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Synthesis of Antimony Sulfide Nanotubes with Ultrathin Walls via Gradual Aspect Ratio Control of Nanoribbons Kang Hyun Park, Jaewon Choi, Hae Jin Kim, Jin Bae Lee, and Seung Uk Son* Department of Chemistry, Sungkyunkwan UniVersity, Suwon 440-746, Korea, and Korea Basic Science Institute, Daejeon 350-3+33, Korea ReceiVed May 11, 2007 ReVised Manuscript ReceiVed July 5, 2007
Nanotubes have captured the imagination of material scientists, due to the well-defined structure and their structurally related properties.1 In conjunction with the discovery and intensive study of carbon nanotubes,2 much effort has been made to develop diverse inorganic nanotubes.3 Actually, the new inorganic nanotubes have opened up diverse research fields including theoretical interpretation of their unique properties and application as electron transport, sensor, separation, and storage materials.4 Among the diverse inorganic nanotubular structures including metal, metal oxide, and metal halide nanotubes, the metal chalcogenide ones have aroused special interest over the last two decades since the discovery of MoS2 and WS2 by Tenne and co-workers.5 These materials showed excellent electrical and mechanical properties which enabled them to be used as scanning probe microscope tips, electrochemical gas storage materials, and alkali metal intercalators.6 Recently, new metal sulfide nanotubes such as TiS2, NbS2, TaS2, ReS2, Bi2S3, and so forth have been prepared.7 Usually, these nanotubes were prepared by gas-gas or solid-gas phase reactions of metal sources with H2S or by heating the premade metal sulfides with/without hydrogen.7 These reactions required quite a high reaction temperature in the region of 800-1000 °C. Thus, the development of a more efficient synthetic route for inorganic nanotubes is very much needed. Moreover, metal sulfide nanotubes are still quite rare and definitely require further exploration. It has been suggested that the rolling process of the layered structure is one of the main mechanisms involved in the formation of tubular structures.8 It is believed that the driving force to reduce the thermodynamically unstable edge area * Corresponding author. E-Mail:
[email protected].
(1) (a) Ajayan, P. M. Chem. ReV. 1999, 99, 1787. (b) Lu, X.; Chen, Z. Chem. ReV. 2005, 105, 3643. (2) Iijima, S. Nature 1991, 354, 56. (3) Tenne, R. Chem.sEur. J. 2002, 8, 5297. (4) (a) Kauser, M. Z.; Ruden, P. P. Appl. Phys. Lett. 2006, 89, 162104. (b) Modi, A.; Koratkar, N.; Lass, E.; Wei, B.; Ajayan, P. M. Nature 2003, 424, 171. (c) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; So¨derlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864. (d) Mpourmpakis, G.; Froudakis, G. E.; Lithoxoos, G. P.; Samios, J. Nano Lett. 2006, 6, 1581. (5) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. (6) (a) Rothschild, R.; Cohen, S. R.; Tenne, R. Appl. Phys. Lett. 1999, 75, 4025. (b) Chen, J.; Kuriyama, N.; Yuan, H.; Takeshita, H. T.; Sakai, T. J. Am. Chem. Soc. 2001, 123, 11813. (c) Zak, A.; Feldman, Y.; Lyakhovitskaya, V.; Leitus, G.; Popovitz-Biro, R.; Wachtel, E.; Cohen, H.; Reich, S.; Tenne, R. J. Am. Chem. Soc. 2002, 124, 4747. (7) (a) Chen, J.; Li, S.-L.; Tao, Z.-L.; Shen, Y.-T.; Cui, C.-X. J. Am. Chem. Soc. 2003, 125, 5284. (b) Nath, M.; Rao, C. N. R. J. Am. Chem. Soc. 2001, 123, 4841. (c) Ye, C.; Meng, G.; Jiang, Z.; Wang, Y.; Wang, G.; Zhang, L. J. Am. Chem. Soc. 2002, 124, 15180. (d) Brorson, M.; Hansen, T. W.; Jacobsen, C. J. H. J. Am. Chem. Soc. 2002, 124, 11582. (8) Tenne, R. Angew. Chem., Int. Ed. 2003, 42, 5124.
Figure 1. Control of aspect ratios of nanoribbons and rolling process to form tubular structures and well-known crystal structure of antimony sulfide (Sb2S3); dark gray colored ball, antimony; sky blue colored ball, sulfur.
induces the connection of each end of the layered structure, which results in the formation of tubes. Thus, it is reasoned that inorganic materials having a layered structural motif can be good candidates for the formation of nanotubes. Antimony sulfide (Sb2S3) usually belongs to the orthorhombic system and forms a quasi-layered structure which shows a strong preference for anisotropic growth along the (001) crystal plane to form nanorods and nanowires (Figure 1).9 As a result of its good electrical properties such as photoconductivity, this material has been widely applied to photovoltaic and thermoelectric devices10 and used as the starting material for the preparation of diverse multi-metallic chalcogenide materials.11 Recently, tubular antimony sulfides which have relatively thick walls (thickness of wall: ca. 100 nm to 10 µm) were prepared by several methods, including the chemical vapor transfer reaction of homemade Sb2S3 powder.12 In this communication, we report the highly reproducible wet chemical synthesis of antimony sulfide nanotubes with ultrathin walls (1.5-2.0 nm) by controlling the aspect ratio of nanoribbons. As far as we are aware, these tubes have the thinnest wall thicknesses among the known metal chalcogenide nanotubes.5,7,8,12 Diverse strategies for the shape-controlled synthesis of semiconductor nanomaterials were extensively developed including the template assisted preparation13 and kinetically controlled growth at low temperature.14 Our research group also has recognized that delicately changing the experimental conditions is a good way to accomplish the shape control of (9) (a) Wang, H.; Zhu, J.-M.; Zhu, J.-J.; Yuan, L.-M.; Chen, H.-Y. Langmuir 2003, 19, 10993. (b) An, C.; Tang, K.; Yang, Q.; Qian, Y. Inorg. Chem. 2003, 42, 8081. (c) Yu, Y.; Wang, R. H.; Chen, Q.; Peng, L.-M. J. Phys. Chem. B 2005, 109, 23312. (d) Jiang, Y.; Zhu, Y. J. J. Phys. Chem. B 2005, 109. 4361. (e) Christian, P.; O’Brien, P. J. Mater. Chem. 2005, 15, 4949. (f) Ota, J.; Srivastava, S. K. Cryst. Growth Des. 2007, 7, 343. (g) Lou, W.; Chen, M.; Wang, X.; Liu, W. Chem. Mater. 2007, 19, 872. (10) Rajpure, K. Y.; Lokhande, C. D.; Bhosale, C. H. Mater. Res. Bull. 1999, 34, 1079. (11) (a) Parise, J. B. Science 1991, 251, 293. (b) Powell, A. V.; Boissie`re, S. Chem. Mater. 2000, 12, 182. (c) Vaqueiro, P.; Chippindale, A. M.; Cowley, A. R.; Powell, A. V. Inorg. Chem. 2003, 42, 7846. (d) Vaqueiro, P.; Chippindale, A. M.; Powell, A. V. Inorg. Chem. 2004, 43, 7963. (12) (a) Zheng, X.; Xie, Y.; Zhu, L.; Jiang, X.; Jia, Y.; Song, W.; Sun, Y. Inorg. Chem. 2002, 41, 455. (b) Yang, J.; Liu, Y.-C.; Lin, H.-M.; Chen, C.-C. AdV. Mater. 2004, 16, 713. (c) Cao, X.; Gu, L.; Wang, W.; Gao, W.; Zhuge, L.; Li, Y. J. Cryst. Growth 2006, 286, 96. (13) (a) Rao, C. N. R.; Govindaraj, A.; Deepak, F. L.; Gunari, N. A.; Nath, M. Appl. Phys. Lett. 2001, 78, 1853. (b) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H.; Yang, P. Nature 2003, 422, 599. (c) Zhang, M.; Cicocan, E.; Bando, Y.; Wada, K.; Cheng, L. L.; Pirouz, P. Appl. Phys. Lett. 2002, 80, 491.
10.1021/cm0712772 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
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Figure 2. Representative TEM image (a), EDS spectrum (b), and Sb 3d orbital peaks of the XPS spectrum (c) of antimony sulfide nanotubes with 1.8 nm thick walls.
semiconductor nanomaterials.15 For example, the aspect ratios of one-dimensional nanomaterials can be controlled by changing the concentration of the chalcogenide source, because each of the lattice planes is likely to have a different reactivity toward the chalcogenide during the growth process. Basically, we utilized this strategy to control the shape of one-dimensional antimony sulfides, wherein the increase in the concentration of sulfur strongly encouraged the growth along the (001) crystal plane. In contrast, decreasing the concentration of sulfur gradually induced the growth of the nanomaterials in the direction vertical to the (001) plane. In this study, we postulated that if the thickness of (quasi) layered structures is thin enough, the delicate control of the aspect ratio can give rise to the formation of tubular structures by the well-known rolling process. Figure 1 illustrates this strategy. In a typical synthesis of antimony sulfide nanotubes, 0.13 g (0.57 mmol) of SbCl3 was carefully dissolved in 2 mL of oleylamine using sonication. To avoid the formation of a turbid solution due to the decomposition of the precursor, the temperature of the solution should be kept sufficiently low. After 0.032 g (1.00 mmol) of sulfur was dissolved in 7 mL of well-dried oleylamine under argon, the two solutions were combined at room temperature, and the reaction temperature was slowly increased to 175 °C over a period of 90 min (1.67 °C/min). Then, the reaction mixture was stirred for an additional 2 h at this temperature. After cooling to room temperature, the reaction mixture was poured into a methanol solution. The brown precipitates that were formed were retrieved by centrifugation.16 Figure 2a shows the representative transmission electron microscopy (TEM) image of the synthesized nanotubes with distinguishably darker walls. The high-resolution transmission electron microscopy (HR-TEM) image in the inset of Figure 2a reveals that the walls have a thickness of 1.8 nm which corresponds to several layers.12b The average diameter of the nanotubes was 10.4 nm, and their length ranged from 100 to 300 nm. The energy dispersive X-ray spectroscopy (EDS) of the nanotubes shows that the atomic ratio of Sb to S is 1:1.51, which supports Sb2S3 species (Figure 2b). The Sb 3d orbital peaks (529.2, 538.3 eV) in the X-ray photoelectron spectroscopy (XPS) spectra confirms that the oxidation state of antimony (14) Pradhan, N.; Xu, H.; Peng, X. Nano Lett. 2006, 6, 720. (15) Park, K. H.; Jang, K.; Kim, S.; Kim, H. J.; Son, S. U. J. Am. Chem. Soc. 2006, 128, 14780. (16) Interestingly, the appearance of powder of nanotubes was much less compact than that of big nanorods, which enable to distinguish roughly the formation of nanotubes from that of big nanorods by eye.
Figure 3. Aspect ratio control experiments: TEM images of very thin antimony sulfide nanoribbons (a), a mixture of nanoribbons having low contrast and nanotubes (b), and nanotubes (c) which were prepared using 2.25, 2.00, and 1.75 equiv of sulfur. Width distribution diagrams (d) of nanoribbons (NR) or nanotubes (NT) obtained from experiments of entries 1 (top), 2 (middle), and 3 (bottom) in Table 1. The dilute blue colored diagram at the bottom of d was calculated from the left distribution diagram.
is Sb(III)17 (Figure 2c). The powder X-ray diffraction pattern was clearly indexed to the orthorhombic phase of Sb2S3 (JCPDS no. 42-1393), which is well-known as the dominant phase of antimony sulfides (Figure 4e).9 The series of experiments in Table 1 and Figure 3 show some information about the formation mechanism of nanotubes. Actually, the nanotubes were synthesized during the delicate aspect ratio control experiments of the onedimensional Sb2S3 nanoribbons by changing the concentration of sulfur (entries 1-4 in Table 1 and Figure 3a-c). Increasing the supply of sulfur induced the growth of Sb2S3 in the longitudinal direction. The very thin and long nanoribbons in TEM image of Figure 3a were obtained by adding 2.25 equiv of sulfur to antimony chloride (0.57 mmol). These nanoribbons have a width average of 2.3 nm, a thickness of 1.8 nm, and a length of 10 µm. The top diagram in Figure 3d reveals size distribution of widths of these nanoribbons. Reducing the number of equivalents of sulfur from 2.25 to 2.0 led to a decrease in the length of the nanoribbons to 500 nm to 1 µm and an increase in the width of them to 17.5 nm (entry 2 in Table 1, Figure 3b, and width distribution diagram in middle of Figure 3d). Interestingly, in addition to the wider nanoribbons, a significant amount of nanotubes with 6.7 nm average width were also observed in some regions. Typical examples were indicated by arrows in Figure 3b. This observation implies that the nanotubes might be formed by the rolling process of the nanoribbons, which has been proposed as the usual mechanism involved in the formation of tubes from lamellar structures.10 It is reasoned that appropriately small aspect ratio is needed to form tubes. When 1.75 equiv18 of sulfur was used, the length of the nanomaterials was shortened to 100-300 nm, and (17) Vasquez, R. P.; Grunthaner, F. J. J. Appl. Phys. 1981, 52, 3509. (18) When the amount of sulfur was reduced to minimum equivalent, 1.50 equiv, to form Sb2S3, the major product was the irregularly shaped antimony oxide which can be formed by reaction of the unreacted SbCl3 with water in methanol.
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Figure 4. TEM images of nanobundles of nanorods formed at 200 °C (a) and rod-like nanbundles formed by the rapid increase of reaction temperature (b), HR-TEM image of one nanorod (c), typical SEM image of rod-like nanbundles (d), and X-ray powder diffraction patterns of nanotubes and nanobundles (e). Table 1. Preparation of Antimony Sulfide Nanomaterialsa products
entry
sulfur (equiv)
tempb (°C)
increase rate (°C/min)
major
1 2 3 4 5 6 7
2.25 2.00 1.75 1.50 1.75 1.75 1.75
175 175 175 175 150 200 175
1.67 1.67 1.67 1.67 1.67 1.67 5.00
nanowires nanoribbons nanotubes nanoparticlesc nanotubes nanobundles nanobundles
minor nanotubes nanorods nanoribbons
a A total of 0.57 mmol of antimony chloride and 9 mL of oleylamine were used. b The temperature was increased from 25 °C. c Irregularly shaped antimony oxide nanoparticles formed as major products.
nearly pure nanotubes were obtained19 (Figure 3c). The measured average diameter of tubes, 10.4 nm, can be transformed to the width of the postulated nanoribbons by multiplying it by π. The dilute blue colored diagram in the bottom of Figure 3d reveals the postulated width distribution of nanotubes which shows the average value, 32.6 nm. Considering the three blue-colored diagrams in Figure 3d, it was concluded that widths of nanoribbons were gradually controlled by variation of sulfur concentration and the appropriate aspect ratios need to form tubes. It is noteworthy that there was no change in thicknesses. Another interesting point is that some experimental conditions are critical to obtain the tubular structure. First, the range of reaction temperature is very important. Increasing the reaction temperature above 200 °C resulted in the formation of interesting nanobundles (Figure 4a). The careful investigation of these nanobundles reveals that they are formed by attachment of many rods. It is well-known that one-dimensional antimony sulfide has a strong preference for attachment to one another in the vertical direction to the (19) It is very noteworthy that the synthesis of the nanotubes is highly reproducible. In the reproducibility test, the nanotubes were prepared five times by two researchers using the optimized experimental procedures described above.
growth direction.9 In contrast, at 150 °C, relatively poor nanotubes could be obtained. Second, the rate of increase of the temperature needs to be sufficiently slow. Figure 4b,d shows the typical TEM and SEM images of the rod-like nanobundles obtained by increasing the reaction temperature at a rate of 5 °C/min (entries 3 and 7 in Table 1). The HRTEM image in Figure 4c reveals that the nanorods grew in the direction of the (001) crystal plane and that the rods have a preference to become attached to each other in the vertical direction of the (001) crystal plane. According to these observations, it was concluded that rapidly increasing the reaction temperature induces inter-rod attachment and a sufficiently low temperature and a slow rate of increase of the reaction temperature are required to induce the intraribbon attachment process required to form the tubular structures. In conclusion, Sb2S3 nanotubes with ultrathin walls (1.52.0 nm) were synthesized at a relatively low temperature (175 °C) by the wet chemical method. The aspect ratios of the Sb2S3 nanoribbons were delicately controlled by changing the concentration of sulfur, which ultimately enabled the formation of the nanotubes. We believe that this strategy can be applied to the preparation of diverse nanotubes from known inorganic nanoribbons having a (quasi) layerstructural motif, and the prepared antimony sulfide nanotubes can be used for the development of gas storage materials. Acknowledgment. This work was supported by the Hydrogen Energy R&D Center, a 21st Century Frontier R&D Program funded by the Ministry of Science and Technology of Korea. K.H.P. is grateful for a grant from the Korea Research Foundation (MOEHRD) (KRF-2005-005J11901). Supporting Information Available: Details of experimental procedures and additional TEM images (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0712772
