J. Phys. Chem. C 2010, 114, 3431–3440
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Optimization of Routes for the Synthesis of Bismuth Nanotubes: Implications for Nanostructure Form and Selectivity Salim Derrouiche, Codruta Zoican Loebick, and Lisa Pfefferle* Department of Chemical Engineering, Yale UniVersity, P.O. Box 208286, New HaVen, Connecticut 06520-8286 ReceiVed: NoVember 17, 2009; ReVised Manuscript ReceiVed: January 9, 2010
In this paper, we demonstrate a new route for synthesis of bismuth nanotubes (BiNTs) based on chemical reduction using sodium borohydride. A comparison and an optimization of different synthesis procedures including hydrothermal and solvothermal synthesis for the production of BiNTs are presented. The effects of parameters such as the bismuth precursor, temperature, and reagent mixing process on nanomaterial structure were evaluated. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy were employed to characterize the products from the different reaction schemes. A postsynthesis sodium borohydride treatment improved both yield and nanotube structure for all syntheses studied. Different morphologies of the bismuth nanostructures were observed depending on the synthesis method selected. 1. Introduction 1
Since the discovery by Iijima in 1991 of carbon nanotubes, much interest has been directed toward the synthesis of nanostructures, especially nanowires, nanopipes, and nanotubes because of their one dimensionality. To date, much effort has been directed to the synthesis of carbon nanotubes, but nanotubes of other compositions are increasingly investigated because of their promising physicochemical and electronic properties. Over the past 15 years, bismuth nanostructures such as thin films,2 nanowires,3 nanolines,4 nanobelts,5 and recently nanopipes and nanotubes6 have been extensively studied because of their very large magneto resistance and excellent thermoelectric properties.7 The electronic structure of bulk bismuth (high anisotropy in the Bi electronic band structure) is now well documented. Due to a small electron effective mass ( solvothermal. A note should be made on the stability against oxidation of the Bi nanotubes synthesized in this study. Slow oxidation of the bismuth nanotubes has been evidenced after a long exposure of the sample to air. One should store samples either in a vacuum chamber in order to increase their lifetime or in a dichloromethane solution. In this study, the time between the synthesis of the bismuth nanotubes and their characterization was kept as short as possible. XRD experiments performed 5 days after the synthesis of bismuth nanotubes exposed to air still exhibit diffraction peaks of the rhombohedral bismuth, indicating that
Synthesis of Bismuth Nanotubes
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Figure 11. TEM images of BiNTs synthesized following route H using NaOH (a-c) and a successive addition of NH4OH and NaOH (d-f) as pH-adjusting reagent: (a-c) (a) before and (b and c) after NaBH4 treatment and (d-f) after NaBH4. (Inset in c and d) HRTEM detail of the tube ends.
the bismuth nanotubes are not significantly affected by the oxygen from air. However, EDS analysis performed 10 days after the synthesis of a bismuth nanotubes sample shows a significant amount of oxygen. This indicates that the bismuth nanotubes are air sensitive. 4. Conclusion In summary, we described a simple new method to obtain bismuth nanotubes with a high yield following chemical reduction (CR) using sodium borohydride as a reducing reagent. Other synthesis routes (solvothermal, S, and hydrothermal, H) from the literature were optimized and compared to the CR method. A postsynthesis treatment consisting of reacting the as-synthesized sample with a sodium borohydride solution greatly improved the nanotubes structure and increased their yield for all processes studied. Among the various methods to synthesize BiNTs, the chemical reduction method has the advantages of being more convenient, cheaper, and more scalable for mass production purpose. Here, we summarize important findings. (1) BiNT arrays were obtained by syntheses S and CR-RT, while isolated single-wall BiNTs were obtained using syntheses H and CR-HT. Temperatures above ambient have been found to be kinetically favorable to the growth process of isolated BiNTs following synthesis CR. (2) We found that the length of the BiNT arrays obtained following synthesis S could be engineered by the correct choice of the reagent mixing process, providing tube arrays in the range 0.02-5 µm in length. The diameter of the BiNTs has been found to be independent of the type of synthesis route, 4-5 nm. (3) Characterization of the synthesized structures through TEM, EDS, XRD, and Raman techniques indicates that BiNT arrays and single-wall BiNTs are consistent with the basic rhombohedral lattice structure of bismuth, indicating a complete reduction Bi3+ f Bi0. (4) The optimized hydrothermal synthesis produced long isolated nanotubes at high yield, allowing the cylindrical tube ends to be observed in TEM. (5) When comparing different synthesis
procedures, it was found that the major Raman peak obtained is red shifted from 198 cm-1 for bulk bismuth to 193 cm-1 for sheet-like structures and then to 185-187 cm-1 for BiNTs. These shifts are small but reproducibly detectable. The position observed at 187 cm-1 was found only for synthesis leading to neat unbundled BiNTs (CR-HT and the hydrothermal route with PR2). To our knowledge, this is the first time that this shift has been observed, and it was tentatively attributed to the formation of 1-D bismuth nanotubes. Acknowledgment. The authors thank Dr. C. Wang and Pr. G. L. Haller of the Department of Chemical Engineering at Yale University for fruitful discussions. We gratefully acknowledge financial support from DOE grant BES DE-FG02-06ER46322 and the Air Force grant FA9550-081-0309. Supporting Information Available: Chemistry of bismuth reduction by sodium borohydride, XRD analysis of BiNT arrays synthesized by the solvothermal-stirring route (S-ST) after treatment with NaBH4, TEM images of bismuth oxide samples, and Bi2O3-n-780, as synthesized and after 24 h of treatment with NaBH4 (route CR-RT). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iijama, I. Nature 1991, 354, 56–58. (2) Kim, D. H.; Lee, S. H.; Kim, J. K.; Lee, G. H. Appl. Surf. Sci. 2006, 252, 3525–3531. (3) Dresselhaus, M. S.; Lin, Y. M.; Rabin, O.; Jorio, A.; Souza Filho, A. G.; Pimenta, M. A.; Saito, R.; Samsonidze, G.; Dresselhaus, G. Mater. Sci. Eng. C 2003, 23, 129–140. (4) Miki, K.; Owen, J. H. G.; Bowler, D. R.; Briggs, G. A. D.; Sakamoto, K. Surf. Sci. 1999, 421, 397–418. (5) Chen, Y.; Gong, R.; Zhang, W.; Xu, X.; Fan, Y.; Liu, W. Mater. Lett. 2005, 59, 909–911. (6) Li, Y.; Wang, J.; Deng, Z.; Wu, Y.; Sun, X.; Yu, D.; Yang, P. J. Am. Chem. Soc. 2001, 123, 9904–9905. (7) Wang, J.; Cao, G.; Li, Y. Mater. Res. Bull. 2003, 38, 1645–1651. (8) Hartman, R. Phys. ReV. 1969, 181, 1070–1086.
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