Synthesis of Wurtzite ZnS Nanowire Bundles Using a Solvothermal

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12658

J. Phys. Chem. C 2007, 111, 12658-12662

Synthesis of Wurtzite ZnS Nanowire Bundles Using a Solvothermal Technique Lanlan Chai, Jin Du, Shenglin Xiong, Haibo Li, Yongchun Zhu, and Yitai Qian* Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China ReceiVed: April 18, 2007

Using hydrazine hydrate (N2H4‚H2O) as the solvent, bundles of wurtzite ZnS nanowires with diameters of 10-25 nm and lengths about 5-8 µm have been synthesized in high yield via a solvothermal route. The formation process of ZnS nanowire bundles was studied, and it was found that the initially formed N2H4 intercalated zinc sulfide served as the precursor to ZnS. Based on the experimental results, the formation mechanism was discussed. The effect of solvent on the determination and evolution of the morphologies of the resulting nanocrystals was investigated. The optical properties of the as-synthesized wurtzite ZnS nanowire bundles were characterized by UV-vis absorption and PL spectrum.

1. Introduction Being an important wide-band gap semiconductor, ZnS has received great attention due to its unique properties and been widely used in optical coatings, solid-state solar cell windows, electrooptic modulators, photoconductors, field effect transistors, sensors, transductors, and light-emitting applications.1 Recently, nanoscale structures of ZnS, particularly in quasi onedimensional (1-D) nanostructures, has undergone intensive investigations as building blocks for nanoelectronic and nanooptic systems.2 From the viewpoint of applications, 1-D ZnS nanostructure arrays with highly aligned and ordered patterns are greatly demanded3 and have been the current focus of research. Highly aligned nanorods,4 nanotube arrays,5-7 bundles of nanowires/nanowire arrays,3,8-12 nanobelt/nanoribbon arrays,12,13 nanocombs,14 nanosaws,15 and nanocantilever arrays on nanoribbons16 of ZnS have been synthesized by chemical vapor deposition (CVD) method,4-6,11 template-assisted route,5-10 electrochemical deposition method,8 and thermal evaporation process.3,10,12-16 ZnS nanowire and nanoribbon arrays grown on the surface of ZnS nanoribbons substrate were prepared by homoepitaxial growth.17 Novel BN-sheathed ZnS branched nanoarchitectures,18 hierarchical saw-like ZnO nanobelt/ZnS nanowire heterostructures,19 and ZnO-coated ZnS nanorod arrays20 have been reported recently. Moreover, ZnS nanorod arrays were also realized by a solvothermal route through thermolysing zinc ethylxanthate (Zn(exan)2) using octylamine (OA) as the precursor solvent and hexadecylamine (HDA) as the main ligand stabilizer.21 By thermal treatment of precursor (ZnS‚0.5en), which was synthesized via a solvothermal route in ethylenediamine (en) solution, ZnS nanosheets composed of quantum wires22 and nanobelt arrays23 have been attained. Realizing synthesis of 1-D nanostructure arrays via selecting some appropriate ligand molecules is exciting and challenging in nanochemistry. On the one hand, it demonstrates the ability to synthesize 1-D nanostructure arrays by a solvothermal process and may provide a more promising technique for preparing 1-D nanostructure arrays than conventional methods in terms of cost and potential for large-scale production; on the other hand, it helps to elucidate the underlying function of ligand molecules * Corresponding author. Phone: +86 551 3601589. Fax: +86 551 3607402. E-mail: [email protected].

on the assembly of nanocrystals. However, to the best of our knowledge, there have been only a few reports in the synthesis of 1-D ZnS nanostructure arrays by a solvothermal process.21-23 Hence, it should be very interesting and important to further develop an effective solvothermal technique for synthesis of 1-D ZnS nanostructure arrays that are highly ordered and have a high aspect ratio. Herein, we report a solvothermal route to synthesize bundles of wurtzite ZnS nanowires with diameters of 10-25 nm and lengths about 5-8 µm using hydrazine hydrate (N2H4‚H2O) as the solvent. The nanowires in the bundle are aligned not only in length direction but also in crystallography orientation. It is anticipated that this ordered nanostructure find applications in nanoelectronics, optics, and other areas based on spatial orientation and arrangement. In addition, this synthetic route may be extended to the synthesis of other chalcogenide nanowire bundles. 2. Experimental Details All reagents used in our experiments were analytically pure grade, purchased from Shanghai Chemical Reagents Company and used without further purification. In a typical procedure, 1 mmol anhydrous zinc chloride (ZnCl2) and 2 mmol thiourea (CS(NH2)2) were dissolved in 40 mL N2H4‚H2O (85 wt %) in a 50 mL Teflon-lined stainless steel autoclave. After being sealed, the autoclave was put into an electric oven. The temperature of the electric oven was increased to 180 °C within 30 min and maintained at this temperature for 30 h. The product was filtered, washed with distilled water and absolute ethanol for several times, and dried in vacuum at 60 °C for 4 h. The X-ray diffraction (XRD) analysis was performed with a Japanese Rigaku D/max-γA rotating-anode X-ray diffractometer equipped with graphite-monochromatized Cu-KR radiation (λ ) 1.54178 Å). Field emission scanning electron microscopic (FESEM) images were collected on a JEOL JSM-6700F SEM. Transmission electron microscopic (TEM) images were measured on a Hitachi Model H-800 instrument at an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) image and selected-area electron diffraction (SAED) pattern were performed with a JEOL-2010 TEM at an acceleration voltage of 200 kV. Element analysis was recorded on an X-ray energy

10.1021/jp073009x CCC: $37.00 © 2007 American Chemical Society Published on Web 08/04/2007

Synthesis of Wurtzite ZnS Nanowire Bundles

Figure 1. (a) XRD pattern and (b) EDS spectrum of the as-synthesized ZnS nanowire bundles.

Figure 2. (a) Low- and (b) high-magnification FESEM images of the as-synthesized ZnS nanowire bundles.

spectrum instrument equipped with INCA300 (Oxford). Fourier transform infrared (FTIR) absorption spectrum was obtained with a Shimadzu IR-440 spectrometer. Thermogravimetric analysis (TGA) measurement of the product was conducted on a Shimadzu TGA-50H analyzer. UV-vis absorption spectrum was taken on a Shimadzu UV-vis spectrophotometer (UV-240). Photoluminescence (PL) measurements were carried out on a Perkin-Elmer LS-55 luminescence spectrometer using a pulsed Xe lamp. 3. Results and Discussion Figure 1a shows the typical XRD pattern of the as-synthesized product. All the diffraction peaks can be readily indexed as wurtzite ZnS, which are close to the literature (JCPDS card no. 36-1450, a ) 3.820 Å, c ) 6.257 Å). The full width of the (002) peak at half-maximum is narrowest in all peaks, suggesting that the as-synthesized wurtzite ZnS is 1-D crystal growing along the [001] direction. The other broadened peaks indicate that the size of some directions of the ZnS crystal may be quite small. The energy dispersion spectroscopy (EDS) analysis has also confirmed that the product is composed of Zn and S elements, as shown in Figure 1b. The signal of Cu element comes from the supporting TEM grid. According to EDS quantitative analysis software, the atomic ratio of Zn to S is 1.07:1, close to the stoichiometry of ZnS. The general morphology of the as-synthesized product was examined by FESEM. As shown in Figure 2a, the product exhibits a 1-D morphology. Higher magnification FESEM image (Figure 2b) indicates that the product is composed of uniform ordered nanowire bundles in high yield with typical lengths of

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Figure 3. (a-c) Typical TEM images, (d) SAED pattern, and (e) HRTEM image of the as-synthesized ZnS nanowire bundles.

5-8 µm and widths of 30-150 nm. The individual nanowires are straight and smooth with an average diameter of approximately 15 nm. Interestingly, the nanowires are parallelly aligned together, side by side, along the bundle. The structure and morphology of the nanowire bundles were further observed by TEM, SAED, and HRTEM. Figure 3a-c show the TEM images of the nanostructure, which confirm that it consists of nanowire bundles in high yield and has lengths of 5-8 µm. The diameters of individual nanowires are in the range of 10-25 nm, which is consistent with the FESEM results. The SAED pattern (Figure 3d) recorded from a bundle shows only one set of diffraction pattern, which reveals that the parallel nanowires in the bundle are single crystals and exhibit the same orientation. All the discrete SAED spots can be well indexed to be wurtzite ZnS. A representative HRTEM image is shown in Figure 3e. It can be seen that there are two nanowires, which are lying side by side in the image. The discriminable lattice fringes verify that the as-synthesized ZnS nanowires are single crystals. The fringe spacing is about 0.310 nm, which is close to the interplanar spacing of the (002) lattice planes of wurtzite ZnS. The studies of the SAED pattern and the HRTEM image demonstrate that the as-synthesized nanowires are single crystalline of wurtzite phase and grow along the [001] direction, which is in good agreement with the results of XRD pattern. The nanowires in the bundle are aligned not only in length direction but also in crystallography orientation, which is a key characteristic of the product. Nanoparticles, especially nanowires, with a distinct anisotropic morphology are indispensable for the fabrication of nanoscale devices, which requires alignment and functionalization processes,24,25 so the nanowire bundles synthesized herein should be a good candidate to the applications in future. To understand the formation mechanism of ZnS nanowire bundles, a number of experiments were performed to follow the nucleation and growth steps by investigating the samples obtained at different stages of the reaction using the XRD and TEM techniques. Figure 4a-e show the XRD patterns of the samples obtained after different reaction times (20 min, 45 min, 1.5, 2, and 6 h). The XRD pattern (Figure 4a) of the sample obtained at 180 °C for 20 min shows a weaker amine-related peak at the 2θ of about 10°.26,27 Taking into consideration that

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Figure 4. XRD patterns of the samples obtained at 180 °C for (a) 20 min, (b) 45 min, (c) 1.5 h, (d) 2 h, and (e) 6 h.

Figure 5. (a) FTIR spectrum and (b) TGA curve of the sample obtained at 180 °C for 20 min.

the medium coordination ability of hydrazine with metallic ion,27-31 the sample obtained at 180 °C for 20 min may be N2H4 intercalated zinc sulfide. Evidence for the presence of N2H4 intercalated can be found in FTIR spectrum (Figure 5a). The broad peak at 3420 cm-1 is assigned to O-H characteristic vibration resulting from small quantity of adsorbed H2O on the sample. Peaks at 3236 and 3103 cm-1 are attributed to the N-H stretching vibration band, and the shift toward lower frequency compared with hydrazine32 may result from the interaction of N2H4 with zinc ion.31 The NH2 scissors and wag, and twist bands

Chai et al.

Figure 6. TEM images of the samples obtained at 180 °C for (a,b) 20 min, (c) 45 min, (d) 1.5 h, (e) 2 h, and (f) 6 h.

appear at 1600, 1334, and 1153 cm-1, respectively. Absorption at 962 cm-1 is often caused by the stretching band ν (N-N) of bridging H2N-NH2 between two metal ions.33 The FTIR characteristics indicate that the N2H4 molecular has intercalated into the complex.28 The TGA curve of the complex obtained at 180 °C for 20 min is shown in Figure 5b. The weight loss before 100 °C could be attributed to the loss of surface H2O molecules adsorbed. The main weight loss starts at 120 °C in N2 stream. Up to 380 °C, the weight loss is about 16.2%. Based on the N2H4 intercalated zinc sulfide, the ratio of ZnS to N2H4 is 1: 0.5. The XRD pattern of the 45 min sample (Figure 4b) is similar to that of the complex obtained at 180 °C for 20 min except for the stronger amine-related peak at the 2θ of about 10°, revealing a significant increase in the crystallinity. When the reaction proceeded for 1.5 h, the peak at 39.8°/2θ specifically due to the wurtzite ZnS can be found in the XRD pattern (Figure 4c). As the reaction proceeded for 2 and 6 h, the corresponding XRD patterns are shown in panels d and e, respectively, of Figure 4, which display only the reflections of wurtzite ZnS. The XRD patterns indicate that the N2H4 intercalated zinc sulfide was formed in the initial stage and gradually reduced accompanied with continuous generation of wurtzite ZnS through decomposition. Figure 6 shows the TEM images of the samples obtained at 180 °C for 20 min, 45 min, 1.5 h, 2 h, and 6 h. These images clearly show the morphology evolution process of the nanowire bundles. The 20 min sample displays a lamellar shape, as shown in Figure 6a,b. There are many folds on the lamella and these folds spontaneously aggregate together. When the reaction proceeded for 45 min, the sample shows a ribbon-like nanostructure with lengths of several hundred nanometers, widths in the range of 30-150 nm, and thickness of about 15 nm (Figure 6c). On the basis of the XRD patterns of Figure 4a,b, we can conclude that these lamella and ribbons are N2H4 intercalated zinc sulfide. As the reaction proceeded for 1.5 h, the shape maintains as nanoribbons (Figure 6d). It is worthy noting that the center of the nanoribbon is thinner than the edge, which is indicated by the different brightness, and the phenomenon is obvious of the 2 h sample (Figure 6e). After 6 h, nanowire bundles with widths of 30-150 nm and lengths of several hundred nanometers formed (Figure 6f). The diameters of individual nanowires are in the range of 10-25 nm. Further extending the age time, uniform nanowire bundles with typical lengths of 5-8 µm were synthesized (Figure 2, 3). On the basis of the above experimental results, we propose a possible formation mechanism of the ZnS nanowire bundles in the following. In the initial stage, hydrazine complexes with metallic Zn2+ ions and forms the transparent soluble complexes solution,31 while thiourea can dissolve in hydrazine hydrate (85

Synthesis of Wurtzite ZnS Nanowire Bundles

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Figure 7. TEM images of the products prepared (a) with 20 mL hydrazine hydrate, (b) with 30 mL hydrazine hydrate, (c) by keeping 20 min N2H4 intercalated zinc sulfide with lamellar shape in absolute ethanol at 180 °C for 30 h, and (d) by keeping 45 min N2H4 intercalated zinc sulfide with ribbon-like shape in absolute ethanol at 180 °C for 30 h.

wt %), supporting a more homogeneous solution environment for the reaction. At elevated temperature, thiourea releases S2ions in solution. The S2- ions released interact with soluble complexes solution formed in the initial stage to form N2H4 intercalated zinc sulfide lamella with many folds on it. After that, these folds on the lamella aggregate together. Then, these aggregated folds break into nanoribbon-like fragments. The similar process that 1-D nanostructures formed through the folding of lamella has been reported.34,35 It was found that many 1-D nanostructures could be related to corresponding layered structures and soft layers might be rolled into 1-D nanostructures like Bi nanotubes,36 W nanowires,37 WS2 nanotubes,38 and MnO2 nanorods/nanowires,35,39 while fragile layers might be broken into CdS and CdSe nanorods.26 Considering that many diamine intercalated materials, including CdS‚0.5en and ZnTe(N2H4), have been investigated to have layered structures,26,27 the present N2H4 intercalated zinc sulfide might have the similar layered structure. Direct evidence for this hypothesis requires structural determination; further work is under way. The N2H4 intercalated zinc sulfide is not stable and can decompose under the present solvothermal conditions. In the solid-solid-phase transition process through decomposition, hydrazine molecules are removed from the complex, leaving ZnS nanowire bundles as the product. In this case, N2H4 intercalated zinc sulfide serves as the real precursor to ZnS. ZnSe and ZnS platelets with striated textures40 and ZnSe nanowire bundles41 have been synthesized through removing en molecules from precursors. These interesting results make us believe that the loss of hydrazine molecules may be responsible for the formation of ZnS nanowire bundles. During the following solvothermal process, due to the presence of a high concentration of coordinating agent (hydrazine), part of the ZnS nanocrystals with smaller sizes, which have high free energies, are gradually dissolved in the solution, then are used as the feed for continuous growth of ZnS nanowire bundles. At the expense of these small nanowires, uniform bundles form via an Ostwald ripening process.42-44 As to the role of hydrazine played in the formation of ZnS nanowire bundles, a series of parallel experiments were carried

Figure 8. (a) UV-vis absorption spectrum and (b) photoluminescence spectrum of the as-synthesized ZnS nanowire bundles.

out to investigate it. It was found that the concentration of hydrazine was crucial for the formation of the nanowire bundles. When 20 mL hydrazine hydrate was mixed with distilled water (the total volume of solvent was fixed at 40 mL), the product is composed of nanoparticles (Figure 7a). Figure 7b shows the TEM image of the product that was prepared with 30 mL hydrazine hydrate. Short nanorods can be seen in the image. With the increase of the concentration of hydrazine, ZnS tend to grow into 1-D nanostructure. When 40 mL hydrazine hydrate (85 wt %) was used as the solvent, the product consists of uniform nanowire bundles (Figure 2, 3). Obviously, the concentration of hydrazine in the reaction solution has an important effect on the determination and evolution of the morphologies of the resulting nanocrystals. The high concentration of hydrazine can slow down the decomposition process, which may separate the decomposition from folding step, favoring the formation of nanowire bundles. To confirm this conclusion, the 20 min N2H4 intercalated zinc sulfide with lamellar shape was kept in absolute ethanol at 180 °C for 30 h. The obtained product comprises of small nanoplates and their assemblies (Figure 7c). When the 45 min N2H4 intercalated zinc sulfide with ribbron-like shape was kept in absolute ethanol at 180 °C for 30 h, the obtained product is composed of nanowire bundles (Figure 7d); however, the surface of the nanowires is not smooth, consisting of small nanoplates. These results coincide with the above conclusion. As is known, study the optical properties is helpful to evaluate the quality of the product and can shed light on its potential applications. The UV-vis absorption peak centers at 312 nm, as shown in Figure 8a. Compared with that of bulk ZnS (335 nm),21 the absorption peak of the product displays a blue shift of 23 nm, indicating the diameter of the nanowires is in or near the quantum-confined regime.45 The room-temperature photo-

12662 J. Phys. Chem. C, Vol. 111, No. 34, 2007 luminescence of the as-synthesized wurtzite ZnS nanowire bundles is shown in Figure 8b, which was carried out with excitation at 350 nm. Two strong emission peaks at 413 and 438 nm in wavelength, accompanied by one weak peak at 465 nm in wavelength, are observed. Many attempts have been made to identify various features seen in the PL spectra of ZnS nanocrystals. Becker and Bard ascribed the blue emissions centered at 428 and 418 nm to the sulfur vacancies and interstitial sulfur lattice defects.46 Denzler et al. have attributed the emission band at 438 nm in ZnS nanocrystals to the transitions involving vacancy states.47 It has been reported that the emission band at around 450 nm is associated with the trapped luminescence arising from the surface states, whereas the emission bands located around 480 nm have traditionally been ascribed to the well-known luminescence of zinc vacancies.48 In our studies, the sulfur vacancies and interstitial sulfur lattice defects could be responsible for the strong emission bands at 413 and 438 nm, whereas the weak peak at 465 nm may arise from the surface states, which is consistent with the result of EDS and the large surface-to-volume ratio in the nanowire bundles configuration. 4. Conclusion In summary, wurtzite ZnS nanowire bundles have been synthesized in high yield via a simple slovothermal route using hydrazine hydrate as the solvent. The nanowires, growing along the [001] direction, are aligned not only in length direction but also in crystallography orientation to form the bundle. The observation of the formation process indicates that their formation involves four stages: (1) the formation of N2H4 intercalated zinc sulfide with lamellar shape; (2) the lamella folding and breaking into ribbron-like nanostructure; (3) the formation of ZnS nanowire bundles via a solid-solid-phase transition process through N2H4 intercalated zinc sulfide decomposition; (4) the growth of ZnS nanowire bundles via an Ostwald ripening process. The solvent has an important effect on the determination and evolution of the morphologies of the resulting nanocrystals. The UV-vis absorption peak of the ZnS nanowire bundles displays a blue shift of 23 nm compared with that of bulk ZnS. The photoluminescence spectrum has one weak peak at 465 nm and two strong peaks located at 413 and 438 nm. Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 20431020) and the 973 Project of China (No. 2005CB623601). References and Notes (1) Nicolau, Y. F.; Dupuy, M.; Bruuel, M. J. Electrochem. Soc. 1990, 137, 2915. (2) Jiang, Y.; Meng, X. M.; Liu, J.; Xie, Z. Y.; Lee, C. S.; Lee, S. T. AdV. Mater. 2003, 15, 323. (3) Moore, D. F.; Ding, Y.; Wang, Z. L. J. Am. Chem. Soc. 2004, 126, 14372. (4) Feng, Q. J.; Shen, D. Z.; Zhang, J. Y.; Liang, H. W.; Zhao, D. X.; Lu, Y. M.; Fan, X. W. J. Cryst. Growth 2005, 285, 561. (5) Shen, X. P.; Han, M.; Hong, J. M.; Xue, Z. L.; Xu, Z. Chem. Vap. Deposition 2005, 11, 250. (6) Zhai, T. Y.; Gu, Z. J.; Ma, Y.; Yang, W. S.; Zhao, L. Y.; Yao, J. N. Mater. Chem. Phys. 2006, 100, 281. (7) Yan, C. L.; Xue, D. F. J. Phys. Chem. B 2006, 110, 25850.

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