm to 1 μm. A continuous reducing of temperature to 120 °C, bamboo-leaflike ZnS nanostructures were obtained and the typical TEM image is depicted in Fig. 3b. To our surprised, when electron beam focused on these bamboo leaves on Fomnvar film coated copper grid, the central part of them melted eventually and hollow bamboo leaves formed at last as shown in Fig. 3b. If the irradiation time of the electron beam was long enough, the bamboo-leaf-like nanostructures would all transfer into hollow ones (image not shown here), which may be due to the poor crystallinity, but the exact explanation is not known currently. The UV–vis spectrum of ZnS nanowires as-prepared is shown in Fig. 4, compared with the bulk ZnS (344 nm), the absorption peaks for
3. Results and discussion Fig. 1 shows the XRD pattern of ZnS nanowires as-synthesized at 160 °C for 12 h with the w0 = 20. The data is in good agreement with that of pure cubic phase zinc blende ZnS (JCPDS No.: 01-0792). The three strong peaks with 2θ values of 28.62, 47.84, and 56.63° correspond to the three crystal plane of (111), (220), and (311) of zinc blende ZnS, respectively. The broadening of the diffraction peaks is due to the small diameter (30–50 nm) of the nanowires. Fig. 2a shows the typical TEM image of the sample prepared at 160 °C for 12 h with the w0 = 20. As shown in the image, the product is primarily composed of nanowires with length up to several micrometers and diameter of 30–50 nm. The diameter is very uniform along the entire nanowire. The corresponding SAED pattern (Fig. 2e) can be indexed as those of the zinc blend phase recorded along the
Fig. 4. UV–vis spectrum of ZnS nanowires obtained at 160 °C for 12 h with the w0 = 20.
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the nanowires are of blue shift. This clearly suggests that these nanowires have obvious quantum confinement effect. For the formation mechanism of the ZnS nanowires, a directional aggregation process, which has been established for the formation of some 1D nanostructure in microemulsions [27], should be responsible for it. It is known that cubic ZnS crystal may be interpreted as stacked {ZnS4} tetrahedra sharing their common corners. The growth direction of ZnS crystal is determined by the relative stacking rate of the constituent tetrahedra in various crystal faces, and the stacking rate is strongly dependent on the bonding force of atoms in the tetrahedra at the interface. The atom at the corner of a tetrahedron has the strongest bonding force (s = 2 vu.) as compared with the atoms at other positions; thus crystals grow fast along the directions in which the tetrahedron corners point. Each tetrahedron has corners in the [111] direction; this favors the growth of ZnS nanocrystals along the [111] axis [28]. A cubic crystal has four [111] axes and eight (111) facets, each of these has three neighboring (111) facets. In the present case, the growth of a given (111) facet may suppress the growth of all three neighboring facets because of the limitation of the size of water pools and selective adsorption of CTAB. Thus, only the two diagonal (111) facets grow, resulting in the formation of nanowires with a growth direction of [111]. When the w0 value is reduced (such as w0 = 10), the low water and reactant content may make the collision and fuse rate between two micelle droplets in the microemulsions slow, which results in the formation of some bamboo-leaf-like ZnS nanostructures. Whereas when the w0 value is as large as 40, larger collision and fuse rate among micelle droplets will lead to the faster nucleation and growth of ZnS, which results in the formation of nanorods. As far as the wide diameter distribution, it is presumed that the larger and uneven micelle size resulted from the larger w0 value should be responsible for it. However, the exact mechanism for the morphological evolution of ZnS nanocrystals is worthy of further investigation. It also remains unclear that how the temperature affects the shape evolution of the products, the determination of the detailed mechanism is still underway. In summary, uniform and long ZnS nanowires have been fabricated by microemulsion-mediated solvothermal method. Short nanorods and bamboo-leaf-like nanostructures were also obtained by controlling the fundamental experimental parameters including the value of w0 and the solvothermal temperature. A directional aggregation process of crystal nuclei along [111] direction could interpret the formation of the onedimension nanostructures. The UV–vis spectrum confirms the quantum confinement effect of ZnS nanowires as-prepared.
Acknowledgments This work was supported by the Natural Science Fund of China (No. 20573017) and Analysis and Testing Foundation of Northeast Normal University.
References [1] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [2] C. Dekler, Phys. Today 52 (1999) 22. [3] Y. Li, H. Liao, Y. Ding, Y. Qian, L. Yang, X. Lin, Chem. Mater. 10 (1998) 2301. [4] S. Yu, Y. Wu, J. Yang, Z. Han, Y. Xie, Y. Qian, X. Lin, Chem. Mater. 10 (1998) 2309. [5] K. Cho, D.V. Talapin, W. Gaschler, C.B. Murray, J. Am. Chem. Soc. 127 (2005) 7140. [6] C. Yang, D.D. Awschalom, G.D. Stucky, Chem. Mater. 14 (2002) 1277. [7] Y. Cui, Z. Zhong, D. Wang, W.U. Wang, C.M. Lieber, Nano Lett. 3 (2003) 149. [8] X. Duan, C. Niu, V. Sahi, J. Chen, J.W. Parce, S. Empedocles, J.L. Goldman, Nature 425 (2003) 274. [9] Y. Huang, X. Duan, Y. Cui, L.J. Lauhon, K.H. Kim, C.M. Lieber, Science 294 (2001) 1313. [10] X. Duan, Y. Huang, C.M. Lieber, Nano Lett. 2 (2002) 487. [11] F. Patolsky, G. Zheng, O. Hayden, M. Lakadamyali, X. Zhuang, C.M. Lieber, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 14017. [12] International Technology Road Map for Semiconductors, 2003 edition. [13] D.F. Moore, Y. Ding, Z.L. Wang, J. Am. Chem. Soc. 126 (2004) 14372. [14] W.T. Yao, S.H. Yu, L. Pan, J. Li, Q.S. Wu, L. Zhuang, J. Jiang, Small 1 (2005) 320. [15] Y.R. Ma, L.M. Qi, J.M. Ma, H.M. Cheng, Langmuir 19 (2003) 4040. [16] L.H. Dong, Y. Chu, F.Y. Yang, Y. Liu, J.L. Liu, Chem. Lett. 34 (2005) 1254. [17] L.W. Yin, Y. Bando, J.H. Zhan, M.S. Li, D. Golberg, Adv. Mater. 17 (2005) 1972. [18] S. Kar, S. Chaudhuri, J. Phys. Chem., B 109 (2005) 3298. [19] H. Zhang, S.Y. Zhang, M. Zuo, G.P. Li, J.G. Hou, Eur. J. Inorg. Chem. (2005) 47. [20] X.S. Fang, C.H. Ye, L.D. Zhang, Y.H. Wang, Y.C. Wu, Adv. Funct. Mater. 15 (2005) 63. [21] Q.S. Wu, N.W. Zheng, Y.P. Ding, Y.D. Li, Inorg. Chem. Commun. 5 (2002) 671. [22] R. Lv, C.B. Cao, H.S. Zhu, Mater. Res. Bull. 39 (2004) 1517. [23] M.H. Cao, C.W. Hu, E.B. Wang, J. Am. Chem. Soc. 125 (2003) 11196. [24] M.H. Cao, Y.H. Wang, C.X. Guo, Y.J. Qi, C.W. Hu, Langmuir 20 (2004) 4784. [25] M.H. Cao, X.L. Wu, X.Y. He, C.W. Hu, Langmuir 21 (2005) 6093. [26] Y. Liu, Y. Chu, Mater. Chem. Phys. 92 (2005) 59. [27] L.M. Qi, J.M. Ma, H.M. Cheng, Z.G. Zhao, J. Phys. Chem., B 101 (1997) 3460. [28] Y.C. Zhu, Y. Bando, D.F. Xue, D. Golberg, J. Am. Chem. Soc. 125 (2003) 16196.
Microemulsion-mediated solvothermal synthesis of ZnS nanowires Lihong Dong a,b , Ying Chu a,⁎, Yanping Zhang a a
Department of Chemistry, Northeast Normal University, Changchun, 130024, PR China b Department of Chemistry, Tonghua Normal College, Tonghua, 134002, PR China Received 20 March 2006; accepted 1 March 2007 Available online 7 March 2007
Abstract Uniform and high-aspect-ratio ZnS nanowires with length up to several micrometers and diameter of 30–50 nm are synthesized by a facile and low-cost microemulsion-mediated solvothermal method. Moreover, ZnS nanorods and bamboo-leaf-like ZnS nanostructures were also obtained by modulating the reaction parameters. Especially, hollow bamboo-leaf-like ZnS nanostructures formed by radiating those bamboo leaves with electron beam. A reasonable mechanism to the formation of the as-prepared one-dimension zinc blend ZnS nanocrystals is also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnS; Nanowires; Nanorods; Bamboo leaves
1. Introduction Nanometer scale one-dimensional structures have attracted considerable attention due to their unique electronic, optical and mechanical properties [1,2]. In recent years, synthesis of onedimensional semiconductor materials such as nanowires, nanorods or nanotubes has been the focus of research work [3–6]. In which semiconductor nanowires may one day be employed in high-performance field-effect transistors [7,8], logic circuits [9], nonvolatile memories [10], and biosensors [11]. They are considered promising candidates to augment or even replace silicon planar technology, which is approaching fundamental scaling limits [12]. As one of the most important semiconductors, ZnS has been known for a long time as a versatile and excellent phosphor host material and it has a wide band-gap of 3.8 eVand a small Bohr radius (2.4 nm), which make it an excellent candidate for exploring the intrinsic recombination processes in dense excitonic systems. Therefore, the synthesis and physical properties of ZnS nanocrystals have been widely investigated. Up to date, ZnS nanocrystals with various morphologies, such as nanowires [13], nanobelts [14], hollow nanospheres [15], hollow nanovessels [16] and nanotubes [17], have been prepared successfully. However, most ZnS nanowires were obtained by chemical vapor deposition (CVD) method [13,18–20], the wet ⁎ Corresponding author. E-mail address: [email protected] (Y. Chu). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.001
chemical route, which is simple, inexpensive and typically scalable for industrial production, has seldom been reported, [21,22] and that uniform, straight and high-aspect-ratio ZnS nanowires with large scale production have not been reported so far to our best knowledge. So it is still a challenge for chemists and materialists to explore facile and low-cost methods to synthesize uniform and high-aspect-ratio ZnS nanowires. In the recent past, surfactant-assisted reverse micelles or microemulsions have been widely and successfully used as an ideal media to prepare inorganic nanoparticles. A reverse micelle or microemulsion is a transparent and isotropic liquid medium with nanosized water pools dispersed in a continuous phase and stabilized by surfactant and cosurfactant molecules at the water/ oil interface. Accordingly, reverse micelles or microemulsions are thermodynamically stable systems and isotropic on a molecular scale and have the ability to solubilize proper solution. As the nanosized water pools, they have been widely used as spatially constrained microreactors for controlled synthesis of nanoparticles with desired narrow size distribution. In addition, the solvothermal method has been widely used to synthesize nanomaterials because of its unique reaction environment. It has been proven that the use of solvothermal method for the synthesis of nanomaterials cannot only decrease reaction temperature of systems but also improve the crystallinity of the products. Whereas the microemulsion-mediated solvothermal method, as a combination of the two methods mentioned above, possesses all the merits of both and has already been approved as
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L. Dong et al. / Materials Letters 61 (2007) 4651–4654
an effective tool to fabricate inorganic nanocrystals with uniform morphology, narrow size distribution and good crystallinity [23–26]. Therefore, in this paper, the microemulsion-mediated solvothermal method is reported to synthesize uniform and long ZnS nanowires with a diameter of 30–50 nm. Moreover, nanorods and bamboo-leaf-like ZnS nanostructures were also obtained by modulating the reaction parameters. 2. Experimental section The reaction to form ZnS in present case can be formulated as the following equation: ZnSO4 þ 3Na2 S2 O4 →ZnS↓ þ Na2 SO4 þ NaS2 O5 þ SO2 Fig. 1. XRD pattern of ZnS nanowires prepared at 160 °C for 12 h with the w0 = 20.
This reaction cannot take place till the solution is boiling, so it was chosen to synthesize ZnS nanocrystals under solvothermal condition. A quaternary microemulsion, cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-hexanol, was
Fig. 2. TEM image of a) ZnS nanowires obtained at 160 °C for 12 h with the w0 = 20; b) ZnS nanowires coexisting with bamboo-leaf-like nanostructures obtained at 160 °C for 12 h with the w0 = 10; c) ZnS round ring-like nanowires obtained at 160 °C for 12 h with the w0 = 10; d) ZnS nanorods obtained at 160 °C for 12 h with the w0 = 40.; e) SAED pattern and f) HRTEM image of a single ZnS nanowire.
L. Dong et al. / Materials Letters 61 (2007) 4651–4654
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Fig. 3. TEM image of a) ZnS nanorods obtained at 140 °C for 12 h with the w0 = 20; b) bamboo-leaf-like ZnS nanostructures obtained at 120 °C for 12 h with the w0 = 20.
selected for this study. As a typical synthesis, two identical solutions were prepared by dissolving CTAB (0.5 g) in 15 mL of cyclohexane and 0.75 mL n-hexanol. The mixing solution was stirred for 30 min until it became transparent. Next, 0.5 mL 0.5 M ZnSO4 aqueous solution or 0.5 mL 1.5 M Na2S2O4 aqueous solution was added into the above solution to make the w0 (w0 is defined as the molar ratio of water to surfactant) equal 20. After substantial stirring, the two optically transparent microemulsion solutions were mixed and stirred for another 10 min. The resulting microemulsion solution was then transferred into a 50 mL stainless Teflon-lined autoclave and heated at 160 °C for 12 h. The as-obtained suspension was naturally cooled to room temperature, and then the precipitates were collected and washed several times with absolute ethanol and distilled water. Finally, the sample was dried in a vacuum at 50 °C. Results with samples prepared with different w0 value and reaction temperature were also compared. The as-prepared powder samples were characterized by Xray powder diffraction (XRD) on a Rigaku X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å). The morphology and size of as-obtained product were observed by transmission electron microscopy (TEM), which was carried out on the Hitachi H-800 transmission electron microscope. High resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) were performed with transmission electron microscope (JEOL JEM-2010, 200 kV). UV–vis absorption was recorded on a SHIMADZU UV-2550 UV–visible spectrophotometer using 1 cm quartz absorption cell.
[011] zone axis. The spot pattern reveals that the nanowire is single crystals in nature, which is further confirmed by the HRTEM image presented in Fig. 2f. The measuring d spacings of (111) planes are 0.312 nm, which is consistent with the ideal values of zinc blende ZnS and this is the growth direction of the nanowires. When the w0 was reduced to 10, as presented in Fig. 2b, besides lots of nanowires, there were also many bamboo-leaf-like nanostructures found. It is very interesting that part of the nanowires curls into round rings as shown in Fig. 2c. An increase of w0 to 40 resulted in the formation of nanorods with wider diameter and length distribution as shown in Fig. 2d. Therefore, it could be deduced that w0 = 20 is the optimal condition to the fabrication of ZnS nanowires. Temperature also played an important role in the morphology of the product. When the w0 value was kept at 20 and the temperature was at 140 °C, only ZnS nanorods were obtained (Fig. 3a). The diameter of the nanorods is about 80 nm and the length ranges from 500 nm to 1 μm. A continuous reducing of temperature to 120 °C, bamboo-leaflike ZnS nanostructures were obtained and the typical TEM image is depicted in Fig. 3b. To our surprised, when electron beam focused on these bamboo leaves on Fomnvar film coated copper grid, the central part of them melted eventually and hollow bamboo leaves formed at last as shown in Fig. 3b. If the irradiation time of the electron beam was long enough, the bamboo-leaf-like nanostructures would all transfer into hollow ones (image not shown here), which may be due to the poor crystallinity, but the exact explanation is not known currently. The UV–vis spectrum of ZnS nanowires as-prepared is shown in Fig. 4, compared with the bulk ZnS (344 nm), the absorption peaks for
3. Results and discussion Fig. 1 shows the XRD pattern of ZnS nanowires as-synthesized at 160 °C for 12 h with the w0 = 20. The data is in good agreement with that of pure cubic phase zinc blende ZnS (JCPDS No.: 01-0792). The three strong peaks with 2θ values of 28.62, 47.84, and 56.63° correspond to the three crystal plane of (111), (220), and (311) of zinc blende ZnS, respectively. The broadening of the diffraction peaks is due to the small diameter (30–50 nm) of the nanowires. Fig. 2a shows the typical TEM image of the sample prepared at 160 °C for 12 h with the w0 = 20. As shown in the image, the product is primarily composed of nanowires with length up to several micrometers and diameter of 30–50 nm. The diameter is very uniform along the entire nanowire. The corresponding SAED pattern (Fig. 2e) can be indexed as those of the zinc blend phase recorded along the
Fig. 4. UV–vis spectrum of ZnS nanowires obtained at 160 °C for 12 h with the w0 = 20.
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the nanowires are of blue shift. This clearly suggests that these nanowires have obvious quantum confinement effect. For the formation mechanism of the ZnS nanowires, a directional aggregation process, which has been established for the formation of some 1D nanostructure in microemulsions [27], should be responsible for it. It is known that cubic ZnS crystal may be interpreted as stacked {ZnS4} tetrahedra sharing their common corners. The growth direction of ZnS crystal is determined by the relative stacking rate of the constituent tetrahedra in various crystal faces, and the stacking rate is strongly dependent on the bonding force of atoms in the tetrahedra at the interface. The atom at the corner of a tetrahedron has the strongest bonding force (s = 2 vu.) as compared with the atoms at other positions; thus crystals grow fast along the directions in which the tetrahedron corners point. Each tetrahedron has corners in the [111] direction; this favors the growth of ZnS nanocrystals along the [111] axis [28]. A cubic crystal has four [111] axes and eight (111) facets, each of these has three neighboring (111) facets. In the present case, the growth of a given (111) facet may suppress the growth of all three neighboring facets because of the limitation of the size of water pools and selective adsorption of CTAB. Thus, only the two diagonal (111) facets grow, resulting in the formation of nanowires with a growth direction of [111]. When the w0 value is reduced (such as w0 = 10), the low water and reactant content may make the collision and fuse rate between two micelle droplets in the microemulsions slow, which results in the formation of some bamboo-leaf-like ZnS nanostructures. Whereas when the w0 value is as large as 40, larger collision and fuse rate among micelle droplets will lead to the faster nucleation and growth of ZnS, which results in the formation of nanorods. As far as the wide diameter distribution, it is presumed that the larger and uneven micelle size resulted from the larger w0 value should be responsible for it. However, the exact mechanism for the morphological evolution of ZnS nanocrystals is worthy of further investigation. It also remains unclear that how the temperature affects the shape evolution of the products, the determination of the detailed mechanism is still underway. In summary, uniform and long ZnS nanowires have been fabricated by microemulsion-mediated solvothermal method. Short nanorods and bamboo-leaf-like nanostructures were also obtained by controlling the fundamental experimental parameters including the value of w0 and the solvothermal temperature. A directional aggregation process of crystal nuclei along [111] direction could interpret the formation of the onedimension nanostructures. The UV–vis spectrum confirms the quantum confinement effect of ZnS nanowires as-prepared.
Acknowledgments This work was supported by the Natural Science Fund of China (No. 20573017) and Analysis and Testing Foundation of Northeast Normal University.
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