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Size- and Shape-Controlled Synthesis of Bismuth Nanoparticles Fudong Wang,§,‡ Rui Tang,§,‡ Heng Yu,§,† Patrick C. Gibbons,#,‡ and William E. Buhro*,§,‡ Department of Chemistry and Physics and Center for Materials InnoVation, Washington UniVersity, Saint Louis, Missouri 63130-4899 ReceiVed February 13, 2008. ReVised Manuscript ReceiVed April 2, 2008
Near-monodisperse Bi dots in the diameter range of 3-115 nm are synthesized by a simple, solutionbased one-step approach by varying the amounts of Bi[N(SiMe3)2]3, Na[N(SiMe3)2], and a polymer surfactant, poly(1-hexadecene)0.67-co-(1-vinylpyrrolidinone)0.33, employed. The reaction conditions are further modified to produce Bi nanorods and nanoplates. Alternatively, near-monodisperse Bi dots in the diameter range of 30-45 nm are synthesized by a secondary-addition technique. With a slight modification of this technique, nanoribbons are obtained. The roles of polymer and Na[N(SiMe3)2] in the size and shape control of these Bi nanoparticles are discussed.
Introduction Metal nanoparticles are of great interest due to their unique size- and shape-dependent optical,1–4 magnetic,5–8 and catalyticproperties,9–11 andpotentialapplicationsinbiosensing,12,13 information storage,5,8 catalysis,9–11 and surface-enhanced Raman scattering (SERS).14,15 Despite the extensive research activities and enormous progress in this field, the main challenges have been and remain size and morphology control, and the lack of sufficient mechanistic understanding to achieve this control.10,16–18 Here we describe synthetic procedures for preparation of size-controlled, spherical Bi * To whom correspondence should be addressed. E-mail:
[email protected]. § Department of Chemistry, Washington University. ‡ Center for Materials Innovation, Washington University. † Current address: MagArray, Inc., 450 El Escarpado, Stanford, CA 943058431. # Department of Physics, Washington University.
(1) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257–264. (2) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 116, 6755–6759. (3) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (4) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487–490. (5) Sun, S.; Murry, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (6) Sun, S.; Fullerton, E. E.; Weller, D.; Murry, C. B. IEEE Trans. Magn. 2001, 37, 1239–1243. (7) Puntes, V.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115– 2117. (8) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821–823. (9) Narayanan, R.;.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663– 12676. (10) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. Nat. Mater. 2007, 6, 692–697. (11) Xiong, Y.; Wiley, B. J.; Xia, Y. Angew. Chem., Int. Ed. 2007, 46, 7157–7159. (12) Nicewarner-Pena, S.; Freeman, R. G.; Reiss, B. D.; He, L.; Pena, D. J.; Walton, I. D.; Cromer, R.; Keating, C. D.; Natan, M. J. Science 2001, 294, 137–141. (13) Cao, Y. C.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536–1540. (14) Nie, S. M.; Emory, S. R. Science 1997, 275, 1102–1106. (15) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 554–9555. (16) Sun, Y.; Xia, Y. Science 2002, 298, 2176–2179. (17) Lisiecki, I. J. Phys. Chem. B 2005, 109, 12231–12244. (18) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Orendorff, C. J. Inorg. Chem. 2006, 45, 7544–7554.
nanoparticles (dots), and strategies for generating rod-, plate-, and ribbon-shaped Bi nanoparticles. This study may also provide a basis for gaining mechanistic insights into size and shape control of metal nanoparticles. We chose to study the Bi system for two reasons. First and most importantly, Bi dots are the best catalysts for the solution-liquid-solid (SLS) growth of diameter-controlled semiconductor quantum wires and rods.19–24 We now use such Bi nanoparticles almost exclusively for the SLS growth of semiconductor nanowires and expect that they will be generally useful to others. The motivation for this study, therefore, is to provide a diameter-controlled synthesis of near-monodispersed Bi dots over a wide diameter range, for SLS growth of near-monodispersed (in diameter) semiconductor nanowires over a similarly wide diameter range. Others are also interested in the electronic properties of Bi nanoparticles. Bulk Bi is a semimetal with unusual electronic properties (i.e., magnetoresistance, thermoelectronic characteristics) due to its highly anisotropic Fermi surface, low carrier densities (105 times smaller than conventional metals at 4.2 K), small carrier effective masses, and long carrier mean free path (as long as a millimeter at 4.2 K).25–28 The electronic properties of Bi are highly susceptible to size-induced quantum confinement effects. For (19) Yu, H.; Li, J.; Loomis, R. A.; Gibbons, O. C.; Wang, L.-W.; Buhro, W. E. J. Am. Chem. Soc. 2003, 125, 16168–16169. (20) Wang, F.; Dong, A.; Sun, J.; Tang, R.; Yu, H.; Buhro, W. E. Inorg. Chem. 2006, 45, 7511–7521. (21) Dong, A.; Wang, F.; Daulton, T. L.; Buhro, W. E. Nano Lett. 2007, 7, 1308–1313. (22) Dong, A.; Tang, R.; Buhro, W. E. J. Am. Chem. Soc. 2007, 129, 12254–12262. (23) Wang, F.; Yu, H.; Li, J.; Hang, Q.; Zemlyanov, D.; Gibbons, P. C.; Wang, L.-W.; Janes, D. B.; Buhro, W. E. J. Am. Chem. Soc. 2007, 129, 14327–14335. (24) Wang, F.; Buhro, W. E. J. Am. Chem. Soc. 2007, 129, 14381–14387. (25) Liu, K.; Chien, C. L. Phys. ReV. B 1998, 58, 681–684. (26) Heremans, J.; Thrush, C. M.; Zhang, Z.; Sun, X.; Dresselhaus, M. S.; Ying, J. Y.; Morelli, D. T. Phys. ReV. B 1998, 58, 91–95. (27) Heremans, J.; Thrush, C. M.; Lin, Y.-M.; Chroin, S.; Zhang, Z.; Dresselhaus, M. S.; Mansfield, J. F. Phys. ReV. B 2000, 61, 2921– 2930. (28) Lin, Y.-M.; Sun, X.; Dresselhaus, M. S. Phys. ReV. B 2000, 62, 4610– 4623.
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example, Bi undergoes a semimetal to semiconductor transition in nanowires when the wire diameter is decreased to about 50 nm.29 Theoretical studies suggest that Bi nanowires may exhibit an enhanced thermoelectric figure of merit, ZT, at 300 K.28 An even larger thermoelectric effect might be achieved under dimensionally more-restricted conditions such as in Bi dots and rods.28,30,31 Xia and co-workers have reported the synthesis of monodisperse Bi dots in the diameter range of 100-600 nm,32 which, however, is not in the generally useful range for the growth of semiconductor quantum wires and rods.19–24 Foos and co-workers reported the synthesis of Bi dots with diameters in the range of 3-10 nm but did not achieve monodispersity.33 In a related study, Wang, Ren, and coworkers prepared Bi dots, nanocubes, nanoplates, and nanobelts (nanoribbons) but did not address size control.34 We previously reported the preparation of near-monodispersed Bi dots using a seeded-growth method, in which very small (d ≈ 1.5 nm) Au nanoclusters served as heterogeneous nucleants for nanoparticle growth.35 This method, however, was somewhat laborious and provided a limited size range (8.5 nm < d < 12.5 nm). We subsequently discovered a convenient one-step synthesis of near-monodisperse Bi dots by using the thermal decomposition of Bi[N(SiMe3)2]3 in the presence of Na[N(SiMe3)2] but did not develop diameter control for a wide range of diameters.36 In the present study, we have modified this one-step method to afford Bi dots in the diameter range of 3-115 nm, with standard deviations in the diameter distributions of 4-19% of the nanoparticle mean diameters. The Bi dots can be made on a large scale and stored for at least a few years under an inert atmosphere for use as needed. The diameters and diameter distributions of the Bi dots are highly dependent on the relative quantities of Bi[N(SiMe3)2]3, Na[N(SiMe3)2], and a polymer stabilizer employed. Under certain conditions, nanoribbons, nanorods, and hexagonal nanoplates are also generated. The possible roles of Na[N(SiMe3)2] and the polymer stabilizer in the size and morphology control are discussed. We expect that such convenient, optimized syntheses of Bi dots will benefit research in the field of semiconductor quantum wires and may provide opportunities for studying the size- and shapedependent electronic properties of Bi nanostructures. Experimental Section Materials. Bi[N(SiMe3)2]3 was prepared according to a literature method.37 Na[N(SiMe3)2] (as a 1.0 M THF solution) was obtained (29) Dresselhaus, M. S.; Lin, Y.-M.; Rabib, O.; Jorio, A.; Souza Filho, A. G.; Pimenta, M. A.; Saito, R.; Samsonidze, G. G.; Dresselhaus, G. Mater. Sci. Eng., C 2003, 23, 129–140. (30) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47, 16631–16634. (31) Heremans, J. S.; Thrush, C. M.; Morelli, D. T.; Wu, M.-C. Phys. ReV. Lett. 2002, 88, 216801. (32) Wang, Y.; Xia, Y. Nano Lett. 2004, 4, 2047–2050. (33) Foos, E. E.; Stroud, R. M.; Berry, A. D.; Snow, A. W.; Armistead, J. P. J. Am. Chem. Soc. 2000, 122, 7114–7115. (34) Wang, W. Z.; Poudel, B.; Ma, Y.; Ren, Z. F. J. Phys. Chem. B 2006, 110, 25702–25706. (35) Yu, H.; Gibbons, P. C.; Kelton, K. F.; Buhro, W. E. J. Am. Chem. Soc. 2001, 123, 9198–9199. (36) Yu, H.; Gibbons, P. C.; Buhro, W. E. J. Mater. Chem. 2004, 14, 595– 602.
from Aldrich packaged under N2 in Sure/Seal bottles. Poly(1hexadecene)0.67-co-(1-vinylpyrrolidinone)0.33 (also known as poly(1vinylpyrrolidone)-graft-(1-hexadecene) by Aldrich) was used as received from Aldrich. The solvent 1,3-diisopropylbenzene (DIPB) was purchased from Aldrich, shaken with concentrated sulfuric acid to remove thiophene, neutralized with K2CO3, washed with water, and distilled over Na.36 A 25 wt % poly(1-hexadecene)0.67-co-(1vinylpyrrolidinone)0.33 solution in DIPB (polymer-DIPB solution) was dried over molecular sieves at least one week with frequent shaking prior to use.36 Polymer-DIPB solutions with lower concentrations were prepared by diluting the 25 wt % stock solution with DIPB. Other reagents were used as received. Synthesis of Bi Dots. All synthetic steps were conducted under dry, O2-free N2(g). The reaction conditions (i.e., quantities of reagents, reaction temperature, and reaction time) for a variety of Bi dot diameters are recorded in Table 1. In a typical synthesis of 7.1 nm diameter (standard deviation ) (11%) Bi dots, Bi[N(SiMe3)2]3 (303 mg, 0.44 mmol) and Na[N(SiMe3)2] (1608 mg of 1.0 THF solution, 1.78 mmol) were combined with the 25 wt % polymer-DIPB solution (10 g) in a Schlenk reaction tube to generate a pale-red solution. This solution was then inserted into a temperature-controlled oil bath at 180 °C with stirring, whereupon the solution turned red then black very quickly (∼1-2 min). The final solution (after 17 h) contained a deep black dispersion of Bi dots. The Bi dots were stored without isolation in the synthesis mixture under inert atmosphere and remained stable for a few years. For the subsequent use in the synthesis of semiconductor quantum wires and rods, the Bi dots were typically not isolated from the reaction mixture. Rather, the mixture was used as a stock solution. The Bi dots could precipitate from the solution but were readily redispersed upon gentle shaking. Synthesis of Bi Nanorods and Nanoplates. The nanorods and nanoplates were synthesized using the same procedure as above. The quantities of reagents and reaction conditions used are recorded in Table S1 (Supporting Information). Synthesis of Bi Dots Using the Secondary-Addition Technique. A mixture containing Bi[N(SiMe3)2]3 (105 mg, 0.15 mmol), Na[N(SiMe3)2] (1023 mg of 1.0 M THF solution, 1.13 mmol), and 25 wt % polymer solution (10 g) was preheated at 210 °C for 2 h before the dropwise addition of a mixture of Bi[N(SiMe3)2]3 (200 mg, 0.29 mmol) and the 25 wt % polymer solution (2 g) from a syringe. The addition was finished in 1 h. Stirring at 210 °C was continued for an additional 15 h, resulting in the 32.1 nm diameter (standard deviation ) (6%) Bi dots. Similarly, for the synthesis of the 44.2 nm diameter (standard deviation ) (6%) Bi dots, a mixture containing Bi[N(SiMe3)2]3 (110 mg, 0.16 mmol), Na[N(SiMe3)2] (205 mg of 1.0 M THF solution, 0.23 mmol), and 25 wt % polymer solution (8 g) was preheated at 210 °C for 1 h before the dropwise addition of a mixture of Bi[N(SiMe3)2]3 (200 mg, 0.29 mmol), and the 25 wt % polymer solution (4 g) from a syringe. The addition was finished in 2.3 h. Stirring at 210 °C was continued for an additional 16 h. Synthesis of Bi Nanoribbons. A mixture containing Bi[N(SiMe3)2]3 (16 mg, 0.023 mmol), Na[N(SiMe3)2] (203 mg of 1.0 M THF solution, 0.22 mmol) and 12.5 wt % polymer solution (8 g) was preheated at 210 °C for 35 min before the dropwise addition of a mixture of Bi[N(SiMe3)2]3 (336 mg, 0.49 mmol) and the 12.5 wt % polymer solution (4 g) from a syringe. The addition was finished in 2.2 h. Stirring at 210 °C was continued for an additional 17 h, resulting in a grayish black suspension. (37) Carmalt, C. J.; Compton, N. A.; Errington, N. J.; Fisher, G. A.; Moenandar, I.; Norman, N. C. Inorg. Synth. 1996, 31, 98–101.
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Figure 2. Bi nanoparticle diameter as a function of Na[N(SiMe3)2]/ Bi[N(SiMe3)2]3 molar ratio at various temperatures. The solid lines refer to experiments using 0.44 mmol of Bi[N(SiMe3)2]3 and the dashed line to one using 0.15 mmol of Bi[N(SiMe3)2]3. Additional ∼1-3 nm diameter Bi dots were formed at the low Na[N(SiMe3)2]/Bi[N(SiMe3)2]3 ratios (
